1、 毕业设计（论文） 外文译文 院 （系）： 机电工程学院 专 业： 机械设计制造及其自动化 姓 名： 学 号： 指导教师评语： 签名： 年 月 日 1 外语文献翻译 原文： 3.4.1CAD HARDWARE Computers There are two major types of hardware used in a CAD system. Computer and input/output (I/O) devices. In the early days of CAD. Some commercial CAD systems used proprietary computers. To
2、day, nearly all CAD software runs on a general-purpose computer. Depending on the complexity of the CAD package, it may require a mainframe computer, a minicomputer, or simply a microcomputer. In general, the more functionalities a CAD system provides, the more powerful the computer needed. Three di
3、mensional solid modelers require much more computing than do two-dimensional drafting systems； thus, they need more powerful computers. Systems that integrate engineering analysis or simulation packages generally are more computation-intensive. A powerful computer not only speeds up the response of
4、a CAD system, but also can support multiple users without significant performance degradation. Several other parameters and components concerning the computer also have to be considered. CAD HARDWARE random-access memory (RAM)capacity permanent disk-storage capacity special graphics accelerator tape
5、 backups RAM is the actual physical memory (vs. virtual memory)of a computer. A small memory capacity means slow processing due to frequent swapping between the physical memory and the virtual memory on disk. Because CAD is extremely storage demanding, disk capacity is important. Small disk capacity
6、 limits the system to the storage of only a few drawings. CAD is graphics-based and requires tremendous data processing. A graphics accelerator can drastically increase the performance of the system. Another component, the hard disk, is typically the major cost of a computer system. Therefore, it is
7、 not cost-justifiable to store all drawings on line on disk. Magnetic tape is still the most economical medium to use for off-line data storage. It is necessary for a CAD system to have a tape backup subsystem. Currently, all levels of computers are used in CAD systems. Personal computers are used i
8、n low-cost,2-D drafting applications, and with the new power of these computers, hey are also being used for some 3-D solid modeling applications. Engineering workstations have been the mainstay in CAD, and are usually a single-user CAD workstation in a network. Minicomputers are used in multiuser C
9、AD systems. Mainframes are used for large multiuser CAD systems to support real time simulation and engineering analysis. Sometimes, in a large corporation, all levels 2 of CAD systems are implemented. All these systems are linked together through a hierarchical computer network. 3.4.2 Input. Output
10、 (I/O)Devices Figure 3.7 shows the typical I/O devices used in a CAD system. Input devices are generally used to transfer information from a human or storage medium to a computer where ”CAD functions” are carried out. A keyboard is the standard input device used to transmit alphanumeric data to the
11、system. Function keypads are also used to make input easier. Joysticks, track balls, and mousse are also used to manipulate a cursor. They can be used to position the graphic cursor(e.g. cross hair)on a monitor and feed back the location of an object on the monitor to the computer. Using these devic
12、es allows an operator to address terminal locations to interactively in reaching an accurate position. Mouses have been used extensively with windows and pull-down menus. They are easy to use for pointing； However, using a mouse to trace a curve is not an easy task. There are three basic approaches
13、to input an existing drawing；(1)model the object on a drawing,(2)digitize the drawing, or (3)scan the object. Digitizing is usually much easier than modeling. A digitizer is a device that translates the X-Y locations on a drawing than modeling. A digitizer is a device that translates the X-Y locatio
14、ns on a drawing into a digital and feeds that signal to a computer. Graphics scanners scan a drawing and convert it to a CAD-system-readable format. Some scanners have built-in character-recognition software that can convert characters on paper into ASCII codes. Another input device is a sketch pad
15、called a graphics tablet. A graphics tablet is a special flat surface on which a user draws with a stylus. The location of the stylus is sent to the computer. The tablet is an absolute coordinate device. It is easy to implement overlay menus on the tablet and pull down menus on the screen to improve
16、 the operation. Currently. the tablet is the most popular input device used in CAD other than the keyboard. The standard output device for CAD is a monitor display. Modern monitor displays are raster-scan display monitors. Similar to a TV monitor, an electron gun(Sony Trinitron uses three guns for t
17、hree basic colors)sends an electronic beam to the front of the monitor. Easy display dot is called a pixel(picture element).For color displays, each pixel is represented by three closely located dots with red, green , and blue colors. The electronic beam selects the color elements and the intensity
18、of each color element. The resolution of the monitor is determined by the number of different colors or gray scales displayable at each pixel, usually measured in bits, A 1-bit display can turn each pixel either ON or OFF.A gray scale has at least v2 bits. A 1-bit display can turn each pixel either
19、ON or OFF.A gray scale has at least 2 bits which allow three light intensities plus an OFF (2*2).Usually, displays are 4 bits(16 colors),8 bits(256 colors),16 bits(thousands of colors),and 24bits (millions of colors).The electronic beam sweeps the screen from the top to the bottom line by line. Because the dot is lit for only a very short period of time, the sweep must be done very quickly in order not to have a flickering image. The refresh rate is a measure of how many times the monitor is redrawn in one second.
