Image forming device, control method and program

Abstract

To prevent a wrong image from being formed on a sheet, when abnormal conveyance is detected, without causing increase in the size of a device and in cost.SOLUTION: An image forming device 1 discharges, during execution of a job, a sheet to a first paper discharge port 51 out of a plurality of paper discharge ports in the case where abnormal conveyance of sheet is not detected. The image forming device 1 discharges, during execution of a job, a sheet whose abnormal conveyance is detected and all the subsequent sheets which have already been fed to a second paper discharge port 52 out of the plurality of paper discharge ports, in the case where abnormal conveyance of sheet is detected.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, a control method, and a program, and more particularly to a control technique when abnormal conveyance of a sheet such as a print paper is detected. [Background technology]

[0002] Conventionally, there is known an image forming apparatus that includes a paper feed section that continuously feeds sheets such as printing paper, an image forming section that forms images on the sheets fed from the paper feed section, and a multi-feed detection section that detects multi-feeding of sheets (for example, Patent Document 1). When this conventional image forming apparatus detects multi-feeding of sheets during sheet transport, it stops forming images on the sheets in the multi-feed state and ejects them to a specified tray, and forms the stopped image on the next sheet that is fed and on which multi-feeding has not been detected (for example, Patent Document 1).

[0003] Further, conventionally, there is known an image forming apparatus that, when detecting a double feed after starting to feed sheets from a paper feed unit, ejects the double-fed sheets via an escape path onto a dedicated escape tray (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2003-263076 A [Patent Document 2] JP 2008-176581 A Summary of the Invention [Problem to be solved by the invention]

[0005] In a job in which multiple sheets are continuously conveyed, an image forming apparatus may insert a partition or a cover by conveying a different type of sheet for each predetermined page. In this case, if the above-mentioned conventional technology is applied, when a multi-feed is detected, the sheet can be discharged to a predetermined paper discharge tray without forming an image on the sheet in which the multi-feed was detected. However, images following the image that was supposed to be formed on the sheet in which the multi-feed was detected are sequentially formed on the sheets following the sheet in which the multi-feed was detected. Therefore, if a sheet used for a partition or cover is included among the sheets that have already been fed when a multi-feed is detected, there is a problem that an image different from the image that is originally intended to be formed on the sheet used for the partition or cover is formed on the sheet used for the cover.

[0006] Furthermore, in the case of the conventional technology of Patent Document 2, when a double feed is detected, it is necessary to provide the image forming device with a dedicated escape path for ejecting the sheet in question to an escape tray, which leads to problems such as an increase in the size of the device and an increase in costs.

[0007] Therefore, the present invention has been made to solve the above problems, and aims to provide an image forming apparatus, a control method, and a program that can prevent the formation of an incorrect image on a sheet without increasing the size of the apparatus or increasing costs. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the invention of claim 1 is an image forming apparatus comprising a plurality of paper feeding units that feed sheets toward a conveying path, an image forming unit that forms an image on the sheet conveyed along the conveying path, an abnormal conveying detection unit that detects abnormal conveying of the sheet, a plurality of paper discharge outlets provided at the end of the conveying path, and a control unit that, if the abnormal conveying detection unit does not detect abnormal conveying, discharges the sheet to a first paper discharge outlet of the plurality of paper discharge outlets, and, if the abnormal conveying detection unit detects abnormal conveying, discharges the sheet for which abnormal conveying has been detected and all subsequent sheets that have already been fed to a second paper discharge outlet of the plurality of paper discharge outlets.

[0009] The invention of claim 2 is an image forming apparatus of claim 1, further comprising a subsequent sheet determination unit which, when the abnormal conveying detection unit detects abnormal conveying, determines whether all sheets that have already been fed and follow the sheet in which abnormal conveying has been detected are of the same type as the sheet in which abnormal conveying has been detected, and the control unit is configured such that, when all sheets following the sheet in which abnormal conveying has been detected are of the same type as the sheet in which abnormal conveying has been detected, only the sheet in which abnormal conveying has been detected is discharged to the second sheet discharge outlet, and the sheets following the sheet in which abnormal conveying has been detected are discharged to the first sheet discharge outlet after an image has been formed on them.

[0010] The invention of claim 3 is characterized in that, in the image forming apparatus of claim 1 or 2, the conveying path has a junction where multiple conveying paths join upstream of the image forming unit, and the abnormal conveying detection unit is provided between the junction and the image forming unit.

[0011] The invention of claim 4 is characterized in that, in the image forming apparatus of claim 2, the subsequent sheet determination unit determines that all sheets following the sheet in which abnormal transport is detected are of the same type as the sheet in which abnormal transport is detected when all sheets following the sheet in which abnormal transport is detected are fed from the same paper feeding unit as the sheet in which abnormal transport is detected.

[0012] The invention of claim 5 is a configuration characterized in that, in the image forming apparatus of claim 2, it further includes a memory unit that stores sheet information of sheets contained in each of the multiple paper feeding units, and the subsequent sheet determination unit reads out from the memory unit sheet information of sheets following the sheet in which abnormal transport was detected, and determines whether all sheets following the sheet in which abnormal transport was detected are of the same type as the sheet in which abnormal transport was detected.

[0013] The invention according to claim 6 is characterized in that the control section does not allow the image forming section to form an image on at least a sheet for which abnormal conveyance has been detected by the abnormal conveyance detection section.

[0014] The invention of claim 7 is an image forming apparatus according to claim 1 or 2, characterized in that when the abnormal transport detection unit detects abnormal transport, the control unit discharges the sheet for which abnormal transport has been detected and all subsequent sheets that have already been fed to the second discharge outlet, and then resumes image formation.

[0015] The invention of claim 8 is an image forming apparatus as defined in claim 1 or 2, wherein the transport path has a reversing path which, after an image is formed on a first side of a sheet, inverts the sheet and transports it again to an image formation position by the image forming unit, and the control unit is configured such that, when abnormal transport is detected by the abnormal transport detection unit during double-sided printing in which a sheet is transported to the reversing path, the control unit does not transport the sheet in respect of which abnormal transport has been detected and any subsequent sheets which have already been fed to the reversing path.

[0016] The invention of claim 9 is an image forming apparatus of claim 8, further comprising a switching judgment unit that judges whether it is possible to discharge the sheet in respect of which abnormal transport has been detected and the subsequent sheets that have already been fed to the second sheet discharge outlet without transporting them to the reverse path, and when the switching judgment unit judges that it is impossible to discharge the sheets to the second sheet discharge outlet without transporting them to the reverse path, the control unit is configured to transport the sheet in respect of which abnormal transport has been detected and the subsequent sheets that have already been fed to the reverse path and then discharge them to the second sheet discharge outlet.

[0017] The invention according to claim 10 is the image forming apparatus according to claim 1 or 2, characterized in that the abnormal transport detection unit detects a double feed state in which a plurality of sheets are transported simultaneously as abnormal transport.

[0018] The invention of claim 11 is characterized in that, in the image forming apparatus of claim 10, the control unit detects the amount of early feed of multiple sheets when a double feed state is detected by the abnormal transport detection unit, and stops the transport of sheets when the amount of early feed is equal to or greater than a predetermined threshold value.

[0019] The invention of claim 12 is characterized in that, in the image forming apparatus of claim 1 or 2, the control unit stops conveying the sheet when the thickness of the sheet in which abnormal conveying is detected is outside a predetermined range.

[0020] The invention according to claim 13 is the image forming apparatus according to claim 1 or 2, characterized in that the abnormal transport detection unit detects skew of a sheet as abnormal transport.

[0021] The invention of claim 14 is characterized in that in the image forming apparatus of claim 13, the control unit stops conveying the sheet when the sheet in which abnormal conveying has been detected is conveyed at an angle of more than a predetermined amount relative to the sheet conveying direction.

[0022] The invention of claim 15 is characterized in that in the image forming apparatus of claim 14, the control unit detects the size of the sheet in which abnormal transport was detected, and stops transport of the sheet if the sheet size is outside a predetermined range.

[0023] The invention of claim 16 is characterized in that in the image forming apparatus of claim 14, the control unit detects the amount of deviation of the sheet in relation to the conveying path in which abnormal conveying has been detected, and stops conveying the sheet if the amount of deviation of the sheet is equal to or greater than a predetermined value.

[0024] The invention of claim 17 is an image forming apparatus as defined in claim 1 or 2, wherein the transport path has an inversion path which inverts the sheet after an image is formed on a first side of the sheet and transports it again to an image formation position by the image forming unit, and the control unit is configured such that when abnormal transport detection unit detects abnormal transport during double-sided printing in which a sheet is transported to the inversion path, the control unit transports the sheet to the inversion path and then discharges it to the second paper output outlet.

[0025] The invention of claim 18 is a control method for an image forming device having a plurality of paper feeding units that feed sheets toward a conveying path, an image forming unit that forms an image on the sheet conveyed along the conveying path, an abnormal conveying detection unit that detects abnormal conveying of the sheet, and a plurality of paper discharge outlets provided at the end of the conveying path, characterized in having a first step of discharging the sheet to a first paper discharge outlet of the plurality of paper discharge outlets when the abnormal conveying detection unit does not detect abnormal conveying, and a second step of discharging the sheet for which abnormal conveying has been detected and all subsequent sheets that have already been fed to a second paper discharge outlet of the plurality of paper discharge outlets when the abnormal conveying detection unit detects abnormal conveying.

[0026] The invention of claim 19 is a control method of claim 18, wherein the second step includes a third step of determining whether, when the abnormal conveying detection unit detects abnormal conveying, all sheets that have already been fed and follow the sheet in which abnormal conveying was detected are of the same type as the sheet in which abnormal conveying was detected, and a fourth step of discharging only the sheet in which abnormal conveying was detected to the second paper discharge outlet, and conveying the sheet following the sheet in which abnormal conveying was detected to the first paper discharge outlet after an image is formed on it, if all sheets following the sheet in which abnormal conveying was detected are of the same type as the sheet in which abnormal conveying was detected.

[0027] The invention of claim 20 is a program executed in an image forming device having a plurality of paper feeding units that feed sheets toward a conveying path, an image forming unit that forms an image on the sheet conveyed along the conveying path, an abnormal conveying detection unit that detects abnormal conveying of the sheet, and a plurality of paper output outlets provided at the end of the conveying path, characterized in that the program causes the image forming device to execute a first step of discharging a sheet to a first paper output outlet of the plurality of paper output outlets when the abnormal conveying detection unit does not detect abnormal conveying, and a second step of discharging the sheet for which abnormal conveying has been detected and all subsequent sheets that have already been fed to a second paper output outlet of the plurality of paper output outlets when the abnormal conveying detection unit detects abnormal conveying.

[0028] The invention of claim 21 is a configuration in which, in the program of claim 20, the second step includes a third step of determining whether, when the abnormal conveying detection unit detects abnormal conveying, all sheets that have already been fed and follow the sheet in which abnormal conveying was detected are of the same type as the sheet in which abnormal conveying was detected, and a fourth step of, if all sheets following the sheet in which abnormal conveying was detected are of the same type as the sheet in which abnormal conveying was detected, discharging only the sheet in which abnormal conveying was detected to the second paper discharge outlet, and conveying the sheet following the sheet in which abnormal conveying was detected to the first paper discharge outlet after an image is formed on it. Effect of the Invention

[0029] According to the present invention, it is possible to prevent an incorrect image from being formed on a sheet, without increasing the size of the apparatus or the cost. [Brief description of the drawings]

[0030] [Figure 1] FIG. 1 illustrates an example of the configuration of an image forming apparatus. [Diagram 2] 3A and 3B are diagrams illustrating configuration examples of a loop roller, a registration roller, and a sheet detection sensor. [Diagram 3] 11A and 11B are diagrams illustrating an example of detection of a sheet by a sensor. [Figure 4] 4 is a block diagram showing a configuration example of a control unit; FIG. [Diagram 5] FIG. 1 is a diagram illustrating an example in which a plurality of sheets are simultaneously conveyed in an image forming apparatus. [Figure 6] 4 is a flowchart showing an example of a basic processing procedure in the image forming apparatus. [Figure 7] 13A and 13B are diagrams illustrating an example of sheet discharge when abnormal conveyance is detected. [Figure 8] FIG. 4 is a diagram showing paper feed information and sheet information. [Figure 9] 10 is a flowchart illustrating an example of a processing procedure in the image forming apparatus when a subsequent sheet determining unit functions. [Figure 10] 13A and 13B are diagrams illustrating an example of sheet discharge when it is determined that a sheet in which abnormal conveyance has been detected and a sheet following it are of the same type. [Figure 11] 10 is a diagram illustrating an example of conveyance positions of a plurality of sheets when a double-sided print job is being executed in the image forming apparatus. [Figure 12] 10 is a flowchart showing a process for determining whether or not to pass through a reversing path during double-sided printing. [Figure 13] 13A and 13B are diagrams illustrating an example of sheet discharge when abnormal sheet transport is detected during execution of a double-sided print job. [Figure 14] 10 is a flowchart illustrating an example of a processing procedure when a double feed of a plurality of sheets is detected as abnormal conveyance. [Figure 15] 10 is a flowchart showing a process for determining whether or not to pass through a reversing path during double-sided printing in accordance with an early feed amount. [Figure 16] 13 is a flowchart showing a processing procedure for stopping a job due to a factor other than the early feed amount. [Figure 17] 13 is a flowchart showing a processing procedure for stopping a job due to a factor other than the early feed amount. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, elements common to each other are designated by the same reference numerals, and redundant description thereof will be omitted.

[0032] 1 is a diagram showing an example of the configuration of an image forming apparatus 1 according to an embodiment of the present invention. This image forming apparatus 1 includes a paper feed unit 2, an image forming unit 3, a paper discharge unit 4, an operation panel 5, and a control unit 6. The image forming apparatus 1 feeds a sheet P such as printing paper from the paper feed unit 2, forms an image on the sheet P in the image forming unit 3, and discharges the sheet with the image formed thereon from the paper discharge unit 4.

[0033] The paper feed unit 2 includes a plurality of paper feed sections 11 and 12. The paper feed section 11 is a first paper feed section. The paper feed section 12 is a second paper feed section. These paper feed sections 11 and 12 include paper feed trays 13 on which sheets P are stacked. For example, different types of sheets P are stacked on the paper feed trays 13 of the first paper feed section 11 and the paper feed trays 13 of the second paper feed section 12. Each of the paper feed sections 11 and 12 includes a pickup roller 14 and a paper feed roller 15. The paper feed unit 2 drives the pickup roller 14 and the paper feed roller 15 of one of the plurality of paper feed sections 11 and 12 to feed the sheet P toward the conveying path 30.

[0034] The transport path 30 is formed inside the paper feed unit 2, the image forming unit 3, and the paper discharge unit 4. The sheet P fed from the paper feed units 11 and 12 is transported along the transport path 30 in the order of the paper feed unit 2, the image forming unit 3, and the paper discharge unit 4. For example, the transport path 30 along which the sheet P fed from the first paper feed unit 11 is transported and the transport path 30 along which the sheet P fed from the second paper feed unit 12 is transported join together at a predetermined position. The transport path 30 transports the sheet P fed from the first paper feed unit 11 and the sheet P fed from the second paper feed unit 12 along the same route downstream of the joining point.

[0035] The image forming unit 3 forms an image on the sheet P transported from the paper feed unit 2 along the transport path 30. The image forming unit 3 includes an image forming section 20, a fixing section 29, a loop roller 42, a registration roller 41, and a sheet detection sensor 60.

[0036] For example, the image forming unit 20 is configured to form an image on the sheet P by an electrophotographic method. The image forming unit 20 includes an intermediate transfer belt 21 and a plurality of image creating units 22. The intermediate transfer belt 21 is an endless belt stretched between a driving roller 23 and a driven roller 24. The driving roller 23 and the driven roller 24 are arranged at a predetermined interval in the vertical direction. The intermediate transfer belt 21 circulates between the driving roller 23 and the driven roller 24 in the vertical direction (clockwise direction). The plurality of image creating units 22 are arranged at a predetermined interval in the vertical direction. These plurality of image creating units 22 are adjacent to the intermediate transfer belt 21. Each image creating unit 22 creates a toner image of each color, for example, Y (yellow), M (magenta), C (cyan), and K (black). Each image creating unit 22 includes an image carrier 25, an exposure unit 26, and a development unit 27. The image carrier 25 is configured by, for example, a photosensitive drum, and is driven to rotate in a predetermined direction. The image carrier 25 has one point on its outer circumferential surface joined to the intermediate transfer belt 21. The exposure unit 26 exposes the surface of the image carrier 25 to light, forming an electrostatic latent image on the surface of the image carrier 25. The development unit 27 develops the electrostatic latent image with toner by applying a developer containing toner to the surface of the image carrier 25, thereby forming a toner image on the image carrier 25. The toner image formed on the surface of the image carrier 25 is primarily transferred to the intermediate transfer belt 21 at a position where the toner image contacts the intermediate transfer belt 21. Each image creation unit 22 forms a color image on the intermediate transfer belt 21 by sequentially primarily transferring a toner image of each color onto the intermediate transfer belt 21.

[0037] The intermediate transfer belt 21 is in contact with the secondary transfer roller 28 in the transport path 30 of the sheet P. The image primarily transferred to the intermediate transfer belt 21 is secondarily transferred to the sheet P when the sheet P transported along the transport path 30 passes between the intermediate transfer belt 21 and the secondary transfer roller 28. Therefore, the contact position between the intermediate transfer belt 21 and the secondary transfer roller 28 is an image forming position where an image is formed on the sheet P.

[0038] The fixing unit 29 is provided downstream of the image forming position in the conveying path 30. The fixing unit 29 fixes the image that has been secondarily transferred onto the sheet P to the sheet P. The fixing unit 29 has, for example, a heating roller and a pressure roller. The fixing unit 29 fixes the image to the sheet P by performing a heat treatment and a pressure treatment on the sheet P on which the image has been formed.

[0039] The loop roller 42, the registration roller 41, and the sheet detection sensor 60 are provided upstream of the image forming position on the transport path 30. The loop roller 42 is provided upstream of the registration roller 41 on the transport path 30. The sheet detection sensor 60 is provided downstream of the loop roller 42 on the transport path 30. Details of the sheet detection sensor 60 will be described later.

[0040] The transport path 30 provided in the image forming unit 3 has a reversing path 31. The reversing path 31 is a transport path that reverses the front and back of the sheet P. For example, when double-sided printing is performed in the image forming apparatus 1, the image forming unit 3 supplies the sheet P with an image formed on a first side of the sheet P to the reversing path 31 and reverses the front and back of the sheet P. The image forming unit 3 supplies the sheet P with the reversed front and back to the image forming position again and forms an image on the second side of the sheet P.

[0041] The conveying path 30 has a switching gate 43 downstream of the fixing unit 29 that can supply the sheet P to the inversion path 31. The switching gate 43 can switch the conveying path of the sheet P to either the downstream side of the conveying path 30 or the inversion path 31. For example, when the sheet P that has passed through the fixing unit 29 is sent to the paper discharge unit 4 without being supplied to the inversion path 31, the image forming unit 3 retracts the switching gate 43 from the conveying path 30. In contrast, when the sheet P that has passed through the fixing unit 29 is supplied to the inversion path 31 in double-sided printing, the image forming unit 3 causes the switching gate 43 to enter the conveying path 30 and guides the sheet P to the inversion path 31. The inversion path 31 is provided with a roller 48 for conveying the sheet P. The roller 48 conveys the sheet P supplied to the inversion path 31 toward the conveying path 30 upstream of the image forming position.

[0042] The sheet P on which an image has been formed in the image forming unit 3 is transported to the paper discharge unit 4. The paper discharge unit 4 includes a first paper discharge outlet 51 and a second paper discharge outlet 52 provided at the end of the transport path 30. The transport path 30 in the paper discharge unit 4 branches into a transport path 32 that guides the sheet P to the first paper discharge outlet 51 and a transport path 33 that guides the sheet P to the second paper discharge outlet 52. The transport path 30 has a roller 44 upstream of the branching portion. The transport path 32 has a roller 45 that transports the sheet P and a discharge roller 46 that discharges the sheet P to the first paper discharge outlet 51. The transport path 33 has a discharge roller 47 that discharges the sheet P to the second paper discharge outlet 52.

[0043] The operation panel 5 is a user interface that can be operated by the user. The operation panel 5 includes a display unit and an operation unit. The display unit displays various operation screens that can be operated by the user. The operation unit accepts operations by the user. For example, the operation panel 5 accepts instructions from the user to execute a job. The operation panel 5 also accepts input of sheet information related to sheets P loaded in paper feed tray 13 of paper feed units 11, 12.

[0044] The control unit 6 comprehensively controls the operation of the image forming apparatus 1. For example, when a job is executed in the image forming apparatus 1, the control unit 6 controls the paper feeding operation in the paper feeding unit 2, the image forming operation in the image forming unit 3, and the paper discharging operation in the paper discharging unit 4. The control unit 6 also controls the operation of conveying the sheet P along the conveying path 30.

[0045] For example, when a job for continuously forming images on a plurality of sheets P is executed in the image forming apparatus 1, the control unit 6 feeds the sheets P designated in the job at a predetermined interval. In this case, the image forming apparatus 1 is in a state where the plurality of sheets P are simultaneously conveyed at a predetermined interval.

[0046] FIG. 2 is a diagram showing an example of the configuration of the loop roller 42, the registration roller 41, and the sheet detection sensor 60. As shown in FIG. 2(a), the sheet detection sensor 60 includes a double feed detection sensor 61, a skew detection sensor 62, a misalignment detection sensor 63, and a timing detection sensor 64. The double feed detection sensor 61 is provided between the registration roller 41 and the loop roller 42. The skew detection sensor 62, the misalignment detection sensor 63, and the timing detection sensor 64 are provided downstream of the registration roller 41 in the conveying direction of the sheet P. For example, the skew detection sensor 62 and the timing detection sensor 64 are configured as reflective sensors. The misalignment detection sensor 63 is configured as a line sensor such as a CIS (Contact Image Sensor). A background plate 65 is provided at the positions where the skew detection sensor 62, the misalignment detection sensor 63, and the timing detection sensor 64 are provided. The upper surface of the background plate 65 is a predetermined color. The background plate 65 is formed with holes that transmit light from the skew detection sensor 62 and the timing detection sensor 64, and is configured to prevent unnecessary reflected light from entering the skew detection sensor 62 and the timing detection sensor 64. Therefore, when the sheet P is transported to the upper surface side of the background plate 65, the skew detection sensor 62, the shift detection sensor 63, and the timing detection sensor 64 detect the sheet P.

[0047] The registration roller 41 corrects the skew of the sheet P conveyed along the conveying path 30. For example, when the sheet P is conveyed along the conveying path 30, the loop roller 42 is rotated. Therefore, the leading edge of the sheet P advances from the loop roller 42 toward the registration roller 41. At this time, the registration roller 41 is not rotated. Therefore, the leading edge of the sheet P stops advancing at a position where it abuts against the nip portion of the registration roller 41. After that, when the rotational drive of the loop roller 42 continues, the sheet P is bent as shown in FIG. 2(a). That is, the sheet P forms a loop between the registration roller 41 and the loop roller 42. By forming a loop in the sheet P, the entire leading edge of the sheet P is in a state of abutting against the nip portion of the registration roller 41 in the width direction of the sheet P. Therefore, even if the sheet P is conveyed to the registration roller 41 in a skewed state, the skew of the sheet P is corrected at the position of the registration roller 41. Thereafter, the registration rollers 41 are rotationally driven at a predetermined timing to transport the sheet P downstream in the transport direction (direction F).

[0048] The double feed detection sensor 61 is composed of, for example, an ultrasonic sensor. The double feed detection sensor 61 has a transmitter 61a and a receiver 61b. The transmitter 61a is provided at an upper position of the transport path 30. The receiver 61b is provided at a lower position of the transport path 30. The transmitter 61a transmits ultrasonic waves toward the receiver 61b. The receiver 61b receives the ultrasonic waves transmitted from the transmitter 61a. As shown in FIG. 2(a), when a sheet P is present between the transmitter 61a and the receiver 61b, the ultrasonic waves received by the receiver 61b are attenuated according to the number of sheets P. Therefore, the double feed detection sensor 61 detects the double feed of sheets P based on the intensity of the ultrasonic waves received by the receiver 61b.

[0049] FIG. 2(b) shows a multi-feed state in which two sheets P1 and P2 are fed at the same time. In the case of a multi-feed state in which multiple sheets P1 and P2 are fed at the same time, the ultrasonic waves emitted from the transmitter 61a pass through the multiple sheets P1 and P2 and reach the receiver 61b. The ultrasonic waves are attenuated when passing through each of the multiple sheets P1 and P2. Therefore, when multiple sheets P1 and P2 are fed at the same time, the ultrasonic intensity received by the receiver 61b is significantly reduced. When the ultrasonic intensity received by the receiver 61b is less than a predetermined value, the multi-feed detection sensor 61 detects that multiple sheets P1 and P2 are fed at the same time and that a multi-feed state has occurred. The multi-feed detection sensor 61 may be configured to detect the thickness of the sheet P.

[0050] The skew detection sensors 62 are provided at two locations on both ends in the width direction of the sheet P on the downstream side of the registration rollers 41. The skew detection sensors 62 detect whether the sheet P sent out from the registration rollers 41 is skewed or not.

[0051] As described above, the skew of the sheet P is corrected to some extent by the registration rollers 41. However, the registration rollers 41 may not be able to completely correct the skew of the sheet P. In that case, the sheet P may be transported in a skewed state toward the downstream side of the registration rollers 41.

[0052] FIG. 3A is a diagram showing an example of detection by the skew detection sensor 62. As shown in FIG. 3A, the skew detection sensor 62 has sensors 62a and 62b provided at two locations on both ends of the width direction of the sheet P. As shown in FIG. 3A, when the sheet P sent out from the registration roller 41 is skewed, the sensor 62a detects the leading edge of the sheet P before the sensor 62b. The sensor 62b detects the leading edge of the sheet P after the sensor 62a detects the leading edge of the sheet P. The skew detection sensor 62 detects whether the sheet P is skewed or not based on the time difference between when the sensors 62a and 62b detect the leading edge of the sheet P. For example, when the time difference between when the sensors 62a and 62b detect the leading edge of the sheet P is less than a predetermined time (T1), the skew detection sensor 62 detects that the sheet P is not skewed. On the other hand, when the time difference between when the sensors 62a and 62b detect the leading edge of the sheet P is equal to or greater than the predetermined time (T1), the skew detection sensor 62 detects that the sheet P is skewed.

[0053] The deviation detection sensors 63 are provided downstream of the skew detection sensor 62 at two locations on both ends in the width direction of the sheet P. The deviation detection sensors 63 detect whether the sheet P sent out from the registration rollers 41 is being conveyed in a state of being biased in the width direction of the conveying path 30.

[0054] FIG. 3B is a diagram showing an example of detection by the misalignment detection sensor 63. As shown in FIG. 3B, the misalignment detection sensor 63 has sensors 63a and 63b provided at two locations on both ends of the width direction of the sheet P. As shown in FIG. 3B, when the sheet P sent out from the registration roller 41 is misaligned to one side, the detection width of the sheet P by the sensor 63b becomes larger than the detection width of the sheet P by the sensor 63a. The misalignment detection sensor 63 detects whether the sheet P is conveyed in a misaligned state based on the difference between the detection widths of the sheet P by the sensors 63a and 63b. For example, when the difference between the detection widths of the sheet P by the sensors 63a and 63b is less than a predetermined value (W1), the misalignment detection sensor 63 detects that the sheet P is not misaligned. On the other hand, when the difference between the detection widths of the sheet P by the sensors 63a and 63b is equal to or greater than the predetermined value (W1), the misalignment detection sensor 63 detects that the sheet P is misaligned. The deviation detection sensor 63 may detect deviation at one location on one end side of the sheet P in the width direction.

[0055] 2, the timing detection sensor 64 is provided upstream of the skew detection sensor 62 and the deviation detection sensor 53. The timing detection sensor 64 detects the operation start timing of the image forming section 20 in order to match the timing at which the toner image primarily transferred to the intermediate transfer belt 21 reaches the image forming position with the timing at which the sheet P transported along the transport path 30 reaches the image forming position. The image forming unit 3 starts the operation of the image forming section 20 at the timing at which the leading edge of the sheet P is detected by the timing detection sensor 64, and forms an image on the sheet P when the sheet P passes the image forming position.

[0056] Furthermore, the timing detection sensor 64 can detect the size in the sheet transport direction of the sheet P. When the timing detection sensor 64 detects the leading edge of the sheet P and turns on, it maintains the on state until the trailing edge of the sheet P passes by. Therefore, the timing detection sensor 64 detects the size of the sheet P in the sheet transport direction by measuring the time that the on state continues.

[0057] 4 is a block diagram showing an example of the configuration of the control unit 6. The control unit 6 includes a CPU 70 and a memory 71. The CPU 70 is a hardware processor that reads out and executes a program 72 stored in the memory 71. The memory 71 is a storage device configured with a hard disk drive (HDD), a solid state drive (SSD), or the like. The memory 71 stores the above-mentioned program 72 in advance. The memory 71 also stores paper feed information 81 or sheet information 82. The sheet information 82 is sheet information input by the user via the operation panel 5.

[0058] The CPU 70 executes the program 72 to cause the control unit 6 to function as a job control unit 73, an abnormal conveyance detection unit 74, a paper discharge control determination unit 75, a subsequent sheet determination unit 76, and a paper discharge control unit 77. However, the subsequent sheet determination unit 76 is an optional function for the control unit 6.

[0059] The job control unit 73 comprehensively controls the execution of jobs in the image forming apparatus 1. For example, the job control unit 73 receives a print job via a network. The job control unit 73 starts execution of the job in a state where various settings specified in the print job are reflected in each of the paper feed unit 2, the image forming unit 3, and the paper discharge unit 4. The job control unit 73 also starts execution of a job when an instruction to execute a print job is received via the operation panel 5. In the following, a case will be exemplified where the job executed by the job control unit 73 is a job to form an image by continuously feeding a plurality of sheets P.

[0060] When the job control unit 73 starts executing a job, it drives one of the first and second feed units 11 and 12, whichever is specified in the job settings, to feed the sheet P. If the job is set to switch the feed unit to be fed every predetermined number of sheets, the job control unit 73 switches the feed unit to feed the sheet P every predetermined number of sheets based on the setting. For example, when the job control unit 73 feeds the sheet P from either the first and second feed units 11 and 12, it records the feed history in the feed information 81 in the memory 71. Therefore, the feed information 81 indicates whether the sheet P being conveyed along the conveyance path 30 was fed from the first and second feed units 11 and 12.

[0061] Fig. 5 is a diagram showing an example in which a plurality of sheets P are simultaneously transported in the image forming apparatus 1. Note that in Fig. 5, components other than the transport path 30 are omitted in order to clearly show the positions of the sheets P1 to P6. Also, X shown in Fig. 5 is an image forming position where an image is formed on the sheet P by the image forming unit 20.

[0062] When the job control unit 73 executes a job to continuously feed a plurality of sheets P, it feeds the sheets P1 to P6 in order from the sheet feed unit 11 or 12. When the sheets P1 to P6 pass the image forming position X, the job control unit 73 forms a pre-specified image on each of the sheets P1 to P6. At this time, the job control unit 73 operates the image forming unit 20 at the timing when the leading edge of the sheet P is detected by the timing detection sensor 64. As a result, the timing when the sheet P arrives at the image forming position X and the timing when the image primarily transferred to the intermediate transfer belt 21 arrives at the image forming position X match, and the image can be secondarily transferred to the sheet P. Thereafter, the job control unit 73 discharges the sheets P1 to P6 on which the images have been formed to a pre-specified paper discharge outlet (for example, the first paper discharge outlet 51).

[0063] The job control unit 73 can roughly recognize the current positions of the multiple sheets P1-P6 being transported along the transport path 30. This is because the sheets P1-P6 are transported along the transport path 30 at a predetermined transport speed. Therefore, the job control unit 73 can recognize the position of each of the sheets P1-P6 on the transport path 30 based on the elapsed time since each of the sheets P1-P6 was fed.

[0064] When a job is being executed by the job control unit 73, the abnormal transport detection unit 74 detects abnormal transport of the sheet P being transported along the transport path 30. The abnormal transport detection unit 74 detects abnormal transport based on the output from the sheet detection sensor 60. For example, when the double feed detection sensor 61 detects a double feed state, the abnormal transport detection unit 74 detects that the sheet P is being abnormally transported. Furthermore, when the skew detection sensor 62 detects skew of the sheet P, the abnormal transport detection unit 74 detects that the sheet P is being abnormally transported. Furthermore, when the deviation detection sensor 63 detects deviation of the sheet P, the abnormal transport detection unit 74 detects that the sheet P is being abnormally transported. Furthermore, when the timing detection sensor 64 detects a sheet size equal to or larger than a predetermined size in the sheet transport direction, the abnormal transport detection unit 74 detects that the sheet P is being abnormally transported.

[0065] The sheet detection sensor 60 is provided upstream of the image forming position X on the conveying path 30. Therefore, the abnormal conveyance detection unit 74 detects whether or not the fed sheet P is abnormally conveyed before the sheet P is conveyed to the image forming position X. Furthermore, when the abnormal conveyance detection unit 74 detects abnormal conveyance of the sheet P, it identifies the abnormally conveyed sheet P. For example, as shown in FIG. 5, when abnormal conveyance is detected when the sheet P4 passes the position of the sheet detection sensor 60, the abnormal conveyance detection unit 74 identifies the sheet P4 as the abnormally conveyed sheet among the multiple sheets P1 to P6 present on the conveying path 30.

[0066] When the abnormal transport detection unit 74 detects abnormal transport of the sheet P, it suspends the execution of the job by the job control unit 73. Furthermore, when the abnormal transport detection unit 74 detects abnormal transport of the sheet P, it causes the sheet discharge control determination unit 75 to function.

[0067] When abnormal conveyance is detected by the abnormal conveyance detection unit 74, the discharge control determination unit 75 determines control to discharge the abnormally conveyed sheet P. For example, as shown in Fig. 5, when abnormal conveyance is detected when the sheet P4 passes the position of the sheet detection sensor 60, the discharge control determination unit 75 determines to discharge the sheet P4 to the second discharge outlet 52. The second discharge outlet 52 is a discharge outlet different from the first discharge outlet 51 to which normal sheets P1 to P3 that are not abnormally conveyed are discharged.

[0068] Incidentally, when a job is being executed in which a plurality of sheets P are continuously conveyed to form an image, sheets P5 and P6 following sheet P4 in which abnormal conveyance was detected may have already been fed from sheet feeding units 11 and 12. In this case, the following sheets P5 and P6 may include a sheet of a different type from sheet P4. If the following sheets P5 and P6 following sheet P4 include a sheet of a different type from sheet P4, job control unit 73 must not form images on sheets P5 and P6. Therefore, discharge control determination unit 75 determines to discharge not only sheet P4 but also sheets P5 and P6 that have already been fed to second discharge outlet 52. This solves the conventional problem.

[0069] The discharge control unit 77 discharges the sheet P in the conveying path 30 based on the control determined by the discharge control determination unit 75. The discharge control unit 77 discharges normal sheets P on which an image has been formed to the first discharge outlet 51. In addition, the discharge control unit 77 discharges the sheet P4 for which abnormal conveyance has been detected, and the sheets P5 and P6 which have already been fed and follow the sheet P4, to the second discharge outlet 52.

[0070] The discharge control unit 77 resumes the execution of the job by the job control unit 73 in conjunction with the start of the discharge of sheets P4, P5, and P6. As a result, the job control unit 73 continues the execution of the job by feeding the same sheet as sheet P4 discharged to the second discharge outlet 52. Note that the discharge control unit 77 may resume the execution of the job by the job control unit 73 after the discharge of sheets P4, P5, and P6 is completed.

[0071] 6 is a flowchart showing an example of a processing procedure in the image forming apparatus 1 for performing the above control. This processing is performed by the CPU 70 of the control unit 6 executing the program 72. When the control unit 6 starts execution of a job (step S10), it determines whether it is time to feed a sheet P (step S11). If it is time to feed a sheet P (YES in step S11), the control unit 6 starts feeding a sheet P from either one of the sheet feed units 11 and 12 (step S12).

[0072] When feeding of the sheet P is started, the control unit 6 judges whether or not abnormal transport of the sheet P is detected (step S13). When abnormal transport of the sheet P is detected (YES in step S13), the control unit 6 suspends the execution of the job (step S14). The control unit 6 identifies the sheet P that is abnormally transported (step S15). The control unit 6 starts the operation of discharging the abnormally transported sheet P to the second discharge outlet 52 (step S16). At this time, the control unit 6 does not form an image on the sheet P to be discharged to the second discharge outlet 52. This allows the control unit 6 to reduce unnecessary toner consumption.

[0073] Next, the control unit 6 judges whether or not a sheet P following the abnormally conveyed sheet P has already been fed (step S17). If the subsequent sheet P has already been fed (YES in step S17), the control unit 6 starts an operation of discharging the already fed subsequent sheet P to the second discharge outlet 52 (step S18). For example, if a plurality of sheets P have already been fed, the control unit 6 discharges all of the plurality of sheets P to the second discharge outlet 52. In this case, too, the control unit 6 does not form an image on the sheet P to be discharged to the second discharge outlet 52. This allows the control unit 6 to reduce unnecessary toner consumption. Also, if a sheet P following the abnormally conveyed sheet P has not been fed (NO in step S17), the control unit 6 skips the process of step S18.

[0074] Next, the control section 6 resumes the execution of the job (step S19). If the control section 6 does not detect abnormal conveyance of the sheet P in step S13 (NO in step S13), the processes in steps S14 to S19 are not performed.

[0075] Next, the control unit 6 judges whether or not there is a next sheet to be fed (step S20). If there is a next sheet to be fed (YES in step S20), the process by the control unit 6 returns to step S11. In this case, when the control unit 6 detects that it is time to feed the next sheet P (YES in step S11), it starts the feeding operation of the sheet P (step S12). Note that if abnormal transport of the sheet P is detected in step S13 (YES in step S13), the control unit 6 judges YES in step S20 after the job execution is resumed, and starts feeding a sheet P to replace the sheet P discharged to the second discharge outlet 52 (step S12).

[0076] On the other hand, if there is no next sheet to be fed (NO in step S20), the control unit 6 judges whether the execution of the job has been completed (step S21). For example, if the last sheet P has not been discharged to the first discharge outlet 51, the control unit 6 judges NO in step S21. In this case, the process by the control unit 6 returns to step S13. The control unit 6 then repeatedly judges whether or not abnormal transport of the sheet P during transport has been detected. Therefore, if the control unit 6 detects abnormal transport of the sheet P when the execution of the job has not been completed, the control unit 6 discharges the sheet P for which abnormal transport has been detected to the second discharge outlet 52.

[0077] Furthermore, when the last sheet P is discharged to the first discharge port 51, the control unit 6 determines that the execution of the job is completed (YES in step S21). In this case, the execution of the job by the image forming apparatus 1 is completed.

[0078] As a result of the control unit 6 performing the above-described process, when the image forming apparatus 1 detects abnormal conveyance of the sheet P during job execution, the image forming apparatus 1 discharges the sheet P for which abnormal conveyance has been detected to the second discharge outlet 52. At this time, the image forming apparatus 1 discharges to the second discharge outlet 52 not only the sheet P for which abnormal conveyance has been detected, but also the sheet P that has already been fed following the sheet P for which abnormal conveyance has been detected.

[0079] FIG. 7 is a diagram showing an example of discharging the sheet P when abnormal conveyance is detected. For example, assume that the sheet detection sensor 60 detects abnormal conveyance of the sheet P4 when six sheets P1 to P6 are being conveyed in the conveyance path 30 as shown in FIG. 5. In this case, the image forming apparatus 1 sequentially forms images on the sheets P1, P2, and P3 conveyed before the sheet P4 for which abnormal conveyance is detected, and discharges them to the first discharge outlet 51. The image forming apparatus 1 also discharges the sheet P4 for which abnormal conveyance is detected, and the succeeding sheets P5 and P6 that have already been fed when the abnormal conveyance of the sheet P4 was detected, to the second discharge outlet 52. At this time, the image forming apparatus 1 does not form images on the sheets P4, P5, and P6. After that, the image forming apparatus 1 feeds the sheet P7. If abnormal conveyance of the sheet P7 is not detected, the image forming apparatus 1 prints the image that should have been printed on the sheet P4 on the sheet P7, and discharges it to the first discharge outlet 51. The same applies to sheets P7 and onwards.

[0080] By performing the above-described discharge process, when sheets P5 and P6 following sheet P4 for which abnormal transport was detected include a sheet used as a partition paper or a cover, image forming apparatus 1 can discharge the sheet to second discharge outlet 52 without forming an image on the sheet used as a partition paper or a cover. Therefore, image forming apparatus 1 can prevent an incorrect image from being formed on a sheet such as a partition paper or a display paper.

[0081] 4, the control unit 6 may determine the types of the sheets P5 and P6 by causing a succeeding sheet determination unit 76 to function. For example, when abnormal conveyance of the sheet P4 is detected, the succeeding sheet determination unit 76 determines whether the types of the sheets P5 and P6 that have been fed and follow the sheet P4 are the same as the type of the sheet P4. If the sheets P5 and P6 are the same type as the sheet P4, the sheets P5 and P6 do not include any special sheets such as partition paper or labels. Therefore, if the sheets P5 and P6 are the same type as the sheet P4, the image forming apparatus 1 may continue the normal conveyance of the sheets P5 and P6 and sequentially form images on each of the sheets P5 and P6.

[0082] For example, the succeeding sheet determination unit 76 reads out the sheet feed information 81 stored in the memory 71 and determines whether the sheet feed unit that fed the sheets P5 and P6 is the same as the sheet feed unit that fed the sheet P4. FIG. 8(a) shows an example of the sheet feed information 81. As shown in FIG. 8(a), the sheet feed information 81 records "YES" for the sheet feed unit that is the source of the sheet feed according to the sheet feed order of the sheet P. Therefore, the succeeding sheet determination unit 76 can determine whether the fed sheets P5 and P6 that follow the sheet P4 in which the abnormal conveyance was detected are sheets that have been fed from the same sheet feed unit as the sheet P4 by referring to the sheet feed information 81. If the sheets P5 and P6 are fed from the same sheet feed unit as the sheet P4, the sheets P5 and P6 are the same type as the sheet P4. Therefore, the subsequent sheet determination unit 76 can determine whether the sheets P5 and P6 are of the same type as the sheet P4 by determining whether the sheets P5 and P6 are fed from the same feed unit as the sheet P4.

[0083] Further, the succeeding sheet determination unit 76 may read the paper feed information 81 and the sheet information 82 from the memory 71 and determine whether the sheets P5 and P6 are of the same type as the sheet P4. FIG. 8B shows an example of the sheet information 82. The sheet information 82 includes sheet information (type, size, etc.) regarding the sheets P loaded on the paper feed tray 13 of the first paper feed unit 11 and sheet information (type, size, etc.) regarding the sheets P loaded on the paper feed tray 13 of the second paper feed unit 12. Therefore, the succeeding sheet determination unit 76 can identify the type of the sheets P fed from the first paper feed unit 11 and the type of the sheets P fed from the second paper feed unit 12. Specifically, even if the paper feed units that fed the multiple sheets are different from each other, the succeeding sheet determination unit 76 can determine whether the multiple sheets are of the same type by referring to the sheet information 82. Therefore, the succeeding sheet determination unit 76 can determine whether the sheets P5 and P6 are of the same type as the sheet P4 by referring to the sheet information 82 corresponding to the paper feed units that fed the sheets P4, P5, and P6, respectively.

[0084] When the succeeding sheet determination unit 76 determines that sheets P5 and P6 are the same type as sheet P4, the discharge control determination unit 75 determines to discharge only sheet P4, for which abnormal conveyance was detected, to the second discharge outlet 52. In addition, the discharge control determination unit 75 determines to normally convey sheets P5 and P6 and to sequentially form images on each of sheets P5 and P6. Then, the discharge control determination unit 75 determines to discharge sheets P5 and P6 on which images have been formed to the first discharge outlet 51.

[0085] The discharge control unit 77 controls the discharge operation of the sheets based on the determination result by the discharge control determination unit 75. That is, the discharge control unit 77 discharges the sheet P4 to the second discharge outlet 52, and discharges the sheets P5 and P6 to the first discharge outlet 51. As a result, the image forming apparatus 1 can continue the execution of the job using the sheets P5 and P6 without discharging the sheets P5 and P6 to the second discharge outlet 52. This improves the efficiency of job execution by the image forming apparatus 1.

[0086] Fig. 9 is a flowchart showing an example of a processing procedure in the image forming apparatus 1 when the succeeding sheet determination unit 76 functions. The flowchart shown in Fig. 9 differs from the flowchart shown in Fig. 6 in steps S25 and S26. Steps S25 and S26 will be described below.

[0087] If the sheets P5 and P6 following the abnormally conveyed sheet P4 have already been fed (YES in step S17), the control unit 6 determines whether the sheets P5 and P6 are the same type as the abnormally conveyed sheet P4 (step S25). At this time, the subsequent sheet determination unit 76 of the control unit 6 functions. The subsequent sheet determination unit 76 refers to the paper feed information 81 or the sheet information 82 and determines whether the sheets P5 and P6 are the same type as the sheet P4.

[0088] If the sheets P5 and P6 are the same type as the sheet P4 (YES in step S25), the image forming apparatus 1 may form images on the sheets P5 and P6 following the sheet P4. Therefore, the control unit 6 continues the normal transport of the sheets P5 and P6 and resumes the execution of the job (step S19).

[0089] On the other hand, if sheets P5 and P6 include a sheet of a different type from sheet P4 (NO in step S25), control unit 6 discharges sheets P5 and P6 to second discharge outlet 52 in the same manner as sheet P4 (step S26). At this time, control unit 6 does not form an image on sheets P5 and P6 discharged to second discharge outlet 52. This allows control unit 6 to reduce unnecessary toner consumption. Also, if no sheet following the abnormally conveyed sheet has been fed (NO in step S17), control unit 6 skips the processes of steps S25 and S26.

[0090] FIG. 10 is a diagram showing an example of discharging the sheet P when it is determined that the sheets P5 and P6 following the sheet P4 for which abnormal conveyance has been detected are the same type as the sheet P4. For example, assume that the sheet detection sensor 60 detects abnormal conveyance of the sheet P4 when six sheets P1 to P6 are being conveyed as shown in FIG. 5. In this case, the image forming apparatus 1 forms images in sequence on the sheets P1, P2, and P3 conveyed before the sheet P4 for which abnormal conveyance has been detected, and normally discharges the sheets to the first discharge outlet 51. Also, the image forming apparatus 1 abnormally discharges only the sheet P4 for which abnormal conveyance has been detected to the second discharge outlet 52. At this time, the image forming apparatus 1 does not form an image on the sheet P4. The following sheets P5 and P6, which have already been fed when abnormal conveyance of the sheet P4 was detected, are the same type as the sheet P4. Therefore, the image forming apparatus 1 normally conveys the sheets P5 and P6 along the conveyance path 30, and resumes the execution of the job. That is, the image forming apparatus 1 forms images on the sheets P5 and P6 in sequence and normally discharges them to the first discharge outlet 51. At this time, the image formed on the sheet P5 is the image that was to be printed on the sheet P4. Therefore, if the sheets P5 and P6 following the sheet P4 in which abnormal transport was detected are of the same type as the sheet P4, the image forming apparatus 1 can reduce the sheets to be abnormally discharged to the second discharge outlet 52 to only the single sheet P4.

[0091] On the other hand, the succeeding sheet determination unit 76 may determine that the sheets P5 and P6 following the sheet P4 in which abnormal conveyance was detected are not the same type as the sheet P4. In this case, the discharge example of the sheets P is the same as the discharge example shown in Fig. 7. Note that when the control unit 6 determines that the sheets P5 and P6 following the sheet P4 include one sheet that is not the same type as the sheet P4, the control unit 6 may discharge the one sheet to the second discharge outlet 52.

[0092] Furthermore, when a double-sided printing job is being executed in the image forming apparatus 1, the preceding sheet P may be transported through the reverse path 31 at the time when abnormal transport is detected. The discharge control determination unit 75 includes a switching determination unit 78. The switching determination unit 78 functions when abnormal transport of the sheet P is detected while a double-sided printing job is being executed. When discharging a sheet to the second discharge outlet 52, the switching determination unit 78 performs a switching determination as to whether to discharge the sheet without passing through the reverse path 31 or to discharge the sheet through the reverse path 31.

[0093] FIG. 11 is a diagram showing an example of the conveying positions of a plurality of sheets P1 to P6 when a double-sided printing job is being executed in the image forming apparatus 1. In the example shown in FIG. 11, the first sheet P1 has already passed the image forming position X via the inversion path 31. Therefore, the first sheet P1 has images formed on both the first and second sides. The second sheet P2 and the third sheet P3 are in a state of being conveyed on the inversion path 31 after an image is formed on the first side. The fourth sheet P4 has passed the position of the sheet detection sensor 60 and is in a state of being directed toward the image forming position X for image formation on the first side. The fifth sheet P5 and the sixth sheet P6 are in a state of being conveyed on the conveying path 30 inside the paper feed unit 2. For example, the second sheet P2 returns to the original conveying path 30 between the sheets P4 and P5. Also, the third sheet P3 returns to the original conveying path 30 between the sheets P5 and P6.

[0094] In the above-mentioned state, it is assumed that the sheet detection sensor 60 detects abnormal conveyance of the sheet P4. When the abnormal conveyance of the sheet P4 is detected, the control unit 6 discharges the sheet P4 to the second discharge outlet 52 as described above. At this time, the switching determination unit 78 selects, as a discharge route to the second discharge outlet 52, either a route that passes through the reverse path 31 or a route that does not pass through the reverse path 31. For example, when the leading edge of the sheet P to be discharged to the second discharge outlet 52 has already reached the switching gate 43 and the switching gate 43 is guiding the leading edge of the sheet P to the reverse path 31, the switching determination unit 78 selects the route that passes through the reverse path 31. On the other hand, when the leading edge of the sheet P to be discharged to the second discharge outlet 52 has not yet reached the switching gate 43 and can be discharged without passing through the reverse path 31, the switching determination unit 78 selects the route that does not pass through the reverse path 31.

[0095] 11, when abnormal conveyance of the sheet P4 is detected, the sheet P4 has not yet reached the image forming position X. Therefore, the switching determination unit 78 selects a route that does not pass through the reverse path 31 as the discharge route for the sheet P4.

[0096] The discharge control determination unit 75 also determines discharge control for sheets P2, P5, P3, and P6 following sheet P4. That is, the discharge control determination unit 75 determines whether or not to discharge each of sheets P2, P5, P3, and P6 to the second discharge outlet 52. At this time, the switching determination unit 78 selects, as a route for discharging the sheet determined to be discharged to the second discharge outlet 52 to the second discharge outlet 52, either a route that passes through the reverse path 31 or a route that does not pass through the reverse path 31.

[0097] Fig. 12 is a flowchart showing a process for determining whether or not to pass through the reversing path 31 during double-sided printing. Steps S27 to S29 shown in Fig. 12 are processes inserted between steps S17 and S18 in the flowchart shown in Fig. 6, for example. Note that the flowchart will be described below by taking as an example a case where abnormal conveyance of sheet P4 is detected in the state shown in Fig. 11.

[0098] During execution of a double-sided printing job, the control unit 6 discharges the sheet P4 for which abnormal transport has been detected to the second discharge outlet 52 (step S16 in FIG. 6). After that, the control unit 6 determines whether or not a sheet following the sheet P4 has already been fed (step S17 in FIG. 6). At this time, the control unit 6 employs one of the following two criteria as a criterion for determining whether or not a sheet is a sheet following the sheet P4.

[0099] The first criterion is a criterion for determining that all sheets passing through the image forming position X after the sheet P4 has passed through the image forming position X are the succeeding sheets. For example, in the state shown in Fig. 11, after the sheet P4 has passed through the image forming position X, the sheets P2, P5, P3, and P6 pass through the image forming position X in order. Therefore, when the first criterion is adopted, the control unit 6 determines that the sheets P2, P5, P3, and P6 are the sheets succeeding the sheet P4.

[0100] The second criterion is a criterion for determining that a sheet fed from the paper feed units 11 and 12 after the sheet P4 is fed is a succeeding sheet. For example, in the state shown in Fig. 11, the sheets fed from the paper feed units 11 and 12 after the sheet P4 is fed are sheets P5 and P6. Therefore, when the second criterion is adopted, the control unit 6 determines that the sheets P5 and P6 are succeeding the sheet P4.

[0101] If a sheet following sheet P4 has already been fed (YES in step S17), control unit 6 determines whether or not the following sheet can be discharged to second discharge outlet 52 without passing through reverse path 31 (step S27). At this time, if there are multiple sheets following sheet P4, control unit 6 may determine whether or not each of the multiple sheets can be discharged without passing through reverse path 31.

[0102] When the sheet can be discharged to the second discharge outlet 52 without passing through the reversing path 31 (YES in step S27), the control unit 6 sets the conveying path 30 that does not pass through the reversing path 31 as a route for discharging to the second discharge outlet 52 (step S28). When the sheet cannot be discharged to the second discharge outlet 52 without passing through the reversing path 31 (NO in step S27), the control unit 6 sets the conveying path 30 that passes through the reversing path 31 as a route for discharging to the second discharge outlet 52 (step S29). The control unit 6 may set such a discharge route for each sheet following the sheet P4.

[0103] Therefore, when the image forming apparatus 1 detects that the sheet P4 is being conveyed abnormally while executing a double-sided printing job, the image forming apparatus 1 selects, as the discharge route for the sheet following the sheet P4, either a route that passes through the reversing path 31 or a route that does not pass through the reversing path 31. This allows the image forming apparatus 1 to discharge the sheet following the sheet P4 to the second discharge outlet 52 without causing a jam in the conveying path 30.

[0104] Furthermore, even when discharging the sheet P4, for which abnormal transport has been detected, to the second discharge outlet 52, the control unit 6 preferably determines whether the sheet P4 can be discharged to the second discharge outlet 52 without passing through the reversing path 31. Based on the determination result, the control unit 6 determines the discharge route for the sheet P4 and discharges it to the second discharge outlet 52.

[0105] Fig. 13 is a diagram showing an example of ejection of sheet P when abnormal conveyance of sheet P4 is detected during execution of a double-sided printing job. For example, assume that when six sheets P1 to P6 are being conveyed as shown in Fig. 11, the sheet detection sensor 60 detects abnormal conveyance of sheet P4. In this case, there are three examples of ejection of sheet P, as shown in Figs. 13(a), (b), and (c).

[0106] FIG. 13(a) shows a first discharge example. In this first discharge example, the above-mentioned first judgment criterion is adopted. In the case of FIG. 13(a), the image forming apparatus 1 forms images (images 1 and 2) on both sides of the sheet P1, which is conveyed before the sheet P4, for which abnormal conveyance has been detected, and normally discharges the sheet P1 to the first discharge outlet 51. Also, the image forming apparatus 1 abnormally discharges the sheet P4, for which abnormal conveyance has been detected, to the second discharge outlet 52. In the state shown in FIG. 11, the sheet P4 has not reached the image forming position X. Therefore, the image forming apparatus 1 does not form an image on the sheet P4. Also, the image forming apparatus 1 conveys the sheet P4 without passing through the reversing path 31, and discharges it to the second discharge outlet 52. Also, the image forming apparatus 1 conveys the sheets P2, P5, P3, and P6 following the sheet P4 to the second discharge outlet 52 in order. In the state shown in FIG. 11, the sheets P2 and P3 have already been conveyed through the reversing path 31. Therefore, the image forming apparatus 1 guides the sheets P2 and P3 to the second discharge outlet 52 via the reversing path 31. The image forming apparatus 1 also guides the sheets P5 and P6 to the second discharge outlet 52 without passing through the reversing path 31.

[0107] Sheet P2 guided to the second discharge outlet 52 has an image (image 3) already formed on a first side. When image forming apparatus 1 transports sheet P2 to the second discharge outlet 52, it does not form an image on the second side of sheet P2. Similarly, sheet P3 guided to the second discharge outlet 52 has an image (image 5) already formed on a first side. When image forming apparatus 1 transports sheet P3 to the second discharge outlet 52, it does not form an image on the second side of sheet P3.

[0108] Thereafter, the image forming apparatus 1 feeds the sheet P7, and forms images (images 3 and 4) on both the first and second sides of the sheet P7, provided that no abnormal conveyance of the sheet P7 is detected. Then, the image forming apparatus 1 conveys the sheet P7 to the first discharge port 51 in the normal manner.

[0109] Next, FIG. 13(b) shows a second discharge example. In this second discharge example, the above-mentioned second judgment criterion is adopted. In the case of FIG. 13(b), the image forming apparatus 1 forms images (images 1 and 2) on both sides of the sheet P1, which is conveyed before the sheet P4, for which abnormal conveyance has been detected, and normally discharges the sheet P1 to the first discharge outlet 51. In addition, the image forming apparatus 1 abnormally discharges the sheet P4, for which abnormal conveyance has been detected, to the second discharge outlet 52. In the state shown in FIG. 11, the sheet P4 has not reached the image forming position X. Therefore, the image forming apparatus 1 does not form an image on the sheet P4. In addition, the image forming apparatus 1 conveys the sheet P4 without passing through the reversing path 31, and discharges the sheet P4 to the second discharge outlet 52.

[0110] The image forming apparatus 1 forms an image (image 4) on the second side of the sheet P2, which is transported to the image forming position X following the sheet P4. As a result, images (images 3 and 4) are formed on both the first and second sides of the sheet P2. Then, the image forming apparatus 1 discharges the sheet P2 to the first discharge outlet 51. Next, the image forming apparatus 1 discharges the sheet P5, which is transported to the image forming position X following the sheet P2, to the second discharge outlet 52. In the state shown in FIG. 11, the sheet P5 has not yet reached the image forming position X. Therefore, the image forming apparatus 1 does not form an image on the sheet P5. Moreover, the image forming apparatus 1 transports the sheet P5 without passing through the reversing path 31, and discharges it to the second discharge outlet 52.

[0111] The image forming apparatus 1 forms an image (image 6) on the second side of the sheet P3, which is transported to the image forming position X following the sheet P5. As a result, images (images 5 and 6) are formed on both the first and second sides of the sheet P3. Then, the image forming apparatus 1 discharges the sheet P3 to the first discharge outlet 51. Next, the image forming apparatus 1 discharges the sheet P6, which is transported to the image forming position X following the sheet P3, to the second discharge outlet 52. In the state shown in FIG. 11, the sheet P6 has not yet reached the image forming position X. Therefore, the image forming apparatus 1 does not form an image on the sheet P6. Moreover, the image forming apparatus 1 transports the sheet P6 without passing through the reversing path 31, and discharges it to the second discharge outlet 52.

[0112] Thereafter, the image forming apparatus 1 feeds the sheet P7, and forms images (images 7 and 8) on both the first and second sides of the sheet P7, provided that no abnormal conveyance of the sheet P7 is detected. Then, the image forming apparatus 1 normally conveys the sheet P7 to the first discharge port 51.

[0113] Next, FIG. 13(c) shows a third discharge example. In this third discharge example, the above-mentioned second judgment criterion is adopted. In addition, in the third discharge example, it is judged whether the sheets P5 and P6 following the sheet P4 are the same type as the sheet P4, and the discharge control is decided. In the case of FIG. 13(c), the image forming apparatus 1 forms images (images 1 and 2) on both sides of the sheet P1, which is conveyed before the sheet P4 in which the abnormal conveyance is detected, and normally discharges the sheet P1 to the first discharge outlet 51. In addition, the image forming apparatus 1 abnormally discharges the sheet P4 in which the abnormal conveyance is detected to the second discharge outlet 52. In the state shown in FIG. 11, the sheet P4 has not reached the image forming position X. Therefore, the image forming apparatus 1 does not form an image on the sheet P4. In addition, the image forming apparatus 1 conveys the sheet P4 without passing through the reversing path 31, and discharges the sheet P4 to the second discharge outlet 52.

[0114] For example, if sheets P5 and P6 are of the same type as sheet P4, image forming apparatus 1 discharges only sheet P4 to second discharge outlet 52. In addition, image forming apparatus 1 determines to perform image formation on sheets P5 and P6.

[0115] Image forming apparatus 1 forms an image (image 4) on the second side of sheet P2, which is transported to image forming position X following sheet P4. As a result, images (images 3 and 4) are formed on both the first and second sides of sheet P2. Then, image forming apparatus 1 discharges sheet P2 to first discharge outlet 51. Image forming apparatus 1 forms an image (image 7) on the first side of sheet P5, which is transported to image forming position X following sheet P2, and guides sheet P5 to reversal path 31.

[0116] The image forming apparatus 1 forms an image (image 6) on the second surface of the sheet P3, which is conveyed to the image forming position X following the sheet P5. As a result, images (images 5 and 6) are formed on both the first and second surfaces of the sheet P3. Then, the image forming apparatus 1 discharges the sheet P3 to the first discharge port 51. The image forming apparatus 1 forms an image (image 9) on the first surface of the sheet P6, which is conveyed to the image forming position X following the sheet P3, and guides the sheet P6 to the reversal path 31. Then, the image forming apparatus 1 forms an image (image 6) on the second surface of the sheet P5. Also, the image forming apparatus 1 forms an image (image 10) on the second surface of the sheet P6. Then, the image forming apparatus 1 discharges the sheets P1, P2, P3, P5, and P6 in order to the first discharge port 51.

[0117] 13C, the number of sheets P discharged to the second discharge outlet 52 can be reduced compared to the first and second discharge examples. Therefore, the image forming apparatus 1 can efficiently execute a double-sided print job.

[0118] In the above, an example has been described in which the sheets are discharged to the second discharge outlet 52 without passing through the reversing path 31 as much as possible. However, this is not limited to this. For example, during double-sided printing, when a sheet for which abnormal transport has been detected and the following sheets are discharged to the second discharge outlet 52, those sheets may be guided to the reversing path 31, passed through the reversing path 31, and then discharged to the second discharge outlet 52.

[0119] Incidentally, when the image forming apparatus 1 conveys a sheet for which abnormal conveyance has been detected toward the second paper discharge port 52, a sheet jam may occur in the middle of the conveyance path 30. Whether or not a sheet jam occurs depends on the degree of the abnormal conveyance. When a sheet jam occurs, it takes time to remove the sheet from the conveyance path 30. Rather, it is easier to remove the sheet if the job is stopped before the sheet becomes jammed. Therefore, when the image forming apparatus 1 detects abnormal conveyance of a sheet, it judges the degree of the abnormal conveyance. If the degree of abnormal conveyance is greater than a predetermined value, it is preferable for the image forming apparatus 1 to stop the execution of the job. This is because the conveyance of the sheet stops when the execution of the job is stopped. Hereinafter, several such examples will be described.

[0120] 14 is a flowchart showing an example of a processing procedure when a double feed of a plurality of sheets is detected as an abnormal conveyance. Steps S30 to S33 shown in FIG. 14 are processes inserted between steps S13 and S14 in the flowchart shown in FIG.

[0121] When the control unit 6 detects abnormal transport of the sheet P during execution of the job (YES in step S13), it determines whether or not a multi-feed state has been detected by the multi-feed detection sensor 61 (step S30). If the detected abnormal transport is not a multi-feed state (NO in step S30), the process by the control unit 6 proceeds to step S14 in Fig. 6. On the other hand, if the detected abnormal transport is a multi-feed state (YES in step S30), the control unit 6 detects the amount of accompanying transport of multiple sheets (step S31).

[0122] For example, when the abnormal conveyance detection unit 74 detects a double feed state based on the detection result of the double feed detection sensor 61, it counts the time during which the timing detection sensor 64 maintains the ON state. As described above, when the timing detection sensor 64 detects the leading edge of a sheet and turns ON, it maintains the ON state until the trailing edge of the sheet passes. In a double feed state in which multiple sheets are simultaneously conveyed, the timing detection sensor 64 turns ON while the multiple sheets being simultaneously conveyed pass. For example, when two sheets are conveyed in a state where they are shifted in the sheet conveyance direction, the time for the two sheets to pass the detection position by the timing detection sensor 64 becomes longer. As a result, the time for which the timing detection sensor 64 is ON also becomes longer. The abnormal conveyance detection unit 74 detects the amount of early feed in the double feed state by counting the time of the ON state. That is, the amount of early feed corresponds to the length of the sheets in the sheet conveyance direction in a state in which multiple sheets are conveyed in a row.

[0123] When the control unit 6 detects the amount of early feed of a plurality of sheets, it judges whether the amount of early feed is equal to or greater than a predetermined threshold V (step S32). If the amount of early feed is equal to or greater than the threshold V, the degree of abnormal conveyance is greater than a predetermined value. In that case, if the sheet for which abnormal conveyance has been detected is conveyed along the conveyance path 30, there is a high possibility that a jam will occur. Therefore, if the amount of early feed is equal to or greater than the threshold V (YES in step S32), the control unit 6 stops execution of the job (step S33). This stops the conveyance of the sheet for which abnormal conveyance has been detected. Therefore, the image forming apparatus 1 can stop execution of the job before a jam occurs.

[0124] Next, Fig. 15 is a flowchart showing a process for determining whether or not to pass through the reversing path 31 during double-sided printing depending on the amount of early feed. Steps S35 to S41 shown in Fig. 15 are processes inserted between steps S17 and S18 in the flowchart shown in Fig. 6, for example. In the following, a case where the abnormal conveyance of the sheet is a double feed state will be described.

[0125] When the control unit 6 determines that the sheet following the sheet for which abnormal transport has been detected has already been fed (YES in step S17), it detects the amount of accompanying feeding of the abnormally transported sheet (step S35). Next, the control unit 6 determines whether or not the sheet following the sheet for which abnormal transport has been detected can be discharged to the second discharge outlet 52 without passing through the reversing path 31 (step S36).

[0126] When it is impossible to discharge the succeeding sheet to the second discharge outlet 52 without passing through the reversing path 31 (NO in step S36), the control unit 6 judges whether the amount of early feed detected in step S35 is equal to or greater than a predetermined threshold value V1 (step S37). When the sheets in the multi-feed state are guided to the reversing path 31, the overlapping sheets may become disintegrated, and a jam is likely to occur. For this reason, the threshold value V1 is preset to a relatively small value. When the amount of early feed is equal to or greater than the threshold value V1 (YES in step S37), the control unit 6 stops the execution of the job (step S39). This allows the image forming apparatus 1 to prevent a jam from occurring. On the other hand, when the amount of early feed is less than the threshold value V1 (NO in step S37), the control unit 6 sets the conveying path 30 to pass through the reversing path 31 as a route for discharging the succeeding sheet to the second discharge outlet 52 (step S38).

[0127] On the other hand, if the succeeding sheet can be discharged to the second discharge port 52 without passing through the reversing path 31 (YES in step S36), the control unit 6 judges whether the amount of early feed detected in step S35 is equal to or greater than a predetermined threshold V2 (step S40). The threshold V2 in this case is a value greater than the threshold V1 described above. When the sheet is conveyed to the second discharge port 52 without passing through the reversing path 31, the possibility that multiple sheets will become scattered is low compared to when the sheet is guided to the reversing path 31. However, when the amount of early feed is considerably large, multiple sheets may become scattered in the conveying path 30. Therefore, the threshold V2 is preset as a value greater than the threshold V1. When the amount of early feed is equal to or greater than the threshold V2 (YES in step S40), the control unit 6 stops the execution of the job (step S39). This allows the image forming apparatus 1 to prevent jamming from occurring. On the other hand, if the amount of early feed is less than the threshold value V2 (NO in step S40), the control unit 6 sets the conveying path 30 that does not pass through the reversing path 31 as a route for discharging the subsequent sheet to the second discharge outlet 52 (step S41).

[0128] Therefore, when the image forming apparatus 1 detects that the sheets are in a multi-feed state while executing a double-sided printing job, it is possible to stop the execution of the job according to the amount of the multi-feed.

[0129] Next, FIGS. 16 and 17 are flowcharts showing a processing procedure for stopping a job due to a factor other than the early feed amount.

[0130] First, Fig. 16(a) is a flowchart for determining whether or not to stop a job depending on the thickness of a sheet. Steps S50 to S53 shown in Fig. 16(a) are processes that are inserted between steps S13 and S14 in the flowchart shown in Fig. 6, for example.

[0131] When the control unit 6 detects abnormal conveyance of a sheet during execution of a job (YES in step S13), it determines whether or not a double feed state has been detected by the double feed detection sensor 61 (step S50). If the detected abnormal conveyance is not a double feed state (NO in step S30), the process by the control unit 6 proceeds to step S14 in FIG.

[0132] On the other hand, if the detected abnormal conveyance is a double feed state (YES in step S50), the control unit 6 detects the thickness of the sheet (step S51). For example, the abnormal conveyance detection unit 74 detects the thickness of the sheet based on the detection result of the double feed detection sensor 61. The control unit 6 judges whether the thickness of the sheet detected by the abnormal conveyance detection unit 74 is outside a predetermined range or not (step S52). For example, if the sheet is thin paper whose thickness does not meet the predetermined range, a jam is likely to occur if the sheet is continued to be conveyed. Also, if the sheet is thick paper whose thickness exceeds the predetermined range, a jam is likely to occur. Therefore, if the sheet thickness is outside the predetermined range (YES in step S52), the control unit 6 stops the execution of the job (step S53). This allows the image forming apparatus 1 to stop the execution of the job before a jam occurs.

[0133] If the thickness of the sheet is within the predetermined range (NO in step S52), the process by the control unit 6 proceeds to step S14 in FIG. 6. In this case, the sheet for which abnormal transport has been detected is transported through the transport path 30 and discharged to the second discharge port 52. Then, the image forming apparatus 1 continues executing the job. Note that the process shown in FIG. 16(a) may be executed in combination with other processes described in this embodiment.

[0134] Fig. 16(b) is a flowchart for determining whether or not to stop a job depending on the inclination of a sheet. Steps S55 to S58 shown in Fig. 16(b) are processes that are inserted between steps S13 and S14 in the flowchart shown in Fig. 6, for example.

[0135] When the control unit 6 detects abnormal conveyance of the sheet during execution of the job (YES in step S13), it determines whether or not skew has been detected by the skew detection sensor 62 (step S55). If the detected abnormal conveyance is not skew (NO in step S55), the process by the control unit 6 proceeds to step S14 in FIG.

[0136] On the other hand, if the detected abnormal conveyance is skew (YES in step S55), the control unit 6 detects the skew of the sheet (step S56). For example, the abnormal conveyance detection unit 74 detects the skew of the sheet based on the detection result of the skew detection sensor 62. The control unit 6 judges whether the skew of the sheet detected by the abnormal conveyance detection unit 74 is equal to or greater than a predetermined amount (step S57). For example, if the skew of the sheet is equal to or greater than the predetermined amount, a jam is likely to occur if the conveyance of the sheet is continued. Therefore, if the skew of the sheet is equal to or greater than the predetermined amount (YES in step S57), the control unit 6 stops the execution of the job (step S58). This allows the image forming apparatus 1 to stop the execution of the job before a jam occurs.

[0137] Furthermore, if the skew of the sheet is less than the predetermined amount (NO in step S57), the process by the control unit 6 proceeds to step S14 in FIG. 6. In this case, the sheet for which abnormal transport has been detected is transported through the transport path 30 and discharged to the second discharge port 52. Then, the image forming apparatus 1 continues executing the job. Note that the process shown in FIG. 16(b) may be executed in combination with other processes described in this embodiment.

[0138] Fig. 16(c) is a flowchart for determining whether or not to stop a job depending on the sheet size. Steps S60 to S63 shown in Fig. 16(c) are processes that are inserted between steps S13 and S14 in the flowchart shown in Fig. 6, for example.

[0139] When the control unit 6 detects abnormal conveyance of the sheet during execution of the job (YES in step S13), it determines whether or not skew has been detected by the skew detection sensor 62 (step S60). If the detected abnormal conveyance is not skew (NO in step S60), the process by the control unit 6 proceeds to step S14 in FIG.

[0140] On the other hand, if the detected abnormal conveyance is skew (YES in step S60), the control unit 6 detects the sheet size (step S61). For example, the abnormal conveyance detection unit 74 detects the sheet length in the sheet conveyance direction based on the detection result of the timing detection sensor 64. The control unit 6 determines whether the sheet size (sheet length) detected by the abnormal conveyance detection unit 74 is outside a predetermined range (step S62). For example, if the sheet size is beyond the predetermined range, a jam is likely to occur if the sheet conveyance is continued. Therefore, if the sheet size is outside the predetermined range (YES in step S62), the control unit 6 stops the execution of the job (step S63). This allows the image forming apparatus 1 to stop the execution of the job before a jam occurs.

[0141] If the sheet size is within the predetermined range (NO in step S62), the process by the control unit 6 proceeds to step S14 in FIG. 6. In this case, the sheet for which abnormal transport has been detected is transported through the transport path 30 and discharged to the second discharge port 52. Then, the image forming apparatus 1 continues executing the job. Note that the process shown in FIG. 16(c) may be executed in combination with other processes described in this embodiment.

[0142] The abnormal transport detection unit 74 is not limited to detecting the sheet length based on the detection result of the timing detection sensor 64. For example, the above-mentioned sheet information 82 includes sheet size information. Therefore, when the detected abnormal transport is skew (YES in step S60), the abnormal transport detection unit 74 may determine whether the sheet size is outside a predetermined range based on the sheet information 82 in step S62.

[0143] Fig. 17(a) is a flowchart for determining whether or not to stop a job depending on the sheet width. Steps S65 to S68 shown in Fig. 17(a) are processes that are inserted between steps S13 and S14 in the flowchart shown in Fig. 6, for example.

[0144] When the control unit 6 detects abnormal conveyance of the sheet during execution of the job (YES in step S13), it determines whether or not skew has been detected by the skew detection sensor 62 (step S65). If the detected abnormal conveyance is not skew (NO in step S65), the process by the control unit 6 proceeds to step S14 in FIG.

[0145] On the other hand, if the detected abnormal conveyance is skew (YES in step S65), the control unit 6 detects the sheet width (step S66). For example, the abnormal conveyance detection unit 74 detects the sheet width in a direction perpendicular to the sheet conveyance direction based on the detection result of the deviation detection sensor 63. The control unit 6 judges whether the sheet width detected by the abnormal conveyance detection unit 74 is equal to or larger than a predetermined width (step S67). For example, if the sheet width is equal to or larger than the predetermined width, a jam is likely to occur if the sheet conveyance is continued. Therefore, if the sheet width is equal to or larger than the predetermined width (YES in step S67), the control unit 6 stops the execution of the job (step S68). This allows the image forming apparatus 1 to stop the execution of the job before a jam occurs.

[0146] Furthermore, if the sheet width is less than the predetermined width (NO in step S67), the process by the control unit 6 proceeds to step S14 in FIG. 6. In this case, the sheet for which abnormal transport has been detected is transported through the transport path 30 and discharged to the second discharge port 52. Then, the image forming apparatus 1 continues executing the job. Note that the process shown in FIG. 17(a) may be executed in combination with other processes described in this embodiment.

[0147] Fig. 17(b) is a flowchart for determining whether or not to stop a job depending on the sheet width. Steps S70 to S73 shown in Fig. 17(b) are processes that are inserted between steps S13 and S14 in the flowchart shown in Fig. 6, for example.

[0148] When the control unit 6 detects abnormal conveyance of the sheet during execution of the job (YES in step S13), it determines whether or not the misalignment of the sheet is detected by the misalignment detection sensor 63 (step S70). If the detected abnormal conveyance is not misalignment of the sheet (NO in step S70), the process by the control unit 6 proceeds to step S14 in FIG.

[0149] On the other hand, if the detected abnormal conveyance is sheet misalignment (YES in step S70), the control unit 6 detects the amount of misalignment of the sheet (step S71). For example, the abnormal conveyance detection unit 74 detects the amount of misalignment of the sheet in the direction perpendicular to the sheet conveying direction based on the detection result of the misalignment detection sensor 63. The control unit 6 judges whether the amount of misalignment of the sheet detected by the abnormal conveyance detection unit 74 is equal to or greater than a predetermined value (step S72). For example, if the amount of misalignment of the sheet is equal to or greater than the predetermined value, a jam is likely to occur if the sheet is continued to be conveyed. Therefore, if the amount of misalignment of the sheet is equal to or greater than the predetermined value (YES in step S72), the control unit 6 stops the execution of the job (step S73). This allows the image forming apparatus 1 to stop the execution of the job before a jam occurs.

[0150] Furthermore, if the amount of deviation of the sheet is less than the predetermined value (NO in step S72), the process by the control unit 6 proceeds to step S14 in FIG. 6. In this case, the sheet for which abnormal transport has been detected is transported through the transport path 30 and discharged to the second discharge port 52. Then, the image forming apparatus 1 continues executing the job. Note that the process shown in FIG. 17(b) may be executed in combination with other processes described in this embodiment.

[0151] As described above, the image forming apparatus 1 of the present embodiment discharges a sheet to the first discharge outlet 51 when abnormal sheet conveyance is not detected during execution of a job. In addition, when abnormal sheet conveyance is detected, the image forming apparatus 1 is configured to be able to discharge the sheet for which abnormal conveyance has been detected and all subsequent sheets that have already been fed to the second discharge outlet 52 different from the first discharge outlet 51. Here, the first discharge outlet 51 and the second discharge outlet 52 are standard discharge outlets mounted on the discharge unit 4. Therefore, the image forming apparatus 1 does not need to provide a dedicated conveyance path for discharging the sheet for which abnormal conveyance has been detected to the second discharge outlet 52. In addition, the image forming apparatus 1 discharges the sheet for which abnormal conveyance has been detected and all subsequent sheets that have already been fed to the second discharge outlet 52. Therefore, the image forming apparatus 1 does not form an incorrect image on the sheet during execution of a job.

[0152] The image forming apparatus 1 of the present embodiment also includes a succeeding sheet determination unit 76. When abnormal conveyance of a sheet is detected, the succeeding sheet determination unit 76 determines whether all sheets that have already been fed and that follow the sheet in which abnormal conveyance is detected are the same type as the sheet in which abnormal conveyance is detected. When all sheets following the sheet in which abnormal conveyance is detected are the same type as the sheet in which abnormal conveyance is detected, the image forming apparatus 1 discharges only the sheet in which abnormal conveyance is detected to the second discharge outlet 52, and discharges the sheets following the sheet in which abnormal conveyance is detected to the first discharge outlet 51 after an image is formed on them. Therefore, when abnormal conveyance is detected during the execution of a job, the image forming apparatus 1 can minimize the number of sheets discharged to the second discharge outlet 52.

[0153] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible.

[0154] For example, in the above embodiment, the image forming unit 20 is configured to form an image on a sheet by an electrophotographic method. However, the image forming unit 20 is not limited to the electrophotographic method. For example, the image forming unit 20 may form an image on a sheet by an inkjet method.

[0155] In the above embodiment, an example has been described in which the program 72 executed by the CPU 70 of the control unit 6 is stored in advance in the memory 71. However, the program 72 is not necessarily limited to being stored in advance in the memory 71. In other words, the program 72 itself can be a subject of trading. Therefore, the program 72 includes both a program provided in a form that can be downloaded via a network such as the Internet and a program provided in a form that is recorded on a computer-readable recording medium such as a CD-ROM. [Explanation of symbols]

[0156] 1. Image forming device 2 Paper feed unit 3 Image forming unit 4 Paper ejection unit 6. Control Unit 11,12 Paper feed section 20 Image forming section 30 Transport Path 31 Reversal Road 51 First paper exit 52 2nd paper exit 72 Programs 73 Job Control Section 74 Abnormal transport detection unit 75 Paper discharge control decision unit 76 Subsequent Sheet Judgment Section 77 Paper ejection control section 78 Switching decision unit

Claims

1. a plurality of paper feed units that feed sheets toward a conveyance path; an image forming unit that forms an image on the sheet transported along the transport path; an abnormal conveyance detection unit that detects abnormal conveyance of a sheet; a plurality of paper discharge ports provided at the end of the transport path; a control unit that, when the abnormal transport detection unit does not detect abnormal transport, discharges the sheet to a first discharge outlet of the plurality of discharge outlets, and, when the abnormal transport detection unit detects abnormal transport, discharges the sheet for which abnormal transport has been detected and all subsequent sheets that have already been fed to a second discharge outlet of the plurality of discharge outlets; An image forming apparatus comprising:

2. a subsequent sheet determination unit that, when the abnormal conveyance detection unit detects abnormal conveyance, determines whether all sheets that have already been fed and that follow the sheet in which the abnormal conveyance has been detected are of the same type as the sheet in which the abnormal conveyance has been detected; Further comprising: The image forming apparatus according to claim 1, characterized in that, when all sheets following the sheet in which abnormal transport is detected are of the same type as the sheet in which abnormal transport is detected, the control unit discharges only the sheet in which abnormal transport is detected to the second paper discharge outlet, and discharges the sheets following the sheet in which abnormal transport is detected to the first paper discharge outlet after an image is formed on them.

3. the conveying path has a junction at which a plurality of conveying paths join together, the junction being located upstream of the image forming unit; 3. The image forming apparatus according to claim 1, wherein the abnormal transport detection unit is provided between the junction and the image forming unit.

4. 3. The image forming apparatus according to claim 2, wherein the subsequent sheet determination unit determines that all sheets following the sheet in which abnormal transport is detected are of the same type as the sheet in which abnormal transport is detected when all sheets following the sheet in which abnormal transport is detected are fed from the same paper feed unit as the sheet in which abnormal transport is detected.

5. a storage unit that stores sheet information of sheets accommodated in each of the plurality of paper feed units; Further comprising: The image forming apparatus according to claim 2, characterized in that the subsequent sheet determination unit reads out sheet information of the sheet following the sheet in which abnormal transport was detected from the memory unit, and determines whether all sheets following the sheet in which abnormal transport was detected are of the same type as the sheet in which abnormal transport was detected.

6. 3. The image forming apparatus according to claim 1, wherein the control unit prevents the image forming unit from forming an image on at least a sheet for which abnormal conveyance has been detected by the abnormal conveyance detection unit.

7. The image forming apparatus according to claim 1 or 2, characterized in that, when the abnormal transport detection unit detects abnormal transport, the control unit ejects the sheet for which abnormal transport has been detected and all subsequent sheets that have already been fed to the second paper ejection outlet, and then resumes image formation.

8. the conveying path includes a reversing path that, after an image is formed on a first surface of the sheet, reverses the sheet and conveys it again to an image forming position by the image forming unit, The image forming apparatus according to claim 1 or 2, characterized in that when abnormal transport is detected by the abnormal transport detection unit during double-sided printing in which a sheet is transported to the reversing path, the control unit does not transport the sheet for which abnormal transport has been detected and any subsequent sheets that have already been fed to the reversing path.

9. a switching determination unit that determines whether or not the sheet in which abnormal conveyance has been detected and the subsequent sheet that has already been fed can be discharged to the second sheet discharge outlet without being conveyed to the reverse path; Further comprising: The image forming apparatus according to claim 8, characterized in that when the switching judgment unit determines that it is impossible to discharge the sheet to the second discharge outlet without transporting the sheet to the reverse path, the control unit transports the sheet in which abnormal transport has been detected and any subsequent sheets that have already been fed to the reverse path and then discharges them to the second discharge outlet.

10. 3. The image forming apparatus according to claim 1, wherein the abnormal transport detection unit detects a double feed state in which a plurality of sheets are simultaneously transported as the abnormal transport.

11. The image forming apparatus according to claim 10, wherein the control unit detects an amount of early feed of multiple sheets when the abnormal transport detection unit detects a double feed state, and stops transport of the sheets when the amount of early feed is equal to or greater than a predetermined threshold value.

12. 3. The image forming apparatus according to claim 1, wherein the control unit stops conveyance of the sheet when the thickness of the sheet in which abnormal conveyance is detected is outside a predetermined range.

13. 3. The image forming apparatus according to claim 1, wherein the abnormal transport detection unit detects skew of the sheet as the abnormal transport.

14. 14. The image forming apparatus according to claim 13, wherein the control unit stops conveyance of the sheet when the sheet in which abnormal conveyance has been detected is conveyed at an angle of a predetermined amount or more with respect to the sheet conveyance direction.

15. 15. The image forming apparatus according to claim 14, wherein the control unit detects a size of the sheet in which abnormal conveyance has been detected, and stops conveyance of the sheet when the size of the sheet is outside a predetermined range.

16. 15. The image forming apparatus according to claim 14, wherein the control unit detects an amount of deviation of the sheet in the conveying path in which abnormal conveyance has been detected, and stops conveying the sheet when the amount of deviation of the sheet is equal to or greater than a predetermined value.

17. the conveying path includes a reversing path that, after an image is formed on a first surface of the sheet, reverses the sheet and conveys it again to an image forming position by the image forming unit, The image forming apparatus according to claim 1 or 2, characterized in that when the abnormal transport detection unit detects abnormal transport during double-sided printing in which a sheet is transported to the reversal path, the control unit transports the sheet to the reversal path and then discharges it from the second paper output outlet.

18. a plurality of paper feed units that feed sheets toward a conveyance path; an image forming unit that forms an image on the sheet transported along the transport path; an abnormal conveyance detection unit that detects abnormal conveyance of a sheet; a plurality of paper discharge ports provided at the end of the transport path; A control method for an image forming apparatus comprising: a first step of discharging the sheet to a first discharge outlet of the plurality of discharge outlets when the abnormal conveyance detection unit does not detect abnormal conveyance; a second step of discharging the sheet for which abnormal conveyance has been detected and all subsequent sheets that have already been fed to a second discharge outlet of the plurality of discharge outlets when the abnormal conveyance detection unit detects abnormal conveyance; A control method comprising the steps of:

19. The second step comprises: a third step of determining whether or not all sheets that have already been fed and that follow the sheet in which abnormal conveyance has been detected are of the same type as the sheet in which abnormal conveyance has been detected, when the abnormal conveyance detection unit detects abnormal conveyance; a fourth step of discharging only the sheet in which abnormal conveyance has been detected to the second discharge outlet when all sheets following the sheet in which abnormal conveyance has been detected are of the same type as the sheet in which abnormal conveyance has been detected, and discharging the sheets following the sheet in which abnormal conveyance has been detected to the first discharge outlet after an image has been formed thereon; 20. The method of claim 18, further comprising:

20. a plurality of paper feed units that feed sheets toward a conveyance path; an image forming unit that forms an image on the sheet transported along the transport path; an abnormal conveyance detection unit that detects abnormal conveyance of a sheet; a plurality of paper discharge ports provided at the end of the transport path; A program executed in an image forming apparatus comprising: a first step of discharging the sheet to a first discharge outlet of the plurality of discharge outlets when the abnormal conveyance detection unit does not detect abnormal conveyance; a second step of discharging the sheet for which abnormal conveyance has been detected and all subsequent sheets that have already been fed to a second discharge outlet of the plurality of discharge outlets when the abnormal conveyance detection unit detects abnormal conveyance; A program characterized by executing the above.

21. The second step comprises: a third step of determining whether or not all sheets that have already been fed and that follow the sheet in which abnormal conveyance has been detected are of the same type as the sheet in which abnormal conveyance has been detected, when the abnormal conveyance detection unit detects abnormal conveyance; a fourth step of discharging only the sheet in which abnormal conveyance has been detected to the second discharge outlet when all sheets following the sheet in which abnormal conveyance has been detected are of the same type as the sheet in which abnormal conveyance has been detected, and discharging the sheets following the sheet in which abnormal conveyance has been detected to the first discharge outlet after an image has been formed thereon; 21. The program according to claim 20, further comprising:

Images