Structural Health Monitoring of Bridges Buildings and Industrial Plant with New Ultra-Long-Range Vibrometer

RSV-150 – The Optical AlternativeOptical measurement of

displacement and velocityNo transducers to attachFast repositioning to different locationsPoint and shootNo targets need to be attached or surface prepStand-off distance to > 100 m

even higher with retro-reflective targetsWorks on all materials and target temperatures High spatial resolution / localization <10mmVoltage output: compatible with all data acquisition systems

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Compared to Accelerometers

Velocity and displacement instead of accelerationRelative measurement instead of absolute

direct DC displacement can be measuredAbsolute directional sensing

in direction of the laser beam, no crosstalkNo temperature sensitivityLaser spot is the sensor – not fixed to structure

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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 (LDV)?

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 ExamplePhoto-detector system

FFT Spectrum

RSV-150: FeaturesLong Range Capabilities due to

High power, but eye-safe & visible laser (class 2)Long Range lens

Easy operation due to:Quick setup:

No need for access to measurement surfacesPoint, focus, measure

Green target laser with good visibility day & nightCoaxial video camera shows measurement locationeven in bright sunlightSignal level indicator to maximize laser return signalOptional zoom scope for wider field of view 3/6/2012 # 8

The RSV-150 System

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RSV-150 SetupOptics

Position using tripod Focus using camera, laser and bargraph

Electronics (Controller)Adjust ranges and output filters16 ranges displacement

< 0.1 µm to > 1 m8 ranges velocity

< 1 µm/s to > 1 m/sbandwidth: 0Hz – 25 kHzVarious high- and low-pass filters

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RSV-150 Sensor HeadComprises

Laser sources for targeting and measurementInline cameraLong range opticsPan/tilt fine adjustment

better than10mm @ 100mfor targeting on fine structures

eg stay cables

Spray water resistant (IP63)Weight < 9 kg

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fine adjustment

scope (option)long range optics

RSV-150 Sensor Head

Properties Spray water resistant (IP63)Manual focusing lens

laser focus = camera focusfocal length approx 1000 mm (equiv. 35mm)

Green laser for targetingMechanical adapter for compensation sensor

compensates for strong ambient vibration of the sensor or tripod

RSV-150: The System

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view through zoom scope

View though internalcamera

RSV-150 ControllerDecoding

decodes high frequency signals for sensor head Output (BNC)

displacement and velocity (+/- 10V)video (PAL, Composite)optical signal level (RSSI)

Power supply100...240VAC (50/60Hz )12-24 V DCapprox. 60W

USB 1.1 for remote control

RSV-150: The System

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RSV-E-150 Controller

ApplicationsApplications

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Condition MonitoringPiping vibration problems

Safe and convenient distances high temperaturesexplosion hazardnuclear radiation

Easy localization by manual “probing” of the pipe lengthEasy assessment of machinery vibration severity level according to ISO 10816-3 for trending with standard data loggers

Suitable for mining environmentsInsensitivity to target surface

Hot targetsMagnetic fields (transformers)High voltage

Applications

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Structural DynamicsEigenfrequency Assessment

bridges, stay cables suitable for high voltage insulators, power poles and cables

Displacement/Deflectionbridges under defined testing loads

Validation of Finite Element ModelsDynamic behavior assessment for update of FE models

Applications

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TroubleshootingPerfect solution when:

Fast response is necessary Contact transducers cannot be mountedAccess is limited, dangerous or impossibleLocation of problem is unknownManual “scanning” can be performed

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Applications Tested Structural Dynamics

TV Tower (Eigenfrequency)Drilling Rig (Troubleshooting)Steel Railroad Bridge (Displacement)Cable Stay Bridges (Tension of stay cables)Bridge bearing integrity (Differential displacement)

Condition MonitoringConveyor belt (bearing condition)High voltage power cables and supports (Eigenfrequency)

Applications

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TV TowerApplications: Structural Dynamics

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Distance 300mLong time average

Coal Mine: Conveyor BeltDistance 8m

meas. point encrusted and rustyTarget: bearing monitoring

red: damagedgreen: good reference

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Power LinesDistance up to 100m

Frequency measurement for condition monitoringEigenfrequencymeasurement for resonance tuningExcitation: Ambient (wind, current)

Applications: Condition Monitoring

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Müngsten BridgeHeight 108mSteel framework -train bridge Distance > 140m

Excitation: train

Applications: Structural Dynamics

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displacement of thebridge during slowtrain pass-by

Concrete BridgeHighway bridgeTarget

EigenfrequencyDisplacements

Application: Structural Dynamics

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1234

Concrete bridgeApplication: Structural Dynamics

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Eigenfrequency5 averages0.031Hz frequency resolution

1

Concrete BridgeApplication: Structural Dynamics

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Displacementtime history during traffic excitation

green targetlaser spot

1

Concrete BridgeApplication: Structural Dynamics

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Displacementtime history during traffic excitationmeasured on pier

View through internal camera

Photopier displacement (in µm)

3

3

NPL Foot Bridge

Built in the mid 1960s, Test bed for many different structural monitoring techniques

Application: Structural Health Monitoring (SHM)

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15 tonnes, 5 meters high, 20 meters long concrete bridge

Courtesy: Elena Barton, NPL, GB

Finite element analysis model of the bridge to further

understand and predict the structural health

Foot Bridge: Modelling and Monitoring Modeling: 3D Laser ScanMonitoring sensors

Acoustic EmissionDigital Image CorrelationDigital LevelingElectro level Beam SensorsFiber Bragg GratingsResistance Strain Gages Time-Domain ReflectometryDistributed Crack Sensor Vibrating Wire Strain Gages Video Gauge Technique Wireless Accelerometer and Magnetic InductionSensors

Laser Doppler Vibrometer

Application: Structural Health Monitoring (SHM)

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Courtesy: Elena Barton, NPL, GB

Foot Bridge: ResultsApplication: Structural Health Monitoring

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Velocity Output

Direct Displacement Output

Max Peak to Peak Displacement 0.534mm

LDV Measurement Advantages

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• Observed LDV and Accelerometer Data Compares Very Well

• Very easy to extract:• FRF - Frequency Response

Function• Time History• It is easy to then relate this

information to any modelling

Special Thanks to Prof. Brownjohn, Sheffield University

Arts Tower Sheffield University

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At 255 feet (78 m) tall it is the second tallest building in Sheffield, England

Has an active condition monitoring system in operation

Vibrometer Measurement Result Arts Tower

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Time Response Velocity Output

Displacement (Velocity Integrated)

FFT

0.53Hz

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Vibrometer Measurement Result Arts Tower

Above: Results from Arts Tower monitoring system:

e then n directions

Polytec Vibrometer Results are Consistent

with Accelerometer-based Condition

Monitoring System

Humber Bridge

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The Humber Bridge is a suspension bridge with the north tower sited on the high water line and the south tower founded in shallow water 500m from the shore.

Measurement Targets

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Primary weight-bearing cablesSmaller suspension cables

Deck

Thanks to Prof. Brownjohn, Sheffield University

Measurement 1- Main Cable Velocity at Clamp

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0.29Hz2.46Hz3.87Hz

Quick SetupGood Signal LevelsResults are plausibleNo one has measured beforeTarget difficult to reachMain cables need more study

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Measurement 2 - Cable Duct

4.34Hz

Some good clean data with Realistic results up to 4 to 5Hz

For Humber Bridge, there would be no other way to measure response of main cables, and theresponse looks realistic.

Measurement 3 – Hanger 6

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For the hangers, the harmonics validate the measurements showing the capability for mass measurement on stay cables/hangers of bridges .

ConclusionLong stand-off distance

Safe, fast and convenient operationQuick setup

No sensor mounting = time saving Easy Troubleshooting

Monitor many locations from one sensor positionTime-saving localization of vibration sources

FlexibilityWorks on all surfacesWorks with all data collectors

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ConclusionHigh stand-off distance

assess many measurement points from one location

Quick setupno sensor mounting = time saving

High resolutiondelivers reliable data for model validation

RSV-150: Summary Structural Dynamics

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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