Sheet conveying device and image forming apparatus

Abstract

To correct skewing of a sheet with high accuracy.SOLUTION: The image forming apparatus includes a first sensor disposed between the first conveyance roller pair and the registration roller pair in the first conveyance path and configured to detect the leading edge of the sheet, and a second sensor disposed between the registration roller pair and the first sensor and configured to detect the leading edge of the sheet. When the sheet is conveyed from the first conveyance roller pair toward the registration roller pair, the control unit conveys the sheet toward the second sensor in a state where a conveyance speed of the first conveyance roller pair is a first speed in a case where a time from a timing at which the feed unit feeds the sheet until the leading edge of the sheet is detected by the first sensor is a first time, when the time from the timing at which the feeding unit feeds the sheet until the leading edge of the sheet is detected by the first sensor is a second time longer than the first time, the sheet is conveyed toward the second sensor in a state where the conveyance speed of the first conveyance roller pair is a second speed higher than the first speed.SELECTED DRAWING: Figure 6

Description

[Technical Field]

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

[0002] Conventionally, image forming apparatuses are provided with a skew correction mechanism that corrects skew in a conveyed sheet in order to align the sheet with the image formed on the sheet. The skew correction mechanism is a mechanism that corrects the posture of a conveyed sheet when it is skewed. A known skew correction mechanism includes a pair of registration rollers and a pair of pre-registration rollers arranged upstream of the pair of registration rollers in the sheet conveyance direction (see Patent Document 1). The pair of registration rollers will hereinafter be referred to as a registration roller pair, and the pair of pre-registration rollers will hereinafter be referred to as a pre-registration roller pair.

[0003] In this skew correction mechanism, the leading edge of the sheet conveyed by the pre-registration roller pair is detected by a registration sensor, and then the leading edge of the sheet is abutted against the stopped registration roller pair, and a loop of a predetermined loop amount is formed to correct the skew of the leading edge of the sheet. After the skew has been corrected, the registration roller pair and the pre-registration roller pair start conveying the sheet at the same speed as the registration roller pair, which are driven in synchronization with an image signal, and the sheet is conveyed to the secondary transfer unit in the image forming unit. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-127753 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the sheet conveying device described in Patent Document 1, the sheet conveying speed in the conveying path upstream of the pair of registration rollers may vary due to the sheet stiffness, the surface properties of the sheet, and the sheet conveying resistance caused by the cross-sectional shape of the conveying path. Such variations cause variations in the timing at which the leading edge of the sheet reaches the pair of registration rollers, resulting in a problem of reduced accuracy in correcting the skew of the sheet and the leading edge position.

[0006] For example, if the sheet conveyance speed is faster than expected, after conveying the preceding sheet, the following sheet may reach the pair of registration rollers before the pair of registration rollers has completely stopped, and the leading edge of the sheet may pass through the nip of the pair of registration rollers before skew correction of the following sheet can begin. On the other hand, if the sheet conveyance speed is slower than expected, the leading edge may not reach the pair of registration rollers in time, and the specified loop amount may not be formed. As a result, the leading edge of the sheet may be conveyed to the secondary transfer unit with its leading edge positioned at an angle, which may result in an image being formed at an angle on the sheet and resulting in poor image positioning. In particular, when the productivity of an image forming apparatus (number of printed sheets per unit time) is high, the conveyance interval between sheets and the sheet waiting time at the pair of registration rollers are often shortened, making these issues more likely to occur.

[0007] SUMMARY OF THE INVENTION An object of the present invention is to provide a sheet conveying device and an image forming apparatus that are capable of correcting skew of a sheet with high accuracy. [Means for solving the problem]

[0008] A first aspect of the present invention includes a feeding section that feeds a sheet from a sheet storage section that stores sheets, a first transport path that transports the sheet fed by the feeding section in a sheet transport direction, a registration roller pair that is disposed on the first transport path and contacts a leading edge of the sheet in the sheet transport direction to correct skew of the sheet and then clamps and transports the sheet, a first transport roller pair that is disposed on the first transport path and transports the sheet to the registration roller pair, a first sensor that is disposed between the first transport roller pair and the registration roller pair on the first transport path and detects the leading edge of the sheet, a second sensor that is disposed between the registration roller pair and the first sensor and detects the leading edge of the sheet, and a control section that controls transport of the sheet, wherein the control section is configured to detect the second sensor. and controls the pair of registration rollers and the first pair of conveying rollers to correct skew of the sheet based on the detection result of the sensor, and when the sheet is conveyed from the first pair of conveying rollers to the pair of registration rollers, if the time from when the feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is a first time, the control unit conveys the sheet toward the second sensor at a conveying speed by the first pair of conveying rollers of a first speed, and if the time from when the feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is a second time that is longer than the first time, the control unit conveys the sheet toward the second sensor at a conveying speed by the first pair of conveying rollers of a second speed that is greater than the first speed.

[0009] A second aspect of the present invention is an image forming apparatus including the above-mentioned sheet conveying device and an image forming section that forms an image on a sheet. [Effects of the Invention]

[0010] According to the present invention, it is possible to achieve highly accurate correction of skew of a sheet. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing an image forming apparatus according to a first embodiment. [Figure 2] FIG. 2 is a block diagram showing a control system of the image forming apparatus according to the first embodiment. [Figure 3] FIG. 2 is a cross-sectional view showing a skew correction unit according to the first embodiment. [Figure 4] 1A and 1B are plan views showing the skew correction unit according to the first embodiment, where (a) shows when the leading edge of the sheet is detected by the first sensor, (b) shows when the leading edge of the sheet is detected by the second sensor, and (c) shows when a loop is formed in the sheet. [Figure 5] This is a time chart showing the control when skew correction is performed by the image forming apparatus of the first embodiment, where (a) shows the sheet position, (b) shows the pre-registration roller speed, and (c) shows the change in the registration roller speed when the sheet arrives at the reference time. [Figure 6] A time chart showing the control when skew correction is performed by the image forming apparatus of the first embodiment, where (a) shows the sheet position, (b) shows the pre-registration roller speed, and (c) shows the change in the registration roller speed when the sheet arrives later than the reference time. [Figure 7] This is a time chart showing the control when skew correction is performed by the image forming apparatus of the first embodiment, where (a) shows the sheet position, (b) shows the pre-registration roller speed, and (c) shows the registration roller speed when the sheet arrives earlier than the reference time. [Figure 8] 6 is a flowchart showing a procedure when skew correction is performed by the image forming apparatus according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] First Embodiment The first embodiment will be described below with reference to the drawings. First, the schematic configuration of an image forming apparatus 1 according to the first embodiment will be described with reference to FIG. 1. FIG. 1 is a cross-sectional view of the schematic configuration of the image forming apparatus 1 according to this embodiment, as viewed from the front side. The image forming apparatus 1 is a tandem-intermediate transfer laser beam printer that uses an electrophotographic process. The image forming apparatus 1 can form and output a full-color or monochrome image corresponding to print image data output from a host device (not shown) connected to a control unit 20 on a sheet S, which is a recording medium.

[0013] [Image forming equipment] The image forming apparatus 1 has an apparatus main body 201A and an image forming section 201B that forms an image on a sheet S. An image reading device 202 that is installed substantially horizontally is provided above the apparatus main body 201A. A discharge space V for sheet discharge is formed between the image reading device 202 and the apparatus main body 201A. In this embodiment, the image forming apparatus 1 is also an example of a sheet conveying device.

[0014] [Feeding unit] The sheets S are stored in feeding cassettes 7A and 7B of cassette feeding units 9A and 9B according to size, for example, or are stacked and supported on a manual feed tray 51 of a manual feed unit 50.

[0015] The cassette feeding units 9A, 9B have feeding cassettes 7A, 7B that store sheets S, and feeding sections 6A, 6B that feed sheets S from the feeding cassettes 7A, 7B. The feeding cassettes 7A, 7B are housed in the apparatus main body 201A and are an example of a sheet storage section that stores sheets S. The feeding sections 6A, 6B are an example of a first feeding section that feeds sheets S stored in the feeding cassettes 7A, 7B to a first pre-registration roller pair 11.

[0016] Although cassette feeding units 9A and 9B accommodate sheets S of different sizes, feeding sections 6A and 6B have the same configuration, so feeding section 6A will be described here. Feeding section 6A has a pickup roller 2 and a separation roller pair 5 consisting of a feed roller 3 (see FIG. 3) and a retard roller 4 (see FIG. 3) for separating sheets S fed from pickup roller 2. Sheets S separated one by one by separation roller pair 5 are conveyed by a first pre-registration roller pair (hereinafter referred to as the first pre-registration roller pair) 11 to a registration roller pair (hereinafter referred to as the registration roller pair) 13. That is, feeding sections 6A and 6B feed sheets S accommodated in feeding cassettes 7A and 7B to first pre-registration roller pair 11.

[0017] To record an image in an appropriate area on the sheet S, the control unit 20 must recognize the size of the stored sheet S. The feed cassettes 7A and 7B are provided with sheet size detection units 8A and 8B. The sheet size detection units 8A and 8B include a rotatable size detection lever that slides in contact with and moves with a side restriction plate that restricts the widthwise position of the sheet S, and multiple sensors or switches. The multiple sensors or switches are provided in positions corresponding to the size detection levers in the loading section where the feed cassettes 7A and 7B are installed. Therefore, when the side restriction plate is moved to match the side edge of the sheet S, the size detection lever rotates in tandem. When the feed cassettes 7A and 7B are installed in the loading section in this state, the size detection lever selectively turns on and off the detection elements of the sensors or switches located in the loading section where the feed cassettes 7A and 7B are installed. This causes signals of different patterns to be sent from the sensors or switches to the control unit 20. Based on the signals, the control unit 20 can recognize the size of the sheets S stored in the feed cassettes 7A and 7B.

[0018] The manual feed unit 50 can be opened and closed by rotating relative to the apparatus main body 201A, can be displaced between a closed position and an open position, and is detachably provided on the apparatus main body 201A. The manual feed unit 50 has a manual feed tray 51 that supports the sheets S, and a feed section 56 that feeds the sheets S from the manual feed tray 51. The manual feed tray 51 supports the sheets S stacked on its upper surface when it is open. The feed section 56 is an example of a second feed section that feeds the sheets S supported by the manual feed tray 51 to the second pre-registration roller pair 12.

[0019] The feeding section 56 has a pickup roller 52 and a separation roller pair 55 consisting of a feed roller 53 (see FIG. 3) and a retard roller 54 (see FIG. 3) for separating the sheets S fed from the pickup roller 52. The sheets S separated one by one by the separation roller pair 55 are conveyed to a registration roller pair 13 by a second pre-registration roller pair (hereinafter referred to as the second pre-registration roller pair) 12. That is, the feeding section 56 feeds the sheets S supported on the manual feed tray 51 to the second pre-registration roller pair 12. In this embodiment, the registration roller pair 13 is a conveyance roller arranged next to the first pre-registration roller pair 11 in the sheet conveyance direction.

[0020] [Image forming section] The image forming unit 201B forms an image on a sheet S fed by a cassette feeding unit 230 or a manual feeding unit 50 and conveyed by a pair of registration rollers 13. The image forming unit 201B includes four process cartridges PY, PM, PC, and PK that form toner images in four colors: yellow (Y), magenta (M), cyan (C), and black (K), an exposure unit 30, and an intermediate transfer belt 36. The four process cartridges PY, PM, PC, and PK have the same configuration except for the colors of the images they form. Therefore, only the configuration and image forming process of the process cartridge PY will be described, and a description of the process cartridges PM, PC, and PK will be omitted. The image forming unit 201B also includes a secondary transfer unit 38 and a fixing unit 201E disposed above the process cartridges PY, PM, PC, and PK. A toner cartridge 35 supplies toner to a developing roller 34.

[0021] The process cartridge PY includes a photosensitive drum 32, which is a drum-shaped electrophotographic photosensitive member, a charging roller 33, and a developing roller 34. The photosensitive drum 32 is rotatably supported by the image forming apparatus 1 and is rotationally driven by a driving unit (not shown). The photosensitive drum 32 is configured by providing a photoconductive layer such as an OPC (organic photoconductor) on the outer surface of an aluminum cylinder. The charging roller 33 charges the photosensitive drum 32. The charging roller 33 is configured by a core metal and a conductive elastic member surrounding the periphery. The charging roller 33 is disposed in contact with the surface of the photosensitive drum 32 and rotates in response to the rotation, and a charging bias is applied to it by a power source (not shown). The developing roller 34 deposits toner of a different color (yellow, magenta, cyan, or black) onto the surface of the photosensitive drum 32 for each process cartridge PY, PM, PC, or PK. The developing roller 34 develops the electrostatic latent image on the surface of the photosensitive drum 32 by attaching toner to the electrostatic latent image, thereby forming a toner image on the surface of the photosensitive drum 32 .

[0022] The exposure unit 30 houses within its housing a laser light source that emits laser light and various optical members for deflection scanning by guiding the laser light emitted from the laser light source to the corresponding photosensitive drum 32. The exposure unit 30 irradiates the charged surface of the photosensitive drum 32 with laser light based on image information, forming an electrostatic latent image.

[0023] The secondary transfer unit 38 includes an intermediate transfer belt 36 wound around a drive roller 36a and a tension roller 36b. A primary transfer roller 39 is provided inside the intermediate transfer belt 36 and abuts against the intermediate transfer belt 36 at a position facing the photosensitive drum 32. The intermediate transfer belt 36 is rotated in the direction of the arrow by the drive roller 36a, which is driven by a drive unit (not shown). A secondary transfer roller 37 that transfers the color image formed on the intermediate transfer belt 36 onto the sheet S is provided at a position facing the drive roller 36a of the secondary transfer unit 38.

[0024] A fixing unit 201E is disposed above the secondary transfer roller 37, and above this fixing unit 201E, a first pair of discharge rollers 225a, a second pair of discharge rollers 225b, and a duplex inverting unit 201F are disposed. The duplex inverting unit 201F is an example of a re-conveying unit, and inverts the sheet S on which an image has been formed by the image forming unit 201B, conveys it upside down, and conveys it to the second pair of pre-registration rollers 12. The duplex inverting unit 201F has a pair of reversible reversing rollers 222 and a re-conveying path 224 that conveys the sheet S on which an image has been formed on its first side back to the image forming unit 201B. An operation unit 23 that accepts user operations is provided on the top of the image forming apparatus 1. The operation unit 23 is a touch panel type that combines display and input functions. The operation unit 23 is connected to the control unit 20 and accepts various user operations, such as various information related to the sheet to be used for printing (e.g., size information, basis weight information, surface property information), and instructions to start or stop printing.

[0025] [Control system] Next, the control unit 20 and control system of the image forming apparatus 1 will be described with reference to Fig. 2. The control unit 20 has various functional units, such as a CPU 21 (Central Processing Unit), a memory 22, an image formation control unit 25, a sheet conveyance control unit 26, and a sensor control unit 27. The CPU 21 executes predetermined control programs and the like to realize various processes performed by the image forming apparatus 1. The memory 22 is, for example, a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores various programs and various data in predetermined storage areas.

[0026] The image formation control unit 25 issues instructions to the image forming unit 201B, including the exposure unit 30, to control image formation. The sheet transport control unit 26 issues instructions to the first pre-registration motor 101, the second pre-registration motor 102, the registration motor 103, etc., and controls the drive of various transport rollers to control the transport of the sheet S. The sensor control unit 27 controls the start or stop of detection by the first sensor S1, the second sensor S2, etc., and receives the detection results of these sensors. It is also possible to configure the computer 24, connected via a network, to receive various information related to the sheet used for printing. The control unit 20 controls the first pre-registration motor 101 to adjust the rotational speed of the first pre-registration roller pair 11, thereby changing the transport speed of the sheet S. The control unit 20 also controls the second pre-registration motor 102 to adjust the rotational speed of the second pre-registration roller pair 12, thereby changing the transport speed of the sheet S.

[0027] [Image formation operation] Next, we will explain the image forming operation of the image forming apparatus 1. First, when the image forming apparatus 1 receives image data of an original to be printed, the image information is processed, converted into an electrical signal, and transmitted to the exposure unit 30 of the image forming section 201B. In the image forming section 201B, a laser is used to sequentially expose the surface of the photosensitive drum 32, the surface of which is uniformly charged to a predetermined polarity and potential by the charging roller 33. As a result, electrostatic latent images of yellow, magenta, cyan, and black are sequentially formed on the photosensitive drum 32 of each process cartridge PY, PM, PC, and PK, respectively.

[0028] Thereafter, the electrostatic latent images are developed and visualized with toner of each color, and the toner images of each color on each photosensitive drum 32 are transferred in order onto the intermediate transfer belt 36 in a superimposed manner by a primary transfer bias applied to the primary transfer roller 39. As a result, a toner image is formed on the intermediate transfer belt 36.

[0029] In parallel with this toner image formation operation, sheets S are fed one by one from cassette feeding units 9A, 9B or manual feeding unit 50. The sheets S fed from cassette feeding units 9A, 9B pass through a first pre-registration roller pair 11 and are transported to a registration roller pair 13. The first pre-registration roller pair 11 and the registration roller pair 13 correct any skew of the sheets S. Specifically, the leading edge of the sheet S being transported is abutted against the nip portion of the registration roller pair 13, which is stopped. The first pre-registration roller pair 11 further transports the sheet S to form a loop, thereby correcting the skew. Similarly, the sheet S fed from manual feeding unit 50 passes through a second pre-registration roller pair 12 and is transported to the registration roller pair 13. The second pre-registration roller pair 12 and the registration roller pair 13 correct any skew of the sheet S. In this embodiment, the first pre-registration roller pair 11, the second pre-registration roller pair 12, the registration roller pair 13, the first sensor S1, and the second sensor S2 are collectively referred to as the skew correction unit 10. Detailed control during loop formation in the skew correction unit 10 will be described later.

[0030] The pair of registration rollers 13 transports the sheet S to the secondary transfer unit 38 in accordance with the timing at which the toner image on the intermediate transfer belt 36 is transferred onto the sheet S. In the secondary transfer unit 38, the toner image is transferred onto the sheet S all at once by a secondary transfer bias applied to the secondary transfer roller 37.

[0031] Next, the sheet S onto which the toner image has been transferred is transported to a fixing unit 201E, where the toners of each color are melted and mixed by heat and pressure in a roller nip formed by a pressure roller 220a and a heating roller 220b, and are fixed as a color image on the sheet S. The sheet S with the fixed image is discharged into a discharge space V by a first discharge roller pair 225a or a second discharge roller pair 225b provided downstream of the fixing unit 201E, and is stacked on a discharge tray 223 protruding from the bottom surface of the discharge space V.

[0032] On the other hand, when forming images on both sides of the sheet S, the sheet S is stopped once with the trailing edge of the sheet S remaining a predetermined distance from the pair of reversing rollers 222, and then the pair of reversing rollers 222 is rotated in the reverse direction, and the sheet S passes through the second pair of pre-registration rollers 12 via a re-conveyance path 224. Then, the sheet S is conveyed again to the pair of registration rollers 13, where skew correction is performed, and the image on the second side is formed and fixed in the image forming section 201B.

[0033] [Skew correction section] Next, the skew correction unit 10 will be described with reference to FIG. 3. FIG. 3 is a cross-sectional view of the skew correction unit 10 as viewed from the front side. The first pre-registration roller pair 11 is disposed downstream in the sheet conveying direction of the feeding units 6A and 6B of the cassette feeding units 9A and 9B. The first pre-registration roller pair 11 includes a driven roller 11a having a roller made of polyacetal (POM) and a drive roller 11b formed of a rubber roller disposed opposite the driven roller 11a. The driven roller 11a is pressed against the drive roller 11b by the elastic force of a spring (not shown). The drive roller 11b is driven by a first pre-registration motor 101. The first pre-registration roller pair 11 is an example of a first conveying roller pair that conveys the sheet S to the registration roller pair 13.

[0034] The pair of registration rollers 13 is provided downstream of the first pair of pre-registration rollers 11 in the sheet conveying direction and functions as a contact portion against which the leading edge of the conveyed sheet abuts to correct skew of the sheet S. The pair of registration rollers 13 has a driven roller 13a and a drive roller 13b. The sheet S abuts against the nip portion between the driven roller 13a and the drive roller 13b so that the leading edge of the sheet S fits along the nip portion, thereby correcting skew of the sheet S. That is, the pair of registration rollers 13 abuts against the downstream end of the sheet S in the sheet conveying direction to correct skew of the sheet S, and then sandwiches and conveys the sheet S. The pair of registration rollers 13 has a driven roller 13a having a roller made of polyacetal (POM) and a drive roller 13b formed of a rubber roller disposed opposite the driven roller 13a. The driven roller 13a is pressed against the drive roller 13b by the elastic force of a spring (not shown). The driving roller 13b is driven by a registration motor 103.

[0035] Between the first pre-registration roller pair 11 and the registration roller pair 13, there are arranged a first guide member 14 that guides the sheet S being conveyed, and a second guide member 15 that faces the first guide member 14. The distance between the first guide member 14 and the second guide member 15 is widened particularly in the center in the sheet conveying direction, and a loop forming portion 16 is formed, which is a space where the sheet S that hits the nip portion of the registration roller pair 13 can form a loop. In this embodiment, the path along which the first pre-registration roller pair 11 and the registration roller pair 13 are arranged and which conveys the sheet S in the sheet conveying direction is referred to as a first conveying path P1.

[0036] On the other hand, the sheet S conveyed from the re-conveyance path 224 or the manual feeding unit 50 is conveyed to the registration roller pair 13 via the second pre-registration roller pair 12. At this time, the conveyed sheet S passes through a junction 17 that joins the first conveyance path P1 between the first pre-registration roller pair 11 and the registration roller pair 13. In this embodiment, the second pre-registration roller pair 12 is arranged to convey the sheet S in the sheet conveyance direction, and the path that joins the junction 17 is defined as the second conveyance path P2.

[0037] The second pre-registration roller pair 12 includes a driven roller 12a having a roller made of polyacetal (POM) and a drive roller 12b formed of a rubber roller disposed opposite the driven roller 12a. The driven roller 12a is pressed against the drive roller 12b by the elastic force of a spring (not shown). The drive roller 12b is driven by a second pre-registration motor 102. The second pre-registration roller pair 12 is an example of a second conveyance roller pair disposed on the second conveyance path P2 and conveys the sheet S from the junction 17 to the registration roller pair 13 via the first conveyance path P1.

[0038] A first sensor S1 is disposed near the downstream side of the junction 17 in the sheet conveying direction to detect the leading edge of the sheet S, i.e., the downstream end in the sheet conveying direction. In this embodiment, the first sensor S1 detects the leading edge of the sheet S at a first detection position S1a located between the pair of registration rollers 13 and the junction 17 on the first conveying path P1. A second sensor S2 is disposed downstream of the first sensor S1 in the sheet conveying direction. The second sensor S2 detects the leading edge of the sheet S at a second detection position S2a located between the first detection position S1a and the pair of registration rollers 13. The distance from the first detection position S1a to the second detection position S2a on the first conveying path P1 is defined as a distance L1. The distance from the second detection position S2a to the nip portion of the pair of registration rollers 13 on the first conveying path P1 is defined as a distance L2. The first sensor S1 and the second sensor S2 are used to control sheet conveyance when forming a loop.

[0039] The first sensor S1 is configured to include, for example, a flag that moves when it comes into contact with the leading edge of the sheet S, and an optical sensor that detects the light beam blocked by the rotation of the flag. This allows for relatively low cost. The second sensor is an optical sensor that has a light-emitting element and a light-receiving element. This allows for highly accurate detection, which can realize highly accurate control when using the detection by the second sensor to determine the timing of feeding the next sheet, for example.

[0040] In this embodiment, the first sensor S1 and the second sensor S2 constitute a detection unit S0. In the first conveying path P1, a third sensor S3 that detects the presence or absence of a sheet S is arranged upstream of the first pre-registration roller pair 11. The third sensor S3 detects the leading edge of the sheet S at a first reference position BP1. A fourth sensor S4 that detects the presence or absence of a sheet S is arranged upstream of the second pre-registration roller pair 12 and downstream of the manual feeding unit 50. The fourth sensor S4 detects the leading edge of the sheet S at a second reference position BP2. A fifth sensor S5 that detects the presence or absence of a sheet S is arranged in the re-conveying path 224 upstream of the second pre-registration roller pair 12. The fifth sensor S5 detects the leading edge of the sheet S at a third reference position BP3.

[0041] [Skew correction operation] Next, the skew correction operation of the sheet S by the skew correction unit 10 will be described with reference to FIGS. 4(a) to 4(c). Here, as shown in FIG. 4(a), the skew correction operation will be described when the sheet S conveyed by the first pre-registration roller pair 11 is skewed clockwise in the sheet conveyance direction DF. FIG. 4(a) shows a state in which the leading edge of the sheet S reaches and is detected by the first sensor S1. In this state, when the first pre-registration roller pair 11 is further driven and the sheet S is conveyed in the sheet conveyance direction DF, the leading edge of the sheet S reaches and is detected by the second sensor S2 as shown in FIG. 4(b). When the sheet S is further conveyed thereafter, the leading edge of the sheet S abuts on the nip portion of the registration roller pair 13. At this time, the registration roller pair 13 stops rotating.

[0042] Further, when the first pre-registration roller pair 11 is driven and the sheet S is conveyed in the sheet conveyance direction DF, a corner Sa at the leading edge of the skewed sheet S abuts against the nip portion of the registration roller pair 13 and is stopped, gradually forming a loop 18 in the sheet S. When the first pre-registration roller pair 11 is further driven, the leading edge of the sheet S turns so that a corner Sb opposite to the corner Sa abutting against the nip portion approaches the nip portion. Then, as shown in FIG. 4(c), the entire leading edge of the sheet S in the sheet conveyance direction DF abuts against the nip portion of the registration roller pair 13. At this time, a loop 18 is formed in the sheet S between the registration roller pair 13 and the pre-registration roller pair 41. As a result, the entire leading edge of the sheet S abuts against the nip portion of the registration roller pair 13, correcting the skew.

[0043] Here, the orientation of the arrangement of each part of the skew correction unit 10 will be described with reference to FIGS. 1 and 3. As shown in FIG. 1, in this embodiment, the conveying path from the feeding units 6A and 6B to the secondary transfer unit 38 is configured as a vertical path facing upward. For this reason, as shown in FIG. 3, the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13 are arranged along the vertical direction. As a result, the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13 are arranged approximately parallel to each other, so that the conveying resistance when the sheet is conveyed from the first pre-registration roller pair 11 to the registration roller pair 13 can be kept small.

[0044] The first guide member 14 is disposed between the pair of registration rollers 13 and the junction 17 on the first conveying path P1 and is arranged so as to intersect with the nip line of the pair of registration rollers 13. Therefore, the leading edge of the sheet S conveyed from the first pair of pre-registration rollers 11 is guided along the second guide member 15 and then guided by the first guide member 14 near the pair of registration rollers 13 and abuts against the nip portion of the pair of registration rollers 13. After abutting against the nip portion of the pair of registration rollers 13, the sheet S is further conveyed by the first pair of pre-registration rollers 11 and comes into contact with the first guide member 14. As the sheet S is further conveyed by the first pair of pre-registration rollers 11, it forms a loop 18 toward the second guide member 15 in the loop forming portion 16. That is, the sheet S that abuts against the pair of registration rollers 13, which is in a stopped state, is positioned along the first guide member 14 and then further conveyed to form the loop 18 in a direction away from the first guide member 14.

[0045] On the other hand, the nip line of the second pre-registration roller pair 12 is arranged along the horizontal direction. That is, the nip line of the second pre-registration roller pair 12 is arranged approximately perpendicular to the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13. This reduces conveyance resistance during conveyance of the sheet S supported on the manual feed tray 51. The leading edge of the sheet S conveyed from the second pre-registration roller pair 12 is guided by the first guide member 14 and abuts against the nip portion of the registration roller pair 13. After abutting against the nip portion of the registration roller pair 13, the sheet S is further conveyed by the second pre-registration roller pair 12, thereby coming into contact with the first guide member 14. The sheet S is further conveyed by the second pre-registration roller pair 12, thereby forming a loop 18 in the loop forming section 16 toward the second guide member 15.

[0046] In this embodiment, the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13 are arranged along the vertical direction due to the vertical path configuration, but this is not limited to this. For example, the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13 can be arranged in a vertical path configuration as long as they are inclined at less than 45 degrees with respect to the vertical direction. Also, although the nip line of the second pre-registration roller pair 12 is arranged along the horizontal direction, this is not limited to this and may be inclined at less than 45 degrees with respect to the horizontal direction, for example.

[0047] Although the present embodiment has been described with reference to a case where the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13 are arranged substantially parallel, this is not limiting. For example, even if the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13 are not parallel, the conveyance resistance can be reduced as long as the nip line of the first pre-registration roller pair 11 is inclined at less than 45 degrees relative to the nip line of the registration roller pair 13. Furthermore, the present embodiment has been described with reference to a case where the nip line of the second pre-registration roller pair 12 is arranged substantially perpendicular to the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13, this is not limiting. For example, the conveyance resistance during conveyance of the sheet S supported on the manual feed tray 51 can be reduced as long as the nip line of the second pre-registration roller pair 12 is inclined at more than 45 degrees relative to the nip line of the registration roller pair 13.

[0048] The set value of the loop amount formed by the sheet S when correcting skew of the sheet S is appropriately set based on sheet information including any one of the size, basis weight, type, etc. of the sheet S. The control unit 20 can determine the set value of the optimal loop amount based on at least one of the sheet information specified by the user via the operation unit 23, the sheet information detected by the sheet size detection units 8A and 8B, or a combination of these. After that, when a toner image is formed on the intermediate transfer belt 36 by the image forming unit 201B, the registration roller pair 13 and the first pre-registration roller pair 11 or the second pre-registration roller pair 12 rotate in synchronization with this, and the sheet S is conveyed with the skew corrected.

[0049] [Loop formation control] Next, loop formation control in the skew correction operation of the sheet S by the skew correction unit 10 will be described with reference to Figures 5(a) to 5(c). Figures 5(a) to 5(c) are timing charts showing the sheet position, pre-registration roller speed, and registration roller speed for each basic time series in this embodiment.

[0050] The sheet S conveyed from the feeding units 6A and 6B is conveyed by the first pre-registration roller pair 11 at a set speed V1, passes through the fourth sensor S4, the first sensor S1, and the second sensor S2, and then reaches the registration roller pair 13. After the sheet S has arrived and the loop formation is completed, the control unit 20 stops the first pre-registration roller pair 11. At this time, the registration roller pair 13 stops at the timing t when the trailing edge of the preceding sheet Q, which was fed before the sheet S, leaves the sheet S. Regi_off The target timing is several tens of milliseconds later than Regi_stop It stops at timing t Regi_stop is calculated based on the detection timing of the sheet by the second sensor S2 and the sheet information. reach is t Regi_stopThe sheet S is positioned after the set speed V1, and the sheet S collides with the stopped registration roller pair 13 to form a loop. Furthermore, the set speed V1 is not changed at timing tS1 when the sheet S is detected by the first sensor S1, but is changed to a loop forming speed VL lower than the set speed V1 at timing tS2 when the sheet S is detected by the second sensor S2. This is to reduce collision noise when the sheet S reaches the registration roller pair 13. In this embodiment, the loop forming speed VL is lower than a set speed V2 (described later) which is higher than the set speed V1, and a set speed V3 (described later) which is lower than the set speed V1.

[0051] The amount of loop formed is managed by the time TL from the timing tS2 when the sheet S passes the second sensor S2 to the time when the first pre-registration roller pair 11 is stopped. The time TL is expressed by the following formula: Here, the loop set amount Lt formed in the sheet S between the registration roller pair 13 and the first pre-registration roller pair 11 is determined based on the sheet information described above. TL = (X2 + Lt) ÷ VL

[0052] The timing at which the loop formation is completed after the time TL has elapsed from the timing tS2 is defined as T Loop_fin After the loop formation is completed, the register ON timing t on At this timing, the sheet starts to be conveyed again from the pair of registration rollers 13 toward the secondary transfer portion 38. on is determined by calculating backwards so that the sheet S arrives in time for the leading edge of the image to reach the secondary transfer unit 44.

[0053] As described above, the amount of loop formed is determined by the timing of passing the second sensor S2 disposed near the pair of registration rollers 13. Therefore, the transport distance from detection by the second sensor S2 to the pair of registration rollers 13 is short, so it is possible to reduce variations in the transport distance from detection to the abutting point due to component tolerances and assembly errors, and variations in the amount of loop formed due to variations in the sheet transport speed.

[0054] [Speed ​​control of the pre-registration roller pair] Next, the speed change control of the first pre-registration roller pair 11 and the second pre-registration roller pair 12 will be described. The timing tS1 of passage of the first sensor S1 mentioned above varies. Causes of this variation include variations in the position of the sheet S in the feeding sections 6A, 6B, and 56, and variations in the conveying speed of the pre-registration roller pairs 11 and 12. Furthermore, the timing changes depending on the thickness, basis weight, and surface properties of the sheet S, changes in the diameter and friction coefficient of the pre-registration roller pairs 11 and 12 due to wear caused by continuous paper feed, the pressure of the pre-registration roller pairs 11 and 12, resistance from the conveying guide, and the driving characteristics of the motor.

[0055] 6(a) to 6(c) are timing charts showing the sheet position, pre-registration roller speed, and registration roller speed when tS1 is delayed by a predetermined time or more from the reference timing T. Here, the reference timing T is assumed to be the timing at which the sheet S passes the first sensor S1 if there is no delay or early arrival of the sheet S. In this embodiment, the reference timing T is the timing at which the sheet S passes the first sensor S1 if there is no delay or early arrival of the sheet S after the leading edge of the sheet S passes any one of the third sensor S3, the fourth sensor S4, and the fifth sensor S5.

[0056] In FIG. 6(a), the dashed line indicates the movement of the leading edge of the sheet SX when the speed change control of the pre-registration roller at the first sensor S1 is not performed, and the solid line indicates the actual movement of the sheet S when the speed change control is performed. When a delay occurs, the arrival time t*S2 of the sheet SX to the second sensor S2 is delayed, and as a result, the loop formation completion time t* Loop_fin is the timing when the register is turned on. on On the other hand, by changing the speed from the set speed V1 to a set speed V2, which is higher than V1, at the timing tS1 when the first sensor S1 passes, the delay of tS2 can be reduced, and the registration ON timing t on It is possible to complete the loop formation by

[0057] 7(a) to 7(c) are timing charts showing the sheet position, pre-registration roller speed, and registration roller speed when tS1 is earlier than the reference timing T by a predetermined time or more. At this time, as shown by the dashed line in FIG. 7(a), the arrival time t* of the sheet SX to the pair of registration rollers 13 reach The timing t Regi_stop As a result, the sheet starts to form a loop after the leading edge of the sheet has passed the pair of registration rollers 13, and it becomes impossible to adjust the leading edge position of the sheet and correct the skew. reach Then, the sheet S reaches the pair of registration rollers 13 after the pair of registration rollers 13 has stopped, so that the loop can be formed appropriately.

[0058] Here, a first reference position BP1 where the third sensor S3 is provided is located upstream of the first detection position S1a on the first transport path P1. The control unit 20 determines whether the sheet S has passed the first reference position BP1 based on the detection result of the third sensor S3. The first detection time is defined as the time from when the leading edge of the first sheet S transported along the first transport path P1 by the first pre-registration roller pair 11 passes the first reference position BP1 to when it is detected by the detection unit S0. Here, the first detection time is the time from when the leading edge of the first sheet S passes the first reference position BP1 to when it is detected by the first sensor S1. At this time, when the first detection time is the first time, the control unit 20 adjusts the rotation speed of the first pre-registration roller pair 11 to set the transport speed of the first sheet S to the first speed (see FIG. 5A). In response to this, when the first detection time for the second sheet S is a second time that is longer than the first time, the control unit 20 changes the conveying speed of the second sheet S to a second speed that is faster than the first speed by adjusting the rotational speed of the first pre-registration roller pair 11 (see Figure 6(a)).

[0059] In this embodiment, the transport control during loop formation and the speed change control of the first pre-registration roller pair 11 are described using an example in which the sheet is fed from the feeding units 6A and 6B, but this is not limiting. For example, similar transport control can be applied using the second pre-registration roller pair 12 to sheets S transported from the duplex reversing unit 201F or the manual feeding unit 50. However, parameters such as the set speed V1, reference times t1 and t2, and set speeds V1 and V2 after speed change in the speed change control of each pre-registration roller pair take different values ​​depending on the upstream transport path. This is because the expected variation in the transport timing of the sheet S varies depending on the upstream transport path.

[0060] For example, the second reference position BP2 where the fourth sensor S4 is provided is located upstream of the first detection position S1a. The control unit 20 determines whether the sheet S has passed the second reference position BP2 based on the detection result of the fourth sensor S4. The second detection time is defined as the time from when the leading edge of the third sheet S, conveyed along the second conveyance path P2 by the second pre-registration roller pair 12, passes the second reference position BP2 until it is detected by the detection unit S0. Here, the second detection time is the time from when the leading edge of the third sheet S passes the second reference position BP2 until it is detected by the first sensor S1. In this case, when the second detection time is the third time, the control unit 20 adjusts the rotation speed of the second pre-registration roller pair 12 to set the conveyance speed of the third sheet S to the third speed (see FIG. 5A). In response to this, when the second detection time for the fourth sheet S is the fourth time, which is longer than the third time, the control unit 20 changes the conveying speed of the fourth sheet S to a fourth speed, which is faster than the third speed, by adjusting the rotational speed of the second pre-registration roller pair 12 (see Figure 6(a)).

[0061] Furthermore, a third reference position BP3 where a fifth sensor S5 is provided is located upstream of the first detection position S1a. The control unit 20 determines whether the sheet S has passed the third reference position BP3 based on the detection result of the fifth sensor S5. The third detection time is defined as the time from when the leading edge of the fifth sheet S, conveyed along the second conveyance path P2 by the second pre-registration roller pair 12, passes the third reference position BP3 until it is detected by the detection unit S0. Here, the third detection time is the time from when the leading edge of the fifth sheet S passes the third reference position BP3 until it is detected by the first sensor S1. If the third detection time is the fifth time, the control unit 20 adjusts the rotational speed of the second pre-registration roller pair 12 to set the conveyance speed of the fifth sheet S to the fifth speed (see FIG. 5(a)). In response to this, when the third detection time for the sixth sheet S is the sixth time, which is longer than the fifth time, the control unit 20 changes the conveying speed of the sixth sheet S to a sixth speed, which is faster than the fifth speed, by adjusting the rotational speed of the second pre-registration roller pair 12 (see Figure 6(a)).

[0062] [Operation procedure] Next, the operation procedure of the skew correction unit 10 will be described with reference to the flowchart shown in Fig. 8. After starting an image forming job, the control unit 20 acquires the transport source, i.e., the cassette feeding units 9A and 9B, the re-feeding path 224, or the manual feeding unit 50, from which the sheet S will be transported (S1). After acquiring the transport source, the control unit 20 starts feeding and detects the leading edge of the sheet S with any of the third sensor S3 to the fifth sensor S5 (S2).

[0063] The control unit 20 rotates the first pre-registration roller pair 11 or the second pre-registration roller pair 12 depending on the conveyance source (S3). After the leading edge of the sheet S is detected by the first sensor S1 (S4), the control unit 20 adjusts the conveyance speed of the sheet S at that time (S5). The adjustment method is as described above. The control unit 20 then determines whether the leading edge of the sheet S is detected by the second sensor S2 (S6). If the control unit 20 determines that the leading edge of the sheet S is not detected by the second sensor S2 (S6; NO), it continues detection. If the control unit 20 determines that the leading edge of the sheet S is detected by the second sensor S2 (S6; YES), it sets the loop formation time T2 based on the detection time t0 obtained from the actual speed (S7). That is, the control unit 20 calculates the time from when the leading edge of the sheet S is detected by the first sensor S1 to when the leading edge of the sheet S is detected by the second sensor S2. Then, based on the calculated time, the control unit 20 adjusts the transport amount of the sheet S by the pre-registration roller pair 11 or the second pre-registration roller pair 12 toward the stopped registration roller pair 13, thereby adjusting the loop amount of the sheet S.

[0064] The control unit 20 continues the rotation of the first pre-registration roller pair 11 or the second pre-registration roller pair 12, and brings the leading edge of the sheet S into contact with the nip portion of the stopped registration roller pair 13 (S8). The control unit 20 further continues the rotation of the first pre-registration roller pair 11 or the second pre-registration roller pair 12, thereby forming a loop 18 in the sheet S (S9). The control unit 20 determines whether the loop formation time T2 has elapsed (S10; NO). If the control unit 20 determines that the loop formation time T2 has not elapsed (S10; YES), the control unit 20 continues forming the loop 18 in the sheet S (S9). If the control unit 20 determines that the loop formation time T2 has elapsed (S10; YES), the control unit 20 stops the rotating pre-registration roller pair (S11) and rotates the registration roller pair 13 in accordance with the timing of image formation (S12).

[0065] As described above, the image forming apparatus 1 of this embodiment adjusts the sheet speed using the detection unit S0 disposed between the pair of registration rollers 13 and each pair of pre-registration rollers 11 and 12. Therefore, by optimizing the timing at which the sheet reaches the pair of registration rollers 13, highly accurate correction of skewed sheet movement can be achieved. That is, even if the conveyance timing of the sheet S detected by the first sensor S1 deviates from the expected timing due to variations in the position of the sheet S in the feeders 6A, 6B, and 56 or variations in the conveyance speed, subsequent variations in conveyance timing can be suppressed. This allows a predetermined amount of loop to be formed with the leading edge of the sheet S stably abutting against the pair of registration rollers 13, thereby improving image position accuracy, such as image skew and leading edge position. In particular, to increase the productivity of the image forming apparatus 1 (number of printed sheets per unit time), shortening the conveyance interval between sheets and the sheet waiting time at the pair of registration rollers 13 is effective. According to this embodiment, the timing t of the registration stop is adjusted. Regi_stop From the register ON timing t on Since the interval between the transfers is shortened, the variation in transfer timing can be reduced.

[0066] Furthermore, according to this embodiment, the detection unit S0 is disposed between the pair of registration rollers 13 and the junction 17. Therefore, by performing speed control based on the detection timing of one sensor for the sheet S transported from a plurality of different upstream transport paths, it is possible to improve image printing accuracy. This allows the number of sensors to be reduced compared to when a sensor is disposed on each of the plurality of upstream transport paths, resulting in cost reduction.

[0067] Furthermore, according to this embodiment, the loop amount of the sheet S is adjusted based on the detection time obtained from the actual speed. Therefore, even if the speed of the sheet S changes due to the stiffness, surface properties, sheet conveyance resistance, etc. of the sheet S, the loop amount of the sheet S during skew correction can be optimized. This makes it possible to prevent a decrease in image formation accuracy due to insufficient loop amount and insufficient skew correction, and to prevent an excessive loop amount from causing noise, sheet wrinkles, or increased torque of the pre-registration motor.

[0068] <Second embodiment> In the first embodiment described above, the control unit 20 determines the rotation speed of the first pre-registration roller pair 11 based on the time from when the leading edge of the sheet is detected by the third sensor S3 to when it is detected by the first sensor S1. In the second embodiment, the control unit 20 determines the rotation speed of the first pre-registration roller pair 11 based on the time from when the feeding units 6A, 6B, and 56 feed the sheet to when the leading edge of the sheet is detected by the first sensor S1. Hereinafter, elements that are given the same reference numerals as those in the first embodiment have basically the same configurations and functions as those described in the first embodiment unless otherwise specified, and differences from the first embodiment will be mainly described.

[0069] In the first embodiment described above, the fourth sensor S4 is disposed downstream of the manual feeding unit 50, and the fifth sensor S5 is disposed in the re-conveying path 224. On the other hand, in the second embodiment, the sheet conveying device does not include the fourth sensor S4 and the fifth sensor S5. In addition, in this embodiment, the third sensor S3 is not necessary for determining the rotation speed of the first pre-registration roller pair 11. Therefore, according to this embodiment, the number of required sensors can be reduced.

[0070] In the second embodiment, the target time is the time from when a signal to start feeding is output from the control unit 20 until the leading edge of the sheet reaches the first sensor S1 without any delay or early arrival. In other words, the reference timing T is the timing that is a predetermined target time after the feeding instruction corresponding to each sheet.

[0071] When a sheet is conveyed from the first pre-registration roller pair 11 to the registration roller pair 13, the target time from when a feeding command is given until the leading edge of the sheet reaches the first sensor S1 is defined as a first target time. If the time from when a feeding command is given for the sheet stored in the feeding cassettes 7A and 7B until the leading edge of the sheet is detected by the first sensor S1 is the same as the first target time (first time), the control unit 20 sets the conveying speed of the first pre-registration roller pair 11 to the first speed (see FIG. 5).

[0072] In contrast, when the time from the instruction to feed the sheet to the detection of the leading edge of the sheet by the first sensor S1 is a second time that is longer than the first target time (first time), the control unit 20 sets the conveying speed of the first pre-registration roller pair 11 to a second speed that is greater than the first speed (see FIG. 6).On the other hand, when the time from the instruction to feed the sheet to the detection of the leading edge of the sheet by the first sensor is a third time that is shorter than the first target time (first time), the control unit 20 sets the conveying speed of the first pre-registration roller pair 11 to a fourth speed that is smaller than the first speed (see FIG. 7).

[0073] That is, when the time from when feeding units 6A, 6B feed a sheet until the leading edge of the sheet is detected by first sensor S1 is a first time, the sheet is conveyed toward the second sensor with the conveying speed by first pre-registration roller pair 11 at a first speed. When the time from when feeding units 6A, 6B feed a sheet until the leading edge of the sheet is detected by first sensor S1 is a second time that is longer than the first time, the sheet is conveyed toward the second sensor with the conveying speed by first pre-registration roller pair 11 at a second speed that is higher than the first speed. When the time from when feeding units 6A, 6B feed a sheet until the leading edge of the sheet is detected by first sensor S1 is a third time that is shorter than the first time, the sheet is conveyed toward the second sensor with the conveying speed by first pre-registration roller pair 11 at a fourth speed that is lower than the first speed.

[0074] In other words, when the time from the instruction to feed the sheet to the detection of the leading edge of the sheet by the first sensor S1 is a second time that is longer than the first time, the control unit 20 increases the conveying speed of the first pre-registration roller pair 11 (see FIG. 6). When the time from the instruction to feed the sheet to the detection of the leading edge of the sheet by the first sensor S1 is a third time that is shorter than the first time, the control unit 20 decreases the conveying speed of the first pre-registration roller pair 11 (see FIG. 7).

[0075] In the sheet conveying direction, the first sensor S1 and the second sensor S2 are arranged between the first pre-registration roller pair 11 and the registration roller pair 13. Based on the time from when the control unit 20 issues a feeding command to when the leading edge of the sheet is detected by the first sensor S1, the sheet conveying speed by the first pre-registration roller pair 11 is adjusted so that the leading edge of the sheet reaches the second sensor S2 and the registration roller pair 13 on time. Furthermore, the control unit 20 controls the formation of a loop based on the passing timing of the second sensor S2 arranged near the registration roller pair 13. This makes it possible to optimize the timing at which the sheet reaches the registration roller pair 13 and reduce variation in the amount of loop formation.

[0076] 6A and 6B, when the time from when the feeding units 6A and 6B feed a sheet to when the leading edge of the sheet is detected by the first sensor S1 is a second time, the first pre-registration roller pair 11 conveys the sheet at a third speed lower than the second speed until the leading edge of the sheet reaches the first sensor S1. That is, the control unit 20 conveys the sheet at the third speed until the leading edge of the sheet reaches the first sensor S1, and then increases the speed from the third speed to the second speed after the sheet reaches the first sensor S1. On the other hand, as shown in FIGS. 7A and 7B, when the time from when the feeding units 6A and 6B feed a sheet to when the leading edge of the sheet is detected by the first sensor S1 is a third time, the first pre-registration roller pair 11 conveys the sheet at a fifth speed higher than the fourth speed until the leading edge of the sheet reaches the first sensor S1. That is, the control unit 20 conveys the sheet at the fifth speed until the leading edge of the sheet reaches the first sensor S1, and after the leading edge of the sheet reaches the first sensor S1, the speed is reduced from the fifth speed to the fourth speed and conveyed.

[0077] Next, a case will be described in which a sheet is conveyed from the second pre-registration roller pair 12 to the registration roller pair 13 via the second conveyance path P2 in order to form an image on the second side of the sheet that has been conveyed by the feeding units 6A and 6B via the first pre-registration roller pair 11 and has an image formed on its first side. When forming an image on the second side of the sheet, the target time from when a sheet is instructed to feed until the leading edge of the sheet reaches the first sensor S1 when the sheet is conveyed from the second pre-registration roller pair 12 to the registration roller pair 13 is set as the second target time. Specifically, the second target time is the time from when an instruction to feed the sheet is issued to form an image on the first side of the sheet, the sheet with the image formed on its first side is conveyed to the re-conveyance path 224, and further until the leading edge of the sheet reaches the first sensor S1, assuming there is no delay or early arrival.

[0078] If the time from the issuance of a sheet feed instruction to the detection by the first sensor S1 is the same as the second target time (fourth time), the control unit 20 sets the conveying speed of the second pre-registration roller pair 12 to a sixth speed (see FIG. 5). On the other hand, if the time from the issuance of a sheet feed instruction to the detection by the first sensor S1 of the leading edge of the sheet is a fifth time, which is longer than the second target time (fourth time), the control unit 20 sets the conveying speed of the second pre-registration roller pair 12 to a seventh speed, which is greater than the sixth speed (see FIG. 6). On the other hand, if the time from the issuance of a sheet feed instruction to the detection by the first sensor S1 of the leading edge of the sheet is a sixth time, which is shorter than the second target time (fourth time), the control unit 20 sets the conveying speed of the second pre-registration roller pair 12 to an eighth speed, which is smaller than the sixth speed (see FIG. 7).

[0079] That is, when forming an image on the first side, the control unit 20 determines the conveying speed of the first pre-registration roller pair 11 based on the time from when the instruction to feed the sheet is issued until the leading edge of the sheet is detected by the first sensor S1, and conveys the sheet toward the second sensor S2. When correcting skew of the sheet to form the second side of the sheet with an image formed on its first side, the control unit 20 determines the conveying speed of the second pre-registration roller pair 12 based on the time from when the instruction to feed the sheet is issued until the leading edge of the sheet with an image formed on its first side is detected by the first sensor S1. In this embodiment, the first sensor S1 and the second sensor S2 are disposed between the registration roller pair 13 and the junction 17. This makes it possible to improve image printing accuracy without disposing sensors in the re-feed path 224.

[0080] Next, a case where a sheet is conveyed from the manual feed tray 51 toward the pair of registration rollers 13 via the second pre-registration roller pair 12 will be described. When a sheet is conveyed from the manual feed tray 51 toward the pair of registration rollers 13, the target time until the leading edge of the sheet reaches the first sensor S1 is set as a third target time. The third target time is the time from when a command to feed a sheet stored in the manual feed tray 51 is issued until the leading edge of the sheet reaches the first sensor S1, assuming there is no delay or early arrival.

[0081] If the time from the issuance of a sheet feed command to the detection by the first sensor S1 is the same as the third target time (seventh time), the control unit 20 sets the conveying speed of the second pre-registration roller pair 12 to a ninth speed (see FIG. 5). On the other hand, if the time from the issuance of a sheet feed command to the detection by the first sensor S1 of the leading edge of the sheet is eighth time, which is longer than the third target time (seventh time), the control unit 20 sets the conveying speed of the second pre-registration roller pair 12 to a tenth speed, which is greater than the ninth speed (see FIG. 6). On the other hand, if the time from the issuance of a sheet feed command to the detection by the first sensor S1 of the leading edge of the sheet is ninth time, which is shorter than the third target time (seventh time), the control unit 20 sets the conveying speed of the first pre-registration roller pair 11 to an eleventh speed, which is smaller than the ninth speed (see FIG. 7). This allows for improved image printing accuracy without disposing a sensor between the separation roller pair 55 and the second pre-registration roller pair 12.

[0082] <Other embodiments> In the above-described embodiment, the image forming apparatus 1 adopts a vertical path configuration in which the transport path from the feeding sections 6A, 6B to the secondary transfer section 38 faces upward, but this is not limited to this and a horizontal path configuration may also be adopted.

[0083] In addition, in each of the above-described embodiments, the image forming apparatus 1 is a color laser printer, but the present invention is not limited to this. For example, the present invention may be applied to an image forming apparatus that is a monochrome laser printer. [Explanation of symbols]

[0084] 1...image forming apparatus (sheet conveying apparatus), 6A, 6B...feeding section (first feeding section), 7A, 7B...feeding cassette (sheet storage section), 56...feeding section (second feeding section), 11...first pre-registration roller pair (first conveying roller pair), 12...second pre-registration roller pair (second conveying roller pair), 13...registration roller pair (registration roller pair), 17...junction section, 20...control section, 51...manual feed tray, 201A...apparatus main body, 201B...image forming section, 201F...duplex reversing section (re-conveying section), P1...first conveying path, P2...second conveying path, S...sheet, S1...first sensor, S2...second sensor

Claims

1. a feeding section that feeds sheets from a sheet storage section that stores sheets; a first conveying path that conveys the sheet fed by the feeding unit in a sheet conveying direction; a pair of registration rollers disposed on the first conveying path, contacting a leading edge of the sheet in the sheet conveying direction to correct skew of the sheet, and then sandwiching and conveying the sheet; a first conveying roller pair disposed on the first conveying path and configured to convey the sheet to the registration roller pair; a first sensor disposed between the first conveying roller pair and the registration roller pair in the first conveying path, the first sensor detecting the leading edge of the sheet; a second sensor disposed between the pair of registration rollers and the first sensor, and configured to detect the leading edge of the sheet; a control unit that controls the conveyance of the sheet, the control unit controls the pair of registration rollers and the pair of first conveying rollers based on a detection result of the second sensor so as to correct skew of the sheet; When the sheet is conveyed from the first conveying roller pair to the registration roller pair, the control unit When a time from when the feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is a first time, the sheet is conveyed toward the second sensor at a conveying speed of the first conveying roller pair at a first speed; When a time from when the feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is a second time that is longer than the first time, the sheet is conveyed toward the second sensor at a conveying speed by the first conveying roller pair at a second speed that is higher than the first speed. A sheet conveying device characterized by:

2. when a time from when the feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is the second time, the control unit causes the first conveying roller pair to convey the sheet at a third speed lower than the second speed until the leading edge of the sheet reaches the first sensor.

2. The sheet transport device according to claim 1.

3. When the sheet is conveyed from the first conveying roller pair to the registration roller pair, the control unit When a time from when the feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is a third time that is shorter than the first time, the sheet is conveyed toward the second sensor at a fourth speed that is lower than the first speed by the first conveying roller pair.

2. The sheet transport device according to claim 1.

4. when a time from when the feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is the third time, the control unit causes the first conveying roller pair to convey the sheet at a fifth speed that is greater than the fourth speed until the leading edge of the sheet reaches the first sensor.

4. The sheet transport device according to claim 3.

5. the first sensor includes a flag that moves when it comes into contact with the sheet, and the second sensor is an optical sensor having a light-emitting portion and a light-receiving portion.

2. The sheet transport device according to claim 1.

6. the registration roller pair is a conveying roller disposed next to the first conveying roller pair in the sheet conveying direction; 2. The sheet transport device according to claim 1.

7. a second conveying path that merges with a junction provided between the pair of registration rollers and the pair of first conveying rollers in the first conveying path; a second conveying roller pair that is disposed on the second conveying path and that conveys the sheet from the junction portion through the first conveying path to the registration roller pair, the first sensor is disposed between the junction and the pair of registration rollers in the sheet conveying direction; 2. The sheet transport device according to claim 1.

8. When the control unit conveys the sheet from the second conveying roller pair to the registration roller pair via the second conveying path in order to form an image on a second surface of the sheet that has been conveyed by the feeding unit via the first conveying roller pair and has an image formed on a first surface thereof, when a time from when the feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is a fourth time, the sheet is conveyed toward the second sensor in a state where the conveying speed by the second conveying roller pair is a sixth speed; When a time from the timing when the feeding unit feeds the sheet to the timing when the leading edge of the sheet is detected by the first sensor is a fifth time that is longer than the fourth time, the sheet is conveyed toward the second sensor at a conveying speed by the second conveying roller pair at a seventh speed that is higher than the sixth speed.

8. The sheet transport device according to claim 7, wherein the sheet transport device is a sheet conveying device.

9. When the control unit conveys the sheet from the second conveying roller pair to the registration roller pair via the second conveying path in order to form an image on the second surface of the sheet that has been conveyed by the feeding unit via the first conveying roller pair and has an image formed on the first surface, When a time from when the feeding unit feeds the sheet until the leading end of the sheet is detected by the first sensor is a sixth time that is shorter than the fourth time, the sheet is conveyed toward the second sensor at an eighth speed that is lower than the sixth speed by the second conveying roller pair.

9. The sheet transport device according to claim 8.

10. The sheet storage unit is stored in the device body, the feeding unit is a first feeding unit that feeds the sheet accommodated in the sheet accommodation unit to the first conveying roller pair, a manual feed tray that is openable and closable relative to the device body and supports sheets stacked on the top surface when open; a second feeding unit that feeds the sheet supported on the manual feed tray to the second conveying roller pair, 8. The sheet transport device according to claim 7.

11. When the sheet is conveyed from the manual feed tray to the pair of registration rollers via the second conveyance roller pair, the control unit: when a time from when the second feeding unit feeds the sheet to when the leading edge of the sheet is detected by the first sensor is seventh time, the sheet is conveyed toward the second sensor at a conveying speed of the second conveying roller pair at a ninth speed, When the time from the timing when the second feeding unit feeds the sheet to the timing when the leading edge of the sheet is detected by the first sensor is an eighth time that is longer than the seventh time, the sheet is conveyed toward the second sensor at a tenth speed that is greater than the ninth speed by the second conveying roller pair.

11. The sheet transport device according to claim 10.

12. When the sheet is conveyed from the manual feed tray to the pair of registration rollers via the second conveyance roller pair, the control unit: When a time from when the second feeding unit feeds the sheet until the leading edge of the sheet is detected by the first sensor is a ninth time that is shorter than the seventh time, the sheet is conveyed toward the second sensor at an eleventh speed that is lower than the ninth speed by the second conveying roller pair.

12. The sheet transport device according to claim 11.

13. the control unit adjusts a conveyance amount of the sheet conveyed by the first conveyance roller pair or the second conveyance roller pair toward the registration roller pair in a stopped state, based on a time from when the leading edge of the sheet is detected by the first sensor to when the leading edge of the sheet is detected by the second sensor, thereby adjusting a loop amount of the sheet.

8. The sheet transport device according to claim 7.

14. a conveying speed of the first conveying roller pair when correcting skew of the sheet after the leading edge of the sheet has passed the second sensor is lower than the first speed and the second speed; 2. The sheet transport device according to claim 1.

15. a sheet conveying device according to any one of claims 1 to 14; an image forming unit that forms an image on a sheet, An image forming apparatus characterized by:

16. a re-conveying unit that inverts the sheet on which the image has been formed by the image forming unit, conveys the sheet, and conveys the sheet to the first conveying path; 16. The image forming apparatus according to claim 15.

Images