Proceedingsofthe2006IEEE/RSJInternationalConferenceonIntelligentRobotsandSystemsOctober9-15,2006,Beijing,ChinaANovelModularFixtureDesignandAssemblySystemBasedonVRPengGaoliang,LiuWenjianSchoolofMechatronicsEngineeringHarbinInstituteofTechnologyHarbin,150001,CAbstract-Modularfixturesareoneoftheimportantaspectsofmanufacturing.ThispaperpresentsadesktopVRsystemformodularfixturedesign.Thevirtualenvironmentisdesignedandthedesignprocedureisproposed.Itassiststhedesignertomakethefeasibledesigndecisionseffectivelyandefficiently.Ahierarchicaldatamodelisproposedtorepresentthemodularfixtureassembly.Basedonthisstructure,theusercanmanipulatethevirtualmodelspreciselyinVEduringthedesignandassemblyprocesses.Moreover,themachiningsimulationformanufacturinginteractioncheckingisdiscussedandimplemented.Finally,thecasestudyhasdemonstratedthefunctionalityoftheproposedsystem.ComparedwiththeimmersiveVRsystem,theproposedsystemhasofferedanaffordableandportablesolutionformodularfixturesdesign.IndexTerms-Modularfixture,desktopVR,assemblydesign,machiningsimlulation.I.INTRODUCTIONModularfixturesareoneoftheimportantaspectsofmanufacturing.Properfixturedesigniscrucialtoproductqualityintermsofprecision,accuracy,andfinishofthemachinedpart.Modularfixtureisasystemofinterchange-eableandhighlystandardizedcomponentsdesignedtosecurelyandaccuratelyposition,hold,andsupporttheworkpiecethroughoutthemachiningprocess1.Tradition-ally,fixturedesignersrelyonexperienceorusetrial-and-errormethodstodetermineanappropriatefixturingscheme.Withtheadventofcomputertechnology,computeraideddesignhasbeenprevalentintheareaofmodularfixturedesign.Ingeneral,theassociatedfixturedesignactivities,namelysetupplanning,fixtureelementdesign,andfixturelayoutdesignareoftendealtwithatthedownstreamendofthemachinetooldevelopmentlife-cycle.Thesepracticesdonotlendthemselveswelltothebridgingofdesignandmanufacturingactivities.Forexample,veryfewsystemshaveincorporatedthefunctionalityofdetectingmachininginterference.Thisleadstoagapbetweenthefixturedesignandmanufacturingoperationswheretheaspectofcutterpathsisnotconsideredduringthedesignstage2.Asaresult,re-designcannotbeavoidedwhenthecutterisfoundtointerferewiththefixturecomponentsinthemanufactu-ringset-up.Therefore,inordertobringmachiningfixturedesignintothearenaofflexiblemanufacturing,amoresystematicandnaturaldesignenvironmentisrequired.Asasynthetic,3D,interactiveenvironmenttypicallygeneratedbyacomputer,VRhasbeenrecognizedasaverypowerfulhuman-computerinterfacefordecades4.VRholdsgreatpotentialinmanufacturingapplicationstosolveproblemsbeforebeingemployedinpracticalmanufacturingtherebypreventingcostlymistakes.TheadvancesinVRtechnologyinthelastdecadehaveprovidedtheimpetusforapplyingVRtodifferentengineeringapplicationssuchasproductdesign5,assembly6,machiningsimulation7,andtraining8.ThegoalofthispaperistodevelopaVR-basedmodularfixturesdesignsystem(VMJFDS).Thisisthefirststeptodevelopanintegratedandimmersiveenvironmentformodularfixturedesign.Thisapplicationhastheadvantagesofmakingthefixturedesigninanaturalandinstructivemanner,providingbettermatchtotheworkingconditions,reducinglead-time,andgenerallyprovidingasignificantenhancementoffixtureproductivityandeconomy.II.OVERVIEWOFTHEPROPOSEDSYSTEMThesystemarchitectureoftheproposeddesktopVRsystemismodularisedbasedonthefunctionalrequirementsofthesystem,whichisshowninFig.1.Atthesystemlevel,threemodulesofproposedsystem,namely,Graphicinterface(GUI),Virtualenvironment(VE)andDatabasemodulesaredesigned.Foreachofthemodules,asetofobjectshasbeenidentifiedtorealizeitsfunctionalrequirements.Thedetailedobjectdesignandimplementationareomittedfromthispaper.Instead,thebriefdescriptionofthesethreemodulesisgivenbelow.1)GraphicInterface(GUI):TheGUIisbasicallyafriendlygraphicinterfacethatisusedtointegratethevirtualenvironmentandmodularfixturedesignactions.2)Virtualenvironment(VE):TheVEprovidestheuserswitha3Ddisplayfornavigatingandmanipulatingthemodelsofmodularfixturesystemanditscomponentsinthevirtualenvironment.AsshowninFig.1,thevirtualenvironmentmodulecomprisestwoparts,namelyassemblydesignenvironmentandmachiningsimulationenvironment.Theuserselectsappropriateelementsandputsdowntheseelementsonthedeskintheassemblydesignarea.Thenheassemblestheselectedelementsonebyonetobuildupthefinalfixturesystemwiththeguidanceofthesystem.1-4244-0259-X/06/$20.00C)2006IEEE2650Authorizedlicenseduselimitedto:NanchangUniversity.DownloadedonDecember20,2009at22:44fromIEEEXplore.Restrictionsapply.Fig.1.OverviewofthedesktopVRbasedmodularfixturedesignsystem.3)Database:Thedatabasedepositallofthemodelsofenvironmentandmodularfixtureelements,aswellasthedomainknowledgeandusefulcases.Thereare5databasesshowninFig.1.Amongthem,knowledge&rulebasegoverningallfixtureplanningprinciplesformsthebrainsofthesystem.III.PROCEDUREOFMODULARFIXTUREDESIGNInthissection,aninstructivemodularfixturedesignprocedurewithinVEispresented.Besidesthe3Ddepththattheusersfeelandthereal-worldlikeoperationprocess,thisprocedurefeaturesintelligenceandintroduction.Duringthedesignprocess,someusefulcasesandsuggestionwillbepresentedtotheuserforreferencebasedonintelligentinferencemethodsuchasCasebasedreasoning(CBR)andRulebasedreasoning(RBR).Furthermore,relativeknowledgeandrulesarepresentedashelppagesthattheusercaneasilybrowsedduringthedesignprocess.OverviewofmodularfixturedesignprocessissummarizedinFig.2.AftertheVEenvironmentisinitialedandtheworkpieceisloaded,thefirststepisfixtureplanning.Inthisstep,theuserfirstdecidesthefixturingscheme,thatisspecifiesthefixturingfacesoftheworkpieceinteractively.Forhelptheusersdecision-making,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.ModularfixturestructureanalysisAfunctionalunit(FU)isacombinationoffixtureelementstoprovideconnectionbetweenthebaseplateandaworkpiece11.Generally,modularfixturestructuremaybedividedintothreefunctionalunitsaccordingtoitsbasicstructurecharacteristics,namelylocatingunit,clampingunit,andsupportingunit.ThenumberoffixtureelementsinaFUmayconsistofoneormoreelements,inwhichonlyoneelementservesasalocator,supportorclamp.Themajortaskofthemodularfixtureassemblyistoselectthesupporting,locating,clampingandaccessoryelementstogeneratethefixtureFUstoconnecttheworkpiecetothebaseplate.Byanalyzingthepracticalapplicationofmodularfixtures,itisfoundthattheassemblyofmodularfixturesbeginsbyselectingthesuitablefixtureelementstoconstructFUs,thensubsequentlymountingtheseFUsonthebaseplate.Therefore,theFUscanberegardedassubassembliesofmodularfixturesystem.Further,thestructureofmodularfixturesystemcanberepresentedasahierarchalstructureasshowninFig.3.2651Authorizedlicenseduselimitedto:NanchangUniversity.DownloadedonDecember20,2009at22:44fromIEEEXplore.Restrictionsapply.UsefTa6*T-siikg&Suggelr,lFixtuieElemenetsrUetrievali0ToolsrKetrieval4Fig.2ModularfixturedesignprocedureinproposedsystemB.HierarchicallystructureddatamodelformodularfixturerepresentationinVEItiscommonthatthecorrespondingvirtualenvironmentmaycontainmillionsofgeometricpolygonprimitives.Overthepastyears,anumberofmodelsub-divisionschemes,suchasBSP-tree10andOctrees,havebeenproposedtoorganizelargepolygonalmodels.However,formodularBa1I_1HsreplalteBansepla1nteElements*LocatngElementsL,catingUnitsAccessoryEllementsClamnpingElemnents!ClampingUnitsSupportingElemntsSupportingUfnitsAccessoryElementsFig.3Hierarchicalstructureofmodularfixturesystemdesignapplications,thesceneisalsodynamicallychanging,duetointeractions.Forexample,indesignprocess,thepartobjectmaychangeitsspatialposition,orientationandassemblyrelations.Thisindicatesthatastaticrepresentation,suchasBSP-tree,isnotsufficient.Furthermore,theabovemodelscanonlyrepresentthetopologystructureoffixturesysteminthecomponentlevel.However,totheassemblyrelationshipamongfixturecomponents,whichreferstothematingrelationshipbetweenassemblyfeaturesthatisnotconcerned.Inthissection,wepresentahierarchicallystructuredandconstraint-baseddatamodelformodularfixturesystemrepresentation,real-timevisualizationandprecise3DmanipulationinVE.AsshowninFig.4,thehigh-levelcomponentbasedmodelisusedforinteractiveoperationsinvolvingassembliesordisassembles.Itprovidesbothtopologicalstructureandlinkrelationsbetweencomponents.Theinformationrepresent-edinthehigh-levelmodelcanbedividedintotwotypes,ponentobjectsandassemblyrelationships.Componentobjectscanbeasubassemblyorapart.Asubassemblyconsistsofindividualpartsandassemblyrelationshipsbetweentheparts.ComponentLevel(PtPartSSubassemblyAssemblyrelationshipFeatureLevelFt3FeatureFeaturematingrelationshipt-tPolygonLevelFZ-ll.PolygonFig.4ThehierarchicalstructuredatamodelinVEThemiddle-levelfeaturebasedmodelisbuiltuponfeaturesandfeatureconstraints.Ingeneral,theassemblyrelationshipoftentreatedasthematingrelationshipsbetweenassemblyfeatures.Thusthefeaturebasedmodelisusedtodescribetheassemblyrelationshipandprovidesnecessaryinformationforspatialrelationshipcalculatingduringassemblyoperation.Inthismodel,onlythefeaturerelationshipsbetweentwodifferentcomponentsareconsidered.Therelationshipbetweenfeaturesofoneelementwillbediscussedinfeaturebasedmodularfixtureelementmodelingbelow.Thelow-levelpolygonbasedmodelcorrespondstotheabovetwolevelmodelsforreal-timevisualizationandinteraction.Itdescribestheentiresurfaceasaninter-connectedtriangularsurfacemesh.Moreabouthowthepolygonsorganizedofasingleelementwillbediscussedisthenextsection.C.ModularfixtureelementsmodelingAsweknow,inVE,thepartisonlyrepresentedasanumberofpolygonprimitives.Thisresultinthetopological2652Authorizedlicenseduselimitedto:NanchangUniversity.DownloadedonDecember20,2009at22:44fromIEEEXplore.Restrictionsapply.relations-hipsandparametricinformationarelostduringthetranslationprocessofmodelsfromCADsystemstoVRsystems.However,thisimportantinformationisnecessaryindesignandassemblyprocess.Inordertofulfilltherequirements,wepresentamodelingschemeforfixtureelementsrepresentationinthissection.Themodularfixtureelementsarepre-manufacturedpartswithstandarddimensions.Afterthefixturingschemedesigned,theleftjobistoselectsuitablestandardelementsandassembletheseelementstoformafixturesysteminafeasibleandeffectivemanner.Therefore,intheproposedsystem,onlytheassemblyfeaturesofthefixtureelementsneedtobeconsidered.Inthispaperanassemblyfeatureisdefinedasapropertyofafixtureelement,whichprovidesrelatedinformationrelevanttomodularfixturedesignandassembly/disassembly.Thefollowingeightfunctionfacesaredefinedasassemblyfeaturesoffixtureelements:supportingfaces,supportedfaces,locatingholes,counterboreholes,screwholes,fixingslots,andscrewbolts.Besidestheinformationaboutthefeatureliketypeanddimension,otherparameters,i.e.therelativepositionandorientationofthefeatureintheelementslocalcoordinatesystemarerecordedwiththegeometricmodelinthefixtureelementdatabase.Whenoneelementassembleswithanother,theinformationaboutthematedfeaturesisretrievedandusedtodecidethespatialrelationshipofthetwoelements.MoreinformationabouttheassemblyfeaturesandtheirmatingrelationshiparediscusseddetailedinRef1.D.ConstraintbasedfixtureassemblyinVE1)AssemblyrelationshipbetweenfixtureelementsMatingrelationshipshavebeenusedtodefineassemblyrelationshipsbetweenpartcomponentsinthefieldofassembly.Accordingtotheassemblyfeaturessummarizedintheabovesection,therearefivetypesofmatingrelationshipsbetweenfixtureelements.Namelyagainst,fit,screwfit,across,andT-slotfit,whichareillustratedinFig.5.Basedonthesematingrelationships,wecanreasonthepossibleassemblyrelationshipofanytwoassembledfixtureelements.2)AssemblyrelationshipreasoningIngeneral,theassemblyrelationshipoftwoassembledpartisrepresentedasthematedassemblyfeaturepairsofthem.Intheabovesection,wedefinedfivebasicmatingrelationshipsbetweenfixtureelements.Therefore,itisenabledtodecidethepossibleassemblyrelationshipsthroughfindingthepossiblematingassemblyfeaturepairs.Thesepossibleassemblyrelationshipsaresavedinassemblyrelationshipsdatabase(ARDB)forfixtureassemblyinnextstage.However,whenthefixtureiscomplicatedandthenumbersofcompositefixtureelementsislarge,thepossibleassemblyrelationshipsaretoomuchtotakemuchtimeforreasoningandtreating.Toavoidthissituation,wefirstdecidethepossibleassembledelementspairs.Thatistoavoidreasoningtheassemblyrelationshipbetweenaclampandthebaseplate,fortheyneverwereassembledtogether.Inthisstage,somerulesareutilizedtofindthepossibleassembledelementspairs.ThealgorithmofassemblyrelationshipsreasoningissimilartowhatdiscussedinRef12.Thusthedetaileddescriptionofthealgorithmisomittedfromthispaper.(a)AIlai.ns.2l.I.FLIiI7Fd)Asicmie1f-isxktElmnFig.5Fivebasicmatingrelationshipsbetweenfixtureelements3)Constraint-basedfixtureassemblyAftercarryingouttheassemblyrelationshipsreasoning,allpossibleassemblyrelationshipsoftheselectedelementsareestablishedandsavedinARDB.Basedontheserelationships