Patent Publication Number: US-8532509-B2

Title: Image forming apparatus and image forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-260330 filed Nov. 13, 2009. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to an image forming apparatus and an image forming method. 
     (ii) Related Art 
     It is known that, in image forming apparatuses that form a color image by forming images of multiple colors with respective toners and superimposing the images of respective colors with each other, a color misregistration of an image transferred onto a recording sheet is caused by relative misregistrations between the images of respective colors. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including a plurality of toner-image forming units that receive image data and form electrostatic latent images on image bearing members in accordance with the image data, and form toner images of respective colors by developing the electrostatic latent images; an intermediate transfer member onto which the toner images are transferred; a transfer unit that transfers the toner images of the respective colors onto the intermediate transfer member; a controller that performs transfer control for changing a transfer pressure applied when the transfer unit transfers the toner images onto the intermediate transfer member; and a misregistration detector that detects a difference of a transfer position of each of the toner images of the respective colors on the intermediate transfer member when the transfer control is performed by the controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic diagram illustrating the structure of an image forming apparatus according to an exemplary embodiment; 
         FIG. 2A  is a partially sectioned front view illustrating a first transfer device according to the exemplary embodiment in a normal-paper receiving state; 
         FIG. 2B  is a partially sectioned side view illustrating the first transfer device according to the exemplary embodiment in the normal-paper receiving state; 
         FIG. 3A  is a partially sectioned front view illustrating the first transfer device according to the exemplary embodiment in an embossed-paper receiving state; 
         FIG. 3B  is a partially sectioned side view illustrating the first transfer device according to the exemplary embodiment in the embossed-paper receiving state; 
         FIG. 4  is a block diagram illustrating the structure of the image forming apparatus according to the exemplary embodiment; 
         FIG. 5  is a diagram illustrating a detection process for detecting a test pattern according to the exemplary embodiment; 
         FIG. 6A  is a diagram illustrating the structure of pattern detectors according to the exemplary embodiment; 
         FIG. 6B  is a diagram illustrating the structure of a light receiving unit according to the exemplary embodiment; 
         FIG. 7  illustrates an operation flow of the overall operation performed by the image forming apparatus according to the exemplary embodiment; 
         FIG. 8  illustrates an operation flow of an image forming process performed by the image forming apparatus according to the exemplary embodiment; 
         FIG. 9  illustrates an operation flow of a first transfer control process performed by the image forming apparatus according to the exemplary embodiment; 
         FIG. 10  illustrates an operation flow of a misregistration detection process performed by the image forming apparatus according to the exemplary embodiment; 
         FIG. 11  illustrates an operation flow of a process for determining a misregistration correction value according to a modification; and 
         FIG. 12  illustrates an operation flow of an image forming process according to the modification. 
     
    
    
     DETAILED DESCRIPTION 
     Structure 
       FIG. 1  is a schematic diagram illustrating the structure of an image forming apparatus  1  according to an exemplary embodiment of the present invention. A cover that presses an original document  2  against a platen glass  5  and an image reading device  4  that reads an image on the original document  2  placed on the platen glass  5  are provided in an upper section of the image forming apparatus  1 . The image reading device  4  emits light toward the original document  2  placed on the platen glass  5  from a light source  6 . The light is reflected by the original document  2  and is then reflected by a full-rate mirror  7  and half-rate mirrors  8  and  9 . Then, the light is guided through a lens  10  to an image reading element  11  including charge coupled devices (CCD). The image reading element  11  converts the image on the original document  2  into red (R), green (G), and blue (B) electrical signals and output the electrical signals to an image processing device  12 . In the present exemplary embodiment, a copying function in which the original document  2  is read by the image reading device  4  is mainly described. However, the image forming apparatus  1  also has a function as a printer in which image data of an image to be printed is received from an apparatus, such as a personal computer (PC), disposed outside the image forming apparatus  1  and is output to the image processing device  12 . 
     The image processing device  12  subjects the image represented by the electrical signals output from the image reading device  4  to image processes, such as shading correction, misregistration correction, brightness/color-space conversion, gamma correction, frame erasing, color and movement editing, and color misregistration correction. The image processing device  12  converts the image that has been subjected to the image processes into image data (raster data) of five colors, which are yellow (Y), magenta (M), cyan (C), black (K), and clear (L). The image data of respective colors is transmitted to exposure devices  14 K,  14 Y,  14 M,  14 C, and  14 L included in image forming units  13 K,  13 Y,  13 M,  13 C, and  13 L for the respective colors. 
     In the drawings and the following description, the letter ‘Y’ is attached to reference numerals that denote components used to form a yellow image. Similarly, the letters ‘M’, ‘C’, ‘K’, and ‘L’ are attached to reference numerals that denote components used to form a magenta image, a cyan image, a black image, and a clear image, respectively. 
     The image forming units  13 K,  13 Y,  13 M,  13 C, and  13 L are units that respectively form yellow, magenta, cyan, black, and clear toner images. The image forming apparatus  1  is provided with attachment sections to which the image forming units  13 K,  13 Y,  13 M,  13 C, and  13 L can be attached. The image forming units  13 K,  13 Y,  13 M,  13 C, and  13 L can be attached to and detached from the image forming apparatus  1 , and are arranged parallel to each other with constant intervals therebetween along a horizontal direction in the image forming apparatus  1 . In the present exemplary embodiment, the five image forming units  13 K,  13 Y,  13 M,  13 C, and  13 L have a similar structure. Therefore, the letters ‘Y’, ‘M’, ‘C’, ‘K’, and ‘L’ will be omitted when the structure of each image forming unit is described. 
     Each image forming unit  13  includes a photosensitive drum  15 , a scorotron  16 , an exposure device  14 , a developing unit  17 , and a cleaning device  18 . The photosensitive drum  15  is an example of an image bearing member, and rotates at a constant rotation speed in a direction indicated by the arrow. The scorotron  16  is used in a first charging process for uniformly charging the surface of the photosensitive drum  15 . The exposure device  14  emits light corresponding to an image of each color toward the surface of the photosensitive drum  15  to form an electrostatic latent image. The developing unit  17  develops the electrostatic latent image formed on the photosensitive drum  15  with toner. The cleaning device  18  removes the toner from the photosensitive drum  15 . 
     The exposure device  14  corresponding to each color emits a laser beam from a laser device  19  in accordance with image data transmitted from the image processing device  12 . The laser beam emitted from the laser device  19  is guided by reflective mirrors  20  and  21  to a rotating polygon mirror  22  having a polygonal shape, which has plural reflective side surfaces, and is reflected by the polygon mirror  22 . The laser beam reflected by the rotating polygon mirror  22  is reflected again by the reflective mirror  21 , and is reflected by plural reflective mirrors  23  and  24  so as to scan the photosensitive drum  15 , which is an image bearing member. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum  15 . Thus, electrostatic latent images are formed on respective photosensitive drums  15 K,  15 Y,  15 M,  15 C, and  15 L, and are developed by developing units  17 K,  17 Y,  17 M,  17 C, and  17 L as black, yellow, magenta, cyan, and clear toner images. 
     The toner images of the respective colors formed on the photosensitive drums  15 K,  15 Y,  15 M,  15 C, and  15 L are transferred in a superimposed manner onto an intermediate transfer belt  25 , which serves as an intermediate transfer member, by first transfer devices  30 K,  30 Y,  30 M,  30 C, and  30 L, which have a similar structure. The intermediate transfer belt  25  is positioned under the image forming units  13 K,  13 Y,  13 M,  13 C, and  13 L. The process of transferring the toner images onto the intermediate transfer belt  25  is hereinafter referred to as a first transfer process. 
     In the present exemplary embodiment, toner images that have been transferred onto the intermediate transfer belt  25  by the first transfer devices  30 K,  30 Y,  30 M,  30 C, and  30 L, which will be described below, in the first transfer process are transferred onto a recording medium (hereinafter referred to as a recording sheet). A transfer pressure applied in the first transfer process is changed in accordance with whether the type (hereinafter referred to as the paper type) of the recording sheet is normal paper or embossed paper. In the present exemplary embodiment, the normal paper and the embossed paper are explained as examples of the types of recording sheets. However, overhead projector (OHP) sheet, for example, may also be used as a recording medium. The first transfer devices  30 K,  30 Y,  30 M,  30 C, and  30 L have a similar structure. Therefore, the letters ‘Y’, ‘M’, ‘C’, ‘K’, and ‘L’ will be omitted when the structure of each first transfer device is described. 
     The intermediate transfer belt  25  is stretched around rollers  40  to  45  with a certain tension applied thereto, and is rotated at a certain speed in the direction indicated by the arrow by the roller  40 , which is rotated by a motor (not shown). In the present exemplary embodiment, the intermediate transfer belt  25  is formed in the shape of an endless belt by, for example, forming a band-shaped flexible synthetic-resin film made of polyimide or the like and connecting ends of the band-shaped flexible synthetic-resin film to each other by welding or the like. 
     The toner images of the respective colors that have been transferred onto the intermediate transfer belt  25  in a superimposed manner are transferred by a second transfer roller  50 , which is pressed against the roller  44 , onto a recording sheet  60  that has been conveyed to the second transfer roller  50 . The process of transferring the toner images onto the recording sheet  60  is hereinafter referred to as a second transfer process. While the recording sheet  60  is being conveyed between the second transfer roller  50  and the roller  44  that is disposed inside the intermediate transfer belt  25 , a second transfer bias is applied to the second transfer roller  50 . The second transfer bias has a polarity opposite to the polarity of the toner that has been transferred onto the intermediate transfer belt  25  in the first transfer process. Therefore, an electrostatic force is applied to the toner on the intermediate transfer belt  25  in the direction from the intermediate transfer belt  25  to the recording sheet  60 , so that the toner is transferred onto the surface of the recording sheet  60  in the second transfer process. 
     The recording sheet  60  onto which the toner images of the respective colors have been transferred in the second transfer process is conveyed to a fixing unit  70  by two conveying rollers  51  and  52 . The recording sheet  60  onto which the toner images have been transferred is subjected to a fixing process in which heat and pressure are applied by the fixing unit  70 , and is then ejected to a paper ejection tray  64 . 
     The recording sheet  60  is fed from one of storage units  61  to  63  for storing recording sheets  60 , and is conveyed to the intermediate transfer belt  25  along sheet-conveying paths (shown by broken lines) including rollers  80 . After the toner image on each photosensitive drum  15  is transferred onto the intermediate transfer belt  25 , residual toner, paper dust, etc., are removed from the photosensitive drum  15  by the cleaning device  18  to be ready for the next image forming process. Residual toner on the intermediate transfer belt  25  is removed by a belt cleaner  90 . 
     Structure of First Transfer Device  30   
     The structure of each first transfer device  30  will now be described. The first transfer device  30  is disposed inside the intermediate transfer belt  25  at a position where the first transfer device  30  is opposed to the corresponding photosensitive drum  15 . The first transfer device  30  includes a first transfer roller  310  disposed at a position where the first transfer roller  310  is opposed to the photosensitive drum  15  and a first transfer bias source  320  that applies a first transfer bias to the first transfer roller  310 . The first transfer roller  310  presses the intermediate transfer belt  25  against the photosensitive drum  15 , and the first transfer bias source  320  changes the first transfer bias applied to the first transfer roller  310 . 
     As illustrated in  FIG. 2A , the first transfer roller  310  includes a roller body  311  and shaft members  312  that extend in an axial direction and project from the roller body  311  at either end thereof. The first transfer roller  310  is disposed in a rectangular housing  330  that is opposed to the photosensitive drum  15  with the intermediate transfer belt  25  disposed therebetween. The housing  330  has an open face at the top, and the first transfer roller  310  can be moved in a vertical direction through the open face. 
     An urging mechanism  340  is provided on an inner bottom surface  331  of the housing  330  at a position corresponding to each of the shaft members  312  of the first transfer roller  310 . Each urging mechanism  340  urges the first transfer roller  310  upward, that is, toward the photosensitive drum  15  to press the intermediate transfer belt  25  against the photosensitive drum  15 . The position at which the first transfer roller  310  and the photosensitive drum  15  are in contact with the intermediate transfer belt  25  corresponds to a first transfer position T 1  (see  FIG. 2B ) at which the first transfer process is performed. The urging force that presses the intermediate transfer belt  25  against the photosensitive drum  15  is adjusted on the basis of a control signal corresponding to the type of the recording sheet, that is, one of the normal paper and the embossed paper. 
     Each urging mechanism  340  includes a bearing  341  that supports the corresponding shaft member  312  of the first transfer roller  310  in a rotatable manner; a pair of guide rails  342  that guide the movement of the bearing  341  in the vertical direction; a pair of discs  343  attached to the respective guide rails  342 ; a base  345  that connects the discs  343  to each other; a first coil spring  346  provided between the base  345  and the bearing  341  to urge the bearing  341  upward; a second coil spring  347  disposed between the base  345  and the bottom surface  331  to urge the base  345  upward; and a moving mechanism  350  that moves the discs  343  in the vertical direction. 
     The bearing  341  has, for example, a rectangular parallelepiped shape and is slidably clamped between the guide rails  342  having an angular U shape at opposite sides thereof. More specifically, the opposite side portions of the bearing  341  are fitted into recesses provided in the angular-U-shaped guide rails  342 . A retaining pin for preventing a displacement of the first transfer roller  310  in the axial direction is inserted into an end portion of the shaft member  312 . 
     The guide rails  342  extend in the vertical direction, and back surfaces of the guide rails  342  are attached to the respective discs  343 . Bottom end portions of the guide rails  342  are in contact with the top surface of the base  345 , which has a plate shape. The base  345  is disposed such that the top surface thereof is parallel to the intermediate transfer belt  25 . The first coil spring  346  is attached to the bottom surface of the bearing  341  at one end thereof, and to the top surface of the base  345  at the other end thereof. The first coil spring  346  is disposed such that, when viewed in a direction of  FIG. 2B  (in a side view), the central axis of the first coil spring  346  coincides with a straight line L 1  that extends in a radial direction of the photosensitive drum  15  (straight line that passes through the center of the photosensitive drum  15  and the center of the first transfer roller  310  in this example). 
     The second coil spring  347  is attached to the bottom surface of the base  345  at one end thereof and to the bottom surface  331  at the other end thereof. Similar to the first coil spring  346 , the second coil spring  347  is also disposed such that, when viewed in a direction of  FIG. 2B  (in a side view), the central axis of the second coil spring  347  coincides with the straight line L 1  that extends in the radial direction of the photosensitive drum  15 . 
     As illustrated in  FIG. 2B , the moving mechanism  350  includes columnar projections  351 , guide grooves  352 , a stepping motor  354 , an extension shaft  355 , and a pair of cams  356 . The projections  351  are provided on outer surfaces of the respective discs  343 . The guide grooves  352  are formed in inner wall surfaces of the housing  330  at positions corresponding to the projections  351 , and guide the movement of the projections  351  fitted in the guide grooves  352  in the vertical direction. The stepping motor  354  is placed on, for example, the top surface of an inverted L-shaped base  353  having a portion fixed to an outer wall surface of the housing  330 . The extension shaft  355  extends from a rotating shaft of the stepping motor  354 . The cams  356  are provided on the extension shaft  355  and move the discs  343  in the vertical direction. The extension shaft  355  is connected to the rotating shaft of the stepping motor  354  with a coupling  357  provided therebetween. The stepping motor  354  is attached to the top surface of the base  353  such that the extension shaft  355  extends parallel to the axial direction of the discs  343  at a position above the discs  343 . The cams  356  are arranged such that the outer peripheral surfaces thereof are in contact with the outer peripheral surfaces of the respective discs  343 . 
     Each first transfer device  30  having the above-described structure changes the transfer pressure at which the intermediate transfer belt  25  is pressed against the photosensitive drum  15  in accordance with the control signal transmitted from a control unit  101 , which will be described below. More specifically, the transfer pressure for when the paper type of the recording sheet on which image data is to be recorded is the embossed paper, which has a larger degree of surface irregularity than that of the normal paper, is set to be lower than the transfer pressure for when the paper type is the normal paper. 
     In the present exemplary embodiment, the state illustrated in  FIGS. 2A and 2B  is the state of each first transfer device  30  for transferring an image onto a sheet of normal paper (hereinafter referred to as a normal-paper receiving state). The state illustrated in  FIGS. 3A and 3B  is the state of each first transfer device  30  for transferring an image on a sheet of embossed paper (hereinafter referred to as an embossed-paper receiving state). The operation of the first transfer device  30  for changing the state thereof from the normal-paper receiving state illustrated in  FIGS. 2A and 2B  to the embossed-paper receiving state illustrated in  FIGS. 3A and 3B  will now be described. 
     In the first transfer device  30 , the stepping motor  354  receives an embossed-paper control signal representing that the state of the first transfer device  30  is to be switched to the embossed-paper receiving state from the control unit  101 . Then, the stepping motor  354  starts to rotate. Accordingly, the cams  356  rotate and the discs  343  are pushed downward by protruding portions of the cams  356  (portions at which the distance from the extension shaft  355  is relatively large). When the discs  343  are pushed downward, the second coil spring  347 , which moves together with the discs  343 , is compressed. 
     The first coil spring  346  continuously urges the first transfer roller  310  upward even when the base  345  is moved downward. However, the transfer pressure at which the intermediate transfer belt  25  is pressed against the photosensitive drum  15  is reduced from that in the normal-paper receiving state. Accordingly, in the embossed-paper receiving state, the force with which the intermediate transfer belt  25  is pressed against the photosensitive drum  15  is lower than that in the normal-paper receiving state. Therefore, the adhesion force applied to the toner images transferred onto the intermediate transfer belt  25  is also reduced. Since the transfer pressure applied in the first transfer process is reduced for the embossed paper, the electrostatic force applied to the toner that has adhered to the intermediate transfer belt  25  is smaller than that in the case of the normal paper. 
     The embossed paper has recessed and projecting portions, and the distance from the recessed portions to the intermediate transfer belt  25  is larger than that from the projecting portions to the intermediate transfer belt  25  in the second transfer process in which the toner images on the intermediate transfer belt  25  are transferred onto the recording sheet  60 . Therefore, a transfer electric field applied to the recessed portions by the second transfer roller  50  in the second transfer process is weaker than that applied to the projecting portions. Accordingly, the electrostatic force that attracts the toner that has adhered to the intermediate transfer belt  25  to the recessed portions is relatively weak. However, as described above, in the case of transferring an image on a sheet of embossed paper, the transfer pressure applied by the first transfer device  30  in the first transfer process is reduced from that applied in the case of transferring an image on a sheet of normal paper. Therefore, the adhesion force applied to the toner images on the intermediate transfer belt  25  is also reduced. As a result, at the position where the second transfer process is performed, the toner that has adhered to the intermediate transfer belt  25  is easily attracted to the recessed portions of the embossed paper when the transfer electric field is applied thereto in the second transfer process. Therefore, the toner images can be reliably transferred to the embossed paper in both the recessed and projecting portions, and the risk that the toner cannot adhere to recessed portions and the corresponding portions of the image will be blank can be reduced. 
     In the case where the paper type of the recording sheet  60  is switched from the embossed paper to the normal paper, the first transfer device  30  rotates the stepping motor  354  in a normal or reverse direction so that the state of the first transfer device  30  switches from the embossed-paper receiving state illustrated in  FIGS. 3A and 3B  to the normal-paper receiving state illustrated in  FIGS. 2A and 2B . 
     In the present exemplary embodiment, the transfer pressure applied by the first transfer device  30  in the first transfer process is changed in accordance with the paper type of the recording sheet  60 . If the transfer pressure is changed while the intermediate transfer belt  25  is in contact with the photosensitive drum  15  without separating the intermediate transfer belt  25  and the photosensitive drum  15  from each other, the tension applied to the intermediate transfer belt  25  varies. As a result, the transfer position at which the toner image formed by the corresponding image forming unit  13  is transferred will be displaced. 
     Therefore, according to the present exemplary embodiment, if the paper type of the recording sheet  60  is switched from the normal paper to the embossed paper or from the embossed paper to the normal paper, the following processes are performed before an image specified by the user (hereinafter referred to as a specified image), which is to be transferred onto the recording sheet  60 , is formed in the image forming unit  13 . That is, a transfer control process is performed to change the transfer pressure applied by the first transfer device  30 . In addition, a misregistration detection process is performed to detect the difference of the transfer position and a correction process is performed to correct the image forming position of the specified image in accordance with the result of the detection. The structure for performing the above-described processes in the image forming apparatus  1  will now be described. 
     The image forming apparatus  1  includes a structure for performing the above-described transfer control process for controlling the transfer pressure applied by each first transfer device  30 , the misregistration detection process, and the correction process in addition to the structure for performing a usual image forming process. The structures of the image forming apparatus  1  will now be described.  FIG. 4  is a block diagram illustrating the structure for performing the transfer control process, the misregistration detection process, and the correction process in the image forming apparatus  1 . As illustrated in  FIG. 4 , the image forming apparatus  1  includes the control unit  101 , a memory unit  102 , an image processing unit  103 , an operation unit  104 , a misregistration detection unit  105 , the image forming units  13 K,  13 Y,  13 M,  13 C, and  13 L, and the above-described first transfer devices  30 K,  30 Y,  30 M,  30 C, and  30 L, which are connected to each other by lines. 
     The control unit  101  includes a central processing unit (CPU)  101 A, a read only memory (ROM)  101 B, and a random access memory (RAM)  101 C. The ROM  101 B stores control programs, and the CPU  101 A executes the control programs using the RAM  101 C as a working area, thereby controlling each part of the image forming apparatus  1  to activate the image forming apparatus  1 . More specifically, the control unit  101  outputs a control signal for carrying out the transfer control process in which the transfer pressure applied by each first transfer device  30  is changed in accordance with the paper type of the recording sheet  60 . In addition, the control unit  101  performs the misregistration detection process for detecting the difference of the transfer position. In an example of the misregistration detection process according to the present exemplary embodiment, each image forming unit  13  receives image data of a test pattern and transfers an image of the test pattern onto the intermediate transfer belt  25 , and the misregistration of the image is detected. In addition, in the present exemplary embodiment, first-transfer-control information representing the control state (the normal-paper receiving state or the embossed-paper receiving state) of each first transfer device  30  set when the toner images have been transferred onto the recording sheet  60  the last time is stored in the memory unit  102 , which will be described below. Then, when the toner images are transferred onto the next recording sheet  60 , the transfer control process for changing the control state of each first transfer device  30  is performed on the basis of the first transfer control information and the paper type. 
     The memory unit  102  is formed of a nonvolatile storage medium, and stores image data of the test pattern (hereinafter referred to as pattern image data) and data of various setting information, such as the first transfer control information, set in the image forming apparatus  1 . The operation unit  104  includes, for example, a touch-panel display device for displaying messages and a menu screen through which the paper type of the recording sheet  60 , for example, can be specified, and receives instructions from the user. 
     The misregistration detection unit  105  detects the test pattern that has been transferred onto the intermediate transfer belt  25  for detecting the difference of the transfer position on the intermediate transfer belt  25 . The misregistration detection unit  105  includes pattern detectors  600 A,  600 B, and  600 C (hereinafter referred to as pattern detectors  600  unless they are distinguished from each other). The misregistration detection process according to the present exemplary embodiment will now be described.  FIG. 5  is a conceptual diagram illustrating the detection of the image of the test pattern transferred onto the intermediate transfer belt  25  with the pattern detectors  600 . 
     According to the present exemplary embodiment, as illustrated in  FIG. 5 , a test pattern  610 , which is a so-called chevron pattern, for detecting the image position is formed on the intermediate transfer belt  25  and is detected by each of the pattern detectors  600 . The pattern detectors  600  are disposed downstream of the image forming unit  13 C in the moving direction of the intermediate transfer belt  25 , and are positioned at respective predetermined measurement reference positions in an OUT section (front section in  FIG. 5 ), a CENTER section (central section in  FIG. 5 ), and an IN section (rear section in  FIG. 5 ) of the image forming apparatus  1  along a main scanning direction. However, four or more pattern detectors  600  may instead be formed with constant intervals therebetween along the width direction of the intermediate transfer belt  25  as necessary. 
     Patterns of various shapes can be used as the test pattern  610 . In the present exemplary embodiment, the test pattern  610  includes angle-shaped marks formed at positions corresponding to the pattern detectors  600 A,  600 B, and  600 C, each angle-shaped mark including straight lines that are connected to each other at the center and inclined leftward and rightward at the same angle. In the test pattern  610  according to the present exemplary embodiment, one of the six colors is set as a reference color, and the angle-shaped marks of respective colors are formed such that the angle-shaped marks are arranged along a sub-scanning direction (moving direction of the intermediate transfer belt  25 ) with predetermined intervals therebetween. 
     The structure of each pattern detector  600  for detecting the test pattern  610  will now be described.  FIGS. 6A and 6B  are schematic diagrams illustrating the structure of each pattern detector  600 . As illustrated in  FIG. 6A , each pattern detector  600  includes light emitting diodes (LED)  620  and  630  which are inclined at predetermined angles and emit light toward the intermediate transfer belt  25  and a light receiving unit  640 . 
     Plural photodiodes, which are light receiving elements, are combined in the light receiving unit  640 . As illustrated in  FIG. 6B , the light receiving unit  640  includes first light receiving elements  641   a  and  641   b  (hereinafter referred to as first light receiving elements  641  unless they are distinguished from each other) and second light receiving elements  642   a  and  642   b  (hereinafter referred to as second light receiving elements  642  unless they are distinguished from each other). The first light receiving elements  641  and the second light receiving elements  642  are inclined at a predetermined angle with respect to the horizontal direction of the intermediate transfer belt  25 , and are arranged symmetrically to each other in the left-right direction. 
     The first and second light receiving elements  641  and  642  receive light emitted by the LEDs  620  and  630  and reflected by the test pattern  610  formed on the intermediate transfer belt  25 , and output signals corresponding to the amounts of the reflected light. In the case where there is no misregistration in the main-scanning direction, the first light receiving element  641   a  and the second light receiving element  642   a  output signals corresponding to the amounts of the reflected light at the same time. Then, after a certain time period from when the signals are output from the first and second light receiving elements  641   a  and  642   a , the first and second light receiving elements  641   b  and  642   b  output signals corresponding to the amounts of the reflected light. 
     The misregistration detection unit  105  compares the signals output from the first and second light receiving elements  641  and  642  with a predetermined threshold. The misregistration detection unit  105  outputs a low-level signal while the waveform of each signal is lower than the threshold and outputs a high-level signal while the waveform of each signal is higher than or equal to the threshold. 
     The image processing unit  103  is included in the image processing device  12 , and subjects the data of the specified image to image processes, such as density adjustment. The image processing unit  103  includes a correction unit  103 A. The correction unit  103 A receives waveform of each detection signal for the reference color from the misregistration detection unit  105 , and detects a time interval from when the detection signal has changed from the low level to the high level the first time to when the detection signal has changed from the low level to the high level the second time. The correction unit  103 A determines the misregistration values of the reference color in the main-scanning and sub-scanning directions on the basis of the detected time intervals, and then determines the misregistration values of the other colors with respect to the misregistration values of the reference color on the bass of the intervals between the images included in the test pattern  610  set in advance. Then, the correction unit  103 A determines correction values for correcting the image forming positions for the image data on the basis of the determined misregistration values. In the present exemplary embodiment, an example in which the image data is corrected on the basis of the determined correction values will be described. However, the image forming positions may instead be corrected by other known methods, such as a method of adjusting the exposure timing, for correcting a color misregistration of an image. 
     The operation of the image forming apparatus  1  according to the present exemplary embodiment will now be described.  FIG. 7  illustrates the operation flow of the overall operation performed by the image forming apparatus  1 . First, the control unit  101  of the image forming apparatus  1  receives a command for specifying the paper type of the recording sheet  60  through the operation unit  104  (step S 11 ). 
     Then, the image reading device  4  reads an original document specified by the user (step S 12 ). The control unit  101  converts the image data of the specified image read by the image processing device  12  in step S 12  into color image data for the five colors, and stores the image data of the respective colors in a storage area of the RAM  101 C (step S 13 ). Then, the control unit  101  performs an image forming process in which the image data of the respective colors corresponding to the specified image, which has been stored in the RAM  101 C in step S 13 , is transferred onto the recording sheet  60  of the paper type specified in step S 11  (step S 14 ). 
       FIG. 8  illustrates the operation flow of the image forming process. The control unit  101  reads the first transfer control information stored in the memory unit  102  (step S 110 ). Then, the control unit  101  determines whether the paper type specified by the user in step S 11  is the normal paper or the embossed paper (step S 120 ). 
     Then, the control unit  101  determines whether or not the control state of each first transfer device  30  is to be changed in accordance with whether or not the previous control state of the first transfer device  30  based on the first transfer control information read in step S 110  is the same as the control state corresponding to the paper type determined in step S 120  (step S 130 ). More specifically, if the paper type of the recording sheet  60  that has been previously subjected to the transfer process is the normal paper, the first transfer device  30  is currently set to the normal-paper receiving state. Therefore, if the paper type of the recording sheet  60  to be subjected to the transfer process next is the embossed paper, it is determined that the control state of the first transfer device  30  is to be changed since the first transfer device  30  is not currently set to the control state corresponding to the embossed paper. If the recording sheet  60  to be subjected to the transfer process next is the normal paper, it is determined that it is not necessary to change the control state of the first transfer device  30  since the first transfer device  30  is currently set to the control state corresponding to the normal paper. Similarly, also in the case where the paper type of the recording sheet  60  that has been previously subjected to the transfer process is the embossed paper, the control state of the first transfer device  30  is changed depending on the paper type of the recording sheet  60  to be subjected to the transfer process next. 
     If the control unit  101  determines that the control state of the first transfer device  30  is to be changed (YES in step S 130 ), a control signal representing the control state corresponding to the paper type determined in step S 120  is transmitted to the first transfer device  30 , and the first transfer control process is performed (step S 140 ). The first transfer control process will now be described in detail with reference to the operation flow illustrated in  FIG. 9 . 
     If the paper type determined in step S 120  in  FIG. 8  is the embossed paper (YES in step S 141 ), the control unit  101  transmits a control signal representing the embossed-paper receiving state to the first transfer device  30  (step S 142 ). Then, when the first transfer device  30  receives the control signal representing the embossed-paper receiving state from the control unit  101 , the first transfer device  30  drives the stepping motor  354  so as to change the state thereof from the normal-paper receiving state illustrated in  FIGS. 2A and 2B  to the embossed-paper receiving state illustrated in  FIGS. 3A and 3B , thereby reducing the transfer pressure of the intermediate transfer belt  25  (step S 143 ). 
     If the paper type determined in step S 120  is the normal paper (NO in step S 141 ), the control unit  101  transmits a control signal representing the normal-paper receiving state to the first transfer device  30  (step S 144 ). Then, the first transfer device  30  drives the stepping motor  354  so as to change the state thereof from the embossed-paper receiving state illustrated in  FIGS. 3A and 3B  to the normal-paper receiving state illustrated in  FIGS. 2A and 2B , thereby controlling the transfer pressure (step S 143 ). 
     Referring to  FIG. 8  again, after the control unit  101  has set the control state of the first transfer device  30  to the control state corresponding to the paper type of the recording sheet  60  in step S 140 , the control unit  101  performs the misregistration detection process for detecting the difference of the transfer position on the intermediate transfer belt  25  (step S 150 ). 
       FIG. 10  illustrates the operation flow of the misregistration detection process performed in step S 150 . The control unit  101  reads the pattern image data from the memory unit  102  and supplies the pattern image data to each image forming unit  13  (step S 151 ). 
     The toner image of the test pattern  610  is transferred onto the intermediate transfer belt  25  by each image forming unit  13  on the basis of the pattern image data supplied from the control unit  101  (step S 152 ). The control unit  101  causes the misregistration detection unit  105  to detect the toner image of the test pattern  610  that has been transferred onto the intermediate transfer belt  25 . Then, the correction unit  103 A of the image processing unit  103  detects detection time intervals for the test pattern  610  on the basis of the detection signals of the test pattern  610  output from the pattern detectors  600  included in the misregistration detection unit  105  (step S 153 ). Then, the misregistration value of each color is determined on the basis of the detection time intervals (step S 154 ). 
     Referring to  FIG. 8  again, the control unit  101  causes the image processing unit  103  to subject the image data of the respective colors stored in the RAM  110 C to the image processes, such as density adjustment. In addition, the control unit  101  determines the correction value for correcting the image forming position of the image data after the image processes on the basis of the misregistration value determined in step S 150 . The control unit  101  corrects the image data on the basis of the correction value, and supplies the corrected image data to each image forming unit  13  (step S 160 ). An electrostatic latent image corresponding to the corrected image data of each color is formed on each photosensitive drum  15  by the corresponding image forming unit  13 , and is developed. Then, the toner image formed on the photosensitive drum  15  by the developing process is transferred onto the intermediate transfer belt  25  by the corresponding first transfer device  30 , which is set to the control state corresponding to the type of the recording sheet  60 . Then, the toner image formed on the intermediate transfer belt  25  is transferred onto the recording sheet  60  by the second transfer roller  50 , and the recording sheet  60  is ejected to the paper ejection tray  64  (step S 170 ). 
     In the image forming apparatus  1  according to the above-described exemplary embodiment, the control state of each first transfer device  30  is changed in accordance with the current control state of the first transfer device  30  and the paper type of the recording sheet  60 . In addition, when the control state of the first transfer device  30  is changed, the misregistration on the intermediate transfer belt  25  caused in the first transfer process is detected before the image data of the original document  2  specified by the user is transferred onto the recording sheet  60  in the second transfer process. Then, the image forming position of the image to be formed on the recording sheet  60  is corrected. Thus, the misregistration owing to the change in the transfer pressure applied by the first transfer device  30  is determined in advance, so that color registration in the image formed on the recording sheet  60  can be reduced. 
     Modifications 
     Although an exemplary embodiment of the present invention is described above, the present invention is not limited to the above-described exemplary embodiment, and the following modifications are included in the scope of the present invention. 
     (1) In the above-described exemplary embodiment, the misregistration detection process is performed each time the paper type is switched between the normal paper and the embossed paper. However, the correction value for correcting the misregistration caused when the paper type is switched from the normal paper to the embossed paper can be stored in advance and used to perform the correction of the image forming positions for the image data. In this case, the correction value used when the paper type is switched from the normal paper to the embossed paper are obtained by measurements and stored by the process illustrated in  FIG. 11 . The operation flow illustrated in  FIG. 11  will now be described. First, an image of the test pattern  610  similar to that in the exemplary embodiment is formed on a sheet of normal paper (step S 21 ). Then, the misregistration value (Rn) of the test pattern on the normal paper is detected by a misregistration detection device provided outside the image forming apparatus  1  (step S 22 ). In addition, an image of the test pattern  610  similar to that in the exemplary embodiment is formed on a sheet of embossed paper (step S 23 ). Then, the misregistration value (Re) of the test pattern on the embossed paper is detected by the misregistration detection device provided outside the image forming apparatus  1  (step S 24 ). Then, the difference between the detected misregistration value (Rn) on the normal paper and the detected misregistration value (Re) on the embossed paper is stored in the memory unit  102  as a misregistration correction value (E). The correction value for when the normal paper is used is determined by detecting the misregistration in accordance with the temperature, the time, and the number of printing sheets, as in the related art. The thus-determined correction value is stored in advance in the memory unit  102  (step S 25 ). The operation for correcting the image forming position using the misregistration correction value stored in advance will now be described with reference to  FIG. 12 . In the following description, it is assumed that the paper type of the recording sheet  60  that has been previously subjected to the transfer process is the normal paper. Referring to  FIG. 12 , first, the control unit  101  reads the previous first transfer control information that is stored in the memory unit  102  (step S 210 ). Then, the control unit  101  determines whether the paper type specified by the user is the normal paper or the embossed paper (step S 220 ). If the paper type specified by the user is the embossed paper (YES in step S 230 ), the control unit  101  reads the misregistration correction value for the normal paper and the misregistration correction value (E) for when the paper type is switched to the embossed paper from the memory unit  102 . Then, the control unit  101  adds the misregistration correction value for the normal paper to the misregistration correction value (E) for the embossed paper (step S 240 ). The control unit  101  performs a process for correcting the image forming position of the image data on the basis of the sum (step S 250 ), and forms an image on a sheet of embossed paper (step S 260 ). If the paper type is normal paper instead of embossed paper in step S 230  (NO in step S 230 ), the image forming process is performed similarly to the previous time the image forming process has been performed (step S 260 ). With this structure, it is not necessary to perform the misregistration detection process when the paper type is switched between the embossed paper and the normal paper. As a result, productivity of the image forming process can be increased. 
     In the above-described example, the misregistration correction value for when the paper type is switched between the normal paper and the embossed paper is stored in advance. However, the misregistration correction value may instead be stored in advance in association with the range of the misregistration value obtained by measurements. In such a case, the misregistration detection process is performed when the paper type of the recording sheet  60  is changed, and the misregistration correction value for the misregistration value range corresponding to the detection result is used to correct the image forming position. 
     (2) According to the above-described exemplary embodiment, the control state of the first transfer device  30  is changed in accordance with the paper type specified by the user through the operation unit  104 . However, the recording sheet  60  may be detected by, for example, an optical sensor and the paper type of the recording sheet  60  may be determined by the control unit  101  on the basis of a signal output by the optical sensor as a result of the detection. More specifically, when the original document specified by the user is read, the amount of light incident on and reflected by the recording sheet  60  is measured by the optical sensor. It is determined that recording sheet  60  is the normal paper if the measured amount of the reflected light is larger than or equal to a threshold (the amount of reflected light corresponding to the degree of surface irregularity of the normal paper) stored in advance in the ROM  101 B. It is determined that the recording sheet  60  is the embossed paper if the measured amount of the reflected light is smaller than the threshold. 
     (3) In addition, in the above-described exemplary embodiment, the transfer pressure applied by the first transfer device  30  is controlled in accordance with the paper type of the recording sheet  60 . However, the first transfer bias applied by the first transfer bias source  320  included in the first transfer device  30  may instead be controlled in accordance with the paper type. When the transfer electric field at the transfer position of the first transfer roller  310  is reduced by controlling the first transfer bias, the electrostatic force that causes the toner to be transferred to the intermediate transfer belt  25  decreases. Therefore, the adhesion force applied to the toner on the intermediate transfer belt  25  decreases. As a result, similar to the above-described exemplary embodiment, the toner image can be reliably transferred, without leaving blank portions, onto the recording sheet  60  such as a sheet of embossed paper that has a larger degree of irregularity than that of the normal paper. In this case, the setting value of the first transfer bias is stored as the first transfer control information. The misregistration detection process similar to that in the above-described exemplary embodiment is performed when the first transfer bias is changed, and the image forming position is corrected accordingly. 
     (4) The programs to be executed by the CPU  101 A may be provided in such a state that the programs are stored in a computer-readable recording medium, such as a magnetic recording medium, an optical recording medium, a magneto-optical recording medium, and a semiconductor memory, and be installed in each apparatus. Examples of magnetic recording media are a magnetic tape and a magnetic disc, such as a hard disk drive (HDD) and a flexible disk (FD). An example of an optical recording medium is an optical disk, such as a compact disc (CD) and a digital versatile disk (DVD). Alternatively, the programs may be downloaded and installed into each apparatus through communication lines. 
     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.