Servo Motor Control Application on a Local Area In

©FreescaleSemiconductor,Inc.,2005.Allrightsreserved.FreescaleSemiconductorApplicationNoteAN2396Rev.1.0,12/20051.0IntroductionThemainpurposeofthisapplicationnoteistoshowonewaytoimplementaclosed-loopcontrolpositionservomotorapplicationwhichutilizestheLocalInterconnectNetwork(LIN)toallowaseriesofsimilarmotorstobeconnectedtogetherandcontrolledfromacentralLINmastercontroller.TheapplicationnoteexplainsabasicdesignwhichusesaDCbrushmotor,feedbackpotentiometer,aFreescaleM68HC08microcontroller,andSmartMOSpowercomponents.ThesoftwaredesignandmessagingstrategyareexplainedstepbysteptoaidinthedevelopmentofanentireLINsubnetworkofservomotors,completewithfullhardwarediagnosticscapability.ServoMotorControlApplicationonaLocalInterconnectNetwork(LIN)ByMattRuff8/16BitDivisionSystemsEngineeringAustin,TexasContents1.0Introduction...........................12.0Improvements,LessonsLearned,andSuggestions..........................163.0Conclusion...........................164.0AppendixA—LINHVACSystemDemonstrationMessagingStrategy.......175.0AppendixB—LINHVACSystemDemonstrationMessagingStrategy—ConfigurationLanguageDescriptionFile..176.0AppendixC—Closed-LoopControlHVACActuatorSourceCode..................17ServoMotorControlApplicationonaLocalInterconnectNetwork(LIN),Rev.1.02FreescaleSemiconductorIntroduction1.1WhatistheDistinctionBetweenOpen-LoopandClosed-LoopControl?Withoutgettingintoadetailedexplanationofthespecificsofcontroltheory,itispossibletomakeasimpledistinctionbetweenopen-loopandclosed-loopsystems.Anopen-loopsystemoperateswithnofeedbackfromtheobjectbeingcontrolled.Thisissortofa“fireandforget”approachtocontrol.Aninputstimulusisprovidedandthecontrollercommandsthesystemtogotoaparticularlocation,speed,whatever,andhopesthatthesystemrespondsaccordingly.Thereisnoinformationfromthesystemundercontroltoindicatethatitevenreceivedthecommand,muchlessacteduponit.Thekeytoaclosed-loopcontrolsystemistheintroductionoffeedback.Ifspeedisbeingcontrolled,ameasureofthecurrentspeedisprovidedbacktothecontroller,allowingittoadjustitscommandsasthesystemrespondstothecommands.Likewiseistruewithposition.Thinkofagymnastonabalancebeam.Theyareconstantlycommandingtheirmusclestoadjustthepressureandpositionoftheirfeettomaintainasetposition.Thisisdonebasedoninputsfromvision,senseofbalance,andeventactilefeedbackfromcontactwiththebeamitself.Thisisaperfectexampleofaclosed-loopsystem.Theperformanceofaclosed-loopsystemispartiallyafunctionofthespeedatwhichthefeedbackisreturnedtothecontroller.Thisclosingoftheloopwillalwaystakeasetamountoftime,andthelongerthattimeisthelessresponsivethecontrollerwillbetofastchangingconditions.Forexample,ifthegymnastsareverytiredtheyarenotabletoperformasadeptlyduetoslowresponsetothefeedbackreceivedandasconditionschangemorerapidlytheyaremorelikelytofalloffthebar.Dependingontheperformancerequirementsoftheapplication,eitheraclosed-looporopen-loopcontrolsystemcanbeusedtocontrolmotorposition,speed,orothersimilarapplication.1.2WhatisaServoMotor?Motorscomeinmanydifferentvarietiesfordifferentapplications.Theterm“servomotor”doesn’treallyapplytothemotoritself,butratherthewayinwhichthemotorisusedandcontrolled.Inapositionservomotorapplication(henceforthjust‘servomotor’),theideaistoholdthetargetload(generallyattachedtothemotorshaftthroughaseriesofgearsforspeedandtorqueadjustment)inagivenposition.Toaccomplishaservomotorfunction,positioninginformationmustbeobtainedfromtheoutputofthemotortoprovidefeedbackforthecontrolsystem.Thiscanbeintheformofapotentiometerattachedsomewhereinthegeartrain,ahalleffectsensormonitoringpassingteethonametalgear,anencoder(opticalormagnetic)mounteddirectlytothemotor,oranyothersuchsensorwhichcanprovidepositionfeedbackofthemotorshaftorconnectedportionofthegeartrain.Itisalsopossibletodeterminemotorshaftpositionbycountingthecommutationpulsesontheterminalsofthemotor.FreescaleSMARTMOSH-Bridgedriversdohavecurrentre-copycapability,whichallowsafractionoftheloadcurrenttobeoutputthroughareferenceresistorServoMotorControlApplicationonaLocalInterconnectNetwork(LIN),Rev.1.0FreescaleSemiconductor3Introductionandmeasuredwithananalog-to-digitalconverter(ADC).Inthisway,itmightbepossibletomonitortheloadcurrentanddetectvariationsintheloadcurrentwhichcorrespondwiththecommutationofthebrushesinthemotor.However,thismethodrequiresadditionalsoftwareandhardwarecomplexitywhichdonotjustifytheirinclusion,giventhatencodersarejustaseffectiveandmuchsimplertoimplement.Thismodificationallowsstreamliningofthelogicneededattheloadtodriveit.1.3WherewouldIuseaDCBrushPositionServoMotor?DCbrushpositionservomotorscanbeusedinmanydifferentapplications.Apartiallistofpossibleapplicationsincludes:•AutomotiveMarket:—Powermirrorpositioning—Powerseatspositioningmotors—Powerdoorandtrunklockmechanisms—Windshieldwipermotors—Heating,Ventilation,andAirConditioning(HVAC)ventcontrols—Powerslidingdoor,sunroof,andconvertibletopactuators—Headlightpositioningandlevellingactuators•IndustrialandConsumerMarkets:—Proportioningvalvesforgassesandliquids—Paperandmaterialshandlingequipment—HVACventi