資源目錄里展示的全都有，所見即所得。下載后全都有,請放心下載。原稿可自行編輯修改=【QQ：401339828 或11970985 有疑問可加】 編號： 畢業(yè)設(shè)計(論文)外文翻譯 （原文） 院（系）： 機電工程學(xué)院 專 業(yè)： 機械設(shè)計制造及其自動化 學(xué)生姓名： 學(xué) 號： 指導(dǎo)教師單位 姓 名 職 稱： 年 6月 3日 Content-based search of mechanical assemblies Abstract Title of Thesis: Degree Candidate: Degree and Year: Thesis directed by: The increased use of 3D CAD systems by product development organizations has resulted in large databases of assemblies; this explosion of assembly data will continue in the future. Currently, there are no effective content-based techniques to search these databases. Ability to perform content- based searches on these databases is expected to help the designers in the following two ways. First, it can facilitate reuse of existing assembly designs, thereby reducing the design time. Second, a lot of useful Design for Manufacturing and Assembly (DFMA) knowledge is embedded in existing assemblies. Therefore a capability to locate existing assemblies and examine them can be used as a learning tool by the designers to learn from the existing assembly designs and hence transfer the best DFMA practices to new designers. This thesis describes a system for performing content-based searches on assembly databases. It lists the templates identified for comprehensive search definitions and describes algorithms to perform content-based searches for mechanical assemblies. The characteristics of mechanical assemblies were identified and categorized based on their similarity and computational complexity to perform comparison. The characteristics were extracted from the CAD data to prepare a CAD independent signature of the assembly. The search methodology consists of exact and approximate string matching, number matching and computing graph compatibility. Various research groups have solved the former two problems. This thesis describes a new algorithm to solve graph compatibility problem using branch and bound search. The performance of this algorithm has been experimentally characterized using randomly generated graphs. This search software provides a CAD format independent tool to perform content based search of assemblies based on the form of assemblies. The capabilities of the search software have been illustrated in this thesis through several examples. This search tool can contribute to significantly reduce the design time and reuse of the knowledge in existing designs. Chapter 1 INTRODUCTION This chapter is arranged in the following manner: Section 1.1 discusses the mechanical assemblies in CAD software systems, Section 1.2 describes the motivation behind the research for assembly search system, Section 1.3 describes the issues involved in the research and Section 1.4 describes the outline of the thesis. Most of the research in this thesis is based on the work reported in [Desh05, Gupt06]. 1.1 Mechanical Assemblies in CAD Over the last ten years, 3D CAD systems have become very popular in the industry. These CAD systems are being used to generate 3D models of parts and assemblies. These models are used as a basis for engineering analysis and generating manufacturing plans. 3D models also allow virtual prototyping and hence reduce the need for physical prototyping. Nowadays, organizations routinely set up databases of CAD models to enable all participants in the product development process to have access to 3D data to support their functions. Specially, design, manufacturing and service engineers are expected to greatly benefit from these databases. Design engineers can access the designs of parts and assemblies in the database to design a product for a similar application. Manufacturing engineers can use these databases to find the manufacturing plans and vendors to manufacture parts and assemblies. Service engineers can use the strategy to disassemble and assemble the products for maintenance and servicing. These databases are updated with the latest versions of parts and assemblies and hence significantly improve information dissemination. CAD databases for even moderate size companies are expected to be large in size. A product assembly can contain many subassemblies and each subassembly can contain 9 many parts. Therefore, even a small organization that has multiple product lines may add hundreds of assemblies to their database every year. The mechanical assemblies consist of either the pointers to or a copy of the geometry of the constituent parts. An assembly in a CAD system can be made of individual parts or subassemblies. The subassemblies are made of its constituent parts. The constituent parts and sub assemblies are represented in a tree structure that represents the bill of materials of the assembly. The constituent parts and subassemblies are placed in specific position using part mating conditions. The assemblies are virtual representation of a product or an important part of a product. Some products have articulations. The articulation is shown in assembly by joints. These joints can be simulated to show the movement of different parts of the assembly. Before a product is manufactured, different analysis are carried out on the parts and assembly. Some examples of such type of analysis are manufacturability, strength and motion analysis. Often the results of such analysis including the product manufacturing (PMI) data are stored with the assemblies. The mechanical assemblies thus contain much more information about the products of an organization as compared to CAD files that only contain the geometry of a part in the product. The assembly databases, besides supporting downstream manufacturing and service operations, can be very useful during the design phase as well. There are two main uses of assembly database during the design stage. ? The first possible usage is to locate existing assemblies that can be reused in a new product. Such reuse of existing designs is beneficial from many different perspectives. It reduces design time by eliminating the need for modeling and analysis for the assembly being reused. Furthermore, the existing assembly is already tested and has an established manufacturing plan. This further reduces the product development time and cost. Sharing assemblies across multiple product lines also allows a company to take advantage of the economy of scale. The design of universal joint shown in the figure 1.1 can be reused to design another universal joint. Figure 1.1: A universal joint – reuse of existing design ? The second possible usage is to provide access to existing design knowledge. Designing assemblies requires considerable effort. Creating good assembly designs require thoughtful analysis and careful application of Design for Manufacturing and Assembly (DFMA) principles. The design thumb rules used in an organization are embedded in the design. These rules include the tweaking of design to suit the manufacturing capabilities in the organization and its vendors. New designers can adopt and copy successful design templates. Moreover, once designers manage to find an assembly with the desired characteristics, they can also access associated data such as cost, reliability, and failure reports. The camera frame assembly shown in the figure 1.2 can be used to access the knowledge about an assembly that fit together tightly. Figure 1.2: A camera frame – use of design knowledge 1.2 Motivation This section describes the motivation behind this work. 1.2.1 Need for assembly search system Saaksvuori reports that up to 70 percent of a designers time can be saved if the existing knowledge base of an organization can be reused for new designs. [Saak04] It is thus very important to have the capability to search the database based on any characteristics for a desired assembly. Currently, content-based search tools do not exist for searching assemblies based on the specified criteria. Therefore, designers locate assemblies by combining the text based and part search methods and manually opening various files and browsing through them using a CAD system. This is a highly inefficient use of designer’s time, and becomes a serious problem as the numbers of assemblies in the database grow. This also requires that designers should have access to the CAD software. 3D CAD software are costly and it would be helpful to search database without requiring to access CAD software. If a designer can access the information of an assembly and study the design without using CAD, it would result in significant cost reduction. 1.2.2 Existing search methods and their limitations Designers have access to several types of search tools. If the assemblies are stored in hard drives, they can use file name-based search tools. This strategy only works if a meaningful file naming convention based on assembly contents is adopted. However, developing and deploying a content- based naming convention is impractical in many large organizations. Many organizations have manufacturing plants located across different geographical locations. The merger of two different organizations in different geographical locations can also lead to two completely different naming conventions that are individually sufficient to cater to the needs of the different manufacturing units but cannot be used as search criteria in a single assembly search system. In such case, information about product designs cannot be used by designers in two different manufacturing units. A newly developed convention cannot be used to search the legacy data in the organization. Any organization usually has very large quantity of legacy data that makes it very difficult to change or implement naming conventions followed in the organization. Another way is to attach text notations to assemblies and store them in a Product Data Management (PDM) database. This scheme only provides limited search capabilities and has a limited discrimination power. Moreover, assemblies need to be manually annotated.The text based search cannot be used to define all attributes of the mechanical assembly. Moreover, manual annotations introduce human errors that cannot be avoided thereby reducing the accuracy of a tool that searches for assemblies based on these text attributes. In the recent past, several geometry-based search tools have emerged. However, these tools, although useful for part searches, are not very effective for assemblies. They can only account for the overall shape of the assembly and cannot account for relationships and structure that exist in assemblies. The overall shape may differ for an articulated assembly. For example, the four bar link mechanism shown in the figure 1.3 occupies different volume in different positions. Only text based search tools and geometry based search tools are clearly insufficient to search for assemblies. This research has been started to provide a content based assembly search tool for designers an organization. Figure 1.3: Slider crank mechanism occupying different volumes due to different position of relative parts 1.3 Research Issues 1.3.1 Desired characteristics of software search system The goal of this research is to develop a content-based assembly search system for searching assemblies from a database of existing assemblies based on different characteristics. The characteristics used by the system to search the database need to be extensive and also include most of the characteristics of a typical assembly. Hence, the system will need to support a comprehensive list of characteristics of assemblies based on which the user can define a search. The characteristics included in the system are enumerated in subsequent chapters. The system should be flexible and allow the user to search based on any combination of the characteristics. It should also handle cases that result in too few or too many search results. Thus, if the search system results are too few then the user should be able to lower the constraints (strictness) of the search criteria by increasing the cut-off values. Also, if the search results are too many then the user should be able to perform iterative refinement. This is achieved by constraining the search by including more assembly characteristics in Search Criteria the search, then performing search and again refining search definition. This iterative refinement is very effective in producing the right number of search results. At any time in the search, the user should be able to exclude any assembly from further search. Finally, the system should have an easy- to-use interface and should be efficient so as to locate assemblies from a database in few seconds. The objectives of this research are summarized in the figure 1.4. Search Algorithms Search Strategies Content based assembly search software Figure 1.4: Objectives of research The thesis describes a system for performing content-based searches on an assembly database. It is followed by the description of the templates for comprehensive search definitions that have been identified after studying various assembly models used in modern CAD systems. It also describes the algorithms developed to perform content- based searches for mechanical assemblies based on these search definition templates. These algorithms have been implemented in a system. The thesis also has illustrations of the possible usages of the prototype system with some examples. 1.3.2 Identifying and extracting characteristics of assemblies for search The initial part of research is to identify a comprehensive set of characteristics of the assembly. These characteristics should cover all possible information about an assembly that can be used by a designer to search for the assembly. To build a list of all the characteristics of an assembly on which search can be performed, an extensive review of existing CAD systems and literature in the assembly modeling field [Anan96, Boot94, Brun00, DeFa87, Home91, Khos89, Lee85, Lee93, Moll93, Shah93] was performed. We decided to base our system with the Pro/Engineer CAD system. Therefore, we also studied the list of all characteristics available in Pro/Engineer. Based on the published assembly characteristics and information available in Pro/Engineer models, we developed a preliminary list of assembly characteristics to support content-based searches. To ensure that these characteristics are not specific to Pro/Engineer, we also studied the assembly characteristics available in another CAD system – Unigraphics. We ensured that our list is compatible with the information available in Unigraphics. This research shows that data used in the search can be extracted from a CAD system. The application programming interface (API) of a CAD system can be used to extract the data in the signature of the assembly before execution. As characteristics used in this research are based on Pro/Engineer, API program for that CAD should be used to demonstrate the capability to extract the signature from the CAD system. This will enable the search system to work independent of any CAD system. The figure 1.5 summarizes the process of identifying assembly characteristics. Assembly Models in Published Literature Verify availability of data Pro/Engineer Unigraphics Add additional assembly characteristics Set of Search Criteria Figure 1.5: Method to identify assembly characteristics An assembly has large quantity of characteristics associated with it. It is important to have an intuitive distinction between different characteristics to develop a search system that is based on all of these characteristics and yet has an intuitive interface to define search. The identified characteristics were categorized into four main categories. The assembly design process was used as the basis of categorization of characteristics. The figure 1.6 shows the top level characteristics of an assembly. Relationships Assembly Parts Joints Mating Figure 1.6: Top level assembly characteristics Then, a suitable format, independent of any CAD system, was developed to store all the characteristics in an assembly as its signature. The assembly format defined in [Gupt01] is used as basis to store the signature of an assembly. In [Gupt01], each part is described by a name, a pointer to the geometry, and a transformation, which places the part in its assembled position in the assembly. Every joint is described by a type, a name, and the names of the base and attached parts forming the joint. The joint and mating condition data stored in the signature can be read to construct a graph. The representation is used to support only the mating conditions and joints that are supported in Pro/Engineer. The signature fully represents an assembly and does not require the use of CAD files. Thus, these assemblies can be viewed and searched independent of any CAD software. As these signatures are not dependent on any single CAD software, they can be used to browse and search 10 assemblies designed using different CAD software. This capability is important for large organizations that use different CAD softwares or for those who collaborate with external organizations in design. 1.3.3 Search methodology for different characteristics Many of the characteristics of assembly are text or geometry based. For search based on text, string matching algorithms can be used. This problem has been studied extensively and various algorithms for exact and approximate matching strings are available. For search based on geometry, various approaches have been suggested for exact geometry match and either of them can be used to compare geometry. This research will undertake experiments to optimize the search time by ranking the efficiency of the each of the criteria. The search based on part mating condition will be based on graph compatibility. The user would define a graph which can be a part of a graph representing mating conditions in an assembly from the database. The two graphs need to be matched with each other and this would require a graph compatibility algorithm. Search based on joint relations uses double, as each relation between two different joints in the query as well as database is stored in a double. It is required to create data structure that can store and provide efficient access to all joint relations defined in either query or database. The search method involves exact string comparison to find the existence of joint relations in query and assemblies from database. 1.4 Outline of Thesis The thesis has been arranged in the following way. The four major categories of assembly characteristics that are used for defining search are dealt with in different chapters as described below. The assembly statistics include assembly characteristics such as size and number of parts in the assembly. Chapter 3 deals with the search based on these statistics. Some parts in an assembly can have specific characteristics with regard to material, size and other characteristics. Use of 19 an uncommon part in an assembly characterizes the assembly and can be used as search criteria. Chapter 4 deals with the searches that utilize characteristics of the constituent parts. The mating conditions in an assembly play an important role in its function. Therefore, we need to support searches based on the type of mating conditions that exist in assemblies. This search is discussed in Chapter 5. Joints in articulated assemblies define the possible mot