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

Abstract

A method is disclosed 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) including at least one print head (16), and the at least first and second printing units (12, 14) are spaced apart from each other in a paper travel direction (20). Further, a printing device (10) is disclosed.

Description

[Technical Field]

[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.The present invention further relates to a printing device.

[0002] A printing device typically has multiple printing units for printing different color planes, and the print medium, in particular a paper web, passes successively through the different printing units.

[0003] To ensure high print quality, the alignment of the color planes must be extremely accurate. In particular, color misalignment of more than 50 microns significantly impacts print quality. Therefore, it is necessary to control the registration of the printing units with respect to horizontal and vertical alignment as well as skew. The vertical direction (or "Y" direction) corresponds to the paper travel direction, and the horizontal direction (or "X" direction) corresponds to the direction perpendicular to the paper travel direction.

[0004] Alignment of a printing unit is typically performed in an automated manner, for example by capturing an image printed by the printhead of the printing unit and inferring, based on the position of the image, whether the printhead is properly aligned or not.

[0005] US Patent Application Publication No. 2012 / 0092403 describes a printing device with printheads arranged in units, each unit printing a different color. A vernier pattern is used to detect and correct misalignment between the printheads of different units. The position of high density regions in the vernier pattern indicates the relative vertical misalignment between the printheads.

[0006] However, known methods are not satisfactory in terms of required accuracy or complexity. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] US Patent Application Publication No. 2012 / 0092403 Summary of the Invention [Problem to be solved by the invention]

[0008] It is therefore an object of the present invention to enable sufficient phase adjustment between printing units of a printing device. [Means for solving the problem]

[0009] To summarize the principles detailed below, this objective is achieved by performing alignment along the vertical direction (i.e., the paper travel direction). To facilitate the reader's understanding, the principles are outlined (incompletely) in the next paragraph and then broadly explained in the following text.

[0010] Vertical alignment is achieved by printing a periodic signal made up of horizontal lines on each printing unit, using a first period on the first printing unit and a second, different period on the second printing unit. The period is given by the distance between two adjacent horizontal lines. The result is "read" by detecting the extrema of a third signal along the vertical direction.

[0011] This object is achieved by a method for adjusting a phase difference between at least a first printing unit and a second printing unit of a printing device, wherein each printing unit comprises at least one print head, and the at least first and second printing units are spaced apart from each other in the paper travel direction. In particular, there is no overlap between the print heads of different printing units in a direction along, i.e., perpendicular to, the paper travel direction. One method step includes printing a first pattern with the first printing unit, the first pattern comprising a plurality of parallel lines extending perpendicular to the paper travel direction and equally spaced from each other by a first distance along the paper travel direction. In particular, the first pattern constitutes a first periodic signal having a first frequency. A further method step includes printing a second pattern with the second printing unit, overlapping the first pattern, the second pattern comprising a plurality of parallel lines extending perpendicular to the paper travel direction and equally spaced from each other by a second distance along the paper travel direction, the second distance being 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 resulting from superimposing the first and second periodic signals is measured with a camera of the printing device, a phase difference between the first and second printing units is estimated based on the measured progression of the third periodic signal, 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 includes adjusting the phase difference by adjusting the timing of ink ejection from at least one of the first and second printing units.

[0012] The interaction of light with ink is assumed to be approximately multiplicative, so the third signal, or at least a portion of the third signal, is the result of multiplying the first signal by the second signal.

[0013] The method of the present invention allows for particularly high alignment accuracy to be achieved with respect to the phase difference between the first and second printing units. The alignment accuracy can be even higher than the pixel size of the camera. For example, with a camera having a pixel size of 90 μm, an alignment accuracy of 10 μm to 50 μm can be achieved. In this application, pixel size refers to the pixel size of the printed image projected onto each pixel of the camera sensor. In other words, pixel size refers to the square of the printed image that can be individually identified by the camera.

[0014] The third periodic signal is measured by roughly reading the captured image to detect the peak of the third periodic signal. This allows the phase difference between the first periodic signal and the second periodic signal to be determined with high accuracy. 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, the phase difference corresponds to a misalignment of the color planes printed by different printing units.

[0015] 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 a reference signal. The phase of the reference signal is known, e.g., zero. In other words, the reference signal is a calculated superposition signal realized by superposing the first and second signals without a phase difference.

[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 travels in a direction along the paper travel direction.

[0018] This method is based on the idea that, rather than measuring the first and second signals directly, a third signal is measured, which is a superposition of the first and second signals, and the relationship between the first and second signals, particularly the phase difference, is evaluated.

[0019] In particular, the phase difference is determined by comparing the measured superimposed signal with a calculated superimposed signal, which represents the signal achieved by superimposing the first and second signals without the phase difference.

[0020] According to one embodiment, the first and / or second distances are determined so that the first or second signal is detected by the camera as a continuously varying signal. In other words, the camera does not see distinct, sharp gaps between the lines, but only changes in signal amplitude that appear as a set of blurred lines. Therefore, the signal is not binary, but is made up of smooth transitions in grayscale values. This allows the first and / or second signal to be detected as a sinusoidal signal rather than as separate lines. This can be achieved as a result of the fact that the edges of printed lines broaden. For example, inkjet-printed lines are typically 30 μm thick, and photons undergo multiple reflections within the paper before leaving the paper and reaching the camera. Furthermore, the limited resolution of the optical system and camera sensor can cause some blurring. As a result, printed lines thicker than 30 μm appear blurred. The combination of both effects can result in a third signal being measured as a superimposed signal when the first and second distances are selected, respectively. The third signal can also be treated as a sinusoidal signal.

[0021] In an exemplary embodiment, a few lines are printed per millimeter, and the difference between the first distance and the second distance is selected to be a few percent. The frequency of the third signal is such that it is easily detectable. For example, the third signal has a frequency of 5 to 10 repetitions per centimeter along the paper travel direction.

[0022] The third periodic signal can be used by measuring the position of at least one maximum and / or the position of at least one minimum of the signal in the paper travel direction, which makes measuring the third signal particularly easy.

[0023] In addition to the first pattern, a first coarse pattern can be printed by the first printing unit, and a second coarse pattern can be printed by the second printing unit, and coarse adjustment is performed based on the positions of the first coarse pattern and the second coarse pattern relative to each other, and the accuracy of the coarse adjustment is at least half of the first distance.

[0024] Because the first and second signals are periodic, there is a position uncertainty given by the period of the signals. A coarse pattern is used to reduce 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. This uncertainty can be avoided by performing a coarse adjustment. In particular, the coarse adjustment must be performed with an accuracy of at least half the first distance to reduce the uncertainty of the first misalignment measurement. This allows for reliable phase adjustment and alignment, especially of the printing units.

[0025] The coarse pattern can be, for example, similar to the first and second patterns, but using thick lines without overlapping and with a large distance between them. From these lines, the misalignment between the printing units is calculated (by any suitable method, here the gaps between the lines are clearly visible). This misalignment has a lower accuracy than that obtained by methods using thinner lines. In particular, the required accuracy obtained with the thick lines must be less than half the initial distance (between the two thin lines). Alternatively, standard alignment marks can be used instead of the thick repeating lines.

[0026] According to one embodiment, a first pattern and a second pattern are printed on the left and right sides of a print head of a printing unit. By comparing a third signal derived 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 third signals on the left and right sides of the print head are out of phase with each other. Rotational misalignment can be corrected by physically rotating the print head or by adjusting the ink ejection on the right side of the print head differently from that on the left side.

[0027] According to another aspect, a first pattern and a second pattern are printed on the left and right sides of a print bar of a printing unit, respectively. By comparing a third signal obtained from the superimposed signals of the first and second patterns on the left and right sides of the print bar, skew misalignment along the paper movement direction can be detected and corrected accordingly. In particular, skew along the paper movement direction is detected when the third signals on the left and right sides of the print bar are out of phase with each other. This skew can be corrected by correcting the timing of ink ejection across the print bar.

[0028] This is 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 having at least one print head, a camera configured to capture images printed by the printing units, and a control unit configured to process the images captured by the camera, wherein the control unit is configured to evaluate the phase difference between the first printing unit and the second printing unit.

[0029] Further features and advantages can be derived from the following description and the enclosed drawings. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 is a diagram illustrating a printing device. [Figure 2] FIG. 2 is a diagram illustrating an image printed by a printing device. [Figure 3] 3A-3C show intermediate stages in printing a section of the image of FIG. 2; [Figure 4] FIG. 3 shows a section of the image of FIG. 2. [Figure 5] FIG. 5 shows the section of FIG. 4 measured by a camera. [Figure 6] FIG. 3 shows a further section of the image of FIG. 2. [Figure 7] 3 is a photograph of an image printed by a printing device as visualized diagrammatically in FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0031] 1 shows a schematic representation of 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.

[0032] The first printing unit 12 and the second printing unit 14 are configured to print different color planes. For simplicity, only two printing units 12, 14 are depicted in Figure 1. However, the printing device 10 can have more than two printing units 12, 14.

[0033] The printing units 12 and 14 are spaced apart from each other in the paper travel direction, and in particular, the printing units 12 and 14 do not overlap with each other in the paper travel direction.

[0034] Each printing unit 12, 14 has a plurality of print heads 16. The print heads 16 are aligned along a line that defines the print bars 13, 15, respectively.

[0035] The print head 16 is mounted on bars 13, 15 which extend perpendicular to the direction of advancement of the printing device 10.

[0036] The print head 16 is movable along the bars 13, 15, or at least rotatable as indicated by the arrow in FIG.

[0037] The forward direction corresponds to the paper travel direction and is indicated by arrow 20 in FIG.

[0038] The bars 13 , 15 are attached to a machine frame 22 of the printing device 10 .

[0039] All printheads 16 attached to one bar 13, 15 are configured to print a single color.

[0040] The printing device comprises a camera 24, for example a 2D-camera or a line camera, arranged to take an image of the sheets printed by the printing units 12,14.

[0041] The camera 24 covers the entire width of the printing units 12, 14. In particular, the camera 24 extends across the entire width of the paper 26 being processed in the printing device 10.

[0042] The camera 24 is positioned downstream of the printing units 12 and 14 in the paper travel direction 20 .

[0043] The printing device 10 further comprises a control unit 28 configured to process images captured by the camera 24 .

[0044] The control unit 28 is further configured to determine the phase difference or misregistration between the printing units 12,14.

[0045] In particular, the control unit 28 is configured to determine the phase difference or misregistration between the printing units 12, 14 based on the image printed by the printing device 10, as will be explained in more detail with respect to the following figures.

[0046] Figure 2 shows an image printed by printing device 10, and in particular by first printing unit 12 and second printing unit 14. The image therefore includes two different colors, which in Figure 2 are two different shades of gray. For example, the two colors are black and magenta.

[0047] The image has different sections and different patterns printed in the different sections, however some sections have the same pattern.

[0048] Sections of the image designated with the same reference number have the same pattern.

[0049] The image has a phase alignment section 30 to allow adjustment of the phase difference.

[0050] Additionally, the imager has a horizontal registration section 32 which is adapted to allow registration of the printing units 12 , 14 in a direction perpendicular to the paper travel direction 20 .

[0051] In the illustrated embodiment, the phase alignment section 30 as well as the horizontal alignment section 32 are printed twice by all print heads 16, specifically on each of the left and right sides of the print heads 16 of each printing unit 12,14.

[0052] The areas 34, 36 covered by one print head 16 in the direction perpendicular to the paper travel direction are designated in Figure 2. 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 area, an overlap section 38 is printed. The overlap section 38 allows for in-color adjustment, in particular the adjustment of the print heads 16 of one printing unit 12, 14 relative to each other.

[0053] Additionally, the image comprises a reference section 40 that is printed twice by all print heads 16, one on each of the left and right sides of the print heads 16, in the embodiment shown.

[0054] Furthermore, the imager comprises a coarse phase alignment section 42. The coarse phase alignment section 42 is adapted to allow a coarse adjustment of the phase difference before a finer adjustment.

[0055] The image further comprises a coarse horizontal registration section 44. The coarse horizontal registration section 44 is adapted to allow for coarse registration of the printing units 12, 14 in a direction perpendicular to the paper travel direction 20, prior to finer adjustments.

[0056] The following describes in more detail the different sections 30, 32, 38, 40, 42, 44, as well as a method for adjusting the phase difference between the first printing unit 12 and the second printing unit 14 based on the printed image, in particular by processing the printed image by the control unit 28.

[0057] Further, a method for aligning the first printing unit 12 and the second printing unit 14 in a direction perpendicular to the paper travel direction based on the printed image, and in particular by processing the printed image by the control unit 28, is described.

[0058] 2 to 5, a method for adjusting the phase difference between the first printing unit 12 and the second printing unit 14 will be described.

[0059] 3-5 show the phase alignment section 30 of the image.

[0060] When printing the phase alignment section 30, in a first step, a first pattern depicted in Figure 3 is printed by the first printing unit 12. The first pattern comprises a plurality of parallel lines 45 extending perpendicular to the paper travel direction 20 and equally spaced apart from one another at a first distance d1 along the paper travel direction 20.

[0061] The first pattern constitutes a first periodic signal S1 having a first frequency f1.

[0062] The distance d1 is kept constant along the first pattern.

[0063] In the next step visualized in Figure 4, a second pattern overlapping the first pattern is printed by the second printing unit 14, the second pattern extending perpendicular to the paper travel direction 20 and comprising a plurality of parallel lines 46 equally spaced from one another along the paper travel direction at a second distance d2, the second distance d2 being different from the first distance d1.

[0064] The second pattern constitutes a second periodic signal S2 having a second frequency f2.

[0065] 3 and 4, lines 45 and 46 can be seen as separate horizontal lines for better understanding, and in particular, line 45 is depicted as a continuous line and line 46 is depicted as a dashed line to make it easier to distinguish the first and second lines 45, 46 from each other.

[0066] However, the first distance d1 and the second distance d2 are determined so that the first signal S1 and the second signal S2, respectively, are detected by the camera 24 as continuously changing signals, for example, several lines printed per mm.

[0067] In particular, S1 and S2 appear as periodic signals, which are sinusoidal in calculations. Furthermore, because the interaction between light and ink on paper is approximately multiplicative, the third signal is the result of multiplying the first and second signals. 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, 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, e.g., the distance between two positions where a line of the first signal aligns with a line of the second signal. Thus, the period of the third signal is selected by setting the frequency of the first signal and the frequency of the second signal to obtain a predetermined frequency of 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 the period of the first signal, and can therefore be greater than 1 mm. As a result, this can be easily detected by a camera.

[0068] For example, the first distance d1 and / or the second distance d2 is not less than 100 μm and not more than 300 μm.

[0069] In one specific embodiment, the size of the ink droplets from the print head 16, i.e., the thickness of the lines 45, 46, is 30 μm. However, at this scale, due to the camera optics and sensor, and due to the photons having multiple reflections within the paper before leaving the paper and reaching the camera, the edges of the printed lines 45, 46 broaden, causing the lines 45, 46 to appear thicker and blurred. Thus, by printing several lines per mm, the signal recorded by the camera 24 can be considered to be a nearly continuously varying signal (and therefore not a binary signal).

[0070] When the first and second patterns, ie the first and second signals S1, S2, are superimposed, a third periodic signal S3 is obtained.

[0071] The third periodic signal S3 obtained by superposing 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 estimated based on the measured third periodic signal S3. The phase is the position of the signal peak.

[0072] FIG. 5 shows the signal S3 detected by the camera 24.

[0073] In particular, the phase of the third signal S3 is determined to derive a value for the phase of the second signal S2.

[0074] The phase difference between the first signal S1 and the second signal S2 can be determined from the phase of the second signal S2.

[0075] If a phase difference is detected, the phase difference is adjusted by adjusting the ejection timing of ink from at least one of the first printing unit 12 and the second printing unit 14.

[0076] The pixel size of the camera 24 is, for example, 90 μm, so the distance d3 corresponds to about 16 pixels.

[0077] 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 and S2 can be evaluated with an accuracy of about 1 / 10 of a pixel, i.e., this method allows the phase difference between the two signals S1 and S2 to be detected with particularly high accuracy.

[0078] The third periodic signal S3 is measured, for example, by measuring the positions of the maximum and minimum values ​​of the signal S3 in the paper travel direction. By knowing the positions of the maximum and minimum values ​​of the third signal S3, the phase of the third signal S3 can be easily evaluated.

[0079] The third signal S3 is used as the reference signal S R By comparing the third signal S3 with the third signal S4, the maximum and minimum of the third signal S3 are detected.

[0080] In particular, the reference signal S R is printed in the criteria section 40.

[0081] Reference signal S R has the same frequency as the third periodic signal S3. R The phase of S1 is set to zero, which corresponds to the phase of signal S3 when signals S1 and S2 are aligned. R When the reference signal S is aligned, the color planes of the printing units are also aligned (along the direction Y). R By using the , it is possible to operate without determining the exact distance of travel of the paper between the printing unit and the camera.

[0082] The phase of the third periodic signal S3 is determined by dividing the third signal S3 by the reference signal S R , the positions of the minimum and maximum values ​​of the third signal S3 are compared with those of the reference signal S R The maximum and minimum values ​​of the third signal S3 are detected by comparing them with the positions of the maximum and minimum values ​​of the reference signal S. R If there is a shift relative to the maximum and minimum values ​​of , a phase difference is detected.

[0083] Reference signal S R The positions of the maximum value MaxR and the minimum value MinR are shown in FIG.

[0084] 2, phase alignment sections 30 present on the left and right sides of print head 16 make it possible to detect rotation of printing units 12 of each print head 16. In particular, rotation exists when the phase difference between first and second signals S1, S2 is different in the two alignment sections 30.

[0085] The phasing sections at the left and right edges of the print sheet make it possible to detect skew of print bar 13 relative to print bar 15. Skew can be caused by imperfections in the parallelism between the different print bars 13, 15.

[0086] Before making a fine adjustment of the phase difference, a coarse phase adjustment is performed.

[0087] The coarse phase adjustment is performed to an accuracy of at least half the first distance d1 to avoid positional uncertainty given by the period of the signals S1, S2.

[0088] Coarse phase adjustment is performed by first and second coarse patterns printed in the coarse phase alignment section 42, as depicted in FIG. 2. The first coarse pattern is printed by the first printing unit 12 and consists of parallel lines 48 extending perpendicular to the paper advance direction, while the second coarse pattern is printed by the second printing unit 14 and consists of parallel lines 50 extending perpendicular to the paper advance direction. Because coarse phase adjustment uses thick lines, there is no overlapping; simply, the position of each line is calculated. Coarse alignment also does not require printing periodic signals. Simple alignment marks printed by each printing unit are also a viable alternative.

[0089] The coarse adjustment is performed based on the positions of the first and second coarse patterns relative to each other, and the accuracy of the coarse adjustment is at least half the first distance d1.

[0090] Print head rotation can be detected by phase alignment sections 30 on each of the left and right sides of print head 16. In particular, rotation is present when the phases of third signals S3 on the left and right sides of print head 16 are different.

[0091] Referring now to FIG. 6, the horizontal alignment section 32 will be described in more detail.

[0092] In the horizontal registration section 32, a first pattern is printed using the first printing unit 12. The first pattern comprises a plurality of parallel lines 52 extending in a direction parallel to the paper travel direction 20 and equally spaced apart from one another at a first distance d1′ in a direction perpendicular to the paper travel direction 20.

[0093] The lines 52 of the first pattern extend continuously 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.

[0094] Furthermore, 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 56 that extend in a direction parallel to the paper travel direction 20 and are spaced apart at an equal distance d2′ from one another, the same as the first distance d1′.

[0095] Line 56 is depicted in dashed form in FIG. 6 to better distinguish lines 52, 56 from one another.

[0096] For example, the first distance d1' and the second distance d2' are between 100 μm and 300 μm.

[0097] The parallel lines 56 of the second pattern form a plurality of bands 53. In particular, the bands 53 are sections that are distinguishable from one another by the different horizontal positions of the lines 56 of the second pattern along the first section 54.

[0098] The bands 53 are arranged one after the other in the paper travel direction 20, in particular in the vertical direction Y. In the illustrated embodiment, the bands 53 are immediately adjacent to one another in the vertical direction, i.e. there is no vertical distance between the bands 53.

[0099] However, there may be vertical distances between the individual bands 53. In that case, the bands 53 can be digitally reassembled by the control unit 28 for further analysis.

[0100] The positions of the lines 56 of the second pattern are shifted between different bands 53. There are at least three bands, and therefore the second pattern is shifted at least twice in the direction perpendicular to the paper travel direction in the first section. In other words, the positions of the in-lines 56 are changed in the direction perpendicular to the paper travel direction. In other words, the positions of the lines 56 are changed across the bands 53.

[0101] In particular, the lines 56 of the second pattern that define the bands 53 are shorter than the lines 52 of the first pattern.

[0102] In the section 54 depicted in FIG. 6, the positions of the lines 56 of the second pattern are shifted seven times along the horizontal direction X of the section 54 .

[0103] The second pattern constitutes a second signal that is a periodic signal S2'.

[0104] Since d2' is equal to d1', the first signal S1' and the second signal S2' have the same frequency along the horizontal direction X.

[0105] Due to the line 56 of the second signal S2' being shifted in section 54, the second signal S2' has a varying phase relative to S1' across the band 53. Within a single band, the phase of the second signal S2' compared to the phase of the first signal S1' is constant.

[0106] The third signal S3', which is a periodic signal along the vertical direction Y, results from the superposition of the first signal S1' and the second signal S2'. Thus, the third signal results from the change in appearance of the band 53. The amount of shift (i.e., phase difference) between S1' and S2' across the band 53 determines the period of the signal S3'. The phase of the signal S3' is determined by the phase difference between the signals S1' and S2' in a given band 53.

[0107] Based on the profile of the measured third signal S3', the misalignment between the first printing unit 12 and the second printing unit 14 in the direction perpendicular to the paper travel direction 20 is evaluated. In particular, based on the positions of the extrema of signal S3', the phase relationship between S1' and S2' can be calculated.

[0108] For example, the line 56 shifts once per centimeter, which means that in this example the band 53 is 1 cm high.

[0109] If misregistration is detected, the misregistration is corrected by shifting the first printing unit 12 and / or the second printing unit 14 in a direction perpendicular to the paper travel direction 20 .

[0110] As described with respect to phase alignment, coarse alignment of the printing units 12, 14 in a direction perpendicular to the paper travel direction 20 can be achieved by the coarse horizontal alignment section 44.

[0111] Similar to the coarse phase alignment section 42, the coarse horizontal alignment section 44 is comprised of a first coarse pattern and a second coarse pattern printed on the coarse horizontal alignment section 44. The first coarse pattern may be printed by the first printing unit 12 and comprised of thick parallel lines 58 extending in a direction parallel to the paper travel direction 20, and the second coarse pattern may be printed by the second printing unit 14 and comprised of thick parallel lines 60 extending in a direction parallel to the paper travel direction. Contrary to the fine adjustment sections 30, 32, the coarse adjustment is not critical and may be performed using any suitable known method.

[0112] As in the case of the coarse phasing, the horizontal course adjustment is performed based on the positions of the first and second coarse patterns relative to each other, and the accuracy of the coarse adjustment is at least half of the first distance d1'.

[0113] Preferably, the first distance d1 of the line 45 extending transversely to the paper travel direction 20 and the first distance d1' of the line 52 extending parallel to the paper travel direction 20, as well as the second distance d2 of the line 46 extending transversely to the paper travel direction 20 and the second distance d2' of the line 56 extending parallel to the paper travel direction 20, are selected so that the frequencies f3, f3' of both the superimposed third signals S3, S3' are the same, thereby making it possible to detect the maximum and minimum values ​​of each of the third signals S3, S3' using the same reference signal SR.

[0114] Figure 7 shows a photograph of the printed image.

[0115] In the photograph 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 illustrated embodiment, the image comprises not only the horizontal alignment section 32 but also the phase alignment section 30. However, depending on requirements, it is also possible that only the phase alignment section 30 or the horizontal alignment section 32 is printed. [Explanation of symbols]

[0117] 10 Printing Devices 12 First printing unit 14 Second printing unit 13,15 Print bar 16 print heads 22 Machine Frame 26 Paper

Claims

1. 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 said printing device (10), wherein each of said printing units (12, 14) comprises at least one print head (16), and at least the first and second printing units (12, 14) are spaced apart from each other in a paper travel direction (20), The method comprises: a step of printing a first pattern by the first printing unit (12), wherein the first pattern extends in a direction perpendicular to the paper running direction (20) and is spaced a first distance (d 1 a printing step including a plurality of parallel lines (45) equally spaced relative to one another in a a step of printing a second pattern by the second printing unit (14) that overlaps the first pattern, the second pattern extending perpendicular to the paper travel direction (20) and extending a second distance (d 2 ), and a plurality of parallel lines (46) equally spaced from each other by the second distance (d 2 ) is the first distance (d 1 ) a printing step, which is different from the A camera (24) is used to capture a first periodic signal (S 1 ) and a second periodic signal (S 2 ) along the paper travel direction (20) obtained by superposing the third periodic signal (S 3 ) measuring the The measured third periodic signal (S 3 ) based on the progress of the third periodic signal (S 3 ) having the same frequency as the reference signal (S R ) is printed, and the third signal (S 3 ) and the reference signal (S R ) to obtain the third periodic signal (S 3 ) the phase of the signal is detected; adjusting a phase difference by adjusting the ejection timing of ink from at least one of the first printing unit (12) and the second printing unit (14); A method comprising:

2. The first distance (d 1 ) and / or the second distance (d 2 ) is the first signal (S 1 ) or a second signal (S 2 2. The method of claim 1, wherein the signal is detected by the camera (24) as a continuously varying signal.

3. The first distance (d 1 ) and / or the second distance (d 2 3. The method of claim 2, wherein the thickness of the first electrode is 100 μm to 300 μm.

4. In addition to the first pattern, a first coarse pattern is printed by the first printing unit (12), and in addition to the second pattern, a second coarse pattern is printed by the second printing unit (14), and a coarse adjustment is performed based on the positions of the first coarse pattern and the second coarse pattern relative to each other, and the accuracy of the coarse adjustment is determined by the first distance (d 1 4. The method according to claim 1, wherein the amount of the hydroxyl group is at least half of the total amount of the hydroxyl group.

5. The third periodic signal (S 3 ) in the paper travel direction (20) 3 5. The method according to claim 1, wherein the position of at least one maximum and / or the position of at least one minimum of the

6. The method according to any one of claims 1 to 5, wherein a first pattern and a second pattern are printed on the left and right sides of the print heads (16) of the printing units (12, 14), respectively.

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 including at least one print head (16); a camera (24) configured to capture images printed by said printing units (12, 14); a control unit (28) configured to process the images captured by the camera (24); Equipped with The printing device (10) is configured to perform the method according to any one of claims 1 to 6, and 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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