METHOD FOR DETECTING PRINTING NOZZLE ERRORS IN AN INKJET PRINTING MACHINE

Abstract

A method for detecting printing nozzle errors in an inkjet printing machine provides a high degree of robustness in the detection of errors by printing a nozzle test pattern in the inkjet printing machine. The test pattern is then digitalized by using a camera and transmitted to a computer for evaluation. There, the recorded test pattern is investigated by using methods of digital image processing, such as a Fourier analysis, and evaluated in the frequency range with regard to specific anticipated printing nozzle errors. Specific printing nozzle errors can be detected especially on the basis of amplitude, phase and variance errors in the signal in the frequency range. Moreover, by using the phase error, it is possible to evaluate whether the two print heads are disposed in an incorrect adjustment position relative to one another by calculating displacements of the phase error in transition regions of two print heads.

Claims

1. A method for detecting printing nozzle errors in an inkjet printing machine by using a computer, the method comprising the following steps: printing a nozzle test pattern; determining precise positions of individual components of the nozzle test pattern; using at least one camera to acquire and record the nozzle test pattern; producing an actual signal from the printed and acquired nozzle test pattern; using the generated actual signal to carry out a Fourier analysis; generating a reference signal with a location frequency of the Fourier-transformed actual signal; generating a correlation signal from the reference and actual signals, the correlation signal describing valid setpoint positions for specific points of the nozzle test pattern; eliminating all positions at edges of the correlation signal not corresponding to any setpoint positions; displacing the reference signal to each of the setpoint positions, resulting in a working point; calculating at least one of amplitude or phase or variance errors from an evaluation of a signal course of the actual signal around the respective working point; and evaluating printing nozzle quality from the calculated amplitude, phase and variance errors.

2. The method according to claim 1, which further comprises providing the nozzle test pattern with a specific number of horizontal rows of periodically vertically printed, equidistant lines, the rows being disposed below one another and limited by horizontal lines, and in each row of the nozzle test pattern, the printing nozzles corresponding to a specific number of the horizontal rows contributing only periodically to the nozzle test pattern.

3. The method according to claim 2, which further comprises determining the positions of the individual nozzle test patterns by detection of horizontal lines and averaging over vertical lines.

4. The method according to claim 2, which further comprises providing the nozzle test pattern with ten horizontal rows of printed patterns with a monotonic autocorrelation function.

5. The method according to claim 4, which further comprises providing the patterns with Barker codes having positive end values respectively at a beginning and an end of a horizontal row.

6. The method according to claim 4, which further comprises providing the pattern as a two-dimensional pattern being formed by two Barker codes perpendicular to one another.

7. The method according to claim 4, which further comprises providing the patterns with Neumann-Hoffman sequences having positive end values respectively at a beginning and an end of a horizontal row.

8. The method according to claim 1, which further comprises printing a nozzle test pattern for each print color involved in a printing process and placing the nozzle test patterns thus produced below one another to form a total test pattern.

9. The method according to claim 2, which further comprises generating the actual signal by averaging all of the horizontal rows of the nozzle test pattern, and then carrying out an interpolation of the actual signal including a reduction of artifacts arising due to a geometrical quantization by using sub-pixeling.

10. The method according to claim 1, which further comprises including a ratio of maximum values of the setpoint signal and the actual signal in the amplitude error, and detecting missing or faintly-printing printing nozzles by an evaluation of the amplitude error.

11. The method according to claim 1, which further comprises using the phase error to describe a deviation of an emphases, in a form of equivalently segmented regions, of the setpoint and the actual signal, and detecting obliquely jetting printing nozzles by an evaluation of the phase error.

12. The method according to claim 1, which further comprises determining a position of at least two print heads from the phase error by calculating displacements of the phase error in transition regions of the at least two print heads, and using the position determination for an evaluation of the print head positions in terms of an incorrect adjustment position of the at least two print heads.

13. The method according to claim 12, which further comprises carrying out the position determination of the at least two print heads by detecting a displacement of base signal values in the generated Fourier-transformed signal in the transition region, and a deviation of the adjustment positions of the two print heads disposed beside one another arising from the numerical displacement of the base signal values in the generated Fourier-transformed signal.

14. The method according to claim 12, which further comprises detecting the position determination of the at least two print heads by a displacement of base signal values in the generated Fourier-transformed signal in the transition region, and a deviation of the adjustment positions of the two print heads disposed beside one another being calculated from the phase error and a filter for the correlation signal.

15. The method according to claim 12, which further comprises using the determined print head positions for the adjustment correction of the at least two print heads at least one of perpendicular to a printing direction corresponding to a hypothetical x-axis, or in a printing direction corresponding to a hypothetical y-axis, or in an angular orientation corresponding to a hypothetical z-axis.

16. The method according to claim 15, which further comprises bringing about the adjustment correction of the at least two print heads perpendicular to the printing direction and in the angular orientation by a mechanical displacement of the at least two print heads, and carrying out the adjustment correction of the at least two print heads in the printing direction electronically by a time-delayed output of printing data to the at least two print heads.

17. The method according to claim 15, which further comprises carrying out the adjustment correction of the at least two print heads perpendicular to the printing direction and in the printing direction by evaluating the periodically vertically printed, equidistant lines being the printed patterns with a monotonic autocorrelation function in the transition region between two print heads, in the case of the adjustment correction of the at least two print heads in the angular orientation, and evaluating the periodically vertically printed, equidistant lines being the printed patterns with the monotonic autocorrelation function in the core region of the at least two print heads.

18. The method according to claim 1, which further comprises using a plurality of sub-cameras for the acquiring and recording of the nozzle test pattern, using individual images resulting from the acquiring and recording to constitute a basis for the method for detecting printing nozzle errors, and determining magnitudes required for the method directly from individual sub-images.

19. The method according to claim 18, which further comprises using printed reference marks to geometrically couple the individual sub-images to one another, causing at least one reference mark to be present in each sub-image and simultaneously using the reference marks as a pattern for a reference system for geometrical calibration of the sub-cameras.

20. The method according to claim 19, which further comprises including a circle in the printed reference mark, and fitting a center point and a diameter of the circle by using detected edge pixels thereof using a regression method.

21. The method according to claim 19, which further comprises providing information from a plurality of printing nozzles in the reference mark, and the plurality of printing nozzles belonging to a single print head.

22. The method according to claim 19, which further comprises integrating the printed reference mark into a printed measurement mark for at least one of color measurement or register control.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0052] FIG. 1 is a diagrammatic, longitudinal-sectional view of a sheet-fed inkjet printing machine;

[0053] FIG. 2 is a plan view of a sheet showing an error image caused by a printing nozzle failure;

[0054] FIG. 3 is a nozzle test pattern for a printing ink;

[0055] FIG. 4 is a diagram of an averaged original signal;

[0056] FIG. 5 is a diagram of an interpolated original signal;

[0057] FIG. 6 is a diagram of a start of an FT correlation signal;

[0058] FIG. 7 is a phase error diagram of an FT actual signal;

[0059] FIG. 8 is an amplitude error diagram of an FT actual signal;

[0060] FIG. 9 is a diagram of an example of an X-stitching error by signal displacement;

[0061] FIG. 10 is a diagram of an example of an X-stitching error by filtration of the correlation;

[0062] FIG. 11 is a diagram of printed Barker sequences of two print heads;

[0063] FIG. 12 is a diagram of a correlation of two Barker sequences; and

[0064] FIG. 13 is a diagram of printed 2D-Barker sequences, normal and sheared.

DETAILED DESCRIPTION OF THE INVENTION

[0065] Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a preferred embodiment in which the area of application is a digital printing machine constructed as a sheet-fed inkjet printing machine 10. An example of the construction of such a machine 10 is represented in FIG. 1. A respective sheet 11 is transported from a feeder 1 in a transport direction T through a printing unit 2 and a drive 6 to a delivery 3. The transport of a respective sheet 11 takes place primarily by using cylinders, i.e. transfer cylinders 5 and a printing cylinder 7. Inkjet print heads 4, which are disposed above the printing cylinder 7, print a sheet 11 which is moved past at a small spacing by the printing cylinder 7. The printing cylinder 7 is therefore also referred to as a jetting cylinder. In the illustrated embodiment, the printing cylinder 7 has three sheet retaining zones 8, which are separated from each other by a respective channel or gap 9.

[0066] When the printing machine 10 is operated, failures of individual printing nozzles in the print heads 4 in the printing unit 2 can occur, as already described at the outset. The consequences then are white lines 13, or in the case of a multicolor print, distorted color values on a print image 12. An example of such a white line 13 is represented in FIG. 2.

[0067] The method according to the invention permits a determination and classification of deviations during printing in the inkjet process. Due to tolerances from manufacture and foreign bodies in the ink, deviations in printing usually occur with all heads 4. Nozzles can completely fail, jet obliquely or in an undefined manner or deposit ink in different thicknesses. Therefore, in order to provide a high-quality print, it is crucial for these errors 13 to be precisely detected and for this information to be sent to a controller, control system or computer 30 of the digital machine 10. The controller 30 is then able in many cases to correct such errors 13 by compensation with ink from adjacent nozzles. An integration of an automatic method for detecting nozzle errors 13 with a feedback to the digital controller 30 is therefore an important element of a digital printing machine 10 and is also known. The method is geared to the known patterns from nozzle monitoring. FIG. 3 shows an example of such a pattern 14 for a specific print color. The pattern 14 is distinguished by equidistant vertical lines 15, which are printed for each color. During the printing by every 10th nozzle, 10 lines with vertical strokes must be printed in order to print with all of the nozzles. In the first line, for example, the first nozzles would be printed {1, 11, 21, . . . }, in the second line all of the second nozzles {2, 12, 22, . . . } etc.

[0068] The method according to the invention is geared to the structure of nozzle patterns 14 and includes the following steps:

1. The position of the pattern 14 is known as a surrounding rectangle on sheet 11 with a small degree of uncertainty. The pattern 14 is limited by horizontal lines 16. During the printing of every n-th nozzle, n patterns 14 are required in order to print all of the nozzles once. All n patterns 14 do not always have to be printed on one sheet. A plurality of patterns 14 form a block. Patterns 14 are placed seamlessly in a row in a block. A block or an individual pattern 14 is separated by a white edge from the subject.
2. A first step determines the precise positions of the individual patterns on the basis of horizontal lines 16. For this purpose, the method averages different vertical lines. The grey-scale value of the color thus clearly appears at points of the horizontal lines. The points between the horizontal lines are less intensely saturated with the paper white due to averaging. An averaged signal 17 can be robustly evaluated by way of a differential filter in order to detect the positions of horizontal lines 16.
3. The method then averages for each pattern all of the horizontal lines to form an actual signal. The overall result is thus a reduction in the signal noise. The total signal is interpolated, since the camera resolution is less than the inkjet resolution and, by using sub-pixeling, artifacts are reduced by a geometrical quantization. For each color, a suitable color channel is selected for the evaluation. Thus, for example, the green channel is taken for the color black. For black, i.e. K, the two other color channels would however also be possible. For the scale colors cyan, magenta and gold, on the other hand, the signal of the respective complementary color is taken. FIG. 3 shows the detected regions in the individual patterns with vertical lines. The averaged original signal 17 is shown in FIG. 4. In FIG. 5, this original signal is interpolated 18.
4. The averaged signal 18 undergoes a Fourier analysis. The equidistant vertical lines in the pattern generate a pronounced local frequency in the frequency domain.
5. A longer reference signal can be generated with this local frequency. The reference signal has an uneven number of extremes. The working point of the reference signal is then the average extreme value.
6. The algorithm then correlates the reference signal and the actual signal. The periodic correlation signal describes valid setpoint positions for the vertical lines. If the reference signal has been selected sufficiently long, local nozzle errors do not have a great effect on the setpoint positions.
7. At the edges of the correlation signal, positions are eliminated that do not correspond to any setpoint positions. These positions arise due to the length of the reference signal and the periodic structure of reference signal and actual signal. FIG. 6 shows the start in a correlation signal 19.
8. The reference signal is then displaced to each setpoint position. The method evaluates the signal course in the actual signal around the working point and basically calculates three characteristic variables:
a) The deviation of the emphases of equivalently segmented regions of the setpoint signal and the actual signal. This deviation is a phase error 22. Obliquely jetting nozzles can be detected with the phase error. FIG. 7 shows such phase errors 22 in a corresponding phase error diagram 20.
b) The ratio of the maximum values of the setpoint signal and the actual signal permits a robust detection of missing or faint nozzles. This error is referred to as an amplitude error 24. The amplitude errors 24 can be seen in FIG. 8 in an exemplary amplitude error diagram 23.
c) An investigation of the scatter of the distribution of the actual signal delivers a further characteristic variable for assessing possible nozzle errors. This error is known as a variance error.
9. Since the resolution of the print head is known exactly and the resolution is retained during printing, enlargement of the camera system can also be determined with the pattern at the same time. The phase error 22 can thus be converted into a metric unit.
10. It is then possible to subject the phase error 22, the amplitude error 24 and the variance error to a robust signal evaluation in order to ascertain significant deviations. The filtering with an averaged absolute deviation from the median (median absolute deviation) delivers a robust assessment of the general signal scatter. If the measured values exceed this limit significantly, the latter are then candidates for possible errors.

[0069] A further, eleventh step also includes a trend adjustment of the averaged values in order to duly take into account individual deviations and measurement errors.

[0070] Furthermore, in a further preferred embodiment variant, no vertically printed, equidistant lines are used for printing nozzle patterns 14, but rather special patterns having autocorrelation functions which are monotonic. These patterns are suitable for measuring spacings precisely, since correlations have the advantage that information concerning entire image areas flows into the result and local errors have only a slight effect on the measurement result. Measurements in the local region at local, vertically printed, equidistant lines, on the other hand, are much more error-sensitive. However, influences of the distortion need to be taken into account, since the correlation patterns extend over a larger area.

[0071] A class of known patterns is the so-called Barker codes 34. Suitable Barker codes 34 must be delimited by color at the ends for printing. Only Barker codes 34 with positive values at both ends thus come into consideration. In contrast with electronic signals with positive and negative components, only color or no color is possible as a signal carrier in printing. The following table shows possible examples of Barker codes 34 to be used:

TABLE-US-00001 S/R to secondary Length Code maxima printable 2 +1−1  −6.0 dB no 3 +1+1−1  −9.5 dB no 4 +1+1−1+1 −12.0 dB yes 5 +1+1+1−1+1 −14.0 dB yes 7 +1+1+1−1−1+1−1 −16.9 dB no 11 +1+1+1−1−1−1+1− −20.8 dB no 1−1+1−1 13 +1+1+1+1+1−1− −22.3 dB yes 1+1+1−1+1−1+1

[0072] Alternative codes from radar technology with similar properties of monotonic autocorrelation functions are the Neuman-Hoffman (NH) sequences. Finally, all of the codes are distinguished in that the correlation functions have unique maxima, which markedly simplifies a signal evaluation. These patterns can be fitted into the central region of a print head 4. The central region contains 1920 nozzles and lies next to the transition regions at the sides of the print head 4. With 1920 nozzles, a unit of a Barker sequence 34 of length 13 can include 147 pixels. This corresponds to a length of 3.112 mm with a printed resolution of 1200 DPI. The correlations between printed Barker sequences 34 from different print heads 4 directly produce a measure for the displacement between print heads 4. FIG. 11 shows Barker codes 34 of length 13 from the above table fitted into the core region of print heads 4. An example of how the codes are segmented in the images and correlated with one another, on the other hand, is shown in FIG. 12. The maximum in a correlated signal 33 of two Barker sequences 34 directly indicates the displacement of the sequences relative to one another in pixel units. Pixels can be converted very precisely with printed equidistant lines into metric coordinates. The method can be used for the determination of the Y-stitchings, whereby the sequences are rotated through 90°. FIG. 13 shows in the left-hand illustration a two-dimensional pattern 28, which is composed of two Barker sequences which are put together perpendicular to one another. With such a sequence 28, it is also possible to detect a rotation of a print head 4. A rotated head leads to shearing of a pattern 29 on a sheet 11. The shearing 29 is shown in the illustration in FIG. 13 on the right-hand side. Since the nozzles in a print head 4 are distributed over a two-dimensional area, gaps in the sheared image 29 emerge when a rotation occurs.

[0073] A plurality of line-scan cameras for printed sheet monitoring are integrated in many printing machines 10. The cameras detect, with small overlaps, a complete printed sheet. Printing of periodic vertical lines, such as are known from patterns for nozzle monitoring, thus permit an adjustment of print heads 4 relative to one another in a further preferred embodiment variant.

[0074] The adjustment of the print heads 4 takes place in the X-direction perpendicular to the printing direction. This process is also called X-stitching. The overlapping regions of the print heads 4 should be aligned in the grid of the print head resolution. An alignment of the grid in the Y-direction and therefore in the printing direction does not take place mechanically, but electronically, in that an output at a print head 4 is delayed (Y-stitching). In many printing machines, moreover, a rotation of individual print heads perpendicular to the X-direction and Y-direction is possible. This adjustment option is called a Z-rotation.

[0075] In addition, a rotation of a print beam with all of the print heads is possible. Furthermore, in the case of the register adjustment, the X-displacements and Y-displacements of the individual color extractions relative to one another should be aligned. A Z-rotation of the entire print beam in turn has an effect on the X-stitching and Y-stitching of the print heads. The X-stitching can be adjusted in a favorable manner by a measurement of periodic vertical, equidistant lines. FIG. 9 represents the deviations between the setpoint and actual positions in a further diagram for phase errors 22. Phase errors 22 are constant for a print head due to the precise division of the print head and a CCD sensor in the core region of a print head 4. A maladjusted print head 4 appears in the transition region by a deviation 21 from X.

[0076] On the other hand, a further preferred embodiment for the position determination of print heads 4 for the adjustment is represented in FIG. 10. This is used if the optics of the camera are not sufficiently precise for the approach represented in FIG. 9. In a transition region 25 between the region of first and second print heads 31, 32, the deviation of X can be determined in a correlation signal 19 by calculation from the phase error and the filter for the correlation. The jump in the correlation signal 19 results due to the deviation of the print heads, which are reflected in a deviation of the equidistant printed lines. In the calculation of the correlation signal 19, a plurality of adjacent equidistant printed lines, acquired by the camera, are evaluated in each case. The deviation of the lines, which are printed in the stitching region by the respectively position-displaced adjacent printing head, cause the jump in the correlation signal 19. This occurs, since ever more adjacent, displaced lines are slowly taken into account in the evaluation, until the signal collapses and is then slowly normalized again, the fewer the lines taken into account by the preceding print head.

[0077] The resolution of a print head 4 is exactly known. A determination of an unknown optical image is possible with lines printed equidistant. A phase error 22 can thus be converted to a precise metric length measure. Suitable methods from signal evaluation permit singular disruptions to be taken into account, such as are caused for example by so-called oblique jetters. Finally, it is crucial for the accuracy of the measurements that many measurements inside the core regions of print heads 4 produce a high measurement accuracy even with a comparatively low camera resolution. Since the transition regions of print heads 4 are known in relation to the camera, interfering influences from these regions can easily be removed. The Y-stitching can be detached with the same principle as the X-stitching, except that the positional errors from different image columns are compared with one another, instead of the positional errors in an image line. In order to ascertain the Z-rotation error, the fact is used that lines printed in the printing direction change their mutual spacings during a Z-rotation. The change in the line spacing can be calculated from the position of the printing nozzles relative to the point of rotation of the print head 4 or of the print beam. In the reversal of motion, the rotation angle can be calculated from the nozzle-accurate measurement of line spacings in a given line pattern. Whereas the X-stitching error scarcely has any influence on the y-stitching error and z-rotation error, Y-stitching and z-rotation have a strong mutual influence on one another. A Z-rotation error of the print beam leads for example to a Y-stitching error variable over the print beam width. Through the use of a regression of the Y-stitching error over the beam width, the Z-rotation error of the print beam can be ascertained and compensated for. The Y-stitching errors of the print heads are then changed by the correction of the z-rotation error of the print beam.

[0078] The printed resolution with many inkjet printing machines 10 is currently higher than an image resolution of the cameras used for the image control. No methods from the prior art thus produce in a first step, from sub-images, a total image which is then evaluated. Due to the lower image resolution, the images have to be aligned with one another with subpixel accuracy. This requires a precise geometrical calibration of the sub-images as well as a large amount of computing time. Such a procedure is acceptable for quality control in the sense of a visual examination of a printed product 11, since a perceptible optical resolution for the user lies well below the printing resolution. However, if the measurements are to be used to correct the printing process, a high measurement accuracy is then required.

[0079] Therefore, in a further preferred embodiment variant, the drawbacks of producing a total image are avoided in that all of the required magnitudes are determined directly from the individual sub-images. A geometrical coupling of the sub-images takes place by way of printed reference marks. Correct assignments of the line elements to specific nozzles are thus also possible with images from a central part of a nozzle pattern. These reference marks are printed with a high resolution and serve as a pattern for reference systems for the geometrical calibration of the cameras. The method determines all of the geometrical magnitudes in relation to printed or otherwise determined reference systems. Such reference systems can arise from limits of the printing substrate or marks in the printing machine. A pixel-accurate detection of geometrical patterns with respect to the reference systems is possible inside a sub-image. The measured values are not distorted by a prior interpolation during a separate alignment of the sub-images to form a total image. Alternatively, a calibration of the cameras relative to one another is possible with special devices. Since the locations of the cameras relative to one another in a printing machines 10 do not change, information concerning reference systems can also be persistently stored.

[0080] The reference marks are detected by the given camera only in a roughly known region. The reference marks are therefore created in such a way that individual nozzle errors do not have a great influence on the position determination of the reference marks. For example, the emphasis of all of the pixels can be formed in the case of a circle. The reference marks can also be used to determine an orientation.

[0081] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: [0082] T transport direction [0083] 1 feeder [0084] 2 printing unit [0085] 3 delivery [0086] 4 inkjet heads [0087] 5 transfer cylinder [0088] 6 drive [0089] 7 printing cylinder (jetting cylinder) [0090] 8 sheet retaining zone [0091] 9 channel [0092] 10 sheet-fed printing machine [0093] 11 sheet [0094] 12 print image [0095] 13 white line [0096] 14 nozzle test pattern for a print colour [0097] 15 vertically printed, equidistant lines [0098] 16 detection of the horizontal line and averaging over the vertical lines [0099] 17 averaged original signal [0100] 18 interpolated original signal [0101] 19 start of a Fourier-transformed correlation signal [0102] 20 phase error diagram of a Fourier-transformed actual signal [0103] 21 offset deviation in the transition region between two print heads [0104] 22 phase error [0105] 23 amplitude error diagram of a Fourier-transformed actual signal [0106] 24 amplitude error [0107] 25 signal region in the transition between two print heads [0108] 28 combined two-dimensional Barker sequence [0109] 29 combined two-dimensional Barker sequence sheared [0110] 30 controller or computer [0111] 31 region of the first print head [0112] 32 region of second print head [0113] 33 representation of a correlation of two Barker sequences [0114] 34 Barker sequence