计算机辅助设计的简要历史在我们讲述CAD的基本理论之前,先说说他的简史是比较合适的。
CAD 是计算机时代的产品.它从早期的计算机绘图系统发展到现在的交互式计算机图形学.两个这样的系统包括:麻省理工学院的Sage Project及Sketchpad。
Sage Project旨在开发CRT显示器及操作系统. Sketchpad是在Sage Project下发展起来的.CRT显示和光笔输入用于与系统进行交互操作.CAD与初次出现的NC和APT(自动编程工具)碰巧同时出世.后来,X-Y绘图仪作为计算机绘图的标准硬拷贝输出装置使用,一个有趣的现象是X-Y绘图仪与NC钻床具有相同的基本机构,除了绘图笔NC机床上的主轴刀具替代之外。
开始，CAD系统仅仅是一个带有内置设计符号的绘图编辑器，供用户使用的几何元素只有直线、圆弧、以及两者的组合。
自由曲线及其曲面的发展，如昆氏嵌面、贝塞尔嵌面以及B-样条曲线，使CAD系统可用于复杂曲线与曲面设计。
三维CAD系统允许设计者步入三维设计空间。
由于一个三维设计模型包含了NC刀具路径编程所需的足够信息，所以能够开发CAD与NC之间联系的系统。
所谓交钥匙的CAD/CAM系统便是根据这一概念开发的，并从20世纪70年代至80年代流行起来。
20世纪70年代，三维实体建模的发明标志着CAD一个新时代的开始。
过去的三维线框模型仅用其边界来表达一个物体。
这在某种意义上是模糊的，一个简单的模型可能有几种解释。
同时也无法获得一个模型的体积信息。
实体模型包含完整的信息，因此，它们不仅可用于生成工程图，而且也可在同一模型上完成工程分析。
后来，开发了许多商业系统和研究系统。
这些系统中相当多的是基于PADL和BUILD系统。
尽管它们在表达上是强有力的，但仍然存在许多缺陷。
例如，这种系统要有极强的计算能力和内存需求，非常规的物体建模方式以及标注公差能力的缺乏，这一切已阻碍了CAD应用。
直到20世纪80年代中期，实体建模开始介入设计环境。
今天实体建模的应用如同绘图和线框模型应用一样普遍。
在个人计算机上，CAD已走向大众化。
这种发展使CAD应用面广并且很经济。
CAD原本作为一种工具仅被航空和其它主要工业企业使用。
诸如AutoCAD、VersaCAD、CADKEY等个人机CAD软件包的引入，使小型公司乃至个人可以拥有并使用CAD系统。
到1988年为止已销售10万个以上的PC CAD 软件包。
今天，基于个人计算机的实体建摸的PC CAD易于获得，并且销售变得更为普及。
由于微型计算机的迅速发展使得个人计算机能够承受实体模型需要的大量计算负荷，所以如今许多实体模型在PC机上运行，并且作为平台已不成为一个问题。
随着标准图形用户界面的发展，CAD系统可以很容易地从一台计算机传送，大多数CAD系统都能在不同平台上运行。
在大型计算机、工作台和基于个人计算机的CAD系统之间几乎没有区别。
计算机辅助设计的结构一个CAD系统包含三个主要部分：（1）硬件计算机及输入/输出装置。
（2）操作系统软件。
（3）应用软件CAD软件包。
硬件主要用于支持软件功能。
在CAD系统中使用着种类繁多的硬件。
操作系统软件是CAD应用软件与硬件之间的界面。
操作系统软件管理着硬件运行并提供许多诸如创建和取消操作任务、控制任务的进程、在任务间分配硬件资源、提供通向软件资源，如文件、编辑器、编译和应用程序的通道等基本功能。
这不仅对CAD软件很重要，而且对非CAD软件也很重要。
应用软件是CAD系统的核心。
它由二维和三维建摸、绘图、工程分析等程序组成。
一个CAD系统的功能便建立在应用软件中。
正是应用软件使一种CAD 软件包区别于另一种，通常应用软件是依赖于操作系统的。
要把在一个操作系统上运行的CAD系统移到另一个操作系统上，并不像编译软件那样微不足道。
因此也必须注意操作系统。
计算机辅助设计计算机辅助设计给了设计者去尝试几个可行的解决方案的能力。
通常还需要某些形式的设计分析计算，而为了这一任务已经编写了许多程序。
计算机为设计者对所建议的各种结构设计的分析和为最终设计准备正式绘图提供了强有力的工具。
在二维绘图领域中，计算机方法能够提供比传统的纸和笔的方法更有意义、更大成本节约的优点，但是一个CAD系统并不仅是一个电子绘图板。
计算机绘图系统可使设计者设计出既快又准确的图形，并且很容易修改。
在涉及到重复性工作时，会戏剧性产生复制产品，因为标准图形只要一次构建成功，就可以从图库中取出。
剪切和粘贴技术作为节约劳动力的辅助工具被使用。
当几个分项目设计人员从事同一个工程时，要建立中心数据库，使得由某一个人绘的细节图可以很容易地合并到其它不同的装配图中。
中心数据库也可作为标准参考零件库使用。
有限元是一项成熟的应力分析技术，它多被土木工程和机械工程所采用。
它由将结构划分成有限个的小单元所组成，并计算每一个单元之间的作用力。
如果被分割的单元足够小，就能对一个结构或实体的内部应力获得一个好的估计。
这些计算机设计惯用于大型结构物的设计，诸如船体、桥梁、飞机机身和海面油井平台。
汽车工业也使用类似的方法来设计和制造车身。
二维绘图CAD使多视图的二维绘图成为可能，视图空间可以从微米到米的比例范围内无限变化。
它提供给机械设计师放大的功能，即使在恰当配合的装配零件中最小的零件也能看清楚，设计程序甚至能自动辨认CAD装配图中的潜在问题。
针对具有不同特征的零件，如运动的或静止的，在显示时可以被指定成不同的颜色。
为了有利于工程设计的变化，可使用带有自动尺寸变化的系统对零件进行尺寸标注。
三维绘图随着三维建模的出现，设计者具有了更多的自由度。
他们可以生成三维零件图并且可以无限制地修改以获得所需的结果。
通过有限元分析，应力加到计算机模型上，并且以图形化的方式显示其结果，在产品物理模型真正产生之前，对设计中的任何内在问题给设计者一个快速的反馈。
三维模型可用线框、曲线或实体方式生成。
在线框模型中，直线和圆弧构成了模型边界。
结果是一个可以从任何位置观察的三维模型，但仍只是一个框架形式。
创建曲面犹如在骨架上包上皮。
一旦这样生成后，模型就可以被渲染，使得图形看上去更逼真。
曲面模型普遍用于构建板金的展开和重叠以用于制造。
实体模型是最复杂的建模层次，并且用于建立实体模型的程序在一段时期内只用在大型计算机上。
只有近年来微型计算机才达到这个能力水平，也可以运行复杂的算法，生成实体模型。
计算机“认为”实体模型是一种具有实体质量的模型，所以它可被“钻孔”“加工”“焊接”，好象它是一个实际的零件。
它能够由任何材料构成并呈现其材料特性，因此，能够进行质量计算。
计算机辅助绘图的好处用计算机完成绘图及设计任务的好处是令人难忘的：提高速度、提高准确性、减少硬拷贝存储空间及易于恢复信息、加强信息传输能力、改善传输质量和便于修改。
速度工业用计算机能以平均每秒3300万次完成一项任务；更新的计算机其速度更快。
用计算机计算零件的变形量是一个重要功绩。
当理论上的载荷力加到零件上时、通过计算机进行有限元分析或者在监视器上显示一个城市的整体规划时，这两者都是既费时又计算量大的任务。
AutoCAD软件可根据需要多次复制所需模型的形状和几何尺寸，快速自动地进行剖面填充及尺寸标注。
准确AutoCAD程序依靠操作系统及计算机平台每点具有14位的精度。
这在用数学计算诸如一个圆的线段数、程序必须圆整线段时是十分重要的。
存储计算机能够在物理空间中存储上千幅图，这空间能够存储上百幅手工图。
而且计算机能够很容易地搜索和找到一幅图，只要操作者拥有正确的文件名。
传输由于计算机的数据是以电子形式存储，它能被送到各种位置。
最明显的位置是监视器。
计算机可以在屏幕上以不同的方式显示数据，如图形，并能方便地将数据转换成可读图形。
这些数据也可被传送给绘图机，打印出常见的图纸，通过直接连接到计算机辅助制造机床或由电话线传到地球的任何地方。
你可以不再冒损失或丢失的危险去邮寄图纸，现在图纸可以通过电信网立即发送到目的地。
A Brief History of ＣＡＤBefore we present the basics of CAD ,it is appropriate to give a brief history . CAD is a product of the computer era. It originated from early computer graphic systems to the development of interactive computer graphics. Two such systems include the Sage Project at the Massachusetts Institute of Technology (MIT) and Sketchpad. The Sage Project was aimed at developing CRT displays and operating systems. Sketchpad was developed under the Sage Project. A CRT display and light pen input were used to interact with the system. This coincidentally happened at about the same time that NC and APT(Automatically Programmed Tool)first appeared. Later, X-Y plotters were used as the standard hard-copy output device for computer graphics. An interesting note is that an X-Y plotter has the same basic structure as a NC drilling machine except that a pen is substituted for the tool on NC spindle.In the beginning, CAD systems were no more than graphics editor with some built-in design symbols. The geometry available to the user was limited to lines, circular arcs, and the combination of the two. The development of free-form curves and surfaces, such as Coon’s patch, Bezier’s patch, and B-spline, enable a CAD system allow to be used for sophisticated curves and surface design. Three-dimensional CAD system allow a designer to move into the third dimension. Because a three-dimensional model contains enough information for NC cutter-path programming, the linkage between CAD and NC can be developed. So called turnkey CAD/CAM systems were developed based on this concept and became popular in the 1970s and 1980s.The 1970s marked the beginning of a new era in CAD-the invention of three-dimensional solid modeling. In the past, three-dimensional, wire-frame models represented an object only by its bounding edges. They are ambiguous in the sense that several interpretations might be possible for a single model. There is also no way to find the volumetric information of a model. Solid models contain complete information; therefore, not only can they be used to produce engineering drawing, but engineering analysis can be performed on the same model as well. Later manycommercial systems and research systems were developed. Quite a few of these systems were based on the PADL and BUILD systems. Although they are powerful in representation, many deficiencies still exist. For example, such systems have extreme computation and resource (memory) requirements, an unconventional way of modeling object and a lack of tolerance capability have all hindered CAD applications. It was not until the mid-1980s that solid modelers made their way into the design environment. Today, their use is as common as drafting and wire-frame model applications.CAD implementations on personal computers (PCs) have brought CAD to the masses. This development has made CAD available and affordable. CAD originally was a tool used only by aerospace and other major industrial corporation. The introduction of PC CAD packages, such as, AutoCAD, VersaCAD, CADKEY, and so on, has made it possible for small companies and even individuals to own and use CAD systems. By1980, more than 100,000 PC CAD packages had been sold. Today PC-based solid modelers are available and are becoming increasingly popular. Because rapid developments in microcomputers have enabled PCs to carry the heavy computational load necessary for solid modeling, many solid modelers now run on PCs, and the platform has become less of an issue. With the standard graphics user interface (GUI), CAD systems can be ported easily from one computer to another , Most major CAD systems are able to run on a variety of platforms. There is little difference between mainframe, workstation, and PC-based CAD systems.The Architecture of CADA CAD system consists of three major parts:(1)Hardware computer and input/output(I/O)devices.(2)Operating system software.(3)Application software CAD package.Hardware is used to support the software functions. A wide range of hardware is used in CAD systems. The operating system software is the interface between the CAD application software and the hardware. It supervises the operation of the hardware and provides basics functions such as creating and removing operation tasks, and providing access to software resources such as files, editors, compilers and utilityprograms. It is important not only for CAD software, but also for non-CAD software.The application software is the heart of a CAD system. It contains of programs that do 2-D and 3-D modeling, drafting, and engineering analysis. The functionality of a CAD system is built into the application software. It is application software that makes one CAD package different form another. Application software is usually operating-system-dependent. To transport a CAD system running in one operating system to another operating system is not as trivial as recompiling the software. Therefore, attention must be given to the operating system as well.Computer Aided DesignComputer aided design gives the designer the ability to experiment with several possible solutions. Usually some forms of design analysis calculations need to be done and many programs have been written for this task. The computer provides the designer with a powerful tool for analyzing proposed designs and for preparing formal drawing of the final design.Two-dimensional drawing is one area in which computer methods can off significant, quantifiable cost advantages over traditional paper and pen methods, but a CAD system is not just an electronic drawing board. Computer drawing systems enable designers to produce fast accurate drawings and easily modify them. Draught productivity rises dramatically when repetitive work is involved, since standard shapes are constructed only once and can be retrieved from a library. Cut and paste techniques are used as labor-saving aids. When several detail drawn by one person can be easily incorporated into different assemble drawing. This central database also serves as a library of standard preferred computers.Finite element is a sophisticated stress analysis technique much used by civil and mechanical engineers. It consists of dividing a structure into small, but finite, components and calculating the force between each element. If the elements are small enough, a good estimate of the internal stresses in a structure or solid body can be obtained. These computer techniques are routinely used in the design of large structure such as ship hulls, bridges, aircraft fuselages and offshore oil rig. The motor car industry also uses similar methods for design and manufacture of car bodies.Two-dimensional DrawingsCAD makes possible multiview 2D drawing, with an endless possibility of views in range of scales from microns to meters to meters. It gives the mechanical designer the ability to magnify even the smallest of components to ascertain if the assembled components fit properly and even to design programs to identify automatically potential problems in CAD assembly. Parts with different characteristics, such as movable or stationary, can be assigned different colors on the display. Parts can be dimensioned with automatic dimensioning changes, allowing for expedient engineering design changes.Three-Dimensional DrawingsDesigners have even more freedom with the advent of 3D modeling. They can 3D parts and manipulate them in endless variations to achieve the desired results. Through finite element analysis FEA), stress can be applied to a computer model and the results graphically displayed, giving the designer guick feedback on any inherent problems in a design before the creation of a physical prototype.3Dmodels can be created in wire-frame, in surfaces or in solid form. In wire-frame, lines and arcs form edges that generate the model. The result is a 3D form that can be viewed from any location but still only a skeletal form. Creating a surface stretches a skin over the skeleton (Fig.8-1b).Once this is done, the model can be rendered so that it appears more tangible. Surface models are commonly used in the creation of sheet metal developments that can be unfolded for manufacture.Solid models are the most complex level of modeling and while the programs to create them have been available for some time on large mainframe computers, it is only recently that microcomputers have reached a level of power that allows the running of the sophisticated algorithms needed to create solid model（Fig.8-2）. The computer “thinks” the solid is sold mass so it can be “drilled”, “machined,”or “welded” as if it were an actual physical part. It can be made out of any material’s characteristics, thereby allowing calculations of mass to be made.CAD’S BenefitsThe benefits of computer use in drafting and design tasks are impressive: increased speed, greater accuracy, reduction of hardcopy storage space as well as better recall, enhanced communication capabilities, improve quality and easiermodification.SpeedA person computer used in industry can perform a task at an average rate of 33 million operations per second; newer computer are even faster. This is an important feat when using it to calculate the amount of deflection of a component, when theoretical physical forces are applied to it, through finite element analysis(FEA) or when displaying an entire city plan on a monitor, both of which are time-consuming and calculation-intensive tasks. AutoCAD software can duplicate any geometry as many times as required and can also perform crosshatching and dimensioning automatically and equally as fast.AccuracyThe AutoCAD program has an accuracy of 14 significant digits of precision for each point, depending on the operating system and computer platform. This extremely important when the program must round off numbers during mathematical calculations such as segmenting a circle.StorageThe computer can store thousands of drawings in the physical space that it would take to store hundreds of manual drawings. Also, the computer can search and find a drawing with ease, as long as the operator possesses the correct.CommunicationBecause the computer’s data is stored in an electronic form, it cam be sent to s variety of locations. The first obvious location is the monitor. The computer can display the data on the screen in different forms such as graphics, easily converting the data into readable drawing. The data can also be a plotter to produce the familiar paper drawing, via a direct link to a computer-aided manufacturing (CAD) machine or via telephone to anywhere around the globe. You no longer have to mail drawing, risking damage and loss; they can not be at their destination instantly via the telecommunications network.第一章. 引言目前，容积式压缩机的全球年产量为1.5亿余台，其中大多数被应用于空气动力和制冷系统。
