Patent Publication Number: US-2023139410-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus including an image detection unit that detects at least a portion of an image on a sheet. 
     Description of the Related Art 
     For example, in some printers, copiers, fax machines, multifunction printers, and other image forming apparatuses, after forming an image on the first side (front side) of a sheet in the image forming unit, the sheet is reversed in the conveyance direction and re-transferred to the image forming unit again to form an image on the second side (back side) of the sheet. In such a system that forms images on both sides of a sheet, errors or variations in conveying speed may cause position deviation of the image formed on a sheet when forming an image on the first side of the sheet. If another image is subsequently formed on the second side, the images on both sides may be misaligned, resulting in a deterioration of the quality of the product. 
     Therefore, an apparatus has been proposed that uses an image detection sensor to detect the distance between the position of the registration mark formed on the first side and the edge of the sheet, and adjusts the image formation position based on the detection result when forming an image on the second side (see JP 2004-279749 A). 
     In recent years, high productivity has been demanded of so-called high-end apparatuses, which require particularly high front-to-back registration accuracy, and sheet conveying speeds have therefore been increased. However, in the detection of images formed on the first side of a sheet, as in JP 2004-279749 A described above, there is a problem that as the sheet conveying speed increases, sheet fluttering or insufficient resolution of the sensor occurs, resulting in a large detection error. If the detection error in detecting the position of the image on the first side is large, the error will affect, for example, the adjustment of the position of the image formed on the second side, resulting in a deterioration of the quality of the product. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, an image forming apparatus includes an image forming unit configured to form an image on a sheet, a reversing unit configured to convey a sheet conveyed from the image forming unit in a first direction, stop the conveyance, and then convey the sheet in a second direction, which is opposite to the first direction, an image detection unit disposed to detect at least a portion of the image on the sheet stopped by the reversing unit, and a control unit configured to control the reversing unit and the image detection unit. The control unit is configured to detect the image on the sheet by the image detection unit while the sheet conveyance is stopped by the reversing unit. 
     According to a second aspect of the present invention, an image forming apparatus includes an image forming unit configured to form an image on a sheet, a conveyance unit configured to convey the sheet on which an image has been formed in the image forming unit and stop the sheet conveyance, an image detection unit configured to detect the image on the sheet, and a control unit configured to control the conveyance unit and the image detection unit. The control unit is configured to detect at least a portion of the image on the sheet by the image detection unit while the sheet conveyance is stopped by the conveyance unit. 
     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 a first embodiment. 
         FIG.  2 A  is a block diagram of a control system of the image forming apparatus according to the first embodiment. 
         FIG.  2 B  is a block diagram of a control unit. 
         FIG.  3 A  illustrates the image position with respect to the sheet before correction of the image by the image forming unit. 
         FIG.  3 B  illustrates the image position with respect to the sheet after correction of the image by the image forming unit. 
         FIG.  4    illustrates the state in which a sheet is conveyed to a second reverse conveyance roller pair in the image forming apparatus according to the first embodiment. 
         FIG.  5    illustrates the state in which a sheet is stopped by the second reverse conveyance roller pair in the image forming apparatus according to the first embodiment. 
         FIG.  6    illustrates the state in which a sheet is reversed and conveyed by the second reverse conveyance roller pair in the image forming apparatus according to the first embodiment. 
         FIG.  7 A  illustrates the positional relationship between the sheet and the image sensor at time points T 1  to T 3  when the sheet is stopped. 
         FIG.  7 B  illustrates the voltage signals of the image sensor at time points T 1  to T 3  when the sheet is stopped. 
         FIG.  7 C  illustrates the signal waveform of the image sensor acquired when the sheet is stopped. 
         FIG.  8 A  is a block diagram of a control system of an image forming apparatus according to a second embodiment. 
         FIG.  8 B  is a block diagram of a control unit. 
         FIG.  9 A  illustrates the image position on the sheet before correction of the conveying speed by the registration roller pair. 
         FIG.  9 B  illustrates the image position on the sheet after correction of the conveying speed by the registration roller pair. 
         FIG.  10    is a schematic cross-sectional view of a sheet pressing mechanism according to a third embodiment. 
         FIG.  11 A  illustrates the positional relationship between the sheet and the image sensor at time points T 1  to T 3  during sheet conveyance. 
         FIG.  11 B  illustrates the voltage signals of the image sensor at time points T 1  to T 3  during sheet conveyance. 
         FIG.  11 C  illustrates the signal waveform of the image sensor acquired during sheet conveyance. 
         FIG.  12 A  is a schematic cross-sectional view of the reversing conveyance path with the trailing edge of the sheet in the first direction inclined away from the image sensor. 
         FIG.  12 B  is a schematic cross-sectional view of the reversing conveyance path with the trailing edge of the sheet in the first direction inclined closer to the image sensor. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     Hereinafter, an image forming apparatus according to the first embodiment will be described with reference to the drawings. The dimensions, materials, shapes, relative arrangements, and others of the components described in the following first embodiment are not intended to limit the scope of application of the present technology only to them unless otherwise specified. 
     Schematic Configuration of Image Forming Apparatus 
       FIG.  1    is a schematic cross-sectional view of the image forming apparatus  100  according to the first embodiment. In the present embodiment, an image forming apparatus  100 , which is a laser beam printer of electrophotographic system, is described as an example of an image forming apparatus, but the image forming apparatus is not limited thereto, and may be an LED printer, an inkjet printer, a sublimation printer, or the like. 
     The image forming apparatus  100  includes an image forming unit  100 A, which is a so-called printer engine, a sheet feeding unit  100 B, a sheet conveying unit  100 C, and a sheet reversing unit  100 D. The image forming unit  100 A includes an optical processing mechanism  101  and a fixing processing mechanism  102  that form an image on a recording material through an image forming process. The sheet feeding unit  100 B feeds a rectangular sheet used as a recording material. The sheet conveying unit  100 C conveys the fed sheet to the image forming unit  100 A and discharges the sheet, or re-conveys the reversed sheet to the image forming unit  100 A. The sheet reversing unit  100 D reverses the conveyance direction of a sheet on which an image has been formed by the image forming unit  100 A, and either re-conveys the sheet to the image forming unit  100 A or reverses it front to back and discharges it. The recording material may be, for example, paper such as plain paper or cardboard, paper with surface treatment such as coated paper or embossed paper, plastic film, or cloth. 
     The optical processing mechanism  101  forms yellow, magenta, cyan, and black toner images and transfers them to an intermediate transfer belt  7 . Specifically, the optical processing mechanism  101  includes laser scanner units  1 Y,  1 M,  1 C, and  1 K, photosensitive drums  2 Y,  2 M,  2 C, and  2 K, charging rollers  3 Y,  3 M,  3 C, and  3 K, developers  4 Y,  4 M,  4 C, and  4 K, and developing sleeves  5 Y,  5 M,  5 C, and  5 K corresponding to respective colors. The optical processing mechanism  101  further includes cleaner units  6 Y,  6 M,  6 C, and  6 K, and primary transfer rollers  8 Y,  8 M,  8 C, and  8 K corresponding to respective colors. The optical processing mechanism  101  further includes the intermediate transfer belt  7 , an intermediate transfer belt driving roller  9 , a cleaner unit  10 , and a secondary transfer roller  11 . On the other hand, the fixing processing mechanism  102  includes a fusing unit  12  as a fixing portion, and the fixing unit  12  has a fixing roller  13  and a pressure roller  14 . 
     The sheet feeding unit  100 B includes sheet feeding cassettes  15   a ,  15   b ,  15   c , and  15   d  that store sheets S, feeding rollers  17   a ,  17   b ,  17   c , and  17   d , separation roller pairs  16   a ,  16   b ,  16   c , and  16   d , and intermediate conveyance roller pairs  20   a ,  20   b ,  20   c , and  20   d . The sheet conveying unit  100 C includes a pre-registration roller pair  19  and a registration roller pair  18 . The sheet conveying unit  100 C further includes a discharge roller pair  21 , a reverse discharge roller pair  26 , and double-sided conveyance roller pairs  23   a ,  23   b ,  23   c , and  23   d . The sheet reversing unit  100 D includes a first reverse conveyance roller pair  22   a  and a second reverse conveyance roller pair  22   b.    
     Image Forming Operation 
     Next, an image forming operation in the image forming apparatus  100  will be described. Each of the photosensitive drums  2 Y,  2 M,  2 C, and  2 K includes an organic photoconductive layer applied to the outer circumference of an aluminum cylinder, which is rotated in a counterclockwise direction in  FIG.  1    by a drive motor (not illustrated). The surfaces of the photosensitive drums  2 Y,  2 M,  2 C, and  2 K are charged by the charging rollers  3 Y,  3 M,  3 C, and  3 K, respectively. The surfaces of the photosensitive drums  2 Y,  2 M,  2 C, and  2 K are then exposed by laser beams emitted from the laser scanner units  1 Y,  1 M,  1 C, and  1 K to form electrostatic latent images on these surfaces based on image data sent from a control unit  200  described below. Then, the respective color toners are transferred to the electrostatic latent images by the developing sleeves  5 Y,  5 M,  5 C, and  5 K of the developers  4 Y,  4 M,  4 C, and  4 K to develop toner images. 
     The intermediate transfer belt  7  is in contact with the photosensitive drums  2 Y,  2 M,  2 C, and  2 K and rotates in a clockwise direction in  FIG.  1    by the intermediate transfer belt driving roller  9 . Then, the toner images of the respective colors are sequentially transferred from the photosensitive drums  2 Y,  2 M,  2 C, and  2 K to the surface of the intermediate transfer belt  7  by the primary transfer rollers  8 Y,  8 M,  8 C, and  8 K under the primary transfer bias to form color toner images. The toner residue remaining on the photosensitive drums  2 Y,  2 M,  2 C, and  2 K without being transferred to the intermediate transfer belt  7  is collected by the cleaner units  6 Y,  6 M,  6 C,  6 K, whereby the photosensitive drums  2 Y,  2 M,  2 C, and  2 K are cleaned. 
     On the other hand, the sheet feeding unit  100 B starts feeding the sheets S from any of the sheet feeding cassettes selected from the sheet feeding cassettes  15   a ,  15   b ,  15   c , and  15   d  by any of the feeding rollers  17   a ,  17   b ,  17   c , and  17   d  corresponding to the selected sheet feeding cassette. The fed sheets S are separated one by one by any of the separation roller pairs  16   a ,  16   b ,  16   c , and  16   d  corresponding to the selected sheet feeding cassette, and then conveyed to the sheet conveying unit  100 C by the intermediate conveyance roller pairs  20   a ,  20   b ,  20   c , and  20   d.    
     In the sheet conveying unit  100 C, the sheet S conveyed by the intermediate conveyance roller pairs  20   a ,  20   b ,  20   c , and  20   d  is conveyed by the pre-registration roller pair  19  toward the registration roller pair  18  via the pre-transfer conveyance path  61 . The registration roller pair  18  conveys the sheet S while adjusting the conveying speed of the sheet S according to the timing when the color toner images transferred on the surface of the intermediate transfer belt  7  reach the secondary transfer roller  11 , that is, the sheet S and the toner images are aligned. 
     Thereafter, the secondary transfer roller  11  and the intermediate transfer belt  7  hold and convey the sheet between them, during which the color toner images on the intermediate transfer belt  7  are superimposed onto the sheet by the secondary transfer bias. The secondary transfer roller  11  is in contact with the intermediate transfer belt  7  while the color toner images are superimposed on the intermediate transfer belt  7 , but is separated from the intermediate transfer belt  7  after the transfer is completed. The toner residue remaining on the intermediate transfer belt  7  without being transferred to the sheet S is collected by the cleaner unit  10 , whereby the intermediate transfer belt  7  is cleaned. 
     The sheet S, to which the color toner images have been transferred by the secondary transfer roller  11 , is conveyed to the fixing unit  12  of the fixing processing mechanism  102  via the fixing conveyance path  62 . The fixing unit  12  heats the sheet S with the fixing roller  13  and applies pressure with the pressure roller  14  to fix the toner images on the sheet S. The fixing roller  13  is formed in a hollow shape and contains a heater (not illustrated) inside. 
     The sheet S that has passed through the fixing unit  12  is guided from the fixing conveyance path  62  to either a discharge conveyance path  63  or a pre-reverse conveyance path  64  by a flapper (not illustrated). The sheet S conveyed to the pre-reverse conveyance path  64  is guided to the sheet reversing unit  100 D, which includes the first reverse conveyance roller pair  22   a , the second reverse conveyance roller pair  22   b , and a reversing conveyance path  65  as a conveyance path. That is, the sheet S conveyed to the pre-reverse conveyance path  64  is guided toward the reversing conveyance path  65  by the first reverse conveyance roller pair  22   a  and/or the second reverse conveyance roller pair  22   b.    
     In the case of double-sided printing, the sheet S with an image on its front side (first side) is conveyed to the reversing conveyance path  65  until its trailing edge passes the entrance of a re-conveyance path  67 . Then, the downstream edge (leading edge) and upstream edge (trailing edge) in the sheet conveyance direction are switched by the switchback operation performed by the second reverse conveyance roller pair  22   b . With the leading and trailing edges switched by the second reverse conveyance roller pair  22   b , the sheet is conveyed to the re-conveyance path  67  and guided by the double-sided conveyance roller pairs  23   a ,  23   b ,  23   c , and  23   d  toward the secondary transfer roller  11  again, where an image is formed on the back side (second side) opposite the front side. 
     The sheet S, on which image formation on one side of the sheet has been completed, or the sheet S, on which image formation on the back side of the sheet in double-sided printing has been completed, is guided to the discharge conveyance path  63 . The sheet S conveyed to the discharge conveyance path  63  is discharged by the discharge roller pair  21  onto a sheet discharge tray  24  provided outside the image forming apparatus  100 . 
     On the other hand, when the sheet S that has passed through the fixing unit  12  is reversed and discharged, the sheet S with an image formed on its surface is guided to the pre-reverse conveyance path  64 . It is then conveyed to the reversing conveyance path  65  until its trailing edge passes the entrance to a reversing discharge path  66 . Then, the downstream edge (leading edge) and upstream edge (trailing edge) in the sheet conveyance direction are switched by the switchback operation performed by the first reverse conveyance roller pair  22   a . The sheet S, whose leading and trailing edges have been switched by the first reverse conveyance roller pair  22   a , is guided to the reversing discharge path  66  and conveyed to the discharge roller pair  21  by the reverse discharge roller pair  26 . The sheet S, which is conveyed to the reversing discharge path  66  after being reversed in this manner, is also discharged by the discharge roller pair  21  with the front and back sides reversed on the sheet discharge tray  24  provided outside the image forming apparatus  100 . 
     Control System Configuration of Image Forming Apparatus 
     Next, the configuration of the control system of the image forming apparatus  100  is described using  FIGS.  2 A and  2 B . The block diagram in  FIG.  2 A  illustrates the control unit  200  as a function, and the block diagram in  FIG.  2 B  illustrates the control unit  200  as a hardware configuration. As illustrated in  FIG.  2 A , in the image forming apparatus  100 , an operation unit  205 , an image sensor  30  as an image detection unit, which will be described below in detail, and the image forming unit  100 A described above are connected to the control unit  200 . The control unit  200  is connected to a computer (PC)  500  or the like disposed outside via an external interface. 
     As illustrated in  FIG.  2 B , the control unit  200  includes a CPU  201 , a ROM  202 , a RAM  203 , and an HDD  204 . The CPU  201  is a calculation unit that controls each unit (see  FIG.  2 A ). The ROM  202  stores control programs for various processes executed by the CPU  201 . The RAM  203  is a system work memory for the CPU  201  to operate. The HDD  204  stores, for example, image data transferred from the computer  500  and setting information input from the operation unit  205 . The HDD  204  stores information in a management table  400 , which will be described below in detail. 
     As illustrated in  FIG.  2 A , the operation unit  205  is an example of a user interface such as an operation panel, which is omitted in  FIG.  1   , and has a display unit and a key input unit. The operation unit  205  accepts setting information and other information entered by the user via the display unit and key input unit, and also displays information to the user via the display unit. The key input unit has, for example, a start key to indicate the start of scanning, copying, or other operations, a stop key to stop scanning, copying, or other operations, and a numeric keypad. 
     The image processing unit  210 , which functions by the control unit  200 , performs various types of image processing on image data to be formed on the sheet S, such as image data transferred from the computer  500 . The image processing unit  210  has an image position correction unit  211 , and the image position correction unit  211  corrects the position of the image formed on the sheet S, as will be described below in detail. The functions of the image processing unit  210  may be realized by an integrated circuit such as an ASIC, or by image data processing by the CPU  201  based on a pre-stored program. 
     The image data generated by the image processing unit  210  is sent to laser scanner units  1 Y,  1 M,  1 C, and  1 K of the image forming unit  100 A. The laser scanner units  1 Y,  1 M,  1 C, and  1 K are controlled based on the image data generated by the image processing unit  210  to expose the surfaces of photosensitive drums  2 Y,  2 M,  2 C, and  2 K. As a result, electrostatic latent images based on the image data generated by the image processing unit  210  are formed on the surfaces of the photosensitive drums  2 Y,  2 M,  2 C, and  2 K. 
     Correction of Image Formation Position on Sheet 
     Next, the correction of the position of the image formation performed by the image position correction unit  211  is described using  FIGS.  2  to  3 B .  FIG.  3 A  illustrates the image position on the sheet before correction of the image by the image forming unit, and  FIG.  3 B  illustrates the image position on the sheet after correction of the image by the image forming unit. 
     The position of the image formed on the sheet S in the image forming operation may not be the ideal position. As shown in  FIG.  3 A , for example, if the sheet S conveyed by the registration roller pair  18  disposed upstream in the conveyance direction of the image forming unit  100 A is inclined, the sheet passes through the secondary transfer roller  11  in an inclined state as it is. As a result, the image a 1  may be formed inclined to the sheet S, and the image a 1  may be formed at a position deviated from the ideal position a 2 . 
     In addition, for example, if the pressure distribution of the rollers of the fixing unit  12  is not uniform, the sheet after passing through the fixing unit  12  may be deformed and the image formed on the sheet S may be inclined. That is, for example, when an image is formed on the surface of the sheet S in double-sided printing, the sheet S expands and contracts due to heating and pressure from the fixing unit  12 . Then, the size of the image formed on the front side of sheet S may be different from the size of the image formed on the back side of the sheet S. In this case, the position of the image formed on the front side of the sheet S is different from the position of the image formed on the back side of sheet S. 
     Therefore, at least one or more distances b 1  and b 2  from the edges of the sheet S, indicated by arrows in  FIG.  3 A , are measured by the image sensor  30 , described below, for the image a 1  deviated from the ideal position a 2 , and the calculation unit  201  calculates the amount of deviation of the image a 1  with respect to the sheet S. The image position correction unit  211  controls the laser scanner units  1 Y,  1 M,  1 C, and  1 K of the image forming unit  100 A so that the image formation position on the sheet is at the ideal position a 2  according to the amount of deviation detected by the image sensor  30  in this manner. In short, the image position correction unit  211  corrects the shape of the toner images formed on the photosensitive drums  2 Y,  2 M,  2 C, and  2 K and transferred to the intermediate transfer belt  7  so that they are in the ideal position a 2 . 
     In this case, the image position correction unit  211  converts the image data based on the conversion formula for correcting the deviation of the image formation position with respect to the sheet stored in the management table  400 . When the image forming unit  100 A forms an image based on the image data converted by the image position correction unit  211 , an image that offsets the deviation of the image formation position with respect to the sheet S is formed on the intermediate transfer belt  7 . 
     The management table  400  stores for each sheet the amount of deviation of image position generated by the calculation unit  201  (described below) and the conversion formula for correcting the amount of deviation. The calculation unit  201  calculates the setting information input from the operation unit  205  and the position information (amount of deviation) of the image obtained by the image sensor  30 , based on the information in the control table  400 , and stores the calculation results in the management table  400 . 
     Problem of Detection Accuracy of Image Sensor During Sheet Conveyance 
     The image sensor  30  described above detects the position of the image transferred and fixed on the sheet S and the edges of the sheet S, and includes, for example, a scanner sensor that scans images, such as a contact image sensor (CIS). With such a scanner sensor, as the conveying speed of the sheet S increases, the measurement error due to insufficient measurement resolution becomes larger when measuring the image position of the sheet S. If the measurement error becomes large, when the image formed on the back side of sheet S is corrected by the image position correction unit  211  described above, an error will occur according to the measurement error, and the quality of the product will be degraded. First, the causes of measurement errors are described below using  FIG.  11   . In the present embodiment, the image sensor  30  includes a CIS, but it may be a scanner sensor such as a charge coupled device (CCD). 
       FIG.  11 A  illustrates the positional relationship between the sheet and the image sensor at time points T 1  to T 3  during sheet conveyance.  FIG.  11 B  illustrates the voltage signals of the image sensor at time points T 1  to T 3  during sheet conveyance.  FIG.  11 C  illustrates the signal waveform of the image sensor acquired during sheet conveyance. 
     As illustrated in  FIG.  11 A , when the sheet S is being conveyed, the sheet S is advancing at a constant speed when time passes at regular time intervals in the order of time points T 1 , T 2 , and T 3 . At this time, the image sensor  30  reads the image on the first side of the sheet S and acquires the distance a 3  from the edge of the sheet S to the edge of the image a 1 . In this case, in detail, the image sensor  30  irradiates light from a light source onto the object to be measured and measures the reflected light as a voltage value V. Then the voltage values V as the signal values read by the image sensor  30  at time points T 1 , T 2 , and T 3  in the length L of the detection range of the image sensor  30  are as illustrated in  FIG.  11 B . That is, they are waveforms whose magnitude changes at the position between the edge of the sheet S and the edge of the image a 1 , i.e., they are signal waveforms indicating the distance a 3 . 
     In this case, the measurement is performed while a certain amount of irradiated light is applied, but only for a moment, there is not enough reflected light to measure color changes such as at the edges of the sheet or in the image, resulting in an extremely small difference in the voltage value V. Therefore, it is necessary to continue measuring the reflected light for a predetermined period of time (expose the camera and charge the voltage) in order to measure sufficient reflected light. 
     Therefore, as illustrated in  FIG.  11 B , when measurement is performed on the sheet S that is being conveyed, there is always a phenomenon where the signal waveform shifts even slightly. The shifted signal waveform is then integrated to obtain the exposed and charged signal waveform as illustrated in  FIG.  11 C . The calculation unit  201  calculates image position information  300  based on the signal waveform, but by measuring at a predetermined time, the signal waveform is not a rectangular wave but a signal with blurred rising and falling edges (afterimage phenomenon). 
     That is, the calculation unit  201  converts this signal waveform into the image position information  300  by setting a threshold value T. However, depending on how this threshold value T is determined and how the signal waveform is blurred, the correct distance a 3  is not obtained and the image position information  300  contains measurement errors. This results in the formation of an image on the second surface that is deviated by the amount of error when correcting the image position on the second side, which degrades the quality of the product. 
     Sheet Conveyance Operation in Sheet Reversing Unit 
     Next, the conveyance operation of the sheet S in the sheet reversing unit  100 D is described.  FIG.  4    illustrates the sheet S being conveyed to the second reverse conveyance roller pair  22   b ,  FIG.  5    illustrates the sheet S stopped by the second reverse conveyance roller pair  22   b , and  FIG.  6    illustrates the sheet S being reversed and conveyed by the second reverse conveyance roller pair  22   b.    
     When double-sided printing is performed as described above, as illustrated in  FIG.  4   , the sheet S is conveyed to the first reverse conveyance roller pair  22   a  and the second reverse conveyance roller pair  22   b  of the sheet reversing unit  100 D via the pre-reverse conveyance path  64  after passing through the fixing unit  12 . The first reverse conveyance roller pair  22   a  and the second reverse conveyance roller pair  22   b  are configured to be rotatable forward and reverse, and they first convey the sheet S in the first direction indicated by the arrow A and guide it into the reversing conveyance path  65 . Then, the sheet S is conveyed by the second reverse conveyance roller pair  22   b  to the reversing conveyance path  65  until the trailing edge of the sheet S in the first direction passes the entrance of the re-conveyance path  67 , which is the branch point between the reversing conveyance path  65  and the re-conveyance path  67 . 
     Then, as illustrated in  FIG.  5   , when the trailing edge of the sheet S in the first direction passes the entrance of the re-conveyance path  67 , the second reverse conveyance roller pair  22   b  is stopped to stop the conveyance of the sheet S. Then, as illustrated in  FIG.  6   , the rotation of the second reverse conveyance roller pair  22   b  is inverted (reversed) and the sheet S is conveyed in the second direction, which is opposite to the first direction indicated by the arrow B, and the sheet S is conveyed to the re-conveyance path  67 . The sheet conveyed to the re-conveyance path  67  as described above is conveyed to the registration roller pair  18  by the double-sided conveyance roller pairs  23   a ,  23   b ,  23   c , and  23   d . The sheet is then conveyed again toward the secondary transfer roller  11  in the image forming unit  100 A to form an image on the second side of the sheet S. 
     When reversing the conveyance direction of the sheet S with the second reverse conveyance roller pair  22   b , the sheet S is always temporarily stopped to drive the second reverse conveyance roller pair  22   b  in reverse. The time depends on the type and performance of the actuator, but in the case of an inexpensive stepping motor, a minimum static period of about 50 ms is required to prevent step-out. In other words, when double-sided printing is performed by the image forming apparatus  100 , the sheet S is always temporarily stopped in the sheet reversing unit  100 D due to its structure. 
     Position and Operation of Image Sensor 
     Next, arrangement and operation of the image sensor  30  will be described with reference to  FIGS.  5  and  7 A to  7 C .  FIG.  7 A  illustrates the positional relationship between the sheet and the image sensor at time points T 1  to T 3  when the sheet is stopped.  FIG.  7 B  illustrates the voltage signals of the image sensor at time points T 1  to T 3  when the sheet is stopped.  FIG.  7 C  illustrates the signal waveform of the image sensor acquired when the sheet of the first embodiment is stopped. 
     As illustrated in  FIG.  5   , the image sensor  30  is disposed at a position facing the first side of the sheet S and capable of detecting at least a portion of the image on the first side formed on the sheet S when the sheet S is stopped by the second reverse conveyance roller pair  22   b  in the sheet reversing unit  100 D. Specifically, as illustrated in  FIG.  7 A , the image sensor  30  is disposed to detect a portion of the image a 1  on the first side of the stopped sheet S and the trailing edge (end) of the sheet S in the first direction. In other words, the image sensor  30  is disposed to detect a portion of the image a 1  on the first side of the stopped sheet S and the leading edge of the sheet S in the second direction (the upstream edge in the second direction). 
     As illustrated in  FIG.  7 A , since the conveyance of the sheet S is stopped by the second reverse conveyance roller pair  22   b , the sheet S does not move at time points T 1 , T 2 , and T 3 . Therefore, as illustrated in  FIG.  7 B , the signal waveform of the voltage value V as the signal value read by the image sensor  30  at time points T 1 , T 2 , and T 3  also does not move in the length L of the detection range of the image sensor  30 . As illustrated in  FIG.  7 C , this allows the integrated signal waveform to be detected as a near rectangular wave signal with almost no blurring, even if the measurement is performed for a specified time so that the difference in the voltage value V can be detected. Therefore, even when converting to the image position information  300  for detecting the distance a 3  in the calculation unit  201 , the image position information  300  with reduced measurement error can be obtained, regardless of the setting of the threshold T and other factors. 
     As described above, in the image forming apparatus  100 , even if the conveying speed of the sheet S is increased to improve productivity, the sheet S is stopped when the image on the first side of the sheet S is detected by the image sensor  30 . This allows stable detection without affecting the detection accuracy with the image sensor  30  and improves the detection accuracy of the formation position of the image a 1  on the sheet S. As a result, in the formation of an image on the second side, when the position of the image a 1  on the first side is fed back to correct the position of the image on the second side, high front-to-back registration accuracy can be achieved, preventing deterioration of the quality of the product. 
     Second Embodiment 
     The second embodiment, which is a partial modification of the first embodiment described above, is then described using  FIGS.  8 A,  8 B,  9 A, and  9 B .  FIG.  8 A  is a block diagram of the control system of the image forming apparatus according to the second embodiment, and  FIG.  8 B  is a block diagram of the control unit.  FIG.  9 A  illustrates the image position on a sheet before correction of the conveying speed by the registration roller pair, and  FIG.  9 B  illustrates the image position on a sheet after correction of the conveying speed by the registration roller pair. In the description of the second embodiment, the same reference numerals are used for parts similar to those in the first embodiment described above, and their description is omitted. 
     In the first embodiment, when adjusting the position of the image formed on the second side of the sheet S, the image is corrected by the image position correction unit  211  (see  FIG.  2 A ), which is then formed on the second side of the sheet S by the image forming unit  100 A. Apart from this, in the second embodiment, the image formation position is adjusted by adjusting the speed of the sheet S and the timing of the sheet S passing through the secondary transfer roller  11  in the image forming unit  100 A. The adjustment of the image formation position according to the second embodiment and the adjustment of the image formation position according to the first embodiment may be performed simultaneously in combination. 
     Specifically, as illustrated in  FIGS.  8 A and  8 B , the image forming apparatus  100  according to the second embodiment includes a sheet position correction unit  212  instead of the image position correction unit  211  (see  FIG.  2 A ) of the image processing unit  210 . The sheet position correction unit  212  controls the speed of the sheet S that is conveyed from the registration roller pair  18  to transfer the toner images on the second side by the secondary transfer roller  11 , so that the position of the image to be formed (toner images to be transferred) is at the target position. 
     The image forming apparatus  100  includes a leading edge detection sensor  31  that detects the tip position of the sheet S between the registration roller pair  18  and the secondary transfer roller  11  in the sheet conveyance direction (see  FIG.  1   ), and outputs a signal indicating that the leading edge of the sheet S is detected to the calculation unit  201 . The leading edge detection sensor includes a scanner sensor such as a contact image sensor (CIS) or a charge coupled device (CCD), or a photoelectric sensor such as a transmission or retro-reflection sensor. 
     Furthermore, the management table  400  in the second embodiment stores a conversion formula (correction amount) of the speed of the registration roller pair  18  for correcting deviation of the image formation position from the ideal position of the image with respect to the sheet S. As in the first embodiment, the management table  400  stores, for each sheet, the amount of deviation of the image formation position calculated by the calculation unit  201  and the conversion formula for correcting the amount of deviation. 
     As described above, the formation position of the image formed on the sheet S by the image forming operation may not be the ideal formation position. As illustrated in  FIG.  9 A , an image c 1  is formed on the first side of the sheet S at a position deviating from the ideal position, and an image c 2  is formed on the second side. When no correction is made, the distance between the trailing edge of sheet S in the first direction (see  FIG.  4   ), i.e., the leading edge in the second direction (see  FIG.  6   ), and the edge of the image c 1  on the first side is the distance a 3 . The distance between the leading edge of the sheet S in the second direction and the edge of the image c 2  on the second side is a distance a 4 . The difference between the distance a 3  and the distance a 4  is the amount of deviation between the front and back, and the front-to-back registration accuracy is not good. 
     Setting information input from the operation unit  205  is input to the calculation unit  201 . In addition, the image position information  300  (the position of the image c 1  formed on the first side of the sheet S, which is the distance a 3 ) detected by the image sensor  30 , the leading edge position of the sheet S detected by the leading edge detection sensor  31 , and other information are input to the calculation unit  201 . The calculation unit  201  stores these pieces of information in the management table  400 . 
     Then, the sheet position correction unit  212  calculates the rotational speed of the registration roller pair  18  by the conversion formula of the management table  400  based on the image position information  300  detected by the image sensor  30  and the information of the leading edge position of the sheet S detected by the leading edge detection sensor  31 . The sheet position correction unit  212  then corrects the rotational speed of the registration roller pair  18  by accelerating or decelerating it to that calculated speed, so that the image c 2  on the second side is aligned with the image c 1  on the first side. In other words, the speed of the registration roller pair  18  is controlled so that the distance a 4  is equal to the distance a 3 . As a result, as illustrated in  FIG.  9 B , the position of the image c 2  (toner images) transferred to the second side of the sheet S by the secondary transfer roller  11  is corrected to obtain good front-to-back registration accuracy. 
     The other configurations, operations, and effects in the second embodiment are the same as those in the first embodiment, so the description thereof is omitted. 
     Third Embodiment 
     The third embodiment, which is a partial modification of the first and second embodiments described above, is then described using  FIGS.  10 ,  12 A, and  12 B .  FIG.  10    is a schematic cross-sectional view of a sheet pressing mechanism according to the third embodiment.  FIG.  12 A  is a schematic cross-sectional view of the reversing conveyance path with the trailing edge of the sheet in the first direction inclined away from the image sensor.  FIG.  12 B  is a schematic cross-sectional view of the reversing conveyance path with the trailing edge of the sheet in the first direction inclined closer to the image sensor. 
     In the first and second embodiments described above, the image on the first side is detected when the sheet S is temporarily stopped during reversal. However, in order to further improve measurement accuracy, it is necessary to take into account the measurement error effect caused by the flutter of the sheet S. As illustrated in  FIGS.  12 A and  12 B , when the sheet S is conveyed through the reversing conveyance path  65  (see  FIG.  1   ), the sheet S is conveyed through a gap G formed between a conveyance guide  41  and a conveyance guide  42  as guide units. Within the gap G, a sheet, especially one with low stiffness, will freely change its posture and shape. A through hole  41   a  is formed in the conveyance guide  41 , and a transparent glass  40  as the transmissive unit of the image sensor  30  is installed in the through hole  41   a  so that it is exposed to the reversing conveyance path  65 . The transparent glass  40  allows the irradiation light of the image sensor  30  to illuminate the sheet S and allows the reflected light to pass through. 
     At this time, as illustrated in  FIG.  12 A , the trailing edge of the sheet S in the first direction may be inclined away from the image sensor  30 . At this time, as illustrated in  FIG.  12 A , the trailing edge of the sheet S in the first direction may be inclined closer to the image sensor  30 . Then, the distance F from the image sensor  30  to the sheet S varies in the sheet conveyance direction. 
     As described above, the image sensor  30  includes, for example, an image sensor such as a CIS, and due to its characteristic of detecting reflected light and converting it into a voltage, changes in the distance and angle to the measurement target will cause variations in the charged voltage due to changes in the size and angle of the reflected light. Therefore, the image sensor generally has a focal length suitable for measurement. 
     Therefore, as illustrated in  FIGS.  12 A and  12 B , when the posture or shape of the sheet changes freely in the gap G, the voltage value measured by the image sensor  30  will vary, resulting in a distance a 3  that is not correct when converted to the image position information  300 . That is, the image position information  300  contains a measurement error. Then, when correcting the image formation position on the second side, an image that is shifted by the amount of the error is formed on the second side, which degrades the quality of the product. 
     Therefore, in the third embodiment, as illustrated in  FIG.  10   , a pressing mechanism  50  is provided to press the sheet S toward the transparent glass  40  of the image sensor  30  to correct the position and posture of the sheet S when the image c 1  of the sheet S is detected by the image sensor  30 . 
     Specifically, as illustrated in  FIG.  10   , the pressing mechanism  50  includes a contact member  51 , which is formed in a cylindrical shape and contacts the sheet S, and an urging member  52 , which includes a coil spring and urges the contact member  51  toward the image sensor  30 . As a result, when the sheet S is stopped by the second reverse conveyance roller pair  22   b , especially the trailing edge of the sheet S in the first direction and the edge of the sheet S with a distance a 3  from the edge of the image are restrained to the transparent glass  40 . This brings the distance F from the image sensor  30  to the sheet S closer to a certain distance. That is, the distance and angle between the image sensor  30  and the first side of the sheet S approaches a constant, which stabilizes the magnitude and angle of the reflected light and reduces the measurement error in the voltage value. Thus, the measurement error of the image sensor  30  is reduced, making it possible to obtain the image position information  300  with a very small measurement error for the correct distance a 3 . Therefore, the pressing mechanism  50  in the third embodiment reduces the fluttering of the sheet S, resulting in good correction of the image formation position in image formation on the second side, and realizing high front-to-back registration accuracy. 
     Possibility of Other Embodiments 
     In the first and third embodiments described above, the image forming apparatus  100  is an image forming apparatus of electrophotographic system. However, the present invention is not limited thereto, and a method of correcting a position at which an image is formed on a sheet by correcting an image to be formed on the sheet or correcting a conveying speed of the sheet can be similarly applied to other types of image forming apparatuses, such as an inkjet image forming apparatus. Therefore, even in such an image forming apparatus, in those that reverse the sheet conveyance direction when printing on both sides of the sheet, the quality of the product can be improved by detecting the image on the sheet with an image sensor while the sheet is temporarily stopped. 
     In the first and third embodiments, the type and position of the image formed on the sheet S is not limited to a specific image. In other words, whether the image is, for example, a cross mark (so-called “registration mark”) or a photographic image, as long as the position of the image relative to the sheet can be detected, there are no restrictions on the type or position of the image. 
     In the first to third embodiments, the sheet is temporarily stopped when reversing the sheet conveyance direction in the sheet reversing unit  100 D, and the image on the sheet is detected by the image sensor  30  at that time. However, as long as the image forming apparatus includes a conveyance unit that conveys a sheet after an image is formed on the sheet by the image forming unit  100 A, and can stop the conveyance of the sheet, the image sensor may detect the image when the conveyance unit temporarily stops the sheet. In this case, the image sensor would be disposed opposite the first side at the position where the sheet is temporarily stopped in the conveyance unit. Examples of cases in which a sheet is temporarily stopped by the conveyance unit include when correcting skew of a sheet or when punching holes in a sheet. 
     In the first to third embodiments, the image sensor  30  detects an image in a range of only the distance L in the sheet conveyance direction, that is, detects an image in a range in which the distance between the trailing edge of the sheet in the first direction and the edge of the image can be detected. However, the embodiments are not limited thereto, and the image may be read from the entire sheet, i.e., any image in any range may be detected as long as the position of the image formed on the sheet can be detected. 
     In the first to third embodiments, the image sensor  30  is disposed at the position to detect the vicinity of the trailing edge of the sheet in the first direction (the leading edge in the second direction) when the sheet is stopped by the second reverse conveyance roller pair  22   b . This is because when correcting the position of the image on the second side of the sheet, the edge of the sheet in the second direction is the leading edge of the sheet, and the distance from the leading edge is adjusted. However, the image sensor  30  may be disposed to detect the vicinity of the leading edge of the sheet in the first direction (trailing edge in the second direction) when the sheet is stopped by the second reverse conveyance roller pair  22   b . That is, if the distance between the leading edge of the sheet in the first direction and the edge of the image from there can be detected, the distance between the trailing edge of the sheet in the first direction and the edge of the image from there may be calculated based on the size of the image formed on the first side of the sheet. 
     In the second embodiment, when adjusting the speed of the sheet at the registration roller pair  18  by the sheet position correction unit  212 , the leading edge detection sensor  31  detects the position of the leading edge of the sheet and adjusts the speed accordingly. However, for example, in a system where the registration roller pair  18  is temporarily stopped to correct skewed sheets at the registration roller pair  18  and then resumes conveyance, the amount of sheet feed from the timing of resumption of conveyance by the registration roller pair  18  may be used as the position of the leading edge of the sheet. 
     In the third embodiment, the contact member  51  of the pressing mechanism  50  is a cylindrical member. However, the contacting member  51  may be any shape such as a flat plate as long as it can stabilize the distance and angle between the image sensor  30  and the sheet S. The contact member  51  may include a single contact member  51  or a plurality of contact members, and their positions may be different from those shown in  FIG.  10   . In short, it is sufficient that the sheet can be pressed toward the image sensor  30 . Although the urging member  52  was described as being a coil spring or the like, it may also be a mechanism that can change the position of the contact member  51  variably, such as a solenoid or cam mechanism, or even a configuration that moves the conveyance guide  42  to narrow the gap G. In addition, although the pressing mechanism  50  is described as an example of a pressing unit, any configuration of the pressing unit may be used as long as the sheet can be pressed against the transmissive unit, for example, by blowing air onto the sheet and pressing it against the transparent glass  40 . 
     The present disclosure can also be realized by supplying a program that realizes one or more functions of the above example to a system or apparatus via a network or storage medium, and processing in which one or more processors in the computer of the system or apparatus read and execute the program. It can also be realized by a circuit that realizes one or more functions (for example, ASIC). 
     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 a ‘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. 2021-178496, filed Nov. 1, 2021, which is hereby incorporated by reference herein in its entirety.