并联机器人原文.docx

并联机器人原文.docx

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并联机器人原文

VirtualPrototypingofaParallelRobotactuatedbyServo-PneumaticDrivesusingADAMS/Controls

WalterKuhlbusch,Dr.RüdigerNeumann,FestoAG&Co.,Germany

Summary

Advancedpneumaticdrivesforservo-pneumaticpositioningallowfornewgenerationsofhandlingsandrobots.Especiallyparallelrobotsactuatedbyservo-pneumaticdrivesallowtherealizationofveryfastpickandplacetasksin3-Dspace.Thedesignofthosemachinesrequiresavirtualprototypingmethodcalledthemechatronicdesign[1].ThemostsuitablesoftwaretoolsareADAMSformechanicsandMatlab/Simulinkfordrivesandcontrollers.Toanalyzetheoverallbehaviortheco-simulationusingADAMS/Controlsisapplied.Thecombinationofthesepowerfulsimulationtoolsguaranteesafastandeffectivedesignofnewmachines.

1.Introduction

Festoisasupplierforpneumaticcomponentsandcontrolsinindustrialautomation.Theutilizationofpneumaticdrivesiswidespreadinindustrywhenworkinginopenloopcontrol.It’slimitedhowever,whenitcomestomultipointmovementorpathcontrol.Thedevelopmenthasbeendriventoservo-pneumaticdrivesthatincludeclosedloopcontrol.Festoservo-pneumaticaxesarequiteaccurate,thustheycanbeemployedasdrivesforsophisticatedtasksinrobotics.Thespecialadvantageofthesedrivesisthelowinitialcostincomparisontoelectricalandhydraulicdrivesystems.Servo-pneumaticdrivenparallelrobotsarenewsystemswithhighpotentialsinapplications.Thedynamicalperformancemeetstheincreasingrequirementstoreducethecycletimes.

Onegoalisthecreationandoptimizationofpneumaticdrivenmulti-axesrobots.Thisallowsustosupportourcustomers,andofcoursetocreatenewstandardhandlingsandrobots（Fig.1）.

Thecomplexityofparallelrobotsrequirestheuseofvirtualprototypingmethods.

Fig.1.Prototypesofservo-pneumaticdrivenmulti-axesmachines

Preferredapplicationsarefastmultipointpositioningtasksin3-Dspace.Freeprogrammablestopsallowaflexibleemploymentofthemachine.Thepointtopoint（ptp）accuracyisabout0.5mm.Thecontinuouspathcontrolguaranteescollisionfreemovementalongatrajectory.

1.1.Whyparallelrobots?

ThemainbenefitsusingparallelinsteadofserialkinematicsisshowninFig.2.

Fig.2.Benefitsofrobotswithparallelkinematics

Highdynamicalperformanceisachievedduetothelowmovedmasses.Whileinserialrobotsthefirstaxishastomoveallthefollowingaxes,theaxesofaparallelrobotcansharethemassoftheworkpiece.Furthermoreserialaxesarestressedbytorquesandbendingmomentswhichreducesthestiffness.Duetotheclosedkinematicsthemovementsofparallelrobotsarevibrationfreeforwhichtheaccuracyisimproved.Finallythemodularconceptallowsacost-effectiveproductionofthemechanicalparts.Ontheotherhandthereisthehigherexpenserelatedtothecontrol.

1.2.WhyPneumaticDrives?

Theadvantagesofservo-pneumaticdrivesare:

•directdrives→highacceleratingpower

•compact（especiallyrodlesscylinderswithintegratedguidance）

•robustandreliable

•cost-effective

Directdrivesimplyahighaccelerationpowerduetothelowequivalentmassinrelationtothedriveforce.Withpneumaticdrivestherelationshipisparticularlyfavorable.Festohasalreadybuiltupsomesystemsolutions,predominantlyparallelrobots（seeFig.1）,todemonstratethetechnicalpotentialofservo-pneumatics.WhichperformancecanbereachedisshowninFig.3.Thisprototypeisequippedwithanadvancedmodelbasedcontrollerthatmakesuseofthecomputedtorquemethod[3].

Fig.3.PerformanceoftheTripod

2.DesignMethod

Thesystemdesign,whereseveralengineeringdisciplinesareinvolvedin,requiresaholisticapproach.Thismethodistheso-calledmechatronicdesign.Thecomponentsofamechatronicsystemarethemechanicalsupportingstructure,theservodrivesaswellasthecontrol.Allthesecomponentsaremappedintothecomputerandoptimizedwithrespecttothemutualinteraction.Thisprocedurecanbeusedtoanalyzeandimproveexistingsystemsaswellastocreatenewsystems.Thetwomainstepsofthemechatronicdesignarefirstbuildingmodelsineachdiscipline,andsecondlytheanalysisandsynthesisofthewholesystem.Thesestepsaredoneinacyclefortheoptimization.

Themodelingcanbecarriedoutintwoways:

Eitheryouapplyonetooltobuildupmodelsinalldisciplines,butwithrestrictions.Theotherwayistousepowerfultoolsineachdisciplineandtoanalyzethewholesystemviaco-simulation.Inthiscaseyouhavetoconsidersomespecialsofthesolvingmethodlikecommunicationstepsizeordirectfeedthroughbehavior.

2.1.WhyCo-Simulation?

Co-simulationisusedbecauseofthepowerfultools,eachspecializedinitsowndiscipline.ADAMSisanexcellenttoolforthemechanicalpartandMatlab/Simulinkisthesuitabletoolforcontrollerdevelopmentandsimulationofpneumatics.

ThebehaviorofthemechanicalpartismodeledatbestusingADAMS/View.TheadvantagesofADAMSare:

•fastphysicallymodelingofrigidandelasticbodies

•extensivefeaturesforparameterization

•animationofsimulationresults

•solvinginversekinematicsby“generalpointmotion”

•visualizationofeigenmodes（ADAMS/LINEAR）

•exportoflinearmodels（A,B,C,D）

Abigadvantageistheautomaticcalculationofthedirectandinversekinematics.Thedirectkinematicsofparallelstructuresoftencannotbesolvedanalytically.Furthermoredifferentkinematicscanbecomparedtoeachotherveryeasilywhenyoudefineatrajectoryoftheend-effectorvia“generalpointmotion”.

Applyingthesetwosoftwaretoolsguaranteesahighflexibilityregardingthedesignofnewsystems.Itisveryimportanttoanalyzetheclosedloopbehavioratanearlystage.Thismakesabigdifferencebetweenthemechatronicdesignandtheconventionaldesign.Furthermorethevisualizationofthemechanicalsystemmakesthediscussionwithinateamveryeasy.

2.2.Restrictions

Adisadvantageisthatthemodelofthemechanicsispurelynumericallyavailable.Howeversomesymboliccodeofthemechanicalsystemisneededforthecontrolhardwarewhenthesystembecomesrealized.Ingeneralwehavetoderivetheequationsoftheinversekinematics,whichareusedinthefeedforwardcontrol.Forspecificrobottypesacontrollerwithdecouplingstructureisnecessaryinordertofulfilltherequirements.Thenthesymboliccodeofthedynamicsisneeded.Forthiswehavetopullupfurthertoolstocompletethetask.

2.3.Whathastobeanalyzed?

Forthedesignofnewrobotsitisimportanttoknowabouttheeffectonthesystemstabilityandaccuracy.ThemainpropertiesthatinfluencestabilityandaccuracyareopposedinTable1fordifferentkinematicalstructures.

Table1:

Propertiesofdifferentkinematicalconfigurations

Withrespecttothecontrolthecartesiantypeisthebestone.Butthemaindisadvantageofaserialrobotcomparedwithaparalleloneisthelowerdynamicsandthelowerstiffness（seeFig.2）.

Dependingontherequirementswithregardtodynamicsandaccuracydifferentcontrolapproachesmustbeapplied.AsmentionedaboveweprefertoemployastandardcontrollerSPC200forasingleaxis.Duetothecouplingoftheaxesthestabilityoftheclosedloopsystemmustbechecked.

3.ModeloftheTripod

ThemodeloftheTripodconsistsofthreeparts:

themechanics,thepneumaticdrives,andthecontroller.

3.1.Mechanics（ADAMS）

Weapplytheso-calleddelta-kinematicswhichcausesapurelytranslationalmovementofthetoolcenterpoint（tcp）.Anadditionalrotarydriveallowstheorientationofthegripperinthehorizontalplane.Togetherwiththerotarydrivethemachinehasfourdegreesoffreedom.

Fig.4.DegreesoffreedomandstructureoftheTripod

Thetripodismodeledusingrigidbodypartswhatisoftensufficientforthepresenttypeofparallelstructure.Theupperandlowerplatesarefixedtoground.Theprofiletubesareconnectedtotheseplatesviafixedjoints.Eachsliderhasonetranslationaldegreeoffreedom.Bothendsofarodareconnectedtotheneighboredpartsbyuniversaljoints.Includingtherotarydrive,themodelverificationresultsinfourGrueblercountsandtherearenoredundantconstraints.Themodelisparameterizedinsuchawaythatdifferentkinematicalconfigurationscanbegeneratedveryeasilybymeansofdesignvariables.Themostimportantparametersaretheradiusesoftheplates（seeFig.4）andthedistancestoeachother.Forinstancethefollowingconfigurationscanbeachievedjustbyvariationoftheseparametersordesignvariables.

Fig.5.Variationofkinematicsby“designvariables”

3.2.Servo-PneumaticDrives（Simulink）

Themodelsoftheservo-pneumaticdrivesaredevelopedbymeansofMatlab/Simulink.Dependingontherequirementsseveralcontrollermodelsweredeveloped.Itiscommontoallthattheyarehighlynon-linear.Mainlythecompressibilityofairmakesamorecomplexcontrolsystemnecessary.AllcontrollermodelsincludingthestandardcontrollerSPC200areavailableasC-codeds-functions.Thisallowstousethesamecodeinthesimulationaswellasonthetargethardware.

AsurveyofthecontrolschemeisshowninFig.6.Forthiscontributionitisimportanttoknowabouttheinterfacefortheco-simulation.Thecalculatedforcesoftheservopneumaticsaretheinputstothemechanics.Thesliderpositionsaretheoutputsofthemechanics.Detailedinformationonthecontrollerscanbefoundin[2]and[3].

Fig.6.Controlstructure

4.Analyzingthebehaviorofthewholesystem

Whenthemodelingisdonewecangoonwiththesecondstepofthemechatronicdesign.InthefollowingitisassumedthattheSPC200contr