Patent Publication Number: US-7587157-B2

Title: Image forming apparatus capable of correcting a rotation speed of an image carrier

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This patent specification is based on Japanese patent application, No. JP2006-015825 filed on Jan. 25, 2006 in the Japan Patent Office, the entire contents of which are incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of forming a quality color image by effectively correcting a variation in surface moving speed of a surface moving member. 
   2. Discussion of the Background 
   Image forming apparatuses include copiers, printers, facsimiles, multi-function devices thereof, etc. Some image forming apparatuses form a color image on a recording member according to an electrophotographic method. Such image forming apparatuses optically write a latent image on an image carrier, develop the latent image with toner, transfer the toner image onto a recording member carried on a surface moving member to obtain a color image. 
   Alternatively, such image forming apparatuses optically write a latent image on a surface moving member, develop the latent image with toner, temporarily transfer the toner image onto a surface moving member and then transfer the toner image onto a recording member carried on another surface moving member to obtain a color image. 
   In recent years, high image quality and high speed processing have been increasingly demanded for the above image forming apparatuses. The image forming apparatuses capable of meeting such demands include a tandem-type image forming apparatus employing a direct transfer method. 
   For example, a tandem-type image forming apparatus employing a direct transfer method forms single-color toner images of black, yellow, magenta, and cyan on photoconductive drums for black, yellow, magenta, and cyan, respectively. Then, the tandem-type image forming apparatus superimposingly transfers the single-color toner images onto a recording member carried on a conveyor belt. 
   However, this tandem-type image forming apparatus employing the direct transfer method may cause a noticeable color misregistration. The color misregistration is caused by a relative displacement in transfer position between the single-color toner images formed on the recording member. Further, the color misregistration may cause bleeding between line images to be precisely superimposed on each other, or a white spot around a black-color character on a multi-color background. The color misregistration may also cause banding, that is, periodic band-shaped unevenness in toner density on a color background. 
   In addition, the image forming apparatuses capable of meeting such demands include a tandem-type image forming apparatus employing an intermediate transfer method. For example, a tandem-type image forming apparatus employing an intermediate transfer method forms single-color toner images of black, yellow, magenta, and cyan on photoconductive drums for black, yellow, magenta, and cyan, respectively. Then, the tandem-type image forming apparatus superimposingly transfers the single-color toner images on an intermediate transfer belt. Then, the tandem-type image forming apparatus transfers a resultant color image onto a recording member. 
   However, this tandem-type image forming apparatus employing the intermediate transfer method may also cause a noticeable color misregistration similar to the tandem-type image forming apparatus employing the direct transfer method. The color misregistration may be caused by a relative displacement in transfer position between the single-color toner images formed on the intermediate transfer belt. 
   The color misregistration is also caused mainly by a relative displacement in transfer position between single-color toner images formed on each of the photoconductive drums. Further, the relative displacement in transfer position between single-color toner images formed on each of the photoconductive drums is caused by a periodic variation in surface moving speed of each of the photoconductive drums. 
   Furthermore, the periodic variation in surface moving speed of each of the photoconductive drums may be significantly influenced by a variation in rotation angular speed of a rotation drive force transmitted to each of the photoconductive drums. The variation in rotation angular speed may be caused by a transmission error due to gear eccentricity or accumulated gear pitch error of a drive transmission system provided at a shaft of the photoconductive drum. The variation in rotation angular speed may also be caused by a transmission error due to shaft inclination or misalignment of a coupling member to allow the photoconductive drum to be attached to and detached from the drive transmission system. 
   Some of the image forming apparatuses are capable of correcting the color misregistration by suppressing the periodic variation in surface moving speed of the photoconductive drum. For example, an image forming apparatus detects a periodic variation in surface moving speed of each of a plurality of photoconductive drums. Based on a detection result of the periodic variation, the image forming apparatus finely adjusts a rotation angular speed of each of the plurality of photoconductive drums so as to suppress the periodic variation. 
   Specifically, a single-color adjustment pattern having a plurality of toner images is formed on each of the photoconductive drums for black, yellow, cyan, and magenta. The single-color adjustment pattern formed on each of the photoconductive drums is transferred onto an intermediate transfer belt so as to be arranged in an order of black, yellow, cyan, and magenta toner images. Then, the resultant multi-color adjustment pattern is sequentially sensed with a pattern sensor in the order of black, yellow, cyan, and magenta toner images. From signals sensed with the pattern sensor, a periodic variation component in surface moving speed having a rotation period of the photoconductive drum are detected as detection data. 
   Based on the detection data, a rotation angular speed of each of the photoconductive drums is finely adjusted so as to suppress the periodic variation in surface moving speed thereof. At this time, the image forming apparatus performs the fine adjustment of rotation angular speed of each of the photoconductive drums in a manner as follows. 
   The above detection data is based on a detection result of the adjustment pattern formed on the intermediate transfer belt while being influenced by the following two variations. The first variation is a variation in surface moving speed of the photoconductive drum at a time when a latent image is written onto the photoconductive drum to form the adjustment pattern. The second variation is a variation in surface moving speed of the photoconductive drum at a time when the adjustment pattern obtained by developing the latent image is transferred onto the intermediate transfer belt. 
   In addition, this image forming apparatus is configured to have a phase difference angle of substantially 180° between a latent-image writing position and a transfer position on the photoconductive drum. Specifically, the phase difference angle is formed with two imaginary lines connecting a rotation center of the photoconductive drum to each of the latent-image writing position and the transfer position on the surface thereof on an imaginary plane perpendicular to a rotation axis of the photoconductive drum. 
   Accordingly, a correction value is obtained by adding up a half gain to the detection data and inverting a phase thereof. Further, the correction value is superposed on an uncorrected target value of the rotation angular speed of the photoconductive drum. Then, drive of the photoconductive drum is controlled according to the corrected target value so as to suppress the periodic variation in surface moving speed of the photoconductive drum having a rotation period thereof. 
   However, as described above, the image forming apparatus is configured to have a phase difference angle of substantially 180° between the latent-image writing position and the transfer position on the photoconductive drum. If the image forming apparatus is configured to have a phase difference angle significantly deviated from 180°, an inaccurate correction value may be obtained, thereby causing an error in drive control of the photoconductive drum. Consequently, some limitation may be imposed on the layout of units in the image forming apparatus. 
   For the above reasons, demand is increasing for an image forming apparatus capable of effectively correcting the periodic variation in surface moving speed of the photoconductive drum with less limitation to the phase difference angle. 
   SUMMARY OF THE INVENTION 
   This patent specification describes an image forming apparatus which can form a quality color image by effectively correcting a variation in surface moving speed of a surface moving member. In one example, an image forming apparatus includes a latent image carrier, a drive control mechanism, a surface moving member, an image forming mechanism, a pattern sensor, and a correction mechanism. The latent image carrier is configured to have a moving surface on which a latent image is formed. The drive control mechanism is configured to control a drive of the latent image carrier so as to match a rotation angular speed of the latent image carrier with a target rotation angular speed thereof. The surface moving member is configured to have a moving surface on which an adjustment pattern for use in controlling the drive of the latent image carrier is formed. The image forming mechanism is configured to develop the latent image with toner and transfer a resultant toner image on the moving surface of the surface moving member so as to form the adjustment pattern along a surface moving direction thereof. The pattern sensor is configured to detect the adjustment pattern. The correction mechanism is configured to correct the target rotation angular speed of the latent image carrier based on an amplitude and a phase of a variable component in a pattern interval of the adjustment pattern detected with the pattern sensor. 

   
     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 diagram illustrating a schematic configuration of an image forming apparatus according to an example embodiment of the present invention; 
       FIG. 2  is an explanatory diagram illustrating a drum drive mechanism to drive a photoconductive drum; 
       FIG. 3  is an explanatory diagram illustrating a pattern sensor to detect an adjustment pattern formed on an intermediate transfer belt; 
       FIG. 4  is an explanatory diagram illustrating a transfer position adjustment pattern; 
       FIG. 5  is an explanatory diagram illustrating an adjustment pattern employed to correct a periodic variation in surface moving speed of the photoconductive drum; 
       FIG. 6  is a block diagram illustrating an electric hardware configuration of the drum drive mechanism; 
       FIG. 7A  is a schematic diagram of a transfer area between the photoconductive drum and the intermediate transfer belt; 
       FIG. 7B  is an explanatory diagram illustrating relationship between toner density and pattern interval of an adjustment pattern formed on the photoconductive drum; 
       FIG. 7C  is an explanatory diagram illustrating an adjustment pattern transferred on the intermediate transfer belt when the surface moving speed of the photoconductive drum is higher than the surface moving speed of the intermediate transfer belt; 
       FIG. 7D  is an explanatory diagram illustrating an adjustment pattern transferred on the intermediate transfer belt when the surface moving speed of the photoconductive drum is lower than the surface moving speed of the intermediate transfer belt; 
       FIG. 8  is a block diagram illustrating a basic configuration of quadrature detection processing employed to calculate an amplitude and a phase of a variable component occurring in a rotation period of the photoconductive drum; 
       FIG. 9  is an explanatory diagram illustrating parameters employed to calculate a drive control correction value for the photoconductive drum; 
       FIG. 10  is a graph illustrating difference in drive control correction values obtained when a phase difference angle between a latent-image writing position and a transfer position on the photoconductive drum is set to 145° and 180°; 
       FIG. 11  is an explanatory diagram illustrating image forming mechanisms  800  according to a variation example 1 of the example embodiment of the present invention; 
       FIG. 12  is an explanatory diagram illustrating image forming mechanisms  900  according to a variation example 2 of the example embodiment of the present invention; and 
       FIG. 13  is an explanatory diagram illustrating an image forming mechanism  1000  according to a variation example 3 of the example embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF 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, particularly to  FIG. 1 , an image forming apparatus  200  according to an example embodiment of the present invention is described. 
   As illustrated in  FIG. 1 , the image forming apparatus  200  includes an intermediate transfer belt  10 , support rollers  7 ,  8 ,  11 , and  12 , image forming mechanisms  300 , a pattern sensor  40 , an optical writing unit  1 , a secondary transfer roller  13 . 
   According to the present example embodiment, the image forming apparatus  200  is provided with the intermediate transfer belt  10  having a endless shape and serving as a surface moving member. The intermediate transfer belt  10  is looped over the support rollers  7 ,  8 ,  11 , and  12 . The intermediate transfer belt has a surface moved in a counterclockwise direction indicated by an arrow A in  FIG. 1 . In the present example embodiment, the support roller  8  is a drive roller (hereinafter, “drive support roller  8 ”). 
   The image forming apparatus  200  also includes an un-illustrated cleaner to remove extra toner remaining on the intermediate transfer belt  10  after image formation. The un-illustrated cleaner is provided at a left side of the support roller  7  in  FIG. 1 . 
   The image forming mechanisms  300  are arranged along a downward surface of the intermediate transfer belt  10  stretched between the support roller  11  and the support roller  12 . The image forming mechanisms  300  forms color images of yellow, cyan, magenta, and black. 
   Each of the image forming mechanisms  300  has a photoconductive drum  2 , a drum drive gear  32 , a bias roller  6 , and a drum position sensor  20  as illustrated in  FIG. 1 . 
   The photoconductive drum  2  serves as a latent image carrier and is rotationally driven in a clockwise direction in  FIG. 1 . The photoconductive drum  2  are surrounded with the unillustrated devices, such as a charger, a developer, and a cleaner. 
   The image forming mechanisms  300  have a similar configuration except for difference in toner color. The bias roller  6  is arranged at a position opposite to the photoconductive drum  2  via the intermediate transfer belt  10 . The intermediate transfer belt  10  is contacted to each of the photoconductive drums  2  with a corresponding one of the bias rollers  6 . 
   Each of the drum drive gears  32  has a marking  4 . The marking  4  is detected with a corresponding one of the drum position sensors  20 . A rotation position of each of the photoconductive drums  2  can be determined based on a detection result by the corresponding one of the drum position sensors  20 . 
   In the image forming apparatus  200 , further, the pattern sensor  40  is provided at a position opposite to a surface of the intermediate transfer belt  10 . The pattern sensor  40  detects an adjustment pattern formed on the surface of the intermediate transfer belt  10 . In the present example embodiment, the two pattern sensors  40  are arrayed along an orthogonal direction (i.e. “belt-width direction”) to the surface moving direction of the intermediate transfer belt  10 . 
   Incidentally, the number of the pattern sensors  40  to be used is not limited to two. An increased number of the pattern sensors  40  may provide higher accuracy in detecting the adjustment pattern, a reduced time for the detecting operation, and higher accuracy in detecting a displacement in a main scanning direction of the intermediate transfer belt  10 . 
   For example, when the number of the pattern sensors  40  is increased up to four, toner images of an adjustment pattern similar in color and shape are detected with the four pattern sensors  40 . Therefore, higher accuracy may be obtained in detecting the adjustment pattern. 
   Furthermore, each of the four pattern sensors  40  detects a corresponding single-color adjustment pattern. Therefore, the adjustment pattern of four color toner images can be detected in a single operation, thereby reducing a time for the detecting operation. 
   Moreover, detection data are obtained from four points in the belt-width direction. Therefore, a positional displacement in the main scanning direction can also be detected. 
   In the image forming apparatus  200 , further, the optical writing unit  1  is provided under the image forming mechanisms  300 . The optical writing unit  1  optically writes an image on the surface of the photoconductive drum  2 . 
   Furthermore, the secondary transfer roller  13  serving as a secondary transfer mechanism is arranged at a position opposite to the drive support roller  8  via the intermediate transfer belt  10 . The secondary transfer roller  13  is in contact with the intermediate transfer belt  10  so as to be pressed toward the drive support roller  8 . 
   A recording sheet is conveyed at a given timing from a lower part in  FIG. 1  to a nip area (i.e. a secondary transfer area) between the secondary transfer roller  13  and the intermediate transfer belt  10 . Then, a color image formed on the surface of the intermediate transfer belt  10  is transferred onto the recording sheet with the secondary transfer roller  13 . Incidentally, the secondary transfer mechanism may include a transfer belt or a non-contact charger. 
   In the image forming apparatus  200 , an un-illustrated fuser is provided above the secondary transfer roller  13  in  FIG. 1 . The un-illustrated fuser fuses and fixes a color image transferred on the recording sheet. 
   Next, an image forming operation of the image forming apparatus  200  is described. 
   When the image forming apparatus  200  is used as a copier, a document is set onto a document plate of an automatic document feeder (not illustrated) provided in an upper part of the image forming apparatus  200 . Alternatively, the document is set onto a contact glass of a scanning part provided under the automatic document feeder. 
   When the document is set onto the automatic document feeder, on pressing an un-illustrated start button, the document is fed to the contact glass and then a scanning unit of the scanning portion is driven. 
   Alternatively, when the document is set onto the contact glass, on pressing the un-illustrated start button, the scanning unit of the scanning portion is driven. 
   On the start of a scanning operation of the scanning unit, the document is illuminated with light from a light source. A reflected light of the document is received at a sensor via an imaging lens. Thereby, the content of the document is converted to image data. Then, the following image forming operation is performed with the image data. 
   In addition, when the image forming apparatus  200  is used as a printer, the image forming apparatus  200  performs the following image forming operation with image data received from an external device, such as a computer or a digital camera. 
   In parallel with the scanning processing of the document and the receiving processing of the image data, the drive support roller  8  is driven with an un-illustrated drive motor. Thereby, the surface of the intermediate transfer belt  10  is moved in the counterclockwise direction indicated by the arrow A in  FIG. 1 . The other support rollers are rotated in conjunction with the surface moving. 
   At this time, in the image forming mechanisms  300 , the photoconductive drums  2  are started to be driven. Then, the optical writing unit  1  emits a light beam to form a latent image on each of the photoconductive drums  2  according to color information of the image data. The latent image is developed with the developer in each of the image forming mechanisms  300 . Thus, a single-color toner image is formed on each of the photoconductive drums  2 . 
   Further, the single-color toner image in each of the photoconductive drums  2  is superimposingly transferred onto the intermediate transfer belt  10  so as to form a composite color image on the intermediate transfer belt  10 . 
   In parallel with the above-described image forming operation, a recording sheet is conveyed to the secondary transfer area at a given timing. Specifically, the recording sheet is picked up from a sheet feed cassette, and is separated sheet by sheet with a separating roller. Then, the recording sheet is sent into a sheet feed path, and is conveyed to a registration roller with a conveyance roller. The recording sheet is stopped by hitting against the registration roller. 
   The registration roller is rotated so that the recording sheet is sent into the secondary transfer roller  13  in accordance with a timing when the composite color image on the intermediate transfer belt  10  is sent into the secondary transfer area. Generally, the registration roller is used while connected to the ground. However, the registration roller may be applied with bias so as to remove dust. 
   In the secondary transfer area, the composite color image is transferred onto the recording sheet by action of a bias applied to the secondary transfer roller  13 . Then, the recording sheet carrying the composite color image is sent into the fuser. The fuser fixes the composite color image on the recording sheet by applying heat and pressure. The recording sheet is output to an sheet output tray via a output roller. 
   In addition, the image forming apparatus  200  is capable of forming a single-color image on a recording sheet. For example, when a black-only image is formed on a recording sheet, the photoconductive drums  2  for yellow, cyan, and magenta colors are preferably detached from the intermediate transfer belt  10  with an un-illustrated detachment mechanism. Further, drive of the photoconductive drums  2  for yellow, cyan, and magenta colors are preferably suspended. 
   The image forming apparatus  200  has a relatively short and simplified sheet-conveyance path from the sheet feed cassette to the sheet output tray. Thereby, higher productivity may be obtained, and an occurrence of sheet jam may be suppressed. 
   In the image forming apparatus  200 , the optical writing unit  1  is arranged under the image forming mechanisms  300  to provide a route through which a recording sheet is conveyed from a lower side to an upper side of the secondary transfer roller  13 . Consequently, toner may be scattered from the image forming mechanisms  300 , the intermediate transfer belt  10 , etc., which are arranged above the optical writing unit  1 . Further, the scattered toner may fall onto the optical writing unit  1 . 
   Therefore, in the present example embodiment, a whole body of the optical writing unit  1  is covered with a cover to suppress getting of the scattered toner into the optical writing unit  1 . However, the optical writing unit  1  emits a light beam onto each of the photoconductive drums  2 . Therefore, the cover has four exit lenses at appropriate portions so that respective light beams emitted from the optical writing unit  1  may pass through the cover. 
   Then, in each of the image forming mechanisms  300 , a writing position of the light beam is configured to deviate from a bottom point of the photoconductive drum  2 . Specifically, the writing position of the light beam is configured so that an angle, φ, formed with a transfer position (i.e. a top point), TP, and the writing position, WP, of the photoconductive drum  2  becomes 145° as illustrated in  FIG. 1 . Thereby, accumulation of the scattered toner on the exit lens may be suppressed. 
   With this configuration, a beam path of the light beam can be inclined relative to a vertical line. Accordingly, a lens surface of the exit lens orthogonally arranged to the beam path can be inclined relative to a horizontal plane. Thus, even when the scattered toner is fell onto the exit lens, the scattered toner is slipped down an inclined surface of the exit lens, thereby suppressing accumulation of the scattered toner on the exit lens. 
   Next, a drum drive mechanism  400  to drive each of the photoconductive drums  2  is described. 
     FIG. 2  is an explanatory diagram illustrating a drum drive mechanism  400  of the photoconductive drum  2  according to the present example embodiment. The drum drive mechanisms  400  have a similar configuration for the photoconductive drums  2  of cyan, magenta, yellow, and black. 
   According to the present example embodiment, a rotation shaft (hereinafter, “drum shaft”) of the photoconductive drum  2  is rotatably supported with a frame of the image forming apparatus  200 . 
   As illustrated in  FIG. 2 , the drum drive mechanism  400  of the present example embodiment includes a coupling  31 , a drum drive gear  32 , a drive motor  33 , a motor shaft gear  34 , a rotary encoder  35 , a motor drive circuit  36 , a controller  37 , and a target value output unit  38 . 
   The drive motor  33  also includes a stepping motor, a DC servo motor, etc. The motor shaft gear  34  is provided for a motor shaft of the drive motor  33 . The drum drive gear  32  is fixed to a drive shaft so as to be engaged with the motor shaft gear  34 . The coupling  31  connects the drive shaft with the drum shaft of the photoconductive drum  2 . 
   According to the present example embodiment, the drum drive mechanism  400  employs a one-step deceleration structure including the motor shaft gear  34  and the drum drive gear  32 . This structure may reduce the count of components, thereby saving manufacturing cost thereof. The one-step deceleration structure may also reduce error factors in gear transmission, such as deviations in gear-teeth shape and gear-core position. 
   In addition, when a relatively high deceleration ratio is set in the one-step deceleration structure, the drum drive gear  32  has a relatively large diameter compared to the diameter of the photoconductive drum  2 . Thus, such a relatively large diameter of the drum drive gear  32  may provide a smaller single-pitch error of the drum drive gear  32  estimated as an error on the photoconductive drum  2 . Thereby, un-evenness in print density in a sub-scanning direction may be suppressed. 
   Incidentally, the deceleration ratio is determined based on a speed range where relatively high efficiency and rotation accuracy are obtained with respect to the target rotation speed and motor characteristics of the photoconductive drum  2 . According to the present example embodiment, a deceleration ratio between the motor shaft gear  34  and the drum drive gear  32  is set to 1:20. 
   The rotary encoder  35  is set to the motor shaft. The rotary encoder  35  detects a rotating state of the drive motor  33 , and feeds a detection signal back to the motor drive circuit  36  of the drive motor  33  through the controller  37 . Thus, the drive motor  33  is controlled to rotate at a desired speed. 
   When the drive motor  33  contains a speed sensor or an encoder, the rotary encoder  35  may be omitted from the drum drive mechanism  400 . The speed sensor may be a printed-coil frequency generator. The encoder may be a magneto-resistive (MR) sensor. 
   The motor drive circuit  36  outputs a given drive current to the drive motor  33 . The rotary encoder  35  detects a rotation angular speed or a rotation angular displacement of the drive motor  33 , and outputs a detection result thereof to the controller  37 . 
   According to the present example embodiment, the drive motor  33  employs a DC servo motor, which is a kind of DC brushless motor. The DC servo motor includes a coil and a rotor to which three phases of U, V, and W are connected in a star configuration. The DC servo motor also includes three hall elements as a rotor position detector to detect a magnetic pole of the rotor. Each of the three hall elements has an output terminal connected to the motor drive circuit  36 . 
   The DC servo motor containing an MR sensor is provided with a rotation speed detection unit having the MR sensor and a magnetic pattern formed around the rotor. The rotation speed detection unit has an output terminal connected to the controller  37 . 
   The motor drive circuit  36  has three higher transistors and three transistors. The three higher transistors are connected to U, V, and W, respectively. The three lower transistors are also connected to U, V, and W, respectively. 
   The motor drive circuit  36  determines a rotor position based on rotor position signals generated from the hall elements, and generates a phase switching signal. An on-and-off state of each of the transistors is controlled based on the phase switching signal. Thus, an exciting phase is sequentially switched, and thereby the rotor is rotated. 
   The controller  37  compares a rotation speed detected in the rotary encoder  35  or the rotation speed detection unit with a target rotation speed. Then, the controller  37  generates a PWM (pulse-width modulated) signal so as to match a detected rotation speed of the motor shaft with the target rotation speed. The PWM signal is ANDed with the phase switching signal generated from the motor drive circuit  36  in an AND gate. Further, the drive current is chopped. Thus, the rotation speed of the drive motor  33  is controlled. 
   The controller  37  may be configured with a PLL control circuit system to compare a pulse signal being output from the rotary encoder  35  or the rotation speed detection unit with a pulse signal being output from the target value output unit  38  with respect to phase and frequency. 
   The target value output unit  38  outputs a pulse signal having a frequency modulated corresponding to a given target rotation speed to correct a variable component in rotation speed having a rotation period of the photoconductive drum  2 . 
   The controller  37  may be an analogue circuit or a digital circuit. For digital processing, the controller  37  measures a period of the pulse signal being output from the rotary encoder  35  or the rotation speed detection unit, and calculates a rotation angular speed. Alternatively, the controller  37  counts the number of pulse signals being output from the rotary encoder  35  or the rotation speed detection unit, and calculates a rotation angular speed based on the counted number of pulse signals in a given time. 
   When the position control system controls a rotation angular displacement instead of the rotation angular speed, the controller  37  counts the number of pulse signals being output from the rotary encoder  35  or the rotation speed detection unit, and calculates a displacement amount of the rotation angle. 
   Then, the controller  37  calculates a difference between the calculated displacement amount and the target amount generated from the target value output unit  38 . The drive of the drive motor  33  is controlled so as to reduce the difference. Generally, the controller  37  contains a PID (proportional integral derivative) control unit, etc., to control a PMW signal so as to suppress a deviation, an overshoot, and an oscillation of the photoconductive drum  2  relative to the target rotation speed. Thus, the PMW signal is output to the motor drive circuit  36 . 
   Next, rotation drive control of the photoconductive drum  2  is described. 
   According to the present example embodiment, a brushless DC servo motor is employed as the drive motor  33  to drive each of the photoconductive drums  2 . On driving each of the photoconductive drums  2 , a variation in surface moving speed of the photoconductive drum  2  may be independently caused by the following two factors. Then, when single-color toner images formed on the photoconductive drums  2  are superimposingly transferred onto the intermediate transfer belt  10 , transfer positions of the single-color toner images may be displaced relative to each other, thereby causing a so-called color displacement. 
   The first factor of the color displacement is a variation in motor rotation caused by torque ripple, etc. Further, the variation in motor rotation may cause a variation in rotation angular speed transmitted to the photoconductive drum  2 . Furthermore, the variation in rotation angular speed causes a variation in surface moving speed of each of the photoconductive drums  2 . As a result, transfer positions at which the single-toner images are transferred on the intermediate transfer belt  10  are displaced in the surface moving direction of the intermediate transfer belt  10  (hereinafter, “positional displacement”). 
   The second factor of the color displacement is a variation in gear transmission caused by gear members. Specifically, the gear members, including the drum drive gear  32 , of the drum drive mechanism  400  may produce an accumulated pitch error. Further, the drum drive gear  32  may have an eccentricity relative to the rotation shaft, thereby causing a variation in rotation angular speed transmitted to the photoconductive drum  2 . Furthermore, the variation in rotation angular speed may cause a variation in surface moving speed of the photoconductive drum  2 , further causing a positional displacement. 
   The variation in surface moving speed of the photoconductive drum  2  caused by the first factor may be suppressed through the above-described feedback control based on a detection result in the rotary encoder  35  provided at the motor shaft. 
   The variation in surface moving speed of the photoconductive drum  2  caused by the second factor may be suppressed by the following control scheme. First, an amplitude and a phase of a variable component of surface moving speed occurring in a rotation period of the photoconductive drum  2  are calculated based on a detection result of an adjustment pattern. Then, the rotation angular speed of the drive motor  33  is controlled so as to suppress the variation in surface moving speed based on calculated amplitude and phase. 
   Next, a detection method of a transfer position adjustment pattern  44  is described. 
     FIG. 3  is an explanatory diagram illustrating a pattern detection mechanism to detect a transfer position adjustment pattern  44  having been formed on the intermediate transfer belt  10  with the image forming mechanisms  300 . In  FIG. 3 , two pattern sensors  40  are arranged at different places from the place of the pattern sensor  40  in  FIG. 1  for convenience of description. 
   According to the present example embodiment, the pattern sensors  40  are arranged along both end portions in the belt-width direction in an image area of the intermediate transfer belt  10 . Each of the pattern sensors  40  includes an LED (light emitting diode) element  41 , a light-sensitive element  42 , and a condenser lens  43 . 
   The LED element  41  serves as an illumination light source and emits a light to the transfer position adjustment pattern  44  formed on the intermediate transfer belt  10 . The condenser lens  43  condenses the light reflected on the transfer position adjustment pattern  44 . 
   The light-sensitive element  42  receives the reflected light condensed through the condenser lens  43 . The light-sensitive element  42  is arranged at a position at which the reflected light is incident through the condenser lens  43 . The light-sensitive element  42  includes a CCD (charge-coupled device) in which a large number of light-sensitive pixels are linearly arranged. 
   As described above, according to the present example embodiment, the pattern sensors  40  are arranged along both ends in the belt-width direction in the image area of the intermediate transfer belt  10 . Thereby, appropriate adjustment can be made for positional displacement in the main scanning direction. In addition, appropriate adjustment can be made for positional displacement, magnification error, and scanning line inclination in the sub-scanning direction. 
     FIG. 4  is an explanatory diagram illustrating a transfer position adjustment pattern  44  according to the present example embodiment. 
   As illustrated in  FIG. 1 , the transfer position adjustment pattern  44  includes toner images in black, cyan, magenta, and yellow. The transfer position adjustment pattern  44  is formed so as to have a so-called chevron shape. Specifically, the toner images in black, cyan, magenta, and yellow are inclined at substantially 45 degree with respect to the sub-scanning direction of the intermediate transfer belt  10  indicated by an arrow B in  FIG. 4 . Accordingly, the toner images in black, cyan, magenta, and yellow are arranged in parallel to each other in a given pitch. 
   The transfer position adjustment pattern  44  is formed on each of both ends of the image area of the intermediate transfer belt  10  in the belt-width direction. The transfer position adjustment pattern  44  is detected with the pattern sensor  40 . At this time, differences in detection time are measured between a black toner image as a reference color and each of the other color toner images. 
   For example, as illustrated in  FIG. 4 , the transfer position adjustment pattern  44  is formed to have toner images in an order of yellow, magenta, cyan, black, black, cyan, magenta, and yellow. When the pattern sensor  40  sequentially detects the toner images, detection time differences, tky, tkm, and tkc, are measured between the black toner image and each of yellow, magenta, and cyan toner images, respectively. 
   Further, a difference between the actually measured value and a corresponding theoretical value is determined for each of the detection time differences, tky, tkm, and tkc. Then, positional displacement amounts in the sub-scanning direction are obtained between the black toner image and each of the other color toner images. 
   Furthermore, based on the detection result with the pattern sensor  40 , detection time differences, tk, tc, tm, and ty, are determined between two toner images different in inclination angle for each color of black, cyan, magenta, and yellow. Then, a difference between the actually measured value and a corresponding theoretical value is determined for each of the detection time differences, tk, tc, tm, and ty. Moreover, based on the difference, positional displacement amounts in the sub-scanning direction are obtained between the black toner image and each of the other color toner images. 
   An inclination amount of the scanning line is determined based on the positional displacement amounts in the sub-scanning direction obtained from two adjustment patterns  44  formed on both ends of the intermediate transfer belt  10 . Further, based on the inclination amount, inclination of a toroidal lens is controlled so as to correct the inclination of the scanning line. 
   In correcting the positional displacement in the sub-scanning direction, a positional displacement amount in the sub-scanning direction is determined based on an average of the detection values. Then, a writing timing is adjusted corresponding to the positional displacement amount in unit of one surface of a polygon mirror, that is, a scanning line pitch. 
   Alternatively, an average rotation angular speed of the drive motor  33  for the photoconductive drum  2  may be adjusted corresponding to the positional displacement amount. At this time, the adjustment is made for a rotation time of the photoconductive drum  2  from the writing position to the transfer position thereon. 
     FIG. 5  is an explanatory diagram illustrating an adjustment pattern  45  employed to suppress a variation in surface moving speed of the photoconductive drum  2  caused by the above-described second factor. 
   The adjustment pattern  45  includes toner images formed in one of black, cyan, magenta, and yellow colors. The toner images have a longer length in the belt-width direction of the intermediate transfer belt  10 . The toner images are arranged at a given pitch in parallel to each other along the surface moving direction of the intermediate transfer belt  10  indicated by an arrow C in  FIG. 5 . 
   The pattern sensor  40  sequentially detects the toner images in an order in which the toner images have been formed along the surface moving direction of the intermediate transfer belt  10 . Then, elapsed times, such as tk 01 , tk 02 , tk 03 , and tk 04 , from a reference time to each detection time of the toner images are determined for each color. 
   According to the present example embodiment, two adjustment patterns  45  different in color are formed on both ends of the intermediate transfer belt  10  in the belt-width direction. Further, as described above, the two pattern sensors  40  are arranged along both ends of the intermediate transfer belt  10  in the belt-width direction. 
   Accordingly, a simultaneous detection for two colors can be performed with the two pattern sensors  40 . That is, according to the present example embodiment, detection of the two adjustment patterns  45  of four colors can be finished by repeating the detecting operation twice, reducing a detection time of the adjustment pattern  45 . 
   Further, according to the present example embodiment, each of the two adjustment patterns  45  is formed in a single color. Therefore, a relatively short interval may be set between toner images included in each of the two adjustment patterns  45 . Thereby, higher accuracy may be obtained in the detection. 
     FIG. 6  is a block diagram illustrating an electrical hardware configuration of the drum drive mechanism  400  of  FIG. 2 . 
   As illustrated in  FIG. 6 , the drum drive mechanism  400  includes, as the electrical hardware configuration, a detection sensor unit  51 , an amplifier (AMP)  52 , a filter  53 , an analog-to-digital (A/D) converter  54 , a FIFO (first-in first-out) memory  55 , a sampling controller  56 , an input-and-output (I/O) port  57 , a CPU (central processing unit)  58 , a ROM (read-only memory)  59 , a RAM (random access memory)  60 , an address bus  61 , a data bus  63 , a light amount controller  64 , and the target-value output unit  38 . 
   A signal generated in the detection sensor unit  51  including the pattern sensor  40  illustrated in  FIG. 3  is amplified with an amplifier  52 . An amplified signal is output to the filter  53 . Then, only a signal component of the transfer position adjustment pattern  44  or the adjustment pattern  45  is passed through the filter  53 . 
   A filtered signal is converted from analog to digital data with the analog-to-digital converter  54 . Data sampling in the analog-to-digital conversion is controlled with the sampling controller  56 . Sampled data are stored into the FIFO memory  55 . 
   After the detection of the adjustment pattern  45 , the stored data are loaded onto the CPU  58  and the RAM  60  via the input-and-output port (I/O)  57  through the data bus  63 . Then, computations are performed in the CPU  58  to calculate various positional displacement amounts as described above. 
   The CPU  58  first changes settings for writing control and drive of an un-illustrated stepping motor serving as a drive source of the intermediate transfer belt  10 . Through changing the settings, the CPU  58  performs skew correction, change of main scanning registration, change of sub-scanning registration, and change of image frequency corresponding to magnification error. The CPU  58  also performs the above correction and changes based on corresponding correction values calculated from a detection signal of the transfer position adjustment pattern  44  as illustrated in  FIG. 4 . 
   For the writing control, the image forming apparatus  200  is provided with a device, such as a clock generator employing a VCO (voltage controlled oscillator), for each color that is capable of finely setting an output frequency and controlling registration in the main scanning direction and the sub-scanning direction. According to the present example embodiment, the image forming apparatus  200  employs an output from the clock generator as an image clock. 
   Then, a drive control value of the drive motor  33  is corrected based on the correction value calculated from the detection signal of the adjustment pattern  45 . The correction is performed so as to reduce a positional displacement amount occurring in a rotation period of the photoconductive drum  2 . A corrected drive control value is set in the target-value output unit  38 . The target-value output unit  38  outputs a rotation-speed target signal to each of the photoconductive drums  2 . Incidentally, the rotation-speed target signal may be a digital data signal or a pulse signal. 
   The CPU  58  also monitors the detection signal from the detection sensor unit  51  at an appropriate interval. Further, the CPU  58  controls a light amount emitted from the optical writing unit  1  via a light amount controller  38 . Thereby, a signal level of received light transmitted from the light-sensitive element  42  of the detection sensor unit  51  is maintained constant. Thus, even when the intermediate transfer belt  10  or the LED element  41  in the detection sensor unit  51  is deteriorated, the adjustment pattern  45  may be appropriately detected. 
   The ROM  59  stores various programs including a program to compute various displacement amounts as described above. A ROM address, a RAM address, and various input-and-output devices are specified via the address bus  61 . 
   Next, a description is given for a configuration and an operation to suppress variation in surface moving speed of the photoconductive drum  2  caused by the above-described second factor. 
   According to the present example embodiment, the adjustment pattern  45  as illustrated in  FIG. 5  is employed in the configuration and the operation to suppress variation in surface moving speed of the photoconductive drum  2  caused by the above-described second factor. The adjustment pattern  45  having one color of yellow, magenta, cyan, and black is sequentially formed on the intermediate transfer belt  10  along the surface movement direction thereof. At this time, the adjustment pattern  45  is formed in relatively large quantity, for example, while the photoconductive drum  2  is rotated a plurality of times. 
   Then, sampling is performed on the adjustment pattern  45  formed on the intermediate transfer belt  10 . Incidentally, a large-quantity formation of the adjustment pattern  45  having a plurality of colors may cause degradation in the adjustment pattern  45 , such as deterioration of toner image due to reverse transfer. 
   Then, according to the present example embodiment, the adjustment pattern  45  is formed in a single color as described above. Accordingly, the above degradation may be suppressed, and the adjustment pattern  45  may be detected with relatively high accuracy. 
   Alternatively, when the above degradation is not so noticeable, the adjustment pattern  45  is formed in an order of yellow, magenta, cyan, and black toner images. Further, the yellow, magenta, cyan, and black toner images are formed so as to be in parallel to each other along the sub-scanning direction. 
   As illustrated in  FIG. 5 , a sampling pattern length, Pa, of the adjustment pattern  45  to be sampled along the surface moving direction of the intermediate transfer belt  10  is set to a length of an integral multiple of a rotation speed variation period. On the setting of the sampling pattern length, Pa, a consideration should be made for other periodic rotation variations in forming and detecting the adjustment pattern  45  on the intermediate transfer belt  10 . 
   Factors influencing the other periodic rotation variations include various frequency components, such as a periodic rotation variation of the drive support roller  8  of the intermediate transfer belt  10 , a pitch error and an eccentric component of the gear to transmit a rotation force of the drive support roller  8 , a walk of the intermediate transfer belt  10 , a deviation distribution of thickness in a circumferential direction of the intermediate transfer belt  10 . 
   All of the above frequency components are superposed in detection data. A variable component having the rotation period of the photoconductive drum  2  should be detected with relatively high accuracy. Then, a pattern interval, Ps, between two adjacent toner images in the adjustment pattern  45  is set so as to be substantially constant. 
   Further, the pattern interval, Ps, is preferably set to be relatively short so as to form the adjustment pattern  45  in relatively dense. However, the pattern interval, Ps, is determined in further consideration with available pattern width, computing time, etc. 
   In addition to the variable components having the rotation period of the photoconductive drum  2 , a variable component having a rotation period of the drive support roller  8  may significantly influence the positional displacement of the adjustment pattern  45 . In this case, the sampling pattern length, Pa, is determined in consideration with the variable component having the rotation period of the drive support roller  8  in addition to the variable components having the rotation period of the photoconductive drum  2 . 
   For example, when a diameter of the photoconductive drum  2  is 40 mm, a rotation period of the photoconductive drum  2  becomes 125.7 mm when converted to a surface moving distance of the intermediate transfer belt  10 . Further, when a diameter of the drive support roller  8  is 30 mm, a rotation period of the drive support roller  8  becomes 94.2 mm when converted to a surface moving distance of the intermediate transfer belt  10 . 
   According to the present example embodiment, the sampling pattern length, Pa, is set to 377 mm, that is, a least common multiple between the above rotation periods of the photoconductive drum  2  and the drive support roller  8 . Further, the pattern interval, Ps, is set to a value so that toner images of the adjustment pattern  45  are arranged at a constant interval with respect to the sampling pattern length, Pa. 
   Thus, the influence of the variable component caused by the drive support roller  8  may be suppressed, and therefore the variable component having the rotation period of the photoconductive drum  2  may be detected with relatively high accuracy. Furthermore, the below-described amplitude and phase of the variable component having the rotation period of the photoconductive drum  2  may be computed with relatively high accuracy. This computation assumes that a term including a variable component caused by the drive motor  8  theoretically becomes zero. 
   Moreover, a periodic rotation variation of the intermediate transfer belt  10  may be caused by the deviation distribution of thickness in the circumferential direction thereof. In this case, the sampling pattern length, Pa, is set so as to be an integral multiple of the rotation period of the photoconductive drum  2  and be nearest to the circumferential length of the intermediate transfer belt  10 . Thus, influence of the periodic variation of the intermediate transfer belt  10  may be suppressed. 
   In addition, a variable component having a more than ten times longer period than a rotation period of the photoconductive drum  2  may influence the positional displacement of the adjustment pattern  45 . An example thereof is a variable component having a rotation period of the drive motor  33 , which serves as the drive source of the drive support roller  8 . Such a variable component can be suppressed with a low pass filter in digital processing. 
   When a feedback control is provided in a drive mechanism of the intermediate transfer belt  10 , relatively high accuracy may be obtained in the detection of the variable component having the rotation period of the photoconductive drum  2 . According to the present example embodiment, a rotary encoder is provided at a rotation shaft of the support roller  12  that is rotated together with the surface moving of the intermediate transfer belt  10 . Based on rotation information being output from the rotary encoder, the drive mechanism of the intermediate transfer belt  10  controls rotation of the un-illustrated drive motor so that a rotation angular speed being output from the rotary encoder is kept substantially constant. Thus, errors resulting from the drive support roller  8  and the drive transmission mechanism, and a belt speed variation resulting from slippage between the drive support roller  8  and the intermediate transfer belt  10  may be effectively suppressed. 
   Then, another one of the periodic rotation variations to be considered is a variation having a rotation period of the support roller  12 . This variation may be mainly caused by eccentricity of the support roller  12  or eccentric installation of the rotary encoder. Accordingly, the variation having the rotation period of the support roller  12  may be detected with relatively high accuracy by setting a sampling pattern length, Pa, to be a length of a least multiple period between a rotation period of the support roller  12  and a rotation period of the photoconductive drum  2 . 
   According to the present example embodiment, effective suppression may be achieved for deterioration in image quality due to a positional displacement that may occur in continued use of the image forming apparatus  200 , in addition to a positional displacement that may occur in factory default setting thereof. 
   Specifically, a position or a size of each of the image forming mechanisms  300  may be subtly changed by a temperature variation inside the image forming apparatus  200  or an external force applied thereto. The temperature variation or the external force may be caused by normal operations, such as a recovery operation from paper jam, part replacement in a maintenance-and-inspection operation, relocation of the image forming apparatus  200 . Therefore, the temperature variation or the external force may distort registration of images formed with the image forming mechanisms  300 . Thus, a factor occurring in continued use of the image forming apparatus  200  may cause a positional displacement, thereby deteriorating image quality. 
   Then, when needed, a sampling operation of the adjustment pattern  45  and a correcting operation based on a result thereof are preferably executed at appropriate timings, such as at turn-on of the image forming apparatus  200 , after a recovery operation from paper jam, before image formation, and during image formation. According to the present example embodiment, each of the sampling operation of the adjustment pattern  45  and the correcting operation based on a result thereof is automatically executed only once at turn-on of the image forming apparatus  200  or after a maintenance-and-inspection operation thereof. 
   Generally, variable components causing the positional displacement and having the rotation period of the photoconductive drum  2  mainly result from errors in dimensional accuracy and positioning accuracy of the photoconductive drum  2 , the drum drive gear  32 , the coupling  31 , etc. On the other hand, the variable components irregularly caused by the temperature variation or the continued use have only small effects to the positional displacement. Therefore, the sampling operation of the adjustment pattern  45  and the correcting operation based on the result thereof may be performed at relatively low frequency as described above. 
   Incidentally, from the viewpoint of increase in detection accuracy, the sampling operation of the adjustment pattern  45  and the correcting operation based on the result thereof are preferably performed after the sampling operation of the transfer position adjustment pattern  44  and the correction operation based on the result thereof. 
   When the sampling operation of the adjustment pattern  45  and the correcting operation based on the result thereof are executed, the CPU  58  illustrated in  FIG. 6  issues instructions to various parts at appropriate timings, such as when the marking  4  illustrated in  FIG. 4  is detected with the drum position sensor  20 . Then, image data of the adjustment pattern  45  for each of the photoconductive drums  2  stored in the ROM  59  is in turns output to a corresponding one of the image forming mechanisms  300 . At this time, image formation is performed in a normal image forming mode of the image forming apparatus  200 . 
   Each of the image forming mechanisms  300  forms the adjustment pattern  45  based on the image data, and superimposingly transfers the adjustment pattern  45  onto the intermediate transfer belt  10 . Thus, two adjustment patterns  45  are formed on the intermediate transfer belt  10 . 
   Further, a detection result of the adjustment pattern  45  in the detection sensor unit  51  is stored in the FIFO memory  55  at an appropriate sampling period having been set in the sampling controller  56 . At this time, the detection result is stored as discrete data having been converted in the analog-to-digital converter  54 . The data stored in the FIFO memory  55  includes a numeric value of output signal corresponding to a light amount that is reflected on the adjustment pattern  45  and is received at the light-sensitive element  42 . The numeric value varies with toner color or toner density of the adjustment pattern  45 . 
   According to the present example embodiment, a passing timing of the adjustment pattern  45  is preferably determined with relatively high accuracy. Therefore, the passing timing of the adjustment pattern  45  is determined according to a peak recognition of the numeric value, not according to a previously defined threshold value. Thereby, effective suppression may be achieved for deterioration of the detection accuracy due to distortion of the adjustment pattern  45  that is caused by a variation in the surface moving speed of the photoconductive drum  2 . Thus, the positional displacement may be detected with relatively high accuracy. Below, a further detailed description is given. 
     FIGS. 7A and 7B  are explanatory diagrams illustrating relationship between surface moving speed of the photoconductive drum  2  and toner density distribution of the adjustment pattern  45  transferred onto the intermediate transfer belt  10 . 
     FIG. 7A  is a schematic diagram of a transfer area between the photoconductive drum  2  and the intermediate transfer belt  10 . In an interface between the photoconductive drum  2  and the intermediate transfer belt  10 , the photoconductive drum  2  and the intermediate transfer belt  10  contact each other. Meanwhile, the photoconductive drum  2  and the intermediate transfer belt  10  slips each other with a toner, a lubricant agent, and a lubricant layer on surfaces thereof. Thus, the surfaces of the photoconductive drum  2  and the intermediate transfer belt  10  move at independent speeds of Vo and Vb, respectively. 
     FIG. 7B  is a graph illustrating relationship between toner density on the vertical axis and interval distance between adjacent toner images on the horizontal axis, with respect to the adjustment pattern  45  formed on the photoconductive drum  2 . According to the present example embodiment, toner images of the adjustment pattern  45  are formed at a substantially constant toner density and at a substantially constant pattern interval, PaN. 
     FIG. 7C  illustrates the adjustment pattern  45  transferred onto the intermediate transfer belt  10  when a surface moving speed, Vo, of the photoconductive drum  2  is higher than a surface moving speed, Vb, of the intermediate transfer belt  10 . In this case, the surface of the photoconductive drum  2  precedes the surface of the intermediate transfer belt  10  at the transfer area. Therefore, a pattern interval, PaH, between adjacent toner images of the adjustment pattern  45  formed on the intermediate transfer belt  10  becomes shorter than the pattern interval, PaN, between adjacent toner images of the adjustment pattern  45  formed on the photoconductive drum  2 . 
   Further, a skirt portion of the toner density line indicated by an arrow, TW, in  FIG. 7C  represents a variation in toner density distribution due to distortion of the adjustment pattern  45  that is caused by the speed difference of the photoconductive drum  2  and the intermediate transfer belt  10 . 
   In the transfer area, a nip portion of substantially 2 mm is formed between the photoconductive drum  2  and the intermediate transfer belt  10  to obtain relatively high transfer efficiency. Therefore, a toner image may be rubbed against each of the photoconductive drum  2  and the intermediate transfer belt  10 . As a result, toner particles aggregating in the toner image may be displaced corresponding to a speed difference between the photoconductive drum  2  and the intermediate transfer belt  10 . Thus, the above variation in toner density distribution may be observed as illustrated in  FIG. 7C . 
   On the other hand,  FIG. 7D  illustrates the adjustment pattern  45  transferred onto the intermediate transfer belt  10  when the surface moving speed, Vo, of the photoconductive drum  2  is lower than the surface moving speed, Vb, of the intermediate transfer belt  10 . In this case, a pattern interval, PaL, between adjacent toner images of the adjustment pattern  45  formed on the intermediate transfer belt  10  becomes longer than the pattern interval, PaN, between adjacent toner images of the adjustment pattern  45  formed on the photoconductive drum  2 . Similar to the case illustrated in  FIG. 7C , a skirt portion of toner density line indicated by an arrow, TW, is also observed in  FIG. 7D . 
   According to the present example embodiment, the pattern intervals, PaH and PaL, are preferably detected with relatively high accuracy. As described above, the moving speed difference between the photoconductive drum  2  and the intermediate transfer belt  10  is periodically changed with a variation in the surface moving speed of the photoconductive drum  2 . In addition, a range of toner density distribution of the adjustment pattern  45  is also periodically changed therewith. 
   In this regard, if an edge portion of the adjustment pattern  45  is determined according to a previously defined threshold value, a non-edge portion may be improperly detected due to the pattern distortion. Then, according to the present example embodiment, a peak value of toner density is employed as a pattern detection timing when the adjustment pattern  45  is detected with the detection sensor unit  51 . 
   Specifically, the CPU  58  selects a peak value in toner density from among a set of signal data having relatively high correlation to toner density, which stored in the FIFO memory  55  at an appropriate sampling period. Then, the CPU  58  stores timing data of the signal data selected as the peak value into the RAM  60 . Thus, the pattern interval, Pah or PaL, may be detected with relatively high accuracy. 
   Detection data (hereinafter “pattern detection data”) of the pattern interval, Pah or PaL, are stored into the RAM  60 , as described above. The pattern detection data varies in a rotation period of the photoconductive drum  2 . According to the present example embodiment, an amplitude and a phase of a variable component having the rotation period of the photoconductive drum  2  are detected based on the pattern detection data. 
   Detecting methods thereof include a method in which the amplitude and the phase of the variable component is detected based on a zero cross point or a peak value of variable assuming an average of all data is zero. However, this method may cause a large error since the pattern detection data is significantly subjected to noise in detection. 
   Then, according to the present example embodiment, a method may be employed in which the amplitude and the phase of a variable component occurring in the rotation period of the photoconductive drum  2  are calculated from the pattern detection data through data processing (i.e. quadrature detection processing) in a quadrature detection scheme. The quadrature detection processing is a signal analysis method generally used in demodulation circuit in the field of communication. 
     FIG. 8  is a block diagram illustrating a basic configuration of quadrature detection processing according to the present example embodiment. 
   As illustrated in  FIG. 8 , the basic configuration of quadrature detection processing includes an oscillator  121 , a quadrature phase shifter  122 , a multiplier  123   a , a multiplier  123   b , an amplitude calculator  124 , a phase calculator  125 , an LPF (low-pass filter)  126   a , and an LPF (low-pass filter)  126   b.    
   An input signal  120  may be generated from the pattern detection data as described above. The pattern detection data includes elapsed times, such as tk 01 , tk 02 , and tk 03 , from a reference time to each detection time of toner images of the adjustment pattern  45 . That is, the pattern detection data includes a monotonic increasing data group in which various variable components are superposed. 
   Then, pattern variation data is obtained by deducting an increasing slope from the pattern detection data. The increasing slope can be calculated from the data group by using least square method. A calculated increasing slope is employed as a magnification correction value. In addition, the pattern variation data is employed as the input signal  120 . 
   The oscillator  121  oscillates a signal at a frequency component to be detected and at a phase based on the given reference timing used in forming the adjustment pattern  45 . The oscillator  121  outputs the signal to the multiplier  123   a  and the quadrature phase shifter  122 . Incidentally, in the present example embodiment, the frequency component to be detected is a frequency, ω 0 /2π, of the rotation period of the photoconductive drum  2 . 
   A rotation period, 2π/ω 0 , of the photoconductive drum  2  can be accurately calculated through measuring a detection signal interval of the marking  4  formed on the dram drive gear  32  for each of the photoconductive drums  2 . 
   The multiplier  123   a  multiplies the input signal  120  by the signal having the frequency oscillated from the oscillator  121 . On the other hand, the multiplier  123   b  multiplies the input signal  120  by a signal being output from the quadrature phase shifter  122 . That is, the input signal  120  is separated into a signal having in-phase component, I(t), and a signal of quadrature component, Q(t), through the multipliers  123   a  and  123   b . The multiplier  123   a  outputs the in-phase component, I(t), to an LPF (low-pass filter)  126   a , while the multiplier  123   b  outputs the quadrature component, Q(t), to an LPF (low-pass filter)  126   b.    
   For a signal multiplied in the multiplier  123   a , the LPF  126   a  causes only a signal in a low frequency band to pass therethrough. Similarly, for a signal multiplied in the multiplier  123   b , the LPF  126   b  causes only a signal in a low frequency band to pass therethrough. 
   According to the present example embodiment, the LPF  126   a  and the LPF  126   b  are designed to smooth data in unit of an integral multiple of an oscillation period, 2π/ω 0 , that is, in unit of a sampling pattern length, Pa. 
   The smoothing of the data in unit of a sampling pattern length, Pa, may reduce a variable component having the rotation period of the drive support roller  8 . 
   Then, the amplitude calculator  124  calculates an amplitude, a(t), based on two inputs, I(t) and Q(t). On the other hand, the phase calculator  125  calculates a phase, b(t), based on two inputs, I(t) and Q(t). The a(t) and b(t) represent the amplitude of the periodic variation of the photoconductive drum  2  and the phase angle from the given reference point, respectively. Incidentally, through setting an oscillation period, ω 0 , to be a high-frequency component of the motor rotation period, similar processing to the above quadrature detection processing may be performed to detect an amplitude and a phase of a variable component having the rotation period of the motor shaft gear  34 . 
   The calculation of the amplitude and the phase by the quadrature detection processing may reduce the amount of pattern detection data to be used, compared to other detection methods employing a zero cross point or a peak value of the variable component, etc. In particular, a pattern interval, Ps, of the adjustment pattern  45  is preferably set so that 4NP toner images (NP representing a natural number) are detected per rotation period of the photoconductive drum  2 . 
   Thus, the amplitude and the phase may be calculated with relatively high accuracy, based on a small number of detected toner images. At this point, since a positional relation between the 4NP toner images of the adjustment pattern  45  is highly discriminative for the variable component, a relatively high sensitivity may be obtained in the calculation. 
   For example, when an adjustment pattern  45  having four toner images is detected, each of the four toner images corresponds to one of zero cross points and peak values of the variable component. Therefore, relatively high sensitivity may be obtained in the detection, compared to when the detected number of toner images is another natural number multiple of four. In addition, even if a deviation is observed between the phases of the four toner images, a positional relation between the four toner images may still provide relatively high sensitivity in the detection. 
   Based on detected data of the amplitude and the phase of the variable component having the rotation period of the photoconductive drum  2 , the CPU  58  calculates a drive control correction value to control the drive of each of the photoconductive drums  2 , and transmits the correction value to the target value output unit  38 . The rotation angular speed of each of the photoconductive drums  2  is finely adjusted with the drive control correction value so as to suppress the variation in surface moving speed of the photoconductive drum  2  corresponding to the variable component. 
   Specifically, as illustrated in  FIG. 7C , when a relatively shorter pattern interval, PaH, of the adjustment pattern  45  is detected since the surface moving speed of the photoconductive drum  2  is higher than the surface moving speed of the intermediate transfer belt  10 , the rotation angular speed of the photoconductive drum  2  is slowed down with the drive control correction value. 
   On the other hand, as illustrated in  FIG. 7D , when a relatively longer pattern interval, PaL, of the adjustment pattern  45  is detected since the surface moving speed of the photoconductive drum  2  is lower than the surface moving speed of the intermediate transfer belt  10 , the rotation angular speed of the photoconductive drum  2  is increased up with the drive control correction value. 
   As described above, data of the amplitude and the phase of the variable component having the rotation period of the photoconductive drum  2  are calculated based on the pattern variation data. Specifically, the data are calculated from a variation in pattern interval on the intermediate transfer belt  10 . The variation in pattern interval on the intermediate transfer belt  10  is caused by the variations in surface moving speed on the writing position, WP, and on the transfer position, TP of the photoconductive drum  2 . 
   Then, referring to  FIG. 9 , description is given to a relationship between variation in rotation angular speed of the photoconductive drum  2  and pattern interval on the intermediate transfer belt  10 . Further description is given to a method of obtaining an appropriate drive control correction value from the pattern variation data based on the pattern detection data as described above. Incidentally, as illustrated in  FIG. 9 , the photoconductive drum  2  is configured to have a phase difference angle, φ, between a writing position, WP, and a transfer position, TP, on the photoconductive drum  2 . 
   Based on a reference timing when the drum position sensor  20  detects the marking  4 , a latent image of the adjustment pattern  45  is written at a constant interval onto the writing position, WP, of the photoconductive drum  2 . At this time, a rotation angular speed, ω, of the photoconductive drum  2  can be expressed by the following Equation 1.
 
ω=ω 0 +Δω cos(ω 0   t   0 +α)  Eq. 1
 
   The second term, Δω cos(ω 0 t 0 +α), of the right-hand side of Equation 1 represents a variation amount in rotation angular speed having the rotation period of the photoconductive drum  2  observed between a reference timing when the drum position sensor  20  detects the marking  4  and a given time, t 0 . Specifically, the second term, Δω cos(ω 0 t 0 +α), of the right-hand side of Equation 1 mainly represents a variation amount of a variable component in rotation angular speed caused by the eccentricity of the drum drive gear  32  attached to the shaft of the photoconductive drum  2 , and the like. 
   In the second term, Δω cos(ω 0 t 0 +α), of the right-hand side in Equation 1, “α” represents a phase of the periodic variation based on the reference time when the drum position sensor  20  detects the marking  4 . Then, a surface moving speed, V SP , of the photoconductive drum  2  can be expressed by the following Equation 2.
 
 V   sp   =R{ω   0 +Δω cos(ω 0   t   0 +α)}  Eq. 2
 
   where “R” represents a radius of the photoconductive drum  2 . 
   Further, a pattern interval, δP 0 , between two given toner images of the adjustment pattern  45  formed on the writing position, WP, at a constant, minute interval, δt, can be expressed by the following Equation 3.
 
 δP   0   =V   SP   δt=R{ω   0 +Δω cos(ω 0   t   0 +α)}δ t   Eq. 3
 
   The adjustment pattern  45  is transferred onto the intermediate transfer belt  10  after a time, T, required for the photoconductive drum  2  to be rotated by an angle, φ, has elapsed. The angle, φ, is formed with an imaginary line connecting between the rotation center and the writing position, WP, of the photoconductive drum  2  and an imaginary line connecting between the rotation center and the transfer position, TP, of the photoconductive drum  2 . 
   A rotation angular speed, ω φ , on transferring the adjustment pattern  45  onto the intermediate transfer belt  10  can be expressed by the following Equation 4.
 
ω φ =ω 0 +Δω cos(ω 0   t   0 +α+φ)  Eq. 4
 
   The second term, Δω cos(ω 0 t 0 +α+φ), of the right-hand side of Equation 4 represents a variable component having a rotation period of the photoconductive drum  2 . Therefore, “φ” represents a phase difference observed after a time, T φ , has elapsed from the writing of the latent image. At this time, a surface moving speed, V TR , of the photoconductive drum  2  can be expressed by the following Equation 5:
 
 V   TR   =R{ω   0 +Δω cos(ω 0   t   0 +α+φ)}  Eq. 5
 
   Furthermore, the following assumptions are made that a surface moving speed of the intermediate transfer belt  10  is substantially equal to an average surface moving speed of the photoconductive drum  2 , and a relation of Vb=Rω 0  is satisfied. 
   In this case, when the surface moving speed of the photoconductive drum  2  is higher than the surface moving speed of the intermediate transfer belt  10 , the pattern interval on the photoconductive drum  2  becomes shorter than when the surface moving speed of the photoconductive drum  2  is substantially equal to the surface moving speed of the intermediate transfer belt  10 . 
   On the other hand, when the surface moving speed of the photoconductive drum  2  is lower than the surface moving speed of the intermediate transfer belt  10 , the pattern interval on the photoconductive drum  2  becomes longer than when the surface moving speed of the photoconductive drum  2  is substantially equal to the surface moving speed of the intermediate transfer belt  10 . 
   Therefore, a pattern interval, P, of the adjustment pattern  45  transferred onto the intermediate transfer belt  10  can be expressed by the following Equation 6. 
   
     
       
         
           
             
               
                 
                   δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   P 
                 
                 = 
                 
                   
                     δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       P 
                       0 
                     
                     ⁢ 
                     
                       
                         V 
                         b 
                       
                       
                         V 
                         TR 
                       
                     
                   
                   = 
                   
                     
                       P 
                       n 
                     
                     ⁢ 
                     
                       
                         
                           ω 
                           0 
                         
                         + 
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             cos 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     ω 
                                     0 
                                   
                                   ⁢ 
                                   
                                     t 
                                     0 
                                   
                                 
                                 + 
                                 α 
                               
                               ) 
                             
                           
                         
                       
                       
                         
                           ω 
                           0 
                         
                         + 
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             cos 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     ω 
                                     0 
                                   
                                   ⁢ 
                                   
                                     t 
                                     0 
                                   
                                 
                                 + 
                                 α 
                                 + 
                                 φ 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 6 
               
             
           
         
       
     
   
   where a relation of P n =Rω 0 δt is satisfied. 
   At this point, the variable component, Δω, is sufficiently small compared to the average angular speed, ω 0 . Therefore, the above Equation 6 can be approximated by the following Equation 7. 
   
     
       
         
           
             
               
                 
                   δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   P 
                 
                 = 
                 
                   
                     P 
                     n 
                   
                   ⁢ 
                   
                     1 
                     
                       ω 
                       0 
                     
                   
                   ⁢ 
                   
                     { 
                     
                       
                         ω 
                         0 
                       
                       + 
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   ω 
                                   0 
                                 
                                 ⁢ 
                                 
                                   t 
                                   0 
                                 
                               
                               + 
                               α 
                             
                             ) 
                           
                         
                       
                       - 
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   ω 
                                   0 
                                 
                                 ⁢ 
                                 
                                   t 
                                   0 
                                 
                               
                               + 
                               α 
                               + 
                               φ 
                             
                             ) 
                           
                         
                       
                     
                     } 
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 7 
               
             
           
         
       
     
   
   Moreover, the above Equation 7 can be converted into the following Equation 8. 
   
     
       
         
           
             
               
                 
                   δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   P 
                 
                 = 
                 
                   
                     P 
                     n 
                   
                   ⁢ 
                   
                     1 
                     
                       ω 
                       0 
                     
                   
                   ⁢ 
                   
                     { 
                     
                       
                         ω 
                         0 
                       
                       + 
                       
                         2 
                         ⁢ 
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               φ 
                               2 
                             
                             ) 
                           
                         
                         ⁢ 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   ω 
                                   0 
                                 
                                 ⁢ 
                                 
                                   t 
                                   0 
                                 
                               
                               + 
                               α 
                               + 
                               
                                 φ 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                     } 
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 8 
               
             
           
         
       
     
   
   Equation 8 represents a pattern interval between two given toner images of the adjustment pattern  45  observed after the adjustment pattern  45  formed on the photoconductive drum  2  at a constant, minute time, δt, is transferred onto the intermediate transfer belt  10 . 
   When the latent image of the adjustment pattern  45  is formed at a constant time interval, the latent image is written at the writing position, WP, at a constant time interval, Te, different from the minute interval, δt. Further, the latent image is transferred onto the intermediate transfer belt  10 , and then a passing timing of the adjustment pattern  45  is detected with the light-sensitive element  42  provided above the intermediate transfer belt  10 . 
   Thus, a detection timing when the adjustment pattern  45  on the intermediate transfer belt  10  is detected is determined. Until this step, the timing when the drum position sensor  20  detects the marking  4  is employed as the reference timing. 
   Where zero represents a reference point at which the adjustment pattern  45  written at the reference timing is detected with the light-sensitive element  42 , a pattern interval, PN, from the reference point to an Nth pattern written at a time, TeN (N represents a natural number), can be expressed by the following Equation 9. 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         P 
                         N 
                       
                       = 
                       
                         
                           ∫ 
                           0 
                           TeN 
                         
                         ⁢ 
                         
                           δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           P 
                           ⁢ 
                           
                             ⅆ 
                             
                               t 
                               0 
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         
                           ∫ 
                           0 
                           TeN 
                         
                         ⁢ 
                         
                           R 
                           ⁢ 
                           
                             { 
                             
                               
                                 ω 
                                 0 
                               
                               + 
                               
                                 2 
                                 ⁢ 
                                 Δω 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   sin 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       φ 
                                       2 
                                     
                                     ) 
                                   
                                 
                                 ⁢ 
                                 
                                   sin 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         
                                           ω 
                                           0 
                                         
                                         ⁢ 
                                         
                                           t 
                                           0 
                                         
                                       
                                       + 
                                       α 
                                       + 
                                       
                                         φ 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             } 
                           
                           ⁢ 
                           
                             ⅆ 
                             
                               t 
                               0 
                             
                           
                         
                       
                     
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 9 
               
             
           
         
       
     
   
   The following Equation 10 can be obtained from the above Equation 9. 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         P 
                         N 
                       
                       = 
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ω 
                             0 
                           
                           ⁢ 
                           TeN 
                         
                         - 
                         
                           2 
                           ⁢ 
                           R 
                           ⁢ 
                           
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               ω 
                             
                             
                               ω 
                               0 
                             
                           
                           ⁢ 
                           
                             sin 
                             ⁡ 
                             
                               ( 
                               
                                 φ 
                                 2 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                             cos 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     ω 
                                     0 
                                   
                                   ⁢ 
                                   TeN 
                                 
                                 + 
                                 α 
                                 + 
                                 
                                   φ 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                         + 
                         C 
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ω 
                             0 
                           
                           ⁢ 
                           TeN 
                         
                         + 
                         
                           2 
                           ⁢ 
                           R 
                           ⁢ 
                           
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               ω 
                             
                             
                               ω 
                               0 
                             
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ( 
                             
                               φ 
                               2 
                             
                             ) 
                           
                           ⁢ 
                           
                             cos 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     ω 
                                     0 
                                   
                                   ⁢ 
                                   TeN 
                                 
                                 + 
                                 α 
                                 + 
                                 
                                   φ 
                                   2 
                                 
                                 + 
                                 π 
                               
                               ) 
                             
                           
                         
                         + 
                         C 
                       
                     
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 10 
               
             
           
         
       
     
   
   where the constant, C, in the above Equation 10 can be expressed by the following Equation 11. 
   
     
       
         
           
             
               
                 C 
                 = 
                 
                   2 
                   ⁢ 
                   R 
                   ⁢ 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                     
                     
                       ω 
                       0 
                     
                   
                   ⁢ 
                   sin 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     
                       φ 
                       2 
                     
                     ) 
                   
                   ⁢ 
                   
                     cos 
                     ⁡ 
                     
                       ( 
                       
                         α 
                         + 
                         
                           φ 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 11 
               
             
           
         
       
     
   
   Thus, toner images of the adjustment pattern  45  are formed on the intermediate transfer belt  10  at a pattern interval expressed by Formula 10, and are detected with the light-sensitive element  42 . 
   As described above, a detected result of the adjustment pattern  45  is stored in the RAM  60  as the pattern detection data. Then, data of the surface moving speed of the intermediate transfer belt  10  included in the pattern detection data are converted into data of the position thereof. 
   Incidentally, the first term of the right-hand side in Equation 10 corresponds to a slope of the pattern detection data, and is used in detecting a magnification error. 
   For the pattern variation data, an amplitude and a phase of a variable component occurring in a rotation period of the photoconductive drum  2  are calculated by the above-described quadrature detection processing. The variable component corresponds to the first term of the right-hand side in Equation 10. 
   The amplitude, A, of the variable component can be expressed by the following Equation 12. The phase, B, of the variable component can be expressed by the following Equation 13. Incidentally, the third term, C, of the right-hand side in Equation 10 represents a constant deviation, and serves to shift a zero level of the periodical variation represented by the second term thereof to the amplitude direction. Therefore, the third term, C, has no effect on the amplitude and the phase detected through quadrature conversion. 
   
     
       
         
           
             
               
                 A 
                 = 
                 
                   2 
                   ⁢ 
                   R 
                   ⁢ 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                     
                     
                       ω 
                       0 
                     
                   
                   ⁢ 
                   
                     sin 
                     ⁡ 
                     
                       ( 
                       
                         φ 
                         2 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 12 
               
             
           
           
             
               
                 B 
                 = 
                 
                   α 
                   + 
                   
                     φ 
                     2 
                   
                   + 
                   π 
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 13 
               
             
           
         
       
     
   
   In this regard, the variable component having a rotation period of the photoconductive drum  2  results from a variation in rotation angular speed of the photoconductive drum  2  represented by the second term of the above Equation 1. Therefore, a drive control correction value to correct the variation in rotation angular speed of the photoconductive drum  2  is set so as to be an inverse number of the second term of Equation 1. 
   Further, based on the amplitude and the phase calculated by the quadrature detection processing, that is, the amplitude and the phase expressed by Equations 12 and 13, respectively, a detected value of the periodic variation of the photoconductive drum  2  is represented by dividing the amplitude by 2×R×sin(φ/2)/ω 0 , and delaying the phase by φ/2+π 
   Therefore, a correction reference signal for use in correcting the periodic variation of the photoconductive drum  2  is preferably corrected through dividing the amplitude calculated from Equation 12 by 2×R×sin(φ/2)/ω 0 , and delaying the phase calculated from Equation 13 by φ/2. At this time, specifically, the phase calculated from Equation 13 is once delayed by φ/2+π and then is further delayed by π. As a result, the phase calculated from Equation 13 is delayed by φ/2. 
   Values for use in correcting the correction reference signal may be calculated in advance from the configuration of the image forming mechanisms  300 . Thereby, the rotation angular speed of the drive motor  33  is controlled so as to cancel the variation in rotation angular speed occurring in the rotation period of the photoconductive drum  2 . Thus, the photoconductive drum  2  may be rotated at a constant rotation angular speed. 
   Incidentally, the controller  37  of  FIG. 2  detects a rotation angular displacement of the photoconductive drum  2 , based on an output pulse count of the rotary encoder  35  serving as a position control mechanism. At this time, a target value of the rotation angular displacement is set in the target value output unit  38 . 
   Then, a drive control correction value to correct the rotation angular displacement of the photoconductive drum  2  is preferably set so as to cancel the variable component, based on a rotation angle, θ, of the photoconductive drum  2 . The rotation angle, θ, can be expressed by the following Equation 14 that is obtained by integrating Equation 1. 
   
     
       
         
           
             
               
                 θ 
                 = 
                 
                   
                     
                       ω 
                       0 
                     
                     ⁢ 
                     
                       t 
                       0 
                     
                   
                   + 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                       
                       
                         ω 
                         0 
                       
                     
                     ⁢ 
                     
                       sin 
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               ω 
                               0 
                             
                             ⁢ 
                             
                               t 
                               0 
                             
                           
                           + 
                           α 
                         
                         ) 
                       
                     
                   
                   + 
                   
                     C 
                     0 
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 14 
               
             
           
         
       
     
   
   In addition, the following Equation 15 is obtained by converting the second term of the right-hand side of Equation 10 into a sine function. 
   
     
       
         
           
             
               
                 
                   P 
                   N 
                 
                 = 
                 
                   
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ω 
                       0 
                     
                     ⁢ 
                     TeN 
                   
                   + 
                   
                     2 
                     ⁢ 
                     R 
                     ⁢ 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                       
                       
                         ω 
                         0 
                       
                     
                     ⁢ 
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       
                         φ 
                         2 
                       
                       ) 
                     
                     ⁢ 
                     
                       sin 
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               ω 
                               0 
                             
                             ⁢ 
                             TeN 
                           
                           + 
                           α 
                           + 
                           
                             φ 
                             2 
                           
                           + 
                           
                             
                               3 
                               2 
                             
                             ⁢ 
                             π 
                           
                         
                         ) 
                       
                     
                   
                   + 
                   C 
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 15 
               
             
           
         
       
     
   
   Then, the drive control correction value to correct the rotation angular displacement of the photoconductive drum  2  is set so as to be an inverse number of the second term of Equation 14. Therefore, the drive control correction value is obtained through dividing the amplitude calculated from Equation 12 by 2×R×sin(φ/2) and delaying the phase calculated from Equation 13 by (φ+3π)/2. 
   In the above description, first, the adjustment pattern  45  is written onto the photoconductive drum  2  based on a timing when the drum position sensor  20  detects the marking  4 . Then, a passing timing of the adjustment pattern  45  is determined based on a reference position at which the adjustment pattern  45  having been transformed on the intermediate transfer belt  10  is detected with the light-sensitive element  42 . 
   However, the surface moving speed of the intermediate transfer belt  10  may be unstable. Further, the average surface moving speed thereof may be uncertain due to expansion or shrink in diameter of the drive support roller  8  that is caused by temperature variation. In such cases, an error may be caused in detecting the above reference position. 
   Then, a reference mark is preferably formed as a reference point of pattern detection separately from the adjustment pattern  45 . The passing timings of toner images of the adjustment pattern  45  formed on the intermediate transfer belt  10  are determined based on a detection result of the reference mark. At this time, a phase relation between a writing timing of the reference mark and a detection timing of the marking  4  detected with the drum position sensor  20  should be reflected to a phase value for use in the drive control correction. 
   According to the present example embodiment, the drive control correction value to correct a variation in surface moving speed of the photoconductive drum  2  may be calculated with relatively high accuracy based on the pattern detection data, regardless of the relative positional relation between the writing position, WP, and the transfer position, TP, on the photoconductive drum  2 . 
   In this regard, as performed in a conventional art, when a drive control correction value is calculated by using 180°, which is different from the actual angle, as a phase difference angle, φ, between the writing position, WP, and the transfer position, TP, an error may be caused in the drive control of the photoconductive drum  2 . The error can be calculated from a difference between two pattern variation values that are obtained by substituting each of an actual phase-difference angle and 180° into “φ” in Equation 10. 
     FIG. 10  illustrates a graph obtained when each of 2.53 rad (i.e. 145°) and 3.14 rad (i.e. 180°) is substituted into “φ” in Equation 10. In this case, a radius, R, of the photoconductive drum  2  is set to 20 mm, a variation rate of rotation angular speed, Δω/ω 0 , is set to 0.1%, and “α” is set to zero. In  FIG. 10 , “ERROR” represents the difference between pattern variation values that are obtained by substituting each of 2.53 rad and 3.14 rad into “φ” in Equation 10. 
   As illustrated in  FIG. 10 , when the phase difference angle between the writing position, WP, and the transfer position, TP, is deviated by 35° from 180°, the small variation rate of 0.1% in rotation angular speed may cause a difference of up to approximately 12 μm in pattern variation amount. The difference indicates an error in the drive control correction. 
   However, according to the present example embodiment, the error may be effectively suppressed as described above, and therefore the drive control correction may be performed with relatively high accuracy. 
   In the above, description has been given to the variation in surface moving speed of the photoconductive drum  2  that may be caused by the variable component in rotation angular speed of the photoconductive drum  2  having a rotation period of the photoconductive drum  2 . However, for a variation in surface moving speed of the photoconductive drum  2  that may be caused by other variable components, a drive control correction value may be calculated in a similar manner to the above. 
   For example, the drive transmission mechanism may include a timing belt stretched over a drive motor shaft pulley and a photoconductive drum shaft pulley. In this case, first, a phase difference angle, φ, between the writing position, WP, and the transfer position, TP, of the photoconductive drum  2  is converted into a phase difference angle, φ tb , on a rotation period of the timing belt. Then, similar processing to the above may be performed with an actual rotation period, ω tb , of the timing belt and the converted phase difference angle, φ tb , of the timing belt. 
   In this case, the timing belt is preferably provided with a marking and a drum position sensor to determine a reference timing of the rotation of the timing belt. However, unless an inappropriate slippage occurs between the photoconductive drum shaft pulley and the timing belt, the reference timing of the rotation of the timing belt may be set based on a detection timing of a marking provided at the photoconductive drum shaft pulley. 
   Variation Example 1 
   Next, a variation example (hereinafter, “variation example 1”) of the present example embodiment is described. 
     FIG. 11  is an explanatory diagram illustrating image forming mechanisms  800  of the variation example 1. 
   Each of the image forming mechanisms  800  of the present variation example 1 employs a photoconductive drum  85  having a relatively larger diameter to obtain high durability in a photoconductive layer of the photoconductive drum  85 . The photoconductive drum  85  is surrounded by a cleaner  81 , a charger  82 , an optical writing unit  83 , and a developer  84 . 
   The cleaner  81 , the charger  82 , the optical writing unit  83 , and the developer  84  are arranged on a left side relative to the photoconductive drum  85  in  FIG. 11 . Thereby, a relatively short interval can be obtained between the adjacent image forming mechanisms  800 . Further, a relatively smaller size can be achieved for a lateral size of the image forming mechanisms  800  as a whole. 
   Thus, the image forming mechanisms  800  have an angle significantly deviated from 180° as a phase difference angle, φ 1 , between a writing position, WP, and a transfer position, TP, on the photoconductive drum  85 . In the variation example 1, φ 1 , is set to 120°. When the image forming apparatus  200  employs the image forming mechanisms  800  instead of the image forming mechanisms  300 , the drive control correction value may also be obtained with relatively high accuracy by performing a similar correction operation to the above-described correction operation. 
   Variation Example 2 
   Next, another variation example (hereinafter, “variation example 2”) of the present example embodiment is described. 
     FIG. 12  is an explanatory diagram illustrating image forming mechanisms  900  of the variation example 2. 
   According to the variation example 2, the image forming mechanisms  900  are arranged in tandem along the intermediate transfer belt  10 . At least one of the image forming mechanisms  900  has a different phase-difference angle between a writing position, WP, and a transfer position, TP, from the others thereof. 
   Generally, monochrome images are more frequently output in an image forming apparatus than color images. Therefore, a photoconductive drum for black image is more frequently used for image formation than any of photoconductive drums for other color images. 
   Then, the image forming mechanisms  900  of the variation example 2 employ a photoconductive drum  92  for black image having a relatively large diameter compared to any of photoconductive drums  91  for other color images. Thereby, relatively high durability may be achieved for the image forming mechanisms  900 . 
   Further, since the photoconductive drum  92  for black image has a relatively large diameter, various surrounding devices are arranged so that each working point thereof onto the photoconductive drum  92  is positioned on the side of photoconductive drum  92  (i.e. the left side in  FIG. 12 ) relative to a tangent plane, D, of the photoconductive drum  92  perpendicular to the surface of the intermediate transfer belt  93 , similar to the variation example 1. Thereby, a relatively small size may be achieved for a lateral size of the image forming mechanisms  900  as a whole. 
   In addition, when similar materials are used for photoconductive layers of the photoconductive drums  91  and  92 , similar phenomena are observed in toner transfer and movement of electric charge in the photoconductive layers. Accordingly, distances among surrounding units of the photoconductive drum  92  are preferably adjusted so as to correspond to distances among surrounding units of each of the photoconductive drums  91 . 
   Then, a phase difference angle, φ 3 , between a writing position, WP, and a transfer position, TP, on the photoconductive drum  92  for black image is configured to be different from a phase difference angle, φ 2 , on between a writing position, WP, and a transfer position, TP, on the photoconductive drum  91  for other color image. 
   The above-described correction operation is independently performed in each of the image forming mechanisms  300 . Therefore, when the image forming apparatus  200  employs the image forming mechanisms  900  instead of the image forming mechanisms  300 , the drive control correction value may also be obtained with relatively high accuracy by performing a similar correction operation to the above-described correction operation. 
   Variable Example 3 
   Next, still another variation example (hereinafter, “variation example 3”) of the present example embodiment is described. 
     FIG. 13  is an explanatory diagram illustrating an image forming mechanism  1000  of the variation example 3. 
   The present example embodiment and the variation examples 1 and 2 thereof describe the cases where drum-shaped photoconductors are employed as image carriers. However, an image carrier to be used in the image forming apparatus  200  is not limited to such drum-shaped photoconductors. 
   As described above, a surface moving member including a latent-image writing position, WT, and a transfer position, TP, may be employed as the image carrier. For example, as described below, an endless-belt-shaped photoconductor may be employed as the image carrier. 
   According to the variation example 3, a photoconductive belt  103  is stretched over three support rollers. One of the three support rollers is configured as a drive roller. The photoconductive belt  103  is endlessly driven with the drive roller in a clockwise direction in  FIG. 13 , similar to an intermediate transfer belt  105 . The photoconductive belt  103  is in contact with the intermediate transfer belt  105  at a portion where the photoconductive belt  103  is supported with the lowest one of the three support rollers. 
   The photoconductive belt  103  is surrounded with a charger  102 , an optical writing unit (not illustrated), a developer  100 , a transfer roller  104 . The charger  102  charges the photoconductive belt  103  at a given electric potential. The optical writing unit emits a laser light  101  according to an image signal to form a latent image on a surface of the photoconductive belt  103 . The developer  100  develops the latent image with charged toner to form a toner image on the photoconductive belt  103 . 
   The transfer roller  104  transfers the toner image from the photoconductive belt  103  onto the intermediate transfer belt  105 . The transfer roller  104  is arranged on an inner side of the photoconductive belt  103  so as to oppose to the lowest support roller of the photoconductive belt  103 . 
   A pattern sensor  106  is provided above the intermediate transfer belt  105 , and detects an adjustment pattern formed on the intermediate transfer belt  105 . 
   For the photoconductive belt  103  having a configuration as described above, a variation in surface moving speed thereof may be caused by eccentricity of the drive roller, deviation in thickness of the photoconductive belt  103 , etc. In this regard, the variation in surface moving speed occurring in a rotation period of the photoconductive belt  103  may be corrected with a drive control correction value in a similar manner as described above. 
   The drive control correction value is calculated based on a rotation angular speed, ω ob , and a phase difference rotation angle, φ ob , in the rotation period of the photoconductive belt  103  between a writing position, WP, of a laser light  101  and a transfer position, TP, at which the intermediate transfer belt  10  is in contact with the photoconductive belt  103 . Parameters corresponding to the radius, R, and the rotation angular speed, ω, of the photoconductive drum  2  as described above can be obtained from a circumferential length and a surface moving speed of the photoconductive belt  103 . 
   Thus, according to the present example embodiment including the variation examples 1, 2, and 3, the drive control correction value may be calculated with relatively high accuracy at any phase difference angle, φ, between the writing position, WP, and the transfer position, TP. Therefore, the units surrounding the photoconductive drum  2  may be arranged so as to significantly deviate the phase difference angle, φ, from 180°. Such a configuration in which the phase difference angle, φ, is significantly deviated from 180° may provide advantages as follows. 
   Incidentally, a conventional configuration where the phase difference angle, φ, is set to 180° is subject to a variation in surface moving speed of the photoconductive drum  2  with respect to positional displacement of image. For example, when a variation in rotation angular speed occurring in a rotation period of the photoconductive drum  2  becomes maximum at the writing position, WP, an adjustment pattern is formed near the writing position, WP, at a larger interval than a target interval. 
   Further, when the adjustment pattern reaches the transfer position, TP, and the phase difference angle, φ, becomes 180°, the variation in rotation angular speed occurring in the rotation period of the photoconductive drum  2  becomes minimum. Thus, the adjustment pattern is transferred onto the intermediate transfer belt  10  at a timing when the surface moving speed of the photoconductive drum  2  is minimum relative to the surface moving speed of the intermediate transfer belt  10 . Thereby, the adjustment pattern is formed on the intermediate transfer belt  10  at a further larger interval than the target interval. 
   On the other hand, the positional displacement of image becomes smaller as the phase difference angle, φ, is more deviated from 180°. According to the present example embodiment, the phase difference angle, φ, can be configured to be significantly deviated from 180°. Therefore, the positional displacement of image may be suppressed. 
   Another advantage of the above configuration is described as follows. 
   When the image forming apparatus  200  illustrated in  FIG. 1  is taken as an example, the image forming apparatus  200  is provided with the secondary transfer roller  13  on the way of the sheet conveyance path to convey a recording sheet from the lower portion to the upper portion of the image forming apparatus  200  in a relatively short distance. In this case, the optical writing unit  1  should be arranged under the image forming mechanisms  300 , as illustrated in  FIG. 1 . However, if the phase difference angle, φ, is set to 180°, accumulation of scattered toner on the exit lens might disturb appropriate latent image formation. 
   Then, in the image forming apparatus  200  of the present example embodiment, the phase difference angle, φ, is set to 145° so that the lens surface of the exit lens is inclined relative to a horizontal plane. Thus, the accumulation of scattered toner on the exit lens may be suppressed, thereby achieving a stable latent image formation. 
   In addition to the above advantages, the advantages described for the variation examples 1, 2, and 3, may be obtained by employing the configuration where the phase difference angle, φ, is significantly deviated from 180°. 
   As described above, in the image forming apparatus  200  according to the present example embodiment, a latent image is formed on a surface of the photoconductive drum  2  serving as an image carrier having a moving surface. The latent image is developed with toner to form a toner image, and the toner image is transferred onto the intermediate transfer belt  10  serving as a surface moving member. The toner image on the intermediate transfer belt  10  is transferred onto a sheet serving as a recording member. Thus, the toner image is formed on the recording sheet. 
   The image forming apparatus  200  includes the drive control mechanism to control the drive of the photoconductive drum  2  so that a rotation angular speed, ω, of the photoconductive drum  2  matches a target rotation angular speed thereof. 
   The image forming apparatus  200  also includes the pattern sensor  40  serving as the pattern detection mechanism. The pattern sensor  40  detects a plurality of toner images of the adjustment pattern  45  formed along the surface moving direction of the intermediate transfer belt  10 . 
   The image forming apparatus  200  further includes the correction mechanism to correct the target rotation angular speed of the photoconductive drum  2 , based on a detection result of the adjustment pattern  45  detected with the pattern sensor  40 . In the correction mechanism, first, an amplitude and a phase of a variable component in pattern interval (i.e. pattern variation data) indicating a periodic variation in surface moving speed of the photoconductive drum  2  are calculated based on the pattern detection data detected with the pattern sensor  40 . 
   Then, the amplitude of the pattern variation data is divided by 2×R×sin(φ/2)ω 0  and the phase thereof is delayed by φ/2 to obtain a correction value. At this time, “ω 0 ” represents an average rotation angular speed of the photoconductive drum  2 , and “R” represents a rotation radius of the photoconductive drum  2 . Further, “φ” represents a phase difference angle formed with an imaginary line connecting between the rotation center and the writing position, WP, of the photoconductive drum  2  and an imaginary line connecting between the rotation center and the transfer position, TP, of the photoconductive drum  2 . 
   Based on the correction value, the correction mechanism corrects the target rotation angular speed of the photoconductive drum  2 . 
   According to the above configuration, regardless of the phase difference angle, φ, the correction value may be accurately calculated based on the detection result of the adjustment pattern  45 . That is, the correction value may cancel an error caused by the periodic variation in rotation angular speed of the photoconductive drum  2 . Therefore, the positional relation of the writing position, WP, and the transfer position, TP is not limited by consideration for calculating an appropriate correction value. Thereby, increased flexibility may be obtained for the arrangement of the units in the image forming apparatus  200 . 
   Further, as described above, when the position control system including the rotary encoder  35  is employed, the target rotation angular displacement may be corrected with a correction value obtained by the amplitude of the pattern variation data by 2×R×sin(φ/2) and delaying the phase of the pattern variation data by (φ+3π)/2. 
   According to the present example embodiment, the correction mechanism calculates the amplitude and the phase of pattern variation data by the quadrature detection processing. Thereby, the amplitude and the phase of pattern variation data may be calculated based on a relatively small+ amount of pattern detection data compared to a method of detecting a zero cross point or a peak value of variable component. 
   In employing the quadrature detection processing, the adjustment pattern  45  is preferably formed by forming a latent image at a constant time interval over a range on the photoconductive drum  2  having a natural-number multiple length of a circumferential length of the photoconductive drum  2 , developing the latent images with toner, and transferring the toner images on the surface of the intermediate transfer belt  10 . Thereby, the correction value may be obtained with relatively high accuracy. 
   In employing the quadrature detection processing, the adjustment pattern  45  is also preferably formed by forming a latent image at a constant time interval over a range on the photoconductive drum  2  having a common multiple length between a circumferential length of the photoconductive drum  2  and a circumferential length of the drive support roller  8 , developing the latent images with toner, and transferring the toner images on the surface of the intermediate transfer belt  10 . Thereby, the correction value may be obtained with relatively high accuracy. 
   In employing the quadrature detection processing, further, the adjustment pattern  45  is preferably formed by forming a latent image at a constant time interval on the photoconductive drum  2  at a rate so that 4NP toner images are formed for a period of a periodic variation in surface moving speed of the photoconductive drum  2 , developing the latent images with toner, and transferring the toner images on the surface of the intermediate transfer belt  10 . Thereby, the amplitude and the phase of the pattern variation data may be obtained with relatively high detection sensitivity. 
   In addition, the reference mark may be formed separately from the adjustment pattern  45  and detected with the pattern sensor  40 . Further, time data that have elapsed from detection of the reference mark to each detection of the toner images of the adjustment pattern  45  may be employed as the detection data. 
   The detection timing of the marking provided on the shaft of the photoconductive drum  2  may be employed to determine a phase relation between the pattern detection data obtained from the detection result of the adjustment pattern  45  and the rotation angle of the photoconductive drum  2 . However, in this case, an error may be caused by a variation in surface moving speed of the intermediate transfer belt  10 . 
   Then, through detecting the reference mark formed as described above, the phase relation between the pattern detection data and the rotation angle of the photoconductive drum  2  may be determined with relatively high accuracy. 
   According to the present example embodiment, the image forming apparatus  200  includes the surface-moving-member drive control mechanism to control drive of the drive support roller  8  based on rotation data of at least one of the drive support roller  8  and the support rollers  7 ,  11 , and  12 . The surface-moving-member drive control mechanism also controls the drive of the drive support roller  8  so that the intermediate transfer belt  10  configured as an endless belt stretched over the drive support roller  8  and the support rollers  7 ,  11 , and  12  moves at a constant speed. 
   In detecting the adjustment pattern  45  formed on the intermediate transfer belt  10 , detection accuracy of the pattern variation data significantly depends on the variation in surface moving speed of the intermediate transfer belt  10 . According to the present example embodiment, the variation of the intermediate transfer belt  10  may be suppressed, and therefore the pattern variation data may be obtained with relatively high accuracy. 
   As described with the variation example 3, when the photoconductive belt  103  is employed as an image carrier, the correction mechanism employs, as the average rotation angular speed, ω 0 , and the rotation radius, R, an average rotation angular speed and a rotation radius that are obtained from the circumferential length and the average surface moving speed of the photoconductive belt  103  by converting the belt-shape of the photoconductive belt  103  into a cylindrical shape. 
   Thus, even in employing the photoconductive belt  103  having relatively high flexibility in layout compared to the photoconductive drum  2 , the periodic variation in surface moving speed of the photoconductive belt  103  may be suppressed, and therefore the positional displacement of image may be suppressed. 
   In employing the photoconductive belt  103 , deviation in thickness along the circumferential direction thereof may cause a variation in surface moving speed thereof. The variation in surface moving speed may be determined based on the detection result of the adjustment pattern  45 , and therefore may be corrected in a similar manner to the above correction operation. 
   On the other hand, the photoconductive drum  2  having a cylindrical shape has relatively high stiffness to load change caused by surrounding devices, such as the developer, the transfer roller, and the cleaner, compared to the belt shaped photoconductor. Therefore, image formation may be performed with relatively high accuracy. 
   As described with the variation example 1, the surrounding devices to form a toner image on the photoconductive drum  85  are arranged so that each working point of the devices onto the photoconductive drum  2  is positioned on the side of the photoconductive drum  85  (i.e. the left side in  FIG. 11 ) relative to a tangent plane of the photoconductive drum  85  perpendicular to the surface of the intermediate transfer belt  86 . 
   Thereby, the image forming apparatus  200  may be effectively downsized. In particular, when the plurality of the photoconductive drums  85  are arranged in tandem in the image forming apparatus  200 , a pitch, LST, between the photoconductive drums  85  may be shorten, and thereby the image forming apparatus  200  may be further effectively downsized. 
   As described with the variation example 2, the image forming apparatus  200  may have a configuration in which the plurality of the photoconductive drums are arranged along the surface moving direction of the intermediate transfer belt  93 , and a circumferential length of at least one of the plurality of the photoconductive drums is different from the other ones thereof. Thereby, a more frequently used one of the photoconductive drums may have a relatively longer circumferential length than a less frequently used one thereof. 
   As described with the present example embodiment, the optical writing unit  8  serving as the latent image writing mechanism is arranged so as to write a latent image by emitting a light obliquely from below the photoconductive drum  2 . Thus, accumulation of scattered toner onto the exit lens may be suppressed, thereby achieving a stable latent image formation. 
   In the above description of the present example embodiment, the image forming apparatus  200  is explained with referring to the tandem-type image forming apparatus employing the intermediate transfer method. However, the image forming apparatus  200  may be configured as a tandem-type image forming apparatus employing a direct transfer method. 
   Further, the image forming apparatus  200  may be configured as an image forming apparatus including only one image carrier, such as a photoconductive drum or a photoconductive belt. For example, in a monochrome image forming apparatus, no positional displacement between color images occurs, but a variation in surface moving speed of the photoconductive drum  2  may expand or contract a portion of image, thereby causing image distortion. 
   However, such image distortion may be suppressed through correcting the variation in surface moving speed of the image carrier by the above-described correction method. Thus, the above-described correction method may be effectively applied to the monochrome image forming apparatus. 
   This invention may be conveniently implemented using a conventional general purpose digital computer programmed according to the teachings of the present specification, as will be apparent to those skilled in the computer art. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art. The present invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be readily apparent to those skilled in the art. 
   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.