JP2012088422A - Image forming apparatus, control method and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of reducing color differences in multicolor.SOLUTION: The image forming apparatus forms a toner pattern on an image carrier 202 and adjusts a charging bias, an exposure amount and a developing bias based on a toner deposition amount of the toner pattern so as to have a ratio of a background potential to a development potential being a predetermined first target ratio. When a saturation of a single color of the toner pattern is out of a predetermined allowable range of saturation of a first color and a second color used for a multicolor, the apparatus increases or decreases the ratio of the background potential to the development potential within a range of a predetermined second target ratio so as to adjust the saturation of the single color of the toner pattern to be within the allowable range.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer.

  When forming a color image in an electrophotographic image forming apparatus, image data expressed in R (red), G (green), and B (blue) is converted into Y (yellow) based on color conversion data. , M (magenta), C (cyan), and K (black) image data. Then, based on the converted image data of Y (yellow), M (magenta), C (cyan), and K (black), respective single color toner images are formed, and the single color toner images are overlapped to form a color image. Form.

  In order to obtain a stable color image density in the above-described image forming apparatus, a toner pattern is formed on the photosensitive member by the same operation as in normal image formation in monochromatic process control, and the amount of toner attached to this toner pattern And a charging bias, a developing bias, a reference exposure amount (hereinafter referred to as “charging bias etc.”) and the like are adjusted based on the sensor detection result (for example, Patent Document 1: Japanese Patent Laid-Open No. Hei 5-). 14729).

  In this type of image forming apparatus, the development potential is obtained from the sensor detection result, a suitable charging bias corresponding to the obtained development potential is specified, and the specified charging bias is adjusted. This makes it possible to maintain the color image density constant even when environmental fluctuations or changes with time occur and development γ (gamma) or the like fluctuates.

  However, in the monochromatic process control described above, image density control is performed for a single color, and image density control is not performed for a multicolor toner image obtained by superimposing single color toner images. For this reason, there is a problem in that the color balance of the multiple colors changes, the color difference between the multiple colors increases, and a shift occurs in the hue direction. For example, even if the image density of a single color toner is within the control range, for example, among the multiple colors represented by two colors, the first color takes the upper limit value of the image density and the second color takes the lower limit value of the image density. This is because the ratio of image density changes. This problem is likely to occur at a high image density of 70% to 100% of the input image density.

  In order to solve the above problem, when controlling multiple colors using two colors, the toner adhesion amount range between the first color and the second color is set so as not to exceed a certain color difference range, and the toner There is a method in which the linear velocity of the developing sleeve is controlled so that the amount of adhesion is within the range (see, for example, Patent Document 2: JP-A-7-271138).

  However, the optimum linear velocity of the developing sleeve differs depending on the paper type and image resolution. If the linear velocity of the developing sleeve is controlled to change the toner adhesion amount, even if the color balance is achieved, the color balance Otherwise, the image quality may be degraded. For example, if the sleeve linear velocity is increased, the toner adhesion amount increases, but if the sleeve linear velocity is too high, the solid image tends to be blurred, so that the image quality is lowered, and as a result, the desired color may be different. is there.

  Further, in order to solve the above problems, the image density is read from the multiple color R, G, B patches with the LED, and the charging bias, the developing bias, the exposure amount, the developing roll rotational speed, the toner are used to increase or decrease the developing γ. There is a method of controlling a supply coefficient or the like (for example, see Patent Document 3: Japanese Patent Application Laid-Open No. 2004-212893).

  The development γ can be changed by controlling the rotation speed of the developing roll. However, controlling the charging bias, developing bias, exposure amount, and toner supply coefficient changes the toner adhesion amount. Therefore, matching the control amount of the developing roll rotation speed with the charging bias, developing bias, exposure amount, and toner supply coefficient. I can't plan.

  In order to solve the above problem, there is a method of adjusting the exposure potential and controlling the color balance when the toner adhesion amount of the first or second color of the multiple colors is different from the appropriate value. (For example, see Patent Document 4: Japanese Patent Laid-Open No. 5-297678).

  However, as shown in FIG. 19, even if the toner adhesion amounts of the two toner patches (density patches) are the same, the shape of the toner dots forming the density patch may differ in dot height and dot diameter. Because there is a problem.

  As a technical document filed by the present applicant prior to the present invention, when using a photoconductor that can greatly change the correspondence between the exposure amount and the exposure portion potential within the exposure amount range used during image formation. There is a document that discloses a technique for determining a suitable reference exposure amount without using a data table (for example, Japanese Patent Application Laid-Open No. 2010-44215).

  Patent Document 5 discloses a technique for determining a reference exposure amount during monochromatic process control. However, in Patent Document 5 described above, since control is not performed on a multiple color toner image in which single color toner images are superimposed, the color balance of the multiple colors changes, the color difference between the multiple colors increases, and the hue direction shifts. There is a problem that will occur.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an image forming apparatus, a control method, and a program capable of reducing the color difference of multiple colors.

  In order to achieve this object, the present invention has the following features.

<Image forming apparatus>
An image forming apparatus according to the present invention includes:
An image bearing member, a charging unit that charges the surface of the image bearing member with a predetermined charging bias, and the surface of the image bearing member charged by the charging unit are exposed with a predetermined exposure amount to form an electrostatic latent image. A predetermined developing bias is applied to the electrostatic latent image formed on the surface of the image carrier, and the toner is attached to the electrostatic latent image, and is applied to the surface of the image carrier. An image forming apparatus for transferring a toner image formed on the surface of the image carrier to a recording medium and forming an image on the recording medium,
Pattern forming means for forming a toner pattern on the image carrier;
Based on the toner adhesion amount of the toner pattern, the electrostatic latent image is not formed with respect to the developing potential that is the difference between the potential of the electrostatic latent image portion where the electrostatic latent image is formed and the developing bias. A single color that adjusts the charging bias, the exposure amount, and the developing bias so that the ratio of the background potential, which is the difference between the potential of the non-electrostatic latent image portion and the developing bias, becomes a predetermined first target ratio. Process control means;
When the saturation of the single color of the toner pattern is not within a predetermined allowable range of the saturation of the first color and the second color used for multiple colors, the ratio of the background potential to the development potential is set to a predetermined second target. A multi-color process control means for increasing / decreasing within a range of the ratio and adjusting the saturation of the single color of the toner pattern to be within the allowable range;
It is characterized by having.

<Control method>
The control method according to the present invention includes:
An image bearing member, a charging unit that charges the surface of the image bearing member with a predetermined charging bias, and the surface of the image bearing member charged by the charging unit are exposed with a predetermined exposure amount to form an electrostatic latent image. A predetermined developing bias is applied to the electrostatic latent image formed on the surface of the image carrier, and the toner is attached to the electrostatic latent image, and is applied to the surface of the image carrier. And a developing means for forming a toner image, transferring the toner image formed on the surface of the image carrier to a recording medium and forming an image on the recording medium. And
A pattern forming step of forming a toner pattern on the image carrier;
Based on the toner adhesion amount of the toner pattern, the electrostatic latent image is not formed with respect to the developing potential that is the difference between the potential of the electrostatic latent image portion where the electrostatic latent image is formed and the developing bias. A single color that adjusts the charging bias, the exposure amount, and the developing bias so that the ratio of the background potential, which is the difference between the potential of the non-electrostatic latent image portion and the developing bias, becomes a predetermined first target ratio. A process control step;
When the saturation of the single color of the toner pattern is not within a predetermined allowable range of the saturation of the first color and the second color used for multiple colors, the ratio of the background potential to the development potential is set to a predetermined second target. A multi-color process control step of increasing or decreasing within a range of the ratio and adjusting the saturation of a single color of the toner pattern to be within the allowable range;
It is characterized by having.

<Program>
The program according to the present invention is:
An image bearing member, a charging unit that charges the surface of the image bearing member with a predetermined charging bias, and the surface of the image bearing member charged by the charging unit are exposed with a predetermined exposure amount to form an electrostatic latent image. A predetermined developing bias is applied to the electrostatic latent image formed on the surface of the image carrier, and the toner is attached to the electrostatic latent image, and is applied to the surface of the image carrier. A program for causing a computer of an image forming apparatus to transfer a toner image formed on the surface of the image carrier to a recording medium and form an image on the recording medium. Because
A pattern forming process for forming a toner pattern on the image carrier;
Based on the toner adhesion amount of the toner pattern, the electrostatic latent image is not formed with respect to the developing potential that is the difference between the potential of the electrostatic latent image portion where the electrostatic latent image is formed and the developing bias. A single color that adjusts the charging bias, the exposure amount, and the developing bias so that the ratio of the background potential, which is the difference between the potential of the non-electrostatic latent image portion and the developing bias, becomes a predetermined first target ratio. Process control processing;
When the saturation of the single color of the toner pattern is not within a predetermined allowable range of the saturation of the first color and the second color used for multiple colors, the ratio of the background potential to the development potential is set to a predetermined second target. A multi-color process control process that increases or decreases within the range of the ratio and adjusts the saturation of the single color of the toner pattern to be within the allowable range;
Is executed by the computer.

  According to the present invention, the color difference of multiple colors can be reduced.

1 is a diagram illustrating a schematic configuration example of an image forming apparatus according to an exemplary embodiment. 1 is a diagram illustrating a schematic configuration example of an image forming unit of an image forming apparatus according to an exemplary embodiment. It is a figure which shows the control system in connection with the monochrome process control of this embodiment. (A) is a figure which shows the schematic structural example of the optical sensor which comprises the image detection part for black, (b) is a schematic structural example of the optical sensor which comprises the image detection part for other colors (color). FIG. It is a figure which shows the example of arrangement | positioning of the optical sensor 301 and the optical sensor 302. FIG. It is a figure which shows the main processing operation examples of the monochrome process control in this embodiment. 6 is a graph showing results of actual measurement of toner adhesion amount with respect to development potential when a toner gradation pattern is formed in a high temperature and high humidity environment and a low temperature and low humidity environment. When the charging potential Vd is 900 [V], 700 [V], or 500 [V], the background potential is changed so that a vertical line of 1 dot width (a line extending in a direction corresponding to the photosensitive body surface moving direction) is obtained. It is a graph of the experimental result which shows the correspondence of the ground potential and line width when image formation is carried out. When the charging potential Vd is 900 [V], 700 [V], and 500 [V], the background potential is changed to change the horizontal line of 1 dot width (the direction corresponding to the direction orthogonal to the photosensitive member surface moving direction). It is a graph of the experimental result which shows the correspondence of the ground potential and line | wire width when image-forming the line extended in FIG. (A) is a graph of experimental results showing the correspondence between the background potential and the halftone density when the charging potential Vd is 900 [V], 700 [V], and 500 [V], and (b) 11 is a graph of experimental results showing the correspondence between the ratio of the background potential to the development potential and the halftone density when the charging potential Vd is 900 [V], 700 [V], and 500 [V]. FIG. 10 is a diagram showing an example of a processing operation for correcting a developing bias Vb ′ for detecting Vr when the charging bias Vg ′ is set to the charging bias upper limit value VgMAX. FIG. 10 is a diagram illustrating a processing operation example for correcting a developing bias Vb ′ for detecting Vr when the charging bias Vg ′ is set to a charging bias lower limit value VgMIN. FIG. 10 is a diagram showing an example of a processing operation for correcting a developing bias Vb ′ for detecting Vr when the charging bias Vg ′ is set between a charging bias upper limit value VgMAX and a charging bias lower limit value VgMIN. (A) is a graph which shows the relationship between the exposure power of the laser diode (LD) after image formation of a predetermined number of times and each exposure part potential VL in a high temperature and high humidity environment, and (b) is a low temperature and low humidity environment. Below, it is a graph which shows the relationship between the exposure power of LD after each image formation of the same frequency, and each exposure part electric potential VL. 6 is a graph showing the relationship between the number of sheets passing through and the residual exposed portion potential Vr in an image forming apparatus using a specific photoconductor 202. FIG. 6 is a diagram illustrating a main processing operation example of multi-color process control for stabilizing the multi-color toner adhesion amount in the present embodiment. 5 is a graph showing the relationship between the toner adhesion amount of a Y density patch and the Y image density proportional to the image reflectance, based on experimental results. It is the graph which showed the relationship between the image density of Y and the saturation of Y from the experimental result. FIG. 6 is a diagram illustrating an example in which the shape of a toner dot forming a density patch may differ in dot height and dot diameter even if the toner adhesion amount of two density patches is the same. It is a graph which shows the tolerance | permissible_range of the single color Y saturation used for multiple colors G, and C saturation. It is a figure which shows the saturation of Y calculated as a result of the toner adhesion amount of Y changing with respect to the background potential coefficient of Y. It is a figure which shows the saturation of C calculated as a result of the toner adhesion amount of C changing with respect to C background potential coefficient. It is a graph which shows the tolerance | permissible_range of the monochrome Y saturation and M saturation used for the multiple color R. FIG. It is a graph which shows the tolerance | permissible_range of the single color M saturation used for the multiple color B, and C saturation.

<Outline of Image Forming Apparatus of Present Embodiment>
First, an outline of the image forming apparatus of the present embodiment will be described with reference to FIGS.

  As shown in FIG. 2, the image forming apparatus of this embodiment includes an image carrier (corresponding to the photoconductor 202) and a charging unit (corresponding to the charging unit 201) that charges the surface of the image carrier 202 with a predetermined charging bias. ), The surface of the image carrier 202 charged by the charging unit 201 is exposed with a predetermined exposure amount to form an electrostatic latent image (corresponding to the writing unit 203), and the surface of the image carrier 202 A developing means (corresponding to the developing unit 205) that applies a predetermined developing bias to the formed electrostatic latent image, attaches toner to the electrostatic latent image, and forms a toner image on the surface of the image carrier 202. 1), and the toner image formed on the surface of the image carrier 202 is transferred to a recording medium (corresponding to the transfer paper 112) and an image is formed on the recording medium 112, as shown in FIG. Forming device.

  As shown in FIG. 3, the image forming apparatus according to the present embodiment is based on a pattern forming unit (corresponding to the control unit 41) that forms a toner pattern on the image carrier 202 and the toner adhesion amount of the toner pattern. This is the difference between the potential of the non-electrostatic latent image portion where no electrostatic latent image is formed and the development bias with respect to the development potential, which is the difference between the potential of the electrostatic latent image portion where the latent image is formed and the development bias. Monochromatic process control means (corresponding to the control unit 41) for adjusting the charging bias, the exposure amount, and the developing bias so that the ratio of the background potential becomes a predetermined first target ratio; When the saturation of the first and second colors used for multiple colors is not within the predetermined allowable range, the ratio of the background potential to the development potential is increased or decreased within the predetermined second target ratio, and the toner pattern Monochromatic saturation of the multicolor process control means adjusted to be within the allowable range (corresponding to the control unit 41), constituting a.

  In the image forming apparatus according to the present embodiment, the ratio of the background potential to the development potential is determined when the saturation of the single color of the toner pattern is not within the predetermined allowable range of the saturation of the first color and the second color used for the multiple colors. Increase / decrease within the range of the predetermined target ratio, and adjust so that the saturation of the single color of the toner pattern falls within the allowable range of the saturation of the first and second colors. Thereby, the color difference of multiple colors can be reduced. Hereinafter, the image forming apparatus of the present embodiment will be described in detail with reference to the accompanying drawings.

<Configuration example of image forming apparatus>
First, a configuration example of the image forming apparatus according to the present embodiment will be described. FIG. 1 is a diagram illustrating a schematic configuration example of the image forming apparatus according to the present exemplary embodiment.

  As shown in FIG. 1, the image forming apparatus according to this embodiment includes image forming units 102Y (yellow), 102M (magenta), and 102C (cyan) along an intermediate transfer belt 101 stretched around a plurality of stretch rollers. , 102K (black). The toner images formed by the image forming units 102Y, 102M, 102C, and 102K are transferred onto the intermediate transfer belt 101 by the primary transfer units 106Y, 106M, 106C, and 106K. An image detection unit 110 for detecting the toner adhesion amount is provided to face the intermediate transfer belt 101. The image detection unit 110 detects the toner adhesion amount of the toner image transferred onto the intermediate transfer belt 101. The toner image on the intermediate transfer belt 101 is transferred to a transfer sheet 112 as a recording medium by a secondary transfer unit 111.

<Configuration Example of Image Forming Units 102Y, 102M, 102C, 102K>
Next, a configuration example of the image forming units 102Y, 102M, 102C, and 102K will be described with reference to FIG. FIG. 2 is a diagram illustrating a schematic configuration example of the image forming units 102Y, 102M, 102C, and 102K. The configuration of each of the image forming units 102Y, 102M, 102C, and 102K is the same, and will be described without distinction from each other.

  Around the photoconductor 202, a charging unit 201 as a charging unit for charging the surface of the photoconductor 202, a writing unit 203 as an exposure unit for writing an electrostatic latent image on the surface of the photoconductor 202 by writing light L, A developing unit 205 as a developing unit that develops an electrostatic latent image with toner, a photoconductor cleaner 206 as a cleaning unit that cleans transfer residual toner and the like on the photoconductor 202, and an erase as a neutralizing unit that neutralizes the surface of the photoconductor 202. A (static elimination unit) 207 and a potential sensor 210 as a potential detecting means are provided.

  The photoconductor 202 of the present embodiment is a high-hardness photoconductor in which a filler is contained in the photoconductor surface layer. The photoconductor 202 of the present embodiment is similar to a general photoconductor that does not contain a filler in the photoconductor surface layer, as shown in FIGS. 14A and 14B, that is, the exposure power LDP. It has a photosensitive characteristic in which the change rate of the exposure portion potential VL with respect to the change gradually decreases as the exposure power LDP increases. Further, as illustrated in FIG. 15, the photoconductor 202 of the present embodiment has a characteristic that the residual exposure portion potential Vr gradually increases with time. For this reason, in the photoconductor 202 of the present embodiment, the correspondence relationship between the exposure power LDP and the exposure portion potential VL within the range of the exposure power LDP used at the time of image formation changes greatly with time.

  The charging unit 201 of the present embodiment is a non-contact type charger composed of a scorotron charger, and sets the grid voltage (charging bias) Vg of the scorotron charger to a target charging potential (in this embodiment, a negative potential) to perform photosensitivity. The surface potential of the body 202 is set to the target charging potential. The charging unit 201 is not limited to this, and other non-contact chargers or contact chargers can also be used.

  The writing unit 203 of the present embodiment uses a laser diode (LD) as a light source, and irradiates intermittent writing light, that is, repetitively pulsed writing light L, so that one dot is formed on the surface of the photoconductor 202. Each electrostatic latent image (1-dot electrostatic latent image) is formed. In this embodiment, by changing the exposure time (unit exposure time) when forming a one-dot electrostatic latent image, the amount of toner attached to the one-dot electrostatic latent image is controlled, and gradation control is performed. be able to. In this embodiment, the maximum unit exposure time is divided into 15 (each unit exposure time is hereinafter referred to as “exposure duty”), and gradation control of 16 gradations can be performed. Therefore, in this embodiment, the exposure duty can be adjusted in 16 steps from 0 (not exposed) to 15 (maximum unit exposure time).

  The developing unit 205 of the present embodiment includes a developing roller as a developer carrying member disposed opposite to the surface of the photosensitive member 202, and includes a toner charged with a predetermined polarity (negative polarity in the present embodiment), a magnetic carrier, and the like. A two-component developer comprising the above is carried on a developing roller, and toner is supplied to the surface of the photoreceptor 202. A developing bias Vb whose absolute value is sufficiently larger than the exposed portion potential VL and sufficiently smaller than the charging potential Vd is applied to the developing roller. As a result, in the development area where the surface of the photoconductor 202 and the developing roller face each other, the toner is moved toward the electrostatic latent image (exposed portion) on the surface of the photoconductor 202, and on the surface of the photoconductor 202. In this non-electrostatic latent image (non-exposed portion), an electric field that does not move the toner can be formed, and the electrostatic latent image can be made into a toner image.

  In the configuration shown in FIG. 2, when image formation is performed, first, the surface of the photosensitive member 202 is charged by the charging unit 201 so that the surface of the photosensitive member 202 is uniformly at a target charging potential (negative potential). Next, writing light L corresponding to the image data is exposed to the photosensitive member 202 from the light source (LD) of the writing unit 203 on the charged surface portion of the photosensitive member 202. As a result, the potential (absolute value) of the exposed portion on the surface of the photoconductor 202 decreases, and an electrostatic latent image is formed on the surface of the photoconductor 202.

  The electrostatic latent image (exposed portion in this embodiment) formed on the photoreceptor 202 is developed into a toner image by toner carried on a developing roller that is a developer carrying member of the developing unit 205. Specifically, a developing bias Vb having an absolute value larger than the exposure portion potential VL and smaller than the charging potential Vd is applied to the developing roller, and the toner charged to a predetermined polarity (negative polarity in this embodiment) is applied. Development is performed by electrostatically attaching to the electrostatic latent image.

  The toner image formed on the photoreceptor 202 is transferred onto the intermediate transfer belt 101 by the primary transfer unit 106. Untransferred toner remaining on the photoconductor 202 without being transferred to the intermediate transfer belt 101 is collected by the photoconductor cleaner 206. In addition, the surface of the photosensitive member 202 after the toner image is transferred onto the intermediate transfer belt 101 is uniformly irradiated with the neutralizing light by the eraser 207, so that the non-electrostatic latent image portion is neutralized and uniform. It will be in the state of being neutralized.

  The toner images of the respective colors formed by the image forming units 102Y, 102M, 102C, and 102K by the above processing are primarily transferred so as to overlap each other on the intermediate transfer belt 101 shown in FIG. Thereafter, each color toner image transferred onto the intermediate transfer belt 101 is transferred from the intermediate transfer belt 101 to the transfer paper 112 by the secondary transfer unit 111, and the toner image is fixed to the transfer paper 112 by a fixing unit (not shown) and a series of printing is performed. Terminate the process.

<Example of monochromatic process control>
Next, in order to stabilize the output image, an example of monochromatic process control for stabilizing the toner adhesion amount with respect to a specified one-dot electrostatic latent image will be described. Here, in order to simplify the description, the description will focus on control for adjusting the charging bias Vg, the developing bias Vb, and the exposure power. Note that the monochrome process control described below includes exposure adjustment control for adjusting the reference exposure used by the writing unit 203 during image formation. The exposure adjustment control is referred to as monochrome process control. May be done separately.

  FIG. 3 is a block diagram showing a control system related to monochromatic process control in the present embodiment. In the monochromatic process control of the present embodiment, first, a density patch 113 (see FIG. 5) as a toner pattern (toner image) is formed on the intermediate transfer belt 101 by a normal image forming operation under a predetermined condition, and this density patch. The toner adhesion amount 113 is detected by an image detection unit 110 including optical sensors 301 and 302 which are optical reflection density sensors. The control unit 41 adjusts the grid voltage (charging bias) Vg of the charging unit 201, the developing bias Vb of the developing unit 205, and the exposure power of the writing unit 203 based on the detection result of the image detecting unit 110.

  4A is a diagram illustrating a schematic configuration example of the optical sensor 301 that configures the black image detection unit 110, and FIG. 4B illustrates the configuration of the image detection unit 110 for another color (color). It is a figure which shows the example of schematic structure of the optical sensor 302 to do.

  The optical sensor 301 includes a light emitting element 303 and a regular reflection light receiving element 304 that receives regular reflection light from the surface of the density patch 113 and the intermediate transfer belt 101. On the other hand, the optical sensor 302 includes a light emitting element 303, a regular reflection light receiving element 304 that receives regular reflection light from the surface of the density patch 113 and the intermediate transfer belt 101, and a density patch 113 and a surface from the surface of the intermediate transfer belt 101. And a diffuse reflection light receiving element 305 that receives diffuse reflection light.

  As shown in FIG. 5, the optical sensor 301 and the optical sensor 302 are respectively arranged at positions that can face the density patch 113 formed on the intermediate transfer belt 101. After starting writing of the writing light L, the control unit 41 starts from the specular reflection light receiving element 304 and the diffuse reflection light receiving element 305 in accordance with the timing when the density patch 113 reaches the position facing the optical sensor 301 and the optical sensor 302. The toner adhesion amount of each density patch 113 is derived by performing an adhesion amount conversion process on the detection result (sensor detection result). For example, a conversion table describing the correspondence between the output voltage and the toner adhesion amount is stored in the ROM 44 in advance, and the toner adhesion amount is derived using this conversion table. Alternatively, the toner adhesion amount is derived by calculating a conversion formula for converting the output voltage into the toner adhesion amount.

FIG. 6 is a flowchart showing the main processing flow of monochromatic process control in this embodiment. In the present embodiment, the target of the toner gradation pattern when performing monochromatic process control so that the adhesion amount conversion processing can be appropriately performed on an image from low density to high density formed on the transfer paper 112. A case where the toner adhesion amount range is in the range from around 0 [mg / cm ² ] to 0.5 [mg / cm ² ] will be described.

  In monochromatic process control, after finishing pre-processing steps such as calibration and abnormality inspection of the image detection unit 110, first, image forming conditions (such as the currently set charging bias Vg0, developing bias Vb0, and exposure power LDP) ( A density patch 113 of 10 gradations is formed on the surface of the photoconductor 202 under the image forming conditions set in the previous monochrome process control (S1). The charging potential (non-exposed portion potential) Vd0 at this time is detected by the potential sensor 210 (S2). Further, the amount of toner attached to the 10-tone gradation patch 113 is detected by the image detection unit 110 (S3). Then, based on the charging potential Vd0 detected in S2 and the detected toner adhesion amount for 10 gradations detected in S3, the current development γ (gamma) is calculated (S4).

  FIG. 7 shows a development potential when a toner gradation pattern is formed in a high temperature and high humidity environment (32 [° C.], 54 [%]) and a low temperature and low humidity environment (10 [° C.], 15 [%]). 6 is a graph showing the results of actual measurement of the toner adhesion amount with respect to. In this graph, the horizontal axis represents development potential, and the vertical axis represents toner adhesion. The development γ is a parameter indicating the slope of this graph, and is a parameter indicating the correspondence between the development potential and the toner adhesion amount. Here, the developing potential indicates a potential difference between the exposed portion potential VL on the photosensitive member 202 and the developing bias Vb. If the developing potential is large, the amount of toner attached to the one-dot electrostatic latent image increases. The image density will increase. The background potential described later indicates a potential difference between the non-exposed portion potential Vd on the photosensitive member 202 and the developing bias Vb. If the background potential is too small, the background stain that causes toner to adhere to the non-exposed portion. When the background potential is too large, carrier adhesion occurs in which the magnetic carrier in the developer adheres to the surface of the photoconductor 202.

In the case of a high temperature and high humidity environment, in order to form the density patch 113 of 0.5 [mg / cm ² ], which is the maximum toner adhesion amount (target maximum toner adhesion amount) in the target toner adhesion amount range in this embodiment, As shown in FIG. 7, a development potential of 360 [V] is required. In contrast, in order to form the density patch 113 of 0.5 [mg / cm ² ] in a low temperature and low humidity environment, a development potential of 500 [V] is required. Thus, the development potential necessary to form the density patch 113 with the same toner adhesion amount of 0.5 [mg / cm ² ] varies depending on the temperature and humidity environment. The reason why the development potential varies depending on the temperature and humidity environment is that the charge amount of the toner varies depending on the temperature and humidity environment. In general, the toner charge amount decreases in a high-temperature and high-humidity environment, so that the toner adhesion amount increases even at the same development potential. .

  As described above, the development potential for obtaining the target image density (target toner adhesion amount) varies depending on the fluctuation of the temperature and humidity environment. Further, the development potential for obtaining the target toner adhesion amount varies depending on factors other than the temperature and humidity environment. Accordingly, the current development γ is confirmed at an appropriate timing, and a development potential for obtaining a target toner adhesion amount is obtained from the development γ, and various image forming conditions (charging bias Vg, development bias Vb, reference exposure amount ( It is necessary to determine the reference exposure power and the reference exposure duty)).

Therefore, in the present embodiment, the development potential VbL for obtaining the toner adhesion amount of 0.5 [mg / cm ² ], which is the target maximum toner adhesion amount, is calculated from the development γ calculated in S4 (S5). Then, in order to obtain the target maximum toner adhesion amount, various image forming conditions are adjusted so that the development potential at the time of image formation becomes the development potential VbL calculated in S5. Hereinafter, this adjustment method will be specifically described.

  In the present embodiment, with the currently set charging bias Vg0 and developing bias Vb0 applied, the exposure power LDP ′ is 1.5 times (150%) the basic exposure power LDP0 and the exposure duty is maximized. With the value (15), the surface of the photoreceptor 202 is exposed to form an electrostatic latent image. Then, the potential of the electrostatic latent image (exposed portion) formed on the surface of the photoreceptor 202 is detected by the potential sensor 210 as a residual exposed portion potential Vr ′ (S6). This residual exposed portion potential Vr 'is used to obtain a developing bias Vb' and a target charging potential Vd 'used when detecting the final residual exposed portion potential Vr.

  Next, a temporary reference exposure portion potential VL0 'at this time is calculated from the residual exposure portion potential Vr' detected in S6 by the following equation (1) (S7). The reference exposure portion potential is an exposure portion potential when exposure is performed with a reference exposure amount (reference exposure power LDP, reference exposure duty).

  VL0 '= Vr'-50 ... Formula (1)

  In the above formula, the value obtained by adding −50 [V] to the residual exposure portion potential Vr ′ is set as the reference exposure portion potential VL0 ′. Generally, the reference exposure portion potential is the residual exposure portion potential Vr. This is because it is empirically recognized that it exists in the vicinity of a value obtained by adding −50 [V] to “. An error between the provisional reference exposure portion potential VL0 'and the actual reference exposure portion potential VL0 is corrected by a correction process described later.

  Next, the development bias Vb ′ for Vr detection used when detecting the final residual exposure portion potential Vr is calculated from the provisional reference exposure portion potential VL0 ′ obtained by the above equation (2). To do.

  Vb '= VbL + VL0' ... Formula (2)

  Next, a target charging potential Vd ′ for Vr detection is calculated by the following equation (3) from the development bias Vb ′ for Vr detection calculated by the above equation (2).

  Vd '= Vb' + Vbg (3)

  Here, the background potential Vbg in the above formula (3) has conventionally been a constant value (for example, 200 [V]). However, in the present embodiment, the background potential Vbg is changed according to the development potential VbL for the reason described later. Variable value. Specifically, the background potential Vbg is calculated from the following mathematical formula (4) (S8).

Vbg = VbL × α (4)
Α in the equation (4) is a parameter indicating a suitable ratio of the background potential Vbg to the development potential VbL, and is hereinafter referred to as a background potential determination coefficient. The background potential determination coefficient α is a value obtained experimentally from the following viewpoints.

  Table 1 below is a table showing experimental conditions and experimental results for obtaining the background potential determination coefficient α.

  In this experiment, as a premise, toner replenishment is not performed, and various image forming conditions are not changed during the experiment. First, the charging potential Vd is adjusted to about 900 [V]. Further, the developing bias Vb and the exposure power LDP are set in accordance with the experimental conditions shown in Table 1 with the background potential being varied as in the experimental conditions shown in Table 1 above. Thereafter, while printing, the toner is forcibly replenished until the image density ID of the solid image becomes 1.6, and an image for measuring the line width of a 1-dot thin line under the condition that the image density ID becomes 1.6. Is printed. Further, a toner image on the surface of the photosensitive member 202 for an image for measuring the line width of a one-dot thin line is collected, and the amount of toner adhesion is measured. Also, a halftone image is printed and the halftone density is measured. This experiment is similarly performed when the charging potential Vd is about 700 [V] and about 500 [V].

  FIGS. 8A and 8B show vertical lines of one dot width (photosensitive member surface) by changing the background potential when the charging potential Vd is 900 [V], 700 [V], and 500 [V]. It is a graph of the said experimental result which shows the correspondence of the ground potential and line width when image-forming the line extended in the direction corresponding to a moving direction). FIG. 8A shows the background potential on the horizontal axis, and FIG. 8B shows the ratio of the background potential to the development potential on the horizontal axis.

  FIGS. 9A and 9B show horizontal lines of 1 dot width by changing the background potential when the charging potential Vd is 900 [V], 700 [V], and 500 [V] (photoconductor surface movement). It is a graph of the said experimental result which shows the correspondence of a ground potential and line | wire width when image-forming the line extended in the direction corresponding to the direction orthogonal to a direction). In FIG. 9A, the horizontal axis indicates the background potential, and in FIG. 9B, the horizontal axis indicates the ratio of the background potential to the development potential.

These graphs are obtained by changing the background potential while adjusting the development γ and the development potential so that the toner adhesion amount (image density) is all the target maximum adhesion amount of 0.5 [mg / cm ² ]. It is.

  As can be seen from FIGS. 8 (a) and 9 (a), even if the development potential is adjusted so that the toner adhesion amount is constant, the fine line The image quality changes that the line width is different. This is due to the influence of the electric field between the non-exposed part and the developing roller (hereinafter referred to as “background part electric field”) on the toner adhering to the boundary part between the exposed part and the non-exposed part in accordance with the difference in background potential. Is presumed to be different. More specifically, when the background electric field for moving the toner to the developing roller side becomes stronger, the effect of removing the toner from the boundary portion with the non-exposed portion in the exposed portion or preventing the toner from adhering to the boundary portion is increased. The toner adhesion area with respect to the area of the portion is reduced. This result is presumed to be manifested as image quality deterioration that the line width of the thin line becomes narrow. On the contrary, if the background electric field for moving the toner to the developing roller side becomes weak, the toner adhesion area with respect to the area of the exposed portion becomes large, and the image quality deterioration is manifested as the line width of the fine line becomes wide.

  Here, FIGS. 8A and 9A are obtained by adjusting the development potential so that the toner adhesion amount is constant. When such adjustment is performed, the background potential is the same as in the conventional case. When the value is fixed (for example, 200 [V]), it can be seen that the line width of the thin line varies depending on the difference in the charging potential Vd. Even in the monochromatic process control of the present embodiment, the development potential is adjusted so that the toner adhesion amount is constant. Therefore, if the background potential is a fixed value, a change in image quality in which the line width of the thin line changes occurs. In particular, in this embodiment, as described above, the photosensitive member 202 having a characteristic that the residual exposure portion potential Vr gradually increases with time is used. Therefore, in order to secure the same toner adhesion amount over time, it is necessary to gradually increase the development bias with time in order to ensure an appropriate development potential. As the developing bias increases, the charging potential Vd is adjusted so as to gradually increase with time in order to maintain a fixed background potential. Therefore, when the background potential is a fixed value (for example, 200 [V]), the image quality is deteriorated such that the line width of the thin line becomes thicker with time.

  On the other hand, when FIG. 8B and FIG. 9B are seen, even when the development potential is adjusted so that the toner adhesion amount is constant, if the ratio of the background potential to the development potential is constant, the charging potential It can be understood that the line width of the thin line does not change even when Vd is different. If this ratio is constant, the effect of the development electric field between the exposure unit and the developing roller on the toner adhering to the boundary portion between the exposure unit and the non-exposure unit, and the non-exposure unit and the development roller It is presumed that the influence of the electric field in the background is stable, and as a result, even if the development potential and the background potential change, the toner adhesion area with respect to the area of the exposed part hardly changes.

  For this reason, a constant image density is obtained by adjusting the background potential so that the ratio of the background potential to the development potential (background potential determination coefficient) is constant while securing the development potential so that the toner adhesion amount is constant. It is possible to suppress a change in image quality that the line width of the thin line changes while maintaining the above.

  FIG. 10A is a graph of the experimental results showing the correspondence between the background potential and the halftone density when the charging potential Vd is 900 [V], 700 [V], and 500 [V]. . FIG. 10B shows the above experimental results showing the correspondence between the ratio of the background potential to the development potential and the halftone density when the charging potential Vd is 900 [V], 700 [V], and 500 [V]. It is a graph of.

  FIG. 10A illustrates a case where the development potential is adjusted so that the toner adhesion amount of the solid image is constant, and the charging potential Vd is obtained when the background potential is a fixed value (for example, 200 [V]) as in the past. It is shown that the halftone density varies depending on the difference. Even in the monochromatic process control of the present embodiment, the development potential is adjusted so that the toner adhesion amount of the solid image is constant. Therefore, if the background potential is a fixed value, an image quality change in which the halftone density changes occurs. End up. In particular, in the present embodiment, since the photosensitive member 202 having the characteristic that the residual exposed portion potential Vr gradually increases with time is used, as described above, the charging potential Vd is adjusted to gradually increase with time. Is done. Therefore, when the background potential is a fixed value (for example, 200 [V]), the image quality is deteriorated such that the halftone density decreases with time.

  On the other hand, as shown in FIG. 10B, when the development potential is adjusted so that the toner adhesion amount of the solid image is constant, the charging potential Vd is different if the ratio of the background potential to the development potential is constant. However, it can be understood that the halftone density does not change. This is considered to be due to the same reason as in the case of the thin line width described above.

  As described above, a constant image density can be obtained by adjusting the background potential so that the ratio of the background potential to the development potential (background potential determination coefficient) is constant while securing the development potential so that the toner adhesion amount is constant. It is possible to suppress a change in image quality in which the halftone density changes while maintaining the image quality.

  From the experimental results described above, the ratio of the background potential to the development potential, that is, the background potential determination coefficient α is in the range of 0.40 to 0.80, preferably in the range of 0.40 to 0.45. If so, the above-described image quality change can be satisfactorily suppressed. In the present embodiment, the background potential determination coefficient α is set to 0.4.

  In this way, the background potential Vbg is calculated by multiplying the development potential VbL calculated in S5 by the background potential determination coefficient α according to the above equation (4). However, as described above, if the background potential Vbg is too small, background contamination occurs, and if the background potential Vbg is too large, carrier adhesion occurs. Therefore, if the background potential Vbg deviating from the range in which the background contamination and the carrier adhesion do not occur is set, a more serious problem than the above-described line width change or halftone density change occurs.

  For this reason, in the present embodiment, the following processing is performed so that the background potential Vbg deviating from the specified range in which background contamination and carrier adhesion do not occur is not set. That is, when the background potential Vbg calculated by the above equation (4) is higher than the upper limit value (VbgMAX) of the specified range, the upper limit value VbgMAX is calculated as the background potential Vbg and used for the subsequent processing. . When the background potential Vbg calculated by the above formula (4) is lower than the lower limit value (VbgMIN) of the specified range, the lower limit value VbgMIN is calculated as the background potential Vbg and used for the subsequent processing. .

  Next, the development bias Vb ′ for Vr detection used when detecting the final residual exposure portion potential Vr is calculated from the provisional reference exposure portion potential VL0 ′ calculated in S7 by the above formula (2) ( S9). Further, using the development bias Vb ′ for Vr detection calculated by the above formula (2) and the background potential Vbg calculated by S8, the target charging potential Vd ′ for Vr detection by the above formula (3). Is calculated (S9).

  Next, the charging bias Vg ′ for detecting Vr is set so that the charging potential becomes the target charging potential Vd ′ for detecting Vr (S10).

  Specifically, first, the charging bias is set to a predetermined fixed value (-550 [V] in the present embodiment), and the developing bias is also set to a predetermined fixed value (-350 [in the present embodiment). V]), the surface of the photosensitive member 202 is charged, and the charged potential at this time is detected by the potential sensor 210. If the detection result is within the target range centered on the target charging potential Vd ′ calculated in S9 (in this embodiment, Vd ′ ± 5 [V]), the fixed value (−550 [−5 [V]) used for this measurement is used. V]) is set to the charging bias Vg ′ for Vr detection.

  On the other hand, when the detection result is out of the target range, the fixed value (−550 [V]) of the charging bias and the detection result (charging potential) and the charging used during the pre-processing of the monochromatic process control are performed. Using the bias (in this embodiment, −700 [V]) and the charging potential detected by the potential sensor 210 at that time, the relationship between the charging bias and the charging potential at the present time is first-order approximated by the least square method. An approximate relational expression (first-order approximate expression) is obtained. Then, a charging bias for Vr detection corresponding to the target charging potential Vd ′ for Vr detection is specified from this first-order approximate expression. Thereafter, the surface of the photosensitive member 202 is charged again using the charging bias for Vr detection specified here, and the charged potential at this time is detected by the potential sensor 210. If the detection result is within the target range, the charging bias for Vr detection specified above is determined as the charging bias Vg ′ for Vr detection. If the detection result is out of the target range, the measurement result at this time is also added to obtain a first-order approximation expression indicating the relationship between the charging bias and the charging potential, and the detection result is within the target range. Repeat the same process until entering.

  Here, the range of the charging bias that can be set is often limited depending on the specifications of the charging unit 201 to be used. In the present embodiment, the settable range of the charging bias is limited to a range of −450 [V] or more and −900 [V] or less. Therefore, in this embodiment, when the charging bias Vg ′ for Vr detection determined as described above exceeds the upper limit value (VgMAX = −900 [V]) of the setting range of the charging bias, the upper limit value VgMAX. Is set as the charging bias Vg ′. On the other hand, when the charging bias Vg ′ for Vr detection determined as described above is below the lower limit value (VgMIN = −450 [V]) of the setting range of the charging bias, the lower limit value VgMIN is set to the charging bias Vg ′. Set as.

  Further, the development bias Vb ′ for Vr detection is corrected and set so that the background potential becomes the background potential Vbg calculated in S8 according to the charging bias Vg ′ for Vr detection set as described above ( S10).

  FIG. 11 is a flowchart showing a flow of processing for correcting the developing bias Vb ′ for detecting Vr when the charging bias Vg ′ is set to the charging bias upper limit value VgMAX. FIG. 12 is a flowchart showing a flow of processing for correcting the developing bias Vb ′ for detecting Vr when the charging bias Vg ′ is set to the charging bias lower limit value VgMIN. FIG. 13 is a flowchart showing a flow of processing for correcting the developing bias Vb ′ for detecting Vr when the charging bias Vg ′ is set between the charging bias upper limit value VgMAX and the charging bias lower limit value VgMIN. is there.

  When the charging bias Vg ′ is set to the charging bias upper limit value VgMAX, as shown in FIG. 11, if the background potential Vbg is set to the upper limit value VbgMAX (S21 / Yes), the background potential Vbg is expressed by the following formula. The value is corrected to the value obtained by (5) (S22).

  Vbg = Vbg1 = VbgMAX− (Vd ′ [calculated value] −VgMAX) × β1 (5)

  The Vd ′ [calculated value] in the equation (5) is the target charging potential Vd ′ for Vr detection calculated in S9 and is distinguished from the charging potential Vd ′ [detected value] detected in S10. It is. Further, β1 in the above formula (5) is a coefficient for making the ratio of the background potential to the development potential constant within the setting range of the charging bias, and is usually set to the same value as the background potential determination coefficient α. Is done.

  On the other hand, if the background potential Vbg is set to the lower limit value VbgMIN (S23 / Yes), the background potential Vbg is not corrected (S24), and the lower limit value VbgMIN is used as it is.

  On the other hand, if the background potential Vbg is set to the upper limit value VbgMAX and the lower limit value VbgMIN (S23 / No), the background potential Vbg is corrected to a value obtained by the following equation (6) (S25).

  Vbg = Vbg2 = Vbg− (Vd ′ [calculated value] −VgMAX) × β1 (6)

  Next, when the background potential Vbg is less than or equal to the lower limit value VbgMIN (S26 / Yes), after correcting the background potential Vbg to the lower limit value VbgMIN (S27), the development bias Vb ′ for detecting Vr is expressed by the following formula ( It is set to the value obtained by 7) (S28).

  Vb '= VgMAX-VbgMIN (7)

  On the other hand, when the background potential Vbg is between the upper limit value VbgMAX and the lower limit value VbgMIN (S26 / No), the developing bias Vb ′ for detecting Vr is set to a value obtained by the following equation (8) (S29). .

  Vb '= VgMAX-Vbg (8)

  When the charging bias Vg ′ is set to the charging bias lower limit value VgMIN, as shown in FIG. 12, if the background potential Vbg is set to the upper limit value VbgMAX (S31 / Yes), the background potential Vbg is corrected. Is not performed (S32), and the upper limit value VbgMAX is used as it is.

  On the other hand, if the background potential Vbg is set to the lower limit value VbgMIN (S33 / Yes), the background potential Vbg is corrected to a value obtained by the following equation (9) (S34).

  Vbg = Vbg3 = VbgMIN− (Vd ′ [calculated value] −VgMIN) × β2 (9)

  Β2 in the above formula (5) is a coefficient for making the ratio of the background potential to the development potential constant within the setting range of the charging bias like the above β1, and is usually the same as the background potential determination coefficient α. Set to a value.

  On the other hand, if the background potential Vbg is set to the upper limit value VbgMAX and the lower limit value VbgMIN (S33 / No), the background potential Vbg is corrected to a value obtained by the following equation (10) (S35).

  Vbg = Vbg4 = Vbg− (Vd ′ [calculated value] −VgMIN) × β2 (10)

  Next, when the background potential Vbg is equal to or higher than the upper limit value VbgMAX (S36 / Yes), the background potential Vbg is re-corrected to the upper limit value VbgMAX (S37), and then the development bias Vb ′ for detecting Vr is expressed by 11) and set to the value obtained (S38).

  Vb ′ = VgMIN−VbgMAX Equation (11)

  On the other hand, when the background potential Vbg is between the upper limit value VbgMAX and the lower limit value VbgMIN (S36 / No), the developing bias Vb ′ for detecting Vr is set to a value obtained by the following equation (12) (S39). .

  Vb ′ = VgMIN−Vbg Expression (12)

  If the charging bias Vg ′ is set between the upper limit value VgMAX and the lower limit value VgMIN of the charging bias, as shown in FIG. 13, if the background potential Vbg is set to the upper limit value VbgMAX (S41 / Yes), the background potential Vbg is corrected to a value obtained by the following equation (13) (S42).

  Vbg = Vbg5 = VbgMAX− (Vd ′ [calculated value] −Vd ′ [detected value]) (Equation 13)

  On the other hand, if the background potential Vbg is set to the lower limit value VbgMIN (S43 / Yes), the background potential Vbg is corrected to a value obtained by the following equation (14) (S44).

  Vbg = Vbg6 = VbgMIN− (Vd ′ [calculated value] −Vd ′ [detected value]) (14)

  On the other hand, if the background potential Vbg is set to the upper limit value VbgMAX and the lower limit value VbgMIN (S43 / No), the background potential Vbg is corrected to a value obtained by the following equation (15) (S45).

  Vbg = Vbg7 = Vbg− (Vd ′ [calculated value] −Vd ′ [detected value]) (15)

  Next, the developing bias Vb 'for detecting Vr is set to a value obtained by the following equation (16) (S46).

  Vb ′ = Vd ′ [detection value] −Vbg (16)

  Next, using the provisional charging bias Vg ′ and development bias Vb ′ for Vr detection set as described above, the same method as in S6, specifically, 1.5 times the basic exposure power LDP0. The surface of the photosensitive member 202 is exposed with the exposure power LDP ′ of (150%) and the exposure duty set to the maximum value (15). Then, the potential of the electrostatic latent image (exposed portion) formed thereby is detected by the potential sensor 210 as the final residual exposed portion potential (detected residual potential) Vr (S11).

  Thereafter, in the present embodiment, a low exposure amount region in which the change rate of the exposed portion potential on the surface of the photoreceptor 202 with respect to the change in exposure amount is large from the target charged potential Vd ′ and the residual exposed portion potential Vr (FIG. 14A). , (B), the adjustment exposure portion potential Vpl belonging to approximately the region extending from the center to the left side of the graph is calculated by the following equation (17) (S12).

  Vpl = (Vd′−Vr) ÷ 3 + Vr (17)

  Next, the temporary charging bias Vg ″ and the developing bias Vb ″ are reset by performing the same processing as the processing from S7 to S10 using the newly detected residual exposure portion potential Vr (S13). ).

  Next, the exposure power for Vpl (pre-reference exposure amount) for obtaining the exposure part potential Vpl for adjustment is specified (S14). However, the adjustment exposure portion potential Vpl of the present embodiment is approximately in the vicinity of 1/3 of the reference exposure portion potential. Therefore, in order to search for the optimum Vpl exposure power in this vicinity, an exposure duty of 5/15, which is 1/3 of the reference exposure amount (exposure duty = 15/15), is used at this time.

  In specifying the exposure power for Vpl for obtaining the exposure part potential Vpl for adjustment, the exposure power is 60%, 80%, 100%, 120% of the basic exposure power LDP0, with the exposure duty fixed at 5/15. The electrostatic latent image (exposure portion) is created by sequentially switching to 150%. The charging bias and the developing bias at this time are the provisional charging bias Vg ″ and the developing bias Vb ″ set in S13. Then, the potential of each exposure unit is detected by the potential sensor 210, and the charging potential Vd at this time is also detected by the potential sensor 210. Then, five data sets showing the correspondence relationship between the exposure power corresponding to each exposure portion, the charging potential Vd at that time, and the residual exposure portion potential Vr and each adjustment exposure portion potential Vpl obtained by the above equation (17). Is calculated. Then, from each data set, the relation between the exposure power and the adjustment exposure portion potential Vpl is first-order approximated by the least square method to obtain an approximate relational expression (first-order approximation expression). The exposure power for Vpl for obtaining the adjustment exposure part potential Vpl calculated in the above is specified.

  Thereafter, the surface of the photosensitive member 202 is exposed using the exposure power for Vpl specified here (exposure duty = 5/15), and the potential of the exposed portion at this time is detected by the potential sensor 210. If this detection result is within the target range (within ± 3 [V] of the adjustment exposure portion potential Vpl calculated in S12), the specified Vpl exposure power specified above is used as it is. On the other hand, when the detection result is out of the target range, the specified Vpl exposure power specified above is further adjusted with a predetermined adjustment value, and the photoconductor is used using the adjusted Vpl exposure power. The process of exposing the surface 202 again and detecting the potential of the exposed portion at this time by the potential sensor 210 is repeated until the detection result falls within the target range.

  When the exposure power for Vpl for obtaining the adjustment exposure part potential Vpl is specified in this way, the exposure power for Vpl is then set to the exposure power of 15/15 exposure duty which is the exposure duty of the reference exposure amount. Conversion is performed (S15).

  In the present embodiment, since the exposure duty used to specify the Vpl exposure power is 1/3 of the exposure duty (15/15) of the reference exposure amount, the exposure power for Vpl specified in S14 is 3 Doubled and converted to exposure power of 15/15 exposure duty.

  Next, the reference exposure power is determined from the converted exposure power obtained by conversion in the above processing (S16). Here, under the conditions of the present embodiment, it has been previously known by experiments and the like that the relationship between the converted exposure power and the reference exposure power is about 2/3. Therefore, in the present embodiment, a value obtained by multiplying the converted exposure power by 2/3 is determined as the reference exposure power. Note that this converted value (2/3 in the present embodiment) is appropriately set by experiment or the like.

  After obtaining the reference exposure power as described above, finally, a correction process for correcting an error between the reference exposure part potential VL0 ′ provisionally determined in S7 and the actual reference exposure part potential VL0 is performed. Do.

  Specifically, first, an electrostatic latent image (exposure portion) is created with the reference exposure amount (reference exposure power determined in S16, 15/15 exposure duty), and the potential of the exposure portion (reference exposure portion potential VL0). ) Is detected by the potential sensor 210 (S17). The charging bias and the developing bias at this time are the provisional charging bias Vg ″ and the developing bias Vb ″ set in S13. A difference ΔVL between the reference exposure part potential VL0 detected in this way and the reference exposure part potential VL0 ′ provisionally determined in S7 is calculated (S18). Then, using the difference ΔVL as a correction value, the provisional charging bias Vg ″ and the developing bias Vb ″ set in S13 are corrected, and the final charging bias Vg and the developing bias Vb are determined (S19). Accordingly, the final charging bias Vg is expressed by the following mathematical formula (18), and the final developing bias Vb is expressed by the following mathematical formula (19). However, when the corrected charging bias Vg and developing bias Vb exceed the preset upper and lower limit values, the charging bias Vg and developing bias Vb before correction are used as final values.

Vg = Vg "-[Delta] VL Expression (18)
Vb = Vb ″ −ΔVL Expression (19)

  In the monochromatic process control of this embodiment, both the development potential and the background potential are adjusted so that the charging bias Vg is not set exceeding the upper limit value VgMAX. Specifically, the ratio of the background potential to the development potential is adjusted to be maintained at the background potential determination coefficient α. By such adjustment, it is possible to suppress changes in image quality such as changes in the line width of thin lines and changes in halftone density while preventing the charging bias Vg from being set exceeding the upper limit value VgMAX. However, in this case, since the development potential is set slightly lower, the image density is slightly lowered, and there is a possibility that some image quality change occurs. For this reason, when priority is given to maintaining the image density, only the background potential may be adjusted for the charge bias Vg exceeding the upper limit value VgMAX. In this case, since the ratio of the background potential to the development potential is slightly changed, there is a possibility that slight changes in image quality such as a change in line width of a thin line and a change in halftone density may occur, but a change in image density can be prevented.

<Example of multi-color process control using background potential coefficient control>
Next, an example of multi-color process control using background potential coefficient control will be described. In the above processing, the description has been given of the single color process control for stabilizing the normal single color toner adhesion amount. However, in the above processing, since the multiple color process control is not performed to stabilize the toner adhesion amount of the multiple colors, the color balance of the multiple colors changes, the color difference of the multiple colors increases, and the hue direction increases. Misalignment will occur. For this reason, in the present embodiment, multi-color process control is performed after single-color process control. The multi-color process control may be performed at the time of process control between sheets during the printing process.

FIG. 16 is a diagram showing a main processing flow of multi-color process control for stabilizing the multi-color toner adhesion amount in the present embodiment. In the present embodiment, the toner gradation pattern when performing multi-color process control so that the adhesion amount conversion process can be appropriately performed on an image from low density to high density formed on the transfer paper 112. The case where the target toner adhesion amount range is in the range from around 0 [mg / cm ² ] to 0.5 [mg / cm ² ] will be described.

  In multi-color process control, after finishing mono-color process control, first, image forming conditions such as the currently set charging bias Vg0, developing bias Vb0, and exposure power LDP (set in the previous mono-color process control). In the image forming conditions), 10-gradation density patches 113 of Y, M, and C are formed on the surface of the photoreceptor 202 (S51). Then, among these 10-gradation density patches 113, the amount of toner attached to the density patch 113 with the input image density of 70% is detected by the image detection unit 110 (S52). The reason why the density patch 113 having the input image density of 70% is selected is that the toner adhesion amount variation at the input image density of 70% is the largest for both single color and multiple colors.

  FIG. 17 is a graph showing the relationship between the toner adhesion amount of the Y density patch 113 and the Y image density proportional to the reflectance of the image based on the experimental results. Then, using FIG. 17, the toner adhesion amount attached to the 10-gradation density patch 113 obtained in step S52 is converted into an image density, and the image density corresponding to the toner adhesion amount is calculated (S53). In place of using FIG. 17, the Y density patch 113 may be measured by an image density sensor or the like to detect the image density.

  FIG. 18 is a graph showing the relationship between the image density of Y and the saturation of Y based on experimental results. Then, using FIG. 18, the Y image density obtained in the step S53 is converted into Y saturation, and the saturation corresponding to the image density is calculated (S54). In place of using FIG. 18, the Y density patch 113 may be measured by a colorimetric sensor or the like to detect the saturation. Similarly, saturation is calculated for M and C.

  There are the following three methods for calculating the saturation.

  1. The toner adhesion amount of the density patch 113 is measured by the image detection unit 110, converted to an image density corresponding to the toner adhesion amount, and converted to a saturation corresponding to the image density. The conversion to the image density according to the toner adhesion amount and the conversion to the saturation according to the image density can be realized by using a conversion algorithm or a conversion table.

  2. An image density sensor for measuring the image density of the density patch 113 is mounted, and the image density of the density patch 113 is measured by the image density sensor and converted to saturation according to the image density. Conversion to saturation according to image density can be realized by using a conversion algorithm or a conversion table.

  3. A colorimetric sensor for measuring the saturation of the density patch 113 is mounted, and the saturation of the density patch 113 is measured by the colorimetric sensor.

  Of the above three methods, the third method is the best. For example, even if the toner adhesion amounts of the two density patches 113 are the same, as shown in FIG. 19, the shape of the toner dots forming the density patch 113 differs in dot height and dot diameter. Because there is. For this reason, the image density and saturation of the density patch 113 may be different depending on whether the measurement is performed or the conversion table is used.

  Multiple colors using two single colors are R, G, and B. Y and M, Y and C, and M and C are used, respectively.

  FIG. 20 is a graph showing an allowable range of single color Y saturation and C saturation used for multiple colors G. When the allowable range is smaller than the range in which the color variation of the density patch 113 of the multi-color G obtained in another image evaluation is smaller, the saturation of the density patch 113 of the monochrome Y and the monochrome C This is a range in which the saturation of the density patch 113 is combined. The overall average line is an approximate straight line that passes through these four points after obtaining the overall average of the density patches 113 every 20, 40, 70, and 100%. That is, when it is within the allowable range (area 1), the change in the color balance of the multiple colors G is suppressed, the color difference is smaller than a certain range, and the deviation in the hue direction is small. Using FIG. 20, the Y saturation and the C saturation calculated in S54 are plotted (S55). In the case of multiple colors R and B, there are graphs showing allowable ranges of Y saturation and M saturation, and M saturation and C saturation, respectively, and the same processing is performed.

  Next, it is confirmed whether or not the Y saturation and the C saturation are within the allowable range (area 1). If both are within the allowable range (S56 / Yes), the feedback is not performed and the multiplexing is performed as it is. End color process control.

  On the other hand, if either or both of the Y saturation and the C saturation are outside the allowable range (areas 2 and 3) (S56 / No), the background potential coefficient control is performed (S57). In FIG. 20, area 2 is above the overall average line excluding area 1, and area 3 is below the overall average line excluding area 1. Also, the background potential coefficient control of S57 is the same as described in the single color process control, by taking advantage of the characteristic that the line width and halftone density of the thin line can be changed by increasing / decreasing the background potential coefficient. This is a control that suppresses the saturation of the monochromatic density patch 113 within an allowable range by adjusting the adhesion amount.

  Hereinafter, the multiple color G will be described as an example. When the Y saturation and the C saturation are in the area 2, the ratio of the Y saturation is large, and the multiple color G using the two-color single color toner becomes a G closer to the color of Y. Therefore, when reducing the Y saturation and reducing the Y toner adhesion amount to adjust the G color balance, the Y background potential coefficient is increased, and conversely the C toner adhesion is increased to increase the C saturation. When adjusting the color balance of G by increasing the amount, the background potential coefficient of C may be reduced.

  On the other hand, when the Y saturation and the C saturation are in the area 3, the ratio of the C saturation is large, and the multiple color G using the two-color single color toner becomes a G closer to the color of C. Therefore, when adjusting the G color balance by increasing the Y toner adhesion amount in order to increase the Y saturation, the C toner adhesion is reduced in order to decrease the Y background potential coefficient and conversely reduce the C saturation. In order to adjust the color balance of G by reducing the amount, the background potential coefficient of C may be increased.

  FIG. 21 shows the saturation of Y calculated as a result of a change in Y toner adhesion amount with respect to the Y background potential coefficient.

  FIG. 22 shows the saturation of C calculated as a result of a change in the toner adhesion amount of C with respect to the background potential coefficient of C.

  21 and FIG. 22, it can be seen how much the background potential coefficient should be changed in order to achieve a desired single color toner saturation. However, as described in the monochromatic process control, if the background potential Vbg is too small, background contamination occurs, and if the background potential Vbg is too large, carrier adhesion occurs. Therefore, the value of the ground potential coefficient should be suppressed to 0.35 to 0.45 within a range in which background dirt and carrier adhesion do not occur.

  After performing the above background potential coefficient control, the process returns to S51 again, and the process of confirming whether or not Y saturation and C saturation are within the allowable range (area 1) is repeated in S56. If this process is repeated 20 times, the background potential coefficient control is terminated, and the multiple color process control is terminated (End). The same multiple color process control is performed for the multiple colors R and B. However, the allowable range of S56 is, in the case of multiple colors G, as shown in FIG. 20, the saturation of single color Y is 64 or more and 71 or less, and the saturation of single color C is 42 or more and 47 or less. In the case of R, as shown in FIG. 23, the saturation of the single color Y is 64 or more and 71 or less, and the saturation of the single color M is 47 or more and 51 or less, and in the case of the multiple color B, as shown in FIG. In addition, the saturation of the single color M is set to 47 to 51, and the saturation of the single color C is set to 42 to 47.

<Operation / Effect of Image Forming Apparatus of this Embodiment>
As described above, the image forming apparatus according to the present exemplary embodiment specifies the saturation of the density patch 113 that is a toner pattern, and the saturation of the density patch 113 indicates the saturation of the first color and the second color used for multiple colors. If it is not within the predetermined allowable range, the ratio of the background potential to the development potential is increased or decreased within the range of the predetermined target ratio (0.35 to 0.45), and the saturation of the density patch 113 is the first color and the second color. Adjust so that the saturation is within the allowable range. Thereby, the color difference of multiple colors can be reduced.

  The above-described embodiment is a preferred embodiment of the present invention, and the scope of the present invention is not limited to the above-described embodiment alone, and various modifications are made without departing from the gist of the present invention. Implementation is possible.

  For example, the series of processing operations of the above-described embodiment need not be performed by only one control unit 41, and can be configured to be performed by a plurality of control units.

  In addition, the control operation of each unit constituting the image forming apparatus of the present embodiment described above can be executed using hardware, software, or a combined configuration of both.

  In the case of executing processing using software, it is possible to install and execute a program in which a processing sequence is recorded in a memory in a computer incorporated in dedicated hardware. Alternatively, the program can be installed and executed on a general-purpose computer capable of executing various processes.

  For example, the program can be recorded in advance on a hard disk or ROM (Read Only Memory) as a recording medium. Alternatively, the program can be stored (recorded) temporarily or permanently in a removable recording medium. Such a removable recording medium can be provided as so-called package software. Examples of the removable recording medium include a floppy (registered trademark) disk, a CD-ROM (Compact Disc Read Only Memory), an MO (Magneto optical) disk, a DVD (Digital Versatile Disc), a magnetic disk, and a semiconductor memory.

  The program is installed in the computer from the removable recording medium as described above. In addition, it is wirelessly transferred from the download site to the computer. In addition, it is transferred to the computer via a network by wire.

  In addition, the image forming apparatus according to the present embodiment is not only executed in time series according to the processing operation described in the above embodiment, but also the processing capability of the unit that executes the process, or in parallel as necessary. It can also be constructed to run individually.

41 control unit 101 intermediate transfer belt 102Y, 102M, 102C, 102K image forming unit 110 image detecting unit 113 density patch 201 charging unit 202 photoconductor 203 writing unit 205 developing unit 210 potential sensor 301, 302 optical sensor

Japanese Patent Laid-Open No. 5-14729 JP 7-271138 A JP 2004-212893 A JP-A-5-297678 JP 2010-44215 A

Claims (10)

An image bearing member, a charging unit that charges the surface of the image bearing member with a predetermined charging bias, and the surface of the image bearing member charged by the charging unit are exposed with a predetermined exposure amount to form an electrostatic latent image. A predetermined developing bias is applied to the electrostatic latent image formed on the surface of the image carrier, and the toner is attached to the electrostatic latent image, and is applied to the surface of the image carrier. An image forming apparatus for transferring a toner image formed on the surface of the image carrier to a recording medium and forming an image on the recording medium,
Pattern forming means for forming a toner pattern on the image carrier;
Based on the toner adhesion amount of the toner pattern, the electrostatic latent image is not formed with respect to the developing potential that is the difference between the potential of the electrostatic latent image portion where the electrostatic latent image is formed and the developing bias. A single color that adjusts the charging bias, the exposure amount, and the developing bias so that the ratio of the background potential, which is the difference between the potential of the non-electrostatic latent image portion and the developing bias, becomes a predetermined first target ratio. Process control means;
When the saturation of the single color of the toner pattern is not within a predetermined allowable range of the saturation of the first color and the second color used for multiple colors, the ratio of the background potential to the development potential is set to a predetermined second target. A multi-color process control means for increasing / decreasing within a range of the ratio and adjusting the saturation of the single color of the toner pattern to be within the allowable range;
An image forming apparatus comprising: The multi-color process control means includes
The ratio of the background potential to the development potential is increased / decreased within the range of the second target ratio, the amount of monochrome toner adhesion of the toner pattern is adjusted, and the monochrome saturation of the toner pattern falls within the allowable range. The image forming apparatus according to claim 1, wherein the adjustment is repeatedly performed until The multi-color process control means includes
3. The image forming apparatus according to claim 2, wherein, even if the adjustment is repeatedly performed a predetermined number of times, if the saturation of a single color of the toner pattern does not fall within the allowable range, the adjustment is terminated. The allowable range is
When the multiple color is R (red), the saturation of the single color Y (yellow) of the toner pattern is 64 or more and 71 or less, and the saturation of the single color M (magenta) of the toner pattern is 47 or more and 51 or less. ,
When the multiple color is G (green), the saturation of the single color Y of the toner pattern is 64 or more and 71 or less, and the saturation of the single color C (cyan) of the toner pattern is 42 or more and 47 or less,
When the multiple color is B (blue), the saturation of the single color M of the toner pattern is 47 or more and 51 or less, and the saturation of the single color C of the toner pattern is 42 or more and 47 or less. Item 4. The image forming apparatus according to any one of Items 1 to 3.   The first target ratio is 0.40 or more and 0.45 or less, and the second target ratio is 0.35 or more and 0.45 or less. 2. The image forming apparatus according to item 1. Image detecting means for reading the toner adhesion amount of the toner pattern;
Density conversion means for converting the image density according to the toner adhesion amount;
Saturation conversion means for converting into saturation according to the image density,
The multi-color process control means includes
The image forming apparatus according to claim 1, wherein a single color saturation of the toner pattern is specified based on the saturation converted by the saturation conversion unit. Density detecting means for reading the image density of the toner pattern;
Saturation conversion means for converting into saturation according to the image density,
The multi-color process control means includes
The image forming apparatus according to claim 1, wherein a single color saturation of the toner pattern is specified based on the saturation converted by the saturation conversion unit. Color detecting means for reading the saturation of the toner pattern;
The multi-color process control means includes
The image forming apparatus according to claim 1, wherein a single color saturation of the toner pattern is specified based on a saturation detected by the color detection unit. An image bearing member, a charging unit that charges the surface of the image bearing member with a predetermined charging bias, and the surface of the image bearing member charged by the charging unit are exposed with a predetermined exposure amount to form an electrostatic latent image. A predetermined developing bias is applied to the electrostatic latent image formed on the surface of the image carrier, and the toner is attached to the electrostatic latent image, and is applied to the surface of the image carrier. And a developing means for forming a toner image, transferring the toner image formed on the surface of the image carrier to a recording medium and forming an image on the recording medium. And
A pattern forming step of forming a toner pattern on the image carrier;
Based on the toner adhesion amount of the toner pattern, the electrostatic latent image is not formed with respect to the developing potential that is the difference between the potential of the electrostatic latent image portion where the electrostatic latent image is formed and the developing bias. A single color that adjusts the charging bias, the exposure amount, and the developing bias so that the ratio of the background potential, which is the difference between the potential of the non-electrostatic latent image portion and the developing bias, becomes a predetermined first target ratio. A process control step;
When the saturation of the single color of the toner pattern is not within a predetermined allowable range of the saturation of the first color and the second color used for multiple colors, the ratio of the background potential to the development potential is set to a predetermined second target. A multi-color process control step of increasing or decreasing within a range of the ratio and adjusting the saturation of a single color of the toner pattern to be within the allowable range;
A control method characterized by comprising: An image bearing member, a charging unit that charges the surface of the image bearing member with a predetermined charging bias, and the surface of the image bearing member charged by the charging unit are exposed with a predetermined exposure amount to form an electrostatic latent image. A predetermined developing bias is applied to the electrostatic latent image formed on the surface of the image carrier, and the toner is attached to the electrostatic latent image, and is applied to the surface of the image carrier. A program for causing a computer of an image forming apparatus to transfer a toner image formed on the surface of the image carrier to a recording medium and form an image on the recording medium. Because
A pattern forming process for forming a toner pattern on the image carrier;
Based on the toner adhesion amount of the toner pattern, the electrostatic latent image is not formed with respect to the developing potential that is the difference between the potential of the electrostatic latent image portion where the electrostatic latent image is formed and the developing bias. A single color that adjusts the charging bias, the exposure amount, and the developing bias so that the ratio of the background potential, which is the difference between the potential of the non-electrostatic latent image portion and the developing bias, becomes a predetermined first target ratio. Process control processing;
When the saturation of the single color of the toner pattern is not within a predetermined allowable range of the saturation of the first color and the second color used for multiple colors, the ratio of the background potential to the development potential is set to a predetermined second target. A multi-color process control process that increases or decreases within the range of the ratio and adjusts the saturation of the single color of the toner pattern to be within the allowable range;
Is executed by the computer.

Images