niweek2011 advanced motion control for machine automation

8/3/2019 NIWeek2011 Advanced Motion Control for Machine Automation

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The Secrets of TuningAdvanced Control Strategies

for Servo Systems

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George Ellis

Chief Engineer, Servo Systems

Kollmorgen Corp.

Alex ZaatariGraduate Student, Modeling of Dynamic Systems and Controls

University of Texas, Austin


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What well talk

about today Classical Control Theory

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Auto Tuning

Four Tuning Secrets

Deploying Your Control System to Hardware


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Some Things Controls Can Help With

Selecting controls equipment

Configuring controllers

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Designing transmissions and end effectors

Diagnosing problems in the field

Predicting performance before a machine is built


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The Simple LoopThe Basis of Classical Controls

R(s) C(s)G(s)

H s

+

-

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R(s) C(s)H(s)G(s)1

G(s)+


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The Simple LoopThe Basis of Classical Controls

R(s) C(s)G(s)

H s

+

-

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R(s) C(s)H(s)G(s)1

G(s)+


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The Basis of Classical Controls

R(s) C(s)G(s)

H (s)

+

-

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R(s) C(s)H(s)G(s)1

G(s)

+

Keep G(s).H(s) far away from 1!


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An Example Control Loop:

A Servo System Velocity LoopVelocity

PICurrent Motor

Filters

Position

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ommanoa

Position

sensorFilters Differentiation

Velocity

Feedback


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G(s)

A Motor Controller

Velocity

CommandPI

Current

Loop

Motor

& LoadFilters

Position

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H(s)

Position



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Calculation:The academic way to get G(s) x H(s)

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H(s)

G(s)

Velocity

Command

P I

C u r r e n t

L o o p

M o t o r

& L o a d

P o s i t i o n

s e n s o r

F i l t e r s

F i l t e r s

D i f f e r e n t i a t i o n


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If calculation isnt practical,

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what about measurement?


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Open Loop Measurement:

(The old way to measure G(s) x H(s))

R(s) C(s)+

-

G(s)Error(s)

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H (s)Feedback(s)

G(s) x H(s) = Feedback(s)Error(s)


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Error(s)Excitation

How We Do It Now!1. Inject an excitation signal into the closed loop

2. Measure Error(s) and Feedback(s)

3. Calculate the open loop

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R(s) C(s)G(s)

H (s)

-

Feedback(s)

G(s) x H(s) = Feedback(s)

Error(s)


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A Demonstration:

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Velocity Command

Well-Behaved: Rapid, Stable Response

2 mSec settling

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VL.KP = 0.1

VL.KI -= 10


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GH for a Well Behaved ServoFocus in

this

range

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Keep G(s).H(s)Keep G(s).H(s)

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ar away rom0dB-180!ar away rom 1!=


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Gain = 0dB

GH for a Well-Behaved Servo

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Phase = -180

Wide separation

(Ratio ~ 3)


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Another Demonstration:Marginal Stability Induced

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with the Phase Lagof a Low-Pass Filter


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G(s)

Velocity

CommandPI

Current

Loop

Motor

& LoadFilters

Add a Low-Pass Filter to the System

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H(s)

Position


350 Hz, 2-pole

Low-pass


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Velocity Command

Velocity Feedback

Adding a Low-Pass Filter

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Gain = 0dB

For reference:

GH for a Well Behaved Servo

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Phase = -180

Wide separation

(Ratio ~ 3)


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Gain = 0dB

GH and Marginal Stability

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Keep G(s).H(s) far away from 0dB-180!

Phase = -180

Almost no separation!


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The 1st Rule of Controls

Minimize phase lag

Specify products with fast sampling

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When tuning, use filters only when you need them


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Rigid vs. Compliant

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Motor Load

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Compliant TransmissionsBelts and pulleysLead screws

Shaft couplings

Gear boxes

Shafts


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A Simple Example:

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7x Load

Rigid coupling


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GH for a (pretty) Well-Behaved Servo

3 mSec settling Velocity FeedbackVelocity Command

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(For ref: retuned & used a 600 Hz Low-Pass Filter)

VL.KP = 0.3

VL.KI -= 10


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Gain = 0dB

Still have a

wide separation

But not in high

Frequencies1400 Hz!

GH for a (pretty) Well-Behaved Servo

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Phase = -180

in low requencies

(For ref: used a 600 Hz Low-Pass Filter)


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(For ref: used a 600 Hz Low-Pass Filter)

0.75ms ~ 1400 Hz!


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Another Simple Example:

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7x Load

Compliant coupling


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GH for a Not-So-Well-Behaved Servo

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This system has the exact same tuning as the previous example!


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Gain = 0dB

GH for a Not-So-Well-Behaved Servo

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Phase = -180

This system has the

exact same tuning asthe previous example!


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Autotuning

The process of

1 Measurin the motor/load

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characteristics2) Applying gains and filter

settings to match the motor &

load


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G(s)

Velocity

CommandPI

Current

Loop

Motor

& LoadFilters

What Autotuning Adjusts

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H(s)

Positionsensor

Filters Differentiation

+ Position Loop Gains

+ Acceleration, Velocity Feed-forward

+


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Q. How Does Autotuning Work?A. It Depends.

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1. Previous Generation Autotune

2. New Generation Autotune


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Previous Generation Autotune

Use the motor to shake the load at low frequency andcalculate the load inertia

Make assumptions about compliance effects at higher

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frequenciesSet the tuning gains based on the measured load

inertia and the assumptions

Results were only as reliable as theassumptions!


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New Generation Autotune

Excite the load at the entire frequency range

Measure the motor/load characteristics across the

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entire range of frequenciesSet the tuning gains based on the full set of

measurements

Results are much more reliable.


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17 mSec settling

A Well-Tuned

(if not so-well-designed) System

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Spring effect


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Autotuning Can Adjust a Lot of Gains

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Gain = 0dB

Results for Auto-Tuned System

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Phase = -180


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Taking Controls to the

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Hardware


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Free standing driveWindows

Host- PC

Drive and

Motor

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Torque

Control

Loop

Position

Control

Loop

Trajectory

Generator

Velocity

Control

Loop


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Deployment Options Motion on WindowsWindows

Host- PC

cRIO Ethernet

ExpansionBackplane

Drive Interface

Hardware

Drive and

Motor

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Position

Control

Loop

Trajectory

GeneratorTorque

Control

Loop

Velocity

Control

Loop


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Four Secrets of Tuning

1. Think in terms of frequency. You can measure thefrequency response with modern drives!

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2. Start with the best mechanical design you can afford.

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3. You need autotuning that works over the entirefrequency range.


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3. You need autotuning that works over the entirefrequency range.

4. You can deploy your control system to standard

hardware


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For more on the topic

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niweek2011 advanced motion control for machine automation

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