JP2011164528A - Image forming apparatus and method for controlling image density - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus and a method for controlling image density, which suppress significant changes in the image density, before and after image density control. <P>SOLUTION: An image density control means calculates an upper-limit value Vp_th, based on a calculated development γ and an upper-limit value Δy_max of the amount of change in thw amount of toner attached which is permissible, before and after process control; when the absolute value ¾ΔVp¾ of the amount of a change in development potential, when a change takes place from the previous development potential Vp (n-1) to a target development potential Vp<SB>0</SB>, is smaller than the calculated upper limit value Vp_th, the development potential is set to the target development potential; and when the change is equal to or larger than the calculated upper limit value Vp_th, and ΔVp is positive, the development potential is set to the value resulting from adding the upper limit value Vp_th to the previous development potential Vp (n-1); and when ΔVp is negative, the development potential is set to a value resulting from subtracting the upper-limit value Vp_th from the previous development potential Vp (n-1). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image density control method.

  In an image forming apparatus such as an electrophotographic copying machine or a laser beam printer, image density control as described in Patent Document 1 is performed in order to always obtain a stable image density. Yes. That is, a gradation pattern composed of a plurality of density detection toner patches formed under different image forming conditions (development potentials) so as to have different toner adhesion amounts on an image carrier such as a photoconductor. The toner adhesion amount of each toner patch is calculated using a detection value obtained by detecting the toner patch as a patch-like toner image by an optical sensor as an adhesion amount detection unit and a predetermined adhesion amount calculation algorithm. Then, a linear equation y = ax + b, which is a development performance line indicating the relationship between the development potential and the toner adhesion amount, is obtained from the relationship between the toner adhesion amount and the development potential of each toner patch. From the obtained linear equation, development γ (development potential is abscissa and inclination a when the toner adhesion amount is ordinate) and development start voltage Vk (development potential is abscissa and toner adhesion amount is ordinate) Find the intercept b). Based on the obtained development γ and development start voltage Vk, a development potential that provides an appropriate toner adhesion amount is set, and image forming conditions such as LD power, charging bias, and development bias are adjusted so that the set development potential is obtained. Control.

  The image density control is configured to be executed when the number of times of image formation reaches a predetermined number. For example, as described in Patent Document 2, when the number of times of image formation reaches a predetermined number during a continuous image forming job, the continuous image forming job is temporarily interrupted, and after performing image density control, the remaining number of sheets An image forming process is performed.

  However, if the developing ability of the developer in the developing device varies greatly between the execution of the previous image density control and the current image density control, before the image density control and after the image density control. The image density sometimes fluctuated greatly. When image density control is performed during a continuous image forming job, if the image density fluctuates significantly before and after image density control, the image density changes abruptly in the middle of the page. A malfunction will occur.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an image forming apparatus and an image density that can suppress a significant change in image density before and after image density control. It is to provide a control method.

In order to achieve the above object, the invention of claim 1 is characterized in that a latent image carrier that carries a latent image, and a latent image carrier that has a charging bias such that the surface of the latent image carrier has a target charging potential. A charging unit that uniformly charges the surface; a latent image forming unit that forms an electrostatic latent image by exposing the surface of the latent image carrier charged by the charging unit; and a surface of the latent image carrier. Developing means for forming a toner image on the surface of the latent image carrier by attaching toner on a developer carrier to which a developing bias is applied to the formed electrostatic latent image; and A plurality of toner images developed under different image forming conditions, and an adhesion amount detecting means for detecting the toner adhesion amount per unit area on the toner image on the surface or the toner image transferred from the latent image carrier to the transfer body A pattern image consisting of The image density for performing image density control for setting the development potential, which is the difference between the potential of the electrostatic latent image portion and the development bias, based on the result of detection of the toner adhesion amount on the plurality of toner images by the adhesion amount detection unit. In the image forming apparatus including the control unit, the image density control unit is set in the previous image density control of the current development potential set in the current image density control based on the result detected by the adhesion amount detection unit. The upper limit value of the change amount with respect to the previous development potential is set, and the current development potential is set so that the change amount of the current development potential with respect to the previous development potential is less than or equal to the above upper limit value. It is.
According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the image density control unit includes an upper limit value of a variation amount of the toner adhesion amount that is allowable before and after the image density control, and the adhesion amount detection unit. The upper limit value of the change amount of the current development potential with respect to the previous development potential is calculated based on the slope of the development performance line indicating the relationship between the development potential and the toner adhesion amount obtained based on the result detected by Image forming apparatus.
According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the image density control unit has a relationship between the development potential and the toner adhesion amount grasped based on a result detected by the adhesion amount detection unit. When the slope of the development performance line shown is larger than a predetermined slope, the upper limit value of the change amount of the current development potential with respect to the previous development potential is set to a value smaller than the upper limit value when the slope is a predetermined value. The upper limit value is set.
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the image density control unit is configured such that the development potential and the toner adhesion amount grasped based on a result detected by the adhesion amount detection unit. If the slope of the development performance line indicating the relationship is smaller than a predetermined slope, the upper limit value of the change amount of the current development potential with respect to the previous development potential is larger than the upper limit value when the slope is a predetermined value. It is characterized by being set to a value.
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the image density control means attaches toner to a predetermined electrostatic latent image based on a result detected by the adhesion amount detection means. When the target development potential is determined so that the amount becomes the target toner adhesion amount and the change amount when changing from the previous development potential to the target development potential is less than the upper limit value, the target development potential is determined. The potential is set as the current development potential, the amount of change when changing from the previous development potential to the target development potential is greater than or equal to the upper limit, and the target development potential is greater than the previous development potential. If the value is larger, the value obtained by adding the upper limit to the previous development potential is set as the current development potential. When the change amount when changing from the previous development potential to the target development potential is equal to or greater than the upper limit and the target development potential is smaller than the previous development potential, the previous development potential is changed to the previous development potential. A value obtained by subtracting the upper limit value is set as the current development potential.
According to a sixth aspect of the present invention, in the image forming apparatus of the fifth aspect, the image density control means interrupts the continuous image forming operation when the number of image forming operations reaches a predetermined number during the continuous image forming operation. When performing image density control, the current development potential is set so that the amount of change with respect to the previous development potential is less than or equal to the upper limit value. In other cases, the upper limit value is not set and the predetermined value is not set. A target development potential such that the toner adhesion amount with respect to the electrostatic latent image becomes the target toner adhesion amount is set as the current development potential.
According to a seventh aspect of the present invention, an electrostatic latent image is formed by exposing a surface of the latent image carrier charged by the charging unit and a charging unit for uniformly charging the surface of the latent image carrier. Using the exposure means and the developing means for forming a toner image on the surface of the latent image carrier by attaching toner to the electrostatic latent image formed on the surface of the latent image carrier. A step of forming a pattern image composed of a plurality of toner images developed under different image forming conditions on the body, a step of detecting an amount of toner adhesion to the plurality of toner images by an adhesion amount detection means, and a result of the detection And a developing potential set for the current image density control based on the result detected by the adhesion amount detecting means. The step of setting the upper limit value of the change amount with respect to the previous development potential set in the previous image density control of the current image, and the change amount of the current development potential with respect to the previous development potential so that the change amount is less than the upper limit value. And a development potential setting step.

According to the present invention, the current development potential is set so that the amount of change of the current development potential set in the current image density control with respect to the previous development potential set in the previous image density control is less than or equal to the upper limit value. The The upper limit value of the change amount of the current development potential relative to the previous development potential is set based on the result detected by the adhesion amount detection means. In this way, by setting the upper limit value of the change amount of the current development potential with respect to the previous development potential based on the result detected by the adhesion amount detection means, as described below, before and after the image density control, Thus, it is possible to suppress a significant difference in image density.
A development performance straight line (y = ax + b (y is the toner adhesion amount, x is the development potential)) is grasped based on the result detected by the adhesion amount detection means. Here, when the fluctuation amount Δy of the toner adhesion amount before and after the image density control is set, the change amount Δx of the current development potential with respect to the previous development potential is
Δx = Δy / a (1)
It can be expressed as. This equation (1) means that the toner adhesion amount changes by Δy when Δx is changed with respect to the previous development potential. Since the relationship between the image density and the toner adhesion amount is proportional, if the amount of change in the toner adhesion amount before and after the image density control is made Δy or less, it is possible to suppress the image density from significantly differing before and after the image density control. . In other words, if the amount of change with respect to the previous development potential is set to Δx or less, the amount of change in toner adhesion before and after image density control can be suppressed to Δy or less, and the change in image density before and after image density control can be suppressed. it can. That is, the change amount Δx when the upper limit value of the toner adhesion amount before and after the image density control is Δy_max becomes the upper limit value of the change amount of the current development potential with respect to the previous development potential.
Here, as can be seen from the above equation 1, when Δy is a predetermined value, the change amount with respect to the previous development potential that can keep the change in the image density below the predetermined value before and after the image density control by the value of the slope a. It can be seen that Δx changes. That is, the upper limit value of the change amount varies depending on the result detected by the adhesion amount detection means. Therefore, as in the present invention, the upper limit value of the change amount of the current development potential with respect to the previous development potential is set based on the result detected by the adhesion amount detection means, and the change of the current development potential with respect to the previous development potential is set. By setting the amount to be equal to or less than the set upper limit value, the image density fluctuation before and after the image density control can be surely kept within a predetermined range. As a result, even if the number of image forming operations reaches a predetermined number during the continuous image forming operation and the continuous image forming operation is temporarily interrupted and the image density control is performed, the image density before and after the image density control is increased. It does not fluctuate significantly. As a result, it is possible to suppress a problem that the image density changes suddenly in the middle of the page.

  According to the present invention, it is possible to keep the image density fluctuation before and after the image density control within an allowable range, and even if the image density control is performed during the continuous image forming operation, the image density changes abruptly in the middle of the page. Can be suppressed.

1 is a schematic configuration diagram illustrating a printer according to an embodiment. The expanded block diagram which shows a process unit. FIG. 3 is an exploded perspective view showing a developing device of the process unit. The schematic sectional drawing of an optical sensor. The block diagram which shows the principal part of an electric circuit. Control flow diagram of process control. FIG. 4 is a schematic diagram showing a gradation pattern on an intermediate transfer belt. 6 is a graph showing the relationship between the toner adhesion amount of a toner patch and Vsp and Vsg. 6 is a graph showing the relationship between the toner adhesion amount of a toner patch, ΔVsp and ΔVsg, and a sensitivity correction coefficient α. 6 is a graph showing the relationship between the toner adhesion amount of a toner patch, a diffuse reflection component, and a regular reflection component. The graph which shows the relationship between the normalization value of the regular reflection component in commercial light shielding, and the output value by the diffused light after a background part correction | amendment correction | amendment. FIG. 6 is a diagram illustrating a relationship between a development potential and a toner adhesion amount. FIG. 4 is a diagram illustrating a relationship between toner adhesion amount and image density. The figure explaining a specific example in case an image density differs significantly before and after process control execution. The figure explaining the specific example in the case of performing the conventional development potential upper limit process. FIG. 10 is a diagram for explaining a specific example in a case where image density is significantly different before and after execution of process control when conventional development potential upper limit processing is executed. 6 is a flowchart of a developing potential upper limit process of the present embodiment. The figure explaining the case where the specific example shown in FIG. 16 is performed by the developing potential upper limit process of this embodiment. The figure explaining the specific example of the developing potential upper limit process of this embodiment. The figure explaining the specific example of the development potential upper limit process of this embodiment when this development (gamma) becomes low compared with the last development (gamma). The figure which shows the result of having compared the development potential upper limit process of this embodiment, and the conventional development potential upper limit process. The control flow figure of the modification of process control.

Hereinafter, an embodiment in which the present invention is applied to an electrophotographic color laser printer (hereinafter referred to as “laser printer”) as an image forming apparatus will be described.
FIG. 1 is a schematic configuration diagram showing a main part of the laser printer according to the present embodiment.
In this laser printer, as image forming means, process units 1Y, M, which are four sets of image forming means for forming images of each color of magenta (M), cyan (C), yellow (Y), and black (K). , C, K (hereinafter, the suffixes Y, C, M, and K of the respective symbols indicate members for yellow, cyan, magenta, and black, respectively). The process units 1Y, 1M, 1C, and 1K, respectively, have photoreceptor units 10Y, 10C, 10M, and 10K having drum-like photoreceptors 11Y, 11M, 10C, and 10K as latent image carriers, and development as development means. Devices 20Y, M, C, and K are provided.

  Above the four color process units 1Y, 1M, 1C, and 1K in the figure, a transfer unit 50 is disposed that is endlessly moved counterclockwise in the figure while stretching the intermediate transfer belt 6 that is an endless moving body. . In addition to the intermediate transfer belt 6, the transfer unit 50 serving as transfer means includes a belt cleaning unit 51, four primary transfer rollers 52 Y, M, C, and K, a secondary transfer backup roller 53, a drive roller 54, and the like. . The intermediate transfer belt 6 is endlessly moved counterclockwise in the figure by the rotational driving of the driving roller 54 while being stretched around these rollers. The four primary transfer rollers 52Y, 52C, 52C, 52M, 52C, 52K, 52C, intermediate transfer belt 6 moved endlessly as described above are sandwiched between the photoreceptors 3Y, 3C, 3M, and 3K to form primary transfer nips. ing. Then, a transfer bias having a polarity opposite to that of the toner (for example, plus) is applied to the back surface (loop inner peripheral surface) of the intermediate transfer belt 6. The intermediate transfer belt 6 sequentially passes through the primary transfer nips for Y, M, C, and K along with the endless movement thereof, and on the photoreceptors 11Y, M, C, and K on the front surface. The Y, M, C, and K toner images are superimposed and primarily transferred. As a result, a four-color superimposed toner image (hereinafter referred to as a color image) is formed on the intermediate transfer belt 6. The color image is conveyed to a secondary transfer portion between the intermediate transfer belt 6 and the secondary transfer roller 3 as the surface of the intermediate transfer belt 6 moves.

  In addition to the process units 1Y, M, C, and K, the laser printer has an optical writing unit (not shown) serving as a latent image forming unit disposed below the process unit 1Y, M, C, and K. Has been placed. A one-dot chain line in FIG. 1 indicates a conveyance path of the transfer paper. The transfer paper fed from the paper feed cassette is transported by a transport roller while being guided by a transport guide (not shown), and is transported to a temporary stop position where the registration roller 5 is provided. The transfer paper is supplied to the secondary transfer unit by the registration roller 5 at a predetermined timing. The color image formed on the intermediate transfer belt 6 is secondarily transferred onto the transfer paper, and a color image is formed on the transfer paper. The transfer paper on which the color image is formed is discharged onto the paper discharge tray 8 after the toner image is fixed by the fixing unit 7.

FIG. 2 is an enlarged view showing a schematic configuration of the yellow process unit 1Y among the process units 1Y, 1M, 1C, and 1K. Since the other process units 1M, 1C, and 1K have the same configuration, their descriptions are omitted.
In FIG. 2, the process unit 1Y includes the photoconductor unit 10Y and the developing device 20Y as developing means as described above. In addition to the photoconductor 11Y, the photoconductor unit 10Y includes a cleaning blade 13Y that cleans the surface of the photoconductor, a charging roller 15Y that is a charging unit that uniformly charges the surface of the photoconductor, and the like. Also provided is a lubricant application / static discharge brush roller 12Y having a function of applying a lubricant to the surface of the photosensitive member and discharging the surface of the photosensitive member. The lubricant application and static elimination brush roller 12Y has a brush portion made of a conductive fiber, and a power supply for static elimination (not shown) for applying a static elimination bias is connected to the cored bar portion.

In the photoreceptor unit 10Y having the above configuration, the surface of the photoreceptor 11Y is uniformly charged by the charging roller 15Y to which a voltage is applied. When the laser beam L _Y that is modulated and deflected by The optical writing unit (not shown) on the surface of the photoreceptor 11Y is irradiated while being scanned, an electrostatic latent image is formed on the surface of the photoreceptor 11Y The The electrostatic latent image on the photoreceptor 11Y is developed by a developing device 20Y described later to become a yellow toner image. The toner image on the photoconductor 11Y is transferred onto the intermediate transfer belt 6 at the primary transfer portion, which is a transfer means where the photoconductor 11Y and the intermediate transfer belt 6 face each other. The surface of the photoconductor 11Y after the toner image is transferred is cleaned by a cleaning blade 13Y as photoconductor cleaning means, and then a predetermined amount of lubricant is applied by a lubricant application / static elimination brush roller 12Y and static elimination is performed. To prepare for the next electrostatic latent image.

  FIG. 3 is an exploded perspective view showing the inside of the developing device 20Y. As shown in FIGS. 2 and 3, the developing device 20Y as the developing means has a first agent containing chamber 29Y in which a first conveying screw 24Y as a developer conveying means is disposed. Further, it also includes a second agent storage chamber 21Y in which a second conveying screw 23Y as a developer conveying means, a developing roll 22Y as a developer carrying member, a doctor blade 25Y as a developer regulating member, and the like are disposed. . In these two agent storage chambers forming the circulation path, a Y developer (not shown) which is a two-component developer composed of a magnetic carrier and a negatively chargeable Y toner is contained. The first conveying screw 24Y is rotationally driven by a driving means (not shown), so that the Y developer in the first agent storage chamber 29Y is moved to the rear side of the printer main body (the side in the direction perpendicular to the drawing sheet in FIG. 2). Transport toward Then, the Y developer transported to the end of the first agent storage chamber 29Y by the first transport screw 24Y enters the second agent storage chamber 21Y through the communication port.

  The second conveying screw 23Y in the second agent storage chamber 21Y is driven to rotate by a driving means (not shown), so that the Y developer is fed to the front side of the printer body (the front side in the direction perpendicular to the drawing sheet in FIG. 2). Transport toward In this manner, the developing roll 22Y is disposed in a posture parallel to the second transport screw 23Y above the second transport screw 21Y that transports the Y developer. The developing roll 22Y includes a magnet roller fixedly disposed in a developing sleeve made of a nonmagnetic sleeve that is driven to rotate clockwise in the drawing. Part of the Y developer transported by the second transport screw 21Y is pumped up to the surface of the developing sleeve by the magnetic force generated by the magnet roller. Then, after the thickness of the layer is regulated by a doctor blade 25Y arranged so as to maintain a predetermined gap from the surface of the developing sleeve, the layer is conveyed to a developing region facing the photoconductor 11Y, and Y on the photoconductor 11Y. Y toner is adhered to the electrostatic latent image. This adhesion forms a Y toner image on the photoreceptor 11Y. The Y developer that has consumed Y toner by the development is returned to the second transport screw 23Y as the developing sleeve rotates. Then, the Y developer transported to the end of the second agent storage chamber 21Y by the second transport screw 23Y returns to the first agent storage chamber 29Y through the communication port. In this way, the Y developer is circulated and conveyed in the developing device.

Since the toner density of the developer in the developing case decreases due to toner consumption accompanying the image formation, the toner cartridge 30Y shown in FIG. 1 may be used by the powder pump 27Y as necessary based on the output value Vt of the toner density sensor 26Y. The toner is replenished to be controlled within an appropriate range. The toner replenishment control is based on the difference value Tn (= Vt _ref −Vt) between the output value Vt and the target output value Vt _ref which is the toner density control reference value, and when the difference value Tn is + (plus), the toner concentration If the difference value Tn is − (minus) and the difference value Tn is − (minus), the toner supply amount is increased as the absolute value of the difference value Tn increases, and the output value Vt becomes the target output value. This is performed so as to approach the value of Vt _ref .

  Of the four photoconductors 11Y, 11M, 11C, and 11K, only the black photoconductor 11K on the most downstream side is always in contact with the intermediate transfer belt 6, and the remaining photoconductors are in contact. 11Y, M, and C can contact and separate from the intermediate transfer belt 6. When a color image is formed on the transfer paper, the four photoconductors 11Y, 11M, 11C, and 11K are in contact with the intermediate transfer belt 6, respectively. On the other hand, when a black monochrome image is formed on the transfer paper, only the black photoconductors 11K on which toner images of black toner are formed by separating the photoconductors 11Y, 11M, and 11C for each color from the intermediate transfer belt 6. Is brought into contact with the intermediate transfer belt 6.

On the upstream side of the secondary transfer portion in the intermediate transfer belt surface movement direction, an optical sensor 69 serving as an adhesion amount detection means is arranged to face the front surface of the intermediate transfer belt 6 with a predetermined gap. Has been.
FIG. 4 is a schematic sectional view of the optical sensor 69. As shown in the figure, the optical sensor 69 includes a light emitting element 311 as a light emitting means, a regular reflection light receiving element 312 as a regular reflection light receiving means for receiving regular reflection light, and a light for receiving diffuse reflection light. And a diffuse reflection light receiving element 313. Each element 311, 312, 313 is mounted on a printed circuit board 314. Each element 311, 312, 313 is enclosed in a case 315. Light emitted from the light emitting element 311 is emitted toward the surface of the intermediate transfer belt 6. The regular reflection light that is regularly reflected by the surface of the intermediate transfer belt 6 and the toner patch transferred to the surface is received by the regular reflection light receiving element 312 and a voltage corresponding to the amount of received light is output. Further, the diffuse reflection light diffusely reflected by the surface of the intermediate transfer belt 6 and the toner patch transferred to the surface is received by the diffuse reflection light receiving element 313, and a voltage corresponding to the amount of received light is output.

  As the light emitting element 311 of the optical sensor, a GaAs light emitting diode having a peak emission wavelength of 940 [nm] is used. Further, as the regular reflection light receiving element 312 and the diffuse reflection light receiving element 313, those having a Si phototransistor having a peak spectral sensitivity wavelength of 850 [nm] are used. That is, this optical sensor detects infrared light of 830 [nm] or more with no significant difference in reflectance by color. By using such an optical sensor, it is possible to detect toner patches of all colors Y, M, C, and K with a single sensor.

  FIG. 5 is a block diagram showing the main part of the electric circuit of the copying machine. In FIG. 1, a control unit 100 as a control means includes a CPU (Central Processing Unit) 101 as a calculation means, a nonvolatile RAM (Random Access Memory) 102 as a data storage means, a ROM (Read Only Memory) 103 as a data storage means, and the like. Have. The control unit 100 is electrically connected to process units 1Y, 1M, 1C, and 1K, an optical writing unit 68, a transfer unit 50, an optical sensor 69, and the like. And the control part 100 controls these various apparatuses based on the control program memorize | stored in RAM102 or ROM103.

  The control unit 100 also controls image forming conditions for forming an image. Specifically, the control unit 100 performs control to individually apply a charging bias to each charging member in the process units 1Y, M, C, and K. As a result, the photoreceptors 11Y, 11M, 11C, and 11K of each color are uniformly charged to the drum charging potential for Y, M, C, and K. The control unit 100 individually controls the powers of the four semiconductor lasers corresponding to the process units 1Y, M, C, and K of the optical writing unit 68. Further, the control unit 100 performs control to apply the developing bias of the developing bias values for Y, M, C, and K to the developing rollers in the process units 1Y, M, C, and K. As a result, a developing potential for electrostatically moving the toner from the sleeve surface side to the photosensitive member side acts between the electrostatic latent images on the photosensitive members 11Y, 11M, 11C, and 11K and the developing sleeve, thereby Develop.

In addition, the control unit 100 executes process control as image density control for optimizing the image density of each color when the power is turned on or every time a predetermined number of prints are performed.
FIG. 6 is a basic flowchart of process control. Note that the process control flow of FIG. 6 is a process flow chart of process control when power is turned on.
First, when the power is turned on and the apparatus is started up (S1), the control unit 100 calibrates the optical sensor 69 (S2). Specifically, the light emission intensity of the light emitting element 311 is adjusted so that the output of the regular reflection light receiving element 312 of the optical sensor 69 becomes a predetermined value (4 V). The optical sensor 69 need not be calibrated.

  Next, the control unit 100 acquires the output value Vt of the toner density sensor 26Y (S3), grasps the toner density in each color developing device, and then intermediates the gradation pattern as shown in FIG. Each color is automatically formed on the transfer belt 6 at a position facing each optical sensor 69 (S4). Each color gradation pattern is composed of about five toner patches with different toner adhesion amounts, and at a patch interval of 5.6 [mm], a K color gradation pattern, a C color gradation pattern, and an M color gradation. The pattern is formed on the intermediate transfer belt 6 in the order of the Y-color gradation pattern. Each toner patch has a width in the main scanning line direction of 10 [mm] and a width in the sub scanning line direction of 14.4 [mm]. In the gradation pattern, the charging and developing bias conditions are changed for each toner patch, and the exposure conditions are formed with a predetermined value (full exposure in which the photosensitive member is sufficiently discharged). The setting of the developing bias and charging bias of each toner patch of the gradation pattern will be described later. The gradation pattern of each color on the intermediate transfer belt is optically detected by the optical sensor 69 (S5).

  Next, using the output value of the light receiving element obtained by detecting each toner patch of the gradation pattern of each color, and an adhesion amount calculation algorithm constructed based on the relationship between the adhesion amount and the output value of the light receiving element Conversion processing is performed on the toner adhesion amount (image density).

  In this embodiment, as described in Japanese Patent Application Laid-Open No. 2006-139180, the toner adhesion amount is calculated using specular reflection light regularly reflected by the toner patch and diffuse reflection light. By calculating the toner adhesion amount using the regular reflection light and the diffuse reflection light, the detection range of the high adhesion amount can be expanded as compared with the case where the toner adhesion amount is calculated using only the regular reflection light. Further, by using the toner adhesion amount calculation algorithm described in Japanese Patent Application Laid-Open No. 2006-139180, the output of the light emitting element or the light receiving element due to temperature change, deterioration with time, or the like, or the light receiving element due to deterioration with time of the intermediate transfer belt 6 Even if the output of the toner changes, an accurate toner adhesion amount can be obtained.

Below, the adhesion amount calculation algorithm in this embodiment is demonstrated concretely. The symbols in the explanation are defined as follows.
Vsg: Output voltage value from the optical sensor that detects the background portion of the transfer belt (background portion detection voltage)
Vsp: Output voltage value from the optical sensor that detects each reference patch (patch detection voltage)
Voffset: Offset voltage (output voltage value when LED is OFF)
_Reg: specular reflection light output (Regular Reflection abbreviation)
_Dif: Diffuse reflected light output (abbreviation for Diffuse Reflection) (cf. JIS Z 8105 color terms)
[N] Number of elements: n array variables

First, an algorithm for calculating the adhesion amount of K toner will be described. I) The offset voltage is subtracted from the regular reflection light using the following equation.
ΔVsg_reg [K] [n] = Vsg_reg [K] [n] −Voffset_reg
ΔVsp_reg [K] = Vsg_reg [K] −Voffset_reg [K]

ii) Normalize regular reflection data.
Normalized value Rn [K] = ΔVsg_reg [K] [n] / ΔVsp_reg [K]

iii) The normalized value is converted into an adhesion amount using an LUT (Look Up Table).
An adhesion amount conversion table corresponding to the normalized value is created in advance, and the adhesion amount is obtained corresponding to the table.
The above is the K toner adhesion amount calculation algorithm.

Next, a color toner adhesion amount calculation algorithm will be described.
The color toner adhesion amount is calculated by a seven-step process of STEP 1 to STEP 7 shown below.

[STEP1]
In STEP 1, data sampling is performed to calculate ΔVsp and ΔVsg. First, for both the regular reflection light output and the diffuse reflection light output, the difference from the offset voltage is calculated for all [n] reference patches. This is because, ultimately, it is desired to express only “the increment of the sensor output by the change due to the amount of adhesion of the color toner”.

また、拡散反射光出力増分については、次のようにして求める。

但し、オフセット出力電圧値（Ｖｏｆｆｓｅｔ＿ｒｅｇ、Ｖｏｆｆｓｅｔ＿ｄｉｆ）が、無視できるレベルに十分に小さい値となるＯＰアンプを用いた場合、この様な差分処理は省略しても構わない。
このようなＳＴＥＰ１により、図８に示す特性曲線を得る。 About the regular reflection light output increment, it calculates | requires as follows.

Further, the diffuse reflected light output increment is obtained as follows.

However, in the case of using an OP amplifier in which the offset output voltage values (Voffset_reg, Voffset_dif) are sufficiently small to a negligible level, such difference processing may be omitted.
By such STEP1, the characteristic curve shown in FIG. 8 is obtained.

このようなＳＴＥＰ２により、図９に示すような特性曲線を得る。なお、感度補正係数αをΔＶｓｐ＿ｒｅｇ［ｎ］とＶｓｐ＿Ｄｉｆ．［ｎ］との最小値としたのは、正反射光出力の正反射成分の最小値がほぼゼロであり、かつ正の値となることがあらかじめわかっているからである。 [STEP2]
In STEP 2, a sensitivity correction coefficient α is calculated. First, ΔVsp_reg. [N] or ΔVsp_dif. From [n], “ΔVsp_reg. [N] / ΔVsp_dif. [N]” is calculated for each reference patch. Then, when performing the component decomposition of the regular reflection light output in STEP 3 described later, a sensitivity correction coefficient α for multiplying the diffused light output (ΔVsp_dif [n]) is calculated as follows.

By such STEP2, a characteristic curve as shown in FIG. 9 is obtained. Note that the sensitivity correction coefficient α is set to ΔVsp_reg [n] and Vsp_Dif. The reason why the minimum value of [n] is set is that it is known in advance that the minimum value of the regular reflection component of the regular reflection light output is almost zero and is a positive value.

また、正反射光出力の正反射成分については、次のようにして求める。

このようにして成分分解を行うと、感度補正係数αが求まるパッチ検知電圧にて、正反射光出力の正反射成分がゼロとなる。そして、図１０に示すように、正反射光出力が正反射光成分と拡散光成分とに成分分解される。 [STEP3]
In STEP3, component decomposition of specularly reflected light is performed.
The diffuse light component of the regular reflection light output is obtained as follows.

Further, the regular reflection component of the regular reflection light output is obtained as follows.

When component decomposition is performed in this way, the regular reflection component of the regular reflection light output becomes zero at the patch detection voltage at which the sensitivity correction coefficient α is obtained. Then, as shown in FIG. 10, the regular reflection light output is decomposed into a regular reflection light component and a diffused light component.

[STEP4]
In STEP 4, the regular reflection component of the regular reflection light output is normalized. As in the following equation, the ratio of the patch detection voltage to the background detection voltage is obtained and converted to a normalized value from 0 to 1.

[STEP5]
In STEP 5, the background portion fluctuation correction of the diffused light output is performed. First, the diffused light output component from the belt background is removed from the diffused light output voltage as shown in the following equation.

[STEP6]
In STEP 6, the sensitivity of the diffused light output is corrected. Specifically, as shown in FIG. 11, by plotting the diffused light output after correction of the background portion variation against the “normalized value of the regular reflection component of regular reflection light”, approximating the plot line, The sensitivity of the diffused light output is obtained, and correction is performed so that the sensitivity becomes a predetermined target sensitivity.

ｍ：データ数
ｘ［ｉ］：正反射光＿正反射成分の正規化値
ｙ［ｉ］：地肌部変動補正後拡散光出力
なお、計算に用いるｘの範囲は、０．１≦ｘ≦１．０である。
上記（１）、（２）、（３）の連立方程式を解くことで、係数ξ１、ξ２、ξ３を求めることができる。 With respect to the “normalized value of regular reflection light (regular reflection component)”, a plot line plotting the diffused light output after correction of background fluctuation is polynomial approximated (in this embodiment, quadratic approximation), and sensitivity is obtained. A correction coefficient η is calculated.
First, the plot line is approximated by a quadratic approximate expression (y = ξ1x2 + ξ2x + ξ3), and coefficients ξ1, ξ2, and ξ3 are obtained by the least square method.

m: number of data x [i]: regular reflection light_normalization value of specular reflection component y [i]: diffused light output after background fluctuation correction The range of x used in the calculation is 0.1 ≦ x ≦ 1 .0.
The coefficients ξ1, ξ2, and ξ3 can be obtained by solving the simultaneous equations (1), (2), and (3).

A sensitivity correction coefficient η is obtained so that a certain normalized value a calculated from the approximated plot line becomes a certain value b.

By multiplying the diffused light output after the background fluctuation correction obtained in STEP5 by the sensitivity correction coefficient η obtained in STEP6, the relationship between the adhesion amount and the diffused output is corrected to be a predetermined relationship.

[STEP7]
In STEP 7, the sensor output value is converted into the toner adhesion amount. By the processing up to STEP 6, all the correction processing for the temporal change of the diffuse reflection output caused by the LED light amount reduction or the like has been performed. Finally, the sensor output value is converted into the toner adhesion amount based on the toner adhesion amount conversion table. It is.
The above is the color toner adhesion amount calculation algorithm.

  When the toner adhesion amount of each toner patch is detected using the above-described toner adhesion amount calculation algorithm, the relationship between the toner adhesion amount of each toner patch and each developing potential when each toner patch is created is shown in FIG. Then, a development potential-toner adhesion amount straight line (y = ax + b), which is a development performance straight line approximated by the least square method, is obtained for each color. The development γ (slope a) and development start voltage Vk (intercept b) are calculated for each color from the development potential-toner adhesion amount straight line (S6).

  Next, the control unit 100 calculates a development bias Vb that matches the development potential set in the development potential upper limit process described later (S7). Further, the control unit 100 determines the charging bias Vc based on the calculated developing bias Vb, and stores the developing bias Vb and the charging bias Vc in a nonvolatile storage unit such as the RAM 102. The charging bias Vc is generally set higher by about 100 to 200 [V] than the developing bias Vb. The developing bias Vb is set in the range of 400 to 700 [V]. That is, even if the calculated development bias is 1 [kV], the development bias Vb is set to 700 [V]. This is because if the setting value of the developing bias exceeds 700 [V], the capacity of the power supply may be exceeded, and the bias may not be stably maintained. If the setting value of the developing bias is less than 400 [V], the charging bias Since the set value becomes too low, the charging tends to be non-uniform, and an abnormal image called “afterimage” that may appear in the next image may occur.

After calculating the development bias Vb, the control unit 100 corrects the toner density control reference value Vt _ref using the development γ and the output value Vt of the toner density detection sensor 26 acquired in S3 (S8). First, a difference value Δγ (Δr = calculated development γ−target development γ) between the target development γ and the calculated development γ is calculated. The target development γ is, for example, 1.0 [(mg / cm ² ) / KV] (when the development start voltage Vk is 0 [V] and the development potential is 1 [kV], the toner adhesion amount is 1.0 [ mg / cm ² ] That is, if the development start voltage Vk = 0 V, the target adhesion amount is 0.5 [mg / cm ² ], and the photosensitive member potential Vl after exposure is 50 V, the target development is achieved. (The developing bias Vb calculated from γ is 550 V.)

When the calculated Δγ is outside the predetermined range, the calculated developing bias Vb may exceed the above-described setting range at the next developing bias adjustment. Therefore, the toner density control reference value Vt _ref is corrected, and correction is performed so that the development γ approaches the target development γ until the next process control. If the toner density control reference value Vt _ref is corrected so that the development γ approaches the target development γ, a prescribed image density cannot be obtained even if an image is formed with the calculated development bias. However, the toner concentration in the developing device does not suddenly become the target toner concentration, and the toner replenishment control is performed so that the toner concentration in the developing device gradually becomes the target toner concentration. It doesn't change. Therefore, even if the toner density control reference value Vt _ref is corrected, a predetermined image density can be obtained with the calculated developing bias at first. Then, it gradually moves away from the prescribed image density. However, the correction amount of the toner density control reference value Vt _ref is not set to such a correction amount that the image density greatly deviates from the specified image density even when an image is formed with the calculated development bias. Therefore, the image does not deteriorate greatly. However, the output value Vt of the toner density detection sensor 26 at the time of creation gradation pattern, when the differ significantly from the toner density control standard value Vt _ref, the results in the correction of the toner density control standard value Vt _ref, Conversely, there is a risk of deviating from the target development γ. Therefore, the relationship between the output value Vt of the toner density detection sensor 26 at the time of gradation pattern creation and the output value Vt of the toner density detection sensor 26 at the time of gradation pattern creation is also taken into consideration, and the toner density control reference value is taken into consideration. Decide whether or not to correct Vtref.
As a specific example, when Δγ ≧ 0.30 [(mg / cm ² ) / kV] and Vt−V _tref ≧ −0.2V, the toner density control reference value V _tref is decreased by 0.2V. Thus, correction is performed to lower the toner density from the present time. Further, when Δγ ≦ −0.30 [(mg / cm ² ) / KV] and Vt−V _tref ≧ 0.2 V, the toner density control reference value V _tref is increased by 0.2 V, and compared with the present time. Perform correction to increase toner density. Further, when −0.30 [(mg / cm ² ) / KV] <Δγ <0.30 [(mg / cm ² ) / KV], the toner density control reference value V _tref is not corrected.
The above is the control flow of process control.

Next, a characteristic configuration of the present embodiment will be described.
FIG. 13 shows the relationship between the toner adhesion amount and the image density. As described above, the adhesion amount and the image density have a substantially linear relationship. For this reason, the image density changes as the amount of adhesion changes. If the amount of adhesion increases, the image density increases, and if the amount of adhesion decreases, the image density decreases.

  If the developing ability of the developer in the developing device varies between the previous process control execution time and the current process control execution time, the image density may be significantly different before and after the process control execution.

This will be specifically described with reference to FIG.
The solid line in FIG. 14 is a development potential-toner adhesion amount straight line grasped by the previous process control (hereinafter referred to as “procon”), and the dotted line is a development potential-toner adhesion amount straight line grasped by the current process control. In the figure, Vp (n-1) is the previous development potential set in the previous process control, and Vp (n) in the figure is the current development potential set in the current process control. At the time of image formation immediately before the current process control, the development potential-toner adhesion amount straight line is almost the development potential-toner adhesion amount straight line grasped by the current process control. However, at this time, image formation is performed with the previous development potential Vp (n−1). Therefore, as shown in the figure, the toner adhesion amount in the image forming operation immediately before the current process control is about 0.55 [mg / cm ² ], and the target toner amount (0.45 [mg / cm ² ]) About 0.1 [mg / cm ² ]. Immediately after the current process control, the target toner amount is obtained, so that the toner adhesion amount changes by about 0.1 [mg / cm ² ] before and after the process control. When the toner adhesion amount changes by 0.1 [mg / cm ² ] from the relationship between the toner adhesion amount and the image density in FIG. 13, the image density fluctuates by about 0.2 before and after the process control, and the level at which the difference can be seen visually. Fluctuations in the image density will occur.

  In the present embodiment, the process control is executed every time a predetermined number of prints are performed. For this reason, if the number of prints reaches a predetermined number during a continuous print job, the continuous print job is temporarily interrupted, the process is executed, the image density is set to an appropriate value, and then the continuous print job is resumed. Such control is performed. When a process is executed during a continuous print job, if there is a change in image density that can be seen by the difference before and after the process, an image with a different image density will be output in the middle of the page. .

Therefore, conventionally, in order to suppress such a rapid change in image density before and after the process control, the following development potential upper limit processing is performed. That is, the previous development potential Vp (n−) of the development potential (hereinafter referred to as the target development potential Vp ₀ ) at which the target toner amount is obtained based on the development potential-toner adhesion amount straight line grasped by the current process computer. weight change to 1), if it is less than the upper limit value, it sets the current development potential Vp (n) to the developing potential Vp ₀ goal. When the obtained target development potential Vp ₀ is equal to or higher than the upper limit value, the following processing is performed. That is, when the target development potential Vp ₀ is larger than the previous development potential Vp (n−1), a value obtained by adding the upper limit value from the previous development potential Vp (n−1) is set to the current development potential Vp ₀ . When the development potential Vp (n) is set and the target development potential Vp ₀ is smaller than the previous development potential Vp (n−1), the upper limit is set from the previous development potential Vp (n−1). The subtracted value is set as the current development potential Vp (n).

In the following, the amount of change of the target development potential Vp ₀ with respect to the previous development potential Vp (n−1) is greater than or equal to the upper limit value and smaller than the previous development potential Vp (n−1). This will be specifically described with reference to FIG.
The solid line shown in FIG. 15 is the development potential-toner adhesion amount straight line grasped by the previous process control, and the dotted line is the development potential-toner adhesion straight line grasped by the current process control. Figure Vp (n-1) is the previous development potential that has been set in the previous process computer, figure Vp ₀ is the development potential is grasped in this process computer - based on the toner adhesion amount straight, determined target This is the target development potential for the toner adhesion amount. Vp_th is an upper limit value of the change amount with respect to the previous development potential Vp (n−1).
As shown in FIG. 15, the change amount of the target development potential Vp ₀ obtained by the current process control with respect to the previous development potential Vp (n−1) is not less than the upper limit value Vp_th and the target development potential Vp _0. Is smaller than the previous development potential Vp (n-1). In this case, the current development potential Vp (n) is set as follows. That is, a value obtained by subtracting the upper limit value Vp_th from the previous development potential Vp (n−1) is set as the current development potential Vp (n) (Vp (n) = Vp (n−1) −Vp_th). . By setting the development potential in this manner, the amount of change in toner adhesion before and after the process control becomes about 0.05 [mg / cm ² ], and the change in image density can be suppressed to about 0.1. .

However, in the conventional upper limit processing of the development potential, depending on the development γ value, the toner adhesion amount largely changes before and after the process control, and the image density may change abruptly before and after the process control.
This will be specifically described below with reference to FIG.
The solid line shown in FIG. 16 is the development potential-toner adhesion amount straight line grasped by the previous process control, and the development potential-toner adhesion amount straight line grasped by the current process control is a straight line as shown by the dotted line in the figure. In this case, as shown in the figure, the amount of change in the toner adhesion amount can be suppressed to about 0.05 [mg / cm ² ] by the above-described upper limit processing. On the other hand, when the development potential-toner adhesion amount straight line grasped by the current process control is larger than the straight line indicated by the dotted line in the drawing, as shown by the alternate long and short dash line in the drawing, it is shown in the figure. As described above, even if the above-described upper limit processing is performed, the toner adhesion amount change of about 0.1 [mg / cm ² ] occurs before and after the process control.

  Further, as a result of intensive studies by the present inventors, it is a problem that the upper limit value of the change amount of the current development potential Vp (n) with respect to the previous development potential Vp (n−1) is fixed. I found out. Therefore, in this embodiment, the upper limit value of the change amount with respect to the previous development potential Vp (n−1) is changed according to the value of development γ obtained by the current process control. This will be specifically described below.

FIG. 17 is a flowchart of the development potential upper limit process of the present embodiment. This process is a process performed between S6 and S7 in the basic flow chart of the process control shown in FIG.
When the development γ and the development start voltage Vk are calculated as indicated by S6 in the basic flow chart of the previous process control, the control unit 100 starts the development potential upper limit process and is stored in the nonvolatile memory such as the ROM 103. The previous development potential Vp (n-1) set by the previous process control is acquired (S11). Next, based on the development γ, a target development potential Vp ₀ such that the toner adhesion amount becomes the target adhesion amount is acquired (S12). The target adhesion amount is determined by the degree of coloring of the toner pigment, but is generally 0.4 to 0.6 [mg / cm ² ]. Next, as shown in equation (2), the developing potential Vp ₀ goal obtained by subtracting the previous development potential Vp (n-1), to change from the previous development potential in developing potential Vp ₀ goal A change amount ΔVp of the developing potential at that time is calculated (S13).
ΔVp = Vp ₀ −Vp (n−1) (2)

  Next, the calculated development γ and the upper limit Δy_max of the fluctuation amount of the toner adhesion amount allowable before and after the process control stored in the nonvolatile memory such as the ROM are acquired (S14), and the development potential with respect to the previous development potential is obtained. The upper limit value Vp_th of the change amount of the development potential this time is calculated (S15).

The calculation of the upper limit value Vp_th will be described.
As described above, when y is the toner adhesion amount, x is the development potential, and a is the development γ, the development potential-toner adhesion amount straight line can be expressed as y = ax + b. Here, assuming that the change in the toner adhesion amount before and after the process control is Δy, it can be expressed as follows.
Δy = aΔx (3)
Δx represents the amount of change with respect to the previous development potential Vp (n−1).

Expression (3) means that when Δx is changed with respect to the previous development potential Vp (n−1), the toner adhesion amount changes by Δy. In other words, in order to make the change amount Δy of the toner adhesion amount equal to or less than a predetermined value, the change amount Δx of the development potential may be set to a predetermined value or less. That is, Δx is Vp_th which is the upper limit value of the development potential change amount when Δy is the upper limit value Δy_max of the variation amount of the toner adhesion amount which is allowable before and after the process control, and can be expressed as in Expression (4). it can.
Vp_th = Δy_max / a = Δy_max / γ (4)

For example, assuming that the calculated value of development γ is 0.8 and the upper limit value Δy_max of the variation amount of the toner adhesion allowable before and after the process control is set to 0.05 [mg / cm ² ], The calculated Vp_th is Vp_th = 0.05 / 0.8 = 0.0625 [kV]. Therefore, the upper limit value Vp_th is set to 0.0625 [kV]. Note that the upper limit value Δy_max of the variation amount of the toner adhesion amount that can be tolerated before and after the process control is obtained in advance by an experiment or the like within a range in which the difference in image density is not apparent before and after the process control.

Table 1 is an example showing each development γ and Vp_th at that time.

  As can be seen from Table 1, when development γ is high (γ = 1.0) with reference to the development γ value of 0.8, Vp_th is smaller than the reference value. Therefore, for example, when Vp_th is fixed to the reference value 0.625 [kV], when development γ becomes 0.2 higher than the reference value development γ (0.8), the previous development potential of this time Even if the fluctuation amount with respect to the development potential is kept within 0.625 [kV], if the fluctuation amount exceeds 0.05 [kV], the image density greatly differs before and after the process control. However, in the present embodiment, when the development γ increases, the value of Vp_th is decreased from 0.0625 [kV] to 0.05 [kV], so that Vp_th is the reference value 0.625 [kV]. Compared with the case where the image density is fixed, it is possible to suppress the image density from greatly differing before and after the process control. Further, as can be seen from Table 1, when the development γ is lower than the reference value (0.8) (γ = 0.5), Vp_th becomes larger than the reference value 0.625 [kV]. Therefore, for example, when Vp_th is fixed to the reference value 0.625 [kV], when the development γ becomes low, it is not possible to achieve sufficient image density recovery by the process control. However, in this embodiment, when the development γ is low, the Vp_th value is increased, so that sufficient image density recovery can be achieved compared to the case where Vp_th is fixed to the reference value 0.625 [kV]. Can be planned.

Thus, when Vp_th is calculated, the absolute value | ΔVp | of the change amount of the development potential when changing from the previous development potential Vp (n−1) to the target development potential Vp ₀ is the upper limit of the calculated change amount. It is determined whether it is less than the value Vp_th (S16). The absolute value of ΔVp is if less than Vp_th (YES in S16), the developing potential Vp ₀ goals set as the current development potential Vp (n). On the other hand, if the absolute value of ΔVp is equal to or greater than Vp_th (NO in S16), it is determined whether ΔVp is positive or negative (S18). That is, it is determined whether the target development potential Vp ₀ is larger or smaller than the previous development potential Vp (n−1). If ΔVp is greater than or equal to 0, that is, the target development potential Vp ₀ is greater than the previous development potential Vp (n−1) (YES in S18), the previous development potential Vp (n The value obtained by adding the upper limit value Vp_th to (-1) is set as the current image potential Vp (n) (S19).
Vp (n) = Vp (n-1) + Vp_th (5)

If ΔVp is less than 0, that is, the target development potential Vp ₀ is smaller than the previous development potential Vp (n−1) (NO in S18), the previous development potential Vp (n The value obtained by subtracting the upper limit value Vp_th from -1) is set as the current image potential Vp (S20).
Vp (n) = Vp (n−1) −Vp_th (6)

  When the development potential upper limit process is completed, the process of step 7 is performed on the basic flow chart of the process control shown in FIG. 6, and the development bias or the like is based on the development potential Vp (n) set in the upper limit process. Is calculated.

  As described above, in this embodiment, the upper limit value Vp_th is set according to the value of the development γ, so that the change amount of the toner adhesion amount before and after the process control is changed to a change amount level that the change in image density cannot be seen visually. It can be surely suppressed.

FIG. 18 is a diagram showing a case where the upper limit processing of the present embodiment is applied to the example shown in the description of FIG. As shown in the figure, when the development potential-toner adhesion amount straight line grasped by the current process computer is a straight line as shown by a dotted line in the figure, the upper limit value Vp_th becomes Vp_th (1), and the upper limit process is performed. The current development potential Vp (n) (1) is set to Vp (n−1) −Vp_th (1). Further, when the development potential-toner adhesion amount straight line grasped by the current process control is a straight line as shown by a one-dot chain line in the figure, the upper limit value Vp_th becomes Vp_th (2), and the upper limit process of the present embodiment. The development potential Vp (n) (2) of this time set by is set to Vp (n−1) −Vp_th (2). As shown in the figure, even when the development potential-toner adhesion amount straight line grasped by the current process control is a one-dot chain line in the figure, the change amount of the toner adhesion before and after the process control is about 0.05 [mg / cm ^2. ] And the change in image density can be suppressed to about 0.1.

Next, a specific example of the development potential upper limit process of the present embodiment will be described with reference to FIG.
In this image forming apparatus, the target toner adhesion amount is set to 0.45 [mg / cm ² ]. Further, the upper limit value Δy_max of the variation amount of the toner adhesion amount allowable before and after the process control stored in the non-volatile memory such as the ROM is 0.05 [mg / cm ² ]. Further, in the previous process control, the grasped development potential-toner adhesion amount straight line is a solid line in the figure, and the y-intercept is 0. In the previous process control, the target development potential Vp ₀ is set as the previous development potential Vp (n−1), and the value of the previous development potential Vp (n−1) is 0.562 [kV]. It is. Further, the development potential-toner adhesion amount straight line grasped by the current process control operation is a dotted line in the figure, and the y-intercept is 0.

When the development γ and the development start voltage Vk are calculated from the development potential-toner adhesion amount straight line in the current process control, the development potential upper limit process is executed, and in S11, the development potential at the previous process control: Vp (n−1): 0.562 [kV] is acquired, and in S12, a target development potential Vp ₀ : 0.409 [kV] is acquired from the development potential-toner adhesion amount straight line. Then, in S13, the change amount ΔVp when changing from the previous development potential Vp (n-1) to the developing potential Vp ₀ goal [kV] = 0.409-0.562 = -0.153 [ kV] the calculate. Next, in step S14, development γ: 1.1 calculated by the current process control is acquired, and in step S15, Vp_th = 0.05 / 1.1 = 0.045 [kV] = 45 [V] is obtained.

  Next, in S16, a determination of | ΔVp | ≦ Vp_th is performed. In this specific example, since | −0.153 |> 0.045 and the condition is not satisfied (NO in S16), the process proceeds to S18. Next, in S18, a determination is made that ΔVp ≧ 0. In this specific example, since −0.153 <0 and the condition is not satisfied (NO in S18), the process proceeds to S20. In step S20, the current development potential Vp (n) is obtained by substituting a value into Vp (n) = Vp (n-1) -Vp_th. In this specific example, Vp = 0.562−0.045 = 0.517 [kV] = 517 [V].

The toner adhesion amount immediately before the process control is y = 1.1 × 0.562 = 0.618 [mg / cm ² ] from y = 1.1x. The toner adhesion amount after the process control is y = 1.1 × 0.517 = 0.568 [mg / cm ² ] from y = 1.1x. Therefore, the change amount of the toner adhesion amount before and after the process control is 0.618−0.568 = 0.05 [mg / cm ² ]. Therefore, as can be seen from FIG. 13, the fluctuation of the image density can be suppressed to about 0.1.

On the other hand, without any development potential limit process, because there is no limit to the amount of change in developing potential, this development potential Vp (n) is set to the development potential Vp ₀ goal. Therefore, the toner adhesion amount after the process control is 0.450 [mg / cm ² ], and the amount of change in the adhesion amount before and after the process control is 0.618−0.450 = 0.168 [mg / cm ² ]. . As a result, as can be seen from FIG. 13, the fluctuation of the image density is about 0.32, and the fluctuation of the image density can be seen at a level where the difference can be seen visually before and after the process control.

  As described above, by performing the image density control method of the present embodiment, it is possible to suppress a sudden change in the toner adhesion amount before and after the process control operation, and thus it is possible to prevent a sudden change in the image density. I understand that

Next, a specific example when the current development γ is lower than the previous development γ will be described with reference to FIG.
Previous development potential Vp (n−1) = 0.45 [kV], calculated target development potential Vp ₀ = 0.642 [kV], current development γ0.7, acceptable toner before and after the process control Assuming that the upper limit value Δy_max of the fluctuation amount of the adhesion amount is 0.05 [mg / cm ² ], the calculated Vp_th is 0.05 / 0.7 = 0.071 [kV]. ΔVp is 0.642−0.45 = 0.192 [kV]. Therefore, since | ΔVp |> Vp_th and ΔVp is positive, the current development potential Vp (n) is 0.45 + 0.071 = 0.521 [kV]. Since the toner adhesion amount immediately before the execution of the process control is y = 0.7x, 0.7 × 045 = 0.315 [mg / cm ² ], and the toner adhesion amount after the execution of the process control is 0.7 × 0.521. = 0.365 [mg / cm ² ]. Therefore, the change amount of the toner adhesion amount before and after the process control can be made 0.05 [mg / cm ² ] or less.

On the other hand, when the upper limit is fixed at 0.05 [kV], the current development potential Vp (n) ′ is 0.45 + 0.05 = 0.5 [kV]. Therefore, the toner adhesion amount after the process control is 0.7 × 0.5 = 0.35 [mg / cm ² ]. In this case, the change amount of the toner adhesion amount before and after the process control is 0.05 [mg / cm ² ]. cm ² ] or less.

However, compared with the case where Vp_th is fixed to 0.05 [kV], the upper limit processing of the present embodiment sets the toner adhesion amount after the process control to the toner target adhesion amount 0.45 [mg / cm ² ]. You can get closer. That is, it is possible to bring the image density closer to a proper value as compared with the case where Vp_th is fixed. As described above, in this embodiment, the fluctuation in image density before and after the process control can be suppressed to a level at which the difference in the image density is not apparent, and the image density after the process control is compared with a case where Vp_th is fixed. Can be brought closer to an appropriate value.

  FIG. 21 shows the result of comparison between the development potential upper limit process (variable upper limit value) of the present embodiment and the conventional development potential upper limit process (fixed upper limit value). The horizontal axis represents the number of images, and the vertical axis represents the image density fluctuation from the start point. As an experimental condition, after executing the previous image density adjustment, a high image area is output, the development γ is increased, a high image density is started, 300 sheets are printed continuously, and the image density is measured on the 200th sheet. Adjustment operation was executed.

  As can be seen from FIG. 21, in the conventional method, the image density greatly fluctuates after execution of the image density adjustment operation, and varies by about 0.2. On the other hand, when the method of the present invention is applied, fluctuations in image density before and after execution of the adjustment operation are suppressed, and fluctuations are about 0.1 or less. Therefore, there is no sudden image density fluctuation before and after execution of the process control during continuous printing.

In the image forming apparatus according to the present embodiment, the process control operation executed by interrupting the continuous printing may be different from the other process control operation.
As shown in FIG. 22, when the development γ and the development start voltage Vk are calculated, it is determined whether or not the process control is executed by interrupting the continuous printing (S36). When interrupting during continuous printing and executing the process control (YES in S36), the above-described development potential upper limit process is performed so that the image density fluctuation before and after the process control is suppressed to 0.1. Set (S37). On the other hand, the process control operation, if not the interrupt during the continuous printing (NO in S36), the toner adhesion amount after process control is set to the development potential Vp ₀ goals the target toner adhering amount 0.45 (S38 ).

When the interruption is not during continuous printing, the output image is often less relevant before and after the process control, so even if the image density before and after the process control changes greatly, there is no significant effect. Therefore, when it is not an interruption during continuous printing, the current development potential Vp (n) is set to the target development potential Vp ₀ without worrying about the image density fluctuation before and after the process control. An image having an appropriate image density can be output. On the other hand, in the case of a process control operation performed by interruption during continuous printing in which images output before and after the process control are related, the process control is performed so that image density fluctuation before and after the process control is suppressed to 0.1. Therefore, the image density does not change abruptly in the middle of the page, and there is no problem.

  In the development potential upper limit processing described above, the upper limit value Vp_th is calculated from the development γ and the upper limit value Δy_max of the allowable amount of toner adhesion before and after the process control, but is not limited thereto. For example, a lookup table that associates the development potential, development γ, and upper limit value Vp_th is stored in a nonvolatile memory such as a ROM, and the upper limit is calculated from the calculated development γ, the previous development potential, and the lookup table. The value Vp_th may be obtained. Also, a reference development γ is defined, and a reference upper limit value Vp_th at the reference development γ is obtained from the reference development γ and the previous development potential. Then, Δγ may be calculated by subtracting the reference development γ from the calculated development γ, and the upper limit value Vp_th may be obtained from the value of Δγ. Specifically, when Δγ is a positive value, the value is changed to an upper limit value Vp_th that is smaller than the reference upper limit value Vp_th according to the value of Δγ. When Δγ is a negative value, the upper limit value Vp_th is changed to a value larger than the reference upper limit value Vp_th corresponding to the reference development γ according to the value of Δγ.

  As described above, according to the image forming apparatus of the present embodiment, the control unit 100 serving as an image density control unit performs process control (hereinafter referred to as image density control) for the current image density based on the development potential-toner adhesion amount straight line as the grasped development performance straight line. The upper limit value Vp_th of the change amount of the current development potential set in the previous process control with respect to the previous development potential set in the previous process control is set. Then, the current development potential is set so that the change amount of the current development potential with respect to the previous development potential is equal to or less than the upper limit value Vp_th. Thus, by setting the upper limit value Vp_th based on the development potential-toner adhesion amount straight line, the image density fluctuation before and after the process control can be surely kept within a predetermined range. As a result, even if the number of image forming operations reaches a predetermined number during the continuous image forming operation and the continuous image forming operation is temporarily interrupted and the process control is performed, the image density greatly fluctuates before and after the process control. Can be suppressed. As a result, it is possible to suppress a problem that the image density changes suddenly in the middle of the page.

  The upper limit value Vp_th may be calculated based on the upper limit value Δy_max of the fluctuation amount of the toner adhesion amount allowable before and after the process control and the development γ that is the slope of the development potential-toner adhesion amount line. Thereby, the fluctuation amount of the toner adhesion amount before and after the process control can be suppressed to Δy_max or less, and it is possible to suppress the image density from largely changing before and after the process control.

  If the change amount ΔVp when changing from the previous development potential to the target development potential calculated based on the development potential-toner adhesion amount straight line is less than the upper limit value Vp_th, the target development potential is set to the current development potential. Set as potential. In this case, it is possible to suppress a large variation in image density before and after the process control and to adjust the image density after the process control to an appropriate image density. On the other hand, when ΔVp is equal to or higher than the upper limit value Vp_th and the target development potential is larger than the previous development potential, a value obtained by adding the upper limit value to the previous development potential is set as the current development potential. When ΔVp is equal to or higher than the upper limit value Vp_th and the target development potential is smaller than the previous development potential, a value obtained by subtracting the upper limit value from the previous development potential is set as the current development potential. Thereby, the image density after the process control can be brought close to an appropriate image density while suppressing a large fluctuation in the image density before and after the process control.

  Further, as shown in FIG. 22, when interrupting the continuous image forming operation and executing the process control, the upper limit value Vp_th is set, and the current development potential is set so as to be equal to or lower than the upper limit value Vp_th. Sets the calculated target development potential as the current development potential without setting the upper limit value Vp_th. In the case of a process control operation performed by an interrupt during a continuous image forming operation in which images output before and after the process control are related, the process control is performed so that the image density fluctuation before and after the process control is suppressed to 0.1. Therefore, the image density does not change abruptly in the middle of the page, and there is no problem. On the other hand, if it is not an interrupt during continuous image forming operation, the output image is often less relevant before and after the process control, so even if the image density before and after the process control changes greatly, there will be a significant impact There is no. Therefore, if it is not an interruption during continuous image forming operation, the density of the image after the process control is set to an appropriate level by setting the current development potential to the target development potential without worrying about the image density fluctuation before and after the process control. The image density can be set.

1: process unit 6: intermediate transfer belt 11: photoconductor 15: charging roller 20: developing device 50: transfer unit 68: optical writing unit 69: optical sensor 100: control unit Vp (n): current development potential Vp ( n-1): previous development potential Vp ₀ : target development potential Vpn development potential Vp_th: upper limit value

JP 2007-79429 A JP 2008-292614 A

Claims (7)

A latent image carrier for carrying a latent image;
Charging means for uniformly charging the surface of the latent image carrier with a charging bias such that the surface of the latent image carrier has a target charging potential;
Latent image forming means for forming an electrostatic latent image by exposing the surface of the latent image carrier charged by the charging means;
Developing means for forming a toner image on the surface of the latent image carrier by attaching toner on the developer carrier to which a developing bias is applied to the electrostatic latent image formed on the surface of the latent image carrier When,
An adhesion amount detection means for detecting a toner adhesion amount per unit area with respect to a toner image on the surface of the latent image carrier or a toner image transferred from the latent image carrier to a transfer body;
Based on the result of detecting the amount of toner attached to the plurality of toner images by the above-mentioned amount-of-attachment detecting means, the potential of the electrostatic latent image portion is formed. In an image forming apparatus comprising image density control means for performing image density control for setting a development potential that is a difference between a development bias and a development bias.
Based on the result detected by the adhesion amount detection means, the image density control means sets the change amount of the current development potential set in the current image density control to the previous development potential set in the previous image density control. An image forming apparatus characterized in that an upper limit is set and the current development potential is set so that a change amount of the current development potential with respect to the previous development potential is not more than the upper limit. The image forming apparatus according to claim 1.
The image density control means includes an upper limit value of a variation amount of the toner adhesion amount allowable before and after the image density control,
Calculate the upper limit of the change amount of the current development potential from the previous development potential based on the slope of the development performance line indicating the relationship between the development potential and the toner adhesion amount obtained based on the result detected by the adhesion amount detection means. And an image forming apparatus. The image forming apparatus according to claim 1 or 2,
If the slope of the development performance line indicating the relationship between the development potential and the toner adhesion amount obtained based on the result detected by the adhesion amount detection means is greater than a predetermined inclination, the image density control means An image forming apparatus, wherein the upper limit value of the change amount of the development potential with respect to the previous development potential is set to a value smaller than the upper limit value when the slope is a predetermined value. The image forming apparatus according to any one of claims 1 to 3,
When the slope of the development performance line indicating the relationship between the development potential and the toner adhesion amount obtained based on the result detected by the adhesion amount detection means is smaller than a predetermined inclination, the image density control means An image forming apparatus, wherein an upper limit value of a change amount of a development potential with respect to a previous development potential is set to a value larger than an upper limit value when the slope is a predetermined value. The image forming apparatus according to claim 1,
The image density control unit obtains a target development potential such that the toner adhesion amount with respect to a predetermined electrostatic latent image becomes a target toner adhesion amount based on the result detected by the adhesion amount detection unit, and the previous development When the change amount when changing from the potential to the target development potential is less than the upper limit value, the target development potential is set as the current development potential, and the target development potential is changed from the previous development potential to the target development potential. When the change amount when changing to is greater than or equal to the above upper limit value and the target development potential is greater than the previous development potential, a value obtained by adding the upper limit value to the previous development potential is set to the current development potential. Set as potential and change from the previous development potential to the target development potential Is equal to or greater than the upper limit value and the target development potential is smaller than the previous development potential, the value obtained by subtracting the upper limit value from the previous development potential is set as the current development potential. An image forming apparatus. The image forming apparatus according to claim 5.
When the image density control unit performs the image density control by interrupting the continuous image forming operation when the number of image forming operations reaches a predetermined number during the continuous image forming operation, the amount of change with respect to the previous development potential is Set the development potential of this time so that it is below the above upper limit,
In other cases, the target development potential is set as the current development potential so that the toner adhesion amount with respect to the predetermined electrostatic latent image becomes the target toner adhesion amount without setting the upper limit value. An image forming apparatus. Charging means for uniformly charging the surface of the latent image carrier, exposure means for forming an electrostatic latent image by exposing the surface of the latent image carrier charged by the charging means, and the latent image carrier The latent image carrier is developed under different image forming conditions by using a developing means for forming a toner image on the surface of the latent image carrier by attaching toner to the electrostatic latent image formed on the surface of the latent image. Forming a pattern image composed of a plurality of toner images,
Detecting a toner adhesion amount with respect to the plurality of toner images by an adhesion amount detection unit;
In an image density control method including a step of setting a development potential based on the detected result,
A step of setting an upper limit value of a change amount of the current development potential set in the current image density control with respect to the previous development potential set in the previous image density control based on a result detected by the adhesion amount detection unit; ,
And a step of setting the current development potential so that the amount of change of the current development potential with respect to the previous development potential is less than or equal to the above upper limit value.

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