Plotter - History

History

A number of printer control languages were created to operate pen plotters, and transmit commands like "lift pen from paper", "place pen on paper", or "draw a line from here to here". Three common ASCII-based plotter control languages are Hewlett-Packard's HP-GL, its successor HP-GL/2 and Houston Instruments DMPL. Here is a simple HP-GL script drawing a line :

SP1; PA500,500; PD; PR0,1000; PU; SP;

This program instructs the plotter, in order, to take the first pen, to go to coordinates X=500, Y=500 on the paper sheet, to lower the pen against the paper, to move 1000 units in the Y direction (thus drawing a vertical line), to lift the pen and finally to put it back in its stall.

Programmers using FORTRAN or BASIC generally did not program these directly, but used software packages, such as the Calcomp library, or device independent graphics packages, such as Hewlett-Packard's AGL libraries or BASIC extensions or high end packages such as DISSPLA. These would establish scaling factors from world coordinates to device coordinates, and translate to the low level device commands. For example, to plot X*X in HP 9830 BASIC, the program would be

10 SCALE -1,1,1,1 20 FOR X =-1 to 1 STEP 0.1 30 PLOT X, X*X 40 NEXT X 50 PEN 60 END

Early pen plotters, e.g., the Calcomp 565 of 1959, worked by placing the paper over a roller that moved the paper back and forth for X motion, while the pen moved back and forth on a track for Y motion. The paper was supplied in roll form and had perforations along both edges that were engaged by sprockets on the rollers.

Another approach, e.g. Computervision's Interact I, involved attaching ball-point pens to drafting pantographs and driving the machines with motors controlled by the computer. This had the disadvantage of being somewhat slow to move, as well as requiring floor space equal to the size of the paper, but could double as a digitizer. A later change was the addition of an electrically controlled clamp to hold the pens, which allowed them to be changed, and thus create multi-colored output.

Hewlett Packard and Tektronix produced small, desktop-sized flatbed plotters in the late 1960s and 1970s. The pens were mounted on a traveling bar, whereby the y-axis was represented by motion up and down the length of the bar and the x-axis was represented by motion of the bar back and forth across the plotting table. Due to the mass of the bar, these plotters operated relatively slowly.

In the 1980s, the small and lightweight HP 7470 introduced the "grit wheel" mechanism, eliminating the need for perforations along the edges, unlike the Calcomp plotters two decades earlier. The grit wheels at opposite edges of the sheet press against resilient urethane-coated rollers and form tiny indentations in the sheet. As the sheet is moved back and forth, the grit wheels keep the sheet in proper registration due to the grit particles falling into the earlier indentations, much like the teeth of two gears meshing. The pen is mounted on a carriage that moves back and forth in a line between the grit wheels, representing the orthogonal axis. These smaller "home-use" plotters became popular for desktop business graphics and in engineering laboratories, but their low speed meant they were not useful for general printing purposes, and different conventional printer would be required for those jobs. One category, introduced by Hewlett Packard's MultiPlot for the HP 2647, was the "word chart", which used the plotter to draw large letters on a transparency. This was the forerunner of the modern Powerpoint chart. With the widespread availability of high-resolution inkjet and laser printers, inexpensive memory and computers fast enough to rasterize color images, pen plotters have all but disappeared. However, the grit wheel mechanism is still found in inkjet-based, large format engineering plotters.

Plotters were also used in the Create-A-Card kiosks that were available for a while in the greeting card area of supermarkets that used the HP 7475 six-pen plotter.

Plotters are used primarily in technical drawing and CAD applications, where they have the advantage of working on very large paper sizes while maintaining high resolution. Another use has been found by replacing the pen with a cutter, and in this form plotters can be found in many garment and sign shops.

If a plotter was commanded to use different colors it had to replace the pen and select the wanted color and/or width.

A niche application of plotters is in creating tactile images for visually handicapped people on special thermal cell paper.

Unlike other printer types, pen plotter speed is measured by pen speed and acceleration rate, instead of by page printing speed. A pen plotter's speed is primarily limited by the type of pen used, so the choice of pen is a key factor in pen plotter output speed. Indeed, most modern pen plotters have commands to control slewing speed, depending on the type of pen currently in use.

There are many types of plotter pen, some of which are no longer mass produced. Technical pen tips are often used, many of which can be renewed using parts and supplies for manual drafting pens. Early HP flatbed and grit wheel plotters used small, proprietary fiber-tipped or plastic nib disposable pens.

One type of plotter pen uses a cellulose fiber rod inserted through a circular foam tube saturated with ink, with the end of the rod sharpened into a conical tip. As the pen moves across the paper surface, capillary wicking draws the ink from the foam, down the rod, and onto the paper. As the ink supply in the foam is depleted, the migration of ink to the tip begins to slow down, resulting in faint lines. Slowing the plotting speed will allow the lines drawn by a worn-out pen to remain dark, but the fading will continue until the foam is completely depleted. Also, as the fiber tip pen is used, the tip slowly wears away on the plotting medium, producing a progressively wider, smudged line.

Ball-point plotter pens with refillable clear plastic ink reservoirs are available. They do not have the fading or wear effects of fiber pens, but are generally more expensive and uncommon. Also, conventional ball-point pens can be modified to work in most pen plotters.