外文翻譯--從 cad 輔助設(shè)計(jì)系統(tǒng)到虛擬裝配 英文版

1、<p>　　A novel data decomposition and information translation</p><p>　　method from CAD system to virtual assembly application</p><p><b>　　Abstract</b></p><p>　　Virtu

2、al assembly (VA) is a key technology for virtual manufacturing systems.</p><p>　　So far, CAD systems are still the main modeling tools for the VA system. There isn’t a</p><p>　　standard data ex-

3、change criterion to transfer the data directly from CAD systems to VA</p><p>　　applications, consequently an original data decomposition and information translation method</p><p>　　(DDITM) was t

4、imely proposed to achieve the decomposition and translation. The information</p><p>　　of the assembly bodies in the CAD system was divided into geometry information, topology</p><p>　　informatio

5、n, and assembly information, etc., which were transferred to the VA application</p><p>　　separately. The geometry information including the surface information was translated by the</p><p>　　dat

6、a translation interface (DTI) developed, the topology information was translated by a</p><p>　　five-hierarchy topology structure (FHTS) constructed, and the assembly information was</p><p>　　tra

7、nslated with database technology. A systematic architecture was formed with the</p><p>　　interaction between the geometry information and the topology information, and the</p><p>　　assembly info

8、rmation and the topology. Finally an experimental VA system was set up to</p><p>　　verify the DDITM, and an assembly simulation was implemented to verify the assembly</p><p>　　information furthe

9、r, which proves the translated information is precise and sufficient.</p><p>　　Keywords CAD ·Data decomposition· Information translation ·Virtual</p><p><b>　　assembly</b&

10、gt;</p><p>　　1 Introduction</p><p>　　Virtual environments (VE) are interactive virtual image graphics displays enhanced by</p><p>　　special processing and non-visual tools, such as

11、auditory and</p><p>　　haptic ones, to</p><p>　　realize the effect of immersion</p><p>　　regarded as a natural extension to</p><p>　　three-dimension</p>

12、<p>　　graphics technology with advanced input and output equipments.</p><p>　　There are four key characteristics – immersion, presence, navigation and</p><p>　　interaction</p&g

13、t;<p><b>　　–</b></p><p>　　that are usually used to measure and classify different VR</p><p>　　systems and the applications [1], which have affected and changed people’s ways o

14、f thought</p><p>　　and action manners greatly. VR has already become one of the most advanced, powerful</p><p>　　technologies, employed in a variety of fields, such as military, medicine, entert

15、ainment,</p><p>　　architecture and mechanical manufacturing, etc.</p><p>　　Virtual assembly (VA) is a concrete application of VR. Assembly technology plays</p><p>　　an important rol

16、e in manufacturing, which is not only a key step in product design and</p><p>　　producing, but also the last step in gaining a whole performance. Some analyses show that</p><p>　　assembly-relate

17、d activities in manufactured goods account for over 50% of the total</p><p>　　production time and 20–40% of the unit production cost [2]. The development of VA based</p><p>　　on VR provides us a

18、 low-cost and rapid method for assembly, which employs visualization</p><p>　　technology, simulation technology, assembly technology and decision-making theory, etc.</p><p>　　After the component

19、s’ designs are finished, the data of these CAD models are transferred to</p><p>　　the VA to implement assembly evaluation, assembly simulation, and assembly planning or</p><p>　　assembly evaluat

20、ion. Then invalid irrational structures are improved, and relative engineering</p><p>　　decisions pertaining to assembly are made, so as to assure a success of actual</p><p>　　assembly process.

21、Consequently the product development cycle is shortened and costs are</p><p>　　reduced, and the assembly efficiency is improved, too.</p><p>　　Assembly modeling is the first step to construct a

22、VA system. Although VR software</p><p>　　has a certain modeling ability</p><p>　　to create some simple geometrical shapes (e.g.,</p><p>　　cylinder, cube, sphere etc.), it cannot mee

23、t the modeling requirements if relying only on VR</p><p>　　software to build complex shapes or if thousands of components are to be built. So far 3D</p><p>　　CAD softwares (e.g., SolidWorks, Pro

24、/Engineer, and Unigraphics, etc.) are the still main</p><p>　　modeling methods for VA system. In this paper we use SolidWorks to build CAD models.</p><p>　　There isn’t a standard data exchange c

25、riterion between the CAD system and VA system,</p><p>　　so the information of CAD models can’t be transferred to VE directly. Although the CAD</p><p>　　system can export the CAD graphical models

26、 in other formats (e.g., WRL,</p><p>　　DXF, 3DS, SLP, etc.) imported into VE and displayed there. However, these</p><p>　　formats can only keep partial geometry information etc. For example, the

27、 whole assembly</p><p>　　body or a single component can be selected, but its surfaces cannot be selected, so the models</p><p>　　in these formats are only of the concept of body and not of that

28、of surface in VE. However</p><p>　　surface is such an important concept in assembly, and it has to serve such functions as</p><p>　　follows: (1) orienting components or assembly bodies, (2) defi

29、ning constraints,</p><p>　　(3) defining joints, (4) defining geometrical and dimensional tolerances, (5) defining</p><p>　　precision or roughness, and (6) defining physical information (e.g., co

30、lors and</p><p>　　materials etc.). Without the surface concept, the assembly concept becomes vague and</p><p>　　the VA system is also reduced to a simple simulation application. In addition, the

31、 VA system</p><p>　　needs not only geometry information, but also topology information, assembly information,</p><p>　　etc. So transferring the information of these CAD models to VA is not only

32、the first step of</p><p>　　constructing a VA system, but also a key step having an influence on the display effect, the</p><p>　　assembly effect, and the precision requirements of the VA system.

33、</p><p>　　In the next section, several data translation methods of assembly-related VE are</p><p>　　introduced. In Sect. 3 an introduction to the DDITM structure is given. In Sect. 4, a concrete

34、</p><p>　　realization algorithm for the DDITM is proposed. In Sect. 5 an experimental VA</p><p>　　system and an assembly simulation system are setup to verify the</p><p>　

35、　translated information. Conclusions are made in Sect. 6.</p><p>　　2 Related works</p><p>　　The previous research on translating the information of CAD models into assembly-related</p>&l

36、t;p>　　VE can be classified into several categories. The first is about the use of the VRML file as a</p><p>　　transformational file, as it is employed in most of the VA systems. One representative work of

37、</p><p>　　this category is from the National Institute of Standards and Technology [3], where they</p><p>　　developed a VRML interface for a system called visual interface to manufacturing (VIM)

38、.</p><p>　　This system provides visual access, using VRML, to a database containing manufacturing</p><p>　　data. Antonishek [4] also used VRML as a bridge between the CAD system and Virtual</

39、p><p>　　Workbench. STEP, a graphical data exchange standard, forms the second category, which is</p><p>　　employed to translate complex assembly data. Mok [5] developed a CAD/CAM/CAE product</p&

40、gt;<p>　　data management (C3P) tool based on a structural product coding system (SPCS). The C3P</p><p>　　system analyzed a product by importing information from its STEP CAD data. Lee [6]</p>&

41、lt;p>　　presented a system focusing on shape representation and interoperability of product models</p><p>　　for distributed virtual prototyping, where STEP was used as a means of transferring and</p>

42、<p>　　sharing product models. Ikonomov et al. [7] proposed a virtual assembly model for</p><p>　　concurrent engineering using STEP data exchange.</p><p>　　The third category integrates th

43、e modeling system with the VA system, both of which</p><p>　　share a common database. Wan et al. [8] described VDVAS, an integrated multi-modal virtual</p><p>　　design and virtual assembly envir

44、onment. One important feature of VDVAS lies in that it</p><p>　　allows designers to modify components of an assembly during the process of assembly</p><p>　　modeling and simulation without the n

45、eed of time-consuming data exchange between the</p><p>　　virtual environment and other CAD applications.</p><p>　　The fourth category focuses on using the interface software. Two typical VA ins

46、tances</p><p>　　are introduced here. These two systems are virtual environment for design and manufacturing</p><p>　　(VEDAM) [9] and virtual assembly development environment (VADE) [10]. VEDAM i

47、s a</p><p>　　very general framework for virtual reality applications in design and manufacturing, whereas</p><p>　　VADE is specifically designed for assembly planning. The two systems have chose

48、n</p><p>　　Pro/Engineering as their modeling systems and obtained the information through automated</p><p>　　transfer from the CAD system using Pro/DEVELOP, the developer’s toolkit for accessing

49、 the</p><p>　　Pro/Engineer database.</p><p>　　Many other formats are also used to transfer the information of CAD models into other</p><p>　　VE, such as “OpenFlight”, “DXF”, “3DS”,

50、“SLP”, and so on. Weyrich et al. [11] presented</p><p>　　an approach of a “virtual workbench” and its application to virtual assembly. The system used</p><p>　　the professional modeling tool Mul

51、tigen II, and the “OpenFlight” format as its data interface</p><p>　　with the virtual environment.</p><p>　　3 Structure of the DDITM</p><p>　　The methods introduced in Sect. 2 have

52、both pros and cons. The first kind is easier to</p><p>　　realize, but the VRML in this way doesn’t provide assembly information and the surface</p><p>　　concept is lost, so it is hard to perform

53、 complex assembly actions in VA. STEP includes</p><p>　　almost all the information of CAD models from design period to assembly period, but its</p><p>　　structure is so complex that the needed i

54、nformation is too difficult to extract. The third kind</p><p>　　spends less time on data translation between the CAD system</p><p>　　and VA system, but it</p><p>　　needs to a create

55、 modeling system itself, so excessive time is spent on developing a</p><p>　　modeling system, moreover the integration between the system and current</p><p>　　CAD system is not good enough. Othe

56、r formats have the same problem as VRML. In this</p><p>　　paper, according to the research on the above techniques, an information decomposition and</p><p>　　translation method (DDITM) is propos

57、ed to translate the information of CAD models to the</p><p>　　VA system.</p><p>　　The DDITM divides the data into several sections: geometry information, topology</p><p>　　informati

58、on and assembly information, which are translated through different methods.</p><p>　　Figure1 shows the translation flowchart of DDITM.</p><p>　　1. As for geometry information, the surfaces of t

59、he CAD models are discretized into triangle</p><p>　　tessellations by means of the CAD forward development method, and then</p><p>　　these tessellations’ information is written into correspondin

60、g VR documents through the</p><p>　　DTI. In VA, when all these triangle tessellations are displayed, continuous geometry entities</p><p>　　are reconstructed. In addition, the surfaces of the CAD

61、 models are treated as separate objects</p><p>　　in these VR documents. By creating the relations between surfaces and tessellations, the</p><p>　　surface concept is constructed in VE.</p>

62、<p>　　2.As for topology information, this paper uses a topology structure named five-hierarchy</p><p>　　topology structure (FHTS) to store the topology information of the CAD models. The FHTS</p>

63、;<p>　　includes five hierarchies, i.e., assembly, subassembly, part, surface, and tessellation. First,</p><p>　　three tables are constructed to realize the FHTS, which are a subassembly table, a part&

64、lt;/p><p>　　table and a surface table. Then each hierarchy’s information extracted from the</p><p>　　CAD system is stored into the corresponding table by using the CAD forward development</p>

65、<p><b>　　method.</b></p><p>　　3. As for assembly information, database technology is adopted. Two tables including mate</p><p>　　table and tolerance table are constructed. Ass

66、embly information (including mate information,</p><p>　　tolerance information) is taken out of the CAD system by using the CAD forward</p><p>　　development method, and then stored into correspon

67、ding tables.</p><p>　　4. The assembly information, the geometry information and the topology information do not</p><p>　　exist separately, instead they interact with each other. The FHTS is a ke

68、rnel part of the</p><p>　　DDITM, which is used as a bridge to link the geometry information and the</p><p>　　assembly information, and both of them interact with the FHTS to share the</p>

69、<p>　　information with each other. After constructing the surface concept, a surface will be treated</p><p>　　as a basic unit to perform VA operations.</p><p>　　4．Realization of the DDITM

70、</p><p>　　4.1 Software platform</p><p>　　To realize the DDITM SolidWorks [12] is selected as the assembly modeling platform,</p><p>　　WorldToolKit (WTK) is selected as the platform

71、for constructing the VA system [13], SQL</p><p>　　Server is selected as the database platform, and Visual C++ is selected as theapplication</p><p>　　developing platform. Both SolidWorks forward

72、development method and the WTK select</p><p>　　Visual C++ as a supporting platform, which can eliminate the compatibility problems</p><p>　　between them.</p><p>　　4.2 Geometry infor

73、mation translation based on surface hierarchy</p><p>　　4.2.1 Selection of VR document format</p><p>　　The DDITM uses the DTI to write the geometry information of SolidWorks models into</p>

74、<p>　　corresponding VR documents loaded into VA as geometry nodes. Then the geometry entities</p><p>　　are displayed in VA with the display mechanism of WTK. VR document interfaces (e.g., NFF,</p&g

75、t;<p>　　3DS, WRL, DXF, and SLP etc.) are used to transfer geometry information from any other</p><p>　　kind of CAD modeling software to VA. Although WTK supports many VR formats, only the</p>&

76、lt;p>　　WRL format can be used to transfer geometry information from SolidWorks to VA, but in a</p><p>　　practical application, the WRL document is of the following limitations in WTK [13]</p><p

77、>　　1. In WRL documents, although CAD models are discretized to triangle tessellations based</p><p>　　on a surface hierarchy, the surfaces have no identities, so WRL documents do not have the</p>&l

78、t;p>　　surface concept, consequently WTK can’t modify the colors or the textures etc. of the</p><p>　　surfaces. Therefore the surfaces in VA can’t be picked up so that the assembly actions based</p>

79、<p>　　on surfaces can’t be performed.</p><p>　　2. WTK ignores scaling factors (if any) within a transform node’s transformation. WTK</p><p>　　can’t perform scale operations on WRL virtual

80、objects, so the size of virtual objects can’t be</p><p>　　changed according to the objects in VA or they can’t make more examples of different sizes.</p><p>　　3. Virtual objects of WRL documents

81、 are of only the body concept and the tessellation</p><p>　　concept, so if precise collision detection is performed, its efficiency will be low, for</p><p>　　intersection test between every two

82、tessellations has to be performed.</p><p>　　WTK Neutral File Format (NFF), another VR document format is adopted to discretize</p><p>　　bodies into aggregates of triangle tessellations. The NFF

83、format, written in ASCII format, is a</p><p>　　neutral file format taken by WTK. Compared with the WRL documents, the NFF documents</p><p>　　include all the geometry information that the WRL doc

84、uments have. In addition, if every</p><p>　　surface is treated as an separate object in NFF documents and identified by a unique identity,</p><p>　　then the surface information is constructed. T

85、he surface information is very important</p><p>　　because assembly information is based on the surfaces, and the assembly information, the</p><p>　　topology information, and the geometry informa

86、tion communicate with each other based on</p><p>　　surface hierarchy, too. Furthermore NFF documents are integrated into WTK closely, so</p><p>　　there aren’t any limitations for them in VA. In

87、short, the NFF is the best VR format to store</p><p>　　the geometry information including the surface information.</p><p>　　4.2.2 Creating NFF documents</p><p>　　Although NFF docume

88、nts are selected to store the geometry in formation, SolidWorks</p><p>　　doesn’t provide the interface to export the NFF documents，hence the DTI is developed here.</p><p>　　It extracts the infor

89、mation of the WRL documents created by SolidWorks and writes this</p><p>　　information into corresponding NFF documents. At the same time, the DTI employs the</p><p>　　CAD forward development me

90、thod to write other geometry information including surface in</p><p>　　formation to corresponding NFF documents. The flow chart of the DTI is as shown in Fig. 2.</p><p>　　First, a pretreatment i

91、s implemented to record the count of discrete points and that of the</p><p>　　triangle tessellations for each surface, which are stored in two variables. Second, write</p><p>　　file heads that i

92、nclude some recognition information of NFF documents (e.g., NFF tag,</p><p>　　NFF version number, position of view point, orientation of viewpoint, etc.). These</p><p>　　file heads are used to m

93、ark the NFF documents. Third, the surfaces’ information is extracted</p><p>　　from WRL documents and written into corresponding NFF documents. In this stage, each</p><p>　　surface of the CAD mod

94、els is treated as an individual object numbered consecutively. We</p><p>　　also need to write the information obtained from the pretreatment stage into a corresponding</p><p>　　NFF document. The

95、n, the concrete information is written into corresponding NFF documents</p><p>　　extracted from the WRL documents by reading these WRL documents line by line. This</p><p>　　information includes

96、 the number of the discrete points, their coordinate values, an</p><p>　　automatic symbol “N” taken to calculate the normal of the tessellation automatically,</p><p>　　the material information a

97、nd tessellations’ information etc. Each triangle tessellation has a</p><p>　　unique identity numbered consecutively. Finally, judge whether it is the last surface, if it is</p><p>　　false, repea

98、t the former process, or else finish reading these WRL documents</p><p><b>　　and</b></p><p>　　creating the NFF documents at last.</p><p>　　In VA, a NFF document is loade

99、d into WTK as a geometry node and all the objects in the</p><p>　　document are displayed in VA respectively. Each surface includes corresponding tessellations.</p><p>　　After constructing the FH

100、TS, the relationship between a surface and tessellations will finally</p><p>　　be constructed. Then the surfaces’ information of these models will be built, and the</p><p>　　operations on surfac

101、es will be converted into the operations on corresponding tessellations.</p><p>　　For example, the change of a surface’s texture will be converted into the change of its</p><p>　　corresponding t

102、essellations’ texture and the change of a surface’s color will be converted into</p><p>　　the change of its corresponding tessellations’ color. Thus through the DTI we translate the</p><p>　　geo

103、metry information based on surface hierarchy from the CAD system to VA system.</p><p>　　4.3 Translation of the topology information</p><p>　　The topology information is the kernel part of the DD

104、ITM, because the topology structure</p><p>　　stores the relationship between assembly body, subassembly bodies, parts, surfaces and</p><p>　　tessellations [14], and both the assembly information

105、 and the geometry information</p><p>　　communicate with the structure. The topology structure is called FHTS, made up of five</p><p>　　hierarchies, i.e., assembly, subassembly, part, surface, an

106、d tessellation. The FHTS is a</p><p>　　relational structure, and among these five hierarchies there exist the following</p><p>　　relationships: an assembly body is an aggregation of subassembly