5-SIGNAL-CONDITIONING-FOR-REACTANCE-VARIATION-SENS

5SIGNALCONDITIONINGFORREACTANCEVARIATIONSENSORSInordertoobtainausefulsignalfromthevariationofacapacitanceoraninductance,weneedatleastanacexcitationvoltageorcurrentforthesensorandsomemethodfordetectingthevariationsduetothemeasurand.5.1PROBLEMSANDALTERNATIVESMeasurementsignals:Analogpresentation,conversiontodigital,conversiontoavariablefrequencysignal,voltagetelemetry,andcurrenttelemetry.Variablereactancesensors:asinglevaryingcapacitanceorinductanceavaryinginductanceplusareferenceinductanceadifferentialcapacitanceorinductance5.1PROBLEMSANDALTERNATIVESSignalconditioningforallthesesensorsmustincludeasuppleofexcitingalternatingcurrent.Capacitivesensors:usuallysmallerthan100pFThesupplyfrequencymustthenbefrom10kHzto100MHzinordertoyieldreasonablevaluesfortheirimpedances.5.1PROBLEMSANDALTERNATIVESInordertoavoidcapacitiveinterferencebecauseoftheirhighoutputimpedance,wefrequentlyconnectcapacitivesensorswithshieldedcables.Thisaddsacapacitanceinparallelwiththatofthesensor:•Reducesensitivityanddecreaselinearity•Anyrelativemovementbetweencableconductorsandtheinsulatingdielectriccanincreaseerrors.Placetheelectroniccircuitascloseaspossibletothesensor.5.1PROBLEMSANDALTERNATIVESIfrequireallthesignalstobeconvertedtodcvoltage:•Peakdetection•rmsmeasurement•MeanvaluecalculationafterrectificationToobtainanelectricsignalfromavariablereactancesensor:Ohm’slaw.QualityfactorQ:itsreactancedividedbyitsresistanceatagivenfrequency.5.1PROBLEMSANDALTERNATIVESInFigure5.1a,constantcurrentsupplySensor:capacitancedisplacement,baseduponthevariationoftheseparationofplatesinaparallelplatecapacitor.Figure5.15.1PROBLEMSANDALTERNATIVESWhenthecapacitancechangesaccordingto(4.4),weobtainIftheopampisassumedidealandRisdisregarded,theoutputvoltageisItisproportionaltothemeasureddistance.xCxdACx110xCCvZZvvexe1005.1PROBLEMSANDALTERNATIVESInFigure5.1b,chargeamplifier,applyaconstantvoltagetothesensor,measuretheresultingcurrent.TheoutputvoltageisItisproportionaltothesensorcapacitance.ThestraycapacitancesCs1andCs2donotcontributetotheoutput.Cs1isinparallelwithvoltagesource.Cs2hasbothendsatthesamevoltage.CCvvxe05.1PROBLEMSANDALTERNATIVESExample5.1Asignalconditionerforthedifferentialcapacitivesensor.5.1PROBLEMSANDALTERNATIVESSinceRisjusttobiastheopamp,wewilldisregardit.TheoutputiswherevAisthevoltageatthemovabeelectrode.AccordingtoKirchoff’scurrentlaw,430CCvvA32121231CCCCCVVVVCjVCjVVCjeAAeAAe5.1PROBLEMSANDALTERNATIVESFromExample4.2wehaveReplacingtheseexpressionsinthoseforvAandv0yielddwhCqhLzhdwCCqhLzhdwCCABAB030'2012222404210122CLxqhdwvCCCvvee5.1PROBLEMSANDALTERNATIVESInordertohaveaFSO=1VweneedForw=8mm,d=0.5mm,h=10mm,andq=1mm,itleadstoC4=5pF.InorderforRnottoaffecttheoutputwaveform,weselectitsimpedancetobe10timesofC4attheexciationfrequency,R=33MxLqhdwV21005.1PROBLEMSANDALTERNATIVESVoltagedividersareanalternativesolutiontointerfacevariablereactancesensors.xxvxZZxZvvee2111000Figure5.25.1PROBLEMSANDALTERNATIVESItisnonlinearwithrespecttox.Straycapacitanceshuntingthesensorwillproduceanoutputerror.Fordifferentialsensors(Figure5.2b),wehave211110000xvxZxZxZvvee5.2acBRIDGES5.2.1SensitivityandLinearityTheclassicalsolutiontocanceltheconstanttermappearingattheoutputofavoltagedividerisabridgecircuit.5.2acBRIDGESWhenthebridgeincludesonlyonesensor,theoutputvoltageisForadifferentialsensorplacedinadjacentarms(Figureb),theoutputisChoosetheimpedancesforthetwootherarms,dependingonthetypeofsensor.xxvve22020xvve5.2acBRIDGESSingleanddifferentialcapacitivesensorshavelargeimpedance,usingresistorsfortheremainingbridgearmsmayproducelargeerrorsduetoparasiticimpedancestoearth.Usingabridgewithtwotightlycoupledinductivearmshavinganaccuratewindingratioandacentralterminalreduceserrorsfromstraycapacitance.5.2acBRIDGESBlumlein(Transformer)bridgesFigure5.4变压器自偶变压器被保护传感器的屏蔽连接等效电路5.2acBRIDGESAdvantages:Forcapacitivesensorconditioning,thetransformer’scentralterminalisusuallygrounded.ItmakesstraycapacitancestogroundCs1andCs2haveanegligibleinfluenceonbridgebalance.ThevoltageorcurrentratioisveryconstantbothwithtimeandtemperaturebecauseitonlydependsonN3/N4.Three-windingtransformers:Providegalvanic(电流)isolationbetweentheoscillatoranddetector.Thispermitsthemtobegroundedatdifferentialpointswithoutrequiringdifferentialmeasurement.5.2acBRIDGESFigure5.4distheequivalentcircuitfortransformerbridges.ForavoltagetransformerhavingN3=N4,thecoilimpedancesareverysmallwehaveZs=Z1||Z2andSupposethatsensorimpedancevariationsarelinearandtheinputimpedanceofthedetectorishigh,thedetectedvoltageis121212222ZZZZvvZZZvveees2xvvvesd5.2acBRIDGESForadifferentialcapacitivesensorbasedonthevariationofeffectiveplatearea,theoutputvoltageisItisbettertousealowinputimpedancedetectorandyieldsxZvied0xZZxZZ1,102012xvves5.2acBRIDGESCurrenttransformer:Capacitivesensorsmayinduceresonance(谐振).Theyaremainlyusedfordifferentialinductivesensorswhoseimpedanceissohighthatstraycapacitancesshouldbeconsideredandtheireffectscanceledbymeansofatr