JPH04309971A - Image forming device - Google Patents

Abstract

PURPOSE:To always obtain an excellent image by accurately detecting the rise of residual potential caused by using a photosensitive body and feeding back the detected value to the control of forming the image in a toner image transfer type image forming device. CONSTITUTION:The surface charge of a photosensitive drum which is electrostatically charged by an electrostatic charger 6 is destaticized by an interimage eraser 10. Two kinds of reversely developing bias having the same polarity as the electrostatically charged polarity of toner and having different voltage are impressed on the destaticized part to form two test toner images. The density of the test toner image is detected by a photosensor and the voltage value of the developing bias at the time of copying processing is controlled in accordance with the detected value of the density.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a toner image transfer type image forming apparatus using electrophotographic copying, and more particularly to an automatic image density control mechanism thereof.

0002

2. Description of the Related Art In recent years, organic photoreceptors have been mainly used in toner image transfer type electrophotographic copying machines. Some types of organic photoreceptors have a characteristic that as the number of times they are used increases, their sensitivity to light does not change much, but their residual potential increases. An increase in the residual potential results in an increase in the potential of the background portion of the image, resulting in frequent occurrence of so-called toner fog.

0003

[Object, structure, and operation of the invention] Therefore, the object of the present invention is to
In a toner image transfer type image forming apparatus, by accurately detecting the increase in residual potential due to the use of a photoreceptor and feeding back the detected value to the control during image formation, it is possible to always obtain copied images of good image quality. It is in. In order to achieve the above object, the image forming apparatus according to the present invention includes a static eliminator that is installed between a charging means and a developing means, and that eliminates the electric charge on the photoconductor, and a developing electrode that has the same charging polarity as the toner. a developing means for applying developing biases of different polarities and at least two voltages to form at least two test toner images of different densities on the surface of the photoreceptor whose charge has been removed by the charge removing means; The toner image forming apparatus includes a density detecting means for detecting the density, and a developing bias voltage controlling means for controlling the voltage value of the developing bias during image formation according to the detected density value of the toner image.

[0004] In the above structure, although the charge on the photoreceptor charged by the charging means is removed by the charge removing means (for example, an LED), some charge remains. Toner is applied to this residual charge portion by a developing means. The adhesion of the toner is carried out by applying to the developing electrode at least two different developing biases having the same polarity as the charging polarity of the toner. In this way, at least two test toner images with different densities are formed, and these densities are determined by the detection means (
for example, a photo sensor). The density of the test toner image is a constant value depending on the residual voltage of the photoreceptor and the voltage of the developing bias, and the residual potential is calculated based on the detected density value. Based on this calculation result, the voltage value of the developing bias to be applied during image formation is controlled. That is, the developing bias is increased in accordance with the increase in the residual potential of the photoreceptor to prevent toner fogging on the background portion of the image.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an image forming apparatus according to the present invention will be described below with reference to the accompanying drawings. In this embodiment, the present invention is applied to a toner image transfer type electrophotographic copying machine. (Schematic configuration and operation of copying machine) As shown in FIG. The image is exposed to the exposure portion X by the optical system 20, and an electrostatic latent image corresponding to the original image is formed on the photoreceptor drum 5.

The optical system 20 includes an exposure lamp 21 and a mirror 22.
It is composed of a to 22d and a projection lens 23. During image exposure, the exposure lamp 21 and mirror 22a move in the direction of arrow b at a speed of v/m (m: copying magnification), and the mirrors 22b and 22c can move at a speed of v/2 m. Around the photosensitive drum 5, there is a charger 6 that uniformly charges the surface to a predetermined potential, an interimage eraser 10 that erases interimage charges in areas other than the latent image forming area, and static electricity formed by an optical system 20. A developing device 7 attaches toner to the electrostatic latent image and visualizes it, and the toner image is transferred to the registration roller 3.
A transfer charger 28 that transfers the image onto the copy paper P fed from 0, a separation charger 29 that separates the copy paper P from the photoreceptor drum 5, a cleaner device 9 that removes residual toner adhering to the photoreceptor drum 5, and a photoreceptor drum. A main eraser 8 for erasing residual charges on the 5 is provided.

(Configuration and operation of developing device) The developing device 7 is
Development is carried out by so-called regular development, in which toner is attached to the charged portions of the latent image passing through the development section Y using a well-known magnetic brush method using a developer made of a mixture of a magnetic carrier and an insulating toner. . Inside the developing tank 70, a developing sleeve 71 containing a magnetic roller 72 and a height regulating plate 73 are provided.
, a bucket roller 74, and a screw roller 75 are installed. The developer is attracted to the outer circumferential surface of the developing sleeve 71 by the magnetic force of the magnetic roller 72 based on the rotation of the bucket roller 74 in the direction of the arrow c, and the developer is attracted to the outer peripheral surface of the developing sleeve 71 by the magnetic force of the magnetic roller 72 based on the rotation of the developing sleeve 71 in the direction of the arrow d or the direction of the arrow d' of the magnetic roller 72. It is transported to the developing section Y based on the rotation. Developing sleeve 7
1 functions as a developing electrode, and during normal copying processing, a developing bias of a predetermined voltage having a polarity opposite to the charged polarity of the toner is applied to the developing sleeve 71 to prevent the toner from covering the background portion of the image. There is.

On the other hand, a toner replenishment roller 77 that is rotationally driven by a replenishment motor 78 is installed at the bottom of a toner tank 76 provided above the developer tank 70 . The toner in the toner tank 76 is replenished onto the screw roller 75 based on the rotation of the replenishment roller 77 , and is agitated and mixed with the already existing developer by the rotation of the roller 75 .
Sent to 4. During stirring and mixing, the toner is triboelectrically charged with the magnetic carrier to a predetermined amount of charge. The toner concentration in the developer is detected by a magnetic sensor (not shown) provided in the developer tank 70, and the replenishment motor 78 is controlled to maintain a constant toner concentration at all times.

(Increase in residual potential of photoreceptor) The photoreceptor used in this copying machine is an organic photoreceptor, and one of its characteristics is that its residual potential increases with use. FIG. 5 shows the surface potential of the photoreceptor with respect to the amount of exposure light, where a curve A1 shows the initial sensitivity characteristics and a curve B1 shows the sensitivity characteristics when the photoreceptor has been used for about half its life. As is clear from Figure 5,
In the initial stage, the residual potential is Vr1, and the residual potential corresponding to the portion where the original image density is 0.07 (the exposure light amount corresponds to 2.0 lux·sec) is Vir1. The developing bias voltage applied to the developing sleeve 71 during development is Vb0, which corresponds to the residual potential of the portion where the original image density is 0.2 (the exposure light amount corresponds to 1.4 lux·sec), and this potential Parts are not developed either. With such initial characteristics, there is a sufficient gap between the developing bias voltage Vb0 and the residual voltage Vir1, and toner does not cover the image background.

On the other hand, after use, the residual potential is Vr
2, and the residual potential corresponding to the portion where the original image density is 0.07 rises to Vir2. In this case, if the developing bias is maintained at the voltage Vb0, the gap between the bias voltage Vb0 and the residual potential Vir2 becomes small, and the background portion of the image is covered with toner. In particular, the original image density is 0.
．． Toner also adheres to the part 2. Therefore, after use, it is necessary to increase the developing bias voltage to Vb0' to prevent toner from covering the image background.

The residual potential Vr2 of the photoreceptor and the developing bias voltage Vb0' necessary to prevent toner fogging have a certain correlation as shown in FIG. Therefore, in order to control the voltage value of the developing bias, the residual potential V of the photoreceptor must be
It is necessary to accurately detect r2. (Detection of residual potential and control of developing bias voltage) In this embodiment, in order to stabilize the image, the charge on the photoreceptor drum 5, which has been uniformly charged by the charger 6, is removed by the inter-image eraser 10. Thus, two latent images of residual potential Vr2 are formed on the photosensitive drum 5. Then, this test latent image is applied to the developing sleeve 71 at voltages Vb1 and Vb2, respectively.
A reverse development bias (opposite polarity to that during normal copying processing, same polarity as the charged polarity of the toner) is applied to force the toner to adhere to the test latent image with residual potential Vr2, forming two test toner images. do. Furthermore, the density of this test toner image is optically detected by a reflective photosensor 19 (see FIG. 1).

FIG. 6 shows the relationship between the voltage value of the reverse developing bias applied to the developing sleeve 71 and the toner adhesion amount (density of the test toner image), where straight line A2 represents the initial characteristic and straight line B represents the initial characteristic.
2 shows the characteristics when the photoreceptor has been used for about half of its life. The slope of the straight line B2 can be determined based on the toner adhesion amounts M1 and M2 corresponding to the reverse development bias voltages Vb1 and Vb2,
The residual potential Vr2 at that time is derived. Also, straight line B2
' represents the toner adhesion amount when the toner charge amount and toner concentration increase when the photoreceptor exhibits the characteristic of straight line B2. Even when such development characteristics change, the residual potential Vr2 can be determined by detecting the densities of test toner images of at least two types of densities.

The method of calculating the residual potential Vr2 is as follows. The function indicating the characteristics is shown by formula (1),
y=ax+b
…(
1) After use, a in formula (1) is represented by formula (2). a=(M2-M1)/(Vb2-Vb1)
(2) On the other hand, the toner adhesion amount M2 when the reverse development bias voltage Vb2 is applied is expressed by equation (3).

M2=a・Vb2+b
…(
3) Here, b in formula (1) is represented by formula (4). b=M2-a・Vb2
…(
4) When y=0, the residual potential becomes Vr2. Therefore,
0=a・Vr2+b
…(
5) By substituting the above equations (2) and (4) into equation (5), the residual potential Vr2 is expressed by equation (6).

Vr2=-b/a=(M2・Vb1−M1・Vb2
)/M2-M1 (6) That is, by detecting the density (adhesion amount) of the test toner image formed under predetermined reverse development bias voltages Vb1 and Vb2, the above formula (
6), the residual potential Vr2 is determined. Next, as shown in FIG.
0' is calculated based on the following equation (7).

Vb0'=k(Vr2-Vr1)+Vb0
…(7)

k: constant If Vb0' calculated here is used as the voltage applied to the developing bias to the developing sleeve 71 in the subsequent copying process, a good copied image without toner fog in the image background can be obtained.

(Specific Example of Image Stabilization Processing) Formation of a test toner image, detection of its density, and control of the developing bias voltage will now be explained in detail. As shown in FIG. 2, a relatively dark test toner image T2 is formed when the reverse development bias of voltage Vb2 is applied, and a relatively thin test toner image T1 is formed when the reverse development bias of voltage Vb1 is applied. These concentrations are detected by a reflective photosensor 19. The photosensor 19 includes a light emitting element 19a and a light receiving element 19b, and is installed at a position facing the surface of the photosensitive drum 5 extending from the transfer section Z to the cleaner device 9 and corresponding to the test toner images T1 and T2. ing. Therefore,
This photosensor 19 can detect the density of the test toner images T1 and T2. If it is dark, the amount of reflected light is small, and the output voltage Vs from the light receiving element 19b becomes low. If it is thin, the amount of reflected light is large, and the output voltage Vs from the light receiving element 19b becomes high.

As shown in FIG. 3, the light emitting element 19a is grounded via the switch SW11 and the variable resistor 19c, and the output voltage Vs of the light receiving element 19b is connected to the microcomputer 2.
The signal is input to the A/D port of 01 (hereinafter referred to as CPU). The input sensor output voltage Vs is converted into a digital signal by an A/D converter, stored in a battery-backed random access memory 202 (hereinafter referred to as RAM) via a data bus 203, and taken out as appropriate for data processing. be done.

A reverse bias control circuit 216 including an arithmetic circuit 211 for calculating a developing bias voltage, a bias switching circuit 214, and a reverse developing bias power supply 215 is connected to the output port of the CPU 201. The bias control circuit 213 includes a developing bias power supply 212, and controls the bias voltage for regular development applied to the developing sleeve 71 during copy processing to a calculated value Vb0'. The reverse bias control circuit 216 controls the development sleeve 71 during image stabilization processing.
The reverse development bias voltages applied to form the test toner image are controlled to Vb1 and Vb2. The bias switching circuit 214 includes switches SW101 and SW102. The switch SW101 switches the voltage between positive and negative, and when turned off, a negative voltage is output (during copying processing), and when turned on, a positive voltage is outputted (during image stabilization processing). When the switch SW101 is turned on, the voltage Vb
2 is output, and a test toner image T2 is formed. Further, when the switch SW102 is turned on, the voltage Vb1 is output and a test toner image T1 is formed.

Next, FIG. 4 shows the process of image stabilization processing.
This will be explained with reference to the time chart. The image stabilization process is first executed when the main switch is turned on and the copying machine is powered on. It is also executed every time a predetermined period of time elapses. When the main switch is turned on, warm-up is performed to start up the fixing device (not shown) for a predetermined time T0. Thereafter, the main motor is turned on and the photoreceptor drum 5 starts rotating, and the charger 6 and the inter-image eraser 10 are turned on and the photoreceptor drum 5 starts rotating.
Once the surface is charged, it is neutralized. When the charge-eliminated portion reaches the developing section Y, the switch SW101 is turned on, a reverse developing bias of voltage Vb2 is applied to the developing sleeve 71, and a test toner image T2 is formed. Thereafter, the switch SW102 is turned on, a reverse developing bias of voltage Vb1 is applied to the developing sleeve 71, and a test toner image T1 is formed.

Switch S for operating the photosensor 19
W11 indicates that the test toner images T2 and T1 are the photosensor 1.
It is turned on just before passing the detection point No. 9 to prepare for measuring the amount of reflected light. When no toner is attached, the light receiving element 19
The output voltage Vs of b is 12V in this embodiment. When the test toner images T2 and T1 pass through the detection point, the output voltage Vs of the light receiving element 19b is increased by 5.
The data are read twice and averaged to obtain the adhesion amounts M2 and M1. Based on this detected value, the residual potential Vr2 is calculated using the equation (6) above.
Further, the developing bias voltage Vb0' used during normal copying processing is calculated using the above equation (7).

(Control Procedure) Next, the control procedure performed by the CPU 201 regarding the image stabilization process described above will be explained with reference to the flowcharts shown in FIGS. 7 and 8. Figure 7 shows CPU2
01 main routine is shown. The power is turned on and the CPU
When the program 201 is reset, the program starts, and in step S1 clears the RAM 202, initializes various registers, and performs initial settings to set each device in an initial mode. Subsequently, warm-up processing is performed in step S2, and image stabilization processing is performed in step S3. Details of the subroutine here will be explained with reference to FIG.

Next, in step S4, a timer T is set, and in step S5, input signals from various keys on an operation panel (not shown) of the copying machine are processed. In step S6, it is determined whether the print switch has been turned on, and if NO, the process returns to step S5. If YES, step S7
The copying operation is processed in step S8, and other processes such as temperature control of the fixing device are performed in step S8. Subsequently, in step S9, it is determined whether or not the timer T has reached a predetermined time T1, and N
If YES, the process returns to step S5, and if YES, the process returns to step S3 to perform image stabilization processing.

FIG. 8 shows a subroutine for image stabilization processing executed in step S3. First, step S1
1, test toner images T1 and T2 are formed in the above-mentioned process,
In step S12, the photosensor 19 detects toner concentration, that is, toner adhesion amounts M1 and M2. Subsequently, in step S13, the residual potential Vr2 is determined based on the equation (6).
is calculated, and in step S14, a developing bias voltage Vb0' corresponding to the residual potential Vr2 is calculated based on the above equation (7). Then, in step S15, the developing bias voltage used for subsequent copying processing is set to the calculated voltage Vb0'.

(Other Embodiments) It should be noted that the image forming apparatus according to the present invention is not limited to the above-mentioned embodiments, and can be variously modified within the scope of the gist thereof. For example, in the embodiment described above, the image stabilization process is executed not only immediately after the power is turned on but also every time a predetermined time T1 elapses.
The process may be executed every time a predetermined number of copies are made or every time the photosensitive drum 5 rotates a predetermined number of times.

[0026]

As is clear from the above description, according to the present invention, at least two test toners are applied to the residual potential portion of the photoreceptor under reverse developing biases of at least two different voltages as image stabilization processing. An image is formed and the detected density values are fed back to the voltage value of the developing bias during image formation, so despite the increase in residual potential due to the use of the photoreceptor, there is no toner fog in the background of the image. You can get a good image.

[Brief explanation of drawings]

The drawing shows an electrophotographic copying machine which is an embodiment of the present invention.

FIG. 1 is a schematic configuration diagram of an electrophotographic copying machine.

FIG. 2 is a chart showing the relationship between the amount of toner adhering to a test toner image and the sensor output voltage.

FIG. 3 is a block diagram of a control circuit.

FIG. 4 is a time chart diagram showing an image stabilization process.

FIG. 5 is a graph showing the surface potential of a photoreceptor relative to the amount of exposure light.

FIG. 6 is a graph showing the relationship between the voltage value of the reverse developing bias and the amount of toner adhesion.

FIG. 7 is a flowchart showing the main routine of the CPU.

FIG. 8 is a flowchart showing a subroutine of image stabilization processing.

[Explanation of symbols]

5...Photoreceptor drum 6...Electrical charger 7...Developing device 10...Inter-image eraser 19...Photo sensor 71...Developing sleeve 201...CPU 202...RAM 211... Arithmetic circuit 214...Bias switching circuit

Claims (1)

[Claims] 1. An electrostatic latent image is formed by an image exposure means on a photoreceptor that is uniformly charged to a predetermined potential by a charging means, and this electrostatic latent image is transferred as a toner image onto copy paper by a developing means. In an image forming apparatus, a static eliminator is installed between a charging means and a developing means, and applies to the developing electrode, a static eliminator that removes the charge on the photoreceptor, and at least two different developing bias voltages having the same polarity as the charging polarity of the toner. a developing means for forming at least two test toner images of different densities on the surface of the photoreceptor whose charge has been removed by the charge removing means; a density detecting means for detecting the density of the test toner image; and a density detecting means for detecting the density of the toner image. An image forming apparatus comprising: a developing bias voltage control means for controlling a developing bias voltage value during image formation according to the developing bias voltage value.