Operational Modal Analysis Case Studies

Operational Modal Analysis

Application cases

2 LMS Web Seminar 2009 – Operational modal analysis Part 2

Overview

Part 1

What is Operational Modal Analysis

Why Operational Modal Analysis ?

What is the difference between ODS, EMA and

OMA ?

LMS PolyMAX Parameter Estimation

Automatic operational modal analysis

Part 2

Applications cases

Conclusion


What is Operational Modal Analysis ?

In-operation Testing

Identification of modal parameters from data

measured on a structure during operational

conditions.

Eigenfrequencies

Damping ratios

Mode shapes

Operational modal analysis = identifying H

Based on Y

Without knowing U (BUT white noise

assumption)

HU Y

Input System Output

White noise

White noise + harmonic



Case Studies


Application example

Civil engineering

Drivers

Artificial excitation problems

Testing complexity

Expensive

Data quality (still ambient

sources active)

High-quality sensors and data

acquisition systems


Advanced algorithms

Commercial software

Real-life references

Forced vibration tests Ambient vibration testsEvolution


Z24-Bridge

Ambient Vibration Test


Quantify variance on modal

parameters due to environmental

influences

Damage detection by changes in

modal parameters

Test Setup

Ambient excitation

170 DOFS

9 setups 0.00 50.00 Hz

500e-18

10.0e-12

Log

g2

0.00

1.00

Am

plit

ude


Z24-Bridge

Operational pole estimation for different runs

0

2

4

6

8

10

12

0 1 2 3 4

mode no.

natu

ral fr

eq

.(H

z)

scatter plot

0

1

2

3

4

5

0 1 2 3 4

mode no.

da

mp

ing

ra

tio

(%

)


Z24-Bridge

Typical synthesis results

3.00 30.00 Linear

Hz

10.0e-15

10.0e-12

Log

g2

deck:120:+Z

Synthesized Crosspow er deck:120:+Z/ref:1:+Z

3.00 30.00 LinearHz

3.00 30.00 Hz

-180.00

180.00

Phase

°

3.00 30.00 Linear

Hz

1.00e-15

10.0e-12

Log

g2

deck:120:+Z

Synthesized Crosspow er deck:120:+Z/ref:2:-Y

3.00 30.00 LinearHz

3.00 30.00 Hz

-180.00

180.00

Phase

°

3.00 30.00 Linear

Hz

100e-15

100e-12

Log

g2

deck:120:+Z

Synthesized Crosspow er deck:120:+Z/ref:2:+Z

3.00 30.00 LinearHz

3.00 30.00 Hz

-180.00

180.00

Phase

°


Z24-Bridge

Merge Modes


Z24-Bridge

Merge Modes


Z24-Bridge

Eigenfrequencies and damping ratios

OMA - Ambient EMA - Shaker

A B

Freq A [Hz]

Freq B [Hz]

Damp. A [%]

Damp. B [%] MAC [%]

Mode 1 Mode 1 3,86 3,87 0,71 1,43 99,44

Mode 2 Mode 2 4,89 4,82 1,30 1,57 94,66

Mode 3 Mode 3 9,75 9,78 1,56 1,56 60,57

Mode 4 Mode 4 10,31 10,46 1,19 1,52 88,97

Mode 5 Mode 5 12,49 12,41 2,66 2,99 96,06

Mode 6 Mode 6 13,43 13,23 3,51 4,58 86,10

Mode 7 Mode 7 17,36 17,52 3,20 3,98 53,03

Mode 8 Mode 8 19,10 19,26 1,73 2,61 6,39

Mode 9 Mode 9 19,84 19,75 2,10 5,08 75,30

Mode 10 Mode 10 26,53 26,65 1,79 3,03 49,88

All 10 modes were found!


Operational Modal Analysis Ambient noise

Experimental Modal Analysis Shaker test

Z24-Bridge

Mode shapes


Bradford & Bingley Stadium

Crowd-structure interaction

Remote Monitoring System

Time histories measured during soccer match

Empty stadium

People filling stadium

People sitting during match

Half-time

Goal

People leaving stadium

Changes in natural frequencies for Mode 1

2,9

2,95

3

3,05

3,1

3,15

3,2

3,25

3,3

3,35

3,4

Mode 1

Fre

qu

en

cy [

Hz] Empty PolyMax

Filling PolyMax

Leaving PolyMax

Half-time PolyMax

Sitting PolyMax

Goal PolyMax

Changes in damping ratios for Mode 1

0,00%

0,50%

1,00%

1,50%

2,00%

2,50%

3,00%

Mode 1

Dam

pin

g r

ati

o

Empty PolyMax

Filling PolyMax

Leaving PolyMax

Half-time PolyMax

Sitting PolyMax

Goal PolyMax


Application example

Aircraft in-flight testing

Test setup

5 flights

30 responses / flight

2 reference points

Different conditions:

Taxi

Climbing

Level flight (3 altitudes, 4 air speeds)

Steady turn

Descend

Objectives

Compare with GVT results

Tracking of frequency and damping



Analysis procedure

Flight data

Correlation functions

Modal parameter

estimation

Modal model

Model Validation



Stabilization diagram

LMS PolyMAX


Aircraft In-flight Testing

Crosspower synthesis

DOF on wing DOF on tail



Evolution of frequency & damping


Flight test

Ground Vibration test


Mode Shapes


Identify modal parameters

Launcher

Main constituents

Using in-flight data

Practical, economical and safety reasons (no GVT)

Validation and updating of FE models

Influence of gradual mass decrease

Damping

Application case

ARIANE-5 Launcher


Flight 501

+- 100 accelerations

booster segments, booster skirts, booster attachments

LH2 and LOX tanks in the main cryogenic stage

MMH and N204 tanks in the EPS stage

VULCAIN and AESTUS engines

ARIANE-5 Launcher


-1.00 620.00 s

-131.45

49.56

Rea

l

(m/s

2 )

0.00

1.00

Am

plitu

de

-1.00 620.00 s

-145.37

90.27

Rea

l

(m/s

2 )

0.00

1.00

Am

plitu

de

-1.00 154.24 s

-147.88

48.41

Rea

l

(m/s

2 )

0.00

1.00

Am

plitu

de

Channels synchronized to common time t0

Non-equidistant

Rounding errors, losses, …

Different sample frequencies

Static components

Drop-outs, spikes

ADC bit resolution

Intensive data preparation

ARIANE-5 Launcher

Characteristics ARIANE-5 Flight Data


8 signals measured at 4 fuel tanks in the

EPS stage

Signals resampled at 93.4Hz

Analyses performed for 4 time data

segments

S1 : 4 - 14s

S2 : 14 - 26s

S3 : 26 - 36s

S4 : 4 - 36s

ARIANE-5 Launcher

Analyses


Integral

Application case

Integral Sine Qualification Test

Multi-shaker single axis qualification test

350 DOF’s

Range 5 - 100 Hz

Repeated modes – symmetric setup

Mode shapes from base-driven test

Y-axis qualification sweep data



Broadband / High-order Analysis

LMS Test.Lab PolyMAX



Crosspower Synthesis

15.06 67.04 Linear

Hz

1.01

45.96

Log

(m2/s

4 )CrossPow er UNIV:796:+X/UNIV:706:+Y

15.06 67.04 LinearHz

15.06 67.04 Hz

-180.00 180.00

Phase

°15.06 67.04 Linear

Hz

1.00

17.01

Log

(m2/s

4 )

CrossPow er UNIV:401:+X/UNIV:706:+Y15.06 67.04 Linear

Hz

15.06 67.04 Hz

-180.00 180.00

Phase

°

15.06 67.04 Linear

Hz

1.03

21.72

Log

(m2/s

4 )

CrossPow er UNIV:313:+Y/UNIV:706:+Y15.06 67.04 Linear

Hz

15.06 67.04 Hz

-180.00 180.00

Phase

°15.06 67.04 Linear

Hz

1.01

181.16

Log

(m2/s

4 )

CrossPow er UNIV:707:+Y/UNIV:706:+Y15.06 67.04 Linear

Hz

15.06 67.04 Hz

-180.00 180.00

Phase

°

Payload module

Service module Antenna

Solar Panel



Mode Shapes


Application

Rotating machinery

Sweeping harmonics (run-up)

Harmonics excite broad

frequency band

Useful excitation – no

filtering!

“End-of-order” effects

Fixed (or slowly varying)

harmonics

Harmonics hamper the

identification process

Have to be removed from

data

He

lic

op

ter

in-f

lig

ht

da

taC

ar

en

gin

e r

un

-up


Industrial examples

Key to the method is the knowledge of the fundamental

frequency of the disturbing harmonics:

Tacho probe during operational test

“Tacho-less rpm extraction”

Hilbert transform

Example: band-pass filtered helicopter roof accelerations

0.50 29.50s

Am

plit

ude

g

0.50 29.50sA

mplit

ude

0.50 29.50s

Am

plit

ude

Hilbert transform magnitude Hilbert transform phase

Instantaneous frequency estimation

derivative


In-flight mode shapes

Industrial example – helicopter in-flight OMA

In-flight acceleration spectrum before

and after applying the harmonic filter


Industrial example – large Diesel engine OMA

Helicopter data: even without filtering, some

modes could be identified

Quasi-stationary running large engine:

Harmonics:

• High-amplitude

• Closely-spaced

OMA: no single structural mode found

Need for harmonic filtering


Industrial example – large Diesel engine OMA

Quasi-stationary running large engine

Filtering the harmonics from the time

series

OMA successfully applied

• in good agreement with the modal

parameters identified using classical

impact testing


Road noise analysis

Booming Noise at 80 Hz

Transfer Path Analysis points to rear

suspension

Experimental Acoustical Cavity Mode: 80 Hz

Acoustic wave is maximum for rear passenger

80 Hz

rear seats

front seats


Road noise analysis

Operational modal analysis of the rear suspension

Rollerbench test with bumps on rollers

Impulsive excitation

High data consistency

2 runs / 45 dofs each

6 ref dofs0.00 21.00 s

-11.00

10.00

Real

(m/s

2 )

0.00

1.00

Ampli

tude

0.00 512.00 Hz

-60.00

-30.00

dBg2

0.00

1.00

Am

plit

ude

F AutoPow er FARG:21:+Z Susp_Run1

F AutoPow er FARG:21:+Z Susp_Run2


Road noise analysis

Stabilization diagram

LMS PolyMAX


Road noise analysis

Crosspower synthesis

0.00 180.00 Linear

Hz

1.00e-6

10.0e-3

Log

g2

Synthesized Crosspow er FARG:21:-X0.00 180.00 Linear

Hz

0.00 180.00 Hz

-180.00

180.00

Phase

°

Synthesized Crosspow er FARG:21:-X

0.00 180.00 Linear

Hz

100e-6

10.0e-3

Log

g2

Synthesized Crosspow er AMAR:813:+Y/FARG:21:+Y0.00 180.00 Linear

Hz

0.00 180.00 Hz

-180.00

180.00

Phase

°

Synthesized Crosspow er AMAR:813:+Y/FARG:21:+Y

0.00 180.00 Linear

Hz

1.00e-6

1.00e-3

Log

g2

Synthesized Crosspow er LONG:302:+Z/FARG:21:+Y0.00 180.00 Linear

Hz

0.00 180.00 Hz

-180.00

180.00

Phase

°

Synthesized Crosspow er LONG:302:+Z/FARG:21:+Y

0.00 180.00 Linear

Hz

10.0e-6

10.0e-3

Log

g2

Synthesized Crosspow er TRAR:608:+Z/FARG:21:+Y0.00 180.00 Linear

Hz

0.00 180.00 Hz

-180.00

180.00

Phase

°

Synthesized Crosspow er TRAR:608:+Z/FARG:21:+Y


Car Road Noise

Results

Source problem: Bending of twist beam

Solution: Tuned Vibration absorber or stiffen twist beam


5 : Belgian blocks

1 : runups

2 : ramps

3 : asphalt

4 : ramps

Car on the road

Road Tests


Car on the road

Run Up Test

Measurements

20 km/h -> 60 km/h on ordinary road in 2nd gear

24 dofs (± 45 sec.)

Different acquisition runs,

Down sampling to 100 Hz

Analysis

Balanced Realization technique

0Hz - 35 Hz: Excitation especially in this frequency band

Mainly rigid body and first flexible modes


Car on the road

6 Important Modes

Torsion mode:

• 30.47 Hz

• Damping: 3.84 %

Rigid body modes

First flexible modes


Car on the road

Belgian Blocks

Constant velocity : 20 km/h

Extra rigid body mode

High level excitation of the first two rigid

body modes


Use of OMA for Updating

Moan Noise

Roller bench testing

320Hz

Brake suspension


Use of OMA for Updating

Moan Noise

Test (320Hz) FE (312Hz)


Damage detection: Sports Car

Fatigue problem attachment power train to body

During test-track endurance test

Detailed test-track data analysis

Laboratory EMA and test-track OMA

Dynamic fatigue calculations

Controlled laboratory endurance tests


Part 1

Mature technology Robust algorithms

Many relevant industrial applications

Complements Experimental Modal Analysis

Better exploitation of operational data

Use PolyMAX to make OMA easy Crystal-clear stabilization diagrams

User-independent results

Part 2

Applications include Troubleshooting

Design verification / FE correlation

Trend analysis

Damage detection / Health monitoring


Conclusion


Invitation

More information on www.lmsintl.com

See LMS website for other web seminarswww.lmsintl.com

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Documents

Operational Modal Analysis Case Studies