Patent Publication Number: US-8120630-B2

Title: Image shift adjusting apparatus of image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from prior U.S. Provisional Application 60/970,474 filed on Sep. 6, 2007, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image shift adjusting apparatus of an image forming apparatus, which adjusts superimposition of plural images for respective color components formed on plural photoreceptors in a color copier or a printer. 
     BACKGROUND 
     As an image forming apparatus, a color image forming apparatus is known in which images of respective colors formed on photoreceptors in plural image formation stations are superimposed on a record medium or a transfer belt. In the image forming apparatus as stated above, it is necessary that plural images formed in the plural image formation stations are accurately superimposed on the transfer belt. 
     Thus, hitherto, in each of the plural image formation stations, an adjustment pattern formed on the transfer belt is detected, and an adjustment value obtained based on the detection result is used to correct an image shift. On the other hand, as an image forming apparatus, there is a color image forming apparatus in which plural process speeds are changed and image formation is performed. In the color image forming apparatus in which the plural process speeds are changed, hitherto, it is necessary that an adjustment value is obtained each time the process speed varies and an image shift is corrected. 
     However, when the adjustment value is obtained each time the process speed varies and the image shift is corrected, each time the process speed is changed, it takes labor to perform the image shift correction, and it takes time to correct the image shift. Thus, it takes time to shift to another process speed, and there is a fear that improvement in productivity is hindered. 
     Then, in an image forming apparatus having plural process speeds, it is desirable to develop an image shift adjusting apparatus of the image forming apparatus, which shortens a time required for image shift correction when the process speed is changed and can improve the productivity of images. 
     SUMMARY 
     According to an aspect of the invention, one process speed is used, and adjustment values for respective plural process speeds are obtained. By this, an operation required for image shift correction when a process speed is changed is simplified, a time required for the image shift correction is shortened, and the productivity of images is improved. 
     According to an embodiment of the invention, an image shift adjusting apparatus of an image forming apparatus includes a running member running at a specified speed, plural image forming units configured to form adjustment patterns different in shape on the running member running at a first speed according to frequencies of exposure beams and by using identical pattern data in an adjustment mode, a detection unit configured to detect the adjustment patterns formed on the running member, and a correction unit configured to correct an image shift caused by the plural image forming units based on detection results of the adjustment patterns obtained by the detection unit. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structural view showing a color copier according to a first embodiment of the invention; 
         FIG. 2A  is a schematic structural view showing a positional relation between a laser exposure device and a photoconductive drum according to the first embodiment of the invention; 
         FIG. 2B  is a schematic structural view showing a laser oscillator according to the first embodiment of the invention; 
         FIG. 3  is a schematic structural view showing a registration sensor according to the first embodiment of the invention; 
         FIG. 4  is a block diagram showing a control system mainly concerned with image shift adjustment according to the first embodiment of the invention; 
         FIG. 5  is a flowchart of forming a first pattern according to the first embodiment of the invention; 
         FIG. 6  is a top view of the first pattern according to the first embodiment of the invention; 
         FIG. 7A  is a schematic explanatory view showing a shape of a generated pattern formed by a first oscillation unit at a first process speed by using a registration pattern according to the first embodiment of the invention; 
         FIG. 7B  is a schematic explanatory view showing a shape of a comparison pattern formed by a second oscillation unit at a first process speed by using the registration pattern according to the first embodiment of the invention; 
         FIG. 8  is a flowchart showing an image shift adjustment using the first pattern according to the first embodiment of the invention; 
         FIG. 9  is an explanatory view of setting an adjustment value of an image inclination from the first pattern according to the first embodiment of the invention; 
         FIG. 10  is an explanatory view of setting an adjustment value of position shift in a sub-scanning direction from the first pattern according to the first embodiment of the invention; 
         FIG. 11  is an explanatory view of setting an adjustment value of position shift in a main scanning direction from the first pattern according to the first embodiment of the invention; 
         FIG. 12  is an explanatory view of setting an adjustment value of magnification error in the main scanning direction from the first pattern according to the first embodiment of the invention; 
         FIG. 13  is an explanatory view showing formation positions of patterns of a case where a first oscillation unit is used and a case where a second oscillation unit is used according to the first embodiment of the invention; 
         FIG. 14  is a flowchart showing an image shift adjustment using a second pattern according to the first embodiment of the invention; 
         FIG. 15  is a top view of the second pattern according to the first embodiment of the invention; 
         FIG. 16  is a flowchart showing an image shift adjustment using the second pattern according to the first embodiment of the invention; 
         FIG. 17  is an explanatory view of setting an adjustment value of position shift in the main scanning direction from the second pattern according to the first embodiment of the invention; 
         FIG. 18  is an explanatory view of setting an adjustment value of magnification error in the main scanning direction from the second pattern according to the first embodiment of the invention; 
         FIG. 19  is a flowchart showing an image formation process at a first process speed according to the first embodiment of the invention; and 
         FIG. 20  is an explanatory view showing a third pattern and positions of registration sensors according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a first embodiment of the invention will be described in detail with reference to the accompanying drawings.  FIG. 1  is a schematic structural view showing a four-tandem color copier  1  as an image forming apparatus of an embodiment of the invention. The color copier  1  switches between two process speeds, that is, a first process speed of a first speed and a second process speed of a second speed, and can form an image. Switching of the process speed may be performed by selecting one of the process speeds by, for example, an operation panel  153  or by setting monochromatic image formation or color image formation. 
     The color copier  1  includes a scanner unit  6 , at an upper part, to read an original document supplied by an auto document feeder  4 . The color copier  1  includes image formation stations  11 Y,  11 M,  11 C and  11 K as four sets of image forming units of yellow (Y), magenta (M), cyan (C) and black (K) arranged in parallel along a transfer belt  10  as a running member. 
     The respective image formation stations  11 Y,  11 M,  11 C and  11 K include photoconductive drums  12 Y,  12 M,  12 C and  12 K. The rotating shafts of the photoconductive drums  12 Y,  12 M,  12 C and  12 K are parallel to a direction (main scanning direction) orthogonal to a running direction (sub-scanning direction) of an arrow n direction of the transfer belt  10 . Further, the respective rotating shafts of the photoconductive drums  12 Y,  12 M,  12 C and  12 K are arranged to be separate from each other at equal intervals along the sub-scanning direction. 
     Charging chargers  13 Y,  13 M,  13 C and  13 K, developing devices  14 Y,  14 M,  14 C and  14 K, and photoreceptor cleaners  16 Y,  16 M,  16 C and  16 K are arranged around the photoconductive drums  12 Y,  12 M,  12 C and  12 K along a rotation direction of an arrow m direction respectively. The developing devices  14 Y,  14 M,  14 C and  14 K respectively have two-component developers made of toners of yellow (Y) magenta (M), cyan (C) and black (K) different in color and carriers, and supply the toners to electrostatic latent images on the photoconductive drums  12 Y,  12 M,  12 C and  12 K. Each of the image formation stations  11 Y,  11 M,  11 C and  11 K can form an image at two process speeds. 
     Exposure lights from a laser exposure device  17  are irradiated between the charging chargers  13 Y,  13 M,  13 C and  13 K and the developing devices  14 Y,  14 M,  14 C and  14 K around the respective photoconductive drums  12 Y,  12 M,  12 C and  12 K, and electrostatic latent images are formed on the photoconductive drums  12 Y,  12 M,  12 C and  12 K respectively. 
     As shown in  FIG. 2A , the laser exposure device  17  includes laser oscillators  27 Y,  27 M,  27 C and  27 K to oscillate laser beams as exposure beams to the photoconductive drums  12 Y,  12 M,  12 C and  12 K respectively. The laser oscillators  27 Y,  27 M,  27 C and  27 K are controlled by laser drivers  28 Y,  28 M,  28 C and  28 K based on data of respective color components of image data read by the scanner unit  6  respectively. 
     As shown in  FIG. 2B , each of the laser oscillators  27 Y,  27 M,  27 C and  27 K includes a first oscillation unit  29   a  as a first oscillator and a second oscillation unit  29   b  as a second oscillator. The first oscillation unit  29   a  oscillates a first laser beam having a clock frequency of, for example, 100 MHz as a first frequency. The second oscillation unit  29   b  oscillates a second laser beam having a clock frequency of, for example, 125 MHZ as a second frequency. The laser drivers  28 Y,  28 M,  28 C and  28 K drive the laser oscillators  27 Y,  27 M,  27 C and  27 K respectively. In the case of the first process speed, the laser drivers  28 Y,  28 M,  28 C and  28 K drive the laser oscillators  27 Y,  27 M,  27 C and  27 K to use the clock of 100 MHz of the first oscillation unit  29   a  respectively. In the case of the second process speed, the laser drivers  28 Y,  28 M,  28 C and  28 K drive the laser oscillators  27 Y,  27 M,  27 C and  27 K to use the clock of 125 MHz of the second oscillation unit  29   b  respectively. 
     The laser beams outputted from the laser oscillators  27 Y,  27 M,  27 C and  27 K are scanned by a polygon mirror  30  in the main scanning direction. Incident angles of the laser beams to the photoconductive drums  12 Y,  12 M,  12 C and  12 K are inclined and adjusted by tilt mirrors  32 Y,  32 M,  32 C and  32 K respectively. The respective tilt mirrors  32 Y,  32 M,  32 C and  32 K are adjusted so that the rotating shafts of the photoconductive drums  12 Y,  12 M,  12 C and  12 K are parallel to the scanning direction of the laser beam. The tilt mirrors  32 Y,  32 M,  32 C and  32 K are adjusted based on the yellow (Y) tilt mirror  32 Y. 
     Horizontal synchronization signal detection sensors  26 Y,  26 M,  26 C and  26 K are provided on extensions of the photoconductive drums  12 Y,  12 M,  12 C and  12 K in the main scanning direction respectively. The horizontal synchronization signal detection sensors  26 Y,  26 M,  26 C and  26 K detect the scanning start of the laser beams outputted from the laser oscillators  27 Y,  27 M,  27 C and  27 K in the main scanning direction, and output horizontal synchronization signals. The polygon mirror  30  is rotated by a polygon mirror motor  33  driven by a polygon mirror motor driver  31 . However there is not necessary to provide a horizontal synchronization signal detection sensor to each laser beam. For example a horizontal synchronization signal detection sensor of yellow is provided as the horizontal synchronization signal detection sensor. In this case a laser beam of yellow (Y) is horizontal synchronized by the horizontal synchronization signal detection sensor of yellow on ahead. After that residual laser beams of magenta (M), cyan (C) and black (K) are horizontal synchronized by leaving a predetermined space from the laser beam of yellow (Y). 
     The transfer belt  10  is supported by a drive roller  20  and a driven roller  21 , and is rotated in the arrow n direction by the driving of the drive roller  20  by a belt motor  10   a . The running speed of the transfer belt  10  can be changed by the drive roller  20 . Toner images formed on the respective photoconductive drums  12 Y,  12 M,  12 C and  12 K are transferred to a sheet paper P conveyed in the arrow n direction by the transfer belt  10  at positions of transfer rollers  15 Y,  15 M,  15 C and  15 K. By this, a color toner image is formed on the sheet paper P conveyed by the transfer belt  10 . 
     The sheet paper P is fed to the transfer belt  10  through a conveyance path  7  from a cassette mechanism  3  including a first and a second paper feed cassettes  3   a  and  3   b . The conveyance path  7  includes pickup rollers  7   a  and  7   b  to take out a sheet paper from the paper feed cassettes  3   a  and  3   b , separation conveyance rollers  7   c  and  7   d , a conveyance roller  7   e  and a register roller  8 . The color toner image is formed on the sheet paper P, and the toner image is fixed by a fixing device  22  to complete the color image, and then, the sheet paper is discharged to a paper discharge tray  25   b  through a paper discharge roller  25   a.    
     After the transfer is ended, the remaining toners on the photoconductive drums  12 Y,  12 M,  12 C and  12 K are cleaned by the photoreceptor cleaners  16 Y,  16 M,  16 C and  16 K, and next printing becomes possible. 
     As shown in  FIG. 3 , a pair of a first registration sensor  36  and a second registration sensor  37  as a detection unit are arranged downstream of the image formation station  11 K of black (K) of the transfer belt  10 . The first registration sensor  36  and the second registration sensor  37  are arranged to be separate from each other by a specified distance in the main scanning direction. 
     Next, a registration mechanism to adjust an image shift will be described.  FIG. 4  is a block diagram showing a control system  100  mainly concerned with the image shift adjustment. A CPU  101  to control the whole color copier  1  in the control system  100  is connected with a laser control ASIC  110  and an engine control ASIC  130 , which are a correction unit, through an input and output interface  105 . The CPU  101  includes a memory  102  to store various settings for controlling the laser control ASIC  110  and the engine control ASIC  130 , and an arithmetic unit  103  to calculate an adjustment value from a detection result of a pattern for adjusting an image shift formed on the transfer belt  10  by using the laser control ASIC  110 . 
     The laser control ASIC  110  includes a RAM  111  to store various settings for controlling the laser drivers  28 Y,  28 M,  28 C and  28 K. Besides, the laser control ASIC  110  is connected with the horizontal synchronization signal detection sensors  26 Y,  26 M,  26 C and  26 K. 
     The engine control ASIC  130  is connected with drum motors  131 Y,  131 M,  131 C and  131 K to drive the photoconductive drums  12 Y,  12 M,  12 C and  12 K respectively, the polygon motor  33  to drive the polygon mirror  30 , the belt motor  10   a  to drive the transfer belt  10 , the tilt mirror motors  132 M,  132 C and  132 K to drive the tilt mirrors  32 M,  32 C and  32 K respectively, and the first and the second registration sensors  36  and  37 . 
     Besides, the laser control ASIC  110  and the engine control ASIC  130  are connected with a print control unit  150  for carrying out image formation in the color copier  1 . The print control unit  150  includes a system unit  151 , an image processing unit  152 , the operation panel  153  and the scanner unit  6 . 
     Next, a process of an image shift adjustment at image formation will be described. In the color copier  1 , when the process speed is changed, the drive speed of the transfer belt  10 , the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the developing devices  14 Y,  14 M,  14 C and  14 K, and the polygon mirror  30  is changed. However, in each of these, the same motor is used for the first process speed and the second process speed, and the rotation speed of the motor is changed according to the process speed. Accordingly, in the drive system of these, the image shift is adjusted in one of the first process speed and the second process speed, and when the process speed is changed, only the rotation ratio of each motor is changed, and it is unnecessary to again adjust the image shift. 
     On the other hand, each of the laser oscillators  27 Y,  27 M,  27 C and  27 K includes two oscillation units, that is, the first oscillation unit  29   a  and the second oscillation unit  29   b . When the process speed is changed, the oscillation unit to be used is switched. Since the different oscillation unit is used as stated above, the characteristic of the oscillation unit is changed, and an adjustment value for correcting an image shift varies between the case of the first process speed and the case of the second process speed. Accordingly, the color copier  1  must have an image shift adjustment value at the first process speed and an image shift adjustment value at the second process speed according to the oscillation unit to be used. 
     In order to set two kinds of image shift adjustment values as stated above, first, in accordance with a flowchart of  FIG. 5 , a description will be given to a formation of a first adjustment pattern on the transfer belt  10 . The first adjustment pattern sets the image shift adjustment value at the first process speed.  FIG. 6  shows wedge-shaped front side first patterns  72 Y,  72 M,  72 C and  72 K and rear side first patterns  73 Y,  73 M,  73 C and  73 K, which are the first adjustment pattern. 
     When the image shift adjustment value at the first process speed is set, it is assumed that the drive speed of the transfer belt  10 , the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the developing devices  14 Y,  14 M,  14 C and  14 K, and the polygon mirror  30  is the first process speed. Besides, each of the laser oscillators  27 Y,  27 M,  27 C and  27 K uses the first oscillation unit  29   a  to oscillate the clock frequency of 100 MHz. 
     At the time of power-on of the color copier  1 , at the time of warm-up after a paper jam process, or at the interval of the paper sheets in the image formation process, the color copier  1  is set to an image shift adjustment mode. In the image shift adjustment mode, although paper feed from the cassette mechanism  3  is not performed, the operation other than that is the same as a normal image formation process. Thus, the front side first patterns  72 Y,  72 M,  72 C and  72 K and the rear side first patterns  73 Y,  73 M,  73 C and  73 K formed on the photoconductive drums  12 Y,  12 M,  12 C and  12 K are directly transferred to the transfer belt  10  running at the first process speed. 
     When the image shift adjustment mode starts, the laser control ASIC  110  reads pattern formation data for forming the front side first patterns  72 Y,  72 M,  72 C and  72 K and the rear side first patterns  73 Y,  73 M,  73 C and  73 K from the memory  102  of the CPU  101 , and stores them in the RAM  111  (Act  200 ). 
     As the pattern formation data, there are, for example, a first registration pattern  70  and a second registration pattern  71  which are horizontally symmetrical and are pattern data. Further, as the pattern formation data, there are instructions of writing positions of the first laser beam for forming the front side first patterns  72 Y,  72 M,  72 C and  72 K on the transfer belt  10  by the first registration pattern  70 , or instructions of writing positions of the second laser beam for forming the rear side first patterns  73 Y,  73 M,  73 C and  73 K on the transfer belt  10  by the second registration pattern  71 . 
     The symmetrical first and second registration patterns  70  and  71  have wedge shapes each formed of two crossing straight lines, and have a specified interval. The first and the second registration patterns  70  and  71  come to have pattern shapes shown in  FIG. 7A  when the pattern formation is performed by the first oscillation units  29   a  of the laser oscillators  27 Y,  27 M,  27 C and  27 K at the first process speed and in a width of 0 to 199 counts at 100 MHz. That is, the shape of the generated pattern  70   a ,  71   a  is such that the apex α of the wedge shape is 45°, and the length in the main scanning direction and the length in the sub-scanning direction are 1:1. 
     Next, in accordance with the pattern formation data read out from the CPU  101 , the laser control ASIC  110  instructs the laser drivers  28 Y,  28 M,  28 C and  28 K about timings when the front side first patterns  72 Y,  72 M,  72 C and  72 K and the rear side first patterns  73 Y,  73 M,  73 C and  73 K are formed using the first and the second registration patterns  70  and  71  (Act  201 ). By this, the front side first patterns  72 Y,  72 M,  72 C and  72 K written using the first registration pattern  70  are positioned in the detection range of the first registration sensor  36 . The rear side first patterns  73 Y,  73 M,  73 C and  73 K written using the second registration pattern  71  are positioned in the detection range of the second registration sensor  37 . 
     For example, in the case of the clock frequency of 100 MHz, it is assumed that the positions of the horizontal synchronization signal detection sensors  26 Y,  26 M,  26 C and  26 K are made reference position L 0 , the first registration sensor  36  is arranged at a position of 150 counts from the reference position L 0 , and the second registration sensor  37  is arranged at a position of 550 counts from the reference position L 0 . At this time, the laser control ASIC  110  instructs the laser drivers  28 Y,  28 M,  28 C and  28 K about the writing start timings of the first and the second registration patterns  70  and  71  by the laser oscillators  27 Y,  27 M,  27 C and  27 K. The timings are the timings when the centers of the front side first patterns  72 Y,  72 M,  72 C and  72 K pass the first registration sensor  36 , and the centers of the rear side first patterns  73 Y,  73 M,  73 C and  73 K pass the second registration sensor  37 . Incidentally, in this embodiment, in the case of the clock frequency of 100 MHz, the position of 150 counts from the reference position L 0  corresponds to a distance of P 1  from the reference position L 0 . Besides, the position of 550 counts from the reference position L 0  is a distance of P 2  from the reference position L 0 . 
     That is, the laser control ASIC  110  receives horizontal synchronization signals from the horizontal synchronization signal detection sensors  26 Y,  26 M,  26 C and  26 K, and then instructs the laser drives  28 Y,  28 M,  28 C and  28 K to start pattern formation using the first registration pattern  70  from 100th count as a first clock number. Further, after receiving the horizontal synchronization signals, the laser control ASIC  110  instructs the laser drives  28 Y,  28 M,  28 C and  28 K to start pattern formation using the second registration pattern  71  from the 500th count as the second clock number. 
     By this, on the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the formation of electrostatic latent images of the front side first patterns  72 Y,  72 M,  72 C and  72 K based on the first registration pattern  70  is started from a position corresponding to a front side first adjustment pattern formation start position L 1  shown in  FIG. 6 . Besides, the formation of electrostatic latent images of the rear side first patterns  73 Y,  73 M,  73 C and  73 K based on the second registration pattern  71  is started from a position corresponding to a rear side first adjustment pattern formation start position L 2 . Thereafter, toner images of the first patterns  72 Y,  72 M,  72 C and  72 K and  73 Y,  73 M,  73 C and  73 K through the developing devices  14 Y,  14 M,  14 C and  14 K are transferred to the transfer belt  10  by the transfer rollers  15 Y,  15 M,  15 C and  15 K. By this, the first patterns  72 Y,  72 M,  72 C and  72 K and  73 Y,  73 M,  73 C and  73 K shown in  FIG. 6  are formed on the transfer belt  10  (Act  202 ). 
     Next, an image shift adjustment at the first process speed will be described with reference to a flowchart of  FIG. 8 . By the start of the image shift adjustment, the first registration sensor  36  detects the front side first patterns  72 Y,  72 M,  72 C and  72 K formed on the transfer belt  10 , and the second registration sensor  37  detects the rear side first patterns  73 Y,  73 M,  73 C and  73 K formed on the transfer belt  10  (Act  210 ). 
     The detection results are inputted to the CPU  101  through the engine control ASIC  130  (Act  211 ). The CPU  101  sets a first adjustment value at the first process speed based on the detection results (Act  212 ). The setting of the first adjustment value is well-known (see, for example, JP-A-8-278680), and various well-known methods can be adopted. 
     For example, as shown in  FIG. 9 , from the detection results of the black (K) front side first pattern  72 K and the rear side first pattern  73 K formed in the image formation station  11 K of black (K), it is assumed that the output start timing is shifted by Δt 1  between the front side and the rear side. By this, the CPU  101  determines that the shaft of the black (K) photoconductive drum  12 K is inclined with respect to the scanning direction of the laser beam by the laser oscillator  27 K. Next, in order to adjust the inclination between both, the CPU  101  sets, as the adjustment value, a rotation amount of the image data corresponding to the inclination amount. 
     Besides, for example, as shown in  FIG. 10 , from the detection results of the first and the second registration sensors  36  and  37 , it is assumed that an interval T 1  between the image formation station  11 C of cyan (C) and the image formation station  11 K of black (K) in the sub-scanning direction is shifted from an interval T 2  between the other image formation stations. The CPU determines that the position of the image formation station  11 K of black (K) shifts in the sub-scanning direction by Δt 2  which is the difference between the interval T 1  and the interval T 2 . Next, in order to adjust the shift in the sub-scanning direction, the CPU  101  sets, as the adjustment value, an image data output timing corresponding to Δt 2 . At this time, the adjustment value of the sum of the inclination amount of  FIG. 9  and the position shift amount in the sub-scanning direction of  FIG. 10  may be set as the image data adjustment value. 
     For example, as shown in  FIG. 11 , from the detection results of the first and the second registration sensors  36  and  37 , it is assumed that the respective image formation stations  11 Y,  11 M,  11  and  11 K cause position shift in the main scanning direction. The CPU  101  determines the position shift of the image in the main scanning direction from differences among detection lengths ΔK 1 , ΔC 1 , ΔM 1  and ΔY 1  of the front side first patterns  72 K,  72 C,  72 M and  72 Y. Next, in order to adjust the position shift, the CPU  101  sets, as the adjustment value, the shift amount of image data in the main scanning direction. The adjustment value is set so that ΔK 1 =ΔC 1  ΔM 1 =ΔY 1  is established. 
     Further, for example, as shown in  FIG. 12 , from the detection results of the first and the second registration sensors  36  and  37 , it is assumed that the respective image formation stations  11 Y,  11 M,  11 C and  11 K cause magnification errors in the main scanning direction. The CPU  101  determines the magnification error in the main scanning direction from detection lengths of the front side first patterns  72 K,  72 C,  72 M and  72 Y and the rear side first patterns  73 K,  73 C,  73 M and  73 Y. 
     For example, the detection lengths of the front side first patterns  72 K,  72 C,  72 M and  72 Y are made ΔK 2 , ΔC 2 , ΔM 2  and ΔY 2 , and the detection lengths of the rear side first patterns  73 K,  73 C,  73 M and  73 Y are made ΔK 3 , ΔC 3 , ΔM 3  and ΔY 3 . The adjustment value is set from the value of the sum of the front side detection length and the rear side detection length for each color. That is, when (ΔK 2 +ΔK 3 )=(ΔC 2 +ΔC 3 )=(ΔM 2 +ΔM 3 )=(ΔY 2 +ΔY 3 ) is established, it is determined that the image magnifications in the main scanning direction of the respective image formation stations  11 K,  11 C,  11 M and  11 Y are the same. Accordingly, the CPU  101  sets, as the adjustment value, the expanded amount or contracted amount of the image data so that the shift amount of the image magnification in the main scanning direction is eliminated. 
     In this embodiment, for example, (ΔK 2 +ΔK 3 )=(ΔC 2 +ΔC 3 )=(ΔM 2 +ΔM 3 )=(ΔY 2 +ΔY 3 )=1 is made a reference value. When (ΔK 2 +ΔK 3 )=(1+R) is established in the black image formation station  11 K, this is larger than the reference value by (R). Accordingly, P 1 ×(correction coefficient)+P 2 ×(correction coefficient)=(R) (where, P 1  is the distance from the reference position L 0  to the first registration sensor  36 . P 2  is the distance from the reference position L 0  to the second registration sensor  37 ). 
     Accordingly, (correction coefficient)=(R)/(P 1 +P 2 ). By using this, the clock frequency is multiplied by (1+correction coefficient) to obtain the adjustment value. 
     The various adjustment values are calculated by the arithmetic unit  103  of the CPU  101  and are set. First adjustment values including the various adjustment values in the main scanning direction and the sub-scanning direction at the set first process speed are stored in the memory  102  of the CPU  101  (Act  213 ). 
     Next, setting of an image shift adjustment value at the second process speed will be described. When the image shift adjustment value at the second process speed is set, it is assumed that the drive speed of the transfer belt  10 , the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the developing devices  14 Y,  14 M,  14 C and  14 K, and the polygon mirror  30  is the first process speed. In each of the laser oscillators  27 Y,  27 M,  27 C and  27 K, the second oscillation unit  29   b  to oscillate the clock frequency of 125 MHz is used. 
     When the image shift adjustment value at the second process speed is set, since the position shift adjustment value in the sub-scanning direction is already set by the setting of the image shift adjustment value at the first process speed, only an image shift adjustment value in the main scanning direction is set. 
     In order to set the image shift adjustment value at the second process speed, similarly to the setting of the image shift adjustment value at the first process speed, second adjustment patterns corresponding to the first patterns  72 Y,  72 M,  72 C and  72 K and  73 Y,  73 M,  73 C and  73 K are formed on the transfer belt  10 . However, at this time, the second oscillation unit  29   b  is used in each of the laser oscillators  27 Y,  27 M,  27 C and  27 K. 
     Thus, even if the patterns are formed on the transfer belt  10  by using the same first and second registration patterns  70  and  71  and at the same process speed, the shapes and formation positions of the second adjustment patterns are different from those of the case where the first oscillation unit  29   a  is used. 
     Next, a description will be given to differences in the shape of a pattern and the formation position of a pattern between the case where the first oscillation unit  29   a  is used and the case where the second oscillation unit  29   b  is used. First, the difference in the shape of the pattern will be described. Patterns are formed in a width of 0 to 199 counts on the transfer belt  10  running at the first process speed by using the first and the second registration patterns  70  and  71  and by using the first oscillation unit  29   a  to oscillate 100 MHz. By this, as shown in  FIG. 7A , the shape of each of the first and the second generated patterns  70   a  and  71   a  is such that the apex α is 45°, and the length in the main scanning direction and the length in the sub-scanning direction are 1:1. 
     On the other hand, patterns are formed in a width of 0 to 199 counts on the transfer belt running at the first process speed by using the same first and the second registration patterns  70  and  71  and by using the second oscillator  29   b  to oscillate 125 MHz. A first and a second comparison patterns  70   b  and  71   b  formed have pattern shapes shown in  FIG. 7B . That is, the shape of each of the comparison patterns  70   b  and  71   b  is such that an apex β of a wedge shape is about 51°, and the length in the main scanning direction is contracted to 0.8 with respect to the length of 1 in the sub-scanning direction. As stated above, the first and the second generated patterns  70   a  and  71   a  formed by using the first oscillation unit  29   a  are different from the first and the second comparison patterns  70   b  and  71   b  formed by using the second oscillation unit  29   b  in the angle of the apex and the length in the main scanning direction. 
     Next, a difference in pattern formation position between the case where the first oscillation unit  29   a  is used, and the case where the second oscillation unit  29   b  is used will be described with reference to  FIG. 13 . The pattern formation on the transfer belt  10  running at the first process speed is started at the 100th count from the reference position L 0  by using the first oscillation unit  29   a  of the clock frequency of 100 MHz and by using the first registration pattern  70 , and the pattern formation is started at the 500th count by using the second registration pattern  71 . At this time, as indicated by dotted lines in  FIG. 13 , the formation start position of the first generated pattern  70   a  formed on the transfer belt  10  is a distance of L 1  from the reference position L 0 . The formation start position of the second generated pattern  71   a  is a distance of L 2  from the reference position L 0 . 
     On the other hand, the second oscillation unit  29   b  of the clock frequency of 125 MHz is used, the formation of the first registration pattern  70  is started at the 100th count from the reference position L 0 , and the formation of the second registration pattern  71  is started at the 500th count. At this time, as shown by solid lines in  FIG. 13 , the formation start position of the first comparison pattern  70   b  formed on the transfer belt  10  is a distance of L 3  from the reference position L 0 . The formation start position of the second comparison pattern  71   b  is a distance of L 4  from the reference position L 0  (where, L 3 =0.8×L 1 , L 4 =0.8×L 2 ). 
     Thus, when the first and the second registration sensors  36  and  37  are arranged to be opposite to the centers (the distance of P 1  from the reference position L 0 , and the distance of P 2  from the reference position L 0 ) of the formation positions of the first and the second generated patterns  70   a  and  71   a  formed on the transfer belt  10 , there is a fear that the first or the second comparison pattern  70   b  or  71   b  goes out of the detection range of the first or the second registration sensor  36  or  37 . In this embodiment, the formation positions of the second adjustment patterns for setting the image shift adjustment value at the second process speed are corrected. By the correction, the second adjustment patterns are formed in the detection ranges of the first and the second registration sensors  36  and  37 . 
     Next, the formation of the second adjustment patterns on the transfer belt  10  running at the first process speed will be described with reference to a flowchart of  FIG. 14 . As shown in  FIG. 15 , the second adjustment patterns include front side second patterns  77 Y,  77 M,  77 C and  77 K and rear side second patterns  78 Y,  78 M,  78 C and  78 K. 
     When an image shift adjustment mode starts, the laser control ASIC  110  reads pattern formation data for forming the front side and the rear side second patterns  77 Y,  77 M,  77 C and  77 K and  78 Y,  78 M,  78 C and  78 K from the memory  102  of the CPU  101 , and stores them in the RAM  111  (Act  300 ). 
     At Act  300 , as the pattern formation data, a first and a second corrected registration patterns are read which are obtained by performing image processing of the first and the second registration patterns  70  and  71  stored in the memory  102  of the CPU  101  by using the first adjustment value at the first process speed, and are stored in the RAM  111 . 
     Further, at Act  300 , instructions of the adjustment pattern formation start positions of the laser oscillators  27 Y,  27 M,  27 C and  27 K for forming the front side second patterns  77 Y,  77 M,  77 C and  77 K and the rear side second patterns  78 Y,  78 M,  78 C and  78 K on the transfer belt  10  by using the first and the second corrected registration patterns are read as the pattern formation data, and are stored in the RAM  111 . When the clock frequency of the second oscillation unit  29   b  is 125 MHz, the instructions of the adjustment pattern formation start positions of the laser oscillators  27 Y,  27 M,  27 C and  27 K are made so that the centers of the front side second patterns  77 Y,  77 M,  77 C and  77 K pass the first registration sensor  36 , and the centers of the rear side second patterns  78 Y,  78 M,  78 C and  78 K pass the second registration sensor  37 . 
     Thus, the timing of the second adjustment pattern formation start of the first registration pattern  70  by the laser oscillators  27 Y,  27 M,  27 C and  27 K is shifted to the rear side by {P 1 −(clock frequency of the first oscillation unit  29   a /clock frequency of the second oscillation unit  29   b )×P 1 } (where, P 1  is the distance from the reference position L 0  to the first registration sensor  36 ). Besides, the timing of the second adjustment pattern formation start of the second registration pattern  71  by the laser oscillators  27 Y,  27 M,  27 C and  27 K is shifted to the rear side by {P 2 −(clock frequency of the first oscillation unit  29   a /clock frequency of the second oscillation unit  29   b )×P 2 } (where, P 2  is the distance from the reference position L 0  to the second registration sensor  37 ). 
     In this embodiment, when the second oscillation unit  29   b  is used, the timing of the start of pattern formation using the first corrected registration pattern by the laser oscillators  27 Y,  27 M,  27 C and  27 K is shifted to the rear side by (150−0.8×150) counts. That is, after the horizontal synchronization signal is received, the pattern formation using the first corrected registration pattern is started from the 130th count. Besides, the timing of the start of pattern formation using the second corrected registration pattern is shifted to the rear side by (550−0.8×550) counts. That is, after the horizontal synchronization signal is received, the pattern formation using the second corrected registration pattern is started from the 610th count. 
     By doing so, the front side second adjustment pattern start positions of the front side second patterns  77 Y,  77 M,  77 C and  77 K are the position of the distance of L 1  from the reference position L 0  which is the same as that of the front side first patterns  72 Y,  72 M,  72 C and  72 K. Besides, the rear side second adjustment pattern start positions of the rear side second patterns  78 Y,  78 M,  78 C and  78 K are the position of the distance of L 2  from the reference position L 0  which is the same as that of the rear side first patterns  73 Y,  73 M,  73 C and  73 K. 
     Accordingly, at Act  300 , the laser control ASIC  110  reads, from the CPU  101 , the count numbers as the timings when the formation of the front side second patterns  77 Y,  77 M,  77 C and  77 K is started from L 1  and the formation of the rear side second patterns  78 Y,  78 M,  78 C and  78 K is started from L 2 , and stores them in the RAM  111 . Next, the laser control ASIC  110  instructs the laser drivers  28 Y,  28 M,  28 C and  28 K about the timings when the front side second patterns  77 Y,  77 M,  77 C and  77 K and the rear side second patterns  78 Y,  78 M,  78 C and  78 K are formed by using the corrected registration patterns (Act  301 ). 
     By the instructions of the timings, the front side second patterns  77 Y,  77 M,  77 C and  77 K formed on the transfer belt  10  are arranged in the detection range of the first registration sensor  36 , and the rear side second patterns  78 Y,  78 M,  78 C and  78 K are arranged in the detection range of the second registration sensor  37 . 
     That is, on the photoconductive drums  12 Y,  12 M,  12 C and  12 K, electrostatic latent images of the front side second patterns  77 Y,  77 M,  77 C and  77 K based on the first corrected registration pattern obtained by performing the image processing of the first register pattern  70  are formed from the front side second adjustment pattern formation start position corresponding to L 1  shown in  FIG. 15 . Besides, electrostatic latent images of the rear side second patterns  78 Y,  78 M,  78 C and  78 K based on the second corrected registration pattern obtained by performing the image processing of the second registration pattern  71  are formed from the rear side second adjustment pattern formation start position corresponding to L 2  shown in  FIG. 15 . 
     Thereafter, the toner images of the front side and the rear side second patterns  77 Y,  77 M,  77 C and  77 K and  78 Y,  78 M,  78 C and  78 K through the developing devices  14 Y,  14 M,  14 C and  14 K are transferred to the transfer belt  10  by the transfer rollers  15 Y,  15 M,  15 C and  15 K. By this, the front side second patterns  77 Y,  77 M,  77 C and  77 K and the rear side second patterns  78 Y,  78 M,  78 C and  78 K shown in  FIG. 15  are formed on the transfer belt  10  (Act  302 ). 
     Next, image shift adjustment at the second process speed will be described with reference to a flowchart of  FIG. 16 . When the image shift adjustment starts, the front side second patterns  77 Y,  77 M,  77 C and  77 K formed on the transfer belt  10  are detected by the first registration sensor  36 , and the rear side second patterns  78 Y,  78 M,  78 C and  78 K are detected by the second registration sensor  37  (Act  310 ). 
     The detection results are inputted to the CPU  101  through the engine control ASIC  130  (Act  311 ). The CPU  101  sets second adjustment values at the second process speed based on the detection results (Act  312 ). The second adjustment values include an adjustment value for adjusting an image shift in the main scanning direction and an adjustment value for adjusting a magnification error in the main scanning direction. The adjustment values in the main scanning direction are set similarly to the setting of the first adjustment values. 
     For example, as shown in  FIG. 17 , from the detection results of the first and the second registration sensors  36  and  37 , it is assumed that position shifts in the main scanning direction occur in the respective image formation stations  11 Y,  11 M,  11 C and  11 K. The CPU  101  determines the position shift of an image in the main scanning direction from differences among detection lengths ΔK 5 , ΔC 5 , ΔM 5  and ΔY 5  of the front side second patterns  77 K,  77 C,  77 M and  77 Y. Next, in order to adjust the position shift, the CPU  101  sets, as an adjustment value, a shift amount of image data in the main scanning direction according to the shift amount of the image in the main scanning direction. The adjustment value is set so that ΔK 5 =ΔC 5 =ΔM 5 =ΔY 5  is established. 
     Next, for example, as shown in  FIG. 18 , from the detection results of the first and the second registration sensors  36  and  37 , it is assumed that the magnification error in the main scanning direction occur in the respective image formation stations  11 Y,  11 M,  11 C and  11 K. The CPU  101  determines the magnification error in the main scanning direction from the detection lengths of the front side second patterns  77 K,  77 C,  77 M and  77 Y and the rear side second patterns  78 K,  78 C,  78 M and  78 Y. 
     For example, the detection lengths of the front side second patterns  77 K,  77 C,  77 M and  77 Y are made ΔK 7 , ΔC 7 , ΔM 7  and ΔY 7 , and the detection lengths of the rear side second patterns  78 K,  78 C,  78 M and  78 Y are made ΔK 8 , ΔC 8 , ΔM 8  and ΔY 8 . The adjustment value is set from the value of the sum of the front side detection length and the rear side detection length for each color. That is, the CPU  101  sets, as the adjustment value, the expanded amount or contracted amount of image data so that (ΔK 7 +ΔK 8 )=(ΔC 7 +ΔC 8 )=(ΔM 7 +ΔM 8 )=(ΔY 7 +ΔY 8 ) is established. 
     In this embodiment, for example, (ΔK 7 +ΔK 8 )=(ΔC 7 +ΔC 8 )=(ΔM 7 +ΔM 8 )=(ΔY 7 +ΔY 8 )=1 is made a reference value. On the other hand, in the black image formation station  11 K, when (ΔK 7 +ΔK 8 )=(1+V) is established, this is larger than the reference value of 1 by (V). However, this is based on the detection in the print result when the pattern is shifted, P 1 ×(H 1 /H 2 ) is used as P 1  in the calculation. Besides, P 2 ×(H 1 /H 2 ) is used as P 2  (where, P 1  is the distance from the reference position L 0  to the first registration sensor  36 , P 2  is the distance from the reference position L 0  to the second registration sensor  37 , H 1  is the clock frequency of the first oscillation unit  29   a , and H 2  is the clock frequency of the second oscillation unit  29   b ). 
     Accordingly, in this case,
 
(correction coefficient)=( V )/( P 1×( H 1/ H 2)+ P 2×( H 1/ H 2))=( V )/( P 1+ P 2)×( H 1/ H 2).
 
     By using this, the clock frequency is multiplied by {1+(correction coefficient)} to obtain the adjustment value. 
     The adjustment value of the image shift in the main scanning direction and the adjustment value of the magnification error, at the second process speed, which are caused by using the second oscillation unit  29   b , are calculated by the arithmetic unit  103  of the CPU  101  and are set. The set second adjustment values including the adjustment values in the main scanning direction at the second process speed are stored in the memory  102  of the CPU  101  (Act  313 ). 
     By this, the memory  102  of the CPU  101  stores the first adjustment values for the image shift adjustment in the main scanning direction and the sub-scanning direction at the first process speed, and the second adjustment values for the adjustment of the position shift and the magnification error in the main scanning direction at the second process speed. Thereafter, the color copier  1  completes the image shift adjustment mode and is put in a print mode. In the print mode, for example, the first process speed is set with priority. 
     Next, an image formation process by a first print mode, which is a first image formation mode, at the first process speed will be described with reference to a flowchart of  FIG. 19 . The image formation process by the first print mode is performed using the first oscillation unit  29   a  to oscillate the clock frequency of 100 MHz of the laser oscillators  27 Y,  27 M,  27 C and  27 K. When the image formation process at the first process speed is started, the laser control ASIC  110  reads the adjustment values for adjusting the position shift in the main scanning direction and the magnification error, which are caused by the use of the first oscillation unit  29   a , from the first adjustment values stored in the memory  102  of the CPU  101 , and stores them in the RAM  111  (Act  400 ). 
     On the other hand, the engine control ASIC  130  reads the adjustment values in the sub-scanning direction, such as the inclination amount and the rotation amount, from the first adjustment values stored in the memory  102  of the CPU  101 , and instructs the image processing unit  152  (Act  410 ). By this, the image data inputted from the scanner unit  6  is adjusted in the sub-scanning direction by the image processing unit  152 , and is inputted to the laser control ASIC  110  (Act  411 ). The laser control ASIC  110  instructs the laser drivers  28 Y,  28 M,  28 C and  28 K to control writing of the image data from the image processing unit  152  in accordance with the adjustment values in the main scanning direction of the first adjustment values (Act  401 ). By this, the laser oscillators  27 Y,  27 M,  27 C and  27 K oscillate the laser beams from the first oscillation units  29   a  at the controlled timings, and form the electrostatic latent images corresponding to the image data on the photoconductive drums  12 Y,  12 M,  12 C and  12 K (Act  402 ). Thereafter, the image formation on the sheet paper P at the first process speed is completed through the developing process, the transfer process, and the fixing process (Act  403 ), and the image formation process is ended. 
     Next, a description will be given to a case where an image formation process is performed at the second process speed in the color copier  1 . The image formation process at the second process speed is performed using the second oscillation unit  29   b  to oscillate the clock frequency of 125 MHz of the laser oscillators  27 Y,  27 M,  27 C and  27 K. When the color copier  1  is set in the first print mode, the mode is switched to a second print mode at the second process speed, which is a second image formation mode, by, for example, the operation panel  153 . By this, the laser control ASIC  110  and the engine control ASIC  130  read the second adjustment values from the memory  102  of the CPU  101  similarly to the first process speed. 
     The adjustment value in the sub-scanning direction in the image shift adjustment at the image formation of the second process speed is identical to that at the first process speed. Accordingly, with respect to the adjustment value in the sub-scanning direction, it is not necessary to again instruct the image processing unit  152 . Similarly to Act  411 , the image processing unit  152  processes the image data inputted from the scanner unit  6  by the adjustment value such as the inclination amount or the rotation amount in the sub-scanning direction indicated by the first adjustment value, and inputs it to the laser control ASIC  110 . However, when the process speed is changed, the engine control ASIC  130  reads the second process speed from the CPU  101 , and controls to change the drive speed of the drum motors  131 Y,  131 M,  131 C and  131 K, the polygon mirror motor  33 , and the belt motor  10   a  to the second process speed. 
     On the other hand, similarly to Act  400 , the laser control ASIC  110  reads the adjustment values for adjusting the position shift in the main scanning direction and the magnification error, which are the second adjustment values stored in the memory  102  of the CPU  101 , and stores them in the RAM  111 . Next, similarly to Act  401 , the laser control ASIC  110  instructs the laser drivers  28 Y,  28 M,  28 C and  28 K to control writing of the image data from the image processing unit  152  in accordance with the second adjustment values. By this, similarly to Act  402 , the laser oscillators  27 Y,  27 M,  27 C and  27 K oscillate laser beams from the second oscillation units  29   b  at the controlled timings, and form electrostatic latent images corresponding to the image data on the photoconductive drums  12 Y,  12 M,  12 C and  12 K. Thereafter, similarly to Act  403 , the image formation on the sheet paper P at the second process speed is completed through the developing process, the transfer process and the fixing process, and the image formation process is ended. 
     Thereafter, when the image formation process is again performed at the first process speed, the print mode is switched to the first print mode by the operation panel  153 . At this time, with respect to the adjustment value in the sub-scanning direction, it is unnecessary to again instruct the image processing unit  152 . However, when the print mode is switched, the engine control ASIC  130  reads the first process speed from the CPU  101 , and controls to change the drive speed of the drum motors  131 Y,  131 M,  131 C and  131 K, the polygon mirror motor  33 , and the belt motor  10   a  to the first process speed. 
     The RAM  111  of the laser control ASIC  110  is again rewritten to the adjustment values for adjusting the position shift in the main scanning direction and the magnification error in the first adjustment values. And then, the image data inputted from the image processing unit  152  is written to the photoconductive drums  12 Y,  12 M,  12 C and  12 K at the frequency oscillated from the first oscillation unit  29   a , and electrostatic latent images are formed. 
     There is a case where an image shift occurs by various factors while the image formation process is being performed at the first process speed or the second process speed. Thus, if necessary, at a specified timing or at warm-up after a maintenance process, the first adjustment values and the second adjustment values are again set. The memory  102  of the CPU  101  is rewritten by the first adjustment values and the second adjustment values which are again set. 
     According to the first embodiment, the first patterns for setting the image shift adjustment values at the first process speed and the second patterns for setting the image shift adjustment values at the second process speed are formed on the transfer belt  10  running at the first process speed. That is, irrespective of the switching of the process speed, the image shift adjustment values in the sub-scanning direction at the first process speed and the second process speed are the same. Accordingly, in the first process speed and the second process speed, the image shift adjustment value in the sub-scanning direction is made common to both, and the image shift adjustment value in the main scanning direction is set for the respective speeds. 
     As a result, in the adjustment mode, when the image shift adjustment values at the second process speed are set, the setting operation is simplified, and the capacity of the memory to store the set values is reduced. Besides, in the print mode, when the process speed is changed, it is unnecessary to perform the image shift adjustment in the sub-scanning direction, and only the image shift adjustment in the main scanning direction is performed. Accordingly, the adjustment operation at the switching of the process speed can be simplified, and the image formation can be speeded up. 
     Besides, as in the first embodiment, the formation positions of the first patterns and the second patterns on the transfer belt  10  for setting the image shift adjustment values are aligned, so that the first patterns and the second patterns can be detected by the same registration sensors  36  and  37 . 
     Next, a second embodiment of the invention will be described. The second embodiment is different from the first embodiment in formation positions of second adjustment patterns. Besides, detection units for detecting the second adjustment patterns are provided. Since the others are the same as the first embodiment, the same structure as the structure explained in the first embodiment is denoted by the same reference numeral and its detailed explanation will be omitted. 
     In the second embodiment, when the second adjustment patterns for setting second adjustment values are formed on the transfer belt  10  running at the first process speed, the count number of the write timing by the second oscillation unit  29   b  of the laser oscillators  27 Y,  27 M,  27 C and  27 K is made equal to the count number of the write timing by the first oscillation unit  29   a  at the first process speed. 
     That is, pattern formation is started at the 100th count from reference position L 0  by using a first corrected registration pattern and pattern formation is started at the 500th count by using a second registration pattern  71 , by the second oscillation unit  29   b . Then, the second adjustment patterns are formed on the transfer belt  10 . By this, the second adjustment patterns are formed at a distance of L 3  from the reference position L 0  and a distance of L 4  from the reference position L 0 , which are the formation positions of the first and the second comparison patterns  70   b  and  71   b  explained in  FIG. 13 . Thus, the second adjustment patterns go out of the detection range of the first or the second registration sensor  36  or  37 . 
     Then, in this embodiment, as shown in  FIG. 20 , a third registration sensor  82  and a fourth registration sensor  83  are disposed in addition to the first and the second registration sensors  36  and  37 . The third registration sensor  82  is at a distance of P 3  from the reference position L 0 , and detects front side third patterns  80 Y,  80 M,  80 C and  80 K which are the second adjustment patterns formed from the pattern formation start position of L 3  from the reference position L 0 . The fourth registration sensor  83  is at a distance of P 4  from the reference position L 0 , and detects rear side third patterns  81 Y,  81 M,  81 C and  81 K which are the second adjustment patterns formed from the pattern formation start position of L 4  from the reference position L 0 . 
     Thereafter, based on the detection results of the front side third patterns  80 Y,  80 M,  80 C and  80 K detected by the third registration sensor  82  and the rear side third patterns  81 Y,  81 M,  81 C and  81 K detected by the fourth registration sensor  83 , similarly to the first embodiment, the second adjustment values (an adjustment value for adjusting an image shift in the main scanning direction and an adjustment value for adjusting a magnification error in the main scanning direction) at the second process speed are determined. 
     According to the second embodiment, similarly to the first embodiment, the first patterns for setting the first adjustment values and the third patterns for setting the second adjustment values are formed on the transfer belt  10  running at the first process speed. That is, irrespective of switching of the process speed, the image shift adjustment values in the sub-scanning direction at the first process speed and the second process speed are equal to each other. Accordingly, in the first process speed and the second process speed, the image shift adjustment value in the sub-scanning direction is made common to both, and the image shift adjustment values in the main scanning direction are respectively set, so that the setting operation is simplified, and the capacity of the memory for storing the set values is reduced. Besides, at switching of the process speed in the print mode, only the image shift adjustment in the main scanning direction is performed. Thus, the adjustment operation at the switching of the process speed can be simplified, and the image formation can be speed up. 
     The present invention is not limited to the above embodiments, but can be variously modified within the scope of the invention. For example, the running speed of the running member is not limited, and the speed can be changed at multiple stages. Similarly, according to the number of running speed switching stages, plural oscillators can be provided. Besides, the clock frequency of the oscillator is not limited. Further, the shape of the pattern data is not limited as long as the image shift can be detected. For example, when Z-shaped pattern data is used, the detection data of the adjustment pattern by the detection unit is increased, and therefore, the image shift adjustment values with higher accuracy can be obtained.