Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-09-03
2001-10-16
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S014000
Reexamination Certificate
active
06302514
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to image forming equipment and is particularly directed to an ink jet printer of the type which uses an optical encoder for carrier position and velocity information. The invention is specifically disclosed as an automatic correction circuit that limits the effect, on print fire timing, of the encoder signal when its most recent time interval is beyond a predetermined tolerance as compared to its previous time interval.
BACKGROUND OF THE INVENTION
Serial line printers that incorporate a moving printhead carrier generally use some type of linear position encoder to provide closed-loop position control for the carrier. It is often desirable to print individual pixels or pels at a resolution greater than the fundamental resolution of the encoder. One conventional technique uses a quadrature signal developed from the base encoder to produce a four-times increase in resolution. While this approach is sufficient to provide high-resolution position information, it lacks the necessary accuracy to generate high-resolution fire pulse timing for the printhead. Typical tolerance values for a quadrature sensor are ±10 degrees on channel phase, and ±15% on duty cycle. These encoder output tolerances force the system designer to use the linear encoder fundamental signal as the base frequency for fire pulse generation.
The derivation of print fire pulse timing from carrier velocity measurements is subject to error from sudden changes in velocity over the distance of the encoder fundamental signal's period. For purposes of print fire pulse derivation, a single period of history of the quadrature signal must be used to provide pulses for the next encoder period. This technique is reliable as long as high frequency perturbations in the velocity do not result in the period being reduced or increased by a significant amount.
If the encoder, for example, produces 150 pulses per inch and the maximum desired print registration or resolution is 1200 pixels (or pels) per inch, the controlling logic must generate eight evenly spaced intervals within the {fraction (1/150)}-inch pulse window as the first step in developing the print fire timing. Individual printing elements, represented by ink jet nozzles in an array, are conventionally staggered within a single pel spacing to minimize the number of elements (i.e., nozzles) that must simultaneously fire. This reduces power requirements and aids in thermal management and fluid dynamics for the ink jet nozzles.
As a result of this nozzle stagger, each of the eight single pel print times must be subdivided into a much larger number of equal time intervals to segregate the specific elements according to their physical locations on the ink jet printhead. Thus, the nozzles are fired over a time lapse based upon the printhead carrier velocity which renders a vertical column on the media from the staggered nozzle array column on the printhead. Margin for error is built in to the system by the fact that some of the available time intervals during the individual pel print times are not needed to fire all of the nozzles.
In an example system of an eight-times expansion in precision to 1200 dpi, a collision between the fire pulses may occur if a subsequent {fraction (1/150)}-inch pulse window time period decreases by more than 12.5% from the previous {fraction (1/150)}-inch measurement. A very large acceleration is necessary to produce such a change in velocity for a typical carrier speed of twenty inches per second (ips), and most printers incorporate a motor system that does not have enough power to produce this acceleration. While this large error is necessary to cause collision between adjacent fire pulses, the drop placement accuracy is effected as soon as the increase in velocity brings the next encoder edge into the final fire window of the previous slice, which will then conflict with the first fire window of the next slice.
In addition, as the carrier moves across the print media, it can experience a rotational motion with respect to the carrier shaft. This rotational motion can cause the encoder strip sensor to see some additional acceleration in the direction of travel of the carrier, resulting from the vibration of the carrier. This new motion is translated into a high frequency disturbance in the velocity. Resultant fire pulse window calculations from such “false” encoder edges can result in pulse timing inaccuracy, and consequently defects in the print-out.
It would be a significant advantage to provide an ink jet printer that can “self-correct” the timing calculations based upon a practical range of carrier acceleration.
SUMMARY OF THE INVENTION
Accordingly, it is a primary advantage of the present invention to provide a programmable self-correcting system for fire pulse calculation based upon signals from a linear encoder system of an image forming apparatus, such as an ink jet printer. It would be a further advantage of the present invention to provide a self-correcting system for fire pulse calculations of an ink jet printer which can selectively ignore “false” transitions of a quadrature linear encoder signal, while maintaining a minimum output fire pulse time signal for the individual nozzles.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, an improved image forming apparatus is provided which includes an ink jet carrier having an array of nozzles with a fire pulse timing circuit that is based upon information gleaned from a linear encoder that measures position and velocity of the carrier along its carrier shaft. The optical encoder that produces the position information uses a quadrature output signal, and one of the channels of that output signal is used to determine “critical edge” transitions that are used to provide the actual print fire timing. Two adjacent encoder periods are needed to determine the acceleration of the carrier. By measuring the time intervals of these two encoder periods, and measuring the difference between these time intervals, the present invention provides correction logic that can determine if the second of the two measurements has been corrupted by motion not relative to carrier speed across the print media.
The correction logic compares the absolute value of the difference between the encoder periods to a programmable number that has been set according to system parameters (including the print resolution being used for a particular document). If the difference between the previous two periods is greater than the preset maximum, the correction logic will limit the new encoder time (i.e., for purposes of fire pulse width calculation) to the old encoder time, plus or minus (plus/minus) the preset limit. In this manner, any significantly large errors in the critical edges of the encoder signals can be temporarily ignored for the fire pulse calculations of the next encoder period, at least within the programmable preset limits. This is particularly useful during printing operations during which the carrier should be traveling at a relatively constant velocity, wherein the false edges that may occur otherwise would significantly degrade the placement of printed pels on the print media if not for the automatic self-correction provided by the present invention.
REFERENCES:
patent: 5861726 (1999-01-01), Uchikata et al.
patent: 5936645 (1999-08-01), Niikura et al.
Eade Thomas Jon
Henry Darrel Lee
Barlow John
Gribbell Frederick H.
Hallacher Craig A.
Lexmark International Inc.
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