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.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-
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