Patent Publication Number: US-9417551-B2

Title: Image forming apparatus for density evenness in a vertical scanning direction

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus. 
     2. Description of Related Art 
     Conventionally, in an electro-photographic type image forming apparatus, when an image is formed on both the front face and the back face of the same sheet, since the toner image formed on the sheet is heated to be fixed, the sheet after forming the image on the front face shrinks about a few %. Therefore, when the image is formed on both faces of the sheet, the image on the back face needs to be formed considering the shrinkage of the sheet, or a problem such as the size of the image being different between the front face and the back face occurs. As one method to solve the above problem, there is a technique to control the magnification of the image by adjusting the rotating speed of the polygon motor for rotating the polygon mirror between the front face and the back face (see Japanese Patent Application Laid-Open Publication No. 2007-179005). 
     Moreover, in an electro-photographic type image forming apparatus, unevenness in density in the vertical scanning direction may occur depending on the rotating cycle of the photoreceptor drum, developer or the like. Usually, the image processing is performed in a line cycle. Therefore, according to a simple configuration using a counter to count up in the line cycle, the position of the photoreceptor drum, etc. in the vertical scanning direction is calculated and the density unevenness in the vertical scanning direction is corrected based on the calculated position (Japanese Patent Application Laid-Open Publication No. 2007-156192). 
     However, if the rotating speed of the polygon motor is changed to adjust the magnification between the front face and the back face, the line cycle changes. With this, the interval of the photoreceptor drum, etc. in the vertical scanning direction which corresponds to the line cycle changes. This causes the problem that the calculated position of the photoreceptor drum, etc. in the vertical scanning direction becomes different in the amount that the magnification is adjusted if the line cycle is merely counted up. Therefore, according to the conventional technique, when the magnification is changed, the space between the sheets needed to be sufficiently opened by, for example, waiting for the position of the photoreceptor drum, etc. to come to its reference position in the vertical scanning direction and resetting the counter, etc. Consequently, high speed processing is not possible. 
     SUMMARY 
     The present invention has been made in consideration of the above problems, and one of the main objects is, in a case where density unevenness in a vertical scanning direction caused by a target component is corrected, to accurately calculate a position of the target component even when a cycle of a signal to calculate the position of the target component in the vertical scanning direction is changed. 
     In order to achieve at least one of the above-described objects, according to an aspect of the present invention, there is provided an image forming apparatus including: 
     an image forming unit which scans a laser light based on image data with a polygon mirror to expose light to a photoreceptor drum to form an electrostatic latent image on the photoreceptor drum and forms an image by attaching toner to the electrostatic latent image; 
     an image magnification changing unit which changes a rotating speed of a polygon motor for rotating the polygon mirror to change magnification of the image; 
     a reference position detecting unit which detects a reference position of a predetermined target component in a vertical scanning direction, the predetermined target component causing density unevenness in the vertical scanning direction; 
     a target component position calculating unit which calculates a position of the target component in the vertical scanning direction based on a detecting result of the reference position of the target component in the vertical scanning direction by the reference position detecting unit and a signal of a predetermined cycle according to the rotating speed of the polygon motor; 
     a storage unit which stores a correction table in which a position of the target component in the vertical scanning direction is corresponded to correction data to correct density unevenness caused by the target component; 
     an image data correcting unit which obtains the correction data corresponded to the position of the target component in the vertical scanning direction from the correction table based on the position of the target component in the vertical scanning direction calculated by the target component position calculating unit and corrects the image data based on the obtained correction data; and 
     a parameter correcting unit which corrects a parameter in the target component position calculating unit according to a timing that the rotating speed of the polygon motor is changed by the image magnification changing unit. 
     Preferably, in the image forming apparatus, 
     the target component position calculating unit includes a counter which is reset when the reference position of the target component in the vertical scanning direction is detected by the reference position detecting unit and adds a predetermined adding value each time based on the signal; and 
     the target component position calculating unit calculates the position of the target component in the vertical scanning direction based on a ratio of a counting value of the counter to a one rotation counting value corresponding to one rotation of the target component. 
     Preferably, in the image forming apparatus, 
     the signal is a signal showing a cycle of each line; and 
     the counter adds the adding value at each cycle of each line. 
     Preferably, in the image forming apparatus, 
     the adding value is a same value between before and after correction by the parameter correcting unit; and 
     the parameter correcting unit corrects the counting value of the counter and the one rotation counting value in the target component position calculating unit depending on the magnification according to the timing that the rotating speed of the polygon motor is changed by the image magnification changing unit. 
     Preferably, in the image forming apparatus, 
     the one rotation counting value is a same value between before and after correction by the parameter correcting unit; and 
     the parameter correcting unit corrects the adding value in the target component position calculating unit depending on the magnification according to the timing that the rotating speed of the polygon motor is changed by the image magnification changing unit. 
     Preferably, in the image forming apparatus, 
     the image magnification changing unit changes the magnification of the image according to whether the image is formed on a front face or a back face of a sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings, and thus are not intended to define the limits of the present invention, and wherein; 
         FIG. 1  is a block diagram showing a functional configuration of an image forming apparatus of the first embodiment; 
         FIG. 2  is a schematic diagram of a configuration of a photoreceptor drum and exposing unit of an image forming unit; 
         FIG. 3  is a functional block diagram of a vertical scanning density unevenness correcting unit; 
         FIG. 4  is a timing chart according to the first embodiment; and 
         FIG. 5  is a timing chart according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     First, the first embodiment of an image forming apparatus of the present invention is described. The image forming apparatus of the present invention is applied in an electro-photographic type copier, and the like. 
       FIG. 1  is a block diagram showing a functional configuration of an image forming apparatus  10  of the first embodiment. 
     As shown in  FIG. 1 , the image forming apparatus  10  includes a CPU (Central Processing Unit)  11 , an image reading unit  12 , an image memory  13 , an image forming unit  14 , an image processing unit  15 , a photoreceptor home sensor  16 , a SOS (Start of Scan) sensor  17 , a ROM (Read Only Memory)  18 , a RAM (Random Access Memory)  19 , an operation unit  20 , a display unit  21 , a storage unit  22 , and the like. 
     The CPU  11  reads various processing programs stored in the ROM  18  and expands the programs in the RAM  19 . According to the expanded programs, the CPU  11  controls each unit of the image forming apparatus  10 . 
     The image reading unit  12  includes a light source, a CCD (Charge Coupled Device) image sensor, an A/D convertor, etc. The image reading unit  12  reads an image of a document by imaging a reflecting light of light illuminated and scanned on a document from a light source and photoelectric conversion of the image. After A/D conversion of the read image, the obtained image data of R (red), G (green), and B (blue) is output to the CPU  11 . 
     The image memory  13  stores the image data obtained by the image reading unit  12 . 
     The CPU  11  synchronizes with the timing that the sheet is conveyed, and transmits image data (RGB data) from the image memory  13  to the image processing unit  15 . 
     The image forming unit  14  performs image forming in an electro-photographic format. The image forming unit  14  includes a photoreceptor drum  141  (see  FIG. 2 ), a charging unit which charges the photoreceptor drum  141 , an exposing unit which exposes and scans the surface of the photoreceptor drum  141  based on the image data and forms an electrostatic latent image (light source  142 , polygon mirror  143 , etc. of  FIG. 2 ), a developing unit which attaches toner to the electrostatic latent image on the photoreceptor drum  141 , a transfer unit which transfers a toner image formed on the photoreceptor drum  141  onto a sheet, a fixing unit which fixes the toner image formed on the sheet, and the like. 
       FIG. 2  shows a schematic configuration of the photoreceptor drum  141  and the exposing unit of the image forming unit  14 . 
     The image forming unit  14  includes, a photoreceptor drum  141 , a light source  142 , a polygon mirror  143 , a polygon motor  144 , a mirror  145 , etc. In the present embodiment, the photoreceptor drum  141  is provided as an example of a predetermined target component which causes density unevenness in the vertical scanning direction and the mechanism of calculating the position of the photoreceptor drum  141  is described. 
     The photoreceptor drum  141  rotates in a certain predetermined cycle, and causes density unevenness in the vertical scanning direction due to characteristics of the photoreceptor drum  141 . 
     The light source  142  is a semiconductor laser which emits laser light based on the image data. 
     The polygon mirror  143  is a polygonal columnar shape in which the side face is a mirror, and reflects laser light emitted from the light source  142 . The polygon mirror  143  rotates around a rotating axis so that the laser light reflected on the mirror scans from one edge of the photoreceptor drum  141  to the other edge and exposes light to the photoreceptor drum  141 . 
     The polygon motor  144  is a motor to rotate the polygon mirror  143 . The rotating speed of the polygon mirror  144  can be changed with the control by the CPU  11 . 
     The mirror  145  reflects the laser light reflected from the polygon mirror  143  and guides the light to the SOS sensor  17 . 
     The image processing unit  15  performs image processing on the image data read and obtained by the image reading unit  12  and outputs the result to the image forming unit  14 . The image processing unit  15  is realized with software processing by the CPU  11  in coordination with a program stored in the ROM  18 . The image processing unit  15  includes a color conversion unit  151  which performs color conversion processing on the image data, a vertical scanning density unevenness correcting unit  152  which performs vertical scanning density unevenness correction processing on the image data based on the correction data to erase density unevenness in the vertical scanning direction, a screen processing unit  153  which performs screen processing to change the image to dots based on the number of screen lines pre-set in the image data, and the like. 
     The image processing unit  15  is composed of a PLD (Programmable Logic Device) such as a FPGA (Field Programmable Gate Array), etc., or an integrated circuit such as an ASIC (Application Specific Integrated Circuit) or a combination of the above, and can perform image processing according to the function included in the circuit. 
     The photoreceptor home sensor  16  is a reference position detecting unit which detects a preset reference position in any position in the vertical scanning direction (circumference direction) of the photoreceptor drum  141 . For example, as shown in  FIG. 2 , a marker M showing the reference position is provided on the photoreceptor drum  141  in any position in the vertical scanning direction. The photoreceptor home sensor  16  outputs to the CPU  11  a home sensor signal showing the reference position of the photoreceptor drum  141  in the vertical scanning direction. 
     The SOS sensor  17  detects the laser light reflected by the mirror  145 , and outputs the SOS signal (horizontal scanning exposure start reference signal) to the CPU  11 . In other words, the SOS sensor  17  detects the timing of writing the line exposed by one edge of the side face (mirror) of the polygon mirror  143 . 
     The ROM  18  includes a nonvolatile semiconductor memory, etc. and stores various processing programs and data and files necessary to perform the programs. 
     The RAM  19  forms a work area to temporarily store the various processing programs read from the ROM  18 , input or output data, and the like when the CPU  11  performs the various processing. 
     The operation unit  20  includes a touch panel formed so as to cover the display screen of the display unit  21 , and various operation buttons such as a numeric button, start button and the like. The operation unit  20  outputs the operation signal according to the operation by the user to the CPU  11 . 
     The display unit  21  includes an LCD (Liquid Crystal Display), and displays various operation buttons, status of the apparatus, operation status of each function, and the like on the display screen according to an instruction of the display signal input from the CPU  11 . 
     The storage unit  22  includes a hard disk, a flash memory, etc., and stores various types of data. Specifically, a vertical scanning density unevenness correction LUT (Look Up Table)  221  is stored in the storage unit  22 . The vertical scanning density unevenness correction LUT  221  is a correction table associating a position of the photoreceptor drum  141  in the vertical scanning direction (reference address) and the correction data to correct the density unevenness caused by the photoreceptor drum  141 . When the address of the vertical scanning density unevenness correction LUT  221  (position of the photoreceptor drum  141  in the vertical scanning direction) is B bits, one rotation of the photoreceptor drum  141  is divided into 256 segments, and correction data to correct the density unevenness in the vertical scanning direction is stored for each segment. 
       FIG. 3  is a functional block diagram of the vertical scanning density unevenness correcting unit  152 . 
     The vertical scanning density unevenness correcting unit  152  includes an image magnification changing unit  31 , a photoreceptor position calculating unit  32 , an image data correcting unit  33 , and a parameter correcting unit  34 . 
     The image magnification changing unit  31  changes the rotating speed of the polygon motor  144  to change the magnification of the image. The image magnification changing unit  31  changes the magnification of the image according to front face or back face of the sheet on which the image is formed. Specifically, the image magnification changing unit  31  increases the rotating speed of the polygon motor  114  so that the rotating speed is faster in the back face processing than the front face processing. With this, the magnification of the image becomes smaller in the back face processing than the front face processing. 
     The photoreceptor position calculating unit  32  calculates the position (phase) of the photoreceptor drum  141  in the vertical scanning direction based on the detecting result of the reference position of the photoreceptor drum  141  in the vertical scanning direction by the photoreceptor home sensor  16  and a signal showing the cycle of each line according to the rotating speed of the polygon motor  144  (line synchronizing signal TG shown in  FIG. 4 ). Specifically, when the rotating speed of the polygon motor  144  becomes fast, the cycle of each line becomes short. 
     The cycle of the SOS signal corresponds to the time being exposed and scanned by one edge of the side face of the polygon motor  144  (when the polygon motor  144  is the hexagonal column, the time that the polygon motor  144  rotates ⅙ of the rotation). When the light source  142  emits n beams of laser light at the same time, the cycle of the SOS signal becomes n times the line cycle. 
     The photoreceptor position calculating unit  32  includes a counter  35  which is reset (0) when the reference position of the receptor drum  141  in the vertical scanning direction is detected by the photoreceptor home sensor  16 , and adds a predetermined adding value  36  for each cycle of each line based on the line synchronizing signal TG. The photoreceptor position calculating unit  32  calculates the position of the photoreceptor drum  141  in the vertical scanning direction based on a ratio of a counting value of the counter  35  to a one rotation counting value  37  corresponding to one rotation of the photoreceptor drum  141 . The address showing the position of the photoreceptor drum  141  in the vertical scanning direction can be obtained by the following formula.
 
address=256*present counting value of counter 35/one rotation counting value 37
 
     The image data correcting unit  33  obtains the correction data corresponding to the position of the photoreceptor drum  141  in the vertical scanning direction from the vertical scanning density unevenness correction LOT  221  based on the position of the photoreceptor drum  144  in the vertical scanning direction calculated by the photoreceptor position calculating unit  32  and corrects the image data based on the obtained correction data. 
     The parameter correcting unit  34  corrects the parameter of the photoreceptor position calculating unit  32  according to the timing that the rotating speed of the polygon motor  144  is changed by the image magnification changing unit  31 . Here, “according to the timing that the rotating speed of the polygon motor  144  is changed” is not limited to when the change of the rotating speed of the polygon motor  144  is detected. Image processing is performed before forming the image, therefore, the parameter of the photoreceptor position calculating unit  32  may be corrected before changing the rotating speed of the polygon motor  144 . The parameter is information showing a numeric value used in calculating the position of the photoreceptor drum  141  in the vertical scanning direction in the photoreceptor position calculating unit  32 . 
       FIG. 4  is a timing chart of the first embodiment. 
       FIG. 4  shows first master index signal MSTIND 1 , second master index signal MSTIND 2 , image valid region signal PAGE, select signal SEL, line synchronizing signal TG, polygon motor rotating speed, home sensor signal, and counter. 
     The first master index signal MSTIND 1  is a signal showing a SOS cycle in the front face processing of the sheet. 
     The second master index signal MSTIND 2  is a signal showing a SOS cycle in the back face processing of the sheet. 
     The image magnification changing unit  31  controls the rotating speed (rotating number) of the polygon motor  144  to match the SOS signal output from the SOS sensor  17  at the cycle of the first master index signal MSTIND 1  or the second master index signal MSTIND 2 . For example, assuming that the sheet shrinks 1% in the vertical scanning direction after forming the image on the front face (100% to 99%), the rotating speed of the polygon motor  144  needs to be corrected 100/99 times with reference to the front face when forming the image on the back face. Therefore, the cycle of the second master index signal MSTIND 2  becomes 1% shorter than that of the first master index signal MSTIND 1 . 
     The image valid region signal PAGE is a signal showing the image valid region in the vertical scanning direction. 
     The select signal SEL is the signal used for selecting the first master index signal MSTIND 1  or the second master index signal MSTIND 2 . 
     The image magnification changing unit  31  selects the first master index signal MSTIND 1  when the select signal SEL is 0, selects the second master index signal MSTIND 2  when the select signal SEL is 1, and generates the line synchronizing signal TG. 
     The line synchronizing signal TG is a signal used in the image processing unit  15 , and the image processing is performed in a cycle of each line synchronizing with the line synchronizing signal TG. When the print head is a 600 dpi/2beam structure, the image magnification changing unit  31  sets the cycle of the first master index signal MSTIND 1  or the second master index signal MSTIND 2  to ½, and generates the line synchronizing signal TG in a unit of 600 dpi. 
     If the line synchronizing signal TG is synchronized with the SOS signal (first master index signal MSTIND 1  or second master index signal MSTIND 2 ), the cycles do not have to be in a relation of 1:1.  FIG. 4  shows an example when the cycles of the line synchronizing signal TG and the SOS signal are in a relation of 1:2. 
     The photoreceptor  141  operates independently from the sheet and is not synchronized with the conveying of the sheet. In other words, the reference position is not always detected by the photoreceptor home sensor  16  between one sheet and the next sheet, and the counter  35  of the photoreceptor position calculating unit  32  is not always reset. The calculating of the position of the photoreceptor drum  141  in the vertical scanning direction by the photoreceptor position calculating unit  32  needs to be performed continuously among sheets even when the image processing is not performed. Here, the reference position of the photoreceptor drum  141  in the vertical scanning direction is detected by the photoreceptor home sensor  16  before the front face processing and then the reference position of the photoreceptor drum  141  in the vertical scanning direction is detected by the photoreceptor home sensor  16  after the back face processing. 
     According to the first embodiment, the adding value  36  used in the photoreceptor calculating unit  32  is the same between before and after the correction by the parameter correcting unit  34 . Here, 1 is used as the adding value  36 . 
     The parameter correcting unit  34  corrects the counting value of the counter  35  and the one rotation counting value  37  in the photoreceptor position calculating unit  32  depending on the magnification according to the timing that the rotating speed of the polygon motor  144  is changed by the image magnification changing unit  31 . Specifically, when the magnification before change is x and the magnification after change is y, the parameter correcting unit  34  multiplies the counting value of the counter  35  and the one rotation counting value  37  by x/y times. 
     The counter  35  operates based on the line synchronizing signal TG. When the reference position of the photoreceptor drum  141  in the vertical scanning direction is detected by the photoreceptor home sensor  16 , the counter  35  is reset to 0. Then, the counter  35  adds 1 each time the line synchronizing signal TG doubling the frequency of the first master index signal MSTIND 1  is input. 
     In an example where the sheet shrinks 1% in the vertical scanning direction after forming the image on the front face, the parameter correcting unit  34  corrects the counting value of the counter  35  by multiplying 100/99 times at the timing that the select signal SEL is changed to 1 when the front face processing ends and the processing advances to the back face processing. Then, the counter  35  adds 1 each time the line synchronizing signal TG doubling the frequency of the second master index signal MSTIND 2  is input. 
     When the counter value is corrected, even after switching to the back face processing, it is as if the counter  35  counts with the line synchronizing signal TG doubling the frequency of the second master index signal MSTIND 2  from the point when the reference position is detected by the photoreceptor home sensor  16  before the front face processing (when the counter  35  is reset). 
     The one rotation counting value  37  of the photoreceptor drum  141  is preset for the front face processing and the back face processing (one rotation counting value A, one rotation counting value B). The parameter correcting unit  34  corrects the one rotation counting value  37  of the photoreceptor position calculating unit  32  to the one rotation counting value A of the front face processing at the timing when the select signal SEL is changed to 0, and corrects the one rotation counting value  37  of the photoreceptor position calculating unit  32  to the one rotation counting value B of the back face processing at the timing when the select signal SEL is changed to 1. The one rotation counting value A of the front face processing and the one rotation counting value B of the back face processing are values according to the ratio of the magnification between the front face and the back face. The values are inversely proportional to the magnification and the cycle of the line synchronizing signal TG, and are in proportion to the rotating speed of the polygon motor  144 . When the sheet shrinks 1% in the vertical scanning direction after the image is formed on the front face, the following is established, one rotation counting value A: one rotation counting value B=99:100. 
     The photoreceptor position calculating unit  32  outputs the address calculated using the following formula in the front face processing to the image data correcting unit  33 .
 
address=256*present counting value of the counter 35/one rotation counting value  A  
 
     Alternatively, the photoreceptor position calculating unit  32  outputs the address calculated using the following formula in the back face processing to the image data correcting unit  33 .
 
address=256*present counting value of the counter 35/one rotation counting value  B  
 
     As described above, according to the first embodiment, the position of the photoreceptor drum  141  can be correctly calculated when the density unevenness in the vertical scanning direction caused by the photoreceptor drum  141  is corrected even if the cycle of the signal (line synchronizing signal TG) for calculating the position of the photoreceptor drum  141  in the vertical scanning direction is changed. 
     Specifically, the position of the photoreceptor drum  141  in the vertical scanning direction can be calculated based on the ratio of the counting value in the counter  35  to the one rotation counting value  37  corresponding to one rotation of the photoreceptor drum  141 . 
     Even if the photoreceptor drum  141  is not in the preset reference position, the position of the photoreceptor drum  141  can be correctly calculated. Therefore, the adjustment of the magnification among sheets can be done reducing the interval between sheets without waiting for the counter  35  to be reset. 
     The adding value  36  is the same between before and after the correction by the parameter correcting unit  34 , and the parameter correcting unit  34  corrects the counting value of the counter  35  and the one rotation counting value  37  in the photoreceptor position calculating unit  32  depending on the magnification according to the timing that the rotating speed of the polygon motor  144  is changed by the image magnification changing unit  31 . Therefore, the position of the photoreceptor drum  141  can be correctly calculated even after the rotating speed of the polygon motor  144  is changed. 
     In the first embodiment, an example using 1 as the adding value  36  is described. However, as long as the value is the same between before and after the correction by the parameter correcting unit  34 , other values can be used as the adding value  36 . 
     Second Embodiment 
     Next, the second embodiment applying the present invention is described. 
     The image forming apparatus of the second embodiment has a configuration similar to that of the image forming apparatus  10  shown in the first embodiment. Therefore,  FIG. 1  to  FIG. 3  are to be referred and the illustration and description of the configuration is omitted. Below, the configuration and the processing characteristic to the second embodiment are described. 
       FIG. 5  is a timing chart of the second embodiment. 
       FIG. 5  shows first master index signal MSTIND 1 , second master index signal MSTIND 2 , image valid region signal PAGE, select signal SEL, line synchronizing signal TG, polygon motor rotating speed, home sensor signal, and counter. 
     The first master index signal MSTIND 1 , the second master index signal MSTIND 2 , the image valid region signal PAGE, the select signal SEL, the line synchronizing signal TG, the polygon motor rotating speed, and the home sensor signal are the same as those of the first embodiment. 
     In the second embodiment, the one rotation counting value  37  used in the photoreceptor position calculating unit  32  is the same between before and after the correction by the parameter correcting unit  34  (one rotation counting value C). 
     The parameter correcting unit  34  corrects the adding value  36  in the photoreceptor position calculating unit  32  depending on the magnification according to the timing that the rotating speed of the polygon motor  144  is changed by the image magnification changing unit  31 . Specifically, when the magnification before change is x and the magnification after change is y, the parameter correcting unit  34  multiplies the adding value  36  by y/x times. 
     The counter  35  operates based on the line synchronizing signal TG. When the reference position of the photoreceptor drum  141  in the vertical scanning direction is detected by the photoreceptor home sensor  16 , the counter  35  is reset to 0. Then, the counter  35  adds the predetermined adding value D each time the line synchronizing signal TG doubling the frequency of the first master index signal MSTIND 1  is input. 
     When the sheet shrinks 1% in the vertical scanning direction after forming the image on the front face, the parameter correcting unit  34  corrects the value to a value multiplying the adding value  36  of the counter  35  by 99/100 times (adding value E) at the timing when the select signal SEL is changed to 1 when the front face processing ends and the processing advances to the back face processing. Then, the counter  35  adds the adding value E each time the line synchronizing signal TG doubling the frequency of the second master index signal MSTIND 2  is input. 
     The adding value D and the adding value E are values according to the ratio of the magnification between the front face and the back face. The values are in proportion to the magnification and the cycle of the line synchronizing signal TG, and are inversely proportional to the rotating speed of the polygon motor  144 . When the sheet shrinks 1% in the vertical scanning direction after the image is formed on the front face, the following is established, adding value D: adding value E=100:99. 
     The adding value  36  is adjusted according to the ratio of the magnification, and therefore, the counting value after one rotation of the photoreceptor drum  141  (one rotation counting value  37 ) does not change. 
     As described above, according to the second embodiment, the position of the photoreceptor drum  141  can be correctly calculated when the density unevenness in the vertical scanning caused by the photoreceptor drum  141  direction is corrected even if the cycle of the signal (line synchronizing signal TG) for calculating the position of the photoreceptor drum  141  in the vertical scanning direction is changed. 
     Specifically, the position of the photoreceptor drum  141  in the vertical scanning direction can be calculated based on the ratio of the counting value in the counter  35  to the one rotation counting value  37  corresponding to one rotation of the photoreceptor drum  141 . 
     Even if the photoreceptor drum  141  is not in the preset reference position, the position of the photoreceptor drum  141  can be correctly calculated. Therefore, the adjustment of the magnification among sheets can be done reducing the interval between sheets without waiting for the counter  35  to be reset. 
     The one rotation counting value  37  is the same between before and after the correction by the parameter correcting unit  34 , and the parameter correcting unit  34  corrects the adding value  36  in the photoreceptor position calculating unit  32  depending on the magnification according to the timing that the rotating speed of the polygon motor  144  is changed by the image magnification changing unit  31 . Therefore, the position of the photoreceptor drum  141  can be correctly calculated even after the rotating speed of the polygon motor is changed. 
     The description of the above described embodiments are examples of the image forming apparatus of the present invention, and the present invention is not limited to the above. The detailed configuration of each unit and the detailed operation composing the apparatus can be suitably modified without leaving the scope of the present invention. 
     For example, in the above described embodiments, the photoreceptor drum  141  is described as an example of the target component which causes the density unevenness in the vertical scanning direction. However, the target component can be a developer, an intermediate transfer belt or the like. 
     In the above described embodiments, the signal showing the cycle of each line (line synchronizing signal TG) is used as the signal with the predetermined cycle according to the rotating speed of the polygon motor  144 , and the counter  35  counts up in a unit of the cycle of each line. However, a signal other than the line synchronizing signal TG can be used if the signal is a cycle corresponding to the rotating speed of the polygon motor  144 . However, preferably a signal with the same cycle as the line synchronizing signal TG or a signal with a shorter cycle is used in order to accurately calculate the position of the photoreceptor drum  141  in the vertical scanning direction. 
     According to the above description, a ROM is used as the computer readable medium storing the program to execute each processing, however, the present invention is not limited to the above. A nonvolatile memory such as a flash memory or a portable recording medium such as a CD-ROM can be used as the computer readable medium. Moreover, as the medium providing the data of the program through communication lines, a carrier wave can be applied. 
     The present U.S. patent application claims priority under the Paris Convention of Japanese Patent Application No. 2014-027279 filed on Feb. 17, 2014 the entirety of which is incorporated herein by reference.