Abstract:
An image forming apparatus includes a plurality of image forming units, a transfer belt that receives toner images from the plurality of image forming units, and a plurality of transfer rollers for rotating the transfer belt in an image forming direction. The image forming apparatus also includes a tensioner having a first end and a second end, and a tension roller coupled to the first end. The tension roller is in contact with the surface of the transfer belt. The tensioner is rotatable at a second end positioned away from a surface of the transfer belt.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to an image forming apparatus and, more particularly, to a system and method for controlling angle fluctuation of a transfer belt in an image forming apparatus. 
   BACKGROUND OF THE INVENTION 
   A color-capable image forming apparatus includes multiple image forming units to be able to reproduce various colors. In particular, to produce the various colors, the image forming apparatus typically includes a cyan, a magenta, a yellow, and a black image forming unit. The colors from these four image forming units are mixed together in different ratios not only to form different colors, but also different gradations of colors, e.g., bright red versus dull red. 
   Each image forming unit generates a toner image that is transferred to an image receiving medium such as paper. Each toner image can be transferred to the paper serially, i.e., one at a time. For example, the image forming units may be rotated and the paper may be passed by each image forming unit after each rotation. Such a rotation of the image forming units can be provided by a barrel-type mechanism that holds each image forming unit and rotates each one to an image forming position at a particular time. The serial transfer can also be performed by placing each image forming unit at an independent position and passing the paper by each image forming unit. 
   Alternatively, the toner images can be transferred to the paper at the same time. To transfer the toner images to the paper at the same time, the toner image of each image forming unit can be transferred to an intermediate transfer unit comprising a transfer belt and a plurality of rollers. Each image forming unit is formed at a particular location along the transfer belt and transfers a toner image to the transfer belt so that the transferred toner image is superimposed on top of any previously transferred toner image (such as from another color). Each image forming unit is typically lined up to be in essentially the same plane. In other words, the transfer belt typically moves in a flat plane to receive each toner image, which simplifies the timing for transferring the toner images from the image forming units to the transfer belt. 
   After each of the toner images has been transferred to the transfer belt, a paper sheet is ejected from a paper supply toward the transfer belt. The paper contacts the transfer belt and deflects it in an image transfer direction, which corresponds to the direction that the transfer belt is moving. The paper is then passed between a pair of transfer rollers so that the toner image on the transfer belt is transferred to the paper. 
   The plurality of rollers in the intermediate transfer unit provide for the movement of the transfer belt. At least one of the rollers is a driving roller, which is a roller that is driven to rotate and thus cause the transfer belt to move. To facilitate the transfer of the toner images from the image forming units to the transfer belt, a roller can be placed opposite each image transfer unit to ensure a proper contact between the image forming unit and the transfer belt. More particularly, the roller is placed so that the transfer belt contacts a drum in the image transfer unit. The drum is a rotatable item in the image transfer unit on which the toner image is formed. In particular, a latent image is formed on the photoelectric drum, and the latent image is converted to the toner image when the latent image receives toner from a toner supply in the image forming unit. In addition to the rollers adjacent to the image forming units, one of the rollers for moving the transfer belt corresponds to one of the pair of transfer rollers used to help transfer the toner image from the transfer belt to the paper. 
   The intermediate transfer unit may also include a tensioner that ensures a proper tension for the transfer belt. The position of the tensioner can be changed to adjust the amount of tension to the transfer belt. The position of the tensioner may be changed, for example, based on the position of other rollers in the intermediate transfer unit and operation conditions, such as humidity and temperature. 
   In operation, when a paper is fed from the paper supply, the paper contacts the transfer belt at a particular belt angle. The belt angle is defined by the angle formed by the transfer belt and a plane defined as intersecting the rotation of axes of the drums of the image forming units. In the conventional image forming apparatus having an intermediate transfer unit, the belt angle is affected significantly by any change in position of the tensioner. The change in the belt angle can have a detrimental effect on the transfer of the toner image to the paper and may increase the likelihood of a paper jam. Accordingly, it would be desirable to have a design for adjusting the position of the tensioner in a manner that reduces the impact on the change to the belt angle. 
   SUMMARY OF THE INVENTION 
   According to an aspect of the invention, an image forming apparatus includes a plurality of image forming units, a transfer belt that receives toner images from the plurality of image forming units, and a plurality of transfer rollers for rotating the transfer belt in an image forming direction. The image forming apparatus also includes a tensioner having a first end and a second end, and a tension roller coupled to the first end. The tension roller is in contact with the surface of the transfer belt. The tensioner is rotatable at a second end positioned away from a surface of the transfer belt. 
   Further features, aspects and advantages of the present invention will become apparent from the detailed description of preferred embodiments that follows, when considered together with the accompanying figures of drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of an image generation system in an image forming apparatus consistent with the present invention. 
       FIG. 2  is a more detailed diagram of the image generation system in the vicinity of the tensioner of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  is a diagram of an image generation system in an image forming apparatus consistent with the present invention. As shown in  FIG. 1 , the image generation system includes a transfer belt  20  positioned around a driving roller  10 , fixed rollers  30  and  32 , image forming unit rollers  22 ,  24 ,  26 , and  28 , transfer roller  34 , and tension roller  40 . Each of the image forming unit rollers  22 ,  24 ,  26 , and  28  is positioned opposite a corresponding one of drums  12 ,  14 ,  16 , and  18 , respectively. The transfer belt  20  lies between the image forming unit rollers  22 ,  24 ,  26 , and  28  and the drums  12 ,  14 ,  16 , and  18 . The tension roller  40  is connected to a tensioner controller  36  by an arm  38 . The combination of the tension roller  40 , the arm  38  and the tensioner controller  36  can be referred to as a tensioner. 
   The drums  12 ,  14 ,  16 , and  18  are preferably part of respective image forming units (not shown). Each image forming unit corresponds to a different color. For example, the image forming units may be a respective one of black (K), cyan (C), magenta (M), or yellow (Y). In operation, the image forming units form a latent image on the drums  12 ,  14 ,  16 , and  18 . The drums  12 ,  14 ,  16 , and  18  are preferably photoelectric drums, and the latent image can be formed by selective application of a laser to the surface of the drums  12 ,  14 ,  16 , and  18  as they rotate. The image forming units also include toner supplies, which supply toner of the applicable color to the latent image to form the toner image. After forming the toner image, the drums  12 ,  14 ,  16 , and  18  continue to rotate and transfer the toner images to the transfer belt  20 . The transfer of the toner images from the drums  12 ,  14 ,  16 , and  18  to the transfer belt  20  is preferably timed so that the toner images are overlaid on top of each other to form a single composite toner image, which will be transferred to an image receiving medium such as paper. 
   As shown in  FIG. 1 , a paper, or other image receiving medium, is provided from a paper supply (not shown). The paper is provided on the side of the transfer belt  20  where the tension roller  40  is located. The paper moves upward toward the transfer roller  34 , which is in same the direction of movement as the transfer belt  20 , i.e., the image forming direction. As the paper reaches the transfer roller  34 , it is pressed against the transfer belt  20  and the transfer roller  34  by another transfer roller (not shown). In other words, the paper is located between the transfer belt  20  and the other transfer roller. The composite image present on the transfer belt  20  is transferred to the paper as the paper and the composite image move between the transfer roller  34  and the other transfer roller. 
   The tensioner, comprising the tension roller  40 , the arm  38 , and the tensioner controller  36 , is controlled to adjust the position of the tension roller  40 , which changes the amount of tension in the transfer belt  20 . More particularly, the tensioner controller  36  controls the positioning of the tension roller  40  with respect to the transfer belt  20 . The tensioner controller  36  is preferably configured to be rotatable around an axis and to be responsive to a control signal, such as from a processor or CPU of the image forming apparatus. The control signal sets the amount and direction of movement of the tensioner controller  36 . In response to the control signal, the tensioner controller  36  rotates in the instructed direction, which moves the arm  38  in the instructed direction and changes the position of the tension roller  40  with respect to the transfer belt  20 . Changing the position of the tension roller  40  results in a change in the tension of the transfer belt  20 . The position of the tension roller  40  may be changed to adjust the tension of the transfer belt  20  to compensate, for example, for changes in conditions, such as humidity and temperature, or changes in operation, such as during image reproduction or in the absence of image reproduction. The position of the tension roller  40  may be changed to keep tension in the transfer belt  20  in accordance with the tolerance of the length of the transfer belt  20 . 
   The position of the tension roller  40  with respect to the transfer belt  40  not only affects the tension of the transfer belt  20 , it also affects the belt angle of transfer belt  20  with respect to a paper being supplied from the paper supply. The belt angle is defined by the angle formed by the transfer belt  20  and, in the case of  FIG. 1 , a horizontal plane. It is also possible to define the plane forming the belt angle as the one intersecting the rotation of axes of the drums  12 ,  14 ,  16 , and  18 . As described previously, the belt angle is affected by changes in the position of the tension roller  40 . It is possible, however, to reduce the impact of changes to the belt angle using a tensioner as shown in  FIG. 1  and controlling the position of the tension roller  40  with respect to the tensioner controller  36  in accordance with the present embodiment of the invention. 
     FIG. 2  is a more detailed diagram of the image generation system in the vicinity of the tensioner shown in  FIG. 1 . As shown in  FIG. 2 , the center coordinate of the tensioner controller  36  is positioned at a coordinate (XT, YT), the center coordinate of the tension roller  40  is positioned at a coordinate (X 2 , Y 2 ), and the center coordinate of the transfer roller  34  is positioned at a coordinate (X 1 , Y 1 ). In general, the center coordinate of the tensioner controller  36 , the tension roller  40 , and the transfer roller  34  coincides with the axis of rotation of each element. In addition, R 1  is the radius of the transfer roller  34 , and R 2  is the radius of the tension roller  40 . 
   As also shown in  FIG. 2 , an angle α corresponds to the belt angle formed by the transfer belt  20  (between the transfer roller  34  and the tension roller  40 ) and a plane P 1 . In one aspect of the present invention, the plane P 1  may be defined as the plane intersecting the axes of rotations of the drums  12 ,  14 ,  16 , and  18  (or a plane parallel thereto). If the drums  12 ,  14 ,  16 , and  18  are arranged horizontally in the image forming apparatus, then the plane P 1  is horizontal, such as shown in  FIG. 2 . It is possible, however, for the drums  12 ,  14 ,  16 , and  18  to be arranged vertically in the image forming apparatus or, less likely, to be slanted at an angle to the horizontal, in which case the plane P 1  would be vertical or would parallel to the angle of slant, respectively. 
   The belt angle α can be determined from the following equation: (1)
 
α=arctan( L 1 T /( R 1 −R 2)−arctan(( X 1 −X 2)/( Y 1 −Y 2)).
 
In equation (1), L 1 T is the length of the belt between the tangents of the transfer roller  34  and the tension roller  40 . The length L 1 T equivalently corresponds to the length of a line parallel to the transfer belt  20  that runs from the center coordinate of the tension roller  40  to a line perpendicular to the transfer belt  20  that intersects the center coordinate of the transfer roller  34 .
 
   In addition to the belt angle α, it is also possible to determine the sensitivity of the fluctuation of the belt angle α with respect to a tensioner angle θ. The tensioner angle θ is the angle formed by the tensioner with respect to a plane P 2 . The plane P 2  intersects the center coordinate of the tensioner controller  36  and is parallel to the plane P 1 . The sensitivity of the fluctuation can be measured by the following equation: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         ⅆ 
                         α 
                       
                       
                         ⅆ 
                         θ 
                       
                     
                     = 
                     
                       A 
                       · 
                       B 
                     
                   
                   , 
                   
                     
 
                   
                   ⁢ 
                   where 
                 
                 ⁢ 
                 
                   
 
                 
                 ⁢ 
                 
                   A 
                   = 
                   
                     1 
                     / 
                     
                       ( 
                       
                         
                           
                             ( 
                             
                               1 
                               + 
                               
                                 arctan 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       ( 
                                       
                                         
                                           X 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                         - 
                                         Xt 
                                         - 
                                         
                                           R 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           cos 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           θ 
                                         
                                       
                                       ) 
                                     
                                     / 
                                     
                                       ( 
                                       
                                         
                                           Y 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                         - 
                                         YT 
                                         - 
                                         
                                           R 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           sin 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           θ 
                                         
                                       
                                       ) 
                                     
                                   
                                   ) 
                                 
                               
                             
                             ) 
                           
                           2 
                         
                         , 
                         
                           
 
                         
                         ⁢ 
                         
                           
                             and 
                             ⁢ 
                             
                               
 
                             
                             ⁢ 
                             B 
                           
                           = 
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 θ 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       ( 
                                       
                                         
                                           Y 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                         - 
                                         YT 
                                         - 
                                         
                                           R 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           sin 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           θ 
                                         
                                       
                                       ) 
                                     
                                     + 
                                     
                                       
                                         
                                           ( 
                                           
                                             
                                               X 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               1 
                                             
                                             - 
                                             Xt 
                                             - 
                                             
                                               R 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               cos 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               θ 
                                             
                                           
                                           ) 
                                         
                                         · 
                                         R 
                                       
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       cos 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       θ 
                                     
                                   
                                   ) 
                                 
                               
                               / 
                               
                                 
                                   ( 
                                   
                                     
                                       X 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                     - 
                                     XT 
                                     - 
                                     
                                       R 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       cos 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       θ 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   In accordance with an embodiment of the present invention, the design of the tensioner enables a reduction in the sensitivity of the belt angle a with respect to a tensioner angle θ. In other words, the design of the tensioner reduces the amount of change in the belt angle α resulting from changes in the tensioner angle θ. There are two factors that contribute to this reduction. First, the tensioner is rotated at the tensioner controller  36 , which is positioned away from the surface of the transfer belt  20 . The position of the tensioner controller  36  is adjacent to, but away from, the surface or side of the transfer belt  20  where a paper is supplied (i.e., paper supply surface). It is also positioned away from the surface of the transfer belt  20  where the drums  12 ,  14 ,  16 , and  18  are located (i.e., the drum surface), as well as away from the surface opposite the drum surface. As shown in  FIG. 2 , the tensioner controller  36  is preferably positioned about midway between plane corresponding to the drum surface and the opposite surface, or positioned closer to the drum surface than the opposite surface. 
   Second, the tensioner angle θ is preferably maintained between 0 and 90 degrees. If the tensioner angle is 0 degrees, then the tensioner is positioned in line with the plane P 2 . If the tensioner angle θ is greater than 0 degrees, then a plane P 3 , which intersects a center of rotation of the tension roller  40  and is parallel to the planes P 1  and P 2 , is positioned ahead of the plane P 2  with respect to the image forming direction of the transfer belt  20 . In other words, as the composite toner image moves along the transfer belt  20  in the image forming direction, the composite toner image intersects the plane P 3  before intersecting the plane P 2 . As shown in  FIG. 2 , when the transfer belt  20  is moving in a counter-clockwise direction, and the drums  12 ,  14 ,  16 , and  18  are positioned horizontally, then the plane P 3  is below the plane P 2  if the tensioner angle θ is greater than 0 degrees. Another way to describe the relative positioning of the plane P 3  when the tensioner angle θ is greater than 0 degrees is for the plane P 3  to intersect the transfer belt  20  between the points where the transfer roller  34  and the tension roller  40  are tangential to the transfer belt  20 . 
   In accordance with the present embodiment, it is possible to have a tensioner that adjusts the tension of a transfer belt  20  by changing a position of a transfer roller  40 , while limiting the resulting amount of change to a belt angle α resulting from the changed position of the transfer roller  40 . More particularly, the transfer roller  40  is preferably positioned in line with or below a horizontal plane intersecting the axis of rotation of the tensioner controller  36 . 
   The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light in the above teachings or may be acquired from practice of the invention. The embodiments (which can be practiced separately or in combination) were chosen and described in order to explain the principles of the invention and as practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.