Abstract:
An imaging subsystem for forming an image on a sheet comprises a tray for holding a supply of sheets, a ramp for directing a sheet from the tray to the imaging subsystem and a drive assembly including a roller for moving a top sheet of the supply of sheets in the tray to the ramp with a reciprocating roller stroke. The drive assembly is configured to maintain a constant roller stroke distance between the roller and the ramp. A sheet feeder comprises a linkage and a roller disposed on the linkage for contacting and driving a sheet from a supply of sheet material by a reciprocal linear movement through a roller stroke distance. The linkage is configured to maintain a constant roller stroke distance between the roller and the ramp. In a method, a roller is moved in a reciprocating stroke to drive a sheet from a supply of sheets to a ramp that directs the sheet to an imaging subsystem. A constant distance is maintained for each stroke as the supply of sheets diminishes.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a sheet feeder, imaging system incorporating the sheet feeder and a method. More particularly, the invention relates to a sheet feeder with a link member that maintains a constant stroke distance between a paper-engaging roller and an inclined paper ramp. The sheet feeder is particularly advantageous when incorporated into an imaging system, such as a printer or photocopier. 
     A quality imaging system requires consistent and error-free feeding of paper from a paper tray. A sheet of paper can become jammed immediately upon exiting the paper tray or at some location downstream in the paper path. Other times two or more pieces of paper are fed simultaneously from the paper tray to cause a jam or other misfunction. A great deal of effort is directed to providing paper imaging system features to avoid jamming or misfunction to overcome these problems. 
     One approach to reducing paper-feed error involves mounting a sheet-separating roller in a freely movable manner in a plane parallel to the stack of paper sheets. The roller moves as a function of the stiffness of the sheets. For example if a top sheet has a high stiffness, then the roller will move rearward until the front edge of the sheet is bent and urged up a receiving ramp. One of the drawbacks of such an approach is the relatively large number of parts required to move a roller proportionally to the stiffness of the sheet. 
     In view of the foregoing, there is a need for a sheet feeder, imaging system and method to feed paper sheets in a consistent and substantially error-free manner. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention relates to a sheet feeder, imaging system and method that provide paper feed in a consistent and substantially error-free manner. In one embodiment, the invention is an imaging subsystem for forming an image on a sheet. The subsystem comprises a tray for holding a supply of sheets, a ramp for directing a sheet from the tray to the imaging subsystem and a drive assembly including a roller for moving a top sheet of the supply of sheets in the tray to the ramp with a reciprocating roller stroke. The drive assembly is configured to maintain a constant roller stroke distance between the roller and the ramp. 
     In another embodiment, the invention relates to a sheet feeder for an imaging system, the sheet feeder comprising a linkage and a roller disposed on the linkage for contacting and driving a sheet from a supply of sheet material by a reciprocal linear movement through a roller stroke distance. The linkage is configured to maintain a constant roller stroke distance between the roller and the ramp. 
     Finally in an embodiment, the invention is a method for feeding sheets in an imaging system. In the method, a roller is moved in a reciprocating stroke to drive a sheet from a supply of sheets to a ramp that directs the sheet to an imaging subsystem. A constant distance is maintain ed for each stroke as the supply of sheets diminishes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an imaging system configured in accordance with the principles of the present invention; 
     FIG. 2 is an elevational view of an exemplary sheet feeder of the present invention, particularly illustrating a linkage in an elevated position when a supply tray is full or nearly full of sheet material; 
     FIG. 3 is a view similar to that of FIG. 2, particularly illustrating the linkage in a position when the supply tray is less than full but not empty; 
     FIG. 4 is a view similar to those of FIGS. 2 and 3, particularly illustrating the linkage in a lowered position with the supply tray empty or nearly empty; 
     FIG. 5 is a schematic view illustrating dimensions of an exemplary embodiment of a linkage of the present invention; and 
     FIG. 6 is an elevational view of an exemplary configuration of a front of a supply tray in relation to a ramp of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the invention, a constant roller stroke distance is maintained between a sheet-feeding roller and a ramp for directing the sheets out of a supply tray. This constant roller stroke distance is maintained regardless of the level of sheet material (e.g., paper) in the supply tray. The principles of the present invention may be applied to imaging systems of all types, such as printers, photocopiers, facsimile machines and so on. 
     In a preferred embodiment, the imaging system is a printer, such as a color laser printer, and the imaging subsystem forms images according to electrophotographic (EPG) principles. The sheet material held in the tray may be paper, transparencies, label sheets, cards, envelopes, and so on. Preferably, the ramp includes one or more low-friction members that provide a smooth contact surface against which the roller may drive the sheets. In addition, the ramp is preferably disposed at an angle so as to further reduce the level of friction the top sheet needs to overcome in moving across the ramp. In this regard, the tray may include retaining structure for holding the supply of sheets in at an angle that corresponds to the angle of the ramp so that a leading edge of each of the sheets in the tray abuts the ramp. 
     According to another preferred embodiment of the invention, the path that a sheet travels before engaging with another component of the imaging system, for example, a transferring unit of the imaging subsystem, is minimized to reduce the likelihood of jamming errors. In this regard, a top edge of the ramp may be disposed at or near the imaging subsystem. To maintain the relatively short distance, the imaging subsystem may be disposed within the imaging system at an angle so that one of the ends may be positioned near the top edge of the ramp. 
     According to another preferred embodiment of the invention, a sheet feeder for an imaging system includes a linkage and a roller. The roller is disposed on the linkage and contacts and drives a sheet from a supply of sheet material. The linkage is configured to move the roller substantially linearly as the supply of sheet material decreases, preferably along an angulated linear path. The sheet feeder may also include a ramp for directing the sheet driven from the supply of sheet material by the roller to an imaging subsystem of the imaging system. In this embodiment, the linkage may be configured to move the roller along a path that is substantially parallel to the ramp. Alternatively, the linkage may configured to maintain the substantially constant roller stroke distance between the roller and the ramp regardless of a level of the supply of sheet material. A motor may be disposed on the linkage for driving the roller. 
     These and other features will become apparent from the drawings and following detailed discussion, which by way of example without limitation describe preferred embodiments of the present invention. 
     Referring to the drawings, an imaging system  50  with a highly stable and essentially error-free sheet feeder  52  is illustrated in FIGS. 1 to  5  according to an exemplary embodiment of the present invention. The principles of the present invention are described herein in the context of an electrophotographic (EPG) imaging system illustrated in the drawings. However, the invention is equally applicable to other devices in which sheets are fed, such as printers and copiers of all types. Therefore, prior to describing the principles of the present invention in detail, an exemplary embodiment of the imaging system  50  will be briefly described to place the sheet feeder  52  in this context. 
     With continued reference to FIGS. 1 to  5 , sheet feeder  52  includes a supply tray  80  for receiving a supply  82  of sheet material and a drive assembly  84  for engaging and driving a top sheet  86  of the supply  82  to a transfer unit along an initial portion of the sheet path S. Sheet feeder  52  may also include a ramp  88  for directing the top sheet  86  to the transfer unit of an imaging subsystem. For the purposes of this description, the term sheet is used herein to indicate any type of substrate on which an image may be formed, such as paper, transparencies, label sheets, envelopes, cards, and the like, either individual substrates (e.g., 8½ by 11 paper) or a continuous roll of substrate material. 
     According to a preferred embodiment, the drive assembly  84  generally includes a roller  90  and a motor disposed at an end  94  of a linkage  96 . Linkage  96  is configured to allow the roller  90  to contact and rest upon the top sheet  86  regardless of the level of the supply  82  of sheets. In addition, exemplary linkage  96  is configured to maintain a substantially constant roller distance between the roller  90  and the ramp  88  so as to define a constant stroke distance d for the roller  90  to drive a top sheet  86  to the ramp  88 . Stroke distance d is the distance of a singular, unbroken linear movement of the reciprocating roller  90 . Further, exemplary linkage  96  is configured to maintain the constant roller distance d between the roller  90  and the ramp  88  regardless of the level of the supply  82  of sheet material. 
     More specifically, with additional reference to FIGS. 2 to  4 , the stroke distance d is defined between the roller  90  and the ramp  88 . When the supply tray  80  is full or nearly full as shown in FIG. 2, the roller  90  is positioned in an upper position near an upper end  98  of the ramp  88 . As the supply of sheets in the supply tray  80  decreases as shown in FIG. 3, the roller is positioned in a mid-level position. And when the supply tray  80  is nearly empty or empty as shown in FIG. 4, the roller  90  is positioned in a lower position near a lower end  100  of the ramp  88 . As shown in these figures, the distance d between the roller  90  and the ramp  88  remains the same regardless of the spatial position of the roller  90 . 
     With particular reference to FIGS. 2 to  5 , linkage  96  according to a preferred embodiment includes three members: a primary member  102 , a secondary member  104 , and a tertiary member  106 . The linkage  96  also includes a first fixed axle  108   a  and a second fixed axle  108   b  and a first free axle  110   a  and a second free axle  110   b.  The respective positions of the fixed axles  108  are fixed to a frame of the imaging system  50  and, accordingly, the ramp  88  and the supply  82  of sheet material. The free axles  110  move spatially with the members  102 ,  104 ,  106 . The frame provides structural support for various components of the imaging system  50 , including the fixed axles  108 . 
     According to the preferred embodiment, the primary member  102  is connected to the free axles  110 , with a pivotal end  114  thereof connected to the second free axle  110   b.  The secondary member  104  is connected to the first fixed axle  108   a  and the first free axle  110   a.  As shown in FIG. 3, the first free axle  110   a  is connected at an intersection of an end  116  of the secondary member  104  and a midportion  118  of the primary member  102 . The tertiary member  106  is connected to the second fixed axle  108   b  and the second free axle  110   b.  Accordingly, the secondary member  104  is pivotal about the first fixed axle  108   a  as shown by arrow B in FIG. 3, and the tertiary member  106  is pivotal about the second fixed axle  108   b  as shown by arrow C in FIG.  3 . 
     This exemplary connection of the members  102 ,  104 ,  106  and the axles  108  and  110  enables the linkage  96  to move the roller  90  up and down along a path parallel to the ramp  88 . In other words, the linkage  96  is configured so that an axis of rotation A of the roller  90  approximates linear motion, particularly linear motion substantially parallel to the ramp  88 . More specifically, as shown in FIG. 2, when the roller  90  is in an elevated position, the second member  104  is angulated between the first fixed axle  108   a  and the first free axle  110   a,  thereby defining an angle α from horizontal. In addition, the tertiary member  106  is angulated between the second fixed axle  108   b  and the second free axle  110   b,  thereby defining an angle β from horizontal. 
     During operation as the supply of sheet material decreases, the primary member  102  is incrementally lowered into the supply tray  80 , with angles α and β incrementally decreasing as well. This results from the secondary member  104  rotating downwardly about the first fixed axle  108   a,  thereby urging the primary member  102  rearward as indicated by arrow R in FIG.  3 . The rearward movement of the primary member  102  as the supply of sheet material decreases in the supply tray  80  is proportional to the inclination of the ramp  88 . More specifically, as shown in FIG. 3, it is preferable for the ramp  80  to be inclined with respect to vertical by an angle γ. In order to maintain the constant gap or distance d with the inclined ramp  88 , the primary member  102  is urged rearward in proportion to the slope or inclination of the ramp  88 . To allow the linear downward movement of the primary member  102  into the supply tray  80 , the tertiary member  106  rotates about the second fixed axle  108   b.    
     Corresponding to the linear movement of the distal end  94  of the primary member  102 , the roller  90  moves substantially linearly. This corresponding linear movement of the roller  90  is indicated in FIG. 4 by path N. For the purposes of this description, the path N is defined as a linear path, preferably an angulated linear path, that the axis of rotation A of the roller  90  follows as the supply  82  of sheet material decreases. The path N is substantially parallel to the contact surface of the ramp  88 . 
     With further reference to FIGS. 2 to  5 , the secondary member  104  includes a pair of transverse portions rotatably mounted on respective sides of the primary member  102  on the first free axle  110   a  and each rotatably mounted on a first fixed axle  108   a   1  and  108   a   2 , respectively. The secondary member  104  may also include a transverse brace connected between the transverse portions to provide stability and rigidity. The terms transverse, longitudinal and normal as used herein, respectively correspond to the x, y, and z axes in standard Cartesian coordinates. 
     To further increase the stability of the linkage  96 , the distal end  94  of the primary member  102  may have a transverse dimension w larger than a proximal end of the primary member  102 . The larger transverse dimension w at the distal end  94  provides greater stability at the connection to a roller housing and greater stability in counteracting any torque produced by the motor. In addition, the transversely broad distal end  94  also provides an adequate platform for supporting the motor. 
     In addition, the midportion  118  of the primary member  102  may be tapered from the distal end  94  to the proximal end. The primary member  102  may include an axle housing projecting transversely therefrom. In addition, the primary member  102  may include one or more cross supports to provide rigidity between the midportion  118  and the axle housing. The primary member  102  may also include a vertex or discontinuity such that the distal end  94  and the midportion  118  are angled with respect to each other. This angle relationship allows the distal end  94  to be substantially horizontal when the supply tray  80  is relatively full as shown in FIG.  2  and to be slightly angled (e.g., less than 30°) when the supply tray  80  is relatively empty as shown in FIG.  5 . Also shown in FIG. 6, according to a preferred embodiment, each of the transverse portions of the secondary member  104  may taper from the first free axle  110   a  to the first fixed axle  108   a.    
     Referencing FIGS. 2 to  5 , for a given angle γ, the distance d between the ramp  88  and the roller  90  is optimized to minimize or prevent the double feeding of paper and, accordingly, paper jams. If the distance d is too small, then a relatively large torque from the motor is needed to drive the roller  90  to engage the top sheet  86  and to urge the top sheet up the ramp  88 . With large torques, there is a tendency for the roller  90  to spin (much like the wheels of a car with high torque), thereby causing the sheet material to buckle and jam. If the distance d is too large, then there is a tendency to double feed sheet material (i.e., engage and drive two or more sheets) because the coefficient of friction between the sheets and the ramp  88  may be less than the coefficient of friction between the top two sheets, thereby causing the top two sheets to feed simultaneously. In view of the foregoing, for a preferred angle γ of about 20 degrees, the distance d may be maintained in a range of about 5 millimeters (mm) and about 56 mm and more preferably in a range of about 25 mm to about 36 mm. If the angle γ is increased, then the distance d may be increased, and vice versa. 
     Exemplary dimensions of a linkage  96  configured for a standard supply tray  80  for holding either 8½-by-11 or A 4  sheet material are shown in FIG.  5 . The exemplary dimensions are given for an angle γ of about 20° and a segment  132  defined between the axis of rotation A of the roller  90  and the second free axle  110   b.  A distance P 1  of a segment defined between the first free axle  110   a  and the second free axle  110   b  may be about 115 mm, and a distance P 2  of a segment defined between the first free axle  110   a  and the axis of rotation A of the roller  90  may be about 155 mm. A distance S of the secondary member  104  defined between the first fixed axle  108   a  and the first free axle  110   a  may be about 89 mm. A distance T of the tertiary member  106  defined between the second fixed axle  108   b  and the second free axle  110   b  may be about 34 mm. In addition, a distance F in the longitudinal direction defined between the locations of the fixed axles  108  may be about 195 mm, and a offset O in the normal direction defined between the locations of the fixed axle  108  may be about 59 mm. Depending upon an angle δ of the discontinuity, a height H defined between the first free axle  110   a  and the segment  132  may be about 13 mm. The angulated linear path N of the axis of rotation A of the roller  90  is also clearly shown in FIG.  5 . 
     Referencing FIGS. 2 to  5 , the ramp  88  is fixed with respect to the housing  54 . For example with additional reference to FIG. 6, the ramp  88  may be attached or integral with the housing  54  and a front  134  of the supply tray  80  may include a bay with a pair of transverse panels  138   a  and  138   b  for accommodating the ramp  88 . Preferably, the bay accommodates the ramp  88  such that the supply  82  of sheets abuts or is urged against the ramp  88 . In this regard, the supply tray  80  may include retaining structure that is preferably disposed within the supply tray at an angle substantially equal to the angle at which the ramp  88  is disposed (i.e., angle γ). The retaining structure is preferably adjustable so that varying sizes of sheet material may be held within the tray  80  with leading edges thereof positioned at or near the ramp  88  or more preferably, abutting the ramp  88 . In addition, the retaining structure is preferably configured so that the leading edges of the sheets are collectively disposed at an angle substantially equal to that of the ramp  88 . 
     As shown in FIGS. 3 to  5  and  8 , according to a preferred embodiment the ramp  88  includes a low-friction contact surface against which the sheet material slides. The low-friction contact surface may include one or more low-friction members  142   a,    142   b,    142   c,  . . . ,  142   n.  The low-friction members  142  are preferably planar and made from a smooth and low-friction material such as a metal alloy (e.g., stainless steel) or a composite material (e.g., ceramic or porcelain). Exemplary members  142  preferably protrude beyond a back portion  144  so that the sheet material contacts the members  142  exclusively. According to a preferred embodiment, the distance d between the ramp  88  and the roller  90  may be defined between the roller  90  and the low-friction members  142 . 
     Referencing FIGS. 3 to  5 , to minimize the likelihood of paper-feeding errors, it is preferable to configure the imaging system  50  so that a top edge of the ramp  88  is disposed at or near the imaging subsystem, particularly the transfer unit. To further minimize paper-feeding errors, with further reference to FIGS. 3 to  5 , exemplary imaging system  50  may include rollers  146  for receiving and engaging the top sheet from the supply  80  of sheets as the top sheet is being driven up the ramp  88 , and for providing the top sheet to the transfer unit. According to a preferred embodiment, a total distance L between the top edge of the ramp  88  and the nip of the transfer unit is less than about 5 centimeters (cm) or 6 cm. For example, a distance l 1  between the top edge of the ramp  88  and the rollers  146  may be about 2 cm, and a distance l 2  between the rollers  146  and the transfer unit  70  may be about 2 cm. 
     Referencing FIGS. 3 to  5 , to minimize the likelihood of paper-feeding errors, it is preferable to configure the imaging system  50  so that a top edge of the ramp  88  is disposed at or near the imaging subsystem, particularly the transfer unit. To further minimize paper-feeding errors, with further reference to FIGS. 3 to  5 , exemplary imaging system  50  may include rollers  146  for receiving and engaging the top sheet from the supply  82  of sheets as the top sheet is being driven up the ramp  88 , and for providing the top sheet to the transfer unit. According to a preferred embodiment, a total distance L between the top edge of the ramp  88  and the nip of the transfer unit is less than about 5 centimeters (cm) or 6 cm. For example, a distance l 1  between the top edge of the ramp  88  and the rollers  146  may be about 2 cm, and a distance l 2  between the rollers  146  and the transfer unit may be about 2 cm. 
     Those skilled in the art will appreciate that the sheet feeder  52  may include other features to enhance user compatibility. For example, the supply tray  80  may include a handle  152  as shown in FIGS. 1 and 3 to  5  for use in removing the supply tray from the system  50 . Those skilled in the art will also appreciate that the imaging subsystem may be an inkjet system, a toner jet system, a laser system, and so on, with the sheet feeder  52  operating in accordance with the principles of the present invention. If the imaging subsystem is configured to carry out an EPG process as described above, then the photoconductor  60  may be a photoreceptive belt or a drum with a photoconductive substrate. 
     Accordingly, sheet-feeding principles of the present invention have been exemplified by the embodiments illustrated in the drawings. These principles focus on a stable and uniform approach to feeding sheets in imaging systems. While preferred embodiments of the invention have been described, the present invention is capable of variation and modification and therefore should not be limited to the precise details of the Examples. The invention includes changes and alterations that fall within the purview of the following claims.