Patent Publication Number: US-7907880-B2

Title: Image forming apparatus with a rotating body controlled in a feedback manner and image forming method using a rotating body controlled in a feedback manner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 11/294,452, filed Dec. 6, 2005, and is based on and claims the benefit of priority from the prior Japanese Patent Application No. 2004-374375, filed on Dec. 24, 2004, the entire disclosures of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus, such as a laser printer, and an image forming method thereof. 
     BACKGROUND 
     For example, a tandem color laser printer is provided with a sheet conveyor belt or an intermediate transfer belt. A color laser printer provided with the sheet conveyor belt adopts a so-called direct transfer method. In this direct transfer method, during conveyance of a sheet by the sheet conveyor belt, toner images of respective colors of yellow, magenta, cyan and black are sequentially superimposed on and transferred to the sheet. Meanwhile, a color laser printer provided with the intermediate transfer belt adopts a so-called indirect transfer method. In this indirect transfer method, after toner images of respective colors of yellow, magenta, cyan and black are sequentially superimposed on and transferred to the intermediate transfer belt, a toner image on the intermediate transfer belt is transferred to a sheet at a time. 
     During the transfer of a toner image of each color, driving of the belts is controlled in a feedback manner such that the speed (traveling speed) of the sheet conveyor belt or the intermediate transfer belt is detected, and the speed of both belts is maintained at constant speed on the basis of the detected speed. If there is irregularity in the speed of the belts, deviation is caused in the transfer position of a toner image of each color on the sheet or the intermediate transfer belt. Therefore, the feedback control requires high precision. 
     As a technique of detecting the speed of each belt, for example, it is considered that an encoder is attached to a belt-supporting roller, the rotational speed of the roller is obtained from output pulses from the encoder, and the speed of the belt is calculated (estimated) on the basis of the obtained rotational speed. However, because the belt is an elastic body, the speed of the belt changes due to micro-vibration caused during traveling of the belt, even when the roller rotates at constant rotational speed. Accordingly, since the speed of the belt calculated from the rotational speed of the roller is not necessarily equal to an actual speed of the belt, the rotational speed cannot be used to control driving of the belt. 
     Thus, for example, JP-A-2004-198624 suggests providing an intermediate transfer belt with a scale in which a number of scale slits are formed at regular intervals, providing a sensor that outputs signals in response to the detection of the scale slits, at a position where the scale can be read, and calculating the speed of the intermediate transfer belt on the basis of the interval (interval from a previous output signal to the next output signal) of output signals from the sensor during driving of the intermediate transfer belt. 
     SUMMARY 
     In considering the expansion and contraction caused by the elasticity of the intermediate transfer belt, the scale should be provided so that its joints (when one scale is wound along the surface of the intermediate transfer belt, the joints are mutually butting opposite ends of the scale, and when a plurality of scales are provided in series, the joints are scales adjacent to each other in a direction that the scales are arrayed) do not overlap each other. However, if such joints exist, when portions of the joints become target positions to be detected by the sensor, the interval of output signals of the sensor becomes long and consequently the speed of the intermediate transfer belt that is lower than the actual speed is detected. 
       FIG. 15  is a graph (the abscissa represents time and the ordinate represents the output time interval of signals from a sensor) showing changes in the interval of output signals of the sensor. If a joint exists in the scale, as shown in the graph, when portions other than the joint become target positions to be detected by the sensor, signals are output from the sensor about every 4.25 msec. However, when the portions of the joint become the target positions to be detected by the sensor (time T), the next signal is output after about 5.3 msec from when a signal is output from the sensor immediately before the time T. As described above, if the interval of output signals of the sensor becomes long and consequently the sensor detects the speed of the intermediate transfer belt which is lower than an actual speed, the rotational speed of a motor that drives the intermediate transfer belt is increased by feedback control. As a result, great irregularity may be caused in the speed of the intermediate transfer belt in front of or behind the positions. 
     Thus, JP-A-2004-198624 suggests determining that, if output signals of the sensor do not change over a predetermined time, a target position to be detected by the sensor is a joint of the scale, and controlling the driving of the intermediate transfer belt in a feedback manner, by using the speed of the intermediate transfer belt that has been just previously detected. However, an immediate value of the speed of the intermediate transfer belt which has been just previously detected is not necessarily detected precisely, but it is often incorrectly detected by influence of noises, which may also result in feedback control that may cause irregularity in the speed of the intermediate transfer belt. 
     The present invention has been made in view of the above circumstances and provides an image forming apparatus and an image forming thereof, which can stably rotate a rotating body, such as belts. 
     According to at least some example aspects of the invention, an image forming apparatus includes a rotating body rotating integrally with a plurality of marks provided at intervals with one another; a sensor that outputs pulses whenever each mark is detected; an actual interval measuring unit that measures an actual interval that is an output interval of the pulses from the sensor; a selecting unit that, if a current actual interval measured by the actual interval measuring unit is within a predetermined normal range, selects the current actual interval as a feedback amount, and that, if a current actual interval measured by the actual interval measuring unit is out of the predetermined normal range, selects a mean value of a plurality of actual intervals measured in the past by the actual interval measuring unit instead of the current actual interval, as a feedback amount; and a control unit that compares a target interval that is a target value of the output interval of the pulses from the sensor with the feedback amount selected by the selecting unit to control rotation of the rotating body in a feedback manner so that a deviation between the target interval and the feedback amount becomes zero. 
     In the above aspect of the invention, the image forming apparatus further includes a storage unit that stores a plurality of actual intervals measured by the actual interval measuring unit during the past predetermined period; and a mean value calculating unit that calculates a mean value of the plurality of actual intervals stored in the storage unit. 
     In the above aspect of the invention, the mean value of the plurality of actual intervals is a mean value of a plurality of actual intervals within the normal range measured in the past by the actual interval measuring unit. 
     In the above aspect of the invention, the mean value of the plurality of actual intervals is a mean value of a plurality of actual intervals measured by the actual interval measuring unit, during a period from when an actual interval out of the normal range is measured by the actual interval measuring unit to when another actual interval out of the normal range is measured next by the actual interval measuring unit. 
     According to an another aspect of the invention, an image forming apparatus includes a rotating body rotating integrally with a plurality of marks provided at intervals with one another; a sensor that outputs pulses whenever each mark is detected; an actual interval measuring unit that measures an actual interval that is an output interval of the pulses from the sensor; a selecting unit that, if a current actual interval measured by the actual interval measuring unit is within a predetermined normal range, selects the current actual interval as a feedback amount, and that, if a current actual interval measured by the actual interval measuring unit is out of the normal range, selects a target interval that is a target value of the output interval of the pulses from the sensor, instead of the current actual interval, as a feedback amount; and a control unit that compares the target interval with the feedback amount selected by the selecting unit to control rotation of the rotating body in a feedback manner so that a deviation between the target interval and the feedback amount becomes zero. 
     In the above aspects of the invention, the normal range is set based on an actual interval measured by the actual interval measuring unit. According to this configuration, the normal range can be a range corresponding to characteristics of each image forming apparatus. 
     In the above aspects of the invention, the predetermined normal range is set to a range that is broader than an error range of the actual interval measured by the actual interval measuring unit. 
     In the above aspects of the invention, the predetermined normal range is set to a range obtained by multiplying the error range of the actual interval measured by the actual interval measuring unit by a predetermined factor. 
     In the above aspects of the invention, the rotating body is one that conveys a recording medium, and the image forming apparatus further includes a supplying unit that supplies a recording medium to the rotating body, and a supply control unit that controls supply start timing of a recording medium by the supplying unit so as to complete conveyance of the recording medium by the rotating body, during a period from when an actual interval out of the normal range is measured by the actual interval measuring unit to when another actual interval out the normal range is measured next by the actual interval measuring unit, if actual intervals measured by the actual interval measuring unit are periodically out of the predetermined normal range. 
     In the above aspects of the invention, the image forming apparatus further includes: a period detecting unit that, when actual intervals measured by the actual interval measuring unit are periodically out of the predetermined normal range, detects the period, and an elapsed time measuring unit that measures an elapsed time after an actual interval out of the normal range is measured by the actual interval measuring unit. If the remaining time obtained by subtracting the time taken from the start of the supply of a recording medium by the supplying unit to the completion of conveyance of the recording medium by the rotating body, from the period detected by the period detecting unit, is longer than the elapsed time measured by the elapsed time measuring unit, the supply control unit states supplying of a recording medium by the supplying unit. 
     In the above aspects of the invention, the image forming apparatus further includes a positional deviation amount calculating unit that calculates, as a positional deviation amount, a cumulative value of deviations between a predetermined basic target interval and actual intervals measured by the actual interval measuring unit, and a positional deviation compensating unit that set, as the target interval, a deviation between the basic target interval and a value obtained by multiplying the positional deviation amount calculated by the positional deviation amount calculating unit by a predetermined proportional gain. 
     In the above aspects of the invention, the rotating body is a conveyor belt that conveys a recording medium, and the apparatus further includes a plurality of image forming unit that are arranged in parallel along a direction in which the recording medium is conveyed by the conveyor belt so as to form images of the recording medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative example structures in accordance with the present invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a side sectional view showing an a color laser printer as an image forming apparatus according to an illustrative example of the invention; 
         FIG. 2  is a side view for explaining the configuration of a scanner unit shown in  FIG. 1 ; 
         FIG. 3  is a block diagram showing a control system for speed control of a conveyor belt shown in  FIG. 1 ; 
         FIG. 4  is a view for explaining the configuration of an encoder shown in  FIG. 3 ; 
         FIG. 5  is a flowchart showing the sequence of speed control of the conveyor belt shown in  FIG. 3 ; 
         FIG. 6  is a flowchart for explaining normal range determination processing; 
         FIG. 7  is a block diagram showing a configuration for setting a target speed; 
         FIG. 8  is a block diagram showing the configuration of a positional deviation amount calculating unit shown in  FIG. 7 ; 
         FIG. 9  is a block diagram showing the configuration of a positional deviation compensating unit shown in  FIG. 7 ; 
         FIG. 10  is a block diagram showing a control system for controlling feeding of a sheet onto the conveyor belt shown in  FIG. 1 ; 
         FIG. 11  is a flowchart showing the control sequence to be executed by a sheet feed control unit shown in  FIG. 10 ; 
         FIG. 12  is a block diagram showing another illustrative example of the invention (in which a mean speed is used) of the control system for speed control of the conveyor belt; 
         FIG. 13  is a flowchart showing the sequence of speed control of the conveyor belt by the control system shown in  FIG. 12 ; 
         FIG. 14  is a flowchart showing still another illustrative example of the invention (in which a feedback amount is determined based on a speed change rate) of the speed control of the conveyor belt by the control system shown in  FIG. 3 ; and 
         FIG. 15  is a graph showing changes in the interval of output signals of a sensor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a side sectional view showing a color laser printer serving as an image forming apparatus according to an example structure. The color laser printer  1  is a tandem color laser printer in which a plurality of process units  16  are arranged in tandem with each other in a horizontal direction. The color laser printer  1  includes, in a box-shaped main casing  2 , a sheet feeding part  4  that feeds a sheet  3  as a recording medium, an image forming part  5  that forms an image on the sheet  3  fed therein, and a sheet discharge part  6  that discharges the sheet  3  on which the image is formed. 
     The sheet feeding part  4  includes a sheet cassette  7  provided at the inner bottom of the main casing  2 , a sheet feeding roller  8  provided on the front upper side (in the following description, the left side in  FIG. 1  is referred to as the rear side and the right side as the front side) of the sheet cassette  7 , a U-shaped path  9  provided on the front upper side of the sheet feeding roller  8 , a pair of conveying rollers  10  provided in the midway of the U-shaped path  9 , and a pair of registration rollers  11  as a supplying unit. 
     A plurality of sheets  3  is stacked within the sheet cassette  7 , and the uppermost sheet  3  in the cassette is delivered to the U-shaped path  9  by the rotation of the sheet feeding roller  8 . The U-shaped path  9  is formed as a substantially U-shaped conveying path for the sheet  3  such that its upstream end is adjacent to the sheet feeding roller  8  on the lower side, and the sheet  3  is fed forward, and its downstream end is adjacent to a conveyor belt  49 , as will be described below, on the upper side, and the sheet  3  is discharged rearward. 
     Then, the sheet  3  delivered to the U-shaped path  9  is conveyed within the U-shaped path  9  by the conveying rollers  10 , and the sheet is discharged rearward by the registration rollers  11  after registration by the registration rollers  11 . The image forming part  5  includes the process units  12  serving as image forming unit, a scanner unit  13 , a transfer part  14 , and a fixing part  15 . 
     A process unit  12  is provided for each toner color of a plurality of toner colors. That is, the process units  12  include four process units, i.e., a yellow process unit  12 Y, a magenta process unit  12 M, a cyan process unit  26 C, and a black process unit  12 K. The process units  12  are sequentially arranged in parallel at intervals with one another from the front to the rear so as to overlap each other in the horizontal direction. 
     Each process unit  12  includes a photosensitive drum  16 , a scorotron charger  17 , and a developing cartridge  18 . 
     Each photosensitive drum  16  is formed in a cylindrical shape, and includes a drum body  19  whose uppermost surface layer is formed by a positively charged photosensitive layer made of polycarbonate, etc., and a drum shaft  20  extending along an axial direction of the drum body  19  on an axis of the drum body  19 . The drum body  19  is rotatably provided with respect to the drum shaft  20 , and the drum shaft  20  is non-rotatably supported by both side walls of the process unit  12  in the width direction (the direction orthogonal to the forward and rearward direction and the vertical direction; this is true of the rest). During image forming, the photosensitive drum  16  is driven to rotate in the same direction (clockwise in the figure) as a circulating direction A of the conveyor belt  49  at a position (image formation position) where the photosensitive drum  16  makes contact with the conveyor belt  49  (as will be described below). 
     The scorotron charger  17  is a positively charged scorotron charger which has wires or grids and causes corona discharge. Behind the photosensitive drum  16 , this scorotron charger is disposed to face the photosensitive drum  16  at a predetermined distance therefrom so as not to make contact with the photosensitive drum  16 . The developing cartridge  18  includes a developing roller  21 , a supply roller  22 , and a layer thickness regulating blade  23  within a casing thereof. 
     The developing roller  21  is disposed to face the photosensitive drum  16  in front of the photosensitive drum  16 , and is pressed against the photosensitive drum  16 . The developing roller  21  is made by covering a metallic roller shaft  24  with a roller part  25  formed of an elastic member, such as conductive rubber material. More specifically, the roller part  25  is formed with a two-layer structure of an elastic roller portion and a coat layer that covers the surface of the roller portion. The elastic roller portion is made of conductive rubber, which contains carbon particles, such as urethane rubber, silicone rubber, and ethylene-propylene-diene-terpolymer (EPDM) rubber. The coat layer is made of urethane rubber, urethane resin, polyimide resin or other materials as a main ingredient. Further, the roller shaft  24  is rotatably supported by both side walls of the process unit  12  in the width direction. 
     The supply roller  22  is disposed to face the developing roller  21  in front of the developing roller  21 , and is pressed against the developing roller  21 . The supply roller  22  is made by covering a metallic roller shaft  26  with a roller part  27  made of conductive sponge member. Further, the roller shaft  26  is rotatably supported by both side walls of the process unit  12  in the width direction. 
     The layer thickness regulating blade  23  is made of metallic leaf spring member, and its tip portion is provided with a pressing member having a semicircular section and made of insulative silicon rubber. Also, the layer thickness regulating blade  23  is supported by the casing of the developing cartridge  18  above the developing roller  21 , and the pressing member at the tip (lower end) is pressed against the roller part  25  of the developing roller  21  from the front upper side. 
     Further, an upper portion of the casing of the developing cartridge  18  is formed as a toner chamber which stores toner. The toner chamber stores toner for each color. Specifically, a positively charged nonmagnetic one-component polymerized toner having a yellow color is stored within a toner chamber of the yellow process unit  12 Y, a positively charged nonmagnetic one-component polymerized toner having a magenta color is stored within a toner chamber of the magenta process unit  12 M, a positively charged nonmagnetic one-component polymerized toner having a cyan color is stored within a toner chamber of the cyan process unit  12 C, and a positively charged nonmagnetic one-component polymerized toner having a black color is stored within a toner chamber of a black process unit  12   k.    
     More specifically, the toner of each color is a polymerized toner having substantially spherical particles obtained through polymerization. The polymerized toner has binder resin as the main ingredient, which is obtained through copolymerization of styrene-based monomers, such as styrene, and acryl-based monomers, such as acrylic acid, alkyl (C1-C4) acrylate, and alkyl (C1-C4) methacrylate, using a known polymerization method, such as suspension polymerization. A coloring agent, a charge control agent, and wax are combined with the polymerized toner to form toner base particles. An external additive is also added to the polymerized toner to improve flowability. 
     As the coloring agent, each coloring agent of yellow, magenta, cyan, and black is combined. As for the charge control agent, combined is a charge control resin obtained through copolymerization of ion-based monomers having an ionized functional group, such as ammonium salt, and monomers that can be copolymerized with ion-based monomers, such as styrene-based monomers and acryl-based monomers. As for the external additive, combined is inorganic powder, such as metallic oxide powder, carbonized powder, and metal salt powder. The metallic oxide powder includes silica, aluminum oxide, titanium oxide, strontium titanate, ceric oxide, and magnesium oxide. 
     In each process unit  12 , during the image forming operation, toner of each color stored in each toner chamber is supplied to the supply roller  22 , and the toner is supplied to the developing roller  21  by the rotation of the supply roller  22 . At this time, the toner is positively friction-charged between the supply roller  22  and the developing roller  21  to which a developing bias is applied. The toner supplied to the developing roller  21  goes in between the pressing member of the layer thickness regulating blade  23  and the developing roller  21  (roller part  25 ) along with the rotation of the developing roller  21 , and then the toner is regulated to a thin layer having uniform thickness and carried on the developing roller  21 . 
     Meanwhile, the scorotron charger  17  causes corona discharge by application of a charging bias so as to uniformly charge the surface of the photosensitive drum  16  positively. After the surface of the photosensitive drum  16  is uniformly charged positively by the scorotron charger  17  along with the rotation of the photosensitive drum  16 , the surface is exposed by high-speed scanning of laser light from the scanner unit  13  as will be described below. As a result, an electrostatic latent image corresponding to an image to be formed on the sheet  3  is formed on the surface. 
     If the photosensitive drum  16  rotates further, the toner carried on the surface of the developing roller  21  and charged positively, is then supplied to the electrostatic latent image formed on the surface of the photosensitive drum  16 , that is, an exposed portion of the uniformly positively charged surface of the photosensitive drum  16 , whose potential is lowered by the exposure with the laser light, when the toner faces and makes contact with the photosensitive drum  16  by the rotation of the developing roller  21 . As a result, the electrostatic latent image on the photosensitive drum  16  is visualized, and then a toner image for each color by reverse development is carried on the surface of the photosensitive drum  16 . 
     As shown in  FIG. 2 , the scanner unit  13  includes a polygon mirror  28 , a black scanning system  29 K and a cyan scanning system  29 C, which are provided behind the polygon mirror  28 , a magenta scanning system  29 M and a yellow scanning system  29 Y, which are provided in front of the polygon mirror  28 , an fθ lens  30  used in collaboration for the black scanning system  29 K and the cyan scanning system  29 C, and an fθ lens  31  used in collaboration for the magenta scanning system  29 M and the yellow scanning system  29 Y. 
     The polygon mirror  28  has a plurality of reflecting surfaces (for instance, six surfaces) at the sides, and is adapted to be rotated at high speed by a polygon motor  32  about the rotation axis extending in a vertical direction. 
     The black scanning system  29 K includes a laser emitting part (not shown), reflecting mirrors  33  and  34 , and a cylindrical lens  35 . In the black scanning system  29 K, a laser beam emitted from the laser emitting part, based on image data, is reflected by the polygon mirror  28 , and passes through the fθ lens  30 , and then is reflected by the reflecting mirrors  33  and  34  and passes through the cylindrical lens  35 , and thus is emitted toward the photosensitive drum  16  of the black process unit  12 K. 
     The cyan scanning system  29 C includes a laser emitting part (not shown), reflecting mirrors  36 ,  37  and  38 , and a cylindrical lens  39 . In the cyan scanning system  29 C, a laser beam emitted from the laser emitting part, based on image data, is reflected by the polygon mirror  28 , and passes through the fθ lens  30 , and then is reflected by the reflecting mirrors  36 ,  37  and  38  and passes through the cylindrical lens  39 , and thus is emitted toward the photosensitive drum  16  of the cyan process unit  12 C. 
     The magenta scanning system  29 M includes a laser emitting part (not shown), reflecting mirrors  40 ,  41  and  42 , and a cylindrical lens  43 . In the magenta scanning system  29 M, a laser beam emitted from the laser emitting part, based on image data, is reflected by the polygon mirror  28 , and passes through the fθ lens  31 , and then is reflected by the reflecting mirrors  40 ,  41  and  42  and passes through the cylindrical lens  43 , and thus is emitted to the photosensitive drum  16  of the magenta process unit  12 M. 
     The yellow scanning system  29 Y includes a laser emitting part (not shown), reflecting mirrors  44  and  45 , and a cylindrical lens  46 . In the yellow scanning system  29 Y, a laser beam emitted from the laser emitting part, based on image data, is reflected by the polygon mirror  28 , and passes through the fθ lens  31 , and then is reflected by the reflecting mirrors  44  and  45  and passes through the cylindrical lens  46 , and thus is emitted toward the photosensitive drum  16  of the yellow process unit  12 Y. 
     Referring to  FIG. 1 , the transfer part  14  is disposed above the sheet cassette  7  and in the forward and rearward direction below the process units  12  within the main casing  2 , and includes a driving roller  47 , a driven roller  48 , the conveyor belt  49  serving as a rotating body, a transfer roller  50 , and a belt cleaning device  51 . 
     The driving roller  47  is disposed at a height position where it does not overlap the photosensitive drum  16  of the black process unit  12 K in the horizontal direction behind the photosensitive drum  16  thereof. During image forming, the driving roller  47  is driven to rotate in a direction (counterclockwise in the figure) opposite to the direction of rotation of the photosensitive drum  16 . 
     The driven roller  48  is disposed at a height position where it does not overlap the photosensitive drum  16  of the yellow process unit  12 Y in the horizontal direction in front of the photosensitive drum  16  thereof. During rotational driving of the driving roller  47 , the driven roller  48  is driven to rotate in the same direction (counterclockwise in the figure) as the circulating direction A of the conveyor belt  49  at a portion where the driven roller  48  makes contact with the conveyor belt  49  as will be described below. 
     The conveyor belt  49  is an endless belt and is formed of conductive resin, such as polycarbonate and polyimide, in which conductive particles, for example, carbon particles, are dispersed. The conveyor belt  49  is wound between the driving roller  47  and the driven roller  48 . The conveyor belt  49  is disposed such that its wound outer contact surface faces and makes contact with all the photosensitive drums  16  of the process units  12 . 
     When the driving roller  47  is driven, the driven roller  48  is rotated accordingly. Then, the conveyor belt  49  is circulated between the driving roller  47  and the driven roller  48  in the direction indicated by the arrow “A” (counterclockwise in the figure) so as to rotate in the same direction as the photosensitive drum  16  of each process unit  12  at the contact surface where the conveyor belt faces and makes contact with the photosensitive drum  16  thereof. The transfer roller  50  is disposed inside the conveyor belt  49  wound between the driving roller  47  and the driven roller  48  so as to face the photosensitive drum  16  of each process unit  12  with the conveyor belt  49  interposed therebetween. The transfer roller  50  is made by covering a metallic roller shaft  52  with a roller part  53  formed of an elastic member, such as conductive rubber material. The roller shaft  52  is disposed so as to extend in the width direction and rotatably supported. The transfer roller  50  rotates in the same direction (clockwise in the figure) as the circulating direction A of the conveyor belt  49  about the roller shaft  52  as a fulcrum, at an image formation position where the transfer roller faces and makes contact with the conveyor belt  49 . During transfer, a transfer bias is applied to the transfer roller  50  by the roller shaft  52 . 
     The sheet  3 , supplied from the sheet feeding part  4 , is conveyed by the conveyor belt  49 , which is circulated by the driving roller  47  and the driven roller  48  so as to sequentially pass through image formation positions between the conveyor belt  49  and the photosensitive drums  16  of the process units  12  from the front toward the rear. While the sheet  3  is conveyed, toner images of respective colors formed on the photosensitive drums  16  of the process units  12  are sequentially transferred to the sheet  3 , thereby forming a color image on the sheet  3 . 
     Specifically, for example, when a yellow toner image carried on the surface of the photosensitive drum  16  of the yellow process unit  12 Y is transferred to the sheet  3 , a magenta toner image carried on the surface the photosensitive drum  16  of the magenta process unit  12 M is then superimposed on and transferred to the sheet  3  where the yellow toner image has already been transferred. In a similar manner, a cyan toner image carried on the surface of the cyan process unit  12 C and a black toner image carried on the surface of the black process unit  12 K are sequentially superimposed on and transferred to the sheet, thereby forming a color image on the sheet  3 . 
     In such color image formation, since the color laser printer  1  is a tandem printer in which a plurality of process units  12  are provided for the respective colors, the toner image of each color can be formed at almost the same speed as that for monochrome image formation, thereby achieving rapid color image formation. Thus, it is possible to form a color image while the printer is made small. 
     The belt cleaning device  51  is disposed in the vicinity of the driving roller  47  below the conveyor belt  49 . The belt cleaning device  51  includes a cleaning member  54  which is disposed in contact with the surface of the conveyor belt  49  to scrape off paper dust or toner adhered to the surface of the conveyor belt  49 , and a cleaning box  55  which collects and reserves the paper dust or toner scraped off by the cleaning member  54 . 
     The fixing part  15  is disposed behind the transfer part  14 . The fixing part  15  includes a heating roller  56  and a pressing roller  57 . 
     The heating roller  56  is made of a metal tube on the surface of which a release layer is formed, and includes a halogen lamp along its axial direction. The surface of the heating roller  56  is heated to a fixing temperature by the halogen lamp. Further, the pressing roller  57  is provided so as to press against the heating roller  56 . 
     The color image transferred onto the sheet  3  is then conveyed to the fixing part  15 , and is fixed on the sheet  3  by being heated and pressed while passing between the heating roller  56  and the pressing roller  69 . 
     The sheet discharge part  6  includes a sheet discharge path  58 , sheet discharge rollers  59 , and a sheet discharge tray  60 . The sheet discharge path  58  is formed as a conveying path for the sheet  3  such that its upstream end is adjacent to the fixing part  15  on the lower side, its downstream end is adjacent to the sheet discharge tray  60  on the upper side, and the sheet is discharged toward the upper side. 
     The sheet discharge rollers  59  are provided as a pair of rollers at the downstream end of the sheet discharge path  58 . The sheet discharge tray  60  is formed on the top surface of the main casing  2  as an inclined wall which is inclined downwardly from the front toward the rear. 
     The sheet conveyed from the fixing part  15  is discharged onto the sheet discharge tray  60  toward the front through the sheet discharge path  58  by the sheet discharge rollers  59 . 
       FIG. 3  is a block diagram showing a control system for speed control of the conveyor belt  49 . 
     The color laser printer  1  includes an encoder  61  which outputs pulse signals along with the circulation of the conveyor belt  49 , a motor  62  which generates a driving force for rotatingly driving the driving roller  47  (conveyor belt  49 ), a motor driver  63  for supplying a driving current to the motor  62 , and an ASIC  64  having a function to control the motor  62  so that the conveyor belt  49  is driven at constant speed by the motor driver  63  on the basis of the output signals from the encoder  61 . 
     As shown in  FIG. 4 , the encoder  61  includes a pattern member  67  having a linear encoder pattern on which a number of marks  66  having optical reflectance, different from the base member  65 , are formed on a strip-shaped base member  65  at equal intervals in the longitudinal direction, and a reflective sensor  70  serving as a sensor configured by a light-emitting element  68  and a light-receiving element  69 . 
     The pattern member  67  is wound around the surface of the conveyor belt  49  at one end of the conveyor belt  49  in the width direction. Both ends of the pattern member  67  are not connected to each other, and the pattern member  67  has a joint of minute width between its both ends. 
     The reflective sensor  70  is disposed such that the light from the light-emitting element  65  is radiated onto the pattern member  67 , and the light reflected by the pattern member  67  enters the light-receiving element  69 . Since the base member  65  and the marks  66  of the pattern member  67  are different in optical reflectance from each other, the level of the output signal from the reflective sensor  70  (light-receiving element  69 ) is switched to either a high level and a low level depending on whether the light from the light-emitting element  68  is reflected on the base member  65  or reflected on the marks  66 . Accordingly, during the circulation (driving) of the conveyor belt  49 , when the pattern member  67  is irradiated with the light from the light-emitting element  68 , the output signals from the reflective sensor  70  are switched alternately between a high level and a low level at timed intervals corresponding to the speed of the conveyor belt  49 . 
     Referring to  FIG. 3 , the ASIC  64  includes a CPU  71 , a register group  72  for storing various kinds of data, an encoder edge detecting unit  73  executed by a hardware configuration or executed in software through processing of programs by the CPU  71 , a speed calculating unit  74  serving as an actual interval measuring unit, a feedback amount selecting unit  75  serving as a selecting unit, a feedback control operating unit  76  serving as a control unit, and a pulse-width-modulation (PWM) generating unit  77 . 
     The output signals of the encoder  61  (reflective sensor  70 ) are input to the encoder edge detecting unit  73 . The encoder edge detecting unit  73  detects an edge (switching from a high level to a low level, or switching from a low level to a high level) of an output signal of the encoder  61 . 
     The speed calculating unit  74  calculates a speed V of the conveyor belt  49  on the basis of the results detected by the encoder edge detecting unit  73 . Specifically, the distance of the marks  66  on the pattern member  67  is divided by an elapsed time from when an edge of an output signal of the encoder  61  is first detected to when the edge of another output signal of the encoder  61  is detected next, whereby the speed V of the conveyor belt  49  is then calculated. 
     The feedback amount selecting unit  75  reads out a upper normal range limit V U , a lower normal range limit V L , and a target speed Vt, which are stored in the register group  72 , compares a current speed V of the conveyor belt  49  calculated by the speed calculating unit  74  with the upper normal range limit V U  and the lower normal range limit V L , respectively, so as to determine whether or not the current speed is greater than the normal range upper or lower limit, and selects any one of the current speed V and the target speed Vt of the conveyor belt  49  as a feedback amount Vf. 
     The speed feedback control operating unit  76  reads out the target speed Vt stored in the register group  72  and calculates a deviation Ve between the target speed Vt and the feedback amount Vf selected by the feedback amount selecting unit  75 . Then, the speed feedback control operating unit  76  reads out a speed FB control parameter stored in the register group  72  so as to perform control operation using the speed FB control parameter to operate an actuation amount U corresponding to the deviation Ve between the target speed Vt and the feedback amount Vf. 
     The PWM generating unit  77  generates a PWM control signal corresponding to the actuation amount U operated by the speed feedback control operating unit  76  so as to give the generated PWM control signal to the motor driver  63 . 
     When the PWM control signal are given to the motor driver  63  from the PWM generating unit  77 , each driving element (for instance, FET) included in the motor driver  63  is turned on/off, and driving power corresponding to the ON/OFF is supplied to the motor  62  from the motor driver  63 . As a result, the motor  62  is rotatingly driven, and the driving roller  47  is then rotated by the driving force of the motor  62  to circulate the conveyor belt  49  at the target speed Vt. 
       FIG. 5  is a flowchart showing a sequence of the speed control of the conveyor belt  49 . 
     During circulation of the conveyor belt  49 , that is, during driving of the motor  62 , the sequence shown in  FIG. 5  is repeatedly executed. 
     If the encoder edge detecting unit  73  detects an edge of an output signal of the encoder  61  (switching from a high level to a low level), the speed calculating unit  74  first obtains an elapsed time from when the edge of the output signal of the encoder  61  has been detected, to calculate the speed V of the conveyor belt  49  from the elapsed time (S 1 ). 
     Next, the feedback amount selecting unit  75  determines whether or not the speed V is included within a normal range defined by the upper normal range limit V U  and the lower normal range limit V L  (S 2 ). In other words, the feedback amount selecting unit  75  determines whether or not the speed V calculated by the speed calculating unit  74  is greater than the lower normal range limit V L  stored in the register group  72  and is smaller than the upper normal range limit V U  stored in the register group  72 . 
     The upper normal range limit V U  and the lower normal range limit V L  are determined by, for example, normal range determination processing as will be described below, and the normal range defined by the upper normal range limit V U  and the lower normal range limit V L  corresponds to the width of the change in the speed V normally calculated by the speed calculating unit  74  when the pattern member  67  is irradiated with the light from the light-emitting element  68  during circulation of the conveyor belt  49 . Accordingly, if the speed V calculated by the speed calculating unit  74  is within the normal range, it can be determined that the speed V is almost equal to an actual speed of the conveyor belt  49 , while if the speed V calculated by the speed calculating unit  74  is out of the normal range, it can be determined that the speed V is not a value obtained by precisely calculating an actual speed of the conveyor belt  49 . For example, because an edge of an output signal of the encoder  61  is not detected over a predetermined time while a joint of the pattern member  67  is irradiated with the light from the light-emitting element  68  or while the toner adhered over a plurality of marks  66  is irradiated with the light from the light-emitting element  68 , the speed V is erroneously calculated by the speed calculating unit  74 . 
     Thus, if the speed V calculated by the speed calculating unit  74  is included within the predetermined normal range (YES in S 2 ), the speed V is set to the feedback amount Vf (S 3 ). Then, the speed feedback control operating unit  76  calculates a deviation Ve between the feedback amount Vf=V and the target speed Vt (S 5 ) to operate an actuation amount U corresponding to the deviation Ve (S 6 ). 
     On the other hand, if the speed V calculated by the speed calculating unit  74  is out of the normal range (NO in S 2 ), the target speed Vt, not the speed V is set to the feedback amount Vf (S 3 ). Then, the speed feedback control operating unit  76  calculates a deviation Ve between the feedback amount Vf=Vt and the target speed Vt (S 5 ) to operate the actuation amount U corresponding to the deviation Ve=0 (S 6 ). 
     If the actuation amount U is operated in this way, the PWM generating unit  77  generates a PWM control signal corresponding to the actuation amount U (S 7 ). Then, this PWM control signal is given to the motor driver  63 , whereby a driving power corresponding to the actuation amount U is supplied to the motor  62  from the motor driver  63 . 
     If the speed V calculated by the speed calculating unit  74  is out of the normal range as described above, the target speed Vt is used as the feedback amount Vf, the actuation amount U corresponding to the deviation Ve=0 between this feedback amount Vf=Vt and the target speed Vt is operated, and a driving power corresponding to this actuation amount U is supplied to the motor  62 . Thus, the speed of the conveyor belt  49  can be prevented from greatly deviating from the target speed Vt. Therefore, even when the pattern member  67  has any joint or toner is adhered over a plurality of marks  66 , the conveyor belt  49  can be stably driven with no irregularity in the speed of the conveyor belt  49 . As a result, occurrence of deviation in a transfer position of a toner image of each color on the sheet  3  can be prevented, and thus a high-quality color image can be formed on the sheet  3 . 
       FIG. 6  is a flowchart for explaining normal range determining processing. 
     The normal range determination processing is executed by the CPU  71 , for example, when the conveyor belt  49  is driven for the first time after power is input to the color laser printer  1 . In the normal range determination processing, the lower normal range limit V L  and the upper normal range limit V U  are respectively determined on the basis of the speed V calculated by the speed calculating unit  74 . 
     Specifically, when the speed V is first calculated by the speed calculating unit  74 , it is determined whether or not the speed V is included within a range defined by a predetermined provisional lower limit Vll and a predetermined provisional upper limit Vul (S 11 ). That is, it is determined whether or not the speed V calculated by the speed calculating unit  74  is greater than the provisional lower limit Vll and smaller than the provisional upper limit Vul. 
     If the speed V is not included within the range defined by the provisional lower limit Vll and the provisional upper limit Vul (NO in S 11 ), in other words, if a joint of the pattern member  67 , etc. is irradiated with the light from the light-emitting element  68  of the reflective sensor  70 , and an abnormal value resulting from this is calculated, the processing does not proceed to the next step. When the speed V is newly calculated by the speed calculating unit  74 , whether or not the calculated speed V is within the range defined by the provisional lower limit Vll and the upper limit value Vul is again determined. 
     If the speed V included within the range defined by the provisional low limit Vll and the provisional upper limit Vul is calculated (YES in S 11 ), data on the speed V is temporarily kept in a buffer memory (not shown) (S 12 ). Thereafter, speeds V calculated by the speed calculating unit  74  are sequentially kept in the buffer memory until a speed V out of the predetermined range defined by the provisional lower limit Vll and the provisional upper limit Vul is calculated, that is, until a joint of the pattern member  67 , etc. is again irradiated with the light from the light-emitting element  68  of the reflective sensor  70  and an abnormal value resulting from this is again calculated. 
     Then, if the speed V out of the range defined by the provisional lower limit Vll and the provisional upper limit Vul is calculated (NO in S 13 ), a maximum value and a minimum value of the speed V kept in the buffer memory till that moment are obtained. Then, the minimum value is subtracted from the maximum value, and a value obtained by dividing a value obtained from the subtraction by two is added to a target speed Vt to obtain an additional value, and the additional value is determined to be the upper normal range limit V U , while a value obtained by subtracting the value, which is obtained by dividing the value obtained from the subtraction by two, from the target speed Vt, is determined to be the lower normal range limit V L  (S 14 ). 
     Since the lower normal range limit V L  and the upper normal range limit V U  are respectively determined on the basis of the speed V calculated by the speed calculating unit  74 , a normal range defined by these upper and lower limits can be used as a range corresponding to characteristics of the color laser printer  1 . Therefore, regardless of differences between individual color laser printers  1 , whether or not the speed V calculated by the speed calculating unit  74  is used as the feedback amount Vf or whether the target speed Vt is used as the feedback amount Vf can be properly selected. Therefore, high-precision feedback control can be achieved and thus the conveyor belt  49  can be driven more stably. 
       FIG. 7  is a block diagram showing a configuration for setting the target speed Vt. 
     The color laser printer  1  includes a positional deviation amount calculating unit  82  as a positional deviation amount calculating unit which calculates, as a positional deviation amount, a cumulative value of deviations between a predetermined basic target speed and speeds (speeds V calculated by the speed calculating unit  74  provided in the ASIC  64 ) of the conveyor belt  49  in order to set a target speed Vt to be used for the speed control of the conveyor belt  49  by the ASIC  64  (speed control system), and a positional deviation amount compensating (control) unit  83  as a positional deviation compensating unit which sets, as the target speed Vt, a deviation between the basic target speed and a value obtained by multiplying the positional deviation amount calculated by the positional deviation amount calculating unit  82  by a predetermined proportional control gain Kp. 
     More specifically, as shown in  FIG. 8 , the positional deviation amount calculating unit  82  includes a deviation operating unit  84  which operates a deviation between a basic target speed and a speed V calculated by the speed calculating unit  74 , and a cumulative operating unit  85  which operates a cumulative value (sum) of deviations to be operated by the cumulative operating unit  85 . The cumulative value to be operated by the cumulative operating unit  85  is used as the positional deviation amount. 
     As shown in  FIG. 9 , the positional deviation amount compensating unit  83  includes a multiplying unit  86  which multiply the positional deviation amount calculated by the positional deviation amount calculating unit  82  by a predetermined proportional control gain Kp, and a deviation operating unit  87  which operates a deviation between a multiplied value obtained by the multiplying unit  86  and the basic target speed. The deviation to be operated by the deviation operating unit  87  is used as the target speed Vt. 
     If deviations between the basic target speed and the speed of the conveyor belt  49  are cumulated, the deviation amount of an actual rotational position with respect to a normal rotational position of the conveyor belt  49  increases accordingly. However, by setting the target speed Vt on the basis of a cumulative value of deviations, any cumulation of the deviations can be prevented, and thus positional deviation of the rotational position of the conveyor belt  49  can be compensated. Therefore, the more stable conveyance of the sheet  3  by the conveyor belt  49  can be achieved. 
       FIG. 10  is a block diagram showing a control system for controlling supply of a sheet onto the conveyor belt  49 . In  FIG. 10 , the same reference numerals as those in  FIG. 3  are given to the parts corresponding to the respective parts shown in  FIG. 3 . 
     The control system shown in  FIG. 10  is incorporated into the ASIC  64 , and includes a period detecting unit  78  as a period detecting unit which detects, when speeds V out of a normal range are periodically calculated by the speed calculating unit  74 , the period T, a timer  79  as an elapsed time measuring unit which measures an elapsed time t from a point of time from when a speed V out of a normal range is calculated, a sheet feed control unit  81  as a supply control unit which controls a registration clutch  80  for switching transmission or interruption of a driving force to the registration rollers  11 . 
     The period detecting unit  78  detects, for example, the time measured by the timer  79  from when a speed V out of the normal range is first calculated by the speed calculating unit  74  to when another speed V out of the normal range is next calculated by the speed calculating unit  74 , as a period T during which a speed V out of the normal range is calculated by the speed calculating unit  74 . 
     Assuming that the time taken until a leading end of a sheet  3  has arrived at the conveyor belt  49  from the start of rotation of the registration rollers  11  is tf, and the time taken for the conveyor belt  49  to convey one sheet  3  (the time taken until a trailing end of the sheet  3  is separated from the conveyor belt  49  after the leading end of the sheet  3  has arrived at the conveyor belt  49 ) is ts, the sheet feed control unit  81 , as shown in  FIG. 11 , subtracts the time tf and the time ts from the period T detected by the period detecting unit  78 , and further subtracts a predetermined extra time tm from the period, and then determines whether or not the resulting remaining time is longer than the elapsed time t measured by the timer  79  (S 21 ). 
     Then, if the remaining time is longer than the elapsed time t, the registration clutch  80  is immediately turned on to start the feeding of the sheet  3  by the registration rollers  11  (S 22 ). On the other hand, if the remaining time is shorter than the elapsed time t, then the sheet feed control unit waits until a speed V out of the predetermined normal range is calculated by the speed calculating unit  74 . Thereafter, If the speed V out of the predetermined normal range is calculated, the sheet feed control unit turns on the registration clutch  80  to start feeding of the sheet  3  by the registration rollers  11  (S 22 ). 
     While the speed V calculated by the speed calculating unit  74  is out of the predetermined normal range, there is a fear that a control is performed which may make the speed of the conveyor belt  49  unstable. Thus, as in the present example structure, if the remaining time obtained by subtracting the time tf and the time ts from the period T, and further subtracting the predetermined extra time tm from the period, is shorter than the elapsed time t after the speed V out of the predetermined normal range is calculated by the speed calculating unit  74 , the sheet feed control unit waits until another speed out of the predetermined normal range is calculated by the speed calculating unit  74  and thereafter starts feeding of the sheet  3  by the registration rollers  11 . As a result, the conveyance of the sheet  3  by the conveyor belt  49  can be completed until another speed V out of the normal range is calculated next by the speed calculating unit  74 . Therefore, better stability and high-precision conveyance of the sheet  3  can be achieved. 
       FIG. 12  is a block diagram showing another example structure of the control system for speed control of the conveyor belt. In  FIG. 10 , the same reference numerals as those in  FIG. 3  are given to the parts corresponding to the respective parts shown in  FIG. 3 . 
     The control system shown in  FIG. 12  includes a mean speed calculating unit  88  as a storage unit and a mean value calculating unit. The mean speed calculating unit  88  stores speeds V calculated by the speed calculating unit  74  from when a speed V out of the normal range is first calculated by the speed calculating unit  74  to when another speed V out of the normal range is next calculated by the speed calculating unit  74 , and calculates a mean speed Vm of a plurality of the stored speeds V. In other words, the mean speed calculating unit  88  uses, as a target, the period from when a speed V out of the normal range is first calculated by the speed calculating unit  74  to when another speed V out of the normal range is next calculated by the speed calculating unit  74 , to store only speeds V included within the normal range during the period, and calculates a mean speed of the speeds V included within the normal range, whereby the mean speed Vm from which influence of the speeds V out of the predetermined normal range is excluded is calculated. 
     The feedback amount selecting unit  75  reads out a upper normal range limit V U  and a lower normal range limit V L  which are stored in the register group  72 , compares a current speed V of the conveyor belt  49  calculated by the speed calculating unit  74  with the upper normal range limit V U  and the lower normal range limit V L , respectively, so as to determine whether or not the current speed is greater than the upper normal range or the lower normal range limit, and selects any one of the current speed V of the conveyor belt  49  and the mean speed Vm calculated by the mean speed calculating unit  88 , as a feedback amount Vf. 
       FIG. 13  is a flowchart showing the sequence of speed control of the conveyor belt  49  by the control system shown in  FIG. 12 . 
     During circulation of the conveyor belt  49 , that is, during driving of the motor  62 , the sequence shown in  FIG. 12  is repeatedly executed. 
     If the encoder edge detecting unit  73  detects an edge (switching from a high level to a low level) of an output signal of the encoder  61 , the speed calculating unit  74  calculates the speed V of the conveyor belt  49  (S 31 ). The calculated speed V is stored in the mean speed calculating unit  88  (S 31 ). 
     Next, the mean speed calculating unit  88  calculates the mean speed Vm (S 32 ). 
     Thereafter, the feedback amount selecting unit  75  determines whether or not the speed V is included within a normal range defined by the upper normal range limit V U  and the lower normal range limit V L  (S 33 ). 
     If the speed V calculated by the speed calculating unit  74  is included within the normal range (YES in S 33 ), the speed V is set to the feedback amount Vf (S 34 ). Then, the speed feedback control operating unit  76  calculates a deviation Ve between the feedback amount Vf=V and the target speed Vt (S 36 ) to operate an actuation amount U corresponding to the deviation Ve (S 37 ). 
     On the other hand, if the speed V calculated by the speed calculating unit  74  is out of the predetermined normal range (NO in S 33 ), a mean speed Vm calculated by the mean speed calculating unit  88 , not the speed V, is set to the feedback amount Vf (S 35 ). Then, the speed feedback control operating unit  76  calculates a deviation Ve between the feedback amount Vf=Vm and the target speed Vt (S 36 ) to operate an actuation amount U corresponding to the deviation Ve (S 37 ). 
     If the actuation amount U is operated in this way, the PWM generating unit  77  generates a PWM control signal corresponding to the actuation amount U (S 38 ). Then, this PWM control signal is given to the motor driver  63 , whereby a driving power corresponding to the actuation amount U is supplied to the motor  62  from the motor driver  63 . 
     The mean speed Vm calculated by the mean speed calculating unit  88  becomes a value from which an instantaneous change in the speed V calculated by the speed calculating unit  74  is excluded, and that is almost equal to the target speed Vt. Thus, if the speed V calculated by the speed calculating unit  74  is out of the normal range, the mean speed Vm is used as the feedback amount Vf, the actuation amount U corresponding to the deviation Ve between this feedback amount Vf=Vm and the target speed Vt is calculated, and a driving power corresponding to this actuation amount U is supplied to the motor  62 . Thus, the speed of conveyor belt  49  can be prevented from greatly deviating from the target speed Vt. Therefore, even when the pattern member  67  has any joint or toner is adhered over a plurality of marks  66 , the conveyor belt  49  can also be driven stably with no irregularity in the speed by the control system of this example structure. As a result, occurrence of deviation in a transfer position of a toner image of each color on the sheet  3  can be prevented, and thus, a high-quality color image can be formed on the sheet  3 . 
     Further, since a plurality of speeds V measured in the past by the speed calculating unit  74  are stored in the mean speed calculating unit  88 , the mean speed Vm can be surely and easily calculated by the mean speed calculating unit  88 . 
     Further, since the period from when a speed V out of the normal range is first calculated by the speed calculating unit  74  to when another speed V out of the normal range is next calculated, is used as the period for which the mean speed Vm is to be calculated, the mean speed Vm of a plurality of speeds V within the normal range can be certainly obtained. Therefore, the mean speed Vm of a plurality of speeds V can be certainly a value within the normal range, and thus, the conveyor belt  49  can be more stably driven by feedback control on the basis of the mean speed Vm. 
       FIG. 14  is a flowchart showing another example structure of the speed control of the conveyor belt  49  by the control system shown in  FIG. 3 . 
     During circulation of the conveyor belt  49 , that is, during driving of the motor  62 , the sequence shown in  FIG. 14  is repeatedly executed. 
     If the encoder edge detecting unit  73  detects an edge (switching from a low level to a high level) of an output signal of the encoder  61 , the speed calculating unit  74  calculates a speed V of the conveyor belt  49  (S 41 ). 
     Next, the feedback amount selecting unit  75  calculates a speed change rate R of the speed V previously calculated by the speed calculating unit  74  (a differential value of the speed V (S 42 ), and then determines whether or not an absolute value of the speed change rate R is smaller than a predetermined threshold value Rt (S 43 ). 
     The threshold value Rt is set to a maximum value of an absolute value of a speed change rate of a speed V normally calculated by the speed calculating unit  74  when the pattern member  67  is irradiated with the light from the light-emitting element  68  during circulation of the conveyor belt  49 . Accordingly, if the absolute value of the speed change rate R is smaller than the threshold value Rt, it can be determined that the speed V calculated by the speed calculating unit  74  at that time is almost equal to an actual speed of the conveyor belt  49 . On the other hand, if the absolute value of the speed change rate R is greater than the threshold value Rt, it can be determined that the speed V calculated by the speed calculating unit  74  at that time is not a value obtained by properly calculating an actual speed of the conveyor belt  49 . For example, because an edge of an output signal of the encoder  61  is not detected over a predetermined time while a joint of the pattern member  67  is irradiated with the light from the light-emitting element  68  or while the toner adhered over the plurality of marks  66  is irradiated with the light from the light-emitting element  68 , the speed V is erroneously calculated by the speed calculating unit  74 . As a result, the speed change rate R becomes more than the threshold value Rt. 
     Thus, if the speed change rate R is smaller than the threshold value Rt (YES in S 43 ), the speed V calculated by the speed calculating unit  74  at that time is set to the feedback amount Vf (S 44 ). Then, the speed feedback control operating unit  76  calculates a deviation Ve between the feedback amount Vf=V and the target speed Vt (S 46 ) to operate the actuation amount U corresponding to the deviation Ve (S 47 ). 
     On the other hand, if the speed change rate R is greater than the threshold value Rt (NO in S 43 ), the target speed Vt, not the speed V calculated by the speed calculating unit  74  at that time, is set to the feedback amount Vf (S 45 ). Then, the speed feedback control operating unit  76  calculates a deviation Ve between the feedback amount Vf=Vt and the target speed Vt (S 46 ) to operate the actuation amount U corresponding to the deviation Ve=0 (S 47 ). 
     If the actuation amount U is operated in this way, the PWM generating unit  77  generates a PWM control signal corresponding to the actuation amount U (S 48 ). Then, this PWM control signal is given to the motor driver  63 , whereby a driving power corresponding to the actuation amount U is supplied to the motor  62  from the motor driver  63 . 
     By the sequence in  FIG. 14 , the effects similar to those by the sequence in  FIG. 3  can also be exhibited. 
     In addition, the configuration in which the speed of the conveyor belt  49  is calculated by the speed calculating unit  74  and feedback control is performed based thereon is exemplified in the above description. However, the following configuration may be adopted, for example. That is, an interval (output interval) from when an edge of an output signal of the encoder  61  is first detected to when an edge of another output signal of the encoder  61  is next detected is measured. Then, if the output interval is within a predetermined normal range, the measured output interval is selected as a feedback amount, while if the output interval is out of the normal range, a target interval that is a target value of an output interval of a signal from the encoder  61 , or a mean value of output intervals is selected as the feedback amount, the actuation amount U is calculated on the basis of a deviation between the target interval and the feedback amount. Since the output interval of signals from the encoder  61  corresponds to the speed of the conveyor belt  49 , this configuration is substantially the same as the configuration of the above-described example structures and thus the effects as described above can be exhibited. 
     Further, the tandem laser printer  1  of the direct transfer type in which transfer is performed on the sheet  3  directly from each photosensitive drum  16  is exemplified. However, the invention is not limited to this type of printer. For example, the invention may be applied to a color laser printer of an intermediate transfer type in which a toner image of each color is transferred once from each photosensitive member to an intermediate transfer belt, and then transferred to a sheet at one time. In this case, the rotating body of the invention may be an intermediate transfer belt. Further, the image forming apparatus of the invention may be a monochrome laser printer. 
     Moreover, a plurality of pattern members  67  may be arranged in parallel on the surface of the conveyor belt  49  along one end of the conveyor belt  49  in its width direction. In this case, the respective pattern members  67  do not overlap each other, but a minute gap may be formed between the pattern members  67 . 
     Further, in the normal range determination processing shown in  FIG. 6 , the following technique may be adopted instead of the determination technique of the predetermined normal range shown in Step S 14 . That is, in the period from when a speed V out of a range defined by the provisional lower limit Vll and the provisional upper limit Vul is first calculated to when another speed V out of such a range is next calculated, a range defined by a maximum value and a minimum value of data of a speed V (data on a speed V included within the range defined by the provision lower limit Vll and the provisional upper limit Vul) temporarily kept in a buffer memory is used as an error range, and an appropriate range including this error range may be used as the normal range. By using a range broader than the error range as the normal range as described above, whether the speed V calculated by the speed calculating unit  74  is used as the feedback amount Vf or whether the target speed Vt or the mean speed Vm is used as the feedback amount Vf can be more precisely selected. Therefore, high-precision feedback control can be achieved, and thus the conveyor belt  49  can be even more stably circulated. 
     Further, a range obtained by multiplying the thus obtained error range by a given factor may be used as the normal range. In this case, the normal range can be surely set to a range broader than the error range. 
     It is considered that the error range varies depending on the environment (temperature, humidity, atmosphere, etc.) of the color laser printer  1  or deterioration degree. Thus, preferably, experiments are made under the various conditions to obtain error ranges under the individual conditions, and on the basis of the greatest error range (maximum error range) of the obtained error ranges, a factor to be multiplied by the error range is determined. 
     According to the above, if a current actual interval measured by the actual interval measuring unit is out of a predetermined normal range, the rotation of the rotating body is controlled in a feedback manner by using a mean value of a plurality of actual intervals measured in the past by the actual interval measuring unit instead of the current actual interval, as a feedback amount. For example, if the position of a mark deviates from a normal position or a developing agent is adhered over a plurality of marks, an actual interval measured by the actual interval measuring unit is out of the predetermined normal range. This actual interval out of the normal range does not precisely correspond to an actual rotational speed of the rotating body. If the rotation of the rotating body is controlled in a feedback manner based on the actual interval, the rotation of the rotating body may become unstable. In order to avoid this defect, it is considered that, if an actual interval measured by the actual interval measuring unit is out of the normal range, the rotation of the rotating body is controlled in a feedback manner, using an immediate value of an actual interval which has just been previously measured, without using the actual interval out of the normal range. However, there is a fear that the immediate value of the actual interval which has just been previously measured does not necessarily correspond precisely to the rotational speed of the rotating body, but it may be out of the normal range. 
     In contrast, a mean value of a plurality of actual intervals measured in the past by the actual interval measuring unit becomes a value from which a change in an immediate value of an actual interval is excluded and that is almost equal to a target value. Thus, if actual intervals are out of a predetermined normal range, a mean value thereof is used as a feedback amount in feedback control of the rotation of the rotating body, so that the rotation of the rotating body can be prevented from becoming unstable. Therefore, even when the position of a mark is out of a normal position or a developing agent is adhered over a plurality of marks, the rotating body can be rotated stably. 
     According to the example structures, since a plurality of actual intervals measured in the past by the actual interval measuring unit are stored in the storage unit, a mean value of the actual intervals can be surely and easily calculated. 
     According to the example structures, since a mean value of a plurality of actual intervals within the normal range is obtained, the mean unit is included within the normal range. Therefore, by using the mean value to control the rotation of the rotating body in the feedback manner, the rotation of the rotating body can be prevented from becoming unstable, and thus the rotating body can be rotated more stably. 
     According to the example structures, since the period for which a mean value of a plurality of actual intervals is to be calculated is used as the period from when an actual interval out of the normal range is first calculated by the actual interval measuring unit to when another actual interval out of the normal range is next calculated, a mean value of a plurality of actual intervals within the normal range can be surely obtained. Therefore, the mean value of a plurality of actual intervals can be certainly a value within the normal range, and thus the rotating body can be more stably rotated by feedback control on the basis of the mean value. 
     According to the example structures, if a current actual interval measured by the actual interval measuring unit is out of a predetermined normal range, the rotation of the rotating body is controlled in a feedback manner by using a target interval that is a target value of the output interval of the pulses from the sensor instead of the current actual interval, as a feedback amount. For example, if the position of a mark is out of a normal position or a developing agent is adhered over a plurality of marks, an actual interval measured by the actual interval measuring unit is out of a predetermined normal range. This actual interval out of the predetermined normal range does not precisely correspond to an actual rotational speed of the rotating body. If the rotation of the rotating body is controlled in a feedback manner based on this actual interval, the rotation of the rotating body may become unstable. In order to avoid this defect, it is considered that, if an actual interval measured by the actual interval measuring unit is out of the predetermined normal range, the rotation of the rotating body is controlled in a feedback manner, using an immediate value of an actual interval which has just been previously measured, without using the actual interval out of the predetermined normal range. However, there is a fear that the immediate value of the actual interval which has just been previously measured does not necessarily correspond precisely to the rotational speed of the rotating body, but it may be out of the normal range. 
     In contrast, if actual intervals are out of a normal range, a target value is used as a feedback amount in feedback control of the rotation of the rotating body, so that the rotation of the rotating body can be prevented from becoming unstable. Therefore, even when the position of a mark is out of a normal position or a developing agent is adhered over a plurality of marks, the rotating body can be stably rotated. 
     According to the example structures, regardless of the differences between individual image forming apparatuses, whether the actual interval calculated by the actual interval measuring unit is used as the feedback amount or the target interval is used as the feedback amount can be properly selected. Therefore, high-precision feedback control can be achieved and thus the rotating body can be driven more stably. 
     According to the example structures, since an actual interval measured by the actual interval measuring unit sometimes includes an error, whether a current actual interval measured by the actual interval measuring unit is used as the feedback amount or a mean interval of a plurality of actual intervals measured in the past or a target interval is used as the feedback amount can be properly selected. Therefore, high-precision feedback control can be achieved, and thus, the rotating body can be stably driven even better. 
     According to the example structures, the normal range can be surely set to a range broader than the error range. 
     According to the example structures, since there is a fear that the rotation of the rotating body, and further the conveyance of a recording medium by the rotating body become unstable during a period for which actual intervals are out of the predetermined normal range, a recording medium is conveyed while avoiding such a period, so that stable and high-precision conveyance of the recording medium can be achieved. 
     According to the example structures, if the remaining time obtained by subtracting the time for conveyance of the recording medium by the rotating body, from the period when an actual interval deviates from the normal range, is longer than the elapsed time from when the actual interval out of the normal range is measured, the conveyance of a recording medium by the rotating body can be completed until another actual interval deviates from the next predetermined normal range, even when the supply of a recording medium is started. Therefore, better stability and high-precision conveyance of the recording medium can be achieved. 
     According to the example structures, if the deviation between the basic target interval and the actual interval is cumulated, the deviation amount of an actual rotational position with respect to a predetermined normal rotational position (target position) of the rotating body increases accordingly. However, by setting the target value on the basis of a cumulative value of deviations, cumulation of the deviations can be prevented, and thus positional deviation of the rotational position of the rotating body can be compensated. Therefore, even better stability and high-precision conveyance of the recording medium can be achieved. 
     According to the example structures, better stability and high-precision conveyance of a recording medium by the conveyor belt can be achieved. Therefore, it is possible to prevent occurrence of deviation in image formation positions by a plurality of image forming unit on a recording medium. As a result, a high-quality image can be formed on a recording medium. 
     The foregoing description of the example structures of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The example structures were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various example structures and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined solely by the following claims and their equivalents.