用低变形传感器监测民用工程结构变形的一致性毕业设计外文翻译.docx

用低变形传感器监测民用工程结构变形的一致性毕业设计外文翻译.docx

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用低变形传感器监测民用工程结构变形的一致性毕业设计外文翻译

Low-coherencedeformationsensorsforthemonitoringofcivil-engineeringstructures

D.Inaudia,A.Elamarib,L.Pfluga,N.Gisinb,J.Breguetb,S.Vurpillota

“IMAC,LaboratoryofStressAnalysis,SwissFederalInstituteofTechnology,CH-1015Lausanne,Switzerland‘GAP,GroupofAppliedPhysics-OpticalSeciion,GenevaUniversityCH-1205Geneva,Switzerland

Rcccivcd25January1993;inrevisedform8March1994;accepted25March1994

Abstract

Anoptical-fiberdeformationsensorwitharesolutionof10pmandanoperationalrangeof60mmhasbeenrealized.Thesystemisbasedonlow-coherenceinterferometryinstandardsingle-modetelecommunicationfibers.Itallowsthemonitoringoflargestructuresoverseveralmonthswithoutnoticeabledrift.Nocontinuousmeasurementisneededandthesystemisinsensitivetovariationsofthefiberlosses.Thistechniquehasbeenappliedtothemonitoringofa20mX5mX0.5m,120tonconcreteslaboversixmonths.Itispossibletomeasuretheshrinkageofconcreteanditselasticcoefficientduringpre-straining,givingreproducibleresultsingoodagreementwiththeoreticalcalculationsandmeasurementsperformedonsmallconcretesamples.Thispaperdescribestheopticalarrangementandtheproceduresusedtoinstallopticalfibersinconcrete.

Keywor&:

Ikformationsensors;Civil-engineeringstructures

1.Introduction

Boththesecurityofcivil-engineeringworksandthelawrequireaperiodicmonitoringofstructures.Themethodsusedforthispurpose,suchastriangulation,waterlevelsorvibratingstrings,areoftenoftediousapplicationandrequireoneormanyspecializedoperators.Thiscomplexityandtheresultingcostslimitthefrequencyofthemeasurements.Furthermore,thespatialresolutionisoftenpoorandtheobservationisusuallyrestrictedtothesurfaceoftheobject.Thereisthusarealdemandforatoolallowinganinternal,automaticandpermanentmonitoringofstructureswithhighaccuracyandstabilityoverperiodstypicallyoftheorderof100yearsforbridges.Inthisframework,fiber-opticsmartstructures（i.e.,structureswithself-testingcapabilities）aregaininginimportanceinmanyfieldsincludingaeronauticsandcompositematerialmonitoring.Thistechnologycanbeappliedincivilengineeringandinparticularfortheshort-andlong-timeobservationoflargestructuressuchasbridges,tallbuildingframes,dams,tunnels,roads,airportrunways,domes,pre-stressingandanchoragecables.Themonitoringofsuchstructuresrequiresthedevelopmentofameasuringtechniquewithhighaccuracy,stabilityandreliabilityoverlongperiods.Ithastobeindependentofvariationsinthefiberlossesandadaptedtotheadverseenvironmentofabuildingsite.Toreducethecostoftheinstrumentation,itisfurthermoredesirabletousethesameportablereadingunitforthemonitoringofmultiplestructures.Wedescribehereasystembasedonlow-coherenceinterferometryrespondingtoalltheserequirements.

2.Experimentalarrangement

Themeasuringtechniquereliesonanarrayofstandardtelecommunicationopticalfibersinmechanicalcontactwithconcrete.Anydeformationofthehoststructureresultsjnachangeintheopticallengthofhefibers.Eachsensorlineconsistsoftwosingle-modeibers:

onemeasurementfiberinmechanicalcontactwiththestructure（gluedorcemented）andareferenceiberplacedloosenearthefirstone（inapipe）inordertobeatthesametemperature.Sincethemeasurementtechniquemonitorsthelengthdifferencebeweenthesetwofibers,onlythemechanicaldeformationwillhaveaneffectontheresultswhileallotherperurbations,suchasthermallyinducedchangesintherefractiveindexofthefibers,willaffectthetwoinanidenticalwayandcanceleachanotherout.Tomeasuretheopticalpathdifferencebetweenthetwofibers,alow-coherencedoubleinterferometerintandemconfigurationhasbeenused（Fig.1）[l].ThesourceisanLED（light-emittingdiode）workingaround1.3pmwithacoherencelengthL,of30pmandaratedpowerof200pW.Theradiationislaunchedintoasingle-modefiberandthendirectedtowardthemeasurementandthereferencefibersbymeansofa50:

50single-modedirectionalcoupler.Attheendsofthefiberstwomirrorsreflectthelightbacktothecoupler,wherethebeamsarcrecombinedwitharelativedelayduetothelengthdifferenceAL,betweenthefibers,andthendirectedtowardsthesecond（reference）interferometer.ThereferenceinterferometerisofMichelsontypewithoneofthearmsendedbyamobilemirrormountedonamicromctricdisplacementtablewitharesolutionof0.1pmandanoperatingrangeof50mm.ItallowstheintroductionofanexactlyknownpathdifFcrenceAL,betweenitstwoarms.Thisfiberinterferometerisportableandneedsnoopticaladjustmentaftertransportation.IthasbeendevelopedbytheGAPwiththesupportoftheSwissPTTforopticalcabletesting[2].

Theintensityattheoutputofthereferenceinter-ferometerismeasuredwithapig-tailphotodiodeandisthengivenby[3]

wherezz,,ristheeffectiverefractiveindexofthefiber,zzgthegrouprefractiveindex（about1%higherthannefrinsilica）,A,thecentralvacuumwavelengthofthelight,zi,,theautocorrelationfunctiontakingthespectralcharacteristicsoftheemissionintoaccountandALthephysicalpathdifferencebetweenthetwointerferingpaths.FurthersimilarinterferencetermsappearinEq.

（1）inthespecialcaseswhenAL,

eitherwithAL,=AL,orwithAL,=-AL,,andthustwointerferencefringepacketsasdescribedbyEq.

（1）.ThemirrorpositioncorrespondingtoAL,=0alsoproducesaninterferenceandisusedasareference.Thesethreefringepacketsarcdetectedbymeansofalock-inamplifiersynchronizedwiththemirrordisplacements.Themirrordisplacementsandthedigitalizationofthelock-inoutputarecarriedoutbymeansofaportablepersonalcomputer.Sincethereferencesignalisgcneratedseparatelyanddoesnothaveaconstantphaserelationtotheinterferencesignal,onlytheenvelopeofthedemodulatedsignalhasaphysicalmeaningandcorrespondstotheenvelopeofthefringepattern.Alock-inplotshowingthethreetypicalpeaksisshowninFig.2.Eachpeakhasawidthofabout30pm.Thecalculationofitscenterofgravitydeterminesitspositionwithaprecisionbetterthan10pm.Thisprecisionisthelimitingfactorofthewholemeasurementtechnique.SinceAL,isknownwithmicrometerprecision,itispossibletofollowAL,withthesameprecision.

Fig.1.Experimentalsetupofthelow-coherencedoubleMichelsoninterferomctcr.D.Innudietal.1Semorsand

Fig.2.Typicalfringecnvclopeasafunctionofthemirrorposition.Thedistancebetweenthecentralandthelateralpeakscorrespondstothelengthdifferencebetweenthemeasurementandthereferencefibersmountedinthetable.Anychangeinthelengthofthestructureresultsinachangeinthepositionofthesepeaks.Anychangeinthelossesofthefiberswillresultinachangeoftheheightofthepeaks.Thecentralpeakisfixedandusedasareference.

ThepathdifferenceAL,isproportionaltothede-formationofthestructureAL,withtherelationbetweenthetwogivenby[4]

wherepisPoisson’sratioandpijisthestrainoptictensor（Pockcl’scoefhcients）.Thecoefficient5takesintoaccountthevariationoftheeffectiveindexneffinafiberunderstrain.Adegradationofoneorbothfibers（duetoaging,forexample）willresultinalowervisibilityofthefringesbutwillnotaffectitsposition.Theinformationaboutthedeformationofthestructureisencodedinthecoherencepropertiesoflightandnotinitsintensityasinthemajorityofthesensorsappliedtodateincivil-engineeringstructures,mostlybasedonmicrobendlossesand/oropticaltime-domainreflectometry（OTDR）techniques.Interferencepeaksresultingfromreflectionsaslowas-30dBofthesourcepowercanbedetectedbyoursystemwithoutphasemodulators.Bymodulatingthephaseinoneofthefourarmsofthetwointerferometers,onecanincreasethedynamicrangeofthedevicetomorethan100dB[5].

Evenifthepolarizationdispersionandbend-inducedbirefringenceinthesensingfiberscouldreducethevisibilityoftheinterferencefringesorevensplitthefringepackets,noneofthoseeffectswasobservedinourexperiment.Noadjustmentofpolarizationbetweenthereferenceandthesensingarmwasthennecessary.Agoodmechanicalcontactbetweenthemeasurementfiberandthestructureundertestisfundamental.Inthisstudyanumberofinstallationprocedureshavebeentestedandoptimizedforthedifferentmeasurements（shrinkage,elasticitymodulus,etc.）.Themountingtechniquescanbedividedintotwomaincategories:

full-lengthcouplingandlocalcoupling.

Duringourtestsfiveoutofsixopticalfiberpairswitha0.9mmnyloncoating,beingmountedontheexternalfaceofa20mlongplasticpipeandprotectedonlywiththinrubberbands（seeFig.3（a））,survivedtheconcretingprocess.Duringthesettingprocesstheconcreteenvelopsthefiberandrealizesthedesiredmechanicalcontact.Thosefibersshowedaminorincreaseinthescatteringlossesandtheappearanceofsmallparasitepeaks.Themeasurementsonthosefiberswereconsistentwiththeresultsobtainedwithotherinstallationtechniques（seebelow）.Itseemsthatforfull-