Patent Publication Number: US-10315871-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2017-005585 filed Jan. 17, 2017. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to an image forming apparatus. 
     (ii) Related Art 
     When color registration is performed, a recording material goes to waste in order to determine whether misregistration between toner images of different colors has occurred on the recording material. In particular, in the case where a continuous sheet is used as a recording material, a large portion of the recording material will go to waste. Accordingly, in order to reduce the amount of waste produced when color registration is performed, it has been proposed to stop transportation of a recording material and perform color registration on an intermediate transfer body. However, when the transportation is stopped, a second transfer roller and the like are separated from the intermediate transfer body, and thus, there is a change in the load applied to the moving intermediate transfer body. As a result, the movement speed of the intermediate transfer body changes between when image formation in which a recording material is transported is performed and when color registration in which transportation of a recording material is stopped is performed, and there is a possibility that misregistration will occur when image formation is performed. 
     In an image forming apparatus that employs a tandem system and directly transfers toner images onto a recording material, it has been proposed to separate a transfer roller of an image engine corresponding to a color that will not be used from a recording material or a transport belt when changing the number of colors to be used. However, similar to the above, there is a possibility that the movement speed of a recording material or the like will change between when the transfer roller is in contact with the recording material or the like and when the transfer roller is separated from the recording material or the like, which in turn results in misregistration. 
     In other words, in the case where an image forming apparatus in which images are formed and sequentially transferred onto a movable body in such a manner as to be superposed with one another by plural image forming units includes a contact and separation unit that comes into and out of contact with the movable body, there is a possibility of misregistration. Although an image forming apparatus that employs an electrophotographic system has been described above as an example, there is a possibility of misregistration also in an image forming apparatus that employs, for example, an ink-jet system when the image forming apparatus includes a contact and separation unit. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including plural image forming units each of which forms an image, a movable body that moves along a movement path extending in a direction in which the plural image forming units are arranged while the images formed by the image forming units are sequentially formed on a surface of the movable body or on a surface of a recording material transported by the movable body in such a manner as to be superposed with one another, a contact and separation unit that is caused to move to a contact position at which the contact and separation unit is in contact with the movable body and to a separation position at which the contact and separation unit is separated from the movable body and that changes a load applied to the movable body, which moves, as a position of the contact and separation unit is changed, and a change unit that changes the load applied to the movable body in a manner opposite to a change in the load upon a change in the position of the contact and separation unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic diagram illustrating an image forming apparatus according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating a speed maintaining function in a state where a second transfer roller is located at an image-formation position; 
         FIG. 3  is a schematic diagram illustrating the speed maintaining function in a state where the second transfer roller is located at a retreat position; 
         FIG. 4  is a diagram illustrating a state in which a second transfer roller according to a second exemplary embodiment of the present invention is located at the image-formation position; 
         FIG. 5  is a diagram illustrating a state in which the second transfer roller according to the second exemplary embodiment is located at the retreat position; 
         FIG. 6  is a diagram illustrating a state in which a second transfer roller according to a third exemplary embodiment of the present invention is located at the image-formation position; 
         FIG. 7  is a diagram illustrating a state in which the second transfer roller according to the third exemplary embodiment is located at the retreat position; 
         FIG. 8  is a diagram illustrating a fourth exemplary embodiment of the present invention; 
         FIG. 9  is a diagram illustrating an image forming apparatus according to a fifth exemplary embodiment of the present invention; and 
         FIG. 10  is a diagram illustrating a speed maintaining function according to the fifth exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. 
       FIG. 1  is a schematic diagram illustrating an image forming apparatus according to a first exemplary embodiment. Note that  FIG. 1  is a conceptual diagram illustrating the configuration of the image forming apparatus, and the arrangement in  FIG. 1  may sometimes be different from a specific arrangement in the actual apparatus. Arrows in  FIG. 1  indicate the movement directions of the corresponding components. 
     An image forming apparatus  1  illustrated in  FIG. 1  is a color printer that employs a so-called tandem system. In the image forming apparatus  1 , for example, a continuous sheet (roll sheet) P is used as a recording material. The continuous sheet P is rolled and wound around a feeding unit  31 , and the feeding unit  31  sends out the continuous sheet P. A portion of the continuous sheet P on which an image has been formed is rolled up by a winding unit  32 . 
     The image forming apparatus  1  includes four image engines that correspond to four colors of, for example, yellow (Y), magenta (M), cyan (C), and black (K). In the first exemplary embodiment, each of the image engines employs a so-called electrophotographic system and forms a toner image. 
     In  FIG. 1 , the image engines are represented by photoconductor drums  10 Y,  10 M,  10 C, and  10 K that are incorporated in the image engines. In the image engines, toner images of the different colors are formed on the photoconductor drums  10 Y,  10 M,  10 C, and  10 K by performing charging, light exposure, and development. 
     The image forming apparatus  1  includes an intermediate transfer belt  20 . The intermediate transfer belt  20  is stretched by a driving roller  21 , a driven roller  22 , and a backup roller  23  and caused to move along a circular path that extends above the image engines by a driving force of the driving roller  21 . The driven roller  22  rotates and is driven by the moving intermediate transfer belt  20 . The intermediate transfer belt  20  corresponds to an example of a movable body according to the present invention. 
     First transfer rollers  11 Y,  11 M,  11 C, and  11 K are disposed at positions facing the photoconductor drums  10 Y,  10 M,  10 C, and  10 K of the image engines with the intermediate transfer belt  20  interposed between the first transfer rollers  11 Y,  11 M,  11 C, and  11 K and the photoconductor drums  10 Y,  10 M,  10 C, and  10 K. Toner images of the different colors formed on the photoconductor drums  10 Y,  10 M,  10 C, and  10 K are sequentially transferred onto the intermediate transfer belt  20  in such a manner as to be superposed with one another by the first transfer rollers  11 Y,  11 M,  11 C, and  11 K. As a result of such transfer of the toner images, a color image is formed on the intermediate transfer belt  20 . A combination of each of the image engines and a corresponding one of the first transfer rollers corresponds to an example of an image forming unit according to the present invention. In the first exemplary embodiment, an image forming operation is performed on the intermediate transfer belt  20 , which is an example of the movable body, by transferring images from the photoconductor drums  10 Y,  10 M,  10 C, and  10 K onto the intermediate transfer belt  20 . 
     The above-mentioned continuous sheet P is pressed against a surface of the intermediate transfer belt  20  by a second transfer roller  40  that faces the backup roller  23 . The second transfer roller  40  transfers a color image formed on the intermediate transfer belt  20  onto the continuous sheet P. In addition, the second transfer roller  40  has contact and separation functions, which are to move toward and away from the intermediate transfer belt  20 . In the first exemplary embodiment, although the contact and separation functions are realized by a cam mechanism, a stepping motor, and the like, which are not illustrated, the contact and separation functions may be realized by a different mechanism that is commonly known. 
     When the second transfer roller  40  moves to a transfer position that is illustrated in  FIG. 1  by using the contact and separation functions, the continuous sheet P is pressed against the intermediate transfer belt  20  by the second transfer roller  40 . When the second transfer roller  40  moves to a retreat position that is spaced apart from the intermediate transfer belt  20 , the continuous sheet P also separates from the intermediate transfer belt  20 . A combination of the second transfer roller  40  and the continuous sheet P corresponds to an example of a contact and separation unit according to the present invention. The state illustrated in  FIG. 1  corresponds to a state where the contact and separation unit according to the present invention is located at a contact position. A state where the second transfer roller  40  has moved to the retreat position as described above corresponds to a state where the contact and separation unit according to the present invention is located at a separation position. 
     The image forming apparatus  1  includes a fixing unit  50 , and a color image that has been transferred to the continuous sheet P by the second transfer roller  40  is fixed onto the continuous sheet P as a result of being heated and pressurized by the fixing unit  50 . 
     The image forming apparatus  1  includes an image sensor  60  that detects a color image on the intermediate transfer belt  20 , a cleaner  70  that scrapes off unwanted substances such as residual toner and paper dust from the intermediate transfer belt  20  by using a blade, and a controller  80  that controls each unit in the image forming apparatus  1 . 
     The image forming apparatus  1  has a function that is so-called color registration and adjusts the timing at which toner images are formed by the image engines such that the toner images are transferred onto the intermediate transfer belt  20  in such a manner as to be superposed with one another without misregistration. More specifically, toner images for use in color registration are formed by the image engines, and the image sensor  60  detects the positions of the toner images on the intermediate transfer belt  20  so as to determine misregistration between the toner images. Then, the controller  80  adjusts the timing at which the image engines form toner images in such a manner that misregistration is reduced. 
     In the image forming apparatus  1  according to the first exemplary embodiment, when color registration is performed, the second transfer roller  40  and the continuous sheet P are separated from the intermediate transfer belt  20 , and transportation of the continuous sheet P is stopped. Consequently, no portions of the continuous sheet P will go to waste when color registration is performed. When image formation is performed on the continuous sheet P after color registration has been completed, the continuous sheet P is pressed against the intermediate transfer belt  20  by the second transfer roller  40 . 
     In general, when there is an object that comes into and out of contact with the intermediate transfer belt  20  as described above, there is a change in the load applied to the intermediate transfer belt  20 , which moves, between when the object is in contact with the intermediate transfer belt  20  and when the object is separated from the intermediate transfer belt  20 , and there is a possibility that the movement speed of the intermediate transfer belt  20  will also change. Such a change in the movement speed causes misregistration when images are formed on the continuous sheet P. Therefore, the image forming apparatus  1  according to the first exemplary embodiment is provided with a speed maintaining function for suppressing changes in the movement speed so as to avoid misregistration. The speed maintaining function will be described in detail below. 
       FIG. 2  and  FIG. 3  are schematic diagrams each illustrating the speed maintaining function.  FIG. 2  illustrates a state in which the second transfer roller  40  is located at an image-formation position, and  FIG. 3  illustrates a state in which the second transfer roller  40  is located at the retreat position. 
     In the first exemplary embodiment, the driving roller  21  is driven by a motor shared by the photoconductor drums  10 Y,  10 M,  10 C, and  10 K of the image engines and the like, and the speed at which the driving roller  21  rotates is kept constant by a speed command unit  81 . The function of the speed command unit  81  is controlled by the controller  80  illustrated in  FIG. 1 . As described above, although the speed at which the driving roller  21  rotates is kept constant, a load is applied by the cleaner  70  and the photoconductor drums  10 Y,  10 M,  10 C, and  10 K to the intermediate transfer belt  20 , which moves. This load is represented by outlined arrows in  FIG. 2  and  FIG. 3 . The load causes the intermediate transfer belt  20  to slide over the driving roller  21 , and there is a speed difference between the driving roller  21  and the intermediate transfer belt  20 . In the image forming apparatus  1  according to the first exemplary embodiment, in order to detect the actual movement speed of the intermediate transfer belt  20 , the speed at which the driven roller  22 , which is driven by the intermediate transfer belt  20 , rotates is detected by a sliding-speed detection unit  83  and converted into the movement speed of the intermediate transfer belt  20 . The function of the sliding-speed detection unit  83  is also controlled by the controller  80  illustrated in  FIG. 1 . 
     As described above, the second transfer roller  40  is caused to move toward and away from the intermediate transfer belt  20  by the cam mechanism and the like. The movement of the second transfer roller  40  toward and away from the intermediate transfer belt  20  is controlled by a contact/separation state control unit  82 , and the function of the contact/separation state control unit  82  is also controlled by the controller  80  illustrated in  FIG. 1 . 
     When an image is formed on the continuous sheet P, as described above, the second transfer roller  40  moves toward the intermediate transfer belt  20 , and the continuous sheet P is pressed against the surface of the intermediate transfer belt  20 . As a result of the second transfer roller  40  pressing the continuous sheet P against the surface of the intermediate transfer belt  20 , there is a change in the load applied to the intermediate transfer belt  20 , which moves. In the first exemplary embodiment, the change in the load is an increase in the load. The sliding-speed detection unit  83  detects the movement speed of the intermediate transfer belt  20  by taking into consideration the influence of the second transfer roller  40 . 
     In the first exemplary embodiment, a case has been described in which the second transfer roller  40  applies a load to the intermediate transfer belt  20 , which moves. However, if the second transfer roller  40  employs a driving system specific to the second transfer roller  40 , there is a case where the second transfer roller  40  assists the intermediate transfer belt  20  in moving. In this case, the load applied to the moving intermediate transfer belt  20  decreases due to the influence of the second transfer roller  40 . Both in the case where the influence of the second transfer roller  40  increases the load and in the case where the influence of the second transfer roller  40  decreases in the load, the sliding-speed detection unit  83  is configured to detect the movement speed of the intermediate transfer belt  20  by taking into consideration the influence of the second transfer roller  40 . 
     When the second transfer roller  40  is located at the image-formation position, since the continuous sheet P is interposed between the second transfer roller  40  and the intermediate transfer belt  20 , the second transfer roller  40  is not directly in contact with the intermediate transfer belt  20 . However, the second transfer roller  40  may also be considered to be indirectly in contact with the intermediate transfer belt  20  with the continuous sheet P interposed therebetween, and thus, a state where the second transfer roller  40  presses the continuous sheet P against the intermediate transfer belt  20  may sometimes be described below as “the second transfer roller  40  is in contact with the intermediate transfer belt  20 ”. 
     The movement speed of the intermediate transfer belt  20  detected by the sliding-speed detection unit  83  is compared with a predetermined reference speed by a sliding-speed-difference calculation unit  84 . As a result, the speed difference between the reference speed and the actual speed is calculated by the sliding-speed-difference calculation unit  84 . The function of the sliding-speed-difference calculation unit  84  is also controlled by the controller  80  illustrated in  FIG. 1 . 
     In the first exemplary embodiment, the first transfer roller  11 K that faces the photoconductor drum  10 K, which is one of the four photoconductor drums  10 Y,  10 M,  10 C, and  10 K and which corresponds to color K, is independently driven by a motor different from the motor that drives the other first transfer rollers, the driving roller  21 , and the like. As a result of a driving torque of the motor that drives the first transfer roller  11 K for color K being increased and decreased by a torque control unit  85 , the load applied to the intermediate transfer belt  20 , which moves, is adjusted, and the movement speed of the intermediate transfer belt  20  is increased and decreased. The torque control unit  85  controls the driving torque so as to reduce the speed difference calculated by the sliding-speed-difference calculation unit  84  as much as possible. By performing this so-called feedback control, the movement speed of the intermediate transfer belt  20  precisely returns to the reference speed and becomes stable. 
     In a state where the driving torque applied to the first transfer roller  11 K for color K is controlled in the manner described above, when the second transfer roller  40  moves to the retreat position and separates from the intermediate transfer belt  20  as illustrated in  FIG. 3 , the load applied to the intermediate transfer belt  20 , which moves, changes. In the first exemplary embodiment, the load is reduced by an amount equal to the load generated by the second transfer roller  40  being in contact with the intermediate transfer belt  20 . At the moment at which the load applied to the moving intermediate transfer belt  20  changes in this manner, the movement speed of the intermediate transfer belt  20  deviates from the reference speed. Such a deviation in speed is detected by the sliding-speed detection unit  83  and the sliding-speed-difference calculation unit  84 , and the driving torque applied to the first transfer roller  11 K for color K is controlled by the torque control unit  85  in such a manner that the deviation is reduced. This causes the movement speed of the intermediate transfer belt  20  to return to the reference speed again. 
     By controlling the driving torque in the manner described above, in the first exemplary embodiment, a load equivalent to the load reduced as a result of the second transfer roller  40  moving away from the intermediate transfer belt  20  is applied to the first transfer roller  11 K for color K. Unlike the first exemplary embodiment, in the case where the second transfer roller  40  employs a driving system specific to the second transfer roller  40  and assists the intermediate transfer belt  20  in moving, the load applied to the moving intermediate transfer belt  20  increases as a result of the second transfer roller  40  moving away from the intermediate transfer belt  20 . In this case, the first transfer roller  11 K for color K is caused to assist the intermediate transfer belt  20  in moving by controlling the driving torque applied to the first transfer roller  11 K for color K in such a manner that the load applied to the moving intermediate transfer belt  20  is reduced by an amount equal to the load increased. 
     In other words, the first transfer roller  11 K for color K changes the load applied to the moving intermediate transfer belt  20  in a manner opposite to the change in the load upon separation of the second transfer roller  40  from the intermediate transfer belt  20 , and the movement speed of the intermediate transfer belt  20  is maintained at the reference speed. The first transfer roller  11 K for color K corresponds to an example of a change unit according to the present invention. In addition, the first transfer roller  11 K for color K corresponds to an example of a second member according to the present invention and also corresponds to an example of a transfer member according to the present invention. That is to say, in the first exemplary embodiment, the first transfer roller  11 K for color K is used both in transfer of a toner image and control of the load applied to the intermediate transfer belt  20 , which moves, so as to make efficient use of the first transfer roller  11 K. The torque applied to the first transfer roller  11 K, which is a roll-shaped member, is stably controlled, and thus, the load applied to the intermediate transfer belt  20 , which moves, is also stably adjusted. 
     Conversely, when the second transfer roller  40  moves from the retreat position to the image-formation position, and the load applied to the moving intermediate transfer belt  20  changes, the torque control unit  85  controls the driving torque such that the first transfer roller  11 K for color K changes the load in a manner opposite to the change upon the movement of the second transfer roller  40  from the retreat position to the image-formation position. The driving torque is controlled in this manner, and as a result, the load applied to the intermediate transfer belt  20 , which moves, is adjusted, so that the differences in the load applied to the moving intermediate transfer belt  20  and the movement speed of the intermediate transfer belt  20  between when the second transfer roller  40  is separated from the intermediate transfer belt  20  and when the second transfer roller  40  is in contact with the intermediate transfer belt  20  are reduced. Therefore, the probability of occurrence of misregistration is reduced even in the case where color registration is performed in the state illustrated in  FIG. 3  in which the second transfer roller  40  is located at the retreat position, and then image formation is performed in the state illustrated in  FIG. 2  in which the second transfer roller  40  has moved to the image-formation position. 
     Note that, in the above-described first exemplary embodiment, although a case has been described in which a continuous sheet is used as a recording material, also in the case where a cut sheet is used as a recording material, there has been proposed a configuration in which, when color registration is performed, the second transfer roller  40  is separated from the intermediate transfer belt  20  in order to avoid contamination of the second transfer roller  40 , and the above-described speed maintaining function is effective for avoiding misregistration. 
     The other exemplary embodiments of the present invention will be described below. 
     A second exemplary embodiment is similar to the first exemplary embodiment, except with regard to a system for controlling the torque applied to the first transfer roller  11 K for color K. The second exemplary embodiment will be described below focusing on the difference from the first exemplary embodiment. 
       FIG. 4  and  FIG. 5  are diagrams each illustrating the second exemplary embodiment.  FIG. 4  illustrates a state in which the second transfer roller  40  is located at the image-formation position, and  FIG. 5  illustrates a state in which the second transfer roller  40  is located at the retreat position. 
     In an image forming apparatus  2  according to the second exemplary embodiment, as a method of controlling the torque applied to the first transfer roller  11 K for color K, a simple switching control is employed instead of feedback control such as that employed in the first exemplary embodiment. In other words, the image forming apparatus  2  according to the second exemplary embodiment includes a contact-state determining unit  86  and a torque control unit  87 , and the contact-state determining unit  86  obtains a control signal for the second transfer roller  40  transmitted by the contact/separation state control unit  82  and determines a contact state or a separation state of the second transfer roller  40 . Then, in accordance with the contact state or the separation state determined by the contact-state determining unit  86 , the torque control unit  87  sets one of two types of driving torques to the driving torque for the first transfer roller  11 K for color K and controls the driving torque. 
     In the state illustrated in  FIG. 4 , the contact-state determining unit  86  determines that the second transfer roller  40  is in the contact state, and for example, the driving torque for the first transfer roller  11 K for color K is controlled by the torque control unit  87  in such a manner that no torque is applied to the intermediate transfer belt  20  by the first transfer roller  11 K for color K. 
     In the state illustrated in  FIG. 5 , the contact-state determining unit  86  determines that the second transfer roller  40  is in the separation state. Then, the driving torque applied to the first transfer roller  11 K for color K is controlled by the torque control unit  87  in such a manner as to, for example, generate a load equal to the load applied to the intermediate transfer belt  20  by the second transfer roller  40  in the state illustrated in  FIG. 4 . 
     As a result of performing such switching control, also in the second exemplary embodiment, the first transfer roller  11 K for color K changes the load applied to the intermediate transfer belt  20  in a manner opposite to the change in the load upon separation of the second transfer roller  40  from the intermediate transfer belt  20 . Therefore, the difference in the load applied to the intermediate transfer belt  20 , which moves, between when the second transfer roller  40  is in contact with the intermediate transfer belt  20  and when the second transfer roller  40  is separated from the intermediate transfer belt  20  is reduced as well as the difference in the movement speed. 
     Such simple switching control according to the second exemplary embodiment is simpler and easier to introduce than the feedback control according to the first exemplary embodiment. 
     A third exemplary embodiment will now be described. The third exemplary embodiment is similar to the first exemplary embodiment, except with regard to the type of the change unit according to the present invention. The third exemplary embodiment will be described below focusing on the difference from the first exemplary embodiment. 
       FIG. 6  and  FIG. 7  are diagrams each illustrating the third exemplary embodiment of the present invention.  FIG. 6  illustrates a state in which the second transfer roller  40  is located at the image-formation position, and  FIG. 7  illustrates a state in which the second transfer roller  40  is located at the retreat position. 
     In an image forming apparatus  3  according to the third exemplary embodiment, the cleaner  70  has a change function for changing a pressing force applied to the blade that is pressed against the intermediate transfer belt  20 . In third exemplary embodiment, although this change function is realized by a cam mechanism, a stepping motor, and the like, which are not illustrated, the change function may be realized by a different mechanism that is commonly known. The cleaner  70  applies a load to the intermediate transfer belt  20 , which moves, as a result of the blade and the intermediate transfer belt  20  rubbing against each other. When the pressing force applied to the blade is changed by the change function, the degree of rubbing between the blade and the intermediate transfer belt  20  is changed, and the load applied to the moving intermediate transfer belt  20  is also changed. 
     As a result of the pressing force applied to the blade, which is pressed against the intermediate transfer belt  20 , being increased and decreased by a pressing-force control unit  88  using the change function, the load applied to the intermediate transfer belt  20 , which moves, is adjusted, and the movement speed of the intermediate transfer belt  20  is increased and decreased. Adjusting the load applied to the moving intermediate transfer belt  20  by controlling the pressing force applied to the blade may accommodate a larger change in the load compared with the case of adjusting the load by, for example, performing torque control in the manner described in the first exemplary embodiment. 
     Similar to the first exemplary embodiment, the image forming apparatus  3  according to the third exemplary embodiment includes the sliding-speed detection unit  83  and the sliding-speed-difference calculation unit  84 . 
     The pressing-force control unit  88  controls the pressing force so as to reduce the speed difference calculated by the sliding-speed-difference calculation unit  84  as much as possible. By performing such control, also in the third exemplary embodiment, the movement speed of the intermediate transfer belt  20  returns to the reference speed and becomes stable. 
     Also in the third exemplary embodiment, when the second transfer roller  40  moves from the image-formation position, which is illustrated in  FIG. 6 , to the retreat position, which is illustrated in  FIG. 7 , in a state where the pressing force applied to the blade is controlled in the manner described above, the load applied to the intermediate transfer belt  20 , which moves, decreases. As a result, the movement speed of the intermediate transfer belt  20  temporarily deviates from the reference speed, and the deviation is detected by the sliding-speed detection unit  83  and the sliding-speed-difference calculation unit  84 . Then, the pressing force applied to the blade is controlled by the pressing-force control unit  88  in such a manner that the deviation is reduced. This causes the movement speed of the intermediate transfer belt  20  to return to the reference speed again. 
     Also when the second transfer roller  40  moves from the retreat position, which is illustrated in  FIG. 7 , to the image-formation position, which is illustrated in  FIG. 6 , deviation of the movement speed of the intermediate transfer belt  20  from the reference speed is detected, and the pressing force applied to the blade is controlled, so that the movement speed of the intermediate transfer belt  20  returns to the reference speed again. 
     As a result of controlling the pressing force applied to the blade in the manner described above, the cleaner  70  changes the load applied to the intermediate transfer belt  20 , which moves, in a manner opposite to the change in the load upon contact or separation of the second transfer roller  40  with or from the intermediate transfer belt  20 . Accordingly, in the third exemplary embodiment, the cleaner  70  corresponds to an example of the change unit according to the present invention. In addition, the blade included in the cleaner  70  corresponds to an example of a first member according to the present invention and also corresponds to an example of a removal member according to the present invention. In other words, in the third exemplary embodiment, the cleaner  70  and the blade are used both in cleaning of the intermediate transfer belt  20  and control of the load applied to the intermediate transfer belt  20 , which moves, so as to make efficient use of the cleaner  70  and the blade. 
     Note that, although the feedback control similar to that in the first exemplary embodiment has been described above as an example of the method for controlling the pressing force applied to the blade of the cleaner  70 , switching control similar to that in the second exemplary embodiment may be employed as the method for controlling the pressing force. 
     A fourth exemplary embodiment will now be described. The fourth exemplary embodiment is similar to the first exemplary embodiment except that a dedicated component has a function of serving as the change unit according to the present invention. The fourth exemplary embodiment will be described below focusing on the difference from the first exemplary embodiment. 
       FIG. 8  is a diagram illustrating the fourth exemplary embodiment of the present invention. 
     An image forming apparatus  4  according to the fourth exemplary embodiment includes a pair of rollers  90  that are driven by a drive motor specific to the pair of rollers  90  so as to rotate and that rotate with the intermediate transfer belt  20  interposed therebetween. A rotational-driving torque that is applied to the pair of rollers  90  is controlled by a torque control unit  89 . 
     Similar to the first exemplary embodiment, the image forming apparatus  4  according to the fourth exemplary embodiment includes the sliding-speed detection unit  83  and the sliding-speed-difference calculation unit  84 . 
     The torque control unit  89  controls the driving torque in a similar manner that the torque control unit  85  according to the first exemplary embodiment controls the driving torque. Thus, the pair of rollers  90  also change the load applied to the intermediate transfer belt  20 , which moves, in a manner opposite to the change in the load upon contact or separation of the second transfer roller  40  with or from the intermediate transfer belt  20 , and the movement speed of the intermediate transfer belt  20  returns to the reference speed whether or not the second transfer roller  40  is in contact with the intermediate transfer belt  20 . In the fourth exemplary embodiment, the pair of rollers  90  correspond to an example of the change unit according to the present invention and are dedicated components for changing the load applied to the intermediate transfer belt  20 , which moves. The pair of rollers  90  also correspond to an example of the second member according to the present invention. As described above, a dedicated component that functions as the change unit according to the present invention is provided, so that members that perform image transfer, belt cleaning, and the like do not need to change the load applied to the moving intermediate transfer belt  20 . In the case where such members change the load applied to the moving intermediate transfer belt  20 , this change operation may have an influence on their roles, which are image transfer and belt cleaning. However, since the members do not need to change the load applied to the moving intermediate transfer belt  20 , image transfer and belt cleaning will not be influenced. 
     Note that, although the feedback control similar to that in the first exemplary embodiment has been described above as an example of the method for controlling the torque applied to the pair of rollers  90 , switching control similar to that in the second exemplary embodiment may be employed as the method for controlling the torque. In addition, although the member that corresponds to an example of the second member according to the present invention has been described above as an example of the dedicated component that functions as the change unit according to the present invention, the member that corresponds to an example of the first member according to the present invention may be used as the dedicated component. 
     A fifth exemplary embodiment will now be described. 
       FIG. 9  is a diagram illustrating an image forming apparatus according to the fifth exemplary embodiment of the present invention. 
     Although an image forming apparatus  5  according to the fifth exemplary embodiment is also a color printer that employs the tandem system, unlike the first exemplary embodiment, cut sheets CP are used as recording sheets. The cut sheets CP are stacked on top of one another in a sheet tray  130 , which is disposed in a lower portion of the image forming apparatus  5 , and are taken out one by one from the sheet tray  130  by a feed roller  131  and separation rollers  132 . 
     The image forming apparatus  5  according to the fifth exemplary embodiment includes six image engines  110 V,  110 V′,  110 Y,  110 M,  110 C, and  110 K that correspond to six colors including, for example, two spot colors and four colors of YMCK. Also in the fifth exemplary embodiment, each of the image engines  110 V to  110 K employs the electrophotographic system and forms a toner image. In the fifth exemplary embodiment, a single exposure unit  112  radiates exposure light beams onto the six image engines  110 V to  110 K. 
     The image forming apparatus  5  according to the fifth exemplary embodiment employs a direct transfer system and includes a sheet-transport belt  120 . The sheet-transport belt  120  is stretched between a driving roller  121  and a driven roller  122  and caused to move along a circular path that extends in the direction in which the image engines  110 V to  110 K are arranged by a driving force of the driving roller  121 . In the fifth exemplary embodiment, the sheet-transport belt  120  corresponds to an example of the movable body according to the present invention. 
     Transfer rollers  111 V,  111 V′,  111 Y,  111 M,  111 C, and  111 K are disposed at positions facing the image engines  110 V to  110 K with the sheet-transport belt  120  interposed between the transfer rollers  111 V to  111 K and the image engines  110 V to  110 K. Each of the cut sheets CP is transported by the sheet-transport belt  120  so as to pass between the image engines  110 V to  110 K and the transfer rollers  111 V to  111 K. Toner images of different colors each of which is formed by a corresponding one of the image engines  110 V to  110 K are sequentially transferred onto one of the cut sheets CP in such a manner as to be superposed with one another by the transfer rollers  111 V to  111 K. As a result of such transfer of the toner images, a color image is formed on the cut sheet CP. A combination of each of the image engines and a corresponding one of the transfer rollers corresponds to an example of an image forming unit according to the present invention. In the fifth exemplary embodiment, image formation is directly performed on each of the cut sheets CP, which are examples of a recording material. 
     The image forming apparatus  5  includes a fixing unit  150 , and a color image formed on one of the cut sheets CP is fixed onto the cut sheet CP as a result of being heated and pressurized by the fixing unit  150 . The image forming apparatus  5  further includes a cleaner  170  that scrapes off unwanted substances, such as paper dust, from the sheet-transport belt  120  by using a blade and a controller  180  that controls each unit in the image forming apparatus  5 . 
     The six image engines  110 V to  110 K included in the image forming apparatus  5  perform color image formation using, for example, four colors of CMYK in addition to color image formation using the six colors including the two spot colors. 
     In the case of performing such image formation using only four of the six colors, the image engines  110 V and  110 V′ and the transfer rollers  111 V and  111 V′ corresponding to the unnecessary colors are moved to retreat positions spaced apart from the sheet-transport belt  120  in order to avoid, for example, contamination and a reduction in the service life thereof. In the case of performing color image formation using the six colors, all the image engines and all the transfer rollers are moved to image-formation positions that are illustrated in  FIG. 9 . 
     As described above, in the fifth exemplary embodiment, the image engines  110 V and  110 V′ and the transfer rollers  111 V and  111 V′ each have contact and separation functions, which are to move into and out of contact with the sheet-transport belt  120 . In the fifth exemplary embodiment, although the contact and separation functions are realized by a link mechanism, a cam mechanism, a stepping motor, and the like, which are not illustrated, the contact and separation functions may be realized by a different mechanism that is commonly known. A combination of each of the image engines  110 V and  110 V′ and a corresponding one of the transfer rollers  111 V and  111 V′ corresponds to an example of a contact and separation unit according to the present invention. The state illustrated in  FIG. 9  corresponds to a state where the contact and separation units according to the present invention are located at contact positions. A state where the image engines  110 V and  110 V′ and the like have moved to the retreat positions as described above corresponds to a state where the contact and separation units according to the present invention are located at separation positions. 
     The image forming apparatus  5  according to the fifth exemplary embodiment also has a color registration function and adjusts the timing at which toner images are formed by the image engines  110 V to  110 K such that the toner images are transferred onto one of the cut sheets CP in such a manner as to be superposed with one another without misregistration. More specifically, toner images for use in color registration are formed by the image engines  110 V to  110 K, and misregistration between the toner images on one of the cut sheets CP is, for example, measured or visually estimated. This misregistration is input to the controller  180 , and the controller  180  adjusts the timing at which the image engines  110 V to  110 K form toner images in such a manner that the misregistration is reduced. 
     As described above, in the fifth exemplary embodiment, when the number of colors used in image formation is changed, the image engines  110 V and  110 V′ and the transfer rollers  111 V and  111 V′ come into or out of contact with the sheet-transport belt  120 . In general, when there is an object that comes into and out of contact with the sheet-transport belt  120  as described above, there is a change in the load applied to the sheet-transport belt  120 , which moves, between when the object is in contact with the sheet-transport belt  120  and when the object is separated from the sheet-transport belt  120 , and there is a possibility that the movement speed of the sheet-transport belt  120  will also change. Such a change in the movement speed causes misregistration, and thus, also the image forming apparatus  5  according to the fifth exemplary embodiment is provided with a speed maintaining function for suppressing changes in the movement speed so as to avoid misregistration. 
       FIG. 10  is a diagram illustrating the speed maintaining function according to the fifth exemplary embodiment. 
     In the fifth exemplary embodiment, the driving roller  121  is driven by a motor shared by photoconductor drums included in the image engines  110 V to  110 K and the like, and the speed at which the driving roller  121  rotates is kept constant. However, a load is applied by the cleaner  170  and the like to the sheet-transport belt  120 , which moves, and this load causes the sheet-transport belt  120  to slide over the driving roller  121 . Consequently, there is a speed difference between the driving roller  121  and the sheet-transport belt  120 . 
     Accordingly, in order to detect the actual movement speed of the sheet-transport belt  120 , the speed at which the driven roller  122 , which is driven by the sheet-transport belt  120 , rotates is detected by a sliding-speed detection unit  183  and converted into the movement speed of the sheet-transport belt  120 . The function of the sliding-speed detection unit  183  is controlled by the controller  180  illustrated in  FIG. 9 . 
     As described above, the image engines  110 V and  110 V′ and the transfer rollers  111 V and  111 V′ each have the contact and separation functions and moves into and out of contact with the sheet-transport belt  120 . The movements of the image engines  110 V and  110 V′ and the like into and out of contact with the sheet-transport belt  120  is controlled by a contact/separation state control unit  182 , and the function of the contact/separation state control unit  182  is also controlled by the controller  180  illustrated in  FIG. 9 . The load applied by the image engines and the like to the sheet-transport belt  120 , which moves, changes as a result of the image engines  110 V and  110 V′ and the like moving into and out of contact with the sheet-transport belt  120 , and the sliding-speed detection unit  183  detects the movement speed of the sheet-transport belt  120  by taking into consideration the influence of the change in the load. 
     The movement speed of the sheet-transport belt  120  detected by the sliding-speed detection unit  183  is compared with a predetermined reference speed by a sliding-speed-difference calculation unit  184 . As a result, the speed difference between the reference speed and the actual speed is calculated by the sliding-speed-difference calculation unit  184 . The function of the sliding-speed-difference calculation unit  184  is also controlled by the controller  180  illustrated in  FIG. 9 . 
     In the fifth exemplary embodiment, the transfer roller  111 K that faces the image engine  110 K, which is one of the six image engines  110 V to  110 K and which corresponds to color K, is independently driven by a motor different from the motor that drives the other transfer rollers, the driving roller  121 , and the like. A driving torque of the motor that drives the image engine  110 K for color K is controlled by a torque control unit  185 . The torque control unit  185  controls the driving torque in a similar manner that the torque control unit  85  according to the first exemplary embodiment controls the driving torque, and the driving torque is increased and decreased in such a manner that the speed difference calculated by the sliding-speed-difference calculation unit  184  is reduced as much as possible, so that the load applied to the sheet-transport belt  120 , which moves, is increased and decreased. As a result, the image engine  110 K for color K changes the load applied to the sheet-transport belt  120  in a manner opposite to the change in the load upon contact or separation of the image engines  110 V and  110 V′ and the transfer rollers  111 V and  111 V′ with or from the sheet-transport belt  120 , which moves, and the movement speed of the sheet-transport belt  120  returns to the reference speed and becomes stable. The image engine  110 K for color K corresponds to an example of the change unit according to the present invention. 
     In the fifth exemplary embodiment, although the cut sheets CP are used as examples of a recording sheet in the image forming apparatus  5 , which employs the direct transfer system, a continuous sheet may be used as a recording sheet also in the direct transfer system. In this case, the continuous sheet corresponds to an example of a movable body according to the present invention. 
     In addition, although the feedback control similar to that in the first exemplary embodiment has been described above as an example of the method for controlling the image engine  110 K for color K, switching control similar to that in the second exemplary embodiment may be employed as the method for controlling the torque. 
     Although the image engine  110 K for color K has been described as an example of the change unit according to the present invention, similar to the third exemplary embodiment, in the image forming apparatus  5 , which employs the direct transfer system, the cleaner  170  may have a function of serving as the change unit according to the present invention. Alternatively, similar to the forth exemplary embodiment, a dedicated component that functions as the change unit according to the present invention may be provided. 
     In the above exemplary embodiments, although a combination of a second transfer roller and a continuous sheet and a combination of some image engines and some transfer rollers have been described as examples of the contact and separation unit according to the present invention, the contact and separation unit according to the present invention is not limited to these and may be any unit as long as the unit changes the load applied to the movable body according to the present invention, which moves, by coming into and out of contact with the movable body. In an indirect transfer system that uses an intermediate transfer body, similar to the fifth exemplary embodiment, there may be a case where some image engines each serve as the contact and separation unit. In addition, there may be a case where both a second transfer roller and an image engine serve as contact and separation units in a single image forming apparatus. In other words, plural contact and separation units according to the present invention may be provided in a single image forming apparatus. 
     In the above exemplary embodiments, although the units each employing an electrophotographic system have been described as examples of the image forming units according to the present invention, the image forming units according to the present invention may each employ an ink-jet system. 
     In the above exemplary embodiments, although a case has been described as an example in which movements of the contact and separation unit and the change unit according to the present invention into and out of contact with the movable body and the loads applied to the movable body by the contact and separation unit and the change unit are controlled by a control unit, the movements and the loads may be manually switched. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention 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 invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.