Patent Publication Number: US-2011069327-A1

Title: Method and apparatus for forming image

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from: U.S. Provisional Application No. 61/244,757 filed on Sep. 22, 2009, the entire contents of each of which are incorporated herein by reference. 
    
    
     FILED 
     Embodiments described herein relates generally to an image forming apparatus and a sheet feeding mechanism. 
     BACKGROUND 
     In an image forming apparatus, movement of a sheet that holds an image affects the quality of the image formed by elements that form the image. One of influences on the image quality includes “nonuniformity of image forming speed” caused when the sheet moves while the elements forming the image form the image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A general architecture that implements the various features of the embodiments will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments and not to limit the scope of the embodiments. 
         FIG. 1  is an exemplary diagram illustrating an apparatus; 
         FIG. 2  is an exemplary diagram illustrating a configuration of an imaging section incorporated in the apparatus shown in  FIG. 1 ; 
         FIG. 3  is an exemplary diagram illustrating a transfer roller at a position for moving a toner image from a transfer belt to a sheet; 
         FIG. 4  is an exemplary diagram illustrating the transfer roller separated from the transfer belt; 
         FIG. 5  is an exemplary diagram illustrating a mechanism configured to move a pusher; 
         FIG. 6  is an exemplary block diagram illustrating a control system of the apparatus; 
         FIG. 7  is an exemplary diagram illustrating a cam and a pusher in a non-pressed state; 
         FIG. 8  is an exemplary diagram illustrating a profile of the cam according to  FIG. 7 ; 
         FIG. 9  is an exemplary diagram illustrating the cam and the pusher in a pressed state; 
         FIG. 10  is an exemplary diagram illustrating a state in which an outer circumference A section of the cam moves to the pusher; 
         FIG. 11  is an exemplary diagram illustrating a state in which an outer circumference C section of the cam moves the pusher; 
         FIG. 12  is an exemplary timing chart illustrating a transfer pressure; 
         FIG. 13  is an exemplary diagram illustrating a state in which an outer circumference A section of another cam moves the pusher; 
         FIG. 14  is an exemplary diagram illustrating a state in which an outer circumference B section of another cam moves the pusher; 
         FIG. 15  is an exemplary diagram illustrating a state in which an outer circumference C section of another cam moves the pusher; 
         FIGS. 16A ,  16 B and  16 C are exemplary diagrams each illustrating test patterns on the transfer belt and detection examples; 
         FIG. 17  is an exemplary timing chart illustrating an imaging and sheet moving position; 
         FIG. 18  is an exemplary diagram illustrating timing of control of the speed of the transfer belt or timing of start of writing of an image; and 
         FIG. 19  is an exemplary diagram illustrating control of the speed of the transfer belt. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to an embodiment, a printer comprising: an image carrier configured to hold an a first color image and a second color image in a state in which the images overlap each other and configured to move the overlapping the first color image and the second color image to a sheet in a transfer position; and controller configured to change speed of forming the first color image and the second color image from a first speed to a second speed higher than the first speed when the thickness of the sheet exceeds a threshold, the first speed including speed obtained by correcting fluctuation in the speed due to the thickness of the sheet with respect to the second speed. 
     Embodiments will now be described hereinafter in detail with reference to the accompanying drawings. 
       FIG. 1  schematically shows an MFP (Multi-Functional Peripheral) to which the embodiment is able to apply. 
     An MFP  101  shown in  FIG. 1  has an image forming section  1  for outputting image information as an output image which is referred to as a hard copy or a print out, for example, a paper supplying section  3  capable of supplying, to the image forming section  1 , paper (an output medium) having an optional size which is used for an image output, and an image reading section  5  for taking, as image data, image information to be an image forming object in the image forming section  1  from an object holding the image information (which will be hereinafter referred to as an original). 
     The image reading section  5  includes an original table (an original glass)  5   a  for supporting an object and an image sensor for converting the image information into image data, for example, a CCD sensor, which will not be described in detail. The image reading section  5  converts reflected light into an image signal through the CCD sensor. The reflected light is obtained by irradiating light from an illuminating apparatus onto the original set into the original table  5   a.    
     Moreover, the image reading section  5  integrally has an automatic document feeder (ADF)  7  for discharging a read original from a reading position to a discharging position and guiding the next original to the reading position after formation of an image output or taking of image information (hereinafter referred to as a read) is ended when the original is a sheet. In place of the ADF  7 , a general original cover may be used. Furthermore, the CCD sensor of the image reading section  5  may be positioned in an optional position in a delivery path through which the original is delivered in the ADF  7  independently of the original table  5   a . The CCD sensor placed in an optional position in the delivery path through which the original is delivered independently of the original table  5   a  reads, as image data, image information included in the original during the delivery without the original positioned on the original table  5   a.    
     An instruction input section for giving an instruction for starting image formation in the image forming section  1  and starting to read image information of the original through the image reading section  5 , that is, a control panel (an operating section)  9  is placed in a strut  9   a  (fixed to the image forming section  1 ) and a swing arm  9   b  in a corner at the left or right side behind the image reading section  5  or the like. 
     The image forming section  1  includes first to fourth photoconductive drums  11   a  to  11   d  for holding latent images, developing devices  13   a  to  13   d  for supplying developers, that is, toners to the latent images held by the photoconductive drums  11   a  to  11   d  and carrying out development, a transfer belt  15  for holding toner images held by the photoconductive drums  11   a  to  11   d  in order, cleaners  17   a  to  17   d  for removing the toner remaining on the photoconductive drums  11   a  to  11   d  from the individual photoconductive drums  11   a  to  11   d,  a transfer device  19  for transferring the toner image held by the transfer belt  15  onto plain paper or a sheet-like transfer medium (hereinafter referred to as a sheet material) such as an OHP sheet to be a transparent resin sheet, a fuser device  21  for fixing the toner image transferred to the sheet material by the transfer device  19  onto the sheet material, and an exposing device  23  for forming latent images on the photoconductive drums  11   a  to  11   d  and the like. 
     The first to fourth developing devices  13   a  to  13   d  store toners having optional colors of Y (yellow), M (magenta), C (cyan) and Bk (black) which are used for obtaining a color image by a subtractive process and visualize a latent image held by each of the photoconductive drums  11   a  to  11   d  in any one of the colors Y, M, C and Bk. The respective colors are determined in predetermined order corresponding to an image forming process or a characteristic of the toner. 
     The transfer belt  15  holds the toner images having the respective colors which are formed by the first to fourth photoconductive drums  11   a  to  11   d  and the corresponding developing devices  13   a  to  13   d  in order (of the formation of the toner images). 
     As explained later with reference to  FIG. 2 , the transfer belt  15  receives pressure from each of a belt opposed member  51  configured to set pressure between the photoconductive drums  11   a  to  11   d  and the transfer belt  15  of the image forming section  1 , a belt cleaner opposed member  55  configured to set pressure applied by a belt cleaner  53  for cleaning the surface of the transfer belt  15 , and a transfer opposed member  57  configured to set pressure applied when the sheet material is brought into contact with the transfer belt  15  by the pressure from the transfer device  19 . 
     The paper supply section  3  supplies, at predetermined timing, the sheet material to be used for transferring the toner image by the transfer device  19 . 
     Cassettes positioned in a plurality of cassette slots  31 , which will not be described in detail, store sheet materials having optional sizes. Depending on an image forming operation which will not be described in detail, a pickup roller  33  takes the sheet material out of the corresponding cassette. The size of the sheet material corresponds to the size of the image of the developer formed by the image forming section  1 . 
     A separating mechanism  35  prevents at least two sheet materials from being taken out of the cassette by the pickup roller  33 . 
     A plurality of delivery rollers  37  feed the sheet material separated to be one sheet by the separating mechanism  35  toward an aligning roller  39 . 
     The aligning roller  39  feeds the sheet material to a transfer position in which the transfer device  19  and the transfer belt  15  come into contact with each other in timing for transferring the image of the developer from the transfer belt  15  by the transfer device  19 . 
     The fuser device  21  fixes the image of the developer (toner) corresponding to the image information onto the sheet material, that is, the output image (hard copy or print out) and feeds the output image to a stock section  47  positioned in a space between the image reading section  5  and the image forming section  1 . 
     The transfer device  19  is positioned in an automatic duplex unit (ADU)  41  for replacing both sides of the sheet material, that is, the output image (hard copy or print out) which has the toner image fixed thereto by the fuser device  21 . A bypass tray  81  is attached to the ADU  41 . 
     The ADU  41  moves to a side (the right side) with respect to the image forming section  1  when the sheet material is jammed between the delivery roller  37  (a final one) and the aligning roller  39  or between the aligning roller  39  and the fuser device  21 , that is, in the transfer device  19  or the fuser device  21 . The ADU  41  integrally has a cleaner  43  for cleaning the transfer device  19 . 
     A media sensor  45 , arranged in a path between the delivery roller  37  and the aligning roller  39 , detects the thickness of the sheet material conveyed to the aligning roller  39 . The media sensor  45  useable an optical type benefit of priority from: U.S. patent application Ser. No. 12/197,880 filed on Aug. 25, 2008 and No. 12/199,424 filed on Aug. 27, 2008 and/or a shift of thickness detecting roller type benefit of priority from: U.S. Provisional Application No. 61/043,801 filed on Apr. 10, 2008, each of which are incorporated. 
       FIG. 2  shows a transfer section of an imaging section in the MFP shown in  FIG. 1 . 
     A bend of a belt surface of the transfer belt  15  is a fixed amount related to tension from at least one tension device. The belt opposed member  51 , the belt cleaner opposed member  55 , and the transfer opposed member  57  are, for example, roller members. The belt opposed member  51  provides the transfer belt  15  (and the photoconductive drums  11   a  to  11   d ) with a transfer voltage (an electrostatic field). The transfer device  19  applies, when the sheet material moves between the transfer device  19  and the transfer belt  15 , pressure for transfer to the sheet material (and the transfer belt  15 ). The transfer device  19  provides the sheet material (and the transfer belt  15 ) with a transfer voltage (an electrostatic field). 
       FIGS. 3 and 4  show the operation (the position) of the transfer device for moving a toner image held by the transfer belt onto the sheet material. 
     The transfer device  19  is held by a supporting member  61  having a fulcrum  61   a.  During non-transfer (non-image formation), as shown in  FIG. 4 , a spring  63  applies a load to the supporting member  61  such that the supporting member  61  is located on a side on which the transfer device  19  (supported by the supporting member  61 ) is not in contact with the transfer opposed member  57  and the transfer belt  15  located on the outer circumference of the transfer opposed member  57  (a separated state or a non-pressed state). During image formation (during a printing sequence), as shown in  FIG. 2 , the supporting member  61  turns around the fulcrum  61   a  such that the transfer device  19  (supported by the supporting member  61 ) is located in a position where the transfer device  19  is in contact with the transfer belt  15  and the transfer opposed member  57  located on the inner side of the transfer belt  15  at a transfer pressure or pressure for the sheet material. Image formation is permitted in a state in which the transfer device  19  supported by the supporting member  61  moves to a position where a segment connecting a center axis  57   a  of the transfer opposed member  57  and a center axis  19   a  of the transfer device  19  is a straight line (a distance between the two center axes is the minimum) or a state in which the transfer device  19  moves to a position where the transfer device  19  turns further to an auxiliary member  59  side, which applies tension to the transfer belt  15  in cooperation with the transfer opposed member  57 , than the position where the distance between the two center axes is the minimum. 
     A pusher  65  linearly moves to apply a propulsion pressure (pressure for turning the supporting member) to the supporting member  61 . 
     The propulsion pressure (i.e., a movement amount of the pusher  65 ) permits the transfer device  19  supported by the supporting member  61  to come into contact with the transfer belt  15  (a contact state). A guide (a pusher supporting member)  67  guides the pusher  65  to linearly (reciprocatingly) move. For example, when the pusher  65  has a long hole (parallel grooves) extending in one direction, the guide  67  is formed in a pin shape. For example, when the pusher  65  is a projection (a pin or a rib), the guide  67  may be formed in a parallel groove or rail shape. 
     A cam  69  sets the movement amount of the pusher  65  according to rotation of the cam  69 . 
     A transfer pressure acting on the sheet material moving between the transfer device  19  and the transfer belt  15  changes according to the movement amount of the pusher  65 . In other words, the transfer pressure acting on the sheet material can be arbitrarily set between the transfer device  19  and the transfer belt  15  by changing the movement amount of the pusher  65 . 
       FIG. 5  shows an example of a mechanism configured to move (reciprocatingly move) the pusher shown in  FIGS. 3 and 4 . 
     The transfer device  19  supported by the supporting member  61  is located in a position for providing the transfer pressure to the transfer belt  15  and the transfer opposed member  57  (the contact state) or a position in the non-pressed state (the separated state) according to the reciprocating movement of the pusher  65  by the rotation of the cam  69 . 
     A shaft  71  supports the cam  69 . The shaft  71  receives rotation of a stepping motor  75  via a gear  73 . The stepping motor  75  rotates in a first direction and a second direction opposite to the first direction. A rotation sensor  77  including an actuator  77   a  held by the shaft  71  and a position detection sensor  77   b  for detecting presence or absence of the actuator  77   a  measures a rotation amount (a rotation angle) of the cam  69 . Specifically, when the actuator  77   a  passes through the position detection sensor  77   b,  the magnitude or the polarity of a sensor signal output by the sensor  77   b  is switched. With this switching as a trigger, the rotation angle (a rotation position) of the cam  69  stops in a specified position. 
     The cam  69  and the actuator  77   a  are positioned with respect to the circumference of the shaft  71 . This makes it possible to accurately set the movement amount given to the supporting member  61  by the pusher  65 . Therefore, it is possible to accurately set the transfer pressure applied to the transfer belt  15  and the transfer opposed member  57  by the transfer device  19 . 
       FIG. 6  shows a control system of the MFP illustrated in  FIGS. 1 to 5 . 
     The MFP (image forming apparatus)  101  includes a system bus line  111 . The system bus line  111  connects a main control block, that is, a main CPU  112  for processing image information of an object to be outputted by an image forming section  1 . The MFP  101  includes the scanner  5  and an image processor  117 . The MFP  101  includes a motor driver  119  which provides electric pulse to rotate the stepping motor  151 . A rotational angle of the stepping motor  151  is proportional to a number of the pulses. A rotation speed is in proportional to a number of the pulses and to control the moving speed of the transfer belt  15  rotates by the first roller  55 . The image forming section  1  includes a motor driver  120  which provides electric pulse to rotate the stepping motor  75 . A rotational angle of the stepping motor  75  is proportional to a number of the pulses. The main control block  112  connects an ROM (Read Only Memory)  113 , an RAM (Random Access Memory)  114 , and a non-volatile RAM  115  for storing a total number of times of image formation, a total operating (working) time or the like, an interface  116  for inputting an output of the media sensor  45  to the main control block  112 , and the operation panel  9 . The image processor  117  connects a page memory  118 . 
       FIG. 7  shows the operation of the pusher  65  and the cam  69  schematically shown in  FIGS. 3 and 4  in a state (of the cam  69 ) immediately before a sheet enters the transfer position where the supporting roller  57  and the transfer roller  19  are in contact with each other.  FIG. 8  shows a state of the cam  69  viewed from a direction orthogonal to an axis of a shaft hole, i.e., a characteristic of a cam outer circumferential shape. 
     The outer circumference of the cam  69  is an oblong shape or an elliptical shape having at least one concave section “b”. An area of the pusher  65  in contact with the outer circumference of the cam  69  is a curved surface of a convex shape. 
     In the cam  69 , a distance “a” from a shaft hole  69   a  to an outer circumference A section (a first convex section) is larger than a distance “c” from the shaft hole  69   a  to an outer circumference C section. The distance “c” from the shaft hole  69   a  to the outer circumference C section (a second convex section) is larger than a distance “b” from the shaft hole  69   a  to an outer circumference B (concave) section. 
     During the printing sequence, as shown in  FIG. 3 , the cam  69  pushes the pusher  65  in the horizontal direction, overcomes the load of the spring  63 , and brings the transfer roller  19  into contact with the supporting roller  57 . 
     At other timing, as shown in  FIG. 4 , an amount with which the cam  69  pushes in the pusher  65  is not large enough for bringing the transfer roller  19  into contact with the supporting roller  57  and spaces the transfer roller  19  away from the supporting roller  57 . 
     In a state in which the sheet is present in a secondary transfer position between the transfer roller  19  and the supporting roller  57  while the cam  69  is maintained in the position shown in  FIG. 3 , moving speed of the transfer belt  15  falls because of the thickness of the sheet. In this case, in the transfer position where the transfer belt  15  and pre-transfer rollers  51   a  to  51   d  are in contact with each other, positions of toners moving to the transfer belt  15  fluctuate (the toners move at an interval different from an original interval of movement conforming to a space between the pre-transfer rollers  51   a  to  51   d ). In this case, a color drift may occur because of superimposition of the toners of four colors. 
     The occurrence of a color drift can be suppressed by depressing a transfer pressure in the secondary transfer position taking into account the presence of the sheet in the secondary transfer position during actual printing. However, it is undesirable to reduce, within an image area, the transfer pressure in the secondary transfer position. 
     Therefore, as shown in  FIG. 12 , it is desirable to maintain the transfer pressure, which is applied to the transfer belt  15  and the supporting roller  57  by the transfer roller  19 , at first magnitude for a predetermined time immediately before the leading end of the sheet having a thickness equal to or smaller than a threshold enters the transfer area and after the trailing end of the sheet exits the transfer area. The thickness as the threshold is, for example, the thickness of a sheet, weight per 1 m 2  of which is 105 g. 
     During (actual) image formation when the sheet is located in the transfer position where the transfer roller  19  and the supporting roller  57  are in contact with each other, load fluctuation occurs when the sheet passes the transfer position. Therefore, as shown in  FIG. 11 , the transfer pressure in the transfer position is set to the first pressure (magnitude) in a cam position [C] of the cam  69  shown in  FIG. 8 . Concerning thick paper having a thickness exceeding the threshold, as shown in  FIG. 10 , the transfer pressure in the transfer position is set to second pressure (magnitude) larger (higher) than the first pressure in a cam position [A] of the cam  69  shown in FIG. B. It is possible to suppress a color drift from occurring irrespective of the thickness of a sheet by setting the two transfer pressures. In each of the cam positions [A] and [C], a different load is assumed according to the thickness of a sheet. During the actual image formation, the cam position [B] is used. The cam positions [A] and [C] are useful for calculating a speed fluctuation amount of the transfer belt  15  in a state close to an actual, printing condition. 
     A speed fluctuation amount of the transfer belt  15  at the time when the sheet (or the thick paper) is located in the transfer area (the transfer position) can be calculated by comparing, during alignment control performed by using the first or second transfer pressure, the speed of the transfer belt  15  obtained by assuming a position of a load during actual printing to which the cam position [B] of the cam  69  shown in  FIG. 8  is applied and a passing time of a test pattern formed on the transfer belt  15  shown in  FIGS. 16A ,  16 B, and  16 C according to the cam position [A (the transfer pressure of the second magnitude obtained by assuming the thick paper) or [C (the transfer pressure of the first magnitude obtained by assuming paper having a thickness smaller than the thickness of the thick paper)]. 
     In other words, as shown in  FIGS. 16A ,  16 B, and  16 C, a speed fluctuation amount of the transfer belt  15  (a sheet) can be calculated by detecting, with a detector  121 , during alignment control for the cam  69 , moving speed of the transfer belt  15  (the number revolutions of the motor  151 ) in a position of “b” (time of contact between the supporting roller  57  and the transfer roller  19 ) and a passing time of a test pattern formed on the transfer belt  15  when pressure provided by “a” of the cam  69  or “c” of the cam  69  is used and comparing the moving speed and the passing time. It is possible to surely suppress a color drift from occurring according to the thickness of a sheet by changing, during an actual image output operation, the detected speed fluctuation amount in conjunction with control of the rotation of the cam  69 , i.e., the pressure between the supporting roller  57  and the transfer roller  19 . 
     More specifically, among exposure of images of first to fourth color components on the photoconductive drums  11   a  to  11   d  (periods  1711  to  1714 ), development of the images of the first to fourth color components held by the photoconductive drums  11   a  to  11   d  (visualization, periods  1721  to  1724 ), transfer of the first to fourth color images held by the photoconductive drums  11   a  to  11   d  (pre-transfer in which superimposition of the color images occurs, periods  1731  to  1734 ) shown in  FIG. 17 , the periods of pre-transfer  1731  to  1734  of arbitrary color images overlap a period of transfer of a pre-transferred image onto a sheet (transfer,  1741 ) in the transfer position, i.e., the position where the supporting roller  57  and the transfer roller  19  are in contact with each other. In other words, when a period required by the transfer  1741  is represented as period  1735  and superimposed on the periods of pre-transfer  1731  to  1734 , the period of pre-transfer  1731  to  1734  in all the colors overlap the period  1735 . For example, in the period of pre-transfer  1731  of an a first color image, a portion closer to the rear end of the image overlaps start timing of the period  1735 . In the periods of pre-transfer  1732  and  1733  of the second and third color images, portions near the centers of the images overlap the start timing. In the period of pre-transfer  1734  of a fourth color image, a portion closer to the front end of the image overlaps the start timing. 
     Since the period  1741  (transfer) is a cause of a speed change of the transfer belt  15 , it is possible to prevent a color drift of the color images from occurring in the periods of pre-transfer  1731  to  1734  by, when a pre-transferred image is transferred onto a sheet, predicting that the speed change of the transfer belt  15  explained with reference to  FIGS. 16A ,  16 B, and  16 C occurs and changing a control amount of the cam  69  shown in  FIG. 12  on the basis of the thickness of the sheet. 
     In an actual operation (image formation of the color components, movement of a sheet, and speed control of the transfer belt), as indicated by a flowchart of an adjustment mode shown in  FIG. 18 , in cam alignment (a rotation amount of the motor  75 ) and image formation (start of writing by a laser) timing control (transfer belt speed adjustment), a main CPU  112  of the MFP  101  sets a rotation angle of the motor  75  such that the cam position [B] of the cam  69  acts on the pusher  65  [ 01 ]. The main CPU  112  detects the speed of the transfer belt  15  [ 02 ]. 
     The main CPU  112  sets the rotation angle of the motor  75  such that the cam position [C] of the cam  69  acts on the pusher  65  [ 03 ]. The main CPU  112  defines a state in which the transfer pressure in the transfer position where the transfer roller  19  and the supporting roller  57  are in contact with each other is equivalent to the first magnitude and detects the speed of the transfer belt  15  [ 04 ]. 
     The main CPU  112  calculates a correction value X (a first correction value) of the number of revolutions of the motor  151  corresponding to a detected speed difference and stores the correction value X in a memory (NVRAM)  115  [ 05 ]. The main CPU  112  may set, on the basis of the detected speed difference, timing when the exposing device  23  starts image formation (timing for starting writing by a laser beam). 
     The main CPU  112  sets the rotation angle of the motor  75  such that the cam position [A] of the cam  69  acts on the pusher  65  [ 06 ]. The main CPU  112  defines a state in which the transfer pressure in the transfer position where the transfer roller  19  and the supporting roller  57  are in contact with each other is equivalent to the second magnitude and detects the speed of the transfer belt  15  [ 07 ]. 
     The main CPU  112  calculates a correction value Y (a second correction value, Y&gt;X) of the number of revolutions of the motor  151  corresponding to a detected speed difference and stores the correction value Y in the memory (NVRAM)  115  [ 08 ]. The main CPU  112  may set, on the basis of the detected speed difference, timing when the exposing device  23  starts image formation (timing for starting writing by a laser beam). 
     During the image formation, as shown in  FIG. 19 , the main CPU  112  sets a correction value M to “M=M←0 (no correction)” at a point when the start of the image formation is instructed [ 11 ]. The main CPU  112  checks, according to an output of the media sensor  45 , the thickness of a sheet used for an image output [ 21 ]. 
     If the thickness of the sheet is equal to or smaller than a threshold [NO in  21 ], the main CPU  112  detects that the leading end of the sheet reaches the transfer position [YES in  22 ] and sets the correction value M to “M=M←X (the first correction value)” [ 23 ]. 
     The main CPU  112  sets, at timing when the trailing end of the sheet passes the transfer position [YES in  24 ], the correction value M to “M=M←0” according to an input sheet size of a sheet size specified by a size detecting function [ 25 ]. 
     When the thickness of the sheet exceeds the threshold [YES in  21 ], the main CPU  112  detects that the leading end of the sheet reaches the transfer position [YES in  26 ] and set the correction value M to “M=M←Y (the second correction value)” [ 27 ]. 
     The main CPU  112  sets, at timing when the trailing end of the sheet passes the transfer position [YES in  28 ], the correction value M to “M=M←0” according to an input sheet size of a sheet size specified by the size detecting function [ 29 ]. 
     The main CPU  112  repeats [ 21 ] to [ 25 ] and [ 26 ] to [ 29 ] until the end of the image formation [YES in  30 ]. When the thickness of the sheet is equal to or smaller than the threshold, the main CPU  112  may set the correction value M to “M=M←0 (no correction)”. 
     The media sensor  45  detects the thickness of the sheet. However, for example, when an instruction of a “thick paper mode” is input from the control panel (the operating section)  9 , the mode is given priority. 
     In this way, it is possible to perform image formation in which a color drift does not occur because of speed fluctuation of the transfer belt  15  that could occur when the sheet reaches the transfer position (the speed fluctuation is sensed as a color drift) in relation to the thickness of the sheet. 
     The control of the speed of the transfer belt  15  is started at timing when the sheet reaches the transfer position during the actual printing operation. However, for example, the speed of the transfer belt  15  can also be controlled by changing the magnitude of an excitation current (increasing the magnitude (the number of pulses) of the excitation current to maintain speed) such that the torque of the motor  151  does not fall. When the sheet reaches the transfer position, since fluctuation in the speed of the transfer belt  15  is also related to the thickness of the sheet, the speed control and the control of writing by a laser beam can be limitedly applied during image formation on the thick paper. 
     In adjusting the position of the start of writing by a laser beam (timing of the start of the image formation), expecting that the speed of the transfer belt  15  fluctuates (delays) when the thick paper reaches the transfer position, the main CPU  112  stops writing of an image by the laser beam (delays the writing) by the number of lines in a main scanning direction equivalent to the delay. The delay in the writing is realized by, for example, delaying timing for reading out image data from the page memory  118  or inserting blank lines equivalent to the delay (the number of lines in the main scanning direction) into image data stored by a page memory  118 . In this case, a delay amount is set for each of the colors. 
     An example of the transfer pressure given to the transfer belt  15  and the supporting roller  57  by the transfer roller  19  is explained below. 
     In the non-pressed state (the separated state), in the cam  69 , as shown in  FIG. 7 , the concave section (the B section shown in  FIG. 8 ) is located on the opposite side of the pusher  65  with respect to the shaft hole  69   a.  In other words, the transfer roller  19  does not apply pressure to the transfer belt  15  and the supporting roller  57 . 
     When image formation is instructed, during a period until the leading end of a sheet moves to the transfer area where the transfer roller  19  and the transfer belt  15  and supporting roller  57  are in contact with each other (during toner image non-transfer), the cam  69  rotates according to the rotation of the stepping motor  75  such that the concave section (the B section) is located on the pusher  65  side with respect to the shaft hole  69   a  as shown in  FIG. 9 . Therefore, pressure of the contact of the transfer roller  19  and the transfer belt  15  in the transfer area during toner image non-transfer is set to pressure of contact of the outer circumference B section and the pusher  65  during image transfer and maintained. 
     On the other hand, during image formation on the thick paper having a thickness larger than the threshold, the speed of the transfer belt  15  (the number of revolutions of the motor  151 ) is changed (increased) according to time when the thick paper enters the transfer area. Alternatively, the number of pulses (to be supplied) or a driving voltage is set (increased) according to the control of the number of pulses such that the torque of the motor  151  increases. 
     This makes it possible to prevent speed fluctuation of the transfer belt  15  that could occur when the thick paper enters the transfer position where the supporting roller  57  and the transfer roller  19  are in contact with each other. In particular, this is useful during color image formation for superimposing the four color images. 
     In this way, loads on the transfer roller  19  and the transfer belt  15  (and the supporting roller  57 ) fluctuate according to the thickness of the sheet. Therefore, alignment is performed in the position of “c” by assuming plain paper (having a thickness equal to or smaller than the threshold) (see  FIG. 11 ). Alignment is performed in the position of “a” by assuming the thick paper (see  FIG. 9 ). This makes it possible to absorb speed fluctuation during image output to the thick paper in which a color drift often occurs compared with image output to the plain paper. The plain paper has, for example, a thickness giving a weight per 1 m 2  equal to or smaller than 105 g. The thick paper has, for example, a thickness giving a weight per 1 m 2  exceeding 105 g. 
     It is possible to obtain functions same as those of the cam  69  explained with reference to  FIG. 8  using a cam  169  in which all the cam positions shown in  FIGS. 13 to 15  are convex or plane (not having a concave section) (B (FIG.  14 &gt;A (FIG.  13 )&gt;C ( FIG. 15 )). When the cam positions do not have a concave section, a degree of freedom of the shape of the pusher  65  is improved. For example, compared with the pusher  65  shown in  FIGS. 3 ,  4 ,  7 , and  9  to  11 , an area in contact with the cam  169  can be increased and a characteristic against abrasion is improved. 
     According to the embodiment explained above, it is possible to prevent a situation in which the speed of the transfer belt  15  changes because of the thickness of a sheet and a color drift occurs during color image output. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.