Patent Publication Number: US-9896292-B2

Title: Sheet conveying apparatus that conveys sheet, and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sheet conveying apparatus that conveys a sheet to a destination, and an image forming apparatus equipped with the sheet conveying apparatus. 
     Description of the Related Art 
     An image forming apparatus includes a sheet conveying apparatus that conveys a recording material (hereinafter referred to as a sheet), and at a branching point of a conveyance path through which the sheet is conveyed by the conveying unit of the sheet conveying apparatus, there is provided a switching member, such as a flapper, for switching a destination to which a sheet is to be conveyed. In the sheet conveying apparatus provided with the flapper, it is required to prevent a sheet from being caught between the flapper and a conveying guide, or from being conveyed to a place other than the destination. To this end, the operation for switching the conveyance path is required to be performed after a trailing end of the preceding sheet has passed the flapper and before a leading end of a sheet following the preceding one reaches the flapper. As a conventional technique concerning the sheet conveying apparatus provided with a flapper, there has been proposed an image forming apparatus in Japanese Laid-Open Patent Publication (Kokai) No. 2015-212717. 
     However, in the above-mentioned conventional technique, depending on the configuration of the switching member for switching a conveyance path, if paper powder and dust, etc., are attached to component members of the switching member, the current state of the switching member cannot be detected, which sometimes requires a retry operation for performing the operation for switching the conveyance path again. If the switching member cannot attain switching by one switching operation, and the switching operation is to be performed again, the switching operation of the switching member is not finished before a sheet reaches the switching position, causing a jam, depending on the conveyance interval of sheets conveyed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sheet conveying apparatus that is capable of continuing conveyance of sheets without causing a jam even when a switching unit performs an operation for switching a conveyance path again, and an image forming apparatus equipped with the sheet conveying apparatus. 
     In a first aspect of the present invention, there is provided a sheet conveying apparatus comprising a conveying unit configured to convey a sheet, a switching member disposed at a branching point of a conveyance path through which the sheet is conveyed by the conveying unit, and configured to switch a destination of conveyance of the sheet, a drive unit configured to operate the switching member, a sensor for detecting a state of the switching member, and a controller configured to cause the drive unit to operate the switching member, to perform a checking operation for checking a state of the drive unit based on an output from the sensor, and to control a sheet conveyance interval between a first sheet conveyed by the conveying unit and a second sheet conveyed following the first sheet, according to a result of the checking operation. 
     In a second aspect of the present invention, there is provided an image forming apparatus comprising a conveying unit configured to convey a sheet, an image forming unit configured to perform image formation on the sheet conveyed by the conveying unit, a switching member disposed at a branching point of a conveyance path through which the sheet is conveyed by the conveying unit, and configured to switch a destination of conveyance of the sheet, a drive unit configured to operate the switching member, a sensor configured to detect a state of the switching member, and a controller configured to cause the drive unit to operate the switching member, to perform a checking operation for checking a state of the drive unit based on an output from the sensor, and to control a sheet conveyance interval between a first sheet conveyed by the conveying unit and a second sheet conveyed following the first sheet, according to a result of the checking operation. 
     According to the present invention, when switching a conveyance path, if detection of the state of the switching member fails, the sheet conveyance interval is increased by taking into account the time required to detect a state of the switching member again, and hence it is possible to continue conveyance of sheets without stopping the apparatus. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to an embodiment. 
         FIG. 2  is a control block diagram of the image forming apparatus shown in  FIG. 1 . 
         FIG. 3  is a flowchart of a sheet conveying process performed by a sheet conveying unit included in the image forming apparatus shown in  FIG. 1 . 
         FIG. 4  is a continuation of  FIG. 3 , performed in a case where it is determined that a retry operation is not required at the activation of the image forming apparatus. 
         FIG. 5  is a continuation of  FIG. 3 , performed in a case where it is determined that a retry operation is required at the activation of the image forming apparatus. 
         FIGS. 6A and 6B  each are a schematic view showing the configuration of a switching member-operating section and operation steps, and a timing diagram of operations. 
         FIG. 7  is a diagram showing a confirmation screen displayed to a user on a display section of a console section. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described in detail below with reference to the accompanying drawings showing embodiments thereof. 
       FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to an embodiment. 
     Referring to  FIG. 1 , the image forming apparatus, denoted by reference numeral  101 , includes an image forming section  20  comprised of a plurality of image forming stations  22  to  25 , and an intermediate transfer belt  26  which is rotated in a state in sliding contact with the image forming stations  22  to  25 . A secondary transfer roller  21   a  is disposed in contact with one tension roller  21   b  that stretches the intermediate transfer belt  26 , via the intermediate transfer belt  26 . Respective contact portions of the intermediate transfer belt  26  and the secondary transfer roller  21   a  form a secondary transfer section  21 . 
     A sheet feeder  14  storing a plurality of sheets P is provided at a lower part of the image forming apparatus  101 . The image forming apparatus  101  includes a sheet conveying unit that conveys sheets P contained in the sheet feeder  14  via the secondary transfer section  21  to the outside of the image forming apparatus  101 . The sheet conveying unit includes a vertical path  10  connected to the sheet feeder  14 , and a horizontal path  31  connected to the vertical path  10 . 
     The horizontal path  31  is branched at an outlet of a fixing section  12  disposed downstream of the secondary transfer section  21  into a straight path  32  and an inversion path  33 . At the branching point, there is disposed a flapper  40  as a member for switching a conveyance path. The inversion path  33  is provided with an inversion discharge path  34  for returning a sheet P having front and reverse sides inverted by the inversion path  33  to the straight path  32 . At a connection portion between the inversion path  33  and the inversion discharge path  34 , there is disposed a flapper  41  as a switching member. 
     On the vertical path  10 , there are disposed a pickup roller  15  for taking in sheets P from the sheet feeder  14 , a separation roller  16  for separating the sheets P one by one, and a vertical path roller  17 . On the horizontal path  31 , there are disposed a registration roller  11 , the fixing section  12 , and a horizontal path roller  13 . Further, on the inversion path  33 , there is disposed an inversion path roller  19 , and on the straight path  32 , there is disposed a discharge roller  18 . 
     The image forming apparatus  101  includes a console section  306  having a display section. The console section  306  receives an instruction from a user, such as an instruction for selecting continuing printing, and further, displays information indicative of an operating state of the apparatus, and so forth, on the display section. 
     Next, a description will be given of the control configuration of the image forming apparatus  101  shown in  FIG. 1 . 
       FIG. 2  is a control block diagram of the image forming apparatus  101  shown in  FIG. 1 . 
     Referring to  FIG. 2 , the control configuration of the image forming apparatus  101  is roughly divided into a controller  200  and a printer controller  201 . 
     The controller  200  includes a CPU  301 . The CPU  301  is connected to a RAM  302  and a ROM  303  via an address bus and a data bus. Further, the CPU  301  is connected to an external interface  307 , a PDL controller  304 , an internal interface  305 , and the console section  306 . The external interface  307  inputs e.g. a print job from an external device, such as a computer. 
     The CPU  301  controls the overall operation of the image forming apparatus  101 , and manages print jobs. The ROM  303  stores e.g. control programs. The RAM  302  stores e.g. data used for processing. The PDL controller  304  is a processing circuit for manipulating and accumulating print data, and performing image processing on print data. The internal interface  305  is a communication circuit for communicating with the printer controller  201 . 
     The console section  306  includes a display device, such as a liquid crystal display device, and a key input device, such as a touch panel. The CPU  301  receives e.g. an instruction for selecting continuing printing from a user via the key input device. Further, the CPU  301  controls the display section of the console section  306  to display information indicative of the operating state of the apparatus, and so forth. 
     The printer controller  201  includes a CPU  311 . The CPU  311  is connected to a RAM  312  and a ROM  313  via an address bus and a data bus. Further, the CPU  311  is connected to a device controller  314  and an internal interface  315 . Further, the CPU  311  is connected to a switching member-operating section  316  via the device controller  314 . 
     The CPU  311  performs basic control of the image forming operation. The ROM  313  stores control programs including a sheet conveying process program for executing a sheet conveying process, described hereinafter. The RAM  312  stores data necessary for image forming processing. The device controller  314  is an electric circuit having input and output ports, etc., for controlling components of the printer section. The internal interface  315  is a communication circuit for transmitting and receiving image signals and timing signals to and from the controller  200 . 
     According to the control programs, the CPU  311  receives image signals from the controller  200 , and controls the device controller  314  to thereby cause the same to perform an image forming operation. Further, the CPU  311  controls the switching member-operating section  316  via the device controller  314  to thereby operate the flappers  40  and  41  as the switching members, and so forth. The image forming apparatus  101  may be configured such that the controller  200  and the printer controller  201  are integrated into one unit, and the image forming apparatus  101  is controlled by one CPU in the one unit. 
     In the image forming apparatus  101  configured as above, the sheets P stored in the sheet feeder  14  are conveyed into the conveyance path by the pickup roller  15  such that after being separated by the separation roller  16  one by one, each sheet P is conveyed into the vertical path  10 . The sheet P conveyed into the vertical path  10  is conveyed into the horizontal path  31  by the vertical path roller  17 . The sheet P conveyed into the horizontal path  31  is conveyed to the secondary transfer section  21  by the registration roller  11  after being adjusted in timing with the intermediate transfer belt  26 . 
     On the other hand, images of respective colors, such as yellow (Y), magenta (M), cyan (C), and black (Bk), which are formed by the image forming stations  22  to  25  of the image forming section  20  using a known method, are transferred onto the intermediate transfer belt  26  to form a color image thereon. The color image formed on the intermediate transfer belt  26  is conveyed, after being adjusted in timing with the sheet P, to the secondary transfer section  21 , where it is transferred onto the sheet P. The sheet P on which the image has been transferred is conveyed into the fixing section  12 , wherein pressure and heat are applied to the sheet P, whereby the transferred image is fixed on the sheet P. The sheet P on which the image has been fixed is conveyed by the horizontal path roller  13 , and is guided into the inversion path  33  or the straight path  32  by switching of the flapper  40 . 
     The sheet P conveyed into the inversion path  33  is switched back by the inversion path roller  19 , and is guided into the inversion discharge path  34  by switching of the flapper  41 . The sheet P having passed through the straight path  32  or the inversion discharge path  34  is discharged to the outside of the image forming apparatus  101  by the discharge roller  18 . 
     Next, a description will be given of a sheet conveying process performed by the sheet conveying unit of the image forming apparatus  101  shown in  FIG. 1 . In the sheet conveying process, it is possible to execute a redetection operation (retry operation) associated with an operation for changing the position of the flapper. Here, the retry operation refers to an operation executed by a sensor (e.g. a sensor  403 , referred to hereinafter), after the sensor has failed in state detection for checking the state of a flapper (e.g. the flapper  40 ) as a switching unit in the conveyance path, to perform the state detection again. The retry operation will be described in detail hereinafter. 
       FIGS. 3, 4, and 5  are a flowchart of the sheet conveying process performed by the sheet conveying unit included in the image forming apparatus  101  shown in  FIG. 1 . This sheet conveying process is performed by the CPU  311  of the printer controller  201  of the image forming apparatus  101  according to a sheet conveying process program stored in the ROM  313 . 
     Referring to  FIG. 3 , first, when the image forming apparatus  101  is activated, the CPU  311  causes the switching member-operating section (hereinafter referred to as the “flapper operating section”)  316  to operate, and performs a checking operation for checking the need of the retry operation before actually starting sheet feeding (step S 602 ). 
     Hereafter, the retry operation performed in the sheet conveying process will be described, while referring to the configuration of the flapper operating section. 
       FIGS. 6A and 6B  each are a schematic view showing the configuration of the flapper operating section  316  and operation steps, and a timing diagram of operations. Although the following description is given of the flapper  40  as the switching member, the switching operation of the flapper  41  is performed in a similar manner. In this case, it is assumed that the flapper  41  is also provided with a sensor corresponding to the sensor  403 , referred to hereinafter. 
       FIGS. 6A and 6B  schematically illustrate the flapper operating section  316 . The flapper operating section  316  is comprised of a disc-shaped light shielding flag member  401  as a plate-shaped rotation member for changing the position of the flapper  40  as the switching member, and a stick-shaped cam  402  which is disposed in a predetermined radial direction passing through the center of the light shielding flag member  401 . The flapper  40  is engaged with the cam  402  via an arm portion  42 , and is repeatedly switched between an urged state in which the flapper  40  is pressed up by the cam  402  which is rotated in accordance with rotation of the light shielding flag member  401  and a released state in which the flapper  40  is released from the pressed state. In the following description, the state in which the flapper  40  is urged by the cam  402  is referred to as the first state, and the released state in which the flapper  40  is released from the urged state is referred to as the second state. 
     The first or second state of the flapper  40  is detected by the sensor  403 . More specifically, an outer peripheral portion of the light shielding flag member  401  is formed with cutouts  404   a  and  404   b  at two locations which are opposed to each other across the center C of rotation of the light shielding flag member  401 , and the sensor  403  for detecting the cutouts is disposed at a location opposed to the outer peripheral portion of the light shielding flag member  401 . The sensor  403  is implemented e.g. by a photo sensor. The flapper operating section  316  includes a stepping motor  316 M for rotating the light shielding flag member  401 . Whether the flapper  40  is in the first state or the second state is determined by detection of the cutout  404   a  or  404   b  of the light shielding flag member  401  being rotated by the stepping motor  316 M, by the sensor  403 . 
     The flapper  40  is repeatedly switched between the first state in which the flapper  40  is pressed by the cam  402  via the arm portion  42  and the second state in which the flapper  40  is released from the pressed state, whenever the light shielding flag member  401  is rotated through a predetermined angle, such as 90 degrees. That is, in a state in which the sensor  403  detects the cutout  404   a  or  404   b  (one of states (a) and (c) in  FIG. 6A ), the flapper  40  is in the second state in which the flapper  40  is not urged. If the light shielding flag member  401  is rotated by the stepping motor  316 M through 90 degrees in a clockwise direction when the flapper  40  is in the second state, the flapper  40  enters the first state in which it is urged by the cam  402  (states (b) and (d) in  FIG. 6A ). In short, the position of the flapper  40  is determined by an output from the sensor  403  and an amount of driving of the stepping motor  316 M. 
     Hereafter, a description will be given of the operation processes of the flapper  40  the position of which is changed in accordance with rotation of the light shielding flag member  401 . 
       FIG. 6A  shows a normal state in which foreign matter, such as paper powder and dust, is not collected in the cutouts  404   a  and  404   b.    
     In the state (a), the cutout  404   a  of the light shielding flag member  401  is in a position opposed to the sensor  403 , and the sensor  403  is in an off-state in which the light shielding flag member  401  is not detected. At this time, the flapper  40  is in the first state in which the flapper  40  is urged by the cam  402 . When the light shielding flag member  401  is further rotated through 90 degrees in the clockwise direction from the state (a), the state of the flapper operating section  316  is changed to a state (b). 
     In the state (b), a portion of the light shielding flag member  401  other than the cutout  404   a  or  404   b  is opposed to the sensor  403 . Therefore, the sensor  403  is in an on-state in which the light shielding flag member  401  is detected. At this time, the flapper  40  is in the second state in which the flapper  40  is released from the state urged by the cam  402 . When the light shielding flag member  401  is further rotated through 90 degrees in the clockwise direction from the state (b), the state of the flapper operating section  316  is changed to a state (c). 
     In the state (c), the cutout  404   b  of the light shielding flag member  401  is opposed to the sensor  403 , and therefore, the sensor  403  is in the off-state in which the light shielding flag member  401  is not detected. At this time, the flapper  40  is in the first state in which the flapper  40  is urged by the cam  402 . When the light shielding flag member  401  is further rotated through 90 degrees in the clockwise direction from the state (c), the state of the flapper operating section  316  is changed to a state (d). 
     In the state (d), a portion of the light shielding flag member  401  other than the cutout  404   a  or  404   b  is opposed to the sensor  403 . Therefore, the sensor  403  is in the on-state in which the light shielding flag member  401  is detected. At this time, the flapper  40  is in the second state in which the flapper  40  is released from the state urged by the cam  402 . When the light shielding flag member  401  is further rotated through 90 degrees in the clockwise direction from the state (d), the state of the flapper operating section  316  is returned to the state (a). 
       FIG. 6B  shows an abnormal state in which foreign matter  405 , such as paper powder and dust, is accumulated in the cutout  404   a  of the light shielding flag member  401 . 
     In a state (e), although the cutout  404   a  of the light shielding flag member  401  is opposed to the sensor  403 , since the foreign matter  405  is accumulated in the cutout  404   a , the sensor  403  is in the on-state, similar to the case where the light shielding flag member  401  is detected. At this time, however, the flapper  40  is in the first state in which the flapper  40  is urged by the cam  402 . 
     In the flapper operating section  316 , after the sensor  403  has detected the cutout  404   a  or  404   b , the light shielding flag member  401  is further rotated through 90 degrees in the clockwise direction, whereby the state of the flapper  40  is changed from the first state to the second state to change the conveyance path. Therefore, as a precondition for switching the conveyance path, the cutout has to be detected by the sensor  403 . 
     However, in the state (e), since the sensor  403  cannot detect the cutout  404   a , the retry operation for rotating the light shielding flag member  401  to a position of the next cutout  404   b  is performed. Then, when the light shielding flag member  401  is rotated through 90 degrees in the clockwise direction from the state (e), the state of the flapper operating section  316  is changed to a state (f). 
     In the state (f), a relationship between the sensor  403  and the light shielding flag member  401 , and the state of the flapper  40  are the same as those in the above-mentioned state (b). However, the retry operation is being performed due to the fact that the cutout  404   a  could not be detected, and hence the light shielding flag member  401  continues rotation without being stopped, until the state of the flapper operating section  316  is changed to a state (g) in which the light shielding flag member  401  has been further rotated through 90 degrees in the clockwise direction from the state (f). 
     In the state (g), the cutout  404   b  of the light shielding flag member  401  is opposed to the sensor  403 . Therefore, the sensor  403  is in the off-state in which the light shielding flag member  401  is not detected. At this time, the flapper  40  is in the first state in which the flapper  40  is urged by the cam  402 . This state is a retry success state in which the cutout  404   b  is detected after performing the retry operation. When the light shielding flag member  401  is further rotated through 90 degrees in the clockwise direction from the state (g), the state of the flapper operating section  316  is changed to a state (h). 
     In the state (h), the relationship between the sensor  403  and the light shielding flag member  401 , and the state of the flapper  40  are the same as those in the above-mentioned state (d). When the light shielding flag member  401  is further rotated through 90 degrees in the clockwise direction from the state (h), the state of the flapper operating section  316  is returned to the state (e). 
     The operation for determining whether or not the sensor  403  detects the cutout  404   a  or  404   b  by causing the light shielding flag member  401  to rotate, as described above, is referred to as the checking operation. Further, in the case where the cutout  404   a  or  404   b  cannot be detected by the sensor  403  even when the light shielding flag member  401  is rotated through 90 degrees in the clockwise direction, the operation performed for further rotating the light shielding flag member  401  through 90 degrees in the clockwise direction to make an attempt of cutout detection is referred to as the retry operation. 
     Referring again to  FIG. 3 , in the step S 602 , the CPU  311  causes the light shielding flag member  401  to perform one rotation, and checks whether or not the cutout  404   a  or  404   b  is detected by the sensor  403 . If both of the cutouts  404   a  and  404   b  are detected by the sensor  403 , it is confirmed that the image forming apparatus  101  at the activation thereof is not in a state requiring the retry operation. On the other hand, if only one of the cutouts  404   a  and  404   b  is detected, it is confirmed that the image forming apparatus  101  at the activation thereof is in a state requiring the retry operation. 
     That is, if both of the cutouts  404   a  and  404   b  are detected by the sensor  403 , the retry operation is not required. On the other hand, if only one of the cutouts  404   a  and  404   b  can be detected by the sensor  403 , it is considered that paper powder or dust is accumulated in one of the cutouts, and hence it is necessary to perform the retry operation in the sheet conveying process which is to be started after this. 
     Note that if neither the cutout  404   a  nor  404   b  is detected, it is considered that the foreign matter  405  is accumulated in both of the cutouts  404   a  and  404   b . Therefore, in this case, it is necessary to stop driving of the apparatus to remove the foreign matter  405 . 
     The CPU  311  determines, based on a result of the checking operation in the step S 602 , whether or not the retry operation is required (step S 603 ). If it is determined in the step S 603  that the retry operation is not required (NO to the step S 603 ), the CPU  311  proceeds to a step S 701  et seq. in  FIG. 4 . On the other hand, if it is determined in the step S 603  that the retry operation is required (YES to the step S 603 ), the CPU  311  proceeds to a step S 801  et seq. in  FIG. 5 . 
     Referring to  FIG. 4 , the CPU  311  determines whether or not printing has been started by the image forming apparatus  101  (step S 701 ). If printing has been started by the image forming apparatus  101  (YES to the step S 701 ), the CPU  311  proceeds to a step S 702 . In the step S 702 , the CPU  311  determines whether or not a jam has occurred at the location of the flapper  40  during conveyance of a sheet P (step S 702 ). Note that after starting printing, if it is required to change the position of the flapper  40  so as to convey a sheet to a designated destination, an operation for changing the position of the flapper  40  is performed. At this time, the sensor  403  performs detection of the cutout  404   a  or  404   b . In a case where the cutout  404   a  or  404   b  is not detected, the retry operation is performed. If it is determined in the step S 702  that a jam has occurred (YES to the step S 702 ), the CPU  311  determines whether or not the retry operation has been performed by the flapper operating section  316  (step S 703 ). If the retry operation has been performed, it is considered that the jam has occurred at the location of the flapper  40  due to the retry operation. 
     If it is determined in the step S 703  that the retry operation has been performed by the flapper operating section  316  (YES to the step S 703 ), the CPU  311  proceeds to a step S 704 . Note that when the retry operation is performed, the CPU  311  increments the count of the number of times of execution of the retry operation by one. The CPU  311  thus calculates the number of times of execution of the retry operation to determine whether or not the number of times of execution of the retry operation has reached a predetermined number, i.e. three (step S 704 ). If it is determined in the step S 704  that the number of times of execution of the retry operation has reached three (YES to the step S 704 ), the CPU  311  proceeds to a step S 705 . In the step S 705 , the CPU  311  displays a confirmation screen on the console section  306 , so as to prompt the user to confirm whether or not to continue printing with reduced productivity, before resuming image formation after jam release processing (step S 705 ). Note that the productivity corresponds to the number of sheets conveyed per unit time or the number of sheets subjected to image formation, and by increasing the sheet conveyance time interval than normal, the productivity is lowered. In the case where the number of times of execution of the retry operation reaches three, it is considered that the foreign matter  405  is accumulated in the cutout  404   a  or  404   b  of the light shielding flag member  401 . Therefore, to prevent a jam from being caused by the same factor during image formation which is to be resumed after releasing the jam, it is effective to reduce the productivity. 
       FIG. 7  is a diagram showing a confirmation screen displayed to a user on the display section of the console section  306 . Referring to  FIG. 7 , the confirmation screen, denoted by reference numeral  900 , displays a message “The cutout of the switching member-operating section cannot be detected by the sensor. Do you wish to continue printing with reduced productivity?”, and selection buttons of “Yes” and “No”. On the confirmation screen  900 , the user selects “Yes” or “No”. 
     Referring again to  FIG. 4 , after the confirmation screen has been displayed for prompting the user to confirm whether or not to continue printing with reduced productivity (step S 705 ), the CPU  311  determines whether or not continuation of printing with reduced productivity is selected by the user (step S 706 ). If it is determined in the step S 706  that continuation of printing with reduced productivity is selected by the user (YES to the step S 706 ), the CPU  311  proceeds to a step S 707 . In the step S 707 , the CPU  311  acquires a time period required to perform the retry operation, which is stored in the ROM  313  in advance (step S 707 ). In the present embodiment, as a drive motor of the flapper operating section  316 , the stepping motor  316 M is used, and the time period required to perform the retry operation is determined in the design stage of the image forming apparatus. 
     After the time period required to perform the retry operation has been acquired (step S 707 ), the CPU  311  causes the device controller  314  to set reduced productivity of printing when resumed, so as to secure the time period required to perform the retry operation (step S 708 ). For example, assuming that the time period required to perform the retry operation is 500 msec, and the sheet conveyance time interval before adjustment of the productivity is 100 msec, it is necessary to secure a sheet conveyance time interval of not shorter than 600 msec. After that, the CPU  311  determines whether or not printing is finished (step S 710 ), and if printing is finished, the CPU  311  clears the count of the number of times of execution of the retry operation in jam occurrence (step S 711 ), followed by terminating the present process. If printing is not finished, the CPU  311  returns to the step S 702 , and repeats the above-described process. 
     Here, the sheet conveyance time interval corresponds to a time interval (time difference) between a time at which a preceding sheet being conveyed passes a predetermined reference position, such as the flapper  40 , and a time at which the following sheet being conveyed next passes the predetermined reference position. The sheet conveyance time interval varies with the sheet conveyance speed. 
     On the other hand, if it is determined in the step S 706  that continuation of printing with reduced productivity is not selected (NO to the step S 706 ), the CPU  311  proceeds to a step S 709 . In the step S 709 , the CPU  311  stops the printing operation, and inhibits reception of a print job thereafter (step S 709 ), followed by terminating the present process. 
     Further, if it is determined in the step S 701  that printing has not been started by the image forming apparatus  101  (NO to the step S 701 ), the CPU  311  proceeds to the step S 710 . Further, if it is determined in the step S 702  that a jam has not occurred (NO to the step S 702 ), the CPU  311  proceeds to the step S 710 . 
     Further, if it is determined in the step S 703  that the retry operation has not been executed (NO to the step S 703 ), the CPU  311  proceeds to the step S 710 . Further, if it is determined in the step S 704  that the number of times of execution of the retry operation is smaller than three (NO to the step S 704 ), the CPU  311  proceeds to the step S 710 . 
     According to the sheet conveying process described with reference to  FIG. 4 , if it is determined at the activation of the image forming apparatus  10  that the retry operation is not required 1, it is determined whether or not a jam has occurred (step S 702 ), and if a jam has occurred, the confirmation screen is displayed so as to prompt the user to confirm whether or not to continue printing with reduced productivity (step S 705 ). Then, if the user selects continuation (resuming) of printing with reduced productivity, the reduced productivity is set (step S 708 ). As a consequence, even when the retry operation is required after activating the image forming apparatus  101 , it is possible to continue sheet conveying processing and image formation processing with the reduced productivity. 
     If it is determined in the step S 603  that the retry operation is required, the CPU  311  proceeds to the step S 801  in  FIG. 5  to display the confirmation screen, shown in  FIG. 7 , so as to prompt the user to confirm whether or not to perform printing with reduced productivity, on the display section of the console section  306 . Then, the CPU  311  determines whether or not printing with reduced productivity is selected by the user (step S 802 ). If it is determined in the step S 802  that printing with reduced productivity is selected by the user (YES to the step S 802 ), the CPU  311  proceeds to a step S 803 . 
     The CPU  311  determines whether or not printing has been started by the image forming apparatus  101 , and waits until printing is started (step S 803 ). After printing has been started by the image forming apparatus  101  (YES to the step S 803 ), the CPU  311  acquires a time period required to execute the retry operation from the ROM  313  which is a storage section (step S 804 ). Then, the CPU  311  causes the device controller  314  to set reduced productivity so as to secure the time period required to execute the retry operation, which is acquired in the step S 804  (step S 805 ), followed by terminating the present process. 
     On the other hand, if it is determined in the step S 802  that printing with reduced productivity is not selected by the user (NO to the step S 802 ), the CPU  311  inhibits reception of a print job thereafter (step S 806 ), followed by terminating the present process. 
     According to the sheet conveying process described with reference to  FIG. 5 , if it is determined at the activation of the image forming apparatus  101  that the retry operation is required, first, the confirmation screen is displayed so as to prompt the user to confirm whether or not to perform printing with reduced productivity (step S 802 ). Then, if continuation of printing with reduced productivity is selected by the user (YES to the step S 802 ), the sheet conveyance time interval is increased by the time period required to execute the retry operation to thereby reduce the productivity (step S 805 ). As a consequence, even when the operation for switching the conveyance path is executed again, it is possible to continue conveyance of sheets without causing a jam. 
     According to the present embodiment, even in a case where the retry operation is performed because the cutout  404   a  or  404   b  of the light shielding flag member  40  is not detected by the sensor  403  when changing the position of the flapper  40  or  41 , it is possible to continue the sheet conveying operation and the image forming operation by reducing productivity. 
     In the present embodiment, it is preferable that the control for increasing a sheet conveyance time interval between a first sheet and a second sheet conveyed following the first sheet, which are conveyed by a conveying unit, by a time period required to execute a retry operation is performed whenever a sheet P passes the flapper  40  or  41 . This makes it possible to convey the sheet P to a destination while avoiding occurrence of a jam. 
     In the present embodiment, to make the sheet conveyance time interval longer than normal, a time interval of feeding sheets from the sheet feeder  14  is made longer than usual (sheet conveyance distance interval between sheets being fed successively is made longer). Instead of changing the sheet conveyance distance interval, the sheet conveyance speed may be lowered. Further, the sheet conveyance distance interval may be increased and the sheet conveyance speed may be lowered. In the case of lowering the sheet conveyance speed, it is also required to reduce the speed of image formation performed by the image forming station. This makes it possible to smoothly perform the image formation processing and processing for transferring a formed image onto a sheet P. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-080406, filed Apr. 13, 2016, and No. 2017-061115, filed Mar. 27, 2017, which are hereby incorporated by reference herein in their entirety.