DE102019211687A1 - Meta information coding for inkjet printing processes - Google Patents

Abstract

Method for printing quality analysis in an inkjet printing machine (4) by means of a computer (3, 6), test patterns (11) being printed during a printing process, measured by means of an image acquisition system (2) and evaluated by the computer (3, 6) and results the evaluation for controlling the inkjet printing machine (4) can be used, printing and evaluation of the test pattern (11) requiring a synchronous process, which is characterized in that codes (9, 13) are integrated into the test pattern (11) by means of local gray value adjustment which are captured by the image acquisition system (2) and evaluated accordingly by the computer (3, 6) for carrying out the print quality analysis.

Description

The invention is concerned with a method for the print quality analysis of an inkjet printing machine by means of the use of coded meta information.

The invention is in the technical field of inkjet printing.

In industrial inkjet printing, the condition of the individual print nozzles of the print heads used in the inkjet printing machine is a decisive criterion for the resulting print quality of the inkjet printing machine used. The condition of the individual pressure nozzles usually changes over time. For example, Dry ink residue in the print nozzles if they will not be used for a while. These dried residues can then cause reduced ink output or e.g. also oblique spraying of the pressure nozzle in question. Even when printing with different loads, the print nozzle often does not behave linearly, which makes its printing behavior difficult to predict. These changes in the status of the printing nozzle and their corresponding effects on the print image to be generated must therefore be checked regularly when operating an inkjet printing machine.

As is known, various types of test patterns are used for this, which are applied to the substrate at regular intervals, in addition to the print image to be generated, and are continuously evaluated accordingly with regard to the current state of the respective pressure nozzles involved in printing the test pattern. The test samples are structured in such a way that all printing nozzles relevant to the printing process are involved in printing these test samples. However, there are different types of test patterns. For example, very widespread Test samples that have solid areas, since in such solid areas one diminishes, crooked or even failed printing nozzles can be recognized very quickly. It is therefore very easy to determine those printing nozzles that are malfunctioning and that have a negative effect on the resulting print image. A disadvantage of these test samples with full-tone areas, however, is that it is often not possible to identify the individual pressure nozzle that is malfunctioning, but usually only the area in which it is likely to be located. Therefore, a second type of test pattern is often additionally printed, in which each pressure nozzle involved a single object, e.g. one line, prints and the individual objects are arranged in such a way that each individual printing nozzle can be clearly identified on the basis of the line printed by it. The evaluation of the object printed by each pressure nozzle also indicates the current status of the pressure nozzle with regard to skewness and pressure strength, also known as phase and amplitude. The reason why you not only print this test pattern and thus evaluate the current status of all printing nozzles is because you can evaluate the status of each individual printing nozzle, but you do not know how this status affects the print image to be generated. After all, not all print nozzles with the same defect pattern have the same effect on the corresponding print pattern. The combination of printing and evaluation of both types of test samples thus results in a relatively reliable evaluation of the state of the printing nozzles used in the inkjet printing machine in question.

A disadvantage of this approach, however, is the effort that must be taken to regularly print and evaluate two different types of test patterns in addition to the print image that is actually to be generated. It would therefore be very advantageous to reduce this effort or waste, which is caused by printing two different test patterns. There are various approaches to this in the prior art. The most obvious solution is certainly to assess the condition of the printing nozzles used on the basis of the actually printed print image. However, these approaches have not proven to be very expedient since, especially in the case of inkjet printing, with its flexibility and its constant change of the subjects to be printed, monitoring the printed image cannot lead to a reliable control of all relevant printing nozzles of the inkjet printing machine. Too many pressure nozzles are simply not involved in the pressure on a regular basis.

An integrated image acquisition system is usually used to evaluate the printed test samples. This is now available inline in many high-quality printing presses. This means that it is installed in or after the last printing unit of the inkjet printing machine. It usually has one or more cameras that monitor the entire print image generated and forward the image data generated in this way to an image processing computer, which then compares the captured image data with digitally provided target image data. With this system, a high-quality print quality analysis of the generated print image is carried out. This system can of course also be used to evaluate the printed test samples. If such a system is not available, the test samples must be examined at regular intervals by the printer either by hand with a magnifying glass or fed to an external image processing system.

When using the integrated image acquisition system, meta information is exchanged between the control system of the print and the inspection system in the printing operation, for example layout information about the type and position of nozzle patterns or image regions which are to be analyzed by the camera. Here, a sequence of layouts is exchanged, which must then be adhered to by the printing sequence. Control commands are typically used to describe a description of the printing sequence, the actual printing is guaranteed with an electronic real-time signal between the printing system and the inspection system. If the electronic signal is disturbed, for example in the way a signal disappears or signals are generated too much due to noise, the synchronization between print layout and actual print is no longer guaranteed.

It could also result in a loss of synchronization by deliberately repeating the printing of a layout by the control electronics in real time, but this real-time behavior cannot be represented by the prefilling in the command interface. An ad hoc repetition of a layout is e.g. desirable if a print is marked as bad and "immediately" it is desired to repeat this print. The reaction time to phase a repetition of a layout synchronized via the command interface is significantly longer.

Another problem with the quality control of the existing pressure nozzles by means of the printing and the evaluation of test patterns is to precisely synchronize the evaluation process of the corresponding test patterns with the printing thereof. For a correct evaluation of the printed test pattern, it must therefore be clearly known which test pattern has just been printed, since a comparison of the recorded test pattern with the wrong digital test pattern available to the computer naturally delivers wrong results.

The object of the present invention is therefore to disclose a method for checking defective printing nozzles in an inkjet printing machine, which method is more efficient and reliable than the previously known methods.

This object is achieved by a method for print quality analysis in an inkjet printing machine using a computer, test patterns being printed during a printing process, measured using an image acquisition system and evaluated by the computer, and results of the evaluation being used to control the inkjet printing machine, printing and evaluation of the test pattern require a synchronous process, which is characterized in that codes are integrated into the test pattern by means of local gray value adjustment, which codes are recorded by the image acquisition system and evaluated accordingly by the computer for carrying out the print quality analysis. The core of the invention is the knowledge that the individual printing nozzles can be controlled in such a way that the local gray values of the pixels to be generated by the printing nozzles are changed. This change can then be used to transmit certain information. Such local gray value changes cannot, of course, be carried out in the print image to be generated, since this changes the desired print image. A change in an appealing test pattern, on the other hand, is far less critical as long as the test pattern can still fulfill its purpose, namely the print quality control. Of course, the local gray value changes do not happen randomly, but correspond to a certain code. This code can then be evaluated using the image acquisition system used for quality control. The information encoded in this way can then be used for various purposes of print quality analysis.

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 Morse codes are used as codes. Of course, many types of codes can be generated using the local gray value change, e.g. QR codes etc. However, since each individual pressure nozzle has vertical objects, e.g. Lines generated in the printing direction mean a local change in the gray value of this printing behavior, of course first of all, a linear printing, weaker or non-printing of a printing nozzle. Morse codes are by far the best option for this printing behavior.

A further preferred development of the method according to the invention is that the computer assigns a layout ID to each test pattern, codes it accordingly and inserts it into the test pattern by means of local gray value adjustment and, when evaluating the measured test patterns, first identifies the test pattern using the detected layout ID and thus printing and evaluation of the test pattern are synchronized. This corresponds to the first application for the codes integrated in the test pattern by means of local gray value adjustment. With this layout ID, the printed test pattern and the evaluation of the same test pattern can be easily and efficiently synchronized. One or more printing nozzles print a certain Morse code in a certain area of the test pattern, which contains the layout ID as information. The image acquisition system, which is responsible for the evaluation knows at which specific position of the test pattern the ID is located and isolates the relevant Morse code, decodes the layout ID and checks whether the ID of the recorded test pattern matches the ID of the internal, digitally available test pattern with which the recorded test pattern is matched, agrees. If this is not the case, a synchronization error is output.

A further preferred development of the method according to the invention is that the layout ID is always placed in the same place in the test pattern by the computer regardless of the structure of the test pattern. As already explained, regardless of the structure of the test pattern, the layout ID must always be placed in the same place on the test pattern. Otherwise the image acquisition system used for evaluation cannot find the Morse code of the layout ID. Because it is of course extremely difficult for the detection system to distinguish the conscious, local gray value adjustments that correspond to the Morse code from random gray value jumps elsewhere in the test pattern. For this, the image acquisition system must know the same place in which the coded layout ID is placed, which also includes the exact size of the area.

A further preferred development of the method according to the invention is that different test patterns are printed which check various possible printing errors by means of the computer-aided evaluation. Using the layout ID, it is also much easier to appropriately assign and synchronize several different test patterns that are used to evaluate possible different printing errors.

A further preferred development of the method according to the invention is that different color separations of the test pattern are iteratively printed as different test patterns, the layout ID also containing the information about the respective color separation. Different color separations of the test pattern in question are an application for different test patterns, since the different color separations must of course always be printed by different print heads in the inkjet printing machine. In order to apply the different inks of the individual color separations, it is necessary to print and evaluate a separate test sample for each color separation. Of course, it would be theoretically conceivable to print a test sample from all existing color separations, either by overprinting or side-by-side printing of all the print heads involved, and then to break down the image digitized by the image acquisition system into the individual color separations. However, this approach is far more error-prone and involves significantly more computing effort. It is therefore preferred to print and evaluate different test samples for the different color separations. In order not to waste additional space on the print substrate, a test pattern with an alternating color separation and the respective print heads involved is always printed and evaluated at the same place. Of course, this results in a certain interval for each print head with its specific color separation until it is checked the next time. However, the frequency of checking each individual printhead is still high enough to ensure an ongoing assessment of the status of all relevant print nozzles.

A further preferred development of the method according to the invention is that the local gray value adjustments for generating the coded layout ID are implemented by using the smallest and largest gray levels of an individual pressure nozzle or by using the smallest and largest gray levels of several adjacent pressure nozzles. The relevant codes, in particular the Morse code, can thus be implemented relatively reliably. Since the Morse code ultimately only consists of printed or non-printed pixels, the use of the smallest and largest gray scale results in an ideal contrast for mapping the Morse code and other possible codes used. Whether the Morse code is implemented by printing a single print nozzle or by printing several print nozzles together depends on various parameters, such as the resolution of the image acquisition system and the space required on the print substrate for the layout ID.

Another preferred development of the method according to the invention is that when a single printing nozzle is used, an information unit of the code is realized by generating a pixel with the smallest and largest gray level or by generating a plurality of pixels with the smallest and largest gray level. If only a single pressure nozzle is to be used to generate a relevant Morse code, this can of course either generate the code with only a single pixel with the contrast between the smallest and largest available gray level, or use several pixels with the smallest or largest gray level for this. Which procedure is preferred depends on the criteria of the respective application. Above all, it is a question of the type of code chosen whether an information unit of the code, e.g. a bit, or a Morse code can be implemented by a single pixel sooner or later.

A further preferred development of the method according to the invention is that In addition or alternatively to the generation of the layout ID, individual printing nozzles of the inkjet printing machine for identification purposes integrate their own printing nozzle number as a code into the test pattern and, when evaluating the measured test pattern, the computer assigns printing errors found to the relevant printing nozzles via this code. This represents the second application of the method according to the invention. In this case, no layout ID of a specific test pattern is generated, which is then used for synchronization between printing and evaluation of the test pattern, but rather the printing of the own pressure nozzle number enables an exact assignment of the pressure nozzle concerned. Of course, this presupposes that the image acquisition system, which is used for the evaluation, is also able to recognize the printed code of an individual printing nozzle, which then contains the printing nozzle number. If this is possible, this offers the great advantage that in a test pattern with full-tone or other gray value areas for evaluating the effect of the condition of a printing nozzle on the resulting print image, there is no longer a need to print a second test pattern to identify the individual printing nozzles. You can simply have the individual print nozzles in a certain area of the test pattern with the full tone or gray value area printed out using the coded, local gray value adjustment of your own print nozzle number. The image acquisition system as an evaluation unit can then decode this number and thus knows immediately which artifact is caused in the continuous test pattern by the pressure nozzle with the relevant pressure nozzle number. Since only a single pressure nozzle can be used here for coding, since the pressure nozzle number in question is to be coded, of course only codes are suitable for this application which can also be implemented with this one-dimensional coding type.

A further preferred development of the method according to the invention is that the image acquisition system is placed inline in the inkjet printing press and has an image acquisition computer which carries out the evaluation of the measured test patterns. An inline image acquisition system, which is placed in the inkjet printing machine, is preferably used to implement the method according to the invention. This has the advantage that an immediate, quasi online evaluation of the printed test samples and thus also an immediate online implementation of the method according to the invention with regard to printing and evaluation of the layout IDs and the printing nozzle numbers is made possible. In addition, the inline image acquisition system, which has print quality control as its main application, is then already present anyway and no additional hardware has to be used to carry out the method according to the invention.

The invention as such and constructively and / or functionally advantageous developments of the invention are described in more detail below with reference to the accompanying drawings using at least one preferred exemplary embodiment. Corresponding elements are provided with the same reference symbols in the drawings.

• 1: ein Beispiel eines verwendeten Inline-Bilderfassungssystems,
• 2: ein Beispiel für den Einsatz der Layout-ID,
• 3: ein Beispiel für die Umsetzung einer per Morse-Code realisierten Druckdüsennummer,
• 4: den schematischen Ablauf zur Synchronisation mittels Layout-ID,
• 5: den schematischen Ablauf zur Identifikation von Druckdüsen mittels Druckdüsennummer.

The figures show:

• 1 : an example of an inline imaging system used,
• 2nd : an example of the use of the layout ID,
• 3rd : an example for the implementation of a pressure nozzle number realized by Morse code,
• 4th : the schematic sequence for synchronization using the layout ID,
• 5 : the schematic sequence for the identification of pressure nozzles by means of the pressure nozzle number.

1 shows an example of an image acquisition system 2nd , which uses the method according to the invention. It consists of at least one image sensor 5 , usually a camera 5 which in the sheet printing machine 4th is integrated. The at least one camera 5 takes that from the press 4th generated print images and sends the data to a computer 3rd , 6 to the results. This calculator 3rd , 6 can have its own separate calculator 6 be, for example, one or more specialized image processing computers 6 , or also with the control computer 3rd the printing press 4th be identical. At least the control computer 3rd the printing press 4th has a display 7 , on which the results of the image inspection are displayed.

When in print meta information 14 next to the regular print image 8th Several methods are conceivable, for example text characters or barcodes. According to the method according to the invention, the coding of information using Morse codes 9 , 13 proposed using the gray levels of a printhead, which is very easy to implement, even from systems that are embedded directly in the printhead control electronics without the complex, typically PC-based, rendering techniques having to be used.

A little complex electronics close to the printhead is able to generate self-generated lines with the possible gray levels of the printhead. This enables this electronics to be located next to the actual print image 8th that it receives and should print the print image 8th with morse codes 9 , 13 to complete.

Furthermore, by reproducing the morse code elements "long / short / nothing" it is also possible for a single printing nozzle to display information, specifically that this printing nozzle, for example, has its own number in the printed image 8th records.

In addition, by using individual pixels with the respective grayscale, it is possible to generate microscopically very small codes, that is to say below 100 μm.

• I) Codierung einer Drucklayout-ID als Metainfo 14 beim Druck eines Testmusters 10, 11 als Morsecode 9
• II) Codierung einer Druckdüsennummer in einem Druckdüsen-Testmuster 11 durch genau diese Druckdüse als Morsecode 13

A distinction should be made between two special applications that are part of the method according to the invention:

• I) Coding of a print layout ID as meta info 14 when printing a test sample 10th , 11 as Morse code 9
• II) Coding a pressure nozzle number in a pressure nozzle test pattern 11 through exactly this pressure nozzle as Morse code 13

To I):

The sequence of the first use case is shown schematically in 4th shown. According to the method according to the invention, a layout ID is generated via the command interface 14 from the control computer 3rd the printing press 4th sent, this layout ID in real time 9 is printed as an element using Morse coding. The imaging system 2nd is now through communication 15 via real-time signal to its image processing computer 6 that the next print triggered the imaging system 2nd happens. Now the next layout from the order of the received layouts is not used, but only the Morse code 9 or the coded layout ID 9 , read out around the layout with the found ID 9 to scan the printed image 8th to use. Here is the omission or repetition of a layout by the control computer 3rd possible because it also means the printed coding 9 on the printed image 8th is included again.

Regardless of a special layout, the coding can always be printed on an identical position defined before printing, so that the parsing of the layout ID 14 It is then always possible to use the layout found to find different elements from the image capture system that are different per print 2nd to edit.

An example here is the monitoring of the print nozzles on each sheet. On each individual sheet there is a pattern that iterates through the colors 10th printed. This turns all / sides into the pattern on a seven-color inkjet press 10th printed for one color. 2nd shows an example of this process. Here as a pattern 10th Register marks 10th printed while the layout ID 9 as the main component of meta information 14 Morse coded according to the register marks 10th is printed. Now comes the synchronization between the printing of the register marks 10th and the imaging system 2nd Checking this, except kick, becomes the pattern 10th analyzed under a wrong assumption of color. The same principle is used when printing nozzle test patterns.

In command of the test pattern 10th then the color is also called. If a trigger signal disappears or a page is repeated, it is possible to determine the correct color using the layout coding.

An alternative variant of I. ) also provides that for coding a layout ID 14 with a morse code, which is generated by a single pressure nozzle with the largest and smallest drop or the smallest and largest gray scale, the morse codes are generated by several combined pressure nozzles. Alternatively, a pressure nozzle can be used that does not use a single pixel, but several pixels for coding the "long" / short "signals. Here it is still given that the coded layout ID 9 can be generated by the electronics close to the printhead, while the printed element can also be generated by an image acquisition system 2nd can be parsed with lower resolution.

It is also possible to use this principle of layout coding to synchronize the print with the image acquisition system 2nd to change to a different coding instead of a Morse code

To II)

The flow of this preferred application is shown schematically in 5 shown. When detecting defective pressure nozzles 16 A line pattern is generated in a print head in which the state of each print nozzle is determined by the position (deviation from the target position) and type of line (eg continuity). One difficulty here is to assign the lines to the actual pressure nozzles. If not only a long line is written for each pressure nozzle, but also thick / thin or no lines using grayscale, the ID of the pressure nozzle can also be read directly. 3rd shows an example of a pressure nozzle test pattern 11 in which in the print line 12th In a single pressure nozzle, the individual numbers of the pressure nozzle number as individual Morse code elements 13a , 13b , 13c , 13d , 13e be encoded.

1. 1. Die Druckdüsennummer wurde durch ein globales Kamerapixel zu Düsenzuordnung per Referenzdüsen und Abzählen im Muster berechnet. Geht durch mechanische Verschiebung des Druckkopfs zum Inspektionssystem nach Messung der Zuordnung und vorm dem Druck des Musters die Zuordnung verloren so werden falsche Druckdüsennummern berechnet.
2. 2. Es ist auch möglich, dass die Zuordnung generell richtig ist, aber durch eine sehr starke Abweichung der Position der Druckdüse im Strichmuster sich Druckdüsen in Ihren Sollpositionen überspringen, sodass auch dadurch die Druckdüsen Durchnummerierung einen Fehler bekommt. Im erfindungsgemäßen Verfahren können solche Probleme nicht auftreten, da die Düsennummer direkt aus den Strichen einer Druckdüse ablesbar ist.

This avoids typical problems:

1. 1. The print nozzle number was calculated using a global camera pixel for nozzle assignment using reference nozzles and counting in the sample. If the assignment is lost due to mechanical displacement of the print head to the inspection system after measurement of the assignment and before the pattern is printed, incorrect print nozzle numbers are calculated.
2. 2. It is also possible that the assignment is generally correct, but due to a very large deviation in the position of the pressure nozzle in the line pattern, pressure nozzles are skipped in their desired positions, so that the numbering of the pressure nozzles also gets an error. Such problems cannot occur in the method according to the invention, since the nozzle number can be read directly from the lines of a pressure nozzle.

In addition to the automatic evaluation by an image acquisition system 2nd in an alternative embodiment variant it is also possible for a human user 1 with a magnifying glass the Morse code 13 reads out so that the number of a pressure nozzle can be read absolutely reliably.

Reference list

1

user

2nd

Imaging system

3rd

Control computer

4th

Printing press

5

Image sensor

6

Image processing computer

7

Display

8th

Printed image

9

Morse coded meta info

10th

Register marks

11

Print nozzle test pattern

12th

Print line of a single pressure nozzle

13, 13a, 13b, 13c, 13d, 13e

Morse coded pressure nozzle number

14

Metainfo

15

Communication between control computer and image processing computer

16

defective pressure nozzles found

Claims (10)

Method for printing quality analysis in an inkjet printing machine (4) by means of a computer (3, 6), test patterns (11) being printed during a printing process, measured by means of an image acquisition system (2) and evaluated by the computer (3, 6) and results the evaluation for controlling the inkjet printing machine (4) are used, printing and evaluation of the test patterns (11) requiring a synchronous process, characterized in that codes (9, 13) are integrated into the test pattern (11) by means of local gray value adjustment, which are captured by the image acquisition system (2) and evaluated accordingly by the computer (3, 6) for carrying out the print quality analysis. Procedure according to Claim 1 , characterized in that Morse codes (9, 13) are used as codes (9, 13). Method according to one of the preceding claims, characterized in that the computer (3, 6) assigns a test ID (11) to each test pattern (11), codes it accordingly and inserts it into the test pattern (11) as a code (9) by means of local gray value adjustment and in the case of the evaluation of the measured test pattern (11), the test pattern (11) is first identified by means of the detected, coded layout ID (9) and thus the printing and evaluation of the test pattern (11) are synchronized. Procedure according to Claim 3 , characterized in that the coded layout ID (9) is always placed in the same position in the test pattern (11) by the computer (3, 6) regardless of the structure of the test pattern (11). Procedure according to Claim 4 , characterized in that different test patterns (11) are printed which check various possible printing errors by means of the computer-aided evaluation. Procedure according to Claim 5 , characterized in that different color separations of the test pattern (11) are iteratively printed as different test patterns (11), the coded layout ID (9) also containing the information about the respective color separation. Method according to one of the preceding claims, characterized in that the local gray value adjustments for generating the encoded Layout ID (9) can be realized by using the smallest and largest gray levels of a single pressure nozzle or by using the smallest and largest gray levels of several adjacent pressure nozzles. Method according to one of the preceding claims, characterized in that when a single printing nozzle is used, an information unit of the code (9) is realized by generating a pixel with the smallest and largest gray level or by generating a plurality of pixels with the smallest and largest gray level. Method according to one of the preceding claims, characterized in that, in addition or as an alternative to generating the layout ID, individual printing nozzles of the inkjet printing machine (4) integrate their own printing nozzle number as a code (13) into the test pattern (11) and during evaluation for identification the measured test pattern (11) of the computers (3, 6) found printing errors (16) by means of this code (13) the relevant printing nozzles. Method according to one of the preceding claims, characterized in that the image acquisition system (2) is placed inline in the inkjet printing machine (4) and has an image acquisition computer (6) which carries out the evaluation of the recorded test patterns (11).

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