新颖的夹具设计和基于虚拟现实的装配系统@中英文翻译@外文翻译

Proceedings ofthe2006 IEEE/RSJInternational Conference on Intelligent Robots and SystemsOctober9- 15, 2006, Beijing, ChinaANovelModularFixtureDesignandAssemblySystemBasedonVRPengGaoliang, LiuWenjianSchoolofMechatronicsEngineeringHarbinInstituteofTechnologyHarbin, 150001, Chinapgl7782a Abstract - Modular fixtures are one oftheimportant aspectsofmanufacturing. This paper presents a desktop VR system formodular fixture design. The virtual environmentis designed andthe design procedure is proposed. It assists the designer to makethe feasible design decisions effectively and efficiently. Ahierarchical data model is proposed to represent the modularfixture assembly. Based on this structure, the user canmanipulate the virtual models precisely in VE during the designand assembly processes. Moreover, the machining simulation formanufacturing interaction checking is discussed andimplemented. Finally, the case study has demonstrated thefunctionality of the proposed system. Compared with theimmersive VR system, the proposed system has offered anaffordable andportable solutionformodularfixtures design.Index Terms - Modularfixture, desktop VR, assembly design,machiningsimlulation.I. INTRODUCTIONModular fixtures are one of the important aspects ofmanufacturing. Proper fixture design is crucial to productquality in terms of precision, accuracy, and finish of themachined part. Modular fixture is a system of interchange-eable and highly standardized components designed tosecurely and accurately position, hold, and support theworkpiece throughout the machining process 1. Tradition-ally, fixture designers rely on experience or use trial-and-error methods to determine an appropriate fixturing scheme.With the advent of computer technology, computer aideddesign has been prevalent in the area of modular fixturedesign.In general, the associated fixture design activities, namelysetup planning, fixture element design, and fixture layoutdesign are often dealt with at the downstream end of themachine tool development life-cycle. These practices do notlend themselves well to the bridging of design andmanufacturing activities. Forexample, very few systems haveincorporated the functionality of detecting machininginterference. This leads to a gap between the fixture designandmanufacturing operationswheretheaspectofcutterpathsis not considered during the design stage 2. As a result, re-designcannotbeavoidedwhenthecutterisfoundtointerferewith the fixture components in the manufactu- ring set-up.Therefore, in orderto bring machining fixture design into thearenaofflexiblemanufacturing, amoresystematicandnaturaldesignenvironmentisrequired.As a synthetic, 3D, interactive environment typicallygenerated by a computer, VR has been recognized as a verypowerful human-computer interface for decades 4. VRholds great potential in manufacturing applications to solveproblems before being employed in practical manufacturingthereby preventing costly mistakes. The advances in VRtechnology in the last decade have provided the impetus forapplying VR to different engineering applications such asproduct design 5, assembly 6, machining simulation 7,andtraining 8. The goal ofthis paper is to develop a VR-basedmodular fixtures design system (VMJFDS). This is thefirststepto develop anintegratedandimmersiveenvironmentfor modular fixture design. This application has theadvantages of making the fixture design in a natural andinstructive manner, providing better match to the workingconditions, reducing lead-time, and generally providing asignificantenhancementoffixtureproductivityandeconomy.II. OVERVIEWOFTHEPROPOSEDSYSTEMThe system architecture of the proposed desktop VRsystemismodularisedbasedonthefunctionalrequirements ofthesystem,whichisshowninFig.1. Atthesystemlevel,threemodules of proposed system, namely, Graphic interface(GUI), Virtual environment (VE) and Database modules aredesigned. For each ofthe modules, a set ofobjects has beenidentified to realize its functional requirements. The detailedobjectdesignandimplementation are omittedfromthispaper.Instead, the briefdescription ofthese three modules is givenbelow.1) Graphic Interface (GUI): The GUI is basically a friendlygraphic interface that is used to integrate the virtualenvironmentandmodularfixturedesignactions.2) Virtual environment (VE): TheVEprovidestheusers witha 3D display for navigating and manipulating the models ofmodular fixture system and its components in the virtualenvironment. As shown in Fig. 1, the virtual environmentmodule comprises two parts, namely assembly designenvironment andmachiningsimulationenvironment. Theuserselects appropriate elements andputs downthese elements onthe desk in the assembly design area. Then he assembles theselected elements one by one to build up the final fixturesystemwiththeguidanceofthesystem.1-4244-0259-X/06/$20.00 C)2006IEEE 2650Authorized licensed use limited to: Nanchang University. Downloaded on December 20, 2009 at 22:44 from IEEE Xplore. Restrictions apply. Fig.1.OverviewofthedesktopVRbasedmodularfixturedesignsystem.3) Database: The database deposit all of the models ofenvironment and modular fixture elements, as well as thedomain knowledge and useful cases. There are 5 databasesshown in Fig.1. Among them, knowledge & rule basegoverning all fixture planning principles forms the brains ofthesystem.III. PROCEDUREOFMODULARFIXTUREDESIGNIn this section, an instructive modular fixture designprocedure within VE is presented. Besides the 3D depth thatthe users feel and the real-world like operation process, thisprocedure features intelligence and introduction. During thedesign process, some useful cases and suggestion will bepresented to the user for reference based on intelligentinference method such as Case based reasoning (CBR) andRule based reasoning (RBR). Further more, relativeknowledge andrules arepresented ashelppages thattheusercaneasilybrowsedduringthedesignprocess.Overview of modular fixture design process issummarized in Fig. 2. After the VE environment is initialedandthe workpiece is loaded, the first step is fixtureplanning.Inthis step, theuserfirstdecides thefixturing scheme, thatisspecifies the fixturing faces of the workpiece interactively.Forhelptheusersdecision-making, someusefulcasesaswellas their fixturing scheme will be presented via the automaticCBR retrieval method. Once the fixturing faces are selected,theusermaybepromptto specifythefixturingpoints. Inthistask, somesuggestions andrulesaregiven.After the fixturing planning, the next step is fixture FUsdesign stage. In this stage, the user may be to select suitablefixture elements andassembletheseindividualparts into FUs.According to the spatial information ofthe fixturingpoints inrelation to the fixture base and the workpiece, some typicalFUs and suggestions may be presented automatically. Thesewillbehelpfulfortheuser. AftertheplanningandFUs designstage, the next stage is interactively assembling the designedfixtureFUstoconnecttheworkpiecetothebaseplate.When the fixture configuration is completed, the resultwill be checked and evaluated within the machiningenvironment. The tasks executed in this environmentincluding assembly planning, machining simulation, andfixture evaluation. Assemblyplanning isusedto gain optimalassembly sequence and assembly path of each component.Machining simulation is responsible for manufacturinginteraction detection. Fixture evaluation will check andevaluate the design result. In conclusion, the whole designprocess isinanaturemannerforthebenefitofVE. Moreover,the presented information of suggestion and knowledge canadvise the user on how to make decisions ofthe best designselection.IV. ASSEMBLYMODELINGOFMODULARFIXTUREA. ModularfixturestructureanalysisA functionalunit(FU) is acombination offixture elementsto provide connectionbetweenthebaseplate and aworkpiece11. Generally, modularfixture structuremaybe dividedintothree functional units according to its basic structurecharacteristics, namely locating unit, clamping unit, andsupporting unit. The number offixture elements in aFU mayconsist ofone or more elements, in which only one elementserves as a locator, support or clamp. The major task ofthemodularfixture assembly is to selectthe supporting, locating,clamping and accessory elements to generate the fixture FUstoconnecttheworkpiecetothebaseplate.By analyzing the practical application ofmodular fixtures,it is found that the assembly ofmodular fixtures begins byselecting the suitable fixture elements to construct FUs, thensubsequentlymountingtheseFUs onthebaseplate. Therefore,the FUs can be regarded as subassemblies ofmodular fixturesystem.Further,thestructureofmodularfixturesystemcanberepresentedasahierarchalstructureasshowninFig.3.2651Authorized licensed use limited to: Nanchang University. Downloaded on December 20, 2009 at 22:44 from IEEE Xplore. Restrictions apply. UsefTa6*T- siikg&Suggelr,lFixtuieElemenetsrUetrieval i0ToolsrKetrieval 4Fig.2ModularfixturedesignprocedureinproposedsystemB. Hierarchically structured data modelfor modularfixturerepresentation in VEIt is common that the corresponding virtual environmentmay contain millions ofgeometric polygon primitives. Overthepastyears, anumberofmodel sub-division schemes, suchasBSP-tree 10 andOctrees,havebeenproposedto organizelargepolygonalmodels.However, formodularBa 1I_ 1 Hsreplalte Bansepla1nte Elements*Locatng ElementsL,cating UnitsAccessoryEllementsClamnpingElemnents!ClampingUnitsSupportingElemntsSupporting UfnitsAccessory ElementsFig. 3Hierarchical structureofmodularfixture systemdesign applications, the scene is also dynamically changing,due to interactions. For example, in design process, the partobject may change its spatial position, orientation andassembly relations. This indicates that a static representation,such as BSP-tree, is not sufficient. Further more, the abovemodels can only represent the topology structure of fixturesystem in the component level. However, to the assemblyrelationship among fixture components, which refers to themating relationship between assembly features that is notconcerned. In this section, we present a hierarchicallystructuredandconstraint-baseddatamodelformodularfixturesystem representation, real-time visualization and precise 3DmanipulationinVE.As shown in Fig.4, the high-level component based modelis used for interactive operations involving assemblies ordisassembles. It provides both topological structure and linkrelationsbetweencomponents. Theinformationrepresent- edin the high-level model can be divided into two types, ponent objects and assembly relationships. Componentobjects can be a subassembly or a part. A subassemblyconsists of individual parts and assembly relationshipsbetweentheparts.Component Level(Pt PartS SubassemblyAssemblyrelationshipFeature LevelFt3 FeatureFeature matingrelationshipt- -tPolygon LevelFZ-ll. PolygonFig.4ThehierarchicalstructuredatamodelinVEThemiddle-levelfeaturebasedmodelisbuiltuponfeaturesand feature constraints. In general, the assembly relationshipoften treated as the mating relationships between assemblyfeatures. Thus the featurebasedmodel isusedto describetheassembly relationship andprovides necessary information forspatial relationship calculating during assembly operation. Inthis model, only the feature relationships between twodifferent components are considered. The relationshipbetween features ofone element will be discussed in featurebasedmodularfixtureelementmodelingbelow.The low-level polygon based model corresponds to theabove two level models for real-time visualization andinteraction. It describes the entire surface as an inter-connected triangular surface mesh. More about how thepolygons organized of a single element will be discussed isthenextsection.C. ModularfixtureelementsmodelingAs we know, in VE, the part is only represented as anumber ofpolygon primitives. This result in the topological2652Authorized licensed use limited to: Nanchang University. Downloaded on December 20, 2009 at 22:44 from IEEE Xplore. Restrictions apply. relations- hips and parametric information are lost during thetranslation process of models from CAD systems to VRsystems. However, this important information is necessary indesign and assembly process. In order to fulfill therequirements, we present a modeling scheme for fixtureelementsrepresentationinthissection.The modular fixture elements are pre-manufactured partswithstandarddimensions. Afterthefixturingschemedesigned,the left job is to select suitable standard elements andassemblethese elements to formafixture systeminafeasibleandeffectivemanner. Therefore, intheproposed system, onlythe assembly features of the fixture elements need to beconsidered.Inthispaperanassemblyfeature isdefinedas apropertyofafixture element, whichprovidesrelatedinformationrelevantto modular fixture design and assembly/disassembly. Thefollowing eight function faces are defined as assemblyfeaturesoffixtureelements: supportingfaces, supportedfaces,locating holes, counterbore holes, screw holes, fixing slots,andscrewbolts. Besidestheinformation aboutthefeatureliketypeanddimension, otherparameters, i.e. therelativepositionandorientationofthe featureintheelements localcoordinatesystem are recorded with the geometric model in the fixtureelement database. When one element assembles with another,the information aboutthematedfeatures isretrieved andusedto decide the spatial relationship ofthe two elements. Moreinformation about the assembly features and their matingrelationship arediscusseddetailedinRef 1.D. Constraintbasedfixtureassemblyin VE1)AssemblyrelationshipbetweenfixtureelementsMating relationships have been used to define assemblyrelationships between part components in the field ofassembly. According to the assembly features summarized inthe above section, there are fivetypes ofmating relationshipsbetween fixture elements. Namely against, fit, screw fit,across, andT-slotfit,which are illustrated inFig. 5. Based onthese mating relationships, we can reason the possibleassemblyrelationshipofanytwoassembledfixtureelements.2)AssemblyrelationshipreasoningIngeneral, the assemblyrelationship oftwo assembledpartisrepresented as thematedassembly featurepairs ofthem. Inthe above section, we defined five basic mating relationshipsbetween fixture elements. Therefore, it is enabled to decidethe possible assembly relationships through finding thepossible mating assembly feature pairs. These possibleassembly relationships are saved in assembly relationshipsdatabase(ARDB)forfixtureassemblyinnextstage.However, when the fixture is complicated and thenumbers ofcomposite fixture elements is large, the possibleassembly relationships are too much to take much time forreasoning andtreating. To avoidthis situation, wefirstdecidethe possible assembled elements pairs. That is to avoidreasoning the assembly relationship between a clamp andthebaseplate, for they never were assembled together. In thisstage, some rules are utilized to find the possible assembledelementspairs.The algorithm of assembly relationships reasoning issimilar to what discussed in Ref 12. Thus the detaileddescriptionofthealgorithmisomittedfromthispaper.(a) AIlai.ns.2l.I.FLIi I7F d) Asicmie 1f-isxkt ElmnFig. 5Fivebasicmatingrelationshipsbetweenfixtureelements3)Constraint-basedfixtureassemblyAftercarrying outthe assemblyrelationships reasoning, allpossible assembly relationships ofthe selected elements areestablishedandsavedinARDB. Basedontheserelationships,the trainee can assemble these individual parts to a fixturesystem. This section is about the discussion of interactiveassembly operation in VE. The process ofa single assemblyoperation is presented in Fig.5 and illustrated by two simplepartsassemblyasshowninFig.6.In general, the assembly operation process is divided intothree steps, namely assembly relationship recognizing,constraint analysis and applying, constraint-based motion.Firstly, the trainee selects an element and moves it to theassembled component. Once an inference between theassembling and assembled component is detected during themoving,the inferredfeatures is checked. Ifthetwo features isone of the assembly relationships in ARDB, they will behighlighted and will await the users confirmation. Once it isconfirmed, the recognized assembly relationship will beappliedby constraint analyzing and solving, that is adjustthetranslationandorientationoftheassemblingelementtosatisfythe position relationship ofthese two components, as well asapplythenew constrainttotheassemblingelement.Whenthenew constraint is applied, the motion of the assemblingelement will be mapped into a constraint space. This is donebytransferring 3Dmotiondatafromtheinputdevicesintotheallowable motions ofthe object. The constraint-based motionnotonlyensuresthattheprecisepositionsofacomponentcanbe obtained, but also guarantee that the existing constraintswill not be violated during the future operations. Theassembling element will reach to the final position throughsuccession assembly relationship recognizing and constraintapplying.2653Ii1-114-(b) F.tAuthorized licensed use limited to: Nanchang University. Downloaded on December 20, 2009 at 22:44 from IEEE Xplore. Restrictions apply. NOAssembly relationship Iis possiblechecking elatioohship?Fig. 6ProcessofassemblyconstraintestablishmentNoV. MACHINING SIMULATIONA. ManufacturinginteractionsDuring the machining process, there are many types ofmanufacturing interactions associated with the fixture mayoccur. These interactions can be divided into two broadcategories illustrated below, namely static interactions anddynamicinteractions.1) Static interactions refer to the interference betweenfixture components, the interference between fixturecomponents and machine tool, and the interference betweenfixture components andmaching feature ofworkpiece duringtheworkpiecesetup.2)Dynamicinteractionsrefertothetool-fixtureinteractions,whic