传感器英文课件第5章-signal-conditioning-for-Reactance-Varia

15SignalConditioningforReactanceVariationSensorsSensorsandSignalConditioning2Variablereactancesensorscanconsistofthefollowing:asinglevaryingcapacitanceorinductanceavaryinginductanceplusareferenceinductance(e.g.,ineddycurrentproximitydetectors);adifferentialcapacitanceorinductanceoravariabletransformerthatyieldsanamplitude-modulatedsignal(e.g.,LVDTs,synchros,andresolvers).5.1ProblemsandAlternatives3Signalconditioningforallofthesesensorsmustincludeasupplyofexcitingalternatingcurrent.Inordertoavoidcapacitiveinterferencebecauseoftheirhighoutputimpedance,wefrequentlyconnectcapacitivesensorswithshieldedcables.Butthisaddsacapacitanceinparallelwiththatofthesensor,whichreducessensitivityanddecreaseslinearity.Furthermore,anyrelativemovementbetweencableconductorsandtheinsulatingdielectriccanincreaseerrors.4Theusualsolutionistoplacetheelectroniccircuitsascloseaspossibletothesensor,thususingshortcablesandevenrigidcables,andtoapplydrivenshieldtechniquesorimpedancetransformers.5Whenthemeasurementsystemrequiresallthesignalstobeconvertedtodcvoltages,someavailableoptionsforthesensorsworkingatalternatingfrequenciesarepeakdetection,rmsmeasurement,and,mostcommonly,meanvaluecalculationafterrectification.6AcommonsolutiontoobtainanelectricsignalfromavariablereactancesensorisjusttoapplyOhm'slaw.Achangeinimpedancecanbedetectedbymeasuringthechangeincurrentwhenaconstantacvoltageissuppliedtoit,orbythechangeinthedropinvoltageacrossitwhendrivenbyaconstantalternatingcurrent.Thereactancemeasurementtechniquesmustconsidersomepossibleinterferences.7ThecircuitinFigure5.1aappliestheconstantcurrentsupplymethodtoacapacitivedisplacementsensorbaseduponthevariationoftheseparationofplatesinaparallelplatecapacitor.Figure5.1(a)8Whenthecapacitancechangesaccordingtothefollowingequation,IftheopampisassumedidealandRisdisregarded,theoutputvoltageis9Hence,itisproportionaltothemeasureddistance,inspiteofthenonlinearrelationshipbetweenthecapacityandthedistance.Risaddedtobiastheopampandshouldbemuchhigherthanthesensorimpedanceattheexcitationfrequency.AnystraycapacitanceshuntingCxcontributesanoutputerror.Hence,itmustbereduced,forexample,byshieldingtheleadsconnectedtothecapacitorplates.10ThecircuitinFigure5.1b,termedachargeamplifier(Section7.3),appliesaconstantvoltagetothesensorandmeasurestheresultingcurrentbyconvertingittoavoltagethroughC.Figure5.1(b)11DisregardingR(opampbias)andstraycapacitanceCs3,theoutputvoltageisThusitisproportionaltothesensorcapacitance.NotethatstraycapacitancesCs1andCs2donotcontributetotheoutput.Cs1isinparallelwithavoltagesourceandCs2hasbothendsatthesamevoltagebecauseoftheopamp.Nevertheless,alargeCs2maycauseoscillation.ShieldingsensorleadsreducesCs3.12Voltagedividersareanalternativesolutiontointerfacevariablereactancesensors.Foracircuitinfigure5.2ayieldsanoutputvoltageFigure5.2(a)nonlinear13Nevertheless,fordifferentialsensors(Figure5.2b)wehaveFigure5.2(b)Nowtheoutputchangeslinearlywithx.14zSensitivityandLinearityFigure5.3(a)Generalacbridgewithonereactivesensor.4.2acBridgesFigure5.3(a)Generalacbridgewithonereactivesensor.Whenthebridgeincludesonlyonesensorwhoseimpedancechangeslinearlywiththemeasurand,Z1=Z0(1+x),(Figure5.3a),andZ0=Z2=Z3=Z4,theoutputvoltageiswhichshowsanonlinearrelationshipwithx.15Butforadifferentialsensorplacedinadjacentarms,Figure5.3(b),theoutputisFigure5.3(b)andthereforevoisproportionaltox.16ThecircuitinFigure5.3chasdoublesensitivity,butitisnonlinear.Figure5.3(c)17transformerbridgeWhenZ1=Z2,U0=0Wheneverresistivelossesinsensorcoilsaresmall(highQ),changesinsensorresistancecanbeneglectedandthecircuityieldsalinearoutput.1819Figure5.20Phaserecoveryprobleminamplitudedemodulation.Incases(a)and(c)thesignalvo(t).isdemodulatedbysimplerectificationandlow-passfiltering,andtheinformationaboutthesignislost.Incases(b)and(d)theinformationaboutthesignisrecoveredthroughphase-sensitivedemodulation.21