Sheet conveyance device and image forming device

Abstract

To more appropriately set a distance at which a sheet is conveyed after a jam occurs.SOLUTION: A sheet conveyance device includes: a conveyance path; conveyance means; sheet detection means; skew detection means; and control means. When a sheet conveyance abnormality is detected while a sheet is held in a nip part, the control means executes conveyance processing to convey the sheet along the conveyance path and then stop the conveyance of the sheet. When a target of a conveyance distance of the sheet from the time point when a tip end of the sheet reaches a detection position to the time point when the conveyance of the sheet is stopped in the conveyance processing, is set as a target conveyance distance, the control means changes the target conveyance distance based on a detection result obtained by the skew detection means so that the target conveyance distance when the sheet is skewed at a first angle is longer than the target conveyance distance when the sheet is skewed at a second angle smaller than the first angle.SELECTED DRAWING: Figure 9

Description

[Technical field]

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

[0002] In an image forming apparatus, if a transport abnormality such as a sheet jam (hereinafter referred to as a jam) occurs during an image forming operation, the image forming operation is interrupted. However, depending on the position of the sheet when the jam occurs, it may be necessary to pull out the sheet with a strong force when removing the sheet stuck in the image forming apparatus (jam clearance), which may make jam clearance difficult. Patent Document 1 describes that if the registration rollers are holding the sheet when a jam occurs, the sheet is transported until it is discharged from the registration rollers. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-227985 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned document, the conveying distance of the sheet after the jam occurs is set regardless of the degree of skew of the sheet. However, in this configuration, it has been found that the distance by which the sheet is conveyed after the jam occurs may be insufficient or excessive depending on the degree of skew of the sheet.

[0005] SUMMARY OF THE PRESENT DISCLOSURE In view of the above, an object of the present invention is to provide an image forming apparatus capable of more appropriately setting the distance by which a sheet is transported after a jam occurs. [Means for solving the problem]

[0006] One aspect of the present invention is a sheet transporting device comprising a transport path along which a sheet is transported, a transport means having a nip portion for clamping the sheet and transporting the sheet, a sheet detection means for detecting the sheet at a detection position downstream of the nip portion in the sheet transport direction, a skew detection means for detecting skew of the sheet, and a control means for controlling the transport of the sheet, wherein when the control means detects a transport abnormality of the sheet while the sheet is clamped in the nip portion, it executes a transport process that transports the sheet along the transport path and then stops the transport of the sheet, and when a target transport distance of the sheet from the point when the leading edge of the sheet reaches the detection position to the point when the transport of the sheet is stopped in the transport process is set as a target transport distance, the control means changes the target transport distance based on the detection result of the skew detection means so that the target transport distance when the sheet is skewed at a first angle is longer than the target transport distance when the sheet is skewed at a second angle smaller than the first angle. Effect of the Invention

[0007] According to the present invention, it is possible to provide an image forming apparatus capable of more appropriately setting the distance by which a sheet is transported after a jam occurs. [Brief description of the drawings]

[0008] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to a first embodiment. [Diagram 2] 5A to 5C are diagrams for explaining a jam at a fixing / discharge section according to the first embodiment. [Diagram 3] 5A to 5C are diagrams for explaining a jam at a fixing / discharge section according to the first embodiment. [Figure 4] 5A and 5B are schematic diagrams illustrating a conveying process after a jam is detected according to the first embodiment. [Diagram 5] 5A and 5B are schematic diagrams illustrating a geometric relationship between a flag of the skew sensor and a skewed sheet according to the first embodiment. [Figure 6]5A and 5B are diagrams illustrating a time difference between detection timings of a registration sensor and a skew sensor according to the first embodiment. [Figure 7] Graph (a) shows the relationship between the skew angle θ of a sheet and the conveying distance D' corresponding to the time difference between the detection timing of the registration sensor and the skew sensor in the first embodiment, and graph (b) shows the relationship between the skew angle θ of the sheet and the skew margin Δ2. [Figure 8] FIG. 1 is a system configuration diagram of an image forming apparatus according to a first embodiment. [Figure 9] 5 is a flowchart showing a control method of the image forming apparatus according to the first embodiment. [Figure 10] 10 is a flowchart showing a control method of an image forming apparatus according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0010] First Embodiment A first embodiment will be described. In this disclosure, the "image forming apparatus" may be a single-function printer equipped with only an image forming function (printing function) for forming an image on a sheet, a copier equipped with a copying function, or a multifunction machine equipped with multiple functions. As the sheet which is the recording material (recording medium), various sheet materials of different sizes and materials can be used, such as paper such as plain paper and thick paper, sheet materials with surface treatment such as coated paper, sheet materials with special shapes such as envelopes and index paper, plastic films, cloth, etc.

[0011] 1 is a schematic diagram showing an image forming apparatus 100 according to a first embodiment. The image forming apparatus 100 is a laser beam printer that forms an image on a sheet P by an electrophotographic process based on image information received from the outside.

[0012] The image forming apparatus 100 includes a process section 102 as an image forming means (image forming unit) and a fixing device 130 as a fixing means. The process section 102 includes a photosensitive drum 122 as an image carrier, a charging roller 123 as a charging means, a scanner 108 as an exposure means, a developing device 125 as a developing means, and a transfer roller 106 as a transfer means. The process section 102 also includes a cleaning blade 126 as a cleaning means, and a waste toner container 127.

[0013] The photosensitive drum 122 is an electrophotographic photosensitive body in which a photosensitive layer is formed of an organic photosensitive body or an amorphous silicon photosensitive body on the outer periphery of a cylindrical base. The scanner 108 includes a light source unit that emits a laser beam, and an exposure optical system that scans the laser beam in the direction of the rotation axis (main scanning direction) of the photosensitive drum 122. The exposure unit may be, for example, an LED exposure device in which LEDs are arranged in the direction of the rotation axis (main scanning direction) of the photosensitive drum 122. The developing device 125 includes a developing roller 121 that carries toner as a developer, and a supply roller 124 that supplies toner to the developing roller 121. The transfer roller 106 is in contact with the photosensitive drum 122, and a transfer nip N2 is formed between the photosensitive drum 122 and the transfer roller 106 as a transfer unit where a toner image is transferred to the sheet P.

[0014] The fixing device 130 has a fixing film 133 as a heating member, a pressure roller 134 as a pressure member, a heater 132 as a heating means, and a thermistor 131 as a temperature detection means. The fixing film 133 is a flexible cylindrical film member. The heater 132 is disposed in the internal space of the fixing film 133. The pressure roller 134 is disposed in contact with the outer circumferential surface of the fixing film 133 and sandwiches the fixing film 133 together with the heater 132. A fixing nip N3 where a toner image is fixed is formed between the pressure roller 134 and the fixing film 133. The thermistor 131 is disposed in contact with the heater 132 and generates a detection signal according to the temperature of the heater 132. The heating member may be, for example, a roller member (fixing roller) having rigidity. The heating member may be a halogen lamp that heats the heating member by radiating heat, or an induction heating coil that generates an alternating magnetic field to heat a conductive layer in the heating member.

[0015] The image forming apparatus 100 also includes a feed tray 140 as a storage unit, a feed roller 101 as a feed unit, a pair of registration rollers (hereinafter referred to as a pair of registration rollers 104), and a pair of discharge rollers 111 as a discharge unit. A plurality of sheets P are stored in a stacked state in the feed tray 140. The pair of registration rollers 104 has a nip portion N1 that holds the sheet P, and is an example of a conveying unit that conveys the sheet P. The pair of registration rollers 104 is disposed downstream of the feed roller 101 in the sheet conveying direction Dc. The pair of registration rollers 104 is disposed between the feed roller 101 and the transfer nip N2 in the sheet conveying direction Dc. The pair of discharge rollers 111 is disposed downstream of the fixing device 130 in the sheet conveying direction Dc.

[0016] The image forming apparatus 100, except for the process unit 102 and the fixing device 130, is an example of a sheet conveying device that performs a sheet conveying operation to convey the sheets P one by one. However, part of the process unit 102 and the fixing device 130 are also involved in the conveyance of the sheets P.

[0017] In this embodiment, the rollers involved in the conveyance of the sheet P (the pair of registration rollers 104, the transfer roller 106, the pressure roller 134, and the pair of discharge rollers 111) are rotated by a single conveyance motor 815 (FIG. 8) which is the same driving source.

[0018] The housing of the image forming apparatus 100 has a sheet transport path CP along which the sheet P fed from the feed roller 101 is transported through a nip portion N1 of the pair of registration rollers 104, a transfer nip N2, a fixing nip N3, and a nip portion N4 of the pair of discharge rollers 111 in this order. The sheet transport path CP is formed by a guide member attached to the frame of the housing. The housing of the image forming apparatus 100 also has an opening (feed opening 300) for receiving the sheet P from the feed tray 140 into the sheet transport path CP, and an opening (discharge opening 301) for discharging the sheet P from the sheet transport path CP toward the discharge tray 112. The pair of discharge rollers 111 is disposed at the discharge opening 301.

[0019] The sheet transport path CP is a transport path that communicates from the feed port 300 to the discharge port 301 and transports the sheet P. The direction in which the sheet P moves along the sheet transport path CP is called a sheet transport direction Dc.

[0020] The image forming apparatus 100 also includes a registration sensor (hereinafter referred to as a registration sensor 105), a skew sensor 135, and a post-fixing sensor 109. The registration sensor 105 is an example of a sheet detection means for detecting the sheet P in the vicinity of the pair of registration rollers 104. The skew sensor 135 is an example of a skew detection means for detecting the skew of the sheet P. The skew sensor 135 may also serve as a sheet width detection means for detecting the length of the sheet in a sheet width direction perpendicular to the sheet conveying direction Dc of the sheet P (hereinafter referred to as a sheet width). The skew sensor 135 will be described in detail later. The post-fixing sensor 109 is an example of a sheet detection means (second sheet detection means) for detecting the sheet P downstream of the fixing device 130 and upstream of the discharge port 301 in the sheet conveying direction Dc. The post-fixing sensor 109 is also used as a jam detection means for detecting a conveying abnormality (jam) of the sheet P.

[0021] In this embodiment, the registration sensor 105 is located downstream of the registration roller pair 104 and upstream of the transfer nip N2 in the sheet conveying direction Dc. In addition, the skew sensor 135 is located downstream of the registration roller pair 104 and upstream of the transfer nip N2 in the sheet conveying direction Dc. In this embodiment, the skew sensor 135 is located near the downstream side of the registration sensor 105 in the sheet conveying direction Dc. In this embodiment, the post-fixing sensor 109 is located downstream of the fixing nip N3 and upstream of the discharge roller pair 111 in the sheet conveying direction Dc.

[0022] The registration sensor 105 and the post-fixing sensor 109 may be configured to use, for example, a sensor configuration in which a flag that rotates when it comes into contact with the sheet P and a photoelectric sensor that detects the rotation of the flag are combined. Alternatively, the registration sensor 105 and the post-fixing sensor 109 may use a photoelectric sensor that emits light toward the sheet transport path CP and detects reflected light from the sheet P.

[0023] (Image formation operation) The image forming operation, which is a series of operations in which the image forming apparatus 100 forms an image on the sheet P while conveying the sheet P, will be described. In the image forming operation, the photosensitive drum 122 is rotated and driven at a predetermined peripheral speed (process speed) in the clockwise direction in the figure. The surface of the photosensitive drum 122 is uniformly charged to a predetermined polarity and potential by the charging roller 123. The charged surface is irradiated with a laser beam output from the scanner 108 for exposure, and an electrostatic latent image is formed on the photosensitive drum 122. The scanner 108 modulates (ON / OFF conversion) the laser beam in response to a time-series digital pixel signal (video signal) input from an image signal generating device such as an image reading device or a computer. The image forming apparatus 100 notifies the image signal generating device of the start timing of scanning and exposure in the sub-scanning direction by a sub-scanning direction synchronization signal. The developing device 125 contains toner, and the developing roller 121 and the supply roller 124 are rotated and driven in a state in which a voltage is applied to each of them and maintained at a predetermined potential. The developing roller 121 rotates while carrying the toner supplied from the supply roller 124, and supplies the toner to the photosensitive drum 122, thereby developing the electrostatic latent image into a toner image.

[0024] A sheet conveying operation is performed in parallel with the creation of the toner image. A feeding roller 101 feeds sheets P one by one from a feeding tray 140. The fed sheets S are conveyed to the transfer nip N2 by a pair of registration rollers 104. At that time, a registration sensor 105 detects that the sheet P has reached the pair of registration rollers 104. The pair of registration rollers 104 conveys the sheet P to the transfer nip N2. The sheet conveying operation and the creation of the toner image are synchronized so that the timing at which the leading edge of the sheet P in the sheet conveying direction Dc enters the transfer nip N2 and the timing at which the toner image carried on the photosensitive drum 122 reaches the transfer nip N2 are aligned.

[0025] A bias electric field is formed in the transfer nip N2 by applying a voltage of a polarity opposite to the normal charging polarity of the toner to the transfer roller 106. This bias electric field causes the toner image to be transferred from the photosensitive drum 122 to the sheet P in the transfer nip N2, forming an image on the sheet P. Residual toner that has not been transferred to the sheet P and remains on the photosensitive drum 122 is scraped off by a cleaning blade 126 and collected in a waste toner container 127.

[0026] The sheet P that has passed through the transfer nip N2 is separated from the photosensitive drum 122 and sent to the fixing device 130, where the toner image is fixed. The fixing device 130 fixes the toner image on the sheet P by heating and pressurizing it while nipping and conveying the sheet P between a fixing film 133 and a pressure roller 134 in the fixing nip N3. The sheet P that has passed through the fixing nip N3 is discharged outside the housing through a discharge port 301 by a pair of discharge rollers 111, and is stacked on a discharge tray 112. At that time, a post-fixing sensor 109 detects that the sheet P has passed through the fixing device 130.

[0027] (Jam detection) A description will now be given of a jam detection method in the image forming apparatus 100 of this embodiment. In the image forming apparatus 100, jam detection is performed using a registration sensor 105 and a post-fixing sensor 109.

[0028] During the image forming operation, when the sheet P having the toner image transferred thereon passes through the fixing device 130 and is discharged from the discharge port 301, the sheet may become stuck between the fixing nip N3 and the pair of discharge rollers 111 for some reason, causing a sheet jam. Such a stuck sheet P is called a fixing discharge section jam. In this embodiment, a fixing discharge section jam is detected as follows.

[0029] 2 is a schematic diagram showing a change (logical change of ON / OFF value) in the detection signal of the post-fixing sensor 109 when the sheet P passes the detection position of the post-fixing sensor 109 in the sheet transport path CP. The detection signal of the post-fixing sensor 109 is ON when the sheet P is at the detection position, and is OFF when the sheet P is not at the detection position. In other words, the detection signal of the post-fixing sensor 109 is OFF before the leading edge of the sheet P reaches the detection position, changes to ON when the leading edge of the sheet P reaches the detection position, and changes to OFF when the trailing edge of the sheet P passes the detection position. In other words, the post-fixing sensor 109 as the second sheet detection means is configured to emit a first signal indicating that a sheet is detected and a second signal indicating that a sheet is not detected.

[0030] 2 shows the transition of the detection signal of the post-fixing sensor 109 when the sheet conveying operation is performed normally. In this case, the estimated time tf during which the detection signal of the post-fixing sensor 109 is ON is equal to the value (sheet length / process speed) obtained by dividing the length of the sheet P in the sheet conveying direction Dc (hereinafter referred to as the sheet length) by the sheet conveying speed (process speed).

[0031] 2 shows an example of transition of the detection signal of the post-fixing sensor 109 when there is an abnormality in the sheet conveying operation. When the sheet P is stuck between the fixing nip N3 and the pair of discharge rollers 111, the detection signal of the post-fixing sensor 109 turns OFF before the expected time tf (predetermined time) during which the post-fixing sensor 109 should be ON has elapsed. This is because the sheet P is bent when stuck, and the sheet P moves out of the range where the post-fixing sensor 109 can detect the sheet P in the thickness direction of the sheet P (the direction perpendicular to both the sheet conveying direction Dc and the sheet width direction).

[0032] The conveyance control unit 805 (FIG. 8), which will be described later, is configured to detect a fixing discharge section jam as a conveyance abnormality based on a detection signal of a post-fixing sensor 109 serving as a second sheet detection means. Specifically, the conveyance control unit 805 measures the time from when the detection signal of the post-fixing sensor 109 turns ON to when it turns OFF each time one sheet P is conveyed in the image forming operation. In addition, the conveyance control unit 805 calculates an estimated time tf from when the detection signal of the post-fixing sensor 109 turns ON to when it turns OFF when the sheet conveyance operation is performed normally, based on the sheet length of the sheet P that is the target of the image forming operation and the setting information of the process speed. If the measured time is shorter than the estimated time tf, it is determined that a fixing discharge section jam has occurred.

[0033] As a result of their own investigation, the inventors have confirmed that a jam at the fixing and discharging section is likely to occur when the sheet P is skewed, and in particular, the greater the degree of skew of the sheet P, the more likely the jam at the fixing and discharging section is to occur.

[0034] Here, the skew of the sheet P means a state in which the leading edge Pa of the sheet P crosses obliquely with respect to an imaginary straight line Lw extending in the sheet width direction Dw when viewed in the thickness direction of the sheet P. The angle between the imaginary straight line Lw and the leading edge Pa of the sheet P is defined as the skew angle θ of the sheet P.

[0035] FIG. 3 is a schematic diagram showing a state of a skewed sheet P immediately before being discharged from the discharge port 301. When the sheet P is skewed, the corner of the sheet P is caught by the edge 301a of the discharge port 301 in the sheet width direction Dw, and the sheet P is likely to be stuck. If the skew angle θ of the sheet P is large, the corner of the sheet P is more likely to be caught. When the sheet P is sent out from the fixing device 130 in a state in which the corner of the sheet P is caught, the sheet P is bent between the fixing nip N3 and the discharge roller pair 111. Then, before the rear end of the sheet P passes through the post-fixing sensor 109, the detection signal of the post-fixing sensor 109 is turned OFF. In this way, when the sheet P is skewed, the sheet P is bent due to the corner being caught, and a jam at the fixing discharge section is likely to occur.

[0036] In other words, after the leading edge of the sheet in the sheet transport direction reaches the second sheet detection means and the second sheet detection means generates the first signal, the sheet gets caught on the edge of the opening in the sheet width direction perpendicular to the sheet transport direction and bends, causing the second sheet detection means to generate the second signal before a predetermined time has elapsed. In such a case, the control means (a control circuit including the transport control unit 805) of this embodiment detects a jam in the fixing discharge section as an example of a transport abnormality.

[0037] (Transportation process after jam detection) When a jam is detected at the fixing / discharging section, the image forming operation is interrupted. In other words, the sheet conveying operation and each step of the electrophotographic process (exposure, development, transfer, and fixing) are basically all stopped immediately after a jam is detected. In addition, the image forming apparatus 100 notifies the user of the occurrence of a jam by, for example, displaying a message on a display unit such as a liquid crystal panel.

[0038] The user removes the jammed sheet P from the sheet transport path CP (jam clearance), thereby clearing the jam. For example, in the case of a jam in the fixing discharge section, when a part of the sheet P is exposed from the discharge port 301, the user tries to grasp the exposed part of the sheet P and pull out the sheet P.

[0039] However, when a user tries to pull out the sheet P while it is being held in the nip portion N1 of the pair of registration rollers 104, the force required to pull out the sheet P (the load on the user) increases due to the pull-out resistance of the pair of registration rollers 104. This reduces the efficiency of the jam clearance process. In addition, depending on the material of the sheet P and the magnitude of the resistance, the sheet P may tear and a piece of the sheet may remain inside the image forming apparatus 100.

[0040] The pull-out resistance of the pair of registration rollers 104 is a friction force in the opposite direction to the sheet conveying direction Dc that the sheet P receives from the pair of registration rollers 104 when the sheet P sandwiched in the nip portion N1 of the pair of registration rollers 104 is pulled out while the conveying motor 815 is stopped. The pull-out resistance of the pair of registration rollers 104 includes friction resistance generated at the bearing portion of the roller pair of the pair of registration rollers 104, inertial resistance of the conveying motor 815, and inertial resistance and friction resistance of a drive transmission portion (such as a gear train) that connects the conveying motor 815 and the pair of registration rollers 104.

[0041] In this embodiment, in order to avoid a decrease in the workability of jam clearance due to the pulling resistance of the pair of registration rollers 104, a conveying process is performed in which the sheet S is conveyed by the pair of registration rollers 104 after the occurrence of a jam is detected (hereinafter referred to as the conveying process after jam detection).

[0042] 4A is a schematic diagram of the image forming apparatus 100 showing a state when a jam occurs at the fixing and discharging section. The sheet P is retained in the sheet conveying path CP in a state where it is bent between the fixing nip N3 and the pair of discharge rollers 111. The leading edge Pa of the sheet P is located downstream of the discharge port 301 in the sheet conveying direction Dc and is exposed to the outside of the image forming apparatus 100. On the other hand, the trailing edge Pb of the sheet P is retained upstream of the nip portion N1 of the pair of registration rollers 104 in the sheet conveying direction Dc. In other words, the sheet P is retained in a state where it is sandwiched between the pair of registration rollers 104.

[0043] 4B is a schematic diagram of the image forming apparatus 100 showing a state after the sheet P has been transported by the transport process after the jam has been detected. In the transport process after the jam has been detected, the sheet P is basically transported so that the rear end Pb of the sheet P passes through the nip portion N1 of the pair of registration rollers 104, and then the transport is stopped. By the rear end Pb of the sheet P passing through the nip portion N1 of the pair of registration rollers 104, the force required to pull out the sheet P is reduced by the pulling resistance of the pair of registration rollers 104, and therefore the workability of the jam clearance can be improved.

[0044] In this embodiment, multiple rollers (the pair of registration rollers 104, the transfer roller 106, the pressure roller 134, and the pair of discharge rollers 111) are driven by a single conveying motor 815 (FIG. 8). In other words, the conveying motor 815, which is the drive source of the pair of registration rollers 104, drives at least one roller selected from the group of the transfer roller, the pressure roller, and the pair of discharge rollers. In this configuration, since the multiple rollers are linked via a drive transmission unit, the pull-out resistance of the pair of registration rollers 104 may be greater than in a configuration in which the pair of registration rollers 104 is driven by a dedicated motor independent of the other rollers.

[0045] Moreover, the pair of registration rollers 104, which is a pair of conveying rollers in this embodiment, is a pair of conveying rollers used for aligning (registering) an image and the sheet P in the sheet conveying direction Dc (sub-scanning direction). Therefore, the contact pressure (nip pressure) between the rollers of the pair of registration rollers 104 at the nip portion N1 is set to a relatively large value so that slippage of the sheet P does not occur. For this reason, it is difficult to pull out the sheet P while causing it to slip relative to the pair of registration rollers 104 at the nip portion N1, and as a result, there are cases in which the pull-out resistance is larger than that of other pairs of conveying rollers.

[0046] Therefore, the conveying process after jam detection according to this embodiment, which will be described below, can be suitably applied to the pair of registration rollers 104 of this embodiment. However, the conveying process after jam detection according to this embodiment may be applied to a pair of conveying rollers having a configuration different from that of the pair of registration rollers 104 of this embodiment.

[0047] (accordion jam) Considering only the ease of jam clearance, it is preferable to continue conveying the sheet P until the detection signal of the registration sensor 105 changes to OFF in the conveying process after the jam is detected so that the sheet P can reliably pass through the pair of registration rollers 104. However, as shown in FIGS. 4A and 4B, the sheet P is bent between the fixing nip N3 and the pair of discharge rollers 111, and therefore, if the conveying distance of the sheet P in the conveying process after the jam is detected is long, the sheet P may be folded in a bellows shape (accordion shape). The phenomenon in which the sheet P is folded in a bellows shape (accordion shape) is called an accordion jam. If the accordion jam progresses, it becomes difficult for the user to remove the sheet P, and the ease of jam clearance may decrease.

[0048] Furthermore, if the conveyance of the sheet P is continued until the detection signal of the registration sensor 105 changes to OFF in the conveyance process after the jam is detected, the following concern arises. That is, at the time when the jam occurs at the fixing discharge section, the sheet P in the double-feed state may be stuck in the sheet conveyance path CP. The double-feed state refers to a state in which a sheet P, which is the original target for image formation, is dragged and fed, and two or more sheets P are conveyed in an overlapping state. In the double-feed state, if the conveyance of the sheet P is continued until the detection signal of the registration sensor 105 changes to OFF in the conveyance process after the jam is detected, the conveyance of the sheet P will continue until all of the overlapping sheets P pass through the nip portion N1 of the pair of registration rollers 104. As a result, the accordion jam may progress, and the workability of the jam clearance may decrease.

[0049] (Details of sheet transport control) Therefore, in this embodiment, a determination process is performed to determine the transport distance of the sheet P in the transport process after the jam is detected, and the transport process is performed based on the determined transport distance (hereinafter, referred to as the target transport distance). The method of determining the transport distance will be described below.

[0050] The transport process after jam detection may be performed by stopping the transport of sheet P once when the jam is detected, and then resuming the transport of sheet P. The transport process after jam detection may also be performed by continuing the transport of sheet P that was being performed when the jam was detected. In this embodiment, the transport process after jam detection is performed by continuing the transport of sheet P that was being performed when the jam was detected. In this case, the transport process after jam detection is a process for the period from when the jam determination unit 806 of the transport control unit 805 (FIG. 8) determines that a jam has occurred until the transport of sheet P is stopped.

[0051] First, the determination process of the target conveying distance is performed immediately after the jam determination unit 806 determines that a jam has occurred. In other words, the determination process may be performed after the conveying process after the jam has been detected is started. Note that, if the conveying of the sheet P is stopped once at the time when the jam has been detected, the determination process may be performed before the conveying of the sheet P is resumed.

[0052] Next, a method for determining the target conveying distance will be described. As described above, if the conveying distance in the conveying process after jam detection is long, there is a concern of an accordion jam, so it is preferable that the target conveying distance is as short as possible. On the other hand, if the trailing end Pb of the sheet P does not pass through the nip portion N1 of the pair of registration rollers 104, the advantage of improved workability in jam clearance cannot be obtained.

[0053] Therefore, in this embodiment, the target conveying distance is determined so that the conveying distance based on the point in time when the detection signal of the registration sensor 105 changes from OFF to ON due to the passage of the leading edge of the sheet P is (sheet length L+margin Δ).

[0054] Under ideal conditions where there is no dimensional error or skew of the sheet P, fluctuation in the sheet conveying speed, or the like, the conveying distance until the detection signal of the registration sensor 105 changes from OFF to ON and then changes back to OFF is equal to the sheet length L of the sheet P. In other words, to make the rear end of the sheet P pass the registration sensor 105 by the conveying process after a jam is detected, the target conveying distance should be set to at least the sheet length L or longer.

[0055] In reality, due to factors such as dimensional errors of the sheet P, the conveying distance from when the detection signal of the registration sensor 105 changes from OFF to ON until it changes back to OFF varies with respect to the sheet length L. Therefore, in this embodiment, the distance obtained by adding a margin Δ to the sheet length L is set as the target conveying distance.

[0056] The sheet length L is basically the sheet length of the sheet P used in the current image formation job. In this embodiment, the sheet length based on the sheet size specified by the user when the image formation job is input is used as the sheet length of the sheet P used in the current image formation job (hereinafter, referred to as the specified sheet length). The image formation job is a series of tasks in which, while continuously feeding the number of sheets P specified by the user, an image forming operation is performed on each sheet P to form an image based on image data on the sheet P, and a product with the image formed is discharged.

[0057] Since the target conveying distance is changed according to the specified sheet length, it is possible to reduce accordion jams, particularly when a conveying process is performed after a jam is detected in a double-feed state. That is, when the target conveying distance is determined regardless of the specified sheet length, the target conveying distance is determined based on the sheet length of the long sheet P so that the sheet P can pass through the pair of registration rollers 104 in the conveying process after a jam is detected even for a long sheet P. However, in this case, the target conveying distance is set unnecessarily long for a short sheet P. Therefore, when a conveying process is performed after a jam is detected when the short sheet P is in a double-feed state, the short sheet P is conveyed an unnecessarily long distance even after the trailing end of the short sheet P passes through the registration sensor 105. As a result, an accordion jam may occur. On the other hand, by changing the target conveying distance according to the specified sheet length, it is possible to set the target conveying distance to an appropriate value according to the sheet length of the sheet P used in the current image forming job, and it is possible to reduce accordion jams.

[0058] Next, a description will be given of the margin Δ. The margin Δ in this embodiment can be divided into the following two elements (Δ1, Δ2).

[0059] The first margin Δ1 is a margin that takes into consideration factors such as the dimensional tolerance of the sheet P and an error in the detection timing of the registration sensor 105 (factors other than the skew of the sheet P). By setting the first margin Δ1, even if factors such as the dimensional tolerance of the sheet P exist, the trailing edge of the sheet P can be made to pass through the nip portion N1 of the pair of registration rollers 104 more reliably by the conveying process after jam detection. The first margin Δ1 is basically independent of the sheet conveying speed. In this embodiment, the first margin Δ1 is set to a uniform value of 5 mm.

[0060] The second margin (hereinafter, referred to as skew margin Δ2) is a margin (skew margin) that assumes skew of the sheet P. As will be described in detail later, if the sheet P is skewed, the rear end of the sheet P may not pass through the nip portion N1 of the pair of registration rollers 104 even if the sheet P is transported by the sheet length L after the detection signal of the registration sensor 105 turns ON. By setting the skew margin Δ2 according to the degree of skew of the sheet P, even if the sheet P is skewed, the rear end of the sheet P can be made to more reliably pass through the nip portion N1 of the pair of registration rollers 104 by the transport process after jam detection.

[0061] As described above, the inventors have conducted research and have found that the greater the degree of skew of the sheet P, the more likely a jam occurs at the fixing and discharging section. Furthermore, if the sheet is skewed, simply adding the first margin Δ1 to the sheet length L may result in the rear end of the sheet P not passing through the nip portion N1 of the pair of registration rollers 104 during the conveying process after jam detection. In this embodiment, the value obtained by adding the first margin Δ1 and the skew margin Δ2 (second margin) to the sheet length L is determined as the target conveying distance, so that the rear end of the sheet P can more reliably pass through the nip portion N1 of the pair of registration rollers 104.

[0062] If the rear end of the sheet P can pass through the nip portion N1 of the pair of registration rollers 104, it is preferable that the target conveying distance is as short as possible, and therefore it is preferable that the skew margin Δ2 is also as small as possible. In this embodiment, the skew margin Δ2 is determined based on the detection result of the skew sensor 135 so that the skew margin Δ2 increases as the skew angle θ of the sheet P increases. More specifically, the skew angle θ of the sheet P is evaluated based on the timing at which the detection signals of the registration sensor 105 and the skew sensor 135 change, and the second margin Δ2 is determined using the evaluated skew angle θ.

[0063] This allows the rear end of the sheet P to pass through the nip portion N1 of the pair of registration rollers 104, while shortening the target conveying distance when the degree of skew is small, compared to when the skew margin Δ2 is set to a uniformly large value based on the degree of skew when the degree of skew is large. In other words, the sheet conveying distance after a jam occurs can be set more appropriately according to the degree of skew of the sheet P.

[0064] (How to determine skew margin) A specific method of determining the skew margin Δ2 in this embodiment will be described with reference to Fig. 5. Fig. 5 is a schematic diagram showing the positional relationship between the skewed sheet P and the flags 135a and 135b of the skew sensor 135.

[0065] The skew sensor 135 has two flags 135a and 135b arranged at an interval in the sheet width direction Dw. In this embodiment, the flags 135a and 135b are arranged on one side and the other side of a conveyance center 501 in the sheet width direction Dw, at positions equidistant from the conveyance center 501. The conveyance center 501 is the center position in the sheet width direction Dw (main scanning direction) of an image formed on the sheet P by the process unit 102. Each of the flags 135a and 135b is a rotating member that can rotate independently of the other flag when it comes into contact with the sheet P. The flag of the registration sensor 105 (not shown in FIG. 5) is arranged on the conveyance center 501.

[0066] The skew sensor 135 includes a photoelectric sensor (photointerrupter) whose detection signal changes in response to the rotation of the flags 135a and 135b. The detection signal of the skew sensor 135 is ON when both the flags 135a and 135b are in contact with the sheet P, and is OFF when at least one of the flags 135a and 135b is not in contact with the sheet P.

[0067] When the sheet P is not skewed, the two flags 135a and 135b come into contact with the leading edge Pa of the sheet P at the same time. However, when the sheet P is skewed as shown in Fig. 5, the leading edge Pa of the sheet P comes into contact with one flag 135a and then comes into contact with the other flag 135b. In other words, there is a time difference between the timing when the leading edge Pa of the sheet P reaches the detection position of the skew sensor 135 on one side and the other side in the sheet width direction Dw with respect to the conveyance center 501.

[0068] For a more specific description, a point of the leading edge Pa that is located on the conveying center 501 is defined as point P0. A point of the leading edge Pa that comes into contact with the flag 135b is defined as point P1. In the example of Fig. 5, point P1 is a delayed point that moves later than point P0 in the sheet conveying direction Dc.

[0069] When the sheet P is not skewed, the points P0 and P1 simultaneously pass the detection position of the skew sensor 135 in the sheet conveying direction Dc. However, when the sheet P is skewed, the point P1 passes the detection position of the skew sensor 135 at a timing delayed by a delay distance d(θ) that depends on the skew angle θ compared to the point P0.

[0070] Here, the skew sensor 135 is located downstream of the registration sensor 105 in the sheet conveying direction Dc, and a distance (sensor distance D) from the detection position of the registration sensor 105 to the detection position of the skew sensor 135 is given. That is, the conveying distance D' of the sheet P from when the detection signal of the registration sensor 105 changes to ON until the detection signal of the skew sensor 135 changes to ON is a value obtained by adding a delay distance d(θ) depending on the skew angle θ to the sensor distance D. That is, D'=D+d(θ). Therefore, as shown in FIG. 6, the skew angle θ of the sheet P can be evaluated based on the delay time td from when the detection signal of the registration sensor 105 changes to ON until the detection signal of the skew sensor 135 changes to ON.

[0071] A method for evaluating the skew angle θ and a method for determining the skew margin in this embodiment will be described in detail. From the geometric relationship shown in Fig. 5, the delay distance d(θ) depending on the skew angle θ is given by the following formula. Equation 1: d(θ)=w×tan(θ) / 2 (where w is the distance between the two flags 135a and 135b of the skew sensor 135.)

[0072] From the formula 1, the conveying distance D' of the sheet P from when the detection signal of the registration sensor 105 changes to ON until the detection signal of the skew sensor 135 changes to ON is expressed as follows. Equation 2: D′=D+w×tan(θ) / 2

[0073] In one example of this embodiment, w=190 (mm) and the sensor distance D=8 (mm). In this case, the relationship between the conveying distance D' and the skew angle θ is as shown in Fig. 7(a). The delay time td described above is equal to the value obtained by dividing the conveying distance D' by the sheet conveying speed V of the pair of registration rollers 104. Equation 3: td=D′ / V

[0074] Using the relationships of Equations 1 to 3, the skew angle θ of the sheet P can be evaluated based on the delay time td from when the detection signal of the registration sensor 105 changes to ON until the detection signal of the skew sensor 135 changes to ON. That is, by solving Equations 1 to 3 for θ, the skew angle θ can be expressed as a function of the delay time td and parameters (w, D, V). Thus, the skew angle θ can be obtained by substituting the measured value of the delay time td and the values ​​of the parameters.

[0075] In addition, when the sheet width of the sheet P is W, the range of the skew angle θ in which the skew sensor 135 can normally detect the skew is θ≦cosec -1 (W / w) and the upper limit of θ is θmax=cosec -1 (W / w). θ>cosec -1 In the case of (W / w), the distance between both ends of the leading edge Pa of the sheet P in the sheet width direction Dw is less than the distance between the flags 135a, 135b of the skew sensor 135. In other words, at least one of the flags 135a, 135b does not contact the leading edge Pa of the sheet P.

[0076] In this embodiment, when the value of the skew angle θ calculated based on the formulas 1 to 3 exceeds the upper limit θmax, the skew angle θ of the sheet P is treated as θmax, and a conveying process after jam detection, which will be described later, is executed. Note that when the value of the skew angle θ calculated by the formulas exceeds the upper limit θmax, the conveying process after jam detection may be immediately interrupted.

[0077] Next, a method for determining a preferable skew margin Δ2 based on the skew angle θ will be described. As shown in Fig. 5, when the sheet P is skewed, the apparent sheet length L' of the sheet P is longer than the sheet length L based on the sheet size specified by the user when inputting the image forming job. The apparent sheet length L' is the length in the sheet conveying direction Dc from the leading edge Pa of the sheet P to the trailing edge Pb of the sheet P at the same point (for example, the position of point P0) in the sheet width direction Dw.

[0078] In this embodiment, the difference between the apparent sheet length L' and the specified sheet length L is set as the skew margin Δ2. In other words, Δ2=L'-L. L' is a function of the skew angle θ, and L'=(1 / cos(θ))L. Therefore, the skew margin Δ2 is given by the following formula 4. Equation 4: Δ2=(1 / cos(θ)-1)L

[0079] The relationship between the skew margin Δ2 and the skew angle θ is expressed using equation 4 as shown in Fig. 7(b). Here, the sheet length L is 297 mm (A4 size, long edge feed). The skew margin Δ2 can be determined based on the skew angle θ evaluated based on the relationships expressed by equations 1 to 3 and Fig. 7(a) and the relationship expressed by equation 4 and Fig. 7(b).

[0080] In this way, the skew sensor 135 serving as a skew detection means is configured to detect the sheet P at a position different from the detection position of the registration sensor 105 serving as a sheet detection means in the sheet width direction Dw perpendicular to the sheet conveying direction Dc. The control means determines the skew angle θ of the sheet based on the time difference (delay time td) from when the sheet detection means issues a detection signal indicating the passage of the leading edge of the sheet to when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet.

[0081] In the above, we have assumed that skew occurs in which the left side of the sheet P is the leading side and the right side is the lagging side in Figure 5. However, the skew margin Δ2 can be determined in a similar manner even if skew occurs in which the left side of the sheet P is the lagging side and the right side is the leading side.

[0082] As described above, in this embodiment, when a jam occurs at the fixing discharge section, a conveying process after the jam is detected is performed to improve the workability of the jam clearance. The target conveying distance, which is the distance to convey the sheet P in the conveying process after the jam is detected, includes a skew margin Δ2 according to the degree of skew (skew angle θ) of the sheet P. As expressed by Equation 4, the value of the skew margin Δ2 increases as the skew angle θ increases.

[0083] In other words, when the control means of this embodiment detects a transport abnormality of the sheet while the sheet is sandwiched in the nip portion, the control means executes a transport process to transport the sheet along the transport path and then stop the transport of the sheet. A target transport distance of the sheet from the time when the leading edge of the sheet reaches a detection position of the sheet detection means to the time when the transport of the sheet is stopped in the transport process is set as a target transport distance. The control means changes the transport distance based on the detection result of the skew detection means so that the target transport distance when the sheet is skewed at a first angle is longer than the target transport distance when the sheet is skewed at a second angle smaller than the first angle.

[0084] This configuration allows the conveying process to be performed with a more appropriate conveying distance according to the skew angle θ of the sheet P. In other words, when the skew angle of the sheet P is small, the target conveying distance in the conveying process after jam detection is short, which reduces the adverse effects (such as accordion jam) of making the target conveying distance unnecessarily long. On the other hand, when the skew angle of the sheet P is large, the target conveying distance in the conveying process after jam detection is long, which reduces the possibility that the sheet P will not pass through the nip portion N1 of the pair of registration rollers 104, thereby reducing the workability of jam clearance.

[0085] That is, according to the present embodiment, it is possible to provide an image forming apparatus capable of more appropriately setting the sheet conveying distance after a jam occurs.

[0086] In addition, based on the knowledge that a jam at the fixing and discharging section is likely to occur when the sheet P is skewed, the value of the target conveying distance is changed according to the skew angle θ. This makes it possible to execute the conveying process with a more appropriate conveying distance according to the skew angle θ of the sheet P when a jam at the fixing and discharging section occurs.

[0087] (Modification regarding the method of determining the skew margin) In this embodiment, the skew angle θ and the skew margin Δ2 are determined using the formulas 1 to 4 based on the geometric relationship shown in FIG. 5, but the skew angle θ and the skew margin Δ2 may be determined by other methods. For example, the relationship between the skew angle θ and the conveying distance D′ (or the relationship between the skew angle θ and the delay time td) shown in FIG. 7(a) may be experimentally obtained in advance and stored as a table in the storage unit of the image forming apparatus 100. Similarly, the relationship between the skew angle θ and the skew margin Δ2 shown in FIG. 7(b) may be experimentally obtained in advance and stored as a table in the storage unit of the image forming apparatus 100. Note that the skew angle θ and the preferred skew margin Δ2 in the actual apparatus may differ from the values ​​(theoretical values) of the skew angle θ and the skew margin Δ2 determined based on the geometric relationship due to various error factors. For this reason, the method of determining the skew margin Δ2 based on the experimentally obtained relationship as in this modified example has the advantage of reducing the influence of such error factors.

[0088] In the present embodiment, the skew angle θ of the sheet P is detected by using the skew sensor 135 that contacts the sheet P at two points in the sheet width direction Dw and the registration sensor 105 that is disposed upstream of the skew sensor 135. However, the present embodiment is not limited to this, and the skew angle of the sheet P can be detected by using only one sensor unit for skew detection. For example, two photoelectric sensors corresponding to the two flags 135a and 135b of the skew sensor 135 of the present embodiment are disposed so that the timing at which the flags 135a and 135b contact the leading edge of the sheet P can be measured. In this case, the time difference between the timing at which the flags 135a and 135b contact the leading edge of the sheet P depends on the skew angle θ of the sheet P. Therefore, the skew angle θ can be evaluated based on the time difference.

[0089] In other words, the skew detection means may be configured to detect the sheet at two detection positions spaced apart from each other in a sheet width direction perpendicular to the sheet conveying direction. In this case, the control means may determine the skew angle of the sheet based on a time difference between when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet at one of the two detection positions and when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet at the other of the two detection positions.

[0090] As another method for detecting the skew angle θ, a line sensor may be used to obtain an image of the sheet P, and the skew angle θ may be evaluated more directly based on the obtained image. That is, image data may be obtained using a line sensor extending in the sheet width direction Dw, edge detection may be performed on the image data to detect the edges of the sheet P, and the skew angle θ may be evaluated based on the detection result.

[0091] In this embodiment, the following two cases are processed as cases in which it is impossible to derive the skew angle θ: when the sheet width W specified by the user when the image formation job is input is equal to or smaller than the interval w between the flags 135a and 135b of the skew sensor 135, or when the detection signal of the skew sensor 135 does not change to ON. In these cases, the control described below is not executed.

[0092] (Block diagram) The system configuration and data flow of the image forming apparatus 100 in this embodiment will be described below. Fig. 8 is a functional block diagram showing the system configuration of the image forming apparatus 100 in this embodiment.

[0093] The image forming apparatus 100 includes a controller 801, a video interface unit 802, and a device control unit 803. The controller 801 is connected to a host computer 800 outside the image forming apparatus 100 so as to be able to communicate with each other. The device control unit 803 is also connected to the device control unit 803 via the video interface unit 802 so as to be able to communicate with each other.

[0094] The controller 801 and the device control section 803 are examples of a control unit that controls the image forming apparatus 100 according to this embodiment.

[0095] The device control unit 803 includes a storage unit 810 such as a ROM or RAM that stores programs and data, and a CPU that reads and executes the programs from the storage unit 810. In FIG. 8, functions achieved by the CPU executing the programs are illustrated as functional blocks of the device control unit 803. The device control unit 803 of this embodiment includes, as functional blocks, at least a print control unit 804, a conveyance control unit 805, and a system timer 809. The conveyance control unit 805 also includes a jam determination unit 806, a sheet information measurement unit 807, and a conveyance distance determination unit 808. The storage unit 810 also includes a sheet information storage unit 811 that stores information about the sheet P.

[0096] When the controller 801 receives an image formation command together with image data from the host computer 800 (i.e., when an image formation job is input), it issues an instruction (print command) to the device control unit 803 to perform an image forming operation. When the device control unit 803 receives a print command from the controller 801, it instructs the print control unit 804 and the transport control unit 805 to perform the operations of each process for realizing the image forming operation described above. The print control unit 804 mainly controls the electrophotographic process such as exposure, development, transfer, and fixing based on the image data input from the host computer 800. The transport control unit 805 mainly controls the sheet transport operation in the image forming operation.

[0097] A jam determination unit 806 of the conveyance control unit 805 monitors the sheet conveyance operation and detects the occurrence of a jam. During the image forming operation, the jam determination unit 806 constantly performs jam determination for the sheet P while checking time information acquired from a system timer 809 and conveyance timing information of the sheet P acquired from the registration sensor 105 and the post-fixing sensor 109. For example, the jam determination unit 806 detects the occurrence of a jam at the fixing and discharging section by the method described above with reference to FIG. 2, based on the detection signal of the post-fixing sensor 109.

[0098] When the jam determination unit 806 outputs the jam occurrence determination result (detects a jam), information indicating the jam occurrence is transmitted to the print control unit 804. Then, the print control unit 804 immediately suspends the electrophotographic process. Also, when the jam determination unit 806 detects a jam other than a jam at the fixing / discharging unit, the conveyance control unit 805 immediately suspends the sheet conveyance operation.

[0099] On the other hand, when the jam determination unit 806 detects a jam at the fixing and discharging section, the transport control unit 805 may execute the transport process after the jam detection described above. That is, when a jam at the fixing and discharging section is detected while the detection signal of the registration sensor 105 is ON, the transport control unit 805 continues the sheet transport operation without interruption, and executes the transport process after the jam detection. On the other hand, when a jam at the fixing and discharging section is detected while the detection signal of the registration sensor 105 is OFF, the transport control unit 805 immediately interrupts the sheet transport operation.

[0100] When a conveying process is performed after a jam is detected, information (conveyance continuation information) indicating execution of the conveying process after a jam is detected is transmitted from the jam determination unit 806 to the sheet information measurement unit 807. In this case, the sheet information measurement unit 807 first acquires sheet size information stored in a sheet information storage unit 811 of a storage unit 810. Thereafter, the sheet information measurement unit 807 evaluates the skew angle θ of the sheet P using the acquired sheet size information and information acquired from the registration sensor 105 and the skew sensor.

[0101] The sheet information measurement unit 807 transmits the sheet size information and the information on the skew angle θ to a conveying distance determination unit 808. The conveying distance determination unit 808 uses the sheet size information and the information on the skew angle θ to determine a target conveying distance in the conveying process after jam detection by the method described above.

[0102] The conveyance control unit 805 converts the target conveyance distance determined by the conveyance distance determination unit 808 into a conveyance time, and drives the conveyance motor 815 for the conveyance time. As a result, in the conveyance process after the jam is detected, the sheet P is conveyed by the target conveyance distance.

[0103] In addition, the transport control unit 805 monitors the detection signal of the registration sensor 105 even while the transport process is being executed after the jam is detected, and immediately interrupts the transport of the sheet P if the detection signal of the registration sensor 105 changes to OFF while the transport process is being executed after the jam is detected.

[0104] (flowchart) A control method for image forming apparatus 100 in this embodiment will be described with reference to the flowchart of Fig. 9. This flowchart is executed for each sheet that is the target of the image forming operation.

[0105] In step S101, when a print command is sent from the controller 801, the device control unit 803 issues instructions to each functional block in the device control unit 803 to start an image forming operation. Here, the device control unit 803 analyzes the setting information of the image forming job, extracts information on the sheet length L and sheet width W specified by the user, and stores it in the sheet information storage unit 811 of the storage unit 810.

[0106] In step S102, the conveyance control unit 805 stores the time when the leading edge of the sheet P reaches the registration sensor 105 (referred to as registration sensor ON time T1). The registration sensor ON time T1 is stored in the sheet information storage unit 811.

[0107] In step S103, the conveyance control unit 805 stores whether or not the skew sensor 135 detects a sheet (changes from OFF to ON), and if a sheet is detected, the time (referred to as skew sensor ON time T2). The sheet information storage unit 811 stores whether or not the skew sensor 135 detects a sheet and the skew sensor ON time T2.

[0108] In step S104, the jam determination unit 806 of the transport control unit 805 determines whether any jam has occurred in the image forming apparatus 100. The jam determination unit 806 constantly performs jam detection during the execution of the image forming operation. That is, the transport control unit 805 detects the occurrence of a jam at the fixing / discharge unit or other jams by the method described above while monitoring various sensor information. If a jam has occurred, the process proceeds to step S106. If no jam has occurred, the jam determination unit 806 continues to detect a jam on the sheet P until the end of the image forming operation. Then, in step S105, the transport control unit 805 determines the end of the image forming operation, and stops transport in step S113.

[0109] In step S106, the jam determination unit 806 determines whether the detected jam is a jam that is to be subjected to a transport process after the jam is detected. In this embodiment, the target of the transport process after the jam is detected is a jam at the fixing / discharging section. Therefore, if the detected jam is a jam at the fixing / discharging section in step S106, it is determined that a transport process after the jam is detected is necessary, and the process proceeds to step S107. If the detected jam is not a jam that is to be subjected to a transport process after the jam is detected in step S106, the transport control unit 805 immediately stops transport of the sheet P in step S113.

[0110] In step S107, the sheet information measurement unit 807 of the conveyance control unit 805 determines whether or not it is possible to derive the skew angle θ of the sheet P from the sheet width W stored in the sheet information storage unit 811 in step S101 and information on the presence or absence of sheet detection by the skew sensor 135. If it is possible to derive the skew angle θ, the process proceeds to step S108, and if it is not possible to derive the skew angle θ, the process proceeds to step S112.

[0111] In step S112, the conveyance control unit 805 continues driving the conveyance motor 815 until it detects that the rear end of the sheet has passed through the registration sensor 105 (until the detection signal of the registration sensor 105 changes to OFF). When the conveyance control unit 805 detects that the rear end of the sheet has passed through the registration sensor 105, it stops conveying the sheet P.

[0112] In other words, if the control means of this embodiment determines that the skew angle θ of sheet P cannot be evaluated, in the transport process after jam detection, based on the detection signal of the registration sensor 105, transports sheet P until the trailing end of the sheet passes through the registration sensor 105.

[0113] In step S108, the sheet information measurement unit 807 derives the skew angle θ of the sheet P by the above-mentioned calculation method using the difference between the registration sensor ON time T1 and the skew sensor ON time T2 (i.e., the delay time td) stored in the above steps S102 and S103. The calculated value of the skew angle θ is stored in the sheet information storage unit 811.

[0114] In step S109, the conveying distance determination unit 808 of the conveying control unit 805 determines a target conveying distance Ds in the conveying process after the jam is detected, using the sheet length L and the skew angle θ. The determined value of the target conveying distance Ds is stored in the sheet information storage unit 811.

[0115] In step S110, the conveyance control unit 805 judges whether the sheet P has been conveyed a target conveyance distance Ds or more since the registration sensor ON time T1 based on the elapsed time from the registration sensor ON time T1. Specifically, the product of the elapsed time from the registration sensor ON time T1 and the sheet conveyance speed (roller peripheral speed) of the pair of registration rollers 104 is compared with the target conveyance distance Ds to make the judgment in S110. If the sheet has been conveyed a target conveyance distance Ds or more, the conveyance control unit 805 immediately stops conveying the sheet P in step S113. This ends the conveyance process (S106Y to S110) after jam detection. In other words, the control unit stops conveying the sheet when the conveyance distance of the sheet from the point when the leading edge of the sheet reaches the detection position after starting the conveyance process becomes the target conveyance distance or more.

[0116] In step S110, if the sheet P has not yet been transported by the target transport distance Ds or more from the registration sensor ON time T1, the transport control unit 805 continues transporting the sheet P. However, if the detection signal of the registration sensor 105 changes from ON to OFF (if the trailing end of the sheet passes through the registration sensor 105) before the transport distance from the registration sensor ON time T1 exceeds the target transport distance Ds, the transport control unit 805 stops transporting the sheet P in S113. In other words, after starting the transport process, the control unit stops transporting the sheet if the sheet detection unit issues a detection signal indicating the passage of the trailing end of the sheet before the transport distance of the sheet from the time when the leading end of the sheet reaches the detection position becomes equal to or more than the target transport distance.

[0117] (Modifications regarding the flow chart) In this embodiment, in S110, S111, and S112, the conveying distance of the sheet P in the conveying process after the jam detection is controlled so that the trailing end of the sheet passes through the registration sensor 105, but the conveying distance of the sheet P may be controlled based on a different standard. For example, the conveying distance of the sheet P in the conveying process after the jam detection may be controlled so that the trailing end of the sheet passes through the transfer nip N2. If the sheet P is conveyed until the trailing end of the sheet passes through the transfer nip N2, the pulling resistance of the sheet P becomes even smaller, and the workability of the jam processing can be further improved. On the other hand, it is necessary to pay attention to the adverse effects (such as accordion jam) of a long conveying distance of the sheet P in the conveying process after the jam detection, and therefore it is preferable to set an appropriate conveying distance according to the situation.

[0118] In addition, in this embodiment, the registration sensor 105 is located near the pair of registration rollers 104, but a sensor may be added downstream of the transfer nip N2. In this case, by using the added sensor instead of the registration sensor 105 of this embodiment to manage the conveying distance of the sheet P, the sheet P can be conveyed so that the rear end of the sheet passes through the transfer nip N2 in the conveying process after a jam is detected.

[0119] In addition, in this embodiment, when it is determined that the skew angle θ of the sheet P cannot be evaluated, the sheet P is transported until the registration sensor 105 detects the trailing end of the sheet (S112), but an upper limit may be set for the transport distance of the sheet P in the above case. That is, when the transport distance of the sheet P from the registration sensor ON time T1 reaches a preset upper limit before the detection signal of the registration sensor 105 changes to OFF in step S112, the transport of the sheet P may be immediately stopped.

[0120] (Verification method) A method for verifying the conveying process after jam detection in this embodiment will be described. When this embodiment is applied, a difference occurs in the conveying distance of the sheet P in the conveying process after jam detection between sheets P with different skew angles θ. The conveying distance of the sheet P here refers to the distance that the rear end of the sheet P moves from a specific reference position in the sheet conveying operation until the conveying of the sheet P is stopped in the conveying process after jam detection. In this embodiment, the specific reference position is the position of the rear end of the sheet P when the leading edge of the sheet P reaches the detection position of the registration sensor 105.

[0121] Specifically, the verification is performed by measuring the transport distance of the sheet P in the transport process after the jam is detected in the following procedure. A sheet length L is set in the image forming apparatus 100, and image forming operations are performed one by one for a plurality of sheets P with different skew angles θ, and a jam in the fixing and discharge section is artificially generated. That is, in the image forming operation for one sheet P, an artificial operation is performed so that the detection signal of the post-fixing sensor 109 is turned OFF after the leading edge of the sheet P reaches the post-fixing sensor 109 and before the trailing edge of the sheet P passes the registration sensor 105. This causes the image forming apparatus 100 to perform the transport process after the jam is detected.

[0122] When the conveying process after the jam detection is completed, the conveying distance of the sheet P (corresponding to the target conveying distance Ds in this embodiment) is measured from the point when the leading edge of the sheet P reaches the detection position of the registration sensor 105. Here, in order to eliminate the case where the conveying of the sheet P ends because the trailing edge of the sheet P has passed the registration sensor 105 (S111Y), an artificial operation may be performed so that the detection signal of the registration sensor 105 is always ON after the start of the conveying process after the jam detection.

[0123] When this embodiment is applied, if the sheet length L is constant, the larger the skew angle θ, the longer the target transport distance of the sheet P in the transport process after jam detection. In other words, the target transport distance when the sheet is skewed at a first angle is longer than the target transport distance when the sheet is skewed at a second angle smaller than the first angle. As a result, the distance the sheet is transported after a jam occurs is more appropriately set according to the skew angle of the sheet.

[0124] Furthermore, when this embodiment is applied, if the skew angle is constant, the longer the sheet length L, the longer the target conveying distance of the sheet P in the conveying process after jam detection. In other words, the target conveying distance when the sheet length is a first length is longer than the target conveying distance when the sheet length is a second length that is shorter than the first length. As a result, the distance by which the sheet is conveyed after a jam occurs is more appropriately set according to the sheet length.

[0125] As a method for measuring the conveying distance of the sheet P, an actual moving distance of the rear end of the sheet P may be measured from the time when the leading end of the sheet P reaches the detection position of the registration sensor 105, or the distance may be calculated based on the driving time of the pair of registration rollers 104 or the conveying motor 815. The driving time of the conveying motor 815 may be measured by monitoring the transmission time of a driving signal for driving the conveying motor 815.

[0126] Furthermore, as long as the target conveying distance in the conveying process after jam detection is changed according to the skew angle and sheet length of the sheet, it is not necessary to calculate the target conveying distance itself as in the present embodiment. For example, after starting the conveying process after jam detection, the conveying control unit 805 may determine the timing to stop conveying the sheet P. In this case, the skew angle and sheet length of the sheet are taken into consideration in determining the stop timing, so that the conveying distance from the registration sensor ON time T1 until the conveying of the sheet P is stopped (i.e., the target conveying distance) may change according to the skew angle and sheet length of the sheet.

[0127] Second Embodiment The second embodiment will be described below. In the following, elements having the same reference symbols as those in the first embodiment have substantially the same configurations and functions as those described in the first embodiment, and differences from the first embodiment will be mainly described.

[0128] In the first embodiment, an example was described in which the sheet length L used to control the conveying process after jam detection uses information on the sheet length L specified by the user when the image formation job was input. In this embodiment, a value derived using the apparent sheet length L' and skew angle θ' calculated for the preceding sheet is used as the sheet length L. The method for controlling the conveying process after jam detection described in this embodiment is applicable to the second and subsequent sheets in an image formation job. The same conveying process after jam detection as described in the first embodiment can be applied to the first sheet in an image formation job.

[0129] 10 is a flowchart showing a control method of the image forming apparatus 100 in this embodiment. The difference from the first embodiment is that a step S214 is added to calculate the sheet length L from the apparent sheet length L' of the preceding sheet and the skew angle θ'. The other steps S201 to S213 are substantially the same as the steps S101 to S113 in the first embodiment (FIG. 9).

[0130] In step S214, the sheet information measuring unit 807 of the conveying control unit 805 calculates the sheet length L of the current sheet from the apparent sheet length L' detected during conveyance of the preceding sheet and the skew angle θ' detected during conveyance of the preceding sheet. From the geometric relationship shown in FIG. 5, the sheet length L is given by the formula L=L'×cos(θ'). It is assumed that the values ​​of θ' and L' detected during conveyance of the preceding sheet are stored in the sheet information storage unit 811. The apparent sheet length L' of the preceding sheet is calculated based on the elapsed time from when the detection signal of the registration sensor 105 changes from OFF to ON until it changes from ON to OFF again, and the sheet conveying speed of the registration roller pair 104. It is also assumed that the image forming operation of the preceding sheet is normally completed without jamming.

[0131] After the sheet length L of the current sheet P is derived in step S214, the same control as in the first embodiment is performed except that the sheet length L is used.

[0132] In this embodiment, too, the target transport distance, which is the distance by which sheet P is transported in the transport process after jam detection, includes a skew margin Δ2 corresponding to the degree of skew of sheet P (skew angle θ), and the larger the skew angle θ, the larger the value of the skew margin Δ2.

[0133] In other words, when the control means of this embodiment detects a transport abnormality of the sheet while the sheet is sandwiched in the nip portion, the control means executes a transport process to transport the sheet along the transport path and then stop the transport of the sheet. A target transport distance of the sheet from the time when the leading edge of the sheet reaches a detection position of the sheet detection means to the time when the transport of the sheet is stopped in the transport process is set as a target transport distance. The control means changes the transport distance based on the detection result of the skew detection means so that the target transport distance when the sheet is skewed at a first angle is longer than the target transport distance when the sheet is skewed at a second angle smaller than the first angle.

[0134] Therefore, it is possible to provide an image forming apparatus capable of more appropriately setting the sheet conveying distance after a jam occurs.

[0135] Furthermore, due to various reasons, such as the skew angle θ of the sheet P being different for each sheet, the actual sheet length L of the sheet P may differ from the sheet length specified by the user when the image forming job was input. In this embodiment, when determining the target conveying distance Ds in the conveying process after a jam is detected, the sheet length L derived based on the apparent sheet length L' and the skew angle θ' of the preceding sheet is used. This makes it possible to determine the target conveying distance Ds in the conveying process after a jam is detected based on the more accurate sheet length L, and to provide an image forming apparatus capable of more appropriately setting the sheet conveying distance.

[0136] (Verification method) A method for verifying the conveying process after jam detection in this embodiment will be described. When this embodiment is applied, if the actual sheet length L of the sheet P differs from the sheet length specified by the user when inputting the image forming job, a difference occurs in the target conveying distance in the conveying process after jam detection between the first sheet P and the second and subsequent sheets P in the job.

[0137] That is, if the actual sheet length L is shorter than the sheet length specified by the user, the target conveying distance for the second and subsequent sheets in the job is shorter than the target conveying distance for the first sheet in the job. If the actual sheet length L is longer than the sheet length specified by the user, the target conveying distance for the second and subsequent sheets in the job is longer than the target conveying distance for the first sheet in the job. Therefore, in a similar manner to that described in the verification method of the first embodiment, a jam is artificially caused in the fixing discharge section for the first sheet or the second and subsequent sheets in the job with the sheet skew angle kept constant. Then, the distance conveyed by the sheet in the conveying process after the jam is detected is measured, and the measured values ​​are compared between the first sheet and the second and subsequent sheets.

[0138] (Modification) In the above-described embodiments, an electrophotographic (direct transfer type) image forming apparatus 100 shown in FIG. 1 has been illustrated as an example of an image forming apparatus equipped with a sheet conveying device. The image forming apparatus is not limited to this, and may be, for example, an intermediate transfer type in which a toner image formed on a photosensitive drum is transferred to a sheet at a transfer section (secondary transfer section) via an intermediate transfer body such as an intermediate transfer belt. In addition, the image forming apparatus may be one in which an image is formed on both sides of a sheet P by passing through the transfer nip N2 and the fixing nip N3, and then the sheet P is turned over and passed through the transfer nip N2 and the fixing nip N3 again.

[0139] Furthermore, the image forming means provided in the image forming apparatus is not limited to an electrophotographic type, but may be, for example, an inkjet type that ejects ink toward the sheet P from a nozzle.

[0140] In addition, in each of the above-mentioned embodiments, the sheet transport device is described as being incorporated as a part of the image forming apparatus, but the sheet transport device is not limited to this. For example, the sheet transport device may be a device connected to the image forming apparatus (such as an automatic document feeder of a document reading device, a large-capacity sheet feeder / optional feeder, or a sheet processing device / finisher). In addition, the sheet transport device may be a device used independently of the image forming apparatus.

[0141] Furthermore, the "conveying means" may be a sheet conveying unit other than the pair of registration rollers.

[0142] In addition, in each of the above-described embodiments, an example has been described in which a conveying process is executed after a jam is detected when a jam is detected at the fixing and discharge section, but the conveying process may also be executed after a jam is detected when a jam occurs in another section.

[0143] (Other embodiments) The present invention can also be realized by a process in which a program for implementing one or more of the functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., ASIC) that implements one or more of the functions.

[0144] Summary of the Disclosure The present disclosure includes at least the following configurations or methods.

[0145] (Configuration 1) a conveying path along which the sheet is conveyed; a conveying means having a nip portion for holding the sheet and for conveying the sheet; a sheet detection unit for detecting the sheet at a detection position downstream of the nip portion in a sheet conveying direction; a skew detection means for detecting skew of the sheet; A control unit for controlling the conveyance of the sheet; A sheet conveying device comprising: when detecting a conveyance abnormality of the sheet while the sheet is held in the nip portion, the control unit executes a conveyance process of conveying the sheet along the conveyance path and then stopping the conveyance of the sheet; When a target conveyance distance of the sheet from the time when the leading edge of the sheet reaches the detection position to the time when the conveyance of the sheet is stopped in the conveyance process is set as a target conveyance distance, the control means changes the target conveying distance based on a detection result of the skew detection means so that the target conveying distance when the sheet is skewed at a first angle is longer than the target conveying distance when the sheet is skewed at a second angle smaller than the first angle. A sheet conveying device comprising:

[0146] (Configuration 2) the control means stops conveying the sheet when a conveying distance of the sheet from a point in time when the leading edge of the sheet reaches the detection position becomes equal to or greater than the target conveying distance after starting the conveying process; the control means stops conveyance of the sheet when the sheet detection means issues a detection signal indicating the passage of the rear end of the sheet before a conveyance distance of the sheet from a point in time when the leading end of the sheet reaches the detection position becomes equal to or greater than the target conveyance distance after starting the conveyance process; 2. The sheet conveying device according to configuration 1,

[0147] (Configuration 3) the control means changes the target conveying distance when the sheet length in the sheet conveying direction is a first length, so that the target conveying distance becomes longer than the target conveying distance when the sheet length is a second length shorter than the first length, when the skew angle of the sheet is constant. 3. The sheet conveying device according to configuration 2.

[0148] (Configuration 4) the target conveying distance is a value obtained by adding a margin according to a skew angle of the sheet to a sheet length in the sheet conveying direction; 2. The sheet conveying device according to configuration 1,

[0149] (Configuration 5) The sheet length is determined based on a sheet size specified by a user. 5. The sheet conveying device according to configuration 4.

[0150] (Configuration 6) The sheet length is calculated based on an apparent sheet length detected by the sheet detection means while the preceding sheet is being conveyed when a plurality of sheets are being conveyed continuously, and a skew angle of the preceding sheet detected by the skew detection means. 5. The sheet conveying device according to configuration 4.

[0151] (Configuration 7) the skew detection means is configured to detect the sheet at a position different from the detection position of the sheet detection means in a sheet width direction perpendicular to the sheet conveying direction, the control means determines a skew angle of the sheet based on a time difference between when the sheet detection means issues a detection signal indicating the passage of the leading edge of the sheet and when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet. 7. The sheet conveying device according to any one of configurations 1 to 6.

[0152] (Configuration 8) the skew detection means is configured to detect the sheet at two detection positions spaced apart from each other in a sheet width direction perpendicular to the sheet conveying direction, the control means determines a skew angle of the sheet based on a time difference between when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet at one of the two detection positions and when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet at the other of the two detection positions. 7. The sheet conveying device according to any one of configurations 1 to 6.

[0153] (Configuration 9) when the control means detects the transport abnormality during the period from when the sheet detection means issues a detection signal indicating the passage of the leading edge of the sheet until when the sheet detection means issues a detection signal indicating the passage of the trailing edge of the sheet, the control means starts the transport process by continuing the transport of the sheet that has been performed before the transport abnormality was detected. 9. The sheet conveying device according to any one of configurations 1 to 8.

[0154] (Configuration 10) A sheet conveying device according to any one of configurations 1 to 9; an image forming unit for forming an image on the sheet conveyed by the sheet conveying device; An image forming apparatus comprising:

[0155] (Configuration 11) the image forming unit includes a transfer unit that transfers a toner image from an image carrier or an intermediate transfer body to the sheet, the conveying means is a pair of registration rollers that conveys the sheet to the transfer section; 11. The image forming apparatus according to claim 10,

[0156] (Configuration 12) a fixing device that fixes the toner image transferred to the sheet in the transfer section onto the sheet; a housing having an outlet through which the sheet that has passed through the fixing device is discharged to the outside of the image forming apparatus; a second sheet detection unit for detecting the sheet downstream of the fixing device and upstream of the discharge port in the sheet conveying direction; Further equipped with the control means detects the transport abnormality based on a detection signal from the second sheet detection means; 12. The image forming apparatus according to claim 11,

[0157] (Configuration 13) the second sheet detection means is configured to generate a first signal indicating that the sheet is detected and a second signal indicating that the sheet is not detected; the control means detects the abnormality in conveyance when, before a predetermined time has elapsed since the second sheet detection means has issued the first signal as a result of the leading edge of the sheet in the sheet conveyance direction reaching the second sheet detection means, the second sheet detection means has issued the second signal as a result of the sheet getting caught on an edge of the discharge port in a sheet width direction perpendicular to the sheet conveyance direction and bending. 13. The image forming apparatus according to claim 12,

[0158] (Configuration 14) A transfer roller forming the transfer portion; a fixing device including a heating member for heating the toner image and a pressure roller for holding the sheet together with the heating member; a pair of discharge rollers for discharging the sheet to the outside of the image forming apparatus; a drive source that drives the pair of registration rollers; Further equipped with the driving source drives at least one roller selected from the group consisting of the transfer roller, the pressure roller, and the pair of discharge rollers; 14. The image forming apparatus according to any one of configurations 11 to 13. [Explanation of symbols]

[0159] 104... conveying means (pair of registration rollers) / 105... sheet detection means (registration sensor) / 135... skew detection means (skew sensor) / 801, 803... control means (controller, device control unit) / CP... conveying path (sheet conveying path)

Claims

1. a conveying path along which the sheet is conveyed; a conveying means having a nip portion for holding the sheet and for conveying the sheet; a sheet detection unit for detecting the sheet at a detection position downstream of the nip portion in a sheet conveying direction; a skew detection means for detecting skew of the sheet; A control unit for controlling the conveyance of the sheet; A sheet conveying device comprising: the control unit, when detecting a conveyance abnormality of the sheet while the sheet is sandwiched in the nip portion, executes a conveyance process of conveying the sheet along the conveyance path and then stopping the conveyance of the sheet; a target conveying distance of the sheet from the time when the leading edge of the sheet reaches the detection position to the time when the conveying of the sheet is stopped in the conveying process is set as a target conveying distance, the control means changes the target conveying distance based on a detection result of the skew detection means so that the target conveying distance when the sheet is skewed at a first angle is longer than the target conveying distance when the sheet is skewed at a second angle smaller than the first angle. A sheet conveying device comprising:

2. the control means stops conveying the sheet when a conveying distance of the sheet from a point in time when the leading edge of the sheet reaches the detection position becomes equal to or greater than the target conveying distance after starting the conveying process; the control means stops conveyance of the sheet when the sheet detection means issues a detection signal indicating the passage of the rear end of the sheet before a conveyance distance of the sheet from a point in time when the leading end of the sheet reaches the detection position becomes equal to or greater than the target conveyance distance after starting the conveyance process; 2. The sheet transport device according to claim 1.

3. the control means changes the target conveying distance when the sheet length in the sheet conveying direction is a first length, so that the target conveying distance becomes longer than the target conveying distance when the sheet length is a second length shorter than the first length, when the skew angle of the sheet is constant.

3. The sheet transport device according to claim 2.

4. the target conveying distance is a value obtained by adding a margin according to a skew angle of the sheet to a sheet length in the sheet conveying direction; 2. The sheet transport device according to claim 1.

5. The sheet length is determined based on a sheet size specified by a user.

5. The sheet transport device according to claim 4.

6. The sheet length is calculated based on an apparent sheet length detected by the sheet detection means while the preceding sheet is being conveyed when a plurality of sheets are being conveyed continuously, and a skew angle of the preceding sheet detected by the skew detection means.

5. The sheet transport device according to claim 4.

7. the skew detection means is configured to detect the sheet at a position different from the detection position of the sheet detection means in a sheet width direction perpendicular to the sheet conveying direction, the control means determines a skew angle of the sheet based on a time difference between when the sheet detection means issues a detection signal indicating the passage of the leading edge of the sheet and when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet.

2. The sheet transport device according to claim 1.

8. the skew detection means is configured to detect the sheet at two detection positions spaced apart from each other in a sheet width direction perpendicular to the sheet conveying direction, the control means determines a skew angle of the sheet based on a time difference between when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet at one of the two detection positions and when the skew detection means issues a detection signal indicating the passage of the leading edge of the sheet at the other of the two detection positions.

2. The sheet transport device according to claim 1.

9. when the control means detects the transport abnormality during the period from when the sheet detection means issues a detection signal indicating the passage of the leading edge of the sheet until when the sheet detection means issues a detection signal indicating the passage of the trailing edge of the sheet, the control means starts the transport process by continuing the transport of the sheet that has been performed before the transport abnormality was detected.

2. The sheet transport device according to claim 1.

10. A sheet conveying device according to any one of claims 1 to 9; an image forming unit for forming an image on the sheet conveyed by the sheet conveying device; An image forming apparatus comprising:

11. the image forming unit includes a transfer unit that transfers a toner image from an image carrier or an intermediate transfer body to the sheet, the conveying means is a pair of registration rollers that conveys the sheet to the transfer section; 11. The image forming apparatus according to claim 10.

12. a fixing device that fixes the toner image transferred to the sheet in the transfer section onto the sheet; a housing having an outlet through which the sheet that has passed through the fixing device is discharged to the outside of the image forming apparatus; a second sheet detection unit for detecting the sheet downstream of the fixing device and upstream of the discharge port in the sheet conveying direction; Further equipped with the control unit detects the transport abnormality based on a detection signal from the second sheet detection unit.

12. The image forming apparatus according to claim 11.

13. the second sheet detection means is configured to generate a first signal indicating that the sheet is detected and a second signal indicating that the sheet is not detected; the control means detects the abnormality in conveyance when, before a predetermined time has elapsed since the second sheet detection means has issued the first signal as a result of the leading edge of the sheet in the sheet conveyance direction reaching the second sheet detection means, the second sheet detection means has issued the second signal as a result of the sheet getting caught on an edge of the discharge port in the sheet width direction perpendicular to the sheet conveyance direction and bending.

13. The image forming apparatus according to claim 12.

14. A transfer roller forming the transfer portion; a fixing device including a heating member for heating the toner image and a pressure roller for holding the sheet together with the heating member; a pair of discharge rollers for discharging the sheet to the outside of the image forming apparatus; a drive source that drives the pair of registration rollers; Further equipped with the driving source drives at least one roller selected from the group consisting of the transfer roller, the pressure roller, and the pair of discharge rollers; 12. The image forming apparatus according to claim 11.

Images