fraunhofer - experimental modal analysis

8/13/2019 Fraunhofer - Experimental Modal Analysis

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Experimental Modal Analysis of a

Machine Tool

LMS User Conference 2007

Stuttgart, 17th and 18th April 2007

Prof. Dr.-Ing. Christian Brecher

Dipl.-Ing. Torsten Gerrath


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Outline

! Fraunhofer-Institute for Production Technology IPT and

Laboratory for Machine Tools and Production Engineering

(WZL)

Introduction

The measurement setup

Approximation of the machine structure

Demonstration of the measurement results and derivation

of improvements


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RWTH Aachen and Fraunhofer-Gesellschaft

RWTH Aachen Unversity

!

Founded in 1870! 30,190 students

Faculty of Mechanical Engineering

! 6,733 students (incl. 1,400 first year

students)! 50 professors

! 2,200 employees

! 170 graduates per year

Fraunhofer-Gesellschaft! 80 institutes und facilities at over 40 locations

in Germany

! 12,500 employees

! approx. 1 billion research funds per year,

over 900 million through research contracts! 3 institutes in Aachen


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Production Technology in Aachen

Laboratory for

Machine Tools and Production Engineering (WZL)

! RWTH Aachen University institute

! Founded in 1906

! 600 employees

! 10,000 m offices and laboratories

Fraunhofer Institute for Production Technology IPT! Fraunhofer-Gesellschaft institute

! Founded in 1980

! 250 employees

! 3,000 m offices and laboratories

!Certified to DIN EN ISO 9001:2000

! Partner in Boston/USA:

Fraunhofer Center for Manufacturing Innovation CMI


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Our Focus

Process Technology

! Machining and material

removal processes

! Laser materials processing

! Forming processes

! CAx, Virtual Reality

Production and Machine

Tools

! Design of production

machines and components! Control technology and

automation

! Component and produc-tion

machines testing

Metrology

! Tactile metrology

! Optical metrology

Management

! Business development

! Technology management

! Innovation management

! Production management

! Quality management

Education

! Professional training

! Executive MBA for

Technology Managers! Conferences, congresses,

seminars

Gearing Technology

! Gear manufacturing

! Gear calculation and

investigation


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Outline

Fraunhofer-Institute for Production Technology IPT and


(WZL)

! Introduction




of improvements


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Outline



(WZL)

Introduction

! The measurement setup



of improvements


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Overview on Different Excitation Strategies for Machine Tools

Anregung undMessung relativ

zwischen Werkzeugund Werkstck

Anregung und Messungabsolut an beliebigenPunkten der Maschine

Relative excitation on TCP Absolute excitation on TCP

Electrohydraulic, piezoelectric,

electrodynamic

Relative against external reference,

impact hammer, electrohydraulic absolute

exciter

Excitation

devices

Clearances are eliminated by static

preload

No preload of the machine tool, therefore

clearances occur as non-linearitiesInfluence of

clearances

Fstat= 0.2 - 5 kNx

F Fstat dyn+

Fdyn

Fdyn

F m= x

Relative excitationbetween tool and

work piece

Absolute excitation atarbitrary points

on the machine tool


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Force Response Function Measurement Setup for Machine Tools

Exciter

! Impact hammer

! Piezoelectric

! Electrohydraulic

Sensors

! Inductive displace-

ment sensor

!Accelerometer

Amplifier Amplifier

A/D-converter

FFT-AnalyserRealteil[m/N]

0,1

0,1

-0,1

-0,1

0.1

0.01

0.001

0 200 800Frequenz [Hz]

Phase[]180

-180

0

Kohrenz

N

achgiebigkeit

[m/N]

1

0

Amplituden- und Phasengang

Ortskurve

Force sensors

! Wire strain gauge

! Piezoelectrics

Locus

Amplitude and phase diagram

Flexibility

Coherence

Phase[]

Frequency [Hz]

Ima

ginarypart

[m/N]

Real part

[m/N]


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The Measurement Setup Grinding Machine and Lathe


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Electro Hydraulic Exciter

0 100 200 300 400 500 600 700 800

Erregerfrequenz f [Hz]

dynamis

cheKraftFdyn

[N]

500

1000

1500

2000Spektrum der Erregerkraft

Static flexibility of analyzedsystem

0,02 m/N

0,05 m/N

Spectrum of excitation force

Dyna

micforceFdyn

[N]

Exciter frequency [Hz]

Technical data:

!

Static preload: Fstat < 7.000 N! Dynamic load: Fdyn < 1.500 N

! Frequency range: 0 - 800 Hz

Force Sensor- Strain gauge


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Types of Excitation Signal

! Periodic signal: sweep-sinus

! Non-periodic signal: impulse

! Noise-signal (statistic distribution of the

frequencies)

Continuous noise-signal

burst random

Continuous noise-signal superposed

with sweep-sinus

Time [s]

0

Ampli

tude

0,5

1

2 2,5 3 3,5 410-1

0

Amplitude 1

-1

0

Amplitude 1

-1

0Amplitude 1

-1

0

Amplitude

1

-1

1,5


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Measurement of the Vibration

Fixing with

! Magnet (a)

! Wax (b)

! Superglue (c)

! Simultaneous acquisition

of the oscillation in three

directions

! Serial measurement of thestructure points

(200 measuring points

would require 600

channels with

simultaneousMeasurement )

ca b

Triaxialaccelerometer

ca

b

-10

dB40

30

20

10

20lgBua(f )

Bua(f0)

0

0,1 kHz2010510,5Frequency

c: Superglue

a: Magnet

b: Wax

Natural f requency of the sensor


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Spatial Excitation

Before measuring it should

be proved

! If all critical resonance

frequencies are excited?

! If the signal quality is good

enough to measure

remote points at the

machine bed?

! If the measurement setupis stable over the

measurement time?

(Torsion of the spindle,

tilting of the exciter,

overload of the drives)

Fdyn

z

x

y z

x

y

Fdyn

Nachgiebigkeits-untersuchung

Modalanalyse

0 100 200-180

0,001

0,01

0,1

0

Frequenz [Hz]Phase[Grad]

Nachgiebigkeit[m/N]

180

300 500 600 700 800

GXX(direkte Anregung)

GXX(rumliche Anregung)

FRF-Analysis Modal analysis

Direct excitation

Spatial excitation

Frequency

Compliance[m/N]

Phase


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Outline



(WZL)

Introduction


! Approximation of the machine structure


of improvements


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Approximation of the Machine Structure (Machining Center)

Building a Model

! Machine bed

! Guideways

! Z-Slide with table

! Column

! Y-Slide

! Headstock

! Tool


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Modal Analysis

Animation of the measurement resultsAnalysis of the measurement data(Curve-Fitting)

Approximation of the machine toolstructure, up to 200 - 300 measurement

points on the machine tool structure

indices:

i: DOF excitation

j: DOF response

v1 v2

x

F

j

i

v3

0Phase

j

-180

-90

Modal parameters: U+jV, !, u*jV = complex amplitude duration

m = damping constant

" = damped eigenfrequency

n = number of eigenfrequencies

k = index of eigenfrequency

Mij = effective mass (""

max)

Modeshape of eigenfrequency

Analysis and eliminationof weak points

ij

n

1k kk

ijkijk

kk

ijkijk

2

ij

ij S)(j

jVU

)(j

jVU

M

1)j(G !"#

$

%&'

(

)"*"+,

""

),*"+,

""

*!,-* .

-


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Selection of Measuring Points (Machining Center)

The possibility for the

identification of weak

spots is dependent on the

position and number ofmeasurement points

! App. 180 Measurement

points

! Minimum 3 measuringpoints for the identification

of bending modes

! Neighbouring points on

both side of interfaces/joints


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Approximation of a Hexapod HOH 600


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Example for Modeling: Hexapod HOH 600

! Approx. 350 measuring points due tothe higher complexity of the machine

structure


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0 200 300 400 500 600-180

0

-90

Frequenz [Hz]

Phase[Gra

d]

Nachgiebigkeit

90

GXX

GYY

GZZ

Mode Shapes of a Machining Center

Bending of the machine column

in x-direction

Flex

ibility[m/N]

Frequency [Hz]

Phase


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0 200 300 400 500 600-180

0

-90

Frequenz [Hz]

Phase[Gra

d]

Nach

giebigkeit

90

GXX

GYY

GZZ


Tilting of the clamping angled

fixture in z-direction

Flexibility[m/N]

Phase

Frequency [Hz]


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0 200 300 400 500 600-180

0

-90

Frequenz [Hz]

Phase[Gra

d]

Nach

giebigkeit

90

GXX

GYY

GZZ


Torsion of the machine

column around the y-axis

Flexibility[m/N]

Frequency [Hz]

Phase


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Derivation of Design Improvements

Problem:

Chatter vibrations of the tool in x-

direction due to the torsion of the

machine column around the y-axis

Improvements:

! Structure optimization of the

machine column to increase the

torsional stiffness

! Damping of the resonance

frequency using auxiliary masses

close to the TCP

fraunhofer - experimental modal analysis

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