According to the requirements of system functions, people describe design problems with limited features, express features with computer data structures through formalization, and store the "feature" data described by data structures in the database. In this way, the "design" process becomes the processing of the database. This process can be regarded as a modeling process. Models are used to represent actual or abstract objects, and are the basis of calculation, analysis, simulation and research on the processed objects. Therefore, the model is the concrete embodiment of the actual structure in the CAD system and the basis for the computer to "identify" the product. We know that the properties ("characteristics") that products and engineering structures can exhibit are infinite, but the available computer resources and human cognitive knowledge are limited. We can only "know" some characteristics of the actual structure, and can only deal with some aspects of product and engineering structure design.
According to different models, CAD systems are generally divided into two-dimensional CAD and three-dimensional CAD systems. Two-dimensional CAD systems generally regard products and engineering design drawings as a collection of geometric elements such as points, lines, circles, arcs, texts, etc. . Any design expressed in the system becomes a geometric figure, and the mathematical model it depends on is the geometric model. The geometric characteristics of these elements are recorded by the system. Two-dimensional CAD system generally consists of graphic input and editing, hardware interface, data interface and secondary development tools.
The core of 3D CAD system is the 3D model of product. Three-dimensional model is to represent the actual shape of the product into a three-dimensional model in the computer, which contains all kinds of information about the points, lines, surfaces and bodies of the geometric structure of the product. The description of computer three-dimensional model has experienced the development from wireframe model, surface model to solid model, and the geometric information expressed is more and more complete and accurate, which can solve a wider range of "design". Among them, the wireframe model only uses the edge line of geometry to represent the shape of geometry, just like the shape built by wireframe, there is no information such as surface and volume in the model. Surface model is to construct the model by using the outer surface of the geometric shape, just like covering a layer of skin on the wireframe model, which makes the geometric shape have a certain outline and can produce effects such as shadow and blanking, but the model lacks the concept of geometric volume, just like an empty shell of geometric body. When the geometric model develops to the solid model stage, the closed geometric surface forms a certain volume, forming the concept of geometric body, just like filling something in the middle of geometric body to make it have the characteristics of weight and density, so that the collision and interference between two geometric bodies can be checked. Because the model of 3D CAD system contains more actual structural features, users can better reflect the actual product structure or manufacturing process when designing product structure with 3D CAD modeling tools.
With the development of CAD technology and the continuous improvement of people's needs, artificial intelligence and other technologies are gradually integrated into CAD systems, forming various knowledge-based CAD systems (or intelligent CAD systems). The application of knowledge greatly improves the "design" function and design automation level of CAD system, greatly strengthens the support for the whole process of product design, and promotes the innovative development of products and engineering.
A stand-alone CAD system is a CAD system installed in a computer and working independently. With the rapid development of economic globalization and network technology, the networked CAD system based on Internet/Intranet has developed rapidly. Networked CAD system can carry out product definition and modeling, product analysis and design, product data management and data exchange by many people in different places in the network environment. It is an important means to realize collaborative design and can provide support for enterprises to use global resources for rapid product development.
Specialized CAD application system develops various professional design techniques into various design tools and knowledge of CAD system by using the secondary development tools or data interface functions provided by general CAD system according to their respective design requirements, so that the design can be directly carried out according to professional design methods, which greatly improves the "design" ability and efficiency of CAD system. However, this CAD system is developed for a specific specialty and is not universal in professional design.
General situation of CAD technology application
CAD/CAM (Computer Aided Design and Manufacturing) technology came into being in the aviation and military industries of developed countries in the late 1950s, and it grew rapidly with the development of computer software and hardware technology and computer graphics technology. 1989, the national academy of engineering named CAD/CAM technology as one of the top ten outstanding achievements of engineering technology (1964- 1989). In the past 30 years, CAD technology and system have developed rapidly, and the application of CAD/CAM has been popularized rapidly. In developed countries, the application of CAD/CAM technology has rapidly expanded from military industry to civil industry, from large enterprises to small and medium-sized enterprises, and from high-tech applications to the design and manufacture of household appliances and light industrial products. This technology is flowing from developed countries to developing countries.
CAD is a very broad concept. Generally speaking, there are two kinds of CAD design objects: one is mechanical, electrical, electronic, light industry and textile products; The other is engineering design products, that is, engineering buildings, which are referred to as AEC (architecture, engineering and construction) in foreign countries. Nowadays, the application scope of CAD technology has been extended to art, film, animation, advertising, entertainment and other fields, which has produced great economic and social benefits and broad application prospects.
CAD is the earliest and most widely used in machinery manufacturing. Using CAD technology to design products can not only make designers get rid of drawing boards, update traditional design ideas, realize design automation, reduce product costs and improve the competitiveness of enterprises and their products in the market; It can also change the original serial operation into parallel operation, establish a brand-new design and production technology management system, shorten the product development cycle and improve labor productivity. Nowadays, the world's major aviation, aerospace and automobile manufacturing giants not only widely use CAD/CAM technology for product design, but also invest a lot of manpower, material resources and funds to develop CAD/CAM software, so as to maintain their leading position in technology and advantages in the international market.
Computer Aided Architectural Design (CAAD) is the application of CAD in the field of architecture, which has brought a real revolution to architectural design. With the development of CAAD software from 2D general drawing software to 3D building model software, CAAD technology has been widely used, which can not only improve the design quality, shorten the project cycle, but also save 2% to 5% of the construction investment. In recent years, China's annual capital construction investment has reached hundreds of billions of yuan. If nearly ten thousand engineering design units in China adopt CAD technology, the investment benefit of capital construction can be greatly improved.
CAD technology is also used in textile and clothing industries. In the past, the pattern design, pattern coordination, color transformation, pattern color separation, sketch drawing and color matching of textiles and garments in China were all done by hand, which was slow and inefficient. At present, the requirements of the international market for textiles and garments are small batch, various colors, high quality and fast delivery, which makes China's textile products uncompetitive in the international market. The adoption of CAD technology has greatly accelerated the pace of China textile and garment enterprises entering the international market.
Nowadays, CAD technology has entered people's daily life and played a great role in the fields of movies, animation, advertising, entertainment and so on. CAD technology has been used in film shooting for more than ten years. Hollywood film companies in the United States mainly use CAD technology to construct scenes, and can use virtual reality to design scenes that cannot be completed manually. This can not only save a lot of manpower and material resources, reduce the shooting cost of the film, but also create a strange and unimaginable environment for the audience and get huge box office income. For example, American sci-fi movies such as Star Wars, Alien, Jurassic Park, and Toy Story, which is made entirely by three-dimensional computer animation, have all achieved great success. The blockbuster Titanic, which caused a sensation all over the world, used a large number of three-dimensional animations to simulate the whole process of Tektronix's voyage and shipwreck with computers. In addition, computer modeling technology is fully used in animation and advertising production, and it is also a virtual reality technology in essence. Virtual reality technology is also used for real-time roaming and entertainment games of various simulators and scenes.
In recent ten years, with the advancement of CIMS engineering and CAD application engineering, the application of computer-aided design technology has become more and more common in China, and more and more design units and enterprises have adopted this technology to improve design efficiency, product quality and working conditions. At present, there are dozens of CAD software imported from abroad in China, and some domestic scientific research institutions, universities and software companies have also developed their own CAD software and put it on the market. The application of CAD technology in China presents a thriving scene.
Second, the technical status in the field of CAD/CAM
1 CAD technology
CAD technology is a comprehensive high-tech in computer and other fields, which integrates computer graphics, database, network communication and other knowledge. It is an important part of advanced manufacturing technology; It is also a key technology to improve design level, shorten product development cycle and enhance industry competitiveness. The characteristics of CAD technology are extensive and complex, rapid technological change, fierce competition, large investment, high risk and high output. The well-known modern CAD/CAM software is huge in scale, multi-functional and complex in system, so it is difficult to keep up with the rapid development of hardware platform and development environment, as well as the ever-changing and growing requirements of users.
2.2 hardware environment. CAD system
At present, the hardware environment of CAD system is mainly engineering workstation and personal computer. Workstation is an interactive computer system with high-speed scientific calculation, rich graphic processing and flexible window and network management functions. Generally, the central processing unit (CPU) with 32-bit or 64-bit word length widely uses RISC, superscalar, super pipeline and super-long instruction technology, has Unix operating system and X-window management system, and runs in a distributed network environment. At present, the popular workstations mainly include: SPARCStation 10 workstation of Sun Company, HP-PA workstation of Hewlett-Packard Company of the United States, Alpha workstation of DEC Company, SGI workstation and RS/6000 workstation of IBM Company. Although the workstation has superior performance and fast graphics processing speed, it is very expensive, which limits the promotion of CAD/CAM technology to some extent at present.
In the past, personal computers were very different from workstations in calculation, graphics, network and parallel processing. The system software such as DOS and Windows running on personal computer is not as good as the system software on workstation in reliability, security and efficiency. However, with the rapid development of computer hardware technology, the continuous improvement of personal computer's computing ability and graphics processing ability and the gradual improvement of operating system, the performance gap between personal computers and professional RISC/Unix workstations has gradually narrowed. In particular, the release of Intel's PentiumPro and Pentium II chips and Microsoft's WindowsNT make personal computers fully compete with low-end workstations in performance, and the low price of personal computers makes them play an irreplaceable role in popularizing CAD applications.