Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-11-07
2002-12-10
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S249000
Reexamination Certificate
active
06493011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color registration deviation correction method for a laser beam printer, a digital copying machine, or the like in which a laser beam is scanned by an optical scanning device provided with a rotating polygon mirror and a multicolor image is formed by an electrophotography system, and an image forming apparatus, and particularly to a color registration deviation correction method having a function of correcting a color registration deviation as a position deviation between respective colors in a sub-scanning direction, and an image forming apparatus.
2. Description of the Related Art
In a multicolor image forming apparatus using an electrophotography system, toners of four colors of black (K), cyan (C), magenta (M), and yellow (Y) are superimposed on a sheet and are transferred, so that multicolor printing is performed. When multicolor printing is performed, if writing positions of the respective colors subtly deviate from each other, a color deviation appears when they are superimposed and transferred on the sheet, and printing quality is degraded. Then, in order to solve this and to obtain a multicolor print of high precision, it becomes necessary to perform writing position control for correcting the color registration deviation.
As a correction of the color registration deviation, there has been conventionally proposed a correction method in which a toner image of a specified pattern is formed on an intermediate transfer body or the like, the pattern is detected by an optical sensor including an optical source, a photodetector, and the like, a color registration deviation amount is calculated, data of a correction amount is transmitted to each correcting system, and correction is made, or a correction method in which a specified pattern is previously prepared in a region outside of an image formation region of an intermediate transfer body or the like, the pattern is detected by a sensor, a color registration deviation amount is calculated from the detection timing of the pattern and the detection timing of a writing timing signal (SOS signal) or the like in a main scanning direction, data of a correction amount is transmitted to each correcting system, and correction is made.
Among various kinds of corrections, especially in the case where writing position correction in a sub-scanning direction is made, high precision correction can be made by combining a section of correcting a writing position in a unit of one line and a section of correcting a writing position in a unit of less than one line. As shown in
FIG. 12A
, a print start signal (image forming signal) is made a trigger, a predetermined number of writing timing signals (SOS signals) in the main scanning direction is counted, and writing timings of the respective colors are made coincident with each other with high precision, so that images are superimposed. Here, if a normal writing timing is made (2) in
FIG. 12A
, when the count number of the SOS signals is increased or decreased ((1) and (2) in FIG.
12
A), as shown in
FIG. 12B
, the writing position is moved up and down in a unit of one line. By using this, the writing timing can be controlled in a unit of one line, and writing position correction can be made.
As a method of correcting a writing position in a unit of less than one line, as shown in
FIG. 13C
, a rotation phase of a rotating polygon mirror (so-called polygon mirror) is changed so that the correction can be made. It is assumed that an output timing of a normal SOS signal is made (2) in FIG.
13
A. When the rotation phase of the polygon mirror is changed ((1) and (2) in FIG.
13
C), as shown in
FIG. 13A
, the generation timing of the SOS signal is changed in accordance with this ((1) and (2) in FIG.
13
A), and as shown in
FIG. 13B
, the writing position in the sub-scanning direction is moved up and down. The rotary phase of the polygon mirror is controlled within the range where the movement of the writing position becomes less than one line, so that the correction can be made.
The polygon mirror is attached to a polygon mirror driving motor. As shown in
FIG. 14A
, PLL (Phase Locked Loop) control is performed by a motor (MOT) control circuit with a reference clock and a rotation frequency signal (FG output or hole element output in a polygon mirror driving motor, or scanning light position detector output by a photodetector arranged outside of a scan surface in an optical scanning device, which becomes a writing position timing signal in the main scanning direction) such as a SOS signal, so that the polygon mirror driving motor rotates at constant speed of rotation with high precision.
As shown in
FIG. 14B
, control of the polygon mirror rotation phase can be made by controlling the phase of the reference clock given to the polygon mirror driving motor for rotating the polygon mirror. That is, the phase of the reference clock is changed, so that a phase difference is generated between the rotation frequency signal and the reference clock, and the rotation speed of the polygon mirror driving motor is changed so as to remove this. With this, the rotation phase of the polygon mirror is also changed. Then, the rotation phase of the polygon mirror is changed, so that the output timing of the writing timing signal in the main scanning direction is also changed with this. Since the writing timing in the main scanning direction is kept constant in accordance with the writing timing signal in the main scanning direction, the writing timing in the sub-scanning direction can be controlled.
As such a correction technique of color registration deviation, especially as a correction technique of less than one line, there is a technique disclosed, for example, Japanese Patent Unexamined Publication No. Hei. 8-152833.
According to the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-152833, in an apparatus which includes one image forming apparatus and in which a multicolor image is formed by superimposing plural toner images on an intermediate transfer body, a phase matching period in which a frequency of a reference clock given to a polygon mirror driving motor at the time of rotation phase control of a polygon mirror is made slightly low or high is provided, and the frequency is returned to the original one after the phase matching period corresponding to a control amount, so that the phase of a mirror surface of the polygon mirror is gradually delayed or advanced, and control is made to obtain a desired phase (see FIGS.
15
A and
15
B). By this, writing timing in the sub-scanning direction is controlled, and color registration position deviation in the sub-scanning direction is corrected, which is proposed in this publication.
Besides, there is also such a technique that in an apparatus in which a multicolor image is formed by plural image forming devices and by sheet conveyance at one time, plural master clocks as references of phase control are previously provided, and when a color registration deviation in a sub-scanning direction is detected, a clock corresponding to a correction amount is selected among the plural master clocks, and further, the frequency of the reference clock given to a polygon mirror driving motor is made slightly low or high, and the frequency is returned to the original one at the point of time when the reference clock and the master clock are coincident with each other, so that writing timing in the sub-scanning direction is controlled and the color registration deviation in the sub-scanning direction is corrected in less than one line (see
FIGS. 14A
,
14
B, and
16
A).
In recent years, in an image forming apparatus such as a digital copying machine or a laser beam printer, high speed and high resolution are demanded. Besides, as described above, the technique of color registration deviation correction is indispensable to a multicolor image forming apparatus to keep a high quality image. In order to keep the high quality image and make printing at high speed,
Fuji 'Xerox Co., Ltd.
Morgan & Lewis & Bockius, LLP
Pendegrass Joan
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