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.
Obrázek1PWDiagram skutečné vlny(2fotografie)
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.
Klasifikace modulace šířky pulsu
Z hlediska polarity modulovaného pulsu lze PWM rozdělit do dvou typů: unipolární a bipolární režim řízení. Základní princip generování unipolárního režimu PWM je zobrazen na obrázku 6.2. alur(Obrázek 6.2(a)),a vygenerujte unipolární PWMpuls(Obrázek6.2(b));potom porovnejte unipolárníPWMpulsnísignál sobrázkem6.2(c )Invertovaný signál UI se násobí, aby se získal kladný a záporný půlvlnný symetrický PWMpulzní signál Ud, jak je znázorněno na obrázku 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 měřící zařízení (4 fotografie)
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. Nastavte periodu čipového časovače/počítače, který poskytuje modulované čtvercové vlny
2.Nastavte čas zapnutí v registru ovládání PWM
3.Nastavte směr výstupu PW. Tento výstup je obecným účelemI/Opin
4. Spusťte časovač
5. Povolte ovladač PWM
V současné době téměř všechny komerčně dostupné jednočipové mikropočítače mají funkce modulu PWM. Pokud ne (např. 8051), lze je implementovat pomocí časovačů a GPIO portů.
1.Povolte související moduly (modul PWM a odpovídající moduly GPIO).
2. Nakonfigurujte funkce modulu PWM, konkrétně:
①: Nastavte periodu časovače PWM. Tento parametr určuje frekvenci tvaru vlny PWM.
②:Nastavte porovnávací hodnotu PWMtimeru. Tento parametr určuje pracovní cyklus tvaru vlny PWM.
③:Nastavte pásmo necitlivosti (pásmo necitlivosti), abyste se vyhnuli přemostění ramene pomocí potřeby nastavit pásmo necitlivosti, obvykle tuto funkci mají mikrokontroléry vyššího konce.
④:Setthefaulthandlingsituation.Generally,thefaultistoblocktheoutputtopreventover-currentfromdamagingthepowertube.ThefaultisgenerallydetectedbyacomparatororADCorGPIO.
⑤:Setthesynchronizationfunction,whichisparticularlyimportantwhenmultiplebridgearms,thatis,multiplePWMmodulesworkincoordination.
3.Setthecorrespondinginterrupt,writeISR,generallyusedforvoltageandcurrentsampling,calculatethedutycycleofthenextcycle,changethedutycycle,thispartwillalsohavethefunctionofPIcontrol.
4.Povolte generování PWMwaveformgeneration.
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.
Metoda EqualpulsewidthPWM
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.
Metoda SPWM
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.
Řízení síťového napětí PWM
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.
Ovládání prouduPWM
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.
Řízení vektoru prostorového napětí 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.
Vektorové řízeníPWM
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.
Nelineární řízeníPWM
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.
Oblasti použití
Servo
Pulsewidthmodulationcanbeusedtocontroltheservomechanism.
Telecom
Intheuseoftelecommunications,pulsewidthmodulationisaformofsignalmodulation.Thewidthofthepulsewavecorrespondstoanotherspecificdatasession.Itisencodedatthetransmittingendanddecodedatthereceivingend.Pulsewavesofdifferentlengths(themessageitselftobetransmitted)willbetransmittedafterafixedperiodoftime(thefrequencyofthecarrierwave).
Přenos energie
PulsewidthmodulationcanbeusedtocontrolhowmuchenergyistransferredtoacarrierwithoutbeingcausedbyimpedanceThelinearenergytransferloss.Thecostofthismethodisthattheenergylostbythecarriersisnotconstantanddiscontinuous(suchasabuckconverter),andtheenergytransferredonthecarriersisnotcontinuous.However,sincethecarriersmaybehigh-frequencyinductance,apassiveelectronicfiltermustbeaddedtomakethesepulsewavessmoothandrestoretheaverageanalogwaveform,sothattheenergyflowsintothecarriers.Itwillbecontinuous.Theenergyflowingfromthesupplyendisnotcontinuous,soinmostcasesadditionalenergystoragespaceisrequired.