搜索
毕业设计（论文）外文文献翻译文献、资料中文题目：脉冲宽度调制（PWM）文献、资料英文题目：文献、资料来源：文献、资料发表（出版）日期：院（部）：专业：班级：姓名：学号：指导教师：翻译日期：2017.02.14外文原文Pulse-widthmodulationPulse-widthmodulation(PWM)isamodulationtechniquethatconformsthewidthofthepulse,formallythepulseduration,basedonmodulatorsignalinformation.Althoughthismodulationtechniquecanbeusedtoencodeinformationfortransmission,itsmainuseistoallowthecontrolofthepowersuppliedtoelectricaldevices,especiallytoinertialloadssuchasmotors.Inaddition,PWMisoneofthetwoprincipalalgorithmsusedinphotovoltaicsolarbatterychargers,[1]Theaveragevalueofvoltage(andcurrent)fedtotheloadiscontrolledbyturningtheswitchbetweensupplyandloadonandoffatafastpace.Thelongertheswitchisoncomparedtotheoffperiods,thehigherthepowersuppliedtotheloadis.ThePWMswitchingfrequencyhastobemuchfasterthanwhatwouldaffecttheload,whichistosaythedevicethatusesthepower.Typicallyswitchingshavetobedoneseveraltimesaminuteinanelectricstove,120Hzinalampdimmer,fromfewkilohertz(kHz)totensofkHzforamotordriveandwellintothetensorhundredsofkHzinaudioamplifiersandcomputerpowersupplies.Thetermdutycycledescribestheproportionof'on'timetotheregularintervalor'period'oftime;alowdutycyclecorrespondstolowpower,becausethepowerisoffformostofthetime.Dutycycleisexpressedinpercent,100%beingfullyon.ThemainadvantageofPWMisthatpowerlossintheswitchingdevicesisverylow.Whenaswitchisoffthereispracticallynocurrent,andwhenitison,thereisalmostnovoltagedropacrosstheswitch.Powerloss,beingtheproductofvoltageandcurrent,isthusinbothcasesclosetozero.PWMalsoworkswellwithdigitalcontrols,which,becauseoftheiron/offnature,caneasilysettheneededdutycycle.PWMhasalsobeenusedincertaincommunicationsystemswhereitsdutycyclehasbeenusedtoconveyinformationoveracommunicationschannel.HistoryInthepast,whenonlypartialpowerwasneeded(suchasforasewingmachinemotor),arheostat(locatedinthesewingmachine'sfootpedal)connectedinserieswiththemotoradjustedtheamountofcurrentflowingthroughthemotor,butalsowastedpowerasheatintheresistorelement.Itwasaninefficientscheme,buttolerablebecausethetotalpowerwaslow.Thiswasoneofseveralmethodsofcontrollingpower.Therewereothers—somestillinuse—suchasvariableautotransformers,includingthetrademarked'Autrastat'fortheatricallighting;andtheVariac,forgeneralACpoweradjustment.Thesewerequiteefficient,butalsorelativelycostly.Foraboutacentury,somevariable-speedelectricmotorshavehaddecentefficiency,buttheyweresomewhatmorecomplexthanconstant-speedmotors,andsometimesrequiredbulkyexternalelectricalapparatus,suchasabankofvariablepowerresistorsorrotatingconvertersuchasWardLeonarddrive.However,inadditiontomotordrivesforfans,pumpsandroboticservos,therewasagreatneedforcompactandlowcostmeansforapplyingadjustablepowerformanydevices,suchaselectricstovesandlampdimmers.OneearlyapplicationofPWMwasintheSinclairX10,a10Waudioamplifieravailableinkitforminthe1960s.AtaroundthesametimePWMstartedtobeusedinACmotorcontrol.Fig.1:apulsewave,showingthedefinitionsof,andD.Pulse-widthmodulationusesarectangularpulsewavewhosepulsewidthismodulatedresultinginthevariationoftheaveragevalueofthewaveform.Ifweconsiderapulsewaveform,withperiod,lowvalue,ahighvalueandadutycycleD(seefigure1),theaveragevalueofthewaveformisgivenby:Asisapulsewave,itsvalueisforandfor.Theaboveexpressionthenbecomes:Thislatterexpressioncanbefairlysimplifiedinmanycaseswhereas.Fromthis,itisobviousthattheaveragevalueofthesignal()isdirectlydependentonthedutycycleDFig.2:AsimplemethodtogeneratethePWMpulsetraincorrespondingtoagivensignalistheintersectivePWM:thesignal(heretheredsinewave)iscomparedwithasawtoothwaveform(blue).Whenthelatterislessthantheformer,thePWMsignal(magenta)isinhighstate(1).Otherwiseitisinthelowstate(0).ThesimplestwaytogenerateaPWMsignalistheintersectivemethod,whichrequiresonlyasawtoothoratrianglewaveform(easilygeneratedusingasimpleoscillator)andacomparator.Whenthevalueofthereferencesignal(theredsinewaveinfigure2)ismorethanthemodulationwaveform(blue),thePWMsignal(magenta)isinthehighstate,otherwiseitisinthelowstate.TimeproportioningManydigitalcircuitscangeneratePWMsignals(e.g.,manymicrocontrollershavePWMoutputs).Theynormallyuseacounterthatincrementsperiodically(itisconnecteddirectlyorindirectlytotheclockofthecircuit)andisresetattheendofeveryperiodofthePWM.Whenthecountervalueismorethanthereferencevalue,thePWMoutputchangesstatefromhightolow(orlowtohigh).[3]Thistechniqueisreferredtoastimeproportioning,particularlyastime-proportioningcontrol[4]–whichproportionofafixedcycletimeisspentinthehighstate.Theincrementedandperiodicallyresetcounteristhediscreteversionoftheintersectingmethod'ssawtooth.Theanalogcomparatoroftheintersectingmethodbecomesasimpleintegercomparisonbetweenthecurrentcountervalueandthedigital(possiblydigitized)referencevalue.Thedutycyclecanonlybevariedindiscretesteps,asafunctionofthecounterresolution.However,ahigh-resolutioncountercanprovidequitesatisfactoryperformance.PWMsamplingtheoremTheprocessofPWMconversionisnon-linearanditisgenerallysupposedthatlowpassfiltersignalrecoveryisimperfectforPWM.ThePWMsamplingtheorem[6]showsthatPWMconversionca