Image forming apparatus and patch forming method

Abstract

To increase the degree of freedom in the arrangement of patches.SOLUTION: An image forming apparatus comprises: an image forming unit that transfers patches Gp formed of toner from a photoconductor drum to a belt; a detection unit that detects the patches on the belt; and a control unit. The controls unit can form a preceding patch Gpf1 in a predetermined color on the belt, and can thereafter form a subsequent patch Gpr1 in a predetermined color on the belt. When forming the subsequent patch Gpr1, the control unit forms the subsequent patch Gpr1 such that a part of the subsequent patch Gpr1 including its rear end overlaps and the other part does not overlap a first range A11 between a first position P1 separated upstream from a leading end of the preceding patch Gpf1 by a drum peripheral length Lc1 being the peripheral length of the photoconductor drum and a second position P2 separated upstream from a rear end of the preceding patch Gpf1 by the drum peripheral length Lc1. A length (Lo) in the direction of movement of the part of the subsequent patch Gpr1 overlapping the first range A11 is 3 mm or less.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to an image forming apparatus and a patch forming method for forming a patch made of toner on a belt.

Background Art

[0002] Conventionally, as an image forming apparatus, when detecting the density of a patch formed on a belt with a sensor, instead of reading the densities of the leading end and the trailing end of the patch with the sensor, the density of an intermediate portion between the leading end and the trailing end is read (see Patent Document 1). In this technique, by not reading the densities of the leading end and the trailing end of the patch where the density becomes unstable due to the edge effect of the electric field, it is possible to accurately read the density of the patch with the sensor.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, after forming a patch at a predetermined position on the photosensitive drum, if a patch is formed again at the predetermined position before the photosensitive drum makes one rotation, there is a problem that the patch cannot be formed neatly because the influence of the previous patch formed at the predetermined position remains. To solve this problem, for a predetermined patch formed on the belt, a range separated by a pitch corresponding to the circumference of the photosensitive drum on the upstream side in the moving direction of the belt, and outside the range corresponding to the contour of the predetermined patch, it is necessary to arrange subsequent patches. However, when arranging subsequent patches in this way, there arises a problem that the degree of freedom in arranging the patches is reduced.

[0005] This problem also occurs in relation to the developing roller. Specifically, after moving the toner at a predetermined portion of the developing roller to the photosensitive drum to form a patch on the photosensitive drum, if the toner at the predetermined portion is moved to the photosensitive drum again before the developing roller makes one rotation, there is a problem that the patch cannot be formed neatly because there is not enough toner on the predetermined portion. To solve this problem, for a predetermined patch formed on the belt, it is necessary to arrange a subsequent patch at a position upstream in the moving direction of the belt, within a range separated by a pitch corresponding to the circumference of the developing roller and outside the range corresponding to the contour of the predetermined patch. However, when arranging subsequent patches in this way, there arises a problem that the degree of freedom in arranging the patches becomes low.

[0006] Therefore, an object of the present invention is to increase the degree of freedom in arranging patches.

Means for Solving the Problems

[0007] To solve the above problems, an image forming apparatus according to the present invention includes an image forming unit, a detection unit, and a control unit. The image forming unit has a photosensitive drum, a developing roller, and a belt. The image forming unit forms the patch on the belt by transferring a patch composed of toner of a predetermined color from the photosensitive drum to the belt. The detection unit detects the patch formed on the belt. After forming a preceding patch of the predetermined color on the belt, the control unit can form a subsequent patch of the predetermined color on the belt with a space upstream in the moving direction of the belt from the preceding patch. When forming the subsequent patch, the control unit forms the subsequent patch such that a part including the leading end or the trailing end of the subsequent patch overlaps and the other part does not overlap within a first range which is a range between a first position separated upstream by the drum circumference which is the circumference of the photosensitive drum from the leading end of the preceding patch and a second position separated upstream by the drum circumference from the trailing end of the preceding patch. The overlapping length Lo, which is the length in the moving direction of the part of the subsequent patch that overlaps with the first range, is 3 mm or less.

[0008] Also, the patch formation method according to the present invention is a patch formation method by the control unit. After forming a leading patch of the predetermined color on the belt, a subsequent patch of the predetermined color is formed on the belt at an interval upstream in the moving direction of the belt from the leading patch. When forming the subsequent patch, a part of the subsequent patch including the leading end or the trailing end overlaps within a first range, which is a range between a first position that is upstream by the drum circumference, which is the circumference of the photosensitive drum, from the leading end of the leading patch, and a second position that is upstream by the drum circumference from the trailing end of the leading patch, and the other part does not overlap, and the subsequent patch is formed. The overlapping length Lo, which is the length in the moving direction of the part of the subsequent patch that overlaps with the first range, is 3 mm or less.

[0009] According to such an image forming apparatus and patch formation method, since a part of the subsequent patch is overlapped within the first range where the influence of the formation of the leading patch remains, the degree of freedom in arranging the patches can be increased. Also, by setting the overlapping length Lo to 3 mm or less, while overlapping the end portions of the patches where the density becomes unstable due to the edge effect or the peripheral speed difference between the photosensitive drum and the developing roller within the first range, the length in the moving direction of the intermediate portion of the patches where there is no influence of the edge effect or the peripheral speed difference between the photosensitive drum and the developing roller outside the first range can be increased, so the number of reading points in the intermediate portion can be increased.

[0010] Also, when the control unit forms the subsequent patch on the belt, the control unit may form the subsequent patch such that a part including the trailing end of the subsequent patch overlaps within the first range.

[0011] According to this configuration, the interval between the leading patch and the subsequent patch can be reduced, so the time required for forming a plurality of patches can be shortened.

[0012] Furthermore, the drum circumferential length is Lc1, and the length from the leading edge of the preceding patch to the trailing edge of the succeeding patch is Le1. <Le1を満たしていてもよい。

[0013] Furthermore, the patch has an intermediate portion that is separated from the leading end and the trailing end of the patch by the overlap length Lo or more, and that is read by the detection unit and acquired by the control unit as information to be used for processing in the control unit, and the length from the leading end of the intermediate portion of the preceding patch to the trailing end of the intermediate portion of the succeeding patch is defined as Lr1, <Lc1を満たしていてもよい。

[0014] Furthermore, when forming the subsequent patch on the belt, the control unit may form the subsequent patch so that a part of the subsequent patch, including the leading end, overlaps the first area.

[0015] Furthermore, the drum circumferential length may be Lc1, the length from the leading edge of the preceding patch to the leading edge of the succeeding patch may be La1, and the length of the patch in the movement direction may be Lg1, so that Lc1+Lg1>La1 may be satisfied.

[0016] Furthermore, the patch may have an intermediate portion that is separated from the leading and trailing ends of the patch by the overlap length Lo or more, and that is read by the detection unit and acquired by the control unit as information to be used for processing in the control unit, and the length from the leading end of the preceding patch to the leading end of the intermediate portion of the following patch may be Lb1, satisfying Lb1 > Lc1 + Lg1.

[0017] An image forming apparatus according to the present invention includes an image forming unit, a detection unit, and a control unit. The image forming unit includes a photosensitive drum, a developing roller, and a belt, and forms a patch made of toner of a predetermined color on the belt by transferring the patch from the photosensitive drum to the belt. The detection unit detects the patch formed on the belt. After the control unit forms a leading patch of the predetermined color on the belt, a subsequent patch of the predetermined color can be formed on the belt at an interval upstream in the moving direction of the belt from the leading patch. When forming the subsequent patch, the control unit forms the subsequent patch such that a part including the leading end or the trailing end of the subsequent patch overlaps within a second range that is a range between a third position that is upstream by a roller circumference, which is the circumference of the developing roller, from the leading end of the leading patch and a fourth position that is upstream by the roller circumference from the trailing end of the leading patch, and another part does not overlap. An overlapping length Lo, which is the length in the moving direction of the part of the subsequent patch that overlaps with the second range, is 3 mm or less.

[0018] Also, the patch forming method according to the present invention is a patch forming method by the control unit. After forming a leading patch of the predetermined color on the belt, a subsequent patch of the predetermined color is formed on the belt at an interval upstream in the moving direction of the belt from the leading patch. When forming the subsequent patch, the subsequent patch is formed such that a part including the leading end or the trailing end of the subsequent patch overlaps within a second range that is a range between a third position that is upstream by a roller circumference, which is the circumference of the developing roller, from the leading end of the leading patch and a fourth position that is upstream by the roller circumference from the trailing end of the leading patch, and another part does not overlap. An overlapping length Lo, which is the length in the moving direction of the part of the subsequent patch that overlaps with the second range, is 3 mm or less.

[0019] According to such an image forming apparatus and patch forming method, since a part of the subsequent patch is overlapped within the second range where the influence of forming the leading patch remains, the degree of freedom in arranging the patches can be increased. Also, by setting the overlapping length Lo to 3 mm or less, while overlapping the end portions of the patches where the density becomes unstable due to the edge effect within the second range, the length in the moving direction of the intermediate portion of the patches where there is no influence of the edge effect outside the second range can be increased, so the number of reading points in the intermediate portion can be increased.

[0020] Further, when forming the subsequent patch on the belt, the control unit may form the subsequent patch such that a part including the trailing end of the subsequent patch overlaps within the second range.

[0021] According to this configuration, since the interval between the preceding patch and the subsequent patch can be reduced, the time required to form a plurality of patches can be shortened.

[0022] Also, with the roller circumference being Lc2 and the length from the leading end of the preceding patch to the trailing end of the subsequent patch being Le2, it may satisfy Lc2 < Le2.

[0023] Further, the patch has an intermediate portion that is separated from the leading end and the trailing end of the patch by a length Lo or more, and is acquired by the control unit as information to be used in the processing by the control unit when read by the detection unit. With the length from the leading end of the intermediate portion of the preceding patch to the trailing end of the intermediate portion of the subsequent patch being Lr2, it may satisfy Lr2 < Lc2.

[0024] Further, when forming the subsequent patch on the belt, the control unit may form the subsequent patch such that a part including the leading end of the subsequent patch overlaps within the second range.

[0025] Also, with the roller circumference being Lc2, the length from the leading end of the preceding patch to the leading end of the subsequent patch being La2, and the length of the patch in the moving direction of the patch being Lg2, it may satisfy Lc2 + Lg2 > La2.

[0026] Further, the patch has an intermediate portion that is separated from the leading end and the trailing end of the patch by a length Lo or more, and is acquired by the control unit as information to be used in the processing by the control unit when read by the detection unit. With the length from the leading end of the preceding patch to the leading end of the intermediate portion of the subsequent patch being Lb2, it may satisfy Lb2 > Lc2 + Lg2.

[0027] Further, the control unit may form a plurality of patches including the preceding patch and the subsequent patch on the belt, and set a developing bias applied to the developing roller based on the result of reading the plurality of patches by the detection unit.

[0028] Further, the control unit may form a plurality of patches including the preceding patch and the subsequent patch on the belt, and set a correspondence relationship between a target density and an exposure density based on the result of reading the plurality of patches by the detection unit.

Advantages of the Invention

[0029] According to the present invention, the degree of freedom in arranging patches can be increased.

Brief Description of the Drawings

[0030]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Best Mode for Carrying Out the Invention

[0031] Hereinafter, an embodiment of the present invention will be described in detail with appropriate reference to the drawings. As shown in FIG. 1, a color printer 1 as an example of an image forming apparatus includes a main body housing 2, a supply unit 3, an image forming unit 4, a detection unit PS, a fixing device 8, a conveyance unit 9, and a control unit 100. The main body housing 2 has a discharge tray 21 on its upper surface.

[0032] The supply unit 3 is provided in the lower part of the main body housing 2 and includes a supply tray 31 for accommodating the sheet S and a supply mechanism 32 for supplying the sheet S in the supply tray 31 to the image forming unit 4. The supply mechanism 32 includes a pickup roller 33, a separation roller 34, a separation pad 35, and a registration roller 36.

[0033] The supply unit 3 supplies the sheet S in the supply tray 31 toward the separation roller 34 by the pickup roller 33 and separates the sheet S into single sheets between the separation roller 34 and the separation pad 35. The registration roller 36 aligns the positions of the leading edges of the sheet S and then conveys the sheet S toward the image forming unit 4.

[0034] The image forming unit 4 includes an exposure device SC, a process unit PU, and a transfer unit 7.

[0035] The exposure device SC is disposed in the upper part of the main body housing 2 and includes a light source (not shown), a polygon mirror, and the like. The exposure device SC exposes the surface of the photosensitive drum 51 by irradiating light onto the surface of the photosensitive drum 51.

[0036] The process unit PU includes a drawer 5 and four developing cartridges 6. The drawer 5 is pullable out from the main body housing 2. The drawer 5 includes four photosensitive drums 51, four chargers 52, four drum cleaners 53, and four charge removal lamps 54, respectively.

[0037] The charger 52 is a member that charges the surface of the photosensitive drum 51. When the surface of the charged photosensitive drum 51 is exposed by the exposure device SC, an electrostatic latent image based on the image data is formed on the surface of the photosensitive drum 51. The drum cleaner 53 is a member that cleans the surface of the photosensitive drum 51. The charge removal lamp 54 is a member that irradiates light on the surface of the photosensitive drum 51 to remove the charge.

[0038] The developing cartridge 6 contains toner. The developing cartridge 6 includes a developing roller 61 that supplies toner to the electrostatic latent image of the photosensitive drum 51 to form an image, which is a toner image, on the photosensitive drum 51.

[0039] The developing cartridges 6 are arranged in the order of 6K, 6C, 6M, 6Y from the downstream side to the upstream side in the conveyance direction of the sheet S, where 6K, 6C, 6M, 6Y indicate the cartridges containing toner of each color, black, cyan, magenta, and yellow. In other words, the developing cartridges 6K, 6C, 6M, 6Y are arranged in the order of 6K, 6C, 6M, 6Y from the downstream side to the upstream side in the moving direction in which the conveyance belt 73 described later moves toward the detection unit PS. Also, the photosensitive drums 51 corresponding to the respective developing cartridges 6 are arranged in the order of K, C, M, Y from the downstream side to the upstream side in the moving direction in which the conveyance belt 73 moves toward the detection unit PS. In the present specification and drawings, when identifying members such as the photosensitive drum 51 and the developing roller 61 corresponding to the color of the toner, symbols K, C, M, Y are assigned corresponding to black, cyan, magenta, and yellow respectively, and the characters "first", "second", "third", "fourth" are added to the beginning of the member name.

[0040] For example, the photosensitive drum 51 and the developing roller 61 corresponding to black are also referred to as the first photosensitive drum 51K and the first developing roller 61K. The photosensitive drum 51 and the developing roller 61 corresponding to cyan are also referred to as the second photosensitive drum 51C and the second developing roller 61C. The photosensitive drum 51 and the developing roller 61 corresponding to magenta are also referred to as the third photosensitive drum 51M and the third developing roller 61M. The photosensitive drum 51 and the developing roller 61 corresponding to yellow are also referred to as the fourth photosensitive drum 51Y and the fourth developing roller 61Y.

[0041] The transfer unit 7 includes a driving roller 71, a driven roller 72, a conveying belt 73 as an example of a belt, four transfer rollers 74, and a belt cleaner BC. The conveying belt 73 is an endless belt. The driving roller 71 and the driven roller 72 are members for rotating the conveying belt 73. The driving roller 71 is arranged downstream of the driven roller 72 in the conveying direction of the sheet S.

[0042] The transfer roller 74 is a member for transferring the image on the photosensitive drum 51 to the sheet S or the conveying belt 73. The transfer roller 74 is arranged inside the conveying belt 73 and sandwiches the conveying belt 73 with the corresponding photosensitive drum 51.

[0043] The belt cleaner BC has a function of cleaning the toner on the conveying belt 73. The belt cleaner BC is located on the side opposite to the photosensitive drum 51 with respect to the conveying belt 73.

[0044] The detection unit PS is a sensor for detecting the printing density of the inspection image formed on the conveying belt 73. The detection unit PS is arranged to face the end of the conveying belt 73 on the driving roller 71 side. The detection unit PS is a light reflection type sensor including a light emitting element such as an LED and a light receiving element such as a phototransistor, and irradiates light on the image on the conveying belt 73 to detect the reflected light.

[0045] The fixing device 8 is a device that thermally fixes the toner image on the sheet S. The fixing device 8 includes a heating roller 81 that heats the sheet S, and a pressure roller 82 that sandwiches the sheet S between the heating roller 81.

[0046] The conveying unit 9 is mainly configured to convey the sheet S discharged from the fixing device 8 outward of the main body housing 2. The conveying unit 9 mainly includes a conveying roller 91 and a discharging roller 92. The sheet S discharged from the fixing device 8 is conveyed to the discharging roller 92 by the conveying roller 91 and discharged to the discharge tray 21 by the discharging roller 92.

[0047] The control unit 100 has a CPU, a ROM, a RAM, etc., and is configured to execute various processes according to a program prepared in advance and in response to receiving a printing command, etc.

[0048] The control unit 100 has a function of executing a printing process for forming an image on the sheet S based on a print job output from a computer (not shown). In the printing process, the control unit 100 executes an exposure process of exposing the photosensitive drum 51 based on a printing pattern image that is a binary image. Specifically, when executing the printing process and printing an image on the sheet S, the control unit 100 selects one printing pattern image from a plurality of printing pattern images with different exposure densities (gradations) so that the printing density of this image becomes a predetermined target density, and executes the exposure process. The printing pattern image is composed of a binary image as shown in FIG. 2. The exposure density refers to the area ratio of the pixels to be exposed in the printing pattern image.

[0049] Here, the printing density refers to the density of the toner image developed by the developing roller 61, which is the density detected by the detection unit PS. The target density refers to the target density indicated by the printing command. For example, when the target density indicated by the printing command is 20%, the control unit 100 selects a printing pattern image with an exposure density of 20% and executes the exposure process using this printing pattern image. However, the relationship between the target density and the exposure density is changed by the gradation setting process described later. For example, when the state is changed from the state where a printing pattern image with an exposure density of 20% is associated with a target density of 20% to the state where a printing pattern image with an exposure density of 10% is associated with a target density of 20%, when forming an image with a target density of 20%, the exposure process is executed using the printing pattern image with an exposure density of 10%.

[0050] As described above, after the control unit 100 selects one printing pattern image corresponding to the target density from among a plurality of printing pattern images, it performs exposure and development using the printing pattern image to form a toner image with the target density on the photosensitive drum 51.

[0051] Note that the plurality of printing pattern images correspond to the exposure density and are stored in the storage unit in advance. Also, the plurality of printing pattern images are stored in the storage unit in advance corresponding to each of the colors yellow, magenta, cyan, and black.

[0052] The control unit 100 is capable of performing image quality adjustment for adjusting the image quality of the printed toner image. The control unit 100 performs image quality adjustment when a predetermined condition is satisfied. Specifically, for example, the control unit 100 can be configured to perform image quality adjustment when any one of the following plurality of conditions is satisfied. First condition: The developing cartridge 6 or the photosensitive drum 51 is new Second condition: An instruction for performing image quality adjustment is received from the user Third condition: The environment such as temperature and humidity has changed by a predetermined value or more since the previous image quality adjustment Condition 4: The number of printed sheets since the last execution of image quality adjustment has reached or exceeded a predetermined number. Condition 5: A developing cartridge 6 with a different serial number is installed.

[0053] In image quality adjustment, the control unit 100 has a function of executing correction image formation processing, bias setting processing, pattern image formation processing, and gradation setting processing for each of the colors yellow, magenta, cyan, and black.

[0054] In the correction image formation processing, after the image forming unit 4 forms a first bias correction image on the conveyance belt 73 with a first developing bias applied to the developing roller 61, the image forming unit 4 forms a second bias correction image on the conveyance belt 73 with a second developing bias having a value different from that of the first developing bias applied to the developing roller 61. Here, the developing bias is the bias applied to the developing roller 61 by a bias application unit (not shown). The image forming unit 4 has a bias application unit.

[0055] The first developing bias and the second developing bias are set to optimal values for each color. The first developing bias may have different values for each color, or may have the same value for some or all of the plurality of colors. For example, the first developing bias corresponding to black may have a different value from the first developing bias corresponding to yellow, or may have the same value. Similarly, the second developing bias may have different values for each color, or may have the same value for some or all of the plurality of colors.

[0056] Both the first bias correction image and the second bias correction image are printing pattern images corresponding to a predetermined exposure density. Specifically, the first bias correction image is formed by forming a printing pattern image with an exposure density of 40% as shown in FIG. 2 using the first developing bias. In this case, the second bias correction image is formed by forming a printing pattern image with an exposure density of 40% using the second developing bias.

[0057] The bias setting process is a process of setting the developing bias based on the results of the detection unit PS reading the first bias correction image and the second bias correction image. In the bias setting process, the control unit 100 converts the intensity of the reflected light from each bias correction image detected by the detection unit PS into the printing density, and calculates the developing bias at which the printing density becomes the target density based on the two types of printing densities and the two types of developing biases. The developing bias set in the bias setting process is set to an optimal value for each color.

[0058] The pattern image forming process is a process of forming a plurality of pattern images on the conveying belt 73 by the image forming unit 4 with the developing bias set in the bias setting process applied to the developing roller 61. Here, the plurality of pattern images are tone setting images for performing the tone setting process. Specifically, the plurality of pattern images are a plurality of printing pattern images corresponding to a plurality of exposure densities. For example, five types of printing pattern images corresponding to exposure densities of 5%, 20%, 40%, 60%, and 80% are used.

[0059] The tone setting process is a process of setting the correspondence relationship between the target density and the exposure density based on the results of the detection unit PS reading the plurality of pattern images. Specifically, in the tone setting process, the control unit 100 sets the correspondence relationship between the target density and the printing pattern image, thereby setting the correspondence relationship between the target density and the density of the toner image actually printed, that is, the exposure density.

[0060] Specifically described, the control unit 100 detects a plurality of pattern images by the detection unit PS, and converts the intensity of the reflected light from each pattern image into the printing density respectively. Then, the control unit 100 compares each calculated printing density with the corresponding exposure density, and when the compared printing density and exposure density are different, corrects the correspondence relationship between the printing density and the exposure density to change the correspondence relationship between the target density and the printing pattern image.

[0061] Specifically, when the tone setting process has never been performed in the past, the correspondence between the target density and the printing pattern image (exposure density) is, for example, the default gamma table shown by the two-dot chain line in FIG. 3. In the following description, the correspondence between the target density and the printing pattern image is also referred to as the "gamma table".

[0062] The gamma table is a table for selecting a printing pattern of a predetermined exposure density corresponding to a predetermined target density when printing at a predetermined target density. The gamma table shown by the two-dot chain line, that is, the default gamma table, has the same value for the target density and the exposure density.

[0063] In the tone setting process, for example, when the detection unit PS detects a printing pattern image corresponding to an exposure density of 20% as shown by the black circle in the figure and the printing density of the image becomes 10%, the control unit 100 stores the printing pattern image again as a pattern image corresponding to a target density of 10%. That is, in the next printing control, when forming an image at a target density of 10%, if the image is formed with the first printing pattern image corresponding to an exposure density of 20%, the printing density will be 10%. Therefore, the control unit 100 associates the printing pattern image with an exposure density of 20% for a target density of 10%.

[0064] Also, in the tone setting process at this time, when the printing density when the printing pattern image with an exposure density of 40% is detected by the detection unit PS is 40%, that is, when the target density, the exposure density, and the printing density match, as shown by the broken line in the figure, the relationship between the target density and the exposure density is linearly interpolated. As a result, for example, in the next printing control, when forming an image at a target density of 20%, the printing pattern image corresponding to the exposure density X corresponding to the intersection of the line of the target density of 20% and the broken line in the figure is selected. At this time, the gamma table becomes a table in which the line with an exposure density of less than 40% in the default gamma table (two-dot chain line) is replaced by the line of the broken line in the figure. That is, as shown by the straight line of the broken line between the black circle shown in the figure and the white circle corresponding to the exposure density of 40%.

[0065] As shown in FIG. 4, in the correction image formation process, the control unit 100 forms a first image group G1 including a plurality of color first bias correction images K1, C1, M1, Y1 and a plurality of color second bias correction images K2, C2, M2, Y2. In the pattern image formation process, the control unit 100 forms a second image group G2 including a plurality of color pattern images Kγ1 to Kγ5, Cγ1 to Cγ5, Mγ1 to Mγ5, Yγ1 to Yγ5. The order of the first bias correction images K1 to Y1, the second bias correction images K2 to Y2, and the pattern images Kγ1 to Yγ5 on the conveyance belt 73 is set in the order shown in the figure so that the time from the start of the correction image formation process to the end of the pattern image formation process is as short as possible. Specifically, the exposure order is determined so that the order of the inspection images on the conveyance belt 73 is the order shown in the figure. The exposure order is stored in a storage unit (not shown). Note that in FIG. 4, each image is shown in a simplified manner.

[0066] Each inspection image is arranged in the moving direction with a space therebetween. Specifically, the first bias correction images K1 to Y1 and the second bias correction images K2 to Y2 are arranged at a first pitch Lp1, respectively. Two adjacent pattern images (for example, Kγ1, Kγ2) among the plurality of pattern images Kγ1 to Yγ5 are arranged at a second pitch Lp2.

[0067] Here, the length of each image in the moving direction is set according to the number of reading points (the number of positions to be read) of the detection unit PS. For example, when the number of reading points in the bias setting process is a first predetermined number, the length of each image in the moving direction constituting the first image group G1 is set to a first length corresponding to the first predetermined number. When the number of reading points in the gradation setting process is a second predetermined number smaller than the first predetermined number, the length of each image in the moving direction constituting the second image group G2 is a second length corresponding to the second predetermined number and smaller than the first length. The interval between each image is set in consideration of variations in color misregistration and reduction in toner consumption.

[0068] Each bias-corrected image is set to be arranged in the order of K1, Y1, M1, K2, C1, Y2, M2, C2 in sequence from the head in the moving direction. The control unit 100 executes correction image formation processing so that the bias-corrected images are arranged in the order described above.

[0069] Each pattern image is set to be arranged in the order of Kγ1, Kγ2, Kγ3, Kγ4, Kγ5, Cγ1, Cγ2, Mγ1, Mγ2, Cγ3, Yγ1, Yγ2, Mγ3, Cγ4, Yγ3, Mγ4, Mγ5, Cγ5, Yγ4, Yγ5 in sequence from the head in the moving direction. The pattern image Kγ3 is arranged after the pattern image Kγ2 with an interval of approximately two pattern images. Specifically, the pattern image Kγ3 is arranged with a pitch of 3×Lp2 with respect to the pattern image Kγ2. Note that the expression of converting the interval between two images into pitch is the same for other images, so it will be omitted in the following description.

[0070] The pattern image Kγ5 is arranged after the pattern image Kγ4 with an interval of approximately two pattern images. The pattern image Cγ1 is arranged after the pattern image Kγ5 with an interval of approximately one pattern image. The pattern image Mγ1 is arranged after the pattern image Cγ2 with an interval of approximately one pattern image. The foremost pattern image Kγ1 in the moving direction is arranged after the second bias-corrected image C2 with an interval of approximately three pattern images. The distance from the front end of the foremost first bias-corrected image K1 to the rear end of the rearmost pattern image Yγ5 in the moving direction is smaller than the circumference of the conveyance belt 73.

[0071] The control unit 100 forms inspection images so as to satisfy the following conditions. In the following description, the inspection images are also referred to as patch Gp.

[0072] As shown in FIG. 6, after forming the black first leading patch Gpf1 on the conveyance belt 73, the control unit 100 can form the black first trailing patch Gpr1 on the conveyance belt 73 at an interval upstream in the moving direction from the first leading patch Gpf1. Here, the first trailing patch Gpr1 means a patch that partially overlaps with a first range A11 described later. In the example of FIG. 6, the first leading patch Gpf1 is the pattern image Kγ1, and the first trailing patch Gpr1 is the pattern image Kγ4.

[0073] When forming the first trailing patch Gpr1, the control unit 100 forms the first trailing patch Gpr1 in a first range A11, which is a range between a first position P1 that is upstream from the tip of the first leading patch Gpf1 by the drum circumference Lc1, which is the circumference of the photosensitive drum 51, and a second position P2 that is upstream from the rear end of the first leading patch Gpf1 by the drum circumference Lc1, such that a part including the rear end of the first trailing patch Gpr1 overlaps and the other part does not overlap. The overlapping length Lo, which is the length in the moving direction of the part of the first trailing patch Gpr1 that overlaps with the first range A11, is 3 mm or less.

[0074] Specifically, the lower limit value of the overlapping length Lo is preferably 0.5 mm, more preferably 1.0 mm. Also, the upper limit value of the overlapping length Lo is preferably 3.0 mm, more preferably 2.5 mm. Note that the combination of the lower limit value and the upper limit value is arbitrary. For example, in the present embodiment, · Total length of patch Gp: 12 mm · Length of intermediate part CP: 7.0 mm · Length of the parts where the density at the front end and the rear end is likely to be unstable: 2.5 mm Therefore, the overlapping length Lo is preferably 0.5 mm or more and 3.0 mm or less, more preferably 1.0 mm or more and 2.5 mm or less. Specifically, in the present embodiment, the length Lo overlapping the first range A11 (photosensitive drum period) is set to 1.3 mm.

[0075] Patch Gp has an intermediate portion CP indicated by diagonal hatching in the figure. The intermediate portion CP is a portion that is separated from the leading and trailing ends of patch Gp by an overlapping length Lo or more. The intermediate portion CP is a portion that is acquired by the control unit 100 as information to be used in the processing by the control unit 100 when read by the detection unit PS. In the present embodiment, the control unit 100 reads the range from the leading end to the trailing end of patch Gp by the detection unit PS, performs each process based on the result of reading the intermediate portion CP, and is configured not to use the result of reading the portion other than the intermediate portion CP for the process. Note that the control unit 100 may be configured to read only the intermediate portion CP of patch Gp by the detection unit PS and not read the portion other than the intermediate portion CP by the detection unit PS.

[0076] The drum circumference Lc1, the length Le1 from the leading end of the first preceding patch Gpf1 to the trailing end of the first succeeding patch Gpr1, and the length Lr1 from the leading end of the intermediate portion CP of the first preceding patch Gpf1 to the trailing end of the intermediate portion CP of the first succeeding patch Gpr1 satisfy the following conditions. Lr1 < Lc1 < Le1

[0077] After forming the black second preceding patch Gpf2 on the conveyor belt 73, the control unit 100 can form the black second succeeding patch Gpr2 on the conveyor belt 73 with a space upstream in the moving direction from the second preceding patch Gpf2. Here, the second succeeding patch Gpr2 means a patch that partially overlaps with a second range A22 described later. In the example of FIG. 6, the second preceding patch Gpf2 is the pattern image Kγ2, and the second succeeding patch Gpr2 is the pattern image Kγ3.

[0078] When forming the second subsequent patch Gpr2, the control unit 100 forms the second subsequent patch Gpr2 in a second range A22, which is the range between a third position P3 that is upstream from the tip of the second preceding patch Gpf2 by the roller circumference Lc2, which is the circumference of the developing roller 61, and a fourth position P4 that is upstream from the trailing end of the second preceding patch Gpf2 by the roller circumference Lc2, such that a part including the tip of the second subsequent patch Gpr2 overlaps and the other part does not overlap. The overlapping length Lo, which is the length in the moving direction of the part of the second subsequent patch Gpr2 that overlaps with the second range A22, is 3 mm or less.

[0079] Specifically, the lower limit value of the overlapping length Lo is preferably 0.5 mm, more preferably 1.0 mm. Also, the upper limit value of the overlapping length Lo is preferably 3.0 mm, more preferably 2.5 mm. Note that the combination of the lower limit value and the upper limit value is arbitrary. For example, in the present embodiment, · Total length of patch Gp: 12 mm · Length of intermediate portion CP: 7.0 mm · Length of portions where the density at the front and rear ends is likely to be unstable: 2.5 mm Therefore, the overlapping length Lo is preferably 0.5 mm or more and 3.0 mm or less, more preferably 1.0 mm or more and 2.5 mm or less. Specifically, in the present embodiment, the length Lo overlapping the second range A22 (developing roller period) is set to 2.4 mm.

[0080] The roller circumference Lc2, the length La2 from the tip of the second preceding patch Gpf2 to the tip of the second subsequent patch Gpr2, the length Lg2 in the moving direction of the patch Gp, and the length Lb2 from the tip of the second preceding patch Gpf2 to the tip of the intermediate portion CP of the second subsequent patch Gpr2 satisfy the following conditions. Lb2 > Lc2 + Lg2 > La2

[0081] The aforementioned first range A11 is the area where the first preceding patch Gpf1 is formed on the photosensitive drum 51. The second range A22 is the portion where the toner for forming the second preceding patch Gpf2 was placed on the developing roller 61. Since the influence caused by forming the previous patch Gp (hereinafter, also referred to as the "influence of the memory effect") remains in the first range A11 or the second range A22, if a patch Gp is formed within the first range A11 or the second range A22, the density of the formed patch Gp may become unstable. Hereinafter, this phenomenon will be described with reference to FIG. 5.

[0082] As shown in FIGS. 5(a) and 5(b), when the first predetermined range R1 of the photosensitive surface of the photosensitive drum 51 is exposed, the toner located in the second predetermined range R2 of the developing roller 61 moves to the first predetermined range R1, and a patch Gp is formed in the first predetermined range R1 of the photosensitive drum 51. As shown in FIG. 5(c), when the developing roller 61 rotates half a turn from the position in FIG. 5(b) and the second predetermined range R2 reaches the developing nip position Pn, which is the contact point between the photosensitive drum 51 and the developing roller 61, again, the amount of toner on the second predetermined range R2 may be less than that when the patch Gp was formed in the first predetermined range R1 of the photosensitive drum 51. At this time, if the toner on the second predetermined range R2 is transferred to the photosensitive drum 51 to form a patch Gp, the density of the patch Gp may become unstable. If the toner on the second predetermined range R2 is transferred to the photosensitive drum 51 to form a patch Gp, the patch Gp transferred to the transfer belt 73 is formed over the entire second range A22 shown in FIG. 6. In the present embodiment, the tip of the second subsequent patch Gpr2 is overlapped with the second range A22. The tip of the second subsequent patch Gpr2 is an area including a portion where the density becomes unstable due to the influence of the edge effect and the peripheral speed difference between the photosensitive drum 51 and the developing roller 61, and is not suitable for density acquisition, so it is not used for control. Therefore, even if the density of this area becomes unstable because the tip of the second subsequent patch Gpr2 overlaps the second range A22, it will not interfere with the control.

[0083] As shown in FIG. 5(d), a first predetermined range R1, which is the exposed range of the photosensitive drum 51, is discharged when passing through the charge-removing lamp 54, but the influence of the previous exposure may remain. If the first predetermined range R1 in a state where the influence of such exposure remains is exposed again, poor formation of an electrostatic latent image may occur, and the density of the patch Gp formed in the first predetermined range R1 may become unstable. If the patch Gp formed in the first predetermined range R1 where the influence of the previous exposure remains is transferred to the conveyance belt 73, the patch Gp is formed over the entire area within the first range A11 shown in FIG. 6. In the present embodiment, the rear end portion of the first subsequent patch Gpr1 is overlapped with the first range A11. The rear end portion of the first subsequent patch Gpr1 is an area including a portion where the density becomes unstable due to the influence of the edge effect and the peripheral speed difference between the photosensitive drum 51 and the developing roller 61, and is not suitable for obtaining the density, and thus is not used for control. Therefore, even if the density of this area becomes unstable because the rear end portion of the first subsequent patch Gpr1 overlaps with the first range A11, it does not interfere with the control.

[0084] As shown in FIG. 6, patches Gp other than the preceding patches Gpf1, Gpf2 and the subsequent patches Gpr1, Gpr2 are formed at positions that do not overlap with the first ranges A11, A12 and the second ranges A21, A22. Here, the first range A12 is an area where the patch Gp of Kγ2 is formed on the photosensitive drum 51. Also, the second range A21 is a portion where the toner for forming the patch Gp of Kγ1 was placed on the developing roller 61.

[0085] As shown in FIG. 4, since the patch lengths (the lengths in the moving direction of the patch Gp) of the patches Gp constituting the first image group G1 are different from those of the patches Gp constituting the second image group G2, there is no patch that satisfies the relationship between the preceding patch and the subsequent patch described above. Among the patches Gp constituting the second image group G2, in addition to the aforementioned black patch Gp shown in FIG. 6, there are some patches Gp that satisfy the relationship between the preceding patch and the subsequent patch. For example, regarding the black patch Gp, in addition to what is shown in FIG. 6, Kγ4 corresponds to the second preceding patch Gpf2, and Kγ5 corresponds to the second subsequent patch Gpr2. Also, regarding the cyan patch Gp, Cγ1 corresponds to the first preceding patch Gpf1, and Cγ3 corresponds to the first subsequent patch Gpr1. Furthermore, regarding the magenta patch Gp, Mγ1 corresponds to Gpf1, and Mγ3 corresponds to Gpr1. Also, Mγ3 corresponds to Gpf2, and Mγ4 corresponds to Gpr2. Regarding the yellow patch Gp, Yγ3 corresponds to Gpf1, and Yγ5 corresponds to Gpr1. Also, Yγ2 corresponds to Gpf2, and Yγ3 corresponds to Gpr2.

[0086] According to the above, the following effects can be obtained in this embodiment. Since a part of the first subsequent patch Gpr1 is overlapped with the first range A11 where the influence of the formation of the first preceding patch Gpf1 remains, the degree of freedom in arranging the patch Gp can be increased. Also, by setting the overlap length Lo to 3 mm or less, while overlapping the end portion of the patch Gp where the density becomes unstable due to the edge effect with the first range A11, the length in the moving direction of the intermediate portion CP of the patch Gp where there is no influence of the edge effect outside the first range A11 can be increased, so the number of reading points at the intermediate portion CP can be increased.

[0087] By overlapping a part including the rear end of the first subsequent patch Gpr1 with the first range A11, the interval between the first preceding patch Gpf1 and the first subsequent patch Gpr1 can be reduced, so the time required for forming a plurality of patches Gp can be shortened.

[0088] Since a part of the second subsequent patch Gpr2 is overlapped with the second range A22 where the influence of the formation of the second preceding patch Gpf2 remains, the degree of freedom in arranging the patch Gp can be increased. Further, by setting the overlap length Lo to 3 mm or less, while overlapping the end portions of the patch Gp where the density becomes unstable due to the edge effect with the second range A22, the length in the moving direction of the intermediate portion CP of the patch Gp where there is no influence of the edge effect can be increased outside the second range A22, so that the number of reading points at the intermediate portion CP can be increased.

[0089] Since the control unit 100 executes the gradation setting process based on the results read by the detection unit PS of a plurality of patches Gp including the preceding patches Gpf1 and Gpf2 and the subsequent patches Gpr1 and Gpr2, the gradation setting process can be accurately performed using the density of the portion not affected by the edge effect or the memory effect of the patch Gp for the gradation setting process.

[0090] Note that the present invention is not limited to the above-described embodiment, and can be used in various forms as exemplified below. In the following description, members, patches, etc. having substantially the same structure as those in the above-described embodiment are denoted by the same reference numerals, and the description thereof is omitted.

[0091] The overlapping method of the preceding patch and the subsequent patch is not limited to the above-described embodiment. For example, as shown in FIG. 7, a part including the rear end of the second subsequent patch Gpr2 may be overlapped with the second range A21. In this form, the roller circumferential length Lc2 is smaller than that in the above-described embodiment. In this form, the second preceding patch Gpf2 is the pattern image Kγ1. Then, the range between the third position P3 that is upstream from the leading end of the pattern image Kγ1 by the roller circumferential length Lc2 and the fourth position P4 that is upstream from the rear end of the pattern image Kγ1 by the roller circumferential length Lc2 is the second range A21. The second subsequent patch Gpr2 becomes the pattern image Kγ2 whose rear end overlaps with the second range A21.

[0092] The roller circumference Lc2, the length Le2 from the leading end of the second leading patch Gpf2 to the trailing end of the second trailing patch Gpr2, and the length Lr2 from the leading end of the intermediate portion CP of the second leading patch Gpf2 to the trailing end of the intermediate portion CP of the second trailing patch Gpr2 satisfy the following conditions. Lr2 < Lc2 < Le2

[0093] In this form, by overlapping a part including the trailing end of the second trailing patch Gpr2 with the second range A21, the interval between the second leading patch Gpf2 and the second trailing patch Gpr2 can be reduced, so that the time required to form the plurality of patches Gp can be shortened.

[0094] As shown in FIG. 8, a part including the leading end of the first trailing patch Gpr1 may be overlapped with the first range A12. In this form, the drum circumference Lc1 is larger than that in the above embodiment. Also, in this form, the pitch of the patch Gp is not constant. In this form, the first leading patch Gpf1 is the pattern image Kγ2. And the range between the first position P1 that is separated upstream from the leading end of the pattern image Kγ2 by the drum circumference Lc1 and the second position P2 that is separated upstream from the trailing end of the pattern image Kγ2 by the drum circumference Lc1 is the first range A12. The first trailing patch Gpr1 becomes the pattern image Kγ5 whose leading end overlaps with the first range A12.

[0095] The drum circumference Lc1, the length La1 from the leading end of the first leading patch Gpf1 to the leading end of the first trailing patch Gpr1, the length Lg1 in the moving direction of the patch Gp, and the length Lb1 from the leading end of the first leading patch Gpf1 to the leading end of the intermediate portion CP of the first trailing patch Gpr1 satisfy the following conditions. Lb1 > Lc1 + Lg1 > La1

[0096] In the above-described embodiment, the first image group G1 is configured not to include the preceding patches Gpf1 and Gpf2 and the succeeding patches Gpr1 and Gpr2. However, the present invention is not limited to this, and the first image group G1 may include the preceding patches Gpf1 and Gpf2 and the succeeding patches Gpr1 and Gpr2. In the following description, the length of the moving direction of the bias correction image is the same as the length of the moving direction of the pattern image, and the first pitch Lp1 has the same value as the second pitch Lp2.

[0097] Specifically, as shown in FIG. 9, by continuously forming the first bias correction image K1 and the second bias correction image K2, K1 may be used as the second preceding patch Gpf2, and K2 may be used as the second succeeding patch Gpr2. In this case, the range between the third position P3 that is separated upstream from the leading end of the first bias correction image K1 by the roller circumference Lc2 and the fourth position P4 that is separated upstream from the trailing end of the first bias correction image K1 by the roller circumference Lc2 becomes the second range A25. Then, the trailing end portion of the second bias correction image K2 overlaps with the second range A25.

[0098] Also, as shown in FIG. 10, by arranging the respective bias correction images in the order of Y1, M1, K2, C1, M2, Y2 in order from the downstream side in the moving direction, Y1 may be used as the first preceding patch Gpf1, and Y2 may be used as the first succeeding patch Gpr1. In this case, the range between the first position P1 that is separated upstream from the leading end of the first bias correction image Y1 by the drum circumference Lc1 and the second position P2 that is separated upstream from the trailing end of the first bias correction image Y1 by the drum circumference Lc1 becomes the first range A13. Then, the trailing end portion of the second bias correction image Y2 overlaps with the first range A13.

[0099] According to the forms of FIGS. 9 and 10, since the control unit 100 executes the bias setting process based on the result of the detection unit PS reading a plurality of patches Gp including the preceding patches Gpf1 and Gpf2 and the succeeding patches Gpr1 and Gpr2, the concentration of the portion not affected by the edge effect or the memory effect of the patch Gp is used for the bias setting process, and the bias setting process can be performed with high accuracy.

[0100] In the above-described embodiment, the conveyance belt 73 was exemplified as an example of the belt. However, the present invention is not limited thereto. For example, the belt may be an intermediate transfer belt.

[0101] In the above-described embodiment, the present invention was applied to the color printer 1. However, the present invention is not limited thereto, and the present invention may be applied to other image forming apparatuses such as copiers and multifunction peripherals. In the above-described embodiment, the color printer 1 exposes the photosensitive drum with a laser scanner. However, as the exposure device, other exposure devices such as an LED exposure head may be used. Further, the first photosensitive drum, the second photosensitive drum, the third photosensitive drum, and the fourth photosensitive drum of the color printer 1 may be supported by one cartridge, or may be supported by individual cartridges.

[0102] Each element described in the above-described embodiment and modification examples may be implemented in any combination.

Explanation of Reference Numerals

[0103] 1 Color printer 4 Image forming unit 51 Photosensitive drum 61 Developing roller 73 Conveyance belt 100 Control unit A11 First range Gp Patch Gpf1 First leading patch Gpr1 First trailing patch Lc1 Drum circumference Lo Overlap length P1 First position P2 Second position PS Detection unit

Claims

1. An image forming unit having a photosensitive drum, a developing roller, and a belt, the image forming unit forming a patch on the belt by transferring a patch composed of toner of a predetermined color from the photosensitive drum to the belt; a detection unit that detects the patch formed on the belt; a control unit, and the control unit after forming a preceding patch of the predetermined color on the belt, can form a subsequent patch of the predetermined color on the belt with a space upstream in the moving direction of the belt from the preceding patch, when forming the subsequent patch, a part including the trailing end of the subsequent patch overlaps, and the other part does not overlap, within a first range that is a range between a first position that is upstream by a drum circumference, which is the circumference of the photosensitive drum, from the leading end of the preceding patch and a second position that is upstream by the drum circumference from the trailing end of the preceding patch, the subsequent patch is formed, An image forming apparatus, wherein an overlapping length Lo, which is a length in the moving direction of the part of the subsequent patch that overlaps with the first range, is 3 mm or less.

2. With the drum circumference being Lc1 and the length from the leading end of the preceding patch to the trailing end of the subsequent patch being Le1, The image forming apparatus according to claim 1, wherein Lc1 < Le1 is satisfied.

3. The patch is an intermediate portion that is separated from the leading end and the trailing end of the patch by a length equal to or greater than the overlapping length Lo, and the intermediate portion is acquired by the control unit as information used for processing in the control unit by being read by the detection unit, With the length from the leading end of the intermediate portion of the preceding patch to the trailing end of the intermediate portion of the subsequent patch being Lr1, The image forming apparatus according to claim 2, wherein Lr1 < Lc1 is satisfied.

4. An image forming unit having a photosensitive drum, a developing roller, and a belt, the image forming unit forming a patch on the belt by transferring a patch composed of toner of a predetermined color from the photosensitive drum to the belt; a detection unit that detects the patch formed on the belt; a control unit, and the control unit after forming a preceding patch of the predetermined color on the belt, can form a subsequent patch of the predetermined color on the belt with a space upstream in the moving direction of the belt from the preceding patch, when forming the subsequent patch, In a first range that is a range between a first position that is separated upstream by a drum circumference, which is the circumference of the photosensitive drum, from the leading end of the preceding patch and a second position that is separated upstream by the drum circumference from the trailing end of the preceding patch, a part of the subsequent patch including the leading end is formed to overlap and another part is formed not to overlap, wherein an overlapping length Lo, which is the length in the moving direction of the part of the subsequent patch that overlaps with the first range, is 3 mm or less, assuming that the drum circumference is Lc1, the length from the leading end of the preceding patch to the leading end of the subsequent patch is La1, and the length of the patch in the moving direction of the patch is Lg1, Lc1 + Lg1 > La1 is satisfied, the patch has an intermediate portion that is separated from the leading end and the trailing end of the patch by a length equal to or greater than the overlapping length Lo and is acquired by the control unit as information to be used for processing in the control unit when read by the detection unit, assuming that the length from the leading end of the preceding patch to the leading end of the intermediate portion of the subsequent patch is Lb1, An image forming apparatus characterized by satisfying Lb1 > Lc1 + Lg1. **Claim 5** An image forming unit having a photosensitive drum, a developing roller, and a belt, the image forming unit forming a patch on the belt by transferring a patch made of toner of a predetermined color from the photosensitive drum to the belt; a detection unit that detects the patch formed on the belt; a control unit, wherein the control unit is capable of forming a subsequent patch of the predetermined color on the belt at an interval upstream in the moving direction of the belt from the preceding patch after forming the preceding patch of the predetermined color on the belt, when forming the subsequent patch, in a second range that is a range between a third position that is separated upstream by a roller circumference, which is the circumference of the developing roller, from the leading end of the preceding patch and a fourth position that is separated upstream by the roller circumference from the trailing end of the preceding patch, a part of the subsequent patch including the leading end or the trailing end is formed to overlap and another part is formed not to overlap, An image forming apparatus characterized in that an overlapping length Lo, which is the length in the moving direction of the part of the subsequent patch that overlaps with the second range, is 3 mm or less. **Claim 6** The control unit when forming the subsequent patch on the belt, The image forming apparatus according to claim 5, wherein a subsequent patch is formed such that a part including the rear end of the subsequent patch overlaps the second range. **Claim 7** With the roller circumference being Lc2 and the length from the leading end of the preceding patch to the trailing end of the subsequent patch being Le2, The image forming apparatus according to claim 6, characterized in that Lc2 < Le2 is satisfied. **Claim 8** The patch is an intermediate portion that is separated from the leading end and the trailing end of the patch by a length of the overlap Lo or more, and is acquired by the control unit as information used for processing in the control unit when read by the detection unit, With the length from the leading end of the intermediate portion of the preceding patch to the trailing end of the intermediate portion of the subsequent patch being Lr2, The image forming apparatus according to claim 7, characterized in that Lr2 < Lc2 is satisfied. **Claim 9** The control unit When forming the subsequent patch on the belt, The image forming apparatus according to claim 5, characterized in that the subsequent patch is formed such that a part including the leading end of the subsequent patch overlaps the second range. **Claim 10** With the roller circumference being Lc2, the length from the leading end of the preceding patch to the leading end of the subsequent patch being La2, and the length of the patch in the moving direction of the patch being Lg2, The image forming apparatus according to claim 9, characterized in that Lc2 + Lg2 > La2 is satisfied. **Claim 11** The patch is an intermediate portion that is separated from the leading end and the trailing end of the patch by a length of the overlap Lo or more, and is acquired by the control unit as information used for processing in the control unit when read by the detection unit, With the length from the leading end of the preceding patch to the leading end of the intermediate portion of the subsequent patch being Lb2, The image forming apparatus according to claim 10, characterized in that Lb2 > Lc2 + Lg2 is satisfied. **Claim 12** An image forming unit having a photosensitive drum, a developing roller, and a belt, the image forming unit forming a patch on the belt by transferring a patch made of toner of a predetermined color from the photosensitive drum to the belt, a detection unit that detects the patch formed on the belt, and a control unit, The control unit After forming a preceding patch of the predetermined color on the belt, it is possible to form a subsequent patch of the predetermined color on the belt at an interval upstream in the moving direction of the belt from the preceding patch, When forming the subsequent patch, In a first range which is a range between a first position that is separated upstream by a drum circumference which is the circumference of the photosensitive drum from the leading end of the preceding patch and a second position that is separated upstream by the drum circumference from the trailing end of the preceding patch, a part of the subsequent patch including the leading end or the trailing end overlaps, and the subsequent patch is formed so that the other part does not overlap. An overlapping length Lo which is the length in the moving direction of the part of the subsequent patch that overlaps the first range is 3 mm or less. The control unit forms a plurality of patches including the preceding patch and the subsequent patch on the belt. An image forming apparatus characterized in that a developing bias applied to the developing roller is set based on a result of reading the plurality of patches by the detection unit.

13. An image forming unit having a photosensitive drum, a developing roller, and a belt, the image forming unit that forms a patch on the belt by transferring a patch made of toner of a predetermined color from the photosensitive drum to the belt, a detection unit that detects the patch formed on the belt, and a control unit, The control unit after forming the preceding patch of the predetermined color on the belt, can form the subsequent patch of the predetermined color on the belt with a space upstream in the moving direction of the belt from the preceding patch. When forming the subsequent patch, In a first range which is a range between a first position that is separated upstream by a drum circumference which is the circumference of the photosensitive drum from the leading end of the preceding patch and a second position that is separated upstream by the drum circumference from the trailing end of the preceding patch, a part of the subsequent patch including the leading end or the trailing end overlaps, and the subsequent patch is formed so that the other part does not overlap. An overlapping length Lo which is the length in the moving direction of the part of the subsequent patch that overlaps the first range is 3 mm or less. The control unit forms a plurality of patches including the preceding patch and the subsequent patch on the belt. An image forming apparatus characterized in that a correspondence relationship between a target density and an exposure density is set based on a result of reading the plurality of patches by the detection unit.

14. An image forming unit that forms a patch made of toner of a predetermined color on a belt, the image forming unit having a photosensitive drum and a developing roller corresponding to the predetermined color, a detection unit that detects the patch formed on the belt, A patch forming method by a control unit of an image forming apparatus including a control unit, After forming a leading patch of the predetermined color on the belt, a subsequent patch of the predetermined color is formed on the belt at an interval upstream in the moving direction of the belt from the leading patch. When forming the subsequent patch, a part of the subsequent patch including the leading end or the trailing end overlaps within a first range which is a range between a first position separated upstream by the drum circumference which is the circumference of the photosensitive drum from the leading end of the leading patch and a second position separated upstream by the drum circumference from the trailing end of the leading patch, and the subsequent patch is formed so that the other part does not overlap. An overlapping length Lo which is the length in the moving direction of the part of the subsequent patch overlapping with the first range is 3 mm or less. A plurality of patches including the leading patch and the subsequent patch are formed on the belt. A patch forming method characterized by setting a developing bias applied to the developing roller based on a result of reading the plurality of patches by the detecting unit.

15. An image forming unit that forms a patch made of toner of a predetermined color on a belt, the image forming unit having a photosensitive drum and a developing roller corresponding to the predetermined color, a detecting unit that detects the patch formed on the belt, a control unit, and a patch forming method by a control unit of an image forming apparatus including the same, After forming a leading patch of the predetermined color on the belt, a subsequent patch of the predetermined color is formed on the belt at an interval upstream in the moving direction of the belt from the leading patch. When forming the subsequent patch, a part of the subsequent patch including the leading end or the trailing end overlaps within a first range which is a range between a first position separated upstream by the drum circumference which is the circumference of the photosensitive drum from the leading end of the leading patch and a second position separated upstream by the drum circumference from the trailing end of the leading patch, and the subsequent patch is formed so that the other part does not overlap. An overlapping length Lo which is the length in the moving direction of the part of the subsequent patch overlapping with the first range is 3 mm or less. A plurality of patches including the leading patch and the subsequent patch are formed on the belt. A patch forming method characterized by setting a correspondence relationship between a target density and an exposure density based on a result of reading the plurality of patches by the detecting unit.

16. An image forming unit that forms a patch made of toner of a predetermined color on a belt, the image forming unit having a photosensitive drum and a developing roller corresponding to the predetermined color, a detection unit that detects the patch formed on the belt, and a control unit, and a patch forming method by a control unit of an image forming apparatus including the above, after forming a preceding patch of the predetermined color on the belt, a succeeding patch of the predetermined color is formed on the belt with a space upstream in the moving direction of the belt from the preceding patch, when forming the succeeding patch, a part including the leading end or the trailing end of the succeeding patch overlaps and the other part does not overlap within a second range that is a range between a third position that is upstream by a roller circumference, which is the circumference of the developing roller, from the leading end of the preceding patch and a fourth position that is upstream by the roller circumference from the trailing end of the preceding patch, and the succeeding patch is formed, A patch forming method, characterized in that an overlapping length Lo, which is a length in the moving direction of the part of the succeeding patch that overlaps the second range, is 3 mm or less.

Images