JP2010134160A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of performing density correction to lessen influence of density irregularity occurring in a periphery direction of an image carrier due to variance of characteristic in the periphery direction and eccentricity of the image carrier, an electrifying member and a developer carrier. <P>SOLUTION: A density pattern K1 is formed all over the developing area in a width direction in order to correct the density irregularity in a shaft direction (main-scanning direction) of a photoreceptor drum 1d. The density pattern K1 is formed at a position that is a 2/3-fold cycle of a rotational cycle of the photoreceptor drum 1d. Thus, the influence of the density irregularity occurring in the rotational cycle of the photoreceptor drum is lessened, and the density correction is accurately performed in both of the shaft direction and the periphery direction (sub-scanning direction). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus including an image reading unit such as a scanner.

  In an image forming apparatus using an electrophotographic system such as a copying machine, a printer, or a FAX, the surface of an image carrier such as a photosensitive drum is uniformly charged by a charging device, and the image is carried by exposure (light irradiation) from an exposure device. In general, the electrostatic latent image formed on the body is visualized by a developing device, the toner image is transferred onto a recording medium, and then a fixing process is performed.

  Here, the photosensitive drum on which the electrostatic latent image is formed has uneven charging characteristics and sensitivity characteristics in the axial direction of the drum surface due to various factors in manufacturing. In addition, when forming a toner image obtained by developing an electrostatic latent image, it is affected by variations in the gap with the developer carrier that supplies toner to the photosensitive drum and variations in the amount of light in the main scanning direction of the exposure means. Thus, density unevenness occurs in the axial direction. Therefore, a method of controlling the surface potential state and density state in the axial direction of the photosensitive drum by changing the exposure amount of the exposure apparatus in the axial direction (main scanning direction) is performed.

  In density control, when the apparatus is started up or when a mode (calibration mode) for appropriately setting image density (gradation) is set, toner is transferred onto a toner carrier or a recording medium and a test pattern ( (Standard image) is formed, and its density is detected to perform tone correction. For example, in the case of a color image forming apparatus, each magenta, cyan, yellow, and black image forming unit forms a test pattern of each color on a toner carrier or a recording medium, and detects the density of each density pattern constituting the test pattern. Then, gradation correction is performed.

  Here, when the test pattern is formed on the recording medium, since the actual image density after the transfer process and the fixing process can be detected, the test pattern is higher than when the test pattern is formed on a toner carrier such as an intermediate transfer belt. Accurate correction is possible. For example, in Patent Document 1, a gradation pattern output on a recording medium is read by an image reading device such as a scanner, a light amount correction amount is calculated from a density difference between the read density and a target density, and image forming conditions are set. An image forming apparatus that improves the stability of image quality by providing feedback is disclosed.

  However, the photosensitive drum has uneven charging characteristics and sensitivity characteristics not only in the axial direction but also in the circumferential direction due to manufacturing problems. Further, when a charging roller (charging member) that is charged in contact with the drum surface as a charging device is used, circumferential charging unevenness also occurs due to eccentricity of the charging roller. Further, uneven transfer in the drum circumferential direction occurs due to fluctuations in the rotation cycle of the intermediate transfer member and the transfer roller.

  On the other hand, when developing an electrostatic latent image, not only in the axial direction but also in the circumferential direction due to the influence of the eccentricity of the developer carrier that supplies toner to the drum surface and the toner charge amount on the developer carrier. Uneven development occurs. In particular, in a developing device that forms only a toner layer on a developer carrier, the toner charge amount on the developer carrier differs between a region where the toner was developed before one rotation and a region where the toner was not developed before one rotation. Differences in development performance occur in these areas, and uneven density tends to occur.

  When density unevenness occurs in the drum circumferential direction for the reasons described above, in the method of Patent Document 1 in which density control in the axial direction is performed using a test pattern, the photosensitive drum, the charging roller, or the developer carrier is used. The density of the test pattern changes depending on the rotation position, and the density detection results vary. As a result, the accuracy of light amount correction is reduced, and it is difficult to ensure image uniformity after correction.

In view of this, a method has been proposed in which density unevenness of the test pattern in the circumferential direction of the photosensitive drum is reduced and density correction is performed with high accuracy. A method is disclosed in which density correction is performed using a test pattern formed by repeating a density pattern N times at a position shifted by 360 / N degrees (1 / N of the circumference) of any rotational phase difference.
JP-A-10-171220 JP 2006-343679 A

  However, in the method of Patent Document 2, when the same test pattern is formed a plurality of times, the density pattern in each test pattern is formed at the same phase position of the image carrier, the charger, or the developer carrier. Therefore, the uneven density in the circumferential direction due to the difference in the circumferential characteristics of the image carrier, the charger, and the developer carrier cannot be removed.

  Even when only one test pattern is formed, the amorphous silicon (a-Si) photoconductor may cause a film formation unevenness of ½ period in production. When = 2, periodic concentration fluctuations could not be removed. On the other hand, when N = 3 or more, the pitch of each density pattern is too narrow on a photoconductor, developer carrier, charging roller, transfer roller, etc. whose diameter has been reduced. There was a problem that the number was limited.

  Further, in the developing device, the toner charge amount differs between the area where the toner was developed before one rotation on the developer carrier and the area where the toner was not developed. For the development of the density pattern, it is preferable to use a region on the developer bearing member where the toner was not developed before one rotation.

  On the other hand, when the same test pattern consisting of a plurality of density patterns with different density input values (tones) or density patterns of a plurality of colors is output to read the image density unevenness and to perform density correction in the photoconductor axis direction, If each density pattern is formed at a different position with respect to the circumferential direction of the photoconductor, uneven charging in the circumferential direction of the photoconductor affects the density of the test pattern, reducing the accuracy of the light amount correction and reducing the density correction. The uniformity of the image will be deteriorated. Also, when the test pattern is formed, the gap between the charger or developer carrier and the photosensitive drum changes if the circumferential position of the charging member or developer carrier faces the photosensitive drum. In the same manner, the uniformity of the image is deteriorated.

  In view of the above problems, the present invention provides a density that reduces the influence of density unevenness generated in the circumferential direction of the image carrier due to variation in characteristics and eccentricity of the image carrier, charging member, and developer carrier in the circumferential direction. An object of the present invention is to provide an image forming apparatus capable of executing correction. Another object of the present invention is to provide an image forming apparatus with small density fluctuation between test patterns when outputting a plurality of test patterns composed of a plurality of density patterns having different density input values or different colors. is there.

  In order to achieve the above object, the present invention provides an electrostatic latent image by exposing an image carrier, a charging member for charging the surface of the image carrier, and the surface of the image carrier charged by the charging member. An image forming unit having an exposure device to be formed and a developing device having a developer carrier disposed opposite to the image carrier and developing an electrostatic latent image formed by the exposure device; An image reading unit capable of reading the density of the test pattern formed on the recording medium in the forming unit, and the image density for each block in the axial direction of the image carrier based on the result of reading the test pattern by the image reading unit. In the image forming apparatus including the density correction unit for correction, the test pattern includes a plurality of density patterns formed with the same pixel configuration, and each density pattern formed with the same pixel configuration is At least one rotation period of at least one of the image carrier, the charging member, and the developer carrier (M and N are relatively prime integers, M ≧ 2, N ≧ 3). It is characterized by being arranged.

  According to the present invention, in the image forming apparatus configured as described above, the test pattern includes a multiple of N density patterns formed with the same pixel configuration.

  Further, according to the present invention, in the image forming apparatus having the above configuration, when outputting a plurality of the test patterns including a plurality of types of density patterns having different pixel configurations, the density patterns formed with the same pixel configuration on the different test patterns. The formation position of the electrostatic latent image is substantially the same region with respect to at least one circumferential direction of the image carrier, the charging member, and the developer carrier.

  In the image forming apparatus having the above-described configuration, the image forming apparatus further includes a position detection mechanism that detects a rotational position of at least one of the image carrier, the charging member, and the developer carrier, and a detection result of the position detection mechanism. The test pattern formation position is controlled based on the above.

  According to the present invention, in the image forming apparatus configured as described above, the image carrier is an amorphous silicon photoconductor.

  According to the present invention, in the image forming apparatus having the above-described configuration, the test pattern is formed of a plurality of color density patterns, and the same color density pattern is formed at a position approximately M / N times the rotation period of the developer carrier. The density pattern of each color in the test pattern is a developer in an area that does not overlap in the circumferential direction on the developer carrier with the density pattern of the same color formed before one rotation of the developer carrier. It is characterized by being formed.

  According to the first configuration of the present invention, when forming a test pattern composed of a plurality of density patterns, each density pattern formed with the same pixel configuration is selected from an image carrier, a charging member, and a developer carrier. In the case of calculating the average density of each density pattern by disposing at approximately M / N times (M and N are prime integers, M ≧ 2, N ≧ 3) at least one rotation period. The influence of density unevenness in the circumferential direction (sub-scanning direction) generated in the rotation cycle of the image carrier, charging member, or developer carrier can be reduced, and density correction can be performed accurately in both the axial direction and the circumferential direction. It can be carried out.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, by forming a test pattern including a multiple of N density patterns formed with the same pixel configuration, the same pixel configuration can be obtained. Since the density pattern is formed using the entire circumferential direction of the photosensitive drum, the average density of each density pattern can be made closer to the theoretical value.

  Further, according to the third configuration of the present invention, when outputting a plurality of test patterns including a plurality of types of density patterns having different pixel configurations in the image forming apparatus having the first or second configuration, different test patterns are provided. By making the electrostatic latent image formation position of each density pattern formed with the same pixel configuration above substantially the same region with respect to at least one circumferential direction of the image carrier, the charging member, and the developer carrier. The density unevenness caused by the difference in the circumferential characteristics of the image carrier, the charging member, or the developer carrier generated between a plurality of test patterns can be minimized.

  According to the fourth configuration of the present invention, in the image forming apparatus having any one of the first to third configurations, the rotational position of at least one of the image carrier, the charging member, and the developer carrier is detected. By controlling the test pattern formation position based on the detection result of the position detection mechanism, the test pattern can be formed at an accurate position according to the rotation period of the image carrier, the charging member, or the developer carrier. it can.

  According to the fifth configuration of the present invention, in the image forming apparatus having any one of the first to fourth configurations, an amorphous silicon photoconductor that easily generates circumferential film formation irregularities during manufacture is used. Thus, it is possible to eliminate periodic density fluctuations in the test pattern caused by the difference in sensitivity characteristics in the circumferential direction.

  According to the sixth configuration of the present invention, in the image forming apparatus having any one of the first to fifth configurations, the test pattern includes a plurality of color density patterns, and the same color density pattern corresponds to the developer. When formed at a position approximately M / N times the rotation period of the carrier, the density pattern of each color in the test pattern is the same as the density pattern of the same color formed one rotation before the developer carrier. By forming with a developer in a region that does not overlap in the circumferential direction on the body, the charge amount of the toner used for forming the density pattern is stabilized, and variations in the toner charge amount that adversely affect the density pattern formation can be eliminated. it can.

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

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The toner images thus transferred are sequentially transferred (primary transfer) onto the intermediate transfer belt 8 that moves in the vicinity of each image forming portion while rotating clockwise in FIG. 1 by a driving means (not shown). The image is transferred onto the paper P at a time (secondary transfer) by the next transfer roller 9 and further fixed on the paper P by the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

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

  The image reading unit 40 includes a scanner lamp that illuminates the document during copying, a scanning optical system equipped with a mirror that changes the optical path of reflected light from the document, and a condensing lens that focuses the reflected light from the document to form an image. , And a CCD sensor or the like (both not shown) that converts the imaged image light into an electrical signal, and reads a document image and converts it into image information. Further, as described later, the image forming units Pa to Pd also have a function of reading the RGB values of the test pattern formed on the paper P and detecting the density.

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

  When the start of image formation is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then the light beam is irradiated by the exposure device 4 to each photosensitive drum 1a. An electrostatic latent image corresponding to the image signal from the image reading unit 40 is formed on ˜1d. Each of the developing devices 3a to 3d includes a developing roller (developer carrying member) disposed opposite to the photosensitive drums 1a to 1d, and toners of colors of yellow, magenta, cyan, and black are respectively supplied by a replenishing device (not shown). A predetermined amount is filled. The toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing devices 3a to 3d, and electrostatically adheres to the electrostatic latent image formed by exposure from the exposure device 4. A toner image is formed.

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

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

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

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

  FIG. 2 is an enlarged view of the vicinity of the image forming portion Pa in FIG. Since the image forming units Pb to Pd have basically the same configuration, description thereof is omitted. Around the photosensitive drum 1a, a charger 2a, a developing device 3a, and a cleaning unit 5a are disposed along the drum rotation direction (counterclockwise in FIG. 2), and the intermediate transfer belt 8 is interposed between the primary transfer roller 6a. Is disposed opposite to the photosensitive drum 1a.

  The charger 2 a includes a charging roller 21 that contacts the photosensitive drum 1 a and applies a charging bias to the drum surface, and a charging cleaning roller 22 for cleaning the charging roller 21. The developing device 4a has two agitating and conveying screws 23, a magnetic roller 24, and a developing roller 25. When positively charged toner is used, a developing bias having the same polarity (positive) as the toner is applied to the developing roller 25. Then, the toner is caused to fly on the drum surface.

  The cleaning unit 5 a includes a rubbing roller 26, a cleaning blade 27, and a recovery screw 28. Residual toner removed from the surface of the photosensitive drum 1 a by the rubbing roller 26 and the cleaning blade 27 is discharged to the outside of the cleaning unit 5 a as the recovery screw 28 rotates.

  FIG. 3 is a side sectional view showing the configuration of the exposure apparatus 4, and also shows the photosensitive drums 1a to 1d disposed in the image forming portions Pa to Pd for convenience of explanation. The exposure device 4 includes a light source (not shown) that emits a light beam 30 modulated based on an image signal, a polygon mirror 31, a scanning lens 32, a correction lens 33, and a folding mirror 34 installed in the optical path of each light beam 30. , 35, 36 and 37 are provided in the housing 4a.

  The exposure apparatus 4 includes four light sources (not shown), and emits light beams 30Y, 30C, 30M, and 30K modulated based on yellow, magenta, cyan, and black image signals, respectively. . The polygon mirror 31 is a light beam deflecting unit, and can rotate the light beams 30Y to 30K incident on the reflecting surface at an equal angular velocity by rotating around the rotation shaft 31a.

  The scanning lens 32 uses the light beams 30Y to 30K deflected at a constant angular velocity by the polygon mirror 31 so that the light beams 30Y to 30K scan the surfaces of the photosensitive drums 1a to 1d at a constant speed in the main scanning direction. To be deflected. The correction lens 33 is a correction unit for the light beams 30Y to 30K, and includes a surface tilt correction function that corrects an error of the light beams 30Y to 30K caused by the surface tilt of the reflection surface of the polygon mirror 22 with respect to the rotation shaft 31a. Yes.

  The folding mirrors 34 to 37 are reflecting means installed in the optical paths of the respective light beams 30Y to 30K. The reflecting mirrors 34 to 37 are composed of thin plate-like mirrors and are arranged in the exposure apparatus 4 while holding both ends thereof. In addition, the number of the folding mirrors 34 to 37 arranged in each optical path and the installation angle of the reflecting surface are appropriately changed.

  The light beam scanning operation by the exposure apparatus 4 configured as described above will be described. First, light beams 30 </ b> Y to 30 </ b> K are incident on the reflection surface of the polygon mirror 31 from four light sources (not shown). At this time, in order to easily separate the optical paths of the four light beams 30Y to 30K deflected by the polygon mirror 31, these light beams 30K to 30Y are incident on the polygon mirror 31 at different angles in the sub-scanning direction. Is configured to do.

  The light beams 30 </ b> Y to 30 </ b> K incident on the polygon mirror 31 are deflected at a constant angular velocity by the polygon mirror 31 and then deflected at a constant velocity by the scanning lens 32. Then, the light beams 30Y to 30K deflected at a constant speed are folded back a predetermined number of times by folding mirrors 34 to 37 arranged in the respective optical paths, and after passing through the correction lens 33, the tilting of the light beams 30Y to 30K is corrected. It is folded back by the last folding mirrors 34a to 37a provided at the end, and light is distributed to the surfaces of the respective photosensitive drums 1a to 1d.

  In addition, in order to set the scanning start timing of each light beam in the main scanning direction, a BD sensor (not shown) for detecting each beam deflected by the polygon mirror 31 is provided outside the effective exposure area of the scanned surface. Yes. By shifting the time to the BD sensor and causing light beams from a plurality of light sources to enter, the synchronous detection signals of the respective light beams are individually generated, and the scanning start timings of the respective light beams are made to coincide with each other with high accuracy. When the BD sensor is provided on the scanning end side, the light beam on the scanning start side is reflected by a BD mirror (not shown) arranged on the scanning start side and enters the BD sensor.

  FIG. 4 is a block diagram showing a control path of the color image forming apparatus of the present invention. Portions common to FIG. 1 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image forming unit Pa to Pd, an image reading unit 40, an AD conversion unit 41, a control unit 42, a storage unit 43, an operation panel 44, a fixing unit 7, an intermediate transfer belt 8, and the like.

  The image signal read by the image reading unit 40 is converted into a digital signal by the AD conversion unit 41 and then sent to the image memory 50 in the storage unit 43. The storage unit 43 includes an image memory 50, a RAM 51, and a ROM 52. The image memory 50 stores an image signal read by the image reading unit 40 and digitally converted by the AD conversion unit 41, and is stored in the control unit 42. Send it out. The RAM 51 and ROM 52 store the processing program, processing content, and the like of the control unit 42.

  Further, the RAM 51 stores a lookup table (RGB-density LUT) in which RGB values of image light read by the image reading unit 40 and output densities corresponding to the RGB values are stored in association with each other, and gradation input values (exposure). A lookup table (tone-density LUT) in which the set amount value) and the output density target value are stored in association with each other, and a γ table in which the tone input value and the actual tone output value are stored in association with each other. Stored. Further, the pixel configuration (density) of the test pattern read by the image reading unit 40 and used for density correction, the gradation input value corresponding to each pixel configuration, the period (pitch) of each density pattern constituting the test pattern, and the like. It is remembered.

  The operation panel 44 includes an operation unit composed of a plurality of operation keys, and a display unit (none of which is shown) that displays setting conditions, apparatus states, and the like, and the user sets printing conditions and the like. In addition, for example, when the image forming apparatus 100 has a facsimile function, it is also used for various settings such as registering a facsimile transmission destination in the storage unit 43 and further reading and rewriting the registered transmission destination.

  The control unit 42 is, for example, a central processing unit (CPU), and according to a set program, the image reading unit 40, the image forming units Pa to Pd, the fixing unit 7, and the sheet P from the sheet cassette 16 (see FIG. 1). In addition to overall control of conveyance and the like, the image signal input from the image reading unit 40 is converted into image data by scaling processing or gradation processing as necessary. The exposure device 4 irradiates a light beam based on the processed image data, and forms latent images on the photosensitive drums 1a to 1d.

  Further, when the calibration mode is set by a key operation or the like on the operation panel 44, the control unit 42 compares the output density of each density pattern in the test pattern read by the image reading unit 40 with a target value, and a γ table. And a function for correcting gradation (density) for each color by adjusting the gradation data (exposure amount) of the image signal stored in the image memory 50 using the created γ table. is doing.

  The rotation position detection sensor 45 detects the rotation positions (rotation angles) of the photosensitive drums 1a to 1d, and the detection result is transmitted to the control unit 42 and used to control the test pattern formation position described later. As the rotational position detection sensor 45, a reflection type optical sensor that detects a mark on the outer peripheral surface by irradiating light to the outer peripheral surface of the drum and reading reflected light, or a detection unit by passing a flag formed on the outer peripheral surface of the drum. A PI (photo interrupter) sensor or the like that detects the rotational position when the light reception signal level is switched to LOW or HIGH is used.

  Next, density correction in the color image forming apparatus of the present invention will be described. 5 and 6 are examples of test patterns used for density correction of the image forming apparatus of the present invention. Here, a black test pattern formed by the image forming unit Pd in FIG. 1 will be described as an example, but yellow, magenta, and cyan test patterns are similarly configured.

  FIG. 5 shows a test pattern T in which black density patterns K1 of a predetermined density are arranged in three places, and FIG. 6 shows black density patterns K1 to K3 arranged in three places in three levels. A test pattern T is shown. Each density pattern K1 to K3 is formed over the entire width direction of the developing area in order to correct density unevenness in the axial direction (main scanning direction) of the photosensitive drum 1d. Each density pattern K1 to K3 is formed at a position having a cycle that is 2/3 times the rotation cycle of the photosensitive drum 1d. That is, assuming that one rotation pitch (peripheral length) of the photosensitive drum 1d is L, the pitch of the density patterns K1 to K3 is 2 / 3L.

  Then, magenta, cyan, yellow and black test patterns T are formed on the paper P, and the density correction of each color is executed. Specifically, the paper P on which the test pattern T is formed is set in the image reading unit 40, the RGB value of each pattern is read, and the output signal is transmitted to the control unit 42. The control unit 42 calculates the output density of each density pattern from the RGB value (usually the most sensitive value of RGB) using an equation or RGB-density LUT.

  At this time, as shown in FIG. 7, the output density is calculated as a three-point average density (white circle in the figure) of the output density (black circle in the figure) measured by the density pattern at three locations. It should be noted that the variation in density pattern formation position (formation timing) with respect to the drum cycle causes a periodic variation (amplitude of density unevenness indicated by a broken line) in the three-point average density. Can be suppressed to about 1/8 of the maximum variation (amplitude of density unevenness indicated by a solid line) in the case of not averaging. Then, the calculated output density is compared with the target value, and a γ table that associates the gradation input value with the output density is created for each color to execute density correction. It should be noted that density correction can be performed for only one to three colors of yellow, magenta, cyan, and black.

  Thereby, when forming a test pattern composed of a plurality of density patterns, it is possible to reduce the influence of density unevenness generated in the rotation cycle of the photosensitive drum, and to reduce the axial direction (main scanning direction) and circumferential direction (sub-scanning). In both directions, density correction can be performed with high accuracy. Further, since the test pattern formation position is controlled based on the detection result of the rotation position detection sensor 45, the test pattern can be formed at an accurate position corresponding to the rotation cycle of the photosensitive drum.

  5 and 6, the pitch of each density pattern is 2/3 times the rotation period of the photosensitive drum, but may be 4/3 times or 3/4 times, for example. That is, the pitch of each density pattern is approximately M / N times the rotation period of the photosensitive drum so that the same density pattern is not formed at the same location on the photosensitive drum (M and N are relatively prime integers, and M It may be formed at a position of ≧ 2, N ≧ 3).

  Then, it is preferable to form the density pattern formed at M / N times as many times as N. For example, since N = 3 in the example of FIGS. 5 and 6, each density pattern K1 to K3 is repeatedly formed three times. Accordingly, the same density pattern is formed using the entire circumferential direction of the photosensitive drum, and the average density of each density pattern can be made closer to the theoretical value.

  FIG. 8 is a diagram showing the same test pattern in which a plurality of sheets (here, two sheets) are formed under different image forming conditions. Here, the second test pattern T2 is output with an exposure amount setting different from that of the first test pattern T1, the test patterns T1 and T2 are read by the image reading unit 40, and the output density is the same between the same density patterns. To adjust the exposure amount for each block divided in the main scanning direction to correct the density unevenness.

  At this time, as shown in FIG. 8, by adjusting the sheet interval S between the test pattern T1 and the test pattern T2, from the last density pattern K3 of the test pattern T1 to the first density pattern K1 of the test pattern T1. Is synchronized with one rotation pitch L of the photosensitive drum 1d. As a result, the formation position of the electrostatic latent image of the density pattern K1 (or K2, K3) formed with the same pixel configuration (density) in the test patterns T1 and T2 is substantially the circumferential direction of the photosensitive drum 1d. It becomes the same position. Therefore, density unevenness caused by a difference in sensitivity characteristics in the circumferential direction of the photosensitive drum that occurs between a plurality of test patterns can be minimized.

  FIG. 9 is a diagram showing another example of forming the test patterns T1 and T2. The test pattern T2 in FIG. 9 is formed at a position delayed by 120 ° (1/3 period) with respect to the rotation period of the photosensitive drum from the test pattern T2 in FIG. That is, the electrostatic latent images of the density patterns K1 to K3 of the test patterns T1 and T2 are formed in substantially the same position with respect to the circumferential direction of the photosensitive drum 1d, although the order is different. By calculating, periodic density unevenness can be eliminated.

  In FIG. 9, the formation timing of the test pattern T2 is delayed by 1/3 period of the photosensitive drum, so that the paper interval S is longer than that in FIG. 8. The space S can be narrowed. That is, by adjusting the test pattern formation timing for the second and subsequent sheets according to the diameter and rotation period of the photosensitive drum, it is possible to arbitrarily set the interval between sheets when outputting a plurality of test patterns.

  FIG. 10 is a diagram illustrating an example in which two test patterns in which density patterns of four colors are arranged is output. FIG. 11 is a diagram in which two test patterns in which two levels of density patterns are arranged for each color are output. It is. Also in these examples, assuming that the rotation pitch (peripheral length) of the photosensitive drums 1a to 1d is L as in FIG. 8, black density patterns K1 and K2, cyan density patterns C1 and C2, and magenta density pattern. The pitches of M1 and M2 and yellow density patterns Y1 and Y2 are 2 / 3L. Thereby, it is possible to reduce the influence of density unevenness generated in the rotation cycle of the photosensitive drum in one test pattern.

  Further, the last density pattern Y of the test pattern T1 to the first density pattern K of the test pattern T2 are synchronized with one rotation pitch L of the photosensitive drums 1a to 1d. Thus, similar to the test pattern of FIG. 8 composed of a single color density pattern, the same color density pattern is formed at the same position on the photosensitive drum between the plurality of test patterns, and the circumferential direction of the photosensitive drum. Concentration unevenness can be eliminated.

  In the test pattern T2 shown in FIGS. 10 and 11, the sheet interval S when outputting the test pattern can be arbitrarily set by delaying or speeding up the formation timing of the test pattern T2.

  The density pattern forming position constituting the test pattern is approximately M / N times the rotation period of the photosensitive drums 1a to 1d (M and N are relatively prime integers, M ≧ 2, N ≧ 3). The method of removing density unevenness caused by the difference in sensitivity characteristics in the drum circumferential direction (sub-scanning direction) by calculating the average density of the density pattern has been described. The charging roller 21 in the chargers 2a to 2d (see FIG. 2). ) Charging characteristics and the developing performance of the developing roller 25 (see FIG. 2) in the developing devices 3a to 3d also cause uneven density in the circumferential direction.

  Therefore, if each density pattern is formed at a position approximately M / N times the rotation period of the charging roller 21 and the developing roller 25 (M and N are prime integers, M ≧ 2 and N ≧ 3), the photosensitive pattern is obtained. As in the case of the body drums 1a to 1d, it is possible to suppress the influence of density unevenness generated in one rotation cycle of the charging roller 21 and the developing roller 25 as much as possible. At this time, if a rotation position detection sensor 45 for detecting the rotation position of the charging roller 21 or the developing roller 25 is provided, a test pattern can be formed at an accurate position according to the rotation cycle of the charging roller 21 or the developing roller 25. Become.

  FIG. 12 shows an example in which the pitch of the three-level density patterns K1 to K3 constituting the black test pattern T is 3/4 times the rotation period Q of the charging roller 21. In FIG. 13, the pitches of the density patterns K, C, M, and Y of the black, cyan, magenta, and yellow constituting the four-color test pattern T are set to 4/3 times the rotation period Q of the charging roller 21. It is an example. In any case, since the density pattern of the same pixel configuration is not formed on the photosensitive drum at a position facing the same portion of the charger, the density unevenness due to the charging characteristic difference is eliminated by calculating the average density of the density pattern. Can do.

  FIG. 14 shows an example in which the pitch of the three-level density patterns K1 to K3 constituting the black test pattern T is set to 3/4 times the rotation period R of the developing roller 29. In FIG. 15, the pitch of the density patterns K, C, M, and Y of the black, cyan, magenta, and yellow constituting the four-color test pattern T is 4/3 times the rotation period R of the developing roller 25. It is an example. In both cases, density patterns having the same pixel configuration are developed in different regions on the developing roller 25 in the circumferential direction. Therefore, density unevenness caused by a difference in development characteristics can be eliminated by calculating the average density of the density patterns.

  In FIG. 15 in which the test pattern T is composed of four color density patterns K, C, M, and Y, each density pattern in the test pattern T and the density of the same color formed before the developing roller 25 is rotated once. The pattern is formed of toner in a region that does not overlap in the circumferential direction on the developing roller 25. For example, the black density pattern K is a region 1/3 from the start of development in the first rotation of the developing roller, a region 1/3 to 2/3 in the second rotation, and a region after 2/3 in the third rotation. Formed of toner.

  As a result, the density patterns K, C, M, and Y of the respective colors are always formed using the area where the toner is not consumed before one rotation of the developing roller 25, so that the charge amount of the toner used for forming the density pattern can be reduced. It is possible to stabilize and eliminate variations in toner charge amount that adversely affect density pattern formation.

  In each of the above-described embodiments, the case where the test pattern formation position is controlled in accordance with the respective rotation cycles of the photosensitive drums 1a to 1d, the charging roller 21, and the developing roller 25 has been described, but the photosensitive drums 1a to 1d are described. By adjusting the rotation cycle of the charging roller 21 and the developing roller 25 and the pitch of the test pattern, the formation position of the test pattern can be controlled by combining these.

  For example, approximately M / N times (M and N are relatively prime integers, M ≧ 2, N ≧ 3) with respect to both the rotation cycle L of the photosensitive drums 1 a to 1 d and the rotation cycle Q of the charging roller 21. If the respective density patterns are formed at the positions, the density unevenness caused by the sensitivity characteristic variation in the circumferential direction of the photosensitive drums 1a to 1d and the charge characteristic variation in the circumferential direction of the charging roller 21 can be eliminated at the same time. . Further, if it is formed at a position that is substantially M / N times the rotation period R of the developing roller 25, density unevenness due to variations in developing characteristics in the circumferential direction of the developing roller 25 can be eliminated at the same time.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, a configuration using an exposure apparatus that scans a light beam using a polygon mirror has been described. However, a configuration in which a photoconductor is exposed using an LED head in which a large number of LEDs are arranged in the main scanning direction. The same applies to. Although only the tandem type color copying machine has been described as the image forming apparatus, the present invention can be applied to other types of copying machines such as a digital copying machine and an analog type monochrome copying machine. .

  INDUSTRIAL APPLICABILITY The present invention can be used in an image forming apparatus that performs density correction by detecting the density of a test pattern output on a recording medium by an image reading unit, and includes a plurality of pixels formed with the same pixel configuration that constitutes a test pattern. A density pattern is a position that is approximately M / N times (M and N are relatively prime integers, M ≧ 2, N ≧ 3) at least one rotation period of at least one of an image carrier, a charging member, and a developer carrier. It is arranged in.

  Thereby, it is possible to reduce the influence of density unevenness in the circumferential direction (sub-scanning direction) generated in the rotation cycle of the image carrier, the charging member, or the developer carrier when calculating the average density of each density pattern. Thus, it is possible to provide an image forming apparatus capable of accurately performing density correction in both the axial direction and the circumferential direction. At this time, if the density pattern formed by the same pixel configuration is included in one test pattern so as to be a multiple of N, the average density can be made closer to the theoretical value.

  In addition, when outputting the same test pattern multiple times with different image formation conditions, the electrostatic latent image formation position of each density pattern formed with the same pixel configuration on the different test patterns can be Differences in the circumferential characteristics of the image carrier, charging member, or developer carrier that occur between multiple test patterns by using substantially the same region in at least one circumferential direction of the member and developer carrier Thus, an image forming apparatus capable of accurately detecting a density change based on a change in image forming conditions is obtained.

  In addition, by providing a position detection mechanism that detects the rotational position of the image carrier, charging member, and developer carrier, the test is performed at an accurate position according to the rotation cycle of the image carrier, charging member, or developer carrier. The image forming apparatus can form a pattern.

  Further, in the case of a test pattern constituted by density patterns of a plurality of colors, the density pattern of each color does not overlap with the density pattern of the same color formed before one rotation of the developer carrier in the circumferential direction on the developer carrier. By forming with the developer in the region, the image forming apparatus can eliminate variations in the toner charge amount that adversely affect the formation of the density pattern.

FIG. 1 is a schematic diagram showing an overall configuration of an image forming apparatus of the present invention. FIG. 2 is a partially enlarged view of the periphery of the image forming unit Pa in FIG. 1. These are side sectional views showing the configuration of an exposure apparatus mounted on the image forming apparatus of the present invention. FIG. 3 is a block diagram showing a control path of the image forming apparatus of the present invention. These are diagrams showing a test pattern T in which black density patterns K1 of a predetermined density are arranged at three locations. FIG. 4 is a diagram showing a test pattern T in which black density patterns K1 to K3 are arranged at three locations at three levels of density. These are graphs showing the relationship between the rotation period of the photoreceptor and the density unevenness. These are diagrams showing an example in which two identical test patterns T1 and T2 in which density patterns K1 to K3 are arranged are output. These are figures which show the other example of formation of the test patterns T1 and T2. FIG. 4 is a diagram illustrating an example in which two identical test patterns in which density patterns K, C, M, and Y of four colors are arranged are output. These are diagrams showing an example in which two identical test patterns in which two levels of density patterns are arranged for each color are output. FIG. 5 is a diagram showing an example in which the pitch of the three-level density patterns K1 to K3 constituting the black test pattern T is 3/4 times the rotation period of the charging roller. FIG. 4 is a diagram showing an example in which the pitch of each density pattern K, C, M, Y constituting the four-color test pattern T is 4/3 times the rotation period of the charging roller. FIG. 4 is a diagram showing an example in which the pitch of the three-level density patterns K1 to K3 constituting the black test pattern T is 3/4 times the rotation cycle of the developing roller. FIG. 4 is a diagram showing an example in which the pitch of each density pattern K, C, M, Y constituting the four-color test pattern T is 4/3 times the rotation cycle of the developing roller.

Explanation of symbols

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
2a to 2d Charger 3a to 3d Developing device 4 Exposure device 5a to 5d Cleaning unit 8 Intermediate transfer belt 21 Charging roller (charging member)
25 Development roller (developer carrier)
40 Image reading unit (image reading means)
42 control unit (density correction means)
45 Rotation position detection sensor (position detection mechanism)
100 Image forming apparatus

Claims (6)

An image carrier, a charging member that charges the surface of the image carrier, an exposure device that forms an electrostatic latent image by exposing the surface of the image carrier charged by the charging member, and the image carrier An image forming unit having a developer carrying member disposed oppositely and having a developing device for developing an electrostatic latent image formed by the exposure device;
Image reading means capable of reading the density of the test pattern formed on the recording medium in the image forming unit;
Density correction means for correcting the image density for each block in the axial direction of the image carrier based on the result of reading the test pattern by the image reading means;
In an image forming apparatus comprising:
The test pattern includes a plurality of density patterns formed with the same pixel configuration, and each density pattern formed with the same pixel configuration includes at least one of the image carrier, the charging member, and the developer carrier. An image forming apparatus, wherein the image forming apparatus is disposed at a position of approximately M / N times the rotation period (M and N are relatively prime integers, M ≧ 2, N ≧ 3).   The image forming apparatus according to claim 1, wherein the test pattern includes a multiple of N density patterns formed with the same pixel configuration.   When a plurality of test patterns including a plurality of types of density patterns having different pixel configurations are output, the electrostatic latent image forming position of each density pattern formed on the different test patterns with the same pixel configuration is determined by the image carrier. 3. The image forming apparatus according to claim 1, wherein the charging member and the developer carrying member are substantially in the same region with respect to a circumferential direction.   A position detection mechanism for detecting a rotational position of at least one of the image carrier, the charging member, and the developer carrier, and controlling a formation position of the test pattern based on a detection result of the position detection mechanism. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   5. The image forming apparatus according to claim 1, wherein the image carrier is an amorphous silicon photoconductor.   The test pattern is composed of density patterns of a plurality of colors, and the density pattern of the same color is formed at a position approximately M / N times the rotation period of the developer carrier, and the density pattern of each color in the test pattern 2. The method according to claim 1, wherein the developer carrying member is formed of a density pattern of the same color formed before one rotation of the developer carrying member and a developer in a region not overlapping in the circumferential direction on the developer carrying member. Item 6. The image forming apparatus according to Item 5.

Images