Patent Publication Number: US-8977167-B2

Title: Image forming apparatus

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus in which a mark formed on an image bearing member is detected to rotational speed control of the image bearing member. Specifically, the present invention relates to a control method capable of realizing stable control with high gain with respect to a wide frequency component of a speed fluctuation without being influenced by an accuracy of the mark. 
     The image forming apparatus in which a toner image carried on the image bearing member (photosensitive member or intermediary transfer member) is transferred onto a recording material and then the image is fixed on a recording material surface by heating and pressing the recording material has been widely used. When the speed fluctuation occurs, the speed fluctuation leads to image density non-uniformity due to sparse/dense of scanning lines and a lowering in accuracy of superposition of respective color images and therefore various proposals as to control of a driving motor for the image bearing member have been made. 
     In Japanese Laid-Open Patent Application (JP-A) 2006-160512, control such that scale-like equidistant marks are formed at edge portions of an intermediary transfer belt and are optically detected to obtain a speed of the intermediary transfer belt in real time and then the real-time speed is fed back to a rotational speed of a driving motor for the intermediary transfer belt is disclosed. However, in this case, a single sensor successively detects the plurality of marks as the same mark and therefore a degree of control of accuracy cannot be enhanced to the extent that the accuracy is not less than mark accuracy. When the intermediary transfer belt is partly elongated to increase a mark interval or a part of the marks is contaminated, a mark detection error occurs and a new speed fluctuation is added to the speed fluctuation of the intermediary transfer belt. 
     On the other hand, in JP-A 2008-276064, control such that the real-time speed of the intermediary transfer belt is obtained by measuring a time when the same mark passes through two sensors spaced with a distance with respect to a rotational direction of the intermediary transfer belt, and then the real-time speed is fed back to the rotational speed of the driving motor is disclosed. In this case, the two sensors detect the same mark and therefore variations or the like in positional accuracy, contamination, and mark interval of individual marks are prevented from influencing the control of the rotational speed. 
     However, a moving speed of a movable member calculated from a passing time of the mark between the two mark detecting means is an average speed of the movable member passing between the two mark detecting means. This average speed is calculated when the mark passes through the second mark detecting means and therefore represents a speed of the movable member at the time before the calculation by a predetermined time, so that detection delay occurs. 
     In the case where the above-described conventional feed-back control is effected by using this average speed, due to the detection delay, phase delay of an integrator occurs and therefore a strong control system cannot be established, so that a servo band cannot be increased. As a result, there arose a problem that control accuracy cannot be enhanced and thus even non-uniformity of a speed at a low frequency cannot be properly controlled. 
     For example, when the movable mark moves to an average speed of 200 mm/sec and a distance between the two mark detecting means is 10 mm, the average speed in a section of 10 mm obtained by the above-described calculating method is close to a speed at the time when the mark passes through a midpoint of 5 mm from each of the two mark detecting means. In this case, the detection delay corresponding to the passing time in about 5 mm occurs. In the case of the average speed of 200 mm/sec, the detection delay of about 25 msec occurs. 
     In a driving system of a certain movable member, when there is no detection delay and a control system is established so that the servo band is approximately 10 Hz, the control is sufficiently stable with a phase margin of about 50 degrees. When the above-described detection delay occurs in this system, the detection delay of 25 msec corresponds to the phase delay of 90 degrees at 10 Hz. Therefore, the phase margin is lost, so that phase inversion is caused at several Hz and thus the control becomes unstable. For that reason, the control system had to be established by narrowing the servo band to stabilize the control, thus sacrificing the control accuracy. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide an image forming apparatus capable of effecting driving speed control of a belt member with high accuracy. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising: a rotatable image bearing member provided with a mark; driving means for rotationally driving the image bearing member; first detecting means for detecting speed information of the image bearing member by detecting the mark moved in a predetermined first distance; second detecting means for detecting speed information of the image bearing member by detecting the mark moved in a second distance shorter than the predetermined first distance; a filter circuit for suppressing a high-frequency part of an output from the first detecting means and a low-frequency part of an output from the second detecting means; a calculating portion for calculating a speed of the image bearing member detected from an output from the filter circuit; and a controller for controlling the driving means so that the calculated speed of the image bearing member is a set speed. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a structure of an image forming apparatus. 
         FIG. 2  is an illustration of a mark detecting sensor. 
         FIG. 3  is a block diagram of intermediary transfer belt drive control in Embodiment 1. 
         FIG. 4  is an illustration of a detection signal of the mark detecting sensor. 
       Part (a) of  FIG. 5  is an illustration of phase delay, and (b) of  FIG. 5  is an illustration of a filter characteristic. 
         FIG. 6  is an illustration of a constitution of a motor designated value operation part. 
         FIG. 7  is an illustration of an effect of control in Embodiment 1. 
         FIG. 8  is a flow chart of control in Modified Embodiment of Embodiment 1. 
         FIG. 9  is a block diagram of intermediary transfer belt drive control in Embodiment 2. 
         FIG. 10  is a flow chart of control in Modified Embodiment of Embodiment 2. 
         FIG. 11  is a block diagram of intermediary transfer belt drive control in Embodiment 3. 
         FIG. 12  is a flow chart of control in Modified Embodiment of Embodiment 3. 
         FIG. 13  is a block diagram of intermediary transfer belt drive control in Embodiment 4. 
         FIG. 14  is an illustration of a mark detecting sensor in Embodiment 4. 
         FIG. 15  is a block diagram of intermediary transfer belt drive control in Embodiment 5. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinbelow, embodiments of the present invention will be described in detail with reference to the drawings. The image forming apparatus according to the present invention can also be carried out in other embodiments in which a part or all of constituents of the respective embodiments are replaced with their alternative constituents so long as a rotational speed of an image bearing member is controlled by using a sensor pair with a long sensor interval and a sensor pair with a short sensor interval. 
     Therefore, when the image forming apparatus uses a photosensitive member, an intermediary transfer member or a recording material conveying member, the present invention can be carried out irrespective of differences of a charging method, an exposure method, a developing method, a transfer method, a tandem/one drum type, an intermediary transfer/recording material conveyance/direct transfer type, and a monochromatic/full color image formation. A member subjected to drive control is not limited to an intermediary transfer belt but may also be rotatable members such as a photosensitive drum, a photosensitive belt and an intermediary transfer drum. 
     In the following embodiments, only a principal portion concerning formation/transfer of the toner image will be described but the present invention can be carried out in various uses including printers, various printing machines, copying machines, facsimile machines, multi-function machines, and so on by adding necessary equipment, options, or casing structures. 
     &lt;Image Forming Apparatus&gt; 
       FIG. 1  is a schematic view for illustrating a structure of an image forming apparatus. 
     As shown in  FIG. 1 , an image forming apparatus  100  is a tandem-type full-color printer of an intermediary transfer type in which four image forming stations (portions) Sa, Sb, Sc and Sd for yellow, magenta, cyan and black, respectively, are arranged along an intermediary transfer belt  31 . 
     In the image forming station Sa, a yellow toner image is formed on a photosensitive drum  11   a  and then is transferred onto the intermediary transfer belt  31 . In the image forming station Sb, a magenta toner image is formed on a photosensitive drum  11   b  and is transferred onto the intermediary transfer belt  31 . In the image forming stations Sc and Sd, a cyan toner image and a black toner image are formed on photosensitive drums  11   c  and  11   d , respectively, and are successively transferred onto the intermediary transfer belt  31 . 
     The four color toner images carried on the intermediary transfer belt  31  are conveyed to a secondary transfer portion T 2 , at which the toner images are collectively secondary-transferred onto a recording material P. 
     The recording material P picked up by a pick-up roller  22  from a recording material cassette  21  is separated one by one by a separating roller  23  and then is fed toward registration rollers  25 . 
     The registration rollers  25  sends the recording material P to the secondary transfer portion T 2  while timing the recording material P to the toner images on the intermediary transfer belt  31 . 
     In a process in which the recording material P and the toner images superposed thereon are nip-conveyed through the secondary transfer portion T 2 , a DC voltage is applied to a secondary transfer roller  37 , so that a full-color toner image is secondary-transferred from the intermediary transfer belt  31  onto the recording material P. Transfer residual toner remaining on the intermediary transfer belt  31  is collected by a belt cleaning device  38 . 
     The recording material P on which the toner images are secondary-transferred is curvature-separated from the intermediary transfer belt  31  and is sent into a fixing device  40  in which the recording material P is, after being subjected to heat and pressure application to fix the toner images on a surface thereof discharged on a discharge tray  48 . In the fixing device  40 , a pressing roller  42  is press-contacted to a fixing roller  41  provided with a heater  43  to form a heating nip. 
     The image forming stations Sa, Sb, Sc and Sd have the same constitution except that colors of the toners used in developing devices  14   a ,  14   b ,  14   c  and  14   d  are yellow, magenta, cyan and black, respectively, which are different from each other. For this reason, in the following, the image forming station Sa will be described and with respect to other image forming stations Sb, Sc and Sb, suffix a of reference numerals (symbols) for representing constituent elements (means) for the image forming station Sa is to be read as b, c and d, respectively, for explanation of associated ones of the constituent elements for the image forming stations Sb, Sc and Sd. 
     At the image forming station Sa, around the photosensitive drum  11   a , a corona charger  12   a , an exposure device  13   a , the developing device  14   a , a transfer roller  35   a  and a drum cleaning device  15   a  are disposed. The photosensitive drum  1   a  is prepared by forming a negatively chargeable photosensitive layer on an outer peripheral surface of an aluminum cylinder and is rotatably supported at end portions, and is driven by an unshown motor to rotate in an arrow R 1  direction. 
     The corona charger  12   a  charges the surface of the photosensitive drum  11   a  to a uniform negative potential. The exposure device  13   a  scans the surface of the photosensitive drum  11   a  with a laser beam obtained by ON/OFF modulation of a scanning signal (image data) obtained by developing a separated color image of yellow, so that an electrostatic image for a yellow image is written (formed) on the photosensitive drum  11   a.    
     The developing device  14   a  supplies the negatively charged toner to the photosensitive drum  11   a  to deposit the toner on an exposed portion of the electrostatic image, thus reversely developing the electrostatic image. 
     The primary transfer roller  35   a  is press-contacted to the intermediary transfer belt  31  toward the photosensitive drum  11   a  to form the primary transfer portion T 1  between the photosensitive drum  11   a  and the intermediary transfer belt  31 . In a process in which the toner image carried on the photosensitive drum  11   a  passes through the primary transfer portion T 1 , a positive DC voltage is applied to the primary transfer roller  35   a , so that the toner image is primary-transferred onto the intermediary transfer belt  31 . 
     The cleaning device  15   a  removes the transfer residual toner which passes through the primary transfer portion T 1  and remains on the surface of the photosensitive drum  11   a.    
     &lt;Intermediary Transfer Belt&gt; 
     The intermediary transfer belt  31  is extended around and supported by a driving roller  32 , a steering roller  33  also functioning as a tension roller, and a back-up roller  34 . The intermediary transfer belt  31  is driven by a motor  36  for rotating the driving roller  32  and is rotated in a direction of an indicated arrow R 2  at a process speed of 200 mm/sec. The motor  36  as an example of a driving means drives and rotates the intermediary transfer belt  31  as an example of the image bearing member. 
     The intermediary transfer belt  31  is formed in an endless form, of a polyimide (PI) resin material containing carbon black for imparting resistivity to the intermediary transfer belt  31 . The intermediary transfer belt  31  may also be formed of polyvinylidene fluoride (PVdF) or the like. 
     The steering roller  33  is tilt-controlled depending on a lateral shift position of the intermediary transfer belt  31 , thus positioning the intermediary transfer belt  31  with respect to a widthwise direction. The back-up roller  34  is connected to the ground potential and bends a circulatory path of the intermediary transfer belt  31  on a downstream side of the secondary transfer portion T 2 , so that the recording material P attached to the intermediary transfer belt  31  is curvature-separated from the intermediary transfer belt  31 . 
     The secondary transfer roller  37  is a rubber roller to which electroconductivity is imparted and is press-contacted to the intermediary transfer belt  31  supported by the back-up roller  34  to form the secondary transfer portion T 2  between the intermediary transfer belt  31  and the secondary transfer roller  37 . 
     A belt cleaning device  38  removes transfer residual toner which passed through a secondary transfer portion T 2  and remains on the intermediary transfer belt  31 . 
     Embodiment 1 
       FIG. 2  is an illustration of a mark detecting sensor.  FIG. 3  is a block diagram of intermediary transfer belt drive control in Embodiment 1.  FIG. 4  is an illustration of a detection signal of the mark detecting sensor. Part ( a ) of  FIG. 5  is an illustration of phase delay, and ( b ) of  FIG. 5  is an illustration of a filter characteristic.  FIG. 6  is an illustration of a constitution of a motor designated value operation part.  FIG. 7  is an illustration of an effect of control in Embodiment 1. 
     As shown in  FIG. 1 , in an image forming apparatus  1 , in order to suppress positional deviation by suppressing a fluctuation in rotational speed of the intermediary transfer belt  31 , a conveyance speed of the intermediary transfer belt  31  is detected to be corrected in real time. In order to detect the conveyance speed of the intermediary transfer belt  31 , scale-like marks  121  are provided on an inner peripheral surface of the intermediary transfer belt  131  so as to be continued over one full circumference. The marks  121  are formed in a scale-like shape along the rotational direction of the intermediary transfer belt  131 . An interval of the marks  121  is 84.6 μm as a scale of a resolution of 300 dpi. 
     As shown in  FIG. 3  with reference to  FIG. 2 , a first mark detecting sensor  122  and a second mark detecting sensor  123  which are an example of a sensor are provided with a distance with respect to the rotational direction of the intermediary transfer belt  131 . An A-phase light receiving portion  304 B and a B-phase light receiving portion  305 B which are an example of another sensor are provided with a distance shorter than that between the first mark detecting sensor  122  and the second mark detecting sensor  123 . An arrangement interval between the A-phase light receiving portion  304 B and the B-phase light receiving portion  305 B is shorter than the interval of the marks  121  adjacent to each other in the scale-like shape. T A-phase light receiving portion  304 B for effecting an output to a sensor section detection operation part  131  also functions as that for effecting an output to an AB-phase section detection operation part  132 . 
     The sensor section detection operation part  131  as an example of a first detecting means detects the same mark formed on the intermediary transfer belt  131  by using the first mark detecting sensor  122  and the second mark detecting sensor  123  to obtain a first detected speed  133  as an example of first speed information. The AB-phase section detection operation part  132  as an example of a second detecting means detects the same mark formed no the intermediary transfer belt  131  by using the A-phase light receiving portion  304 B and the B-phase light receiving portion  305 B to obtain a second detected speed  134  as an example of second speed information. 
     In order to detect the mark  121  on the intermediary transfer belt  131 , the two mark detecting sensors  122  and  123  are provided on a supporting member  124  along a movement direction of the intermediary transfer belt  131 . The first mark detecting sensor  122  is disposed upstream of the second mark detecting sensor  123  with respect to the rotational direction of the intermediary transfer belt  131  which rotates in the arrow R 2  direction while opposing the first mark detecting sensor  122 . The first mark detecting sensor  122  and the second mark detecting sensor  123  include light emitting portions  302 A and  302 B, respectively, for an LED and light receiving portions  303 A and  303 B, respectively, for a photodiode which are provided fixedly on casings  301 A and  301 B, respectively. 
     The A-phase light receiving portion  304 A of the first mark detecting sensor  122  and the A-phase light receiving portion  304 B of the second mark detecting sensor  123  are provided with an interval of 10 mm. 
     When the intermediary transfer belt  131  is conveyed (rotated), the marks  121  equidistantly arranged on the intermediary transfer belt  131  successively pass through opposing positions to the first mark detecting sensor  122  and the second mark detecting sensor  123 . At that time, an A-phase signal  124  is outputted from the A-phase light receiving portion  304 A of the first mark detecting sensor  122 , and an A-phase signal  126  is outputted from the A-phase light receiving portion  304 B of the second mark detecting sensor  123 . 
     As shown in  FIG. 4 , the A-phase signal  124  of the first mark detecting sensor  122  and the A-phase signal  126  of the second mark detecting sensor  123  are inputted into the sensor section detection operation part  131  with a delay time corresponding to the interval of 10 mm. The sensor section detection operation part  131  selects the A-phase signals  124  and  126  for the same mark and calculates the “delay time corresponding to the interval of 10 mm”. 
     As a method of discriminating the A-phase signals for the same mark, there is also a method in which an intrinsic characteristic signal pattern is outputted with respect to each of marks different in thickness or arrangement interval. In this embodiment, an intrinsic origin mark present with one full circumference is detected by the first mark detecting sensor  122  and the second mark detecting sensor  123 , so that the respective marks are numbered with the intrinsic origin mark as a starting point, thus being discriminated. 
     The sensor section detection operation part  131  calculates a first detected speed  133 , based on sensor section detection, from the A-phase signal  124  of the first mark detecting sensor  122  and the A-phase signal  126  of the second mark detecting sensor  123 . The sensor section detection operation part  131  detects passing times of the marks  121  from passing of the A-phase light receiving portion  304 A of the first mark detecting sensor  122  to passing of the A-phase light receiving portion  304 B of the second mark detecting sensor  123  successively as t 12 ( 1 ), t 12 ( 2 ), . . . , as shown in  FIG. 4 . The sensor section detection operation part  131  calculates the first detected speeds  133  as V 1 ( 1 )=10 (mm)/t 12 ( 1 ), V 1 ( 2 )=10 (mm)/t 12 ( 2 ), . . . , from the detected passing times. 
     In Embodiment 1, the conveyance speed of the intermediary transfer belt  131  is corrected by drive control and therefore a controller  200  generates a motor command signal by composite operation of the first detected speed  133  and a second detected speed  134 . A motor driver  160  drives a motor  36  on the basis of the generated motor command signal. The motor  36  rotates the driving roller  32  via a speed reducer  39 , so that the intermediary transfer belt  31  is rotated. 
     As shown in  FIG. 2 , the light receiving portion  303 B of the downstream second mark detecting sensor  123  includes the A-phase light receiving portion  304 B and the B-phase light receiving portion  305 B. The A-phase light receiving portion  304 B and the B-phase light receiving portion  305 B of the second mark detecting sensor  123  are provided with an interval of 21.15 μm. This is because when one period of an output signal of the second mark detecting sensor  123  for detecting the marks  121  with the interval of 84.6 μm is 360 degrees, a phase difference of 90 deg. is provided between the A-phase signal and the B-phase signal.
 
84.6(μ m )/(360 deg./90 deg.)=21.5(μ m )
 
     As shown in  FIG. 3 , the A-phase light receiving portion  304 B and the B-phase light receiving portion  305 B of the second mark detecting sensor  123  detect the mark  121  on the intermediary transfer belt  131  to output an A-phase signal  126  and a B-phase signal  127  which are different in rising timing of a pulse. 
     Simultaneously with input of the A-phase signals  124  and  126  of the first and second mark detecting sensors  122  and  123  into the sensor section detection operation part  131 , the A-phase signal  126  and the B-phase signal  127  of the second mark detecting sensor  123  are outputted into the AB-phase section detection operation part  132 . The AB-phase section detection operation part  132  calculates the second detected speed  134  from the A-phase signal  126  from the A-phase light receiving portion  304 B and the B-phase signal  127  from the B-phase light receiving portion  305 B. 
     The AB-phase section detection operation part  132  detects passing times of the marks  121  from passing of the A-phase light receiving portion  304 B of the second mark detecting sensor  123  to passing of the B-phase light receiving portion  305 B successively as tAB( 1 ), tAB( 2 ), . . . , as shown in  FIG. 4 . 
     The AB-phase section detection operation part  132  calculates the second detected speeds  134  by the following equations.
 
 V 2(1)=21.15(μ m )/ tAB   (1)
 
 V 2(2)=21.15(μ m )/ tAB   (2)
 
 V 2(3)=21.15(μ m )/ tAB   (3)
 
     The controller  200  has an electric circuit as shown in  FIG. 3  and is provided on a control substrate (board)  90  shown in  FIG. 1 . The controller  200  includes a low-pass filter  141 , a high-pass filter  142 , a signal composite (synthesizing) operation part  143 , a target speed generator  151 , a speed deviation operation part  152  and a motor command value operation part  153 . 
     The controller  200  as an example of a control means feed backs the first detected speed  133  and the second detected speed  134 , thus controlling the motor  36 . The controller  200  feeds back the second detected speed  134  with a proportion higher than that of the first detected speed  133  with respect to a speed fluctuation of not less than a predetermined frequency of the intermediary transfer belt  131 . However, with respect to the speed fluctuation of less than the predetermined frequency, the controller feed backs the first detected speed  134  with a proportion higher than that of the second detected speed  134 . The predetermined frequency is, as described later in Comparative Embodiment, lower than a frequency of the speed fluctuation of the intermediary transfer belt  131  caused in the case where only the first detected speed  133  is fed back thereby to control the motor  36 . 
     The low-pass filter  141  attenuates, when the frequency at which the influence of the “delay time corresponding to the interval of 10 mm” appears is defined as a cut-off frequency, a signal component of not less than the cut-off frequency. The low-pass filter  141  extracts, from the first detected speed  133 , a signal in a low frequency range including a DC component less affected by the “delay time corresponding to the interval of 10 mm”. 
     The high-pass filter  142  has the same cut-off frequency as the low-pass filter  141  and attenuates a signal component of not more than the cut-off frequency. The high-pass filter  142  extracts, from the second detected speed  134 , a signal in a frequency range which includes a boundary frequency range of a servo band and in which the control is influenced by the “delay time corresponding to the interval of 10 mm”. 
     Specifically, with respect to the first detected speed  133  with the detection delay of 25 msec which is ½ of an average movement time of 50 msec due to the “delay time corresponding to the interval of 10 mm”, as shown in  FIG. 5 , a phase delay remarkably appears at a signal of 1 Hz or more. Therefore, as shown in (b) of  FIG. 5 , the low-pass filter  141  and the high-pass filter  142  which have the cut-off frequency of 1 Hz. 
     The signal composite operation part  143  generates a composite signal  144  by adding a low-pass filter passing signal  171  obtained by filter operation of the first detected speed  133  and a high-pass filter passing signal  172  obtained by filter operation of the second detected speed  134  in real time. Then, the composite signal  144  is increased in feed-back ratio of the first detected speed  133  more than the second detected speed  134  in the frequency range, lower than 1 Hz, including a DC component. Further, in the frequency range, higher than 1 Hz, including the boundary frequency of the servo band, the composite signal  144  is increased in feed-back ratio of the second detected speed  134  more than the first detected speed  133 . As a result, the composite signal  144  with substantially no detection delay due to the “delay time corresponding to the interval of 10 mm” is generated. 
     The speed deviation operation part  152  operates, in order to operate an increase/decrease of the motor command value from a difference between a current detected speed and a target speed, a deviation between the composite signal  144  and the target speed generated by the target speed generation or  151 , thus calculating a control operation input signal  155 . 
     As shown in  FIG. 6 , the motor command value operation part  153  performs operations of PI control and upper and lower limiting control to generate a motor driving command signal  156  to be inputted into a motor driver  160 . The motor driving command signal  156  is a PWM pulse signal, and the motor driver  160  drives, on the basis of this PWM pulse signal, the motor  36  which is a DC servo motor. 
     A PI control proportion operation part  181  calculates a PI control proportion operation value  191  by multiplying the control operation input signal  155  by a proportional parameter. A PI control integral operation part  182  calculates a PI control integral operation value  192  by subjecting to integral operation a value obtained by multiplying the control operation input signal  155  by an integral parameter. A PI control synthesizing portion  183  calculates a PI control composite value by adding the PI control proportion operation value  191  and the PI control integral operation value  192 . 
     A motor drive command value  194  is a PWM duty ratio and therefore has to be a value in a range from 0 to 1. Therefore, an upper and lower limiting controller  184  sets the motor drive command value  194  at 0 when the PI control composite value  193  is a negative value and sets the motor drive command value  194  at 1 when the PI control composite value  193  is a large value. Further, when the PI control composite value  193  is a value between 0 and 1, the motor drive command value  194  is made equal to the PI control composite value  193 . A PWM pulse signal generator  185  generates, as the motor drive command signal  156 , a pulse signal providing the PWM duty ratio of the motor drive command value  194 . 
     As shown in  FIG. 7 , the image forming apparatus  1  was actuated and then rising of the peripheral speed of the intermediary transfer belt  31  was measured. In the control in this embodiment, the intermediary transfer belt  31  is subjected to the speed control by using the composite signal  144  which is not substantially influenced by the detection delay due to the “delay time corresponding to the interval of 10 mm”. For this reason, similarly as in the case where the speed signal with no detection delay, the speed of the intermediary transfer belt  31  can be caused to converge to 200 mm/sec. 
     Comparative Embodiment 
     In Comparative Embodiment, the controller  200  generates the motor drive command signal  156  by effecting PI control such that only the first detected speed  133  is fed back to the rotational speed of the motor  36 . 
     As is understood from the calculating method of the passing times t 12 ( 1 ), t 12 ( 2 ), . . . , the first detected speed  133  is the average speed of the mark  121  passing between the A-phase light receiving portion  304 A of the first mark detecting sensor and the A-phase light receiving portion  304 B of the second mark detecting sensor. This average speed in the 10 mm section is close to the value of the speed of the mark  121  passing through the point of 5 mm which is ½ of the section (10 mm). 
     Further, the first detected speed  133  is operated and updated by using the A-phase signal  126  when the mark  121  passes through the second mark detecting sensor  123  and therefore is calculated with a delay, corresponding to a passing time of about 5 mm, from the above-described average speed. 
     This results in the detection delay of 25 msec when the speed of the intermediary transfer belt  31  is 200 mm/sec. Therefore, the first detected speed  133  has a phase delay characteristic due to the delay time as shown in  FIG. 5 . 
     In the case where the feed-back control is effected by using only the first detected speed  133 , due to the phase delay, phase delay of the integrator occurs and therefore a strong control system cannot be established, so that the servo band cannot be increased. 
     In the case of the image forming apparatus  1 , the intermediary transfer belt  31  is moved at the average speed of 200 mm/sec, and the distance between the first mark detecting sensor  122  and the second mark detecting sensor  123  is 10 mm. In this case, the average speed of the intermediary transfer belt  31  in the 10 mm section is close to the movement speed of the mark when the mark passes through the point of 5 mm which is ½ of the section. For this reason, a delay corresponding to a time when the intermediary transfer belt  31  is moved by 5 mm occurs. 
     The intermediary transfer belt  31  passes through the 10 mm section in 50 msec at the movement speed of 200 mm/sec and therefore the detection delay of about 25 msec occurs. 
     Assuming that a time constant of the driving system of the intermediary transfer belt  31  is 0.16 sec, in the case where the driving system is established so that the servo band is approximately 10 Hz by the PI control, when there is no detection delay, it is possible to provide the phase margin of about 50 deg. as shown in  FIG. 7 . When there is no detection delay, even at the servo band of about 10 Hz, the phase margin is about 50 deg. and the control is sufficiently stable. 
     However, in the case where the servo band is 10 Hz, 200 mm corresponds to 10×360 deg. and therefore 360 deg. corresponds to 20 mm, so that the detection delay of 5 mm (25 msec) corresponds to the phase delay of 90 deg. Therefore, the phase margin is lost, so that phase inversion occurs at several Hz and thus the control becomes unstable. When the above-described delay of 25 msec is added to this system, the phase delay of 90 deg. at 10 Hz is added, so that the phase margin is lost and thus the control becomes unstable. 
     As a result, as shown in  FIG. 7 , the control diverges. When there is no detection delay, the speed of the intermediary transfer belt  31  can be converged to 200 mm/sec. However, when the feed-back control is effected by using only the first detected speed  133  with the detection delay, the speed is largely increased and decreased to cause oscillation. 
     For that reason, in Comparative Embodiment, the servo band is designed by being narrowed to 1 Hz to be stabilized, so that accuracy of the speed control had to be sacrificed. 
     On the other hand, in Embodiment 1, as described above, by effecting the signal synthesizing method by the controller  200 , the influence of the detection delay is suppressed, so that the speed control accuracy is improved. As a result, as shown in  FIG. 7 , the control is converged substantially similarly as in the case where there is no detection delay. 
     According to the control in Embodiment 1, the single mark is detected and therefore even when there is a mark interval error, the control accuracy can be improved. Further, the second detected speed  134  is obtained by using the AB-phase section detection of the second mark detecting sensor  123  and therefore the influence of the mark interval error is eliminated, so that the control accuracy particularly in the low frequency range can be improved. 
     In addition, the detection delay influence of the section detection by the two mark detecting sensors is suppressed and therefore the servo band which is a frequency range of followable speed fluctuation can be increased. As a result, the rotational speed fluctuation of the intermediary transfer belt  31  can be suppressed to reduce a degree of the positional deviation during transfer, so that it is possible to prevent a lowering in image quality. 
     Incidentally, in Embodiment 1, the second detected speed  134  was calculated by the AB-phase section detection of the second mark detecting sensor  123 . However, the second detected speed B 4  may also be, as shown in  FIG. 4 , replaced with detected speeds V 3  which are shown below and are calculated from time intervals tAA( 1 ), tAA( 2 ), . . . of the marks  121  successively passing through the second mark detecting sensor  123 .
 
 V 3(1)=84.6(μ m )/ tAA   (1)
 
 V 3(2)=84.6(μ m )/ tAA   (2)
 
 V 3(3)=84.6(μ m )/ tAA   (3)
 
     However, it is difficult to dispose consecutive mark  121  at a constant interval of 84.6 μm and therefore an interval error of the marks  121  is included in the speed obtained by encoder detection. By passing the second detected speed  134  through the high-pass filter  142 , the mark interval error at a low frequency can be reduced. However, the mark interval error is not included in the detected speed by the above-described AB-phase section detection and therefore when the speed obtained by the AB-phase section detection is used, the control accuracy particularly at a low frequency can be improved. 
     Further, in Embodiment 1, the second detected speed  134  was detected by using the single second mark detecting sensor  123 . However, a plurality of second mark detecting sensors  123  may also be arranged along the rotational direction of the intermediary transfer belt  31  to obtain an average of outputs from the second mark detecting sensors  123 , so that an error with a short sensor interval may also be reduced. 
     Further, in Embodiment 1, the calculating method of the first detected speed  133  by the sensor section detection is described based on the passing times between the A phases of the two sensors but the first detected speed  133  may also be calculated from the passing times between the B phases of the two sensors. Similarly, the calculating method of the second detected speed  134  by the AB-phase section detection is described based on the time difference between the A and B phases of the second mark detecting sensor  123  but the second detected speed  134  may also be calculated from the time difference between the A and B phases of the first mark detecting sensor  122 . 
     Further, in Embodiment 1, an example in which the signal band separation is made by using the filters is shown but the filters may also be replaced with a circuit element capable of making a similar band separation, such as an integrating circuit described later. 
     Further, in Embodiment 1, the control operation by the motor command value operation part  153  is described as an example of the PT control but may also be effected by another feed-back control method such as simple P control or the like. 
     Further, in Embodiment 1, the passing speed is controlled by converting the data of the detected passing time into the passing speed but may also be controlled by using the data of the passing time as it is. 
     Modified Embodiment of Embodiment 1 
       FIG. 8  is a flow chart of control in Modified Embodiment of Embodiment 1. The electric circuit shown in  FIG. 3  may preferably be realized by using a program operation of a micro-computer circuit as a part thereof. In this case, the controller  200  includes the micro-computer circuit, a small number of dedicated integrated circuits and a program stored in a non-volatile memory and is provided on the control substrate  90  shown in  FIG. 1 . Processing for generating the motor drive command signal by the controller  200  is carried out by a CPU of the micro-computer circuit on the basis of the control program stored in the non-volatile memory provided on the control substrate  90 . 
     As shown in  FIG. 8  with reference to  FIG. 3 , the controller  200  discriminates whether or not a drive ON signal of the intermediary transfer belt  31  outputted from the CPU which manages the control of respective parts of the image forming apparatus  1  is effective (S 410 ). In the case where the ON signal is not effective (drive OFF) (NO of S 410 ), zero is stored as a PWM duty ratio D (S 510 ). However, in the case where the ON signal is effective (drive ON) (YES of S 410 ), the first detected speed  133  is calculated by the operation and then extracts and stores a calculated speed value V 1  (S 420 ). The passing time of the mark  121  from passing of the A-phase light receiving portion  304 A of the first mark detecting sensor  122  to passing of the A-phase light receiving portion  304 B of the second mark detecting sensor  123  is detected and then the first detected speed  133  is calculated from this passing time. The calculated value of the first detected speed  133  is extracted from the sensor section detection operation part  131  and is stored. 
     The controller  200  operates the second detected speed  134  and then extracts and stores the calculated speed value V 2  (S 430 ). The passing time of the mark  121  from passing of the A-phase light receiving portion  304 B of the second mark detecting sensor  123  to passing of the B-phase light receiving portion  305 B of the second mark detecting sensor  123  is detected and then the second detected speed  134  is calculated from this passing time. The calculated value of the second detected speed  134  is extracted from the AB-phase section detecting operation part  132  and is stored. 
     The controller  200  performs the operation of the low-pass filter by using, as an input, the speed value V 1  stored in the step S 420 , and then stores a calculated value Vf 1  (S 440 ). Further, the controller  200  performs the operation of the high-pass filter by using, as an input, the speed value V 2  stored in the step S 430 , and then stores a calculated value Vf 2  (S 450 ). 
     The controller  200  adds the value Vf 1  stored in the step S 440  and the value Vf 2  stored in the step S 450  and then stores a calculated value Vs (S 460 ). Further, the controller  200  operates a target speed at a subsequent sampling time and then stores a calculated value Vo (S 470 ). 
     The controller  200  subtracts the value Vs stored in the step S 460  from the speed value Vo stored in the step S 470  and then stores a calculated speed deviation Vd (S 480 ). Further, the PI control operation is performed by using, as an input, the speed deviation stored in the step S 480  (S 490 ). Then, the controller performs, after calculating a proportional operation value P by multiplying the speed deviation Vd by a proportional parameter, integral operation to calculate an integral operation value I. This integral operation is performed by accumulating values each obtained by multiplying the speed deviation Vd by the integral parameter and the sampling time. Then, the proportional operation value P and the integral operation value I are added to calculate a control value C. 
     The controller  200  stores zero as the PWM duty ratio D when the control value C calculated in the step S 490  is a negative value, and stores 1 as the PWM duty ratio D when the control value C is a value larger than 1. Further, when the control value C is a value between 0 and 1, the controller  200  stores the control value C as the PWM duty ratio D (S 500 ). 
     The controller  200  generates a pulse signal with the PWM duty ratio D stored in the step S 500  or the step S 510  (S 520 ). Thereafter, every sampling time of the controller  200 , the flow up to the steps  520  is periodically repeated to effect the drive control. 
     Embodiment 2 
       FIG. 9  is a block diagram of intermediary transfer belt drive control in Embodiment 2. In Embodiment 1, the method in which the frequency ranges of the detection signals are cut by using the filters and are synthesized to generate the speed signal from which the detection delay is eliminated was described. In Embodiment 2, a method in which a speed signal from which the detection delay is eliminated by integral operation of the first detected speed will be described. This is because the integral operation has a characteristic such that a signal amplification in a low frequency range and a signal attenuation in a high frequency range are realized. 
     As shown in  FIG. 9 , a control constitution in this embodiment is the same as that in Embodiment 1 except for an interior constitution of a controller  210 . Further, a part of the interior constitution of the controller  210  is also the same as that in Embodiment 1. For this reason, in  FIG. 9 , constituent elements (parts) which are the same as those in  FIG. 3  of Embodiment 1 are represented by the same reference numerals and will be omitted from redundant description. 
     The controller  210  is provided, as the electric circuit, on the control substrate  90 . The controller  210  includes the signal synthesizing operation part  143 , the integrating operation part  145 , the target speed generator  151 , speed deviation operation parts  152 A and  152 B, and the motor command value operation part  153 . 
     The speed deviation operation part  152 A of the controller  210  operates, in order to operate the fluctuation (increase/decrease) of the motor command value from the difference between the current detected speed and the target speed, the deviation between the first detected speed  133  obtained by the sensor section detection and the target speed generated by the target speed generator  151 . 
     The integrating operation part  145  uses the frequency, at which the detection delay influence appears, as a zero cross frequency as indicated by broken lines in (b) of  FIG. 5 , and amplitudes the signal component not more than the zero cross frequency and attenuates the signal component not less than the zero cross frequency. The integrating operation part  145  generates, from the above deviation, a deviation integration signal  146  by amplifying the low frequency range signal including the DC component with less detection delay influence and by attenuating a signal in a frequency range, influenced by the detection delay, including the boundary frequency range of the servo band. 
     As shown in (a) of  FIG. 5 , the first detected speed  133  has the detection delay of 25 msec and therefore as described in Embodiment 1, the phase delay remarkably appears with respect to the signal of not less than 1 Hz. For this reason, as shown in (b) of  FIG. 5 , in the integrating operation part  145 , the first detected speed  133  is subjected to integral operation and multiplication (×2.0×π×1 (Hz)) so that 1 Hz becomes the zero-cross frequency. As a result, the deviation integral signal  146  becomes a signal which is amplified in the frequency range lower than 1 Hz and is attenuated in the frequency range higher than 1 Hz. 
     The speed deviation operation part  152  generates a deviation signal  147  by operating the deviation between the second detected speed  134  obtained by the AB-phase section detection and the target speed generated by the target speed generator  151 . The signal synthesizing operation part  143  calculates the control operation input signal  155  by adding the deviation integral signal  146  and the deviation signal  147  in real time. 
     When the deviation integral signal  146  and the deviation signal  147  are added, their composite signal  144  is increased in ratio of the first detected speed  133  more than the second detected speed  134  in the frequency range, lower than 1 Hz, including a DC component. Further, in the frequency range, higher than 1 Hz, including the boundary frequency of the servo band, the composite signal  144  is increased in ratio of the second detected speed  134  more than the first detected speed  133 . As a result, the control operation input signal  155  with substantially no detection delay is generated. 
     As shown in  FIG. 6 , the motor command value operation part  153  performs operations of PI control and upper and lower limiting control to generate a motor driving command signal  156  to be inputted into a motor driver  160 . The motor driving command signal  156  is a PWM pulse signal, and the motor driver  160  drives) the motor  36  on the basis of this PWM pulse signal. 
     In the constitution of Embodiment 2, the integrator is used for the band separation and therefore a gain particularly in a low frequency range becomes large. Therefore, in the low frequency range, the speed control accuracy of the intermediary transfer belt  31  can be improved. 
     In Embodiment 2, an example in which the signal band separation is made by using the integral operation is shown but the band separation for the first detected speed  133  may also be made by using a control element having a similar function. 
     Modified Embodiment of Embodiment 2 
       FIG. 10  is a flow chart of control in Modified Embodiment of Embodiment 1. The electric circuit shown in  FIG. 9  includes the micro-computer circuit, a small number of dedicated integrated circuits and a program stored in a non-volatile memory and is provided on the control substrate  90  shown in  FIG. 1 . Processing for generating the motor drive command signal by the controller  210  is carried out by a CPU of the micro-computer circuit on the basis of the control program stored in the non-volatile memory provided on the control substrate  90 . 
     As shown in  FIG. 10  with reference to  FIG. 9 , the controller  210  discriminates whether or not a drive ON signal of the intermediary transfer belt  31  outputted from the CPU which manages the control of respective parts of the image forming apparatus  1  is effective (S 610 ). In the case where the ON signal is not effective (drive OFF) (NO of S 610 ), zero is stored as a PWM duty ratio D (S 710 ). However, in the case where the ON signal is effective (drive ON) (YES of S 610 ), the first detected speed  133  is detected and then extracts and stores a detected speed value V 1  (S 620 ). The passing time of the mark  121  from passing of the A-phase light receiving portion  304 A of the first mark detecting sensor  122  to passing of the A-phase light receiving portion  304 B of the second mark detecting sensor  123  is detected and then the first detected speed  133  is calculated from this passing time. The calculated value of the first detected speed  133  is extracted from the sensor section detection operation part  131  and is stored. 
     The controller  210  detects the second detected speed  134  and then extracts and stores the detected speed value V 2  (S 630 ). The passing time of the mark  121  from passing of the A-phase light receiving portion  304 B of the second mark detecting sensor  123  to passing of the B-phase light receiving portion  305 B of the second mark detecting sensor  123  is detected and then the second detected speed  134  is calculated from this passing time. The calculated value of the second detected speed  134  is extracted from the AB-phase section detecting operation part  132  and is stored. 
     The controller  210  operates a target speed at a subsequent sampling time and then stores a calculated value Vo (S 640 ). Then, the speed value V 1  stored in the step S 620  is subtracted from the stored speed value Vo and the thus calculated speed deviation Vd 1  is stored (S 650 ). Further, the controller  210  subtracts the step value V 2  stored in the step S 630  from the stored speed value Vo and then stores a calculated speed deviation Vd 2  (S 660 ). The controller  210  performs the integral operation of the speed deviation Vd 1  stored in the step S 650  and stores a calculated deviation integration value Xd (S 670 ). This integral operation is performed by accumulating values each obtained by multiplying the speed deviation Vd 1  by the integral parameter and the sampling time. 
     The controller  210  adds the speed deviation Vd 2  stored in the step S 660  and the deviation integration value Xd stored in the step S 670  and stores a calculated value S (S 680 ). Then, the PI control operation is performed by using the stored value S as an input (S 690 ). 
     Specifically, a proportional operation value P is calculated by multiplying the value S by a proportion parameter. Then, the value S is subjected to the integral operation to calculate an integral operation value I. This integral operation is performed by accumulating values each obtained by multiplying the value S by an integration parameter and a sampling time. Then, the proportional operation value P and the integral operation value I are added to calculate a control value C. 
     The controller  210  stores zero as the PWM duty ratio D when the control value C calculated in the step S 690  is a negative value, and stores 1 as the PWM duty ratio D when the control value C is a value larger than 1. Then, when the control value C is a value between 0 and 1, the controller  210  stores the control value C as the PWM duty ratio D (S 700 ). Then, the controller  210  generates a pulse signal with the PWM duty ratio D stored in the step S 700  or the step S 710  (S 720 ). Thereafter, every sampling time of the controller  210 , the flow up to the steps  720  is periodically repeated to effect the drive control of the intermediary transfer belt  31 . 
     Embodiment 3 
       FIG. 11  is a block diagram of intermediary transfer belt drive control in Embodiment 3. In Embodiment 3, another method in which a speed signal from which the detection delay is eliminated by integral operation of the first detected speed will be described. Embodiment 3 is also an example in which the detection delay is suppressed by synthesizing a closed-loop signal by using respective detection signals. 
     As shown in  FIG. 11 , a control constitution in this embodiment is the same as that in Embodiment 1 except for an interior constitution of a controller  220 . Further, a part of the interior constitution of the controller  220  is also the same as that in Embodiment 1. For this reason, in  FIG. 11 , constituent elements (parts) which are the same as those in  FIG. 3  of Embodiment 1 are represented by the same reference numerals and will be omitted from redundant description. 
     The controller  220  is provided, as the electric circuit, on the control substrate  90 . The controller  210  includes the signal synthesizing operation part  143 , the integrating operation part  145 , the target speed generator  151 , speed deviation operation part  152  the motor command value operation part  153 , and a sign inversion operation part  154 . 
     The speed deviation operation part  152  of the controller  220  operates, in order to operate the fluctuation (increase/decrease) of the motor command value from the difference between the current detected speed and the target speed, the deviation between the first detected speed  133  obtained by the sensor section detection and the target speed generated by the target speed generator  151 . 
     The integrating operation part  145  uses the frequency, at which the detection delay influence appears, as a zero cross frequency, and amplitudes the signal component not more than the zero cross frequency and attenuates the signal component not less than the zero cross frequency. The integrating operation part  145  generates, from the above deviation, a deviation integration signal  149  by amplifying the low frequency range signal including the DC component with less detection delay influence and by attenuating a signal in a frequency range, influenced by the detection delay, including the boundary frequency range of the servo band. 
     As described in Embodiment 2, with respect to the first detected speed  133  having the detection delay of 25 msec, as shown in (a) of  FIG. 5 , the phase delay remarkably appears with respect to the signal of not less than 1 Hz. Therefore, in the integrating operation part  145 , the first detected speed  133  is subjected to integral operation and multiplication (×2.0×π×1 (Hz)) so that 1 Hz becomes the zero cross frequency. As a result, the deviation integral signal  149  becomes a deviation integration signal  149  which is amplified in the frequency range lower than 1 Hz and is attenuated in the frequency range higher than 1 Hz. 
     This deviation integration signal  149  is treated as a target value of a closed loop of the second detected speed  134  obtained by the AB-phase section detection. For sign inversion operation part  154  inverts the sign of a signal for the second detected speed  134  obtained by the AB-phase section detection so that the sign (positive/negative) of the difference between the current detected speed and the target speed coincides with the sign of the fluctuation of the motor command value. The signal synthesizing operation part  143  calculates the control operation output signal  155  by synthesizing the sign-inverted signal and the deviation integration signal  149 . The second detected speed  134  is multiplied by −1 in the sign inversion operation part  154  and is synthesized with the deviation integration signal  149  in the signal synthesizing operation part  134 . The calculated control operation signal  155  is increased in ratio of the first detected speed  133  more than the second detected speed  134  in the frequency range, lower than 1 Hz, including a DC component. Further, in the frequency range, higher than 1 Hz, including the boundary frequency of the servo band, the composite signal  144  is increased in ratio of the second detected speed  134  more than the first detected speed  133 . As a result, the control operation input signal  155  with substantially no detection delay is generated. 
     As shown in  FIG. 6 , the motor command value operation part  153  performs operations of PI control and upper and lower limiting control to generate a motor driving command signal  156  to be inputted into a motor driver  160 . The motor driving command signal  156  is a PWM pulse signal, and the motor driver  160  drives) the motor  36  on the basis of this PWM pulse signal. 
     In the constitution of Embodiment 3, the integrator is used for the band separation and therefore a gain particularly in a low frequency range becomes large. Therefore, in the low frequency range, the speed control accuracy of the intermediary transfer belt  31  can be improved. 
     In Embodiment 3, an example in which the signal band separation is made by using the integral operation is shown but the band separation for the first detected speed  133  may also be made by using a control element having a similar function. 
     Modified Embodiment of Embodiment 3 
       FIG. 12  is a flow chart of control in Modified Embodiment of Embodiment 1. The electric circuit shown in  FIG. 11  includes the micro-computer circuit, a small number of dedicated integrated circuits and a program stored in a non-volatile memory and is provided on the control substrate  90  shown in  FIG. 1 . Processing for generating the motor drive command signal by the controller  220  is carried out by a CPU of the micro-computer circuit on the basis of the control program stored in the non-volatile memory provided on the control substrate  90 . 
     As shown in  FIG. 12  with reference to  FIG. 11 , the controller  220  discriminates whether or not a drive ON signal of the intermediary transfer belt  31  outputted from the CPU which manages the control of respective parts of the image forming apparatus  1  is effective (S 810 ). In the case where the ON signal is not effective (drive OFF) (NO of S 810 ), zero is stored as a PWM duty ratio D (S 910 ). However, in the case where the ON signal is effective (drive ON) (YES of S 810 ), the first detected speed  133  is detected and then extracts and stores a detected speed value V 1  (S 820 ). The passing time of the mark  121  from passing of the A-phase light receiving portion  304 A of the first mark detecting sensor  122  to passing of the A-phase light receiving portion  304 B of the second mark detecting sensor  123  is detected and then the first detected speed  133  is calculated from this passing time. The calculated value of the first detected speed  133  is extracted from the sensor section detection operation part  131  and is stored. 
     The controller  220  detects the second detected speed  134  and then extracts and stores the detected speed value V 2  (S 830 ). The passing time of the mark  121  from passing of the A-phase light receiving portion  304 B of the second mark detecting sensor  123  to passing of the B-phase light receiving portion  305 B of the second mark detecting sensor  123  is detected and then the second detected speed  134  is calculated from this passing time. The calculated value of the second detected speed  134  is extracted from the AB-phase section detecting operation part  132  and is stored. 
     The controller  220  operates a target speed at a subsequent sampling time and then stores a calculated value Vo (S 840 ). Then, the speed value V 1  stored in the step S 620  is subtracted from the stored speed value Vo to obtain a speed deviation Vd 1 , which is stored (S 850 ). The controller  220  performs the integral operation of the speed deviation Vd 1  stored in the step S 850  and stores a calculated deviation integration value Xd (S 860 ). This integral operation is performed by accumulating values each obtained by multiplying the speed deviation Vd 1  by the integral parameter and the sampling time. 
     The controller  200  multiplies the speed value V 2  stored in the step S 830  by −1 and then stores a calculated value Vm (S 870 ). Then, the controller  220  adds the speed deviation integration value Xd stored in the step S 860  and the value Vm stored in the step S 870  and stores a calculated value S (S 880 ). Then, the controller  220  performs the PI control operation by using the value S stored in the step S 880  as an input (S 890 ). First, a proportional operation value P is calculated by multiplying the value S by a proportion parameter. Then, the value S is subjected to the integral operation to calculate an integral operation value I. This integral operation is performed by accumulating values each obtained by multiplying the value S by an integration parameter and a sampling time. Then, the proportional operation value P and the integral operation value I are added to calculate a control value C. 
     The controller  220  stores zero as the PWM duty ratio D when the control value C calculated in the step S 890  is a negative value, and stores 1 as the PWM duty ratio D when the control value C is a value larger than 1. Then, when the control value C is a value between 0 and 1, the controller  210  stores the control value C as the PWM duty ratio D (S 900 ). 
     The controller  220  generates a pulse signal with the PWM duty ratio D stored in the step S 900  or the step S 910  (S 920 ). Thereafter, every sampling time of the controller  220 , the flow up to the steps  920  is periodically repeated to effect the drive control of the intermediary transfer belt  31 . 
     Embodiment 4 
       FIG. 13  is a block diagram of intermediary transfer belt drive control in Embodiment 4. In Embodiments 1 to 3, the method in which the speed calculated by the AB-phase section detection by the second mark detecting sensor  123  was used as the second detected speed  134  was described. In Embodiment 4, a method in which a speed calculated by encoder detection by the second mark detecting sensor  123  is used as the second detected speed  134  will be described. That is, in Embodiments 1 to 3, the second mark detecting sensor had the constitution in which it included the plurality of light receiving portions and detected the belt movement speed from the mark passing time. On the other hand, in this embodiment, such a constitution that a single light receiving portion detects the belt movement speed from the mark passing time in the mark is employed. 
     As shown in  FIG. 14 , the control of Embodiment 4 is the same as that in Embodiment 1 except for an encoder detection operation part  135 . 
     As shown in  FIGS. 13 and 14 , the encoder detection operation part  135  as an example of a second detecting means detects a plurality of marks  121  formed on the intermediary transfer belt  31  by using the A-phase light receiving portion  304 B to obtain the second detected speed  134  as an example of second speed information. The A-phase light receiving portion  304 B for performing the output to the encoder detection operation part  135  also has the function of performing the output to the sensor section detection operation part  131 . 
     Simultaneously with input of the A-phase signals  124  and  126  of the first and second mark detecting sensors  122  and  123  into the sensor section detection operation part  131 , the A-phase signal  126  of the second mark detecting sensor  123  is outputted into the encoder detect ion operation part  135 . The encoder detection operation part  135  calculates the second detected speed  134  from the A-phase signal  126  from the A-phase light receiving portion  304 B. 
     The encoder detection operation part  135  detects passing time intervals of the marks  121  with the interval of 84.6 mm successively passing through the A-phase light receiving portion  304 B of the second mark detecting sensor  123  as tAA( 1 ), tAA( 2 ), . . . , as shown in  FIG. 4 . 
     The encoder detection operation part  135  calculates the second detected speeds  134  by the following equations.
 
 V 3(1)=84.6(μ m )/ tAA   (1)
 
 V 3(2)=84.6(μ m )/ tAA   (2)
 
 V 3(3)=84.6(μ m )/ tAA   (3)
 
     The controller  200  as an example of a control means feed backs the first detected speed  133  and the second detected speed  134 , thus controlling the motor  36 . 
     According to the control in Embodiment 4, as the second detected speed  134 , a speed obtained by passing the encoder detection value of the second mark detecting sensor  123  through the high-pass filter  142  and therefore the influence of the mark interval error can be made small. 
     In addition, the detection delay influence of the section detection by the two mark detecting sensors is suppressed and therefore the servo band which is a frequency range of followable speed fluctuation can be increased. As a result, the rotational speed fluctuation of the intermediary transfer belt  31  can be suppressed to reduce a degree of the positional deviation during transfer, so that it is possible to prevent a lowering in image quality. 
     Further, in Embodiment 4, the second detected speed  134  was detected by using the single second mark detecting sensor  123 . However, a plurality of second mark detecting sensors  123  may also be arranged along the rotational direction of the intermediary transfer belt  31  to obtain an average of outputs from the second mark detecting sensors  123 , so that an error variation in interval may also be reduced. 
     Further, in Embodiment 4, the calculating method of the second detected speed  134  by the encoder detection is described based on the time interval of the A-phase signal of the second mark detecting sensor  123  but the second detected speed  134  may also be calculated from the time interval of the B-phase signal of the second mark detecting sensor  123 , the time interval of the A-phase signal of the first mark detecting sensor  122 , or the time interval of the B-phase signal of the first mark detecting sensor  122 . 
     Further, in Embodiment 4, the control constitution is described based on the controller  200  which is the same as that in Embodiment 1 but the controller  200  may also be replaced with the controller  210  which is the same as that in Embodiment 2 or the controller  220  which is the same as that in Embodiment 3. 
     Embodiment 5 
       FIG. 15  is a block diagram of intermediary transfer belt drive control in Embodiment 5. In Embodiments 1 to 3, the method in which the speed calculated by the AB-phase section detection by the second mark detecting sensor  123  was used as the second detected speed  134  was described. In Embodiment 5, a method in which a speed calculated by encoder detection by a rotary encoder  128  is used as the second detected speed  134  will be described. 
     As shown in  FIG. 15 , the constitution of Embodiment 5 is the same as those in Embodiments 1 and 4 except for the rotary encoder  128  and an encoder detection operation part  136 . For this reason, in  FIG. 15 , constituent elements (means) identical to those in  FIG. 3  are represented by the same reference numerals (symbols) and will be omitted from redundant description. 
     As shown in  FIG. 15 , the rotary encoder  128  is mounted on the rotation shaft of the driving roller  32  for rotating the intermediary transfer belt  31 . The rotary encoder  128  outputs a pulse signal  129  depending on an angle of rotation of the rotation shaft. The rotary encoder  128  outputs, when the rotation shaft is rotated, e.g., one turn, 600 pulse signals  129 . 
     The rotary encoder  128  may only be required to be mounted on the rotation shaft which rotates in interrelation with the rotation of the intermediary transfer belt  31  and may also be mounted on a shaft of any one of the steering roller  33 , the back-up roller  34  and the motor  36  for rotating the driving roller  32 . 
     The rotary encoder detection operation part  136  calculates the second detected speed  134  from the pulse signal  129  from the rotary encoder  128 . 
     The rotary encoder detection operation part  136  successively detects time intervals f pulse edges of the pulse signals  129  as tRe( 1 ), tRE( 2 ), . . . , respectively. 
     The rotary encoder detection operation part  136  calculates the second detected speed  134 , according to equations shown below, based on the number of pulses per one full circumference of the driving roller  32  and a feeding radius of the intermediary transfer belt (the sum of the radius of the driving roller  32  and ½ of the thickness of the intermediary transfer belt  31 ) of, e.g., 15 mm.
 
 V 4(1)=2×π×15( mm )/600/ tRE   (1)
 
 V 4(2)=2×π×15( mm )/600/ tRE   (2)
 
 V 4(3)=2×π×15( mm )/600/ tRE   (3)
 
     The controller  200  as an example of a control means feed backs the first detected speed  133  and the second detected speed  134 , thus controlling the motor  36 . 
     According to the control in Embodiment 5, as the second detected speed  134 , a speed obtained by passing the encoder detection value of the rotary encoder  128  through the high-pass filter  142  and therefore the influence of the detection error in a low frequency range of the rotary encoder  128  can be made small. 
     In addition, the detection delay influence of the section detection by the two mark detecting sensors is suppressed and therefore the servo band which is a frequency range of followable speed fluctuation can be increased. As a result, the rotational speed fluctuation of the intermediary transfer belt  31  can be suppressed to reduce a degree of the positional deviation during transfer, so that it is possible to prevent a lowering in image quality. 
     In Embodiment 5, the control constitution is described based on the controller  200  which is the same as that in Embodiment 1 but the controller  200  may also be replaced with the controller  210  which is the same as that in Embodiment 2 or the controller  220  which is the same as that in Embodiment 3. 
     The embodiments of the present invention are described above but the present invention can also be carried out by employing the same constitutions as those in Embodiments 1 to 5 with respect to not only the intermediary transfer belt  31  but also the photosensitive drums  11   a ,  11   b ,  11   c  and  11   d . Further, in the same constitutions as those in Embodiments 1 to 5, the present invention can also be carried out with respect to speed control of other image bearing members such as a photosensitive belt an intermediary transfer drum, a recording material conveying belt and a recording material conveying drum. As a result, it is possible to realize an image forming apparatus capable of outputting a high quality image by enhancing followability of speed control with respect to a speed fluctuation at a higher frequency without impairing the speed control accuracy with respect to the speed fluctuation at the normal frequency for the recording material conveying belt or the like. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 093903/2011 filed Apr. 20, 2011, which is hereby incorporated by reference.