Domov Technika Pulzní šířková modulace

Pulzní šířková modulace



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,andtheareaof​​therectangularpulseandthecorrespondingsinepart(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,sothattheareaof​​theoutputpulsevoltageisthesameasthatofthesinewave.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

Thebasicideaof​​currentcontrolPWMistousethedesiredoutputcurrentwaveformasthecommandsignalandtheactualcurrentwaveformasthefeedbacksignal.Theon-offofeachswitchingdeviceisdeterminedbythecomparisonoftheinstantaneousvalues​​ofthetwo,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),isanewtypeThebasicideaof​​nonlinearcontroltechnologyistocontrolthedutycycleoftheswitch,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.Thebasicideaof​​resonantsoft-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.

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