Patent Publication Number: US-7215907-B2

Title: Method and apparatus for image forming capable of effectively eliminating color displacements

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Applications No. 2003-192821 filed on Jul. 7, 2003, No. 2003-408291 filed on Dec. 5, 2003, and No. 2004-114717 filed on Apr. 8, 2004 in the Japanese Patent Office, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming method and apparatus, and more particularly to a method and apparatus for image forming capable of effectively eliminating color displacement by controlling a clock control motor controlled by a command clock signal and a feedback signal, in accordance with a velocity curve. 
   2. Discussion of the Background 
   Background image forming apparatuses are commonly known as electrophotographic copying machines, printing machines, facsimile machines, and multi-functional apparatuses having at least two functions of copying, printing and facsimile functions. Some of the background apparatuses use an intermediate transfer method, and some use a direct transfer method. 
   The background image forming apparatus using the intermediate transfer method is referred to as an “intermediate transfer image forming apparatus”, and transfers an electrostatic latent image formed on a photoconductor onto an intermediate transfer member before transferring the electrostatic latent image onto a recording medium. 
   The background image forming apparatus using the direct transfer method is referred to as a “direct transfer image forming apparatus”, and directly transfers the electrostatic latent image onto the recording medium which is conveyed by a recording medium bearing member. 
   In both background image forming apparatuses, the photoconductor is driven by a photoconductor motor to rotate, and the intermediate transfer member and the recording medium bearing member are driven by a drive motor to rotate. 
   The photoconductor and the intermediate transfer member rotate while they are held in contact to each other, a surface linear velocity of the photoconductor is required to have the same rate as that of the intermediate transfer member. In a case where the photoconductor rotates at a different rate from the intermediate transfer member, a surface of the photoconductor rubs a surface of the intermediate transfer member, hastening their surface wear. 
   To prevent the wearing of the surfaces, the intermediate transfer image forming apparatus has employed a stepping motor as the photoconductor motor and the drive motor for controlling the number of input pulses of the stepping motor to synchronize the surface linear velocities of the photoconductor and the intermediate transfer member. Also, the direct transfer image forming apparatus has employed the stopping motor for synchronizing the surface linear velocities of the photoconductor and the recording medium bearing member. 
   The stepping motor, however, generally consumes a large amount of electric power and produces a loud noise. Therefore, a clock control motor such as a direct current (DC) brushless motor is used as an alternative to the stepping motor. The DC brushless motor is controlled by a command clock signal and a feedback signal, and can reduce the power consumption and the loud noise. 
   The DC brushless motor, however, may vary its rotation speed particularly when it is started and stopped. In a case where the DC brushless motor is used as the photoconductor motor and the drive motor, the surface linear velocity of the photoconductor may be greatly different from that of the intermediate transfer member or that of the recording medium bearing member, which results in significant wear that shortens its life. Consequently, the DC brushless motor has been thought to be unsuitable for the background image forming apparatus. 
     FIG. 1  shows an example of the command clock signal of the DC brushless motor. The rotation of the DC brushless motor is controlled by the command clock signal having a predetermined number of clock pulses, as shown in  FIG. 1 , and the feedback signal output from the DC brushless motor. 
     FIG. 2  shows an example of the surface linear velocities of the photoconductor and the intermediate transfer member when the DC brushless motors are started. The DC brushless motor works as the photoconductor motor which rotates the photoconductor and the drive motor which rotates the intermediate transfer member. The solid line represents the surface linear velocity of the photoconductor, and the alternate long and short dash line represents the surface linear velocity of the intermediate transfer member. The photoconductor motor and the drive motor are controlled by a command clock signal same as the command clock signal shown in  FIG. 1 . However, when DC brushless motor is started, a significant difference between the surface linear velocity of the photoconductor and the surface linear velocity of the intermediate transfer member may be caused due to a property of the DC brushless motor, loads applied to the photoconductor and the intermediate transfer member, and the difference of the inertias of the photoconductor, as shown in  FIG. 2 . 
     FIG. 3  shows a graph of the command clock signal when the DC brushless motor is stopped, and  FIG. 4  shows a graph of the surface linear velocity of the photoconductor and the intermediate transfer member when the DC brushless motor is stopped. 
   When a motor stop signal is issued to stop inputting the command clock signal to the photoconductor motor and the drive motor as shown in  FIG. 3 , the surface linear velocities of the photoconductor and the intermediate transfer member driven by the DC brushless motor start to decrease down to a level, as shown in  FIG. 4 , at which the photoconductor and the intermediate transfer member stop as shown in  FIG. 4 . At this time, a significant difference between the surface linear velocity of the photoconductor and the surface linear velocity of the intermediate transfer member may also be caused due to a property of the DC brushless motor, loads applied to the photoconductor and the intermediate transfer member, and the difference of the inertias of the photoconductor, as indicated by the solid line and the alternate long and short dash line shown in  FIG. 4 . 
   As described above, the significant difference between the surface linear velocity of the photoconductor and the surface linear velocity of the intermediate transfer member may cause damages such as scratches on the surfaces thereof and defects such as streaks on an image, resulting in a deterioration of the image. The defects may be observed when the DC brushless motor is used as the drive motor for the recording medium bearing member. Due to the drawbacks as described above, the stepping motor has preferably been used, without solving the problems of high power consumption and loud noise. 
   SUMMARY OF THE INVENTION 
   The present invention has been made under the above-described circumstances. 
   An object of the present invention is to provide a novel image forming apparatus which can control a clock control motor controlled by a command clock signal and a feedback signal, in accordance with the velocity curve. 
   In one exemplary embodiment, a novel image forming apparatus includes at least one image bearing member, a transferring member, at least one first motor, a second motor, and a control mechanism. The at least one image bearing member is configured to bear a toner image on a surface thereof. The transferring member is arranged close to or in contact with the at least one image bearing member and is configured to rotate in substantially synchronism with the at least one image bearing member to transfer the toner image born on the at least one image bearing member onto a recording medium. The at least one first motor rotates the at least one image bearing member. The second motor rotates the transferring member. The control mechanism is configured to control a rotation number of at least one of the at least one first motor and the second motor during at least one of rise and fall time periods with a command clock signal and a feedback signal in accordance with a predetermined velocity curve. 
   A novel image forming apparatus includes at least one image bearing member, an intermediate transfer member, a third motor, a fourth motor, a transfer mechanism, and a control mechanism. The at least one image bearing member is configured to bear a toner image on a surface thereof. The intermediate transfer member is configured to receive the toner image from the at least one image bearing member. The third motor rotates the at least one image bearing member. The fourth motor rotates the intermediate transfer member. The transfer mechanism is configured to transfer the toner image from the intermediate transfer member to a recording medium. The control mechanism is configured to control rotations of the third and fourth motors. At least one of the third and fourth motors includes a clock control motor controlled by a command clock signal and a feedback signal. The control mechanism controls a rotation number of the clock control motor in accordance with a predetermined velocity curve during at least one of rise and fall time periods of the clock control motor. 
   The third motor may include the clock control motor, and the fourth motor may include a stepping motor. 
   Each of the third and fourth motors may include the clock control motor. 
   The clock control motor may be controlled to be rotated by the command clock signal having the clock number in accordance with the predetermined velocity curve during the at least one of rise and fall time periods of the clock control motor. 
   The clock control motor may be controlled to be rotated by the command clock signal having a gradually increasing pulse number during the rise time period, having a substantially constant pulse number during a steady rotation time period, and having a gradually decreasing pulse number during the fall time period. 
   The image forming apparatus may further include a braking mechanism configured to forcedly reduce a rotation number of the clock control motor during the fall time period of the clock control motor. 
   The rotation number of the clock control motor may be controlled by changing a pulse number of the command clock signal in steps during the at least one of rise and fall time periods of the clock control motor. 
   The predetermined velocity curve may be stored in a memory and may be changed by controlling an operation panel of the image forming apparatus or a connecting terminal of the image forming apparatus. 
   The clock control motor may include a direct current brushless motor. 
   A novel image forming method includes the steps of driving an image bearing member with a primary driving member, driving an overlaying member with a secondary driving member, forming a toner image on the image bearing member, moving the toner image with the image bearing member to a primary transfer position, overlaying at least one toner image formed on the bearing member into a single toner image at the primary transfer position, transporting the single toner image to a secondary transfer position, transferring the single toner image transported to the secondary transfer position by the transporting step onto a recording medium, and controlling a rotation number of at least one of the primary and secondary driving members with a command clock signal and a feedback signal in accordance with a predetermined velocity curve. 
   The controlling step may control the rotation number of the at least one of the primary and secondary driving members during at least one of rise and fall time periods with the command clock signal and the feedback signal in accordance with the predetermined velocity curve. 
   A novel image forming apparatus includes at least one image bearing member, a recording medium bearing member, a fifth motor, a sixth motor, a transfer mechanism, and a control mechanism. The at least one image bearing member is configured to bear a toner image on a surface thereof. The recording medium bearing member is configured to carry a recording medium to receive the toner image from the at least one image bearing member. The fifth motor rotates the at least one image bearing member. The sixth motor rotates the recording medium bearing member. The transfer mechanism is configured to transfer the toner image from the image bearing member to a recording medium. The control mechanism is configured to control rotations of the fifth and sixth motors. At least one of the fifth and sixth motors includes a clock control motor controlled by a command clock signal and a feedback signal. The control mechanism controls a rotation number of the clock control motor in accordance with a predetermined velocity curve during at least one of rise and fall time periods of the clock control motor. 
   The fifth motor may include the clock control motor, and the sixth motor includes a stepping motor. 
   Each of the fifth and sixth motors may include the clock control motor. 
   The clock control motor may be controlled to be rotated by the command clock signal having the clock number in accordance with the predetermined velocity curve during the at least one of the rise and fall time periods of the clock control motor. 
   The clock control motor may be controlled to be rotated by the command clock signal having a gradually increasing pulse number during the rise time period, having a substantially constant pulse number during a steady rotation time period, and having a gradually decreasing pulse number during the fall time period. 
   The novel image forming apparatus may further include a braking mechanism configured to forcedly reduce a rotation number of the clock control motor during the fall time period of the clock control motor. 
   The rotation number of the clock control motor may be controlled by changing a pulse number of the command clock signal in steps during the at least one of the rise and fall time periods of the clock control motor. 
   The predetermined velocity curve may be stored in a memory and can be changed by controlling an operation panel of the image forming apparatus or a connecting terminal of the image forming apparatus. 
   The clock control motor may include a direct current brushless motor. 
   A novel image forming method includes the steps of energizing an image bearing member with a primary driving member, driving an overlaying member with a secondary driving member, forming a toner image on the image bearing member, moving the toner image with the image bearing member to a transfer position, transferring at least one toner image formed on the bearing member onto the recording sheet driven by the driving step in a single overlaid toner image at the transfer position, and controlling a rotation number of at least one of the primary and secondary driving members with a command clock signal and a feedback signal in accordance with a predetermined velocity curve. 
   A novel image forming apparatus includes a plurality of color image bearing members, a monochrome image bearing member, an intermediate transfer member, a first gear, a second gear, a seventh motor, an eighth motor, a ninth motor, a transfer mechanism, and a control mechanism. The plurality of color image bearing members have surfaces to bear a plurality of color toner images. The monochrome image bearing member has a surface to bear a monochrome toner image. The intermediate transfer member is configured to receive the plurality of color toner images from the plurality of color image bearing members and the monochrome toner image from the monochrome image bearing member. The first gear is coupled with at least one of the plurality of color image bearing members. The plurality of a second gear coupled with the monochrome image bearing member. The seventh motor includes the clock control motor rotating the at least one of the plurality of color image bearing members via the first gear. The eighth motor includes the clock control motor rotating the monochrome image bearing member via the second gear. The ninth motor rotates the intermediate transfer member. The transfer mechanism is configured to transfer the toner image from the intermediate transfer member to a recording medium. And, the control mechanism is configured to control rotations of the seventh, eighth and ninth motors. The control mechanism controls rotation numbers of the clock control motors during at least one of rise and fall time periods in accordance with a predetermined velocity curve. 
   A rotation number of at least one of the clock control motors of the seventh and eighth motors may be controlled to be changed to set positions of the first and second gears to have a predetermined phase relationship therebetween, after completion of the rise time periods of the seventh and eighth motors and before start of a subsequent image forming operation. 
   The control mechanism may have a plurality of operation modes which are selectable and bi-directionally switchable without stopping the eighth and ninth motors. The plurality of operation modes may include a color mode and a monochrome mode. The color mode has a function of producing a full-color image by sequentially overlaying the plurality of color toner images formed on the surfaces of the plurality of color image bearing members and the monochrome toner image formed on the surface of the monochrome image bearing member onto the intermediate transfer member, and onto the recording medium. The monochrome mode has a function of producing a monochrome image by stopping rotations of the plurality of color image bearing members, separating the intermediate transfer member from the plurality of color image bearing members, rotating the monochrome image bearing member, and transferring the monochrome toner image onto the intermediate transfer member, and onto the recording medium. 
   A rotation number of the at least one of the clock control motors of the seventh and eighth motors may be controlled to be changed to set positions of the first and second gears to have a predetermined phase relationship therebetween, before the subsequent image forming operation starts in the color mode which is previously switched from the monochrome mode. 
   The control mechanism may have a plurality of switchable surface linear velocities and a plurality of speed modes. The plurality of switchable surface linear velocities may include a first surface linear velocity, and a second surface linear velocity which is slower than the first surface linear velocity, The plurality of speed modes may include a full speed color mode, a low speed color mode, a full speed monochrome mode, and a low speed monochrome mode. The full speed color mode may have a function of rotating the plurality of color image bearing members, the monochrome image bearing member and the intermediate transfer member at the first surface linear velocity in the color mode. The full speed monochrome mode may have a function of rotating the monochrome image bearing member and the intermediate transfer member at the first surface linear velocity in the monochrome mode. The low speed color mode may have a function of rotating the plurality of color image bearing members, the monochrome image bearing member and the intermediate transfer member at the second surface linear velocity in the color mode. The low speed monochrome mode may have a function of rotating the monochrome image bearing member and the intermediate transfer member at the second surface linear velocity in the monochrome mode. The rotation number of the at least one of the clock control motors of the seventh and eighth motors is controlled to be changed to set positions of the first and second gears to have a predetermined phase relationship therebetween, before the subsequent image forming operation starts in one of the full speed color mode and the low speed color mode which is previously changed from different one of the full speed color mode, the low speed color mode, the full speed monochrome mode and the low speed monochrome mode. 
   The novel image forming apparatus may further include a first sensor and a second sensor. The first sensor is configured to detect a first position of the first gear in a circumferential direction of the first gear. The second sensor is configured to detect a second position of the second gear in a circumferential direction of the second gear. A rotation number of at least one the clock control motors of the seventh and eight motors may be controlled in accordance with a detection time difference between a first time period in which the first sensor detects the first position and a second time period in which the second sensor detects the second position, when the predetermined phase relationship between the first and second gears is adjusted. 
   The novel image forming apparatus may further include a third sensor, a fourth sensor and a second sensor. The third sensor is configured to detect a third position of the first gear in a circumferential direction of the first gear. The fourth sensor is configured to detect a fourth position of the second gear in a circumferential direction of the second gear. A rotation number of at least one of the clock control motors of the seventh and eight motors may be controlled in accordance with a value obtained by adding a predetermined correction value to a detection time difference between a third time period in which the third sensor detects the third position and a fourth time period in which the fourth sensor detects the fourth position, when the predetermined phase relationship between the first and second gears is adjusted. 
   The novel image forming apparatus may further include a third sensor and a fourth sensor. The third sensor may be configured to detect a third position of the first gear in a circumferential direction of the first gear. The fourth sensor may be configured to detect a fourth position of the second gear in a circumferential direction of the second gear. A rotation number of at least one of the clock control motors of the seventh and eight motors may be controlled in accordance with a value obtained by adding a predetermined correction value to a detection time difference between a third time period in which the third sensor detects the third position and a fourth time period in which the fourth sensor detects the fourth position, when the predetermined phase relationship between the first and second gears is adjusted. 
   A rotation number of at least one of the clock control motors of the tenth and eleventh motors may be controlled to be changed to set positions of the third and fourth gears to have a predetermined phase relationship, after completion of the rise time period of the tenth and eleventh motors and before start of a subsequent image forming operation. 
   The control mechanism may have a plurality of operation modes which are selectable and bi-directionally switchable without stopping the eleventh and twelfth motors. The plurality of operation modes may include a color mode and a monochrome mode. The color mode may have a function of producing a full-color image by sequentially overlaying the plurality of color toner images formed on the surfaces of the plurality of color image bearing members and the monochrome toner image formed on the surface of the monochrome image bearing member onto the recording medium carried by the recording medium bearing member. The monochrome mode may have a function of producing a monochrome image by stopping rotations of the plurality of color image bearing members, separating the recording medium bearing member from the plurality of color image bearing members, rotating the monochrome image bearing member, and transferring the monochrome toner image onto the recording medium carried by the recording medium bearing member. 
   A rotation number of the at least one of the clock control motors of the tenth and eleventh motors may be controlled to be changed to set positions of the third and fourth gears to have a predetermined phase relationship, before the subsequent image forming operation starts in the color mode which is previously switched from the monochrome mode. 
   The control mechanism may have a plurality of switchable surface linear velocities and a plurality of speed modes. The plurality of switchable surface linear velocities may include a third surface linear velocity, and a fourth surface linear velocity which is slower than the third surface linear velocity. The plurality of speed modes may include a full speed color mode, a low speed color mode, a full speed monochrome mode, and a low speed monochrome mode. The full speed color mode may have a function of rotating the plurality of color image bearing members, the monochrome image bearing member and the recording medium bearing member at the third surface linear velocity in the color mode. The full speed monochrome mode may have a function of rotating the monochrome image bearing member and the recording medium bearing member at the third surface linear velocity in the monochrome mode. The low speed color mode may have a function of rotating the plurality of color image bearing members, the monochrome image bearing member and the recording medium bearing member at the fourth surface linear velocity in the color mode. The low speed monochrome mode may have a function of rotating the monochrome image bearing member and the recording medium bearing member at the fourth surface linear velocity in the monochrome mode. A rotation number of the at least one of the clock control motors of the tenth and eleventh motors may be controlled to be changed to set positions of the third and fourth gears to have a predetermined phase relationship, before the subsequent image forming operation starts in one of the full speed color mode and the low speed color mode which is previously changed from different one of the full speed color mode, the low speed color mode, the full speed monochrome mode and the low speed monochrome mode. 
   The novel image forming apparatus further include a fifth sensor and a sixth sensor. The fifth sensor may be configured to detect a fifth position of the third gear in a circumferential direction of the third gear. The sixth sensor may be configured to detect a sixth position of the fourth gear in a circumferential direction of the fourth gear. A rotation number of at least one of the clock control motors of the tenth and eleventh motors may be controlled in accordance with a detection time difference between a fifth time period in which the fifth sensor detects the fifth position and a sixth time period in which the sixth sensor detects the sixth position, when the predetermined phase relationship between the third and fourth gears is adjusted. 
   The novel image forming apparatus may further include a seventh sensor and an eighth sensor. The seventh sensor may be configured to detect a seventh position of the third gear in a circumferential direction of the third gear. The eighth sensor may be configured to detect an eighth position of the fourth gear in a circumferential direction of the fourth gear. A rotation number of at least one of the clock control motors of the tenth and eleventh motors may be controlled in accordance with a value obtained by adding a predetermined correction value to a detection time difference between a seventh time period in which the seventh sensor detects the seventh position and an eighth time period in which the eighth sensor detects the eighth position, when the predetermined phase relationship between the third and fourth gears is adjusted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a graph showing a command clock signal at a start of a DC brushless motor used in a background image forming apparatus; 
       FIG. 2  is a graph showing surface linear velocities at the start of a photoconductor and an intermediate transfer member driven by the DC brushless motor of  FIG. 1 ; 
       FIG. 3  is a graph showing a command clock signal at a stop of the DC brushless motor; 
       FIG. 4  is a graph showing surface linear velocities at the stop of the photoconductor and the intermediate transfer member driven by the DC brushless motor of  FIG. 3 ; 
       FIG. 5  is a drawing of a schematic structure of an image forming apparatus provided with an intermediate transfer member according to an exemplary embodiment of the present invention when the image forming apparatus is in a color mode; 
       FIG. 6  is a drawing of a schematic structure of the image forming apparatus of  FIG. 5  when the image forming apparatus is in a black-and-white mode; 
       FIG. 7  is a drawing of a schematic structure of an image forming apparatus provided with a recording medium bearing member according to an exemplary embodiment of the present invention when the image forming apparatus; 
       FIG. 8  is a schematic structure of drive circuits driving the photoconductors and the intermediate transfer member of the image bearing member of  FIG. 5 ; 
       FIG. 9  is a schematic structure of a positional relationship of the photoconductor and gears provided for driving the photoconductor; 
       FIG. 10  is a graph showing motor rotations of photoconductor motors and a drive motor of the image forming apparatus of  FIG. 5 ; 
       FIG. 11  is a graph showing motor rotations of the drive motor during a fall time period of the drive motor; 
       FIGS. 12A ,  12 B and  12 C are drawings illustrating circuits of a braking mechanism of the DC brushless motor; 
       FIG. 13  is a graph showing surface linear velocities of two photoconductor motors and the drive motor during a rise time period; 
       FIG. 14  is a graph showing surface linear velocities of the two photoconductor motors and the drive motor during the rise time period, a steady rotation time period and the fall time period; 
       FIG. 15  is a graph showing surface linear velocities of the two photoconductor motors during the rise time period; 
       FIG. 16  is a graph showing surface linear velocities of the two photoconductor motors during the rise time period, the steady rotation time period and the fall time period; 
       FIG. 17  is a schematic structure of a phase relationship of a plurality of gears; 
       FIGS. 18A and 18B  are flowcharts showing an adjustment of the plurality of gears; 
       FIG. 19  is a graph of a control of motor rotations of the photoconductor motors; 
       FIG. 20  is a graph of another control of motor rotations of the photoconductor motors; 
       FIG. 21  is a graph of a surface linear velocity of the photoconductor motors when they are switched from a full speed mode to a low speed mode; 
       FIG. 22  is a graph of surface linear velocities of the photoconductor motors and the drive motors when they are switched between a color mode and a black-and-white mode; 
       FIG. 23  is a graph showing a curve of a deflection of a pitch circle of a black-and-white gear in a radius direction thereof; 
       FIG. 24  is a graph showing a curve of a deflection of a pitch circle of a color gear in a radius direction thereof; 
       FIG. 25  is a graph showing a difference between the curves of the deflections of the pitch circles of the black-and-white gear and the color gear shown in  FIGS. 24 and 25 ; 
       FIG. 26  is a graph showing another difference between the curves of the deflections of the pitch circles of the black-and-white gear and the color gear; 
       FIG. 27  is a graph showing a curve of a deflection when one of the curve of the deflections shown in  FIG. 26  is shifted; 
       FIG. 28  is a graph showing a command clock signal at a start of a DC brushless motor used in the image forming apparatus of  FIG. 5 ; 
       FIG. 29  is a graph showing surface linear velocities of the photoconductor and the drive motor during the rise time period; 
       FIG. 30  is a graph showing another command clock signal input to the DC brushless motor during the rise time period; 
       FIG. 31  is a graph showing another command clock signal input to the DC brushless motor during the fall time period; 
       FIG. 32  is a graph showing surface linear velocities of the photoconductor motor and the drive motor during the fall time period; 
       FIG. 33  is a schematic structure of an image forming portion of a tandem image forming apparatus; and 
       FIG. 34  is a schematic structure of an image forming portion of an image forming apparatus provided with one photoconductor. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
   Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, preferred embodiments of the present invention are described. 
     FIG. 5  shows a schematic cross sectional view of an image forming apparatus  1 . The image forming apparatus  1  of  FIG. 5  is a printer using an intermediate transfer method. The image forming apparatus  1  includes four photoconductors  2   y ,  2   c ,  2   m  and  2   bk , and an intermediate transfer member  3 . The photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are in a cylindrical shape, and have an outer diameter. The intermediate transfer member  3  forms an endless belt extended with supporting rollers  4 ,  5 , and  6 . The photoconductors  2   y ,  2   c ,  2   m  and  2   bk  have surfaces that are held in contact with a surface of the intermediate transfer member  3  when the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are activated for image forming. The photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are driven by a photoconductor motor, which will be described below, in a direction indicated by arrows in  FIG. 5 . The intermediate transfer member  3  is rotated by a drive motor, which will also be described below, in a direction A, indicated by an arrow in  FIG. 5 . 
   As described above, the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are held in contact with the intermediate transfer member  3 , and are rotated in a same direction that the intermediate transfer member  3  travels in  FIG. 5 . Since the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  have structures and functions similar to each other, except that the toners contained therein are of different colors, the discussion below with respect to  FIGS. 6–9  and  33  uses reference numerals for specifying components of the image forming apparatus  1  without suffixes of colors such as y, c, m and bk. In other words, the photoconductor  2  of  FIG. 6 , for example, can be any one of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk.    
   The photoconductor  2  has image forming components for forming an image around it. A charging unit including a charging roller  7  is applied with a charged voltage. When the photoconductor  2  is driven to rotate clockwise in  FIG. 5 , the charging unit applies the charged voltage to the photoconductor  2  to uniformly charge the surface of the photoconductor  2  to a predetermined polarity. An optical writing unit  8  emits a laser beam L, which is optically modulated. The laser beam L irradiates the photoconductor  2  so that an electrostatic latent image is formed on the charged surface of the photoconductor  2 . A developing unit  9  visualizes the electrostatic latent image formed on the surface of the photoconductor  2  as a single color toner image. Thus, the toner image is formed on the surface of the photoconductor  2 . 
   The intermediate transfer member  3  is held in contact with a primary transfer roller  10  (namely  10   y,    10   c,    10   m,  and  10   bk ) corresponding to the photoconductor  2 . The primary transfer roller  10  is disposed opposite to the photoconductor  2 , sandwiching the intermediate transfer member  3 . The primary transfer roller  10  receives a transfer voltage to transfer the color toner image onto the surface of the intermediate transfer member  3  which is rotated in the direction A. After the toner image formed on the surface of the photoconductor  2  is transferred onto the surface of the intermediate transfer member  3 , a cleaning unit  11  removes residual toner on the surface of the photoconductor  2 . 
   Through the operations similar to those as described above, yellow, cyan, magenta and black images are formed on the surfaces of the respective photoconductors  2   y ,  2   c ,  2   m  and  2   bk . Those color toner images are sequentially overlaid on the surface of the intermediate transfer member  3 , such that a full-color toner image is formed on the surface of the intermediate transfer member  3 . 
   In  FIG. 5 , a sheet feeding unit  14  is provided at a lower portion of the image forming apparatus  1 . The sheet feeding unit  14  includes a sheet feeding cassette  12  and a sheet feeding roller  13 . The sheet feeding cassette  12  accommodates a plurality of recording media such as transfer sheets and resin sheets that include a recording medium P. When the sheet feeding roller  13  is rotated by a drive motor (not shown), the recording medium P placed on the top of a stack of transfer sheets in the sheet feeding cassette  12  is fed and conveyed in a direction B in  FIG. 5 . The recording medium P is conveyed to a portion between rollers of a registration roller pair  15 . The registration roller pair  15  stops and feeds the recording medium P in synchronization with a movement of the full-color toner image towards a portion between the supporting roller  4  held in contact with the intermediate transfer member  3  and a secondary transfer unit including a secondary transfer roller  16 . At this time, the secondary transfer roller  16  is applied with an adequate predetermined transfer voltage to a predetermined polarity such that the full-color toner image, formed on the surface of the intermediate transfer member  3 , is transferred on the recording medium P. 
   The recording medium P that has the full-color toner image thereon is conveyed further upward and passes between a pair of fixing rollers of a fixing unit  17 . The fixing unit  17  includes a heat roller  18  having a heater therein and a pressure roller  19  for pressing the recording medium P for fixing the full-color toner image. The fixing unit  17  fixes the full-color toner image to the recording medium P by applying heat and pressure. After the recording medium P passes the fixing unit  17 , the recording medium P is discharged by a sheet discharging roller pair  20  to a sheet discharging tray  21  provided at the upper portion of the image forming apparatus  1 . After the full-color toner image is transferred onto the recording medium P, a transfer member cleaning unit  22  removes residual toner adhering on the surface of the intermediate transfer member  3 . As described above, the image forming apparatus  1  of this embodiment of the present invention performs its image forming operation such that the full-color toner image formed on the photoconductor  2  is transferred onto the intermediate transfer member  3  and then onto the recording medium P to obtain a recorded image. 
   The above-described image forming operations are performed in a color mode for producing a full-color image on the recording medium P. The image forming apparatus  1  also performs image forming operations in a black-and-white mode for producing a single black-and-white toner image on the recording medium P. 
   Referring to  FIG. 6 , the image forming apparatus  1  in the black-and-white mode is described. 
   In the black-and-white mode, the intermediate transfer member  3  is detached from the surfaces of the photoconductors  2   y ,  2   c  and  2   m  used for producing a full-color toner image and is held in contact with the photoconductor  2   bk  used for producing a black-and-white toner image. In the black-and-white mode, the photoconductors  2   y ,  2   c , and  2   m  are not rotated while the photoconductor  2   bk  is rotated. 
   The black-and-white toner image is formed on the photoconductor  2   bk  through the same operations as those for the full-color toner image. The black-and-white toner image formed on the photoconductor  2   bk  is transferred onto the surface of the intermediate transfer member  3  that is rotated in the direction A in  FIG. 6 . 
   The recording medium P is also fed from the sheet feeding unit  14 , is fed and stopped in synchronization with the registration roller pair  15 , and is conveyed to the portion between the supporting roller  4  held in contact with the intermediate transfer member  3  and the secondary transfer roller  16 . Consequently, the black-and-white toner image is transferred onto the recording medium P at the portion. The recording medium P also passes through the fixing unit  17 . At this time, the black-and-white toner image on the recording medium P is fixed, and is then discharged to the sheet discharging tray  21 . In the black-and-white mode, the photoconductors  2   y ,  2   c , and  2   m  do not operate and are not held in contact with the intermediate transfer member  3 . As a result, the photoconductors  2   y ,  2   c , and  2   m  may be used longer, compared to a case where the photoconductors  2   y ,  2   c , and  2   m  are held in contact with the intermediate transfer member  3  during an image forming operation of a black-and-white toner image. 
   The image forming apparatus  1  using the intermediate transfer method as shown in  FIG. 5  has a structure, in which a plurality of photoconductors carry their toner image which are different in colors from each other, transfer the respective toner images onto the intermediate transfer member  3  to form an overlaid full-color toner image, and then transfer the overlaid full-color toner image onto the recording medium P. As an alternative, the image forming apparatus  1  may have a structure in which one photoconductor carries one toner image in one cycle of a plurality of toner images with different colors from each other, such as yellow, cyan, magenta and black toner images, on a surface thereof, sequentially transfers toner images one after another onto the intermediate transfer member to form an overlaid full-color toner image, and then transfer the overlaid full-color toner image onto the recording medium P. In this case, merely one photoconductor is used for the image forming operation. 
   As described above, the image forming apparatus using the intermediate transfer method according to this embodiment of the present invention includes at least one photoconductor for bearing a toner image and an intermediate transfer member for receiving the toner image formed on the photoconductor, so that the toner image transferred onto the intermediate transfer member onto a recording medium to obtain a recorded image. 
   Referring to  FIG. 7 , a structure of an exemplary image forming apparatus  101  with a direct transfer method is described. When components included in the image forming apparatus  101  have structures and functions same as those of the image forming apparatus  1  of  FIG. 5 , the reference numerals for specifying the components of the image forming apparatus  1  are applied to the respective components of the image forming apparatus  101 , except for the image forming apparatus  101  and a recording medium bearing member  103 . 
   In  FIG. 7 , similar to the image forming apparatus with the intermediate transfer method, the image forming apparatus with the direct transfer method also includes four photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and a recording medium bearing member  103 . The photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are in a cylindrical shape, and have an outer diameter, The recording medium bearing member  103  forms an endless belt extended with supporting rollers  4 ,  5 , and  6 . The photoconductor  2   y ,  2   c ,  2   m  and  2   bk  are held in contact with the recording medium bearing member  103  and are rotated in a same direction that the intermediate transfer member  3  travels in  FIG. 7 . 
   Through the operations similar to those as described in the discussion of  FIG. 5 , yellow, cyan, magenta and black images are formed on the surfaces of the respective photoconductors  2   y ,  2   c ,  2   m  and  2   bk . The recording medium P fed from the sheet feeding cassette  14  is conveyed by the recording medium bearing member  103  and sequentially passes through portions between the respective photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the recording medium bearing member  103  so that respective color toner images formed on the respective photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are sequentially overlaid onto the recording medium P. The overlaid color toner image formed on the recording medium P is fixed to the recording medium P by the fixing unit  17 . After passing through the fixing unit  17 , the recording medium P is discharged to the sheet discharging tray  21 . 
   As described above, the image forming apparatus  101  with the direct transfer method of  FIG. 7  includes the recording medium bearing member  103 , and has a structure in which the recording medium bearing member  103  conveys a recording medium so that respective color toner images formed on the respective photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are transferred onto the recording medium. The image forming apparatus  1  with the intermediate transfer method of  FIG. 5 , on the other hand, transfers the respective color toner images formed on the respective photoconductors  2   y ,  2   c ,  2   m  and  2   bk  onto the intermediate transfer member  3  and then onto the recording medium. The difference described above is a basic difference between the image forming apparatus with the intermediate transfer method and that with the direct transfer method. 
   The image forming apparatus  101  of  FIG. 7  with the direct transfer method also has a commonly known structure with one photoconductor, which is same as that of the image forming apparatus  1  of  FIG. 5  with the intermediate transfer method. In this structure, the image forming apparatus  101  with the direct transfer method includes one photoconductor  2 . The one photoconductor  2  bears one toner image in one cycle of a plurality of toner images with different colors from each other on a surface thereof, sequentially transfers toner images one after another onto the recording medium P carried by the recording medium bearing member  103  to form an overlaid full-color toner image. This structure may also be applied to the present invention. Further, the image forming apparatus  101  with the direct transfer method may also have a structure in which a single toner image is formed on the photoconductor  2 , and is transferred onto a recording medium P carried by a recording medium bearing member  103 , so as to obtain a single color image. This structure may also be applied to the present invention, 
   As described above, the image forming apparatus  101  using the direct transfer method according to this embodiment of the present invention includes at least one photoconductor for bearing a toner image and a recording medium bearing member for carrying a recording medium for receive the toner image formed on the photoconductor, so that the toner image is directly transferred onto the recording medium bearing member to obtain a recorded image. 
   Hereinafter, the discussion will be made mainly for structures and functions with respect to the image forming apparatus with the intermediate transfer method. However, structures and functions with respect to the image forming apparatus with the direct transfer method may also be applied to the present invention. 
   Referring to  FIG. 8 , a structure of an image forming system driving the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the intermediate transfer member  3  is described with respect to the image forming apparatus with the intermediate transfer method of  FIG. 5  according to an exemplary embodiment of the present invention. The image forming system of  FIG. 8  is included in the image forming apparatus  1  of  FIG. 5 , and can also be applied to the image forming apparatus  101  of  FIG. 7 . 
   As shown in  FIG. 8 , the image forming apparatus  1  with the intermediate transfer method includes photoconductor motors M 1  and M 2  which drive the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  to rotate clockwise in  FIG. 5 , and a drive motor DM which drives the intermediate transfer member  3  to rotate in a direction A. The photoconductor motor M 1  of  FIG. 8  drives the photoconductors  2   y ,  2   c  and  2   m  to rotate for forming yellow, cyan and magenta toner images, respectively. The photoconductor motor M 2  of  FIG. 8  drives the photoconductor  2   bk  to rotate for forming a black-and-white toner image. 
   The image forming apparatus  101  of  FIG. 7  with the direct transfer method also includes the photoconductor motors M 1  and M 2  which drive the photoconductors  2   y,    2   c,    2   m  and  2   bk  to rotate, and the drive motor DM which drives the recording medium bearing member  103  to rotate. The photoconductor motors M 1  and M 2  and the drive motor DM included in the image forming apparatus  101  of  FIG. 7  with the direct transfer method have the same structures and functions as those of the photoconductor motors M 1  and M 2  and the drive motor DM included in the image forming apparatus  1  of  FIG. 5  with the intermediate transfer method, so that they drive the photoconductors  2   y,    2   c,    2   m  and  2   bk  and the recording medium bearing member  103  to rotate. 
   The photoconductors  2   y,    2   c,    2   m  and  2   bk  include gears  23   y,    23   c,    23   m  and  23   bk,  respectively. The gears  23   y,    23   c,    23   m  and  23   bk,  which are concentrically coupled with the respective photoconductors  2   y,    2   c,    2   m  and  2   bk  have a common radius and a common number of teeth. 
   Referring to  FIG. 9 , an alignment of a gear attached to a photoconductor is described. As previously indicated, the photoconductors  2   y,    2   c,    2   m  and  2   bk  have structures and functions similar to each other, except that the toners contained therein are of different colors, so the discussion with respect to  FIG. 9  uses reference numerals for specifying components of the image forming apparatus  1  without suffixes of colors such as y, c, m and bk. 
   The photoconductor  2  is supported by a photoconductor shaft  40  which is concentrically fixed thereto. The photoconductor shaft  40  is connected with a drive shaft  42  via a joint set  41 . The joint set  41  includes a first joint set  41   a  and a second joint set  41   b . The first joint set  41   a  is attached onto a portion of the photoconductor shaft  40  on the side close to the photoconductor  2 , and the second joint set  41   b  is attached onto a portion of the photoconductor shaft  40  on the side close to the gear  23 . The drive shaft  42  is concentrically mounted to the photoconductor shaft  40 , and is rotatably supported by a frame of the image forming apparatus  1  via first and second shaft bearings  43   a  and  43   b . The drive shaft  42  is also provided with the gear  23  that is also shown in  FIG. 8 . The gear  23  includes an adequate material such as a metal and resin. In this embodiment, the gear  23  includes a resin. 
   The photoconductor shaft  40  is rotatably mounted to a housing  45  via a third shaft bearing  44 . A process cartridge  46  is formed by a component at least one of the photoconductor  2 , the photoconductor shaft  40  corresponding to the photoconductor  2 , and the housing  45 . In  FIG. 9 , a charging roller  7  is also rotatably mounted to the housing  45 , as one component of the process cartridge  46 . As shown in  FIG. 9 , the process cartridge  46  is detachably provided to the image forming apparatus  1 . When the process cartridge  46  is removed from the image forming apparatus  1 , the first and second joint members  41   a  and  41   b  of the joint set  41  are detached from the photoconductor shaft  42 . 
   As shown in  FIG. 8 , the gear  23   y  coupled with the photoconductor  2   y,  and the gear  23   c  coupled with the photoconductor  2   c  are meshed with an intermediate gear  24 . That is, the gears  23   y  and  23   c  are in mesh via the intermediate gear  24 . The photoconductor motor M 1  includes an output shaft having a first output gear  25  fixed thereto. The first output gear  25  is meshed with the gear  23   c,  which is coupled with the photoconductor  2   c,  and the gear  23   m,  which is coupled with the photoconductor  2   m.  The second photoconductor motor M 2  includes an output shaft (not shown) having a second output gear  26  fixed thereto. The second output gear  26  is meshed with the gear  23   bk,  which is coupled with the photoconductor  2   bk.    
   When the photoconductor motor M 1  starts, the first output gear  25  rotates counterclockwise in  FIG. 8 , as indicated by an arrow shown in  FIG. 8 . Then, the gears  23   c  and  23   m,  which are meshed with the first output gear  25 , are rotated clockwise in  FIG. 8 , as indicated by arrows shown in  FIG. 8 . Consequently, the photoconductors  2   c  and  2   m  are rotated in a same direction as that of the gears  23   c  and  23   m,  and at a same number of rotations as that of the gears  23   c  and  23   m.    
   When the photoconductors  2   c  and  2   m  are rotated, the gear  23   y,  which is meshed with the gear  23   c  via the intermediate gear  24 , is also rotated. Accordingly, the photoconductor  2   y  is rotated in a same direction as that of the gear  23   y  and at a same number of rotations as that of the gear  23   y.  The photoconductor  2   y  has the same number of rotations as those of the photoconductors  2   c  and  2   m.    
   Further, when the photoconductor motor M 2  starts, the second output gear  26  rotates counterclockwise in  FIG. 8 , as indicated by an arrow shown in  FIG. 8 . Then, the gear  23   bk,  which is meshed with the second output gear  26 , is rotated clockwise in  FIG. 8 , as indicated by an arrow in  FIG. 8 . Consequently, the photoconductor  2   y  is rotated in a same direction as that of the gear  23   bk  and at a same number of rotations as that of the gear  23   bk.    
   In a case where needed, each of the gears  23   y ,  23   c  and  23   m  coupled with the photoconductors  2   y ,  2   c  and  2   m , respectively, is hereinafter referred to as a “color gear”, and the gear  23   bk  coupled with the photoconductor  2   bk  is hereinafter referred to as a “black-and-white gear.” 
   Further, as shown in  FIG. 8 , the supporting roller  4  that supports the intermediate transfer member  3  is integrally coupled with a first timing pulley  27  that is concentrically provided to the supporting roller  4 . The first timing pulley  27  and a second timing pulley  28 , which is fixed to an output shaft (not shown) of the drive motor DM, extendedly support a timing belt  29  which includes an endless belt. When the drive motor DM starts, the second timing. pulley  28  is rotated counterclockwise, as indicated by an arrow in  FIG. 8 . A driving force generated by the second timing pulley  28  is transmitted to the first timing pulley  27  via the timing belt  29 . Then, the supporting roller  4  is rotated counterclockwise, which is a same direction that the first timing pulley  27  is rotated, at a same number of rotations as that of the first timing pulley  27 . Consequently, the intermediate transfer member  3  is driven to rotate in a direction A as shown in  FIG. 8 . As described above, the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the intermediate transfer member  3  are driven to rotate, so that the above-described image forming operations are performed. 
   In  FIG. 8 , the image forming system includes a control circuit  30  and first and second drive circuits  31  and  32 . The control circuit  30  controls rotations of the photoconductor motors M 1  and M 2 , and the drive motor DM. The first and second drive circuits  31  and  32  are circuits for driving the photoconductor motors M 1  and M 2 , and the drive motor DM. 
   In the image forming system of  FIG. 8 , at least one motor of the photoconductor motors M 1  and M 2  and the drive motor DM includes a clock control motor. The clock control motor is controlled by a command clock signal and a feedback signal. In  FIG. 8 , the photoconductor motors M 1  and M 2  include the clock control motor, and the drive motor DM includes a stepping motor. A clock control motor that is commonly known is a direct current (DC) brushless motor. When the photoconductor motors M 1  and M 2  employ the DC brushless motor, the image forming system can reduce its power consumption and noise when compared to the photoconductor motors M 1  and M 2  employing the stepping motor. 
   In addition to the photoconductor motors M 1  and M 2 , the drive motor DM may also include the clock control motor employing the DC brushless motor. By doing so, the above-described power consumption and noise may further be reduced. Nevertheless, the image forming apparatus  1  of the present invention uses a stepping motor for the drive motor OM because of reasons described below. 
   Generally, the intermediate transfer member  3  and the recording medium bearing member  103  can be rotated with a small amount of driving force. Accordingly, a small motor is required for the drive motor DM. However, a DC brushless motor which is compact in size and less expensive in cost is not in the market at the present time, so a small-sized stepping motor is reasonable for the driving motor DM to reduce manufacturing costs of the image forming apparatus  1 . That is why the stepping motor is employed as the drive motor DM for the image forming apparatus  1 . 
   By controlling the number of input pulses, the stepping motor can correctly control the rotation numbers during a rise time period, a fall time period, and a steady rotation time of the stepping motor. 
   On the contrary, it is difficult to correctly control the number of rotations of the DC brushless motor during the rise and fall time periods to obtain a desired number of rotations. When a background image forming apparatus uses the DC brushless motor for driving a photoconductor and an intermediate transfer member, a surface linear velocity of the photoconductor and that of the intermediate transfer member contacting the photoconductor may be substantially different during the rise and fall time periods. That is, a surface of the photoconductor rubs that of the intermediate transfer member extremely hard, and thereby the surfaces thereof may be worn away. 
   To eliminate the problem, tests were conducted and it was found that if the DC brushless motor is controlled to rotate according to a predetermined velocity curve, a substantially desired rotation rate may be obtained during a steady rotation time, a rise time period and a fall time period of the DC brushless motor. That is, the DC brushless motor that rotates at a rate according to the number of clocks of the command clock signal may be constructed such that the DC brushless motor is controlled to rotate during its rise and fall time periods by the command clock signal having the number of input pulses according to the predetermined velocity curve. The number of input pulses represents the number of input pulses generated in a unit time, that is a frequency. 
   Specifically, the image forming system of  FIG. 8  operates as follows. A memory.  33  of  FIG. 8  includes data of the predetermined velocity curve. The command clock signal according to the velocity curve is output from the control circuit  30  to drive the photoconductor motors M 1  and M 2  to rotate including the DC brushless motor at a rotation rate according to the number of input pulses. Feedback signals FB 1  and FB 2  that are output from the photoconductor motors M 1  and M 2 , respectively, are compared with the above-described command clock signal to control the numbers of rotations of the photoconductor motors M 1  and M 2 . The feedback signals FB 1  and FB 2  are pulse signals according to the numbers of rotations of the photoconductor motors M 1  and M 2 . A feedback signal can be detected according to the number of rotation of a component which is rotated by the photoconductor motors M 1  and M 2 , such as the photoconductors  2   y ,  2   c ,  2   m  and  2   bk . With this structure, the clock control motor is controlled by the command clock signal and the feedback signal. 
   In the image forming system of the image forming apparatus  1  shown in  FIG. 8 , the drive motor DM includes a stepping motor. Therefore, the command clock signal synchronized with the rotation of the drive motor DM needs to be input to the photoconductor motors M 1  and M 2  such that surface linear velocities of the photoconductors  2   y,    2   c,    2   m  and  2   bk  may be approximately the same as that of the intermediate transfer member  3 . To prevent an easy wearing of the photoconductors  2   y,    2   c,    2   m  and  2   bk  and the intermediate transfer member  3 , the rotation of the DC brushless motor is controlled as follows. During the rise time period, the number of input pulses (frequency) of the command clock signal is continuously or gradually increased. During the fall time period, the number of input pulses of the command clock signal is continuously or gradually decreased. During the steady rotation time, the number of input pulses of the command clock signal is in a constant rate. Thus, the rotation of the DC brushless motor is controlled. By doing so, the intermediate transfer member  3  and the photoconductors  2   y,    2   c,    2   m  and  2   bk  which rotatably contact with the intermediate transfer member  3  during the rise and fall time periods of the photoconductor motors M 1  and M 2  and the drive motor DM may rotate at an approximately same surface linear velocity, and thereby the surfaces thereof are prevented from the easy wearing. 
   The easy wearing of the surfaces of the intermediate transfer member  3  and the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  may also be reduced even if the above-described controls are performed during one of the rise and fall time periods. That is, at least one motor of the photoconductor motors M 1  and M 2  and the drive motor DM includes the clock control motor, more specifically the DC brushless motor, and a control unit for controlling the number of the clock control motor according to a predetermined velocity curve during at least one of the rise and fall time periods. By using the control unit, the wearing of the intermediate transfer member  3  and the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  may be reduced and, at the same time, the power consumption and the operation noise may also be reduced. In the image forming apparatus  1 , the control circuit  30  and the memory  33  of  FIG. 8  represent the above-described control unit. 
   As described above, the rotation of the clock control motor is controlled by the command clock signal having the number of input pulses according to the above-described velocity curve during at least one of the rise and fall time periods. More preferably, the rotation of the clock control motor is controlled by the command clock signal having the gradually increasing number of input pulses during the rise time period, by the command clock signal having the constant number of clocks during the steady rotation time, and by the command clock signal having the gradually decreasing number of input pulses during the fall time period. The above-described structure is also applied to the image forming apparatus  101  with the direct transfer method. 
   Next, a detailed example of the above-described embodiment of the image forming apparatus  1  shown in  FIG. 5  is described. 
   The drive motor DM is a stepping motor having specifications shown in Table 1 as described below. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Excitation Method 
               Unipolar, 1–2 phase 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               Motor rotations 
               During steady 
               2255.423 
               PPS 
             
             
                 
               (PPS, pulse per 
               rotation time 
             
             
                 
               sec) 
               At start 
               786 
               PPS 
             
             
                 
                 
               At stop 
               786 
               PPS 
             
             
                 
               Number of steps 
               At start 
               100 
               steps 
             
             
                 
                 
               At stop 
               100 
               steps 
             
             
                 
               Transition time 
               Rise time period 
               1000 
               mm/sec 
             
             
                 
               period 
               Fall time period 
               1000 
               mm/sec 
             
          
         
         
             
             
             
             
          
             
                 
               Surface linear velocity of intermediate 
               155 
               mm/sec 
             
             
                 
               transfer member in steady rotation time 
             
             
                 
                 
             
          
         
       
     
   
   The photoconductor motors M 1  and M 2  are DC brushless motors. Rotations of the DC brushless motor are controlled according to a velocity curve corresponding to the specifications of the stepping motor that is shown in Table 1. 
   Generally, a primary frequency F (Hz) is obtained by a formula of:
 
 F=N*Fd; 
 
   where “N” represents a natural number, and “Fd” represents a dividing frequency based on the primary frequency. According to the above-described formula, a relationship between a fundamental frequency F (Hz) and a predetermined dividing frequency Fd (Hz) of the image forming apparatus  1  is defined as the above-described formula, F=N*Fd, that is, Fd=F/N. 
   On the other hand, the dividing frequencies Fd (Hz) of the photoconductor motors M 1  and M 2  that include the DC brushless motors are obtained by a formula of:
 
 Fd=R*P/ 60( s );
 
   where “R” represents the number of rotations of the DC brushless motor (rpm), and “P” represents the number of frequency generation (FG) pulses to rotate the DC brushless motor for one cycle. According to the above-described formulae, the primary frequency F (Hz) can be obtained by a formula of:
 
 F=N*R*P/ 60( s ).
 
That is, the number of rotations of the DC brushless motor (rpm) can be obtained by a formula of:
 
 R=F* 60( s )/( P*N ).
 
   According to the relationships as described above, the rotation numbers of the photoconductor motors M 1  and M 2  can be modified by changing the natural number N. Further, by changing the number of pulses (FG pulses) of the command clock signal supplied to the photoconductor motors M 1  and M 2 , the dividing frequency Fd can be controlled to set the rotation numbers of the respective photoconductor motors M 1  and M 2  to respective desired numbers. Thus, the rotation numbers of the photoconductor motors M 1  and M 2  are controlled to adjust the surface linear velocities of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk.    
   As an example of the surface linear velocities of the stepping motor used for the image forming apparatus  1 , it was assumed the fundamental frequency F is 9830400 (Hz), and the number of FG pulses P is 45. Table 2 shows exemplary results according to the formulae as described above. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
               Common 
                 
                 
               Surface linear 
             
             
                 
               denominator 
               Dividing 
                 
               velocity of 
             
             
                 
               (Natural 
               frequency 
               Motor speed 
               photoconductor 
             
             
                 
               number) 
               (Hz) 
               (rpm) 
               (mm/sec) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               8310 
               1182.960289 
               1577.280385 
               155.1588 
             
             
                 
               8311 
               1182.817952 
               1577.090603 
               155.1918 
             
             
                 
               8312 
               1182.67565 
               1576.900866 
               155.1731 
             
             
                 
               8313 
               1182.533381 
               1576.711175 
               155.1544 
             
             
                 
               8314 
               1182.391147 
               1576.52153 
               155.1358 
             
             
                 
               8315 
               1182.248948 
               1576.33193 
               155.1171 
             
             
                 
               8316 
               1182.106782 
               1576.142376 
               155.0985 
             
             
                 
               8317 
               1181.964651 
               1575.952868 
               155.0798 
             
             
                 
               8318 
               1181.822553 
               1575.763405 
               155.0612 
             
             
                 
               8319 
               1181.68049 
               1575.573987 
               155.0425 
             
             
                 
               8320 
               1181.538462 
               1575.384615 
               155.0239 
             
             
                 
               8321 
               1181.396467 
               1575.195289 
               155.0053 
             
             
                 
               8322 
               1181.254506 
               1575.006008 
               154.9866 
             
             
                 
               8323 
               1181.11258 
               1575.816773 
               154.9680 
             
             
                 
               8324 
               1180.970687 
               1574.627583 
               154.9494 
             
             
                 
                 
             
          
         
       
     
   
   Referring to  FIG. 10 , a schematic graph of velocity curves of the drive motor DM including the stepping motor and the first and second photoconductor motors M 1  and M 2  including the DC brushless motor are described. A vertical axis of the graph indicates-the number of motor rotations, and a horizontal axis of the graph-indicates time. A velocity curve A indicates the number of pulses of the drive motor DM. A velocity curve B indicates the number of the pulses of the first photoconductor motor M 1 , and a velocity curve C indicates the number of pulses of the second photoconductor motor M 2 . The velocity curve A of  FIG. 10  includes the number of pulses S 0  which indicates the number of pulses at a start of the drive motor DM. The number of pulses S 0  is 786 PPS, as shown in Table 1. Table 1 also indicates that periods required to the drive motor DM during the rise and fall time periods are 1000 msec each, the numbers of steps required at that time are 100 steps each, and the number of pulses during the steady rotation is 2255.423 PPS. 
   The rotation speeds of the first and second photoconductor motors M 1  and M 2  shown as the velocity curves B and C of  FIG. 10  are controlled according to the velocity curve of the stepping motor indicated as the velocity curve A of  FIG. 10 . The numbers of pulses S 1  and S 2  indicate the number of pulses at a start of the photoconductor motors M 1  and M 2  respectively. Here, the natural number described above is set to 23800 so that the numbers of pulses S 1  and S 2  may become 550.7 rpm. The settings are made as described above because the photoconductor motors M 1  and M 2  may not be correctly rotated even if the clock having the number below the number of rotations during the steady rotation time is given at the start of the photoconductor motors M 1  and M 2 . 
   A time required for the rise and fall time periods of the first and second photoconductor motors M 1  and M 2  is 1000 msec, which is the same as the time required to the drive motor DM. The DC brushless motor generally completes its rise time period of approximately 400 msec when a load to the motor drive shaft is 0.8 kgfcm. However, as shown in  FIG. 10 , by setting the rise and fall time periods of the photoconductor motors M 1  and M 2  to 1000 msec, which is far longer than 4000 msec, the velocity curves of the photoconductor motor M 1  and M 2  may be close to the velocity curve of the drive motor DM including the stepping motor with a higher precision, and thereby the wearing of the surfaces of the photoconductors  2   y,    2   c,    2   m  and  2   bk  and the intermediate transfer member  3  may effectively be reduced. 
   In this example, the number of rotations of the photoconductor motors M 1  and M 2  during the steady rotation time is approximately 1576.33. Accordingly, as shown in Table 2, the natural number during-the steady rotation time of the photoconductor motors M 1  and M 2  is 8315, the divided frequency is approximately 1182.2489, and the surface linear velocities of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are 155.12 mm/sec. 
   By controlling the number of clocks of the command clock signal to be supplied to the photoconductor motors M 1  and M 2  as described above, the surface linear velocities of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  may be substantially equal to that of the intermediate transfer member  3  during the steady rotation time, the rise time period, and the fall time period. 
   When the number of rotations of the DC brushless motor become below a predetermined number of rotation, its control becomes difficult even during the fall time period. To eliminate the problem, as shown in  FIG. 8 , a feeler is provided to a gear attached to a photoconductor producing a color toner image. In this example, a feeler Fm is provided to the gear  23   m  attached for the photoconductor  2   m  producing a magenta toner image, and a feeler Fbk is provided to the gear  23   bk  attached for the photoconductor  2   bk  producing a black toner image. And, first and second sensors  34   m  and  34   bk  are fixedly disposed at the gears  23   m  and  23   bk , respectively. These sensors  34   m  and  34   bk  includes a photo sensor, for example. 
   Referring to  FIG. 11 , the numbers of rotations of the photoconductor motors M 1  and M 2  including the DC brushless motor during the fall time period are described.  FIG. 11  shows that when the numbers of rotations of the photoconductor motors M 1  and M 2  reach their respective predetermined values, the first and second sensors  34   m  and  34   bk  of  FIG. 8  are started for checking. In this example, when the photoconductor motors M 1  and M 2  rotate at 550.7 rpm (the above-described natural number 23800), the first and second sensors  34   m  and  34   bk  are started. The numbers of clocks of the command clock signal which are input to the photoconductor motors M 1  and M 2  during the fall time period gradually decreases, as indicated by a dashed line in  FIG. 11 . When the photoconductor motors M 1  and M 2  rotate at the speed of 550.7 rpm, the input of clocks of the command clock signal to the photoconductor motors M 1  and M 2  is stopped. After the input of the clocks is stopped, if the first and second sensors  34   m  and  34   bk  detect the feelers Fm and Fbk, respectively, the speeds of the photoconductor motors M 1  and M 2  are forcedly decreased by applying the brakes so as to stop the photoconductor motors M 1  and M 2 . Such control is made every time the clock pulses of the photoconductor motors M 1  and M 2  fall, both in the color mode and in the black-and-white mode. Since the photoconductor motors are forcedly stopped, the number of rotations of the photoconductor motors M 1  and M 2  may easily become close to or meet with the number of rotations of the drive motor MD. 
   Referring to  FIGS. 12A ,  12 B and  12 C, states of a braking unit that applies the brakes onto the photoconductor motors M 1  and M 2  are described. A coil  35  of  FIG. 12A  represents a winding of the DC brushless motor included in the photoconductor motors M 1  and M 2 . When the DC brushless motor rotates, a counter electromotive voltage is generated. Although the counter electromotive voltage and its action cannot be seen, it is illustrated in  FIG. 12 , represented by a symbol of a direct current having a reference numeral as a “counter electromotive voltage 36”. When the DC brushless motor rotates, an electric current I flows in a direction indicated by an arrow in  FIG. 12B . At this time, the DC brushless motor rotates clockwise. Under the status as shown in  FIG. 12B , a short brake SB is turned on as shown in  FIG. 12C , the counter electromotive voltage  36  is generated, and the electric current I flows oppositely. At this time, the DC brushless motor tries to rotate counterclockwise, so that the brake is applied to the DC brushless motor included in the photoconductor motors M 1  and M 2 . Since the counter electromotive voltage becomes proportional to the number of rotations of a motor, when the number of rotations becomes 0 rpm, the counter electromotive voltage becomes 0V, and the motor stops without rotating counterclockwise. 
   As described above, the image forming apparatus  1  of the present invention includes the braking unit forcedly decreasing the speed of the clock control motor, when the number of rotations of the clock control motor becomes equal to or less than a predetermined value at the stop of the clock control motor including the DC brushless motor. 
   Referring to  FIG. 13 , a test result examined at the start of the photoconductor motors M 1  and M 2  and the drive motor DM using the image forming apparatus  1  of  FIGS. 5 to 8 . The horizontal axis shows time, and the vertical axis surface linear velocities of the photoconductors  2   m  and  2   bk  and that of the intermediate transfer member  3 . A solid line represents an actual measured value of the intermediate transfer member  3 , a dashed line represents an actual measured value of the photoconductor  2   bk , and a short and long dash line represents an actual measured value of the photoconductor  2   m , which are common to  FIG. 14 . 
   As shown in  FIG. 13 , the photoconductor motors M 1  and M 2  and the drive motor DM start at a speed of 1000 msec. If such a long period of time is taken for the start, a slope for the surface linear velocity at the start does not change, when a load to the motor driving shaft of the photoconductor motors M 1  and M 2  vary at a value between 0 to 0.8 kgfcm. 
   Referring to  FIG. 14 , another test result is described. Tests were conducted under a condition that the photoconductor motors M 1  and M 2  and drive motor DM start and stop at a speed of 1000 msec, and steadily rotate at a speed of 6000 msec. As shown in  FIG. 13 , a solid line represents an actual measured value of the intermediate transfer member  3 , a dashed line represents an actual measured value of the photoconductor  2   bk , and a short and long dash line represents an actual measured value of the photoconductor  2   m .  FIG. 14  can tell that the photoconductor motors M 1  and M 2  including the DC brushless motor can be controlled at the start and stop thereof. 
   In  FIG. 13 , the supply of the command clock signal is continuously increased at the start of the photoconductor motors M 1  and M 2 . By doing so, the surface linear velocities of the photoconductors  2   m  and  2   bk  linearly start as well. The status is same as a status at the start shown in  FIG. 14 . However, if the photoconductor motors M 1  and M 2  are controlled at the start and stop thereof, in a same manner as described above, a large amount of memory is required, and thereby a cost of the image forming apparatus  1  may be increased. 
   Hence, in a period at least one of the start and stop of the clock control motor including the DC brushless motor, the number of clocks of the command clock signal is changed in stages to control the number of rotations of the clock control motor. By doing so, an excessive amount of memory is not required and the cost of the image forming apparatus may be reduced. 
   Referring to  FIG. 15 , an example of the test that the clocks of the command clock signal is changed in twenty stages when the photoconductor motors M 1  and M 2  are started. In the test, the number of clocks of the command clock signal to be supplied to the photoconductor motors M 1  and M 2  is incremented by one per one step. In this case, the command clock signal to the first and second photoconductor motors M 1  and M 2  is supplied from the same source as before, the surface linear velocities of the photoconductors  2   m  and  2   bk  have a substantially same curve at the start. When the photoconductor motors M 1  and M 2  are stopped, the motors M 1  and M 2  can be controlled as described above. 
   As previously described, the image forming apparatus  1  shown in  FIGS. 5 to 8  includes the photoconductors  2   y ,  2   c  and  2   m  for producing color toner images, the gears  23   y ,  23   c  and  23   m  coupled with the photoconductors  2   y ,  2   c  and  2   m , respectively, the photoconductor  2   bk  for producing a black-and-white toner image, the gear  23   bk  coupled with the photoconductor  2   bk , the first photoconductor motor M 1  including the clock control motor which rotates the photoconductors  2   y ,  2   c  and  2   m  via the gears  23   y ,  23   c  and  23   m , respectively, and the second photoconductor motor M 2  including the clock control motor which rotates the photoconductor  2   bk  via the gear  23   bk . Both of the clock control motors for color and black-and-white images include the DC brushless motor. 
   When the above described gears  23   y ,  23   c ,  23   m  and  23   bk  include a resin material, it is generally mandatory that they have eccentricity to their respective shafts. With such eccentricity, an overlaid full-color image transferred from the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  onto the intermediate transfer member  3  may have color shift therein. Hence, in the image forming apparatus  1  of the present invention, to prevent the color shift of the overlaid full-color image, the gears  23   y ,  23   c ,  23   m  and  23   bk  are disposed to have their predetermined phases in the rotation direction of the gears  23   y ,  23   c ,  23   m  and  23   bk . It is commonly known that background image forming apparatuses have such structure as described above. 
   Referring to  FIG. 17 , positions and phases of the gears  23   y ,  23   c ,  23   m  and  23   bk  and the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  corresponding to the gears  23   y ,  23   c ,  23   m  and  23   bk  are described. The photoconductors  2   y ,  2   c ,  2   m  and  2   bk  have a portion contacting the intermediate transfer member  3  for transferring respective single color toner images formed on the surfaced thereon onto the surface of the intermediate transfer member  3 . The portion is referred to as a “transfer portion”. A distance from the transfer portion of one photoconductor to that of another photoconductor mounted next to the one photoconductor is referred to as a “distance PT”. That is, the distance PT is formed between the photoconductors  2   y  and  2   c , between the photoconductors  2   c  and  2   m , and between the photoconductors  2   m  and  2   bk . In addition, a reference position is provided to each of the gears  23   y ,  23   c ,  23   m  and  23   bk  which have an eccentricity equal to each other, and the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  corresponding to the gears  23   y ,  23   c ,  23   m  and  23   bk  in the circumferential direction thereof. The reference position is referred to as a “reference position X”, and is arranged at a portion farthest from the center of the shaft of the gears  23   y ,  23   c ,  23   m  and  23   bk , and that of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  corresponding to the gears  23   y ,  23   c ,  23   m  and  23   bk , respectively, in the circumferential direction. 
     FIG. 17  shows a status that the reference position X of the photoconductor  2   y  for a yellow toner image is at the transferring portion, that is, a status that the yellow toner image formed on the surface of the photoconductor  2   y  is transferred onto the intermediate transfer member  3 . In  FIG. 17 , the photoconductors  2   y  and  2   c  are arranged adjacent to each other with the distance PT. That is, the reference position X of the photoconductor  2   c  is located upstream from its transfer portion by the distance PT in the rotation direction of the photoconductor  2   c . Similar to the photoconductor  2   c , the reference position X of the photoconductor  2   m  is located upstream from its transfer portion by approximately twice the distance PT, and the reference position X of the photoconductor  2   bk  is located upstream from its transfer position by approximately three times the distance PT. 
   As shown in  FIG. 8 , the gears  23   y ,  23   c ,  23   m  and  23   bk  are in mesh with the intermediate gear  24  and the first and second output gears  25  and  26 . However,  FIG. 17  shows, as a matter of convenience, that the intermediate gear  24  and the first and second output gears  25  and  26  which drive the gears  23   y ,  23   c ,  23   m  and  23   bk  are in mesh with the gears  23   y ,  23   c ,  23   m  and  23   bk  at identical positions in the circumferential direction thereof. 
   As described above, the circumferential phases of the gears  23   y ,  23   c ,  23   m  and  23   bk  and the meshing positions of the intermediate gear  24  and the first and second output gears  25  and  26  that drive the gears  23   y ,  23   c ,  23   m  and  23   bk  are specified. With this structure, even if the gears  23   y ,  23   c ,  23   m  and  23   bk  have a slight eccentricity, the overlaid full-color toner image transferred onto the intermediate transfer member  3  may be prevented from color shift. The circumferential phases of the gears  23   y ,  23   c ,  23   m  and  23   bk  and the meshing positions of the intermediate gear  24  and the first and second output gears  25  and  26  that drive the gears  23   y ,  23   c ,  23   m  and  23   bk , as shown in  FIG. 8 , are relatively specified so as to obtain the same effect as that shown in  FIG. 17 . That is, the gears  23   y ,  23   c ,  23   m  and  23   bk  have respective-mounting angles to prevent the color shift on a full-color image completely produced. 
   Here, in the image forming apparatus  1  of the present invention, a color image is produced in the color mode and a black-and-white image is produced in the black-and-white mode, as previously described. In an image forming operation in the color mode, the first photoconductor motor M 1  drives the photoconductors  2   y ,  2   c  and  2   m  to-rotate for forming respective single color toner images on the surfaces thereon, and the second photoconductor motor M 2  drives the photoconductor  2   bk  to rotate for forming a black-and-white toner image on the surface thereon. The respective single color toner images and the black-and-white toner image are then transferred onto the intermediate transfer member  3 , and onto the recording medium P to obtain a full-color image. Further, in an image forming operation in the black-and-white mode, the first photoconductor motor M 1  does not operate the photoconductors  2   y ,  2   c  and  2   m  while the second photoconductor motor M 2  drives the photoconductor  2   bk  to rotate for forming a black-and-white toner image on the surface thereon. The black-and-white toner image is then transferred onto the intermediate transfer member  3 , and onto the recording medium P to obtain a black-and-white image. Specifically, while the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are held in contact with the intermediate transfer member  3  in the color mode, the photoconductors  2   y ,  2   c , and  2   m  are separated from the intermediate transfer member  3  and the photoconductor  2   bk  is held in contact with the intermediate transfer member  3  in the black-and-white mode. The color mode and the black-and-white mode are selectably provided to the image forming apparatus  1  of the present invention. 
   As previously described, when the image forming operation is performed in-the black-and-white mode, only the photoconductor  2   bk  is rotated but the photoconductors  2   y ,  2   c  and  2   m  are stopped. Therefore, the gears  23   y ,  23   c ,  23   m  and  23   bk  shown in  FIG. 17  may be out of phase in the circumferential direction thereof. 
   However, the image forming apparatus  1  of the present invention is provided with the feelers Fm and Fbk, and the first and second sensors  34   m  and  34   bk . And, the image forming apparatus  1  also applies the brake on the first and second photoconductor motors M 1  and M 2  including the DC brushless motor at the stop thereof in the color mode, and it also applies the brake on the second photoconductor motor M 2  in the black-and-white mode. Therefore, the gears  23   y ,  23   c ,  23   m  and  23   bk  and the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  can be stopped at an approximately same position. By doing so, the previously described relationship of the gears  23   y ,  23   c ,  23   m  and  23   bk  is prevented from significantly being out of the above-described phase. 
   However, it is difficult for the above-described braking unit to maintain the relationship of phases of the gears  23   y ,  23   c ,  23   m  and  23   bk  with a high precision. Therefore, another structure instead of the above-described braking unit is preferably employed for adjusting the relationship of phases-of the gears  23   y ,  23   c ,  23   m  and  23   bk.    
   As previously described with reference to  FIG. 8 , the image forming apparatus  1  includes the first and second sensors  34   m  and  34   bk  for detecting the feelers Fm and Fbk provided to the gears  23   m  and  23   bk . The first sensor  34   m  detects a first position, which corresponds to the position of the feeler Fm, of the gear  23   m  in the circumferential direction of the gear  23   m , and the second sensor  34   bk  detects a second position, which corresponds to the position of the feeler Fbk, of the gear  23   bk  in the circumferential direction of the gear  23   bk . As an alternative, the feelers Fm and Fbk may be provided at the first and second positions, respectively, of the photoconductors  2   m  and  2   bk , respectively, so that the first and second sensors  34   m  and  34   bk  can detect the feelers Fm and Fbk. 
   As described above, the image forming apparatus  1  includes the first sensor  34   m  for detecting the first position in the circumferential direction of the gear  23   m  (in  FIG. 8 ) for a color image, and the second sensor  34   bk  for detecting the second position in the circumferential direction of the gear  23   bk  for a black-and-white image. In the image forming apparatus  1 , the phases of the respective gears  23   y ,  23   c  and  23   m  for the color images and that of the gear  23   bk  for the black-and-white are adjusted in a period after the first and second photoconductor motors M 1  and M 2  are stopped and before the next image forming operation is started. That is, the relationship of the phases is adjusted in a period before the first and second photoconductor motors M 1  and M 2  steadily rotate. At this time, a time lag may be generated between a time when the first sensor  34   m  detects the first position that is the position of the feeler Fm and that when the second sensor  34   bk  detect the second position that is the position of the feeler Fbk, which is represented by “Δt”. According to the time lag Δt, the number of rotations of at least one photoconductor motor of the first and second photoconductor motors M 1  and M 2  may be controlled, and the gears  23   y ,  23   c ,  23   m  and  23   bk  maintain or become close to the above-described relationship of the phases. 
   More specifically, when the color gears  23   y ,  23   c  and  23   m  and the black-and-white gear  23   bk  are correctly arranged to maintain the above-described respective predetermined phases for preventing the color shift and are rotated at the steady rotation, a reference time lag generated between a time when the first sensor  34   m  detects the feeler Fm and a time when the second sensor  34   bk  detects the feeler Fbk, which is defined as “ΔT”. The time lag ΔT may include an appropriate number including zero (0). In this example, the reference time lag ΔT is set to zero. And, before adjusting the actual phases, according to a time difference between the time lag Δt and the reference time lag ΔT (zero in this example), the number of clocks of-the command clock signal to be supplied from the control circuit  30  to the first and second photoconductor motors M 1  and M 2  is increased or decreased. By doing so, the number of the photoconductor motors M 1  and M 2  can be controlled and the relationship of the phases of the gears  23   y ,  23   c ,  23   m  and  23   bk  are adjusted as described above. Then, the numbers of rotations of the photoconductor motors M 1  and M 2  are returned to those for the steady rotations to perform the image forming operations. With this structure, a color shift may be reduced and a high quality image may be obtained. When the time difference between the time lag Δt and the reference time lag ΔT is defined as a sensor detection time lag ΔS, the sensor detection time lag ΔS of the image forming apparatus  1  of the present invention may be equal to the time lag Δt. 
   As described above, the control unit including the control circuit  30  is configured such that when adjusting the relationship of the phases of the color gears  23   y ,  23   c  and  23   m  and the black-and-white gear  23   bk , according to the time lag generated between a time when the first sensor  34   m  detects the first position and a time when the second sensor  34   bk  detects the second position, the number of rotations of at least one of the photoconductor motors M 1  and M 2 . The control unit controls by changing the number of rotations of at least one of the first and second photoconductor motors M 1  and M 2  the color photoconductors  2   y ,  2   c  and  2   m , so that the predetermined relationship of the phases of the color gears  23   y ,  23   c  and  23   m  and the black-and-white gear  23   bk  may be obtained in a period after the first and second photoconductor motors M 1  and M 2  are stopped and before the next image forming operation is started, that is, before the first and second photoconductor motors M 1  and M 2  steadily rotate. 
   Referring to  FIG. 18 , a detailed example of the phase. adjusting operation of the relationship of the above-described phases is described. 
   In Step S 1  of  FIG. 18 , rotations of the first and second photoconductor motors M 1  and M 2  are started. In Step S 2 , it is determined whether 1000 msec, which is a rise time period of the photoconductor motors M 1  and M 2 , has passed. When 1000 msec has not passed and when the determination result in Step S 2  is NO, the process of Step S 2  repeats until the rotation speeds of the photoconductor motors M 1  and M 2  exceed 1000 msec. When 1000 msec has passed and the determination result in Step S 2  is YES, the first and second sensors  34   m  and  34   bk  are started to be checked. In Step S 3 , it is determined whether the second sensor  34   bk  detects the feeler Fbk, which is the second position of the black-and-white gear  23   bk , before the first sensor  34   m  detects the feeler Fm. When the second sensor  34   bk  detects the feeler Fbk before the first sensor  34   m  detects the feeler Fm and when the determination result in Step S 3  is YES, the procedure goes to Steps S 4  through S 11  of  FIG. 18 . (When the second sensor  34   bk  does not detect the feeler Fbk before the first sensor  34   m  detects the feeler Fm and when the determination result in Step S 3  is NO, the procedure goes to Step S 12 .) 
   In Step S 4  of  FIG. 18 , it is determined whether the above-described sensor detection time lag ΔS is less than 40 ms. When the sensor detection time lag ΔS is less than. 40 ms and when the determination result in Step S 4  is YES, the phase adjusting operation is completed. When the sensor detection time lag ΔS is equal to or more than 40 ms and when the determination result in Step S 4  is NO, the procedure goes to Step S 5 . 
   In Step S 5  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 40 ms and less than 80 ms. When the sensor detection time lag ΔS is equal to or more than 40 ms and less than 80 ms and when the determination result in Step S 5  is YES, the procedure goes to a process C 1  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 40 ms and not less than 80 ms and when the determination result in Step S 5  is NO, the procedure goes to Step S 6 . 
   In Step S 6  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 80 ms and less than 152 ms when the sensor detection time lag ΔS is equal to or more than 80ms and less than 152 ms and when the determination result in Step S 6  is YES, the procedure goes to a process C 2  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 80 ms and not less than 152 ms and when the determination result in Step S 6  is NO, the procedure goes to Step S 7 . 
   In Step S 7  of  FIG. 18 , it is determined whether the sensor detection time lag Δs is equal to or more than 152 ms and less than 305 ms. When the sensor detection time lag ΔS is equal to or more than 152 ms and less than 305 ms and when the determination result in Step S 7  is YES, the procedure goes to a process C 3  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 152 ms and not less than 305 ms and when the determination result in Step S 7  is NO, the procedure goes to Step S 8 . 
   In Step S 8  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 305 ms and less than 458 ms. When the sensor detection time lag ΔS is equal to or more than 305 ms and less than 458 ms and when the determination result in Step S 8  is YES, the procedure goes to a process C 4  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 305 ms and not less than 458 ms and when the determination result in Step S 8  is NO, the procedure goes to Step S 9 . 
   In Step S 9  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 458 ms and less than 530 ms. When the sensor detection time lag ΔS is equal to or more than 458 ms and less than 530 ms and when the determination result in Step S 9  is YES, the procedure goes to a process C 5  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 458 ms and not less than 530 ms and when the determination result in Step S 9  is NO, the procedure goes to Step S 10 . 
   In Step S 10  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 530 ms and less than 570 ms. When the sensor detection time lag ΔS is equal to or more than 530 ms and less than 570 ms and when the determination result in Step S 10  is YES, the procedure goes to a process C 6  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 530 ms and not less than 570 ms and when the determination result in Step S 10  is NO, the procedure goes to Step S 11 . 
   In Step S 11  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS Is equal to or more than 570 ms and less than 610 ms. When the sensor detection time lag Δs is equal to or more than 570 ms and less than 610 ms and when the determination result in Step S 11  is YES, the phase adjusting operation is completed. When the sensor detection time lag ΔS is equal to or more than 610 ms and when the determination result in Step S 11  is NO, the procedure goes to an error handling operation. 
   For example, when the sensor detection time lag ΔS is less than 40 ms in Step S 4  or when the sensor detection time lag ΔS is equal to or more than 570 ms and less than 610 ms, the gears  23   y ,  23   c ,  23   m  and  23   bk  are, fox example, approximately ±22.5 degrees and are rarely out of phases. Accordingly, it is determined that the operation states of the gears  23   y ,  23   c ,  23   m  and  23   bk  are regarded as being within a regular range and the process is completed. Here, a time of 610 ms indicates a time required for one cycle of the photoconductor  2   bk . When the sensor detection time lag ΔS makes any value indicated in Steps S 5  through  10 , one of the following processes C 1  through C 6  is performed according to the value. Rates (%) indicated below represent a rotation rate of each photoconductor during the steady rotation time:
     Process C1: Number of Rotations of Photoconductor 2BK−5%,
       Number of Rotations of Photoconductor 2M+5%;   
       Process C2: Number of Rotations of Photoconductor 2BK−10%,
       Number of Rotations of Photoconductor 2M+10%;   
       Process C3: Number of Rotations of Photoconductor 2BK−16%,
       Number of Rotations of Photoconductor 2M+16%;   
       Process C4: Number of Rotations of Photoconductor 2BK+16%,
       Number of Rotations of Photoconductor 2M−16%;   
       Process C5: Number of Rotations of Photoconductor 2BK+10%,
       Number of Rotations of Photoconductor 2M−10%;   
       Process C6: Number of Rotations of Photoconductor 2BK+5%,
       Number of Rotations of Photoconductor 2M−5%.   
       

   As described above, when the second sensor  34   bk  does not detect the feeler Fbk before the first sensor  34   m  detects the feeler Fm and when the determination result in Step S 3  is NO, the procedure goes to Step S 12 . 
   In Step S 12 , it is determined whether the first sensor  34   m  detects the feeler Fm before the second sensor  34   bk  detects the feeler Fbk. When the first sensor  34   m  detects the feeler Fm before the second sensor  34   bk  detects the feeler Fbk and when the determination result in Step S 12  is YES, the procedure goes to Steps S 13  through  520  of  FIG. 18 , When the first sensor  34   m  does not detect the feeler Fm before the second sensor  34   bk  detects the feeler Fbk and when the determination result in Step S 12  is NO, the process of Step S 12  goes back to a procedure before Step S 3  and repeats until the first sensor  34   m  detects the feeler m before the second sensor  34   bk  detects the feeler Fbk. 
   In Step S 13  of  FIG. 18 , it is determined whether the above-described sensor detection time lag ΔS is less than 40 ms. When the sensor detection time lag ΔS is less than 40 ms and when the determination result in Step S 13  is YES, the phase adjusting operation is completed. When the sensor detection time lag ΔS is equal to or more than 40 ms and when the determination result in Step S 13  is NO, the procedure goes to Step S 14 . 
   In Step S 14  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 40 ms and less than 80 ms. When the sensor detection time lag ΔS is equal to or more than 40 ms and less than 80 ms and when the determination result in Step S 14  is YES, the procedure goes to a process B 1  (see below for details). When the sensor detection time lag Δs is not equal to or more than 40 ms and not less than 80 ms and when the determination result in Step S 14  is NO, the procedure goes to Step S 15 . 
   In Step S 15  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 80 ms and less than 152 ms. When the sensor detection time lag ΔS is equal to or more than 80 ms and less than 152 ms and when the determination result in Step S 15  is YES, the procedure goes to a process B 2  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 80 ms and not less than 152 ms and when the determination result in Step S 15  is NO, the procedure goes to Step S 16 . 
   In Step S 16  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 152 ms and less than 305 ms. When the sensor detection time lag ΔS is equal to or more than 152 ms and less than 305 ms and when the determination result in Step S 16  is YES, the procedure goes to a process B 3  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 152 ms and not less than 305 ms and when the determination result in Step S 16  is NO, the procedure goes to Step S 17 . 
   In Step S 17  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 305 ms and less than 458 ms. When the sensor detection time lag ΔS is equal to or more than 305 ms and less than 458 ms and when the determination result in Step S 17  is YES, the procedure goes to a process B 4  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 305 ms and not less than 458 ms and when the determination result in Step S 17  is NO, the procedure goes to Step S 18 . 
   In Step S 18  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 458 ms and less than 530 ms. When the sensor detection time lag ΔS is equal to or more than 458 ms and less than 530 ms and when the determination result in Step S 18  is YES, the procedure goes to a process B 5  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 458 ms and not less than 530 ms and when the determination result in Step S 18  is NO, the procedure goes to Step S 19 . 
   In Step S 19  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 530 ms and less than 570 ms. When the sensor detection time lag ΔS is equal to or more than 530 ms and less than 570 ms and when the determination result in Step S 19  is YES, the procedure goes to a process B 6  (see below for details). When the sensor detection time lag ΔS is not equal to or more than 530 ms and not less than 570 ms and when the determination result in Step S 19  is NO, the procedure goes to Step S 20 . 
   In Step S 20  of  FIG. 18 , it is determined whether the sensor detection time lag ΔS is equal to or more than 570 ms and less than 610 ms. When the sensor detection time lag ΔS is equal to or more than 570 ms and less than 610 ms and when the determination result in Step S 20  is YES, the phase adjusting operation is completed. When the sensor detection time lag ΔS is equal to or more than 610 ms and when the determination result in Step S 20  is NO, the procedure goes to an error handling operation. 
   Similar to the processes of Steps S 4  through S 11 , when the sensor detection time lag ΔS makes any value indicated in Steps S 14  through  19 , one of the following processes B 1  through B 6  is performed according to the value. When the sensor detection time lag ΔS is less than 40 ms and when the sensor detection time lag ΔS is equal to or more than 570 ms and less than 610 ms, the phase adjusting process is completed.
     Process B1: Number of Rotations of Photoconductor 2BK+5%,
       Number of Rotations of Photoconductor 2M−5%;   
       Process B2: Number of Rotations of Photoconductor 2BK+10%,
       Number of Rotations of Photoconductor 2M−10%;   
       Process B3: Number of Rotations of Photoconductor 2BK+16%,
       Number of Rotations of Photoconductor 2M−16%;   
       Process B4: Number of Rotations of Photoconductor 2BK−16%,
       Number of Rotations of Photoconductor 2M+16%;   
       Process B5: Number of Rotations of Photoconductor 2BK−10%,
       Number of Rotations of Photoconductor 2M+10%;   
       Process B6: Number of Rotations of Photoconductor 2BK−5%,
       Number of Rotations of Photoconductor 2M+5%.   
       

   As previously described, to increase and decrease the numbers of rotations of the gears  23   y ,  23   c ,  23   m  and  23   bk  and the respective photoconductors  2   y ,  2   c ,  2   m  and  2   bk , the numbers of rotations of the first and second photoconductor motors M 1  and M 2  during the steady rotation time are controlled to be changed. The photoconductor motors M 1  and M 2  are then rotated at the changed numbers of rotations to adjust the phases of the gears  23   y ,  23   c ,  23   m  and  23   bk . After adjusting the phases of the gears  23   y ,  23   c ,  23   m  and  23   bk , the changed numbers of rotations of the photoconductor motors M 1  and M 2  are changed back to their original numbers of rotations during the steady rotation time to perform the image forming operations. 
   Table 3 shows the above-described sensor detection time lag ΔS, an angular difference with respect to the sensor detection time lag ΔS, and fluctuation in the numbers of rotations of the respective photoconductor motors for correcting the sensor detection time lag ΔS. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
                 
                 
               Fluctuation in 
             
             
                 
                 
                 
               Rotations of 
             
             
                 
               Angular Difference 
               ΔS 
               Photoconductor 
             
             
                 
                 
             
           
          
             
                 
               Equal to or more 
               Equal to or more 
               ±16% 
             
             
                 
               than ±90 degrees 
               than ±152 ms to 
             
             
                 
               to equal to or 
               equal to or less 
             
             
                 
               less than 180 
               than 305 ms 
             
             
                 
               degrees 
             
             
                 
               Equal to or more 
               Equal to or more 
               ±10% 
             
             
                 
               than ±45 degrees 
               than ±80 ms to less 
             
             
                 
               to less than 90 
               than 152 ms 
             
             
                 
               degrees 
             
             
                 
               Equal to or more 
               Equal to or more 
                ±5% 
             
             
                 
               than ±22.5 degrees 
               than ±40 ms to less 
             
             
                 
               to less than 45 
               than 80 ms 
             
             
                 
               degrees 
             
             
                 
               Equal to or more 
               Equal to or more 
               0 
             
             
                 
               than ±0 degree to 
               than ±0 ms to less 
             
             
                 
               equal to or less 
               than 40 ms 
             
             
                 
               than 22.5 degree 
             
             
                 
                 
             
          
         
       
     
   
   Referring to  FIG. 19 , an example of controlling the rotations of the photoconductor motors M 1  and M 2  is described. 
   As shown in  FIG. 19 , in a case where the sensor detection time lag ΔS is detected after the first and second photoconductor motors M 1  and M 2  are started, the numbers of rotations of the photoconductor motors M 1  and M 2  are changed at a time T 1  to respective values with respect to the steady rotation time, When the sensor detection time lag ΔS is detected again, the numbers of rotations of the photoconductor motors M 1  and M 2  are changed at a time T 2 . The number of rotations may be changed every time the sensor detection time lag ΔS is detected, to make the number of rotations set back to the number of rotations of the photoconductor motors M 1  and M 2  for their steady rotation time. In  FIG. 19 , the numbers of rotations of the. photoconductor motors M 1  and M 2  are changed by 16% on the first attempt, and by 10% on the second attempt, to the number of rotations thereof during the steady rotation time, so that the numbers of rotations of the photoconductor motors M 1  and M 2  are set back to that during the steady rotation time (a rated number of rotations). 
   In the example as described above, the numbers of rotations of the first and second photoconductor motors. M 1  and M 2  are controlled according to the values of the sensor detection time lag ΔS to adjust the phases of the gears  23   y ,  23   c ,  23   m , and  23   bk  to the predetermined states at short times. As an alternative, the number of rotations of one of the photoconductor motors M 1  and M 2  may be controlled. Table 4 shows the sensor detection time lag ΔS, an angular difference with respect to the sensor detection time lag ΔS, and fluctuation in the number of rotations of the photoconductor motor for correcting the sensor detection time lag ΔS. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 4 
             
             
                 
                 
             
             
                 
                 
                 
               Fluctuation in 
             
             
                 
                 
                 
               Rotations of 
             
             
                 
               Angular Difference 
               ΔS 
               Photoconductor 
             
             
                 
                 
             
           
          
             
                 
               Equal to or more 
               Equal to or more 
               ±32% 
             
             
                 
               than ±90 degrees 
               than ±152 ms to 
             
             
                 
               to equal to or 
               equal to or less 
             
             
                 
               less than 180 
               than 305 ms 
             
             
                 
               degrees 
             
             
                 
               Equal to or more 
               Equal to or more 
               ±20% 
             
             
                 
               than ±45 degrees 
               than ±80 ms to less 
             
             
                 
               to less than 90 
               than 152 ms 
             
             
                 
               degrees 
             
             
                 
               Equal to or more 
               Equal to or more 
               ±10% 
             
             
                 
               than ±22.5 degrees 
               than ±40 ms to less 
             
             
                 
               to less than 45 
               than 80 ms 
             
             
                 
               degrees 
             
             
                 
               Equal to or more 
               Equal to or more 
               0 
             
             
                 
               than ±0 degree to 
               than ±0 ms to less 
             
             
                 
               equal to or less 
               than 40 ms 
             
             
                 
               than 22.5 degree 
             
             
                 
                 
             
          
         
       
     
   
   Referring to  FIG. 20 , an example of controlling the rotation of the photoconductor motor M 1  is described. 
   The number of rotation may be changed every time the sensor detection time lag ΔS is detected, to make the number of rotation set back to the number of rotation of the photoconductor motor M 1  for its steady rotation time (a rated number of rotations). 
   Referring to  FIG. 16 , a graph of phase adjustments of the gears  23   y ,  23   c ,  23   m  and  23   bk  is described. After the first and second photoconductor motors M 1  and M 2  are started, the numbers of rotations of the photoconductor motors M 1  and M 2  are controlled according to the values of the sensor detection time lag ΔS to adjust the phases of the gears  23   y ,  23   c ,  23   m  and  23   bk.    
   The above-described phase adjustment may be performed when the image forming operation in the black-and white mode is completed and that in the color mode is restarted. However, when the phase adjustment is performed when the image forming operation is started in the color mode and in the black-and-white mode, the gears  23   y ,  23   c ,  23   m  and  23   bk  may be configured to constantly have their desired phases, and thereby the image produced may be of high quality. 
   When the above-described braking unit is employed, the braking unit may stop the first position of the gear  23   m  in the vicinity of the first sensor  34   m  when the photoconductor motor M 1  stops, and may stop the second position of the gear  23   bk  in the vicinity of the second sensor  34   bk  when the photoconductor motor M 2  stops. Accordingly, if the braking unit and the above-described phase adjusting structure may be used together, when the photoconductor motors M 1  and M 2  start their rotations, the first and second positions of the gears  23   m  and  23   bk  are disposed at respective positions close to the first and second sensors  34   m  and  34   bk , respectively. With this structure, the sensors  34   m  and  34   bk  detect the first and second positions, respectively, at short times. Thereby, the phases of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  may be adjusted at short times. 
   The image forming apparatus  1  of the present invention is Delectably provided with the color mode and the black-and-white mode, as described above. With a background image forming apparatus, a plurality of image forming operations including some jobs in the color mode and other Jobs in the black-and-white mode cannot sequentially be performed. That is, when a job performed in the color mode is completed, the photoconductor motors M 1  and M 2  and the drive motor DM are stopped once. Next, the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the intermediate transfer member  3  are stopped. After that, the second photoconductor motor M 2  and the drive motor DM are started again to start another job in the black-and-white mode. This structure, however, increases the number of ON and OFF operations to start the photoconductor motors M 1  and M 2  and the drive motor DM. Every time the ON and OFF operations are performed, the gears  23   y ,  23   c ,  23   m  and  23   bk  receive impacts caused by the ON and OFF operations, and thereby the gears  23   y ,  23   c ,  23   m  and  23   bk  may deteriorate in durability. 
   To eliminate the above-described inconvenience, the image forming apparatus of the present invention includes a structure such that the mode may bi-directionally be switched between the color mode and the black-and-white mode without stopping the second photoconductor motor M 2  and the drive motor DM. 
   For example, assume that ten jobs of the image forming operations are sequentially performed, where the first five jobs are performed in the color mode before the other five jobs are performed in the black-and-white mode. Firstly, the first and second photoconductor motors M 1  and M 2  and the drive motor DM of  FIG. 8  are started, and the first five jobs of the image forming operations are sequentially performed. Subsequently, the first photoconductor motor M 1  stops while the second photoconductor motor M 2  and the drive motor DM maintains their operations, and then the other five jobs are performed in the black-and-white mode. 
   When switching the mode from the black-and-white mode to the color mode, the second photoconductor motor M 2  and the drive motor DM are started, and the image forming operations are performed in the black-and-white mode. After the jobs in the black-and-white mode are completed, the first photoconductor motor M 1  is started while the second photoconductor motor M 2  and the drive motor DM keeps their rotations, and then the jobs are performed in the color mode. 
   With the structure as described above, the number of the ON and OFF operations and the impacts made to the resin-based gears  23   y ,  23   c ,  23   m  and  23   bk  may be reduced, and thereby the lives of the gears  23   y ,  23   c ,  23   m  and  23   bk  may be made long. 
   Further, the image forming apparatus  1  with the direct transfer method shown in  FIG. 7  includes motors and gears that are not shown in the figure. That is, photoconductors  2   y ,  2   c  and  2   m  for producing color toner images, the gears  23   y ,  23   c  and  23   m  coupled with the photoconductors  2   y ,  2   c  and  2   m , respectively, the photoconductor  2   bk  for producing a black-and-white toner image, the gear  23   bk  coupled with the photoconductor  2   bk , the first photoconductor motor M 1  including the clock control motor which rotates the photoconductors  2   y ,  2   c  and  2   m  via the gears  23   y ,  23   c  and  23   m , respectively, and the second photoconductor motor M 2  including the clock control motor which rotates the photoconductor  2   bk  via the gear  23   bk . The image forming apparatus  1  also includes the color mode and the black-and-white mode. In the color mode, respective single color toner images formed on the surfaces of the photoconductors  2   y ,  2   c  and  2   m  and the black-and-white toner image formed on the surface of the photoconductor  2   bk  are sequentially transferred onto the recording medium P carried by the recording medium bearing member  103  to obtain a full-color image. In an image forming operation in the black-and-white mode, the photoconductors  2   y ,  2   c , and  2   m  are separated from the recording medium bearing member  103  and the photoconductor  2   bk  is held in contact with the recording medium bearing member  103 . With this structure, the black toner image formed on the surface of the photoconductor  2   bk  are transferred onto the recording medium P carried by the recording medium bearing member  103  to obtain a black-and-white image. The color mode and the black-and-white mode are selectably provided to the image forming apparatus  1 . Also in this example, both of the first and second photoconductor motors M 1  and M 2  include the DC brushless motor. The image forming apparatus  1  also has a structure such that the mode may bi-directionally be switched between the color mode and the black-and-white mode without stopping the second photoconductor motor M 2  and the drive motor DM, and thereby the lives of the gears  23   y ,  23   c ,  23   m  and  23   bk  may be made long. 
   Assuming that the image forming mode is switched from the black-and-white mode to the color mode without stopping the second photoconductor motor M 2  and the drive motor DM, as described above if the drive unit has a structure that the number of rotations of one of the first and second photoconductor motor M 1  and M 2  may be controlled to obtain the predetermined phases of the color gears  23   y,    23   c  and  23   m  before starting the image forming operation in the color mode, the image forming operation in the color mode may produce a full-color image without the color shift. The phase adjusting operation may be performed in the same manner as the operations previously described with regard to  FIGS. 16 ,  18  and  20 . However, this operation is performed after the image forming mode is switched to the color mode. The phase adjusting operations for the gears  23   y,    23   c,    23   m  and  23   bk  are performed as described above, before starting the image forming operation in the color mode. 
   The image forming apparatus  1  shown in  FIG. 5  may also include a structure such that surface linear velocities of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk , and the intermediate transfer member  3  can separately be switched. The structure may selectably be provided with a full speed mode and a low speed mode. In the full speed mode, the image forming operation is performed by rotatably driving the photoconductor and the intermediate transfer member  3  at a first surface linear velocity. In the low speed mode, the image forming operation is performed by rotatably driving the photoconductor and the intermediate transfer member  3  at a second surface linear velocity, which is lower than the first surface linear velocity. The full speed mode may speed up the image forming operation when compared with that performed in the low speed mode. On the other hand, the operation performed in the low speed mode may obtain an image with a high image density, compared with that performed in the full speed mode. 
   Referring to  FIG. 21 , a surface linear velocity of a photoconductor in the color mode is described. The surface linear velocity in  FIG. 21  is obtained when a speed mode of the photoconductor is changed from a high speed mode HM to a low speed mode LM in the middle of the image forming operation performed in the color mode. The solid line represents surface linear velocities of the photoconductors  2   y ,  2   c  and  2   m , and the dashed line represents a surface linear velocity of the photoconductor  2   bk . A value of “V 1 ” represents a surface linear velocity obtained in the high speed mode, and a value of “V 2 ” represents a surface linear velocity obtained in the low speed mode. 
   When the speed mode is changed from the high speed mode HM to the low speed mode LM, the first and second photoconductor motors M 1  and M 2  and the drive motor DM are still activated without stopping. At this time, in a period IS, which is a predetermined period before the surface linear velocity of the photoconductor is stably controlled to the low speed V 2 , the surface linear velocities of the photoconductors  2   y ,  2   c  and  2   m  and that of the photoconductor  2   bk  may become drastically different to each other, according to an over shoot of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk . When such difference occurs, the gears  23   y ,  23   c ,  23   m  and  23   bk  may drastically be out of phase, and the color shift may occur in the subsequent color mode. The above-described inconvenience may occur when the speed mode is changed from the low speed mode to the high speed mode. 
   Accordingly, when the image forming operation is performed in the color mode, by changing the speed mode without stopping the second photoconductor motor M 2  and the drive motor DM, the phase adjustment of the gears  23   y ,  23   c ,  23   m  and  23   bk  needs to be done. To avoid the above-described necessity, the image forming apparatus  1  of the present invention has the structure as described below. 
   The image forming apparatus  1  of  FIG. 5  includes a copy mode selection of the color mode and the black-and-white mode, and a speed selection of the high speed mode and the low speed mode. These modes can be flexibly combined to make four selective modes; a full speed color mode, a full speed black-and-white mode, a low speed color mode, and a low speed black-and-white mode. The full speed color mode may be selected for performing a copy job in the color mode by rotating the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the intermediate transfer member  3  at the first surface linear velocity. The full speed black-and-white mode may be selected for performing a copy job in the black-and-white mode by rotating the photoconductor  2   bk  and the intermediate transfer member  3  at the first surface linear velocity. The low speed color mode may be selected for performing a copy job in the color mode by rotating the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the intermediate transfer member  3  at the second surface linear velocity. The low speed black-and-white mode may be selected for performing a copy job in the black-and-white mode by rotating the photoconductor  2   bk  and the intermediate transfer member  3  at the second surface linear velocity. 
   As previously described, the mode may be changed without stopping the second photoconductor motor M 2  and the drive motor DM. When the changed mode is the full speed color mode or the low speed color mode, the control unit may be configured to control the change of the rotation number-of at least one motor of the first and second photoconductor motors M 1  and M 2  to obtain the predetermined phases of the gears  23   y ,  23   c ,  23   m  and  23   bk  before starting the image forming operation in the changed mode. 
   With the above-described structure, the full-color image produced at the last stage of the image forming operation may be prevented from the color shift even when the mode is changed from the black-and-white mode to the color mode. 
   Referring to  FIG. 22 , an example of an operation of the structure of  FIG. 21  is described. The vertical axis shows the surface linear velocities of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the intermediate transfer member  3 , and the horizontal axis shows the time. The solid line represents the surface linear velocity of the intermediate transfer member  3   m , and the dashed line represents the surface linear velocity of the photoconductor  2   y ,  2   c  and  2   m , and the short and long dashed line represents the surface linear velocity of the photoconductor  2   bk . The first surface linear velocity V 1 , which is a basic surface linear velocity of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the intermediate transfer member  3 , is 155 mm/sec, and the second surface linear velocity V 2  is 77.5 mm/sec, which is half of the first surface linear velocity V 1 . 
   At t 0  of  FIG. 22 , the first and second photoconductor motors M 1  and M 2  and the drive motor DM are started. At t 1 , the first and second photoconductor motors M 1  and M 2  and the drive motor DM complete the starting operation. In a period of the starting operation, the intermediate transfer member  3  and the photoconductors  2   y,    2   c,    2   m  and  2   bk  increase their speeds at the substantially the same surface linear velocity. The time required for the starting operation is approximately 1000 msec. 
   During a period of t 3 , which is a time after the starting operation of the photoconductor motors M 1  and M 2  and the drive motor DM are completed, the phase adjusting operations of the gears  23   y ,  23   c ,  23   m  and  23   bk  are performed, which is same as shown in  FIGS. 16 ,  18  to  20 . During a period of t 4 , the image forming operation is performed in the full speed color mode, which is a combination of the high speed mode and the color mode. 
   At t 5 , the numbers of rotations of the first and second photoconductor motors M 1  and M 2  and the drive motor DM are decreased so that the surface linear velocities of the photoconductors  2   y,    2   c  and  2   m  and the intermediate transfer member  3  the second surface linear velocity V 2 . In a period of t 6 , the phase adjusting operations of the gears  23   y,    23   c,    23   m  and  23   bk  are performed. In the example shown in  FIG. 22 , the gears  23   y,    23   c,    23   m  and  23   bk  are in the predetermined phases even when the speeds of the photoconductor motors M 1  and M 2  and the drive motor DM are decreased. Since no gears are out of phase, a phase adjusting operation is not performed to control the actual speeds of the photoconductor motors M 1  and M 2  and the drive motor DM. 
   In a period of t 7 , the image forming operation is performed in the low speed color mode, which is a combination of the low speed mode and the color mode. At t 8 , as shown in  FIG. 6 , the intermediate transfer member  3  is detached from the photoconductors  2   y ,  2   c ,  2   m  and  2   bk . At t 9 , the surface linear velocities of the photoconductors  2   y ,  2   c  and  2   m  are decreased, the first photoconductor motor M 1  is stopped, and then the rotations of the photoconductors  2   y ,  2   c  and  2   m  are stopped. 
   Subsequently, in a period of t 10 , the image forming operation is performed in the low speed black-and-white mode, which is a combination of the low speed mode and the black-and-white mode. During the period of t 10 , the phase adjusting operation of the gears  2   y ,  2   c  and  2   m  are not performed before this image forming operation. 
   Next, at t 11 , the surface linear velocities of the photoconductor  2   bk  and the intermediate transfer member  3  are started to increase. At t 12 , the surface linear velocities of the photoconductor  2   bk  and the intermediate transfer member  3  are returned to the first surface linear velocity V 1 . At this moment, the phase adjusting operation of the photoconductor  2   bk  and the intermediate transfer member  3  is not performed. Subsequently, in a period of t 13 , the image forming operation is performed in the full speed black-and-white mode, which is a combination of the high speed and the black-and-white mode. 
   At t 14 , the first photoconductor motor M 1  starts the rotation, and at tl 5 , the starting operation of the photoconductor motor M 1  completes. The starting operation at t 5  also takes approximately 1000 msec. Subsequently, in a period of t 16 , the phase adjusting operation of the gears  23   y,    23   c,    23   m  and  23   bk  is performed. At t 17 , the intermediate transfer member  3  contacts the photoconductors  2   y,    2   c  and  2   m.  After the intermediate transfer member  3  and the photoconductors  2   y,    2   c  and  2   m  are held in contact with each other at t 17 , the image forming operation is performed in the full speed color mode, which is a combination of the high speed mode and the color mode. 
   The intermediate transfer member  3  may contact with the photoconductors  2   y ,  2   c  and  2   m  while the phase adjusting operation is performed. With the structure, however, a great impact is given onto the surfaces of the gears  23   y ,  23   c ,  23   m  and  23   bk  to promote the wearing. Accordingly, as shown in  FIG. 22 , it is preferable to contact the intermediate transfer member  3  with the photoconductors  2   y ,  2   c  and  2   m  after the phase adjusting operation is performed. 
   The above-described structure may be applied to the image forming apparatus  1  with the direct transfer method as shown in  FIG. 7 . That is, this structure is provided with a function that the mode can be changed without stopping the second photoconductor motor M 2  and the drive motor DM, and another function that surface linear velocities of the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the recording medium bearing member  103  can be switched. Also, this structure includes a full speed color mode, a full speed black-and-white mode, a low speed color mode, and a low speed black-and-white mode. The full speed color mode may be selected for performing a copy job in the color mode by rotating the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the recording medium bearing member  103  at the first surface linear velocity. The full speed black-and-white mode may be selected for performing a copy job in the black-and-white mode by rotating the photoconductor  2   bk  and the recording medium bearing member  103  at the first surface linear velocity. The low speed color mode may be selected for performing a copy job in the color mode by rotating the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  and the recording medium bearing member  103  at the second surface linear velocity. The low speed black-and-white mode may be selected for performing a copy job in the black-and-white mode by rotating the photoconductor  2   bk  and the recording medium bearing member  103  at the second surface linear velocity. When the changed mode is the full speed color mode or the low speed color mode, the control unit may be configured to control the change of the rotation number of at least one motor of the first and second photoconductor motors M 1  and M 2  to obtain the predetermined phases of the gears  23   y ,  23   c ,  23   m  and  23   bk  before starting the image forming operation in the changed mode. 
   Referring to  FIGS. 23 and 24 , deflections of pitch circles of the gears  23   bk  and  23   m  of  FIG. 8  in the radius direction thereof are described. A curve C 1  shown in  FIG. 23  and a curve C 2  shown in  FIG. 24  represent the above-described deflections observed when the gears  23   bk  and  23   m,  respectively, are rotated by one cycle. Since the rotations of a single gear cannot be measured, the deflection is substituted for the volume of rotations of the single gear. When pitch radiuses of the gears  23   bk  and  23   m  at their maximum values (+) are engaged with the output gears  26  and  25 , respectively, angular velocities of the gears  23   bk  and  23   m  are at their minimum. When pitch radiuses of the gears  23   bk  and  23   m  at their minimum values (−) are engaged with the output gear  26  and the intermediate gear  24 , respectively, angular velocities of the gears  23   bk  and  23   m  are at their maximum. 
   Here, the curve C 1  of  FIG. 23  and the curve C 2  of  FIG. 24  are approximated to each other. When the phases of the gears  23   y  and  23   bk  are correctly adjusted as described above, a difference AC between the curves C 1  and C 2  becomes minimal, as shown in  FIG. 25 . Therefore, when the phase adjusting operation is performed as described above, an occurrence of the color shift may effectively be restrained. 
   In fact, the curves representing the deflections of the pitch circles of the gears  23   bk  and  23   m  rarely approximate to each other as shown in  FIGS. 23 and 24 . In most cases, as shown in  FIG. 26 , curves C 3  and C 4  representing deflection of the pitch circle of the gears  23   bk  and  23   m  may have a large difference therebetween. In such cases, when the phase adjusting operation is performed, the difference ΔC between the curves C 3  and C 4  becomes large as shown in  FIG. 26 . 
   In such cases, when the phase of the curve C 4  is shifted by an amount of a color shift angle Y as shown in  FIG. 27 , the difference ΔC between the curves C 3  and C 4  becomes small. That is, the gears  23   y,    23   c,    23   m  and  23   bk  are preferably measured before assembling them to the image forming apparatus  1 . By doing so, the color shift angle Y of the phase having a smallest difference C may be previously measured, a corrective value according to the optical color shift angle Y, and the phase adjusting operation may be performed as described above. After the phase adjusting operation, the control unit is configured to control the rotation number of at least one of the first and second photoconductor motors M 1  and M 2  according to a value obtained by adding the above-described predetermined corrected value to a time difference between a time in which the first sensor  34   m  detects the first position (the feeler Fm) and a time in which the second sensor  34   bk  detects the second position (the feeler Fbk). By doing so, the color shift produced on a final color image may be further reduced, and thereby the image quality of the final color image may be increased. 
   More specifically, the image forming operation may be controlled as shown in Table 5 described below instead of Table 3 which is previously described. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 5 
             
             
                 
                 
             
             
                 
                 
               Fluctuation in 
             
             
                 
                 
               Rotations of 
             
             
                 
               Angular Difference 
               Photoconductor 
             
             
                 
                 
             
           
          
             
                 
               Equal to or more than ±90 degrees to 
               16% 
             
             
                 
               equal to or less than 180 degrees 
             
             
                 
               Equal to or more than 45 degrees to less 
               10% 
             
             
                 
               than 90 degrees 
             
             
                 
               Equal to or more than 22.5 degrees to 
                5% 
             
             
                 
               less than 45 degrees 
             
             
                 
               Equal to or more than 0 degree to equal to 
               0 
             
             
                 
               or less than 22.5 degree 
             
             
                 
                 
             
          
         
       
     
   
   Referring to  FIG. 28 , the command clock signal produced when the photoconductor motors M 1  and M 2  and the drive motor DM are started is described. 
   As previously described, the photoconductor motors M 1  and M 2  and the drive motor DM may include the DC brushless motor. In this case, when the photoconductor motors M 1  and M 2  and the drive motor DM are started, the command clock signal having the number of clocks gradually increasing as shown in  FIG. 28  is input to the photoconductor motors M 1  and M 2  and the drive motor DM. After the command clock signal is input to each motor, the surface linear velocities of the photoconductor and the intermediate transfer member  3  or those of the photoconductor and the recording medium bearing member  103  may be controlled as indicated by a solid line and a short and long dashed line shown in  FIG. 29 . Further, an amount of difference between an overshoot volume represented by a reference character e and an undershoot volume represented by a reference character f may be reduced. 
   Referring to  FIG. 30 , an example of the command clock signal produced when the photoconductor motors M 1  and M 2  and the drive motor DM area started is described. 
   When the photoconductor motors M 1  and M 2  and the drive motor DM including the DC brushless motor are started, the command clock signal having the number of clocks gradually increasing as indicated by reference characters g, h and i as shown in  FIG. 30  is input to the photoconductor motors M 1  and M 2  and the drive motor DM. By doing so, similar to the case shown in  FIG. 29 , after the command clock signal is input to each motor, the surface linear velocities of the photoconductor and the intermediate transfer member  3  or those of the photoconductor and the recording medium bearing member  103  may be controlled to avoid a great difference. 
   Referring to  FIG. 31 , an example of the command clock signal produced when the photoconductor motors M 1  and M 2  and the drive motor DM including the DC brushless motor are stopped. When the photoconductor motors M 1  and M 2  and the drive motor DM are stopped, the command clock signal having the number of clocks gradually decreasing is input. After the command clock signal is input to each motor, the surface linear velocities of the photoconductor and the intermediate transfer member  3  (or those of the photoconductor and the recording medium bearing member  103 ) may be controlled as indicated by a solid line and a short and long dashed line shown in  FIG. 32 . Further, an amount of speed difference between them may be reduced or be eliminated. 
   In the above-described examples, the first photoconductor motor M 1  controls the rotations of the photoconductors  2   y ,  2   c  and  2   m , the second photoconductor motor M 2  controls the rotation of the photoconductor  2   bk . As an alternative, a drive method of each photoconductor may have another drive method. For example, as shown in  FIG. 33 , that the gears  23   y ,  23   c ,  23   m  and  23   bk  concentrically coupled with the photoconductors  2   y ,  2   c ,  2   m  and  2   bk , respectively, may be engaged with the output gears  25   y ,  25   c ,  25   m  and  25   bk  of the photoconductor motors M 3 , M 4 , M 5  and M 6 , respectively. The gears  23   y ,  23   c ,  23   m  and  23   bk  and the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are rotated, different color toner images formed on the photoconductors  2   y ,  2   c ,  2   m  and  2   bk  are transferred onto the intermediate transfer member  3  which moves in a direction A. The image forming apparatus  1  having the above-described structure may also be applied. 
   In the image forming apparatus  1  as shown in  FIG. 33 , the intermediate transfer member  3  is supported by supporting rollers  4 ,  5 ,  5   a  and  6 . An output gear  28   a  of the drive motor DM is engaged with a gear  27   a  which is concentrically fixed to the supporting roller  4 . The rotation of the drive motor DM is transmitted to the supporting roller  4  via the output gear  28   a  and the gear  27   a . Then, the intermediate transfer member  3  is rotated in the direction A. 
   At least one motor of the above-described photoconductor motors M 3 , M 4 , M 5  and M 6  and the drive motor DM includes the clock control motor including the DC brushless motor, and the DC brushless motor is controlled as described above. With this structure, when the photoconductor motors M 3 , M 4 , M 5  and M 6  and the drive motor DM are started and stopped, a significantly different value between the surface linear velocities of the photoconductors  2   y,    2   c,    2   m  and  2   bk  and that of the intermediate transfer member  3  are prevented. Other basic structures are the same as the structures of the image forming apparatus as shown in  FIGS. 5 to 9 . In  FIG. 33 , the same reference numerals are applied to elements corresponding to the respective element as shown in  FIG. 8 . 
   In addition, the present invention may be applied to the image forming apparatus  1  which forms a single toner image on one photoconductor, transfers the single toner image onto a recording medium carried by the recording medium bearing member, and repeats the same image forming operations for four times to complete one full-color toner image. 
   Referring to  FIG. 34 , an exemplary structure of an image forming portion of the above-described image forming apparatus with one photoconductor is described. 
   The image forming apparatus described here, which includes a gear  27  concentrically fixed to the photoconductor  2 , is engaged with an output gear  25  of the photoconductor motor M. The photoconductor motor M drives the photoconductor  2  clockwise in  FIG. 34 , so that a single color toner image is formed on a surface of the photoconductor  2 . 
   A recording medium bearing member  3   b  which is an endless belt extended by supporting rollers  4   a  and  5   a . The supporting roller  5   a  includes a gear  27   b  which is concentrically coupled threrewith. The gear  27   b  is engaged with an output gear  28   b  of the drive motor DM. The drive motor DM drives the recording medium bearing member  3   b  in a direction A as shown in  FIG. 34 . 
   A recording medium P which is fed from a sheet feeding unit (not shown) is carried by the recording medium bearing member  3   b  and is conveyed to a transferring unit (not shown). The transferring unit transfers the single color toner image formed on the surface of the photoconductor  2  onto the recording medium P. After the image forming operations for transferring the different single color toner images onto the recording medium P are performed for four times and the full-color toner image is formed on the recording medium P, the recording medium P is separated from the recording medium bearing member  3   b  and passes through a fixing unit, where the full-color toner image is fixed onto the recording medium P. 
   At least one motor of the photoconductor motor M and the drive motor DM includes a clock control motor including a DC brushless motor, and the DC brushless motor is controlled the same way as previously described. With this structure, when the photoconductor motor M and the drive motor DM are started, stopped, and stably rotated, a significantly different value between the surface linear velocities of the photoconductor  2  and that of the recording medium bearing member  3   b  is prevented. 
   In the image forming apparatus as described above, the number of rotations of the DC brushless motor is controlled according to a predetermined velocity curve. The predetermined velocity curve is recorded in the memory  33 , for example, a nonvolatile memory, as shown in  FIG. 8 . At this time, when the properties of the elements of the image forming apparatus may be changed with age, the surface linear velocities of the photoconductor and the intermediate transfer member or the recording medium bearing member may be significantly different, Therefore, it is preferable to have a structure such that the velocity curve can be changed by controlling an operation panel (not shown) of the image forming apparatus or a connecting terminal, such as a personal computer, of the image forming apparatus. By doing so, a large difference between the surface linear velocities of the photoconductor and the intermediate transfer member or the recording medium bearing member, the velocity curve may be changed to a smaller value for making the difference smaller. 
   The present invention may be widely used for an image forming apparatus other than a printer, that is, a copying machine, a facsimile machine, and a multifunction machine. 
   Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.