JP5272105B2 - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that reduces time for density correction as much as possible to improve efficiency of image formation and prevent the occurrence of image failure with a simple configuration, and maintains high accuracy of density correction. <P>SOLUTION: A control unit 32 sets a circumferential velocity ratio R of developing rollers 3aa to 3dd corresponding to photoreceptor drums 1a to 1d. The control unit 32 sets the circumferential velocity ratio R to a first circumferential velocity ratio R1 during image formation, and sets the circumferential velocity ratio R to a second circumferential velocity ratio R2 that is smaller than the first circumferential velocity ratio R1 for solid density correction. The control unit 32 changes the circumferential velocity ratio R from the second circumferential velocity ratio R2 to the first circumferential velocity ratio R1 after the solid density correction. <P>COPYRIGHT: (C)2013,JPO&amp;INPIT

Description

  The present invention relates to an image forming apparatus using electrophotography, and more particularly to a density correction method for an output image in the image forming apparatus.

  In an image forming apparatus using an electrophotographic process, the density is corrected by transferring the toner directly onto the toner carrier when the apparatus is activated or when a mode (calibration mode) for setting the image density appropriately is set. It is common to form a pattern (reference image), detect the density, and perform density correction. For example, in the case of a color image forming apparatus, a reference image of each color is formed on an image carrier by magenta, cyan, yellow, and black image forming units, and the reference image transferred on a toner carrier such as an intermediate transfer belt The density is corrected by detecting the density by the detecting means.

  Examples of the image density adjustment method include a method of adjusting the charging potential of the photosensitive member, the developing bias potential, the exposure amount by the exposure device, and the like based on the detected image density. In the color image forming apparatus, calibration is performed. Plays an important role in matching colors. In order to stably output high-quality images including solid density areas (solid areas) and halftone density areas (halftone areas), solid densities and intermediate It is necessary to improve the correction accuracy of both tonal densities. There is also a demand for efficient calibration without reducing productivity (image formation efficiency).

  For example, in Patent Document 1, first, solid density control is performed by controlling the peripheral speed ratio of the developer carrier by image density detection using a test solid patch, and laser power is then detected by image density detection using a test halftone patch. A method is disclosed in which the toner adhesion amount from halftone to solid can be accurately performed by controlling and controlling the halftone density.

  Further, Patent Document 2 discloses that the toner adhering amount of solid is controlled by detecting the toner adhering amount of the test solid patch by the peripheral speed ratio variable setting means to control the peripheral speed ratio of the developer carrying carrier to the image carrier during image formation. By controlling the peripheral speed of the developer transport carrier of the development device, and then performing line width control on the image carrier, development conditions can be variably set in response to environmental changes during image formation And a method that makes it possible to obtain a desired line width is disclosed.

  In Patent Documents 3 and 4, the photosensitive drum and the toner on the drum are irradiated with polarized light, and the same polarized light as the irradiated light is emitted by the first light receiving unit, and the polarized light different from the irradiated light is applied. The light receiving unit 2 receives light, and Patent Document 3 discloses a method for accurately measuring the toner adhesion amount by measuring the toner adhesion amount with the first and second light receiving units. Yes. In Patent Document 4, the output accuracy of the first light receiving unit and the output signal of the second light receiving unit are switched and output at a predetermined adhesion amount, so that the detection accuracy is maintained even if the toner adhesion amount increases. A method for enabling is disclosed.

JP-A-8-297384 Japanese Patent Laid-Open No. 9-50155 Japanese Patent No. 2729976 JP 2002-169345 A

  However, in the methods of Patent Documents 1 and 2, the peripheral speed ratio of the developer carrier to the image carrier is set based on the solid density detection result, and halftone density correction is performed at the peripheral speed ratio set at the time of the solid density correction. Therefore, it is necessary to change various peripheral speed ratios when correcting the solid density. For this reason, there is a possibility that it is difficult to perform density correction quickly. In addition, if the peripheral speed ratio is changed many times, there is a possibility of causing a defect of an image such as a jitter image. In addition, such a change in the peripheral speed ratio may cause defects in the image itself such as fine line reproducibility and dot reproducibility.

  Further, in the methods of Patent Documents 3 and 4, not only the reflected light is detected in two components, but also the density correction is performed using the detection result from the reflected light of the two components, so that the density correction control is complicated. Therefore, it may be difficult to perform density correction quickly.

  In view of the above problems, the present invention improves the image forming efficiency by shortening the density correction time as much as possible with a simple configuration, prevents the occurrence of image quality problems, and also provides high levels of solid density and halftone density correction. An object of the present invention is to provide an image forming apparatus that can be maintained by the above.

  In order to achieve the above object, the present invention provides an image carrier, an exposure device that forms an electrostatic latent image on the image carrier, and a developer carrier that has an electrostatic latent image formed by the exposure device. An image forming unit including a developing device that develops an image; a detecting unit that detects a density of a reference image formed on a print medium by the image forming unit; and a solid density correction and an intermediate level based on a detection result of the detecting unit An image forming apparatus comprising: a control unit that performs tone adjustment correction; and a control unit that sets a peripheral speed ratio of the developer carrier to the image carrier during image formation to be a predetermined first peripheral speed ratio. Performs a solid density correction by changing the circumferential speed ratio to a predetermined second circumferential speed ratio smaller than the first circumferential speed ratio, and after correcting the solid density, the circumferential speed ratio is changed to the second circumferential speed ratio. The halftone density correction is performed by changing the first peripheral speed ratio to the first peripheral speed ratio. There.

  According to the present invention, in the image forming apparatus having the above-described configuration, the control unit forms a first reference image having a density pattern of a predetermined density by stepwise changing a developing bias application condition applied to the developing apparatus. At the same time, a development bias level is set based on the density detection result of the first reference image, the solid density correction is performed, and the development bias level set in the solid density correction is used to form a gradation pattern in a predetermined order. A second reference image is formed and halftone density correction is performed based on the density detection result of the second reference image.

  Further, the present invention is characterized in that the control means performs the halftone density correction using the second reference image formed by changing the exposure amount of the exposure apparatus stepwise.

  According to the present invention, in the image forming apparatus having the above-described configuration, the detection unit irradiates the print medium with measurement light, receives regular reflection light and irregular reflection light among reflected light from the print medium, and controls the control unit. And the control means performs the solid density correction based on the output value of one of the regular reflection light and the irregular reflection light.

  According to the first configuration of the present invention, when the peripheral speed ratio of the developer carrier to the image carrier at the time of image formation is the predetermined first peripheral speed ratio, the control means sets the peripheral speed ratio to the first peripheral speed ratio. The solid density correction is performed by changing to a predetermined second peripheral speed ratio smaller than the speed ratio, and after correcting the solid density, the peripheral speed ratio is changed from the second peripheral speed ratio to the first peripheral speed ratio to perform intermediate density correction. By doing so, the number of changes in the peripheral speed ratio in density correction can be reduced, so that the density correction time can be shortened as much as possible with a simple configuration to improve image formation efficiency, and image defects such as jitter images occur. Can be prevented.

  Also, by reducing the peripheral speed ratio at the time of solid density correction, the output characteristics of the detection means can be stabilized, so that the solid density correction accuracy can be maintained at a high level, and the halftone density correction is performed after the solid density correction. As a result, the halftone correction accuracy can be maintained at a high level.

  Further, according to the second configuration of the present invention, in the image forming apparatus having the first configuration, the control unit varies the application condition of the developing bias applied to the developing device step by step, and the density of a predetermined density. A first reference image composed of a pattern is formed, a development bias level is set based on a density detection result of the first reference image, solid density correction is performed, and the development bias level set in the solid density correction is used in a predetermined order. By forming a second reference image having a gradation pattern and performing halftone density correction based on the density detection result of the second reference image, it is possible to set development bias application conditions more quickly and with high accuracy. .

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the control unit is configured to change the exposure amount of the exposure apparatus in a stepwise manner. By performing halftone density correction using, exposure conditions can be set more quickly and with high accuracy.

  According to the fourth configuration of the present invention, in the image forming apparatus having the first to third configurations, the detection unit irradiates the print medium with the measurement light, and the specular reflection of the reflected light from the print medium. Light and diffusely reflected light can be received and output to the control means, and the control means performs complex control by performing solid density correction based on the output value of one of the regular reflected light and the irregularly reflected light. Therefore, since the solid density correction can be performed with a simple configuration, the density correction time can be further shortened and the cost can be reduced.

FIG. 1 is a schematic diagram showing an overall configuration of a color image forming apparatus of the present invention. These are the schematic side views which show the structure of the detection sensor 21 used for the color image forming apparatus of this invention. FIG. 3 is a block diagram showing a control path of the color image forming apparatus of the present invention. These are graphs showing the relationship between the developing bias level and the sensor output value at the first peripheral speed ratio R1 and the second peripheral speed ratio R2. These are flowcharts for explaining density correction control executed in the image forming apparatus of the present invention. These are figures which show an example of the density correction pattern used by solid density control. These are diagrams showing an example of a density correction pattern used in halftone density control.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a color image forming apparatus of the present invention. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and magenta, cyan, and yellow are respectively subjected to charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums (image carriers) 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and these photosensitive drums 1a. 1d are sequentially transferred onto an intermediate transfer belt (print medium) 8 that moves adjacent to each image forming portion while rotating clockwise in FIG. 1 by a driving means (not shown). After (primary transfer), the image is transferred onto the paper 28 at a time (secondary transfer) by the secondary transfer roller 9, and further fixed on the paper 28 by the fixing unit 7 and then discharged from the apparatus main body. It has become. While the photosensitive drums 1a to 1d are rotated counterclockwise at a peripheral speed D in FIG. 1, an image forming process for the photosensitive drums 1a to 1d is executed.

  The paper 28 onto which the toner image is transferred is accommodated in the paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. Further, a cleaning blade 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure device 4 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the start of image formation is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure device 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. The developing devices 3a to 3d include developing rollers (developer carrying members) 3aa, 3ba, 3ca, and 3da that are arranged to face the photosensitive drums 1a to 1d, and toners of magenta, cyan, yellow, and black, respectively. A predetermined amount is filled by a replenishing device (not shown).

  The toner is carried on the developing rollers 3aa to 3da rotating at the peripheral speed S, and is supplied onto the photosensitive drums 1a to 1d by applying a developing bias to the developing rollers 3aa to 3da, and is electrostatically attached thereto. Thereby, a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 4 is formed. The peripheral speed ratio R (S / D) of the developing rollers 3aa to 3da with respect to the photosensitive drums 1a to 1d is set to a predetermined first peripheral speed ratio R1.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, magenta, cyan, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 8 by the primary transfer rollers 6a to 6d. Primary transferred onto. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 is rotated by a drive motor (not shown). Then, the sheet 28 is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the sheet 28 is conveyed at the nip portion (secondary transfer nip portion) with the intermediate transfer belt 8. A full color image is secondarily transferred onto 28. The paper 28 to which the toner image has been transferred is conveyed to the fixing unit 7.

  The paper 28 conveyed to the fixing unit 7 is heated and pressurized when passing through the nip part (fixing nip part) of the pair of fixing rollers 13 to fix the toner image on the surface of the paper 28, and a predetermined full-color image is formed. It is formed. The paper 28 on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the paper 28, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when images are formed on both sides of the paper 28, a part of the paper 28 that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the paper 28 is distributed to the paper conveyance path 18 by the branching portion 14 by rotating the discharge roller 15 in the reverse direction, and is conveyed again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the paper 28 where the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  A density detection sensor 21 is disposed on the downstream side of the image forming unit Pd and the upstream side of the secondary transfer roller 9. The density detection sensor 21 irradiates the density correction pattern formed on the intermediate transfer belt 8 in the image forming portions Pa to Pd with measurement light, receives the reflected light from the pattern, photoelectrically converts it, and outputs a received light output signal. The output value is A / D converted and then transmitted to the control unit 32 described later as a sensor output value (output value).

  As the density detection sensor 21, an optical sensor including a light emitting element composed of an LED or the like and a light receiving element composed of a photodiode or the like is generally used. When measuring the density of the density correction pattern, when the measurement light is sequentially irradiated from the light emitting element to each patch image on the intermediate transfer belt 8, the measurement light is reflected as light reflected by the toner and light reflected by the belt surface. Incident on the light receiving element.

  When the toner adhesion amount is large, the reflected light from the belt surface is blocked by the toner, so that the light reception amount of the light receiving element is reduced. On the other hand, when the adhesion amount of toner is small, conversely, the amount of reflected light from the belt surface increases, resulting in an increase in the amount of light received by the light receiving element. Accordingly, the toner adhesion amount (toner density) of the density correction pattern of each color is detected from the output value based on the received reflected light amount, and the development bias, exposure characteristic value, etc. are adjusted in comparison with a predetermined target density. Thus, density correction is performed for each color.

  FIG. 2 is a schematic side view showing the configuration of the detection sensor 21 used in the color image forming apparatus of the present invention. As shown in FIG. 2, the density detection sensor 21 includes a light emitting unit 22 that irradiates the intermediate transfer belt 8, a first light receiving unit 23 and a second light receiving unit 24 that receive reflected light from the intermediate transfer belt 8, and a beam. A splitter 25 can also be used.

  The light emitting unit 22 includes a light emitting element 22a composed of an LED or the like disposed at an angle inclined by a predetermined amount with respect to the surface of the intermediate transfer belt 8, and P of the light emitted from the light emitting element 22a parallel to the scanning direction. The first polarizing filter 22b that transmits only the wave component is disposed, and only the P-polarized measuring light is incident on the intermediate transfer belt 8 from the light emitting unit 22.

  A beam splitter 25 is disposed between the intermediate transfer belt 8 and the first and second light receiving portions 23 and 24. The beam splitter 25 is for separating the reflected light reflected from the intermediate transfer belt 8 into reflected light toward the first light receiving unit 23 and reflected light toward the second light receiving unit 24.

  The first light receiving unit 23 includes a first light receiving element 23 a made of a photodiode that receives reflected light from the intermediate transfer belt 8, and a second polarized light disposed between the first light receiving element 23 a and the beam splitter 25. And a filter 23b. The second polarizing filter 23b is for transmitting the P wave component (regular reflection light) parallel to the scanning direction out of the reflected light separated by the beam splitter 25. Thereby, the 1st light receiving element 23a can receive only regular reflection light among reflected light.

  The second light receiving unit 24 includes a second light receiving element 24 a made of a photodiode or the like that receives reflected light from the intermediate transfer belt 8, and a third polarized light disposed between the second light receiving element 24 a and the beam splitter 25. And a filter 24b. The third polarizing filter 24b is for transmitting the S wave component (diffuse reflected light) perpendicular to the scanning direction out of the reflected light separated by the beam splitter 25. Thereby, the 2nd light receiving element 23a can receive only irregular reflection light among reflected light.

  The 1st and 2nd light-receiving parts 23 and 24 each transmit the received regular reflection light and irregular reflection light to the control part 32 (refer FIG. 3) as a sensor output value (output value). Then, at the time of solid density correction, the control unit 30 stores in advance using the sensor output value of one of the first light receiving unit 23 and the second light receiving unit 24 (here, the first light receiving unit 23 is used). The solid density correction is performed in comparison with the target value stored in the unit 32 (see FIG. 3).

  On the other hand, at the time of halftone density correction, the control unit 32 obtains the ratio of the sensor output values of the first light receiving unit 23 and the second light receiving unit 24, and is stored in advance in the storage unit 32 (see FIG. 3). Compared with the table, halftone density correction is performed.

  Since the density detection sensor 21 needs to strictly define the distance to the measurement object, as shown in FIG. 1, the density detection sensor 21 faces the driving roller 11 with a small distance fluctuation to the surface of the intermediate transfer belt 8. The intermediate transfer belt 8 is positioned in the width direction according to the density correction pattern formation position on the intermediate transfer belt 8.

  The density detection sensor 21 may be disposed at another position where the density correction pattern on the intermediate transfer belt 8 can be detected. However, when the density detection sensor 21 is disposed downstream of the secondary transfer roller 9, for example, the image forming unit Pa. The time from when the density correction pattern is formed by .about.Pd to when the density detection is performed becomes longer, and the density correction pattern may come into contact with the secondary transfer roller 9 to change the surface state of the density correction pattern. . Therefore, as shown in FIG. 1, it is preferable to dispose on the downstream side of the image forming unit Pd and the upstream side of the contact position of the secondary transfer roller 9.

  FIG. 3 is a block diagram showing a control path of the color image forming apparatus of the present invention. Portions common to FIGS. 1 and 2 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image forming unit Pa to Pd, an image input unit 30, an AD conversion unit 31, a control unit 32, a storage unit 33, an operation panel 34, a fixing unit 7, an intermediate transfer belt 8, a density detection sensor 21, and the like. It is the composition which includes.

  When the image forming apparatus 100 is a color copying machine, the image input unit 30 includes a scanning lamp mounted with a scanner lamp that illuminates the original during copying and a mirror that changes the optical path of reflected light from the original, and reflection from the original. 1 is an image reading unit that includes a condenser lens that collects light to form an image and a CCD that converts the formed image light into an electrical signal. In the case of a printer, the receiving unit receives image data transmitted from a personal computer or the like. The image signal input from the image input unit 30 is converted into a digital signal by the AD conversion unit 31 and then sent to the image memory 40 in the storage unit 33.

  The storage unit 33 stores print image data input from the image input unit 30 and digitally converted by the AD conversion unit 31 in units of pages, necessary data generated during control of the image forming apparatus 100, and image formation. A readable / writable RAM (Random Access Memory) 41 in which data and the like temporarily required for the control of the apparatus 100 are stored, and the image forming apparatus 100 such as a control program for the image forming apparatus 100 and numerical values necessary for the control. A read-only ROM (Read Only Memory) 42 that stores data that is not changed during use is provided.

  In addition, the RAM 41 includes parameters associated with the first peripheral speed ratio R1, the second peripheral speed ratio R2, the sensor output value of the first peripheral speed ratio R1 and the sensor output value of the second peripheral speed ratio R2, Also stored are a target value (target density) of the sensor output value at the time of density correction, a γ table storing the density input value and the actual density output value (sensor output value) in association with each other.

  The operation panel 34 displays the state of the image forming apparatus 100, the image forming status, and the number of copies, and as a touch panel, functions such as double-sided printing and black-and-white reversal, and various settings such as magnification setting and density setting, the number of printing copies When the image forming apparatus 100 has a FAX function, a numeric keypad for inputting the FAX number of the other party, a start button for instructing the user to start image formation, and when stopping image formation. A stop / clear button, a reset button used when setting various settings of the image forming apparatus 100 to a default state, and the like are provided. The user operates the operation panel 34 to input an instruction, whereby the image forming apparatus 100 is set. Are set to execute various functions such as image formation.

  The control unit 32 is, for example, a central processing unit (CPU), and according to a set program, the image input unit 30, the image forming units Pa to Pd, the fixing unit 7, and the sheet 28 from the sheet cassette 16 (see FIG. 1). In addition to overall control of conveyance and the like, the image signal input from the image input unit 30 is converted into image data by scaling processing or gradation processing as necessary. The exposure device 4 irradiates laser light based on the processed image data, and forms latent images on the photosensitive drums 1a to 1d.

  Further, when the calibration mode is set, the control unit 32 receives the function of setting the peripheral speed ratio R and the sensor output value from the density detection sensor 21, and each patch of the solid density correction pattern (first reference image). A function for comparing the sensor output value of the image with the target value, a function for performing solid density correction for setting the development bias level according to the comparison result, and a halftone density correction pattern (second reference image) at the set development bias level And has a function of performing halftone density correction using the γ table from the sensor output value of each patch image of the gradation pattern.

  Next, density correction control in the color image forming apparatus of the present invention will be described. FIG. 4 is a graph showing the relationship between the developing bias level and the sensor output value at the first peripheral speed ratio R1 and the second peripheral speed ratio R2. As shown in FIG. 4, the peripheral speed ratio R is changed from the first peripheral speed ratio R1 (indicated by □ in the figure) to a predetermined second peripheral speed ratio R2 (indicated by ● in the figure) that is smaller than the first peripheral speed ratio R1. The sensor output value decreases to substantially the same at each development bias level of 200 V, 250 V, 300 V, and 350 V, and the curve obtained with the second circumferential speed ratio R2 is the curve obtained with the first circumferential speed ratio R1. And almost parallel. Here, the first peripheral speed ratio R1 is 1.4, and the second peripheral speed ratio R2 is 1.1.

  Thus, even if the first peripheral speed ratio R1 is lowered to the second peripheral speed ratio R2, the relative relationship between the developing bias level and the sensor output value hardly changes. Further, in a solid image (high density portion) with a large amount of toner adhering to the photosensitive drums 1a to 1d, the change in the amount of specular reflection light with respect to the change in the toner amount becomes small. Is difficult to stabilize, and the solid density correction accuracy may be reduced.

  Therefore, the peripheral speed ratio R is decreased from the first peripheral speed ratio R1 to the second peripheral speed ratio R2, the amount of toner adhering to the photosensitive drums 1a to 1d is reduced, and solid density correction is performed. The detection accuracy of specular reflection light was improved. Furthermore, if the number of changes in the peripheral speed ratio R increases, there is a risk that an image defect such as a jitter image may occur. Therefore, the peripheral speed ratio R is changed only during the solid density correction, and after the solid density correction is performed. In the halftone density correction, the first peripheral speed ratio R1 is returned to the same as in the image formation.

  Note that when the peripheral speed D of the photosensitive drums 1a to 1d is increased, a load on a drive motor (not shown) increases. Therefore, the change from the first peripheral speed ratio R1 to the second peripheral speed ratio R2 is changed to the developing roller 3aa. It is preferable to reduce the peripheral speed S of ˜3 da. Accordingly, here, the first peripheral speed ratio R1 is changed to the second peripheral speed ratio R2 by keeping the peripheral speed D of the photosensitive drums 1a to 1d constant and decreasing the peripheral speed S of the developing rollers 3aa to 3da. It was decided to. Further, the relationship between the developing bias level and the sensor output value at the first peripheral speed ratio R1 and the second peripheral speed ratio R2 can be obtained by a preliminary experiment or the like.

  Next, density correction control in the color image forming apparatus of the present invention will be described. FIG. 5 is a flowchart showing density correction control executed by the image forming apparatus of the present invention. The density correction control procedure will be described along the steps of FIG. 5 with reference to FIGS.

  First, when printing is started by a user operation (step S1), the control unit 32 sets the circumferential speed ratio R of the developing rollers 3aa to 3da to the photosensitive drums 1a to 1d to the first circumferential speed ratio R1 during image formation. To the second peripheral speed ratio R2 (step S2), and the light receiving unit 23 of the detection sensor 21 starts detection (step S3).

  Next, solid density correction is executed, and the control unit 32 instructs formation of a solid density correction pattern (step S4). An example of a density correction pattern used for solid density correction is shown in FIG. In the present invention, with respect to each of the colors magenta, cyan, yellow and black, the density bias pattern of a predetermined density (here, 100%) is applied to the developing rollers 3aa to 3da (see FIG. 1) and the developing bias application conditions are shown in the above diagram. As shown in FIG. 4, a plurality of sets are formed by changing stepwise from 200V to 350V every 50V.

  A developing bias in which an AC bias Vac is superimposed on a DC bias Vdc is applied to the developing rollers 3aa to 3da. In the present embodiment, the DC bias value is changed to a total of four stages of 200V, 250V, 300V, and 350V. Although the DC bias value is changed stepwise here, at least one of the AC bias peak-to-peak value, frequency, and duty ratio is changed stepwise instead of or together with the DC bias value. A development bias level may be set.

  FIG. 6 shows a solid density correction pattern for each color at a predetermined development bias level (for example, 200 V), and a total of four 100% patterns are formed for the four colors magenta, cyan, yellow, and black. Note that the solid density correction patterns similar to those shown in FIG. 6 are formed when the developing bias level is 250 V, 300 V, and 350 V, so that a total of 4 × 4 = 16 patterns are formed on the intermediate transfer belt 8. become. The density setting of the solid density correction pattern is not limited to 100%, and is not particularly limited as long as it is detected by the first light receiving unit 23 and can be corrected by the control unit 32.

  Next, the density of the solid density correction pattern formed in step S4 is detected by the first light receiving unit 23 of the density detection sensor 21, and a development bias level is set for each color so that the sensor output value becomes the target value (step). S5). Here, it is assumed that the developing bias level of each color is set to 300V.

  After performing the solid density correction in this way, the control unit 32 performs setting to return the circumferential speed ratio R from the second circumferential speed ratio R2 to the first circumferential speed ratio R1 (step S6), and the detection sensor 21 performs the first adjustment. The first light receiving unit 23 and the second light receiving unit 24 start detection (step S7). Further, the control unit 32 instructs the formation of a halftone density pattern by the development bias level 300 V set in step S6, and performs halftone density correction (step S8).

  An example of a density correction pattern used for halftone density correction is shown in FIG. In the present invention, for each color of magenta, cyan, yellow, and black, a gradation pattern of density steps (here, 5 steps of 20%, 40%, 60%, 80%, and 100%) in a predetermined order is applied to the exposure apparatus 4. The exposure amount (laser output level) is changed stepwise to form a plurality of sets. Here, a total of 4 × 5 = 20 patterns are formed on the intermediate transfer belt 8.

  Then, the density of the halftone density pattern is detected by the first light receiving unit 23 and the second light receiving unit 24, and halftone density correction is performed using the γ table stored in the RAM 41 based on the density detection result. Thereafter, it is determined whether or not printing has been completed (step S9). If printing continues, the process returns to step S2, and the same procedure (steps S3 to S8) is repeated thereafter.

  By performing density correction according to the above procedure, the peripheral speed ratio R is changed from the first peripheral speed ratio R1 to the second peripheral speed ratio R2 only at the time of solid density correction. It is possible to shorten the time as much as possible to improve the image forming efficiency and to prevent the occurrence of image defects such as jitter images.

  Further, by reducing the peripheral speed ratio R during the solid density correction, the output characteristics of the detection sensor 21 can be stabilized, so that the solid density correction accuracy can be maintained at a high level. Further, by performing halftone density correction after solid density correction, halftone density stability and gradation can be ensured, so halftone density correction can also be maintained at a high level.

  In the present embodiment, the application conditions of the development bias applied to the developing devices 3a to 3d are changed stepwise to form a solid density correction pattern, and the development bias level is set based on the density detection result of the solid density correction pattern. A solid density correction was performed by setting, and a halftone density correction pattern composed of gradation patterns in a predetermined order was formed using the set development bias level, and a halftone density correction was performed.

  As a result, the developing bias application condition can be set more quickly and with high accuracy, and the halftone density pattern can be formed and the halftone density can be corrected more quickly and with high accuracy. In the present embodiment, since the halftone density correction is performed using the halftone density correction pattern formed by changing the exposure amount of the exposure apparatus 4 stepwise, the exposure condition is set more quickly and with high accuracy. be able to.

  Further, in the present embodiment, the first light receiving unit 23 and the second light receiving unit 24 are provided in the detection unit 21 and the solid density correction is performed based on the detection result of the first light receiving unit 23, so that complicated control is not performed. Solid density correction can be performed with a simple configuration, and the density correction time can be further shortened and the cost can be further reduced.

  The calibration mode is set when the apparatus is turned on or by a user operation, or when the cumulative number of printed sheets from the previous density correction reaches a predetermined number. You can also.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, the target density of the solid density pattern can be the same for each color, or can be set individually for each color. Further, here, the density stage of the development Bayus level for forming the solid image density pattern is set to 4 stages, and the density stage of the gradation pattern for forming the halftone density pattern is set to 5 stages. If there are, it is good also as three steps or less, and good also as five steps or more.

  It is also possible to make the number of density steps of the development bay level equal to the number of density steps of the gradation pattern. As the development bias level and the density pattern of the gradation pattern become multistage, the relationship with the corresponding image density can be accurately grasped, but the number of patterns to be formed increases and the time required for density correction also increases. Therefore, about 4 to 5 steps are preferable as in the above embodiment.

  In the above embodiment, the first peripheral speed ratio R1 is 1.4 and the second peripheral speed ratio R2 is 1.1. However, the first and second peripheral speed ratios R1 and R2 are particularly limited to these. It is not something. The first peripheral speed ratio R1 can be appropriately set according to image forming conditions and the like, and can be set to about 1.2 to 2, for example.

  Further, if the second peripheral speed ratio R2 is increased, the toner adhesion amount on the photosensitive drums 1a to 1d may not be sufficiently reduced, and the detection accuracy may not be sufficiently improved. If the second peripheral speed ratio R2 is decreased, the toner adhesion amount is too small. May become difficult. Therefore, for example, the second peripheral speed ratio R2 can be appropriately set in consideration of such a viewpoint. For example, the second peripheral speed ratio R2 is set to about 60 to 80% with respect to the first peripheral speed ratio R1. It is preferable.

  In the present embodiment, the density correction pattern formed on the intermediate transfer belt 8 is detected. However, in the case of using a direct transfer type tandem color image forming apparatus, a conveyance belt for conveying paper is used as a print medium. The density correction pattern formed on the transport belt can also be detected.

  The present invention is not limited to a tandem color image forming apparatus that transfers a full color image formed by sequentially laminating toner images of each color onto the intermediate transfer belt 8 as shown in FIG. Of course, the present invention can be applied to various image forming apparatuses such as rotary color image forming apparatuses, monochrome copying machines, monochrome printers, and facsimiles.

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
2a to 2d charger 3a to 3d developing device 3aa to 3da developing roller 4 exposure device 6a to 6d primary transfer roller 7 fixing unit 8 intermediate transfer belt (printing medium)
9 Secondary transfer roller 21 Concentration detection sensor (detection means)
22 light emitting unit 23 first light receiving unit 24 second light receiving unit 32 control unit (control means)
33 Storage Unit 34 Operation Panel 41 RAM
42 ROM
100 Image forming apparatus

Claims (4)

Image formation comprising an image carrier, an exposure device that forms an electrostatic latent image on the image carrier, and a development device that has a developer carrier and develops the electrostatic latent image formed by the exposure device And
Detecting means for detecting a density of a reference image formed on the print medium by the image forming unit;
Control means for performing solid density correction and halftone density correction based on the detection result of the detection means;
In an image forming apparatus comprising:
The control means sets the peripheral speed ratio of the developer carrier to the image carrier at the time of image formation to a first peripheral speed ratio;
When the solid density correction, the control means switches the peripheral speed ratio to a second peripheral speed ratio smaller than the first peripheral speed ratio to form a plurality of first reference images while changing the developing bias, An image forming apparatus that detects these densities, determines a developing bias from which the target density is obtained based on the detected densities, and returns the peripheral speed ratio to the first peripheral speed ratio to perform halftone density correction. . Wherein, after the solid density correction, and characterized in that the half-tone density correction based on a density detection result of the second reference image to form a second reference image comprising a gradation pattern having a predetermined order The image forming apparatus according to claim 1.   The image forming apparatus according to claim 2, wherein the control unit performs the halftone density correction using the second reference image formed by changing the exposure amount of the exposure apparatus in a stepwise manner. . The detection means can irradiate the print medium with measurement light, receive regular reflection light and irregular reflection light among the reflected light from the print medium, and output to the control means.
The image forming apparatus according to claim 1, wherein the control unit performs the solid density correction based on an output value of one of the regular reflection light and the irregular reflection light. .