Image forming device

Abstract

To effectively prevent a variation in the magnification of an image on a recording medium.SOLUTION: An image forming apparatus 100 has: an image carrier 204; a transfer member 206; a first driving unit M1 that provides a driving force to the image carrier 204; a second driving unit M2 that provides a driving force to the transfer member 206; an acquisition unit 513 that acquires information on the distribution of a toner image transferred to a recording medium P at a transfer part N2 in a surface of the recording medium P; and a control unit 309 that controls the second driving unit M2. The control unit 309 can change the number of rotations of the transfer member 206 driven by the second driving unit M2 while the recording medium P passes through the transfer unit N2 based on the information on the distribution.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, printer, facsimile machine, or multifunction machine that uses an electrophotographic or electrostatic recording method, or that has a plurality of functions selected from these. [Background technology]

[0002] In conventional image forming apparatuses, such as electrophotographic copying machines, a toner image formed on an image carrier is transferred to a recording medium such as paper. In intermediate transfer image forming apparatuses, a toner image formed on a photosensitive member serving as a first image carrier is primarily transferred to an intermediate transfer member serving as a second image carrier, and then secondarily transferred to a recording medium such as paper. The following description mainly focuses on an intermediate transfer image forming apparatus having an intermediate transfer belt serving as an intermediate transfer member.

[0003] The secondary transfer is performed in a secondary transfer portion (secondary transfer nip portion) formed by pressing an outer secondary transfer roller (outer transfer member, transfer member) provided on the outer peripheral surface of the intermediate transfer belt against an inner secondary transfer roller (inner transfer member) provided on the inner peripheral surface of the intermediate transfer belt. There is also a configuration in which the outer secondary transfer roller is pressed against the intermediate transfer belt via an outer secondary transfer belt (outer transfer member, transfer member) to form the secondary transfer portion.

[0004] Here, various problems may arise during secondary transfer depending on the characteristics (rigidity, basis weight, surface properties, etc.) of the recording medium used for image formation. For example, when recording media with different surface properties are used, the conveying force applied to the recording medium from the intermediate transfer belt may differ, which may cause the conveying speed of the recording medium to change. When the conveying speed of the recording medium relative to the intermediate transfer belt differs in this way, the length of the image on the recording medium in the conveying direction of the recording medium (the sub-scanning direction of the image) may change.

[0005] The conveyance speed of the recording medium also varies depending on the toner present between the intermediate transfer belt and the recording medium at the secondary transfer unit. This is because the presence of toner changes the coefficient of friction and electrostatic state between the intermediate transfer belt and the recording medium, causing fluctuations in the conveyance force. In particular, if the toner image is unevenly distributed on a single recording medium or if the amount of toner applied varies, the conveyance speed of the recording medium may fluctuate even while the recording medium passes through the secondary transfer unit. This can cause the expansion / contraction ratio of the image in the conveyance direction of the recording medium (the sub-scanning direction of the image) to fluctuate (partial magnification fluctuation) while the recording medium passes through the secondary transfer unit, resulting in localized changes in the length of the image in the sub-scanning direction.

[0006] To address this issue, Patent Document 1 proposes setting the speed of the secondary transfer outer belt so that the magnification of the image on the recording medium matches when no toner image is placed and when a halftone toner image is placed. This is intended to prevent image misalignment due to the presence or absence of a toner image. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5864867 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, in the configuration described in Patent Document 1, the speed of the secondary transfer outer belt is adjusted so that the lengths of images on the recording medium are consistent under different conditions of toner application amount (no toner, halftone), preventing misalignment of the image. In this case, in order to determine a single speed of the secondary transfer outer belt that will result in a constant amount of expansion / contraction of the image for any amount of toner application, the amount of toner application and the amount of expansion / contraction of the image must be proportional to each other.

[0009] However, in the secondary transfer section, the electrostatic attraction force between the intermediate transfer belt and the recording medium and the frictional force via the toner act in a complex manner, so the relationship between the amount of toner applied and the amount of image expansion / contraction is not necessarily proportional, and it has been found that there are cases where the speed of the transfer member cannot be uniquely determined.

[0010] SUMMARY OF THE INVENTION It is therefore an object of the present invention to effectively suppress fluctuations in the magnification of an image on a recording medium. [Means for solving the problem]

[0011] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus including a rotatable image carrier that carries a toner image, a rotatable transfer member that contacts the image carrier to form a transfer section and transfers the toner image from the image carrier to a recording medium that passes through the transfer section, a first drive section that applies a drive force to the image carrier, a second drive section that applies a drive force to the transfer member, an acquisition section that acquires information about the distribution of the toner image transferred to the recording medium in the transfer section, and a control section that controls the second drive section, and the control section is capable of changing the number of rotations of the transfer member driven by the second drive section while the recording medium passes through the transfer section based on the information about the distribution. the acquiring unit acquires, as information about the distribution, an index value correlated with the amount of toner in a toner image to be transferred to each of a plurality of divided areas obtained by dividing the surface of the recording medium into a plurality of areas in a conveying direction of the recording medium, the control unit sets, based on the index value for each of the divided areas, a driving rotation speed of the transfer member by the second driving unit when each of the divided areas passes through the transfer unit, and the relationship between the index value for each of the divided areas and the driving rotation speed is nonlinear; The image forming apparatus is characterized by the above. According to another aspect of the present invention, a recording medium transfer device includes a rotatable image carrier that carries a toner image, a rotatable transfer member that contacts the image carrier to form a transfer section and transfers the toner image from the image carrier to a recording medium that passes through the transfer section, a first drive section that applies a driving force to the image carrier, a second drive section that applies a driving force to the transfer member, an acquisition section that acquires information about a distribution of the toner image transferred to the recording medium in the transfer section, and a control section that controls the second drive section, wherein the control section is capable of changing the number of rotations of the transfer member driven by the second drive section while the recording medium passes through the transfer section based on the information about the distribution, and the acquisition section acquires information about the distribution. An image forming apparatus is provided, characterized in that: as information, an index value correlated with the amount of toner in a toner image to be transferred to each divided area obtained by dividing the surface of a recording medium into a plurality of areas in the conveying direction of the recording medium is acquired; the control unit sets the number of drive rotations of the transfer member by the second drive unit when each divided area passes through the transfer unit based on the index value for each divided area; and the index value for each divided area indicates a weighted average value obtained by weighting and averaging index values ​​related to the amount of toner in a toner image to be transferred to each of a plurality of areas in each divided area in a direction perpendicular to the conveying direction based on the index value for each of the plurality of areas. According to another aspect of the present invention, a printing apparatus includes a rotatable image carrier that carries a toner image, a rotatable transfer member that contacts the image carrier to form a transfer section and transfers the toner image from the image carrier to a recording medium that passes through the transfer section, a first drive section that applies a drive force to the image carrier, a second drive section that applies a drive force to the transfer member, an acquisition section that acquires information about the distribution of the toner image transferred to the recording medium in the transfer section, and a control section that controls the second drive section, wherein the control section is capable of changing the number of drive rotations of the transfer member driven by the second drive section while the recording medium passes through the transfer section based on the information about the distribution, and the acquisition section acquires information about the distribution of the toner image in the plane of the recording medium as the recording medium passes through the transfer section. An image forming apparatus is provided, characterized in that an index value correlating with the amount of toner in a toner image to be transferred to each of a plurality of divided areas divided in a feed direction is obtained, and the control unit sets the number of drive rotations of the transfer member by the second drive unit when each of the divided areas passes through the transfer unit based on the index value for each of the divided areas, and the index value for each of the divided areas indicates a weighted average value obtained by weighting and averaging index values ​​related to the amount of toner in a toner image to be transferred to each of a plurality of areas in a direction perpendicular to the feed direction in each of the divided areas based on the index value for each of the plurality of areas and information regarding the position of each of the plurality of areas in the direction perpendicular to the feed direction. According to another aspect of the present invention, a recording medium transfer device includes a rotatable image carrier that carries a toner image, a rotatable transfer member that contacts the image carrier to form a transfer section and transfers the toner image from the image carrier to a recording medium that passes through the transfer section, a first drive section that applies a drive force to the image carrier, a second drive section that applies a drive force to the transfer member, an acquisition section that acquires information about a distribution of the toner image to be transferred to the recording medium in the transfer section, and a control section that controls the second drive section, and the control section controls the second drive section based on the information about the distribution. An image forming apparatus is provided, characterized in that the number of rotations driven by the second drive unit to the transfer member can be changed, the acquisition unit acquires, as information about the distribution, an index value correlated with the amount of toner in the toner image to be transferred to each of the divided areas obtained by dividing the surface of the recording medium into multiple areas in the conveying direction of the recording medium, the control unit sets the number of rotations driven by the second drive unit to drive the transfer member when each of the divided areas passes through the transfer unit based on the index value for each of the divided areas, and the index value correlated with the amount of toner is the area ratio of the toner image per unit area. According to another aspect of the present invention, a recording medium transfer system includes a rotatable image carrier that carries a toner image, a rotatable transfer member that contacts the image carrier to form a transfer section and transfers the toner image from the image carrier to a recording medium that passes through the transfer section, a first drive section that applies a drive force to the image carrier, a second drive section that applies a drive force to the transfer member, an acquisition section that acquires information about a distribution of the toner image to be transferred to the recording medium in the transfer section, and a control section that controls the second drive section, and the control section controls the transfer of the toner image by the second drive section while the recording medium passes through the transfer section based on the information about the distribution. The image forming apparatus is characterized in that the driving rotation speed of the member is changeable, the acquisition unit acquires, as information regarding the distribution, an index value correlated with the amount of toner in the toner image transferred to each divided area obtained by dividing the surface of the recording medium into multiple areas in the conveying direction of the recording medium, the control unit sets the driving rotation speed of the transfer member by the second drive unit when each divided area passes through the transfer unit based on the index value for each divided area, and the width of each divided area in the conveying direction is greater than the length of the contact area between the image carrier and the recording medium in the conveying direction. [Effects of the Invention]

[0012] According to the present invention, it is possible to effectively suppress fluctuations in the magnification of an image on a recording medium. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 10 is a graph showing the relationship between the amount of applied toner and the image magnification. [Figure 3] 10 is a graph showing the relationship between the driving rotation speed of the outer secondary transfer roller and image magnification. [Figure 4] 10A and 10B are schematic diagrams showing examples of image patterns for measuring image magnification. [Figure 5] FIG. 2 is a block diagram illustrating a control mode of the image forming apparatus. [Figure 6] 10A and 10B are schematic diagrams for explaining a method of dividing a CMYK image to calculate a divided video count value. [Figure 7] 10 is a graph showing the relationship between the amount of applied toner and the number of rotations of the outer secondary transfer roller to achieve a predetermined image magnification. [Figure 8] 5 is a schematic diagram for explaining a method for calculating the driving rotation speed of the outer secondary transfer roller in the first embodiment. FIG. [Figure 9] 10 is a schematic diagram for explaining a method for calculating the driving rotation speed of the outer secondary transfer roller in the second embodiment. FIG. [Figure 10] 10A and 10B are explanatory diagrams showing examples of contributions of the toner image position in the main scanning direction to the image magnification; [Figure 11] 10A and 10B are schematic diagrams illustrating another example of a transfer member. DETAILED DESCRIPTION OF THE INVENTION

[0014] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0015] [Example 1] 1. Configuration and operation of image forming apparatus 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. In this embodiment, the image forming apparatus 100 is a tandem multifunction machine (having the functions of a copier, printer, and facsimile machine) that employs an intermediate transfer method and is capable of forming full-color images using an electrophotographic method.

[0016] The image forming apparatus 100 has four image forming units (stations), Sa, Sb, Sc, and Sd, which form toner images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Elements in the image forming units Sa, Sb, Sc, and Sd that have the same or corresponding functions or configurations will be generally described by omitting the suffixes a, b, c, and d indicating that the element is for one of the colors. In this embodiment, the image forming unit S includes a photosensitive drum 200, a charging roller 201, an exposure device 310 that exposes each photosensitive drum 200 to light, a developing device 202, a primary transfer roller 203, and the like, which will be described later.

[0017] The photosensitive drum 200, a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as a first image carrier, is driven to rotate counterclockwise in FIG. 1. The surface of the rotating photosensitive drum 200 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging roller 201, a roller-type charging member serving as a charging means. The charged surface of the photosensitive drum 200 is irradiated and scanned with a laser beam from an exposure device (polygon laser scanner) 310 serving as an exposure means, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 200. The exposure device 310 scans the laser beam in response to an image signal received by a control unit 309 from an external device (host device) 602 (FIG. 5) such as a personal computer, and irradiates the surface of the photosensitive drum 200 with the laser beam. In this embodiment, the exposure device 310 is configured as a single unit that exposes the photosensitive drums 200a to 200d of the image forming units Sa to Sd.

[0018] The electrostatic latent image formed on the photosensitive drum 200 is developed (visualized) by the developing device 202 as a developing means, which supplies toner as a developer, and a toner image (toner image, developer image) is formed on the photosensitive drum 200. In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 200 (negative polarity in this embodiment) adheres to the exposed portion (image portion) on the photosensitive drum 200, which has been uniformly charged and then exposed to light, thereby reducing the absolute value of the potential (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during the development process, is negative polarity.

[0019] An intermediate transfer belt 204, which is an intermediate transfer body formed of an endless belt and serves as a second image carrier, is disposed facing the photosensitive drums 200a-200d of each of the image forming units Sa-Sd. The intermediate transfer belt 204 is stretched over a plurality of tension rollers (support rollers), including a drive roller 211, a tension roller 212, and a secondary transfer inner roller 205, and is stretched with a predetermined tension. The drive roller 211 is driven to rotate by a belt drive motor M1 (FIG. 5) serving as a drive source constituting a first drive unit, and a driving force is transmitted to the intermediate transfer belt 204, causing it to rotate (circulate) clockwise in FIG. 1. Primary transfer rollers 203a-203d, which are roller-type transfer members serving as primary transfer means, are disposed on the inner circumferential surface of the intermediate transfer belt 204, corresponding to each of the photosensitive drums 200a-200d. The primary transfer roller 203 comes into contact with the inner circumferential surface of the intermediate transfer belt 204 and presses the intermediate transfer belt 204 toward the photosensitive drum 200, forming a primary transfer portion (primary transfer nip portion) N1 where the photosensitive drum 200 and the intermediate transfer belt 204 come into contact. The tension rollers other than the drive roller 211 and each primary transfer roller 203 are rotated in accordance with the rotation of the intermediate transfer belt 204.

[0020] The toner image formed on the photosensitive drum 200 is primarily transferred onto the rotating intermediate transfer belt 204 at the primary transfer portion N1 by the action of the primary transfer roller 203. During the primary transfer process, a primary transfer voltage (primary transfer bias) of a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) is applied to the primary transfer roller 203. For example, when a full-color image is formed, the toner images of yellow, magenta, cyan, and black formed on each of the photosensitive drums 200a to 200d are transferred onto the intermediate transfer belt 204 in succession so as to be superimposed on top of each other.

[0021] On the outer peripheral surface side of the intermediate transfer belt 204, a secondary transfer outer roller (transfer outer member) 206, which is a roller-type transfer member serving as a secondary transfer means, is disposed at a position facing the secondary transfer inner roller (transfer inner member) 205. The secondary transfer outer roller 206 is pressed toward the secondary transfer inner roller 205 and abuts against the secondary transfer inner roller 205 via the intermediate transfer belt 204, forming a secondary transfer portion (secondary transfer nip portion) N2 where the intermediate transfer belt 204 and the secondary transfer outer roller 206 come into contact. The secondary transfer outer roller 206 abuts against the intermediate transfer belt 204 and rotates counterclockwise in FIG. 1. The secondary transfer outer roller 206 can be rotated by a secondary transfer outer roller drive motor M2 (FIG. 5) serving as a drive source constituting a second drive portion. At the secondary transfer section N2, the toner image on the intermediate transfer belt 204 is secondarily transferred onto a recording medium P, such as paper, that is sandwiched and transported between the intermediate transfer belt 204 and the outer secondary transfer roller 206 by the action of the outer secondary transfer roller 206 and other components. During the secondary transfer process, a secondary transfer voltage (secondary transfer bias) having a polarity opposite to the normal charging polarity of the toner is applied to the outer secondary transfer roller 206. In this embodiment, the inner secondary transfer roller 205 is electrically grounded. Note that the outer transfer member corresponding to the outer secondary transfer roller 206 in this embodiment may be electrically grounded, and a transfer voltage having the same polarity as the normal charging polarity of the toner may be applied to the inner transfer member corresponding to the inner secondary transfer roller 205 in this embodiment during the secondary transfer process.

[0022] Recording media (recording materials, sheets, paper) P are stored in a cassette 214 serving as a storage unit. The recording media P in the cassette 214 are fed out one by one by a feeding member such as a feed roller 215, and are transported to a registration unit (pair of registration rollers) 208 by a transport member such as a transport roller 216. After being made to wait in the registration unit 208, the timing of the recording media P is controlled by a control unit (CPU) 309 to align the position of the toner image on the intermediate transfer belt 204 with the position of the recording media P, and the recording media P is then transported from the registration unit 208 to a secondary transfer unit N2.

[0023] The recording medium P onto which the toner image has been transferred is transported to a fixing device 700, which serves as an image heating device. The fixing device 700 fixes (melts and adheres) the toner image onto the recording medium P by applying heat and pressure to the recording medium P bearing the unfixed toner image.

[0024] When printing on one side of the recording medium P, the recording medium P on which the toner image has been fixed as described above is discharged (output) to the outside of the image forming apparatus main body (hereinafter also simply referred to as the "apparatus main body") 400. When printing on both sides of the recording medium P, the recording medium P on which the toner image has been fixed on the first side is conveyed to a reversing unit 209 provided inside the apparatus main body 400, stopped once, and then the conveying direction is reversed and conveyed to a reversing path. Thereafter, this recording medium P is sent to a secondary transfer unit N2 in an inverted state via a double-sided conveying unit 210, and after the toner image has been secondarily transferred to the second side, it is discharged to the outside of the apparatus via a fixing device 700.

[0025] Meanwhile, toner remaining on the surface of the photosensitive drum 200 after the primary transfer step (primary transfer residual toner) is removed from the surface of the photosensitive drum 200 and collected by a drum cleaner 207 serving as a photosensitive body cleaning means. This cleans the surface of the photosensitive drum 200. Furthermore, toner remaining on the surface of the intermediate transfer belt 204 after the secondary transfer step (secondary transfer residual toner) and deposits such as paper dust are removed from the surface of the intermediate transfer belt 204 and collected by a belt cleaner 217 serving as an intermediate transfer body cleaning means. This cleans the surface of the intermediate transfer belt 204.

[0026] FIG. 5 is a block diagram showing the control mode of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 has a control unit (CPU) 309 in the apparatus main body 400. The control unit 309 is configured with various functional blocks, such as an image processing unit 500, an engine control unit 502, a drive control unit 503, and an arithmetic unit 504. The image processing unit 500 is also configured with various functional blocks, such as an image generation unit 511, a color conversion processing unit 512, and a video count unit 513. In this embodiment, the above-mentioned functional blocks are realized by the control unit 309 executing a program stored in a storage unit 501, which is configured with ROM and RAM provided in the apparatus main body 400. The control unit 309 is connected to various parts of the image forming apparatus 100, including a belt drive motor M1 and a secondary transfer outer roller drive motor M2. The control unit 309 comprehensively controls the operation of each part of the image forming apparatus 100. The control unit 309 is also connected to an operation unit 601 provided in the image forming apparatus 100, the image reading device 300, and a host device 602 external to the image forming apparatus 100. The engine control unit 309 of the control unit 309 controls the image formation process as described above. The drive control unit 503 of the control unit 309 controls the drive rotation speed of the outer secondary transfer roller 206 as described later. The calculation unit 504 of the control unit 309 controls the drive rotation speed of the outer secondary transfer roller 206 based on the toner image distribution as described later. The image processing unit 500 will be described later.

[0027] 2. Image magnification fluctuations in the secondary transfer area When the toner image is transferred from the intermediate transfer belt 204 to the recording medium P at the secondary transfer portion N2, a speed difference may occur between the surface speed of the intermediate transfer belt 204 and the conveying speed of the recording medium P. In this case, if the conveying speed of the recording medium P is faster than the surface speed of the intermediate transfer belt 204, the toner image on the recording medium P will stretch, and the magnification of the image on the recording medium P (here, also simply referred to as "image magnification") will increase. Conversely, if the conveying speed of the recording medium P is slower than the surface speed of the intermediate transfer belt 204, the toner image on the recording medium P will shrink, and the image magnification on the recording medium P will decrease. When the image magnification changes in this way, expansion / contraction, positional deviation, etc. of the image on the recording medium P may occur.

[0028] 3. Influence of toner amount on intermediate transfer belt The intermediate transfer belt 204 applies a conveying force to the recording medium P by the combined action of the electrostatic adsorption force and frictional force due to the secondary transfer bias, and the intermolecular force of the toner. Furthermore, if toner is present between the intermediate transfer belt 204 and the recording medium P, these forces change depending on the amount of toner applied. Therefore, depending on the distribution of the toner image transferred to one sheet of recording medium P, the conveying force applied from the intermediate transfer belt 204 to the recording medium P changes while the recording medium P passes through the secondary transfer section N2. This causes the conveying speed of the recording medium P to change continuously, and the magnification of the image transferred to the recording medium P also changes within one sheet of recording medium P (partial magnification variation).

[0029] 4. Effect of the outer secondary transfer roller In the case where the outer secondary transfer roller 206 is configured to rotate by being driven without being externally driven, the outer secondary transfer roller 206 is driven to rotate by the intermediate transfer belt 204 via the recording medium P. Therefore, the greater the rotational load on the outer secondary transfer roller 206, the greater the resistance to conveyance of the recording medium P, and the lower the conveyance speed of the recording medium P.

[0030] In contrast, when the outer secondary transfer roller 206 is configured to be driven by an external drive motor or the like, the recording medium P receives a conveying force from the intermediate transfer belt 204 from the transfer surface side, and also receives a conveying force from the outer secondary transfer roller 206 from the non-transfer surface side. In this case, if the rotation speed of the outer secondary transfer roller 206 is increased so that the surface speed (circumferential speed) of the outer secondary transfer roller 206 is faster than when the outer secondary transfer roller 206 is driven, the recording medium P receives a conveying force from the outer secondary transfer roller 206 in the conveying direction, and the conveying speed of the recording medium P increases. Conversely, if the rotation speed of the outer secondary transfer roller 206 is decreased so that the surface speed of the outer secondary transfer roller 206 is slower than when the outer secondary transfer roller 206 is driven, the outer secondary transfer roller 206 applies a conveying force to the recording medium P in the direction opposite to the conveying direction. In this case, the outer secondary transfer roller 206 acts as a conveying resistance, and the conveying speed of the recording medium P decreases.

[0031] Therefore, by changing the rotation speed of the outer secondary transfer roller 206, the conveying speed of the recording medium P can be changed, and as a result, the image magnification on the recording medium P can be changed.

[0032] 5. Correction of image magnification using the outer secondary transfer roller As described above, the image magnification on the recording medium P varies depending on the amount of applied toner and the rotational speed of the outer secondary transfer roller 206. FIG. 2 shows the relationship between the amount of applied toner and the image magnification for each rotational speed of the outer secondary transfer roller 206. In FIG. 2, the horizontal axis represents the amount of applied toner, and the vertical axis represents the image magnification (which increases toward the top of the figure). FIG. 3 also shows the relationship between the rotational speed of the outer secondary transfer roller 206 and the image magnification for each amount of applied toner. In FIG. 3, the horizontal axis represents the rotational speed of the outer secondary transfer roller 206 (which increases toward the right of the figure), and the vertical axis represents the image magnification (which increases toward the top of the figure). In FIG. 2(a), the relationship between the amount of applied toner and the image magnification is a curve that maximizes the image magnification at a certain amount of applied toner and then inflects as the amount of applied toner changes. In contrast, FIG. 2(b) shows, as a "comparison example," the relationship between the amount of applied toner and the image magnification when the image magnification is proportional to the amount of applied toner.

[0033] As described above, the higher the driving rotation speed of the outer secondary transfer roller 206, the faster the conveying speed of the conveying member P and the larger the image magnification. Therefore, both the curve in Fig. 2(a) and the straight line in Fig. 2(b) move upward in the figure as the driving rotation speed of the outer secondary transfer roller 206 increases. Also, the greater the amount of toner carried, the more easily the image magnification changes in response to changes in the driving rotation speed of the outer secondary transfer roller 206, so the gap between the curve and the straight line tends to widen.

[0034] 3 shows the relationship between the image magnification and the driving rotation speed of the outer secondary transfer roller 206 for three levels of toner amount: 0%, 20%, and 100%. As shown in FIG. 3, the image magnification changes approximately linearly (is proportional) to the driving rotation speed of the outer secondary transfer roller 206.

[0035] When the image magnification is proportional to the amount of applied toner, as in the "comparative example" shown in FIG. 2B, there exists a driving rotation speed of the outer secondary transfer roller 206 (here, also referred to as the "neutral rotation speed") at which the image magnification is constant (shows a horizontal straight line) relative to the amount of applied toner in FIG. 2B. In addition, in FIG. 3B, which shows the relationship between the driving rotation speed of the outer secondary transfer roller 206 and the image magnification in the "comparative example," the lines for each amount of applied toner intersect at a single point when the driving rotation speed of the outer secondary transfer roller 206 is the neutral rotation speed. Therefore, in a "comparative example" where the image magnification is proportional to the amount of applied toner, the neutral rotation speed can be determined as follows. That is, the neutral rotation speed can be determined by measuring the image magnification for four combinations of conditions, including a minimum of two conditions for the driving rotation speed of the outer secondary transfer roller 206 and a minimum of two conditions for the amount of applied toner, based on the relationship in FIG. 3B.

[0036] The image magnification can be calculated by outputting an image with feature points in the transport direction (sub-scanning direction) as shown in Fig. 4 and reading the distance between the feature points. Alternatively, the transport speed of the recording medium P can be measured using a speed sensor or the like and converted into the image magnification.

[0037] On the other hand, when the relationship between the amount of applied toner and the image magnification is not proportional, as shown in FIG. 2A, there is no neutral rotation speed of the outer secondary transfer roller 206 at which the image magnification is constant relative to the amount of applied toner. Therefore, as shown in FIG. 3A, the straight lines for each amount of applied toner intersect at different points. As a result, a method of measuring the image magnification under several conditions that combine the driving rotation speed of the outer secondary transfer roller 206 and the amount of applied toner cannot determine an appropriate driving rotation speed of the outer secondary transfer roller 206 to achieve a predetermined image magnification.

[0038] Therefore, even if the relationship between the image magnification and the amount of toner applied is not proportional, in order to achieve a predetermined image magnification for an image with an arbitrary amount of toner applied, it is desirable to drive the secondary transfer outer roller 206 at different driving rotation speeds depending on the amount of toner applied.

[0039] 6. Configuration of processing unit related to image formation Next, the configuration and operation of the processing units involved in image formation in the image forming apparatus 100 of this embodiment will be described with reference to FIG.

[0040] <Image Generation Section> The image generation unit 511 generates a raster image that can be printed based on print data (including image data) received from a host device 602 or the like located outside the device main body 400, and outputs it for each pixel as RGB data and attribute data indicating the data attributes.

[0041] Here, the image data received by the image generation unit 511 may be image data read by the image reading device 300 provided in the image forming apparatus 100. The image reading device 300 is a device that reads an original placed on a platen glass, and has a function of converting the original into a raster image that can be printed (reader image processing unit). Alternatively, the image data received by the image generation unit 511 may be image data received from an image reading means arranged outside the apparatus main body 400 via an interface (not shown) provided in the image forming apparatus 100.

[0042] <Color conversion processing section> The RGB data generated by the image generation unit 511 is sent to the color conversion processing unit 512. The color conversion processing unit 512 converts the RGB data to match the toner colors and generates CMYK data. The CMYK data has a value corresponding to the amount of toner applied for each pixel of CMYK, and each color is expressed in 8 bits, for example, from 0 to 255. The amount of toner applied for each pixel is the sum of the amounts of toner applied for each CMYK color.

[0043] <Video Counting Section> The CMYK data color-converted by the color conversion processing unit 512 is transmitted to the video counting unit 513. The video counting unit 513 detects the amount of applied toner for pixels in each divided area when the CMYK data (areas within the page to be printed out of the image data) generated by the color conversion processing unit 512 is divided into predetermined areas (hereinafter also referred to as "divided areas").

[0044] Fig. 6 shows an example in which the image data is divided into widths of Ly in the transport direction (sub-scanning direction) and widths of Lx in the main scanning direction. In Fig. 6, the division number in the main scanning direction (x direction in the figure) is i (i = 1 to m), and the division number in the transport direction (y direction in the figure) is j (j = 1 to n).

[0045] Based on the amount of toner applied to the pixels in each divided area, the divided video count value Vc of each divided area is calculated. ij Here, the divided video count value Vc ij is a representative value of the values ​​detected by the video count unit 513. The representative value is determined as, for example, the simple sum of the toner amount for each pixel in each divided area, the sum of the maximum values ​​per unit area in each divided area, or the area ratio (coverage rate, area ratio) of the toner in each divided area.

[0046] Hereafter, the average toner amount (0 to 1023) in each divided area is calculated as the divided video count value Vc ij 2, 3, 7, etc., the amount of applied toner (%) represents the ratio of the amount of applied toner of each color to the total amount of applied toner when the amount of applied toner of each pixel is at its maximum.

[0047] 7. Configuration of the calculation unit for the driving rotation speed of the outer secondary transfer roller Next, the configuration and operation of the calculation unit for the driving rotation number of the outer secondary transfer roller 206 in the image forming apparatus 100 of this embodiment will be described with reference to FIG.

[0048] <Arithmetic section> The calculation unit 504 suppresses image magnification fluctuations caused by toner image distribution on one sheet of recording medium P, and determines a command value (drive command value) for the drive rotation speed of the outer secondary transfer roller 206 to achieve a desired image magnification.

[0049] FIG. 7 shows the relationship between the toner amount and the drive rotation speed of the outer secondary transfer roller 206 to achieve a predetermined image magnification, in the relationship between the toner amount and the drive rotation speed of the outer secondary transfer roller 206 and the image magnification shown in FIGS. 2(a) and 3(a). In FIG. 7, the horizontal axis represents the toner amount, and the vertical axis represents the drive rotation speed of the outer secondary transfer roller 206 (which increases upward in the figure). FIG. 7 also shows the above relationship for each target image magnification. A positive target image magnification indicates that the image will expand, and a negative target image magnification indicates that the image will shrink. As shown in FIG. 7, it can be seen that the amount of change in the drive rotation speed of the outer secondary transfer roller 206 required to accommodate changes in the toner amount varies depending on the target image magnification.

[0050] Here, while one sheet of recording medium P passes through the secondary transfer section N2, the amount of toner carried on the toner image to be secondarily transferred changes from moment to moment, so if the drive rotation speed of the outer secondary transfer roller 206 is changed too much, there are concerns from the viewpoint of drive control performance and vibration. Therefore, it is desirable to set the target image magnification so that the range of change in the drive rotation speed of the outer secondary transfer roller 206 in response to changes in the amount of toner carried is small. However, when the target image magnification is set in this way, the image size does not necessarily match the original image. Therefore, regarding the magnification error with respect to the original image that occurs when a predetermined target image magnification is set, it is desirable to change the magnification (magnification conversion) of the original image data in advance to cancel this magnification error.

[0051] The relationship between the amount of applied toner and the number of rotations of the outer secondary transfer roller 206 shown in FIG. 7 is stored in the storage unit 501. The calculation unit 504 calculates the divided video count value Vc for each divided area sent from the video count unit 513. ij The number of rotations of the outer secondary transfer roller 206 corresponding to the average amount of applied toner is determined for each divided area based on the above relationship read out from the storage unit 501.

[0052] The relationship between the amount of toner applied and the number of rotations of the outer secondary transfer roller 206 as shown in FIG. 7 may be set for each type of recording medium P and stored in the storage unit 501. The calculation unit 504 may then select and use the corresponding relationship based on information about the recording medium P onto which the toner image is transferred to determine the number of rotations of the outer secondary transfer roller 206. Here, the information about the recording medium P includes any information that can distinguish the recording medium P, such as attributes based on general characteristics (so-called paper type categories) such as plain paper, fine paper, glossy paper, glossy paper, coated paper, embossed paper, thick paper, thin paper, synthetic paper, and label paper; numerical values ​​and numerical ranges for physical properties such as basis weight, thickness, surface glossiness, whiteness, and rigidity; and brand (including manufacturer, product name, and model number). Each recording medium P distinguished by the information about the recording medium P can be considered to constitute a type of recording medium P. In addition, information regarding the recording medium P may be included in or replaced by print mode information that specifies the operational settings of the image forming device 100, such as "plain paper mode," "thick paper mode," or "thin paper mode."

[0053] 8. Operation of the secondary transfer unit for each page The following describes the operation around the secondary transfer portion N2 to suppress image magnification fluctuations when different toner image patterns are transferred to the recording medium P for each page. As described above, the secondary transfer portion (secondary transfer nip portion) N2, where the intermediate transfer belt 204 and the outer secondary transfer roller 206 come into contact, is formed by the inner secondary transfer roller 205 and the outer secondary transfer roller 206, which are arranged to face each other with the intermediate transfer belt 204 in between.

[0054] In this embodiment, the outer secondary transfer roller 206 is configured to be switchable between a contact state in which it contacts the intermediate transfer belt 204 and a spaced state in which it is spaced from the intermediate transfer belt 204. The image forming apparatus 100 is provided with a contact / separation mechanism (not shown) for switching the contact state of the outer secondary transfer roller 206 with respect to the intermediate transfer belt 204 and the spaced state. The contact / separation mechanism is controlled by the control unit 309 to switch the contact state of the outer secondary transfer roller 206 with respect to the intermediate transfer belt 204 and the spaced state. In this embodiment, the outer secondary transfer roller 206 starts rotating in the spaced state before the first recording medium P of a job (a series of operations for forming images on one or more recording media P, started by a single start command) is transported from the registration unit 208 to the secondary transfer unit N2. The outer secondary transfer roller 206 then contacts the intermediate transfer belt 204 before the recording medium P reaches the secondary transfer unit N2. At this time, the driving rotation number of the outer secondary transfer roller 206 is set to a driving rotation number N ITB is.

[0055] The recording medium P fed from cassette 214 waits in registration unit 208, and then is transported from registration unit 208 with the timing controlled by control unit (CPU) 309 so as to align the position of the toner image with that of the recording medium P. Thereafter, in synchronization with the timing at which the recording medium P enters secondary transfer unit N2, drive control unit 503 of control unit 309 controls secondary transfer outer roller drive motor M2 so as to change the drive rotation speed of secondary transfer outer roller 206 in accordance with the toner image distribution. The drive rotation speed of secondary transfer outer roller 206 when the toner image at a position distance y from the leading edge of the recording medium P in the transport direction is transferred is controlled as the drive rotation speed at time t when the recording medium P is transported by distance y from the leading edge of the recording medium P in the transport direction at secondary transfer unit N2.

[0056] 9. Control according to the toner amount distribution in the sub-scanning direction averaged in the main scanning direction Using the image shown in FIG. 8A as an example, the determination of the driving rotation speed of the outer secondary transfer roller 206 for suppressing image magnification fluctuations due to toner amount distribution will be described.

[0057] The image shown in FIG. 8A is converted into the toner amount for each pixel by the color conversion processing unit 512, and the video count unit 513 converts the divided video count value Vc for each predetermined region (divided region). j In this embodiment, as shown in FIG. 8B, the divided areas for calculating the divided video count value are those in which the image data of each page is divided only in the sub-scanning direction out of the main scanning direction and the sub-scanning direction. The average amount of applied toner in each divided area is calculated as the divided video count value Vc j Therefore, in each divided region in the sub-scanning direction, the amount of applied toner is averaged in the main scanning direction. Note that, as described above, the average amount of applied toner for each divided region in the main scanning direction and the sub-scanning direction may be calculated, and the average amount of applied toner for each divided region in the sub-scanning direction may be averaged in the main scanning direction.

[0058] The calculation unit 504 calculates the divided video count value Vc for each divided area in the sub-scanning direction. jis applied to information (relational formula, table, etc.) showing the relationship between the amount of applied toner and the driving rotation speed of the outer secondary transfer roller 206 shown in Fig. 7. As a result, the calculation unit 504 obtains a command value (nj) for the driving rotation speed of the outer secondary transfer roller 206 to correct the image magnification to a predetermined value (Fig. 8(c)).

[0059] In this way, by changing the driving rotation speed of the outer secondary transfer roller 206 based on the toner amount distribution of the image transferred onto one sheet of recording medium P, it is possible to suppress fluctuations in image magnification.

[0060] 8C, since the command value for the number of drive rotations of the outer secondary transfer roller 206 based on the toner amount distribution corresponds to each divided area in the sub-scanning direction, the number of drive rotations of the outer secondary transfer roller 206 may be discontinuous in the sub-scanning direction. Therefore, in order to avoid a sudden acceleration / deceleration of the number of drive rotations of the outer secondary transfer roller 206, the command value for the number of drive rotations of the outer secondary transfer roller 206 may be averaged in the sub-scanning direction by, for example, finding a moving average in the sub-scanning direction (conveyance direction).

[0061] Furthermore, because the driving rotation speed of the outer secondary transfer roller 206 is calculated for each divided area in the sub-scanning direction, narrower the width of each divided area in the sub-scanning direction, the better the tracking ability to changes in the toner application amount distribution. However, at the secondary transfer unit N2, the toner image is transferred sequentially to the recording medium P in the transport direction (sub-scanning direction) within the area sandwiched between the intermediate transfer belt 204 and the outer secondary transfer roller 206. At this time, changes in the toner application amount within a width smaller than the length of the recording medium P in the transport direction where it contacts the intermediate transfer belt 204 are unlikely to appear as partial magnification errors in the image. Furthermore, dividing the divided areas for calculating the driving rotation speed of the outer secondary transfer roller 206 from the divided video count values ​​into excessively small areas undesirably increases the amount of calculation processing. For this reason, the effect of making the width of the divided areas in the sub-scanning direction narrower than the contact length between the intermediate transfer belt 204 and the recording medium P at the secondary transfer unit N2 gradually decreases. In other words, it is desirable that the width of the divided regions in the sub-scanning direction is greater than the contact length between the intermediate transfer belt 204 and the recording medium P at the secondary transfer portion N2.

[0062] In this embodiment, the rotational speed of the outer secondary transfer roller 206 is changed in accordance with the distribution of the toner amount, thereby stabilizing the conveyance speed of the recording medium P and suppressing fluctuations in image magnification. However, the present invention is not limited to this configuration, and may be configured as shown in FIG. 11, for example. That is, the outer secondary transfer roller 206 is pressed against the intermediate transfer belt 204 via the outer secondary transfer belt (outer transfer member) 218. The recording medium P is then sandwiched between the intermediate transfer belt 204 and the outer secondary transfer belt 218, and the toner image is secondarily transferred to the recording medium P. The outer secondary transfer belt 218 is stretched over multiple stretching rollers, including the outer secondary transfer belt drive roller 219, and the outer secondary transfer roller 206 is disposed on the inner circumferential side of the outer secondary transfer belt 219. In this case, the rotational speed of the outer secondary transfer belt drive roller 219, which drives the outer secondary transfer belt 218 to rotate, is controlled in accordance with the distribution of the toner amount, thereby stabilizing the conveyance speed of the recording medium P and suppressing fluctuations in image magnification.

[0063] Thus, in this embodiment, the image forming apparatus 100 includes a rotatable image carrier 204 that carries a toner image, a rotatable transfer member 206 that abuts against the image carrier 204 to form a transfer section N2 and transfers the toner image from the image carrier 204 to a recording medium P passing through the transfer section N2, a first drive unit M1 that applies a driving force to the image carrier 204, a second drive unit M2 that applies a driving force to the transfer member 206, an acquisition unit (video count unit) 513 that acquires information regarding the distribution of the toner image transferred to the recording medium P at the transfer section N2 within the surface of the recording medium P, and a control unit 309 (more specifically, a drive control unit 503 and a calculation unit 504) that controls the second drive unit M2, and the control unit 309 is capable of changing the number of rotations of the transfer member 206 driven by the second drive unit M2 while the recording medium P passes through the transfer section N2 based on the information regarding the distribution. In this embodiment, the acquisition unit 513 acquires, as information regarding the distribution, an index value correlating with the amount of toner in the toner image to be transferred to each of the divided regions obtained by dividing the surface of the recording medium P in the conveyance direction of the recording medium. The control unit 309 then sets the number of rotations of the transfer member 206 driven by the second drive unit M2 when each of the divided regions passes through the transfer unit N2 based on the index value for each of the divided regions. In this embodiment, the relationship between the index value for each of the divided regions and the number of rotations is nonlinear. In this embodiment, the index value for each of the divided regions represents an average value obtained by averaging, in a direction perpendicular to the conveyance direction, the index value correlating with the amount of toner in the toner image to be transferred to each of the divided regions. Here, the index value correlating with the amount of toner is typically the area ratio of the toner image per unit area. The width of each of the divided regions in the conveyance direction is preferably greater than the length of the contact area between the image carrier 204 and the recording medium P in the conveyance direction. The transfer member 206 may be a roller or an endless belt.

[0064] As described above, in this embodiment, while the toner image is being transferred to the recording medium P at the secondary transfer unit N2, the driving rotation speed of the outer secondary transfer roller 206 is changed within the plane according to the toner image distribution of the image to be formed on the recording medium P. This makes it possible to suppress fluctuations in image magnification due to the toner image distribution. According to this embodiment, even if the amount of toner applied is not proportional to the amount of expansion or contraction of the image, it is possible to suppress fluctuations in image magnification by appropriately controlling the driving rotation speed of the outer secondary transfer roller 206. Therefore, according to this embodiment, even for images with various different toner image distributions, it is possible to effectively suppress fluctuations in the magnification of the image on the recording medium P and suppress expansion or contraction, misalignment, etc. of the image on the recording medium P.

[0065] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed description thereof will be omitted.

[0066] In the first embodiment, the distribution of the amount of applied toner in the main scanning direction is treated as an average, but even if the average value is the same, there may be a difference in the appropriate number of rotations for driving the outer secondary transfer roller 206 depending on whether the amount of applied toner in the main scanning direction is relatively uniform or uneven. Therefore, in this embodiment, the distribution of the amount of applied toner in the main scanning direction is also taken into consideration when determining the number of rotations for driving the outer secondary transfer roller 206. Note that, since the calculation for determining the number of rotations for driving the outer secondary transfer roller 206 is the same as in the first embodiment, a description thereof will be omitted where appropriate.

[0067] 1. Control that takes into account the distribution of toner amount in the main scanning direction Using the image shown in FIG. 9A as an example, the determination of the driving rotation speed of the outer secondary transfer roller 206 for suppressing image magnification fluctuations due to toner amount distribution in this embodiment will be described.

[0068] In this embodiment, the video count unit 513 calculates a divided video count value Vc for each divided area divided into predetermined sizes in the main scanning direction and the sub-scanning direction as shown in FIG. 9(b). ij is required.

[0069] Next, consider a row of divided areas aligned in the main scanning direction corresponding to one divided area (for example, the jth area) in the sub-scanning direction. The toner amount distribution in the main scanning direction, that is, the divided video count value Vc ij In this case, the image magnification is equivalent to the amount of toner applied in the main scanning direction (divided video count value Vc j ) can be assumed. Specifically, the divided video count values ​​Vc ij A weight (weighting coefficient) w is set according to the size of (i=1 to m), and is regarded as follows:

[0070]

number

[0071] 2A, the weight w is set so that the toner amount increases in the vicinity of 20% along this curve. In other words, the weight w is set so that the toner amount with a relatively large image magnification fluctuation is larger than the toner amount with a relatively small image magnification fluctuation.

[0072] Based on this concept, the driving rotation speed of the outer secondary transfer roller 206 can be determined as follows. That is, the relationship between the driving rotation speed of the outer secondary transfer roller 206 and the image magnification for the divided area in the sub-scanning direction where the distribution of the toner amount in the main scanning direction is biased can be determined by dividing the divided video count value Vc ijThe command value for the number of rotations of the outer secondary transfer roller 206 to correct the image magnification to a predetermined value can be calculated by weighting the number of rotations with a weight w corresponding to the magnitude of (i = 1 to m). From the linear equation obtained in this way, the number of rotations of the outer secondary transfer roller 206 to correct the image magnification to a predetermined value can be uniquely calculated. In other words, from among the relational expressions between the number of rotations and the image magnification for each weighted average of the amount of applied toner in the main scanning direction based on the amount of applied toner, the relational expression corresponding to the weighted average calculated as described above can be used to calculate the command value for the number of rotations of the outer secondary transfer roller 206 to correct the image magnification to a predetermined value. Note that information on the above relational expression (linear expression) can be stored in the storage unit 501. Information on this relational expression (linear expression) may be set for each type of recording medium P and stored in the storage unit 501.

[0073] By performing the above process sequentially for each divided area divided in the sub-scanning direction, it is possible to determine the command value for the drive rotation speed of the outer secondary transfer roller 206 corresponding to each divided area in the sub-scanning direction, which corrects the image magnification in the transport direction, as shown in Figure 9(c).

[0074] In this embodiment, the image pattern is converted into a one-dimensional image based on the relationship between the change in image density in the main scanning direction and the change in image magnification (i.e., the change in image magnification is converted into an equivalent change in image density in the sub-scanning direction). The speed of the outer secondary transfer roller 206 is then controlled to cancel the change in image magnification due to the change in image density in the sub-scanning direction. That is, in this embodiment, the image area is divided in the main scanning direction, and the weight of the image density on the image magnification in the sub-scanning direction for each divided image area is estimated. The image pattern is then converted into a uniform image pattern in the main scanning direction with equivalent sensitivity to the image magnification in the sub-scanning direction. The speed of the outer secondary transfer roller 206 required to achieve a predetermined image magnification is calculated based on the relationship between the speed of the outer secondary transfer roller 206 and the image magnification for each toner amount calculated in advance. This process is performed for each image area divided in the sub-scanning direction. As a result, for any image pattern, the speed of the outer secondary transfer roller 206 is variably controlled within the plane (from the time the leading edge of the recording medium P enters the secondary transfer unit N2 until the trailing edge exits) so that the partial magnification in the transport direction is stabilized, taking into account the distribution of toner images within the plane. As a result, it is possible to reduce the fluctuation in image magnification within the plane in the transport direction for any image pattern.

[0075] 9C, since the command value for the number of drive rotations of the outer secondary transfer roller 206 based on the distribution of applied toner amount corresponds to each divided region in the sub-scanning direction, the number of drive rotations of the outer secondary transfer roller 206 may be discontinuous in the sub-scanning direction. Therefore, in order to avoid a sudden acceleration / deceleration of the number of drive rotations of the outer secondary transfer roller 206, the command value for the number of drive rotations of the outer secondary transfer roller 206 may be averaged in the sub-scanning direction by, for example, finding a moving average in the sub-scanning direction (conveyance direction).

[0076] 2. When considering the toner image position in the main scanning direction Furthermore, when there is a bias in the distribution of toner application amount within the divided regions in the main scanning direction as described above, the image magnification may also change depending on the position of the toner image in the main scanning direction. For example, there are cases where the toner image is positioned near the center in the main scanning direction and cases where it is positioned near the edge of the recording medium P in the main scanning direction. In such cases, the drive rotation speed of the outer secondary transfer roller 206 can be determined taking into account the position of the toner image in the main scanning direction.

[0077] FIG. 10 shows an example of the rate of change of image magnification with respect to the position of the toner image in the main scanning direction. In the example shown in FIG. 10, the more distant the position is from the center in the main scanning direction, the greater the influence on the image magnification, even for the same toner image. Here, the contribution to the image magnification at position x from the center in the longitudinal direction (approximately parallel to the main scanning direction) of the outer secondary transfer roller 206 is expressed as the divided video count value Vc ij Similarly, the video signal is divided into multiple divided regions in the longitudinal direction corresponding to the divided video count values. i (i=1~m).

[0078] In addition, as in the case of weighting by the amount of applied toner described above, a uniform amount of applied toner in the main scanning direction (divided video count value Vc j ) can be assumed. Therefore, similarly to the case of weighting by the amount of applied toner described above, a weighted average by the amount of applied toner and the position of the toner image can be obtained.

[0079] The driving rotation speed of the outer secondary transfer roller 206 can be calculated as follows. That is, the relationship between the driving rotation speed of the outer secondary transfer roller 206 and the image magnification for the divided area in the sub-scanning direction is calculated by multiplying the divided video count value Vc ij A weight w according to the size of (i=1 to m) and a weight f according to the position x from the center in the longitudinal direction of the outer secondary transfer roller 206 iand a weighted average of the above. From the linear equation obtained in this manner, the number of drive rotations of the outer secondary transfer roller 206 for correcting the image magnification to a predetermined value can be uniquely determined. In other words, from among the relational expressions between the number of drive rotations and the image magnification for each weighted average of the toner application amount in the main scanning direction depending on the toner application amount and the position of the toner image, a relational expression corresponding to the weighted average obtained as described above can be used to determine a command value for the number of drive rotations of the outer secondary transfer roller 206 for correcting the image magnification to a predetermined value. Note that information on the above relational expression (linear expression) can be stored in the storage unit 501. Furthermore, information on this relational expression (linear expression) may be set for each type of recording medium P and stored in the storage unit 501.

[0080] By performing the above process sequentially for each divided area divided in the sub-scanning direction, it is possible to determine the command value for the drive rotation speed of the outer secondary transfer roller 206 corresponding to each divided area in the sub-scanning direction, which corrects the image magnification in the transport direction, as in Figure 9(c).

[0081] In this way, the image pattern can be made one-dimensional based on the relationship between image density change and image magnification change, taking into account the image position in the main scanning direction (i.e., converted into an image density change in the sub-scanning direction equivalent to the image magnification change). The speed of the outer secondary transfer roller 206 can then be controlled to cancel image magnification change due to image density change in the sub-scanning direction. That is, the image area can be divided in the main scanning direction, and the weight of the image density for each divided image area and its position in the main scanning direction on the image magnification in the sub-scanning direction can be estimated. The image pattern can then be converted into one that is uniform in the main scanning direction and has equivalent sensitivity to image magnification in the sub-scanning direction. This allows the speed of the outer secondary transfer roller 206 to be variably controlled within the plane so that the partial magnification in the transport direction is stabilized in a manner more in line with the in-plane toner image distribution for any image pattern. As a result, the in-plane image magnification change in the transport direction can be further reduced for any image pattern.

[0082] Even in this case, if the driving rotation speed of the outer secondary transfer roller 206 is calculated based on the toner amount distribution for each divided region divided in the sub-scanning direction, the driving rotation speed of the outer secondary transfer roller 206 may become discontinuous in the sub-scanning direction. Therefore, in order to avoid a sudden acceleration / deceleration of the driving rotation speed of the outer secondary transfer roller 206, the command value for the driving rotation speed of the outer secondary transfer roller 206 may be averaged in the sub-scanning direction by, for example, finding a moving average in the sub-scanning direction (conveyance direction).

[0083] 10 shows an example of a characteristic in which the contribution of the toner image position in the main scanning direction to image magnification increases the further away from the center of the outer secondary transfer roller 206, but this is not the only characteristic. The contribution of the toner image position in the main scanning direction to image magnification varies depending on the device configuration, such as the shape and material of the outer secondary transfer roller and the distribution of the clamping force of the recording medium P at the secondary transfer unit N2. Therefore, the drive speed of the outer secondary transfer roller 206 can be determined taking into account the contribution to image magnification specific to each device configuration.

[0084] As in the first embodiment, the acquisition unit 513 acquires, as information about the distribution, an index value correlating with the amount of toner in the toner image to be transferred to each of the divided regions obtained by dividing the surface of the recording medium P into a plurality of regions in the conveyance direction of the recording medium. The control unit 309 sets the number of rotations of the transfer member 206 driven by the second drive unit M2 when each of the divided regions passes through the transfer unit N2 based on the index value for each of the divided regions. In this embodiment, the index value for each of the divided regions represents a weighted average value obtained by weighting and averaging index values ​​related to the amount of toner in the toner image to be transferred to each of the divided regions in the direction perpendicular to the conveyance direction of the recording medium P based on the index value for each of the divided regions. Alternatively, in this embodiment, the index value for each of the divided regions represents a weighted average value obtained by weighting and averaging index values ​​related to the amount of toner in the toner image to be transferred to each of the divided regions in the direction perpendicular to the conveyance direction based on the index value for each of the divided regions and information about the position of each of the divided regions in the direction perpendicular to the conveyance direction. In this case, the information about the position may indicate the distance from the center of the recording medium P in a direction perpendicular to the transport direction.

[0085] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0086] In the above embodiment, the present invention is applied to a tandem color image forming apparatus equipped with multiple photosensitive members, but the present invention is not limited to such an embodiment. The present invention can also be applied to, for example, a single-drum color image forming apparatus or a monochrome image forming apparatus in which a multi-color toner image is formed on a single photosensitive member and transferred to a recording medium via an intermediate transfer member. [Explanation of symbols]

[0087] 100 Image forming device 204 Intermediate transfer belt 205 Secondary transfer inner roller 206 Secondary transfer outer roller 309 Control Unit 500 Image Processing Unit 503 Drive control unit 504 Arithmetic section M1 Belt Drive Motor M2 Secondary transfer outer roller drive motor

Claims

1. A method of manufacturing a toner image forming apparatus, comprising: a rotatable image carrier that carries a toner image; a rotatable transfer member that contacts the image carrier to form a transfer section and transfers a toner image from the image carrier to a recording medium that passes through the transfer section; a first driving unit that applies a driving force to the image carrier; a second driving unit that applies a driving force to the transfer member; an acquiring unit that acquires information regarding the distribution of the toner image transferred onto the recording medium by the transfer unit; a control unit that controls the second driving unit; and the control unit is capable of changing the number of rotations of the transfer member driven by the second drive unit while the recording medium passes through the transfer unit based on information about the distribution; the acquiring unit acquires, as information about the distribution, an index value correlated with the amount of toner in a toner image to be transferred to each of a plurality of divided areas obtained by dividing the surface of the recording medium in a conveying direction of the recording medium, the control unit sets a driving rotation number of the transfer member by the second driving unit when each of the divided areas passes through the transfer unit based on the index value for each of the divided areas; the relationship between the index value for each of the divided regions and the driving rotation speed is nonlinear; An image forming apparatus characterized by:

2. 2. The image forming apparatus according to claim 1, wherein the index value for each of the divided areas indicates an average value obtained by averaging index values ​​correlated with the amount of toner in the toner image transferred to each of the divided areas in a direction perpendicular to the transport direction.

3. A rotatable image carrier that carries a toner image; a rotatable transfer member that contacts the image carrier to form a transfer section and transfers a toner image from the image carrier to a recording medium that passes through the transfer section; a first driving unit that applies a driving force to the image carrier; a second driving unit that applies a driving force to the transfer member; an acquiring unit that acquires information regarding the distribution of the toner image transferred onto the recording medium by the transfer unit; a control unit that controls the second driving unit; and the control unit is capable of changing the number of rotations of the transfer member driven by the second drive unit while the recording medium passes through the transfer unit based on information about the distribution; the acquiring unit acquires, as information about the distribution, an index value correlated with the amount of toner in a toner image to be transferred to each of a plurality of divided areas obtained by dividing the surface of the recording medium in a conveying direction of the recording medium, the control unit sets a driving rotation number of the transfer member by the second driving unit when each of the divided areas passes through the transfer unit based on the index value for each of the divided areas; the index value for each of the divided regions indicates a weighted average value obtained by weighting and averaging index values ​​relating to the toner amounts of the toner images transferred to a plurality of regions in each of the divided regions in a direction perpendicular to the transport direction, based on the index values ​​for each of the plurality of regions; An image forming apparatus characterized by:

4. A rotatable image carrier that carries a toner image; a rotatable transfer member that contacts the image carrier to form a transfer section and transfers a toner image from the image carrier to a recording medium that passes through the transfer section; a first driving unit that applies a driving force to the image carrier; a second driving unit that applies a driving force to the transfer member; an acquiring unit that acquires information regarding the distribution of the toner image transferred onto the recording medium by the transfer unit; a control unit that controls the second driving unit; and the control unit is capable of changing the number of rotations of the transfer member driven by the second drive unit while the recording medium passes through the transfer unit based on information about the distribution; the acquiring unit acquires, as information about the distribution, an index value correlated with the amount of toner in a toner image to be transferred to each of a plurality of divided areas obtained by dividing the surface of the recording medium in a conveying direction of the recording medium, the control unit sets a driving rotation number of the transfer member by the second driving unit when each of the divided areas passes through the transfer unit based on the index value for each of the divided areas; the index value for each of the divided regions indicates a weighted average value obtained by weighting and averaging index values ​​relating to the toner amounts of the toner images transferred to a plurality of regions in each of the divided regions in a direction perpendicular to the transport direction, based on the index values ​​for each of the plurality of regions and information relating to the positions of each of the plurality of regions in the direction perpendicular to the transport direction; An image forming apparatus characterized by:

5. 5. The image forming apparatus according to claim 4, wherein the information about the position indicates a distance from the center of the recording medium in a direction perpendicular to the transport direction.

6. An image forming apparatus as described in any one of claims 3 to 5, characterized in that the relationship between the index value for each of the divided areas and the drive rotation speed is nonlinear.

7. 7. The image forming apparatus according to claim 1, wherein the index value correlated with the amount of toner is an area ratio of the toner image per unit area.

8. A rotatable image carrier that carries a toner image; a rotatable transfer member that contacts the image carrier to form a transfer section and transfers a toner image from the image carrier to a recording medium that passes through the transfer section; a first driving unit that applies a driving force to the image carrier; a second driving unit that applies a driving force to the transfer member; an acquiring unit that acquires information regarding the distribution of the toner image transferred onto the recording medium by the transfer unit; a control unit that controls the second driving unit; and the control unit is capable of changing the number of rotations of the transfer member driven by the second drive unit while the recording medium passes through the transfer unit based on information about the distribution; the acquiring unit acquires, as information about the distribution, an index value correlated with the amount of toner in a toner image to be transferred to each of a plurality of divided areas obtained by dividing the surface of the recording medium in a conveying direction of the recording medium, the control unit sets a driving rotation number of the transfer member by the second driving unit when each of the divided areas passes through the transfer unit based on the index value for each of the divided areas; The index value correlated with the toner amount is the area ratio of the toner image per unit area, An image forming apparatus characterized by:

9. 9. The image forming apparatus according to claim 1, wherein the width of each of the divided areas in the transport direction is greater than the length of a contact area between the image carrier and the recording medium in the transport direction.

10. A rotatable image carrier that carries a toner image; a rotatable transfer member that contacts the image carrier to form a transfer section and transfers a toner image from the image carrier to a recording medium that passes through the transfer section; a first driving unit that applies a driving force to the image carrier; a second driving unit that applies a driving force to the transfer member; an acquiring unit that acquires information regarding the distribution of the toner image transferred onto the recording medium by the transfer unit; a control unit that controls the second driving unit; and the control unit is capable of changing the number of rotations of the transfer member driven by the second drive unit while the recording medium passes through the transfer unit based on information about the distribution; the acquiring unit acquires, as information about the distribution, an index value correlated with the amount of toner in a toner image to be transferred to each of a plurality of divided areas obtained by dividing the surface of the recording medium in a conveying direction of the recording medium, the control unit sets a driving rotation number of the transfer member by the second driving unit when each of the divided areas passes through the transfer unit based on the index value for each of the divided areas; a width of each of the divided regions in the transport direction is greater than a length of a contact region between the image carrier and the recording medium in the transport direction; An image forming apparatus characterized by:

11. 11. The image forming apparatus according to claim 1, wherein the transfer member is a roller or an endless belt.

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