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1、184IEEETRANSACTIONSONMAGNETICS,VOL.45,NO.1,JANUARY2009StudyofLift-OffInvarianceforPulsedEddy-CurrentSignalsGuiYunTian ,YongLi ,andCatalinMandacheSchoolofElectrical,ElectronicandComputerEngineeringUniversityofNewcastleUponTyne,NewcastleUponTyneNE17RU,U.K.InstituteforAerospace
2、Research,NationalResearchCouncilCanada,Ottawa,ONK1A0R6,CanadaLift-offinvariance(LOI)isgettingmuchattentionfromresearchersinthefieldofelectromagneticnondestructiveevaluation(ENDE)because,attheLOIpoint,eddy-currentsignalsfordifferentlift-offsintersectandthesignalamplitudeisindep
3、endentofthelift-offvariance.WediscussourongoingresearchintoLOI,startingfromanoverviewofthestate-of-theartofpulsededdy-currenttesting(PEC)systemsandtheiruseintheeliminationofthelift-offeffect.WehaveinvestigatedLOIcharacteristicswithrespecttovariationintheconfigurationofthePECpr
4、obeinatheoreticalstudy,implementedbyextendedtruncatedregioneigenfunctionexpansion(ETREE)modeling.Wefoundthat:1)theLOIoccurswhenthefirst-ordertimederivativeofthemagneticfieldsignalsareacquiredfromHallsensors;2)anLOIrangeinsteadofasingleLOIpoint,whenmultiplelift-offsareintroduced
5、throughbothexperimentalandtheoreticalstudies.TheLOIrangevarieswiththeHallsensorpositionintheprobeassemblyandtheconductivityofthesamplesunderinspection,whichareimportantparametersforthedesignanddevelopmentofPECsystems.Basedonthisunderstanding,weinvestigatednewapproachesusingth
6、eoreticalcomputationandmultiplelift-off,ormagneticsensorarrays,forconductivityandlift-offestimation.Ourstudycanbeextendedfordesignanddevelopmentofmultipurposeeddy-currentsensorsystemsforsurfaceformmeasurementanddefectdetection.IndexTerms—Inverseprocess,lift-off,lift-offcompen
7、sation,lift-offinvariance,normalization,pulsededdycurrent.I.INTRODUCTIONTABLEIPECSYSTEMCOMPARISONHEmaindifferencebetweenpulsededdy-currenttestingT(PEC)andsweep-frequencyorconventionaleddy-currenttestingtechniquesliesinthatPECemploysapulsedexcitationcurrentinsteadofsinusoidalc
8、urrentinsinglefrequencyormul-tiplefrequencies.Theuseofexcitationcurr
