一种新的阻抗测量系统外文翻译.docx
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一种新的阻抗测量系统外文翻译.docx
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一种新的阻抗测量系统外文翻译
ANewImpedanceMeasurementSystemfor
PZT-BasedStructuralHealthMonitoring
Abstract—Recentresearchhasshownthatelectromechanical(E/M)impedance-basedtechniquesareanefficientstrategyfordamagesensinginthecontextofstructuralhealthmonitoring(SHM).Thebasicprincipleistoexcitetheleadzirconateti-tanate(PZT)sensorsattachedtothestructureandtomeasurethecorrespondingelectricalimpedance.Avariationintheelectricalimpedanceindicatesavariationinthemechanicalimpedanceand,consequently,structuraldamage.Tomeasuretheelectricalimpedance,mostuniversitiesandresearchcentersuseexpensive,bulky,heavy,andhigh-poweredimpedanceanalyzers,whichlimitnewdevelopmentsandpracticalapplications.Toovercometheseissues,thispaperpresentsanefficientandinexpensivemethodol-ogyforelectricalimpedancemeasurement.Toverifytheefficiencyandaccuracyofthenewmethodology,practicaltestswerecarriedout,andtheresultswerecomparedwiththoseobtainedusingaconventionalimpedanceanalyzer.
IndexTerms—Impedanceanalyzer,smartstructures,structuralhealthmonitoring(SHM),virtualinstrumentation.
1.INTRODUCTION
THEFIELDOFstructuralhealthmonitoring(SHM)hasreceivedattentionfromnotonlytheacademiccommunitybutalsotheprivateindustry.TheuseofSHMtodetectdamagesinconcretestructures,suchasbridgesandlargebuildings,orinmechanicalstructuresissomethingrecentandpromising.Theobjectiveofanyapplicationistoofferahighdegreeofsecurityandtosignificantlyreducethecostofmaintenance[1]–[3].TherearedifferentmethodologiestodevelopanSHMsystem[4]–[6],butspecialattentionisgiventothosebasedonelectromechanical(E/M)impedanceobtainedfrompiezoelec-trictransducers,suchasleadzirconatetitanate(PZT)ceramics,bondedinthestructuretobemonitored.Themaincharacteristicofthesetransducersisthepiezoelectriceffect.Fromthisprop-erty,thereisadirectrelationshipbetweenelectricalimpedanceandmechanicalimpedance.Consequently,astructuralfailurewouldcauseachangeinthemechanicalimpedance,whichinturnwillchangetheelectricalimpedanceobtainedfromthePZTbondedinthestructure[7]–[9].
ThePZTsensorsareinexpensive,small,andlightweight,andrequirelowpower;theyhavealowsensitivitytotemperaturevariationsandhavealinearresponseunderlowelectricfields.
ManuscriptreceivedApril9,2008;revisedJuly23,2008.FirstpublishedAugust7,2009;currentversionpublishedSeptember16,2009.ThisworkwassupportedinpartbyBrazil’sAgencyfortheFinancingStudiesandProjects(FINEP)andinpartbyBrazil’sNationalCouncilforScientificandTechnologicalDevelopment(CNPq).TheAssociateEditorcoordinatingthereviewprocessforthispaperwasDr.JenniferMichaels.
TheauthorsarewiththeDepartmentofElectricalEngineering,SãoPauloStateUniversity(UNESP),SãoPaulo15385-000,Brazil(e-mail:
*********************.unesp.br;**************.unesp.br).DigitalObjectIdentifier10.1109/TIM.2009.2018693
Fig.1.Circuitproposedin[11]and[12]forPZTimpedanceassessmentthroughaspectrumanalyzer.
[10].ThesepropertiesmaketheE/Mimpedancetechniqueeffi-cient,accurate,andnondestructiveforSHMapplication.How-ever,theelectricalimpedancemeasurementisusuallymadewithexpensive,bulky,heavy,andhigh-powerinstruments,suchasimpedanceanalyzers(e.g.,HP4194AandHP4192A),whicharetraditionallyusedinuniversitiesandresearchcenters.Thesefactorswouldcertainlylimittheuseoflarge-scaleE/Mimpedance-basedtechniques,suchasinindustrialandfieldapplicationsorbyuniversitieswithfewfinancialresources.
ManyresearchershaveproposedothersystemstomeasuretheelectricalimpedanceinSHMapplications.Amethodsug-gestedin[11]and[12]usesaspectrumanalyzerandasim-pleandinexpensiveauxiliarycircuit,asillustratedinFig.1.Accordingtotheauthors,iftheresistanceRSisconsideredsmall,thenthePZTimpedanceisapproximatelygivenby
˙˙˙˙˙
Z=RSVi/Vo,whereViistheexcitationsignal,andVoistheresponsesignal,beingbothvectors.Toobtaintheimpedanceinthefrequencydomain,thediscreteFouriertransforms(DFTs)ofsignalsViandVo(consideredasdiscrete)arecalculatedbyusingaspectrumanalyzer.Inadditiontothesimplicityofthecircuitanditslowcost,thismethodpossessesthefollowingdisadvantages:
1)TheimpedanceisjustanapproximationthatdependsontheresistanceRSandtendstopresentahighdiscrepancywhenthefrequencyincreasesbecausethePZTispredominantlycapacitive.2)IftheresistanceRSisverysmall,thenitisnecessarytouseahigh-gainamplifier(R2/R1)thatcanreducethebandwidth.3)Thespectrumanalyzerisstillanexpensiveinstrument.
Theimpedanceanalyzerproposedin[13]onlyusesaresistorasanauxiliarycircuit,andthesignalresponseacquisitionisperformedbyadataacquisition(DAQ)device,whichiscontrolledbythesoftwareLabVIEW.Italsoseemsthatitismoreaccuratethantheimpedanceanalyzersuggestedin[11]and[12].However,theexcitationsignalisprovidedbyanexternalfunctiongeneratorthatneedsanadditionalgeneralpurposeinterfacebuscard.Inadditiontoincreasingthecost,thesedevicesmakethesystemlessversatile.
Fig.2.CompletesystemproposedforPZTimpedancemeasurement.
Recently,low-cost,low-power,andwirelessimpedancede-vicesweredeveloped.Thesystemproposedin[14]isbasedonaDSPandusesanewalgorithmthateliminatestheuseofanalog-to-digitalconverters(ADCs)anddigital-to-analogconverters(DACs).However,therangeandthefrequencyresolutionarelimitedbythesmallmemoryspaceoftheDSP.
Asaconsequence,aconventionalandexpensiveimpedanceanalyzerisstillnecessarytodeterminethefrequencyrangesensitivetostructuraldamage.Thedevicesproposedin[15]and[16]arebasedonAD5933,whichisaminiaturizedimpedance-measuringchipmadebyAnalogDevices.In[15],astatisticaltechniquedeterminesthemaincomponentsoftheimpedancesensitivetostructuraldamage,andin[16],theauthorsuseamultiplexerforsevensensors,anautoragingcapabilityfordifferentimpedancevalues,andmoreversatilesleepmodes.Nevertheless,theAD5933allowsimpedancemeasurementsonlygoingupto100kHz,whichcanlimitmanySHMapplica-tions.In[17],theauthorsproposedacomplexcircuittodesignaself-containedSHMsystem,butthesystemwastestedusingaDAQdeviceconnectedtoaPCrunningLabVIEW.In[18],theauthorsproposedthecircuitdevelopedin[17]tobeusedinaspecificremoteapplicationreplacingtheDAQdevicebyothercomplexhardwarebasedonafield-programmablegatearray.
Finally,itseemsthatthemostrecentsystemsarebeingproposedforspecificapplicationspreviouslystudiedinthelaboratory—eventheonesbasedonwirelesstechnology.There-fore,itisstillnecessarytouseevaluationboardsordedicatedprintedcircuitboards.
Toeliminatesomeofthementioneddrawbacks,thispaperpresentsaversatile,efficient,andinexpensivemethodology,inwhichtheaccuracyoftheimpedancecalculationdoesnotdependoneitherthefrequencyrangeorthecomponentsusedinauxiliarycircuits.TheproposedsystemsuccessfullyreplacesanarrayofequipmentusedtodevelopSHMapplications,suchasimpedanceanalyzers,spectrumanalyzers,andsignalgenerators.Themaincharacteristicsofthesystemarethefollowing:
1)ItonlyusesaDAQdeviceandaresistortocon-nectthePZTsensor.2)ItcangeneratedifferentsignalstoexcitethePZT.3)ThereisnoDAQoftheexcitationsig-nalduringthemeasurements.4)Itisaveryaccurateim-pedanceanalyzerbecauseitwasvalidatedinpracticalcircuitswithnoSHMapplication.5)Itisverysimpletoimplementdifferentmetricstoevaluatethestructuraldamage.6)Fi-nally,theoverallsystemcaneasilybeadaptedforreal-timeapplications.
2..PROPOSEDMETHODOLOGY
Thebasicprincipleofoperationisbasedonthefrequencyresponsefunction(FRF)obtainedbyDFTwithregardtotheexcitationandresponsesignalsofthePZTcircuit.UsingtheFRFandconsideringthedetailedparametersoftheanalyzedcircuit,itispossibletoobtaintheaccuratePZTimpedance.ThecompletesystemisdepictedinFig.2.Thehardwareof
thesystemconsistsofaninexpensiveDAQdevicemodelUSB-6211fromNationalInstrumentsandapersonalmicrocomputer(PC)laptop.ThesoftwarewasdevelopedinLabVIEW,whichisagraphicalprogrammingenvironment.ThecommunicationbetweentheDAQandthePCisperformedthroughauniversalserialbus(USB)port,whichprovidesversatilitytothesystem.
TheDAQdeviceismultifunctionalwithanalogchannelsbasedon16-bitADCand16-bitDAC.TheADChasadiffer-entialinput,whichprovidesgoodnoiseimmunityandallowstheacquisitionofsmall-amplitudesignals(afewmillivolts).Thisavoidstheuseofinputamplifiersintheauxiliarycircuit,whichwouldaffecttheperformanceofthesystemathigher
frequencies.
Theexcitationsignalisgeneratedbysoftware,whichallowsthechoiceofawidevarietyofsignals,suchasChirp,sine,tri-angular,square,random,etc.,andeliminatestheDAQprocessofthesignal.Thisispossiblebecausethereisacalibrationbetweenthesignalgeneratedbysoftwarex[n]andthesignalx(t)thatreachestheauxiliarycircuit,whichisperformedasfollows:
Beforeanymeasurementandafteralltheparametershavebeendefined,theDACoutputandtheADCinputareinternallyconnectedintheDAQdevice,andtheexcitationsig-nalissampledandstoredoncetoreplacethesignalgeneratedbythesoftware.Becausetheconvertersarenotidealdevices,thisproceduresignificantlyreducesthediscrep
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