Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1998-09-28
2002-03-26
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06361137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ink-jet hard copy apparatus and, more particularly, to the art of generating control signals for firing ink droplets from a scanning ink-jet printhead and, specifically to methods and apparatus for compensating for variations in printhead-to-media spacing and printhead scanning velocity.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the
Hewlett-Packard Journal,
Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in
Output Hardcopy [sic] Devices,
chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1
depicts an ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer,
101
. A housing
103
encloses the electrical and mechanical operating mechanisms of the printer
101
. Operation is administrated by an electronic controller (usually a microprocessor-controlled printed circuit board, not shown) connected by appropriate cabling to a computer (not shown). Cut-sheet print media
105
, loaded by the end-user onto an input tray
107
, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station where graphical images or alphanumeric text is created. A carriage
109
, mounted on a slider
111
, scans the print medium. An encoder strip
113
and appurtenant devices (not shown) are provided for keeping track of the position of the carriage
109
at any given time. The fundamentals of encoder tracking are set out in U.S. Pat. Nos. 4,786,803 and 4,789,874 (Majette, et al.) (assigned to the common assignee hereof and incorporated herein by reference in their entireties). A set
115
of ink-jet pens, or print cartridges,
117
A-
117
D are releasable mounted in the carriage
109
for easy access. In pen-type hard copy apparatus, separate, replaceable or refillable, ink reservoirs (not shown) are located within the housing
103
and appropriately coupled to the pen set
115
via ink conduits (not shown). Once a printed page is completed, the print medium is ejected onto an output tray
119
.
An ink-jet pen includes a printhead which consists of a number of columns of ink nozzles. The columns of nozzles fire ink droplets that are used to create a print column of dots on an adjacently positioned print media as the pen is scanned across the media. A given nozzle of the printhead is used to address a given vertical column position, referred to as a picture element, or “pixel,” on the print media. Horizontal positions on the print media are addressed by repeatedly firing a given nozzle as the pen is scanned. Thus, a single sweep scan of the pen can print a swath of dots. The print media is stepped to permit a series of scans. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Generally, the pen scanning axis is referred to as the x-axis, the print media transport axis is referred to as the y-axis, and the ink drop firing direction is referred to as the z-axis.
Note that when a nozzle is fired, the ink is ejected from the pen at a finite velocity and it must travel a finite distance along the z-axis between the pen and the print media (for convenience and without limitation to the scope of the invention, the word “paper” will be used hereinafter to mean any form of print media). Since the pen is not stopped at each position during scanning in the x-axis, a fired ink droplet will also have a velocity in the x-axis direction as it traverses the distance to the paper surface. Thus, in order to hit a target pixel, any given nozzle should be fired a finite time before the pen positions the nozzle directly over the location where the dot is intended to be printed. However, in the art it is often generally assumed that all drops will have the same offset and thus, without such time of drop firing compensation, overall print quality is not affected even though the image is shifted as a whole. If at all compensated, an average advanced time of the firing signal is calculated by using the expected flight time of the drop and the current pen velocity, each of which is known from the design of a specific implementation of ink-jet hard copy apparatus (e.g., it is known that the maximum allowable carriage speed without print quality degradation is calculated by taking the time it takes for pen control logic circuitry to shift one set of data up to the pen and fire divided by the pen nozzle stagger distance (explained hereinbelow); the flight time is calculated by dividing the nozzle-to-paper spacing by the velocity of the ink drop.
A typical prior art drop firing encoder is shown in
FIG. 1A
with a timing diagram therefor shown in FIG.
1
B. An encoder
113
provides two output timing signals, “EncA” and “EncB,” which are decoded
121
as fundamental coarse position indicators of where the carriage
109
is during a scan. The leading and trailing edge of each encoder signal can thus be used in conjunction with a counter
122
to generate carriage position, tracking carriage movement in units such as {fraction (1/150)}th inch (this value will be used throughout as an exemplary embodiment herein; no limitation on the scope of the invention is intended thereby nor should any be inferred therefrom). A series of fire timing pulses, “FTP”_COUNT, is generated for each position signal, allowing the FIRE pulse actually to trigger firing of a plurality of nozzles in the printhead. Fire timing pulses are generated continuously by a clock during normal printing and used in accordance with the number of nozzles arrays in a particular printhead design as needed. The Fire Position circuitry
123
combines the position information with a value for a nozzle firing register
123
to generate a nozzle firing pulse, “FIRE,” e.g., every period comprising movement of the carriage {fraction (1/150)}th inch. The leading or trailing edge is also used in a Period_Counter
124
to determine the carriage velocity. Dividing
125
the period by a predetermined number (e.g., 100, taken from an extrapolation_division register (not shown)—a value related to the number of nozzle firing desired per period for a particular printhead implementation, the FTP_COUNT pulses) provides an extrapolation for the timing of the FTP_COUNT pulses. That is, an extrapolator latch
126
_counter
127
takes the measurement of the carriage period as measured in clock cycles divided by the value kept in the extrapolation-division register. The FTP_COUNT pulses are also provided
128
as fine position indicator for carriage position.
However, the horizontal distance from the actual advanced firing position of a given nozzle to where the drop actually lands is dependent on the scanning velocity of the pen. Knowing the total flight time of the ink drop and the pen scan velocity, the distance can be calculated by multiplying these two values. If the scan velocity of the pen is constant, the amount by which the firing signal precedes each pixel position is a constant. As discussed above, in this case the whole printed image is just shifted by a constant amount; that is, the image is moved by the number of dot positions that equal the over-shoot distance. Compensation in the foregoing manner moves the whole rendered image to attempt to compensate simply for this error. However, this does methodology does nothing to improve instantaneous drop placement accuracy within each scan swath.
In fact, when a pen is scanned across the paper, its velocity is not constant. Also, there are pen acceleration and deceleration ramps
Eaton John T.
Quintana Jason
Thomsen Carl Eric
Barlow John
Hewlett--Packard Company
Huffman Julian D.
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