Tone reproduction curve control method

Electrophotography – Control of electrophotography process – Of plural processes

Reexamination Certificate

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C399S027000

Reexamination Certificate

active

06741816

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling a printer, and more particularly, to an efficient tone reproduction curve (TRC) control method for providing images of high quality in response to environmental changes. The present application is based on Korean Patent Application No. 2001-48522, filed Aug. 11, 2001, which is incorporated herein by reference.
2. Description of the Related Art
A usual electrophotographic process applied in printers includes charging a photosensitive body with electricity, exposing the charged photosensitive body to form a latent image in a particular image area, adhering a developer to an area of the latent image on the charged photosensitive body using a developing device, transferring the developed image to a sheet, and fixing the image using a fusing roller.
In the charging process, the quality of the image can be increased by uniformly charging the photosensitive body with electricity. Accordingly, it is necessary to control the electric potential of the photosensitive body to be uniform during the charging. When the charging potential is low, contamination can occur in a non-image area. When the charging potential is high, developed mass per area (DMA) changes. When the charging potential is excessively high, the photosensitive body is permanently damaged.
Since the quality of an output image is related to the potential of a photosensitive body, it is necessary to maintain the potential of the photosensitive body within a predetermined range in order to obtain images of high quality. An electrostatic voltmeter (ESV) or an electrometer is used for measuring the potential of the photosensitive body. The ESV is disposed near the surface of a photoreceptor belt so that it can measure the potential of the photosensitive body when the photoreceptor belt passes an electrostatic electrode.
The potential of the photosensitive body is decreased to a predetermined potential referred to as an exposure potential to form the latent image during the exposure. The development device comes to have a development potential such that the potential of the developer is higher than the potential of a portion on which the latent image is formed, that is, the exposure potential, and lower than the potential of the other portion of the photoreceptor belt on which the latent image is not formed. As a result, the developer is made to adhere to an area of the latent image and thus development is performed.
The DMA of the developer during development is influenced not only by the charging potential, as described above, but also by the exposure potential and the development potential.
When the exposure potential is low, the difference between the exposure potential and the development potential is very big even if the development potential is maintained uniform. As a result, the amount of absorbed developer increases. When the exposure potential is high, the difference between the exposure potential and the development potential is small even if the development potential is maintained uniform. As a result, the amount of absorbed developer decreases, thereby producing a blurred image.
Similarly, under the condition that a constant charging potential and a constant exposure potential are applied to the photosensitive body, when a development potential applied to the photosensitive body is very high, the small difference between the development potential and the exposure potential causes the developer to be excessively absorbed. In contrast, when a development potential applied to the photosensitive body is very low, the big difference between the development potential and the exposure potential causes the developer to be poorly absorbed, thereby blurring an image.
FIG. 1
shows a method of controlling DMA to correct development errors in order to obtain an image of high quality, which is disclosed in U.S. Pat. No. 5,749,021. According to the method, the charging potential, exposure potential, and development potential are controlled based on internal process parameters known as a discharge ratio, a cleaning potential and a development potential.
This conventional method increases the quality of a printed image by controlling DMA in a process control loop. An area in which an image is formed is referred to as an image area. A test patch is usually provided between image areas to measure DMA. The measured DMA is compared with a target value, an error signal is transmitted to a controller, and the internal process parameters are adjusted, thereby correcting errors. In other words, a grid potential and an average beam power of an exposure system are calculated using the internal processes parameters to control the system of a printer.
Referring to
FIG. 1
, a level
1
controller
120
provides suitable control signals U
g
and U
l
to an electrostatic charging and exposure system
122
to control the electrostatic charging and exposure system
122
. Reference numeral
124
denotes a charging potential value V
h
and an exposure potential value V
l
of the electrostatic charging and exposure system
122
, which are measured by an ESV. Comparators
126
a
and
126
b
compare the values V
h
and V
l
with target values V
h
T
and V
l
T
denoted by reference numeral
128
for a charging potential and an exposure potential, respectively, and transmit error signals E
h
and E
l
, respectively, denoted by reference numeral
129
to the level
1
controller
120
. The gain of a level
1
loop is obtained from the error signals such that the potentials of a photosensitive body can converge to target values within a predetermined range.
The target values V
h
T
and V
l
T
for the charging potential and exposure potential provided to the level
1
controller
120
and the electrostatic charging and exposure system
122
are generated from a level
2
controller
130
. Comparators
136
a
,
136
b
, and
136
c
compare DMA sensor values D
l
, D
m
, and D
h
denoted by reference numeral
134
, which are measured from test patches provided according to a toner area coverage in a color toner density (CTD) sensor, with target values D
l
T
, D
m
T
, and D
h
T
denoted by reference numeral
138
, respectively, and transmit error signals
139
to the level
2
controller
130
. In addition, the level
2
controller
130
generates a signal V
T
d
for controlling a development system
132
.
In other words, in the conventional DMA control method, DMA values measured by a CTD sensor are compared with target DMA values to generate differences therebetween. The differences are transmitted to a level
2
controller. The level
2
controller linearizes internal process parameters, i.e., a discharge ratio, a cleaning potential, and a development potential and extracts target values for control parameters, i.e., a charging potential, an exposure potential, and a development potential, from the linearized discharge ratio, cleaning potential and development potential to control a level
1
controller and charging, exposure, and development systems.
The conventional DMA control method disclosed in U.S. Pat. No. 5,749,021 uses not only a CTD sensor but also an electrostatic sensor in order to diagnose the status of all parameters influencing an electrophotographic process, which complicate measurement. In addition, the conventional DMA control method is not adaptive in a state in which a charging, exposure, or development system changes due to changes in external environmental parameters such as temperature and humidity of a printer, or due to internal environmental changes such as replacement or supplement of substances such as a developer or a photosensitive body included in the printer. Moreover, the conventional DMA control method has a disadvantage of individually linearizing a discharge ratio, a cleaning potential, and a development potential in order to control a charging potential, an exposure potential, and a development potential based on internal process parameters, i.e., a discharge ratio, a cleaning potential, and a development potenti

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