Image formation apparatus

Abstract

To improve positional accuracy in a sheet width direction of an image formed on a sheet.SOLUTION: An image formation apparatus includes an image formation part that forms an image on a sheet S, a first conveyance part 70a that conveys the sheet S to the image formation part, a CIS 91 that detects a position of the sheet S in a sheet width direction intersecting a sheet conveyance direction, and a control part that adjusts a position at which the image is formed on the sheet S on the basis of a first width position (L1m) of the sheet in the sheet width direction at a first detection position located in a central region among regions obtained by equally dividing, into three parts in the sheet conveyance direction, the sheet S conveyed by the first conveyance part 70a, acquired on the basis of a detection result of the CIS 91.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that forms an image on a sheet. [Background technology]

[0002] Conventionally, image forming devices that form images on sheets require high positional accuracy between the sheet and the image written on the sheet. In particular, the following image forming device has been developed as a means for correcting misalignment in the sheet width direction, which is perpendicular to the sheet conveyance direction (see Patent Document 1). This image forming device is equipped with a detection unit that detects the side edge position in the sheet width direction, near a pair of registration rollers that are arranged upstream of a transfer unit that transfers a toner image on an intermediate transfer body to a sheet. Then, a latent image is formed on an image carrier based on the sheet side edge position detected by the detection unit, and the writing position is corrected to match the sheet position. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-293280 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the image forming apparatus described in Patent Document 1, the position of the side edge of the sheet at the leading edge of the sheet is read by a CIS sensor, and the writing position is controlled based on the detection result. Therefore, if the image forming position is adjusted to match the side edge position of the leading edge of the sheet, there is a risk that the image at the trailing edge of the sheet will be more misaligned than at the leading edge. In particular, if the sheet cutting accuracy is poor, image misalignment in the sheet width direction at the trailing edge of the sheet is likely to occur. Furthermore, when double-sided printing is performed, the sheet transport direction is reversed, which may result in misalignment of the images on the front and back of the sheet width direction.

[0005] SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that improves the positional accuracy of an image formed on a sheet in the sheet width direction. [Means for solving the problem]

[0006] A first aspect of the present invention is an image forming apparatus comprising: an image forming unit that forms an image on a sheet; a conveying unit that conveys the sheet to the image forming unit; a width position detection unit that detects the position of the sheet in a sheet width direction that intersects with the sheet conveying direction; and a control unit that adjusts the position at which the image is formed on the sheet based on a first width position of the sheet in the sheet width direction at a first detection position located in a central area of ​​three equal areas obtained by dividing the sheet conveyed by the conveying unit in the sheet conveying direction based on the detection result of the width position detection unit. [Effects of the Invention]

[0007] According to the present invention, it is possible to improve the positional accuracy of an image formed on a sheet in the sheet width direction. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view showing an image forming apparatus according to a first embodiment. [Figure 2] FIG. 2 is a block diagram showing a control system of the image forming apparatus according to the first embodiment. [Figure 3] 1A and 1B are diagrams showing a sheet conveying section according to the first embodiment, in which (a) shows when the leading edge of a sheet is detected in the first conveying section, and (b) shows when the leading edge of a sheet is detected in the second conveying section. [Figure 4] 1A and 1B are diagrams showing a sheet conveying section according to a first embodiment, in which (a) shows when the center position of the sheet is detected in the first conveying section, and (b) shows when the center position of the sheet is detected in the second conveying section. [Figure 5] 5A and 5B are diagrams showing a first conveying unit according to the first embodiment, in which (a) shows when the leading edge of a sheet is detected, and (b) shows when an image is transferred onto the sheet. [Figure 6]5A and 5B are diagrams showing a first conveying section according to the first embodiment, in which (a) shows when the central position of a sheet is detected, and (b) shows when the trailing edge of the sheet is detected. [Figure 7] 10 is a flowchart showing the first half of a procedure for double-sided printing by the image forming apparatus according to the first embodiment. [Figure 8] 10 is a flowchart showing the second half of the procedure when double-sided printing is performed by the image forming apparatus according to the first embodiment. [Figure 9] 10A and 10B are diagrams showing a first conveying section according to a second embodiment, in which (a) shows when the leading edge of a sheet is detected, and (b) shows when the trailing edge of a sheet is detected. [Figure 10] 10 is a flowchart showing the first half of a procedure for double-sided printing by an image forming apparatus according to a third embodiment. [Figure 11] 10 is a flowchart showing the second half of the procedure when double-sided printing is performed by the image forming apparatus according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0010] First Embodiment An image forming apparatus 1 according to a first embodiment will be described with reference to FIGS. 1 to 8. First, the schematic configuration of the image forming apparatus 1 according to this embodiment will be described with reference to FIG. 1. FIG. 1 is a cross-sectional view showing the schematic configuration of the image forming apparatus 1 according to this embodiment as seen from the front side. The depth direction of the paper when viewed from the front side F of the main body is the rear side B of the main body. The image forming apparatus 1 is a tandem-intermediate transfer laser beam printer that uses an electrophotographic process. The image forming apparatus 1 can form and output a full-color or monochrome image corresponding to print image data output from an external computer 204 (see FIG. 2) connected to a control unit 200 on a sheet S, which is a recording medium.

[0011] [Image forming device] The image forming apparatus 1 includes an apparatus main body 1A, an image forming section 2 that forms an image on a sheet S, a secondary transfer section 3, and a sheet conveying section 4.

[0012] [Image forming section] The image forming unit 2 forms an image on the conveyed sheet S. The image forming unit 2 has four process cartridges PY, PM, PC, and PK that form toner images of four colors, yellow (Y), magenta (M), cyan (C), and black (K), respectively, an exposure unit 13, and an intermediate transfer belt 21. The four process cartridges PY, PM, PC, and PK have the same configuration except for the colors of the images they form. Therefore, only the configuration and image forming process of the process cartridge PY will be described, and a description of the process cartridges PM, PC, and PK will be omitted. The image forming unit 2 also has a secondary transfer inner roller 22 and a primary transfer roller 25.

[0013] The process cartridge PY includes a photosensitive drum 11, which is a drum-shaped electrophotographic photosensitive member, a charging roller 12, and a developing roller 14. The photosensitive drum 11 is rotatably supported by the image forming apparatus 1 and is rotationally driven by a driving unit (not shown). The photosensitive drum 11 is configured by providing a photoconductive layer such as an OPC (organic photoconductor) on the outer surface of an aluminum cylinder. The charging roller 12 charges the photosensitive drum 11. The charging roller 12 is configured by a core metal and a conductive elastic member surrounding the periphery. The charging roller 12 is disposed in contact with the surface of the photosensitive drum 11 and rotates in response to the rotation, and a charging bias is applied to it from a power source (not shown). The developing roller 14 deposits toner of a different color (yellow, magenta, cyan, or black) onto the surface of the photosensitive drum 11 for each of the process cartridges PY, PM, PC, and PK. The developing roller 14 develops the electrostatic latent image on the surface of the photosensitive drum 11 by attaching toner to the electrostatic latent image on the surface of the photosensitive drum 11, thereby forming a toner image on the surface of the photosensitive drum 11. Toner is supplied from a toner cartridge 15 to each developing roller 14.

[0014] The exposure unit 13 houses within its housing a laser light source that emits laser light and various optical members for guiding the laser light emitted from the laser light source to the corresponding photosensitive drum 11 for deflection scanning. The exposure unit 13 irradiates the charged surface of the photosensitive drum 11 with laser light based on image information, forming an electrostatic latent image. Thereafter, a predetermined pressure and electrostatic bias are applied by the primary transfer roller 25, and the toner image is transferred onto the intermediate transfer belt 21.

[0015] The intermediate transfer belt 21 is stretched by rollers such as a drive roller 23, a tension roller 24, and a secondary transfer inner roller 22, and is driven to be transported in a direction D2. The image formation processes for each color of Y, M, C, and K, which are processed in parallel, are performed at timings such that they are superimposed on the toner images of the upstream colors that have been primarily transferred onto the intermediate transfer belt 21. As a result, a full-color toner image is finally formed on the intermediate transfer belt 21, and this toner image is transported to the secondary transfer unit 3.

[0016] The secondary transfer unit 3 has an intermediate transfer belt 21 and an outer secondary transfer roller 44 disposed opposite the inner secondary transfer roller 22 with the intermediate transfer belt 21 sandwiched therebetween. The secondary transfer unit 3 has a transfer nip formed by the opposing inner secondary transfer roller 22 and outer secondary transfer roller 44, and performs transfer (secondary transfer) of a toner image onto the sheet S by applying a predetermined pressure force and electrostatic bias. The secondary transfer unit 3 is an example of an image forming position, and is a position where an image is formed on a sheet by the image forming unit 2.

[0017] [Sheet transport device] The sheet conveying unit 4 includes sheet cassettes 31 and 32, feeding units 31a and 32a, a first conveying unit 70a, and a second conveying unit 70b. Each sheet cassette 31 and 32 is provided with a sheet size sensor 31d or 32d that detects the size of the stored sheet. The sheet size sensors 31d and 32d are an example of a length detection unit and detect the length of the sheet S in the sheet conveying direction. The first conveying unit 70a conveys the sheet S when an image is formed on the front side (first side) of the sheet S. The second conveying unit 70b inverts the sheet S that has passed through the first conveying unit 70a and conveys the sheet S when an image is formed on the back side (second side) of the sheet S that is opposite to the first side. The first conveying unit 70a is an example of a conveying unit that conveys the sheet S to the image forming unit 2, and includes a pre-registration roller pair 41 and a registration roller pair 42. The pre-registration roller pair 41 and the registration roller pair 42 are disposed upstream of the secondary transfer unit 3 in the sheet conveying direction.

[0018] The sheet S is fed from the feeding unit 31a, passes through the pre-registration roller pair 41, and is conveyed to the registration roller pair 42. The pre-registration roller pair 41 and the registration roller pair 42 correct skew of the sheet S. Specifically, the leading edge of the sheet S being conveyed is abutted against the nip portion of the registration roller pair 42, which is stopped. In this way, the pre-registration roller pair 41 loops the sheet S to correct skew. The registration roller pair 42 conveys the sheet S to the secondary transfer unit 3 in synchronization with the transfer of the toner image on the intermediate transfer belt 21 to the sheet S. That is, the registration roller pair 42, while stopping its rotation, abuts the leading edge of the sheet S in the sheet conveyance direction against the registration roller pair 42 to correct skew of the sheet S, and then sandwiches and conveys the sheet S.

[0019] After the toner image is transferred to the sheet S in the secondary transfer unit 3, the sheet S is transported to a fixing device 50, where the toner image is melted and fixed to the sheet S by heat and pressure. After fixing, the sheet S is transported to an ejection conveyance unit 60, and a switching member 64 switches the conveyance path depending on whether the paper passing mode is single-sided or double-sided. In the single-sided mode, the sheet S is stacked on an ejection tray 80 by an ejection roller pair 62.

[0020] On the other hand, in the case of double-sided paper feeding, the sheet S is stopped once with the trailing edge remaining a predetermined distance from the reversing roller pair 71, and then the reversing roller pair 71 is rotated in the reverse direction. The reversing roller pair 71 is an example of an inverting unit, and inverts the sheet S on which an image has been formed in the image forming unit 2. The sheet S is then transported again to the registration roller pair 42 via the second transport unit 70b, where image formation and fixing on the second side are performed in the image forming unit 2. The second transport unit 70b is an example of a re-transport path, and transports the sheet S that has been inverted by the reversing roller pair 71 back to the registration roller pair 42. After fixing, the sheet S is transported via the switching member 64 and stacked on the discharge trays 80, 82 by the discharge roller pairs 62, 71.

[0021] [Control system] Next, the control unit 200 and control system of the image forming apparatus 1 will be described with reference to Fig. 2. The control unit 200 has various functional units, such as a CPU 201 (Central Processing Unit), a memory 202, an image formation control unit 205, a sheet conveyance control unit 206, a sensor control unit 207, and a sheet side edge position control unit 208. The CPU 201 executes predetermined control programs and the like to realize various processes performed by the image forming apparatus 1. The memory 202 is, for example, a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores various programs and various data in predetermined storage areas.

[0022] The operation unit 203, which may be a touch panel mounted on the top of the device main body 1A, accepts various user operations, such as inputting various information about the sheets used for printing (e.g., size, basis weight, and surface properties) and issuing instructions to start or stop printing. The image formation control unit 205 issues instructions to the image forming unit 2, including the exposure unit 13, to control image formation. The sheet transport control unit 206 issues instructions to the feed motor 101, registration motor 102, and duplex motor 103, and controls the drive of various transport roller pairs to control the transport of the sheet S. The sensor control unit 207 controls the start or stop of detection by the sheet size sensors 31d and 32d, the registration sensor 43, and other sensors, and accepts the detection results of these sensors. The sheet side edge position control unit 208 accepts detection signal values ​​from the CISs 91 and 92 (described later) and converts them into sheet side edge positions. It is also possible to configure the device to receive various information about the sheets used for printing via a computer 204 connected via a network, for example.

[0023] [Seat position detection in the sheet width direction] Next, a configuration for detecting the side edge positions in the sheet width direction (the end positions in the sheet width direction) will be described as a specific configuration of the sheet conveying unit 4. Note that the image forming apparatus 1 according to this embodiment will be described as adopting, as an example, a center-based sheet conveying method in which the center line in the width direction of the sheet, which is perpendicular to the sheet conveying direction in the sheet conveying path, is aligned with the center line in the sheet width direction of the sheet, to convey the sheet.

[0024] Sheets S, which are transfer materials, are stacked and stored by size in sheet cassettes 31, 32 or manual feed section 33, and in order to form an image in an appropriate range on sheets S, it is necessary to make image forming apparatus 1 recognize the size of the stored sheets S. Sheet cassettes 31, 32 have side regulation plates that regulate the position of the sheet in the width direction, which makes it possible to suppress skew and widthwise positional deviation of sheets S that occur between sheet S feed sections 31a, 32a and each conveyance roller pair.

[0025] However, in reality, there is a risk that a small gap may occur between the side regulating plate and the sheet S. If a gap occurs, the sheet S may become skewed or may become misaligned in the sheet width direction when being fed and conveyed. In conventional image forming apparatuses, the sheet may be conveyed to the secondary transfer unit 3 while being misaligned in the sheet width direction, and the position of the image formed on the first side of the sheet S may become misaligned.

[0026] Furthermore, the sheet S that has been switched back and reversed by the reversing roller pair 71 is transported again to the image forming unit 2 via the second transport section 70b. At this time, when the sheet S is nipped and transported by one roller pair at the reversing roller pair 71, both the roller pair and the sheet S may be misaligned in the sheet width direction. Furthermore, before the switchback, the sheet S is transported from the fixing device 50 to the reversing roller pair 71, and after the switchback, the sheet S is transported from the reversing roller pair 71 to the double-sided transport roller pairs 72 and 73, where it is nipped and transported by multiple roller pairs. For this reason, the sheet S may be misaligned or skewed in the width direction due to misalignment between the roller pairs. In this case, there is a risk that the image formed on the second side of the sheet S may be misaligned in the sheet width direction.

[0027] As such, there is a risk that the image formed on the sheet S will be misaligned in the width direction on both the first and second sides. To address this, by providing CISs 91 and 92 that detect the side edge positions on both the first and second sides, the amount of misalignment in the side edge positions in the sheet width direction can be detected, and a latent image is formed on the image carrier using the sheet side edge positions based on the detected amount as a reference. This makes it possible to correct the image formation position so that it is written in accordance with the sheet position.

[0028] However, when reading the side edge of a sheet, the reading position differs between the first and second surfaces, resulting in a problem of misalignment of the image position in the main scanning direction between the front and back sides. Furthermore, when reading the side edge of a sheet, if the cutting accuracy of the sheet S is poor, forming an image aligned with the side edge position of the leading edge of the sheet results in a large misalignment between the sheet S and the image in the sheet width direction at the trailing edge of the sheet. Therefore, in this embodiment, the position of the side edge of the sheet at the center position in the sheet conveyance direction is used as a reference to correct the writing position, thereby reducing the misalignment of the image in the sheet width direction relative to the sheet S.

[0029] 3(a) and 3(b) illustrate the difference in the reading positions between the first and second sides, which is one of the issues discussed above. The dashed-dotted lines in the center of FIGS. 3(a) and 3(b) represent the center in the sheet width direction. Based on the detection results of CIS 91 and 92, the control unit 200 calculates the amount of deviation between the nominal position (design target position) and the detection results. By controlling the amount of image writing correction in the exposure unit 13 based on the deviation amount, it is possible to improve the accuracy of the image positions on the first and second sides formed on the sheet S. In other words, the image forming unit 2 can change the position at which the image is formed on the sheet S, and the control unit 200 adjusts the position at which the image is formed on the sheet S in the image forming unit 2.

[0030] As shown in FIG. 3A, the first conveying unit 70a includes a CIS 91 that detects the side edge position of the first surface of the sheet S and a registration sensor 43. The CIS 91 is disposed upstream of the registration roller pair 42 in the sheet conveying direction. In FIG. 3A, the CIS 91 detects the side edge position of the leading edge of the first surface of the sheet S, i.e., the misalignment amount L1t in the sheet width direction. In this embodiment, the CIS 91 and the registration sensor 43 are arranged side by side in a direction perpendicular to the sheet conveying direction. This allows the CIS 91 to detect the side edge position of the leading edge or trailing edge of the sheet by synchronizing the detection timing of the registration sensor 43 with the detection timing of the leading edge or trailing edge of the sheet. Here, the CIS 91 detects the side edge position of the first surface of the sheet S, but it is also possible to detect the side edge position of the second surface of the sheet S. That is, the CIS 91 is an example of a width position detection unit and a first width position detection unit, and detects the position of the sheet S in the sheet width direction intersecting the sheet conveyance direction.

[0031] The CIS 91 is provided at a position offset to one side from the center of the sheet width direction of the sheet S in the sheet conveyance direction indicated by direction D1, relative to the sheet S nipped and conveyed between the pre-registration roller pair 41; in this embodiment, the CIS 91 is provided on the rear side B of the main body. This is because, in correcting the image formation position on the sheet S, it is only necessary to detect the side edge position of one side of the sheet S. By providing the CIS 91 on the rear side B of the main body, the left side in the sheet conveyance direction can be used as the reference for the image formed on the sheet S. Furthermore, the CIS 91 is positioned so that it can detect the side edge positions of both the smallest and largest sheet sizes compatible with the image forming apparatus 1.

[0032] On the other hand, as shown in Fig. 3(b), the second conveying section 70b has a CIS 92 that detects the side edge position of the second surface of the sheet S, after the sheet S has been switchback-reversed by the reversing roller pair 71, before being conveyed again to the image forming section 2. In Fig. 3(b), the CIS 92 detects the amount of positional deviation L2t in the sheet width direction of the leading edge of the second surface of the sheet S. The CIS 92 is an example of a second width position detection section, and is disposed in the second conveying section 70b to detect the position of the sheet S in the sheet width direction.

[0033] The CIS 92 is provided at a position offset to one side from the center of the sheet width direction of the sheet S in the sheet conveyance direction indicated by direction D3, relative to the sheet S nipped and conveyed between the pair of double-sided conveyance rollers 72; in this embodiment, the CIS 92 is provided on the rear side B of the main body. This is because, as with the first side, it is only necessary to detect the side edge position of one side of the sheet S when correcting the image formation position on the sheet S. By providing the CIS 92 on the rear side B of the main body, the left edge in the sheet conveyance direction can be used as the reference for the image formed on the sheet S. Furthermore, the CIS 92 is positioned so that it can detect the side edge positions of both the smallest and largest sheet sizes compatible with the image forming apparatus 1.

[0034] Here, it can be seen that the positional deviation amount L1t of the sheet leading edge, which is the detection value detected by CIS 91 shown in FIG. 3(a), is different from the positional deviation amount L2t of the sheet leading edge, which is the detection value detected by CIS 92 shown in FIG. 3(b). This can be caused by a case where the sheet S is skewed or where the sheet edge is not perpendicular due to the cutting characteristics of the sheet S. In such a sheet state, the leading and trailing edges of the sheet S are reversed when the sheet S is nipped and conveyed by the pair of pre-registration rollers 41 and when the sheet S is switchback-reversed by the pair of reversing rollers 71. This causes a deviation in the reading positions of the CISs 91 and 92.

[0035] To address the issue of misalignment between the first and second sides when the detection positions of CIS 91 and 92 shown in Figures 3(a) and (b) are set at the leading edge of the sheet edge, in the example shown in Figures 4(a) and (b), the detection positions of CIS 91 and 92 are set at the center of the sheet conveyance direction of sheet S. As shown in Figures 3(a) and (b), when sheet position misalignment of the side edges of the first and second sides is detected at the leading edge of the sheet, misalignment of the reading positions of CIS 91 and 92 occurs when sheet S is running skew or when the perpendicularity of the sheet edge is poor due to the cutting characteristics of sheet S. If feedback control is performed on the image formation position when this misalignment in the reading positions of CIS 91 and 92 occurs, image registration on the front and back sides will also be misaligned, resulting in poor accuracy.

[0036] The leading and trailing edges of the first and second sides are reversed. Therefore, performing detection by CIS 91 and 92 at the center position in the sheet transport direction reduces the amount of deviation in the detection position in the width direction between the front and back of the first and second sides of the sheet L1m and L2m, compared to detecting the side edges at the leading edge of the sheet. In addition, CIS 91 and CIS 92 are both located at the rear side B of the main body, and by aligning their detection standards as sensors, this is advantageous in terms of detection accuracy. In other words, CIS 91 and CIS 92 are located on the same side in the sheet width direction with respect to the center of the sheet transport area through which sheet S is transported (see the dashed line in Figure 3(a)).

[0037] [Procedure for detecting the side edge position in the sheet width direction and correcting the image formation position during double-sided printing] 5(a), (b) and 6(a), (b) are diagrams for explaining a method for detecting the side edge position in the width direction of the sheet S and correcting the image forming position in the sheet conveying section 4. Hereinafter, a processing procedure for detecting the side edge position in the width direction of the sheet S and correcting the image forming position in the image forming apparatus 1 will be explained using the flowcharts shown in FIGS.

[0038] The control unit 200 starts a print job upon receiving a print execution instruction from the user via the operation unit 203 or the computer 204 (S101). The user can specify the number of copies to be printed and also the type of sheets to be used for printing. The control unit 200 also uses the sheet size sensors 31d and 32d to obtain information on the sizes of the sheets stored in the sheet cassettes 31 and 32. Alternatively, the control unit 200 may obtain information based on the sheet size information input by the user via the operation unit 203.

[0039] The control unit 200 starts feeding the sheet S (S102) and determines whether printing is being performed on the first side of the print job (S103). If the control unit 200 determines that printing is being performed on the first side (S103: YES), it sets the g1n position, which is the image writing position for the first side, to a preset g1 and writes an image at the g1n position (S104). Here, g1, which is the g1n position for the first side of the first sheet, is a value based on the results of write position adjustment performed at the time of factory shipment or the like, and is stored in the memory 202 as a fixed value specific to the main body. Furthermore, g1n for the second and subsequent sheets when sheets are fed continuously is calculated by feedback control, which will be described later. Note that n here refers to printing the nth sheet in the print job.

[0040] Next, the sheet S is conveyed to the registration roller pair 42 in the first conveying section 70a. Here, when viewed from the image forming section 2 side, the conveyed sheet S is rotated counterclockwise with respect to the sheet conveying direction, skewed, and shifted to the rear side B as shown in FIG. 5(a). The dashed rectangle shown in FIG. 5(a) schematically shows a state in which the leading edge of the sheet S, which has been conveyed without skew or lateral shift, abuts on the nip portion of the registration roller pair 42, and the side edge position in the sheet width direction at this time is set as the zero point, and the rear side B is set as the positive direction.

[0041] The control unit 200 acquires the detection result of the registration sensor 43 for the leading edge of the conveyed sheet S (S105), and the leading edge of the sheet S abuts against the nip portion of the stopped registration roller pair 42. Thereafter, the pre-registration roller pair 41 feeds the sheet S by a set feed amount, thereby forming a predetermined amount of slack in the sheet S (S106). In this way, the control unit 200 corrects the skew of the sheet S, and further starts the rotational driving of the registration roller pair 42 (S107).

[0042] The control unit 200 continues the conveyance by the registration roller pair 42, and transfers an image (toner image) onto the sheet S at the secondary transfer unit 3 (S109). As shown in FIG. 6(a), the control unit 200 detects the side edge position at the center position of the sheet S by the CIS 91 (S110), and stores the detection result L1m in, for example, the memory 202.

[0043] Here, the central position will be described. In this embodiment, the central position of the sheet S in the sheet conveying direction is defined as the first detection position. The control unit 200 adjusts the position at which an image is formed on the sheet S, as will be described later, based on a first width position of the sheet in the sheet width direction at the first detection position. Note that the central position here refers to the central position of the sheet S in the sheet conveying direction. For example, after detecting the leading edge of the sheet with the registration sensor 43, the control unit 200 detects the side edge position with the CIS 91 at the timing when the sheet S has been conveyed a distance half the sheet length previously acquired, and acquires this as the first width position. That is, the control unit 200 detects the first width position at the first detection position with the CIS 91 based on the length of the sheet S detected by the sheet size sensors 31d and 32d.

[0044] Here, the control of detecting the side edge position at the center of the sheet S is one of the characteristic controls of the image forming apparatus 1 according to this embodiment. Generally, the perpendicularity of a sheet itself, i.e., the angle between the leading edge and the left edge in the sheet conveyance direction, may not be exactly 90°. In this embodiment, skew correction is performed based on the leading edge of the sheet S. Therefore, if the perpendicular line drawn from the leading edge along the sheet conveyance direction is not parallel to the left edge, even if the image writing position in the sheet width direction relative to the sheet S is corrected by image position correction, gradual misalignment in the sheet width direction occurs as the sheet is conveyed toward the rear edge. In other words, even if the image forming position in the sheet width direction can be corrected to a position approximately close to g1 at the leading edge of the sheet S, there is a risk of it being misaligned to a position L1b at the rear edge of the sheet S. In contrast, in this embodiment, by detecting the side edge position at the center of the sheet S, significant image misalignment relative to the sheet S can be prevented even at the leading and trailing edges of the sheet S.

[0045] The control unit 200 fixes the toner image via the fixing device 50 (S112). If the print job is single-sided printing, the sheet S with the fixed toner image is discharged to the discharge tray 80, whereas if the print job is double-sided printing, the sheet S is inverted for image formation on the second side (S113). The control unit 200 determines whether there are any subsequent prints (S114). If the control unit 200 determines that there are no subsequent sheets (S114: NO), it ends the print job (S115).

[0046] If the control unit 200 determines that there is a subsequent sheet (S114: YES), it calculates the image position correction in the sheet width direction for the first side based on the detection result of L1m stored in the memory 202 (S116). Here, the control unit 200 passes the correction amount, taking into account the center position detection of g1n←L1m-g1(n-1), to the subsequent sheet, and returns to the processing of step S103. That is, the control unit 200 adjusts the position where the image is formed on the subsequent sheet, the second sheet, which is conveyed after the first sheet, based on the first width position (L1m) of the preceding sheet, the first sheet, at the first detection position. Note that the first and second sheets are not limited to being conveyed consecutively. When sheets of different sizes are mixed, the same image formation position adjustment may be performed only on sheets of the same size.

[0047] When the control unit 200 determines that the print job is printing on the second side (S103: NO), the control unit 200 causes the sheet S to be switched back by the reversing roller pair 71 and then conveyed again by the second conveying unit 70b to the pre-registration roller pair 41. When the second conveying unit 70b conveys the sheet S, the control unit 200 detects the side edge position (L2m) of the center position in the sheet conveying direction of the second side of the sheet S using the CIS 92 (S117). That is, when the first sheet is conveyed to the second conveying unit 70b, the control unit 200 acquires the sixth width position (L2m) of the sheet in the sheet width direction at the third detection position, which is the center position, based on the detection result of the CIS 92. Then, the control unit 200 adjusts the position where an image is formed on the second side of the first sheet in the sheet width direction, based on the sixth width position. That is, the control unit 200 calculates the image position correction in the sheet width direction on the second surface based on the obtained detection result of L2m, and writes the image at the calculated image position g2n (S118).

[0048] The control unit 200 acquires the detection result of the registration sensor 43 for the leading edge of the conveyed sheet S (S119), and the leading edge of the sheet S abuts against the nip portion of the stopped registration roller pair 42. Thereafter, the pre-registration roller pair 41 feeds the sheet S by a set feed amount, thereby forming a predetermined amount of slack in the sheet S (S120). In this way, the control unit 200 corrects the skew of the sheet S, and further starts the rotational driving of the registration roller pair 42 (S121).

[0049] The control unit 200 continues the conveyance by the registration roller pair 42, and transfers an image (toner image) onto the sheet S in the secondary transfer unit 3 (S122). The subsequent processing is the same as that from S112 onwards.

[0050] As described above, according to the image forming apparatus 1 of this embodiment, the control unit 200 adjusts the position at which an image is formed on the sheet S based on the first width position of the sheet at the first central position. Therefore, compared to when the position at which an image is formed on the sheet S is adjusted based on the side edge position of the leading edge of the sheet, it is possible to reduce positional deviation of the image in the sheet width direction relative to the sheet at the trailing edge of the sheet, etc. This makes it possible to improve the positional accuracy of the image formed on the sheet S in the sheet width direction.

[0051] <Second embodiment> Next, a second embodiment of the present invention will be described in detail with reference to FIG. 9. This embodiment differs from the first embodiment in that the side edge position of the central position is obtained using the side edge positions of the leading edge and trailing edge of the sheet. However, other configurations are the same as those of the first embodiment, so the same reference numerals are used and detailed description will be omitted. In the first embodiment, the first width position is directly read by the CIS 91, but this is not limiting. In other words, the side edge position may be read by the CIS 91 at a position other than the first detection position and calculated based on the detection results, and this case will be described in this embodiment.

[0052] 9(a) shows the detection of the side edge of the leading edge of the sheet on the first side, and FIG. 9(b) shows the detection of the side edge of the trailing edge of the sheet on the first side. In detecting the side edge of the leading edge of the sheet, the control unit 200 synchronizes the detection timing of the CIS 91 with the detection of the arrival of the leading edge of the sheet by the registration sensor 43, thereby enabling the CIS 91 to detect the side edge of the leading edge of the sheet with high accuracy. That is, the control unit 200 uses the CIS 91 to detect a second width position in the sheet width direction at the first conveyance direction position of the sheet S in the sheet conveyance direction (leading edge of the sheet S). Similarly, in detecting the side edge of the trailing edge of the sheet, the control unit 200 synchronizes the detection timing of the CIS 91 with the detection of the passage of the trailing edge of the sheet by the registration sensor 43, thereby enabling the CIS 91 to detect the side edge of the trailing edge of the sheet with high accuracy. That is, the control unit 200 detects a third width position in the sheet width direction at the second conveying direction position (the rear end portion of the sheet S) of the sheet S in the sheet conveying direction by the CIS 91. Then, the control unit 200 calculates the first width position at the first detection position based on the second width position and the third width position.

[0053] In this embodiment, the first conveying direction position is the leading edge of the sheet, and the second conveying direction position is the trailing edge of the sheet. However, this is not limited to this, and as long as the width positions can be detected at at least two conveying direction positions, not just the leading edge or the trailing edge, the side edge position of the center position can be calculated.

[0054] According to this embodiment, the side edge position of the center position of the sheet S is obtained by calculating it from the side edge positions of the leading edge and trailing edge of the sheet without directly measuring the center position. Since the side edge positions of the leading edge and trailing edge of the sheet can be obtained with high accuracy, it becomes possible to obtain the side edge position of the center position with high accuracy, and it is possible to correct the image position with high accuracy.

[0055] <Third embodiment> Next, a third embodiment of the present invention will be described in detail with reference to Figures 10 and 11. This embodiment differs from the first embodiment in that it does not have a CIS 92. However, other configurations are the same as those of the first embodiment, so the same reference numerals will be used and detailed description will be omitted.

[0056] Hereinafter, a processing procedure for detecting the side edge position in the width direction of the sheet S and correcting the image forming position in the image forming apparatus 1 will be described with reference to the flowcharts shown in FIGS.

[0057] Steps S101 to S110 are the same as those in the first embodiment. That is, the control unit 200 acquires L1m as the first width position based on the detection result of the CIS 91 for the first side of the first sheet (S110). After the control unit 200 detects the side edge position at the center position using the CIS 91 in step S110, the control unit 200 detects the side edge position at the trailing edge of the sheet using the CIS 91 (S111), as shown in FIG. 6(b), and stores the detection result L1b in, for example, the memory 202. Note that this detection is not performed for the second and subsequent sheets. Subsequently, steps S112 to S115 are the same as those in the first embodiment.

[0058] If the control unit 200 determines that the print job is printing on the second side (S103: NO), the sheet is reversed by a switchback at the reversing roller pair 71, and is then transported again by the second transport unit 70b to the pre-registration roller pair 41. The switchback reversal is characterized by the fact that the trailing edge of the first side switches to the leading edge of the second side, and it is known that if the perpendicularity of the sheet S is poor, the detection result L1b of the side edge position on the trailing edge of the first side correlates with the side edge position on the leading edge of the second side. In practice, it is common for deviations during transport by the second transport unit 70b to also be added.

[0059] In consideration of the above phenomenon, in this embodiment, the image writing position g2n for the second side is calculated based on the detection result L1b of the side edge position on the trailing edge side of the first side, and g2n = L1b for the second side of the first sheet (S123). That is, in this embodiment, when forming an image on the first side of the first sheet, the control unit 200 acquires a fourth width position in the sheet width direction of the trailing edge of the first sheet in the sheet conveying direction based on the detection result of the CIS 91. Then, the control unit 200 adjusts the position where the image is formed in the sheet width direction on the second side of the first sheet, which is opposite the first side, based on the fourth width position.

[0060] Thereafter, steps S119 to S122 are the same as those in the first embodiment. As shown in Fig. 6(a), the control unit 200 detects the side edge position at the central position using the CIS 91 (S125), and stores the detection result L2m in, for example, the memory 202. That is, the control unit 200 acquires L2m as the fifth width position of the sheet in the sheet width direction at the second detection position, which is the central position of the sheet S, based on the detection result of the CIS 91 on the second side opposite to the first side of the first sheet (S125).

[0061] Based on the obtained first width position (L1m), the control unit 200 adjusts the position where an image is formed on the first side of the second sheet, which is conveyed after the first sheet (S116). Based on the obtained fifth width position (L2m), the control unit 200 adjusts the position where an image is formed on the second side of the second sheet, which is opposite to the first side (S116). Specifically, the control unit 200 passes on to the subsequent sheets the amount of correction that takes into account the center position detection of g2n←L2m-L1m as the image position correction in the width direction on the second side of the second sheet and thereafter (S116).

[0062] As described above, according to the image forming apparatus 1 of this embodiment, the CIS 91 detects the side edge positions at the center of both sides, so the number of sensors can be reduced to one, thereby reducing the number of parts and costs.

[0063] <Other embodiments> In the above-described embodiments, the central position for detecting the first width position is the central position of the sheet S in the sheet conveyance direction, but this is not limited to this. For example, the central position may be located in the central region of three equal regions obtained by dividing the conveyed sheet S in the sheet conveyance direction. This can reduce misalignment of the image in the sheet width direction relative to the sheet at the rear end of the sheet, etc., compared to when detecting the side edge position at the leading edge of the sheet.

[0064] In addition, in each of the above-described embodiments, image formation is performed from the first sheet S, but this is not limited to this, and the first sheet may be passed through simply for the purpose of detecting the side edge position without image formation. In this case, image formation begins from the second sheet passed through, and highly accurate positioning can be achieved from the first sheet on which an image is formed.

[0065] In addition, in each of the above-described embodiments, the control unit 200 adjusts the position where an image is formed on the sheet S in the image forming unit 2, but this is not limiting. For example, a movement mechanism that can adjust the position of the sheet S in the width direction may be provided, and the position of the sheet S may be adjusted based on the side edge position of the acquired central position.

[0066] In addition, in each of the above-described embodiments, the case where the position where an image is formed on the sheet S in the image forming unit 2 is adjusted in the sheet width direction has been described, but this is not limiting. That is, the image forming position may be adjustable not only in the sheet width direction but also in the sheet conveyance direction or rotation direction, and an optimal adjustment method may be selected depending on the shape (distortion) of the sheet.

[0067] Furthermore, in the above-described embodiment, the image forming apparatus 1 adopts a vertical path configuration in which the transport path from the feeding sections 31a, 32a to the secondary transfer section 3 faces upward, but this is not limited to this and a horizontal path configuration may also be adopted.

[0068] In addition, in each of the above-described embodiments, the image forming apparatus 1 is a color laser printer, but the present invention is not limited to this. For example, the present invention may be applied to an image forming apparatus that is a monochrome laser printer. Alternatively, the present invention may be applied to an inkjet printer. [Explanation of symbols]

[0069] 1...image forming apparatus, 2...image forming section, 3...secondary transfer section (image forming position), 31d, 32d...sheet size sensor (length detection section), 42...pair of registration rollers, 70a...first conveying section (conveying section), 70b...second conveying section (re-conveying path), 71...pair of reversing rollers (reversing section), 91...CIS (width position detection section, first width position detection section), 92...CIS (second width position detection section), 200...control section

Claims

1. an image forming unit that forms an image on a sheet; a conveying section that conveys a sheet to the image forming section; a width position detection unit that detects a position of the sheet in a sheet width direction that intersects with the sheet conveying direction; a control unit that adjusts a position where an image is to be formed on a sheet based on a first width position of the sheet in the sheet width direction at a first detection position located in a central region of three equal regions obtained by dividing the sheet conveyed by the conveying unit into three equal regions in the sheet conveying direction, the first detection position being acquired based on a detection result of the width position detection unit, An image forming apparatus characterized by:

2. the first detection position is a center position of the sheet in the sheet conveying direction; 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. a length detection unit that detects the length of the sheet in the sheet conveying direction, the control unit detects the first width position at the first detection position using the width position detection unit based on the length of the sheet detected by the length detection unit; 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. the control unit detects, by the width position detection unit, a second width position in the sheet width direction at a first conveying direction position of the sheet in the sheet conveying direction and a third width position in the sheet width direction at a second conveying direction position different from the first conveying direction position in the sheet conveying direction, and calculates the first width position at the first detection position based on the second width position and the third width position.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

5. the first conveying direction position is a leading edge of the sheet in the sheet conveying direction, the second conveying direction position is a rear end of the sheet in the sheet conveying direction; 5. The image forming apparatus according to claim 4.

6. the conveying unit is disposed upstream in the sheet conveying direction of an image forming position where an image is formed on a sheet by the image forming unit, and includes a pair of registration rollers that, while rotation is stopped, contact a leading edge of the sheet in the sheet conveying direction to correct skew of the sheet, and then sandwich and convey the sheet.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

7. the width position detection unit is disposed upstream of the pair of registration rollers in the sheet conveying direction, and detects the position of a side edge of the sheet; 7. The image forming apparatus according to claim 6, wherein the image forming apparatus is a recording medium.

8. the image forming unit is capable of changing a position where an image is formed on a sheet; the control unit adjusts a position where an image is formed on a sheet in the image forming unit.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

9. the control unit adjusts a position where an image is formed on a sheet in the image forming unit in the sheet width direction.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

10. the control unit adjusts a position where an image is to be formed on a second sheet that is conveyed after the first sheet, based on the first width position of the first sheet at the first detection position.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

11. an inversion unit that inverts a sheet on which an image has been formed in the image forming unit; a re-conveyance path for re-conveying the sheet that has been inverted by the reversing unit to the pair of registration rollers, The control unit When forming an image on a first surface of a first sheet, a fourth width position in the sheet width direction of a trailing end of the first sheet in the sheet conveying direction is acquired based on a detection result of the width position detection unit; a position where an image is to be formed in the sheet width direction on a second surface of the first sheet opposite to the first surface, based on the fourth width position; 7. The image forming apparatus according to claim 6, wherein the image forming apparatus is a recording medium.

12. The control unit On a first surface of the first sheet, the first width position is acquired based on a detection result of the width position detection unit; On a second surface of the first sheet opposite to the first surface, a fifth width position of the sheet in the sheet width direction is acquired at a second detection position located in a central region of three equal regions obtained by dividing the sheet conveyed by the conveying unit in the sheet conveying direction, based on the detection result of the width position detection unit; adjusting a position where an image is to be formed on a first side of a second sheet that is conveyed after the first sheet based on the first width position; adjusting a position where an image is to be formed on a second surface of the second sheet opposite to the first surface based on the fifth width position; 12. The image forming apparatus according to claim 11.

13. The control unit When forming an image on a first surface of a first sheet, the first width position is acquired based on a detection result of the width position detection unit; When an image is formed on a second surface of the first sheet opposite to the first surface, a fifth width position of the sheet in the sheet width direction at a second detection position located in a central region of three equal regions obtained by dividing the sheet conveyed by the conveying unit in the sheet conveying direction is acquired based on a detection result of the width position detection unit; adjusting a position where an image is to be formed on a first side of a second sheet that is conveyed after the first sheet based on the first width position; adjusting a position where an image is to be formed on a second surface of the second sheet opposite to the first surface based on the fifth width position; 12. The image forming apparatus according to claim 11.

14. the width position detection unit is a first width position detection unit, an inversion unit that inverts a sheet on which an image has been formed in the image forming unit; a re-conveyance path for re-conveying the sheet that has been inverted by the reversing unit to the pair of registration rollers; a second width position detection unit disposed on the re-feeding path and configured to detect a position of the sheet in the sheet width direction, The control unit When forming an image on a first surface of a first sheet, the first width position is acquired based on a detection result of the first width position detection unit; When the first sheet is conveyed to the re-conveying path, a sixth width position of the sheet in the sheet width direction is acquired based on a detection result of the second width position detection unit, the sixth width position being acquired at a third detection position located in a central region of three equal regions obtained by dividing a second surface of the first sheet conveyed on the re-conveying path, the second surface being opposite to the first surface, into three equal regions in the sheet conveying direction; adjusting a position where an image is to be formed on the second surface of the first sheet in the sheet width direction based on the sixth width position; 7. The image forming apparatus according to claim 6, wherein the image forming apparatus is a recording medium.

15. The control unit When forming an image on a first surface of a first sheet, the first width position is acquired based on a detection result of the first width position detection unit; adjusting a position where an image is to be formed on a first surface of a second sheet that is conveyed after the first sheet based on the first width position; 15. The image forming apparatus according to claim 14.

16. the first width position detection unit and the second width position detection unit are disposed on the same side with respect to the center of a sheet conveying area in the sheet width direction, 15. The image forming apparatus according to claim 14.

Images