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§6 Applications of fiber optic sensors§6 Applications of fiber optic sensors

Strain sensors and temperature Strain sensors and temperature sensorssensors

Acoustic sensors, displacement sensorsAcoustic sensors, displacement sensors Electric current sensorsElectric current sensors Rotation sensorsRotation sensors Chemical sensorsChemical sensors Sensors for mechanical and civil Sensors for mechanical and civil

structuresstructures

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Three application examples using Three application examples using FBG sensors FBG sensors

Sensing mechanismSensing mechanism

Bragg resonance wavelength: Bragg resonance wavelength: BB=2n=2n BB=0.78x10=0.78x10-6-6BB. 1. 1 1pm 1pm BB=6.7x10=6.7x10-6 -6 BBT. 1°C T. 1°C 10pm 10pm

Bragggrating

fibercore

fibercladdingI

input spectrum

measurandinduced

shift

ITransmitted signal

I

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Wavelength detection using passive optical filtersWavelength detection using passive optical filters ( (Melle et alMelle et al) )

broadbandsource

I

I

input spectrum

gratingsensor

coupler

strain-induced shift

lllllllllll

I/Iref

edgefilter

photodetectorIIref

Output

decreasingstrain

increasing strain

wavelength

Outp

ut

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Wavelength detection using tunable optical filters(Wavelength detection using tunable optical filters(Kersey et aKersey et al)l)

Well suited for WDMWell suited for WDM Resolution demonstrated: Resolution demonstrated: 11 and and 0.2°C 0.2°C Types of TOFs: Fabry-Perot filters, FBG- based filters Types of TOFs: Fabry-Perot filters, FBG- based filters

Broadbandsource

output

couplerlllllllllllllllllllllllllllllllll

1

FiberFabry-Perot

Photodetector

.....2 N

VFP

scanningwaveform

VFP

Output

. . .

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Application 1:Application 1: Internal strain monitoring of Internal strain monitoring of structural materialsstructural materials

Schematic of the test specimen showing the location of the FBG Schematic of the test specimen showing the location of the FBG sensors and loading setupsensors and loading setup

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Result of strain measurement at position 3 (Result of strain measurement at position 3 ( ) ) strain measured by grating, (o) strain measured strain measured by grating, (o) strain measured for a strain gauge attached at the surfacefor a strain gauge attached at the surface

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 5 10 15 20 25 30 35 40

Applied load (kN)

Str

ain

()

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Result of strain measurement at position 1 (Result of strain measurement at position 1 ( ) strain measured by grating, (o) ) strain measured by grating, (o) strain measured for a strain gauge attached at the surfacestrain measured for a strain gauge attached at the surface

0

1000

2000

3000

4000

5000

6000

0 5 10 15 20 25 30 35 40

Applied load (kN)

Str

ain

()

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Structure under investigation with FBG sensors

FBGsensing unit

U

xy

SignalConditioner

Hot wire

Square Cylinder

d = 6 mm

x/d = 2.0

FBG sensor

FBG sensor

DataAcquisition

System

y/d = 1.5

FBGsensing unit

U

U x

y

x

y

Configuration ofwake generator

d

d

Optical Interrogation System

Wind Tunnel

Computer Data Acquisition System

Application 2: Measurement of flow-induced vibration

Experimental setup

THE HONG KONG POLYTECHNIC UNIVERSITY香港理工大學

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a) Strain measured from FBGb) Displacement measured from vibrometerc) Wind velocity measured from hot-wire

10 20 30 40 500

10

20

30

40

50

Xrm

s(

m)

( b )

Ur

0

10

20

30

40

50

Yrm

s(

m)

( a )

= 0 = 15 = 30 = 45

Dependence of the vibration amplitude of the square cylinder on Ur: (a) Yrms, (b) Xrms.

Results

THE HONG KONG POLYTECHNIC UNIVERSITY香港理工大學

See: Zhou Y, Wang ZJ, So RMC, Xu SJ, and Jin W, JOURNAL OF FLUID MECHANICS 443: 197-229 SEP 25 2001

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Application 3: Investigation on Structural Fatigue Problems of the KCR Vehicles

• Fiber Bragg Gratings Interrogator is used to monitor the strain and stress in the bodyshell of the KCRC compartments in order to identify the possible fatigue problems that may affect the life expectancy.

SG1FBG1

SG2

SG3

Strain gauge (SG)Optical sensor (FBG)

SG4FBG2

SG5FBG3

THE HONG KONG POLYTECHNIC UNIVERSITY香港理工大學

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Results of static and dynamic load test of the FBG sensors

• The results from FBG sensors agree well with the electrical strain gauges.

-800

-600

-400

-200

0

200

400

600

800

2.5 3 3.5 4 4.5 5 5.5

Pressure Transducer Output (V)

Mic

ro-S

trai

n

FBG1

FBG2

FBG3

THE HONG KONG POLYTECHNIC UNIVERSITY香港理工大學

See: W. Jin et al., “Structural strain and temperature measurements using FBG sensors,” Chapter in book: Guided wave optical components and devices, B. P. Pal (Eds), Elsevier,Burlington, USA, pp.389-400. 2005