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1 Basic Concepts of Robust Design Optimization (RDO) WOST15, 2018 Veit Bayer Christian Bucher Dynardo, Weimar, Vienna

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Page 1: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

1

Basic Concepts of Robust Design

Optimization (RDO)

WOST15 2018

Veit Bayer

Christian Bucher

Dynardo Weimar Vienna

2Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview

Introduction

Definitions of Probability

Random Variables and Vectors

Simulation

Estimation

Robustness and Reliability Analysis

Variance-based sigma levels reliability-based

Methods of Reliability Analysis

Examples

3Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Introduction

4Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

Design ImprovementOptimize design performance

copy Dynardo GmbH

5Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

1 Search optima with

flat surrounding

(stability)

6Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

2 Keep safety distance

from infeasible domain

(safety quality)

7Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Probability

8Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Consider a sample space the set of all possible events (or all possible outcomes of basic variables)

bull Event that one realization of parameters falls into subdomain

Kolmogorov axioms

Events and Probabilities

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

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Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

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Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

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Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

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Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

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Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

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Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

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Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 2: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

2Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview

Introduction

Definitions of Probability

Random Variables and Vectors

Simulation

Estimation

Robustness and Reliability Analysis

Variance-based sigma levels reliability-based

Methods of Reliability Analysis

Examples

3Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Introduction

4Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

Design ImprovementOptimize design performance

copy Dynardo GmbH

5Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

1 Search optima with

flat surrounding

(stability)

6Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

2 Keep safety distance

from infeasible domain

(safety quality)

7Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Probability

8Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Consider a sample space the set of all possible events (or all possible outcomes of basic variables)

bull Event that one realization of parameters falls into subdomain

Kolmogorov axioms

Events and Probabilities

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 3: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

3Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Introduction

4Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

Design ImprovementOptimize design performance

copy Dynardo GmbH

5Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

1 Search optima with

flat surrounding

(stability)

6Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

2 Keep safety distance

from infeasible domain

(safety quality)

7Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Probability

8Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Consider a sample space the set of all possible events (or all possible outcomes of basic variables)

bull Event that one realization of parameters falls into subdomain

Kolmogorov axioms

Events and Probabilities

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 4: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

4Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

Design ImprovementOptimize design performance

copy Dynardo GmbH

5Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

1 Search optima with

flat surrounding

(stability)

6Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

2 Keep safety distance

from infeasible domain

(safety quality)

7Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Probability

8Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Consider a sample space the set of all possible events (or all possible outcomes of basic variables)

bull Event that one realization of parameters falls into subdomain

Kolmogorov axioms

Events and Probabilities

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 5: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

5Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

1 Search optima with

flat surrounding

(stability)

6Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

2 Keep safety distance

from infeasible domain

(safety quality)

7Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Probability

8Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Consider a sample space the set of all possible events (or all possible outcomes of basic variables)

bull Event that one realization of parameters falls into subdomain

Kolmogorov axioms

Events and Probabilities

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 6: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

6Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robust Design Optimization (RDO) optimizes the design performance while taking into account scatter of design (optimization) variables and other tolerances or uncertainties

bull As a consequence of input scatter the location of the optima as well as the contour lines of constraints may vary

Uncertainties in Optimization

2 Keep safety distance

from infeasible domain

(safety quality)

7Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Probability

8Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Consider a sample space the set of all possible events (or all possible outcomes of basic variables)

bull Event that one realization of parameters falls into subdomain

Kolmogorov axioms

Events and Probabilities

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 7: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

7Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Probability

8Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Consider a sample space the set of all possible events (or all possible outcomes of basic variables)

bull Event that one realization of parameters falls into subdomain

Kolmogorov axioms

Events and Probabilities

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 8: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

8Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Consider a sample space the set of all possible events (or all possible outcomes of basic variables)

bull Event that one realization of parameters falls into subdomain

Kolmogorov axioms

Events and Probabilities

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 9: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

9Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Complimentary events

bull An event can happen ( ) or not happen ( )

bull Complimentary events cannot happen at the same time

Events and Probabilities

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 10: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

10Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Conditional Probability

Independence

Events and Probabilities

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

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Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

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Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

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Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

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Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

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Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

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Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

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Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

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Further Reading

Page 11: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

11Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Total Probability

Bayesrsquo Theorem

Purpose Testing model updating conclude from a measurement eg to product safety or quality

Decomposition of Event Space

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

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Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 12: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

12Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Random Variables

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 13: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

13Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Statistical Characterization of Random Variables

bull Expectation operator

bull Mean value

bull Variance

bull Coefficient of variation

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 14: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

14Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Probability Distribution and Density Function

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 15: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

15Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Distribution Types

Uniform Normal Log-normal

Exponential Weibull Rayleigh

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 16: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

16Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Statistical Characterization of Random Vectors

bull Arbitrary number k of random variables can be arranged in a vector

bull Mean value vector

bull Coefficient of correlation between two random variables

bull Covariance matrix of a random vector

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 17: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

17Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Random generators produce numbers uniformly distributed in [01]

bull Mapping to prescribed marginal distribution

copy Dynardo GmbH

Simulation of Random Variables

fU

u FX

fX

x

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 18: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

18Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull For each random variable the original marginal distribution is

transformed to an uncorrelated standard normal variable by the CDF

bull Assume a correlated joint Normal distribution for the random vector

bull Iterate the correlation coefficients of Zi Zj to match the original ones

copy Dynardo GmbH

Simulation of Random Vectors

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 19: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

19Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Example

copy Dynardo GmbH

Simulation of Random Vectors

Standard normal space Original space

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 20: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

20Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Estimation

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 21: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

21Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Estimate an unknown parameter from independent observations

bull Example mean value

bull Consistency

bull (Asymptotic) Unbiasedness

Remarks

bull The true parameter is usu not known the available information is the

sample

bull Any estimate from a finite sample contains statistical uncertainty

which can be reduced by an increased sample size

copy Dynardo GmbH

Estimation

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 22: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

22Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull An estimator from a random sample is a random variable by itself

bull Variance of the estimator

bull Estimator for variance

bull Estimate the variance of the estimator

serves to assess the confidence of the estimate

copy Dynardo GmbH

Estimator Variance

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 23: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

23Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Statistical error (or standard error) of the estimator

bull If the distribution of the error is known (eg assume Normal)

then the confidence interval can be established

copy Dynardo GmbH

Confidence Interval

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 24: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

24Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robustness Analysis

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 25: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

25Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Intuitively The performance of a robust design is largely unaffected by random perturbations

bull Variance indicator The coefficient of variation (CV) of the objective function andor constraint values is not greater than the CV of the input variables

bull Sigma level The interval mean+- sigma level does not reach an undesired performance (eg design for six-sigma)

bull Probability indicator The probability of reaching undesired performance is smaller than an acceptable value

How to Define the Robustness of a Design

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 26: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

26Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Robustness in terms of limits

bull Safety margin (sigma level) of one or more responses y

bull Reliability (failure probability) with respect to given limit state

Robustness in terms of stability

bull Performance (objective) of robust optimum is less sensitive to input uncertainties

bull Minimization of statistical evaluation of objective function f (eg minimize mean andor standard deviation)

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 27: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

27Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Taguchi loss functions

bull Target value m is optimal (k scaling factor for costs)

bull Minimum is optimal (requires positive objective)

bull Maximum is optimal (requires strictly positive objective)

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 28: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

28Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Variance based Robustness Analysis

1) Define the robustness space using scatter range distribution and correlation

2) Scan the robustness space by producing and evaluating ndesigns

3) Check the variation 4) Check the

explainability of the model

5) Identify the most important scattering variables

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 29: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

29Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Exceedance Probability

bull Probability of reaching values above a limit for Gaussian distribution

m x

fX(x)

x

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 30: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

30Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Sigma Level vs Failure Probability

bull The sigma level can be used to estimate the probability of exceeding

a certain response limit

bull Since the distribution type of the response is generally unknown

this estimate may be very inaccurate for small probabilities

(sigma levels larger than 3)

bull The sigma level deals with single limit values whereas the failure

probability quantifies the event that any of several limits is exceeded

Reliability analysis should be applied to proof the required safety level

Distribution Required sigma level (CV=20)

pF = 10-2 pF = 10-3 pF = 10-6

Normal 232 309 475

Log-normal 277 404 757

Rayleigh 272 376 611

Weibull 203 254 349

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 31: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

31Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Example Optimized Damped Oscillator

bull Robustness evaluation at

the deterministic optimum

bull Mass m damping ratio D stiffness k and initial kinetic energy Ekin

taken as normally distributed random variables

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 32: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

32Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull Robustness analysis with respect to damped eigen-frequency and

maximum amplitude

ndash Check CV of objective and constraints

ndash Check if safety constraint safety = 85 rads

is outside of 45 level

ndash Check importance of input variables

ndash Check explainability by MOPCoP

Example Damped OscillatorVariance based Robustness Analysis

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 33: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

33Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Constraint equation (omega)

bull CoD and CoP is 100

bull k is most important m is minor

bull Mean is close to deterministic

value

bull CV is 27

bull Safety limit is 238 which is

smaller as the required 45

Optimum is not robust in terms of

the constraint condition

Example Damped Oscillator

238

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 34: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

34Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Objective function (xmax)

bull CoD and CoP is 92

bull D is most important Ekin

and k are minor important

bull Mean is not close to

deterministic value

bull CV is 110

Optimum is not robust in terms

of the objective function

Example Damped Oscillator

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 35: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

35Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Reliability Analysis

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 36: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

36Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Concept of Safety

bull Failure occurs if loading S exceeds the resistance R

bull Probability of failure

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 37: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

37Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbHcopy Dynardo GmbH

Partial Safety Factors

bull Definition of characteristical values for loading Sk and resistance Rk

bull Design values are obtained by

using partial safety factors

bull Final safety proof

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 38: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

38Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

bull The complementary of reliability the probability of failure PF is computed (for numerical reasons)

bull PF is the probability of the event that a set of (stochastic) input parameters leads to an inadmissible state of the analyzed system

(eg exceedance of allowable stress)

bull The limit state function g(x) separates the random variable space Xinto a safe domain g(x) gt 0 and failure domain g(x) le 0

bull Multiple failure criteria (limit state functions) are possible

bull Series system

fails if one single component fails

g(x) = mini (gi (x))

bull Parallel system

fails if all components fail

g(x) = maxi (gi (x))

copy Dynardo GmbH

Reliability Analysis

FF

G

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 39: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

39Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Failure Probability

bull The probability of failure is the integral of the joint probability density

function over the failure domain

bull By introducing an indicator function

I(g(x)) = 1 if (g(x)) lt 0 I(g(x)) = 0 else

this can be computed as the expected value of I

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 40: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

40Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Monte Carlo Simulation

bull Robust for arbitrary limit state functions

bull Confidence of the estimate is very low for small failure probabilities

Sigma level le 2

Independent of number of random variables

X1

X2

g=0

Sigma

level

PF N for cov(PF) = 10

2 23E-2 4 400

3 13E-3 74 000

45 34E-6 29 500 000

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 41: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

41Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

First Order Reliability Method (FORM)

bull Operates in the space of

standardized Gaussian variables

bull Search for failure point with

maximum probability density

(design point)

bull Equals the point in U on the limit state surface with minimal

distance to origin

bull Limit state function is linearized

around design point

bull Then failure probability can be

calculated analytically

bull Distance to origin (in U) is called

reliability index b

bull Can be interpreted as

generalization of sigma level

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 42: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

42Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling

bull Guide the sampling by making use of information about the failure

domain in order to increase the amount of failure events

bull To warrant correct statistics each sample is weighted by the ratio of

original to sampling density

bull Different strategies exist to estimate an ldquooptimalrdquo sampling density

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 43: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

43Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Importance Sampling Using Desing Point (ISPUD)

bull Based on FORM

bull Sampling density is centered at the design point

Requires continuously differentiable limit state function

Multiple design points (local minima) are not supported

May be able to mitigate error due to linearization in FORM

(oscillating limit state surface)

Moderate number of random variables

g(X) = 0

design point

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 44: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

44Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Importance Sampling

bull Sampling density is defined by mean value vector and covariance

matrix of samples in the failure domain

bull Search for dominant failure region by 2-3 sampling iterations

Applicable for non-smooth and even discontinuous limit state functions

Limited to small to medium number of random variables

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 45: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

45Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Directional Sampling

bull Radial search for multiple ldquostar-shapedrdquo failure regions

Applicable for non-smooth and even discontinuous limit state functions

Limited to small number of random variables

Few unsuccessful solver calls possible (as long as search is successful)

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 46: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

46Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Adaptive Response Surface Method

bull The limit state function is approximated by an Adaptive Response

Surface Method using a Moving Least Squares model

bull Directional Sampling is performed on the Response Surface

bull Additional supports are added near the limit state surface in regions of

high probability density

Applicable to a wide range of limit state functions

Efficient for a moderately high number of random variables

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 47: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

47Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Overview of Methods

Recommended area of application

Approach Non-linearity Failure domains No parameters No solver runs

Monte Carlo

Simulation

arbitrary arbitrary many gt10^4 (3 sigma)

gt10^7 (5 sigma)

Directional

Sampling

arbitrary arbitrary lt= 10 1000-5000

Adaptive Importance

Sampling

arbitrary one dominant lt= 10 500-1000

FORM SORM

ISPUD

monotonic one dominant lt= 20 200-500

Adaptive Response

Surface Method

continuous few dominant lt= 20 200-500

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 48: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

48Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVerification of Robust Design by Reliability Analysis

bull Safety margin of 45 is equivalent to a failure probability of 3410-6

if responses were normally distributed

Reliability

Method Samples Failure probability Error Beta

FORM 65 1310-6 - 47

Adaptive Sampling 1500 1310-6 8410-8 47

Directional Sampling 600 1310-6 4910-7 47

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 49: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

49Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for Best Practice

bull If there is no qualified information about the random parametersonly variance-based robustness evaluation is justified

bull Results with high sigma levels must be verified by reliability analysis

bull Choose proper reliability method due to dimension reliability level solver behavior

bull Reliability results shall be confirmed by a second method

bull When a reduced parameter set is used a confirmation with full parameter set is required

bull Use MOP (based on robustness samples) in order to

bull Monitor sampling

bull Monitor solver behavior

bull Analyze cause for non-robustness

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 50: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

50Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Recommendations for best practice

bull The Robustness Wizard of optiSLang guides through the setup and helps with the decision for the method based on

bull Knowledge about uncertainty

bull Number of failed designs

bull Solver behavior

bull Sigma level

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 51: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

51Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Robust Design Optimization

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 52: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

52Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Model CalibrationsIdentify important model parameter

for the best fit between simulation

and measurement

Design ImprovementOptimize design performance

Design QualityEnsure design robustness

and reliability

Design QualityEnsure design robustness

and reliability

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

Design UnderstandingInvestigate parameter sensitivities

reduce complexity and

generate best possible meta models

CAE-Data

Measurement

Data

Robust Design

copy Dynardo GmbH

Design ImprovementOptimize design performance

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 53: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

53Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Iterative Robust Design Optimization

bull Decoupled optimization and

robustnessreliability analysis

bull For each optimization run the

safety margins are adjusted for

the critical model responses

bull Applicable to variance- and

reliability-based RDO

In our implementation variance-

based robustness analysis is

used inside the iteration and a

final reliability proof is performed

for the final design

Definition of

design and

stochastic

variables

Sensitivity

analysis

Design

failure

Update

constraints

Deterministic

optimization

Variance-

based

robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 54: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

54Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Iterative RDO

bull Safety margin of 45s to safety limit safety=85 rads

Sigma level requires (safety-mean) ge 45

Deterministic constraint is modified iteratively

Step 1 Step 2 Step 3 hellip

Optimization (global ARSM) Robustness (100 ALHS)

Constraint m k xmax Mean Sigma Sigma level

le 8 078 500 799 025 799 022 233

le 7 104 500 694 029 694 019 82

le 763 086 491 756 028 755 020 466

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 55: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

55Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled Robust Design Optimization

bull Fully coupled optimization and robustnessreliability analysis

bull For each design during the optimization procedure (nominal design)

the robustnessreliability analysis is performed

bull Applicable to variance- reliability- and Taguchi-based RDO

Our efficient implementation uses small sample variance-based

robustness measures during the optimization and a final

(more accurate) reliability proof

But still the procedure is often not applicable to complex CAE models

Definition of

design and

stochastic

variables

Sensitivity

analysisOptimization

Robustness

evaluation

Final

reliability

proof

Optimal and

robust

design

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 56: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

56Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Coupled RDO in optiSLang

bull Nested loop enables the coupled RDO

bull Optimizer has to handle statistical

errors of inner robustness analysis

bull Sigma level as constraint

See tutorial HelpTutorialsOscillatorOscillator_Robustness

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 57: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

57Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

Example Damped OscillatorVariance-based Fully Coupled RDO

bull ARSM + robustness analysis

bull For each design 1+20 solver runs

bull Definition of robustness constraint for optimization procedure (safety-mean)-45s ge 0

Robust Design Optimization (ARSM+LHS)

m k xmax Mean Sigma Sigma level

ARSM+20LHS 087 500 028 76 020 45

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 58: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

58Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

copy Dynardo GmbH

bull Approximation of model responses in

mixed optimizationstochastic space

bull Simultaneous RDO is performed on

a global response surface

bull Applicable to variance- reliability-

and Taguchi-based RDO

bull Approximation quality significantly

influences RDO results

Final robustnessreliability proof

is required

bull Pure stochastic variables have small

influence compared to design variables

Important local effects in the stochastic

space may be not represented

RDO on Global Response Surface

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading

Page 59: Basic Concepts of Robust Design Optimization (RDO) · Basics Concepts of Robust Design Optimization 4 WOST 15, Weimar, June 21, 2018 ... Taguchi loss functions • Target value m

59Basics Concepts of Robust Design Optimization

WOST 15 Weimar June 21 2018

C Bucher Computational Analysis of Randomness in Structural

Mechanics Taylor amp Francis 2009

copy Dynardo GmbH

Further Reading