JPH028301B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image density control device for an electrophotographic apparatus, and particularly to one suitable for preventing density unevenness in formed images.

For example, with a microfilm reader printer, the original image taken on microfilm may be a negative image or a positive image. Regardless of the original image, the copy obtained from it must be a positive image. To form a positive image from a negative image using an electrophotographic device, reversal development is performed. Many laser beam printers use a method in which a laser oscillates and scans a certain part of the image, and the exposed part is reversely developed.

The image forming process when such reversal development is performed using the electrophotographic apparatus shown in FIG. 1 will be described.
A photoreceptor 1 having a conductor covered with a photoconductive layer is uniformly charged, for example, negatively, by a primary charger 2 in a dark place, and then a negative original image light 3 is projected to form a negative electrostatic latent image. do. This electrostatic latent image is developed with a toner image supplied from a developing device 4. The toner in the developing device 4 is negatively charged due to mutual friction or friction with the developing sleeve 4a. A bias voltage (biased AC voltage) in which a negative DC voltage is superimposed on an AC voltage is applied to the sleeve 4a and the blade 4b.
Jump to and develop. The obtained positive image is transferred by applying positive corona discharge from the back side of the transfer material P by the transfer charger 5. The image on the transfer material P is fixed to obtain a hard copy. On the other hand, after the transfer, the residual toner on the photoreceptor 1 is cleaned by a cleaner device 6, and the residual charge is irradiated with uniform light 7 to eliminate short circuits (static charge removal).
Then, the next image forming process begins.

In order to improve the transfer efficiency, the positive operating voltage of the charger 5 is increased. However, when this positive voltage is increased, the potential of the photoreceptor 1 also becomes positive. Therefore, the charging characteristic (negative) of the photoreceptor 1 is opposite to that of the photoreceptor 1, and the residual charge cannot be removed sufficiently unless the amount of charge removal light is considerably increased. In particular, in the part where the transfer material P is interrupted, the photoconductor 1 is directly charged with positive corona ions, so the potential of the photoconductor 1 becomes higher than the part corona-charged via the transfer material P, and the charge is increased. Many will remain. According to experiments, corona discharge was applied directly to the surface of a photoreceptor whose primary charging potential was -800V without passing through the transfer material using a transfer charger voltage that caused corona discharge to reach +80V through the transfer material. , it became +500V. In such a residual charge state, even if the amount of the static eliminating light 7 is increased, the static eliminating state will become uneven. If the non-uniformly charged state is primary charged again in the next image forming step, the primary charging potential will become non-uniform. Usually, the outer circumference of the photoreceptor 1 is often shorter than the length of the transfer material P. Therefore, one rotation of the photoreceptor 1 does not complete the copying of one sheet, and it may take two or several times. From the second rotation onward, the photoreceptor 1 has a history of receiving transfer corona discharge during the previous rotation.
In one copy, there are areas in which images are formed with different states of primary charging, resulting in uneven image density.

The present invention has been made in view of the above situation, and an object of the present invention is to provide an image density control device for electrophotography that can obtain a uniform image density.

The present invention achieves this object in an electrophotographic apparatus that transfers a toner image formed on a rotating photoreceptor onto a sheet, and develops the image using a toner having a charge of the same polarity as the electrostatic latent image charge on the photoreceptor. a developing means, a bias voltage means for applying a developing bias voltage to the developing means, a corona discharge means for applying a corona discharge to the photoreceptor at a transfer position for transferring the toner image on the photoreceptor to a sheet, and the corona discharge of the photoreceptor. a detection means for detecting an area that has received corona discharge by means of the corona discharge means; a detection means for developing an electrostatic latent image formed in the area of the photoreceptor that has received corona discharge by the corona discharge means; Means for switching the developing bias voltage based on the output of the detection means so that the developing bias voltage is different when developing the formed electrostatic latent image, and adjusting the developing bias voltage for areas receiving corona discharge and areas not receiving corona discharge. An image density control device for an electrophotographic apparatus, characterized in that the image density control device has an adjusting means for controlling the image density of an electrophotographic device.

Examples of the present invention will be described in detail below.

FIG. 1 shows an electrophotographic apparatus controlled by an image density control device to which the present invention is applied. In the figure, 8 is a photoelectric sensor that detects the presence or absence of the transfer material P. The other parts are as described above, and will not be explained again. In addition, 9 is slit,
10 is a shutter.

FIG. 2 is a circuit block diagram of an image density control device to which the present invention is applied. In the figure, 20 is a microcomputer with a central processing unit CPU,
The storage device ROM/RAM, input/output section I/O, etc. are all integrated into one chip. 21 is the drive circuit,
Based on the command from the microcomputer 20, the charger 2.
Drive voltages HV-, HV+, and HAC are sent to the charger 5, developing sleeve 4a, and blade 4b, respectively. 22
A waveform shaping circuit shapes the waveform of the signal from the sensor 8 that detects the presence or absence of the transfer material P and sends it to the microcomputer 20. 23 is a voltage setting circuit, a variable resistor
The DC component voltage (hereinafter referred to as "developing bias voltage") of the biased AC voltage HAC is adjusted by VR, and the remote signal V _REM is sent to the drive circuit 21. By adjusting the variable resistor VR, the density of the developed image can be adjusted arbitrarily.

Details of the bias voltage setting circuit 23 are shown in FIG. In the circuit shown in the figure, when relay RY1 is on, constant voltage V _CC is connected to resistor R1 and variable resistor.
The voltage is divided by VR and resistor R2 and output to the drive circuit 21 as a remote signal _VREM . When relay RY1 is off, voltage V _CC is connected to resistor R3
The voltage is divided by R1, variable resistor VR, and resistor R2, and output to the drive circuit 21 as a remote signal V _REM .
Therefore, unless the image density adjustment (adjustment using the variable resistor VR) is changed, the remote signal V _REM will be lower when the relay RY1 is off. The degree of reduction varies depending on the degree of image density adjustment (resistance division ratio of variable resistor VR). When the voltage of the remote signal V _REM is high, it drops significantly,
When it's low, it doesn't drop much.

The variation of the developing bias voltage V _DC with the voltage of the remote signal V _REM is shown in FIG. relay
When RY1 is on (output signal out is low)
Remote signal V _REM voltage is set to 10V by variable resistor VR
When the developing bias voltage V _DC is adjusted to -
It is 400V. When relay RY1 is turned off (output signal out is high), the remote signal V _REM voltage drops to 9V and the developing bias voltage V _DC also drops to -360V. relay
When RY1 is on, the remote signal V _REM voltage is adjusted to a slightly lower 7.5V, and the developing bias voltage
V _DC is -300V, and when relay RY1 is turned off, the remote signal V _REM voltage drops to 6.75V and the developing bias voltage V _DC also drops to -360V. Depending on the adjustment value of the remote signal V _REM voltage, the amount of drop in the remote signal V _REM voltage changes, and at the same time, the amount of drop in the developing bias voltage also changes.

Each function of the microcomputer 20 operates according to program procedures stored in its ROM area. FIG. 6 shows a flowchart for executing a program for a portion of the image forming sequence that includes the configuration of the present invention. Note that this program is an example for copying one sheet. The operation will be explained below according to this flowchart.

First, in a series of image forming sequences, the rotating photoreceptor 1 is primarily charged. A negative high voltage HV- is applied to the primary charger 2 by the drive circuit 21 (step 101). When the shutter 10 opens and starts image exposure, the counter of the microcomputer 20 starts counting the clock CL in step 102. After the start, the time is preset based on the rotational speed of the photoreceptor 1 and the position of the developing device 4.
After T ₁ has elapsed (step 103), the count of the clock CL is stopped at one end in step 104, and then a signal for applying the biased AC voltage HAC is sent to the driver 21.
(Step 105). At this time, output out
Since the signal from VDC remains low, relay RY1 is energized, and the developing bias voltage V _DC is high. Next, in step 106, the photoelectric sensor 8 is operated to detect the transfer material P (step 106).
107), and starts counting the clock CL (step 108). After the start, when a time _T2 preset from the feeding speed of the transfer material P (rotational speed of the photoreceptor 1) and the distance between the sensor 12 and the transfer charger 5 has elapsed (step 109), the transfer charger 5 positive high voltage HV
A signal for applying + is sent to the drive circuit 21 (step 110). Similarly, when the clock CL passes a time _T3 preset from the rotational speed of the photoreceptor 1 and the distance between the sensor 12 and the developing unit 4 (step
111), output to the setting circuit 23 in step 112
Make the out signal high. Then, relay RY1 is de-energized, remote signal V _REM drops, and developing bias voltage V _DC drops. That is, the area on the photoreceptor 1 that has been transferred and charged is once again primarily charged and then exposed to image light to form an electrostatic latent image.
When it reaches the position, the developing bias voltage V _DC becomes low.

FIG. 5 is a curve (EV characteristic curve) showing the relationship between the exposure amount and the surface potential when the photoreceptor 1 is primarily charged by the primary charger 2 and exposed to light. Curve a is the EV characteristic of the area not subjected to transfer charging. Note that since the curve a is dark decay, the surface potential is lower than the primary charging potential of -800V even when the exposure amount is 0.
Curve b indicates that the primary charger 2
E- when exposed after applying the same voltage to
This is a V characteristic. A high-density original film and a low-density original film are exposed to a photoreceptor 1 using a light source (not shown) with the same amount of light. The points on the EV characteristic curve a of the highest density part (dark part) and the lowest density part (light part) included in the high density film are respectively defined as Pd ₁ and Pl ₁ , and the developed density at that time is appropriate. Let the developing bias voltage V _DC be V _DC1 . EV characteristic curve a of dark and light areas of low density film
Let the upper points be Pd ₂ and Pl ₂ , respectively, and let the development bias voltage V _DC at which the development density at that time becomes appropriate be V _DC2 .
On curve b, there are dark areas of high-density film.
Pd ₁ ′, light area Pl ₁ ′, dark area of low density film
Pd ₂ ′ and light part Pl ₂ ′. The developed density is determined by the difference V- between the developing bias voltage V _DC and the potential V on the surface of the photoreceptor.
The development bias voltages V _DC that can maintain the same development density are V _DC1 ′ and V _DC2 ′ _, respectively. Development bias voltage V _DC1 and V _DC1 ′ in dark area
The difference between V ₁ and the developing bias voltage V _DC2 of the light section is
The difference between V _DC2 ′ is greater than v ₂ . That is, the lower the density of the film, the more the potential corresponding to the original image moves to the right on the characteristic curve, and the developing bias voltage V _DC decreases. Curves a and b show that the lower the surface potential (+
direction), the difference becomes smaller. Therefore, the lower the film density, the smaller the amount by which the developing bias voltage needs to be changed.

In the image density control device of the present invention, when the remote signal V _REM voltage increases or decreases due to the output signal out of the microcomputer 20, which is generated when the transferred charged area comes to a position facing the developing device 4, the drive circuit 2 The developing bias voltage drop amount also increases or decreases accordingly. Therefore, when the EV characteristic curve a for the area not subjected to transfer charging changes to the curve b for the area subjected to transfer charging, the developing bias voltage V _DC1 .
Change V _DC2 to V _DC1 ′ and V _DC2 ′ respectively. In this way, the development density is controlled to be optimal. Further, since the transfer charger 5 discharges only when the transfer material P is interposed between the photoreceptor 1 and the photoreceptor 1,
There is little difference in residual charge between the area that received the transfer charge and the area that did not, and the difference in potential after charge removal is also small. Therefore, since the difference between curves a and b is relatively small, the developed density remains constant even if the amount of change in the developing bias voltage is small.

Note that in the above examples, the characteristics and characteristics of the photoreceptor are not limited to the voltage values specifically shown.
It can be modified and applied as appropriate depending on the conditions of use. In this example, a negative voltage was applied as primary charging, but
Even when a photoreceptor having positive characteristics is used and a positive voltage is applied as primary charging to form an image, the present invention can be applied by reversing the positive and negative changes in the developing bias voltage V _DC . Further, although the timing for changing the developing bias voltage was calculated by software processing using a counter built into a microcomputer, it may also be a hardware processing that takes the timing from an encoder such as a photointerrupter provided on a rotating photoreceptor.
The developing bias voltage setting circuit can be applied to circuits other than those described above, as long as the amount of switching change changes depending on the setting of the developing bias voltage V _DC , such as an A/D converter or a D/A converter. You may also be able to set it using .

As described above, according to the electrophotographic apparatus equipped with the image density control device of the present invention, extremely high quality copied images can be obtained with uniform image density.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an electrophotographic apparatus to which the present invention can be applied, FIG. 2 is a block diagram of an image density control apparatus to which the present invention is applied, FIG. 3 is a circuit diagram of the main part thereof, and FIG.
FIG. 5 is an EV characteristic curve diagram, and FIG. 6 is a flowchart of the control device. 1 is a photoreceptor, 2 is a primary charger, 4 is a developer, 5
is a transfer charger, 8 is a transfer material detection sensor, 20 is a microcomputer, 21 is a drive circuit, 23
is a developing bias setting circuit.

Claims (1)

[Claims] 1. In an electrophotographic device that transfers a toner image formed on a rotating photoreceptor onto a sheet, there is a developing means that performs development with toner having a charge of the same polarity as the electrostatic latent image charge on the photoreceptor, and a developing means that performs development with a toner having the same polarity as the electrostatic latent image charge on the photoreceptor. A bias voltage means for applying a bias voltage, a corona discharge means for applying a corona discharge to the photoreceptor at a transfer position for transferring the toner image on the photoreceptor to a sheet, and an area of the photoreceptor that receives corona discharge by the corona discharge means. a detection means for detecting the corona discharge, and a detection means for developing an electrostatic latent image formed in an area of the photoreceptor that has been subjected to corona discharge by the corona discharge means, and a detection means for developing an electrostatic latent image formed in an area that has not received corona discharge. means for switching the developing bias voltage based on the output of the detecting means so that the developing bias voltage is different depending on when developing;
An image density control device for an electrophotographic apparatus, comprising an adjusting means for adjusting a developing bias voltage in an area that receives corona discharge and an area that does not receive corona discharge.