Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-08-16
2001-03-20
Nguyen, Thinh (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S009000, C347S019000
Reexamination Certificate
active
06203140
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of compensating for the failure of a dot generating unit in a printing system including a multiple-unit printhead scanning an image receiving medium in line direction and capable of printing several lines at a time, wherein multi-pass printing and interleaved line feed are employed and wherein as a unit fails the image information associated with that unit for each pass of the printhead is printed with at least one other unit of the printhead during at least one of the other passes.
A multiple-unit printhead comprises a plurality of dot generating units arranged in an array which extends perpendicular to the direction of the printing lines on the image receiving medium. Thus, image dots can be printed simultaneously in a plurality of lines, while the printhead performs a single scanning stroke or pass across the image receiving medium. For example, in the case of an ink jet printer, each dot generating unit is formed by a single nozzle and an associated actuator system by which ink droplets are jetted out from the nozzle in response to a drop demand signal supplied thereto in accordance with the image information to be printed.
Multi-pass printing means that only part of the image information is printed in a single pass of the printhead, i.e. during the movement of the printhead in a unique direction over the entire length of the line, and the printed line is completed in one or more subsequent passes. For example, in case of a two-pass system, every second dot or pixel is printed during the forward pass of the printhead and the missing dots are inserted during the second pass.
Interleaved line feed means that at least two different units or nozzles of the printhead contribute to the printing of each image line. This is achieved by feeding the image recording medium in a direction normal to the image lines in steps that have a width smaller than the extension of the nozzle array in that direction, so that the nozzle array sweeps at least twice over each location on the receiving medium.
An example of a printing system with the above features is disclosed in US-A-5,359,355. When, in this system, one of the dot generating units of the printhead becomes inoperative, e.g. because the nozzle has become clogged or air is trapped in the actuator system, the image information can not be printed completely. If, for example, the printhead has eight nozzles and one of them is inoperative, then, in a single-pass system or a system in which no interleaved line feed is employed, every eighth line will be missing on the printed document. In the case of a two-pass system with interleaved line feed, every second dot or pixel will be missing in every fourth line.
JP-A-
60-104 335
discloses an ink jet printer, in which additional nozzles are provided in reserve on the printhead. If one of the regular nozzles fails, the printing pattern is changed, so that one or more of the reserve nozzles are activated in order to compensate for the failure. In this system, however, the number of usable nozzles is always limited to the maximal number of consecutive nozzles in the array that are operative. As a result, a certain loss of production of the printer must generally be expected, depending on the location where the nozzle failure occurs.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for compensating failure of a dot generating unit with minimised losses in image quality and production.
According to the present invention, when n is the number of passes per scan cycle, the unit which prints also the image information associated with the inoperative unit is operated with n times the operating frequency of the other units, and the printhead is moved in the scanning direction with the same speed as in the case where all units are operative.
Thus, when an individual nozzle becomes inoperative, the task of this nozzle is taken over by one of the other regular nozzles of the printhead. It is accordingly not necessary to provide spare nozzles on the printhead, so that a cost reduction can be realised. Most importantly, however, the failure of the nozzle does not lead to a reduced productivity of the printer because the image information that normally had to be printed by the inoperative nozzle is printed during one of the other passes that would have been performed anyway. Thus, if the printhead has eight nozzles for example, and one of these nozzles becomes clogged, the printing process can be continued with printing the full image information of eight lines in each multi-pass cycle, and it is not necessary to perform any extra scan passes.
It will be understood that the nozzle or nozzles that have to take over the task of the inoperative nozzle will be required to generate more dots than during normal operation. This means that either the scanning speed of the printhead must be reduced or the nozzle must be capable of generating dots at a higher frequency than under normal conditions. However, with existing printheads, e.g. ink jet printheads with piezoelectric actuators, it is generally possible to increase the operating frequency of a few of the nozzles without causing a significant loss of image quality. The reason is that the upper limit for the operating frequency of the dot generating units of a multiple-unit printhead is generally imposed by cross-talk among the various units. For example, in case of an ink jet printhead with piezoelectric actuators, the printhead has to absorb reaction forces caused by the piezoelectric actuators, and this leads to a certain noise which adversely influences the performance of the neighbouring nozzles. However, this phenomenon becomes significant only when almost all of the nozzles operate at a high frequency. When only one or two out of the plurality of nozzles are operated at a higher frequency, as is the case in the present invention, then the cross-talk effects are generally tolerable. Thus, it is possible according to the present invention to compensate for the failure of one or a few nozzles of the printhead by increasing the operating frequency of some of the other nozzles, so that the printing speed need not be reduced and, nevertheless, the printed image will have a satisfactory quality.
As a result, the present invention permits the maintenance intervals for the printer to be significantly extended, especially for a color printer in which the likelihood of nozzle failure is multiplied because one printhead must be provided for each color.
In a preferred embodiment, the function of the dot generating units is automatically checked at regular intervals, for example after each scan cycle or after a certain number of scan cycles, and when it has been detected that an individual unit has become inoperative, the compensation procedure according to the present invention is initiated automatically by appropriately changing the timing in which the units are activated and the pattern in which the image information is supplied thereto. This function can easily be implemented in the control software of the printer and can be initiated “on the fly”, i.e. without interrupting the operation of the printer.
An example of a system for automatically detecting the failure of a nozzle in an ink jet printer is disclosed in US-A-4,498,088.
REFERENCES:
patent: 4498088 (1985-02-01), Kanayama
patent: 5359355 (1994-10-01), Nagoshi et al.
patent: 5675365 (1997-10-01), Becerra et al.
patent: 5796418 (1998-08-01), Silverbrook
patent: 0616893A2 (1994-09-01), None
patent: 0677390A1 (1995-10-01), None
patent: 0694396A2 (1996-01-01), None
patent: 0729838A1 (1996-09-01), None
patent: 0783973A2 (1997-07-01), None
patent: 60-104335 (1995-06-01), None
Birch & Stewart Kolasch & Birch, LLP
Nguyen Thinh
Oce--Technologies B.V.
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