外文原文：
Computer and Design
The computer has had a major impact on the way everyday tasks associated with design are accomplished .It can be used in many ways to do many things. However, all design tasks accomplished using a computer fall into one of four broad categories:
*design modeling *design analysis *design documentation 1. Design Modeling
In CAD/CAM design modeling, a geometric model of a product is developed that describes the part mathematically. This mathematical description is converted to graphic form and displayed on a cathode ray tube (CRT). The geometric model also allows the graphic image to be easily edited and manipulated once displayed.
The process begins when a designer creates a graphic image using a special ICG software package. The image is created by entering commands that cause the computer to construct the image out of points, lines, circles, and curves. To create a graphic image, the computer must translate the geometric characters into a corresponding mathematical model. What appears to the designer as a graphic image is stored in the computer as a series of mathematical coordinates.
As the designer issues commands to edit or manipulate a graphic image, the computer revises the mathematical model .The computer must first change the geometric model before it can change the graphic image, and it must calculate the mathematical coordinates of a geometric character before it can display that character.
Graphic images are displayed in one of three formats: ．two-dimensional or 2-D
．two-and-a-half-dimensional or 21/2-D ．three-dimensional or 3-D
These three formats are illustrated in Fig.2.3 , 2.4 and 2.5 .A2-D graphic image shows an orthographic(flat) representation and usually shows two or more views (e.g. ,top ,front ,and right side).A 21/2-D graphic image is an oblique representation .
A 3-D graphic image may be a wireframe model or a true 3-D solid model (Fig2.6).
Communication can be enhanced further when graphic images are displayed in color .Many capabilities of CAD/CAM, including finite-element analysis, wireframe
3-D, and solids modeling , are more easily understood when the graphic images are
Fig 1
displayed in color .Specific applications such as piping drawings are also easier to read when displayed in an appropriate variety of colors .
When graphic images are displayed in only one color ,it is sometimes difficult to determine when lines cross over or under ,what is near and what is far away ,what is the original image and what is the distorted image (as in the case of finite-element analysis), and which layer a given component is on .The use of color allows for a more definitive distinction between over and under ,near and far ,and original and distorted .It also helps clarify objects consisting of numerous surfaces or several component parts .
2. Design Analysis
The computer has simplified the design analysis stage of the design process significantly .Once a proposed design has been developed, it is necessary to analyze how it will stand up to the conditions to which it will be subjected .Such analysis methods as heat transfer and stress-strain calculations are time-consuming and complex .With CAD/CAM, special computer programs written specifically for analysis purposes are available.
One such program that has simplified the analysis of manufactured products is called finite-element analysis .Finite-element analysis involves breaking an object up into many small rectangular or triangular elements (Fig.2.7),then analyzing each individual element by computer .This approach gives a thorough analysis that might not be feasible without the aid of a computer .It also offers the advantage of specifically pinpointing the locations of problems so that design corrections can be more easily made .
With finite-element analysis , the computer must have a powerful processing capability .The computer analyzes the whole object by analyzing each individual interconnected finite element .By analyzing the response of each finite element of the object to the stress ,strain ,heat ,or other force acting on it ,the computer can predict the reaction of the whole object .
Modern CAD/CAM systems with finite-element analysis capability make the process simple to achieve .Users define the area that is to be divided .The computer
Fig 2
计算机科学与技术专业外文翻译--计算机与设计
