JP4280588B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a tandem color image forming apparatus such as a copying machine or a laser beam printer, and more particularly to a color image forming apparatus that detects toner density and controls image density using the result.

  In recent years, there has been an increasing demand for high productivity for image forming apparatuses that perform color image output by an electrophotographic method. As an image forming method that meets this demand, an image forming unit including a plurality of image carriers and developing devices is used as a transfer belt. An image forming apparatus called a tandem method, which is arranged in parallel at opposed positions and sequentially transfers toner images on an image carrier onto transfer paper (or transfer belt), has come to dominate.

  In such an image forming apparatus, a reference toner patch is formed on an image carrier or an intermediate transfer member, and image density control is performed based on the measurement result of the toner density using an optical toner density sensor. The technology has been put into practical use (for example, see Patent Document 1).

JP 2000-206761 A

  However, such an image forming apparatus has the following problems.

  (1) The positions of the toner density sensors arranged for the respective photoconductors are all set to the same position with respect to the width direction of the photoconductor (the width direction of the intermediate transfer belt) so that the toner density sensors can be detected When one or a plurality of reference toner patches are formed in parallel at the same position in the width direction of the photoreceptor, the reference toner patches of the respective colors are overlapped on the intermediate transfer belt, and the intermediate transfer belt cleaning and the secondary transfer cleaning are performed. As a result, the amount of toner input to the portion increases, resulting in poor cleaning and toner scattering, and the image quality thereafter deteriorates.

  (2) Further, in such an image forming apparatus, first, after the image formation of one or a plurality of reference toner patches of one color is completed so that the reference toner patches do not overlap on the intermediate transfer belt, the next There is a method of forming a reference toner patch of four colors, but this method requires an idle rotation time, and when using four colors, a method of forming images of four color reference toner patches in parallel is used. Compared to this, it takes nearly four times longer, leading to an increase in user waiting time and a decrease in productivity.

  (3) An apparatus for detecting the toner density of the reference toner patch by shifting the installation position of the toner density sensor corresponding to each color and the image forming position of the reference toner patch with respect to the width direction of the intermediate transfer belt is disclosed. However, the intermediate transfer belt is easily scratched because it rubs against the transfer paper. As a result, the surface state of the transfer belt becomes non-uniform, and a plurality of (each color) at different positions in the width direction of the intermediate transfer belt. Variations in detection between toner density sensors become large, and detection is not stable.

  (4) In the image forming apparatus that detects the toner density on the intermediate transfer belt, when the toner density changes, whether the cause is the toner density in the developing device or the potential of the photosensitive member, to the intermediate transfer belt. For example, the toner replenishment amount in the developing device is reduced even if the transfer rate to the intermediate transfer belt is abnormal and the toner density on the intermediate transfer belt is low. Increasing the toner density by increasing it may cause toner scattering. Thus, a problem arises when detecting the toner density on the intermediate transfer belt.

  Therefore, an image forming apparatus that solves the above problem has been proposed. That is, a detector for optically detecting a toner image in an image forming apparatus for transferring a toner image formed by an electrophotographic method onto a photosensitive member as a plurality of rotationally driven image bearing members to an intermediate transfer member to obtain a color image Are arranged to be shifted from each other in the axial direction of the photosensitive member so as to face the photosensitive member. Then, the reference toner patches are formed by shifting in the axial direction of the photosensitive member for each photosensitive member, and the density of the formed reference toner patch is detected by a detector corresponding to each reference toner patch. Then, image density control is executed based on the detection results of these detectors. Accordingly, it is possible to provide an image forming apparatus that does not cause a cleaning failure and can detect toner density stably.

  By the way, in this image forming apparatus, a detector for detecting a toner image on the photoconductor is required for each photoconductor as an image carrier. For example, the light receiving characteristics and detection characteristics of each detector and the photoconductor If the surface property value of the toner changes due to aging or environmental changes, even if the toner adhesion amount is detected for each photoconductor and the image density is controlled, there is a possibility that variation occurs between colors, resulting in fluctuations in image density. There is. Such a problem occurs not only in an image forming apparatus in which a plurality of detectors for detecting a toner image on a photoconductor are shifted from each other but also in an image forming apparatus in which a plurality of detectors are not shifted from each other. is there.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that prevents image density defects due to differences in detection characteristics of a plurality of detectors that detect the density of a toner image, and has a stable image density.

  An object of the present invention is to provide an image forming apparatus that does not cause a cleaning failure over time and that can stably detect a toner density.

  An object of the present invention is to make it possible to detect a stable toner density without causing poor cleaning over time, thereby reducing the waiting time for the user, increasing the productivity, and stabilizing the image quality. Is to provide a device.

  The invention according to claim 1 is an image forming apparatus in which a toner image formed by electrophotography on a plurality of rotationally driven image carriers is transferred to an intermediate transfer member to obtain a color image. A plurality of first detectors optically detecting the density of a toner image formed on the opposed image carrier, and provided opposite to the intermediate transfer member; A second detector for optically detecting the density of the toner image transferred to the intermediate transfer member, and a toner patch for forming a reference toner patch, which is a toner image, on each image carrier and transferring the toner image to the intermediate transfer member; Forming means; correction means for correcting the detection result of the first detector based on the detection result of the second detector; and image density control based on the corrected detection result of the first detector. Image density control means for executing

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, at least two of the first detectors are arranged so as to be shifted from each other in an axial direction of the image carrier, and the toner patch. The forming unit forms the reference toner patch in accordance with the arrangement of the first detector.

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the second detector is shifted in the axial direction of the image carrier with respect to all the first detectors. The toner patch forming means is arranged to form the reference toner patch according to the arrangement of the first and second detectors.

  According to a fourth aspect of the present invention, in the image forming apparatus according to the first, second, or third aspect, the toner patch forming means uses a plurality of the first detections as a reference toner patch detected by the second detector. The image forming condition is the same as that of at least one of the reference toner patches detected by the device.

  According to a fifth aspect of the present invention, in the image forming apparatus according to the first, second, third or fourth aspect, the image carrier is provided for each of a plurality of toner colors including black, and a black toner image is provided. The first detector and the second detector corresponding to the image carrier on which is formed are the image carrier among all the first detectors and the second detectors. It is arrange | positioned in the most distant position in the axial center direction.

  According to the first aspect of the present invention, even if the detection characteristics of the first detector provided for each of the plurality of image carriers are different, the detection result of the second detector detects the toner density on the intermediate transfer member. Therefore, it is possible to prevent an image density defect caused by a difference in detection characteristics of a plurality of detectors, and to provide an image forming apparatus having a stable image density. it can. Accordingly, it is possible to provide an image forming apparatus having a stable image density over time even when the environment changes.

  According to the second aspect of the present invention, the reference toner patches corresponding to at least two image carriers are formed so as to be shifted from each other, and accordingly, these reference toner patches are transferred to be shifted from each other on the intermediate transfer belt. In addition, it is possible to prevent occurrence of poor cleaning in intermediate transfer cleaning and secondary transfer cleaning, and toner scattering due to transfer scattering in each transfer process. Further, since the reference toner patch can be created in parallel by at least two image forming units, it is possible to reduce the waiting time of the user when performing a self-check such as potential control.

  According to the third aspect of the present invention, the reference toner patch detected by the first detector and the reference toner patch detected by the second detector can be formed in parallel. It is possible to reduce the waiting time of the user when executing image density control such as potential control accompanied by toner patch creation.

  According to the fourth aspect of the present invention, the correction is performed based on the reference toner patches having the same image forming conditions, so that the correction accuracy can be increased.

  According to the fifth aspect of the present invention, the positional relationship between the first detector and the second detector corresponding to the other color toner that contributes more to the color variation than the black toner is black. Since the positional relationship between the corresponding first detector and the second detector becomes closer, it is possible to reduce the difference due to the image forming position of the reference toner patch and increase the correction accuracy.

  An embodiment of the present invention will be described with reference to the drawings. The present embodiment is an application example to a tandem full-color electrophotographic copying machine (hereinafter simply referred to as “copying machine”) as an image forming apparatus.

  First, the configuration of the entire copying machine according to the present embodiment will be described. FIG. 1 is a schematic configuration diagram showing the entire copying machine of the present embodiment. The copying machine includes a copying machine main body 100 that forms an image, a paper feeding device 200 on which the copying machine main body 100 is mounted and that supplies transfer paper 5 as a recording material to the copying machine main body 100, and a copying machine main body. The scanner 300 is mounted on the scanner 100 and reads a document image, and the automatic document feeder (ADF) 400 is mounted on the scanner 300. The copying machine main body 100 is provided with a manual feed tray 6 for manually feeding the transfer paper 5 and a paper discharge tray 7 for discharging the transfer paper 5 on which an image has been formed.

  FIG. 2 is an enlarged view showing the configuration of the copying machine main body 100. The copying machine main body 100 is provided with an endless belt-like intermediate transfer belt 10 which is an intermediate transfer member. As shown in FIG. 3, the intermediate transfer belt 10 has a three-layer structure including a base layer 11, an elastic layer 12, and a coat layer 13. The base layer 11 is configured, for example, by combining a fluorine resin having a small elongation or a rubber material having a large elongation with a material that hardly stretches such as a canvas. The elastic layer 12 is made of, for example, fluorine rubber or acrylonitrile-butadiene copolymer rubber, and is formed on the base layer 11. The coat layer 13 is formed by coating the surface of the elastic layer 12 with, for example, a fluorine resin. The intermediate transfer belt 10 is rotationally driven in the clockwise direction in FIG. 2 while being stretched around the three support rollers 14, 15 and 16.

  As shown in FIG. 2, the belt stretched portion between the first support roller 14 and the second support roller 15 among the support rollers 14, 15, 16 has yellow, cyan, magenta, and black (black). Four image forming units 18Y, 18C, 18M, and 18K corresponding to the four colors are arranged side by side. Above these image forming units 18Y, 18C, 18M, and 18K, an exposure device 21 is provided as shown in FIG. The exposure apparatus 21 emits writing light by driving a semiconductor laser (not shown) by a laser control unit (not shown) based on the image information of the document read by the scanner 300, and each image forming unit. This is for forming an electrostatic latent image on the photoconductive drums 20Y, 20C, 20M, and 20K as image carriers provided on 18Y, 18C, 18M, and 18K. Here, the emission of the writing light is not limited to the laser, but may be an LED, for example.

  The configuration of the image forming units 18Y, 18C, 18M, and 18K will be described. In the following description, the image forming unit 18K that forms a black toner image will be described as an example, but the other image forming units 18Y, 18C, and 18M have the same configuration. Here, FIG. 4 is an enlarged view showing a configuration of two adjacent image forming units 18M and 18K. In addition, in the code | symbol in a figure, the symbol of "M" and "K" which shows distinction of a color is abbreviate | omitted, and a symbol is abbreviate | omitted suitably also in the following description.

  In the image forming unit 18, a charging device 60, a developing device 61, a photoconductor cleaning device 63, and a charge removal device 64 are provided around the photoconductor drum 20. Further, a primary transfer device 62 is provided at a position facing the photosensitive drum 20 via the intermediate transfer belt 10.

  The charging device 60 is of a contact charging type that employs a charging roller, and uniformly charges the surface of the photosensitive drum 20 by applying a voltage in contact with the photosensitive drum 20. As the charging device 60, a non-contact charging type using a non-contact scorotron charger or the like can be used.

  The developing device 61 uses a two-component developer composed of a magnetic carrier and a nonmagnetic toner. As the developer, a one-component developer may be used. The developing device 61 can be broadly divided into a stirring unit 66 and a developing unit 67 provided in the developing case 70. In the agitating unit 66, a two-component developer (hereinafter simply referred to as “developer”) is conveyed while being agitated and supplied onto a developing sleeve 65, which will be described later, as a developer carrying member. The stirring unit 66 is provided with two parallel screws 68, and a partition plate is provided between the two screws 68 for partitioning so that both ends communicate with each other. Further, a toner concentration sensor 71 for detecting the toner concentration of the developer in the developing device 61 is attached to the developing case 70. On the other hand, in the developing unit 67, the toner of the developer attached to the developing sleeve 65 is transferred to the photosensitive drum 20. The developing portion 67 is provided with a developing sleeve 65 that faces the photosensitive drum 20 through the opening of the developing case 70, and a magnet (not shown) is fixedly disposed in the developing sleeve 65. Further, a doctor blade 73 is provided so that the tip approaches the developing sleeve 65. In the present embodiment, the distance at the closest portion between the doctor blade 73 and the developing sleeve 65 is set to 0.9 mm.

  In the developing device 61, the developer is conveyed and circulated while being stirred by two screws 68, and is supplied to the developing sleeve 65. The developer supplied to the developing sleeve 65 is pumped and held by a magnet. The developer pumped up by the developing sleeve 65 is conveyed along with the rotation of the developing sleeve 65 and is regulated to an appropriate amount by the doctor blade 73. The regulated developer is returned to the stirring unit 66. Thus, the developer transported to the developing area facing the photosensitive drum 20 is brought into a spiked state by the magnet and forms a magnetic brush. In the developing region, a developing electric field for moving the toner in the developer to the electrostatic latent image portion on the photosensitive drum 20 is formed by the developing bias applied to the developing sleeve 65. As a result, the toner in the developer is transferred to the electrostatic latent image portion on the photosensitive drum 20, and the electrostatic latent image on the photosensitive drum 20 is visualized to form a toner image. The developer that has passed through the developing region is transported to a portion where the magnetic force of the magnet is weak, and thus is separated from the developing sleeve 65 and returned to the stirring unit 66. When the toner concentration in the stirring unit 66 becomes light by repeating such an operation, the toner concentration sensor 71 detects this, and the toner is supplied to the stirring unit 66 based on the detection result.

  The primary transfer device 62 employs a primary transfer roller, and is installed so as to be pressed against the photosensitive drum 20 with the intermediate transfer belt 10 interposed therebetween. The primary transfer device 62 may not be a roller shape, but may be a conductive brush shape, a non-contact corona charger, or the like.

  The photoconductor cleaning device 63 includes a cleaning blade 75 made of, for example, polyurethane rubber, which is disposed so that the front end is pressed against the photoconductor drum 20. In the present embodiment, a conductive fur brush 76 that contacts the photosensitive drum 20 is used in combination to improve the cleaning performance. A bias is applied to the fur brush 76 from a metal electric field roller 77, and the tip of a scraper 78 is pressed against the electric field roller 77. The toner removed from the photoconductor drum 20 by the cleaning blade 75 and the fur brush 76 is accommodated in the photoconductor cleaning device 63. Thereafter, the toner is brought to one side of the photoconductor cleaning device 63 by the recovery screw 79, returned to the developing device 61 through the toner recycling device, and reused.

  The static eliminator 64 is composed of a static elimination lamp, and irradiates light to initialize the surface potential of the photosensitive drum 20.

  Further, the image forming unit 18 is provided with a toner density sensor 310 and a potential sensor 320 which are first detectors corresponding to the respective photosensitive drums 20. Specifically, as shown in FIG. 5, the toner density sensor 310 (310Y, 310C, 310M, 310K) is provided so as to face the photosensitive drum 20 for each photosensitive drum 20, and the shaft of the photosensitive drum 20 is mutually connected. They are arranged shifted in the direction of the center 90. The toner density sensor 310 is an optical infrared light reflection type sensor, and optically detects the density of the toner image formed on the surface of the photosensitive drum 20. Similarly, the potential sensor 320 (320Y, 320C, 320M, 320K) is also provided so as to face the photosensitive drum 20 for each photosensitive drum 20, and is arranged so as to be shifted from each other in the direction of the axial center 90 of the photosensitive drum 20. . These potential sensors 320 detect the potential on the surface of the photosensitive drum 20. Here, in the present embodiment, the developing sleeve 65 (see FIG. 4) is about 10 degrees above the horizontal line passing through the axis 90 of the photosensitive drum 20 in order to secure a space for disposing the toner density sensor 310. It is arranged.

  Specific settings of the image forming unit 18 will be described. The diameter of the photosensitive drum 20 is 60 mm, and the photosensitive drum 20 is driven at a linear speed of 282 mm / s. The developing sleeve 65 has a diameter of 25 mm, and the developing sleeve 65 is driven at a linear speed of 564 mm / s. Further, the charge amount of the toner in the developer supplied to the development region is preferably in the range of about − (minus) 10 to −30 μC / g. The development gap, which is the gap between the photosensitive drum 20 and the development sleeve 65, can be set in the range of 0.5 to 0.3 mm, and the development efficiency can be improved by reducing the value. Further, the photosensitive layer of the photosensitive drum 20 has a thickness of 30 μm, the beam spot diameter of the optical system of the exposure device 21 is 50 × 60 μm, and the light quantity is about 0.47 mW. As an example, the surface of the photosensitive drum 20 is uniformly charged to −700 V by the charging device 60, and the potential of the electrostatic latent image portion irradiated with the laser by the exposure device 21 is −120 V. On the other hand, the developing bias voltage is set to -470V, and a developing potential of 350V is secured. Such process conditions are appropriately changed according to the result of the potential control.

  In the image forming unit 18 having the above configuration, first, the surface of the photosensitive drum 20 is uniformly charged by the charging device 60 as the photosensitive drum 20 rotates. Next, based on the image information read by the scanner 300, the exposure light is irradiated from the exposure device 21 to form an electrostatic latent image on the photosensitive drum 20. Thereafter, the electrostatic latent image is visualized by the developing device 61 to form a toner image. This toner image is primarily transferred onto the intermediate transfer belt 10 by the primary transfer device 62. The transfer residual toner remaining on the surface of the photosensitive drum 20 after the primary transfer is removed by the photosensitive member cleaning device 63, and thereafter, the surface of the photosensitive drum 20 is discharged by the static eliminating device 64, and the next image formation is performed. Provided.

  Next, as shown in FIG. 2, a secondary transfer roller 24 as a secondary transfer device is provided at a position facing the third support roller 16 among the support rollers. When the toner image on the intermediate transfer belt 10 is secondarily transferred onto the transfer paper 5, the secondary transfer roller 24 is pressed against the portion of the intermediate transfer belt 10 wound around the third support roller 16. Next transfer is performed. The secondary transfer device may not be configured using the secondary transfer roller 24 but may be configured using, for example, a transfer belt or a non-contact transfer charger. The secondary transfer roller 24 is in contact with a roller cleaning unit 91 that cleans toner adhering to the secondary transfer roller 24.

  Further, on the downstream side of the secondary transfer roller 24 in the transfer paper conveyance direction, an endless belt-like conveyance belt 22 is stretched between the two rollers 23a and 23b. Further, a fixing device 25 for fixing the toner image transferred onto the transfer paper 5 is provided further downstream in the transport direction. The fixing device 25 has a configuration in which a pressure roller 27 is pressed against a heating roller 26. Further, a belt cleaning device 17 is provided at a position facing the second support roller 15 among the support rollers of the intermediate transfer belt 10. The belt cleaning device 17 is for removing residual toner remaining on the intermediate transfer belt 10 after the toner image on the intermediate transfer belt 10 is transferred to the transfer paper 5.

  Further, a correction sensor 330 serving as a second detector is provided opposite to the intermediate transfer belt 10 in the intermediate transfer belt 10 on the downstream side of all the photosensitive drums 20 in the intermediate transfer belt rotation direction. Specifically, the correction sensor 330 is disposed at a position facing the first support roller 14 via the intermediate transfer belt 10. The correction sensor 330 has the same structure as the toner density sensor 310 and is an optical infrared reflection type sensor that optically detects the density of the toner image formed on the surface of the intermediate transfer belt 10. . Here, FIG. 5 schematically illustrates the correction sensor 330 in order to explain the positional relationship between the correction sensor 330 and the toner density sensor 310. As shown in FIG. 5, the correction sensor 330 is arranged so as to be shifted in the direction of the axis 90 of the photosensitive drum 20 with respect to the toner density sensors 310Y, 310C, 310M, and 310K. Further, the correction sensor 330 and the toner density sensors 310Y, 310C, 310M, and 310K are arranged in the direction of the axial center 90 of the photosensitive drum 20, the correction sensor 330, the yellow toner density sensor 310Y, and the cyan color sensor. The toner density sensor 310C, the magenta toner density sensor 310M, and the black (black) toner density sensor 310K are arranged in this order. In other words, the correction sensor 330 and the black toner density sensor 310K are arranged at positions farthest in the direction of the axis 90 of the photosensitive drum 20 among all the toner density sensors 310 and the correction sensor 330. Yes.

  Further, as shown in FIG. 1, the copying machine main body 100 is provided with a conveyance path 48 that guides the transfer paper 5 fed from the paper feeding device 200 to the paper discharge tray 7 via the secondary transfer roller 24. Along the conveyance path 48, a conveyance roller 49a, a registration roller 49b, a discharge roller 56, and the like are provided. A switching claw 55 is provided on the downstream side of the transport path 48 to switch the transport direction of the transfer paper 5 after transfer to the paper discharge tray 7 or the paper reversing device 93. The paper reversing device 93 reverses the transfer paper 5 and sends it again toward the secondary transfer roller 24. Further, the copying machine main body 100 is provided with a manual feed path 53 that joins from the manual feed tray 6 to the transport path 48, and the transfer paper 5 set in the manual feed tray 6 is located upstream of the manual feed path 53. A sheet feeding roller 50 and a separation roller 51 are provided for feeding sheets one by one.

  The paper feeding device 200 includes a plurality of paper feeding cassettes 44 that store the transfer paper 5, a paper feeding roller 42 and a separation roller 45 that feed the transfer papers stored in these paper feeding cassettes 44 one by one, and the fed transfer paper Is composed of a transport roller 47 that transports the paper along the paper feed path 46. The paper feed path 46 is connected to the conveyance path 48 of the copying machine main body 100.

  The scanner 300 will be briefly described with reference to FIG. In the scanner 300, first and second traveling bodies 33 and 34 mounted with a document illumination light source and a mirror reciprocate in order to read and scan a document (not shown) placed on the contact glass 31. To do. The image information scanned by the traveling bodies 33 and 34 is collected by the imaging lens 35 on the imaging surface of the reading sensor 36 installed behind the imaging lens 35 and read by the reading sensor 36 as an image signal.

  FIG. 6 is a block diagram showing the electrical connection of each part provided in the copying machine of the present embodiment. As shown in FIG. 6, the copying machine of the present embodiment is provided with a main control unit 500 having a computer configuration, and the main control unit 500 drives and controls each unit. The main control unit 500 includes a ROM (Read Only Memory) 503 that stores in advance fixed data such as a computer program via a bus line 502 in a CPU (Central Processing Unit) 501 that executes various calculations and drive control of each unit. A RAM (Random Access Memory) 504 functioning as a work area for storing various data in a rewritable manner is connected.

  The ROM 503 stores a conversion table (not shown) that stores information related to conversion of the toner density sensor 310 and the correction sensor 330 into the toner adhesion amount per unit area.

  Connected to the main controller 500 are each part of the copying machine main body 100, a paper feeding device 200, a scanner 300, and an automatic document feeder 400. Here, the toner density sensor 310, the potential sensor 320, and the correction sensor 330 of the copying machine main body 100 send the detected information to the main control unit 500.

  Next, the operation of the copying machine of this embodiment will be described. When copying a document using the copying machine having the above configuration, first, the document is set on the document table 30 of the automatic document feeder 400. Alternatively, the automatic document feeder 400 is opened, a document is set on the contact glass 31 of the scanner 300, and the automatic document feeder 400 is closed and pressed by it. Thereafter, when the user presses a start switch (not shown), when the document is set on the automatic document feeder 400, the document is conveyed onto the contact glass 31. Then, the scanner 300 is driven and the first traveling body 33 and the second traveling body 34 start traveling. Thereby, the light from the first traveling body 33 is reflected by the document on the contact glass 31, and the reflected light is reflected by the mirror of the second traveling body 34 and guided to the reading sensor 36 through the imaging lens 35. . In this way, the image information of the original is read.

  When the start switch is pressed by the user, a drive motor (not shown) is driven, and one of the support rollers 14, 15, 16 is rotationally driven to rotate the intermediate transfer belt 10. At the same time, the photosensitive drums 20Y, 20C, 20M, and 20K of the image forming units 18Y, 18C, 18M, and 18K are also rotationally driven. Thereafter, based on the image information read by the reading sensor 36 of the scanner 300, writing light is emitted from the exposure device 21 onto the photosensitive drums 20Y, 20C, 20M, and 20K of the image forming units 18Y, 18C, 18M, and 18K. Each is irradiated. As a result, electrostatic latent images are formed on the respective photosensitive drums 20Y, 20C, 20M, and 20K, and are visualized by the developing devices 61Y, 61C, 61M, and 61K. Then, yellow, cyan, magenta, and black toner images are formed on the photosensitive drums 20Y, 20C, 20M, and 20K, respectively.

  Each color toner image formed in this way is primarily transferred by the primary transfer devices 62Y, 62C, 62M, and 62K so as to sequentially overlap each other on the intermediate transfer belt 10. As a result, a composite toner image in which the toner images of the respective colors overlap is formed on the intermediate transfer belt 10. The transfer residual toner remaining on the intermediate transfer belt 10 after the secondary transfer is removed by the belt cleaning device 17.

  When the user presses the start switch, the paper feed roller 42 of the paper feed device 200 corresponding to the transfer paper 5 selected by the user rotates, and the transfer paper 5 is sent out from one of the paper feed cassettes 44. The transferred transfer paper 5 is separated into one sheet by the separation roller 45 and enters the paper feed path 46, and is conveyed by the conveyance roller 47 to the conveyance path 48 in the copying machine main body 100. The transfer sheet 5 thus transported is stopped when it abuts against the registration roller 49b. When the transfer paper 5 not set in the paper feed cassette 44 is used, the transfer paper 5 set on the manual feed tray 6 is fed out by the paper feed roller 50 and separated into one sheet by the separation roller 52, and then manually fed. It is conveyed through the paper path 53. Then, it stops when it hits the registration roller 49b.

  The registration roller 49 b starts to rotate in accordance with the timing at which the composite toner image formed on the intermediate transfer belt 10 as described above is conveyed to the secondary transfer unit facing the secondary transfer roller 24. Here, the registration roller 49b is generally used while being grounded, but a bias may be applied to remove the paper dust of the transfer paper 5. The transfer paper 5 sent out by the registration roller 49b is sent between the intermediate transfer belt 10 and the secondary transfer roller 24, and the secondary transfer roller 24 causes the composite toner image on the intermediate transfer belt 10 to be transferred onto the transfer paper 5. Secondary transfer is performed. Thereafter, the transfer paper 5 is conveyed to the fixing device 25 while being attracted to the secondary transfer roller 24, and heat and pressure are applied by the fixing device 25 to perform a toner image fixing process. The transfer paper 5 that has passed through the fixing device 25 is discharged to the paper discharge tray 7 by the discharge roller 56 and stacked. When image formation is performed also on the back surface of the surface on which the toner image is fixed, the transfer paper 5 that has passed through the fixing device 25 is switched by the switching claw 55 and sent to the paper reversing device 93. The transfer paper 5 is then reversed and guided to the secondary transfer roller 24 again.

  Next, potential control processing, which is image density control and self-check performed by the CPU 501 of the present embodiment based on a computer program, will be described with reference to FIGS. 7 is a graph showing the relationship between the toner adhesion amount on the photosensitive drum 20 and the sensor output, FIG. 8 is a graph showing the relationship between the surface roughness of the photosensitive drum 20 and the sensor output, and FIG. 9 is a potential control. FIG. 10 is a diagram illustrating a patch pattern formed on the photosensitive drum 20, FIG. 11 is a plan view illustrating a patch pattern transferred onto the intermediate transfer belt 10, and FIG. 12 illustrates the patch pattern. FIG. 15 is a graph showing the relationship between the potential data at the time of potential control and the toner adhesion amount data in each patch pattern. FIG. 16 is a linear approximation of the potential data and the control potential data with respect to the toner adhesion amount data at the potential control. FIG. 17 is a schematic diagram showing a potential control table.

  The potential control process is a process for controlling the potential based on the detection result of the toner density sensor 310, but the output characteristics of the toner density sensor 310 may change due to various factors. Therefore, in the potential control process of the present embodiment, the detection result of the toner density sensor 310 is corrected based on the detection result of the correction sensor 330, and the process is executed using the corrected detection result.

  Here, a change in the output characteristics of the toner density sensor 310 will be described with an example. When the photosensitive drum 20 is new, the sensor characteristic shows a line a in FIG. However, when the surface of the photoconductive drum 20 becomes rough with time, the light reflection on the surface of the photoconductive drum 20 increases as shown in FIG. 8, thereby increasing the sensor output of the toner density sensor 310. . As a result, the sensor output characteristics are as shown by a line b in FIG. In the copying machine according to the present embodiment, Vsg adjustment is periodically performed so that the output of the toner density sensor 310 with respect to the photosensitive drum 20 becomes constant as will be described later. It is adjusted to be equivalent to the line a. At this time, since the LED light emission amount of the toner density sensor 310 is adjusted so as to decrease the output of the toner density sensor 310, the sensor output characteristic of the toner density sensor 310 is a characteristic as indicated by a line c in FIG. It becomes. As a result, for the same sensor output, the characteristic changes from line a to line c, so that the calculated toner adhesion amount changes. If this change occurs at the same time for all colors, the color balance does not change greatly. However, in the case of a configuration having different photosensitive drums 20 for each color as in the copying machine of the present embodiment, each of the photosensitive drums 20 The method of surface degradation is also different. Further, even if the surface of the photosensitive drum 20 is deteriorated in the same manner, the calculated toner adhesion amount varies depending on the error in each color of the voltage after Vsg adjustment.

  In the potential control routine shown in FIG. 9, basically, at the time of starting the copying machine, a predetermined number of copies is made for each copy (that is, between the image forming operation and the image forming operation during the continuous image forming operation). This is done as needed, such as every hour. Here, the execution operation at the time of activation will be described. First, in order to distinguish the power-on state from an abnormal process such as a jam, the fixing temperature of the fixing device 25 is detected as a potential control execution condition in step S701. Based on the input signal from the fixing temperature sensor, it is determined whether or not the fixing temperature of the fixing device 25 exceeds 100 ° C. If the fixing temperature of the fixing device 25 exceeds 100 ° C. (in step S701). N), it is determined as abnormal, and the process is terminated without executing the potential control.

  If the fixing temperature of the fixing device 25 does not exceed 100 ° C. (Y in step S701), the surface potential of each photosensitive drum 20 uniformly charged under a predetermined condition is checked by the potential sensor 320 (step S701). Next, Vsg adjustment is performed in step S703 (step S703). In this Vsg adjustment, an output value for the background portion (surface) of the photosensitive drum 20 is taken from the toner concentration sensor 310, and the reflected light of the light irradiated from the toner concentration sensor 310 to the background portion of the photosensitive drum 20 becomes a constant value. The light emission amount of the toner density sensor 310 is adjusted so that Here, in steps S702 to S703, the image forming units 18 of the respective colors perform parallel processing.

  In step S704, it is checked whether there is any abnormality in the processing in steps S702 to S703. If there is an abnormality (Y in step S704), the process proceeds to step S718, an error code is set, and the process ends.

  If it is determined in step S704 that there is no abnormality in the processing in steps S702 to S703 (Y in step S704), it is determined whether the potential control method is set to automatic or fixed instead of automatic. (Step S705).

  In steps S703 to S704, the operation is performed prior to step S706 for use in other toner supply control regardless of the potential control method.

  If it is determined in step S705 that the potential control method is not automatic but fixed (N in step S705), an error code is set in step S718, and the process ends. On the other hand, if it is determined in step S705 that the potential control method is automatic (Y in step S705), the processes in steps S706 to S707 are performed on the image forming units 18 of the respective colors in parallel.

  In step S706, as shown in FIG. 5, a patch pattern (latent image pattern) 600 is formed on each photosensitive drum 20 as a reference toner patch that is a toner image. The patch pattern 600 is formed by shifting each color with respect to the direction of the axis 90 (width direction) of the photosensitive drum 20. In this embodiment, N patch patterns 600 (600a, 600b, 600c,...) That are electrostatic latent images having N gradation densities are exposed for each color, for example, as shown in FIG. They are formed at predetermined intervals along the rotation direction of the body drum 20. In the present embodiment, 16 rectangular patch patterns 600 (600a, 600b, 600c,...) Each having a different gradation density and each side of 15 × 20 mm are arranged in the rotational direction of the photosensitive drum 20. Are formed at intervals of 10 mm. Further, the interval between the patch patterns 600 for these colors in the direction of the axis 90 of the photosensitive drum 20 is set to 5 mm (see FIG. 12). In this way, the patch patterns 600 are formed close to each other so that the overall width thereof is within the A6 width.

  Further, as shown in FIG. 5, in addition to these patch patterns 600, a patch pattern 601 which is a reference toner for correction is formed on the photosensitive drum 20 for each color. These patch patterns 601 are formed so as to be shifted from the patch pattern 600 in the direction of the axis 90 of the photosensitive drum 20. Specifically, the patch pattern 601 is formed outside the region occupied by the patch patterns 600Y, 600C, 600M, and 600K in the direction of the axial center 90 of the photosensitive drum 20 when transferred to the intermediate transfer belt 10. At this time, the arrangement of the patch patterns 600 and 601 in the axial direction of the photosensitive drum 20 is the patch pattern 601, the patch pattern 600Y, the patch pattern 600C, the patch pattern 600M, and the patch pattern 600K. Note that the size of the patch pattern 601 and the interval between them are the same as those of the patch pattern 600, and the image forming conditions of the patch pattern 601 are the same as the image forming conditions of the patch pattern 600. Here, the function of the toner patch forming means is executed in step S706.

  By forming the patch patterns 600 and 601 in this way, when the patch patterns 600 and 601 are transferred to the intermediate transfer belt 10, as shown in FIG. 601 is formed without overlapping. Here, FIG. 12 shows an enlarged view of patch patterns 600 and 601 on the intermediate transfer belt 10.

  In the next step S 707, the output value of the potential sensor 320 with respect to the potential of these patch patterns 600 on the photosensitive drum 20 is read and stored in the RAM 504. Then, the patch patterns 600 and 601 for the four colors on the photosensitive drum 20 are developed in the black developing device 61K, the cyan developing device 61C, the magenta developing device 61M, and the yellow developing device 61Y in a parallel operation to be visualized. Thus, a toner image of each color is obtained.

  Next, the CPU 501 performs toner density detection by the toner density sensor 310 for the patch pattern 600 of the photosensitive drum 20 and density detection by the correction sensor for the patch pattern 601 transferred to the intermediate transfer belt 10 (step S708). . In this toner density detection, the output values of the toner density sensor 310 and the correction sensor 330 for the patch patterns 600 and 601 that are toner images of the respective colors are stored in the RAM 504 as Vpi (i = 1 to N) for each color.

  Next, the toner adhesion amount is calculated (step S709). That is, referring to the conversion table stored in the ROM 503, the output values of the toner density sensor 310 and the correction sensor 330 stored in the RAM 504 are converted into the toner adhesion amount per unit area and stored in the RAM 504 again.

Next, the toner adhesion amount on the photosensitive drum 20 based on the detection value of the toner density sensor 310 calculated in step S709 is corrected based on the detection value of the correction sensor 330 (step S710, correction means). First, the previously determined toner adhesion amount on the photosensitive drum 20 (hereinafter also referred to as toner adhesion amount on the drum) and toner adhesion amount on the intermediate transfer belt 10 (hereinafter also referred to as toner adhesion amount on the belt). The relationship between the four toner density sensors 310Y, 310C, 310M, and 310K is that the coefficient corresponding to the deviation from the true value of the toner adhesion amount is A, and the toner density from the photosensitive drum 20 to the intermediate transfer belt 10 is A. When the transfer rate (%) is α, it is expressed by the following formula.
Amount of toner on drum (K) = Amount of toner on belt (K) / α ----------- (1)
Amount of toner on drum (M) = Amount of toner on belt (M) / α ----------- (2)
Toner adhesion amount on drum (C) = Toner adhesion amount on belt (C) / α ----------- (3)
Amount of toner on drum (Y) = Amount of toner on belt (Y) / α ----------- (4)

However, since each detection error is added to the actual detection result, it is as follows. Here, a coefficient corresponding to a deviation from each true toner adhesion amount with respect to the correction sensor 330 is B.
Amount of toner on drum (K) x A (K) = Amount of toner on belt (K) x B (K) / α ----- (5)
Amount of toner on drum (M) x A (M) = Amount of toner on belt (M) x B (M) / α ----- (6)
Amount of toner on drum (C) x A (C) = Amount of toner on belt (C) x B (C) / α ----- (7)
Amount of toner on drum (Y) x A (Y) = Amount of toner on belt (Y) x B (Y) / α ----- (8)

Regarding B, since the correction sensor 330 is single, it can be said that there is no difference between colors. The transfer rate α is about 95% in the present embodiment. So if we consider B = 1,
Toner adhesion on drum (K) x A (K) = Toner adhesion on belt (K) / α --------- (9)
Amount of toner on drum (M) x A (M) = Amount of toner on belt (M) / α --------- (10)
Amount of toner on drum (C) x A (C) = Amount of toner on belt (C) / α --------- (11)
Amount of toner on drum (Y) x A (Y) = Amount of toner on belt (Y) / α --------- (12)
It becomes.

  Next, based on these formulas, the toner adhesion amount on the photosensitive drum 20 is corrected from the toner adhesion amount on the photosensitive drum 20 and the toner adhesion amount on the intermediate transfer belt 10 detected so far. The corrected toner adhesion amount is obtained. First, black (K) will be described as an example.

  In black (K), the output of the toner density sensor 310K and the correction sensor 330 is saturated in the high toner adhesion amount portion, so that the patch patterns 600 and 601 are created so as to obtain data of the low toner adhesion region. Set the image conditions and create an image. Table 1 shows the detection results of the toner density sensor 310K and the correction sensor 330 at this time. The result is shown in a graph in FIG.

Here, A (K) = 1.0497 is obtained.

  Next, Table 2 shows the detection results of the toner density sensor 310M and the correction sensor 330 regarding magenta (M). FIG. 14 shows the result as a graph.

Here, A (M) = 1.025 is obtained. Similarly, A (C) = 1.057 and A (Y) = 1.002.

In the present embodiment, the toner adhesion amount on the photosensitive drum 20 is calculated based on the following formula obtained by modifying the above formulas (9) to (12).
Toner adhering amount on drum (K) = Toner adhering amount on belt (K) / α / A (K) --------- (13)
Toner adhesion amount on drum (M) = Toner adhesion amount on belt (M) / α / A (M) --------- (14)
Amount of toner on drum (C) = Amount of toner on belt (C) / α / A (C) --------- (15)
Toner adhesion amount on drum (Y) = Toner adhesion amount on belt (Y) / α / A (Y) --------- (16)

If the toner adhesion amount on the belt is a true value, the corrected toner adhesion amount on the drum can be obtained by the following equation.
Amount of toner adhesion on drum after correction (K) = Amount of toner adhesion on drum (K) x α x A (K)-(17)
Toner adhering amount on drum after correction (M) = Toner adhering amount on drum (M) x α x A (M) --- (18)
Amount of toner adhering to drum after correction (C) = Amount of toner adhering to drum (C) x α x A (C) ---- (19)
Amount of toner adhesion on drum after correction (Y) = Amount of toner adhesion on drum (Y) x α x A (Y) --- (20)

For example, when the toner adhesion amount detection result for (M) on the photosensitive drum 20 is 0.506, the corrected toner adhesion amount on the photosensitive drum 20 is 0.506 × 1.025 × 0.95 = 0.493 according to the equation (18). (mg / cm ² ).

  Here, another form of correction will be described. In this embodiment, in consideration of the case where the detection accuracy of the color toner adhesion amount detection on the intermediate transfer belt 10 is low, the color adhesion amount is set based on (M) toner. This maintains the balance of color adhesion detection. Specifically, A (M) = 1, A (C) '= A (C) / A (M) = 1.057 / 1.025 = 1.031, A (Y)' = A (Y) / A (M) = 1.002 / 1.025 = 0.978. In the embodiment, (K) correction which does not affect the color balance is not performed.

Here, when the toner adhesion amount on the intermediate transfer belt 10 of (M) is considered as a reference, the toner adhesion amount on the photosensitive drum 20 after correction is obtained by the following equation.
Toner adhering amount on drum after correction (M) = Toner adhering amount on drum (M) x α x A (M) / A (M)-(21)
Toner adhering amount on drum after correction (C) = Toner adhering amount on drum (C) x α x A (C) / A (M)-(22)
Amount of toner adhesion on drum after correction (Y) = Amount of toner adhesion on drum (Y) x α x A (Y) / A (M)-(23)

For example, when the toner adhesion amount detection result of (C) on the photosensitive drum 20 is 0.456, the corrected toner adhesion amount on the drum of (C) is 0.456 × 1.031 × 0.95 = 0.447 (0.447) according to the equation (22). mg / cm ² ).

  In this way, after correcting and calculating the toner adhesion amount of each color by the above two correction methods, steps S711 to S713 are executed. Hereinafter, these steps will be described in detail.

Here, FIG. 15 shows the relationship in the patch patterns 600 (600a, 600b, 600c,...) Between the potential data of the patch pattern 600 on the photosensitive drum 20 and the toner adhesion amount data obtained by the above processing. It is plotted on the XY plane. The X axis indicates the potential (difference between the developing bias potential VB and the surface potential of the photosensitive drum 20) (unit V), and the Y axis indicates the toner adhesion amount (mg / cm ² ) per unit area. In this embodiment, as described above, the toner density sensor 310 is configured by an optical sensor such as an infrared light reflection type sensor. The infrared light reflection type sensor is generally shown in FIG. As shown, saturation characteristics are exhibited in a multi-adhesion portion where the toner adhesion amount is large, and the obtained detection value does not correspond to the actual toner adhesion amount. For this reason, if the toner adhesion amount is calculated by using the detection value of the toner density sensor 310 obtained in the multiple adhesion portion as it is, an adhesion amount different from the actual adhesion amount is obtained, and this toner adhesion is obtained. The toner replenishment control based on the amount cannot be performed accurately.

  Therefore, the CPU 501 according to the present embodiment uses the patch pattern 600 (600a, 600b, 600c) obtained from the potential sensor 320 and the toner density sensor 310 for each color patch pattern 600 (600a, 600b, 600c...). )) And the toner adhesion amount data after the visualization, as described later, the relationship between the potential data Xn (n = 1 to 10) and the toner adhesion amount data Yn (development of the developing device 61) By selecting only a straight section of (γ characteristics) and applying the least square method to the data in this section, a linear approximation of the developing characteristics of each developing device 61 is performed by a method as described later, and an approximate linear equation of the developing characteristics is obtained. Is obtained for each color, and the control potential is calculated for each color by this approximate linear equation.

The following formula is used for the calculation of the least square method.
Xave = ΣXn / k -------- (31)
Yave = ΣYn / k --------- (32)
Sx = Σ (Xn-Xave) × (Xn-Xave) ---------- (33)
Sy = Σ (Yn-Yave) × (Yn-Yave) ---------- (34)
Sxy = Σ (Xn-Xave) × (Yn-Yave) -------- (35)
An approximate linear equation obtained from the data of the potential of the patch pattern 600 (600a, 600b, 600c...) Obtained from the potential sensor 320 and the toner density sensor 310 and the toner adhesion amount after the visualization is expressed as Y = A1 × X + B1. Where the coefficients A1 and B1 use the above variables,
A1 = Sxy / Sx ------------------ (36)
B1 = Yave-A1 × Xave ----- (37)
It can be expressed.

The correlation coefficient R of the approximate linear equation is
R × R = (Sxy × Sxy) / (Sx × Sy) -------- (38)
It can be expressed as In the present embodiment, until step S709, the CPU 501 has the potential data Xn of the patch pattern 600 (600a, 600b, 600c...) Obtained from the potential sensor 320 and the toner density sensor 310 for each color, and the visible image. A set of five data from the younger numerical value of the toner adhesion amount data Yn after conversion,
(X1-X5, Y1-Y5)
(X2-X6, Y2-Y6)
(X3-X7, Y3-Y7)
(X4 to X8, Y4 to Y8)
(X5 to X9, Y5 to Y9)
(X6-X10, Y6-Y10)
And performing linear approximation calculation according to the above equations (31) to (38) and calculating the correlation coefficient R to obtain the following six sets of approximate linear equations and correlation coefficients (39) to (44) Get.
Y11 = A11 × X + B11; R11 --------- (39)
Y12 = A12 × X + B12; R12 -------- (40)
Y13 = A13 × X + B13; R13 -------- (41)
Y14 = A14 × X + B14; R14 -------- (42)
Y15 = A15 × X + B15; R15 -------- (43)
Y16 = A16 × X + B16; R16 -------- (44)
The CPU 501 selects, as an approximate linear equation, one set of approximate linear equations corresponding to the maximum value in the correlation coefficients R11 to R16 from the obtained six sets of approximate linear equations.

Next, in step S711, the main control unit 500 (CPU 501) uses the approximate linear equation selected for each color in the above-described approximate linear equation, as shown in FIG. 16, when the Y value becomes the required maximum toner adhesion amount Mmax. A value, that is, a development potential value Vmax is calculated. The developing bias potential VB of the black developing device 61K, the cyan developing device 61C, the magenta developing device 61M, and the yellow developing device 61Y and the surface potential (exposure potential) VL by image exposure of each color on the photosensitive drum 20 are obtained from the above formula. It is given by the following equations (45) and (46).
Vmax = (Mmax-B1) / A1 ------ (45)
VB-VL = Vmax = (Mmax-B1) / A1 ------ (46)
The relationship between VB and VL can be expressed using the coefficient of the approximate linear method (E). Therefore, equation (46) is
Mmax = A1 * Vmax + B1 --------- (47)
It becomes.

Here, the relationship between the charging potential VD before exposure of the photosensitive drum 20 and the developing bias potential VB is a linear equation as shown in FIG.
Y = A2 * X + B2 -------- (48)
From the X coordinate VK (development start voltage of the developing device 61) of the intersection of the X axis and the scumming margin voltage Vα obtained experimentally,
VD-VB = VK + Vα -------- (49)
Given in.

  Therefore, the relationship between Vmax, VD, VB, and VL is determined by the equations (46) and (49). In this example, using Vmax as a reference value, the relationship between this and each control voltage VD, VB, VL is obtained in advance through experiments or the like, tabulated as shown in FIG. 17, and stored in the ROM 503 as a potential control table T1.

  In step S712, the CPU 501 selects Vmax closest to the calculated Vmax for each color from the potential control table T1, and sets each control voltage (potential) VB, VD, VL corresponding to the selected Vmax as a target potential. To do.

  Next, in step S712, the laser emission power of the semiconductor laser is controlled to the maximum light amount via the laser control unit of the exposure apparatus 21, and the residual potential of the photosensitive drum 20 is acquired by taking in the output value of the potential sensor 320. Is detected (step S713). In step S714, when the residual potential is not zero, the target potentials VB, VD, and VL determined in step S712 are corrected to the target potential.

  In step S715, it is determined whether or not there is an error in steps S705 to S714. If there is an error even for one color (N in step S715), the image density fluctuation becomes large even if only the other colors are controlled, and the operation in step S716 to be performed thereafter is wasted. A code is set (step S718), and the process ends. In this case, the image forming conditions are not updated, and image forming is performed under the same image forming conditions as before until the next self-check is successful.

  If it is determined in step S715 that there is no error (Y in step S715), in step S716, a power supply circuit (FIG. 5) is arranged so that the charging potential VD by the charging device 60 of the photosensitive drum 20 becomes the target potential in parallel with each color. (Not shown), the laser emission power of the semiconductor laser is adjusted via a laser control unit (not shown) so that the surface potential VL of the photosensitive drum 20 becomes the target potential, and the black developing device 61K. The power supply circuit is adjusted so that the developing bias potentials VB of the cyan developing device 61C, the magenta developing device 61M, and the yellow developing device 61Y become the target potential, respectively (image density control means).

  In step S717, it is determined whether or not there is an error in step S716. If there is no error in step S716 (Y in step S717), the process ends. On the other hand, if there is an error in step S716 (N in step S717), the process proceeds to step S718, an error code is set, and the process is terminated.

  Such potential control is an important control particularly in a color image forming apparatus in order to maintain a constant image quality.

  As described above, in the present exemplary embodiment, even if the detection characteristics of the toner density sensors 310Y, 310C, 310M, and 310K provided for each of the plurality of photosensitive drums 20 are different, the detection result is the intermediate transfer belt. 10 is corrected based on the detection result of the correction sensor 330 for detecting the toner density on the image 10, so that it is possible to prevent the occurrence of an image density defect due to a difference in detection characteristics among the plurality of toner density sensors 310Y, 310C, 310M, and 310K. And a copying machine with a stable image density can be provided. As a result, it is possible to provide a copying machine having a stable image density over time even when the environment changes.

  In the present embodiment, the patch patterns 600 corresponding to the photosensitive drums 20 for executing the potential control that is the image density control are shifted from each other on the intermediate transfer belt 10 as shown in FIG. As a result, the load applied to the belt cleaning device 17 and the roller cleaning unit 91 is reduced, so that it is possible to prevent the occurrence of cleaning failure and toner scattering due to transfer scattering in each transfer process. . In addition, since the patch patterns 600 of the respective colors can be formed in parallel on the respective photosensitive drums 20, it is possible to reduce the waiting time of the user when executing the potential control.

  Further, in the present embodiment, the patch pattern 600 and the patch pattern 601 can be formed in parallel, thereby executing image density control such as potential control accompanying the creation of the patch pattern 600. User waiting time can be reduced.

  In the present embodiment, the toner density is corrected based on the patch patterns 600 and 601 having the same image forming conditions, so that the correction accuracy can be increased.

  In the present embodiment, the toner density sensor 310K corresponding to black (K) is disposed at a position farther from the correction sensor 330 than the other toner density sensors 310Y, 310C, 310M. The positional relationship between the correction sensor 330 and the toner density sensors 310Y, 310C, 310M corresponding to the other color (Y, C, M) toners that greatly contribute to the color fluctuation compared to the (K) toner is black. Since the positional relationship between the toner density sensor 310K corresponding to (K) and the correction sensor 330 is closer, this can reduce the difference due to the image forming positions of the patch patterns 600 and 601 and increase the correction accuracy. it can.

  In the present embodiment, since the entire patch pattern 600 for each color is within the A6 width, an image that is normally output is A6 width or larger, and this causes a variation in the position of the patch pattern 600 for each color. Therefore, the detection error for each color due to the detection position is reduced, and a high-speed and high-precision copying machine can be provided.

  In the present embodiment, in the potential control process that is the image density control, the outputs of the potential sensor 320 and the toner density sensor 310 are processed in parallel for the four colors, so that an independent sampling cycle of 4 msec is applied to each channel. A / D is adopted.

  In this embodiment, the copying machine is described as an example of the image forming apparatus. However, the present invention is not limited to this, and a printer or the like may be used.

1 is a schematic configuration diagram illustrating an entire copying machine according to an embodiment of the present invention. FIG. 2 is an enlarged view showing a configuration of a copying machine main body. FIG. 3 is a cross-sectional view illustrating a structure of an intermediate transfer belt. 2 is an enlarged view showing a configuration of two adjacent image forming units. FIG. FIG. 3 is a plan view mainly showing a toner density sensor, a potential sensor, and a photosensitive member. FIG. 2 is a block diagram showing electrical connections of respective units provided in the copying machine. 6 is a graph showing the relationship between the toner adhesion amount on the photosensitive drum and the sensor output. It is a graph which shows the relationship between the surface roughness of a photoreceptor drum, and a sensor output. It is a flowchart which shows an electric potential control routine. It is explanatory drawing for demonstrating the patch pattern formed on a photoconductor drum. FIG. 3 is a plan view showing a patch pattern transferred onto an intermediate transfer belt. It is an enlarged view which shows the patch pattern. 6 is a graph showing the relationship between the toner adhesion amount on the photosensitive drum and the toner adhesion amount on the intermediate transfer belt for black. 6 is a graph showing a relationship between a toner adhesion amount on a photosensitive drum and a toner adhesion amount on an intermediate transfer belt with respect to magenta. 6 is a graph showing a relationship between potential data and toner adhesion amount data for each patch pattern during potential control. 6 is a graph showing linear approximation of potential data and control potential data with respect to toner adhesion amount data during potential control. It is a schematic diagram which shows an electric potential control table.

Explanation of symbols

10 Intermediate transfer belt (intermediate transfer member)
20 Photosensitive drum (image carrier)
90 Axle 310 Toner concentration sensor (first detector)
330 Detector for correction (second detector)
600, 601 Patch pattern (reference toner patch)
S706 Toner patch forming means S710 Correction means S716 Image density control means

Claims (5)

In an image forming apparatus that obtains a color image by transferring a toner image formed by electrophotography on a plurality of rotationally driven image carriers to an intermediate transfer member,
A plurality of first detectors that are provided to face the image carrier for each image carrier and optically detect the density of a toner image formed on the opposite image carrier;
A second detector provided opposite to the intermediate transfer member and optically detecting the density of the toner image transferred to the intermediate transfer member;
A toner patch forming means for forming a reference toner patch, which is a toner image, on each of the image carriers and transferring it to an intermediate transfer member;
Correction means for correcting the detection result of the first detector based on the detection result of the second detector;
Image density control means for performing image density control based on the corrected detection result of the first detector;
An image forming apparatus comprising: At least two of the first detectors are arranged offset from each other in the axial direction of the image carrier;
The image forming apparatus according to claim 1, wherein the toner patch forming unit forms the reference toner patch in accordance with an arrangement of the first detector. The second detector is arranged so as to be shifted in the axial direction of the image carrier with respect to all the first detectors,
The image forming apparatus according to claim 1, wherein the toner patch forming unit forms the reference toner patch in accordance with an arrangement of the first and second detectors.   The toner patch forming means forms the reference toner patch detected by the second detector under the same image forming condition as at least one of the reference toner patches detected by the plurality of first detectors. The image forming apparatus according to claim 1, 2 or 3. The image carrier is provided for each of a plurality of toner colors including black,
The first detector and the second detector corresponding to the image carrier on which a black toner image is formed are among all the first detectors and the second detectors. The image forming apparatus according to claim 1, wherein the image forming apparatus is disposed at a position farthest in the axial direction of the image carrier.

Images