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Plotting on the straight and narrow.

Many people, having discovered word processing, would not dream of using a typewriter to compose a letter. Similarly, many engineers who have discovered computer-aided design shun manual drafting techniques. At some point, however, electronic files have to be reproduced in hard copy. The device that creates the output largely determines the quality of the copy.

Output devices for engineering applications are generally referred to as plotters. While some types of plotters, such as laser and ink-jet plotters, are similar to the business printers from which they were derived, they have different requirements for data input and output. Plotters have to accept vector CAD files or possess some scheme for converting vector files into raster files. Printers generally handle text files or graphics files such as those in PostScript. Output from plotters can be A-size (8.5 by 11 inches) through E-size (34 by 44 inches) or larger. Printers are usually built to handle A- or B-size (11- by 17-inch) media. Furthermore, high-resolution final plots of 400 dots per inch (dpi) or more are often stored as the permanent record of a job and have to be of exceptional quality.

The Plot Thickens

Pen-plotters are ubiquitous denizens of most engineering offices that have implemented CAD. According to a recent study conducted by Answers Research (Solana Beach, Calif.), a market research and consulting firm, over 65 percent of respondents use pen-plotters for final output and about 55 percent use them routinely for check plots. The pens produce high-quality output, and the use of colored pens allows multicolored drawings.

Pen-plotters have been around for three decades. In that time, they have changed very little conceptually. In addition to being highly accurate and possessing color capabilities, pen-plotters tend to be among the most inexpensive of CAD output devices. The HP DraftPro series of pen-plotters from Hewlett-Packard Co. (Palo Alto, Calif.) starts at $3795 for a version that handles C- and D-size drawings only. On the high end, the DPX-4600 from Roland Digital Group (Irvine, Calif.), a subsidiary of Roland Corp. (Hamamatsu, Japan), handles A- through E-size drawings and draws with a mechanical resolution of 0.0001 inch. It costs $15,000. Intermediate-level pen-plotters are available in the $5000-to-$10,000 price range.

Pen-plotters receive vector data as input directly from CAD packages through one of several file formats including Hewlett-Packard Graphics Language (HPGL), HPGL/2, and DMPL from Houston Instrument (Austin, Tex.). Because they use vector data, pen-plotters read files that are much smaller than raster-based output devices such as laser printers. The smaller the file, the sooner the computer sending the information to the plotter can be freed to do other tasks. Vector files also allow users to turn off CAD layers they do not want to plot and change to the scale of layers they do want to see in hard copy.

On the downside, pen-plotters have a relatively low output speed compared to nonmechanical devices such as electrostatic, thermal, and ink-jet plotters. An E-size monochrome plot of moderate complexity can take up to an hour to complete on a pen-plotter. Time increases with added color and complexity. Dealing with pen-plotter consumables such as media and pens is also necessary.

The Mighty Pen

Pen-plotters generally accept bond paper, vellum, tracing paper, plastic films, and some specialty media including double-matte polyester film for double-sided plotting or media with cotton fibers that resist yellowing for archival use. Depending on the make and model, a pen-plotter will accept a single sheet fed manually into the system or will feed itself from a roll of media. Roll-fed plotters are able to make a number of plots in an unattended mode for as long as the roll lasts. A standard 150-foot roll of paper will generally produce between 30 and 35 E-size plots. Roll-fed plotters can also produce oversized drawings--100 feet or longer--on a single sheet. Some roll-fed plotters are equipped with automatic cutting devices that cut sheets from the roll into the proper sizes and stack them after plotting.

Like manual pens, plotting pens also run out of ink. The performance of each pen varies with the supplier. Moreover, because the mechanical arm of a basic pen-plotter does not discern whether its pens are filled with ink or not, often it goes through the motions of following a CAD file's vectors with an empty pen as precisely as with a full one. "It's like going for a long drive," said Ruth Rhoades, product manager for plotters at Houston Instrument. "You should make sure you have a full tank before starting out."

Features have been added to some models in order to increase the ability of pen-plotters to operate unattended. Pen grouping, for example, is a common technique to avoid running out of ink. Under pen grouping, the pen-plotter will accept the assignment of more than one pen to a particular color. Plotters generally hold up to eight pens. If more than one pen has been grouped under a given color, the plotter will alternate from pen to pen within a grouping at certain intervals. This way, all pens of the same color are used evenly, reducing the probability that any one pen will run dry.

Most manufacturers of pen-plotters include pen grouping in all but the most entry-level models. The Artisan Plus series of low-end pen-plotters from CalComp (Anaheim, Calif.) mounts pens in an eight-place turret. When a new pen is needed, either for a color change or to rotate pens in a group, the pen holder moves to the turret and exchanges pens. The replaced pen is immediately capped to prevent drying. Sensors in the pen holder detect pen force, velocity, and acceleration and make adjustments to maintain the consistency of the drawing. The Houston Instrument DMP-160 series of plotters allows pens to be clustered in groups of two, four, or eight. Each pen is used for 100 meters of drawing before the plotter cycles to the next pen in the group to ensure that the pens wear evenly.

Hewlett-Packard has developed an optical line sensor for its DraftMaster series of pen-plotters that looks at the line being drawn to see if a pen needs to be replaced. The SurePlot drawing system uses a sensing LED to make sure that the contrast between the line and the medium is sufficient. The system can be programmed to check the line periodically, for example, every three meters. If the line is unacceptably light, the plotter goes back to the last verified point and redraws the portion with a new pen from the same group. The HP SurePlot system can also correct for pen clogging, a plotter malady resulting from the accumulation of paper fibers.

Mutoh America Inc. (Mt. Prospect, Ill.), a subsidiary of Mutoh Industries Inc. (Tokyo), addressed the problem of ink mileage by designing its F-920 series and XP-500 series plotters to use pencils as well as pens. According to Mutoh, which pioneered the use of pencil plotting in the United States, pencils are widely used in Japan. The plotters are equipped with an automatic lead loader that holds up to 260 leads of various colors, hardnesses, and thicknesses. The lead loader continuously feeds pencil leads into the drawing cartridges. In addition to being more reliable than pens, since they do not dry up or leak, pencil leads cost about 15 cents apiece compared to between $2 and $20 for pens.

High Output, Big Bucks

Electrostatic plotters are high-resolution high-output devices that solve many of the shortfalls of pen-plotters, particularly in the area of speed. The Versatec 8600E series electrostatic plotters from Xerox Engineering Systems (Santa Clara, Calif.), are monochrome output devices that can plot an E-size drawing with a resolution of 400 dots per inch in about one minute. While this compares favorably to the hour it takes for a pen-plotter to do the same job, speed does not come cheaply. The Versatec 8636E-HR roll-fed electrostatic plotter costs $28,000. The basic Xerox color electrostatic plotter, the Versatec CADcolor system, which can produce color output at 300 dpi, costs $42,000. Most electrostatic plotters on the market fall in the $25,000-to-$50,000 price range.

Electrostatic plotters require expensive and environmentally hazardous chemicals to produce images on special electrostatic media. Opaque, translucent, vellum, and clear film electrostatic media are available. Pigment particles for color plots and carbon particles for monochrome plots are held suspended in a paraffin-based toner during the imaging process. The toner is then washed away by a solvent leaving the pigments or carbon. The plotter's toner heads require almost daily cleaning and attention.

The toners and replenishers used by electrostatic plotters are petroleum-based hydrocarbons and are classified as hazardous materials by the Environmental Protection Agency because they have flash points between 100[degrees] and 140[degrees]F. Used toners and replenishers are classified as hazardous waste and there are laws regulating their disposal. Firms that use electrostatic plotters are required to get an EPA small-quantity hazardous waste generator identification number.

Trey Welliver, environmental administrator for CalComp, recommends that users of electrostatic plotters contract waste disposal firms that specialize in removing flammable materials to handle their plotter waste chemicals. The EPA generator ID number will be required when hiring such a firm. Welliver said that waste materials generally have to be picked up from the job site once or twice a year.

In addition to the issue of disposal is the issue of costs for paper and film, which can add up quickly. A roll of 36-inch paper for a pen-plotter is priced at approximately 10 cents per foot. However, 36-inch polyester film costs approximately $3 per foot. A roll of 36-inch electrostatic paper costs about 20 cents per foot, and 36-inch electrostatic flim costs about $3 per foot.

Special subsystems have been developed to make electrostatic plotters more conducive to office environments. The CalComp 68000 series color electrostatic plotters employ a closed-loop toner application and vacuum process to minimize the threat posed by harmful chemicals. The plotter uses premixed toner that is recycled to the source bottle rather than dumped into a waste bottle.

Hot Plotting

One method for increasing the throughput of plots that does not require the fuss and muss of pen or toner is thermal-imaging technology. Thermal plotters, also called direct-image plotters, produce images by heating special paper with a thermal printhead. The places where the paper is exposed to the heated printhead turn black. High-resolution thermal plotters capable of producing drawings with resolutions up to 406 dpi are available. Because the systems do not plot mechanically, they have throughput speeds similar to electrostatic plotters. The cost of this special 36-inch thermal premium-grade paper is about 20 cents per foot; 36-inch thermal film can cost up to $5 per foot.

In general, thermal plotters are monochrome devices that require special thermal paper or transparent thermal film. They can produce images with complex shading by changing the density of dots in filled-in areas. The GS-600 series of thermal plotters from Oyo Geospace (Houston) will produce photo-realistic images with up to 16 shades of gray. CalComp has developed a two-color thermal paper for its DrawingMaster Plus direct-imaging plotter that creates images in black and red, depending on the temperature of the printhead.

While thermal plotters have a higher throughput than pen-plotters and have filled-area capabilities, they are limited to output in one or two colors and are more expensive. The Oyo Geospace GS-612, which uses 12-inch media, costs $9600. The GS-624, using 24-inch media, costs $19,600, and the GS-636, using 36-inch media, costs $26,000. CalComp's DrawingMaster Plus two-color plotter costs $23,000. Roland Digital's LTX-320 thermal plotter, which uses 24-inch media, costs $10,000, and the company's LTX-420 plotter for 36-inch media costs $13,000.

Another thermal technology, called thermal transfer, is used to produce color output. Unlike direct imaging, thermal-transfer plotters melt pigments from a thermally sensitive ribbon onto standard bond paper or transparency film. The nature of the technology makes thermal-transfer plotters more suitable for filled-area drawings than for precise line drawings. Thus they are useful for presentation graphics. The ColorMaster Plus VRC thermal-tranfer plotters from CalComp will convert vector files into raster files for CAD applications such as rendering solids models.

Ink-Jets Take Off

Although the Answers Research survey found that ink-jet plotters account for only about 1 percent of the final plots and 2 percent of the check plots currently produced, several companies are exploring the potential of bringing this well-established document-output technology to CAE. According to International Data Corp. (Cambridge, Mass.), there is a "high degree" of dissatisfaction among workstation users in pen-plotter throughput. IDC and Answers Research agree that there is a need in the marketplace for traditional printing devices that can handle large formats (B- to E-size) and media.

Steve Sakumoto, Hewlett-Packard's manager for large-format plotters, said his company's research has shown that 80 percent of all plotter output is monochrome only. Furthermore, HP said that 80 to 90 percent of plotter output is line drawings as opposed to filled-area drawings. Output involving shading is usually reserved for visualization and presentation graphics, which are typically thrown out after they are used. Output for mapping and geographic information system applications are an exception to this rule; they require gray-scale and color shading with photo-realistic quality.

"A lot of companies have already implemented CAD and have a pen-plotter," said Sakumoto. "We're hoping ink-jet plotters appeal to companies that need to increase the number of drawings they output."

Last November, Hewlett-Packard introduced its DesignJet ink-jet plotter for CAE applications. Using the same physical ink-jet tehnnology as its DeskJet printers, the DesignJet is able to handle A- through E-size bond paper, vellum, tracing paper, and plastic film. The system can produce an E-size drawing with a resolution of 300 dpi in about six minutes.

The DesignJet will accept vector files in HPGL/2 format as well as raster files in a Hewlett-Packard Raster Transfer Language (HP/RTL) format. The plotter has an internal rasterizer based on Intel's 32-bit i960 RISC processor. The DesignJet will accept sheet or roll media and is equipped with a cut and stack system to accommodate up to 20 drawings. The basic system costs $11,000. The DesignJet is a monochrome output device, however, Sakumoto said color inkjet technology, similar to the HP PaintJet printer, may be used on future ink-jet plotters.

The potential of the ink-jet plotter market is drawing both new and established manufacturers of output devices. This month, Pacific Data Products (San Diego) will unveil an entry-level ink-jet plotter that is capable of handling A- through C-size media. The ProTracer ink-jet, based on a Canon ink-jet engine, can output a C-size drawing with a resolution of 360 dpi in about five minutes. Pacific Data is positioning the ProtTracer as a "personal plotter" for engineers who need to produce their own check plots quickly. The basic ProTracer model, priced at $1500, is equipped with an AutoDesk interface driver and handles AutoCAD files exclusively. An HPGL emulation card and a 2-megabyte memory expansion board can be added for $700. A PostScript language emulation card and an 8-megabyte memory expansion board can be added for $1400. ProTracer accepts single-sheet and cut-sheet medial only.

And Still Champion

Most output devices that were developed after the pen-plotter, except for high-end systems like color electrostatic plotters, were designed to do some of the work of the pen-plotter without replacing it. Ink-jet and thermal plotters produce drawings faster than pen-plotters but in return generally give up color or resolution or both. Color electrostatic plotters are priced beyond the reach of most average engineering firms.

Pen-plotters are widely regarded as the most economical way to provide good-quality color output where quantity is not a consideration. These plotters can handle all types of media and there are models available that handle all standard format sizes. Pen-plotters do not need vector-to-raster conversion capabilities because they plot CAD files directly. Software and hardware improvements have increased the ability of pen-plotters to operate unattended. For these reasons, the original engineering output device, after the manual pencil, will like be producing drawings for the foreseeable future.
COPYRIGHT 1992 American Society of Mechanical Engineers
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Title Annotation:computer aided engineering applications
Author:Puttre, Michael
Publication:Mechanical Engineering-CIME
Date:Mar 1, 1992
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