Background
Withthedevelopmentofelectronictechnology,avarietyofpulsewidthmodulation(PWM)technologieshaveemerged,including:phasevoltagecontrolPWM,pulsewidthPWMmethod,RandomPWM,SPWMmethod,linevoltagecontrolPWM,etc.,andthepulsewidthPWMmethodusedintheNi-MHbatteryintelligentcharger,itusesthepulsetrainwithequalpulsewidthasthePWMwaveform,bychangingtheperiodofthepulsetrainThefrequencycanbeadjusted,andthevoltagecanbeadjustedbychangingthepulsewidthordutycycle,andthevoltageandfrequencycanbechangedinacoordinatedmannerbyadoptinganappropriatecontrolmethod.ThepurposeofcontrollingthechargingcurrentcanbeachievedbyadjustingtheperiodofthePWMandthedutyratioofthePWM.
Thevalueoftheanalogsignalcanbechangedcontinuously,andtheresolutionofitstimeandamplitudeisnotlimited.A9Vbatteryisananalogdevice,becauseitsoutputvoltageisnotexactlyequalto9V,butchangesovertime,andcantakeanyrealvalue.Similarly,thecurrentdrawnfromthebatteryisnotlimitedtoasetofpossiblevalues.Thedifferencebetweenananalogsignalandadigitalsignalisthatthevalueofthelattercanusuallyonlybelongtoapredeterminedsetofpossiblevalues,forexample,avalueinthesetof{0V,5V}.
Theanalogvoltageandcurrentcanbedirectlyusedforcontrol,suchascontrollingthevolumeofacarradio.Inasimpleanalogradio,thevolumeknobisconnectedtoavariableresistor.Whentheknobisturned,theresistancevaluebecomeslargerorsmaller;thecurrentflowingthroughthisresistancealsoincreasesordecreases,therebychangingthecurrentvaluedrivingthespeaker,makingthevolumecorrespondinglylargerorsmaller.Likearadio,theoutputofananalogcircuitislinearlyproportionaltotheinput.
Althoughanalogcontrolmayseemintuitiveandsimple,itisnotalwaysveryeconomicalorfeasible.Oneofthemisthatanalogcircuitstendtodriftovertimeandarethereforedifficulttoadjust.Thesophisticatedanalogcircuitsthatcansolvethisproblemcanbeverylarge,bulky(suchasold-fashionedhomestereoequipment),andexpensive.Theanalogcircuitmayalsogeneratesevereheat,anditspowerconsumptionisproportionaltotheproductofthevoltageandcurrentatbothendsoftheworkingelement.Theanalogcircuitmayalsobeverysensitivetonoise,anydisturbanceornoisewilldefinitelychangethemagnitudeofthecurrentvalue.
Bycontrollingtheanalogcircuitdigitally,thecostandpowerconsumptionofthesystemcanbegreatlyreduced.Inaddition,manymicrocontrollersandDSPshaveincludedPWMcontrollersonthechip,whichmakestherealizationofdigitalcontroleasier.
Basicprinciple
Thebasicprincipleofpulsewidthmodulation(PWM):Thecontrolmethodistocontroltheonandoffoftheswitchingdevicesoftheinvertercircuit,sothattheoutputterminalobtainsaseriesofequalamplitudebutForpulseswithinconsistentwidths,usethesepulsestoreplacethesinewaveorthedesiredwaveform.Thatis,multiplepulsesaregeneratedinthehalfcycleoftheoutputwaveform,sothattheequivalentvoltageofeachpulseisasinusoidalwaveform,andtheobtainedoutputissmoothandhaslesslow-orderharmonics.Thewidthofeachpulseismodulatedaccordingtocertainrules,whichcanchangetheoutputvoltageandoutputfrequencyoftheinvertercircuit.
Kuva 1PWTodellinen aaltomuotokaavio (2 kuvaa)
Forexample,ifthesinehalf-wavewaveformisdividedintoNequalparts,thesinehalf-wavecanberegardedasaAwaveformcomposedoftwoconnectedpulses.Thesepulseshavethesamewidthandareequaltoπ/n,buttheamplitudesarenotequal,andthetopofthepulseisnotahorizontalstraightline,butacurve,andtheamplitudeofeachpulsechangesaccordingtothesinelaw.Iftheabove-mentionedpulsesequenceisreplacedwiththesamenumberofrectangularpulsesequencesofequalamplitudebutnotequalwidth,themidpointoftherectangularpulseandthemidpointofthecorrespondingsineequaldivisionarecoincident,andtheareaoftherectangularpulseandthecorrespondingsinepart(i.e.impulse)Iftheyareequal,asetofpulsesequencesisobtained,whichisthePWMwaveform.Itcanbeseenthatthewidthofeachpulsechangesaccordingtothesinelaw.Accordingtotheprincipleofequalimpulseandsameeffect,PWMwaveformandsinehalfwaveareequivalent.Forthenegativehalfcycleofthesine,thesamemethodcanbeusedtoobtainthePWMwaveform.InthePWMwaveform,theamplitudeofeachpulseisequal.Tochangetheamplitudeoftheequivalentoutputsinewave,youonlyneedtochangethewidthofeachpulseaccordingtothesameproportionalcoefficient.Therefore,intheAC-DC-ACfrequencyconversionIntheconverter,thepulsevoltageoutputbythePWMinvertercircuitistheamplitudeoftheDCsidevoltage.
Accordingtotheabove-mentionedprinciple,thewidthandintervalofeachpulseofthePWMwaveformcanbeaccuratelycalculatedafterthesinewavefrequency,amplitudeandthenumberofpulsesinahalfcyclearegiven.Accordingtothecalculationresult,theon-offofeachswitchingdeviceinthecircuitiscontrolled,andtherequiredPWMwaveformcanbeobtained.Figure1showsthereal-timewaveformofthePWMwaveoutputbytheinverter.
Pulssinleveyden modulaatioluokitus
Moduloidun pulssin polariteetin näkökulmasta PWM voidaan jakaa kahteen tyyppiin:yksinapainen ja kaksinapainen ohjausmoodi.Yksinapaisen PWM-tilan luomisen perusperiaate on esitetty Kuvassa 6.2. ationsignalur(Kuva6.2(a)),ja generoi yksinapainen PWM-pulssi(Kuva6.2(b));vertaa sitten yksinapaista PWM-pulssisignaalia kuvassa 6.2(c) )Käänteinen signaali on kerrottu saadakseenpositiivisen ja negatiivisen puoliaaltosymmetrisen PWM-pulssisignaalinUd, kuten kuvassa 6.2(d).
ThebipolarPWMcontrolmodeusespositiveandnegativealternatingbipolartriangularcarrierutandmodulatedwaveur,asshowninFigure6.3,thebipolarcanbedirectlyobtainedbycomparingutandur,PWMpulsewithouttheneedforinvertercircuit.
Inadditiontotheabovetwoclassificationsofmodulationmethodsfromtheperspectiveofdifferentprinciples,inrecentyears,thedirectpulsewidthmodulationmethodusingthechiphasbeenacceptedbymoreusers.Thefieldofsignalconditioningoftenneedstofaceproblemssuchasthetransmission,acquisition,andcontrolofanalogsignals.Traditionalsignalchaincircuitsincludeanalog-to-digitalconverters(ADC),digital-to-analogconverters(DAC),operationalamplifiers(OpAmp),andcomparators(Comparator)andsoon,theyplaythemainroleofanalogsignalprocessing.Thefunctionofthesignalchainchipisbasicandpowerful.Aftercarefuldesign,avarietyofexcellentsignalprocessingcircuitscanbeformed.However,evenso,inmanyapplicationfields,therearestillbottlenecksandconstraints,andtheidealcircuitperformanceandindicatorscannotbeachieved.Therefore,moreinnovativeanalogcircuitprocessingtechnologiesandchipproductsareeagertoappearinthefieldofsignalchain.Anewtypeofanalogsignalprocessingchip,whichrealizesthehigh-precisionconversionfunctionofanalogsignaltoPWMsignal,wecallitAPC(AnaloguetoPWMConvertor).
Harmonicspectrum
AssumingthatthecarrierfrequencyoftheSPWMwaveisfcandthefundamentalfrequencyisfs,fc/fsiscalledthecarrierratioN.Forathree-phaseinverter,whenNisWhenitisanintegermultipleof3,theoutputdoesnotcontainthethirdharmonicandtheintegermultipleof3harmonic.Andtheharmonicconcentrationisnearanintegermultipleofthecarrierfrequency,thatis,theharmonicorderis:kfc±mfs,andkandmareintegers.
Figure2isaMatlabsimulationdiagramofthewaveformandfrequencyspectrumoftheSPWMwithfundamentalfrequencyfs=50Hz,carrierfrequencyfc=3kHz,andmodulationratio0.8.
PWM-mittauslaite (4 kuvaa)
Theamplitudesofthe58thand60thharmonicsinFigure2are27.8%and27.7%,respectively,withthelargestcontentTheharmonicsarethe119thand121stharmonics,andtheharmonicamplitudesare39.1%and39.3%,respectively.Thatis,themaximumharmonicisaroundtwicethecarrierfrequency.Withtheincreaseoftheharmonicfrequency,theoverallharmonicamplitudeshowsadownwardtrend.AccordingtotheGB/T22670inverter-poweredthree-phasecageinductionmotortestmethod,thebandwidthofthefrequencyconversionpowertransmittershouldbeatthecarrierfrequencyWhenthecarrierfrequencyis3kHz,thebandwidthisatleast18kHz.Inactualuse,itisrecommendedtouseafrequencyconversionpowersensorandafrequencyconversionpoweranalyzerwithabandwidthof30kHzormore.
ThecarrierratiooftheactualSPWMwaveisnotnecessarilyaninteger.Atthistime,inordertoreducethespectrumleakage,thelengthoftheFourierwindowcanbeappropriatelyincreased,andtheFouriertransformofthePWMwithmultiplefundamentalcyclescanbeperformed.(FFTorDFT).
Specificprocess
Pulsewidthmodulation(PWM)isamethodofdigitallyencodinganalogsignallevels.Throughtheuseofhigh-resolutioncounters,thedutycycleofthesquarewaveismodulatedtoencodethelevelofaspecificanalogsignal.ThePWMsignalisstilldigital,becauseatanygivenmoment,thefull-scaleDCpowersupplyiseithercompletely(ON)orcompletelyabsent(OFF).Thevoltageorcurrentsourceisappliedtotheanalogloadinarepetitivepulsesequenceofon(ON)oroff(OFF).Whenitison,itiswhentheDCpowersupplyisaddedtotheload,andwhenitisoff,itiswhenthepowersupplyisdisconnected.Aslongasthebandwidthissufficient,anyanalogvaluecanbeencodedusingPWM.
Mostloads(whetherinductiveorcapacitive)requireamodulationfrequencyhigherthan10Hz,usuallybetween1kHzand200kHz.ManymicrocontrollersincludeaPWMcontrollerinside.Forexample,Microchip'sPIC16C67containstwoPWMcontrollers,eachofwhichcanselecttheturn-ontimeandperiod.Thedutycycleistheratiooftheon-timetotheperiod;themodulationfrequencyisthereciprocaloftheperiod.BeforeperformingthePWMoperation,thiskindofmicroprocessorrequiresthefollowingtaskstobecompletedinthesoftware:
1.Aseta piiriajastimen/laskurin jakso, joka tuottaa moduloidun neliöaallon
2.Aseta päällekytkentäaikaPWMcontrol-rekisterissä
3.MääritäPWM-ulostulon suunta. Tämä tulos on yleiskäyttöinen I/Opin
4. Käynnistä mittari
5.Ota PWM-ohjain käyttöön
Nykyään melkein kaikissa kaupallisesti saatavissa yksisiruisissa mikrotietokoneissa on PWM-moduulitoimintoja. Jos ne eivät toimi (kuten aikaisin8051), ne voidaan myös toteuttaa ajastimien ja GPIO-porttien avulla. sarja):
1.Ota käyttöön liittyvät moduulit (PWM-moduuli ja vastaavat nastatGPIO-moduuli).
2.MääritäPWM-moduulin toiminnot erityisesti:
①: AsetaPWM-ajastimen jakso. Tämä parametri määrittää PWM-aaltomuodon taajuuden.
②: AsetaPWM-ajastimen vertailuarvo. Tämä parametri määrittää PWM-aaltomuodon käyttöjakson.
③:Asetakuollut kaista(kuollut kaista),jotta välttääsiltavarrenläpiasettaakuollut kaista,yleensä korkeammalla mikro-ohjaimella on tämä toiminto.
④:Setthefaulthandlingsituation.Generally,thefaultistoblocktheoutputtopreventover-currentfromdamagingthepowertube.ThefaultisgenerallydetectedbyacomparatororADCorGPIO.
⑤:Setthesynchronizationfunction,whichisparticularlyimportantwhenmultiplebridgearms,thatis,multiplePWMmodulesworkincoordination.
3.Setthecorrespondinginterrupt,writeISR,generallyusedforvoltageandcurrentsampling,calculatethedutycycleofthenextcycle,changethedutycycle,thispartwillalsohavethefunctionofPIcontrol.
4.Ota PWM-aaltomuodon luominen käyttöön.
Advantages
OneoftheadvantagesofPWMisthatthesignalsfromtheprocessortothecontrolledsystemareindigitalform,withoutdigital-to-analogconversion.Keepingthesignalindigitalformminimizestheeffectofnoise.Noisecanonlyaffectdigitalsignalswhenitisstrongenoughtochangelogic1tologic0orchangelogic0tologic1.
TheenhancementofnoiseresistanceisanotheradvantageofPWMoveranalogcontrol,andthisisalsothemainreasonwhyPWMisusedforcommunicationinsomecases.SwitchingfromanalogsignaltoPWMcangreatlyextendthecommunicationdistance.Atthereceivingend,asuitableRCorLCnetworkcanfilteroutthemodulatedhigh-frequencysquarewaveandrestorethesignaltoananalogform.Inshort,PWMiseconomical,space-saving,andhasstronganti-noiseperformance.Itisaneffectivetechnologythatisworthyofbeingusedbyengineersinmanydesignapplications.
Controlmethod
Thereisanimportantconclusioninthesamplingcontroltheory:whennarrowpulseswiththesameimpulsebutdifferentshapesareappliedtothelinkwithinertia,theeffectisbasicallythesame.ThePWMcontroltechnologyisbasedonthisconclusiontocontroltheturn-onandturn-offofthesemiconductorswitchingdevice,sothattheoutputterminalcanobtainaseriesofpulsesofequalamplitudebutunequalwidth,andusethesepulsestoreplacesinewavesorotherpulses.Thedesiredwaveform.Thewidthofeachpulseismodulatedaccordingtocertainrules,whichcanchangethesizeoftheoutputvoltageoftheinvertercircuitandtheoutputfrequency.
ThebasicprincipleofPWMcontrolhasbeenputforwardveryearly,butduetotherestrictionofthedevelopmentlevelofpowerelectronicdevices,ithasnotbeenrealizeduntilthe1980s.Untilthe1980s,withtheemergenceandrapiddevelopmentoffully-controlledpowerelectronicdevices,PWMcontroltechnologywastrulyapplied.Withthedevelopmentofpowerelectronicstechnology,microelectronictechnologyandautomaticcontroltechnology,aswellastheapplicationofvariousnewtheoreticalmethods,suchasmoderncontroltheory,nonlinearsystemcontrolideas,PWMcontroltechnologyhasachievedunprecedenteddevelopment,andavarietyofPWMcontroltechnology,accordingtothecharacteristicsofPWMcontroltechnology,mainlyhasthefollowing8typesofmethods.
EqualpulsewidthPWM-menetelmä
VVVF(VariableVoltageVariableFrequency)devicewasrealizedbyPAM(PulseAmplitudeModulation)controltechnologyintheearlydays.Theinverterpartcanonlyoutputsquarewavevoltagewithadjustablefrequencyandcannotregulatevoltage.TheequalpulsewidthPWMmethodisdevelopedtoovercomethisshortcomingofthePAMmethod,anditisthesimplestkindofPWMmethod.ItusesapulsetrainwiththesamewidthofeachpulseasaPWMwave,andachievestheeffectoffrequencymodulationbychangingitsperiod.Thevoltagecanbeadjustedbychangingthepulsewidthordutycycle,andthevoltageandfrequencycanbecoordinatedtochangebyadoptinganappropriatecontrolmethod.ComparedwiththePAMmethod,theadvantageofthismethodisthatitsimplifiesthecircuitstructureandimprovesthepowerfactoroftheinputend,butatthesametime,therearealsolargeharmoniccomponentsintheoutputvoltageinadditiontothefundamentalwave.
RandomPWM
Fromthe1970stotheearly1980s,becausethehigh-powertransistorsatthattimeweremainlybipolarDarlingtontransistors,Thecarrierfrequencygenerallydoesnotexceed5kHz,andthevibrationcausedbytheelectromagneticnoiseandharmonicsofthemotorwindingshasattractedpeople'sattention.Inordertoimprove,therandomPWMmethodcameintobeing.Theprincipleistorandomlychangetheswitchingfrequencytomaketheelectromagneticnoiseofthemotorapproximatetoband-limitedwhitenoise(inalinearfrequencycoordinatesystem,theenergydistributionofeachfrequencyisuniform),althoughthetotaldecibelofnoisehasnotchanged,itischaracterizedbyafixedswitchingfrequencyTheintensityofcolorednoiseisgreatlyreduced.Becauseofthis,eventhoughIGBTshavebeenwidelyused,randomPWMstillhasitsspecialvalueforoccasionswherethecarrierfrequencymustbelimitedtoalowerfrequency;ontheotherhand,itshowsthatthebestwaytoeliminatemechanicalandelectromagneticnoiseisnotblindlyToimprovetheoperatingfrequency,therandomPWMtechnologyprovidesananalysisandanewwaytosolvethisproblem.
SPWM-menetelmä
SPWM(SinusoidalPWM)methodisarelativelymaturemethod,andnowadaysthePWMmethodiswidelyused.Animportantconclusionintheaforementionedsamplingcontroltheory:whennarrowpulseswiththesameimpulseanddifferentshapesareappliedtoalinkwithinertia,theeffectisbasicallythesame.TheSPWMmethodisbasedonthisconclusion.ThepulsewidthchangesaccordingtothesinelawandthePWMwaveformequivalenttothesinewave,thatis,theSPWMwaveformcontrolstheonandoffoftheswitchingdevicesintheinvertercircuit,sothattheareaoftheoutputpulsevoltageisthesameasthatofthesinewave.Itishopedthattheoutputsinewavehasthesameareainthecorrespondinginterval,andthefrequencyandamplitudeoftheoutputvoltageoftheinvertercircuitcanbeadjustedbychangingthefrequencyandamplitudeofthemodulatingwave.SPWMreferstoamodulationmethodinwhichtheoutputamplitudeofthevariablefrequencypowersupplyisequalandthedutyratioofthesequencepulsechangesaccordingtothelawofthesinefunctionduringthemodulationperiod.Thelargerthevalueofthesinefunction,thelargerthecorrespondingpulsedutycycle,andthesmallertheintervalbetweenadjacentpulses.Correspondingly,whenthevalueofthesinefunctionissmaller,thepulsedutycycleisalsosmaller,andtheintervalbetweenadjacentpulsesislarger.Therealizationofthismethodhasthefollowingschemes.
Equalareamethod:ThisschemeisactuallyadirectexplanationoftheprincipleoftheSPWMmethod.Replacethesinewavewiththesamenumberofrectangularpulsetrainsofequalamplitudebutnotequalwidth,andthencalculateThewidthandintervalofeachpulsearestoredinthemicrocomputer,andthePWMsignalisgeneratedtocontroltheonandoffoftheswitchingdevicebythewayoflookingupthetable,soastoachievetheexpectedpurpose.SincethismethodisbasedonthebasicprincipleofSPWMcontrol,itcanaccuratelycalculatetheon-offtimeofeachswitchingdevice.Thewaveformobtainedisveryclosetoasinewave,butitscalculationiscumbersomeandthedataoccupiesalargememory,whichcannotbecontrolledinrealtime.Shortcomings.
Hardwaremodulationmethod:Thehardwaremodulationmethodisproposedtosolvetheshortcomingsofthecumbersomecalculationoftheequalareamethod.Thesignalisusedasthecarrier,andthedesiredPWMwaveformisobtainedbymodulatingthecarrier.Usuallyanisoscelestrianglewaveisusedasthecarrierwave.Whenthemodulatingsignalwaveisasinewave,theSPWMwaveformisobtained.Therealizationmethodissimple.Ananalogcircuitcanbeusedtoformatriangularwavecarrierandasinemodulatedwavegeneratingcircuit,andacomparatorcanbeusedtodeterminetheirintersectionpoint,andtheon-offoftheswitchingdevicecanbecontrolledatthetimeoftheintersectionpointtogeneratetheSPWMwave.However,thestructureofthisanalogcircuitiscomplex,anditisdifficulttoachieveprecisecontrol.
Softwaregenerationmethod:AsthedevelopmentofmicrocomputertechnologymakesiteasiertogenerateSPWMwaveformswithsoftware,thesoftwaregenerationmethodhasemergedasthetimesrequire.Thesoftwaregenerationmethodisactuallythemethodofusingsoftwaretorealizethemodulation.Therearetwobasicalgorithms,namelythenaturalsamplingmethodandtheregularsamplingmethod.
Naturalsamplingmethod:Takeasinewaveasthemodulatingwaveandanisoscelestrianglewaveasthecarrierforcomparison,andcontroltheonandoffoftheswitchingdeviceatthenaturalintersectionofthetwowaveforms.ThisisNaturalsamplingmethod.TheadvantageisthattheobtainedSPWMwaveformisclosesttothesinewave,butduetothearbitraryintersectionofthetrianglewaveandthesinewave,thepulsecenterisnotequidistantwithinonecycle,sothepulsewidthexpressionisatranscendentequation,whichiscomplicatedtocalculateanddifficulttocontrolinrealtime.
Regularsamplingmethod:Regularsamplingmethodisawidelyusedengineeringpracticalmethod,generallyusingtrianglewaveascarrier.Theprincipleistosampleasinewavewithatrianglewavetoobtainastepwave,andthenusetheintersectionofthestepwaveandthetrianglewavetocontroltheonandoffoftheswitchingdevice,soastorealizetheSPWMmethod.Whenthetrianglewaveonlysamplesthesinewaveatthetop(orbottom)positionofthetrianglewave,thepulsewidthdeterminedbytheintersectionpointofthestepwaveandthetrianglewaveissymmetricalwithinacarrierperiod(iesamplingperiod).Themethodiscalledsymmetricregularsampling.Whenthetrianglewavesamplesthesinewaveatbothitsapexandthebottompoint,thepulsewidthdeterminedbytheintersectionofthestaircasewaveandthetrianglewaveisgenerallynotatthesamepositionwithinacarriercycle(thistimeistwicethesamplingcycle).Asymmetry,thismethodiscalledasymmetricregularsampling.Theregularsamplingmethodisanimprovementofthenaturalsamplingmethod.Itsmainadvantageisthatitissimpletocalculateandconvenientforonlinereal-timeoperation.Amongthem,theasymmetricregularsamplingmethodisclosertosineduetoitsmanyorders.ThedisadvantageisthattheDCvoltageutilizationrateislowandthelinearcontrolrangeissmall.Theabovetwomethodsareonlyapplicabletosynchronousmodulationmethods.
Low-orderharmoniceliminationmethod:Thelow-orderharmoniceliminationmethodisamethodforeliminatingsomemainlow-orderharmonicsinthePWMwaveform.TheprincipleistoexpandtheoutputvoltagewaveformaccordingtotheFourierseries,expressedasu(ωt)=ansinnωt,firstdeterminethevalueofthefundamentalcomponenta1,andthenmaketwodifferentan=0.Threeequationscanbeestablishedandsolvedsimultaneouslytoobtaina1,a2anda3,sothattheharmonicsofthetwofrequenciescanbeeliminated.Althoughthismethodcaneliminatethespecifiedlow-orderharmonicswell,theamplitudeoftheremainingun-eliminatedlower-orderharmonicsmaybequitelarge,anditalsohasthedisadvantageofcomplicatedcalculations.Thismethodisalsoonlyapplicabletosynchronousmodulation.
Comparisonmethodoftrapezoidalwaveandtrianglewave:Thevariousmethodsintroducedabovearemainlyaimedatoutputtingthewaveformasclosetothesinewaveaspossible,thusneglectingtheutilizationofDCvoltage,suchasSPWMmethod,itsDCvoltageutilizationrateisonly86.6%.Therefore,inordertoimprovetheutilizationrateofDCvoltage,anewmethod-thecomparisonmethodoftrapezoidalwaveandtrianglewaveisproposed.Inthismethod,atrapezoidalwaveisusedasamodulationsignal,atriangularwaveisusedasacarrierwave,andtheamplitudesofthetwowavesareequalized,andtheon-offoftheswitchingdeviceiscontrolledatthemomentofintersectionofthetwowavestorealizePWMcontrol.Becausewhentheamplitudeofthetrapezoidalwaveandtheamplitudeofthetriangularwaveareequal,theamplitudeofthefundamentalwavecomponentcontainedinithasexceededtheamplitudeofthetriangularwave,whichcaneffectivelyimprovetheutilizationrateoftheDCvoltage.However,sincethetrapezoidalwaveitselfcontainslow-orderharmonics,theoutputwaveformcontains5th,7thandotherlow-orderharmonics.
LinevoltagecontrolPWM
WhenthevariousPWMcontrolmethodsintroducedaboveareusedinathree-phaseinvertercircuit,theyareallrelatedtothethree-phaseoutputphase.Thevoltageiscontrolledseparatelysothatitsoutputisclosetoasinewave.However,forathree-phasesymmetricalloadlikeathree-phaseasynchronousmotorwithoutaneutralline,theinverteroutputdoesnotneedtopursueaphasevoltageclosetoasinewave,butcanfocusonmakingthelinevoltagetendtoSine.Therefore,thelinevoltagecontrolPWMisproposed,andtherearemainlythefollowingtwomethods.
Saddlewaveandtrianglewavecomparisonmethod:ThesaddlewaveandtrianglewavecomparisonmethodisalsotheharmonicinjectionPWMmethod(HIPWM),theprincipleistoaddacertainproportiontothesinewaveThemodulationsignalissaddle-shaped,andtheamplitudeissignificantlyreduced.Therefore,whentheamplitudeofthemodulationsignaldoesnotexceedtheamplitudeofthecarrier,theamplitudeofthefundamentalwavecanexceedtheamplitudeofthetriangularwave,andtheDCvoltagecanbeincreased.Utilizationrate.Inathree-phasenon-neutralsystem,sincethethirdharmoniccurrenthasnopath,thethreelinevoltagesandlinecurrentsdonotcontainthethirdharmonic.Inadditiontoinjectingthirdharmonics,otherwaveformswith3timesthefrequencyofthesinewavesignalcanalsobeinjected,andthesesignalswillnotaffectthelinevoltage.ThisisbecausethephasevoltageoutputbytheinvertercircuitafterPWMmodulationmustalsocontainthecorrespondingharmonicsthatare3timesthesinewavesignal.However,whenthelinevoltageissynthesized,theseharmonicsinthephasevoltageswillcanceleachother.Sothatthelinevoltageisstillasinewave.
Unitpulsewidthmodulationmethod:Becausethethree-phasesymmetricallinevoltagehastherelationshipofUuv+Uvw+Uwu=0,therefore,acertainlinevoltageisequaltotheothertwoatanytimeThesumofnegativelinevoltages.Nowadays,acycleisequallydividedinto6intervals,eachintervalis60°.ForacertainlinevoltagesuchasUuv,the60°intervalonbothsidesofthehalfcycleisrepresentedbyUuvitself,andthemiddle60°intervalisrepresentedby-(Uvw+Uwu).WhenUvwandUwuareprocessedinthesameway,thethree-phaselinevoltagewaveformcanbeobtainedwithonlytwowaveformshapesinthe60°intervalonbothsideswithinahalfcycle,andtherearepositiveandnegativewaveforms.Usingthisvoltagewaveformasareferencesignalforpulsewidthmodulation,thecarrierstillusesatrianglewave,andthecurveofeachintervalisapproximatedwithastraightline(practiceshowsthattheerrorcausedbythisisnotlargeanditiscompletelyfeasible),andthepulseofthelinevoltagecanbeobtained.Thewaveformiscompletelysymmetricalandhasstrongregularity.Thenegativehalfcycleistheinverseofthecorrespondingpulsetrainofthepositivehalfcycle.Therefore,aslongasthepulsetraininthe60°intervalonbothsidesofthehalfcycleisdetermined,themodulationpulsewaveformofthelinevoltageisuniqueSure.Thispulseisnotthedrivingpulsesignaloftheswitchingdevice,butsincethepulseoperatingmodeofthethree-phaselinevoltageisknown,thedrivingpulsesignaloftheswitchingdevicecanbedetermined.Thismethodcannotonlysuppressmorelow-orderharmonics,butalsoreduceswitchinglossandwidenthelinearcontrolarea,andatthesametimebringtheconvenienceofmicrocomputercontrol,butthismethodisonlysuitableforasynchronousmotorsandhasasmallapplicationrange.
CurrentcontrolPWM
ThebasicideaofcurrentcontrolPWMistousethedesiredoutputcurrentwaveformasthecommandsignalandtheactualcurrentwaveformasthefeedbacksignal.Theon-offofeachswitchingdeviceisdeterminedbythecomparisonoftheinstantaneousvaluesofthetwo,sothattheactualoutputchangeswiththechangeofthecommandsignal.Therealizationschememainlyhasthefollowing3kinds.
Hysteresiscomparisonmethod:ThisisaPWMcontrolmethodwithfeedback,thatis,thecurrentofeachphaseisfedbackandthecurrentsetvalueispassedthroughthehysteresiscomparatortoobtainthecorrespondingTheswitchingstateofthebridgearmswitchingdevicemakestheactualcurrenttrackthechangeofagivencurrent.Theadvantageofthismethodisthatthecircuitissimple,thedynamicperformanceisgood,andtheoutputvoltagedoesnotcontainharmoniccomponentsofaspecificfrequency.Thedisadvantageisthattheswitchingfrequencyisnotfixedandcausesmoreseriousnoise.Comparedwithothermethods,theoutputcurrentcontainsmoreharmonicsatthesameswitchingfrequency.
Triangularwavecomparisonmethod:ThismethodisdifferentfromthetriangularwavecomparisonmethodintheSPWMmethod.Here,thecommandcurrentiscomparedwiththeactualoutputcurrenttoobtainthedeviationcurrent,whichisamplifiedbytheamplifierThencomparewiththetrianglewavetogeneratethePWMwave.Atthistime,theswitchingfrequencyisfixed,thusovercomingtheshortcomingsofthenon-fixedfrequencyofthehysteresiscomparisonmethod.However,thecurrentresponseinthiswayisnotasfastasthehysteresiscomparisonmethod.
Predictivecurrentcontrolmethod:Predictivecurrentcontrolistopredictthecurrenterrorvectortrendbasedontheactualcurrenterror,loadparametersandotherloadvariablesatthebeginningofeachadjustmentcycle.Therefore,thevoltagevectorgeneratedbyPWMinthenextregulationperiodwilldefinitelyreducethepredictederror.Theadvantageofthismethodisthatifyougivetheregulatormoreinformationbesidestheerror,youcangetafasterandmoreaccurateresponse.Thelimitationofthistypeofregulatoristheresponsespeedandtheaccuracyoftheprocessmodelcoefficientparameters.
SpacevoltagevectorcontrolPWM
SpacevoltagevectorcontrolPWM(SVPWM)isalsocalledfluxsinusoidalPWMmethod.Itisbasedontheoverallthree-phasewaveformgenerationeffect,andaimstoapproximatetheidealcircularrotatingmagneticfieldtrajectoryofthemotorairgap.Theactualmagneticfluxgeneratedbythedifferentswitchingmodesoftheinverterisusedtoapproximatethereferencecircularmagneticflux.ThecomparisonresultdeterminestheswitchingoftheinverterandformsthePWMwaveform.Fromtheperspectiveofthemotor,thismethodregardstheinverterandthemotorasawhole,andcontrolstheinscribedpolygontoapproximateacircle,sothatthemotorobtainsacircularmagneticfield(sineflux)withaconstantamplitude.Thespecificmethodisdividedintoopen-loopmagneticfluxandclosed-loopmagneticflux.Theopen-loopmagneticfluxmethodusestwonon-zerovectorsandazerovectortosynthesizeanequivalentvoltagevector.Ifthesamplingtimeissmallenough,anyvoltagevectorcanbesynthesized.Theoutputvoltageofthismethodis15%higherthanthatofsinewavemodulation,andthesumoftheeffectivevalueoftheharmoniccurrentisclosetotheminimum.Themagneticfluxclosedlooptypeintroducesmagneticfluxfeedbacktocontrolthesizeandchangespeedofthemagneticflux.Aftercomparingtheestimatedmagneticfluxandthegivenmagneticflux,thenextvoltagevectorisdeterminedaccordingtotheerrortoformthePWMwaveform.Thismethodovercomestheshortcomingsoftheopen-loopmagneticfluxmethod,solvestheproblemofthelargeinfluenceofthestatorresistancewhenthemotorisatlowspeed,andreducesthepulsationandnoiseofthemotor.However,becausethetorqueadjustmentisnotintroduced,thesystemperformancehasnotbeenfundamentallyimproved.
VectorcontrolPWM
Vectorcontrolisalsocalledfield-orientedcontrol.ItsprincipleistochangethestatorcurrentIa,Iboftheasynchronousmotorinthethree-phasecoordinatesystem.AndIc,throughthethree-phase/two-phasetransformation,areequivalenttothealternatingcurrentIa1andIb1inthetwo-phasestationarycoordinatesystem,andthenthroughtherotationtransformationaccordingtotherotormagneticfieldorientation,itisequivalenttothedirectcurrentIm1andIt1inthesynchronousrotatingcoordinatesystem(Im1isequivalenttotheexcitationcurrentoftheDCmotor;It1isequivalenttothearmaturecurrentproportionaltothetorque),andthenimitatethecontrolmethodoftheDCmotortorealizethecontroloftheACmotor.TheessenceistoequateanACmotortoaDCmotor,andindependentlycontrolthetwocomponentsofspeedandmagneticfield.Bycontrollingtherotorflux,andthendecomposingthestatorcurrenttoobtainthetwocomponentsoftorqueandmagneticfield,throughcoordinatetransformation,orthogonalordecouplingcontrolisrealized.However,duetothedifficultyofaccuratelyobservingtherotorfluxlinkageandthecomplexityofthevectortransformation,theactualcontroleffectisoftendifficulttoachievetheeffectoftheoreticalanalysis.Thisistheshortcomingofthevectorcontroltechnologyinpractice.Inaddition,itmustdirectlyorindirectlyobtainthepositionoftherotorfluxlinkageinspacetorealizethestatorcurrentdecouplingcontrol.Inthisvectorcontrolsystem,itisnecessarytoconfiguretherotorpositionorspeedsensor,whichobviouslybringsinconveniencetomanyapplications.
DirectTorqueControlPWM
In1985,ProfessorDepenbrockofRuhrUniversityinGermanyfirstproposedtheDirectTorqueControlTheory(DirectTorqueControlforshortDTC).Directtorquecontrolisdifferentfromvectorcontrol.Itdoesnotindirectlycontrolthetorquebycontrollingthecurrentandflux,butitcontrolsthetorquedirectlyasthecontrolledquantity.Itdoesnotneedtodecouplethemotormodel,butinCalculatetheactualvalueofthemotorfluxandtorqueinthestationarycoordinatesystem,andthengeneratethePWMsignalthroughtheBand-Bandcontrolofthefluxlinkageandtorquetooptimallycontroltheswitchingstateoftheinverter,therebytoalargeextentItsolvestheabove-mentionedshortcomingsofvectorcontrol,caneasilyrealizespeedsensorless,hasfasttorqueresponsespeedandhighspeedandtorquecontrolaccuracy,andadoptsnovelcontrolideas,simpleandclearsystemstructure,andexcellentThedynamicandstaticperformanceoftheproducthasbeenrapidlydeveloped.Butdirecttorquecontrolalsohasshortcomings,suchastheincreaseininverterswitchingfrequencyislimited.
Epälineaarinen ohjausPWM
Singlecyclecontrolmethod[7],alsoknownasIntegrationResetControl(IRC),isanewtypeThebasicideaofnonlinearcontroltechnologyistocontrolthedutycycleoftheswitch,sothattheaveragevalueoftheswitchvariableisequaltoorproportionaltothecontrolreferencevoltageineachcycle.Thistechnologyhasthedualityofmodulationandcontrolatthesametime.Itachievesthepurposeoftrackingthecommandsignalbyresettingtheswitch,integrator,triggercircuit,andcomparator.Theone-cyclecontrolleriscomposedofacontroller,acomparator,anintegratorandaclock,andthecontrollercanbeanRSflip-flop.
Single-cyclecontroldoesnotrequireerrorsynthesisinthecontrolcircuit.Itcanautomaticallyeliminatesteady-stateandtransienterrorsinonecycle,sothattheerrorsfromthepreviouscyclewillnotbecarriedtothenextcycle.Althoughthehardwarecircuitismorecomplicated,itovercomestheshortcomingsofthetraditionalPWMcontrolmethod,andissuitableforvariouspulsewidthmodulationsoftswitchinginverters.Ithastheadvantagesoffastresponse,constantswitchingfrequency,andstrongrobustness.Inaddition,onecycleControlcanalsooptimizesystemresponse,reducedistortionandsuppresspowerinterference,whichisaverypromisingcontrolmethod.
Resonantsoft-switchingPWM
InthetraditionalPWMinvertercircuit,theworkingmodeofhardswitchingofpowerelectronicswitchingdevices,thelargeswitchingvoltageandcurrentstressandHighdu/dtanddi/dtlimittheimprovementoftheoperatingfrequencyofswitchingdevices,andhighfrequencyisoneofthemaindevelopmenttrendsofpowerelectronics,whichcanreducethevolumeoftheconverter,reducetheweight,reducethecost,andimprovetheperformance(especiallyWhentheswitchingfrequencyisabove18kHz),thevibrationisreduced,makinganoiselesstransmissionsystempossible.Thebasicideaofresonantsoft-switchingPWMistoaddaresonantnetworkonthebasisoftheconventionalPWMconvertertopology.Theresonantnetworkisgenerallycomposedofaresonantinductor,aresonantcapacitorandapowerswitch.Duringswitching,theresonantnetworkworkstoenablethepowerelectronicdevicetoachieveasoftswitchingprocessattheswitchingpoint.TheresonantprocessisextremelyshortanddoesnotbasicallyaffecttherealizationofthePWMtechnology.SoastomaintainthecharacteristicsofPWMtechnology,butalsorealizethesoftswitchingtechnology.However,theexistenceoftheresonantnetworkinthecircuitwillinevitablyproduceresonantlosses,andthecircuitwillbeaffectedbyinherentproblems,thuslimitingtheapplicationofthismethod.
Sovellusalueet
Servo
Pulsewidthmodulationcanbeusedtocontroltheservomechanism.
Televiestintä
Intheuseoftelecommunications,pulsewidthmodulationisaformofsignalmodulation.Thewidthofthepulsewavecorrespondstoanotherspecificdatasession.Itisencodedatthetransmittingendanddecodedatthereceivingend.Pulsewavesofdifferentlengths(themessageitselftobetransmitted)willbetransmittedafterafixedperiodoftime(thefrequencyofthecarrierwave).
Energiansiirto
PulsewidthmodulationcanbeusedtocontrolhowmuchenergyistransferredtoacarrierwithoutbeingcausedbyimpedanceThelinearenergytransferloss.Thecostofthismethodisthattheenergylostbythecarriersisnotconstantanddiscontinuous(suchasabuckconverter),andtheenergytransferredonthecarriersisnotcontinuous.However,sincethecarriersmaybehigh-frequencyinductance,apassiveelectronicfiltermustbeaddedtomakethesepulsewavessmoothandrestoretheaverageanalogwaveform,sothattheenergyflowsintothecarriers.Itwillbecontinuous.Theenergyflowingfromthesupplyendisnotcontinuous,soinmostcasesadditionalenergystoragespaceisrequired.