An Introduction to Scanning Laser Vibrometry for Non-Contact Vibration Measurement

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An Introduction to Scanning Laser Vibrometry for Non-Contact Vibration

Measurement

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Presentation Outline Deflection Shape and Experimental Modal

Analysis used in the product development cycle

Traditional test methods versus Scanning laser vibrometry

Principles of 1-D and 3-D laser vibrometry Applications examples Accessories Conclusions

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Why Can’t We Rely 100% on the Computer Model?

Material properties Tolerances in manufacturing

Complex joint dynamics Damping Boundary conditions Changes in stiffness during rotation

→ → → FEM Validation and Updating

Uncertainties in:

Impossible or Difficult to Model:

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PSV-500-3D

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Other Examples of the Need to Perform an Experimental Modal Test

Build an experimental model Compute structural modifications

Understand the dynamics of a structure (of a competitor for example) for which we do not have a finite element model

Troubleshooting:

Benchmarking:

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Full Field - Traditional Approach Issues with Contact Transducers

Mass loading Damping and stiffness errors

Time consuming Limited working environments Complicated cabling Safety Coarse spatial resolution Coordinate correlation to FEM

geometry cumbersome Transducer calibration Limited bandwidth Poor Modal Assurance Criteria

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A Better SolutionScanning Laser Vibrometer

Laser has no mass Faster - laser can be scanned Long standoffs, through windows,

fluids, hi temps No cabling on structure Spot dia typically 10s of µm FEM geometry import for easy

point definition One (3) transducer(s) to calibrate

(no book keeping) Wide bandwidth – no accel rolloff or

resonance - to 1.2GHz!! Reduced chance of spatial aliasing

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Spatial Aliasing19 MP

-4.00E-08

-3.00E-08

-2.00E-08

-1.00E-08

0.00E+00

1.00E-08

2.00E-08

3.00E-08

0.23

4668

0.18

6155

0.13

7642

0.08

9129

0.04

06156

-0.00

7897

-0.05

641

-0.10

4923

-0.15

3436

-0.20

1949

9 MP

-4.00E-08

-3.00E-08

-2.00E-08

-1.00E-08

0.00E+00

1.00E-08

2.00E-08

3.00E-08

0.23467 0.1619 0.08913 0.01636 -0.05641 -0.12918 -0.20195

5 MP

-2.00E-08

-1.50E-08

-1.00E-08

-5.00E-09

0.00E+00

5.00E-09

1.00E-08

0.234668 0.113385 0.0163594 -0.104923 -0.201949

19 point profile

19 points

9 points

5 points

Low point density:amplitude errordistorted shape

514 Hz

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PSV Configurations

1D Scanning

Simple structuresODS

Acoustics – noise source identification

Rotating

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3D Scanning

Complex structuresStructural Dynamics

Modal analysis

FEM validation

Stress & strain

Robot-integrated

Polytec Scanning Vibrometer (PSV)

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Front-end Data ManagementSystem

HD Video

LDVSensor

ScanningMirrors

ScanElectronics

Stitched data

Sensor Head Test Object

Test Object

1-D Scanning Vibrometry (PSV)

Up to 250,000 locations scanned point-by-point – vibration velocity or displacement

Software for exciter signal generation, data acquisition, display & manipulation

Geometry file imported or measured

Animated data visualization

Data export for modal analysis or FEM validation

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Operational steps:Define measurement points

using video image and draw programby geometry import

Excite structureusing internal or external function generator signal

Scan to acquire vibration response at each point time history orFFT spectrum

Visualizeanimated operating deflection shapes at selected frequencies of interesttime domain animations

Export for post-processing (e.g. modal analysis or FEM validation)

PSV Operational Steps

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PSV Working Principle

Measure V and at many points relative to a reference signal Amplitude and relative phase at each scan point

Spatial vibration pattern of measurement surface

Scan signal

Ref signal

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Operating Deflection Shapes

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Wide Ranging Applications

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Typical 1-D Scanning Vibrometer

PSV-500-H 4 (or 8) channel data

acquisition with 80 kHz bandwidth

4 or 8 reference channels 50° x 40° scan angle 13 velocity ranges up to 10

m/s Resolution to <0.01 µm/s/√Hz Autofocus MIMO with multi-shaker

excitation, 4 independent generator channels; Principal Component Analysis

Ethernet connection to DMS Coherence Optimizer

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Example measurement on black loudspeaker cone

480 pm deflection1775 Hz

8 averages

New Technology - New Quality

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Digital Broadband Decoder

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Scan Modes

Time

Frequency

FFT

Zoom FFT

Time Domain

FastScan

Time

Frequency

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Time Domain Animations

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Acoustics

Acoustics Configuration50 kHz vibrometer bandwidth digital

7 or 10 measurement ranges

1 channel for reference sensor (digital)

VibroLink Ethernet connection

Optional signal generator

Geometry Scan Unit optional

Software for acoustic measurements – live or stored data – headphones include

Configuration Examples

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Requirements for Quiet Products

Sound propagationFrom source to receiver

Transmission (structure-borne noise)

Leakage (airborne noise)

Secondary noise (structure-borne noise)

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PSV-500 and its Applications

F

Structural dynamics

EmissionSurface velocity

Sound pressure

Fluid-structure-coupling Courtesy : P. Zeller; Handbuch

Fahrzeugakustik

Operating Load

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Laser vibrometers measure vibration in laser direction only

Data needs careful interpretation due to different vibration directions, surface shape and varying scan angles

Modal analysis software expects 3-D data

FE models produce 3-D data

Limitations of a 1-D Scanning Vibrometer

Test ObjectLaser Vibrometer Θ

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3-D Scanning Laser Vibrometry

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x

yz

3-D object coordinate system defined by means of a minimum of 3 known reference points

Laser beam unit vectors for all scan heads are determined using the transformation matrix:

Principles of 3-D Laser Vibrometry

3

2

1

1

333

222

111

v

v

v

lll

lll

lll

v

v

v

zyx

zyx

zyx

z

y

x

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If the object geometry is known, all 3 lasers intersect for all (accessible) scan points

3 simultaneously acquired vibration signals are transformed into the object‘s coordinate system by software

x

y z

Principles of 3-D Laser Vibrometry

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Data import (via UFF):

a) Other test software packages

b) Finite Element Model Geometry File

c) Extracted from other methods such as CAT scan

What if there is no geometry file?

Options for Obtaining Geometry File

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Options for Obtaining Geometry File

When there is no geometry file:

a) Alignment of all 3 laser beams manually for every scan point

b) Measure geometry for every scan point

GEOMETRY SCAN UNIT

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PSV-500 Geometry Scan Unit

More compact, more stable, no moving parts, fasterFor eg: 844 points in 3mins 30secs

Higher sensitivity and wider optical dynamic range Handles larger range of surfaces and variations in surface finish

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Beam Positioning Accuracy

Reasons for poor overlap: Inaccurate “3D-Alignment” during set up procedureImperfect geometry data

Why must beams overlap?Lasers must measure at same point for optimal lateral resolutionMore critical for strain measurements - geometry must be known for transformation to in-plane and out-of-plane data

standard

with triangulation during scan

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Modifies the 3D coordinates by overlapping the 3 beams at one point.

Result: Impoved geometry as input data L for strain calculation

Result: accurate geometry

PSV-S-TRIA Video Triangulation

][m

m

L

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PSV-S-TRIAVideo Triangulation during Scan

before triangulation

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PSV-S-TRIAVideo Triangulation during Scan

after triangulation

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Structural Dynamics (3D)

3D Structural Dynamics Configuration100 kHz vibrometer bandwidth (digital)

13 measuring ranges

8 reference channels (integrated)

MIMO with multi-shaker excitation up to 4 shakers (integrated)

Principal Component Analysis (PCA)

Geometry Scan Unit

Coherence Optimizer

VibroLink Ethernet connection

StrainProcessor optional

Configuration Examples

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Deflection shape at 82 Hz showing individual measurement points

Station Wagon 3-D Vibration Data

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Faster set-up According to a European car manufacturer just the mounting and cabling of the accels and dummy

masses plus the coordinate definitions for a car body usually takes around 4 – 5 days

FEM mesh (coarsened) can be imported Manual identification of points relative to FEM nodes not necessary All points correlate exactly with FE model

Faster data acquisition No moving of dummy masses, accels and retesting

Spatially more detailed data Important for FEM correlation, model updating, and acoustics

Benefits compared to Accelerometers Insensitive to crosstalk when rotational vibration is present Calibration not affected by temperature Flat frequency response

Software is easy to learn and system easy to use

Other time savings for design (FEM) department No modeling of dummy masses necessary

A major European car manufacturer estimates total time savings of > 50%

Benefits of Scanning Vibrometry for Modal Analysis

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RoboVib

CA

E w

ork

flow

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Results Exampledeflection shape at 38 Hz

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Case Study – Transmission Case

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Element type Tetrahedral (solid 185)

Number of nodes 19994

Number of elements 69019

Material Aluminum casting alloy

Case Study – Finite Element Model

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Results

Example: deflection shape at 592 Hz

Case Study – Polytec Data

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Frequency Response Functions

Case Study – Test Data

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Circle Fitting

Resonant Frequency

Damping

Case Study – Curve Fitting

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Case Study – Polytec MAC Data

Accel Data < 0.64

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Am

plit

ude (

dB

)

Frequency (Hz)

Frequency Shift

Density (% Change) = -1.3 %Elasticity Modulus (% Change) = -5.7 %

EMA

Case Study – Model Update

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Traditional - Fixed CostsAerospace GVT Example

Large Size >400 Channels 2-8 Reference Channels (MIMO)

Medium Size 100-400 Channels 2 Reference Channels (MIMO)

Small Size <100 Channels 1 or 2 Reference Channels

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Number of Channels

Time & Complexity

$$$

Small Medium Large

Total Cost of Modal Test Add in the variable cost of personnel on a daily rate

Add in time delay, impacts on production schedule!

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Many Accessories Enable New Applications

Rotating Structures

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Bladed Disk: 10,900 rpm

725 Hz 800 Hz

845 Hz 895 Hz

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Many Accessories Enable New Applications

Small parts at high spatial resolution, high frequencies, strain

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Stress distribution across the surface of a turbine blade in the X direction

Dynamic Stress and Strain

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FEA / Experimental Correlation

Strain in x direction

Strain in y direction

Shear Strain

Results courtesy of the University of Adelaide ME DepartmentPSV data

PSV data

PSV data

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Many Accessories Enable New Applications

Brake squeal studies

Entomology

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More comprehensive vibration testing in a fraction of the time and effort with enhanced detail and accuracy compared to traditional techniques easily integrated with CAE/FEM

FE Model Validations can be made using non-contact 3-D (and 1-D) laser measurement

Many accessories are available to extend the range of applications

Engineering services and rentals are available

Conclusions

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Engineering Services and Rental Program

Advanced non-contact vibration and surface metrology measurements available for every budget

Measurements using Polytec’s latest, non-contact measurement technology

Skilled and experienced applications engineers to operate the measurement system to its fullest potential

Convenience of testing at the customer’s facility or in a Polytec lab Short-notice, critical measurements Scheduled, occasional measurements Build justification prior to purchase Save cost by renting instead of buying Budget flexibility, rent-to-buy