1、Proceedingsofthe2006IEEE/RSJ InternationalConferenceonIntelligentRobotsandSystems October9-15,2006,Beijing,China ANovelModularFixtureDesignandAssemblySystem BasedonVR PengGaoliang,LiuWenjian School ofMechatronicsEngineering HarbinInstituteofTechnology Harbin,150001,China Abstract-Modularfixturesareo

2、neoftheimportantaspects ofmanufacturing.ThispaperpresentsadesktopVRsystemfor modularfixturedesign.Thevirtualenvironmentisdesignedand thedesignprocedureisproposed.Itassiststhedesignertomake thefeasibledesigndecisionseffectivelyandefficiently.A hierarchicaldatamodelisproposedtorepresentthemodular fixt

3、ureassembly.Basedonthisstructure,theusercan manipulatethevirtualmodelspreciselyinVEduringthedesign andassemblyprocesses.Moreover,themachiningsimulationfor manufacturinginteractioncheckingisdiscussedand implemented.Finally,thecasestudyhasdemonstratedthe functionalityoftheproposedsystem.Comparedwithth

4、e immersiveVRsystem,theproposedsystemhasofferedan affordableandportablesolutionformodularfixturesdesign. IndexTerms-Modularfixture,desktopVR,assemblydesign, machiningsimlulation. I.INTRODUCTION Modularfixturesareoneoftheimportantaspectsof manufacturing.Properfixturedesigniscrucialtoproduct qualityin

5、termsofprecision,accuracy,andfinishofthe machinedpart.Modularfixtureisasystemofinterchange- eableandhighlystandardizedcomponentsdesignedto securelyandaccuratelyposition,hold,andsupportthe workpiecethroughoutthemachiningprocess1.Tradition- ally,fixturedesignersrelyonexperienceorusetrial-and- errormet

6、hodstodetermineanappropriatefixturingscheme. Withtheadventofcomputertechnology,computeraided designhasbeenprevalentintheareaofmodularfixture design. Ingeneral,theassociatedfixturedesignactivities,namely setupplanning,fixtureelementdesign,andfixturelayout designareoftendealtwithatthedownstreamendofth

7、e machinetooldevelopmentlife-cycle.Thesepracticesdonot lendthemselveswelltothebridgingofdesignand manufacturingactivities.Forexample,veryfewsystemshave incorporatedthefunctionalityofdetectingmachining interference.Thisleadstoagapbetweenthefixturedesign andmanufacturingoperationswheretheaspectofcutte

8、rpaths isnotconsideredduringthedesignstage2.Asaresult,re- designcannotbeavoidedwhenthecutterisfoundtointerfere withthefixturecomponentsinthemanufactu-ringset-up. Therefore,inordertobringmachiningfixturedesignintothe arenaofflexiblemanufacturing,amoresystematicandnatural designenvironmentisrequired.

9、Asasynthetic,3D,interactiveenvironmenttypically generatedbyacomputer,VRhasbeenrecognizedasavery powerfulhuman-computerinterfacefordecades4.VR holdsgreatpotentialinmanufacturingapplicationstosolve problemsbeforebeingemployedinpracticalmanufacturing therebypreventingcostlymistakes.TheadvancesinVR tech

10、nologyinthelastdecadehaveprovidedtheimpetusfor applyingVRtodifferentengineeringapplicationssuchas productdesign5,assembly6,machiningsimulation7, andtraining8.ThegoalofthispaperistodevelopaVR- basedmodularfixturesdesignsystem(VMJFDS).Thisisthe firststeptodevelopanintegratedandimmersiveenvironment for

11、modularfixturedesign.Thisapplicationhasthe advantagesofmakingthefixturedesigninanaturaland instructivemanner,providingbettermatchtotheworking conditions,reducinglead-time,andgenerallyprovidinga significantenhancementoffixtureproductivityandeconomy. II.OVERVIEWOFTHEPROPOSEDSYSTEM Thesystemarchitectur

12、eoftheproposeddesktopVR systemismodularisedbasedonthefunctionalrequirementsof thesystem,whichisshowninFig.1.Atthesystemlevel,three modulesofproposedsystem,namely,Graphicinterface (GUI),Virtualenvironment(VE)andDatabasemodulesare designed.Foreachofthemodules,asetofobjectshasbeen identifiedtorealizeit

13、sfunctionalrequirements.Thedetailed objectdesignandimplementationareomittedfromthispaper. Instead,thebriefdescriptionofthesethreemodulesisgiven below. 1)GraphicInterface(GUI):TheGUIisbasicallyafriendly graphicinterfacethatisusedtointegratethevirtual environmentandmodularfixturedesignactions. 2)Virtu

14、alenvironment(VE):TheVEprovidestheuserswith a3Ddisplayfornavigatingandmanipulatingthemodelsof modularfixturesystemanditscomponentsinthevirtual environment.AsshowninFig.1,thevirtualenvironment modulecomprisestwoparts,namelyassemblydesign environmentandmachiningsimulationenvironment.Theuser selectsapp

15、ropriateelementsandputsdowntheseelementson thedeskintheassemblydesignarea.Thenheassemblesthe selectedelementsonebyonetobuildupthefinalfixture systemwiththeguidanceofthesystem. 1-4244-0259-X/06/$20.00C)2006IEEE 2650 Authorized licensed use limited to: Nanchang University. Downloaded on December 20, 2

16、009 at 22:44 from IEEE Xplore. Restrictions apply. Fig.1.OverviewofthedesktopVRbasedmodularfixturedesignsystem. 3)Database:Thedatabasedepositallofthemodelsof environmentandmodularfixtureelements,aswellasthe domainknowledgeandusefulcases.Thereare 5databases showninFig.1.Amongthem,knowledge&rulebase g

17、overningallfixtureplanningprinciplesformsthebrainsof thesystem. III.PROCEDUREOFMODULARFIXTUREDESIGN Inthissection,aninstructivemodularfixturedesign procedurewithinVEispresented.Besidesthe3Ddepththat theusersfeelandthereal-worldlikeoperationprocess,this procedurefeaturesintelligenceandintroduction.Du

18、ringthe designprocess,someusefulcasesandsuggestionwillbe presentedtotheuserforreferencebasedonintelligent inferencemethodsuchasCasebasedreasoning(CBR)and Rulebasedreasoning(RBR).Furthermore,relative knowledgeandrulesarepresentedashelppagesthattheuser caneasilybrowsedduringthedesignprocess. Overviewo

19、fmodularfixturedesignprocessis summarizedinFig.2.AftertheVEenvironmentisinitialed andtheworkpieceisloaded,thefirststepisfixtureplanning. Inthisstep,theuserfirstdecidesthefixturingscheme,thatis specifiesthefixturingfacesoftheworkpieceinteractively. Forhelptheusersdecision-making,someusefulcasesaswell

20、 astheirfixturingschemewillbepresentedviatheautomatic CBRretrievalmethod.Oncethefixturingfacesareselected, theusermaybeprompttospecifythefixturingpoints.Inthis task,somesuggestionsandrulesaregiven. Afterthefixturingplanning,thenextstepisfixtureFUs designstage.Inthisstage,theusermaybetoselectsuitable

21、 fixtureelementsandassembletheseindividualpartsintoFUs. Accordingtothespatialinformationofthefixturingpointsin relationtothefixturebaseandtheworkpiece,sometypical FUsandsuggestionsmaybepresentedautomatically.These willbehelpfulfortheuser.AftertheplanningandFUsdesign stage,thenextstageisinteractively

22、assemblingthedesigned fixtureFUstoconnecttheworkpiecetothebaseplate. Whenthefixtureconfigurationiscompleted,theresult willbecheckedandevaluatedwithinthemachining environment.Thetasksexecutedinthisenvironment includingassemblyplanning,machiningsimulation,and fixtureevaluation.Assemblyplanningisusedto

23、gainoptimal assemblysequenceandassemblypathofeachcomponent. Machiningsimulationisresponsibleformanufacturing interactiondetection.Fixtureevaluationwillcheckand evaluatethedesignresult.Inconclusion,thewholedesign processisinanaturemannerforthebenefitofVE.Moreover, thepresentedinformationofsuggestiona

24、ndknowledgecan advisetheuseronhowtomakedecisionsofthebestdesign selection. IV.ASSEMBLY MODELINGOFMODULARFIXTURE A.Modularfixturestructureanalysis Afunctionalunit(FU)isacombinationoffixtureelements toprovideconnectionbetweenthebaseplateandaworkpiece 11.Generally,modularfixturestructuremaybedividedint

25、o threefunctionalunitsaccordingtoitsbasicstructure characteristics,namelylocatingunit,clampingunit,and supportingunit.ThenumberoffixtureelementsinaFUmay consistofoneormoreelements,inwhichonlyoneelement servesasalocator,supportorclamp.Themajortaskofthe modularfixtureassemblyistoselectthesupporting,lo

26、cating, clampingandaccessoryelementstogeneratethefixtureFUs toconnecttheworkpiecetothebaseplate. Byanalyzingthepracticalapplicationofmodularfixtures, itisfoundthattheassemblyofmodularfixturesbeginsby selectingthesuitablefixtureelementstoconstructFUs,then subsequentlymountingtheseFUsonthebaseplate.Th

27、erefore, 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 ap

28、ply. Usef Ta6 *T- siikg& Sugge lr,l Fixtui e Elemenets rUetrieval i0 Tools rKetrieval 4 Fig.2Modularfixturedesignprocedureinproposedsystem B.Hierarchicallystructureddatamodelformodularfixture representationinVE Itiscommonthatthecorrespondingvirtualenvironment maycontainmillionsofgeometricpolygonprim

29、itives.Over thepastyears,anumberofmodelsub-divisionschemes,such asBSP-tree10andOctrees,havebeenproposedtoorganize largepolygonalmodels.However,formodular Ba 1I_1Hsreplalte Bansepla1nteElements *LocatngElements L,catingUnits AccessoryEllements ClamnpingElemnents !ClampingUnits SupportingElemnts Suppo

30、rtingUfnits AccessoryElements Fig.3Hierarchicalstructureofmodularfixturesystem designapplications,thesceneisalsodynamicallychanging, duetointeractions.Forexample,indesignprocess,thepart objectmaychangeitsspatialposition,orientationand assemblyrelations.Thisindicatesthatastaticrepresentation, suchasB

31、SP-tree,isnotsufficient.Furthermore,theabove modelscanonlyrepresentthetopologystructureoffixture systeminthecomponentlevel.However,totheassembly relationshipamongfixturecomponents,whichreferstothe matingrelationshipbetweenassemblyfeaturesthatisnot concerned.Inthissection,wepresentahierarchically str

32、ucturedandconstraint-baseddatamodelformodularfixture systemrepresentation,real-timevisualizationandprecise3D manipulationinVE. AsshowninFig.4,thehigh-levelcomponentbasedmodel isusedforinteractiveoperationsinvolvingassembliesor disassembles.Itprovidesbothtopologicalstructureandlink relationsbetweenco

33、mponents.Theinformationrepresent-ed inthehigh-levelmodelcanbedividedintotwotypes,i.e. componentobjectsandassemblyrelationships.Component objectscanbeasubassemblyorapart.Asubassembly consistsofindividualpartsandassemblyrelationships betweentheparts. ComponentLevel (Pt Part SSubassembly Assembly relat

34、ionship FeatureLevel Ft3Feature Featuremating relationship t-t PolygonLevel FZ-ll.Polygon Fig.4ThehierarchicalstructuredatamodelinVE Themiddle-levelfeaturebasedmodelisbuiltuponfeatures andfeatureconstraints.Ingeneral,theassemblyrelationship oftentreatedasthematingrelationshipsbetweenassembly feature

35、s.Thusthefeaturebasedmodelisusedtodescribethe assemblyrelationshipandprovidesnecessaryinformationfor spatialrelationshipcalculatingduringassemblyoperation.In thismodel,onlythefeaturerelationshipsbetweentwo differentcomponentsareconsidered.Therelationship betweenfeaturesofoneelementwillbediscussedinf

36、eature basedmodularfixtureelementmodelingbelow. Thelow-levelpolygonbasedmodelcorrespondstothe abovetwolevelmodelsforreal-timevisualizationand interaction.Itdescribestheentiresurfaceasaninter- connectedtriangularsurfacemesh.Moreabouthowthe polygonsorganizedofasingleelementwillbediscussedis thenextsec

37、tion. 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-hipsandparametr

38、icinformationarelostduringthe translationprocessofmodelsfromCADsystemstoVR systems.However,thisimportantinformationisnecessaryin designandassemblyprocess.Inordertofulfillthe requirements,wepresentamodelingschemeforfixture elementsrepresentationinthissection. Themodularfixtureelementsarepre-manufactu

39、redparts withstandarddimensions.Afterthefixturingschemedesigned, theleftjobistoselectsuitablestandardelementsand assembletheseelementstoformafixturesysteminafeasible andeffectivemanner.Therefore,intheproposedsystem,only theassemblyfeaturesofthefixtureelementsneedtobe considered. Inthispaperanassembl

40、yfeatureisdefinedasapropertyof afixtureelement,whichprovidesrelatedinformationrelevant tomodularfixturedesignandassembly/disassembly.The followingeightfunctionfacesaredefinedasassembly featuresoffixtureelements:supportingfaces,supportedfaces, locatingholes,counterboreholes,screwholes,fixingslots, an

41、dscrewbolts.Besidestheinformationaboutthefeaturelike typeanddimension,otherparameters,i.e.therelativeposition andorientationofthefeatureintheelementslocalcoordinate systemarerecordedwiththegeometricmodelinthefixture elementdatabase.Whenoneelementassembleswithanother, theinformationaboutthematedfeatu

42、resisretrievedandused todecidethespatialrelationshipofthetwoelements.More informationabouttheassemblyfeaturesandtheirmating relationshiparediscusseddetailedinRef1. D.ConstraintbasedfixtureassemblyinVE 1)Assemblyrelationshipbetweenfixtureelements Matingrelationshipshavebeenusedtodefineassembly relati

43、onshipsbetweenpartcomponentsinthefieldof assembly.Accordingtotheassemblyfeaturessummarizedin theabovesection,therearefivetypesofmatingrelationships betweenfixtureelements.Namelyagainst,fit,screwfit, across,andT-slotfit,whichareillustratedinFig.5.Basedon thesematingrelationships,wecanreasonthepossibl

44、e assemblyrelationshipofanytwoassembledfixtureelements. 2)Assemblyrelationshipreasoning Ingeneral,theassemblyrelationshipoftwoassembledpart isrepresentedasthematedassemblyfeaturepairsofthem.In theabovesection,wedefinedfivebasicmatingrelationships betweenfixtureelements.Therefore,itisenabledtodecide

45、thepossibleassemblyrelationshipsthroughfindingthe possiblematingassemblyfeaturepairs.Thesepossible assemblyrelationshipsaresavedinassemblyrelationships database(ARDB)forfixtureassemblyinnextstage. However,whenthefixtureiscomplicatedandthe numbersofcompositefixtureelementsislarge,thepossible assembly

46、relationshipsaretoomuchtotakemuchtimefor reasoningandtreating.Toavoidthissituation,wefirstdecide thepossibleassembledelementspairs.Thatistoavoid reasoningtheassemblyrelationshipbetweenaclampandthe baseplate,fortheyneverwereassembledtogether.Inthis stage,somerulesareutilizedtofindthepossibleassembled

47、 elementspairs. Thealgorithmofassemblyrelationshipsreasoningis similartowhatdiscussedinRef12.Thusthedetailed descriptionofthealgorithmisomittedfromthispaper. (a)AIlai.ns .2 l.I.F LIiI7 Fd)Asicmie1f-isxktElmn Fig.5Fivebasicmatingrelationshipsbetweenfixtureelements 3)Constraint-basedfixtureassembly Af

48、tercarryingouttheassemblyrelationshipsreasoning,all possibleassemblyrelationshipsoftheselectedelementsare establishedandsavedinARDB.Basedontheserelationships, thetraineecanassembletheseindividualpartstoafixture system.Thissectionisaboutthediscussionofinteractive assemblyoperationinVE.Theprocessofasi

49、ngleassembly operationispresentedinFig.5andillustratedbytwosimple partsassemblyasshowninFig.6. Ingeneral,theassemblyoperationprocessisdividedinto threesteps,namelyassemblyrelationshiprecognizing, constraintanalysisandapplying,constraint-basedmotion. Firstly,thetraineeselectsanelementandmovesittothe

50、assembledcomponent.Onceaninferencebetweenthe assemblingandassembledcomponentisdetectedduringthe moving,theinferredfeaturesischecked.Ifthetwofeaturesis oneoftheassemblyrelationshipsinARDB,theywillbe highlightedandwillawaittheusersconfirmation.Onceitis confirmed,therecognizedassemblyrelationshipwillbe

51、 appliedbyconstraintanalyzingandsolving,thatisadjustthe translationandorientationoftheassemblingelementtosatisfy thepositionrelationshipofthesetwocomponents,aswellas applythenewconstrainttotheassemblingelement.Whenthe newconstraintisapplied,themotionoftheassembling elementwillbemappedintoaconstraint

52、space.Thisisdone bytransferring3Dmotiondatafromtheinputdevicesintothe allowablemotionsoftheobject.Theconstraint-basedmotion notonlyensuresthattheprecisepositionsofacomponentcan beobtained,butalsoguaranteethattheexistingconstraints willnotbeviolatedduringthefutureoperations.The assemblingelementwillr

53、eachtothefinalpositionthrough 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 checkingel

54、atioohship? Fig.6Processofassemblyconstraintestablishment No V.MACHININGSIMULATION A.Manufacturinginteractions Duringthemachiningprocess,therearemanytypesof manufacturinginteractionsassociatedwiththefixturemay occur.Theseinteractionscanbedividedintotwobroad categoriesillustratedbelow,namelystaticint

55、eractionsand dynamicinteractions. 1)Staticinteractionsrefertotheinterferencebetween fixturecomponents,theinterferencebetweenfixture componentsandmachinetool,andtheinterferencebetween fixturecomponentsandmachingfeatureofworkpieceduring theworkpiecesetup. 2)Dynamicinteractionsrefertothetool-fixtureint

56、eractions, whichoccurwithinasingleoperationwhenthetoolandthe fixtureusedinthatoperationmaycollideduringcutting. Generally,theaspectsofmachiningprocessandcutter pathsarenotconsideredduringthefixturedesignstage.Asa result,theseinteractionsmayoftenoccurduringthepractical manufacturing.Thusthehumanmachi

57、nistshavetospend muchoftheirtimeidentifyingtheseinteractionsandresolving them.Itisoftenresultsinmodificationorre-designoffixture system.Thatistediousandtimecostly. B.Interferencedetection Althoughthecurrentlycommercialsoftware,like VERICUT,cansimulatesNCmachiningtodetecttoolpath errorsandinefficient

58、motionpriortomachininganactual workpiece.Itisavailabletoeliminateerrorsthatcouldruin thepart,damagethefixture,breakthecuttingtool,orcrash themachineduringthepartprogrammingstage.However, thesesoftwareareexpensiveandorientedtoNCprogram- mertherebynotsuitableforfixturedesigners. Duringthefixturedesign

59、stage,itshouldbeensuredthat theassociatedfixtureinteractionscanbeavoided.Inthis system,afterthefixtureconfigurationiscomplete,the machiningsimulationmoduleispresentedtotheuserto identifytheinteractionsandresolvethem. Withinthemachiningsimulationenvironment,the3D digitalmodelofmachinetoolispresented.Thecanassemble thefixturecomponentsontheworkbenchandsetupthe workpiece,justaswhatthemachiningengineersdointhe actualsite.Duringthesetup,thefixturecomponentsandthe workpiecearemovetotheirassemblypositionunder manipulation.Theinterferencecheckingmoduleiscarriedout. Ifinterferenceoccurs,thei