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.
Фигура 1PWДиаграма на действителната форма на вълната (2 снимки)
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.
Класификация на широчинноимпулсната модулация
От гледна точка на полярността на модулирания импулс, PWM може да се раздели на два типа: униполярни и биполярни режими на управление. Основният принцип за генериране на униполярен режим на PWM е показан на Фигура 6.2. модулационния сигнал (Фигура 6.2(a)) и генерирайте еднополярен PWM импулс (Фигура 6.2 (b)); след това сравнете униполярния PWM импулсен сигнал с този, показан на Фигура 6.2 (c) ) Инвертираният сигнал UI се умножава, за да се получи положителен и отрицателен полувълнов симетричен PWM импулсен сигнал Ud, както е показано на Фигура 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.
Устройство за измерване на ШИМ (4 снимки)
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. Задайте периода на таймера/брояча на чип, който осигурява модулирана квадратна вълна
2. Задайте времето за включване в контролния регистър на ШИМ
3. Задайте посоката на PW изхода. Този изход е I/Opin с общо предназначение
4. Стартирайте таймера
5.Активирайте PWM контролера
В днешно време почти всички налични в търговската мрежа едночипови микрокомпютри имат PWMмодулни функции. Ако не го направят (като ранния 8051), те също могат да бъдат внедрени с помощта на таймери и GPIOпортове. LM серия):
1.Активиране на свързани модули (PWMмодул и съответстващи пиновеGPIOмодул).
2. Конфигурирайте функцията на PWMмодула, по-специално:
①: Задаване на периода на PWM таймера. Този параметър определя честотата на формата на PWM вълната.
②: Задайте сравнителна стойност на PWM таймера. Този параметър определя работния цикъл на формата на вълната на PWM.
③: Задаване на мъртва зона (мъртва зона), за да избегнете рамото на моста чрез необходимостта да зададете мъртва зона, обикновено микроконтролерите от по-висок клас имат тази функция.
④:Setthefaulthandlingsituation.Generally,thefaultistoblocktheoutputtopreventover-currentfromdamagingthepowertube.ThefaultisgenerallydetectedbyacomparatororADCorGPIO.
⑤:Setthesynchronizationfunction,whichisparticularlyimportantwhenmultiplebridgearms,thatis,multiplePWMmodulesworkincoordination.
3.Setthecorrespondinginterrupt,writeISR,generallyusedforvoltageandcurrentsampling,calculatethedutycycleofthenextcycle,changethedutycycle,thispartwillalsohavethefunctionofPIcontrol.
4.Активирайте генерирането на PWM вълна.
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.
Метод EqualpulsewidthPWM
VVVF(VariableVoltageVariableFrequency)devicewasrealizedbyPAM(PulseAmplitudeModulation)controltechnologyintheearlydays.Theinverterpartcanonlyoutputsquarewavevoltagewithadjustablefrequencyandcannotregulatevoltage.TheequalpulsewidthPWMmethodisdevelopedtoovercomethisshortcomingofthePAMmethod,anditisthesimplestkindofPWMmethod.ItusesapulsetrainwiththesamewidthofeachpulseasaPWMwave,andachievestheeffectoffrequencymodulationbychangingitsperiod.Thevoltagecanbeadjustedbychangingthepulsewidthordutycycle,andthevoltageandfrequencycanbecoordinatedtochangebyadoptinganappropriatecontrolmethod.ComparedwiththePAMmethod,theadvantageofthismethodisthatitsimplifiesthecircuitstructureandimprovesthepowerfactoroftheinputend,butatthesametime,therearealsolargeharmoniccomponentsintheoutputvoltageinadditiontothefundamentalwave.
Случайна ШИМ
Fromthe1970stotheearly1980s,becausethehigh-powertransistorsatthattimeweremainlybipolarDarlingtontransistors,Thecarrierfrequencygenerallydoesnotexceed5kHz,andthevibrationcausedbytheelectromagneticnoiseandharmonicsofthemotorwindingshasattractedpeople'sattention.Inordertoimprove,therandomPWMmethodcameintobeing.Theprincipleistorandomlychangetheswitchingfrequencytomaketheelectromagneticnoiseofthemotorapproximatetoband-limitedwhitenoise(inalinearfrequencycoordinatesystem,theenergydistributionofeachfrequencyisuniform),althoughthetotaldecibelofnoisehasnotchanged,itischaracterizedbyafixedswitchingfrequencyTheintensityofcolorednoiseisgreatlyreduced.Becauseofthis,eventhoughIGBTshavebeenwidelyused,randomPWMstillhasitsspecialvalueforoccasionswherethecarrierfrequencymustbelimitedtoalowerfrequency;ontheotherhand,itshowsthatthebestwaytoeliminatemechanicalandelectromagneticnoiseisnotblindlyToimprovetheoperatingfrequency,therandomPWMtechnologyprovidesananalysisandanewwaytosolvethisproblem.
SPWM метод
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.
Векторно управление на космическото напрежение PWM
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.
Нелинейно управление с ШИМ
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.
Резонансна ШИМ с меко превключване
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.
Области на приложение
Серво
Pulsewidthmodulationcanbeusedtocontroltheservomechanism.
Телеком
Intheuseoftelecommunications,pulsewidthmodulationisaformofsignalmodulation.Thewidthofthepulsewavecorrespondstoanotherspecificdatasession.Itisencodedatthetransmittingendanddecodedatthereceivingend.Pulsewavesofdifferentlengths(themessageitselftobetransmitted)willbetransmittedafterafixedperiodoftime(thefrequencyofthecarrierwave).
Трансфер на енергия
PulsewidthmodulationcanbeusedtocontrolhowmuchenergyistransferredtoacarrierwithoutbeingcausedbyimpedanceThelinearenergytransferloss.Thecostofthismethodisthattheenergylostbythecarriersisnotconstantanddiscontinuous(suchasabuckconverter),andtheenergytransferredonthecarriersisnotcontinuous.However,sincethecarriersmaybehigh-frequencyinductance,apassiveelectronicfiltermustbeaddedtomakethesepulsewavessmoothandrestoretheaverageanalogwaveform,sothattheenergyflowsintothecarriers.Itwillbecontinuous.Theenergyflowingfromthesupplyendisnotcontinuous,soinmostcasesadditionalenergystoragespaceisrequired.