Image forming device

Abstract

To improve alignment accuracy between an image formed on a first surface of a sheet and an image formed on a second surface thereof.SOLUTION: An image forming apparatus controls an arrival timing t8 to cause a sheet S to arrive at a secondary transfer nip N when transferring a transfer image 611 to a second surface of the sheet S, based on a passage timing t4 at which a patch sensor detects a downstream patch 602 and a passage timing t5 at which a registration sensor 59 detects a rear end of the sheet S when transferring the transfer image 611 to a first surface of the sheet, and a passage timing t6 at which the patch sensor 58 detects an upstream patch 601 and a passage timing t7 at which the registration sensor 59 detects a leading end of the sheet S when transferring the transfer image 611 to the second surface of the sheet S.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that transfers an image from an image carrier to a sheet. [Background technology]

[0002] For example, some image forming devices, such as copiers, printers, fax machines, and multifunction peripherals, primarily transfer a toner image from a photosensitive drum to an intermediate transfer belt, and then secondary transfer and fix the toner image to a sheet to form an image on the sheet. In such image forming devices, environmental changes, such as temperature changes, can cause the rotation speed of the intermediate transfer belt to change, the intermediate transfer belt to shrink, or the laser imaging position relative to the photosensitive drum to change. If these environmental changes cause changes in various parts of the device, the position of the image transferred to the sheet may be shifted.

[0003] To address this issue, a system has been proposed in which a registration image is formed on the intermediate transfer belt separately from the toner image to be transferred to the sheet, and the registration image is detected by a sensor while the leading edge of the sheet is also detected by a sensor. Then, a position for changing the sheet return speed is set according to the timing of these detections, thereby adjusting the sheet transport timing (see Patent Document 1). This makes it possible to align the sheet with the toner image on the intermediate transfer belt with high precision. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-139677 Summary of the Invention [Problem to be solved by the invention]

[0005] In a system like the one described in Patent Document 1, which detects a registration image and adjusts the sheet transport timing, the leading edge of the sheet and the leading edge of the transferred image can be aligned with high precision. However, the length of the sheet in the transport direction varies depending on the cutting accuracy even within the same package, and also varies depending on manufacturing conditions such as so-called lot differences, and can also vary due to moisture absorption in the feed cassette. Therefore, the length in the transport direction between the trailing edge of the image formed on the first side of the sheet and the trailing edge of the sheet (margin length) is not necessarily constant, and errors can occur.

[0006] During double-sided printing, the leading and trailing edges of the sheet with an image formed on the first side are inverted and re-transported to the secondary transfer unit. Therefore, even if the alignment image is detected and the timing of sheet transport is adjusted as described above, there is a problem that an error occurs in the alignment between the image formed on the first side and the image formed on the second side.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming apparatus that can improve the accuracy of alignment between an image formed on a first side of a sheet and an image formed on a second side of the sheet. [Means for solving the problem]

[0008] One aspect of the present invention is an image forming apparatus including an image carrier that rotates while carrying an image, an image forming unit that forms an image to be carried on the image carrier, a transfer unit that transfers the image carried on the image carrier to a sheet, a transport unit that transports the sheet to the transfer unit, a reverse re-transport unit that reverses the transport direction of the sheet with the image formed on a first side and transports it again to the transport unit, a control unit that controls the image forming unit and the transport unit, a first sensor that detects the image carried on the image carrier, and a second sensor that detects an edge of the sheet transported by the transport unit, and the control unit controls the first transferred image that is transferred to the first side of the sheet by the image forming unit onto the image carrier, and the reverse re-transport unit that reverses the transport direction of the sheet with the image formed on a first side and transports it again to the transport unit. Upstream a first index image, a second transfer image to be transferred to the second surface of the sheet, and a rotation direction of the second transfer image of the image carrier. downstreamand a second index image formed on the first surface of the sheet, and based on a first timing when the first index image is detected by the first sensor when the first transferred image is transferred to the first surface of the sheet, a second timing when the rear end of the sheet is detected by the second sensor, a third timing when the second index image is detected by the first sensor when the second transferred image is transferred to the second surface of the sheet, and a fourth timing when the leading end of the sheet is detected by the second sensor when the second transferred image is transferred to the second surface of the sheet, the image forming apparatus is characterized in that

[0009] One aspect of the present invention is an image forming apparatus including an image carrier that rotates while carrying an image, an image forming unit that forms an image to be carried on the image carrier, a transfer unit that transfers the image carried on the image carrier to a sheet, a transport unit that transports the sheet to the transfer unit, a reverse re-transport unit that reverses the transport direction of a sheet with an image formed on a first side and transports it again to the transport unit, a control unit that controls the image forming unit and the transport unit, a first sensor that detects the image carried on the image carrier, and a second sensor that detects an edge of a sheet transported by the transport unit, and the control unit controls the image to be transferred to the sheet by the image forming unit on the image carrier, and the reverse re-transport unit that reverses the transport direction of the sheet with an image formed on a first side and transports it again to the transport unit. Upstream and an index image is formed on the sheet, and when the transfer image is transferred to the sheet, the length error in the conveying direction between the transfer image on the sheet and the trailing edge of the sheet is calculated based on a first timing when the index image is detected by the first sensor and a second timing when the trailing edge of the sheet is detected by the second sensor. [Effects of the Invention]

[0010] According to the present invention, it is possible to improve the accuracy of alignment between an image formed on the first surface of a sheet and an image formed on the second surface of the sheet. [Brief explanation of the drawings]

[0011] [Figure 1]1 is a schematic cross-sectional view showing an image forming apparatus according to an embodiment of the present invention. [Figure 2] 2 is an enlarged schematic view showing the periphery of an intermediate transfer belt of the image forming apparatus according to the present embodiment; [Figure 3] FIG. 2 is a block diagram showing a control system of the image forming apparatus according to the present embodiment. [Figure 4] FIG. 10 is a schematic diagram showing a state in which a downstream patch has reached a patch sensor. [Figure 5] FIG. 10 is a schematic diagram illustrating a state in which the leading edge of the sheet has reached a registration sensor. [Figure 6] FIG. 10 is a schematic diagram showing a state in which an upstream patch has reached a patch sensor. [Figure 7] FIG. 10 is a schematic diagram illustrating a state in which the trailing edge of the sheet has reached the registration sensor. [Figure 8] 10 is a time chart showing the positions of a downstream patch, a transferred image, the leading edge of a sheet, the trailing edge of a sheet, and an upstream patch. DETAILED DESCRIPTION OF THE INVENTION

[0012] An image forming apparatus according to the present embodiment will be described below with reference to the drawings. The dimensions, materials, shapes, relative positions, and the like of the components described in the following embodiments are not intended to limit the scope of application of the present technology unless otherwise specified.

[0013] [Schematic configuration of image forming device] Fig. 1 is a schematic diagram showing an image forming apparatus 100 according to this embodiment. Fig. 2 is an enlarged schematic diagram showing the periphery of an intermediate transfer belt of the image forming apparatus according to this embodiment. Note that the image forming apparatus 100 according to this embodiment is an electrophotographic laser beam printer.

[0014] As shown in FIG. 1, the image forming apparatus 100 includes an image forming unit 100A, which is a printer engine, a sheet feeding unit 100B, a sheet conveying unit 100C, and a sheet reversing and re-conveying unit 100D. The image forming unit 100A includes a developing mechanism 101, a transfer mechanism 102, and a fixing mechanism 103, which serve as image forming units for forming an image on an intermediate transfer belt 7 (described later) to be transferred to a recording material through an image formation process. The sheet feeding unit 100B feeds rectangular sheets used as recording materials. The sheet conveying unit 100C, which serves as a conveying unit, conveys the fed sheets to the image forming unit 100A and discharges them. The sheet reversing and re-conveying unit 100D reverses the conveying direction of the sheets on which the images have been formed by the image forming unit 100A and re-conveys them to the image forming unit 100A, or discharges them upside down. As the recording material, paper such as plain paper or thick paper, surface-treated paper such as coated paper or embossed paper, plastic film, cloth, or other sheets can be used.

[0015] The developing mechanism 101 forms toner images of yellow, magenta, cyan, and black on photosensitive drums 2Y, 2M, 2C, and 2K, respectively, and develops the toner images. Specifically, the developing mechanism 101 includes laser scanner units 1Y, 1M, 1C, and 1K, developer containers 9Y, 9M, 9C, and 9K, and image forming units 4Y, 4M, 4C, and 4K. Stations 4Y, 4M, 4C, and 4K include photosensitive drums 2Y, 2M, 2C, and 2K, charging rollers 3Y, 3M, 3C, and 3K, and developing sleeves 5Y, 5M, 5C, and 5K, respectively. The transfer mechanism 102 includes cleaner units 6Y, 6M, 6C, and 6K, and primary transfer rollers 8Y, 8M, 8C, and 8K, respectively. Furthermore, the transfer mechanism 102 includes an intermediate transfer belt 7 as an image carrier or intermediate transfer body, a cleaner unit 10, and a pair of secondary transfer rollers 11 as a transfer unit. On the other hand, the fixing process mechanism 103 includes a fixing device 12 as a fixing unit, and the fixing device 12 has a fixing roller 13 and a pressure roller 14.

[0016] The sheet feeding section 100B includes feeding cassettes 15a, 15b, and 15c that store sheets S, feeding rollers 17a, 17b, and 17c, separation roller pairs 16a, 16b, and 16c, and intermediate conveying roller pairs 20a, 20b, and 20c. The sheet conveying section 100C includes a pre-registration roller pair (hereinafter referred to as a "pre-registration roller pair") 19, a registration roller pair (hereinafter referred to as a "registration roller pair") 18, and a discharge roller pair 21. The sheet reversing and re-conveying section 100D includes a first reversing roller pair 22a, a second reversing roller pair 22b, and duplex conveying roller pairs 23a, 23b, 23c, and 23d.

[0017] [Image formation operation] Next, the image forming operation of the image forming apparatus 100 will be described. The photoconductor drums 2Y, 2M, 2C, and 2K are configured by coating the outer periphery of an aluminum cylinder with an organic photoconductive layer, and are rotated counterclockwise in FIG. 1 by a drive motor (not shown). The surfaces of the photoconductor drums 2Y, 2M, 2C, and 2K are charged by charging rollers 3Y, 3M, 3C, and 3K. Then, the surfaces of the photoconductor drums 2Y, 2M, 2C, and 2K are exposed to laser light emitted from laser scanner units 1Y, 1M, 1C, and 1K, forming electrostatic latent images on the surfaces of the photoconductor drums 2Y, 2M, 2C, and 2K based on image data sent from a control unit 300 (described below). Then, developing sleeves 5Y, 5M, 5C, and 5K of stations 4Y, 4M, 4C, and 4K transfer toner of each color supplied from storage containers 9Y, 9M, 9C, and 9K to the electrostatic latent images, thereby developing the toner images.

[0018] As shown in FIG. 2, the intermediate transfer belt 7 is stretched over a drive roller 31, idler rollers 32, 33, and 35, a tension roller 34, and an inner secondary transfer roller 11B. The intermediate transfer belt 7 is rotated in the direction of arrow R1 by the drive roller 31, which is driven by a belt drive device (not shown). The intermediate transfer belt 7 contacts the photosensitive drums 2Y, 2M, 2C, and 2K, and forms primary transfer nips NY, NM, NC, and NK at the portions where the intermediate transfer belt 7 is sandwiched between the primary transfer rollers 8Y, 8M, 8C, and 8K. Toner images of each color are sequentially transferred from the photosensitive drums 2Y, 2M, 2C, and 2K to the surface of the intermediate transfer belt 7 at the primary transfer nips NY, NM, NC, and NK by the primary transfer bias of the primary transfer rollers 8Y, 8M, 8C, and 8K. This forms a color toner image. Residual toner that is not transferred from the photosensitive drums 2Y, 2M, 2C, and 2K to the intermediate transfer belt 7 is collected by the cleaner units 6Y, 6M, 6C, and 6K shown in FIG. 1, and the intermediate transfer belt 7 is cleaned.

[0019] 1, the sheet feeding unit 100B starts feeding the sheets S from a selected one of the feeding cassettes 15a, 15b, and 15c by one of the feeding rollers 17a, 17b, and 17c corresponding to the selected feeding cassette. The fed sheets S are separated one by one by one of the separation roller pairs 16a, 16b, and 16c corresponding to the selected feeding cassette, and then conveyed to the sheet conveying unit 100C by the intermediate conveying roller pairs 20a, 20b, and 20c.

[0020] In the sheet conveying section 100C, the sheet S conveyed by the intermediate conveying roller pair 20a, 20b, and 20c is conveyed by the pre-registration roller pair 19 toward the registration roller pair 18 via the pre-transfer conveying path 61. The registration roller pair 18 conveys the sheet S while adjusting the conveying speed of the sheet S in accordance with the timing at which the color toner image transferred onto the surface of the intermediate transfer belt 7 reaches the secondary transfer roller pair 11, that is, aligns the sheet S with the toner image.

[0021] 2, the registration roller pair 18 is composed of a drive roller 18A and a driven roller 18B. The secondary transfer roller pair 11 is composed of a secondary transfer outer roller 11A and a secondary transfer inner roller 11B, and forms a secondary transfer nip N at the portion where the intermediate transfer belt 7 is sandwiched.

[0022] Thereafter, the secondary transfer roller pair 11 sandwiches and conveys the sheet S together with the intermediate transfer belt 7, and the color toner image on the intermediate transfer belt 7 is transferred and superimposed on the sheet S by a secondary transfer bias at the secondary transfer nip N. Note that residual toner that remains on the intermediate transfer belt 7 without being transferred to the sheet S is collected by a cleaner unit 10 shown in FIG. 1, and the intermediate transfer belt 7 is cleaned.

[0023] 1, the sheet S onto which the color toner image has been transferred by the secondary transfer roller pair 11 is transported to a fixing device 12 of a fixing processing mechanism 103 via a fixing transport path 62. The fixing device 12 heats the sheet S with a fixing roller 13 and presses the sheet S with a pressure roller 14, thereby fixing the toner image onto the sheet S. The fixing roller 13 is formed hollow and has a heater (not shown) built in it.

[0024] The sheet S that has passed through the fixing device 12 is guided from the fixing conveyance path 62 by a flapper (not shown) to either a discharge conveyance path 63 or a pre-reverse conveyance path 64. The sheet S that has been conveyed into the pre-reverse conveyance path 64 is guided to a sheet reversal re-conveyance section 100D that is composed of a first reversal roller pair 22a, a second reversal roller pair 22b, and a reversal conveyance path 65 as a conveyance path. That is, the sheet S that has been conveyed into the pre-reversal conveyance path 64 is guided toward the reversal conveyance path 65 by the first reversal roller pair 22a and / or the second reversal roller pair 22b.

[0025] In the case of double-sided printing, the sheet S, on which an image has been formed on its front side (first side), is conveyed into the reversing conveying path 65 until its rear end passes the entrance of the re-conveying path 67. Then, the downstream end (leading end) and the upstream end (trailing end) in the sheet conveying direction are swapped by a switchback operation performed by the second reversing roller pair 22b. With the leading and trailing ends swapped by the second reversing roller pair 22b, the sheet is conveyed to the re-conveying path 67 and is guided again by the double-sided conveying roller pairs 23a, 23b, 23c, and 23d toward the secondary transfer roller pair 11, where an image is formed on the rear side (second side) opposite to the front side.

[0026] Then, the sheet S on which image formation has been completed in single-sided printing or the sheet S on which image formation has been completed in double-sided printing is guided to the discharge conveyance path 63. The sheet S conveyed to the discharge conveyance path 63 is conveyed by a pair of discharge rollers 21 to a conveyance path 201 of the processing device 200 connected to the image forming apparatus 100. Then, the sheet S is discharged onto a discharge tray 202 provided outside the processing device 200.

[0027] On the other hand, when the sheet S that has passed through the fixing device 12 is reversed and discharged, the sheet S with an image formed on its front side is guided to the pre-reverse conveying path 64. Thereafter, the sheet S is conveyed into the reversing conveying path 65 until its rear end passes the entrance of the reversing discharge path 66. Then, the downstream end (leading end) and the upstream end (trailing end) in the sheet conveying direction are swapped by a switchback operation performed by the first reversing roller pair 22a. The sheet S whose leading and trailing ends have been swapped by the first reversing roller pair 22a is guided to the reversing discharge path 66 and conveyed to the discharge roller pair 21 by the reversing discharge roller pair 26. Then, the sheet S conveyed to the reversing discharge path 66 after being reversed in this way is also discharged by the discharge roller pair 21 in an inverted state onto a discharge tray 202 provided outside the processing device 200. Note that, in the description of the present embodiment, a description of the processing device 200 is omitted, but examples of the processing device 200 include a binding device that staples sheets, a folding device that folds sheets, and a hole punching device that punches holes in sheets.

[0028] [Configuration of the control system of the image forming device] Next, the configuration of the control system of image forming apparatus 100 will be described with reference to Fig. 3. Note that the block diagram in Fig. 3 shows the functions of each part of control unit 300, and the actual hardware configuration of control unit 300 includes a CPU, ROM, RAM, HDD, etc.

[0029] 3, the control unit 300 is connected to an operation unit 320 and an external interface (IF) 310 provided in the image forming apparatus 100, and is connected to an externally placed computer or the like via the external interface. An image signal including information about an image to be formed on a sheet by the image forming apparatus 100 is input from this external interface 310.

[0030] 1, and includes a display unit and a key input unit. The operation unit 320 receives setting information and the like input by the user via the display unit and key input unit, and displays information to the user via the display unit. The key input unit includes, for example, a start key for instructing the start of an operation such as scanning or copying, a stop key for instructing the stop of an operation such as scanning or copying, and a numeric keypad.

[0031] The control unit 300 is also connected to the laser scanner units 1Y, 1M, 1C, and 1K, and the registration motor M1 that drives the drive roller 18A of the registration roller pair 18. The control unit 300 also controls ... downstream Patch 601 (see Figure 4) and Upstream The control unit 300 is connected to a patch sensor 58 (see FIG. 2) as a first sensor that detects a patch 602 (see FIG. 6). The control unit 300 is also connected to a registration sensor 59 (see FIG. 2) as a second sensor that detects the leading and trailing ends of the sheet S. In addition to these, the control unit 300 is also connected to, for example, a belt motor M2 that drives the intermediate transfer belt 7, a feed motor M3 that drives the feed rollers 17a, 17b, and 17c, and motors and sensors of various parts not shown.

[0032] The control unit 300 functions as an image signal processing unit 301, a printer engine control unit 302, a storage unit 303, etc. The image signal processing unit 301 generates information on an image to be formed on a sheet based on an image signal received from a computer or the like via an external interface 310, for example, and temporarily stores the information in the storage unit 303. The image signal processing unit 301 also functions as a printer engine control unit 302, which will be described in detail later. downstream Patch 601 (see Figure 4) and Upstream Image information for carrying the patch 602 (see FIG. 6) on the surface of the intermediate transfer belt 7 is generated and stored in the storage unit 303. At this time, in this embodiment, downstream Patch 601 and Upstream To prevent the patch 602 from being transferred to the sheet, downstream Patch 601 and Upstream The image information of the patch 602 is generated with a margin or more away from the image information to be formed on the sheet.

[0033] The printer engine control unit 302 controls the laser scanner units 1Y, 1M, 1C, and 1K based on the image information stored in the storage unit 303. That is, the printer engine control unit 302 controls the laser scanner units 1Y, 1M, 1C, and 1K based on the image information stored in the storage unit 303. downstream Patch 601 and Upstream Based on the image information of patch 602, electrostatic latent images of the image are formed on the surfaces of photosensitive drums 2Y, 2M, 2C, and 2K. Note that, although the present embodiment describes a case where image information is temporarily stored in storage unit 303, image information may be output directly from image signal processing unit 301 to printer engine control unit 302, meaning that image information does not have to be stored in storage unit 303. In this case, image information may be stored in storage unit 303 only when, for example, a jam or the like occurs, allowing reprinting when recovery is achieved.

[0034] As will be described in detail later, the timing calculation unit 305 calculates the timing for conveying the leading edge of the sheet S to the secondary transfer nip N, specifically the timing for slowing down the conveying speed of the sheet S. Then, the sheet conveyance control unit 304 controls the registration motor M1 to control the speed at which the sheet S is conveyed to the secondary transfer nip N in accordance with the timing calculated by the timing calculation unit 305.

[0035] [Sheet leading edge positioning when transferring the image on the first side] Next, the alignment of the leading edge of the sheet S with the image transferred from the intermediate transfer belt 7 to the sheet when transferring (printing) an image on the first side (front side) of the sheet will be described with reference to FIGS. 4, 5, and 8. downstream FIG. 5 is a schematic diagram showing a state where the patch has reached the patch sensor, and FIG. 8 is a schematic diagram showing a state where the leading edge of the sheet has reached the registration sensor. downstream Patch, transfer image, leading edge of sheet, trailing edge of sheet, Upstream 10 is a time chart showing the position of a patch.

[0036] 8, the horizontal axis represents time, and the vertical axis represents the position in the conveying direction, with the transfer position (position of the secondary transfer nip N) set at 0, and the slope represents the conveying speed. downstream The patch 601, the leading and trailing edges of the transferred image 611, Upstream The patch 602 indicates the distance along the conveying path of the intermediate transfer belt 7. Also, for the leading and trailing ends of the sheet S, the distance along the pre-transfer conveying path 61 (see FIG. 1) is indicated. Upstream In the positive range on the vertical axis, the transfer image 611 is transferred onto the sheet S. downstream Patch 601, Upstream The patch 602 indicates the distance along the conveyance path of the intermediate transfer belt 7. The leading and trailing edges of the sheet S and the leading and trailing edges of the transferred image 611 indicate the distance along the fixing conveyance path 62 (see FIG. 1).

[0037] 4 and 5, the control unit 300 controls the laser scanner units 1Y, 1M, 1C, and 1K as described above to form an image 611 to be transferred from the intermediate transfer belt 7 to the first side of the sheet S (hereinafter referred to as a "transfer image") on the surface of the intermediate transfer belt 7. The control unit 300 also controls the laser scanner units 1Y, 1M, 1C, and 1K to form an image 611 on the surface of the intermediate transfer belt 7 in the rotation direction indicated by the arrow R1 of the intermediate transfer belt 7. downstream Patch image as downstreamA patch 601 is formed on the surface of the intermediate transfer belt 7 .

[0038] In this embodiment, the patch images are formed one by one between a plurality of transferred images that are continuously printed. Therefore, even if the patch image is the same, the transfer image is used as a reference and the patch image is formed in the rotation direction of the intermediate transfer belt 7. downstream If it is in downstream It is called patch 601, and it is the rotation direction of the intermediate transfer belt 7 based on the transferred image. Upstream If it is in Upstream Say patch 602. In other words, Upstream Patch 602 is based on the following transfer image: downstream Patch 601 came out. downstream Patch 601 is based on the previous transfer image. Upstream In this embodiment, the transferred image 611 transferred to the first surface is the first transferred image, Upstream Located in Upstream The patch 602 is the first index image. The transfer image transferred to the second surface is the second transfer image, and the rotation direction of the second transfer image is downstream Located in downstream The patch 601 becomes the second index image.

[0039] In addition, in the description of this embodiment, since it is difficult to define the size of the image in the transfer image 611 that is actually transferred to the sheet and has what margins, the transfer image 611 will be described as having a size that includes margins.

[0040] As shown in FIG. 4, the patch sensor 58 is formed on the surface of the intermediate transfer belt 7. downstream The patch 601 is read by the patch sensor 58, and the image shown in FIG. downstreamThe timing calculation unit 305 detects the passing timing t1 of the patch 601. Based on the passing timing t1, the timing calculation unit 305 calculates the arrival timing t3 at which the leading edge of the transferred image 611 reaches the secondary transfer nip N. As shown in FIG. 5, the registration sensor 59 detects the leading edge of the sheet S, and detects the passing timing t2 of the leading edge of the sheet S shown in FIG. 8. Then, the sheet conveyance control unit 304 controls the registration motor M1 to downstream The registration roller pair 18 is controlled based on the passing timing t1 of the patch 601 and the passing timing t2 of the leading edge of the sheet S. That is, the conveying speed of the sheet S is controlled so that the leading edge of the sheet S reaches the secondary transfer nip N at the arrival timing t3.

[0041] 8, the sheet S is first conveyed by the pair of registration rollers 18 at a first conveying speed V1, and when the sheet S reaches the speed change position Δt2 after the leading edge of the sheet S passes the registration sensor 59, the sheet S is conveyed at a second conveying speed V2 (decelerated). This second conveying speed V2 is slower than the first conveying speed and is the same as the rotation speed of the intermediate transfer belt 7. Therefore, downstream Patch 601, Transfer Image 611, Upstream The patch 602 is transported on the surface of the intermediate transfer belt 7 at a second transport speed V2.

[0042] Here, the deceleration timing Δt2, which is the time it takes to reach this speed change position, will be described. downstream It is determined from the passing timing t1 when the patch 601 is detected and the passing timing t2 when the registration sensor 59 detects the leading edge of the sheet S.

[0043] That is, each parameter is L1: distance from patch sensor 58 to secondary transfer nip N (transfer position) (first distance) L2: Distance (second distance) from the registration sensor 59 to the secondary transfer nip N (transfer position) L3: The conveyance distance of the sheet S from the detection of the leading edge of the sheet S by the registration sensor 59 to the deceleration L4: downstreamThe distance (third distance) from the patch 601 to the leading edge of the transferred image 611 (the position corresponding to the leading edge of the sheet S) t1: downstream Timing of Patch 601 passing t2: Timing when the leading edge of sheet S passes Δt2: Time from detection of the leading edge of sheet S to the start of deceleration Then, if the arrival times at the respective transfer positions are placed on the left and right sides so that the leading edge of the transferred image 611 conveyed by the intermediate transfer belt 7 and the leading edge of the sheet S conveyed on the pre-transfer conveying path 61 arrive at the transfer position simultaneously, the following formula (1) is obtained. t1+(L1+L4) / V2=t2+L3 / V1+(L2-L3) / V2...(1)

[0044] Solving this for L3 and then converting it into its transport time gives: L3=(((L1+L4-L2)+(t1-t2)V2)V1) / (V2-V1)...(2) Δt2=L3 / V1=((L1+L4-L2)+(t1-t2)V2) / (V2-V1)...(3) This becomes:

[0045] As described above, the timing calculation unit 305 calculates the time Δt2 from when the leading edge of the sheet S passes the registration sensor 59 until deceleration begins. Then, the sheet conveyance control unit 304 decelerates the conveyance speed from the first conveyance speed to the second conveyance speed after the time Δt2 has elapsed since the leading edge of the sheet S passed the registration sensor 59. This aligns the leading edge of the sheet S so that it reaches the transfer position in time with arrival timing t3 when the leading edge of the transferred image reaches the transfer position.

[0046] Note that the above formula (3) is a theoretical value, and strictly speaking, it would be better to also take into account the speed change time of the registration roller pair 18, but for the sake of convenience, this explanation will be omitted. By controlling in the above manner, when transferring an image on the first side of the sheet S, the position of the transferred image 611 and the position of the sheet S can be aligned with high precision.

[0047] [Calculation of length error on the rear edge side when transferring the image on the first side] Next, calculation of the error in length between the transferred image on the rear edge side and the rear edge of the sheet S when transferring an image on the first side of the sheet S will be described with reference to FIGS. 6, 7, and 8. Upstream FIG. 7 is a schematic diagram showing a state in which the patch has reached the patch sensor, and FIG. 8 is a schematic diagram showing a state in which the trailing edge of the sheet has reached the registration sensor.

[0048] As shown in FIGS. 6 and 7, the image signal processing unit 301 detects the rotation direction of the transferred image 611. Upstream Also Upstream 6, the patch sensor 58 is formed on the surface of the intermediate transfer belt 7. Upstream The patch 602 is read by the patch sensor 58 and the first timing is shown in FIG. Upstream It detects the passing timing t4 of the patch 602. Furthermore, as shown in Fig. 8, the registration sensor 59 detects the trailing edge of the sheet S, and detects the passing timing t5 of the trailing edge of the sheet S shown in Fig. 8 as the second timing.

[0049] The timing calculation unit 305 calculates the time by the patch sensor 58. Upstream A calculation is performed to estimate the length of the extra margin at the trailing edge from the timing t4 when the patch 602 is detected and the timing t5 when the registration sensor 59 detects the trailing edge of the sheet S.

[0050] That is, each parameter is L5: Upstream Distance from the patch 602 to the rear end of the transferred image 611 (the position corresponding to the rear end of the sheet S) t4: Upstream Timing of Patch 602 passing t5: Timing when the rear end of sheet S passes ΔLp: Length error at the rear end of the first side of sheet S (excess margin) Then, at the transfer position, theoretically, the rear end of the transferred image 611 and the position of the nominal length of the sheet S (i.e., the position obtained by subtracting the error in the length of the sheet S from the rear end of the transferred image 611) are aligned, and the following formula (4) is obtained. t4+(L1-L5) / V2=t5+(L2-ΔLp) / V2...(4)

[0051] When this is converted into the length error ΔLp, which is the excess margin, ΔLp=V2(t5-t4)+L2-L1+L5...(5) This becomes:

[0052] As described above, there are cases where the length of the sheet S is not the nominal length, or where the transferred image 611 is misaligned due to expansion or contraction of the intermediate transfer belt 7, etc. Even in such cases, the patch sensor 58 detects the position of the transferred image 611. Upstream Based on the passing timing t4 of the patch 602 and the passing timing t5 of the rear end of the sheet S detected by the registration sensor 59, the length error (excess margin) can be calculated with high accuracy.

[0053] [Sheet leading edge positioning when transferring image to the second side] Next, the alignment of the leading edge of the sheet S when the sheet S is turned over and an image is transferred onto the second surface will be described.

[0054] When the length error ΔLp, which is the excess margin calculated by the above formula (5), is converted into the transport time, Δtp=(ΔLp) / V2=t5-t4+(L2-L1+L5) / V2...(6) Therefore, the timing at which the leading edge of sheet S reaches the transfer position is shifted by Δtp relative to the leading edge of sheet S when the sheet S is turned over and an image is formed on the second side. This causes the front and back positions (registration positions) of the image formed on the first side and the image to be formed on the second side to match.

[0055] In detail, the image signal processing unit 301 determines the rotation direction of the transfer image 611 to be transferred onto the second surface (different from the transfer image 611 transferred onto the first surface). downstream Also downstream The patch sensor 58 forms a patch image as a patch 601. downstreamThe patch 601 is read by the patch sensor 58 and the third timing is shown in FIG. downstream The registration sensor 59 detects the passing timing t6 of the patch 601. The registration sensor 59 also detects the leading edge of the sheet S, and detects the passing timing t7 of the leading edge of the sheet S shown in FIG.

[0056] That is, each parameter is L6: The conveyance distance of the sheet S from the detection of the leading edge of the sheet S by the registration sensor 59 to the deceleration when transferring the image on the second side t6: When transferring the image on the second side downstream Arrival timing of patch 601 at patch sensor 58 t7: Timing at which the leading edge of the sheet S reaches the registration sensor 59 during image transfer on the second side Δt6: Time from detection of the leading edge of the sheet S to the start of deceleration when transferring the image on the second side Then, at the transfer position, the leading edge of the transferred image 611 on the second surface and the position obtained by adding the length error ΔLp to the leading edge of the sheet S are aligned, and therefore the following formula (7) is obtained. t6+(L1+L4) / V2=t7+L6 / V1+((L2+ΔLp)-L6) / V2...(7)

[0057] Converting this to L6 and then to transportation time gives us: L6=(((L1+L4-L2-ΔL_p )+(t6-t7)V2)V1) / (V2-V1)...(8) Δt6=L6 / V1 =((L1+L4-L2-ΔLp)+(t6-t7)V2) / (V2-V1) =((L4+L5)+(t4-t5+t6-t7)V2) / (V2-V1)...(9) This becomes:

[0058] As described above, the timing calculation unit 305 takes into account the length error ΔLp, downstreamBased on the timing t6 of passing the patch 601 and the timing t7 of passing the leading edge of the sheet S, the time Δt6 from when the leading edge of the sheet S passes the registration sensor 59 until deceleration starts is calculated. This time Δt6 is the time for adjusting the conveying speed of the sheet S, and in other words, it is synonymous with the arrival timing of the leading edge of the sheet S at the transfer position when transferring the image on the second side. Note that the length error ΔLp is, as described above, Upstream The calculation is based on the timing t4 when the patch 602 passes and the timing t5 when the rear end of the sheet S passes.

[0059] Then, the sheet conveying control unit 304 decelerates the conveying speed from the first conveying speed to the second conveying speed after a time Δt6 has elapsed since the leading edge of the sheet S passed the registration sensor 59. As a result, even if there is a length error (excess margin) on the rear end side of the sheet S when transferring the image on the first side, the leading edge of the sheet S is aligned to reach the transfer position at arrival timing t8 when the leading edge of the transferred image on the second side reaches the transfer position.

[0060] [Summary of this embodiment] As explained above, downstream The timing t6 when the patch 601 passes, the timing t7 when the leading edge of the sheet S passes, Upstream The timing t4 when the patch 602 passes and the timing t5 when the trailing edge of the sheet S passes are detected. Then, based on these timings, the timing when the sheet S reaches the secondary transfer nip N by the registration roller pair 18 when the transfer image is transferred onto the second side of the sheet S is controlled. This allows the leading edge of the sheet S to reach the secondary transfer nip N when the image on the second side is transferred, taking into account the length error ΔLp on the trailing edge side of the sheet S when the image on the first side is transferred. Therefore, even if there is an error in the length of the sheet S or an error in the position of the transfer image on the intermediate transfer belt 7, it is possible to improve the alignment accuracy between the image formed on the first side of the sheet S and the image formed on the second side.

[0061] [Possibility of alternative embodiments] In the present embodiment described above, Upstream Patch 602 and subsequent transfer images downstream Located in downstream In the above example, a single patch image is used for both the patch 601 and the previous transfer image. Upstream to Upstream Patch 602 and subsequent transfer images downstream to downstream Even if the patch 601 is formed separately, the images can be aligned front to back in the same manner.

[0062] In addition, in this embodiment, downstream Patches and Upstream In the above description, the patch image constituting the patch is formed at a distance of at least the transfer image and its margin so as not to be transferred onto the sheet. However, this is not limiting, and for example, when cutting a sheet, the patch image may be transferred onto the margin of the sheet to be cut.

[0063] Furthermore, in this embodiment, the patch image is assumed to be small in size, and therefore the timing for detecting the patch image has been described as a single point. However, this is not limiting, and if the patch image has a certain length in the transport direction, the leading edge or trailing edge of the patch image may be accurately detected. In this case, the various calculations performed by the timing calculation unit 305 described above must also take the length of the patch image into account to accurately calculate the distance and timing.

[0064] In the present embodiment, when a transfer image is transferred onto the first surface of a sheet, the error in the length between the rear end of the sheet and the rear end of the transfer image in the conveying direction is calculated. However, as shown in Equation (9), for example, Upstream By substituting the timing t4 at which the patch passes and the timing t5 at which the trailing edge of the sheet passes into the formula, it is not necessary to calculate the error in this length.

[0065] Furthermore, in the present embodiment, when a transfer image is transferred onto the first side of a sheet, the error in the length in the transport direction between the rear end of the sheet and the rear end of the transferred image is calculated (i.e., the passing times t4 and t5 are detected), and the error is used to align the transfer image on the second side. However, this is not limiting, and the calculation result of the error in the length in the transport direction may be used for any control, such as calculating the position of the image on the first side when cutting the sheet. [Explanation of symbols]

[0066] 2Y, 2M, 2C, 2K... Photosensitive drum (photosensitive drum) / 4Y, 4M, 4C, 4K... Station (image forming unit) / 7... Intermediate transfer belt (image carrier, intermediate transfer body) / 11... Secondary transfer roller pair (transfer unit) / 12... Fixing device (fixing unit) / 58... Patch sensor (first sensor) / 59... Registration sensor (second sensor) / 100... Image forming device / 101... Imaging unit (developing processing mechanism) / 100C... Sheet conveying unit (conveying unit) / 100D... Sheet reversing and re-conveying unit (reversing and re-conveying unit) / 300... Control unit / 601... downstream Patch (second index image) / 602… Upstream Patch (first index image, index image) / 611... Transfer image (first transfer image, second transfer image) / L1... Distance (first distance) / L2... Distance (second distance) / L4... Distance (third distance) / ΔLp... Error / S... Sheet / t1... Passing timing (third timing) / t2... Passing timing (fourth timing) / t3... Arrival timing / t4... Passing timing (first timing) / t5... Passing timing (second timing) / V1... First conveying speed / V2... Second conveying speed

Claims

1. an image carrier that rotates while carrying an image; an image forming unit that forms an image to be carried on the image carrier; a transfer unit that transfers the image carried on the image carrier onto a sheet; a conveying unit that conveys a sheet to the transfer unit; a reverse re-conveyance unit that reverses the conveyance direction of the sheet having the image formed on the first surface thereof and re-conveys the sheet to the conveyance unit; a control unit that controls the image forming unit and the conveying unit; a first sensor for detecting an image carried on the image carrier; a second sensor that detects an edge of the sheet being conveyed by the conveying unit, The control unit the image forming unit forms, on the image carrier, a first transfer image to be transferred to a first side of the sheet, a first index image upstream of the first transfer image of the image carrier in a rotation direction, a second transfer image to be transferred to a second side of the sheet, and a second index image downstream of the second transfer image of the image carrier in the rotation direction; a first timing when the first sensor detects a first index image and a second timing when the second sensor detects a trailing edge of the sheet when a first transfer image is transferred onto a first surface of the sheet; When a second transfer image is transferred onto the second surface of the sheet, based on a third timing at which the first sensor detects a second index image and a fourth timing at which the second sensor detects a leading edge of the sheet, controlling arrival timing of the sheet at the transfer unit by the conveying unit when a second transfer image is transferred onto a second surface of the sheet; An image forming apparatus characterized by:

2. the control unit calculates a length error in the conveyance direction between a first transferred image on a first surface of the sheet and a trailing edge of the sheet based on the first timing and the second timing; controlling the arrival timing based on the error; 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. the control unit controls the arrival timing based on a first distance between the first sensor and the transfer unit, a second distance between the second sensor and the transfer unit, a third distance between a second index image and a second transfer image, the error on the first surface of the sheet, the third timing, and the fourth timing.

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

4. the control unit controls the arrival timing by causing the conveying unit to convey the sheet at a first conveying speed, and changing the conveying speed to a second conveying speed different from the first conveying speed when the sheet reaches a speed change position.

4. The image forming apparatus according to claim 1, wherein the image forming apparatus comprises: a first fixing member;

5. The second conveying speed is slower than the first conveying speed.

5. The image forming apparatus according to claim 4.

6. the image forming unit has a plurality of image forming units each including a photosensitive member and developing a latent image formed on the photosensitive member into a toner image; the image carrier is an intermediate transfer body that carries the toner images transferred from the photosensitive bodies of the plurality of image forming units as the images and transports them to the transfer section; 6. The image forming apparatus according to claim 1, wherein the image forming apparatus comprises: a first fixing member;

7. a fixing unit that fixes the image transferred to the sheet by the transfer unit, 7. The image forming apparatus according to claim 1, wherein the image forming apparatus comprises: a first fixing member;

8. an image carrier that rotates while carrying an image; an image forming unit that forms an image to be carried on the image carrier; a transfer unit that transfers the image carried on the image carrier onto a sheet; a conveying unit that conveys a sheet to the transfer unit; a reverse re-conveyance unit that reverses the conveyance direction of the sheet having the image formed on the first surface thereof and re-conveys the sheet to the conveyance unit; a control unit that controls the image forming unit and the conveying unit; a first sensor for detecting an image carried on the image carrier; a second sensor that detects an edge of the sheet being conveyed by the conveying unit, The control unit a transfer image to be transferred onto a sheet and an index image formed on the image carrier by the image forming unit, the index image being located upstream of the transfer image in a rotation direction of the image carrier; When a transfer image is transferred onto a sheet, based on a first timing at which the first sensor detects an index image and a second timing at which the second sensor detects a trailing edge of the sheet, calculating a length error between the transferred image on the sheet and the trailing edge of the sheet in the conveying direction; An image forming apparatus characterized by:

Images