Patent Publication Number: US-9891557-B2

Title: Image formation apparatus having intermediate transfer belt speed control

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. 2015-181972 filed on Sep. 15, 2015, entitled “IMAGE FORMATION APPARATUS”, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This disclosure relates to an image formation apparatus and is suitably applied to, for example, an electrophotographic printer. 
     2. Description of the Related Art 
     There has been an image formation apparatus that prints an image, for example, in a way that: a toner image is generated by an exposure device and then is carried by a traveling intermediate transfer belt, while a sheet serving as a medium is conveyed by a conveyer section including rollers and the like; and then the toner image is transferred from the intermediate transfer belt onto the sheet, and finally is fixed to the sheet with application of heat and pressure to the sheet. 
     When printing the image on the sheet, the image formation apparatus needs to transfer the toner image onto the sheet with the position of the sheet and the position of the toner image carried on the intermediate transfer belt aligned with each other and the traveling speed of the sheet and the traveling speed of the intermediate transfer belt equalized to each other. 
     To this end, there has been proposed an image formation apparatus including an image sensor that detects the traveling speed of the intermediate transfer belt and the position of the toner image, and a sheet sensor that detects the traveling speed and the position of the sheet. On the basis of the detection results of the sensors, the image formation apparatus adjusts the conveyance speed of the sheet and aligns the position of the sheet with the position of the toner image (see, for example, Japanese Patent Application Publication No. 2010-277038 (FIGS. 6 and 7)). 
     SUMMARY OF THE INVENTION 
     However, in most of conventional image formation apparatuses, it is difficult to adjust the traveling speed of the sheet because of constraints of the conveyer section and the like. In this case, it is likely that accuracy is deteriorated in the adjustment of the traveling speeds of the sheet and the intermediate transfer belt, and the alignment of the position of the toner image with the position of the sheet and the quality of the image to be formed is degraded. 
     An object of an embodiment of the invention is to provide an image formation apparatus that can form a high-quality image on a medium. 
     An aspect of the invention is an image formation apparatus that includes: an intermediate transfer belt that carries and conveys a developer image formed by an image formation section; a driver that conveys the intermediate transfer belt in a predetermined direction; a transfer device that transfers the developer image carried on the intermediate transfer belt onto a predetermined medium; a first detector that detects the belt conveyance speed, which is the speed of the intermediate transfer belt; a controller that controls the driver; a conveyer section that conveys the medium to the transfer device; and a second detector that detects the medium conveyance speed, which is the speed of the medium being conveyed by the conveyer section. The controller controls the driving of the driver on the basis of the belt conveyance speed and the medium conveyance speed. 
     According to this aspect of the invention, it is possible to adjust the conveyance speed of the intermediate transfer belt, which carries the developer image, to the conveyance speed of the medium. Therefore, any extension and contraction of the developer image in the conveying direction are not caused in the transfer device. It is possible to transfer the developer image with a high degree of accuracy, while maintaining the quality of the printing. 
     Therefore, it is possible to provide an image formation apparatus capable of forming a high-quality image on a medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the configuration of an image formation apparatus according to a first embodiment; 
         FIG. 2  is a schematic diagram illustrating the configuration of a primary transfer section; 
         FIG. 3  is a block diagram illustrating the block configuration of the image formation apparatus according to the first embodiment; 
         FIG. 4  illustrates schematic diagrams (A) and (B) for explaining the writing-start-position alignment processing; 
         FIG. 5  is a schematic diagram illustrating the conveyance speeds of an intermediate transfer belt and a sheet; 
         FIG. 6  is a flowchart for explaining an image formation processing procedure according to the first embodiment; 
         FIG. 7  is a schematic diagram illustrating the configuration of an image formation apparatus according to a second embodiment; 
         FIG. 8  is a block diagram illustrating the block configuration of the image formation apparatus according to the second embodiment; and 
         FIG. 9  is a flowchart for explaining an image formation processing procedure according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Descriptions are provided hereinbelow for embodiments based on the drawings. In the respective drawings referenced herein, the same constituents are designated by the same reference numerals and duplicate explanation concerning the same constituents is omitted. All of the drawings are provided to illustrate the respective examples only. 
     1. First Embodiment 
     1-1. Configuration of an Image Formation Apparatus 
     As illustrated in  FIG. 1 , image formation apparatus  1  according to a first embodiment is configured as an electrophotographic printer. Image formation apparatus  1  is configured to print a desired color image on, for example, a long sheet P. Image formation apparatus  1  mainly includes main body section  2  disposed on the back side, and roll feeder unit  3  disposed on the front side. Controller  5  that collectively controls the entire image formation apparatus  1  is provided on the inside of main body section  2 . 
     For convenience of explanation, in the following explanation, the roll feeder unit  3  side is defined as the front side, the main body section  2  side is defined as the back side, the near side of the paper surface in  FIG. 1  is defined as the left side, the depth side of the paper surface is defined as the right side, and the upper side and the lower side are further defined. 
     Sheet P serving as a medium is wound to turn around the circumferential side surface of a core material (not illustrated in the figure) extending along the left-right direction and is formed in a roll shape (this portion is hereinafter referred to as roll section PR). During printing, one end is peeled from the outermost circumference of roll section PR. Roll feeder unit  3  includes housing  10  disposed in the center thereof and is formed in a relatively small rectangular parallelepiped shape. In housing  10 , roll section PR of sheet P is rotatably held by roll holder  11  provided on the front upper side of housing  10 . 
     Roll conveyance guide roller  12  is provided on the back lower side of roll holder  11  in housing  10 . When one end is peeled from the outermost circumference of roll section PR, sheet P is pulled out in a back downwardly direction along arrow E 1 , passes the lower side of roll conveyance guide roller  12  to change the traveling direction thereof to the back upper direction and travel in a direction of arrow E 2 , and is taken into main body section  2 . 
     When sheet P travels to the main body section  2  side, roll section PR rotates in an arrow R 2  direction. On the other hand, roll feeder unit  3  applies a driving force of back tension motor  13  to roll holder  11  via torque limiter  14  to thereby apply force to roll section PR in an arrow R 1  direction. 
     In main body section  2 , four primary transfer sections  21 Y,  21 M,  21 C, and  21 K are disposed to be arrayed along the front-back direction near the top on the inside of housing  20  formed in a rectangular parallelepiped shape. Primary transfer sections  21 Y,  21 M,  21 C, and  21 K respectively correspond to the colors of yellow (Y), magenta (M), cyan (C), and black (K). 
     Primary transfer sections  21 Y,  21 M,  21 C, and  21 K (hereinafter collectively referred to as primary transfer sections  21  as well) have the same configuration except that only the colors are different. As illustrated in  FIG. 2 , primary transfer section  21  includes image formation unit  22  disposed in the center and functioning as an image formation section, and also includes toner cartridge  23 , toner duct  24 , and primary transfer roller  25  that are disposed around image formation unit  22 . 
     Toner cartridge  23  stores toner serving as a developer. Toner cartridge  23  is disposed on the upper side of image formation unit  22  and is attached to image formation unit  22  via toner duct  24 . Toner cartridge  23  supplies the toner to toner storage section  31  of image formation unit  22  via toner duct  24 . 
     In image formation unit  22 , besides toner storage section  31 , the following components are also incorporated therein: supply roller  32 , development roller  33 , development blade  34 , photosensitive drum  35 , charge roller  36 , LED (Light Emitting Diode) head  37 , and photosensitive drum cleaner blade  38 . A driving force is supplied to image formation unit  22  from image formation motor  27  ( FIG. 1 ), whereby supply roller  32 , development roller  33 , and charge roller  36  are rotated in the arrow R 1  direction and photosensitive drum  35  is rotated in the arrow R 2  direction. 
     A predetermined bias voltage is applied to supply roller  32 . Supply roller  32  causes the toner in toner storage section  31  to adhere to the circumferential side surface of supply roller  32  and rotates to thereby cause the toner to adhere to the circumferential side surface of development roller  33 . The predetermined bias voltage is applied to development roller  33  as well. After excess toner is removed from the circumferential side surface of development roller  33  by development blade  34 , development roller  33  brings its circumferential side surface into contact with the circumferential side surface of photosensitive drum  35 . 
     On the other hand, in a state where the predetermined bias voltage is applied to charge roller  36 , charge roller  36  comes into contact with photosensitive drum  35  to thereby uniformly charge the circumferential side surface of photosensitive drum  35 . In LED head  37 , LED chips are linearly disposed along the left-right direction. The LED chips emit light at predetermined time intervals in a light emission pattern based on image data supplied from controller  5  ( FIG. 1 ). Consequently, an electrostatic latent image is formed on the circumferential side surface near the upper end of photosensitive drum  35 . 
     Subsequently, photosensitive drum  35  rotates in the arrow R 2  direction to thereby bring a part where the electrostatic latent image is formed into contact with development roller  33 . Consequently, the toner adheres to the circumferential side surface of photosensitive drum  35  on the basis of the electrostatic latent image. A toner image based on the image data is developed. 
     Primary transfer roller  25  serving as a primary transfer device is disposed on the lower side of photosensitive drum  35 . The vicinity of the upper end on the circumferential side surface of primary transfer roller  25  is in contact with the vicinity of the lower end of photosensitive drum  35 . Intermediate transfer belt  41  (explained in detail below) is held between primary transfer roller  25  and photosensitive drum  35 . Thus, primary transfer roller  25  is rotated by the movement of intermediate transfer belt  41 , which is driven by the rotation of photosensitive drum  35  and transfer-belt drive roller  43 . That is, primary transfer roller  25  rotates in the arrow R 1  direction with the predetermined bias voltage applied. Therefore, primary transfer roller  25  can transfer a toner image developed on the circumferential side surface of photosensitive drum  35  onto intermediate transfer belt  41 . Consequently, intermediate transfer belt  41  carries the toner image. The toner remaining on the circumferential side surface of photosensitive drum  35  is scraped off by photosensitive drum cleaner blade  38 . 
     Intermediate transfer section  40  is disposed on the lower side of primary transfer sections  21  ( 21 Y,  21 M,  21 C and  21 K) on the inside of housing  20  ( FIG. 1 ). Intermediate transfer section includes intermediate transfer belt  41  and intermediate-transfer-belt travel section  42  for causing intermediate transfer belt  41  to travel. 
     Intermediate transfer belt  41  is made of a material having flexibility and configured as an endless belt. Intermediate-transfer-belt travel section  42  includes rollers such as transfer-belt drive roller  43  on the front side, transfer-belt driven rollers  44  and  45  on the back side, and transfer-belt driven roller  46  on the lower side. Intermediate transfer belt  41  is stretched and suspended to surround the rollers (i.e., transfer-belt drive roller  43  and transfer-belt driven rollers  44 ,  45 , and  46 ) of intermediate-transfer-belt travel section  42  and backup roller  62  of secondary transfer device  60  explained below. 
     Transfer-belt drive roller  43  is driven to rotate by belt motor  47  functioning as a drive section via a gear and the like (not illustrated in the figures). The rotation of transfer-belt drive roller  43  drives intermediate transfer belt  41  to travel in the arrow B 1  direction. The travel movement of intermediate transfer belt  41  drives the rollers of intermediate-transfer-belt travel section  42  and backup roller  62  respectively to rotate. Note that secondary transfer roller  61 , which is pressed against backup roller  62  with transfer intermediate transfer belt  41  therebetween, is also rotated by the travel movement of intermediate transfer belt  41 . 
     Incidentally, belt motor  47  is a DC (Direct Current) brushless motor. The rotating speed of belt motor  47  is controlled according to a cycle of a pulse included in a pulse signal supplied from controller  5 . Therefore, in a state where belt motor  47  is subjected to a constant speed control to a predetermined speed, a traveling distance per one pulse is uniquely decided. Controller  5  controls the cycle of the pulse in the pulse signal supplied to belt motor  47  and counts the number of pulses supplied to belt motor  47 . Therefore, in the state where belt motor  47  is subjected to the constant speed control, controller  5  can calculate a traveling distance of intermediate transfer belt  41 , that is, a conveyance distance of a toner image, by multiplying the counted number of pulses with a predetermined coefficient. 
     In intermediate transfer section  40 , image conveyance speed sensor  48  is provided on the lower side of intermediate transfer belt  41  between transfer-belt drive roller  43  and transfer-belt driven roller  46 . In image conveyance speed sensor  48  functioning as a first detector, an image sensor (an image pickup element) is incorporated. Image conveyance speed sensor  48  picks up, with the image sensor, at predetermined time intervals, a toner image transferred onto intermediate transfer belt  41 . Subsequently, image conveyance speed sensor  48  applies a predetermined speed detection processing to sequentially obtained images to thereby detect the moving speed of the toner image, that is, the conveyance speed of intermediate transfer belt  41 , and supplies the moving speed to controller  5 . 
     Further, on the lower side of intermediate transfer section  40  on the inside of housing  20 , central conveyance section  50 , secondary transfer section  60 , fuser section  70 , and discharge roller pair  80  are disposed in this order from the front side toward the back side in a place substantially in the center in the up-down direction in housing  20 . 
     In central conveyer section  50  functioning as a conveyer section, three conveyance roller pairs  51 A,  51 B, and  51 C are disposed to be spaced apart from one another in the front-back direction. Each of conveyance roller pairs  51 A,  51 B, and  51 C (hereinafter collectively referred to as conveyance roller pairs  51 ) includes a set of two rollers. The two rollers are disposed to sandwich a conveyance path of sheet P from above and below. A driving force is transmitted to at least one of the rollers of conveyance roller pair  51  from conveyance motor  53  via a gear, a belt, and the like (not illustrated in the figures). The roller rotates with the driving force and conveys sheet P along the conveyance path. 
     Incidentally, conveyance motor  53  is a pulse motor (i.e., a stepping motor). Conveyance motor  53  is closely subjected to a driving control according to a pulse signal supplied from controller  5 . Therefore, a traveling distance per one pulse of conveyance motor  53  is uniquely decided irrespective of the rotating speed of conveyance motor  53 . As in the case of belt motor  47 , controller  5  controls a cycle of a pulse in the pulse signal supplied to conveyance motor  53  and counts the number of pulses supplied to conveyance motor  53 . Therefore, controller  5  can calculate a conveyance distance of sheet P by conveyance roller pair  51  by multiplying the counted number of pulses with a predetermined coefficient. 
     Provided in central conveyer section  50  are three sensors, that is, inlet sensor  54 , bite sensor  55 , and write-start sensor  56  that detect the presence or absence of sheet P. Inlet sensor  54  is disposed on the front side of conveyance roller pair  51 A disposed on the front most side and detects the leading end of sheet P. Bite sensor  55  is disposed on the back side of conveyance roller pair  51 A and detects whether sheet P is bitten by conveyance roller pair  51 A. Write-start sensor  56  is disposed on the back side of conveyance roller pair  51 C located on the back most side and is used for the purpose of aligning the position of the toner image carried on intermediate transfer belt  41  and the position of sheet P. The output signals of the sensors are OFF when sheet P is not detected and are ON when sheet P is detected. 
     Further on the back side than conveyance roller pair  51 C on the back-most side in central conveyer section  50 , and slightly further on the front side than secondary transfer section  60 , sheet conveyance speed sensor  58  is provided on the lower side of the conveyance path of sheet P. In sheet conveyance speed sensor  58  functioning as a second detector, as in image conveyance speed sensor  48 , an image sensor is incorporated. Sheet conveyance speed sensor  58  detects the conveyance speed of sheet P on the basis of an image obtained by picking up an image of the lower surface of sheet P and supplies the conveyance speed to controller  5 . 
     Secondary transfer device  60  functioning as a transfer device includes secondary transfer roller  61  and backup roller  62 , both of which are formed in a cylindrical shape with a center axis directed in the left-right direction. Secondary transfer roller  61  is located on the lower side of intermediate transfer belt  41  and sheet P. The predetermined bias voltage is applied to secondary transfer roller  61 . Backup roller  62  is located substantially right above secondary transfer roller  61 . Intermediate transfer belt  41  carrying the toner image and sheet P are held between backup roller  62  and secondary transfer roller  61 . Incidentally, backup roller  62  is made of, for example, a resin material. 
     In a state where intermediate transfer belt  41  and sheet P are held between secondary transfer roller  61  and backup roller  62 , secondary transfer device  60  rotates secondary transfer roller  61  and backup roller  62  in the arrow R 1  direction and the arrow R 2  direction, respectively, to thereby transfer the toner image from intermediate transfer belt  41  onto sheet P. At this point, intermediate transfer belt  41  is bent along the circumferential side surface of backup roller  62 . Intermediate transfer belt  41  approaches sheet P from a position away from sheet P. After coming into contact with sheet P, intermediate transfer belt  41  moves away from sheet P again. For convenience of explanation, a position where intermediate transfer belt  41  and sheet P are in contact with each other and the toner image is transferred is referred to as secondary transfer position Q. 
     Intermediate-transfer-belt cleaner blade  66  is provided on the back upper side of secondary transfer device  60 . Waste toner box  67  is disposed below intermediate-transfer-belt cleaner blade  66 . Intermediate-transfer-belt cleaner blade  66  is in contact with intermediate transfer belt  41 . Intermediate-transfer-belt cleaner blade  66  scrapes off toner adhering to (i.e., remaining on) intermediate transfer belt  41  without being transferred from intermediate transfer belt  41  onto sheet P in secondary transfer device  60 , and stores the toner in waste toner box  67 . Consequently, intermediate transfer belt  41  changes to a state where the toner does not adhere to intermediate transfer belt  41 , that is, a state where a new toner image can be transferred onto intermediate transfer belt  41  in primary transfer section  21 . 
     In fuser section  70  serving as a fixation device or a fixation unit, heat roller  71  and press roller  72  are disposed to hold sheet P from above and below. Heat roller  71  is formed in a cylindrical shape with a center axis directed in the left-right direction. A heater is provided on the inside of heat roller  71 . Press roller  72  is formed in a cylindrical shape, the same as the cylindrical shape of heat roller  71 . A heater is provided on the inside of press roller  72 . Press roller  72  presses the surface on the upper side of press roller  72  against the surface on the lower side in heat roller  71  with a predetermined pressing force. A driving force is transmitted to heat roller  71  and press roller  72  from fuse-discharge motor  73  via a gear, a belt, and the like (not illustrated in the figures), whereby heat roller  71  and press roller  72  respectively rotate in the arrow R 1  direction and the arrow R 2  direction. Incidentally, a temperature detector (not illustrated in the figures) that detects temperature is provided in fuser section  70 . The temperature detector detects temperatures of heat roller  71  and press roller  72  and notifies controller  5  of the temperatures. 
     Fuser section  70  heats heat roller  71  and rotates heat roller  71  and press roller  72  respectively in predetermined directions on the basis of the control by controller  5  to thereby apply heat and pressure to sheet P and fix the toner image and passes sheet P to discharge roller pair  80  in the back. Consequently, an image based on the image data is formed on sheet P. 
     Like conveyance roller pair  51  of central conveyer section  50 , discharge roller pair  80  is disposed to hold sheet P from above and below with two rollers. A driving force is transmitted to the rollers of discharge roller pair  80  from fuse-discharge motor  73  via a gear, a belt, and the like (not illustrated in the figures). Consequently, discharge roller pair  80  can discharge sheet P to the back of main body section  2 . Incidentally, sheet P discharged to the back from main body section  2  is wound by a sheet winder (not illustrated in the figures) set on the back side of main body section  2 . 
     As illustrated in  FIG. 3 , controller  5  includes a not-illustrated CPU (Central Processing Unit) in the center thereof. Controller  5  reads out predetermined computer programs from a ROM (Read Only Memory), a flash memory, and the like (not illustrated in the figures) and executes the computer programs to thereby perform various kinds of processing concerning printing. Controller  5  includes storage  5 M including a RAM (Random Access Memory), a hard disk drive, and a flash memory and causes storage  5 M to store various kinds of information. 
     Display  6 , high-voltage power supply  8 , and the like are also connected to controller  5 . Display  6  includes a liquid crystal panel and displays various kinds of information that should be notified to a user. High-voltage power supply  8  applies the predetermined bias voltage to supply roller  32 , development roller  33 , and charge roller  36  of image formation unit  22  ( FIG. 2 ), primary transfer roller  25 , and secondary transfer roller  61  and backup roller  62  of secondary transfer device  60  ( FIG. 1 ), respectively, at predetermined timings. 
     In this way, image formation apparatus  1  transfers, with primary transfer section  21 , the toner image onto intermediate transfer belt  41  which is caused to travel by intermediate-transfer-belt travel section  42 , conveys sheet P with central conveyer section  50 , transfers the toner image from intermediate transfer belt  41  onto sheet P with secondary transfer device  60 , and fixes the toner image with fuser section  70  to perform printing on sheet P. 
     1-2. Image Formation Processing 
     In image formation apparatus  1  ( FIG. 1 ), when print processing is started for the first time, a distal end portion of sheet P is pulled out by manual work of the user from roll section PR of sheet P held by roll holder  11 , caused to pass on the lower side of roll conveyance guide roller  12 , and then inserted into main body section  2  from the front side of central conveyer section  50 . 
     At this point, when inlet sensor  54  detects the leading end of sheet P, controller  5  drives conveyance motor  53  to rotate conveyance roller pair  51  and to thereby convey sheet P backwards. At a stage when bite sensor  55  detects the leading end of sheet P, that is, at a stage when the vicinity of the leading end of sheet P is held by conveyance roller pair  51 A, controller  5  stops the conveyance. 
     Roll feeder unit  3  causes a certain degree of tension to sheet P by applying a driving force of back tension motor  13  to roll holder  11  via torque limiter  14 . Consequently, roll feeder unit  3  can generate an appropriate tension and prevent the occurrence of creases and the like in sheet P without hindering the advancement of sheet P to main body section  2 . 
     Controller  5  ( FIGS. 1 and 3 ) is connected to a host apparatus (not illustrated in the figures), such as a personal computer, by radio or wire via a not-illustrated communication processor. When image data representing a printing target image is given and a printing of the image data is instructed from the host apparatus, controller  5  starts the print processing for forming the image on the surface of sheet P. Incidentally, controller  5  sets, as a start condition for the printing operation, the detection of sheet P by bite sensor  55 . 
     First, controller  5  heats heat roller  71  and press roller  72  of fuser section  70  and controls the heating according to a temperature received from a temperature detector (not illustrated in the figures) to thereby adjust the temperature to a predetermined temperature. Controller  5  supplies a driving force from belt motor  47  of intermediate transfer section  40  to intermediate-transfer-belt travel section  42  to thereby cause intermediate transfer belt  41  to move or travel. 
     Subsequently, after applying a predetermined image processing and the like to the image data acquired from the host apparatus, controller  5  decomposes the image data into image data of the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) and supplies the image data to LED heads  37  in image formation units  22  ( FIG. 2 ) of the respective colors, respectively. According to the supply of the respective image data, image formation units  22  rotate supply rollers  32 , photosensitive drums  35 , and the like with a driving force from image formation motor  27  ( FIG. 1 ), cause LED heads  37  to emit lights in a light emission pattern based on the supplied image data, to form toner images on photosensitive drums  35 , and to sequentially transfer toner images onto intermediate transfer belt  41 . Consequently, toners for the four colors are sequentially superimposed and the toner images are carried on intermediate transfer belt  41 . 
     After a predetermined time elapses from a point in time when the light emission of LED head  37  is started in image formation unit  22 , controller  5  supplies a driving force from conveyance motor  53  to conveyance roller pair  51 . This starts the conveyance of sheet P, and advances sheet P to secondary transfer device  60 . Controller  5  then performs a type of processing called write-start-position alignment processing. 
     Specifically, first, when detecting the leading end of sheet P with write-start sensor  56 , controller  5  recognizes a positional relation between sheet P and the toner image on intermediate transfer belt  41  and detects a difference amount of position between the positions of sheet P and the toner image using a traveling distance and the like of intermediate transfer belt  41  obtained from a driving amount of belt motor  47 . The difference amount between the positions represents a distance of preceding sheet P from the toner image. Subsequently, controller  5  calculates a time with an arithmetic processing explained below and then controls conveyance motor  53  to temporarily reduce the conveyance speed of sheet P and thereafter increase the conveyance speed again to thereby align the positions of sheet P and the toner image in secondary transfer device  60 . 
     After transferring the toner image from intermediate transfer belt  41  onto sheet P in secondary transfer position Q with secondary transfer device  60 , controller  5  heats and pressurizes sheet P in fuser section  70  to thereby fix the toner image. Sheet P is discharged to the back of main body section  2  by discharge roller pair  80 . 
     1-3. Write-Start-Position Alignment Processing 
     The write-start-position alignment processing performed by controller  5  in the printing operation (i.e., the image formation processing) explained above is further explained with reference to  FIG. 4(A) , which is an extraction of a part of  FIG. 1 . First, controller  5  conveys sheet P to precede the toner image to a certain degree. At this point, a preceding distance of sheet P from the toner image is also referred to as an adjustment distance. Incidentally, it is likely that, when the adjustment distance is too short, an adjustment range of positions is narrowed and, when the adjustment distance is too long, the adjustment distance leads to a decrease in adjustment accuracy and the like. Therefore, the adjustment distance is desirably approximately 15 mm to 35 mm. 
     Controller  5  rotates conveyance motor  53  of central conveyer section  50  at a normal rotating speed to thereby convey sheet P at a sheet conveyance speed Vf, which is the normal conveyance speed, with conveyance roller pairs  51 . Thereafter, at a point in time when write-start sensor  56  detects the leading end of sheet P (hereinafter referred to as write-start point in time), controller  5  controls the rotating speed of conveyance motor  53 . As illustrated in  FIG. 4(B) , controller  5  reduces the conveyance speed of sheet P to a sheet adjustment speed Vs that is lower than the sheet conveyance speed Vf. Incidentally,  FIG. 4(B)  is a waveform representing the conveyance speed of sheet P. The abscissa represents the position of the leading end of sheet P to correspond to  FIG. 4(A) . The ordinate represents the magnitude of the conveyance speed. 
     Controller  5  counts the number of pulses in a pulse signal supplied to conveyance motor  53 . The number of pulses is equivalent to the conveyance distance of sheet P. The number of pulses can be converted into the conveyance distance [mm] of sheet P by multiplying the number of pulses with a ratio explained below. Further, controller  5  calculates, according to a calculation method explained below, re-acceleration pulse value Xp, which is the number of pulses to a point in time when the conveyance speed of sheet P is started to be increased again, based on the write-start point in time. 
     Then, at a point in time when the number of pulses in the pulse signal supplied to conveyance motor  53  reaches re-acceleration pulse value Xp after the write-start point in time, controller  5  increases the conveyance speed of sheet P and resets the conveyance speed from sheet adjustment speed Vs to sheet conveyance speed Vf. Consequently, controller  5  can align the position of the toner image on intermediate transfer belt  41  and the position of sheet P with each other. 
     Re-acceleration pulse value Xp is explained. For convenience of explanation, various values are defined in advance. In primary transfer section  21 Y located on the most upstream side, a place where photosensitive drum  35  is exposed to light by LED head  37  is referred to as most upstream exposure position E. 
     Image conveyance distance Limg is a conveyance distance [mm] of the toner image from most upstream exposure position E to secondary transfer position Q. Image conveyance position Dimg is a conveyance distance [mm] of the toner image from most upstream exposure position E at the write-start point in time. Incidentally, image conveyance position Dimg can be obtained by multiplying the number of pulses in the pulse signal supplied to belt motor  47  ( FIG. 1 ) from most upstream exposure position E until write-start sensor  56  detects the leading end of sheet P (e.g., detects when the write-start point in time comes) with a coefficient representing a belt traveling distance per one pulse [mm/pulse]. 
     Distance Dsns is a distance [mm] from write-start sensor  56  to secondary transfer position Q. Distance Ddec is a distance [mm] in which sheet P is conveyed from a deceleration start until deceleration is completed, by conveyance motor  53 . Distance Dacc is a distance [mm] in which sheet P is conveyed from an acceleration start until acceleration is completed, by conveyance motor  53 . Distance Dmgn is the distance between the leading end of sheet P and secondary transfer position Q at a point in time when the acceleration is completed. 
     Time Tdec is a time [s] in which conveyance motor  53  is decelerated. Time Tacc is a time [s] in which conveyance motor  53  is accelerated. Distance pulse ratio Pf is a conveyance distance [mm/pulse] of sheet P by conveyance roller pair  51  per one pulse in a pulse signal supplied to conveyance motor  53  ( FIG. 1 ). 
     Sheet conveyance speed Vf is the conveyance speed [mm/s] of sheet P by conveyance roller pair  51  at a normal time. Sheet adjustment speed Vs is the conveyance speed [mm/s] of sheet P by conveyance roller pair  51  at a deceleration time. Belt conveyance speed Vb is the conveyance speed [mm/s] of intermediate transfer belt  41 . Re-acceleration distance X is a distance [mm] from the position of write-start sensor  56  to the position of sheet P at a point in time when the conveyance speed is started to be increased again. Re-acceleration distance X is a value obtained by converting re-acceleration pulse value Xp, which is the number of pulses, into a distance. 
     Distance Ddst is a distance [mm] between places where photosensitive drums  35  are respectively exposed to light by LED heads  37  in image formation units  22  adjacent to each other. LED head light emission interval Tdst is a time [s] from light emission of LED head  37  on the upstream side until light emission of LED head  37  on the downstream side when the same image data is formed in image formation units  22  adjacent to each other. Photosensitive drum speed Vd is the speed of the circumferential side surface (the surface) in photosensitive drum  35 . Transfer accuracy of the toner image onto intermediate transfer belt  41  is increased by setting photosensitive drum speed Vd slightly lower than belt conveyance speed Vb. 
     When the values defined as explained above are used, time T 1  from the write-start point in time until the toner image reaches secondary transfer position Q can be represented by the following Expression (1): 
     
       
         
           
             
               
                 
                   
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   = 
                   
                     
                       Limg 
                       - 
                       Dimg 
                     
                     Vb 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Time T 2  from the write-start point in time until sheet P reaches secondary transfer position Q can be represented by the following Expression (2): 
     
       
         
           
             
               
                 
                   
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   = 
                   
                     Tdec 
                     + 
                     
                       
                         X 
                         - 
                         Ddec 
                       
                       Vs 
                     
                     + 
                     Tacc 
                     + 
                     
                       
                         Dsns 
                         - 
                         X 
                         - 
                         Dacc 
                       
                       Vf 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Since time T 1  is equal to time T 2 , when the expressions are arranged by putting the expressions as (1)=(2), re-acceleration distance X can be represented by the following Expression (3) by using parameters C 1  and C 2 . Parameters C 1  and C 2  are respectively represented as Expression (4) and Expression (5): 
     
       
         
           
             
               
                 
                   X 
                   = 
                   
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                       ⁢ 
                       
                         ( 
                         
                           Limg 
                           - 
                           Dimg 
                         
                         ) 
                       
                     
                     + 
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   = 
                   
                     
                       Vf 
                       · 
                       Vs 
                     
                     
                       Vb 
                       ⁡ 
                       
                         ( 
                         
                           Vf 
                           - 
                           Vs 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   = 
                   
                     
                       
                         Vf 
                         · 
                         
                           Vs 
                           ⁡ 
                           
                             ( 
                             
                               Tacc 
                               + 
                               Tdec 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         Vs 
                         ⁡ 
                         
                           ( 
                           
                             Dsns 
                             - 
                             Dacc 
                           
                           ) 
                         
                       
                       - 
                       
                         Vf 
                         · 
                         Ddec 
                       
                     
                     
                       Vs 
                       - 
                       Vf 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     A relation of the following Expression (6) holds among re-acceleration distance X, re-acceleration pulse value Xp, and distance pulse ratio Pf. 
     
       
         
           
             
               
                 
                   Xp 
                   = 
                   
                     X 
                     Pf 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Controller  5  can obtain re-acceleration pulse value Xp by substituting re-acceleration distance X obtained from Expression (3), Expression (4), and Expression (5) in Expression (6). 
     Incidentally, in Expressions (1) to (5), belt conveyance speed Vb and sheet conveyance speed Vf are treated as different values. This is because the thickness and the bend of intermediate transfer belt  41  are taken into account. 
     In  FIG. 5  in which a part of  FIG. 4(A)  is enlarged, intermediate transfer belt  41  has a sufficient thickness  41 T. Therefore, imaginary center line  41 C representing the center in the thickness direction is assumed. Both of lower surface  41 U and center line  41 C of intermediate transfer belt  41  advance at belt conveyance speed Vb in a portion where intermediate transfer belt  41  linearly advances, for example, a portion where intermediate transfer belt  41  is stretched and suspended between transfer-belt drive roller  43  ( FIG. 4(A) ) and transfer-belt driven roller  46 . 
     On the other hand, intermediate transfer belt  41  advances to bend along the outer circumferential surface of backup roller  62  in the vicinity of secondary transfer position Q. That is, in intermediate transfer belt  41 , distances from center point  62 X, which is a rotation center of backup roller  62 , to center line  41 C and lower surface  41 U are different from each other. 
     Therefore, when intermediate transfer belt  41  advances while bending along the outer circumferential surface of backup roller  62 , center line  41 C is advanced generally at belt conveyance speed Vb. However, lower surface  41 U is advanced at belt surface conveyance speed Vb′, higher than belt conveyance speed Vb. In intermediate transfer belt  41 , lower surface  41 U carries the toner image and is in contact with sheet P. 
     Therefore, controller  5  can transfer the toner image onto sheet P without deteriorating the image quality by matching belt surface conveyance speed Vb′, which is the speed of lower surface  41 U, with sheet conveyance speed Vf. Since the radius of backup roller  62  is a fixed value, a ratio of belt conveyance speed Vb to belt surface conveyance speed Vb′ is also a fixed value smaller than 1. In other words, in image formation apparatus  1 , when the ratio of the speed (Vb/Vf) reaches a predetermined target value (hereinafter referred to as target speed ratio Abf), a relation of “belt surface conveyance speed Vb′=sheet conveyance speed Vf” is established. 
     1-4. Correction of Conveyance Speed During a Printing Operation 
     Incidentally, in image formation apparatus  1 , even if belt conveyance speed Vb and sheet conveyance speed Vf are set taking into account the bend of intermediate transfer belt  41  along backup roller  62  as explained above, a deviation sometimes occurs between belt surface conveyance speed Vb′ and sheet conveyance speed Vf because of the thickness and the type of sheet P, manufacturing errors of the rollers and the belts, and the like. In such a case, in image formation apparatus  1 , the toner image transferred onto sheet P in secondary transfer device  60  is extended or reduced more than the original toner image along the conveyance direction of sheet P. As a result, image quality is deteriorated. 
     Therefore, in image formation apparatus  1 , conveyance speed correction processing for correcting belt conveyance speed Vb according to sheet conveyance speed Vf is performed during a printing operation according to the control by controller  5 . For convenience in the following explanation, belt conveyance speed Vb that should be corrected, that is, a target value of belt conveyance speed Vb, is represented as belt conveyance correction value Vbt. A proper value in design in belt conveyance speed Vb is represented as belt conveyance reference speed Vbb. 
     When receiving an instruction for printing from the host apparatus (not illustrated in the figures), controller  5  reads out an image formation program from storage  5 M and executes the image formation program to thereby start the image formation processing illustrated in  FIG. 6  and shift to first step SP 1 . In step SP 1 , controller  5  starts an acquisition processing for image data from the host apparatus, sequentially supplies acquired image data to LED heads  37  of the respective colors, and shifts to the next step SP 2 . 
     In step SP 2 , controller  5  starts a conveyance operation of sheet P to thereby perform the write-start-position alignment processing and then starts a transfer processing of the toner image from intermediate transfer belt  41  onto sheet P by secondary transfer device  60  and shifts to the next step SP 3 . In step SP 3 , controller  5  detects belt conveyance speed Vb and sheet conveyance speed Vf respectively with image conveyance speed sensor  48  ( FIG. 1 ) and sheet conveyance speed sensor  58 , calculates belt conveyance corrected speed Vbt according to the following Expression (7), and shifts to the next step SP 4 .
 
 Vbt=Abf×Vf   (7)
 
     In step SP 4 , controller  5  determines whether belt conveyance corrected speed Vbt exceeds a range of ±0.1% from the present belt conveyance speed Vb. When an affirmative result is obtained, this indicates that a difference between belt conveyance corrected speed Vbt and the present belt conveyance speed Vb is sufficiently large and exceeds a range in which the difference can be regarded as an error and it is necessary to correct belt conveyance speed Vb. At this point, controller  5  shifts to the next step SP 5 . 
     In step SP 5 , controller  5  determines whether belt conveyance corrected speed Vbt is within a range of ±1% of belt conveyance reference speed Vbb. When an affirmative result is obtained, this indicates that, even if the conveyance speed of intermediate transfer belt  41  is corrected to belt conveyance corrected speed Vbt, since a difference between belt conveyance corrected speed Vbt and belt conveyance reference speed Vbb, which is the reference value in design, is relatively small, it is estimated that the likelihood of an occurrence of deficiencies in the components is extremely low. At this point, controller  5  shifts to the next step SP 6 . 
     In step SP 6 , first, controller  5  controls the rotating speed of belt motor  47  such that belt conveyance speed Vb is corrected to belt conveyance corrected speed Vbt. Subsequently, controller  5  calculates LED head light emission interval Tdst according to the following Expression (8) using belt conveyance speed Vb after the correction (i.e., using belt conveyance corrected value Vbt) and corrects LED head light emission interval Tdst to an obtained value. 
     
       
         
           
             
               
                 
                   Tdst 
                   = 
                   
                     Ddst 
                     Vb 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Further, controller  5  controls the rotating speed of image formation motor  27  such that photosensitive drum speed Vd, which is the speed of the circumferential side surface (the surface) of photosensitive drum  35 , is matched with belt conveyance speed Vb after the correction (i.e., belt conveyance corrected speed Vbt) and shifts to the next step SP 8 . Incidentally, at this point, in image formation unit  22  ( FIG. 2 ), the rotating speed is also corrected in the other rollers that receive the supply of the driving force from image formation motor  27 . 
     On the other hand, when a negative result is obtained in step SP 5 , this indicates that, since a difference between belt conveyance corrected speed Vbt and belt conveyance reference speed Vbb, which is the reference value in design, is relatively large, it is estimated that the likelihood of an occurrence of deficiencies in the components is high and indicates that belt conveyance speed Vb should not be corrected to belt conveyance corrected speed Vbt. At this point, controller  5  shifts to the next step SP 7 . 
     In step SP 7 , controller  5  causes display  6  ( FIG. 3 ) to display a predetermined warning screen to thereby notify the user that it is likely that some failure occurs in central conveyer section  50  or the like and shifts to the next step SP 8  without correcting belt conveyance speed Vb. 
     When a negative result is obtained in step SP 4 , this indicates that, since a difference between belt conveyance corrected speed Vbt and present belt conveyance speed Vb is relatively small and is in a range in which the difference can be regarded as an error, it is unnecessary to correct belt conveyance speed Vb. At this point, controller  5  shifts to the next step SP 8  without correcting belt conveyance speed Vb. 
     In step SP 8 , controller  5  determines whether all of the toner images based on the image data acquired from the host apparatus are finished being transferred onto sheet P, that is, the print processing based on the image data is ended. When a negative result is obtained, controller  5  returns to step SP 3  to thereby repeat the series of processing while the remaining image data is printed. 
     On the other hand, when an affirmative result is obtained in step SP 8 , this indicates that advance preparation should be performed for the write-start-position alignment processing performed during a start of the print processing. At this point, controller  5  shifts to the next step SP 9 . After updating parameter C 1  according to Expression (4) described above using the latest belt conveyance speed Vb at an end time of the print processing, controller  5  shifts to the next step SP 10  and ends image formation processing procedure RT 1 . 
     1-5. Operations and Effects 
     In the above configuration, after starting the print processing of the image data, controller  5  of image formation apparatus  1  according to the first embodiment acquires sheet conveyance speed Vf and calculates belt conveyance corrected speed Vbt using target speed ratio Abf. Further, controller  5  controls the rotating speed of belt motor  47  such that belt conveyance speed Vb is corrected to belt conveyance corrected speed Vbt. 
     Therefore, during the execution of the print processing, image formation apparatus  1  can match belt surface conveyance speed Vb′, which is the conveyance speed of lower surface  41 U ( FIG. 5 ) in intermediate transfer belt  41 , with sheet conveyance speed Vf in the vicinity of secondary transfer device  60 . Consequently, image formation apparatus  1  can transfer with high accuracy, in secondary transfer device  60 , the toner image from lower surface  41 U of intermediate transfer belt  41  onto sheet P, the speeds of which substantially coincide with each other, without extending or compressing the toner image in the conveying direction. 
     From another viewpoint, even when the thickness or the type of sheet P changes or a manufacturing error of the rollers, the belts, and the like occurs, image formation apparatus  1  can correct belt conveyance speed Vb to absorb the change or the manufacturing error. Therefore, when the toner image is transferred in secondary transfer device  60 , extension and contraction of an image do not occur and it is possible to maintain high image quality. 
     Incidentally, in image formation apparatus  1 , as a method of aligning and adjusting the positions and the speeds of the image data on intermediate transfer belt  41  and sheet P, it is also conceivable to adjust the conveyance speed of sheet P. 
     However, as explained above, image formation apparatus  1  urges, with back tension motor  13  ( FIG. 1 ), the back tension in the arrow R 1  direction, which is the opposite direction of the rotating direction in pulling out sheet P, against roll section PR of sheet P. Consequently, image formation apparatus  1  can apply an appropriate tension to sheet P being conveyed. As a result, the occurrence of creases, damage to sheet P, and the like are prevented. 
     Therefore, if the conveyance speed of sheet P is adjusted, from the viewpoint of preventing the occurrence of creases and the like, image formation apparatus  1  needs to simultaneously and appropriately finely control the rotating speeds and back tensions in a large number of conveyance roller pairs and roll sections PR. However, such fine control in image formation apparatus  1  is extremely difficult. Therefore, in image formation apparatus  1 , when the fine control cannot be appropriately performed, it is likely that, for example, excessively large tension is applied to sheet P to damage sheet P and creases occur in sheet P. 
     In view of this point, image formation apparatus  1  can match belt surface conveyance speed Vb′ and sheet conveyance speed Vf while stably conveying sheet P, the handling of which is difficult because of its large length, and thus prevent the occurrence of problems by correcting belt conveyance speed Vb, which is the conveyance speed of intermediate transfer belt  41 . 
     Image formation apparatus  1  corrects photosensitive drum speed Vd and LED head light emission interval Tdst of photosensitive drum  35  in addition to belt conveyance speed Vb of intermediate transfer belt  41 . Therefore, even at a stage when the toner image is primarily transferred from photosensitive drum  35  to intermediate transfer belt  41  in primary transfer section  21 , image formation apparatus  1  can maintain a high image quality without causing extension and compression of an image and without causing color drift and the like. 
     Further, if belt conveyance corrected speed Vbt calculated on the basis of sheet conveyance speed Vf is within a range of ±0.1% from belt conveyance speed Vb at that point in time, image formation apparatus  1  regards a difference between belt conveyance corrected speed Vbt and belt conveyance speed Vb as being within a range of error and does not correct belt conveyance speed Vb. Consequently, image formation apparatus  1  does not degrade image quality to the contrary by correcting belt conveyance speed Vb at an excessive frequency. 
     Moreover, if belt conveyance corrected speed Vbt calculated on the basis of sheet conveyance speed Vf exceeds a range of ±1% of belt conveyance reference speed Vbb, image formation apparatus  1  regards that some failure occurs in, for example, central conveyer section  50 , which conveys sheet P at sheet conveyance speed Vf, and displays a warning screen without correcting belt conveyance speed Vb. That is, image formation apparatus  1  can detect, on the basis of belt conveyance corrected speed Vbt calculated to correct belt conveyance speed Vb, the occurrence of some failure in central conveyer section  50  and the like originally unrelated to the conveyance by intermediate transfer belt  41  and give notice of the occurrence. 
     At a point in time when the printing of the image data ends and it is unnecessary to correct belt conveyance speed Vb, image formation apparatus  1  updates parameter C 1  using a latest value of belt conveyance speed Vb according to Expression (4). In the write-start-position alignment processing performed when starting the printing of the image data next, image formation apparatus  1  can accurately align the leading end of sheet P with the position of the toner image in intermediate transfer belt  41  according to the most recent states in intermediate transfer section  40  and central conveyer section  50  by calculating and using re-acceleration pulse value Xp using the latest parameter C 1 . 
     Incidentally, if sheet conveyance speed sensor  58  is disposed between conveyance roller pairs  51 B and  51 C, for example, when slack occurs in sheet P between conveyance roller pairs  51 B and  51 C, it is likely that image formation apparatus  1  cannot correctly detect the speed of sheet P immediately before secondary transfer device  60 . In this regard, in actual image formation apparatus  1 , sheet conveyance speed sensor  58  is disposed further on the back side than conveyance roller pair  51 C on the back most side in central conveyer section  50  and slightly further on the front side than secondary transfer device  60 . Therefore, image formation apparatus  1  can accurately detect the conveyance speed in sheet P immediately before sheet P reaches secondary transfer device  60 . 
     In image formation apparatus  1 , sheet conveyance speed sensor  58 , on the upper surface of which the image pickup element is disposed, is set on the lower side of sheet P ( FIG. 1 ). Consequently, in image formation apparatus  1 , even if toner particles drop from the toner image adhering to the outer circumferential surface side of intermediate transfer belt  41  because of, for example, the movement or traveling of intermediate transfer belt  41 , since sheet P covers the upper side of sheet conveyance speed sensor  58 , that is, the image pickup element side, it is possible to prevent any deterioration in picked-up image quality, that is, a decrease in the detection accuracy of sheet conveyance speed Vf due to a deposit of the toner particles. 
     According to the configuration explained above, after starting the print processing of the image data, controller  5  of image formation apparatus  1  acquires sheet conveyance speed Vf and calculates belt conveyance corrected speed Vbt using target speed ratio Abf. Thereafter, controller  5  controls the rotating speed of belt motor  47  and corrects belt conveyance speed Vb to belt conveyance corrected speed Vbt. Consequently, during the execution of the print processing, image formation apparatus  1  does not need to change the sheet conveyance speed Vf of sheet P, which is the long paper, and can match the belt surface conveyance speed Vb′ with sheet conveyance speed Vf in the vicinity of secondary transfer device  60 . As a result, image formation apparatus  1  can transfer with high accuracy the toner image from intermediate transfer belt  41  onto sheet P without extending or compressing the toner image in the conveying direction and realize a high-quality printing. 
     2. Second Embodiment 
     As illustrated in  FIGS. 7 and 8  respectively corresponding to  FIGS. 1 and 3 , image formation apparatus  101  according to a second embodiment is the same as image formation apparatus  1  according to the first embodiment except that image formation apparatus  101  includes controller  105  and secondary transfer device  160  instead of controller  5  and secondary transfer device  60 . 
     Like controller  5 , controller  105  includes a not-illustrated CPU in the center thereof. Controller  105  reads out predetermined computer programs from a not-illustrated ROM and the like and executes the predetermined computer programs to perform various kinds of processing concerning printing. Controller  105  includes storage  105 M formed of a RAM and the like and causes storage  105 M to store various kinds of information. 
     Secondary transfer device  160  is different from secondary transfer device  60  ( FIGS. 1 and 3 , etc.) according to the first embodiment in that temperature sensor  163  is added. However, secondary transfer device  160  has the same configuration as secondary transfer device  60  concerning secondary transfer roller  61  and backup roller  62 . Temperature sensor  163  is disposed in the vicinity of backup roller  62 . Temperature sensor  163  detects the ambient temperature, that is, generally the temperature of backup roller  62 , and notifies controller  105  of the temperature. 
     Incidentally, backup roller  62  is made of the resin material as explained above and expands or contracts according to a change in temperature. According to the expansion or contraction of backup roller  62 , in image formation apparatus  101 , the target speed ratio Abf, which is the target value of the ratio of the conveyance speeds (belt conveyance speed Vb/sheet conveyance speed Vf) in intermediate transfer belt  41  and sheet P, changes. 
     For example, backup roller  62  expands and its apparent radius increases when the temperature is relatively high. At this point, in image formation apparatus  101 , target speed ratio Abf is a relatively large value. On the other hand, backup roller  62  contracts and the apparent radius decreases when the temperature is relatively low. At this point, in image formation apparatus  101 , target speed ratio Abf is a relatively small value. In this way, in image formation apparatus  101 , an appropriate value of target speed ratio Abf changes according to the temperature change of backup roller  62 . 
     Therefore, controller  105  causes storage  105 M to store in advance a target speed ratio table that associates the temperature of backup roller  62  and a value of target speed ratio Abf. Then, controller  105  detects the temperature of backup roller  62  with temperature sensor  163  in the image formation processing and reads out target speed ratio Abf corresponding to the detected temperature from storage  105 M and uses that read out target speed ratio Abf. 
     Specifically, when performing the image formation processing, controller  105  reads out an image formation program from storage  105 M and executes the image formation program to thereby start image formation processing procedure RT 2  illustrated in  FIG. 9  instead of image formation processing procedure RT 1  ( FIG. 6 ) in the first embodiment, and shifts to step SP 21 . 
     In steps SP 21  and SP 22 , controller  105  performs kinds of processing respectively the same as the kinds of processing in steps SP 1  and SP 2  and shifts to the next step SP 23 . In step SP 23 , controller  105  acquires the temperature detected by temperature sensor  163  and shifts to the next step SP 24 . 
     In step SP 24 , controller  105  reads out, from the target speed ratio table stored in storage  105 M, target speed ratio Abf corresponding to the temperature detected in step SP 23  and shifts to the next step SP 25 . 
     In step SP 25 , as in step SP 3  of image formation processing procedure RT 1  ( FIG. 3 ), controller  105  detects belt conveyance speed Vb and sheet conveyance speed Vf respectively with image conveyance speed sensor  48  ( FIG. 1 ) and sheet conveyance speed sensor  58  and calculates belt conveyance corrected speed Vbt according to Expression (7) described above. However, at this point, controller  105  calculates belt conveyance corrected speed Vbt using target speed ratio Abf read out from storage  105 M in step SP 24 , that is, corresponding to the detected temperature. 
     Therefore, in steps SP 26  to SP 31 , controller  105  performs kinds of processing respectively the same as the kinds of processing in steps SP 4  to SP 9  of image formation processing procedure RT 1  ( FIG. 3 ), shifts to the next step SP 3 , and ends image formation processing procedure RT 2 . 
     In the configuration explained above, as in the first embodiment, after starting the print processing of image data, controller  105  of image formation apparatus  101  according to the second embodiment acquires sheet conveyance speed Vf and calculates belt conveyance corrected speed Vbt using target speed ratio Abf. At this point, controller  105  can calculate, by using target speed ratio Abf corresponding to the temperature detected by temperature sensor  163 , belt conveyance corrected speed Vbt that takes into account the expansion or contraction corresponding to the temperature of backup roller  62  and is more accurate than belt conveyance corrected speed Vbt in the first embodiment. 
     Thereafter, as in the first embodiment, controller  105  controls the rotating speed of belt motor  47  and corrects belt conveyance speed Vb to belt conveyance corrected speed Vbt. Consequently, image formation apparatus  101  can transfer, in secondary transfer section  60 , the toner image from lower surface  41 U ( FIG. 5 ) of intermediate transfer belt  41  onto sheet P, the speeds of which substantially coincide with each other, more accurately than in the first embodiment without extending or compressing the toner image in the conveying direction. 
     In other points, image formation apparatus  101  according to the second embodiment can achieve the same action and effects as the action and effects of image formation apparatus  1  according to the first embodiment. 
     According to the configuration explained above, after starting the print processing of the image data, controller  105  of image formation apparatus  101  acquires sheet conveyance speed Vf and acquires the temperature in the vicinity of backup roller  62  and calculates belt conveyance corrected speed Vbt using target speed ratio Abf corresponding to the temperature. Subsequently, controller  105  controls the rotating speed of belt motor  47  and corrects belt conveyance speed Vb to belt conveyance corrected speed Vbt. Consequently, during the execution of the print processing, image formation apparatus  101  can match belt surface conveyance speed Vb′ with sheet conveyance speed Vf in the vicinity of secondary transfer device  60 , and highly accurately transfer the toner image from intermediate transfer belt  41  onto sheet P, and thereby realize a high-quality printing. 
     3. Other Embodiments 
     Note that, in the first embodiment, taking into account the fact that intermediate transfer belt  41  bends along the outer circumferential surface of backup roller  62  ( FIG. 5 ), belt conveyance speed Vb is corrected to match belt surface conveyance speed Vb′, which is the speed on lower surface  41 U rather than in center line  41 C, with sheet conveyance speed Vf. However, the invention is not limited to this. For example, when a difference between belt conveyance speed Vb and belt surface conveyance speed Vb′ is a little (is small), belt conveyance speed Vb may be corrected to the same value as sheet conveyance speed Vf. The same applies to the second embodiment. 
     In the first embodiment, image conveyance speed sensor  48  is disposed in the place where intermediate transfer belt  41  linearly travels. Image conveyance speed sensor  48  detects belt conveyance speed Vb. However, the invention is not limited to this. For example, belt surface conveyance speed Vb′ may be directly detected by setting the radius and the material of transfer-belt drive roller  43  the same as the radius and the material of backup roller  62  and detecting the conveyance speed of intermediate transfer belt  41  at the surface of intermediate transfer belt  41  bent by transfer-belt drive roller  43 . The same applies to the second embodiment. 
     Further, in the first embodiment, in the vicinity of secondary transfer device  60 , sheet P is linearly conveyed and intermediate transfer belt  41  is caused to travel so to bend along the circumferential side surface of backup roller  62 . However, the invention is not limited to this. For example, in the vicinity of secondary transfer device  60 , sheet P may be advanced to bend along the circumferential side surface of secondary transfer roller  61  and intermediate transfer belt  41  may be linearly advanced. In this case, belt conveyance corrected speed Vbt only has to be calculated by taking into account the speed of the surface of sheet P obtained on the basis of the radius of secondary transfer roller  61  and the thickness of sheet P. Alternatively, for example, in the vicinity of secondary transfer device  60 , sheet P may be advanced to bend along the circumferential surface of secondary transfer roller  61  and intermediate transfer belt  41  may be caused to travel to bend along the circumferential side surface of backup roller  62 . In this case, by appropriately setting the radiuses of secondary transfer roller  61  and backup roller  62 , it is possible to match a ratio of belt conveyance speed Vb and sheet conveyance speed Vf with a ratio of the speed of the surface of intermediate transfer belt  41  and the speed of the surface of sheet P, and simplify the arithmetic processing. The same applies to the second embodiment. 
     In the first embodiment, sheet conveyance speed sensor  58  is disposed on the back side of conveyance roller pair  51 C and slightly on the front side of secondary transfer device  60 . However, the invention is not limited to this. Sheet conveyance speed sensor  58  may be disposed in other various places such as a place between conveyance roller pair  51 B and conveyance roller pair  51 C. In these cases, in short, the conveyance speed of sheet P only has to be able to be detected. However, it is desirable to detect a value as close as possible to the conveyance speed of sheet P in secondary transfer position Q by disposing sheet conveyance speed sensor  58  as close as possible to secondary transfer device  60 . The same applies to the second embodiment. 
     In the first embodiment, sheet conveyance speed sensor  58  is disposed on the lower side of the conveyance path of sheet P. However, the invention is not limited to this. Sheet conveyance speed sensor  58  may be disposed, for example, on the upper side of the conveyance path of sheet P. In this case, the image sensor only has to be disposed on the lower side, which is the sheet P side. Consequently, it is possible to theoretically eliminate deposition of toner particles and the like on the image sensor and a cover and the like that protect the image sensor. This makes it possible to maintain a high detection accuracy of sheet conveyance speed Vf. 
     In the first embodiment, the image sensors are mounted on image conveyance speed sensor  48  and sheet conveyance speed sensor  58  and the conveyance speed of the toner image or sheet P is detected on the basis of images picked up at predetermined time intervals. However, the invention is not limited to this. Sensors corresponding to well-known various speed detecting methods may be mounted on image conveyance speed sensor  48  and sheet conveyance speed sensor  58  and the conveyance speed may be detected according to the speed detecting methods. Sensors corresponding to speed detecting methods different from each other may be respectively mounted on image conveyance speed sensor  48  and sheet conveyance speed sensor  58 . The same applies to the second embodiment. 
     In the first embodiment, image conveyance speed sensor  48  picks up the toner image on intermediate transfer belt  41  and detects the speed of the toner image. However, the invention is not limited to this. For example, image conveyance speed sensor  48  may detect the speed of the surface itself of intermediate transfer belt  41 . Alternatively, a mark, unevenness, and the like may be formed outside a range in which the toner image is formed and images of the mark, the unevenness, and the like may be picked up to detect the conveyance speed of the mark, the unevenness, and the like. The same applies to the second embodiment. 
     In the first embodiment, belt conveyance speed Vb is corrected when belt conveyance corrected speed Vbt exceeds the range of ±0.1% from the present belt conveyance speed Vb in step SP 4  and the like of image formation processing procedure RT 1  (FIG.  6 ). However, the invention is not limited to this. Belt conveyance speed Vb may be corrected when other various conditions, such as a condition that belt conveyance corrected speed Vbt deviates to outside a range of ±0.01% from the present belt conveyance speed Vb, are satisfied. Alternatively, belt conveyance speed Vb may be corrected irrespective of a relation between belt conveyance corrected speed Vbt and present belt conveyance speed Vb. The same applies to the second embodiment. 
     In the first embodiment, belt conveyance speed Vb is corrected when belt conveyance corrected speed Vbt is within the range of ±1% of belt conveyance reference speed Vbb in step SP 5  and the like of image formation processing procedure RT 1  ( FIG. 6 ). However, the invention is not limited to this. Belt conveyance speed Vb may be corrected when other various conditions, such as a condition that belt conveyance corrected speed Vbt is within a range of ±2% of belt conveyance reference speed Vbb, are satisfied. The warning screen may not be displayed on the display  6  when these conditions are not satisfied. Alternatively, belt conveyance speed Vb may be corrected irrespective of a relation between belt conveyance corrected speed Vbt and belt conveyance reference speed Vbb. The same applies to the second embodiment. 
     In the first embodiment, the values of LED head light emission interval Tdst and photosensitive drum speed Vd are corrected when belt conveyance speed Vb is corrected in step SP 6  of image formation processing procedure RT 1  ( FIG. 6 ). However, the invention is not limited to this. For example, a value of photosensitive drum speed Vd may not be corrected when a relation with belt conveyance speed Vb after correction, for example, a speed ratio, a speed difference, or the like, is within a predetermined range. LED head light emission interval Tdst may not be corrected, for example, when a change amount in the case of the correction is smaller than a predetermined threshold. The same applies to the second embodiment. 
     Further, in the second embodiment, temperature sensor  163  is provided in secondary transfer device  60  to detect the temperature of backup roller  62 , target speed ratio Abf corresponding to the detected temperature is read out from storage  105 M, and belt conveyance corrected speed Vbt is calculated using target speed ratio Abf. However, the invention is not limited to this. A sensor for detecting a value representing an environment in the vicinity of backup roller  62  such as humidity may be provided. Belt conveyance corrected speed Vbt only has to be calculated using target speed ratio Abf depending on the value detected by the sensor. 
     In the second embodiment, controller  105  causes storage  105 M to store the target speed ratio table that associates the temperature of backup roller  62  and a value of target speed ratio Abf. Controller  105  reads out and uses the target speed ratio Abf corresponding to the temperature detected by temperature sensor  163 . However, the invention is not limited to this. Target speed ratio Abf corresponding to the temperature of backup roller  62  may be obtained by various methods, such as a method of obtaining target speed ratio Abf by creating in advance a function representing a relation between a value of the temperature and a value of target speed ratio Abf and performing an arithmetic processing for applying the detected temperature to the function. 
     In the first embodiment, the invention is applied when image formation apparatus  1  forms an image (i.e., performs print processing) on a medium formed by long paper. However, the invention is not limited to this. The invention may be applied when images are formed on media formed in various shapes, such as cut paper of the A4 size and the like. In particular, the invention is effective, for example, when an image is formed on a “sturdy” medium such as thick paper and on a medium such as a slippery film having a smooth surface. In these cases, roll feeder unit  3  may be omitted and, instead of roll feeder unit  3 , supply mechanisms and conveyance mechanisms suitable for the medium, such as a sheet cassette and a paper feeding mechanism, may be built in the lower side in main body section  2 . Incidentally, when the cut paper of the thick paper is used as a medium, sheet conveyance speed sensor  58  is desirably set on the upstream side (i.e., the front side) of secondary transfer device  60  and within a range of distance Dmgn ( FIG. 5 ). When sheet conveyance speed sensor  58  is set outside the range, it is likely that an accurate sheet conveyance speed Vf cannot be detected. The same applies to the second embodiment. 
     In the first embodiment, primary transfer sections  21  ( 21 Y,  21 M,  21 C, and  21 K) for the four colors are provided in image formation apparatus  1  and the toner images for the four colors are sequentially superimposed and carried on intermediate transfer belt  41 . However, the invention is not limited to this. Primary transfer sections  21  for one to three or five or more colors may be provided in image formation apparatus  1 . Toner images for the number of colors may be sequentially superimposed and carried on intermediate transfer belt  41 . The same applies to the second embodiment. 
     In the first embodiment, the invention is applied to image formation apparatus  1 , which is the electrophotographic printer. However, the invention is not limited to this. Image formation apparatus  1  may be applied to, for example, electrophotographic apparatuses such as a copying machine, and a facsimile apparatus, and a multifunction printer having functions of the two in combination. The same applies to the second embodiment. 
     The invention is not limited to the embodiments explained above and other embodiments. That is, the application range of the invention covers embodiments obtained by optionally combining a part or all of the embodiments and the other embodiments and embodiments obtained by extracting a part of the embodiments and the other embodiments. 
     In the first embodiment, image formation apparatus  1  includes intermediate transfer belt  41  functioning as the intermediate transfer belt, belt motor  47  functioning as the driver, secondary transfer device  60  functioning as the transfer device, image conveyance speed sensor  48  functioning as the first detector, controller  5  functioning as the controller, central conveyer section  50  functioning as the conveyer section, and sheet conveyance speed sensor  58  functioning as the second detector. However, the invention is not limited to this. The image formation apparatus may include an intermediate transfer belt, a driver, a transfer device, a first detector, a controller, a conveyer section, and a second detector having any of other various configurations. 
     The invention can be used in, for example, an electrophotographic image formation apparatus adopting a system for transferring a toner image onto a sheet via an intermediate transfer belt. 
     The invention includes other embodiments in addition to the above-described embodiments without departing from the spirit of the invention. The embodiments are to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. Hence, all configurations including the meaning and range within equivalent arrangements of the claims are intended to be embraced in the invention.