Motor Control Sensor Feedback Circuits.docx
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Motor Control Sensor Feedback Circuits.docx
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MotorControlSensorFeedbackCircuits
MotorControlSensorFeedbackCircuits
INTRODUCTION
Sensorsareacriticalcomponentinamotorcontrolsystem.Theyareusedtosensethecurrent,position,speedanddirectionoftherotatingmotor.Recentadvancementsinsensortechnologyhaveimprovedtheaccuracyandreliabilityofsensors,whilereducingthecost.Manysensorsarenowavailablethatintegratethesensorandsignal-conditioningcircuitryintoasinglepackage.
Inmostmotorcontrolsystems,severalsensorsareusedtoprovidefeedbackinformationonthemotor.Thesesensorsareusedinthecontrolloopandtoimprovethereliabilitybydetectingfaultconditionsthatmaydamagethemotor.Asanexample,Figure1providesablockdiagramofaDCmotorcontrolsystemtoshowthesensorfeedbackprovidedforatypicalmotorcontrol.
Alistofthesensorsthatcanbeusedtofeedbackinformationtoamicrocontrollerarelistedbelow:
•Currentsensors
-Shuntresistor
-Current-sensingtransformer
-Halleffectcurrentsensor
CURRENTSENSORS
Thethreemostpopularcurrentsensorsinmotorcontrolapplicationsare:
•Shuntresistors
•Halleffectsensors
•Currenttransformers
Shuntresistorsarepopularcurrentsensorsbecausetheyprovideanaccuratemeasurementatalowcost.Halleffectcurrentsensorsarewidelyusedbecausetheyprovideanon-intrusivemeasurementandareavailableinasmallICpackagethatcombinesthesensorandsignal-conditioningcircuit.Current-sensingtransformersarealsoapopularsensortechnology,especiallyinhigh-currentorACline-monitoringapplications.AsummaryoftheadvantagesanddisadvantagesofeachofthecurrentsensorsisprovidedinTable1.
Figure2showsanexampleofanACmotorpoweredbyathree-phaseinverterbridgecircuit.ThisexampleshowsthatthecompositecurrentofallthreeInsulatedGateBipolarTransistor(IGBT)circuitlegscanbemeasuredwithasingleshuntresistor,orthatthecurrentineachindividuallegcanbedeterminedwiththreeshuntresistors.Figure2showsasystemthatusesshuntresistors.However,Halleffectandcurrentsensingtransformerscanalsobeusedtoprovidethecurrentmeasurement.
ShuntResistors
Shuntresistorsareapopularcurrent-sensingsensorbecauseoftheirlowcostandgoodaccuracy.Thevoltagedropacrossaknownlowvalueresistorismonitoredinordertodeterminethecurrentflowingthroughtheload.Iftheresistorissmallinmagnitude,thevoltagedropwillbesmallandthemeasurementwillnothaveamajoreffectonthemotorcircuit.Thepowerdissipationoftheresistancemakescurrentshuntsimpracticalformeasurementsofmorethanapproximately20amperes.
Theselectioncriteriaofashuntcurrentresistorrequirestheevaluationofseveraltrade-offs,including:
•IncreasingRSENSEincreasestheVSENSEvoltage,whichmakesthevoltageoffset(VOS)andinputbiascurrentoffset(IOS)amplifiererrorslesssignificant.
•AlargeRSENSEvaluecausesavoltagelossandareductioninthepowerefficiencyduetotheI2xRlossoftheresistor.
•AlargeRSENSEvaluewillcauseavoltageoffsettotheloadinalow-sidemeasurementthatmayimpacttheEMIcharacteristicsandnoisesensitivityofthesystem.
•Special-purpose,lowinductanceresistorsarerequiredifthecurrenthasahigh-frequencycontent.
•ThepowerratingofRSENSEmustbeevaluatedbecausetheI2xRpowerdissipationcanproduceselfheatingandachangeinthenominalresistanceoftheshunt.
Special-purpose,shuntcurrentmeasurementresistorsareavailablefromanumberofvendors.Ifstandardresistorsareused,itisrecommendedthatmetal-filmresistorsbeusedratherthanwire-woundresistorsthathavearelativelylargeinductance.
AshuntresistorcanalsobecreatedfromthetraceresistanceonaPCB,asshowninFigure3.PCBshuntresistorsofferalowcostalternativetodiscreteresistors.However,theiraccuracyoverawidetemperaturerangeispoorwhencomparedtoadiscreteresistor.ThetemperaturecoefficientofacopperPCBtraceshuntresistorisequaltoapproximately+0.39%/C.FurtherdetailsonPCBtraceresistorsaregiveninreference
(2).
FIGURE3:
PCBShuntResistor.
High-Sidevs.Low-SideCurrentShuntMeasurements
SYSTEMINTEGRATIONISSUES
Shuntresistorscanprovideeitherahigh-sideorlowsidemeasurementofthecurrentthroughtheload,asshowninFigure4.Ahigh-sidemonitorhastheresistorconnectedinserieswiththepowersource,whilethelow-sidemonitorlocatestheresistorbetweentheloadandthegroundcurrentreturnpath.Bothapproachesposeatrade-offtothedesigner.Theattributesofthetwomethods,alongwiththetypicalmonitorcircuits,willbeshowninthefollowingsections.Reference(3)providesmoredetailsonhigh-sideandlow-sideshunts.
High-sidecurrentmeasurementsarethepreferredmethodfromasystem-integrationstandpointbecausetheyarelessintrusivethanlow-sidemeasurements.Thetrade-offwiththehigh-sidemeasurementisthatthecircuitryismorecomplexthanthelow-sidemethod.
High-sideresistiveshuntmeasurementswillnothaveasignificantimpactonthesystemifthesensingresistorissmallandtheresultingvoltagedropacrosstheshuntissmallcomparedtothesupplyvoltage.Incontrast,low-sidemonitoringdisruptsthegroundpathoftheload,whichcancausenoiseandEMIproblemsinthesystem.
Low-sidecurrentmeasurementsareoftenchosenbecauselowvoltageopampscanbeusedtosensethevoltageacrosstheshuntresistor.Notethatlow-sidemonitoringisnotpossibleinsomeapplicationsbecausethegroundconnectionismadeviathemechanicalmountingofthemotoronthechassisormetalframe.Forsystemspoweredviaasinglewireconnection,itmaynotbepracticaltoinsertashuntresistorbetweenthedeviceandthechassisthatfunctionsasthegroundwire.
FIGURE4:
High-SideandLow-SideResistiveCurrentShunts.
HIGH-SIDECURRENTSHUNTMEASUREMENTS
High-sidecurrentmeasurementscanbeimplementedwithadifferentialamplifiercircuitthatproducesanoutputvoltagethatisproportionaltoVSENSEorthecurrentflowingthroughtheload.Figure5providesanexampleofahigh-sideshuntcircuit.ThedifferentialamplifiercircuitcanbeimplementedwithanopampanddiscreteresistorsorwithanintegratedICdevice.IntegrateddifferentialamplifierICsareavailablefromanumberofsemiconductorvendorsandofferaconvenientsolutionbecausetheamplifierandwellmatchedresistorsarecombinedinasingledevice.
Theattributesofhigh-sidemonitoringarelistedbelow:
Advantages:
•Lessintrusivethanlow-sidemonitorsandwillnotaffecttheEMIcharacteristicsofthesystem.
•Candetectovercurrentfaultsthatcanoccurbyshortcircuitsorinadvertentgroundpathsthatcanincreasetheloadcurrenttoadangerouslevel.
•Adifferentialamplifiercircuitwillfilterundesirablenoiseviathecommon-mode-rejection-ratio(CMRR)oftheamplifier.
•Aresistivenetworkcanbeusedtoreducethevoltageattheamplifier’sinputterminals.Forexample,ifRIN=R*,theinputvoltagewillbereducedinhalfandtheamplifierwillbebiasedatVS/2.Notethattheamplifiergainwillbeequaltooneandthatasecondamplifiermaybeneededtoincreasethesensor’soutputvoltage.
Disadvantages:
•TheVSENSEvoltageisapproximatelyequaltothesupplyvoltage,whichmaybebeyondthemaximuminputvoltagerangeoftheoperationalamplifier.
•Adifferentialamplifier’sCMRRwillbedegradedbymismatchesintheamplifierresistors.
•Theinputimpedanceofthedifferentialcircuitisrelativelylowandisasymmetrical.Theinputimpedanceattheamplifier’snon-invertinginputisequaltoRIN+R*,whiletheimpedanceattheinvertingterminalisequaltoRIN.
•Mayrequirerail-to-rail-inputopampsbecauseofthehighvoltageleveloftheinputsignal.
Thehigh-sideshuntcircuitrequiresahigh-voltageamplifierthatcanwithstandahighcommonmodevoltage.Inaddition,thekeyamplifierspecificationsareahighCMRRandalowVOSbecauseoftherelativelysmallmagnitudeofVSENSE.HighvoltageopampsandintegrateddifferentialamplifierICsareavailableforsystemsthathaveamaximumvoltageofapproximately60V.Forvoltagerequirementsbeyond60V,acurrentmirrorcircuitcanbeusedtosensethecurrent.Acurrentmirrorcanbeimplementedwithreadilyavailable,high-voltagetransistors.References
(1)and(5)provideexamplesofhigh-voltage,high-sidecurrentmonitorcircuits.
Table2providesalistoftherecommendedMicrochipopampsthatcanbeusedinahigh-sidecircuit.
FIGURE5:
High-SideResistiveCurrentMeasurementCircuit.
LOW-SIDECURRENTMEASUREMENT
Low-sidecurrentmeasurementsoffertheadvantagethatthecircuitrycanbeimplementedwithalowvoltageopampbecausethemeasurementisreferencedtoground.Thelow-sidemeasurementcircuitcanuseanon-invertingamplifier,asshowninFigure6.
Thelow-sidecurrentmonitorcanalsobeimplementedwithadifferentialamplifier.TheadvantagesofdifferentialamplificationarelimitedbecauseRSENSEisconnectedtogroundandthecommonmodevoltageisverysmall.NotethatintegratedIClow-sidemonitorsthatcombinetheopampandresistorsarenotreadilyavailablebecauseofthesimplicityofthecircuitthatcanbeimplementedwithafewdiscreteresistorsandlowvoltageopamp.
Theattributesoflow-sidemonitoringare:
Advantages
•VSENSEisreferencedtoground.Therefore,alowvoltageamplifiercanbeused.
•Anon-invertingamplifiercanbeusedandtheinputimpedanceofthecircuitwillbeequaltothelargeinputimpedanceoftheamplifier.
Disadvantages
•Thelow-sideresistordisruptsthegroundpathandtheaddedresistancetothegr
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