Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-07-14
2001-08-28
Royer, William J. (Department: 2852)
Electrophotography
Image formation
Transfer
C347S116000
Reexamination Certificate
active
06282396
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color image forming apparatus and method of forming a color image using an electrophotographic process.
2. Discussion of the Background
A so-called tandem-type method is used in a color image forming apparatus in which a color image is obtained by transferring by superposition respective color images formed with an electrophotographic processing section of the apparatus onto a recording sheet.
FIG. 1
is a side view of a color image forming apparatus of the tandem-type method. As shown in
FIG. 1
, a sheet conveying path
4
is provided for guiding a transfer sheet
1
, as a recording sheet, from a sheet feeding section
2
through a sheet discharging section
3
in the color image forming apparatus. The sheet conveying path
4
includes a conveying belt
7
that is movably positioned between a belt drive roller
5
that is rotated by drive power from a drive power source (not shown) and a belt driven roller
6
coupled to the drive power source. Further, on the conveying belt
7
, four electrophotographic processing sections
8
Y,
8
M,
8
C, and
8
K, for yellow, magenta, cyan, and black are respectively disposed in order. These electrophotographic processing sections respectively include a photoconductive drum
9
, as a photoconductive element, that contacts the conveying belt
7
, as well as a charging device
10
, an exposing device
11
, a developing device
12
, a transferring device
13
, and a photoconductive element cleaner
14
each being disposed in order around the photoconductive element
9
. In addition, the conveying path
4
is provided with a fixing unit
15
at a position just after the conveying belt
7
, as shown.
Typically, the color image forming apparatus that has such a construction feeds an uppermost transfer sheet
1
from a sheet feeding section
2
towards the sheet conveying path
4
, and is conveyed with the conveying belt
7
. During the sheet conveying process, an image forming operation for each of the four colors is performed by each electrophotographic processing section, using electrophotographic processes, i.e., charging, exposing, developing, and transferring processes. A color toner image is transferred onto the transfer sheet
1
, and fixed thereon by being heated and pressed with the fixing unit
15
. This is a principle of image forming by the color image forming apparatus of the tandem-type method as shown in FIG.
1
.
FIG. 2
illustrates from a perspective view the conveying belt
7
and respective drums
9
. As will be discussed herein, a main scanning direction is indicated by a mark B, and a sub-scanning direction is indicated by a mark C.
Even though the color image forming apparatus of the tandem-type method has an advantage of high printing speed, the present inventors recognized that this conventional apparatus has a shortcoming in that an alignment of each of the colors is difficult to achieve and maintain. Therefore, for example, a slight positional deviation often occurs when a user or a service engineer moves a part of the electrophotographic processing section from a proper position when removing a jammed sheet or repairing the apparatus, and this slight positional deviation causes a color deviation between the respective colors.
Several approaches to preventing the color image positional deviation have been proposed in recent years.
For example, as discussed in reference to
FIGS. 3 and 4
, an image positional deviation detecting method is disclosed in Japanese Laid-Open Patent Publication NO. 6-18796/1994. Image positional deviation detecting sensors
102
, which include two CCD line sensors
101
(in FIG.
4
), are positioned so as to face the conveying belt
7
. Image positional deviation detecting marks
103
are formed on the conveying belt
7
by the electrophotographic processing section before the image forming operation is performed. The detecting marks
103
are positioned in areas where the CCD line sensors
101
can read them such that an amount of image positional deviation corresponding to the electrophotographic processing sections
8
Y,
8
M,
8
C, and
8
K can then be detected by reading the positional deviation detecting marks
103
by the CCD line sensors
101
as shown in FIG.
3
. The image positional deviation detecting sensor
102
includes a light source
104
and a light collecting lens
105
for collecting and providing reflection light to the CCD line sensor
101
, reflected by the conveying belt
7
, which is emitted from the light source
104
as shown in FIG.
4
.
However, as presently recognized, the image positional deviation detecting method disclosed in Japanese Laid-Open Patent Publication No. 6-18796/1994 has some problems in that parts costs are greater than desired due to the inclusion of the expensive CCD line sensor
101
or light collecting lens
105
. Furthermore, focusing of the reflection light from the conveying belt
7
must be adjusted by the light collecting lens
105
, and therefore the successful operation of the apparatus becomes troublesome.
In reference to FIGS.
5
through
8
(
b
), and in light of the limitations of the above-mentioned method, a device is described in Japanese Laid-Open Patent Publication No. 6-118735/1994 as detecting the color image positional deviation using an inexpensive reflection-type optical sensor
204
composed of a light source
201
, and a slit
202
, and a light accepting element
203
. Namely, V-shaped image positional deviation detecting marks
205
are formed on the conveying belt
7
, and a leading edge and a trailing edge thereof are detected with two reflection-type optical sensors
204
, as shown in FIG.
6
. For example, in the case of detecting the image positional deviation between a black electrophotographic processing section
8
K (
FIG. 1
) and a magenta electrophotographic processing section
8
M (FIG.
1
), two black lines K
1
and K
2
which compose each edge of the first V-shaped mark, two magenta lines M
1
and M
2
which compose each edge of the second V-shaped mark, a black line K
3
which composes one edge of the third V-shaped mark, and a magenta line M
3
which composes another edge of the third V-shaped mark are formed on the conveying belt
7
, as shown.
FIG. 7
shows an example in which the magenta electrophotographic processing section
8
M deviates in a sub-scanning direction. Namely, when the image positional deviation detecting mark
205
is detected with respective reflection-type optical sensors
204
, an output signal from one side of the reflection-type optical sensor
204
a
(lower part of
FIG. 7
) is represented in FIG.
8
(
a
), and another side of the reflection-type optical sensor
204
b
(upper part of
FIG. 7
) is represented by a diagram in FIG.
8
(
b
). Thus, if a time difference between pulses based on a signal of one side reflection-type optical sensor
204
is not constant {FIG.
8
(
a
)}, and a time difference between pulses based on a signal of another side reflection-type optical sensor
204
is constant {FIG.
8
(
b
)}, an electrophotographic processing section of a certain color is judged to have deviated in a sub-scanning direction.
In light of the above description regarding deviation in the subscanning direction, it is possible for deviations to occur in the main scanning direction. More particularly, when an electrophotographic processing section of a certain color deviates in position along the main scanning direction, the timing of output signals from two reflection-type optical sensors
204
a
,
204
b
deviates. For example, if the image positional deviation detecting mark
205
, composed of two black lines K
1
and K
2
which construct each edge of the first V-shaped mark, deviates upwards, it is assumed that a pulse based on the output signal of the reflection-type optical sensor
204
b
{FIG.
8
(
b
)} precedes a pulse based on the output signal of the reflection type optical sensor
204
a
{FIG.
8
(
a
)}. Therefore, the image positional deviatio
Iwata Nobuo
Satoh Toshiya
Shinohara Tadashi
Shio Yutaka
Sugiyama Mitsugu
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
Royer William J.
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