Patent Publication Number: US-7594647-B2

Title: Pick mechanism with stack height dependent force for use in an image forming device

Description:
BACKGROUND 
   Media sheets for use in an image forming device are initially stored in an input area. The input area is sized to hold a predetermined number of media sheets that are stacked together. A pick mechanism is positioned adjacent to the input tray to pick individual media sheets from the stack and deliver them into a media path. The pick mechanism should accurately deliver one sheet from the input area, and should deliver the sheet in a timely manner. 
   The pick mechanism includes a pivoting arm having a pick roller at the distal end. The pick roller rests on the stack and rotates to drive the top-most sheet from the stack into the media path. The arm applies a downward force onto the media stack. This force applied through the roller increases the friction between the roller and top-most sheet such that the sheet is delivered to the media path by rotation of the roller. 
   One prior art device limited the amount of force applied to the media stack. A drawback of applying a limited force is that the roller may slip during rotation. Roller slip causes a delay in picking the media sheet from the stack and introducing the sheet into the media path. This delay may cause print errors as the toner image is not accurately aligned with the top edge of the media sheet. 
   Another prior art device increased the amount of force applied to the media sheet to prevent roller slip. However, increased force caused the pick roller to move multiple sheets from the media stack into the media path. This double feed results in a media jam as the combined sheets cannot be moved as a unit through the device. The jam required the operator to locate the jam, remove the media sheets, reset the device, and then resume image formation. 
   SUMMARY 
   The present application is directed to embodiments of a pick mechanism for use in an image forming device. In one embodiment, a first mechanism individually moves each of the media sheets from a stack in the input area thereby gradually decreasing a height of the stack. The first mechanism applies a first force profile to the stack while individually moving each of the plurality of media sheets. As the media sheets are moved, the height of the stack gradually decreases from a first height to a second height. As the stack decreases below the second height, a second force profile is applied to the stack. The second force profile is different from the first profile. The first and second force profiles prevent slip as the media sheets are fed from the input area, and also prevent double sheet feeds. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view illustrating a pick mechanism according to one embodiment of the present invention; 
       FIG. 2  is a schematic view illustrating an image forming device according to one embodiment of the present invention; 
       FIG. 3  is a schematic view illustrating the pick mechanism according to one embodiment of the present invention; 
       FIG. 4  is a side view illustrating the pick mechanism and a substantially full stack of media sheets within an input tray according to one embodiment of the present invention; 
       FIG. 5  is a side view illustrating the pick mechanism and a partially depleted stack of media sheets within an input tray according to one embodiment of the present invention; 
       FIG. 6  is a side view illustrating the pick mechanism and a depleted stack of media sheets within an input tray according to one embodiment of the present invention; 
       FIG. 7  is a graph illustrating a normal force applied to the media stack by the pick mechanism according to one embodiment of the present invention; and 
       FIG. 8  is a graph illustrating a normal force applied to the media stack by the pick mechanism according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present application is directed to embodiments of a pick mechanism for applying a force to a media sheet within an image forming device. The pick mechanism, generally illustrated as numeral  20  in  FIG. 1 , includes a pick arm  21 , pick roller  22 , and a biasing mechanism  23 . The pick arm  21  is pivotally positioned at point  24  such that the pick rollers  22  rest on a top-most media sheet within a stack. The pick arm  21  applies a downward force onto the media stack. When the media stack is above a predetermined level, a first amount of force is applied to the stack. As the media stack decreases, the arm  21  pivots about point  24 . The biasing mechanism  23  engages and applies a force thereby reducing the force applied by the pick arm  21 . 
   The pick mechanism  20  is positioned within an image forming device  100  as illustrated in  FIG. 2 . An input tray  101  is sized to contain a stack of media sheets. The pick mechanism  20  is positioned with the pick roller  22  resting on the top-most sheet of the stack. A drive mechanism  102  is operatively connected to a gear train  29  extending through the arm  21  that causes rotation of the pick rollers  22 . Rotation causes the top-most sheet to be moved from the stack and into the media path. 
   The device  100  includes a plurality of removable image formation cartridges  103 , each with a similar construction but distinguished by the toner color contained therein. In one embodiment, the device  100  includes a black cartridge (K), a magenta cartridge (M), a cyan cartridge (C), and a yellow cartridge (Y). Each cartridge  103  includes a reservoir holding a supply of toner, a developer roller for applying toner to develop a latent image on a photoconductive drum, and a photoconductive (PC) member  104 . Each cartridge  103  forms an individual monocolor image on the PC member  104  that is combined in layered fashion on an intermediate transfer mechanism (ITM) belt  105 . The ITM belt  105  is endless and rotates in the direction indicated by arrow G around a series of rollers adjacent to the PC members  104 . Toner is deposited from each PC member  104  as needed to create a full color image on the ITM belt  105 . The ITM belt  105  and each PC drum  104  are synchronized so that the toner from each PC drum  104  precisely aligns on the ITM belt  105  during a single pass. 
   As the toner images are being formed on the ITM belt  105 , the pick mechanism  20  picks a media sheet from the input tray  101 . The media sheet is transported to a transfer location  106  where it intersects the toner images on the ITM belt  105 . The sheet and attached toner next travel through a fuser  107  having a pair of rollers and a heating element that heats and fuses the toner to the sheet. The sheet with fused image is then either transported out of the device  100 , or forwarded to a duplex path for image formation on a second side of the media sheet. 
   The pick mechanism  20  should accurately introduce the media sheet into the media path. Too much force applied to the media stack by the pick mechanism may cause a double feed resulting in a media jam as the media sheets move into or along the media path. Too little force applied to the media stack by the pick mechanism  20  may result in the pick rollers  22  slipping on the top-most sheet. Slipping causes the media sheet to be delayed in the input tray  101  and delivered late to the media path and ultimately to the transfer location  106 . As a result, the media sheet does not align with the toner images on the ITM belt  105 . In one embodiment, the toner images are transferred to the media sheet too close to the leading edge (i.e., the toner images are not centered on the media sheet). Therefore, proper operation of the pick mechanism  20  is important. 
   The force applied by the pick mechanism  20  is a function in part of the weight of the pick mechanism  20 , and the angle of the pick arm  21 .  FIG. 1  illustrates a perspective view of one embodiment of the pick mechanism  20 , and  FIG. 3  illustrates a schematic illustration. The arm  21  is pivotally positioned within the device  100  at a pivotal attachment  24 . The arm  21  is positioned adjacent to the input tray  101  for the rollers  22  to remain in contact with the top-most media sheet in the stack. The arm  21  forms an angle α with a plane formed by the top-most media sheet. When the input tray  101  is full of stacked media, the angle α is small or even zero if the arm is parallel to the top-most sheet. The angle α increases as the stack is depleted. 
   A gear train  29  extends through the arm  21  and includes an input gear  29   a  (i.e., first gear) and an output gear  29   b  (i.e., last gear). An input torque supplied by the driving mechanism  102  is transferred through the gear train  29  ultimately causing rotation of the rollers  22 . Each gear in the gear train  29  includes a number of teeth that mesh with the adjacent gears to transfer the torque and rotate the rollers  22 . 
   The following equations govern the function of the force applied by the pick mechanism  20  to the media sheets:
 
 F   s   =T   i   N   o ( Eff   n )/ N   i   R   o   (Eq. 1)
 
 F   N   =W+[T   i +( F   s ( L  sin α+ R   o ))/ L  cos α]  (Eq. 2)
 
where
     F s =tangential force exerted on a media sheet by the pick roller;   T i =input torque to the pick arm gears from the motor;   N o =number of teeth on the output gear;   Eff=gear mesh efficiency;   n=number of gear meshes;   N i =number of teeth on the input gear;   R o =radius of the pick roller;   F N =normal force exerted on the pick roller by the media sheet;   W=normal force exerted on the media sheet by the pick roller;   L=length of the pick arm; and   α=angled formed between a plane of the top-most media sheet and the arm.   

   The force applied through the pick rollers  22  to the media stack is dependent upon the angle α. When the media stack is full, the force applied to the media sheets is small thus increasing the possibility of roller slippage. When the media stack is low, the force applied is greater thus increasing the possibility of double feeds. To compensate for this, the biasing mechanism  23  is attached to the arm  21 . 
   The biasing mechanism  23  has a first end connected to the arm  21  and a second end connected to a body  150  of the device  100 . The biasing mechanism  23  is extendable from a non-engaged orientation to an engaged orientation. In the non-engaged orientation, the biasing mechanism  23  does not apply an upward force to the arm  21 . Once the biasing mechanism  23  engages, it applies an upward force. During the initial stages of engagement, the amount of force is not as great as during further stages of engagement. Therefore, as the angle α of the arm  21  becomes larger, the amount of force applied by the biasing mechanism  23  becomes greater. In one embodiment, the biasing mechanism  23  is a spring. 
   When the media stack is full and the angle α is large, the biasing mechanism  23  is not engaged. Therefore, the force applied to the media stack is defined by the above equations. However, as the media stack is depleted below a predetermined amount, the biasing mechanism  23  becomes engaged and counteracts the applied force. As the media stack becomes more depleted and the angle α becomes larger, the biasing mechanism applies a greater counteracting force. In this manner, the force applied to the media stack is regulated to prevent too great or too small of a force and prevent double feeds and roller slippage. 
     FIGS. 4 ,  5 , and  6  illustrate the affects of the biasing mechanism  23  as media sheets are picked from the input tray  101  and the stack height is reduced.  FIG. 4  illustrates the input tray  101  accommodating a full stack of media sheets having a stack height H. The biasing mechanism  23  includes a first end attached to the arm  21  and a second end attached to the body  150 . With the arm  21  being nearly horizontal, the distance x between the first and second ends of the biasing mechanism  23  is relatively small. The biasing mechanism  23  therefore has not become engaged and does not apply a counterbalance force to the arm  21 . Therefore, the force applied through the roller  22  to the top-most sheet in the stack is defined by equations 1 and 2 stated above. 
     FIG. 5  illustrates a state when a number of sheets have been removed from the input tray  101  and the stack height reduced to height h. The arm  21  has pivoted downward with the angle α becoming larger. As a result of the pivoting action, the distance x between the first and second ends of the biasing mechanism  23  has increased. The biasing mechanism  23  is now engaged and applies a counterbalance force to the arm  21 . Therefore, the overall force applied to the top-most media sheet through the rollers  22  is the force as defined in equations 1 and 2, less the counterbalance force applied by the biasing mechanism  23 . 
     FIG. 6  illustrates a state with almost the entire stack of media sheets having been depleted from the input tray  101 . The stack has been reduced to a height h′. The arm  21  has pivoted an additional amount with the distance x between the first and second ends of the biasing mechanism  23  becoming larger. This results in an additional amount of counterbalance force being applied to the arm  21 . 
     FIG. 7  illustrates the amount of normal force applied by the pick mechanism  20  to the top-most media sheet. The force is substantially constant as the media stack is depleted from a full amount to some predetermined amount. In this embodiment, the input tray  101  is able to accommodate a media stack having a height of about 55 mm. The pick mechanism  20  applies a normal force of about 50 grams until the media stack has become depleted to a height of about 45 mm. Point A indicates a substantially full stack height as discussed in the embodiment of  FIG. 4 . 
   At a stack height of about 45 mm, the biasing mechanism  23  begins to engage and apply a counterbalance force. As the stack height decreases and the angle α becomes larger, the biasing mechanism  23  applies a greater force. The overall force applied to the media sheets gradually decreases as the stack height is diminished. Point B correlates to the embodiment illustrated in  FIG. 5  with a stack height of about 40 mm and a force applied of about 48 grams. Point C correlates to the embodiment illustrated in  FIG. 6  with a stack height of about 5 mm and an overall force of about 17 grams. 
   The force profiles may vary as necessary to reduce or eliminate roller slippage and double feeds.  FIG. 8  illustrates another embodiment. During the first profile Q the media sheets are depleted but the biasing mechanism  23  does not become engaged. During this depletion, the angle α of the arm  21  is increasing and thus the force applied to the media sheets increases. At some predetermined height, the biasing mechanism  23  becomes engaged and begins to offset the force applied by the arm  21 . This is illustrated in profile J. The point where the biasing mechanism  23  engages, and the amount of force applied at each height may vary depending upon the application. 
   In the embodiment illustrated in  FIG. 1 , two rollers  22  are positioned towards an end of the pick arm  21 . Various numbers and sizes of rollers  22  may be used again depending upon the application. 
   The term “image forming device” and the like is used generally herein as a device that produces images on a media sheet. Examples include but are not limited to a laser printer, ink-jet printer, fax machine, copier, and a multi-functional machine. Examples of an image forming device include Model Nos. C750 and C752 available from Lexmark International, Inc. of Lexington, Ky. 
   The embodiments illustrated in  FIGS. 2 ,  4 ,  5 , and  6  illustrate the input area comprising an input tray  101  having a bottom and side walls sized to contain the sheets. The input area may also include a manual feed area  109  where the media sheets are placed in a stacked orientation that are fed into the media path. 
   These embodiments may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.