JPH028302B2 - - 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 toner supplied from the 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 voltage in which a negative DC voltage is superimposed on an AC voltage (biased AC bias voltage) is applied to the sleeve 4a and the blade 4b, and the negatively charged toner is transferred to the image-exposed portion of the photoreceptor 1 (surface potential approximately 0V). Jump 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 be short-circuited (discharged), and 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 onwards, 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; An image density control device for an electrophotographic apparatus, comprising means for switching the developing bias voltage based on the output of the detecting means so that the developing bias voltage is different when developing the formed electrostatic latent image. .

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 bias voltage HAC is adjusted by VR, and the remote signal V _REM is sent to the drive circuit 21.
It is sent to 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, constant voltage V _CC is divided by resistor R1, variable resistor VR, and resistor R2. D1, D2
is a diode. The relay RY1 is turned on and off by a signal from the output OUT of the microcomputer 20, which will be described later. When relay RY1 is on, the divided voltage of remote signal V _REM is output through its contacts. Relay RY1
When is off, the divided voltage drops by the forward voltage drop of the diode D1 (approximately 0.1V) and is output as the remote signal V _REM . This drop in remote signal V _REM is constant regardless of the degree of image density adjustment (adjustment by variable resistor VR).

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 energized (output signal out is low), remote signal V _REM voltage is set by variable resistor VR.
If adjusted to 7.5V, the developing bias voltage V _DC will be -
It is 300V. When relay RY1 is de-energized (output signal out is high), remote signal V _REM voltage is
The voltage becomes 6.5V, and the developing bias voltage V _DC becomes -260V.

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 V DC remains low, relay RY1 is energized, so the developing bias voltage V _DC is −
It is 300V. Next, in step 106, the photoelectric sensor 8
When the transfer material P is detected (step
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 is 5 positive high voltage
A signal for applying HV+ 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, the relay RY1 is de-energized, the remote signal V _REM drops, and the developing bias voltage shifts from -300V to -260V. That is, the area on the photoreceptor 1 that has been transferred is once again primarily charged and then imagewise exposed to form an electrostatic latent image, and when it reaches the position of the developer 4, the developing bias voltage is lowered.

FIG. 5 shows a curve (E-V
characteristic curve). Curve a is E-V when the area that has not received transfer charging is primarily charged to -800V.
It is a characteristic. Note that the surface potential of curve a is lower than -800V even when the exposure amount is 0 because of dark decay. Light amount E
When an electrostatic latent image with a surface potential V ₁ when exposed to light is developed with a developing bias _DC voltage V DC of Va, the developed density corresponds to the potential difference v _p between Va and V ₁ . The larger the potential difference v _p is, the higher the development density is. When the same voltage is applied to the primary charger 2 to primary charge the area that has received the transfer charge, the EV characteristic polarity is curve b.
Then, the surface potential decreases while remaining substantially parallel to curve a. At the same exposure amount E, the surface potential changes from V ₁ to V ₂ and the potential difference with Va is v, and v>v _p , so the developed density increases. However, due to the above-mentioned control, when the transfer-charged area reaches the position of the developing device 4, the developing bias voltage V _DC changes from Va to Vb.
Therefore, the potential difference with the surface potential V ₂ is maintained at v _p . As a result, the developed density becomes constant. Further, the discharge by the transfer charger 5 is caused by the transfer material P
is present between the photoconductor 1 and the photoconductor 1. Therefore, there is little difference in residual charge between the area on the photoconductor 1 that has received transfer charge and the area that has not received transfer charge, and the difference in potential after charge removal is also small. few. 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 shift direction of the developing bias voltage V _DC . Furthermore, although the timing for changing the developing bias 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 may be a circuit other than the above example as long as it can be switched so as to shift the level at a constant level. For example, a Zener diode may be used instead of the diode D1.

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 develops the toner image with a charge of the same polarity as the electrostatic latent image charge on the photoreceptor, and a developing means. a bias voltage means for applying a developing bias voltage; a corona discharge means for applying a corona discharge to the photoreceptor at a transfer position in order to transfer the toner image on the photoreceptor to a sheet; A detection means for detecting an area, a case for developing an electrostatic latent image formed in an area subjected to corona discharge by the corona discharge means of the photoreceptor, and an electrostatic latent image formed in an area not subjected to corona discharge. 1. An image density control device for an electrophotographic apparatus, comprising means for switching a developing bias voltage based on an output of a detecting means so that the developing bias voltage is different depending on when the image is developed.