Abstract:
An image forming apparatus includes a plurality of image forming means; transferring means for superimposingly transferring the developed images; a detector for detecting light reflected by the developed image; a position detector for detecting a position of the developed image; memory for storing for each color of the developer, information relating to a correction amount for correcting an error between a position obtained by the position detector and an actual position, a corrector for correcting the position of the developed image formed on the image bearing member by the image forming means, on the basis of positions of the developed images of the respective color detected by the position detector and information relating to the correction amount stored in the memory.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention primarily relates to an image forming apparatus which uses an electrophotographic process. In particular, it relates to a method for controlling an electrophotographic image forming apparatus so that when images, different in color, formed on the image bearing members of the apparatus are transferred onto the intermediary transferring member of the apparatus, or directly onto a sheet of final recording medium such as a sheet of recording paper, the images will precisely align among themselves. 
         [0002]    A color image forming apparatus equipped with multiple photosensitive drums is designed so that images, different in color, formed on its photosensitive drums, one for one, do not fail to align among themselves as they are transferred onto the subsequent image bearing means. In reality, however, when the images, different in color, formed on the photosensitive drums are transferred onto the subsequent image bearing means, the images tend to fail to precisely align among themselves, because of mechanical errors, such as the one that occurs when the photosensitive drums are attached to the frame of the image forming apparatus, the error in the length of the path of the laser beam for writing each of the electrostatic latent images for forming the multiple monochromatic developer images, different in color, one for one, the changes which occur to the laser beam paths, etc. Thus, various methods for minimizing a color image forming apparatus in the amount by which misalignment occurs among the multiple monochromatic color images when the images are transferred onto the subsequent image bearing member. 
         [0003]    One such method is disclosed in Japanese Laid-open Patent Application 2002-023445. According to this patent application, images for minimizing a color image forming apparatus in the amount by which multiple images, different in color, fail to align among themselves as they are transferred onto an intermediary transfer belt, is formed on the intermediary transfer belt. Then, the positioning of the images for minimizing the image forming apparatus in the amount of misalignment among the multiple images, different in color, on the intermediary transfer belt, is detected in order to minimizing the image forming apparatus in the amount of the misalignment, or making the image forming apparatus virtually free of the misalignment. 
         [0004]    Japanese Laid-open Patent Application 2002-023445 discloses a method for detecting the amount of the positional deviation of each of the multiple monochromatic toner images different in color, from a referential positional deviation detection image, by detecting the light which is regularly reflected by the positional deviation detection image. In the case of this patent application, the means for detecting the positional deviation detection image is made up of a light emitting element such as an infrared light emitting diode, a light sensing element, such as a photo-transistor, for catching the light regularly reflected by the positional deviation detection image. The positional deviation detection image is such an image that is made of two sections having two referential sections of a specific color, and another section which is different in color from the referential sections and is sandwiched between the two referential sections. The amount of the positional deviation among the two referential sections and the other section is calculated based on the amount of positional deviation between the center of the two referential section and the center of the other section. Then, the image forming apparatus is adjusted in image formation settings such as the timing with which developer (toner) images, different in color, begin to be written, image formation clock, and/or the like, based on the calculated amount of the positional deviation among the two referential sections of the positional deviation detection image, and another section of the positional deviation detection image, in order to correct the apparatus in terms of the positional deviation among the multiple images, different in color, which the apparatus forms to yield a multicolor image. 
         [0005]    Japanese Laid-open Patent Application 2009-93155 discloses another method for detecting the amount of the positional deviation among the multiple developer (toner) images, which an electrophotographic image forming apparatus forms to yield a multicolor image. According to this patent application, positional deviation detection images are formed on the intermediary transfer member of the apparatus, and the amount of the light which is diffusely reflected by the positional deviation detection images on the intermediary transfer belt is detected by the light sensing element of the sensor unit for detecting the positional deviation detection images, because the amount by which light is diffusely reflected by the intermediary transfer belt is not affected by the surface condition of the belt as much as the amount by which light is regularly reflected by the intermediary transfer belt. More specifically, in a case where the amount of the positional deviation among the positional deviation detection images is obtained by detecting the amount by which light is diffusely reflected by the positional deviation detection images, the amount by which light is diffusely reflected by a black developer image (image formed of black developer) formed on the intermediary transfer belt is as small as the amount by which light is diffusely reflected by the intermediary transfer belt itself. Therefore, the positional deviation detection images are formed as shown in  FIG. 14 . That is, the positional deviation detection image of black color is formed on each of the three positional deviation detection images which are different in color from the black positional deviation detection images. More concretely, yellow, magenta and cyan developer images  1401 ,  1402  and  1403 , respectively, are formed on the intermediary transfer belt, and three black developer images are formed on the yellow, magenta and cyan developer images  1401 ,  1402  and  1403 , one for one. Thus, even the black developer image, which is very small in the amount by which it diffusely reflects light, can be detected. 
         [0006]    Further, Japanese Laid-open Patent Applications 2007-272111 and 2005-234238 describe so-called “trailing edge piling”, which is a phenomenon that when an electrostatic image on the peripheral surface of a photosensitive member (drum) is developed during an electrophotographic image forming process, developer (toner) unintendedly piles along the trailing edge portion of an exposed area of the peripheral surface of a photosensitive member (drum). To describe how and why the “trailing edge piling” occurs with reference to  FIG. 15 , when an electrostatic latent image on a photosensitive drum is developed by a development roller  1502 , the developer (toner) particles  1504  on the peripheral surface of the electrophotographic drum  1502 , which are facing the area of the peripheral surface of the photosensitive drum  1501 , which is between the exposed area  1503  and unexposed area  1505 , and the area adjacent thereto, jump onto the exposed area  1503  of the peripheral surface of the photosensitive drum  1501 , which is lower in electrical potential. That is, the upstream edge portion of the exposed area of the photosensitive drum  1501  in terms of the rotational direction of the photosensitive drum  1501  receives developer (toner) by an amount greater than the theoretically correct amount. In other words, the developer (toner) piles up across the trailing edge portion of the exposed area of the peripheral surface of the photosensitive drum  1501  as shown ( 1601 ) in  FIG. 16 . Thus, in order to minimize an electrophotographic image forming apparatus in “trailing edge piling”, that is, in the amount by which developer (toner) piles up along the trailing edge of the exposed portion of the peripheral surface of the photosensitive drum  1501 , the fourth of the abovementioned four patent applications proposes the following method. That is, the method extracts the information about the contour of the image to be formed, from the information about the image to be formed. Then, it modifies (adjusts) the original image formation data in such a manner that the trailing edge portion of the image to be formed, will be theoretically less in image density. 
         [0007]    Japanese Laid-open Patent Application H7-134479 describes the relationship between the ambient temperature of an image forming apparatus and the extent of the “trailing edge piling”. More specifically, when an electrophotographic image forming apparatus is operated in an environment which is low in temperature and humidity, the tribo-electric charge which the developer (toner) acquires is relatively narrow in terms of the range, and therefore, the “trailing edge piling” is small in the amount. However, there is such a tendency that as the environment in which an electrophotographic image forming apparatus is being operated increases in temperature and humidity, the level to which the developer (toner) becomes triboelectrically charged becomes wider in range in proportion to the amount by which temperature and humidity increase. 
         [0008]    When an electrophotographic image forming apparatus is operated in an environment in which the “trailing edge piling” occurs, the “trailing edge piling” occurs to the positional deviation detection images while they are formed on the photosensitive drums to be transferred onto the intermediary transfer member. In a case where the position of the edge of the positional deviation detection images, along which developer (toner) has piled is detected by a sensor, it cannot be accurately detected, which is problematic in that the detected position of the trailing edge of the positional deviation detection image is incorrect. 
         [0009]    For example, in a case where a sensor structured to detect the amount of the positional deviation of an image, by detecting the amount of the light diffusely reflected by the surface of the image, a positional deviation detection image is formed as shown in  FIG. 17(A) . That is, first, a referential developer image  1701 , that is, a positional deviation detection image of yellow color (referential color) is formed, and a black developer image  1702 , that is, the image, the position of which is detected, formed on the referential (yellow) developer image  1701 . In this case, the trailing edge of the resultant positional deviation detection image is excessive in the amount of developer, being therefore higher in density than the theoretically correct amount, because of the effect of the “trail edge piling”, as shown in  FIG. 17(B) . Thus, the analog signal outputted by the sensor, which shows the intensity (amount) of the light diffusely reflected by the positional deviation detection image, becomes as shown in  FIG. 17(C) . More specifically, the analog signal related to the referential developer image, or the color (yellow) image, has the pattern represented by a solid line  1706 , being slightly higher in amplitude at the trailing edge. As for the analog signal related to the black developer image, the black developer image absorbs light, being therefore small in the amount (intensity) by which it diffusely reflect light. Therefore, the analog signal from the sensor, which shows the amount by which light is diffusely reflected by the black developer, is hardly effected by the “trail end piling”, even thought the “trailing edge piling” occurs to the black developer image. 
         [0010]    The amount of positional deviation of the positional deviation detection image is calculated as follows. The analog signal outputted by the sensor is digitized with reference to a preset threshold value. Then, the amount of positional deviation of the positional deviation detection image is calculated based on a point in time at which the thus obtained digital signal steps up and a point in time at which the digital signal steps down. More concretely, referring to  FIG. 17(D) , the position of the center of the background color developer image (pattern)  1701  is obtained (calculated) based on a point ty 11  in time at which the digital signal steps up in value and a point ty 12  in time at which the digital signal steps down in value. Similarly, the position of the center of the black developer image is obtained by calculation based on a point tk 11  in time at which the digital signal steps down, and a point tk 12  in time at which the digital signal steps up. Then, a distance Δdy between the position of the center of the background color image and the position of the center of the black developer image is calculated as the amount of positional deviation of the trailing edge of the background color image. 
         [0011]    When the “trailing edge piling” does not occur, the analog signal outputted by the sensor, and the digital signal obtained from the analog signal with the use of a preset threshold value, become as indicated by a broken line  1705  in  FIG. 17(C)  and a broken line in  17 (D), respectively. Therefore, when there is no positional deviation between the background color image and black developer image, the amount Δdy of the positional deviation is zero (Δdy=0) as shown in  FIG. 17(E) . However, when the “trailing edge piling” occurs, the tailing edge portion of the background developer image becomes higher in density. Therefore, the analog signal outputted by the sensor, and the digital signal obtained from the analog signal with the use of a preset threshold value, become as indicated by a solid line  1706  in  FIG. 17(C)  and a solid line in  17 (D), respectively. Therefore, even when there is no positional deviation between the background color image (pattern) and black developer pattern, the amount Δdy of the positional deviation is not zero (Δdy′≠0) as shown in  FIG. 17(E) . That is, the position of the trailing edge of the background image is detected as if it has deviated in position. 
         [0012]    As the “trailing edge piling” occurs, the amount of the positional deviation among the positional deviation detection images formed on the intermediary transfer belt cannot be accurately detected. Thus, it is impossible to precisely correct an electrophotographic image forming apparatus in the positional deviation among the positional deviation detection images, based on the results of the detection of the images by the sensor. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention was made in consideration of the above-described issue. Thus, the primary object of the present invention is to provide a technology for precisely correcting an electrophotographic image forming apparatus which uses multiple developers, different in color, to form a multicolor image, in the positional deviation among the multiple monochromatic images formed on the multiple image bearing members of the apparatus, one for one. 
         [0014]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is an example of a table which shows the relationship between the temperature/humidity range (having three sub-ranges), and the amounts PDe_ky, PDe_my and PDe_cy, by which the position of the trailing edge of the positional deviation detection images of black, magenta, and cyan colors, respectively, are to be corrected. 
           [0016]      FIG. 2  is a schematic sectional view of the image forming apparatus in the first and second embodiments of the present invention, and shows the positioning of the various components of the apparatus, which are related to the present invention. 
           [0017]      FIG. 3  is a schematic drawing of the sensor units in the first and second embodiments, and shows the structure of the units. 
           [0018]      FIG. 4  is a drawing of the circuit for detecting the amount of light reflected by the positional deviation detection image, in the first and second embodiments. 
           [0019]      FIG. 5  is a drawing of the positional deviation detection images, in the first and second embodiments. 
           [0020]      FIG. 6  is a drawing which shows the pattern in which the positional deviation detection images are arranged in the first and second embodiments. 
           [0021]      FIG. 7  is a drawing which shows the pattern in which the positional deviation detection images are arranged, and the signals outputted by the sensor for detecting the tailing edge of each of the positional deviation detection images, in the first and second embodiments. 
           [0022]      FIG. 8  also is a drawing which shows the pattern in which the positional deviation detection images are arranged, and the signals outputted by the sensor for detecting the trailing edge of each of the positional deviation detection images, in the first and second embodiments. 
           [0023]      FIG. 9  is a graph which shows how the ambient temperature and humidity ranges of the image forming apparatus are divided in the first embodiment. 
           [0024]      FIG. 10  is a flowchart of the control sequence for correcting the image forming apparatus in the positional deviation among monochromatic images, in the first and second embodiments. 
           [0025]      FIG. 11  is a graph which shows the division of the temperature range of the environment in which the image forming apparatus is operated, in the second embodiment. 
           [0026]      FIG. 12  is an example of a table which shows the relationship between the temperature/humidity range (having three sub-ranges), and the amounts PDet_ky, PDet_my and PDet_cy, by which the position of the trailing edge of the positional deviation detection images of black, magenta, and cyan colors, respectively, are to be corrected. 
           [0027]      FIG. 13  is a drawing which shows the division of the humidity range of the environment in which the image forming apparatus is operated, in the second embodiment. 
           [0028]      FIG. 14  is a drawing which shows the positional deviation detection images in accordance with the background art, and the pattern in which the images are arranged. 
           [0029]      FIG. 15  is a drawing for describing the principle based on which the “trailing edge piling” occur. 
           [0030]      FIG. 16  is a drawing for showing the piling of developer (toner) along the trailing edge of the exposed area of the peripheral surface of the photosensitive drum. 
           [0031]      FIG. 17  is a drawing of the positional deviation detection image in accordance with the prior art, and the waveform of the signal outputted by the sensor for detecting the amount by which light is reflected by the surface of the positional deviation detection image. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Hereinafter, the embodiments of the present invention are described with reference to the appended drawings. The following embodiments of the present invention are examples of the embodiment of the present invention. They are not intended to limit the present invention in scope. 
       Embodiment 1 
       [0033]    Next, the first embodiment of the present invention is described. 
         [0034]    [Description of Image Forming Apparatus] 
         [0035]      FIG. 2  is a schematic sectional view of the image forming apparatus  201 , more specifically, a color laser beam printer. It shows the structure of the apparatus. The image forming apparatus in this embodiment is capable of forming a multicolor image by layering two or more of yellow (Y), magenta (M), cyan (C) and black (Bk) images. Thus, it has four image forming stations, different in the color of the image they form. 
         [0036]    The image forming operation of the color laser beam printer  201  is as follows. As the printer  201  receives image formation data  203  from a host computer  202 , it converts the image data  203  into video signal data with the use of its printable image generating section  204 , and generates video signals for forming the image to be formed. The control section  206  of the printer  201  has a computing means such as a CPU  209 . As the CPU  209  receives the video signal  205  generated by the printable image generating section  204 , it drives multiple laser diodes  211 , as laser light emitting elements, which are in the scanner units  210 , in response to the video signal  205 . 
         [0037]    The beams  212   y ,  212   m ,  212   c  and  212   k  of laser light (which hereafter will be referred to as laser beam  212 ) emitted by the laser diodes  211 , one for one, are deflected by a polygonal mirror  207 , are transmitted through lenses  213   y ,  213   m ,  213   c  and  213   k  (which hereafter will be referred to as lens  213 ), are deflected by deflection mirrors  214   y ,  214   m ,  214   c  and  214   k  (which hereafter will be referred to as deflection mirror  214 ), so that they are projected upon the peripheral surface of the photosensitive drums  215   y ,  215   m ,  215   c  and  215   k  (which will be referred to as photosensitive drum  215 ), respectively. The photosensitive drums  215   y ,  215   m ,  215   c  and  215   k , which are image bearing members, are charged by charging means  216   y ,  216   m ,  216   c  and  216   k  (which hereafter will be referred to as charging means  216 ), respectively. 
         [0038]    As the laser beam  212  is projected upon the charged area of the peripheral surface of the photosensitive drum  215 , the points of the charged area of the peripheral surface of the photosensitive drum  215 , upon which the laser beam  212  were projected, reduce in surface potential level. Consequently, an electrostatic latent image is effected on the peripheral surface of the photosensitive drum  215 . This electrostatic latent image is developed by one of developing means  217   y ,  217   m ,  217   c  and  217   k  (which hereafter will be referred to as developing means  217 ) into visible image formed of toner (developer), which reflects the electrostatic latent image. That is, the image forming apparatus  201  has image forming means which form four toner images, different in color, on its four photosensitive drums  215 , one for one. The toner image formed on the photosensitive drum  215  is transferred (primary transfer) onto an intermediary transfer belt  219 , by a proper amount of bias voltage applied to primary transfer members  218   y ,  218   m ,  218   c  and  218   k  (which hereafter will be referred to as primary transfer member  218 ). The intermediary transfer belt  219  is an intermediary transfer member, and is in the form of an endless belt which is circularly movable. 
         [0039]    To describe in more detail the primary transfer of a toner image, first, the yellow toner image is transferred onto the intermediary transfer belt  219 , and then, the magenta, cyan and black toner images are sequentially transferred in layers onto the yellow toner image on the intermediary transfer belt  219 . Consequently, a multicolor image is effected on the intermediary transfer belt  219 . As described above, the image forming apparatus  201  has also a transferring means which sequentially transfers the toner images formed on the photosensitive drums  215 , one for one, onto the intermediary transfer member. 
         [0040]    The intermediary transfer belt  219  is circularly moved by an intermediary transfer belt driving roller  226  which is under the control of the control sections  206 . The sheets  221  of recording medium stored in layers in a cassette  220  are picked up one by one by a sheet feeding roller  222 , and are conveyed to a secondary transfer roller  223  in synchronism with the arrival of the images having transferred onto the intermediary transfer belt  219  at the second transfer roller  223 . Then, the images on the intermediary transfer belt  219  are transferred together (secondary transfer) onto the sheet  221  of recording medium by the secondary transfer roller  223 . During the secondary transfer, a proper bias voltage is applied to the secondary transfer roller  223  to increase the efficiency with which the images are transferred. 
         [0041]    After the secondary transfer of the toner images onto the sheet  221  of recording medium, the toner images are thermally fixed to the sheet  221  by the heat and pressure in a fixing device  224 . Then, the sheet  221  is discharged into a delivery tray which is an integral part of the top wall of the external shell of the image forming apparatus. Designated by a referential code  225  in  FIG. 1  is a sensor unit  225  for detecting the trailing edge of each of the positional deviation detection images, different in color, transferred onto the intermediary transfer belt  219 . The sensor unit  225  detects the amount of the beam of light projected upon each of the positional deviation detection images, different in color, formed on the intermediary transfer belt  219 , and reflected by each positional deviation detection image. Then, it sends the results of its detection to the control section  206 . The control section  206  determines the position of each of positional deviation detection images (formed of developer) on the intermediary transfer belt  219 , based on the results of detection of the positional deviation detection images by the sensor unit  225 , and corrects the image forming apparatus in terms of the positional deviation among the multiple images, different in color, formed to yield a multicolor image, based on the determined position of each image. 
         [0042]    An environment detection sensor  227  has a temperature detection element and a humidity detection element, such as a thermistor. It is an environment detecting means which detects the temperature and humidity (which will be referred to as temperature/humidity, hereafter) of the environment in which the image forming apparatus  201  is being operated. It sends the detected temperature and humidity to the control section  206 . The environment detection sensor  227  is positioned in the image forming apparatus  201 , more specifically, in an area where it is unlikely to be affected by the heat generated within the image forming apparatus  201  and also, where it can accurately detect the temperature and humidity of the environment in which the image forming apparatus  201  is being operated. 
         [0043]    The control section  206  adjusts the parameters for controlling pressure, fixation process, positional deviation correction process, etc., based on the temperature/humidity detected by the environment sensor  227 . In this embodiment, the environment sensor  227  is positioned in an internal area of the image forming apparatus  201 , in which the sensor  227  can accurately detect the temperature/humidity of the environment in which the apparatus  201  is being operated. However, the environment sensor  227  may be positioned in an internal area of the apparatus  201  so that it can accurately detect the temperature/humidity of a specific point in the apparatus  201 . 
         [0044]    [Structure of Sensor Unit] 
         [0045]      FIG. 3  shows the structure of the sensor unit  225 . The sensor unit  225  has a pair of optical sensors  301  and  302 , which are aligned in the direction perpendicular to the direction A in which the intermediary transfer belt  219  is moved, in order to detect the amount of image magnification in the primary scan direction, and the image angle relative to the secondary scan direction. The optical sensors  301  and  302  detect the amount by which a beam of light is diffusely reflected by the intermediary transfer belt  219  and each positional deviation detection image on the intermediary transfer belt  219 . Each of the optical sensors  301  and  302  has a light emitting element  303  and a light sensing element  304 . The light emitting element  303  is positioned so that the beam of infrared light emitted by the element  303  hits the surface of the intermediary transfer belt  219  at an angle of 15° relative to a line perpendicular to the surface of the intermediary transfer belt  219 . 
         [0046]    The light sensing element  304  which is for detecting the portion of the beam of infrared light emitted by the light emitting element  303  and diffusely reflected by the surface of the intermediary transfer belt  219  and the positional deviation detection images  305  on the intermediary transfer belt  219 . It is positioned so that the angle between the line which connects the center of the light sensing element  304  and the point on the intermediary transfer belt  219  which the beam of infrared light emitted by the light emitting element  303  hits, and the line perpendicular to the surface of the intermediary transfer belt  219  becomes 45°. As the intermediary transfer belt  219  is circularly moved, the beam of infrared light emitted by the light emitting element  303  hits the surface of the intermediary transfer belt  219  and each of the positional deviation detection image  305 , different in color, on the intermediary transfer belt  219 , and the light sensing element  304  catches a portion of the beam of infrared light emitted by the light emitting element  303  and diffusely reflected by the surface of the intermediary transfer belt  219  and each of the positional deviation detection image  305  on the intermediary transfer belt  219 . 
         [0047]      FIG. 4  shows the circuit for driving the sensor unit  225 . The light emitting element  303  is turned on and off by the light emitting element driving signal Vledon from the control section  206 . More specifically, a switching element  404  such as a transistor is driven by the light emitting element driving signal Vledon through a base resistor  405 , whereby the current which flows to the light emitting element  303  is controlled by a current regulator resistor  405 , turning on or off the light emitting element  303 . As the light sensing element  304  receives the portion of the beam of infrared light emitted by the light emitting element  303  and diffusely reflected by the surface of the intermediary transfer belt  219  and each positional deviation image on the intermediary transfer belt  219 , electric current flows through the resistor  401  by an amount proportional to the amount of light received by the light sensing element  304 . Thus, the amount of the diffusely reflected light received by the light sensing element  303  is outputted in the form of an analog signal. 
         [0048]    The above-described analog signal which shows the amount of the diffusely reflected light is converted into a digital signal Vdout, by comparing the analog signal in voltage with a preset threshold voltage, the value of which is set by a pair of voltage dividing resistors  406  and  407 . The control section  206  detects the points in time at which the digital signal steps up in value (voltage) and the points in time at which the digital signal steps down in value (voltage) by sampling the digital signal Vout with preset intervals, and sequentially stores the points in time at which each edge was detected, in an unshown storage device. 
         [0049]    [Positional Deviation Detection Images] 
         [0050]    Described next are a set of positional deviation detection images in this embodiment, an example of the pattern in which the positional deviation detection images are arranged, and a method for correcting the image forming apparatus  201  in the positional deviation which occurs among multiple developer images (toner images) different in color as the images are transferred onto the intermediary transfer belt  219 . 
         [0051]      FIG. 5  shows a set of positional deviation detection images, and the pattern in which the images are arranged.  FIG. 5  does not show the trailing edge buildup of developer (toner). The positional deviation detection image set is made up of yellow developer images  501   y  and  502   y , magenta developer images  501   m  and  502   m , cyan developer images  501   c  and  502   c , and black developer images  501   k  and  502   k . As is evident from  FIG. 5 , the positional deviation detection image set has a front half, which includes images  501   y ,  501   m ,  501   c  and  501   k , and a rear half, which includes images  502   y ,  502   m ,  502   c  and  502   k . The front and rear halves are symmetrical with reference to the line which coincides with the center of the set, in terms of the moving direction of the intermediary transfer belt  219  and is perpendicular to the moving direction of the intermediary transfer belt  219 . 
         [0052]    The amount of positional deviation of this positional deviation detection set can be determined by detecting the amount of positional deviation of each developer image in the set, in terms of both the primary and secondary scan directions, by the sensor unit  225 . It should be noted here that the black developer image is layered upon the yellow developer image, because the light sensing element  304  of the sensor unit  225  used in this embodiment is such a light sensing element that senses the portion of the beam of light emitted from the light emitting element  303  and diffusely reflected by the positional deviation detection images. 
         [0053]    The light which was diffusely reflected by the black developer image on the intermediary transfer belt  219  is as low in intensity as the light which was diffusely reflected by the intermediary transfer belt  219  itself. Thus, if the black developer image is formed directly on the intermediary transfer belt  219 , the difference in intensity between the light which was diffusely reflected by the black developer image and the light which was diffusely reflected by the intermediary transfer belt  219  itself is too small for the sensor unit  225  to detect the edge (pattern) of the black developer image. Therefore, the black developer image, which is small in the amount by which it diffusely reflect light, is formed on the color developer image which is larger in the amount by which it diffusely reflect light, so that the edge of the black developer image can be detected by the sensor unit  225 . It does not need to be the yellow developer image that the black developer image is to be formed. That is, it may be the cyan developer image or magenta developer images. 
         [0054]      FIG. 6  is a drawing which shows the pattern in which the positional deviation detection images are formed on the intermediary transfer belt  219 . In  FIG. 6 , the intermediary transfer belt  219  which is an endless belt is drawn as if it were an ordinary long belt. The positional deviation detection images PL 1 -PL 6 , and PR 1 -PR 6 , in  FIG. 6  correspond to the positional deviation detection images of the positional deviation detection image set in  FIG. 5 , respectively. In the case of  FIG. 6 , a set of six positional deviation detection images (PL 1 -PL 6 ), which are to be detected by the optical sensor  301 , and another set of six positional deviation detection images (PR 1 -PR 6 ), which are to be detected by the optical sensor  302 , are formed on the intermediary transfer belt  219 , being evenly spaced across the entire circumference of the intermediary transfer belt  219 . That is, a total of  12  positional deviation detection images are formed on the intermediary transfer belt  219 . Thus, the fluctuation in the peripheral velocity, fluctuation in the moving speed of the intermediary transfer belt  219 , and the like, are cancelled. As the intermediary transfer belt  219  is moved in the direction indicated by an arrow mark in  FIG. 6 , the positional deviation detection images on the intermediary transfer belt  219  are sequentially detected by the optical sensors  301  and  302 . 
         [0055]      FIG. 7(C)  shows an example of waveform of the analog signal outputted by the sensor unit  225  when the positional deviation detection images were detected by the sensor unit  225 . The analog signal outputted by the sensor unit  225  when the unit  225  detects the color development images in the positional deviation detection image set includes a large amount of light diffusely reflected by the color development images. Thus, its peak voltage is greater in value than the threshold voltage. On the other hand, the analog signals outputted by the sensor unit  225  when the unit  225  detects the black developer image and the intermediary transfer belt  219  itself do not include a large amount of components diffusely reflected by the black developer image and intermediary transfer belt  219  itself. Therefore, the portions of signals which correspond to the black developer image and intermediary transfer belt  219  itself, are lower in amplitude than the threshold voltage. 
         [0056]      FIG. 7(D)  shows an example of the waveform of the digital signal obtained by digitizing the analog signal outputted by the sensor unit  225 , based on the relationship between the peak and valley portions of the analog signal and the threshold voltage, with the use of a comparator or the like device. The position of the edge of each of the color developer images, and that of the black developer image, can be detected based on this digital signal. 
         [0057]    [Description of Method for Detecting Positional Deviation] 
         [0058]    Next, the method for calculating the amount of positional deviation among the images for detecting the positional deviation, based on the results of the detection of the position of the edge of each positional deviation detection image, is described. The computation for determining the amount of positional deviation with the use of the following mathematical equations is carried out by the control section  206 . 
         [0059]    In this embodiment, the amount by which positional deviation will possibly occur among the images, different in color, for forming a multicolor image when the images are transferred onto the intermediary transfer belt  219  is obtained by computing the amount of positional deviation among the referential positional deviation detection image having the referential color, and each of the positional deviation detection images, which is different in color from the referential image. In the case of this embodiment, the amount of positional deviation of the magenta, cyan, and black images for positional deviation detection, relative to the yellow image for positional deviation detection, is calculated.  FIG. 8(C)  shows the points in time which correspond to the front edge, center, and trailing edge of each positional deviation detection images, computationally obtained by the control section  206  based on the digital signal obtained through the conversion of the analog signal outputted by the sensor unit  225  when the positional deviation detection images shown in  FIGS. 8(A) and 8(B)  were detected by the sensor unit  225 . What the referential codes in  FIG. 8(C)  stand for are as follows: 
         [0060]    ty 11 , ty 12  and ty 1 : positions of front edge, trailing edge, and center of first yellow developer image, respectively, 
         [0061]    tk 11 , tk 12  and tk 1 : positions of front edge, trailing edge, and center of first black developer image, respectively, 
         [0062]    tm 11 , tm 12  and tm 1 : positions of front edge, trailing edge, and center of first magenta developer image, respectively, 
         [0063]    tc 11 , tc 12  and tc 1 : positions of front edge, trailing edge, and center of first cyan developer image, respectively, 
         [0064]    ty 21 , ty 22  and ty 2 : positions of front edge, trailing edge, and center of second yellow developer image, respectively, 
         [0065]    tk 21 , tk 22  and tk 2 : positions of front edge, trailing edge, and center of second black developer image, respectively, 
         [0066]    tm 21 , tm 22  and tm 2 : positions of front edge, trailing edge, and center of second magenta developer image, respectively, 
         [0067]    tc 21 , tc 22  and tc 2 : positions of front edge, trailing edge, and center of second cyan developer image, respectively. 
         [0068]    The position of the center of each positional deviation detection image can be obtained by following mathematical equations: 
         [0000]        tk 1=( tk 11+ tk 12)/2 
         [0000]        ty 1=( ty 11+ ty 12)/2 
         [0000]        tm 1=( tm 11+ tm 12)/2 
         [0000]        tc 1=( tc 11+ tc 12)/2 
         [0000]        tk 2=( tk 21+ tk 22)/2 
         [0000]        ty 2=( ty 21+ ty 22)/2 
         [0000]        tm 2=( tm 21+ tm 22)/2 
         [0000]        tc 2=( tc 21+ tc 22)/2 
         [0069]    The difference in the length of time between the yellow developer image, or the referential developer image, and each of the rest of the developer images which are different in color, can be calculated with the use of the following mathematical equations, based on the calculated position of the center of each positional deviation detection image: 
         [0070]    Amount of difference, in terms of length of time, of the black developer image: PDt_ky=((tk 1 −ty 1 )+(tk 2 −ty 2 ))/2, 
         [0071]    Amount of difference, in terms of length of time, of the magenta developer image: PDt_my=((tm 1 −ty 1 )+(tm 2 −ty 2 ))/2, and 
         [0072]    Amount of difference, in terms of length of time, of the cyan developer image: PDt_cy=((tc 1 −ty 1 )+(tc 2 −ty 2 ))/2. 
         [0073]    The point in time which corresponds to the positions of the front edge, trailing edge, and center of each positional deviation detection image corresponds to the elapsed length of time from a preset referential point in time (for example, point in time at which timer is started). The control section  206  calculates the amount of positional deviation of each of the positional deviation detection images other than the yellow positional deviation detection image, or the referential image, relative to the yellow positional deviation detection image, with the use of the following equations, based on the speed PS of the intermediary transfer belt  219  and the calculated amount, in length in time, of the positional deviation: 
         [0074]    Amount of positional deviation of black developer image in terms of the secondary scan direction: Photosensitive drum 1 _ky=PS×PDt_ky, 
         [0075]    Amount of positional deviation of magenta developer image in terms of the secondary scan direction: Photosensitive drum 1 _my=PS×PDt_my, and 
         [0076]    Amount of positional deviation of cyan developer image in terms of the secondary scan direction: Photosensitive drum 1 _my=PS×PDt_cy. 
         [0077]    The control section  206  carries out the above described computation for each positional deviation detection image set, and obtains the average amount of positional deviation of all the sets, obtaining thereby the amount of positional difference between the point in time at which each of positional deviation detection images other than the yellow positional deviation detection image, that is, the referential positional deviation detection image, begins to be formed (written) and the point in time at which the yellow positional deviation detection image begins to be formed (written). Here, the calculated amounts PDd 1 _ky, PDd 1 _my and PDd 1 _cy will be referred to as the first amount of positional deviation. If the first positional deviation is positive in value, it means that the point in time at which the black, magenta, and/or cyan positional deviation detection image began to be written is later relative to the point in time at which the yellow positional deviation detection image, or the referential positional deviation detection image, began to be written. On the other hand, if the first position deviation is negative in value, it means that the point in time at which the black, magenta, and/or cyan positional deviation detection image began to be written is earlier than the point in time at which the yellow positional deviation detection image, or the referential positional deviation detection image, began to be written. 
         [0078]    [Errors Attributable to Trail Edge Piling] 
         [0079]    In a case where the “trailing edge piling”, that is, the phenomenon which results in increase in the image density of the downstream edge portion of a toner image, in terms of the secondary scan direction, formed on the intermediary transfer belt  219 , did not occur, the waveform of the analog signal outputted by the sensor unit  225  is as indicated by a broken line in  FIG. 7(C) . In comparison, when the “trailing edge piling” occurred, the analog signal is as indicated by a solid line in  FIG. 7(D) . That is, it is later in terms of the point in time at which its steps down in potential for the following reason. That is, as the “trailing edge piling” occurs to the positional deviation detection images, the positional deviation detection images become higher in density across their trailing edge portion, which in turn increase the amount by which the light emitted by the light emitting element  303  and diffusely reflected by the positional deviation detection image is detected by the sensor unit  225 . 
         [0080]    In such a case, as shown in  FIG. 7(D) , the position of the trailing edge of the positional deviation detection image, which is determined based on the signal outputted by the sensor unit  225 , is offset downstream compared to the detected position (actual position) of the trailing edge of the positional deviation detection image to which the “trail edge piling” did not occur. In  FIG. 7(D) , the difference in length of time which occurs between the actual and calculated positions of the trailing edge of each positional deviation detection image is shown by a referential code Δty, Δtm or Δtc. Because of this error in the detection of the position of the trailing edge of a positional deviation detection image, the position of the center of each positional deviation detection image, which is calculated based on the output signal of the sensor unit  225 , is offset downstream by an amount proportional to the amount of increase, in density, of the trailing edge portion of the positional deviation detection image, which is attributable to the “trailing edge piling”. 
         [0081]    As described above, the position of the trailing edge of each positional deviation detection image, which is determined by calculation based on the output signal from the sensor unit  225  includes the error attributable to the “trailing edge piling”. That is, the greater the amount of “trailing edge piling”, the greater the error in the calculated position of the trailing edge of the positional deviation detection image, which is determined by calculation based on the output signal from the sensor unit  225 , and therefore, the greater the error in the position of the center of the positional deviation detection image, which is calculated based on the calculated position of the trailing edge of the positional deviation detection image. Therefore, in order to accurately control the image forming apparatus  201  to correct the apparatus  201  in the positional deviation among multiple developer (toner) images which are different in color, it is necessary to take into consideration, the deviation of the calculated position of the center of each positional deviation detection image, from the actual center of each positional deviation detection image, which is attributable to the “trailing edge piling”. 
         [0082]    One of the thinkable solutions to this problem is to use the value obtainable by subtracting the amount of error in the position of the trailing edge of the positional deviation detection image, which is attributable to the “trailing edge piling”, from the value which indicates the position of the trailing edge of the positional deviation detection image, which is determined by calculation based on the output signal from the sensor unit  225 . In this case, the position of the center of each of the positional deviation detection images is obtained based on the corrected position of the tail end edge of the positional deviation detection image. Then, the amount of positional deviation among the positional deviation detection images which are different in color are obtained by comparing the positional deviation detection images in terms of the position of their center. Then, the image forming apparatus  201  is corrected in the positional deviation which occurs as multiple color developer images are sequentially transferred onto its intermediary transfer belt  219 . Therefore, even if the “trailing edge piling” occurs, it is possible to accurately and precisely correct the image forming apparatus  201  in the positional deviation among multiple monochromatic developer images, different in color, which are formed to yield a multicolor image. 
         [0083]    [Effect of Environment and Color upon Amount of “Trailing Edge Filing”] 
         [0084]    The amount of “trailing edge piling” is affected by the changes in the temperature/humidity of the environment in which the image forming apparatus  201  is being used. Thus, the error in the detected amount of positional deviation among the positional deviation detection images reflects the changes in the temperature/humidity of the environment in which the image forming apparatus  201  is being used. More specifically, in an environment which is low in temperature/humidity, the triboelectric charge given to developer is narrow in its range of potential, and therefore, the amount of “trailing edge piling” is small. Therefore, the detected positional deviation among the positional deviation detection images is small in the amount of error. In comparison, as the environment in which the image forming apparatus  201  is being operated increases in temperature/humidity, the triboelectric charge which the developer acquires widens in the range of its potential, which in turn increases the amount of the “trailing edge piling”. Therefore, the calculated amount of the positional deviation among the positional deviation detection images is more erroneous than when the environment in which the image forming apparatus  201  is being operated is low in temperature/humidity. 
         [0085]    In this embodiment, therefore, the amount of error in the calculated position of the center of each positional deviation detection image, which is attributable to the “trailing edge piling” is estimated based on the ambient temperature/humidity of the image forming apparatus  201 . Then, this estimated error is taken into consideration when the amount of positional deviation of each positional deviation detection image, relative to the referential positional deviation detection image, in terms of the direction of the secondary scan, which is calculated based on the output signal from the sensor unit  225 , is corrected. The thus obtained amount of positional deviation will be referred to as “second amount of positional deviation”, hereafter. Then, the image forming apparatus  201  is corrected in the positional deviation, based on the second amount of positional deviation. Thus, even if the “trailing edge piling” occurs to the positional deviation detection images, that is, even if developer piles along the trailing edge of the positional deviation detection image while the positional deviation detection image is formed, the error attributable to the “trailing edge piling” is minimized in its effect upon the calculation of the position of the trailing edge of the positional deviation detection image. Therefore, the image forming apparatus  201  can be accurately corrected in the positional deviation among the multiple color images, different in color, which are formed to yield a multicolor image. 
         [0086]    Next, the method for estimating the amount of the error attributable to the effect of the ambient temperature/humidity of the image forming apparatus  201  upon the amount of the “trailing edge piling”, is described. Referring to  FIG. 9 , in this embodiment, each of the temperature range and humidity range is divided into three sub-ranges (Sub-ranges A-C). The amount by which developer piles up along the trailing edge of the positional deviation detection image while the positional deviation detection image is formed is affected by the color of the positional deviation detection image. In this embodiment, therefore, the position of the trailing edge of each positional deviation detection image is calculated based on the output signal from the sensor unit  225 , under each sub-range of the temperature/humidity range, and the amount of the error in the calculated position of the positional deviation detection image, which is attributable to the “trailing edge piling” is found out in advance. Then, this information about the error is organized into a table which shows the relationship between the ambient temperature/humidity range (three sub-ranges) and developer color, and is stored in advance in a storage means  230 . This information about the error is such information that shows the correlation between the environment in which the image forming apparatus  201  is being operated, and the error, and is used as the amount by which the first amount of positional deviation is corrected (amount for compensating for “trailing edge piling”). 
         [0087]      FIG. 1  is an example of a table which shows the relationship between the temperature/humidity range (having three sub-ranges), and the amounts PDe_ky, PDe_my and PDe_cy, by which the position of the trailing edge of the positional deviation detection image of black, magenta, and cyan colors, respectively, are to be corrected. In this embodiment, the temperature/humidity range is divided into three sub-ranges. However, it may be divided into four or more smaller sub-ranges. Further, instead of a table such as the one in  FIG. 1 , a formula for calculating the amount by which compensation is to be made for the “trailing edge piling” based on the temperature/humidity detected by the environment sensor  227  may be provided. Further,  FIG. 1  is for the case in which the calculated position of each of the positional deviation detection images of black, magenta, and cyan colors, relative to the calculated position of the positional deviation detection image of yellow color, that is, the referential positional deviation detection image, is corrected. However, it is not mandatory that the color of the referential positional deviation detection image is yellow. That is, in a case where color other than yellow is chosen as the color for the referential positional deviation detection image, the table in  FIG. 1  is to be modified so that it shows the relationship between the temperature/humidity range (three sub-ranges) and the amount by which the calculated position of the trailing edge of each of the positional deviation detection images, which is different in color from the referential positional deviation detection image, in terms of the amount by which the calculated position of each positional deviation detection image is corrected. 
         [0088]    The control section  206  uses the amount of the positional deviation among multiple (four) developer (toner) images, different in color, which is determined based on the output signal from the sensor unit  225 , and the amount of the “trailing edge piling”, which is affected by the ambient temperature/humidity detected by the environment sensor  227 . More specifically, the control section  206  adjusts the image forming apparatus  201  in the timing with which each developer (toner) image is formed by the image forming means. 
         [0089]    [Control Sequence for Correcting Image Forming Apparatus in Positional Deviation among Multiple Developer Images] 
         [0090]      FIG. 10  is a flowchart of the control sequence for correcting the image forming apparatus  201  in the positional deviation among multiple monochromatic developer images which the image forming apparatus  201  forms to yield a multicolor image. Hereafter, this control sequence will be referred to simply as “positional deviation control sequence”. In Step  1001 , the printable image data generation section  204  receives a calibration command, which is a command for making the image forming apparatus  201  carry out the positional deviation control sequence. As the section  204  receives the calibration command, the control section  206  obtains the values of the ambient temperature/humidity detected by the environment sensor  227 . 
         [0091]    In Step  1002 , the control section  206  obtains, from the table for compensating for the “trailing edge piling”, the amount by which the calculated position of the trailing edge of each positional deviation detection image is to be corrected according to the temperature/humidity detected by the environment sensor  227 . 
         [0092]    In Step  1003 , the control section  206  determines (calculates) the amount of the positional deviation. More specifically, it makes the image forming apparatus  201  form positional deviation detection images on the intermediary transfer belt  219 , obtains the output signal of the sensor unit  225 , and calculates the front edge position, trailing edge position, and center position of each positional deviation detection image, based on the output signal of the sensor unit  225 . 
         [0093]    In Step  1004 , the control section  206  calculates the first amounts PDd 1 _ky, PDd 1 _my and PDd 1 _cy of positional deviation, from the position of the center of each positional deviation detection image, calculated based on the output signal of the sensor unit  225 . 
         [0094]    In Step  1005 , the control section  206  calculates the amounts PDe_ky, PDe_my and PDe_cy, by which the position of the trailing edge of the positional deviation detection image of black, magenta, and cyan colors, respectively, are to be corrected according to the ambient temperature/humidity detected by the environment sensor  227 , with reference to the compensation table for the “trailing edge piling”. Then, the control section  206  calculates the second amounts PDd 2 _ky, PDd 2 _km and PDd 2 _kc of positional deviation, from the calculated amount of compensation for the “trailing edge piling”, and the first amounts of positional deviation, using the following mathematical equations: 
         [0000]        PDd 2 —   ky=PDd 1+ ky−PDe   —   ky,   
         [0000]        PDd 2 —   my=PDd 1+ my−PDe   —   my,   
         [0000]        PDd 2 —   cy=PDd 1+ cy−PDe   —   cy.   
         [0095]    In Step  1006 , the control section  206  transmits the second amounts of positional deviation to the printable image generation section  204 , and makes the section  204  adjust, in proportion to the second amounts of the positional deviation, the position, in terms of the secondary scan direction, at which multiple images, different in color, begin to be written. Then, the control section  206  controls the image formation stations so that they begin to write multiple images, different in color, one for one, with the timings which correspond to the second amount of positional deviation. This correctional control sequence is only one of many correctional control sequences available for correcting the image forming apparatus  201 , more specifically, the image forming stations (image forming means), in the position of the developer images formed on the image bearing members, one for one, by the image forming means, based on the second amount of positional deviation, in order to ensure that the registration of the monochromatic color toner images, different in color, formed on the photosensitive drums  215 , one for one, will be such that the monochromatic color toner images properly align on the intermediary transfer belt  219 , or the medium, onto which the monochromatic toner images are transferred in layers. As for the other correctional control sequences, there are a method which electrically corrects image formation signals, a method which drives the mirror, which is positioned in the laser beam path, in order to compensate for the error in the length and/or angle of the laser beam path for each color, which is attributable to the mechanical error, such as the error in the distance among the photosensitive drums, which is attributable to the attachment of the photosensitive drums, and the like methods. In the case of such methods, the control section  206  corrects the image formation signal, based on the table stored in the ROM of the control section  206 , in order to correct the image forming apparatus  201  in terms of the error in the registration (positional deviation) of each monochromatic color image on the photosensitive drum. 
         [0096]    In this embodiment, the position of the trailing edge of each positional deviation detection image, which is calculated based on the output signal from the sensor unit  225 , is corrected in consideration of the error attributable to the “trailing edge piling”. Then, the image forming apparatus  201  is corrected in the image formation operation, based on the corrected position of the trailing edge of the positional deviation detection image, so that the apparatus  201  is minimized in the amount of positional deviation among multiple monochromatic image, different in color, which are formed to yield a multicolor image. That is, the information related to the amount by which compensation is to be made for the error in the position of the trailing edge of the positional deviation detection image, which is determined based on the output signal of the sensor unit  225 , that is, the distance between the position of the trailing edge of the positional deviation detection image, which is determined based on the output signal of the sensor unit  225  and the actual position of the trailing edge of the positional deviation detection image, which is attributable to the unintended increase in image density which occurs to the downstream edge portion of the positional deviation detection image, in terms of the direction in which recording medium is conveyed or the intermediary transfer belt  219  is moved, when a monochromatic developer image is transferred onto a sheet of recording medium or the intermediary transfer belt  219 , is stored in advance for each color. Then, the image forming apparatus  201  is corrected in the position in which the developer image is formed by the image forming means, based on the information about the position of each of multiple positional deviation detection images, different in color, formed on a sheet of recording medium or the intermediary transfer belt  219 , and the information about the amount by which the position of the positional deviation detection image is to be corrected, in order to minimize the image forming apparatus  201  in terms of the positional deviation among the multiple monochromatic developer images, different in color, which are formed to yield a multicolor image. Further, the effect of the temperature/humidity (environmental factors) of the environment in which the image forming apparatus  201  is being operated, upon the amount of the “trailing edge piling” is taken into consideration. Therefore, even if the “trailing edge piling” occurs, that is, even if developer particles pile along the trailing edge of the exposed portion of the peripheral surface of the photosensitive drum  215 , the image forming apparatus  201  is highly precisely corrected in the positional deviation among the multiple monochromatic developer images formed to yield a multicolor image. 
         [0097]    In this embodiment, first, the amount (first amount) of positional deviation of each positional deviation detection image, relative to the referential positional deviation detection image (positional deviation detection image of yellow color in this embodiment), in terms of the secondary scan direction, is calculated, and then, the first amount of positional deviation is adjusted based on the information about the error which is attributable to the “trailing edge piling” and may be in the calculated position of the trailing edge of the positional deviation detection image. However, the method for compensating for the error in the position of the trailing edge of the positional deviation detection image, which is attributable to the “trailing edge piling”, based on the information about the amount by which the first amount of positional deviation is to be corrected, is not limited to the above described one. For example, the amount of the positional deviation, which does not include the error attributable to the “trailing edge piling”, may be calculated by correcting the amount of the positional deviation of the trailing edge of the positional deviation detection image, which is calculated from the output signal from the sensor unit  225 , using the information about the amount by which the correction is to be made. Further, it may be calculated by correcting the position of the trailing edge of the positional deviation detection image calculated from the output signal from the sensor unit  225 , based on the information about the amount by which the correction is to be made, and then, calculating the amount of positional deviation, from the corrected position of the trailing edge of the positional deviation detection image. The values in  FIG. 1  which are for the amount by which the trailing edge position of the positional deviation detection image calculated from the output signal from the sensor unit  225  is to be adjusted, have only to be set and stored in advance, according to what kind of factor is to be adjusted in value. 
       Embodiment 2 
       [0098]    Next, the second embodiment of the present invention is described. The basic structure of the sensor unit, and the basic design and arrangement of the positional deviation detection images, in this embodiment are the same as those in the first embodiment. Therefore, they are not going to be described in detail here. 
         [0099]    The environment sensor  227  positioned in the image forming apparatus  201  in this embodiment is a temperature detection element such as a thermistor. More specifically, the environment sensor  227  is placed with the image forming apparatus  201 , in the area which is unlikely to be affected by the heat generated within the image forming apparatus  201 , and yet, in which the environment sensor  227  can accurately detect the ambient temperature of the image forming apparatus  201 . The environment sensor  227  detects the ambient temperature of the image forming apparatus  201 . In this embodiment, the environment sensor  227  is positioned inside the image forming apparatus  201 , in the area in which the environment sensor  227  can accurately detect the ambient temperature of the image forming apparatus  201 . However, it is not mandatory that the environment sensor  227  is placed in the above described location. That is, the position in which the environment sensor  227  is to be placed may be any location as long as the location allows the environment sensor  227  to accurately detect the ambient temperature of the image forming apparatus  201 . 
         [0100]    The amount by which the error attributable to the “trailing edge piling” is to be corrected is set based on the temperature level detected by the environment sensor  227 . Referring to  FIG. 11 , the range into which the temperature detected by the environment sensor  227  is expected to fall is divided into three sub-ranges A-C, and the amount by which the error attributable to the “trailing edge piling” is set in advance for each sub-range. Further, since the amount of the “trailing edge piling”, that is, the amount by which developer piles along the trailing edge of an exposed area of the peripheral surface of the photosensitive drum  215  is affected by the color of the image to be formed on the photosensitive drum  215 . Therefore, the amount by which the error attributable to the “trailing edge piling” is to be corrected is set in advance for each color for the positional deviation detection image.  FIG. 12  is an example of a table which shows the relationship in terms of the amounts PDet_ky, PDet_my and PDet_cy by which the error attributable to the “trailing edge piling” is to be corrected, with respect to the temperature (three sub-ranges) detected by the see  227  and the colors of the positional deviation detection images. In this embodiment, the temperature range is divided into three sub-ranges. However, the temperature range may be divided into four or more narrower sub-ranges. Further, instead of relying on a table such as the one in  FIG. 12 , a formula for computing the amount by which compensation is made for the error attributable to the “trailing edge piling”, based on the ambient temperature of the image forming apparatus  201  may be created so that the error attributable to the “trailing edge piling” can be calculated based on the ambient temperature of the image forming apparatus  201 . That is, the information about the error attributable to the “trailing edge piling” to be stored in the storage means  230  may be a table like the one in  FIG. 1 , or a formula usable to compute the amount of error attributable to the “trailing edge piling”, based on the ambient temperature of the image forming apparatus  201  and/or the color of the developer. The control section control section  206  obtains the amount by which the error attributable to the trailing edge piling to be corrected, with reference to the table or formula stored in the storage means  230 . 
         [0101]    The operational sequence for correcting the image forming apparatus in the positional deviation among the multiple developer images, different in color, which are formed to yield a multicolor image is the same as the one in the first embodiment. That is, it is the same as Steps  1001 - 1006 . In this embodiment, the second amounts PDd 2 _ky, PDd 2 _my and PDd 2 _cy of the positional deviation are calculated with the use of the following equations: 
         [0000]        PDd 2 —   ky=PDd 1 —   ky+PDet   —   ky,   
         [0000]        PDd 2 —   my=PDd 1 —   my+PDet   —   my,   
         [0000]        PDd 2 —   cy=PDd 1 —   cy+PDet   —   cy.   
         [0102]    In this embodiment, the environment sensor  227  is a temperature detection element. However, it may be a humidity detection sensor. In a case where it is a humidity detection sensor, the humidity range is divided into three sub-ranges A-C, as shown in  FIG. 13 , and a table such as the one in  FIG. 12  which shows the relationship between the humidity range (having three sub-ranges), and the amounts by which the position of the trailing edge of the positional deviation detection image of black, magenta, and cyan colors, respectively, are to be adjusted, is stored in the storage means  230 . Then, the amount by which the position of the trailing edge of the positional deviation detection image, which is calculated based on the output signal from the sensor unit  225 , is to be corrected in terms of the error attributable to the “trailing edge piling” is calculated based on the detected humidity, with reference to the table in the storage means  230 . In other words, the second amount of positional deviation may be calculated from the first amount of positional deviation and the amount by which the position of the trailing edge of the positional deviation detection image calculated from the output signal of the sensor unit  225  is to be adjusted in terms of the error attributable to the “trailing edge piling”, as in the first embodiment. 
         [0103]    The control section  206  makes the image formation stations start writing images, different in color, one for one, with the timings based on the second amount of positional deviation. 
         [0104]    In this embodiment, the image forming apparatus  201  is more simply structured than the one in the first embodiment. That is, the condition of the environment in which the image forming apparatus  201  is being used is detected by using only the temperature sensor or humidity sensor, and the amount by which the trailing edge position of each positional deviation detection image detected by the sensor unit  225  is to be corrected in terms of the error attributable to the “trailing edge piling” is calculated based on the ambient humidity detected by the humidity sensor. Then, the point at which each of multiple developer images, different in color, is to begin to be written is adjusted by the calculated amount by which the trailing edge position of each positional deviation detection image detected by the sensor unit  225  is to be corrected in terms of the error attributable to the “trailing edge piling”. Therefore, even in a case where the “trailing edge piling” occurs, that is, developer piles along the trailing edge of an exposed portion of the peripheral surface of the photosensitive drum  215 , the image forming apparatus  201  is highly precise in the position at which each of multiple developer images, different in color, is to begin to be written. 
         [0105]    In the preceding embodiments, the amount (first amount) by which the position of the trailing edge of each positional deviation detection image, which is detected by the sensor unit  225  and includes the error attributable to the “trailing edge piling”, is to be adjusted in consideration of the error attributable to the “trailing edge piling” to obtain the second amount by which the position at which each of the multiple images, different in color, is to begin to be written is adjusted. Then, based on the second amount of positional deviation, the image forming apparatus  201  is adjusted in the timing with which each of the multiple images, different in color, is to begin to be written (by laser). In the present invention, however, the timing with which the writing of the positional deviation detection image on the intermediary transfer belt  219  ends, that is, the timing with which the trailing edge of the positional deviation detection image is written on the intermediary transfer belt  219 , may be advanced by the amount equivalent to the amount of error attributable to the “trailing edge piling”. That is, the image forming apparatus  201  may be adjusted in the registration, in advance for the image forming means, by the amount equivalent to the error attributable to the “trailing edge piling”, in order to make the timing with which the writing of the trailing edge of the positional deviation detection image is to be ended, earlier than the original timing set by the information about the image to be formed, by modifying the control program and/or table to be used to control the operation of the image forming means based on the information about the image to be formed. 
         [0106]    As the image forming means is controlled as described above, the portion of the intermediary transfer belt  219 , on which the trailing edge of the positional deviation detection image is to be formed, shifts frontward (upstream in terms of moving direction of intermediary transfer belt  219 ) of the original portion of the intermediary transfer belt  219 , on which the trailing edge of the positional deviation detection image is to be formed. On the other hand, as described above, due to the error attributable to the “trailing edge piling”, the position of the trailing edge of the positional deviation detection image calculated based on the output signal from the sensor unit  225  deviates rearward (downstream in terms of moving direction of intermediary transfer belt  219 ). Thus, if the timing with which the writing of each positional deviation detection image ends is advanced so that the frontward deviation and rearward deviation cancel each other, it is possible to make the trailing edge position of the positional deviation detection image calculated from the output signal from the sensor unit  225  precisely coincide with the point at which the trailing edge of the positional deviation detection image will be actually. 
         [0107]    In this case, the amount (first amount) of positional deviation to be calculated based on the position of the trailing edge of the positional deviation detection image calculated based on the output signal from the sensor unit  225  will have been rid of the error attributable to the “trailing edge piling”. Therefore, the image forming apparatus  201  is more precisely corrected in the amount of positional deviation among the multiple developer images, different in color, than in a case where it is corrected based on the first amount of the positional deviation in the preceding embodiments. 
         [0108]    Further, in the preceding embodiments, the image forming apparatus  201  was structured so that developer images were transferred (primary transfer) from the photosensitive drums  215 , as image bearing members, onto the intermediary transfer belt  219 , as intermediary transfer medium, and then, the developer images on the intermediary transfer belt  219  are transferred (secondary transfer) onto a sheet of recording paper. However, the present invention is also applicable to an image forming apparatus equipped with a transferring means which directly transfers developer images onto a sheet of recording paper, as final recording medium, from the photosensitive drums  215 . In a case where the present invention is applied to such an image forming apparatus, the sensor unit  225  detects the position of the trailing edge of positional deviation detection images formed on a sheet of recording paper. 
         [0109]    Further, in each of the preceding embodiments, the photosensitive drums  215  were fixed in position, and the intermediary transfer belt  219  was circularly moved. Therefore, the developer images, different in color, are different in position at which they are transferred onto the intermediary transfer belt  219 . However, the present invention is also applicable to an image forming apparatus structured so that its multiple photosensitive drums are sequentially moved into a single location in which they are transferred onto the intermediary transferring member (primary transferring member) or a sheet of recording medium (paper) (secondary transfer member). 
         [0110]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0111]    This application claims priority from Japanese Patent Application No. 270296/2011 filed Dec. 9, 2011 which is hereby incorporated by reference.