JP3023139B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus that is effective when applied to a full-color copying machine.

[Conventional technology]

2. Description of the Related Art In an image forming apparatus such as a copying machine, a predetermined visual pattern formed on a photoreceptor is read by a photo sensor in order to prevent a decrease in image quality, for example, a decrease in image quality due to background contamination, and the density of the photo sensor is detected. The potential at the time of image formation of the photoconductor, the developing bias,
Alternatively, there is one that corrects the exposure amount.

In such an image forming apparatus, when the developing pattern is formed while the developing bias is kept constant, not only the toner consumption increases with the rise in the residual potential of the photoconductor, but also the residual potential of the photoconductor becomes a constant value. At this point, the density of the visualized pattern reaches the saturation amount, so that the residual potential cannot be accurately detected based on the density detection level.

For this reason, conventionally, for example, a visible image pattern is formed by a developing bias corrected based on the immediately preceding density detection level, thereby improving the detection accuracy of the residual potential of the photoconductor (Japanese Patent Application Laid-Open No. Sho 63/63). -142370).

[Problems to be solved by the invention]

However, in such a conventional image forming apparatus, when applied to a full-color copying machine, the toner density of the developer (especially cyan and yellow) is high especially in a low-temperature and low-humidity environment, so that the image density is not sufficiently high. In order to obtain the toner density, the toner concentration must be increased, so that the toner holding power of the carrier is reduced and so-called sleeve contamination is easily generated, and the charge of the toner deposited on the sleeve shifts to the effective developing bias. As a result, there is a problem that the entire surface is stained and the image quality is reduced.

[Object of the invention]

Accordingly, an object of the present invention is to improve the image quality by making the developing bias variable and keeping the effective developing bias constant with respect to the charged potential of the photoconductor.

[Means for solving the problem]

In order to achieve the above object, the present invention provides an electrostatic latent image formed by charging and exposing a surface of a photoreceptor, and developing the electrostatic latent image by a developing device using a developer containing at least a toner. An image forming apparatus for forming a visible image on the photoconductor by forming a predetermined visual pattern on the photoconductor and having a density detection unit for reading the visual image density by a photo sensor,
In an image forming apparatus in which at least one of a developing bias, a charging potential, and an exposure amount is changed based on a detection value of the photosensor, an effective developing bias with respect to a background potential of the photoconductor is changed with respect to a background potential of the photoconductor. A developing bias applying means for applying a developing bias at the time of correction that reverses the magnitude relationship of the effective developing bias at the time of image forming; and developing toner at a background potential portion on the photoreceptor by the developing bias applying means at the time of correction. First control means for detecting the developed toner image by the density detection means and controlling a correction developing bias so that the detected value becomes constant; and effective developing during image formation based on the first control means. And second control means for controlling the bias.

 Hereinafter, the present invention will be described based on examples.

 1 to 3 are views showing an embodiment of the present invention.

In FIG. 2, reference numeral 10 denotes a color copying machine as an image forming apparatus. The color copying machine 10 electrically connects a scanner unit 11 for reading an image and an image signal output from the scanner unit 11 as a digital signal. An image processing unit 12 (variable control means) for processing and a printer unit 13 for forming an image on recording paper based on image recording information of each color from the image processing unit 12 are provided.

The scanner section 11 has a lamp 15 for scanning and illuminating the original surface of the original sheet placed on the original placing table 14, and reflected light from the original illuminated by the lamp 15 is reflected by mirrors 16, 17, and 18.
And is incident on the imaging lens 19. Imaging lens
Reference numeral 19 forms an image of the original reflected light on a dichroic prism 20, and the dichroic prism 20 disperses the incident light into light having three wavelengths, for example, red (R), green (G), and blue (B). Light is incident on CCDs (Charge Coupled Devices) 21R, 21G, and 21B. These CCDs 21R, 21G, and 21B convert incident light into digital signals and output the digital signals. The output is subjected to necessary processing in an image processing unit 12, and is, for example, yellow (Y), magenta (M), cyan (C) and The writing unit of the printer unit 13 is converted into image forming information of each color of black (BK).
22 (developed image pattern forming means). Writing unit
Reference numeral 22 denotes a recording unit that outputs laser beams 22Y, 22M, 22C, and 22BK including image information of each color based on an output signal of the image processing unit 12.
The light is emitted to Y, 23M, 23C, and 23BK, and the recording units 23Y, 23M, 23C, and 23BK are arranged at regular intervals on the same plane. Each recording unit has the same configuration of the electrophotographic system. For example, the recording unit 23C uniformly charges the photosensitive drum 24C at a potential corresponding to any of the gradations 1 to 7 shown in FIG. The photosensitive drum 24C is charged and irradiated with modulated light corresponding to image information by a laser beam 22C from the writing unit 22, thereby exposing the photosensitive drum 24C.
Developing unit for latent image of cyan light image formed on 4C
Development is performed by 26C to form a visible image (toner image).
The image processing unit 12 controls the operation of each unit of the printer unit 13 according to a predetermined program stored in the internal memory, and writes the amount of light (laser power) written by the writing unit 22.
Can be changed from gradation 1 to gradation 7, while the writing unit 22 can output a signal for forming a sensor pattern. The laser power at the time of writing the sensor pattern is, for example, gradation 4.

Further, the printer unit 13 is provided with a paper feeding unit,
The paper feeding means adjusts the timing at which the toner image by the development reaches the transfer position, and sends out the transfer paper P in the paper feed cassette 29 to the outside by any one of the plurality of paper feed rollers 28. Align the tip with 30,
The sheet is sent to the transfer belt 31 at the same timing. The transfer paper P conveyed in the left direction in the figure by the transfer belt 31 is a photosensitive drum 24BK, 24C, 24M, on which a visible image is formed.
After being conveyed to the 24Y side to transfer the visible image and fixing the transferred image by the fixing roller 32, the sheet is discharged by the discharge roller 33. The transfer belt 31 is supported by a belt driving roller 34 and a driven roller 35 so as to maintain a predetermined tension, and the toner adhered on the transfer belt 31 at the time of transfer is removed by a cleaning unit 36. Also,
Each of the developing units 26BK, 26C, 26M, and 26Y (hereinafter, also simply referred to as the developing unit 26) is a developing sleeve 3
7, a replenishing roller 38 and a toner container 39 are provided.Although not shown in detail, a stirring member, a group of fixed magnets, a doctor blade, and the like are provided, and the toner T in the toner container 39 is self-weighted. The charged toner adheres onto the supply roller 38, and the charged toner is sent to the developing sleeve 37 side by the rotation of the supply roller 38, and the photosensitive drum 24BK, 24C, 24M or 24Y ( Hereinafter, this is also referred to simply as the photosensitive drum 24). The developing bias VB of each developing sleeve 37 is variably controlled by the image processing unit 12.

On the other hand, 41Y, 41M, 41C and 41BK
The reflective photosensors 41Y, 41M, 41C, and 41BK are provided in correspondence with the photoconductive drums 24Y, 24M, 24C, and 24BK. That is, the toner concentration TC of the visual image is detected. As shown in FIG. 3, the photo sensors 41Y, 41M, 41C, and 41BK (hereinafter, also simply referred to as the photo sensor 41, and the photo sensor 41 forms a density detecting means)
Light-emitting element 43 and light-receiving element 44 mounted on (wiring board)
And a dustproof filter 45, and a storage member 46 that supports the dustproof filter 45 and stores the light emitting element 43 and the light receiving element 44.
The storage member 46 has a tapered narrowed portion 46a, 4
6b is formed. Further, the photo sensor 41 reads the sensor pattern written by the writing unit 22, and the image processing unit 12 determines the developing bias of the developing unit 26 and the photosensitive drum based on the detection voltage of the sensor pattern.
At least one of the charging potential of 24 or the writing light amount (exposure amount) of the writing unit 22 is changed.

In such a configuration, an image signal is output as a digital signal from the scanner unit 11, and when this image signal is electrically processed by the image processing unit 12, based on the image recording information of each color from the image processing unit 12, Then, the printer unit 13 forms an image on the transfer paper P.

In addition, the image processing unit is similar to the normal image forming.
A predetermined sensor pattern is formed by a sensor pattern forming means including the writing unit 12, the writing unit 22, the charging charger 25, and the developing unit 26, and this is read by the photo sensor 41. Then, based on the detection information of the sensor pattern, at least one of the developing bias of the developing unit 26, the charged potential of the photosensitive drum 24, or the writing light amount of the writing unit 22 is appropriately controlled.

On the other hand, the image processing unit 12 forms a correction developing bias applying unit, a first control unit, and a second control unit, and controls the developing bias of the developing unit 26 after or before the image forming operation. Specifically, as shown in FIG.
The background potential VO of 24 is small in the direction that reverses the magnitude relationship during normal imaging (VB indicated by the solid line is larger than the minus potential VO), for example, a potential difference ΔV of about 1/5 or less of the imaging potential. _The correction-time developing bias having the _OB is applied to the developing sleeve 37 of the developing unit 26 by the correction-time developing bias applying means, whereby the toner is developed on the photosensitive drum 24. The output VK of the photo sensor 41 that reads this toner image (detection voltage at the time of minute potential)
Are constant so that the developing bias VB is equal to the arrows S ₁ and S _{2 in the} figure.
Is shifted by the first control means. That is, in this embodiment, the shift amount VBS is considered as a difference between the effective developing bias and the output developing bias, and this is added to the developing bias at the time of image formation by the second control means.

Specifically, the developing bias VB is considered as the sum of the bias value VB (target value) when there is no effective bias shift and the value VBS for canceling the effective bias shift, and the background potential of the photosensitive drum 24 is considered. When the effective bias shift is obtained based on VO, the following is obtained.

VB = VB (target value) + VBS (1) VB (target value) = VO + VBK (2) VB = VO + VBK + VBS (3) where VO: charged potential of photosensitive drum 24 (ground potential), VBK : VK imaging potential (for example, set to 24 Volt) The output of the photosensor 41 at this time is defined as VK, and VB is set so that the photosensor output VK becomes the target value VKO.
Is shifted by the effective bias deviation (in other words,
The optimum shift amount can be understood.

In this embodiment, eight moving averages of VK are taken and compared with VKO. When the difference between VK and VKO is less than 0.1 volt (0.2 volt in the case of black), control is not performed, and VB is not controlled.
Even if K is as small as 24 volts, it is hard to be affected by uneven charging or the like.

| VK−VKO | <0.1volt (4) That is, the target voltage of the control pattern section of the toner concentration TC is V
Assuming that TC and the bias shift amount target voltage are VKO, and the nth detection voltage of the photosensor 41 is TC control pattern section detection voltage: VSP (n), and bias shift amount detection voltage: VK (n), the normal toner density In the case of the control state, the following equation holds for almost all n.

| VSP (n) −VTC | <0.2volt (5) (0.4 volt in the case of black) In this case, the moving average of the bias shift amount detection voltage VK (n) is set as the shift amount VK, for example, calculated by the following equation. I do.

On the other hand, when the toner density control state is abnormal, (4)
The expression does not hold, and for some n or all n, | VSP (n) −VTC |> 0.2 volt (7) (0.4 volt for black). In this case, VKO, which is the target value of VK, is substituted for n for which inequality (7) holds, instead of VK (n).

VK (n) = VKO (8) Then, the moving average of the shift amount VK is calculated from the equation (6) using VK (n).

In the present embodiment, the VK imaging potential VB
An electric field is applied in the forward direction (the direction in which normal toner is developed) to reduce the effect of the oppositely charged toner (the oppositely charged toner is not developed for the forward electric field). VBK is set to a level higher than the amount of non-charged toner adhered to remove the effect of background contamination on the photosensitive drum 24. This prevents the so-called runaway state due to an infinitely large shift amount, and also prevents a detection error of the photo sensor 41.

In addition, the toner concentration deviated from the target value due to a defective toner supply or a defective toner end detection (VSP in FIG.
If the deviation is large, VK is detected to be small because the developing ability is reduced. However, the correction of VK (shift of bias VB) is also reduced, or the correction is not performed when the deviation is large. That is, in the process in which the toner density control shifts from a normal state to an abnormal state, the VK correction amount is gradually changed in accordance with the degree of the abnormality. The correction amount is zero, but if the ripple is simply large (it becomes remarkable when the developer is used in a fatigued state and under high-temperature and high-humidity conditions), the correction works but the degree Become smaller.

As described above, in the present embodiment, the toner is developed on the photosensitive drum 24 by the correction-time developing bias having a small potential difference in a direction opposite to the magnitude relationship during image formation with respect to the background potential VO of the photosensitive drum 24. Then, the developing bias VB is shifted so that the density detection value VK of the photosensor 41 for this toner image becomes constant. Therefore, the developing bias VB is kept constant with respect to the charging potential VO of the photosensitive drum 24, and the deviation of the effective developing bias is prevented, and the image quality is improved.

〔effect〕

According to the present invention, the image is developed on the photoconductor by a correction developing bias having a minute potential difference in a direction opposite to the magnitude relationship during image formation with respect to the background potential of the photoconductor, and the detection value of the photosensor for this image is obtained. The developing bias is changed so as to maintain a constant value, so that the effective developing bias can be kept constant with respect to the charged potential of the photoconductor to prevent deviation of the effective developing bias, thereby improving image quality. Can be.

[Brief description of the drawings]

1 to 3 show an embodiment of the image forming apparatus according to the present invention. FIG. 1 shows the relationship between the output of the photo sensor, the dynamic range of the electrostatic latent image and the developing characteristics, and the control of the developing bias. FIG. 2 is a schematic configuration diagram of the color copying machine, and FIG. 3 is a configuration diagram of the photosensor. 10: color copier (image forming apparatus), 12: image processing unit (correction developing bias applying means, first
Control means, second control means), 22 writing unit, 24Y, 24M, 24C, 24BK photosensitive drum, 25Y, 25M, 25C, 25BK charging charger 26Y, 26M, 26C, 26BK Developing unit, 37… Developing sleeve, 41Y, 41M, 41C, 41BK… Photo sensor (density detecting means), VB… Developing bias, VK… Density detection value at minute potential, VO… photosensitive drum charging potential (Background potential), ΔV _OB … a minute potential difference.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Katsuhiro Aoki 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd. (72) Takayuki Maruta 1-3-6 Nakamagome, Ota-ku, Tokyo Ricoh Co., Ltd. (56) References JP-A-58-60758 (JP, A) JP-A-63-167386 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 13 / 06 G03G 15/00 303 G03G 15/06-15/06 102 G03G 15/36 G03G 21/00 370-540 G03G 21/02-21/04 G03G 21/14 G03G 21/20

Claims (1)

(57) [Claims] An electrostatic latent image is formed by charging and exposing a surface portion of a photoreceptor, and the electrostatic latent image is developed by a developing device using a developer containing at least a toner to form a latent image on the photoreceptor. An image forming apparatus for forming an image, comprising density detection means for forming a predetermined visual pattern on a photoreceptor and reading the visual density by a photo sensor, based on a detection value of the photo sensor In an image forming apparatus in which at least one of a developing bias, a charging potential, and an exposure amount is changed, a magnitude relationship between an effective developing bias with respect to a background potential of the photoconductor and an effective developing bias at the time of image formation with respect to a background potential of the photoconductor is provided. A developing bias applying means for applying a developing bias at the time of correction such that the developing bias is reversed, and developing toner at a background potential portion on the photoconductor by the developing bias applying means at the time of correction. First control means for performing detection by the density detection means and controlling the developing bias at the time of correction so that the detected value is constant; and second control for controlling the effective developing bias at the time of image formation based on the first control means. And an image forming apparatus.