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英文原文DSPBasedHighSpeedSensorlessControlforaBrushlessDCMotorUsingaDCLinkVoltageControlDepartmentofElectricalEngineeringKoreaAdvancedInstituteofScienceandTechnologyTaejon,KoreaADSPbasedhighspeedsensorlesscontrolforabrushlessDCBLDCmotorusingaDClinkvoltagecontrolschemeispresented.ByoperatingtheBLDCmotorinahighspeedrange,thedrivesystemcanhaveasmallsizeandbelightweightatthesameoutputrating.Intheexistingsensorlesscontrolschemes,thePWMtechniqueisgenerallyusedasaspeedcontrol.However,sincethePWMandinvertercommutationcannotbeperformedindependently,asignificantcommutationdelaymayexistinahighspeedregion.Ontheotherhand,usingtheDClinkvoltagecontrolscheme,theinverterisoperatedwiththesquarewaveof120°conductionandthespeedcontrolisachievedbyregulatingtheDClinkinputvoltageoftheinverterthroughthechopper.Byusingthistechnique,sincethevoltagecontrolandcommutationcanbeachievedindependently,acommutatingdelaydoesnotexisteveninahighspeedregion.Also,thephasecurrentcanhaveawaveformsimilartotherectangularwaveandtheterminalvoltageismoreecienttodealwithinthepositiondetectioncircuits.Thepracticalimplementationissuesconcerningthecommutationdelayinahighspeedsensorlesscontrolarediscussed.ThewholecontrolsystemisimplementedonaBLDCmotorusingDSPTMS320C240andtheeŒectivenessisveriedthroughthecomparativesimulationsandexperiments.KeywordsbrushlessDCmotor,sensorlesscontrol,DSPcontrol1.IntroductionInmanyindustrialelds,theinstallationofashaftsensormaysignicantlyincreasethedrivecostaswellascomplicatethemotorconfiguration1.Inparticular,foramotorbuiltinacompletelysealedcompressor,ashaftsensorisdifficulttoapplyduetothedegradationofthesensorreliabilityinhightemperatureandtheneedforextraleadwires.Furthermore,thesesensors,particularlyHallsensors,aretemperaturesensitive,limitingtheoperationofthemotortobelowabout75°C1.Anabsolutesensorisgenerallyspeedlimitedtoabout6000rpmandaresolverneedsaspecialexternalcircuit.Also,thesensoraccuracymaybeaffectedbytheaccuracyofthemounting.Toovercomethesedrawbacks,sensorlesscontroltechniquesforaBLDCmotorhavebeenproposed1_5.Therearetwocategoriesofpositiondetectionschemes,namely,themethodusingthebackEMFofthemotor2andthemethodbasedonthedetectionoftheconductingintervaloffreewheelingdiodes3.Intheexistingsensorlesscontrolschemes,thePWMtechniqueisgenerallyusedforaspeedcontrol.However,sincethePWMandinvertercommutationcannotbeperformedindependently,asignicantcommutationdelaymayexistinahighspeedregion.Recently,toimprovethedriveeffciencyandprovidethedesiredcurrentwaveform,asensorlesscontrolschemeusingaquasicurrentsourceinverterhasbeenproposed6.SuchacircuitarrangementisknownasavariableDClinkinverter7.Inthisscheme,theinverterfrequencyiscontrolledtosupplythreephaserectangularcurrentwithapulsewidthof120°andthemotorvoltageforthespeedcontrolisregulatedbyusingastepdownchopperactingasabuckconverter.However,someadvantagesoftheDClinkvoltagecontrolschemeovertheconventional2phasePWMschemeinthehighspeedsensorlesscontrolhavenotbeenaddressed.ThisarticlepresentsaDSPbasedhighspeedsensorlesscontrolforaBLDCmotorusingaDClinkvoltagecontrolscheme.BydrivingtheBLDCmotorathighspeed,theoveralldrivesystemcanhaveasmallsizeandalightweightatthesamepowerrating.TocontroltheBLDCmotorathighspeedwithoutashaftsensor,aDSPbasedcontrollerisdevelopedusingTMS320C240.UsingtheDClinkvoltagecontrolscheme,theinverterisoperatedwiththesquarewaveof120°conductionintervalandthespeedcontrolisachievedbyregulatingtheDClinkinputvoltageofinverterthroughthechopper.Byusingthistechnique,sincethevoltagecontrolandcommutationcanbeachievedindependently,thecommutatingdelaysuchasintheconventional2phaseexcitationPWMmethodsdoesnotexisteveninahighspeedregion,whichwillbediscussedinthelatersection.TherotorpositioninformationisdetectedusingthebackEMFfromtheterminalvoltagesofthemotorandtheswitchingsequenceoftheinverter2.ThesensedbackEMFisusedintheintegrationandcomparisoncircuitstoobtainthecommutationsignals.ThedetectedcommutationsignalsareusedtoapplythepropernextsequenceofinverterandobtaintherotationalspeedwithinaDSP.ThecalculatedspeediscontrolledbyadigitalPIcontrolalgorithmandthecontrolleroutputisappliedtothechopper.Thepracticalimplementationissuesconcerningthecommutationdelayofthe2phaseexcitationPWMschemesathighspeedarediscussedandsomeadvantagesoftheDClinkvoltagecontrolschemeinahighspeedsensorlesscontrolarementioned.ThewholecontrolsystemisimplementedonaBLDCmotorusingDSPTMS320C240andtheeŒectivenessisveriedthroughthecomparativesimulationsandexperiments.2.SensorlessControlofBLDCMotorABLDCmotorconsideredinthispaperconsistsofpermanentmagnetsmountedontherotorsurfaceandthreephaseconcentratedstatorwindingsdisplacedby120°.Thestatorcurrentsaresuppliedbythe2phaseexcitationschemewhereonlytwoofthethreephasesareexcitedatanyinstantoftimeandonephaseisconductedduring120°period8.Thisexcitationschemedoesnotrequiredeadtimeofthepowerdevices,andfurthermore,theunconductingopenphasecanbeusefullyutilizedtoobtaintherotorpositioninformation.TherotorpositioninformationaregenerallyobtainedfromtheindirectdetectionmethodusingthemotorbackEMF1_4.In2,therotorpositionhasbeenestimatedfromtheintegrationofthebackEMFwaveform.Thismethodisknowntoprovidetheadvantagessuchasthereducedswitchingnoisesensitivityandautomaticadjustmentoftheswitchinginstantswithoutthephaseshiftof30°degrees.Thus,thisdetectionschemeisemployedinthispaper.Thespeedinformationcanbeobtainedfromthederivativeofthedetectedpositionsignals.SincethecommutationsignalsarefedintoaDSPevery60°period,ifthecounterclockinDSPisTCandthenumberofcountduring60degreesisa,themechanicalrotorspeedcanbecomputedinrpmasfollowswherePisthenumberofpoles.3.ProblemsofExistingSensorlessSpeedControlSchemesIntheexistingsensorlesscontrolschemes,the2phaseexcitationPWMtechniqueisgenerallyemployedforaspeedcontrol.BasedonthemethodexecutingthePWM,PWMschemescanbeclassifiedastheunipolarandbipolarswitchingmethods.Intheunipolarswitchingmethod,thePWMissuperimposedononeofthetwoactiveswitchesinonstate,whiletheotherswitchremainsonstate.Ontheotherhand,inthebipolarswitchingmethod,thetwoactiveswitchesexecutethePWMatthesametime.Sincetheunipolarswitchinghasanadvantageofthereducedswitchingloss,thisschemeisgenerallypreferred4.Also,basedonthepositionthatthePWMissuperimposedon,theunipolarswitchingmethodisclassiedastheongoingphasePWM,offgoingphasePWM,upperswitchPWM,andlowerswitchPWMschemes.IntheongoingphasePWMscheme,eachswitchexecutesthePWMduringtherst60¯degreesofactiveintervalandisheldinonstateduringthesecond60°interval,andintheoffgoingphasePWMscheme,viceversa3,4.IntheupperswitchPWMscheme,thePWMisexecutedonlyontheupperoneoftwoactiveswitches,andinthelowerswitchPWMscheme,viceversa.DependingontheusedPWMscheme,thiscontroltechniquemaycauseacommutationdelayoranirregularswitchingfrequencyofthepowerdevicesinahighspeedsensorlesscontrol.Figure1showstherelationbetweenthePWMswitchingperiodandcommutatinginstantinthe2phaseexcitationPWMscheme.InFigure1,TsandfsdenotethePWMswitchingperiodandfrequency,respectively.Figure1ashowsacaseoftheidealcommutation.Ascanbeseeninthegure,ifthecommutatinginstantissynchronizedwiththeendofthePWMswitchingperiod,anidealcommutationcanbeobtainedwithoutanydelayintheinvertersequencechange.However,sincethecommutatinginstantdependsontherotorposition,itdoesnotusuallycoincidewiththeendofthePWMperiod.Inthiscase,thecommutationcanbeperformedsynchronizedwiththeendofthepresentPWMperiodtostartanextinvertersequenceasFigure1b,whichisthenormallyusedmethod.ThisresultsinanundesirablecommutationdelayandthemaximumvalueofthisdelaybecomesthePWMswitchingperiod.Iftheswitchingfrequencyischosenas16kHz,themaximumvalueofthecommutationdelaywillbe62.5sec.Eventhoughthiscommutationdelaycanbeneglectedforamediumspeedrange,ithassignificantinuencesonthephasecurrentresponseanddriveperformanceathighspeedsincethe60degreeintervalthatthecommutationarisesinisrelativelysmall.Forexample,whena2polemotorisrotatingat50,000rpm,60degreeintervalbecomes200sec.ThiscommutatingdelaycanbereducedbyincreasingthePWMswitchingfrequency.Inpractice,however,theswitchingfrequencycannotbeincreasedwithoutlimitbecauseoftheincreasedswitchingloss.Also,theswitchingfrequencyofcommerciallyavailablepowerdevicesislessthan20kHz.Thus,toavoidanundesirablecommutationdelay,thenextinvertersequencehastobeappliedassoonasthecommutationsignalinterruptoccurs.Then,thepresentPWMperiodhastobeterminatedandthenewPWMperiodsynchronizedwiththecommutationinterruptsignalmustbestarted.IntheupperandlowerswitchPWMschemes,thismayyieldanirregularswitchingfrequencymuchlargerthanfsunderahighdutyconditionasshowninFigure1c.IntheongoingandoffgoingphasePWMschemes,thisirregularswitchingfrequencydoesnotoccursincethephaseexecutingthePWMiscontinuallychangedevery60°interval.Thus,theongoingandoffgoingphasePWMschemeswiththemethodinFigure1ccanbeapreferredwayforahighspeedsensorlesscontrol.Nevertheless,thereisstillaproblem.Athighspeed,onlyafewPWMpulsescanbeusedforthespeedcontrolduringa60°interval.Sincea60°intervalofa2polemotorbecomes200secat50,000rpm,iftheswitchingfrequencyischosenas16kHz,thenumberofPWMpulsesduring60¯isonly3.2,whichresultsinanunequalnumberofPWMpulses3or4duringa60°interval.Unlesstheresolutionofthepulsewidthisconsiderablyhigh,thismayresultinaspeedrippleatsteadystateanddegradetheaccuracyofthepositionsignaldetection.ThisproblemismoreseriousatahigherspeedregionandcanbeeffectivelyovercomebycontrollingthevoltageandfrequencyindependentlybytheDClinkvoltagecontrolscheme.
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