Computer-aided design (CAD)

Type of physical science: Computation

Field of study: Artificial intelligence

In computer-aided design, engineers use television-type cathode-ray tube displays to work in concert with the computer in originating and modifying the blueprints and production plans for any new components or machines to be made. Rapid advancements in digital computer technology have made the design of large, complex systems routine in cases that were impossible to complete by hand.

Overview

Computer-aided design (CAD) is a technique in which people and computers are blended into a problem-solving team, intimately coupling the best characteristics of each. The result of this combination works well. Furthermore, CAD offers the advantages of integrated teamwork.

In CAD, the computer is not used when the human designer is most effective, and vice versa. Naturally, it is useful to examine the individual characteristics of people and computers in order to identify which processes can best be separately performed by each and where one can aid the other. The capabilities of people and computers can be compared in terms of the method of logic and reasoning, the level of intelligence, the method of information input, the organization of information, the effort involved in organizing information, the storage of detailed information, the tolerance for repetitious and mundane work, the ability to extract significant information, the production of errors, the tolerance for erroneous information, the method of error detection, the method of editing information, and analysis capabilities. In most cases, people and computers are complementary; for some tasks, a person is far superior to the computer, while in others, the computer excels. Therefore, it is the marriage of the characteristics of each that is so important in CAD. The characteristics affect the design of a CAD system in four areas: design construction logic (the method of constructing the design), information handling (the storing and communication of design information), modification (the handling of errors and design changes), and analysis (the examination of the design and factors influencing it).

In design construction logic, the use of experience combined with judgment is a necessary ingredient of the design process. The design construction must therefore be controlled by the designer and aided by the computer. Consequently, designers must have the flexibility to work on various parts of the design at any time and in any sequence and must be able to follow their own intuitive design logic rather than a stylized computer logic. Designers can learn from past designs, while the computer can provide rapid recall of old designs for reference. Thus, one designer can pass on experience to the computer, and other designers in the same project can then have access to this experience.

In information handling, the design solution stage can proceed based on the requirement specification. Similarly, when the design solution is complete, information must be outputted in order for the design to be manufactured. In the conventional design process, information is assimilated by the designer from the input specification. The design solution process then takes place, whereby information is passed from the designer to paper and back again in the form of sketches and calculations. When this process is completed, manufacturing information in the form of drawing and instructions is produced. In the design process using CAD techniques, the design solution stage now includes a flow of information between the designer and computer in the form of graphics and alphanumeric characters. The initial specification is usually input to the computer by the designer, and the computer used is usually a workstation.

During the design process using CAD techniques, the human brain is able to store and retrieve information in an intuitively ordered manner, and the information is not entirely retained as time passes. By contrast, computers have large permanent storage capacities and have the capability to store a large number of complex pictures. Therefore, a pictorial database is an important research topic that studies how to store and retrieve pictorial information effectively and efficiently. Thus, the pictorial database is an important part of CAD, especially for projects involving a large number of complex pictures.

The output of manufacturing information, from the finding solution stage of the design process, usually involves the production of drawings. This is a slow and mundane process when carried out manually but is quite suitable for execution by the computer using a plotter.

Furthermore, drawings can first be displayed on the screen. The designer can modify the drawings on the screen through the use of the computer until the drawings are satisfactory. Then, hard copies can be plotted on paper.

In the modification process, design-descriptive information usually is modified to correct errors, to make design changes, and to produce new designs from previous ones. In terms of design-error detection, the computer has the capability of detecting errors that are systematically definable; whereas designers can exercise an intuitive approach. For example, the computer can calculate the torque capacity of a shaft, while a designer can tell from experience that the calculated shaft is too small or too large, thus concluding that errors must have occurred.

It is generally difficult for a computer to perform design-error correction automatically.

Therefore, it is usually left to the designer to carry out the correction of errors.

Finally, in the analysis process, a computer can perform analytical calculations of a numerical analysis nature, which people usually find time-consuming and tedious. Thus, the numerical analysis involved in the design should be done by the computer, leaving the designer free to make decisions based on the results of the calculations and past experience.

The computer has four main functions: to serve as an extension to the memory of the designer, to enhance the analytical and logical power of the designer, to relieve the designer from routine repetitious tasks, and to organize the design information and data in such a way that the designer will be able to retrieve the information effectively and efficiently.

The designer is left to perform the following activities: to control the design process in terms of information generation and flow; to apply creativity, ingenuity, and experience; and to learn from previous designs.

The main purpose of CAD is to produce a definition of the part or system to be manufactured in the form of a pictorial (geometric) database, or a drawing derived from this database, which establishes the physical configuration of the part or system. On the other hand, the purpose of computer-aided manufacture (CAM) is to translate this definition into tangible hardware based on that database. Furthermore, in CAM, the computer plays many diverse roles in manufacturing functions, such as numerical control, process planning, robotics, and factory management. A CAD system allows a user to interact with a computer through a graphics terminal to define a design configuration, analyze the structure and its mechanical behavior, perform kinematic study and model testing, and produce engineering drawings automatically.

Production can then make use of the geometric description provided by CAD as a starting point in CAM. Integrated CAD/CAM systems will eventually lead to the completely automated factory.

The main benefits and advantages of CAD are that it improves efficiency and productivity and at the same time improves quality. CAD can drastically reduce the number of steps involved in the design process for a particular product and can also make each design step much easier and less tedious for the designer to perform. As a result, an immense increase in the work output of a designer can be made possible and an enormous amount of time-saving can also be achieved between the initial conception of an idea and its final implementation. CAD enables an accurate representation of a design and provides the designer with versatile tools to manipulate it graphically. With this flexibility, the designer can obtain an insight into complex problems arising from the design. This insight will help the designer make better decisions and will reduce the possibility of errors, which may be difficult to spot by the conventional method.

Hence, the designer will be more likely to arrive at an optimal solution that will eventually lead to an improvement in the quality of the final product, making it more competitive in the market.

In addition, all information is stored in the computer instead of on paper. Thus, the transfer of data from department to department is quicker, more reliable, and less redundant.

Applications

Interactive computer graphics has been widely applied in many different areas of science and technology where drawings are a vital element for illustrating new scientific or design ideas. In electronic circuit design, computers have been used to assist engineers in designing circuits for other computers. Several hundred programs are available to help electronic engineers in their circuit design work. An engineer defines the circuit requirements and then the computer develops, analyzes, and evaluates trial designs that may meet the requirements. A trial design may be modified by the engineer as required. The computer then analyzes and evaluates the modification. CAD is also used to plan the layout of integrated circuits, the location of circuit boards in the computer, and the ways in which these boards will be interconnected. An example of a CAD system is SCALD (Structural Computer-Aided Logic Design), which was developed as part of a U.S. Navy effort at Lawrence Livermore Laboratory in California. The goal of the Navy team was to put the supercomputers on one to four chips. While conventional large-scale computer design efforts (also using computers) may take many years to complete, a large Navy processor called the S-1 was completed using SCALD with only two years of design effort.

Computers may do much of the detailed design work in ship design, such as determining the positioning of hull-reinforcing members, determining welding requirements, and generating numerical control programs to control the flame-cutting machines that are used to shape steel plates.

In aircraft design, aircraft designers working at a CRT display can draw the shape of a fuselage and have the computer analyze the physical characteristics of the shape; they can also vary the position, angle, and length of the wings and have the computer report on structural strength and life characteristics. In addition to receiving data from orbiting satellites, computers can be used to design the satellites themselves. The Marisat communications satellite has been used by the U.S. Navy and international commercial maritime operators to provide an instant link between ship and shore. A computer at Hughes Aircraft Company was used for the structural analysis, thermal analysis, and simulation tests of this satellite. For example, the computer helped to determine how structurally sound each design was during the planning stages and what sort of temperature ranges the Marisat could handle.

The Ohio Highway Department created a computer system that enabled design engineers to determine quickly the social and economic impact of proposed road construction.

The system permitted engineers to consider factors such as alternate routes, the amount of earth to be moved or added, the number of citizens that will be forced to relocate, and the construction costs of alternate routes. Aerial photographs were converted by computer into three-dimensional topographical maps. Proposed routes were then plotted on these maps, and a computer was used to evaluate the alternatives.

New building designs can also be analyzed and evaluated by computer. Space planning--that is, the process of locating rooms and/or workstations in a structure--can be facilitated by the computer testing of different approaches to determine the optimum locations for various users of the building. Physical characteristics of planned buildings can also be evaluated. For example, the computer might be asked to work out the acoustical characteristics of a theater that the designer has sketched. The sketch may then be modified by the architect on the basis of the computer analysis or the computer may be instructed to display a theater with more desirable acoustics.

Automobile manufacturers can use computers to evaluate the structural characteristics of alternative designs. Engineers can "assemble" models of the components in a car and then "road test" the proposed car design on a simulated drive route. A chassis cross-member, for example, can be redesigned to reduce weight, and the effects of the change can be determined by a computer program. This design approach significantly reduces the costly and time-consuming process of making and testing a series of prototype parts until the desired results are obtained.

Other areas of computer applications include the study of molecular structures in chemistry, medical research, animation, and aircraft flight simulation. Modern aircraft pilots are trained not only in a real aircraft in the air but also on the ground at the controls of a flight simulator, which resembles an aircraft flight deck in both its functions and its appearance. A flight simulator contains all the usual control instrumentation, together with television screens displaying computer-generated views of the terrain visible on takeoff and touchdown. These views change instantaneously according to the actions taken by the trainee pilot to control the "aircraft" so as to keep a precise impression continuously of the aircraft's motion. The major advantages of using flight simulators for training pilots are the cost savings on fuel, better safety, and the ability to prepare the trainee pilot to fly an aircraft.

Context

In the nineteenth century, the industrial revolution considerably enhanced the physical power of humans. In the twentieth century, a second industrial revolution took place, with computers offering an enhancement of mental capabilities. Beginning in the late 1950's, the applications of computers and computing techniques in all types of engineering disciplines increased dramatically because computers became larger in memory capacity and faster in processing speed. Therefore, more complex problems could be solved and more calculations could be performed in a given time. More important, with the advent of microelectronics, such as "very large-scale integration" (VLSI) technology, computer hardware became less expensive and within the financial reach of most industrial companies. Because of VLSI technology, computer hardware became smaller in size, so its applications were increasingly spread to other areas of industry that previously could not use the computers. As a result of these developments in computer science and computer engineering, CAD was conceived and has been rapidly gaining acceptance in engineering industries for its ability to create major increases in productivity.

CAD is essentially based on a versatile and powerful technique called computer graphics, the creation and manipulation of pictures on a display device with the aid of a computer. Computer graphics originated at the Massachusetts Institute of Technology (MIT) in 1950, when the first computer-driven display, linked to a Whirlwind 1 computer, was used to generate simple pictures. The first important step toward computer graphics came in 1963 when a system called SKETCHPAD was demonstrated at the Lincoln Laboratory of MIT. This system consisted of a cathode-ray tube (CRT) driven by a TX2 computer. The CRT had a keyboard and a light pen. Pictures could be drawn on the screen and then manipulated interactively by the user via the light pen. This demonstration clearly showed that the CRT had the potential to be used as a designer's electronic drawing board with common graphic operations such as scaling, translation, rotation, animation, and simulation automatically performed at the "push of a button." At this time, SKETCHPADs were very expensive; therefore, they were adopted only in such major industries as the aircraft and automotive industries where their use in design justified the high capital costs.

A crucial factor preventing computer graphics from being generally applied to engineering industries was that there was a lack of appropriate graphics and application software to run on these systems. Nevertheless, a computer-based design system was clearly emerging.

Since these pioneering developments in computer graphics, new and improved hardware--which is faster in processing speed, larger in memory, cheaper in cost, and smaller in size--has become widely available. Sophisticated software techniques and packages have also gradually been developed. Consequently, the application of CAD in industry has grown rapidly. Initially, CAD systems primarily were automated drafting stations in which computer-controlled plotters produced engineering drawings. The systems were later linked to graphic display terminals, where geometric models describing part dimensions were created, and the resulting database in the computer was then used to produce drawings. Today, CAD systems can do much more than mere drafting. Some systems have analytical capabilities that allow parts to be evaluated with techniques such as the finite element method. There are also kinematic analysis programs that enable the motion of mechanisms to be studied. In addition, CAD systems include testing techniques to perform model analysis on structures and to evaluate their response to pinpoint any possible defects.

Principal terms

CLIPPING: a process that divides each element of a picture into its visible and invisible portions, allowing the invisible portion to be discarded

COMPUTER GRAPHICS: the creation, storage; and manipulation of models of objects and their pictures via computer

DISPLAY TUBE: an electronic device which can present information visually; display screens are used in computers and television sets, which have cathode-ray display tubes (CRTs)

INTERACTIVE COMPUTER GRAPHICS: computer graphics with which a user dynamically controls the picture's content, format, size, or colors on a display surface by means of interaction devices such as a keyboard, lever, joystick, or mouse

SIMULATION AND ANIMATION: computer-produced animated films of the time-varying behavior of real or simulated objects

STORAGE TUBE: a display tube incorporating a physical system for storing the picture automatically at the front of the tube

VERY LARGE-SCALE INTEGRATION (VLSI) CHIPS: integrated circuit chips that consists of more than five thousand gates (transistors)

WORKSTATION: an area with equipment that consists of a microcomputer system, a graphics and alphanumeric display unit, a printer, a plotter, a keyboard, a hard disk, a mouse, and other interactive and storage devices

Bibliography

Barr, P. C., R. L. Krimper, M. R. Lazear, and C. Stammen. CAD: PRINCIPLES AND APPLICATIONS. Englewood Cliffs, N.J.: Prentice-Hall, 1985. This book argues that there is growing need to provide a basis and structure for CAD. Attempts to categorize CAD into its principal parts from a computer graphics point of view and from an applications point of view.

Besant, C. B., and C. W. K. Lui. COMPUTER-AIDED DESIGN AND MANUFACTURE. 3d ed. Chichester, England: Ellis Horwood, 1986. This book contributes valuable knowledge and know-how to the CAD industry. The practical aspects of CAD/CAM are emphasized for those who have no in-depth knowledge of computing.

Encarnacao, Jose, and E. G. Schlechtendahl. COMPUTER AIDED DESIGN: FUNDAMENTALS AND SYSTEM ARCHITECTURES. New York: Springer-Verlag, 1983. This book introduces the fundamentals of CAD. Design is interpreted as an interactive process involving specification, synthesis, analysis, and evaluation, with CAD as a tool to provide computer assistance in all these phases. Economic, ergonomic, and social aspects are considered as well.

Gardan, Yvon, and Michel Lucas. INTERACTIVE GRAPHICS IN CAD. Translated by Meg Tombs. London: Kogan Page, 1984. This book focuses on the discussion that because of the developments in hardware and software, a designer is now able to make decisions based on the information presented with the help of interactive, graphics techniques.

Hyman, Anthony. THE COMPUTER IN DESIGN. London: Studio Vista, 1973. Geared for a wide audience. It is easy to understand, no mathematics, and many interesting computer-generated pictures are included.

Lee, Edward T. "Similarity Retrieval for Pictorial Databases." In MANAGEMENT AND OFFICE INFORMATION SYSTEMS, edited by S. K. Chang. New York: Plenum Press, 1984. Chapter 15 discusses pictures and figures that play fundamental and important roles in computer-aided design. Similarity retrieval, picture representation, picture processing, picture storage, and pictorial database organization are thoroughly discussed.

Majchrzak, Ann, et al. HUMAN ASPECTS OF COMPUTER-AIDED DESIGN. London: Taylor & Francis, 1987. Presents an integrated discussion of the technical, operator, and management factors that arise when introducing CAD systems.

Sata, Toshio, and Ernest Warman, eds. MAN-MACHINE COMMUNICATION IN CAD/CAM. New York: Elsevier, 1981. Many models of man-machine interaction in CAD/CAM are proposed. Applications and environmental influence are emphasized. Stimulating reading.

Vlietstra, J., and R. F. Wielinga, eds. COMPUTER-AIDED DESIGN. Amsterdam: North-Holland, 1973. Discusses the definition and analysis of CAD, systems software and applications, languages in which CAD systems are written, input/output organization, the economics of CAD, hardware configuration options, and analysis of particular CAD applications.

Zobrist, George W., ed. PROGRESS IN COMPUTER-AIDED VLSI DESIGN. Vol. 1, TOOLS. Norwood, N.J.: Ablex, 1989. Various tools and environments for computer-aided VLSI design, such as integration of VLSI CAD tools based on cell-objects, a design environment for development of microprogrammed processors, and a VLSI module layout generator are described in detail. Furthermore, the concept and design of an extensible multilevel logic simulator and demand-driven simulation are discussed.