This application claims priority to Dutch Patent Application No. NL2027178, filed on Dec. 21, 2020, the entirety of which is expressly incorporated herein by reference.
The present invention generally pertains to detecting ejection abnormalities in an inkjet print head, in particular a piezo-actuated inkjet print head.
During the execution of an ink jet print process, using a page wide array of ejection units, several faults can disturb the jetting of a drop from a nozzle, being the part of an ejection unit where a drop originates, leading to ejection abnormalities. For example, blocking of an ink nozzle due to the presence of a dirt particle is one of the most common causes of malfunction in ink jetting. In many printers, nozzle failures are detected by printing a test chart and optically checking the result. It is usually no problem to find an exact nozzle number of a failing nozzle, since only one of a plurality of nozzles has fired, and the number is inferred from a position of a missing dot. However, it is necessary to print this dedicated test chart.
It is also a known procedure to optically scan a printed image in order to infer the presence of failing nozzles from stripes occurring in a transport direction. However, the accurate nozzle number is often difficult to determine, due to the limited optical resolution of the scanner, the high density of ejection units and alignment differences between the print head arrays and the scanner.
As a consequence, it is desired to have a method for detecting failing nozzles in an array of ejection units during printing of an arbitrary image, such as a print job from a customer, wherein an accurate nozzle number of a failing nozzle is determined.
In an aspect of the present invention, a method of detecting failing nozzles in an array of ejection units during the printing of an image onto a recording medium, the method comprising the steps according to claim 1 is provided. The recording medium and the one or more ejection units are arranged to be moved relative to one another in a transport direction perpendicular to a page width direction. In an embodiment, said ejection units are arranged to eject droplets of a liquid and comprise one or more of nozzles, one or more liquid ducts each connected to one of the one or more nozzles, and one or more electro-mechanical transducers each arranged to create an acoustic pressure wave in the liquid in one or more ducts. In an embodiment, the ejection unit is further arranged to sense a residual pressure wave in the liquid in each of the one or more ducts.
The method of the present invention comprises creating a halftone mask for the object of the print job that specifies the droplets to be ejected by each of the one or more nozzles onto the recording medium during the printing of the image, wherein the halftone mask is created such that a different variation is introduced in the page width direction depending upon which of the one or more nozzles in one or more ejection units is failing. In a further embodiment, a number of parameters that are used in the analysis of a residual pressure wave are determined with the aid of the determination of failing nozzles along the steps of the invented method.
In another step, the method of the present invention comprises actuating the electro-mechanical transducer to generate a pressure wave in the liquid in one of the one or more liquid ducts such that droplets are ejected by each of the one or more nozzles in one or more ejection units according to the halftone mask created in the step of creating a halftone mask.
In another step, the method of the present invention comprises scanning the recording medium to analyze the image printed onto the recording medium. Said scanning process is usually an optical scanning process. The optical resolution of the scanning process is limited allowing only a coarse determination of a failing nozzle number. The summing method provides an exact determination of this number. This is for example the case when the density of dots within a row amounts to 1200 dpi and the groups of dots comprise 5 columns and about 50 rows.
In another step, the method of the present invention comprises detecting failing nozzles amongst the one or more nozzles in one or more ejection units from the image resulting from the scanning step.
In an embodiment, the method of the present invention comprises that the halftone mask created for the object of the print job that specifies the droplets to be ejected by each of the one or more nozzles onto the recording medium is substituted by a standard halftone mask depending upon the content of the image to be printed onto the recording medium. This allows not using the created halftone mask but a standard one (as shown in
In an embodiment, detecting failing nozzles amongst the one or more nozzles in one or more ejection units from the image resulting from the scanning step comprises several steps. One step comprises defining one or more groups in the transport direction and one or more groups in the page width direction. In another step, the amount of ejected droplets for each of the one or more groups in the transport direction is counted. This counting process takes place for those nozzles detected to be failing nozzles by analyzing the scanned image. In an embodiment, droplets are not directly counted, but certain areas are averaged (as observed below in
In an embodiment, the method of the present invention comprises that the halftone mask created for the object of the print job that specifies the droplets to be ejected by each of the one or more nozzles onto the recording medium is substituted by a standard halftone mask, and the method further comprises a post-processing step that alters the previous actuation (step b) such that droplets ejected by each of the one or more nozzles are displaced depending upon the group in the transport direction to which they belong. For this end, the droplet ejection device or the printing system of the present invention may also contain a content detector that analyzes the content of the image to be printed onto the recording medium in order to determine whether using a standard halftone mask together with a post-processing step would be more beneficial.
In an embodiment, the method of the present invention comprises sensing a residual pressure wave in the liquid in each of the one or more liquid ducts. Further, it comprises comparing the residual pressure wave sensed in the liquid in each of the one or more liquid ducts with the residual pressure wave of a correctly functioning unit by determining the difference of one or more parameters of the residual pressure wave sensed with the same one or more parameters of a correctly functioning unit such that failing nozzles are detected. Further, it also comprises using the result of the comparison between residual pressure waves for improving the detection of failing nozzles.
In an embodiment, the method of the present invention comprises that the halftone mask for the object of the print job that specifies the droplets to be ejected by each of the one or more nozzles has coverage between 5% and 100%. The lower the amount of correction, the more difficult it is detecting the correct nozzle failure number, but a lower amount of correction leads to less distortion in the print. So it is needed to reduce the amount of correction until the distortions in the prints are not visible any more. This depends on the print process situation (needed quality, print resolution, droplet sizes, etc.)
In an embodiment, the method of the present invention comprises that the halftone mask for the object of the print job that specifies the droplets to be ejected by each of the one or more nozzles has an amount of correction between 5% and 100%, wherein the amount of correction specifies the number of droplets ejected by the one or more of nozzles of the one or more ejection units that change their ejection position in comparison with a standard halftone mask.
The present invention also pertains to a droplet ejection device comprising a number of ejection units arranged to eject droplets of a liquid and each comprising a nozzle, a liquid duct connected to the nozzle, and an electro-mechanical transducer arranged to create an acoustic pressure wave in the liquid in the duct, wherein each of the ejection units is associated with a processor configured to perform the method according to any of the methods of the present invention.
Further, the present invention relates to a printing system comprising the droplet ejection device of the present invention as an ink jet print head and a control unit comprising a processor suitable for executing the method according to any of the methods of the present invention.
Also, the present invention relates to a software product comprising program code on a machine-readable non transitory medium, the program code, when loaded into a control unit of the printing system of the present invention, causes the control unit to execute any of the methods of the present invention.
The present invention will become more fully understood from the detailed description given below, and the accompanying drawings which are given by way of illustration only, and are thus not limitative of the present invention, and wherein:
The present invention will now be described with reference to the accompanying drawings, wherein the same or similar elements are identified with the same reference numeral.
A single ejection unit of an ink jet print head is shown in
A recess that forms an ink duct 16 is formed in the face of the wafer 10 that engages the membrane 14, e.g. the bottom face in
An opposite end of the ink duct 16, on the right side in
Adjacent to the membrane 14 and separated from the chamber 20, the support member 12 forms another cavity 26 accommodating a piezoelectric actuator 28 that is bonded to the membrane 14.
An ink supply system which has not been shown here keeps the pressure of the liquid ink in the ink duct 16 slightly below the atmospheric pressure, so as to prevent the ink from leaking out through the nozzle 22.
The nozzle face 24 is made of or coated with a material which is wetted by the ink, so that adhesion forces cause a pool 30 of ink to be formed on the nozzle face 24 around .the nozzle 22. The pool 30 is delimited on the outward (bottom) side by a meniscus 32a.
The piezoelectric transducer 28 has electrodes 34 that are connected to an electronic circuit that has been shown in the lower part of
When an ink droplet is to be expelled from the nozzle 22, the processor 50 sends a command to the controller 48 which outputs a digital signal that causes the D/A-converter 46 and the amplifier 40 to apply an actuation pulse to the transducer 28. This voltage pulse causes the transducer to deform in a bending mode. More specifically, the transducer 28 is caused to flex downward, so that the membrane 14 which is bonded to the transducer 28 will also flex downward, thereby to increase the volume of the ink duct 16. As a consequence, additional ink will be sucked-in via the supply line 18. Then, when the voltage pulse falls off again, the membrane 14 will flex back into the original state, so that a positive acoustic pressure wave is generated in the liquid ink in the duct 16. This pressure wave propagates to the nozzle 22 and causes an ink droplet to be expelled. The pressure wave will then be reflected at the meniscus 32a and will oscillate in the cavity formed between the meniscus and the left end of the duct 16 in
The electrodes 34 of the transducer 28 are also connected to an A/D converter 52 which measures a voltage drop across the transducer and also a voltage drop across the resistor 38 and thereby implicitly the current flowing through the transducer.
Corresponding digital signals S are forwarded to the controller 48 which can derive the impedance of the transducer 28 from these signals. The measured electric response (current, voltage, impedance, etc.) is signaled to the processor 50 where the electric response is processed further.
A diagram of a printing system is shown in
A diagram of a printing system is shown in
“-” represents the position where a dot will be placed in a standard halftone mask, but it is empty in this mask.
“X” (capital x) represents a moved dot position (one pixel to the left or right side).
“#” represents the horizontal position where a nozzle failure is present.
In this process a plurality of groups are defined, which in the example of
If the amount of droplets within each group vertically is counted, the result shows that there are 0 to 3 droplets within each group. Subsequently, the total number of droplets within a plurality of columns can be added (5 in the example of
In the example of
The method described is not always able to pinpoint exactly which nozzle is not ejecting correctly due to inaccuracies in the alignment between scanner and print head, but reaches an accuracy which depends upon the number of groups created (e.g. with 5 groups and accuracy of −2 of +2 nozzles is reached).
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
Number | Date | Country | Kind |
---|---|---|---|
2027178 | Dec 2020 | NL | national |