JP2009048137A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of reducing the density unevenness of an image, with simple control, by coping with various generation causes thereof. <P>SOLUTION: The image forming apparatus forms a test image, detects densities in a plurality of portions of the test image, by a density detector 68, calculates a density difference between the maximum density and the minimum density out of the plurality of detected densities, judges whether the calculated density difference is a preset prescribed value or less, and controls a developing toner amount supplied from a developing device 18 to an electrostatic latent image carrier 4, in order to bring the density difference into the prescribed value or less, when the density difference is determined to be higher than the prescribed value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as an electrophotographic system, an electrostatic recording system, an ionography, or a magnetic recording system, and in particular, an image of a copying machine, a printer, a facsimile, or a multifunction machine having these functions combined. The present invention relates to a forming apparatus.

  An image created using an image forming apparatus may have uneven density. The main causes of density unevenness may be vibration associated with driving of the electrostatic latent image carrier, the intermediate transfer member, or the developing device, and uneven charging of the electrostatic latent image carrier.

  For example, Patent Documents 1 to 4 propose techniques for preventing this density unevenness.

  In the technique of Patent Document 1, a member having viscoelasticity is interposed between an end surface of a photosensitive drum and a housing of a cartridge surrounding the photosensitive drum.

  The technique of Patent Document 2 measures and stores a change (uneven driving) in the rotation speed of the photosensitive drum, and controls the exposure timing in accordance with the uneven driving of the photosensitive drum.

  The technique of Patent Document 3 is a technique for correcting the potential unevenness on the surface of the photoconductor due to the manufacturing variation of the photoconductor. Specifically, the potential nonuniformity on the surface of the photoconductor is measured and the potential unevenness is canceled out. The charging bias applied to the charging member is controlled.

The technique of Patent Document 4 detects density of a test image formed on a photoreceptor, an intermediate transfer body, or a recording sheet at a plurality of locations, and performs Fourier transform on the detected value to perform frequency analysis, thereby generating density unevenness. The cause is specified, and control is performed according to the specified cause of occurrence.
JP-A-8-30058 JP-A-9-81006 JP 2002-207350 A JP 2006-106556 A

  However, the technique of Patent Document 1 can prevent density unevenness due to vibration of the photosensitive drum drive system, but density unevenness caused by vibration of other drive systems (for example, the drive system of the developing device). In addition, uneven density due to uneven charging of the photoreceptor cannot be eliminated.

  Further, although the technique of Patent Document 2 can prevent uneven image density due to uneven driving of the photosensitive drum, it cannot prevent uneven density due to other causes as in the technique of Patent Document 1.

  Furthermore, the technique of Patent Document 3 can prevent uneven image density due to uneven charging of the photoconductor, but cannot eliminate uneven density due to vibration of the drive system.

  Furthermore, in the technique of Patent Document 4, it may be possible to eliminate the density unevenness of the image corresponding to various causes. However, in order to identify the cause of the density unevenness, it is necessary to perform frequency analysis. It requires complicated control. In addition, even if a plurality of causes are identified, the image creation process is changed according to the causes, and extremely complicated control is required.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of suppressing unevenness in image density corresponding to various causes of occurrence by simple control.

In order to solve the above problems, an image forming apparatus according to the present invention provides:
An electrostatic latent image carrier carrying an electrostatic latent image;
A developing device for forming a toner image by attaching toner to the electrostatic latent image;
A transferred member to which the toner image formed on the electrostatic latent image carrier is transferred directly or via another member;
A density detection device for detecting the density of one of the toner image on the electrostatic latent image carrier or the toner image on the transfer member;
An image forming apparatus including a control unit that performs various types of control relating to an image forming operation,
The control unit
A test image forming unit for forming a test image;
A density difference calculating unit that detects a density at the plurality of locations of the test image by the density detection device and calculates a density difference between the maximum density and the minimum density among the detected density;
A determination unit for determining whether the concentration difference calculated by the concentration calculation unit is equal to or less than a predetermined value set in advance;
When the determination unit determines that the density difference is greater than the predetermined value, the developing toner amount supplied from the developing device to the electrostatic latent image carrier is less than the predetermined value. And a developing toner amount control unit for controlling in this manner.

  According to the present invention, the density of the test image is detected at a plurality of locations, and it is determined that the density difference between the maximum density and the minimum density among the detected plurality of densities is greater than a predetermined value set in advance. In this case, the developing toner amount is controlled so that the density difference becomes a predetermined value or less. Therefore, by such simple control, it is possible to suppress uneven density of the image corresponding to various causes of occurrence.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings. In the following description, terms indicating a specific direction (for example, “up”, “down”, “left”, “right”, and other terms including them, “clockwise direction”, “counterclockwise” ”) Is used to facilitate understanding of the invention with reference to the drawings, and the present invention should not be construed as being limited by the meaning of these terms. In the image forming apparatus described below, the same reference numerals are used for the same or similar components.

  FIG. 1 shows a schematic configuration of an image forming apparatus 100 according to an embodiment of the present invention. The image forming apparatus 100 is an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multifunction machine having a combination of these functions. Currently, various types of electrophotographic image forming apparatuses have been proposed. The illustrated image forming apparatus is a so-called tandem color image forming apparatus. However, the present invention is not applied only to this type of image forming apparatus. For example, a so-called four-cycle color image forming apparatus or a toner image on an electrostatic latent image carrier is directly applied to a recording sheet. The present invention is equally applicable to a direct transfer type color image forming apparatus for transferring. Furthermore, the present invention can be applied to a monochrome image forming apparatus having only one developing device. Furthermore, the present invention is not limited to an electrophotographic image forming apparatus, but can be applied to various types of image forming apparatuses such as an electrostatic recording system, ionography, or magnetic recording system.

  The image forming apparatus 100 includes an image reading unit 102 for reading a document image, and a printing unit 104 that prints an image read by the image reading unit 102 on a medium such as paper.

  The image reading unit 102 includes a known document reading mechanism that optically reads an image of a document placed on a document table (platen). The image reading unit 102 reads red (R), green (G), blue ( In addition to color separation into three colors B), image data of red (R), green (G), and blue (B) can be created. The image reading unit 102 is communicably connected to a control unit 70 which will be described later, and the image reading unit 102 transmits red (R), green (G), and blue (B) image data to the control unit 70 for control. The unit 70 converts the image data into yellow (Y), magenta (M), cyan (C), and black (K) image data.

  The print unit 104 includes an endless intermediate transfer belt 30 (a member to be transferred). The intermediate transfer belt 30 is wound around a plurality of rollers 32, 33, and 34. The roller 32 on the right side of the drawing is drivingly connected to a motor (not shown), and the rotation transmitted from the motor to the roller 32 is transmitted to the rollers 33 and 34 via the belt 30. Four image forming units 3 (3Y, 3M, 3C, 3K) are arranged in the order from the left side to the right side in the drawing in the vicinity of the belt portion extending from the roller 34 at the lower left side of the drawing to the roller 32 on the right side of the drawing. Has been placed.

  As shown in FIG. 2, each image forming unit 3 includes a cylindrical photosensitive member 4 as an electrostatic latent image carrier. Around the photoreceptor 4, in order along the rotation direction (clockwise direction in the drawing), a charging device 8 that charges the outer peripheral surface of the photoreceptor 4, and an image is projected onto the photoreceptor 4 to electrostatically charge each color. An exposure device 10 that forms a latent image, a developing device 18 that visualizes the electrostatic latent image on the photosensitive member 4 with a developer, and a developer image that is formed on the photosensitive member 4 inside the intermediate transfer belt 30 A primary transfer roller 14 for transferring the toner to the intermediate transfer belt 30 and a cleaning member 16 for collecting the developer transferred to the intermediate transfer belt 30 and remaining on the photoreceptor 4 in a misalignment are disposed.

  The developing device 18 includes a housing 19 that accommodates toner as a developer. In the developing device housings 19 of the four image forming units 3 (3Y, 3M, 3C, 3K), different color developers, that is, yellow (Y), magenta (M), cyan (C), black (K), respectively. ) Toner is contained. The developing device 18 includes a developing roller 20 that is disposed opposite to the outer peripheral surface of the photoreceptor 4. A nip portion between the developing roller 20 and the photosensitive member 4 forms a developing region 21 for supplying toner carried on the developing roller 20 to the photosensitive member 4. A power supply 22 is connected to the developing roller 20 via a control circuit 24, and a developing bias is applied from the power supply 22 to the developing roller 20 via the control circuit 24, whereby the developing roller 20 is directed toward the photoreceptor 4. Thus, an electric field for electrically energizing the toner is formed between the developing roller 20 and the photosensitive member 4.

  The primary transfer roller 14 is disposed inside the endless intermediate transfer belt 30, can be brought into and out of contact with the photoreceptor 4 via the intermediate transfer belt 30, and is in contact with the belt 30 while being in pressure contact with the intermediate transfer belt 30. A primary transfer region 15 for transferring (primary transfer) the toner image on the photoconductor 30 to the intermediate transfer belt 30 is formed at a contact portion (nip portion) with the photoconductor 4. In order to promote this transfer, the primary transfer roller 14 is connected to a power source (primary transfer bias applying device) (not shown).

  Returning to FIG. 1, a secondary transfer roller 40 that presses the sheet of the recording medium against the belt 30 is provided outside the belt portion supported by the driving roller 32 arranged on the right side of the drawing. The secondary transfer roller 40 is provided so as to be able to come into contact with and separate from the outer peripheral surface of the belt 30, and transfers the four-color toner images superimposed on the belt 30 to the recording sheet by primary transfer while in contact with the outer peripheral surface of the belt. A secondary transfer region 41 for (secondary transfer) is formed. In order to promote this transfer, the secondary transfer roller 40 is connected to a power source (secondary transfer bias applying device) (not shown).

  An intermediate transfer belt cleaning member 64 that collects untransferred toner remaining on the intermediate transfer belt 30 after the secondary transfer is provided outside the belt portion supported by the roller 34 disposed in the lower left of the drawing. ing. The cleaning member 64 is disposed in contact with the intermediate transfer belt 30, and a contact portion (nip portion) between the cleaning member 64 and the belt 30 forms a cleaning region 65.

  In the vicinity of the upper side of the belt portion moving from the right roller 32 to the upper left roller 33 in the drawing, an image density detection sensor (density detection means) is used as a density detection device for detecting the density of the toner image formed on the belt 30. 68 is provided. Although not shown, the image density detection sensor 68 includes, for example, a light emitting unit that illuminates a toner image formed on the outer peripheral surface of the belt 30 and a light receiving unit that receives light reflected by the toner image. The density of the toner image is detected according to the amount. However, the density detection device according to the present invention is not limited to the optical image density detection sensor described above.

  The image forming apparatus 100 includes a control unit 70 that comprehensively controls the various apparatuses described above in relation to the image forming operation. A specific configuration of the control unit 70 related to the present invention will be described later.

  A paper feeding cassette 44 of the paper feeding device is detachably provided below the printing unit 104. A paper feed roller 52 is provided in the vicinity of the paper feed cassette 44. A recording sheet conveyance path 50 is provided above the paper feed roller 52. The conveyance path 50 extends upward via a transfer region (secondary transfer region) 41 and reaches a paper discharge unit 61 provided between the document reading unit 102 and the print unit 104. In the middle of the conveyance path 50, a timing roller 54 is disposed between the paper feed roller 52 and the transfer area 41, and a fixing device 56 is provided between the transfer area 41 and the paper discharge unit 61.

  The operation of the image forming apparatus 100 having the above configuration will be described. First, image data created by the image forming apparatus 100 is acquired by the image reading unit 102 and transmitted to the control unit 70. When the image forming apparatus 100 functions as a printer connected to a computer (not shown), image data output from the computer is transmitted to the control unit 70. The control unit 70 that has acquired the image data drives a necessary apparatus to create a target image on the recording medium.

  Specifically, when image formation starts, the roller 32 rotates counterclockwise based on the drive of the drive motor. The rotation of the roller 32 is transmitted to the intermediate transfer belt 30, and the intermediate transfer belt 30 and the rollers 33 and 34 rotate counterclockwise. When creating a full-color image, the photoreceptor 4 of each image forming unit 3 rotates in the clockwise direction. Further, the outer peripheral surface of each photoconductor 4 is charged to a predetermined potential by the charging device 8. The outer peripheral surface of the charged photoconductor 4 is exposed by light emitted from an exposure device 10 driven based on a command from the control unit 70, and an electrostatic latent image corresponding to image data to be reproduced is formed. . When the electrostatic latent image passes through the developing area 21, toner of a corresponding color is supplied from the developing device 18, and a full color toner image corresponding to the image data is formed. Each toner image is transferred and superimposed on the outer peripheral surface of the intermediate transfer belt 30 passing therethrough in the transfer region 15 to form a target full-color toner image. The toner that has passed through the transfer region 15 without being transferred to the intermediate transfer belt 30 is collected by the cleaning device 16.

  The recording sheet 36 to which the full color toner image is transferred is sent to the conveyance path 50 one by one based on the rotation of the paper feed roller 52, and is sent to the transfer area 41 with the timing adjusted by the timing roller 54. In 41, the toner image is transferred from the intermediate transfer belt 30. The recording sheet 36 to which the toner image has been transferred is conveyed to a fixing device 56 where the toner image is fixed on the recording sheet 36 and then discharged to a paper discharge unit 61. The toner that has passed through the transfer region 41 without being transferred to the recording sheet 36 is collected by the cleaning device 64.

[First Embodiment]
FIG. 3 is a block diagram of the first embodiment of the image forming apparatus 100. As shown in FIG. 3, the image forming apparatus 100 includes an operation unit 80, a communication unit 82, and a storage unit 84 in addition to the above-described components. The operation unit 80 includes, for example, a switch (not shown) and a liquid crystal (LCD) display. The switch of the operation unit 80 includes a switch for selecting a print mode (monochrome, full color, single-sided, double-sided, etc.), a switch for inputting a recording sheet type, and the like, and a switch for switching between a normal image forming mode and a test image forming mode. Etc. The display unit of the operation unit 80 displays an image forming mode, internal information of the image forming apparatus, and the like. The communication unit 82 can communicate with a host computer via a bidirectional communication medium. The communication unit 82 receives various setting information or image data transmitted from the host computer, and transmits the internal state of the apparatus 2 or a determination result by the determination unit 76 described later to the host computer. The storage unit 84 is composed of, for example, a non-volatile memory such as an HDD, and stores various setting information, image data and the like input by the operation unit 80 or the host computer. The print unit 104, the control unit 70, the operation unit 80, the communication unit 82, and the storage unit 84 are connected via a system bus 86.

  The control unit 70 includes, for example, a CPU, a ROM, and a RAM (not shown), and comprehensively controls each unit connected to the system bus 86 to perform various controls relating to the image forming operation. For example, the control unit 70 captures various setting information input by the operation unit 80 or the host computer, performs data check, converts the information into a predetermined form, and saves it in the storage unit 84. The control unit 70 forms a test image (specifically, for example, an image pattern with an area ratio of 100%) formed of an image that should have a constant density (an image that has a constant density unless density unevenness occurs). 72 and a plurality of locations of the test image, the density is detected by the image density detection sensor 68, and the density difference between the maximum density IDdmax and the minimum density IDdmin (hereinafter referred to as “maximum density difference DGmax”). And a determination unit 76 for determining whether or not the maximum density difference DGmax calculated by the density difference calculation unit 74 is equal to or less than a preset target density difference DGtrg. When the determination unit 76 determines that the maximum density difference DGtrg is larger than the target density difference DGtrg, the current density supplied from the developing device 18 to the photosensitive member 4 is determined. The amount of toner, including the developing toner amount control unit 78 the maximum density difference DGmax is controlled to be equal to or less than the target density difference DGtrg.

  In the present embodiment, image stabilization control is performed in the test image formation mode. Specifically, in the image stabilization control, as shown in FIG. 4, image density detection sensor light emission amount adjustment control (step S1), toner adhesion amount adjustment control (step S2), density difference suppression control (step S3), line The width adjustment control (step S4) and the gradation correction control (step S5) are sequentially performed.

  In the image density detection sensor light emission amount adjustment control in step S1, the light emission amount from the light emitting unit of the image density detection sensor is adjusted so that the detection value of the image density detection sensor becomes a preset value.

  In the toner adhesion amount adjustment control in step S2, each color (yellow, magenta, cyan, black) is set so that the toner adhesion amount when forming an image with a 100% area ratio (solid image) becomes a preset amount. Development bias is adjusted. Specifically, by varying the development bias for each color, a plurality of levels of toner adhesion amount patterns (area ratio 100%) are formed on the belt 30, and the toner adhesion amount of each level of toner adhesion amount pattern is It is detected by an image density detection sensor. By this detection, a relationship between the development bias and the toner adhesion amount (see FIG. 11) is obtained, and based on the obtained relationship, a development bias capable of obtaining a preset toner adhesion amount is calculated, and the calculated value is calculated. Is set as the developing bias.

  The density difference suppression control in step S3 will be described in detail later.

  In the line width adjustment control in step S4, the exposure amount is adjusted so that the line width of each color becomes a preset width. Specifically, by varying the exposure amount for each color, a plurality of levels of one dot line are formed on the belt 30 with an area ratio of 25%, and the line width of each level of one dot line is the image density detection sensor. Is detected. By this detection, a relationship between the exposure amount and the line width is obtained, and based on the obtained relationship, an exposure amount that can obtain a preset line width is calculated, and the calculated value is set as the exposure amount. .

  In the gradation correction control in step S5, gradation correction of each color (correction of the one-dimensional lookup table) is performed. Specifically, for each color, a gradation pattern of 256 levels is formed on the belt 30, the density of the gradation pattern of each stage is detected by an image density detection sensor, and gradation correction is performed based on these detection values. Is called.

  The flow of each process of density difference suppression control (step S3 in FIG. 4) will be described with reference to FIG.

  First, the process of step S <b> 11 is performed by the test image forming unit 72. In step S <b> 11, a test image for one round of the belt is formed on the belt 30 for each color of yellow, magenta, cyan, and black. As the test image, for example, an image with an area ratio of 100% is formed under the condition of the developing bias set by the toner adhesion amount adjustment control (step S2 in FIG. 4). However, the test image may be formed under a developing bias condition other than the developing bias set by the toner adhesion amount adjustment control. Further, the test image does not necessarily have to be formed for one round of the belt, and may be formed only on a portion on the belt 30 where the image is formed.

  The processing of step S12 and step S13 is performed by the density difference calculation unit 74.

  In step S12, for each color, the density of the test image on the belt 30 is continuously detected from one end of the test image to the other end in the circumferential direction of the belt by the image density detection sensor. However, if the density of the test image is detected at a plurality of locations within the range of the test image, it is not always necessary to detect the density continuously from one end to the other end of the test image. You may make it detect intermittently at intervals in a direction. When the density is detected by the image density detection sensor, specifically, it is calculated based on the output (output voltage) of the image density detection sensor and the relationship between the preset output and density as shown in FIG. The detected density becomes the detected density. A specific example of the detection result of the test image density is shown in FIG.

  In step S13, a density difference (maximum density difference DGmax) between the maximum density IDdmax and the minimum density IDdmin among the densities detected in step S12 is calculated.

  In step S14, the determination unit 76 determines whether or not the maximum density difference DGmax calculated in step S13 is equal to or smaller than a preset target density difference DGtrg. If it is determined in step S14 that the maximum density difference DGmax is less than or equal to the target density difference DGtrg, the density difference suppression control is terminated, and if it is determined that the maximum density difference DGmax is greater than the target density difference DGtrg, step S15 is performed. Proceed to

  The processing in steps S15 and S16 is performed by the developing toner amount control unit 78.

  In step S15, a development bias increase / decrease value corresponding to the detected density is calculated for each image portion. Specifically, as shown in FIG. 7, the outer peripheral surface of the belt 30 is divided into, for example, 19 equal areas arranged in order in the circumferential direction of the belt 30, and image portions corresponding to these areas of the belt 30. Every time, the increase / decrease value of the development bias is calculated. More specifically, in the case of the detection density shown in FIG. 7, an image portion where the average value of the detection density is lower than the target density IDtrg set in advance corresponding to the test image, that is, the region 3 shown in FIG. 7. 7 and the image portions corresponding to the belt regions of the regions 11 to 16, the amount of the developing toner is increased, and the image portion having a high average value of the detected density, that is, the regions 1, 2, 6 to 10, For the image portions corresponding to the belt regions 17 to 19, the developing toner amount is decreased. The increase / decrease value of the development bias is a relationship between the difference between the detected density IDd and the target density IDtrg (IDd−IDtrg) set in advance for each image portion corresponding to each belt region and the increase amount of the development bias (for example, FIG. It is calculated based on 8).

  In step S16, the development bias is calculated for each portion of the image based on the increase / decrease value calculated in step S15, and after the calculated value is set as the development bias, the density difference suppression control ends. By setting the developing bias in this manner, the amount of developing toner supplied from the developing device 18 to the photosensitive member 4 is controlled to an amount that makes the maximum density difference DGmax equal to or less than the target density difference DGtrg. Uneven density can be suppressed.

[Second Embodiment]
FIG. 9 is a block diagram of the second embodiment of the image forming apparatus 100. As shown in FIG. 9, in the second embodiment, the control unit 70 includes a test image forming unit 72, a density difference calculation unit 74, a determination unit 76, and a developing toner amount control unit 78 similar to those in the first embodiment. In addition, a determination unit 79 is provided.

  The determination unit 79 sets the entire image so that the image portion corresponding to the belt portion for which the minimum density IDdmin is detected by the developing toner amount control unit 78 becomes the target density IDtrg set in advance corresponding to the test image. When it is assumed that the development toner amount is controlled to increase uniformly over the range, the density of the image portion where the density is maximum (hereinafter referred to as “maximum density IDmax”) is the image on the recording sheet 36. It is configured to determine whether or not the density is lower than an upper limit value (hereinafter referred to as “limit density IDlmt”) that can be properly fixed.

  With reference to FIG. 10, the flow of each process of density difference suppression control (step S3 in FIG. 4) according to the second embodiment will be described.

  First, steps S21 to S24 are performed in the same manner as steps S11 to S14 of the first embodiment. In step S24, when it is determined that the maximum density difference DGmax of the test image on the belt 30 is equal to or less than the preset target density difference DGtrg, the density difference suppression control ends, and the maximum density difference DGmax is the target density difference. If it is determined that it is greater than DGtrg, the process proceeds to step S25.

  Unlike the first embodiment, the processes in steps S25 to S29 are based on the premise that the entire image is formed under a common development bias condition, and is a process for setting the common development bias.

  The processing in step S25 is performed by the developing toner amount control unit 78.

  In step S25, a developing bias Vtrg that increases the density of the image portion corresponding to the belt area (for example, the belt area 14 in FIG. 7) in which the minimum density IDdmin is detected in step S22 to the target density IDtrg is calculated. . Specifically, the density IDdmin having the minimum detected density IDd is set based on a preset relationship (IDd−IDtrg) between the detected density IDd and the target density IDtrg (for example, FIG. 8). After the development bias increase amount is calculated, the increase amount is added to the development bias Vo set in the toner adhesion amount adjustment control (step S2 in FIG. 4), so that the development bias Vtrg is Calculated.

  The processes in steps S26 and S27 are performed by the determination unit 79.

  In step S26, when it is assumed that a solid image (test image) is formed with the development bias Vtrg set for the entire image, the density (maximum density IDmax) of the portion having the maximum density in the entire solid image to be formed. Is calculated. Specifically, the maximum density IDmax is calculated on the assumption that the image part whose density is the maximum density IDmax is the same as the image part in which the maximum density IDdmax is detected in step S22. More specifically, for the image portion where the maximum density IDdmax is detected, the development bias is set based on the relationship between the development bias and the density (see FIG. 11) obtained in the toner adhesion amount adjustment control (step S2 in FIG. 4). The density when Vo is increased from Vtrg is calculated, and the calculated value is set as the maximum density IDmax.

  In step S27, it is determined whether or not the maximum density IDmax calculated in step S26 is equal to or less than a preset limit density IDlmt. If it is determined in step S27 that the maximum density IDmax is less than or equal to the limit density IDlmt, the process proceeds to step S28. If it is determined that the maximum density IDmax is greater than the limit density IDlmt, the process proceeds to step S29.

  The processing of step S28 and step S29 is performed by the developing toner amount control unit 78.

  In step S28, after the developing bias is set to Vtrg, the density difference suppression control ends. By setting the developing bias to Vtrg, the target density IDtrg can be achieved even for the image portion where the minimum density IDdmin is detected in step S22. On the other hand, since the image portion in which the maximum density IDdmax is detected in step S22 can be suppressed to a density equal to or lower than the limit density IDlmt, the image can be appropriately fixed on the recording sheet 36.

  In step S29, the developing bias is set to a bias Vlmt corresponding to the limit density IDlmt. Specifically, the development bias increase amount is calculated such that the density of the image portion where the maximum density IDdmax is detected in step S22 becomes the limit density IDlmt, and the calculated increase amount is added to the development bias Vo. As a result, the development bias Vlmt is calculated. The development bias increase amount is calculated based on a relationship (not shown) between a preset difference between the detected density IDd and the limit density IDlmt (IDd−IDlmt) and the development bias increase amount. By setting the developing bias to Vlmt, the density of the image can be made not more than the limit density IDlmt over the entire image, and the image can be properly fixed on the recording sheet 36. Further, since the density can be increased to the vicinity of the target density IDtrg for the image portion in which the minimum density IDdmin is detected in step S22, the density unevenness can be suppressed.

  As described above, the density difference suppression control according to the second embodiment has a configuration in which the development bias is uniformly increased over the entire image. Therefore, the control is simpler than that of the first embodiment. Uneven density can be suppressed. On the other hand, in the control of the second embodiment, not only the image portion having a low density but also the image portion having a high density compared to the target density IDtrg at the time of density detection (step S22 in FIG. 10) Since the toner adhesion amount is increased uniformly, the density of the image portion that was high at the time of density detection further increases. However, as shown in FIG. 12, since the density change when the toner adhesion amount changes is smaller as the density is higher, the density change due to the density difference suppression control is changed in the image portion that was higher in density detection. It is small and does not prevent suppression of uneven density.

  While the present invention has been described with reference to the above-described embodiments, the present invention is not limited to the above-described embodiments.

  For example, in the above-described embodiment, the configuration in which the density of the test image on the intermediate transfer belt 30 is detected by the density detection device (image density detection sensor 68) has been described. The density of the test image on the latent image carrier or the density of the test image on the recording sheet 36 before or after fixing may be detected.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to the present invention. FIG. 4 is a diagram illustrating an intermediate transfer belt and members around the intermediate transfer belt. 1 is a block diagram of a first embodiment of an image forming apparatus. It is a flowchart which shows the flow of a process of image stabilization control. It is a flowchart which shows the flow of the process of the density | concentration difference suppression control which concerns on 1st Embodiment. It is a graph which shows the relationship between the output and density of an image density detection sensor. It is a graph which shows the specific example of the detection density of a test image. It is a graph which shows the relationship between the difference of a detection density and a target density, and the increase amount of developing bias. It is a block diagram of 2nd Embodiment of an image forming apparatus. It is a flowchart which shows the flow of a process of the density | concentration difference suppression control which concerns on 2nd Embodiment. It is a graph which shows the relationship between development bias and density. 6 is a graph showing a relationship between toner adhesion amount and density.

Explanation of symbols

4: photosensitive member (electrostatic latent image carrier), 14: primary transfer roller, 15: primary transfer region, 18: developing device, 20: developing roller (developer carrier), 22: power supply (developing bias applying device) 24: control circuit, 30: intermediate transfer belt (transfer member), 36: recording sheet, 40: secondary transfer roller, 41: secondary transfer area, 68: image density detection sensor (density detection device), 70: Control unit 72: Test image forming unit 74: Density difference calculating unit 76: Determination unit 78: Development toner amount control unit 79: Determination unit 100: Image forming apparatus

Claims (5)

An electrostatic latent image carrier carrying an electrostatic latent image;
A developing device for forming a toner image by attaching toner to the electrostatic latent image;
A transferred member to which the toner image formed on the electrostatic latent image carrier is transferred directly or via another member;
A density detection device for detecting the density of one of the toner image on the electrostatic latent image carrier or the toner image on the transfer member;
An image forming apparatus including a control unit that performs various types of control relating to an image forming operation,
The control unit
A test image forming unit for forming a test image;
A density difference calculating unit that detects a density by the density detection device at a plurality of locations of the test image and calculates a density difference between the maximum density and the minimum density among the detected density;
A determination unit for determining whether or not the concentration difference calculated by the concentration calculation unit is equal to or less than a predetermined value set in advance;
When the determination unit determines that the density difference is greater than the predetermined value, the developing toner amount supplied from the developing device to the electrostatic latent image carrier is less than the predetermined value. An image forming apparatus comprising: a developing toner amount control unit that controls the developing toner amount.   The developing toner amount control unit, when controlling the developing toner amount, compares the developing toner amount for the image portion with the detected low density compared with a target density set in advance corresponding to the test image. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled to increase and to reduce the amount of the developing toner for the image portion having the detected high density.   The image forming apparatus according to claim 1, wherein the developing toner amount control unit controls the developing toner amount so that the developing toner amount is uniformly increased over the entire image when the developing toner amount is controlled. . The control unit
The developing toner amount is uniformly distributed over the entire image so that the image portion in which the minimum density is detected by the developing toner amount control unit has a target density set in advance corresponding to the test image. A determination unit that determines whether or not the density of the image portion having the maximum density is equal to or less than an upper limit value of the density that can properly fix the image on the recording sheet. Prepared,
When the determination unit determines that the density of the maximum image portion is equal to or less than the upper limit value, the development toner amount control unit determines that the image portion where the minimum density is detected is the test image. The developing toner amount is uniformly increased over the entire image so as to obtain a preset target density corresponding to the image density, and the determination unit causes the maximum density of the image portion to be higher than the upper limit value. If it is determined that the density is too large, the developing toner amount is controlled to be increased uniformly over the entire image so that the density of the maximum image portion becomes the upper limit value. The image forming apparatus according to claim 3. The developing device is
A developer carrying body disposed opposite to the electrostatic latent image carrying body and carrying a developer containing toner;
A developing bias applying device for applying a developing bias to the developer carrying member,
The image forming apparatus according to claim 1, wherein the developing toner amount control unit controls the developing toner amount by controlling the developing bias.

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