JPS60209764A - Image density controller of electrophotographic device - Google Patents

Abstract

PURPOSE:To control a developed image to constant density and to improve its picture quality by varying a bias voltage applied to a developing means synchronously with areas on a photosensitive body which are charged electrostatically for transfer and not charged when a positive image is formed from an original negative image. CONSTITUTION:A driving circuit 21 applies driving voltage HV-, HV+, and HAC to a charger 2, charger 5, and developing sleeve 4a and blade 4b under the command to a microcomputer 20. A waveform shaping circuit 22 shapes the waveform of the signal of a sensor 8 which detects whether a transfer material P is present or not and sends the waveform-shaped signal to the microcomputer 20. A bias voltage setting circuit 23 divides a constant voltage Vcc through a resistance Ra, variable resistance VR, and resistance R2. When its relay RY1 is on, the divided voltage is outputted as it is as a remote signal VREM through the contact of the relay and the developing bias voltage VDC is varied to adjust image density.

Description

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

For example, the original image taken on microfilm by a microfilm league printer 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 in the electrophotographic apparatus shown in FIG. 1 will be explained below.

A photoreceptor l having a conductor covered with a photoconductive layer is uniformly charged, for example, negatively, by a primary band detector 2 in a dark place, and then a negative original image light 3 is projected to form a negative electrostatic latent image. form. This electrostatic latent image is developed with toner supplied from the developing device 4.

The toner in the developing device 4 is transferred to phase 1j-friction or developing sleeve 4.
becomes negatively charged due to friction with a. 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 l (surface potential approximately OV). 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 photoreceptor l
The potential of will also become 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, at the part where the transfer material P is broken, the photoreceptor L is charged by direct pressure of corona ions, so the potential of the photoreceptor L becomes higher than the part corona-charged through the transfer material P, and the charge is increased. Many remain. According to an experiment, a corona discharge was applied directly to the surface of a photoreceptor whose negative charge potential was 1800V without using a transfer material, using a voltage of a transfer charger that caused a corona discharge to reach +80V through the transfer material. Where, ÷ 500v
Became. In such a state of residual charge, even if the amount of light of the static elimination light 7 is increased, the state of static elimination will become uneven.

While the state of non-uniform charge remains, it is re-injected in the next image forming process.
When the next charge is performed, the negative charge potential becomes non-uniform.

Usually, the outer circumference of the photoreceptor l is often shorter than the length of the transfer material P. Therefore, one rotation of the photoreceptor l does not complete the copying of one sheet, and it may take two or several times. After the second rotation, the photoreceptor 1 has a history of being subjected to transfer corona discharge during the previous rotation, and there are areas in one copy where images are formed with different primary charging states. This results 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 by means of a developing means, in which an electrostatic latent image formed on an area of a photoconductor that has been transferred and charged and an electrostatic latent image formed on an area that has not been transferred and charged alternately. An image density control device for an electrophotographic apparatus, characterized in that the bias voltage applied to the developing means is changed in synchronization with the development means facing each other, and the density of the developed image formed in both the areas is controlled to be constant. It is.

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. Other parts are
Since this is the same as described above, further explanation will be omitted. Note that 9 is a slit and 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, which includes a central processing unit CPU and a storage device ROM-RA.
M, human output unit I10, etc. are all on one chip. 21
is a drive circuit, which sends driving voltages HV--HV+ and HAC to the charger 2, charger 5, developing sleeve 4a, and blade 4b in response to instructions from the microcomputer 20. 2
A waveform shaping circuit 2 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 that adjusts the DC division voltage (hereinafter referred to as "developing bias voltage") of the biased AC bias voltage HAC using a variable resistor VR, and outputs the remote signal VaEr+.
is sent to the drive circuit 21. This variable resistance V
By adjusting R, 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 same figure, constant voltage Vcc is connected to resistor R1 and variable resistor VR.
・Voltage is divided by resistor R2. Di.D2 is a diode. Relay RYI is turned on and off by a signal from the output OUT of microcomputer 20, which will be described later. When the relay RYI is on, the remote signal VRE11 is output as a divided voltage through its contacts.

When relay RYI is off, the divided voltage drops by the forward voltage drop of diode Di (approximately 0° tV) and is output as remote signal VRat1. This drop in remote signal VRE11 is constant regardless of the degree of image density adjustment (adjustment by variable resistor VR).

FIG. 4 shows the variation of the developing bias voltage Voc depending on the voltage of the remote signal VRE. When the relay RYI is energized (output signal out is low), if the voltage of the remote signal VREM is adjusted by 7.5V by the variable resistor VR, the developing voltage (Iasumi pressure Dc) is -300V.

When the relay RYI is de-energized (the output signal out is high), the remote signal VREM voltage becomes 6.5V and the developing bias voltage VDC 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 l is primarily charged. A negative high voltage HV- is applied to the primary charger 2 by the drive circuit 21 (step 1ot). When the shutter IO opens and starts image exposure, the counter of the microcomputer 20 starts counting the clock CL in step 102. After the start, when a time T preset from the rotational speed of the photoreceptor l and the position of the developing device 4 has elapsed (step 103), step 1
After stopping the count of the lock CL at 04, a signal for applying the biased AC voltage HAC is output to the driver 21 (step 105).

At this time, the signal from the output out remains low, so
Since the relay RYI is energized, the developing bias voltage vDC is -300V. Next, in step 106, the photoelectric sensor 8 is operated, and when the transfer material P is detected (step 107), counting of the clock CL is started (step 108). After the start, the feeding speed of the transfer material P (photoreceptor 1
When a preset time T2 has elapsed based on the rotational speed of the sensor 12 and the transfer charger 5 (step 10
9) A signal for applying a positive high voltage HV+ to the transfer charger 5 is sent to the drive circuit 21 (step 110). Similarly, when the clock CL passes a time T3 preset based on the rotational speed of the photoreceptor l and the distance between the sensor 12 and the developing device 4 (step 111), the setting circuit 23 is activated in step 112.
The signal of the output (out) for the signal is set high. Then, the relay RYl is de-energized, the remote signal VRE11 drops, and the developing bias voltage changes from 1300v to 1260v.
shift to. That is, the area on the photoreceptor l that has been transferred is charged 11 degrees 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 is a curve (EV characteristic curve) showing the relationship between the amount of mist light and the surface potential when the photoreceptor 1 is subjected to primary (1) electrical mist light. Curve a is the EV characteristic when the area that has not been subjected to transfer charging is primarily charged to -800V. In addition, since the song MAa has dark decay, the surface potential is -8 even when the exposure amount is 0.
It becomes lower than 00■. Developing the electrostatic latent image of surface potential ■1 when exposed with light amount E, the bias DC component voltage VDC is Va
When developed with , the resulting gls degree is ■al! It corresponds to the potential difference Vo of l:v□. The larger the upper difference v, ) is, the higher the development density is. When the same voltage is applied to the negative charger 2 to primary charge the transfer-charged area, the EV characteristic polarity is a curve, and the surface potential becomes low while remaining substantially parallel to the curve a. At the same exposure amount E, the surface potential changes from vI to v2, and the potential difference with Va is V, and v>v, so the developed density increases. However, due to the above control,
When the transfer-charged area reaches the position of the developing device 4, the developing bias voltage Voc decreases from Va to vb, so the potential difference with the surface potential v2 is V. will be maintained as is. As a result, the developed density becomes constant. In addition, since the discharge by the transfer charger 5 occurs only when the transfer material P is interposed between the photoreceptor l, the areas on the photoreceptor l that have been transferred and the areas that have not been In this case, the difference in residual charge is small, and the difference in potential after 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 embodiments, the voltage values are not limited to those specifically shown, and can be changed as appropriate depending on the characteristics of the photoreceptor, usage conditions, etc. In this embodiment, a negative voltage was applied as the primary charge, but even if a photoreceptor with positive characteristics is used and a positive voltage is applied as the -order charge to form an image, the developing bias voltage Voc may be shifted. It can be applied in reverse. Further, 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 timing from an encoder such as a photointerrupter provided on a rotating photoreceptor.

The developing bias voltage setting circuit can be applied to any circuit other than the above-mentioned inverted arrangement as long as it can be switched so as to shift the level at a constant level. For example, a Zener diode can be used instead of the diode Di.

As explained above, according to the electrophotographic apparatus equipped with the image density control device of the present invention, extremely high quality copied images (!) can be produced with uniform image density.

[Brief explanation of the drawing]

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 device to which the present invention is applied, FIG. 3 is a circuit diagram of its main parts, and FIG. A diagram explaining changes in developing bias, Figure 5 is an EV characteristic curve diagram, and Figure 6 is an EV characteristic curve diagram.
The figure is a flowchart diagram 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, and 23 is a development bias setting circuit.

Claims (1)

[Claims] (1) Transfer of “photoreceptor”? The electrostatic latent image formed in the area that received the transfer band '1E and the electrostatic latent image formed in the area that did not receive the transfer band '1E are synchronously opposed to the development stage L. An image 6 degree control device for an electronic 11 degree device, characterized in that the density of the developed image formed in the circular area is controlled to be constant by changing the bias voltage applied to the developing means.