Method and apparatus for compensation for time varying nozzle misalignment in a drop on demand printhead
View Patent ↗A method and apparatus is provided for compensating for variable overlap between segments of a page width print head system to reduce visually perceptible artifacts due to misalignment of adjacent overlapped segments. One method employs a summator which sums a current dither value from a dither matrix with an overlap signal to provide an output value which is then compared in a comparator with an input continuous tone data value providing an output compensated dither value to control nozzles in the overlap region of the segments. Another method uses a software program to provide the compensated dither matrix. A sensor provides a measure of the degree of overlap of the segments to generate the overlap signal. The sensor may sense temperature or relative displacement of the segments. The degree of overlap may be determined for various temperatures and stored in a ROM.
1. In an ink ejection print head comprising a plurality of overlapping print head segments, wherein the spatial relationship between adjacent segments is variable with time, a method for controlling the firing of nozzles within the overlapped segments comprising the steps of:
(a) determining a measure of the overlap between a first print head segment adjacent to a second print head segment, by measuring a temperature of the first print head segment and a temperature of the second print head segment to determine a relative displacement of the first print head segment and the second print head segment;
(b) creating a half toning pattern for the nozzles in the region of overlap of the first print head segment and the second print head segment using a dither matrix with an interpolation function, such that at least one nozzle to be fired from the first print head segment and at least one nozzle to be fired from the second print head segment are coextensive in the region of overlap between the first print head segment and the second print head segment; and
(c) adjusting said half toning pattern as a function of said measure in the overlap between the first print head segment and the second print head segment to reduce artifacts produced by the overlapping of the first print head segment and the second print head segment, such that V A +V B is greater than V where V A and V B are respective independent dither matrix values of the two adjacent segments and V is a continuous tone value to be represented.
2. A method for controlling the firing of nozzles as claimed in claim 1 further including the step of adjusting the firing of nozzles in adjacent segments such that corresponding nozzles of respective segments never both fire.
3. A method for controlling the firing of nozzles as claimed in claim 1 wherein said further step includes the step of assigning an output dot to one or the other nozzle stochastically according to a probability given by a current interpolation factor of said interpolation function.
4. A method as claimed in claim 1 further including the step of ensuring that corresponding nozzles of adjacent overlapping segments never both produce an output.
5. An ink ejection print head system comprising:
a plurality of spaced apart spatially overlapping print head segments including a first print head segment adjacent to a second print head segment;
at least one means for measuring a temperature of the first print head segment and a temperature of the second print head segment to determine a relative displacement of the first print head segment and the second print head segment;
means for providing a half toning of a continuous tone image for the nozzles in the region of overlap of the first print head segment and the second print head segment, such that at least one nozzle to be fired from the first print head segment and at least one nozzle to be fired from the second print head segment are coextensive in the region of overlap between the first print head segment and the second print head segment, and means for adjusting said half toning means in the region of overlap between the first print head segment and the second print head segment to reduce artifacts between the first print head segment and the second print head segment,
wherein said means for providing the half toning of a continuous tone image includes a dither matrix and said means for adjusting said half toning means includes a summation means having two inputs, one input being the output of said dither matrix and said other input being an overlap signal derived from said at least one means for measuring.
6. An ink ejection print head system as claimed in claim 5 further including a comparator means for comparing the output of said summation means and a continuous tone data input, the output of said comparator means being the half tone data for corresponding nozzles of adjacent print head segments.
7. An ink ejection print head system as claimed in claim 5 further including a means for subtracting the output of said summation means from a continuous tone data input signal to produce a half tone data value for driving the nozzles of adjacent print head segments.