Method for aligning printing units of printing device and printing device

Abstract

The invention relates to a method for aligning at least a first printing unit (12) of a printing device (10) and at least a second printing unit (14) of the printing device (10) with respect to a direction transverse to a paper travel direction (20), each printing unit comprising at least one printing head (16), wherein the at least first and second printing units (12, 14) are arranged at a distance from each other with respect to the sheet travel direction (20). Furthermore, a printing device (10) is disclosed.

Description

[0001] The present invention relates to a method for aligning at least a first printing unit of a printing device and at least a second printing unit of the printing device in a direction relative to the paper travel direction or transverse to the paper travel direction. The phase difference represents the magnitude of the misalignment between the printing units. Furthermore, the present invention relates to a printing device.

[0002] Printing devices typically have several printing units for printing different color planes. The printing medium (in particular a paper web) passes successively through the different printing units.

[0003] The alignment of the color planes must be extremely accurate to ensure high printing quality. In particular, a color misalignment of more than 50 micrometers will significantly affect the printing quality. Therefore, this requires registering the printing units with respect to horizontal and vertical alignment as well as with respect to skew. The vertical direction (or "Y" direction) is aligned with the paper travel direction, while the horizontal direction (or "X" direction) is aligned with the direction transverse to the paper travel direction. The alignment of the printing units typically takes place in an automated manner. For example, an image printed by the print head of a printing unit is captured and it is estimated based on the position of the image whether the print head is properly aligned.

[0004] EP0955177B1 discloses a method for aligning two print heads in the horizontal direction. For this purpose, each print head prints a repeating pattern, where the phase of the pattern for one print head is gradually shifted relative to the other. The pattern consists of vertical rows printed in different zones adjacent to each other in the horizontal direction.

[0005] However, the known methods are not satisfactory in terms of the required accuracy or complexity.

[0006] Therefore, the object of the present invention is to achieve sufficient alignment between the printing units of a printing device.

[0007] To outline the principle detailed below, the object is achieved by performing the alignment along the horizontal direction of the first and second printing units. For the sake of the reader's understanding, we outline (in an incomplete manner) the principle in the next paragraph and describe the principle extensively below.

[0008] Alignment along the horizontal direction is performed by using two printing units of the same period, with each printing unit printing a periodic signal composed of small vertical lines. The period is given by the distance between two adjacent vertical lines. The above-mentioned set of vertical lines constitutes a (horizontal) band. A number of bands are printed along the vertical direction. In each band, the horizontal position of one of the two signals is shifted by a (preferably) fixed amount, which changes the appearance of each band. This object is achieved by a method for reading the result by detecting the extreme values of a third signal along the vertical direction, the method being for aligning at least a first printing unit of a printing device and at least a second printing unit of the printing device relative to a direction transverse to the paper travel direction. Each printing unit comprises at least one print head, wherein at least the first and second printing units are arranged at a distance from each other relative to the paper travel direction. In particular, there is no overlap between the print heads of different printing units in the direction along the paper travel direction (i.e., the vertical direction). The method comprises printing a first pattern with the first printing unit, the first pattern comprising a plurality of parallel lines that extend in a direction parallel to the paper travel direction and are equidistantly spaced from each other by a first distance in a direction transverse to the paper travel direction, wherein the lines of the first pattern extend continuously over at least a first section in the paper travel direction, and the first pattern constitutes a first signal. Another method step comprises printing a second pattern overlapping the first pattern with the second printing unit, the second pattern comprising a plurality of parallel lines that extend in a direction parallel to the paper travel direction and are arranged at an equal distance from each other equal to the first distance. The second pattern constitutes a second signal. The superposition of the first pattern and the second pattern constitutes a band in the first section. Another method step comprises repeating the printing of the first pattern with the first printing unit and the printing of the second pattern with the second printing unit for creating a number of bands along the paper travel direction, wherein the position of the lines of the first pattern or the second pattern is shifted in each band relative to the direction transverse to the paper travel direction. The string of bands constitutes a third periodic signal. The third periodic signal is measured by a camera of the printing device along the paper travel direction. The misalignment between the first printing unit and the second printing unit in the direction transverse to the paper travel direction is evaluated based on the measured third signal. The misalignment is adjusted by shifting the first printing unit and / or the second printing unit in the direction transverse to the paper travel direction.

[0009] In particular, the position of the lines of the first or second pattern is not shifted such that they have the same horizontal position in each band. In the case where there is no vertical distance between the bands, the lines of the first or second pattern extend continuously over all the bands.

[0010] In a creative way, the adjustment of the printing units relative to the direction transverse to the paper travel direction can achieve a particularly high alignment accuracy.

[0011] In particular, the phase relationship between the first signal and the second signal can be evaluated by measuring the third periodic signal, and the relative position between the first printing unit and the second printing unit in a direction transverse to the paper travel direction can be determined based on the third signal.

[0012] For example, the strips are arranged one after the other with respect to the paper running direction. Thus, the third periodic signal can be processed particularly easily.

[0013] Assuming that the interaction of light with ink is approximately multiplication, the third signal or at least part of the third signal is the result of multiplication of the first signal and the second signal.

[0014] The alignment accuracy can be even higher than the pixel size of the camera. For example, an alignment accuracy of 10 μm to 50 μm can be achieved with a camera with a pixel size of 90 μm. In the sense of the application, the pixel size means the pixel size of the printed image, which is projected onto the individual pixels of the camera sensor. In other words, the pixel size is the square of the printed image that can be individually distinguished by the camera.

[0015] To measure the third periodic signal, the peak of the third periodic signal is detected by a rough reading of the image. This allows a very precise phase difference between the first periodic signal and the second periodic signal to be determined. In particular, the phase difference between the first periodic signal and the second periodic signal determines the position of the peak of the third periodic signal. Furthermore, the phase difference between the first periodic signal and the second periodic signal corresponds to a phase mismatch between the printing units, or in other words, to a mismatch in the alignment of the color planes printed by the different printing units.

[0016] When the phase difference between the first periodic signal and the second periodic signal is zero, the distance between the printing units is properly adjusted.

[0017] The third periodic signal runs in a direction along the paper traveling direction.

[0018] The method is based on the idea of not measuring the first signal and the second signal directly, but measuring a third signal in which the first and second signals are superimposed in order to assess the relationship, in particular the misalignment, between the first signal and the second signal.

[0019] In particular, misalignment is determined by comparing the measured superimposed signal with the calculated superimposed signal. The calculated superimposed signal represents the signal achieved by the superposition of the first and second signals in the absence of any misalignment. According to one aspect, the first distance and / or the second distance are defined such that the first signal and the second signal are respectively detected by the camera as continuously varying signals. In other words, the camera does not see a distinct and clear gap between the lines; it only sees a change in the signal amplitude, which appears as a set of blurred lines. Thus, the signals are not binary, but rather consist of a smooth variation of gray values. As a result, the first and / or second signal can be detected as a sine signal rather than as individual lines. This can be achieved by spreading the edges of the printed lines. For example, the thickness of a line printed by inkjet is typically 30 μm, but before the paper leaves and reaches the camera, photons undergo multiple reflections within the paper. The optics of the camera sensor and the limited resolution can cause some additional blurring. Therefore, the printed lines appear as blurred with a thickness greater than 30 μm. The combination of the two effects results in the possibility of measuring a third superimposed signal when the first and second distances are respectively selected. The third signal can also be regarded as a sine signal.

[0020] According to an exemplary embodiment, a number of lines are printed per millimeter. The frequency of the third signal is such that it can be easily detected.

[0021] The third periodic signal can be used by measuring the position of at least one maximum and / or at least one minimum of the signal in the paper travel direction. Thus, measuring the third signal is particularly easy.

[0022] For example, a reference signal having the same frequency as the third periodic signal is printed, and the same phase of the third periodic signal is detected by comparing the third signal with the reference signal. In particular, the phase of the third signal is detected by comparing the positions of the minimum and maximum of the third signal with those of the reference signal. The phase of the reference signal is known and is, for example, zero. In other words, the reference signal is the calculated superimposed signal achieved by the superposition of the first and second signals in the absence of any phase difference.

[0023] In addition to the first pattern, a first rough pattern can be printed by the first printing unit, and a second rough pattern can be printed by the second printing unit, wherein a rough adjustment is performed based on the positions of the first and second rough patterns relative to each other.

[0024] The accuracy of the rough adjustment can be at least half of the first distance.

[0025] Because the first and second signals are periodic signals, the positions given by the periods of the signals have an uncertainty. A coarse pattern is used to mitigate this uncertainty. For example, if the second signal is shifted by a distance equal to the first distance, the third signal will be the same. By making a coarse adjustment, this uncertainty can be avoided. In particular, the coarse adjustment must be made with an accuracy of at least half of the first distance to mitigate the uncertainty of the first misalignment measurement. Thereby, a particularly reliable phase adjustment and alignment of the printing unit can be achieved.

[0026] For example, the coarse pattern can be similar to the first and second patterns, but use thicker lines, and there is a larger distance between the thicker lines without overlap. The positional mismatch between the printing units is calculated from these lines (by any suitable method; here the gaps between the lines are clearly visible). The accuracy of this positional mismatch is worse than that obtained by using thinner lines. In particular, the required accuracy obtained with the thicker lines must be less than half of the first distance (between two thin lines). As an alternative, standard alignment marks can be used instead of the thick repeating lines.

[0027] According to one aspect, the first and second patterns are printed on each of the left and right sides of the print head of the printing unit. By comparing the third signal generated from the superimposed signals of the first and second patterns on the left and right sides of the print head, rotational misalignment can be detected and corrected accordingly. In particular, when the phases of the third signals on the left and right sides of the printing unit are different, rotation is detected. The rotational misalignment is corrected by physically rotating the print head, or by adjusting the ink distribution differently on the right side of the print head compared to the left side.

[0028] According to another aspect, the first and second patterns are printed on each of the left and right sides of the printing bar of the printing unit. By comparing the third signal generated from the superimposed signals of the first and second patterns on the left and right sides of the printing bar, skew misalignment along the direction of paper movement can be detected and corrected accordingly. In particular, when the phases of the third signals on the left and right sides of the printing bar are different, skew along the direction of paper movement is detected. The skew can be corrected by correcting the rhythm of the ink distribution on the printing bar.

[0029] This object is further achieved by a printing device, in particular an inkjet printing device, comprising: at least a first printing unit and a second printing unit, each printing unit comprising at least one print head, a camera configured to capture an image printed by the printing unit, and a control unit configured to process the image captured by the camera, wherein the printing device is configured to perform the inventive method, and wherein the control unit is configured to evaluate the misalignment between the first printing unit and the second printing unit in a direction transverse to the paper travel direction.

[0030] Further features and advantages can be derived from the following description and the drawings. In the drawings:

[0031] - Figure 1 shows a printing device schematically,

[0032] - Figure 2 schematically shows an image printed by the printing device,

[0033] - Figure 3 shows an intermediate stage when printing Figure 2 a section of the image,

[0034] - Figure 4 shows Figure 2 a section of the image,

[0035] - Figure 5 shows a section measured by a camera of Figure 4 the image,

[0036] - Figure 6 shows Figure 2 another section of the image, and

[0037] - Figure 7 shows Figure 2 a photograph of the image printed by the printing device visually schematized in

[0038] Figure 1 Schematically shows a printing device 10 having a first printing unit 12 and a second printing unit 14. The printing device 10 is an inkjet printing device, in particular a digital printer.

[0039] The first printing unit 12 and the second printing unit 14 are configured to print different color planes. For simplicity, Figure 1 only two printing units 12, 14 are depicted in

[0040] However, the printing device 10 may have more than two printing units 12, 14.

[0041] Each printing unit 12, 14 has a plurality of print heads 16. The print heads 16 are aligned along a line that respectively forms a printing bar 13, 15.

[0042] The print heads 16 are attached to the bars 13, 15, and the bars 13, 15 extend transversely to the advancing direction of the printing device 10.

[0043] The print heads 16 can move or rotate along the bars 13, 15, as Figure 1 indicated by the arrows in

[0044] The advancing direction corresponds to the paper traveling direction and is indicated by arrow 20 in Figure 1 the figure.

[0045] The rods 13, 15 are attached to the machine frame 22 of the printing device 10.

[0046] All the printing heads 16 attached to one of the rods 13, 15 are configured to print a single color.

[0047] The printing device includes a camera 24, for example, a 2D camera or a line array camera, which is configured to capture an image of the paper printed by the printing units 12, 14.

[0048] The camera 24 covers the entire width of the printing units 12, 14. In particular, the camera 24 extends over the entire width of the paper 26 processed in the printing device 10.

[0049] The camera 24 is positioned downstream of the printing units 12, 14 with respect to the paper traveling direction 20.

[0050] The printing device 10 further includes a control unit 28, which is configured to process the image captured by the camera 24.

[0051] The control unit 28 is further configured to determine the phase difference or misalignment between the printing units 12, 14. In particular, the control unit 28 is configured to determine the phase difference or misalignment between the printing units 12, 14 based on the image printed by the printing device 10, which will be further described in detail with reference to the following drawings.

[0052] Figure 2 An image printed by the printing device 10 (in particular by the first printing unit 12 and the second printing unit 14) is shown. Thus, the image contains two different colors, which are Figure 2 two different shades of gray in the figure. For example, the two colors are black and magenta.

[0053] The image has different sections, in which different patterns are printed. However, some sections have the same pattern.

[0054] The sections of the image designated by the same reference numerals are similar, i.e., have the same pattern.

[0055] The image has a phase alignment section 30, which is suitable for realizing the adjustment of the phase difference.

[0056] In addition, the image has a horizontal alignment section 32, which is suitable for realizing the alignment of the printing units 12, 14 with respect to the direction transverse to the paper traveling direction 20.

[0057] In the depicted embodiment, the phase alignment section 30 and the horizontal alignment section 32 are printed twice by each printhead 16, specifically on each side of the left and right sides of the printheads 16 of the respective printing units 12, 14.

[0058] In Figure 2 the areas 34, 36 covered by one printhead 16 in a direction transverse to the paper travel direction are specified. At the transition between the two printheads 16 of a printing unit 12, 14, the printheads 16 overlap to some extent. In this area, a print overlap section 38 is printed. The overlap section 38 allows color adjustment, in particular the adjustment of the printheads 16 of one printing unit 12, 14 relative to each other.

[0059] Furthermore, in the depicted embodiment, the image includes a reference section 40, which is printed twice by each printhead 16 on each side of the left and right sides of the printhead 16.

[0060] Furthermore, the image includes a coarse phase alignment section 42. The coarse phase alignment section 42 is suitable for achieving a coarse adjustment of the phase difference before a more precise adjustment.

[0061] Furthermore, the image includes a coarse horizontal alignment section 44. The coarse horizontal alignment section 44 is suitable for achieving a coarse alignment of the printing units 12, 14 relative to the direction transverse to the paper travel direction 20 before a more precise adjustment.

[0062] Hereinafter, the different sections 30, 32, 38, 40, 42, 44 and a method for adjusting the phase difference between the first printing unit 12 and the second printing unit 14 based on a printed image (in particular by processing the printed image with the aid of the control unit 28) will be described in more detail.

[0063] Furthermore, a method for aligning the first printing unit 12 and the second printing unit 14 relative to the direction transverse to the paper travel direction based on a printed image (in particular by processing the printed image with the aid of the control unit 28) will be described in detail.

[0064] Reference Figures 2 to 5 will be made to a method for adjusting the phase difference between the first printing unit 12 and the second printing unit 14.

[0065] Figures 3 to 5 The phase alignment section 30 of the image is shown.

[0066] When printing the phase alignment section 30, in the first step, the first printing unit 12 prints Figure 3The first pattern depicted in . The first pattern includes a plurality of parallel lines 45 that extend in a direction transverse to the paper travel direction 20 and are equally spaced from each other along the paper travel direction 20 by a first distance d1. The first pattern constitutes a first periodic signal S1 having a first frequency f1.

[0067] The distance d1 remains constant along the first pattern.

[0068] In Figure 4 In the following steps visualized in , a second pattern overlapping the first pattern is printed with a second printing unit 14. The second pattern includes a plurality of parallel lines 46 that extend in a direction transverse to the paper travel direction 20 and are equally spaced from each other along the paper travel direction by a second distance d2, where the second distance d2 is different from the first distance d1.

[0069] The second pattern constitutes a second periodic signal S2 having a second frequency f2.

[0070] In Figure 3 and Figure 4 In and , for better understanding, the lines 45 and 46 can be recognized as separate horizontal lines. In particular, to better distinguish the first and second lines 45, 46 from each other, the line 45 is depicted as a continuous line, while the line 46 is depicted as a dashed line.

[0071] However, the first distance d1 and the second distance d2 are defined such that the first signal S1 and the second signal S2 are respectively detected by the camera 24 as continuously varying signals, for example, by printing a certain number of lines per millimeter.

[0072] In particular, S1 and S2 appear as sinusoidal periodic signals in the calculation. In addition, the interaction between light and the ink on the paper is approximately multiplicative, and the third signal is the result of the multiplication of the first signal and the second signal. Therefore, the frequency of the third signal is the difference between the frequency of the first signal and the frequency of the second signal. In other words, since the third signal results from the superposition of the first and second signals, the period of the third signal is the (minimum) distance between two positions where the first and second signals overlap in the same way, for example, the distance between two positions where the lines of the first signal are aligned with the lines of the second signal. Therefore, the period of the third signal is selected by setting the frequencies of the first signal and the second signal to obtain a predetermined frequency for the third signal. For example, if the periods of the first signal S1 and the second signal S2 differ by 10%, the period of the third signal S2 is 10 times larger than the period of the first signal and can thus be made greater than 1 mm. Subsequently, it can be easily detected by the camera. For example, the first distance d1 and / or the second distance d2 is between 100 and 300 μm.

[0073] In a specific embodiment, the size of the ink droplets dripping from the print head 16 (i.e., the thickness of lines 45, 46) is 30 μm. However, at this size, due to the camera optics and sensors, and due to the multiple reflections that photons undergo within the paper before leaving the paper and reaching the camera, the edges of the printed lines 45, 46 spread out, causing lines 45, 46 to appear with a thicker thickness and to be blurred. Therefore, by printing a number of lines per mm, the signal recorded by the camera 24 can be approximately regarded as a continuously varying signal (and thus not a binary signal).

[0074] When the first and second patterns (i.e., the first and second signals S1, S2) are superimposed, a third periodic signal S3 is generated.

[0075] The third periodic signal S3 generated from the superposition of the first and second periodic signals S1, S2 is measured by the camera 24 of the printing apparatus 10, and the phase difference between the first printing unit 12 and the second printing unit 14 is evaluated based on the measured third periodic signal S3. The phase is the position of the peak of the signal.

[0076] Figure 5 Shows the signal S3 detected by the camera 24.

[0077] In particular, the phase of the third signal S3 is determined to derive the value of the phase of the second signal S2.

[0078] The phase difference between the first signal S1 and the second signal S2 can be determined from the phase of the second signal S2.

[0079] If a phase difference is detected, the phase difference is adjusted by adjusting the rhythm of the ink distribution from at least one of the first printing unit 12 and the second printing unit 14.

[0080] For example, the pixel size of the camera 24 is 90 μm. Therefore, the distance d3 represents approximately 16 pixels. By detecting the position of the maximum value of the third signal S3 with an accuracy of one pixel, the value of the phase difference between the two signals S1, S2 can be evaluated with an accuracy of approximately 1 / 10 of a pixel. In other words, according to the described method, the phase difference between the two signals S1, S2 can be detected with particularly high accuracy.

[0081] For example, the third periodic signal S3 is measured by measuring the positions of the maximum and minimum values of the signal S3 in the paper travel direction. Given the positions of the maximum and minimum values of the third signal S3, the phase of the third signal S3 can be evaluated in an easy manner.

[0082] By comparing the third signal S3 with the reference signal S R The maximum and minimum values of the third signal S3 are detected. In particular, the reference signal S R Is printed in the reference section 40.

[0083] Reference signal S R has the same frequency as the third periodic signal S3. In particular, S R is set to have a phase of zero, which corresponds to the phase of signal S3 when signals S1 and S2 are aligned. Thus, when S3 and S R are aligned, the color planes of the printing unit are also aligned (along direction Y). Using reference signal S R it is possible to operate without precisely determining the distance the paper travels between the printing unit and the camera.

[0084] By comparing the third signal S3 with the reference signal S R the phase of the third periodic signal S3 is detected, in particular by comparing the positions of the minimum and maximum values of the third signal S3 with the positions of the maximum and minimum values of the reference signal S R If the minimum and maximum values of the third signal S3 are shifted relative to the maximum and minimum values of the reference signal S R a phase difference is detected.

[0085] In Figure 2 the maximum value Max R of the reference signal S R and the minimum value Min R are indicated.

[0086] As Figure 2 depicted, by means of the phase alignment sections 30 present on each of the left and right sides of the print head 16, the rotation of the printing unit 12, print head 16 can be detected separately. In particular, rotation occurs when the phase difference between the first and second signals S1, S2 in the two alignment sections 30 is different.

[0087] By means of the phase alignment sections present at the leftmost and rightmost parts of the printed paper, the skew of the printing bar 13 relative to the printing bar 15 can be detected. The skew can be caused by defects in the parallelism between different printing bars 13, 15.

[0088] Before performing a fine adjustment of the phase difference, a coarse phase adjustment is performed.

[0089] The coarse phase adjustment is performed with an accuracy of at least half of the first distance d1 to avoid the uncertainty of the positions given by the periods of signals S1, S2.

[0090] As Figure 2As depicted, the coarse phase adjustment is performed by means of a first coarse pattern and a second coarse pattern printed in the coarse phase alignment section 42. The first coarse pattern is printed by the first printing unit 12 and consists of parallel lines 48 extending in a direction transverse to the paper travel direction, and the second coarse pattern is printed by the second printing unit 14 and consists of parallel lines 50 extending in a direction transverse to the paper travel direction. Since the coarse phase adjustment uses thicker lines, we do not superimpose them: we only calculate the position of each line. For coarse alignment, we do not even need to print a periodic signal. Simple alignment marks printed by each printing unit would also be a possible alternative.

[0091] The coarse adjustment is performed based on the relative position of the first and second coarse patterns with respect to each other, where the accuracy of the coarse adjustment is at least half of the first distance d1.

[0092] By means of the phase alignment sections 30 present on each of the left and right sides of the print head 16, the rotation of the print head can be detected. In particular, rotation occurs when the phases of the third signal S3 on the left and right sides of the print head 16 are different.

[0093] Reference Figure 6 , describes the horizontal alignment section 32 in more detail.

[0094] In the horizontal alignment section 32, the first printing unit 12 prints a first pattern. The first pattern includes a plurality of parallel lines 52 that extend in a direction parallel to the paper travel direction 20 and are equally spaced from each other at a first distance d1' in a direction transverse to the paper travel direction 20.

[0095] The lines 52 of the first pattern continuously extend over at least a first section 54 in the paper travel direction 20. The first pattern constitutes a first signal, in particular a continuous signal S1'. In other words, the signal S1' has a constant phase.

[0096] In addition, the second printing unit 14 prints a second pattern that overlaps the first pattern. The second pattern includes a plurality of parallel lines 56 that extend in a direction parallel to the paper travel direction 20 and are arranged at an equal distance d2' from each other that is the same as the first distance d1'.

[0097] In Figure 6 , the lines 56 are depicted as dashed lines to better distinguish the lines 52, 56 from each other.

[0098] For example, the first distance d1' and the second distance d2' are between 100 and 300 μm.

[0099] The parallel lines 56 of the second pattern form a plurality of bands 53. In particular, the bands 53 are sections along the first section 54, which can be distinguished from each other due to the different horizontal positions of the lines 56 of the second pattern.

[0100] The bands 53 are arranged successively with respect to the paper travel direction 20, in particular with respect to the vertical direction Y. In the illustrated embodiment, the bands 53 are directly adjacent to each other in the vertical direction, i.e., there is no vertical distance between the bands 53.

[0101] However, there may be a vertical distance between individual bands 53. In this case, the bands 53 can be digitally reorganized by means of the control unit 28 for further analysis.

[0102] The positions of the lines 56 of the second pattern are shifted between different bands 53. There are at least three bands, so the second pattern is shifted at least twice in the direction transverse to the paper travel direction in the first section. In other words, the position of the line 56 is modified in the direction transverse to the paper travel direction. In other words, the position of the line 56 is modified on the bands 53.

[0103] In particular, the lines 56 of the second pattern defining the bands 53 are shorter than the lines 52 of the first pattern.

[0104] In Figure 6 the section 54 depicted, the position of the line 56 of the second pattern is shifted seven times along the horizontal direction X in the section 54.

[0105] The second pattern constitutes a second signal of the periodic signal S2'.

[0106] Since d2' is equal to d1', the first signal S1' and the second signal S2' have the same frequency along the horizontal direction X. Since the lines 56 of the second signal S2' are shifted in the section 54, the second signal S2' has a varying phase with respect to S1' on the bands 53. Within a single band, the phase of the second signal S2' is constant compared to the phase of the first signal S1'.

[0107] The third signal S3' is a periodic signal along the vertical direction Y, and the third signal S3' is generated by superimposing the first signal S1' and the second signal S2'. Therefore, the third signal results from the change in the appearance of the bands 53. The period of the signal S3' is determined by the amount of shift (i.e., phase difference) between S1' and S2' on the bands 53. The phase of the signal S3' is determined by the phase difference between S1' and S2' on a predetermined band 53.

[0108] Based on the path of the measured third signal S3', the misalignment between the first printing unit 12 and the second printing unit 14 in the direction transverse to the paper travel direction 20 is evaluated. In particular, based on the position of the limit value of the signal S3', the phase relationship between S1' and S2' can be calculated.

[0109] For example, the line 56 is shifted once per centimeter. In other words, in this example, the height of the tape 53 is one centimeter. If misalignment is detected, the misalignment is adjusted by shifting the first printing unit 12 and / or the second printing unit 14 in a direction transverse to the paper travel direction 20.

[0110] As described with respect to phase alignment, with regard to the misalignment of the printing units 12, 14 in a direction transverse to the paper travel direction 20, rough alignment can be performed by means of the rough horizontal alignment section 44.

[0111] As the rough phase alignment section 42, the rough horizontal alignment section 44 consists of a first rough pattern and a second rough pattern printed in the rough horizontal alignment section 44. The first rough pattern is printed by the first printing unit 12 and can consist of thick parallel lines 58 extending in the direction along the paper travel direction 20, and the second rough pattern is printed by the second printing unit 14 and can consist of thick parallel lines 60 extending in the direction along the paper travel direction. Contrary to the finer adjustment sections 30, 32, the rough adjustment is less critical and can be performed using any suitable known method.

[0112] Similar to the rough phase alignment, the horizontal path adjustment is performed based on the relative positions of the first and second rough patterns with respect to each other, where the accuracy of the rough adjustment is at least half of the first distance d1'.

[0113] Preferably, the first distance d1 of the line 45 extending in a direction transverse to the paper travel direction 20, the first distance d1' of the line 52 extending in a direction parallel to the paper travel direction 20, the second distance d2 of the line 46 extending in a direction transverse to the paper travel direction 20, and the second distance d2' of the line 56 extending in a direction parallel to the travel direction 20 are selected such that the frequencies f3, f3' of two of the superimposed third signals S3, S3' are the same. Thus, the same reference signal S R can be used to detect the maximum and minimum values of the third signals S3, S3' respectively.

[0114] In Figure 7 a photo of the printed image is shown.

[0115] In the photo, the positions of the maxima Max of the third signals S3, S3' in the phase alignment section 30 and the horizontal alignment section 32 are marked.

[0116] In the depicted embodiment, the image includes a phase alignment section 30 and a horizontal alignment section 32. However, depending on requirements, only the phase alignment section 30 or the horizontal alignment section 32 can be printed.

Claims

1. A method for aligning at least a first printing unit (12) of a printing device (10) and at least a second printing unit (14) of the printing device (10) relative to a direction transverse to the paper travel direction (20), each printing unit comprising at least one print head (16), wherein the at least first and second printing units (12, 14) are arranged at a distance from each other relative to the paper travel direction (20), the method comprising: - Printing a first pattern with the first printing unit (12), the first pattern comprising a plurality of parallel lines (52), the lines (52) extending in a direction parallel to the paper travel direction (20) and being equidistantly spaced from each other by a first distance (d1') in the direction transverse to the paper travel direction (20), wherein the lines of the first pattern extend continuously over at least a first section (54) in the paper travel direction (20), the first pattern constituting a first signal (S1'); - Printing a second pattern overlapping the first pattern with the second printing unit (14), the second pattern comprising a plurality of parallel lines (56), the lines (56) extending in a direction parallel to the paper travel direction (20) and being arranged at an equal distance (d2') equal to the first distance (d1') relative to each other, the second pattern constituting a second signal (S2'), wherein the superposition of the first pattern and the second pattern constitutes a band (53) in the first section (54); - Repeating the printing of the first pattern with the first printing unit (12) and the printing of the second pattern with the second printing unit (14) to create a plurality of bands (53) along the paper travel direction (20), wherein the positions of the lines (50, 52) of the first pattern or the second pattern are shifted relative to the direction transverse to the paper travel direction (20) in each band (53), and the consecutive bands (53) constitute a third periodic signal (S3'); - Measuring the third periodic signal (S3') along the paper travel direction (20); - Evaluating the misalignment between the first printing unit (12) and the second printing unit (14) in the direction transverse to the paper travel direction (20) based on the path of the measured third signal (S3'); and - Adjusting the misalignment by shifting the first printing unit (12) and / or the second printing unit (14) in the direction transverse to the paper travel direction (20).

2. The method according to claim 1, wherein The first distance (d1') and / or the second distance (d2') are defined such that the first signal (S1') and the second signal (S2') are respectively detected by the camera (24) as continuously varying signals.

3. The method according to claim 2, characterized in that, The first distance (d1') and / or the second distance (d2') are between 100 and 300 μm.

4. The method according to any one of the preceding claims, wherein the bands (53) are arranged successively relative to the paper travel direction (20).

5. The method according to any one of the preceding claims, wherein, in addition to the first pattern, a first rough pattern is printed by the first printing unit (12), and, in addition to the second pattern, a second rough pattern is printed by the second printing unit (14), wherein a rough adjustment is performed on the basis of the positions of the first and second rough patterns relative to each other, and wherein the accuracy of the rough adjustment is at least half of the first distance (d1').

6. The method according to any one of the preceding claims, wherein the third periodic signal (S3') is measured by measuring the position of at least one maximum value and / or the position of at least one minimum value of the signal (S3') in the paper travel direction (20).

7. The method according to any one of the preceding claims, wherein a reference signal (S R ) having the same frequency as the third periodic signal (S3') is printed, and the phase of the third periodic signal (S3') is detected by comparing the third signal (S3') with the reference signal (S R ).

8. The method according to any one of the preceding claims, wherein the first and second patterns are printed on each of the left and right sides of the print heads (16) of the printing units (12, 14).

9. A printing device (10), in particular an inkjet printing device, comprising at least a first printing unit (12) and a second printing unit (14), each printing unit (12, 14) comprising at least one print head (16), a camera (24) configured to capture an image printed by the printing units (12, 14), and a control unit (28) configured to process the image captured by the camera (24), wherein the printing device (10) is configured to perform the method according to any one of the preceding claims, and wherein the control unit (28) is configured to evaluate a misalignment between the first printing unit (12) and the second printing unit (14) in the direction transverse to the paper travel direction (20).

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