JP2013104957A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus which is capable of reducing a toner consumption amount in an image stabilization control, and performing the appropriate image stabilization control even when a ghost is generated in the image stabilization control.SOLUTION: An image forming apparatus according to the present invention comprises: an image forming part; a density detection part which detects a density of a patch constituting a toner pattern formed on an image carrier; and a gradation correction part which generates gradation correction data based on a detection result by the density detection part, and performs a gradation correction using the gradation correction data. The toner pattern is constituted to include a plurality of gradation patches whose density gradually vary along the traveling direction of the image carrier, and reference patches in the same density arranged having at least one of the plurality of gradation patches therebetween. The gradation correction part corrects detection densities of the gradation patches according to detection densities of the reference patches by the density detection part, and generates gradation correction data according to the corrected detection densities.

Description

  The present invention relates to an electrophotographic image forming apparatus.

  In general, an electrophotographic image forming apparatus (printer, copying machine, facsimile, etc.) forms an electrostatic latent image by irradiating (exposing) laser light based on image data to a charged photosensitive drum. Then, the electrostatic latent image is visualized by attaching toner as colored particles to the electrostatic latent image to form a toner image. Then, the toner image is transferred directly or indirectly to the paper, and then heated and pressed in a fixing device to be fixed, thereby forming an image on the paper.

In such an image forming apparatus, the image quality of the output image (image formed on the paper) is deteriorated due to deterioration over time of the photosensitive drum, developer, etc., environment around the apparatus (temperature and humidity fluctuations), and the like. There is a problem. Specifically, a phenomenon occurs in which the gradation of the input image is not faithfully reproduced in the output image. Therefore, in a conventional image forming apparatus, image stabilization control is performed to stably reproduce the gradation of an input image on an output image (for example, Patent Documents 1 and 2).
As image stabilization control, for example, the density of each CMYK toner pattern formed on an intermediate transfer belt as an image carrier is detected by a photosensor, and tone correction data is based on the detection result (tone characteristics). There is a gradation correction that generates a (so-called gamma correction curve) and feeds it back to image forming conditions such as a charging potential, a developing potential, and an exposure amount.

In Japanese Patent Laid-Open No. 2004-228688, a header portion made of a solid black image is formed in front of the gradation image portion where the gradation level gradually changes, and the circumference of the toner carrier (developing roller) is longer than the circumference. An image forming apparatus is disclosed that corrects the density detection result at each position in the gradation image portion using the density detection result, and performs gradation correction based on the corrected density detection result.
In Patent Document 2, a highlight part gradation patch and a shadow part gradation patch are formed, and the density detection results of these two gradation patches are used as the density detection of the two gradation patches in the previous gradation correction. An image forming apparatus that performs gradation correction by selecting the number of gradation patches to be formed for gradation correction and the gradation in accordance with the comparison result in comparison with the result is disclosed.

JP 2005-49425 A JP 2010-262243 A

  In the image stabilization control described above, it is necessary to accurately detect the density of the toner pattern formed on the image carrier and accurately grasp the current gradation characteristics. However, when image formation is repeatedly performed by the image forming apparatus, the durability of the developer decreases, and accordingly, from the downstream (first image forming side) to the upstream (later image forming side) in the sub-scanning direction. A so-called ghost in which the concentration decreases toward the surface tends to occur. If a ghost occurs during image stabilization control, the influence of the ghost is included in the density detection result of the toner pattern. Therefore, the gradation characteristic calculated from the density detection result is not accurate. Even if tone correction is performed based on this, appropriate image stabilization control is not achieved.

  For example, in an image forming apparatus equipped with a two-component developing type developing device that uses toner and a carrier (magnetic material) as the main components of the developer, the toner concentration in the developer decreases (becomes carrier rich), resulting in ghosting. Occurs. Further, for example, in an image forming apparatus including a one-component developing type developing device that uses only toner as a main component of a developer, a ghost is generated due to insufficient supply capability of toner to a developing roller.

  This ghost is particularly likely to occur after an image having a high density (for example, a black solid image) is formed. When a black solid image is formed as a part of the toner pattern with a circumference equal to or longer than the circumference of the toner carrier as in Patent Document 1, there is a high possibility that a ghost will occur. In addition, in the image stabilization control disclosed in Patent Document 1, a header portion made of a solid black image is formed with a length equal to or longer than the circumference of the image carrier, and thus toner consumption increases.

  The present invention has been made to solve the above-described problems, and can reduce the amount of toner consumed in the image stabilization control, and can perform appropriate image stabilization control even when a ghost occurs during the image stabilization control. It is an object of the present invention to provide an image forming apparatus that can be used.

The image forming apparatus according to the present invention develops the electrostatic latent image by forming an electrostatic latent image on the photosensitive member by an exposure device based on input image data, and attaching toner to the photosensitive member by a developing device. An image forming unit for forming a toner image and transferring the toner image formed on the photoconductor to an image carrier;
A density detector for detecting the density of a patch constituting the toner pattern formed on the image carrier;
A gradation correction unit that generates gradation correction data based on a detection result by the density detection unit and performs gradation correction using the gradation correction data;
The toner pattern includes a plurality of gradation patches whose density changes stepwise along the traveling direction of the image carrier, and a reference patch having the same density arranged across at least one of the plurality of gradation patches. And comprising
The gradation correction unit determines whether a ghost has occurred in the toner pattern based on the detected density of the reference patch by the density detection unit. The gradation correction data is generated based on the corrected detection density after correcting the detection density of the tone patch.

  According to the present invention, it is possible to realize an image forming apparatus that can reduce the amount of toner consumed in image stabilization control and can perform appropriate image stabilization control even when a ghost is generated during image stabilization control.

1 is a diagram schematically showing an overall configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a main part of a control system of the image forming apparatus according to the embodiment. FIG. 6 is a diagram illustrating a positional relationship between a density detection sensor and a tone correction toner pattern. FIG. 3 is a diagram illustrating a patch configuration of a toner pattern used in an embodiment. FIG. 6 is a diagram illustrating an example of sensor output values of patches when a ghost is generated in a toner pattern. FIG. 6 is a diagram illustrating an example of sensor output values of patches when a ghost is not generated in a toner pattern. It is a flowchart which shows an example of a gradation correction data generation process. It is a figure which shows the relationship between a patch position and the influence of a ghost. It is a figure which shows the gradation correction curve employ | adopted by embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram schematically showing an overall configuration of an image forming apparatus 1 according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a main part of a control system of the image forming apparatus 1 according to the embodiment.
An image forming apparatus 1 shown in FIGS. 1 and 2 is an intermediate transfer type color image forming apparatus using electrophotographic process technology. That is, the image forming apparatus 1 transfers the toner images of C (cyan), M (magenta), Y (yellow), and K (black) formed on the photosensitive drum 413 to the intermediate transfer belt 421 (primary transfer). Then, after superposing four color toner images on the intermediate transfer belt 421, the toner images are transferred (secondary transfer) to a sheet to form an image.
In the image forming apparatus 1, the photosensitive drums 413 corresponding to the four colors of CMYK are arranged in series in the running direction of the intermediate transfer belt 421, and the respective color toner images are sequentially transferred to the intermediate transfer belt 421 in one procedure. Tandem system is adopted.

  As illustrated in FIGS. 1 and 2, the image forming apparatus 1 includes a control unit 10, an operation display unit 20, an image processing unit 30, an image forming unit 40, a conveyance unit 50, and a fixing unit 60.

  The control unit 10 includes a CPU (Central Processing Unit) 11, a ROM (Read Only Memory) 12, a RAM (Random Access Memory) 13, and the like. The CPU 11 reads a program corresponding to the processing content from the ROM 12 and develops it in the RAM 13, and centrally controls the operation of each block of the image forming apparatus 1 in cooperation with the developed program. At this time, various data stored in the storage unit 72 is referred to. The storage unit 72 includes, for example, a nonvolatile semiconductor memory (so-called flash memory) or a hard disk drive.

  The control unit 10 transmits and receives various data to and from an external device (for example, a personal computer) connected to a communication network such as a LAN (Local Area Network) or a WAN (Wide Area Network) via the communication unit 71. Do. For example, the control unit 10 receives image data transmitted from an external device, and forms an image on a sheet based on the image data (input image data). The communication unit 71 is composed of a communication control card such as a LAN card, for example.

  The operation display unit 20 is configured by, for example, a liquid crystal display (LCD) with a touch panel, and functions as a display unit 21 and an operation unit 22. The display unit 21 displays various operation screens, an image status display, an operation status of each function, and the like according to a display control signal input from the control unit 10. The operation unit 22 includes various operation keys such as a numeric keypad and a start key, receives various input operations by the user, and outputs an operation signal to the control unit 10.

The image processing unit 30 includes a circuit that performs digital image processing on input image data according to initial settings or user settings. For example, the image processing unit 30 performs gradation correction based on the gradation correction data (gradation correction table) under the control of the control unit 10. In other words, the gradation correction unit according to the present invention includes the control unit 10 and the image processing unit 30.
Further, the image processing unit 30 performs various correction processes such as color correction and shading correction, a compression process, and the like on the input image data in addition to the gradation correction. The image forming unit 40 is controlled based on the digital image data subjected to these processes.

  The image forming unit 40 is based on the input image data, and image forming units 41Y, 41M, 41C, 41K, and an intermediate transfer unit 42 for forming an image using colored toners of Y component, M component, C component, and K component. And a concentration detection sensor 43 and the like.

  The Y component, M component, C component, and K component image forming units 41Y, 41M, 41C, and 41K have the same configuration. For convenience of illustration and explanation, common constituent elements are denoted by the same reference numerals, and Y, M, C, or K are added to the reference numerals when distinguished from each other. In FIG. 1, only the components of the image forming unit 41Y for the Y component are denoted by reference numerals, and the components of the other image forming units 41M, 41C, and 41K are omitted.

  The image forming unit 41 includes an exposure device 411, a developing device 412, a photosensitive drum 413, a charging device 414, a drum cleaning device 415, and the like.

  The photosensitive drum 413 has, for example, an undercoat layer (UCL), a charge generation layer (CGL), a charge transport layer (CGL) on the peripheral surface of an aluminum conductive cylinder (aluminum tube). It is a negatively charged organic photoreceptor (OPC: Organic Photo-conductor) in which CTL (Charge Transport Layer) is sequentially laminated.

The charging device 414 uniformly charges the surface of the photoconductive drum 413 to a negative polarity.
The exposure device 411 is composed of, for example, a semiconductor laser, and irradiates the photosensitive drum 413 with laser light corresponding to the image of each color component. When the photosensitive drum 413 is irradiated with laser light, a positive charge is generated in the charge generation layer of the photosensitive drum 413 and is transported to the surface of the charge transport layer. Then, the surface charge (negative charge) of the photosensitive drum 413 is neutralized. An electrostatic latent image of each color component is formed on the surface of the photosensitive drum 413 due to a potential difference from the surroundings.

  The developing device 412 contains a developer of each color component (for example, a two-component developer composed of a toner having a small particle diameter and a carrier), and the toner of each color component is attached to the surface of the photosensitive drum 413. The electrostatic latent image is visualized to form a toner image. The detailed configuration of the developing device 412 will be described later.

  The drum cleaning device 415 includes a drum cleaning blade that is in sliding contact with the surface of the photosensitive drum 413. Transfer residual toner remaining on the surface of the photosensitive drum 413 after the primary transfer is scraped off and removed by a drum cleaning blade.

  The intermediate transfer unit 42 includes an intermediate transfer belt 421 serving as an intermediate transfer member, a primary transfer roller 422, a secondary transfer roller 423, a driving roller 424, a driven roller 425, a belt cleaning device 426, and the like.

The intermediate transfer belt 421 is an endless belt, and is stretched around a driving roller 424 and a driven roller 425. The intermediate transfer belt 421 travels at a constant speed in the direction of arrow A by the rotation of the driving roller 424. When the intermediate transfer belt 421 is pressed against the photosensitive drum 413 by the primary transfer roller 422, the toner images of the respective colors are sequentially superimposed on the intermediate transfer belt 421 and primarily transferred. When the intermediate transfer belt 421 is pressed against the sheet S by the secondary transfer roller 423, the toner image primarily transferred to the intermediate transfer belt 421 is secondarily transferred to the sheet S.
The belt cleaning device 426 includes a belt cleaning blade that is in sliding contact with the surface of the intermediate transfer belt 421. Transfer residual toner remaining on the surface of the intermediate transfer belt 421 after the secondary transfer is scraped off and removed by a belt cleaning blade.

  The density detection sensor 43 is disposed opposite to the intermediate transfer belt 421 on the upstream side in the running direction (sub-scanning direction) of the intermediate transfer belt 421 from the secondary transfer position where the toner image is secondarily transferred to the paper S. . For example, two density detection sensors 43 are arranged opposite to both sides in the width direction of the intermediate transfer belt 421 (the direction perpendicular to the traveling direction of the intermediate transfer belt 421, the main scanning direction) (see FIG. 3). . The density detection sensor 43 is used when generating gradation correction data, and is used for the gradation correction toner pattern formed in the non-image forming area of the intermediate transfer belt 421 (both sides in the width direction of the intermediate transfer belt 421). Detect concentration.

The density detection sensor 43 includes, for example, a light emitting element such as a light emitting diode (LED) and a light receiving element such as a photodiode (PD), and is a reflective optical sensor that detects the reflection density of the toner pattern. Can be applied. The reflection density of the toner pattern, the amount of light incident on the measurement object I _0, when the amount of reflected light from the measuring object was I, represented by -log (I / I _0).
The higher the density of the toner image formed on the intermediate transfer belt 421 (for example, a black solid image), the smaller the reflected light amount I, and the greater the reflection density, that is, the sensor output value output from the density detection sensor 43. Conversely, the lower the density of the toner image formed on the intermediate transfer belt 421 (for example, the base surface on which the toner image is not formed), the greater the reflected light amount I, the reflected density, that is, the sensor output value output from the density detection sensor 43. Becomes smaller.

The number and arrangement of the density detection sensors 43 are not limited to the above-described mode, and it is sufficient that the density of the toner pattern formed on the intermediate transfer belt 421 can be detected.
Further, when the intermediate transfer belt 421 is made of a translucent material, a transmission type optical sensor in which a light emitting element and a light receiving element are opposed to each other with the intermediate transfer belt 421 interposed therebetween is used as the density detection sensor 43. Can be applied.

  The fixing unit 60 includes a pressure unit (for example, a fixing roller on the fixing surface (image forming surface) side and a pressure roller on the back surface side) that forms a nip portion (fixing nip) for nipping and conveying the paper S. And a heating unit (for example, a fixing roller) that contacts the sheet S on which the toner image is transferred and heats the sheet at a fixing temperature. A known technique can be applied to the fixing unit 60. The fixing unit 60 fixes the toner image on the paper S by heating and pressurizing the conveyed paper S at the nip portion.

  The transport unit 50 includes a paper feed unit 51, a transport mechanism 52, a paper discharge unit 53, and the like. In the two paper feed tray units 51a and 51b constituting the paper feed unit 51, paper (standard paper, special paper) S identified based on the basis weight, size, etc. of the paper is stored for each preset type. Is done.

The sheets S accommodated in the sheet feed tray units 51a and 51b are sent out one by one from the top, and are conveyed to the image forming unit 40 by a conveyance mechanism 52 having a plurality of conveyance rollers such as registration rollers 52a. At this time, the registration section provided with the registration rollers 52a corrects the inclination of the fed paper S and adjusts the conveyance timing.
In the image forming unit 40, the toner image on the intermediate transfer belt 421 is secondarily transferred to one side (image forming surface) of the sheet S at a time, and a fixing process is performed in the fixing unit 60. The sheet S on which the image has been formed is discharged out of the apparatus by a discharge unit 53 having a discharge roller 53a.

As described above, the image forming apparatus 1 forms an electrostatic latent image on the photosensitive drum 413 (photosensitive member) by the exposure device 411 based on the input image data, and attaches toner to the photosensitive drum 413 by the developing device 412. Thus, the electrostatic latent image is developed to form a toner image, and the image forming unit 40 is provided to transfer the toner image formed on the photosensitive drum 413 to the intermediate transfer belt 421 (image carrier).
In addition, a density detection sensor 43 (density detection unit) that detects the density of a patch that forms a toner pattern formed on the intermediate transfer belt 421, and tone correction data are generated based on the detection result of the density detection sensor 43. A gradation correction unit (image processing unit 30 and control unit 10) that performs gradation correction using the gradation correction data is provided.

FIG. 3 is a diagram showing a positional relationship between the density detection sensor 43 and the tone correction toner pattern TP. In FIG. 3, an image forming area is an area where a toner image can be formed by the image forming unit 41, and a non-image forming area is an area where a toner image is not formed by the image forming unit 41.
As shown in FIG. 3, the toner patterns TP for CMYK colors are formed on the non-image forming area of the intermediate transfer belt 421, that is, on both sides in the width direction of the intermediate transfer belt 421. The width of the patch constituting the toner pattern TP is set to be equal to or larger than the detection width of the density detection sensor 43 so that the density of each patch is accurately detected by the density detection sensor 43.

FIG. 4 is a diagram illustrating a patch configuration of the toner pattern TP used in the embodiment.
As shown in FIG. 4, the toner pattern TP has a configuration in which twelve patches P <b> 0 to P <b> 11 are continuously arranged along the traveling direction of the intermediate transfer belt 421. The patches P0 to P11 are formed on the intermediate transfer belt 421 in order from the first patch P0, and the density (the density of the actually formed toner image, the output gradation value) is sequentially detected by the density detection sensor 43.

  The patches P0 to P4 and P6 to P10 are gradation patches for generating gradation correction data, and the density (the density of the input image for forming each patch, the input gradation value) changes stepwise. . The input gradation values of the patches P0 to P4 and P6 to P10 change stepwise from 255 to 0 when, for example, the gradation change is expressed by 256 gradations.

  A patch P0 arranged at the front end of the toner pattern TP, P5 arranged in the middle of the toner pattern TP, and P11 arranged at the rear end of the toner pattern TP are reference patches for detecting the occurrence of a ghost, and are the same. Are formed (input gradation value). Further, when a ghost is generated in the toner pattern TP, the gradation patches P0 to P4 and P6 to P10 are detected based on the detected densities (output gradation values and sensor output values) of the reference patches P0, P5, and P11. The detected density is corrected.

  In the present embodiment, a black solid image having the highest density (input gradation value is 255) is used as the reference patches P0, P5, and P11, and the reference patch and the gradation patch are used for the patch P0. Thereby, the amount of toner consumed to form the toner pattern TP can be suppressed.

Further, reference patches P0 and P11 are arranged at the front and rear ends of the toner pattern TP. Thereby, it can be easily determined whether or not a ghost is generated as a whole in the toner pattern TP.
Further, a reference patch P5 is arranged in the middle of the toner pattern TP. Thereby, the influence of the ghost generated in the toner pattern TP can be accurately estimated.

  FIG. 5 is a diagram illustrating an example of sensor output values of the patches P0 to P11 when a ghost is generated in the toner pattern TP. FIG. 6 is a diagram illustrating an example of sensor output values of the patches P0 to P11 when no ghost is generated in the toner pattern TP.

As shown in FIG. 6, when no ghost is generated in the toner pattern TP, the sensor output values V _P0 , V _P5 , and V _P11 of the reference patches P0, P5, and P11 having the same input gradation value are almost the same. become. Further, the sensor output values V _{P0 to} V _P4 and V _{P6 to} V _P10 of the gradation patches _{P0 to P4} and _{P6 to P10} in which the input gradation values are decreased stepwise increase. In this case, the input gradation values of the gradation patches _{P0 to P4} and _{P6 to P10} and the output gradation values calculated from the sensor output values V _{P0 to} V _P4 and V _{P6 to} V _P10 are associated with each other to obtain a desired level. A tonal characteristic curve is generated.

On the other hand, as shown in FIG. 5, when a ghost is generated in the toner pattern TP, since the density of the patch formed later on the intermediate transfer belt 421 becomes lighter, the reference patches P0, P5, and P11 are used. Sensor output values V _P0 , V _P5 , and V _P11 increase in the order of the reference patches P0, P5, and P11.

Further, in FIG. 5, the sensor output values V _{P0 to} V _P4 and V _{P6 to} V _P10 of the gradation patches _{P0 to P4} and _{P6 to P10} increase in a stepwise manner, as in the case where no ghost occurs. Although there are sensor output values V _{P0 to} V _P4 and V _{P6 to} V _P10 , the influence of ghost is included.

Therefore, in the present embodiment, it is determined whether or not a ghost has occurred in the toner pattern TP by comparing the sensor output values V _P0 , V _P5 , and V _P11 of the reference patches P0, P5, and P11. When a ghost is generated, the sensor output values V _{P0 to} V _P4 , V of the gradation patches _{P0 to P4} and P6 to P10 are considered in consideration of the influence of the ghost (increase of the sensor output value due to the ghost). correcting the _P6 ~V _P10, generate an appropriate tone correction data (gradation correction data generation process). Specifically, gradation correction data generation processing is performed according to the flowchart shown in FIG.

FIG. 7 is a flowchart illustrating an example of the gradation correction data generation process. The gradation correction data generation process shown in FIG. 7 is realized by the CPU 11 executing a predetermined program stored in the ROM 12 when the image forming apparatus 1 is turned on, for example.
The gradation correction data generation process is performed periodically when a predetermined time has elapsed after the previous gradation correction data generation, when a predetermined number of image formations have been completed, or when the toner in the developing device 412 has been replaced. It is desirable that this is done. Further, during the gradation correction data generation processing, the secondary transfer roller 423 and the driving roller 424 are separated from each other.

  In step S101 in FIG. 7, the control unit 10 forms toner patterns TP of the respective colors on the intermediate transfer belt 421 by the image forming units 41Y, 41M, 41C, and 41K based on the image data for gradation correction (FIG. 7). 3 and 4). Image data for gradation correction is stored in the ROM 12, for example.

In step S102, the control unit 10 acquires the detection result (corresponding to the sensor output value _Vbase and the gradation value 0 (white solid image)) of the base surface (the area where the toner image is not formed) by the density detection sensor 43. To do. The acquired sensor output value V _base is temporarily stored in the RAM 13.

In step S103, the control unit 10 acquires the detection results (sensor output values V _{P0 to} V _P11 ) of the patches _{P0 to P11} by the density detection sensor 43. The acquired sensor output values V _{P0 to} V _P11 are temporarily stored in the RAM 13. The relationship shown in FIG. 5 or 6 is obtained by steps S102 and S103. The toner pattern TP formed on the intermediate transfer belt 421 is removed by the belt cleaning device 426 after passing through the detection area of the density detection sensor 43.

In step S104, the control unit 10 compares the sensor output values V _P0 , V _P5 , and V _P11 of the reference patches P0, P5, and P11 to determine whether or not a ghost has occurred in the toner pattern TP. For example, when the difference (ΔV in FIG. 5) between the sensor output values V _P0 and V _P11 of the reference patches P0 and P11 exceeds a predetermined value, the control unit 10 determines that a ghost has occurred in the toner pattern TP. To do.
If the controller 10 determines that a ghost has occurred in the toner pattern TP, the process proceeds to step S105. If the controller 10 determines that no ghost has occurred, the controller 10 proceeds to step S107. .

In step S105, the control unit 10 calculates correction amounts for the gradation patches P0 to P4 and P6 to P10. When the ghost is generated in the toner pattern TP, the sensor output values V _P5 and V _{P11 of} the reference patches P5 and _P11 are naturally increased by the influence of the ghost. The difference between the sensor output value V _P0 of the reference patch _P0 and the sensor output value V _P5 of the reference patch P5 corresponds to the influence of the ghost in the reference patch P5, and the sensor output value V _P0 of the reference patch _P0 and the sensor output value of the reference patch P11. The difference of V _P11 corresponds to the ghost effect in the reference patch P11.

Therefore, by performing approximate calculation using the sensor output values V _P0 , V _P5 , and V _P11 of the reference patches P0, P5, and P11, the relationship between the patch position and the influence of the ghost (increment of the sensor output value due to the ghost) is obtained. (See FIG. 8). From this relationship, the influence of the ghost in the gradation patches _{P0 to P4 and P6 to P10} , that is, correction amounts V _{ref (P0) to} V _{ref (P4)} V _{ref (P6) to} V _{ref (P10)} are calculated. Can do.

Although FIG. 8 shows a case where linear approximation is performed based on the sensor output values V _P0 , V _P5 , and V _P11 of the reference patches P 0, P 5, and P ₁₁ , it is optimal for the process characteristics of the image forming apparatus 1. It is desirable to use an approximation method. For example, the relationship between the patch position and the influence (correction amount) of the ghost may be calculated by high-order equation approximation, logarithmic approximation, power approximation, or other approximation calculation.

In step S106 of FIG. 7, the control unit 10 uses the correction amount V _{ref (Pn)} calculated in step S105 to output the sensor output values V _{P0 to} V _P4 , V of the gradation patches _{P0 to P4} and P6 to P10. Correct _{P6 to} V _P10 . For convenience, the sensor output value after correction of the gradation patch Pn (n = 0 to 4, 6 to 10) is assumed to be V _Pn ′. Since the correction amount V _{ref (P0)} in the gradation patch P0 is 0, V _P0 = V _P0 ′.

In step S107, the control unit 10 calculates output gradation values of the gradation patches P0 to P4 and P5 to P10. Specifically, the control unit 10 uses the sensor output values V _Pn (V _{P0 to} V _P4 , V _{P6 to} V _P10 ) of the gradation patches P0 to _P4 and P5 to _P10 , and the normalized value of the reference patch P0 is 0. Then, the standardized value of the base surface is normalized by 8 bits according to the following equation (1) so that the normalized value is represented by 255, and this normalized value is converted into an output gradation value.
Normalized value = {(V _Pn −V _P0 ) / (V _base −V _P0 )} × 255 (1)

At this time, if the ghost occurs in the toner pattern TP ( "YES" in step S104), as the sensor output value V _Pn, the sensor output value V _Pn after correction calculated 'is used in step S106. That is, the output gradation value is calculated based on the sensor output value from which the influence of the ghost has been eliminated. On the other hand, when no ghost is generated in the toner pattern TP, the sensor output value V _Pn actually detected is used as the sensor output value V _Pn .

Incidentally, in the above equation (1), the sensor output value V _P0 of the highest concentration of patches P0, depending on the process characteristics of the image forming apparatus 1, the reference patch P0, P5, P11 sensor output value V _P0, V _P5 , _VP11 average value, maximum value, or minimum value may be adopted.

  In step S108, the control unit 10 generates a tone characteristic curve by associating the input tone values of the tone patches P0 to P4 and P5 to P10 with the output tone value calculated in step S107. This gradation characteristic curve is represented, for example, by a curve L1 in FIG.

In step S109, the control unit 10 faithfully reproduces the gradation of the input image on the output image based on the gradation characteristic curve L1 generated in step S108, that is, the target gradation characteristic L0 shown in FIG. 9 is obtained. Thus, a tone correction curve (gamma correction curve) L2 for correcting the tone value of the input image is generated. The gradation correction curve L2 is represented by a curve that is line-symmetric with the gradation characteristic curve L1 with respect to the target gradation characteristic L0. When the input gradation value is x, the output gradation value is y, and the gradation characteristic curve L1 is represented by y = f (x), the target gradation characteristic L0 is y = x, and therefore the gradation correction curve L2 Is an inverse function (y = f ⁻¹ (x)) of the gradation characteristic curve L1.

In step S110, the control unit 10 determines, based on the gradation correction curve L2 generated in step S109, a gradation in which the input gradation value is associated with the correction gradation value for which the input gradation value is to be corrected. Correction data (tone correction table) is generated and updated. The generated gradation correction data is stored in the RAM 13, for example.
In subsequent image formation, gradation correction is performed with reference to the updated gradation correction data, and the image formation conditions are determined based on the corrected gradation value.

As described above, in the image forming apparatus 1, the toner pattern TP used to generate the gradation correction data has a plurality of levels whose density changes stepwise along the traveling direction of the intermediate transfer belt 421 (image carrier). Tone patches P0 to P4 and P6 to P10, and reference patches P0, P5, and P11 having the same density arranged with at least one of the plurality of tone patches P0 to P4 and P6 to P10 interposed therebetween. The
Then, the control unit 10 (tone correction unit) is based on the sensor output values V _P0 , V _P5 , and V _P11 (detected densities) of the reference patches P0, P5, and P11 by the density detection sensor 43 (density detection unit). It is determined whether or not a ghost has occurred in the toner pattern TP (step S104 in FIG. 7), and if it is determined that a ghost has occurred (“YES” in step S104 in FIG. 7), the gradation patches P0 to P0. The sensor output values V _{P0 to} V _P4 and V _{P6 to} V _P10 (detected concentrations) of _P4 and _{P6 to P10} are corrected (steps S105 and S106 in FIG. 7), and the corrected sensor output values V _P0 'to V _P4 ' , Tone correction data is generated based on V _P6 ′ to V _P10 ′ (detected density) (steps S107 to S110 in FIG. 7).

According to the image forming apparatus 1, even if a ghost is generated in the toner pattern TP for generating the gradation correction data, a desired gradation characteristic curve L1 excluding the influence of the ghost can be obtained, and an appropriate value can be obtained. Tone correction data can be generated. Therefore, appropriate image stabilization control is performed, and an image that faithfully reproduces the input image can be formed.
Further, unlike Patent Document 1, it is not necessary to form a solid black image with a circumferential length of the toner carrying member (developing roller) or more, so that it is possible to suppress toner consumption in image stabilization control.

As mentioned above, the invention made by the present inventor has been specifically described based on the embodiment. However, the present invention is not limited to the above embodiment, and can be changed without departing from the gist thereof.
For example, the number and arrangement of the reference patches constituting the toner pattern TP are not limited to those shown in the embodiment (see FIG. 4), and the reference patches having the same density so as to sandwich at least one of the gradation patches. Should just be arranged. However, in order to accurately determine the presence / absence of a ghost, it is desirable to configure the reference patch with a high density patch image.

In the embodiment, the case where the image carrier in the present invention is configured by the intermediate transfer belt 421 has been described. However, the present invention is also applicable to the case where the intermediate transfer drum or the sheet is the image carrier. Can do.
The present invention can also be applied to a monochrome image forming apparatus that forms a single-color image or a rotary (4-cycle) image forming apparatus that uses an image forming unit in which four CMYK colors are integrated.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

1 Image forming apparatus 10 Control unit (tone correction unit)
20 operation display unit 21 display unit 22 operation unit 30 image processing unit (gradation correction unit)
40 Image forming unit 41, 41Y, 41M, 41C, 41K Image forming unit 411 Exposure device 412 Developing device 413 Photoconductor drum 414 Charging device 415 Drum cleaning device 42 Intermediate transfer unit 421 Intermediate transfer belt 422 Primary transfer roller 423 Secondary transfer roller 424 Driving roller 425 Driven roller 426 Belt cleaning device 43 Concentration detection sensor (concentration detection unit)
50 transport unit 51 paper feed unit 51a, 51b paper feed tray unit 52 transport mechanism 52a registration roller 53 paper discharge unit 53a paper discharge roller 60 fixing unit 71 communication unit 72 storage unit S paper TP toner pattern P0, P5, P11 reference patch P0 ~ P4, P6 ~ P10 gradation patch

Claims (5)

Based on the input image data, an exposure device forms an electrostatic latent image on the photoconductor, and a developing device attaches toner to the photoconductor to develop the electrostatic latent image to form a toner image. An image forming unit for transferring the toner image formed on the photoconductor to an image carrier;
A density detector for detecting the density of a patch constituting the toner pattern formed on the image carrier;
A gradation correction unit that generates gradation correction data based on a detection result by the density detection unit and performs gradation correction using the gradation correction data;
The toner pattern includes a plurality of gradation patches whose density changes stepwise along the traveling direction of the image carrier, and a reference patch having the same density arranged across at least one of the plurality of gradation patches. And comprising
The gradation correction unit determines whether a ghost has occurred in the toner pattern based on the detected density of the reference patch by the density detection unit. An image forming apparatus, wherein the gradation correction data is generated based on a corrected detected density of a tone patch and a corrected detected density.   The image forming apparatus according to claim 1, wherein the reference patch is disposed at a front end and a rear end of the toner pattern.   The image forming apparatus according to claim 2, wherein the reference patch is disposed in the middle of the toner pattern.   The image forming apparatus according to claim 1, wherein the reference patch also serves as a part of the gradation patch.   The image forming apparatus according to claim 1, wherein the reference patch is a black solid image having the highest density.