6英文资料圆钢送料机械手上下料机械手毕业设计.docx
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6英文资料圆钢送料机械手上下料机械手毕业设计.docx
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6英文资料圆钢送料机械手上下料机械手毕业设计
英文资料
Thecontraltechniquesdrivesandcontralshandbook
ChapterA4
Torque,speedandpositioncontrol
A4.1Generalprinciples
A4.1.1Theidealcontrolsystem
Manyapplicationsexistwheresomethinghastobecontrolledtofollowareferencequantity.Forexample,thespeedofalargemotormaybesetfromalow-powercontrolsignal.Thiscanbeachievedusingavariable-speeddriveasdescribedinthefollowing.
Ideally,therelationshipbetweenthereferenceandthemotorspeedshouldbelinear,andthespeedshouldchangeinstantlywithchangesinthereference.AnycontrolsystemcanberepresentedasinFigureA4.1b,withaninputreferencesignal,atransferfunctionFandanoutput.Forthesystemtobeideal,thetransferfunctionFwouldbeasimpleconstant,sothattheoutputwouldbeproportionaltothereferencewithnodelay.
FigureA4.1Variable-speeddriveandmotor
A4.1.2Open-loopcontrol
Unfortunately,thetransferfunctionofmanypracticalsystemsisnotaconstant,andsowithoutanyformoffeedbackfromtheoutputtocorrectforthenon-idealnatureofthetransferfunction,theoutputdoesnotfollowthedemandasrequired.Usinganinductionmotorsuppliedbyasimpleopen-loopvariable-speeddriveasanexample,thefollowingillustratessomeunwantedeffectsthatcanoccurinpracticalsystems:
Speedregulation.Theoutputofasimpleopen-loopdriveisafixedfrequencythatisproportionaltothespeedreference,andsothefrequencyappliedtothemotorremainsconstantforaconstantspeedreference.Thespeedofthemotordropsasloadisappliedbecauseoftheslipcharacteristicofthemotor,andsothespeeddoesnotremainattherequiredlevel.
Instability.Itispossibleundercertainloadconditionsandatcertainfrequenciesforthemotorspeedtooscillatearoundtherequiredspeed,eventhoughtheappliedfrequencyisconstant.Anothermajorsourceofinstabilityin
1
rotatingmechanicalsystemsislow-losselasticcouplingsandshafts.
Non-linearity.Therearemanypossiblesourcesofnon-linearity.If,forexample,themotorisconnectedtoagearbox,thespeedattheoutputofthegearboxcouldbeaffectedbybacklashbetweenthegears.
Variationswithtemperature.Someaspectsofthesystemtransferfunctionmayvarywithtemperature.Forexample,theslipofaninductionmotorincreasesasthemotorheatsup,andsoforagivenloadthemotorspeedmayreducefromthestartingspeedwhenthemotorwascold.
Delay.Withasimpleopen-loopinverterandinductionmotortherecanbeadelaybeforethemotorspeedreachesthedemandedlevelafterachangeinthespeedreference.Inverysimpleapplicationssuchascontrollingthespeedofaconveyorbelt,thistypeofdelaymaynotbeaproblem.Inmorecomplexsystems,suchasonamachinetoolaxis,delayshaveasignificanteffectonthequalityofthesystem.
Thesearejustsomeoftheunwantedeffectsthatcanbeproducedifanopen-loopcontrolsystemisused.Onemethodthatimprovesthequalityofthecontrolleristouseameasureoftheoutputquantitytoapplysomefeedbacktogiveclosed-loopcontrol.
A4.1.3Closed-loopcontrol
Thesimpleopen-loopdriveofSectionA4.1.2canbereplacedwithacontrolsystemasinFigureA4.2.Thiscontrolsystemnotonlyprovidesameanstocorrectforanyerrorintheoutputvariable,butalsoenableastableresponsecharacteristicA4.2Controllersinadrive
A4.2.1General
Althoughamodernvariable-speeddriveincludesmanyfeatures,thebasicfunctionofthedriveistocontroltorque(orforce),speedorposition.Beforeproceedingtothespecificdetailsofhowdifferenttypesofvariable-speeddrivefunction,thetheoryofcontrolforeachofthesequantitiesisdiscussed.ApositioncontrolsystemisshowninFigureA4.5.Thisincludesaninnerspeedcontroller,andwithinthespeedcontrollerthereisaninnertorquecontroller.Itispossibletocreateasystemwherethepositioncontrollerdeterminesthemechanicaltorquethatisappliedtotheloaddirectlywithouttheinnerspeedandtorqueloops.However,thepositioncontrollerwouldneedtobeabletocontrolthecomplex
combinedtransferfunctionofthemotorwindings,themechanicalloadandtheconversionfromspeedtoposition.
ThereforeitismoreusualtousetheformatshowninFigureA4.5.Theotheradvantageofthisapproachisthatlimitscanbeappliedtotherangeorrateofchangeofspeedandtorquebetweeneachofthecontrollers.Whenasystemisrequiredtocontrolspeedonly,thepositioncontrollerisomitted,andwhenasystemisrequiredtocontroltorqueonly,thepositionandspeedcontrollersareomitted.
Apositionsensorisshownprovidingfeedbackforthesystem,butthismaybereplacedbyaspeedsensororitmaybeomittedaltogetherasfollows.Positioninformationisrequiredbythetorquecontrollertofunctioninana.c.motordrive(seethedottedline).Ifpositionfeedbackisprovidedthespeedfeed-backisderivedasthechangeofspeedoverafixedsampleperiod.Sensorlessschemesarepossibleforspeedandtorquecontrolofa.c.motors,inwhichcasethesensorisnotrequired.
Positionfeedbackisnotnecessaryforthetorquecontrollerinad.c.motordrive,soaspeedfeedbackdevicesuchasatacho-generatorcanbeusedtoprovidethefeed-backforthespeedcontroller.Again,sensorlessschemesarepossiblewhereaspeedfeedbackdeviceisnotrequired.
A4.2.2Torquecontrol
Atorquecontrollerforarotarymotor,oraforcecontrollerforalinearmotor,isthebasicinnerloopofmostvariable-speeddrives.Onlytorquecontrolisdiscussedhere,buttheprinciplesalsoapplytoforcecontrolforalinearapplication.Inordertoexplaintheprinciplesoftorquecontrol,thesimpled.c.motorsysteminFigureA4.6isusedasanexample.Theanalysisoftorquecontrolinana.c.motorcanbedoneinexactlythesameway,providedsuitabletransformationsarecarriedoutinthedrive.Thesetransformationswillbediscussedlater.
Thetorquedemandorreference(Te*)isconvertedbythetorquecontrollerintoacurrentinthemotorarmature,andthemotoritselfconvertsthecurrentintotorque
FigureA4.6Torqueandcurrentcontrollersinad.c.motordrive:
(a)torquecontrol;
(b)currentcontroltodrivethemechanicalload.FigureA4.6bshowsthesystemrequiredtoconvertthetorquereferenceintomotorcurrent.Thetorquereference(Te*)isfirsttransformedintoacurrentreference(ia*)byincludingthescalingeffectofthemotorflux.Themotorflux,controlledbythemotorfieldcurrent(if),isnormallyreducedfromitsratedlevelathigherspeedswhentheterminalvoltagewouldexceedthemaximumpossibleoutputvoltageofthepowercircuitwithoutthisadjustment.Currentlimitsarethenappliedtothecurrentreferencesothattherequiredcurrentdoesnotexceedthecapa-bilitiesofthedrive.Thecurrentreference(limitedtoamaximumlevel)becomestheinputforthePIcontroller.Theelectricalequivalentcircuitofthemotorconsistsofaresistance(Ra),aninductance(La)andabackemfthatisproportionaltofluxandspeed
(Kevc/crated).
ThePIcontrolleralonecouldsuccessfullycontrolthecurrentinthiscircuitbecauseasthespeedincreases,thevoltagerequiredtoovercomethebackemfwouldbepro-videdbytheintegralterm.Theintegralcontrolislikelytoberelativelyslow,sotoimprovetheperformanceduringtransientspeedchangesavoltagefeed-forwardtermequivalenttoKevc/cratedisincluded.ThecombinedoutputofthePIcontrollerandthevoltagefeed-forwardtermformthevoltagereference(va*),andinresponsetothisthepowercircuitappliesavoltage(va)tothemotor’selectricalcircuittogiveacurrent(ia).Thecurrentismeasuredbyasensorandusedasfeedbackforthecurrentcontroller.
AswellasthelinearcomponentsshowninFigureA4.6,thecurrentcontrolloopinadigitaldriveincludessampledelaysaswellasdelayscausedbythepowercircuit.Inpractice,theresponseofthecontrollerisdominatedbytheproportionalgain.Inparticular,ifavoltagefeed-forwardtermisused,theintegraltermhasverylittleeffectonthetransientresponse.
Settingofthecontrolloopgainsisclearlyveryimportantinoptimisingtheper-formanceofthecontrolloop.Oneofthesimplestmethodstodetermineasuitableproportionalgainistousethefollowingequation:
whereLaisthemotorinductanceandTsthecurrentcontrollersampletime.Kisacon-stantthatisrelatedtothecurrentandvoltagescaling,andthedelayspresentinthecontrolsystemandpowercircuit.Mostmodernvariable-speeddrivesincludeauto-tuningalgorithmsbasedonmeasurementoftheelectricalparametersofthe
motortakenbythedriveitself,andsotheuserdoesnotnormallyneedtoadjustthecurrentcontrollergains.
Itisusefultoknowtheclosed-looptransferfunctionofthetorquecontroller(i.e.Te/Te*)sothattheresponseofastand-alonetorquecontroller,ortheeffectofaninnertorquecontrolleronouterloopssuchasaspeedcontroller,canbepredicted.Astheresponseisdominatedbythesystemdelaysitisappropriatetorepresenttheclosed-loopresponseassimplegainsandaunitygaintransportdelayasshowninFigureA4.7.
ThetorquereferencecouldbeinNm,butitismoreconventionaltouseavaluethatisapercentageoftheratedmotortorque.FigureA4.7agivesthetransferfunctionwhenthetorquecontrollerisusedalone.KtisthetorqueconstantofthemotorinNmA21.Ifthetorquecontrollerisusedwithanouterspeedcontrolleraslightlydifferentrepresentationmustbeused,asinFigureA4.7b.Thespeedcontrollerpro-ducesatorquereferencewhereavalueofunitycorrespond
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