Hewlett-packard Company

Gregory D Raskin

William C Hilliard

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.

Position leading, delay and timing uncertainty to improve position &

In bidirectional inkjet printing, in certain situations the printer uses relatively large amounts of ink--in relation to the amount of liquid carrier that can be absorbed by or evaporated from the printing medium that is in use. For example, this occurs when a printer does double-ink-drop printing on transparency stock, particularly for particular colors (e.g., cyan). In such situations an undesirable, unesthetic mottling effect can arise. It has been discovered that, 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--actually the equivalent of about one eighth of a column width.

Encoder-signal timing uncertainty adjusted to improve esthetic quality

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.

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San Diego, CA, United States of America

Gregory D Raskin