Abstract:
An apparatus and method for delivery of sheet media to a printer includes a chassis, a drive motor, a media tray with elevatable pressure plate, a plurality of pick rollers situated opposite movable pinch rollers that come into contact with the pick rollers to form a roller assembly therebetween when in an extended position. A movable separator forms a throat between one of the pick rollers and the leading edge of the separator when it is in an extended position. A common transmission is used to simultaneously position the pinch rollers, separator, and the pressure plate. The apparatus also includes a media retarder for preventing multiple sheets of media from advancing through the apparatus at the same time. The method includes delaying the rotation of the pick rollers until a stack of media has been raised to press against the pick rollers, and the pinch rollers and separator are in an extended position. The pinch rollers and separator are retracted, and the transmission is disengaged from the pick rollers once the media sheet is delivered to a printer feed system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to printing systems, and more particularly relates to sheet feeder apparatus for delivery of print media to printers. 
     BACKGROUND OF THE INVENTION 
     Sheet media delivery apparatus are used to deliver print media to printers. These apparatus are often incorporated into auxiliary bin sheet feeders, which are typically used to increase the media delivery capacity of nominally low-capacity inkjet or laser printers. They also are commonly used as part of an overall media transport system in large-capacity office printers and copier systems. 
     Various auxiliary bin sheet feeders were developed for many of the early laser printers, such as the Hewlett-Packard “LASERJET” II and “LASERJET” III printers. Since these printers were fairly large (in comparison to today&#39;s laser printers), the overall size of the auxiliary bin sheet feeders could also be fairly large without appearing out of place or taking up excessive room on a desk or printer stand that was sized to accommodate the printer. 
     Today&#39;s contemporary laser printers and inkjet printers are much smaller than their predecessors. However, the demand for increased media-handling capacities remains as strong as ever. In parallel with this reduction in printer size, office and consumer markets have also demanded a reduction in the size of the auxiliary bin sheet feeders that work with the newer printers. 
     A reduction in overall size of an auxiliary bin sheet feeder requires a reduction in the media delivery system. The media delivery system typically performs two functions. First, a sheet of media must be “picked” off of a stack of media. It is desired to pick a single sheet of media at time, which is known as “singulation.” The second function of the media delivery system is to advance the picked sheet of media into a printer media feed system. 
     A common problem generally encountered in media delivery systems is “multipicking.” Multi-picking may occur when underlying sheets in the stack are partially “dragged” out of the tray by the picking of sheets above. Many conventional media delivery systems are designed to provide minimal drag on the media sheet after it has entered the feed mechanism of the printer. While minimizing drag improves media alignment and positional accuracy in the printer, retracting the separator pad can create another type of multi-picking known as a “trailing pick.” A trailing pick is caused when an underlying sheet is dragged by a sheet above it (generally the picked sheet) as the picked sheet is transported through the media delivery system. 
     A common technique for performing the pick operation employs a D-shaped wheel (D-wheel) that is rotated to cause a media sheet pick action. As the D-wheel is rotated, its curved portion contacts the media, urging it forward. As the D-wheel is further rotated it falls out of contact with the media at its flattened portion, allowing an upstream media transport mechanism to advance the media without resistance from the D-wheel. This arrangement is satisfactory so long as the media sheet is not bent around the D-wheel shaft during a feed operation. This situation may occur when the media tray is positioned at an angle relative to the feed mechanism. If the media sheet presses against the D-shaped wheel, significant drag on the media sheet results. Therefore, D-wheel systems are generally impractical for use in feed systems that require the media to be bent as it is fed to the printer, a common condition when auxiliary bin feeders are used with today&#39;s smaller printers. 
     The use of a D-wheel is also impractical for use in a low-profile auxiliary feed system applications that require the sheet media to be advanced a fair distance prior to entering the printer feed mechanism. For instance, a typical feed system may employ a D-wheel with a nominal diameter of about 2 inches (50 mm). Since about ¾ of the perimeter of the D-wheel contacts the media during a feed operation, a D-wheel of this size can advance the media about 5 inches. In comparison, the D-wheel diameter of a low-profile auxiliary feed mechanism may be limited to about half this size. Such a reduced-size wheel can only advance the media about 2.5 inches, which is insufficient in most applications. 
     Another typical pick and feed arrangement is shown in FIGS. 1-3, which illustrate a pick roller system employed in a media sheet feed mechanism manufactured by the Epson Corporation. As shown in FIG. 1, the system comprises a pick roller  216  that is driven by a drive gear  210  mounted on a shaft  212 , which in turn is coupled to a drive motor (not shown). A pivot arm  214  is mounted for rotation about shaft  212  and encloses a rubber pick roller  216 . A driven gear  218  mates with drive gear  210 , is rigidly connected to pick roller  216 , and is mounted for rotation on a shaft  220 . A spring washer  222  is positioned between an inner surface of arm  214  and driven gear  218 , and performs a friction clutch function. 
     The pick and feed system is positioned above a media tray including a pressure plate  224  which supports a stack of media sheets  226 . The tray is biased by a spring  228  into contact with the pick roller  216 . An edge separator  230  is positioned to maintain an uppermost sheet on stack  226  in place until operation of the rubber pick roller  216 . 
     The pick operation is illustrated in FIGS. 2 and 3. To implement a pick operation, drive gear  210  is driven in a counterclockwise (CCW) direction, thereby causing driven gear  218  to rotate in a clockwise (CW) direction. Due to the friction exerted by spring washer  222 , arm  214  and pick roller  216  are caused to rotate in a CCW direction until arm  214  hits stop  232 . This action causes pick roller  216  to come into contact with a top sheet  234  of stack  226 . The top sheet is forced against pick roller  216  through the action of spring  228  on tray  224 . Continued clockwise rotation of the pick roller  216  feeds the top sheet  234  from the stack  226 . 
     As shown in FIG. 3, when the top sheet  234  is grabbed by a pair of feed rollers  236 , the direction of rotation of the driven gear  210  is reversed to a CW direction, thereby causing the arm  214  and pick roller  216  to rotate in a CCW direction and out of engagement with the top sheet. The CCW rotation of pick roller  216  is required as the clutching action of spring washer  222  otherwise would cause pick roller  216  to impede the feeding of the top sheet. The CCW rotation of the arm  214  and pick roller  216  continues until the arm  214  hits a second stop  238 . 
     The prior art device of FIGS. 1-3 is generally too large to be used in a low-profile auxiliary bin sheet feeder. In order to meet the height restrictions necessitated by the lower profile, the size of the pick roller and drive gear must be reduced, which adversely impacts the pick and feed performance. Furthermore, the positioning of the feed rollers adds extra length to the overall size of the feed system. 
     Reduced-size media delivery systems present other problems that are not generally encountered with larger systems. One such problem is that the size (diameter) of the drive motor on these systems may be limited. A drive motor with a 50% reduction in diameter may have 25% of the torque of a comparable full-size motor. As a result, the torque available to drive the system may be dramatically reduced. 
     In addition to the foregoing problems, the media feed system needs to be able to handle media that is not completely flat. Humidity will often cause media sheets to become corrugated in that the sheets have ripples or waves formed in them and are no longer flat. Corrugated sheets pose a problem for conventional feed systems because they have a tendency to jam, rip or become skewed when they enter a feed-roller assembly subsequent to being picked. 
     Therefore a need exists for an improved reduced-size auxiliary bin feeder and associated media delivery system. It is further desired to have a media delivery system that reduces multi-picks and trailing picks, and reduces the adverse effect of wavy media sheets. It is additionally desired to provide a media delivery system that requires less motor torque. 
     SUMMARY OF THE INVENTION 
     The above and other desired features are achieved in accordance with the present invention which, according to a first aspect of the invention, is exemplified by an apparatus for delivering a sheet media to a printer. The apparatus includes a plurality of pick rollers, a plurality of moveable pinch rollers, and a mechanism for raising a stack of sheet media. In a preferred operational sequence, the stack of sheet media is raised so that the top sheet of media is urged into pressure contact with the pick rollers, thereby flattening the leading edge of the media in the areas that contact the pick rollers. Preferably in conjunction with the raising of the media, the pinch rollers are extended until they contact the pick rollers, thereby forming a roller assembly with a nip therebetween. The pick rollers are then rotated so as to pick the top sheet of media, and continued to be rotated so as to transport the top sheet through the roller assembly into a printer. 
     According to another aspect of the invention, an auxiliary bin sheet feeder is disclosed comprising a chassis having a drive motor and a media tray. A plurality of pick rollers are mounted to the chassis adjacent to opposing extendable pinch rollers, forming a roller assembly therebetween when the pinch rollers are extended to be in contact with the pick rollers. The media tray includes a pressure plate that can be raised to lift a stack of media so that it may be “picked” by the pick rollers. An extendable media separator is situated adjacent to one of the pick rollers, and serves the purpose of reducing multiple picks. The motor preferably is connected to a transmission including multiple gear trains for driving the pick rollers, pinch roller and separator extension mechanisms, and a pressure plate lifting mechanism. The pinch roller and separation extension mechanisms preferably comprise a common camshaft that causes the pinch rollers and the separator to extend when the camshaft is rotated. The pressure plate lifting mechanism preferably comprises a linkage connected to the transmission. A transmission crank gear preferably is commonly connected to the camshaft and the lift linkage. The transmission preferably is designed so that the pick rollers do not rotate until the pressure plate is raised and the pinch rollers and separator are in their extended positions. In addition, the transmission is preferably designed to disengage the pinch roller and separator extension mechanisms once the pinch roller and separator are extended, permitting the majority of the motor torque to be available to advance the media through the feeder. A media retarder, situated in close proximity to the separator, preferably is used to prevent more than one sheet of media at a time from advancing through the roller assembly subsequent to a pick operation. A paper kicker may optionally be used to push back the leading edges of the media sheets toward the top of the media stack so that the media sheets come into contact with the pick rollers at a consistent position. The paper kicker also is used to push sheets off of the media retarder and separator when additional sheets of media are added to the media tray. 
     In a preferred operational sequence of the sheet feeder, the pressure plate is lifted to raise a stack of sheet media, urging a top sheet of the media into contact with the pick rollers. Preferably in conjunction with the lifting operation, the pinch rollers are extended so that they come into contact with the pick rollers. At the same time, the separator is extended so that a rearward portion of the separator comes into contact with one of the pick rollers so as to form a throat between a forward portion of the separator and the pick roller. Once all of the foregoing events have occurred, the pinch roller and separator extension mechanisms are disengaged from the transmission, as well as the pressure plate lifting mechanism, and the transmission engages and drives the pick rollers so as to pick the top sheet of media in the media stack. As the media is picked, its leading edge is moved into the throat, whereby “singulation” is performed. The leading edge then proceeds through the roller assembly so as to transfer the media sheet towards a printer feed mechanism. An edge sensor preferably is used to sense when the media sheet has entered the printer feed mechanism. At this point the drive motor is reversed so as to cause the transmission to disengage the pick rollers and reengage the mechanisms to retract the pinch rollers and separator so that the printer feed mechanism can advance the media sheet without resistance from the roller assembly. 
     The invention can be used in an auxiliary media feed apparatus that is attachable to a printer, or as part of a built-in printer feed system. 
     These and other features and advantages of the invention are described in detail below in the following detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a prior art pick roller mechanism. 
     FIG. 2 is a schematic side view of the pick roller mechanism of FIG. 1 during a pick action. 
     FIG. 3 is a schematic side view of the pick roller mechanism of FIG. 1 after the pick action and during a sheet feed. 
     FIG. 4 is an isometric view showing an exemplary media delivery apparatus according to the present invention. 
     FIG. 5 is an isometric view of the drive mechanism of the FIG. 4 apparatus. 
     FIGS.  6 ( a ) and  6 ( b ) respectively show isometric and elevational views of the drive assembly and pressure plate of the FIG. 4 apparatus, with the pressure plate in the paper-down position. 
     FIGS.  7 ( a ) and  7 ( b ) respectively show isometric and elevational views of the drive assembly and pressure plate of the FIG. 4 apparatus, with the pressure plate in the paper-up position. 
     FIGS.  8 ( a ) and  8 ( b ) show elevation views of the drive mechanism gear assembly from the front and back sides, respectively, when the pressure plate is in the paper-down position. 
     FIGS.  9 ( a ) and  9 ( b ) show elevation views of the drive mechanism gear assembly from the front and back sides respectively as the lifter  28  is being rotated to lift the pressure plate  11  to the paper-up position. 
     FIG. 10 shows an isometric view of the backside of the drive mechanism gear assembly. 
     FIG. 11 is an isometric view of one of the pinch roller assemblies of the FIG. 4 apparatus. 
     FIG. 12 is an isometric view of the separator assembly. 
     FIGS.  13 ( a ) and  13 ( b ) are elevational views of the media delivery path during a pick operation. 
     FIG.  13 ( c ) is an elevational view for illustrating the media incidence angle. 
     FIGS.  14 ( a ) and  14 ( b ) are elevational views of the media delivery path showing the effect of the media retarder. 
     FIG. 15 is an isometric view of the separator and media retarder. 
    
    
     DETAILED DESCRIPTION 
     An exemplary media delivery apparatus in accordance with the present invention is shown in FIG.  4 . The apparatus includes a chassis  10 , which rotatably supports a pick roller shaft  16  at opposite ends of the shaft. Multiple pick rollers  12  are  30  mounted on the pick roller shaft  16 , and a pick gear  18  is connected to the shaft at one of its ends. A media separator  14  is pivotally coupled to the chassis  10 , and situated adjacent one of the pick rollers  12 . A gear train  20 , including a crank gear  22 , is coupled to a link  24 , which in turn is coupled to a rocker  26 . The rocker  26  is coupled to a shaft  31 , which in turn is coupled to a lifter  28  through a torsion spring  27 . The apparatus also optionally includes a paper kicker  29  that is pivotally mounted to the chassis  10 . 
     FIG. 5 shows an isometric view of the gear train  20 . Motor  30  turns drive gear  32 , which drives speed-reducing gears  34  and  36 . The gear  36  drives gears  38  and  40 . The gears  38  and  40  are concentric gears that are joined so that they rotate in tandem about a common shaft  42  that is affixed to motor mount  44 . Gear  40  simultaneously drives pinion gear  46  and gear  48 . Gears  48 ,  50 ,  52 , and  54  form a gear train with gear  54  as the output drive gear. The gears  46 ,  48 ,  50 ,  52 , and  54  are commonly mounted to crank arm  56 . Pinion gear  58  is mounted on pick arm  60  and driven by gear  38 . Pinion gear  58  engages pick gear  18  (see FIG. 4) when the pick arm  60  is rotated to a pick gear driving position, as explained below. 
     The media delivery apparatus additionally includes a pressure plate  11  that is used to support a stack of media, as shown in FIGS.  6 ( a )- 7 ( b ). The forward end of the pressure plate  11  is supported by the lifter  28 , enabling the pressure plate to be raised to a “paper-up” position (FIGS.  7 ( a )-( b )) and lowered to a “paper-down” position (FIGS.  6 ( a )-( b )) as the lifter  28  is rotated about axis  68 . The link  24  is connected to the crank gear  22  at pivot  64 , and connected to the rocker  26  at pivot  66 . The rocker  26  is connected to the shaft  31 , and shaft  31  is coupled to lifter  28  through torsion spring  27 , causing the lifter  28  to pivot about axis  68 . The combination of the crank gear  22 , the link  24 , the rocker  26 , and their associated pivots form a four-bar linkage. 
     The following discusses the operation of the gear assembly during the lifting of the pressure plate  11 . With reference to FIGS. 5,  8 ( a ) and  10 , a relief  62  is cut into the crank gear  22  at about one-half of the gear teeth depth so that gear  54  cannot drive the crank gear  22  when the pressure plate is in a paper-down position. To lift the pressure plate, the motor  30  is rotated counterclockwise as viewed in FIG.  5 . The angular direction and velocity of the motor  30  is controlled by a motor controller which may be located in the media delivery apparatus, or in a printer to which the apparatus is coupled. Rotating the motor  30  counterclockwise causes the gears in the assembly to be driven so as to rotate crank arm  56  clockwise about shaft  42 , as shown in FIG.  9 ( b ) and FIG.  5 . As a result, the pinion gear  46  engages the crank gear  22 , causing the crank gear  22  to rotate clockwise. As the crank gear  22  rotates clockwise, the link  24  is driven towards the right so as to raise the lifter  28 , thereby raising the forward end of pressure plate  11  as shown in FIG.  7 ( a ). The forces created in the gear train also apply a clockwise rotational force to pick arm  60 , urging engagement of the pinion gear  58  with the pick gear  18 . However, the pick arm  60  is prevented from rotating enough to engage the gears by means of lower arm  70 , which rides along a ridge  72  formed on the backside of the crank gear  22  along a portion of the gear&#39;s outer circumference as shown in FIGS.  8 ( a ) and  10 . 
     Pinion gear  46  continues to drive crank gear  22  until the lifter  28  is fully-raised, as shown in FIGS.  7 ( a )-( b ) and  9 ( a )-( b ). As the lifter  28  is being raised, the pinion gear  46  continues to rotate the crank gear  22  until pinion gear  46  reaches a recess  74 , which is formed in the crank gear  22  at about one-half of the gear teeth depth. At this point, the crank gear  22  can no longer be driven forward by gear  46 . In synchrony, the pick arm  60  rotates about the shaft  42  to engage the pinion gear  58  with the pick gear  18 . This is shown in FIGS.  7 ( a )-( b ) and  9 ( a )-( b ). At this point in its rotation, the ridge  72  on crank gear  22  has rotated relative to the lower arm  70  so that it no longer prevents lower arm  70  (and therefore pick arm  60 ) from rotating, permitting the pick arm  60  to rotate until lower arm  70  rides along inner ridge  76 , whereby the pinion gear  58  and the pick gear  18  are engaged. A detent  78  (FIG. 10) is cut into inner ridge  76  so that a protrusion  80  along leaf spring  82  engages the detent  78 , thereby locking the rotation of the crank gear  22  (the leaf spring and protrusion are removed from FIG. 10 for clarity). 
     Once the pressure plate  11  has been raised to a paper-up position, the motor continues to rotate in the counterclockwise direction, causing the gear train to rotate pick gear  18  in the clockwise direction, thereby rotating pick rollers  12  via shaft  16 . 
     The operation of the pinch roller assembly will now be described. Referring to FIG. 11, a camshaft  84  is concentrically mounted to crank gear  22 , and rotates about bearings at its respective ends (not shown). Pinch roller frame  90  is pivotally mounted to chassis  10  (not shown) at pivots  94 . A leaf spring  96  is mounted to the underside of pinch-roller frame  90 . As the camshaft  84  is rotated clockwise a cam surface  98  engages the leaf spring  96 , causing the roller frame  90  to rotate about pivots  94  in a counterclockwise direction until a pinch roller  100  comes into contact with a pick roller  12 . When the crank gear  22  is locked in the paper-up position the cam surface  98  displaces the leaf spring  96  so that a constant force is applied by the pinch roller  100  to the pick roller  12  along a line of contact formed between the two cylindrical bodies. 
     The operation of the separator assembly is similar to the pinch roller assembly operation. Details of the separator assembly are shown in FIGS. 12 and 15. Referring to FIG. 12, the camshaft  84  has a cam surface  102  formed on its underside, which allows lift  104  to rotate about pivots  106  in a counterclockwise direction as camshaft  84  is rotated clockwise. Lift  104  is biased to rotate counterclockwise about pivots  106  by spring  108  which is situated in a well  128  formed in the chassis  10  (FIG.  15 ). The lift  104  contacts the separator  14  at its underside, urging the separator  14  to rotate about a pair of opposed pivots that are suitably mounted in the chassis  10  (not shown) in a counterclockwise direction as the lift  104  is raised. 
     As the pressure plate  11  is being raised by the lifter  28 , the top leading edge of a top sheet of media  114  is pushed against the underside of the pick rollers  12 , as shown in FIGS.  13 ( a )-( b ). As shown in FIGS.  6 ( a )- 7 ( b ), the rocker  26  is connected to a shaft  31 . The lifter  28  is connected to a torsion spring  27  (FIG. 4) which is situated concentrically about the shaft  31  and connected to the rocker  26  so that a bias spring force is applied to a media stack  115  (and thus to media sheet  114 ) when the media stack  115  presses against the pick rollers  12 . When the lift  104  is at its maximum height position, a throat  118  (FIG.  13 ( a )) is formed between a separator pad  15 , which is situated on top of the separator  14 , and one of the pick rollers  12 , as shown in FIG.  13 ( b ). The pick roller  12  contacts the separator pad  15  to form the back of the throat  118 . 
     As shown in FIG.  13 ( a ), the pressure plate  11  is in the paper-up position, with the top sheet pressed against the pick rollers  12 . In FIG.  13 ( b ) the top sheet  114  is pulled into the throat  118  by the pick roller  12 , eventually passing over the separator pad  15 . The separator pad  15  is used to prevent more than one sheet of media from advancing through the throat  118  at a time. The separator pad  15  is preferably made of an elastomer such as rubber, with a Shore-A durometer of 50-90. The pick roller  12  is preferably made of an ethylene-propylene diene monomer with a durometer of about  35 . The softer pick roller  12  creates more friction against the top surface of the top sheet  114  than is created between the bottom of the top sheet and separator pad  15  so that the top sheet  114  is pulled past the separator pad  15 , whereby its leading edge enters the nip formed between the pick roller  12  and the pinch rollers  100  and is advanced upward against frame  124 . 
     As the top sheet  114  is dragged by the pick roller, frictional forces between the top sheet and underlying sheets may urge the underlying sheets to enter the throat  118 , as shown in FIG.  13 ( b ). At this point, the rubber surface of the separator pad  15  creates enough friction between itself and the underlying media sheets to overcome the friction created between the top sheet and any underlying sheets, thereby separating the sheets of media. 
     The separator performance is highly dependent on the size of the throat  118 , and the media incidence angle  119  (FIG.  13 ( c )), which is the angle between the entering media and the separator pad at the point of tangency to the pick roller. There is a tradeoff between the size of the front of the throat opening and the angle of incidence  119 . A larger throat opening admits thicker or wavier media, but usually requires a steeper incidence angle, which leads to media “stubbing,” as explained below. 
     In many conventional feeders the pick roller is about 52 mm in diameter, which results in a smaller incidence angle for the same size throat opening. Conversely, the pick roller  12  is preferably about 22 mm in diameter, which requires the use of a steeper incidence angle  119  to create a throat opening that is large enough to provide proper separation. Even with the steeper incidence angle, the throat opening is still limited by the smaller-size pick roller. As a result, imperfect media has a tendency to “stub” as it passes over the separator  14  and separator pad  15 . For example, if the leading edge of a media sheet is wavy, the sheet may tend to stub against the front surface  121  of the separator  14  as it is urged forward by the pick roller  12 . In order to reduce this “stubbing” effect, the front surface  121  must be made of a low-friction material, preferably a polymer such as nylon. Such a low-friction material allows the media to slip over the front part of the separator  14  without stubbing. 
     A second type of stubbing is caused by the larger incidence angle. In this instance, the larger incidence angle tends to cause the leading edge of the sheet to curl downward and stub against the separator pad as it enters the back of the throat. In order to overcome this problem, the front edge of the media needs to be flattened, which is accomplished when the media stack is pressed against the underside of the pick rollers prior to picking the media. 
     As the leading edge of the top sheet is pulled upwardly it is fed into a second printer feed mechanism (not shown). In order to prevent skewing and other adverse feeding effects, it is desirable to remove the frictional forces acting on the media at the separator pad and between the pick rollers and pinch rollers. Thus, as the media is fed into the second printer feed mechanism, the media separator  14  and pinch rollers  100  are retracted so that they are no longer in contact with the pick rollers  12 , as shown in FIGS.  14 ( a )-( b ). The effect of retracting the media separator  14  is shown in FIG.  14 ( a ). As the front of the media separator  14  is lowered, there may be enough friction between the first sheet  114  and the second sheet  123  to cause the second sheet to pass over the separator pad  15  (which has been retracted from the pick roller  12 ), and continue up the media transport path, resulting in a trailing pick. One device for reducing trailing picks, found in conventional feeders having large (i.e. about 52 mm) pick rollers, is a media separator that has a front surface  121  (FIGS.  13 ( a )-( b )) made of an elastomer. As the second sheet is dragged by the first, it first must pass over the front surface  121  of the separator. Because this surface is made of an elastomer, the friction between the bottom side of the second sheet exceeds the friction between the two sheets, thereby retarding advancement of the second sheet up the media transport path. However, as discussed above, the reduced throat size necessitates the use of a low-friction material for the front surface  121 . A downside of the low-friction surface is that it does not provide enough friction to prevent trailing picks. 
     In order to reduce the likelihood of trailing picks, the preferred embodiment incorporates the use of the media retarder  120 , which protrudes above the level of the retracted separator pad, as shown in FIGS.  14 ( b ) and  15 . The media retarder is preferably made of an elastomer with a durometer in the range of 50-90. As the second sheet  123  is dragged by the top sheet  114  it is stopped by the friction between the media retarder  120  and itself, which is greater than the friction between the two sheets of media. 
     As the top sheet advances through the media transport path a media edge sensor (not shown) is used to determine when the media has advanced into engagement with a conventional printer media feed mechanism (not shown) associated with the printer. At this point, the sensor signals the motor controller to rotate the motor clockwise, causing the pinch rollers  100  and the separator  14  and the pressure plate  11  to be retracted, and disengaging the pick gear  18  so that the sheet can be advanced into the printer with minimal drag forces caused by the media delivery apparatus. 
     Retraction of the pinch rollers  100  and the separator  14  is accomplished by rotating the crank gear  22  clockwise, which in turn rotates the camshaft  84  clockwise. As the camshaft rotates, cam surfaces  98  (FIG. 11) provide a decrease in the cam diameter adjacent to the pinch roller frames  94  and cam surface  102  provides an increase in the cam diameter adjacent to the lift  104 , thereby causing the pinch rollers  100  and separator  14  to retract. In synchrony with this action, the lifter  28  and pressure plate II are lowered. The rotation of the crank gear  22  is shown in FIGS.  6 ( a )- 9 ( b ). The motor  30  is rotated clockwise, causing the crank arm  56  to rotate about the shaft  42  so that the gear  54  engages with the crank gear  22 . A notch  124  is cut into the backside of the crank gear  22  so that a farside protrusion  126  can swing into a groove  128  that is formed in the backside of crank gear  22 , as shown in FIG.  10 . As the crank arm  56  rotates, a tab  130  on the crank arm contacts the wall of a recess  132  in the pick arm  60  (see FIG. 10) so as to cause the pick arm  60  to rotate counter-clockwise about the shaft  42 , disengaging the pinion gear  58  from the pick gear  18 . This allows the pick rollers  12  to freely rotate with little resistance. As the motor  30  rotates clockwise the resultant action of the gear train causes the gear  54  to rotate counterclockwise, causing the crank gear  22  to rotate clockwise. As the crank gear  22  rotates clockwise, the pivot  64  moves to the left (as viewed in FIGS.  6 ( a )- 7 ( b )) so as to lower the lifter  28  and pressure plate  11 . At the same time, the camshaft  84  is rotated clockwise, rotating the cam surfaces  98  and  102 , thereby causing the pinch rollers  100  and the separator  14  to be retracted away from the pick rollers  12 . The crank gear  22  continues to rotate clockwise until pinion gear  54  reaches relief  62 , at which point the protrusion  80  on leaf spring  82  falls into a second detent  136  (see FIGS.  8 ( a ) and  10 ). This locks the crank gear  22  in the paper-down position. 
     As shown in FIG. 4, the media delivery apparatus may also incorporate the use of a paper kicker  29 , which is used to align the leading edges of the media sheets. The paper kicker  29  is hingedly mounted in the chassis  10 , and spring-biased in an upward position. The position of the paper kicker  29  is controlled by the position of the cam  84 , whereby the cam  84  provides a cam surface  126  (see FIG. 12) for actuating the paper kicker  29 . 
     In the illustrated embodiment, all of the gears preferably are made from an acetal polymer such as Delrin, with the exception of the drive gear  32 , which is preferably made of metal. The majority of all of the other components preferably are made from suitable engineering thermoplastics by injection molding or other suitable processes, except for the springs, which are made out of suitable spring steels, and the motor mount, which is made of metal. 
     Experimental Results 
     Experimental results have shown significant improvements over the prior art. When the leading edge of the media stack is pressed against the pick rollers prior to being picked, the pick rollers flatten out any waves that are in the areas where the pick rollers contact the media. As a result, sheet media damage incidence has been reduced by a factor of 20. In addition, the life of the separator pad and its adjacent pick roller have been significantly extended because the pick roller is no longer rubbing against the separator pad for an extended period prior to picking a media sheet. Experimental results have also shown that the peak torque loading requirement has been reduced by 30%. Since the pick rollers are not driven until the pressure plate is fully lifted, the media stack-lift and media feed loads are no longer superimposed. The reduced torque load requirements provides for increased motor life and/or reduction in motor size and energy requirements. Experimental results have also shown a significant reduction in multi-picks and trailing picks due to the combined use of the media separator and the media retarder. 
     Having described the principles of our invention with reference to a preferred embodiment, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. For example, while the illustrated embodiment uses a crank arm and pick arm to engage and disengage the crank gear and the pick gear, respectively, this could also be accomplished by providing electronic clutches somewhere in the gear trains that drive the crank gear and the pick gear. Additionally, the gear trains could be replaced by drive belts and pulleys. Many other such variations will be apparent to those skilled in the art. 
     In view of the many embodiments to which the principles of our invention can be applied, it should be understood that the detailed embodiment is exemplary only and should not be taken as limiting the scope of our invention. Rather, we claim as our invention all such embodiments as may fall within the scope and spirit of the following claims, and equivalents thereto.