Image forming apparatus and control method therefor

Details Image forming apparatus and control method therefor Image forming apparatus and control method therefor

1998-12-21

2001-05-22

Brase, Sandra (Department: 2852)

Incremental printing of symbolic information

Electric marking apparatus or processes

Electrostatic

C347S129000

Reexamination Certificate

active

06236415

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus for forming a multi-colored image by superimposing a plurality of plane images. The invention also relates to a control method for the above type of image forming apparatus.
2. Description of the Related Art
As color-image forming apparatuses for printing color image data, laser beam printers (LBPs) are known. In LBPs, scanning is performed on a photosensitive member by reflecting a laser beam on a rotating polygonal member, and latent images, each corresponding to one line of a plane image, are sequentially formed on the photosensitive member. One of the developers (toners), such as magenta (M), cyan (C), yellow (Y), and black (BK), is then attached to the latent images for a plurality of lines (latent images for one frame), thereby forming a plane image for one color. The plane image for one color is transferred to paper fixed on a transfer drum. Then, printing an image for one color is completed. Subsequently, printing operations on the remaining three colors are sequentially performed. Finally, a multi-colored image of four colors is ready to be printed.
Alternatively, another printing method is available. Plane images of the respective four colors formed on a photosensitive member are temporarily superimposed on each other on an intermediate transfer member, and then, the superimposed plane images on the intermediate transfer member are transferred to paper simultaneously.
In the above-described image forming apparatuses, such as LBPs, a plurality of plane images are sequentially superimposed while they are being driven in a sub-scanning direction. More specifically, the photosensitive member, the transfer member, and the intermediate transfer member are driven at a constant velocity in the direction (sub-scanning direction) perpendicular to a main scanning direction. The individual plane images formed on the photosensitive member are transferred to the transfer member or the intermediate transfer member and are superimposed in synchronization with a sub-scanning start signal, which is generated every time the transfer member or the intermediate transfer member is rotated one turn. With this arrangement, positional offsets of the plane images can be reduced.
Alternatively, instead of transferring each plane image for one color to the subsequent stage from the photosensitive member, latent images for four colors may be sequentially formed on the photosensitive member to create the respective plane images for four colors. In this case, the resulting color images formed on the photosensitive member are simultaneously transferred to the subsequent transfer material (paper).
In the aforementioned color-image forming techniques, it is desirable that the individual plane images be superimposed with a minimal amount of positional offset so as to obtain a printed multi-colored image of high quality.
Hitherto, various methods for reducing the amount of the above positional offset have been considered. For example, the number of main-scanning start signals (beam detection (BD) signals) which are obtained while the photosensitive member is rotated one turn is adjusted to be an integer (FIG.
18
B). With this arrangement, the operation of a motor for driving the photosensitive member is synchronized with the operation of a scanner motor for driving the main scanning operation.
The aforementioned method is discussed in detail below with reference to
FIGS. 18A and 18B
.
FIG. 18A and 18B
schematically illustrate main scanning lines formed on a photosensitive member or an intermediate transfer member of a conventional image forming apparatus.
In
FIGS. 18A and 18B
, reference numeral
801
indicates an image carrier, such as a photosensitive member or an intermediate transfer member, which will hereinafter be described as a photosensitive member. An ITOP (a signal indicating the top position of a recording sheet) sensor
802
detects a sensor flag (not shown), which is disposed at a predetermined position on a lateral surface of the photosensitive member
801
, while the photosensitive member
801
is being rotated one turn, and generates a sub-scanning start signal (ITOP signal).
FIG. 18A
illustrates the main scanning lines when the number of main-scanning start signals (BD signals) generated while the photosensitive member
801
is rotated one turn is set to be n+½. That is,
FIG. 18A
illustrates an example in which the individual plane images fail to be precisely superimposed on one another. In
FIG. 18A
, there are shown positions of the main-scanning recording line signals, i.e., the first line, the second line, the third line, the (n−1)-th line, the n-th line of the first rotation, and the first line and the second line of the second rotation of the photosensitive member
801
.
FIG. 18A
illustrates the main scanning lines up to the third lines of the second rotation of the photosensitive member
801
.
FIG. 18A
reveals that every time the photosensitive member
801
is rotated one turn, i.e., every time one sub-scanning start signal is generated, the first line of the first rotation of the photosensitive member
801
is offset with respect to the first line of the second rotation by “0.5” lines. Similarly, for every rotation of the photosensitive member
801
, such as the third time, the fourth time, . . . , the (n−1)-th time, and the n-th time, “0.5” lines of offsets are generated between the same order of lines of the (n−1)-th rotation and the n-th rotation.
FIG. 18B
illustrates the main scanning lines when the number of main-scanning start signals (BD signals) generated while the photosensitive member
801
is rotated one turn is set to be “n”. That is,
FIG. 18B
illustrates an example in which the individual plane images can be precisely superimposed on each other.
FIG. 18B
illustrates the main scanning lines up to the third lines of the second rotation of the photosensitive member
801
.
FIG. 18B
shows that even after the photosensitive member
801
has been rotated a few turns, the same order of lines of the respective rotations, for example, the first line of the first rotation and the second line of the second rotation, theoretically match.
In conventional image forming apparatuses, the following methods are known as methods for providing synchronization between the operation of a motor for driving the photosensitive member or the intermediate transfer member and the operation of the scanner motor for driving the main scanning operation.
In a first method, a signal obtained by scaling a BD signal is used as a reference clock for a motor for driving the photosensitive member or the intermediate transfer member. In the second method, the reference clock for a drum motor for driving the photosensitive member or the intermediate transfer member and the reference clock for a scanner motor for driving the main scanning operation are generated in the same oscillator. Examples of the above-described methods are discussed in detail below.
FIG. 19
illustrates a conventional image forming apparatus constructed in accordance with the aforementioned first method.
In
FIG. 19
, a photosensitive member
901
is rotated by a photosensitive-member driving motor
907
via a driving belt
908
. A scanner motor
902
is controlled to operate at a constant velocity by a phase-locked loop (PLL) circuit
910
based on a reference clock supplied from an oscillator
911
, thereby driving a polygonal mirror
903
. The polygonal mirror
903
deflects on its mirror planes (for example, eight planes) a laser beam applied from a laser
904
and line-scans the photosensitive member
901
in a planar form via a lens
905
.
A beam detect (BD) sensor
906
is disposed on a portion free from an image forming region on the scanning lines of a laser beam, and generates a main-scanning start signal (BD signal) synchronized with the operation of the scanner motor
902
, i.e., synchronized with every laser scanning line. For every rotation of the polygonal mi

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