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The word "plotter" can mean several things in the printer industry. Small plotters to collect data from anaolgue instruments were once common - and are still the basis of vehicle tachographs. Data collectors which store signals direct to memory have largely taken over from this kind of device.
Today's plotters are usually wide-format printers. These are printers for A0 - A1 - A2 paper sizes rather than the normal office A4. Printers like this have two or three main purposes:
| Spatial information. Printing large swathes of information, often in considerable detail for engineering diagrams and maps. This "technical documentation" role can require high definition printing as well as large format. |
| Decorative surfaces. Printing cloth for banners, vinyl signs for vehicles and shopfronts, playbills and posters. The "display graphics" sector often doesn't need high resolution - a poster may look fine at just 20 dpi. | |
| Photography. There are lots of uses for large photographs. Clearly there is overlap with the spatial task and with decoration. What is different is that large -format photo printers need very good greyscale and a wide colour gamut and do need high resolution so this is one of the most exacting tasks. |
These are rather different markets with some overlap.
Market
HP reputedly has about 90% of the technical documentation plotter market. This may be because they have a lead on two of the key innovations making today's plotters possible - the grit-wheel paper drive and the inkjet cartridge. Both were HP innovations. Most of todays "plotters" are basically inkjet printers with a long carriage and mutltiple rubber rollers or grit-wheel paper movement.
Inkjet mechanisms are by far the most common and they come in great variety: sizes from 11 inch A3 large format printers through to 100 inches plus - the US market is dominant so plotter dimensions tend to be given in inches.Speeds vary greatly as well. For instance the 24 inch HP Designjet 130 takes about 12 minutes to print a page of A1 glossy media. The 42 inch HP Designjet 4500 can manage 100 such pages per hour; it's a dual swath printer which uses two sets of four printheads to give it a speed of 93 metres squared per hour.
Speed isn't the only criterion for large format machines and sometimes it is not particularly important as a machine will be set away doing a job unattended. Colour gamut is important, so some printers have multiple inks. The users might want to print on stiff or heavy materials so the feed path may be straight and capable of handling media with a weight of 300 grammes per square metre. A common requirement is for prints that are much longer than usual, used for advertising banners. Banners for outdoor use can't use conventional water-based inks without lamination. One answer to this is lamination, another is to use UV setting polymers or solvent based inks.
Technical & Spatial Information.
Flatbed pen devices were common in the 1980s. A flatbed plotter is basically an engineers drawing table, but instead of the pulleys and a moving rule that keeps the lines straight the pen plotter has a motor driving the ruler and another driving the pen. The result is an X-Y plotter that can be computer driven.
At one time these machines were just about the only way to get graphical information from a computer onto paper. They could get pretty elaborate. The Tektronix 4602 A3 flatbed had electrostatic paper grip and a carousel for 6 coloured pens. The HP7475 likewise. Some pen plotters had A0 or larger beds.
Plotters naturally use a vector language. The motors might be stepper or DC-encoder types, either way they are paired with a counter which the plotters internal logic can use to determine the current position and move it to a new position. HP used the HP-GL language which took two-letter mnemonic commands such as PU0,0;PD100,0,100,100,0,100,0,0; which should move to the origin and draw a box.HP-GL was later extended with pen widths and an ability to send the file in binary ather than ASCII, creating HP/GL-2. Other manufacturers tended to adopt HP-GL for compatability and it is still widely used.
Watching a pen plotter work was fun. Unfortunately users got a lot of opportunity to watch pen plotters because whilst they were faster than people at a drawing board they couldn't work much more quickly or the fibre-tipped pen would skip or miss. Putting a big engineering diagram on paper could take a pen-plotter over an hour.
Pen plotters were big as well. An A0 plotter needed a huge A0 bed for the paper, just over a metre long on one side and just under on the other. The motors driving the rule and the pen had to be correspondingly chunky because there are several metres of wire rope running round inside.
Grit wheels do away with the huge flat bed.
The grit wheel has rubber platen rollers like a typewriter or dot-matrix printer on one side of the paper and a wheel or roller with a surface like ather sharp sandpaper on the other.
The grit wheel grabs the paper and moves it back and forth inside a long thin plotter mechanism that only has to move the pen back and forth. Paper tends to be light (an A0 sheet is typically 80 grammes) and the grit-wheel grips it firmly so a sheet or roll can move more accurately and quickly. The grit-wheel can mark the back of the paper a bit but that doesn't matter because scarcely anyone uses a double-sided plot. Grit-wheels tend not to be used in other printers - even when they aren't duplex - because they don't entirely suit automatic sheet feeding.
The pen remains a problem, it still has to roll backwards and forwards to do the plotting and it makes one line at a time. It makes sense to replace the pen with an inkjet cartridge.
Inkjet mechanisms are still sometimes refered to as pens - notably in HP literature. Indeed part of the purpose of the inkjet invention was a portable plotter-like printer. One of the early sales successes was in the "HP PaintJet" colour printer designed by the plotter division.
In an ordinary inkjet printer the pen raster scans back and forth over the 210 mm (8 inch) pagewidth of A4. There is no great problem putting it on longer ails and having it span the 841 mm of A0. The printhead cable is longer and the printer needs more memory or clever algortithms to create the larger pagewidth bitmap but mechanically there is no real problem. The least competent inkjet (the HP "St Helens") can draw 12 lines at once - handy for text.
Most plotters today are inkjets with a wide carriage. They can be used in construction, engineering and design. They can be used to prototype wallpaper and cloth. They can make banners and outdoor posters. Recent printheads have over a thousand nozzles printing a swath of over an inch. Plotters with thousands of heads can be very fast and accurate.
One of the things thermal inkjets aren't very good at is outdoor inks. The thermal inkjet really wants an aqueous ink, which will naturally tend to run if it gets wet again. This can be overcome by things like ultraviolet fixing but if something hardwaring is wanted thermal resins tend to be better.
Display Graphics Plotters.
The display graphics sector produces big things like banners, posters, shop and van signage - as well as small things like computer badges and trophy centres.
Display graphics can be produced using a technical plotter, although using the driver at its native resolution there may be an unwanted focus on quality at the expense of speed. Older inkjet plotters also tend to have rather small inkjet cartridges, which can make a printout expensive.
Thermal resin plotters are similar to the inkjet design - grit wheel Y axis, carriage driven by a belt. The printhead tends to be a small thermal element with a foil.
Plotters can also sometimes do unusual things. Fitted with a scalpel a plotter can trim vinyl to shape for signmaking.
Roland have products in this line.
Laser Cutters and Engravers
There are several types of laser engraver, mostly using CO2 lasers to burn an image into plastic or laquer. CO2 lasers can't directly cut metal - the energy is at the wrong wavelength and tends to reflect - but metals can be treated so that the laser has an effect.
Production lines commonly use a machine which scans using a rotating polygon mirror and some sort of carriage feed mechanism - not unlike a giant high powered office printer. This kind of gadget tends to have a price in the £50,000 class and is quite commonly used in electronics - it can laser-trim resistors and serial-number assemblies.
Flatbed plotters have a successor in the laser engraver, which is a pen-plotter with a laser scalpel.
Mounting an actual laser like a pen is not (yet) practical. 15 Watt CO2 lasers tend to be about 300mm long with 2,000 Volt Supplies and 50 Watt lasers run to nearly a metre long with higher supply voltages.
The laser itself is usually mounted in the rear or base of the machine and aimed at the work area through a sequence of mirrors. Each arm of the plotter carries a mirror that -with careful trimming - aims the laser at the next step of it's journey. The pen itself has one last mirror facing onto the work-piece with a lense to give the required focus.
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© Graham Huskinson 2010
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