Patent Publication Number: US-2016236888-A1

Title: Conveyance control device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-025470 filed Feb. 12, 2015. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a conveyance control device and an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, there is provided a conveyance control device including: 
     a driving unit that conveys an object to be conveyed along a predetermined conveyance path; 
     a detection unit that detects a periodic signal for every integer cycle of a cycling member that cycles with following the object to be conveyed which is conveyed along the conveyance path; 
     an acquisition unit that acquires conveyance speed information of the object to be conveyed based on the periodic signal detected by the detection unit and a length of predetermined integer cycles; and 
     a correction unit that corrects an operation timing at which an operation is performed while conveying the object to be conveyed, based on the conveyance speed information acquired by the acquisition unit. 
    
    
     
       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 schematic diagram of an image forming apparatus according to the present exemplary embodiment; 
         FIG. 2  is a schematic diagram of an image forming unit according to the present exemplary embodiment; 
         FIG. 3A  is a perspective view of a position detection roller according to the present exemplary embodiment, and  FIG. 3B  is a front view when seen from an arrow IIIB direction of  FIG. 3A ; 
         FIG. 4  is a functional block diagram for performing correction control of an image formation timing according to a conveyance speed variation of a recording medium P on a driving roller in a main controller according to the present exemplary embodiment; 
         FIGS. 5A to 5C  are timing charts illustrating a relationship between a rotation position of a position detection roller and a periodic signal; 
         FIG. 6  is a flow chart illustrating a flow of an image formation timing correction control routine based on the detection of a conveyance speed according to the present exemplary embodiment; 
         FIG. 7A  is a front view of the position detection roller when plural periodic signals are fetched, and  FIG. 7B  is a timing chart illustrating a relationship between a rotation position of the position detection roller in  FIG. 7A  and a periodic signal; 
         FIGS. 8A to 8C  illustrate modification examples of a position detection plate;  FIGS. 8A and 8B  illustrate a case where a shielding plate is used, and  FIG. 8C  illustrates a case where a detection mark member and a reflection type sensor are combined with each other; 
         FIGS. 9A and 9B  are functional block diagrams illustrating correction control when a table storage unit storing a correction coefficient for slip compensation is provided; 
         FIG. 10  is a schematic diagram of an image forming apparatus according to Modification Example 1; and 
         FIG. 11  is a schematic diagram of an image forming apparatus according to Modification Example 2. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an outline of an image forming apparatus  10  according to the present exemplary embodiment. 
     A recording medium P as an example of an object to be conveyed is taken up around a paper feeding roller  16  of a paper feeding unit  14  in the form of a layer in advance. Meanwhile, a typical example of the recording medium P includes a sheet material including paper and a resin film. 
     The recording medium P taken up around the paper feeding roller  16  is extracted from the outermost layer of the paper feeding roller  16 , is wound around plural winding rollers  18 , and is sent out to an image forcing unit  20 . The recording medium P on which an image is formed by the image forming unit  20  is taken up around a take-up roller  17  of an accommodation unit  15 . The take-up roller  17  rotates so as to take up the recording medium F in the form of a layer. 
     In addition, a portion of the winding roller  18  serves as a driving roller, and is taken up around the take-up roller  17  while adjusting tension of the recording medium P between the rollers. 
     The image forming apparatus  10  includes a main controller  100 . The main controller  100  includes the paper feeding unit  14 , the image forming unit  20 , a driving control unit  102  of a driving system that controls the driving of a driving system (mainly, a motor) conveying the recording medium P by the accommodation unit  15 , and an image forming control unit  104  that acquires image data from the outside, converts the image data to exposure data, and controls an image forming process in the image forming unit  20 . 
     The image forming apparatus  10  of the present invention transfers and fixes a toner image constituted by a developer G (see  FIG. 2 ) to the surface of the recording medium P to thereby form an image on the surface of the recording medium P. 
     The image forming unit  20  has a function of forming a toner image using the developer G, transferring the toner image to the surface of the recording medium P, and fixing a toner onto the surface of the recording medium P to thereby form an image on the surface of the recording medium P. In the image forming unit  20 , image forming units  60 C,  60 M,  60 Y, and  60 K are disposed in a vertical direction (height direction of the apparatus) of  FIG. 1 , and driving rollers  106  and  108  are provided on the upstream side and downstream side of the image forming units  60 C,  60 M,  60 Y, and  60 K, respectively, as examples of driving units. 
     The driving rollers  106  and  108  are configured such that the rotation speeds thereof are independently controlled by the driving control unit  102  of the main controller  100 . For example, a conveyance speed according to the driving roller  108  on the downstream side is higher than a conveyance speed according to the driving roller  106  on the upstream side so that tension of the recording medium P during conveyance is maintained within a predetermined range. 
     The image forming units  60 C,  60 M,  60 Y, and  60 K have functions of forming toner images of the respective colors and transferring the toner images of the respective colors to the recording medium P conveyed. The image forming units  60 C,  60 M,  60 Y, and  60 K are disposed in this order along a conveyance path of the recording medium P from the upstream side to the downstream side (from top to bottom in  FIG. 1 ) in the conveyance direction of the recording medium P. 
     Here, a suffix “C” means cyan, a suffix “M” means magenta, a suffix “Y” means yellow, and a suffix “K” means black. The image forming units  60 C,  60 M,  60 Y, and  60 K form toner images of a C color, an M color, a Y color, and a K color, respectively. In addition, the image forming units  60 C,  60 M,  60 Y, and  60 K have the same configuration except for toner colors included in the developer G used. 
     Accordingly, an image forming unit  60  will be described in detail with reference to  FIG. 2 , but the description will be given by omitting the suffixes C, M, Y, and K. 
     As illustrated in  FIG. 2 , the image forming unit  60  includes a developer supply unit  70  and a transfer unit  80 . 
     The developer supply unit  70  accommodates the developer G, and has a function of supplying the developer G to the transfer unit  80 . The developer supply unit  70  includes a container  72  and a supply roll  74 . Meanwhile, a portion of the supply roll  74  is immersed in the developer G accommodated in the container  72 . 
     The container  72  is connected to an external tank (not shown) and is replenished with the developer G stored in the external tank. 
     The supply roll  14  draws up the developer G accommodated in the container  72  while rotating to thereby supply the developer G to a developing roll  85  to be described later. Here, the layer thickness of the developer G is adjusted by a blade (not shown). The developer is charged to have a positive polarity, as an example, and is supplied to the developing roll  85 . 
     The transfer unit  80  transfers a toner image formed on a photoreceptor  82  to be described later to the recording medium P using the developer G. The transfer unit  80  includes the photoreceptor  82 , a charging device  83 , an exposure device  84 , the developing roll  85 , a transfer drum  86 , and a transfer roll  88 . 
     The photoreceptor  82  has a function of holding a latent image, and the charging device  83  has a function of charging the photoreceptor  82 . 
     The exposure device  84  has a function of forming a latent image on the photoreceptor  82  charged by the charging device  83 , and the developing roll  85  has a function of developing the latent image held by the photoreceptor  82  as a toner image using the developer G supplied from the developer supply unit  70 . 
     The developing roll  85  forms a nip N 1  in conjunction with the photoreceptor  82 . The developing roll  85  is applied with a voltage while rotating, and develops the latent image held by the photoreceptor  82  as a toner image by using an electric field formed in the nip N 1 . 
     The transfer drum  86  has a function of primarily transferring the toner image formed on the photoreceptor  82  to the outer peripheral surface of the transfer drum  86  and holding the transferred toner image. The transfer drum  86  forms a nip N 2  in conjunction with the photoreceptor  82 . The transfer drum  86  is applied with a voltage while rotating and primarily transfers the toner image on the photoreceptor  82  to the outer peripheral surface thereof by using an electric field formed in the nip N 2 . 
     The transfer roll  88  has a function of secondarily transferring the toner image held by the outer peripheral surface of the transfer drum  86  to the recording medium P conveyed. The transfer roll  88  is disposed on the opposite side to the transfer drum  86  with the conveyance path of the recording medium P interposed therebetween and forms a nip N 3  in conjunction with the transfer drum  86 . The transfer roll  88  is applied with a voltage while rotating and secondarily transfers the toner image held by the outer peripheral surface of the transfer drum  86  to the recording medium P by using an electric field formed in the nip N 3 . 
     As illustrated in  FIG. 1 , a fixing device  90  is provided on the downstream side (downstream side of the driving roller  108 ) of the image forming unit  60 . The fixing device  90  includes a heating roll  92  and a pressure roll  94 . 
     The fixing device  90  has a function of fixing the toner image formed by the image forming unit  60  onto the surface of the recording medium P by heating and pressing toner images of many colors formed on the surface of the recording medium P by the image forming unit  60 . 
     Conveyance Control 
     Here, in the present exemplary embodiment, the driving of the driving roller  106  provided on the upstream side of the image forming unit  20  and the driving of the driving roller  108  provided on the downstream side are independently controlled by the driving control unit  102  functioning as a portion of the main controller  10  so that a conveyance speed of the recording medium P and the tension (tensioning force) of the recording medium P are controlled within a predetermined range. 
     The conveyance speed of the recording medium P may become uneven due to factors including expansion and contraction based on physical properties of the recording medium P itself and a slip from a roller which is wound with the recording medium. Such speed unevenness affects distortion at the time of transferring (secondarily transferring) each color. 
     Consequently, in the present exemplary embodiment, as illustrated in  FIG. 1 , a conveyance speed detection roller  110  is disposed at the final stags of the image forming unit  60  (between the image forming unit  60 K of the K color and the driving roller  108  on the downstream side in the present exemplary embodiment). A rotation state of the conveyance speed detection roller  110  is detected by a position detection sensor  116  to be described later and is fed back to the driving control unit  102  so as to control the rotation speeds of the driving rollers  106  and  108 . The conveyance speed detection roller  110  and the position detection sensor  116  function as examples of detection units. 
     Conveyance Speed Detection Roller  110   
     As illustrated in  FIG. 1 , the recording medium P is wound around the conveyance speed detection roller  110 . As a result, the conveyance direction of the recording medium F is turned around by 90 degrees from the upper direction to the right direction of  FIG. 1 . In other words, the recording medium P is wound around one fourth of the whole circumference of the conveyance speed detection roller  110 . The conveyance speed detection roller  110  rotates at the same linear speed as the conveyance speed of the recording medium P without slipping between the speed detection roller and the recording medium P by such a winding amount. 
     Meanwhile, the winding amount which is one fourth of the whole circumference is a criterion of a winding amount that does not cause a slip between the conveyance speed detection roller  110  and the recording medium P, and the winding amount is not limited to a fourth part of the whole circumference. 
     For example, the winding amount may be increased or decreased in consideration of the physical properties (coefficients of friction) of the conveyance speed detection roller  110  and recording medium P, an installation space (including the volume and spatial shape) of the conveyance speed detection roller  110 , and a target conveyance changing angle. 
     As illustrated in  FIGS. 3A and 3B , a position detection plate  114  is attached to a rotation axis  112  of the conveyance speed detection roller  110 . The position detection plate  114  in the present exemplary embodiment has a disc shape and rotates centering on the rotation axis  112  (rotates on a concentric circle) in conjunction with the conveyance speed detection roller  110 . 
     A notch portion  114 A is formed at one location of a peripheral edge of the position detection plate  114 . In addition, the position detection sensor  116  is disposed at one portion of the peripheral edge of the position detection plate  114 . In the position detection sensor  116 , a pair of leg portions  116 B and  116 C protrude from a main body portion  116 A having a rectangular block shape, and the peripheral edge of the position detection plate  114  passes through a gap between the pair of leg portions  116 B and  116 C. 
     As illustrated in  FIG. 4 , the main body portion  116 A of the position detection sensor  116  is provided with a light detection control unit  118 . In addition, the leg portion  116 B is provided with a light projection unit  120 , and the leg portion  116 C is provided with a light receiving unit  122 . The light detection control unit  118  has a function of performing irradiation with specific light from the light projection unit  20  and outputting an electric signal having been subjected to photoelectric conversion according to whether the light receiving unit  122  receives specific light. 
     For example, the light detection control unit  118  outputs a low-level signal (L signal, 0 V) when the light receiving unit  122  has not received specific light, and outputs a high-level signal (H signal, 3.3 V or 5.0 V) when the light receiving unit  122  has received specific light. That is, an H signal is output as a pulse signal for each cycle. 
     Here, as illustrated in  FIGS. 3A and 3B , the notch portion  114 A is formed in the peripheral edge of the position detection plate  114  as described above. The notch portion  114 A passes through the pair of leg portions  116 B and  116 C whenever the position detection plate  114  rotates one turn. That is, an H signal is output from the position detection sensor  116  whenever the notch portion  114 A passes between the pair of leg portions  116 B and  116 C (whenever the conveyance speed detection roller  110  rotates one turn) (see  FIGS. 5A to 5C ). 
     Incidentally, it is premised that the conveyance speed detection roller  110  is concentric with the rotation axis  112  and the position detection plate  114 , but the conveyance speed detection roller may be eccentric in the manufacture thereof or due to deterioration over time. 
     When there is an eccentricity, a speed transmitted to the peripheral surface of the conveyance speed detection roller  110 , the rotation axis, and the position detection plate  114  from the recording medium P varies. For example, as illustrated in  FIGS. 5A to 5C , a rotational error (eccentricity) of the position detection roller  110  which overlaps the speed of the recording medium may be detected. 
     That is, a result of the speed detection of the recording medium P in the position detection roller  110  shows that a cycle depending on the speed of the recording medium of  FIG. 5A  (relatively long cycle, and referred to as “cycle A” below) overlaps a cycle depending on the rotational error of the position detection roller  110  of  FIG. 5B  (relatively short cycle, and referred to as “cycle B” below) (see  FIG. 5C ). 
     On the other hand, from the viewpoint of the above-mentioned color distortion, conveyance may be preferably performed with a target conveyance amount at least at a pitch between colors of the image forming unit  60  (pitch of a secondary transfer position). However, in order to form a peripheral length of the conveyance speed detection roller  110  in compliance with the pitch of the secondary transfer position, more highly accurate manufacture is required than the manufacture of a normal roller. 
     Consequently, in the present exemplary embodiment, a rotation speed (cycle) for each turn which does not need to take a variation in conveyance speed, occurring when the number of turns of the position detection roller  110  is one or less, into consideration is monitored. That is, in the case of N-round (N is an integer, and N=1 in the present exemplary embodiment) units, even when a rotation speed up to N cycles varies due to the eccentricity of the position detection roller  110  (cycle B), a conveyance length for detecting the rotation speed becomes constant as long as a slip (sliding) does not occur between the position detection roller  110  and the recording medium P, and thus it is possible to obtain an accurate average rotation speed (cycle) for every N cycles. 
       FIG. 4  is a functional block diagram for performing correction control of an image formation timing, which is targeted for the driving rollers  106  and  108 , corresponding to a conveyance speed variation of the recording medium P in the main controller  100 . Meanwhile,  FIG. 4  does not limit hardware configuration of the main controller  100 . 
     A user interface  150  is connected to the image forming control unit  104  of the main controller  100 . The image forming control unit  104  includes an image data processing unit  152  and an image formation execution unit  154 . 
     The image data processing unit  152  has a function of receiving image data from the outside and a function of generating exposure data for each color from the received image data. 
     The exposure data generated by the image data processing unit  152  is sent out to the image formation execution unit  154 . The exposure data is sent out with measuring an image formation timing for each color, and an instruction for conveying the recording medium P at a predetermined conveyance speed is given to the driving control unit  102 . 
     The driving control unit  102  controls driving with respect to each driving roller including the driving rollers  106  and  108 . Meanwhile, the driving control unit  102  sends out a signal from a sensor detecting the recording medium P provided at each location of a device to the image forming control unit  104 , and controls an image formation time. 
     The driving control unit  102  is provided with a signal reception unit  156  and is connected to the light detection control unit  118  of the position detection sensor  116 . The signal reception unit  156  receives a periodic signal (see  FIGS. 5A to 5C ) which is output from the light detection control unit  118 . 
     The signal reception unit  156  is connected to a cycle computation unit  158 , and the periodic signal received from the light detection control unit  118  is sent out to the cycle computation unit  158 . The cycle computation unit  158  computes a rotation cycle of the conveyance speed detection roller  110  based on the periodic signal and sends out the computed rotation cycle to an actual conveyance speed computation unit  160  as an example of an acquisition unit. The actual conveyance speed computation unit  160  computes an actual conveyance speed of the recording medium P based on the peripheral length (length of one turn) of the conveyance speed detection roller  110  and the rotation cycle, and sends out the computed conveyance speed to a difference computation unit  162 . 
     In addition, the actual conveyance speed computation unit  160  sends out a request signal to a target conveyance speed reading unit  164  at a point of time when information of the actual conveyance speed is sent out to the difference computation unit  162 . 
     The target conveyance speed reading unit  164  reads out information of a target conveyance speed of the recording medium P from the image formation execution unit  154  of the image forming control unit  104  based on the request signal, and sends out the information to the difference computation unit  162 . 
     The difference computation unit  162  computes a difference between the information of the actual conveyance speed and the information of the target conveyance speed, and feeds the difference back to an image formation timing correction unit  166 , as an example of a correction unit, of the image forming control unit  104 . 
     In the present exemplary embodiment, image formation timings of the other colors (M, Y, and K colors) are corrected based on a C color. 
     That is, the image formation timing correction unit  166  is connected to the image formation execution unit  154 , and sends out information of the correction of the image formation timings of M, Y, and K colors to the image formation execution unit  154  based on the information received from the difference computation unit  162 . 
     The image formation execution unit  154  having received the information of the correction changes the image formation timings of M, Y, and K colors and sends out exposure data to each image forming unit. 
     For example, when the actual conveyance speed is lower than the target conveyance speed, it takes a long time for the recording medium P to move between the image forming units  60 , and thus an image formation timing is delayed. 
     Hereinafter, operations of the present exemplary embodiment will be described. 
     Flow of Image Formation 
     First, a flow of processing for image formation in the image forming apparatus  10  will be described. 
     When the main controller  100  receives image data, the image data is converted into exposure data of each color, and the converted exposure data of each color is transmitted to the exposure device  84  constituting the image forming unit  60 . 
     Subsequently, in the image forming unit  60 , the photoreceptor  82  is charged by a charging device  83 C based on an instruction for the execution of image formation, and a charged photoreceptor  82 C is exposed by an exposure device  84 C, thereby forming a latent image for a C color on the photoreceptor  82 C. The latent image for a C color is developed as a toner image of a C color by a developing apparatus  85 C supplied with a developer G of a C color from a developer supply unit  70 C. 
     Subsequently, the toner image of a C color reaches the nip N 2  by the rotation of the photoreceptor  82 C and is primarily transferred to a transfer drum  86 C. Further, the toner image of a C color which is transferred to the transfer drum  86 C reaches the nip N 3  by the rotation of the transfer drum  86 C. The toner image of a C color which has reached the nip N 3  is secondarily transferred to the surface of the recording medium P conveyed by a transfer roll  88 C. 
     Similarly, in the image forming units  60 M,  60 Y and  60 K constituting the image forming unit  60 , the toner images of M, Y, and K colors are secondarily transferred to the surface of the recording medium P from transfer drums  86 M,  86 Y and  86 K in a sequential order so as to overlap the toner image of a C color which is secondarily transferred to the surface of the recording medium P. 
     Subsequently, the recording medium P having a surface on which the toner image of each color is formed by the image forming unit  60  reaches the fixing device  90 . The toner image of each color on the surface of the recording medium P is heated and pressed by a fixing device  90 A, and is fixed onto the surface of the recording medium P. 
     Control of Conveyance Speed 
     Incidentally, the conveyance speed of the recording medium P may become uneven and affect distortion at the time of transferring (secondarily transferring) each color. 
     Consequently, in the present exemplary embodiment, the conveyance speed detection roller  110  detecting the conveyance speed of the recording medium P is disposed, and the feed-back control of an image formation timing is performed based on the periodic signal received from the conveyance speed detection roller  110 . 
     Hereinafter, a flow of an image formation timing correction control routine based on the detection of a conveyance speed will be described with reference to a flow chart of  FIG. 6 . 
     In step  200 , a periodic signal is received from the position detection sensor  116 . The periodic signal is a pulse signal which is inverted from an L signal to an H signal for each cycle in the present exemplary embodiment and moves by one turn even when eccentricity occurs in the position detection roller  110 . Accordingly, a speed variation in the middle of the movement is offset, and thus the periodic signal may be applied as an accurate periodic signal. 
     In the subsequent step  202 , a cycle t is computed based on the received periodic signal. That is, the cycle is a time between H signals. The time between H signals is theoretically constant all the time, but it is preferable to compute an average value of the times obtained plural times. 
     Meanwhile, as illustrated in  FIGS. 7A and 7B , a cycle may be obtained by forming plural notch portions  114 A,  114 B,  1140 , and  114 D in the peripheral edge of the position detection plate  114 , individually measuring cycles tn (n is an integer, t 1 , t 2 , t 3 , and t 4  in  FIG. 7B ) of the respective notch portions  114 A,  114 B,  114 C, and  114 D, and computing an average value of the cycles. In other words, it is also possible to apply a signal for each slit which is selected from an existing pulse encoder. 
     In addition, as the number of opportunities to detect a cycle increases (as n of the cycle tn increases) in one turn, it is possible to suspect an error, for example, when an image is formed while accelerating or decelerating the recording medium P. 
     In the subsequent step  204 , a peripheral length L of a position detection roller is read out. Then, the flow chart proceeds to step  206  to compute an actual conveyance speed VJ (average speed) of the recording medium P (VJ=L/t). 
     In the subsequent step  208 , a target conveyance speed VM is read out from the image forming control unit  104 . The target conveyance speed VM is a conveyance speed of the recording medium P which, is recognized by the image forming control unit  104 . The image formation execution unit  154  of the image forming control unit  104  determines an image formation timing of each color based on the target conveyance speed VM. 
     In the subsequent step  210 , a difference ΔV (=VM−VJ) between the actual conveyance speed VJ and the target conveyance speed VM is computed, and then the flow chart proceeds to step  212 . 
     In step  212 , an instruction for the correction of the image formation timing determined based on the target conveyance speed VM is given based on the difference ΔV, and the routine is terminated. 
     The image formation execution unit  154  of the image forming control unit  104  corrects the image formation timing when an instruction for correction is given. In the present exemplary embodiment, the correction of an image formation timing is performed based on a C color. Hereinafter, the correction of an image formation timing of an M color will be described as an example. 
     Hereinafter, when an image formation timing is set as tcm, a distance between image formation positions (secondary transfer positions) of Y and M colors is set as Xcm, and a target conveyance speed is set as VM, the image formation timing tcm is normally set by the following Expression (1). 
       tcm=Xcm/ VM   (1)
 
     Here, when there is a difference ΔV between a target conveyance speed VM and an actual conveyance speed VJ, the image formation timing is delayed by Xcm/ΔV when ΔV is positive. Meanwhile, the image formation timing may be advanced when ΔV is negative. 
     Meanwhile, in the above description, the image formation timing is computed in advance based on the target conveyance speed VM, and correction is performed based on ΔV. However, the image formation timing may be directly computed based on the actual conveyance speed VJ. 
     Meanwhile, when the image formation timing is computed through correction based on ΔV mentioned above, an image forming process may be performed based on the target conveyance speed VM as an emergency measure even when malfunction of the position detection sensor  116  or the like occurs. 
     Meanwhile, in the present exemplary embodiment, a circular plate material is used as the position detection plate  114 , but light shielding plates  184  and  186  may be provided as illustrated in  FIGS. 8A and 8B . In addition, as illustrated in  FIG. 8C , a detection mark member  188  may be provided on the peripheral surface of the position detection roller  110 , and a signal for each turn may be obtained using a reflection type sensor  190 . 
     Machine Compensation in Manufacture and Installation 
     When the actual conveyance speed VJ is obtained, a peripheral length L of the position detection roller  110  and a distance (secondary transfer pitch) between image formations of the respective colors have a manufacturing error. Consequently, an image forming process is performed at the time of manufacture or the installation of the image forming apparatus  10 , an image formation timing is adjusted through feedback, and a conveyance speed of the recording medium P at that time is set as a target conveyance speed VM, thereby computing the image formation timing. Thereafter, the image formation timing may be corrected based on the difference ΔV between the target conveyance speed VM and the actual conveyance speed VJ. 
     Temperature Compensation 
     When the actual conveyance speed VJ is obtained, the peripheral, length L of the position detection roller  110  and the distance (secondary transfer pitch) between image formations of the respective colors may expand or contract and vary due to temperature changes within the apparatus. 
     In this case, the peripheral length and the distance may be manually adjusted by an operator from the user interface  150 . Alternatively, a temperature sensor may be provided in the image forming apparatus  10 , and a correction coefficient based on a temperature detected by the temperature sensor may be added to Expression (1) mentioned above. It is preferable that the temperature sensor is located in the vicinity of the position detection roller  110 . 
     Slip Compensation 
     For example, when a winding amount of the recording medium P with respect to the position detection roller  110  is insufficient due to the space of the image forming apparatus  10 , a slip may occur between the position detection roller  110  and the recording medium P. In addition, a minute slip may occur in spite of a sufficient winding amount. It is known that this slip depends on a conveyance speed, and particularly becomes remarkable when an image is formed while accelerating or decelerating the recording medium P. 
     Consequently, as illustrated in  FIGS. 9A and 9B , a table storage unit  192  is provided in the image forming control unit  104 , and a relationship between a conveyance speed (actual conveyance speed VJ) and a correction coefficient α based on a slip is tabled in advance and is stored in the table storage unit  192 . The correction coefficient may be added to Expression (1) mentioned ahove as a slip correction coefficient when the image formation execution unit  154  computes an image formation timing. 
       tcm=α×(Xcm/ VM )  (2)
 
       FIG. 9B  is an example of a table showing an actual conveyance speed VJ and a slip correction coefficient α which is stored in the table storage unit  192 . 
     Modification Example 1 
     In the present exemplary embodiment, as illustrated in  FIG. 1 , the image forming apparatus  10  that forms an image on one surface of a recording medium P is illustrated. However, as illustrated in  FIG. 10 , it is also possible to use a double-surface adaptable image forming apparatus  10 R that forms an image on one surface of a recording medium P and then inverts the recording medium P by an inversion device  168  to form an image on the other surface. In this case, the position detection rollers  110 , which are disposed in the respective image forming units, may detect an actual conveyance speed VJ independently on the front and back surfaces thereof and may correct an image formation timing independently on the front and back surfaces thereof. 
     Modification Example 2 
     In the present exemplary embodiment and Modification Example 1, the electrophotographic image forming apparatus  10  ( 10 R) has been described as an example. However, as illustrated in  FIG. 11 , it is also possible to use an ink jet type image forming apparatus  10 I. Hereinafter, an outline of the ink jet type image forming apparatus  10 I will be described. 
       FIG. 11  illustrates an outline of the ink jet type image forming apparatus  10 I according to Modification Example 2 of the present exemplary embodiment (hereinafter, simply referred to as “image forming apparatus  10 I”). 
     The image forming apparatus  10 I according to the present exemplary embodiment ejects ink onto a recording medium  12  by a so-called ink jet method using ink to record (print) an image and dries the ink. 
     As illustrated in  FIG. 11 , the recording medium P is taken up around a paper feeding roller  171  of a paper feeding unit  170  in the form of a layer in advance. 
     The recording medium P taken up around the paper feeding roller  171  is extracted from the outermost layer of the paper feeding roller  171 , is wound around plural winding rollers  172 , and is taken up around a take-up roller  174  of an accommodation unit  173 . The take-up roller  174  is provided with a motor (not shown) to rotate the take-up roller  174  so as to take up the recording medium P in the form of a layer. 
     In  FIG. 11 , a region for conveyance is provided and serves as an image forming unit  180 . 
     In addition, a region in which the recording medium P is vertically and linearly conveyed is provided on a downstream side of the image forming unit  180  and serves as a drying unit  181 . 
     Ink jet heads  182 Y,  182 M,  182 C, and  182 K of yellow (Y), magenta (M), cyan (C), and black (K) colors are arrayed in the image forming unit  180 . Hereinafter, the ink jet heads will be collectively referred as an ink jet head  182 . 
     The ink jet head  182  brings ink which is stored in, for example, an ink cassette (not shown) and ejects droplets from nozzles toward a recording medium  12  confronting the nozzles by pressure control, ultrasonic control, or the like. 
     In the image forming apparatus  10 I according to the present exemplary embodiment, nozzles are arrayed in the entire region in a main scanning direction of the recording medium P, the ink jet heads  182  of the respective colors are arrayed in a sub scanning direction of the recording medium P, and droplets having the amount of ink according to image data are ejected from the nozzles of the respective ink jet heads  182  in synchronization with the conveyance speed of the recording medium P. 
     Meanwhile, this array configuration is not limited, and a configuration may also be adopted in which an ink jet head  27  is moved in a main scanning direction. 
     In the drying unit  181 , the recording medium P on which an image is formed by the image forming unit  180  is conveyed from the top to the bottom of  FIG. 11 , and surfaces having the respective images formed thereon sequentially confront a dried region in accordance with the conveyance. 
     The position detection roller  110  is provided on the downstream side of a driving roller  176  of the image forming apparatus  10 I and on the upstream side of the image forming unit  180 , and the recording medium P is wound around the position detection roller. 
     Similarly to  FIGS. 3A and 3B , the position detection roller  110  is provided with the position detection plate  114  and is disposed corresponding to the position detection sensor  116 . 
     The position detection sensor  116  outputs a pulse signal (H signal) for each turn in association with the conveyance of the recording medium P, and controls (corrects) an image formation timing in the image forming unit  180  based on the periodic signal thereof. 
     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 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.