Abstract
When duplex printing non-rectangular sheets of printable media, the sheets require proper registration to prevent misalignment of front and back images. This is ensured by: determining relative orientations of three different edges of a sheet being transported towards print station on a first pass; registering the sheet on the first pass by aligning one of the edges with respect to a predetermined alignment direction; printing a first image on a first side of the registered sheet; flipping the sheet on a second pass; registering the flipped sheet by aligning one of the edges with respect to a predetermined alignment direction; and printing a second image on a second side of the registered, flipped sheet aligned with the first image based on the determined relative orientations.
Claims
1. A method for registering sheets of printable media, comprising the steps of: determining relative orientations of three different edges of a sheet being transported towards a print station on a first pass; registering the sheet on the first pass by aligning one of said edges with respect to a predetermined alignment direction; printing a first image on a first side of the registered sheet; flipping the sheet on a second pass; registering the flipped sheet by aligning one of said edges with respect to a predetermined alignment direction; and printing a second image on a second side of the registered, flipped sheet aligned with the first image based on the determined relative orientations.
2. The method according to claim 1, wherein the alignment direction for registering the sheet is parallel to a transport direction of the sheet relative to the print station.
3. The method according to claim 1, further comprising: sensing an orientation of each of the three edges of the sheet, wherein the orientation of the aligned edge is sensed perpetually during registration.
4. The method according to claim 1, further comprising a step of minimizing a distance between consecutive sheets after registration.
5. The method according to claim 1, wherein a same lateral edge of the sheet is aligned with respect to a predetermined alignment direction on both the first and the second pass of the sheet after registration.
6. The method according to claim 1, further comprising the steps of: determining a printable area aligned on both sides of the sheet based on the determined relative orientations; and printing the first and second images on the registered sheet within said printable area.
7. The method according to claim 2, wherein a same lateral edge of the sheet is positioned at a same lateral position on both the first and the second pass of the sheet after registration.
8. The method according to claim 7, wherein the step of printing the first image comprises printing the first image offset by a predetermined spacing with respect to a reference point on one of said edges, and wherein the step of printing the second image comprises applying the relative orientations of the three different edges to convert the predetermined spacing for offsetting the second image with respect to a reference point on an edge opposite said one of said edges, such that the first and second images are aligned with one another.
9. The method according to claim 6, wherein the step of determining the printable area comprises maximizing the printable area within the surface of the sheet.
10. The method according to claim 9, wherein the printable area is a rectangle extending perpendicular to a lateral edge of the sheet.
11. The method according to claim 10, wherein two corners of the printable area are positioned on the aligned lateral edge, and: when the two corners of the sheet along the aligned lateral edge are greater than 90°, a third corner of the printable area is positioned on the other lateral edge of the sheet opposite the aligned lateral edge; or when at least one of the two corners of the sheet along the aligned lateral edge is less than 90°, a third corner of the printable area is positioned on the longitudinal edge of the sheet connected to the one of the two corners of the sheet, which has the smallest angle.
12. A printer comprising: a sheet registration device, the sheet registration device comprising: a sensor assembly configured for sensing relative orientations of three different edges of a sheet being transported through the registration device; and an actuator assembly configured for re-orienting the sheet, such that one of the edges of the sheet is aligned with a predetermined alignment direction; and a controller configured to: determine relative angle parameters corresponding to angles between at least two different pairs of said edges from data from the sensor assembly; and printing a second image on a second side of the sheet opposite a first side printed with a first image, wherein the second image is aligned with the first image based on the relative angle parameters.
13. The printer according to claim 12, wherein the sensor assembly comprises a pair of linear detector arrays spaced apart from one another in the transport direction and extending perpendicular to the transport direction for sensing a lateral edge of the sheet during registration.
14. The printer according to claim 13, wherein the controller is configured to determine a printable area by maximizing an area of a rectangle positioned within the sheet's edges and extending perpendicular to the one edge to be aligned with the alignment direction during registration.
15. The printer according to claim 14, wherein the actuator assembly comprises a pair of independently drivable steering rollers.
16. A sheet registration device for the printer according to claim 12, which comprises: a sensor assembly configured for sensing relative orientations of three different edges of a sheet being transported through the registration device; an actuator assembly configured for re-orienting the sheet, such that one of the edges of the sheet is aligned with a predetermined alignment direction; and a controller configured to: determine relative angle parameters corresponding to angles between at least two different pairs of said edges from data from the sensor assembly; and orienting the sheet on a second pass of the sheet along the sheet registration device, such that, when printing a second image on a second side of the sheet opposite a first side printed with a first image, the second image is aligned with the first image based on the relative angle parameters.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:
[0044] FIGS. 1A-2B schematically illustrate misalignment errors which may occur when using a sheet registration device according to the prior art;
[0045] FIG. 3 schematically illustrates a sheet registration device according to the present invention;
[0046] FIGS. 4A-4D schematically illustrate the steps by which the sheet registration device of FIG. 3 registers a sheet on both passes of the sheet;
[0047] FIG. 5 schematically illustrate the step of determining a printable area on the sheet;
[0048] FIG. 6A-B schematically illustrate the printable area determined by the step of FIG. 5;
[0049] FIG. 7 schematically illustrates the step of minimizing the distance between consecutive sheets; and
[0050] FIG. 8 is a block diagram schematically illustrating the steps of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0051] The present invention will now be described with reference to the accompanying drawings, wherein the same reference numerals have been used to identify the same or similar elements throughout the several views. It will appreciated that in the drawing the deformation of the sheets S has been exaggerated for illustrative purposes.
[0052] FIG. 1A illustrates the issues which occur when using a registering method according to the prior art. The sheet S in FIG. 1A is non-rectangular, for example due deviations during its cutting process. The sheet S in FIG. 1A has three non-straight corners, the bottom right corner being less than 90°. A front side image I1 has been printed on the sheet S after the sheet S was registered by aligning its leading edge (top edge in FIG. 1A) parallel to the lateral direction Y, which is perpendicular to the transport direction X in which the sheet S is conveyed along its transport path towards the print station (not shown). After printing the front side image I1, the sheet S is flipped, usually along a lateral axis in the direction Y, such that the unprinted side will face the print station. Again, the flipped sheet S is registered before reaching the print station by aligning its current leading edge (which was the trailing edge during its first pass along the print station) in the lateral direction Y. A back side image I2 is then printed on the registered, flipped sheet S. However, since the leading and trailing edges of the sheet S are not parallel, the back side image I2 is printed rotationally displaced with respect to the front side image I1. This rotational deviation will result in a visually unappealable print product. When for example processing the sheets S into a book, the pages will be cut again and bound together, but only the front side images I1 or only the back side images I2 will be properly aligned with the book's spine. The effect becomes even more obvious when using (semi)transparent print media, such as low grammage paper.
[0053] FIGS. 2A and 2B illustrate that front and back side image misalignment will still occur even when the trailing and leading edges of the sheet S are parallel. In FIG. 2A the lateral edges of the sheet S are not perpendicular to the trailing and leading edges. This results in a lateral shift of the back side image I2 with respect to the front side image I1, as the lateral edge position as well as the leading edge is used to determine the starting point for printing the images I1, I2.
[0054] The present invention proposes a sheet registration device 1, as shown in FIG. 3, which is able to properly align the front and back side images I1, I2 on non-rectangular sheets S, regardless of the relatively orientations of the different edges of the sheet S. The sheet registration device 1 comprises a sheet registration unit 6, 7 configured to re-orient and/or laterally shift the sheet S. The registration unit 6, 7 is preferably configured to rotate the sheet S around an axis perpendicular to the plane of the sheet S and/or to adjust the lateral position of the sheet S in the direction Y. The sheet registration device 6,7 in FIG. 3 comprises a pair of undependably drivable steering rollers 6, 7. By driving the steering rollers 6, 7 at different velocities, the sheet S may be rotated, as well as moved further along in the transport direction X. In this manner, the lateral position of the sheet S may be changed as well. The steering rollers 6, 7 may further be configured to rotate around an axis perpendicular to the directions X, Y to improve the efficiency of the registration movement. FIG. 3 further illustrates a pair of feed rollers 8, 9 upstream of the steering rollers 6, 7 in the transport direction X, which feed rollers 8, 9 are arranged to transport a sheet S onto the steering rollers 6, 7. It will be appreciated that every roller 6-9 may be part of a pinch, wherein a further roller is positioned opposite said roller 6-9, such that the sheet S is pinched between said roller 6-9 and the further roller for improved grip on and control of the sheet S.
[0055] A sensor assembly 2-5 is provided at the registration unit 6, 7 to determine the orientations of three or more different edges of the sheet S. Orientation is preferably defined as the direction in which the respective edge extends, specifically within the plane X, Y of the sheet S on the sheet registration device 1. The orientation may be expressed for example in terms of an angle or tangent with respect to the X or Y axis, or by any other suitable parameter.
[0056] The sensor assembly 2-5 comprises longitudinal edge sensors 2, 3 arranged to determine the orientation and/or direction of the trailing edge TE and/or leading edge LE of the sheet S. This is achieved by two spaced apart edge detection sensors 2, 3 provided at predetermined positions. The edge detection sensors 2, 3 may be formed by a light, such as a photodiode, and a (photo)detector to determine the passage of a longitudinal edge over said edge detection sensor 2, 3. From the timing of the passage of the respective edge and the predetermined positions of the longitudinal edge detection sensors 2, 3, the orientation of the leading edge LE may be determined. Similarly, the orientation of the trailing edge TE may be determined as it passes over the edge detection sensors 2, 3. Other suitable sensors such as contact sensors or (CCD) cameras may be applied for determining the orientation of the longitudinal edges LE, TE.
[0057] It will be appreciated that the sheet registration unit 6, 7 is used as an example and that other means for registering the sheet S may be applied within the present invention. These means may include for example an alignment wall protruding from a sheet support surface. The alignment wall is positionable at the intended lateral position and the sheet S is moved against the alignment with its lateral edge, such that said lateral edge orients parallel to the alignment wall. In another example, the alignment wall is moveable to shift and rotate the sheets S into the intended orientation and position.
[0058] The sensor assembly 2-5 further comprises lateral edge sensors 4, 5 arranged to determine the orientation and/or direction of at least one lateral edge SE (or side edge SE) of the sheet S. In FIG. 3, the lateral edge sensors 4, 5 are illustrated as linear detector arrays 4, 5, which are at different positions in the transport direction X. The lateral edge sensors 4, 5 extend in the lateral direction Y. In FIG. 3, the lateral edge sensors 4, 5 are formed by linear detector arrays 4, 5. These detector arrays 4,5 allow for the detection of the lateral sheet edge SE at different positions, such that the orientation of the lateral edge SE can be tracked during the registration movement. The initial orientation in which the lateral edge SE arrives at the linear detector arrays 4, 5 can be determined. The completion of aligning the lateral edge SE in the transport direction X can also be confirmed, as well as the sensing of any orientation in between these two states. It will be appreciated that different sensor assemblies may be applied for tracking the orientation of the lateral edge, such as a camera system combined with image recognition software.
[0059] The method of registering a sheet S by means of the sheet registration device 1 will be explained with regard to FIGS. 4A to 4D. The steps of the method are further illustrated in the block diagram in FIG. 8.
[0060] The non-rectangular sheet S arrives at the sheet registration device 1 in an initial orientation. The feed rollers 8, 9 receive the sheet S from an upstream portion of the transport path and move the sheet S further towards the steering rollers 6, 7. As the sheet S passes over the lateral edge sensors 4, 5 in FIG. 4A, the orientation of the lateral edge SE is detected. This orientation is stored on a memory of a controller of the respective printer, for example as a vector or an angle with respect to the X or Y axis.
[0061] When the leading edge LE passes over the longitudinal edge sensors 2, 3, the orientation of the leading edge LE with respect to the lateral edge SE is determined. This can be done by e.g. comparing the vectors of both edges SE, LE to one another to determine a parameter corresponding to the relative angle A1 between the leading edge LE and the lateral edge SE. It will be appreciate that the positioning of the linear sensor arrays 4, 5 allows both edges SE, LE to be detected simultaneously, thus eliminating any errors due to displacement of the sheet S between measurements. This positioning of the sensors 2-5 allows the registration movement to start as soon as the sheet S reaches the steering rollers 6, 7. The trailing edge TE is preferably sensed after the lateral edge SE has been aligned to the transport direction X, allowing for easy determining of the angle between these edges, SE, TE based on the data from the longitudinal edge sensors 2, 3.
[0062] In FIG. 4B, based on the determined initial orientation of the lateral edge SE, the controller instructs the steering rollers 6, 7 to apply suitable velocity profiles to different portions of the sheet S to align the lateral edge SE with the desired alignment direction. In this example, the alignment direction is parallel to the transport direction X for both passes of the sheet S. Herein the lateral edge SE is also positioned at a predetermined lateral position. During registration, the orientation of the lateral edge SE may be continuously tracked by the lateral edge sensors 4, 5 to verify its final alignment and/or perform additional corrections. It will be appreciate that the sheet S is in perpetual forward motion while in the sheet registration device 1 to achieve high throughput.
[0063] In FIG. 4C, the lateral edge SE has been position in the lateral direction Y and aligned parallel to the transport direction X. In FIG. 4C, the trailing edge TE passes over the longitudinal edge sensors 2, 3, such that the orientation of the trailing edge TE with respect to the lateral edge SE can be determined. This information is stored in a suitable parameter, for example as the relative angle A2. Additionally, a length (L in FIG. 5) of the lateral edge SE may be determined by the longitudinal edge sensors 4, 5 and stored onto a memory of the controller. In case the lateral edge SE has not been fully registered at the intended lateral position and/or orientation, the final orientation and/or lateral position of the lateral edge SE is detected by the lateral edge sensors 4, 5 and stored on a memory of the controller. The trailing edge TE is then sensed using longitudinal edge sensors 2, 3 and the determined orientation with respect to the stored orientation of the lateral edge SE can be determined. In consequence, the back side image I2 can be properly aligned regardless whether the registration of the sheet S is executed fully as intended. Any deviations from the intended registration can be corrected using data from the sensors 2-5. It will be appreciated that additional longitudinal edge sensors may be provided on the sheet registration device.
[0064] Based on the parameters for the relative angles A1, A2 and the length L of the lateral edge LE, the printable area (PA in FIG. 5) for both sides of the sheet S is determined and preferably maximized. This step will be illustrated in detail with regard to FIG. 5 later on. After the sheet S has passed the sheet registration device 1 it progresses towards the print station, which is preferably positioned adjacent and/or (directly) downstream of the sheet registration device 1. The print station then prints a front side image (I1 in FIG. 5) on the first side of the registered sheet S within the determined printable area PA. In FIG. 4C, a starting point SP, for example the top-right corner, of the front side image I1 is indicated.
[0065] After front side printing, the sheet S is flipped along the lateral axis Y and returned to the sheet registration device 1 via a duplex pass of the printer. Generally, the sheet S is flipped such that its leading and trailing edges change places, while the lateral edges SE maintain the same left or right positions as on the earlier simplex pass.
[0066] FIG. 4D indicates the step of registering the flipped, printed sheet S. The same lateral edge SE as in FIG. 4B is aligned with the transport direction X and positioned at the same lateral position. As will be shown in FIG. 5, the determined printable PA then defines the same area as on the first pass of the sheet S in FIG. 4B. Subsequently, the registered sheet S is passed to the print station, wherein the back side image (I2 in FIG. 5) is printed on the second side of the sheet S. Therein, the position of the starting point SP of the front side image I1 is taken into account, along with the relative angles A1, A2, the length L of the lateral edge E, and a dimension such as the height of the first image I1, such that the back side image I2 is printed aligned with the front side image I1. Specifically, the controller determines the offset O of the starting point with respect to a reference point, for example as a vector extending between the top-right corner of the sheet S and the intended position of the top-right corner of the front side image I1. In the example of FIG. 4C, the offset O is the displacement vector between the point on the leading edge LE detected by one of the longitudinal edge sensors 2, 3 and the to top-right corner of the be formed image I1. On the duplex pass, the same lateral edge SE is registered similarly to the first pass, such that the lateral edge SE is at the same lateral position and extends in the same direction X as on the first pass. By applying the determined relative angles of the different edges, the offset O can be converted to determine a corresponding offset O′ for the back side image I2 with respect to the point where said one of the longitudinal edge sensors 2, 3 will detect the new leading edge of the sheet S, such that the front and back side image I1, I2 will be aligned. The position of the image I1 on the sheet S with respect to the edges LE, SE, TE is known. The corresponding offset O′ in FIG. 4C-D extends between the bottom right corner and the reference point where the longitudinal edge sensor 3 will pass below the respective edge. The corresponding offset can be deduced from the angles A1, A2, the sheet length L, and the length of the image I1 in the transport direction X. Since the sheet S is flipped on the second pass so the bottom right corner of the first image I1 in FIG. 4C becomes the top right corner on the second pass. The corresponding offset O′ can be used to start the printing of the second image I2, such that its top right corner is positioned at the one of the first image I1. Thus, the timing of starting the printing of each image I1, I2 can be timed with respect to the print station, such that the images I, I2 will overlap fully. For example, the bottom right corner of the back side image I2 will be at the same position as the corner of the front side image I1 which was printed at the starting point SP. The front back side images I1, I2 will further be parallel to one another (no rotational deviation as shown in FIG. 1B), since the same lateral edge SE is used to register the sheet S on both passes.
[0067] An additional advantage of the above described method is that when a stack of sheets is formed is this manner all images in the stack are aligned with one another. The printed stack is generally processed further, for example by cutting the stack as a whole to a predetermined size and binding the sheets S together. Such processing operations generally use one corner of the stack as a reference. When deviations as shown in FIGS. 1B and 2B are present in the stack, these will be visible in the final product (or require manually aligning the individual sheets in the stack). The proposed method aligns the images with respect to the same corner, such that the images are suitably aligned for further processing after the stack has been formed by the printer.
[0068] FIG. 5 illustrates the step of determining the printable area PA of the sheet S. The printable area PA is in this example derived from the relative angles A1, A2 between the lateral edge SE and respectively the trailing edge TE1 and the leading edge LE1 (on the first pass of the sheet S) as well as from the length L of the lateral edge SE. FIG. 5 illustrates the sheet S on its first pass after having been registered parallel to the transport direction X (solid line). In dashed lines, the flipped sheet S on its second pass is indicated after having been registered at the same lateral position as during the first pass. So the sheet S in its original state and in its flipped state is has its lateral edge SE parallel to the transport direction X and at the same position along the lateral axis Y. The sheet S and the flipped sheet S can however be offset from one another in the transport direction X by a distance D. Distance D is indicated as the distance between the top-right corners of the sheet S in its original and flipped state. The distance D can be used and/or varied to maximize the printable area PA.
[0069] The printable area PA is FIG. 5 is formed by the largest possible rectangle starting at and perpendicular to the lateral edge SE within the outer edges of the sheet S in its original and in its flipped state. P1 in FIG. 5 is the intersection between the axes X, Y and the vector of the leading edge LE1 of the sheet S on its first pass. The leading edge LE2 of the flipped sheet S passes through axis X a distance D from the axis Y at point P2 on the lateral edge SE. In turn point P2 defines a point P3. Point P3 is the intersection nearest the lateral edge SE of either leading LE1 or flipped leading edge LE2 with a line perpendicular to the lateral edge SE and passing through point PE. The tangents or vectors for leading LE1 or flipped leading edge LE2 are derived from their respective relative angles A1, A2. Similarly, point P4 is defined by distance L from P1 along the axis X. The point P5 is found by determining the intersection nearest the lateral edge SE of a line parallel to the axis Y and passing through point P4 and one of trailing edges TE1, TE2 on the first and second passes of the sheet S. Again, the tangents of the trailing edges TE1, TE2 are derived from the relative angles A1, A2. The shortest of distance P2-P3 and P5-P5 is taken as the width W of the printable area PA. Multiplying said width W by the height H (which is equal to L-D in FIG. 5) of the printable area PA yields the surface area of the printable area PA. This surface area may be maximized with respect to the distance D to find the maximum printable area PA. This distance D is stored and applied to correctly align the front and back side image I1, I2, when printing these within the printable area PA. It will be apparent that in the example in FIG. 5 the distance D can also be varied in the opposite direction (above the axis Y), with suitable amendments, as the point P2 will be fixed at the position of P1, while point P4 will be determined by a distance L-D. Maximization can be achieved computationally and/or mathematically. For reference the sheet S with the printable area PA on its first pass is shown in FIG. 6A, while the sheet S with the printable area PA on its second pass is shown in FIG. 6B.
[0070] FIG. 7 illustrates the step of minimizing the inter-sheet distance between subsequent sheets S after registration. Since the relative orientations of the sheets S are known, the distance between them can be minimized without the sheets S overlapping. This provides high throughput through the print station. FIG. 7 illustrates the example where the sheets S have been similarly cut, which is common for batch processed sheets S. The sheets S are oriented in such a manner that they touch each other only at the adjacent corners. The sheets S do not overlap anywhere else. The orientations are used to adjust the distance between subsequent sheets S, such that subsequent trailing and leading edges intersection at a single point without the sheets S overlapping. It will be appreciate that in this example the minimum inter-sheet distance is taken as zero, but instead of actual zero a minimal non-zero distance value may be applied, for example for safety reasons.
[0071] FIG. 8 schematically indicates the step of the proposed method of registering sheets S. In step i, at least three edges of the sheet S are sensed. This sensor data is applied in step ii to determine the relative orientations and/or angles of said edges LE, TE, SE with respect to one another. In step iii, the printable area PA of the sheet S is maximized based on the determined orientations and/or relative angles A1, A2. It will be appreciated that step ii may be omitted and the method may proceed from step ii to step iv, which is the registering of the sheet S by aligning its lateral edge SE with the transport direction X (or another desired direction). In step v a front side image I1 is printed on the sheet S, preferably within the printable area PA. Subsequently, the printed sheet S is flipped such that its unprinted side will face the print station when it returns there. In step vi, the flipped sheet S is registered again by aligning the same lateral edge SE as in its first pass with the transport direction X. In step vii, the back side image is printed onto the unprinted side of the sheet S. Therein, the relative orientations and/or angles of said edges LE, TE, SE are used to correctly align the front side image I1 with the back side image I2. By aligning the same lateral edge SE on both passes of the sheet S, alignment of the images I1, I2 in the lateral direction Y is ensured. The relative orientations and/or angles of the edges LE, TE, SE are used to correctly time the start of the print process, such that the first line of the back side image I2 is printed overlapping the last printed line of the front side image I1.
[0072] Although specific embodiments of the invention are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations exist. It should be appreciated that the exemplary embodiment or exemplary embodiments are examples only and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents. Generally, this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.
[0073] It will also be appreciated that in this document the terms “comprise”, “comprising”, “include”, “including”, “contain”, “containing”, “have”, “having”, and any variations thereof, are intended to be understood in an inclusive (i.e. non-exclusive) sense, such that the process, method, device, apparatus or system described herein is not limited to those features or parts or elements or steps recited but may include other elements, features, parts or steps not expressly listed or inherent to such process, method, article, or apparatus. Furthermore, the terms “a” and “an” used herein are intended to be understood as meaning one or more unless explicitly stated otherwise. Moreover, the terms “first”, “second”, “third”, etc. are used merely as labels, and are not intended to impose numerical requirements on or to establish a certain ranking of importance of their objects.
[0074] The present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
