Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
2001-01-16
2002-08-13
Chen, Sophia S. (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
C250S339110, C250S341800
Reexamination Certificate
active
06434347
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a printer for printing full color images through a superposed transfer of different color images by use of a plurality of electrostatic recording units equipped with an electrophotographic record printing function, and to a toner density measuring method for use therein. More particularly, the present invention relates to a printer in which the toner density is measured by optically detecting Y, M, C and K toners transferred onto the surface of a conveyance belt, as well as to a toner density measuring method for use therein.
2. Description of the Related Arts
In the conventional color printers using electrophotographic recording, four electrostatic recording units for black (K), cyan (C), magenta (M) and yellow (Y) are arranged in tandem in the direction where record sheets are conveyed. The electrostatic recording units for four colors each optically scan a photosensitive drum on the basis of image data to form a latent image thereon, develop the latent image with a color toner in the developing device, and thereafter transfer the toner image onto record sheets being conveyed at a constant speed, in a superposed manner in the order of yellow (Y), magenta (M), cyan (C) and black (K), and finally perform a thermal fixation of the transferred image through a fixing device. It is necessary for the printers having Y, M, C and K electrostatic recording units arranged in tandem in the record sheet conveying direction to reduce the positional offset of the toner image transferred onto the record sheets in motion by each electrostatic recording unit to thereby enhance the color matching accuracy in order to improve the color printing quality. To this end, such tandem-type printers perform a color matching process in which a toner mark is transferred onto the surface of the conveyance belt upon the power on or the cover opening or closing, the toner mark being read by e.g., a sensor to vary the write timing of the LEDs or laser diodes making up an exposure device to thereby regulate the color drift and the toner density.
The conventional printers use a sensor as depicted in
FIG. 1
to detect the toner density. The sensor designated at
104
comprises an LED lamp
106
for issuing a spot light of the order of 8 millimeters in diameter on the surface of the belt
108
and a photodiode
112
for receiving a diffusely reflected light from the toner
110
lying on the spot light region. In the sensor circuit, as shown in
FIG. 2
the diffusely reflected light from the toner
110
on the belt surface
108
originating from the LED lamp
106
is received by the photodiode
112
and thereafter amplified by an amplifier
114
having a negative feedback circuit composed of a resistor
116
, and further amplified by amplifiers
118
and
120
, sampled by an A/D converter
122
and entered as light reception data into an MPU
124
to determine the density control parameter such as the emission time of the exposure LEDs corresponding to the toner density from the light reception data. The toner density regulation includes setting the density control parameter which is optimum for the default, e.g., the optimum emission time in four stages as 8 &mgr;s, 13 &mgr;s, 18 &mgr;s and 23 &mgr;s, transferring the toner in each emission time to determine the density, converting the measured density into the emission time to obtain a lag relative to the optimum emission time, and correcting the optimum emission time from this lag to regulate the toner density upon the printing to the optimum value.
However, such a conventional method for measuring the toner density from the diffusely reflected light from the toner transferred onto the belt surface has a deficiency that the Y, M and C color toners have a different reflectance from that of the K black toner so that the density of black toner cannot be detected. For example, assume as in
FIG. 3A
that slanted lines are transferred at a constant pitch by the LED exposure device, with the LEDs' emission current unvaried and with the emission time varied to e.g., 8, 13, 18 and 23 to increase the line width so that toner transfer patterns
128
,
130
,
132
,
134
are transferred onto the belt surface in the order of thinness. When this toner transfer pattern is read by the sensor
104
of
FIG. 1
, light reception signals corresponding to the toner density (amount of adhesion of toner) are obtained as in
FIG. 3B
in the case of the Y, M and C color toners. More specifically, in the case of the Y, M and C color toners although there exists a slight difference thereamong, when a light from the LED lamp
106
is irradiated on the toners, the light diffusely reflected on the Y, M and C toners impinges on the photodiode
112
. At that time, a higher density with a large amount of adhesion of toner results in a larger quantity-of-light diffusely reflected on the toner, whereas a lower density with a small amount of adhesion of toner results in a smaller quantity-of-light diffusely reflected on the toner, whereby the Y, M and C toner density can be measured from the magnitude of the diffusely reflected light received by the photodiode
106
as shown by way of an example in
FIGS. 3A and 3B
, where toner marks
128
,
130
,
132
and
134
with progressively increasing density being respectively shown to have increasing magnitude of output signals
136
,
138
,
140
and
142
. However, the K black toner exceptionally has a high light absorbance and presents an extremely less variation of the diffusely reflected light arising from the amount of adhesion of toner as shown in FIG.
5
C. For this reason, a problem is posed that the sensor of
FIG. 1
is unable to determine the density of the K black toner. On the contrary, in order to obtain only the K black toner density, it will suffice to irradiate the toner transfer surface with the LED lamp to receive the regularly reflected light therefrom, where the higher the toner density, the lower is the reflection, as shown by way of an example where the toner mark density is increased from signal
136
to
138
in
FIG. 3B
, but the reflection output signal is decreased to a lower output signal level
144
in FIG.
3
C. However, this approach may be problematic in that there is a need to provide two different types of sensors, i.e., the sensor for black toner and the sensor for color toners as shown in
FIG. 1
, which leads to a complicated sensor arrangement, sensor circuit and sensor structure, resulting in a rise of costs.
In recent years, as shown in
FIG. 4
, a sensor
146
is provided with a photodiode
112
for receiving a diffusely reflected light from the LED lamp
106
and with a photodiode
148
for receiving a regularly reflected light therefrom to provide an integrated sensor structure allowing the simultaneous measurement of the Y, M and C color toners and K black toner. However, this sensor also necessitates the two photodiodes
112
and
148
for the black toner and for color toners, with the need for two reception light amplifying circuits of FIG.
1
. Thus, the same deficiency still remains left as in the case of use of the two sensors for black toner and the color toners excepting that the single sensor placement is sufficient.
SUMMARY OF THE INVENTION
According to the present invention there are provided a printing apparatus capable of measuring the density of both Y, M and C color toners and K black toner by the combination of a single light emitting element and a single light receiving element and equipped with a sensor function having a simple structure and circuit configuration and easy to install, and a toner density measuring method for use therein.
The present invention is directed to a printing apparatus comprising a belt unit which conveys record sheets on a belt at a constant speed, the record sheets being electrostatically adhered to the surface of the belt; and a plurality of electrostatic recording units arranged in the direction of conveyance of the record sheets, the electrostatic recording
Armstrong Westerman & Hattori, LLP
Chen Sophia S.
Fujitsu Limited
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