Abstract:
A media feed mechanism includes a picking device, a first feedroller and a second feedroller. The picking device picks a sheet of media from a media source. The first feedroller moves the sheet of media along a feed media path. During a skew correction phase, the first feedroller rolls in a forward direction feeding the sheet of media forward and the second feedroller turns in a reverse direction preventing the sheet of media from progressing past a nip of the second feedroller. This results in skew correction. After skew correction is performed, the second feedroller turns in the forward direction advancing the sheet of media for printing.

Description:
BACKGROUND  
         [0001]    The present invention relates to sheet feeding of media and pertains particularly to skew correction for a media feed mechanism.  
           [0002]    In printers and other devices that require sheet feeding of media, it is very important to position the image relative to the edges of the media. Some printers use active skew correction during media feeding. When positioning media, many considerations need to be taken into account for optimal performance.  
           [0003]    For example, a feeding mechanism needs to provide correct positioning of media for printing. Top skew correction is necessary to line up the image relative to the top of the media. Side skew correction is necessary to line up the image relative to the side of the media. Heavy weight media or sticky media may require greater amount of skew correction. Lighter weight media can be permanently damaged by skew correction that is too rough.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with the preferred embodiment of the present invention, a media feed mechanism includes a picking device, a first feedroller and a second feedroller. The picking device picks a sheet of media from a media source. The first feedroller moves the sheet of media along a feed media path. During a skew correction phase, the first feedroller rolls in a forward direction feeding the sheet of media forward and the second feedroller turns in a reverse direction preventing the sheet of media from progressing past a nip of the second feedroller. This results in skew correction. After skew correction is performed, the second feedroller turns in the forward direction advancing the sheet of media for printing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a side view of a printer simplified to illustrate media feed and skew correction in accordance with an embodiment of the present invention.  
         [0006]    [0006]FIG. 2 is a side view of the printer shown in FIG. 1 after a feedroller transmission changes gear in accordance with an embodiment of the present invention.  
         [0007]    [0007]FIG. 3 is a perspective view of a portion of the printer shown in FIG. 1 in accordance with an embodiment of the present invention.  
         [0008]    [0008]FIG. 4 is another perspective view of the portion of the printer shown in FIG. 3 in accordance with an embodiment of the present invention.  
         [0009]    [0009]FIG. 5 is a flowchart that illustrates operation of media feed in accordance with an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    [0010]FIG. 1 is a side view of a printer  10  simplified to illustrate feeding and skew correction of a sheet of media  13 .  
         [0011]    In one embodiment of the present invention, media feed begins when a feedroller  21  rotates in reverse and acts as a drive mechanism for activating a pick tire  12  and a feedroller  14 . Pick tire  12  is used to forward sheet of media  13  from a media stack  11  to feedroller  14 . Feedroller  14  feeds sheet of media  13  around a media path bounded by a cleanout guide  16 , an upper media guide  18  and a platen  17 . Sheet of media is thus redirected 180 degrees and guided into feedroller nip  19  at a pinch roller  20 . The length of the media path from pick tire  12  to pinch roller nip  19  is short enough (e.g., less than or equal to five inches) to ensure that another sheet of media is not picked before skew correction is completed and another pick cycle is initiated.  
         [0012]    The pinch force created by feedroller  21  is greater than the pinch force created by feedroller  14 . Since feedroller  21  rotates in reverse, sheet of media  13  will not feed past feedroller nip  19 . Feedroller  14  overdrives sheet of media  13  into feedroller nip  19  of reversing feedroller  21 , actively squaring sheet of media  13  relative to feedroller nip  19 . This active squaring of sheet of media  13  occurs within a skew correction phase. The force of feedroller nip  19  is greater than feedroller  14  enabling sheet of media  13  not to push past feedroller nip  19  and to enable sheet of media  13  to slip back through a feedroller pinch  15 . Space  23  is provided between feedroller  14  and feedroller  21  for lighter weight media to form a large buckle. This is useful when using lighter weight media that is not stiff enough to be pushed back past feedroller  14 .  
         [0013]    Feedroller  21  reverses direction to advance sheet of media  13  to top of form. For the first 6 millimeters (mm) of the feedroller advance, feedroller  14  loses motion while feedroller transmission  22  disengages from one gear and engages a different gear. The new position of feedroller transmission  22  is shown in FIG. 2.  
         [0014]    The lost motion of feedroller  14  that occurs when feedroller transmission  22  disengages from one gear and engages a different gear enables sheet of media  13  to pull away from cleanout guide  16  at the top of the media path. When feedroller  14  is engaged once again, feedroller  14  continues to feed sheet of media  13  forward at the same rate as feedroller  21 , ensuring (for most types of media) that sheet of media  13  does not drag on the surface of cleanout guide  16  at the top of the media path surface and does not drag on platen  17  at the bottom of the media path.  
         [0015]    In an alternative embodiment of the present invention, feedroller  21  rotates in reverse and acts as the drive mechanism for activating pick tire  12  and feedroller  14 . Sheet of media  13  is picked by pick tire  12  and fed to feedroller  14 . The motion of feedroller  21  is reversed (so that feedroller  21  is rotating in the forward direction). This disengages and re-engages feedroller  14 . Feedroller  14  always drives in the same forward direction.  
         [0016]    Feedroller  14  redirects sheet of media  13  on a media path that turns sheet of media  13  180 degrees and just past feedroller nip  19 . Feedroller  21  is reversed once again (so that feedroller  21  is rotating in the reverse direction) to drive sheet of media  13  back through feedroller nip  19 . While feedroller  14  is disengaging and reengaging sheet of media  13  is able to get all the way back in front of feedroller nip  19  before feedroller  14  re-engages and drives sheet of media  13  into feedroller nip  19 . This driving of sheet of media  13  into feedroller nip  19  by feedroller  14  corrects skew.  
         [0017]    The pinch force of feedroller  21  is greater than the pinch force of feedroller  14 . When feedroller  21  rotates in reverse, sheet of media  13  cannot feed past feedroller nip  19 . Feedroller  14  overdrives sheet of media  13  into feedroller nip  19  of reversing feedroller  21 , actively squaring sheet of media  13  relative to feedroller nip  19 .  
         [0018]    The force of feedroller nip  19  is greater than the force of feedroller  14 , enabling sheet of media  13  not to push past feedroller nip  19  and to enable sheet of media  13  (provided sheet of media  13  is stiff enough) to slip back through feedroller pinch  15 . Space is provided in the media path between feedroller  14  and feedroller  21  so that if sheet of media  13  is of lighter weight and not stiff enough to slip back through feedroller pinch  15 , there will be room within the media path for the resulting buckle in sheet of media  13 .  
         [0019]    After skew correction, feedroller  21  reverses direction (so that feedroller  21  is rotating in the forward direction) to advance sheet of media  13  to top of form. For the first six millimeters (mm) of the advance of feedroller  21 , feedroller  14  loses motion while feedroller transmission  22  disengages from one gear and engages with a different gear. This lost motion enables sheet of media  13  to pull away from cleanout guide  16  at the top of the media path. When feedroller  14  is engaged once again, feedroller  21  continues to feed sheet of media  13  forward at the same rate as feedroller  21 , insuring sheet of media  13  does not drag on the surface of cleanout guide  16  at the top of the media path surface and does not drag on platen  17  at the bottom of the media path.  
         [0020]    In printer  10 , space  23  is sufficiently large so that when a lighter weight sheet of media buckles, there is room for the buckle without resulting in a permanent crease in the sheet of media. The large expanse of space  23  also enables printer  10  to correct for a greater amount of skew.  
         [0021]    The action of feedroller transmission  22  results in lost motion of feedroller  14  whenever feedroller  21  reverses direction. The lost motion of the feedroller  14  enables sheet of media  13  to pull away from cleanout guide  16  at the top of the media path as feedroller  21  advances sheet of media  13  from feedroller nip  19  to the top of the media. Space  23  is sufficiently large to ensure that sheet of media  13  (for most types of media) does not touch the surface of cleanout guide  16  at the top of the media path surface or platen  17  at the bottom of the media path while sheet of media  13  is fed through. This eliminates unpredictable drag that exists between different types of media and thus improves the accuracy of positioning sheet of media  13  from top of the page to the bottom of the page.  
         [0022]    The force of feedroller  14  force is sufficiently low to allow media to slip enabling media to be overdriven into feedroller nip  19  and to correct for a greater amount of skew.  
         [0023]    The design of printer  10  allows sheet of media  13  to be feed continuously from pick directly into feedroller nip  19 , reducing the time required to perform active skew correction.  
         [0024]    During skew correction, printer  10  is programmed to ignore motor stalls. That is, when sheet of media  13  makes it into feedroller nip  19 , printer  10  ignores motor stall of feedroller  14  as media is squared in feedroller nip  19  by overdriving feedroller  14 . This is particularly important for the case when heavy or sticky media is used resulting in motor stall. Once the move is complete the firmware of printer  10  is again enabled to monitor motor stalls.  
         [0025]    The design of printer  10 , particularly the enlargement of space  23 , prevents damage that can happen when media of lighter weight buckles. Vertical positioning of media is very accurate. Media throughput is fast. Printer  10  corrects for a large amount of top skew (image relative to top edge of sheet of media  13 ) and side skew (image relative to side of sheet of media  13 ). This skew correction eliminates adverse effects of customer loading. There is a large amount of skew available for media that is heavy or sticky. The skew correction of printer  10  is much better than skew performance of many high end printers.  
         [0026]    [0026]FIG. 3 is a perspective view of a portion  30  of printer  10 . Feedroller  21 , pinch roller  20  and feedroller transmission  22  are shown.  
         [0027]    [0027]FIG. 4 is another perspective view of portion  30  of printer  10 . Feedroller transmission  22  is shown.  
         [0028]    [0028]FIG. 5 is a flowchart that illustrates operation of media feed. In a block  41 , the job starts. At this point, retries equals zero. In a block  42 , pick tire  12  engages sheet of media  13  and begins to move sheet of media  13  from media tray  11  past feedroller  14 . In a block  43 , error detection on the media axis is turned off. Error detection on the media axis indicates, for example, when feedroller  14  is stalled as the result of a media jam.  
         [0029]    In a block  44 , sheet of media  13  is forced against feedroller nip  19  at pinchroller  20 . Since feedroller  21  rotates in reverse, sheet of media  13  will not feed past feedroller nip  19 . This causes sheet of media  13  to buckle on top and will force the front edge of media  13  to sit against pinch roller  20 . This move may cause the motor driving feedroller  14  to stall. This is acceptable because sheet of media  13  is being purposely overdriven into pinch roller  20 . The potential of a motor stall is why error detection was turned off in block  43 .  
         [0030]    In a block  45 , a check is made to see whether sheet of media  13  moved a desired amount past a media sensor. If sheet of media  13  moved the desired amount past a media sensor, this indicates a successful feed. In a block  46 , any motor stall is cleared. In a block  47 , error detection on the media axis is turned back on. In a block  48 , sheet of media  13  is advanced forward to the first printable position. This move engages sheet of media  13  into pinch rollers  20  and feedroller  21 . This pulls sheet of media  13  out straight, and takes up any slack created when sheet of media  13  was pushed into feedroller  21  when feedroller  21  was moving in reverse. In a block  49 , printing is begun.  
         [0031]    In block  45 , when the check shows sheet of media  13  has not moved a desired amount past a media sensor, this indicates an unsuccessful feed. In a block  50 , a check is made to see if retries is greater than or equal to two. If not, in a block  51 , sheet of media  13  is ejected. In a block  52 , retries is incremented. Then the process is repeated beginning in block  42 .  
         [0032]    If in block  50  retries is greater than or equal to two, in a block  53 , a media jam is reported. This is a print failure.  
         [0033]    The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.