Patent Publication Number: US-2023152743-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, and a multifunction peripheral having a plurality of functions thereof. 
     Description of the Related Art 
     In an image forming apparatus, an image formed on an image bearing member such as a photosensitive drum or an intermediate transfer belt is transferred to a sheet in a transfer portion, the sheet to which the image has been transferred is conveyed to a fixing device, and the image is fixed to the sheet. At this time, a loop is formed on the sheet so that the sheet is not pulled between the transfer portion and the fixing device, and the loop is detected by a sensor to control the sheet conveyance speed of the transfer portion and the fixing device (see JP 2007-233372 A). In addition, JP 2007-233372 A discloses that a sheet remaining in a fixing device is detected by a loop sensor for detecting a loop. 
     As described above, in the case of the configuration described in JP 2007-233372 A, the loop detection unit also detects the residual sheet, but in order to detect the small-sized sheet, it is required to bring the contact portion of the loop detection unit, which is contact with the sheet, close to the inlet of the nip portion of the fixing device as much as possible. On the other hand, in the loop detection, since the deformation amount of the sheet is detected, it is required to bring the contact portion into contact with the sheet on the upstream of the position where the residual sheet detection is performed. In addition, when the residual sheet is detected by the loop detection unit, it is difficult to accurately detect the residual sheet. 
     SUMMARY OF THE INVENTION 
     The present invention provides a configuration capable of detecting a loop of a sheet and capable of improving detection accuracy of a sheet remaining in a fixing device. 
     According to one aspect of the present invention, an image forming apparatus includes a transfer portion configured to transfer a toner image onto a recording material, a first rotary member that is rotatable and includes a heat source, a second rotary member configured to form a nip portion by being in contact with an outer peripheral surface of the first rotary member, the second rotary member being configured to fix the toner image by applying heat and pressure while nipping and conveying the recording material together with the first rotary member, a contact member configured to come into contact with a back surface between the nip portion and the transfer portion in a conveyance direction of the recording material in a case where a surface of the recording material that comes into contact with the first rotary member is referred to a front surface and a surface of the recording material that comes into contact with the second rotary member is referred to the back surface when the recording material is nipped and conveyed by the nip portion, the contact member being capable of being located at a first position, a second position, and a third position, and, a detection unit configured to detect a position of the contact member so that the detection unit can detect the first position, the second position, and the third position. The first position is a position where the contact member is located in a case where the contact member is not in contact with the back surface. The second position and the third position are positions where the contact member is positioned in a case where the contact member comes into contact with the back surface. The contact member reaches the third position by being biased by the recording material from the second position. 
     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 view of an image forming apparatus according to a first embodiment. 
         FIG.  2    is a schematic configuration cross-sectional view of a fixing device according to the first embodiment. 
         FIG.  3    is a diagram illustrating a loop detection state of a sheet according to the first embodiment. 
         FIG.  4    is a diagram illustrating a residual detection state of a sheet according to the first embodiment. 
         FIG.  5    is a flowchart of a loop detection operation of a sheet according to the first embodiment. 
         FIG.  6    is a flowchart of a residual detection operation of a sheet according to the first embodiment. 
         FIG.  7    is a diagram illustrating a residual detection state of a sheet according to a comparative example. 
         FIG.  8    is a diagram illustrating a loop detection state of a sheet according to the comparative example. 
         FIG.  9 A  is a diagram for explaining a state in which a sheet is looped according to a second embodiment. 
         FIG.  9 B  is a diagram for explaining a state in which the loop of the sheet is eliminated according to the second embodiment. 
         FIG.  10 A  is a diagram for explaining a state in which a residual sheet detection flag is in contact with a trailing edge of a residual sheet according to the second embodiment. 
         FIG.  10 B  is a diagram for explaining a state in which a residual sheet is detected by a residual sheet detection unit according to the second embodiment. 
         FIG.  11    is a diagram illustrating a relationship between a conveyance locus of a sheet and a nip portion according to a second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     A first embodiment will be described with reference to  FIGS.  1  to  6   . First, a schematic configuration of an image forming apparatus according to the present embodiment will be described with reference to  FIG.  1   . 
     Image Forming Apparatus 
     The image forming apparatus  100  of the present embodiment is an electrophotographic tandem type full color printer including four image forming units Pa, Pb, Pc, and Pd each including photosensitive drums  3   a ,  3   b ,  3   c , and  3   d  serving as photosensitive members. The image forming apparatus  100  forms a toner image on a sheet (recording material) according to an image signal from a document reading device (not illustrated) connected to an image forming apparatus main body  100 A or a host device such as a personal computer communicably connected to the image forming apparatus main body  100 A. Examples of the sheet include paper, a plastic film, and cloth. The image forming units Pa, Pb, Pc, and Pd form toner images of yellow, magenta, cyan, and black, respectively. 
     Note that the four image forming units Pa, Pb, Pc, and Pd included in the image forming apparatus  100  have substantially the same configuration except that the developed colors are different. Therefore, the image forming unit Pa will be described as a representative, and description of other image forming units will be omitted. 
     As illustrated in  FIG.  1   , a cylindrical photosensitive member, that is, a photosensitive drum  3   a  is disposed serving as an image bearing member in the image forming unit Pa. The photosensitive drum  3   a  is rotationally driven in an arrow direction in the drawing. A charging roller  2   a  serving as a charging unit, a developing unit  1   a , a primary transfer roller  6   a  serving as a transfer member, and a cleaning device  4   a  serving as a cleaning unit are disposed around the photosensitive drum  3   a . An exposing unit (laser scanner in the present embodiment)  5   a  serving as an exposure unit is disposed above the photosensitive drum  3   a  in the drawing. 
     An endless intermediate transfer belt  20  serving as an image bearing member that carries an image and an intermediate transfer member is disposed below each image forming unit in  FIG.  1   . The intermediate transfer belt  20  is stretched over the plurality of rollers  13 ,  14 , and  15 , and is configured to circulate (rotate) in a direction of an arrow A. Specifically, when the driving roller  13  is rotationally driven by a motor (not illustrated), the intermediate transfer belt  20  rotates. Then, the intermediate transfer belt  20  carries and conveys the toner image primarily transferred to the intermediate transfer belt  20  as described below. A secondary transfer outer roller  11  serving as a transfer member is disposed at a position facing a secondary transfer inner roller  14  with the intermediate transfer belt  20  interposed therebetween among the rollers stretching the intermediate transfer belt  20 , and forms a secondary transfer portion T 2  that transfers the toner image on the intermediate transfer belt  20  to the sheet P. That is, in the secondary transfer portion T 2 , the secondary transfer outer roller  11  conveys the sheet while nipping the sheet between the intermediate transfer belt  20 , and transfers the toner image on the intermediate transfer belt  20  (on the image bearing member) to the sheet P. The fixing device  9  is disposed downstream of the secondary transfer portion T 2  in a sheet conveyance direction. 
     A cassette  10  in which the sheet P is accommodated is disposed below the image forming apparatus  100 . The sheet P fed from the cassette  10  is conveyed toward a registration roller  12  by a feed roller (not illustrated). Then, a leading edge of the sheet P abuts against the registration roller  12  in the stopped state, and a skew feeding of the sheet P is corrected by forming a loop. Thereafter, the registration roller  12  is started to rotate in synchronization with the toner image on the intermediate transfer belt  20 , and the sheet P is conveyed to the secondary transfer portion T 2 . 
     A process of forming, for example, a four-color full-color image by the image forming apparatus  100  configured as described above will be described. First, when the image forming operation is started, a surface of the rotating photosensitive drum  3   a  is uniformly charged by the charging roller  2   a . Next, the photosensitive drum  3   a  is exposed by a laser beam corresponding to an image signal emitted from the exposing unit  5   a . As a result, an electrostatic latent image corresponding to the image signal is formed on the photosensitive drum  3   a . The electrostatic latent image on the photosensitive drum  3   a  is visualized by toner as a developer stored in the developing unit  1   a , and becomes a visible image. 
     The toner image formed on the photosensitive drum  3   a  is primarily transferred to the intermediate transfer belt  20  at a primary transfer portion formed between the photosensitive drum  3   a  and the primary transfer roller  6   a  disposed with the intermediate transfer belt  20  interposed therebetween. At this time, a primary transfer bias is applied to the primary transfer roller  6   a . The toner (transfer residual toner) remaining on the surface of the photosensitive drum  3   a  after the primary transfer is removed by the cleaning device  4   a.    
     Such an operation is sequentially performed in each of the yellow, magenta, cyan, and black image forming units, and toner images of four colors are superimposed on the intermediate transfer belt  20 . Thereafter, the sheet P accommodated in the cassette  10  is conveyed to the secondary transfer portion T 2  in accordance with the formation timing of the toner image. Then, by applying a secondary transfer bias to the secondary transfer outer roller  11 , the toner images of the four colors on the intermediate transfer belt  20  are secondarily transferred collectively onto the sheet P. The toner remaining on the intermediate transfer belt  20  without being transferred in the secondary transfer portion T 2  is removed by an intermediate transfer belt cleaner  24 . 
     Next, the sheet P is conveyed to the fixing device  9  via a belt conveyance device  30 . The belt conveyance device  30  is, for example, a conveyor belt that adsorbs and conveys the sheet P, and is disposed between the secondary transfer portion T 2  and the fixing device  9 . The belt conveyance device  30  is disposed to feed the sheet P discharged from the secondary transfer portion T 2  to the fixing device  9  while assisting the conveying posture of the sheet P. In the present embodiment, the belt conveyance device  30  is formed of a belt stretched between rollers, but may have a configuration of only a guide member that guides the sheet P. The sheet P having passed through the secondary transfer portion T 2  is delivered to the belt conveyance device  30 , and is conveyed to the fixing device  9  by the belt conveyance device  30 . The belt conveyance device  30  may be omitted, and the sheet P having passed through the secondary transfer portion T 2  may be directly conveyed to the fixing device  9 . 
     The fixing device  9  includes a fixing film  91  (first rotary member) and a pressure roller  92  (second rotary member) serving as a pair of rotary members, and a fixing nip portion is formed by the fixing film  91  and the pressure roller  92 . The sheet P to which the toner image has been transferred is caused to pass through the fixing nip portion of the fixing device  9 , whereby the sheet P is heated and pressurized. Then, the toner on the sheet is melted and mixed, and fixed to the sheet P as a full-color image. Thereafter, the sheet P is discharged to the outside of the apparatus by a discharge roller. 
     On the other hand, in a case where images are formed on both sides of the sheet P (double-sided printing), the sheet P is switched back to a switchback conveyance path  17  by switching a switching member  16 , and the sheet P is conveyed to a duplex conveyance path  19  by a conveyance roller  18 . The sheet P is conveyed from the duplex conveyance path  19  to the registration roller  12  in a state where the front and back sides of the sheet are reversed, and a toner image is formed on the back surface of the sheet P as described above. As a result, a series of image forming processes ends. 
     Note that the image forming apparatus  100  according to the present embodiment can also form a monochrome or multicolor image using an image forming unit for a desired monochrome or some of four colors, such as a black monochrome image. 
     In addition, the image forming apparatus  100  is provided with an operation panel S serving as an operation unit, and the operation panel S includes various buttons and operation switches operated by a user, and a display unit that displays a message to the user, an error such as occurrence of a jam, and the like. The image forming apparatus  100  includes a plurality of sheet detection sensors capable of detecting a sheet. Among these sheet detection sensors, a post-registration sensor  21  is a sensor for detecting that the leading edge of the sheet P having passed through the registration roller  12  has reached. A post-secondary transfer sensor  22  is a sensor for detecting that the leading edge of the sheet P has passed through the secondary transfer portion T 2 . A discharge sensor  23  is a sensor for detecting the sheet P on the side of the fixing device  9  from which the sheet P is discharged. 
     Signals from these sensors are transmitted to a control unit  110 . The control unit  110  receives signals of various sensors including these sheet detection sensors and controls each unit. In addition, the entire image forming apparatus  100  is controlled according to input job information such as the number of sheets on which an image is formed and the type of the sheet P, and an image is formed. Such a control unit  110  includes a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM). The CPU controls each unit while reading a program corresponding to the control procedure stored in the ROM. In addition, work data and input data are stored in the RAM, and the CPU performs control with reference to data stored in the RAM on the basis of the above-described program or the like. 
     Fixing Device 
     Details of the fixing device  9  will be described with reference to  FIG.  2   . The fixing device  9  includes the fixing film  91  serving as a fixing member and the pressure roller  92  serving as a pressure member. A heating member  102  for heating is disposed inside the fixing film  91  to heat the fixing film  91  to a predetermined temperature. In the fixing film  91 , a stay  104  is disposed so as to penetrate, and a support member  105  supporting the heating member  102  is biased toward the pressure roller  92  by an urging member such as a spring (not illustrated) via the stay  104 . Thus, a fixing nip portion N for nipping and conveying the sheet P is formed between the fixing film  91  and the pressure roller  92 . 
     The heating member  102  is, for example, a plate-shaped ceramic heater, and is disposed at the fixing nip portion N. That is, the heating member  102  is supported by the support member  105  so as to face the pressure roller  92  via the fixing film  91 . Further, the fixing film  91  rotates following the pressure roller  92  as the pressure roller  92  is rotationally driven. The fixing film  91  and the pressure roller  92  are disposed inside a casing  90 . 
     That is, the fixing device  9  includes a casing  90 , the fixing film  91  and the pressure roller  92  serving as a pair of rotary members disposed inside the casing  90 , an inlet guide  93 , and a sheet detection unit  50 . In the casing  90 , an inlet and an outlet of the sheet P are opened, and the fixing film  91 , the pressure roller  92 , the inlet guide  93 , and the sheet detection unit  50  are disposed therein. The inlet guide  93  is disposed between the inlet of the casing  90  and the fixing nip portion N, and guides the sheet P enters from the inlet of the casing  90  and bearing the unfixed toner image to the inlet of the fixing nip portion N. The sheet P guided along the inlet guide  93  is heated and pressurized while being nipped and conveyed by the fixing film  91  and the pressure roller  92  at the fixing nip portion N. As a result, the toner image is fixed to the sheet P. The sheet detection unit  50  serves as both a loop detection sensor that detects that a loop is formed on the sheet P and a residual sheet detection sensor that detects that a sheet remains in the fixing device  9 . A detailed description will be described below. 
     A fixing motor M capable of changing a rotational speed is attached to the fixing device  9 , and the pressure roller  92  is rotationally driven by the fixing motor M. The control unit  110  can control the conveyance speed of the sheet P at the fixing nip portion N by controlling the rotational speed of the fixing motor M. In the fixing device  9  of the present embodiment, the pair of rotary members forming the fixing nip portion N is configured by the fixing film  91  and the pressure roller  92 , but the configuration of the pair of rotary members is not limited thereto. For example, both of the pair of rotary members may be rollers, or both may be endless belts stretched by a plurality of rollers. Furthermore, one may be an endless belt, and the other may be a roller. The heating method is not limited to the ceramic heater described above, and for example, a halogen heater may be disposed in a roller, or an electromagnetic induction heating (IH) may be used. 
     Sheet Detection Unit 
     Next, a configuration of the sheet detection unit  50  will be described with reference to  FIG.  2   . The sheet detection unit  50  is disposed between the secondary transfer portion T 2 , the fixing film  91 , and the pressure roller  92 , and can detect a sheet. Specifically, the sheet detection unit  50  is disposed between the belt conveyance device  30  serving as a pre-fixing conveyance unit and the fixing nip portion N. The sheet detection unit  50  includes a flag  51  serving as a moving member and a swing member, and a detection unit  500  that detects the position of the flag  51 . The flag  51  moves in contact with the sheet. In the present embodiment, the flag  51  is a swing member that can swing about a rotation shaft  50   a  serving as a swing center. The detection unit  500  includes a residual sheet detection sensor S 2  serving as a first sensor and a loop detection sensor S 1  serving as a second sensor. 
     The flag  51  serving as a contact member includes a contact portion  52  that can come into contact with the sheet, a first flag portion  53 , and a second flag portion  54 . The contact portion  52  is disposed such that a distal end portion is located on the fixing nip portion N side (nip portion side) with respect to the swing center, and is biased in a direction opposite to a pressing direction by the sheet P by a spring (not illustrated). When the sheet P is nipped and conveyed by the fixing nip portion N, in a case where a surface of the sheet P in contact with the fixing film  91  is referred to a front surface and a surface of the sheet P in contact with the pressure roller  92  is referred to a back surface, the contact portion  52  of the flag  51  is in contact with the back surface of the sheet P between the fixing nip portion N and the secondary transfer portion T 2  in the conveyance direction of the sheet P. Specifically, the sheet detection unit  50  is disposed below the inlet guide  93 . The inlet guide  93  is, for example, a plate-like member having an opening or a notch through which the flag  51  can pass, and the contact portion  52  protrudes upward from the opening or the notch. When the contact portion  52  comes into contact with the sheet P conveyed from the belt conveyance device  30 , the contact portion  52  is pressed downward by the sheet P, so that the contact portion rotates in a counterclockwise direction in  FIG.  2    around the rotation shaft  50   a . The spring (not illustrated) biases the contact portion  52  to rotate in a clockwise direction in  FIG.  2   . 
     The flag  51  as described above can be located at a first position, a second position, and a third position. The first position is a position where the flag  51  is located when the flag  51  is not in contact with the back surface of the sheet P. The second position and the third position are positions where the flag  51  is located when the flag  51  comes into contact with the back surface of the sheet P. The flag  51  reaches the third position by being biased to the sheet P from the second position. Specifically, the first flag portion  53  and the second flag portion  54  are provided so as to protrude from the rotation shaft  50   a  at positions different from the contact portion  52 , and can swing about the rotation shaft  50   a  together with the contact portion  52 . The first flag portion  53  and the second flag portion  54  are arranged at positions having different phases in a rotation direction around the rotation shaft  50   a , and the contact portion  52 , the first flag portion  53 , and the second flag portion  54  swing around the rotation shaft  50   a  while maintaining their phase positions. The first flag portion  53  is provided at a position where the loop detection sensor S 1  can detect, and the second flag portion  54  is provided at a position where the residual sheet detection sensor S 2  can detect. 
     When the flag  51  moves to the third position, the loop detection sensor S 1  detects that the sheet conveyed by the secondary transfer portion T 2 , the fixing film  91 , and the pressure roller  92  forms a loop. The residual sheet detection sensor S 2  detects that a sheet remains in the fixing device  9  when the flag  51  moves to the second position different from the third position. Each of the loop detection sensor S 1  and the residual sheet detection sensor S 2  is a photo-interrupter in which a light emitting portion and a light receiving portion are arranged to face each other. That is, the light emitted from the light-emitting portion is shielded by the flag, so that rotation positions of the first flag portion  53  and the second flag portion  54  can be detected. Signals detected by the loop detection sensor S 1  and the residual sheet detection sensor S 2  are sent to the control unit  110 , and the control unit  110  performs control as described below based on the received signals. 
     Loop Detection and Residual Sheet Detection 
     Next, loop detection and residual sheet detection in the present embodiment will be described with reference to  FIGS.  3  and  4   .  FIG.  3    illustrates a state in which the loop detection is performed by the sheet detection unit  50 .  FIG.  3    illustrates a state L 1  in which a loop is generated in the sheet P due to a difference in sheet conveyance speed between the belt conveyance device  30  and the fixing device  9  and a state L 2  in which the loop is eliminated. The belt conveyance device  30  is designed to convey a sheet at substantially the same speed as a sheet conveyance speed in the secondary transfer portion T 2 . Similarly, a sheet conveyance speed in the fixing device  9  is designed to convey the sheet at substantially the same speed. However, in the fixing device  9 , when the pressure roller  92  is rotationally driven, the fixing film  91  is driven to rotate. The fixing film  91  rotates following the rotational drive of the pressure roller  92 . Then, a slight slip occurs between the pressure roller  92  and the fixing film  91 . As a result, an error occurs in the rotational speed of the belt. The same applies to the belt conveyance device  30 . Therefore, there is a possibility that the sheet conveyance speed in the fixing device  9  and the sheet conveyance speed in the secondary transfer portion T 2  are different. Therefore, when the sheet conveyance speed in the fixing device  9  is lower than the sheet conveyance speed in the belt conveyance device  30 , a loop in which the sheet is bent occurs between the fixing device  9  and the belt conveyance device  30 , and the sheet is not pulled between the fixing device  9  and the secondary transfer portion T 2 . On the other hand, when the sheet conveyance speed in the fixing device  9  is faster than the sheet conveyance speed in the belt conveyance device  30  in a state where the loop is not formed, there is a possibility that the sheet is pulled between the fixing device  9  and the secondary transfer portion T 2  and a misalignment occurs in the transferred image in the secondary transfer portion T 2 . 
     Therefore, in the present embodiment, the loop detection sensor S 1  is disposed so that the first flag portion  53  can be detected when the sheet P loops. That is, in a state (broken line) where the contact portion  52  of the flag  51  is in contact with the sheet P in the state L 2 , the first flag portion  53  does not shield the loop detection sensor S 1 . On the other hand, when the contact portion  52  comes into contact with the sheet P in the state L 1  to swing in the counterclockwise direction and is located at the third position (solid line), the first flag portion  53  shields the loop detection sensor S 1  (turns on the sensor). 
     In  FIG.  3   , when the flag  51  is located at the third position (solid line), the second flag portion  54  shields the residual sheet detection sensor S 2  (turns on the sensor). Therefore, when both the loop detection sensor S 1  and the residual sheet detection sensor S 2  are turned on, the control unit  110  determines that a loop of the sheet P is formed between the belt conveyance device  30  and the fixing nip portion N. The positions of the second flag portion  54  and the residual sheet detection sensor S 2  may be set such that the residual sheet detection sensor S 2  is not turned on in this state. 
       FIG.  4    illustrates a state in which the residual sheet detection is performed by the sheet detection unit  50 . In a case where the sheet P remains in the fixing nip portion N at the time of the jam, the contact portion  52  of the flag  51  comes into contact with the trailing edge of the remaining sheet P, and thus, in a case where the flag  51  rotates in the counterclockwise direction and is located at the second position (solid line), the second flag portion  54  shields the residual sheet detection sensor S 2  (sensor is turned on). At this time, the first flag portion  53  does not shield the loop detection sensor S 1 . Therefore, the control unit  110  determines that the loop detection sensor S 1  is turned off, the residual sheet detection sensor S 2  is turned on, and the sheet remains in the fixing nip portion N. A broken line indicates a state in which the contact portion  52  of the flag  51  does not detect the sheet P. That is, the position of the broken line in  FIG.  4    indicates the first position. 
     In a case where the sheet remains in the fixing nip portion N as described above, if the fixing device  9  is heated without detecting the residual sheet, the toner melts at the fixing nip portion N, and the residual sheet sticks to the fixing film  91 , which may cause malfunction of the fixing device  9 . For this reason, the sheet remaining in the fixing nip portion is generally detected by a sensor disposed on the conveyance path of the fixing device. Then, as the sensor arranged in front (upstream) of the fixing nip portion and the sensor arranged in rear (downstream) of the fixing nip portion are closer to the fixing nip portion, it is possible to detect the residual of the sheet having a shorter length in the conveyance direction. 
     Therefore, as in the present embodiment, in a case where the sheet detection unit  50  performs both the loop detection of the sheet and the residual sheet detection of the fixing nip portion, the flag  51  is preferably arranged at a position as close as possible to the fixing nip portion N. If the distal end of the contact portion  52  of the flag  51  is disposed at a position close to the inlet of the fixing nip portion N, even when a sheet having a short length in the conveyance direction remains in the fixing nip portion N, the residual sheet can be easily detected. 
     Therefore, in the present embodiment, the loop detection sensor S 1  is disposed so as to be able to detect that the sheet forms a loop when the flag  51  moves to the third position, and the residual sheet detection sensor S 2  is disposed so as to be able to detect that the sheet remains in the fixing device  9  when the flag  51  moves to the second position different from the third position. Therefore, the loop detection and the residual sheet detection can be performed in a state where the flag  51  is at a position suitable for the residual sheet detection and a position suitable for the loop detection, respectively. Therefore, it is possible to improve the accuracy of the detection of the loop of the sheet and the detection of the sheet remaining in the fixing device by one flag  51  without increasing the number of components. 
     Further, in the present embodiment, the sheet detection unit  50 , that is, the flag  51  and the detection unit  500  are detachable integrally with the fixing device  9  with respect to the image forming apparatus main body  100 A. Further, the sheet detection unit  50  is disposed inside the casing  90  of the fixing device  9 . Specifically, the rotation shaft  50   a  of the flag  51  is supported directly on a frame of the fixing device  9  or on the inlet guide  93  provided on the frame. The fixing film  91  and the pressure roller  92  are also supported by the frame. 
     By making the sheet detection unit  50  detachable integrally with the fixing device  9  in this manner, the contact portion  52  of the flag  51  can be easily brought close to the fixing nip portion N. That is, in a case where the sheet detection unit  50  and the fixing device  9  are configured to be separately attachable to and detachable from the image forming apparatus main body  100 A, a plurality of components are interposed between a support portion of the flag  51  with the contact portion  52  and a support portion of the fixing film  91  and the pressure roller  92 . Therefore, it is necessary to define the positional relationship between the contact portion  52 , the fixing film  91 , and the pressure roller  92  in consideration of the tolerance of the plurality of components, and it is difficult to bring the contact portion  52  close to the fixing nip portion N. 
     On the other hand, by making the sheet detection unit  50  detachable integrally with the fixing device  9  as in the present embodiment, it is possible to reduce the number of components existing between the support portion with the contact portion  52  of the flag  51  and the support portion of the fixing film  91  and the pressure roller  92 . Therefore, the number of parts considering tolerance is reduced, and the contact portion  52  can be easily brought close to the fixing nip portion N. In particular, by supporting the rotation shaft  50   a  of the flag  51 , the fixing film  91 , and the pressure roller  92  on a common frame, the positional relationship between the contact portion  52  of the flag  51  and the fixing nip portion N can be more easily ensured, and the contact portion  52  can be easily brought close to the fixing nip portion N. Note that, even if the rotation shaft  50   a  is supported by the inlet guide  93  provided in the frame, the number of components increases due to the presence of the inlet guide  93 , but the contact portion  52  can be easily brought close to the fixing nip portion N as compared with a case where the rotation shaft  50   a  is supported at a portion other than each component of the fixing device  9 . As a result, even if the length of the sheet remaining in the fixing nip portion N is short, the residual sheet can be easily detected by the flag  51 , and the detection accuracy of the sheet remaining in the fixing device  9  can be improved with the configuration capable of detecting the loop of the sheet. 
     In the above description, the sheet detection unit  50  includes two sensors S 1  and S 2 , but the number of sensors to be arranged may be increased in order to improve the position detection accuracy of the flag. In addition, in the above description, a flag  51  provided with two flag portions (light shielding portions, the first flag portion  53  and the second flag portion  54 ) to be detected by the photo-interrupter is illustrated, but the flag  51  may be provided with one flag portion. That is, the sensors S 1  and S 2  may be shielded by one flag portion. In the above description, the residual sheet detection sensor S 2  is used for detecting the residual sheet, but may be used as a second loop detection sensor that detects a loop state different from the state detected by the loop detection sensor S 1 . 
     Loop Control 
     Here, the loop control of detecting the loop state of the sheet using the above-described sheet detection unit  50  and controlling the sheet conveyance speed of the fixing device  9  will be described. When the loop detection sensor S 1  does not detect a loop of the sheet at the time of conveying the sheet, the control unit  110  sets the conveyance speed of the sheet by the fixing film  91  and the pressure roller  92  to a first speed. On the other hand, when the loop detection sensor S 1  detects the loop of the sheet, the control unit  110  sets the sheet conveyance speed by the fixing film  91  and the pressure roller  92  to a second speed faster than the first speed. 
     That is, in a state where the sheet is not looped, the control unit  110  slows the rotational speed of the fixing motor M that drives the pressure roller  92  to lower the speed of conveying the sheet by the fixing film  91  and the pressure roller  92  than the speed of conveying the sheet by the belt conveyance device  30 . As a result, a loop is formed on the sheet conveyed by the belt conveyance device  30  and the fixing nip portion N. On the other hand, in a state where the sheet is looped, the control unit  110  increases the rotational speed of the fixing motor M that drives the pressure roller  92 , so that the speed at which the sheet is conveyed by the fixing film  91  and the pressure roller  92  is faster than the speed at which the sheet is conveyed by the belt conveyance device  30 . As a result, the conveyance speed of the sheet is secured without making the loop of the sheet too large. 
     Such loop control of the present embodiment will be specifically described with reference to a flowchart of  FIG.  5   . First, when a job is started, the sheet P is fed from the cassette  10  and conveyed toward the registration roller  12  (S 101 ). Next, an image is formed in each image forming unit in synchronization with feeding of the sheet, and the image is transferred to the sheet P conveyed from the registration roller  12  in the secondary transfer portion T 2  (S 102 ). The sheet P to which the image has been transferred is conveyed by the rotation of the secondary transfer portion T 2  and the belt conveyance device  30 , and reaches the fixing device  9 . 
     At this time, the conveyance speed of the fixing device  9  (the sheet conveyance speed by the fixing film  91  and the pressure roller  92 ) is set to VL (first speed) which is 2% slower than the sheet conveyance speed in the secondary transfer portion T 2 . As a result, as the sheet P is conveyed, a loop of the sheet is formed between the belt conveyance device  30  and the fixing nip portion N of the fixing device  9 . 
     Then, the control unit  110  determines whether or not a loop is formed on the sheet from the situation of the sheet detection unit  50  (S 103 ). Specifically, it is determined whether the sheet P passes through the fixing nip portion N in a state where the output of the loop detection sensor S 1  is off, that is, in a state where the sheet P does not form a loop, or whether the sheet P passes through the fixing nip portion N in a state where the sheet P forms a loop between the belt conveyance device  30  and the fixing nip portion N (the output of the loop detection sensor S 1  is on). 
     When the sheet P passes through the fixing nip portion N without forming a loop (No in S 103 ), the rotation of the fixing motor M is controlled to set the conveyance speed of the fixing device  9  to VL (first speed) (S 104 ). That is, when the sheet conveyance speed of the fixing device is VL, this VL is maintained, and when the sheet conveyance speed of the fixing device is VH to be described below, the sheet conveyance speed of the fixing device is reduced from VH to VL. 
     On the other hand, in a case where a loop is formed on the sheet in S 103  (Yes in S 103 ), the rotation of the fixing motor M is controlled for a certain period of time h in order to set the conveyance speed of the fixing device  9  to VH (second speed) which is 2% faster than the sheet conveyance speed in the secondary transfer portion T 2  (S 105 ). That is, the sheet P is conveyed for the certain period of time h (0.5 seconds in the present embodiment) in a state where the sheet conveyance speed of the fixing device  9  is VH, and the loop of the sheet is eliminated. Thereafter, the control of S 103  to S 105  is repeated until the post-secondary transfer sensor  22  detects that the trailing edge of the sheet P has passed through the secondary transfer portion T 2  (S 106 ). Note that the loop detection sensor may determine that the sensor signal is in the loop state when the on state of the sensor signal continues for a certain period of time (for example, 0.1 seconds) in order to prevent erroneous detection. 
     Residual Sheet Detection Control 
     Next, residual sheet detection control of detecting a sheet remaining in the fixing nip portion N using the above-described sheet detection unit  50  and notifying an error will be described. In a case where the sheet detection unit  50  detects a sheet when the power of the apparatus is turned on, the control unit  110  outputs information indicating that a sheet remains in the fixing device  9 . That is, in a case where the residual sheet detection sensor S 2  is turned on when the power of the image forming apparatus  100  is turned on, the control unit  110  displays an error notification indicating that a sheet remains in the fixing device  9  on the operation panel S. Note that the control unit  110  may output the error notification to an external terminal such as a personal computer connected to the image forming apparatus  100 . Further, even if the residual sheet detection sensor S 2  is not turned on, when the loop detection sensor S 1  is turned on, an error notification indicating that a sheet remains in the fixing device  9  may be displayed on the operation panel S. 
     Such residual sheet detection control of the present embodiment will be specifically described with reference to a flowchart of  FIG.  6   .  FIG.  6    is a flowchart for explaining a process of detecting remaining of a sheet P in a power-on state of the image forming apparatus  100  and in an idle state in which a print job is not executed. When the image forming apparatus  100  is powered on, the control unit  110  checks whether the residual sheet detection sensor S 2  is turned on, that is, whether the sheet P is detected (S 201 ). In the turn-on state (Yes in S 201 ), the control unit  110  determines that there is a residual sheet in the fixing device  9 , and notifies the user of an error indicating that there is a residual sheet (S 202 ). 
     On the other hand, when the residual sheet detection sensor S 2  does not detect the sheet P at the time of power-on in S 201  (No in S 201 ), the processing proceeds to normal initialization processing, image forming job reception, and image forming job execution processing (S 203 ). Note that the image forming job is an operation from the start of image formation to the completion of image formation based on a print signal (image forming signal) for forming an image on a sheet. The initialization processing is processing of starting rotation of the photosensitive drum, sequentially raising various voltages, adjusting various voltages, and the like as a preparation operation before the image forming operation, and is so-called pre-rotation processing. 
     Next, the control unit  110  determines whether an image forming job not being executed is in a standby state (S 204 ). If it is in the standby state (Yes in S 204 ), the process proceeds to S 201 , and the control unit  110  determines whether to notify the user of the jam error by determining that there is a residual sheet similarly to when the power is turned on. When the process is not in the standby state in S 204  (No in S 204 ), the process returns to S 203  to continue the normal processing. As a result, it is possible to determine whether the sheet P remains in a state where the sheet P is not conveyed. 
     In the present embodiment, with a configuration in which two contact portions are provided in one flag (moving member), loop detection and residual detection can be performed. As a result, the number of components can be reduced as compared with a configuration in which the loop detection and the residual detection are performed by independent flags (moving members). 
     Second Embodiment 
     A second embodiment will be described with reference to  FIGS.  7  to  11   . In the first embodiment described above, the configuration in which both the sheet on which the loop is formed and the sheet remaining in the fixing device  9  are detected by the distal end portion of the contact portion  52  of the flag  51  has been described. On the other hand, in the present embodiment, the sheet on which the loop is formed and the sheet remaining in the fixing device  9  are detected at two different places of a flag  51 A. 
     Comparative Example 
     First, a comparative example illustrated in  FIGS.  7  and  8    will be described. As described above, in the residual sheet detection, if the distal end portion of the contact portion  52  of the flag  51  is caught on the trailing edge of the sheet remaining in the fixing nip portion N, the residual sheet can be detected. Therefore, it is desirable that the distal end of the contact portion  52  is located near the fixing nip portion N. 
     However, as illustrated in the comparative example of  FIG.  7   , when the distal end of the contact portion  52  is brought close to the fixing nip portion N by extending the flag  51  from the rotation shaft  50   a  to improve the detection performance of the residual sheet, the following problem may occur. That is, as illustrated in  FIG.  8   , when the distal end of the contact portion  52  is brought close to the fixing nip portion N, the loop determination is performed in a region where the difference in the loci between the state L 1  in which the loop of the sheet occurs in the vicinity of the fixing nip portion N and the state L 2  in which the loop is eliminated is small. For this reason, there is a possibility that it is difficult to discriminate between a minute behavior change and occurrence of a loop at the time of conveying the sheet P. 
     Configuration of Present Embodiment 
     Therefore, in the present embodiment, as illustrated in  FIGS.  9 A to  10 B , the distal end portion of the flag  51 A of the sheet detection unit  50 A is bent, and a bent portion  52   b  is selectively used as the loop detection position and a distal end portion  52   a  is selectively used as the residual sheet detection position, thereby achieving both the loop detection performance and the residual sheet detection performance. That is, in the sheet detection unit  50 A of the present embodiment, the shape of the flag  51 A is different from that of the first embodiment. The sheet detection unit  50 A is similar to the first embodiment in that the sheet detection unit includes a first flag portion  53 , a second flag portion  54 , and a detection unit  500  (a loop detection sensor S 1  and a residual sheet detection sensor S 2 ). The flag  51 A of the present embodiment has a bent portion  52   b  bent such that the distal end portion  52   a  is bent in a direction extending from the rotation shaft  50   a  serving as a swing center. The bent portion  52   b  is a first contact portion for detecting that a loop is formed, and the distal end portion  52   a  is a second contact portion for detecting that a sheet remains in the fixing device  9 . 
     In the flag  51 A, the bent portion  52   b  is arranged such that the bent portion  52   b  is closest to the conveyance path at a position where the loop detection sensor S 1  is switched on by the first flag portion (light shielding portion)  53  provided in the flag  51 A, that is, at a position where the loop is detected. That is, when the flag  51 A is located at the position where the loop is detected (third position, position in contact with sheet P in L 1  state in  FIG.  9 A , position indicated by solid line), the bent portion  52   b  is located closer to the path along which the sheet is conveyed than the distal end portion  52   a . As a result, the sheet can be detected by the bent portion  52   b  at the time of detecting the loop, and the sheet can be detected by the distal end portion  52   a  at the time of detecting the residual sheet (in a state of being in contact with the sheet P remaining in the fixing nip portion N in  FIG.  10 B ). As a result, even if the distal end portion of the flag  51 A is brought close to the fixing nip portion N to increase the residual sheet detection performance, it is possible to suppress deterioration of the loop detection performance. Hereinafter, a specific description will be given. 
     Loop Detection Operation and Residual Sheet Detection Operation 
     Next, a loop detection operation and a residual sheet detection operation performed using the sheet detection unit  50 A of the present embodiment will be described with reference to  FIGS.  9 A to  10 B . 
     When the sheet P passes on the inlet guide  93 , the flag  51 A of the sheet detection unit  50 A is pressed by the sheet P and rotates about the rotation shaft  50   a . The flag  51 A is provided with a rotational force in a direction opposite to the pressing by the sheet P (clockwise direction in  FIGS.  9 A,  9 B,  10 A, and  10 B ) by a turning spring (not illustrated). 
     The distal end portion  52   a  of the flag  51 A pressed by the sheet P is disposed so as to be rotatable between the state L 1  (see  FIG.  9 A ) in which a loop is generated due to a difference between a conveyance speed of the sheet P conveyed by the intermediate transfer belt  20  and the belt conveyance device  30  and a conveyance speed of the sheet P conveyed by the fixing device  9  and the state L 2  (see  FIG.  9 B ) in which the loop is eliminated. The belt conveyance device  30  is designed to convey a sheet at substantially the same speed as a sheet conveyance speed in the secondary transfer portion T 2 . Similarly, a sheet conveyance speed in the fixing device  9  is designed to convey the sheet at substantially the same speed. However, the fixing device  9  rotates the fixing film  91  by rotationally driving the pressure roller  92 . As the fixing film  91  rotates following the rotational drive of the pressure roller  92 , a slight slip occurs between the pressure roller  92  and the fixing film  91 . As a result, an error occurs in the rotational speed of the fixing film  91 . The same applies to the belt conveyance device  30 . Therefore, there is a possibility that the sheet conveyance speed in the fixing device  9  and the sheet conveyance speed in the secondary transfer portion T 2  are different. 
     In a case where the secondary transfer portion T 2  and the fixing nip portion N nip and convey the sheet at the same time and the sheet conveyance speed in the fixing device  9  becomes faster than the sheet conveyance speed in the secondary transfer portion T 2 , there is a possibility that the fixing device  9  pulls the sheet and the image is not transferred to a desired region on the sheet (transfer deviation). In order to suppress the transfer deviation, the loop detection sensor detects the loop amount of the sheet and controls the sheet conveyance speed in the fixing device. Details thereof will be described below. 
     In the loop detection sensor S 1 , the loop detection sensor S 1  is turned on when light of the loop detection sensor S 1  is shielded, and the loop detection sensor S 1  is turned off when light of the loop detection sensor S 1  is transmitted. Therefore, as illustrated in  FIG.  9 A , when the bent portion  52   b  of the flag  51 A reaches the position of the state L 1  where the loop has occurred, the position of the first flag portion  53  is defined such that the first flag portion  53  turns on the loop detection sensor S 1 . 
     Next, as illustrated in the state L 2  of  FIG.  9 B , when the loop of the sheet is eliminated, the first flag portion  53  transmits the light of the loop detection sensor S 1 , and the loop detection sensor S 1  is turned off. Then, the sheet conveyance speed of the fixing device  9  is reduced, and the loop is formed again. 
     Next, as illustrated in  FIG.  10 A , when the sheet P remains in the fixing nip portion N at the time of a jam or the like, the trailing edge of the sheet P is detected by the distal end portion  52   a  of the flag  51 A. In the present embodiment, even when a small-sized sheet such as a postcard or an envelope having a length of about 150 mm in the sheet conveyance direction remains in the fixing nip portion N, the distal end portion  52   a  of the flag  51 A is extended to the vicinity of the inlet of the fixing nip portion N so that the residual sheet can be detected. 
     Next, as illustrated in  FIG.  10 B , when the sheet P remains in the fixing nip portion N due to occurrence of a jam or the like, the sheet P follows the direction of the nip surface of the fixing nip portion N, so that the trailing edge of the sheet P is lowered from the broken line position to the solid line position. Then, the distal end portion  52   a  in contact with the trailing edge of the sheet P rotates counterclockwise, and the second flag portion  54  shields the light of the residual sheet detection sensor S 2 . Even in this state, the first flag portion  53  does not shield the light of the loop detection sensor S 1 . Since the second flag portion  54  shields the light of the residual sheet detection sensor S 2 , the residual sheet detection sensor S 2  is turned on, and it is detected that the sheet P remains in the fixing nip portion N. 
     In the above description, the loop detection position and the residual sheet detection position are provided by providing the bent portion  52   b  in the flag  51 A. However, a loop detection unit (first contact portion) and a residual sheet detection unit (second contact portion) may be provided in the flag by using a configuration in which a convex portion is provided in the middle of the flag instead of the bent portion  52   b  or a configuration in which the flag is bifurcated. 
     As illustrated in  FIG.  11   , the fixing nip portion N is preferably disposed with an inclination θ with respect to an imaginary line L 3  is a line extended which is extended a sheet conveyance locus of the belt conveyance device  30  disposed on the upstream of the fixing device  9  such that the downstream of the fixing nip portion N is away from the flag  51 A. That is, when the conveyance locus of the sheet conveyed by the belt conveyance device  30  is extended, the direction of conveying the sheet in the fixing nip portion N is preferably inclined with respect to the imaginary line L 3  such that the outlet of the fixing nip portion N is separated from the imaginary line L 3  than the inlet of the fixing nip portion N. As a result, since the sheet remaining in the fixing nip portion N is held in the fixing nip portion N so as to tilt the flag  51 A downward, the flag  51 A facilitates detection of the residual sheet. Note that 0 is desirably set in a range of 0 to 40 degrees. 
     Other Embodiments 
     In the above-described embodiments, the moving member detected by the loop detection sensor S 1  and the residual sheet detection sensor S 2  is the flag  51  serving as the swing member, but the moving member may slide by coming into contact with the sheet instead of swinging. For example, the moving member is arranged so as to be movable in the vertical direction in  FIGS.  3  and  4   , and the loop detection sensor S 1  and the residual sheet detection sensor S 2  are arranged vertically. At this time, if the loop detection sensor S 1  is disposed below the residual sheet detection sensor S 2 , both the loop detection and the residual sheet detection can be performed by one moving member as in the case illustrated in  FIGS.  3  and  4   . 
     In addition, in the above-described embodiment, the intermediate transfer method of transferring a toner image from the intermediate transfer belt  20  serving as an image bearing member to a sheet has been described, but the present invention is also applicable to a direct transfer method of directly transferring a toner image from a photosensitive drum to a sheet. In this case, the photosensitive drum corresponds to the image bearing member. 
     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. 2021-184628, filed Nov. 12, 2021, which is hereby incorporated by reference herein in its entirety.