Research on Three Dimensional Computer Assistance Assembly Process Design System

Research on Three Dimensional Computer Assistance AssemblyProcess Design SystemHOU Wenjun ,YAN Yaoqi ,DUAN Wenjia SUN Hanxu( Automation School , Bijing University of Posts & Telecommunications ,Beijing 100876 China E-mail :Wenjunh21 13@263. net )Abstract : The computer aided process planning rwill certainly play a significant role in the success of enterprise infon matiom-izatiom. 3-dimensional design will promote Tri -dimensional process planning . This article analysis nowadays situation and prob-lems of assembly process planning , gives a 3-dimensional computer aided process planning system ( 3D- VAPP ) , and researches onthe product in formation extraction , assembly sequence and path planning in visual interactive assembly process design , dynamicemulation of assembly and process verification , assembly animation outputs and automatic ex ploding view generation，interactivecraft filling and crafi knorcledge management , etc. It also gives a muli-laser collision detect and multi perspectire automaticcamera switching algorithm. Experiments were cdone to validate the feasibility of such technology and algorithm，which establishedthe foundation of tri-dimensional computer aided process planning .Key words : assemblycraft ; information extraction ; collision detect ; craft plan ; components position and posture1 IntroductionAs an important part of enterprise informationization , Computer- Aided -Process Planning( CAPP ) willcertainly playing a significant role in the success of enterprise informationization. A great strength has been putin the research of CAPP system and so far developed many experimental or commercial products. But in general ,there are still plenty of problems in the direction of research and development. From the application perspective ,the interests archived contrast sharply with the investment. For a long time , the aim of CAPP system has beendeveloping a totally automatic system but an assistant system , to take place of traditional manual process plan-ning , which is much unilateral. It took too much attention on automation，which induces long developing cycle ,high costs , bad flexibility. And on the other hand , most of current CAPP systems are still two dimension ,which cannot refer design information. With the popularization of 3D CAD software , such system will not beable to satisfy the requirements of enterprise informationization. As a result , the tri-dimensional designing basedCAPP system with fine human interface is the only solutiorfLarge enterprises whose main business are manufacture and assemble , always focus on the process and se-quence in the whole assembling of their products but the detailed parameters of one particular process ，becausetheir products are so complicated that require high assembly quality and long time. During the process planning,it needs a large amount of data , including blueprint , model , technical specification , ready craft files and commonfiles. Because of the characteristics of assembly process planning , a process planning system dealing with thewhole process of it is badly needed. Nowadays research reflects such characteristics :.1. Too much research has been put on virtual assembly , while few researches were put on the combinationof assembly knowledge and virtual assembly. Such systems can describe sequence and posture , but doing nothingwith process specifications during the corresponding assembly.2. Many such systems are isolated assembly presentation or verifying systems , which cannot finely integratethe for -ground CAD platforms and back ground Product Data Management Systems( PDM ), and so，cannotshare information among them.3. Such systems always use immersion virtual reality environments . such environments may be not suitablefor virtual assembly systems , because there are many literal inputs and out- puts. It is hard for the immersionvirtual reality environments to deal with such information and thus it is not practical. .4. Many systems require expensive hardware which partl|中国煤化工2 The Architecture of 3-Dimensional ComputerYHC N M H Gcess Planning2.1 System FunctionThis article researches on applying VR technology on assembly sequence planning. This is the project forshanghai jiansheluqiao co. ltd , the 3-Dimensional Computer Aided Assembly Process Planning system( 3D-- 108方数据VAPP ). The system constructs the virtual assembly system by using OPEN INVENTOR , Visual C+ + 6.0and Access as database , its function is as follows in Fig. 1.InformatinextractonStructure analysisAsembl:DisassemblyL sequence plan」Assembly sequece adjustmenRecord assembly pathI Asembly pauhprocessModty assembly pathplan_planningC Addcraf seeificaionProcess planC Modify craft specificationTidimensi-Environment gettingonal CAPPTexture eitingsystem厂VirtualAssembly erulationemulation andollapse detectionverficationSinge stereo displayStereo displayDesktop stereo display_Animation recordExploding viewReport fom generate[ Report manage ]Report printAssemblyCraft knowledge consutresouroe andKnowledgeCraft knowledge modifiyenancecraft flemainteCraft knowlodge addmanagementCraft fleCaf ie Tok卫manageCraft file outputFig. 1 System module and architectureThe system extends the tri-dimensional data stream from the CAD area to the process planning area. Withthe mechanism of combining knowledge based assistant decision and fine human interface , the user can performvisual analysis and designing，assembly sequence and path planning , dynamic reverse emulating and craft verifi-cation , and so , manage the assembly craft files and knowledge in a scientific way.The system includes mainly four modules : information- extraction , interactive assembly process planning ,virtual assembly emulation and verification , assembly process resources and files management.2.2 Key System TechnologyThe key technology of the 3D-V APP system mainly includes : information extraction of product model ,as-sembly sequence and path planning and optimization in visual and inter- active environments , dynamic emulateand craft verification in virtual environments , real- time rendering and stereo display in dynamic assembly , as-sembly animation and automatic exploding view generating , interactive craft flling and craft knowledge manage-ment.3 Key Technology Research3.1 Production Information ExtractionCAD model commonly use precise math expression to describe geometric information. Such expression re-quires huge system resources to display it and it is hard for real-time interactive operation when the model iscomplicated. Model using triangle strip is easy to display，render and process when performing collapse- detect-ing. It needs less calculation and on the other hand , current graphic hardware supports acceleration of drawingtri- angles. As a result ，modeling by triangle strips well satisfies our demand for real-time characteristics in as-sembly emulation and verification. Fortunately ,most CAD systems support such file formats. So it is possible toprocess files generated by different CAD systems. However , Model using triangle strips losses information of as-sembly hiberarchy , craft like the material , parts list ,etc. To solve this problem , what information we extractnot only includes the geometry information but also the assembly hiberarchy. How we rebuild the assembly in-formation of product model is as follows in Fig.2 :中国煤化工Information extract include following procedures :( 1 ) Extract geometry information from VRML ModelMHCNMHG( 2 ) Extract position and posture describing matrix :一1089 一Roo Ro1 Ro2 0]「Roo Ro1 Ro27[ T]=R1o R1 R12 0as| R1o R1i R12 describes the posture of the parts in the assembly coor-R20 R21 R22 (R20 R21 R22dinates and[ X Y Z ]describes the position.(3 ) Extract all the position and posture information of parts in the assembler part as the final data of posi-tion and posture which will be stored in the information database in the system.(4 ) By using layered architecture , from top to bottom , extract the hiberarchy of the assembler part. Thestructure of the tri- dimensional assembly model is as follows in Fig.3 :Proceas11 TrianglelVirtualRoo node| CADJVRMLdesign meshenviromenlmodel!T modelenvironmentL modelmodelChild nodeChild node]CADredeve-DataI FeaturelTranslation ,Rolation iModel| Information....Translation、Rotation'Model[ Information| rebuild |Fig.3 Structure of the scene of 3- dimentional assembly modelFig.2 Rebuild interface of product model3.2 Assembly Sequence and Path OptimizationThe visual interactive process planning module provides a environment for process planner to disassembleand assemble virtually. In such a environment , with the help of knowledge- based decision assistant , planner' sown experience , and the information of the product ( including assembler part assembly view and part assemblyview )at the same time , planner can perform precise disassembly and path optimization and then , the systemoutputs it in reverse order. The interface of the software is as follows in Fig.4 :1 )Assembly sequence planningThinking of all the assembly sequence solving methodsand taking the certain circumstances of the enterprise intoaccount , the article brings a theory combining two analysismethods :( 1 )Layered assembly sequence solving methodAccording to the types of the assembler part ,generate油the assembly sequence layer by layer after resolving the hib-erachy.( 2 ) Manual disassembly methodGenerally , assembly and disassembly are reversibleFig.4 Software interfaceprocessesWith the engineer' s the own experience and analysis and fine interactive interface provided by the system ,the assembler parts can be disassembled manually , thus the gives the assembly sequence.2 ) Assembly path optimizationAssembly path optimization is the path which the part follows from the free position to the restricted posi-tion in the assembler part. Path optimization only deals with geometry information in the working space，andthe paths are for the every part without collapses. The path can be simple lines , and complicated curves as well.In the virtual assembly , the results of path , including position and direction ,are recorded in discrete pointswhich are organized by lists.As the large investment on the hardware based on VR for assembly path planning ,and immature interactivemethods on nowadays hardware configuration , we use a CAD-based path planning method. The assembly pathcan be expressed as follows :F = {F F2.......F;={S'Pi,Q'}As the Q' represents the assembly path of part P; , static child assembler narts are represented as S; ,thefree parts to be assembled are represented as P;. The assembly中国煤化工cided by the initial and .final position of the part.TYRCNMHG3 ) Interactive assembly process planning algorithm flowThe assembly sequence and path planning includes five procedures : specifying moving direction , the modelselect , moving pattern select , move , and release . The flow chart is as right in Fig. 5 :After specifying the assembly sequence and path , the Gantt chart of the assembly sequence can be automati-- 10丽方数据cally generated. By flling the craft specifications , the(二Start)whole chart is finished and then can be used to generateConstruct hiberarchyassembly craft cards.4 ) Multilayer mesh mechanical model assembly[Select base partverification technology+[ Specifty mannerDuring the planning of assembly sequence andSpecily directionpath , collapses always take place between currentlyCSelect objectmoving objects ( including parts , assembler , tools , fix-tures ) and static objects. Only by performing collapseSwitchmanneGain tansfomation matix」detecting dynamically during assembly can guarantee thedragSwtich directionvalidity of the path . At the same time , the collapse de-tecting algorithm must improve greatly the efficiency asTans OR rotate?trang| rotatewell as the precise because of real-time demands of theCalculatesystem.L trang yectorLrotate matrixAs the reason rose forwardly , from the perspective[ M=M x tanslationM=M x rotationof application demands and specific circumstances , thearticle uses a static collapse detect algorithm based o1discrete points because the time slice is very short ( lessSave curent matritthan 30 fps) in the virtual assembly environment. Todirection OR~NoDiassembled?improve the efficiency ，considering the problems ofmanner. switchYesFmodel represented by polygons , we bring a layered preSequence adjustcise collapse detecting algorithm facing mechanical virtu-| problemal assembly based on the current collapse detecting algo-Path adjustrithm. The algorithm includes four layer : part bound-ing-box layer , mesh bounding-box layer , triangle stripVerifcationlayer , feature filter layer2J as follows in Fig. 6 :Result oulputThe algorithm is as follows : when two objects arefar away from each other，the bounding-box algorithmCEnd( there are many methods and improving strategy )is ap-Fig.5 The algorithm of visual interactiveplied to decide whether collapse will take place. After aassembly process planningcollapse is detected , the triangle mesh surface collapsedetect is applied to solve the amount of collapse. But if ageometric restriction is detected( for example , a hole andFirstPart bounding -boxlayera shaft , because their diameters equals , and their approxi-letectmate representation by polygons cause the system to recog-nize it wrongly as a collapse ) , precise collapse detect is ap-Half-Strip bounding -boxlayerprecise 1plied to filter the collapsed surface.3.3 Emulation of AssemblyTriangle mesh layerEmulation of the assembly process is an important wayPreciseto accelerate assembly ，verify assembly craft. The emula-detecttion includes : while user is assembling , the system will au-| Characterstic filter layertomatically record the moving and sweeping path of parts ,transform of view points ，collapse detecting ，and onlineediting. The system can reappearance the assembly processCollapse dectectedvividly , providing animation record and exploding view au-Fig. 6 Multi-layer collapse detecttomatically to guide the workers and engineers.The presentation and transformation of parts during assembly in the tri- dimentional space is the hinge inemulating assembly process correctly. To represent the arbitra-tion,weusethe4X4posture describing matrix[ P ]:中国煤化工Roo Rou Ro2 0]YHCNMHG[P]=R1o R11 R12 0[ T]=R3R4Rs(R2oR21R220R。R7Rg(XYZ1」Lx’Y’Z’1一1091 一Here , the 3X 3 sub-matrix in the upper-left corner of[ P ] is the posture describe matrix of the part in thecoordinates of assembler part and( X ,Y ,Z )is the position of its origin in the coordinates. .While the part to be assembled moves from one key point Kp to anotherK p i.e ,the posture mat- rix[ P]has been transformed by a matrix[ T ] and reached a new posture[ P ] as follows in Fig.7 :thatis[P]=[ P ]●[ T]asis[ T]alsoa matrix :The 3X3 matrix in the upper-left corner of[ T ] represents the rotation of the part. ( X' ,Y' ,Z' )repre-sents the transformation along the axis x心z. To transform a part from one posture to another，the problemlies on how toget[ T ].As there may be block between parts during the emulation , the automatic moving of observe camera tcavoid assembly details being blocked by other static object must be taken into consideration. So we bring an algo-rithm for dynamic multi- perspective camera switching. It works by matching the camera with the object to beassembled. It makes a line between the object and the camera. By deciding whether the line is through some ob-ject , it makes decision whether to use this camera or the next one.The system realized real. time rendering , which can apply textures to objects to archive vivid representation.It also realized extracting bounding- box from the triangle mesh to generate exploding view.3.4 Building Assembly Craft Knowledge and Craft Files ManagementAs to VAPP system , craft files management is required. With craft management module of the system , theuser can browse and edit the information of parts property and assembly craf( including selecting fixture , fllingassembly operation specifications etc. ) conveniently.At the same time，the system can output the craft data generated in inter- active designing module by reportforms , assembly craft cards , fixture lists , and other documents that can be used by assembly workers. On theother hand , the module is also responsible for managing assembly animation and exploding view ever created.Users can consult them whenever they want.The process flow is as follows in Fig.8 :Process fowProcess Environment| Generate enaft dataDatabaseDesign file templateMiscroft excelFill in craf data|yprenerate craft fileCraft flesmanagemet moduleShare,consult,outpu,elc.Fig7 Transformation of partFig.8 The work flow of assembly craft files processing4 ConclusionsThe system realized a series of full function that is , model extraction , assembly analysis , assembly sequenceand path planning , dynamic assembly emulation and verification , assembly craft knowledge maintenance and as-sembly file management. The innovation lies in : combine the computer aided process planning and VR-technolo-gy in the perspective of integration , and realized the seamless integration of CAPP and CAD , extended the tri-dimensional product data application to the CAPP area and combined the knowledge based decision assistant andhuman-computer interactivity. The system plays a significant assistant role in enterprise informationization.But ,in the generalized perspective , assembly process planning includes not only the selection of assembly se-quence , but also overall consideration of working place , tools ,and workers ,etc. So ,we can make full useof vi-sual interactive tools provided by virtual assembly system to research on how to analysis assembly tolerance , howto plan the position and configuration of assembly production unit and stream lines. Thus , we make out a com-plete craft specification.References中国煤化工[1] TIAN Lizhong , FU Yili ,MA Yulin ,CHU Linbo. Finding DiserHc N M H Grence Paning[J] Jourmalof Computer- Aided Design & Computer Graphics. 2001 ,13( 12)( it[2] ZHENG Yi ,NING Ruxin , LIU Jianhua , YAO Jun. Research on Fast Collision Detection Method in Virtual Assembly Envi-ronment[J ] Journal of System Simulation , 2004 &( in Chinese ).[3] Suri S , Hubbard P M , HughesJ J. Collision Detection in Aspect and Scale Bounded Polyhedra[ A] Proc. 9th ACM- SIAMSympos. Discrete Algorithms ,1998.- 10互方数据