METHOD AND SYSTEM FOR THE DETECTION AND CORRECTION OF PRINTING DEFECTS OF A PRINTING DEVICE

Abstract

Method and respective device for the detection and correction of printing defects of a digital printing device (A) comprising the following steps: —arranging a digital printing device (A) equipped with one or more print heads (H) with a plurality of dispensing nozzles (N); —continuously controlling said plurality of dispensing nozzles (N) to realize a real printing pattern (RPP) on a printing medium (P); —continuously acquiring the real printing pattern (RPP); —detecting at least one printing defect (DF); —elaborating, onboard a control unit of the digital printing device (A), at least one compensation scheme to mask a detected printing defect (DF,); —applying said compensation device during the realization of said real printing pattern (RPP).

Claims

1. A method for the detection and correction of printing defects of a digital printing device comprising the following steps: arranging a digital printing device equipped with one or more print heads with a plurality of dispensing nozzles placed along a printing area; arranging an optical detection system, placed next to said printing area with reference to a printing direction; controlling said plurality of dispensing nozzles for dispensing ink onto a printing medium, relatively movable with respect to said printing area, said dispensing being performed based on a predefined ideal printing pattern comprising one or more levels of grey, said dispensing realizing a real printing pattern on said printing medium; acquiring the real printing pattern through said optical detection system; detecting at least one printing defect defined by a difference of said real printing pattern acquired by the optical detection system with respect to said ideal printing pattern, elaborating, on board a control unit of the digital printing device, at least one compensation scheme to mask a detected printing defect; applying said compensation scheme during the realization of said real printing pattern, after realizing the real printing pattern with the applied compensation scheme, acquiring again the real printing pattern through said optical detection system; modifying said compensation scheme in feedback control to make the visual appearance of said real printing pattern similar to the visual appearance of said ideal printing pattern.

2. The method for the detection and correction of printing defects according to claim 1, wherein the steps of realizing the real printing pattern with the applied compensation scheme, acquiring again the real printing pattern and modifying said compensation scheme in feedback control are repeated until a desired degree of similarity is achieved between said real printing pattern and said ideal printing pattern.

3. The method for the detection and correction of printing defects of a digital printing device according to claim 2, wherein at the end of the step of modifying said compensation scheme in feedback control, an optimal compensation scheme is obtained, which is applicable in subsequent printing operations of the digital printing device.

4. The method for the detection and correction of printing defects of a digital printing device according to claim 1, wherein the step of detecting at least one printing defect comprises the sub-step of identifying at least one defective dispensing nozzle responsible for the printing defect, and wherein the elaborated compensation scheme provides for the dispensing variation of one or more dispensing nozzles close to said defective dispensing nozzle.

5. The method for the detection and correction of printing defects of a digital printing device according to claim 1, wherein the step of detecting at least one printing defect comprises the sub-step of identifying a tone unevenness in the real printing pattern.

6. The method for the detection and correction of printing defects of a digital printing device according to claim 5, wherein the elaborated compensation scheme provides for the dispensing variation of one or more dispensing nozzles at the area affected by the tone unevenness, by intervening on the drop volume assigned to the single dispensing nozzles or on printing head adjustment parameters that affect an entire module of dispensing nozzles.

7. The method for the detection and correction of printing defects of a digital printing device according to claim 5, wherein the elaborated compensation scheme acts on the image processing, through the printing RIP, modifying the density of the printing pixels in the areas affected by the tone unevenness.

8. The method for the detection and correction of printing defects of a digital printing device according to claim 1, further comprising a step of detecting at least one setup information starting from the analysis of said real printing pattern, and a step of setting up the device during start-up phase based on said detected setup information, wherein the detected setup information starting from the analysis of said real printing pattern is an information on the overlap between the different adjacent modules of dispensing nozzles and/or the alignment between the different modules of dispensing nozzles related to the different colours.

9. The method for the detection and correction of printing defects of a digital printing device according to one of the previous claim 1, wherein at least the steps of detecting a printing defect, of elaborating a compensation scheme and of applying the compensation scheme can be carried out during a production cycle of the digital printing device.

10. The method for the detection and correction of printing defects of a digital printing device according to claim 9, wherein during the production cycle the feed speed of the printing medium with respect to the printing area is greater than 80 m/min.

11. The method for the detection and correction of printing defects of a digital printing device according to claim 1, wherein said optical detection system provides for a plurality of scanning sensors which are linearly distributed along the printing area.

12. The method for the detection and correction of printing defects of a digital printing device according to claim 11, wherein a single scanning sensor is placed in linear correspondence with a single dispensing nozzle.

13. The method for the detection and correction of printing defects of a digital printing device according to claim 11, wherein a single scanning sensor is placed in linear correspondence with two or more adjacent dispensing nozzles of the print head, said ideal printing pattern comprising two or more subsequent modules of lines, each line being at a determined dispensing nozzle, the lines corresponding to the adjacent dispensing nozzles in a linear correspondence with a single scanning sensor being realized in distinct modules of lines.

14. The method for the detection and correction of printing defects of a digital printing device according to claim 11, wherein said scanning sensors are CIS sensors.

15. A system for the detection and correction of printing defects of a digital printing device comprising: a digital printing device equipped with one or more print heads with a plurality of dispensing nozzles placed along a printing area, arranged to dispense ink onto a printing medium, relatively movable with respect to said printing area, based on a predefined ideal printing pattern comprising one or more levels of grey, realizing a real printing pattern on said printing medium; an optical detection system, placed next to said printing area with reference to a printing direction, arranged to acquire the real printing pattern; a control unit arranged for: detecting: at least one printing defect defined by a difference of said real printing pattern acquired by the optical detection system with respect to said ideal printing pattern; elaborating, on board a control unit digital printing device, at least one compensation scheme to mask a detected printing defect; applying said compensation scheme during the realization of said real printing pattern, after realizing the real printing pattern with the applied compensation scheme, acquiring again the real printing pattern through said optical detection system; modifying said compensation scheme in feedback control to make the visual appearance of said real printing pattern similar to the visual appearance of said ideal printing pattern.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] FIG. 1 schematically shows an ideal printing pattern used by the method and system for the detection and correction of printing defects of a digital printing device according to the present invention;

[0072] FIG. 2 schematically shows a system for the detection and correction of printing defects of the method according to the present invention;

[0073] FIG. 3 schematically shows the system in a correction step of the error detected in the step shown in FIG. 2;

[0074] FIG. 4 schematically shows a first variant of the system of FIG. 2, in which in particular the sensors that constitute the optical detection system are highlighted;

[0075] FIG. 5 schematically shows a second variant of the system of FIG. 2, in which in particular the sensors that constitute the optical detection system are highlighted;

[0076] FIG. 6 schematically shows the system of FIG. 2 during another detection step of the method according to the present invention;

[0077] FIG. 7 schematically shows the system according to the present invention during a correction step of the error detected in the step shown in FIG. 6.

DETAILED DESCRIPTION

[0078] With reference to the enclosed FIGS. 2-7, S generally identifies a system for the detection and correction of printing defects.

[0079] The system S comprises in particular a digital printing device A, preferably for flat printing, comprising one or more print heads H which are side by side to each other so as to define a printing area. Preferably, the defined printing area is of a large dimension, in particular equal to or greater than two meters.

[0080] For the sake of simplicity of representation, in the enclosed figures a unique print head H is represented.

[0081] The print head H, of a per se known type, has a plurality of dispensing nozzles N arranged to dispense a volume of ink which is adjustable in a fine manner and in real time. The instantaneous dispensing of a single nozzle N onto a printing medium P locally defines an ink point IP; the different ink points IP cooperate to define a printing image.

[0082] The dispensing nozzles N are preferably arranged in an offset manner, for reasons linked to dimensions and to the internal structure of the print head itself. Nevertheless, to each single dispensing nozzle N a specific position on the printing area corresponds, so that said dispensing nozzles N can be ideally considered to be side by side to each other. In the following of the present description, the term “adjacent” related to the dispensing nozzles N refers to said ideal linear arrangement along the printing area.

[0083] The system S also comprises means for moving the aforementioned printing medium P, which are arranged for advancing said printing medium P in a feed direction orthogonal to the printing area, herein called printing direction R.

[0084] The system S also comprises an optical detection system OD, comprising a plurality of scanning sensors C, preferably of the CIS type, which are linearly side by side to each other along a direction parallel to the printing area.

[0085] The optical detection system OD is arranged downstream the digital printing device A and is arranged to acquire the image freshly printed said printing device A.

[0086] In particular, it should be noted that the optical detection system OD is able to discriminate the position of the ink points IP deposited by adjacent nozzles.

[0087] In a first variant, illustrated in FIG. 4, a single scanning sensor C is arranged at each dispensing nozzle N of the digital printing device A.

[0088] In a second variant, illustrated in FIG. 5, a scanning sensor C is instead arranged in front of each pair of adjacent dispensing nozzles N.sub.1, N.sub.2, which are offset to each other along the printing direction R.

[0089] The system S also comprises a control unit CU, which on the one hand controls the operation of the dispensing nozzles N, on the other hand acquires the signal of the optical detection system OD.

[0090] Said control unit CU, which can be realized by one or more computers, by one or more microprocessors or by analogous known devices, may comprise or be connected to a storage medium in which ideal compensation schemes, obtained from the method described below, will be stored.

[0091] The above system S is able to perform a method for the detection and correction of printing defects DF, DF2 and/or the setup during start-up phase of the previously described digital printing device.

[0092] A first printing defect DF detectable and correctable with the method according to the present invention is the presence of defective dispensing nozzles N.sub.f.

[0093] An example of how said defect DF can be detected and corrected is hereinafter provided, with specific reference to FIGS. 1-3.

[0094] In a first step, the digital printing device A is controlled to perform a print aimed at identifying and correcting the printing defects DF linked to the presence of defective dispensing nozzles N.sub.f.

[0095] Said printing is preferably realized based on an ideal printing pattern IPP, namely a digital image made of a pixel array, illustrated in FIG. 1.

[0096] Said ideal printing pattern IPP comprises a plurality of subsequent bands L1, L2, each band comprising pixels of homogeneous colour intensity, particularly to each band a different level of grey corresponds.

[0097] The digital printing device A, as represented in FIG. 2, will thus print a real printing pattern RPP defined by the deposition of the ink points IP onto the printing medium P.

[0098] At defective dispensing nozzles N.sub.f, the real printing pattern RPP will locally have a printing defect DF, represented in this case by a missing column or having a colour density less than the one of the corresponding area of the ideal printing pattern IPP.

[0099] Through the optical detection system OD, the control unit CU is able to identify the specific defective dispensing nozzle N.sub.f.

[0100] In the specific case of the variant of FIG. 4, the position of the defective dispensing nozzle N.sub.f is immediately deducible from the position of the scanning sensor C which detects the defect.

[0101] Instead, in the case of the variant of FIG. 5, it will be preferable to perform a different or further print test in which the two dispensing nozzles relating to each single sensor C are made to print—preferably: continuous lines—in two subsequent steps. In this way, the sensor resolution will be sufficient to identify the defective nozzle N.sub.f in a timely manner.

[0102] Once the defective dispensing nozzle N.sub.f has been identified, the control unit CU modifies the dispensing of the nozzles based on an initial compensation scheme, which substantially reassigns the volume of ink attributed to the defective dispensing nozzle N.sub.f distributing it to the adjacent dispensing nozzles N.sub.1, N.sub.2.

[0103] At this point, in a preferred embodiment with error feedback control, the optical detection system OD continues its acquisition and will detect a real printing pattern RPP modified by the compensation scheme.

[0104] The control unit CU goes back comparing said real printing pattern RPP with the expected result, i.e. with the ideal printing pattern IPP.

[0105] Depending on the residual discrepancies, said control unit CU will modify the adopted compensation scheme and so on, until an acceptable overlap between real printing pattern RPP and ideal printing pattern IPP is achieved.

[0106] Thus, a feedback control of the adopted compensation scheme is carried out, which advantageously allows achieving an optimal compensation for each single printing defect DF that has been found.

[0107] In another embodiment, the compensation scheme can be adopted without a further feedback control, for instance during a production cycle following a preliminary detection printing.

[0108] A second printing defect DF2 detectable and correctable with the method according to the present invention is the presence of banding, i.e. areas having tone unevenness.

[0109] An example of how said defect DF can be detected and corrected is hereinafter provided, with specific reference to FIGS. 6-7.

[0110] In this case, the defect detection illustrated in FIG. 6 leads to a correction of the drop volume assigned to the dispensing nozzles N in the area affected by the phenomenon, thus realizing the correction visible in FIG. 7.

[0111] Other correction systems of banding can be employed, as widely described in the summary of the present invention.

[0112] The method and device according to the present invention are moreover able to detect, still with excellent performances and the possibility of a real time correction, other printing defects known in the field, besides automatically performing the setup of the printing device during a start-up phase.

[0113] Obviously, a skilled person can make several changes and variants to the above described invention, in order to meet contingent and specific needs, all of them by the way contained in the scope of protection of the invention as defined by the following claims.