EP0401691B1 - Method and device for detecting registration marks in a multicolour printing press - Google Patents

Abstract

The method involves detecting a printed registration mark (1) composed of signs (3a to 3e) and of colour printing flags (4a, 4c), comparing the image of the signs (3a to 3e) of the printed registration mark (1) with the signs of a virtual reference registration mark, then subsequently defining a base area (5a to 5c) for each sign (3a to 3e), calculating the geometrical centres (6a to 6c) of each base area (5a to 5c), detecting a colour printing flag (4a to 4c) in these base areas, calculating and storing their geometrical centre (7a to 7c), measuring the position deviations ( DELTA x to DELTA x<2> and DELTA y to DELTA y<2>) between the geometrical centres (6a to 6c) of the base areas (5a to 5c) and the geometrical centres (7a to 7c) of the colour printing flags (4a to 4c), and finally using these results in order to control the member for correcting the registration errors of a printing press. <??>The method is used in the sector of the control of the registration of colours in multicolour printing presses. <IMAGE>

Description

The subject of the present invention is a method and a device for detecting marks for printing in a multi-color printing press working on a material, in strip or in sheets, having an image printing area and an area for printing signs of print quality control.

The problem posed by the identification of the colors printed by the different groups of a multi-color printer, in particular the different printing groups of an offset press, is well known to the manufacturers of these machines who have already sought to detect and then correct these errors, in order to avoid, as much as possible, printing defects caused by incorrect color registration.

One of the solutions which has been adopted consists in printing one or more registration marks, in relation to the printed image, in an unprinted zone of the material, in strip or in sheets, worked by the printing press, by example in the edge of the sheet or the strip or sometimes, in a space separating two successive impressions. These registration marks, which must also serve to control the longitudinal and lateral registration errors, are generally trapezoidal in shape and are detected using a reading device. To check the position of the registration mark affixed by a printing group with the position of the registration mark applied by another printing group, it is first necessary to open a reading window in the area where the 'The location of the tracking mark is anticipated. It is then necessary to define the position of this reading window and to recognize, within this one, an image which is effectively the mark of reference. The following operation consists in measuring the dimension of the mark, in an identical zone for all the successive marks parading in front of the reading device, so as to know the existing locating error. Finally, it is necessary to transform this measurement information into a corrective signal which can be used by the marking correction bodies. In this solution, the size of the registration marks is relatively large, in order to allow good reading. A device for obtaining a reading window is described in detail in patent CH 548933 and a device for reading and recognizing registration marks is the subject of patent CH 556738.

As we have already mentioned, this procedure requires large marks to ensure acceptable detection of errors in lateral and longitudinal position of the print. However, the fact of admitting large marks inevitably leads to a significant loss of material. Indeed, by the current printing technique, the registration marks must occupy an area without printing, in order to be able to be differentiated for sure from the printing itself. The printing area of the registration marks cannot remain on the finished product and it is therefore necessary to plan for its elimination, and therefore the loss of material.

The object of the present invention is to provide a solution to the problem of waste material when using registration marks to correct the longitudinal and lateral errors of printing, while considerably increasing the reliability of detection of registration marks. .

To this end, the invention proposes a detection method in accordance with what is stated in claim 1, as well as a device for implementing the method in accordance with what is stated in claim 5.

• La figure 1 représente l'aspect d'une première marque de repérage,
• La figure 2 représente une seconde marque de repérage,
• La figure 3 représente l'image d'une marque de repérage virtuelle,
• La figure 4 est une vue schématique d'un dispositif de détection des marques de repérage,
• La figure 5 est un schéma bloc du dispositif de la figure 4, et
• La figure 6 est un organigramme fonctionnel du dispositif de la figure 4.

The accompanying drawing illustrates, by way of example, an embodiment of an embodiment of the invention. In this drawing,

• FIG. 1 represents the appearance of a first registration mark,
• FIG. 2 represents a second registration mark,
• FIG. 3 represents the image of a virtual registration mark,
• FIG. 4 is a schematic view of a device for detecting registration marks,
• FIG. 5 is a block diagram of the device of FIG. 4, and
• FIG. 6 is a functional flow diagram of the device of FIG. 4.

Figure 1 shows the appearance of a first printed registration mark 1 carried by a strip material 2 such as, for example, cardboard. This printed registration mark 1 is affixed in an area called the print quality control signs of the print quality area, for example the area between two images printed on the strip 2 or else in an area on the margin of strip 2 to be printed. The reason for printing the printed registration mark 1 in the area reserved for print control signs is motivated by the fact that the print control signs are still on strip 2 and that, by placing the printed registration marks 1 there, the area is removed specific area reserved exclusively for printing registration marks and thereby eliminating waste, also increasing the useful surface to be printed.

The printed marking mark 1 in FIG. 1 is made up of 5 signs 3a to 3e. These signs 3a to 3e are represented here as being points distributed in staggered rows, in the manner of the number five carried by the face of a die.

The printed registration mark 1 also includes three color printing indicators 4a to 4c. The color printing indicator 4a being that corresponding to the printing group applying the blue color, the color printing indicator 4b being that corresponding to the printing group applying the red color and the color printing indicator 4c being that corresponding to the printing group applying the color yellow. The printing group applying the black color, in this case being the first printing group, will be the reference group and its color printing indicator will be represented by the five signs 3a to 3e of the printed registration mark 1.

It is understood that the order in which the colors are printed may be different. FIG. 1 also shows, in phantom, basic fields 5a to 5c which are assigned to each of the color print indicators 4a to 4c. One can define, in these basic fields 5a to 5c represented here in a substantially square shape, a geometric center 6a to 6c for each of them. It is also possible to define the geometric center 7a to 7c of each of the color printing indicators 4a to 4c. We can therefore practice measuring the position error, on the abscissa and on the ordinate, of each of the color printing witnesses 4a to 4c with respect to the geometric centers 6a to 6c of each of the basic fields 5a to 5c. These position errors will take, on the abscissa, the values Δx to Δx₂, and, on the ordinate, the values Δy to Δy₂.

2 shows a second registration mark 8, identical in all respects to the registration mark 1 in FIG. 1 as regards the arrangement of the signs 9a to 9e. The second registration mark differs from the first by the position occupied by the colored printing indicators 10 to 10c which are arranged here outside the five signs 9a to 9e making up the registration mark 8. This arrangement of the registration indicators color printing 10a to 10c is encountered, for example, when the machine is started, which can be assimilated to a transitional period during which the tracking error can be greater than in normal cruising conditions. However, it is necessary to be able to detect these errors in the position of the color printing indicators 10a to 10c in order to switch the device from the transient regime to the cruising regime. To do this, we proceed to the enlargement of the basic fields 5a to 5c of FIG. 1. This enlargement will define the enlarged fields 11 to 11c. The position of the geometric centers 12a to 12c of each of the color printing indicators 10a to 10c will also be defined. It will then be possible to calculate, on the abscissa and on the ordinate, the position errors of each color printing witness 10a to 10c with respect to the geometric centers 6a to 6c of each of the basic fields 5a to 5c. These position errors will take, on the abscissa, the values Δx¹ to Δx¹₂ and, on the ordinate, the values Δy¹ to Δy¹₂.

FIG. 3 represents the image of a reference virtual reference mark 13 which has been placed, for reasons of clarity of the figure, in a square in dotted lines 14. This virtual reference reference mark 13 comprises the same number of signs 15a to 15e as the printed registration mark 1 or 8. The arrangement of these signs 15a to 15e, in the reference virtual reference mark 13, is identical to that of the printed registration mark 1 or 8 There may, however, be a difference in position between the signs of the reference virtual registration mark 13 and the signs of the printed registration mark 1 or 8. In this case, as we will see later, an adaptation of the image comparator will be carried out in order to support an admissible difference value between these different signs.

Figure 4 is a schematic view of a device for detecting registration marks intended for processing a double-sided printed web. To this end, the device comprises a first high speed camera and low optical aberration 16 associated with a flash 17 for the front of the strip, and a second camera 23 associated with a flash 24 for the back of the strip. The cameras 16 and 23 are connected to an image processing device 18 equipped with an image viewing monitor 19. The image processing device is connected to a central unit 20 advantageously chosen as being a personal computer of the type PC - AT or similar. The central unit 20 comprises an interface 21 for processing information relating to the operating conditions of the machine which are themselves recorded in a conditioning unit 22 for "machine" information. This conditioning member 22 receives, from the machine, "INDEX" information originating from a pulse generator mounted on the printing cylinder of the machine. This "INDEX" information corresponds to a position "zero" of the pulse generator and will be used to calculate, using a pulse counter, the number of pulses separating this "INDEX" position relative to the image of the printing cylinder photograph. The pulse generator and the counter have not been shown in this figure.

The conditioning unit 22 also receives information P relating to the moment at which the flash 17 must be controlled respectively 24. This information comes from a pulse generator coupled to the counter (not shown) and will be defined as a function of the value issued for the "INDEX" information. The conditioning unit 22 will emit, simultaneously and in order, a signal for choosing the camera SCAM, a signal for requesting a flash command DCFL and will receive, if the conditions are acceptable, a release signal SQ. The receipt signal SQ having been received, the conditioning unit 22 will then issue the OFL₁ flash order or the OFL₂ flash order depending on whether camera 16 or camera 23 has been selected. The OFL₁ signal will be transmitted to the control circuit 25 of the flash 17 and the OFL₂ signal will be transmitted to the control circuit 26 of the flash 24. The connection between the image processing device 18 and the central unit 20 is made at l using an RS 232 cable while the central unit 20 is connected to the conditioning unit 22 using a BNET network.

Figure 5 is a block diagram of the device of Figure 4, showing in detail the elements constituting the device for detecting registration marks. This figure shows for memory the reading devices of the marking marks, that is to say the cameras 16 and 23, their respective flash 17 and 24 as well as their control circuit 25 and 26. The central unit 20, its interface 21 and the packaging member 22 are also shown.

The image processing device 18 is shown in dotted lines. This image processing device 18 comprises a digitizer 27 connected to a memory 28 in which a generator circuit 29 is provided for the reference virtual reference mark 13. The image processing device 18 also comprises a discrimination unit 30 composed of a basic field generator 31, a basic field enlarger 32, an image comparator 33, a calculator of the geometric base field center 34, a color print indicator detector 35, a calculator of the geometric center of color print witness 36 and, finally, a position comparator of geometric centers 37 to establish the recognition of position errors between the geometric center of the base field and the geometric center of the color print witness . The discrimination unit 30 is followed by a signal transmitter 38 which will serve as its output stage.

Figure 6 is a functional flowchart of the device of Figure 4 to which we will refer below to explain the operation of the device for detecting registration marks. This operation is as follows:
First, read the printed registration mark. To do this, the flash and the camera must be controlled, which will be done using signals from the conditioning unit 22 (see FIG. 5). The image that has been read will then be digitized. We will then search, on the basis of this digitization, if it is indeed a registration mark, by comparing the digital image with a reference virtual registration mark. A read security mark will be given for all the signs making up the reference virtual reference mark, so as to admit a certain distortion between the read image and the reference image. So, referring to Figures 1 and 3, it will be necessary, so that one is in the presence of a registration mark, that the signs 3a to 3e of the printed registration mark 1 are in correspondence with the signs 15a to 15e of the registration mark reference virtual 13. This correspondence having been established, we will admit that the reference mark has been found. It will then be necessary to define basic fields 5a to 5c in order to be able to search in them for the existence of a color printing indicator 4a to 4c.

Once the basic fields 5a to 5c have been defined, the position of their geometric centers 6a to 6c will be calculated and stored. Next, in each basic field 5a to 5c, we will look for the existence of a color printing indicator 4a to 4c. The color printing indicators 4a to 4c having been found, it will be necessary to define their geometric center 7a to 7c. Finally, we will calculate the position deviations Δx to Δx₂ and Δy to Δy₂ and we will send the result of these measurements to the central unit 20 which will act on the device for correcting the location error of the printing press. .

In the case of the transient regime, that is to say when starting the printing press, it will be necessary to enlarge the basic fields 5a to 5c, so as to be able to detect color print indicators situated outside from the base fields area 5a to 5c. Thus, the existence of the registration mark having been found and recognized as conforming, the basic fields 5a to 5c defined and their geometric centers 6a to 6c calculated, the system having given information "color print witness not found" , it will be necessary to enlarge the basic fields 5a to 5c, so as to transform them into enlarged fields 11a to 11c (see Figure 2). We will then look for, within these enlarged fields 11a to 11c, the existence of color printing indicators 10a to 10c. These having been found, we will define their center 12a to 12c and the position deviations Δx¹ to Δx¹₂ and Δy¹ to Δy¹₂ of the geometric centers 12a to 12c of the color print indicators 10a to 10c will be calculated relative to the geometric centers 6a to 6c of the basic fields 5a to 5c. These results will then be sent to the central unit 20 which will control the device for correcting the location of the printing press. The steady state or cruising speed will be established automatically thereafter, when the color printing indicators are detected in the areas defined by the basic fields 5a to 5c.

In the event that the printed registration mark 1 cannot be compared with the reference virtual registration mark 13, the system will issue a message "mark not found" signifying the end of the procedure which must then be repeated.

In the event that, even after enlarging the basic fields 5a to 5c, no color printing indicator 4a to 4c is found, the system will issue a message "indicator not found" also signifying the end of the procedure which must be repeated.

In the present description, a high speed and low optical aberration camera was used for reading the printed registration marks, the printed registration marks being still visible to the naked eye. One could, however, imagine using even more powerful cameras which would then allow the reading of printed cue marks that the human eye would be unable to detect.

Claims (7)

1. Method for detecting registration marks in a multicolour printing press processing webs or sheets with an area for printing the image and another area for printing the marks allowing to check the print accuracy, characterized by its being successively capable of:

   - comparing a printed registration mark (1) with at least two recognition signs (3a to 3e) and a colour pilot mark (4a to 4c or 10a to 10c) with a virtual referential registration mark (13) of similar configuration, though including only the recognition signs (15a to 15e);

   - determining at least a basic area (5a to 5c) for scanning the colour pilot mark (4a to 4c or 10a to 10c) for each recognition sign (3a to 3e or 9a to 9e);

   - calculating and memorizing the geometric centre (6a to 6c) of every basic area (5a to 5c) for scanning the colour pilot mark (4a to 4c or 10a to 10c);

   - successively detecting the colour pilot marks (4a to 4c or 10a to 10c) within every basic area (5a to 5c);

   - calculating and memorizing the geometric centre (7a to 7c or 12a to 12c) of every colour pilot mark (4a to 4c or 10a to 10c) detected;

   - measuring along both the ordinate and abscissa axes the off-line positions (Δx, Δy) between the geometric centre (6a to 6c) of the basic area (5a to 5c), and the geometric centre (7a to 7c or 12a to 12c) of the colour pilot mark (4a to 4c or 10a to 10c);

   - converting the measurments taken from the off-line positions (Δx, Δy) into control values to be used for acting on the misregister correction appliance.
2. Method according to claim 1, characterized by the comparison of registration marks (1) printed into the print area containing the print accuracy control signs to virtual referential registration marks (13) previously memorized.
3. Method according to claim 2, characterized by the comparison of registration marks (1) invisible to the naked eye to virtual referential registration marks (13) previously memorized.
4. Method according to claim 1, characterized by the fact that after determining a basic area (5a to 5c) for scanning the colour pilot mark (4a to 4c or 10a to 10c), the said basic area (5a to 5c) is enlarged, though with geometric centre (6a to 6c) of the basic area (5a to 5c) kept as centre of the new area (11a to 11c) thus determined.
5. Device for the execution of the method according to the claims 1 to 4, characterized by its comprising means for illuminating and scanning the printed registration mark (1), means for generating and memorizing a virtual referential registration mark (13), means for comparison enabling the recognition between the printed registration mark (1) and the virtual referential registration mark (13), means for determining within the limits of the means for comparison, at least one basic area (5a to 5c) for scanning a colour pilot mark (4a to 4c or 10a to 10c), means for calculating and memorizing the geometric centre (6a to 6c) of every basic area (5a to 5c), means for detecting in every basic area (5a to 5c) the said colour pilot mark (4a to 4c or 10a to 10c), means for calculating and memorizing the geometric centre (7a to 7c or 12a to 12c) of the colour pilot mark (4a to 4c or 10a to 10c) detected, means for measuring both along in the abscissa and ordinate axes the off-line positions (Δx, Δy) between the geometric centre (6a to 6c) of the basic area (5a to 5c) and the geometric centre (7a to 7c or 12a to 12c) of the colour pilot mark (4a to 4c or 10a to 10c) detected in the latter area, means for enlarging the said basic area (5a to 5c) and means for converting the off-line position measurements (Δx, Δy) gathered between the geometric centre (6a to 6c) of the basic area (5a to 5c) and the geometric centre (7a to 7c or 12a to 12c) of the colour pilot mark (4a to 4c or 10a to 10c) into control values to be used for acting on the misregister correction appliance.
6. Device according to claim 5, characterized by its means enabling the illumination and scanning of the registration marks (1) consisting of a flash projector (17, 24) operating jointly with a low-aberration high-speed camera (16, 23).
7. Device according to claim 5, characterized by its means for generating and memorizing virtual referential registration marks (13), means for comparing registration marks (1) with virtual referential registration marks (13), means for determining, within the limits of the said means for comparison, a basic area (5a to 5c) for scanning a colour pilot mark (4a to 4c or 10a to 10c) and means for enlarging the said basic area (5a to 5c) consist of an image processor (18) comprising a digitaliser (27) acting jointly with a memory (28) and including a generator (29) generating virtual referential registration marks (13), the said memory being connected to an image comparator (33) receiving the images of the digitalised registration marks (1) printed and the image of the virtual referential registration marks (13) and emitting a signal of similarity of the compared images, which signal, in case of similarity, is transmitted to a generator (31) for the basic area (5a to 5c), connected to a calculator (34) for the geometric centre (6a to 6c) of the said basic area (5a to 5c) and memorizing the data concerning the position of the said geometric centre (6a to 6c), the said generator (31) for the basic area (5a to 5c) being coupled, on the one hand, to a detector (35) of colour pilot marks (4a to 4c or 10a to 10c), emitting signals indicating the presence of a colour pilot mark (4a to 4c or 10a to 10c) within the basic area (5a to 5c) and retransmitted to a calculator (36) for the geometric centre (7a to 7c or 12a to 12c) of the colour pilot mark (4a to 4c or 10a to 10c), memorizing such signals, and, on the other hand, to an enlarger (32) for the basic area (5a to 5c), the said detector (35) for the colour pilot mark (4a to 4c or 10a to 10c) being connected to a position comparator (37) for the geometric centres (6a to 6c) of the basic areas (5a to 5c) and for the geometric centres (7a to 7c or 12a to 12c) of the colour pilot marks (4a to 4c or 10a to 10c), the said position comparator (37) transmitting the off-line positions (Δx, Δx', Δy, Δy') appearing between the geometric centres (6a to 6c, 7a to 7c or 12a to 12c) to a signal transmitter (38) connected to a central unit (20) controlling a misregister correction appliance through a conditioner (22).

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