Direction-independent encoder reading; position leading and delay, and uncertainty to improve bidirectional printing

摘要

Inversion of the encoder signal--during pen-carriage operation in just one of two printing directions--advantageously causes development of the position-signal pulse at each encoder bar to be generated from the same edge of each bar, even though the pulse-using circuit is always triggered from the same apparent waveform feature (e. g., a so-called "falling edge"). As a result, the position at which ink is fired from a pen on the carriage is independent of tolerances in bar width. Further asymmetry of timing, provided by addressing each position based on an earlier-arriving encoder-signal pulse and passing that pulse through a delay line, is preferably used to compensate for the fact that ink-drop time-of-flight acts in opposite senses, during pen scanning in the two different printing directions respectively. This time-of-flight effect, for the bidirectionally flying ink drops, produces undesirable offset of the actually printed ink position in opposite directions from the nominal ink- firing point. The invention uses asymmetrical timing in such a way that the ink-firing points, in the two directions respectively, bracket each common, desired mark location; the bidirectionally flying drops then "lead" or approach each common mark location from opposite directions and can be aligned precisely. Another technique is useful when the printer uses large amounts of ink--relative to the amount of liquid carrier that can be absorbed by or evaporated from the printing medium--as for example, when a printer does double-ink-drop printing on transparency stock. In this case print quality can be improved by deliberately selecting a relatively large amount of jitter or random variation in firing time within each pixel column. A preferred amount corresponds to about one eighth of a column width.