Abstract:
The present invention provides an image forming apparatus including: a rotating body that holds recording medias on a circumferential surface; an image forming section that forms an image; a generating section that generates a pulse and a reference pulse; a measuring section that measures the number of pulses; a computing section that each time when predetermined number of pulses from generation of the reference pulse has been measured, divides a pulse width of a pulse before predetermined number of pulses by a first value to compute a pulse division time; and a controlling section that each time when predetermined number of pulses are measured from generation of the reference pulse, multiplies the pulse division time by a second predetermined value, thereby obtaining a forming start time, and that controls to start image formation when the forming start time has elapsed from the measurement of predetermined number of pulses.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-330713 filed Dec. 25, 2008. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an image forming apparatus, an image forming method, and a computer-readable medium storing a program. 
     SUMMARY 
     According to an aspect of the present invention, there is provided an image forming apparatus including: a rotating body that rotates and holds a plurality of recording media on a circumferential surface of the rotating body; an image forming section that forms an image on the recording media held on the rotating body when rotating; a generating section that generates a pulse signal according to the rotation amount of the rotating body and generates a reference pulse for every rotation cycle of the rotating body; a measuring section that measures the number of pulses of the pulse signal; a computing section that each time when a predetermined number of pulses from the time of generation of the reference pulse has been measured, divides a time of a pulse width of a pulse before the predetermined number of pulses by a first value to compute a pulse division time, the predetermined number of pulses being predetermined for each of the plurality of recording media; and a controlling section that each time when the predetermined number of pulses are measured from the time of generation of the reference pulse, multiplies the pulse division time by a second predetermined value, thereby obtaining a forming start time, and that controls the image forming section so as to start image formation when the forming start time has elapsed from the time of measurement of the predetermined number of pulses, the second predetermined value being predetermined for each of the plurality of recording media. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a diagram illustrating the configuration of an image forming apparatus according to the exemplary embodiment; 
         FIG. 2  is a diagram illustrating the structure of an image forming drum of the exemplary embodiment; 
         FIG. 3  is a block diagram illustrating the essential configuration of the electric system of the image forming apparatus according to the exemplary embodiment; 
         FIG. 4  is a flowchart of an image forming control process executed by a CPU of the image forming apparatus of the exemplary embodiment; 
         FIG. 5  is a flowchart of the image forming control process executed by the CPU of the image forming apparatus of the exemplary embodiment; 
         FIG. 6  is a timing chart for explaining the operation of the image forming apparatus of the exemplary embodiment; 
         FIG. 7  is a diagram illustrating an example of a procedure for determining Na, Nb, ia, and ib; 
         FIG. 8  is a diagram for explaining an example of the procedure for determining Na, Nb, ia, and ib. 
     
    
    
     DETAILED DESCRIPTION 
     Herebelow, an example of an exemplary embodiment of the present invention will be described in detail with reference to the drawings. 
     In the exemplary embodiment, the present invention is applied to an image forming apparatus of an ink jet type.  FIG. 1  is a diagram illustrating the configuration of an image forming apparatus  10  according to the exemplary embodiment. 
     As illustrated in  FIG. 1 , the image forming apparatus  10  has a sheet feeding/conveying section  12  that feeds and conveys a recording sheet W which is a recording medium. A treatment liquid applying section  14 , an image forming section  16 , an ink drying section  18 , an image fixing section  20 , and a discharging and conveying section  24  are provided along the conveying direction of the recording sheet W at a downstream side of the sheet feeding/conveying section  12 . The treatment liquid applying section  14  applies a treatment liquid onto the recording side (surface) of the recording sheet W. The image forming section  16  forms an image on the recording side of the recording sheet W. The ink drying section  18  dries the image formed on the recording side. The image fixing section  20  fixes the dried image onto the recording sheet W. The discharging/conveying section  24  conveys the image fixed recording sheet W to a discharging section  22 . 
     The sheet feeding/conveying section  12  has a storing section  26  that stores the recording sheet W. The storing section  26  has a motor  30 . Further, the storing section  26  has a sheet feeding device (not illustrated). The sheet feeding device conveys the recording sheet W from the storing section  26  to the treatment liquid applying section  14 . 
     The treatment liquid applying section  14  has an intermediate conveying drum  28 A and a treatment liquid applying drum  36 . The intermediate conveying drum  28 A is rotatably disposed between the storing section  26  and the treatment liquid applying drum  36 . A belt  32  is extended on the rotational shaft of the intermediate conveying drum  28 A and the rotational shaft of the motor  30 . Accordingly, the rotational driving force of the motor  30  is transmitted via the belt  32  to the intermediate conveying drum  28 A. Due thereto, the intermediate conveying drum  28 A is rotated in the arcuate arrow A direction as shown in  FIG. 1 . 
     Further, the intermediate conveying drum  28 A is provided with sheet grippers  34  which are holding members that grippes the leading section of the recording sheet W, to hold the recording sheet W. The recording sheet W conveyed from the storing section  26  to the treatment liquid applying section  14  is held via the sheet grippers  34  onto the circumferential surface of the intermediate conveying drum  28 A. Then, the recording sheet W is conveyed to the treatment liquid applying drum  36  by rotation of the intermediate conveying drum  28 A. 
     As in the intermediate conveying drum  28 A, the sheet grippers  34  are provided in later-described intermediate conveying drums  28 B,  28 C,  28 D, and  28 E, the treatment liquid applying drum  36 , an image forming drum  44 , an ink drying drum  56 , an image fixing drum  62 , and a discharging/conveying drum  68 . The recording sheet W is conveyed by the sheet grippers  34  from the upstream drum to the downstream drum. 
     The treatment liquid applying drum  36  is coupled to the intermediate conveying drum  28 A by a gear (not illustrated) and is rotated upon receipt of a rotational force. 
     The recording sheet W conveyed by the intermediate conveying drum  28 A is conveyed to the treatment liquid applying drum  36  via the sheet grippers  34  of the treatment liquid applying drum  36 . Then, the recording sheet W held onto the circumferential surface of the treatment liquid applying drum  36  is conveyed. 
     In the upper section of the treatment liquid applying drum  36 , a treatment liquid applying roller  38  is disposed to be contacted with the circumferential surface of the treatment liquid applying drum  36 . The treatment liquid applying roller  38  applies a treatment liquid onto the recording side of the recording sheet W on the circumferential surface of the treatment liquid applying drum  36 . The treatment liquid reacts with ink to coagulate a coloring material (pigment) to promote separation of the coloring material and a solvent. 
     The recording sheet W, onto which the treatment liquid is applied by the treatment liquid applying section  14 , is conveyed to the image forming section  16  by rotation of the treatment liquid applying drum  36 . 
     The image forming section  16  has the intermediate conveying drum  28 B and the image forming drum  44 . The intermediate conveying drum  28 B is coupled to the intermediate conveying drum  28 A by a gear (not illustrated). The intermediate conveying drum  28 B is rotated upon receipt of the rotational force of the gear. 
     The recording sheet W conveyed by the treatment liquid applying drum  36  is conveyed to the intermediate conveying drum  28 B via the sheet grippers  34  of the intermediate conveying drum  28 B of the image forming section  16 . The recording sheet W held onto the circumferential surface of the intermediate conveying drum  28 B is then conveyed. 
     The image forming drum  44  is coupled to the intermediate conveying drum  28 A by a gear (not illustrated). Accordingly, the image forming drum  44  rotates upon receipt of the rotational force of the gear. 
     As illustrated in  FIG. 1 , the recording sheet W conveyed by the intermediate conveying drum  28 B is conveyed to the image forming drum  44  via the sheet grippers  34  of the image forming drum  44 . Then, the recording sheet W held onto the circumferential surface of the image forming drum  44  is conveyed. As illustrated in  FIG. 2 , the circumferential surface has an A side and a B side. Due thereto, each time the image forming drum  44  is rotated through one revolution, an image is formed by an ink jet recording head  48  (the detail will be described below) on the two recording sheets W held onto the A side and the B side. 
     A head unit  46  is disposed above the image forming drum  44  so as to be close to the circumferential surface of the image forming drum  44 . The head unit  46  has the four ink jet recording heads  48  corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K). The recording heads  48  include nozzles (not illustrated) as plural image forming elements forming a dot that configures an image. The recording heads  48  are arrayed along the circumferential direction of the image forming drum  44 . The recording head  48  jets an ink droplet from the nozzle on a treatment liquid layer, which is formed on the recording side of the recording sheet W by the treatment liquid applying section  14 , thereby forming an image. The image forming drum  44  may hold the plural recording sheets W (in this exemplary embodiment, the two recording sheets W on the A side and the B side) onto the circumferential surface thereof. The image forming drum  44  holding the recording sheets W is rotated so as to form the image on each of the plural recording sheets W by the ink jet recording head  48 . 
     The image forming drum  44  includes a rotary encoder  52 . The rotary encoder  52 , according to the exemplary embodiment, generates and outputs a pulse signal due to the rotation of the image forming drum  44 . One pulse of the pulse signal corresponds to a predetermined rotation angle θ 0  (for instance, 1.257 milli-radians). Further, the rotary encoder  52  according to the exemplary embodiment generates and outputs a reference signal, that is, an index pulse (reference pulse), indicating that the reference point of the image forming drum  44  has been passed by rotation of the image forming drum  44 . Namely, the rotary encoder  52  generates the pulse signal according to the predetermined rotation amount (rotation angle) θ 0  of the image forming drum  44 , and generates the reference pulse for every rotation cycle of the image forming drum. 
     The recording sheet W, with the image formed on the recording side by the image forming section  16 , is conveyed to the ink drying section  18  by rotation of the image forming drum  44 . 
     The ink drying section  18  includes the intermediate conveying drum  28 C and the ink drying drum  56 . The intermediate conveying drum  28 C is coupled to the intermediate conveying drum  28 A by a gear (not illustrated). Accordingly, the intermediate conveying drum  28 C rotates upon receipt of the rotational force of the gear. 
     The recording sheet W conveyed by the image forming drum  44  is conveyed to the intermediate conveying drum  28 C via the sheet grippers  34  of the intermediate conveying drum  28 C. The recording sheet W, held onto the circumferential surface of the intermediate conveying drum  28 C, is then conveyed. 
     The ink drying drum  56  is coupled to the intermediate conveying drum  28 A by a gear (not illustrated). Accordingly, the ink drying drum  56  rotates upon receipt of the rotational force of the gear. 
     The recording sheet W conveyed by the intermediate conveying drum  28 C is conveyed to the ink drying drum  56  via the sheet grippers  34  of the ink drying drum  56 . The recording sheet W, held onto the circumferential surface of the ink drying drum  56 , is then conveyed. 
     Above the ink drying drum  56 , hot air heaters  58  are disposed close to the circumferential surface of the ink drying drum  56 . The unnecessary solvent on the image formed on the recording sheet W is then removed by hot air from the hot air heaters  58 . The dried recording sheet W is then conveyed to the image fixing section  20  by rotation of the ink drying drum  56 . 
     The image fixing section  20  includes the intermediate conveying drum  28 D and the image fixing drum  62 . The intermediate conveying drum  28 D is coupled to the intermediate conveying drum  28 A by a gear (not illustrated). Accordingly, the intermediate conveying drum  28 D rotates upon receipt of the rotational force of the gear. 
     The recording sheet W conveyed by the ink drying drum  56  is conveyed to the intermediate conveying drum  28 D via the sheet grippers  34  of the intermediate conveying drum  28 D. The recording sheet W, held onto the circumferential surface of the intermediate conveying drum  28 D, is then conveyed. 
     The image fixing drum  62  is coupled to the intermediate conveying drum  28 A by a gear (not illustrated). The image fixing drum  62  rotates upon receipt of the rotational force of the gear. 
     The recording sheet W conveyed by the intermediate conveying drum  28 D is then conveyed to the image fixing drum  62  via the sheet grippers  34  of the image fixing drum  62 . The recording sheet W, held onto the circumferential surface of the image fixing drum  62 , is then conveyed. 
     In the upper section of the image fixing drum  62 , a fixing roller  64  having a heater therein is disposed to press into contact with the circumferential surface of the image fixing drum  62 . The recording sheet W held onto the circumferential surface of the image fixing drum  62  pressed into contact with the fixing roller  64 , and is heated by the heater. Accordingly, the coloring material of the image formed on the recording side of the recording sheet W melts onto the recording sheet W to fix the image. The image fixed recording sheet W is then conveyed to the discharging/conveying section  24  by rotation of the image fixing drum  62 . 
     The discharging/conveying section  24  has the intermediate conveying drum  28 E and the discharging/conveying drum  68 . The intermediate conveying drum  28 E is coupled to the intermediate conveying drum  28 A by a gear (not illustrated). Accordingly, the intermediate conveying drum  28 E rotates upon receipt of the rotational force of the gear. 
     The recording sheet W conveyed by the image fixing drum  62  is conveyed to the intermediate conveying drum  28 E via the sheet grippers  34  of the intermediate conveying drum  28 E. The recording sheet W held onto the circumferential surface of the intermediate conveying drum  28 E is then conveyed. 
     The discharging/conveying drum  68  is coupled to the intermediate conveying drum  28 A by a gear (not illustrated). Accordingly, the discharging/conveying drum  68  rotates upon reception of the rotational force of the gear. 
     The recording sheet W conveyed by the intermediate conveying drum  28 E is conveyed to the discharging/conveying drum  68  via the sheet grippers  34  of the discharging/conveying drum  68 . The recording sheet W, held onto the circumferential surface of the discharging/conveying drum  68 , is then conveyed to the discharging section  22 . 
       FIG. 3  is a block diagram illustrating the essential configuration of the electric system of the image forming apparatus  10  according to the exemplary embodiment. 
     The image forming apparatus  10  includes a computer (not illustrated). As illustrated in  FIG. 3 , the computer includes a CPU (Central Processing Unit)  70 , a ROM (Read Only Memory)  72 , a RAM (Random Access Memory)  74 , an NVM (Non Volatile Memory)  76 , a UI (User Interface) panel  78 , and a communication I/F (Communication Interface)  80 . 
     The CPU  70  controls the operation of the entire image forming apparatus  10 . The CPU  70  reads a program from the ROM  72  to execute an image forming control process. 
     The ROM  72 , as a memory section, stores programs for executing the image forming control process that controls the operation of the image forming apparatus  10  (the detail will be described below), and further stores various parameters. 
     The RAM  74  is used as a work area for executing various programs. The NVM  76  stores various information that needs to be held even when the power switch of the device is turned OFF. 
     The UI panel  78  includes a touch panel display configured by providing a transmissive touch panel on a display. The UI panel  78  displays various information on the display surface of the display, and inputs desired information and instructions based on the touching of the touch panel by the user. 
     The communication interface  80  is connected to a terminal device  82  such as a personal computer, or the like. The communication interface  80  receives image information and various information showing an image that is to be formed on the recording sheet W, from the terminal device  82 . 
     The CPU  70 , the ROM  72 , the RAM  74 , the NVM  76 , the UI panel  78 , and the communication interface  80  are mutually connected via a BUS (system bus). Therefore, the CPU  70  accesses the ROM  72 , the RAM  74 , and the NVM  76 . The CPU  70  displays various information on the UI panel  78  and grasps the contents of an operation instruction of the user to the UI panel  78 . The CPU  70  receives various information from the terminal device  82  via the communication interface  80 . 
     Further, the image forming apparatus  10  includes a recording head controller  84  and a motor controller  86 . 
     The recording head controller  84  controls the operation of the ink jet recording head  48  according to an instruction of the CPU  70 . The motor controller  86  controls the operation of the motor  30 . 
     Both, the recording head controller  84  and the motor controller  86  are connected to the BUS. The CPU  70  controls the operation of the recording head controller  84  and the motor controller  86 . 
     The rotary encoder  52  is connected to the BUS. The CPU  70  receives the pulse signals generated by the rotary encoder  52 . The CPU  70  receives the index pulses generated by the rotary encoder  52 . 
     The operation of the image forming apparatus  10  according to the exemplary embodiment will be described with reference to the drawings. 
     In the image forming apparatus  10  according to the exemplary embodiment, the recording sheet W is conveyed by the sheet feeding device from the storing section  26  to the intermediate conveying drum  28 . The recording sheet W is then conveyed to the image forming drum  44  via the intermediate conveying drum  28 , the treatment liquid applying drum  36 , and the intermediate conveying drum  28 . Then, the recording sheet W is held onto the circumferential surface of the image forming drum  44 . According to image information, an ink droplet is jetted from the nozzle of the ink jet recording head  48  onto the recording sheet W on the image forming drum  44 . Due thereto, the image shown by the image information is formed on the recording sheet W. 
     The image forming control process executed by the CPU  70  of the image forming apparatus  10  will be described with reference to  FIG. 4  and  FIG. 5 . In the exemplary embodiment, an instruction for executing an image forming process for forming an image on the recording sheet W, and image information expressing the subject of the image to be formed, are inputted from the terminal device  82  and via the communication I/F  80 . When it is determined that the execution instruction and image information have been inputted, the CPU  70  executes the image forming control process. 
     In step  100 , whether the index pulse has been received from the rotary encoder  52  is determined. If it is determined that the index pulse has not been received in step  100 , the determination process in step  100  is performed again. On the other hand, if it is determined that the index pulse has been received in step  100 , the routine proceeds to step  102 . Note that, the timing of reception of the index pulse is the reference timing necessary for performing image formation by the ink jet recording head  48 . More specifically, the timing of detection of the rise of the index pulse (the timing of generation of the index pulse) is the reference timing. 
     In step  102 , a measuring process for monitoring the pulse signal from the rotary encoder  52  to measure the number of pulses starts. While the process is performed in steps  102  to  122 , the measuring process continues. 
     In step  104 , it is determined whether the number of measured pulses has reached a first predetermined number Na. If it is determined that the number of measured pulses has not reached the first predetermined number Na in step  104 , the routine returns to step  104  and performs the determination process again. On the other hand, if it is determined in step  104  that the number of measured pulses has reached the first predetermined number Na, the routine proceeds step  106 . 
     In step  106 , a time Pa corresponding to the width (pulse width) of a pulse which is a second predetermined number G of pulses before the first predetermined number Na (for instance, a pulse which is one pulse before the first predetermined number Na), is computed. The value of the second number G is not limited to one, and may be two or more. 
     In step  108 , the time Pa is divided by a first predetermined value K (for instance, 256) to compute a time Ts (=Pa/K) which is shorter than the time Pa that corresponds to the pulse width. 
     In step  110 , after it is determined that the number of pulses measured in step  104  has reached the pulse number of first predetermined number Na, whether a time (ia×(Pa/K)) obtained by multiplying the time Ts (=Pa/K) by a second predetermined value ia corresponding to the A side has elapsed, is determined. If a negative determination is output (if the time (ia×(Pa/K)) has not elapsed), the determination process of step  110  will be performed again. On the other hand, If an affirmative determination is output (if the time (ia×(Pa/K)) has elapsed) the routine proceeds to step  112 . 
     In step  112 , in order to form an image according to the image information (by forming a dot corresponding to image information) on the recording sheet W on the A side, an instruction is outputted to the recording head controller  84  so as to control the ink jet recording head  48 . Accordingly, the recording head controller  84  controls the ink jet recording head  48  to start image forming, according to the image information, on the recording sheet W on the A side. 
     The process of steps  100  to  112  will be described with reference to the timing chart of  FIG. 6 . 
     As illustrated in  FIG. 6 , if it is determined in step  100  that the index pulse (reference pulse) has been received, the pulses are measured (counted) until the number of pulses reaches the first number Na of steps  102  and  104 . In steps  106  and  108 , the time Pa corresponding to the width (pulse width) of a pulse which is a second predetermined number G of pulses before the first predetermined number Na (for instance, a pulse which is one pulse before the first predetermined number Na), is computed. Then, the time Pa is divided by the first predetermined value K (for instance, 256) to compute the Ts (=Pa/K). In steps  110  and  112 , after the timing when the pulse number of the first predetermined number Na is measured, when the time (ia×(Pa/K)), which is obtained by multiplying the time Ts (=Pa/K) by the second predetermined value ia that corresponds to the A side, has elapsed, the recording head  48  as the image forming section is controlled to start image forming on the recording sheet W on the A side. 
     Here, the image forming control process will be described with reference to  FIG. 5 . In step  114 , it is determined whether the number of measured pulses has reached a third predetermined number Nb. In step  114 , if it is determined that the number of measured pulses has not reached the third predetermined number Nb, the routine returns to step  114  and performs the determination process again. On the other hand, if it is determined that the number of measured pulses has reached the third predetermined number Nb in step  114 , the routine proceeds to step  116 . 
     In step  116 , a time Pb corresponding to the width of a pulse which is a fourth predetermined number D of pulses before the third predetermined number Nb (for instance, a pulse which is one pulse before the third predetermined number Nb), is computed. The value of the fourth number D is not limited to one, and may be plural of two or more. 
     In step  118 , the Pb relative to the pulse width is divided by the first predetermined value K (for instance, 256) to compute a time T&#39;s (=Pb/K) which is shorter than the time Pb that corresponds to the pulse width. 
     In step  120 , after it is determined that the number of pulses measured in step  114  has reached the pulse number of the third predetermined number Nb, whether a time (ib×(Pb/K)) obtained by multiplying the time T&#39;s (=Pb/K) by a second predetermined value ib corresponding to the B side has elapsed, is determined. If a negative determination is output (if the time (ib×(Pb/K)) has not elapsed), the determination process of step  120  is performed again. If an affirmative determination is output (if the time (ib×(Pb/K)) has elapsed), the routine proceeds to step  122 . 
     In step  122 , an instruction is outputted to the recording head controller  84  to control the ink jet recording head  48  (to form a dot corresponding to image information) to start image forming, according to the image information, on the recording sheet W on the B side. Due thereto, the recording head controller  84  controls the ink jet recording head  48  so as to start image formation, according to the image information, on the recording sheet W on the B side. 
     The processes of steps  100  and  114  to  122  will be described with reference to the timing chart of  FIG. 6 . 
     As illustrated in  FIG. 6 , if it is determined that the index pulse has been received in step  100 , the pulses generated by the rotary encoder  52  (as the generating section that generates the pulse signal according to the rotation amount θ 0  of the rotating drum  44 ) are measured (counted) till it reaches the pulse number of the third number Nb, in step  114 . In steps  116  and  118 , the time Pb shown by the width of the fourth predetermined number D (for instance, one) of pulses, before the timing when the pulse number of the third number Nb pulses is measured, is computed. The time Pb is divided by the first predetermined value K (for instance, 256) to compute the time T&#39;s (=Pb/K). In steps  120  and  122 , after the timing when the pulse number of the third predetermined number Nb pulses is measured, when the time (ib×(Pb/K)), obtained by multiplying the time T&#39;s (Pb/K) by the second predetermined value ib corresponding to the B side, has elapsed, the recording head  48  as the image forming section is controlled to start image formation on the recording sheet W on the B side. 
     As described above, image forming on the recording sheet W on the A side starts when the timing of Ja (Ta+(ia×(Pa/K))) seconds has elapsed, from the rise (the reference timing) of the index pulse (reference pulse). Further, the image forming on the recording sheet W on the B side starts when the timing of Jb (Tb+(ib×(Pb/K))) seconds has elapsed, from the rise of the index pulse. Ta (sec) is the time required for counting Na pulses form the rise of the index pulse as a trigger. Tb (sec) is the time required for counting Nb pulses form the rise of the index pulse as a trigger. 
     The image forming apparatus  10  of the exemplary embodiment starts the measuring process for measuring the number of pulses of the pulse signal in step  102 . Then, each time a predetermined number of pulses (Na pulses for the recording sheet W on the A side and Nb pulses for the recording sheet W on the B side) are measured for each of the plural recording sheets W (in the exemplary embodiment, the two recording sheets W on the A side and the B side) from the reference timing, the time (Pa for the recording sheet W on the A side and Pb for the recording sheet W on the B side) shown by a pulse width before the predetermined number of pulses measured by the measuring process is divided by the first predetermined value K (for instance, 256). The time ((Pa/K) for the recording sheet W on the A side and (Pb/K) for the recording sheet W on the B side) which is shorter than the time shown by the pulse width is computed in steps  108  and  118 . Then, step  112  and the following processes are performed. Note that, the next process is performed each time the predetermined numbers of pulses are measured for each of the plural recording sheets W from the timing of generation of the index pulse. Namely, after the timing of measurement of the predetermined number of pulses, a time obtained by multiplying the computed time ((Pa/K) for the recording sheet W on the A side and (Pb/K) for the recording sheet W on the B side) which is shorter than the times Pa and Pb shown by the pulse width by the second predetermined value for each of the plural recording media W (ia for the recording sheet W on the A side and ib for the recording sheet W on the B side) corresponding to the recording sheet W to be image-formed, is calculated. The recording head  48  is controlled in step  122  so as to start image forming on the corresponding recording sheet W when the calculated time elapses. 
     The Na, Nb, ia, and ib are experimentally determined to appropriately form an image on both the recording sheets W on the A side and the B side. 
     An example of a procedure for determining the Na, Nb, ia, and ib will be described with reference to  FIG. 7  and  FIG. 8 . In the following description: “Lo” denotes a distance (mm) from the leading position of the recording sheet to an ideal printing start position; “R” denotes a radius (mm) of the image forming drum; “P” denotes a sheet thickness (mm); “E” denotes the number of encoder pulses (ppr); and “K” denotes a division number. Note that, the above mentioned values are previously set. 
     In step  200 , temporarily, an initial value Nao is set to the variable Na, an initial value Nbo is set to the variable Nb, an initial value iao is set to the variable ia, and an initial value ibo is set to the variable ib. Note that, these initial values may be any value. However, to simplify the process, it is preferred to set the initial value to a value that is close to an ideal value. 
     In step  202 , a line, which is in parallel with the leading position of the recording sheet, is printed (image formed) to each of the sheets on the A side and the B side. 
     In step  204 , a distance La (mm) from the leading position of the sheet on the A side to the printed line, is measured. Note that, the distance La may be directly measured or may be measured by image processing an image obtained by a photographing section such as a camera. 
     In step  206 , difference between the distance Lo and the distance La is calculated as a difference ΔLa. 
     In step  208 , whether ΔLa is within the error range, is determined. A certain value δ is previously determined. If, −δ≦ΔLa≦δ, it is determined that ΔLa is within the error range. If a negative determination is output, it is determined that ΔLa is not within the error range. 
     If it is determined in step  208  that ΔLa is within the error range, the routine proceeds to step  214 . In step  214 , again, the initial value Nao is set to the variable Na and the initial value iao is set to the variable ia. Note that, the step  214  may be omitted. 
     If it is determined in step  208  that ΔLa is not within the error range, the routine proceeds to step  210 . In step  210 , ΔNa and Δia that satisfies ΔLa≈2π(R+P)/E*{ΔNa+Δia/K} are searched. Note that, Δia=ΔNa/K. 
     In step  212 , (Nao−ΔNa) is set to the variable Na and (iao−Δia) is set to the variable ia. 
     In step  216 , a distance Lb (mm) from the leading position of the sheet on the B side to the printed line is measured. Note that, the distance Lb may be directly measured or may be measured by image processing an image obtained by a photographing section such as a camera. 
     In step  218 , a difference between the distance Lo and the distance Lb is calculated as a difference ΔLb. 
     In step  220 , whether ΔLb is within the error range, is determined. The certain value δ is previously determined. If, −δ≦ΔLb≦δ, it is determined that ΔLb is within the error range. If a negative determination is output, it is determined that ΔLb is not within the error range. 
     If it is determined in step  220  that ΔLb is within the error range, the routine proceeds to step  226 . In step  226 , again, the initial value Nbo is set to the variable Nb and the initial value ibo is set to the variable ib. Note that, step  226  may be omitted. 
     If it is determined in step  220  that ΔLb is not within the error range, the routine proceeds to step  222 . In step  222 , ΔNb and Δib that satisfies ΔLb≈2π(R+P)/E*{ΔNb+Δib/K} are searched. Note that, Δib=ΔNb/K. 
     In step  224 , (Nbo−ΔNb) is set to the variable Nb and (ibo−Δib) is set to the variable ib. 
     In step  228 , a line, which is parallel with the leading position of the recording sheet, is printed (image formed) to each of the sheets on the A side and the B side. 
     In step  230 , the distance La (mm) from the leading position of the sheet on the A side to the printed line is measured, and the distance Lb (mm) from the leading position of the sheet on the B side to the printed line is measured. 
     In step  232 , the difference between the distance Lo and the distance La measured in step  230  is calculated as the difference ΔLa. Together therewith, in step  232 , the difference between the distance Lo and the distance Lb measured in step  230  is calculated as the difference ΔLb. 
     In step  234 , whether the difference ΔLa calculated in step  232  is 0 or a value close to 0 (−δ≦ΔLa≦δ), and whether the difference ΔLb calculated in step  232  is 0 or a value close to 0 (−δ≦ΔLb≦δ), is determined. If a negative determination is output in step  234 , the routine returns to step  208 . If an affirmative determination is output in step  234 , the values of the variables Na, Nb, ia, and ib are set as the first number Na, the third number Nb, the second value ia that corresponds to the recording sheet W on the A side, and the second value ib that corresponds to the recording sheet W on the B side, respectively. Due thereto, the first number Na, the third number Nb, the second value ia, and the second value ib are determined. Accordingly, this routine finishes. 
     In the exemplary embodiment, the image forming apparatus that directly forms an image on the recording sheet W by the ink jet recording head has been described. However, the invention is not limited to this. The image forming apparatus may be one that forms an image on the recording sheet W via an intermediate transfer member as the rotating body. As an example of such an image forming apparatus, a recording head, as an image forming portion, having a light emitting element (a light emitting section), such as an LED which emits a light beam, may form a latent image on the peripheral surface (a predetermined side) of a photosensitive drum which is a rotating body, the latent image may be made into a toner image using a material (for instance, toner) for forming an image, and the toner image may be transferred onto the surface of the recording sheet. In this form, the latent image according to image information is formed on the circumferential surface by the light beam, emitted from the light emitting section, according to the image information. The material for forming an image is attached to the formed latent image. Then, the image formed on the circumferential surface is transferred onto the recording sheet via the rotating photosensitive drum. The invention may be applied to the image forming apparatus having the rotating photosensitive drum. Namely, the photosensitive drum is the rotating body that functions as the transfer member that transfers each of plural images formed on the circumferential surface onto each of plural recording sheets. 
     The configuration of the image forming apparatus  10  described in the exemplary embodiment is an example and may be modified according to the conditions within the range without departing from the purpose of the invention. 
     The equations described in the exemplary embodiment are an example. Therefore, unnecessary parameters may be deleted, and new parameters may be added. 
     The flow of the process of various processing programs described in the exemplary embodiments is an example. Within the range without departing the purpose of the invention, an unnecessary step may be deleted, a new step may be added, and the process of orders may be replaced. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.