Sheet conveying device and image forming apparatus

Abstract

To optimize a loop amount of a loop formed by a sheet abutted against a pair of registration rollers.SOLUTION: A sheet conveying device according to the present invention comprises: a second conveying path P2 merging with a merging portion 17 of a first conveying path P1; a first sensor S1 detecting the tip of a sheet S at a first detection position S1a located between a pair of registration rollers 13 and a merging portion 17 in the first conveying path P1; a second sensor S2 detecting the tip of the sheet S at a second detection position S2a located between the first detection position S1a and the pair of registration rollers 13; and a control unit for adjusting the amount of a loop of the sheet S by adjusting the amount of conveyance of the sheet S by a first pair of pre-registration rollers 11 or a second pair of pre-registration rollers 12 toward the pair of registration rollers 13 in a stopped state based on a detection time, which is a time from the detection of the tip of the sheet S by the first sensor S1 to the detection of the tip of the sheet S by the second sensor S2.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a sheet conveyance device for conveying a sheet, and an image forming apparatus including the same.

Background Art

[0002] Conventionally, in an image forming apparatus, a skew correction mechanism for correcting the skew of a conveyed sheet is provided in order to align the position of the sheet and the image formed on the sheet. The skew correction mechanism is a mechanism that corrects the posture straight when the conveyed sheet is skewed. As the skew correction mechanism, one having a registration roller pair and a preregistration roller pair disposed upstream of the registration roller pair in the sheet conveyance direction is known (see Patent Document 1). Hereinafter, the registration roller pair is referred to as a registration roller pair, and the preregistration roller pair is referred to as a preregistration roller pair.

[0003] In this skew correction mechanism, after the leading end of the sheet conveyed by the preregistration roller pair is detected by the registration sensor, it is abutted against the registration roller pair that has stopped, and further, a loop having a predetermined loop amount is formed to correct the skew of the leading end portion of the sheet. After the skew of the sheet is corrected, the sheet is conveyed at the same speed by the driving of the registration roller pair synchronized with the image signal, and the registration roller pair and the preregistration roller pair start conveying, and the sheet is conveyed to the secondary transfer portion in the image forming portion.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the skew correction mechanism described in Patent Document 1, after a sheet is detected by a registration sensor and the sheet is abutted against a registration roller pair by a pre-registration roller pair, when forming a loop with a predetermined loop amount, the following problems may occur.

[0006] The actual conveyance speed of the sheet actually conveyed with respect to a predetermined rotation speed of the pre-registration roller pair may vary and is not constant. The reasons include, for example, the stiffness of the sheet, the surface properties of the sheet, and the sheet conveyance resistance associated with the cross-sectional shape of the conveyance path upstream of the pre-registration roller pair in the sheet conveyance direction. Therefore, the actual conveyance speed may become lower than the assumed reference speed. In this case, the actual loop amount actually formed becomes smaller than the predetermined loop amount required to correct the skew, and there is a risk that the skew correction becomes insufficient. As a result, the sheet is conveyed to the secondary transfer portion in an inclined state, and there is a risk that an image is formed obliquely on the sheet and the image formation accuracy is reduced. Also, the actual conveyance speed may become higher than the reference speed. In this case, there is a risk that noise is generated because the loop amount becomes excessive and the sheet strongly hits the conveyance guide, or that wrinkles are generated because the loop amount becomes excessive and fills the pass space, increasing the load acting on the sheet locally.

[0007] An object of the present invention is to provide a sheet conveyance device and an image forming device that can optimize the loop amount of a loop formed by a sheet abutted against a registration roller pair.

Means for Solving the Problems

[0008] A first aspect of the present invention includes a first conveyance path for conveying a sheet in a sheet conveyance direction; a registration roller pair disposed on the first conveyance path, which abuts against the downstream end of the sheet in the sheet conveyance direction to correct the skew of the sheet and then sandwiches and conveys the sheet; a first conveyance roller pair disposed on the first conveyance path for conveying the sheet to the registration roller pair; a second conveyance path that merges into a merging portion provided between the registration roller pair and the first conveyance roller pair in the first conveyance path; a second conveyance roller pair disposed on the second conveyance path for conveying the sheet from the merging portion to the registration roller pair via the first conveyance path; a first sensor for detecting the leading edge of the sheet at a first detection position located between the registration roller pair and the merging portion in the first conveyance path; a second sensor for detecting the leading edge of the sheet at a second detection position located between the first detection position and the registration roller pair; and a control unit that adjusts the conveyance amount of the sheet conveyed by the first conveyance roller pair or the second conveyance roller pair toward the stopped registration roller pair based on a detection time, which is the time from when the leading edge of the sheet is detected by the first sensor until it is detected by the second sensor, to adjust the loop amount of the sheet. The sheet conveyance apparatus is characterized by comprising the above components.

[0009] A second aspect of the present invention includes a first conveyance path for conveying a sheet in a sheet conveyance direction, a registration roller pair disposed on the first conveyance path, which abuts against the downstream end of the sheet in the sheet conveyance direction to correct the skew of the sheet and then sandwiches and conveys the sheet, a first conveyance roller pair disposed on the first conveyance path for conveying the sheet to the registration roller pair, a second conveyance path that merges into a merging portion provided between the registration roller pair and the first conveyance roller pair in the first conveyance path, a second conveyance roller pair disposed on the second conveyance path for conveying the sheet from the merging portion through the first conveyance path to the registration roller pair, a detection unit for detecting the conveyance speed of the sheet by detecting the leading edge of the sheet at a detection position located between the registration roller pair and the merging portion in the first conveyance path, and a control unit for adjusting the conveyance amount of the sheet conveyed by the first conveyance roller pair or the second conveyance roller pair toward the stopped registration roller pair based on the conveyance speed detected by the detection unit to adjust the loop amount of the sheet. The sheet conveyance device is characterized by comprising these components.

[0010] A third aspect of the present invention is an image forming apparatus characterized by comprising the above-described sheet conveyance device and an image forming unit that forms an image on the sheet conveyed by the registration roller pair.

Advantages of the Invention

[0011] According to the present invention, it is possible to optimize the loop amount of the loop formed by the sheet abutted against the registration roller pair.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0013] Hereinafter, this embodiment will be described with reference to the drawings. First, the schematic configuration of the image forming apparatus 1 according to this embodiment will be described with reference to FIG. 1. FIG. 1 is a cross-sectional view taken from the front side showing the schematic configuration of the image forming apparatus 1 according to this embodiment. The image forming apparatus 1 is a tandem type - intermediate transfer type laser beam printer using an electrophotographic process. The image forming apparatus 1 can form and output a full-color or monochrome image corresponding to the print image data output from a host apparatus (not shown) connected to the control unit 20 on a sheet S which is a recording medium.

[0014] [Image Forming Apparatus] The image forming apparatus 1 includes an apparatus main body 201A and an image forming unit 201B that forms an image on the sheet S. An image reading apparatus 202 installed substantially horizontally is provided above the apparatus main body 201A. A discharge space V for discharging the sheet is formed between the image reading apparatus 202 and the apparatus main body 201A. In this embodiment, the image forming apparatus 1 is also an example of a sheet conveying apparatus.

[0015] [Feeding Unit] The sheet S is stored, for example, in the feeding cassettes 7A and 7B of the cassette feeding units 9A and 9B for each size, or is loaded and supported on the manual feed tray 51 of the manual feeding unit 50.

[0016] The cassette feeding units 9A and 9B each include a feeding cassette 7A or 7B for accommodating the sheet S, and a feeding unit 6A or 6B for feeding the sheet S from the feeding cassette 7A or 7B. The feeding cassettes 7A and 7B are accommodated in the apparatus main body 201A and are an example of a sheet accommodating portion for accommodating the sheet S. The feeding units 6A and 6B are an example of a first feeding unit for feeding the sheet S accommodated in the feeding cassettes 7A and 7B to the first pre-registration roller pair 11.

[0017] Although the cassette feeding units 9A and 9B accommodate sheets S of different sizes, the configurations of the respective feeding units 6A and 6B are the same. Therefore, the feeding unit 6A will be described here. The feeding unit 6A includes a pickup roller 2, and a separation roller pair 5 including a feed roller 3 (see FIG. 3) and a retard roller 4 (see FIG. 3) for separating the sheet S sent out from the pickup roller 2. The sheets S separated one by one by the separation roller pair 5 are conveyed to a registration roller pair 13 (hereinafter referred to as the first pre-registration roller pair 11). That is, the feeding units 6A and 6B feed the sheet S accommodated in the feeding cassettes 7A and 7B to the first pre-registration roller pair 11.

[0018] Here, in order to record an image within an appropriate range of the sheet S, it is necessary to cause the control unit 20 to recognize the size of the sheet S stored therein. The sheet size detection units 8A and 8B are provided in the paper feed cassettes 7A and 7B. The sheet size detection units 8A and 8B include a size detection lever that is rotatable in conjunction with being in sliding contact with a side regulation plate that regulates the position of the sheet S in the width direction, and a plurality of sensors or switches. The plurality of sensors or switches are provided at positions corresponding to the size detection lever in the mounting portion where the paper feed cassettes 7A and 7B are mounted. Therefore, when the side regulation plate is moved in accordance with the side end portion of the sheet S, the size detection lever rotates in conjunction. When the paper feed cassettes 7A and 7B are mounted on the mounting portion in that state, the size detection lever selectively turns on and off the detection elements of the sensors or switches arranged in the mounting portion where the paper feed cassettes 7A and 7B are mounted. As a result, signals of different patterns are transmitted from the sensors or switches to the control unit 20. Then, the control unit 20 can recognize the size of the sheet S stored in the paper feed cassettes 7A and 7B based on the signals.

[0019] The manual feed unit 50 is openable and closable by rotating with respect to the apparatus main body 201A, is displaceable between a closed position and an open position, and is detachably provided on the apparatus main body 201A. The manual feed unit 50 includes a manual tray 51 that is a means for supporting the sheet S, and a feed unit 56 that feeds the sheet S from the manual tray 51. The manual tray 51 supports the sheet S loaded on the upper surface in the open state. The feed unit 56 is an example of a second feed unit that feeds the sheet S supported by the manual tray 51 to the second pre-registration roller pair 12.

[0020] The feeding unit 56 includes a separation roller pair 55 composed of a pickup roller 52, a feed roller 53 (see FIG. 3) and a retard roller 54 (see FIG. 3) for separating the sheet S fed out from the pickup roller 52. The sheets S separated one by one by the separation roller pair 55 are conveyed to the registration roller pair 13 by a second pre-registration roller pair (hereinafter referred to as the second pre-reg roller pair) 12. That is, the feeding unit 56 feeds the sheet S supported by the manual feed tray 51 to the second pre-reg roller pair 12.

[0021] [Image forming unit] The image forming unit 201B forms an image on the sheet S fed by the cassette feeding unit 230 or the manual feeding unit 50 and conveyed by the registration roller pair 13. The image forming unit 201B includes four process cartridges PY, PM, PC, and PK that form toner images of four colors: yellow (Y), magenta (M), cyan (C), and black (K), an exposure unit 30, and an intermediate transfer belt 36. The four process cartridges PY, PM, PC, and PK have the same configuration except that the colors of the images they form are different. Therefore, only the configuration of the process cartridge PY and the image forming process will be described, and the description of the process cartridges PM, PC, and PK will be omitted. Further, the image forming unit 201B includes a secondary transfer unit 38 disposed above the process cartridges PY, PM, PC, and PK, and a fixing unit 201E. The toner cartridge 35 supplies toner to the developing roller 34.

[0022] The process cartridge PY has a photosensitive drum 32 which is a drum-type electrophotographic photoreceptor, a charging roller 33, and a developing roller 34. The photosensitive drum 32 is rotatably supported by the image forming apparatus 1 and is rotationally driven by a driving means (not shown). The photosensitive drum 32 is configured by providing a photoconductive layer such as OPC (organic photoconductor) on the outer peripheral surface of an aluminum cylinder. The charging roller 33 charges the photosensitive drum 32. The charging roller 33 is composed of a core bar and a conductive elastic member surrounding the core bar, is disposed in contact with the surface of the photosensitive drum 32 and rotates passively, and a charging bias is applied by a power source (not shown). The developing roller 34 attaches toners of different colors (yellow, magenta, cyan, black) to the surface of the photosensitive drum 32 for each of the process cartridges PY, PM, PC, and PK. The developing roller 34 attaches toner to the electrostatic latent image on the surface of the photosensitive drum 32 to develop the electrostatic latent image and forms a toner image on the surface of the photosensitive drum 32.

[0023] The exposure unit 30 houses a laser light source that emits laser light inside the housing and various optical members for guiding the laser light emitted from the laser light source to the corresponding photosensitive drum 32 for performing deflection scanning. The exposure unit 30 irradiates the surface of the charged photosensitive drum 32 with laser light based on image information to form an electrostatic latent image.

[0024] The secondary transfer unit 38 includes an intermediate transfer belt 36 wound around a driving roller 36a and a tension roller 36b. Inside the intermediate transfer belt 36, a primary transfer roller 39 that abuts against the intermediate transfer belt 36 is provided at a position facing the photosensitive drum 32. Here, the intermediate transfer belt 36 rotates in the direction of the arrow by the driving roller 36a driven by a driving unit (not shown). At a position facing the driving roller 36a of the secondary transfer unit 38, a secondary transfer roller 37 for transferring the color image formed on the intermediate transfer belt 36 to the sheet S is provided.

[0025] Above the secondary transfer roller 37, a fixing unit 201E is arranged. Above this fixing unit 201E, a first discharge roller pair 225a, a second discharge roller pair 225b, and a duplex reversing unit 201F are arranged. The duplex reversing unit 201F is an example of a re-conveying unit, which reverses the front and back of the sheet S on which an image has been formed by the image forming unit 201B and conveys it, and conveys it to the second pre-registration roller pair 12. The duplex reversing unit 201F has a reversible reversing roller pair 222 and a re-conveying path 224 that conveys the sheet S with an image formed on the first side back to the image forming unit 201B. On the upper part of the image forming apparatus 1, an operation unit 23 for receiving operations from the user is provided. As the operation unit 23, a touch panel method that combines a display function and an input function is adopted. The operation unit 23 is connected to the control unit 20 and receives various information (such as size information, basis weight information, surface property information, etc.) about the sheet used by the user for printing, and various operations performed by the user such as an instruction to execute printing or interrupt it.

[0026] [Control System] Next, the control unit 20 and the control system of the image forming apparatus 1 will be described with reference to FIG. 2. The control unit 20 has functional units such as a CPU 21 (Central Processing Unit), a memory 22, an image forming control unit 25, a sheet conveyance control unit 26, and a sensor control unit 27. The CPU 21 realizes various processes performed by the image forming apparatus 1 by executing a predetermined control program and the like. The memory 22 is, for example, a RAM (Random Access Memory), a ROM (Read only memory), etc., and stores various programs and various data in a predetermined storage area.

[0027] The image formation control unit 25 issues instructions to the image formation unit 201B including the exposure unit 30 and controls image formation. The sheet conveyance control unit 26 issues instructions to the first pre-registration motor 101, the second pre-registration motor 102, the registration motor 103, etc., controls the driving of various conveyance rollers, and controls the conveyance of the sheet S. The sensor control unit 27 controls the start or stop of detection by the first sensor S1, the second sensor S2, etc., and receives the detection results of these sensors. Note that it can also be configured to be able to receive various information regarding the sheet used for printing via a computer 24 connected via a network, for example.

[0028] [Image formation operation] Next, the image formation operation of the image forming apparatus 1 will be described. First, when the image forming apparatus 1 receives the image data of the document to be printed, the image information is processed and then converted into an electrical signal and transmitted to the exposure unit 30 of the image forming unit 201B. In the image forming unit 201B, the surface of the photosensitive drum 32 whose surface is uniformly charged to a predetermined polarity and potential by the charging roller 33 is sequentially exposed by a laser. As a result, electrostatic latent images of yellow, magenta, cyan, and black are sequentially formed on the photosensitive drums 32 of the respective process cartridges PY, PM, PC, and PK.

[0029] Thereafter, this electrostatic latent image is developed and visualized with each color toner, and the toner images of each color on each photosensitive drum 32 are sequentially superimposed and transferred onto the intermediate transfer belt 36 by the primary transfer bias applied to the primary transfer roller 39. As a result, a toner image is formed on the intermediate transfer belt 36.

[0030] Also, in parallel with this toner image forming operation, the sheet S is fed one by one in the cassette feeding units 9A and 9B or the manual feeding unit 50. The sheet S fed from the cassette feeding units 9A and 9B passes through the first pre-registration roller pair 11 and is conveyed to the registration roller pair 13. The skew of the sheet S is corrected by the first pre-registration roller pair 11 and the registration roller pair 13. Specifically, the leading edge of the conveyed sheet S is abutted against the nip portion of the registration roller pair 13 that is stopped. The first pre-registration roller pair 11 further conveys the sheet S to form a loop for skew correction. Similarly, the sheet S fed from the manual feeding unit 50 passes through the second pre-registration roller pair 12 and is conveyed to the registration roller pair 13. The skew of the sheet S is corrected by the second pre-registration roller pair 12 and the registration roller pair 13. In the present embodiment, the skew correction unit 10 includes the first pre-registration roller pair 11, the second pre-registration roller pair 12, the registration roller pair 13, the first sensor S1, and the second sensor S2. Details of the control during loop formation in the skew correction unit 10 will be described later.

[0031] The registration roller pair 13 conveys the sheet S to the secondary transfer unit 38 in accordance with the timing when the toner image on the intermediate transfer belt 36 is transferred to the sheet S. In the secondary transfer unit 38, the toner image is collectively transferred onto the sheet S by the secondary transfer bias applied to the secondary transfer roller 37.

[0032] Next, the sheet S onto which the toner image has been transferred is conveyed to the fixing unit 201E, and in the roller nip portion formed by the pressure roller 220a and the heating roller 220b, it receives heat and pressure, and the toners of each color are melted and mixed, and are fixed as a color image on the sheet S. The sheet S on which the image has been fixed is discharged into the discharge space V by the first discharge roller pair 225a or the second discharge roller pair 225b provided downstream of the fixing unit 201E, and is stacked on the discharge tray 223 protruding from the bottom surface of the discharge space V.

[0033] When forming images on both sides of the sheet S, after the trailing edge of the sheet S is left at a predetermined distance from the pair of reversing rollers 222 and once stopped, the pair of reversing rollers 222 is reversed, and the sheet is passed through the second pre-registration roller pair 12 via the re-conveying path 224. Then, it is conveyed to the registration roller pair 13 again for skew correction, and the second image formation and fixing are performed by the image forming unit 201B.

[0034] [Skew correction unit] Next, the skew correction unit 10 will be described with reference to FIG. 3. FIG. 3 is a cross-sectional view when viewed from the front side showing the skew correction unit 10. The first pre-registration roller pair 11 is arranged downstream in the sheet conveying direction of the feeding parts 6A and 6B of the cassette feeding units 9A and 9B. The first pre-registration roller pair 11 includes a driven roller 11a having a roller made of polyacetal (POM) and a driving roller 11b formed of a rubber roller arranged opposite to the driven roller 11a. The driven roller 11a is pressed against the driving roller 11b by the elastic force of a spring (not shown). The driving roller 11b is driven by the first pre-registration motor 101. The first pre-registration roller pair 11 is an example of the first conveying roller pair that conveys the sheet S to the registration roller pair 13.

[0035] The registration roller pair 13 is provided downstream in the sheet conveying direction of the first pre-registration roller pair 11 and functions as a contact portion where the leading edge of the conveyed sheet abuts to correct the skew of the sheet S. The registration roller pair 13 includes a driven roller 13a and a driving roller 13b. The skew of the sheet S is corrected by the sheet S abutting along the leading edge of the sheet S at the nip portion between the driven roller 13a and the driving roller 13b. That is, after abutting against the downstream end of the sheet S in the sheet conveying direction to correct the skew of the sheet S, the registration roller pair 13 sandwiches and conveys the sheet S. The registration roller pair 13 includes a driven roller 13a having a roller made of polyacetal (POM) and a driving roller 13b formed of a rubber roller arranged opposite to the driven roller 13a. The driven roller 13a is pressed against the driving roller 13b by the elastic force of a spring (not shown). The driving roller 13b is driven by the registration motor 103.

[0036] Between the first pre-registration roller pair 11 and the registration roller pair 13, a first guide member 14 for guiding the conveyed sheet S and a second guide member 15 facing the first guide member 14 are arranged. The first guide member 14 and the second guide member 15 are spaced apart particularly at the central portion in the sheet conveyance direction, and constitute a loop forming portion 16 which is a space in which the sheet S abutted against the nip portion of the registration roller pair 13 can form a loop. Further, in the present embodiment, the first pre-registration roller pair 11 and the registration roller pair 13 are arranged, and the path for conveying the sheet S in the sheet conveyance direction is defined as the first conveyance path P1.

[0037] On the other hand, the sheet S conveyed from the re-conveyance passage 224 or the manual feeding unit 50 is conveyed to the registration roller pair 13 via the second pre-registration roller pair 12. At this time, the conveyed sheet S passes through a confluence portion 17 where it merges into the first conveyance path P1 between the first pre-registration roller pair 11 and the registration roller pair 13. In the present embodiment, the second pre-registration roller pair 12 is arranged, and the path for conveying the sheet S in the sheet conveyance direction and merging into the confluence portion 17 is defined as the second conveyance path P2.

[0038] The second pre-registration roller pair 12 includes a driven roller 12a having a roller made of polyacetal (POM) and a driving roller 12b formed of a rubber roller arranged to face the driven roller 12a. The driven roller 12a is pressed against the driving roller 12b by the elastic force of a spring (not shown). The driving roller 12b is driven by a second pre-registration motor 102. The second pre-registration roller pair 12 is an example of a second conveyance roller pair that conveys the sheet S from the confluence portion 17 to the registration roller pair 13 via the first conveyance path P1.

[0039] In the vicinity downstream in the sheet conveyance direction of the confluence portion 17, a first sensor S1 for detecting the leading end of the sheet S, that is, the downstream end in the sheet conveyance direction, is disposed. In the present embodiment, the first sensor S1 detects the leading end of the sheet S at a first detection position S1a located between the registration roller pair 13 and the confluence portion 17 in the first conveyance path P1. A second sensor S2 is disposed downstream of the first sensor S1 in the sheet conveyance direction. The second sensor S2 detects the leading end of the sheet S at a second detection position S2a located between the first detection position S1a and the registration roller pair 13. The distance in the first conveyance path P1 from the first detection position S1a to the second detection position S2a is defined as a distance L1. Also, the distance in the first conveyance path P1 from the second detection position S2a to the nip portion of the registration roller pair 13 is defined as a distance L2. The first sensor S1 and the second sensor S2 are used for sheet conveyance control during loop formation.

[0040] The first sensor S1 is configured to include, for example, a flag that moves by contacting the leading end of the sheet S, and an optical sensor that detects a light beam blocked by the rotation of the flag. Therefore, it can be made relatively inexpensive. The second sensor applies an optical sensor having a light emitting portion and a light receiving portion. Therefore, highly accurate detection can be performed, and for example, highly accurate control can be realized when the detection by the second sensor is used for the feeding timing of the next sheet.

[0041] [Oblique correction operation] Next, the skew correction operation of the sheet S by the skew correction unit 10 will be described with reference to FIGS. 4(a) to 4(c). Here, as shown in FIG. 4(a), the skew correction operation will be described when the sheet S conveyed by the first pre-reg roller pair 11 is skewed clockwise in the drawing with respect to the sheet conveyance direction DF. FIG. 4(a) shows a state where the leading end of the sheet S has reached and been detected by the first sensor S1. In this state, when the first pre-reg roller pair 11 is further driven and the sheet S is conveyed in the sheet conveyance direction DF, as shown in FIG. 4(b), the leading end of the sheet S reaches and is detected by the second sensor S2. After that, when the sheet S is further conveyed, the leading end of the sheet S abuts against the nip portion of the registration roller pair 13. At this time, the registration roller pair 13 has stopped rotating.

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

[0043] Here, the orientation of the arrangement of each part of the skew correction unit 10 will be described with reference to FIGS. 1 and 3. As shown in FIG. 1, in the present embodiment, the conveyance path from the feeding units 6A and 6B to the secondary transfer unit 38 has a configuration of a vertical path that is upward. Therefore, as shown in FIG. 3, the nip lines of the first pre-reg roller pair 11 and the registration roller pair 13 are provided along the vertical direction. As a result, the nip lines of the first pre-reg roller pair 11 and the registration roller pair 13 are provided substantially in parallel, so that the conveyance resistance during sheet conveyance from the first pre-reg roller pair 11 to the registration roller pair 13 can be suppressed to be small.

[0044] The first guide member 14 is an example of a guide member, which is disposed between the registration roller pair 13 and the confluence portion 17 in the first conveyance path P1 and is provided so as to intersect the nip line of the registration roller pair 13. For this reason, the leading end of the sheet S conveyed from the first pre-registration roller pair 11 is guided along the second guide member 15, and then is guided by the first guide member 14 in the vicinity of the registration roller pair 13 and abuts against the nip portion of the registration roller pair 13. After the sheet S abuts against the nip portion of the registration roller pair 13, the sheet S is further conveyed by the first pre-registration roller pair 11, so that the sheet S is in contact with the first guide member 14. The sheet S is further conveyed by the first pre-registration roller pair 11, so that a loop 18 is formed toward the second guide member 15 in the loop forming portion 16. That is, the sheet S abutted against the stopped registration roller pair 13 is positioned along the first guide member 14 and then is further conveyed to form a loop 18 in a direction away from the first guide member 14.

[0045] On the other hand, the nip line of the second pre-registration roller pair 12 is provided along the horizontal direction. That is, the nip line of the second pre-registration roller pair 12 is provided substantially orthogonally to the nip lines of the first pre-registration roller pair 11 and the registration roller pair 13. For this reason, it is possible to suppress the conveyance resistance during conveyance of the sheet S supported by the manual feed tray 51. The leading end of the sheet S conveyed from the second pre-registration roller pair 12 is guided by the first guide member 14 and abuts against the nip portion of the registration roller pair 13. After the sheet S abuts against the nip portion of the registration roller pair 13, the sheet S is further conveyed by the second pre-registration roller pair 12, so that the sheet S is in contact with the first guide member 14. The sheet S is further conveyed by the second pre-registration roller pair 12, so that a loop 18 is formed toward the second guide member 15 in the loop forming portion 16.

[0046] In this embodiment, since it is a vertical path configuration, the nip lines of the first pre-register roller pair 11 and the register roller pair 13 are described as being provided along the vertical direction, but this is not limiting. For example, if each nip line of the first pre-register roller pair 11 and the register roller pair 13 has an inclination of less than 45 degrees with respect to the vertical direction, it can be a vertical path configuration. Also, the case where the nip line of the second pre-register roller pair 12 is provided along the horizontal direction is described, but this is not limiting. For example, it may have an inclination of less than 45 degrees with respect to the horizontal direction.

[0047] Also, in this embodiment, the case where the nip lines of the first pre-register roller pair 11 and the register roller pair 13 are provided substantially in parallel is described, but this is not limiting. For example, even if the nip lines of the first pre-register roller pair 11 and the register roller pair 13 are not parallel, if the nip line of the first pre-register roller pair 11 has an inclination of less than 45 degrees with respect to the nip line of the register roller pair 13, the conveying resistance can be suppressed to a small value. Also, in this embodiment, the case where the nip line of the second pre-register roller pair 12 is provided substantially perpendicular to the nip lines of the first pre-register roller pair 11 and the register roller pair 13 is described, but this is not limiting. For example, if the nip line of the second pre-register roller pair 12 has an inclination of more than 45 degrees with respect to the nip line of the register roller pair 13, the conveying resistance during the conveyance of the sheet S supported by the manual feed tray 51 can be suppressed to a small value.

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

[0049] Here, since the basis weight and type of the sheet S are different, the loop amount during skew correction is different. For example, in the case of the same conveyance path, the loop amount of the sheet S such as thick paper with high rigidity is larger than that of the sheet S such as thin paper with low rigidity. That is, for example, when adjusting so that the loop amount becomes the first loop amount when the first sheet is conveyed, when the second sheet with a different sheet type from the first sheet is conveyed through the same conveyance path, the loop amount is adjusted to be the second loop amount different from the first loop amount.

[0050] [Set loop amount] Next, a method for setting the set loop amount LS when presetting the loop amount formed during skew correction will be described. Here, the case of conveying the sheet S from the cassette feeding units 9A and 9B will be described, but the set loop amount LS can be set in the same manner for the sheet S conveyed from the re-conveyance path 224 or the manual feeding unit 50.

[0051] The control unit 20 conveys the sheet S at the set speed V1 by the first pre-register roller pair 11, reaches the register roller pair 13 via the first sensor S1 and the second sensor S2, forms the loop 18, and then stops the first pre-register roller pair 11. In this embodiment, it is assumed that the sheet S is conveyed at a constant speed of the set speed V1. For this reason, the set loop amount LS is managed by the time T1 from when the sheet S passes through the second sensor S2 until the first pre-register roller pair 11 stops.

[0052] When the sheet S is conveyed at an ideal speed at the set speed V1, the time T1 is calculated by the formula (1). T1 = (L2 + LS) / V1 ···(Formula 1)

[0053] However, even when the first pre-register roller pair 11 is rotated at a conveyance speed that becomes the set speed V1, the actual conveyance speed V2, which is the speed at which the sheet S is actually conveyed, is generally known to be slower than the set speed V1. This is because a slight slip occurs between the first pre-register roller pair 11 and the sheet S. The main factor of this slip is due to the conveyance resistance received from the guide member when conveying the sheet S, and it is known that a particularly large conveyance resistance is received when conveying a thick sheet S with high rigidity of the sheet S to a curved guide member. Also, depending on the combination of the surface properties of the guide member and the surface properties of the sheet S, the conveyance resistance changes to be larger or smaller.

[0054] Therefore, the actual conveyance speed V2 when the first pre-register roller pair 11 conveys the sheet S can be calculated by the following method. First, when the time when the sheet S conveyed by the first pre-register roller pair 11 passes through the first sensor S1 is t1 and the time when it then passes through the second sensor S2 is t2, the actual conveyance speed V2 of the sheet S is calculated by the formula (2). Here, the time (t2 - t1) from when the tip of the sheet S is detected by the first sensor S1 until the tip of the sheet S is detected by the second sensor S2 is defined as the detection time t0. V2 = L1 / (t2 - t1) ···(Formula 2)

[0055] Based on the calculated actual conveyance speed V2, the loop formation time T2 from when the sheet S passes through the second sensor S2 until the first pre-regulator pair 11 stops is calculated by Equation (3). T2 = (L2 + LS) / V2 = (L2 + LS) / (L1 / (t2 - t1)) ···(Equation 3)

[0056] By setting the loop formation time from when the sheet S passes through the second sensor S2 until the first pre-regulator pair 11 stops to T2, the actual loop amount formed can be made closer to the set loop amount LS. The control unit 20 adjusts the loop amount of the sheet S by adjusting the loop formation time T2 while keeping the conveyance speed at which the sheet S is conveyed by the first pre-regulator pair 11 constant toward the stopped regulator pair 13 based on the detection time t0. Here, the loop formation time T2 is calculated based on the detection time t0, but it is not limited to this. For example, a table showing the relationship between the detection time t0 and the loop formation time T2 may be prepared and the loop formation time T2 may be obtained using that table. As described above, the first sensor S1 measures the start time of the detection time t0, and the second sensor S2 measures the end time of the detection time t0. That is, the second sensor S2 is an example of a detection unit, and detects the conveyance speed of the sheet by detecting the leading end of the sheet S at the second detection position S2a (detection position).

[0057] [Operation Procedure] Next, the operation procedure of the skew correction unit 10 will be described with reference to the flowchart shown in FIG. 5. After the start of an image forming job, the control unit 20 acquires the conveyance source of which sheet S among the cassette feeding units 9A and 9B, the re-conveyance path 224, and the manual feeding unit 50 is to be conveyed (step S1). After acquiring the conveyance source, the control unit 20 acquires necessary sheet information (step S2). The control unit 20 sets a set loop amount LS to be formed by the loop forming unit 16 based on the sheet information (step S3). In the present embodiment, the control unit 20 adjusts the set loop amount LS based on the detection time t0 and the sheet information regarding the sheet S to be conveyed by the method described above.

[0058] The control unit 20 rotates the first pre-registration roller pair 11 or the second pre-registration roller pair 12 according to the conveyance source (step S4). After the leading edge of the sheet S is detected by the first sensor S1 (step S5), the control unit 20 determines whether or not it has been detected by the second sensor S2 (step S6). If the control unit 20 determines that the leading edge of the sheet S has not been detected by the second sensor S2 (step S6; NO), the detection continues. If the control unit 20 determines that the leading edge of the sheet S has been detected by the second sensor S2 (step S6; YES), it sets a loop forming time T2 (step S7).

[0059] The control unit 20 continues the rotation of the first pre-registration roller pair 11 or the second pre-registration roller pair 12, and causes the leading edge of the sheet S to abut against the nip portion of the registration roller pair 13 that is stopped (step S8). The control unit 20 forms a loop 18 of the sheet S by further continuing the rotation of the first pre-registration roller pair 11 or the second pre-registration roller pair 12 (step S9). The control unit 20 determines whether or not the loop formation time T2 has elapsed (step S10). When the control unit 20 determines that the loop formation time T2 has not elapsed (step S10; NO), the control unit 20 continues to form the loop 18 of the sheet S (step S9). When the control unit 20 determines that the loop formation time T2 has elapsed (step S10; YES), the control unit 20 stops the rotating pre-registration roller pair (step S11), and rotates the registration roller pair 13 in accordance with the timing of image formation (step S12).

[0060] As described above, according to the image forming apparatus 1 of the present embodiment, the loop amount of the sheet S is adjusted based on the detection time t0 obtained from the actual speed. Therefore, even when the speed of the sheet S changes due to the stiffness and surface properties of the sheet S, the sheet conveyance resistance, etc., the loop amount of the sheet S during skew correction can be optimized. As a result, it is possible to suppress a decrease in image forming accuracy due to insufficient loop amount and insufficient skew correction, and the occurrence of noise, sheet wrinkles, and an increase in torque of the pre-registration motor due to an excessive loop amount.

[0061] Further, according to the image forming apparatus 1 of the present embodiment, the second sensor S2 is disposed on the downstream side of the confluence portion 17. Therefore, the number of sensors can be reduced as compared with the case where sensors are respectively disposed in a plurality of upstream conveyance paths, and cost reduction and miniaturization of the image forming apparatus 1 can be achieved.

[0062] Further, according to the image forming apparatus 1 of the present embodiment, at the time of skew correction, the loop formation time T2 is adjusted while keeping the conveyance speed at which the sheet S is conveyed by the first pre-registration roller pair 11 or the second pre-registration roller pair 12 toward the stopped registration roller pair 13 constant. Therefore, the calculation accuracy of the set loop amount LS can be increased as compared with the case where the sheet S is conveyed while increasing or decreasing the conveyance speed.

[0063] <Other Embodiments> In the above-described embodiment, at the time of skew correction, the loop formation time T2 is adjusted while keeping the conveyance speed at which the sheet S is conveyed by the first pre-registration roller pair 11 or the second pre-registration roller pair 12 toward the stopped registration roller pair 13 constant. However, the present invention is not limited to this, and the conveyance speed at which the sheet S is conveyed by the first pre-registration roller pair 11 or the second pre-registration roller pair 12 toward the stopped registration roller pair 13 may be increased or decreased according to, for example, the delay of the sheet S. Also in this case, the control unit 20 can adjust the conveyance amount of the sheet S conveyed by the first pre-registration roller pair 11 or the second pre-registration roller pair 12 toward the stopped registration roller pair 13 based on the detection time t0 to adjust the loop amount of the sheet S.

[0064] In the above-described embodiment, the detection time t0 is calculated using two sensors, i.e., the first sensor S1 and the second sensor S2, and the actual conveyance speed of the sheet S is calculated based on the detection time t0. However, the present invention is not limited to this. For example, the actual conveyance speed may be detected using a speed sensor capable of directly detecting the conveyance speed of the sheet S at a detection position located between the merging unit 17 and the registration roller pair 13. Alternatively, the actual conveyance speed may be detected using a movement amount sensor capable of measuring the conveyance distance of the sheet S at the detection position and a timer. In these cases, these speed sensors and movement amount sensors are examples of the detection unit, and the conveyance speed of the sheet is detected by detecting the leading edge of the sheet S at the detection position on the first conveyance path P1. Therefore, the detection position located between the merging unit 17 and the registration roller pair 13 can be made one, and the number of sensors can be further reduced as compared with the above-described embodiment.

[0065] Also, in the above-described embodiments, the image forming apparatus 1 adopts a vertical path configuration in which the conveyance path from the feeding units 6A and 6B to the secondary transfer unit 38 is upward, but the present invention is not limited to this, and a horizontal path configuration may be adopted instead.

[0066] Also, in each of the above-described embodiments, the image forming apparatus 1 which is a color laser printer has been described as an example, but the present invention is not limited to this. For example, the present invention may be applied to an image forming apparatus which is a monochrome laser printer.

Explanation of Reference Numerals

[0067] 1... Image forming apparatus (sheet conveyance apparatus), 6A, 6B... Feeding units (first feeding units), 7A, 7B... Feeding cassettes (sheet storage units), 11... First pre-registration roller pair (first conveyance roller pair), 12... Second pre-registration roller pair (second conveyance roller pair), 13... Registration roller pair (registration roller pair), 14... First guide member (guide member), 18... Loop, 20... Control unit, 51... Manual feed tray, 56... Feeding unit (second feeding unit), 201A... Apparatus main body, 201B... Image forming unit, 201F... Duplex reversing unit (re-conveyance unit), P1... First conveyance path, P2... Second conveyance path, S... Sheet, S1... First sensor, S1a... First detection position, S2... Second sensor (detection unit), S2a... Second detection position (detection position), t0... Detection time, T2... Loop formation time

Claims

1. A first conveyance path for conveying a sheet in the sheet conveyance direction, a registration roller pair disposed in the first conveyance path, contacting the downstream end of the sheet in the sheet conveyance direction to correct the skew of the sheet, and then sandwiching and conveying the sheet, a first conveyance roller pair disposed in the first conveyance path for conveying the sheet to the registration roller pair, a second conveyance path that merges into a merging portion provided between the registration roller pair and the first conveyance roller pair in the first conveyance path, a second conveyance roller pair disposed in the second conveyance path for conveying the sheet from the merging portion to the registration roller pair via the first conveyance path, a first sensor for detecting the leading edge of the sheet at a first detection position located between the registration roller pair and the merging portion in the first conveyance path, a second sensor for detecting the leading edge of the sheet at a second detection position located between the first detection position and the registration roller pair, a control unit that adjusts the conveyance amount of the sheet by the first conveyance roller pair or the second conveyance roller pair toward the stopped registration roller pair based on a detection time, which is the time from when the leading edge of the sheet is detected by the first sensor until the leading edge of the sheet is detected by the second sensor, to adjust the loop amount of the sheet. A sheet conveyance device characterized by the above.

2. The control unit adjusts so that the loop amount becomes a first loop amount when the first sheet is conveyed, and adjusts so that the loop amount becomes a second loop amount different from the first loop amount when a second sheet having a sheet type different from that of the first sheet is conveyed along the same conveyance path as the first sheet. The sheet conveyance device according to claim 1, characterized by the above.

3. The control unit adjusts the loop amount based on the detection time and sheet information regarding the sheet to be conveyed. The sheet conveyance device according to claim 1, characterized by the above.

4. The control unit adjusts the loop amount of the sheet by adjusting a loop formation time, which is the time for conveying the sheet while keeping the conveyance speed of the sheet conveyed by the first conveyance roller pair or the second conveyance roller pair toward the stopped registration roller pair constant based on the detection time. The sheet conveyance device according to claim 3, characterized by the above.

5. The sheet information is information including any one of the size, basis weight, and type of the sheet. The sheet conveying device according to claim 3, characterized in that.

6. A guide member disposed between the registration roller pair and the confluence portion in the first conveyance path and intersecting the nip line of the registration roller pair is provided. The sheet abutted against the stopped registration roller pair is positioned along the guide member and then further conveyed to form a loop in a direction away from the guide member. The sheet conveying device according to claim 1, characterized in that.

7. The nip line of the first conveyance roller pair has an inclination of less than 45 degrees with respect to the nip line of the registration roller pair. The nip line of the second conveyance roller pair has an inclination of more than 45 degrees with respect to the nip line of the registration roller pair. The sheet conveying device according to claim 1, characterized in that.

8. The nip line of the registration roller pair has an inclination of less than 45 degrees with respect to the vertical direction. The sheet conveying device according to claim 7, characterized in that.

9. A sheet storage portion housed in the apparatus main body and storing sheets, A first feeding portion for feeding the sheets stored in the sheet storage portion to the first conveyance roller pair, A manual feed tray that is openable and closable with respect to the apparatus main body and supports the sheets stacked on the upper surface in the open state, A second feeding portion for feeding the sheets supported by the manual feed tray to the second conveyance roller pair is provided. The sheet conveying device according to claim 1, characterized in that.

10. The sheet conveying device according to claim 1, An image forming portion for forming an image on the sheet conveyed by the registration roller pair is provided. An image forming apparatus, characterized in that.

11. A re-conveyance portion that reverses the front and back of the sheet on which the image has been formed by the image forming portion and conveys it to the second conveyance roller pair is provided. The image forming apparatus according to claim 10, characterized in that.

12. A first conveyance path for conveying the sheet in the sheet conveyance direction, A registration roller pair disposed in the first conveyance path, abutting against the downstream end of the sheet in the sheet conveyance direction to correct the skew of the sheet, and then sandwiching and conveying the sheet, A first conveyance roller pair disposed in the first conveyance path for conveying the sheet to the registration roller pair, A second conveyance path that merges into a merging section provided between the registration roller pair and the first conveyance roller pair in the first conveyance path; A second conveyance roller pair that is disposed in the second conveyance path and conveys a sheet from the merging section to the registration roller pair via the first conveyance path; A detection unit that detects the conveyance speed of the sheet by detecting the leading end of the sheet at a detection position located between the registration roller pair and the merging section in the first conveyance path; A control unit that adjusts the conveyance amount of the sheet by the first conveyance roller pair or the second conveyance roller pair toward the stopped registration roller pair based on the conveyance speed detected by the detection unit, and adjusts the loop amount of the sheet. A sheet conveyance device characterized by the above.

Images