Method for adjusting phase difference between printing units of printing device and printing device

Abstract

A method for adjusting a phase difference between at least a first printing unit (12) of a printing device (10) and a second printing unit (14) of the printing device (10), each printing unit (12, 14) comprising at least one printing head (16), where the at least first and second printing units (12, 14) are arranged at a distance from each other with respect to a paper travel direction (20). Furthermore, a printing device (10) is disclosed.

Description

[0001] The present invention relates to a method for adjusting a phase difference between at least a first printing unit of a printing device and a second printing unit of the printing device. Furthermore, the present invention relates to a printing device.

[0002] Printing devices typically have a number of printing units for printing different color planes. A 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 whether the print head is properly aligned based on the position of the image.

[0004] US2012 / 0092403A1 describes a printing device having print heads arranged in units, where each unit prints a different color. Vernier patterns are used to detect and compensate for misalignment between the print heads of different units. The position of the dense regions in the vernier pattern indicates the relative vertical misalignment between the print heads. However, the known methods are not satisfactory in terms of the required accuracy or complexity.

[0005] Therefore, the object of the present invention is to achieve sufficient phase adjustment between the printing units of a printing device.

[0006] To outline the principle detailed below, the object is achieved by performing alignment along the vertical direction (i.e., the paper travel direction). For the ease of understanding of the reader, we outline the principle (in an incomplete manner) in the next paragraph and describe the principle extensively below.

[0007] Alignment along the vertical direction is performed by printing a periodic signal consisting of horizontal lines with each printing unit, using a first period of a first printing unit and a different second period of a second printing unit. The period is given by the distance between two adjacent horizontal lines. The result is "read" by detecting the extreme values of a third signal along the vertical direction.

[0008] This object is achieved by a method for adjusting the phase difference between at least a first printing unit of a printing device and a second printing unit of the printing device, each printing unit comprising at least one print head, wherein at least the first and second printing units are arranged at a distance from each other with respect 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). A method step comprises printing a first pattern with the first printing unit, the first pattern comprising a plurality of parallel lines that extend in a direction transverse to the paper travel direction and are equally spaced from each other at a first distance along the paper travel direction. In particular, the first pattern constitutes a first periodic signal having a first frequency. Another method step comprises printing a second pattern that overlaps the first pattern with the second printing unit, the second pattern comprising a plurality of parallel lines that extend in a direction transverse to the paper travel direction and are equally spaced from each other at a second distance along the paper travel direction, wherein the second distance is different from the first distance. In particular, the second pattern constitutes a second periodic signal having a second frequency. After printing the first and second patterns, a third periodic signal generated from the superposition of the first and second periodic signals is measured with a camera of the printing device, and the phase difference between the first printing unit and the second printing unit is evaluated based on the path of the measured third periodic signal, wherein a reference signal having the same frequency as the third periodic signal is printed, and the phase of the third periodic signal is detected by comparing the third signal with the reference signal. The method further comprises adjusting the phase difference by adjusting the ink distribution rhythm from at least one of the first and second printing units. It is assumed that the interaction between light and ink is approximately multiplicative. Thus, the third signal or at least part of the third signal is the result of the multiplication of the first signal and the second signal.

[0009] By means of the inventive method, a particularly high alignment accuracy with respect to the phase difference between the first printing unit and the second printing unit can be achieved. The alignment accuracy can even be 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 having a pixel size of 90 μm. In the context of the application, the pixel size means the pixel size of the printed image, and the printed image 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. To measure the third periodic signal, the peak of the third periodic signal is detected by a rough reading of the image. This allows for the determination of a very precise phase difference between the first periodic signal and the second periodic signal. 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 the phase mismatch between the printing units, or in other words, this phase difference corresponds to the misalignment of the color planes printed by different printing units.

[0010] In particular, the phase of the third signal is detected by comparing the positions of the minimum and maximum values of the third signal with those of the reference signal. The phase of the reference signal is known and, for example, is zero. In other words, the reference signal is a superimposed signal of a calculation achieved by superimposing the first and second signals without any phase difference. When the phase difference between the first periodic signal and the second periodic signal is zero, the distance between the printing units is appropriately adjusted.

[0011] The third periodic signal runs in the direction along the paper travel direction.

[0012] This method is based on the idea that instead of directly measuring the first and second signals, the third signal resulting from the superposition of the first and second signals is measured to evaluate the relationship, in particular the phase difference between the first and second signals.

[0013] In particular, the phase difference 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 without any phase difference. 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 the change in the signal amplitude, which appears as a set of blurred lines. Thus, the signals are not binary, but consist of a smooth change in 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 the superimposed third signal when the first and second distances are respectively selected. The third signal can also be regarded as a sine signal.

[0014] According to an exemplary embodiment, we print a certain number of lines per millimeter and select the difference between the first distance and the second distance to be a certain percentage. The frequency of the third signal is such that it can be easily detected. For example, the third signal has a frequency that repeats five to ten times per cm along the paper travel direction.

[0015] The third periodic signal can be used by measuring the position of at least one maximum value and / or at least one minimum value of the signal in the paper travel direction. Thus, measuring the third signal is particularly easy.

[0016] Except for the first pattern, a first rough pattern can be printed by a first printing unit, and a second rough pattern can be printed by a second printing unit, wherein a rough adjustment is performed based on the positions of the first and second rough patterns relative to each other, and the accuracy of the rough adjustment is at least half of a first distance.

[0017] Since the first and second signals are periodic signals, the positions given by the periods of the signals have uncertainties. The rough patterns are 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 performing the rough adjustment, this uncertainty can be avoided. In particular, the rough adjustment must be performed 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 units can be achieved.

[0018] For example, the rough patterns 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.

[0019] 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, rotation is detected when the phases of the third signals on the left and right sides of the printing unit are different. 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.

[0020] According to another aspect, the first and second patterns are printed on each of the left and right sides of the print 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 print bar, skew misalignment along the direction of paper movement can be detected and corrected accordingly. In particular, skew along the direction of paper movement is detected when the phases of the third signals on the left and right sides of the print bar are different. The skew can be corrected by correcting the rhythm of the ink distribution on the print bar.

[0021] 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 a method with inventive step, and wherein the control unit is configured to evaluate the phase difference between the first printing unit and the second printing unit.

[0022] Further features and advantages can be derived from the following description and the drawings. In the drawings:

[0023] - Figure 1 shows the printing device schematically,

[0024] - Figure 2 schematically shows the image printed by the printing device,

[0025] - Figure 3 shows an intermediate stage when printing Figure 2 a section of the image,

[0026] - Figure 4 shows Figure 2 a section of the image,

[0027] - Figure 5 shows the section measured by the camera Figure 4 of,

[0028] - Figure 6 shows Figure 2 another section of the image, and

[0029] - Figure 7 shows Figure 2 a photograph of the image printed by the printing device visualised schematically in.

[0030] 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.

[0031] 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. However, the printing device 10 may have more than two printing units 12, 14.

[0032] The printing units 12, 14 are arranged at a distance from each other with respect to the paper travel direction. In particular, there is no overlap between the printing units 12, 14 along the paper travel direction.

[0033] Each printing unit 12, 14 has a plurality of print heads 16. The print heads 16 are aligned along a line which respectively constitutes a printing bar 13, 15.

[0034] The print heads 16 are attached to the bars 13, 15, and the bars 13, 15 extend in a direction transverse to the advancing direction of the printing device 10.

[0035] The print heads 16 are movable or rotatable along the bars 13, 15, at least to some extent as indicated by the arrows in Figure 1 the figure.

[0036] The advancing direction corresponds to the paper travel direction and is indicated by the arrow 20 in Figure 1 the figure.

[0037] The bars 13, 15 are attached to the machine frame 22 of the printing device 10.

[0038] All the print heads 16 attached to one bar 13, 15 are configured to print a single color.

[0039] 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.

[0040] 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.

[0041] The camera 24 is positioned downstream of the printing units 12, 14 with respect to the paper travel direction 20.

[0042] The printing device 10 further includes a control unit 28 which is configured to process the image captured by the camera 24.

[0043] 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 described in further detail with reference to the following drawings.

[0044] 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 two different shades of gray in Figure 2 the figure. For example, the two colors are black and magenta.

[0045] The image has different sections in which different patterns are printed. However, some sections have the same pattern.

[0046] Sections of the image designated by the same reference numeral are similar, i.e., have the same pattern.

[0047] The image has a phase alignment section 30, which is suitable for implementing adjustment of the phase difference.

[0048] In addition, the image has a horizontal alignment section 32, which is suitable for implementing alignment of the printing units 12, 14 in a direction transverse to the paper travel direction 20.

[0049] In the depicted embodiment, the phase alignment section 30 and the horizontal alignment section 32 are printed twice by each print head 16, specifically on each side of the left and right sides of the print heads 16 of the respective printing units 12, 14.

[0050] In Figure 2 Regions 34, 36 designated by one print head 16 in a direction transverse to the paper travel direction are specified. At the transition between the two print heads 16 of one printing unit 12, 14, the print heads 16 overlap to some extent. In this region, a print overlap section 38 is printed. The overlap section 38 allows color adjustment, in particular adjustment of the print heads 16 of one printing unit 12, 14 relative to each other.

[0051] In addition, in the depicted embodiment, the image includes a reference section 40, which is printed twice by each print head 16 on each side of the left and right sides of the print head 16.

[0052] In addition, the image includes a coarse phase alignment section 42. The coarse phase alignment section 42 is suitable for implementing a coarse adjustment of the phase difference before a more precise adjustment.

[0053] In addition, the image includes a coarse horizontal alignment section 44. The coarse horizontal alignment section 44 is suitable for implementing a coarse alignment of the printing units 12, 14 in a direction transverse to the paper travel direction 20 before a more precise adjustment.

[0054] 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.

[0055] In addition, a method for aligning the first printing unit 12 and the second printing unit 14 in a 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.

[0056] Reference Figures 2 to 5, a method will be described for adjusting the phase difference between a first printing unit 12 and a second printing unit 14.

[0057] Figures 3 to 5 The phase alignment section 30 of the image is shown.

[0058] When printing the phase alignment section 30, in a first step, the first printing unit 12 prints Figure 3 the 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 apart 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.

[0059] The distance d1 remains constant along the first pattern.

[0060] In Figure 4 the following step visualized in, a second pattern that overlaps the first pattern is printed by the 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 apart 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.

[0061] The second pattern constitutes a second periodic signal S2 having a second frequency f2.

[0062] In Figure 3 and Figure 4 , 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.

[0063] 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.

[0064] Specifically, S1 and S2 appear as sinusoidal periodic signals in the calculation. In addition, the interaction between light and ink on the paper is approximately multiplicative, and the third signal is the result of multiplying the first signal by 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, by setting the frequencies of the first signal and the second signal, the period of the third signal is selected 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 are between 100 and 300 μm.

[0065] In a specific embodiment, the size of the ink droplets (i.e., the thickness of lines 45, 46) dripping from the print head 16 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 the lines 45, 46 to appear to have a thicker thickness and to be blurred. Therefore, by printing several 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).

[0066] When the first and second patterns (i.e., the first and second signals S1, S2) are superimposed, a third periodic signal S3 is generated.

[0067] 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 device 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.

[0068] Figure 5 Shows the signal S3 detected by the camera 24.

[0069] Specifically, the phase of the third signal S3 is determined to derive the value of the phase of the second signal S2.

[0070] The phase difference between the first signal S1 and the second signal S2 can be determined from the phase of the second signal S2.

[0071] 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.

[0072] For example, the pixel size of the camera 24 is 90 μm. Thus, the distance d3 represents approximately 16 pixels. By detecting the position of the maximum value of the third signal S3 with a precision of one pixel, the value of the phase difference between the two signals S1 and S2 can be evaluated with a precision of approximately 1 / 10 of a pixel. In other words, according to the described method, the phase difference between the two signals S1 and S2 can be detected with particularly high accuracy.

[0073] 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. When the positions of the maximum and minimum values of the third signal S3 are known, the phase of the third signal S3 can be evaluated in an easy manner.

[0074] 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.

[0075] The reference signal S R has the same frequency as the third periodic signal S3. In particular, the phase of S R is set to zero, which corresponds to the phase of the signal S3 when the 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 the direction Y). Using the reference signal S R the operation can be carried out without precisely determining the distance traveled by the paper between the printing unit and the camera.

[0076] 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.

[0077] In Figure 2 the positions of the maximum value Max R and the minimum value Min R of the reference signal S R are indicated.

[0078] 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 print head 16 of the printing unit 12 can be detected separately. In particular, rotation occurs when the phase difference between the first and second signals S1 and S2 in the two alignment sections 30 is different.

[0079] By means of phase alignment sections present at the leftmost and rightmost portions of the printed sheet, 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.

[0080] Before performing the fine adjustment of the phase difference, a rough phase adjustment is carried out.

[0081] The rough phase adjustment is carried out with an accuracy of at least half of the first distance d1 to avoid the uncertainty of the position given by the periods of the signals S1, S2.

[0082] As Figure 2 depicted, the rough phase adjustment is carried out by means of a first rough pattern and a second rough pattern printed in the rough phase alignment section 42. The first rough 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 rough 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 rough phase adjustment uses thicker lines, we do not superimpose them: we only calculate the position of each line. For rough 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.

[0083] The rough adjustment is carried out 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.

[0084] By means of 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.

[0085] Reference Figure 6 is made to describe the horizontal alignment section 32 in more detail.

[0086] 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 equidistantly spaced from each other at a first distance d1' in a direction transverse to the paper travel direction 20.

[0087] 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.

[0088] In addition, a second pattern overlapping the first pattern is printed by the second printing unit 14. 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 equal distances d2' from each other that are the same as the first distance d1'.

[0089] In Figure 6 the lines 56 are depicted as dashed lines to better distinguish the lines 52, 56 from each other.

[0090] For example, the first distance d1' and the second distance d2' are between 100 and 300 μm.

[0091] 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 that can be distinguished from each other due to the different horizontal positions of the lines 56 of the second pattern.

[0092] 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 depicted 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.

[0093] However, there can be a vertical distance between individual bands 53. In this case, the bands 53 can be digitally reorganized by the control unit 28 for further analysis.

[0094] 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 first section with respect to the direction transverse to the paper travel direction. In other words, the positions of the lines 56 are modified in the direction transverse to the paper travel direction. In other words, the positions of the lines 56 are modified on the bands 53.

[0095] In particular, the lines 56 of the second pattern defining the bands 53 are shorter than the lines 52 of the first pattern.

[0096] In Figure 6 the depicted section 54, the positions of the lines 56 of the second pattern are shifted seven times along the horizontal direction X in the section 54.

[0097] The second pattern constitutes a second signal of the periodic signal S2'.

[0098] 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'.

[0099] 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 originates from the change in the appearance of the belt 53. The period of the signal S3' is determined by the shift amount (i.e., phase difference) between S1' and S2' on the belt 53. The phase of the signal S3' is determined by the phase difference between the signal S1' and S2' on the predetermined belt 53.

[0100] Based on the route 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.

[0101] For example, the line 56 shifts once per centimeter. In other words, in this example, the height of the belt 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 the direction transverse to the paper travel direction 20.

[0102] As described with respect to phase alignment, with regard to the misalignment of the printing units 12, 14 in the direction transverse to the paper travel direction 20, it can be roughly aligned by means of the rough horizontal alignment section 44.

[0103] As the rough phase alignment section 42, the rough horizontal alignment section 44 is composed 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 be composed 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 be composed 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.

[0104] Similar to the rough phase alignment, the horizontal route adjustment is performed based on the relative positions of the first and second rough patterns to each other, where the accuracy of the rough adjustment is at least half of the first distance d1'.

[0105] Preferably, the first distance d1 of the line 45 extending in the direction transverse to the paper travel direction 20 and the first distance d1' of the line 52 extending in the direction parallel to the paper travel direction 20, and the second distance d2 of the line 46 extending in the direction transverse to the paper travel direction 20 and the second distance d2' of the line 56 extending in the direction parallel to the travel direction 20 are selected such that the frequencies f3, f3' of both of the superimposed third signals S3, S3' are the same. Thus, the same reference signal S RCan be used to detect the maximum and minimum values of the third signals S3 and S3' respectively.

[0106] In Figure 7 a photograph of the printed image is shown.

[0107] In the photograph, the positions of the maximum values Max of the third signals S3 and S3' in the phase alignment section 30 and the horizontal alignment section 32 are marked.

[0108] In the illustrated 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 may be printed.

Claims

1. A method for adjusting the phase difference between at least a first printing unit (12) of a printing device (10) and a second printing unit (14) of the printing device (10), each printing unit (12, 14) 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 the steps of: - printing a first pattern with the first printing unit (12), the first pattern comprising a plurality of parallel lines (45), the lines (45) extending in a direction transverse to the paper travel direction (20) and being equidistantly spaced from each other along the paper travel direction (20) by a first distance (d1), - printing a second pattern overlapping the first pattern with the second printing unit (14), the second pattern comprising a plurality of parallel lines (46), the lines (46) extending in a direction transverse to the paper travel direction (20) and being equidistantly spaced from each other along the paper travel direction (20) by a second distance (d2), wherein the second distance (d2) is different from the first distance (d1), - measuring, with a camera (24) along the paper travel direction (20), a third periodic signal (S3) resulting from the superposition of the first and the second periodic signals (S1, S2), - Evaluate the phase difference between the first printing unit (12) and the second printing unit (12) based on the route of the measured third periodic signal (S3), where 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 ), and - adjusting the phase difference by adjusting the ink dispensing rhythm from at least one of the first and the second printing units (12, 14).

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), 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 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 relative positions of the first and second rough patterns with respect to each other, wherein the accuracy of the rough adjustment is at least half of the first distance (d1).

5. 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 and / or the position of at least one minimum of the signal (S3) in the paper travel direction (20).

6. 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).

7. 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 phase difference between the first printing unit (12) and the second printing unit (12).

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