Abstract:
An image forming apparatus having: a rotating member adapted to support an image; a first drive source configured to generate a drive force; a transmission mechanism configured to transmit the generated drive force toward the rotating member, the mechanism including an upstream gear and a downstream gear configured to receive the drive force therefrom; a sensor configured to output a signal indicating a rotational status of the rotating member; a control circuit configured to generate a control signal appropriate for non-uniform rotation of the rotating member, on the basis of the signal outputted by the sensor; and an actuator configured to cause a rotation axis of the first drive source or the transmission mechanism to pivot, in accordance with the control signal, thereby changing a force with which the upstream gear pushes the downstream gear into rotation such that the non-uniform rotation is cancelled out.

Description:
This application is based on Japanese Patent Application No. 2012-206690 filed on Sep. 20, 2012, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus including a rotating member adapted to support an image and a transmission mechanism that transmits a drive force generated by a drive source to the rotating member. 
     2. Description of Related Art 
     An image forming apparatus as mentioned above is disclosed in Japanese Patent Laid-Open Publication No. 2010-102247, for example. In Japanese Patent Laid-Open Publication No. 2010-102247, the transmission mechanism includes a drum drive gear, a coupling disc, an encoder head, and a control circuit. 
     The drum drive gear is positioned coaxially with a rotating shaft of a photoreceptor drum, which is an example of the rotating member. The coupling disc engages with the photoreceptor drum and the drum drive gear. The encoder head detects information about the rotation of the coupling disc. The control circuit controls the rotation of the photoreceptor drum on the basis of the rotation information detected by the encoder head. 
     Here, the drum drive gear and the coupling disc engage with each other via a viscoelastic member. By passive vibration control using the viscoelastic member, a resonant frequency of the photoreceptor drum or the like is shifted from original value, thereby reducing transmissibility of input vibrations having the certain frequency. 
     However, such passive vibration control simply shifts the resonant frequencies, so that resonance characteristics of other frequency bands (e.g., a low-frequency band) persist. The persistence of the resonance characteristics causes a problem where conventional drive force transmission mechanisms cannot suppress the vibration of the photoreceptor drum (i.e., the rotating member) upon input of vibration that is difficult to predict. 
     SUMMARY OF THE INVENTION 
     An image forming apparatus according to an embodiment of the present invention includes: a rotating member adapted to support an image; a first drive source configured to generate a drive force; a transmission mechanism configured to transmit the drive force generated by the first drive source toward the rotating member, the mechanism including an upstream gear and a downstream gear configured to receive the drive force from the upstream gear; a sensor configured to output a signal indicating a rotational status of the rotating member; a control circuit configured to generate a control signal appropriate for non-uniform rotation of the rotating member, on the basis of the signal outputted by the sensor; and an actuator configured to cause a rotation axis of the first drive source or the transmission mechanism to pivot, in accordance with the control signal generated by the control circuit, thereby changing a force with which the upstream gear pushes the downstream gear into rotation such that the non-uniform rotation of the rotating member is cancelled out. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the configuration of an image forming apparatus according to an embodiment; 
         FIG. 2  is an oblique view illustrating a first configuration example of a drive system for a photoreceptor drum in  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating the configuration of a substantial part of the drive system in  FIG. 2 ; 
         FIG. 4  is a diagram outlining the operation of the drive system in  FIG. 2   
         FIG. 5  is a diagram illustrating suppression of non-uniform rotation of the photoreceptor drum; 
         FIG. 6  is a graph showing frequency response functions for the drive system in  FIG. 2 , a conventional drive system, and a drive system without vibration control; 
         FIG. 7  is an oblique view illustrating a second configuration example of the drive system for the photoreceptor drum in  FIG. 1 ; 
         FIG. 8  is a block diagram illustrating the configuration of a substantial part of the drive system in  FIG. 7 ; 
         FIG. 9  is a diagram illustrating a third configuration example of the drive system for the photoreceptor drum in  FIG. 1 ; 
         FIG. 10  is a diagram illustrating a substantial part of a fourth configuration example of the drive system for the photoreceptor drum in  FIG. 1 ; and 
         FIG. 11  is a diagram illustrating a substantial part of a fifth configuration example of the drive system for the photoreceptor drum in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described. 
     Preliminary Notes 
     First, the X-, Y-, and Z-axes shown in some figures will be defined. The X-axis represents the left-right (width) direction of the image forming apparatus, and the Y-axis represents the front-back direction of the image forming apparatus. Moreover, the Z-axis represents the top-bottom (height) direction of the image forming apparatus. 
     Furthermore, for some components, the suffix A, B, C, or D is assigned at the ends of their reference numerals. The suffixes A, B, C, and D represent yellow (Y), magenta (M), cyan (C), and black (Bk), respectively. For example, an imaging unit  11 A is intended to mean an imaging unit  11  for yellow. Moreover, in the case where none of the suffixes is assigned to a reference numeral which can be assigned any one of the suffixes, the reference numeral is intended for collective reference to all colors. For example, an imaging unit  11  is intended to mean an imaging unit for any one of the colors Y, M, C, and Bk. 
     General Configuration of Image Forming Apparatus 
       FIG. 1  is a schematic diagram illustrating the configuration of an image forming apparatus according to an embodiment. In  FIG. 1 , the image forming apparatus is, for example, a multifunction printer (MFP) of a tandem type employing electrophotography, which forms a full-color image on a sheet S (such as paper). To this end, the image forming apparatus generally includes a supply unit  1 , an image forming unit  2 , and an output tray  3 . 
     The supply unit  1  has unprinted sheets S stacked in an unillustrated supply tray. From the supply tray, the sheets S are fed one by one from the top by a supply roller (not shown) that is rotating, toward the uppermost stream of a feeding path P (see the long dashed short dashed line). 
     The image forming unit  2  includes imaging units  11 A to  11 D. The image forming unit  2  further includes a scanning optical system  12 , primary transfer rollers  13 A to  13 D, an intermediate transfer belt  14 , rollers  15  and  16 , a secondary transfer roller  17 , a fusing unit  18 , and an ejection roller pair  19 . 
     The imaging units  11  are arranged in the left-right direction immediately below the intermediate transfer belt  14  to be described later. Each imaging unit  11  has a photoreceptor drum  110 , which is a typical example of the rotating member. There are arranged different types of components around each photoreceptor drum  110 , such that one component from each type is provided, e.g., one charger, one developer, etc., are arranged around the photoreceptor drum  110 . Each charger charges the circumferential surface of the photoreceptor drum  110  for its corresponding color. The charged circumferential surface of the photoreceptor drum  110  is irradiated with an optical beam for the corresponding color generated by the scanning optical system  12 . As a result, an electrostatic latent image in the corresponding color is formed and supported on the circumferential surface of the photoreceptor drum. The developer supplies toner onto the circumferential surface of the photoreceptor drum for the corresponding color and develops the electrostatic latent image. As a result, a toner image in the corresponding color is formed on the circumferential surface of the photoreceptor drum. 
     The intermediate transfer belt  14  is stretched around the rollers  15  and  16 , etc., in a looped form, so as to contact the circumferential surfaces of the photoreceptor drums. The intermediate transfer belt  14  is rotated in the direction of arrow α by the rollers  15  and  16  being rotated by drive forces provided by unillustrated motors. 
     Each primary transfer roller  13  is disposed so as to be opposed vertically to the photoreceptor drum for its corresponding color with respect to the intermediate transfer belt  14 . The primary transfer roller  13  transfers a toner image supported on the photoreceptor drum for the corresponding color onto the intermediate transfer belt  14  moving in the direction of arrow α, approximately in the same position (i.e., primary transfer). Ultimately, a composite toner image (i.e., a full-color image) composed of overlapping toner images in their respective colors is formed on the surface of the intermediate transfer belt  14 . Moreover, the composite toner image supported on the intermediate transfer belt  14  is carried to the position of a transfer nip to be described later. 
     Furthermore, the secondary transfer roller  17  is disposed so as to be opposed to the roller  16  with respect to the intermediate transfer belt  14 . The secondary transfer roller  17  and the intermediate transfer belt  14  are in contact with each other so that there is a transfer nip formed therebetween. A sheet S fed by the supply unit  1  as mentioned above is introduced to the transfer nip. Moreover, a transfer bias voltage is applied to the secondary transfer roller  17 , so that the composite toner image is attracted to the secondary transfer roller  17  by the transfer bias voltage, and is transferred onto the sheet S introduced to the transfer nip (secondary transfer). The sheet S subjected to the secondary transfer is forwarded from the transfer nip toward the fusing unit  18 . 
     Upon the introduction of the sheet S subjected to secondary transfer, the fusing unit  18  heats and presses the sheet S, thereby fixing the composite toner image on the sheet S. The sheet S subjected to the fixing process is forwarded as a print from the fusing unit  18  being rotated, and thereafter ejected by the ejection roller pair  19  being rotated counterclockwise, into the output tray  3  provided above the image forming unit  2 . 
     First Configuration Example of Drive System 
       FIG. 2  is an oblique view illustrating a first configuration example of a drive system for the photoreceptor drum  110  shown in  FIG. 1 .  FIG. 3  is a block diagram illustrating the configuration of a substantial part of the drive system in  FIG. 2 . 
     In  FIG. 2 , the drive system includes a motor  21 , which is a typical example of a first drive source, a transmission mechanism  22 , a joint member  23 , and a coupling member  24 . 
     The motor  21  is secured to, for example, the frame of the image forming unit  2 . The motor  21  rotates its own rotating shaft under control of a control circuit  28 . 
     Note that to reduce the number of parts, in some cases, the motor  21  might be shared between photoreceptor drums  110  for a plurality of colors. In such a case, a drive system for at least one of the colors Y, M, C, and Bk has the configuration shown in  FIG. 2 , and a drive system for the remaining colors does not have its own motor  21  but receives a drive force from the motor  21  included in the other drive system via gears or the like. 
     The transmission mechanism  22  consists of a gear  22   a , which is provided on the rotating shaft of the motor  21 , a small-diameter gear  22   b , a two-stage gear unit  22   c , and a large-diameter gear  22   d.    
     The gear  22   a  is provided on the rotating shaft of the motor  21  so as to rotate in synchronization therewith. As a result, the drive force generated by the motor  21  is inputted to the transmission mechanism  22 . The gear  22   a  is provided at the uppermost stream of the transmission mechanism  22 , so as to transmit the inputted drive force from the motor  21 , toward the downstream. The drive force is used at least for rotating the photoreceptor drum  110  for a corresponding color. 
     The small-diameter gear  22   b  is provided immediately downstream from the gear  22   a , so as to mesh with the gear  22   a . The small-diameter gear  22   b  is rotated about its own shaft by the drive force transmitted from the gear  22   a.    
     The two-stage gear unit  22   c  is provided immediately downstream from the small-diameter gear  22   b , and includes an input gear and an output gear. The input gear and the output gear are provided coaxially. Moreover, in the example illustrated in the figure, the input gear has a larger diameter than the output gear. The input gear meshes with the small-diameter gear  22   b , and is rotated about the shaft of the two-stage gear unit  22   c  by the drive force transmitted by the small-diameter gear  22   b . On the other hand, the output gear is rotated about the two-stage gear unit  22   c  at the same angular velocity as the input gear. 
     The large-diameter gear  22   d  is provided immediately downstream from the two-stage gear unit  22   c . In the present embodiment, the large-diameter gear  22   d  is provided at the lowermost stream of the transmission mechanism  22 . The large-diameter gear  22   d  meshes with the output gear of the two-stage gear unit  22   c , so as to be rotated about its own shaft by the drive force transmitted by the output gear. 
     The joint member  23  has an approximately cylindrical shape. The joint member  23  is fixed at one end to the shaft of the large-diameter gear  22   d . Moreover, the joint member  23  has a protrusion or a groove formed at the other end. The joint member  23  is rotated at the same rotational speed as the large-diameter gear  22   d.    
     The coupling member  24  has an approximately cylindrical shape. The coupling member  24  has a protrusion or a groove formed at one end so that it can engage with the protrusion or groove of the joint member  23 . Moreover, the coupling member  24  is fixed at the other end to the rotating shaft of the photoreceptor drum  110 . 
     With the above configuration, the drive force generated by the motor  21  is transmitted to the photoreceptor drum  110  via the transmission mechanism  22 , the joint member  23 , and the coupling member  24 . The photoreceptor drum  110  is rotated at a predetermined rotational speed by the drive force transmitted thereto. 
     Such a drive system has a problem in that non-uniform rotation (i.e., a variation in rotational speed) occurs due to the vibration generated by meshing of the gears  22   a  to  22   d  in the transmission mechanism  22  and also due to the vibration per rotational cycle of the photoreceptor drums  110 . 
     To suppress such non-uniform rotation, the drive system includes an encoder  25 , a hinge member  26 , a piezoelectric element  27 , which is a typical example of an actuator, and a control circuit  28 , as shown in  FIGS. 2 and 3 . 
     The encoder  25  is a sensor that outputs a signal indicating the rotational status of the photoreceptor drum  110 . More specifically, the encoder  25  outputs a signal indicating non-uniform rotation for a rotational cycle of the photoreceptor drum  110 . The encoder  25  thus configured is attached to the rotating shaft of the photoreceptor drum  110 . 
     The hinge member  26  is a plate-like member having a predetermined shape (in the example of  FIG. 2 , oval). The hinge member  26  has a first through-hole  26   a  and a second through-hole  26   b  provided therein, the first through-hole  26   a  has approximately the same diameter as the shaft of the large-diameter gear  22   d  located on the downstream side, and the second through-hole  26   b  has approximately the same diameter as the shaft of the two-stage gear unit  22   c  located upstream from the large-diameter gear  22   d . The first through-hole  26   a  has the shaft of the large-diameter gear  22   d  inserted therein. Moreover, the second through-hole  26   b  has the shaft of the two-stage gear unit  22   c  inserted therein. Here, the shaft of the large-diameter gear  22   d  is not fixed to the first through-hole  26   a , and the shaft of the two-stage gear unit  22   c  is not fixed to the second through-hole  26   b.    
     The piezoelectric element  27  is, for example, of a laminated type, and it extends and contracts in the direction of the lamination upon application of a voltage. Here, the amount of extension/contraction of the piezoelectric element  27  is about 5 μm. The piezoelectric element  27  is preferably positioned as described below. The piezoelectric element  27  is fixed at one end in the direction of the lamination to, for example, the frame of the image forming apparatus. Moreover, the piezoelectric element  27  is fixed at the other end in the direction of the lamination to the hinge member  26 . In addition, the piezoelectric element  27  is oriented so as to extend and contract in direction γ perpendicular to line β extending between the centers of the through-holes  26   a  and  26   b.    
     By positioning the piezoelectric element  27  as above, the hinge member  26  vibrates clockwise or counterclockwise about the shaft of the large-diameter gear  22   d , as indicated by arrow  8 , in synchronization with the extension and contraction of the piezoelectric element  27 . The vibration instantaneously strengthens or weakens the force with which the teeth of the output gear of the two-stage gear unit  22   c  push the teeth of the large-diameter gear  22   d . This instantaneously accelerates or decelerates the rotational speed of the large-diameter gear  22   d , hence the rotational speed of the photoreceptor drum  110 . Here, in the case where the amount of extension/contraction of the piezoelectric element  27  is about 5 μm, the photoreceptor drum  110  is rotated instantaneously faster or slower within the range of ±1 μm in the rotational direction. 
     The control circuit  28  is configured by a processor, random-access memory (RAM), etc. To suppress non-uniform rotation, the control circuit  28  receives an output signal from the encoder  25 . From the received signal, the control circuit  28  reads information about non-uniform rotation for a rotational cycle of the photoreceptor drum  110 . The control circuit  28  generates a control signal in opposite phase to the non-uniform rotation according to the obtained information, and applies the signal to the piezoelectric element  27 . 
     Note that the control signal does not have to be in complete opposite phase to non-uniform rotation, and it simply deals with non-uniform rotation so that the actuator (i.e., the piezoelectric element  27 ) can eliminate the non-uniform rotation substantially. This also applies to second through fifth configuration examples to be described later. 
     Furthermore, the control signal may be updated every rotational cycle of the photoreceptor drum  110 , or it may be left unupdated for more than one rotation for which the degree of non-uniform rotation can be considered completely insignificant. 
     Operation of First Configuration Example 
     In the present drive system, because of the positions of the hinge member  26  and the piezoelectric element  27 , as well as the control signal from the control circuit  28 , the transmission mechanism  22  experiences a rotational variation in opposite phase to non-uniform rotation for a rotational cycle of the photoreceptor drum  110 . As a result, the non-uniform rotation of the photoreceptor drum  110  can be cancelled out by the rotational variation caused to the transmission mechanism  22 , leading to suppression of the non-uniform rotation of the photoreceptor drum  110 . 
     Here, it is assumed that none of the hinge member  26 , the piezoelectric element  27 , and the control signal is provided. In such a case, the teeth of the output gear in the two-stage gear unit mesh with the teeth of the large-diameter gear without jostling or coming out of contact with each other, as shown at the top right panel of  FIG. 4 . Moreover, as illustrated at the top center panel of  FIG. 4 , non-uniform rotation (a variation in rotational speed) might occur every rotational cycle of the photoreceptor drum. 
     On the other hand, in the case of the drive system shown in  FIGS. 2 and 3 , when the rotational speed of the photoreceptor drum  110  is slowed, as shown at the middle center panel of  FIG. 4 , the control circuit  28  causes the piezoelectric element  27  to extend (see the middle left panel of  FIG. 4 ) by applying a control signal thereto, thereby rotating the hinge member  26 . This strengthens the force with which a tooth of the output gear located on the upstream side presses a tooth of the large-diameter gear  22   d  located on the downstream side (see the middle right panel of  FIG. 4 ), thereby eliminating the delay in the rotational speed of the photoreceptor drum  110 . 
     Assuming here that the rotation angle of the large-diameter gear  22   d  is Y, and the rotation angle of the hinge member  26  is X, Y is represented by equation (1) below:
 
 Y =( Z 4/ Z 3)·( Z 2/ Z 1)· X   (1),
 
where Z 1  is the number of teeth of the large-diameter gear  22   d , Z 2  is the number of teeth of the output gear in the two-stage gear unit  22   c , Z 3  is the number of teeth of the input gear in the two-stage gear unit  22   c , and Z 4  is the number of teeth of the small-diameter gear  22   b.  
 
     In the case where there is an increase in the rotational speed of the photoreceptor drum  110 , as illustrated at the bottom center panel of  FIG. 4 , the control circuit  28  causes the piezoelectric element  27  to contract (see the bottom left panel of  FIG. 4 ) by applying a control signal thereto, thereby weakening the force with which a tooth of the output gear located on the upstream side presses a tooth of the large-diameter gear  22   d  located on the downstream side (see the bottom right panel of  FIG. 4 ). This eliminates the increase in the rotational speed of the photoreceptor drum  110 . 
     Note that in the state shown at the bottom right panel of  FIG. 4 , the weakening of the pressing force results in the tooth of the output gear located on the upstream side coming out of contact with the tooth of the large-diameter gear  22   d  located on the downstream side, so that the pressing force is reduced instantaneously to zero. 
     Effects 
     Drive systems without vibration control are prone to non-uniform rotation (a variation in speed) every rotational cycle of the photoreceptor drum, as shown at the top panel of  FIG. 5 . However, by equipping the image forming apparatus with the drive system described above, it is rendered possible to suppress non-uniform rotation every rotational cycle of the photoreceptor drum  110 , as shown at the bottom panel of  FIG. 5 . 
     Furthermore, in general, the drive system has such a frequency characteristic that the level of vibration transmission varies depending on an input vibration frequency. A quantified version of such a frequency characteristic is called a frequency response function. In the frequency response function, the frequency at which the level of vibration transmission is maximized is a resonant frequency, and the level of vibration transmission at the resonant frequency is called resonance magnification.  FIG. 6  is a graph showing frequency response functions where inputs are vibrations of the motor, and outputs are vibrations of the photoreceptor drum. In the figure, curve C 1  represents the frequency response function for the drive system of the present embodiment, curve C 2  represents the frequency response function for a conventional drive system with passive vibration control, and curve C 3  represents the frequency response function for a drive system without vibration control. 
     In the case of the drive system without vibration control, the level of vibration transmission for the drive system peaks at the resonant frequency, as indicated by curve C 3 . For the conventional drive system with passive vibration control, for example, the resonant frequency of the drive system is shifted to the lower side of the frequency, and the level of vibration transmission is reduced, as indicated by curve C 2 . Accordingly, upon input of vibration that is not expected by design, the passive vibration control, in some cases, might not be able to suppress the vibration completely. On the other hand, as for the drive system of the present embodiment indicated by curve C 1 , the control circuit  28  reads information about non-uniform rotation of the photoreceptor drum  110  from an output signal of the encoder  25 . Through the piezoelectric element  27 , the control circuit  28  provides the transmission mechanism  22  with a rotational variation in opposite phase to the non-uniform rotation according to the obtained information. As a result, the image forming apparatus can appropriately suppress vibration within a wide range of frequencies. 
     Furthermore, for the present drive system, vibration control is performed by a simple configuration using the encoder  25 , the piezoelectric element  27 , and the control circuit  28 , which can contribute to cost reduction of the image forming apparatus. 
     Second Configuration Example of Drive System 
       FIG. 7  is an oblique view of a second configuration example of the drive system for the photoreceptor drum  110  shown in  FIG. 1 .  FIG. 8  is a block diagram illustrating the configuration of a substantial part of the drive system in  FIG. 7 . 
     The second configuration example shown in  FIG. 7  differs from the first configuration example shown in  FIG. 2  in that a hinge member  31 , a motor  32 , which is a typical example of a second drive source, and a control circuit  33  are provided in place of the hinge member  26 , the piezoelectric element  27 , and the control circuit  28 . Since there is no other difference between these configuration examples, elements in  FIG. 7  that correspond to those in  FIG. 2  are denoted by the same reference numerals, and any descriptions thereof will be omitted. 
     The hinge member  31  is a plate-like member having a predetermined shape. The hinge member  31  has a first through-hole  31   a  and a second through-hole  31   b  provided therein, the first through-hole  31   a  has inserted therein the shaft of the large-diameter gear  22   d  located on the downstream side, and the second through-hole  31   b  has inserted therein the shaft of the two-stage gear unit  22   c  located upstream from the large-diameter gear  22   d . The shaft of the large-diameter gear  22   d  is not fixed to the first through-hole  31   a , and the shaft of the two-stage gear unit  22   c  is not fixed to the second through-hole  31   b.    
     Here, the direction from the center of the through-hole  31   a  toward the center of the through-hole  31   b  is denoted by β. The hinge member  31  is toothed at the edge in direction β. The toothed edge will be referred to below as a rack gear  31   c.    
     The motor  32  is preferably an ultrasonic motor, assuming that the drive system receives high-frequency vibration. The motor  32  rotates its own rotating shaft in response to a control signal from the control circuit  33 . The rotating shaft has a gear  32   a  provided thereon. The gear  32   a  meshes with the rack gear  31   c . Here, for the dimensions and the number of teeth, the gear  32   a  and the rack gear  31   c  are designed to have values such that the photoreceptor drum  110  can be rotated faster or slower within the range of about 1 μm in the rotational direction. 
     With the above configuration, the hinge member  31  pivots clockwise or counterclockwise on the shaft of the large-diameter gear  22   d , as indicated by arrow δ, in synchronization with the forward or backward rotation of the motor  32 . This pivoting action instantaneously accelerates or decelerates the rotational speed of the photoreceptor drum  110  in the same manner as described in conjunction with the first configuration example. 
     The control circuit  33  is configured by a processor, RAM, etc. To suppress non-uniform rotation, the control circuit  33  generates a control signal in opposite phase to non-uniform rotation for a rotational cycle of the photoreceptor drum  110 , on the basis of an output signal from the encoder  25 , and the control circuit  33  outputs the generated signal to the motor  32 . 
     Operation and Effects of Second Configuration Example 
     In the present drive system, through the hinge member  31  and the motor  32 , the transmission mechanism  22  receives a rotational variation in opposite phase to non-uniform rotation for a rotational cycle of the photoreceptor drum  110 , in accordance with the control signal from the control circuit  33 . Thus, as in the first configuration example, the non-uniform rotation of the photoreceptor drum  110  can be cancelled out by the rotational variation caused to the transmission mechanism  22 , leading to suppression of the non-uniform rotation of the photoreceptor drum  110 . 
     Third Configuration Example of Drive System 
       FIG. 9  is a diagram illustrating a substantial part of a third configuration example of the drive system for the photoreceptor drum  110  shown in  FIG. 1 . The third configuration example shown in  FIG. 9  differs from the first configuration example shown in  FIG. 2  in that a transmission mechanism  41 , a piezoelectric element  42 , and a control circuit  43  are provided in place of the transmission mechanism  22 , the hinge member  26 , the piezoelectric element  27 , and the control circuit  28 . Since there is no other difference between these configuration examples, elements in  FIG. 9  that correspond to those in  FIG. 2  are denoted by the same reference numerals, and any descriptions thereof will be omitted. Note that  FIG. 9  shows a top view of the transmission mechanism  41  and the piezoelectric element  42 . 
     The transmission mechanism  41  differs from the transmission mechanism  22  in  FIG. 2  in structure, and includes a two-stage gear unit  41   a  and a large-diameter gear  41   b  in place of the two-stage gear unit  22   c  and the large-diameter gear  22   d  of the transmission mechanism  22 . 
     The two-stage gear unit  41   a  has an input gear and an output gear. The input gear and the output gear are provided coaxially with each other. The input gear meshes with the small-diameter gear  22   b  provided upstream therefrom, and is caused to rotate about the shaft of the two-stage gear unit  41   a  by a drive force transmitted from the small-diameter gear  22   b . On the other hand, the output gear is a helical gear that rotates about the shaft of the two-stage gear unit  41   a  at the same angular velocity as the input gear. The two-stage gear unit  41   a  thus configured is attached to, for example, the frame of the image forming apparatus so that it can be displaced in the direction of the rotating shaft. The amount of such displacement is about 5 μm. 
     The large-diameter gear  42   b  is a helical gear that meshes with the output gear of the two-stage gear unit  41   a  provided upstream therefrom and is caused to rotate about its own shaft by a drive force transmitted from the output gear. Here, the large-diameter gear  42   b  is attached to, for example, the frame of the image forming apparatus, such that, unlike the two-stage gear unit  41   a , it cannot be displaced in the direction of its own rotating shaft. 
     The piezoelectric element  42  is, for example, of a laminated type, and it extends and contracts in the direction of the lamination (indicated by arrow β in the figure) upon application of a voltage. The amount of extension/contraction of the piezoelectric element  42  is about 5 μm. The piezoelectric element  42  thus configured is preferably positioned as described below. The piezoelectric element  42  is fixed at one end in the direction of the lamination to, for example, the frame of the image forming apparatus. Moreover, the piezoelectric element  42  is fixed at the other end in the direction of the lamination to the two-stage gear unit  41   a . In addition, the piezoelectric element  42  is oriented so as to extend and contract in the direction of the rotating shaft of the two-stage gear unit  41   a  (the direction of arrow γ). 
     With the above configuration, the two-stage gear unit  41   a  vibrates in the direction of its own rotational shaft, in synchronization with the extension and contraction of the piezoelectric element  42 . Due to this vibration, the tooth of the output gear in the two-stage gear unit  41   a  located on the upstream side instantaneously pushes the tooth of the large-diameter gear  41   b  located downstream therefrom, in the rotational direction of the large-diameter gear  41   b , or it instantaneously comes out of contact therewith. Note that the displacement of the large-diameter gear  41   b  in the direction of the rotating shaft is restricted. Consequently, the foregoing action instantaneously accelerates or decelerates the rotational speed of the large-diameter gear  41   b , hence the rotational speed of the photoreceptor drum  110 . In this manner, in the third configuration example, as in the first configuration example, the rotational speed of the photoreceptor drum  110  is accelerated or decelerated instantaneously. 
     The control circuit  43  is configured by a processor, RAM, etc. To suppress non-uniform rotation, the control circuit  43  generates a control signal in opposite phase to non-uniform rotation for a rotational cycle of the photoreceptor drum  110 , on the basis of an output signal from the encoder  25 , and the control circuit  43  applies the generated signal to the piezoelectric element  42 . 
     Operation and Effects of Third Configuration Example 
     In the present drive system, through the piezoelectric element  42 , the transmission mechanism  41  receives a rotational variation in opposite phase to non-uniform rotation for a rotational cycle of the photoreceptor drum  110 , in accordance with the control signal from the control circuit  43 . Thus, as in the first configuration example, the non-uniform rotation of the photoreceptor drum  110  can be cancelled out by the rotational variation caused to the transmission mechanism  41 , leading to suppression of the non-uniform rotation of the photoreceptor drum  110 . 
     Fourth Configuration Example of Drive System 
       FIG. 10  is a diagram illustrating a substantial part of a fourth configuration example of the drive system for the photoreceptor drum  110  shown in  FIG. 1 . The fourth configuration example shown in  FIG. 10  differs from the first configuration example shown in  FIG. 2  in that a piezoelectric element  51  and a control circuit  52  are provided in place of the hinge member  26 , the piezoelectric element  27 , and the control circuit  28 . Since there is no other difference between these configuration examples, elements in  FIG. 10  that correspond to those in  FIG. 2  are denoted by the same reference numerals, and any descriptions thereof will be omitted. 
     The piezoelectric element  51  is, for example, of a laminated type, and it extends and contracts in the direction of the lamination (indicated by arrow β in the figure) upon application of a voltage. The amount of extension/contraction of the piezoelectric element  51  is about 5 μm. The piezoelectric element  51  thus configured is preferably positioned as described below. The piezoelectric element  51  is fixed at one end in the direction of the lamination to, for example, the frame of the image forming apparatus. Moreover, the piezoelectric element  51  is fixed at the other end in the direction of the lamination to an attachment plate  21   a  of the motor  21 . 
     With the above configuration, the attachment plate  21   a  pivots on the rotating shaft of the small-diameter gear  22   b  provided downstream from the gear  22   a , in synchronization with the extension and contraction of the piezoelectric element  51 . Due to this pivoting action, the tooth of the gear  22   a  located on the upstream side instantaneously pushes the tooth of the small-diameter gear  22   b  located downstream therefrom, in the rotational direction of the small-diameter gear  22   b , or it instantaneously comes out of contact therewith. The variation in speed of the small-diameter gear  22   b  due to such vibration is transmitted to the large-diameter gear  22   d , and further to the photoreceptor drum  110 . As a result, the rotational speed of the photoreceptor drum  110  is accelerated or decelerated instantaneously. In other words, in the fourth configuration example, the rotational speed of the photoreceptor drum  110  is accelerated or decelerated instantaneously, in the same manner as described in conjunction with the first configuration example. 
     The control circuit  52  is configured by a processor, RAM, etc. To suppress non-uniform rotation, the control circuit  52  generates a control signal in opposite phase to non-uniform rotation for a rotational cycle of the photoreceptor drum  110 , on the basis of an output signal from the encoder  25 , and the control circuit  52  applies the generated signal to the piezoelectric element  51 . 
     Operation and Effects of Fourth Configuration Example 
     In the present drive system, through the piezoelectric element  51 , the transmission mechanism  22  receives a rotational variation in opposite phase to non-uniform rotation for a rotational cycle of the photoreceptor drum  110 , in accordance with the control signal from the control circuit  52 . Thus, as in the first configuration example, the non-uniform rotation of the photoreceptor drum  110  can be cancelled out by the rotational variation caused to the transmission mechanism  22 , leading to suppression of the non-uniform rotation of the photoreceptor drum  110 . 
     Fifth Configuration Example of Drive System 
     In the fourth configuration example, the piezoelectric element  51  extends and contracts to vibrate the attachment plate  21   a  of the motor  21 . However, this is not restrictive, and the piezoelectric element  51  may vibrate a hinge member  61  fixed to the attachment plate  21   a , as shown in  FIG. 11 . In this case also, non-uniform rotation of the photoreceptor drum  110  can be suppressed, as in the fourth configuration example. 
     Supplementary 
     The foregoing has been described with respect to suppression of non-uniform rotation of the photoreceptor drum  110 , which is a typical example of the rotating member. However, a similar technical problem might occur to the intermediate transfer belt  14  with a toner image supported thereon. Accordingly, each of the above configuration examples may be provided to suppress non-uniform rotation of the intermediate transfer belt  14 , which is another example of the rotating member. 
     Although the present invention has been described in connection with the preferred embodiment above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the invention.