Patent Publication Number: US-11640272-B2

Title: Image forming system that executes recovery process

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming system that performs a recovery process when a product defect occurs. 
     Description of the Related Art 
     Japanese Patent Laid-Open No. 2004-020650 discloses an image forming apparatus that performs a recovery process when a product defect occurs. Here, product defects include an image defect of a formed image and a defect caused to a sheet such as folding of the sheet at the time of conveyance. That is, product defects refer to any defect caused to a sheet on which an image, which is an output of the image forming apparatus, is formed (i.e., a product). The recovery process refers to a process of resuming image formation from an image formed on a sheet having a product defect. 
     In the configuration of Japanese Patent Laid-Open No. 2004-020650, even if the recovery process is performed, a product defect may occur again after the image formation is restarted. Specifically, product defects can be classified into sporadic defects such as folding of a sheet and recurring defects that can be caused by a rise in temperature and the like of the image forming apparatus. Generally, in order to suppress recurring defects, it is necessary to perform the operation of adjusting the image forming apparatus. On the other hand, since sporadic defects occur singly, the operation of adjusting the image forming apparatus is unnecessary. Therefore, when a recurring defect occurs, with the configuration of Japanese Patent Laid-Open No. 2004-020650 there is a high possibility that a product defect may occur again even if the recovery process is performed, and the time required for image formation may become long. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique for preventing an increase in the time required for image formation when a product defect occurs. 
     According to a present disclosure, an image forming system includes: an image forming unit configured to form an image on a sheet; an inspection unit configured to inspect the sheet on which the image has been formed by the image forming unit and determine whether or not the sheet has a product defect; a first discharge unit to which the sheet on which the image has been formed by the image forming unit is discharged; a second discharge unit to which the sheet on which the image has been formed by the image forming unit is discharged; and a control unit configured to discharge to the first discharge unit the sheet determined by the inspection unit to not have the product defect and discharge to the second discharge unit the sheet determined by the inspection unit to have the product defect, wherein the control unit is configured to, in a case where the inspection unit determines that a first sheet on which the image has been formed by the image forming unit has the product defect, execute a recovery process in which the image formed on the first sheet is formed on a second sheet by the image forming unit, and the control unit is configured to, in a case where the inspection unit determines that the first sheet has the product defect, execute an adjustment operation of adjusting an image forming condition for the image forming unit prior to executing the recovery process. 
     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 configuration diagram of an image forming system according to an embodiment. 
         FIG.  2    is a configuration diagram of an image forming apparatus according to the embodiment. 
         FIG.  3    is a configuration diagram of the image forming apparatus according to the embodiment. 
         FIG.  4    is a configuration diagram of a development station according to the embodiment. 
         FIG.  5    is a diagram illustrating a print job management table according to the embodiment. 
         FIGS.  6 A and  6 B  are explanatory diagrams of examples of product defect determination. 
         FIG.  7    is a diagram illustrating an example of an image for adjustment. 
         FIGS.  8 A to  8 D  are explanatory diagrams of examples of correction information calculation. 
         FIGS.  9 A and  9 B  are explanatory diagrams of examples of correction information calculation. 
         FIG.  10    is a flowchart of a process executed by the control unit according to the embodiment. 
         FIG.  11    is a flowchart of a process executed by the control unit according to the embodiment. 
         FIG.  12    is a diagram illustrating an example of a selection screen. 
         FIG.  13    is a diagram illustrating an example of an image for measuring maximum density. 
         FIG.  14    is a diagram illustrating an example of an image for measuring gradation. 
         FIG.  15    is a flowchart of a process executed by the control unit according to the embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. Note, the following embodiments are not intended to limit the scope of the claimed invention. Multiple features are described in the embodiments, but limitation is not made to an invention that requires all such features, and multiple such features may be combined as appropriate. Furthermore, in the attached drawings, the same reference numerals are given to the same or similar configurations, and redundant description thereof is omitted. 
     First Embodiment 
     An image forming system according to the present embodiment, upon detecting a product defect, executes an adjustment operation for preventing the occurrence of product defects before performing a recovery process and performs the recovery process after the completion of the adjustment operation. As a result, even if a product defect occurs, the time required for image formation is prevented from becoming long. Note that as described above, when the product defect is a sporadic defect, the adjustment operation is generally unnecessary. Therefore, in the present embodiment, a process of determining whether or not the caused product defect is a recurring defect is provided, and when it is determined that the defect is a recurring defect, the recovery process is performed. However, it is also possible to configure such that the recovery process is always performed when a product defect is detected. Hereinafter, the present embodiment will be described in detail. 
       FIG.  1    illustrates the image forming system according to the present embodiment. According to  FIG.  1   , an image forming apparatus  100 , a host computer  101 , and an inspection apparatus  150  are configured to be able to communicate with each other via a network  105 . The network  105  can be, for example, a wired or wireless LAN (local area network) or a wide area network (WAN). First, a control unit  110  of the image forming apparatus  100  will be described. A communication controller  111  controls communication via the network  105 . A non-volatile memory  112  stores programs such as a control program, data, and the like to be stored even when the power of the image forming apparatus  100  is turned off. A CPU  114  executes control programs and controls the entire image forming apparatus  100 . Note that at that time, the CPU  114  uses a RAM  113  as a work area. An HDD  115  is a storage device used for temporary or long-term storage of large amounts of data, such as image data and various kinds of setting data. A communication port  116  performs a process of communicating with the inspection apparatus  150 . Each functional block of the control unit  110  is connected to each other via a system bus  117 . Furthermore, the system bus  117  is also connected to an operation panel  120  and a printer engine  130 . 
     The host computer  101 , when causing the image forming apparatus  100  to form an image, creates a print job and transmits it to the control unit  110  of the image forming apparatus  100 . The control unit  110 , upon receiving a print job from the host computer  101 , controls the printer engine  130  in accordance with the print job to form an image on the sheet. Though each member (hardware configuration) for forming an image on a sheet by the printer engine  130  will be described later with reference to  FIGS.  2  and  3   , the members include an image capturing unit  131 . The image capturing unit  131  captures a sheet on which an image has been formed and transmits the image data of the captured sheet to the inspection apparatus  150 . The operation panel  120  has, for example, a touch panel, and provides a user interface for the user to operate the image forming apparatus  100 . Note that the operation panel  120  displays the state of the image forming apparatus  100  to the user. 
     Note that in  FIG.  1   , the image capturing unit  131  is configured to directly transmit image data to the inspection apparatus  150  but may be configured to transmit image data to the inspection apparatus  150  via the printer engine  130  and the communication port  116 . In addition, the image capturing unit  131  may be configured to transmit image data to the inspection apparatus  150  through the network  105  via the printer engine  130  and the communication controller  111 . In  FIG.  1   , the control unit  110  is configured to be able to communicate with the inspection apparatus  150  via the communication port  116  but may be configured to communicate with the inspection apparatus  150  only via the network  105  without the communication port  116  being provided. 
       FIGS.  2  and  3    are configuration diagrams of the image forming apparatus  100 . The image forming apparatus  100  includes an image forming unit  200  ( FIG.  2   ) that forms images on sheets and the image capturing unit  131 , a stacker  240 , and a finisher  250  that are disposed downstream of the image forming unit  200  in a conveyance direction of sheets ( FIG.  3   ). 
     First, the image forming unit  200  will be described. The development stations  204 Y,  204 M,  204 C and  204 K respectively form toner images of yellow, magenta, cyan, and black and transfer these to an intermediate transfer belt  208 . Note that by transferring the toner images of the respective colors to the intermediate transfer belt  208  in a superimposed manner, a full-color toner image can be formed on the intermediate transfer belt  208 . The configurations of development stations  204 Y,  204 M,  204 C and  204 K are similar and are illustrated in  FIG.  4   . A photosensitive body  2041  is driven to rotate in a counterclockwise direction in the figure at the time of image formation. A charging roller  2042  charges the surface of the photosensitive body  2041  to a uniform potential. An exposure apparatus  2043  forms an electrostatic latent image on the photosensitive body  2041  by scanning the charged and rotating photosensitive body  2041  in a main scanning direction with light. Note that the main scanning direction is a direction parallel to the rotational axis of the photosensitive body  2041 . Further, the circumferential direction of the photosensitive body  2041  will be referred to as a sub-scanning direction. A development roller  2044  causes toner to adhere to the electrostatic latent image of the photosensitive body  2041  by outputting a development bias voltage, thereby forming a toner image on the photosensitive body  2041 . Note that the toner is supplied from a toner container  2045 . A primary transfer roller  2045  transfers the toner image of the photosensitive body  2041  onto the intermediate transfer belt  208  by outputting a primary transfer bias voltage. 
     Returning to  FIG.  2   , the intermediate transfer belt  208  is driven to rotate in a clockwise direction in the figure at the time of image formation. Accordingly, the toner image on the intermediate transfer belt  208  is conveyed to a position facing a secondary transfer roller  209 . The secondary transfer roller  209  transfers the toner image of the intermediate transfer belt  208  onto a sheet conveyed from a cassette  201  or a cassette  202  along a conveyance path  203  by outputting a secondary transfer bias voltage. After the toner image is transferred, the sheet is conveyed to a fixing unit  211 . The fixing unit  211  causes the toner image to be fixed to the sheet by heating and pressurizing the sheet. The sheet on which the toner image is fixed by the fixing unit  211  is conveyed to a position  214 . Note that depending on the type of sheet, further fixing by a fixing unit  213  is necessary, and such sheets are conveyed to the position  214  via the fixing unit  213 . Meanwhile, sheets that do not require further fixing by the fixing unit  213  are conveyed to the position  214  via a conveyance path  212 . 
     When forming an image on only one side of a sheet, the sheet is conveyed from the position  214  to a position  215  and then conveyed to the image capturing unit  131  on the downstream side ( FIG.  3   ). Meanwhile, when forming an image on both sides of a sheet, the sheet is conveyed to a reversing path  216  via the position  214  and then conveyed again to the position facing the secondary transfer roller  209  via a conveyance path  217 . Thereafter, the sheet on which the images have been formed on both sides is conveyed to the image capturing unit  131  via the position  215 . In the following description, when an image is formed on both sides, the surface of the sheet on which the image is first formed will be referred to as a first surface, and the surface of the sheet on which the image is last formed will be referred to as a second surface. When an image is formed on only one surface, an image is formed on the first surface, and an image is not formed on the second surface. 
     Proceeding to  FIG.  3   , the sheet conveyed to the image capturing unit  131  is conveyed to the stacker  240  via a conveyance path  233 . The image capturing unit  131  includes image capturing devices  231  and  232 . The image capturing device  231  captures one surface of the sheet, and the image capturing device  232  captures the other surface of the sheet. Image data of the sheet surfaces that the image capturing devices  231  and  232  have captured is transmitted to the inspection apparatus  150  as illustrated in  FIG.  1   . The inspection apparatus  150  determines a product defect (i.e., an image defect of an image formed on the sheet or a defect caused to the sheet itself) based on the image data and feeds back the determination result to the control unit  110  of the image forming apparatus  100 . 
     The stacker  240  has an escape tray  246  which is a discharge tray. The escape tray  246  serves as a discharge destination for sheets determined to be a defective product by the inspection apparatus  150 . When outputting a sheet to the escape tray  246 , the sheet is discharged to the escape tray  246  via a conveyance path  244  and a conveyance path  247 . The stacker  240  has a stack tray  241  on which a large number of sheets can be stacked. When stacking sheets on the stack tray  241 , the sheets conveyed to the stacker  240  are conveyed to a reversing path  249  and then discharged to the stack tray  241  via the conveyance paths  244  and  245 . Meanwhile, when conveying a sheet to the finisher  250  on the downstream side, the sheet is conveyed via the conveyance path  244  and a conveyance path  248 . 
     The finisher  250  performs a stapling process, a punching (hole punching) process, a binding process, and the like according to the specifications by the user. After the processes in the finisher  250 , the sheet is discharged to one of a discharge tray  251 , a discharge tray  252 , and a saddle stitch binding tray  258 . 
       FIG.  5    illustrates a print job management table managed by the control unit  110 . The control unit  110 , upon receiving the print job, adds an entry (row) for the management table based on the content of the print job. In job ID fields of  FIG.  5   , identifiers of print jobs that the control unit  110  allocated to the print jobs when the print jobs were received are stored. In sheet number fields, numbers assigned in order of image formation for the sheets on which images will be formed in each print job are stored. Here, the control unit  110  adds an entry for the management table in the numerical order (ascending order) of sheets. Cassette fields store information indicating the cassette  201  or the cassette  202  in which the corresponding sheet is stored. Page number fields contain the page numbers of images. For example, in the case of double-sided printing, a page number is assigned to each of the first side and the second side, and in the case of single-sided printing, a page number is assigned to only the first side and the page number of the second side is “none”. Inspection result fields store the inspection results of the sheets inspected by the inspection apparatus  150 . Note that when a print job is received and an entry is added, the initial value of the inspection result field is “none”. Then, “NG” is stored in the inspection result field of a sheet determined by the inspection apparatus  150  to be a defective product, and “OK” is stored in the inspection result field of a sheet not determined to be a defective product. A pointer  310  indicates a sheet to be fed from the cassette  201  or  202  next. The control unit  110 , upon feeding the sheet of the entry indicated by the pointer  310 , advances the entry that the pointer  310  indicates to the next entry (in  FIG.  5   , one entry below). 
     Next, a process performed by the inspection apparatus  150  will be described. The inspection apparatus  150  determines whether or not a product defect has occurred by analyzing image data sent from the image forming apparatus  100  in accordance with preset inspection information. Hereinafter, an example of an inspection will be described.  FIGS.  6 A and  6 B  illustrate images captured by the image capturing unit  131 . Note that  FIG.  6 A  is an image of the first surface, and  FIG.  6 B  is an image of the second surface. Reference numerals  410 ,  421 , and  422  in  FIGS.  6 A and  6 B  are inspection areas indicated by inspection information. The inspection apparatus  150  determines whether or not a bar code in the inspection area  410  can be read in accordance with the inspection information. If the bar code cannot be read, the inspection apparatus  150  determines that the product is a defective product. When the bar code can be read, the inspection apparatus  150  determines a numerical value (character string) in each of the inspection area  421  and the inspection area  422  by image recognition in accordance with the inspection information. Note that the image forming apparatus  100  prints the same numerical value in the inspection area  421  and the inspection area  422 . When the numerical values determined from the inspection area  421  and the inspection area  422  by image recognition are different, the inspection apparatus  150  determines that the product is a defective product. On the other hand, when the bar code in the inspection area  410  can be read, and the numerical values in the inspection area  421  and the inspection area  422  match, the inspection apparatus  150  determines that the product is normal. 
     Note that the inspection described with reference to  FIGS.  6 A and  6 B  is an example, and the inspection apparatus  150  can determine various types of product defects. Examples of the types of product defects are, for example, a shift in position at which to form an image on a sheet (shift in formation position), an overlap of sheets, missing of sheets, a color misregistration, a tone shift, and the like. For example, when a product defect is determined by comparing image data (reference image) used to form an image with image data of a sheet captured by the image capturing unit  131 , the control unit  110  also transmits the image data used to form the image to the inspection apparatus  150 . In addition, the inspection apparatus  150  can determine not only one type of product defect but also a plurality of types of product defects individually. As described above, the content of the process executed by the inspection apparatus  150  to determine the product defect of each type is set in advance as the inspection information in the inspection apparatus  150  and the image forming apparatus  100 . 
       FIG.  7    illustrates an image for adjustment used in the operation of adjusting a formation position performed by the image forming apparatus  100  when the inspection apparatus  150  determines that a product defect of the formation position shift has occurred. Although  FIG.  7    illustrates only the first surface of the sheet, the same image is also formed on the second surface of the sheet. As illustrated in  FIG.  7   , the image for adjustment has four marks  820  formed in predetermined positions on a sheet. In this example, it is assumed that the marks are formed such that the lengths D 3  to D 10  in  FIG.  7    are 1 cm, respectively. The image capturing unit  131  captures the first side and the second side of a sheet on which the image for adjustment has been formed and transmits the captured image data to the inspection apparatus  150 . The inspection apparatus  150  determines two lengths, a length D 1  of the sheet in a direction perpendicular to the conveyance direction and a length D 2  of the sheet in the conveyance direction, based on the image data received from the image forming apparatus  100 . Further, the inspection apparatus  150  determines the marks  820  based on the image data received from the image forming apparatus  100  and determines the eight lengths D 3  to D 10  on the first surface of the sheet. Further, the inspection apparatus  150 , also on the second surface of the sheet, determines four marks formed on the second surface and determines the eight lengths in the same manner as the first surface. That is, the inspection apparatus determines a total of 18 lengths. The inspection apparatus  150  transmits the determined 18 lengths to the image forming apparatus  100 . 
     The control unit  110  obtains correction information for reducing a shift in formation position based on the 18 lengths received from the inspection apparatus  150 , and stores the correction information in the RAM  113 . Hereinafter, the correction information and the transformation of the image data for reducing a shift in formation position based on the correction information will be described with reference to  FIGS.  8 A to  8 D  and  FIGS.  9 A and  9 B . In  FIGS.  8 A to  8 D  and  FIGS.  9 A and  9 B , bold square frames indicate the sides of sheets. In  FIGS.  8 A to  8 D  and  FIGS.  9 A and  9 B , the upper left corner is the origin, the left-right direction of the figure is the X direction, and the vertical direction is the Y direction. Note that it is assumed that the value of X increases from left to right, and the value of Y increases from top to bottom. The X direction corresponds to a direction of scanning (main scanning direction) by the exposure apparatus  2043 . The Y direction corresponds to the sub-scanning direction.  FIG.  8 A  illustrates four points C 1  to C 4  obtained by the lengths D 1  to D 10  received from the inspection apparatus  150 . The coordinates of the point C 1  are (D 3 , D 4 ); the coordinates of the point C 2  are (D 1 -D 7 , D 8 ); the coordinates of the point C 3  are (D 5 , D 2 -D 6 ); and the coordinates of the point C 4  are (D 1 -D 9 , D 2 -D 10 ). 
     First, a right-angle correction for making a straight line connecting the points C 1  and C 2  perpendicular to a straight line connecting the points C 1  and C 3  is performed. As illustrated in  FIG.  8 B , the right-angle correction is performed by obtaining a write start position of the image in each scan in the main scanning direction so that each of points C 1  to C 4  becomes points C 5  to C 8  with respect to a center position of a straight line connecting points C 1  and C 2 . Subsequently, a trapezoidal correction for making a straight line connecting the points C 7  and C 8  perpendicular to a straight line connecting the points C 5  and C 7  is performed. As illustrated in  FIG.  8 C , the trapezoidal correction is performed by obtaining a scale factor in the sub-scanning direction so that the points C 7  and C 8  become the points C 9  and C 10 , respectively, with respect to the center position of the straight line connecting the point C 7  and C 8 . Subsequently, in order to make the lengths of the image in the main scanning direction and the sub-scanning direction an ideal length as illustrated in  FIG.  8 D , a scale correction is performed so that points C 5 , C 6 , C 9 , and C 10  become points C 11  to C 14 , respectively, with respect to the center of the image. Note that in this example, since the lengths D 3  to D 10  are 1 cm, the ideal lengths of the image in the main scanning direction and the sub-scanning direction are shorter than D 1  and D 2  by 2 cm, respectively. 
     Subsequently, as illustrated in  FIG.  9 A , a straight line connecting points C 11  and C 12  is rotated with the point C 11  as the rotational axis so as to be parallel to the X direction, and a skew is corrected so that each of points C 12  to C 14  becomes points C 15  to C 17 . Finally, as illustrated in  FIG.  9 B , the write start positions are corrected so that the center of a rectangle connecting the points C 11 , C 15 , C 16 , and C 17  is the center of the sheet. 
     The control unit  110  stores the write start position of the image in each scan in the main scanning direction, the scale factor in the sub-scanning direction, the scale factor of the scale correction, the rotational angle in the skew correction, and the like as correction information in the RAM  113 . Then, at the time of image formation, the control unit  110  sets image forming conditions based on the correction information and performs transformation of image data. 
       FIG.  10    is a flowchart of a process executed by the control unit  110  upon receiving a print job. Note that in the example of  FIG.  10   , it is assumed that the inspection apparatus  150  determines a shift in formation position of an image. Further, as described above, the control unit  110 , upon receiving the print job, adds an entry to the management table of  FIG.  5   . If no other job is ongoing when a print job is received, the pointer  310  is moved to a position of an entry for a sheet on which an image will be formed first in the print job. In step S 100 , the control unit  110  starts forming an image from a sheet corresponding to an entry in the management table indicated by the pointer  310 . As described above, the control unit  110  causes the image capturing unit  131  to capture a sheet on which an image has been formed and transmits the image data to the inspection apparatus  150 . Then, the control unit  110  acquires a determination result as to whether a product defect has occurred from the inspection apparatus  150 . 
     In step S 101 , the control unit  110  determines whether a product defect has occurred to the sheet based on the determination result from the inspection apparatus  150 . If a product defect has not occurred, the control unit  110  determines in step S 111  whether formation of all images have been completed by the print job. If image formation is not completed, the control unit  110  repeats the process from step S 101 . Meanwhile, if the image formation is completed, the control unit  110  ends the process of  FIG.  10   . 
     Further, if a product defect has occurred in step S 101 , the control unit  110  moves the pointer  310  to an entry of the sheet having the product defect and sets the inspection result field of the entry to “NG” in step S 102 , and stops (suspends) image formation in step S 103 . Note that as described above, a sheet having a product defect is discharged to the escape tray  246 . Subsequently, the control unit  110  initializes a counter C to 0 in step S 104  and determines whether there is a succeeding sheet in step S 105 . Note that in the present embodiment, a succeeding sheet refers to a sheet succeeding the sheet in which a product defect has occurred and having already been fed from the cassette  201  or  202  to the image forming apparatus  100  or on which an image has already been transferred when the image formation was stopped in step S 102 . 
     When there are succeeding sheets, the control unit  110  causes the inspection apparatus  150  to also inspect the succeeding sheets and determines in step S 106  whether or not a product defect has occurred. Then, the counter C is incremented by 1 in step S 107  each time there is a sheet having a product defect. Note that all of these succeeding sheets are discharged to the escape tray  246  regardless of whether or not a product defect has occurred. 
     When all the subsequent sheets have been discharged to the escape tray  246 , the control unit  110  determines in step S 108  whether or not a condition to start the adjustment operation is satisfied. For example, configuration may be taken such that if the value of the counter C is greater than or equal to a threshold, the condition to start the adjustment operation is satisfied. For example, the threshold may be 2. The value of the counter C being the threshold of 2 or more means that a product defect has occurred in two or more succeeding sheets, and thus it is for enabling to determine that a defect is not a sporadic defect but a recurring defect. Note that if the total number of succeeding sheets is less than the threshold, it is not determined that a recurring defect is occurring even if a recurring defect is occurring. Therefore, configuration may be taken such that the condition to start the adjustment operation is satisfied even when the total number of succeeding sheets is less than the threshold. 
     If the condition for starting the adjustment operation is not satisfied, the control unit  110  determines that the defect is a sporadic defect and repeats the process from step S 100 . Meanwhile, when the condition for starting the adjustment operation is satisfied, the control unit  110  executes the adjustment operation for reducing a shift in formation position in order to reduce the occurrence of the product defect in step S 109 . For example, the adjustment image illustrated in  FIG.  7    is formed, and a sheet on which an adjustment image is formed is captured by the image capturing unit  131 , and image data of the sheet is transmitted to the inspection apparatus  150 , and correction information is obtained and updated based on information fed back from the inspection apparatus  150 . Thereafter, the control unit  110  initializes the counter C to 0 in step S 110  and repeats the process from step S 100 . Note that since the pointer  310  is indicating an entry of a sheet having a product defect in the process in step  102 , image formation is resumed from the image formed on the sheet having a product defect in the later step S 100  (recovery process). That is, it is possible to print an image corresponding to the image of the sheet having a product defect on another sheet (second sheet) by the recovery process after the sheet (first sheet) having a product defect is discharged to the escape tray  246 . 
     As described above, in the present embodiment, when a product defect is detected it is determined whether or not a sheet succeeding such a sheet also has a product defect, and based on the determination result, it is determined whether the defect is a recurring defect or a sporadic defect. If it is determined that the defect is a recurring defect, the adjustment operation is executed. With this configuration, it is possible to prevent the product defect from occurring again after the recovery process, thereby preventing the time required for image formation from becoming long. In addition, usability can be improved because the user does not need to determine whether or not the defect is a recurring defect. Further, since in the case where it is determined that the defect is a sporadic defect, unnecessary adjustment operation is not executed, it is possible to prevent the time required for image formation from becoming long. 
     Note that in the flowchart of  FIG.  10   , when a product defect is detected, it is determined in the process in steps S 104  to S 108  whether the defect is a recurring defect or a sporadic defect, and when it is determined that the defect is a recurring defect, the adjustment operation is executed. However, as described above, it is also possible to configure such that the adjustment operation is always performed when a product defect is detected. In this case, a flowchart is such that steps S 104  to S 108  and S 110  of the flowchart of  FIG.  10    are omitted; after step S 103 , the process in step S 109  is executed; and then the process is repeated from step S 100 . A possibility that a product defect may occur again after the recovery process is reduced by executing the adjustment operation before the recovery process, and thus, it is possible to prevent a time required for image formation from becoming long. 
     Second Embodiment 
     Next, the second embodiment will be described focusing on the difference from the first embodiment. In the first embodiment, when the condition for starting the adjustment operation is satisfied, the control unit  110  executes the adjustment operation. However, when the user can determine that the product defect is due to a mistake of the user when setting a sheet or a blot on a sheet that was there from the beginning, there is no need to perform the adjustment operation. In the present embodiment, it is possible for the user to select whether or not to actually execute the adjustment operation when the condition for starting the adjustment operation is satisfied. 
       FIG.  11    is a flowchart of a process executed by the control unit  110  upon receiving a print job. Note that the same step numbers are assigned to the processing steps that are similar to those in the flowchart of the first embodiment illustrated in  FIG.  10   , and the description thereof will be omitted. 
     In the present embodiment, when the condition for starting the adjustment operation is satisfied in step S 108 , the control unit  110  displays a selection screen for the user to select whether or not to execute the adjustment operation on the operation panel  120  in step S 200  and then determines a user input in step S 201 .  FIG.  12    illustrates an example of the selection screen. When the user permits the adjustment operation to be performed by, for example, touching “start” in  FIG.  12   , the control unit  110  performs the adjustment operation in step S 109 . Meanwhile, if the user does not allow the adjustment operation to be executed by touching “cancel” in  FIG.  12   , for example, the control unit  110  repeats the process from step S 100 . 
     As described above, in the present embodiment, when the condition for starting the adjustment operation is satisfied, the user is prompted to input whether or not to execute the adjustment operation instead of automatically starting the adjustment operation. According to this configuration, even if the condition for starting the adjustment operation is satisfied, the adjustment operation is not performed in a case where the user can determine that the defect is a sporadic defect, and the time required for image formation can be prevented from becoming long. 
     Note that in the present embodiment, steps S 104  to S 108  and S 110  of  FIG.  11    can be omitted similarly to the first embodiment. That is, configuration may be taken such that when a product defect is detected, the selection screen of  FIG.  12    is displayed, and when the user permits execution of the adjustment operation, the adjustment operation is executed, and when the user does not permit execution of the adjustment operation, the adjustment operation is not executed. 
     Third Embodiment 
     Next, the third embodiment will be described focusing on the difference from the first and second embodiments. In the first and the second embodiments, the inspection apparatus  150  determines a shift in formation position as a product defect, and when the condition for starting the adjustment operation is satisfied, the image forming apparatus  100  performs the operation of adjusting the formation position. In the present embodiment, the adjustment operation for tones, that is, the respective gradations of yellow, magenta, cyan, and black, and the operation of adjusting the formation position are selectively executed. 
     First, the operation of adjusting gradation will be described. The operation of adjusting gradation includes a process of determining emission luminance of the exposure apparatus  2043  for obtaining a predetermined maximum density and a process of creating a gradation correction table. The image forming apparatus  100  forms an image for measuring maximum density illustrated in  FIG.  13    on a sheet in order to determine the emission luminance of the exposure apparatus  2043  for obtaining the predetermined maximum density. According to  FIG.  13   , the image for measuring maximum density includes five patch images formed by scanning the photosensitive body  2041  with five different levels of emission luminance. Specifically, the lowest level of emission luminance is A, and the other four levels of emission luminance are A+α, A+2α, A+3α, and A+4α. The image capturing unit  131  captures a sheet on which the image for measuring maximum density is formed. The control unit  110  determines the density of each patch image based on the image data captured by the image capturing unit  131 . Then, the control unit  110  determines the emission luminance of the exposure apparatus  2043  for obtaining maximum density based on the emission luminance used for forming each patch image and the density of each patch image. Note that in the present embodiment, the maximum density is controlled by adjusting the emission luminance of the exposure apparatus  2043 , but configuration may be taken such that the maximum density is controlled by adjusting other parameters related to density. For example, the maximum density can be controlled by the development bias voltage outputted from the development roller  2044 . 
     After determining the emission luminance of the exposure apparatus  2043  for maximum density, the control unit  110  forms an image for measuring gradation illustrated in  FIG.  14    on a sheet. The image for measuring gradation includes a plurality of patch images of different densities. In  FIG.  14   , the number of gradations is 10, but the number of gradations may be other than 10. The pixel value (value of image data) of each pixel of the same region is the same. The image capturing unit  131  captures a sheet on which the image for measuring gradation is formed. The control unit  110  determines the density of each patch image based on the image data captured by the image capturing unit  131 . Then, based on the pixel value of each patch image and the density of each patch image, the control unit  110  creates a gradation correction table, which is data for correcting pixel values in order to bringing the relationship between the pixel value and the density closer to an ideal state. 
       FIG.  15    is a flowchart of a process executed by the control unit  110  upon receiving a print job. Note that the same step numbers are assigned to the processing steps that are similar to those in the flowchart of the first embodiment illustrated in  FIG.  10   , and the description thereof will be omitted. In the present embodiment, the inspection apparatus  150  determines whether a product defect is a shift in position at which to form an image or a tone shift and notifies the control unit  110  of the determination result, that is, the type of product defect. Therefore, in step S 101 , the control unit  110  acquires information about the type of product defect from the inspection apparatus  150  in addition to whether or not a product defect has occurred. 
     The control unit  110 , upon suspending image formation in step S 103 , discharges the succeeding sheets to the escape tray  246  in step S 300 . Then, in step S 301 , the control unit  110  determines whether a shift in formation position is notified as a product defect. When the shift in formation position is not notified, the control unit  110  advances the process to step S 303 . Meanwhile, when the shift in formation position is notified, the control unit  110  executes the operation of adjusting the formation position in step S 302  in the same manner as in step S 109  of the first embodiment and then advances the process to step  303 . Then, in step S 303 , the control unit  110  determines whether a tone shift is notified as a product defect. If a tone shift is not notified, the control unit  110  repeats the process from step S 100 . Meanwhile, if a tone shift is notified, the control unit  110 , in step S 304 , performs the operation of adjusting gradation described with reference to  FIGS.  13  and  14    and repeats the process from step S 100 . 
     As described above, in the present embodiment, the adjustment operation corresponding to the type of product defect detected by the inspection apparatus  150  is executed. Therefore, unnecessary adjustment operation is not performed, and the time required for image formation can be prevented from becoming long, and usability can be improved. 
     Note that although the inspection apparatus  150  is an external apparatus of the image forming apparatus  100  in each of the above embodiments, the inspection apparatus  150  may be a component of the image forming apparatus  100 . That is, the image forming apparatus  100  may be configured to execute the inspection function executed by the inspection apparatus  150 . In addition, the first and the second embodiments can be combined with the third embodiment. For example, when a product defect of a first type occurs, the control unit  100  may be configured to inspect the succeeding sheet and, if the product defect of the first type has occurred to the succeeding sheets that are greater than or equal to the threshold, perform the adjustment operation for suppressing the product defect of the first type. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     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. 2020-203652, filed Dec. 8, 2020, which is hereby incorporated by reference herein in its entirety.