Patent Publication Number: US-9846399-B2

Title: Position detection apparatus that detects position of target

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a position detection apparatus that detects a position of a target. 
     Description of the Related Art 
     There is a known image forming apparatus that primarily transfers toner images respectively formed on a plurality of photosensitive members to an intermediate transfer belt and secondarily transfers a color image composited on the intermediate transfer belt to a recording sheet. 
     Incidentally, when the intermediate transfer belt (a target) in the image forming apparatus is deviated (moves) in a width direction that intersects perpendicularly with a belt conveying direction, color misregistration in which the toner images of the plurality of colors on the intermediate transfer belt are deviated may occur. In order to prevent such color misregistration, a belt-deviation-amount detection technique that detects a deviation amount of the intermediate transfer belt in the width direction that intersects perpendicularly with the belt conveying direction is proposed. 
     As an apparatus that detects a deviation amount of an intermediate transfer belt, there is a known apparatus that is provided with a swinging arm of which one end is in contact with an edge of the intermediate transfer belt to swing and two transmission optical sensors disposed on the other end of the swinging arm, for example. This deviation amount detection apparatus detects the deviation amount of the belt corresponding to light amount variation due to change in a shield factor with using the fact that the shield factors of the two transmission optical sensors vary corresponding to the swinging angle of the swinging arm (for example, see U.S. Pat. No. 8,412,081). 
     However, the above-mentioned belt-deviation-amount detection technique cannot detect breakage (hereinafter referred to as failure) even if any one of the optical sensors used for detecting the deviation amount (moving amount) of the belt has broken. Then, if the position of the target is corrected on the basis of the erroneously detected position while one of the sensors fails, an error may occur or the target may break. 
     SUMMARY OF THE INVENTION 
     Accordingly, a first aspect of the present invention provides a position detection apparatus that detects a position of a target in a predetermined direction, the position detection apparatus including a swinging member of which one end is in contact with the target in the predetermined direction, a moving member that is in contact with the other end of the swinging member, a plurality of sensors that are arranged in a direction that intersects a moving direction of the moving member and output signals corresponding to a position of the moving member that corresponds to a swinging amount of the swinging member, and a detection unit configured to detect the position of the target based on output signals of the sensors. The moving member has a plurality of measured parts disposed on the moving member along a plurality of loci of measuring positions of the sensors formed on the moving member during movement of the moving member. The measured parts are disposed so that the sum total of the output signals of the sensors becomes an even number. The detection unit determines that any one of the sensors failed in a case where the sum total of the output signals of the sensors is an odd number. 
     Accordingly, a second aspect of the present invention provides a position detection apparatus that detects a position of a target in a predetermined direction, the position detection apparatus including a swinging member of which one end is in contact with the target in the predetermined direction, a moving member that is in contact with the other end of the swinging member, a plurality of sensors that are arranged in a direction that intersects a moving direction of the moving member and output signals corresponding to a position of the moving member that corresponds to a swinging amount of the swinging member, and a detection unit configured to detect the position of the target based on output signals of the sensors. The moving member has a plurality of measured parts disposed on the moving member along a plurality of loci of measuring positions of the sensors formed on the moving member during movement of the moving member. The measured parts are disposed so that the sum total of the output signals of the sensors becomes an odd number. The detection unit determines that any one of the sensors failed in a case where the sum total of the output signals of the sensors is an even number. 
     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 sectional view schematically showing a configuration of an image forming apparatus according to a first embodiment. 
         FIG. 2  is a perspective view showing an intermediate transfer mechanism in the image forming apparatus in  FIG. 1 . 
         FIG. 3A  and  FIG. 3B  are views schematically showing a configuration of a belt-deviation-amount detection apparatus in the image forming apparatus in  FIG. 1 . 
         FIG. 4  is a view showing an example of an arrangement of projection groups on a rotating member of the belt-deviation-amount detection apparatus in  FIG. 3A  and  FIG. 3B . 
         FIG. 5A  through  FIG. 5H  are views showing rotating positions of a rotating member where rotating areas respectively face transmission optical sensors of the belt-deviation-amount detection apparatus in  FIG. 3A  and  FIG. 3B . 
         FIG. 6  is a flowchart showing procedures of a sensor-failure detection process executed by the image forming apparatus shown in  FIG. 1 . 
         FIG. 7A ,  FIG. 7B , and  FIG. 7C  are views schematically showing a configuration of a belt-deviation-amount detection apparatus in a second embodiment. 
         FIG. 8  is a view showing an example of an arrangement of projection groups on a slide member of the belt-deviation-amount detection apparatus in the second embodiment. 
         FIG. 9A  through  FIG. 9H  are views showing slide positions of the slide member where slide areas respectively face transmission optical sensors of the belt-deviation-amount detection apparatus in the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereafter, embodiments according to the present invention will be described in detail with reference to the drawings. 
       FIG. 1  is a sectional view schematically showing a configuration of an image forming apparatus according to a first embodiment. As shown in  FIG. 1 , the image forming apparatus  100  is provided with an intermediate transfer belt  6  as a target of position detection and a plurality of image forming stations  10 Y,  10 M,  10 C, and  10 K that are arranged along a horizontal part of the intermediate transfer belt  6 . 
     The image forming stations  10 Y,  10 M,  10 C, and  10 K are respectively provided with photosensitive drums  2 Y,  2 M,  2 C, and  2 K as photosensitive members, charging rollers  3 Y,  3 M,  3 C, and  3 K that are respectively arranged around the photosensitive drums  2 Y,  2 M,  2 C, and  2 K, and laser scanner units  1 Y,  1 M,  1 C, and  1 K. Each of the photosensitive drums  2 Y,  2 M,  2 C, and  2 K is configured by applying an organic photoconductive layer to a periphery of an aluminum cylinder, and is rotated counterclockwise in  FIG. 1  by a driving force transferred from a driving motor (not shown). 
     The charging rollers  3 Y,  3 M,  3 C, and  3 K electrify uniformly the surfaces of the corresponding photosensitive drums  2 Y,  2 M,  2 C, and  2 K, respectively. The laser scanner units  1 Y,  1 M,  1 C, and  1 K respectively form electrostatic latent images on the surfaces of the corresponding photosensitive drums  2 Y,  2 M,  2 C, and  2 K by exposing the photosensitive drums  2 Y,  2 M,  2 C, and  2 K selectively on the basis of image data sent from a controller (not shown). 
     The image forming stations  10 Y,  10 M,  10 C, and  10 K are respectively provided with development devices  4 Y,  4 M,  4 C, and  4 K, drum cleaners  5 Y,  5 M,  5 C, and  5 K, and primary transfer rollers  7 Y,  7 M,  7 C, and  7 K that are disposed oppositely to the photosensitive drums through the intermediate transfer belt  6 , respectively. The development devices  4 Y,  4 M,  4 C, and  4 K are respectively provided with developing sleeves and stirring conveyance members which stir developer, and develop electrostatic latent images by supplying developer to the surfaces of the photosensitive drums  2 Y,  2 M,  2 C, and  2 K. The drum cleaners  5 Y,  5 M,  5 C, and  5 K respectively collect residual toners on the surface of the photosensitive drums  2 Y,  2 M,  2 C, and  2 K after primarily transferring. The collected residual toners are stored in a cleaner container (not shown). 
     The intermediate transfer belt  6  is an endless belt, and is looped over a plurality of rollers including a driving roller  8 , deviation control roller  9 , and secondary transfer internal roller  12 . The intermediate transfer belt  6  is in slidably contact with the photosensitive drums  2 Y,  2 M,  2 C, and  2 K, is rotatably driven in clockwise in  FIG. 1 , and receives transfer of visible images from the photosensitive drums  2 Y,  2 M,  2 C, and  2 K. The visible images transferred to the intermediate transfer belt  6  are superimposed to form a color image. 
     A secondary transfer external roller  11  is arranged oppositely to the secondary transfer internal roller  12 . The contact part of the secondary transfer internal roller  12  and secondary transfer external roller  11  becomes a secondary transfer area. A transfer sheet is conveyed to the secondary transfer area so as to synchronize with the color image famed on the intermediate transfer belt  6  that is rotating, and the color image on the intermediate transfer belt  6  is transferred to the transfer sheet. The secondary transfer external roller  11  is in contact with the intermediate transfer belt  6  while the color image is formed on the intermediate transfer belt  6 , and detaches from the intermediate transfer belt  6  after completing the transfer. 
     A belt cleaner  16  that cleans the intermediate transfer belt  6  is arranged oppositely to the driving roller  8  through the intermediate transfer belt  6 . The belt cleaner  16  collects residual toner on the intermediate transfer belt  6  after the secondary transfer. The collected residual toner is stored in a cleaner container (not shown). 
     Next, an intermediate transfer mechanism of the image forming apparatus in  FIG. 1  will be described. 
       FIG. 2  is a perspective view showing the intermediate transfer mechanism in the image forming apparatus in  FIG. 1 . 
     As shown in  FIG. 2 , the intermediate transfer belt  6  is looped over the driving roller  8 , the deviation control roller  9 , the secondary transfer internal roller  12 , idler rollers  13  through  15 , etc. The intermediate transfer belt  6  rotates so as to be in slidably contact with the primary transfer rollers  7 Y,  7 M,  7 C, and  7 K of the image forming stations  10 Y,  10 M,  10 C, and  10 K corresponding to colors of yellow (Y), magenta (M), cyan (C), and black (K). 
     The surface of the driving roller  8  is formed by a rubber layer. The driving roller  8  is rotated clockwise by a driving motor  8   a , and rotates the intermediate transfer belt  6  by the friction between the rubber layer and the internal surface of the intermediate transfer belt  6 . Moreover, the driving roller  8  functions as a counter roller of the belt cleaner  16  ( FIG. 1 ), and receives pressure of a cleaning blade. 
     The deviation control roller  9  corrects deviation of the intermediate transfer belt  6 . The far side of the deviation control roller  9  in the longitudinal direction thereof is fixed. Rotation of a deviation correction cam  18  changes inclination of the deviation control roller  9  through a deviation correction arm  17  to correct the deviation of the intermediate transfer belt  6 . Moreover, a tension spring  19  (a far side is not shown) pressurizes the deviation control roller  9  in the outside direction of the intermediate transfer belt  6 , which stretches the intermediate transfer belt  6 . 
     The secondary transfer internal roller  12  is a counter roller that backs up the secondary transfer external roller  11  at the time of transferring the color image formed on the intermediate transfer belt  6  to the transfer sheet. The idler rollers  13  through  15  are stretching rollers that stretch the intermediate transfer belt  6 . Particularly, the idler roller  13  is adjusting the posture of the intermediate transfer belt  6  so that the transfer sheet enters into the secondary transfer area along the intermediate transfer belt  6 . Moreover, the idler rollers  14  and  15  are adjusting the posture of the intermediate transfer belt  6  so that the plurality of primarily transferring positions famed at the contact parts between the photosensitive drums  2 Y,  2 M,  2 C, and  2 K and the primary transfer rollers  7 Y,  7 M,  7 C, and  7 K may be maintained in approximately linear shapes. 
     The intermediate transfer mechanism has an inclination correction motor  31 , an inclination-correction-motor HP sensor  32 , and a CPU  20  that controls them. The CPU  20  detects a deviation amount of the intermediate transfer belt  6  (a moving amount of a target) on the basis of detection results of a belt-deviation-amount detection apparatus (a position detection apparatus) mentioned later, and corrects the deviation of the intermediate transfer belt  6  by controlling the inclination correction motor. Moreover, the CPU  20  detects failure of an optical sensor that detects the deviation on the basis of the detection result of the belt-deviation-amount detection apparatus. 
     Next, the belt-deviation-amount detection apparatus that detects the deviation amount of the intermediate transfer belt in the image forming apparatus  100  will be described. 
       FIG. 3A  and  FIG. 3B  are views schematically showing a configuration of the belt-deviation-amount detection apparatus in the image forming apparatus in  FIG. 1 .  FIG. 3A  is a sectional view that is vertical to the belt conveyance direction, and  FIG. 3B  is a plan view showing a rotating member  23  in  FIG. 3A  viewed in a direction of an arrow Z. It should be noted that an arrow IF indicates a direction of applied force that occurs when the intermediate transfer belt  6  deviates leftward in  FIG. 3A , and an arrow IR indicates a direction of applied force that occurs when the intermediate transfer belt  6  deviates rightward in  FIG. 3A . 
     In  FIG. 3A  and  FIG. 3B , the rotating member  23  as a moving member formed in a fan shape in a plan view is rotatably arranged under the intermediate transfer belt  6 . Two sides  23   a  and  23   b  of the rotating member  23  forms 90 degrees, for example. A pivot of the fan shape that is an intersection of the sides  23   a  and  23   b  serves as a rotating shaft  24 . A plurality of (N pieces of) optical sensors are arranged over the rotating member  23  in the direction that intersects the rotating direction (moving direction) of the rotating member  23 . In this example, four transmission optical sensors  22 A,  22 B,  22 C, and  22 D are arranged in the longitudinal direction of the side  23   a.    
     A plurality of projection groups  26 A,  26 B,  26 C, and  26 D are disposed on the rotating member  23  along a plurality of loci of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D that are formed on the rotating member  23  by rotating the rotating member  23  around the rotating shaft  24 . It should be noted that the projection group  26 A has one projection on the same circumference. Similarly, the projection group  26 B has two projections, the projection group  26 C has four projections, and the projection group  26 D has three projections. The projection groups  26 A,  26 B,  26 C, and  26 D disposed on the moving member (the rotating member  23 ) function as shading member groups to the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. It should be noted that the rotating member  23  is made from optically transparent material. Four light sources are disposed under the rotating member  23  so as to be arranged oppositely to the transmission optical sensors  22 A,  22 B,  22 C, and  22 D, respectively, through the rotating member  23 . The light sources respectively irradiate the transmission optical sensors  22 A,  22 B,  22 C, and  22 D with lights that transmit the rotating member  23 . 
     The rotating member  23  of such a configuration is divided into eight rotating areas θ 1  through θ 8  corresponding to unit arcs that divide a circular arc portion  23   c  into eight equally, for example (see  FIG. 4  and  FIG. 5A  through  FIG. 5H  mentioned later). The reason why the rotating member  23  is divided into the eight rotating areas θ 1  through  88  will be described in detail with reference to  FIG. 4  and  FIG. 5A  through  FIG. 5H  later. 
     The projection groups  26 A,  26 B,  26 C, and  26 D disposed on the rotating member  23  along the loci of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D are arranged so that a combination of output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D at the time of reading is different for every rotating area among the rotating areas θ 1  through θ 8 . Arrangement of the projection groups will be described later with reference to  FIG. 4 . 
     One end of a swinging arm  21  as a swinging member is in contact with the edge of the intermediate transfer belt  6  in the width direction that intersects perpendicularly with the rotating direction of the intermediate transfer belt  6 . The other end across a swinging shaft  21   a  is in contact with a contact surface  25  of the rotating member  23 . The contact surface  25  is disposed at a side surface near the circular arc  23   c  of the fan-shaped rotating member  23 . 
     The swinging arm  21  swings around the swinging shaft  21   a  corresponding to the deviation amount of the intermediate transfer belt  6 , and the other end that is in contact with the contact surface  25  pushes the contact surface  25  and rotates the rotating member  23  in the direction of the arrow IF, for example. It should be noted that the rotating member  23  is always energized in the direction of the arrow IR by the spring member in  FIG. 3B . The combination of the projections of the projection groups  26 A,  26 B,  26 C, and  26 D that respectively face the transmission optical sensors  22 A,  22 B,  22 C, and  22 D vary corresponding to the rotation angle A of the rotating member  23 . As a result of this, the combination of the output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D varies. 
     The transmission optical sensors  22 A,  22 B,  22 C, and  22 D shall output an output signal “1”, for example, when the projections of the projection groups  26 A,  26 B,  26 C, and  26 D as shading member groups shield the incident lights. On the other hand, the transmission optical sensors  22 A,  22 B,  22 C, and  22 D shall output an output signal “0”, for example, when the projection groups do not shield the incident lights (i.e., when the incident lights are received). 
       FIG. 4  is a view showing an example of an arrangement of the projection groups  26 A,  26 B,  26 C, and  26 D on the rotating member  23 . 
     In  FIG. 4 , the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D are arranged sequentially from the position near the rotating shaft  24  over the rotating member  23  along the side  23   a  in the radius direction of the fan shape. The distance from the rotating shaft  24  to the transmission optical sensors  22 A,  22 B,  22 C, and  22 D are Ra, Rb, Rc, and Rd, respectively. The projections of the arc-shaped projection groups  26 A,  26 B,  26 C, and  26 D are disposed on the rotating member  23  at the radius positions that respectively correspond to the transmission optical sensors  22 A,  22 B,  22 C, and  22 D so that the combination of the projections is different for every rotating area among the rotating areas θ 1  through θ 8 . 
     The projection of the projection group  26 A that corresponds to the transmission optical sensor  22 A is formed in the rotating areas θ 1  through θ 4   a  at the position of the radius Ra from the rotating shaft  24 . Moreover, the projections of the projection group  26 B that correspond to the transmission optical sensor  22 B are formed in the rotating areas θ 1 , θ 2 , θ 5 , and θ 6  at the positions of the radius Rb from the rotating shaft  24 . Moreover, the projections of the projection group  26 C that correspond to the transmission optical sensor  22 C are formed in the rotating areas θ 1 , θ 3 , θ 5 , and θ 7  at the positions of the radius Rc from the rotating shaft  24 . Moreover, the projections of the projection group  26 D that correspond to the transmission optical sensor  22 D are formed in the rotating areas θ 1 , θ 4 , θ 6 , and θ 7  at the positions of the radius Rd from the rotating shaft  24 . 
     The following table 1 shows the output signals of the three transmission optical sensors  22 A,  22 B, and  22 C among the transmission optical sensors  22 A,  22 B,  22 C, and  22 D in  FIG. 4  for each of the rotating areas θ 1  through θ 8 . 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 22A 
                 22B 
                 22C 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 θ1 
                 1 
                 1 
                 1 
               
               
                   
                 θ2 
                 1 
                 1 
                 0 
               
               
                   
                 θ3 
                 1 
                 0 
                 1 
               
               
                   
                 θ4 
                 1 
                 0 
                 0 
               
               
                   
                 θ5 
                 0 
                 1 
                 1 
               
               
                   
                 θ6 
                 0 
                 1 
                 0 
               
               
                   
                 θ7 
                 0 
                 0 
                 1 
               
               
                   
                 θ8 
                 0 
                 0 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     In the table 1, the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C is different in each of the eight rotating areas θ 1  through θ 8 . Accordingly, it is understood that the projection groups  26 A,  26 B, and  26 C are arranged so that the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C is different for every rotating area. 
     Moreover,  FIG. 5A  through  FIG. 5H  are views showing the rotating positions of the rotating member  23  where the rotating areas θ 1  through θ 8  face the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. 
       FIG. 5A  shows the rotating position of the rotating member  23  where the rotating area θ 1  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D.  FIG. 5B  shows the rotating position of the rotating member  23  where the rotating area θ 2  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. Moreover,  FIG. 5C  shows the rotating position of the rotating member  23  where the rotating area θ 3  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D.  FIG. 5D  shows the rotating position of the rotating member  23  where the rotating area θ 4  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. Moreover,  FIG. 5E  shows the rotating position of the rotating member  23  where the rotating area θ 5  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D.  FIG. 5F  shows the rotating position of the rotating member  23  where the rotating area θ 6  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. Furthermore,  FIG. 5G  shows the rotating position of the rotating member  23  where the rotating area θ 7  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D.  FIG. 5H  shows the rotating position of the rotating member  23  where the rotating area θ 8  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. 
     In the belt-deviation-amount detection apparatus equipped with the rotating member  23  and the transmission optical sensors  22 A,  22 B,  22 C, and  22 D of such a configuration, the deviation amount of the intermediate transfer belt  6  is detected with using the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C. Namely, the deviation amount of the intermediate transfer belt  6  is detected with using the combination of the output signals of M types (three types) of the transmission optical sensors corresponding to M types (three types) of the projection groups  26 A,  26 B, and  26 C except one type among N types (four types) of the projection groups in the embodiment. 
     As shown in  FIG. 4  and  FIG. 5A  through  FIG. 5H , the projection of the projection group  26 A is disposed in the rotating areas θ 1  through  84 , the projections of the projection group  26 B are disposed in the rotating areas θ 1 , θ 2 , θ 5 , and θ 6 , and the projections of the projection group  26 C are disposed in the rotating areas θ 1 , θ 3 , θ 5 , and θ 7 . 
     Hereinafter, the reason why the rotating member  23  is divided into the eight rotating areas θ 1  through  88 , and the reason why the projection groups  26 A,  26 B, and  26 C are arranged as mentioned above are described. 
     As mentioned above, the rotating angle Δθ of the rotating member  23  that corresponds to the deviation amount of the intermediate transfer belt  6  is detected with using the three transmission optical sensors  22 A,  22 B, and  22 C among the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D in the embodiment. 
     Accordingly, it is first considered how many combinations the output signals of the three transmission optical sensors  22 A,  22 B, and  22 C give. One sensor is able to output two statuses of ON and OFF. There are three sensors. Accordingly, the output signals of three sensors give eight combinations (i.e., 2 3 =8). 
     However, if one of the plurality of sensors used for detecting the deviation amount of the intermediate transfer belt  6  fails, the combination of the output signals differs from that shown in the table 1. Accordingly, a correct rotating-angle A of the rotating member  23  is no longer obtained in such a case. In this case, the deviation amount of the intermediate transfer belt  6  is detected erroneously. Then, when an erroneous belt-deviation correction control is performed on the basis of the erroneous detection result, an excessive deviation error may occur or the belt may break. 
     Consequently, failure of a sensor is detected with using the rotating member  23  and the transmission optical sensors  22 A,  22 B,  22 C, and  22 D, which prevents the erroneous belt-deviation correction control on the basis of the erroneous detection result in the embodiment. 
     Hereinafter, a sensor-failure detection process executed by the CPU  20  in  FIG. 2  for detecting failure of a transmission optical sensor will be described. 
       FIG. 6  is a flowchart showing procedures of the sensor-failure detection process executed in the image forming apparatus  100  shown in  FIG. 1 . The CPU  20  of the image forming apparatus  100  performs the sensor-failure detection process according to a sensor-failure detection program stored in a ROM (not shown). It should be noted that the sensor-failure detection process is performed repeatedly at fixed time intervals during the image forming process. 
     As shown in  FIG. 6 , when the sensor-failure detection process is started, the CPU  20  reads the output signals of the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D (step S 101 ). Next, the CPU  20  calculates the sum total of the output signals (output values) of the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D (step S 102 ). After calculating the sum total of the output signals, the CPU  20  determines whether the calculated sum total of the output signals is an odd number (step S 103 ). 
     As a result of the determination in the step S 103 , when the sum total of the output signals is an odd number (“YES” in the step S 103 ), the CPU  20  determines that one of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D failed (step S 104 ). 
     In this embodiment, failure of a sensor is detected with using the projection group  26 D and the transmission optical sensor  22 D corresponding to the projection group  26 D. The projection group  26 D is a shading member group other than M types of shading member groups applied to detect the deviation amount of the intermediate transfer belt  6  among the four projection groups  26 A,  26 B,  26 C, and  26 D of the rotating member  23 . 
     That is, the projection group  26 D is disposed so that the sum total of the output signals of the three transmission optical sensors  22 A,  22 B, and  22 C and the output signal of the transmission optical sensor  22 D that faces the projection group  26 D becomes an even number, for example. The following table 2 shows examples of the combinations of the output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D and the sum totals of the output signals. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 22A 
                 22B 
                 22C 
                 22D 
                 TOTAL OUTPUT 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 θ1 
                 1 
                 1 
                 1 
                 1 
                 4 
               
               
                   
                 θ2 
                 1 
                 1 
                 0 
                 0 
                 2 
               
               
                   
                 θ3 
                 1 
                 0 
                 1 
                 0 
                 2 
               
               
                   
                 θ4 
                 1 
                 0 
                 0 
                 1 
                 2 
               
               
                   
                 θ5 
                 0 
                 1 
                 1 
                 0 
                 2 
               
               
                   
                 θ6 
                 0 
                 1 
                 0 
                 1 
                 2 
               
               
                   
                 θ7 
                 0 
                 0 
                 1 
                 1 
                 2 
               
               
                   
                 θ8 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     As shown in the table 2, the combinations of the output signals of the transmission optical sensor  22 A,  22 B, and  22 C corresponding to the rotating areas θ 1  through  88  are different mutually, and the sum totals of the output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D corresponding to the rotating areas θ 1  through  88  are even numbers. In this case, the projections of the projection group  26 D are disposed in the rotating areas θ 1 , θ 4 , θ 6 , and θ 7  (see  FIG. 4  and  FIG. 5A  through  FIG. 5H ). 
     In the belt-deviation-amount detection apparatus constituted thus, when any one of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D fails, the sum total of the four sensor output signals may be an odd number. Accordingly, failure of a sensor is detectable by detecting that the sum total of the output signals of the transmission optical sensors becomes an odd number. It should be noted that there is an extremely low possibility that two sensors among a limited plural number of sensors (the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D in this case) fail simultaneously. Accordingly, when the sum total of the output signals varies from an even number to an odd number, it is determined that any one of the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D failed in the embodiment. 
     Hereinafter, concrete examples in which one of four transmission optical sensors failed will be described. It should be noted that an output signal shall be always “0” when a transmission optical sensor failed. 
     The following table 3 shows the combinations of the output signals of the sensors corresponding to the rotating areas when the transmission optical sensor  22 B among the transmission optical sensors  22 A,  22 B,  22 C, and  22 D failed. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 22A 
                 22B 
                 22C 
                 22D 
                 TOTAL OUTPUT 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 θ1 
                 1 
                 0 
                 1 
                 1 
                 3 (ODD)  
               
               
                   
                 θ2 
                 1 
                 0 
                 0 
                 0 
                 1 (ODD)  
               
               
                   
                 θ3 
                 1 
                 0 
                 1 
                 0 
                 2 (EVEN) 
               
               
                   
                 θ4 
                 1 
                 0 
                 0 
                 1 
                 2 (EVEN) 
               
               
                   
                 θ5 
                 0 
                 0 
                 1 
                 0 
                 1 (ODD)  
               
               
                   
                 θ6 
                 0 
                 0 
                 0 
                 1 
                 1 (ODD)  
               
               
                   
                 θ7 
                 0 
                 0 
                 1 
                 1 
                 2 (EVEN) 
               
               
                   
                 θ8 
                 0 
                 0 
                 0 
                 0 
                 0 (EVEN) 
               
               
                   
                   
               
            
           
         
       
     
     As shown in the table 3, since the transmission optical sensor  22 B failed, the output signal of the transmission optical sensor  22 B is always “0”. In this case, the sum total of the output signals varies from an even number to an odd number in the rotating areas θ 1 , θ 2 , θ 5  and θ 6  as shown in the table 3. 
     Accordingly, when the rotating member  23  rotates according to the deviation amount of the intermediate transfer belt  6 , and when the transmission optical sensors  22 A,  22 B,  22 C, and  22 D face the rotating area θ 1 , θ 2 , θ 5 , or θ 6 , it is determined that any one of the transmission optical sensors failed. 
     On the other hand, when the sensors face the rotating area θ 3 , θ 4 , θ 7 , or θ 8 , the combination of the output signals of the transmission optical sensors is not different from that in the normal state because the output signal of the transmission optical sensor  22 B is “0” even if it does not fail. In the rotating areas θ 3 , θ 4 , θ 7 , and θ 8 , since the sum total does not vary from an even number to an odd number, failure of a transmission optical sensor is undetectable. However, the rotating area that faces the sensors is specified on the basis of the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C. Accordingly, even if the transmission optical sensor  22 B fails, the rotating angle A of the rotating member  23  is detectable in these rotating areas θ 3 , θ 4 , θ 7 , and θ 8 . 
     Moreover, when the transmission optical sensors  22 A,  22 B,  22 C, and  22 D face the rotating area θ 1 , θ 2 , θ 5 , or θ 6  of the rotating member  23  according to the variation of the deviation amount of the intermediate transfer belt  6 , the sum total of the output signals becomes an odd number. Accordingly, failure of a transmission optical sensor is detectable at this point of time. 
     It should be noted that the rotating areas θ 1  and θ 3  of which the combinations of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C are identical are distinguished on the basis of whether the sum total of the output signals is an odd number or an even number. The rotating areas θ 2  and θ 4 , the rotating areas θ 5  and θ 7 , the rotating areas θ 6  and θ 8  are also distinguished in the same manner, respectively. The belt-deviation-amount detection method, which combines the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C and the determination of whether the sum total of the output signals is an odd number or an even number, will be mentioned later with reference to table 4. 
     Referring back to  FIG. 6 , after detecting the failure of the transmission optical sensor (step S 104 ), the CPU  20  displays an error message showing that one of the plurality of sensors failed on a display unit (not shown) in step S 105 , and finishes this process after that. 
     On the other hand, as a result of the determination in the step S 103 , when the sum total of the output signals is not an odd number (“NO” in the step S 103 ), the CPU  20  determines that failure of a transmission optical sensor is not detected in step S 106 , and finishes this process after that. 
     As mentioned above, the transmission optical sensors  22 A,  22 B,  22 C, and  22 D are arranged at the side of the rotating member  23  so that each of the sensors outputs “1” when a projection as a shading part is detected and outputs “0” when a projection is not detected. Moreover, the projections of the projection groups  26 A,  26 B,  26 C, and  26 D are disposed so that the combination of the output signals of the sensors differs for each of the rotating areas θ 1  through θ 8  and so that the sum total of the output signals of the sensors becomes an even number. Then, according to the process in  FIG. 6 , failure of a sensor is presumed when the sum total of the output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D varies from an even number to an odd number. This enables to detect failure of any one sensor among the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. 
     Next, the deviation detection method for the intermediate transfer belt  6  using the sensor-fault detection will be described. 
     The following table 4 shows the output signals when the transmission optical sensor  22 A among the transmission optical sensors  22 A,  22 B,  22 C, and  22 D failed. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 22A 
                 22B 
                 22C 
                 22D 
                 TOTAL OUTPUT 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 θ1 
                 0 
                 1 
                 1 
                 1 
                 3 (ODD)  
               
               
                   
                 θ2 
                 0 
                 1 
                 0 
                 0 
                 1 (ODD)  
               
               
                   
                 θ3 
                 0 
                 0 
                 1 
                 0 
                 1 (ODD)  
               
               
                   
                 θ4 
                 0 
                 0 
                 0 
                 1 
                 1 (ODD)  
               
               
                   
                 θ5 
                 0 
                 1 
                 1 
                 0 
                 2 (EVEN) 
               
               
                   
                 θ6 
                 0 
                 1 
                 0 
                 1 
                 2 (EVEN) 
               
               
                   
                 θ7 
                 0 
                 0 
                 1 
                 1 
                 2 (EVEN) 
               
               
                   
                 θ8 
                 0 
                 0 
                 0 
                 0 
                 0 (EVEN) 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 4, when the transmission optical sensor  22 A failed, the rotating areas θ 1  and θ 5  cannot be distinguished on the basis of the output signals of the three transmission optical sensors  22 A,  22 B, and  22 C. In the same manner, the rotating area θ 2  and θ 6 , the rotating areas θ 3  and θ 7 , and the rotating areas θ 4  and θ 8  cannot be distinguished without using the output signal of the transmission optical sensor  22 D. Accordingly, even when the transmission optical sensors  22 A,  22 B,  22 C, and  22 D actually face the rotating area θ 1 , it may be erroneously detected that the sensors face the rotating area θ 5 . In this case, it is determined that the rotating member  23  rotated quickly until the rotating area facing the transmission optical sensors varied from θ 1  to θ 5  in the IF direction in  FIG. 5A , for example. As a result, it is erroneously detected that the intermediate transfer belt  6  was deviated in the IF direction quickly. 
     Then, the deviation correction control is performed so that the deviation of the intermediate transfer belt  6  is corrected by the deviation control roller  9  in the IR direction that is opposite to the IF direction. However, since the actual deviation amount of the intermediate transfer belt  6  is an amount equivalent to the rotating area θ 1 , the position of the intermediate transfer belt  6  after the correction will excessively deviate in the IR direction as a result. Moreover, an excessive deviation error may occur due to excessive deviation of the intermediate transfer belt  6 , and the intermediate transfer belt  6  may run on an edge member and corrupt. 
     Consequently, the deviation amount of the intermediate transfer belt  6  is detected with using the sensor-failure detection result in the embodiment. That is, as shown in the table 4, the rotating areas θ 1  and θ 5  cannot be distinguished on the basis of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C. In the same manner, the rotating area θ 2  and θ 6 , the rotating areas θ 3  and θ 7 , and the rotating areas θ 4  and θ 8  cannot be distinguished, respectively. However, the sum totals of the output signals in the rotating areas θ 1 , θ 2 , θ 3 , and θ 4  are odd numbers, respectively, and the sum totals of the output signals in the rotating areas θ 5 , θ 6 , θ 7 , and θ 8  are even numbers, respectively. Accordingly, when the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C and the sum total of the output signals are combined, the rotating areas θ 1  and θ 5 , the rotating area θ 2  and θ 6 , the rotating areas θ 3  and θ 7 , and the rotating areas θ 4  and θ 8  are able to be distinguished, respectively. This avoids erroneous detection of the rotating angle of the rotating member  23 . 
     According to the embodiment, the rotating angle Δθ of the rotating member  23  is detected without erroneous detection on the basis of the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C and the determination of whether the sum total of the output signals of the transmission optical sensor  22 A,  22 B,  22 C, and  22 D is an odd number or an even number. Then, the deviation amount of the intermediate transfer belt  6  is found using the detected rotating angle Δθ of the rotating member  23 . Moreover, when the deviation of the intermediate transfer belt  6  is corrected on the basis of the found deviation amount, the erroneous correction control for the intermediate transfer belt  6  on the basis of the erroneous detection is prevented, which avoids an excessive deviation error and corruption of the belt, etc. 
     Although the deviation amount of the intermediate transfer belt  6  is detected by detecting the rotating angle A of the rotating member  23  from the eight (=2 3 ) rotating areas with using the three transmission optical sensors in the embodiment, the number of the transmission optical sensors is not limited particularly. When the number of the transmission optical sensors is increased and the rotating member  23  is divided into more areas correspondingly, the resolution of the detectable belt deviation amount is improved. 
     In the embodiment, failure of a sensor is detected when the sum total of the output signals of the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D varies from an even number to an odd number. On the other hand, the projection group  26 D as the shading member group may be arranged so that the sum total of the output signals of the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D becomes an odd number. In such a case, failure of a sensor is detectable when the sum total of the output signals of the four sensors varies from an odd number to an even number. 
     Next, a second embodiment of the present invention will be described. 
       FIG. 7A ,  FIG. 7B , and  FIG. 7C  are views schematically showing a configuration of a belt-deviation-amount detection apparatus in a second embodiment.  FIG. 7A  is a sectional view that is vertical to the belt conveying direction.  FIG. 7B  is a plan view showing a slide member shown in  FIG. 7A  viewed in a direction of an arrow Z.  FIG. 7C  is a side view showing the slide member shown in  FIG. 7A  viewed in a direction of an arrow X. It should be noted that an arrow IF in  FIG. 7A  and  FIG. 7B  indicates a direction of applied force that is generated when the intermediate transfer belt  6  deviates leftward in  FIG. 7A , and an arrow IR indicates a direction of applied force that is generated when the intermediate transfer belt  6  deviates rightward in  FIG. 7A . 
     As shown in  FIG. 7A ,  FIG. 7B , and  FIG. 7C , a tabular slide member  27 , which appears a rectangle in the plan view ( FIG. 7B ), is arranged under the intermediate transfer belt  6  as a moving member so as to be movable in a predetermined direction, i.e., a longitudinal direction of the rectangle. The four transmission optical sensors  22 A,  22 B,  22 C, and  22 D are arranged over a short side  27   a  that intersects perpendicularly with the moving direction of the slide member  27  along the short side  27   a . The configurations of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D are the same as that of the first embodiment mentioned above. 
     A plurality of projection groups  29 A,  29 B,  29 C, and  29 D are disposed on the slide member  27  along a plurality of loci of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D that are formed on the slide member  27  by sliding the slide member  23  in the IF direction or the IR direction in  FIG. 7A . The projection groups  29 A,  29 B,  29 C, and  29 D function as the shading member groups to the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. It should be noted that the slide member  27  is made from optically transparent material. Four light sources are disposed under the slide member  27  so as to irradiate the transmission optical sensors  22 A,  22 B,  22 C, and  22 D with light, respectively. 
     The slide member  27  of such a configuration is equally divided into eight slide areas x 1  through x 8  in the slide direction (moving direction) of the slide member  27 , as shown in  FIG. 8  mentioned later. The reason why the slide member is divided into the eight slide areas is the same as that of the first embodiment. Accordingly, the description is omitted. 
     The projection groups  29 A,  29 B,  29 C, and  20 D disposed on the slide member  27  along the loci of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D are arranged so that a combination of output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D at the time of reading is different for every slide area among the slide areas x 1  through x 8 . The arrangements of the projection groups will be described later with reference to  FIG. 8 . 
     One end of the swinging arm  21  (a swinging member) is in contact with the edge of the intermediate transfer belt  6  in the width direction that intersects perpendicularly with the rotating direction of the intermediate transfer belt  6 . The other end is in contact with a contact surface  28  of the slide member  27 . The contact surface  28  is a side surface of the slide member  27 . 
     The swinging arm  21  swings around the swinging shaft  21   a  corresponding to the deviation amount of the intermediate transfer belt  6 , and the other end that is in contact with the contact surface  28  pushes the slide member  27  and moves the slide member  27  rightward in  FIG. 7B , for example. It should be noted that the slide member  27  is always energized leftward in  FIG. 7B  by the spring member. The combination of the projections of the projection groups  29 A,  29 B,  29 C, and  29 D that face the transmission optical sensors  22 A,  22 B,  22 C, and  22 D vary corresponding to the slide amount of the slide member  27 . As a result of this, the combination of the output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D varies among a plurality of combinations. 
     The transmission optical sensors  22 A,  22 B,  22 C, and  22 D shall output an output signal “1” as a detection result of “ON”, for example, when detecting the projection groups  29 A,  29 B,  29 C, and  29 D as the shading member groups. On the other hand, the transmission optical sensors  22 A,  22 B,  22 C, and  22 D shall output an output signal “0” as a detection result of “OFF”, for example, when not detecting the projection groups. 
       FIG. 8  is a view showing an example of an arrangement of the projection groups  29 A,  29 B,  29 C, and  29 D on the slide member  27 . 
     As shown in  FIG. 8 , the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D are arranged sequentially from the position near the contact surface  28  over the slide member  27  along the short side  27   a  that intersects perpendicularly with the slide direction of the side member  27 . The projections of the four projection groups  29 A,  29 B,  29 C, and  29 D are disposed on the slide member  27  at the positions that respectively correspond to the transmission optical sensors  22 A,  22 B,  22 C, and  22 D so that the combination of the projections is different for every slide area among the slide areas x 1  through x 8 . 
     The projection of the projection group  29 A corresponding to the transmission optical sensor  22 A is formed in the slide areas x 1  through x 4 . Moreover, the projections of the projection group  29 B corresponding to the transmission optical sensor  22 B are formed in the slide areas x 1 , x 2 , x 5 , and x 6 . Moreover, the projections of the projection group  29 C corresponding to the transmission optical sensor  22 C are formed in the slide areas x 1 , x 3 , x 5 , and x 7 . Moreover, the projections of the projection group  29 D corresponding to the transmission optical sensor  22 D are formed in the slide areas x 1 , x 4 , x 6 , and x 7 . 
     The following table 5 shows the output signals of the three transmission optical sensors  22 A,  22 B, and  22 C among the transmission optical sensors  22 A,  22 B,  22 C, and  22 D in  FIG. 8  for each of the slide areas x 1  through x 8 . 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 22A 
                 22B 
                 22C 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 x1 
                 1 
                 1 
                 1 
               
               
                   
                 x2 
                 1 
                 1 
                 0 
               
               
                   
                 x3 
                 1 
                 0 
                 1 
               
               
                   
                 x4 
                 1 
                 0 
                 0 
               
               
                   
                 x5 
                 0 
                 1 
                 1 
               
               
                   
                 x6 
                 0 
                 1 
                 0 
               
               
                   
                 x7 
                 0 
                 0 
                 1 
               
               
                   
                 x8 
                 0 
                 0 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     As shown in the table 5, the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C is different in each of the eight slide areas x 1  through x 8 . Accordingly, it is understood that the projection groups  29 A,  29 B, and  29 C are arranged so that the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C is different for every slide area. 
       FIG. 9A  through  FIG. 9H  are views showing slide positions of the slide member  27  where the slide areas x 1  through x 8  respectively face the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. 
       FIG. 9A  shows the slide position of the slide member  27  where the slide area x 1  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D.  FIG. 9B  shows the slide position of the slide member  27  where the slide area x 2  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. Moreover,  FIG. 9C  shows the slide position of the slide member  27  where the slide area x 3  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D.  FIG. 9D  shows the slide position of the slide member  27  where the slide area x 4  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. Moreover,  FIG. 9E  shows the slide position of the slide member  27  where the slide area x 5  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D.  FIG. 9F  shows the slide position of the slide member  27  where the slide area x 6  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. Furthermore,  FIG. 9G  shows the slide position of the slide member  27  where the slide area x 7  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D.  FIG. 9H  shows the slide position of the slide member  27  where the slide area x 8  faces the transmission optical sensors  22 A,  22 B,  22 C, and  22 D. 
     As shown in  FIG. 8  and  FIG. 9A  through  FIG. 9H , the projection of the group  26 A is disposed in the slide areas x 1  through x 4 , the projections of the projection group  26 B are disposed in the slide areas x 1 , x 2 , x 5 , and x 6 , and the projections of the projection group  29 C are disposed in the slide areas x 1 , x 3 , x 5 , and x 7 . 
     In the belt-deviation-amount detection apparatus of such a configuration, the deviation amount of the intermediate transfer belt  6  is detected on the basis of the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C. That is, the slide amount of the slide member  27  is detected by using the fact that the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C differs for each of the eight slide areas of the slide member  27 . Then, the deviation amount of the intermediate transfer belt  6  is detected on the basis of the slide amount of the slide member  27 . 
     However, if one of the plurality of sensors used for detecting the deviation amount of the intermediate transfer belt fails, the combination of the output signals differs from that shown in the table 5. Accordingly, correct moving amount Δx of the slide member  27  will be no longer obtained, and the deviation amount of the intermediate transfer belt  6  will be erroneously detected. Then, when an erroneous belt-deviation correction control is performed on the basis of the erroneous detection result, an excessive deviation error may occur or the belt may break. 
     Consequently, failure of a sensor is detected with using the slide member  27  and the transmission optical sensors, which prevents the erroneous belt-deviation correction control on the basis of the erroneous detection result in the embodiment. 
     In this embodiment, failure of a sensor is detected with using the projection group  29 D and the transmission optical sensor  22 D corresponding to the projection group  29 D. The projection group  29 D is not applied to detect the deviation amount of the intermediate transfer belt  6  among the four projection groups  26 A,  26 B,  26 C, and  26 D of the rotating member  27 . In this example, the projections of the projection group  29 D are disposed in the slide areas x 1 , x 4 , x 6 , and x 7  (see  FIG. 8 ). 
     In this embodiments, the projection group  26 D is arranged so that the sum total of the output signals of the three transmission optical sensors  22 A,  22 B, and  22 C and the output signal of the transmission optical sensor  22 D that faces the projection group  26 D becomes an even number, for example. The following table 6 shows examples of the combinations of the output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D and the sum totals of the output signals. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 22A 
                 22B 
                 22C 
                 22D 
                 TOTAL OUTPUT 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 x1 
                 1 
                 1 
                 1 
                 1 
                 4 
               
               
                   
                 x2 
                 1 
                 1 
                 0 
                 0 
                 2 
               
               
                   
                 x3 
                 1 
                 0 
                 1 
                 0 
                 2 
               
               
                   
                 x4 
                 1 
                 0 
                 0 
                 1 
                 2 
               
               
                   
                 x5 
                 0 
                 1 
                 1 
                 0 
                 2 
               
               
                   
                 x6 
                 0 
                 1 
                 0 
                 1 
                 2 
               
               
                   
                 x7 
                 0 
                 0 
                 1 
                 1 
                 2 
               
               
                   
                 x8 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     As shown in the table 6, the combinations of the output signals of the transmission optical sensor  22 A,  22 B, and  22 C corresponding to the side areas x 1  through x 8  are different mutually, and the sum totals of the output signals of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D corresponding to the slide areas x 1  through x 8  are even numbers. 
     In the belt-deviation-amount detection apparatus of such a configuration, when any one of the transmission optical sensors  22 A,  22 B,  22 C, and  22 D fails, the sum total of the four sensor output signals may be an odd number. Accordingly, failure of a sensor is detectable by detecting that the sum total of the output signals of the transmission optical sensors becomes an odd number. 
     The procedure of the sensor-failure detection process is the same as that of the flowchart in  FIG. 6  mentioned above. Accordingly, the description is omitted. Moreover, the detection method for the deviation amount of the intermediate transfer belt  6  with using the sensor-failure detection result is also performed in the same manner as the first embodiment mentioned above. 
     In the second embodiment, the combination of the projections as shading parts of the projection groups  29 A,  29 B, and  29 C varies for each of the slide areas x 1  through x 8 , the combination is detected by the plurality of transmission optical sensors  22 A,  22 B, and  22 C, and the moving amount Δx of the slide member  27  is detected on the basis of the combination of the output signals. Moreover, the projection group  29 D is constituted so that the sum total of the output signals of the transmission optical sensor  22 A,  22 B,  22 C, and  22 D becomes an even number. When the sum total of the output signals becomes an odd number, failure of a transmission optical sensor is detected. Then, the erroneous detection of the moving amount Δx of the slide member  27  that corresponds to the deviation amount of the intermediate transfer belt  6  is avoided by combining the combination of the output signals of the transmission optical sensors  22 A,  22 B, and  22 C and the sum total of the output signals. Moreover, since this prevents the erroneous belt-deviation correction control on the basis of the erroneous detection result, the deviation is corrected satisfactorily without causing an excessive deviation error, breakage of the belt, etc. 
     In the embodiment, when the number of the transmission optical sensors is increased and the slide member  27  is divided into more slide areas correspondingly, the resolution of the detectable deviation amount of the intermediate transfer belt  6  is improved. Moreover, failure of a sensor may be detected when the sum total of the output signals of the four transmission optical sensors  22 A,  22 B,  22 C, and  22 D varies from an odd number to an even number. 
     OTHER EMBODIMENTS 
     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. 2015-242983, filed Dec. 14, 2015, which is hereby incorporated by reference herein in its entirety.