The flatbed plotter looks somewhat like a pool table (Figure 11.4). It has a large flat surface for holding the drawing medium. Flatbed plotters typically have table sizes and, hence, drawing sizes ranging from 30" x 30" to over 60" x 80". The drawing medium is held in place through minute holes in the table surface by a vacuum system. The flatbed plotter is popular in engineering and architectural applications where large drawings must be made. The flatbed plotter also allows a wide variety of drawing media (i.e. vellum, tracings, transparencies, etc.) to be used. Across the width of the plotter is a bridge. Thebridge can move along the length of the plotting surface (the x-axis). This movement is controlled by a special type of motor called a stepping motor. The stepping motor moves a fixed increment every time it is energized. This fixed increment regulates the resolution of any line that can be drawn. Typical flatbed resolutions range from .01" to .0001". Also important is the rate at which the motors can be incremented, or the speed of the plotting. Speeds of 1.5 to 40 inches per second are common. The bridge has a carriage containing one or more pens attached to it. The carriage can move along the bridge (the y-axis) and it's movement is controlled by another stepping motor.
The drum plotter is similar in operation to the flatbed plotter. It has a bridge with a carriage pen riding on it providing y-axis movement. Typical widths of drum plotters range from twelve to thirty inches. The main difference between the drum and flatbed is that the drawing surface is wrapped around the perimeter of a drum (Figure 11.5). The drum is rotated by a stepping motor providing x-axis movement. Since the drawing medium can be wrapped around the drum many times, a very long drawing is possible. Some drum plotters state the capability of making drawings 180 feet long!
Drum plotters are generally smaller than flatbed plotters. Their drum based designs make them more compact, more portable and less expensive. They usually have lower resolutions that flatbeds.
The operation of an incremental pen plotter is very simple; there are only three basic actions that the user can take: pen selection, pen up/down, and pen movement. Through pen selection commands, the user can specify which of the pens in a multiple pen carriage he wishes to use next. The pens can be of different types, line thickness, ink color or type, etc. Upon selection of a new pen, that pen is moved to the current drawing position automatically. A pen can also be moved "up" or "down". When a pen is down, it is in contact with the drawing surface. Any movement of the pen will cause a line to be drawn. If a pen is in the up position, pen movements merely change the location of the pen with respect to the plotting surface and nothing is drawn. Finally, the pen can be moved across the plotting surface.
Incremental pen plotters allow only eight possible directions of motion as shown in Figure 11.6. The pen can be moved along the x- or y- axes by activating either the x- or y- axis stepping motor in the positive or negative direction. Diagonal movements are produced by simultaneously energizing the x- and y- axis stepping motors. Each step when the pen is down produces a straight line equal to the resolution of the plotting device (perhaps as small as .0001). Long lines are thus composed of thousands of these steps or increments. A virtually unlimited number of graphic images can be produced by specifying these simple actions in the appropriate sequences.
http://ltl13.exp.sis.pitt. edu/Website/Courses/Graphics/b ook/Chapter11.htm
Answered by
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2:58 PM on August 30, 2008
