Tandem printing system with fine paper-position correction

Information

  • Patent Grant
  • 6823786
  • Patent Number
    6,823,786
  • Date Filed
    Monday, August 5, 2002
    21 years ago
  • Date Issued
    Tuesday, November 30, 2004
    19 years ago
Abstract
A tandem printer with a mechanism for fine substrate-position correction, comprising: a first printing station; a second printing station; a rotatable element, rotating at a given rotation rate, that receives the substrate after printing thereon by the first printing station and transfer the substrate toward the second printing station; a sensor which measure the position of an edge of the substrate during its transfer from the first printing station to the second printing station; and a controller, which applies a corrective step change in angular position of the rotatable element responsive to the measurements of the sensor, without changing the general rotation rate of the rotatable element.
Description




FIELD OF THE INVENTION




The present invention relates generally to printing systems and more particularly to tandem printing systems for printing variable information using two or more printing stations and including a paper-position correction mechanism.




BACKGROUND OF THE INVENTION




Tandem printing systems, that is printing systems with two or more printing engines, are well known, both for duplex printing and for multi-colour, single-side printing, with each colour being printed with a different one of a tandem series of printing engines. Such systems are known both for conventional and electronic printing. However, such systems depend on a very accurate transmission and edge alignment of the paper.




In systems of rollers and belts, a mismatch in transmission and edge alignment may occur for various reasons. The rollers may be slightly off centre, the paper may slip or creep off is its hold, there may be diameter variations, for example, because of different paper thickness and the belts may stretch as a result of heat or age. To achieve the accuracy required in tandem printing, these misalignments must be corrected.




SUMMARY OF THE INVENTION




One aspect of some preferred embodiments of the invention relates to a tandem printer with a mechanism for fine paper-position correction.




Preferably, the tandem printer has a motive system comprising a rotatable element which rotates at a given rotation rate and on which the paper is mounted, wherein a step angular displacement to the rotatable element brings the paper into alignment, without changing the rotation rate of the rotatable element.




Preferably, a paper sensor measures the position of the paper. The measurements are reported to a controller which applies the step, angular displacement to the rotatable element, in addition to the continuous rotation, responsive to the measurements of the sensor.




Preferably, the motive system also comprises a flexible strip, for example, a timing belt, which transfers motion at a constant rotation rate from a driving roller to the rotatable element, wherein a step displacement to the flexible strip induces the step angular displacement to the rotatable element. The axis of the flexible strip is defined as the line connecting the centre of the rotatable element and the driving roller.




Preferably, the step displacement of the flexible strip is provided by linear motion of two pulleys located upstream and downstream of the rotatable element, wherein as one pulley presses onto the flexible strip, requiring slack, the other pulley pulls away from the flexible strip, releasing slack. The transfer of flexible-strip slack from one pulley to the other provides the step displacement of the flexible strip.




Preferably, the tandem printer comprises means for providing the required linear motion to the pulleys.




In some preferred embodiments of the invention, means for providing linear motion to the pulleys comprises a rod having two edges on which the two pulleys are mounted. The rod is preferably situated perpendicular to the axis of the flexible strip, with one pulley, at one edge, pressing against the flexible strip upstream of the rotatable element, at a first point, and the other pulley, at the other edge, pressing against the flexible strip downstream of the rotatable element, at a second point. Movement of the rod up and down in a direction generally, perpendicular to the axis of the flexible strip provides the required linear motion to the pulleys.




Alternatively, means for providing linear motion to the pulleys comprises a shaft on which one pulley is mounted, pressing against the flexible strip at a first point and a spring-loaded device (for example, a spring-loaded piston-cylinder device) on which the other pulley is mounted, in partial compression, pressing against the flexible strip at a second point, wherein the first and second points are upstream and downstream of the rotatable element, in any order, reasonably far from any rotating elements associated with the flexible strip. Linear movement of the shaft provides the linear motion to the pulley mounted on it. The response of the spring-loaded device to the release or demand in slack provides the motion of the other pulley.




Preferably, a stepper motor provides the motion for the pulleys. In some preferred embodiments an eccentric shaft is used to convert the motor motion to linear motion. Alternatively, any of a slider-crank mechanism, a piston-cylinder mechanism, or a turning-screw mechanism may be used. Alternatively still, any other method of providing linear motion, known to persons versed in kinematics, may be used.




Preferably, the motor is activated by a controller which determines when the paper is out of alignment and the magnitude and direction of the misalignment.




In some preferred embodiments, the section of the flexible strip adjacent to the first point and the section of the flexible strip adjacent to the second point are parallel. For small displacements, the step angular displacement of the rotatable element is symmetric for upward and downward linear displacements of the pulleys.




Preferably, the step angular displacement of the rotatable element is given as a function (which may be empirical) of the linear displacement of the pulleys. Alternatively, a lookup table is used.




In some preferred embodiments of the invention, the tandem printer comprises a duplex printer for printing on both sides of paper while inverting it. Alternatively, the tandem printer comprises a multicolour printer of single side printing, with each colour being printed with a different one of the tandem series of printing engines.




In some preferred embodiments of the invention, the tandem printer comprises any conventional printer, such as a printer which prints directly from plates. Alternatively, the tandem printer comprises any of a lithographic printer, an electrostatic printer, or an electronic printer.




There is thus provided, in accordance with a preferred embodiment of the invention a tandem printer with a mechanism for fine substrate-position correction, comprising:




a first printing station;




a second printing station;




a rotatable element, rotating at a given rotation rate, that receives the substrate after printing thereon by the first printing station and transfers the substrate toward the second printing station;




a sensor which measures the position of an edge of the substrate during its transfer from the first printing station to the second printing station; and




a controller, which applies a corrective step change in angular position of the rotatable element responsive to the measurements of the sensor, without changing the general rotation rate of the rotatable element.




Preferably, the sensor which measures the position of an edge of the substrate is situated on the rotatable element.




Alternatively, the sensor which measures the position of an edge of the substrate is adjacent to the rotatable element.




In a preferred embodiment of the invention, the tandem printer also comprises:




a transfer system which transfers the substrate from the first printing station to the second printing station, in which the rotatable element is comprised;




the transfer system further comprising:




a flexible strip, travelling at a given rate and providing motion to the rotatable element, wherein a corrective step displacement of the flexible strip induces the corrective step change in angular position of the rotatable element.




Preferably, the flexible strip rotates at a constant rate.




Preferably, the flexible strip is a timing belt.




In a preferred embodiment of the invention, the tandem printer also comprises at least one pulley that provides the corrective step displacement of the flexible strip.




In a preferred embodiment of the invention, the at least one pulley comprises:




two pulleys, situated along the flexible strip, one upstream and one downstream of the rotatable element, said pulleys pressing into the flexible strip at a first point and a second point, respectively, wherein when pressure of one pulley is partially released, the other pulley takes up the thus produced slack, providing the corrective step displacement of the flexible strip.




In a preferred embodiment of the invention, the tandem printer also comprises a rod, comprising two points, to which the two pulleys are attached, one at each edge, wherein linear movement of the rod provides the motion of the pulleys into and away from the flexible strip.




Preferably, the tandem printer also includes a motion provider for the rod, comprising:




an eccentric shaft to which the rod is attached; and




a motor which provides motion to the eccentric shaft,




wherein the motor is activated by the controller.




Alternatively, the tandem printer includes a motion provider for the rod, comprising:




a slider-crank mechanism, wherein the rod is attached to the slider and moves in the same direction as the slider; and




a motor which provides motion to the slider-crank mechanism,




wherein the motor is activated by the controller.




Alternatively, the tandem printer includes a motion provider for the rod, comprising:




a piston-cylinder mechanism, wherein the rod is attached to the piston and moves in the same direction as the piston; and




a motor which provides motion to the piston-cylinder mechanism,




wherein the motor is activated by the controller.




Alternatively still, the tandem printer includes a motion provider for the rod, comprising:




a turning-screw mechanism, wherein the rod is attached to the screw and moves in the same direction as the screw; and




a motor which provides motion to the turning-screw mechanism,




wherein the motor is activated by the controller.




Preferably, the motor is a stepper motor.




In another preferred embodiment of the invention, the tandem printer also comprises:




a shaft on which one of the two pulleys is mounted, pressing against the flexible strip at a first point; and




a resilient device on which the other pulley is mounted, resiliently pressing against the flexible strip at a second point,




wherein linear movement of the shaft provides motion of the pulley at the first point, and the response of the resilient device to release or demand in slack provides motion of the pulley at the second point.




Preferably, the shaft is an eccentric shaft and including:




a motor which provides motion to the eccentric shaft,




wherein the motor is activated by the controller.




Preferably, the tandem printer also includes a motion provider for the shaft, comprising:




a slider-crank mechanism, wherein the shaft is connected to the slider and moves in the same direction as the slider, and




a motor which provides motion to the slider-crank mechanism,




wherein the motor is activated by the controller.




Alternatively, the tandem printer also includes a motion provider for the shaft, comprising:




a piston-cylinder mechanism, wherein the shaft is connected to the piston and moves in the same direction as the piston; and




a motor which provides motion to the piston-cylinder mechanism,




wherein the motor is activated by the controller.




Alternatively still, the tandem printer also includes a motion provider for the shaft, comprising:




a turning-screw mechanism, wherein the shaft is connected to the screw and moves in the same direction as the screw; and




a motor which provides motion to the turning-screw mechanism,




wherein the motor is activated by the controller.




Preferably, the motor is a stepper motor.




In a preferred embodiment of the invention, the two pulleys are substantially identical.




In a preferred embodiment of the invention, the section of the flexible strip adjacent to the first point and a section of the flexible strip adjacent to the second point are parallel to each other.




In a preferred embodiment of the invention, the tandem printer comprises a duplex printer for printing on both sides of the paper while inverting it.




Alternatively, the tandem printer comprises a multicolour printer of single-side printing, with each colour being printed with a different one of the tandem series of printing engines.




In a preferred embodiment of the invention, the tandem printer is a conventional printer which uses plates.




Alternatively, the tandem printer is an electrostatic printer.




Alternatively, the tandem printer is an electronic printer.




Alternatively, the tandem printer is a lithographic printer.




In a preferred embodiment of the invention, a multi-engine printer with a mechanism for fine substrate-position correction, is provided, comprising at least 3 printing engines, wherein each adjacent pair of printing engines comprises a first printing station and a second printing station, of the tandem printer described herein.




There is further provided, in accordance with a preferred embodiment of the invention a tandem printing method while applying a fine positional correction to a substrate, comprising:




printing on a substrate by a first printing station;




transferring the substrate from the first printing station toward a second printing station, comprising:




mounting the substrate on a rotatable element of a substrate-transfer system; and




moving the substrate by rotating the rotatable element at a given rotation rate;




measuring the angular position of an edge of the substrate on the rotatable element; and




applying a step angular displacement to the rotatable element, responsive to the measurement, without changing the rotation rate of the rotatable element.




Preferably, rotating the rotatable element at a given rotation rate comprises rotating the rotatable element by a flexible strip; and




applying the step angular displacement to the rotatable element comprises applying a step displacement to the flexible strip.




Preferably, applying the step displacement to the flexible strip comprises any of a positive and negative step displacements to the flexible strip, thus inducing any of a clockwise and a counterclockwise step angular displacements to the rotatable element.




Preferably, applying the positive step displacement to the flexible strip comprises:




releasing flexible strip slack upstream of the rotatable element; and




taking up flexible strip slack downstream of the rotatable element; and




applying the negative step displacement to the flexible strip comprises:




releasing flexible strip slack downstream of the rotatable element; and




taking up flexible strip slack upstream of the rotatable element.




Preferably, the method also includes using a lookup table to calculate a necessary step displacement of the flexible strip in order to achieve a desired step angular displacement of the rotatable element











BRIEF DESCRIPTION OF THE DRAWINGS




The present invention will be more clearly understood from the following detailed description of the preferred embodiments of the invention and from the attached drawings, in which same number designations are maintained throughout the figures for each element and in which:





FIG. 1

is a schematic illustration of a tandem printer comprising a correctional mechanism for correcting a paper position, in accordance with a preferred embodiment of the invention; and





FIG. 2

is a schematic illustration of a tandem printer comprising another correctional mechanism for correcting a paper position, in accordance with another preferred embodiment of the invention.











DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS




Reference is now made to

FIG. 1

which is a schematic illustration of a tandem printer


10


having a roller assembly


20


for paper inversion and transfer and a correctional mechanism


30


, in accordance with a preferred embodiment of the invention.




Preferably, tandem printer


10


comprises a first printing station


11


, comprising an impression roller


12


and at least one first printing engine


16


associated with it, and a second printing station


13


, comprising an impression roller


14


and at least one second printing engine


18


associated with it. In the system shown, each of printing engines


16


and


18


comprises an intermediate transfer member (ITM)


15


and


17


respectively. The image is transferred to ITM or


17


and then to paper


40


on the respective impression roller. Alternatively, no ITM is used, and each of printing engine


16


and


18


comprises a photoconductive drum


15


or


17


respectively. The image is preferably formed on photoconductive drum


15


or


17


and transferred to paper


40


on the impression roller. For two side printing, one side of a paper


40


is printed while it is on impression roller


12


, by first printing engine


16


, and an opposite side of paper


40


is printed while it is on impression roller


14


, by second printing engine


18


. For multicolour, single side printing, each colour is printed with a different one of the tandem series of printing engines, such as printing engines


16


and


18


.




In a preferred embodiment of the invention, roller assembly


20


inverts the paper and transfers it from impression roller


12


to impression roller


14


. In a preferred embodiment of the invention, the “rollers” of roller assembly


20


are rotary arms, rather than rollers, each with vacuum nipples that attach themselves to the paper. Generally, the nipples are evenly distributed along the length of the paper and extend through the width of it. The paper pick-off system, which removes the paper from one roller and transfers it to the other, comprises a vacuum pick-off system, as one set of nipples lets go and the other takes over. The rotary-arm “rollers” and the vacuum nipples and vacuum paper pick-off system associated with them are described in PCT patent application, PCT/IL98/00553, “Printing System” filed on Nov. 11, 1998, in the Israel receiving office, whose disclosure is incorporated herein by reference. Alternatively, conventional rollers may be used. The exact configuration of the paper transfer and (or) inversion system may differ from that shown, since the inversion is applicable to many transfer/inversion and perfecta or perfecta-like systems known in the art.




Typically, after the first image (which may be coloured or black and white) is transferred onto the first side of paper


40


, paper


40


is transferred from impression roller


12


to a roller R


1


of assembly


20


. From roller R


1


, which rotates in a clockwise direction, paper


40


is transferred to a perfecting roller R


2


, which, during the transfer from roller R


1


to roller R


2


, rotates in the counterclockwise direction.




Preferably, perfecting roller R


2


is controlled by a servo motor (not shown) capable of moving at different angular velocities, clockwise and counterclockwise. Preferably, the purpose of perfecting roller R


2


is to invert the paper. Additionally, perfecting roller R


2


corrects the position of paper


40


for the following reason: Impression rollers


12


and


14


are each controlled by a separate engine and slight errors are introduced by a system that controls their relative rotation. Perfecting roller R


2


corrects for slight variations in angular velocities and in phases, responsive to measurements of a first paper sensor


22


which senses the position of the leading edge of paper


40


(before inversion). An error of 1-2 mm in the position of the paper may be encountered and corrected by the servo motor using the following method:




After receiving paper


40


, servo-controlled roller R


2


changes its direction to clockwise, and changes its velocity to correct for any error in the position of paper


40


, bringing the trailing edge of paper


40


exactly to a pick-up point of a roller R


3


, rotatable counterclockwise.




As roller R


3


picks up the trailing edge of paper


40


, the trailing edge becomes the new leading edge, and paper


40


is inverted. It is noted that the paper is still referenced to its original leading edge.




The next transfer, from roller R


3


, rotatable counterclockwise to a roller R


4


, rotatable clockwise, involves a second paper sensor


24


which determines the position of the new tailing edge (previous leading edge) of paper


40


, close to the transfer point from R


3


, as it is transferred onto roller R


4


. Second paper sensor


24


may be situated on roller R


4


or it may be adjacent to roller R


4


. From roller R


4


, the paper is transferred to impression roller


14


. Exact synchronisation between the image on impression roller


14


and paper


40


cannot be performed when paper


40


reaches impression roller


14


; in general, the image is already on ITM (or photoconductive drum)


17


to produce an image on the second side of paper


40


when it reaches impression roller


14


at a precise time and angular position of impression roller


14


. Thus, any correction in synchronisation must be made to the position of paper


40


before it reaches impression roller


14


.




In the following discussion, the term pulley, as used here, refers to a wheel, possibly with a grooved rim, in direct contact with the flexible strip, preferably, a timing belt, to drive it,or to be driven by it. The term tension pulley refers to a wheel, possibly with a grooved rim, in direct contact with the flexible strip, to keep the flexible strip under tension. Preferably, roller R


4


and a roller R


5


are in communication with each other through a flexible strip


26


, wherein a driving pulley


28


is mounted on roller R


5


and moving continuously with it at a constant rotation rate, and a driven pulley


32


is mounted on roller R


4


and moving continuously with it at the same (or proportional) constant rotation rate as that of R


5


. As used herein, the generic term flexible strip means a smooth belt, or a timing belt, or a cable, or a bead cable, or an endless chain. The term pulley may mean a sprocket




An axis


27


of flexible strip


26


is defined as the line connecting the centres of R


4


and R


5


. A tension pulley


34


serves as a tension to flexible strip


26


. (Tension pulley


34


is optional, and may be eliminated.) Alternatively, two or more tension pulleys


34


may be used.) Correctional mechanism


30


is associated with flexible strip


26


and roller R


4


. Note that correction mechanism


30


does not affect roller R


5


, the driver, since the motion of roller R


5


is controlled by a driving mechanism (not shown), which is commonly driven by, or synchronously driven with impression rollers


14


and/or


12


, at a constant rotation rate. Only driven pulley


32


of roller R


4


is free to respond to corrections.




Preferably, correctional mechanism


30


comprises a correctional tension pulley


38


, mounted on an shaft


46


. Motion is provided by a stepper motor


42


, which moves shaft


46


and correctional tension pulley


38


so that they travel up or down a specific amount. At an “equilibrium position”, correctional tension pulley


38


presses against flexible strip


26


at a first point, downstream of roller R


4


, producing an indentation in the profile of flexible strip


26


. A tension pulley


44


mounted on a spring-loaded device


45


in partial compression, presses against flexible strip


26


at a second point, upstream of roller R


4


, and produces a second indentation in the profile of flexible strip


26


. Spring loaded device


45


may be a spring-loaded piston-cylinder device. Alternatively, another spring-loaded device may be used. Alternatively, another method of resiliently pressing tension pulley


44


against flexible strip


26


may be used.




Preferably, when second paper sensor


24


determines that a positional correction to paper


40


is required, the controller activates stepper motor


42


which drives shaft


46


and correctional tension pulley


38


. Step angular displacement of roller R


4


is provided as follows:




When shaft


46


moves up, pressing correctional tension pulley


38


deeper against flexible strip


26


at the first point, downstream of roller R


4


, a demand for slack at the first point is created. In response, spring-loaded device


45


on which tension pulley


44


is mounted, compresses, pulling tension pulley


44


away from flexible strip


26


and releasing the slack that is to needed downstream. Flexible strip


26


moves in a clockwise direction, producing a step angular displacement to roller R


4


in the clockwise direction.




When shaft


46


drives correctional tension pulley


38


away from flexible strip


26


, slack in flexible strip


26


is created at the first point, downstream of roller R


4


. Spring compression of spring-loaded device


45


is released somewhat; tension pulley


44


presses deeper against flexible is strip


26


at the second point, gathering the slack that was released downstream. Flexible strip


26


moves a step in a counterclockwise direction, producing a step angular displacement to roller R


4


in the counterclockwise direction. Note that the pressure of spring-loaded tension pulley


44


on flexible strip


26


is such that pulley


38


is always in contact with flexible strip


26


. Note also that the situation described may be reversed; pulley


38


my be spring loaded or otherwise resiliently pressed against flexible strip


26


and tension pulley


44


may be driven by shaft


46


.




Reference is now made to

FIG. 2

which is a schematic illustration of a tandem printer


60


having a roller assembly


20


for paper inversion and transfer and another correctional mechanism


50


, in accordance with another preferred embodiment of the invention.




In

FIG. 2

, the step linear displacement of flexible strip


26


is provided by two correctional tension pulleys


48


and


52


, mounted on the two ends of a rod


54


and connected to a shaft (not shown) which is driven by stepper motor


42


. Rod


54


is situated inside flexible strip


26


, perpendicular to axis


27


, with correctional tension pulleys


48


and


52


pressing against flexible strip


26


at two points, A and B, upstream and downstream of roller R


4


.




Preferably, step angular displacement of roller R


4


is provided as follows:




When rod


54


moves up or down, one or the other of correction pulleys


48


or


52


is pressed deeper against flexible strip


26


, requiring more slack, while the other is pulled away from flexible strip


26


, releasing slack The transfer of slack provides the step displacement of flexible strip


26


, in the direction of the increased indentation, producing a step angular displacement to roller R


4


in that direction.




In some preferred embodiments of the invention, as shown, correctional tension pulleys


48


and


52


are external to flexible strip


26


. Alternatively, they are internal to flexible strip


26


. Note that pulleys


48


and


52


are always in contact with flexible strip


26


. In some preferred embodiments of the invention, two tension pulleys


34


are used. Alternatively, only one tension pulley


34


is used. Alternatively still, no tension pulley is used.




The following discussion applies to the embodiments of both

FIGS. 1 and 2

.




Preferably, the step angular displacement of roller R


4


is given as a function (which may be empirical) of the linear displacement. Alternatively, the step angular displacement of roller R


4


is determined from a lookup table 100.




In some preferred embodiments, the section of the flexible strip adjacent to the first point and the section of the flexible strip adjacent to the second point are parallel.




For some configurations, small step angular displacements of roller R


4


are symmetric for upward and downward displacements of tension pulleys


38


and


44


or tension pulleys


48


and;


52


. More generally, they are not.




Preferably, there is only one sheet of paper on roller R


4


at any time. Preferably, the total travel time of paper


40


on roller R


4


is about 0.4-0.5 seconds, and the order of magnitude of the correction time by correctional mechanism


30


is 0.05-0.1 seconds. Preferably, the order of magnitude of the positional correction of paper


40


by correctional mechanism


30


is about 0.5 mm.




In some preferred embodiments, roller assembly


20


comprises more rollers or fewer rollers, depending on the distances between impression roller


12


and impressions roller


14


.




In some preferred embodiments, a multi-printing system, comprising more than two printing engines, may be used, wherein a correctional mechanism such as correctional mechanism


30


or correctional mechanism


50


is positioned between any two adjacent printing engines.




In some preferred embodiments an eccentric shaft is used to convert the motion of stepper motor


42


to linear motion. Alternatively, any of a slider-crank mechanism, a piston-cylinder mechanism, or a turning-screw mechanism may be used. Alternatively still, any other method of providing linear motion, known to persons versed in kinematics, may be used.




In some preferred embodiments of the invention, as shown in

FIGS. 1 and 2

, the tandem printer comprises a duplex printer for printing on both sides of paper while inverting it Alternatively, the tandem printer comprises a multicolour printer of single side printing, with each colour being printed with a different one of the tandem series of printing engines. Where the tandem printer is a multicolour printer of single side printing, perfecting roller R


2


and first paper sensor


22


are eliminated as the paper is not inverted. Alternatively, an additional roller (not shown) is provided and an operator may choose, preferably, with a keystroke at a control panel, whether to invert the paper, wherein the paper then passes through perfecting roller R


2


, or not to invert the paper, wherein the paper then passes through the additional roller.




In some preferred embodiments of the invention, the tandem printer comprises any conventional printer, such as a printer which prints directly from plates. Alternatively, the tandem printer comprises any of a lithographic printer, an electrostatic printer, or an electronic printer.




The present invention has been described using non-limiting detailed descriptions of preferred embodiments thereof that are provided by way of examples and are not intended to limit the scope of the invention. Variations of embodiments described will occur to persons of the art. Similarly, combinations of features of different embodiments within the scope of the claims will occur to persons of the art. These are still within the scope of the invention. The terms “comprise,” include,” and “have” or their conjugates, when used herein, mean “including but not necessarily limited to.” The scope of the invention is limited only by the following claims:



Claims
  • 1. A tandem printer with a mechanism for fine substrate-position correction, comprising:a fit printing station; a second printing station; a transfer system which transfers the substrate from the first printing station to the second printing station, comprising: a rotatable element, rotating at a given rotation rate, that receives the substrate after printing thereon by the first printing station and transfers the substrate toward the second printing station; a sensor which measures the position of an edge of the substrate during its transfer from the first printing station to the second printing station; a flexible strip, traveling at a given rate and providing motion to the rotatable element, wherein a corrective step displacement of the flexible strip induces a corrective step change in angular position of the rotatable element; a controller, which applies a corrective step change in angular position of the rotatable element responsive to the measurements of the sensor, without changing the general rotation rate of the rotatable element via a step displacement of the flexible strip; and two pulleys, situated along the flexible strip; one upstream and one downstream of the rotatable element, said pulleys pressing into the flexible strip at a first point and a second point, respectively, wherein when pressure of one pulley is partially released, the other pulley takes up the thus produced slack, providing a corrective step displacement of the flexible strip.
  • 2. A tandem printer according to claim 1 wherein the sensor which measures the position of an edge of the substrate is situated on the rotatable element.
  • 3. A tandem printer according to claim 1 wherein the sensor which measure the position of an edge of the substrate is adjacent to the rotatable element.
  • 4. A tandem printer according to claim 1 wherein the flexible strip rotates at a constant rate.
  • 5. A tandem printer according to claim 1 wherein the flexible strip is a timing belt.
  • 6. A tandem printer according to claim 1 and comprising:a rod, comprising two points, to which the two pulleys are attached, one at each edge, wherein linear movement of the rod provides the motion of the pulleys into and away from the flexible strip.
  • 7. A tandem printer according to claim 6 and including a motion provider for the rod, comprising:an eccentric shaft to which the rod is attached; and a motor which provides motion to the eccentric shaft, wherein the motor is activated by the controller.
  • 8. A tandem printer according to claim 6 and including a motion provider for the rod, comprising:a slider-crank mechanism, wherein the rod is attached to the slider and moves in the same direction as the slider; and a motor which provides motion to the slider-crank mechanism, wherein the motor is activated by the controller.
  • 9. A tandem printer according to claim 6 and including a motion provider for the rod, comprising:a piston-cylinder mechanism, wherein the rod is attached to the piston and moves in the same direction as the piston; and a motor which provides motion to the piston-cylinder mechanism, wherein the motor is activated by the controller.
  • 10. A tandem printer according to claim 6 and including a motion provider for the rod, comprising:a tuning-screw mechanism, wherein the rod is attached to the screw and moves in the same direction as the screw; and a motor which provides motion to the turning-screw mechanism, wherein the motor is activated by the controller.
  • 11. A tandem printer according to claim 1 and comprising:a shaft on which one of the two pulleys is mounted pressing against the flexible strip at a first point; and a resilient device on which the other pulley is mounted, resiliently pressing against the flexible strip at a second point, wherein linear movement of the shaft provides motion of the pulley at the first point, and the response of the resilient device to release or demand in slack provides motion of the pulley at the second point.
  • 12. A tandem printer according to claim 11, wherein the shaft is an eccentric shaft and including:a motor which provides motion to the eccentric shaft, wherein the motor is activated by the controller.
  • 13. A tandem printer according to claim 11 and including a motion provider for the shaft, comprising:a slider-crank mechanism, wherein the shaft is connected to the slider and moves in the same direction as the slider; and a motor which provides motion to the slider-crank mechanism, wherein the motor is activated by the controller.
  • 14. A tandem printer according to claim 11 and including a motion provider for the shaft, comprising:a piston-cylinder mechanism, wherein the shaft is connected to the piston and moves in the same direction as the piston; and a motor which provides motion to the piston-cylinder mechanism, wherein the motor is activated by the controller.
  • 15. A tandem printer according to claim 11 and including a motion provider for the shaft, comprising:a turning-screw mechanism, wherein the shaft is connected to the screw and moves in the same direction as the screw; and a motor which provides motion to the turning-screw mechanism, wherein the motor is activated by the controller.
  • 16. A tandem printer according to claim 1 wherein the two pulleys are substantially identical.
  • 17. A tandem printer according to claim 1, wherein the section of the flexible strip adjacent to the first point and a section of the flexible strip adjacent to the second point are parallel to each other.
  • 18. A tandem printer according to claim 1, wherein the tandem printer comprises a duplex printer for printing on both sides of the paper while inverting it.
  • 19. A tandem printer according to claim 1, wherein the tandem printer comprises a multicolour printer of single-side printing, with each colour being printed with a different one of the tandem series of printing engines.
  • 20. A tandem printer according to claim 1, wherein the tandem printer is a conventional printer which uses plates.
  • 21. A tandem printer according to claim 1, wherein the tandem printer is an electrostatic printer.
  • 22. A tandem printer according to claim 1, wherein the tandem printer is an electronic printer.
  • 23. A tandem printer according to claim 1, wherein the tandem printer is a lithographic printer.
  • 24. A method of applying a fine positional correction to a substrate on a tandem printer, comprising:printing on a substrate by a first printing station; transferring the substrate from the first printing station toward a second printing station, comprising: mounting the substrate on a rotatable element of a substrate-transfer system; and moving the substrate by rotating the rotatable element at a given rotation rate; measuring the angular position of an edge of the substrate on the rotatable element; and applying a step angular displacement to the rotatable element, responsive to the measurement, without changing the rotation rate of the rotatable element, wherein rotating the rotatable element at a given rotation rate comprises rotating the rotatable element by a flexible strip; wherein applying the step angular displacement to the rotatable element comprises applying a step displacement to the flexible strip, comprising either a positive and negative step displacement to the flexible strip, thus inducing a clockwise and a counterclockwise step angular displacements to the rotatable element; and applying the positive step displacement to the flexible strip comprises: releasing flexible strip slack upstream of the rotatable element; and taking up flexible strip slack downstream of the rotatable element; and wherein applying the negative step displacement to the flexible strip comprises: releasing flexible strip slack downstream of the rotatable element; and taking up flexible strip slack upstream of the rotatable element.
  • 25. A method according to claim 24 and including using a lookup table to calculate a necessary step displacement of the flexible strip in order to achieve a desired step angular displacement of the rotatable element.
  • 26. A method according to claim 24 wherein the flexible strip is driven by a driving pulley, rotating at a constant rotation rate.
  • 27. A method according to claim 24 wherein the flexible strip is a timing belt.
RELATED APPLICATIONS

The present application is a U.S. national application of PCT/IL99/00600, filed 7 Nov. 1999.

PCT Information
Filing Document Filing Date Country Kind
PCT/IL99/00600 WO 00
Publishing Document Publishing Date Country Kind
WO01/34397 5/17/2001 WO A
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