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Laser Vibrometry: An Introduction to Non-contact Vibration Measurement

AutomobilesAirplanesHome AppliancesData Storage and SemiconductorMicrostructures, MEMS and NanotechnologyEntertainmentLife Sciences (Biology, Medical)Production Testing & Quality Control UltrasonicsOptical Industry & Telecommunications Defense equipmentCivil Structures

Vibration is Everywhere

The TechnologyPoint & shootEye-safe visible laser probeDetects Doppler shiftAnalogous to acoustic Doppler effectHigh-precision laser interferometerDecodes information to give both displacement and velocity signalsEasy-to-use data acquisition & software processingHardware and software for full-field scanning

TimeHistory

Impulse Response

Frequency Response

Vibrometers convert changes in the laser frequency to a voltage output proportional

to instantaneous velocityChanges in optical phase are converted to

signals proportional to displacement

ODS and modal analysis

What does Laser Vibrometry Provide?

Laser Doppler Vibrometry is a non-contact, "point and shoot" technology that directly measures the vibration of a test object using the Doppler effect.

Emitted frequency f

vObserved frequency fD

c: velocity of the sound waveλ: emitted wavelength

f: emitted frequency

Analogy: Acoustic Doppler EffectFor car moving at velocity V, the observer hears the frequency fD = c/(λ -V/f).

Sound emitted from stationary car has frequency f = c/λ

What is Laser Doppler Vibrometry? PDV-100 Portable Digital

Vibrometer

c

λ

For a vibrometer: Δ fD α VΔ fD = 2V/λ

Frequency Modulated signal

40 MHz ± fD

Bragg cellf0 ± fD

f0

f0 + 40 MHz

He-Ne Laser<1mw (633nm)

x(t)v(t)

Photo-detector

The Heterodyne Interferometer

Measurement Beam

Reflected Beam

Δ fD = 2V/λ

Signal Demodulation

Voltage ~Velocity

Voltage ~ Displacement

FM Doppler signal

AM electrical signal

Controller

Photo-detector system

FFT Spectrum

Single Point

Multi Point Scanning

3 Axis

Differential 3 AxisScanning

Many Vibrometer Types

Long Range

Why Laser Vibrometry?

Polytec’s non-contact laser vibrometers are revolutionizing dynamic testing in all kinds of applications because of their:

Higher accuracyCost and time savingsWider range of operation and applications

Higher accuracy –no mass or stiffness added

Delicate, lightly damped structuresfrequency and temperature independentfine spatial resolution

Down to <1µmSmall structuresMeasurements closer to edges

digital decodingAchieves sub-pm displacement resolution!!

What are the Benefits of Laser Doppler Vibrometry?

What are the Benefits of Laser Doppler Vibrometry?Cost and time savings –

point and shootTarget almost anything quickly and easilyEasier set-up, nothing to attach, close or far

laser may be moved around with ease or scanned (PSV/MSA) for rapid full-field data

Amplitude and phase spectra at up to >250k pointsData for modal analysis Only one transducer to calibrate, connectNo cables over the structure to secure or failNo phase-matching issuesRoboVib is the ultimate solution

What are the Benefits of Laser Doppler Vibrometry?

Wider range of operation and applications – Part 1 of 3

wide frequency range (DC – 1,200 MHz)large amplitude (to 30 m/s)hot (no surface temperature limit)

What are the Benefits of Laser Doppler Vibrometry?

Wider range of operation and applications – Part 2 of 3

submergedthrough windowsdelicate

What are the Benefits of Laser Doppler Vibrometry?

Wider range of operation and applications – Part 3 of 3

long range rotating inaccessible microscopic

All Kinds of ApplicationsSame technology packaged for different applications:

Macro-Structures

Micro-Structures

SmallStructures

Measure on macro- and micro-structures Example - OFV-534 with integrated video and illumination for sensor positioning and sample viewingTailored solutions for:

Research, production, industrial and in-the-field environmentsUltrasonic and high amplitude applications

Single Point Measurement

VibraLight

Resonance Testing of Disk-drive components

OFV-534 sensor head

Objective lens

Controller

Disk-drive

Vertical test stand

Applications Example

VibSoft

Differential Measurement

Example: HSV-2000

Engine valve-train testing:

- relative movement between two surfaces

- removes background vibration of engine block

- 12 kHz valve bounce revealed

Car engine

Two optical sensor heads

Beam steering probes

Rotational MeasurementRLV-5500 - Rotational vibrometer measures pure angular motion –independent of shape or any lateral vibrations

Measured outputs:1. DC rpm value2. Angular velocity signal3. Angular displacement

Example applications:Order tracking analysisCrankshaftsPropshafts

RotVib Features

Two interferometers

Measures angular velocity and displacement from -8,000 to +20,000 RPM

Eliminates down-time associated with contact methods

d = 7.5mm or 24mm

3-D Single Point Vibrometry

Integrates 3 independent vibrometers

Simultaneous measurement of all axes of motion at a single point

Vx, Vy & Vz

Full Field - Traditional Approach Using Contact TransducersMass loading

Damping and stiffness errors

Time consumingCoarse spatial resolutionLimited working environmentsComplicated cablingCumbersome correlation of coordinates to FEM geometryTransducer re-calibration (book keeping)Limited bandwidth

LDV Controller

Data ManagementSystem

Video

LDVSensor

ScanningMirrors

ScanElectronics

Stitched data

Sensor Head Test Object

Test Object

Up to 250,000 points scannedEasy-to-use software for data acquisition, display &

manipulationAnimated data visualizationEfficient interfaces for modal analysis or FEM

validationGeometry file imported or measuredInternal waveform generator

Scanning Vibrometry (PSV)

3-D Scanning MeasurementDeflection Shape

Full 3-D vibration response over measurement surface (>2,500 points)Integrated data stitching provides complete coverage with single systemPrecise data for modal analysis and model verification

Correlation with accelerometer data out-of-plane

RoboVib

CAE w

ork

flow

Micro System AnalyzerMSA-500 - Powerful all-in-one measurement workstation for 3-D static and dynamic characterization of MEMS and MOEMS microstructures

Characterization of out-of-plane vibrations by scanning vibrometry

PLUS:

Measurement of in-plane motion and vibration by stroboscopic video microscopy

Determination of surface topography by white light interferometry

Three

Measurement

Technologies

One

Integrated

Package

Customer Support

Five Territory Sales Offices

Software Maintenance Contracts

Three Application Engineering Support Facilities

Engineering Services and Rentals

On-site Training

Training and Technical Seminars

Two Service Centers

Hardware Maintenance and Calibration Programs

Engineering Services and Rental Program

Advanced non-contact vibration measurements available for every budget

Measurements using Polytec’s latest, non-contact measurement technologyApplication engineers to operate the measurement system to its fullest potentialRental of any of Polytec‘s productsConvenience of testing at the customer’s facility or in a Polytec labComplete measurement data for further processingShort-notice, critical measurementsOccasional measurementsExtended evaluation prior to purchaseBudget Flexibility