Sheet conveying device and image forming apparatus

Abstract

To reduce the occurrence of misalignment formed on the front and back of a sheet.SOLUTION: The passage of a leading edge of a sheet S is detected at a first detection position P1 and a second detection position P2 and the passage of a trailing edge of the sheet S is detected at a third detection position P3. On the basis of a first time which is a difference in timing for the leading edge of the sheet S to pass through the first detection position P1 and the second detection position P2, and a first distance L12 between the first detection position P1 and the second detection position P2, the transport speed of the sheet S is calculated. On the basis of the transport speed, a second time which is a difference in timing for the leading edge of the sheet S to pass through the second detection position P2 and the trailing edge of the sheet S to pass through the third detection position P3, and a second distance L23 between the second detection position P2 and the third detection position P3, a length of the sheet S is calculated. The conveyance timing of the sheet S by a second conveyance roller pair is controlled on the basis of information on the length of the sheet S.SELECTED DRAWING: Figure 5

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, there are image forming apparatuses that have the function of printing images on both sides of a sheet. In such image forming apparatuses, when forming images on both sides of a sheet, an image is first formed on a first side (front side) of the sheet. Then, after the sheet with the image formed on the first side is turned over, an image is formed on a second side (back side) of the sheet. A switchback method is commonly used as a method for turning over a sheet. However, with the switchback method, positional misalignment occurs between the front and back sides of the image formed on the sheet in the sheet conveyance direction.

[0003] In Patent Document 1, a pair of upstream conveying rollers and a pair of downstream conveying rollers are provided at different positions in the sheet conveying direction in a sheet conveying path after the sheet has been switched back. Furthermore, a plurality of sensors are provided at different positions in the sheet conveying direction between the pair of upstream conveying rollers and the pair of downstream conveying rollers.

[0004] The sheet conveyance speed is calculated by measuring the time it takes for the sheet conveyed by the conveyance rollers to pass through multiple sensors, and the length of the sheet in the conveyance direction is calculated. This allows an image on the second side to be formed in accordance with the length of the sheet, and a configuration is disclosed that corrects positional misalignment between the front and back of the sheet. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-4137 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the configuration disclosed above, the sensor detects the trailing edge of the sheet after it passes through the pair of upstream conveying rollers. The conveyed sheet receives an impact when it enters the nip between the pair of downstream conveying rollers, which may temporarily reduce the sheet conveyance speed. In this case, the sheet length in the sheet conveyance direction is affected by variations in the sheet conveyance speed, which may result in variations in the sheet length and, as a result, misalignment between the front and back of the image formed on the sheet.

[0007] An object of the present invention is to accurately calculate the length of a sheet in the conveying direction while it is being conveyed, and to reduce the occurrence of misalignment formed on the front and back surfaces of the sheet. [Means for solving the problem]

[0008] In one aspect of the present invention, a sheet conveying device includes a first conveying roller pair that rotates while sandwiching a sheet and conveys the sheet, an upstream conveying roller pair that is disposed upstream of the first conveying roller pair in a sheet conveying direction and conveys the sheet, a downstream conveying roller pair that is disposed downstream of the first conveying roller pair in the conveying direction and conveys the sheet, a second conveying roller pair that is disposed downstream of the downstream conveying roller pair in the conveying direction and conveys the sheet, a first detection unit that is provided downstream of the first conveying roller pair in the conveying direction and at a first detection position upstream of the downstream conveying roller pair and that detects passage of a leading edge of the sheet conveyed by the first conveying roller pair, a second detection unit that is provided downstream of the first conveying roller pair in the conveying direction and at an upstream side of the downstream conveying roller pair and that is different from the first detection position and that detects passage of a leading edge of the sheet conveyed by the first conveying roller pair, and a second detection unit that is provided upstream of the first conveying roller pair in the conveying direction and at a second detection position downstream of the first conveying roller pair and upstream of the downstream conveying roller pair and that is different from the first detection position and that detects passage of a leading edge of the sheet conveyed by the first conveying roller pair. versusand a control unit that performs calculations in response to signals from the first detection unit, the second detection unit, and the third detection unit, wherein the control unit calculates a sheet conveyance speed based on a first time that is the difference between the timing at which the leading edge of the sheet passes the first detection position and the timing at which the leading edge of the sheet passes the second detection position, and a first distance between the first detection position and the second detection position in the conveyance direction, calculates a sheet length in the conveyance direction based on the conveyance speed, a second time that is the difference between the timing at which the leading edge of the sheet passes the second detection position and the timing at which the trailing edge of the sheet passes the third detection position, and a second distance between the second detection position and the third detection position in the conveyance direction, and controls the timing at which the sheet is conveyed by the second conveyance roller pair based on information about the sheet length.

[0009] In one aspect of the present invention, a sheet conveying device includes a first conveying roller pair that rotates while sandwiching a sheet and conveys the sheet, an upstream conveying roller pair that is arranged upstream of the first conveying roller pair in a sheet conveying direction and conveys the sheet, a downstream conveying roller pair that is arranged downstream of the first conveying roller pair in the conveying direction and conveys the sheet, a second conveying roller pair that is arranged downstream of the downstream conveying roller pair in the conveying direction and conveys the sheet, and No. 1 a first detection unit provided at a first detection position upstream of the pair of conveying rollers and downstream of the upstream pair of conveying rollers, the first detection unit detecting the passage of a trailing edge of the sheet conveyed by the first pair of conveying rollers; Upstream a second detection unit provided downstream of the pair of conveying rollers and at a second detection position different from the first detection position, the second detection unit detecting the passage of a trailing edge of the sheet conveyed by the pair of first conveying rollers; downstreama third detection unit provided at a third detection position upstream of the conveying roller pair and configured to detect the passage of a leading edge of a sheet conveyed by the first conveying roller pair; and a control unit configured to perform calculations in response to signals from the first detection unit, the second detection unit, and the third detection unit, wherein when the leading edge of a sheet conveyed by the first conveying roller pair is detected by the third detection unit, the trailing edge of the sheet has passed through the upstream conveying roller pair, and when the trailing edge of the sheet conveyed by the first conveying roller pair is detected by the first detection unit and the second detection unit, the leading edge of the sheet has not yet reached the downstream conveying roller pair, and the control unit detects that the trailing edge of the sheet has passed through the first detection position a sheet conveying speed is calculated based on a first time that is a difference between a timing at which the leading edge of the sheet passes the third detection position and a timing at which the trailing edge of the sheet passes the second detection position, and a first distance between the first detection position and the second detection position in the conveying direction; a sheet length in the conveying direction is calculated based on the conveying speed, a second time that is a difference between a timing at which the leading edge of the sheet passes the third detection position and a timing at which the trailing edge of the sheet passes the second detection position, and a second distance between the second detection position and the third detection position in the conveying direction; and a timing at which the sheet is conveyed by the second conveying roller pair is controlled based on the information on the length of the sheet.

[0010] According to another aspect of the present invention, there is provided a sheet conveying device including a first conveying roller pair that rotates while sandwiching a sheet and conveys the sheet; an upstream conveying roller pair that is disposed upstream of the first conveying roller pair in a sheet conveying direction and conveys the sheet; a downstream conveying roller pair that is disposed downstream of the first conveying roller pair in the sheet conveying direction and conveys the sheet; a second conveying roller pair that is disposed downstream of the downstream conveying roller pair in the sheet conveying direction and conveys the sheet; and a second conveying roller pair that is disposed downstream of the first conveying roller pair in the sheet conveying direction and upstream of the downstream conveying roller pair in the sheet conveying direction, extending along the sheet conveying direction, and versusa second reading unit disposed upstream of the first conveying roller pair in the conveying direction and downstream of the upstream conveying roller pair in the conveying direction, extending along the conveying direction, and reading an image of the trailing edge of the sheet conveyed by the first conveying roller pair; and a control unit that performs calculations in accordance with the images read by the first reading unit and the second reading unit, wherein when an image of the leading edge of the sheet conveyed by the first conveying roller pair is detected by the first reading unit, the trailing edge of the sheet has passed the upstream conveying roller pair, and when an image of the trailing edge of the sheet conveyed by the first conveying roller pair is detected by the second reading unit, the leading edge of the sheet has not reached the downstream conveying roller pair, and the control unit calculates a sheet conveying speed based on a fifth time that is the difference in timing between the multiple images read by the first reading unit and a fifth distance that is the difference in position of the leading edge of the sheet in the multiple images, calculates a length of the sheet in the conveying direction based on the conveying speed, a sixth time that is the difference in timing between the images read by the first reading unit and the images read by the second reading unit, and a sixth distance that is the difference in position of the sheet in the images read by the first reading unit and the images read by the second reading unit, and controls the timing of conveying the sheet by the second conveying roller pair based on the information on the length of the sheet. [Effects of the Invention]

[0011] According to the present invention, it is possible to reduce the occurrence of misalignment formed on the front and back surfaces of a sheet. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram illustrating the configuration of an image forming apparatus according to a first embodiment. [Figure 2] 10A and 10B are diagrams showing a configuration for calculating the sheet conveying speed and the sheet length in the conveying direction in a reference example. [Figure 3] FIG. 10 is a signal output diagram in a configuration for detecting the passage of a sheet according to a reference example. [Figure 4]10A, 10B, 10C, and 10D are cross-sectional views for explaining the behavior of the sheet S in the reference example. [Figure 5] FIG. 2 is a top view showing the configuration of the sheet detection unit according to the first embodiment. [Figure 6] 5A to 5E are diagrams showing the sheet conveyance behavior of the sheet detection unit of the first embodiment. [Figure 7] 5A and 5B are diagrams showing changes in signals in the sheet detection unit according to the first embodiment. [Figure 8] FIG. 10 is a top view showing the configuration of a sheet detection unit according to a modified example of the first embodiment. [Figure 9] 10A to 10E are diagrams showing the sheet conveyance behavior of the sheet detection unit according to a modified example of the first embodiment. [Figure 10] FIG. 10 is a top view showing the configuration of a sheet detection unit according to a second embodiment. [Figure 11] 10A and 10B are diagrams showing changes in signals in the sheet detection unit according to the second embodiment. [Figure 12] FIG. 11 is a top view showing the configuration of a sheet detection unit according to a third embodiment. [Figure 13] 10A and 10B are diagrams showing changes in signals in the sheet detection unit according to the third embodiment. [Figure 14] FIG. 10 is a top view showing the configuration of a sheet detection unit according to a fourth embodiment. [Figure 15] FIG. 13 is a top view showing a sheet conveyance mode in a modified example of the fourth embodiment. [Figure 16] FIG. 13 is a diagram showing a change in a signal in a sheet detection unit according to a modified example of the fourth embodiment. [Figure 17] FIG. 13 is a top view showing the configuration of a sheet detection unit according to a fifth embodiment. [Figure 18] 10A is a diagram showing an example of an image read by the first reading unit of the fifth embodiment, and FIG. 10B is a diagram showing another example of an image read by the second reading unit of the fifth embodiment. [Figure 19] 10A is a plot of the relationship between the timing of reading by the first reading unit and the position of the edge of the sheet in the fifth embodiment, and FIG. 10B is a plot of the relationship between the timing of reading by the second reading unit and the position of the edge of the sheet in the fifth embodiment. [Figure 20]10A is a top view showing the detection of the leading edge of a sheet in the fifth embodiment, and FIG. 10B is a top view showing the detection of the trailing edge of a sheet in the fifth embodiment. [Figure 21] FIG. 1 is a control block diagram according to first to fifth embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings.

[0014] [Embodiment 1] <Overall configuration of image forming apparatus> First, a schematic configuration of a printer 1 as an example of an image forming apparatus equipped with a sheet conveying device according to the first embodiment will be described. FIG. 1 is a schematic diagram of the printer 1. The printer 1 includes a control unit 9 that controls the overall operation of the printer 1 based on image information input from an external PC or image information read from an original. The printer 1 is a device such as a copier, facsimile, or multifunction peripheral that forms images on sheets used as recording media. The printer 1 is also capable of printing for purposes other than general office use, and various sheets can be used as recording media, including paper such as paper and envelopes, glossy paper, plastic film such as overhead projector sheets, and cloth. The printer 1 includes a device main body 100A that houses a feed cassette 51 for storing sheets S and an image forming engine 513 that forms images on sheets S fed from the feed cassette 51. The image forming engine 513, an example of an image forming unit, includes four image forming units PY, PM, PC, and PK that form yellow, magenta, cyan, and black toner images, respectively, and an intermediate transfer belt 506. The image forming engine 513 forms an image on the sheet S by a tandem intermediate transfer method. The image forming stations PY to PK are electrophotographic units having photosensitive drums 1Y, 1M, 1C, and 1K, respectively, which are photosensitive members.

[0015] The image forming units PY to PK share a common configuration except for the different colors of toner used for development. Here, the configuration of the image forming engine 513 and the toner image formation process will be described using the yellow image forming unit PY as an example. In addition to the photosensitive drum 1Y, the image forming unit PY includes an exposure device 511, a development device 510, and a drum cleaner 509. The photosensitive drum 1Y is a drum-shaped photosensitive body having a photosensitive layer on its outer periphery. It rotates in a direction (arrow A in FIG. 1) that is parallel to the rotation direction of the intermediate transfer belt 506 (arrow B in FIG. 1). The surface of the photosensitive drum 1Y is charged by receiving electrical charge from a charging unit such as a charging roller. The exposure device 511 irradiates the photosensitive drum 1Y with laser light modulated according to image information, and scans the photosensitive drum 1Y using an optical system including a reflection device 512 to write an electrostatic latent image on the surface of the photosensitive drum 1Y. The development device 510 contains a developer containing toner, and supplies the toner to the photosensitive drum 1Y to visualize the electrostatic latent image into a toner image. The toner image formed on the photosensitive drum 1Y is primarily transferred to the intermediate transfer belt 506 at a primary transfer portion, which is a nip portion between the primary transfer roller 507 and the intermediate transfer belt 506. Residual toner remaining on the photosensitive drum 1Y after transfer is removed by a drum cleaner 509.

[0016] Intermediate transfer belt 506 is wound around drive roller 504, driven roller 505, inner secondary transfer roller 503, and primary transfer roller 507, and is driven to rotate in the clockwise direction (arrow B) in FIG. 1 by drive roller 504. The image formation process described above is carried out in parallel in each image forming station PY to PK, and four color toner images are transferred in a superimposed manner to form a full-color toner image on intermediate transfer belt 506. This toner image is transported to secondary transfer station 100C while still supported by intermediate transfer belt 506. Secondary transfer station 100C is configured as a nip between secondary transfer roller 56 and inner secondary transfer roller 503, which serve as transfer means. A bias voltage of a polarity opposite to the charge polarity of the toner is applied to secondary transfer roller 56, thereby secondarily transferring the toner image to sheet S. Residual toner remaining on intermediate transfer belt 506 after transfer is removed by a belt cleaner.

[0017] The sheet S onto which the toner image has been transferred is delivered to a fixing unit 58 by a pre-fixing conveyance section 57. The fixing unit 58 has a pair of fixing rollers that sandwich and convey the sheet S, and a heat source such as a halogen heater, and applies pressure and heat to the toner image carried on the sheet S. This melts and fixes the toner particles to the sheet S.

[0018] Next, a sheet conveying process for conveying sheets will be described. A sheet conveying system 100D, which serves as a sheet conveying device in this embodiment, feeds sheets S stored in a feed cassette 51 and discharges the sheets S on which images have been formed to the outside of the apparatus main body 1A. The sheet conveying system 100D includes a sheet feeding section 53, a sheet conveying section 54, a skew correction section 55, a branching conveying section 59, a reversing conveying section 501, and a duplex conveying section 502. The feed cassette 51 is attached to the apparatus main body 100A so as to be removable, and stores sheets S stacked on a lift plate 52 that can be raised and lowered. The sheets S are fed one by one by the sheet feeding section 53. Examples of the sheet feeding section 53 include a belt system in which a suction fan attracts the sheets S to a belt member for conveyance, and a friction separation system using a roller or pad. The sheet S sent out from the sheet feeding unit 53 is conveyed along a feeding path 54 a by a pair of conveying rollers of the sheet conveying unit 54 and delivered to the skew correction unit 55 .

[0019] The sheet S delivered to the skew correction unit 55 undergoes skew correction and timing correction before being conveyed toward the secondary transfer unit 100C. At this time, a pair of registration rollers 7, which serves as a second conveyance roller pair included in the skew correction unit 55, sends the sheet S to the secondary transfer unit 100C at a timing that matches the progress of the image formation process by the image forming units PY to PK. After the toner image is transferred to the sheet S in the secondary transfer unit 100C and the image is fixed by a fixing unit 58, the sheet S is conveyed to a branch conveyance unit 59, which branches off the conveyance path of the sheet S. When image formation on the sheet S is complete, the sheet S is discharged by a pair of discharge rollers to an output tray 500 located outside the apparatus main body 1A. When an image is to be formed on the back side of the sheet S, the sheet S is delivered to a duplex conveyance unit 502 via a reverse conveyance unit 501. The reverse conveyance unit 501 has a pair of reversing rollers that can rotate forward and backward, and delivers the sheet S to the duplex conveyance unit 502 in a reversed state using a switchback method that reverses the front and back of the sheet S. The double-sided conveying section 502 conveys the sheet S again via the sheet conveying section 54 toward the skew correction section 55. Then, the sheet S is discharged onto the discharge tray 500 after an image is formed on the back side thereof.

[0020] As described above, the printer 1 achieves the formation of an image on a sheet by operating the "image forming process" and the "sheet conveying process" in conjunction with each other.

[0021] <Configuration for calculating sheet length in the conveying direction in the reference example> Conventionally, the switchback method described above has been commonly used to reverse a sheet in an image forming apparatus because of its simple configuration and space-saving advantages. However, with the switchback method, the leading and trailing edges of the sheet are swapped, resulting in misalignment of the image formed on the front and back of the sheet in the sheet transport direction, even if a mechanism for correcting sheet skew is provided. This is due to variations in sheet dimensions caused by variations in sheet cutting and the shrinkage and expansion of sheet fibers due to the amount of moisture absorbed from the air. In particular, when an image is formed on the back of a sheet after an image is formed on the front of the sheet, the sheet is easily shrunk because it is heated and pressurized in the fixing unit. In this case, simply aligning the timing of the toner image and the leading edge of the sheet based on the leading edge of the image formed on the front of the sheet when transferring the image to the back of the sheet results in misalignment of the image formed on the front and back of the sheet. Such misalignment can result in image loss during post-printing processes such as trimming and folding, or blank spaces on the next page, resulting in a decrease in the quality of the printed output.

[0022] In response to this, as shown in Figure 2, a known configuration is one in which sensors that detect the passage of a sheet are provided at two detection units SN1A and SN2A in the duplex conveying unit of an image forming apparatus, and the sheet conveying speed and sheet length in the conveying direction are calculated based on signals emitted by the sensors. After calculating the sheet length, the control unit controls the timing of conveying the sheet to the secondary transfer unit by the skew correction unit based on the calculated sheet length information. Specifically, if the control unit determines that the calculated sheet length is shorter than the sheet length information, the control unit delays the timing of conveying the sheet to the secondary transfer unit by the skew correction unit. Conversely, if the control unit determines that the calculated sheet length is longer than the sheet length information, the control unit advances the timing of conveying the sheet to the secondary transfer unit by the skew correction unit.

[0023] By detecting the length of the sheet in this way and controlling the timing of sheet transport by the skew correction unit, it is possible to know the reference position of the edge of the sheet when forming an image on the first side (front side) even if the leading and trailing edges of the sheet are swapped when forming an image on the second side (back side).As a result, since the position of the image formed on the first side (front side) is known, it is possible to form the image on the second side (back side) in accordance with the reference position of the edge of the sheet when forming the image on the first side (front side), thereby preventing misalignment between the front and back of the image formed on the sheet.

[0024] Fig. 2 is a top view illustrating a configuration for detecting the passage of a sheet in a conventional image forming apparatus as a reference example. Fig. 2 shows, as a reference example, conveyance rollers 5 and 6 arranged in a duplex conveyance unit of a conventional image forming apparatus, and detection units SN1A and SN2A arranged between the conveyance rollers 5 and 6 in the conveyance direction. Detector SN1A, which is arranged upstream in the conveyance direction, is provided with two sensors SN1 and SN2 arranged at a distance d between them across the center of the width direction (hereinafter referred to as conveyance center C) of the sheet conveyance path, which is perpendicular to the conveyance direction. Similarly, detector SN2A, which is arranged downstream of detector SN1A in the conveyance direction, is provided with two sensors SN3 and SN4 arranged at a distance d between them across the conveyance center C.

[0025] When a sheet S conveyed in the direction of arrow FX in FIG. 2 passes through detection units SN1A and SN2A, sensors SN1, SN2, SN3, and SN4 output signals as shown in FIG. 3. FIG. 3 is a signal output diagram for a reference example of a configuration for detecting sheet passage. In FIG. 3, T1 and T2 are the times when sensors SN1 and SN2 of detection unit SN1A detect the passage of the leading edge of the sheet. T3 and T4 are the times when sensors SN3 and SN4 of detection unit SN2A detect the passage of the leading edge of the sheet. T1' and T2' are the times when sensors SN1 and SN2 detect the passage of the trailing edge of the sheet. T3' and T4' are the times when sensors SN3 and SN4 detect the passage of the trailing edge of the sheet. In FIG. 3, it is assumed that the sheet is conveyed by conveyance rollers 5 and 6 in FIG. 2 so that the leading and trailing edges of the sheet are perpendicular to the straight line indicating the conveyance center C.

[0026] Here, if the time required for the leading edge of sheet S to pass between sensors SN1 and SN3 is time F, then time F = T3 - T1. Meanwhile, if the time required for the leading edge of sheet S to pass between sensors SN2 and SN4 is time E, then time E = T4 - T2. Furthermore, if the time required for the trailing edge of sheet S to pass between sensors SN1 and SN3 is time H, then time H = T3' - T1'. Meanwhile, if the time required for the trailing edge of sheet S to pass between sensors SN2 and SN4 is time G, then time G = T4' - T2'. The conveying speed of sheet S is then calculated based on times E, F, G, and H, and the distance D between detection units SN1A and SN2A. When calculating the conveying speed VEX of sheet S, in order to average out the effects of conveying rollers 5, 6, etc., the average value Avg(E, F, G, H) of times E, F, G, and H is taken as the time it takes for sheet S to be conveyed the distance D, and the conveying speed VEX is calculated using (Equation 1). Conveying speed VEX=D / Avg(E, F, G, H) (Equation 1)

[0027] Furthermore, if the time required for the leading edge to the trailing edge of sheet S to pass sensor SN1 is time AX, and the time required for the leading edge to the trailing edge of sheet S to pass sensor SN2 is time BX, then time AX = T1' - T1 and time BX = T2' - T2. If the time required for the leading edge to the trailing edge of sheet S to pass sensor SN2 is time CX, and the time required for the entire sheet S to pass sensor SN4 is time DX, then time CX = T3' - T3 and time DX = T4' - T4. To average out the effects of conveyance rollers 5 and 6, etc., the length L of sheet S in the conveyance direction is calculated using Equation 2 based on the average value Avg(AX, BX, CX, DX) of times AX, BX, CX, and DX and the conveyance speed VEX calculated using Equation 1. Length L = VEX × Avg(AX, BX, CX, DX) (Equation 2)

[0028] Next, the behavior of a sheet S as it passes through the conveyance rollers 5 and 6 of the reference example will be described with reference to FIGS. 4A to 4D. FIGS. 4A to 4D are cross-sectional views illustrating the behavior of the sheet S in the reference example. As shown in FIG. 4A, when the edge of the sheet S is detected by the detectors SN1A and SN2A, the sheet S is being conveyed toward the conveyance roller 6 while being sandwiched between the conveyance rollers 5. As shown in FIG. 4B, when the sheet S enters the conveyance roller 6, a "bouncing behavior" is observed, in which the conveyance roller 6 bounces in the direction indicated by the dashed line due to the impact of the entry. In FIG. 4B, the position of the conveyance roller 6 where the bounding behavior occurs due to the entry of the sheet S is indicated by the dashed line. Note that this bouncing behavior becomes more pronounced as the conveyance speed is increased to ensure productivity in the sheet conveyance operation or as the thickness of the sheet increases. When the bouncing behavior occurs, the sheet S and the conveyance roller 6 slip the instant the clamping pressure between the conveyance rollers 6 and the sheet S is released. As a result of the slip, the conveyance rollers 6 may pinch the sheet S at a different position than when there is no slip, as shown in FIG. 4C. Then, when the detection units SN1A and SN2A detect the passage of the trailing edge of the sheet, the sheet S is conveyed in a slipped state (FIG. 4D). Therefore, the slip between the sheet S and the conveyance rollers 6 causes a misalignment of the sheet. As a result, an error occurs when detecting the passage of the trailing edge of the sheet S, which may reduce the accuracy of measuring the length of the sheet S in the conveyance direction. This accuracy of measuring the length of the sheet S affects the timing at which the skew correction unit conveys the sheet to the secondary transfer unit, leading to misalignment of the images formed on the front and back of the sheet.

[0029] <Configuration for Calculating Sheet Length in the Conveying Direction in the First Embodiment> Next, a configuration for detecting the length of a sheet in the conveying direction will be described in embodiment 1. Fig. 5 is a top view showing the configuration of a sheet detection unit 10 for detecting the length of a sheet in the conveying direction in embodiment 1. The description will be given assuming that the sheet detection unit 10 is disposed in the duplex conveying section 502, but the sheet detection unit 10 can be disposed in a location other than the duplex conveying section 502 as long as it is on the conveying path along which the sheet S is conveyed in the printer 1.

[0030] The sheet detection unit 10 includes a pair of conveying rollers 11 as a first pair of conveying rollers that conveys the sheet S, and a first detector S1, a second detector S2, and a third detector S3 that detect the passage of the end portions (leading and trailing ends) of the sheet. In the conveying roller pair 11 of this embodiment, the peripheral surfaces of each roller that contacts the sheet are blasted and made of metal. Therefore, the outer diameter of the pair of conveying rollers 11 changes less with temperature and humidity than commonly used rubber rollers, making it less likely for the sheet S to slip. This allows for more accurate and stable control of the sheet conveyance. Furthermore, the first detector S1 and the second detector S2 are located downstream of the pair of conveying rollers 11 in the sheet conveyance direction D1. The third detector S3 is located upstream of the pair of conveying rollers 11 in the conveyance direction D1. An upstream pair of conveying rollers 12 that conveys the sheet is located upstream of the third detector S3, and a downstream pair of conveying rollers 13 that conveys the sheet is located downstream of the first detector S1.

[0031] The first detection unit S1 detects the passage of the edge (leading edge and trailing edge) of the sheet at a first detection position P1 downstream of the pair of conveying rollers 11 in the conveying direction D1. The second detection unit S2 detects the passage of the edge (leading edge and trailing edge) of the sheet at a second detection position P2 downstream of the pair of conveying rollers 11 in the conveying direction D1 and different from the first detection position P1. The third detection unit S3 detects the passage of the edge (leading edge and trailing edge) of the sheet at a third detection position P3 upstream of the pair of conveying rollers 11 in the conveying direction D1. FIG. 5 shows an example in which the first detection position P1, the second detection position P2, and the third detection position P3 are each disposed at the center of the sheet width direction perpendicular to the conveying direction D1.

[0032] 5, the distance between the first detection position P1 and the second detection position P2 in the conveying direction D1 is indicated as L12, and the distance between the second detection position P2 and the third detection position P3 in the conveying direction D1 is indicated as L23. The first detection unit S1 includes an optical sensor that outputs a low signal when no sheet is present at the first detection position P1 and a high signal when a sheet is present. The second detection unit S2 and the third detection unit S3 also use the same sensor as the first detection unit S1. Therefore, when the leading edge of the sheet S passes, the signals output from the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3 switch from low to high. When the trailing edge of the sheet S passes, the signals output from the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3 switch from high to low. Therefore, the control unit 9 can recognize the timing when the leading edge or trailing edge of the sheet passes through the first detection position P1, the second detection position P2, and the third detection position P3, based on the signals output from the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3. Furthermore, the control unit 9 can calculate the conveying speed V of the sheet S and the length of the sheet S in the conveying direction D1, based on the timing when the leading edge or trailing edge of the sheet passes through the first detection position P1, the second detection position P2, and the third detection position P3.

[0033] Furthermore, if the length of the sheet S is L, the length between the conveying roller pair 11 and the upstream conveying roller pair 12 is L(11-12), the length between the downstream conveying roller pair 13 and the conveying roller pair 11 is L(13-11), the length between the second detection unit S2 and the upstream conveying roller pair 12 is L(2-12), and the length between the downstream conveying roller pair 13 and the third detection unit S3 is L(13-3), the rollers are arranged so that the following relationship holds. L>L(L11-12) and L>(L13-11) (Equation 3) ·L(2-12)>L and L(13-3)>L···(Equation 4)

[0034] By arranging each conveying roller pair and each detecting unit so as to satisfy the relationships of (Equation 3) and (Equation 4) above, the sheet detection unit 10 can detect the leading and trailing ends of the sheet using the first detecting unit, the second detecting unit, and the third detecting unit while the sheet is being conveyed by the conveying roller pair 11 and in a state where the sheet is not being nipped by the upstream conveying roller pair or the downstream conveying roller pair. In other words, when calculating the length of the sheet, it is possible to perform the calculation before the leading end of the sheet is nipped by the downstream conveying roller pair. In other words, it is possible to suppress vibration of the sheet caused by the leading end of the sheet being nipped by the downstream conveying roller pair, and it is possible to detect the length of the sheet with high accuracy.

[0035] Next, with reference to FIG. 6, the sheet conveyance behavior of the sheet detection unit in this embodiment will be described. FIG. 6(A) is a diagram showing a sheet of length L being nipped between the upstream conveyance roller pair 12 and being conveyed. This is the timing when the leading edge of the sheet is detected by the third detection unit S3. FIG. 6(B) is a diagram showing a sheet being nipped between the conveyance roller pair 11 and being conveyed. This is the timing when the leading edge of the sheet is detected by the second detection unit S2. At this time, the trailing edge of the sheet has passed through the upstream conveyance roller pair 12. As will be described later, time measurement is started at this timing to calculate the sheet speed being conveyed (start timing of the first time). Time measurement is also started to calculate the length of the sheet (start timing of the second time). FIG. 6(C) is a diagram showing a sheet being nipped between the conveyance roller pair 11 and being conveyed. This is the timing when the leading edge of the sheet is detected by the first detection unit S1 (start timing of the third time). At this time, the trailing edge of the sheet has passed through the upstream conveyance roller pair 12, and the third detection unit S3 has detected the sheet. As will be described later, at this timing, time measurement ends (end of first time) to calculate the speed of the sheet being conveyed. Time measurement also starts to calculate the length of the sheet. FIG. 6D shows a diagram in which the sheet is nipped between the pair of conveying rollers 11 and being conveyed. The leading edge of the sheet has not yet reached the pair of downstream conveying rollers 13, and the trailing edge of the sheet has been detected by the third detection unit S3. As will be described later, at this timing, time measurement ends (end of second time and third time) for the times at which time measurement started in FIGS. 6B and 6C to calculate the length of the sheet. FIG. 6E shows a diagram in which the leading edge of the sheet is nipped between the pair of downstream conveying rollers 13. The trailing edge of the sheet is being nipped between the pair of conveying rollers 11 and being conveyed. As described above, the sheet detection unit in this embodiment detects the leading and trailing edges of the sheet while conveying it.

[0036] Next, a method for calculating the length of the sheet S in the conveying direction D1 in this embodiment will be described in detail with reference to FIGS. 5, 6, and 7. FIG. 7 is a diagram showing changes in the signals output from the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3 when the sheet S passes through the sheet detection unit 10 shown in FIGS. 5 and 6. In FIG. 7, the timings when the leading edge of the sheet S passes the second detection position P2 and the first detection position P1 are indicated as T2t and T1t, respectively. Also, the timing when the trailing edge of the sheet S passes the third detection position P3 is indicated as T3h. First, the conveying speed V of the sheet S is calculated from the timings T2t, T1t, and T3h and the relative positions of the first detection position P1, the second detection position P2, and the third detection position P3 in the conveying direction D1, as shown in Equation 1-1. Conveying speed V = L12 / (T1t-T2t) (Equation 1-1) In (Equation 1-1), the conveying speed V is calculated based on the difference between the timing T1t when the leading edge of the sheet passes the first detection position P1 and the timing T2t when it passes the second detection position P2, and the distance L12. The difference between the timing T1t when the leading edge of the sheet passes the first detection position P1 and the timing T2t when it passes the second detection position P2 is the first time in this embodiment, and the distance L12 is the first distance in this embodiment.

[0037] Furthermore, the length L of the sheet in the conveying direction D1 is calculated as in (Equation 1-2). Length L' = L23 + (T3h - T2t) × V Length L" = L23 + L12 + (T3h-T1t) × V Length L = (L' + L") / 2 (Equation 1-2) In (Equation 1-2), the length L' of the sheet in the conveying direction D1 is calculated based on the difference between the timing T2t when the leading edge of the sheet passes the second detection position P2 and the timing T3h when the trailing edge of the sheet passes the third detection position P3, and the distance L23. Also, the length L" of the sheet in the conveying direction D1 is calculated based on the difference between the timing T1t when the leading edge of the sheet passes the first detection position P1 and the timing T3h when the trailing edge of the sheet passes the third detection position P3, and the distance (L23+L12).

[0038] The difference between the timing T2t when the leading edge of the sheet passes the second detection position P2 and the timing T3h when the trailing edge of the sheet passes the third detection position P3 is the second time in this embodiment, and the distance L23 is the second distance in this embodiment.

[0039] Furthermore, the difference between the timing T1t when the leading edge of the sheet passes the first detection position P1 and the timing T3h when the trailing edge of the sheet passes the third detection position P3 is the third time in this embodiment, and the distance (L23+L12) is the third distance in this embodiment.

[0040] Here, the sheet length L is calculated from the average of the length L' as the first length in this embodiment and the length L" as the second length. This makes it possible to correct variations in the detection of the edge of the sheet S by each of the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3, thereby reducing the calculation error of the length of the sheet S. Therefore, the length L of the sheet in the conveying direction D1 may be calculated based on only either the length L' or the length L". In this embodiment, for example, the conveying speed V of the sheet S calculated by (Equation 1-1) may be substituted into the equation (Equation 1-2) length L" = L23 + L12 + (T3h - T1t) × V, and the length L of the sheet in the conveying direction D1 may be calculated as length L = length L".

[0041] In this manner, in this embodiment, the length L is calculated using the timing (T2t, T1t) at which the leading edge of the sheet passes after it has passed through the pair of conveying rollers 11 and the timing (T3h) at which the trailing edge of the sheet passes before it passes through the pair of conveying rollers 11. Therefore, the length L of the sheet S in the conveying direction D1 can be calculated with high accuracy without being affected by the sheet entering the downstream conveying roller pair 13. In other words, by controlling the timing at which the skew correction unit conveys the sheet based on the length L of the sheet S, it is possible to suppress misalignment between the front and back of the image formed on the sheet S.

[0042] Next, modifications of Embodiment 1 are shown in FIGS. 8 and 9. Regarding FIG. 8, a state is assumed in which after the leading end of the sheet is nipped by the downstream conveyance roller pair, the trailing end of the sheet is detected by the third detection unit. That is, regarding the length L of the sheet, it is longer than that in Embodiment 1, and it is the case when the positional relationship between each conveyance roller pair and each detection unit is as follows (Equation 4)'. ·L(2 - 12) < L, and L(13 - 3) < L ··· (Equation 4)'

[0043] As in the above Embodiment 1, by arranging the trailing end of the sheet to be detected by the third detection unit before the leading end of the sheet is nipped by the downstream conveyance roller pair, it is possible to surely prevent the impact when the leading end of the sheet is nipped by the downstream conveyance roller pair. However, even when the positional relationship is as in the above (Equation 4)' as in this modification, the length L of the sheet S can be accurately calculated for the following reasons. FIG. 9 is a diagram showing the sheet conveyance behavior of the sheet detection unit in the arrangement of FIG. 8, and this will be used for explanation. FIG. 9(A) is a diagram showing that a sheet of length L is sandwiched by the upstream conveyance roller pair 12 and being conveyed. This is the timing when the leading end of the sheet is detected by the third detection unit S3. FIG. 9(B) is a diagram showing that the sheet is sandwiched by the conveyance roller pair 11 and being conveyed. This is the timing when the leading end of the sheet is detected by the second detection unit S2. At this time, the trailing end of the sheet is in a state of being sandwiched by the upstream conveyance roller pair 12. FIG. 9(C) is a diagram showing that the sheet is sandwiched by the conveyance roller pair 11 and being conveyed. This is the timing when the leading end of the sheet is detected by the first detection unit S1. At this time, the trailing end of the sheet is after passing through the upstream conveyance roller pair 12 and the third detection unit S3 is in a state of detecting the sheet.

[0044] In this modified example, the sheet conveyance speed is calculated when the leading edge of the sheet passes the second and first detection units. When the leading edge of the sheet passes the second detection unit, the trailing edge of the sheet is sandwiched between the upstream conveyance roller pair 12. When the leading edge of the sheet passes the third detection unit, the trailing edge of the sheet has passed the upstream conveyance roller pair 12. Generally, when the number of sandwiching conveyance roller pairs changes, the sheet conveyance speed may vary significantly. However, in this modified example, the leading edge of the sheet is sandwiched between the blast-treated conveyance roller pair 11, so the impact is small. Figure 9(D) shows the timing when the leading edge of the sheet reaches the downstream conveyance roller pair 13. At this time, as described above, the leading edge of the sheet receives an impact from the downstream conveyance roller pair 13. However, in this modified example, because the trailing edge of the sheet is sandwiched between the conveyance roller pair 11, sheet positional deviation is unlikely to occur upstream of the conveyance roller pair 11. Note that this impact may be expected for the sheet between the conveyance roller pair 11 and the downstream conveyance roller pair 13. 9(E) shows the timing when the trailing edge of the sheet is detected by the third detection unit. The trailing edge of the sheet is detected by the third detection unit while being nipped and conveyed between the pair of conveying rollers 11. In other words, the trailing edge of the sheet can be detected without being affected by the impact when the leading edge of the sheet enters the pair of downstream conveying rollers 13.

[0045] As described above, the sheet receives an impact when the leading edge of the sheet is nipped by the downstream conveyance roller pair, but the trailing edge of the sheet is sandwiched between the conveyance roller pair 11. While the sheet is sandwiched between the conveyance roller pair 11, the trailing edge of the sheet is detected by the third detection unit. This makes it possible to detect the impact that the leading edge of the sheet receives from the downstream conveyance roller pair 13 without the trailing edge of the sheet receiving the impact. In other words, it becomes possible to accurately calculate the conveyance speed and length of the sheet being conveyed. Note that in a configuration in which the trailing edge of the sheet is detected downstream of the conveyance roller pair 11 as in the conventional example, the detection is performed in a state that includes the impact that the leading edge of the sheet received from the downstream conveyance roller pair 13, so naturally the detection accuracy decreases.

[0046] [Embodiment 2] <Configuration for Calculating Sheet Length in the Conveying Direction in the Second Embodiment> Next, a description will be given of a configuration for calculating the length of a sheet in the conveying direction in embodiment 2. Fig. 10 is a top view showing the configuration of a sheet detection unit 10 for calculating the length of a sheet in the conveying direction in embodiment 2.

[0047] The difference from the first embodiment is the number of detection units arranged upstream and downstream of the conveyance roller pair 11. Specifically, in this embodiment, two detection units are arranged upstream of the conveyance roller pair 11 and downstream of the upstream conveyance roller pair 12, and one detection unit is arranged downstream of the conveyance roller and upstream of the downstream conveyance roller pair 13. Note that the configurations of the printer 1, the conveyance roller pair 11, the upstream conveyance roller pair 12, and the downstream conveyance roller pair 13 are the same as those of the first embodiment, so redundant description will be omitted. The sheet detection unit 10 will be described assuming that it is arranged in the duplex conveyance section 502, but it can be arranged anywhere on the conveyance path along which the sheet S is conveyed in the printer 1, other than the duplex conveyance section 502. The sheet detection unit 10 has the conveyance roller pair 11 that conveys the sheet S, and a first detection unit S1, a second detection unit S2, and a third detection unit S3 that detect the passage of an edge of the sheet.

[0048] The first detection unit S1 detects the passage of an edge of a sheet at a first detection position P1 that is upstream of the conveying roller pair 11 in the conveying direction D1 and downstream of the upstream conveying roller pair 12. The second detection unit S2 detects the passage of an edge of a sheet at a second detection position P2 that is upstream of the conveying roller pair 11 in the conveying direction D1 and different from the first detection position P1. The third detection unit S3 detects the passage of an edge of a sheet at a third detection position P3 that is downstream of the conveying roller pair 11 in the conveying direction D1 and upstream of the downstream conveying roller pair 13.

[0049] FIG. 8 shows an example in which the first detection position P1, the second detection position P2, and the third detection position P3 are each located at the center of the sheet width direction perpendicular to the conveying direction D1. Also, in FIG. 8, the distance between the first detection position P1 and the second detection position P2 in the conveying direction D1 is shown as L12, and the distance between the second detection position P2 and the third detection position P3 in the conveying direction D1 is shown as L23. The first detection unit S1 includes an optical sensor that outputs a low signal when no sheet is present at the first detection position P1 and a high signal when a sheet is present. The second detection unit S2 and the third detection unit S3 also use the same sensor as the first detection unit S1. Therefore, when the leading edge of the sheet S passes, the signals output from the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3 switch from low to high. Furthermore, when the trailing edge of the sheet S passes, the signals output from the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3 switch from High to Low. Therefore, the control unit 9 can recognize the timing when the leading or trailing edge of the sheet passes the first detection position P1, the second detection position P2, and the third detection position P3, based on the signals output from the sensors of the first detection unit S1, the second detection position P2, and the third detection position P3. The control unit 9 can also calculate the conveying speed V of the sheet S and the length of the sheet S in the conveying direction D1 based on the timing when the trailing or leading edge of the sheet passes the first detection position P1, the second detection position P2, and the third detection position P3.

[0050] Furthermore, if the length of the sheet S is L, the length between the conveying roller pair 11 and the upstream conveying roller pair 12 is L(11-12), the length between the downstream conveying roller pair 13 and the conveying roller pair 11 is L(13-11), the length between the third detection unit S3 and the upstream conveying roller pair 12 is L(3-12), and the length between the downstream conveying roller pair 13 and the second detection unit S2 is L(13-2), the rollers are arranged so that the following relationship holds. L>L(L11-12) and L>(L13-11) (Equation 5) ·L(3-12)>L and L(13-2)>L···(Equation 6) By arranging each conveying roller pair and each detecting unit so as to satisfy the relationships of (Equation 5) and (Equation 6) above, the sheet detection unit 10 can detect the leading and trailing ends of the sheet using the first detecting unit, the second detecting unit, and the third detecting unit while the sheet is being conveyed by the conveying roller pair 11 and in a state where the sheet is not being nipped by the upstream conveying roller pair or the downstream conveying roller pair. In other words, when calculating the length of the sheet, it is possible to perform the calculation before the leading end of the sheet is nipped by the downstream conveying roller pair. In other words, it is possible to suppress vibration of the sheet caused by the leading end of the sheet being nipped by the downstream conveying roller pair, and it is possible to detect the length of the sheet with high accuracy.

[0051] Next, a method for calculating the length of the sheet S in the conveying direction D1 in this embodiment will be described with reference to FIGS. 10 and 11. FIG. 11 is a diagram showing changes in the signals output from the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3 when the sheet S passes through the sheet detection unit 10 of FIG. 10. In FIG. 11, the timings when the trailing edge of the sheet S passes through the first detection position P1 and the second detection position P2 are indicated as T1h and T2h, respectively. Also, the timing when the leading edge of the sheet S passes through the third detection position P3 is indicated as T3t. First, the conveying speed V of the sheet S is calculated from the timings T1h, T2h, and T3t and the relative positions of the first detection position P1, the second detection position P2, and the third detection position P3 in the conveying direction D1, as shown in Equation 2-1. Conveying speed V = L12 / (T2h-T1h) (Equation 2-1) In (Equation 2-1), the conveying speed V is calculated based on the difference between the timing T1h when the trailing edge of the sheet passes the first detection position P1 and the timing T2h when it passes the second detection position P2, and the distance L12. The difference between the timing T1h when the trailing edge of the sheet passes the first detection position P1 and the timing T2h when it passes the second detection position P2 is the first time in this embodiment, and the distance L12 is the first distance in this embodiment.

[0052] Furthermore, the length L of the sheet in the conveying direction D1 is calculated as in (Equation 2-2). Length L' = L12 + L23 + (T1h - T3t) × V Length L" = L23 + (T2h - T3t) × V Length L = (L' + L") / 2 (Equation 2-2) In (Equation 2-2), the length L' of the sheet in the conveying direction D1 is calculated based on the difference between the timing T1h when the rear end of the sheet passes the first detection position P1 and the timing T3t when the front end of the sheet passes the third detection position P3, and the distance (L23+L12). In addition, the length L" of the sheet in the conveying direction D1 is calculated based on the difference between the timing T2h when the trailing end of the sheet passes the second detection position P2 and the timing T3t when the leading end of the sheet passes the third detection position P3, and the distance L23. The difference between the timing T1h when the trailing end of the sheet passes the first detection position P1 and the timing T3t when the leading end of the sheet passes the third detection position P3 is the third time in this embodiment, and the distance (L23+L12) is the third distance in this embodiment. In addition, the difference between the timing T2h when the trailing end of the sheet passes the second detection position P2 and the timing T3t when the leading end of the sheet passes the third detection position P3 is the second time in this embodiment, and the distance L23 is the second distance in this embodiment.

[0053] Here, the sheet length L is calculated from the average of the length L' as the first length in this embodiment and the length L" as the second length. This makes it possible to correct variations in the detection of the edge of the sheet S by each of the sensors of the first detection unit S1, the second detection unit S2, and the third detection unit S3, thereby reducing calculation errors in the length of the sheet S. Therefore, the length L of the sheet in the conveying direction D1 may be calculated based on only either the length L' or the length L". In this embodiment, for example, the conveying speed V of the sheet S calculated by (Equation 2-1) may be substituted into the equation (Equation 2-2) length L' = L12 + L23 + (T1h - T3t) × V, and the length L of the sheet in the conveying direction D1 may be calculated as length L = length L'.

[0054] As described above, in this embodiment, the length L is calculated using the timing (times T1h and T2h) when the rear end of the sheet passes before the leading edge of the sheet passes through the pair of conveying rollers 11, and the timing (time T3t) when the rear end of the sheet passes after the leading edge of the sheet passes through the pair of conveying rollers 11. Therefore, the length L of the sheet S in the conveying direction D1 can be calculated with high accuracy without being affected by the sheet entering the pair of downstream conveying rollers 13. In other words, by controlling the timing of conveying the sheet by the skew correction unit based on the length L of the sheet S, it is possible to suppress misalignment between the front and back sides of the image formed on the sheet S. Note that as a modification of the second embodiment, a configuration similar to that of the modification of the first embodiment may also be used. That is, the rear end of the sheet may be detected by the second detection unit after the leading edge of the sheet is sandwiched between the pair of downstream conveying rollers 13.

[0055] [Embodiment 3] <Configuration for Calculating Sheet Length in the Conveying Direction in the Third Embodiment> Next, a description will be given of a configuration for calculating the length of a sheet in the conveying direction in embodiment 3. Fig. 12 is a top view showing the configuration of a sheet detection unit 10 for calculating the length of a sheet in the conveying direction in embodiment 3.

[0056] The difference between the first and second embodiments is the number of detection units arranged upstream and downstream of the conveyance roller pair 11. Specifically, in this embodiment, two detection units are arranged upstream of the conveyance roller pair 11 and downstream of the upstream conveyance roller pair 12, and two detection units are arranged downstream of the conveyance roller pair and upstream of the downstream conveyance roller pair 13. Note that the configurations of the printer 1, the conveyance roller pair 11, the upstream conveyance roller pair 12, and the downstream conveyance roller pair 13 are the same as in the first embodiment, so redundant description will be omitted. The sheet detection unit 10 will be described assuming that it is arranged in the duplex conveyance section 502, but it can be arranged anywhere on the conveyance path along which the sheet S is conveyed in the printer 1, other than the duplex conveyance section 502. The sheet detection unit 10 has the conveyance roller pair 11 that conveys the sheet S, and a first detection unit S1, a second detection unit S2, a third detection unit S3, and a fourth detection unit S4 that detect the passage of an edge of the sheet.

[0057] The first detection unit S1 detects the passage of an edge of a sheet at a first detection position P1, which is upstream of the pair of conveying rollers 11 in the conveying direction D1 and downstream of the pair of upstream conveying rollers 12. The second detection unit S2 detects the passage of an edge of a sheet at a second detection position P2, which is upstream of the pair of conveying rollers 11 in the conveying direction D1 and different from the first detection position P1. The third detection unit S3 detects the passage of an edge of a sheet at a third detection position P3, which is downstream of the pair of conveying rollers 11 in the conveying direction D1 and upstream of the pair of downstream conveying rollers 13. The fourth detection unit S4 detects the passage of an edge of a sheet at a fourth detection position P4, which is downstream of the pair of conveying rollers in the conveying direction D1 and different from the third detection position P3.

[0058] 10 shows an example in which the first detection position P1, the second detection position P2, the third detection position P3, and the fourth detection position P4 are each disposed at the center of the sheet width direction perpendicular to the conveying direction D1. Also, in FIG. 12, the distance between the first detection position P1 and the second detection position P2 in the conveying direction D1 is shown as L12, the distance between the second detection position P2 and the third detection position P3 in the conveying direction D1 is shown as L23, and the distance between the third detection position P3 and the fourth detection position P4 in the conveying direction D1 is shown as L34. The first detection unit S1 includes an optical sensor that outputs a low signal when no sheet is present at the first detection position P1 and a high signal when a sheet is present. The second detection unit S2, the third detection unit S3, and the fourth detection unit S4 also use the same sensor as the first detection unit S1. Therefore, when the leading edge of the sheet S passes, the signals output from the sensors of the first detector S1, the second detector S2, the third detector S3, and the fourth detector S4 switch from low to high. When the trailing edge of the sheet S passes, the signals output from the sensors of the first detector S1, the second detector S2, the third detector S3, and the fourth detector S4 switch from high to low. Therefore, the control unit 9 can recognize the timing of the leading edge or trailing edge of the sheet passing through the first detection position P1 based on the signal output from the sensor of the first detector S1. The control unit 9 can also recognize the timing of the leading edge or trailing edge of the sheet passing through the second detection position P2, the third detection position P3, and the fourth detection position P4 for the second detector S2, the third detector S3, and the fourth detector S4, just like the first detector S1. In addition, the control unit 9 can calculate the conveying speed V of the sheet S and the length of the sheet S in the conveying direction D1 based on the timing at which the trailing or leading end of the sheet passes through the first detection position P1, the second detection position P2, the third detection position P3, and the fourth detection position P4.

[0059] Furthermore, if the length of the sheet S is L, the length between the conveying roller pair 11 and the upstream conveying roller pair 12 is L(11-12), the length between the downstream conveying roller pair 13 and the conveying roller pair 11 is L(13-11), the length between the third detection unit S3 and the upstream conveying roller pair 12 is L(3-12), and the length between the downstream conveying roller pair 13 and the second detection unit S2 is L(13-2), the rollers are arranged so that the following relationship holds. L>L(L11-12) and L>(L13-11) (Equation 7) ·L(3-12)>L and L(13-2)>L···(Equation 8)

[0060] By arranging each conveying roller pair and each detector so as to satisfy the relationships of (Equation 7) and (Equation 8) above, the sheet detection unit 10 can detect the leading and trailing edges of the sheet using the first, second, third, and fourth detectors while the sheet is being conveyed by the conveying roller pair 11 and without the sheet being nipped by the upstream conveying roller pair or the downstream conveying roller pair. In other words, when calculating the length of the sheet, it is possible to perform the calculation before the leading edge of the sheet is nipped by the downstream conveying roller pair. In other words, it is possible to suppress vibration of the sheet caused by the leading edge of the sheet being nipped by the downstream conveying roller pair, and it is possible to detect the length of the sheet with high accuracy.

[0061] Next, a method for calculating the length of the sheet S in the conveying direction D1 in this embodiment will be described with reference to FIGS. 12 and 13. FIG. 13 is a diagram showing changes in signals output from the sensors of the first detection unit S1, the second detection unit S2, the third detection unit S3, and the fourth detection unit S4 when the sheet S passes through the sheet detection unit 10 of FIG. 12. In FIG. 10, the timings when the trailing edge of the sheet S passes through the first detection position P1 and the second detection position P2 are indicated as T1h and T2h, respectively. Furthermore, the timings when the leading edge of the sheet S passes through the third detection position P3 and the fourth detection position P4 are indicated as T3t and T4t, respectively. First, the conveying speed V of the sheet S is calculated from the timings T1h, T2h, T3t, and T4t and the relative positions of the first detection position P1, the second detection position P2, the third detection position P3, and the fourth detection position P4 in the conveying direction D1, as shown in Equation 3-1. Speed ​​V'=L34 / (T4t-T3t) Speed ​​V”=L12 / (T2h-T1h) Conveying speed V = (V' + V") / 2 (Equation 3-1) In (Equation 3-1), a speed V" is calculated as the first speed in this embodiment based on the difference between the timing T1h when the trailing end of the sheet passes through the first detection position P1 and the timing T2h when it passes through the second detection position P2, and the distance L12. The difference between the timing T1h when the trailing end of the sheet passes through the first detection position P1 and the timing T2h when it passes through the second detection position P2 is the first time in this embodiment, and the distance L12 is the first distance in this embodiment. The difference between the timing T3t when the leading end of the sheet passes through the third detection position P3 and the timing T4t when it passes through the fourth detection position P4 is the fourth time in this embodiment, and the distance L34 is the fourth distance in this embodiment.

[0062] Furthermore, in (Equation 3-1), the speed V' as the second speed in this embodiment is calculated based on the difference between the timing T3t when the leading edge of the sheet passes the third detection position P3 and the timing T4t when the leading edge of the sheet passes the fourth detection position P4, and the distance L34. Here, by calculating the conveying speed V from the average of the speeds V' and V", it is possible to reduce calculation errors due to variations in detection of the edge of the sheet S by the sensors of the first detection unit S1, the second detection unit S2, the third detection unit S3, and the fourth detection unit S4.

[0063] Furthermore, the length L of the sheet in the conveying direction D1 is calculated as in (Equation 3-2). Length L' = L12 + L23 + L34 + (T1h - T4t) × V Length L" = L12 + L23 + (T1h-T3t) × V Length L"' = L23 + L34 + (T2h - T4t) × V Length L"" = L23 + (T2h - T3t) x V Length L = (L' + L" + L"' + L"") / 4 (Equation 3-2) In (Equation 3-2), the length L' of the sheet in the conveying direction D1 is calculated based on the difference between the timing T1h when the trailing edge of the sheet passes the first detection position P1 and the timing T4t when the leading edge of the sheet passes the fourth detection position P4, and the distance (L12+L23+L34). Also, the length L" of the sheet in the conveying direction D1 is calculated based on the difference between the timing T1h when the trailing edge of the sheet passes the first detection position P1 and the timing T3t when the leading edge of the sheet passes the third detection position P3, and the distance (L12+L23). Also, the length L"' of the sheet in the conveying direction D1 is calculated based on the difference between the timing T2h when the trailing edge of the sheet passes the second detection position P2 and the timing T4t when the leading edge of the sheet passes the fourth detection position P4, and the distance (L23+L34). Furthermore, the length L'''' of the sheet in the conveying direction D1 is calculated based on the difference between the timing T2h when the rear end of the sheet passes the second detection position P2 and the timing T3t when the front end of the sheet passes the third detection position P3, and the distance L23.

[0064] The difference between the timing T1h when the trailing edge of the sheet passes the first detection position P1 and the timing T3t when the leading edge of the sheet passes the third detection position P3 is the third time in this embodiment, and the distance (L23+L12) is the third distance in this embodiment. Also, the difference between the timing T2h when the trailing edge of the sheet passes the second detection position P2 and the timing T3t when the leading edge of the sheet passes the third detection position P3 is the second time in this embodiment, and the distance L23 is the second distance in this embodiment.

[0065] Here, the sheet length L is calculated from the average of the length L' as the first length in this embodiment, and the lengths L", L'", and L"" as the second lengths. This makes it possible to reduce calculation errors in the length of the sheet S due to variations in detection of the edge of the sheet S by the sensors of the first detection unit S1, the second detection unit S2, the third detection unit S3, and the fourth detection unit S4. Note that the length L of the sheet in the conveying direction D1 may be calculated based on only one of the length L' or the length L", or on the average of two or more of the lengths L', the length L", the length L"', and the length L"". In this embodiment, for example, the conveying speed V of the sheet S calculated by (Equation 3-1) may be substituted into the equation (Equation 3-2) of length L" = L12 + L23 + (T1h - T3t) × V to calculate the length L of the sheet in the conveying direction D1 as length L = length L".

[0066] As described above, in this embodiment, the length L is calculated from the timing (T1h, T2h) at which the rear end of the sheet passes before passing through the pair of conveying rollers 11 and the timing (T3t, T4t) at which the leading edge of the sheet passes through the pair of conveying rollers 11. Therefore, the length L of the sheet S in the conveying direction D1 can be calculated with high accuracy without being affected by the sheet entering the pair of downstream conveying rollers 13. In other words, by controlling the timing at which the skew correction unit conveys the sheet based on the length L of the sheet S, it is possible to suppress misalignment between the front and back sides of the image formed on the sheet S. Note that, as a modification of the third embodiment, a configuration similar to that of the modifications of the first and second embodiments may be used. That is, the trailing edge of the sheet may be detected by the second detection unit after the leading edge of the sheet is sandwiched between the pair of downstream conveying rollers 13.

[0067] [Embodiment 4] <Configuration for Calculating Sheet Length in the Conveying Direction in the Fourth Embodiment> Next, a description will be given of a configuration for calculating the length of a sheet in the conveying direction in embodiment 4. Fig. 14 is a top view showing the configuration of a sheet detection unit 10 for calculating the length of a sheet in the conveying direction in embodiment 4.

[0068] The difference from the first, second, and third embodiments is that a plurality of detection units are arranged in the width direction of the sheet, upstream and downstream of the transport roller pair 11. Note that the configurations of the printer 1, the transport roller pair 11, the upstream transport roller pair 12, the downstream transport roller pair 13, the first detection unit S1, the second detection unit S2, and the third detection unit S3 are the same as those of the first embodiment, so redundant explanations will be omitted. The sheet detection unit 10 will be described assuming that it is arranged in the duplex transport section 502, but it can be arranged in a location other than the duplex transport section 502 as long as it is on the transport path along which the sheet S is transported in the printer 1.

[0069] The first detection unit S1 has sensors S1A and S1B that detect the passage of an edge of a sheet at a first detection position P1 that is downstream of the pair of conveying rollers 11 in the conveying direction D1 and upstream of the pair of downstream conveying rollers 13. The sensors S1A and S1B are a pair of sensors arranged at an interval in the width direction W perpendicular to the conveying direction D1. The second detection unit S2 has sensors S2A and S2B that are downstream of the pair of conveying rollers 11 in the conveying direction D1 and detect the passage of an edge of a sheet at a second detection position P2 that is different from the first detection position P1. The sensors S2A and S2B are a pair of sensors arranged at an interval in the width direction W perpendicular to the conveying direction D1. The third detection unit S3 has sensors S3A and S3B that detect the passage of an edge of a sheet at a third detection position P3 that is upstream of the pair of conveying rollers 11 in the conveying direction D1 and downstream of the pair of upstream conveying rollers 12. The sensors S3A and S3B are a pair of sensors spaced apart in the width direction W perpendicular to the conveying direction D1.

[0070] 14, the distance between the first detection position P1 and the second detection position P2 in the conveying direction D1 is indicated as L12, and the distance between the second detection position P2 and the third detection position P3 in the conveying direction D1 is indicated as L23. The sensors S1A and S1B constituting the first detection unit S1 are optical sensors that output a low signal when no sheet is present at the first detection position P1 and a high signal when a sheet is present. The same sensors as those used for the first detection unit S1 are also used for the second detection unit S2 and the third detection unit S3. Therefore, when the leading edge of the sheet S passes, the signals output from the first detection unit S1, the second detection unit S2, and the third detection unit S3 switch from low to high.

[0071] Furthermore, when the trailing edge of the sheet S passes, the signals output from the first detection unit S1, the second detection unit S2, and the third detection unit S3 switch from High to Low. Therefore, the control unit 9 can recognize the timing when the leading edge or trailing edge of the sheet passes the first detection position P1, the second detection position P2, and the third detection position P3, based on the signals output from the first detection unit S1, the second detection position P2, and the third detection position P3. The control unit 9 can also calculate the conveying speed V of the sheet S and the length of the sheet S in the conveying direction D1 based on the timing when the leading edge or trailing edge of the sheet passes the first detection position P1, the second detection position P2, and the third detection position P3.

[0072] Furthermore, if the length of the sheet S is L, the length between the conveying roller pair 11 and the upstream conveying roller pair 12 is L(11-12), the length between the downstream conveying roller pair 13 and the conveying roller pair 11 is L(13-11), the length between the second detection unit S2 and the upstream conveying roller pair 12 is L(2-12), and the length between the downstream conveying roller pair 13 and the third detection unit S3 is L(13-3), the rollers are arranged so that the following relationship holds. L>L(L11-12) and L>(L13-11) (Equation 9) ·L(2-12)>L and L(13-3)>L···(Eq. 10)

[0073] By arranging each conveying roller pair and each detecting unit so as to satisfy the relationships of (Equation 9) and (Equation 10) above, the sheet detection unit 10 can detect the leading and trailing ends of the sheet using the first detecting unit, the second detecting unit, and the third detecting unit while the sheet is being conveyed by the conveying roller pair 11 and in a state where the sheet is not being nipped by the upstream conveying roller pair or the downstream conveying roller pair. In other words, when calculating the length of the sheet, it is possible to perform the calculation before the leading end of the sheet is nipped by the downstream conveying roller pair. In other words, it is possible to suppress vibration of the sheet caused by the leading end of the sheet being nipped by the downstream conveying roller pair, and it is possible to detect the length of the sheet with high accuracy.

[0074] 14, the first detection unit S1 is configured with sensors S1A and S1B that are arranged at overlapping positions in the conveying direction D1 and at different positions in the width direction W. The sensors S2A, S2B, S3A, and S3B are also in the same positional relationship as the sensors S1A and S1B. Therefore, the conveying speed and the length of the sheet S in the conveying direction D1 can be obtained from each of the sensors S1B, S2B, and S3B arranged on one side in the width direction W, for example, the sensors S1B, S2B, and S3B arranged on the upper side in FIG. 14, and the sensors S1A, S2A, and S3A arranged on the lower side.

[0075] Specifically, by applying the configuration of the first embodiment, the speed VB is calculated based on the signals from the sensors S1B, S2B, and S3B arranged on the upper side of Fig. 14, and the speed VA is calculated based on the signals from the sensors S1A, S2A, and S3A arranged on the lower side. Then, the conveying speed V of the sheet S is calculated from the average of the speeds VA and VB using (Equation 4-1). Conveying speed V = (VA + VB) / 2 (Equation 4-1) Next, the length LB of the sheet S is calculated based on the calculated sheet conveying speed V and the signals of the sensors S1B, S2B, and S3B arranged above. The length LA of the sheet S is also calculated based on the calculated conveying speed V and the signals of the sensors S1A, S2A, and S3A arranged below. Then, the length L of the sheet S in the conveying direction D1 is calculated from the average of the lengths LA and LB using (Equation 4-2). Length L = (LA + LB) / 2 (Equation 4-2) In this manner, in this embodiment, by arranging two sensors in the width direction W, it becomes possible to use twice as much data, and the calculation error due to the variation in detection of the edge of the sheet S can be further reduced.

[0076] <Modification of the fourth embodiment> Next, a method for calculating the length of the sheet S in the conveying direction D1 as a modified example of the fourth embodiment will be described with reference to FIGS. 15 and 16. FIG. 15 is a top view showing a state in which the sheet S is conveyed in a state inclined by a skew angle θ in the sheet detection unit 10 of FIG. 14. FIG. 16 is a diagram showing changes in signals output from sensors S2A and S2B of the second detection unit S2 in the state shown in FIG. 15. In FIG. 16, T2At and T2Bt indicate the timings at which the leading edge of the sheet S passes through sensors S2A and S2B, respectively. As shown in FIG. 15, when the sheet S is conveyed skewed, the length of the sheet S in the conveying direction calculated by (Equation 4-2) becomes the sheet length (length L') measured along the conveying center C in the width direction W. In contrast, in this embodiment, the skew angle θ of the sheet S is calculated based on changes in the signals output from sensors S2A and S2B among the first detection unit S1, the second detection unit S2, and the third detection unit S3, for example, to determine the length of the sheet S in the conveying direction.

[0077] As shown in Fig. 15, when the sheet S is conveyed at a skew angle θ with respect to the conveying direction D1, the timings at which the signals of the sensors S2A and S2B become High differ by a time (T2Bt-T2At) as shown in Fig. 16. The control unit 9 calculates the skew angle θ by performing a calculation as shown in (Equation 4-3) based on the conveying speed V obtained from (Equation 4-1), the time (T2At-T2Bt), and the length of the interval W1 between the sensors S2A and S2B. Slant angle θ=tan^(-1){(T2Bt-T2At)×V / W} (Equation 4-3) Then, based on the skew angle θ calculated in this way, the length L' calculated by (Equation 4-2) is corrected using (Equation 4-4) to obtain the length L of the sheet S in the conveying direction D1. Length L = L'cosθ (Equation 4-4) As described above, in this embodiment, even if the sheet S is skewed, the length L of the sheet S in the conveying direction D1 can be calculated with high accuracy without being affected when the sheet enters the downstream conveying roller pair 13. In reality, the sheet being conveyed is somewhat skewed, so by implementing this embodiment, the sheet conveying speed and sheet length can be calculated more accurately. In other words, by controlling the sheet conveying timing by the skew correction unit based on the length L of the sheet S, it is possible to suppress misalignment between the front and back of the image formed on the sheet S.

[0078] [Embodiment 5] <Configuration for Calculating Sheet Length in the Conveying Direction in the Fifth Embodiment> Next, a description will be given of a configuration for calculating the length of a sheet in the conveying direction in embodiment 5. Fig. 17 is a top view showing the configuration of a sheet detection unit 10 for calculating the length of a sheet in the conveying direction in embodiment 5.

[0079] The difference between the first, second, third, and fourth embodiments is the type of detection unit disposed upstream and downstream of the conveyance roller pair 11. Specifically, a CIS (Contact Image Sensor) capable of reading the edge of a sheet is disposed. The configurations of the printer 1, the conveyance roller pair 11, the upstream conveyance roller pair 12, and the downstream conveyance roller pair 13 are the same as those of the first embodiment, so a redundant description will be omitted. The following description will be given assuming that the sheet detection unit 10 is disposed in the duplex conveyance section 502; however, the sheet detection unit 10 can be disposed anywhere on the conveyance path along which the sheet S is conveyed in the printer 1, other than the duplex conveyance section 502. The sheet detection unit 10 includes the conveyance roller pair 11 that conveys the sheet S, and a first reading unit S10 and a second reading unit S20 that extend along the conveyance direction D1. The first reading unit S10 is disposed either upstream or downstream of the conveyance roller pair 11 in the conveyance direction D1, and reads the edge of the sheet S conveyed through the sheet detection unit 10. The second reading unit S20 is provided on the other of the upstream and downstream sides of the conveying roller pair 11 in the conveying direction D1, and reads the edge of the sheet S conveyed through the sheet detection unit 10. In FIG. 15, the first reading unit S10 is disposed downstream of the conveying roller pair 11 in the conveying direction D1 and upstream of the downstream conveying roller pair 13. Also, a configuration is illustrated in which the second reading unit S20 is disposed upstream of the conveying roller pair 11 in the conveying direction D1 and downstream of the upstream conveying roller pair 12. In FIG. 17, the distance from the upstream end of the first reading unit S10 to the upstream end of the second reading unit S20 in the conveying direction D1 is shown as distance L120. As the first reading unit S10 and the second reading unit S20, for example, a CIS (Contact Image Sensor) is used.

[0080] Fig. 18(A) is a diagram showing an example of multiple images read by the first reading unit S10. Fig. 19(A) is a diagram plotting the relationship between the reading timing T of multiple images read by the first reading unit S10 and the position X of the sheet edge in each of the multiple images. As shown in Figs. 18(A) and 19(A), the first reading unit S10 can read the image of the edge of a sheet conveyed through the sheet detection unit 10 at regular intervals. In Figs. 18(A) and 19(A), the position X of the sheet edge read by the first reading unit S10 is X11 at time T11, X12 at time T12, ..., and X1m at time T1m.

[0081] 18(B) is a diagram showing an example of a plurality of images read by the second reading unit S20. FIG. 19(B) is a diagram plotting the relationship between the read timing T of a plurality of images read by the second reading unit S20 and the position X of the sheet edge in each of the plurality of images. As shown in FIGS. 18(B) and 19(B), the second reading unit S20 can read the image of the edge of a sheet conveyed through the sheet detection unit 10 at regular intervals. In FIGS. 18(B) and 19(B), the position X of the sheet edge read by the second reading unit S20 is X21 at time T21, X22 at time T22, ..., and X2n at time T2n.

[0082] 18(A, B), the area where the sheet is detected is shown in white, and the area where the sheet is not detected is shown in black. In this way, the first reading unit S10 and the second reading unit S20 can read the change in the position of the edge of the sheet as the edge of the sheet passes through the sheet detection unit 10 as a series of multiple images at regular intervals. The images read by the first reading unit S10 and the second reading unit S20 are transmitted to the control unit 9. The control unit 9 calculates the sheet conveying speed and the length of the sheet in the conveying direction based on the images read by the first reading unit S10 and the second reading unit S20.

[0083] Furthermore, if the length of the sheet S is L, the length between the conveying roller pair 11 and the upstream conveying roller pair 12 is L(11-12), the length between the downstream conveying roller pair 13 and the conveying roller pair 11 is L(13-11), the length between the upstream end of the first reading unit S10 and the upstream conveying roller pair 12 is L(10-12), and the length between the downstream conveying roller pair 13 and the downstream end of the second reading unit S20 is L(12-20), the sheets are arranged so that the following relationship holds. L>L(L11-12) and L>(L13-11) (Equation 11) ·L(10-12)>L and L(12-20)>L···(Equation 12) By arranging each conveying roller pair and each reading unit so as to satisfy the relationship between the above formulas (11) and (12), the sheet detection unit 10 can detect the leading and trailing ends of the sheet using the first reading unit and the second reading unit while the sheet is being conveyed by the conveying roller pair 11 and without the sheet being nipped by the upstream conveying roller pair or the downstream conveying roller pair. In other words, the sheet length can be calculated before the leading end of the sheet is nipped by the downstream conveying roller pair. This means that vibration of the sheet caused by the leading end of the sheet being nipped by the downstream conveying roller pair can be suppressed, making it possible to accurately detect the length of the sheet. Note that, as a modification of the fifth embodiment, the same configuration as the modifications of the first, second, and third embodiments may also be used. That is, the trailing end of the sheet may be detected by the second reading unit after the leading end of the sheet is nipped by the downstream conveying roller pair 13.

[0084] The control unit 9 plots the relationship between the read timing T of multiple images based on the images read by the first reading unit S10 and the second reading unit S20 and the sheet edge position X for each of the multiple images, and performs a linear approximation using the least squares method. The slope of the line in the plot after linear approximation is then determined as the sheet conveyance speed. As shown in FIG. 19(A), V1 is the slope of the line obtained by plotting the timing difference between any two points T11, T12, and T1m and the difference in the sheet edge positions for images X11, X12, and X1m read at each time. As shown in FIG. 19(B), V2 is the slope of the line obtained by plotting the timing difference between any two points T21, T22, and T2n and the difference in the sheet edge positions for images X21, X22, and X2n read at each time. Then, assuming that the sheet conveying speed is V1 from the slope of the straight line obtained in Figure 19(A) and the sheet conveying speed is V2 from the slope of the straight line obtained in Figure 19(B), the sheet conveying speed V is calculated as shown in (Equation 5-1). Conveying speed V=(V1+V2) / 2 (Formula 5-1) The difference in timing between any two points T11, T12, . . . , T1m is an example of a fifth time in this embodiment. The difference in the position of the sheet edge in images X11, X12, . . . , X1m read at each time is an example of a fifth distance in this embodiment. The conveying speed V1 is the third speed in this embodiment. The difference in timing between any two points T21, T22, . . . , T2n is an example of a seventh time in this embodiment, and the difference in the position of the sheet edge in images X21, X22, . . . , X2n read at each time is an example of a seventh distance in this embodiment. The conveying speed V2 is the fourth speed in this embodiment.

[0085] Next, a method for calculating the length L of a sheet in the conveying direction D1 using the sheet conveying speed V will be described. As shown in Fig. 20(A), the position of the leading edge of the sheet read by the first reading unit S10 at time T = T1i is defined as X1i. As shown in Fig. 20(B), the position of the trailing edge of the sheet read by the second reading unit S20 at time T = T2j is defined as X2j. In this case, the distance conveyed by the sheet between time T = T1i and T2j can be expressed as (T2j - T1i) × V.

[0086] The sixth time in this embodiment corresponds to the timing difference between the reading time T1i of the first reading unit S10 and the reading time T1j of the second reading unit S20. The sixth distance in this embodiment corresponds to the difference between the position X1i of the leading edge of the sheet read by the first reading unit S10 and the position X2j of the trailing edge of the sheet read by the second reading unit S20.

[0087] The control unit 9 calculates the sheet length Lij from the data at time T=T1i and time T=T2j based on the distance ((T2j-T1i)×V), the distance L120, the position X1i of the sheet end, and the position X2j of the sheet end, as shown in (Equation 5-2). Length Lij = (T2j - T1i) × V + L120 + X1i - X2j (Equation 5-2) Then, based on all the images read by the first reading unit S10 and the second reading unit S20, the length L of the sheet in the conveying direction D1 is calculated as shown in (Equation 5-3). Length L=Σ[i=1→m]Σ[j=1→n] {(T2j-T1i)×V+L120+X1i-X2j} / m×n (Formula 5-3)

[0088] As described above, in this embodiment, the length L of the sheet is calculated based on the position change of the rear end of the sheet before the leading edge of the sheet passes through the pair of downstream conveying rollers 13 and the position change of the rear end of the sheet after the leading edge of the sheet passes through the pair of conveying rollers 11. Therefore, the length L of the sheet S in the conveying direction D1 can be calculated with high accuracy without being affected by the sheet entering the pair of downstream conveying rollers 13. In other words, by controlling the timing of conveying the sheet by the skew correction unit based on the length L of the sheet S, it is possible to suppress misalignment between the front and back of the image formed on the sheet S.

[0089] <Other embodiments> The printer 1 of the first to fifth embodiments is an example of an image forming apparatus, and may be an image forming apparatus equipped with an inkjet image forming means instead of an electrophotographic image forming means. Also, there are image forming apparatuses equipped with accessory devices such as an optional feeder and a sheet processing device in addition to the main body equipped with the image forming means, and the configuration equivalent to the sheet conveying device described in the first to fifth embodiments may be used to convey the sheet S in such accessory devices.

[0090] FIG. 21 shows a control block diagram for embodiments 1 to 5. The control unit 9 in embodiments 1 to 5 includes a central processing unit (CPU) and memory. The CPU loads and executes programs stored in the memory, and controls the printer 1 in cooperation with each functional unit that performs a specific function. The memory includes non-volatile storage media such as read-only memory (ROM) and volatile storage media such as random access memory (RAM), and serves as a storage location for programs and data and as a work area for the CPU when executing the programs. The memory is an example of a non-transitory storage medium that stores programs for controlling the printer 1. The control unit 9 may be implemented on the control unit circuit as independent hardware such as an ASIC, or may be implemented in software as a functional unit of a program executed by the CPU or another processing unit. The control unit rotates each motor connected to each roller based on information from the first to fourth detection units and the first and second reading units. The rotation of the motors can also be controlled in terms of timing and speed, and is controlled based on information from each detection unit and reading unit. [Explanation of symbols]

[0091] 1 Printer (image forming device) / 7 Registration roller pair (second conveying roller pair) / 9 Control unit / 11 Conveying roller pair (first conveying roller pair) / 12 Upstream conveying roller pair / 13 Downstream conveying roller pair / 100D Sheet conveying system (sheet conveying device) / 502 Double-sided conveying unit / 513 Image forming engine (image forming means) / P1 First detection position / P2 Second detection position / P3 Third detection position / P4 Fourth detection position / S1 First detection unit / S1A, S1B Sensor / S2 Second detection unit / S2A, S2B Sensor / S3 Third detection unit / S3A, S3B Sensor / S4 Fourth detection unit / S10 First reading unit / S20 Second reading unit

Claims

1. a first conveying roller pair that rotates while sandwiching a sheet and conveys the sheet; an upstream conveying roller pair disposed upstream of the first conveying roller pair in a sheet conveying direction and configured to convey the sheet; a downstream conveying roller pair disposed downstream of the first conveying roller pair in the conveying direction and configured to convey a sheet; a second conveying roller pair disposed downstream of the downstream conveying roller pair in the conveying direction and configured to convey a sheet; a first detection unit that is provided at a first detection position downstream of the first conveying roller pair and upstream of the downstream conveying roller pair in the conveying direction, and that detects passage of a leading edge of the sheet conveyed by the first conveying roller pair; a second detection unit that is provided at a second detection position that is downstream of the first conveying roller pair and upstream of the downstream conveying roller pair in the conveying direction, and that is different from the first detection position, and that detects passage of a leading edge of the sheet conveyed by the first conveying roller pair; a third detection unit provided at a third detection position upstream of the first conveying roller pair and downstream of the upstream conveying roller pair in the conveying direction, the third detection unit detecting passage of a trailing end of the sheet conveyed by the first conveying roller pair; a control unit that performs calculations in response to signals from the first detection unit, the second detection unit, and the third detection unit, the control unit calculates a sheet conveying speed based on a first time that is a difference between a timing at which a leading edge of the sheet passes the first detection position and a timing at which the leading edge of the sheet passes the second detection position, and a first distance between the first detection position and the second detection position in the conveying direction; calculating a length of the sheet in the conveying direction based on the conveying speed, a second time that is a difference between a timing when the leading edge of the sheet passes the second detection position and a timing when the trailing edge of the sheet passes the third detection position, and a second distance between the second detection position and the third detection position in the conveying direction; controlling a timing for conveying the sheet by the second conveying roller pair based on the information about the length of the sheet; A sheet conveying device characterized by:

2. when the leading edge of the sheet conveyed by the first conveying roller pair is detected by the first detecting unit and the second detecting unit, the trailing edge of the sheet has passed through the upstream conveying roller pair, when the rear end of the sheet conveyed by the first conveying roller pair is detected by the third detection unit, the leading end of the sheet has not reached the downstream conveying roller pair; 2. The sheet transport device according to claim 1.

3. the control unit sets the calculated length of the sheet as a first length, calculating a second length of the sheet in the conveying direction based on the conveying speed, a third time that is a difference between a timing when the leading edge of the sheet passes the first detection position and a timing when the trailing edge of the sheet passes the third detection position, and a third distance between the first detection position and the third detection position in the conveying direction; calculating a length of the sheet in the conveying direction based on the first length and the second length, and controlling a timing of conveying the sheet by the second conveying roller pair based on information about the length of the sheet; 2. The sheet transport device according to claim 1.

4. a first conveying roller pair that rotates while sandwiching a sheet and conveys the sheet; an upstream conveying roller pair disposed upstream of the first conveying roller pair in a sheet conveying direction and configured to convey the sheet; a downstream conveying roller pair disposed downstream of the first conveying roller pair in the conveying direction and configured to convey a sheet; a second conveying roller pair disposed downstream of the downstream conveying roller pair in the conveying direction and configured to convey a sheet; a first detection unit that is provided at a first detection position upstream of the first conveying roller pair and downstream of the upstream conveying roller pair in the conveying direction, and that detects passage of a trailing end of the sheet conveyed by the first conveying roller pair; a second detection unit that is provided at a second detection position that is upstream of the first conveying roller pair and downstream of the upstream conveying roller pair in the conveying direction and that is different from the first detection position, and that detects passage of a trailing end portion of the sheet conveyed by the first conveying roller pair; a third detection unit that is provided at a third detection position downstream of the first conveying roller pair and upstream of the downstream conveying roller pair in the conveying direction, and that detects passage of a leading edge of the sheet conveyed by the first conveying roller pair; a control unit that performs calculations in response to signals from the first detection unit, the second detection unit, and the third detection unit, when the leading edge of the sheet conveyed by the first conveying roller pair is detected by the third detection unit, the trailing edge of the sheet has passed through the upstream conveying roller pair, When the trailing edge of the sheet conveyed by the first conveying roller pair is detected by the first detecting unit and the second detecting unit, the leading edge of the sheet has not yet reached the downstream conveying roller pair, the control unit calculates a sheet conveying speed based on a first time that is a difference between a timing at which the trailing edge of the sheet passes the first detection position and a timing at which the trailing edge of the sheet passes the second detection position, and a first distance between the first detection position and the second detection position in the conveying direction; calculating a length of the sheet in the conveying direction based on the conveying speed, a second time that is a difference between a timing when the leading edge of the sheet passes the third detection position and a timing when the trailing edge of the sheet passes the second detection position, and a second distance between the second detection position and the third detection position in the conveying direction; controlling a timing for conveying the sheet by the second conveying roller pair based on the information about the length of the sheet; A sheet conveying device characterized by:

5. the control unit sets the calculated length of the sheet as a first length, calculating a second length of the sheet in the conveying direction based on the conveying speed, a third time that is a difference between a timing when the leading edge of the sheet passes the third detection position and a timing when the trailing edge of the sheet passes the first detection position, and a third distance between the first detection position and the third detection position in the conveying direction; calculating a length of the sheet in the conveying direction based on the first length and the second length, and controlling a timing of conveying the sheet by the second conveying roller pair based on information about the length of the sheet; 5. The sheet transport device according to claim 4.

6. a fourth detection unit that is provided at a fourth detection position that is located downstream of the first conveying roller pair and upstream of the downstream conveying roller pair in the conveying direction and that is different from the third detection position, and that detects passage of a leading edge of the sheet conveyed by the first conveying roller pair; the control unit sets the calculated sheet conveying speed as a first speed, calculating a second speed based on a fourth time that is a difference between a timing at which the leading edge of the sheet passes the third detection position and a timing at which the leading edge of the sheet passes the fourth detection position, and a fourth distance between the third detection position and the fourth detection position in the conveying direction; calculating a sheet conveying speed based on the first speed and the second speed; 6. The sheet conveying device according to claim 4 or 5.

7. the first detection unit, the second detection unit, and the third detection unit are a plurality of detection units arranged at intervals in a width direction of the sheet that is perpendicular to the conveying direction, the control unit calculates a skew angle of the sheet based on a difference between a timing at which the leading edge of the sheet passes one of the plurality of detection units and a timing at which the leading edge of the sheet passes the other of the plurality of detection units, and a length of the interval in the width direction, and corrects a length of the sheet based on the skew angle.

7. The sheet conveying device according to claim 1, wherein the sheet conveying device is a sheet conveying device.

8. a first conveying roller pair that rotates while sandwiching a sheet and conveys the sheet; an upstream conveying roller pair disposed upstream of the first conveying roller pair in a sheet conveying direction and configured to convey the sheet; a downstream conveying roller pair disposed downstream of the first conveying roller pair in the conveying direction and configured to convey a sheet; a second conveying roller pair disposed downstream of the downstream conveying roller pair in the conveying direction and configured to convey a sheet; a first reading unit that is disposed downstream of the first conveying roller pair and upstream of the downstream conveying roller pair in the conveying direction, extending along the conveying direction, and that reads an image of a leading edge of a sheet conveyed by the first conveying roller pair; a second reading unit disposed upstream of the first conveying roller pair and downstream of the upstream conveying roller pair in the conveying direction, extending along the conveying direction, and configured to read an image at a rear end of a sheet conveyed by the first conveying roller pair; a control unit that performs calculations in accordance with the images read by the first reading unit and the second reading unit, when an image on the leading edge of the sheet conveyed by the first conveying roller pair is detected by the first reading unit, the trailing edge of the sheet has passed through the upstream conveying roller pair, When the image at the rear end of the sheet conveyed by the first conveying roller pair is detected by the second reading unit, the leading end of the sheet has not yet reached the downstream conveying roller pair, the control unit calculates a sheet conveying speed based on a fifth time that is a difference in timing between the plurality of images read by the first reading unit and a fifth distance that is a difference in position of a leading edge of the sheet in the plurality of images; calculating a length of the sheet in the conveying direction based on the conveying speed, a sixth time that is a timing difference between the image read by the first reading unit and the image read by the second reading unit, and a sixth distance that is a difference in sheet position between the image read by the first reading unit and the image read by the second reading unit; controlling a timing for conveying the sheet by the second conveying roller pair based on the information about the length of the sheet; A sheet conveying device characterized by:

9. the control unit sets the calculated sheet conveying speed as a third speed, calculating a fourth speed based on a seventh time that is a difference in timing between the plurality of images read by the second reading unit and a seventh distance that is a difference in position of an edge of the sheet in the plurality of images; calculating a sheet conveying speed based on the third speed and the fourth speed; 9. The sheet transport device according to claim 8.

10. the first conveying roller pair has a blast-treated peripheral surface that contacts the sheet; 10. The sheet conveying device according to claim 1, wherein the sheet conveying device is a sheet conveying device.

11. a sheet conveying device according to any one of claims 1 to 10; an image forming means for forming an image on the sheet conveyed by the sheet conveying device, An image forming apparatus characterized by:

12. a double-sided conveying unit that reverses the sheet on which the image has been formed by the image forming means and conveys the sheet to the image forming means again; the sheet conveying device calculates the length of the sheet being conveyed through the double-sided conveying unit; 12. The image forming apparatus according to claim 11.

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