Method for assessing the condition and improving the printing quality of printing nozzles in printheads of an inkjet printing machine and improved printing nozzle test chart

Abstract

A method for assessing the condition and improving the printing quality of printing nozzles in printheads of an inkjet printing machine includes printing a nozzle test chart formed of a specific number of horizontal rows of equidistant vertical lines printed periodically underneath one another, with only every n.sup.th printing nozzle contributing to the test chart in every row of the test chart and n corresponding to a different number between one and the specific number of horizontal rows, recording and digitizing the test chart and analyzing the test chart regarding the condition of the contributing nozzles by using a computer. The nozzles printing horizontal lines between the horizontal rows complementing the equidistant vertical lines to form quadrangular objects used by the computer to analyze the condition of the contributing nozzles. Measures are taken to improve the printing quality of the nozzles depending on the condition of the contributing nozzles.

Claims

1. A method for assessing the condition and improving the printing quality of printing nozzles in printheads of an inkjet printing machine, the method comprising the following steps: printing at least one nozzle test chart formed of a specific number of horizontal rows of equidistant vertical lines printed periodically and disposed underneath one another, with only every n.sup.th printing nozzle in a width direction of a printhead contributing to the nozzle test chart in each respective row of the nozzle test chart and with n corresponding to a different number between one and the specific number of horizontal rows for every row; using a computer to record and digitize the printed nozzle test chart and analyze the nozzle test chart in terms of a condition of contributing printing nozzles; using the printing nozzles to additionally print horizontal lines between the horizontal rows, the horizontal lines complementing the equidistant printed vertical lines to form quadrangular objects being specifically used by the computer to analyze the condition of the contributing printing nozzles; using the computer to employ the quadrangular objects to draw conclusions on the condition of the printing nozzles that contribute to the nozzle test chart by applying image processing algorithms to check: whether all equidistant lines printed vertically exist by comparing the number of detected quadrangular objects to the number of expected equidistant lines printed vertically, whether the quadrangular objects that have been formed are complete and do not have any gaps, whether the equidistant lines printed vertically have been printed continuously and are not porous, and whether the detected quadrangular objects have an expected regular size; and then taking measures to improve the printing quality of the printing nozzles depending on the condition of the contributing printing nozzles.

2. The method according to claim 1, which further comprises using an in-line image processing system to record the printed nozzle test chart, the computer corresponding to an image processor of the in-line image processing system.

3. The method according to claim 1, which further comprises carrying out the measures to improve the printing quality of the printing nozzles by switching off printing nozzles having a condition not within desired printing quality parameters and compensating for those printing nozzles by using adjacent printing nozzles and, for printing nozzles having a print dot deviating in a printing direction, varying an instant in time at which printing takes place.

4. A method for assessing the condition and improving the printing quality of printing nozzles in printheads of an inkjet printing machine, the method comprising the following steps: printing at least one nozzle test chart formed of a specific number of horizontal rows of equidistant vertical lines printed periodically and disposed underneath one another, with only every n.sup.th printing nozzle in a width direction of a printhead contributing to the nozzle test chart in each respective row of the nozzle test chart and with n corresponding to a different number between one and the specific number of horizontal rows for every row; using a computer to record and digitize the printed nozzle test chart and analyze the nozzle test chart in terms of a condition of contributing printing nozzles; using the printing nozzles to additionally print horizontal lines between the horizontal rows, the horizontal lines complementing the equidistant printed vertical lines to form quadrangular objects being specifically used by the computer to analyze the condition of the contributing printing nozzles; printing the horizontal lines with a line thickness of multiple pixels and every contributing printing nozzle applying a larger ink drop volume than an ink drop volume applied for the vertical lines; and then taking measures to improve the printing quality of the printing nozzles depending on the condition of the contributing printing nozzles.

5. A method for assessing the condition and improving the printing quality of printing nozzles in printheads of an inkjet printing machine, the method comprising the following steps: printing at least one nozzle test chart formed of a specific number of horizontal rows of equidistant vertical lines printed periodically and disposed underneath one another, with only every n.sup.th printing nozzle in a width direction of a printhead contributing to the nozzle test chart in each respective row of the nozzle test chart and with n corresponding to a different number between one and the specific number of horizontal rows for every row; using a computer to record and digitize the printed nozzle test chart and analyze the nozzle test chart in terms of a condition of contributing printing nozzles; carrying out the step of recording the printed nozzle test chart by slightly overexposing the nozzle test chart to make porous equidistant vertical lines caused by weak nozzles more visible; using the printing nozzles to additionally print horizontal lines between the horizontal rows, the horizontal lines complementing the equidistant printed vertical lines to form quadrangular objects being specifically used by the computer to analyze the condition of the contributing printing nozzles; and then taking measures to improve the printing quality of the printing nozzles depending on the condition of the contributing printing nozzles.

6. A method for assessing the condition and improving the printing quality of printing nozzles in printheads of an inkjet printing machine, the method comprising the following steps: printing at least one nozzle test chart formed of a specific number of horizontal rows of equidistant vertical lines printed periodically and disposed underneath one another, with only every n.sup.th printing nozzle in a width direction of a printhead contributing to the nozzle test chart in each respective row of the nozzle test chart and with n corresponding to a different number between one and the specific number of horizontal rows for every row; using a computer to record and digitize the printed nozzle test chart and analyze the nozzle test chart in terms of a condition of contributing printing nozzles; using the printing nozzles to additionally print horizontal lines between the horizontal rows, the horizontal lines complementing the equidistant printed vertical lines to form quadrangular objects being specifically used by the computer to analyze the condition of the contributing printing nozzles; using the computer to binarize the recorded and digitized nozzle test chart for analysis by the computer in accordance with a dynamically detected threshold and using the computer to implement object recognition based on the thus obtained binarized nozzle test chart to detect the quadrangular objects; and then taking measures to improve the printing quality of the printing nozzles depending on the condition of the contributing printing nozzles.

7. The method according to claim 6, which further comprises determining the condition of the printing nozzles contributing to the nozzle test chart by analyzing the quadrangular objects, statistically determining an average size of the quadrangular objects, and using the computer to conclude from an existence of a regular quadrangle that a respective printing nozzle works correctly.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a diagrammatic, longitudinal-sectional view of an example of the structure of a sheet-fed inkjet printing machine;

(2) FIG. 2 is a plan view of a printing substrate having a white line caused by a missing nozzle;

(3) FIG. 3 is a plan view illustrating a section of two rows with an every-10.sup.th-nozzle pattern in a nozzle test chart;

(4) FIG. 4 is a plan view illustrating an object detection of quadrangles in a section;

(5) FIG. 5 is a plan view illustrating a detection of a phase defect by using a defective quadrangle;

(6) FIG. 6 is a plan view illustrating an object detection of a phase defect by using a segmented defective quadrangle; and

(7) FIG. 7 is a flow chart of the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(8) Referring now in detail to the figures of the drawings, in which mutually corresponding elements have the same reference symbols, and first, particularly, to FIG. 1 thereof, it is seen that the field of application of the preferred exemplary embodiment is an inkjet printing machine 7. FIG. 1 shows an example of the fundamental construction of such a machine 7, including a feeder 1 for feeding a printing substrate 2 to a printing unit 4, where it receives an image printed by printheads 5, and a delivery 3 receiving the printing substrate 2. The machine is a sheet-fed inkjet printing machine 7 controlled by a control unit, controller or computer 6. While this printing machine 7 is in operation, individual printing nozzles in the printheads 5 in the printing unit 4 may fail as described above. Such a failure results in white lines 9 or, in the case of multicolor printing, in distorted color values. An example of such a white line 9 in a printed image 8 is shown in FIG. 2.

(9) In order to detect such white lines 9, nozzle test charts 11 including multiple objects 12, which are mostly in the form of lines 12 in the printing direction that are printed in multiple rows, are printed at regular intervals. The individual printing nozzles alternatingly contribute to every row of objects 12. A printed nozzle test chart 11a is then recorded and digitized and the individual objects 12 are measured and analyzed by the computer 6. Based on the characteristics of the objects 12, the computer 6 assesses a condition 17 of the printing nozzles in terms of parameters such as amplitude and phase. The computer may be the control unit 6 of the printing machine. In most cases, however, it is the computer of an image recording system 10 used to record and digitize the printed nozzle test charts 11a. The image recording system 10 is preferably disposed in-line and downstream of the printheads 5 in the inkjet printing machine 7.

(10) In terms of analyzing re-digitized nozzle test charts 11b, the invention makes use of an image processing effect, namely the object segmentation effect. FIG. 7 is a schematic flow chart of a preferred embodiment of the method of the invention. The nozzle test chart 11 that is used is similar to the line-shaped test chart described above. Every printing nozzle prints a vertical line 12. Since the printing nozzles are spaced apart by a distance N in one row, there are N rows disposed underneath one another. The printing nozzle prints a small available dot. In addition, the entirety of the nozzles seamlessly prints horizontal lines 18 between the rows between the vertical lines/stripes 12 disposed in rows. These lines 18 are formed of multiple pixels. If necessary, a larger ink volume is used for these pixels, causing the printing nozzles to print a large dot. This prevents missing printing nozzles from creating gaps in the horizontal lines 18. In a regular case, the horizontal lines 18 complement the vertical lines 12 to form quadrangles 14 in the nozzle test chart 11. This is shown by way of example in FIG. 3. Prior art methods do not include this horizontal line 18. It is true that there are prior art methods that may have horizontal lines, but those horizontal lines are optionally used for a better distinction between the rows of stripes and they do not form closed quadrangles 14 in the nozzle test chart 11. The printed nozzle test chart 11a is then recorded by the image recording system 10. The image recording system preferably slightly overexposes the nozzle test chart to make porous lines printed by weak nozzles more pronounced. Then a computer 6 binarizes the re-digitized nozzle test chart 11b in accordance with a dynamically detected threshold to obtain a binarized nozzle test chart 11c that is easier to analyze.

(11) Then the computer 6 specifically looks for and analyzes the quadrangles 14 in the binarized nozzle test chart 11c by using image processing algorithms. For this purpose, the average size of the quadrangles 14 is calculated statistically as shown in FIG. 4. Specific printing nozzle conditions 17 result in specific defects in the quadrangles 14. For instance, if a printing nozzle fails completely, no line 12 is printed and the expected closed quadrangle 14 is not formed at this location in the nozzle test chart. Instead, the computer 6 will find a quadrangle that is approximately twice the expected size and may thus conclude that the nozzle in question is defective. However, if the nozzle has a phase defect 13, i.e. if it exhibits a deviation in the Y/printing direction, there will be no closed quadrangle 14 because a gap is created, as seen in FIG. 5. Instead, an open quadrangle 15 is created. Thus, the computer 6 or rather the image processing algorithm realizes that there is no closed quadrangle 14 but a different object 15 distinguished from an expected regular quadrangle 14 and may conclude that there is a phase defect 13. If a printing nozzle defect causes a nozzle to print intermittently, the resultant amplitude defect may be detected due to a porous vertical line, i.e. a line that is not continuous. If a printing nozzle prints at an angle in the x direction, i.e. in a direction transverse to the printing direction, two closed quadrangles 14 that deviate slightly from the regular size are created. If the printing nozzle slightly deviates to the left, the quadrangle 14 to the left is slightly smaller. If the printing nozzle deviates to the right, the quadrangle 14 to the right is correspondingly smaller. Deviations of the contributing printing nozzles may be detected based on these size deviations of the quadrangles 14.

(12) Missing printing nozzles, amplitude defects, and phase defects in the x direction may in principle be detected by using normal printing nozzle test charts without the quadrangles of the invention. A major advantage of the nozzle test chart 11 of the invention including quadrangles 14 and of the method of the invention for analyzing the latter is a much more efficient detection of phase defects 13 in the y direction. Such defects are difficult or even impossible to detect using conventional nozzle test charts.

(13) Moreover, the embodiment pertaining to object recognition of the quadrangles 14 described above, in particular for detecting phase defects 13 in the y-direction, may even be further improved. This is done in a further preferred embodiment in which the computer 6 divides the nozzle chart rows into smaller horizontal areas 16 as shown in FIG. 6. The computer 6 determines an amplitude signal for every area 16 and analyzes it as follows: the cell in which the vertical printing nozzle line 12 is expected is used to determine the amplitude signal. If necessary, only an inner region 19 of a nozzle line is used to determine the signal to prevent the horizontal lines 18 from interfering with the signal. The inner area 19 of the cell is the area in which the vertical printing nozzle line 12 is expected. Then the inner area 19 is horizontally divided into area parts. If the free area around the horizontal lines 18 is used, too, or if an area part is used to determine an amplitude signal, the amplitude signal generally ought to be of the same strength or even stronger due to interference. However, if the amplitude signal in one of the two area parts at the horizontal separators is weaker than the actual inner region 19 for determining the amplitude signal, this indicates a phase defect 13 of the printing nozzle.

LIST OF REFERENCE NUMERALS

(14) 1 feeder 2 current printing substrate/current print sheet 3 delivery 4 inkjet printing unit 5 ink jet printhead 6 computer 7 inkjet printing machine 8 printed image on the current print sheet 9 white line 10 in-line image recording system 11 nozzle test chart 11a printed nozzle test chart 11b recorded re-digitized nozzle test chart 11c binarized nozzle test chart 12 vertical printing nozzle line 13 printing nozzle line with a phase defect in the printing direction 14 closed quadrangle detected by object recognition 15 open quadrangle 16 smaller region for analysis in the quadrangle 17 established printing nozzle condition 18 horizontal line in the nozzle test chart 19 inner region for determining the amplitude signal