CAD/CAM

Hardware in CADGraphics Display

The graphics display of a workstation is considered its most important component because the quality of the displayed image influence the perception of generated designs on the CAD/CAM system.

The graphics display enables the user to communicate with the displayed image by adding, deleting or moving graphics entities on the display screen.

Various Display Technologies IncludeCathode ray tube (CRT)Liquid crystal display (LCD)PlasmaLight emitting diodes (LED)

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Cathode Ray Tube (CRT)It is the most dominating and has produced a wide range of extremely effective graphics displays. The figure shows a schematic diagram of a typical CRT. The operation of the CRT is based on the concept of energizing an electron beamthat strikes the phosphor coating at very high speed. The energy transfer from the electron to the phosphor due to the impact causes it to illuminate and glow.The electrons are generated via the electron gun that contains the cathode and are focused into a beam via the focusing unit shown

By controlling the beam direction and intensity in a way related to the graphics information generated in the computer, meaningful and desired graphics can be displayed on the screen.

The deflection system of the CRT controls the x and y, or the horizontal and vertical, positions of the beam which in turn are related to the graphics information through the display controller, which typically sits between the computer and CRT.

There are two basic techniques used for generating the image on the CRT screen. They are:

1- Stroke writing2- Raster scan

CRT

Stroke writing

The other names for the stroke-writing technique include line drawing, random position and directed beam.

The stroke writing system uses an electron beam which operates like a pencil to create a line image on the CRT screen. The image is constructed out of a sequence of straight line segments.

Each line segment is drawn on the screen by directing the beam to move from one point on the screen to the next, where each point is defined by its x and y coordinates.

Although the procedure results in images composed of only straight lines, smooth curves can be approximated by making the connecting line segments short enough.

Stroke Writing

In this approach, the viewing screen is divided into a large number of discrete phosphor picture element, called pixels. The matrix of pixels constitutes the raster. The number of separate pixels in the raster display might typically range from 256 * 256 (a total of 65,000) to 1024 * 1024 (a total over 1,000,000 points). Each pixel on the screen can be made to glow with a different brightness. Colour screens provide for the pixels to have different colours as well as brightness.

During operation , an electron beam creates the image by sweeping along a horizontal line on the screen from left to right and energizing the pixels in that line during the sweep. When the sweep of one line is completed, the electron beam moves to the next line below and proceeds in fixed pattern as indicated in figure. After sweeping the entire screen, the process is repeated at a rate of 30 to 60 entire scans of the screen per second.

Raster Scan

Raster Scan

Raster Scan

Plasma Panel DisplayThough the CRT display is highly refined, it is sometimes not suitable particularly for portable applications because of the depth that is necessary for the cathode ray tube. In such situations, the plasma panel has been found to be useful though in a limited sense. Plasma uses a neon gas in a glass envelope with electrodes to display the image. Though they are small and flat, they consume a large amount of power and also the resolution is not very good.

These rare gases actually have a life and fade over time. The life of these phosphors is around 25,000 to 30,000 hours (not replaceable)

Liquid Crystal Display (LCD)

Plotters

There are various types of output devices used in conjunction with a computer aided design system. These output devices include:

Pen plottersHard copy unitsElectrostatic plottersComputer output to microfilm units

PlottersPen Plotters: The accuracy and quality of the hard copy plot produced by a pen plotter is considerably greater than the apparent accuracy and quality of the corresponding image on the CRT screen.

In this case of the CRT image, the quality of the picture is degraded because of the lack of resolution and because of losses in the digital to analog conversion through the display generator. On the other hand the precision pen plotter is capable of achieving a hard-copy drawing whose accuracy is nearly consistent with the digital definitions in the CAD data base.

PlottersDrum Plotter: It is generally the least expensive. It uses a round drum, usually mounted horizontally, and a slide which can be moved along the track mounted axially with respect to the drum. The paper is attached to the drum and the pen is mounted on the slide. The relative motion between pen and paper is achieved by coordinating the rotation of the drum with the motion of the slides. The drum plotter is fast and it can make drawings of virtually unlimited length. The width however, is limited by the length of the drum. These lengths typically range between 216mm and 1067mm

Plotters

Flat Bed PlotterIt is more expensive It uses a flat drawing surface to which the paper is attached. On some models, the surface is horizontal, while other models use a drawing surface which is mounted in nearly vertical orientation to conserve floor space.

Hard-copy units are relatively fast but their accuracy and resolution are poor. Pen plotters are highly accurate but plotting time can take many minutes (up to a half hour or longer for complicated drawings).

The electrostatic plotter offers a compromise between these two types in terms of speed and accuracy. It is almost as fast as the hard copy unit and almost as accurate as the pen plotter

Plotters

PlotterThe electrostatic copier consists of a series of wire styli mounted on a bar which spans the width of the charge sensitive paper. The styli have a density of up to 200 per liner inch. The paper is gradually moved past the bar and certain styli are activated to place dots on the paper. By coordinating the generation of the dots with paper travel, the image is progressively transferred from the data base into hard-copy form. The dots overlap each other slightly to achieve continuity. For example, a series of adjacent dots gives the appearance of a continuous line.

The limitation of the electrostatic plotter is that it must be in the raster format (in the same format used to derive the raster-type CRT) in order to be readily convert into hard copy using the electrostatic method. If the data are not in raster format, some type of conversion is required to change them into the required format. The conversion mechanism is usually based on the combination of software and hardware

The advantage of the electrostatic plotter which is shared with the drum type pen plotter is that length of the paper is virtually unlimited. Another advantage is hat the electrostatic plotter can be utilized as a high speed line printer, capable of up to 1200 lines of text per minute.

Dot matrix printer

Computer Output to MicrofilmCOM units reproduce the drawings on microfilms rather than as full size engineering drawings. It is used to be an expensive equipment. However, there are several important advantages.

Storage benefits

Speed

Disadvantages: Cannot write notes, enlargements are not of high quality

Hardcopy UnitsEach workstation in a CAD/CAM system should have its own hardcopy unit to produce quick low-quality copies of screen images To provide a copy of user input and system output information which is always useful in tracking errors and mistakes.

Black and white PrintersDot matrix printer (inexpensive but slow, 75dpi)Laser printers (expensive, faster, 600 dpi)A laser source is turned on and off. The laser beam bounces off a spinning mirror and exposes a photosensitive drum covered with an even charge. When the drum is exposed to the beam, the charge on the drum surface changes which attracts the toner. The toner is then transferred to the paper to produce the image

Colour PrintersImpact, photographic, electrophotographic, electrostatic, thermal-transfer and inkjet printers

Hardware integration and networking

The integration and networking between the various components and peripherals of a system ensures the success of CAD/CAM system

Hardware integration and networkingStorage of design and manufacturing dataCommunication between users and devices (LAN, specialized LAN cards, RS 232)TCP/IP protocolsCommunication in CAM