Image forming device

Abstract

To improve the accuracy of an image formation position compared with a prior art.SOLUTION: A test chart has a first section, a second section, and a third section. The first section is a section where an image is formed while a sheet is sandwiched by first conveying means and image forming means. The second section is a section where an image is formed while the sheet is conveyed only by the image forming means. The third section is a section where an image is formed while the sheet is sandwiched by the image forming means and second conveying means. Control means determines the amount of correction of image shift by using a first measurement result indicating image shift in the first section, a second measurement result indicating image shift in the second section, and a third measurement result indicating image shift in the third section.SELECTED DRAWING: Figure 21

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] When a sheet is skewed, the position or posture of an image printed by an image forming device may shift relative to the sheet. This occurs because the relative positional relationship between multiple conveyance rollers that convey the sheet deviates from the positional relationship assumed in the design. This shift occurs due to the accumulation of tolerances of parts and assembly positions. In the manufacturing factory of the image forming device, the relative position is properly adjusted using dedicated assembly tools. However, slight relative positional shifts may occur due to transportation from the factory to the customer, installation in the customer's room, part replacement, etc. Patent Document 1 proposes forming patch images at the four corners of the sheet P, calculating the amount of relative positional shift based on the patch images, and correcting the formation position of the image on the sheet P. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-103805 Summary of the Invention [Problem to be solved by the invention]

[0004] Meanwhile, a conveyance path of an image forming apparatus includes multiple conveyance rollers (e.g., registration rollers, transfer rollers, and fixing rollers) that convey a sheet P from upstream to downstream while transferring the sheet P. The conveyance direction of the sheet P is dominated by the conveyance roller with the greater conveyance force (nip pressure and frictional force) between the upstream and downstream conveyance rollers that simultaneously hold the sheet. In Patent Document 1, the amount of misalignment is determined from patch images formed at the four corners of the sheet, but it is not specified which of the multiple conveyance rollers is responsible for this misalignment. Therefore, the technology of Patent Document 1 leaves room for improvement in the accuracy of image formation position. Therefore, an object of the present invention is to improve the accuracy of image formation position compared to conventional techniques. [Means for solving the problem]

[0005] The present invention is, for example, an image forming means for conveying a sheet while nipping the sheet to form an image on the sheet; a first conveying means provided upstream of the image forming means in a sheet conveying direction, the first conveying means conveying the sheet while nipping the sheet; a second conveying means provided downstream of the image forming means in a sheet conveying direction, the second conveying means configured to convey the sheet while nipping the sheet; a control means for controlling the image forming means to generate a test chart, which is a sheet on which a test image is formed, and for executing a process to correct a deviation in the formation position of the image formed on the sheet by the image forming means based on a reading result of the test image; The test chart is a first section in which an image is formed on the sheet while the sheet is sandwiched between the first conveying means and the image forming means; a second section in which an image is formed on the sheet while the sheet is being transported only by the image forming means; a third section in which an image is formed on the sheet while the sheet is sandwiched between the image forming means and the second conveying means, The image forming apparatus is characterized in that the control means determines the amount of correction for the image misalignment using a first measurement result indicating the image misalignment in the first section, a second measurement result indicating the image misalignment in the second section, and a third measurement result indicating the image misalignment in the third section. [Effects of the Invention]

[0006] According to the present invention, the accuracy of the image formation position is improved compared to the prior art. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Figure 2] FIG. 10 is a diagram illustrating the cause of image misalignment. [Figure 3] FIG. 10 is a diagram illustrating the cause of image misalignment. [Figure 4] 10A and 10B are diagrams illustrating image deviations for each section. [Figure 5] 10A and 10B are diagrams illustrating image deviations for each section before and after correction. [Figure 6] 10A and 10B are diagrams illustrating image deviations for each section before and after correction. [Figure 7] FIG. 10 is a diagram illustrating the cause of image misalignment. [Figure 8] FIG. 10 is a diagram illustrating the cause of image misalignment. [Figure 9] 10A and 10B are diagrams illustrating image deviations for each section. [Figure 10] 10A and 10B are diagrams illustrating image deviations for each section before and after correction. [Figure 11] FIG. 10 is a diagram illustrating the cause of image misalignment. [Figure 12] FIG. 10 is a diagram illustrating the cause of image misalignment. [Figure 13] FIG. 10 is a diagram illustrating the cause of image misalignment. [Figure 14] 10A and 10B are diagrams illustrating image deviations for each section. [Figure 15] A diagram explaining the separation mechanism [Figure 16] 10A and 10B are diagrams illustrating image deviations for each section. [Figure 17] FIG. 2 is a diagram illustrating an image reading device. [Figure 18] FIG. 2 is a diagram illustrating an image reading device. [Figure 19] FIG. 4 is a diagram illustrating a mechanical adjustment mechanism. [Figure 20] FIG. [Figure 21] FIG. [Figure 22] FIG. [Figure 23] 10 is a flowchart illustrating a correction method. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0009] <Image forming device> Image forming apparatus 100 forms a toner image on sheet P in accordance with YMCK color signals. The letters YMCK attached to the reference symbols represent the toner colors yellow, magenta, cyan, and black. When matters common to all four colors are explained, the letters YMCK are omitted from the reference symbols.

[0010] Photoconductor 1 is an image carrier that carries an electrostatic latent image and a toner image. Charging unit 2 uniformly charges the surface of photoconductor 1. Exposure unit 3 irradiates photoconductor 1 with laser light corresponding to a color signal to form an electrostatic latent image. Developer 4 develops the electrostatic latent image with toner to form a toner image. Primary transfer roller 5 transfers the toner image from photoconductor 1 to intermediate transfer belt 6. Here, the YMCK toner images are superimposed to form a color image. Intermediate transfer belt 6 transports the toner image to secondary transfer unit 7. Secondary transfer unit 7 is provided with opposing roller 10 and secondary transfer roller 11 that rotate while sandwiching intermediate transfer belt 6 between them.

[0011] Sheet cassette 15 is a storage container that holds a large number of sheets P. Paper feed roller 17 sends sheets P stored in sheet cassette 15 to a conveyance path. Separation roller 18 separates and conveys one sheet P from the multiple sheets P. Conveyance roller pair 20a, 20b conveys sheet P to registration roller pair 19. Registration roller pair 19 is a roller pair consisting of two rollers and conveys sheet P to secondary transfer unit 7. Secondary transfer unit 7 includes opposing roller 10, intermediate transfer belt 6, and secondary transfer roller 11. The opposing roller 10 or intermediate transfer belt 6 and secondary transfer roller 11 form a single roller pair. In the following, the secondary transfer unit 7 may refer to this roller pair. In secondary transfer unit 7, the intermediate transfer belt 6 and secondary transfer roller 11 sandwich and convey sheet P, thereby transferring a toner image from the intermediate transfer belt 6 to sheet P. The intermediate transfer belt 6 and secondary transfer roller 11 convey sheet P to fuser 12.

[0012] The fixing device 12 fixes the toner image onto the sheet P by conveying the sheet P while sandwiching it between the fixing roller 13 and the pressure roller 14. The fixing roller 13 and the pressure roller 14 form a pair of rollers. In the following, the fixing device 12 may refer to this pair of rollers. The fixing roller 13 generates heat using a heater provided inside and applies the heat to the sheet P. The pressure roller 14 is configured to press the sheet P against the fixing roller 13.

[0013] Thereafter, the conveying roller pair 20c and 20d convey the sheet P and discharge it outside the image forming apparatus 100. When images are to be formed on both sides of the sheet P, the conveying roller pair 20e and 20f pull the sheet P in and switch back the sheet P to turn it over. Thereafter, the conveying roller pairs 20g to 20j convey the sheet P and hand it over to the conveying roller pair 20a.

[0014] 1, La indicates the distance from the center of the nip portion of the secondary transfer unit 7 to the center of the nip portion of the fixing device 12. Lb indicates the distance from the center of the nip portion of the registration roller pair 19 to the center of the nip portion of the secondary transfer unit 7.

[0015] <Four illustrative cases> The causes of image misalignment and correction using a test chart will be explained for each of the four cases below. Image misalignment refers to the deviation of the image formation position from the nominal position in design. (1) The size of the sheet P is A3 size. The conveyance direction of the sheet P and the long side of the sheet P are roughly parallel. The direction of the relative positional deviation of the registration roller pair 19 with respect to the secondary transfer roller 11 is different from the direction of the relative positional deviation of the fixing device 12. The relative positional deviation refers to, for example, the deviation of the rotation axis of one rotating body with respect to the rotation axis of the other rotating body. (2) The size of the sheet P is A3 size. The conveyance direction of the sheet P is generally parallel to the long side of the sheet P. The direction of the relative positional deviation of the registration roller pair 19 with respect to the secondary transfer roller 11 is the same as the direction of the relative positional deviation of the fixing device 12. (3) The size of the sheet P is A4 size. The conveyance direction of the sheet P is generally parallel to the short side of the sheet P. The direction of the relative positional deviation of the registration roller pair 19 with respect to the secondary transfer roller 11 is different from the direction of the relative positional deviation of the fixing unit 12. (4) The size of the sheet P is A3 size. The conveying direction of the sheet P and the long side of the sheet P are generally parallel. The direction of the relative positional deviation of the registration roller pair 19 with respect to the secondary transfer roller 11 is different from the direction of the relative positional deviation of the fixing device 12. The registration roller pair 19 has a pressure release mechanism (separation mechanism).

[0016] <Cause of image misalignment in case (1)> 2A shows that the direction of positional deviation of the registration roller pair 19 relative to the secondary transfer roller 11 is different from the direction of positional deviation of the fixing device 12 relative to the secondary transfer roller 11. In FIG. 2A, the dashed lines indicate the nominal positions of each rotation shaft (ideal positions assumed in the design).

[0017] A sheet P fed from the sheet cassette 15 is conveyed to the registration roller pair 19 by the conveyance roller pair 20a and the conveyance roller pair 20b. At this time, the registration roller pair 19 is stopped and not rotating. When the leading edge of the sheet P hits the registration roller pair 19, the skew of the sheet P is mechanically corrected. In other words, the rotation axis of the registration roller pair 19 and the edge of the leading edge of the sheet P become parallel. Note that the cause of the skew at this stage may be that the sheet P is stacked at an angle relative to the sheet cassette 15, or that the sheet P becomes skewed during conveyance by the sheet feed roller 17, the conveyance roller pair 20a, and the conveyance roller pair 20b.

[0018] If the nip pressure of the pair of registration rollers 19 is low, the leading edge of the sheet P passes through the nip portion of the pair of registration rollers 19, resulting in failure of skew correction. Therefore, the nip pressure of the pair of registration rollers 19 is set sufficiently high so that the leading edge of the sheet P does not pass through the nip portion.

[0019] The timing at which the toner image arrives at the secondary transfer unit 7 and the timing at which the sheet P arrives at the secondary transfer unit 7 must coincide. Therefore, the pair of registration rollers 19 starts rotating in synchronization with the exposure by the exposure unit 3. As a result, the sheet P is transported to the secondary transfer roller 11.

[0020] As shown in Figure 2(A), the relative positional deviation between the registration roller pair 19 and the secondary transfer roller 11 is +θ. Therefore, the conveyance distance on the front side of the sheet P is Lb1, and the conveyance distance on the rear side of the sheet P is Lb2 (Lb1>Lb2). As a result, as shown in Figure 2(B), the leading edge on the rear side of the sheet P leads the leading edge on the front side.

[0021] When the sheet P passes through the nip between the intermediate transfer belt 6 and the secondary transfer roller 11, a toner image is transferred onto the sheet P. There are various types (basis weight, material) of sheets P on the market. In order to maintain high transfer performance for various types of sheets P, the nip pressure of the secondary transfer unit 7 is set high. The pair of registration rollers 19 and the secondary transfer roller 11 transport the sheet P while transferring the toner image onto it. As a result, the leading edge of the sheet P heads toward the fixing unit 12.

[0022] 3A, the relative positional deviation of the fixing device 12 with respect to the secondary transfer roller 11 is angle -β. Therefore, the conveyance distance of the front side of the sheet P is La1, and the conveyance distance of the rear side of the sheet P is La2 (La1>La2). Therefore, the leading edge of the rear side of the sheet P enters the fixing device 12 ahead of the leading edge of the front side.

[0023] A high nip pressure is set in the fixing device 12 in order to fix the toner image on the sheet P. In the state shown in FIG. 3A, the sheet P is conveyed by the registration roller pair 19, the secondary transfer roller 11, and the fixing device 12.

[0024] 3B, the rear end of the sheet P passes through the registration roller pair 19. Therefore, the sheet P is transported by the secondary transfer roller 11 and the fixing device 12. Depending on the relative positional deviation of the fixing device 12 with respect to the secondary transfer roller 11, the sheet P is skewed.

[0025] Fig. 4 is a diagram illustrating image misalignment in case (1). In Fig. 4, the misalignment amount (partial skew amount) is the amount of misalignment in the conveyance direction of sheet P, and is the difference between the actual image formation position and the nominal position on the front side of sheet P. In this case, the image formed on sheet P is divided into three sections along the conveyance direction of sheet P.

[0026] The first section is a section in which a toner image is transferred onto the sheet P while the sheet P is sandwiched between the registration roller pair 19 and the secondary transfer roller 11. In the first section, the sheet P is skewed due to a relative positional deviation of the registration roller pair 19 with respect to the secondary transfer roller 11. Because the relative positional deviation of the registration roller pair 19 is +θ, the amount of deviation gradually increases.

[0027] The second section is a section in which a toner image is transferred while the sheet P is sandwiched between the registration roller pair 19, the secondary transfer roller 11, and the fuser 12. In the second section, the direction of misalignment of the registration roller pair 19 relative to the secondary transfer roller 11 differs from the direction of misalignment of the fuser 12. Here, of the three roller pairs, the roller pair with the highest conveying force is the fuser 12. Therefore, in the second section, the fuser 12 has a large effect on skew. Because the relative misalignment of the fuser 12 is -β, the amount of misalignment gradually changes in the negative direction.

[0028] The third section is a section in which the sheet P is conveyed while being sandwiched between the secondary transfer roller 11 and the fixing device 12. In the third section, the sheet P is skewed in accordance with the relative positional deviation of the fixing device 12 with respect to the secondary transfer roller 11. Since the relative positional deviation of the fixing device 12 is −β, the amount of deviation gradually changes in the negative direction.

[0029] <Image misalignment correction for case (1)> FIG. 5 is a diagram illustrating image misalignment correction using a basic test chart Tc0. Test chart Tc0 is a sheet P on which four reference marks M1 to M4 are formed. The reference marks M1 to M4 are formed at the four corners of the sheet P. The user measures the positions at which the reference marks M1 to M4 are formed and determines the amount of misalignment relative to each nominal position.

[0030] The amount of misalignment before correction in Fig. 5 is the actual amount of misalignment on the front side of the test chart Tc0. Note that the amount of misalignment before correction in Fig. 5 is not calculated from the reference marks M1 to M4.

[0031] The measurement results of the fiducial marks M1 to M4 only reveal the amount of misalignment at the four corners of the test chart Tc0. Therefore, when the measurement results of the fiducial marks M1 to M4 are used to correct the image misalignment, the amount of misalignment at the leading and trailing edges of the sheet P is reduced to zero, but the amount of misalignment in other sections is not adequately corrected. In fact, in this example, the amount of misalignment actually increases in some sections. This is because the misalignment is corrected while ignoring the fact that the trends in the amount of misalignment are different in each of the three sections. Therefore, there is room for improvement in both the basic test chart Tc0 and the correction method.

[0032] ●Improved test chart Tc1 FIG. 6 shows an improved test chart Tc1. The test chart Tc1 has four reference marks M for each of three sections with different roller clamping relationships. Reference marks M1 to M4 are provided at the four corners of the first section. Image misalignment in the first section is corrected using the measurement results of the reference marks M1 to M4. Reference marks M5 to M8 are provided at the four corners of the second section. Image misalignment in the second section is corrected using the measurement results of the reference marks M5 to M8. Reference marks M9 to M12 are provided at the four corners of the third section. Image misalignment in the third section is corrected using the measurement results of the reference marks M9 to M12. Note that if the number of sections with different roller clamping relationships for the sheet P is n (n is an integer greater than or equal to 2), the total number m of reference marks M in the test chart Tc1 is 4×n. The length of the first section in the conveyance direction (sub-scanning direction) of the sheet P basically coincides with the distance Lb between the center of the nip portion of the registration roller pair 19 and the center of the nip portion of the secondary transfer unit 7. The length of the second section basically corresponds to the distance (La+Lb) between the center of the nip portion of the registration roller pair 19 and the center of the nip portion of the fixing device 12. The length of the third section basically corresponds to the distance La between the center of the nip portion of the secondary transfer unit 7 and the center of the nip portion of the fixing device 12.

[0033] In this way, by using the improved test chart Tc1, it is possible to accurately measure the position of the reference mark M for each section where the clamping relationship between the roller pairs is different. In addition, the amount of image misalignment for each section can be accurately determined from the measurement results of the four reference marks M for each section. Furthermore, since the image misalignment is corrected for each section, the image misalignment is accurately corrected for all sections of the sheet P. As shown in Figure 6, by performing image misalignment correction for each section using the test chart Tc1, it can be seen that the amount of misalignment after correction is very small.

[0034] Image misalignment can be corrected using either hardware or software. Hardware correction involves adjusting the alignment of each roller (adjusting the assembly position) and the front and rear nip pressures of each roller pair. Alignment and nip pressure can be adjusted using a motor or manually. Software correction involves correcting the image data itself (correcting pixel position) and correcting the image writing position in the sub-scanning and main-scanning directions. One method of correcting pixel position is to offset the misalignment by shifting the pixel position in the opposite direction in advance based on the misalignment amount for each section acquired using the fiducial mark M. Methods for acquiring the misalignment amount include using an image reader to read a test chart and measuring the position of the fiducial mark M, or manually measuring the position of the fiducial mark M.

[0035] <Cause of image misalignment in case (2)> 7A, in case (2), the direction of deviation of the registration roller pair 19 relative to the secondary transfer roller 11 is the same as the direction of deviation of the fixing device 12. As described above, the leading edge of the sheet P abuts against the nip of the registration roller pair 19, and the skew of the leading edge of the sheet P is corrected.

[0036] As shown in Figures 7(A) and 7(B), the pair of registration rollers 19 is misaligned with the secondary transfer roller 11 by an angle of +θ. As a result, the distance Lb2 on the rear side is shorter than the distance Lb1 on the front side, and the leading edge of the sheet P on the rear side precedes the leading edge of the sheet P on the front side. The pair of registration rollers 19 and the secondary transfer roller 11 sandwich and transport the sheet P, while a toner image is transferred onto the sheet P. Then, the leading edge of the sheet P heads toward the fixing device 12. As shown in Figures 7(A) and 7(B), the fixing device 12 is misaligned with the secondary transfer roller 11 by an angle of +β.

[0037] 8A, when θ=β, the leading edge of the sheet P on the rear side and the leading edge of the sheet P on the front side enter the fixing device 12 at almost the same time. At this time, the sheet P is conveyed while being sandwiched between the registration roller pair 19, the secondary transfer roller 11, and the fixing device 12.

[0038] 8(B), the rear end of the sheet P passes through the registration roller pair 19. Therefore, the sheet P is conveyed while being sandwiched between the secondary transfer roller 11 and the fixing device 12. The sheet P is skewed by the amount of the relative positional deviation (angle + β) between the secondary transfer roller 11 and the fixing device 12.

[0039] Figure 9 shows the amount of misalignment for each section for case (2). Compared to Figure 4, the amount of misalignment in Figure 9 gradually increases. In other words, the amount of misalignment changes almost linearly. This is because the direction of misalignment of the registration roller pair 19 relative to the secondary transfer roller 11 is the same as the direction of misalignment of the fixing unit 12.

[0040] <Image misalignment correction using test chart Tc0> FIG. 10 shows the results of image misalignment correction using test chart Tc0. In case (2), the misalignment amount after correction is generally smaller than the misalignment amount before correction. This is because the misalignment direction of the registration roller pair 19 relative to the secondary transfer roller 11 is the same as the misalignment direction of the fixing unit 12, and β and θ are approximately equal. In this way, in case (2), where the misalignment amount changes approximately linearly, the misalignment amount can be reduced even when test chart Tc0 is used.

[0041] In case (2), the size of the sheet P is assumed to be A3 size. However, if the test chart Tc0 is used for a sheet P of another size, image misalignment may not be corrected sufficiently.

[0042] <Cause of image misalignment in case (3)> In case (3), the size of the sheet P is A4 size. As shown in Figure 11(A), the leading edge of the A4 size sheet P hits the nip of the pair of registration rollers 19, and the skew of the sheet P that occurred in the conveyance path from the sheet cassette 15 to the pair of registration rollers 19 is corrected. However, at this point, the leading edge of the sheet P simply becomes parallel to the rotation axis of the pair of registration rollers 19.

[0043] 11(B), the pair of registration rollers 19 is shifted by an angle +θ with respect to the secondary transfer roller 11. Therefore, the leading edge of the sheet P on the rear side precedes the leading edge on the front side. While the pair of registration rollers 19 and the secondary transfer roller 11 convey the sheet P while nipping it, a toner image is transferred onto the sheet P at the secondary transfer unit 7.

[0044] 12A, while the sheet P is being sandwiched between the secondary transfer roller 11 and the intermediate transfer belt 6, the rear end of the sheet P leaves the pair of registration rollers 19. At this stage, the secondary transfer roller 11 alone transports the sheet P to the fixing device 12.

[0045] As shown in FIG. 12(B), the fixing device 12 is inclined at an angle -β with respect to the secondary transfer roller 11. Therefore, the leading edge of the sheet P on the rear side enters the fixing device 12 first. This is because the direction in which the registration roller pair 19 shifts relative to the secondary transfer roller 11 is different from the direction in which the fixing device 12 shifts relative to the secondary transfer roller 11. The sheet P is transported by the secondary transfer roller 11 and the fixing device 12. Here, the sheet P is skewed depending on the relative positional deviation between the secondary transfer roller 11 and the fixing device 12. As shown in FIG. 13, the trailing edge of the sheet P leaves the secondary transfer roller 11, and the fixing device 12 transports the sheet P by itself while nipping it.

[0046] 14 shows image misalignment occurring on an A4-sized sheet P in case (3). The first section is a section in which a toner image is transferred onto the sheet P while the pair of registration rollers 19 and the secondary transfer roller 11 sandwich and transport the sheet P. In the first section, the sheet P is skewed due to a relative positional deviation of the pair of registration rollers 19 with respect to the secondary transfer roller 11, causing image misalignment in the toner image.

[0047] The second section is a section in which the secondary transfer roller 11 transports the sheet P alone while the toner image is transferred onto the sheet P. In the second section, the sheet P is skewed due to the difference in nip pressure between the front side and rear side of the secondary transfer roller 11 and misalignment between the opposing roller 10 and the secondary transfer roller 11. In other words, these factors cause image misalignment.

[0048] The third section is a section in which the toner image is transferred onto the sheet P while the secondary transfer roller 11 and the fixing device 12 sandwich and transport the sheet P. In the third section, the sheet P is skewed due to a relative positional deviation between the secondary transfer roller 11 and the fixing device 12. In other words, the cause of the image deviation in the third section is the relative positional deviation between the secondary transfer roller 11 and the fixing device 12.

[0049] When a sheet P having a short length in the transport direction is used, a section occurs in which the toner image is transferred onto the sheet P while the secondary transfer roller 11 transports the sheet P alone. Here, the transport force of the fixation device 12 is the strongest, the transport force of the registration roller pair 19 is medium, and the transport force of the secondary transfer roller 11 is the weakest. Therefore, the influence of the registration roller pair 19 is dominant in the first section, and the influence of the fixation device 12 is dominant in the third section. Therefore, when test chart Tc0 is used, it is not possible to measure the amount of skew of the sheet P caused by the secondary transfer roller 11 alone in the first and third sections. Therefore, by measuring the amount of skew in the second section using test chart Tc1, it is possible to extract the amount of skew caused by the secondary transfer roller 11 alone. In other words, using test chart Tc1 allows image misalignment caused by the amount of skew caused by the secondary transfer roller 11 alone to be properly corrected.

[0050] <Cause of image misalignment in case (4)> 15(A) and 15(B) show a separation mechanism 1500 on the front side of the registration roller pair 19. The registration roller pair 19 has an upper roller 191 and a lower roller 192. The separation mechanism 1500 separates the upper roller 191 from the lower roller 192, or brings the upper roller 191 and the lower roller 192 into contact with each other. When the upper roller 191 and the lower roller 192 are separated from each other, the clamping force of the registration roller pair 19 on the sheet P becomes zero.

[0051] An input gear 1502 is fixed to the rotating shaft of motor 1501. The input gear 1502 meshes with an idler gear 1503. The idler gear 1503 meshes with a cam input gear 1504. The cam input gear 1504 meshes with a cam gear 1505. The rotational force generated by motor 1501 is input to input gear 1502 and transmitted to cam gear 1505 via idler gear 1503 and cam input gear 1504.

[0052] A similar spacing mechanism may also be provided on the rear side of the registration roller pair 19. A rotational force is transmitted to this rear spacing mechanism via a rotation shaft 1610 coupled to the cam input gear 1504. The outer diameter of the cam of the cam gear 1505 is eccentric with respect to the rotation shaft of the cam gear 1505. As shown in FIG. 17(A), when the distance from the rotation shaft to the cam surface becomes less than a predetermined distance, the upper roller 191 and the lower roller 192 come into contact with each other. As shown in FIG. 17(B), when the distance from the rotation shaft to the cam surface becomes equal to or greater than the predetermined distance, the cam surface of the cam gear 1505 pushes up the cam follower 1506. The cam follower 1506 is rotatably supported by the upper roller 191. The upper roller 191 is pushed up by the cam follower 1506. As a result, the upper roller 191 is separated from the lower roller 192. The separated state is maintained even when the driving force of the motor 1501 is turned off. The cam gear 1505 is rotatably supported by a bearing or the like on the rotation shaft of the lower roller 192 .

[0053] When the leading edge of the sheet P conveyed by the registration roller pair 19 is nipped by the secondary transfer unit 7, the separation mechanism 1500 separates the registration roller pair 19. When the registration roller pair 19 is in the separated state, the sheet P is conveyed only by the secondary transfer roller 11. Therefore, in this state, the sheet P is not affected by the registration roller pair 19.

[0054] 16A is a diagram illustrating image misalignment when a separation mechanism 1500 for the registration roller pair 19 is provided. The first section is a section in which the registration roller pair 19 and the secondary transfer roller 11 convey the sheet P while nipping it between them. When the sheet P is sufficiently nipped by the secondary transfer roller 11, the separation mechanism 1500 separates the registration roller pair 19, making the first section a very short section. Note that the cause of image misalignment in the first section is skew of the sheet P due to a relative positional deviation of the registration roller pair 19 with respect to the secondary transfer roller 11.

[0055] The second section is a section in which the secondary transfer roller 11 alone transports the sheet P. The cause of the image misalignment in the second section is the difference in nip pressure between the front side and the rear side of the secondary transfer roller 11, and skew of the sheet P caused by misalignment between the opposing roller 10 and the secondary transfer roller 11.

[0056] The third section is a section in which the sheet P is conveyed while being sandwiched between the secondary transfer roller 11 and the fixing device 12. The cause of the image misalignment in the third section is the skew of the sheet P caused by the relative positional deviation between the secondary transfer roller 11 and the fixing device 12.

[0057] When the separating mechanism 1500 is present, the first section is very short. Therefore, image misalignment occurring in the first section may be ignored. However, if image misalignment correction is also applied to the first section, the image misalignment can be corrected with higher accuracy.

[0058] However, if image shift correction is performed based on the measurement results of the shift amount obtained from a short section, the measurement results will vary greatly, which may result in a decrease in correction accuracy.

[0059] Therefore, as shown in FIG. 16B, when creating the test chart Tc1, the separation timing of the registration roller pair 19 is delayed compared to the separation timing when printing the user image. In other words, when creating the test chart Tc1, the first section is ensured to be longer. This ensures that the lengths of the first section, second section, and third section are each sufficiently long so as to increase the accuracy of measuring the amount of misalignment. The difference between the normal separation timing and the separation timing of the registration roller pair 19 can be determined through experiments or simulations so as to ensure sufficient accuracy in measuring the amount of misalignment.

[0060] In this way, by using the separation mechanism 1500, even if the length of the sheet P in the conveying direction is long, the skew component caused solely by the secondary transfer roller 11 can be accurately extracted. As a result, the amount of image misalignment can also be accurately obtained. In particular, the separation timing of the registration roller pair 19 when forming the test chart Tc1 is delayed compared to normal separation timing. As a result, by simply outputting one test chart Tc1, the correction amounts (adjustment amounts) of the registration roller pair 19, the secondary transfer roller 11, and the fixing unit 12 can be accurately obtained.

[0061] <Image reader> 17 shows an example of an image reading device 1700 capable of reading the test chart Tc1. The image reading device 1700 may be a stand-alone type, or may be mounted on or connected to an image forming apparatus 100 that functions as a copier. The image reading device 1700 has a reading unit 1701 and an ADF 1750.

[0062] The ADF 1750 is an automatic document feeder that automatically feeds documents (such as test charts). A pickup roller 1711 picks up a document placed on a document tray 1702 and sends it down a conveyance path. A separation roller pair 1712 separates a single document from multiple documents and conveys it further downstream. Multiple conveyance roller pairs 1713, 1714, and 1715 convey the document further downstream. A discharge roller pair 1716 discharges the document onto a discharge tray 1703.

[0063] The reading unit 1701 has a first reading unit 1719, a second reading unit 1720, a platen glass 1717, and a document table glass 1718. The platen glass 1717 is provided between the pair of transport rollers 1714 and the pair of transport rollers 1715. The first reading unit 1719 is provided below the platen glass 1717. The first reading unit 1719 reads the first side of a document transported by the ADF 1750 while stopped below the platen glass 1717. The second reading unit 1720 reads the second side of a document transported by the ADF 1750. The first reading unit 1719 can read a document placed on the platen glass 1717 by moving in the sub-scanning direction. The first reading unit 1719 and the second reading unit 1720 each incorporate a light source that illuminates the document, a photoelectric conversion element that converts light from the document into an electrical signal, and the like. The photoelectric conversion element is, for example, a CCD sensor or a CMOS sensor. This provides image data that is the result of reading the test chart Tc1. CCD is an abbreviation for charge-coupled device. CMOS is an abbreviation for complementary metal oxide silicon. Furthermore, the amount of image misalignment is calculated from the image data. The user may manually measure the position of the reference mark on the test chart Tc1 using a scale or the like. However, by using the image reading device 1700, it is possible to calculate the amount of misalignment more accurately and in a shorter time.

[0064] As shown in FIG. 18, the image reading device 1700 may be connected to the image forming device 100. Elements already described are given the same reference numerals, and the description thereof is incorporated herein by reference. The test chart Tc1 created by the image forming device 100 is delivered to the image reading device 1700. The image reading device 1700 reads the test chart Tc1 with a first reading unit 1719 while conveying the test chart Tc1 with conveyance roller pairs 1713 to 1715. In this case, the user does not need to place the test chart Tc1 on the platen 1702 or platen glass 1718, thereby saving the user time and effort.

[0065] <Image misalignment correction details> The following two image correction methods are described below. (1) Hardware correction (mechanical correction) (2) Software correction (image processing correction or electrical correction) Here, mechanical correction refers to a method of adjusting the roller mounting position (alignment adjustment) or adjusting the nip pressure on the front and rear sides of the roller pair. Mechanical correction can be manual correction, in which the user makes adjustments manually, or automatic correction, in which a driving source such as a motor is used.

[0066] Image processing correction refers to a method of correcting the image data itself, while electrical correction refers to a correction such as adjusting the timing at which the laser beam starts to irradiate in the main scanning direction or the sub-scanning direction, or adjusting the drive speed of the intermediate transfer belt.

[0067] Either hardware correction or software correction may be performed, or both may be performed. In the latter case, hardware correction is first performed based on the measurement results using the test chart Tc1. Next, the test chart Tc1 is output again, and software correction is performed based on the measurement results. This allows image misalignment to be corrected with great precision.

[0068] ●Hardware correction (1) Pressure adjustment mechanism FIG. 19A shows a pressure adjustment mechanism 1900 that mechanically adjusts the nip pressure of the secondary transfer unit 7. The secondary transfer roller 11 is rotatably supported by a rotating frame 1904. A rotating shaft 1903 is provided at one end of the rotating frame 1904. The rotating frame 1904 is rotatable around the rotating shaft 1903. A motor 1905 rotates in accordance with a control signal supplied from an external device. A pressure cam 1906 is fixed to the rotating shaft of the motor 1905. When the motor 1905 rotates the pressure cam 1906, the rotating frame 1904 rotates around the rotating shaft 1903, thereby raising or lowering the secondary transfer roller 11. This adjusts the nip pressure in the secondary transfer unit 7.

[0069] 19A, the pressure adjustment mechanism 1900 is provided at one end of the secondary transfer roller 11, but this is merely an example. The pressure adjustment mechanism 1900 may also be provided at the other end. Alternatively, the pressure adjustment mechanism 1900 may be provided only at one end, and the other end of the rotation shaft of the secondary transfer roller 11 may simply be rotatably supported by a bearing.

[0070] The rotation angle of the motor 1905 is determined so that the image shift is sufficiently small. In other words, the rotation angle (correction amount) may be determined according to the amount of image shift measured using the test chart Tc1.

[0071] (2) Alignment adjustment mechanism FIG. 19B shows an alignment adjustment mechanism 1910 for the secondary transfer roller 11. A shaft end 1911 on the front side of the secondary transfer roller 11 is rotatably supported by a long bearing 1912 fixed to a side plate (not shown). The long bearing 1912 has an elongated hole. The shaft end 1911 is inserted into this elongated hole and is supported so as to be movable along the elongated hole in the direction of arrow C. The shaft end 1911 is further rotatably supported by a bearing 1913. The shaft end 1911 is movable together with the bearing 1913 in the direction of arrow C. A motor 1914 is fixed to the side plate (not shown). An output shaft 1915 is provided at the tip of the motor 1914. A lead is provided on the output shaft 1915. The tip of the lead abuts against the bearing 1913. A spring member (not shown) is provided on the opposite side of the bearing 1913. The bearing 1913 is biased in the direction of arrow D by a spring member. In other words, the bearing 1913 is pressed against the output shaft 1915. Therefore, when the motor 1914 rotates in the direction of arrow M1 by a predetermined number of steps, the tip of the lead of the output shaft 1915 moves in the direction of arrow C1 by a predetermined amount. In other words, the bearing 1913 also moves in the direction of arrow C1 by a predetermined amount. Conversely, when the motor 1914 rotates in the direction of arrow M2 by a predetermined number of steps, the tip of the output shaft 1915 moves in the direction of arrow C2 by a predetermined amount. In other words, the bearing 1913 also moves in the direction of arrow C2 by a predetermined amount. In this way, the shaft end 1911 on the front side of the secondary transfer roller 11 moves along the direction of arrow C. As a result, the alignment of the secondary transfer roller 11 is adjusted. The number of steps of the motor 1914 is determined so that image misalignment is sufficiently small. In other words, the number of steps (correction amount) can be determined according to the amount of image misalignment measured using the test chart Tc1.

[0072] (3) Other 19(A) and 19(B) illustrate the adjustment mechanism for the secondary transfer roller 11. However, the pressure adjustment mechanism 1900 and the alignment adjustment mechanism 1910 can also be applied to the registration roller pair 19 and the fixing roller 13 of the fixing unit 12.

[0073] 19(A) and 19(B) use actuators such as motors, but a manual adjustment mechanism may also be employed. The front and rear sides of a frame unit that supports the roller pairs are fastened to the main body frame of the image forming apparatus 100 with screws. A maintenance worker loosens the screws, moves the frame unit so that the amount of image misalignment is reduced, and then fastens the screws again. In this way, the alignment of each roller pair may be manually adjusted.

[0074] ● Software correction Software correction includes image processing correction and electrical correction. First, the method for measuring the amount of misalignment will be described in detail. Misalignment generally occurs in the sub-scanning direction (lead position misalignment) and the main scanning direction (side position misalignment). The sub-scanning direction is generally parallel to the transport direction of the sheet P. The main scanning direction is perpendicular to the sub-scanning direction. The lead position is the starting position for printing an image, starting from the leading edge of the sheet P in the transport direction. Electrical correction involves adjusting the timing at which the laser beam emitted from the exposure device 3 onto the photoconductor 1 begins to irradiate the photoconductor 1. This changes the starting position for printing an image in the transport direction of the sheet P. The side position is the starting position for printing an image, starting from the left edge of the sheet P in the transport direction. Electrical correction involves adjusting the timing at which the laser beam emitted from the exposure device 3 onto the photoconductor 1 begins to irradiate the photoconductor 1. This changes the starting position for printing an image, starting from the left edge of the sheet P.

[0075] Further deviation amounts include the deviation amount of the image length in the sub-scanning direction (sub-scanning magnification deviation amount) and the deviation amount of the image length in the main scanning direction (main scanning magnification deviation amount). In electrical correction, the sub-scanning magnification is adjusted by controlling the drive speed of the intermediate transfer belt 6. If the drive speed is increased, the image length in the sub-scanning direction becomes shorter. If the drive speed is decreased, the image length in the sub-scanning direction becomes longer. The main scanning magnification is adjusted by controlling the frequency of the image clock. The image clock is a control parameter that controls the irradiation time per pixel in the exposure unit 3. If the frequency is increased, the image length in the main scanning direction becomes shorter. If the frequency is decreased, the image length in the main scanning direction becomes longer. The image clock is also called the main scanning synchronization signal.

[0076] In image processing correction, the position of each pixel constituting the image data is shifted as necessary to reduce the misalignment of the side and lead positions. The amount of shift may be set based on the measurement results of the test chart Tc1, or may be manually input by the user after confirming the amount of misalignment.

[0077] <controller> 20 shows a control board 2000 mounted on the image forming apparatus 100. The control board 2000 is mounted with a CPU 2001 and a memory 2050. The memory 2050 includes a ROM (non-volatile memory), a RAM (volatile memory), an HDD (hard disk drive), an SSD (solid state drive), etc. The CPU 2001 realizes various functions by executing control programs stored in the memory 2050. Note that all or part of the functions described below may be implemented by hardware circuits other than the CPU 2001.

[0078] The pattern generator 2002 generates an image signal based on test image data 2051 that forms the basis of the test chart Tc1, and outputs the image signal to the exposure device 3 of the printer engine 2080. The printer engine 2080 is a mechanism that prints an image on a sheet P in the image forming apparatus 100. The printer engine 2080 drives the exposure device 3 in accordance with the image signal to form a toner image, which is then transferred and fixed onto the sheet P. This generates the test chart Tc1. Note that motors 1501, 1905, 1914a, and 1914b are connected to the printer engine 2080. The motors 1914a and 1914b correspond to the motor 1914 of the alignment adjustment mechanism 1910 described above. The motor 1914a adjusts the alignment of the registration roller pair 19. The motor 1914b adjusts the alignment of the fixing device 12.

[0079] The image processing unit 2003 applies color space conversion and gradation correction to image data input by the user to generate an image signal. The position correction unit 2004 is an optional function that operates when the image processing correction described above is executed. Based on the correction amount calculated from the amount of misalignment, the position correction unit 2004 shifts the position of each pixel in the image data or adjusts the area per pixel, thereby correcting the position of the toner image formed on the sheet P.

[0080] The mark detection unit 2005 detects the position of the mark M and the positions of the four sides of the test chart Tc1 by performing edge detection or pattern matching on the image data generated by reading the test chart Tc1 with the image reading device 1700. Note that the mark M is formed using black toner to increase detection accuracy. That is, a toner color is selected that maximizes the difference in reflectance between the surface of the sheet P and the reflectance of the mark M. The measurement unit 2006 measures the amount of misalignment of each mark M based on the position of the mark M and the positions of the four sides detected by the mark detection unit 2005. Note that the reference position of the mark M is obtained from the test image data 2051. The difference between the position (nominal position) of each mark M in the test image data 2051 and the measurement result of the position of each mark M is calculated as the amount of misalignment.

[0081] The determination unit 2007 determines the correction amount based on the amount of deviation. For example, the determination unit 2007 may calculate the correction amount by substituting the amount of deviation into a predetermined mathematical formula. The predetermined mathematical formula is assumed to be obtained in advance by conducting experiments or simulations during the design or manufacturing process of the image forming apparatus 100, and to be stored in the memory 2050.

[0082] In the case of hardware correction, the number of steps of motor 1501 and motor 1914, etc. are determined, and the direction and amount of movement of the frame unit are also determined. The notification unit 2008 outputs a guidance message for the user or maintenance worker to the display device of the operation unit 2060. The guidance message may include the direction and amount of movement of the frame unit, etc.

[0083] The determination unit 2007 may store the correction amount in association with the type of sheet A (basis weight, presence or absence of gloss, size) in a sheet management table 2052 stored in the memory 2050. For example, the CPU 2001 may extract a correction amount corresponding to the type of sheet A specified by the user via the operation unit 2060 from the sheet management table 2052 and set the correction amount in the position correction unit 2004. Alternatively, the CPU 2001 may set the number of steps of the motor 1501 and the motor 1914, etc., according to the correction amount extracted from the sheet management table 2052.

[0084] (1) How to calculate the amount of deviation FIG. 21A shows the measurement values ​​of each mark M on test chart Tc0. a is the length of test chart Tc0 in the transport direction (sub-scanning direction). b is the length of test chart Tc0 in the main scanning direction. c is the distance from the edge of mark M1 in the sub-scanning direction. d is the distance from the edge of mark M1 in the main scanning direction. e is the distance from the edge of mark M2 in the sub-scanning direction. f is the distance from the edge of mark M2 in the main scanning direction. g is the distance from the edge of mark M3 in the sub-scanning direction. h is the distance from the edge of mark M3 in the main scanning direction. i is the distance from the edge of mark M4 in the sub-scanning direction. j is the distance from the edge of mark M4 in the main scanning direction.

[0085] Figure 22 shows the formula for calculating the measurement value and deviation from the measurement results of mark M. The measurement value of the lead position is half the sum of distance c and distance e. The deviation is the difference between the measurement value and the nominal value (e.g., 1 cm). The measurement value of the side position is half the sum of distance f and distance j. The deviation is the difference between the measurement value and the nominal value (e.g., 1 cm). The measurement value of the main scanning magnification is calculated by subtracting the sum of distance d and distance f from distance b to obtain the first difference, subtracting the sum of distance h and distance j from distance b to obtain the second difference, adding the first difference and the second difference to obtain a sum, and halving this sum. The deviation amount of the main scanning magnification is calculated by dividing the difference between the measurement value and the nominal value by the nominal value. The measurement value of the sub-scanning magnification is calculated by subtracting the sum of distance c and distance g from distance a to obtain the first difference, subtracting the sum of distance e and distance i from distance a to obtain the second difference, adding the first difference and the second difference to obtain a sum, and halving this sum. The deviation amount of the sub-scanning magnification is obtained by dividing the difference between the measured value and the nominal value by the nominal value.

[0086] In this way, for the basic test chart Tc0, the amount of deviation for the lead position, side position, overall main scanning magnification, and overall sub-scanning magnification is calculated from the four reference marks M. Furthermore, the amount of correction (adjustment direction and adjustment value) for hardware correction or the amount of correction (pixel shift amount or timing adjustment amount) for software correction is calculated from each deviation amount.

[0087] FIG. 21B shows the measurement values ​​of each mark M on the test chart Tc1. As described above, the test chart Tc1 is divided into three sections according to the distance between the three roller pairs. The CPU 2001 calculates the amount of misalignment and the amount of correction for each section using the same method as for the test chart Tc0. As shown in FIG. 21B, the measurement values ​​for the xth section are ax to jx. As shown in FIG. 21B, x is 1, 2, or 3. For each section, the measurement values ​​of the lead position, side position, main-scanning overall magnification, and sub-scanning overall magnification, as well as the respective amounts of misalignment, are calculated using the formulas shown in FIG. 22. Furthermore, the amount of correction is calculated from the amount of misalignment for each section. The formulas or program code for converting the amount of misalignment into the amount of correction are determined in advance through experiments or simulations and are stored in the memory 2050.

[0088] 22 omits the index x. The calculated correction amount is registered in the sheet management table 2052 as attribute information of the sheet P. When both hardware correction and software correction are performed, the correction amount for software correction calculated from the test chart Tc1 created immediately after the hardware correction is completed is registered in the sheet management table 2052.

[0089] <Flowchart> 23 shows a method for correcting image misalignment that is executed by the CPU 2001 in accordance with a control program. Here, the CPU 2001 executes the following process by determining that a predetermined correction start condition is met. The predetermined correction start condition may be, for example, that an instruction to execute correction is input from the operation unit 2060, or that the cumulative number of image formation sheets exceeds a threshold value.

[0090] In S2201, the CPU 2001 outputs the test chart Tc1. As described above, the CPU 2001 (pattern generator 2002) outputs an image signal based on the test image data 2051 to the printer engine 2080. As a result, the image forming apparatus 100 creates the test chart Tc1 by printing four marks M for each section on the sheet P. Note that the position correction unit 2004 may be disabled when creating the test chart Tc1. Note that if the number of sections is N, the total number of reference marks M is 4×N.

[0091] In S2202, the CPU 2001 determines whether to perform the measurement manually. For example, the CPU 2001 may display a message on the operation unit 2060 inquiring whether to perform the measurement manually and accept a Yes / No input via the operation unit 2060. Alternatively, whether to perform the manual measurement may be selected using default settings stored in the memory 2050. If the measurement is to be performed manually, the CPU 2001 proceeds to S2203. In S2203, the CPU 2001 accepts input of the measurement results (e.g., distances ax to jx) for the test chart Tc1 via the operation unit 2060. On the other hand, if the measurement is not to be performed manually (i.e., if automatic measurement is selected), the CPU 2001 proceeds to S2211. In S2211, the CPU 2001 controls the image reading device 1700 to read the test chart Tc1 and receives the reading results (e.g., image data) from the image reading device 1700. In S2212, based on the read results, the CPU 2001 calculates the measurement results for each mark M. As described above, the mark detection unit 2005 detects the four sides of the test chart Tc1 and the marks M, and the measurement unit 2006 measures the distances ax to jx.

[0092] In S2204, the CPU 2001 (measurement unit 2006) calculates the amount of deviation based on the distances ax to jx. As described above, the formulas shown in Fig. 22 are applied for each section to calculate the amount of deviation for each side position, lead position, main scanning magnification, and sub scanning magnification.

[0093] In S2205, the CPU 2001 determines whether image misalignment correction is necessary based on each misalignment amount. This determination is made for each section. The CPU 2001 compares each misalignment amount for the side position, lead position, main scanning magnification, and sub-scanning magnification with the corresponding threshold value. The threshold value is assumed to be stored in advance in the memory 2050. The CPU 2001 may determine that correction should be performed if at least one of the four misalignment amounts exceeds the threshold value. Alternatively, the CPU 2001 may determine that correction should be performed if at least two of the four misalignment amounts exceed the threshold value. If correction is not necessary for all sections, the CPU 2001 terminates the correction process. On the other hand, if correction is necessary for even one section, the CPU 2001 proceeds to S2206.

[0094] In S2206, the CPU 2001 (decision unit 2007) calculates the amount of correction based on the amount of misalignment for each section. For example, if the correction for the first section is achieved by adjusting the alignment of the registration roller pair 19, the decision unit 2007 decides the amount of adjustment for the registration roller pair 19 based on the amount of misalignment calculated from the distances a1 to j1. If the correction for the second section is achieved by adjusting the nip pressure of the secondary transfer roller 11, the decision unit 2007 decides the amount of adjustment for the nip pressure on the near side and the far side of the secondary transfer roller 11 based on the amount of misalignment calculated from the distances a2 to j2. If the adjustment for the third section is achieved by adjusting the alignment of the fixation unit 12, the decision unit 2007 decides the amount of adjustment for the alignment of the fixation unit 12 based on the amount of misalignment calculated from the distances a3 to j3.

[0095] In S2207, the CPU 2001 determines whether to perform automatic correction. Automatic correction refers to all of the software corrections described above and hardware correction methods that use actuators. Manual correction refers to correction methods in which a user or maintenance worker loosens screws, etc. If automatic correction is to be performed, the CPU 2001 proceeds to S2208. In S2208, the CPU 2001 performs automatic correction. For software correction, the CPU 2001 registers the correction amount in the sheet management table 2052. For hardware correction, the CPU 2001 drives the motors 1914 and 1905 according to the correction amount. On the other hand, if manual correction is selected, the CPU 2001 proceeds to S2221. In S2221, the CPU 2001 (notification unit 2008) displays guidance on the manual correction method on the display device of the operation unit 2060. The guidance includes the part to be adjusted, the direction and amount of adjustment, etc.

[0096] <Technical ideas derived from examples> [Point 1] The secondary transfer unit 7 is an example of an image forming unit that conveys a sheet while nipping it to form an image on it. The registration roller pair 19 is an example of a first conveying unit that is located upstream of the image forming unit in the sheet conveyance direction and conveys the sheet while nipping it. The fixing unit 12 is an example of a second conveying unit that is located downstream of the image forming unit in the sheet conveyance direction and conveys the sheet while nipping it. The CPU 2001 is an example of a control unit that controls the image forming unit to generate a test chart, which is a sheet on which a test image is formed. The control unit may execute a process to correct the misalignment of the image formed on the sheet by the image forming unit based on the reading result of the test image. Such a process includes the automatic correction described above and a display process that assists manual correction. As illustrated in FIG. 21B, the test chart Tc1 has a first section, a second section, and a third section. The first section is a section in which an image is formed on the sheet while it is nipped between the first conveying unit and the image forming unit. The second section is a section in which an image is formed on the sheet while it is conveyed only by the image forming unit. The third section is a section in which an image is formed while the sheet is sandwiched between the image forming unit and the second conveying unit. The CPU 2001 may determine the amount of correction for the image misalignment using a first measurement result indicating the image misalignment in the first section, a second measurement result indicating the image misalignment in the second section, and a third measurement result indicating the image misalignment in the third section. The correction amount may be used for the automatic correction described above or for manual correction. In this manner, in the above embodiment, the test chart is logically divided into multiple sections, and the image misalignment is measured for each section. Therefore, the above embodiment can improve the accuracy of the image formation position compared to conventional techniques.

[0097] [Point 2] 21(B) and the like, a reference mark M serving as a reference for measuring image misalignment may be formed in each of the first, second, and third sections of the test chart, thereby enabling accurate measurement of the amount of image misalignment for each section.

[0098] [Point 3] 21(B), the reference marks M may be provided at the four corners of the first section, the four corners of the second section, and the four corners of the third section. The four corners of each section are positions that are likely to reveal the characteristics of the image shift in each section. Therefore, by using the measurement results of the reference marks M provided at the four corners, the amount of image shift in each section can be accurately measured.

[0099] [Point 4] 15, the separation mechanism 1500 is an example of a switching unit that switches the first conveying unit between a contact state where the sheet can be clamped and a separation state where the sheet is not clamped. The CPU 2001 controls the switching unit to switch the first conveying unit from the contact state to the separation state when the sheet on which the test chart is formed is passing through the first conveying unit. This makes it possible to move the test chart from the first section to the second section.

[0100] [Point 5] When forming a test chart, there is a timing when the first conveying means switches from the contact state to the separated state. This timing is later than the timing when the first conveying means switches from the contact state to the separated state when forming an arbitrary image prepared by the user on a sheet. This makes it possible to secure a longer first section, thereby improving the measurement results for the first section.

[0101] [Points 6 and 7] The alignment adjustment mechanism 1910 is an example of a first adjustment unit that reduces image misalignment in the first section by adjusting the first conveying unit according to a correction amount determined based on the misalignment amount measured in the first section. This allows image misalignment in the first section to be accurately reduced. The first adjustment unit may include a first motor (e.g., motor 1914) and a first adjustment mechanism (e.g., output shaft 1915) driven by the first motor to adjust the fixed position of the first conveying unit. The output shaft 1915 has a conversion mechanism, such as a ball screw mechanism, that converts rotational motion into linear motion.

[0102] [Points 8 and 9] The pressure adjustment mechanism 1900 is an example of a second adjustment unit that reduces image misalignment that occurs in the second section by adjusting the image forming unit according to a correction amount determined based on the misalignment amount measured in the second section. This allows image misalignment in the second section to be accurately reduced. The second adjustment unit may include a second motor (motor 1905) and a second adjustment mechanism (e.g., pressure cam 1906) that is driven by the second motor to adjust the balance between the nip pressure at one end and the nip pressure at the other end of the image forming unit.

[0103] [Points 10 and 11] The alignment adjustment mechanism 1910 is an example of a third adjustment unit that reduces image misalignment in the third section by adjusting the second conveying unit according to a correction amount determined based on the misalignment amount measured from the third section. This allows image misalignment in the third section to be accurately reduced. The third adjustment unit may include a third motor (e.g., motor 1914) and a third adjustment mechanism (e.g., output shaft 1915) that is driven by the third motor to adjust the fixed position of the second conveying unit.

[0104] [Point 12] As shown in FIG. 1 and other figures, the second conveying unit may be a fixing unit that fixes an image onto a sheet while conveying the sheet. The fixing unit 12 conveys the sheet in cooperation with the secondary transfer unit 7. Therefore, misalignment of the fixing unit 12 causes image misalignment in the third section. Therefore, by adjusting the alignment of the fixing unit 12, image misalignment in the third section can be accurately corrected.

[0105] [Points 13-15] The operation unit 2060 is an example of a first display unit that displays the amount of manual correction for the first conveying means determined based on the amount of misalignment measured from the first section. The operation unit 2060 is an example of a second display unit that displays the amount of manual correction for the image forming means determined based on the amount of misalignment measured from the second section. The operation unit 2060 is an example of a third display unit that displays the amount of manual correction for the second conveying means determined based on the amount of misalignment measured from the third section. By notifying the maintenance worker or the like of the amount of correction (adjustment amount) in this way, the maintenance worker can easily correct the image misalignment. The information displayed here may include the adjustment direction and amount of adjustment.

[0106] [Point 16] The CPU 2001 is an example of an image correction unit that reduces image misalignment by pre-correcting the position of each pixel that constitutes an image. This correction is performed based on the amount of misalignment measured from the first section, the second section, and the third section. The misalignment measurement results indicate the deviation of each pixel's position from its nominal position. Therefore, by pre-shifting the pixel position in the opposite direction to the measurement results (pre-distortion), the image misalignment on the sheet is offset.

[0107] [Point 17] The CPU 2001 is an example of a correction unit that reduces image misalignment by correcting the image writing start position and magnification in each section of the sheet. This correction is also performed based on the misalignment amount measured from the first section, the second section, and the third section. In this way, the CPU 2001 may electrically reduce image misalignment by adjusting the timing at which the laser beam from the exposure device 3 starts irradiating the image or by adjusting the frequency of the image clock.

[0108] [Point 18] Hardware correction is an example of a first correction mode. In the first correction mode, a first test chart, which is a sheet on which a test image is formed by controlling the image forming means, is generated, and a reading result of the test image on the first test chart is obtained. Furthermore, in the first correction mode, the first conveying means, the image forming means, and the second conveying means are adjusted based on the reading result, thereby mechanically correcting the image misalignment. The software correction is an example of the second correction mode. In the second correction mode, the image forming unit is controlled to generate a second test chart, which is a sheet on which a test image is formed, and the reading result of the test image on the second test chart is obtained. Furthermore, in the second correction mode, In this case, the image misalignment is electrically corrected by pre-correcting the positions of the multiple pixels that make up the image based on the reading results. The CPU 2001 may execute the first correction mode to mechanically correct the image misalignment, and then execute the second correction mode to electrically correct the remaining image misalignment. This will further reduce image misalignment compared to when hardware correction and software correction are executed separately.

[0109] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0110] 11: secondary transfer roller, 12: fixing unit 12, 19: pair of registration rollers, 2001: CPU

Claims

1. an image forming means for conveying a sheet while nipping the sheet to form an image on the sheet; a first conveying means provided upstream of the image forming means in a sheet conveying direction, the first conveying means conveying the sheet while nipping the sheet; a second conveying means provided downstream of the image forming means in a sheet conveying direction, the second conveying means configured to convey the sheet while nipping the sheet; a control means for controlling the image forming means to generate a test chart, which is a sheet on which a test image is formed, and for executing a process to correct a deviation in the formation position of the image formed on the sheet by the image forming means based on a reading result of the test image; The test chart is a first section in which an image is formed on the sheet while the sheet is sandwiched between the first conveying means and the image forming means; a second section in which an image is formed on the sheet while the sheet is being transported only by the image forming means; a third section in which an image is formed on the sheet while the sheet is sandwiched between the image forming means and the second conveying means, an image forming apparatus characterized in that the control means determines a correction amount for the image misalignment using a first measurement result indicating the image misalignment in the first section, a second measurement result indicating the image misalignment in the second section, and a third measurement result indicating the image misalignment in the third section.

2. 2. The image forming apparatus according to claim 1, wherein a reference mark serving as a reference for measuring the image misalignment is formed in each of the first section, the second section, and the third section of the test chart.

3. 3. The image forming apparatus according to claim 2, wherein the reference marks are provided at four corners of the first section, four corners of the second section, and four corners of the third section.

4. a switching means for switching the first conveying means between a contact state in which the sheet can be clamped and a separated state in which the sheet is not clamped, 4. The image forming apparatus according to claim 1, wherein the control unit controls the switching unit to switch the first conveying unit from the contact state to the separation state when the sheet on which the test chart is formed is passing through the first conveying unit, thereby moving the test chart from the first section to the second section.

5. 5. The image forming apparatus according to claim 4, wherein the timing at which the first conveying means is switched from the contact state to the separated state when the test chart is being formed is later than the timing at which the first conveying means is switched from the contact state to the separated state when an arbitrary image prepared by a user is being formed on a sheet.

6. 6. An image forming apparatus according to claim 1, further comprising a first adjustment means for reducing image misalignment in the first section by adjusting the first conveying means according to a correction amount determined based on the amount of misalignment measured from the first section.

7. The first adjustment means is A first motor; 7. The image forming apparatus according to claim 6, further comprising: a first adjustment mechanism that is driven by the first motor to adjust a fixed position of the first conveying means.

8. An image forming apparatus according to any one of claims 1 to 7, further comprising a second adjustment means for reducing image misalignment occurring in the second section by adjusting the image forming means according to a correction amount determined based on the amount of misalignment measured from the second section.

9. The second adjustment means is A second motor; 9. The image forming apparatus according to claim 8, further comprising: a second adjustment mechanism driven by the second motor to adjust the balance between the nip pressure at one end and the nip pressure at the other end of the image forming means.

10. An image forming apparatus as described in any one of claims 1 to 5, further comprising a third adjustment means for reducing image misalignment in the third section by adjusting the second conveying means according to a correction amount determined based on the amount of misalignment measured from the third section.

11. The third adjustment means is a third motor; 11. The image forming apparatus according to claim 10, further comprising: a third adjustment mechanism that is driven by the third motor to adjust a fixed position of the second conveying means.

12. 12. The image forming apparatus according to claim 1, wherein the second conveying unit is a fixing unit that fixes an image on the sheet while conveying the sheet.

13. 6. The image forming apparatus according to claim 1, further comprising a first display unit that displays a manual correction amount for the first conveying unit determined based on the deviation amount measured from the first section.

14. 7. The image forming apparatus according to claim 1, further comprising a second display unit that displays a manual correction amount for the image forming unit determined based on the deviation amount measured from the second section.

15. 8. The image forming apparatus according to claim 1, further comprising a third display unit that displays a manual correction amount for the second conveying means determined based on the deviation amount measured from the third section.

16. The image forming apparatus according to any one of claims 1 to 5, characterized in that the control means further has an image correction means for reducing image misalignment by pre-correcting the position of each pixel constituting the image based on the amount of misalignment measured from the first section, the amount of misalignment measured from the second section, and the amount of misalignment measured from the third section.

17. The image forming apparatus according to any one of claims 1 to 5, characterized in that the control means further has a correction means for reducing image misalignment by correcting the image writing position and magnification in each section of the sheet based on the amount of misalignment measured from the first section, the amount of misalignment measured from the second section, and the amount of misalignment measured from the third section.

18. The control means a first correction mode in which a first test chart is generated as a sheet on which a test image is formed by controlling the image forming means, and image misalignment is mechanically corrected by adjusting the first conveying means, the image forming means, and the second conveying means based on a reading result of the test image on the first test chart; a second correction mode in which the image forming means is controlled to generate a second test chart, which is a sheet on which a test image is formed, and image misalignment is electrically corrected by pre-correcting the positions of a plurality of pixels constituting the image based on the reading result of the test image on the second test chart, 18. The image forming apparatus according to claim 1, wherein the control unit executes the first correction mode to mechanically correct image misalignment, and then executes the second correction mode to electrically correct the remaining image misalignment.

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