Media path for a small, low-cost, color thermal inkjet printer - includes related article on modeling a permanent magnet stepper motor - design of HP DeskJet 1200C and HP DeskJet 1200C/PS printers - Technical

Hewlett-Packard Journal, Feb, 1994 by Damon W. Broder, David C. Burney, Shelley I. Moore, Stephen B. Witte

The Deskjet 1200C media path is heated for media independence, requiring development of a new grit drive roller and pinch wheel combination. A new stepper motor was developed to attain the target speed and accuracy. Media flatteners and precise gearing with an antibacklash device contribute to accuracy.

The media path is the part of a printer that moves media past the print cartridges, stopping and accurately locating the media for printing. The media path also constrains the media in a plane a fixed distance from the print cartridges. A media path should advance media accurately and quickly, be quiet, be inexpensive, hold the media flat, and keep the media the correct distance from the print cartridges.

Fig. 1 shows an overview of the media path of the HP DeskJet 1200C printer. Media stacked in the input tray (1) is individually picked by a media pick roller (not shown) and driven around the curved preheat zone (2) where it is preconditioned (moisture is driven off and the temperature is raised). When the page reaches the pinch/drive rollers (3), the main drive system (4) takes over from the pick roller drive (not shown). Once in the print zone (5) the media is heated further and ink is sprayed onto the page. The heating, soaking, and drying causes the media to move out of its plane, but the media control shims (6) help hold it flat for better print quality. The page is then incrementally advanced and printed upon until the entire page has been printed. Finally, the page is fed out into the output tray (7) and the process is ready to repeat.

Design Approach

For the office printer market, the Deskjet 1200C is designed to support a wide variety of plain papers, to be HP LaserJet printer compatible, to print text very quickly, to print high-quality graphics, and to be cost competitive. These characteristics forced the design team to face the following challenges:

* Constrain plain papers flat even though plain paper tends to cockle and curl in various directions when ink is sprayed on and heated.

* Print to 50-dot row margins for LaserJet compatibility, even though such small margins allow very little control over media flatness.

* Move media very quickly through the print area while maintaining high placement accuracy for good graphics print quality.

* Keep the price low.

The parts of the media path (Fig. 1) that drive and meter the media include the stepper motor, the drive pinions, the drive gears, the antibacklash device, the shaft bushings, the adjustable plate, the drive rollers, and the pinch rollers. While driving the media these components work together to ensure fast, accurate movement of the print media, which in turn provides the best possible throughput.

The stepper motor provides the fastest response available in an inexpensive motor. Although stepper motors aren't always the most accurate type of motor, the accuracy of the system can be optimized by designing the system so that the motor always moves in multiples of four half steps. Moves of four half steps cancel out all of the error caused by manufacturing variation except the locations of the stator teeth which are formed out of the sheet-metal case of the motor. Used in this way, the stepper motor can provide very fast and accurate moves and is well-suited for driving the media in an inkjet printer.

The gearing system in the Deskjet 1200C is designed to optimize the accuracy of the media drive. A single-reduction drive is used because there are fewer components in the gear train, so this type of drive is more accurate. However, accuracy is not the only reason to use a single reduction. The size and spacing of the drive rollers relative to the print cartridges and heater also pushed the design towards a single-reduction gear train. The Deskjet 1200C relies on a heated media path to dry the ink as it is being printed. To make room for the heater while maintaining control of the print media, the drive rollers had to be much smaller than other inkjet printers had typically used. By using smaller drive rollers, we were able to use a single-reduction gear train.

The spacing of the drive rollers was a critical issue in the design of the Deskjet 1200C. The spacing is constrained by the size of the print cartridges and heater. Thus, we had to trade off room for the heater and print cartridges against control over the leading edge (the star wheels hold the media down against the output drive roller) and print quality in the bottom margins (the main drive roller is more accurate than the output drive roller). As the space between the rollers increases, there is a longer span of media at the top of the page to control with the star wheels and a longer space at the page bottom where the media is driven by the output roller.

A major problem in reducing the shaft spacing was reducing the size of the drive roller. Drive rollers in inkjet printers have typically been rather large elastomer-coated shafts, 40 to 70 mm in diameter. To shrink the roller to a size we could use (< 20 mm) we had to find a different process, one that was new to inkjet printing. We decided to try the grit drive system used in several HP plotters, which consists of a gritcoated metal drive wheel and an elastomer pinch wheel. By using a grit system, we could minimize manufacturing errors associated with the size and shape of elastomer rollers and reduce the size to one we could use. Of course, we then faced many problems adapting the grit drive system to a heated media path system.