Color image forming apparatus

Details Color image forming apparatus Color image forming apparatus

1999-09-09

2001-09-04

Chen, Sophia S. (Department: 2852)

Electrophotography

Image formation

Transfer

C399S302000

Reexamination Certificate

active

06285849

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrophotographic color image forming apparatus having a plurality of photosensitive bodies, and more particular, to a technique of detecting misregistering of respective color toner images formed on the photosensitive bodies and precisely registering the images on a recording medium to make transfer.
2. Description of the Related Art
Conventionally, in a color image forming apparatus employing an electrophotographic technique, a photosensitive body serving as an image carrying body is charged by electrifying means, light is irradiated to form a latent image on the charged photographic body in accordance with image information, the image is developed by a developing means, and the developed toner image is transferred to a sheet material or the like to form an image.
Meanwhile, with the spread of color images, a tandem system color image forming apparatus has been proposed for forming a full-color image by preparing a plurality of image stations, in which such a series of image forming processes are performed, forming respective images of cyan image, magenta image, yellow image and preferably black image as well on respective image carrying bodies, and transferring respective color images to a sheet material at respective transfer positions of the respective image carrying bodies in superposing manner. Such tandem type color image forming apparatus is advantageous in attainment of high speed operation since it includes respective image forming units for respective color images.
Such color image forming apparatus, however, offers a problem in how to favorably execute registering of respective images formed in different image forming units. This is because misregistration of image forming positions for four color images transferred to a sheet or the like eventually leads to positional misregistration or as a change in color tone.
Meanwhile, misregistering of transferred images includes, as shown in
FIGS. 17A
to
17
E, misregistering in a moving direction (a direction of an arrow A in
FIG. 17A
) of a transfer material (herein-after, referred to as “subscanning misregistering”; see FIG.
17
A), misregistering in a scanning direction (perpendicular to the direction of an arrow A in
FIG. 17B
) (hereinafter, referred to as “primary scanning misregistering”; see FIG.
17
B), misregistering in a skew direction (hereinafter, referred to as “skew misregistering”; see FIG.
17
C), differences in magnification error (see
FIG. 17D
) and differences in curvature error (see FIG.
17
E). Actually, the misregistering of transferred images amounts to superposed misregistering and differences of the above five kinds.
In the case of subscanning misregistering in
FIG. 17A
, the main cause for misregistering results from misregistering in mounting of respective image stations or scanning optical systems and in mounting of a lens or a mirror (not shown) in a scanning optical system. This is the same with the case of primary scanning misregistering in FIG.
17
B.
Also, the main cause for skew misregistering in the skew direction in
FIG. 17C
results from angular misregistering of rotating shafts on photosensitive drums in the image stations and angular misregistering of scanning optical systems. The main cause for misregistering due to differences in magnification error shown in
FIG. 17D
results from differences in scanning length due to errors in optical length from respective scanning optical systems to the photosensitive drums in the image stations. The main cause for misregistering due to differences in curvature error in
FIG. 17E
results from errors in assembling of lenses or the like in the respective scanning optical systems.
To correct these five kinds of misregistering or differences, it has been proposed to depict a reference pattern (hereinafter, referred to as “registering pattern”) in advance, to detect this by means of a plurality of sensors (misregistering detection), to calculate an amount of misregistering from the detected result and to register respective images (correction of misregistering) in accordance with the calculated amount of misregistering.
Hereinafter, a conventional detection of a registering pattern and a correcting operation of misregistering will be described.
FIG. 18
is an illustration showing a construction of a conventional registering pattern detecting means (hereinafter, referred to as “pattern detecting means”).
FIG. 19
is an illustration showing an arrangement of a conventional registering pattern on an intermediate transfer belt and conventional pattern detecting means.
FIGS. 20 and 21
are illustrations showing an arrangement of conventional registering patterns on an intermediate transfer belt and conventional pattern detecting means and output signals of the pattern detecting means.
As shown in
FIG. 18
, the pattern detecting means
40
comprises an image sensor (hereinafter, referred to as “CCD”)
41
, a light source such as a lamp
42
and a cell focussing lens array
43
for focussing reflected light on the CCD
41
. Such pattern detecting means
40
a
and
40
b
are so disposed that picture elements in the CCDs
41
a
and
41
b
arranged in a line are positioned on a line perpendicular to a conveying direction A of the intermediate transfer belt
12
as shown in FIG.
19
. As shown in the drawings, two detecting means in total are disposed one near each of two ends of the intermediate transfer belt
12
in a widthwise direction perpendicular to the conveying direction A.
With the above arrangement, detecting/correcting actions of registering patterns comprises forming predetermined registering patterns such as straight lines, figures or the like (e.g., toner images
44
,
45
,
46
and
47
for respective colors at predetermined spacings on a line perpendicular to the conveying direction A of the intermediate transfer belt
12
) and measuring misregistering of respective colors (misregistering) by means of the pattern detecting means
40
a
and
40
b
as shown in FIG.
19
.
Hereupon, as shown in FIG.
20
(
a
), subscanning misregistering shown in
FIG. 17A
can be obtained by calculating misregistering for respective colors (&Dgr;Y
1
=&Dgr;T
1
·v) from time lags (&Dgr;T
1
=T−T
1
, T is a predetermined design value) between points of time, at which registering patterns
44
,
45
,
46
and
47
of respective colors on the intermediate transfer belt
12
pass the CCD
41
a
in the pattern detecting means and predetermined design values and from the conveying speed v of the intermediate transfer belt
12
.
As shown in
FIG. 21A
, the primary scanning misregistering shown in
FIG. 17B
can be obtained by calculating misregistering for respective colors from differences of positions of those picture elements (&Dgr;X
1
) when scanning start positions of registering patterns
44
,
45
,
46
and
47
of respective colors on the intermediate transfer belt
12
pass the CCD
41
a
in the pattern detecting means.
As shown in FIG.
20
(
b
), the skew error shown in
FIG. 17C
can be obtained by calculating skew errors (&Dgr;Y
2
=&Dgr;T
2
·v) of respective colors from time lags (&Dgr;T
2
) between points of time, at which the registering patterns
44
,
45
,
46
and
47
of the same color formed on both widthwise sides of the intermediate transfer belt
12
pass the CCD
41
a
and CCD
41
b
in the pattern detecting means, and from the conveying speed v of the intermediate transfer belt
12
.
As shown in FIGS.
21
(
a
) and (
b
), the magnification error shown in
FIG. 17D
can be obtained by calculating magnification errors (&Dgr;X
3
=&Dgr;X
2
−&Dgr;X
1
) of respective colors from differences (&Dgr;X
2
, &Dgr;X
1
) of positions of those picture elements when scanning start positions and scanning terminating positions of registering patterns
44
,
45
,
46
and
47
of the same color on the intermediate transfer belt
12
pass the CCD
41
a
and CCD
41
b
in the respective pattern detecting means.
Thus, misregisterin

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