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Proceedingsofthe2006IEEE/RSJInternationalConferenceonIntelligentRobotsandSystemsOctober915,2006,Beijing,ChinaANovelModularFixtureDesignandAssemblySystemBasedonVRPengGaoliang,LiuWenjianSchoolofMechatronicsEngineeringHarbinInstituteofTechnologyHarbin,150001,Chinapgl7782ahit.edu.cnAbstractModularfixturesareoneoftheimportantaspectsofmanufacturing.ThispaperpresentsadesktopVRsystemformodularfixturedesign.Thevirtualenvironmentisdesignedandthedesignprocedureisproposed.Itassiststhedesignertomakethefeasibledesigndecisionseffectivelyandefficiently.Ahierarchicaldatamodelisproposedtorepresentthemodularfixtureassembly.Basedonthisstructure,theusercanmanipulatethevirtualmodelspreciselyinVEduringthedesignandassemblyprocesses.Moreover,themachiningsimulationformanufacturinginteractioncheckingisdiscussedandimplemented.Finally,thecasestudyhasdemonstratedthefunctionalityoftheproposedsystem.ComparedwiththeimmersiveVRsystem,theproposedsystemhasofferedanaffordableandportablesolutionformodularfixturesdesign.IndexTermsModularfixture,desktopVR,assemblydesign,machiningsimlulation.I.INTRODUCTIONModularfixturesareoneoftheimportantaspectsofmanufacturing.Properfixturedesigniscrucialtoproductqualityintermsofprecision,accuracy,andfinishofthemachinedpart.Modularfixtureisasystemofinterchangeeableandhighlystandardizedcomponentsdesignedtosecurelyandaccuratelyposition,hold,andsupporttheworkpiecethroughoutthemachiningprocess1.Traditionally,fixturedesignersrelyonexperienceorusetrialanderrormethodstodetermineanappropriatefixturingscheme.Withtheadventofcomputertechnology,computeraideddesignhasbeenprevalentintheareaofmodularfixturedesign.Ingeneral,theassociatedfixturedesignactivities,namelysetupplanning,fixtureelementdesign,andfixturelayoutdesignareoftendealtwithatthedownstreamendofthemachinetooldevelopmentlifecycle.Thesepracticesdonotlendthemselveswelltothebridgingofdesignandmanufacturingactivities.Forexample,veryfewsystemshaveincorporatedthefunctionalityofdetectingmachininginterference.Thisleadstoagapbetweenthefixturedesignandmanufacturingoperationswheretheaspectofcutterpathsisnotconsideredduringthedesignstage2.Asaresult,redesigncannotbeavoidedwhenthecutterisfoundtointerferewiththefixturecomponentsinthemanufacturingsetup.Therefore,inordertobringmachiningfixturedesignintothearenaofflexiblemanufacturing,amoresystematicandnaturaldesignenvironmentisrequired.Asasynthetic,3D,interactiveenvironmenttypicallygeneratedbyacomputer,VRhasbeenrecognizedasaverypowerfulhumancomputerinterfacefordecades4.VRholdsgreatpotentialinmanufacturingapplicationstosolveproblemsbeforebeingemployedinpracticalmanufacturingtherebypreventingcostlymistakes.TheadvancesinVRtechnologyinthelastdecadehaveprovidedtheimpetusforapplyingVRtodifferentengineeringapplicationssuchasproductdesign5,assembly6,machiningsimulation7,andtraining8.ThegoalofthispaperistodevelopaVRbasedmodularfixturesdesignsystemVMJFDS.Thisisthefirststeptodevelopanintegratedandimmersiveenvironmentformodularfixturedesign.Thisapplicationhastheadvantagesofmakingthefixturedesigninanaturalandinstructivemanner,providingbettermatchtotheworkingconditions,reducingleadtime,andgenerallyprovidingasignificantenhancementoffixtureproductivityandeconomy.II.OVERVIEWOFTHEPROPOSEDSYSTEMThesystemarchitectureoftheproposeddesktopVRsystemismodularisedbasedonthefunctionalrequirementsofthesystem,whichisshowninFig.1.Atthesystemlevel,threemodulesofproposedsystem,namely,GraphicinterfaceGUI,VirtualenvironmentVEandDatabasemodulesaredesigned.Foreachofthemodules,asetofobjectshasbeenidentifiedtorealizeitsfunctionalrequirements.Thedetailedobjectdesignandimplementationareomittedfromthispaper.Instead,thebriefdescriptionofthesethreemodulesisgivenbelow.1GraphicInterfaceGUITheGUIisbasicallyafriendlygraphicinterfacethatisusedtointegratethevirtualenvironmentandmodularfixturedesignactions.2VirtualenvironmentVETheVEprovidestheuserswitha3Ddisplayfornavigatingandmanipulatingthemodelsofmodularfixturesystemanditscomponentsinthevirtualenvironment.AsshowninFig.1,thevirtualenvironmentmodulecomprisestwoparts,namelyassemblydesignenvironmentandmachiningsimulationenvironment.Theuserselectsappropriateelementsandputsdowntheseelementsonthedeskintheassemblydesignarea.Thenheassemblestheselectedelementsonebyonetobuildupthefinalfixturesystemwiththeguidanceofthesystem.142440259X/06/20.00C2006IEEE2650AuthorizedlicenseduselimitedtoNanchangUniversity.DownloadedonDecember20,2009at2244fromIEEEXplore.Restrictionsapply.Fig.1.OverviewofthedesktopVRbasedmodularfixturedesignsystem.3DatabaseThedatabasedepositallofthemodelsofenvironmentandmodularfixtureelements,aswellasthedomainknowledgeandusefulcases.Thereare5databasesshowninFig.1.Amongthem,knowledgerulebasegoverningallfixtureplanningprinciplesformsthebrainsofthesystem.III.PROCEDUREOFMODULARFIXTUREDESIGNInthissection,aninstructivemodularfixturedesignprocedurewithinVEispresented.Besidesthe3Ddepththattheusersfeelandtherealworldlikeoperationprocess,thisprocedurefeaturesintelligenceandintroduction.Duringthedesignprocess,someusefulcasesandsuggestionwillbepresentedtotheuserforreferencebasedonintelligentinferencemethodsuchasCasebasedreasoningCBRandRulebasedreasoningRBR.Furthermore,relativeknowledgeandrulesarepresentedashelppagesthattheusercaneasilybrowsedduringthedesignprocess.OverviewofmodularfixturedesignprocessissummarizedinFig.2.AftertheVEenvironmentisinitialedandtheworkpieceisloaded,thefirststepisfixtureplanning.Inthisstep,theuserfirstdecidesthefixturingscheme,thatisspecifiesthefixturingfacesoftheworkpieceinteractively.Forhelptheusersdecisionmaking,someusefulcasesaswellastheirfixturingschemewillbepresentedviatheautomaticCBRretrievalmethod.Oncethefixturingfacesareselected,theusermaybeprompttospecifythefixturingpoints.Inthistask,somesuggestionsandrulesaregiven.Afterthefixturingplanning,thenextstepisfixtureFUsdesignstage.Inthisstage,theusermaybetoselectsuitablefixtureelementsandassembletheseindividualpartsintoFUs.Accordingtothespatialinformationofthefixturingpointsinrelationtothefixturebaseandtheworkpiece,sometypicalFUsandsuggestionsmaybepresentedautomatically.Thesewillbehelpfulfortheuser.AftertheplanningandFUsdesignstage,thenextstageisinteractivelyassemblingthedesignedfixtureFUstoconnecttheworkpiecetothebaseplate.Whenthefixtureconfigurationiscompleted,theresultwillbecheckedandevaluatedwithinthemachiningenvironment.Thetasksexecutedinthisenvironmentincludingassemblyplanning,machiningsimulation,andfixtureevaluation.Assemblyplanningisusedtogainoptimalassemblysequenceandassemblypathofeachcomponent.Machiningsimulationisresponsibleformanufacturinginteractiondetection.Fixtureevaluationwillcheckandevaluatethedesignresult.Inconclusion,thewholedesignprocessisinanaturemannerforthebenefitofVE.Moreover,thepresentedinformationofsuggestionandknowledgecanadvisetheuseronhowtomakedecisionsofthebestdesignselection.IV.ASSEMBLYMODELINGOFMODULARFIXTUREA.ModularfixturestructureanalysisAfunctionalunitFUisacombinationoffixtureelementstoprovideconnectionbetweenthebaseplateandaworkpiece11.Generally,modularfixturestructuremaybedividedintothreefunctionalunitsaccordingtoitsbasicstructurecharacteristics,namelylocatingunit,clampingunit,andsupportingunit.ThenumberoffixtureelementsinaFUmayconsistofoneormoreelements,inwhichonlyoneelementservesasalocator,supportorclamp.Themajortaskofthemodularfixtureassemblyistoselectthesupporting,locating,clampingandaccessoryelementstogeneratethefixtureFUstoconnecttheworkpiecetothebaseplate.Byanalyzingthepracticalapplicationofmodularfixtures,itisfoundthattheassemblyofmodularfixturesbeginsbyselectingthesuitablefixtureelementstoconstructFUs,thensubsequentlymountingtheseFUsonthebaseplate.Therefore,theFUscanberegardedassubassembliesofmodularfixturesystem.Further,thestructureofmodularfixturesystemcanberepresentedasahierarchalstructureasshowninFig.3.2651AuthorizedlicenseduselimitedtoNanchangUniversity.DownloadedonDecember20,2009at2244fromIEEEXplore.Restrictionsapply.UsefTa§6TsiikgSuggelr,lFixtuieElemenetsrUetrievali0ToolsrKetrieval4Fig.2ModularfixturedesignprocedureinproposedsystemB.HierarchicallystructureddatamodelformodularfixturerepresentationinVEItiscommonthatthecorrespondingvirtualenvironmentmaycontainmillionsofgeometricpolygonprimitives.Overthepastyears,anumberofmodelsubdivisionschemes,suchasBSPtree10andOctrees,havebeenproposedtoorganizelargepolygonalmodels.However,formodularBa1I_}1HsreplalteBansepla1nteElementsLocatngElementsL,catingUnitsAccessoryEllementsClamnpingElemnentsClampingUnitsSupportingElemntsSupportingUfnitsAccessoryElementsFig.3Hierarchicalstructureofmodularfixturesystemdesignapplications,thesceneisalsodynamicallychanging,duetointeractions.Forexample,indesignprocess,thepartobjectmaychangeitsspatialposition,orientationandassemblyrelations.Thisindicatesthatastaticrepresentation,suchasBSPtree,isnotsufficient.Furthermore,theabovemodelscanonlyrepresentthetopologystructureoffixturesysteminthecomponentlevel.However,totheassemblyrelationshipamongfixturecomponents,whichreferstothematingrelationshipbetweenassemblyfeaturesthatisnotconcerned.Inthissection,wepresentahierarchicallystructuredandconstraintbaseddatamodelformodularfixturesystemrepresentation,realtimevisualizationandprecise3DmanipulationinVE.AsshowninFig.4,thehighlevelcomponentbasedmodelisusedforinteractiveoperationsinvolvingassembliesordisassembles.Itprovidesbothtopologicalstructureandlinkrelationsbetweencomponents.Theinformationrepresentedinthehighlevelmodelcanbedividedintotwotypes,i.e.componentobjectsandassemblyrelationships.Componentobjectscanbeasubassemblyorapart.Asubassemblyconsistsofindividualpartsandassemblyrelationshipsbetweentheparts.ComponentLevelPtPartSSubassemblyAssemblyrelationshipFeatureLevelFt3FeatureFeaturematingrelationshipttPolygonLevelFZll.PolygonFig.4ThehierarchicalstructuredatamodelinVEThemiddlelevelfeaturebasedmodelisbuiltuponfeaturesandfeatureconstraints.Ingeneral,theassemblyrelationshipoftentreatedasthematingrelationshipsbetweenassemblyfeatures.Thusthefeaturebasedmodelisusedtodescribetheassemblyrelationshipandprovidesnecessaryinformationforspatialrelationshipcalculatingduringassemblyoperation.Inthismodel,onlythefeaturerelationshipsbetweentwodifferentcomponentsareconsidered.Therelationshipbetweenfeaturesofoneelementwillbediscussedinfeaturebasedmodularfixtureelementmodelingbelow.Thelowlevelpolygonbasedmodelcorrespondstotheabovetwolevelmodelsforrealtimevisualizationandinteraction.Itdescribestheentiresurfaceasaninterconnectedtriangularsurfacemesh.Moreabouthowthepolygonsorganizedofasingleelementwillbediscussedisthenextsection.C.ModularfixtureelementsmodelingAsweknow,inVE,thepartisonlyrepresentedasanumberofpolygonprimitives.Thisresultinthetopological2652AuthorizedlicenseduselimitedtoNanchangUniversity.DownloadedonDecember20,2009at2244fromIEEEXplore.Restrictionsapply.relationshipsandparametricinformationarelostduringthetranslationprocessofmodelsfromCADsystemstoVRsystems.However,thisimportantinformationisnecessaryindesignandassemblyprocess.Inordertofulfilltherequirements,wepresentamodelingschemeforfixtureelementsrepresentationinthissection.Themodularfixtureelementsarepremanufacturedpartswithstandarddimensions.Afterthefixturingschemedesigned,theleftjobistoselectsuitablestandardelementsandassembletheseelementstoformafixturesysteminafeasibleandeffectivemanner.Therefore,intheproposedsystem,onlytheassemblyfeaturesofthefixtureelementsneedtobeconsidered.Inthispaperanassemblyfeatureisdefinedasapropertyofafixtureelement,whichprovidesrelatedinformationrelevanttomodularfixturedesignandassembly/disassembly.Thefollowingeightfunctionfacesaredefinedasassemblyfeaturesoffixtureelementssupportingfaces,supportedfaces,locatingholes,counterboreholes,screwholes,fixingslots,andscrewbolts.Besidestheinformationaboutthefeatureliketypeanddimension,otherparameters,i.e.therelativepositionandorientationofthefeatureintheelementslocalcoordinatesystemarerecordedwiththegeometricmodelinthefixtureelementdatabase.Whenoneelementassembleswithanother,theinformationaboutthematedfeaturesisretrievedandusedtodecidethespatialrelationshipofthetwoelements.MoreinformationabouttheassemblyfeaturesandtheirmatingrelationshiparediscusseddetailedinRef1.D.ConstraintbasedfixtureassemblyinVE1AssemblyrelationshipbetweenfixtureelementsMatingrelationshipshavebeenusedtodefineassemblyrelationshipsbetweenpartcomponentsinthefieldofassembly.Accordingtotheassemblyfeaturessummarizedintheabovesection,therearefivetypesofmatingrelationshipsbetweenfixtureelements.Namelyagainst,fit,screwfit,across,andTslotfit,whichareillustratedinFig.5.Basedonthesematingrelationships,wecanreasonthepossibleassemblyrelationshipofanytwoassembledfixtureelements.2AssemblyrelationshipreasoningIngeneral,theassemblyrelationshipoftwoassembledpartisrepresentedasthematedassemblyfeaturepairsofthem.Intheabovesection,wedefinedfivebasicmatingrelationshipsbetweenfixtureelements.Therefore,itisenabledtodecidethepossibleassemblyrelationshipsthroughfindingthepossiblematingassemblyfeaturepairs.ThesepossibleassemblyrelationshipsaresavedinassemblyrelationshipsdatabaseARDBforfixtureassemblyinnextstage.However,whenthefixtureiscomplicatedandthenumbersofcompositefixtureelementsislarge,thepossibleassemblyrelationshipsaretoomuchtotakemuchtimeforreasoningandtreating.Toavoidthissituation,wefirstdecidethepossibleassembledelementspairs.Thatistoavoidreasoningtheassemblyrelationshipbetweenaclampandthebaseplate,fortheyneverwereassembledtogether.Inthisstage,somerulesareutilizedtofindthepossibleassembledelementspairs.ThealgorithmofassemblyrelationshipsreasoningissimilartowhatdiscussedinRef12.Thusthedetaileddescriptionofthealgorithmisomittedfromthispaper.aAIlai.ns..2l.I.FLIiI7FdAsicmie1fisxktElmnFig.5Fivebasicmatingrelationshipsbetweenfixtureelements3ConstraintbasedfixtureassemblyAftercarryingouttheassemblyrelationshipsreasoning,allpossibleassemblyrelationshipsoftheselectedelementsareestablishedandsavedinARDB.Basedontheserelationships,thetraineecanassembletheseindividualpartstoafixturesystem.ThissectionisaboutthediscussionofinteractiveassemblyoperationinVE.TheprocessofasingleassemblyoperationispresentedinFig.5andillustratedbytwosimplepartsassemblyasshowninFig.6.Ingeneral,theassemblyoperationprocessisdividedintothreesteps,namelyassemblyrelationshiprecognizing,constraintanalysisandapplying,constraintbasedmotion.Firstly,thetraineeselectsanelementandmovesittotheassembledcomponent.Onceaninferencebetweentheassemblingandassembledcomponentisdetectedduringthemoving,theinferredfeaturesischecked.IfthetwofeaturesisoneoftheassemblyrelationshipsinARDB,theywillbehighlightedandwillawaittheusersconfirmation.Onceitisconfirmed,therecognizedassemblyrelationshipwillbeappliedbyconstraintanalyzingandsolving,thatisadjustthetranslationandorientationoftheassemblingelementtosatisfythepositionrelationshipofthesetwocomponents,aswellasapplythenewconstrainttotheassemblingelement.Whenthenewconstraintisapplied,themotionoftheassemblingelementwillbemappedintoaconstraintspace.Thisisdonebytransferring3Dmotiondatafromtheinputdevicesintotheallowablemotionsoftheobject.Theconstraintbasedmotionnotonlyensuresthattheprecisepositionsofacomponentcanbeobtained,butalsoguaranteethattheexistingconstraintswillnotbeviolatedduringthefutureoperations.Theassemblingelementwillreachtothefinalpositionthroughsuccessionassemblyrelationshiprecognizingandconstraintapplying.2653Ii1114bF.tAuthorizedlicenseduselimitedtoNanchangUniversity.DownloadedonDecember20,2009at2244fromIEEEXplore.Restrictionsapply.
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