Position leading, delay and timing uncertainty to improve position amp; quality in bidirectional printing

摘要

In bidirectional printing, ink-drop time-of-flight effects undesirably operate in opposite senses, during operation in the two different printing directions respectively, to offset the actually printed ink position in opposite directions from any nominal ink-firing point. When a common firing point is used for marks that should be aligned, during bidirectional scanning, the two resulting sets of image features are misaligned. To compensate for this adverse phenomenon, the firing points, in the two directions respectively, are made to bracket each common, desired mark location; the bidirectionally flying drops thus "lead" or approach each desired common mark location from opposite directions and can be made to align precisely. This can be done by addressing each position based on an earlier-arriving encoder-signal pulse and passing the signal through a delay line--during pen movement in just one of the two directions. A related approach is to use a subpixel spacing feature generally provided in the pen-positioning system, to back the firing position off in for example units of about 1/24 millimeter (1/600 inch)-- but during scanning in only one of two directions--to roughly align the marks. The asymmetrical earlier-pulse selection (or "backing off") and delay improve alignment. Another technique is useful for certain situations in which the printer uses large amounts of ink-- relative to the amount of liquid that can be absorbed by or evaporated from the printing medium that is in use--for example, when doing double- ink-drop printing on transparency stock. An unesthetic mottling effect arises in such situations. It has been discovered that, in this case, print quality is improved by purposely choosing relatively large jitter or random variation in firing time in each pixel column.