Abstract:
A pressure is exerted on a paper to flatten the paper being fed in the paper feeder for an image reader and to avoid distortion due to paper warping. The pressure is exerted along the line where the incident light is reflected from the paper.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention is related to image reader, in particular to the paper feeder of the image reader. 
     (2) Description of the Related Art 
     An image reader is usually used in a scanner, a copier, a facsimile machine, etc. All these equipment contain an image reader. To increase the throughput, automatic document feeders (ADF) are customarily used. 
     FIG. 1 shows such an ADF, including a feeding roller  162  for picking up a sheet of paper  10  to be scanned, a taking up roller  164  to take up the paper  10  after scanning, an upper guide plate  12 ; a left lower guide plate  142 ; a central lower guide plate  144 ; a right lower guide plate  146 . The three lower guide plates and the upper guide plate  12  form a curved conduit for the paper to move and to read the image. The bottom of the central guide plate  133  is transparent and is used to read the image at the bottom of the paper  10 . 
     An light ray  152  is incident on the bottom side of the paper and is reflected as a ray  154  to an image sensor  18  for image processing. 
     FIG. 2 shows another common practice. The paper feeding roller  162  , the take-up roller  164  and the upper paper guide  12  function in a similar way as in FIG. 1 for feeding the paper  10 . However the lower paper guides  242 ,  244  and  246  form a unitary transparent arc. 
     Both the paper guides shown in FIG.  1  and FIG. 2 have a common drawback as shown in FIG.  3 . Due to the finite distance between the upper paper guide  12  and the lower paper guide  144 , the paper fed between these guides may wrap in the lateral direction orthogonal the direction of the paper feed. The warping may be due to warping of the paper, paper rubbing, air disturbance in the paper passage, or jittering of the rollers  162 ,  164  when the rollers are turn on and off. 
     FIG. 4 shows another cause of paper distortion when the paper  10  wrinkles along lines perpendicular to the direction of the paper movement between the upper guide plate  12  and the lower guide plate  144 . 
     In FIG.  3  and FIG. 4, the image on the paper is read by means of the reflected light rays  1641 ,  1642  and  1643  incident on the image sensor  18 , from which the image is processed. Due to paper, an original straight line on the image becomes crooked or distorted. Such a distorted reproduction is not satisfactory to high quality image reproduction. 
     High quality reproduction especially important for digital processing, such as pattern recognition, digital camera and digital drawings. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to improve a paper feeder structure for high quality reproduction of a scanned paper. Another object of this invention is to avoid paper warping in a paper feeder. 
     These objects are achieved in this invention by putting pressure at the line where the incident light ray is reflected. By exerting pressure, the paper is flattened at the points of light incidence. The pressure may be exerted by a spring, a weight or a roller. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 shows prior art paper feeder design. 
     FIG. 2 shows another prior art paper feeder design. 
     FIG. 3 shows a paper warped in a lateral direction 
     FIG. 4 shows a paper warped in a longitudinal direction. 
     FIG. 5 shows a first embodiment of the present invention. 
     FIG. 6 shows how the paper is prevented from warping. 
     FIG. 7 shows a roller for flattening a paper, 
     FIG. 8 shows a short arm roller design of the present invention. 
     FIG. 9 shows a long arm roller design of the present invention. 
     FIG. 10 shows a measurement of the distortion due to paper warping. 
     FIG. 11 shows the measurement of the with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 5 shows a first embodiment of the present invention. All the parts with the same reference numerals as in FIG. 1 correspond to the same function. The difference is that at a point  122  at the upper guide plate  12  is attached a flat pressure spring  20 . The end of this flat pressure spring  20  presses the paper  10  against the transparent window  144  of the lower center guide plate at the point where the incident light ray  152  is reflected as light ray  154 , which sensitizes the image sensor  18 . 
     FIG. 6 show the front view of FIG.  5 . The spring  20  presses against the paper at line  144 . Because of this pressure, there is a tiny crevice  202  between the end of the flat spring  20  and the paper  10  is flattened. When a light ray is incident at this pressure line, the reflected light rays  2641 ,  2642  and  2643  are not distorted. 
     FIG. 7 shows different versions for exerting pressure for flattening the paper. In one version as shown in FIGS. 6 and 7, a pressure spring  20  is attached at one end to the upper guide plate  12  in FIG.  5  and the other end  204  presses against the paper where the paper is sensed. In a second version, the pressure spring  30  is attached at one end  302  to a upper guide plate  12  and is folded along line  304  so that the other end of the pressure spring has a large area of the flattened paper. In a third version A roller  40  is placed above the paper  10  at the point of light incidence  404 . The roller  404  is hinged along axial  402 . The weight of the roller  40  flattens the paper  10 , against which the roller presses. As a variation, the roller  50  is not supported axially but is hinged at points  502 . The roller  50  presses against the paper  10  at line  504 . 
     FIG. 8 shows the cross-sectional view of the short arm roller  40  shown in FIG.  7 . The structure of FIG. 1 is modified with the additional of the pressure roller  40 . The ends of the roller  40  is axially supported on the frame of the paper feeder. 
     FIG. 9 shows the cross-sectional view of the long arm roller  50  shown in FIG.  7 . The roller is supported at the two ends by two long arms  502  which are attached to the upper guide plate  12 . 
     FIG. 10 shows a measurement of the unevenness of a scanned lines along a vertical direction of the typical scanner. Note that the deviations measure two divisions FIG. 11 shows the measurement using the novel paper feeding using the present invention. Note that the vertical lines are uniformly straight. 
     While the preferred embodiment of the invention has been described, it will be apparent to those skilled in the art that various modifications may be made in the embodiments for the paper feeder without departing from the spirit of the present invention. Such modifications are all within the scope of this invention.