EP3427957B1 - Detecting of failed print nozzles at the printing margin - Google Patents

Description

The present invention relates to a method for detecting failed printing 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 through printheads with one or more rows of printing nozzles, the so-called "missing nozzles" can develop an error. This means that individual pressure nozzles no longer work properly. This can manifest itself, 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 single-color print at this point in the print image and, in the case of a multi-color print, is expressed by a color distortion.

The reason for a missing nozzle is usually that the print nozzle is clogged and no more ink is emitted. Damage in the supply line of the ink to the pressure nozzle can theoretically lead to this error pattern. Furthermore, partial clogging is also possible, as a result of which the printing nozzle ejects a reduced amount of ink and / or the ink jet deviates from its desired printing point at a certain angle, as a result of which the printing nozzle ultimately prints obliquely.

There are various approaches in the prior art for compensating for such missing nozzles. For example, the use of redundant pressure nozzles is known, so that if individual pressure nozzles fail, other pressure nozzles that cover the same location in the printed image can take over.

Since this redundancy leads to increased costs for the printing press, another approach provides that neighboring printing nozzles of the missing nozzle, which are still printing properly, can cover the white line caused by the missing nozzle by an increased ink application.

Another approach provides, in the case of multi-color printing, to cover a failed printing nozzle of a certain color by a previously calculated combination of available printing colors of the printing nozzles of the other color separations printing at the same position. As a result, the color value at the location of the missing nozzle is also changed here, but a specifically calculated, deviating color value in the entire printed image is still significantly better in perceptibility than an uncontrolled white line.

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

However, in order to be able to carry out such compensation methods, the missing nozzle must first be determined in an exact position. A variety of approaches are known in the prior art for this purpose. One of the most common approaches is to print test patterns that can be used to precisely assign certain printing objects, such as dots or lines, to a printing nozzle. One such known test pattern is, for example, to have vertical lines printed by every tenth print nozzle in the print head. In a first horizontal row, every first, eleventh, twentieth, etc. printing nozzle will print a corresponding vertical line, in a second horizontal row every second, twelfth, twenty-second, etc., up to the tenth row with the tenth, twentieth, thirtieth printing nozzles . The number of rows to be printed one below the other corresponds to the distance between the individual printing nozzles that print in the test pattern. Other distances can also be selected, 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 printing 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 still the case that the digital cameras used to evaluate the text samples have a lower resolution than can be achieved with the print head of the inkjet printing machine. As a result, it does not make sense to print test samples where all printing nozzles are directly in a row. The pressure nozzles can also realize different drop sizes. For example, if you want to print with the largest possible drop size, it can happen that the image objects printed by each printing nozzle influence each other, for example because the ink runs together. This also makes test patterns with a number impractical.

After the nozzle test pattern has been printed, it is measured by the digital camera, it being possible for the digital camera to be integrated, for example, as part of an image inspection system in the inkjet printing machine. Alternatively, the nozzle test pattern is first printed and later examined by an external camera. By means of a computer-aided evaluation of the nozzle test pattern for missing image objects, missing nozzles can then be precisely detected, since the distance between the individual printing nozzles and thus between the printed image objects is known.

A problem with this approach, however, is that if the printing nozzles fail at the outermost edge of the printhead, this cannot be identified unambiguously during the detection by printing a test pattern and subsequent digital evaluation. The computer which evaluates the digital image recognizes a number of image objects which have been printed by the individual printing nozzles in the nozzle test pattern. However, if he analyzes the series of vertical lines in the first horizontal row of the nozzle test pattern, for example, he does not recognize that the first line in the event of the failure of the first pressure nozzle is actually not the first line of the nozzle test pattern, but is the second line. This is because the detection algorithm in the evaluation computer does not per se know the exact position of the first printing nozzle in the digital print image. The failure of the first or last The print nozzle in the print head cannot be determined by means of the standard method for the detection of failed print nozzles via the known nozzle test pattern. If the second, third or any other printing nozzle located further inside the print head failed, the detection algorithm would recognize that the distance between the line caused to the left and right of the failed printing nozzle is larger than it would be given the known distance between the individual printing nozzles would be expected.

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

From the European patent application EP 2042324 A2 there is also known a print nozzle test pattern which is recorded on a recording medium by means of a line head with a plurality of recording elements by causing the plurality of recording elements to carry out a recording operation while the recording medium and the line head are moved relative to one another in a relative movement direction, whereby the print nozzle test image has a line pattern block having a plurality of line patterns each corresponding to the plurality of recording elements, the plurality of line patterns being arranged at a predetermined interval or further so that they are separated from each other, the print nozzle test image being separated in the width direction is in a plurality of test image zones of smaller width than the recording width (Wh) of the line head corresponding to the width of the test image, and the plurality of print nozzle test image zones form a print nozzle test image which sows corresponds to all the recording elements of the line head, the plurality of test image zones containing reference line patterns which are arranged at both end zones, and the Reference Line Patterns Have line characteristics that are different from the rest of the multiple line patterns.

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

The solution to the problem is a method according to claim 1. The key of the method according to the invention lies in the fact that the computer or the detection algorithm, when evaluating the digital image of the nozzle test pattern, always examines where the first line of the underlying row of vertical lines is located. The reason for this is that in the nozzle test pattern used, which consists of x different rows at intervals of x printing nozzles, the beginning of each row inevitably has a certain geometric deviation. For example, if the first row consists of the first, eleventh, and twenty-first pressure nozzles and the second row of the corresponding twelfth, twenty-second, etc., the beginning of the second row is shifted by the horizontal distance between the first and second pressure nozzles. In the event that, for example, the first pressure nozzle fails, the first row begins at the position of the eleventh pressure nozzle, but the second row, if the second pressure nozzle functions correctly, begins at a distance of nine pressure nozzles further to the left. If all pressure nozzles function 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 by the distance between the first and second pressure nozzles. Since the image sensors that record the printed nozzle test pattern have a lower resolution than the distance between two printing nozzles, it is not possible to test for the expected distance between the first and second printing nozzles. Instead, the real distance between the second and eleventh pressure nozzles is tested if the first pressure nozzle fails. The resolution of the image sensors used is large enough for this distance of nine printing nozzles. If such a too great distance of nine pressure nozzles occurs between the beginning of the first and second rows, 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 start of the second, third, fourth, fifth, etc. row. The decisive evaluation is that of Distance between the beginning of a row and the beginning of the row below it. If a distance of nine pressure nozzles is measured here, the first pressure nozzle in the row has failed. If the distance is even greater, for example nineteen pressure nozzle distances, even the first two pressure nozzles in the row have failed. Since each vertical line can be assigned to a specific pressure nozzle in the nozzle test pattern, one or more failed pressure nozzles can be reliably detected at the edge of the print.

Advantageous and therefore preferred developments of the method result from the associated subclaims and from the description with the associated drawings.

A preferred development of the method according to the invention is that the computer records and evaluates the distance between the first line of the row of nozzle test patterns below it until it is ensured that no first line of the row below of the nozzle test pattern lies horizontally to the left of the respectively examined first Line of a row. For the special case that several immediately following pressure nozzles fail at the printing edge, for example the first and the second pressure nozzle, the positions of the further first lines of the corresponding rows must also be recorded. For example, in the case in which the first and second pressure nozzles have failed, the method according to the invention would not be able to detect the failure of the first pressure nozzle if one only looked at the beginning of the second row. Since the second pressure nozzle has also failed here, the start of the second row, which in the case corresponds to the twelfth pressure nozzle, is again only shifted by one nozzle distance, namely to the eleventh pressure nozzle, which marks the beginning of the first row here. The detection algorithm must also look at 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 is again nine nozzle positions, and accordingly the distance between the beginning of the first row and the beginning of the third row is eight nozzle positions. With this knowledge, the detection algorithm recognizes that the first and the second pressure nozzle have failed. The detection algorithm must be like this Long compare the beginning of a row with the beginning of the several rows below it, until it finds a corresponding horizontal left beginning of an underlying row. In this case, he can assume that all rows up to the row whose beginning is further horizontally to the left have a missing nozzle in their first pressure nozzle. If he does not find such a horizontally left beginning of a row below, he can assume that the pressure nozzles 1 to 10 lying on the outermost pressure edge of the nozzle test pattern have not failed. An exception is the case where the first nine pressure nozzles have failed, but the tenth works. In this case, the beginning of the tenth row would only deviate from the first line of the first row by one nozzle distance to the left, since in this case the beginning of the first row would be the eleventh pressure nozzle, but the beginning of the tenth row would be the tenth pressure nozzle. This distance is then too short horizontally in order to be able to detect it according to the invention. However, the failure of the first nine pressure nozzles at a time is extremely unlikely. On the other hand, such a serious mistake would also be noticed immediately in later printing. Naturally, the principle according to the invention also applies to all other nozzle test patterns which work with a number of rows other than ten.

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

A further preferred development of the method according to the invention is that the computer carries out the same method point-symmetrically at the end of a row in order to determine failed print nozzles on the opposite edge of the print head. The method according to the invention described so far can be carried out correspondingly point symmetrically for the printing nozzles on the opposite edge of the printing head. This allows both edges of the nozzle test pattern to be examined for the failure of the first x pressure nozzles on the outside.

A further preferred development of the method according to the invention is that the image sensor is a digital camera mounted in the inkjet printing machine in order to carry out the method by means of inline measurements. For a computer-aided implementation of the method according to the invention, it is extremely advantageous if the digital camera for recording the image data of the nozzle test pattern is a digital camera installed in the inkjet printing machine, which in each case carries out corresponding inline measurements. This means that the nozzle test pattern is printed in the inkjet printing machine and then immediately measured inline by the digital camera installed in the inkjet printing machine.

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

Another preferred development of the method according to the invention is that the information about the detected, failed pressure nozzles is sent from the computer Compensation algorithm is transferred, which compensates for the detected, failed print nozzles by adapting the print image data or controlling the neighboring, functioning print nozzles. The failed pressure nozzles detected by the method according to the invention are transferred by the computer to a compensation algorithm, which then compensates for the failed pressure nozzles as well as possible using one of the compensation methods known from the prior art. This can then be carried out by the prepress computer, for example, in the case of adaptation of the print image data in the prepress. In this case, the information about the missing nozzles detected would be transmitted from the computer to the pre-stage computer. In the event of compensation in the printing press by activating the adjacent, functioning printing nozzles of a missing nozzle, it 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 press is identical to the computer that carries out 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:

Figure 1:

a schematic representation of a sheet inkjet printing machine

Figure 2:

the error pattern of a white line, caused by a missing nozzle

Figure 3

a used, multi-row nozzle test pattern

Figure 4:

is a schematic representation of the beginning and end of such a nozzle test pattern

Figure 5:

schematic sequence of the method according to the invention

The field of application of the preferred embodiment variant is an inkjet printing machine 7. An example of the basic structure of such a machine 7, consisting of feeder 1 for feeding the printing substrate 2 into the printing unit 4, where it is printed by the print heads 5, up to Boom 3 is in Figure 1 shown. This is a sheet inkjet printing machine 7, which is controlled by a control computer 6. When operating this printing press 7, as already described, individual print nozzles in the print heads 5 can 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 Figure 2 shown.

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

The method according to the invention is matched to the structure of the nozzle test patterns 14, 16. In Figure 5 the sequence of the method is briefly shown in a schematic overview. In a first step, the digitally available nozzle test pattern 14 is printed. It can lie both on the print substrate 2 next to the print image 12 that is actually to be produced, and can also be placed there individually. The printed print nozzle test 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 print images 12 generated. The captured digital print image 10 with the pressure nozzle test pattern 16 is then forwarded to the computer 6 for evaluation.

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

If all print nozzles in a row fail at the print edge, in Figure 5 accordingly, the pressure nozzles, a 'to' j ',, k' to 't'), the detection algorithm recognizes that the measured distance, k 'to' t 'is smaller than the expected, known distance of the pressure nozzles, a 'to' t '. In this case, an error message can be issued because so many nozzles cannot be compensated.

The relationship is also point-symmetrical for the lines of the pressure nozzles 'k' to 't' on the right-hand side at the print edge 8 of the nozzle test pattern 16.

Detected missing nozzles 11 are forwarded by the detection algorithm to the control computer 6 of the printing press 7, which initiates any necessary compensation for the determined failed printing nozzle 11. In the case of larger print heads 5 with more than 20 print nozzles, inner print nozzles are also present in the nozzle test pattern 14, 16. In addition to the detection algorithm for the pressure nozzles at the printing edge 8, 9, the image 10 generated by the digital camera must also be tested for missing nozzles for the inner pressure nozzles in a further detection algorithm. It is sufficient here to use methods known from the prior art which test for gaps or deviations in the printed nozzle test pattern 16.

Reference list

1

Investors

2nd

Printing substrate

3rd

boom

4th

Inkjet printing unit

5

Inkjet print head

6

computer

7

Inkjet printing machine

8th

right pressure nozzle edge in the pressure nozzle test pattern

9

left pressure nozzle edge in the pressure nozzle test pattern

10th

captured digital print image

11

found missing nozzle

12th

selected print image

13

white line

14

digital nozzle test pattern

15

Pressure nozzle test line

16

printed nozzle test pattern

Claims (7)

1. Method for detecting failed printing nozzles (11) in an inkjet printing machine (7) by means of a computer (6) wherein a digital nozzle test chart (14) is printed for every colour separation, said nozzle test chart consisting of a specific number of horizontal rows of equidistant vertical lines (15) printed periodically and disposed underneath one another, wherein in every row of the digital nozzle test chart, periodically only those printing nozzles that correspond to the specific number of horizontal rows contribute to the digital nozzle test chart (14), wherein in every row, a different nozzle marks the beginning of the row and wherein for the purpose of analysis, the printed nozzle test chart (16) is recorded by an image sensor, digitized and transmitted to the computer (6) for analysis,
   characterized
   in that to identify failed printing nozzles (11) at a left-hand or right-hand margin of a printing head (5) during the analysis of the printed nozzle test chart (16), the computer (6) checks the left-hand or right-hand beginning, respectively, of the horizontal rows (8, 9) of the printed nozzle test chart (16) to find out how far the lines (15) at the left-hand or right-hand beginning of the respective next row underneath in the printed nozzle test chart (16) deviate geometrically, wherein the computer (6) checks the geometric deviation of the lines (15) to find out the size of the horizontal distance between the first line (15) in a row of the printed nozzle test chart (16) and the first line (15) of the next row underneath in the nozzle test chart (16).
2. Method according to claim 1,
   characterized in
   that the computer (6) continues to record and analyse the distance of the first line (15) of the respective next row located underneath in the printed nozzle test chart (16) until it is ensured that no first line (15) of the respective next row underneath in the printed nozzle test chart (16) is horizontally located to the left of the respective examined first line (15) of a row.
3. Method according to claim 2,
   characterized
   in that the computer detects a case in which all first printing nozzles at the margin of the printing head (8, 9) have failed by recognizing that the total distance between the first line (15) of the first row of the printed nozzle test chart (16) and the last line (15) of the last row of the nozzle test chart (16) of a row of the printed nozzle test chart is smaller than the distance between the first and last printing nozzle of the printing head (8, 9).
4. Method according to any one of claims 1 to 3,
   characterized
   in that to detect failed printing nozzles (11) at the opposite margin of the printing head (8, 9), the computer (6) executes the same method in a centrosymmetric way at the end of a row.
5. Method according to any one of the preceding claims,
   characterized
   in that to execute the method by means of in-line measurements, the image sensor is a digital camera provided in the inkjet printing machine (7).
6. Method according to any one of the preceding claims,
   characterized
   in that in addition to executing the method for detecting failed printing nozzles (11) at the print margin (8, 9), a further method is executed to detect printing nozzles that are located further inward.
7. Method according to any one of the preceding claims,
   characterized
   in that the computer (6) transfers the information on the detected failed printing nozzles (11) to a compensation algorithm that compensates for the detected failed printing nozzles (11) by adapting the print image data (12) or actuating the adjacent functional printing nozzles.

Images