Image forming apparatus that performs image density control

Details Image forming apparatus that performs image density control Image forming apparatus that performs image density control

1999-04-06

2001-05-01

Grimley, Arthur T. (Department: 2852)

Electrophotography

Control of electrophotography process

Of plural processes

C399S055000

Reexamination Certificate

active

06226466

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system or an electrostatic recording system employed for, for instance, a printer or a copying machine, and more particularly to density control therefor.
2. Related Background Art
As an example of conventional image forming apparatuses, a color image forming apparatus of an electrophotographic system is illustrated in FIG.
8
.
The color image forming apparatus has a photosensitive drum
1
as an image bearing member. The photosensitive drum
1
is rotated in the direction shown by an arrow mark by a driving means not shown. The surface of the photosensitive drum
1
is uniformly charged by a primary charging roller
2
serving as a charging means abutting against the photosensitive drum
1
during its rotation. Then, the surface of the photosensitive drum
1
is irradiated with a laser beam L in accordance with a yellow image pattern by an exposure device
3
(laser scanner) so that an electrostatic latent image is formed on the surface of the photosensitive drum
1
. In this case, the charging roller
2
and the exposure device
3
serve as an electrostatic image forming means for the photosensitive drum
1
.
A latent image formed on the photosensitive drum
1
is reversely developed by a developing device y with yellow toner charged with negativity contained therein, which is previously opposed to the photosensitive drum
1
, as the photosensitive drum
1
rotates. On a rotary support member
5
(rotary drum) are supported four developing devices
4
y
,
4
m
,
4
c
and
4
k
. Before a development operation, a prescribed developing device is rotated and moved to a developing position opposed to the photosensitive drum
1
. The latent image is visualized as a yellow toner image in accordance with the development.
The toner image obtained on the photosensitive drum
1
is transferred (primary transfer) onto the surface of an intermediate transfer belt
6
rotating in the direction shown by the arrow mark at the substantially same speed as that of the photosensitive drum
1
by a primary transfer roller
7
a
to which primary transfer bias is applied. The toner remaining on the surface of the photosensitive drum
1
after transfer is removed by a cleaning means such as a blade.
A process comprising charging, an exposure, a development and a primary transfer as described above is carried out for each of colors including magenta, cyan and black subsequently to yellow, hence a multicolor image obtained by superposing together the toner images of four colors is formed on the intermediate transfer belt
6
.
The multicolor image formed on the intermediate color transfer belt
6
is transferred onto the surface of a transfer material serving as a recording material which is completely conveyed to the intermediate transfer belt
6
by conveying means such as pick-up rollers
9
at a prescribed timing by a secondary transfer roller
7
b
to which a secondary transfer bias is applied (secondary transfer).
The transfer material to which the multicolor image is conveyed to a fixing device
11
by a conveyor belt
10
in which toner is melted and fixed to the transfer material under heating and pressure, hence the multicolor changes to a final color image.
Upon the use of the image forming apparatus described above, is required such maintenance as the replenishment of toner, the treatment of waste toner, the replacement of a worn (consumed) photosensitive drum
1
by a new drum. In this example of the prior art, the photosensitive drum
1
, the charging roller
2
and the cleaning means
8
are formed as an integrated process cartridge
13
. Further, the developing devices
4
y
,
4
m
,
4
c
and
4
k
are each formed also as a developing process cartridge and are respectively readily detachably attachable to an apparatus main body, so that a user can perform a maintenance of them with ease.
Image forming apparatuses as well as the image forming apparatus of this example are generally provided with adjusting mechanisms for adjusting the density of an output image. Most of them have density control means for automatically controlling the output image to have proper density. Especially, in the image forming apparatus for outputting a full color image as in the present example, a more accurate density control has been demanded for each of the colors of yellow, magenta, cyan and black in order to obtain a desired color balance.
According to this example, the density of the output image is detected in such a manner that the toner image of a specific halftone pattern due to area gradation is formed on the photosensitive drum
1
and the amount of reflection light of the halftone pattern on the photosensitive drum
1
is measured by a reflection light amount sensor
12
which comprises a light emitting element and a light receiving element. Since the density of an image is controlled on the basis of image forming conditions such as the charging potential of the photosensitive drum
1
, exposure potential after the exposure of laser, developing bias potential, etc., a plurality of halftone patterns are formed by changing stepwise or gradually one or the combinations of a plurality of conditions of the image forming conditions and the reflection light amount of them is respectively measured by the reflection light amount sensor
12
. Thus, based on the measured reflection light amount, an image forming condition from which it is estimated that a desired constant density (reflection light amount) can be obtained is obtained.
In this connection, the reflection light amount sensor
12
employs infrared light and is designed to estimate the quantity of toner on the photosensitive drum
1
regardless of the color of toner. Although the amount of infrared light
3
received by the reflection light amount sensor
12
is substantially directly proportional to or inversely proportional to the quantity of toner sticking to the photosensitive drum
1
, the quantity of toner sticking to the drum is not ordinarily proportional to the density of an output image. However, since the quantity of toner sticking to the photosensitive drum is correlated with the density of the output image in the ratio 1:1, the density of the output image can be estimated from the measured value of the reflection light amount sensor
12
.
A density control for the image forming apparatus of the present example will be described in detail hereinafter. In the present example of the prior art, it is assumed that the surface of the photosensitive drum
1
is charged with electricity so that the surface potential of the photosensitive drum
1
reaches −600V and that the sensitivity of the photosensitive drum
1
and the exposure amount of laser are adjusted so that the potential of a laser exposure part reaches about −200V under normal temperature and normal humidity (23° C., 60%Rh). Further, as a detecting pattern image, is used a halftone pattern (9/16) for printing 9 dots of the matrix of 4×4 dots as shown in FIG.
5
. At this time, the developing bias formed by superimposing the AC (alternative current) voltage of rectangular wave (frequency of 2000 Hz, amplitude of 1600 Vpp) upon DC (direct current) voltage as shown in
FIG. 4
is employed and a DC voltage component Vdc is changed so that the development amount of toner is controlled.
Prior to a normal image forming, as shown in
FIG. 6
a plurality of image patches with the above described halftone pattern patches of square with side of 30 mm are printed at intervals in a section in which the reflection light amount sensor
12
is disposed. Each of the image patches is developed with the developing bias of a respectively different DC voltage component and the reflection light amount of each of the image patches is measured by the reflection light amount sensor
12
. In this example, the number of the image patches is five and the DC voltage component Vdc of the developing bias is changed at intervals of 50V from −300V to

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