Sheet transport device and image forming apparatus

Abstract

This helps to suppress the increase in size of the equipment. [Solution] The system includes a third sensor S3 that detects a sheet S being transported in the sheet transport direction D1 at a third detection position S3a, a fourth sensor S4 that detects a sheet S being transported in the sheet transport direction D1 at a fourth detection position S4a downstream of the third detection position S3a, a second transport roller pair 602 positioned upstream of the third detection position S3a and transporting the sheet S in the sheet transport direction D1, and a control unit that calculates the sheet length in the sheet transport direction D1 based on the detection result of the third sensor S3 on the sheet S being transported by the second transport roller pair 602 and the transport speed of the sheet S being transported by the second transport roller pair 602, and calculates the amount of curl of the sheet S being transported by the second transport roller pair 602 based on the detection result of the fourth sensor S4 on the sheet S being transported by the second transport roller pair 602.

Description

Technical Field

[0001] The present invention relates to a sheet conveying device for conveying a sheet and an image forming apparatus to which the same is applied.

Background Art

[0002] In recent years, image forming apparatuses such as copiers, printers, and facsimiles using an electrophotographic method have become widespread. At the time of image formation in such an image forming apparatus, a toner image is transferred to a sheet being conveyed with reference to the leading edge of the sheet. However, when there is a deviation in the length of the sheet, a deviation occurs in the distance (margin) between the trailing edge side of the sheet and the image end. Further, when forming images on both sides of the sheet, the leading edge serving as a reference for the image position on the sheet is switched between the front side and the back side during transfer, leading to a positional deviation of the image between the front and the back. For this reason, as a method for detecting the length of the sheet, a configuration in which a plurality of sensors are provided so as to sandwich a pair of conveying rollers at different positions in the conveying direction of the sheet is known (see Patent Document 1). In this configuration, by measuring the time it takes for the sheet to pass through these sensors, the conveying speed of the sheet is calculated, and the length of the sheet in the conveying direction is calculated. Thereby, by adjusting the size and position of the image according to the length of the sheet, it becomes possible to form an image at an accurate position.

[0003] Furthermore, in electrophotographic image forming apparatuses, various types of curls occur, such as heat curl (downward curl) caused by passing through a fixing device that applies heat to fix the toner image to the sheet, and toner curl (upward curl) caused by the contraction of the toner. To suppress the occurrence of curls, some apparatuses are equipped with curl correction means to correct the curls that occur in the sheet. As an example of such curl correction means, a pair of rollers with different hardnesses configured to have a variable pressing force is known, but even with the same type of sheet, the amount of curl that occurs may differ depending on the manufacturing lot. Therefore, even if the amount of curl that occurs is predicted from various parameters and the curl correction force is determined accordingly, some curl may remain or be overcorrected. To address this, a means has been proposed to detect the state of the sheet that has passed through the curl correction means and to correct the curl correction force of the next sheet based on the detected amount of remaining curl (see Patent Document 2). Two sensors that intersect each other are placed in the transport path, and the direction and height of the curl are detected using the timing information of their detection. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-42963 [Patent Document 2] Japanese Patent Publication No. 2008-24518 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, in order to achieve high quality in printed materials, it is desirable to have both a sheet length detection unit and a curl detection unit. However, if the two detection units are placed separately on the transport path, space is required for each, which leads to the problem of the device becoming larger.

[0006] The present invention aims to provide a sheet transport device and an image forming device that can suppress the increase in size of the device. [Means for solving the problem]

[0007] One aspect of the present invention is a sheet conveying device comprising: a first sheet detection unit that detects a sheet being conveyed in the sheet conveying direction at a first sheet detection position; a second sheet detection unit that detects a sheet being conveyed in the sheet conveying direction at a second sheet detection position downstream of the first sheet detection position in the sheet conveying direction; a pair of conveying rollers arranged upstream of the first sheet detection position in the sheet conveying direction and conveying the sheet in the sheet conveying direction; and a control unit that calculates the sheet length in the sheet conveying direction based on the detection result of the first sheet detection unit on the sheet being conveyed by the pair of conveying rollers and the conveying speed of the sheet being conveyed by the pair of conveying rollers, and calculates the amount of curl of the sheet being conveyed by the pair of conveying rollers based on the detection result of the second sheet detection unit on the sheet being conveyed by the pair of conveying rollers.

[0008] Another aspect of the present invention is a sheet conveying device comprising: a pair of conveying rollers for conveying a sheet in the sheet conveying direction; a first sheet detection unit for detecting a sheet being conveyed by the pair of conveying rollers at a first sheet detection position located downstream of the pair of conveying rollers in the sheet conveying direction in order to obtain the sheet length of the sheet being conveyed by the pair of conveying rollers; and a second sheet detection unit for detecting a sheet being conveyed by the pair of conveying rollers at a second sheet detection position located downstream of the first sheet detection position in the sheet conveying direction in order to obtain the amount of curl of the sheet being conveyed by the pair of conveying rollers.

[0009] Another aspect of the present invention is an image forming apparatus characterized by comprising: an image forming unit for forming an image on a sheet; a curl correction device for correcting the curl of the sheet on which the image has been formed by the image forming unit; and the sheet conveying device for conveying the sheet. [Effects of the Invention]

[0010] According to the present invention, it is possible to suppress the increase in size of the device. [Brief explanation of the drawing]

[0011] [Figure 1] This is a cross-sectional view showing an image forming apparatus according to this embodiment. [Figure 2] This is a perspective view showing the curl that occurs in the sheet according to the first embodiment, where (a) is an upward curl and (b) is a downward curl. [Figure 3] This is a cross-sectional view showing the curl-correcting section according to the first embodiment. [Figure 4] This is a cross-sectional view showing the default state of the curl straightening section according to the first embodiment. [Figure 5] This is a cross-sectional view showing the state when the curl-correcting part according to the first embodiment corrects the lower curl. [Figure 6] This is a cross-sectional view showing the state when the curl-correcting unit according to the first embodiment corrects the upper curl. [Figure 7] This is a block diagram showing the control system of an image forming apparatus according to the first embodiment. [Figure 8] This is a cross-sectional view showing the sheet length detection unit and curl detection unit according to the first embodiment. [Figure 9] This is a cross-sectional view showing the sheet length detection unit and curl detection unit according to the first embodiment. (a) shows the state when the leading edge of the sheet is detected by the third sensor, (b) shows the state when the trailing edge of the sheet is detected by the first sensor, and (c) shows the state when the trailing edge of the sheet is detected by the second sensor. [Figure 10] This is a time chart showing the signals output by each sensor when a sheet is transported in the sheet length detection unit according to the first embodiment. [Figure 11] This is a cross-sectional view showing the curl detection unit according to the first embodiment, where (a) is a sheet without curl, (b) is a sheet with a small upward curl, (c) is a sheet with a large upward curl, and (d) is a sheet with a small downward curl. [Figure 12]A time chart showing signals output by each sensor in the sheet length detection unit according to the first embodiment, where (a) is a sheet without curl, (b) is a sheet with a small upward curl, (c) is a sheet with a large upward curl, and (d) is a sheet with a small downward curl. [Figure 13] A plan view showing a curl detection unit according to a modification of the first embodiment. [Figure 14] A cross-sectional view showing a curl detection unit according to the second embodiment, where (a) is a sheet without curl, (b) is a sheet with a small upward curl, (c) is a sheet with a large upward curl, and (d) is a sheet with a small downward curl. [Figure 15] A time chart showing signals output by each sensor in the sheet length detection unit according to the second embodiment, where (a) is a sheet without curl, (b) is a sheet with a small upward curl, (c) is a sheet with a large upward curl, and (d) is a sheet with a small downward curl.

Modes for Carrying Out the Invention

[0012] <The First Embodiment> Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings. In the first embodiment, an image forming apparatus such as a copying machine, a facsimile machine, a printer, or a multifunction machine having a plurality of these functions is exemplified, in which an electrostatic latent image is formed on an image carrier by an electrophotographic method or an electrostatic recording method, and this electrostatic latent image is transferred as a toner image onto a sheet by a developer. Further, an image forming apparatus 1 which is an intermediate transfer tandem type full-color printer is applied as the image forming apparatus, but it is not limited thereto, and other types of image forming apparatuses may be used. Also, it is not limited to being full-color, and it can also be applied to monochrome, monocolor, or an inkjet printer.

[0013] [Overall Configuration of the Image Forming Apparatus] First, the overall configuration of the image forming apparatus 1 according to the first embodiment will be described using FIG. 1. FIG. 1 is a cross-sectional view of the image forming apparatus 1. The image forming apparatus 1 is a device such as a copier, a facsimile machine, and a multifunction machine that forms an image on a sheet used as a recording medium. Further, the image forming apparatus 1 is capable of printing other than for general office use, and as the recording medium, various sheets such as paper including paper and envelopes, glossy paper, plastic films such as overhead projector sheets, and cloth can be used.

[0014] In the apparatus main body 1A of the image forming apparatus 1, a feed cassette 51 for storing the sheet S and an image forming unit 513 for forming an image on the sheet S fed from the feed cassette 51 are accommodated. The image forming unit 513 includes four process units PY, PM, PC, and PK that respectively form yellow, magenta, cyan, and black toner images, and an intermediate transfer belt 506. The image forming unit 513 forms an image on the sheet S by an intermediate transfer tandem method. The process units PY, PM, PC, and PK are electrophotographic units for each color.

[0015] Process units PY, PM, PC, and PK share a common configuration except for the color of the toner used for development. Here, the configuration of the image forming unit 513 and the image formation process of the toner image will be explained using the yellow process unit PY as an example. Process unit PY includes a photosensitive drum 508, an exposure device 511, a developing device 510, and a drum cleaner 509. The photosensitive drum 508 is a drum-shaped photoreceptor having a photosensitive layer on its outer circumference, and rotates in a direction (R2 direction in Figure 1) along the rotation direction of the intermediate transfer belt 506 (R1 direction in Figure 1). The surface of the photosensitive drum 508 is charged by receiving charge from a charging part such as a charging roller. The exposure device 511 irradiates the photosensitive drum 508 with a laser beam modulated according to the image information, and scans the photosensitive drum 508 with an optical system including a reflector 512 to draw an electrostatic latent image on the surface of the photosensitive drum 508. The developing device 510 contains a developer containing toner and supplies toner to the photosensitive drum 508 to make the electrostatic latent image visible as a toner image. The toner image formed on the photosensitive drum 508 is first transferred to the intermediate transfer belt 506 in the primary transfer section, which is the nip between the primary transfer roller 507 and the intermediate transfer belt 506. Any remaining toner on the photosensitive drum 508 after the transfer is removed by the drum cleaner 509.

[0016] The intermediate transfer belt 506 is wrapped around the drive roller 504, the driven roller 505, the secondary transfer inner roller 503, and the primary transfer roller 507, and is rotationally driven by the drive roller 504 in the R2 direction in Figure 1. The image forming process described above proceeds in parallel in each process unit PY, PM, PC, and PK, and the four-color toner images are superimposed by multiple transfers to form a full-color toner image on the intermediate transfer belt 506. This toner image is transported to the secondary transfer section 514 while still supported on the intermediate transfer belt 506. The secondary transfer section 514 is configured as a nip section between the secondary transfer roller 56 and the secondary transfer inner roller 503. A bias voltage with the opposite polarity to the charge polarity of the toner is applied to the secondary transfer roller 56, thereby secondary transferring the toner image to the sheet S. After transfer, any remaining toner on the intermediate transfer belt 506 is removed by a belt cleaner.

[0017] The sheet S onto which the toner image has been transferred is passed to the fuser unit 58 by the pre-fixing transport unit 57. The fuser unit 58 has a pair of fuser rollers that grip and transport the sheet S, and a heat source such as a halogen heater, and applies heat and pressure to the toner image supported on the sheet S. As a result, the toner particles melt and solidify, fixing the toner image onto the sheet S.

[0018] Next, the sheet transport process for transporting the sheets will be described. In the first embodiment, the sheet transport device 515 transports the sheets S contained in the feed cassette 51 and discharges the sheets S with the formed image to the outside of the device body 1A. The sheet transport device 515 has a sheet feed unit 53, a sheet transport unit 54, a skew correction unit 55, a sheet shape acquisition unit 600, a branch transport unit 59, a reversal transport unit 501, and a double-sided transport unit 502. The feed cassette 51 is removably mounted on the device body 1A and stores the sheets S loaded on a lifting plate 52 that can be raised and lowered. The sheets S stored in the feed cassette 51 are fed one by one by the sheet feed unit 53. The sheet feed unit 53 can be a belt system that uses a suction fan to attract and transport the sheets S to a belt member, or a friction separation system using rollers or pads. The sheet S, which is fed out from the sheet feeding unit 53, is transported along the feeding path 54a by the transport roller pair of the sheet transport unit 54 and handed over to the skew correction unit 55.

[0019] The sheet S, which has been handed over to the skew correction unit 55, is transported to the secondary transfer unit 514 after skew correction and timing correction are performed. At this time, the registration roller pair 7 included in the skew correction unit 55 feeds the sheet S to the secondary transfer unit 514 at a timing that matches the progress of the image formation process by the process units PY, PM, PC, and PK. After the toner image is transferred in the secondary transfer unit 514 and the image is fixed by the fuser 58, the curl of the sheet S is corrected by the curl correction device 800 (described later), and the curl height and sheet length are measured by the sheet shape acquisition unit 600. After that, it is transported to the branching transport unit 59 which branches the transport path. If image formation on the sheet S is complete, the sheet S is discharged by the discharge roller pair to the discharge tray 500 located outside the main body 1A of the device.

[0020] When forming an image on the back surface of sheet S, sheet S is transferred to the double-sided transfer unit 502 via the reversal transfer unit 501. The reversal transfer unit 501 has a pair of reversible rollers that can rotate in both forward and reverse directions, and transfers sheet S to the double-sided transfer unit 502 with its front and back surfaces reversed using a switchback method. The double-sided transfer unit 502 then transfers sheet S again towards the skew correction unit 55 via the sheet transfer unit 54. After the image is formed on the back surface of sheet S, it is discharged into the discharge tray 500.

[0021] The image forming apparatus 1 has a control unit 9 that controls the overall operation of the image forming apparatus 1 based on image information input from an external PC and image information read from a document. The control unit 9 has a CPU and memory and performs the control of each part of the image forming apparatus 1 as described above. The CPU outputs output signals to each electrical component to operate the electrical component at the desired timing and required control amount based on detection signals input from each sensor and information stored in memory. The memory stores data necessary for controlling each unit, and the CPU reads data stored in the memory and writes data to the memory.

[0022] In Figure 1, the image forming apparatus 1 is shown as an example in which the curl correction device 800 and the sheet shape acquisition unit 600 are placed on the transport path upstream of the sheet transport direction D1 of the branch transport unit 59, but the placement is not limited to this.

[0023] [The principle behind the formation of curls] Next, we will explain the curl that occurs in the sheet using Figures 2(a) and 2(b). Figure 2(a) illustrates a state in which a downward curl (heat curl) occurs, in which the downstream end and upstream end of the sheet in the sheet transport direction D1 are lower than the center. Figure 2(b) illustrates a state in which an upward curl (toner curl) occurs, in which the downstream end and upstream end of the sheet in the sheet transport direction D1 are higher than the center.

[0024] As the sheet S passes through the fuser 58, it is heated by a heat source such as a heater, and a temperature difference generally occurs between the image-forming surface, which becomes hot to melt the toner, and its back surface. As a result, moisture movement occurs within the sheet S in the nip of the fuser 58, and the moisture content on the back surface becomes greater than that on the image-forming surface, which becomes hotter. After passing through the fuser 58, when the sheet S comes into contact with air, moisture evaporates from the hot sheet S, but more evaporation occurs from the back surface, which has a higher moisture content. If the material of the sheet S is pulp, the greater the evaporation, the greater the shrinkage, so the back surface, which shrinks more, pulls downwards, causing the downstream and upstream ends in the sheet transport direction D1 to droop downwards compared to the center, resulting in the state shown in Figure 2(a).

[0025] Furthermore, the toner and sheet S, which are melted and fixed at high temperature in the fuser 58, cool down when exposed to air after passing through the fuser 58, and shrink as a result. Since the constituent materials of the toner and the material of the sheet S generally have different coefficients of thermal expansion, the amount of shrinkage in response to the decrease in temperature differs between the front and back sides. Generally, the amount of shrinkage of the toner layer on the surface is greater, resulting in an upward curl at the edges of the sheet S, as shown in Figure 2(b). The upward curl also differs depending on the image density, as the thickness of the toner layer changes with the image density.

[0026] Ultimately, the sum of the downward and upward curl determines the curl state, which includes the amount of curl at the edge of the sheet relative to the center of the sheet in the sheet transport direction D1, and the direction in which the edge of the sheet curls relative to the center of the sheet. For this reason, the amount of curl changes depending on various parameters such as the sheet material, thickness, stiffness, moisture content, temperature, ambient temperature, ambient humidity, fixing temperature, and image density. Furthermore, the moisture content of the sheet changes under the influence of constantly changing conditions (ambient temperature, ambient humidity, fixing temperature, etc.). Therefore, even with the same printing conditions (sheet properties, image), there will be differences in the process of how the amount of curl changes over time, and consequently, differences in the final deformation amount.

[0027] The curl correction device 800 cancels out the curl that has occurred by applying deformation in the opposite direction. That is, the curl correction device 800 corrects the curl of the sheet S on which the image has been formed by the image forming unit 513. Therefore, it is desirable for the curl correction device 700 to predict in advance the amount of curl that will occur under such various conditions and set an appropriate amount of correction accordingly.

[0028] [Curl Correction Department] The curl straightening device 800 in the first embodiment will be described with reference to Figures 3 to 7. Figure 3 is a cross-sectional view showing the configuration of the curl straightening device 800. As shown in Figure 3, the curl straightening device 800 comprises a frame (not shown), a transport path 830, an upward straightening section 840a located on the upstream side of the sheet transport direction D1 of the transport path 830, and a downward straightening section 840b located on the downstream side.

[0029] The upward straightening section 840a has a sponge roller 801a and a metal roller 803a arranged substantially opposite each other. The metal roller 803a is rotatably fixed to the frame via a metal roller shaft 804a. The sponge roller 801a is rotatably fixed to the oscillating arm 805a via a sponge roller shaft 802a. One end of the oscillating arm 805a is pivotably connected to the frame via an oscillating shaft 806a, and a follower 807a is rotatably fixed to the other end via a follower shaft 808a. In addition, the cam 809a is rotatably fixed to the frame by an eccentric cam shaft 810a, and the surface of the cam 809a is in contact with the follower 807a. The angle of the cam 809a is controlled by a cam motor M1a (see Figure 7) which is commanded by the control unit 9 (see Figure 7). As the relative distance between the camshaft 810a and the follower shaft 808a changes, the angle of the oscillating arm 805a changes, causing the sponge roller 801a to be displaced in the thickness direction of the sheet S. This makes it possible to change the pressing force between the sponge roller 801a and the metal roller 803a.

[0030] In the upward straightening section 840a, the sponge roller 801a and the metal roller 803a are pressed together by a predetermined pressing force, and the harder metal roller 803a bites into the less hard sponge roller 801a, forming a curved nip. As the sheet S passes through this nip, it is forcibly deformed along the metal roller 803a, which has a smaller radius, and thus curved. The greater the clamping pressure, the more the sponge roller 801a bites into the metal roller 803a and deforms, and the wider the width of the nip in the sheet conveying direction D1 becomes. As a result, the time during which the sheet S is forcibly deformed as it passes through the nip becomes longer, making it possible to correct the curl more strongly. In this way, the curl straightening device 800 can adjust the amount of straightening applied to the sheet S by the upward straightening section 840a by controlling the rotation angle of the cam 809a with the cam motor M1a.

[0031] The metal roller 803a is rotationally driven in the sheet transport direction D1 by the curl straightening section transport motor M2a (see Figure 7). The sponge roller 801a rotates by being pressed against the metal roller 803a. In the curl straightening device 800, the sheet S is held between the sponge roller 801a and the metal roller 803a, thereby obtaining a transport force and being transported downstream in the sheet transport direction D1.

[0032] The upward straightening section 840a has been described above. The downward straightening section 840b has the same configuration as the upward straightening section 840a, except that the top and bottom are reversed. Therefore, for the downward straightening section 840b, the description of the upward straightening section 840a will be used as a substitute by replacing the symbol a with the symbol b.

[0033] The specific operation of the curl straightening device 800 will now be described. Figure 4 is a cross-sectional view showing the default state of the curl straightening device 800. As shown in Figure 4, the default state for the upward straightening section 840a and the downward straightening section 840b is when the pressing force between the sponge rollers 801a, 801b and the metal rollers 803a, 803b is at its minimum. Even when the sheet S is passed through in this state, the curl straightening device 800 does not cause forced deformation of the sheet S.

[0034] Figure 5 is a cross-sectional view showing the state of the curl straightening device 800 when correcting a downward curl. As shown in Figure 5, when a downward curl occurs in the sheet S, the curl straightening device 800 controls the angle of the cam 809a of the upward straightening section 840a and changes the clamping pressure of the sponge roller 801a and the metal roller 803a to a predetermined value. As a result, a downwardly convex curved nip is formed in the upward straightening section 840a, and an upward forced deformation is applied to the sheet S as it passes through the curl straightening device 800, resulting in the sheet S being output in a flat state with the downward curl corrected.

[0035] Figure 6 is a cross-sectional view showing the state of the curl straightening device 800 when correcting an upward curl. As shown in Figure 6, when an upward curl occurs in the sheet S, the curl straightening device 800 controls the angle of the cam 809b of the downward straightening section 840b and changes the clamping pressure of the sponge roller 801b and the metal roller 803b to a predetermined value. As a result, an upwardly convex curved nip is formed in the downward straightening section 840b, and a downward forced deformation is applied to the sheet S as it passes through the curl straightening device 800, resulting in the sheet S being output in a flat state with the upward curl corrected.

[0036] Thus, in the curl straightening device 800 of the first embodiment, the pressing force between the sponge rollers 801a, 801b and the metal rollers 803a, 803b of the upward straightening section 840a and the downward straightening section 840b is controlled. As a result, the curl straightening device 800 can apply a forced deformation to counteract any amount and direction of curl, thereby straightening the curl of the sheet S. Note that it is sufficient for either the sponge rollers or the metal rollers to be movable relative to the other. Furthermore, although the curl straightening device 800 of the first embodiment uses sponge rollers 801a, 801b and metal rollers 803a, 803b, it is not limited to this. For example, the curl straightening device 800 may use a belt stretched over multiple rollers and rollers. In this case, the belt and rollers form a nip section for gripping and conveying the sheet S, and the rollers contact the belt in such a way that the belt bends.

[0037] [Control Block] Figure 7 is a control block diagram of the image forming apparatus 1. As shown in Figure 7, the control unit 9 of the image forming apparatus 1 is connected to the first sensors S1 to the fifth sensors S5, cam motors M1a and M1b, curl straightening section transport motors M2a and M2b, first transport motor M11, and second transport motor M12, which will be described later.

[0038] [Sheet shape acquisition unit] Next, the sheet shape acquisition unit 600 will be described in detail using Figures 8 to 12. Figure 8 shows a cross-sectional view of the sheet shape acquisition unit 600 in this embodiment. The sheet shape acquisition unit 600 has a sheet length detection unit 620 and a curl detection unit 630, and the sheet length detection unit 620 and the curl detection unit 630 are arranged in that order from the upstream side in the sheet transport direction D1. Since the curl of the sheet S changes with changes in the moisture content and temperature of the sheet after passing through the fuser 58, the curl detection unit 630 can be detected with higher accuracy if it is placed on the side away from the fuser 58. In this embodiment, the curl correction device 800 and the sheet shape acquisition unit 600 are arranged between the fuser 58 and the branch transport unit 59 (see Figure 1). However, it is not limited to this, and they may be arranged at locations other than between the fuser 58 and the branch transport unit 59, as long as it is on the transport path through which the sheet S is transported in the image forming apparatus 1.

[0039] [Sheet length detection unit] The sheet length detection unit 620 will be explained using Figures 8 to 10. The sheet length detection unit 620 includes a first transport roller pair 601 driven by a first transport motor M11 (see Figure 7) and a second transport roller pair 602 driven by a second transport motor M12 (see Figure 7). The sheet length detection unit 620 also includes a first sensor S1, a second sensor S2, and a third sensor S3 that detect the passage of the ends (front end Sa and rear end Sb) of the sheet S. A linear first guide member 611 is positioned between the first transport roller pair 601 and the second transport roller pair 602, facing each other across a horizontal transport path. Specifically, the first guide member 611 is positioned upstream of the second transport roller pair 602 in the sheet transport direction D1, and is positioned facing each other across the transport path in an orthogonal direction D3 that is perpendicular to the sheet transport direction D1 and the width direction D2 which is perpendicular to the sheet transport direction D1.

[0040] The second transport roller pair 602 is an example of a transport roller pair, positioned upstream of the third detection position S3a of the third sensor S3 in the sheet transport direction D1, and transports the sheet S in the sheet transport direction D1. The outer circumferential surface of the second transport roller pair 602 that contacts the sheet is blast-treated, and metal is used as the material. That is, the second transport roller pair 602 has a blast-treated metallic surface. Therefore, the outer diameter changes due to temperature, humidity, and wear are smaller than those of commonly used rubber rollers, and slippage of the sheet S is less likely to occur, so the transport of the sheet S can be controlled with higher precision and stability.

[0041] The first sensor S1, the second sensor S2, and the third sensor S3 are optical sensors that output a High signal if a sheet is present and a Low signal if a sheet is not present. In the sheet transport direction D1, the first sensor S1 is positioned upstream of the second transport roller pair 602 and downstream of the first transport roller pair 601, and the second sensor S2 is positioned upstream of the second transport roller pair 602 and downstream of the first sensor S1. In the sheet transport direction D1, the third sensor S3 is positioned downstream of the second transport roller pair 602.

[0042] The third sensor S3 is an example of the first sheet detection unit and detects a sheet S being transported in the sheet transport direction D1 at the third detection position S3a, which is the first sheet detection position. The first sensor S1 is an example of the third sheet detection unit and detects a sheet S being transported by the second transport roller pair 602 at the first detection position S1a, which is the third sheet detection position different from the third detection position S3a in the sheet transport direction D1. The second sensor S2 is an example of the fourth sheet detection unit and detects a sheet S being transported by the second transport roller pair 602 at the second detection position S2a, which is the fourth sheet detection position different from the third detection position S3a and the first detection position S1a in the sheet transport direction D1.

[0043] L12 is defined as the distance in the transport path between the first detection position S1a, which is the sheet detection position of the first sensor S1 along the sheet transport direction D1, and the second detection position S2a, which is the sheet detection position of the second sensor S2. L23 is defined as the distance in the transport path between the second detection position S2a and the third detection position S3a, which is the sheet detection position of the third sensor S3. The control unit 9 detects the passage of the sheet S from the outputs of the first sensor S1, the second sensor S2, and the third sensor S3, and calculates the transport speed V and sheet length Ls of the sheet S from the relative timing. Specifically, the third sensor S3 detects the sheet S being transported by the second transport roller pair 602 at the third detection position S3a in order to obtain the sheet length Ls in the sheet transport direction D1 of the sheet S being transported by the second transport roller pair 602.

[0044] Next, using Figures 9(a) to 10, we will explain the changes in sensor output when the sheet S is transported and the procedure for detecting the sheet length Ls using the sensor output. As shown in Figure 9(a), when the leading edge Sa of the sheet S passes the third detection position S3a of the third sensor S3 while the sheet S is held between the second transport roller pair 602, the output of the third sensor S3 changes from Low (L) to High (H) (time T3t in Figure 10). Subsequently, when the rear end Sb of the sheet S passes the first detection position S1a of the first sensor S1, the output of the first sensor S1 changes from High to Low (time T1h in Figure 10). Furthermore, when the rear end Sb of the sheet S passes the second detection position S2a of the second sensor S2, the output of the second sensor S2 changes from High to Low (time T2h).

[0045] Based on the detected timing and the relative positions of each sensor as described above, the conveying speed Vs of the sheet S is determined as follows. Specifically, the control unit 9 calculates the conveying speed Vs of the sheet S being conveyed by the second conveying roller pair 602 based on the difference between the time when the first sensor S1 detects the edge of the sheet and the time when the second sensor S2 detects the edge of the sheet, and the distance L12. Conveying speed Vs = L12 / (T2h - T1h) ... (Equation 1)

[0046] Although this explanation describes the calculation of the transport speed Vs using the first sensor S1 and the second sensor S2, it is not limited to this method. For example, the transport speed Vs may be calculated using the first sensor S1 and the third sensor S3. In that case, the control unit 9 calculates the sheet transport speed Vs based on the difference between the time when the third sensor S3 detects the leading edge Sa of the sheet and the time when the first sensor S1 detects the leading edge Sa of the sheet, and the distance L12 + L23.

[0047] Furthermore, the sheet length Ls of sheet S in the sheet transport direction D1 can be determined as follows. First sheet length L1 = L12 + L23 + (T1h - T3t) × Vs Second sheet length L2 = L23 + (T2h - T3t) × Vs Sheet length Ls = (L1 + L2) / 2 ... (Formula 2)

[0048] As described above, by calculating the sheet length Ls using the timing of the passage of the leading edge Sa and trailing edge Sb of the sheet S, which is detected while the sheet S is held between the second transport roller pair 602, which are high-precision transport members, it becomes possible to detect the sheet length Ls with high accuracy. In this embodiment, the sheet length Ls is calculated by adding and averaging the first sheet length L1 and the second sheet length L2. This makes it possible to correct for variations in the detection of the edges of the sheet S by each of the first sensor S1, second sensor S2, and third sensor S3 by canceling them out, thereby reducing the calculation error of the sheet length S.

[0049] In this embodiment, the sheet length Ls is calculated from the average value of the first sheet length L1 calculated from the timings acquired by the first sensor S1 and the third sensor S3, and the second sheet length L2 calculated from the timings acquired by the second sensor S2 and the third sensor S3. That is, the control unit 9 calculates the first sheet length L1 based on the time when the third sensor S3 detects the leading edge Sa, which is the first end of the sheet, the time when the first sensor S1 detects the trailing edge Sb, which is the second end of the sheet, the distance L12 + L23, and the sheet transport speed Vs. The control unit 9 also calculates the second sheet length L2 based on the time when the third sensor S3 detects the leading edge Sa, the time when the second sensor S2 detects the trailing edge Sb, the distance L23, and the sheet transport speed Vs.

[0050] However, this is not the only option. For example, the sheet length Ls may be calculated by substituting the sheet transport speed Vs calculated using formula 1 into formula 2 to obtain the first sheet length L1. In this case, the sheet length Ls will be calculated based on the timings acquired by the first sensor S1 and the third sensor S3. In this case, the control unit 9 calculates the sheet length Ls in the sheet transport direction D1 based on the time when the third sensor S3 detects the leading edge Sa of the sheet, the time when the first sensor S1 detects the trailing edge Sb of the sheet, the distance L12 + L23, and the sheet transport speed Vs. Alternatively, the calculated second sheet length L2 may be used as the sheet length Ls. In this case, the sheet length Ls will be calculated based on the timings acquired by the second sensor S2 and the third sensor S3.

[0051] In this case, since the sheet length detection is based on the timing difference between multiple sensors, variations in the sheet S transport speed Vs will result in variations in the curl detection. In contrast, in this embodiment, the second transport roller pair 602 that transports the sheet S while sheet length detection is being performed is made of metal with a blast-treated outer surface, and can maintain a highly accurate and stable transport speed Vs, making it suitable for sheet length detection.

[0052] [Curl detection unit] Next, the curl detection unit 630 will be explained using Figures 11(a) to 12(d). Figures 11(a) to 11(d) show cross-sectional views of the curl detection unit 630. The curl detection unit 630 includes a second transport roller pair 602, a third sensor S3, a fourth sensor S4, and a second guide member 612. The third sensor S3 and the fourth sensor S4 are arranged at only one location in the width direction D2.

[0053] The second guide member 612 is positioned downstream of the second transport roller pair 602 in the sheet transport direction D1, and is positioned opposite to it across the transport path in the orthogonal direction D3. The spacing of the second guide members 612 in the orthogonal direction D3 is wider than the spacing of the first guide members 611. The second guide member 612 is provided downstream of the third detection position S3a of the third sensor S3 in the sheet transport direction D1, and the distance between the upper and lower guides of the transport path is wider than at other positions. In this embodiment, the distance between the upper and lower guides of the first guide member 611 is set to approximately 3 mm, while the distance between the upper and lower guides of the second guide member 612 is set to approximately 10 mm to 20 mm. In this way, the system is configured so as to not suppress the curl shape of the sheet S as it passes through.

[0054] The fourth sensor S4 detects the sheet S at the fourth detection position S4a, which is downstream of the third detection position S3a in the sheet transport direction D1. The fourth sensor S4 is an example of the second sheet detection unit, and the fourth detection position S4a is an example of the second sheet detection position. The fourth detection position S4a is located between the second guide members 612. The fourth sensor S4 is an optical sensor that detects the passage of the ends (front end Sa and rear end Sb) of the sheet S, and is installed at an angle to the orthogonal direction D3. The fourth detection position S4a of the fourth sensor S4 is positioned further upstream in the sheet transport direction D1 as it moves upward in the orthogonal direction D3, and further downstream as it moves downward in the orthogonal direction D3, so as to detect the sheet S (see Figure 11(a)). The control unit 9 can detect the passage of the sheet S from the outputs of the third sensor S3 and the fourth sensor S4, and can calculate the amount of curl of the sheet S from the relative timing. In other words, the fourth sensor S4 detects the sheet S being conveyed by the second conveyor roller pair 602 at the fourth detection position S4a in order to acquire the amount of curl of the sheet S being conveyed by the second conveyor roller pair 602.

[0055] Figure 11(a) shows the state when the leading edge Sa of an uncurled sheet S passes the fourth detection position S4a during transport. Figure 12(a) shows the changes in the outputs of the third sensor S3 and the fourth sensor S4 during the operation shown in Figure 11(a). When the sheet S is transported by the second transport roller pair 602, the output of the third sensor S3 changes from Low (L) to High (H) when the leading edge Sa of the sheet S passes the third detection position S3a of the third sensor S3 (time T3t in Figure 12(a)). The output of the fourth sensor S4 changes from Low to High when the leading edge Sa of the sheet S passes the fourth detection position S4a of the fourth sensor S4 (time T4t in Figure 12(a)). At this time, since the fourth detection position S4a of the fourth sensor S4 is inclined with respect to the sheet transport direction D1 and the orthogonal direction D3, the timing of time T4t relative to time T3t changes depending on the height at which the sheet S passes through the second guide member 612 of the transport path. Let ΔT0 (=T4t-T3t) be the difference in detection timing between the third sensor S3 and the fourth sensor S4 when an uncurled sheet S passes through.

[0056] Next, Figure 11(b) shows the state in which the tip Sa of the sheet S that is curled upward passes through the fourth detection position S4a. Also, Figure 12(b) shows the changes in the output of the third sensor S3 and the fourth sensor S4 during the operation shown in Figure 11(b). The tip Sa of the sheet S that is curled upward passes through the upper region of the second guide member 612 compared to when the sheet S that is not curled passes through. Therefore, as shown in Figure 12(b), the detection timing difference ΔT1 (=T4t-T3t) becomes smaller than ΔT0.

[0057] Figure 11(c) shows the state in which the leading edge Sa of the sheet S, which is curled upward more than in Figure 11(b), passes the fourth detection position S4a. Figure 12(c) shows the changes in the output of the third sensor S3 and the fourth sensor S4 during the operation in Figure 11(c). If a sheet that is curled upward even more than in Figure 11(c) passes, the detection timing difference ΔT2 (=T4t-T3t) becomes even smaller than ΔT1.

[0058] On the other hand, Figure 11(d) shows the state in which the tip Sa of the sheet S, which is curled downward, passes through the fourth detection position S4a. Also, Figure 12(d) shows the changes in the output of the third sensor S3 and the fourth sensor S4 during the operation shown in Figure 11(d). In this case, since the tip Sa of the sheet S passes through a lower region than when it is not curled, the detection timing difference ΔT3 (=T4t-T3t) becomes larger than ΔT0.

[0059] As described above, by evaluating the magnitude of the detection timing difference between the third sensor S3 and the fourth sensor S4, it becomes possible to determine the direction and amount of curl at the leading edge Sa of the sheet S. Specifically, the control unit 9 calculates the amount of curl of the sheet S based on the time elapsed from when the third sensor S3 detects the end (leading edge Sa) of the sheet S until the fourth sensor S4 detects the end (leading edge Sa) of the sheet S, and the conveying speed Vs of the sheet S.

[0060] In this case, curl detection is based on the timing difference between multiple sensors, so if the conveying speed Vs of the sheet S varies, the curl detection results will also vary. In contrast, in this embodiment, the second conveying roller pair 602 that conveys the sheet S when curl detection is performed is made of metal with a blast-treated outer surface, and can maintain a highly accurate and stable conveying speed Vs, making it suitable for detecting the amount of curl.

[0061] In this embodiment, the control unit 9 calculates the sheet length Ls in the sheet transport direction D1 based on the detection result of the third sensor S3 on the sheet S being transported by the second transport roller pair 602 and the transport speed Vs of the sheet S being transported by the second transport roller pair 602. The control unit 9 also calculates the amount of curl of the sheet S being transported by the second transport roller pair 602 based on the detection result of the fourth sensor S4 on the sheet S being transported by the second transport roller pair 602. In this way, the control unit 9 calculates and acquires the sheet length Ls and the amount of curl using the sheet shape acquisition unit 600.

[0062] As described above, according to this embodiment, the second transport roller pair 602 and the third sensor S3 for acquiring the sheet shape are used in common by the sheet length detection unit 620 and the curl detection unit 630. Therefore, compared to the case where the sheet length detection unit 620 and the curl detection unit 630 are provided separately, the number of parts for the second transport roller pair 602 and the third sensor S3 is reduced, and the overall size of the device is suppressed and miniaturized while acquiring the sheet shape.

[0063] Furthermore, according to this embodiment, by standardizing the transport components used for sheet length detection and curl detection, it becomes possible to miniaturize the device and reduce costs while detecting sheet length and curl amount with high accuracy. As a result, it is possible to improve print quality and increase productivity by reducing adjustment downtime. For example, when connecting a paper finisher that folds the sheet at a predetermined position as a post-processing device, it is necessary to fold the sheet S at a predetermined position, such as the center of the sheet S, according to the size of the printed sheet S. According to the image forming apparatus 1 of this embodiment, the sheet length after printing can be automatically acquired, so it is possible to eliminate the need for the operator to adjust the folding position.

[0064] In the embodiment described above, the third sensor S3 and the fourth sensor S4 are placed at only one location in the width direction D2, but this is not limited to this. For example, as shown in Figure 13, multiple sensors may be provided at different locations in the width direction D2. If two sensors are provided in addition to the third sensor S3, then a sixth sensor S6 and an eighth sensor S8 may be provided. The sixth sensor S6 is an example of a sixth sheet detection unit, and detects a sheet at a sixth detection position S6a, which overlaps with the third detection position S3a in the sheet transport direction D1 and is a sixth sheet detection position different from the third detection position S3a in the width direction D2. The eighth sensor S8 detects a sheet at an eighth detection position S8a, which overlaps with the third detection position S3a in the sheet transport direction D1 and is a sixth sheet detection position different from the third detection position S3a in the width direction D2.

[0065] Furthermore, for example, if two additional sensors are to be provided besides the fourth sensor S4, a seventh sensor S7 and a ninth sensor S9 are provided. The seventh sensor S7 is an example of a seventh sheet detection unit, and detects a sheet at the seventh detection position S7a, which is a seventh sheet detection position that overlaps with the fourth detection position S4a in the sheet transport direction D1 and is different from the fourth detection position S4a in the width direction D2. The ninth sensor S9 detects a sheet at the ninth detection position S9a, which overlaps with the fourth detection position S4a in the sheet transport direction D1 and is different from the fourth detection position S4a in the width direction D2. By providing multiple sensors in the width direction D2 in this way, it becomes possible to handle various curl shapes, such as when the curl of the leading edge Sa of the sheet occurs only at the corners, and curl can be detected with higher accuracy.

[0066] Furthermore, in the above-described embodiment, the control unit 9 calculates the transport speed Vs based on the detection timing of the first sensor S1 and the second sensor S2 and the distance between the first sensor S1 and the second sensor S2, but is not limited to this. For example, a separately provided speed sensor may be used, or the transport speed Vs may be calculated from the rotational speed of the second transport motor M12. The rotational speed of the second transport motor M12 may be calculated using a rotational speed sensor, or based on the signal given by the control unit 9 when instructing the second transport motor M12 to rotate.

[0067] Furthermore, in the embodiments described above, the third sensor S3 was described as being used in common by the sheet length detection unit 620 and the curl detection unit 630, but this is not the only case. For example, if the second transport roller pair 602 is shared, the third sensor S3 does not necessarily have to be shared (see the second embodiment).

[0068] Furthermore, although the above-described embodiment described a case in which optical sensors were used as the first sensor S1 to the fourth sensor S4, it is not limited to this. For example, mechanical switches, laser sensors, CCDs, CISs, etc., can be used as the first sensor S1 to the fourth sensor S4.

[0069] <Second Embodiment> Next, a second embodiment will be described using Figures 14(a) to 15(d). This embodiment differs from the first embodiment in that a fifth sensor S5 is added to the curl detection unit 630A. However, other components, such as the sheet length detection unit 620, are the same as in the first embodiment, so the same reference numerals are used and detailed explanations are omitted.

[0070] [Curl detection unit] Figures 14(a) to (d) show cross-sectional views of the curl detection unit 630A. The curl detection unit 630A includes a second transport roller pair 602, a third sensor S3, a fourth sensor S4, a fifth sensor S5, and a second guide member 612. The fourth sensor S4 and the fifth sensor S5 are positioned at only one location in the width direction D2.

[0071] The fifth sensor S5 detects the sheet S at the fifth detection position S5a, which is downstream of the third detection position S3a in the sheet transport direction D1. The fifth sensor S5 is an example of the fifth sheet detection unit, and the fifth detection position S5a is an example of the fifth sheet detection position. The fifth detection position S5a is located between the second guide members 612. The fifth sensor S5 is an optical sensor that detects the passage of the ends (front Sa and rear Sb) of the sheet S, and is installed in an inclined state with respect to the orthogonal direction D3. The fifth detection position S5a of the fifth sensor S5 is positioned further downstream in the sheet transport direction D1 as it goes up in the orthogonal direction D3, and further upstream as it goes down, so as to detect the sheet S (see Figure 14(a)). The fourth sensor S4 and the fifth sensor S5 are arranged so that the fourth detection position S4a and the fifth detection position S5a intersect approximately in the center of the vertical direction of the second guide member 612.

[0072] The control unit 9 can detect the passage of the sheet S from the outputs of the fourth sensor S4 and the fifth sensor S5, and can calculate the amount of curl of the sheet S from the relative timing.

[0073] Figure 14(a) shows the state in which the leading edge Sa of an uncurled sheet S passes through the fourth detection position S4a and the fifth detection position S5a during transport. Figure 15(a) shows the change in output of the fourth sensor S4 and the fifth sensor S5 during the operation shown in Figure 14(a). When the sheet S is transported by the second transport roller pair 602, the leading edge Sa of the sheet S passes approximately in the vertical center of the second guide member 612, so the fourth sensor S4 and the fifth sensor S5 change from Low to High at approximately the same time (times T4t and T5t). Therefore, the detection timing difference ΔT0 (=T5t - T4t) for an uncurled sheet is 0.

[0074] Next, Figure 14(b) shows the state in which the tip Sa of the sheet S that is curled upward passes through the fourth detection position S4a and the fifth detection position S5a. Also, Figure 15(b) shows the change in output of the fourth sensor S4 and the fifth sensor S5 during the operation shown in Figure 14(b). The tip Sa of the sheet S that is curled upward passes through the upper region of the second guide member 612 compared to when the sheet S that is not curled passes through. Therefore, as shown in Figure 15(b), the fourth sensor S4 detects first (time T4t) and the fifth sensor S5 detects later (time T5t), so the detection timing difference ΔT1 is a positive value.

[0075] Figure 14(c) shows the state in which the leading edge Sa of the sheet S, which is curled upward more significantly than in Figure 14(b), passes through the fourth detection position S4a and the fifth detection position S5a. Figure 15(c) shows the change in output of the fourth sensor S4 and the fifth sensor S5 during the operation in Figure 14(c). If a sheet that is curled upward even more significantly, as in Figure 14(c), passes through, the detection timing difference ΔT2 shown in Figure 15(c) becomes a positive value and is greater than ΔT1.

[0076] On the other hand, Figure 14(d) shows the state in which the tip Sa of the sheet S, which is curled downwards, passes through the fourth detection position S4a and the fifth detection position S5a. Also, Figure 15(d) shows the change in output of the fourth sensor S4 and the fifth sensor S5 during the operation shown in Figure 14(d). In this case, the tip Sa of the sheet S passes through a lower region than in the uncurled state. Therefore, as shown in Figure 15(d), the fifth sensor S5 detects first (time T5t) and the fourth sensor S4 detects later (time T4t), so the detection timing difference ΔT3 is a negative value.

[0077] As described above, by evaluating the magnitude of the difference in detection timing between the fourth sensor S4 and the fifth sensor S5, it becomes possible to determine the direction and amount of curl at the leading edge Sa of the sheet S. Specifically, the control unit 9 calculates the amount of curl of the sheet based on the difference between the time when the fourth sensor S4 detects the end of the sheet (leading edge Sa) and the time when the fifth sensor S5 detects the leading edge Sa of the sheet (leading edge Sa), and the conveying speed Vs of the sheet.

[0078] As described above, according to this embodiment, the second transport roller pair 602 for acquiring the sheet shape is used in common by the sheet length detection unit 620 and the curl detection unit 630A. Therefore, compared to the case where the sheet length detection unit 620 and the curl detection unit 630A are provided separately, the number of parts due to the second transport roller pair 602 is reduced, and the overall size of the device can be suppressed and miniaturized while acquiring the sheet shape.

[0079] In the embodiments described above, the fourth sensor S4 and the fifth sensor S5 are placed at only one location in the width direction D2, but this is not limited to this. For example, multiple sensors may be provided at different positions in the width direction D2, as in the modified example of the first embodiment (see Figure 13).

[0080] Furthermore, while the embodiments described above describe the case where the sheet shape acquisition unit 600 is located on a transport path within the image forming apparatus 1, the invention is not limited to this. For example, the sheet shape acquisition unit 600 may be configured as a standalone optional device connected to the image forming apparatus 1. In this case, by connecting the sheet shape acquisition device to an image forming apparatus that does not have a sheet shape acquisition unit 600 inside, it becomes possible to add the function of calculating the curl shape and length of the sheet S. In this case as well, miniaturization of the sheet shape acquisition device can be achieved. [Explanation of Symbols]

[0081] 1...Image forming apparatus, 9...Control unit, 513...Image forming unit, 515...Sheet transport device, 602...Second transport roller pair (transport roller pair), 611...First guide member, 612...Second guide member, D1...Sheet transport direction, D2...Width direction, D3...Orthogonal direction, S...Sheet, Sa...Front end (first end), Sb...Rear end (second end), S1...First sensor (third sheet detection unit), S1a...First detection position (third sheet detection position), S2...Second sensor (fourth sheet detection unit), S2a...Second detection position (fourth sheet S3...Third sensor (first sheet detection unit), S3a...Third detection position (first sheet detection position), S4...Fourth sensor (second sheet detection unit), S4a...Fourth detection position (second sheet detection position), S5...Fifth sensor (fifth sheet detection unit), S5a...Fifth detection position (fifth sheet detection position), S6...Sixth sensor (sixth sheet detection unit), S6a...Sixth detection position (sixth sheet detection position), S7...Seventh sensor (seventh sheet detection unit), S7a...Seventh detection position (seventh sheet detection position)

Claims

1. A first sheet detection unit detects a sheet being transported in the sheet transport direction at a first sheet detection position, A second sheet detection unit detects a sheet being transported in the sheet transport direction at a second sheet detection position downstream of the first sheet detection position in the sheet transport direction, A pair of conveying rollers is positioned upstream of the first sheet detection position in the sheet conveying direction and conveys the sheet in the sheet conveying direction, The system includes a control unit that calculates the sheet length in the sheet transport direction based on the detection result of the first sheet detection unit on the sheet transported by the transport roller pair and the transport speed of the sheet transported by the transport roller pair, and calculates the amount of curl of the sheet transported by the transport roller pair based on the detection result of the second sheet detection unit on the sheet transported by the transport roller pair. A sheet conveying device characterized by the following features.

2. The second sheet detection unit is provided in a state inclined with respect to the sheet transport direction and the width direction perpendicular to the sheet transport direction, The sheet conveying device according to feature 1.

3. The control unit calculates the amount of curl of the sheet based on the time elapsed from when the first sheet detection unit detects the edge of the sheet until the second sheet detection unit detects the edge of the sheet, and the conveying speed of the sheet. The sheet conveying device according to feature 2.

4. In the sheet transport direction, a first guide member is positioned upstream of the transport roller pair and opposite each other across the transport path in the orthogonal direction, In the sheet transport direction, a second guide member is provided, which is positioned downstream of the transport roller pair and is positioned opposite to the transport path and the second sheet detection position in the orthogonal direction, The spacing of the second guide members in the orthogonal direction is wider than the spacing of the first guide members. The sheet conveying device according to feature 2.

5. The unit includes a third sheet detection unit that detects a sheet being transported by the transport roller pair at a third sheet detection position different from the first sheet detection position in the sheet transport direction, The control unit calculates the conveying speed of the sheet being conveyed by the conveying roller pair based on the difference between the time when the first sheet detection unit detects the edge of the sheet and the time when the third sheet detection unit detects the edge of the sheet, and the distance between the first sheet detection position and the third sheet detection position in the conveying path. The sheet conveying device according to feature 1.

6. The control unit calculates the sheet length in the sheet transport direction based on the time when the first sheet detection unit detects the first end of the sheet, the time when the third sheet detection unit detects a second end of the sheet that is different from the first end of the sheet, the distance between the first sheet detection unit and the third sheet detection unit in the transport path, and the transport speed of the sheet being transported by the transport roller pair. The sheet conveying device according to feature 5.

7. A third sheet detection unit detects a sheet being transported by the transport roller pair at a third sheet detection position different from the first sheet detection position in the sheet transport direction, The system includes a fourth sheet detection unit that detects a sheet being transported by the transport roller pair at a fourth sheet detection position different from the first and third sheet detection positions in the sheet transport direction, The control unit calculates the conveying speed of the sheet being conveyed by the conveying roller pair based on the difference between the time when the third sheet detection unit detects the edge of the sheet and the time when the fourth sheet detection unit detects the edge of the sheet, and the distance between the third sheet detection position and the fourth sheet detection position in the conveying path. The sheet conveying device according to feature 1.

8. The control unit, The first sheet length in the sheet transport direction is calculated based on the time when the first sheet detection unit detects the first end of the sheet, the time when the third sheet detection unit detects a second end of the sheet that is different from the first end of the sheet, the distance between the first sheet detection unit and the third sheet detection unit in the transport path, and the transport speed of the sheet being transported by the transport roller pair. The second sheet length in the sheet transport direction is calculated based on the time when the first sheet detection unit detects the first end of the sheet, the time when the fourth sheet detection unit detects the second end of the sheet, the distance between the first sheet detection unit and the fourth sheet detection unit in the transport path, and the transport speed of the sheet being transported by the transport roller pair. The sheet length is calculated based on the average value. The sheet conveying device according to feature 7.

9. A fifth sheet detection unit is provided, which is inclined at a different angle from the second sheet detection unit with respect to the orthogonal direction, and detects a sheet being transported in the sheet transport direction at a fifth sheet detection position downstream of the first sheet detection position in the sheet transport direction. The control unit calculates the amount of curl of the sheet based on the difference between the time when the second sheet detection unit detects the edge of the sheet and the time when the fifth sheet detection unit detects the edge of the sheet, and the conveying speed of the sheet. The sheet conveying device according to feature 2.

10. A sixth sheet detection unit detects a sheet being transported by the transport roller pair at a sixth sheet detection position that overlaps with the first sheet detection position in the sheet transport direction and differs from the first sheet detection position in the width direction perpendicular to the sheet transport direction. The system includes a seventh sheet detection unit that detects a sheet being transported by the transport roller pair at a seventh sheet detection position that overlaps with the second sheet detection position in the sheet transport direction and differs from the second sheet detection position in the width direction. The sheet conveying device according to feature 1.

11. The transport roller pair has a blasted metal surface layer, The sheet conveying device according to feature 1.

12. A pair of conveying rollers that transport the sheet in the sheet transport direction, In order to obtain the sheet length in the sheet transport direction of the sheet being transported by the transport roller pair, a first sheet detection unit detects the sheet being transported by the transport roller pair at a first sheet detection position located downstream of the transport roller pair in the sheet transport direction, To obtain the amount of curl of the sheet being conveyed by the conveying roller pair, the system includes a second sheet detection unit that detects the sheet being conveyed by the conveying roller pair at a second sheet detection position located downstream of the first sheet detection position in the sheet conveying direction. A sheet conveying device characterized by the following features.

13. An image forming unit that forms an image on a sheet, A curl correction device for correcting the curl of a sheet on which an image has been formed by the image forming unit, A sheet conveying device according to any one of claims 1 to 12 for conveying sheets, comprising: An image forming apparatus characterized by the following:

Images