Brushless DC Motor Control using

AVR194:BrushlessDCMotorControlusingATmega32M1Features•BLDCMotorBasics•HardwareImplementation•CodeExampleReferences[1]ATmega32/64/M1/C1datasheet[2]AVR138:ATmega32M1familyPSCCookbook[3]AVR430:MC300HardwareUserGuide[4]AVR470:MC310HardwareUserGuide1.IntroductionThisapplicationnotedescribeshowtoimplementabrushlessDCmotorcontrolinsensormodeusingtheATmega32M1AVRmicrocontroller.ThehighperformanceAVRcorefittedwithPowerStageControllermoduleofATmega32M1allowstodesignhighspeedbrushlessDCmotorapplications.Inthisdocument,wewillgiveashortdescriptionofbrushlessDCmotortheoryofoperations,wewilldetailhowtocontrolabrushlessDCmotorinsensormodeandwewillalsogiveashortdescriptionoftheATAVRMC310andATAVRMC300boardsusedinthisapplicationnote.SoftwareimplementationisalsodiscussedwithsoftwarecontrolloopusingaPIDfilter.ThisapplicationnotedealsonlywithBLDCmotorcontrolapplicationusingHalleffectpositionsensorstocontrolcommutationsequence.2.TheoryofOperationBrushlessDCmotorsareusedinagrowingnumberofmotorapplicationsastheyhavemanyadvantages:Theyhavenobrushessotheyrequirelittleornomaintenance.TheygeneratelessacousticandelectricalnoisethanuniversalbrushedDCmotors.Theycanbeusedinhazardousoperationenvironments(withflammableproducts).8-bitMicrocontrollerApplicationNote8138A–AVR–04/0828138A–AVR–04/08AVR194Theyalsohaveagoodweight/sizetopowerratio.Suchmotorshavelittlerotorinertia.Thecoilsareattachedtothestator.Thecommutationiscontrolledbyelectronics.CommutationtimesareprovidedeitherbypositionsensorsorbycoilsBackElectromotiveForcemeasurements.Insensormode,BrushlessDCmotorsusuallyconsistofthreemainparts:aStator,aRotorandHallSensors.2.1StatorAbasicthreephasesBLDCmotorStatorhasthreecoils.Inmanymotorsthenumberofcoilsisreplicatedtohaveasmallertorqueripple.Figure1showstheelectricalschematicofthestator.Itconsistsofthreecoilseachincludingthreeelementsinseries,aninductance,aresistanceandonebackelectromotiveforce.Figure1.StatorElectricalConfiguration(Threephases,threecoils)2.2RotorTherotorinaBLDCmotorconsistsofanevennumberofpermanentmagnets.Thenumberofmagneticpolesintherotoralsoaffectsthestepsizeandtorquerippleofthemotor.Morepolesgivesmallerstepsandlesstorqueripple.Thepermanentmagnetsgofrom1to5pairsofpoles.Incertaincasesitcangoupto8pairsofpoles.(Figure2).Figure2.Threephase,threecoilBLDCmotorstatorandrotorL,RL,RL,RBemfUBemfVBemfWUVWNScoilUcoilWcoilVRotorStatorNScoilUcoilWcoilVRotorStatorNS1pairofpoles2pairsofpoles38138A–AVR–04/08AVR194Thecoilsarestationarywhilethemagnetisrotating.TherotoroftheBLDCmotorislighterthantherotorofaconventionaluniversalDCmotorwherethecoilsareplacedontherotor.2.3HallSensorFortheestimationoftherotorposition,themotorisequippedwiththreehallsensors.Thesehallsensorsareplacedevery120°.Withthesesensors,6differentcommutationsarepossible.Phasecommutationdependsonhallsensorvalues.Powersupplytothecoilschangeswhenhallsensorvalueschange.Withrightsynchronizedcommutations,thetorqueremainsnearlyconstantandhigh.Figure3.HallSensorssignalsforCWrotation2.4PhaseCommutationsTosimplifytheexplanationofhowtooperateathreephaseBLDCmotor,atypicalBLDCmotorwithonlythreecoilsisconsidered.Aspreviouslyshown,phasescommutationdependsonthehallsensorvalues.Whenmotorcoilsarecorrectlysupplied,amagneticfieldiscreatedandtherotormoves.ThemostelementarycommutationdrivingmethodusedforBLDCmotorsisanon-offscheme:acoiliseitherconductingornotconducting.Onlytwowindingsaresuppliedatthesametimeandthethirdwindingisfloating.Connectingthecoilstothepowerandneutralbusinducesthecurrentflow.Thisisreferredtoastrapezoidalcommutationorblockcommutation.TocommandbrushlessDCmotors,apowerstagemadeof3halfbridgesisused.Figure4belowshowsa3halfbridgeschematic.tttHall_1Hall_2Hall_3tttCoil_UCoil_VCoil_W48138A–AVR–04/08AVR194Figure4.PowerStageReadinghallsensorvaluesindicateswhichswitchshouldbeclosed.Formotorswithmultiplepolestheelectricalrotationdoesnotcorrespondtoamechanicalrota-tion.AfourpoleBLDCmotorusesfourelectricalrotationcyclestohaveonemechanicalrotation.Thestrengthofthemagneticfielddeterminestheforceandspeedofthemotor.Byvaryingthecurrentflowthroughthecoils,thespeedandtorqueofthemotorcanbeadjusted.Themostcommonwaytocontrolthecurrentflowistocontroltheaveragecurrentflowthroughthecoils.PWM(PulseWidthModulation)isusedtoadjusttheaveragevoltageandtherebytheaveragecurrent,inducingthespeed.Forexample,thePWMfrequencyselectedistherangefrom10kHzto200kHzaccordingtotheapplication(commutationlosses,audiblefrequency...).Forathreephase,threecoilBLDCmotor,therotatingfieldisdescribedinFigure5.Table1.SwitchescommutationforCWrotationHallSensorsValue(H3H2H1)PhaseSwitches101U-VQ1;Q4001U-WQ1;Q6011V-WQ3;Q6010V-UQ3;Q2110W-UQ5;Q2100W-VQ5;Q4UVWVDCQ1Q5Q6Q3Q4Q2+V058138A–AVR–04/08AVR194Figure5.CommutationstepsandrotatingfieldCommutationcreatesarotatingfield.Step1PhaseUisconnectedtothepositiveDCbusvolt-agebyQ1andPhaseVisconnectedtogroundbyQ4,PhaseWisunpowered.TwofluxvectorsaregeneratedbyphaseU(redarrow)andphaseV(bluearrow).Thesumofthetwovectorsgivethestatorfluxvector(greenarrow).Thentherotortriestofollowthestatorflux.Assoonasthero