METHOD FOR FINE ADJUSTMENT OF THE POSITION OF INK DROPS PRINTED BY AT LEAST ONE PRINTING HEAD OF A PRINTING DEVICE AND PRINTING DEVICE

Abstract

A method for fine adjustment of the position of ink drops printed by at least one printing head (12) of a printing device, the printing head (12) being a two-dimensional printing head (12) comprising a plurality of nozzles (30). In one method step, an ink drop is dispensed at a plurality of nozzles (30) of the at least one printing head (12) simultaneously to print an image, preferably at all nozzles (30) of the printing head (12). In another method step, the printed image is captured by a camera (20), wherein the camera (20) captures the pattern of dots printed by the printing head (12) and the captured pattern is compared to a pattern (36) of the nozzles (30) of the printing head (12). In a further method step, the dispensation of an ink drop from at least one nozzle (30) is modified in terms of timing with respect to the dispense of ink drops from the other nozzles (30) of the printing head (12) for the subsequent printing process if a deviation has been detected between the pattern (36) of dots printed by the printing head (12) and the nozzle pattern (36). Furthermore, a printing device is provided.

Claims

1. A method for fine adjustment of a position of ink drops printed by at least one printing head of a printing device, the printing head being a two-dimensional printing head comprising a plurality of nozzles, each printing head has nozzles arranged in columns and rows, wherein each column and row has at least two nozzles, the method comprising: dispensing an ink drop at a plurality of nozzles of the at least one printing head simultaneously to print an image, preferably at all nozzles of the printing head, capturing the printed image by a camera, wherein the camera captures a pattern of dots printed by the printing head and the captured pattern is compared to a pattern of the nozzles of the printing head, and modifying the dispensation of an ink drop from at least one nozzle in terms of timing with respect to the dispense of ink drops from other nozzles of the printing head for a subsequent printing process if a deviation has been detected between the pattern of dots printed by the printing head and the nozzle pattern.

2. The method according to claim 1, further comprising: determining the position and/or orientation of the at least one printing head based on the image captured by the camera, determining the deviation of the position and/or orientation of the printing head from a target position and/or orientation, and adjusting the position and/or the orientation of the printing head based on the determined deviation.

3. The method according to claim 2, further comprising: adjusting the position of the at least one printing head mechanically in a direction traverse to an advance direction of the printing device.

4. The method according to claim 1, wherein the printing device comprises at least two printing heads and a delay parameter is determined for at least one printing head in order to compensate a misalignment of the printing head along an advance direction of the printing device.

5. The method according to claim 1, wherein the camera covers can entire width of the at least one printing head.

6. A printing device, in particular an ink jet printing device, comprising: at least one printing head, the printing head being a two-dimensional printing head comprising a plurality of nozzles, each printing head has nozzles arranged in columns and rows, wherein each column and row has at least two nozzles, a camera configured to capture an image printed by the at least one printing head, and a control unit configured for processing the image captured by the camera and for comparing a pattern of printed dots to a pattern of the nozzles of the printing head and detecting a deviation between the pattern of dots printed by the printing head and the nozzle pattern, wherein the control unit is configured for controlling a timing of a dispensation of ink drops from every nozzle of the at least one printing head individually if a deviation has been detected between the pattern of dots printed by the printing head and the nozzle pattern.

7. Printing device according to claim 6, wherein the control unit is configured for determining a deviation of a position and/or orientation of the at least one printing head from a target position, and wherein the printing device comprises an adjustment unit configured to adjust a position and/or orientation of the at least one printing head based on a deviation determined by the control unit.

8. The printing device according to claim 6, wherein a pixel size of the camera is less than half of a distance between two adjacent nozzles of the at least one printing head and bigger than a required measurement accuracy, in particular bigger than 50 m.

9. The printing device according to claim 6, wherein the at least one printing head includes at least two printing heads, wherein the control unit is configured to control a timing of a dispensation of ink drops from different printing heads.

10. The printing device according to claim 6, wherein a position of the at least one printing head is adjustable in a direction transverse to an advance direction of the printing device and the position of the at least one printing head is fixed with respect to the advance direction.

11. The printing device according to claim 7, wherein the at least one printing head is mounted rotatably.

12. The printing device according to claim 6, wherein the at least one printing head has nozzles arranged in columns and rows, wherein each column and row has at least two nozzles.

13. The printing device according to claim 12, wherein the columns and rows are arranged in form of a parallelogram.

14. The printing device according to claim 6, wherein the camera is a linear camera covering an entire width of the at least one printing head.

15. The printing device according to claim 6, wherein the at least one printing head is attached to a bar extending transverse to an advance direction of the printing device.

Description

[0047] Further features and advantages of the invention become apparent from the following description and the enclosed figures. In the figures:

[0048] FIG. 1 shows an inventive printing device in a schematic view seen from below,

[0049] FIG. 2 shows printing heads of the inventive printing device of FIG. 1,

[0050] FIG. 3 shows a representation of a nozzle pattern of one of the printing heads of FIG. 2 seen from inside the printing head,

[0051] FIG. 4 schematically shows panels of two printing heads seen from inside the printing head, and

[0052] FIG. 5 shows a pattern visualizing the dots printed by a printing head compared to a nozzle pattern.

[0053] FIG. 1 shows a printing device 10 comprising a plurality of printing heads 12 in a schematic view. In the depicted embodiment, seven printing heads 12 are shown, however, the number of printing heads 12 may vary.

[0054] The printing device 10 is an ink jet printing device, in particular a digital printer.

[0055] The printing device 10 has a resolution of 30 m or an even better resolution. The resolution means the minimal distance between two printed dots.

[0056] The printing heads 12 are attached to a bar 14 extending transverse to an advance direction of the printing device 10.

[0057] The bar 14 is attached to a machine frame 16 of the printing device 10.

[0058] The advance direction corresponds to a paper/substrate travel direction and is indicated in FIG. 1 by arrow 18.

[0059] All the printing heads 12 attached to one bar 14 are configured to print a single colour. Thus, for printing different colours the printing device 10 comprises several printing bars 14 with attached printing heads 12 that are arranged along the advance direction. For reasons of simplicity, only one printing bar 14 is depicted in FIG. 1.

[0060] The printing device comprises a camera 20, which is for example a 2D-camera, in particular a linear camera, that is configured to capture an image printed by the printing heads 12.

[0061] The camera 20 covers the whole width of the printing heads 12. In particular, the camera 20 extends over the whole width of a paper/substrate 22 that is processed in the printing device 10.

[0062] The camera is positioned downstream of the printing heads 12 with respect to the paper/substrate travel direction 18.

[0063] The printing device 10 further comprises a control unit 24 configured for processing the image captured by the camera 20.

[0064] The control unit 24 is further configured for determining a deviation of the position and/or orientation of the printing heads 12 from a target position.

[0065] The target position is a position in which the printing heads 12 attached to one bar 14 are aligned with respect to each other in such a way that an image printed by the printing device 10 is printed with the required accuracy i.e. such that no transition zone between two printing heads 12 is visible on the printed image.

[0066] In order to align the printing heads 12 with respect to each other, the printing device comprises an alignment unit 26.

[0067] The alignment unit 26 is configured to adjust a position and/or orientation of the printing heads 12 based on a deviation determined by the control unit 24.

[0068] The position of the printing heads 12 is adjustable in a direction transverse to the advance direction 18 of the printing device 10, in particular by means of the alignment unit 26.

[0069] Furthermore, the printing heads 12 are mounted rotatably.

[0070] The position of the printing heads 12 with respect to the advance direction 18 is fixed.

[0071] For example, the alignment unit 26 comprises alignment means 28 assigned to each printing head 12.

[0072] The alignment means 28 may comprise a linear drive and/or a rotational drive in order to adjust the position and/or orientation of the printing heads 12.

[0073] The printing heads 12 are two-dimensional printing heads.

[0074] FIG. 2 shows three printing heads 12 arranged in a row in a view from below such that the nozzles 30 of the printing heads 12 are visible.

[0075] Each of the printing heads 12 comprises a plurality of nozzles 30 (see also FIGS. 3 and 4).

[0076] More precisely, each printing head 12 comprises a printing section 32 in which the nozzles 30 are arranged.

[0077] Each nozzle 30 can be addressed individually.

[0078] Also, the amount of ink ejected from a nozzle 30 can be controlled individually.

[0079] The nozzles 30 are produced in a panel 34 which is inserted in the printing head 12.

[0080] In order to print an image, drops of ink are dispensed from the nozzles 30 in order to form dots on a paper/substrate 22 while the paper/substrate 22 travels along the advance direction.

[0081] The control unit 24 is configured to control the timing when a drop is dispensed from a nozzle 30. In particular, the control unit 24 is configured to delay a dispensation of ink drops from a nozzle.

[0082] According to one aspect, a global delay can be achieved by means of the control unit 24. That means that the control unit 24 adapts the timing for dispensation of ink from all nozzles 30 of a printing head 12 in the same manner.

[0083] According to another aspect an individual delay can be achieved, which means that the timing for dispensation of ink from the nozzles is controlled individually for every single nozzle 30 of a printing head 12.

[0084] FIG. 3 shows a nozzle pattern 36 of a printing head 12. The pattern 36 depicted in FIG. 3 may be present twice on each printing head 12, as it is apparent in FIG. 2.

[0085] The position of the nozzles 30 in the printing head 12 can be manufactured with a high accuracy, in particular with an accuracy in the submicron range. For example, the position of the nozzles 30 is produced with an accuracy of 80 to 100 nm.

[0086] The nozzles 30 are arranged in columns and rows, wherein each column and row has a plurality of nozzles 30.

[0087] More precisely, the columns and rows are arranged in form of a parallelogram.

[0088] The specific pattern of the nozzles 30 facilitates a high resolution of an image printed by the printing device.

[0089] In one exemplary embodiment, the printer can print a dot every 21.16 m while a diameter of the printed dot is 30 m.

[0090] FIG. 4 schematically shows the panels 34 of two printing heads 12 comprising the nozzle pattern 36 of FIG. 3.

[0091] However, the nozzle pattern 36 of FIG. 3 is comprised twice by each printing head 12, wherein there is a distance between the patterns 36.

[0092] The parallelograms formed by the columns and rows of nozzles 30 are tilted with respect to the outer boundaries of the printing heads 12.

[0093] More precisely, a row formed by the outermost nozzles 30 of the nozzle pattern 36 is inclined with respect to an edge of the printing head 12 extending in a direction transverse to the advance direction 18. This tilted arrangement ensures a continuous printing dot coverage capability in a direction transverse to an advance direction of the printing device 10 despite a small (adjustable) gap between the printing heads 12. In other words, the leftmost nozzle in a printing head is located to the left of the rightmost nozzle of its (closest) neighbouring printing head when there is no gap between the printing heads, the right and left direction being measured along direction transverse to an advance direction 18 of the printing device 10. Thanks to the inclination of the outermost nozzles 30 of the nozzle pattern 36, the largest acceptable gap between the printing heads 12 that keeps a continuous printing dot coverage is increased. In particular, due to the tilted arrangement, two neighbouring printing heads 12 can print with a slight overlap to avoid a visible gap in the printed image even if there is a slight distance between the printing heads 12.

[0094] The pixel size of the camera 20 is related to the nozzle pattern 36, in particular to a distance of the nozzles 30.

[0095] The pixel size of the camera 20 is less than half of the distance between two adjacent nozzles 30 of a printing head 12, for example 0.4 times the distance between two adjacent nozzles 30. Thus, the camera 20 comprises at least two pixels for one dot printed by a printing head 12.

[0096] Yet, the pixel size of the camera 20 is bigger than a required measurement accuracy, in particular bigger than 50 m.

[0097] In an exemplary embodiment, the pixel size is 90 m.

[0098] FIG. 5 shows a printing pattern printed by a printing head 12 compared to a nozzle pattern 36.

[0099] The filled dots visualize the position of the nozzles 30. The unfilled dots visualize the position of the dots printed by the printing head 12.

[0100] In an ideal scenario, when every ink drop lands exactly at the desired position on the paper/substrate 22, the arrangement of the printed dots corresponds to the nozzle pattern 36.

[0101] In FIG. 5, however, it is obvious that the arrangement of the dots does not completely correspond to the nozzle pattern 36. This deviation is due to different random factors, like for example, small differences in reaction time of each individual nozzle, small differences in ejection pressure and speed or differences in the angle of ejection of the ink drops.

[0102] Such a deviation can be compensated along the advance direction 18 of the printing device 10 by means of the control unit 24 controlling the timing of a dispensation of ink drops from every nozzle of a printing head individually.

[0103] The compensation becomes effective for images printed after the compensation happened.

[0104] In the following, a method for aligning the printing heads 12 of a printing device is described. In practice, the fine adjustment of the position of ink drops method disclosed in this invention is applied after the print heads 12 are aligned

[0105] An alignment of the printing heads 12 is necessary before the first use of the printing device 10 or after a printing head 12 has been exchanged or reinstalled, for example after maintenance. When all printing heads 12 are properly aligned, a high-quality print can be achieved.

[0106] Firstly, the printing device 10 is started and a paper/substrate 22 travels along the advance direction 18.

[0107] While the paper/substrate 22 travels along the advance direction 18, an ink drop is dispensed at a plurality of nozzles 30 of every printing head 12 simultaneously to print an image, preferably at all nozzles 30 of every printing head 12.

[0108] However, it is also possible that ink is dispensed only at a determined group of nozzles 30. For example, ink may be dispensed from every nozzle 30 except the outermost nozzles 30 of the nozzle pattern 36. In another example, the ink may be dispensed from every third (or n-th) nozzle 30 of the nozzle pattern 36 (in both directions).

[0109] By dispensing an ink drop from each nozzle 30 simultaneously, the printed dots allow conclusions about the positions of the printing heads 12 with respect to each other. The simultaneous dispensation is only necessary for a calibration process, in the regular operation of the printing device, a simultaneous dispense is not required.

[0110] While the paper/substrate 22 travels further along the advance direction 18, the printed image is captured by a camera 20.

[0111] Because of the specific pixel size already discussed above, the camera 20 can distinguish all the dots printed by the printing heads 12.

[0112] For each dot of the image that is captured by the camera 20, it is estimated by the control unit 24 from which nozzle 30 the ink producing the dot has most likely been ejected.

[0113] Based on the image captured by the camera 20, the position and/or orientation of each of the printing heads 12 is determined.

[0114] The position and/or orientation of the printing head 12 is for example estimated by means of bundle adjustment. For example, an iteratively reweighted square method can be used.

[0115] Optionally, a scale parameter could be added in the estimation.

[0116] Optionally, a skew parameter could be added in the estimation. En ralit: 2 translation, 2 scales, 1 skew et 1 rotation, rajouter cela dans l'autre brevet?

[0117] Afterwards, the deviation of the position and/or orientation of each printing head 12 from a target position and/or orientation is determined, in particular by means of the control unit 24.

[0118] For example, information about a target position of the printing heads 12 is saved in a memory of the control unit 24.

[0119] If a deviation has been detected, the position and/or the orientation of the printing heads 12 is adjusted, in particular by means of the adjustment unit 26.

[0120] When a deviation has been detected regarding a position of a printing head 12 in a direction traverse to an advance direction 18 of the printing device 10, the position of the printing heads 12 is adjusted mechanically in the respective direction.

[0121] If a deviation has been detected regarding a rotational orientation of a printing head 12, the orientation is adjusted mechanically.

[0122] The position of the printing heads 12 can be adjusted with an accuracy of at least 5 m.

[0123] However, if a deviation has been detected regarding a position of a printing head 12 in the advance direction 18 with respect to the neighbouring printing heads, the misalignment is compensated by means of the control unit 24 determining a delay parameter for the respective printing head 12. In particular, the control unit 24 effects a global delay.

[0124] Moreover, in order to compensate for the misalignment of individual dots of an image with respect to the nozzles 30 of the printing head 12, the dispensation of an ink drop from a respective nozzle 30 is modified in terms of timing with respect to the dispense of ink drops from the other nozzles 30 of the printing head 12 for the subsequent printing process.

[0125] The dispensation timing of each individual nozzle 30 is controlled by the control unit 24.