Abstract:
A document scanner with automatic dust avoidance includes an automated document feeder; a transparent plate; an illumination source for illuminating the document through the transparent plate; at least one mirror for reflecting light reflected from the document; at least one sensor for imaging light from the at least one mirror; a microprocessor for determining the presence of dust on the transparent plate; and a motor for tilting the at least one mirror when dust is detected on the transparent plate.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention concerns improvements to a document scanner to eliminate image artifacts caused by the collection of dust or damage in the optical path. More particularly, the invention concerns such scanners in which documents are automatically fed past one or more stationary imagers comprising a sensor, lens, and mirrors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Sheet fed scanners have become a popular computing accessory both in the home and the office. With respect to sheet fed scanners, an image forming subsystem, such as a camera, typically a charged couple device (CCD) and a lens in combination with an illumination source, sits in a stationary position and scans an image as a sheet of paper is moved past the camera, through a narrow transport path, by a paper transport mechanism. Individual raster lines are imaged by the camera and then pieced together to create a two-dimensional (2D) image representation of the original document. The camera is basically imaging one sliver of the document many times as the document is moved past the camera. The paper motion supplies one dimension of the document image, while the width is supplied by the camera. The in-paper travel direction and the width of the document are determined by the optics magnification and the dimensions of the CCD within the image forming subsystem. 
         [0003]    In some cases, the shape of the sheet fed scanners paper path is semi-circular. For example, some scanners have a semi-circular paper path wherein sheets can be fed from a tray on top and exit beneath, or vice versa. In other cases, the paper path is “straight through.” In some cases, the scanner has two cameras, one for imaging the front side of the sheet or document, the other for imaging the rear side of the sheet or document. 
         [0004]    A portion of the transport path is made of a transparent material, such as glass or plastic, so that an illumination source can illuminate the documents and so that the cameras can capture images of the documents. Dust can build up on the transparent surface, which can cause artifacts in the captured image of the document. For example, as the document is moved past the transparent surface, if there is a dust particle on the transparent surface, it may be imaged by the CCD. If the dust particle remains on the transparent surface for the full duration of scanning a document, the captured image will have a streak in the image at the location of the dust particle. 
         [0005]    U.S. Pat. No. 7,058,236 describes an attempt to solve this problem by moving the illumination system and one or more camera mirrors behind the transparent plate. If dust is detected, the illumination and mirrors are repositioned behind the transparent plate to avoid imaging the document through the dust. A disadvantage of this method is that it requires relatively large motors and complex linear slide mechanisms to independently move these components. Also, power cables for the illumination must be made flexible and carefully routed to prevent damage from repeated movement. 
         [0006]    In another prior art example, the camera is comprised of a contact image sensor (CIS) that is movable behind the transparent surface. If dust is detected, the CIS is repositioned behind the transparent plate to avoid imaging the document through the dust. A disadvantage of this method is that the CIS includes the sensor and its circuit board as well as the entire illumination system. This requires relatively large motors and a complex linear slide mechanism to move these components. Also, power and data wires for the sensor and illumination must be made flexible and carefully routed to prevent damage from repeated movement. 
         [0007]    A simple and inexpensive dust avoidance mechanism is desired that works in a scanner having stationary imagers comprising a sensor, lens and mirrors. 
       SUMMARY OF THE INVENTION 
       [0008]    Briefly, according to one aspect of the present invention a document scanner with automatic dust avoidance includes an automated document feeder; a transparent plate; an illumination source for illuminating the document through the transparent plate; at least one mirror for reflecting light reflected from the document; at least one sensor for imaging light from the at least one mirror; a microprocessor for determining the presence of dust on the transparent plate; and a motor for tilting the at least one mirror when dust is detected on the transparent plate. 
         [0009]    The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a better understanding of the invention, reference is made to the following detailed description taken in conjunction with the appended drawings in which: 
           [0011]      FIG. 1  is a perspective view of a sheet fed scanner according to the prior art; 
           [0012]      FIG. 2  is a side sectional view of a sheet fed scanner illustrating the paper path, illumination source, camera location, input and output trays according to the prior art; 
           [0013]      FIG. 3  is a detailed view of a sheet fed scanner paper path and sheet drive means according to the prior art; 
           [0014]      FIG. 4  is a detailed view of the auto feeder portion of a sheet fed scanner paper transport according to the prior art; 
           [0015]      FIGS. 5-7  are the upper paper path portion of the sheet fed scanner with a view of the upper camera attached according to the prior art; 
           [0016]      FIG. 8  is a schematic view of the optical fold and major elements of the upper camera, illumination source and paper path of a prior art sheet fed scanner; 
           [0017]      FIG. 9  is a schematic view of the optical fold and major elements of the upper camera, illumination source and paper path of the preferred embodiment of the invention; 
           [0018]      FIG. 10  is a schematic view of the optical fold and major elements of the upper camera, illumination source and paper path of the preferred embodiment of the invention with a particle of dust on the transparent plate in a location where it will be imaged by the camera; 
           [0019]      FIG. 11  is a schematic view of the optical fold and major elements of the upper camera, illumination source and paper path of the preferred embodiment of the invention when the pivoting mirror is rotated to redirect the optical fold away from a particle of dust; 
           [0020]      FIG. 12  is a perspective view of the optical fold and major elements of the upper camera, illumination source and paper path of the preferred embodiment of the invention with the camera housing not shown; 
           [0021]      FIG. 13  is a perspective view of the optical fold and major elements of the upper camera, illumination source and paper path of the preferred embodiment of the invention with the camera housing shown; 
           [0022]      FIG. 14  is a block diagram showing an arrangement of a control system according to the present invention; 
           [0023]      FIG. 15  is a schematic view of the mirror pivot motor, gears and cam according to the present invention; 
           [0024]      FIG. 16  is a graph showing scan position as a function of cam angle for the preferred embodiment—cam and yoke relationships are shown at various positions for clarity; 
           [0025]      FIG. 17  is a perspective view of the optical fold and major elements of the upper camera, illumination source and paper path of the preferred embodiment of the invention showing the optical folded path imaging a calibration patch; 
           [0026]      FIG. 18  is a schematic view showing three scan positions through a glass window having a calibration patch with angled edge; and 
       
    
    
       [0027]    Corresponding numerals and references in the detailed description correspond to like parts in the figures unless otherwise indicated. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
         [0029]    In one embodiment a modular scanner according to the present invention comprises a sheet fed scanning unit. As shown in  FIGS. 1-4 , in the sheet fed transport scanning unit  101 , sheets  221  are fed into a paper path with the use of an auto feeder mechanism  105 . The auto feeder consists of a stack support or input tray  103 , an urging or picker roller  110 , a pressure roller  106 , a feed roller  109 , a pre-separation pad  107 , and a separation roller  108 . When commanded to feed, the scanner&#39;s auto feeder  105  advances sheets  221  from a stack  222  placed on the stack support  103  into the scanners paper path. The functions of the urging or picker roller  110 , separation roller  108 , and pre-separation pad  107  are commonly found within the art of friction feeding devices as they serve to singularly advance the intended sheet to be imaged into the transport and hold back or retard any subsequent non-intended sheets. The stack support  103  can be referred to as an entrance, supply, or input tray. A side view of the auto feeder  105  is shown in  FIG. 4 . 
         [0030]    The paper path consists of an upper and lower portion  111 ,  112 . The lower paper path portion  112  contains drive rollers  113  which move the sheets  221 . The upper paper path portion  111  housing idler or normal force rollers  114  provide the necessary contact forces for the drive rollers. The drive rollers  113  are in turn driven through a series of timing belts  115  which are connected through a pulley arrangement to a drive motor  116 , shown in  FIG. 3 . 
         [0031]    The upper and lower paper paths  111  and  112  are also constructed with an upper and lower clear aperture area or glass window  117  and  118 . The upper paper path consists of a substantially flat paper path portion and a structural supporting frame  159  that is screwed to the path. This upper path is pivotably mounted to the lower paper path and in turn, the lower paper path is attached to a base structure. The upper path normal force rollers  114  are aligned and in contact with the lower paper path drive rollers  113  thereby forming a nip line through which drive is imparted to the sheets  221 . The upper paper path rollers  114  are idlers that are spring loaded against the drive rollers  113  when the upper paper path is pivoted down and secured by the latching mechanism  120 , as shown in  FIGS. 5 and 6 . 
         [0032]    Mounted to the upper paper path  111  are an upper illumination source (lamp)  121  and an upper camera  123 , shown in  FIGS. 6-8 . As sheets  221  are moved through the paper path, an image of the top side of the sheet is formed by reflected light from the lamp off the sheet. The reflected light then bounces off a series of fold mirrors  127  and through a lens  126  where it is focused onto a charged coupled device (CCD) imager  125 . The camera housing  128  hat holds the mirrors  127 , lens  126 , CCD  125 , and CCD circuit board that make up the upper camera. The CCD width or (number of pixels) makes up one dimension of the image and the paper travel and successive lines of CCD output form the other dimension of the two-dimensional image. This method of creating images is universally applied by scanners of the sheet fed variety where the illumination and camera are held stationary and the paper is moved past them. These images are then output from the scanner to a connected computer where they are either stored or manipulated for customer use. In the present invention, the illumination source is a pair of light emitting diode (LED) arrays but this approach could be accomplished by any suitable light source such Xenon gas fluorescent lamps, Hg fluorescent, and halogen. In the present invention the CCD/lens reduction camera is of a four channel design (red, green, blue, and black) but could also be of a single channel design or any other desired makeup. In the making of a color imager, one could use a single channel CCD and then alternate their illumination source to be of red, green, and then blue light as an alternative to a multi-channel CCD with a “white” light source. The CCD/lens reduction camera (imager) could also be replaced with a contact array or other such imaging device. 
         [0033]    In the same manner as described for the upper camera, the lower camera  124  produces images of the bottom side of the sheets being fed as they pass over the lower paper path clear aperture. Also mounted to the base unit is a sheet support that receives sheets as they exit the paper path. This support is commonly referred to as an exit or output tray  104  as shown in  FIG. 1 . 
         [0034]    To summarize, the sheet fed scanning for a single sheet, the process begins with a command to feed, the sheet passes the clear apertures and is imaged on both top and bottom sides and then it is deposited in the output tray. For a stack of documents, this process is repeated until the stack is depleted. 
         [0035]    A sheet fed scanner having a camera of the type having one or more stationary imagers comprising a sensor, lens, and mirrors is shown in  FIG. 8 . An upper camera  123  is comprised of a CCD imager  125 , a lens  126  and one or more stationary mirrors  127 . The lens  126  is used to focus an image of a narrow portion of document  221  onto CCD  125  as the document  221  is fed past glass window  117 , between upper paper path  111  and lower paper path  112 , and illuminated by illumination sources  121 ,  122 . The focused beam is reflected off of five mirrors to create an optical folded path  208  between the document  221  and CCD  125 . This allows the camera to be more compact than if no mirrors were used. 
         [0036]      FIGS. 9-13  show the preferred embodiment of the invention, in which the camera has been adapted for streak removal. In the invention, the first mirror in the optical folded path is not fixed. In  FIGS. 9 ,  12 , and  13  pivoting mirror  200  is mounted to yoke  216  and end cap  217 . Mirror pivot bearings  202  are also attached to yoke  216  and end cap  217  and rotate about pivot axis  218 , formed by two bearing holes  219  in camera housing  128 . Cam  214  is rotated by pivot motor  210 , through gear train  212 . Yoke  216  acts as a cam follower and is rotated about pivot axis  218  as cam  214  rotates. Cam  214 , gears  212 , and pivot motor  210  are mounted to components that are not shown in the figures for clarity. In  FIG. 9 , pivoting mirror  200  is tilted to the point in the middle of its full range, causing folded optical path  208  to intersect glass window  117  at a central scan position  220 . 
         [0037]    If, as shown in  FIG. 10 , a dust particle  230  becomes stuck on glass window  117  and coincident with folded optical path  208 , a streak will be formed on the CCD image of the document at a position coincident with the dust particle. As shown in  FIG. 14 , raw camera data is sent to image correction circuit  232 . Corrected image data is sent to an image analysis algorithm  234  and then to image processing software  236 . After being processed, the final image data output can be stored or sent to another device such as a connected computer. If image analysis algorithm  234  detects a streak in the corrected image data, it predicts a new dust free scan position. This new position may be arbitrary or may be determined by analyzing previous scans. For example, positions may be avoided if streaks were found previously at those positions. Before the next document is scanned, motor control circuit  238  causes pivot motor  210  to drive pivoting mirror  200  to a new scan position. The goal is to find a dust free scan position. Subsequent document scans are subjected to the same process, and mirror corrections are made until a dust free scan position is determined. 
         [0038]    The process of determining a dust free scan position is iterative. During this process, scanned images may contain streaks caused by dust. Image processing software  236  may include algorithms to remove these streaks by interpolation or pixel replacement. To minimize the number of iterative steps, the image analysis algorithm  234  specifies mirror corrections that move the scan position a distance larger than an average dust particle. 
         [0039]    In  FIG. 11 , pivot motor  210  has rotated cam  214  to rotate yoke  216  and tilt pivoting mirror  200  to alter folded optical path  208  to be directed to dust-free scan position  223 . Cam  214  is configured to provide equal mirror angular changes for given cam angle changes. Also, cam  214  includes a plurality of detent notches  215  that are engaged by detent pawl  211  to lock cam  214  into discrete angular positions. These positions correspond to scan positions that are separated by a distance of approximately two times the size of an average dust particle. Thus when dust is detected the mirror is pivoted to yield a new scan position that is unlikely to have a streak caused by the same piece of dust. 
         [0040]      FIG. 16  shows how scan position varies with cam angle in the preferred embodiment that has a scan position range equal to 4.8 mm. When at zero degrees, the cam positions the yoke at an angle such the scan position is at its nominal or zero mm. If the cam is rotated counter-clockwise by 90 degrees, the scan position is moved to the extreme negative value of −2.4 mm. Cam angles of 45 degrees and 135 degrees result in the same scan position of −1.2 mm. Cam angles of 0 degrees and 180 degrees result in the same central (0 degree) scan position. As the cam revolves between 180 degrees and 360 degrees, the scan position ranges from 0 mm to +2.4 mm. 
         [0041]    Because the illumination source  121  remains stationary, there is a change in illumination intensity of the document being scanned as scan position is changed. Scan positions further from the illumination focus point will have lower illumination intensity.  FIGS. 17-18 , a calibration patch  224 , having angled edge  225 , is printed onto glass window  117  at a location beyond where documents are typically scanned. As shown in  FIG. 18 , at maximum lower scan position  240 , more CCD pixels image the white patch than at maximum upper scan position  242 . Image analysis algorithm  234  counts the number if CCD pixels that image the white patch  224  at both the maximum lower scan position  240  and maximum upper scan position  242 . The central scan position  220  is then determined by image analysis algorithm  234  as the point midway between positions  240  and  242 . After image analysis algorithm  234  determines the central scan position  220 , a look-up table  235  is created in image analysis algorithm  234  that maps illumination intensity to scan position. Therefore, gains can be applied to the image data to correct for changes in illumination intensity as a result of changes in scan position. 
         [0042]    During normal operation, the scanner starts scanning at central scan position  220 . This position provides the most illumination. If a dust particle is detected the mirror is pivoted to yield a new scan position that is unlikely to have a streak caused by the same piece of dust. A gain is applied to the image data by image processing software  236 , based upon the look-up table  235 , to correct for illumination intensity changes at the new scan position. 
         [0043]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
       PARTS LIST 
       [0000]    
       
           101  sheet fed transport scanning unit 
           103  stack support (input tray) 
           104  output tray (exit hopper) 
           105  auto feeder mechanism
         106  pressure roller   
     
           107  pre-separation pad 
           108  separation roller 
           109  feed roller 
           110  urging (picker) roller 
           111  paper path (upper) 
           112  paper path (lower) 
           113  drive rollers 
           114  idler (normal force) rollers 
           115  timing belts (sheet fed transport) 
           116  drive motor (sheet fed transport) 
           117  glass window (clear aperture) upper camera 
           118  glass window (clear aperture) lower camera 
           119  idler (normal force) roller springs 
           120  latching mechanism 
           121  illumination source (LED) upper 
           122  illumination source (LED) lower 
           123  upper camera 
           124  lower camera 
           125  CCD imager 
           126  lens 
           127  fixed fold mirrors 
           128  camera housing 
           159  structural supporting frame 
           200  pivoting mirror 
           202  mirror pivot bearings 
           208  folded optical path 
           210  pivot motor (mirror) 
           211  detent pawl 
           212  gear train (mirror) 
           214  cam 
           215  detent notches (cam gear) 
           216  yoke (mirror pivot) 
           217  end cap (mirror pivot) 
           218  pivot axis (mirror) 
           219  bearing holes 
           220  central scan position 
           221  sheet (document) 
           222  stack (documents) 
           223  dust-free scan position 
           224  calibration patch 
           225  angled edge 
           230  dust particle 
           232  image correction circuit 
           234  image analysis algorithm 
           235  look-up table 
           236  image processing software 
           238  motor control circuit 
           240  maximum lower scan position 
           242  maximum upper scan position