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Proceedingsofthe2006IEEE/RSJ InternationalConferenceonIntelligentRobotsandSystems October9-15,2006,Beijing,China ANovelModularFixtureDesignandAssemblySystem BasedonVR PengGaoliang,LiuWenjian School ofMechatronicsEngineering HarbinInstituteofTechnology Harbin,150001,China Abstract-Modularfixturesareoneoftheimportantaspects ofmanufacturing.ThispaperpresentsadesktopVRsystemfor modularfixturedesign.Thevirtualenvironmentisdesignedand thedesignprocedureisproposed.Itassiststhedesignertomake thefeasibledesigndecisionseffectivelyandefficiently.A hierarchicaldatamodelisproposedtorepresentthemodular fixtureassembly.Basedonthisstructure,theusercan manipulatethevirtualmodelspreciselyinVEduringthedesign andassemblyprocesses.Moreover,themachiningsimulationfor manufacturinginteractioncheckingisdiscussedand implemented.Finally,thecasestudyhasdemonstratedthe functionalityoftheproposedsystem.Comparedwiththe immersiveVRsystem,theproposedsystemhasofferedan affordableandportablesolutionformodularfixturesdesign. IndexTerms-Modularfixture,desktopVR,assemblydesign, machiningsimlulation. I.INTRODUCTION Modularfixturesareoneoftheimportantaspectsof manufacturing.Properfixturedesigniscrucialtoproduct qualityintermsofprecision,accuracy,andfinishofthe machinedpart.Modularfixtureisasystemofinterchange- eableandhighlystandardizedcomponentsdesignedto securelyandaccuratelyposition,hold,andsupportthe workpiecethroughoutthemachiningprocess1.Tradition- ally,fixturedesignersrelyonexperienceorusetrial-and- errormethodstodetermineanappropriatefixturingscheme. Withtheadventofcomputertechnology,computeraided designhasbeenprevalentintheareaofmodularfixture design. Ingeneral,theassociatedfixturedesignactivities,namely setupplanning,fixtureelementdesign,andfixturelayout designareoftendealtwithatthedownstreamendofthe machinetooldevelopmentlife-cycle.Thesepracticesdonot lendthemselveswelltothebridgingofdesignand manufacturingactivities.Forexample,veryfewsystemshave incorporatedthefunctionalityofdetectingmachining interference.Thisleadstoagapbetweenthefixturedesign andmanufacturingoperationswheretheaspectofcutterpaths isnotconsideredduringthedesignstage2.Asaresult,re- designcannotbeavoidedwhenthecutterisfoundtointerfere withthefixturecomponentsinthemanufactu-ringset-up. Therefore,inordertobringmachiningfixturedesignintothe arenaofflexiblemanufacturing,amoresystematicandnatural designenvironmentisrequired. Asasynthetic,3D,interactiveenvironmenttypically generatedbyacomputer,VRhasbeenrecognizedasavery powerfulhuman-computerinterfacefordecades4.VR holdsgreatpotentialinmanufacturingapplicationstosolve problemsbeforebeingemployedinpracticalmanufacturing therebypreventingcostlymistakes.TheadvancesinVR technologyinthelastdecadehaveprovidedtheimpetusfor applyingVRtodifferentengineeringapplicationssuchas productdesign5,assembly6,machiningsimulation7, andtraining8.ThegoalofthispaperistodevelopaVR- basedmodularfixturesdesignsystem(VMJFDS).Thisisthe firststeptodevelopanintegratedandimmersiveenvironment formodularfixturedesign.Thisapplicationhasthe advantagesofmakingthefixturedesigninanaturaland instructivemanner,providingbettermatchtotheworking conditions,reducinglead-time,andgenerallyprovidinga significantenhancementoffixtureproductivityandeconomy. II.OVERVIEWOFTHEPROPOSEDSYSTEM ThesystemarchitectureoftheproposeddesktopVR systemismodularisedbasedonthefunctionalrequirementsof thesystem,whichisshowninFig.1.Atthesystemlevel,three modulesofproposedsystem,namely,Graphicinterface (GUI),Virtualenvironment(VE)andDatabasemodulesare designed.Foreachofthemodules,asetofobjectshasbeen identifiedtorealizeitsfunctionalrequirements.Thedetailed objectdesignandimplementationareomittedfromthispaper. Instead,thebriefdescriptionofthesethreemodulesisgiven below. 1)GraphicInterface(GUI):TheGUIisbasicallyafriendly graphicinterfacethatisusedtointegratethevirtual environmentandmodularfixturedesignactions. 2)Virtualenvironment(VE):TheVEprovidestheuserswith a3Ddisplayfornavigatingandmanipulatingthemodelsof modularfixturesystemanditscomponentsinthevirtual environment.AsshowninFig.1,thevirtualenvironment modulecomprisestwoparts,namelyassemblydesign environmentandmachiningsimulationenvironment.Theuser selectsappropriateelementsandputsdowntheseelementson thedeskintheassemblydesignarea.Thenheassemblesthe selectedelementsonebyonetobuildupthefinalfixture systemwiththeguidanceofthesystem. 1-4244-0259-X/06/$20.00C)2006IEEE 2650 Authorized licensed use limited to: Nanchang University. Downloaded on December 20, 2009 at 22:44 from IEEE Xplore. Restrictions apply. Fig.1.OverviewofthedesktopVRbasedmodularfixturedesignsystem. 3)Database:Thedatabasedepositallofthemodelsof environmentandmodularfixtureelements,aswellasthe domainknowledgeandusefulcases.Thereare 5databases showninFig.1.Amongthem,knowledge&rulebase governingallfixtureplanningprinciplesformsthebrainsof thesystem. III.PROCEDUREOFMODULARFIXTUREDESIGN Inthissection,aninstructivemodularfixturedesign procedurewithinVEispresented.Besidesthe3Ddepththat theusersfeelandthereal-worldlikeoperationprocess,this procedurefeaturesintelligenceandintroduction.Duringthe designprocess,someusefulcasesandsuggestionwillbe presentedtotheuserforreferencebasedonintelligent inferencemethodsuchasCasebasedreasoning(CBR)and Rulebasedreasoning(RBR).Furthermore,relative knowledgeandrulesarepresentedashelppagesthattheuser caneasilybrowsedduringthedesignprocess. Overviewofmodularfixturedesignprocessis summarizedinFig.2.AftertheVEenvironmentisinitialed andtheworkpieceisloaded,thefirststepisfixtureplanning. Inthisstep,theuserfirstdecidesthefixturingscheme,thatis specifiesthefixturingfacesoftheworkpieceinteractively. Forhelptheusersdecision-making,someusefulcasesaswell astheirfixturingschemewillbepresentedviatheautomatic CBRretrievalmethod.Oncethefixturingfacesareselected, theusermaybeprompttospecifythefixturingpoints.Inthis task,somesuggestionsandrulesaregiven. Afterthefixturingplanning,thenextstepisfixtureFUs designstage.Inthisstage,theusermaybetoselectsuitable fixtureelementsandassembletheseindividualpartsintoFUs. Accordingtothespatialinformationofthefixturingpointsin relationtothefixturebaseandtheworkpiece,sometypical FUsandsuggestionsmaybepresentedautomatically.These willbehelpfulfortheuser.AftertheplanningandFUsdesign stage,thenextstageisinteractivelyassemblingthedesigned fixtureFUstoconnecttheworkpiecetothebaseplate. Whenthefixtureconfigurationiscompleted,theresult willbecheckedandevaluatedwithinthemachining environment.Thetasksexecutedinthisenvironment includingassemblyplanning,machiningsimulation,and fixtureevaluation.Assemblyplanningisusedtogainoptimal assemblysequenceandassemblypathofeachcomponent. Machiningsimulationisresponsibleformanufacturing interactiondetection.Fixtureevaluationwillcheckand evaluatethedesignresult.Inconclusion,thewholedesign processisinanaturemannerforthebenefitofVE.Moreover, thepresentedinformationofsuggestionandknowledgecan advisetheuseronhowtomakedecisionsofthebestdesign selection. IV.ASSEMBLY MODELINGOFMODULARFIXTURE A.Modularfixturestructureanalysis Afunctionalunit(FU)isacombinationoffixtureelements toprovideconnectionbetweenthebaseplateandaworkpiece 11.Generally,modularfixturestructuremaybedividedinto threefunctionalunitsaccordingtoitsbasicstructure characteristics,namelylocatingunit,clampingunit,and supportingunit.ThenumberoffixtureelementsinaFUmay consistofoneormoreelements,inwhichonlyoneelement servesasalocator,supportorclamp.Themajortaskofthe modularfixtureassemblyistoselectthesupporting,locating, clampingandaccessoryelementstogeneratethefixtureFUs toconnecttheworkpiecetothebaseplate. Byanalyzingthepracticalapplicationofmodularfixtures, itisfoundthattheassemblyofmodularfixturesbeginsby selectingthesuitablefixtureelementstoconstructFUs,then subsequentlymountingtheseFUsonthebaseplate.Therefore, theFUscanberegardedassubassembliesofmodularfixture system.Further,thestructureofmodularfixturesystemcanbe representedasahierarchalstructureasshowninFig.3. 2651 Authorized licensed use limited to: Nanchang University. Downloaded on December 20, 2009 at 22:44 from IEEE Xplore. Restrictions apply. Usef Ta6 *T- siikg& Sugge lr,l Fixtui e Elemenets rUetrieval i0 Tools rKetrieval 4 Fig.2Modularfixturedesignprocedureinproposedsystem B.Hierarchicallystructureddatamodelformodularfixture representationinVE Itiscommonthatthecorrespondingvirtualenvironment maycontainmillionsofgeometricpolygonprimitives.Over thepastyears,anumberofmodelsub-divisionschemes,such asBSP-tree10andOctrees,havebeenproposedtoorganize largepolygonalmodels.However,formodular Ba 1I_1Hsreplalte Bansepla1nteElements *LocatngElements L,catingUnits AccessoryEllements ClamnpingElemnents !ClampingUnits SupportingElemnts SupportingUfnits AccessoryElements Fig.3Hierarchicalstructureofmodularfixturesystem designapplications,thesceneisalsodynamicallychanging, duetointeractions.Forexample,indesignprocess,thepart objectmaychangeitsspatialposition,orientationand assemblyrelations.Thisindicatesthatastaticrepresentation, suchasBSP-tree,isnotsufficient.Furthermore,theabove modelscanonlyrepresentthetopologystructureoffixture systeminthecomponentlevel.However,totheassembly relationshipamongfixturecomponents,whichreferstothe matingrelationshipbetweenassemblyfeaturesthatisnot concerned.Inthissection,wepresentahierarchically structuredandconstraint-baseddatamodelformodularfixture systemrepresentation,real-timevisualizationandprecise3D manipulationinVE. AsshowninFig.4,thehigh-levelcomponentbasedmodel isusedforinteractiveoperationsinvolvingassembliesor disassembles.Itprovidesbothtopologicalstructureandlink relationsbetweencomponents.Theinformationrepresent-ed inthehigh-levelmodelcanbedividedintotwotypes,i.e. componentobjectsandassemblyrelationships.Component objectscanbeasubassemblyorapart.Asubassembly consistsofindividualpartsandassemblyrelationships betweentheparts. ComponentLevel (Pt Part SSubassembly Assembly relationship FeatureLevel Ft3Feature Featuremating relationship t-t PolygonLevel FZ-ll.Polygon Fig.4ThehierarchicalstructuredatamodelinVE Themiddle-levelfeaturebasedmodelisbuiltuponfeatures andfeatureconstraints.Ingeneral,theassemblyrelationship oftentreatedasthematingrelationshipsbetweenassembly features.Thusthefeaturebasedmodelisusedtodescribethe assemblyrelationshipandprovidesnecessaryinformationfor spatialrelationshipcalculatingduringassemblyoperation.In thismodel,onlythefeaturerelationshipsbetweentwo differentcomponentsareconsidered.Therelationship betweenfeaturesofoneelementwillbediscussedinfeature basedmodularfixtureelementmodelingbelow. Thelow-levelpolygonbasedmodelcorrespondstothe abovetwolevelmodelsforreal-timevisualizationand interaction.Itdescribestheentiresurfaceasaninter- connectedtriangularsurfacemesh.Moreabouthowthe polygonsorganizedofasingleelementwillbediscussedis thenextsection. C.Modularfixtureelementsmodeling Asweknow,inVE,thepartisonlyrepresentedasa numberofpolygonprimitives.Thisresultinthetopological 2652 Authorized licensed use limited to: Nanchang University. Downloaded on December 20, 2009 at 22:44 from IEEE Xplore. Restrictions apply. relations-hipsandparametricinformationarelostduringthe translationprocessofmodelsfromCADsystemstoVR systems.However,thisimportantinformationisnecessaryin designandassemblyprocess.Inordertofulfillthe requirements,wepresentamodelingschemeforfixture elementsrepresentationinthissection. Themodularfixtureelementsarepre-manufacturedparts withstandarddimensions.Afterthefixturingschemedesigned, theleftjobistoselectsuitablestandardelementsand assembletheseelementstoformafixturesysteminafeasible andeffectivemanner.Therefore,intheproposedsystem,only theassemblyfeaturesofthefixtureelementsneedtobe considered. Inthispaperanassemblyfeatureisdefinedasapropertyof afixtureelement,whichprovidesrelatedinformationrelevant tomodularfixturedesignandassembly/disassembly.The followingeightfunctionfacesaredefinedasassembly featuresoffixtureelements:supportingfaces,supportedfaces, locatingholes,counterboreholes,screwholes,fixingslots, andscrewbolts.Besidestheinformationaboutthefeaturelike typeanddimension,otherparameters,i.e.therelativeposition andorientationofthefeatureintheelementslocalcoordinate systemarerecordedwiththegeometricmodelinthefixture elementdatabase.Whenoneelementassembleswithanother, theinformationaboutthematedfeaturesisretrievedandused todecidethespatialrelationshipofthetwoelements.More informationabouttheassemblyfeaturesandtheirmating relationshiparediscusseddetailedinRef1. D.ConstraintbasedfixtureassemblyinVE 1)Assemblyrelationshipbetweenfixtureelements Matingrelationshipshavebeenusedtodefineassembly relationshipsbetweenpartcomponentsinthefieldof assembly.Accordingtotheassemblyfeaturessummarizedin theabovesection,therearefivetypesofmatingrelationships betweenfixtureelements.Namelyagainst,fit,screwfit, across,andT-slotfit,whichareillustratedinFig.5.Basedon thesematingrelationships,wecanreasonthepossible assemblyrelationshipofanytwoassembledfixtureelements. 2)Assemblyrelationshipreasoning Ingeneral,theassemblyrelationshipoftwoassembledpart isrepresentedasthematedassemblyfeaturepairsofthem.In theabovesection,wedefinedfivebasicmatingrelationships betweenfixtureelements.Therefore,itisenabledtodecide thepossibleassemblyrelationshipsthroughfindingthe possiblematingassemblyfeaturepairs.Thesepossible assemblyrelationshipsaresavedinassemblyrelationships database(ARDB)forfixtureassemblyinnextstage. However,whenthefixtureiscomplicatedandthe numbersofcompositefixtureelementsislarge,thepossible assemblyrelationshipsaretoomuchtotakemuchtimefor reasoningandtreating.Toavoidthissituation,wefirstdecide thepossibleassembledelementspairs.Thatistoavoid reasoningtheassemblyrelationshipbetweenaclampandthe baseplate,fortheyneverwereassembledtogether.Inthis stage,somerulesareutilizedtofindthepossibleassembled elementspairs. Thealgorithmofassemblyrelationshipsreasoningis similartowhatdiscussedinRef12.Thusthedetailed descriptionofthealgorithmisomittedfromthispaper. (a)AIlai.ns .2 l.I.F LIiI7 Fd)Asicmie1f-isxktElmn Fig.5Fivebasicmatingrelationshipsbetweenfixtureelements 3)Constraint-basedfixtureassembly Aftercarryingouttheassemblyrelationshipsreasoning,all possibleassemblyrelationshipsoftheselectedelementsare establishedandsavedinARDB.Basedontheserelationships, thetraineecanassembletheseindividualpartstoafixture system.Thissectionisaboutthediscussionofinteractive assemblyoperationinVE.Theprocessofasingleassembly operationispresentedinFig.5andillustratedbytwosimple partsassemblyasshowninFig.6. Ingeneral,theassemblyoperationprocessisdividedinto threesteps,namelyassemblyrelationshiprecognizing, constraintanalysisandapplying,constraint-basedmotion. Firstly,thetraineeselectsanelementandmovesittothe assembledcomponent.Onceaninferencebetweenthe assemblingandassembledcomponentisdetectedduringthe moving,theinferredfeaturesischecked.Ifthetwofeaturesis oneoftheassemblyrelationshipsinARDB,theywillbe highlightedandwillawaittheusersconfirmation.Onceitis confirmed,therecognizedassemblyrelationshipwillbe appliedbyconstraintanalyzingandsolving,thatisadjustthe translationandorientationoftheassemblingelementtosatisfy thepositionrelationshipofthesetwocomponents,aswellas applythenewconstrainttotheassemblingelement.Whenthe newconstraintisapplied,themotionoftheassembling elementwillbemappedintoaconstraintspace.Thisisdone bytransferring3Dmotiondatafromtheinputdevicesintothe allowablemotionsoftheobject.Theconstraint-basedmotion notonlyensuresthattheprecisepositionsofacomponentcan beobtained,butalsoguaranteethattheexistingconstraints willnotbeviolatedduringthefutureoperations.The assemblingelementwillreachtothefinalpositionthrough successionassemblyrelationshiprecognizingandconstraint applying. 2653 I i 1-11 4- (b)F.t Authorized licensed use limited to: Nanchang University. Downloaded on December 20, 2009 at 22:44 from IEEE Xplore. Restrictions apply. NO AssemblyrelationshipIispossible checkingelatioohship? Fig.6Processofassemblyconstraintestablishment No V.MACHININGSIMULATION A.Manufacturinginteractions Duringthemachiningprocess,therearemanytypesof manufacturinginteractionsassociatedwiththefixturemay occur.Theseinteractionscanbedividedintotwobroad categoriesillustratedbelow,namelystaticinteractionsand dynamicinteractions. 1)Staticinteractionsrefertotheinterferencebetween fixturecomponents,theinterferencebetweenfixture componentsandmachinetool,andtheinterferencebetween fixturecomponentsandmachingfeatureofworkpieceduring theworkpiecesetup. 2)Dynamicinteractionsrefertothetool-fixtureinteractions, whichoccurwithinasingleoperationwhenthetoolandthe fixtureusedinthatoperationmaycollideduringcutting. Generally,theaspectsofmachiningprocessandcutter pathsarenotconsideredduringthefixturedesignstage.Asa result,theseinteractionsmayoftenoccurduringthepractical manufacturing.Thusthehumanmachinistshavetospend muchoftheirtimeidentifyingtheseinteractionsandresolving them.Itisoftenresultsinmodificationorre-designoffixture system.Thatistediousandtimecostly. B.Interferencedetection Althoughthecurrentlycommercialsoftware,like VERICUT,cansimulatesNCmachiningtodetecttoolpath errorsandinefficientmotionpriortomachininganactual workpiece.Itisavailabletoeliminateerrorsthatcouldruin thepart,damagethefixture,breakthecuttingtool,orcrash themachineduringthepartprogrammingstage.However, thesesoftwareareexpensiveandorientedtoNCprogram- mertherebynotsuitableforfixturedesigners. Duringthefixturedesignstage,itshouldbeensuredthat theassociatedfixtureinteractionscanbeavoided.Inthis system,afterthefixtureconfigurationiscomplete,the machiningsimulationmoduleispresentedtotheuserto identifytheinteractionsandresolvethem. Withinthemachiningsimulationenvironment,the3D digitalmodelofmachinetoolispresented.Thecanassemble thefixturecomponentsontheworkbenchandsetupthe workpiece,justaswhatthemachiningengineersdointhe actualsite.Duringthesetup,thefixturecomponentsandthe workpiecearemovetotheirassemblypositionunder manipulation.Theinterferencecheckingmoduleisc