Abstract:
An image-forming apparatus includes: a forming unit that forms an image on an image holder that is a rotating body holding an image; a measuring unit that measures a rotation amount of the image holder with reference to a certain position on the image holder; a determining unit that: identifies a position on the image holder from which an image is to be formed by the forming unit, according to the rotation amount measured by the measuring unit; if, while a plurality of images are successively formed on the image holder, an image for density correction is to be formed, postpones forming of the image for density correction until a timing arrives at which one image of the plurality of images is to be formed from a predetermined position on the image holder, and when the timing arrives at which the one image of the plurality of images is to be formed from the predetermined position on the image holder, determines to start forming of the image for density correction from the predetermined position, instead of forming the one image of the plurality of images; and a correction unit that obtains a reading result of the image for density correction formed by the forming unit from the predetermined position, and corrects a density of at least one of the plurality of images on the basis of the obtained reading result.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-152112 filed on Jun. 26, 2009. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an image-forming apparatus and an image-forming method. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image-forming apparatus including: a forming unit that forms an image on an image holder that is a rotating body holding an image; a measuring unit that measures a rotation amount of the image holder with reference to a certain position on the image holder; a determining unit that: identifies a position on the image holder from which an image is to be formed by the forming unit, according to the rotation amount measured by the measuring unit; if, while a plurality of images are successively formed on the image holder, an image for density correction is to be formed, postpones forming of the image for density correction until a timing arrives at which one image of the plurality of images is to be formed from a predetermined position on the image holder, and when the timing arrives at which the one image of the plurality of images is to be formed from the predetermined position on the image holder, determines to start forming of the image for density correction from the predetermined position, instead of forming the one image of the plurality of images; and a correction unit that obtains a reading result of the image for density correction formed by the forming unit from the predetermined position, and corrects a density of at least one of the plurality of images on the basis of the obtained reading result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a block diagram that shows the configuration of an image-forming apparatus according to an exemplary embodiment; 
         FIG. 2  shows the configuration of an image-forming unit and a density sensor unit included in the image-forming apparatus; 
         FIGS. 3A to 3D  illustrate, when successively forming multiple images, the relationship between placement phases and images on a photosensitive drum; 
         FIGS. 4A to 4D  illustrate, when successively forming multiple images, the relationship between placement phases and images on a photosensitive drum; 
         FIGS. 5A to 5D  illustrate, when successively forming multiple images, the relationship between placement phases and images on a photosensitive drum; 
         FIG. 6  is a flowchart that shows the operation of a controller; and 
         FIG. 7  is a flowchart that shows the operation of a controller according to a modified example. 
     
    
    
     DETAILED DESCRIPTION 
     (1) Configuration 
       FIG. 1  is a block diagram that shows the configuration of an image-forming apparatus  1  according to an exemplary embodiment of the invention. The image-forming apparatus  1  includes a controller  100 , an image-forming unit  200 , and a density sensor unit  400 . The controller  100  includes a computing apparatus such as a CPU (Central Processing Unit) or an ASIC (Application Specific Integrated Circuit), and various memories, and controls operation of the image-forming apparatus  1 . The image-forming unit  200  is an example of a forming unit that electrophotographically forms an image. Images formed by the image-forming unit  200  include images formed on a recording medium such as paper according to image information that the controller  100  has acquired from an external host apparatus or the like (hereinafter referred to as ordinary images), and images formed on an intermediate transfer belt described below in order to correct the density of the ordinary images (hereinafter referred to as density correction images). The density sensor unit  400  is, for example, an optical sensor, and reads the density of a density correction image formed by the image-forming unit  200 , and supplies the results of that reading to the controller  100 . The controller  100 , in order to correct the density of images formed by the image-forming unit  200  based on those reading results, for example, corrects exposure conditions or a charging potential in the image-forming unit  200 , or alternatively, corrects the contents of a look-up table for density correction. 
       FIG. 2  shows the structure of the image-forming unit  200  and the density sensor unit  400 . As shown in  FIG. 2 , the image-forming unit  200  includes photosensitive drums  210 Y,  210 M,  210 C, and  210 K, charging units  220 Y,  220 M,  220 C, and  220 K, exposing units  230 Y,  230 M,  230 C, and  230 K, development units  240 Y,  240 M,  240 C, and  240 K, a transfer unit  250 , a fixing unit  290 , and phase sensor units  300 Y,  300 M,  300 C, and  300 K. The transfer unit  250  has an intermediate transfer belt  255 , multiple rotating rollers  251 , primary transfer rollers  260 Y,  260 M,  260 C, and  260 K, a secondary transfer roller  270 , a backup roller  271 , and multiple delivery rollers  280 . Among the reference numerals assigned to the configurations included in the image-forming unit  200 , the reference numerals with a letter (Y, M, C, or K) appended indicate that the corresponding configuration is related to image-forming in a color corresponding to the letter. For example, the photosensitive drum  210 Y, the charging unit  220 Y, the exposing unit  230 Y, and the development unit  240 Y are for forming a Y (yellow) developer image in cooperation with the intermediate transfer belt  255 . Note that M indicates magenta, C indicates cyan, and K indicates black. Furthermore, reference numerals that differ only by the appended letter have the same basic configuration, although their positions and the developer used are different. Below, when it is not particularly necessary to distinguish between such respective configurations, notation of Y, M, C, or K is omitted, as in the “photosensitive drums  210 ” or the “charging units  220 ”. 
     The photosensitive drums  210  are cylindrical rotating bodies having a photoconductive film layered on their surface, and are an example of an image holder that holds an image. The photosensitive drums  210 , when in a state contacting the intermediate transfer belt  255 , are rotated in the direction of arrow A in  FIG. 2  with the cylinder center as an axis, along with movement of the intermediate transfer belt  255 . The charging unit  220  charges the photoconductive film of the photosensitive drums  210  to a predetermined potential. The exposing units  230  irradiate (i.e., expose) an amount of light controlled by the controller  100  on the charged photosensitive drums  210  to form an electrostatic latent image. The development units  240  develop the electrostatic latent image formed on the photosensitive drums  210  with a developer. The intermediate transfer belt  255  of the transfer unit  250  is an endless belt-like member, and moves so as to turn in the direction of arrow B in  FIG. 2  while in contact with the rotating rollers  251 , the primary transfer rollers  260 , and the backup roller  271 . The rotating rollers  251  are cylindrical members that support movement of the intermediate transfer belt  255 , and rotate with a cylinder center as an axis. 
     The primary transfer rollers  260  are cylindrical members that face the photosensitive drums  210  while sandwiching the intermediate transfer belt  255 , and produce a potential difference from the photosensitive drums  210  to transfer the image on the surface of the photosensitive drums  210  to the surface of the intermediate transfer belt  255 . The secondary transfer roller  270  is a cylindrical member that faces the backup roller  271  while sandwiching the intermediate transfer belt  255 , and produces a potential difference from the backup roller  271  to transfer the image on the surface of the intermediate transfer belt  255  to paper. The delivery rollers  280  are cylindrical members that carry paper to a position where the secondary transfer roller  270  performs transfer, and carry paper to which an image has been transferred to the position where the fixing unit  290  is provided. The fixing unit  290  applies heat and pressure to the paper to which an image has been transferred to fix the image on the paper. That is, a paper transport path is as indicated by arrow C with a broken line in  FIG. 2 . The density sensor unit  400  is provided at a position facing the intermediate transfer belt  255 , and reads the density of a density correction image that has been formed on the surface of the intermediate transfer belt  255 . 
     In the photosensitive drums  210 Y,  210 M,  210 C, and  210 K, in order to specify a position where an image is formed, markers referred to as placement phases are prescribed, such as origin placement phases  310 Y,  310 M,  310 C, and  310 K, second placement phases  320 Y,  320 M,  320 C, and  320 K, third placement phases  330 Y,  330 M,  330 C, and  330 K, and fourth placement phases  340 Y,  340 M,  340 C, and  340 K. The origin placement phases  310  are provided at one predetermined location of the photosensitive drums  210 . The second placement phases  320  are provided at a position advanced by a center angle of 90° in the direction of reverse rotation of the photosensitive drums  210  from the origin placement phases  310 . The third placement phases  330  are provided at a position advanced by a center angle of 180° in the direction of reverse rotation of the photosensitive drums  210  from the origin placement phases  310 . The fourth placement phases  340  are provided at a position advanced by a center angle of 270° in the direction of reverse rotation of the photosensitive drums  210  from the origin placement phases  310 . 
     The photosensitive drums  210  are manufactured such that they have properties as uniform as possible throughout their entire surface, but in the manufacturing process of the photosensitive drums  210  some amount of difference in film thickness occurs, and bias in properties of that surface occurs as a result of effects over time due to passing through many instances of the image-forming process, and thus a bias in charging properties or development properties may occur. Consequently, by the controller  100  starting formation of an ordinary image or a density correction image from any of the origin placement phase  310 , the second placement phase  320 , the third placement phase  330 , or the fourth placement phase  340 , effects of variation of properties of the surface of the photosensitive drum  210  as described above are suppressed as much as possible. For example, if the controller  100  forms ordinary images from the position of four phases, i.e. the origin placement phase  310 , the second placement phase  320 , the third placement phase  330 , and the fourth placement phase  340 , variation in image quality of the ordinary images is limited as much as possible to the four phases. On the other hand, the controller  100  starts formation of a density correction image from only any one (here, the origin placement phase  310 ) of the origin placement phase  310 , the second placement phase  320 , the third placement phase  330 , and the fourth placement phase  340 . In order to improve the accuracy of density correction, it is necessary to suppress as much as possible the effects of variation of the properties of the surface of the photosensitive drums  210 , and so it is desirable to use a density correction image formed in a specific region of the photosensitive drums  210 . 
     The phase sensor unit  300 , for example, is a rotary encoder, and converts a rotation displacement amount of the photosensitive drums  210  to an electrical signal and supplies that signal to the controller  100 . Based on this electrical signal, the controller  100  measures a rotation amount of the photosensitive drums  210  using the origin placement phase  310  as a reference, and specifies the rotational state of the photosensitive drums  210 . That is, the phase sensor unit  300  and the controller  100  function as an example of a measuring unit that measures the rotation amount of the photosensitive drums  210 , using a particular position (here, the origin placement phase) on the photosensitive drums  210  as a reference. 
     (2) Operation 
     Next is a description of operation in this exemplary embodiment. 
       FIGS. 3A to 3D  illustrate, when successively forming multiple ordinary images, the relationship between placement phases and ordinary images on a photosensitive drum  210 . On the horizontal axis in  FIGS. 3A to 3D , one circumference of the photosensitive drum  210  is spread out, and the circumferential length of one circumference is successively joined for multiple circumferences. 
       FIG. 3A  shows an example case in which the length in a sub-scanning direction (circumferential direction of the photosensitive drum  210 ) of an ordinary image is at least ¾ and less than one times the circumferential length of the photosensitive drum. This is referred to below as forming ordinary images at a 1 drum interval. In  FIGS. 3A to 3D , an image indicated by “1” is an ordinary image initially formed in the photosensitive drum  210 , and an image indicated by “2” is an ordinary image formed next in the photosensitive drum  210 . This is likewise true for an image indicated by “3” and subsequent numbers. Also, in  FIGS. 3A to 3D , corresponding to the image indicated by “1”, reference numerals of the origin placement phase  310 , the second placement phase  320 , the third placement phase  330 , and the fourth placement phase  340  are respectively added, and the positional relationship of these placement phases is the same for the images indicated by “2” or subsequent numbers. The same manner of representation as in  FIG. 3  is also used in  FIGS. 4 and 5 . 
     As shown in  FIG. 3A , when forming the ordinary image “1” that is formed first, the controller  100  starts that formation from the origin placement phase  310  of the photosensitive drum  210 , and finishes that formation before the origin placement phase  310  of the next circumference. Likewise, when forming the ordinary image “2” that is formed second, the controller  100  starts that formation from the origin placement phase  310  of the photosensitive drum  210 , and finishes that formation before the origin placement phase  310  of the next circumference. This is likewise true for the ordinary image “3” and subsequent ordinary images. 
     Next,  FIG. 3B  shows an example case in which ordinary images are formed at a 1.25 drum interval. As shown in  FIG. 3B , when forming the ordinary image “1” formed first, the controller  100  starts that formation from the origin placement phase  310  of the photosensitive drum  210 , and furthermore, exceeding one circumference of the photosensitive drum  210 , finishes that formation before the second placement phase  320  of the second circumference. When forming the ordinary image “2” that is formed second, the controller  100  starts that formation from the second placement phase  320  of the second circumference of the photosensitive drum  210 , and finishes that formation before the third placement phase  330  of the third circumference. This is likewise true for the ordinary image “3” and subsequent ordinary images. 
     Next,  FIG. 3C  shows an example case in which ordinary images are formed at a 1.5 drum interval. As shown in  FIG. 3C , when forming the ordinary image “1” that is formed first, the controller  100  starts that formation from the origin placement phase  310  of the photosensitive drum  210 , and furthermore, exceeding one circumference of the photosensitive drum  210 , finishes that formation before the third placement phase  330  of the second circumference. When forming the ordinary image “2” that is formed second, the controller  100  starts that formation from the third placement phase  330  of the second circumference of the photosensitive drum  210 , and finishes that formation before the origin placement phase  310  of the fourth circumference. This is likewise true for the ordinary image “3” and subsequent ordinary images. 
       FIG. 3D  shows an example case in which ordinary images are formed at a 2 drum interval. As shown in  FIG. 3D , when forming the ordinary image “1” that is formed first, the controller  100  starts that formation from the origin placement phase  310  of the photosensitive drum  210 , and finishes that formation by the origin placement phase  310  of the third circumference. When forming the ordinary image “2” that is formed second, the controller  100  starts that formation from the origin placement phase  310  of the third circumference of the photosensitive drum  210 , and finishes that formation by the origin placement phase  310  of the fifth circumference. This is likewise true for the ordinary image “3” and subsequent ordinary images. 
       FIGS. 4A to 4D  illustrate, when forming a density correction image between two ordinary images in a so-called inter-image region, the relationship between placement phases and ordinary images and density correction images on the photosensitive drum  210 . An inter-image region is, when ordinary images are continuously formed, a region from the end of a particular ordinary image to the start of the next ordinary image. In this example, the length in the sub-scanning direction of the density correction image is assumed to be the same as that length for the above-described ordinary image. 
       FIG. 4A  shows an example in which, when forming ordinary images at a 1 drum interval, a density correction image is formed in the inter-image region. Processing is repeated in which, when forming the ordinary image “1” that is formed first, the controller  100  starts that formation from the origin placement phase  310  of the photosensitive drum  210 , and finishes that formation before the origin placement phase  310  of the next circumference. When the controller  100  forms the density correction image, as described above, it is necessary for the density correction image to be formed from the origin placement phase  310 , and in this example, because a schedule is adopted in which an ordinary image is also formed from the origin placement phase  310 , formation of the density correction image is also started from the origin placement phase  310 , same as for the ordinary image, and finished before the origin placement phase  310  of the next circumference. 
     On the other hand,  FIG. 4B  shows an example in which, when forming ordinary images at a 1.25 drum interval, a density correction image is formed in the inter-image region. In this case as well, it is necessary to start formation of the density correction image from the origin placement phase  310 , so when, for example, the controller  100  forms a density correction image in an inter-image region between an ordinary image “5” and an ordinary image “6”, it is necessary to wait until arrival of the origin placement phase  310 , so a period occurs in which an image is not being formed (a non-image-forming period). Such a non-image-forming period also occurs in the same manner in the case of a 1.5 drum interval as shown in  FIG. 4C , but does not occur in the case of a 2 drum interval as shown in  FIG. 4D . When such a non-image-forming period occurs, the total time needed to form one sequence of a group of ordinary images becomes longer, so there is a decrease in time efficiency related to image-forming. 
     In order to suppress such a decrease in time efficiency, the controller  100  performs the following sort of processing. 
       FIGS. 5A to 5D  illustrate the relationship between placement phases and ordinary images and density correction images on the photosensitive drum  210 , and  FIG. 6  is a flowchart that shows operation of the controller  100 . 
       FIG. 5A  shows an example of forming a density correction image in the inter-image region when forming ordinary images at a 1 drum interval. First to fifth ordinary images “1” to “5” shown in  FIG. 5A  are formed with the same timing as the first to fifth ordinary images “1” to “5” illustrated in  FIG. 4A , so a detailed description thereof is omitted here. Here, an example is described of a case in which the time when a density correction image is formed has arrived after finishing formation of the fifth ordinary image “5”. The time when the density correction image is formed, for example, may arrive at each occurrence of a predetermined period, or may arrive at each occurrence of a predetermined image-forming amount, or may arrive in response to an instruction from a user of the image-forming apparatus. 
     First, the controller  100  judges whether or not the time for forming the density correction image has arrived (Step S 10 ). Here, when the controller  100  judges that the time for forming the density correction image has arrived (Step S 10 ; YES), the controller  100  specifies a placement phase where forming of the next planned ordinary image will start, based on an electrical signal from the phase sensor unit  300  (Step S 20 ). The placement phase specified at this time, when stated according to the example in  FIG. 5A , is the placement phase where formation of a sixth ordinary image “6” will be started, i.e., the origin placement phase  310 . Next, the controller  100  judges whether or not the placement phase specified in Step S 20  is the origin placement phase  310  (Step S 30 ). Here, the controller  100  judges that the placement phase of the next planned ordinary image is the origin placement phase  310  (Step S 30 ; YES), so the controller  100  determines that the density correction image will be formed from the origin placement phase  310  (Step S 40 ). That is, the controller  100  determines that instead of forming the next planned ordinary image, formation of the density correction image will be started from the origin placement phase  310 . Along with this determination, the image-forming unit  200  forms the density correction image. 
     Next, the controller  100  judges whether or not to end ordinary image formation (Step S 60 ). Here, formation of the sixth and subsequent ordinary images is not yet completed, so the controller  100  judges that ordinary image formation is not finished (Step S 60 ; NO), and returning to Step S 10 , the controller  100  judges whether or not to perform density correction. Here, the controller  100  judges not to perform density correction (Step S 10 ; NO), and determines the placement phase where the sixth ordinary image “6” will be formed (Step S 50 ). Thereafter, the above processing is repeated. When the controller  100  has judged in Step S 60  to finish ordinary image formation (Step S 60 ; YES), image-forming by the image-forming unit  200  ends. 
     Thus, in the example in  FIG. 5A , formation of the density correction image is started from the position of the origin placement phase  310  after formation of the fifth ordinary image ends, so a non-image-forming period as illustrated in  FIGS. 4B and 4C  does not occur. This is also true when forming a density correction image in the case of forming ordinary images at a 2 drum interval, as in  FIG. 5D . 
     Next,  FIG. 5B  shows an example of forming a density correction image in the inter-image region when forming ordinary images at a 1.25 drum interval. In  FIG. 5B , an example is illustrated of a case in which the time when a density correction image is formed has arrived after finishing formation of the fifth ordinary image “5”. 
     In  FIG. 6 , when the controller  100  judges that the time for forming the density correction image has arrived (Step S 10 ; YES), the controller  100  specifies a placement phase where forming of the next planned ordinary image will start (Step S 20 ). The placement phase specified in the example in  FIG. 5B  is the placement phase of the sixth ordinary image “6”, and this is the second placement phase  320 . Next, the controller  100  judges whether or not the placement phase specified in Step S 20  is the origin placement phase  310  (Step S 30 ). In the example in  FIG. 5B , the controller  100  judges that the placement phase of the next planned ordinary image is not the origin placement phase  310  (Step S 30 ; NO), so processing proceeds to Step S 50 , in which the controller  100  determines that the forming position of the sixth ordinary image “6” is the second placement phase  320 , and ordinary image-forming is performed. Next, via the processing of Steps S 60  and S 10 , the controller  100  again specifies the placement phase where formation of the next planned ordinary image will be started (Step S 20 ). The placement phase specified in the example in  FIG. 5B  is the placement phase of a seventh ordinary image “7”, and this is the third placement phase  330 , so this placement phase also is judged to not be the origin placement phase  310  (Step S 30 ; NO). 
     Next, via the processing of Steps S 50 , S 60  and S 10 , the controller  100  again specifies the placement phase where formation of the next planned ordinary image will be started (Step S 20 ). The placement phase specified in the example in  FIG. 5B  is the placement phase of an eighth ordinary image “8”, and this is the fourth placement phase  340 , so this placement phase also is judged to not be the origin placement phase  310  (Step S 30 ; NO). Next, via the processing of Steps S 50 , S 60  and S 10 , the controller  100  again specifies the placement phase where formation of the next planned ordinary image will be started (Step S 20 ). The placement phase specified in the example in  FIG. 5B  is the placement phase of a ninth ordinary image “9”, and the controller  100  judges this placement phase to be the origin placement phase  310  (Step S 30 ; YES), and determines that instead of the ninth ordinary image “9”, a density correction image will be formed from the origin placement phase  310  (Step S 40 ). Thus, the controller  100  specifies the placement phase at which an ordinary image that is formed first from among the ordinary images included in a group of ordinary images will be formed at a time when it was judged to form a density correction image or thereafter, and when the specified placement phase is the origin placement phase  310 , the controller  100  determines to start formation of a density correction image from the origin placement phase  310  instead of forming the ordinary image that is formed first. On the other hand, when the specified placement phase is not the origin placement phase  310 , the controller  100  specifies an ordinary image formed earliest from the origin placement phase from among a group of ordinary images formed at a time when it was judged to form a density correction image or thereafter, and determines to start formation of a density correction image from the origin placement phase  310  instead of forming the specified ordinary image. That is, the time for formation of a density correction image is delayed until the arrival of the time for formation of an ordinary image from the origin placement phase  310 , so a non-image-forming period as illustrated in  FIGS. 4B and 4C  does not occur. 
       FIG. 5C  shows an example of forming a density correction image in the inter-image region when forming ordinary images at a 1.5 drum interval, and in this case as well, the position where a density correction image will be formed is determined with the same reasoning as stated above. Specifically, formation of a density correction image is started after finishing formation of the sixth ordinary image “6”. 
     (3) Modifications 
     Modified Example 1 
     In this exemplary embodiment, the controller  100  delays the time for formation of a density correction image until arrival of the time for formation of an ordinary image that is formed from the origin placement phase  310 , but when instructed to immediately perform density correction, even if the placement phase is other than the origin placement phase  310 , a placement phase of the photosensitive drum  210  at which it is possible to form a density correction image in the shortest period from the time of that instruction may be determined to be the placement phase at which formation of a density correction image will be started. 
       FIG. 7  is a flowchart that shows operation of a controller  100  according to this modified example. In  FIG. 7 , the processing of Step S 70  is added to the processing shown in  FIG. 6 . That is, when the placement phase of a planned ordinary image is not the origin placement phase  310  (Step S 30 ; NO), the controller  100  judges whether or not to immediately form a density correction image (Step S 70 ), and when a judgment has been made to immediately form a density correction image (Step S 70 ; YES), the placement phase at which a density correction image will be formed is specified (Step S 40 ). 
     Modified Example 2 
     The density sensor unit  400  according to this exemplary embodiment detects the density of a density correction image that has been transferred to the intermediate transfer belt  255 , but this is not a limitation. For example, the density sensor unit  400  may detect the density of a density correction image that has been formed on the photosensitive drum  210 , or may detect the density of a density correction image that has been transferred to a recording medium such as paper. 
     Modified Example 3 
     In this exemplary embodiment, the placement phases include four phases: the origin placement phase  310 , the second placement phase  320 , the third placement phase  330 , and the fourth placement phase  340 , but this is not a limitation on the number of placement phases. Also, the placement phase at which a density correction image is formed does not have to be one placement phase. When a high accuracy of density correction is not sought, or when a decrease in time efficiency due to delaying formation of a density correction image is not allowable, the number of placement phases at which a density correction image is formed may be at least one placement phase and less than the total number of placement phases. 
     Also, in this exemplary embodiment, there are four intervals at which images are formed: a 1 drum interval, a 1.25 drum interval, a 1.5 drum interval, and a 2 drum interval, but this is not a limitation. 
     Modified Example 4 
     The image-forming apparatus  1  according to this exemplary embodiment includes the photosensitive drums  210 Y,  210 M,  210 C, and  210 K, and developer of the colors yellow (Y), magenta (M), cyan (C), and black (K) is used, but this is not a limitation. For example, the image-forming apparatus may be configured such that developer of one color is used by one photosensitive drum. 
     Modified Example 5 
     The phase sensor unit  300  may be any sensor unit or the like that detects the amount of rotation of a photosensitive drum  210  with reference to a particular position on that photosensitive drum  210 . Also, the amount of rotation stated here may be a value that indicates the amount that the photosensitive drum  210  has rotated, i.e., a rotation angle when the photosensitive drum  210  has rotated, the amount of movement of the surface of the photosensitive drum  210  when the photosensitive drum  210  has rotated, or the like. 
     The foregoing description of the exemplary embodiment 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. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment were chosen and described 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 be defined by the following claims and their equivalents.