Sheet transport device

Abstract

It corrects sheet skew and misalignment with high precision. [Solution] A sheet conveying device comprising: a feeding unit for feeding sheets; an image forming unit for forming an image on the sheet fed by the feeding unit at an image forming position; a first conveying unit positioned between the feeding unit and the image forming position in the sheet conveying direction for conveying sheets; and a second conveying unit positioned between the first conveying unit and the image forming position in the sheet conveying direction for conveying sheets, wherein the first conveying unit is capable of performing a first skew correction process for correcting the skew of the sheet and a moving process for moving the sheet that has undergone the first skew correction process in a width direction perpendicular to the sheet conveying direction; and the second conveying unit is capable of performing a second skew correction process for correcting the skew of the sheet that has undergone the first skew correction process and the moving process by the first conveying unit.

Description

Technical Field

[0001] The present invention relates to a sheet conveying device for conveying a sheet.

Background Art

[0002] According to Patent Document 1, an image forming apparatus provided with a registration unit for correcting skew of a sheet is disclosed. The registration unit has a conveying unit for conveying the sheet in the sheet conveying direction, and a skew correction unit for conveying the sheet obliquely with respect to the sheet conveying direction. The skew correction unit has a reference member and a skew feed roller for obliquely conveying the sheet toward the reference member. The sheet is conveyed while abutting against the reference member by the skew feed roller, and the skew of the sheet is corrected.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The registration unit described in Patent Document 1 employs so-called side registration type skew correction, and a long linear conveyance path is required in the registration unit. For this reason, a device capable of correcting sheet skew and misalignment with high precision by a method different from the side registration method is desired.

[0005] An object of the present invention is to provide a form of a sheet conveying device capable of correcting sheet skew and misalignment with high precision.

Means for Solving the Problems

[0006] The present invention relates to a sheet conveying device comprising: a feeding unit for feeding sheets; an image forming unit for forming an image on a sheet fed by the feeding unit at an image forming position; a first conveying unit disposed between the feeding unit and the image forming position in the sheet conveying direction for conveying sheets; and a second conveying unit disposed between the first conveying unit and the image forming position in the sheet conveying direction for conveying sheets, wherein the first conveying unit is capable of performing a first skew correction process for correcting the skewness of a sheet and a moving process for moving a sheet that has undergone the first skew correction process in a width direction perpendicular to the sheet conveying direction; and the second conveying unit is capable of performing a second skew correction process for correcting the skewness of a sheet that has undergone the first skew correction process and the moving process by the first conveying unit. [Effects of the Invention]

[0007] According to the present invention, the skewness and misalignment of the sheet can be corrected with high precision. [Brief explanation of the drawing]

[0008] [Figure 1] A schematic diagram showing the image forming system according to this embodiment. [Figure 2] Cross-sectional and plan views showing the registration unit. [Figure 3] A block diagram showing the control blocks of an image forming system. [Figure 4] A flowchart illustrating sheet transport control. [Figure 5] A flowchart illustrating sheet transport control. [Figure 6] A cross-sectional and plan view showing the sheet reaching the nip of the first pair of registration rollers. [Figure 7] A cross-sectional and plan view showing the sheet reaching directly in front of the first shift roller pair. [Figure 8] A cross-sectional view and a plan view showing the state after the first movement process has been performed on the sheet. [Figure 9]Cross-sectional and plan views showing the sheet reaching the nip of the second pair of registration rollers. [Figure 10] Cross-sectional and plan views showing the second lateral displacement of the sheet. [Figure 11] Cross-sectional and plan views showing how the sheet has shifted in the width direction. [Modes for carrying out the invention]

[0009] Hereinafter, this embodiment will be described in detail with reference to the drawings. As shown in Figure 1, the image forming system 10 as a sheet transport device includes an image forming apparatus 1 and a sheet feeding apparatus 2. The image forming apparatus 1 includes a printer, copier, facsimile, and multifunction device, and forms an image on a sheet used as a recording medium based on image information input from an external PC or image information read from an original document. In this embodiment, an electrophotographic full-color laser printer is used as the image forming apparatus 1. Although an electrophotographic full-color laser printer is used in the image forming apparatus 1 of this embodiment, for example, an image forming apparatus equipped with an inkjet image forming means instead of an electrophotographic one may also be used.

[0010] [Overview of Image Forming Apparatus] Image forming apparatus 1 is a POD machine capable of handling printing for purposes other than general office use, and can use various sheets as recording media, such as paper and envelopes, glossy paper, plastic films such as overhead projector sheets (OHT), and cloth. The main body 1A of image forming apparatus 1 houses a feeding unit 40H as a main feeding unit for feeding sheets S, and an image forming unit 513 that forms an image on the sheet S fed from the feeding unit 40H. The image forming unit 513 is a tandem type intermediate transfer system equipped with four process units PY, PM, PC, PK that form yellow, magenta, cyan, and black toner images, and an intermediate transfer belt 506 which is an intermediate transfer body. Process units PY to PK are electrophotographic units each having a photosensitive drum 1Y, 1M, 1C, 1K which is a photosensitive element.

[0011] Process units PY to PK are configured similarly except for the color of the toner used for development. Therefore, the configuration of the process unit and the toner image formation process (image formation operation) will be explained using the yellow process unit PY as an example. Process unit PY includes a photosensitive drum 1Y, an exposure unit 511, a developing unit 510, and a drum cleaner 509. The photosensitive drum 1Y is a drum-shaped photoreceptor having a photosensitive layer on its outer circumference and rotates in a direction along the rotation direction of the intermediate transfer belt 506. The surface of the photosensitive drum 1Y is charged by being supplied with charge from a charging means such as a charging roller.

[0012] The exposure unit 511 emits a laser beam modulated according to image information and scans the photosensitive drum 1Y with an optical system including a reflector 512, thereby imprinting an electrostatic latent image on the surface of the photosensitive drum 1Y. The developing unit 510 contains a developer containing toner and supplies toner to the photosensitive drum 1Y to develop the electrostatic latent image into a toner image. The toner image formed on the photosensitive drum 1Y is first transferred to the intermediate transfer belt 506 by the primary transfer roller 507. Any remaining toner on the photosensitive drum 1Y after the transfer is removed by the drum cleaner 509.

[0013] The intermediate transfer belt 506 is wound around a driving roller 504, a driven roller 505, an inner secondary transfer roller 503, and a plurality of primary transfer rollers 507, and is rotationally driven in the clockwise direction in the figure by the driving roller 504. The above-described image forming operation proceeds in parallel in each of the process units PY to PK, and the four-color toner images are multi-transferred so as to overlap each other, whereby a full-color toner image is formed on the intermediate transfer belt 506. This toner image is carried on the intermediate transfer belt 506 and conveyed to the secondary transfer nip N1. The secondary transfer nip N1 is formed by a secondary transfer roller 56 and the inner secondary transfer roller 503. By applying a bias voltage having a polarity opposite to the charging polarity of the toner to the secondary transfer roller 56, the toner image is secondarily transferred to the sheet S at the image forming position. That is, the secondary transfer roller 56 is an example of a transfer unit that transfers the image on the intermediate transfer belt 506 to the sheet S. The residual toner remaining on the intermediate transfer belt 506 after transfer is removed by the belt cleaner 508.

[0014] The sheet S onto which the toner image has been transferred is delivered to the fixing unit 58 by the pre-fixing conveyance unit 57. The fixing unit 58 has a pair of fixing rollers that sandwich and convey the sheet S, and a heat source such as a halogen heater, and heats and pressurizes the toner image carried on the sheet S. Thereby, the toner particles are melted and fixed, and a fixed image fixed on the sheet S is obtained.

[0015] [Sheet conveyance system of the image forming apparatus] Next, the configuration and operation of a sheet conveyance system that feeds the sheet S housed in the feed cassette 50H of the feed unit 40H and discharges the sheet S on which an image has been formed to the outside of the apparatus will be described. The sheet conveyance system roughly includes a feed unit 40H, a conveyance unit 54, a registration unit 100, a branch conveyance unit 59, and a re-conveyance unit 550.

[0016] The feeding unit 40H has a feeding cassette 50H that is removably attached to the apparatus main body 1A, a tray 52H that is vertically supported by the feeding cassette 50H, an air blowing unit 51H, and a suction feeding unit 53H. The tray 52H is vertically movable with respect to the feeding cassette 50H so that the uppermost sheet among the stack of sheets loaded on the tray 52H maintains a predetermined height. The air blowing unit 51H blows air onto a plurality of sheets located above the stack of sheets supported by the tray 52H. That is, the air blowing unit 51H blows air onto the end of the sheet S to lift the sheet S.

[0017] The suction feeding unit 53H has a suction belt provided with a plurality of holes and a suction fan that drives the inside of the suction belt to a negative pressure. The uppermost sheet S lifted by the air blowing unit 51H is adsorbed by the suction belt of the suction feeding unit 53H and fed one by one as the suction belt rotates. Note that the sheet feeding method by the feeding unit 40H is not limited to the above-described belt suction method, and a friction separation method using rollers or pads may be applied.

[0018] The conveyance unit 54 is an example of a conveyance unit that forms a feeding path 54a from the feeding cassette 50H toward the image forming unit 513 and conveys the sheet in the feeding path 54a. The sheet S sent out from the feeding unit 53 is conveyed along the feeding path 54a and the vertical path 111 continuous with the feeding path 54a and delivered to the registration unit 100.

[0019] After skew correction is performed on the sheet S in the registration unit 100 as the second conveyance unit, the sheet is sent to the secondary transfer nip N1. The configuration and operation of the registration unit 100 will be described later.

[0020] The sheet S, on which the toner image has been transferred in the secondary transfer nip N1 and the image has been fixed by the fixing unit 58, is transported to a branch transport unit 59 which has a switching member that can switch the transport path of the sheet S. If image formation on the sheet S is complete, the sheet S is discharged by the discharge roller pair of the branch transport unit 59 into a discharge tray 500 located outside the main body 1A of the device.

[0021] Furthermore, when the printed surface of the sheet S is to be placed on the underside of the sheet S and discharged to the discharge tray 500, the sheet S is first transported to the reversal transport unit 501, switched back, and then discharged to the discharge tray 500.

[0022] When an image is formed on the back surface of sheet S, sheet S is transported to the re-transport unit 550. The re-transport unit 550 has an inversion transport unit 501 and a double-sided transport unit 502, and transports the sheet with the image formed by the image forming unit 513 inverted to its front and back sides, and merges with the vertical path 111. The re-transport unit 550 has an inversion path 54d, which is an inversion path that reverses the transport direction of sheet S, and a re-transport path 54e, which is a double-sided transport path. The re-transport path 54e merges the sheet S transported from the inversion path 54d with the vertical path 111.

[0023] The sheet S transported from the branching transport unit 59 is transferred to the double-sided transport unit 502, which has a re-transport path 54e, via the reversing transport unit 501, which has a reversing path 54d. The reversing transport unit 501 has a pair of reversing rollers that can rotate in both forward and reverse directions, and switches back the sheet S to transfer it to the double-sided transport unit 502. The sheet S transported to the double-sided transport unit 502 has its leading edge detected by a detection unit 4 provided in the re-transport path 54e, and is stopped within the re-transport path 54e, waiting until it is re-transported to form the image on the second surface (double-sided waiting).

[0024] For the detection unit 4, for example, an optical sensor can be applied. The double-sided transport unit 502 transports the sheet S toward the registration unit 100 in accordance with the timing of image formation on the second surface. That is, the re-transport unit 550 stops the sheet once in the re-transport path 54e and then starts transporting the sheet S toward the registration unit 100. After an image is formed on the back surface of the sheet S, it is discharged into the discharge tray 500.

[0025] [Sheet feeding device] As shown in Figure 1, a sheet feeding device 2 is connected to the upstream side of the inlet path 110 of the image forming apparatus 1. The sheet feeding device 2 has three feeding units 40L, 40M, and 40U arranged vertically, each having the same configuration as the feeding unit 40H of the image forming apparatus 1. Specifically, the feeding unit 40L has a feeding deck 50L that is removably attached to the main body 2A of the sheet feeding device 2, a tray 52L, an air blowing unit 51L, and a suction feeding unit 53L. The feeding unit 40M has a feeding deck 50M that is removably attached to the main body 2A of the sheet feeding device 2, a tray 52M, an air blowing unit 51M, and a suction feeding unit 53M. The feeding unit 40U includes a feeding deck 50U that is removablely attached to the main body 2A of the sheet feeding device 2, a tray 52U as a sheet support, an air blowing section 51U, and a suction feeding section 53U.

[0026] The air blowing units 51L, 51M, and 51U blow air onto multiple sheets located at the top of the sheet bundles supported on each tray. That is, the air blowing units 51L, 51M, and 51U blow air onto the edges of the sheets S to make them float. Each of the suction feeding units 53L, 53M, and 53U has a suction belt with multiple holes and a suction fan that is driven to create negative pressure inside the suction belt. The top sheet S, which has been made floated by the air blowing units 51L, 51M, and 51U, is then attracted to the suction belts of the suction feeding units 53L, 53M, and 53U, respectively, and is fed one sheet at a time as the suction belts rotate.

[0027] Similar to the image forming apparatus 1, the sheet S is fed by the feeding units 40L, 40M, and 40U in accordance with the image forming timing of the image forming apparatus 1. The sheet feeding apparatus 2 is equipped with a registration unit 240, and the sheet S fed from the feeding units 40L, 40M, and 40U is transported by the registration unit 240 to the inlet path 110 of the image forming apparatus 1.

[0028] More specifically, the sheets S fed from the feeding deck 50M of the feeding unit 40M and the feeding deck 50U of the feeding unit 40U are transported via the upper transport path 241 to the inlet path 110 of the image forming apparatus 1 by the registration unit 240. The sheets S fed from the feeding deck 50L are transported via the lower transport path 242 to the inlet path 110 of the image forming apparatus 1 by the registration unit 240.

[0029] A horizontal path 270 extending horizontally from the confluence of the upper transport path 241 and the lower transport path 242 is provided with a first pre-shift sensor S2 for detecting the leading edge of the sheet S. For example, an optical sensor can be used as the first pre-shift sensor S2. When the leading edge of the sheet S is detected by the first pre-shift sensor S2, the registration unit 240 temporarily stops the sheet S.

[0030] Then, the registration unit 240, acting as the first transport unit, re-feeds the sheet S in accordance with the image formation timing of the image forming apparatus 1 and transports the sheet S to the image forming apparatus 1. This makes it possible to correct for variations in transport timing that occurred in the feed units 40L, 40M, and 40U. The feed units 40L, 40M, and 40U of the sheet feed device 2 and the registration unit 240 are controllable by the control unit 9 of the image forming apparatus 1. The detailed configuration of the registration unit 240 will be described later.

[0031] The sheet S, transported along the horizontal path 270, is transferred to the inlet path 110 of the image forming apparatus 1. The vertical path 111 also merges with the inlet path 110. The transport path from the horizontal path 270 to the discharge tray 500 of the sheet feeding device 2 extends in a nearly straight horizontal direction. This reduces the transport resistance of the sheet, thereby reducing the load and damage to the sheet. [Registration Unit]

[0032] Next, the configurations of the registration unit 240 of the sheet feeding device 2 and the registration unit 100 of the image forming apparatus 1 will be explained using Figure 2. Figure 2 is a cross-sectional view and a plan view showing the registration units 240 and 100.

[0033] First, the configuration of the registration unit 240 of the sheet feeding device 2 will be described. As shown in Figure 2, the registration unit 240 has an upper transport path 241, a lower transport path 242, and a horizontal path 270. The upper transport path 241 is formed by a curved guide 301b as a first guide and a oscillating guide 301a as a second guide that is curved along the upper transport path 241 and opposite to the curved guide 301b. The curved guide 301b and the oscillating guide 301a constitute a guide section that guides the sheet toward the first registration roller pair 243 while curving it.

[0034] The oscillating guide 301a is configured to swing around the oscillating axis 301c and is biased to a standby position by a spring 301d. In the standby position, the oscillating guide 301a forms an upper transport path 241 together with the curved guide 301b, and by swinging from the standby position to the retracted position against the biasing force of the spring 301d, it is possible to form a loop space SP1 in the upper transport path 241.

[0035] The lower transport path 242 is formed by a pair of curved guides 302a and 302b. A portion of the curved guide 302a bulges outward relative to the curved guide 302b, forming a loop space SP2.

[0036] The horizontal path 270 is formed by guides 303a, 303b, 304a, 304b, 305a, 305b, and 306a, 306b, which are opposite to each other. Guides 303a, 303b are located upstream of the first registration roller pair 243 in the sheet transport direction D. Guides 304a, 304b and 305a, 305b are located between the first registration roller pair 243 and the first shift roller pair 244 in the sheet transport direction D. Guides 306a, 306b are located downstream of the first shift roller pair 244 in the sheet transport direction D. Guide 304a bulges upward relative to guide 304b, forming a loop space SP3.

[0037] Furthermore, the registration unit 240 includes an upper transport roller pair 245 provided on the upper transport path 241, a lower transport roller pair 246 provided on the lower transport path 242, a first registration roller pair 243 and a first shift roller pair 244 provided on the horizontal path 270. In addition, the registration unit 240 includes a first pre-registration sensor S1, a first pre-shift sensor S2, and a first line sensor S3 provided on the horizontal path 270.

[0038] The first pre-registration sensor S1 is positioned upstream of the first registration roller pair 243 and downstream of the confluence of the upper transport path 241 and the lower transport path 242 in the sheet transport direction D. The first pre-shift sensor S2 and the first line sensor S3 are positioned between the first registration roller pair 243 and the first shift roller pair 244 in the sheet transport direction D. The first pre-registration sensor S1 and the first pre-shift sensor S2 are positioned in the center of the horizontal path 270 in the width direction W perpendicular to the sheet transport direction D and detect the sheet S. The first line sensor S3 is positioned to one side of the center of the horizontal path 270 in the width direction W and detects the position of the side edge SE of the sheet S in the width direction W. The first line sensor S3 is composed of, for example, a contact image sensor.

[0039] Next, the configuration of the registration unit 100 of the image forming apparatus 1 will be described. The registration unit 100 has a horizontal path 541 that extends substantially horizontally from the confluence 540 of the vertical path 111 and the inlet path 110. The horizontal path 541 is formed by guides 120, 121, and 122. Guide 120 is positioned between the transport roller pair 101 and the second registration roller pair 102 in the sheet transport direction D, and is configured to form a loop space SP4. Guides 121 and 122 are positioned between the second registration roller pair 102 and the secondary transfer nip N1 in the sheet transport direction D.

[0040] The registration unit 100 also includes a transport roller pair 101 and a second registration roller pair 102. The transport roller pair 101 is positioned downstream of the confluence 540 in the sheet transport direction D and transports the sheet S toward the second registration roller pair 102. The second registration roller pair 102 is provided in the horizontal path 541.

[0041] Furthermore, the registration unit 100 includes a second pre-registration sensor S4, a second line sensor S5, and a tip sensor S6, all located on the horizontal path 541. The second pre-registration sensor S4 and the second line sensor S5 are positioned between the transport roller pair 101 and the second registration roller pair 102 in the sheet transport direction D. The tip sensor S6 is positioned between the second registration roller pair 102 and the secondary transfer nip N1 in the sheet transport direction D.

[0042] The second register front sensor S4 and the tip sensor S6 are positioned in the center of the horizontal path 541 in the width direction W and detect the sheet S. In particular, the tip sensor S6 detects the tip of the sheet S. The second line sensor S5, like the first line sensor S3, is positioned to one side of the center of the horizontal path 541 in the width direction W and detects the position of the side edge SE of the sheet S in the width direction W. The second line sensor S5 is composed of, for example, a contact image sensor.

[0043] [Control Block] Figure 3 is a block diagram showing the control block of the image forming system 10. As shown in Figures 1 and 3, the image forming apparatus 1 of the image forming system 10 has a control unit 9. The control unit 9 has a CPU (Central Processing Unit) 91, a ROM (Read Only Memory) 92, and a RAM (Random Access Memory) 93. The CPU 91 reads and executes various programs stored in the ROM 92. The RAM 93 is used as a workspace for the CPU 91.

[0044] The control unit 9 is connected to an operation unit 94 equipped with a touch panel and physical keys, which allows input of various settings for the image forming system 10 and instructions to start print jobs. The input side of the control unit 9 is connected to a first register sensor S1, a first shift sensor S2, a first line sensor S3, a second register sensor S4, a second line sensor S5, and an end-point sensor S6.

[0045] The output side of the control unit 9 is connected to the feed motor 250, the first upper roller drive motor MD1U, the first upper roller separation motor MA1U, the first lower roller drive motor MD1L, the first lower roller separation motor MA1L, the first resistor drive motor MD2, and the first resistor separation motor MA2. Furthermore, the output side of the control unit 9 is connected to the first shift drive motor MD3, the first shift separation motor MA3, the first shift motor MS3, the second roller drive motor MD4, the second roller separation motor MA4, the second resistor drive motor MD5, the second resistor separation motor MA5, and the second shift motor MS5.

[0046] The supply motor 250 is configured to drive the suction belts of each suction supply unit. The fans of each air blowing unit and the suction fans of each suction supply unit may be driven by the driving force of the supply motor 250 or by other motors.

[0047] As shown in Figures 2 and 3, the upper conveyor roller pair 245, which is an upstream conveyor roller pair, is driven by the first upper roller drive motor MD1U and can release the nip by the first upper roller separation motor MA1U. In the following, when a roller pair is driven, it means that the drive roller, among the drive roller and driven roller that make up the roller pair, is driven by the input of motor drive. Also, in the following, when the nip of a roller pair is released, it means that the nip of the roller pair is released when the driven roller of the roller pair separates from the drive roller.

[0048] The lower transport roller pair 246 is driven by the first lower roller drive motor MD1L and can release the nip by the first lower roller separation motor MA1L. The first registration roller pair 243 is driven by the first register drive motor MD2 and can release the nip by the first register separation motor MA2. The first shift roller pair 244 is driven by the first shift drive motor MD3 and can release the nip NP1 by the first shift separation motor MA3. Furthermore, the first shift roller pair 244 can move in the width direction W while the sheet S is gripped by the nip NP1 by the first shift motor MS3.

[0049] The transport roller pair 101 is driven by the second roller drive motor MD4 and can release the nip by the second roller separation motor MA4. The second registration roller pair 102 is driven by the second register drive motor MD5 and can release the nip NP2 by the second register separation motor MA5. Furthermore, the second registration roller pair 102 can move in the width direction W while the sheet S is held between the nip NP2 by the second shift motor MS5.

[0050] [Skew correction control] Next, the sheet transport control of the image forming system 10 will be described with reference to Figures 4 to 11. Figures 4 and 5 are flowcharts of the sheet transport control. Figure 6 is a cross-sectional view and a plan view showing the sheet S reaching the nip NP1 of the first registration roller pair 243. Figure 7 is a cross-sectional view and a plan view showing the sheet S reaching just before the first shift roller pair 244. Figure 8 is a cross-sectional view and a plan view showing the sheet S after the first movement process has been performed. Figure 9 is a cross-sectional view and a plan view showing the sheet S reaching the nip NP2 of the second registration roller pair 102. Figure 10 is a cross-sectional view and a plan view showing the second lateral displacement Δ2 of the sheet S. Figure 11 is a cross-sectional view and a plan view showing the sheet S shifted in the width direction W.

[0051] As shown in Figure 4, when a print job is input as a job from the operation unit 94 or an external computer connected to the image forming system 10, the control unit 9 starts sheet transport control. The print job includes the number of copies to be printed and the size of the sheets to be printed, as specified by the user.

[0052] When sheet transport control is started, the control unit 9 feeds the sheet S from the designated feed source corresponding to the print job (step S11). The image forming system 10 has a feed cassette 50H and feed decks 50L, 50M, and 50U as feed sources, but the following explanation will use the case where the sheet S is fed from the feed deck 50U as an example. The sheet S fed from the feed deck 50U passes through the upper transport path 241 and is transported by the upper transport roller pair 245 toward the first registration roller pair 243.

[0053] Then, as shown in Figure 6, after the leading edge of the sheet S is detected by the first pre-registration sensor S1, it reaches the nip NP1 of the first registration roller pair 243, which is in a stopped state (step S12). Next, the control unit 9 performs the first skew correction process (step S13). Here, it is assumed that when the sheet S reaches the nip NP1 of the first registration roller pair 243, it is skewed by an angle θ1 with respect to the sheet transport direction D.

[0054] In the first oblique correction process, the control unit 9 stops the first register drive motor MD2 and stops driving the first registration roller pair 243, and then drives the first upper roller drive motor MD1U based on the detection result of the first register front sensor S1. As a result, the sheet S forms a loop as its leading edge abuts against the nip NP1 of the first registration roller pair 243, which is part of the registration roller pair, and is then conveyed by the upper convey roller pair 245.

[0055] When a loop is formed in the sheet S, the surface of the sheet S with the loop formed on it presses against the rocking guide 301a, causing the rocking guide 301a to move from the standby position to the retracted position. As a result, a loop space SP1 is formed in the upper transport path 241, the leading edge of the sheet S follows the nip NP1, and the oblique movement of the sheet S is corrected. That is, the sheet S rotates counterclockwise by an angle θ1 as shown in Figure 6.

[0056] After the skew of the sheet S is corrected, the control unit 9 drives the first register drive motor MD2 to transport the sheet S in the sheet transport direction D using the first registration roller pair 243. Then, after the sheet S has been transported a predetermined distance by the first registration roller pair 243, the control unit 9 drives the first upper roller separation motor MA1U to release the nip of the upper transport roller pair 245.

[0057] Then, based on the detection result of the first pre-shift sensor S2, the control unit 9 stops the sheet S in front of the first shift roller pair 244 (step S14). Next, the control unit 9 re-transports the sheet S in accordance with the exposure timing of the photosensitive drum by the exposure unit 511. Alternatively, the control unit 9 may not stop the sheet S in front of the first shift roller pair 244, but instead control the transport speed of the sheet S in accordance with the image transfer timing at the secondary transfer nip N1.

[0058] The position of the side edge SE of the sheet S in the width direction W is detected by the side edge detection unit and the first line sensor S3, which acts as the first side edge detection unit (step S15). Based on the detection result of the first line sensor S3, the control unit 9 calculates the first lateral displacement amount Δ1 of the side edge SE of the sheet S from the reference position ST1 in the width direction W, as shown in Figure 7. The reference position ST1 is pre-stored in the ROM 92 for each size of the sheet S.

[0059] Next, as shown in Figure 8, the control unit 9 drives the first register separation motor MA2 after the sheet S has been transported a predetermined distance by the first shift roller pair 244 to release the nip NP1 of the first registration roller pair 243 (step S16). Note that the timing of releasing the nip of the upper transport roller pair 245 may be synchronized with the timing of releasing the nip NP1 of the first registration roller pair 243. That is, the timing of releasing the nip of the upper transport roller pair 245 may be anytime after the sheet S has been gripped by the first registration roller pair 243. Also, the timing of releasing the nip NP1 of the first registration roller pair 243 may be anytime after the sheet S has been gripped by the first shift roller pair 244.

[0060] Furthermore, the control unit 9 performs a first movement process (movement process) in which the first shift roller pair 244, which is a shift roller pair holding the sheet S, is moved (shifted) by a first lateral displacement amount Δ1 in the width direction W by the first shift motor MS3 (step S17). At this time, because the upper transport path 241 is curved, the sheet S may rotate and move diagonally with respect to the sheet transport direction D, as shown in Figure 9, a so-called diagonal return may occur. In this embodiment, the first shift roller pair 244 is driven by the first shift drive motor MD3 and moved in the width direction W by the first shift motor MS3, but is not limited to this. For example, the first shift roller pair 244 may be moved in the width direction W by the first shift motor MS3 while the first shift drive motor MD3 is stopped.

[0061] The sheet S is then transferred to the inlet path 110 of the image forming apparatus 1 by the first shift roller pair 244 and transported toward the second registration roller pair 102 by the first shift roller pair 244 and the transport roller pair 101. Next, the control unit 9 performs a second skew correction process (step S19). Here, as shown in Figure 9, it is assumed that the sheet S, when it reaches the nip NP2 of the second registration roller pair 102, is skewed by an angle θ2 with respect to the sheet transport direction D.

[0062] In the second skew correction process, the control unit 9 stops the second register drive motor MD5 and stops driving the second registration roller pair 102, and then drives the second roller drive motor MD4 based on the detection result of the second register front sensor S4. As a result, the sheet S forms a loop as its tip abuts against the nip NP2 of the second registration roller pair 102 and is then transported by the transport roller pair 101. The loop formed on the sheet S can enter the loop space SP4 formed in the guide 120. The tip of the sheet S then follows the nip NP2, and the skew of the sheet S is corrected. That is, the sheet S rotates clockwise by an angle θ2 in Figure 9.

[0063] The position of the side edge SE of the sheet S in the width direction W, which has been corrected for skewness by the second skew correction process, is detected by the second line sensor S5, which acts as a second side edge detection unit (step S20). Based on the detection result of the second line sensor S5, the control unit 9 calculates the second lateral displacement amount Δ2 of the side edge SE of the sheet S from the reference position ST1 in the width direction W, as shown in Figure 10.

[0064] Next, the control unit 9 determines whether the sheet S is the first sheet of the print job, as shown in Figure 5 (step S21). If it is determined that the sheet S is the first sheet of the print job (step S21: YES), the control unit 9 stops the sheet S with its leading edge abutting against the nip NP2 of the second registration roller pair (step S22). At this time, the second registration drive motor MD5 is stopped.

[0065] Next, the control unit 9 sets the exposure position on the photosensitive drum by the exposure unit 511 to a shifted position, which is moved by a second lateral displacement amount Δ2 from the normal position (step S23). That is, the exposure unit 511 performs a process to adjust the position of the electrostatic latent image in the width direction W based on the detection result of the second line sensor S5. The normal position is the position where the transport center of the horizontal paths 270, 541 coincides with the center of the image formed at the normal position.

[0066] In this print job, the exposure position on the photosensitive drum by the exposure unit 511 will be the shifted position. That is, the image on the second side of the first print job, and the images formed on the second and subsequent print jobs, will be formed at the shifted position. The reference position of the second line sensor S5 will be the reference position ST2 (not shown), which is moved in the width direction W by a second lateral displacement amount Δ2 from the reference position ST1. Note that the image formation on the photosensitive drum by the exposure unit 511 may be performed before step S22. That is, after the position of the side edge SE of the sheet S is detected by the second line sensor S5 in step S20, the image formation on the photosensitive drum by the exposure unit 511 may be performed at any timing.

[0067] Next, as shown in Figure 10, the control unit 9 drives the second register drive motor MD5 to drive the second registration roller pair 102 and re-transports the sheet S (step S24). After the sheet S has been transported a predetermined distance by the second registration roller pair 102, the control unit 9 drives the first shift separation motor MA3 and the second roller separation motor MA4 to release the nip of the first shift roller pair 244 and the nip of the transport roller pair 101 (step S25).

[0068] On the other hand, if it is determined in step S21 that sheet S is not the first sheet of the print job (step S21: NO), the control unit 9 drives the second register drive motor MD5 (step S27). As a result, sheet S, which has been corrected for skew by the second skew correction process, is re-transported by the second registration roller pair 102. Next, the control unit 9 drives the first shift separation motor MA3 and the second roller separation motor MA4, similar to step S25, to release the nip of the first shift roller pair 244 and the nip of the transport roller pair 101 (step S28).

[0069] Next, the control unit 9 calculates the displacement between the updated reference position ST2 and the position of the side edge SE of the sheet S in the width direction W detected by the second line sensor S5 in step S20, i.e., the amount of deviation between the reference position ST2 and the detected position of the side edge SE. Then, the control unit 9 executes a second movement process in which the second registration roller pair 102, which is holding the sheet S, is moved in the width direction W by the amount of the above-mentioned deviation using the second shift motor MS5 (step S29). As a result, the sheet S moves in the width direction W by the amount of the deviation, but this amount of deviation is very small. Therefore, there is almost no diagonal return associated with the movement process of the sheet S in step S29.

[0070] Here, the skewness and misalignment in the width direction W of the conveyed sheet S often occur due to wear of the conveying roller pair or tolerances of the guides that guide the sheet. In other words, the skewness and misalignment in the width direction W that occurs in the first sheet of the print job often occur to the same extent in the second and subsequent sheets of the print job. For this reason, in this embodiment, the second image of the first sheet of the print job and the image formed on the second sheet of the print job are formed at the shift position as described above. This makes it possible to suppress the amount of shift in the width direction W of the sheet S by the second registration roller pair 102 in step S29, and to suppress the return of skewness due to the shift movement of the sheet. Furthermore, for the second sheet S of the print job, the image is formed at the shift position based on the position of the side edge SE of the first sheet S of the print job detected by the second line sensor S5. Therefore, the image formation operation on the photosensitive drum by the exposure unit 511 can be performed at an earlier timing than when printing on the first sheet of the print job, which reduces the time when sheet S is stopped, such as in step S22, and improves productivity.

[0071] After steps S25 and S29, the control unit 9 controls the speed of the second registration roller pair 102 in accordance with the image transfer timing at the secondary transfer nip N1, based on the detection result of the tip sensor S6, and controls the transport speed of the sheet S (step S26).

[0072] As shown in Figure 11, the toner image is transferred to the sheet S at the secondary transfer nip N1 (step S30). Next, the control unit 9 drives the second register separation motor MA5 to release the nip NP2 of the second registration roller pair 102 (step S31). Furthermore, the control unit 9 fixes the toner image to the sheet S at the fixing unit 58 (step S32) and discharges the sheet S to the discharge tray 500 (step S33). With this, the sheet transport control is completed.

[0073] Furthermore, when forming an image on the second side of sheet S during double-sided printing, sheet S is transported from the vertical path 111 to the registration unit 100 without passing through the registration unit 240 of the sheet feeding device 2. For this reason, steps S18-S21 and S27-S33 of the flowchart in Figures 4 and 5 are executed on the sheet S that will be printed on both sides.

[0074] The above explanation used an example where the sheet S is fed from the feeding deck 50U, but the same control is performed as in the flowcharts in Figures 4 and 5 above when the sheet S is fed from the feeding deck 50M. Furthermore, when the sheet S is fed from the feeding deck 50L, the control is the same as above, except that the sheet S is transported by the lower transport roller pair 246 via the lower transport path 242 instead of the upper transport path 241.

[0075] Furthermore, when a sheet S is fed from the feed cassette 50H of the image forming apparatus 1, the sheet S is transported from the vertical path 111 to the registration unit 100, similar to the control for forming an image on the second side of the sheet S in double-sided printing. In the registration unit 100, skew correction and movement (shift) of the sheet S in the width direction W are performed. In other words, the sheet S fed from the feed unit 40H of the image forming apparatus 1 does not pass through the registration unit 240, but is transported by the registration unit 100 before an image is formed by the image forming unit 513.

[0076] As described above, in this embodiment, the sheets S fed from the feeding decks 50L, 50M, and 50U of the sheet feeding device 2 undergo a first skew correction process and a first movement process by the registration unit 240 of the sheet feeding device 2. Furthermore, the sheets S undergo at least a second skew correction process by the registration unit 100 of the image forming apparatus 1. As a result, large skew and positional deviations in the width direction W are corrected by the first skew correction process and the first movement process.

[0077] Furthermore, the execution of the first movement process may cause a phenomenon known as "skew return," where the sheet S becomes skewed. This is particularly due to the fact that a portion of the sheet S being subjected to the first movement process is located on a curved transport path (for example, the upper transport path 241 or the lower transport path 242). However, even if a skew return occurs and the sheet S becomes skewed again, the skew of the sheet S is corrected by the second skew correction process. Therefore, the skew and misalignment of the sheet S can be corrected with high precision.

[0078] Furthermore, this embodiment does not employ the so-called side registration method, which corrects the skewness of the sheet S by bringing it against a reference member using skewed rollers. Instead, the skewness of the sheet S is corrected by bringing it into contact with the nips NP1 and NP2 of the first registration roller pair 243 and the second registration roller pair 102. As a result, it does not require a long, straight transport path like the side registration method, and the device can be made smaller. For example, by forming the upper transport path 241 and the lower transport path 242 through which the sheet S fed from the feed decks 50L, 50M, and 50U passes in a curved shape, the horizontal size of the sheet feed device 2 can be reduced.

[0079] Furthermore, in this embodiment, the sheet S is fed by an air-blowing unit and an adsorption feeding unit. By applying such an air-feeding system, various types of sheets S can be fed, increasing the range of compatible media. Moreover, when an air-feeding system is applied, the sheet S may be skewed or misaligned during feeding. However, as in this embodiment, by performing at least a first skew correction process, a first movement process, and a second skew correction process, the skew and misalignment of the sheet S can be corrected with high precision. In other words, according to this embodiment, it is possible to provide an image forming system 10 that can handle various media, can be miniaturized, and can correct large skew and misalignment in the width direction W with high precision.

[0080] <Other Embodiments> In the embodiment described above, the registration unit 240 had a first registration roller pair 243 and a first shift roller pair 244, but is not limited thereto. For example, the first shift roller pair 244 may be omitted, and the first registration roller pair 243 may perform the first skew correction process and the first movement process. Also, the registration unit 100 had a second registration roller pair 102, and the second registration roller pair 102 was configured to perform the second skew correction process and the second movement process, but is not limited thereto. For example, the registration unit 100 may have a registration roller pair that performs the second skew correction process and a shift roller pair that performs the second movement process.

[0081] Furthermore, in the above-described embodiment, the skew correction process was performed by abutting the leading edge of the sheet against the nip of the registration roller pair, but it is not limited to this. For example, a shutter having abutting surface is provided upstream of the nip of the registration roller pair in the sheet transport direction, and the skew correction process may be performed by abutting the leading edge of the sheet against the abutting surface of the shutter.

[0082] Furthermore, in the above-described embodiment, the exposure position on the photosensitive drum by the exposure unit 511 was adjusted based on the detection result of the second line sensor S5, but this is not limited to this. For example, it is not necessary to change the exposure position on the photosensitive drum by the exposure unit 511.

[0083] Furthermore, in the above-described embodiment, the registration unit 240 was provided in the sheet feeding device 2 and the registration unit 100 was provided in the image forming apparatus 1, but the invention is not limited to this. For example, both registration units 240 and 100 may be provided in the sheet feeding device 2 or the image forming apparatus 1.

[0084] Furthermore, while the above-described embodiment employs an air supply method in which the sheet is supplied by an air blowing unit or an adsorption supply unit, the invention is not limited to this. For example, a friction separation method using rollers or pads may also be employed.

[0085] Furthermore, in the above-described embodiment, the first line sensor S3 was positioned upstream of the first shift roller pair 244 in the sheet transport direction D, but is not limited to this. For example, the first line sensor S3 may be positioned downstream of the first shift roller pair 244 in the sheet transport direction D. Similarly, the second line sensor S5 was positioned upstream of the second registration roller pair 102 in the sheet transport direction D, but is not limited to this. For example, the second line sensor S5 may be positioned downstream of the second registration roller pair 102 in the sheet transport direction D.

[0086] Furthermore, in the above-described embodiment, the position of the side edge SE of the first sheet S of the print job is detected by the second line sensor S5, and the exposure position of the second image of the first sheet of the print job and the images formed on the second and subsequent sheets of the print job are adjusted using this detection result. However, the position of the side edge SE of each sheet S being transported may be detected by the second line sensor S5, and the exposure position of the photosensitive drum by the exposure unit 511 may be adjusted using the detection result of the second line sensor S5 for each sheet.

[0087] Furthermore, in the above-described embodiment, the second movement process by the second registration roller pair 102 was not performed on the first sheet S of the print job, but the embodiment is not limited to this. For example, a second movement process similar to that in step S29 may be performed after step S25.

[0088] The present invention can also be realized by supplying a program that implements one or more of the functions of the above embodiments to a system or device via a network or storage medium, and by having one or more processors in the computer of that system or device read and execute the program. It can also be realized by a circuit (e.g., an ASIC) that implements one or more functions. [Explanation of Symbols]

[0089] 1: Image forming apparatus / 1Y: Photoreceptor (photosensitive drum) / 2: Sheet feeding device / 10: Sheet transport device (image forming system) / 40H: Main feeding unit (feeding unit) / 40L, 40M, 40U: Feeding unit / 51U: Air blowing unit / 52U: Sheet support unit (tray) / 53U: Suction feeding unit / 100: Second transport unit (registration unit) / 102: Second registration roller pair / 240: First transport unit (registration unit) / 243: Registration roller Registration roller pair, first registration roller pair / 244: Shift roller pair (first shift roller pair) / 245: Upstream transport roller pair (upper transport roller pair) / 301a: Guide section, second guide (oscillating guide) / 301b: Guide section, first guide (curved guide) / 510: Developing section / 511: Exposure section / 513: Image forming section / D: Sheet transport direction / S: Sheet / SE: Side edge / S3: Side edge detection section, first side edge detection section (first line sensor) / S5: Second side edge detection section (second line sensor) / W: Width direction

Claims

1. A feeding unit that feeds sheets, An image forming unit that forms an image on a sheet fed by the aforementioned feeding unit at an image forming position, A first conveying unit is positioned between the feeding unit and the image forming position in the sheet conveying direction and conveys the sheet, The system comprises a second transport unit positioned between the first transport unit and the image forming position in the sheet transport direction, which transports the sheet. The first transport unit is capable of performing a first skew correction process to correct the skew of the sheet, and a move process to move the sheet that has undergone the first skew correction process in a width direction perpendicular to the sheet transport direction. The second transport unit is capable of performing a second skew correction process to correct the skew of the sheet that has undergone the first skew correction process and the movement process by the first transport unit. A sheet conveying device characterized by the following features.

2. An image forming apparatus including the second transport unit and the image forming unit, A sheet feeding device, which includes the feeding unit and the first transport unit and is connected to the image forming apparatus, is provided. The sheet conveying device according to feature 1.

3. The image forming apparatus has a main feeding unit for feeding sheets, The sheet fed from the main supply unit does not pass through the first transport unit, but is transported by the second transport unit, after which an image is formed by the image forming unit. The sheet conveying device according to feature 2.

4. The first transport unit includes a pair of registration rollers that perform the first skew correction process when the leading edge of the sheet abuts against it, a side edge detection unit that detects the position of the side edge in the width direction of the sheet that has undergone the first skew correction process, and a pair of shift rollers that perform the movement process by moving in the width direction while gripping the sheet based on the detection result of the side edge detection unit. A sheet conveying device according to any one of claims 1 to 3.

5. The first transport unit includes an upstream transport roller pair positioned upstream of the registration roller pair in the sheet transport direction, and a guide portion that guides the sheet being transported by the upstream transport roller pair toward the registration roller pair while curving it. The sheet conveying device according to feature 4.

6. The guide portion comprises a first guide and a second guide that faces the first guide and is movable away from the first guide when pressed by a sheet that abuts against the registration roller pair. The sheet conveying device according to feature 5.

7. The feeding unit comprises a sheet support section for supporting a sheet, an air blowing section for blowing air onto the edges of the sheet supported by the sheet support section to make the sheet float, and an adsorption feeding section for adsorbing and feeding the sheet that has been made floated by the air blowing section. The sheet conveying device according to feature 1.

8. The aforementioned movement process is a first movement process, The second transport unit is capable of performing a second movement process to move the sheet that has undergone the second skew correction process in the width direction. The sheet conveying device according to feature 4.

9. The registration roller pair and the side end detection unit are, respectively, a first registration roller pair and a first side end detection unit. The second transport unit includes a pair of second registration rollers that perform the second skew correction process when the leading edge of the sheet abuts against them, and a second side edge detection unit that detects the position of the side edge in the width direction of the sheet that has undergone the second skew correction process. The second pair of registration rollers executes the second movement process by moving in the width direction while gripping the sheet, based on the detection result of the second side end detection unit. The sheet conveying device according to feature 8.

10. The image forming unit comprises a photoreceptor, an exposure unit that forms an electrostatic latent image on the photoreceptor by exposing it to light, and a developing unit that develops the electrostatic latent image as a toner image. The exposure unit is capable of performing a process to adjust the position of the electrostatic latent image in the width direction based on the detection result of the second side edge detection unit. The sheet conveying device according to feature 9.

11. The second transport unit does not perform the second movement process on the first sheet of a job in which the sheets are fed by the feeding unit, but performs the second movement process on the second and subsequent sheets of the job. The sheet conveying device according to feature 8.

Images