《现代传感器》课件：Lecture5

ModernSensors

Lecture5X.WuHandina3to5projectreport

(30%towardyourgrade)Topic:High-resolutionsensor:force,motion,…Workingprinciple:equationsandcalculation(MathCADorMatlab),>5equationlines2Schematicdrawingstoexplaintheoperationprinciple:noengineeringdrawingsrequiredIssuestoaddressHowcanyouobtaintheresolutionand/ordynamicrange?noisefloorestimation:Johnsonnoise,op-ampnoise,ACpulsenoise…etc.showyourworkintheformofequations/calculationsJustifymaterialselectioninyourstructureReferencesDue:beforefinalexam.AReviewofLecture4CapacitivesensorHighsensitivityRelativelyhighcostACorswitchedcapacitivemeasurementcircuitProvideanoverviewofaccelerometer.ClassificationbyapplicablerangesExclusivelyfocusoncapacitiveaccelerometerUsefulrange<0,Z=a/02ApplicationsLecture5:BasicintentProvidesomegeneralexamplesofaccelerometers.AcarefulattentionwillbegiventoaparticularcommercialproductbytheAnalogDevicesPiezoelectricityPiezoelectricSensorsPiezoelectricaccelerometerTrendsinaccelerometerdesignExample:massonacantileverConfiguration:1mgmasslocated1mmfromthesupportofthecantileverThecantileveris100micronslongand10micronsthick.adopedsiliconstraingaugeResonantfreq:1kHzThena1milli-gaccelerationwouldcauseachangeofRby1ppm–prettysmall!SiliconAccelerometerSiliconaccelerometerwithadopedsiliconstraingaugeMinimumresolution:afew

milli-gs,andatcostof10-50dollarsperdevice.moresophisticatedaccelerometertechnologyreliesontheuseofcapacitivedisplacementtransducerswithinamicromachinedsiliconstructureAccelerometersforAirBagsafetysystem

InitiallymadeavailablebyChryslerinthemid1980s,nowastandarditemEarlyair-bagsystems:RolamitesFrictionlessmachinefromrollersandbandstworollersheldinatrackonoppositesidesofanS-shapedbandofspringymetal,therollersglideeffortlesslyinthetrackbecausethebandmoveswiththemastheyrollalong.Sincethebandandrollersarebothmovingatthesamespeed,thereisnoslipordragbetweenthemandthereforevirtuallynofriction.RolamitesAirbagG-gaugealittleclumsy,butwithsomepackaging,canperformreliablythroughoutanautomotivelifetimewhichmightbemorethan10years,andincludeexposuretoextremecoldandheatforextendedperiodsoftime.Onepossibleproblemhasbeenthetendencytodeployupontheencounterofalong,deeppothole.Unnecessarydeploymentissomewhatdangerous,itbrieflyincapacitatesthedriver.itcausessomeexpensivedamagetothevehicledashboard,andoccasionallyinjuresthedriver.numerousreportsofhearinglossduetotheabruptchangeinpressurecausedbydeploymentinasealedcar.Today’sairbagsensor:MEMSAirbagAccelerometersmanufacturers:EG&GICSensors,AnalogDevices,Motorolaairbag-likedevicesforaeroplanesasearlyasthe1940s,thefirstactualexampleinaproductioncarwasthe1981Mercedes-BenzS-Class.airbagtriggeringalgorithmsarebecomingmoreandmorecomplex.ThemostcommonMEMSaccelerometerinuseistheADXL-50byAnalogDevices,butthereareotherMEMSmanufacturersaswell.ADXL50ADXL50measuresthedifferencebetweenthetwocapacitorsADXL50:FunctionalBlockDiagramPrincipleofOperationThedetectionschemeworksbydetectingthedifferencebetweentwocapacitances.Thisdifferencesignalisamplified,andcomparedwithathresholddetector,whichsendspositiveornegativepulsestothecontrolelectrodes

Bycountingthedifferencebetweenpositiveandnegativepulses,itispossibletomeasuretheaccelerationofthesystem.Afully-integrateddeviceAfterallthedepositionandetchingstepsarecomplete,themasselementsarereleasedbyabriefdipinHF,andthewafersarediced,mounted,andpackagedinordinarytransistorheaders.ADXL50:PerformanceFull-ScaleMeasurementRange:50gSelf-TestonDigitalCommand+5VSingleSupplyOperationSensitivityPrecalibratedto19mV/gInternalBufferAmplifierforUserAdjustableSensitivityandZero-gLevelFrequencyResponse:DCto10kHzPostFilteringwithExternalPassiveComponentsHighShockSurvival:>2000gUnpoweredTest:ADchipforvehiclenavigationPrimarysourceofinfotocalculateposition:GPS(globalpositioningbysatellite)24lowearthorbitingsatellietesAffectedbythelocationofthereceiverw/respecttoenvironmentTwotypesoflimitations:Selectiveavailability:signalsdeliberatelyalteredbythemilitary,limitingitsaccuracyMultipathreflections:introducingerrorsduetoinaccuratedistancesandtimesusedinthecomputationofposition.Deadreckoning:startingcoordinates,direction,speedTest:ADchipsforvehiclenavigationDirection:gyroscopeSpeed:accelerometerTestADXL202：2g3dBbandwidthsetaround10Hz,RMSnoisefigureof1.9mg,~resolution103Zerodriftover24hrs:2.4mg，～10ppm/24hrsTest:ADchipsforvehiclenavigationFieldtestGPSonandoffforcorrelation30mawayfromref.WithoutGPS:Stateofart:capacitivemeasurementGermanycompany:PI(PhysikInstrumentation)Resolution0.01nmMeasurablerange:15-100mZerodrift:0.1nm/hrTempdrift:-30ppm/KNoisefactor:0.115ppm/Hz106distinguishablestepsPIcapacitivepositionsensorPiezoelectricityaphenomenoninwhichforcesappliedtoasegmentofmaterialleadtotheappearanceofelectricalchargeonthesurfacesofthesegment.Thispolarizationofthecrystalleadstoanaccumulationofcharge:Q(charge)(1x3matrix)=d(piezoelectriccoefficient3x3matrix)F(Force1x3matrix)PiezoelectricMaterialsManypolymers,ceramics,andmoleculessuchaswaterarepermanentlypolarized:inE-fieldthesepolarizedmoleculeswillalignthemselveswiththeelectricfield,resultingininduceddipolesApermanently-polarizedmaterialsuchasquartz(SiO2)orbariumtitanate(BaTiO3)willproduceanelectricfieldwhenthematerialchangesdimensionsasaresultofanimposedmechanicalforce:piezoelectriceffect.Conversely,anappliedelectricfieldcancauseapiezoelectricmaterialtochangedimensions.Thisphenomenonisknownaselectrostriction,orthereversepiezoelectriceffect.

PiezoelectricEffect

ReversePiezoelectricEffect

PiezoelectricityTypicalvaluesofthepiezoelectricchargecoefficientsare

1-100pico-coulombs/N.a1cmx1cmslabof1mmthickPZT(leadzirconiumtitanate).A1Nforceisappliedalongthezaxis,whichisthe1mmdimension.Whatvoltageappearsacrosselectrodesonthelargesurfaces?Itisinterestingthatthedimensionsoftheobjectcompletelycancelouttoproducealargervoltage,weneedtoreducethecapacitanceofthisstructure.Theeasiestwaytodothisistoreducethearea.Twotypesofdevices:resonantandnon-resonantNon-resonant:generator-likeLowsensitivitySimpledesignPowerlessdeviceResonant:lookingforchangeinresonancefrequencyResearchstartedasearlyas1930sHighsensitivityQuartz:<20%changeofresonancefreq.Silicon:>200%changeallowedQuartz:cutrelatedpropertiesY-cut:largetempcoefficientofresonantfreq:80-100ppm/C,canmeasure10-6C;LC-cut:10-3

Cmeasurementfrom80-230C;All-quartzstructurepressuresensor:Resolves10-7offullscale,Record-maker!ShearModeMostcommonconstructionforpiezoelectricaccelerometerisshearmode.

fashionedinconfigurations:tri-shear,planarshearannularshear.

Significantbenefitsoveroriginalcompressiondesignsinthatthecrystalsensingelementisbetterisolatedfrombasestraininputsandthermaltransientsbymountingthecrystalonaninternalpost.

Sheardesignsalsotypicallyexhibitlowertransversesensitivityandbasestrainsensitivity,andhavebetterthermalstability.

CompressionModeTheoriginalcompressiondesignsforaccelerometerscombinedanXcutquartzcrystal(orceramicelement)preloadedwithaseismicmass.

Thebenefitsofthesimple,reliabledesignweregoodsensitivityandahighnaturalfrequencyprovidingabroaduseablefrequencyrange.

Drawbacksincludedsensitivitytobasestraineffectsandathermaltransientsensitivityalsomanifestedthroughstraineffects.

Withaslightmodificationtotheoriginalcompressionmode,

theinvertedcompressionmodedesignevolvedinordertoimprovetheselimitations.

Essentially,theinvertedcompressiondesign“hangs”thecompressionmodesensingelementfromthetopoftheaccelerometerpackage.

Thisdesigneliminatesmuchoftheseundesirablestraineffectsonthesensingelement.

Bothofthesecompressionmodedesignshaveessentiallybecomeobsolete.FlexureBeamFlexurebeamisanothergeometricvariationofsensingelementstructure.Whileithasoneofthehighestoutputsforlowprofile,lowcostandlightweightdesigns,itislessusedduetochallengeswithshocksurvivabilityandthermaltransients.

Inapplicationswhereextremelylowprofileelementsprevail,someflexurebeamdesignsarestilltheonlyavailablesolution.

QuartzbasedsensorsVariousDesigns:fromquartztosiliconUSpatent5396798132,138,140:siliconarms148:piezoelectricresistor141:drivecapacitorPiezoleakage

piezoelectricsarenotgenerallyverygooddielectrics.Inparticular,piezoelectricmaterialsaresomewhatleakyachargeplacedonapairofelectrodesgraduallyleaksaway.thereisatimeconstantfortheretentionofavoltageonthepiezoelectricaftertheapplicationofaforce.Thistimeconstantdependsonthecapacitanceoftheelement,andtheleakageresistance.Typicaltimeconstantsareoforder1sec.Becauseofthiseffect,piezoelectricsarenotveryusefulforthedetectionofstaticquantities,suchastheweightofanobject.EquivalentCircuit

Equivalentelectricalcircuitfor

piezomeasurementcircuit.

Rx=Resistanceofpiezo

Cx=Capacitanceofpiezo

Cc=Capacitanceofcable

Ca=Capacitanceofamplifiercircuit

Ra=ResistanceofamplifiercircuitAdvantagesanddisadvantagesAdvantagesoverothersensingmechanismthedevicegeneratesitsownvoltage.Becauseofthis,thesensorelementdoesnotneedtohavepowerappliedtoitinordertofunction.Forapplicationswherepowerconsumptionisasignificantconstraint,piezoelectricdevicescanbeveryvaluable.someinterestingscalinglawswhichsuggestitisusefulinsmalldevices.Theprimarydisadvantagesinherentlysensitiveonlytotimevaryingsignals.theCurietemperature.IfthecrystaliseverheatedtoneartheCurietemperature,itcanbecome`de-poled'whichcanresultinalossofpiezoelectricsensitivity.Forvariousmaterials,thisCurietemperaturecanbeashighas600Coraslowas50C.Theneedtostaybelowthistemperaturecanimposeseriousconstraintsontheapplicabilityofthesesensors.UsefulFrequencyRangeAsthenaturalfrequencyisapproached,thefrequencyresponseofthesensorincreaseslogarithmically.Itisarisingresponse,ratherthanadecreasingresponsethatlimitsthefrequencyresponseattheupperlimit.Lowfrequencyresponseislimitedbytheinteractionoftheelectronicsinthecouplerorsignalconditonerintypicalmeasurementsystems.Whentousepiezoelectricsensor?Nevertheless,ifyouhaveafasttime-varyingsignal,youshouldgiveseriousthoughttotheuseofpiezoelectricsensingelements.Example:PiezoelectricAccelerometer10gmmassrestingonaslabofPZT.Thepiezoslabhasdimensionsof1squarecminarea,and1mmthick.Find:VasafunctionofforceandV(1mili-g)V(1milli-g)~10-5VQ1:measurable?Q2:improvements?ImprovementsThepiezoelectricelementcanhaveasmallerareaandalargerthickness.ApreampResonantfreq:200KHzCommercialPiezoelectricAccelerometerPiezoelectricaccelerometersareonthemarket,Primarilyofferedforvibrationmeasurement.Formoderatesignals(milli-gs),fairlysmalldeviceswithsimplecircuitsarequitesufficientTrendsinAccelerometerDesignTrendsinaccelerometerrequirementsformilitaryandaerospace

IncreasedHigh-andLow-FrequencyResponseRangeThecurrentlimitationattheNationalInstituteofStandardsandTechnology(NIST)is15,000Hz,butworkisinprogressthatisexpectedtoraisethislimitto20,000Hz.UpperLimitPyroshock:short-duration,high-amplitude,high-frequency,transientstructuralresponsesinaerospacevehicles.Pyroshockonrocketormissilesystems:explosiveboltsandnuts,pinpullers,separationofspentrocketboosterstages,linearcuttingofthestructure,andotheractionsthatproduceanear-instantaneousreleaseofstrainenergy.Tosupportstructuralanalysisoftypicalmilitaryandaerospacesystems,However,thehigherresonantfrequenciespossiblewithMEMStechnologyoftenincreasetheaccelerometer'mercialpiezoelectricaccelerometerstypicallyhaveamaximumresonantfrequency1/5thatcurrentlyachievablewithMEMStechnology.IncreasedHigh-andLow-FrequencyResponseLowerlimitExperimentalmodalanalysis,ortheabilitytocharacterizestructuresempiricallyintermsoftheirdamping,resonantfrequencies,andvibratorymodeshapes,ThefirstvibratorymodeofatransportaircraftsuchastheBoeing737orMcDonnell-DouglasDC9occursat<1Hz.Thisisthewing"butterfly"mode.Thespacestationandotherlargespacestructureshaveevenlowerstructuralfrequencies.ExperimentalcharacterizationofthesestructuresisnowpossibleusingvariablecapacitanceMEMSaccelerometersaswellaspiezoelectricaccelerometers.Piezoelectrictypesarenotcapableoffrequencyresponseto0Hz.Improvedhousingisolationfromthermalandacousticstimuli,however,resultsindesignswith

responsesto<0.1HzIncreasedReliabilityThelimitednumberofmeasurementsandthehighcostofthesystemundertesthavehistoricallygenerateddemandsonthepartofmilitaryandaerospacecustomersforahighdegreeofreliabilityinaccelerometerperformance.Whentheaccelerometerispermanentlyintegratedintothesystemforcontrolandmonitoringfunctions,thereliabilityrequirementsareintensified.Anumberoffactorshavecontributedtoincreasedreliability.Theelasticsiliconflexuralelementcontainingdiffusedgauges,inherentinMEMSaccelerometerdesign,hasimprovedreliabilityoverthatofferedbytheearliermetallicflexureswithbondedbulksemiconductorgauges.SiliconmicromachininghasenabledpreciseplacementofoverrangestopsinbothresistiveandcapacitiveMEMS-typeaccelerometers,ensuringsu