外文翻译使用双光纤布拉格光栅传感器和互相关技术的水流量计Word文档下载推荐.docx

外文翻译使用双光纤布拉格光栅传感器和互相关技术的水流量计Word文档下载推荐.docx

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oftheFBGsensorwiththeinterferometricdetectionwas4×

10−4pm/（Hz）1/2correspondingto0.33nε/（Hz）1/2.Awaterflowexperiment

showedthattheflowmeterhadalinearcharacteristicatvelocityrangefrom0to1.0m/sandtheminimumdetectablevelocityof0.05m/s.

1.Introduction

FiberBragggrating（FBG）sensorshavevariousadvantagessuchassmallsize,simplicityinsensingprinciple,electromagneticinterference（EMI）immunityandcapabilityofmultiplexing.Becauseoftheseadvantages,anumberofbasicresearchesandapplicationsonFBGsensorshavebeenmade[1–3].Intelecommunicationsystems,FBGsareusedasadd-dropmultiplexersbecauseoftheirnarrowbandwidth（typically0.1nm）.TheFBGapplicationtoopticaltunablefiltersisalsousefulfordiscriminationofthesignalsinFBGsensorsystems[4].Theapplicationstosmartstructuresandhealthmonitoringareattractiveandhavebeeninvestigatedactively[5,6].FBGsareembeddedincompositematerials

andusedasstrainandtemperaturesensorsintheapplication.Inthefieldofcivilengineering,strainmeasurementsforbridgesandbuildingsaremadeusingFBGsensorarrayswithwavelengthdivisionmultiplexing（WDM）andtimedivisionmultiplexing（TDM）[7].

IntheFBGsensorapplications,thechoiceofthewavelength-shiftdetectionmethodisveryimportantbecausethenoiselevelandthemeasurementbandwidthofthesystemaremainlydeterminedbythedetectionmethod.ThemostcommonlyuseddetectionmethodisthetunablefilterdetectionusingFabry–Perotfilter.Thismethodisthestandardtechniqueandprovidesstaticorquasi-staticmeasurementwithastrainresolutionof1_ε.Anotherpromisingmethodistheinterferometricdetection[8,9].Thismethodhasthecapabilityofdynamicmeasurementwithhighstrainresolutionintheorderofnε/（Hz）1/2.TherearesomereportsaboutthenoiseestimationoftheFBGsensorwithinterferometricdetection[10–12].

OursubjectofresearchistheFBGapplicationtoawaterflowmeter.Therearevariouskindsofflowmetersincludingturbineflowmeters,vortexflowmetersanddifferentialpressuretypeflowmeters.Measurandsofflowmetersarerangingovervariousflowincludingwaterflow,gasflowandmultiphaseflow.Cross-correlationflowmeter,whichutilizesatimedelayofsignalsbycoherentstructuresincludingvorticesandnaturallyexistingunsteadypressurefield,isusuallyusedforpipeflowmeasurement.Theadvantageofthecross-correlationflowmeterisitssimplicityinsensingprinciple.Theonlyparameterrequiredtotheflowmeteristhedistancebetweentwosensors.Inthecross-correlationflowmeter,twopairofaultrasonictransmitterandareceiverareusuallyusedbecauseoftheirnon-intrusivenesstotheflow[13].Theflowmeterusingtheultrasonictransducershasagoodlinearityatwidevelocityrange.Theproblemwiththeflowmeteriscomplexityofthesensingpartbecausethesystemneedsatleastfourtransducers.Thecross-correlationflowmeterreportedbyDyakowskiandWilliams[14]uses16lightrays（eightpairs）todetectflowsignalsingas–solidmixture.Thevelocitiesareobtainedfromcross-correlationoftheintensitymodulatedlightsignals,andtheaveragevelocityandthevelocitydistributioninthepipearethenobtainedbycombiningcalculatedvelocities.ThisflowmeterisattractivebecauseofEMIimmunityandthepassivenature.However,thesystemneedsparticles,whichreflectorscatterthelightrays,inthefluidandtheapplicationislimited.Therearefewreportsconcerningthecross-correlationflowmeterusingopticalsensors,notlightrayorlaserbeam,suitedforwaterflowmeasurement.

Inthispaper,wepresentawaterflowmeterusingdualFBGsensorsandcross-correlationtechnique.Theflowmeterhasnoelectronicsandnomechanicalpartsinitssensingpart,andthusthestructureissimple.Atfirst,weexplaintheprincipleandtheschematicdiagramoftheflowmeter.Next,wepresentthenoiseestimationoftheFBGsensorwiththeinterferometricdetectionusingaMach–Zehnderinterferometercomprisedofa2×

2anda3×

3couplers[9].Finally,wedescribeexperimentalperformancesoftheFBGsensorandtheflowmeter.

2.Across-correlationflowmeterusingFBGsensors

Fig.1showstheprincipleoftheflowmeter.Thecross-correlationflowmeterpresentedhereusesFBGstrainsensorscomprisedofFBGsandmetalcantilevers.Intheflowmeasurementsection,theFBGsensorsandabluffbodyareused.Thebluffbodywhoseshapeisarectangularcolumngeneratesstablevortices.ThetimedelaybetweenthevortexsignalsdetectedbytheFBGsensorsareestimatedusingthesmoothedcoherencetransformRSCOT（τ）[15].ThefunctionRSCOT（τ）isexpressedasfollows:

（1）

whereGxx（f）andGyy（f）arethepowerspectraoftheupstreamanddownstreamsensorsignals,Gxy（f）isthecross-spectrumoftwosignalsandF−1denotestheinverseFouriertransform.ThefunctionRSCOT（τ）isacross-correlationfunctionweightedwiththecoherenceofthesignalsandcandetectthetimedelaymorepreciselyandrobustlythanthesimplecross-correlationfunction.ThemaximumofRSCOT（τ）isthebestestimate_tofthetimedelaybetweentwoFBGsensors.Themeasuredvelocityvmeasisthencalculatedfromthefollowingsimpleequation:

（2）

wheredsisthedistancebetweentwosensors.

Fig.2illustratestheschematicdiagramofthewholesystem.Weusedanamplifiedspontaneousemission（ASE）asanopticalsourceofthesystem.TheASEhasoutputpowerof22dBmandfullwidthathalfmaximum（FWHM）of50nmatC-band.ThelightfromtheASEisseparatedbyanoptical3dBcouplerandthenilluminatestwoFBGsensorsinstalledtothePVCpipewhoseinnerdiameteris20mm.ThelightreflectedbytheFBGsensorsisfedtoMach–Zehnderinterferometers,whicharecomprisedofa2×

2anda3×

3couplers,withtheopticalpathdifferencesof1.635and3.169mm.Inthe3×

3coupler,threefibersarearrangedinatriangulararray.Theseinterferometersareusedaswavelength-shiftdetectorsforinterferometricdetection.Theincidentlightisphase-modulatedbytheMach–Zehnderinterferometerandthenconvertedtovoltagesignalsbyphotodetectors.SixoutputsignalsaresimultaneouslydigitizedbyanA/Dconverterwitharesolutionof16bitandsamplingfrequencyof10kHz,andthedetectedsignalsarethenprocessedtoobtainthetimedelay.

TheFBGreflectsthelightwavewithacertainwavelengthλBcalledBraggwavelengthandthewavelengthisthenexpressedasfollows:

，（3）

wherenistheeffectiverefractiveindexoftheFBGandΛisthemodulationpitchoftherefractiveindexoftheFBG.TheBraggwavelengthλBchangesbylongitudinalstrainεzappliedtotheFBG,andtheBraggwavelength-shiftδλBisexpressedasfollows[3]:

（4）

wherep12istheelasto-opticconstantoftheopticalfiberandisapproximately0.22.Thisyieldsthestrainsensitivityof1.2pm/_ε.

ToobtaintheshiftδλB,theoutputsoftheinterferometerareused,andtheoutputsVm（m=1,2,3）areexpressedasfollows[9]:

Vm=αmVin+Re[Γ（τ）]=αmVin[1+γcos（θMZI+θm）],（5）

whereΓ（τ）istheauto-correlationfunctionoflightwavereflectedbytheFBGsensor,VinisthevoltagecorrespondingtoopticalpowerreflectedbytheFBGsensor,andαmisthecoefficientcompensatingdifferencesofphotodetectorsensitivitiesandobtainedfrompreliminaryexperiments.Ifthesplitratiosofthe2×

2and3×

3couplersare1:

1and1:

1:

1,respectively,onecanobtainθ1=0,θ2=2π/3andθ3=−2π/3,andtheoutputsV1,V2andV3arederivedasfollows:

（6）

ThesignalθMZIcanbethencalculatedusingthefollowingequation:

（7）

whereListheopticalpathdifferenceoftheinterferometer.TherelationshipbetweenthesignalvariationδθMZIandtheshiftδλBisexpressedasfollows:

（8）

wheretheterm（λB+δλB）wasassumedtobenearlyequaltoλBbecausetheshiftδλBissignificantlysmallerthanλB.Anaccidentallossofopticalpowerwhilemeasurement,whichcausesproblemsinopticalintensitymodulationtypesensors,isadmissibleinsomemeasurebecausethewavelength-to-phasesensitivity（=−2πL/λ2B）dependsononlythepathdifferenceL.

3.NoiseestimationoftheFBGsensorwithinterferometricdetection

TherearesomereportsaboutthenoiseestimationofFBGsensorswithinterferometricdetection.Howeverthenoiseestimationreportsabouttheinterferometricdetectionusinga2×

3couplershavenotbeenpresented.

3.1.Noiseofthephotodetector

Fig.3showsthecircuitdiagramofthephotodetector.Thenoiseofthephotodetectorisdefinedbythenoiseofaphotodiodeandatransimpedanceamplifier.ThenoiseofthephotodetectorisdeterminedbythermalnoisesduetothefeedbackresistanceRfofthetransimpedanceamplifierRfandshuntresistanceRshofthephotodiodeandbyshotnoiseduetotheoutputcurrentim（=Vm/Rf）ofthephotodiode.Theequivalentinputnoisevoltageandcurrentof