How Cad Developed
Early CAD systems like SKETCHPAD and the others which followed needed the power of mainframe computers-expensive units that might cost up to a million dollars. Because of the expense, and the necessity to invest in a large mainframe computer, most of the first major CAD users were manufacturers in such fields as aerospace, automobiles, and electronics. They saw the benefits of CAD use in increased productivity and better design, and had the capital to make the necessary investment. In addition, their needs were extensive enough to justify the cost economically.
During the 1960s and 1970s, the cost of computing power decreased, as electronics manufacturers developed the ability to put more circuitry into an integrated circuit chip. Mini- and microcomputers began to be developed, and display screens became cheaper. Consequently, CAD became more popular-even commonplace-for product design, drafting, and related engineering analysis.
In the early 1980s, taking advantage of the phenomenal rise in popularity of the personal computer, CAD systems began to be developed that could run on desktop computers, while still connecting to other systems.
How Cad Works
The key to CAD is computer graphics-the use of the computer to display graphic images. The images are based on mathematical coordinates, which are just like points drawn on a graph for a geometry class. This descriptive information exists in the computer as digital electronic data. The computer makes it possible to store, retrieve, transmit, and process this data quickly and accurately, and the monitor, or display screen, shows the data as drawings.
"In the CAD world, we take a pencil out of the hand of the operator and place it into the hand of the computer," explains Marmon Pine, president of CAD Design Systems, Inc. "We do exactly the same things designers and drafting people have always done with a pencil and paper. But now, we communicate with the computer, so that the messages we used to send to our fingers to direct the pencil are now sent in computer language."
At the heart of every CAD system is a computer, which works with the data the CAD operator supplies. There are several methods for giving data to the computer. One technique uses a mouse-a small, hand-held device that's moved either mechanically or optically across an array of lines on a small pad. As the mouse "runs" across the lines, it counts them, sending information to the computer about how fast it is moving and how many times it has moved. The computer uses this information to move an indicator (typically a cursor, cross-hair, or indicator circle) around the screen.
Another type of input device is called a digitizing tablet, or a stylus and tablet. The stylus is a pencil-like device that the CAD operator moves around on a tablet. The tablet varies in size. Typically, it's 11 inches x 11 inches, but it can be as large as 4 to 5 feet wide. The position of the stylus is electronically detected and sent to the computer. Like the information sent by the mouse, the information sent by the stylus is used to move the cursor, cross-hair, or indicator circle around on the screen. The computer translates the position of the stylus into a series of points that have precise coordinates.
It's possible to talk to the computer verbally, but this system is seldom used in modern CAD, since it's not as quick or efficient as the mouse or digitizing pad.
Of course, the CAD operator can type on a keyboard, sending information about the coordinates and typing in commands, just as he or she would type a letter.
Another device used to input data to the computer is a touch screen-a sensing device on the screen of the monitor (or display screen). As the CAD operator moves a finger around the screen of the monitor, the sensing device tells the computer where the finger is. It's possible to point directly to things on the screen, much as you might touch the controls of a microwave oven.
At a relatively high level of sophistication, it's possible to send information from another computer to the computer the CAD operator is working on, connecting the computers directly or using a modem- a device which enables computer equipment to talk to other equipment, often through phone lines. Such information is "read" by the CAD operator's computer just as if the operator had put it into the computer with a mouse, stylus/digitizing pad, or other input device.
The computer gives back the information it's received, using various forms of output devices to let the CAD operator see how the computer is manipulating the data. Usually the CAD operator sits in front of a monitor, which looks somewhat like a television screen. Another common device is a graphics plotter. A simple form of graphics plotter may be a normal dot matrix printer, operating in the "graphics mode," drawing points on a piece of paper, one point at a time. A more complex plotter might use a technical pen which follows the computer's commands and draws a picture on a piece of paper. Sometimes the paper is stationary, and the pen moves in both directions. At other times, depending upon the particular plotter, the paper may move in one direction and the pen may move in another.
Small plotters are standard sheet-carrying devices, usually 8 inches x 11 inches. Large, computer-driven plotters can even plot on 50 inch by 60 inch sheets of paper.