Image forming apparatus

Abstract

To suppress image defects.SOLUTION: Provided is an image forming apparatus comprising: an image carrier that bears a toner image; a belt rotating in a predetermined rotation direction; an outer roller that is in contact with an outer circumferential surface of the belt; an inner roller that forms a transfer nip together with the outer roller; a tension roller; a conveying roller pair that conveys a sheet toward the transfer nip; a guide member that is disposed between a virtual line passing through a nip of the conveying roller pair and the transfer nip, and a virtual line extending along the outer circumferential surface of the belt stretched between the tension roller and the inner roller and has a projection projecting toward the outer circumferential surface of the belt; a belt drive motor; a torque sensor; and a control unit that executes conveyance control which, after the sheet reaches the transfer nip, controls a rotational speed of the conveying roller pair such that a first conveyance speed of the sheet by the transfer nip becomes higher than a second conveyance speed of the sheet by the conveying roller pair, based on a detection result of the torque sensor.SELECTED DRAWING: Figure 7

Description

[Technical Field]

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

[0002] In general, image forming devices transfer a toner image formed on an intermediate transfer belt or photosensitive drum to a sheet in a transfer section, then fix the toner image to the sheet using heat and pressure. A pair of registration rollers (hereinafter referred to as the registration roller pair) is located upstream of the transfer section, and a state in which the sheet straddles both the transfer section and the registration roller pair occurs. It is generally known that when a sheet straddles these rollers, slight differences in the rotational speeds of the two rollers can cause the registration roller pair to pull or push the sheet. As a result, the sheet's position becomes unstable upstream of the transfer section, resulting in image defects. For example, the outer diameter tolerance of the registration roller pair can cause the sheet transport speed to differ between the registration roller pair and the transfer section.

[0003] Therefore, Patent Document 1 proposes an image forming apparatus that stores a set torque for the secondary transfer roller when a sheet is transported only by the transfer unit.The image forming apparatus then subtracts the actual torque of the secondary transfer roller when the sheet is transported by both the registration roller pair and the transfer unit from the set torque to obtain a torque deviation, and controls the rotation speed of the registration roller pair so that the torque deviation becomes zero.This prevents the registration roller pair from pulling or pushing the sheet. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-091347 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the case of a sheet with low stiffness, although the difference in the sheet conveying speed between the transfer section and the pair of registration rollers is likely to affect the sheet posture, it is unlikely to affect the fluctuation in the driving torque of the secondary transfer roller when the sheet is pressed into the transfer section. This is because a loop is formed in the sheet when the sheet is pressed into the transfer section. Even if the rotation speed of the pair of registration rollers is controlled so that the torque deviation becomes zero as in Patent Document 1, there is a risk that the sheet posture may not be properly stabilized, particularly in the case of a sheet with low stiffness.

[0006] SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that can stabilize the posture of a sheet conveyed to a transfer nip and suppress image defects. [Means for solving the problem]

[0007] The present invention relates to an image forming apparatus including an image carrier that carries a toner image, a belt onto which the toner image is transferred from the image carrier while rotating in a predetermined rotation direction, an outer roller that contacts the outer peripheral surface of the belt, an inner roller that is disposed opposite the outer roller across the belt and that contacts the inner peripheral surface of the belt to form a transfer nip together with the outer roller, a tension roller that contacts the inner peripheral surface of the belt and is disposed upstream of the inner roller in the rotation direction, a pair of conveying rollers that convey a sheet toward the transfer nip, and a guide member that guides the sheet conveyed by the pair of conveying rollers toward the transfer nip, wherein the nip of the pair of conveying rollers and the a guide member disposed between a virtual line passing through the transfer nip and a virtual line extending along the outer peripheral surface of the belt stretched between the tension roller and the inner roller, the guide member having a convex portion protruding toward the outer peripheral surface of the belt; a belt drive motor for driving the belt; a torque sensor for detecting the torque of the belt drive motor; and a control unit that performs conveying control to control the rotational speed of the conveying roller pair based on the detection result of the torque sensor after the sheet reaches the transfer nip so that a first conveying speed of the sheet by the transfer nip is faster than a second conveying speed of the sheet by the conveying roller pair.

[0008] The present invention also provides an image forming apparatus including an image carrier that carries a toner image, a belt onto which the toner image is transferred from the image carrier while rotating in a predetermined rotation direction, an outer roller that contacts the outer peripheral surface of the belt, an inner roller that is disposed opposite the outer roller across the belt and that contacts the inner peripheral surface of the belt to form a transfer nip together with the outer roller, a tension roller that contacts the inner peripheral surface of the belt and is disposed upstream of the inner roller in the rotation direction, a pair of conveying rollers that convey a sheet toward the transfer nip, and a guide member that guides the sheet conveyed by the pair of conveying rollers toward the transfer nip, the guide member including a virtual conveying roller that passes through the nip of the pair of conveying rollers and the transfer nip. and a virtual line extending along the outer peripheral surface of the belt stretched between the tension roller and the inner roller, and a guide member having a convex portion protruding toward the outer peripheral surface of the belt; a belt drive motor for driving the belt; a torque sensor for detecting the torque of the belt drive motor; and a control unit that performs transport control that controls the rotational speed of the transport roller pair so that the torque detected by the torque sensor when the sheet is sandwiched between the transport roller pair and the transfer nip is greater than the torque detected by the torque sensor when the sheet is sandwiched and transported only by the transfer nip. [Effects of the Invention]

[0009] According to the present invention, it is possible to suppress image defects. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic overall view showing a cross-sectional configuration of an image forming apparatus. [Figure 2] FIG. 2 is a schematic diagram showing the peripheral configuration of a transfer nip. [Figure 3] FIG. 2 is a block diagram showing a control block of a control unit provided in the image forming apparatus. [Figure 4] 6 is a graph showing torque fluctuations of a belt drive motor. [Figure 5](a) is a schematic diagram showing the behavior of the sheet, and (b) is an enlarged view of Figure 5(a). [Figure 6] 10 is a flowchart showing print control. [Figure 7] 10 is a flowchart showing print control. [Figure 8] 8(a) is a schematic diagram showing a guide member according to a modified example, and FIG. 8(b) is an enlarged view of FIG. 8(a) according to the modified example. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Overall configuration] 1 is a schematic overall view showing a cross-sectional configuration of an image forming apparatus 100 according to the present embodiment. The image forming apparatus 100 has an image forming section 140 that forms an image on a sheet P, which is a recording material, a feeding unit 117, a fixing device 150, a sheet discharge device 200, and an image reading device 102. The image forming apparatus 100 also has an apparatus main body 101, which is a housing that houses the image forming section 140.

[0012] The image forming apparatus includes printers, copiers, facsimiles, and multifunction peripherals, and refers to an apparatus that 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. In addition, an image forming apparatus may be connected to accessories such as an optional feeder, an image reader, and a sheet processing device in addition to the main body having an image forming function, and the entire system connected with such accessories is also a type of image forming apparatus.

[0013] The image forming section 140 is an electrophotographic unit of an intermediate transfer tandem type in which image forming stations Y, M, C, and Bk that form toner images of four colors are arranged along an intermediate transfer belt 145 .

[0014] The sheets P are stored in a cassette 116 provided at the bottom of the apparatus main body 101, and are fed one by one by a feeding unit 117. The feeding unit 117 may include, for example, a feeding roller that feeds the sheets P, and a separation roller that is disposed in contact with the feeding roller and separates the sheets P fed by the feeding roller from the other sheets P by applying a frictional force to the sheets P. Note that the sheets P, which are recording materials, can be a variety of sheets of different sizes and materials, such as paper such as plain paper and cardboard, surface-treated sheet materials such as plastic film, cloth, and coated paper, and sheet materials with special shapes such as envelopes and index paper.

[0015] A sheet P fed from a feeding unit 117 is conveyed by a pre-registration roller pair (hereinafter referred to as a pre-registration roller pair) 110 to a registration roller pair (hereinafter referred to as a registration roller pair) 120. Then, the sheet P has its skew corrected by the registration roller pair 120, which serves as a conveying roller pair, and is conveyed toward a transfer nip 130 at a timing synchronized with a toner image formation process by an image forming section 140.

[0016] The image forming unit 140 is provided with an intermediate transfer belt 145 onto which a toner image is transferred from the photosensitive drum 141, and a secondary outer transfer roller 132 serving as an outer roller that abuts against an outer peripheral surface 145b (see FIG. 2) of the intermediate transfer belt 145. The image forming unit 140 is also provided with a drive roller 131 that is disposed opposite the secondary outer transfer roller 132 across the intermediate transfer belt 145, abuts against an inner peripheral surface 145c of the intermediate transfer belt 145, and forms a transfer nip 130 together with the secondary outer transfer roller 132. The transfer nip 130 conveys a sheet while sandwiching it. The image forming unit 140 is also provided with a tension roller 147 that abuts against the inner peripheral surface 145c (see FIG. 2) of the intermediate transfer belt 145 and is disposed upstream of the drive roller 131 in the direction of arrow A.

[0017] The intermediate transfer belt 145 is wound around a drive roller 131, a tension roller 146, a plurality of primary inner rollers 144, and a tension roller 147, and is stretched with a predetermined tension. When the drive roller 131 is driven, the intermediate transfer belt 145 rotates in the direction of arrow A. As a result, the intermediate transfer belt 145 is positioned between the tension roller 147 and the drive roller 131 in the direction of arrow A, and forms a belt inclined surface 145a that guides the sheet P toward the transfer nip 130. The belt inclined surface 145a is part of the outer peripheral surface 145b. The tension roller 147 is positioned upstream of the drive roller 131 in the direction of arrow A.

[0018] In parallel with the above-described process of transporting the sheet P to the transfer nip 130, the image forming unit 140 executes a toner image formation process. Each of the image forming stations Y, M, C, and Bk of the image forming unit 140 includes a photosensitive drum 141, which is a drum-shaped image carrier (electrophotographic photosensitive member), a charging unit such as a charging roller, and a developing unit 143, which serves as a developing unit. The image forming unit 140 also includes an exposure device 142 disposed below the four photosensitive drums 141. In the toner image formation process, the charging unit uniformly charges the surface of the photosensitive drum 141, and the exposure device 142 exposes the photosensitive drum 141 based on a signal representing image information to be formed, thereby writing an electrostatic latent image on the surface of the photosensitive drum 141. This electrostatic latent image is developed into a single-color toner image with toner supplied from the developing unit 143. As a result, toner images of four colors, yellow, magenta, cyan, and black, are formed on the surfaces of the four photosensitive drums 141.

[0019] The intermediate transfer belt 145 as a belt is driven to rotate in the direction of arrow A, which is a predetermined rotation direction in FIG. 1. The toner images carried on the four photosensitive drums 141 are sequentially transferred by the primary inner roller 144 to the intermediate transfer belt 145 so as to be superimposed on one another. As a result, a full-color toner image is finally formed on the intermediate transfer belt 145, and is carried by the intermediate transfer belt 145 and transported to the transfer nip 130. Then, the toner image is secondarily transferred from the intermediate transfer belt 145 to the sheet P by the pressure and transfer bias in the transfer nip 130. When the toner image is secondarily transferred to the sheet P, a secondary transfer voltage is applied from a high-voltage power supply to either the drive roller 131 or the secondary outer transfer roller 132.

[0020] The sheet P that has passed through the transfer nip 130 is transported to the fuser 150. The fuser 150 has a fuser roller 155 that has a built-in heater, and a pressure roller 156 that contacts the fuser roller 155 with a predetermined pressure. The fuser roller 155 is driven by a fuser motor 154 (see FIG. 3 ), which will be described later, and the pressure roller 156 is rotated by the fuser roller 155. The fuser 150 applies pressure and heat to the toner image on the sheet P while nipping and transporting the sheet P with a fuser nip 157 that serves as a fuser formed by the fuser roller 155 and the pressure roller 156. This melts the toner, and the toner adheres after passing through the fuser nip, resulting in a fixed image on the sheet P.

[0021] The sheet P that has passed through the fixing unit 150 is guided by a first guide member 151 to either a first discharge path 230 toward a first discharge roller pair 160 or a second discharge path 240 toward a second discharge roller pair 161. When images are formed on both sides of the sheet P, the sheet P with an image formed on its first side is guided by the first guide member 151 toward the second discharge roller pair 161 and conveyed toward the outside of the apparatus by the second discharge roller pair 161. When the trailing end of the sheet P in the conveying direction passes through the second guide member 152, the second discharge roller pair 161 reverses the conveying direction of the sheet P and sends the sheet P into a double-sided conveying path 180. The portion of the sheet P that protrudes outside the apparatus main body 101 during the reversing operation by the second discharge roller pair 161 is supported by a second discharge tray 171. The sheet P reaches the pair of registration rollers 120 again via the double-sided conveying path 180, and after skew correction and timing correction, passes through the transfer nip 130 and the fixing device 150, where an image is formed on the second side.

[0022] A sheet discharge device 200 is provided downstream of the fixing device 150 in the sheet conveying direction, which discharges the sheet P outside the apparatus, i.e., outside the apparatus main body 101. The sheet discharge device 200 has a first discharge roller pair 160, a second discharge roller pair 161, a first discharge tray 170, and a second discharge tray 171.

[0023] When discharging the sheet P, the sheet P sent out from the fixing unit 150 is guided by the first guide member 151 to the first discharge roller pair 160, and is discharged to the outside of the apparatus main body 101 by the first discharge roller pair 160. A first discharge tray 170 is provided on the upper part of the apparatus main body 101, and the sheet P discharged by the first discharge roller pair 160 is stacked (supported) on the first discharge tray 170. The sheet P stacked on the first discharge tray 170 or the second discharge tray 171 slides along the inclined surface of the first discharge tray 170 or the second discharge tray 171 due to its own weight, and is aligned when its rear end hits an alignment surface of the apparatus main body 101.

[0024] The image forming apparatus 100 includes an image reading device 102 mounted on an upper portion of the apparatus main body 101. The image reading device 102 includes a platen glass on which an original document is placed and an image sensor that reads an image of the original document through the platen glass. The image reading device 102 also includes an automatic document feeder that feeds original documents set in an original tray one by one and has the image read by the image sensor. The image forming apparatus 100 of this embodiment has a so-called internal discharge type configuration in which an internal discharge space 190 for sheets P is provided between the image forming unit 140 and the image reading device 102 in the vertical direction. The internal discharge type configuration has the advantage of being able to reduce the footprint of the image forming apparatus 100 as viewed from above, compared to a configuration in which, for example, the first discharge tray 170 is provided on the side of the apparatus main body 101 and the sheet discharge space is located to the side of the apparatus main body 101.

[0025] Furthermore, the image forming unit 140 described above is an example of an image forming unit, and may be, for example, a direct transfer type electrophotographic unit in which a toner image formed on a photosensitive member is transferred to a sheet without an intermediate transfer member. Furthermore, the sheet P may be fed from a multi-tray 113 that is openably and closably provided on the side of the apparatus main body 101. The sheet P stacked on the multi-tray 113 is conveyed to a pull-off roller pair 115 by a conveyance roller pair 114. The sheet P is then conveyed to a registration roller pair 120 by the pull-off roller pair 115, where a toner image is formed as described above.

[0026] [Transfer nip peripheral configuration] Next, the peripheral configuration of the transfer nip 130 will be described in more detail with reference to FIG. 2. FIG. 2 is a schematic diagram showing the peripheral configuration of the transfer nip 130. As shown in FIG. 2, a pre-registration sensor 111 is disposed between the pre-registration roller pair and the registration roller pair 120. The leading edge of the sheet P conveyed by the pre-registration roller pair 110 abuts against and follows the nip of the registration roller pair 120, which is in a stopped state, thereby correcting skew of the sheet P. The registration roller pair 120 is driven in accordance with the timing at which the toner image is transferred to the sheet, based on the timing at which the pre-registration sensor 111 detects the leading edge of the sheet P.

[0027] A guide member 301 is disposed between the pair of registration rollers 120 and the transfer nip 130 to guide the sheet P conveyed by the pair of registration rollers 120 toward the transfer nip 130. The guide member 301 has a protrusion 301a that protrudes toward the belt slope 145a of the intermediate transfer belt 145. The protrusion 301a is disposed in an area AR between an imaginary line L1 that passes through the nip 120a of the pair of registration rollers 120 and the transfer nip 130, and an imaginary line L2 that extends along the belt slope 145a. The imaginary line L2 is also a common tangent to the drive roller 131 and the outer secondary transfer roller 132 at the transfer nip 130.

[0028] The guide member 301 is disposed on the opposite side of the imaginary line L2 from the drive roller 131, i.e., on the non-image side. As will be described later, the sheet P sandwiched between the transfer nip 130 and the pair of registration rollers 120 is conveyed so as to slide on the convex portion 301a of the guide member 301. Therefore, the guide member 301 acts as a conveying load when conveying the sheet P. In addition, a fixing loop sensor 153 is disposed between the transfer nip 130 and the fixing unit 150, which detects the amount of slack (hereinafter also referred to as loop) of the sheet P.

[0029] [Control Block] 3 is a block diagram showing a control block of the control unit 210 provided in the image forming apparatus 100. As shown in FIG.

[0030] The CPU (Central Processing Unit) 211 is a device that performs arithmetic processing. The memory 212 includes a ROM (Read Only Memory) and a RAM (Random Access Memory). The RAM stores information input to the control unit 210, information detected by various sensors, and calculation results. The ROM stores control programs, pre-determined data, and the like. The CPU 211 and memory 212 are capable of transferring and reading data from and to each other. The interface unit (I / F) 213 controls the input and output of signals between the control unit 210 and devices connected to the control unit 210.

[0031] The control unit 210 is connected to various parts of the image forming apparatus 100 (image forming unit 140, drive devices for members related to the conveyance of intermediate transfer belt 145 and sheet P, various power supplies, etc.). For example, the control unit 210 is connected to a belt drive motor 133, various high-voltage power supplies (charging voltage, developing voltage, primary transfer voltage, secondary transfer voltage), etc. (not shown). In addition, signals (output values) indicating detection results such as the current value of each drive motor are input to the control unit 210, and are stored in memory 212.

[0032] The control unit 210 is also connected to a pre-registration drive motor 112, a registration drive motor 121, and a fixing motor 154. The pre-registration drive motor 112 drives the pre-registration roller pair 110. The registration drive motor 121, which serves as a conveying motor, drives the registration roller pair 120. The fixing motor 154 drives the fixing roller 155. The pre-registration roller pair 110 and the registration roller pair 120 each have a drive roller and a driven roller that rotates following the drive roller, and these drive rollers are driven by the pre-registration drive motor 112 and the registration drive motor 121, respectively. The pressure roller 156 of the fixing unit 150 rotates following the fixing roller 155. The fixing motor 154 may drive the pressure roller 156, and the fixing roller 155 may rotate following the pressure roller.

[0033] The belt drive motor 133 drives the drive roller 131 to rotate the intermediate transfer belt 145 in the direction of arrow A (FIG. 1). The belt drive motor 133 is also provided with a torque sensor 134 that can continuously detect the drive load (torque) of the belt drive motor 133 over any time interval. In this embodiment, a current detection unit that detects the current flowing through the belt drive motor 133 is used as an example of the torque sensor 134. Torque fluctuation information of the belt drive motor 133 collected using the torque sensor 134 is stored in memory 212.

[0034] An operation unit 220 is also connected to the control unit 210. The operation unit 220 has a display unit such as a display panel that displays information, and an input unit that inputs information to the control unit 210 through operations by an operator such as a user or a service representative. The operation unit 220 may be configured with a touch panel that has the functions of a display unit and an input unit. An external device such as an image reading device 102 or a personal computer connected to the image forming apparatus 100 may also be connected to the control unit 210.

[0035] Control unit 210 controls each unit of image forming apparatus 100 based on job information to form an image. Job information includes information (command signals) related to the printing operation, such as the paper feed tray, start instruction (start signal), image information, and attributes of sheet P, which are input from operation unit 220 or an external device. Note that information related to the attributes of sheet P (also simply referred to as "sheet information") includes any information that can distinguish sheets, such as the type of sheet (so-called paper type category) such as plain paper, fine paper, glossy paper, coated paper, embossed paper, thick paper, thin paper, etc., numerical values ​​or numerical ranges for basis weight, thickness, size, or brand (including manufacturer, product number, etc.).

[0036] In this embodiment, the information about the attributes of the sheet P includes information related to the stiffness of the sheet P, for example, information about the basis weight of the sheet P. When information about the printing operation conditions is input from the operation unit 220, the operation unit 220 functions as an input unit that inputs information about the basis weight of the sheet P onto which the toner image is transferred to the control unit 210. When information about the printing operation conditions is input from an external device such as a personal computer, the interface unit 213 functions as an input unit that inputs information about the basis weight of the sheet P onto which the toner image is transferred to the control unit 210.

[0037] The image forming apparatus 100 executes a job (print job) that is a series of operations that starts with a single start instruction and forms and outputs an image on one or multiple sheets P. A job generally includes an image formation process (printing operation), a pre-rotation process, a sheet-to-sheet process when forming images on multiple sheets P, and a post-rotation process. The image formation process is a process of forming an electrostatic image of the image that will actually be formed and output on the sheet P, forming a toner image, and performing primary and secondary transfer of the toner image, and the image formation time refers to this process period. The pre-rotation process is a process of performing preparatory operations before the image formation process, from when a start instruction is input until the actual start of image formation. The sheet-to-sheet process is a process corresponding to the interval between sheets P when image formation is performed continuously on multiple sheets P (continuous image formation). The post-rotation process is a process of performing organizing operations (preparatory operations) after the image formation process. Non-image formation time (non-image formation section) is a section other than image formation time, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and also the pre-multi-rotation process, which is a preparatory operation when the image forming device 100 is turned on or when it returns from a sleep state.

[0038] The sleep state (hibernation state) is a state in which the supply of power to each unit of image forming apparatus 100 other than control unit 210 (or a part thereof) is stopped, and power consumption is reduced compared to the standby state. In this embodiment, the current value of belt drive motor 133 is always stored during the image formation process. This current value is then stored in memory 212 as torque fluctuation data, i.e., fluctuation data of torque Tq of belt drive motor 133.

[0039] [Torque fluctuations in belt drive motors] Next, torque fluctuations of the belt drive motor 133 detected by the torque sensor 134 will be described with reference to Fig. 4. Fig. 4 is a graph showing torque fluctuations of the belt drive motor 133. Note that times t1 to t5 described below are calculated based on the distance between each roller, the length of the sheet P in the conveying direction, and the rotational speed of each roller, with time t0 when the sheet P reaches the transfer nip 130 as the reference time.

[0040] As shown in FIG. 4, torque fluctuations occur in the belt drive motor 133 due to fluctuations in the load received via the sheet P passing through the transfer nip 130, the intermediate transfer belt 145, and the drive roller 131. The torque Tq of the belt drive motor 133 changes depending on which section of the conveying path the sheet P passes through. In this embodiment, time t0 to time t2 is defined as section A, and time t2 to time t4 is defined as section B. Section A is the section from when the leading edge of the sheet P reaches the transfer nip 130 (time t0) to when it reaches the fixing nip 157 (time t2). Section B is the section from when the leading edge of the sheet P reaches the fixing nip 157 (time t2) to when the trailing edge of the sheet P reaches the nip of the pre-registration roller pair 110 (time t4). During section B, after time t3 when the leading edge of sheet P reaches fixing loop sensor 153, control unit 210 performs loop control to control the amount of loop of sheet P between transfer nip 130 and fixing nip 157 by controlling fixing motor 154.

[0041] Figure 4 shows, from top to bottom, the rotation speed of the fixing motor 154, the period during which the sheet P is transported by the pre-registration roller pair 110, the period during which the sheet P is transported by the registration roller pair 120, the period during which the sheet P is transported by the transfer nip 130, the period during which the sheet P is transported by the fixing roller 155, and the torque fluctuations of the belt drive motor 133.

[0042] 4, the idling torque value Tq_ST is the steady torque value at a time (t0-Δt) Δt before time t0, i.e., before the leading edge of the sheet P reaches the transfer nip 130. In other words, the idling torque value Tq_ST is the driving load (torque) when driving the belt drive motor 133 in a state where the sheet P is not sandwiched in the transfer nip 130.

[0043] After the leading edge of the sheet P reaches the transfer nip 130 at time t0, the torque Tq remains below the idling torque value Tq_ST in section A. This is because, as the sheet P is transported to the transfer nip 130, the transport force from the pair of registration rollers 120 is applied to the belt drive motor 133 as torque for driving the intermediate transfer belt 145, and the torque Tq is reduced accordingly.

[0044] In section B after section A, the leading edge of the sheet P reaches the fixing nip 157, increasing the conveying resistance and requiring a larger torque to convey the sheet P. As a result, the torque Tq of the belt drive motor 133 increases and remains at a value that is approximately greater than the idling torque value Tq_ST.

[0045] Then, at time t4, when the trailing edge of sheet P passes through the pair of pre-registration rollers 110, the conveying force of the pair of pre-registration rollers 110 used to convey sheet P is lost, and the torque Tq of belt drive motor 133 increases. Next, the trailing edge of sheet P passes through convex portion 301a of guide member 301 (see FIG. 2) and pair of registration rollers 120 in that order (time t5), and the torque Tq increases. After that, at time t6 when the trailing edge of sheet P passes through transfer nip 130, the torque Tq of belt drive motor 133 becomes a steady idling torque value Tq_ST'.

[0046] As described above, fluctuations in the torque Tq of the belt drive motor 133 occur when the leading edge of the sheet P reaches the nip of each roller pair on the conveyance path, or when the trailing edge of the sheet P passes through the nip of each roller pair. Generally, the torque Tq also differs depending on the attributes of the sheet P being conveyed (e.g., basis weight, stiffness, etc.). Here, by subtracting the idling torque value Tq_ST from the torque Tq of the belt drive motor 133, it is possible to remove forces other than those caused by the sheet P, such as the sliding load of the rollers, and hereinafter this is referred to as the paper conveyance torque Tq_p. In other words, the paper conveyance torque Tq_p is the torque Tq minus the idling torque value Tq_ST.

[0047] [Sheet behavior before transfer nip] Next, the behavior of the sheet P before the transfer nip 130 will be described with reference to Figures 5(a) and 5(b). Figure 5(a) is a schematic diagram showing the behavior of the sheet P, and Figure 5(b) is an enlarged view of Figure 5(a). As shown in Figure 5(a), a guide member 301 is disposed between the pair of registration rollers 120 and the transfer nip 130, and the sheet P slides against the guide member 301, thereby acting as a transport load when the sheet P is transported. The sheet P is guided by the guide member 301 and transported while forming a loop so as to approach the intermediate transfer belt 145.

[0048] For example, if the outer diameter of the pair of registration rollers 120 is the nominal dimension (value in the drawing), the sheet P conveyed between the pair of registration rollers 120 and the transfer nip 130 is conveyed in posture P1. However, if the outer diameter of the pair of registration rollers 120 is smaller than the nominal dimension, when the pair of registration rollers 120 rotates at the nominal speed, the sheet P is conveyed in posture P2. In posture P2, the sheet P has a smaller loop amount than in posture P1. This is because the conveying speed of the sheet P by the pair of registration rollers 120 decreases when the outer diameter of the pair of registration rollers 120 is smaller than the nominal dimension.

[0049] Furthermore, if the outer diameter of the registration roller pair 120 is larger than the nominal dimension, when the registration roller pair 120 rotates at the nominal speed, the sheet P is conveyed in posture P3. In posture P3, the sheet P has a larger loop amount than in posture P1. This is because the conveying speed of the sheet P by the registration roller pair 120 increases when the outer diameter of the registration roller pair 120 is larger than the nominal dimension.

[0050] At this time, if the distance over which the sheet P and the belt slope 145a of the intermediate transfer belt 145 are in frictional contact is defined as distance X, then as shown in Figure 5(b), distance X becomes distances X1, X2, and X3 when the posture of the sheet P is postures P1, P2, and P3, respectively. Distance X2 is shorter than distance X1, and distance X3 is longer than distance X2.

[0051] In this embodiment, the appropriate distance X is greater than or equal to distance X2 and less than or equal to distance X3. If distance X is smaller than distance X2, sheet P may not align with belt inclined surface 145a. This reduces the load on the secondary transfer power source that applies the secondary transfer voltage to outer secondary transfer roller 132, which may cause the secondary transfer voltage to increase more than expected. If the secondary transfer voltage increases more than expected, abnormal discharge to sheet P may occur, which may result in image defects such as image distortion. On the other hand, if distance X is greater than distance X3, sheet P may rub against belt inclined surface 145a more than necessary, which may result in image defects such as image rubbing.

[0052] In the following, it is assumed that the sheet P is in a pulling state when the sheet conveyance speed by the pair of registration rollers 120 is slower than the sheet conveyance speed by the transfer nip 130. Also, it is assumed that the sheet P is in a pushing state when the sheet conveyance speed by the pair of registration rollers 120 is faster than the sheet conveyance speed by the transfer nip 130. Generally, when the sheet P is in a pulling state, the conveyance resistance of the sheet P increases, and therefore the above-mentioned paper conveyance torque Tq_p increases. On the other hand, when the sheet P is in a pushing state, the conveyance force from the pair of registration rollers 120 is applied to the belt drive motor 133 as torque for driving the intermediate transfer belt 145, and therefore the paper conveyance torque Tq_p decreases.

[0053] Note that, compared to sheets with low stiffness, sheets with high stiffness, such as cardboard, experience a higher reaction force against the conveying force from the pair of registration rollers 120 and the transfer nip 130, as well as a higher reaction force when a loop is formed in the sheet. Therefore, a sheet with high stiffness is less likely to change its posture and the fluctuation range of the distance X is also small, even if the conveying speed of the sheet P at the pair of registration rollers 120 changes relative to the nominal conveying speed due to the intersection of the outer diameter dimensions of the pair of registration rollers 120. In other words, whether the sheet is in a pulled state or a pushed state, the sensitivity of the paper conveying torque Tq_p to the difference in sheet conveying speed between the pair of registration rollers 120 and the transfer nip 130 remains the same.

[0054] On the other hand, a sheet with low stiffness, such as thin paper, is prone to changes in sheet posture due to the intersection of the outer diameter dimensions of the registration roller pair 120. In particular, when the sheet is in a pushed-in state, the sensitivity of the paper conveyance torque Tq_p to the difference in sheet conveyance speed between the registration roller pair 120 and the transfer nip 130 decreases. This is because the registration roller pair 120 pushes the sheet, forming a loop in the sheet, and the sheet conveyance force of the registration roller pair 120 is transmitted to the belt drive motor 133 in an attenuated state.

[0055] For this reason, it is necessary to maintain the distance X within a certain range, especially for sheets with relatively low stiffness, to prevent image defects from occurring. The ideal range of the distance X is determined by factors such as the material of the intermediate transfer belt 145 and the voltage value applied to the sheet.

[0056] Therefore, in this embodiment, the control unit 210 performs the transport control shown in FIG. 6 so that the posture of the sheet upstream of the transfer nip 130 is stabilized to the extent that no image defects occur, even for a sheet with low rigidity. [Register speed control]

[0057] Next, registration speed control as a conveyance control in which the control unit 210 controls the rotation speed of the registration roller pair 120 will be described with reference to Figures 6 and 7. Figures 6 and 7 are flowcharts showing print control including the registration speed control. In the present embodiment, the registration speed control is performed in section B shown in Figure 4, that is, from time t0 to time t4, but is not limited to this. In other words, the registration speed control only needs to be performed during at least a part of the period between time t0, when the leading edge of the sheet P reaches the transfer nip 130, and time t4, when the trailing edge of the sheet P passes through the pre-registration roller pair 110.

[0058] The following describes an example in which an operator causes image forming apparatus 100 to execute a print job from operation unit 220. Note that Figures 6 and 7 show print control that focuses on registration speed control in this embodiment, and omit many other operations that are normally required to execute a job and output an image.

[0059] 6, the operator inputs information such as the size and basis weight of the sheet S to be used from the operation unit 220 of the image forming apparatus 100 (S1). After that, the operator executes a print job from the operation unit 220 (S2). The job information sent to the control unit 210 here includes information on the attributes of the sheet P. In this embodiment, the information on the attributes of the sheet P includes at least information on the basis weight and size of the sheet P.

[0060] When a print job is executed, the control unit 210 retrieves from the memory 212 the registration speed correction value v1(n-1) for the sheet immediately preceding the sheet that is the target of registration speed control, and the target torque Tloop, which will be described later (S3). The sheet that is the target of registration speed control is the nth sheet, and the sheet immediately preceding the nth sheet is the (n-1)th sheet. The memory 212 stores a table showing information related to sheet attributes and the corresponding target torque Tloop. The control unit 210 also sets predetermined printing operation conditions for each attribute of the sheet P.

[0061] Next, the control unit 210 starts rotating the pair of registration rollers 120 after the skew of the sheet P has been corrected by the pair of registration rollers 120 (S4). At this time, which is time t1 in FIG. 4, the pair of registration rollers 120 rotates at a speed V1+v1(n-1). Note that the speed V1 is the nominal rotation speed of the pair of registration rollers 120.

[0062] Next, the control unit 210 drives the belt drive motor 133 so that the intermediate transfer belt 145 moves at a speed VPS (S5). The control unit 210 also starts obtaining the torque Tq of the belt drive motor 133 from the torque sensor 134 (S6).

[0063] Next, the control unit 210 determines whether the time has come to t0-Δt (S7). If it is determined that the time has come to t0-Δt (S7: Yes), the control unit 210 acquires the idling torque value Tq_ST when the sheet P is not sandwiched in the transfer nip 130 (S8). Then, at time t0, the sheet P reaches the transfer nip 130 (S9).

[0064] 7, the control unit 210 determines whether time t2 has arrived (S10). At time t2, the leading edge of the sheet P reaches the fixing nip 157. If it is determined that time t2 has arrived (S10: Yes), the control unit 210 executes registration speed control to control the registration speed, which is the rotation speed of the registration roller pair 120, using the target torque Tloop called in step S3, the idling torque value Tq_ST acquired in step S8, and the torque Tq acquired in step S11.

[0065] First, the paper conveying torque Tq_p is calculated from the idling torque value Tq_ST and the torque Tq detected by the torque sensor 134. The paper conveying torque Tq_p as the conveying torque is calculated by subtracting the idling torque value Tq_ST from the torque Tq. The torque Tq is a first torque of the belt driving motor 133 when a sheet is sandwiched by the transfer nip 130. The idling torque value Tq_ST is a second torque of the belt driving motor 133 when a sheet is not sandwiched by the transfer nip 130.

[0066] Here, the target torque Tloop will be described. The target torque Tloop is an ideal value of the paper conveyance torque Tq_p in section B of FIG. 4 in the registration speed control of this embodiment. The target torque Tloop is a positive value that causes the sheet P to be in a state of tension between the pair of registration rollers 120 and the transfer nip 130. In other words, the target torque is the torque of the belt drive motor 133 that is set so that the sheet conveyance speed (first conveyance speed) by the transfer nip 130 is faster than the sheet conveyance speed (second conveyance speed) by the pair of registration rollers 120. However, the target torque Tloop is set to a value that causes the sheet P to be in a state of tension to the extent that it is not damaged, and different values ​​can be set for different types and sizes of sheets.

[0067] Then, the control unit 210 determines whether the paper transport torque Tq_p is greater than the target torque Tloop (S11). If the paper transport torque Tq_p is greater than the target torque Tloop (S11: Yes), the control unit 210 reduces the rotational speed of the registration roller pair 120 by, for example, a predetermined value f (S12). On the other hand, if the paper transport torque Tq_p is equal to or less than the target torque Tloop (S11: Yes), the control unit 210 increases the rotational speed of the registration roller pair 120 by the value f (S13). Note that the value f used to increase or decrease the registration speed does not necessarily have to be constant, and the value f1 used to decrease the registration speed and the value f2 used to increase the registration speed may be different from each other.

[0068] Next, the control unit 210 determines whether time t4 has arrived (S14). If it is determined that time t4 has not arrived (S14: No), the process returns to step S11. That is, until time t4 arrives, the paper conveying torque Tq_p is compared with the target torque Tloop, and the registration speed, which is the rotation speed of the registration roller pair 120, is feedback-controlled. As a result, the sheet P is controlled so as to be in a tensioned state between the registration roller pair 120 and the transfer nip 130.

[0069] As the sheet P is pulled, the non-image side of the sheet P (the side opposite to the side onto which the toner image is transferred) contacts the convex portion 301a of the guide member 301. Therefore, the sheet P is supported at three points: the pair of registration rollers 120, the convex portion 301a, and the transfer nip 130. In other words, the registration speed is controlled so that the sheet P is supported at three points: the pair of registration rollers 120, the convex portion 301a, and the transfer nip 130. That is, in the registration speed control, the rotation speed of the pair of registration rollers 120 is controlled based on the detection result of the torque sensor 134 so that the first conveyance speed of the sheet by the transfer nip 130 is faster than the second conveyance speed of the sheet by the pair of registration rollers 120. At this time, the convex portion 301a is positioned so that the distance X at which the sheet P and the belt inclined surface 145a of the intermediate transfer belt 145 make rubbing contact with each other is within the range of distances X2 to X3.

[0070] In this embodiment, if the convex portion 301a is positioned in the area AR between the imaginary line L1 passing through the nip 120a of the registration roller pair 120 and the transfer nip 130, and the imaginary line L2 extending along the belt inclined surface 145a, the distance X is configured to be in the range of distances X2 to X3.

[0071] 8(a) and 8(b), the guide member 401 may have a protrusion 401a, and the protrusion 401a may be arranged so that the distance X at which the sheet P rubs against the belt slope 145a of the intermediate transfer belt 145 is an ideal distance X1. At this time, between the protrusion 401a and the transfer nip 130, the sheet P is in a position that is approximately in line with the position P1.

[0072] Returning to the explanation of Fig. 6, if it is determined that time t4 has arrived (S14: Yes), the control unit 210 stores the registration speed correction value v1(n) in the memory 212. The registration speed correction value v1(n) is calculated, for example, by the following equation (1). v1(n)=(-f·m1+f·m2) / (m1+m2) ···(1) m1: The number of times the register speed is decelerated in step S12 during the registration speed control m2: The number of times the speed is increased in step S13 during the registration speed control

[0073] The registration speed correction value v1(n) is called from the memory 212 by the control unit 210 in step S3 when the next sheet, i.e., the (n+1)th sheet, is conveyed. Then, the control unit 210 determines whether all the number of prints specified in the print job have been printed (S16). If it is determined that all the number of prints specified in the print job have not been printed (S16: No), the process returns to step S3.

[0074] If it is determined that all the sheets specified in the print job have been printed (S16: Yes), the control unit 210 stops the rotation of each roller (S17) and ends the print job (S18). This ends the print control.

[0075] As described above, in this embodiment, the target torque Tloop is set so that the sheet P is in a state of tension between the pair of registration rollers 120 and the transfer nip 130. Therefore, the sheet P is supported at three points, namely, the pair of registration rollers 120, the convex portion 301a, and the transfer nip 130, and the posture of the sheet conveyed to the transfer nip 130 is stabilized.

[0076] Furthermore, even for sheets with relatively low stiffness, such as thin paper, by tensioning the sheet, registration speed control can be performed within a range in which the paper conveyance torque Tq_p is highly sensitive to the difference in sheet conveyance speed between the pair of registration rollers 120 and the transfer nip 130. This allows for good responsiveness of the increase or decrease in registration speed to the sheet's posture, and the sheet can be conveyed in an appropriate posture. This allows the distance X between the sheet P and the belt slope 145a of the intermediate transfer belt 145 to be kept within an appropriate range in which image defects do not occur. This makes it possible to suppress image defects.

[0077] Furthermore, when the sheet P is in a position where it slides on the convex portion 301a of the guide member 301, the distance X is within an appropriate range where image defects do not occur, thereby suppressing image defects. Also, since the image forming apparatus 100 can be manufactured while allowing for the outer diameter dimensions of the registration roller pair 120 to be crossed, it is possible to suppress image defects while reducing costs. Furthermore, since image defects can be suppressed even with thin paper with low stiffness, the number of compatible media can be increased, improving usability.

[0078] <Other embodiments> In the above embodiment, a current detection unit that measures the current value of the belt drive motor 133 is used as the torque sensor 134, but the present invention is not limited to this. For example, a strain gauge that detects the torque of the belt drive motor 133, a capacitance torque sensor, or the like may be used.

[0079] Furthermore, the torque sensor 134 may detect the torque of any member on the drive transmission path between the belt drive motor 133 and the drive roller 131 instead of the torque of the belt drive motor 133 .

[0080] In the above-described embodiment, the rotational speed of the pair of registration rollers 120 is controlled based on the detection result of the torque sensor 134 in the registration speed control so that the first conveyance speed of the sheet by the transfer nip 130 is faster than the second conveyance speed of the sheet by the pair of registration rollers 120. However, the present invention is not limited to this. That is, the torque (reference torque) detected by the torque sensor 134 when the sheet is sandwiched and conveyed only by the transfer nip 130 may be stored in advance in the memory 212. Then, in the registration drive control, the rotational speed of the pair of registration rollers 120 may be controlled so that the torque detected by the torque sensor 134 when the sheet is sandwiched between the pair of registration rollers 120 and the transfer nip 130 is greater than the reference torque.

[0081] In the above embodiment, the registration roller pair 120 is driven by the registration drive motor 121 separate from the belt drive motor 133, but this is not limited to this. For example, instead of the registration drive motor 121, a speed change mechanism may be used that can change the rotation speed of the registration roller pair 120 by changing the speed of the drive from the belt drive motor 133 and transmitting it to the registration roller pair 120.

[0082] In the above-described embodiment, the registration speed control is performed regardless of the sheet attribute, but the present invention is not limited to this. For example, the registration speed control may be configured not to be performed on a sheet having a first stiffness (e.g., cardboard), but to be performed on a sheet having a second stiffness (e.g., thin paper) that is smaller than the first stiffness.

[0083] The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that realizes one or more functions. [Explanation of symbols]

[0084] 100: Image forming apparatus / 120: Pair of conveying rollers (pair of registration rollers) / 121: Conveying motor (registration drive motor) / 132: Outer roller (secondary transfer outer roller) / 133: Belt drive motor / 134: Torque sensor, current detection unit / 141: Image carrier (photosensitive drum) / 145: Belt (intermediate transfer belt) / 145a: / 145b: Outer surface / 145c: Inner surface / 147: Tension roller / 301, 401: Guide member / 301a: Convex portion A: Rotation direction (direction) / L1, L2: Virtual line / Tq: Torque, first torque / Tq_p: Transport torque (paper transport torque) / Tq_ST: Second torque (idle rotation torque)

Claims

1. an image carrier that carries a toner image; a belt onto which a toner image is transferred from the image carrier while rotating in a predetermined rotation direction; an outer roller that contacts the outer peripheral surface of the belt; an inner roller disposed opposite the outer roller with the belt interposed therebetween, in contact with the inner circumferential surface of the belt, and forming a transfer nip together with the outer roller; a tension roller that contacts the inner circumferential surface of the belt and is disposed upstream of the inner roller in the rotation direction; a pair of conveying rollers that convey a sheet toward the transfer nip; a guide member that guides the sheet conveyed by the conveying roller pair toward the transfer nip, the guide member being disposed between an imaginary line passing through the nip of the conveying roller pair and the transfer nip and an imaginary line extending along the outer peripheral surface of the belt stretched between the tension roller and the inner roller, and having a protrusion that protrudes toward the outer peripheral surface of the belt; a belt drive motor for driving the belt; a torque sensor that detects the torque of the belt drive motor; a control unit that executes conveyance control to control the rotation speed of the conveyance roller pair based on a detection result of the torque sensor after the sheet reaches the transfer nip so that a first conveyance speed of the sheet by the transfer nip is faster than a second conveyance speed of the sheet by the conveyance roller pair, An image forming apparatus characterized by:

2. the convex portion of the guide member is configured to be in frictional contact with the sheet sandwiched between the pair of conveying rollers and the transfer nip when the conveying control is being executed; 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. the control unit acquires a target torque of the belt drive motor, which is set so that the first conveying speed is faster than the second conveying speed, and a conveying torque obtained by subtracting a second torque of the belt drive motor when the sheet is not sandwiched by the transfer nip from the first torque of the belt drive motor when the sheet is sandwiched by the transfer nip, and in the conveying control, controls the rotation speed of the conveying roller pair so that the second conveying speed is decelerated when the conveying torque is larger than the target torque, and controls the rotation speed of the conveying roller pair so that the second conveying speed is increased when the conveying torque is smaller than the target torque.

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

4. the target torque is set in accordance with the attributes of the sheet to be conveyed; 4. The image forming apparatus according to claim 3, wherein the image forming apparatus is a recording medium.

5. the torque sensor includes a current detection unit that detects a current flowing through the belt drive motor; 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

6. the control unit executes the conveyance control during at least a part of a period from when the sheet reaches the transfer nip until when the sheet passes through the pair of conveyance rollers.

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

7. a conveying motor for driving the pair of conveying rollers; the control unit controls the conveying motor to control the rotation speed of the pair of conveying rollers.

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

8. an image carrier that carries a toner image; a belt onto which a toner image is transferred from the image carrier while rotating in a predetermined rotation direction; an outer roller that contacts the outer peripheral surface of the belt; an inner roller disposed opposite the outer roller with the belt interposed therebetween, in contact with the inner circumferential surface of the belt, and forming a transfer nip together with the outer roller; a tension roller that contacts the inner circumferential surface of the belt and is disposed upstream of the inner roller in the rotation direction; a pair of conveying rollers that convey a sheet toward the transfer nip; a guide member that guides the sheet conveyed by the conveying roller pair toward the transfer nip, the guide member being disposed between an imaginary line passing through the nip of the conveying roller pair and the transfer nip and an imaginary line extending along the outer peripheral surface of the belt stretched between the tension roller and the inner roller, and having a protrusion that protrudes toward the outer peripheral surface of the belt; a belt drive motor for driving the belt; a torque sensor that detects the torque of the belt drive motor; a control unit that executes conveyance control to control the rotation speed of the conveyance roller pair so that the torque detected by the torque sensor when the sheet is sandwiched between the conveyance roller pair and the transfer nip is larger than the torque detected by the torque sensor when the sheet is sandwiched and conveyed only by the transfer nip, An image forming apparatus characterized by:

Images