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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.Anothermajorsourceofinstabilityinrotatingmechanicalsystemsislow-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,butalsoenableastableresponsecharacteristic

A4.2Controllersinadrive

A4.2.1General

Althoughamodernvariable-speeddriveincludesmanyfeatures,thebasicfunctionofthedriveistocontroltorque（orforce）,speedorposition.Beforeproceedingtothespecificdetailsofhowdifferenttypesofvariable-speeddrivefunction,thetheoryofcontrolforeachofthesequantitiesisdiscussed.ApositioncontrolsystemisshowninFigureA4.5.Thisincludesaninnerspeedcontroller,andwithinthespeedcontrollerthereisaninnertorquecontroller.Itispossibletocreateasystemwherethepositioncontrollerdeterminesthemechanicaltorquethatisappliedtotheloaddirectlywithouttheinnerspeedandtorqueloops.However,thepositioncontrollerwouldneedtobeabletocontrolthecomplexcombinedtransferfunctionofthemotorwindings,themechanicalloadandtheconversionfromspeedtoposition.

ThereforeitismoreusualtousetheformatshowninFigureA4.5.Theotheradvantageofthisapproachisthatlimitscanbeappliedtotherangeorrateofchangeofspeedandtorquebetweeneachofthecontrollers.Whenasystemisrequiredtocontrolspeedonly,thepositioncontrollerisomitted,andwhenasystemisrequiredtocontroltorqueonly,thepositionandspeedcontrollersareomitted.

Apositionsensorisshownprovidingfeedbackforthesystem,butthismaybereplacedbyaspeedsensororitmaybeomittedaltogetherasfollows.

Positioninformationisrequiredbythetorquecontrollertofunctioninan