Abstract:
Apparatus for rotating a sheet moving in a first direction, the rotator comprising: 
     at least one first roller that rotates against a sheet first side, the at least one first roller having a first drive; 
     at least one second roller that rotates against the sheet first side, the at least one second roller having a second drive that is capable of rotating the second roller independently of the first roller, the second roller being spaced a distance from the at least one first roller in a direction perpendicular to the first direction; and 
     a controller that controls the first and second drives to rotate the sheet around an axis substantially perpendicular to the plane of the sheet.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a rotator on a duplex imager that rotates a sheet following inversion. 
   BACKGROUND OF THE INVENTION 
   To produce accurately positioned duplex (two sided) images, whether by a printer or copier, the front side and rear side images are usually referenced from a same edge of a sheet on which they are printed. Since many inverters invert the sheet so the leading edge (from which the front image is referenced), becomes the trailing edge and since most printers reference the current leading edge, the rear side imager lacks a reference to the image on the front side. 
   Some prior art duplex imaging systems use relatively complex measurement systems to determine the position of the current trailing edge and use that edge as a reference for the printing of the rear image. Other prior art systems use bulky and/or complex mechanisms to rotate an inverted sheet to restore its reference edge to the lead position; one such system comprising an arm that grabs the sheet, rotates the arm 180 degrees about an end of the arm remote from the sheet and releases the sheet. 
   A skewed image, i.e., an image whose edges are slanted with respect to the edges of the sheet on which they are printed, is another shortcoming of prior art imagers. As a sheet moves along a printer or copier, it may be subject to air turbulence that causes misalignment. To correct the misalignment, in some printers, a side edge of the moving sheet contacts stationary guide rails along its path so the sheet straightens prior to reaching an imaging station. However, in high speed imaging, the contact time may not be sufficient to straighten the sheet and a skewed image may result. 
   Occasionally, grossly misaligned sheets override the guide rails, especially if they are too close to the guide rail. Rather than straightening, these sheets remain grossly misaligned and often jam in the next station, for example an imaging station or a sheet inverter. A jam in a station results in wasted time while the imager is shut down to clear the jam. 
   U.S. Pat. No. RE 37,007 describes a system for de-skewing in which rollers are configured to selectively drive a sheet to correct skew. The rollers are all driven by a common drive mechanism and contact with the sheet is controlled by counter rollers. 
   SUMMARY OF THE INVENTION 
   An aspect of some embodiments of the present invention relates to a rotator that rotates an inverted sheet utilizing spaced rollers. In an exemplary embodiment, at least two spaced, driven rollers contact a surface of a sheet and rotate in opposite directions, causing the sheet to revolve around an axis perpendicular to the sheet, thereby reversing the leading and trailing edges. 
   Optionally, the rotator includes at least one counter roller that presses the sheet against at least one driven roller, thereby preventing the sheet from slipping during rotation. Optionally, the at least one counter roller is friction driven by its friction with the moving sheet. Optionally, the at least one counter roller has more than one degree of rotational freedom. In an embodiment of the invention, counter rollers are provided for each of the driven rollers. Optionally, the rollers are independently driven. 
   An aspect of some embodiments of the present invention provides a skewed sheet correction system comprising two or more sensors spaced away from each other, the sensors being operationally linked to a controller that controls a sheet rotator. In an exemplary embodiment, the two or more sensors sense a degree of skew along the leading edge of a sheet and provide signals to the controller that directs skew-correcting rotation by the rotator. Optionally, the sheet rotator comprises at least two driven rollers spaced from each other. 
   An aspect of some embodiments of the present invention provides a sheet trailing edge sensor operationally linked to a controller that controls a sheet rotator. In an exemplary embodiment, the trailing edge sensor senses the trailing edge of a sheet, and directs the rotator to rotate the sheet 180 degrees, bringing the trailing edge to the lead. 
   An aspect of some embodiments of the present invention provides a system for realigning grossly misaligned sheets. 
   As in the prior art system described above, an exemplary embodiment of an inventive system comprises a guide rail aligned with a station entry and an optional sheet side offset mechanism. The system also includes a trajectory offset mechanism that acts on a sheet to offset the trajectory of a first side edge away from the guide rail with sufficient offset between the first side edge and the rail so that even a grossly skewed sheet does not override the rail. Optionally, prior to entering a station, the sheet side offset mechanism presses against a second side edge causing the first side edge to contact the guide rail, thereby aligning the sheet with the station entry. 
   Optionally, the trajectory offset mechanism comprises at least two driven rollers, spaced away from each other, that contact the sheet surface. In an exemplary embodiment, the at least two rollers rotate at different speeds to offset an edge of the sheet from the rail. Alternatively, the at least two driven rollers rotate around a point that is offset a distance from the sheet center, thereby offsetting the edge as the sheet is rotated. 
   There is thus provided, in accordance with an embodiment of the invention, apparatus for rotating a sheet moving in a first direction, the rotator comprising: 
   at least one first roller that rotates against a sheet first side, the at least one first roller having a first drive; 
   at least one second roller that rotates against the sheet first side, the at least one second roller having a second drive that is capable of rotating the second roller independently of the first roller, the second roller being spaced a distance from the at least one first roller in a direction perpendicular to the first direction; and 
   a controller that controls the first and second drives to rotate the sheet around an axis substantially perpendicular to the plane of the sheet. 
   Optionally, the apparatus comprises at least one counter roller adapted to contact a second side of the sheet opposite at least one of the first and second rollers. Optionally, the at least one counter roller is friction driven. Optionally, the at least one counter roller has freedom of motion along at least two axes. 
   In an embodiment of the invention, the controller selectively operates the rollers in at least two modes, a first mode in which the rollers rotate with opposite senses, thereby rotating the sheet and a second mode in which the rollers operate with a same sense, thereby advancing the sheet. Optionally, the controller is operative, in a skew correction mode, to rotate the rollers at different rates to correct skew in the sheet. Optionally, the apparatus comprises at least one skew sensor connected to the controller, the at least one skew sensor being adapted to sense skew of the sheet. 
   In an embodiment of the invention, the controller is operative, in a skew correction mode to rotate the rollers at different rates to correct skew in the sheet. Optionally, the apparatus includes at least one skew sensor connected to the controller, the at least one skew sensor being adapted to sense skew of the sheet. 
   In an embodiment of the invention, the apparatus includes a trailing edge sensor, the sensor sensing a trailing edge of the sheet as it moves in the given direction. Optionally, the controller causes the rollers to rotate the sheet 180 degrees in response to said sensing, such that leading and trailing edges of the sheet are interchanged. 
   In a embodiment of the invention, the controller controls the rotation speed of the at least one first roller to differ from the rotation speed of the at least one second roller operative to offset the sheet laterally to the first direction. 
   In an embodiment of the invention, the center of the sheet as it moves in the first direction is laterally offset to the first direction from the midpoint of the rollers, such that the lateral position of the sheet with respect to a general transport direction is changed during said rotation. 
   In an embodiment of the invention, the controller causes the rollers to rotate the sheet 180 degrees, such that leading and trailing edges of the sheet are interchanged. 
   There is further provided, in accordance with an embodiment of the invention, alignment apparatus for laterally aligning a sheet moving in a first direction, the system comprising: 
   an alignment surface, defining a side boundary; 
   a sheet edge offset mechanism that offsets a sheet so that it is further from the rail; and 
   an alignment mechanism operative to press the side edge of the sheet against the alignment surface so the sheet side edge substantially aligns with the side boundary. 
   Optionally, the sheet offset mechanism comprises apparatus for rotating a sheet according to an embodiment of the invention. 
   There is further provided, in accordance with an embodiment of the invention, apparatus for reversing the leading and trailing edges of a sheet moving in a given direction, comprising: 
   at least one trailing edge sensor that determines the position of a trailing edge of a sheet traveling along a sheet conveyor; 
   a rotator that rotates a sheet 180 degrees; and 
   a controller that receives signals from the at least one trailing edge sensor and signals the rotator to rotate the sheet responsive to the passage of the sheet trailing edge. 
   Optionally, the rotator comprises apparatus for rotating a sheet according to the invention. 
   There is further provided, in accordance with an embodiment of the invention, duplex printing apparatus comprising: 
   a first printing engine; 
   a second printing engine; 
   a sheet transport system that transports a sheet from the first printing engine after printing on a first side thereof to the second printing engine for printing on the second side, the sheet transport system comprising: 
   a sheet turner which turns over the sheet while exchanging the leading and tailing edges thereof, and 
   one or more of sheet rotating apparatus, alignment apparatus and apparatus for reversing the leading and trailing edges of a sheet according to the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description of non-limiting exemplary embodiments of the present invention should be read in conjunction with the drawings. Corresponding structures in different drawings are indicated with the same reference numeral. The drawings are: 
       FIG. 1A  is a schematic aerial view of a sheet rotator, in accordance with an embodiment of the invention; 
       FIG. 1B  is a side view of a portion of the sheet rotator of  FIG. 1A , in accordance with an embodiment of the invention; 
       FIG. 2  is a schematic aerial view of a skewed edge sensor system, in accordance with an embodiment of the invention; 
       FIG. 3  is a schematic aerial view of a trailing edge sensor system, in accordance with an embodiment of the invention; and 
       FIG. 4  is an aerial view of a sheet alignment mechanism, according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1A  is a schematic aerial view of a sheet rotator  100  located between a turn-over drum  320  and a rear side imager  332  along a sheet conveyor  102 , in accordance with an embodiment of the invention. The general direction of a sheet  154  is shown by an arrow  101 . After sheet  154  is imaged on a first side by a front side imager shown schematically by box  330 , optionally referenced to an edge  152 , drum  320  grabs sheet  154  by reference edge  152  and turns the sheet over as indicated by arrow  310 . Sheet  154  rolls over drum  320  so that the rear surface becomes uppermost. However, during this flipping action, a trailing edge  150  of the sheet flips forward of reference edge  152 . The trailing edge thus becomes the leading edge. As used herein, the terms “turn over” and “flipping” are used interchangeably to denote the act of turning over the sheet so that the positions of the surfaces of the sheet are exchanged. The term “inverted” or “rotated” are used to denote interchanging of the leading and trailing edges. These changes in orientation sometime occur together. Sometimes only one of the changes occurs, such as for example when the leading and trailing edges are interchanged without turning over the sheet. 
   While a turn-over drum  320  is depicted, the present invention is operable with many alternative prior art flippers, including curved plate inverters or any other sheet flipping mechanism that reverses the leading and trailing edges. The invention is also useful for any other situation in which it is desired to reverse leading and trailing edges, without flipping the sheet. 
   Following turn-over and inversion by drum  320 , sheet  154  moves in direction  101  over driven rollers  110  and  120  of a rotator system  100 . Rollers  110  and  120  are optionally overlaid by counter pressure rollers  190  and  195  respectively, to assure that rollers  110  and  120  drive sheet  154 . Until sheet  154  is positioned for inversion of the leading and trailing edges or partial rotation, as described below, the sheet is optionally driven by rollers  110  and  120  in direction  101 . 
   When the sheet is positioned for inversion of the leading and tailing edges of sheet  154 , rollers  110  and  120  are rotated such that they locally drive the sheet in directions  112  and  122 , causing sheet  154  to rotate in a direction  130 . With 180 degrees of rotation, reference edge  152  is restored to the lead position. 
   Optionally, after inverting the leading and trailing edges rollers  110  and  120  both rotate together in a direction to drive sheet  154  in direction  101 , until trailing edge  150  is released by rollers  110  and  120 . Alternatively or additionally, sheet  154  may be conveyed directly after rotation by other means for example, by conveyor  102 . Conveyor  102  may comprise a series of rollers, one or more moving belts or any of the many known conveyor systems. 
   The variety of desirable motions is facilitated if rollers  110  and  120  are independently rotatable and/or driven. 
     FIG. 1B  is a side view of a portion of rotator  100 , showing roller  110  positioned against sheet  154  and counter roll  190  pressing sheet  154  against roller  110 , thereby preventing slippage of sheet  154  as roller  110  rotates. In an exemplary embodiment, counter roller  190  is driveless, rotating as a result of friction with sheet  154 . Optionally, counter roller  190  may have two or more degrees of freedom and, for example, may have a spherical surface, to avoid slippage as sheet  154  is rotated. 
   During conveying, sheet  154  may be skewed, especially as the sheet moves at high speeds. When skewing occurs prior to entering an imager, for example front side imager  330 , the resultant image is skewed with respect to sheet  154 . 
     FIG. 2  is a schematic aerial view of a skewed edge sensor system  200  comprising sensors  210  and  220  that sense the position of leading edge  152  after inversion of the leading and trailing edges. In an exemplary embodiment, sensors  220  and  210  are connected to a controller  230  that controls the rotation of independently driven rollers  110  and  120 . When controller  230  senses a skew along reference edge  152  (for example, determining that the sheet passes the sensors at different times) controller  230  directs rollers  110  and/or  120  to correct the skew. For example, when corner  252  is forward of corner  254 , controller  230  directs roller  110  to briefly drive the sheet in direction  112  and/or roller  120  to briefly drive the sheet in direction  122 . As above. Sheet  154  rotates in direction  130  until reference edge  152  is no longer skewed. 
   While skewed edge sensor system  200  and rotator  100  are shown located upstream of rear side imager  330 , they could be located anywhere along conveyor  102 . For example they may be located prior to rear imager  332  ( FIG. 1A ) or prior to any station, a station comprising any sheet processor, for example inverter  320  or a sheet stacker mechanism (not shown). 
   Reversing the leading and trailing edges using rollers  110  and  120  can take with the sheet located at substantially any position along the length of sheet  154 . If only a single size sheet is used, then, in an embodiment of the invention, a sensor or sensors, such as sensors  210 ,  220  of  FIG. 2  are used to sense when the leading and trailing edges should be reversed. Until the sheet reaches the sensor(s), rollers  110  and  120  both drive the sheet in direction  101 , moving the sheet forward. When the leading edge is sensed by the sensor(s), the direction of rotation of one of the rollers is reversed, reversing the leading and trailing edges, as described above. For sheets of nominal length, after this rotation, the new leading edge will be substantially in the same place as the previous leading edge. 
   However, when sheet  154  has a different length other than nominal, after rotation, edge  150  is in a different position formerly occupied by reference edge  152 . As a result, the front and rear images may be imaged at different distances from reference edge  152 , unless an additional step of leading edge alignment is carried out. Usually, the longest length to be printed is the “nominal” and sheets that are not nominal are shorter. 
     FIG. 3  is an aerial view of a system utilizing a trailing edge sensor  310  located along conveyor  102  in a duplex imager, in accordance with an exemplary embodiment of the present invention. In the illustration sheet  354  is a “short” sheet. Following reversal of the leading and trailing edges during a prior flipping of the sheet, a trailing edge  350  passes trailing edge sensor  310 . The passage generates a signal that controller  230  utilizes to initiate rotation of short sheet  354  by 180 degrees, using rollers  120  and  110 . Solid lines show the position of short sheet  354  and edge  350  prior to rotation while broken lines show the position of short sheet  354 A and edge  350 A following rotation. 
   When a trailing edge sensor is used to time the rotation, then after rotation, the position of the leading edge after rotation of the sheet will be the same irrespective of the length of the sheet. This is useful to reduce the amount (and time) of travel and to provide a common timing for the fault determination and subsequent alignment steps (if any), independent of the length of the sheet. 
   This invariance of the position of the leading edge after rotation can be illustrated by considering the distances  360  and  370 . Distance  360  is the distance of the trailing edge from the rollers  110 ,  120 , when rotation is instituted by trailing edge sensor  310 . After rotation, the edge  350  has been repositioned to position  350 A, a distance  370  from the rollers. Since  360  is substantially the same as distance  370  and since the distance  360  is not dependent on the length of the sheet, position  350 A will not depend on the length of the sheet. 
     FIG. 4  is an aerial view of a system  400  for aligning sheets  154 , even when grossly misaligned. System  100  comprises a trajectory offset mechanism  100  and an alignment mechanism  450 . 
   Alignment mechanism  450  comprises a guide  140  aligned with imager  332 , and a sheet transverse offset mechanism  448 , which pushes sheet  154  against guide rail, so that the sheet enters imager  332  at a correct transverse (to motion direction  101 ) position. The inventors have found that to facilitate transverse alignment of the sheet, the sheet should be at least some minimum distance (designated as  446  on  FIG. 4 ) from guide  140 . When this distance is too small, there is a tendency for the sheet to override guide  140  or be otherwise unaligned. Such lack of alignment can cause jamming of sheet  154  in imager  332  or improper placement of images on sheet  154 . 
   In an exemplary embodiment, trajectory offset mechanism  100  acts on sheet  154  to offset a first side edge  444  from guide rail  140  by an offset distance  446 . In an exemplary embodiment, offset mechanism  100  creates sufficient offset distance  446  between edge  444  and rail  140  so that even a grossly skewed sheet is properly positioned. Prior to entering imager  332 , sheet side offset mechanism  448  presses against side  402  of sheet  154 , causing side  444  of the sheet to contact guide rail  140 , and to be aligned with guide rail  140  and also with imager  332 . 
   The means by which transverse offset mechanism  100  offsets sheet  154  from rail  140  may comprise any of a number of options. For example, the midpoint between rollers  110  and  120  may not align with the midpoint of sheet  154  as it enters these rollers. As rollers  110  and  120  rotate sheet  154  by 180 degrees, sheet  154  is offset laterally to the general direction of motion  101 . Alternatively, the rollers can be made to rotate at different rotation rates, such that the sheet rotates about a point that is not at the midpoint between rollers  110  and  120 . This will also cause transverse offset of the sheet. 
   Mechanism  100  can also be used to provide offset, without inverting leading and trailing edges. For example, if one of the rollers is rotated at a speed that is faster than the speed of the other roller, the sheet will be skewed. If the sheet is driven for a period of time in the direction of the skewed leading edge and then deskewed, an offset in the sheet will be generated. 
   While, alignment system  400  is shown prior to imager  332 , transverse sheet offset and alignment can be produced anywhere in the paper path, when needed to provide transverse sheet alignment. 
   In some embodiments of the invention, other methods of lateral moving of the sheet may be implementing prior to side alignment. Such methods may include physical lateral transport of the sheet and may include methods as are known in the art. 
   While the present invention has been described with respect to exemplary embodiments thereof, these embodiments are presented by way of example only and are not meant to limit the scope of the invention which is defined by the claims. For example, the functions of offset can be carried out independently, by separate mechanisms or, a combination of two or more of rotation, de-skewing and lateral offset can be performed simultaneously in a single station. 
   Furthermore, embodiments of the invention may incorporate some but not all features of the above exemplary embodiments and may include combinations of features from different embodiments. As used in the claims the terms “comprise” or “include” and their conjugations shall mean “including but not necessarily limited to.” 
   It will be appreciated by a person skilled in the art that the present invention is not limited by what has thus far been described. Rather, the scope of the present invention is limited only by the following claims.