Mechanical design of a color graphics printer

Hewlett-Packard Journal, August, 1988 by Chuong Cam Ta, Lawrence W. Chan, P. Jeffrey Wield, Ruben Nevarez

Measurement and Simulation

Visual appearance of the graphics output shows how well the media drive advances paper, but to analyze and improve the system, a measurement technique was developed using a 50,800-count encoder coupled to the drive roller. Measurement accuracy of 1/100 degree is possible.

This measurement was used to confirm the gear model, but it also revealed another source of error in the motor. Step motors have a step accuracy specification, and the effect of this error was easily predicted as it was attenuated through the gear train. The measured error was something else. A strong every-other-swath event was observed. Its cause was found by Fourier analysis of the drive roller position output. The Fourier analysis converted the encoder signal into a frequency-versus-magnitude map of the drive roller's angular position. Each gear showed a peak at its predictable frequency. The only component matching the frequency of the every-other-swath error was the alternating phases of the step motor. Each phase exerted a different torque and the dynamic effect of this difference resulted in alternating overshoot and undershoot of the correct step size. Implementation of a motor deceleration algorithm solved the problem by minimizing the "kick" applied to the drive roller at the last step. The torque difference between phases then became insignificant.

How much step accuracy is required? Simulation of various inaccuracies involving the frequency and magnitude of each error was done by a combination of modeling and measurement. With the encoder mounted to the drive roller, the angular position of the roller could be measured very accurately. With the appropriate high-resolution input we could rotate the drive roller to any desired position, perfect or not. A step motor with a 1000-to-1 gear ratio was set in motion to drive the roller. When the right position was reached the step motor was turned off and a graphics swath was printed. Although slow. 10 to 12 hours per page, the process produced a well-defined typical graphics output that could be evaluated by a panel of judges for its appearance.

Using the model, worst-case errors caused by individual mechanism components were anzlyzed to assess their effect on the printed page. This ensured high-quality output in even a worst-case scenario. Exhaustive testing then proved we had a cost-effective, highly accurate media drive.

Primer Design

Whenever the PaintJet printer's output is unacceptable, the user needs a quick and easy way to correct the problem. Also, when a user opens a new cartridge, it needs to be serviced to begin printing. The primer (Fig. 10) and wiper have been designed for these situations.

Typical examples of degradation are missing drops and missing colors, which can be caused by trapped air bubbles, contaminants in and on the nozzles, and shock to the cartridge. To correct these problems, the user can take the cartridge out of the carriage, open the primer lid by rotating it 180 degrees, and insert the pen into the primer lid. The next step is to rotate the lid 90 degrees so that the pen cartridge stands up and the primer lid windows are facing upward. The primer lid is then pushed down against a spring and held down until the windows are filled with ink. This should take a few seconds. Once the windows are filled with ink, the lid can be allowed to come up and the pen can be taken out and wiped with a wiper located under the carriage lid. The problem should now be corrected and the cartridge ready for printing. However, it may occasionally be necessary to repeat the procedure to solve all of the printing problems.