Patent Publication Number: US-11662681-B2

Title: Image forming apparatus and movement amount detection device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2021-137626 filed Aug. 25, 2021. 
     BACKGROUND 
     (i) Technical Field 
     The present disclosure relates to an image forming apparatus and a movement amount detection device. 
     (ii) Related Art 
     Japanese Unexamined Patent Application Publication No. 2010-256789 discloses a belt meandering amount measurement device that measures a meandering amount of an endless belt that is wound around multiple rollers and rotated. In this belt meandering amount measurement device, a measurement reference portion extending in a rotating direction of a belt and having a predetermined length, a first measurement unit extending parallel in the rotating direction of the belt while being shifted from the measurement reference portion in a width direction of the belt, and a second measurement unit extending parallel in the rotating direction of the belt while being shifted from the measurement reference portion and the first measurement unit in the width direction of the belt are formed at one side end portion in the width direction of the belt. A sensor that outputs voltages in accordance with positions of the measurement reference portion, the first measurement unit, and the second measurement unit in the width direction of the belt is disposed near the one side end portion in the width direction of the belt. As pre-use data setting of the belt, in a state in which the belt rotates without meandering, voltages at the positions of the measurement reference portion, the first measurement unit, and the second measurement unit are measured with the measurement reference portion serving as a reference position. A conversion expression representing a relationship between the measurement result and distances of the first measurement unit and the second measurement unit from the reference position in the width direction of the belt is obtained. When the belt is used, the sensor measures a voltage at a predetermined interval at any position of the measurement reference portion, the first measurement unit, and the second measurement unit, converts the voltage into a distance from the reference position in the width direction of the belt by the conversion expression, and obtains a difference of the distance from the reference position obtained at the predetermined interval to measure a meandering amount of the belt. 
     SUMMARY 
     There is an image forming apparatus in which an image forming body that forms an image on a belt and a moving mechanism that moves the belt in a thickness direction thereof are provided in the vicinity of the transported belt. Further, in this image forming apparatus, a rotational member that is rotatable around a rotation shaft in a transport direction of the belt is brought into contact with a side surface of the belt, and a movement amount of the belt in a width direction is detected based on a physical amount that changes in accordance with a rotation angle of the rotational member. 
     In this image forming apparatus, when the position of the belt in the thickness direction changes, the rotation angle of the rotational member per unit movement amount in the width direction of the belt changes. Thus, this image forming apparatus is not able to accurately detect the movement amount in the width direction of the belt whose position in the thickness direction changes. 
     Aspects of non-limiting embodiments of the present disclosure relate to accurately detecting a movement amount in a width direction of a belt whose position in a thickness direction changes, as compared with a case where a movement amount in a width direction of a belt is detected based on a physical amount that changes due to rotation of a rotational member that is rotatable around a rotation shaft in a transport direction of the belt. 
     Aspects of certain non-limiting embodiments of the present disclosure overcome the above disadvantages and/or other disadvantages not described above. However, aspects of the non-limiting embodiments are not required to overcome the disadvantages described above, and aspects of the non-limiting embodiments of the present disclosure may not overcome any of the disadvantages described above. 
     According to an aspect of the present disclosure, there is provided an image forming apparatus including: a belt to be transported; an image forming body that forms an image on the belt or a recording medium that is transported by the belt; a moving mechanism that moves the belt in a movement direction extending in a thickness direction of the belt; a rotational member including a rotation shaft extending in the movement direction and a contact portion that is rotatable around the rotation shaft and that is in contact with a side surface of the belt regardless of a position of the belt in the movement direction; an acquirer that acquires a physical amount that changes due to rotation of the contact portion around the rotation shaft when the belt moves in a width direction; and a detector that detects a movement amount of the belt in the width direction based on the physical amount acquired by the acquirer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present disclosure will be described in detail based on the following figures, wherein: 
         FIG.  1    is a general configuration diagram illustrating an image forming apparatus according to a present exemplary embodiment; 
         FIG.  2    is a schematic side view illustrating a transfer belt, a first photoreceptor drum, a second photoreceptor drum, a first transfer roller, and a rotational member according to the exemplary embodiment; 
         FIG.  3    is a schematic side view illustrating the transfer belt and the rotational member according to the exemplary embodiment; 
         FIG.  4    is a perspective view illustrating a movement amount detection device of the exemplary embodiment; 
         FIG.  5    is a cross-sectional view of the movement amount detection device taken along line V-V in  FIG.  4   ; 
         FIG.  6    is a cross-sectional view of the rotational member taken along line VI-VI in  FIG.  3   ; 
         FIG.  7    is a cross-sectional view of the rotational member taken along line VII-VII in  FIG.  3   ; 
         FIG.  8    is a cross-sectional view of the rotational member taken along line VIII-VIII in  FIG.  3   ; 
         FIG.  9    is a schematic plan view of the rotational member and the transfer belt according to the exemplary embodiment; 
         FIG.  10    is a schematic plan view of a rotational member and a transfer belt according to a comparative example; and 
         FIG.  11    is a schematic plan view of a rotational member, a rotation shaft, and a transfer belt according to a modification. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an exemplary embodiment according to the disclosure will be described in detail with reference to the drawings. Hereinafter, an upstream side in a transport direction of recording paper P as an example of a recording medium may be simply referred to as an “upstream side”, and a downstream side in the transport direction may be simply referred to as a “downstream side”. Similarly, an upstream side in a circling direction of a transfer belt (belt) (formation target body)  52  may be simply referred to as an “upstream side”, and a downstream side in the circling direction (transport direction) may be simply referred to as a “downstream side”. In the following description, a reference position of the “upstream side” and the “downstream side” of the transfer belt is a second transfer position T 2  (nip region Np) described later. That is, a direction from the second transfer position T 2  toward a pressing roller  49  after passing through a driving roller  44  is the “downstream side” of the transfer belt, and a direction from the second transfer position T 2  toward a second photoreceptor unit  30 K after passing through a retract roller  47  is the “upstream side” of the transfer belt. 
     As illustrated in  FIG.  1   , an image forming apparatus  10  according to the present exemplary embodiment is of an electrophotographic system that forms a toner image (an example of an image) on recording paper P. The image forming apparatus  10  includes an image forming section  12 , a storage section  14 , a transport section  16 , and a fixing device  18  in an apparatus body (not illustrated). Hereinafter, each component of the image forming apparatus  10  will be described. 
     In the following description, a width direction (horizontal direction) of the apparatus body is defined as an X direction, an up-down direction (vertical direction) of the apparatus body is defined as a Y direction, and a front-rear direction (a direction orthogonal to a paper surface) orthogonal to the X direction and the Y direction is defined as a Z direction. In  FIG.  1   , the near side of the paper surface is a front side, and the far side of the paper surface is a rear side. 
     Image Forming Section 
     The image forming section  12  has a function of forming a toner image on recording paper P. The image forming section  12  includes a first photoreceptor unit  20 , a second photoreceptor unit  30 , and a transfer device  50 . 
     Photoreceptor Units 
     As illustrated in  FIG.  1   , two first photoreceptor units  20  and two second photoreceptor units  30  are provided. Each first photoreceptor unit  20  and each second photoreceptor unit  30  are attachable to and detachable from the apparatus body. The image forming apparatus  10  includes first photoreceptor units  20 Y and  20 M for yellow (Y) and magenta (M) and second photoreceptor units  30 C and  30 K for cyan (C) and black (K). 
     In the following description, in a case where it is necessary to distinguish each color of yellow (Y), magenta (M), cyan (C), or black (K), an alphabet of Y, M, C, or K is added after a reference numeral of each member. In a case where it is not necessary to distinguish each color, an alphabet of Y, M, C, or K may be omitted. 
     The transfer belt  52  made of an elastic material of the transfer device  50  described later includes two straight portions that are straight-line shaped when viewed in the Z direction. The two straight portions are an upper portion  52 A and a lower portion  52 B. When viewed in the Z direction, the upper portion  52 A extends in the X direction, and the lower portion  52 B is inclined with respect to the X direction. That is, when viewed in the Z direction, an angle θB (see  FIG.  1   ) defined by the lower portion  52 B and the X direction is an acute angle, and the angle θB is larger than an angle θA (not illustrated) defined by the upper portion  52 A and the X direction. Note that the angle θA is 0° or an acute angle slightly larger than 0°. When viewed in the Z direction, the upper portion  52 A and the lower portion  52 B are mutually arranged in the Y direction. The “straight portion” in the present specification and claims is not limited to a completely straight-line-shaped portion. For example, in the upper portion  52 A located between a retract roller  39  and a retract roller  48  described later, portions pressed by the two first photoreceptor drums  22  and first transfer rollers  41  are slightly recessed; however, the upper portion  52 A corresponds to the “straight portion”. Similarly, in the lower portion  52 B located between a retract roller  40  and the retract roller  47 , portions pressed by the two second photoreceptor drums  32  and first transfer rollers  41  are slightly recessed; however, the lower portion  52 B corresponds to the “straight portion”. A width direction of the transfer belt  52  extends in the Z direction. 
     The two first photoreceptor units  20  face an outer peripheral surface (upper surface) of the upper portion  52 A and are arranged in the X direction along the upper portion  52 A. Each first photoreceptor unit  20  includes the first photoreceptor drum  22  that rotates in one direction (for example, the counterclockwise direction in  FIG.  1   ). Each first photoreceptor drum  22  is rotatable around a rotation axis  20 X extending in the Z direction. Each first photoreceptor unit  20  includes a first charging portion  24 , a first exposure portion  25 , a first developing portion  26 , and a first removing portion  27  in this order from the upstream side in the rotation direction of the first photoreceptor drum  22 . Each first photoreceptor unit  20  further includes a pair of support plates  28  separated from each other in the Z direction. One of the support plates  28  is not illustrated in  FIG.  1   . The first charging portion  24 , the first exposure portion  25 , the first developing portion  26 , and the first removing portion  27  are members extending in the Z direction. Both end portions of the first charging portion  24 , the first exposure portion  25 , the first developing portion  26 , and the first removing portion  27  in the Z direction are supported by the pair of support plates  28 . Further, relative movement of the pair of support plates  28  is restricted. As illustrated in  FIG.  1   , the dimension of each first photoreceptor unit  20  in the X direction is a horizontal dimension  20 L. 
     The two second photoreceptor units  30  face an outer peripheral surface (lower surface) of the lower portion  52 B and are arranged along the lower portion  52 B. Each second photoreceptor unit  30  includes a second photoreceptor drum  32  that rotates in one direction (for example, the counterclockwise direction in  FIG.  1   ). Each second photoreceptor drum  32  is rotatable around a rotation axis  30 X extending in the Z direction. Each second photoreceptor unit  30  includes a second charging portion  34 , a second exposure portion  35 , a second developing portion  36 , and a second removing portion  37  in this order from the upstream side in the rotation direction of the second photoreceptor drum  32 . Each second photoreceptor unit  30  further includes a pair of second support plates  38  separated from each other in the Z direction. One of the second support plates  38  is not illustrated in  FIG.  1   . The second charging portion  34 , the second exposure portion  35 , the second developing portion  36 , and the second removing portion  37  are members extending in the Z direction. Both end portions of the second charging portion  34 , the second exposure portion  35 , the second developing portion  36 , and the second removing portion  37  in the Z direction are supported by the pair of second support plates  38 . Further, relative movement of the pair of second support plates  38  is restricted. As illustrated in  FIG.  1   , the dimension of each second photoreceptor unit  30  in the X direction is a horizontal dimension  30 L. 
     In the present specification and claims, a term “image forming body” refers to a body that causes toner or ink to adhere to a formation target body (for example, the transfer belt  52 ). That is, the first photoreceptor drum  22  of the first photoreceptor unit  20  corresponds to the “image forming body”, and the second photoreceptor drum  32  of the second photoreceptor unit  30  corresponds to the “image forming body”. That is, the first charging portion  24 , the first exposure portion  25 , the first developing portion  26 , and the first removing portion  27  do not correspond to the “image forming body”. Similarly, the second charging portion  34 , the second exposure portion  35 , the second developing portion  36 , and the second removing portion  37  do not correspond to the “image forming body”. As will be described later, when the image forming apparatus  10  is of an inkjet type, an inkjet head corresponds to the “image forming body”. 
     A first distance  20 B is a distance (adjacent distance) between two portions on which images are formed by the two first photoreceptor drums  22  or two inkjet heads on the outer peripheral surface of the upper portion  52 A when viewed in the Z direction. When the first photoreceptor drums  22  correspond to the “image forming bodies”, two line segments connecting the first photoreceptor drums  22  and the first transfer rollers  41  respectively corresponding to the first photoreceptor drums  22  intersect with the outer peripheral surface of the upper portion  52 A at two intersection portions of the outer peripheral surface. When the first photoreceptor drums  22  correspond to the “image forming bodies”, the first distance  20 B is a distance between the two intersection portions when viewed in the Z direction. When the image forming apparatus  10  is of an inkjet type, the first distance  20 B is a distance between center portions of inkjet heads (image forming bodies) corresponding to the first photoreceptor units  20 . 
     Further, a second distance  30 B is a distance between two portions of the outer peripheral surface of the lower portion  52 B on which images are formed by the two second photoreceptor units  30  or two inkjet heads when viewed in the Z direction. When the second photoreceptor drums  32  correspond to the “image forming bodies”, two line segments connecting the second photoreceptor drums  32  and the first transfer rollers  41  respectively corresponding to the second photoreceptor drums  32  intersect with the outer peripheral surface of the lower portion  52 B at two intersection portions of the outer peripheral surface. When the second photoreceptor drums  32  correspond to the “image forming bodies”, the second distance  30 B is a distance between the two intersection portions when viewed in the Z direction. When the image forming apparatus  10  is of an inkjet type, the second distance  30 B is a distance between center portions of inkjet heads (image forming bodies) corresponding to the second photoreceptor units  30 . 
     As illustrated in  FIG.  1   , a developing roller  26 A, a recovery auger  26 B, a supply auger  26 C, and a stirring auger  26 D are provided inside the first developing portion  26 . Similarly, a developing roller  36 A, a recovery auger  36 B, a supply auger  36 C, and a stirring auger  36 D are provided inside the second developing portion  36 . The supply auger  26 C and the stirring auger  26 D are arranged in the X direction. In contrast, the supply auger  36 C and the stirring auger  36 D are arranged in the Y direction. Hence, the horizontal dimension of the second developing portion  36  is shorter than the horizontal dimension of the first developing portion  26 . Thus, the horizontal dimension  30 L is shorter than the horizontal dimension  20 L. 
     As illustrated in  FIG.  1   , the two first photoreceptor units  20  are arranged in the X direction when viewed in the Z direction. That is, the two first photoreceptor units  20  are not arranged in the Y direction. In contrast, when viewed in the Z direction, portions of the two second photoreceptor units  30  are arranged in the Y direction. A horizontal dimension  30 V illustrated in  FIG.  1    is an X-direction dimension of the portions of the two second photoreceptor units  30 . A horizontal dimension  30 E illustrated in  FIG.  1    is a horizontal dimension of a portion including the two second photoreceptor units  30 . A horizontal dimension  30 G illustrated in  FIG.  1    is a horizontal dimension of a portion including the lower portion  52 B and the two second photoreceptor units  30 . 
     The first charging portion  24  of each first photoreceptor unit  20  charges an outer peripheral surface of the first photoreceptor drum  22 . Then, the first exposure portion  25  exposes the outer peripheral surface of the first photoreceptor drum  22  charged by the first charging portion  24  to light to form an electrostatic latent image on the outer peripheral surface of the first photoreceptor drum  22 . The first developing portion  26  develops the electrostatic latent image formed on the outer peripheral surface of the first photoreceptor drum  22  by the first exposure portion  25  to form a toner image. After the toner image is transferred to the transfer belt  52 , the first removing portion  27  removes the toner remaining on the outer peripheral surface of the first photoreceptor drum  22 . 
     The second charging portion  34  of each second photoreceptor unit  30  charges an outer peripheral surface of the second photoreceptor drum  32 . Then, the second exposure portion  35  exposes the outer peripheral surface of the second photoreceptor drum  32  charged by the second charging portion  34  to light to form an electrostatic latent image on the outer peripheral surface of the second photoreceptor drum  32 . The second developing portion  36  develops the electrostatic latent image formed on the outer peripheral surface of the second photoreceptor drum  32  by the second exposure portion  35  to form a toner image. After the toner image is transferred to the transfer belt  52 , the second removing portion  37  removes the toner remaining on the outer peripheral surface of the second photoreceptor drum  32 . 
     Transfer Device 
     As illustrated in  FIG.  1   , the transfer device  50  includes the four first transfer rollers  41  serving as first transfer bodies, the transfer belt  52  serving as an intermediate transfer body, and a transfer cylinder  85  serving as a second transfer body. That is, the transfer device  50  first transfers the toner images formed on the outer peripheral surfaces of the respective first photoreceptor drums  22  to the transfer belt  52  in a superimposed manner, and second transfers the superimposed toner images to recording paper P. 
     First Transfer Roller 
     As illustrated in  FIG.  1   , each first transfer roller  41  facing the upper portion  52 A transfers the toner image formed on the outer peripheral surface of the corresponding first photoreceptor drum  22  to the outer peripheral surface of the transfer belt  52  at a first transfer position T 1  between the first photoreceptor drum  22  and the first transfer roller  41 . Each first transfer roller  41  facing the lower portion  52 B transfers the toner image formed on the outer peripheral surface of the corresponding second photoreceptor drum  32  to the outer peripheral surface of the transfer belt  52  at a first transfer position T 1  between the second photoreceptor drum  32  and the first transfer roller  41 . In the present exemplary embodiment, a first transfer voltage is applied between the first transfer roller  41  and the first photoreceptor drum  22 , and hence the toner image formed on the outer peripheral surface of the first photoreceptor drum  22  is transferred to the outer peripheral surface of the transfer belt  52  at the first transfer position T 1 . Similarly, a first transfer voltage is applied between the first transfer roller  41  and the second photoreceptor drum  32 , and hence the toner image formed on the outer peripheral surface of the second photoreceptor drum  32  is transferred to the outer peripheral surface of the transfer belt  52  at the first transfer position T 1 . 
     Each first transfer roller  41  is movable in a thickness direction TD (see an arrow in  FIG.  1   ) of the transfer belt  52 . The thickness direction TD of the transfer belt  52  in this specification refers to a thickness direction of the transfer belt  52  when each of retract rollers  39 ,  40 ,  47 , and  48  described later is located at a pressing position. Further, a rotation shaft of the first transfer roller  41  is urged by an urging member (not illustrated) in a direction toward an inner peripheral surface of the transfer belt  52 . 
     Transfer Belt 
     The annular transfer belt  52  illustrated in  FIG.  1    is wound around the four retract rollers  39 ,  40 ,  47 , and  48 , the driving roller  44 , a steering roller  45 , a backup roller  46 , and a pressing roller  49 , and hence the posture is determined. 
     Each of the retract rollers  39 ,  40 ,  47 , and  48 , which are moving mechanisms of the present exemplary embodiment, is rotatably in contact with the inner peripheral surface of the transfer belt  52  and is movable in a predetermined advance-retract direction RD. Each of the retract rollers  39 ,  40 ,  47 , and  48  is movable in the advance-retract direction RD between a pressing position and a retracted position that is a position on the inner peripheral side of the transfer belt  52  with respect to the pressing position. The retract roller  39  is located on the downstream side of the first photoreceptor unit  20 Y and located on the upstream side of the steering roller  45 . The retract roller  40  is located on the upstream side of the second photoreceptor unit  30 C and located on the downstream side of the steering roller  45 . The retract roller  47  is located on the downstream side of the second photoreceptor unit  30 K and located on the upstream side of the backup roller  46 . The retract roller  48  is located on the upstream side of the first photoreceptor unit  20 Y and located on the downstream side of the driving roller  44 . 
     The upper portion  52 A and the lower portion  52 B of the transfer belt  52  are movable in a movement direction MD (see  FIG.  1   ) extending in the thickness direction TD. As illustrated in  FIG.  2   , when the transfer belt  52  moves in the movement direction MD, the first transfer roller  41  moves in the thickness direction TD following the transfer belt  52 . 
     For example, when the retract rollers  40  and  47  are located at the pressing positions, the lower portion  52 B is located at a first transport position PM 1  indicated by a solid line in  FIGS.  2  and  3   . At this time, the second photoreceptor unit  30 C and the second photoreceptor unit  30 K may transfer the toner images to the transfer belt  52 . When the retract rollers  40  and  47  are located at the retracted positions, the lower portion  52 B is located at a second transport position PM 2  indicated by an imaginary line in  FIGS.  2  and  3   . At this time, the second photoreceptor unit  30 C and the second photoreceptor unit  30 K are not able to transfer the toner images to the transfer belt  52 . Although illustration is omitted, when the retract roller  39  and the retract roller  48  are located at the pressing positions, the upper portion  52 A is located at a first transport position PM 1  corresponding to the first transport position PM 1  in  FIGS.  2  and  3   . At this time, the first photoreceptor unit  20 Y and the first photoreceptor unit  20 M may transfer the toner images to the transfer belt  52 . When the retract roller  39  and the retract roller  48  are located at the retracted positions, the upper portion  52 A is located at a second transport position PM 2  corresponding to the second transport position PM 2  in  FIGS.  2  and  3   . At this time, the first photoreceptor unit  20 Y and the first photoreceptor unit  20 M are not able to transfer the toner images to the transfer belt  52 . By individually controlling the positions of the respective retract rollers  39 ,  40 ,  47 , and  48 , only any one to three of the first and second photoreceptor units  20  and  30  may be brought into a state in which transfer to the transfer belt  52  is possible. 
     The movement direction MD that is a direction extending in the thickness direction TD of the transfer belt  52  includes a direction completely parallel to the thickness direction TD and a direction slightly inclined with respect to the thickness direction TD. In a case where the movement direction MD is inclined with respect to the thickness direction TD, an inclination angle defined by the movement direction MD and the thickness direction TD when viewed in the Z direction is any angle of 10° or less. 
     The driving roller  44  having a circular cross section is configured to be rotationally driven around an axis  44 X extending in the Z direction by a driver (not illustrated), and causes the transfer belt  52  to circle at a predetermined speed in a circling direction indicated by arrow A. 
     The diameter of the steering roller  45  having a circular cross section is the same as the diameter of the driving roller  44  within a range of tolerance. In other words, an outer peripheral length  45 C of the steering roller  45  and an outer peripheral length  44 C of the driving roller  44  are the same within a range of tolerance. The steering roller  45  is rotatable around a rotation axis  45 X extending in one direction. The steering roller  45  is an example of a change roller. Further, the steering roller  45  is rotatable around a rotation center shaft that is provided at a center portion of the steering roller  45  in a direction along the rotation axis  45 X and that intersects with the rotation axis  45 X. The position of the steering roller  45  in the rotation direction around the rotation center shaft when the rotation axis  45 X is parallel to the Z direction is a neutral position of the steering roller  45 . Further, the transfer device  50  includes a driving mechanism (not illustrated) that rotates the steering roller  45  by applying a driving force to the steering roller  45 . When the driving mechanism applies, to the steering roller  45 , a driving force corresponding to a movement amount (meandering amount) of the transfer belt  52  in the width direction detected by movement amount detection devices  17 A and  17 B described later, the meandering of the transfer belt  52  is suppressed by the steering roller  45  that is rotated. 
     The first distance  20 B of the two first photoreceptor drums  22  and the second distance  30 B of the two second photoreceptor drums  32  are set to be integral multiples of the outer peripheral length  44 C of the driving roller  44  and the outer peripheral length  45 C of the steering roller  45 . The second distance  30 B is shorter than the first distance  20 B. For example, the first distance  20 B of the present exemplary embodiment is set to be four times the outer peripheral length  44 C and the outer peripheral length  45 C, and the second distance  30 B is set to be three times the outer peripheral length  44 C and the outer peripheral length  45 C. 
     A distance along the transfer belt  52  between the first transfer position T 1  of the first photoreceptor drum  22  on the downstream side and the first transfer position T 1  of the second photoreceptor drum  32  on the upstream side is different from the first distance  20 B and the second distance  30 B. The distance along the transfer belt  52  between the first transfer position T 1  of the first photoreceptor drum  22  on the downstream side and the first transfer position T 1  of the second photoreceptor drum  32  on the upstream side is also set to an integral multiple of the outer peripheral length  44 C of the driving roller  44  and the outer peripheral length  45 C of the steering roller  45 . 
     The backup roller  46  faces the transfer cylinder  85  with the transfer belt  52  interposed therebetween. A region where the transfer cylinder  85  and the transfer belt  52  are in contact with each other is the nip region Np (see  FIG.  1   ). The nip region Np is the second transfer position T 2  where the toner images are transferred from the transfer belt  52  to the recording paper P. 
     Further, the pressing roller  49  located on the upstream side of the retract roller  48  and on the downstream side of the driving roller  44  is rotatably in contact with the outer peripheral surface of the transfer belt  52  and presses the transfer belt  52  toward the inner peripheral side. 
     Movement Amount Detection Device 
     A base plate  50 A (not illustrated in  FIG.  1   , see  FIG.  4   ) on which the first transfer rollers  41 , the retract rollers  39 ,  40 ,  47 , and  48 , and the driving roller  44  are supported is provided with two movement amount detection devices  17 A and  17 B. One movement amount detection device  17 A is disposed in the vicinity of one side surface  52 G in the width direction of the upper portion  52 A, and is located on the downstream side of the first photoreceptor unit  20 Y and on the upstream side of the first photoreceptor unit  20 M. The other movement amount detection device  17 B is disposed in the vicinity of one side surface  52 G in the width direction of the lower portion  52 B, and is located on the downstream side of the second photoreceptor unit  30 C and on the upstream side of the second photoreceptor unit  30 K. As illustrated in  FIGS.  4  and  5   , the movement amount detection devices  17 A and  17 B each include a rotation unit  55  and a detection unit  60 . 
     The rotation unit  55  includes a support member  56 , a rotational member  57 , a rotation shaft  58 , and a coil spring  59 . The support member  56 , which is a metallic press-formed product, includes a first connecting portion  56 A, a fixed portion  56 B, and a second connecting portion  56 C. The first connecting portion  56 A and the fixed portion  56 B intersect with each other, and the first connecting portion  56 A and the second connecting portion  56 C intersect with each other. 
     The rotational member  57 , which is a metallic press-formed product, includes a base portion  57 A, a coupling portion  57 B, a rotating portion  57 C, and a pressing portion  57 D. The base portion  57 A includes a vertical wall portion  57 A 1 , a lower portion  57 A 2 , and an upper portion  57 A 3 . The lower portion  57 A 2  and the upper portion  57 A 3  are respectively connected to both end portions of the vertical wall portion  57 A 1 . The lower portion  57 A 2  and the upper portion  57 A 3  intersect with the vertical wall portion  57 A 1 . That is, the cross-sectional shape of the base portion  57 A is substantially U-shaped. 
     One end portion of a metallic rotation shaft  58  extending in a direction intersecting with the Z direction is fixed to the first connecting portion  56 A. The rotation shaft  58  penetrates through the lower portion  57 A 2  and the upper portion  57 A 3 . The base portion  57 A (rotational member  57 ) is rotatable relative to the support member  56  around the rotation shaft  58 . 
     One end portion of the coupling portion  57 B having a flat plate shape and extending in one direction is connected to the upper portion  57 A 3 . The upper portion  57 A 3  and the coupling portion  57 B are located on the same plane. One end portion of the rotating portion  57 C extending in a direction intersecting with the coupling portion  57 B is connected to the other end portion of the coupling portion  57 B. When viewed along the rotation shaft  58 , the rotating portion  57 C is located on the outer peripheral side of the rotation shaft  58 . 
     The rotating portion  57 C includes a first plate-shaped portion  57 C 1 , a second plate-shaped portion  57 C 2 , and a contact portion  57 C 3 . The rotating portion  57 C (the first plate-shaped portion  57 C 1 , the second plate-shaped portion  57 C 2 , and the contact portion  57 C 3 ) extends in a direction along the rotation shaft  58 . In this case, the expression “extends in a direction along the rotation shaft  58 ” includes that the rotating portion  57 C extends in a direction completely parallel to the rotation shaft  58  and that the rotating portion  57 C extends in a direction slightly inclined with respect to the rotation shaft  58 . In a case where the rotating portion  57 C is inclined with respect to the rotation shaft  58 , an inclination angle defined by the rotating portion  57 C and the rotation shaft  58  when viewed in the Z direction is any angle of 10° or less. 
     As illustrated in  FIGS.  6  to  8   , the first plate-shaped portion  57 C 1  and the second plate-shaped portion  57 C 2  intersect with each other when viewed along the rotation shaft  58 . An intersection angle θC between the first plate-shaped portion  57 C 1  and the second plate-shaped portion  57 C 2  is an obtuse angle. The first plate-shaped portion  57 C 1  includes a wide portion  57 C 1   a  and a narrow portion  57 C 1   b . The wide portion  57 C 1   a  is one end portion of the first plate-shaped portion  57 C 1  in the longitudinal direction. The narrow portion  57 C 1   b , which is a portion of the first plate-shaped portion  57 C 1  excluding one end portion, is narrower than the wide portion  57 C 1   a . The second plate-shaped portion  57 C 2  includes a wide portion  57 C 2   a  and a narrow portion  57 C 2   b . The narrow portion  57 C 2   b  is narrower than the wide portion  57 C 2   a . The longitudinal dimension of the wide portion  57 C 2   a  is larger than the longitudinal dimension of the wide portion  57 C 1   a . The contact portion  57 C 3  is formed at a portion of one surface of the rotating portion  57 C. As described later, the one surface of the rotating portion  57 C faces the one side surface  52 G of the transfer belt  52 . The straight-line-shaped contact portion  57 C 3  extending in a direction along the rotation shaft  58  is a connecting portion between the first plate-shaped portion  57 C 1  and the second plate-shaped portion  57 C 2 . As illustrated in  FIGS.  6  to  8   , the contact portion  57 C 3  is a round surface. 
     One end portion of the pressing portion  57 D having a flat plate shape and extending in one direction is connected to the upper portion  57 A 3 . That is, the upper portion  57 A 3 , the coupling portion  57 B, and the pressing portion  57 D are located on the same plane. 
     As illustrated in  FIG.  4   , both end portions of the coil spring (second urging member)  59  are respectively fixed to the first connecting portion  56 A of the support member  56  and the vertical wall portion  57 A 1  of the rotational member  57 . The coil spring  59  is normally elastically deformed. Thus, the rotational member  57  is urged to rotate in the counterclockwise direction in  FIG.  9    by the urging force generated by the coil spring  59 . 
     The detection unit  60  includes a case  61 , an optical sensor  67 , an interlocking member  72 , and a coil spring (first urging member)  77 . 
     The case  61  has a shape obtained by processing a portion of a rectangular parallelepiped. That is, a recessed portion  62  is formed in one surface (left side surface in  FIG.  5   ) of the case  61 . Both end portions in the Z direction of one end portion  62 A of the recessed portion  62  in a direction along the rotation shaft  58  are closed by a pair of support walls  63 . Although not illustrated, bearing portions are formed at the pair of support walls  63 . In contrast, both end portions in the Z direction of a portion of the recessed portion  62  excluding the one end portion  62 A are opened. Further, a space  64  is formed inside the case  61 . As illustrated in  FIG.  5   , the space  64  communicates with the portion of the recessed portion  62  excluding the one end portion  62 A. The optical sensor  67  is fixed to an inner surface of the space  64 . The optical sensor  67  includes a light emitting element  68  and a light receiving element  69  that face each other. Inspection light emitted from the light emitting element  68  is received by the light receiving element  69 . The light emitting element  68 , the light receiving element  69 , and the above-described driving mechanism are connected to a control device (detector)  80  illustrated in  FIG.  5   . 
     The control device  80  includes a central processing unit (CPU, or processor), a read only memory (ROM), a random access memory (RAM), a storage, a communication interface (I/F), and an input/output I/F. The CPU, the ROM, the RAM, the storage, the communication I/F, and the input/output I/F are communicably connected to one another via a bus. The CPU is a central processing unit and executes various programs and controls each component. That is, the CPU reads a program from the ROM or the storage, and executes the program using the RAM as a work area. The CPU controls the driving mechanism and performs various types of calculation processing in accordance with the program. This calculation processing includes calculation processing of the movement amount in the width direction of the transfer belt  52  based on the light amount of the inspection light received by the light receiving element  69 . 
     An interlocking member  72  is an integrally molded product including a supported shaft  73 , a pressed portion  74 , and a detected portion  75 . One end portion of the pressed portion  74  and one end portion of the detected portion  75  are connected to the supported shaft  73  extending in the Z direction. As illustrated in  FIG.  5   , the pressed portion  74  has a substantially V-shaped cross section, and the detected portion  75  has a substantially L-shaped cross section. 
     As illustrated in  FIG.  5   , both end portions of the interlocking member  72  are rotatably supported by the bearing portions of the pair of support walls  63 . A distal end portion of the detected portion  75  is located inside the space  64 . Most of the pressed portion  74  is located outside the case  61 . 
     Further, as illustrated in  FIG.  5   , both end portions of the coil spring  77  are respectively fixed to an inner surface of the one end portion  62 A and the supported shaft  73 . The coil spring  77  is normally elastically deformed. Thus, the interlocking member  72  is urged to rotate in the clockwise direction in  FIG.  5    by the urging force generated by the coil spring  77 . The urging force of the coil spring  77  is smaller than the urging force of the coil spring  59 . 
     As illustrated in  FIGS.  4  and  5   , the rotation unit  55  and the detection unit  60  are connected to each other. To be specific, the first connecting portion  56 A of the support member  56  is fixed to one surface (an upper surface in  FIG.  5   ) of the case  61 , and the second connecting portion  56 C is fixed to another surface (a left side surface in  FIG.  5   ) of the case  61 . When the rotation unit  55  and the detection unit  60  are connected to each other, the pressing portion  57 D of the rotational member  57  and the pressed portion  74  of the interlocking member  72  come into contact with each other. As described above, the urging force of the coil spring  77  is smaller than the urging force of the coil spring  59 . Thus, the pressing portion  57 D (rotational member  57 ) is rotated in the counterclockwise direction in  FIG.  9    by the coil spring  59 , and the interlocking member  72  (pressed portion  74 ) in contact with the pressing portion  57 D is rotated in the counterclockwise direction in  FIG.  5    against the urging force of the coil spring  77 . When external forces other than those of the coil spring  59  and the coil spring  77  are not applied to the rotational member  57  and the interlocking member  72 , the rotational member  57  is located at an initial position  57 IP indicated by an imaginary line in  FIG.  9   , and the interlocking member  72  is located at an initial position  72 IP indicated by an imaginary line in  FIG.  5   . The rotation unit  55  is provided with a stopper (not illustrated) for restricting counterclockwise rotation of the rotational member  57  around the rotation shaft  58  at the initial position  57 IP. Further, the fixed portion  56 B of the support member  56  of each of the movement amount detection device  17 A and the movement amount detection device  17 B each constituted by connecting the rotation unit  55  and the detection unit  60  is fixed to the base plate  50 A by bolts or the like. 
     When the fixed portion  56 B is fixed to the base plate  50 A, an extension direction of the rotation shaft  58  becomes a direction extending in the movement direction MD. In other words, the extension direction of the rotating portion  57 C and the rotation shaft  58  intersects with the transport direction of the transfer belt  52 . In this case, the expression “direction extending in the movement direction MD” includes a direction completely parallel to the movement direction MD and a direction slightly inclined with respect to the movement direction MD. In a case where the rotation shaft  58  is inclined with respect to the movement direction MD, an inclination angle defined by the movement direction MD and the axis of the rotation shaft  58  when viewed in the Z direction is any angle of 10° or less. 
     When the detected portion  75  is not located between the light emitting element  68  and the light receiving element  69 , the light receiving element  69  receives all inspection light emitted by the light emitting element  68 . When the detected portion  75  is located between the light emitting element  68  and the light receiving element  69 , the light receiving element  69  is not able to receive all or part of the inspection light emitted by the light emitting element  68 . That is, the light amount of the inspection light received by the light receiving element  69  changes in accordance with the rotation angle of the detected portion  75  (interlocking member  72 ) around the supported shaft  73 . In other words, the light amount of the inspection light received by the light receiving element  69  changes in accordance with the rotation angle of the rotational member  57  (rotating portion  57 C) rotating in association with the detected portion  75  around the rotation shaft  58 . 
     As illustrated in  FIGS.  1  and  9   , the contact portion  57 C 3  of the rotating portion  57 C of the movement amount detection device  17 A disposed in the vicinity of the upper portion  52 A is in contact with the side surface  52 G of the upper portion  52 A. Similarly, as illustrated in  FIGS.  2 ,  3   , and  9 , the contact portion  57 C 3  of the rotating portion  57 C of the movement amount detection device  17 B disposed in the vicinity of the lower portion  52 B is in contact with the side surface  52 G of the lower portion  52 B. Further, even when the retract rollers  39 ,  40 ,  47 , and  48  are located at any positions in the movement direction MD, the contact state between the contact portion  57 C 3  of the movement amount detection device  17 A and the side surface  52 G of the upper portion  52 A is maintained, and the contact state between the contact portion  57 C 3  of the movement amount detection device  17 B and the side surface  52 G of the lower portion  52 B is maintained. 
     In this case, a position in the width direction of the transfer belt  52  indicated by a solid line in  FIG.  9    is defined as a reference position  52 SP. When the transfer belt  52  is located at the reference position  52 SP, the rotational member  57  (rotating portion  57 C) is located at a reference rotational position  57 SP indicated by a solid line in  FIG.  9    in a plan view. The reference rotational position  57 SP is a position rotated by only a predetermined angle in the clockwise direction in plan view from the initial position  57 IP indicated by an imaginary line in  FIG.  9   . At this time, the interlocking member  72  is located at a reference rotational position  72 SP indicated by a solid line in  FIG.  5   . 
     The transfer belt  52  is movable in the width direction. That is, the transfer belt  52  is movable in the up-down direction in  FIG.  9    from the reference position  52 SP. When the transfer belt  52  moves to a first position  52 P 1  indicated by an imaginary line in  FIG.  9   , the rotating portions  57 C (contact portions  57 C 3 ) of the movement amount detection devices  17 A and  17 B that is in contact with the side surface  52 G of the transfer belt  52  each move from the reference rotational position  57 SP to a first rotational position  57 P 1  indicated by the imaginary line following the transfer belt  52 . As a result, the interlocking member  72  moves from the reference rotational position  72 SP to a first rotational position  72 P 1  indicated by an imaginary line in  FIG.  5   . In contrast, when the transfer belt  52  moves to a second position  52 P 2  indicated by an imaginary line in FIG.  9 , the rotating portion  57 C (contact portion  57 C 3 ) of the movement amount detection device  17 A that is in contact with the side surface  52 G of the transfer belt  52  moves from the reference rotational position  57 SP to a second rotational position  57 P 2  indicated by the imaginary line following the transfer belt  52 . As a result, the interlocking member  72  moves from the reference rotational position  72 SP to a second rotational position  72 P 2  indicated by an imaginary line in  FIG.  5   . The first position  52 P 1  is a position of the transfer belt  52  when the transfer belt  52  according to the present exemplary embodiment is maximally moved upward in  FIG.  9   . The second position  52 P 2  is a position of the transfer belt  52  when the transfer belt  52  according to the present exemplary embodiment is maximally moved downward in  FIG.  9   . In this manner, the urging forces of the coil spring  59  and the coil spring  77  are used to maintain the contact state between the contact portion  57 C 3  of the rotational member  57  and the side surface  52 G of the transfer belt  52  and the contact state between the pressing portion  57 D and the interlocking member  72  (pressed portion  74 ). The rotational member  57  and the interlocking member  72  are rotated in association with the movement of the belt  52  in the width direction. 
     Transport Section 
     As illustrated in  FIG.  1   , the transport section  16  includes a transport device (not illustrated) that transports recording paper P sent out from the storage section  14  in a direction of arrow B. The recording paper P sent out from the storage section  14  is transported to the transfer cylinder  85  by the transport device. The recording paper P on which a toner image has been second-transferred by passing through the transfer cylinder  85  (second transfer position T 2 ) is transported to the fixing device  18  by the transport device. 
     Fixing Device 
     As illustrated in  FIG.  1   , the fixing device  18  includes a heating roller  42  as an example of a heating member and a pressure roller  43  as an example of a pressure member. The fixing device  18  fixes the toner image transferred to the recording paper P by the transfer cylinder  85  to the recording paper P by sandwiching the recording paper P between the heating roller  42  and the pressure roller  43  and heating and pressing the recording paper P. 
     Next, operations and effects of the image forming apparatus  10  configured as described above will be described in detail. 
     In the image forming apparatus  10  according to the present exemplary embodiment, the transfer belt  52  that is circled in the arrow A direction by the driving force generated by the driving roller  44  may meander in the width direction. That is, the transfer belt  52  may move from the reference position  52 SP toward the first position  52 P 1  and toward the second position  52 P 2 , and the rotational member  57  (rotating portion  57 C) may rotate from the reference rotational position  57 SP to the first rotational position  57 P 1  and the second rotational position  57 P 2 . In other words, the interlocking member  72  may rotate from the reference rotational position  72 SP to the first rotational position  72 P 1  and the second rotational position  72 P 2 . Consequently, as illustrated in  FIG.  5   , the position of the detected portion  75  in the rotation direction changes. Thus, the light amount (physical amount) of the inspection light received by the light receiving element  69  changes. That is, the light amount of the inspection light received by the light receiving element  69  changes due to the rotation of the rotating portion  57 C around the rotation shaft  58 . That is, the light amount of the inspection light received by the light receiving element  69  changes due to the movement amount of the transfer belt  52  in the width direction from the reference position  52 SP. 
     Information relating to the light amount of the inspection light received by the light receiving element  69  is transmitted from the light receiving element  69  to the control device  80 . The control device  80  that has received this information calculates the movement amount of the transfer belt  52  in the width direction from the reference position  52 SP based on the information and controls the above-described driving mechanism. 
     In the image forming apparatus  10  according to the present exemplary embodiment, the upper portion  52 A and the lower portion  52 B of the transfer belt  52  are movable between the first transport position PM 1  and the second transport position PM 2  in the movement direction MD. As described above, even when the retract rollers  39 ,  40 ,  47 , and  48  are located at any positions in the movement direction MD, the contact state between the contact portion  57 C 3  of the movement amount detection device  17 A and the side surface  52 G of the upper portion  52 A is maintained, and the contact state between the contact portion  57 C 3  of the movement amount detection device  17 B and the side surface  52 G of the lower portion  52 B is maintained. Further, the straight-line-shaped contact portion  57 C 3  of the rotating portion  57 C extends in the direction along the rotation shaft  58 . Thus, even when the position of the transfer belt  52  in the thickness direction TD changes, the magnitude of the rotation angle of the rotational member  57  (rotating portion  57 C) per unit movement amount of the transfer belt  52  in the width direction is constant. For this reason, the movement amount detection devices  17 A and  17 B (optical sensor  67 ) of the present exemplary embodiment may accurately acquire a physical amount (light amount) as compared with a case of acquiring a physical amount (light amount) that changes due to rotation of a rotating portion that is rotatable around a rotation shaft extending in the transport direction of the transfer belt  52 . Thus, the image forming apparatus  10  according to the present exemplary embodiment may accurately detect the movement amount in the width direction of the transfer belt  52  whose position in the thickness direction TD changes, as compared with the case of acquiring the physical amount (light amount) that changes due to the rotation of the rotating portion that is rotatable around the rotation shaft extending in the transport direction of the transfer belt  52 . 
       FIG.  10    illustrates a rotational member  90  according to a comparative example of the present disclosure. A rotating portion  91  of the rotational member  90  is a plate-shaped member having a rectangular cross-sectional shape. That is, an outer peripheral surface of the rotating portion  91  is constituted by four flat surfaces. A first corner portion  93  is formed between a first surface  92 , which is one of the four surfaces, and a second surface  95  adjacent to the first surface  92 . Similarly, a second corner portion  94  is formed between the first surface  92  and a third surface  96  adjacent to the first surface  92 . The rotational member  90  extends in a direction parallel to a rotation shaft  58 . As indicated by a solid line in  FIG.  10   , when the transfer belt  52  is located at a second position  52 P 2 , the first corner portion  93  of the rotating portion  91  is in contact with a side surface  52 G of a transfer belt  52 . Further, when the transfer belt  52  is located at a first position  52 P 1 , the second corner portion  94  of the rotating portion  91  is in contact with the side surface  52 G. 
     A line segment Dm 1  illustrated in  FIG.  10    connects the rotation shaft  58  and the first corner portion  93 , and a line segment Dm 2  connects the rotation shaft  58  and the second corner portion  94 . The line segment Dm 1  is longer than the line segment Dm 2 . That is, the length of the line segment Dm 1  differs from the length of the line segment Dm 2 . Hence, the rotation angle of the rotating portion  91  around the rotation shaft  58  when the transfer belt  52  moves from the second position  52 P 2  in the width direction by only a unit movement amount differs from the rotation angle of the rotating portion  91  around the rotation shaft  58  when the transfer belt  52  moves from the first position  52 P 1  in the width direction by only the unit movement amount. Hence, there is a possibility that the optical sensor  67  of this comparative example is not able to accurately acquire the amount of received light (physical amount) that changes due to the rotation of the rotational member  90 . Thus, there is a possibility that the accuracy of the calculation amount (the movement amount of the transfer device  50  in the width direction) obtained by the control device  80  based on the amount of received light is lowered. 
     In contrast, the contact portion  57 C 3  of the rotating portion  57 C of the image forming apparatus  10  according to the present exemplary embodiment has a straight-line shape (round surface shape) extending in the direction along the rotation shaft  58 . Hence, even when the rotational position of the rotating portion  57 C changes, the length of the line segment connecting the rotation shaft  58  and the contact position between the contact portion  57 C 3  and the side surface  52 G of the transfer belt  52  does not change. Thus, the image forming apparatus  10  according to the present exemplary embodiment may more accurately detect the movement amount of the transfer belt  52  in the width direction than a case where a portion of the flat-surface-shaped first surface  92  is brought into contact with the side surface  52 G of the transfer belt  52 . 
     The first corner portion  93  and the second corner portion  94  in  FIG.  10    are constituted by corner portions of a plate-shaped member. Hence, when the first corner portion  93  and the second corner portion  94  come into contact with the side surface  52 G of the transfer belt  52 , the transfer belt  52  is easily damaged. In contrast, the contact portion  57 C 3  of the rotating portion  57 C of the image forming apparatus  10  according to the present exemplary embodiment has a round surface shape. Thus, the rotating portion  57 C of the rotational member  57  according to the present exemplary embodiment is less likely to damage the transfer belt  52  than a case where the first corner portion  93  and the second corner portion  94  constituted by the corner portions of the plate-shaped member are brought into contact with the side surface  52 G of the transfer belt  52 . 
     Further, in the image forming apparatus  10  according to the present exemplary embodiment, the cross section of the rotating portion  57 C intersecting with the direction along the rotation shaft  58  has different areas depending on the position of the rotating portion  57 C in the direction. That is, in this case, a proximal end portion of the rotating portion  57 C that is an end portion connected to the coupling portion  57 B is defined as “one end portion”, and an end portion of the rotating portion  57 C provided with the narrow portion  57 C 2   b  is defined as “the other end portion”. In this case, as is clear from  FIGS.  6  to  8   , the area of a cross section of a portion of the rotating portion  57 C excluding the other end portion of the rotating portion  57 C is larger than the area of a cross section of the other end portion of the rotating portion  57 C. In other words, the area of the cross section of the portion illustrated in  FIGS.  7  and  8    is larger than the area of the cross section of the portion illustrated in  FIG.  6   . The mechanical strength of the rotating portion  57 C having such a configuration is higher than that in a case where the area of the cross section of the entire rotating portion  57 C intersecting with the direction along the rotation shaft  58  is the same as the area of the cross section of the other end portion. Further, as compared with a case where the area of the cross section of the entire rotating portion  57 C intersecting with the direction along the rotation shaft  58  is the same as the area of the cross section of the other end portion, the rotational operation of the rotating portion  57 C having such a configuration around the rotation shaft  58  is stabilized. 
     Further, the rotating portion  57 C of the image forming apparatus  10  according to the present exemplary embodiment has a plate-shaped structure. For this reason, the rotational operation of the rotating portion  57 C of the image forming apparatus  10  is smooth as compared with a case where the rotating portion is a block body. 
     Further, the image forming apparatus  10  according to the present exemplary embodiment includes the coil spring  77  that applies a force to the interlocking member  72  in a direction in which the interlocking member  72  is brought into contact with the rotational member  57 . Thus, the structure of the image forming apparatus  10  according to the present exemplary embodiment is simpler than that in a case where the rotational member  57  and the interlocking member  72  are coupled via a link member. 
     Further, the image forming apparatus  10  according to the present exemplary embodiment includes the coil spring  59  that applies a force to the rotational member  57  in a direction in which the contact portion  57 C 3  is brought into contact with the side surface  52 G of the transfer belt  52 . Thus, the structure of the image forming apparatus  10  according to the present exemplary embodiment is simpler than that in a case where the contact portion  57 C 3  (rotational member  57 ) and the transfer belt  52  are connected by a relatively displaceable mechanism. 
     Further, the rotational member  57  of the image forming apparatus  10  according to the present exemplary embodiment includes the pressing portion  57 D that may come into contact with the pressed portion  74  of the detection unit  60 . Accordingly, the image forming apparatus  10  according to the present exemplary embodiment may detect the movement amount of the transfer belt  52  in the width direction in which the position in the thickness direction TD changes by using the detection unit  60  including the interlocking member  72  and the optical sensor  67 . 
     The second distance (adjacent distance)  30 B, which is the distance between the rotation axes  30 X of the two second photoreceptor drums  32  (image forming bodies) located on the downstream side of the steering roller  45  and on the upstream side of the transfer position to recording paper P, is an integral multiple of the outer peripheral length  45 C of the steering roller  45 . Thus, as compared with a case where the second distance  30 B is different from an integral multiple of the outer peripheral length  45 C, an increase in displacement amount of registration of toner images formed on the transfer belt (formation target body)  52  by the two second photoreceptor drums  32  located on the downstream side of the steering roller  45  is suppressed. 
     Further, in the image forming apparatus  10 , the first distance  20 B between the two first photoreceptor drums  22  and the second distance  30 B between the two second photoreceptor drums  32  each are set to be an integral multiple of the outer peripheral length  44 C of the driving roller  44 . Thus, as compared with a case where the first distance  20 B and the second distance  30 B are set to lengths different from integral multiples of the outer peripheral length  44 C, an increase in displacement amount of registration of toner images formed on the transfer belt (formation target body)  52  by the two second photoreceptor drums  32  located on the downstream side of the steering roller  45  is suppressed. 
     Further, the second distance  30 B between the two second photoreceptor drums  32  located on the downstream side of the first photoreceptor drums  22  is shorter than the first distance  20 B. In a comparative example (not illustrated) in which the second distance  30 B is set to be longer than or equal to the first distance  20 B, the second distance  30 B is set to meet the first distance  20 B. Thus, the distance along the transfer belt  52  from the driving roller  44  to the second photoreceptor unit  30 K is shorter in this exemplary embodiment than that in the comparative example. As the distance increases, the cumulative amounts of the speed fluctuation of the transfer belt  52  and the error of the adjacent distance increase. Thus, in the comparative example, the displacement amount of the registration of the toner images between the second photoreceptor unit  30 C and the second photoreceptor unit  30 K is likely to be larger than the displacement amount of the registration of the toner images between the first photoreceptor unit  20 Y and the first photoreceptor unit  20 M. In contrast, in the exemplary embodiment, since the distance (second distance  30 B) between the second photoreceptor unit  30 C and the second photoreceptor unit  30 K is shorter than that in the comparative example, the cumulative amounts of the speed fluctuation and the error of the adjacent distance are smaller than those in the comparative example. Thus, in the present exemplary embodiment, as compared with a case where the second distance  30 B is set to be a length longer than or equal to the first distance  20 B, an increase in the displacement amount of the registration of the toner images is suppressed as the position of the photoreceptor drum is located on the downstream side of the transfer belt  52 . 
     Although the image forming apparatus  10  and the movement amount detection devices  17 A and  17 B according to the present exemplary embodiment have been described above based on the drawings, the image forming apparatus  10  and the movement amount detection devices  17 A and  17 B according to the present exemplary embodiment are not limited to those illustrated in the drawings, and may be appropriately changed in design without departing from the gist of the present disclosure. 
     For example, the image forming apparatus  10  may be configured such that each of the first photoreceptor units  20  and each of the second photoreceptor units  30  form toner images on recording paper P (formation target body) transported by a transport belt (not illustrated) provided instead of the transfer belt  52 . 
     In the present exemplary embodiment, a toner image is described as an example of an image. In this case, the toner image is formed by a dry electrophotographic system, but the present disclosure is not limited to this. For example, an image of the present disclosure may be a toner image formed by a wet electrophotographic system or an image formed by an inkjet system. 
     Further, the image forming apparatus  10  may be configured such that an ink image or a toner image is formed on an elongated non-annular continuous paper (formation target body) that is wound around multiple rotating bodies including the driving roller  44  and that is transported by the driving roller  44  and the rotating bodies while having a shape having at least one straight portion when viewed in the Z direction, and such that the steering roller (change roller)  45  is rotatably in contact with the inner peripheral surface of the continuous paper. 
     When the transfer belt  52  is located at the reference position  52 SP, an angle formed by a straight line (not illustrated) connecting the contact portion  57 C 3  and the rotation shaft  58  in plan view and the transport direction A of the transfer belt  52  may be as small as possible. That is, when this angle is small, the difference between the rotation amount of the rotating portion  57 C around the rotation shaft  58  when the transfer belt  52  moves from the reference position  52 SP in the width direction by only the unit movement amount and the rotation amount of the rotating portion  57 C around the rotation shaft  58  when the transfer belt  52  moves from the first position  52 P 1  or the second position  52 P 2  in the width direction by only the unit movement amount becomes small. That is, as the angle is smaller, a sensor (for example, the optical sensor  67 ) that acquires the physical amount may more accurately acquire the physical amount that changes due to the rotation of the rotating portion  57 C. Thus, for example, the present disclosure may be implemented in an aspect of a modification illustrated in  FIG.  11   . The flat-surface shape of a coupling portion  57 B of this modification is a V-shape. In the example illustrated in  FIG.  11   , a portion of the coupling portion  57 B is located directly below the upper portion  52 A. In the example illustrated in  FIG.  11   , when the transfer belt  52  is located at the reference position  52 SP, an angle formed by a straight line connecting the contact portion  57 C 3  and the rotation shaft  58  in plan view and the transport direction A of the transfer belt  52  is substantially 0°. 
     One of the movement amount detection device  17 A and the movement amount detection device  17 B may be omitted from the image forming apparatus  10 . 
     The image forming apparatus  10  may be provided with another movement amount detection device in addition to the movement amount detection device  17 A and the movement amount detection device  17 B. 
     The number of colors of images (toner images or ink images) formed on a formation target body (transfer belt  52  or recording paper P) need not be four. For example, the number of colors of images may be six. 
     For example, three or more image forming bodies may be arranged along the upper portion  52 A. Similarly, three or more image forming bodies may be arranged along the lower portion  52 B. 
     The foregoing description of the exemplary embodiments of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical applications, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the following claims and their equivalents.