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商业计算机-电脑辅助设计(CAD)

2005-12-21 00:00Wikipedia

Computer-aided design (CAD)

    Computer-aided design (CAD) is the use of a wide range of computer-based tools that assist engineers, architects and other design professionals in their design activities. It is the main geometry authoring tool within the Product Lifecycle Management process and involves both software and sometimes special-purpose hardware. Current packages range from 2D vector based drafting systems to 3D parametric surface and solid design modellers.

    CAD is sometimes translated as "computer-assisted", "computer-aided drafting", or a similar phrase. Related acronyms are CADD, which stands for "computer-aided design and drafting", CAID for Computer-aided Industrial Design and CAAD, for "computer-aided architectural design". All these terms are essentially synonymous, but there are some subtle differences in meaning and application. Contents

    Introduction

    CAD is used to design and develop products, these can be goods used by end consumers or intermediate goods used in other products. CAD is also extensively used in the design of tools and machinery used in the manufacture of components. CAD is used throughout the engineering process from conceptual design and layout, through detailed engineering and analysis of components to definition of manufacturing methods.

    Fields of use AEC Architecture Engineering and Construction MCAD Mechanical Automotive Aerospace Consumer Goods Machinery Ship Building ECAD Electronic and Electrical Manufacturing process planning

    Architecture

    The software package may produce its results in several formats, but typically provides a graphically-based result which is then able to be used to create concept sketches for assessment and approval, and eventually working drawings. An example would be a structural design package used to assess the integrity of a steel-framed building by performing all the calculations necessary to determine the size and strength of the components, and the effect of such things as wind-loading. The output commonly is a schedule of materials and some basic sketches which can be transferred to a computer-aided drafting package for final production of construction working drawings.

    Computer-aided drafting, however, commonly refers to the actual technical drawing component of the project, using a computer rather than a traditional drawing board. The input into this aspect of the design process may come from specialised calculation packages, from pre-existing component drawings, from graphical images such as maps, from photos and other media, or simply from hand-drawn sketches done by the designer. The operator's task is to use the CAD software to meld all the relevant components together to produce drawings and specifications which can then be used to estimate quantities of materials, determine the cost of the project and ultimately provide the detailed drawings necessary to build it.

    The spectrum of architectural and engineering projects commonly documented with computer-aided drafting is broad, and includes architectural, mechanical, electrical, structural, hydraulic, interior design, civil construction. CAD may also provide input to other forms of design communication such as 3D visualisations, model construction, animated fly-throughs, to name a few.

    Computer-aided drafting software is also a basic tool used in other disciplines related to Architecture, for example Civil Engineering, for site design, for instance roads, grading and drainage, in mapping and cartography, in the production of plans and sketches for a variety of other purposes (such surveyor's plans and legal descriptions of land), and as the input format to geographic and facilities information systems. Additionally, landscape architecture and interior design is often also commonly performed using CAD software

    Mechanical

    CAD is used in a variety of ways within engineering companies. At its simplest level it is a 2D Wireframe package that is used to create engineering drawings. This has however over the last 20 years been overtaken by 3D parametric feature based modelling. Component forms are created either using Freeform surface modelling or solid modelling or a hybrid of the two. These individual components are then assembled into a 3D representation of the final product; this is called bottom-up design. These assembly models can be used to perform analysis to assess if the components can be assembled and fit together as well as for simulating the dynamics of the product. FEA can also be performed on the components and assemblies to assess their strength. Over the last few years, methods and technology have been developed to do top-down design within CAD. This involves starting with a layout diagram of the product; which is broken down into sub-systems with ever increasing detail until the level of single components is reached; geometry in each level being associative with the level above. Detailed design of the individual components is then completed before building up the final product assembly. In general the 3D models are used to generate a 2D technical drawing, this has, however, been slowly replaced by direct transfer of the data to CAM, CNC , Rapid prototyping and Product visualization systems, non geometric information being communicated to down-stream processes with the aid of PMI.

    Electrical and Electronic

    Electronic design automation (EDA) includes PCB design, intelligent wiring diagrams (routing) and component connection management.

    Manufacturing process planning

    2D and 3D CAD systems are sometimes used for graphically represented of plant layout, usually with the aid of specific machine geometry libraries and layout tools. Although this is often done with specialist real-time process simulation tools based on Product visualization and Manufacturing Process Management technologies.

    History

    Designers have long used computers for their calculations. Initial developments were carried out in the 1960s within the aircraft and automotive industries in the area of 3D surface construction and NC programming, most of it independent of one another and often not publicly published until much later. Some of the mathematical description work on curves was developed in the early 1940s by Isaac Jacob Schoenberg, Apalatequi (Douglas Aircraft) and Roy Liming (North American Aircraft), however probably the most important work on polynomial curves and sculptured surface was done by Pierre Bezier (Renault), Paul de Casteljau (Citroen), S.A. Coons (MIT, Ford), James Ferguson (Boeing), Carl de Boor(GM), Birkhoff(GM) and Garabedian(GM) in the 1960s and W. Gordon (GM) and R. Riesenfeld in the 1970s.

    It is argued that a turning point was the development of SKETCHPAD system in MIT in 1963 by Ivan Sutherland (who latter created a graphics technology company with Dr. David Evans). The distinctive feature of SKETCHPAD was that it allowed the designer to interact with computer graphically: the design can be fed into the computer by drawing on a CRT monitor with a light pen. Effectively, it was a prototype of graphical user interface, an indispensable feature of modern CAD.

    First commercial applications of CAD were in large companies in the automotive and aerospace industries, as well as in electronics. Only large corporations could afford the computers capable of performing the calculations. Notable company projects were at GM (Dr. Patrick J.Hanratty) with DAC-1 (Design Augmented by Computer) 1964; Lockhead projects; Bell GRAPHIC 1 and at Renault (Bezier) 每 UNISURF 1971 car body design and tooling.

    The most influential event in the development of CAD was the founding of MCS (Manufacturing and Consulting Services Inc.) in 1971 by Dr. P. J. Hanratty, who wrote the system ADAM (Automated Drafting And Machining) but more importantly supplied code to companies such as McDonnell Douglas (Unigraphics) Computervision(CADDS), Calma, Gerber, Autotrol and Control Data.

    As computers became more affordable, the application areas have gradually expanded. The development of CAD software for personal desk-top computers was the impetus for almost universal application in all areas of construction.

    Other key points in the 1960s and 1970s would be the foundation of CAD systems United Computing, Intergraph, IBM, Intergraph IGDS in 1974 (which led to Bentley MicroStation in 1984)

    CAD implementations have evolved dramatically since then. Initially, with 2D in the 1970s, it was typically limited to producing drawings similar to hand-drafted drawings. Advances in programming and computer hardware, notably solid modelling in the 1980s, have allowed more versatile applications of computers in design activities. Key product for 1981 were the solid modelling packages - Romulus (ShapeData) and Uni-Solid (Unigraphics) based on PADL-2 and the release of the surface modeler Catia (Dassault). Autodesk was founded 1982 by John Walker, which led to the 2D system AutoCAD. The next milestone was the release of Pro/Engineer in 1988, which heralded greater usage of feature based modeling methods. Also of importance to the development of CAD was the development of the B-rep solid modeling kernels (graphics engines) Parasolid(ShapeData) and ACIS (Spatial Technologies) at the end of the 1980s beginning of the 1990s, both inspired by the work of Ian Braid. This led to the release of mid-range packages such as SolidWorks in 1995 SolidEdge (Intergraph) in 1996.

    Today CAD is not limited to drafting and rendering, and it ventures into many more "intellectual" areas of a designer's expertise.

    Software providers today

    This is an ever changing industry with many well know products and companies being taken over and merged with others. There are many CAD software products currently on the market. More than half of the market is however covered by the four main PLM corporations Autodesk, Dassault Systemes, PTC, and UGS Corp., but there are many other CAD packages with smaller user bases or covering niche user areas.

    Packages are can be classified into 3 types: 2D drafting systems (e.g. AutoCAD, Microstation); mid-range 3D solid feature modellers (e.g. SolidWorks, SolidEdge, Alibre); and high-end 3D hybrid systems (e.g. CATIA, NX (Unigraphics)). However these classifications cannot be too strictly taken as many 2D systems have 3D modules, the mid-range systems are increasing their surface functionality, and the high-end systems have developed their user interface in the direction of interactive Windows systems.

    Capabilities

    The capabilities of modern CAD systems include: Wireframe geometry creation 3D parametric feature based modelling, Solid modelling Freeform surface modelling Automated design of assemblies, which are collections of parts and/or other assemblies create Engineering drawings from the solid models Reuse of design components Ease of modification of designs of model and the production of multiple versions Automatic generation of standard components of the design Validation/verification of designs against specifications and design rules Simulation of designs without building a physical prototype Output of engineering documentation, such as manufacturing drawings, and Bills of Materials to reflect the BOM required to build the product Import/Export routines to exchange data with other software packages Output of design data directly to manufacturing facilities Output directly to a Rapid Prototyping or Rapid Manufacture Machine for industrial prototypes maintain libraries of parts and assemblies calculate mass properties of parts and assemblies aid visualization with shading, rotating, hidden line removal, etc……

    Bi-directional parametric associatively (modification of any feature is reflected in all information relying on that feature; drawings, mass properties, assemblies, etc…… and counter wise) kinematics, interference and clearance checking of assemblies sheet metal hose/cable routing electrical component packaging inclusion of programming code in a model to control and relate desired attributes of the model Programmable design studies and optimization Sophisticated visual analysis routines, for draft, curvature, curvature continuity……

    Software technologies

    Originally software for CAD systems were developed with computer language such as Fortran, but with the advancement of Object-oriented programming methods this has over the last decade or so radically changed. The development of a typical modern Parametric feature based modeler and freeform surface systems are built around a number of key, C programming language, modules with their own APIs. A CAD system can be seen as built up from the interaction a GUI with an Associative engine and Geometry constraint engine controlling BREP, CSG and NURBS geometry via a Geometric modeling kernel.

    Hardware and OS technologies

    Today most CAD computer workstations are Windows based PCs; some CAD systems also run on hardware running with one of the Unix operating systems and a few with Linux. Generally no special hardware is required with the exception of a high end OpenGL based Graphics card; however for complex product design machines with high speed (and possibly multiple) CPUs and large amount of RAM are recommended. The human-machine interface is generally via a computer mouse but can also be via a pen and digitizing graphics tablet. Manipulation of the view of the model on the screen is also sometimes done with the use of a spacemouse/spaceball. Some systems also support stereoscopic glasses for viewing the 3D model.

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