Abstract:
A method and apparatus for end-to-end advancement of sheet media in an automatic document feeder suitable for use with a scanner are disclosed. The method involves driving a drive roller in a forward direction to advance a sheet medium section-by-section at a substantially uniform speed across a scan zone of the scanner. Such driving involves accelerating the sheet medium to reach the uniform speed, maintaining the speed while a section is scanned, decelerating the sheet medium to a stop after the section is scanned and reversing the sheet medium by a predetermined distance to allow it to be subsequently accelerated. To prevent an overlap of sheet media during such back and forth advancement of a sheet medium, a pick roller is prevented from reversing when the drive roller is reversed. When the drive roller is driven in the forward direction, the pick roller is only driven after a predetermined delay.

Description:
FIELD OF INVENTION 
     This invention relates to a subsystem in an automatic document feeder for feeding of sheet media. More particularly, this invention relates to a subsystem in an automatic document feeder suitable for use with a scanner wherein a scanned sheet is moved back and forth during scanning. 
     BACKGROUND 
     The throughput of office equipment such as photocopiers, scanners and fax machines is measured by the number of sheet media that can be processed by these equipment. One factor that determines the throughput of these equipment is the throughput of an automatic document feeder that is usually attached to these equipment for feeding sheet media to these equipment for processing. The throughput of the automatic document feeder should be as high as that of the equipment so as to make the most efficient use of the equipment. 
     Currently, the throughput of some scanners is limited by the throughput of an automatic document feeder that is attached to the scanners. In these scanners, the complicated back and forth transportation of sheet medium allows sheet media to be processed only one at a time. The automatic document feeder is designed to be able to pick and feed a new sheet medium only after the preceding sheet medium is fed through to the end of a document path. Such a method of sheet processing is inefficient and greatly limits the throughput of the document feeder. End-to-end feeding of the sheets is used in higher-end scanners to increase throughput. In end-to-end feeding, as soon as a sheet medium leaves a media tray, the next topmost sheet medium is picked and advanced to closely follow the preceding sheet medium. 
     Such end-to-end feeding of sheet media is easily accomplished in the high-end scanners. These high-end scanners usually have an abundance of memory for capturing scanned image of an entire sheet medium in one continuous pass of the sheet medium. However, low-cost units do not have the luxury of such a large memory. To completely capture the image on a sheet medium, each sheet medium is divided into contiguous sections. The size of each section is determined by the amount of available memory in the units. Each section of the sheet medium is separately scanned and the data of the scanned image on the section is stored in the available memory for uploading to a computer. Once the data in the memory is uploaded, the memory becomes available to capture another set of data. 
     The quality of a scanned sheet depends on several factors, one of which is the speed at which the sheet medium is passed over a scan zone of a scanner. To achieve a higher quality scanned image, the sheet medium is advanced over the scan zone at a uniform speed. In order to get a subsequent section across the scan zone at a uniform speed after a section is scanned, the sheet medium will have to be retracted, or reversed, in the document path. This reversing of sheet medium is required to allow the sheet medium to be accelerated to reach the uniform speed when the subsequent section reaches the scan zone. Such forwarding and reversing of a sheet medium poses a challenge in the picking and advancing of the sheet medium in a low-cost document feeder as illustrated in FIGS. 1A and 1B. 
     FIGS. 1A and 1B show a media feed subsystem  2  of an automatic document feeder wherein a single motor (not shown) is used to drive both a pick roller  4  and a drive roller  6 . The motor drives a main gear  8 , which in turn drives the pick and drive rollers  4 ,  6  via free gears, generally illustrated in the figures as free gears  10 ,  12 . FIG. 1A shows that the main gear  8  being driven in a counterclockwise direction to advance a first sheet medium  14  in a forward direction as indicated by arrow F. The first sheet medium  14  is shown  30  to have been advanced beyond the pick roller  4 . Using an appropriate gear ratio the drive roller  6  is driven faster than the pick roller  4  to create a gap  16  between a trailing edge  18  of the first sheet medium  14  and the leading edge of a second sheet medium  20 . FIG. 1A shows the second sheet medium  20  being urged against the pick roller  4  for picking. Driving the main gear to reverse the drive roller  6  would also cause pick roller  4  to reverse. This reversing of the pick roller  4  would push the second sheet medium  20  away from under the pick roller  4  as shown in FIG.  1 B. There is thus a tendency during subsequent forwarding of the pick roller  4  that a third sheet medium is pick in place of the second sheet medium, thereby disrupting the sequence of media feeding. 
     This problem is solved by the introduction of a slip clutch (not shown) that is attached to a shaft (not shown) of the pick roller  4  to allow unidirectional rotation of the pick roller  4 . FIG. 2A shows a subsystem similar to that in FIGS. 1A and 1B with the addition of such a slip clutch. When the drive roller  6  is reversed, the slip clutch prevents the pick roller  4  from rotating in the reverse direction so that the pick roller  4  continues to bear upon the second sheet medium  20 . This solution creates another problem. As the drive roller  6  and pick roller  4  are driven in the forward direction, there is a tendency for the two sheet media  14 ,  20  to overlap if the first sheet medium  14  is reversed by a amount that the differential in speeds of the two rollers  4 ,  6  cannot correct. The overlapping of sheet media is unacceptable because image on the overlapped portion cannot be scanned and the scanner is also unable to easily identify the sheet media boundaries. The pre-selection of gear ratio to ensure a speed differential to cater to the worst case scenario would compromise throughput as the gap between two sheet media could be large. 
     The prior art therefore creates the need for a method and apparatus for sequencing sheet media to increase throughput in a scanner or the like without increasing the cost of prior art apparatus excessively. 
     SUMMARY 
     In one aspect of the present invention, a method of driving a sheet media feed subsystem for substantially end-to-end advancement of sheet media in an automatic document feeder suitable for use with a scanner involves driving a drive roller to advance a sheet medium section-by-section across a scan zone of the scanner. In advancing a section of the sheet medium across the scan zone, the method involves accelerating the drive roller in a forward direction to attain a substantially uniform speed when the section reaches the scan zone. Thereafter, the drive roller is driven at the substantially uniform speed to advance the section across the scan zone. When the section is scanned, the drive roller is decelerated to bring the drive roller to a stop. The drive roller is then reversed to reverse the sheet medium by a predetermined distance so as to allow the drive roller to be subsequently accelerated to bring a next section to the substantially uniform speed when the next section reaches the scan zone. As the drive roller is reversed, a pick roller is prevented from reversing. As the drive roller is driven in the forward direction following a reversed rotation, the pick roller is similarly driven in the forward direction after a predetermined period. When a preceding sheet medium leaves the pick roller, the pick roller picks and advances a next sheet medium to follow the preceding sheet medium closely to leave a gap between the two sheet media. The delay in driving the pick roller prevents overlapping of the two sheet media. 
     In another aspect of the present invention, a media feed subsystem according to a preferred embodiment that is suitable for implementing the method above has a drive roller, a pick roller, a slip clutch and a drive system. The drive and pick rollers are driven by the drive system according to the method above. The slip clutch allows the pick roller to be driven in a single direction and prevents the pick roller from reversing when the drive roller is reversed. The drive system includes a delay mechanism that allows the pick roller to be driven after the drive roller is driven forward for a predetermined period following the reversing of the drive roller. 
     Preferably, the drive system includes a single motor that drives the pick roller and the drive roller via a gear train. The delay mechanism preferably includes a delay gear for driving the pick roller. This delay gear is attached to a second shaft to allow the second shaft to rotate in one direction to engageably drive the delay gear and to rotate in the other direction to disengage the delay gear to delay its driving by the predetermined period. 
     Preferably, the delay gear has an aperture for receiving the second shaft. The delay gear has a gap defined therein for receiving a stub that is fixedly attached to the second shaft. The movement of the stub within the gap allows the second shaft to be rotated without rotating the delay gear. However, when the second shaft is rotated to allow the stub to engage a wall that defines the gap, further rotation of the second shaft will also cause the delay gear to rotate. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be better understood with reference to the drawings, in which: 
     FIGS. 1A and 1B are side views of a prior art pick roller, drive roller and associated gear train for driving the pick and drive rollers. FIG. 1A shows the rollers being driven in a forward direction to advance and pick a first sheet medium and a second sheet medium respectively. FIG. 1B shows the rollers being driven in a reverse direction to cause the first sheet medium to be reversed. The second sheet medium is shown to be pushed away from under the pick roller. 
     FIGS. 2A and 2B are side views similar to FIGS. 1A and 1B except the pick roller is prevented from rotating in the reverse direction by a slip clutch (not shown) to hold the second sheet medium under the pick roller. FIG. 2A shows a warped first sheet medium between the pick and drive rollers. FIG. 2B shows overlapping of two sheet media when the pick and drive rollers are driven in the forward direction after the reversing action of the drive roller in FIG.  2 A. 
     FIGS. 3A-3H are side views similar to FIGS. 2A and 2B illustrating the sequence of driving a pick roller and a drive roller according to the present invention. These figures show an example of a mechanism, in this case a delay gear, which can be used to delay the driving of the pick roller after the drive roller is driven in a forward direction following a reverse rotation. The delay is proportional to the angular distance covered in the reverse rotation of the drive roller. 
     FIG. 3A shows the drive roller being driven at a faster speed than the pick roller to introduce a gap between a first and a second sheet medium. FIG. 3A also shows a shaft of the delay gear having a stub that engages a wall of the delay gear to drive the delay gear. 
     FIG. 3B shows that when the driver roller is reversed, the delay gear shaft is rotated to disengage the stub from the wall of the delay gear so that the delay gear and pick roller remain stationary. 
     FIG. 3C shows that when the drive roller is again driven in the forward direction after the reverse rotation shown in FIG. 3B, the pick roller remains stationary until the stub has been rotated to again engage the wall of the delay gear. This delay in driving the pick roller allows the drive roller to advance the first sheet medium to restore the gap between the first and second sheet medium. 
     FIGS. 3D and 3E show that as the pick and drive rollers are further driven in the forward direction, the gap between the first and second sheet medium widens. 
     FIG. 3F is similar to FIG. 3B showing the drive roller being driven in the reverse direction. FIG. 3F however shows an overlap of two sheet media in a portion of the document path between the pick and drive rollers. 
     FIG. 3G is similar to FIG. 3C showing the gap in FIG. 3E restored after the drive roller is driven in the forward direction subsequent to the reverse rotation as shown in FIG.  3 F. 
     FIG. 3H shows a maximum gap that is attained between two sheet media when the second sheet medium reaches the drive roller. 
     FIG. 4A is an isometric drawing of an embodiment of a pick roller on a pick roller shaft. A pick roller gear drives the pick roller shaft via a slip clutch. The delay gear of FIGS. 3A-3H is shown in engagement with the pick roller gear. 
     FIG. 4B is an exploded isometric drawing of the delay gear in FIG. 4A as seen in the direction of an arrow A in FIG.  4 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3A is a side view of an example of a media feed subsystem  23  that is driven according to the present invention. The media feed subsystem  23  has a pick roller  4  and a drive roller  6 . A single stepper motor (not shown) drives the rollers  4 ,  6  via a gear train  24  that is illustrated in dotted lines in FIG.  3 A. This gear train  24  is illustrated to include a main gear  8 , a free gear  12  and a delay gear  26 . The stepper motor (not shown) drives the main gear  8 , which in turn drives the free gear  12  and delay gear  26  to ultimately drive the drive roller  6  and pick roller  4  respectively. The ratios of the gears are selected to drive the rollers  4 ,  6  at the required speeds and forces. In this particular embodiment, the drive roller  6  is driven at a faster speed than the pick roller  4  to create a gap  16  between sequentially fed sheet media. 
     In an automatic document feeder in which this media subsystem  23  can be used, the pick roller  4  is positioned over and in constant engagement with a stack of sheet media  28  on a pressure plate (not shown). The media stack  28  is urged against the pick roller  4  by the pressure plate (not shown) to allow the pick roller  4  to separate a sheet medium on top of the media stack  28  to advance it along a document path in the automatic document feeder. The pressure plate has an idle roller (not shown) mounted thereon. In the absence of sheet medium between the pick roller  4  and the pressure plate, the pick roller  4  rides on the idle roller to relieve pressure that would otherwise build up where the pick roller meets the pressure plate. 
     The drive roller  6  is positioned downstream along the document path. When a sheet medium is sufficiently advanced along the document path to come under the drive roller  6 , the faster drive roller  6  pulls the sheet medium as opposed to the pick roller  4  pushing the sheet medium. The pick roller  4  is driven via a slip clutch  30  (FIG. 4A) that allows the pick roller  4  to rotate or freewheel at the increased speed of the drive roller  6  when a single sheet of sheet medium simultaneously contacts both rollers  4 ,  6 . 
     When attached to a scanner (not shown), the drive roller  6  advances the sheet medium over a scan zone (generally indicated by arrow X in FIG. 3A) for scanning. As previously described, there is usually insufficient memory in a low-cost scanner unit to completely capture the image on a sheet medium in a single continuous pass. This memory is a critical resource in the low-cost scanner. It has also been described that the sheet medium is divided into several contiguous sections for separate scanning. And to ensure that the scanned image is of a sufficiently high quality, the drive roller  6  is required to advance each section to be scanned across the scan zone at a substantially uniform speed. To ensure such an advancement of the sections of a sheet medium, after one section is scanned, the sheet medium is decelerated to come to a stop, reversed and forwarded to allow the sheet medium to accelerate to the substantially uniform speed when the next section reaches the scan zone. Preferably, the next section should reach the scan zone when the memory is again available. 
     Typically, scanning can be performed at resolutions in the range of 60-1200 dots-per-inch (dpi). The length of the sections is inversely proportional to the resolution of scanning. Scanning at the lowest resolution allows the length of the section to be the largest. In the preferred embodiment, this worst case length is about 7.8 mm. 
     The reversing and forwarding of the sheet medium in such an application is mandated by the recommended driving profile of the stepper motor to ensure that the stepper motor does not miss steps. Those skilled in the art would be familiar with such a stepper motor driving profile. 
     In a document feeder, it is also important to have a gap between sheet media to denote the boundary of each sheet medium. During the reversing of a sheet medium, the pick roller  4  is prevented from rotating by the slip clutch  30  (FIG.  4 A). Once the sheet medium is reversed the desired distance, the drive roller  6  is again driven in the forward direction to bring a subsequent section for scanning at the scan zone. When the drive roller  6  is driven in the forward direction, the pick roller  4  is not immediately driven but only after a delay that is proportional to the distance the sheet medium is reversed. This delay allows the gap between the two sheet media prior to the reversing to be restored. 
     FIG. 4A is an isometric drawing of a mechanism  32  that can be used to implement the delay in the driving of the pick roller  4 . FIG. 4A shows the pick roller  4  and the delay gear  26  of FIGS. 3A-3H. The pick roller  4  is mounted on a shaft  34  that is attached to a pick roller gear  36  via the slip clutch  30 . The delay gear  26  meshes with the pick roller gear  36  to drive it. FIG. 4B is an exploded isometric drawing of the delay gear  26  assembly as seen from a direction according to an arrow A in FIG.  4 A. 
     The delay gear  26  has an aperture  38  for receiving a shaft  40  that is attached to a free gear  42  (FIG.  4 A). On the edge of this aperture  38  are several, preferably three, axially extending resilient arms  44 . To attach the delay gear  26  to the shaft  40 , the shaft  40  is inserted into the aperture defined by the resilient arms  44 . In doing so, the resilient arms  44  flex radially outwards. The delay gear  26  is slipped along the shaft  40  until latching tips  46  of the resilient arms  44  are over an annular groove  48  on the shaft  40 . In this position, the resilient arms  44  collapse to allow the latching tips  46  to engage the groove  48  to hold the delay gear  26  in place on the shaft  40 . When in this position, a stub  50  on the shaft  40  is positioned in a gap  52  defined between two resilient arms  44  on the delay gear  26 . This shaft  40  is free to rotate within boundaries defined by the gap  52  without rotating the delay gear  26 . The delay gear  26  will start to rotate when the stub  50  engages blocking walls  54  that defines the boundaries of the gap  52 . 
     The operation of this delay gear  26  for use in the current invention is next described. FIG. 3A shows the stub  50  urging against a blocking wall  54  of the delay gear  26  to drive the delay gear  26 . In this figure, the delay gear  26  meshes with the pick roller gear  36  to drive the pick roller  4  to rotate it in a forward direction (arrow F) as described previously. 
     FIG. 3B shows that when the driver roller  6  is reversed, the slip clutch  30  on the pick roller shaft  34  prevents the pick roller  4  from rotating. Such an action will not cause a sheet medium  20  to be pushed from under the pick roller  4 . The first sheet medium  14  that is reversed is allowed to warp. When the main gear  8  is driven to reverse the first sheet medium  14 , the free gear  42  is rotated in an anti-clockwise direction. The stub  50  on the shaft  40  will move away from the blocking wall  54  without rotating the delay gear  26 . The angular distance moved by the stub  50  in the gap  52  corresponds to the distance the first sheet medium  14  is reversed. The minimum length of the gap  52  should correspond to the maximum reverse distance of a sheet medium  14  under the drive roller  6 . This angular distance is easily determined by those skilled in the art. 
     FIG. 3C shows the drive roller  6  again being driven to advance the first sheet medium  14  in the forward direction as indicated by the arrow F. The pick roller  4  remains stationary until the stub  50  again engages the blocking wall  54 . This delay in driving the pick roller  4  allows the drive roller  6  to advance the first sheet medium  14  to restore the gap between the first and second sheet medium  14 ,  20 . 
     Once the stub  50  engages the blocking wall  54 , the delay gear  26  is again rotated to advance the second sheet medium  20  towards the drive roller  6  as shown in FIGS. 3D and 3E. As the drive roller  6  is driven at a faster speed, the gap between the two sheet media  14 ,  20  widens. 
     FIG. 3F shows the overlap of sheet media during the reversing of a sheet medium. However, with the delay in driving the pick roller  4  after the drive roller  6  is again driven in the forward direction, the gap between the two sheet media will again be restored as shown in FIG. 3G when the rollers are again driven in the forward direction. 
     FIG. 3H shows a maximum gap width between the two sheet media when the second sheet medium reaches the drive roller  6 . When the second sheet medium  20  is moved to such a position in the document path, it will be advanced and reversed in tandem with the first sheet medium  14 . 
     The invention should not be construed to be limited to the use of the delay gear  26  as described above. Other means of introducing a delay in the driving of the pick roller  4  will be readily known to a person skilled in the art. For example, two separate motors, a pick motor and a drive motor, can be used to drive the pick and the drive rollers  4 ,  6  respectively. The delay is achieved by activating the pick motor after a predetermined period after the drive motor is driven. 
     As another example when only a single motor is used to drive both the pick and drive rollers, a free gear can be switch in and out of position to engage and disengage the pick roller gear. Such a gear can be switched by means of a solenoid.