外文翻译--自动控制器.doc

附加外文翻译外文文献I,robotcontrollerAsophisticatedapproachtokinematicsiswhatdifferentiatesrobotcontrollersfrommoregeneralpurposemotionequipment.Aninterestingsituationemergedrecentlywhenamanufacturertriedtoputavisionsystemonanassemblyline.Theideawastolocatepartsonamovingconveyorwithavisionsystem,thenpositionaroboticsarmtopickthemuplineatatime.Engineerstherediligentlyworkedoutnumerousdisplacementfudgefactorstorelatethelocationsoftheconveyorendeffectorsandpartsimagedbythecamera.Thefudgefactorsletthemotioncontrollerinferthephysicallocationofapartfromthevisionsystemdata,thendirectthearmtotherightplacetopickitup.Problemwas,therelativepositionofthevariouscomponentsallchangedeverytimetheconveyorwentbackonlineafterservicingormaintenance.Thefactorssocarefullycomputedbecameuseless.Thisnecessitatedregularroundsofrecalculatingnewdisplacements.Attherootofthesedifficultiesweresomefundamentalmisunderstandingsabouthowgeneral-purposemotioncontrollersdifferfrommorespecializedrobotcontrollers.Hardwarewise,thetwocanlooksimilar.BothfrequentlyemployPentium-basedprocessorsoradoptahybridapproachwithageneralCPUsupervisingoneormoredigitalsignalprocessorsdedicatedtoservoloops.However,thesoftwarearchitectureofarobotcontrollerdiffersdramaticallyfromthatofanordinarymotion-controllersoftware:Itgenerallyconsistsofaroutineforclosed-looppositionorvvelocitycontrol,operatorinterfacefunctions,androutineforsupervisorytasks.Animportantpointtonoteisatthesupervisorylevelofcontrol.Taskstherethatrelatetohandlingmotiondonotextendmuchpastsimplyissuingpositioncommandsandindividualaxes.Inotherword,thesupervisorylevelisrelativelysimply.Thesupervisorylevelofrobotcontrollerismoresophisticated.Foronething,itiswrittenwiththeideathat,postroboticsystemsincorporatefeedbackfromhigh-levelsensorsthatresideoutsidetheposition-encoder-feedbackservoloopsofindividualaxes.Typicalexamplesincludeindustrialvisionsystemandforcesensors.Mostroboticworkinvolvesusinginformationfromthesesensorstocalculatethetrajectoryofarobotarm.Tohandlethiscalculationprocess,supervisorylevelsoftwareimplementsatrajectoryplanningalgorithm.Thisalgorithmrelatesthephysicallocationofpositioningelements,sensorfeedback,andtheobjectsbeingpositionedintermsofwhatscalledaworldcoordinatesystem.Thisisincontrasttogeneral-purposemotionequipmentwhichtendstouseaseparatereferenceframeforeachaxisofmotion.Onebenefitofaworld-coordinatesystemisthatitcaneliminatetheneedforfudgefactorsrelatingsensordatatothepositionofvariouscomponents.Thestateoftheartissuchthatstraightforwardsetuproutinescancomputesuckinformationautomatically.Moreover,datagatheredduringsetupgoesintotransformationcalculationsthatdetermineworldcoordinatesandwhicharemoreprecisethananymanuallydeducedfudgefactors.REFEENCEKINEMATICSItisusefultobrieflyreviewthewayarobotcontrollerimplementsworldcoordinates.Readerswillprobablyrecallfromengineeringmechanicsthatthepositionofanarbitrarypointexpressedinonecoordinatesystemcanbemappedinanotherthroughuseofa4x4transformation.InthecaseofScasarobotarm,thepositionofapointattheendofthearmcanbeexpressedintermsoftheproductof4x4matrices,onematrixforeachlinkintherobotarm.Matrixcoefficientsforthearmitselfaredeterminedbylinklengthandgeometry,andjointangle.Obviouslylinkgeometryisknown.Jointanglecoefficientscomefromfeedbackprovidedbyjointservoencoders.Inananalogousmanner,thecoordinatesinferredfromtheimageofanindustrialvisionsystemcanbeexpressedinworldcoordinatesviaanothersetof4x4transformations.Thecoefficientsforthetransformationmatricescomefrominformationdeterminedduringequipmentsetup.Take,asanexample,thecaseofpartslayingonaconveyor.Therobotarmwilllocatethreepointsontheconveyoraspartofthesetupprocess.Thesepoints,ofcourse,definetheconveyorplane.Therobotcontrollerusesthisinformationtodeducethetransformationcoefficientsthatwillrelateconveyorpositioninworldcoordinates.Apointtonoteisthateveniftheconveyorisonananglethisfactwillbereflectedinthetransformationcoefficientscalculatedautomaticallyduringsetup.Thereisnoneedforcomputingadditionaldisplacementsorothercompensatingoffsets.Modernrobotcontrollersuseprogramminglanguagesthatalsoworkinworldcoordinates.Toolcommands,visioncommands,andconveyordefinitionsallgetexpressedthisway.Putanotherway,theworld-coordinatesystemandthetransformationsthatmakeitpossibleareembeddedinthecontrollerprogramminglanguage.Oneadditionalmanifestationofthisapproachisthatwhenprogrammingmovesoperatorsofsucksystemsneednotconcernthemselveswithtimingrelationshipsattheoperating-systemlevel.Thisisalsoonereasonwhyrobotcontrollerscanimplementahigh-levelcalibrationmethodology.Oncerepositioned,arobotanditsancillarysystemscanfindtheirbearingsthroughuseofafewsoftwaresetuputilitiesthatrecalculatetransformationcoefficients.Thisprocesscontrastswiththatnecessaryformoregeneral-purposemotioncontrollers.Thoughthesesystemsalsotendtoemployspecial-purposeautomationsoftware,positioningcommandstendtoassumecoordinatesystemsthatcenteroneachaxisofmotion.Itiscertainlypossibletodefineworld-coordinatesystemforthesecontrollers.ButcontrolvendersgenerallyleavethistasktoOEMshandingapplicationswhereitspecificallycomesinhandy.Thereasonisthatroboticpositioningisaspecialcaseofmotioncontrol.WorldcoordinatesofferlimitedutilityinsimplerbutmoretypicalpositioningapplicationsthatcanrangefromconvertingmachinestocardreadersinATMs.Allinall,theprocessoffittinggeneral-purposecontrollerstoroboticapplicationsputstheburdenonOEMsforcoordinatetransformationrelationshipsalreadyavailableinrobotcontrollers.Alternativelytheycansimplytrytomakedowithaseriesoflossrobustphysicaloffsetsanddisplacements.TRAJECTORYPLANNINGTheservoloopsoftwarethatpositionsanaxisonarobotisfairlyconventional.Eachaxishasitsownservoloop.Anerrortermdrivesaxismotion,derivedfromthedifferencebetweenpositionfeedbackandapositioncommand.Theremaybefeedforwardconstantstoadjustthepositionerrorundercertainconditions.Andaswithgeneral-purposemotioncontrollers,roboticservoloopsexecuteontheorderofonceeverymillisecond.Thesoftwarethatfeedspositioncommandstoeachservoloopiscalledthetrajectoryplanner.Itisthetrajectoryplannerthatcomputesamodelofwhereatooltipmustgofromwhereitcurrentlyresides.Todoso,itmusttakeinformationoutofworld-coordinateformandtranslateitintojointangles(foraScasarobot)orintodisplacementsformoregeneral-purposeautomationequipment.Thetrajectoryplannerrepeatsthisprocessaboutevery16-mses.Trajectoryandservocycletimeentersintonotonlysystembandwidthconcerns,butalsosafetyconsiderations.Theroboticindustryhasissuedstrictsafetystandardsthatdictateminimumlevelsofperformanceinemergencysituations.Perhapsmostobviousoftheseistheemergencystop.Roboticcontrollersemployanemergencystopalgorithmthatbypassesthetrajectoryplannerandits16-msescycletimeandexecutescontrolled-stoproutinesinfirmware.Thispowersdowneachaxistoacontrolledstopinafewmilliseconds.Thisfast-but-controlledemergencystopcanbecontrastedwiththetechniqueusedbymanygeneral-purposemotioncontrollers.Thesimplerapproachistojustdrivealargemomentarysurgeofnegative(halt)powertotheamplifier.Thiscertainlystopsthepositioningequipment.Butinthecaseofarobot,itcouldeasilysnapoffawristmechanismifthereisenoughinertia.Finally,robotcontrollersemployothersafety,measuresthatarecommonlyfoundinNCequipmentbutwhicharerareinmoregeneral-purposepositions.Forexample,lossofencoderfeedbackwillgenerateanemergencyst