Sheet feeding device, and image forming device

Abstract

To provide a sheet feeding device capable of suppressing dispersion of the amount of deflection formed on a sheet while feeding the sheet, and an image forming device provided with the sheet feeding device.SOLUTION: A register control section 95 for an image forming device 10 controls rotational drive of a register roller 80A so as to feed out a sheet correspondingly to register start timing as deflecting the sheet whose tip has reached nip parts of a pair of register rollers 80 in a state the register roller 80 are stopped. A feeding speed control section 96 controls rotational speed of a feeding roller 23A to control feeding speed of the sheet such that a feeding amount of the sheet after a tip detected time on a basis of an elapsed time Δt when a scan start time is earlier than the tip detected time becomes a reference feeding amount.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a sheet conveying device capable of conveying a sheet, and an image forming apparatus including the sheet conveying device. [Background technology]

[0002] Image forming devices such as printers, copiers, facsimiles, and multifunction devices equipped with these functions are equipped with a sheet transport device that transports sheets such as print paper to an image transfer position. Conventional sheet transport devices include a pair of registration rollers that perform a registration operation (also called "registration") on the sheet. The registration operation involves causing the leading edge of the sheet to abut against the nip between the pair of registration rollers that are stopped, and then continuing to transport the sheet using the upstream transport rollers in that state, thereby creating a flexure in the sheet and correcting any skew in the sheet during transport.

[0003] When a deflection sufficient to correct the inclination of the sheet is formed, the conveying roller is stopped, and then the pair of resist rollers and the conveying roller are rotated so as to feed the sheet at a predetermined feed timing that coincides with the transfer start timing at which image transfer to the sheet begins at the image transfer position.

[0004] Furthermore, a conventional image forming apparatus (see Patent Document 1) has been disclosed in which the transport roller is decelerated by the time of the sending-out timing, thereby driving the pair of registration rollers to rotate at the sending-out timing, and the transport roller continues to rotate in synchronization with the rotation speed of the pair of registration rollers without stopping. By controlling the rotation of the transport roller in this way, time loss when the transport roller is driven again after a temporary stop is eliminated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-209297 Summary of the Invention [Problem to be solved by the invention]

[0006] However, conventional image forming devices change the start timing of the slowdown control depending on when the resist sensor detects the leading edge of the sheet, so the degree of delay due to slippage or the like before the sheet reaches the resist sensor is not reflected in the change process, resulting in a problem in which the amount of sheet deflection varies from sheet to sheet.

[0007] SUMMARY OF THE INVENTION An object of the present invention is to provide a sheet conveying device capable of suppressing variations in the amount of flexure formed in a sheet when the sheet is conveyed, and an image forming apparatus including the sheet conveying device. [Means for solving the problem]

[0008] A sheet conveying device according to one aspect of the present invention includes a pair of conveying rollers that convey a sheet toward an image transfer position where a toner image formed in an image forming unit is transferred onto the sheet, and a pair of registration rollers that are provided between the image transfer position and the pair of conveying rollers; the sheet sensor is provided between the pair of registration rollers and the pair of conveying rollers and detects the leading edge of the sheet; a registration control unit that controls the rotational drive of the pair of registration rollers so that the sheet is conveyed by the pair of conveying rollers while the pair of registration rollers is stopped, and the sheet whose leading edge has reached the pair of registration rollers is bent, and then sends out the sheet at a predetermined sending timing in accordance with a transfer start timing at which transfer of the toner image to the sheet at the image transfer position is started; and a conveying speed control unit that controls the rotational speed of the pair of conveying rollers based on the elapsed time from a predetermined write start timing at which optical scanning of a photosensitive drum included in the image forming unit is started to a leading edge detection timing at which the leading edge of the sheet is detected by the sheet sensor, so that the sheet feed amount after the leading edge detection timing becomes a predetermined standard conveying amount.

[0009] An image forming apparatus according to another aspect of the present invention includes the sheet transporting device described above, and is configured to transfer a toner image onto a sheet transported to an image transfer position by the sheet transporting device. [Effects of the Invention]

[0010] According to the present invention, it is possible to suppress variations in the amount of sagging formed in a sheet when the sheet is conveyed. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a perspective view showing the configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the internal configuration of the image forming apparatus. [Figure 3] FIG. 3 is a schematic diagram showing the configuration of the periphery of a sheet conveying path of the image forming apparatus. [Figure 4] FIG. 4 is a block diagram showing the configuration of the image forming apparatus. [Figure 5] FIG. 5 is a graph showing the sheet conveying speed V when the sheet is conveyed from the sheet storage unit in an ideal state. [Figure 6] FIG. 6 is a graph showing the sheet conveyance speed V when the leading edge of a sheet conveyed at a constant initial speed is detected at time T11 after the elapse of time Δt (=td1) from reference time T0. [Figure 7] FIG. 7 is a graph showing the sheet conveying speed V when the leading edge of a sheet conveyed at a constant initial speed is detected at time T12 after the elapse of time Δt (=td2) from the reference time T0. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. The embodiment described below is merely an example of the present invention, and is not intended to limit the technical scope of the present invention.

[0013] Fig. 1 is a perspective view showing the configuration of an image forming apparatus 10 according to an embodiment of the present invention. Fig. 2 is a schematic diagram showing the internal configuration of the image forming apparatus 10. In Fig. 2, the image reading unit 12 is not shown. In the following description, a vertical direction D1 is defined based on the state in which the image forming apparatus 10 is installed and ready for use (the state in Fig. 1), a front-to-rear direction D2 is defined with the front side (front face) of the image forming apparatus 10 as the front, and a left-to-right direction D3 is defined when the image forming apparatus 10 is viewed from the front side (front face).

[0014] [Image forming apparatus 10] 1, image forming apparatus 10 is a multifunction device capable of printing images on sheets such as printing paper, and has various functions such as a print function, a copy function, a facsimile function, and a scan function. Image forming apparatus 10 is not limited to the multifunction device, and may be any device that has a print function for printing images on conveyed sheets, such as a printer, a copier, or a fax machine.

[0015] The image forming apparatus 10 includes an image reading unit 12 and an image forming unit 14. The image reading unit 12 performs image reading processing to read an image of a document, and is provided in the upper part of the image forming apparatus 10. The image forming unit 14 performs image forming processing to form a color image based on an electrophotographic method, and is provided in the lower part of the image forming apparatus 10. In addition, a sheet discharge unit 15 is provided to the right of the image forming unit 14.

[0016] A discharge space 21 is provided above the image forming unit 14. The sheet discharge unit 15 forms the discharge space 21 between the image forming unit 14 and the image reading unit 12, and connects the image forming unit 14 and the image reading unit 12 vertically.

[0017] The sheet discharge section 15 discharges the sheet after image formation into the discharge space 21. A sheet discharge outlet 15A (see FIG. 2) is formed on the left side surface of the sheet discharge section 15 on the discharge space 21 side. The sheet is discharged from the sheet discharge outlet 15A.

[0018] Image forming unit 14 includes housing 11 as the device main body. Each component that constitutes image forming unit 14 is disposed inside housing 11. Housing 11 includes an exterior frame that covers the entire image forming unit 14, and an internal frame that supports each component that constitutes image forming unit 14.

[0019] The image forming section 14 forms a color image on a sheet such as printing paper using a so-called tandem system. As shown in FIG. 2, the image forming section 14 includes a plurality of image forming units 4, an intermediate transfer unit 5, an optical scanning device 13, a secondary transfer roller 20, a fixing device 16, a sheet tray 18, a sheet storage section 27, a feeding unit 28, an operation display section 17 (see FIG. 1), a sheet transport path 26 (hereinafter simply referred to as the transport path 26), a sheet transport unit 23, a registration roller unit 60, a toner container 3, and a control section 90 (see FIG. 4). Note that the image forming section 14 may be configured to form a single-color image on a sheet using a single image forming unit 4. In this case, the intermediate transfer unit 5 is not required.

[0020] 2, the sheet storage unit 27 is provided at the bottom of the image forming apparatus 10. The sheet storage unit 27 stores a plurality of sheets and is formed, for example, in the shape of a tray with an open top. The sheet storage unit 27 is supported by the housing 11.

[0021] The feeding unit 28 picks up multiple sheets stacked in the sheet storage section 27 one by one and feeds the sheets toward the conveying path 26. The feeding unit 28 includes a pickup roller 29, a feeding roller 30, and a separation roller 31. The pickup roller 29 and the feeding roller 30 are provided on the upper side of the right side of the sheet storage section 27. The separation roller 31 is provided below the feeding roller 30, in contact with the roller surface of the feeding roller 30. Because the separation roller 31 is provided, even if multiple sheets are picked up by the pickup roller 29, only the uppermost sheet to be fed is separated from the other sheets by the separation roller 31.

[0022] 3 is a schematic diagram showing the configuration of the vicinity of the conveying path 26. The feed roller 30 conveys the sheet downstream in the conveying direction D11 by receiving a rotational driving force from a conveying motor 56 (see FIG. 4). A drive transmission mechanism (not shown) that transmits the rotation of the feed roller 30 to the pickup roller 29 is provided between the feed roller 30 and the pickup roller 29. The pickup roller 29 and the feed roller 30 are connected via the drive transmission mechanism. When the feed roller 30 is rotated by the conveying motor 56, the drive transmission mechanism also rotates the pickup roller 29 in the same direction and at the same peripheral speed as the feed roller 30.

[0023] When an instruction signal to start a sheet feeding operation is input to the image forming apparatus 10, the feed roller 30 and the pickup roller 29 are rotated by the rotational driving force of the conveyance motor 56, and the sheet is fed from the sheet storage unit 27 to the conveyance path 26. Specifically, the pickup roller 29 picks up a sheet in the sheet storage unit 27 and sends the sheet downstream in the feeding direction, and when the leading edge of the sheet reaches the nip portion between the feed roller 30 and the separation roller 31, the feed roller 30 conveys the sheet to the conveyance path 26.

[0024] The conveying path 26 is a guide path that guides the sheet fed by the feeding roller 30 to the sheet discharge opening 15A. As shown in Fig. 2, the conveying path 26 curves upward from the feeding roller 30 and then extends upward, passing through the secondary transfer roller 20 and reaching the sheet discharge opening 15A.

[0025] As shown in FIG. 3, a sheet transport unit 23 and a registration roller unit 60 are provided in the transport path 26.

[0026] The sheet transport unit 23 transports the sheet fed to the transport path 26 by the feeding unit 28 in a transport direction D11 toward an image transfer position P0. The image transfer position P0 is a position where the toner image on the transfer belt 5A is transferred to the sheet, and is a position where the drive roller 5B and the secondary transfer roller 20 face each other. The sheet transport unit 23 receives a rotational driving force from a transport motor 57 (see FIG. 4) to transport the sheet downstream in the transport direction D11. The configuration of the sheet transport unit 23 will be described later.

[0027] The registration roller unit 60 is disposed on the conveying path 26 upstream of the image transfer position P0 in the conveying direction D11 and downstream of the sheet conveying unit 23 in the conveying direction D11. In other words, the registration roller unit 60 is provided between the image transfer position P0 and the sheet conveying unit 23. The registration roller unit 60 corrects the inclination of the sheet that is conveyed on the conveying path 26 in a state inclined with respect to the conveying direction D11, and conveys the corrected sheet downstream in the conveying direction D11. The configuration of the registration roller unit 60 will be described later.

[0028] As shown in FIG. 2, each image forming unit 4 is provided below the intermediate transfer unit 5. Each image forming unit 4 performs an image formation process to form a toner image on the surface of the transfer belt 5A based on image data input from the outside. The multiple image forming units 4 are arranged side by side in the running direction of the transfer belt 5A (the direction indicated by arrow D10). From left to right on the transfer belt 5A, a yellow image forming unit 4Y, a cyan image forming unit 4C, a magenta image forming unit 4M, and a black image forming unit 4K are arranged in a row in that order.

[0029] Each image forming unit 4 includes a photosensitive drum 41, a charging device 42, a developing device 44, a primary transfer roller 45, etc. Image forming unit 4Y forms a toner image on the surface of photosensitive drum 41 using yellow toner. Image forming unit 4C forms a toner image on the surface of photosensitive drum 41 using cyan toner, image forming unit 4M forms a toner image on the surface of photosensitive drum 41 using magenta toner, and image forming unit 4K forms a toner image on the surface of photosensitive drum 41 using black toner. The toner images on photosensitive drum 41 are developed by developing device 44.

[0030] The intermediate transfer unit 5 includes a transfer belt 5A, a drive roller 5B, and a driven roller 5C. The transfer belt 5A is a belt member onto which the toner images of each color formed on the photosensitive drum 41 of each image forming unit 4 are transferred. The transfer belt 5A is provided above the photosensitive drum 41. The transfer belt 5A is an endless circular belt. The transfer belt 5A is rotatably supported by a drive roller 5B and a driven roller 5C spaced apart in the left-right direction D3. The transfer belt 5A is supported so as to be stretched over the drive roller 5B and the driven roller 5C. As the surface of the transfer belt 5A passes between the photosensitive drum 41 and the primary transfer roller 45, the toner images from each photosensitive drum 41 are transferred onto the transfer belt 5A in order, superimposed on top of each other. The toner images transferred to the transfer belt 5A are transported to the image transfer position P0 and transferred to a sheet at the image transfer position P0.

[0031] The optical scanning device 13 irradiates the photosensitive drum 41 of each image forming unit 4 with laser light based on the input image data for each color. This forms an electrostatic latent image on each photosensitive drum 41. When image forming instructions for forming images on multiple sheets are input to the image forming device 10 along with image data corresponding to each sheet, the control unit 90 determines the scanning start timing (corresponding to the writing start timing of the present invention) at which laser scanning of the photosensitive drum 41 with laser light based on the image data begins, and irradiates the photosensitive drum 41 with the laser light at the scanning start timing determined for each of the multiple sheets.

[0032] The secondary transfer roller 20 is disposed opposite the drive roller 5B across the vertically extending transport path 26. The secondary transfer roller 20 performs a transfer process in which the toner image on the transfer belt 5A is transferred to a sheet by a transfer potential applied to the secondary transfer roller 20. The position at which the toner image is transferred to the sheet by the secondary transfer roller 20 is the image transfer position P0. The sheet to which the toner image has been transferred is transported to the fixing device 16.

[0033] The fixing device 16 heats the toner image transferred to the sheet to fix it to the sheet, and includes a heating roller 16A and a pressure roller 16B. The sheet transported to the fixing device 16 is conveyed while being sandwiched between the heating roller 16A and the pressure roller 16B. During this conveyance, heat is transferred from the heating roller 16A to the toner image transferred to the sheet, heating the toner image. This fixes the toner image to the sheet.

[0034] The sheet on which the image has been formed and which has passed through the fixing device 16 is discharged from the sheet discharge port 15A onto the sheet tray 18 by a discharge roller 24 provided at the most downstream end of the conveying path 26.

[0035] In the image forming apparatus 10, when an image is formed on both sides of a sheet, the sheet having an image formed on one side thereof that has passed through the fixing device 16 is inverted and then conveyed again upstream of the secondary transfer roller 20. Specifically, the discharge rollers 24 are stopped in a state in which the leading edge of the sheet having an image formed on one side thereof is exposed to the outside through the sheet discharge opening 15A. At this time, the trailing edge of the sheet is held in a state in which it is clamped by the discharge rollers 24. Thereafter, the discharge rollers 24 are driven in the reverse direction, causing the sheet to switch back and be fed backward.

[0036] As shown in FIG. 2, the image forming apparatus 10 includes a reverse conveyance path 25 that branches off from a branch point P1 on the conveyance path 26 downstream of the fixing device 16 and connects to a junction point P2 on the conveyance path 26. The junction point P2 is located upstream of the sheet conveyance unit 23 on the conveyance path 26. A sheet conveyed in reverse from the sheet discharge port 15A is guided to the reverse conveyance path 25, conveyed by conveyance rollers 25A provided on the reverse conveyance path 25, passes through the reverse conveyance path 25, merges with the conveyance path 26, and is conveyed again to the secondary transfer roller 20. When the sheet subsequently reaches the secondary transfer roller 20, a toner image is transferred onto the back surface of the sheet, and the sheet passes through the fixing device 16, forming an image on the back surface of the print paper. The sheet with images formed on both sides is discharged from the sheet discharge port 15A to the sheet tray 18 by the discharge rollers 24, which are driven back to normal rotation.

[0037] [Sheet transport unit 23] 3, the sheet transport unit 23 has a transport roller 23A that is rotationally driven by a driving force from a transport motor 57 (see FIG. 4), and a driven roller 23B that is disposed in contact with the outer circumferential surface of the transport roller 23A. The driving force from the transport motor 57 is transmitted to a rotation shaft 47 of the transport roller 23A. The transport roller pair of the present invention is realized by the transport roller 23A and the driven roller 23B.

[0038] The conveying motor 57 is, for example, a stepping motor or an inner brushless motor. These motors have high speed response and high positional accuracy, but require a certain waiting time after stopping before they can be driven again.

[0039] Rotation shaft 49 of driven roller 23B is biased by spring 23C with a predetermined elastic force (spring force) toward transport roller 23A, so that driven roller 23B is pressed against transport roller 23A. When transport roller 23A is rotationally driven in this state, driven roller 23B is driven.

[0040] The rotation shaft 49 of the driven roller 23B is supported by a support portion 51 provided on a guide member or the like of the transport path 26. In this embodiment, the driven roller 23B is supported by the support portion 51 so as to be movable between a contact position where it contacts the transport roller 23A and a contact release position where it is separated from the transport roller 23A.

[0041] 4 is a block diagram showing the configuration of image forming apparatus 10. A solenoid 64 is provided inside housing 11. Solenoid 64 is connected to control unit 90 and is activated when power is supplied by control unit 90. A plunger of solenoid 64 is connected to support unit 51 via a link member (not shown). When power is supplied to solenoid 64, the plunger is actuated to move support unit 51 from the contact position to the contact release position. When power is removed from solenoid 64, the plunger is returned to its original position by a tension spring provided in solenoid 64, and support unit 51 is returned to the contact position by the spring force of spring 23C.

[0042] 3, in the conveying path 26, a leading edge detection sensor 61 (an example of a sheet sensor of the present invention) is provided upstream of the registration roller unit 60 in the conveying direction D11 and downstream of the sheet conveying unit 23. In other words, the leading edge detection sensor 61 is provided between the registration roller unit 60 and the sheet conveying unit 23. In this embodiment, the leading edge detection sensor 61 is provided at a position close to the registration roller unit 60.

[0043] The leading edge detection sensor 61 is provided near the center of the conveying path 26 in the width direction D2. The leading edge detection sensor 61 detects the leading edge of a sheet that has passed through the sheet conveying unit 23. The leading edge detection sensor 61 is, for example, a reflective optical sensor. The leading edge detection sensor 61 is connected to the control unit 90, and a detection signal from the leading edge detection sensor 61 is sent to the control unit 90. The control unit 90 detects the leading edge of the sheet being conveyed on the conveying path 26 based on a change in the detection signal sent from the leading edge detection sensor 61. Note that such detection methods are conventionally known, and therefore a detailed description thereof will be omitted.

[0044] [Registration Roller Unit 60] 3, the registration roller unit 60 is provided on the conveying path 26. The registration roller unit 60 is provided on the conveying path 26 upstream of the image transfer position P0 in the conveying direction D11 and downstream of the leading edge detection sensor 61 in the conveying direction D11.

[0045] The registration roller unit 60 corrects the inclination (conveyance deviation) of the sheet that is conveyed by the sheet conveying unit 23 in a state inclined with respect to the conveying direction D11, and conveys the corrected sheet in the conveying direction D11.

[0046] As shown in Fig. 3, the registration roller unit 60 includes a registration roller pair 80. The registration roller pair 80 includes a registration roller 80A that is rotated by receiving a rotational driving force from a transport motor 58 (see Fig. 4), and a driven roller 80B that is disposed in contact with the outer peripheral surface of the registration roller 80A. The driving force from the transport motor 58 is transmitted to the rotation shaft of the registration roller 80A.

[0047] The driven roller 80B is biased toward the registration roller 80A by a spring 80C. This causes the driven roller 80B to be pressed against the registration roller 80A. When the registration roller 80A is rotated in this state, the driven roller 80B is driven.

[0048] [Control unit 90] The control unit 90 performs overall control of the image forming apparatus 10, controls the operation of the sheet transport unit 23 and the operation of the registration roller unit 60, and also controls the sheet transport speed of the transport roller 23A.

[0049] 4, the control unit 90 is composed of a CPU 91, a ROM 92, a RAM 93, a storage unit 94, etc. The control unit 90 is electrically connected to the motors 56, 57, 58, the tip detection sensor 61, the solenoid 64, etc. via signal lines, etc. The motors 56, 57, 58 are connected to the control unit 90, and are driven and controlled by receiving individual control signals from the control unit 90.

[0050] The CPU 91 is a processor that executes computer programs to perform various data processing and predetermined control. The RAM 93 is a computer-readable volatile or nonvolatile storage device. The RAM 93 temporarily stores the computer programs executed by the CPU 91 and data output or referenced by the CPU 91 during the execution of various processes. The ROM 92 is a nonvolatile storage device that pre-stores control programs such as a BIOS and an OS that cause the CPU 91 to execute various arithmetic processes. The storage unit 94 is a flash memory that stores various information. The storage unit 94 stores control programs for executing various processes by the control unit 90, as well as data, thresholds, reference values, and the like used in the various processes. The storage unit 94 may also be a nonvolatile storage device such as an HDD or SSD connected to the control unit 90.

[0051] Incidentally, in the image forming apparatus 10, there are cases where slow-down control is performed to slow down the sheet conveyance speed at a predetermined timing when the sheet enters the registration roller pair 80. For example, if the start timing of the slow-down control is changed depending on the timing when the leading edge detection sensor 61 detects the leading edge of the sheet, the degree of delay due to slippage or the like that occurs before the sheet reaches the leading edge detection sensor 61 is not reflected in the process of changing the start timing, which can cause a problem in that the amount of sagging of the sheet varies from sheet to sheet.

[0052] In this embodiment, the deceleration start timing at which the sheet starts to decelerate is determined by the control unit 90. This makes it possible to prevent variations in the amount of sagging formed during the conveyance of the sheet.

[0053] The control unit 90 executes a sheet conveying process for conveying a sheet from the sheet storage unit 27 toward the image transfer position P0 by causing the CPU 91 to execute various control programs stored in advance in the ROM 92 or the storage unit 94.

[0054] 4, the control unit 90 includes various processing units such as a registration control unit 95 and a conveying speed control unit 96. Note that in this embodiment, not all of these processing units are necessarily required, and some may be omitted.

[0055] The control unit 90 functions as various processing units such as a registration control unit 95 and a conveyance speed control unit 96 by the CPU 91 executing various arithmetic processes in accordance with the control program. The control unit 90 or the CPU 91 is an example of a computer or processor that executes the control program. Note that some or all of the processing units included in the control unit 90 may be configured with electronic circuits such as motor drivers. The control program may also be a program that causes multiple processors to function as the various processing units.

[0056] The registration control unit 95 controls the rotational drive of the registration roller 80A so that the sheet is conveyed by the sheet conveying unit 23 while the registration roller 80A of the registration roller unit 60 is stopped, and the sheet whose leading edge has reached the nip portion of the registration roller pair 80 is bent, and then the sheet is sent out at a predetermined sending timing that coincides with the transfer start timing at which the transfer of the toner image onto the sheet begins at the image transfer position P0.

[0057] In addition, when the scanning start timing is earlier than the leading edge detection timing at which the leading edge of the sheet is detected by the leading edge detection sensor 61, the conveying speed control unit 96 controls the rotation speed of the conveying roller 23A and controls the conveying speed of the sheet based on the elapsed time Δt from the scanning start timing to the leading edge detection timing so that the sheet feed amount after the leading edge detection timing becomes a predetermined reference conveying amount.

[0058] 5 is a graph showing the sheet transport speed V when the sheet is transported under ideal conditions, without delay due to slippage or the like, and without being accelerated due to positional misalignment during feeding or the like. In this case, as shown in FIG. 5, the sheet is transported at a constant speed of a predetermined initial speed Vd, and the leading edge of the sheet is detected by the leading edge detection sensor 61 at time T1. Time T1 in this case is an example of the reference timing of the present invention. Time T1 is the timing when an elapsed time Δt (=time td0) has elapsed since reference time T0, which is the previous scanning start timing.

[0059] Here, the reference time point T0 is the timing at which laser scanning with a laser beam starts on the photosensitive drum 41 of the image forming unit 4K located at the most downstream side in the running direction D10 of the transfer belt 5A.

[0060] In the example shown in Figure 5, when the leading edge of the sheet being transported at the constant speed of the initial speed Vd is detected at time T1 after the elapse of the elapsed time Δt (= time td0) from the reference time point T0, the control unit 90 transports the sheet at the initial speed Vd until time T2 (an example of the reference deceleration start timing of the present invention) after the elapse of the set time x (= td1) from time T1, starts deceleration control at time T2 to linearly decelerate the sheet transport speed V from the initial speed Vd to speed Vr (low set speed), and transports the sheet at the constant speed of the speed Vr from time T3 when the deceleration control ends until time T4, which is the feed timing.

[0061] Then, at time T4, the control unit 90 starts increasing the speed of the sheet by the transport roller 23A, and drives the registration roller pair 80, which had been stopped, to start transporting the sheet by the registration roller pair 80. At this time, the control unit 90 controls the drive of each of the transport roller 23A and the registration roller 80A, and linearly increases the sheet transport speed V until it reaches a speed Vp (transfer transport speed) that is faster than the initial speed Vd. Thereafter, the sheet, whose speed has been increased to the speed Vp, is fed into the image transfer position P0 while being maintained at the speed Vp.

[0062] Here, the time from the time point T3 to the time point T4 is a low-speed setting time tdr determined for conveying the sheet at the low speed Vr, and this is an invariable time.

[0063] Furthermore, if the time required for the sheet to be transported from the start of laser scanning at the reference time T0 to the time T4 is defined as ta, the time required from the time T4 to the transfer start timing when the image is transferred at the image transfer position P0 and the time required until the transfer start timing when the leading edge of the toner image on the transfer belt 5A reaches the image transfer position P0 are constant, and therefore the required time ta is an invariant time that does not change as long as the rotation speed of the photosensitive drum 41 and the movement speed of the transfer belt 5A are constant.

[0064] As shown in Figure 5, the registration roller 80A is stopped until time T4, so if the amount of deflection of the sheet in this case is F, the amount of deflection F can be expressed as the sum of the areas of areas A1, A2, and A3 on the graph in Figure 5, that is, it can be expressed by the following calculation formula (1).

[0065] F=Vd·x+(Vd+Vr)(ta-tdr-x)+Vr·tdr =-Vr x + Vd Ta - Vd tdr + Vr Ta =-Vr·x+K (K=Vd·Ta-Vd·tdr+Vr·Ta)···(1)

[0066] The calculation formula (1) is a linear function with the set time x from the time T1 to the time T2 as a variable. In this embodiment, the control unit 90 uses the calculation formula (1) to find the set time x at which the amount of slack F becomes a predetermined set amount of slack (a constant amount) even if the timing of detecting the leading edge varies due to delays in the arrival of the sheet, and determines the deceleration start timing at which the amount of slack F becomes the set amount of slack. Then, the control unit 90 gradually decelerates the conveying speed of the sheet to the time T3 at the determined deceleration start timing, and performs speed control after the time T3. As a result, even if the timing of detecting the leading edge varies, the sheet always becomes slackened by the set amount of slack.

[0067] In this embodiment, in the example shown in Figure 5, when the leading edge detection timing is the time T1 (reference timing), the control unit 90 calculates the set time x using the calculation formula (1), determines the time T2 (reference deceleration start timing) obtained by adding the elapsed time Δt (= td0) and the set time x to the reference time T0 as the deceleration start timing, and controls the rotation of the conveying roller 23A so that the sheet conveying speed V starts to decelerate at the time T2.

[0068] FIG. 6 is a graph showing the conveyance speed V of the sheet when the leading end of the sheet being conveyed at a constant speed at the initial speed Vd is detected at time T11 after the elapse of the elapsed time Δt (= td1 < td0) from the reference time T0. In other words, FIG. 6 is a graph showing the conveyance speed V of the sheet when the leading end of the sheet is detected earlier than time T1 with reference to the reference time T0.

[0069] In the example shown in FIG. 6, the leading end detection timing is time T11 which is earlier than time T1 (reference timing). In this case, the control unit 90 obtains the set time x using the calculation formula (1), and determines a time T21 (first deceleration start timing) which is earlier than time T2 (reference deceleration start timing) by a time corresponding to the elapsed time Δt (= td1). Specifically, the control unit 90 determines the time T21 obtained by adding the elapsed time Δt (= td1) and the set time x to the reference time T0 as a new deceleration start timing, and rotationally controls the conveyance roller 2?? so as to start decelerating the conveyance speed V of the sheet at time T21. Also in this case, a deflection of the set deflection amount is formed in the sheet.

[0070] FIG. 7 is a graph showing the conveyance speed V of the sheet when the leading end of the sheet being conveyed at a constant speed at the initial speed Vd is detected at time T12 after the elapse of the elapsed time Δt (= td2 > td0) from the reference time T0. In other words, FIG. 7 is a graph showing the conveyance speed V of the sheet when the leading end of the sheet is detected after time T1 with reference to the reference time T0.

[0071] 7, the leading edge detection timing is time T12, which is later than time T1 (reference timing). In this case, the control unit 90 calculates the set time x using the calculation formula (1) and determines T22 (second deceleration start timing), which is earlier than time T2 (reference deceleration start timing) by a time corresponding to the elapsed time Δt (= td2). Specifically, the control unit 90 determines time T22, which is obtained by adding the elapsed time Δt (= td2) and the set time x to the reference time T0, as the new deceleration start timing, and controls the rotation of the conveyance roller 23A so that the sheet conveyance speed V starts to decelerate at time T22. In this case, the sheet also becomes sagged by the set amount of sagging.

[0072] In the above-described embodiment, the image forming apparatus 10 is exemplified as an example of the image forming apparatus of the present invention, but the present invention can also be understood as a sheet conveying apparatus equipped with a sheet conveying unit 23, a registration roller unit 60, and a control unit 90.

[0073] [Notes on the Invention] The following is a summary of the invention extracted from the above-described embodiment. Note that the configurations and processing functions described in the following supplementary notes can be selected and combined as desired.

[0074] <Appendix 1> a pair of conveying rollers that convey the sheet toward an image transfer position where a toner image formed in the image forming unit is transferred onto the sheet; a pair of registration rollers provided between the image transfer position and the pair of transport rollers; a sheet sensor provided between the pair of registration rollers and the pair of conveyance rollers, the sheet sensor detecting the leading edge of the sheet; a registration control unit that controls the rotational drive of the registration roller pair so that the sheet is conveyed by the conveying roller pair while the registration roller pair is stopped, and the sheet whose leading edge has reached the registration roller pair is bent, and then the sheet is fed at a predetermined feed timing that matches a transfer start timing at which transfer of the toner image onto the sheet is started at the image transfer position; a conveyance speed control unit that controls the rotation speed of the conveyance roller pair based on the elapsed time from a predetermined write start timing at which optical scanning of the photosensitive drum included in the image forming unit is started to a leading edge detection timing at which the leading edge of the sheet is detected by the sheet sensor, so that the sheet feed amount after the leading edge detection timing becomes a predetermined reference conveyance amount; A sheet conveying device comprising:

[0075] <Appendix 2> The conveying speed control unit controlling the rotation of the conveying roller pair so as to convey the sheet at a predetermined initial speed Vd until the leading edge detection timing; When the leading edge detection timing is a predetermined reference timing, the rotation of the conveying roller pair is controlled so that the sheet conveying speed is gradually reduced from the initial speed Vd to a low set speed Vr that is slower than the initial speed Vd at a predetermined reference deceleration start timing; If the leading edge detection timing is earlier than the reference timing, a first deceleration start timing is determined that is earlier than the reference deceleration start timing by a time period corresponding to the elapsed time, and the pair of conveying rollers is controlled to rotate so that the sheet conveying speed is gradually reduced from the initial speed Vd to the low set speed Vr at the first deceleration start timing; A sheet conveying device as described in Appendix 1, wherein, when the leading edge detection timing is later than the reference timing, a second deceleration start timing is determined that is later than the reference deceleration start timing by a time corresponding to the elapsed time, and the pair of conveying rollers is controlled to rotate so as to gradually reduce the sheet conveying speed from the initial speed Vd to the low set speed Vr at the second deceleration start timing.

[0076] <Appendix 3> the registration control unit controls the rotation of the registration roller pair so as to transport the sheet at a transfer transport speed Vp that is faster than the initial speed Vd after the sending-out timing, 3. The sheet transporting device according to claim 2, wherein the transport speed control unit controls the rotation of the transport roller pair so as to transport the sheet at the transfer transport speed Vp after the sending-out timing.

[0077] <Appendix 4> A sheet conveying device according to any one of Supplementary Note 1 to 3, an image forming apparatus that transfers a toner image onto a sheet that is transported to an image transfer position by the sheet transport device; [Explanation of symbols]

[0078] 4: Image forming unit 10: Image forming device 13: Optical scanning device 14: Image forming unit 20: Secondary transfer roller 23: Sheet transport unit 23A: Transport roller 23B: Driven roller 25: Reversing conveyance path 26: Sheet transport path 27: Sheet storage area 28: Feeding unit 30: Feed roller 31: Separation roller 41: Photosensitive drum 56-58: Transport motor 60: Registration roller unit 61: Tip detection sensor 80: Registration roller pair 80A: Registration roller 90: Control unit 95: Resist control section 96: Conveying speed control section

Claims

1. a pair of conveying rollers that convey the sheet toward an image transfer position where a toner image formed in the image forming unit is transferred onto the sheet; a pair of registration rollers provided between the image transfer position and the pair of transport rollers; a sheet sensor provided between the pair of registration rollers and the pair of conveyance rollers, the sheet sensor detecting the leading edge of the sheet; a registration control unit that controls the rotational drive of the registration roller pair so that the sheet is conveyed by the conveying roller pair while the registration roller pair is stopped, and the sheet whose leading edge has reached the registration roller pair is bent, and then the sheet is fed at a predetermined feed timing that matches a transfer start timing at which transfer of the toner image onto the sheet is started at the image transfer position; a conveyance speed control unit that controls the rotation speed of the conveyance roller pair based on the elapsed time from a predetermined write start timing at which optical scanning of the photosensitive drum included in the image forming unit is started to a leading edge detection timing at which the leading edge of the sheet is detected by the sheet sensor, so that the sheet feed amount after the leading edge detection timing becomes a predetermined reference conveyance amount; A sheet conveying device comprising:

2. The conveying speed control unit controlling the rotation of the conveying roller pair so as to convey the sheet at a predetermined initial speed Vd until the leading edge detection timing; When the leading edge detection timing is a predetermined reference timing, the pair of conveying rollers are controlled to rotate so as to gradually reduce the sheet conveying speed from the initial speed Vd to a low set speed Vr that is slower than the initial speed Vd at a predetermined reference deceleration start timing; If the leading edge detection timing is earlier than the reference timing, a first deceleration start timing is determined that is earlier than the reference deceleration start timing by a time period corresponding to the elapsed time, and the pair of conveying rollers is controlled to rotate so that the sheet conveying speed is gradually reduced from the initial speed Vd to the low set speed Vr at the first deceleration start timing; 2. The sheet conveying device according to claim 1, wherein, when the leading edge detection timing is later than the reference timing, a second deceleration start timing is determined that is later than the reference deceleration start timing by a time corresponding to the elapsed time, and the pair of conveying rollers is controlled to rotate so as to gradually reduce the sheet conveying speed from the initial speed Vd to the low set speed Vr at the second deceleration start timing.

3. the registration control unit controls the rotation of the registration roller pair so as to transport the sheet at a transfer transport speed Vp that is faster than the initial speed Vd after the sending-out timing, The sheet transport device according to claim 2 , wherein the transport speed control unit controls the rotation of the transport roller pair so that the sheet is transported at the transfer transport speed Vp after the sending-out timing.

4. The sheet conveying device according to claim 1 or 2, an image forming apparatus that transfers a toner image onto a sheet that is transported to an image transfer position by the sheet transport device;

Images