Paper rotation method and apparatus

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; anda controller that controls the first and second drives to rotate the sheet around an axis substantially perpendicular to the plane of the sheet.

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.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1Ais a schematic aerial view of a sheet rotator100located between a turn-over drum320and a rear side imager332along a sheet conveyor102, in accordance with an embodiment of the invention. The general direction of a sheet154is shown by an arrow101. After sheet154is imaged on a first side by a front side imager shown schematically by box330, optionally referenced to an edge152, drum320grabs sheet154by reference edge152and turns the sheet over as indicated by arrow310. Sheet154rolls over drum320so that the rear surface becomes uppermost. However, during this flipping action, a trailing edge150of the sheet flips forward of reference edge152. 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 drum320is 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 drum320, sheet154moves in direction101over driven rollers110and120of a rotator system100. Rollers110and120are optionally overlaid by counter pressure rollers190and195respectively, to assure that rollers110and120drive sheet154. Until sheet154is positioned for inversion of the leading and trailing edges or partial rotation, as described below, the sheet is optionally driven by rollers110and120in direction101.

When the sheet is positioned for inversion of the leading and tailing edges of sheet154, rollers110and120are rotated such that they locally drive the sheet in directions112and122, causing sheet154to rotate in a direction130. With 180 degrees of rotation, reference edge152is restored to the lead position.

Optionally, after inverting the leading and trailing edges rollers110and120both rotate together in a direction to drive sheet154in direction101, until trailing edge150is released by rollers110and120. Alternatively or additionally, sheet154may be conveyed directly after rotation by other means for example, by conveyor102. Conveyor102may 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 rollers110and120are independently rotatable and/or driven.

FIG. 1Bis a side view of a portion of rotator100, showing roller110positioned against sheet154and counter roll190pressing sheet154against roller110, thereby preventing slippage of sheet154as roller110rotates. In an exemplary embodiment, counter roller190is driveless, rotating as a result of friction with sheet154. Optionally, counter roller190may have two or more degrees of freedom and, for example, may have a spherical surface, to avoid slippage as sheet154is rotated.

During conveying, sheet154may be skewed, especially as the sheet moves at high speeds. When skewing occurs prior to entering an imager, for example front side imager330, the resultant image is skewed with respect to sheet154.

FIG. 2is a schematic aerial view of a skewed edge sensor system200comprising sensors210and220that sense the position of leading edge152after inversion of the leading and trailing edges. In an exemplary embodiment, sensors220and210are connected to a controller230that controls the rotation of independently driven rollers110and120. When controller230senses a skew along reference edge152(for example, determining that the sheet passes the sensors at different times) controller230directs rollers110and/or120to correct the skew. For example, when corner252is forward of corner254, controller230directs roller110to briefly drive the sheet in direction112and/or roller120to briefly drive the sheet in direction122. As above. Sheet154rotates in direction130until reference edge152is no longer skewed.

While skewed edge sensor system200and rotator100are shown located upstream of rear side imager330, they could be located anywhere along conveyor102. For example they may be located prior to rear imager332(FIG. 1A) or prior to any station, a station comprising any sheet processor, for example inverter320or a sheet stacker mechanism (not shown).

Reversing the leading and trailing edges using rollers110and120can take with the sheet located at substantially any position along the length of sheet154. If only a single size sheet is used, then, in an embodiment of the invention, a sensor or sensors, such as sensors210,220ofFIG. 2are used to sense when the leading and trailing edges should be reversed. Until the sheet reaches the sensor(s), rollers110and120both drive the sheet in direction101, 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 sheet154has a different length other than nominal, after rotation, edge150is in a different position formerly occupied by reference edge152. As a result, the front and rear images may be imaged at different distances from reference edge152, 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. 3is an aerial view of a system utilizing a trailing edge sensor310located along conveyor102in a duplex imager, in accordance with an exemplary embodiment of the present invention. In the illustration sheet354is a “short” sheet. Following reversal of the leading and trailing edges during a prior flipping of the sheet, a trailing edge350passes trailing edge sensor310. The passage generates a signal that controller230utilizes to initiate rotation of short sheet354by 180 degrees, using rollers120and110. Solid lines show the position of short sheet354and edge350prior to rotation while broken lines show the position of short sheet354A and edge350A 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 distances360and370. Distance360is the distance of the trailing edge from the rollers110,120, when rotation is instituted by trailing edge sensor310. After rotation, the edge350has been repositioned to position350A, a distance370from the rollers. Since360is substantially the same as distance370and since the distance360is not dependent on the length of the sheet, position350A will not depend on the length of the sheet.

FIG. 4is an aerial view of a system400for aligning sheets154, even when grossly misaligned. System100comprises a trajectory offset mechanism100and an alignment mechanism450.

Alignment mechanism450comprises a guide140aligned with imager332, and a sheet transverse offset mechanism448, which pushes sheet154against guide rail, so that the sheet enters imager332at a correct transverse (to motion direction101) position. The inventors have found that to facilitate transverse alignment of the sheet, the sheet should be at least some minimum distance (designated as446onFIG. 4) from guide140. When this distance is too small, there is a tendency for the sheet to override guide140or be otherwise unaligned. Such lack of alignment can cause jamming of sheet154in imager332or improper placement of images on sheet154.

In an exemplary embodiment, trajectory offset mechanism100acts on sheet154to offset a first side edge444from guide rail140by an offset distance446. In an exemplary embodiment, offset mechanism100creates sufficient offset distance446between edge444and rail140so that even a grossly skewed sheet is properly positioned. Prior to entering imager332, sheet side offset mechanism448presses against side402of sheet154, causing side444of the sheet to contact guide rail140, and to be aligned with guide rail140and also with imager332.

The means by which transverse offset mechanism100offsets sheet154from rail140may comprise any of a number of options. For example, the midpoint between rollers110and120may not align with the midpoint of sheet154as it enters these rollers. As rollers110and120rotate sheet154by 180 degrees, sheet154is offset laterally to the general direction of motion101. 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 rollers110and120. This will also cause transverse offset of the sheet.

Mechanism100can 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 system400is shown prior to imager332, 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.