MRvsHall_technicalanalysis

MRvs.HallEffectforPositionSensingHowmagnetoresistivesensorsandintegratedHallsensorsdifferandwhatthismeansfordesignersofmagneticangleandlinearpositionsensors.WhilebothmagnetoresistiveandintegratedHallsensorsmeasurethestrengthofmagneticfieldcomponents,theydosoindifferentwaysandthiscandirectlyaffecttheirsuccessfuluseinapositionsensingapplication.Choosingwhichofthetwoisabetterfitforyourapplicationmayrequireyoutoconsidersensorcharacteristicsthatdon'tshowuponmanufacturers'datasheets.Thisarticleisdesignedtohelpyourdecisionprocessbyreviewingtwoanglesensors—ananisotropicmagnetoresistive(AMR)sensorandaHallsensor—inaposition-sensingapplicationexample.TheTheoreticalBasicsFigure1showshowthetwosensortypesreactinthepresenceofanexternalmagneticfield.Amagnetoresistive(MR)sensorrecordsaxisormagnetangleinformationviathefieldvectorinthechipplane.TheAMReffectisaresistoreffectbasedonthedependenceofelectricalresistanceontheanglebetweenthedirectionsofthecurrentflowandmagnetizationofaferromagneticmaterial.AnexternalmagneticfieldcanswitchtheinternaldirectionofmagnetizationoftheAMRmaterialandthuseffectchangesinresistance(ΔR/R)thataretypicallyontheorderofafewpercent.Figure1:HereweseethebasicprinciplesofoperationforAMRandHalleffectsensors.SeveralresistorsconnectedinaWheatstonebridgeareusedtomeasureangles.Eachcomponentresistorcontributestoachangeinthedifferentialoutputvoltageofthebridge,dependingontheX,Yfielddirectioninthesensorplaneandonthedirectionofcurrentflowintheresistors.Thisdirectionofcurrentflowiseithergivenbythealignmentoftheresistorpathsorisforcedinanotherdirectionbyshort-circuitcontacts(barberpoles).ForoursampleAMRsensor(seeitsspecificationsinFigure2),thebridgeprovidesadifferentialsinevoltageof70mVwithacyclicrepetitionevery180°.Afieldalignedintheoppositedirectionproducesthesameresistiveeffect.Figure2:Selectedelectricalcharacteristicsofthesensorsfromthemanufacturers'datasheets.Toevaluatethisbridgevoltageweneedausefulreference—preferablyareferencevoltage.Ideally,thisisasecondbridgesetatatilttothefirstbridgeandgeneratingadifferentialcosinevoltage.Wecannowdeterminetheangletobemeasuredfromtherelationshipbetweenthetwobridgevoltages.Sinceanychangeinsensitivity(causedbytemperature,forexample)willaffectbothvoltagesinasimilarway,thesechangesarenotimportantfordeterminingtheangle.ForoursampleHallsensor,we'reinterestedinthefieldcomponentverticaltothechip.TheHalleffectgeneratesadirectedvoltage,typicallyofjustafewmillivolts,whichisdependentonthestrengthoftheZcomponentofamagneticfield.Consequently,asingleHallelementseesonlythedistancebetweenitselfandthemagnet.Ifseveralelementsareused,eachrecordingtheZfieldatvariouspositions,thenevaluatinglocalgradientsprovidesinformationontheangle.ThearrangementofHallelementsonthechipdetermineswhichmagnettousesincethesensormustbeabletodetectthebentnearmagneticfield.Externalhomogenousstrayfields,suchasthosefrombrushlessDCmotorassemblies,cannotcauseinterference.BecausethepolarityoftheHallvoltagefollowsthefielddirection,amagneticnorthpolecanbedistinguishedfromasouthpole.Therangeofanglemeasurementisextendedto360°butwithhalfthesensitivityforanglechangeswithin180°incomparisontoMRsensors.Tosummarize,adesignercanexpectthefollowingcharacteristicsfromthetwotypesofsensors:FortheAMRsensor:1.Highsensitivityandalow-noisesignal.2.Alargeoperatingdistancefromthemagnetispossible.3.Angleofmeasurementisupto180°butwithhigheranglesensitivity4.Possibledisturbancefromexternalstrayfields.FortheHallsensor:1.Becauseofinternalpreamplification,sensitivityiscomparable2.Sensorfunctionsonlyinthenearfieldofthemagnet2.Angleofmeasurementisupto360°3.Possibledisturbancefromexternalstraydipolefields.ComparingSensorSpecificationsForourexperiment,thetwosamplesensorsaretheKMZ43TMRanglesensorfromPhilipsandtheiC-MAHallsensorfromiC-Haus.WeshallexcludethevariousintegratedevaluationfunctionsoftheHallsensorfromthisstudy(forfurtherdiscussionseesidebariC-MASensorFunctions)andconcentrateinsteadontheanalogsensorsignalswhichhavebeenpreconditionedonthechip.Intheexperimenttheseanalogsignalsarelinkedtotheevaluationcircuitry.Manufacturers'datasheets,asshowninFigure2,don'tusuallyspecifythequalityoftheanalogsine/cosinesensorsignalswithrespecttoharmonic-freewaveform,amplitudesynchronism,phaseprecision,andoffsetnordotheyentirelyspecifychangesinfieldstrengthdependencyortemperature.Theinformationprovidedis,however,sufficienttoletusadapttheevaluationcircuitrytothesensorandtocorrectfirst-ordersignalerrors.Wewantareasonablesignallevel.TheMRsensor'stypicalsignalamplitudeof70mVrequirespreamplification;incontrast,theHallsensorprovidespreamplificationonchip.Nonmatchingamplitudesarenotacceptable:adifferencebetweenthesineandcosinesignalamplitudecausesanangleerrorof~0.2°(withrespecttothesignalperiod)atadifferenceinamplitudesof0.7%.WhiletheMRsensordoesnotsufferfromnonmatchingamplitudes,theHallsensorwillneedtobecalibratedduetothe5%signal-referencedspecification.Calibrationoftheoffsetisevenmoreimportantbecauseaslittleasa1%errorforboththesineandcosinesignalleadstoanangleerrorof~0.8°