EP3427957A1 - Detecting of failed print nozzles at the printing margin - Google Patents

Abstract

Method for detecting failed printing nozzles (11) in an inkjet printing press (7) by a computer (6), wherein for each color separation a nozzle test pattern (14) is printed, which consists of a certain number of horizontal rows of periodically vertically printed, equidistant lines (15 ) arranged one below the other, wherein in each row of the nozzle check pattern (14) only periodically the print nozzles contribute to the nozzle check pattern (14) corresponding to the determined number of horizontal rows and wherein the printed nozzle check pattern (16) is for evaluation by an image sensor is detected, digitized and transmitted to the computer (6) for evaluation, and which is characterized in that for determining failed print nozzles (11) on an edge of the print head (8, 9) in the evaluation of the nozzle check pattern (16) by the computer (6) examining the beginning of the horizontal rows (8, 9) of the nozzle check pattern (16) as shown in FIG t the lines (15) at the beginning of the respectively underlying row of the nozzle check pattern (16) differ geometrically, wherein the computer (6) examines the geometric deviation of the lines (15) to what extent the horizontal distance of the first line (15) of a row of the nozzle check pattern (16) from the first line (15) of the lower row of the nozzle check pattern (16).

Description

The present invention relates to a method for detecting failed nozzles in an inkjet printing machine at the printing edge.

The invention is in the technical field of digital printing.

When operating inkjet printing machines, which eject droplets of ink via printheads with one or more rows of printing nozzles in a targeted manner, the error image of the so-called "missing nozzles" can occur. This means that individual pressure nozzles no longer work properly. This can be expressed, for example, in that a so-called "white line" occurs at the point in the printed image where the defective printing nozzle is responsible. The white line consists of a stripe-like artifact in the printing direction, which is white in the case of a monochrome print at this point in the print image and manifests itself in the case of multi-color printing by a color distortion.

The cause of a missing nozzle is usually that the pressure nozzle is clogged and emits no more ink. Also, damage in the supply of the ink to the pressure nozzle can theoretically lead to this error. Furthermore, a partial blockage is possible, whereby the pressure nozzle ejects a reduced amount of ink and / or the ink jet deviates at a certain angle from its desired pressure point, whereby the pressure nozzle presses in the end obliquely.

To compensate for such missing nozzles, there are various approaches in the prior art. For example, the use of redundant pressure nozzles is known, so that if individual pressure nozzles fail, other pressure nozzles which cover the same location in the printed image can be inserted.

As this redundancy leads to increased costs for the printing press, another approach is that adjacent nozzles of the missing nozzle, which still print properly, can cover the white line caused by the missing nozzle by increased ink application.

Another approach, in the case of multi-color printing, is to cover a failed printing nozzle of a particular color by a previously calculated combination of available inks of the printing nozzles of the other separations that print at the same position. As a result, the color value at the location of the missing nozzle is also changed here, but a deliberately calculated, deviating color value in the entire print image is still significantly better in terms of perceptibility than an uncoated white line.

A completely different approach, in turn, is to compensate the announcement of the location of a missing nozzle, the print image in the preliminary stage of the printing process to be adjusted so that at the location of the failed print nozzle in the print head to be printed color values are adjusted so that a compensation of the missing nozzle becomes possible. The change of the printed image with regard to its geometric position in the printed image can also help to reduce the effect of a missing nozzle.

However, in order to be able to carry out such compensation methods, the missing nozzle first has to be determined with exact position accuracy. For this purpose, a large number of approaches are known in the prior art. One of the most common approaches is to print test patterns that accurately pinpoint specific print objects, such as dots or dashes, to a print nozzle. For example, such a known test pattern is to print vertical bars of every tenth printing nozzle in the printhead. In this case, in a first horizontal row, each first, eleventh, twentieth, etc. printing nozzle will print a corresponding vertical line, in a second horizontal row every second, twelfth, twenty-second, etc., all the way to the tenth, tenth, twentieth, thirtyth nozzle , The number of rows to be printed together corresponds to the distance between the individual printing nozzles that print in the test pattern. Other distances can be chosen, such as every fifth or every twentieth pressure nozzle. The number of rows then varies accordingly. The decisive parameter for the selection of the distance between the individual pressure nozzles in the test pattern lies in the resolution of the camera with which the nozzle test pattern is measured. In the current state of the art, it is currently still the case that the digital cameras used to evaluate the text patterns have a lower resolution than can be achieved with the print head of the inkjet printing press. Thus, it does not make sense to print test patterns where all print nozzles are directly in line. Also, the pressure nozzles can realize different drop sizes. For example, if you want to print with the largest possible droplet size, it can happen that the image objects printed by each print nozzle influence each other, because the ink runs together, for example. Again, making test patterns with a number impractical.

After printing the nozzle check pattern, this is measured by the digital camera, wherein the digital camera, for example, can be integrated as part of an image inspection system in the inkjet printing press. Alternatively, the nozzle check pattern is first printed and later examined by an external camera. By means of a computer-aided evaluation of the nozzle check pattern for missing image objects, pinpoint missing nozzles can be detected, since the distance between the individual print nozzles and thus between the printed image objects is known.

A problem with this approach, however, is that if the print nozzles fail each at the outermost edge of the printhead, it can not be accurately detected during detection by printing a test pattern and then digitally evaluating. The computer that evaluates the digital image recognizes a series of image objects that have been printed by the individual print nozzles in the nozzle check pattern. However, he does not recognize, for example, when analyzing the series of vertical bars in the first horizontal row of the nozzle check pattern that the first bar in the event of the failure of the first print nozzle is in fact not the first bar of the nozzle check pattern, but rather the second stroke. This is because the exact position of the first print nozzle in the digital print image per se is not known to the detection algorithm in the evaluation computer. The failure of each first or last Pressure nozzle in the print head can therefore not be determined by means of the standard method for detecting failed nozzles via the known nozzle test pattern. If the second, third or another pressure nozzle located further inside the printhead would fail, the detection algorithm would recognize that the distance between the line caused left and right of the failed printing nozzle is greater than that given the known distance between the individual printing nozzles would be expected.

From the prior art is known to solve this problem, so-called "locator marks" to use. In the process, certain position markers of known dimension and position are added to the nozzle test pattern, so that the detection algorithm can always recognize during the evaluation of the digital image of the nozzle test pattern at which point the horizontal rows of the vertical image objects of the nozzle test pattern begin or end. However, these locator marks have the disadvantage that they influence the structure of the nozzle check pattern and thus impair the evaluation for detecting the missing nozzles.

The object of the present invention is therefore to find a method for detecting failed nozzles by means of the pressure and the evaluation of a nozzle test pattern, which does not require locator marks and overcomes the disadvantages known from the prior art.

The solution to the problem is a method for detecting failed nozzles in an inkjet printing machine by a computer, wherein for each color separation, a nozzle test pattern is printed, which consists of a certain number of horizontal rows of periodically vertically printed, equidistant lines, which are arranged one below the other, wherein in each row of the nozzle check pattern, only the periodic pressure nozzles contribute to the nozzle check pattern corresponding to the determined number of horizontal rows and wherein the printed nozzle check pattern is detected, digitized and transmitted to the computer for evaluation by an image sensor and characterized in that for the determination of failed printing nozzles at an edge of the print head in the evaluation of the Nozzle check pattern is examined by the computer, the beginning of the horizontal rows of the nozzle check pattern to how far the lines at the beginning of the respective underlying row of the nozzle check pattern geometrically differ, the computer examines the geometric deviation of the lines to how large the horizontal distance of the first line of a Row of the nozzle check pattern from the first line of the underlying row of the nozzle check pattern is. The key of the method according to the invention is that the computer, or the detection algorithm, always examines where the respective first line of the underlying row of vertical lines is located when evaluating the digital image of the nozzle check pattern. The reason for this is that in the nozzle check pattern used, which consists of x different rows at intervals of x printing nozzles printed lines, the beginning of each row inevitably has a certain geometric deviation. For example, if the first row consists of the first, eleventh, twenty-first, and second row of the twelfth, twenty-second, and so on, the beginning of the second row is shifted by the horizontal distance between the first and second nozzles. For example, if now the first pressure nozzle fails, the first row starts at the position of the eleventh pressure nozzle, the second row, with the correct function of the second pressure nozzle, but begins by the distance of nine pressure nozzles farther left. However, if all pressure nozzles are functioning correctly, the distance between the beginning of the first row and the beginning of the second row would have to be much smaller - namely, only the distance between the first and second pressure nozzles. Since the image sensors that record the printed nozzle check pattern have a lower resolution than the distance between two print nozzles, the expected distance between the first and second print nozzles can not be tested. Instead, the system tests for the actual gap between the second and the eleventh pressure nozzle when the first pressure nozzle fails. For this distance of nine printing nozzles, the resolution of the image sensors used is large enough. If such an excessive distance of nine pressure nozzles now occurs between the beginning of the first and second series, the compensation algorithm can unerringly detect the failure of the first pressure nozzle. The method applies analogously to the second, third, fourth and fifth, etc., pressure nozzle, which marks the corresponding beginning of the second, third, fourth, fifth, etc. series. The decisive evaluation is that of the Distance between the beginning of a row and the beginning of the row below. If a distance of nine pressure nozzles is measured here, then in each case the first pressure nozzle of the row has failed. If the distance is even greater, for example nineteen pressure nozzle distances, then even the first two pressure nozzles of the row have failed. Since each vertical line of a particular pressure nozzle can be assigned in the nozzle test pattern, one or more failed pressure nozzles can thus be reliably detected at the pressure edge.

Advantageous and therefore preferred developments of the method will become apparent from the accompanying dependent claims and from the description with the accompanying drawings.

A preferred development of the method according to the invention is that the computer records and evaluates the distance of the first line of the respectively underlying row of the nozzle check pattern until it is ensured that no first line of the respectively underlying row of the nozzle check pattern lies horizontally to the left of the respectively examined first Line of a row is located. For the special case that each of several directly following pressure nozzles fail at the pressure edge, for example, the first and the second pressure nozzle, the positions of the further underlying first lines of the corresponding rows must also be detected. Thus, for example, in the case in question that the first and the second pressure nozzle have failed, the failure of the first pressure nozzle could not be detected with the method according to the invention if only the beginning of the second row is considered. Since here also the second pressure nozzle has failed, the beginning of the second row, which corresponds in the case of the twelfth pressure nozzle, again only by a nozzle spacing, namely the eleventh pressure nozzle, which here yes marked the beginning of the first row, moved. The detection algorithm also has to look where the beginning of the third row is. Only here will he notice that the distance between the beginning of the second row and the beginning of the third row each again nine nozzle positions and corresponding to the distance between the beginning of the first row and the beginning of the third row eight nozzle positions. With this knowledge, the detection algorithm recognizes that the first and second pressure nozzles have failed. The detection algorithm must be so long compare the beginning of a row with the beginning of each of several underlying rows until it finds a corresponding beginning of an underlying row beginning horizontally to the left. In this case, he can assume that all rows up to the row, the beginning of which lies horizontally further to the left, have a missing nozzle in their respective first pressure nozzle. If he does not find such a horizontally left-hand beginning of an underlying row, he can assume that the pressure nozzles 1 to 10, which are located at the extreme pressure edge of the nozzle check pattern, have not failed. An exceptional case is the case that the first nine pressure nozzles have failed, but the tenth works. In this case, the beginning of the tenth row would differ only by a nozzle distance to the left of the first line of the first row, since the beginning of the first row would be the eleventh pressure nozzle, the beginning of the tenth row would be the tenth pressure nozzle. This distance is then horizontally too low to be able to detect it according to the invention. The failure of the first nine pressure nozzles in one piece, however, is extremely unlikely. On the other hand, such a serious error would immediately attract attention even in the later production run. Naturally, the principle according to the invention also applies to all other nozzle test patterns which operate with a different number of rows than ten.

A further preferred development of the method according to the invention is that the computer detects the case where all the first print nozzles have failed at the edge of the print head by the fact that the total distance of the first line of the first row of the nozzle check pattern and the last line of the last row of the nozzle check pattern a row of the nozzle check pattern is smaller than the distance of the first and last print nozzle of the print head. The extremely unlikely case that all the first printing nozzles which print the respective first lines of the first rows have failed, the detection algorithm can in this case be detected by the distance of the first line of the first row and the last line of the last row with the known total distance between the first and last pressure nozzle compares. If this distance correspondingly much smaller than the expected distance from the first and last pressure nozzle, so it can be concluded in the event of failure of the entire first pressure nozzles on the pressure edge. Also, the special case mentioned that all the first pressure nozzles have failed at the pressure edge except the pressure nozzle of the last row, which, due to the low resolution of the image sensors is no longer directly detectable, can be detected with this approach.

A further preferred embodiment of the method according to the invention is that for determining failed print nozzles on the opposite edge of the print head of the computer performs the same method each point-symmetrical at the end of a row. The inventive method described so far can be carried out correspondingly point-symmetrically for the printing nozzles on the opposite edge of the print head. This allows both edges of the nozzle test pattern to be examined for the failure of the respective outer x first pressure nozzles.

A further preferred development of the method according to the invention is that for carrying out the method by means of inline measurements, the image sensor is a digital camera mounted in the inkjet printing press. For a computer-aided implementation of the method according to the invention, it is extremely advantageous if the digital camera for taking the image data of the nozzle test pattern is a digital camera mounted in the inkjet printing press, which respectively performs inline measurements. This means that the nozzle check pattern is printed on the inkjet press and then immediately measured inline by the digital camera mounted on the inkjet press.

A further preferred development of the method according to the invention is that, in addition to carrying out the method for detecting failed nozzles at the pressure edge, a further method for detecting nozzles located further inside is also carried out. Since with the method according to the invention only missing nozzles are detected at the pressure edge, a normal method for the detection of missing nozzles, which are not at the pressure edge, must of course also be carried out at the same time. For this purpose, then a standard method is sufficient, such as the evaluation of the horizontal distances of the vertical lines in the nozzle check pattern.

A further preferred development of the method according to the invention is that the information about the detected, failed pressure nozzles from the computer a Compensation algorithm is passed, which compensates by adapting the print image data or control of the adjacent, functioning pressure nozzles, the detected, failed pressure nozzles. The precipitated pressure nozzles at the pressure edge detected by the method according to the invention are transferred by the computer to a compensation algorithm, which then compensates the failed pressure nozzles as well as possible with one of the compensation methods known from the prior art. This can be done, for example, in the case of adaptation of the print image data in the preliminary stage then from the precursor computer. In this case, the information about the detected missing nozzles would be transmitted from the computer to the precursor computer. In the case of a compensation in the printing press by the control of the respectively adjacent, functioning printing nozzles of a missing nozzle is conceivable to send the information directly to the control computer of the printing press. In the event that the control computer of the printing machine is identical to the computer, which performs the detection method, there is an internal transfer to the compensation algorithm.

Figur 1:

eine schematische Darstellung einer Bogen-Inkjet-Druckmaschine

Figur 2:

das Fehlerbild einer white line, verursacht durch eine missing nozzle

Figur 3

ein verwendetes, mehrreihiges Düsentestmuster

Figur 4:

eine schematische Darstellung des Beginns und Endes eines solchen Düsentestmusters

Figur 5:

schematischen Ablauf des erfindungsgemäßen Verfahrens

The drawings show:

FIG. 1:

a schematic representation of a sheet-fed inkjet printing machine

FIG. 2:

the defect image of a white line caused by a missing nozzle

FIG. 3

a used, multi-row nozzle test pattern

FIG. 4:

a schematic representation of the beginning and end of such a nozzle check pattern

FIG. 5:

schematic sequence of the method according to the invention

The field of application of the preferred embodiment is an inkjet printing machine 7. An example of the basic structure of such a machine 7, consisting of feeder 1 for the supply of the printing substrate 2 in the printing unit 4, where it is printed by the printheads 5, to Boom 3, is in FIG. 1 shown. This is a sheet-fed inkjet printing machine 7, which is controlled by a control computer 6. During operation of this printing machine 7, as already described, it is possible for individual printing nozzles in the printing heads 5 to fail Printing unit 4 come. The result is then white lines 13 in the printed image 12, or in the case of multicolored printing, distorted color values. An example of such a white line 13 in a printed image 12 is shown in FIG FIG. 2 shown.

FIG. 3 shows an example of a printed nozzle check pattern 16 for a particular ink. Such a pattern 14 is printed for each ink 16. It is characterized by equidistant vertical lines 15. When printing every 10th nozzle, 10 lines must be printed with vertical lines or nozzle nozzles 15 to print with all nozzles. In the first line, for example, the one-ss nozzles would print {1, 11, 21, ...} in the next line all two-jets {2, 12, 22, ...} etc .. Good are also in FIG. 3 to recognize the consequence of failed printing nozzles in the form of white lines 13, since at these points in the printed nozzle test pattern the printing nozzle test line is missing.

The inventive method is adapted to the structure of the nozzle test pattern 14, 16. In FIG. 5 the sequence of the procedure is shown briefly in a schematic overview. In a first step, the digitally present nozzle check pattern 14 is printed. It can be both on the print substrate 2 next to the actually created print image 12, as well as be placed there individually. The printed printing nozzle check pattern 16 is then recorded by a camera which is installed after the last printing unit 4 of the inkjet printing machine 7 and detects the generated print images 12. The detected, digital print image 10 with the pressure nozzle test pattern 16 is then forwarded to the computer 6 for evaluation.

FIG. 4 FIG. 12 shows a simplified example of a printed nozzle check pattern 16 with only 20 vertical lines of print nozzles, here denoted as nozzles, a 'on the left print edge 9a, t' on the right print edge 8. If pressure nozzle 'a' on the left pressure edge 9 fails, the detection algorithm recognizes the line of 'k' and then the two lines of 'b' and '1'. The image resolution of the digital cameras used and thus the generated digital images, which the detection algorithm evaluates is limited. Nevertheless, the detection algorithm recognizes that 'b' is located horizontally to the left and below 'k' and '1' below 'k'. From this, the detection algorithm can conclude that the Pressure nozzle, a 'missing. If, on the other hand, the first line on the left edge of the picture is recognized below the line of '1', then the corresponding print nozzles 'a' and 'b' are missing. The process continues to the line of '1'. If the detection algorithm does not find a horizontal line to the left, it can assume that the pressure nozzles 'a' to 'j' lying at the extreme left pressure edge 8 of the nozzle check pattern have not failed.

If all the print nozzles in a row fall off the pressure edge, in FIG. 5 corresponding to the print nozzles, a 'to' j ',, k' to, t '), then the detection algorithm recognizes that the measured distance, k' to, t 'is less than the expected, known distance of the print nozzles, a 'to' t '. In this case, an error message may be issued because so many nozzles can not be compensated.

The relationship is point-symmetric also valid for the lines of the printing nozzles, k 'to' t 'on the right side at the pressure edge 8 of the nozzle check pattern 16.

Detected missing nozzles 11 are forwarded by the detection algorithm to the control computer 6 of the printing press 7, which initiates an ev. Necessary compensation of the determined failed pressure nozzle 11. For larger printheads 5 with more than 20 pressure nozzles, internal pressure nozzles are also present in the nozzle test pattern 14, 16. Here, in addition to the detection algorithm for the printing nozzles at the printing edge 8, 9, the image 10 produced by the digital camera must also be tested for missing nozzles in a further detection algorithm for the inner printing nozzles. Here, it suffices to use known methods according to the prior art which test for gaps or deviations in the printed nozzle test pattern 16.

LIST OF REFERENCE NUMBERS

1

investor

2

printing substrate

3

boom

4

Inkjet printing unit

5

Inkjet printhead

6

calculator

7

Inkjet press

8th

Right pressure nozzle edge in the pressure nozzle test pattern

9

left pressure nozzle edge in the pressure nozzle test pattern

10

recorded, digital print image

11

determined missing nozzle

12

selected print image

13

white line

14

digital pressure nozzle test pattern

15

Pressure nozzle test line

16

printed pressure nozzle test pattern

Claims (7)

Method for detecting failed printing nozzles (11) in an inkjet printing press (7) by a computer (6), wherein for each color separation a nozzle test pattern (14) is printed, which consists of a certain number of horizontal rows of periodically vertically printed, equidistant lines (15 ) arranged one below the other, wherein in each row of the nozzle check pattern (14) only periodically the print nozzles contribute to the nozzle check pattern (14) corresponding to the determined number of horizontal rows and wherein the printed nozzle check pattern (16) is for evaluation by an image sensor recorded, digitized and transmitted to the computer (6) for evaluation,
characterized,
in that the start of the horizontal rows (8, 9) of the nozzle check pattern (16) is investigated to determine failed nozzle nozzles (11) at an edge of the print head (8, 9) during the evaluation of the nozzle test pattern (16) by the computer (6) how far the lines (15) at the beginning of the respective underlying row of the nozzle check pattern (16) geometrically differ, wherein the computer (6) examines the geometric deviation of the lines (15) to what extent the horizontal distance of the first line (15) a row of the nozzle check pattern (16) from the first line (15) of the lower row of the nozzle check pattern (16). Method according to claim 1,
characterized,
that the computer (6) as long as the distance between the first line (15) of each row below the nozzle check pattern (16) and evaluates, until it is ensured that there is no first line (15) of each row below the nozzle check pattern (16) horizontally to the left of the first line (15) of each row examined. Method according to claim 2,
characterized,
in that the computer (6) detects the case that all first printing nozzles have failed at the edge of the printing head (8, 9), in that the total distance of the first line (15) of the first row of the nozzle test pattern (16) and the last line ( 15) of the last row of the nozzle check pattern (16) of a row of the nozzle check pattern (16) is smaller than the distance of the first and last print nozzles of the print head (8, 9). Method according to one of claims 1 to 3,
characterized,
in that for determining failed printing nozzles (11) at the opposite edge of the print head (8, 9), the computer (6) carries out the same method in each case point-symmetrically at the end of a row. Method according to one of the preceding claims,
characterized,
in that for carrying out the method by means of inline measurements, the image sensor is a digital camera mounted in the inkjet printing press (7). Method according to one of the preceding claims,
characterized,
in addition to carrying out the method for detecting failed pressure nozzles (11) at the pressure edge (8, 9), a further method for detecting nozzles located further inside is also carried out. Method according to one of the preceding claims,
characterized,
that the information about the detected, the failed pressure nozzles (11) by the computer (6) is passed to a compensation algorithm that by adapting the print image data (12) or control of the adjacent working pressure nozzles compensates for the detected, the failed pressure nozzles (11).

Images