PRINTING PLATE, METHOD FOR DETECTING A POSITION OF A PRINTING PLATE, CONTROL UNIT FOR A SYSTEM FOR DETECTING A POSITION OF A PRINTING PLATE, SYSTEM FOR DETECTING A POSITION OF A PRINTING PLATE, AND COMPUTER PROGRAM

Abstract

A printing plate (10) having at least one optical marker (12, 14) for detecting a position of the printing plate (10) is described. The optical marker (12, 14) comprises an optically detectable structure (18) being positioned within a periphery (20) of the marker (12, 14). Furthermore, a method for detecting a position of a printing plate (10) is presented. Additionally, a control unit (38) for a system (36) for detecting a position of a printing plate (10) and a system (36) for detecting a position of a printing plate (10) are explained. Moreover, a computer program comprising computer readable program code means for causing steps of the method is defined.

Claims

1. A printing plate comprising: at least one optical marker for detecting a position of the printing plate, wherein the at least one optical marker comprises an optically detectable structure being positioned within a periphery of the at least one optical marker.

2. The printing plate according to claim 1, wherein the optically detectable structure comprises at least two areas differing in brightness and/or in spatial frequency, especially wherein the at least two areas differ in their respective grey scale.

3. The printing plate according to claim 2, wherein a cumulated length of all border lines between the at least two areas exceeds a length of the periphery of the at least one optical marker, especially wherein the cumulated length of all border lines exceeds the length of the periphery by at least 50%.

4. The printing plate according to claim 2, wherein at least one of the at least two areas has a form of a ring segment or a polygon segment, especially wherein the area is ring-shaped or polygon-shaped.

5. The printing plate according to claim 4, wherein at least two of the at least two areas are ring-shaped or polygon-shaped, wherein the at least two of the at least two areas are arranged concentrically.

6. The printing plate according to claim 1, wherein the at least one optical marker is integrally formed with the printing plate.

7. The printing plate according to claim 1, wherein the at least one optical marker includes a plurality of optical markers, especially wherein the plurality of optical markers are arranged along a straight line.

8. A method for detecting a position of a printing plate, the method comprising: providing a printing plate according to claim 1, capturing an image of the printing plate or at least a portion thereof comprising the at least one optical marker by a camera unit, determining a position of the at least one optical marker in the captured image by identifying a structure positioned within the periphery of the at least one optical marker in the captured image, and deriving a position of the printing plate from the determined position of the at least one optical marker in the captured image.

9. The method according to claim 8, wherein the structure is identified by evaluating a grey scale distribution and/or a spatial frequency distribution of the captured image.

10. The method according to claim 8, wherein the structure is identified by applying one or more of a pattern recognition technique, an object recognition technique, or a digital image correlation technique to the captured image.

11. The method according to claim 8, wherein an edge detection technique is applied to a portion of the captured image comprising the at least one optical marker.

12. The method according to claim 11, wherein a cumulative length of the detected edges of the structure is calculated, especially wherein the cumulative length is taken as an indicator for detection quality.

13. A system for detecting a position of a printing plate, the system comprising: a control unit for detecting a position of a printing plate according to claim 1, wherein the control unit is configured to perform a method including: capturing an image of the printing plate or at least a portion thereof comprising the at least one optical marker by a camera unit, determining a position of the at least one optical marker in the captured image by identifying a structure positioned within the periphery of the at least one optical marker in the captured image, and deriving a position of the printing plate from the determined position of the at least one optical marker in the captured image.

14. The system according to claim 13, further comprising: the camera unit, wherein the camera unit is coupled to the control unit such that an image captured by the camera unit is provided to the control unit.

15. A non-transitory computer-readable medium storing instructions that, when executed by a processor, perform operations including: capturing an image of a printing plate according to claim 1 or at least a portion thereof comprising the at least one optical marker by a camera unit, determining a position of the at least one optical marker in the captured image by identifying a structure positioned within the periphery of the at least one optical marker in the captured image, and deriving a position of the printing plate from the determined position of the at least one optical marker in the captured image.

Description

[0050] The invention will now be described with reference to the enclosed drawings. In the drawings,

[0051] FIG. 1 shows a system according to the invention for detecting a position of a printing plate, comprising a control unit according to the invention on which a computer program according to the invention is stored and executable, and a printing plate according to the invention which is mounted on a corresponding printing cylinder,

[0052] FIG. 2 shows the printing plate of FIG. 1 in a state where it is not mounted on the corresponding printing cylinder,

[0053] FIG. 3 shows a portion of the printing plate of FIGS. 1 and 2 comprising a marker according to a first variant,

[0054] FIG. 4 shows a grey value distribution in a zone IV of FIG. 3,

[0055] FIG. 5 shows a portion of an alternative printing plate, wherein the view corresponds to the view of FIG. 3 and the same marker as in FIG. 3 is provided on the printing plate,

[0056] FIG. 6 shows a grey value distribution in a zone VI of FIG. 5,

[0057] FIG. 7 shows a portion of the printing plate of FIGS. 1 and 2 comprising a marker according to a second variant,

[0058] FIG. 8 shows a grey value distribution in a zone VIII of FIG. 7,

[0059] FIG. 9 shows a portion of the printing plate of FIG. 5, wherein the marker according to the second variant is provided on the printing plate,

[0060] FIG. 10 shows a grey value distribution in a zone X of FIG. 9,

[0061] FIG. 11 shows a portion of a printing plate being equipped with a marker according to a third variant, and

[0062] FIG. 12 shows a portion of a printing plate being equipped with a marker according to a fourth variant.

[0063] In FIGS. 1 and 2 a printing plate 10 is shown which comprises two optical markers 12, 14.

[0064] The optical markers 12, 14 are arranged along a straight line 16 if the printing plate 10 is in a flat position (cf. FIG. 2).

[0065] The printing plate 10 is a flexible relief printing plate, wherein the relief is formed in a layer of UV-cured polymer.

[0066] The optical markers 12, 14 are integrally formed in this layer.

[0067] In the following, the optical markers 12, 14 will be explained in detail. However, for the ease of explanation, reference will only be made to marker 12. The following explanation apply to optical marker 14 mutatis mutandis.

[0068] FIG. 3 shows an optical marker 12 according to a first variant which is formed on a smooth surface of the printing plate 10.

[0069] The optical marker 12 is substantially square-shaped and comprises an optically detectable structure 18 within its periphery 20.

[0070] This structure is made up of six areas 22a, 22b, 22c, 22d, 22e, 22f, wherein the areas 22a, 22b, 22c, 22d, 22e are substantially shaped as closed polygonal strips having the form of a thick square line and the area 22f is formed as a square.

[0071] All areas 22a, 22b, 22c, 22d, 22e, 22f are arranged concentrically.

[0072] As can be directly seen from FIG. 3 and in more detail in FIG. 4, the areas 22a, 22b, 22c, 22d, 22e, 22f differ in respect of their grey scale, wherein areas of high grey scale and low grey scale are arranged in an alternating manner along a direction extending from the periphery 20 of the marker towards its center.

[0073] In order to map the marker of FIG. 3 on the diagram of FIG. 4, two auxiliary lines 24, 26 are represented in both Figures. Additionally, the reference signs of the corresponding areas are noted besides the peaks in the grey scale distribution.

[0074] In FIG. 4 the grey value is represented in a relative manner ranging from 0% to 100%. The terms grey scale and grey value are synonyms.

[0075] Border lines between two areas of differing grey value are defined as edges and it clearly results from FIG. 3 that a cumulative length of all edges is a multiple of the length of the periphery 20 of the marker 12.

[0076] FIG. 5 also shows an optical marker 12 according to the first variant, thus the marker 12 is substantially the same as in FIG. 3. In the following, only the differences as compared to the printing plate 10 of FIG. 3 will be explained.

[0077] The marker 12 is now formed on a rough surface of the printing plate 10 offering less contrast as compared to the printing plate 10 disclosed in FIG. 3.

[0078] This results in a different grey scale distribution as can be seen from FIG. 5 and in more detail from FIG. 6.

[0079] FIG. 7 shows the printing plate 10 having an optical marker 12 according to a second variant, which is applied to a substantially smooth surface of the printing plate 10. Again, only the differences with respect to the embodiment of FIG. 3 will be explained.

[0080] The optical marker 12 is substantially round, i.e. its periphery 20 is substantially a circle line.

[0081] The optically detectable structure 18 within the periphery 20 is made up of eight areas 22a, 22b, 22c, 22d, 22e, 22f, 22g, 22h wherein the areas 22a, 22b, 22c, 22d, 22e, 22f, 22g are substantially shaped as circular strips and the area 22h is formed as a circle.

[0082] All areas 22a-22h are arranged concentrically.

[0083] As can be directly seen from FIG. 7 and in more detail in FIG. 8, the areas 22a, 22b, 22c, 22d, 22e, 22f, 22g, 22h differ in respect of their grey scale, wherein areas of high grey value and low grey value are arranged in an alternating manner when considering a direction extending from the periphery 20 of the marker towards its center.

[0084] In order to map the marker of FIG. 7 on the diagram of FIG. 8, two auxiliary lines 24, 26 are represented in both Figures. Additionally, the reference signs of the areas are noted besides the corresponding peaks in the grey scale distribution.

[0085] If again border lines between two areas of differing grey value are defined as edges, it clearly results from FIG. 7 that a cumulative length of all edges is a multiple of the length of the periphery 20 of the marker 12.

[0086] FIG. 9 also shows an optical marker 12 according to the second variant, thus the marker 12 is substantially the same as in FIG. 7. In the following, only the differences as compared to the printing plate of FIG. 7 will be explained.

[0087] The marker 12 is now formed on a rough surface of the printing plate 10 having less contrast that the printing plate 10 shown in FIG. 7.

[0088] This results in a different grey scale distribution as can be seen from FIG. 9 and in more detail from FIG. 10.

[0089] FIG. 11 shows a portion of a printing plate 10 having an optical marker 12 according to a third variant. The marker 12 according to the third variant differs from the marker according to the second variant in that instead of eight areas differing in grey value only four areas 22a, 22b, 22c, 22d are provided.

[0090] The areas 22a, 22b, 22c are formed as continuous circular strips and the area 22d is formed as a circle.

[0091] If analyzed along a line extending from the periphery 20 to a center of the marker 12, the areas 22a, 22b, 22c differ in width. In the example shown, the areas 22a, 22c are wider than the areas 22b, 22d. In other words, the areas 22a, 22b, 22c, 22d differ in spatial frequency.

[0092] Beyond that, the explanations concerning the marker 12 according to the second variant also apply to the marker 12 according to the third variant.

[0093] FIG. 12 shows a printing plate 10 having an optical marker 12 according to a fourth variant.

[0094] The optical marker 12 is substantially square-shaped.

[0095] The inner structure 18 is made up of five areas 22a, 22b, 22c, 22d, 22e, wherein the areas 22a, 22b, 22c, 22d are substantially shaped as polygonal strips having the form of a thick square line and the area 22e is formed as a square.

[0096] However, the areas of low grey value are provided with interruptions 28.

[0097] In detail, the areas 22b and 22d are not formed by a closed polygonal strip, rather is the polygonal strip interrupted once on each side. The portions of the polygonal strip extending between two interruptions 28 may also be designated polygon segments.

[0098] Beyond that, reference is made to the explanations concerning the marker 12 according to the first variant.

[0099] A position of the printing plate 10 comprising the marker 12 can be detected by performing the following method.

[0100] At first, the printing plate 10 comprising the marker 12 needs to be provided. In this context the printing plate 10 may be provided on a corresponding printing cylinder 30 as shown in FIG. 1.

[0101] Then an image of the printing plate 10 or at least a portion thereof comprising the marker 12 is taken by a camera unit 32.

[0102] Subsequently, a position of the marker 12 in the captured image is determined by identifying the structure 18 being positioned within the periphery 20 of the marker in the captured image.

[0103] In the present embodiment, this is done by evaluating the grey scale distribution as shown in FIGS. 4, 6, 8, and 10.

[0104] As can be seen from a comparison of these grey scale distributions and the corresponding printing plates 10 represented in FIGS. 3, 5, 7, and 9 respectively, the inner structure 18 has the effect that also in cases where the periphery 20 of the marker 12 cannot be clearly detected (cf. for example FIGS. 5 and 6 or 9 and 10), the inner structure 18 is still detectable.

[0105] As a consequences thereof, in all cases represented in FIGS. 3 to 10 the position of the marker 12 in the image can be determined with high reliability.

[0106] In addition to the evaluation of the grey scale distribution, an edge detection technique is applied to the image. As a result thereof, edges, i.e. border lines between areas 22a-22h of differing grey scale are determined and a corresponding length of these edges is calculated.

[0107] The lengths of all edges is cumulated and the cumulated length is taken as an indicator for detection quality. This means that a big cumulated length indicates high detection quality and a short cumulated length indicates low detection quality.

[0108] Having regard to FIGS. 3 to 6, for example the edges between the areas 22a and 22b on the right side of FIG. 5 are not detectable. Consequently, a cumulated length of detected edges in the example of FIG. 5 is lower than a cumulated length of detected edges in the example of FIG. 3.

[0109] Based on the determination of the position of the marker 12 in the captured image, the position of the printing plate 10 is derived.

[0110] Substantially two alternatives exist.

[0111] The position of the printing plate 10 can be derived with respect to an additional marker 34 which is not placed on the printing plate 10, but for example on the printing cylinder 30. This marker 34 needs to be represented in the captured image.

[0112] Alternatively, the position of the printing plate 10 can be derived by using a calibrated camera unit 32 being in a known position. As a consequence thereof, a position of at least some portions of the captured image, e.g. single pixels or pixel groups, is known and the position of the printing plate 10 can be derived on this basis.

[0113] It is noted that the position of the printing plate can be represented using a coordinate system attributed to a room, a printing machine or a printing cylinder.

[0114] FIG. 1 also shows a system 36 for detecting the position of the printing plate 10.

[0115] In addition to the camera unit 32 which has already been mentioned, this system 36 comprises a control unit 38 which is a computer in the example shown.

[0116] The control unit 38 and the camera unit 32 are coupled via a data line 40 such that an image captured by the camera unit 32 may be provided to the control unit 38.

[0117] The control unit 38 further comprises a data storage unit 42 and a data processing unit 44.

[0118] A computer program comprising computer readable program code means is stored on the data storage unit 42 and may be executed by the data processing unit 44.

[0119] The control unit 38 and the computer program executable thereon are configured for causing the method step of capturing an image of at least a portion of the printing plate 10 comprising the marker 12 by the camera unit 32.

[0120] To this end, a signal is sent from the processing unit 44 to the camera unit 32 via the data line 40 triggering the camera unit to capture an image and send the captured image to the control unit 38 via the data line 40.

[0121] The captured image is then stored in the data storage unit 42.

[0122] The control unit 38 and the computer program executable thereon are further configured for performing the step of determining the position of the marker 12 in the captured image by identifying the structure 18 being positioned within the periphery 20 of the marker 12 in the captured image.

[0123] To this end the grey scale distribution in the captured image is analyzed using the data processing unit 44.

[0124] The resulting position of the marker 12 is stored on the data storage unit 42.

[0125] Furthermore, the control unit 38 and the computer program executable thereon are configured for deriving the position of the printing plate 10 from the position of the marker 12 in the captured image.

[0126] The control unit 38 and the computer program executable thereon is configured according to one of the alternatives mentioned above in this respect.

[0127] Either a position of the further marker 34 has also been determined in the captured image and stored in the data storage unit 42.

[0128] Then the position of the printing plate 10 is derived by comparing the position of the marker 34 and the marker 12.

[0129] Alternatively, the camera unit 32 is calibrated such that at least for a portion of the captured image a corresponding position is known.

[0130] In this case, the position of the printing plate 10 is derived by comparing the position of the marker 12 to the known position of the portion of the captured image.

[0131] The derived position of the printing plate 10 may be used for determining an amount and/or a direction by which the position of the printing plate 10 shall be corrected.

[0132] The detection of the position of the printing plate 10 and a corresponding correction thereof can be performed in a closed loop control system.