ModernSensors

Lecture3X.WuLecture3BasicIntentReviewsomebackgroundonelectricalmeasurementofsensoroutputsSomedetailsregardingthebehaviorofsimplepassivefiltersandoperationalamplifiersProvideanoverviewofpiezoresistivedevices.SomeexamplesareworkedoutusingthissensingtechniqueAReviewofLecture2CharacteristicsofsensorsTransferfunctionSensitivityDynamicRangeHysteresisTemperatureCoefficientLinearityAccuracyNoiseResolutionBandwidthIntroductiontoSensorsElectronicsTheelectronicswhichgoalongwiththephysicalsensorelementareveryimportant:Limittheperformance,cost,andrangeofapplicabilityIfcarriedoutproperly,thedesigncanimprovethecharacteristicsoftheentiredevice,FocusonbasictechniquesfroprocessingthesignalsmosttypicallyproducedbyasensorMostsensoractlikepassivedeviceResistiveCapacitiveInductiveResistiveSensorCircuitsResistivesensorsobeyOhm’slawHowtogetavoltagesignaloutofthesensor?NeedaconstantcurrentsourceTheeasiestwaytobuildacurrentsource:voltagedividerCondition:loadR>>sensorRShortcoming:smallsignalmightneedsomeamplificationManysensorsrespondtophysicalsignalsbyproducingachangeincapacitanceImpedance:VerymuchlikearesistoratAC,maymeasurecapacitancebybuildingvoltagedividercircuits,useeitherresistororcapacitorastheloadresistanceResistorshavemuchsmallertempcoeff.thancaps:0.3ppm/oCv.s.200ppm/oCCapacitancemeasuringcircuitsCapacitancemeasuringcircuitsAsubstantialhassle:providinganACbiasConvertingtheACformicroprocessorinterfaceUseclocksignalorintegratedclock/samplingcircuitModulatedsignalcreatesanopportunityforuseofsomeadvancedsamplingandprocessingtechniquesLock-in:biasthesensorandtriggerthesampling,getthelownoisesignalDisadvantage:clockedswitchinjectnoisechargeintocircuitVeryaccuratecapacitancemeasurementstillrequiresexpensiveprecisioncircuitry

InductancemeasurementcircuitsImpedance:iL->essentiallyresistiveelements

Inductivesensorsgenerallyrequireexpensivetechniquesforthefabricationofthesensormechanicalstructure:3DstructureInexpensivecircuitsarenotofmuchuse,expensiveanyway!LimitationsLimitationstoresistancemeasurementLeadresistace->4-wireconfigurationOutputimpedanceThemeasuringnetworkresistanceplacesalowerlimitonthevalueofaresistancewhichmayacrosstheoutputterminalsAnexample:10Kthermister+1Mload,ifconnectedtoan1Kmeasuringinstrument->outputvoltagewouldbereducedby~90%LimitationstomeasurementofcapacitanceStrayCapacitanceAppearasadditionalcapacitancesinthemeasuringcircuitWiresmovingaboutwithrespecttoground,causingcapacitancefluctuationsTheseeffectsareduetopressure-inducedvibrationsinthepositionsofobjects,referredtoasmicrophonics.Piezoresistivedevices–anoverviewSilicon-basedSpecificadvantagesare:Highsensitivity,>0.5mV/mbarGoodlinearityatconstanttemperature

Abilitytotrackpressurechangeswithoutsignalhysteresis,uptothedestructivelimitStructureandAssemblyPrincipleofOperationDeformationbyappliedpressurecauseshighlevelsofmechanicaltensionattheedgeofdiaphragmSemiconductorresistorsonthefrontsidetransducethistensionintoresistancechangesbymeansofthepiezoresistiveeffect..SpecSheetNominalPressureRange(mbar)1002004001000Sensitivity(mV/mbar)0.50.250.120.06Linearity(%FSO)<1

BridgeResistance(k)5.6

ChipSize(mm3)3x3x1

2.2x2.2x1

DiaphragmSize(mm2)2x21.5x1.51.1x1.10.8x0.8BASICPROCESSINGSTEPSPressureandTemperatureSensorCluster

Diffusion(n-typeandp-type)MetallizationAnisotropicback-sideetching(byusingwellknownchemicalsasTMAHorKOH)Theoreticalbackground:piezoresistanceApiezoresistor:adevicewhichexhibitsachangeinresistancewhenitisstrained.Therearetwocomponentsofthepiezoresistiveeffectthegeometriccomponent

theresistivecomponent.

Thegeometriccomponentofpiezoresistivity:astrainedelementundergoesachangeindimension.Thesechangesincrosssectionalareaandlengthaffecttheresistanceofthedevice.

Strain:DefinitionStrainistheamountofdeformationofabodyduetoappliedforceDimensionless:mm/mmstrainisoftenexpressedasmicrostrain(),whichisx10-6.

Poisson’sRatio

Poisson’sratioWhenabarisstrainedwithauniaxialforce,aphenomenonknownasPoissonStraincausesthegirthofthebar,D,tocontractinthetransversedirection.ThemagnitudeofthistransversecontractionisamaterialpropertyindicatedbyitsPoisson'sRatio:

Definedasthenegativeratioofthestraininthetransversedirection(perpendiculartotheforce)tothestrainintheaxialdirection(paralleltotheforce),Poisson’sratio=eT/eLPoisson'sRatioforsteel,forexample,rangesfrom0.25to0.3

ClassicalDevice:MercuryTube

Anelastictubefilledwithaincompressibleconductivefluid,suchasmercury(really!)R=(Resistivityofmercury)(lengthoftube)/(crosssectionalareaoftube)

Gaugefactor:K=2forliquidstraingaugeWhatdoesitmean?ifaliquidstraingaugeisstretchedby1%,itsresistanceincreasesby2%.

MetalwirebasedstraingaugeMetalwires:stretchingofthewirechangesthegeometryofthewireinawaywhichactstoincreasetheresistance:GaugefactorK=2~4=Poisson’sratioTofindK:MetalwirebasedstraingaugeIssuesindesignwewouldprefertohavealargechangeinresistancetosimplifythedesignoftherestofasensinginstrument,sowegenerallytrytochoosesmalldiameters,smallyoung'smodulus,andlargegagefactorswhenpossible.Theelasticlimitsofmostmaterialsarebelow1%,sowearegenerallytalkingaboutresistancechangeswhichareinthe1%-0.001%range.Clearly,themeasurementofsuchresistancesisnottrivial,andweoftenseeresistancebridgesdesignedtoproducevoltageswhichcanbefedintoamplificationcircuits.WheatstonebridgeTheWheatstonebridgeiswidelyusedforprecisionmeasurementsofresistanceHowtochooseR?Rx=R+RR1=R2=R3V=-R*(V/4R)

MetalWireStrainGauge:thinfilmpattern

Themetallicstraingaugeconsistsofaveryfinewireor,morecommonly,metallicfoilarrangedinagridpattern.ThegridpatternmaximizestheamountofmetallicwireorfoilsubjecttostrainintheparalleldirectionAvarietyofshapesavailableStrainGaugeMeasurementInpractice,thestrainmeasurementsrarelyinvolvequantitieslargerthanafewmillistrain(x10-3).

Tomeasurethestrainrequiresaccuratemeasurementofverysmallchangesinresistance

Forexample,supposeatestspecimenundergoesastrainof500m.Astraingaugewithagaugefactorof2willexhibitachangeinelectricalresistanceofonly2(500x10-6)=0.1%.Fora120gauge,thisisachangeofonly0.12.

FoilStrainGaugeGaugefactor:alittleover2Outputsinglegauge+3dummyresistorsArea:2-10mm2Measurement:Quarter-bridgecircuitIfthenominalresistanceofthestraingaugeisdesignatedasRG,thenthestrain-inducedchangeinresistance,R,canbeexpressedas

R=RG·K·.AssumingthatR1=R2andR3=RG,VO/VEX=-K/4(1+K·/2)4(1+K·/2)termthatindicatesthenonlinearityofthequarter-bridgeoutputwithrespecttostrainTackleTemperatureEffectStraingaugematerialalsorespondtochangesintemperatureMinimizesensitivitytotemperaturebyprocessingthegaugematerialUsingtwostraingaugesinthebridge,theeffectoftemperaturecanbefurtherminimized

IncreaseSensitivityHalf-bridgecircuitFull-bridgeStrainGaugeinIndustryadfSignalConditioningBridgecompletionExcitationRemotesensingAmplificationFilteringOffsetShuntcalibrationStrainGaugeinIndustryPackagedfoilstraingaugeSpecificationsPerformance

Hysteresis<0.02%RatedOutput(R.O.)LongTermStability<0.04%RatedOutput(R.O.)Nonlinearrity<0.1%R.O./YearNonRepeatability<0.01%R.OCreep/CreepRecovery,20minutes<0.05%R.O.Temp.EffectonZeroBalanceStandard<0.03%R.O./°COptional<0.004%R.O./°CTemp.EffectonOutput

Standard<0.025%Reading/°COptional<0.002%Reading/°CAsopposetoP33.2007handbookBridgeCompletionUnlessyouareusingafull-bridgestraingaugesensorwithfouractivegauges,youwillneedtocompletethebridgewithreferenceresistors.Therefore,straingaugesignalconditionerstypicallyprovidehalf-bridgecompletionnetworksconsistingofhigh-precisionreferenceresistors.

OtherIssuesExcitation–typicallyprovideaconstantvoltagesourcetopowerthebridge.3~10Varecommon.

Whileahigherexcitationvoltagegeneratesaproportionatelyhigheroutputvoltage,thehighervoltagecanalsocauselargererrorsbecauseofself-heating..

RemotesensingLongleadneedswirecompensationOtherApplication:DataStorage

A100micron-longpiezoresistivecantileverisdraggedalongapolycarbonatediskat10mm/s,bouncingupanddownasitpassesoversub-micronindentationsinthesurfaceofthedisk.Thisideaisessentiallyahigh-perfo