Abstract:
An image forming apparatus includes: a plurality of image forming units as defined herein; an image carrier as defined herein; a first operation controlling unit as defined herein; a second operation controlling unit as defined herein; a density detecting sensor that detects densities of the images formed on the image carrier; a measuring unit that binarizes a density detection output of the image position deviation correction patterns formed on the image carrier, from the density detecting sensor, and that measures intervals between color patterns of the image position deviation correction patterns, based on the binarized output; and a position deviation correction controlling unit that performs a position deviation correcting control of, when the intervals measured by the measuring unit and of the color patterns in the image position deviation correction patterns are deviated from a predetermined range, correcting position deviation of the toner images formed by the image forming units.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-050650 filed on Mar. 4, 2009. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an image forming apparatus. 
     2. Related Art 
     As an image forming apparatus which performs color printing, there is a tandem type apparatus in which image forming sections (photosensitive members, exposing devices, and the like) that transfer toner images of colors (Y (yellow), M (magenta), C (cyan), and K (black)) to a sheet are provided respectively for the colors. 
     In the tandem type image forming apparatus, the respective image forming sections perform a printing operation on one sheet to form a color image configured by toner images of the four colors. 
     When transfer timings of the image forming sections are caused to be delicately deviated from one another by a temperature rise in the apparatus, a temporal change, or the like, therefore, so-called misregistration in which only a specific color is printed while being deviated in one image occurs. 
     As a registration adjusting technique for correcting the misregistration, known is a technique in which each image forming section forms a registration adjustment patch (hereinafter, referred to as regi adjustment patch) configured by a toner image on a transfer belt, the regi adjustment patches for the colors on the transfer belt are detected by a sensor, the amount of deviating from a reference position is calculated, and the writing timing of the image forming section or the like is corrected so as to eliminate the misregistration amount. 
     SUMMARY 
     However, a density sensor which detects a regi adjustment patch has a low responsibility. When the printing speed is high and a transfer/conveyor belt on which a patch is formed is moved at a high speed, therefore, the output of a sensor which measures the patch often fails to have a value necessary for detecting the patch. 
     As shown in  FIG. 10 , in the case the output of the sensor which measures the patch cannot obtain a sufficient value which is equal to or larger than a threshold, as in Y (yellow) (reference numeral  1001 ) or C (cyan) (reference numeral  1002 ), when the output value is binarized, the patch cannot detected. 
     According to an aspect of the invention, there is provided an image forming apparatus including: a plurality of image forming units which form toner images of different colors, respectively; an image carrier onto which the color toner images respectively formed by the image forming units are transferred, and which carries the color toner images; a first operation controlling unit which performs a first operation control of driving the image forming units and the image carrier in accordance with a printing speed that is selectively designated from a plurality of printing speeds, and transferring the color toner images formed by the image forming units to a recording medium; a second operation controlling unit which performs a second operation control of activating a position deviation correction mode where position deviations of the toner images formed by the image forming units are corrected, driving the image forming units and the image carrier, and forming image position deviation correction patterns of the colors on the image carrier, the patterns having a size and axis-to-axis distance that correspond to the currently set printing speed; a density detecting sensor which detects densities of the images formed on the image carrier; a measuring unit which binarizes a density detection output of the image position deviation correction patterns formed on the image carrier, from the density detecting sensor, and which measures intervals between color patterns of the image position deviation correction patterns, based on the binarized output; and a position deviation correction controlling unit which performs a position deviation correcting control of, when the intervals measured by the measuring unit and of the color patterns in the image position deviation correction patterns are deviated from a predetermined range, correcting position deviation of the toner images formed by the image forming units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a diagram showing a functional configuration of an image forming apparatus  1 ; 
         FIG. 2  is a view showing a process of calculating misregistration amounts from regi adjustment patches; 
         FIGS. 3A and 3B  are views showing respectively regi adjustment patches for a high-speed mode and regi adjustment patches for a low-speed mode; 
         FIG. 4  is a view showing the lengths and intervals of regi adjustment patches for the high- and low-speed modes; 
         FIG. 5  is a flowchart showing a process in the image forming apparatus  1  (Example 1); 
         FIGS. 6A and 6B  are views showing a process of obtaining a misregistration amount (Example 1); 
         FIGS. 7A to 7C  are views showing the lengths and intervals of regi adjustment patches corresponding to printing speeds; 
         FIG. 8  is a flowchart showing a process in the image forming apparatus  1  (Example 2); 
         FIGS. 9A to 9C  are views showing a process of obtaining a misregistration amount (Example 2) (all references for ( 1/1)V, (½)V, and (⅓)V are equal to one another); and 
         FIG. 10  is a view showing a sensor output in which conventional patch detection fails. 
     
    
    
     DESCRIPTION OF REFERENCE NUMBERALS AND SIGNS 
     
         
           1  image forming apparatus 
           7  exposing device 
           11  transfer/conveyor belt 
           14  controlling portion 
           18  binarizing portion 
           19  patch interval measuring portion 
           20  registration adjusting portion 
           21  image writing controlling portion 
           22  density sensor 
           23  setting information storing portion 
       
    
     DETAILED DESCRIPTION 
     Hereinafter, examples of the invention will be described in detail with reference to the accompanying drawings. 
     First, an image forming apparatus  1  will be described with reference to  FIG. 1 . 
       FIG. 1  is a diagram showing a functional configuration of the image forming apparatus  1 . 
     As shown in  FIG. 1 , the image forming apparatus  1  has an operating/displaying section  2 , a sheet feeding tray  3 , a pickup roller  4 , photosensitive members  5 , charging devices  6 , exposing devices  7 , developing devices  8 , transferring devices  9 , cleaners  10 , a transfer/conveyor belt  11 , a fixing device  12 , a control board  13 , and a density sensor  22 . 
     The image forming apparatus  1  is a printing apparatus which prints a print job that is received from the outside. Alternatively, the image forming apparatus may be a multifunction apparatus having functions of, for example, forming an image of copied data on a sheet, and forming an image of facsimile data on a sheet. 
     The operating/displaying section  2  is configured by a liquid crystal touch panel, displays information to the user, and receives instructions from the user. 
     The sheet feeding tray  3  stores sheets which are to be used in the printing in the image forming apparatus  1 . 
     The pickup roller  4  feeds the sheets stored in the sheet feeding tray  3 , to a conveying path. 
     In each of the photosensitive members  5 , an electrostatic latent image formed by the corresponding exposing device  7  is developed to a toner image by the corresponding developing device  8 . 
     The toner image formed on the photosensitive member  5  is transferred to a sheet conveyed on the transfer/conveyor belt  11 , by the corresponding transferring device  9 . 
     The toner which has not been transferred to the sheet to remain on the photosensitive member  5  is cleaned by the corresponding cleaner  10 . 
     The image forming apparatus  1  is a tandem type apparatus, and, as shown in  FIG. 1 , four kinds of image forming sections each configured by the photosensitive member  5 , the charging device  6 , the exposing device  7 , the developing device  8 , the transferring device  9 , and the cleaner  10  are used dedicatedly for Y (yellow), M (magenta), C (cyan), and K (black), respectively. 
     The toner images of Y (yellow). M (magenta), C (cyan), and K (black) are sequentially transferred from the photosensitive members  5  for respective colors onto the sheet conveyed by the transfer/conveyor belt  11 . 
     With respect to regi adjustment patches, the patch for Y (yellow) is transferred from the photosensitive member  5  for Y (yellow) to the transfer/conveyor belt  11 , the patch for M (magenta) is transferred from the photosensitive member  5  for M (magenta) to the transfer/conveyor belt  11 , the patch for C (cyan) is transferred from the photosensitive member  5  for C (cyan) to the transfer/conveyor belt  11 , and the patch for K (black) is transferred from the photosensitive member  5  for K (black) to the transfer/conveyor belt  11 . 
     The regi adjustment patches transferred to the transfer/conveyor belt  11  are moved below the density sensor  22  in accordance with the movement of the transfer/conveyor belt  11 , and measured by the density sensor  22 . 
     The sheet onto which the toner images are transferred is sent to the fixing device  12 , the toner images are fixed to the sheet, and then the sheet is discharged to the outside of the apparatus. 
     The control board  13  includes a controlling portion  14 , a ROM (Read Only Memory)  15 , a RAM (Random Access Memory)  16 , an image producing portion  17 , a binarizing portion  18 , a patch interval measuring portion  19 , a registration adjusting portion  20 , an image writing controlling portion  21 , and a setting information storing portion  23 . 
     The controlling portion  14  is configured by a CPU (Central Processing Unit), and generally controls the image forming apparatus  1 . 
     The ROM  15  stores firmware for operating the image forming apparatus  1 , and the like. 
     The RAM  16  functions as work area for storing various kinds of information such as system data for controlling the operation of the apparatus. 
     The image producing portion  17  produces a bitmap image for producing an image on a sheet. 
     The binarizing portion  18  receives data detected from the regi adjustment patches, from the density sensor  22 , binarizes the data with respect to a predetermined threshold, and sends the binarized data to the patch interval measuring portion  19 . 
     The patch interval measuring portion  19  measures patch intervals of the regi adjustment patches on the transfer/conveyor belt, on the basis of the data sent from the binarizing portion  18 . 
     Based on the regi adjustment patch intervals measured by the patch interval measuring portion  19 , the registration adjusting portion  20  calculates misregistration amounts respectively indicating the degrees by which the intervals of the regi adjustment patches deviate from a predetermined reference value, and controls the image writing controlling portion  21  so as to eliminate the misregistration amounts. 
     By the control which is performed on the image writing controlling portion  21  by the registration adjusting portion  20 , a control of the writing timing of the exposing device  7  dedicated for the color to be adjusted, and the like is performed, so that the deviation of the writing timing of the color is corrected. 
     In order to allow the bitmap image produced by the image producing portion  17  to be printed, the image writing controlling portion  21  controls electrostatic-latent image writings in which the exposing devices  7  dedicated for the respective colors form electrostatic latent images on the photosensitive members  5  dedicated for the respective colors. 
     The setting information storing portion  23  stores data related to the registration adjustment, such as the sizes of the regi adjustment patches, the reference value of patch intervals, etc. 
     The density sensor  22  measures the density of a patch which is formed on the transfer/conveyor belt, and may be configured by, for example, an optical density sensor which receives reflected light to measure the density. 
     The density sensor  22  is used also for measuring the density of a patch for process control adjustment. 
     In the thus configured image forming apparatus  1 , there is a case where the timings of writing the electrostatic latent images onto the photosensitive members  5  are delicately deviated from one another by a temperature rise in the image forming apparatus  1 , a temporal change, or the like, and hence so-called misregistration in which only a specific color is printed while being deviated occurs. In order to correct the misregistration, color registration adjustment is performed. 
     For example, the color registration adjustment is performed in the following manner. The regi adjustment patches for Y (yellow), M (magenta), C (cyan), and K (black) are formed on the transfer/conveyor belt  11  by image producing sections for the respective colors. The regi adjustment patches are measured by the density sensor  22 . The amounts by which the intervals of the measured regi adjustment patches deviate from the reference value are calculated. The registration adjusting portion  20  controls the writing timings of the image writing controlling portion  21  so as to eliminate the misregistration amounts. 
     Next, the process of calculating the misregistration amounts from the regi adjustment patches which are formed on the transfer/conveyor belt  11  in the image forming apparatus  1  will be described in more detail with reference to  FIG. 2 . 
     In  FIG. 2 , (a) is a diagram of the regi adjustment patches formed on the transfer/conveyor belt  11 , (b) is a graph showing data output from the density sensor, and (c) is a graph showing data which are obtained by binarizing the data output from the density sensor  22 , in the binarizing portion  18 . 
     In the case where color registration adjustment is necessary, the exposing devices  7  respectively dedicated for the colors write electrostatic latent images of regi adjustment patches onto the respective photosensitive members  5  of a predetermined timing, toner images are developed, and regi adjustment patches for Y (yellow), M (magenta), C (cyan), and K (black) are formed on the transfer/conveyor belt  11  ((a)  FIG. 2 ). 
     The image writing controlling portion  21  controls the timings when the exposing devices  7  write the electrostatic latent images so that the intervals of the color regi adjustment patches formed on the transfer/conveyor belt  11  coincide with the predetermined reference value. 
     The regi adjustment patches formed on the transfer/conveyor belt  11  are measured by the density sensor  22  in accordance with the movement of the transfer/conveyor belt  11 . 
     Analog data which are measured by the density sensor  22  as shown in (b) of  FIG. 2  are supplied to the binarizing portion  18 . 
     The data supplied to the binarizing portion  18  are binarized with respect to the threshold, so that portions where a regi adjustment patch exceeding the predetermined threshold is detected are set to “1” in binary notation, and the other portions are set to “0”. 
     On the basis of the binary data which are output from the binarizing portion  18  in this way, as shown in (c) of  FIG. 2 , for example, the length from the position where the regi adjustment patch for K (black) starts to be written to that where the regi adjustment patch for Y (yellow) starts to be written is measured. 
     When the length from the position where the regi adjustment patch for K (black) starts to be written to that where the regi adjustment patch for Y (yellow) starts to be written is measured as shown in (c) of  FIG. 2 , the measurement value is compared with the initially assumed reference value, and the difference between the measured value and the reference value is calculated as a misregistration amount. 
     The registration adjusting portion  20  adjusts the image writing controlling portion  21  so that the misregistration amount by which the measurement value is deviated from the reference value is corrected, thereby performing the color registration adjustment. 
     In the image forming apparatus  1 , a plurality of printing speeds can be set. In the case where printing on a large number of sheets is performed for a short time period, for example, the printing speed is set to be high, and, in the case where priority is given to the printing quality than the printing speed, the printing speed is set to be low. 
     The printing speed can be changed by, for example, changing the moving speeds of the transfer/conveyor belt  11  and the photosensitive members  5 . 
     Hereinafter, the image forming apparatus  1  will be described by way of illustrating Examples 1 and 2 which are different from each other in kind of the regi adjustment patches formed on the transfer/conveyor belt  11 . 
     In Example 1, the image forming apparatus  1  will be described in which regi adjustment patches are formed in different manners in cases where the printing speed is set to a high-speed mode and the moving speed of the transfer/conveyor belt  11  is high, and where the printing speed is set to a low-speed mode and the moving speed of the transfer/conveyor belt  11  is low. 
     In Example 2, the image forming apparatus  1  will be described in which, when the maximum moving speed of the transfer/conveyor belt  11  is indicated by V, there exist regi adjustment patches in the case where the moving speed of the transfer/conveyor belt  11  is V, those in the case where the moving speed of the transfer/conveyor belt  11  is (½)V, and those in the case where the moving speed of the transfer/conveyor belt  11  is (⅓)V. 
     EXAMPLE 1 
     The image forming apparatus  1  in which there are regi adjustment patches for the high-speed mode where the printing speed is high, and those for the low-speed mode where the printing speed is low will be described. 
     The regi adjustment patches in the case where the printing speed is set to the high-speed mode (the case where the transfer/conveyor belt  11  is moved at a high speed), and those in the case where the printing speed is set to the low-speed mode (the case where the transfer/conveyor belt  11  is moved at a low speed) will be summarily described with reference to  FIGS. 3A and 3B . 
       FIGS. 3A and 3B  are diagrams showing the regi adjustment patches for the high- and low-speed modes,  FIG. 3A  is a diagram showing the regi adjustment patches for the high-speed mode in the case where the printing speed is set to the high-speed mode, and  FIG. 3B  is a diagram showing those for the low-speed mode in the case where the printing speed is set to the low-speed mode. 
     As shown in  FIGS. 3A and 3B , the regi adjustment patches for the high-speed mode are formed so that their lengths are longer in the sheet conveying direction (the arrow  301 ) than those of the regi adjustment patches for the low-speed mode, in order that, even when the transfer/conveyor belt  11  is moved at a higher speed, the patches are detected by the binarizing portion  18 . 
     As described above, the regi adjustment patches for the high-speed mode are formed longer than those for the low-speed mode. Even when the printing speed of the image forming apparatus  1  is high and the transfer/conveyor belt  11  is moved at a higher speed, therefore, the output of the density sensor which measures the regi adjustment patches has a value sufficient for patch detection which is performed after the data are binarized in the binarizing portion  18 . 
     Next, the regi adjustment patches for the high- and low-speed modes will be described in more detail with reference to  FIG. 4 . 
       FIG. 4  is a diagram showing the lengths of the regi adjustment patches for the high- and low-speed modes. 
     As shown in  FIG. 4 , in the regi adjustment patches for the high-speed mode, the length of a patch of K (black) in the sheet conveying direction is indicated by Dk(a), and the patch interval between the patch of K (black) and a patch of Y (yellow) is indicated by Dk_y(a). In the regi adjustment patches for the low-speed mode, the length of a patch of K (black) in the sheet conveying direction is indicated by Dk(b), and the patch interval between the patch of K (black) and a patch of Y (yellow) is indicated by Dk_y(b). 
     As shown in  FIG. 4 , the regi adjustment patches for the high- and low-speed modes have relationships of “Dk(a)&gt;Dk(b)” and “Dk_y(a)&gt;Dk_y(b)”. 
     The above-described relationships between the regi adjustment patches for the high- and low-speed modes similarly hold not only for K (black) and Y (yellow), but also for K (black) and M (magenta), and K (black) and C (cyan). 
     The lengths Dk(a) and Dk(b) and the intervals Dk_y(a) and Dk y(b) are configured so that, in accordance with the moving speed (printing speed) of the transfer/conveyor belt  11  and the response speed of the density sensor  22 , an amplitude sufficient for detection of a regi adjustment patch in the binarizing portion  18  can be obtained from the output value of the density sensor. Also with respect to the regi adjustment patches for Y (yellow), M (magenta), and C (cyan), and the patch intervals between K (black) and M (magenta), and K (black) and C (cyan), the lengths and intervals of the regi adjustment patches are similarly configured so that, in accordance with the moving speed (printing speed) of the transfer/conveyor belt  11  and the response speed of the density sensor  22 , an amplitude sufficient for detection of a regi adjustment patch in the binarizing portion  18  can be obtained from the output value of the density sensor  22 . 
     Next, a process of performing the registration adjustment in the image forming apparatus  1  will be described with reference to  FIG. 5 . 
       FIG. 5  is a flowchart showing the process of the registration adjustment which is performed in the image forming apparatus  1 . 
     Upon receiving a request for the registration adjustment, the controlling portion  14  activates a registration adjustment mode, and checks whether the speed of printing which is performed in the image forming apparatus  1  is in the high-speed mode or in the low-speed mode (step  501 ). 
     If it is checked that the printing speed is in the high-speed mode (HIGH-SPEED MODE in step  501 ), the image writing controlling portion  21  controls the exposing device  7  so as to form the regi adjustment patches for the high-speed mode on the transfer/conveyor belt  11  (step  502 ). 
     Then, the regi adjustment patches for the high-speed mode formed on the transfer/conveyor belt  11  are measured by the density sensor  22 , and the data measured by the density sensor  22  are binarized in the binarizing portion  18 , and thereafter sent to the patch interval measuring portion  19 . 
     In the patch interval measuring portion  19 , it is determined whether the used regi adjustment patches are patches for the high-speed mode or those for the low-speed mode (step  504 ). 
     If the regi adjustment patches for the high-speed mode are used, as shown in  FIG. 6A , the patch interval measuring portion  19  compares the measurement value (reference numeral  601 ) which is in the data sent from the binarizing portion  18 , and which is obtained by measuring the length from the position where K (black) starts to be written to that where Y (yellow) starts to be written, with the reference value (reference numeral  602 ) for the high-speed mode to calculate the misregistration amount (reference numeral  603 ) (step  505 ). 
     The misregistration amount is calculated not only with respect to K (black) and Y (yellow), but also with respect to K (black) and M (magenta), and K (black) and C (cyan), as far as a misregistration amount exists therebetween. 
     The misregistration amount calculated by the patch interval measuring portion  19  is sent to the registration adjusting portion  20 . The registration adjusting portion  20  performs controls such as that the writing timing of the image writing controlling portion  21  is adjusted so as to eliminate the misregistration amount (step  506 ). 
     If it is checked in step  501  that the printing speed is set to the low-speed mode (LOW-SPEED MODE in step  501 ), the image writing controlling portion  21  controls the exposing device  7  so as to form the regi adjustment patches for the low-speed mode on the transfer/conveyor belt  11  (step  507 ). 
     Then, the regi adjustment patches for the low-speed mode formed on the transfer/conveyor belt  11  are measured by the density sensor  22 , and the data measured by the density sensor  22  are binarized in the binarizing portion  18 , and thereafter sent to the patch interval measuring portion  19 . 
     In the patch interval measuring portion  19 , it is determined whether the used regi adjustment patches are patches for the high-speed mode or those for the low-speed mode (step  504 ). If the regi adjustment patches for the low-speed mode are used, as shown in  FIG. 6B , the patch interval measuring portion  19  compares the measurement value (reference numeral  604 ) which is in the data sent from the binarizing portion  18 , and which is obtained by measuring the length from the position where K (black) starts to be written to that where Y (yellow) starts to be written, with the reference value (reference numeral  605 ) for the low-speed mode to calculate the misregistration amount (reference numeral  606 ) (step  508 ). 
     The misregistration amount is calculated not only with respect to K (black) and Y (yellow), but also with respect to K (black) and M (magenta), and K (black) and C (cyan), as far as a misregistration amount exists therebetween. 
     The misregistration amount calculated by the patch interval measuring portion  19  is sent to the registration adjusting portion  20 . The registration adjusting portion  20  performs controls such as that the writing timing of the image writing controlling portion  21  is adjusted so as to eliminate the misregistration amount (step  506 ). 
     The adjustment of eliminating the misregistration amount calculated from the regi adjustment patches for the high-speed mode can be reflected also in the printing process in the low-speed mode, and that of eliminating the misregistration amount calculated from the regi adjustment patches for the low-speed mode can be reflected also in the printing process in the high-speed mode. 
     The setting information storing portion  23  previously stores: the lengths Dk(a), Dy(a), Dm(a), and Dc(a) of K (black), Y (yellow), M (magenta), and C (cyan) of the regi adjustment patches for the high-speed mode; in the regi adjustment patches for the high-speed mode, the patch interval Dk_y(a) between K (black) and Y (yellow), the patch interval Dk_m(a) between K (black) and M (magenta), and the patch interval Dk_c(a) between K (black) and C (cyan); the lengths Dk(b), Dy(b), Dm(b), and Dc(b) of K (black), Y (yellow), M (magenta), and C (cyan) of the regi adjustment patches for the low-speed mode; and, in the regi adjustment patches for the low-speed mode, the patch interval Dk_y(b) between K (black) and Y (yellow), the patch interval Dk_m(b) between K (black) and M (magenta), and the patch interval Dk_c(b) between K (black) and C (cyan). 
     In step  504 , alternatively, it may not be determined whether the used regi adjustment patches are patches for the high-speed mode or those for the low-speed mode, but instead it may be determined whether the printing speed of the image forming apparatus  1  is set to the high-speed mode or the low-speed mode. If the printing speed is set to the high-speed mode, it may be determined that the patches for the high-speed mode are used, and the process may then proceed to step  505 , and, if the printing speed is set to the low-speed mode, it may be determined that the patches for the low-speed mode are used, and the process may then proceed to step  508 . 
     Although the image forming apparatus  1  has been described as a tandem type image forming apparatus using the direct transfer system, a tandem type image forming apparatus using the secondary transfer system may be used. 
     EXAMPLE 2 
     The image forming apparatus  1  will be described in which, in the case where the printing speed of the image forming apparatus  1  is deemed as the moving speed of the transfer/conveyor belt  11  and the maximum moving speed of the transfer/conveyor belt  11  is indicated by V, there exist regi adjustment patches in the case where the moving speed of the transfer/conveyor belt  11  is V, those in the case where the moving speed of the transfer/conveyor belt  11  is (½)V, and those in the case where the moving speed of the transfer/conveyor belt  11  is (⅓)V, according to a change of the printing speed. 
       FIGS. 7A to 7C  are diagrams showing regi adjustment patches in the respective printing speeds.  FIG. 7A  is a diagram showing the regi adjustment patches in the case where the moving speed of the transfer/conveyor belt  11  is V,  FIG. 7B  is a diagram showing those in the case where the moving speed of the transfer/conveyor belt  11  is (½)V, and  FIG. 7C  is a diagram showing those in the case where the moving speed of the transfer/conveyor belt  11  is (⅓)V. 
     As shown in  FIGS. 7A to 7C , in the regi adjustment patches, the length of a patch of K (black) in the sheet conveying direction in the case where the moving speed of the transfer/conveyor belt  11  is maximum or V is indicated by Dk(c), that of a patch of K (black) in the sheet conveying direction in the case where the moving speed of the transfer/conveyor belt  11  is (½)V is indicated by Dk(d), and that of a patch of K (black) in the sheet conveying direction in the case where the moving speed of the transfer/conveyor belt  11  is (⅓)V is indicated by Dk(e). 
     Then, Dk(c), Dk(d), and Dk(e) have relationships of “Dk(d)=[Dk(c)/2]” and “Dk(e)=[Dk(c)/3]”. 
     In the case where the moving speed of the transfer/conveyor belt  11  is V′, the length of a patch of K (black) of the regi adjustment patches in the sheet conveying direction is indicated by Dk(x), and Dk(c) and Dk(x) have a relationship of “Dk(x)=(V′/V)×Dk(c)”. 
     As described above, the moving speed (printing speed) of the transfer/conveyor belt  11  is in proportional relationship with the length of a patch of K (black) in the regi adjustment patches in the sheet conveying direction. 
     The above-described relationships between the moving speed (printing speed) of the transfer/conveyor belt  11  and the lengths of the regi adjustment patches hold not only for K (black)), but also for Y (yellow), M (magenta), and C (cyan). 
     As shown in  FIGS. 7A to 7C , in the case where the moving speed of the transfer/conveyor belt  11  is maximum or V, the interval between the regi adjustment patch of K (black) and that of Y (yellow) is indicated by Dk_y(c), and, in the case where the moving speed of the transfer/conveyor belt  11  is (½)V, the interval between the regi adjustment patch of K (black) and that of Y (yellow) is indicated by Dk_y(d), and, in the case where the moving speed of the transfer/conveyor belt  11  is (⅓)V, the interval between the regi adjustment patch of K (black) and that of Y (yellow) is indicated by Dk_y(e). 
     Then, Dk_y(c), Dk_y(d), and Dk_y(e) have relationships of “Dk_y(d)=[Dk_y(c)]/2” and “Dk_y(e)=[Dk_y(c)]/3”. 
     In the case where the moving speed of the transfer/conveyor belt  11  is V′, the interval between the regi adjustment patch of K (black) and that of Y (yellow) is indicated by Dk_y(x), and Dk_y(c) and Dk_y(x) have a relationship of “Dk_y(x)=(V′/V)×Dk_y(c)”. 
     As described above, the moving speed (printing speed) of the transfer/conveyor belt  11  is in proportional relationship with the interval between the regi adjustment patch of K (black) and that of Y (yellow). 
     The above-described relationships between the moving speed (printing speed) of the transfer/conveyor belt  11  and the interval between the regi adjustment patches hold not only for K (black) and Y (yellow), but also for K (black) and M (magenta), and K (black) and C (cyan). 
     As described above, when the printing speed is reduced to ½ or ⅓, also the lengths and intervals of the regi adjustment patches are reduced to ½ or ⅓. 
     The length (Dk(c)) and interval (Dk_y(c)) of the regi adjustment patches in case where the moving speed of the transfer/conveyor belt  11  is maximum or V are configured so that, in accordance with the moving speed V of the transfer/conveyor belt  11  and the response speed of the density sensor  22 , an amplitude sufficient for detection of a regi adjustment patch in the binarizing portion  18  can be obtained from the output value of the density sensor  22 . Also with respect to the regi adjustment patches for Y (yellow), M (magenta), and C (cyan), and the patch intervals between K (black) and M (magenta), and K (black) and C (cyan), the lengths and intervals of the regi adjustment patches in case where the moving speed of the transfer/conveyor belt  11  is maximum or V are similarly configured so that, in accordance with the moving speed V of the transfer/conveyor belt  11  and the response speed of the density sensor  22 , an amplitude sufficient for detection of a regi adjustment patch in the binarizing portion  18  can be obtained from the output value of the density sensor  22 . 
     These lengths and intervals of regi adjustment patches in the sheet conveying direction corresponding to the moving speed of the transfer/conveyor belt  11  are previously stored in the setting information storing portion  23 . 
     Next, a process of performing the registration adjustment in the image forming apparatus  1  will be described with reference to  FIG. 8 . 
       FIG. 8  is a flowchart showing the process of the registration adjustment which is performed in the image forming apparatus  1 . 
     Upon receiving a request for the registration adjustment, the controlling portion  14  activates a registration adjustment mode, and checks whether, when the maximum speed is indicated by V, the speed of the printing which is currently performed in the image forming apparatus  1  is in a V mode, a (½)V mode, or a (⅓)V mode. 
     The V mode, the (½)V mode, and the (⅓)V mode are previously set in the image forming apparatus  1 , and regi adjustment patch lengths and intervals corresponding to the speeds of the modes are previously stored in the setting information storing portion  23 . 
     If it is checked by the controlling portion  14  that the printing speed is set to the V mode (V MODE in step  801 ), the image writing controlling portion  21  controls the exposing device  7  so that the regi adjustment patches for the moving speed V of the transfer/conveyor belt  11  are formed on the transfer/conveyor belt  11  (step  802 ). 
     Then, the regi adjustment patches for the moving speed V of the transfer/conveyor belt  11  which are formed on the transfer/conveyor belt  11  are measured by the density sensor  22 , and the data measured by the density sensor  22  are binarized in the binarizing portion  18 , and thereafter sent to the patch interval measuring portion  19  (step  803 ). 
     As shown in  FIG. 9A , the patch interval measuring portion  19  compares the measurement value (reference numeral  901 ) which is in the data sent from the binarizing portion  18 , and which is obtained by measuring the length from the position where K (black) starts to be written to that where Y (yellow) starts to be written, with a reference value (reference numeral  902 ) which is common for the all modes (the V mode, the (½)V mode, and the (⅓)V mode) to calculate the misregistration amount (reference numeral  903 ) (step  804 ). 
     The misregistration amount is calculated not only with respect to K (black) and Y (yellow), but also with respect to K (black) and M (magenta), and K (black) and C (cyan), as far as a misregistration amount exists therebetween. 
     The misregistration amount calculated by the patch interval measuring portion  19  is sent to the registration adjusting portion  20 . The registration adjusting portion  20  performs controls such as that the writing timing of the image writing controlling portion  21  is adjusted so as to eliminate the misregistration amount (step  805 ). 
     If it is checked in step  801  that the printing speed is set to the (½)V mode ((½)V MODE in step  801 ), the image writing controlling portion  21  controls the exposing device  7  so that the regi adjustment patches for the moving speed (½)V of the transfer/conveyor belt  11  are formed on the transfer/conveyor belt  11  (step  806 ). 
     Then, the regi adjustment patches for the moving speed (½)V formed on the transfer/conveyor belt  11  are measured by the density sensor  22 , and the data measured by the density sensor  22  are binarized in the binarizing portion  18 , and thereafter sent to the patch interval measuring portion  19  (step  803 ). 
     As shown in  FIG. 9B , the patch interval measuring portion  19  compares the measurement value (reference numeral  904 ) which is in the data sent from the binarizing portion is, and which is obtained by measuring the length from the position where K (black) starts to be written to that where Y (yellow) starts to be written, with the reference value (reference numeral  905 ) which is common for the all modes (the V mode, the (½)V mode, and the (⅓)V mode) to calculate the misregistration amount (reference numeral  906 ) (step  804 ). 
     The misregistration amount is calculated not only with respect to K (black) and Y (yellow), but also with respect to K (black) and M (magenta), and K (black) and C (cyan), as far as a misregistration amount exists therebetween. 
     The misregistration amount calculated by the patch interval measuring portion  19  is sent to the registration adjusting portion  20 . The registration adjusting portion  20  performs controls such as that the writing timing of the image writing controlling portion  21  is adjusted so as to eliminate the misregistration amount (step  805 ). 
     If it is checked in step  801  that the printing speed is set to the (⅓)V mode ((⅓)V MODE in step  801 ), the image writing controlling portion  21  controls the exposing device  7  so that the regi adjustment patches for the moving speed (⅓)V of the transfer/conveyor belt  11  are formed on the transfer/conveyor belt  11  (step  807 ). 
     Then, the regi adjustment patches for the moving speed (⅓)V formed on the transfer/conveyor belt  11  are measured by the density sensor  22 , and the data measured by the density sensor  22  are binarized in the binarizing portion  18 , and thereafter sent to the patch interval measuring portion  19  (step  803 ). 
     As shown in  FIG. 9C , the patch interval measuring portion  19  compares the measurement value (reference numeral  907 ) which is in the data sent from the binarizing portion  18 , and which is obtained by measuring the length from the position where K (black) starts to be written to that where Y (yellow) starts to be written, with the reference value (reference numeral  908 ) which is common for the all modes (the V mode, the (½)V mode, and the (⅓)V mode) to calculate the misregistration amount (reference numeral  909 ) (step  804 ). 
     The misregistration amount is calculated not only with respect to K (black) and Y (yellow), but also with respect to K (black) and M (magenta), and K (black) and C (cyan), as far as a misregistration amount exists therebetween. 
     The misregistration amount calculated by the patch interval measuring portion  19  is sent to the registration adjusting portion  20 . The registration adjusting portion  20  performs controls such as that the writing timing of the image writing controlling portion  21  is adjusted so as to eliminate the misregistration amount (step  805 ). 
     The control in which the misregistration amount is calculated from the regi adjustment patches in the case where the moving speed of the transfer/conveyor belt  11  is V and the writing timing or the like is controlled can be reflected also in the printing process in another printing speed. 
     Moreover, the control in which the misregistration amount is calculated from the regi adjustment patches in the case where the moving speed of the transfer/conveyor belt  11  is (½)V and the writing timing or the like is controlled can be reflected also in the printing process in another printing speed. 
     Furthermore, the control in which the misregistration amount is calculated from the regi adjustment patches in the case where the moving speed of the transfer/conveyor belt  11  is (⅓)V and the writing timing or the like is controlled can be reflected also in the printing process in another printing speed. 
     The control in which the misregistration amount is calculated from the regi adjustment patches corresponding to the moving speed of the transfer/conveyor belt  11  and the writing timing or the like is controlled can be reflected also in the printing process in another printing speed. 
     Alternatively, the registration adjustment may be performed in the following manner. The lengths and intervals of regi adjustment patches corresponding to the moving speed of the transfer/conveyor belt  11  are not previously stored in the setting information storing portion  23 . On the basis of the moving speed V″ of the transfer/conveyor belt  11 , the length of a regi adjustment patch of K (black) is calculated as “(V″/V)×Dk(c)”, and the patch interval between the patch of K (black) and the patch of Y (yellow) is calculated as “(V″/V)×Dk_y(c)”. The lengths of regi adjustment patches for the other colors and the patch intervals are similarly calculated. Regi adjustment patches are formed on the basis of the calculated values. Also in the alternative, the registration adjustment is performed by conducting the processes of steps  803 ,  804 , and  805 . 
     Although the image forming apparatus  1  has been described as a tandem type image forming apparatus using the direct transfer system, a tandem type image forming apparatus using the secondary transfer system may be used. 
     The invention is useful in an image forming apparatus. 
     The foregoing description of the embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obyiously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention defined by the following claims and their equivalents.