Patent Publication Number: US-8526868-B2

Title: Image alignment adjusting apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Provisional U.S. application No. 61/300,158 filed on Feb. 1, 2010 and No. 61/300,166 filed on Feb. 1, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to image alignment in an image forming apparatus that superimposes plural images such as a color copying machine or a MFP (multi-functional peripheral) 
     BACKGROUND 
     A color image forming apparatus that superimposes plural images to obtain a color image performs alignment of the plural images, prevents blurs and bleeding of the images, and maintains satisfactory image quality. An image forming apparatus that obtains a color image using a traveling belt images an adjustment pattern for alignment adjustment on the belt and aligns plural images using a detection result obtained by detecting the adjustment pattern. The thickness of the belt varies depending on regions of the belt. Since the image forming apparatus aligns the images taking into account the thickness that varies depending on the regions of the belt, during alignment adjustment, the image forming apparatus images plural adjustment patterns over the entire circumference of the belt. The image forming apparatus averages detection results obtained by detecting the plural adjustment patterns imaged over the entire circumference of the belt. The image forming apparatus aligns the plural images using an average obtained by averaging the detection results to thereby improve accuracy of the alignment. 
     However, if the image forming apparatus images the plural adjustment patterns over the entire circumference of the belt and obtains an average of the plural adjustment patterns every time the image forming apparatus performs the image alignment adjustment, time required for the image alignment adjustment is long. Therefore, it is likely that the image forming apparatus keeps a user waiting during the image alignment adjustment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a main part of a color printer according to a first embodiment; 
         FIG. 2  is a schematic block diagram of a control system configured to mainly perform alignment adjustment in a sub-scanning direction in the first embodiment; 
         FIG. 3  is a schematic diagram for explaining an example of adjustment patterns imaged on a transfer belt during initial alignment adjustment and timing for image formation of the adjustment patterns and detection of the adjustment patterns in the first embodiment; 
         FIG. 4  is a flowchart for explaining the initial alignment adjustment in the first embodiment; 
         FIG. 5  is a schematic diagram for explaining an example of distance data between black (K) images and cyan (C) images of the adjustment patterns imaged on the transfer belt in the first embodiment; 
         FIG. 6  is a flowchart for explaining image print in the first embodiment; 
         FIG. 7  is a schematic diagram for explaining an example of black (K) and cyan (C) images of print images printed on the transfer belt in the first embodiment; 
         FIG. 8  is a flowchart for explaining intermediate alignment adjustment in the first embodiment; 
         FIG. 9  is a schematic diagram for explaining an example of adjustment patterns imaged on a transfer belt during the intermediate alignment adjustment and timing for image formation of the adjustment patterns and detection of the adjustment patterns in the first embodiment; 
         FIG. 10  is a schematic diagram for explaining comparison of distance data and intermediate distance data imaged on the transfer belt in the first embodiment; 
         FIG. 11  is a schematic diagram for explaining an example of intermediate alignment adjustment during continuous print in the first embodiment; 
         FIG. 12  is a flowchart for explaining switching of a mode of a color printer according to a second embodiment; 
         FIG. 13  is a flowchart for explaining initial alignment adjustment in an image quality priority print mode in the second embodiment; 
         FIG. 14  is a schematic diagram for explaining an example of distance data between black (K) images and cyan (C) images of adjustment patterns imaged on a transfer belt in the second embodiment; 
         FIG. 15  is a schematic diagram for explaining a position of blur occurrence due to a projection of the transfer belt in the second embodiment; 
         FIG. 16  is a schematic diagram for explaining detection of a position of blur occurrence due to the projection of the transfer belt in the second embodiment; 
         FIG. 17  is a diagram for explaining detection timing for a position of blur occurrence due to the projection of the transfer belt in the second embodiment; and 
         FIG. 18  is a flowchart for explaining an image quality priority print mode in the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an image alignment adjusting apparatus includes: an endless traveling belt; a pattern sensor configured to detect an adjustment pattern including plural colors imaged on the traveling belt; and a correcting unit configured to use, in initial adjustment, for image alignment adjustment by an image forming unit configured to image the adjustment pattern, an initial adjustment value obtained by detecting, with the pattern sensor, a plurality of sets of the adjustment pattern imaged over the entire circumference of the traveling belt and use, in intermediate adjustment, for the image alignment adjustment by the image forming unit, an intermediate adjustment value obtained by correcting the initial adjustment value using an intermediate detection value obtained by detecting, with the pattern sensor, one set of the adjustment pattern imaged on the traveling belt. 
     Embodiments are explained below. 
     First Embodiment 
       FIG. 1  is a schematic diagram of a main part of a color printer  1  of a tandem type, which is an image forming apparatus according to a first embodiment. The color printer  1  includes four sets of image forming stations  13 K,  13 C,  13 M, and  13 Y arranged in parallel along the lower side of a transfer belt  12 , which is an endless traveling belt. The image forming stations  13 K,  13 C,  13 M, and  13 Y respectively include photoconductive drums  14 K,  14 C,  14 M, and  14 Y. Rotation axes of the photoconductive drums  14 K,  14 C,  14 M, and  14 Y are parallel to a direction (a main scanning direction) orthogonal to a traveling direction (a sub-scanning direction) in an arrow f direction of the transfer belt  12 . The rotation axes of the photoconductive drums  14 K,  14 C,  14 M, and  14 Y are arranged at equal intervals from one another along the sub-scanning direction of the transfer belt  12 . 
     The image forming stations  13 K,  13 C,  13 M, and  13 Y respectively include, around the photoconductive drums  14 K,  14 C,  14 M, and  14 K which rotate in an arrow g direction, chargers  16 K,  16 C,  16 M, and  16 Y, developing devices  17 K,  17 C,  17 M, and  17 Y, and photoconductive cleaners  18 K,  18 C,  18 M, and  18 Y. 
     The image forming stations  13 K,  13 C,  13 M, and  13 Y respectively include, around the photoconductive drums  14 K,  14 C,  14 M, and  14 K, chargers  16 K,  16 C,  16 M, and  16 Y, developing devices  17 K,  17 C,  17 M, and  17 Y, and photoconductive cleaners  18 K,  18 C,  18 M, and  18 Y. 
     The color printer  1  includes a laser exposing device  20 . The laser exposing device  20  and the image forming stations  13 K,  13 C,  13 M, and  13 Y configure an image forming unit. The laser exposing device  20  irradiates exposure lights corresponding to the respective colors to sections between the chargers  16 K,  16 C,  16 M, and  16 Y and the developing devices  17 K,  17 C,  17 M, and  17 Y around the photoconductive drums  14 K,  14 C,  14 M, and  14 Y. The laser exposing device  20  forms electrostatic latent images based on image data or data of respective color components of the adjustment patterns on the photoconductive drums  14 K,  14 C,  14 M, and  14 Y. The laser exposing device  20  includes laser oscillators  21 K,  21 C,  21 M, and  21 Y for the respective color components of black (K), cyan (C), magenta (M), and yellow (Y). The developing devices  17 K,  17 C,  17 M, and  17 Y respectively form toner images or adjustment patterns of black (K), cyan (C), magenta (M), and yellow (Y) on the photoconductive drums  14 K,  14 C,  14 M, and  14 Y. 
     The color printer  1  includes a driving roller  12   a  and a driven roller  12   b  configured to support the transfer belt  12 . The driving roller  12   a  and the driven roller  12   b  cause the transfer belt  12  to travel in the arrow f direction. The transfer belt  12  includes a belt marker  22  on the inner circumference thereof. The belt marker  22  is formed of a reflection tape that reflects light. The color printer  1  includes, on the inside of the transfer belt  12 , a belt sensor  23  configured to detect the belt marker  22 . 
     The color printer  1  includes primary transfer rollers  26 K,  26 C,  26 M, and  26 Y respectively in positions opposed to the photoconductive drums  14 K,  14 C,  14 M, and  14 Y via the transfer belt  12 . The primary transfer rollers  26 K,  26 C,  26 M, and  26 Y respectively primarily transfer toner images formed on the photoconductive drums  14 K,  14 C,  14 M, and  14 Y to superimpose the toner images one on top of another on the transfer belt  12 . The photoconductive cleaners  18 K,  18 C,  18 M, and  18 Y respectively remove and collect toners remaining on the photoconductive drums  14 K,  14 C,  14 M, and  14 Y after the primary transfer. 
     The color printer  1  includes a secondary transfer roller  27  in a secondary transfer position opposed to the driving roller  12   a  via the transfer belt  12 . The color printer  1  collectively secondarily transfers, in a nip between the transfer belt  12  and the secondary transfer roller  27 , the toner images on the transfer belt  12  onto a sheet P fed from a paper feeding unit  28 . 
     The color printer  1  includes a fixing device  30  and a paper discharge roller  31  further downstream than the secondary transfer roller  27  along a conveying direction of the sheet P. The color printer  1  fixes the toner images on the sheet P with the fixing device  30  and discharges the sheet P with the paper discharge roller  31 . 
     The transfer belt  12  includes a belt cleaner  12   c . The belt cleaner  12   c  removes the adjustment patterns imaged on the transfer belt  12  and the toners remaining on the transfer belt  12  after a print image is secondarily transferred. 
     When the color printer  1  of the tandem type superimposes plural images one on top of another on the transfer belt  12 , a positional shift (a superimposition shift) tends to occur. When the positions of the plural images shift from one another, it is likely that a bleeding image is formed and image quality is deteriorated. As the positional shift of the images, there is, for example, (1) a shift in the main scanning direction (2) a shift in the sub-scanning direction, (3) a shift of image magnifications, or (4) a tilt of the images. The color printer  1  needs to perform alignment adjustment in order to correct the positional shift of the images. 
     The color printer  1  includes a front pattern sensor  37  and a rear pattern sensor  38  for detecting adjustment patterns imaged on the transfer belt  12  for alignment adjustment. The front pattern sensor  37  and the rear pattern sensor  38  are present around the transfer belt  12  and downstream of the image forming station  13 K for black (K). The front pattern sensor  37  detects a front side adjustment pattern formed in a front area that is parallel to a traveling direction of the transfer belt  12 . The rear pattern sensor  38  detects a rear side adjustment pattern formed in a rear area that is parallel to the traveling direction of the transfer belt  12 . 
     The color printer  1  calculates, using detection results of the front pattern sensor  37  and the rear pattern sensor  38 , an adjustment value for adjusting (1) the shift in the main scanning direction, (2) the shift in the sub-scanning direction, (3) the shift of image magnifications, or (4) the tilt of the images. If the images positionally shift from one another in the main scanning direction or the sub-scanning direction, the color printer  1  calculates a shift of output timings of lasers in the main scanning direction or the sub-scanning direction as the adjustment value and shifts the output timings of the lasers in the main scanning direction or the sub-scanning direction. If the magnifications of the images shift from one another, the color printer  1  calculates shift amounts of clock speeds of the lasers as the adjustment value and shifts clock frequencies of the lasers. If the images tilt, the color printer  1  calculates shift amounts of the tilts as the adjustment value and shifts the tilt of a tilt mirror of an optical system. 
     A block diagram of a control system  100  configured to mainly perform alignment adjustment in the sub-scanning direction of the color printer  1  is shown in  FIG. 2 . The front pattern sensor  37 , the rear pattern sensor  38 , and the belt sensor  23  are connected to a CPU  101  configured to control the entire color printer  1 . The CPU  101  is connected to a laser control unit  110  and a print control unit  120 . The CPU  101  includes a memory  102 , a calculating unit  103 , and an alignment counter  104 . 
     The memory  102  stores, for example, various settings for controlling the laser control unit  110  and the print control unit  120 . The memory  102  stores, for example, theoretical values of distance data of adjustment patterns  50  explained later or theoretical values of timings from detection of the belt marker  22  until detection of the adjustment patterns  50 . The calculating unit  103  calculates, for example, from pattern information obtained from the front pattern sensor  37  or the rear pattern sensor  38 , an image shift in the sub-scanning direction and calculates an alignment adjustment value of the laser control unit  110 . The alignment counter  104  counts, for example, the number of times of detection of the belt marker  22  by the belt sensor  23 . Alternatively, the alignment counter  104  may count, for example, the number of sheets. 
     The laser control unit  110  controls, for example, the laser oscillators  21 K,  21 C,  21 M, and  21 Y for the respective color components via a laser driver  21 . The laser driver  21  controls writing start timings of the laser oscillators  21 K,  21 C,  21 M, and  21 Y for the respective color components of the laser exposing device  20 . 
     The print control unit  120  controls, for example, the photoconductive drums  14 K,  14 C,  14 M, and  14 Y, the transfer belt  12 , the chargers  16 K,  16 C,  16 M, and  16 Y, the developing devices  17 K,  17 C,  17 M, and  17 Y, the photoconductive cleaners  18 K,  18 C,  18 M, and  18 Y, and the fixing device  30 . 
     An example of the adjustment patterns  50  imaged over the entire circumference of the transfer belt  12  during alignment adjustment is shown in  FIG. 3 . The adjustment patterns  50  are, for example, wedge-type patterns including patterns of the four colors K, C, M, and Y as one set. As a reference, each of the wedge-type patterns of the four colors K, C, M, and Y is apart from the wedge-type pattern adjacent thereto by, for example, 10 mm as a theoretical space. (If a space between each of the wedge-type patterns of the four colors K, C, M, and Y and the wedge-type pattern adjacent thereto is 10 mm, which is a theoretical reference value, the positions in the sub-scanning direction of the wedge-type patterns coincide with each other.) 
     The alignment adjustment of the color printer  1  includes initial adjustment and intermediate adjustment. In the initial adjustment, the color printer  1  adjusts (1) the shift in the main scanning direction, (2) the shift in the sub-scanning direction, (3) the shift of magnifications, and (4) the tilt of images. The color printer  1  performs the initial adjustment, for example, during warm-up by power-on of the color printer  1 , during return from a sleep mode for interrupting power supply to a heating source of the fixing device  30  or according to a request from an operator even during ready. 
     In the intermediate adjustment, the color printer  1  adjusts a shift in the sub-scanning direction of images. The color printer  1  performs the intermediate adjustment during a print mode in which print of the images can be immediately started when a print request for the images is sent to the color printer  1 . The color printer  1  desirably periodically performs intermediate alignment adjustment during the ready after performing the initial adjustment. 
     The color printer  1  images, for example, eight sets of the adjustment patterns  50  from a first pattern  51  to an eighth pattern  58  on the transfer belt  12  during the initial adjustment. The color printer  1  images, with the detection of the belt marker  22  by the belt sensor  23  as a start point, eight sets of front adjustment patterns  50   a  on the front side of the transfer belt  12 . The color printer  1  images eight sets of rear adjustment patterns  50   b  on the rear side of the transfer belt  12 . 
     During the alignment adjustment, the front pattern sensor  37  detects the front adjustment patterns  50   a  and the rear pattern sensor  38  detects the rear adjustment patterns  50   b.    
     (I) Initial Alignment Adjustment (Alignment in the Sub-Scanning Direction) 
     The initial alignment adjustment in the sub-scanning direction is explained below. When a power supply is turned on, the color printer  1  starts warm-up and starts the initial alignment adjustment in the sub-scanning direction shown in  FIG. 4 . When the initial alignment adjustment is started, the print control unit  120  controls the transfer belt  12  to travel in the arrow f direction. When the belt sensor  23  detects the belt marker  22  of the transfer belt  12  (ACT  200 ), the CPU  101  instructs the laser control unit  110  and the print control unit  120  to image the adjustment patterns  50 . The color printer  1  images, with the position of the belt marker  22  as a reference, the eight sets of the adjustment patterns  50  from the first pattern  51  to the eighth pattern  58  shown in  FIG. 3  over the entire circumference of the transfer belt  12  (ACT  201 ). 
     The front pattern sensor  37  detects the front adjustment patterns  50   a  and the rear pattern sensor  38  detects the rear adjustment patterns  50   b  (ACT  202 ). In the transfer belt  12 , in some case, a projection or fluctuation in thickness occurs during manufacturing. Fluctuation in the thickness direction of the transfer belt  12  causes a positional shift of images in the sub-scanning directions. When the adjustment patterns  50  are imaged over the entire circumference of the transfer belt  12 , in some case, a positional shift of the adjustment patterns  50  occurs in an area where fluctuation in the thickness occurs in the transfer belt  12 . If the adjustment patterns  50  positionally shift from one another, distance data among toner images of the respective colors of eight sets of the adjustment patterns  50  detected by the front pattern sensor  37  or the rear pattern sensor  38  are different. For the alignment adjustment in the sub-scanning direction, the calculating unit  103  calculates distance data of the detected eight sets of the adjustment patterns  50  (ACT  203 ). 
     The calculating unit  103  calculates an average of the calculated distance data and calculates an adjustment value (ACT  204 ). The CPU  101  updates an adjustment value stored in the memory  102  to the calculated adjustment value (ACT  205 ). 
     Further, the CPU  101  checks whether the adjustment value updated in ACT  205  is correct. The CPU  101  adjusts output timings of lasers of the laser oscillators  21 K,  21 C,  21 M, and  21 Y using the updated adjustment value updated in ACT  205  and images eight sets of the adjustment patterns  50  from the first pattern  51  to the eighth pattern  58  shown in  FIG. 3  over the entire circumference of the transfer belt  12  (ACT  220 ). 
     As in ACT  202  to ACT  204 , the CPU  101  detects the eight sets of the adjustment patterns  50  imaged anew (ACT  221 ) and calculates distance data of toner images of the respective colors of the eight sets of the adjustment patterns  50  (ACT  222 ). The calculating unit  103  calculates an adjustment value for check from the calculated distance data of the toner images of the respective colors (ACT  223 ). 
     The CPU  101  checks, from the calculated adjustment value for check, whether a distance among the toner images of the respective colors of the adjustment patterns  50  is within a tolerance of 10 mm as the theoretical reference value (ACT  224 ). The CPU  101  updates the adjustment value stored in the memory  102  to the adjustment value for check (ACT  226 ) and finishes the initial alignment adjustment. If the distance among the toners of the respective colors deviates from the tolerance of the theoretical reference value in ACT  224 , the CPU  101  may perform the check in ACT  220  to ACT  224  again. During the initial alignment adjustment, the check of patterns after adjustment is repeated, whereby the alignment adjustment accuracy of the color printer  1  is further improved. 
     For example, as shown in  FIG. 5 , distance data between images Kn of black (K) and images Cn of cyan (C) from the first pattern  51  to the eighth pattern  58  is represented as (an). For the alignment adjustment in the sub-scanning direction, the calculating unit  103  calculates an average (X) obtained by averaging the distance data (an) of the images Kn of black (K) and the images Cn of cyan (C) and sets the average (X) as an initial adjustment value (h 0 ). 
     In the same manner as the alignment adjustment for black (K) and cyan (C), the calculating unit  103  calculates an initial adjustment value for alignment adjustment between cyan (C) and magenta (M) and between magenta (M) and yellow (Y). A reference for the alignment adjustment is not limited to black (K). 
     When the color printer  1  finishes warm-up operation including the initial alignment adjustment, the color printer  1  switches to a ready mode. When a print request for images is received, the color printer  1  starts print operation shown in  FIG. 6 . The CPU  101  recognizes, from detection information of the belt marker  22 , respective image forming positions for the first pattern  51  to the eighth pattern  58  on the transfer belt  12 . The CPU  101  recognizes, as image shift amounts in the sub-scanning direction, differences between the distance data (an) of the images Kn of black (K) and the images Cn of cyan (C) and the initial adjustment value (h 0 ) in the positions of the transfer belt  12 . (Concerning each of differences between cyan (C) and magenta (M) and between magenta (M) and yellow (Y), the CPU  101  recognizes image shift amounts in the sub-scanning direction in the same manner.) 
     During print of the images, the CPU  101  shifts, to correspond to the positions of the transfer belt  12 , output timing of the laser oscillator  21 C for cyan (C) with respect to oscillation timing of the laser oscillator  21 K for black (K) according to the initial adjustment value (h 0 ) (ACT  211 ). 
     For example, the distance data (an) of the first pattern  51 , the distance data (an) of the second pattern  52 , the distance data (an) of the third pattern  53 , and the like calculated in ACT  222  are respectively represented as a 1 =10, a 2 =11, a 3 =12, and the like and the average (X) from the first pattern  51  to the eighth pattern calculated in ACT  223  is represented as initial adjustment value (h 0 )=11. (A difference between the initial adjustment value (h 0 ) and the distance data (an))=an image shift amount is (−1), (0), (+1), and the like respectively in the first pattern  51 , the second pattern  52 , the third pattern  53 , and the like. 
     An example of a print image obtained in ACT  212  after adjusting the output timing of the laser oscillator  21 C for cyan (C) using the initial adjustment value (h 0 )=11 is shown in  FIG. 7 . In the position of the first pattern  51 , a cyan (C) image shifts by −1 with respect to a black (K) image. In the position of the second pattern  52 , the black (K) image and the cyan (C) image coincide with each other. In the position of the third pattern  53 , the cyan (C) image shifts by +1 with respect to the black (K) image. In the position of the eighth pattern  58 , the cyan (C) image shifts by −1 with respect to the black (K) image. 
     After alignment adjustment, when a print request is received, the color printer  1  subjects the images to print processing (ACT  212 ). During print of the images, the image shift amounts of the black (K) image and the cyan (C) image of the transfer belt  12  are averaged as shown in  FIG. 7 . 
     While the image print is performed, the alignment counter  104  sequentially counts up the number of times of detection of the belt marker  22  by the belt sensor  23  (ACT  213 ). While the image print is performed, if the number counted by the alignment counter  104  reaches a predefined number of sheets (Yes in Act  214 ), the color printer  1  performs the intermediate alignment adjustment explained later (ACT  216 ). In ACT  217 , the color printer  1  clears a count value of the alignment counter  104  and proceeds to ACT  218 . 
     If the number counted by the alignment counter  104  does not reach the predefined number of sheets (No in ACT  214 ), the color printer  1  proceeds to ACT  218 . If image print end conditions are not satisfied (No in ACT  218 ), the color printer  1  repeats ACT  211  to ACT  218 . If the color printer  1  finishes the image print (Yes in ACT  218 ), the color printer  1  stands by for the next print. 
     (II) Intermediate Alignment Adjustment (Alignment in the Sub-Scanning Direction) 
     The intermediate alignment adjustment in the sub-scanning direction is explained. Even after the color printer  1  finishes the initial adjustment, a positional shift of toner images tends to occur because of a change in environmental characteristics in the apparatus. Even during print, the color printer  1  periodically performs alignment adjustment as indicated by ACT  216 . However, the positional shift of the toner images during the print is considered to be mainly caused by fluctuation in characteristics of the optical system of the laser exposing device  20  due to a temperature rise in the apparatus. Fluctuation in the transfer belt  12  due to the temperature rise in the apparatus can be generally neglected. The positional shift of the toner images due to the temperature rise in the apparatus is unrelated to a region of the transfer belt  12 . The positional shift of the toner images due to the temperature rise in the apparatus appears in common over the entire circumference of the transfer belt  12 . 
     Therefore, it is unnecessary to detect the positional shift of the toner images due to the temperature rise in the apparatus by imaging plural sets of adjustment patterns on the transfer belt  12 . An adjustment value for the positional shift of the toner images due to the temperature rise in the apparatus can be obtained by imaging one set of an adjustment pattern on the transfer belt  12  and detecting the imaged one set of the adjustment pattern. An imaging position of the one set of the adjustment pattern imaged on the transfer belt  12  for the intermediate alignment adjustment is not limited. For the intermediate alignment adjustment, the color printer  1  may image the one set of the adjustment pattern in any position of the transfer belt  12 . 
     When the intermediate alignment adjustment shown in  FIG. 8  is started and the belt sensor  23  detects the belt marker  22  (ACT  230 ), the color printer  1  images one set of a ninth pattern  59  shown in  FIG. 9  on the transfer belt  12  (ACT  231 ). As the ninth pattern  59 , a pattern having a shape the same as that of the first pattern  51  is imaged in a position the same as the position of the first pattern  51 . The front pattern sensor  37  and the rear pattern sensor  38  detect the ninth pattern  59  (ACT  232 ). 
     For the alignment adjustment in the sub-scanning direction, the calculating unit  103  calculates distance data of the first pattern  51  for initial adjustment and intermediate distance data among toner images of the respective colors as intermediate detection values of the ninth pattern  59  for intermediate adjustment (ACT  233 ). There is a difference between the distance data of the first pattern  51  and the intermediate distance data of the ninth pattern  59  because of fluctuation in the characteristics of the optical system of the laser exposing device  20  due to the temperature rise inside the color printer  1 . 
     For example, as shown in  FIG. 10 , distance data a 1  between a pattern K 1  of black (K) and a pattern C 1  of cyan (C) of the first pattern  51  and intermediate distance data b 1  between a pattern K 9  of black (K) and a pattern C 9  of cyan (C) of the ninth pattern  59  are compared. For the alignment adjustment in the sub-scanning direction, the calculating unit  103  adds a difference (b 1 −a 1 ) between the distance data a 1  between the image K 1  of black (K) and the image C 1  of cyan (C) of the first pattern  51  and the intermediate distance data b 1  between the image K 9  of black (K) and the image C 9  of cyan (C) of the ninth pattern  59  to the initial adjustment value (h 0 ). The calculating unit  103  sets, as an intermediate adjustment value (h 2 ), a value (h 0 +(b 1 −a 1 )) obtained by adding the difference (b 1 −a 1 ) between the distance data a 1  of the first pattern  51  and the intermediate distance data b 1  of the ninth pattern  59  to the initial adjustment value (h 0 ) (ACT  234 ). The intermediate adjustment value (h 2 )=h 0 +(b 1 −a 1 ) is common over the entire circumference of the transfer belt  12 . The CPU  101  updates the adjustment value stored in the memory  102  to the calculated intermediate adjustment value (h 2 ) (ACT  236 ). 
     The CPU  101  recognizes, as image shift amounts, differences between the distance data (an) between a pattern Kn of black (K) and a pattern Cn of cyan (C) and the intermediate adjustment value (h 2 ) in the positions of the transfer belt  12  using the intermediate adjustment value (h 2 ) instead of the initial adjustment value (h 0 ). During printing of the images, the CPU  101  shifts, to correspond to the positions of the intermediate belt  12 , output timing of the laser oscillator  21 C for cyan (C) with respect to oscillation timing of the laser oscillator  21 K for black (K) according to the intermediate adjustment value (h 2 ). 
     For example, in ACT  233 , the calculating unit  103  sets the intermediate distance data b 1  of the ninth pattern  59  to 10.5. In ACT  234 , the calculating unit  103  adds a difference (0.5) between the intermediate distance data b 1 =10.5 of the ninth pattern  59  and the distance data a 1 =10 of the first pattern  51  to the initial adjustment value (h 0 )=11 and obtains an intermediate adjustment value h 2 =11.5. 
     During the printing of the images, the CPU  101  shifts, to correspond to the positions of the transfer belt  12 , output timing of the laser oscillator  21 C for cyan (C) with respect to oscillation timing of the laser oscillator  21 K for black (K) according to the image shift amounts. 
     In the intermediate adjustment, the CPU  101  performs the intermediate alignment adjustment between black (K) and cyan (C) over the entire circumference of the transfer belt  12  using the intermediate adjustment value (h 2 ) obtained from the one set of the ninth pattern  59  imaged on the transfer belt  12 . 
     As in the intermediate alignment adjustment between black (K) and cyan (C), the CPU  101  performs the intermediate alignment adjustment between cyan (C) and magenta (M) and between magenta (M) and yellow (Y) according to the one set of the ninth pattern  59  imaged on the transfer belt  12 . 
     When a print request is received, the color printer  1  adjusts output timings of lasers of the laser oscillators  21 K,  21 C,  21 M, and  21 Y using the intermediate adjustment value (h 2 ) and prints images (ACT  212 ). After printing the images, the color printer  1  periodically repeats the intermediate alignment adjustment in ACT  230  to ACT  236 . 
     One set of an adjustment pattern used for the intermediate adjustment is not limited to the ninth pattern  59  corresponding to the first pattern  51 . For example, if the number counted by the alignment counter  104  reaches the predefined number of sheets (Yes in ACT  214 ) during continuous print for the A4 size (the JIS standard), the color printer  1  temporarily suspends the continuous print and performs the intermediate adjustment. For example, as shown in  FIG. 11 , the color printer  1  performs the intermediate alignment adjustment in a space (S) between print P 1  before the continuous print is suspended and print P 2  at the time when the continuous print is resumed. 
     If a position (S 1 ) where the continuous print is suspended is before an image forming position of the fourth pattern  54  of the initial adjustment for the transfer belt  12 , the color printer  1  images one set of a twelfth pattern  62  in the image forming position of the fourth pattern  54 . The shape of the twelfth pattern  62  is the same as the shape of the fourth pattern  54  in the initial adjustment. The twelfth pattern  62  is an intermediate pattern that can be imaged first in the space (S) after the suspension of the continuous print. 
     The calculating unit  103  sets, as the intermediate adjustment value (h 2 ), a value obtained by adding a difference (b 12 −a 4 ) between the distance data (a 4 ) of the fourth pattern  54  for the initial adjustment and intermediate distance data (b 12 ) of the twelfth pattern for the intermediate adjustment to the initial adjustment value (h 0 ). The same result is obtained even though an adjustment pattern in any position corresponding to the first pattern  51  to the eighth pattern  58  is used as one set of an adjustment pattern used for the intermediate adjustment value (h 2 ). 
     After imaging the twelfth pattern  62 , the color printer  1  resumes the continuous print from, for example, (S 2 ) of the transfer belt  12 . After the suspension of the continuous print, the color printer  1  can immediately image patterns for the intermediate alignment adjustment on the transfer belt  12  and perform the intermediate alignment adjustment without waiting for the transfer belt  12  to reach the image forming position of the first pattern  51  for the initial adjustment. During the intermediate adjustment, the color printer  1  images one set of an adjustment pattern in the space (S) to thereby reduce suspension time in performing the intermediate adjustment during the continuous print. 
     According to the first embodiment, in the initial alignment adjustment, the color printer  1  calculates an average (X) of the eight sets of the adjustment patterns  50  and obtains the initial adjustment value (h 0 ). The color printer  1  adjusts output timings of laser oscillators  21 K,  21 C,  21 M, and  21 Y according to image shift amounts obtained from the initial adjustment value (h 0 ) and performs the alignment adjustment. In the intermediate alignment adjustment, the color printer  1  obtains the intermediate adjustment value (h 2 ) from the ninth pattern  59  imaged in a position of the transfer belt  12  the same as the position of the first pattern  51  and in the shape the same as the shape of the first pattern  51 . The color printer  1  adjusts output timings of the laser oscillators  21 K,  21 C,  21 M, and  21 Y and performs the alignment adjustment according to image shift amounts obtained from the intermediate adjustment value (h 2 ). In the intermediate alignment adjustment, the color printer  1  can perform the alignment adjustment simply by imaging the one set of the ninth pattern  59  on the transfer belt  12 . In the intermediate alignment adjustment, it is unnecessary to image plural sets of adjustment patterns over the entire circumference of the transfer belt  12 . Therefore, alignment adjustment time is reduced. 
     Second Embodiment 
     In a second embodiment, two initial adjustment values are switched in the first embodiment. In the second embodiment, a print area of images is switched in the first embodiment. In the second embodiments, components the same as those explained in the first embodiment are denoted by the same reference numerals and signs and detailed explanation of the components is omitted. 
     The color printer  1  has a speed priority print mode for giving priority to print speed and an image quality priority print mode for giving priority to print image quality. An operator switches the speed priority print mode and the image quality priority print mode from, for example, a control panel of the color printer  1 . During a standby mode of the color printer  1 , for example, as shown in  FIG. 12 , the operator selects various modes from the control panel. 
     If the operator selects print (Yes in ACT  240 ), the color printer  1  switches to a print mode. The print mode is the speed priority print mode. If the operator selects the image quality priority print (Yes in ACT  241 ), the color printer  1  switches from the speed priority print mode to the image quality priority print mode. If the operator selects filing (Yes in ACT  242 ), the color printer  1  switches to a filing mode. If the operator selects scan (Yes in ACT  243 ), the color printer  1  switches to a scan mode. If the operator selects facsimile (Yes in ACT  244 ), the color printer  1  switches to a facsimile mode. In a state of the standby mode, if a predefined time elapses (Yes in ACT  246 ), the color printer  1  switches to a sleep mode. 
     In some case, the transfer belt  12  has an area where an image blur is conspicuous because of a projection or fluctuation in thickness that occurs during manufacturing. When the area with the conspicuous image blur is present in the transfer belt  12 , if an initial adjustment value is calculated targeting the entire area of the transfer belt  12 , it is likely that accuracy of the initial adjustment value falls. In the second embodiment, an initial adjustment value is calculated targeting an area excluding the area with the conspicuous image blur of the transfer belt  12  to improve the accuracy of the initial adjustment value. In the second embodiment, the area with the conspicuous image blur of the transfer belt  12  is not used for print of images, whereby a higher-quality print image is obtained. 
     The color printer  1  according to the second embodiment has a speed priority adjustment value (H 1 ) and an image quality priority adjustment value (H 2 ) as the initial adjustment value. The speed priority adjustment value (H 1 ) refers to the initial adjustment value (h 0 ) obtained in the initial alignment adjustment of (I) explained above. The initial adjustment value (h 0 ) as the speed priority adjustment value (H 1 ) is an adjustment value obtained by averaging all the distance data (an) of the eight sets of the adjustment patterns  50  imaged over the entire circumference of the transfer belt  12  during the initial alignment adjustment. 
     The color printer  1  according to the second embodiment includes, as image print areas, an area including the entire circumference of the transfer belt  12  and an OK area excluding an NG area, which is an image formation inhibited area, from the entire circumference of the transfer belt  12 . 
     In the speed priority print mode, the color printer  1  performs the initial alignment adjustment of (I) and the intermediate alignment adjustment of (II) using the speed priority adjustment value (H 1 ) as the initial adjustment value. In the speed priority print mode, the color printer  1  obtains a print image using the entire circumference of the transfer belt  12 . 
     (III) Initial Alignment Adjustment (Alignment in the Sub-Scanning Direction) in the Image Quality Priority Print Mode 
     If the operator selects the image quality priority print mode during the standby mode (Yes in ACT  241 ), the color printer  1  starts the initial alignment adjustment in the sub-scanning direction shown in  FIG. 13 . However, the color printer  1  does not have to perform the initial alignment adjustment in the image quality priority print mode every time the operator selects the image quality priority print mode. For example, during the initial alignment adjustment of (I) in the first embodiment, the initial alignment adjustment in the image quality priority print mode is set in advance. If the operator selects the image quality priority print mode, the color printer  1  obtains an image quality priority print image using the already-set initial alignment adjustment in the image quality priority print mode. 
     In the initial alignment adjustment of the image quality priority print mode, when the belt sensor  23  detects the belt marker  22  of the transfer belt  12  (ACT  300 ), the color printer  1  images, with the position of the belt marker  22  as a reference, the eight sets of the adjustment patterns  50  from the first pattern  51  to the eighth pattern  58  shown in  FIG. 3  on the entire circumference of the transfer belt  12  (ACT  301 ). 
     The front pattern sensor  37  and the rear pattern sensor  38  detect the eight sets of the adjustment patterns  50  (ACT  302 ). For the alignment adjustment in the sub-scanning direction, the calculating unit  103  calculates distance data among toner images of the respective colors of the detected eight sets of the adjustment patterns  50  (ACT  303 ). 
     The calculating unit  103  calculates a total average of the distance data of the eight sets of the adjustment patterns  50  and calculates an adjustment value (h 1 ) (ACT  304 ). The CPU  101  updates the adjustment value stored in the memory  102  to the calculated adjustment value (h 1 ) (ACT  305 ). 
     As in the first embodiment, for the alignment adjustment in the sub-scanning direction, the calculating unit  103  calculates a total average (X)=(adjustment value−1) of the distance data (an) between the images Kn of black (K) and the images Cn of cyan (C). The calculating unit  103  calculates (adjustment value−1) for the alignment adjustment between cyan (C) and magenta (M) and between magenta (M) and yellow (Y) in the same manner. 
     The CPU  101  finds a NG pattern from the distance data (an) of the eight sets of the adjustment patterns  50  calculated in ACT  303  (ACT  307 ). The CPU  101  compares the adjustment value (h 1 ) updated in ACT  305  and the distance data (an) and sets an adjustment pattern having a largest difference from the adjustment value (h 1 ) as the NG pattern. 
     For example, as shown in  FIG. 14 , the distance data (an) of the first pattern  51 , the distance data (an) of the second pattern  52 , the distance data (an) of the third pattern  53 , and the distance data (an) of the fourth pattern  54  to the eighth pattern  58  calculated in ACT  303  are respectively represented as a 1 =10, a 2 =11, a 3 =12, and (a 4  to a 8 )=10. The average (X) from the first pattern  51  to the eighth pattern  58  calculated in ACT  204  is the adjustment value (h 1 )=10.375. Distance data having a largest difference from the adjustment value (h 1 )=10.375 is the third pattern  53 , the distance data (an) of which is a 3 =12. Therefore, the third pattern  53  is set as the NG pattern. 
     In ACT  308 , the CPU  101  determines a NG area of the transfer belt  12  and stores the NG area in the memory  102 . The NG area is an area in which an image blur is conspicuous and a toner image is not printed during the image quality priority print mode. For example, as shown in  FIG. 15 , when a projection  70  that occurs in the transfer belt  12  passes through the secondary transfer roller  27 , the traveling speed of the transfer belt  12  fluctuates. When the speed of the transfer belt  12  fluctuates in this way, blurs occur in toner images being transferred from the photoconductive drums  14 K,  14 C,  14 M, and  14 Y to the transfer belt  12 . 
     In  FIG. 15 , when the projection  70  passes through the secondary transfer roller  27 , an image of black (K) blurs in G 1  of the transfer belt  12 , an image of cyan (C) blurs in G 2  of the transfer belt  12 , an image of magenta (M) blurs in G 3  of the transfer belt  12 , and an image of yellow (Y) blurs in G 4  of the transfer belt  12 . In G 1  to G 4  of the transfer belt  12 , a toner image of one color in which a blur occurs and toner images of the remaining three colors cause a positional shift and deteriorates image quality. Therefore, a section from G 1  to G 4  of the transfer belt  12  is set as the NG area. 
     Detection of the NG area is explained below. For example, as shown in  FIG. 16 , a distance among the photoconductive drums  14 K,  14 C,  14 M, and  14 Y is represented as Ld (mm), a distance from the photoconductive drum  14 K for black (K) to the front pattern sensor  37  and the rear pattern sensor  38  is represented as Ls (mm), a distance from the front pattern sensor  37  and the rear pattern sensor  38  to the secondary transfer roller  27  is represented as Lr (mm), and a distance from the distal end of the belt marker  22  to the projection  70  is represented as Xb (mm). Ld, Ls, and Lr are peculiar values of the color printer  1 . Xb is different depending on the transfer belt  12 . 
     When the transfer belt  12  moves by [Lr+Xb] (mm) with reading of the distal end of the belt marker  22  by the belt sensor  23  as a start point, the projection  70  reaches the secondary transfer roller  27 . Traveling times of the transfer belt  12  respectively corresponding to the distances Ld (mm), Ls (mm), Lr (mm), and Xb (mm) are represented as tLd (sec), tLs (sec), tLr (sec), and tXb (sec). When traveling speed (process speed) of the transfer belt  12  is represented as Vd (mm/s), tLd=Ld/Vd (sec), tLs=Ls/Vd (sec), tLr=Lr/Vd (sec), and tXb=Xb/Vd (sec). 
     Time from detection of the belt marker  22  by the belt sensor  23  until the section from G 1  to G 4  as the NG area of the transfer belt  12  reaches the front pattern sensor  37  or the rear pattern sensor  38  is calculated and the section from G 1  to G 4  of the transfer belt  12  is detected. As shown in  FIG. 17 , the blur occurrence position G 1  of the image of black (K) is in a position of (tLr+tXb+tLs) (sec) after the detection of the belt marker  22  by the belt sensor  23 . The blur occurrence position G 2  of the image of cyan (C) is in a position of (tLr+tXb+tLs+tLd) (sec) after the detection of the belt marker  22  by the belt sensor  23 . The blur occurrence position G 3  of the image of the magenta (M) is in a position of (tLr+tXb+tLs+2tLd) (sec) after the detection of the belt marker  22  by the belt sensor  23 . The blur occurrence position G 4  of the image of yellow (Y) is in a position of (tLr+tXb+tLs+3tLd) (sec) after the detection of the belt marker  22  by the belt sensor  23 . 
     The CPU  101  determines a section from (tLr+tXb+tLs) (sec) to (tLr+tXb+tLs+3tLd) (sec) after the detection of the belt marker  22  by the belt sensor  23  as an NG area of the transfer belt  12  and stores the NG area in the memory  102  (ACT  308 ). 
     In ACT  310 , the calculating unit  103  calculates the image quality priority adjustment value (H 2 ). The calculating unit  103  excludes the NG pattern (the third pattern  53 ) found in ACT  307  from the eight sets of the adjustment patterns  50  from the first pattern  51  to the eighth pattern  58 . The calculating unit  103  calculates an average of the distance data of the remaining seven sets of the adjustment patterns  50  excluding the NG pattern and calculates the image quality priority adjustment value (H 2 ). The CPU  101  updates the adjustment value stored in the memory  102  to the image quality priority adjustment value (H 2 ) (ACT  311 ). 
     For example, the calculating unit  103  calculates an average (X) of the distance data (an)=a 1 =10 of the first pattern  51 , the distance data (an)=a 2 =11 of the second pattern  52 , and the distance data (an)=(a 4  to a 8 )=10 of the fourth pattern  54  to the eighth pattern  58  excluding the third pattern  53 . The average (X)=(the image quality priority adjustment value (H 2 ))=10.143. 
     The CPU  101  further checks the image quality priority adjustment value (H 2 ) updated in ACT  311 . The CPU  101  adjusts output timings of the lasers of the laser oscillators  21 K,  21 C,  21 M, and  21 Y using the adjustment value updated in ACT  311  and images the eight sets of the adjustment patterns  50  from the first pattern  51  to the eighth pattern  58  shown in  FIG. 3  on the entire circumference of the transfer belt  12  again (ACT  312 ). The CPU  101  detects the eight sets of the adjustment patterns  50  in the same manner as ACT  302  to ACT  304  (ACT  313 ) and calculates distance data among the toner images of the respective colors of the eight sets of the adjustment patterns  50  (ACT  314 ). The calculating unit  103  calculates an adjustment value for check from the calculated distance data (ACT  315 ). 
     The CPU  101  checks, from the calculated adjustment value for check, whether the distance among the toner images of the respective colors of the adjustment patterns  50  is within the tolerance of 10 mm as the theoretical reference value (ACT  316 ). If the CPU  101  repeats, a predefined number of times, operation for checking whether the distance among the toner images of the respective colors of the adjustment patterns  50  is within the tolerance of 10 mm as the theoretical reference value in ACT  312  to ACT  316  (Yes in ACT  317 ), the CPU  101  updates the adjustment value stored in the memory  102  to the adjustment value for check (ACT  318 ). The CPU  101  finishes the initial alignment adjustment in the image quality priority print mode. 
     When the color printer  1  finishes the initial alignment adjustment in the image quality priority print mode, the color printer  1  starts the image quality priority print mode shown in  FIG. 18 . In the image quality priority print mode, the color printer  1  performs image blur adjustment using the image quality priority adjustment value (H 2 ) as an initial adjustment value. During the image quality priority print mode, the CPU  101  sets output timings of the laser oscillators  21 K,  21 C,  21 M, and  21 Y for the color components of black (K), cyan (C), magenta (M), and yellow (Y) according to the image quality priority adjustment value (H 2 ) updated in ACT  318  (ACT  320 ). 
     The CPU  101  detects the belt marker  22  with the belt sensor  23  (ACT  321 ). The color printer  1  performs print processing to transfer image quality priority toner images onto a print OK area of the transfer belt  12  (Yes in ACT  322 ) (ACT  323 ). If a transfer area of the image quality priority toner images extends to the print NG area of the transfer belt  12  (the section from G 1  to G 4  of the transfer belt  12 ) (No in ACT  322 ), the color printer  1  stands by for the image quality priority print mode until the print NG area passes. The color printer  1  transfers, after the NG area of the transfer belt  12  passes, the image quality priority images onto the print OK area of the transfer belt  12  and performs the print processing (ACT  323 ). If image quality priority print finish conditions are not satisfied (No in ACT  324 ), the color printer  1  repeats ACT  320  to ACT  324 . When the color printer  1  finishes the image quality priority image print (Yes in ACT  324 ), the color printer  1  stands by for the next operation. 
     If the operator selects print while the color printer  1  stands by for the next operation (Yes in ACT  240 ), the color printer  1  switches to the print mode in the speed priority print mode. If the operator selects the image quality priority print mode (Yes in ACT  241 ), the color printer  1  switches from the speed priority print mode to the image quality priority print mode. 
     During the image quality priority print mode, the CPU  101  more highly accurately sets output timings of the laser oscillators  21 K,  21 C,  21 M, and  21 Y for the respective color components according to the image quality priority adjustment value (H 2 ). During the image quality priority print mode, the color printer  1  prints images avoiding the NG area of the transfer belt  12 . In the image quality priority print mode, since the NG area of the transfer belt  12  is excluded from a print area, print speed falls. However, in the image quality priority print mode, a higher definition print image in which an image blur is less likely to occur can be obtained. 
     While in the speed priority print mode, when the color printer  1  obtains a print image using the entire circumference of the transfer belt  12 , the color printer  1  may use the image quality priority adjustment value (H 2 ) as the initial adjustment value. If the color printer  1  uses the image quality priority adjustment value (H 2 ) as the initial adjustment value, the color printer  1  obtains a print image at high speed. Further, color printer  1  obtains a higher definition print image. 
     According to the second embodiment, in the initial alignment adjustment in the image quality priority print mode, the CPU  101  sets, as the NG pattern, the adjustment pattern having the largest difference between (adjustment value−1) and the distance data (an) among the eight sets of adjustment patterns imaged over the entire circumference of the transfer belt  12 . The CPU  101  calculates an average of the remaining seven sets of adjustment patterns excluding the NG pattern and obtains the image quality priority adjustment value (H 2 ). The CPU  101  adjusts output timings of the laser oscillators  21 K,  21 C,  21 M, and  21 Y according to the image quality priority adjustment value (H 2 ). The color printer  1  obtains more highly accurate alignment adjustment. 
     According to the second embodiment, in the image quality priority print mode, the color printer  1  prints images avoiding the NG area G 1  to G 4  of the transfer belt  12  in which a blur is likely to occur. In the image quality priority print mode, the color printer  1  obtains a higher definition print image in which an image blur is less likely to occur. 
     According to the second embodiment, in the speed priority print mode, the CPU  101  averages all distance data of eight sets of adjustment patterns imaged over the entire circumference of the transfer belt  12  and obtains the speed priority adjustment value (H 1 ). In the speed priority print mode, the color printer  1  prints images using the entire circumference of the transfer belt  12 . In the speed priority print mode, the color printer  1  obtains a print image at high speed. 
     While certain embodiments have been described these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel apparatus and methods described herein may be embodied in a variety of other forms: furthermore various omissions, substitutions and changes in the form of the apparatus and methods described herein may be made without departing from the spirit of the inventions. The accompanying claims and there equivalents are intended to cover such forms of modifications as would fall within the scope and spirit of the invention.