EP1219420B1 - Device for scanning register marks in a multicolour printing machine - Google Patents

Description

The present invention relates to a marking mark reading device in a polychromatic printing machine working a material strip or sheets. This material, or printing medium, generally has an area for printing the image and a print area reserved for the signs of quality control, signs usually known by the name of marks of registration or register, with reference to the marking of the different colors of the printing.

Such machines have in fact several printer groups placed one after the other; each printing on the support, with the aid of an engraved cylinder or a forme cylinder, for example, the same pattern of a different color. To obtain a perfect final image, it is necessary that all the different color prints are superimposed exactly. The control of the marking of these prints is done by means of registration marks printed by each printing cylinder in the area reserved for the signs of the quality control, generally in the margin of the support worked. With a reading device, these marks make it possible to determine the error of marking of each color with respect to the color of the first printing unit taken generally as a reference. To compensate for these offsets, a correction order is then generated and acts either on the print medium path or on the position of the corresponding print cylinders.

Many known devices, such as those described in the documents CH690096 , EP0401691 and US5747795 , allow to detect and read these signs printed on elements in sheet or band parading in front of a light source. However, these devices can generally read only one registration mark at a time which, in the case of a full-color printing, requires as many reading devices as there are marks or colors in the print.

Some devices, such as the one described in the document EP0214214 , make it possible to take an image of a whole group of marks using a CCD video camera, then to subject this image to an analog-digital conversion, to center this scanned image on a window of analysis and to determine deviations from marks of references. A white light source provides sufficient illumination of the medium filmed by the video camera. This light source can be derived from a strobe which, thanks to its repeated flashes, makes it possible to take still images of the moving medium at high speed.

Other devices, such as the one described in the document EP0512448 , propose to solve problems of recognition of registration marks which have the particularity of being weakly contrasted with respect to the background color of the medium on which they are printed; This is often the case when the print colors are pale, such as pastel yellow, cream or sky blue. The cited device reads only one mark at a time, which is illuminated by a white light source. The light reflected by this mark is separated by two optical fiber channels at the end of which are placed two different color filters placed in front of two photosensitive units. Each of these photosensitive units is specially receptive in a frequency band of a different color and generates an electrical signal when the registration mark passes. The mark is detected by means of a comparator / selector device distinguishing, among the electrical pulses generated, the pulse most representative of the color mark.

When it is intended to simultaneously detect several registration marks with the same device, the lighting of these marks becomes an increasingly significant component, especially when these marks can no longer be made visible by a single light source, white or monochrome. Indeed, according to the color of the printed marks, the latter may, under such lighting, not be sufficiently contrasted and appear invisible or pose problems on the contrary glare or reflection in the presence of specular colors such as gilding for example.

In the case where the colors used are intense and clearly distinguish the marks printed by well-defined contours, the simultaneous detection, by the same device, of several brands with such colors would cause a priori no particular problem; the latter can be easily identified under a single white light as shown in the case of the document device CH686501 .

But in a good third of cases, the printed colors are not so clear of each other and require specific lighting to improve the perceived contrast between them or with respect to the background color of the printed pattern. Thus, a mark with a predominance of green, purple or orange will appear all the more contrasted that its lighting color is rich in complementary color, respectively red, yellow or blue in these cases.

In order to guarantee the reliability and the performance of the detection systems, it is also necessary to make these distinctive signs quite apparent. Indeed, when starting the printing machine, the first phase is to look for the initially unknown positions of each of the registration marks. This task can be greatly facilitated if the marks in question are each made clearly visible by a light source of adequate color. Similarly, when these marks scroll at high speeds of up to 20 m / s, it is easy to understand that it is also advantageous, even necessary, that these marks can be perceived without any ambiguity possible.

Currently, the simultaneous detection of two or three low contrast color registration marks must be performed by as many reading devices; each of them to be equipped with a specific lighting according to the color of the mark for which it is intended. However, such a plurality of devices increases the costs of installation and maintenance of the printing machine, requires more space and makes the detection system more difficult to manage as a whole by proportionally increasing the risk of breakdowns incurred.

The object of the present invention is to overcome these drawbacks by proposing a compact reading device making it possible, with a minimum of one read head, to simultaneously detect several registration marks, while the latter would each require a reading device equipped with a reader. particular lighting to provide sufficient contrast necessary for their detection. The device according to the invention thus has the advantage of being able to detect offsets between the prints of each color after simultaneous reading of a reference mark and one or more registration marks by one and the same reading head.

This goal is achieved thanks to a reading head equipped with one or more rows of photosensitive elements, generally identical, and a lighting source whose color and / or intensity are alternately modulated. The use of a plurality of different elements sensitive to particular colors as a function of those used in the printing, provides the same effect and can be considered as another embodiment of the proposed device.

During their passage time under said reading head, the registration marks are scanned by the photosensitive elements and read by scanning into a multitude of fine independent cuts which, successively arranged one after the other, reconstitute in the form of images. striated the images of the read marks. The modulation of the illumination source provides an alternation of colors and / or of intensity making it possible to obtain a line-by-line coloring cycle during the scanning of the image, or group of lines per group of lines, even image by image.

The simultaneous or almost simultaneous reading of these marks by this device does not depend on the shape of these marks, their size, or their disposition to each other. Thus, the detection of low-contrast concentric registration marks can be read simultaneously without difficulty by the device of the present invention which will alternately modulate the color of its lighting according to that of the marks read so as to render them, in turn, well visible.

Advantageously, it may be possible, according to the form known in advance of the marks to be detected, to vary the periodicity of the alternations of the lighting in time or to vary the extent of the illuminated beaches in turn. Thus, it may be advantageous to define and program different lighting sequences that are specifically suited to the geometry of a certain type of chosen mark. By way of example, a sequence of such sequences could comprise the reading of a group of several successive lines illuminated under the same color, then that of a succession of lines taken one by one alternately in one color and then in another, before going back to reading a group of several illuminated lines under the same lighting.

In the case where the marking marks are of the same color, the device as expected would not be disturbed by a lack of alternations in the lighting colors. According to the invention, the device in question is not a priori not limited by the possible number of lighting colors. As previously suggested, in combination or as a substitute for alternating lighting colors, the intensity of this lighting could also be modulated and used to detect the desired brands. Usefully, the use of filters generating different colors from a white light for example could also replace the diversity of light sources without bringing a new feature. Finally, according to various conceivable embodiments, the number of rows of photosensitive elements in no way limits the applications offered by this device as described above.

• La figure 1 est une représentation schématique en perspective dudit dispositif de lecture disposé au-dessus d'un support imprimé de marques de repérage,
• la figure 2 est une représentation fortement agrandie d'un exemple d'un couple de marques de repérage telles qu'imprimées sur un support par une machine polychrome,
• la figure 3 est une représentation fortement agrandie d'un exemple de deux marques de repérage concentriques telles qu'imprimées sur un support par une machine polychrome,
• les figures 4 et 5, sont des représentations des marques de repérage des figures respectives 2 et 3 telles qu'elles apparaissent sous des bandes d'éclairage, dans deux modes de détection illustratifs différents, à leur lecture simultanée par le dispositif de l'invention.

The invention will be better understood from the study of an embodiment taken in no way limiting and illustrated by the appended figures in which:

• The figure 1 is a diagrammatic representation in perspective of said reading device disposed above a printed marking mark medium,
• the figure 2 is a greatly enlarged representation of an example of a pair of registration marks as printed on a support by a polychrome machine,
• the figure 3 is a greatly enlarged representation of an example of two concentric registration marks as printed on a support by a polychrome machine,
• the Figures 4 and 5 , are representations of the marking marks of the respective figures 2 and 3 as they appear under lighting strips, in two different illustrative detection modes, at their simultaneous reading by the device of the invention.

The figure 1 gives a schematic representation in perspective of the device 1 of the present invention. This device is placed above a support 2, scrolling in a full-color printing machine, so that it can easily detect the passage of registration marks 21, 22 printed on this support. The reading device 1 consists of a housing, drawn in broken lines and in partial section, in which there are at least two light sources 3, 4 allowing the one and the other to project alternately on the support 2 a lighting zone 5 covering at least the area occupied by the registration marks 21, 22. Each lighting source 3, 4 may be constituted by one or more light diodes 13, 14 as illustrated by way of example in FIG. the figure 1 . The reading device 1 also comprises an optics 6 allowing projecting, on at least one photosensitive element 7, the image of a portion 8 of the surface of the support 2 which is illuminated in zone 5. The reading portion 8 corresponds to a strip whose dimensions are mainly chosen as a function of those of the marking marks and the content of the specifications of the reading device; the latter defining in particular the resolution of the image of said scanned registration marks, as well as the speed of acquisition of these images by the reading device as a function of the speed of displacement of the support 2. The photosensitive element 7 may be a CCD sensor composed of a large number of pixels 17 generating electrical pulses and forming, according to their number and their arrangement, one or more photosensitive strips placed side by side. In the figure 1 only one band of pixels 17 constitutes the photosensitive element 7 as illustrated. The latter is connected, like the reading sources 3 and 4, to a microprocessor 9, in particular for controlling the lighting of these sources in a programmed mode and for processing the pulses delivered by each pixel 17.

The principle of operation of this device aims to scan the registration marks 21, 22 in their entirety by successively capturing adjacent image portions 8, alternately illuminated in one color and then in another thanks to the light sources 3, 4. Each portion 8 of registration marks is preferably read only once under the light of one of the light sources, which sources follow lighting sequences governed in time and in their duration according to the programmed mode in the microprocessor 9. The final image of the registration marks taken by this device can be easily recomposed by assembling successively all the read portions 8 in the same order as naturally defined during the acquisition by the displacement of the support 2 Once recomposed, this image, or the data it contains, can then be used to determine any discrepancies between the colors of the colors. impressions during the operation of the registration of the corresponding printing cylinders.

The Figures 2 and 3 each give examples of two pairs of registration marks 21, 22 and 31, 32 as could be found on the support 2 after it has been printed by a polychrome machine. In these illustrations, the dimensions of these marks are greatly enlarged; in reality they each occupy only a surface of a few square millimeters. Marking marks of the figure 2 correspond to those who are illustrated in the figure 1 , but it goes without saying that other marks of identification, like those of the figure 3 for example, could be represented. Each mark of a couple is of a different color; thus the color of the mark 21 or 31 is not the same as that of the mark 22 or respectively 32. As illustrated in these figures, the marks 21 and 22 of triangular shapes oppose exactly by their points and are so perfectly in register with each other, just like the marks 31 and 32 which are perfectly concentric. In order to be able to measure possible offsets in the relative positions of two marks of the same pair, the latter are scanned by the reading device 1 of the present invention. Being of different color, often pale and low contrast, and although read simultaneously, each of these marking marks needs to be illuminated by a complementary color light to be made visible. This makes it possible to improve the contrast and to ensure that it can be correctly detected by the photosensitive element 7. For this purpose, the reading device according to the invention makes it possible to respond fully to these requirements by means of the alternating illumination illustrated in Figures 4 and 5 on the marking marks of the corresponding figures 2 and 3.

The figure 4 gives, in a plan view, an example of a representation of the illumination of the reading device 1 applied to the triangular registration marks 21, 22. The presence of these marking marks can then be made clearly visible by the succession of portions 8, or reading slices, generated by the device 1. The latter appear alternately in one color and then in another forming thin strips 8a, 8b of identical dimensions. These colors correspond to those of the different light sources 3, 4 which are chosen according to the colors of the read marks. Thus, the strips 8a, 8b each have a color that reveals a strong contrast with that of the registration marks 21, 22 taken respectively. Under this zebra light, it becomes possible to clearly highlight the contours of the two tracking marks read simultaneously. The contours of each of these marks are drawn as a dashed line which easily allows complete reconstruction of the shape of the mark and perfect detection thereof during the stamping operation of the printing rolls.

The figure 5 offers a reconstruction of the image of the concentric locating marks of the figure 3 from the 8 portions of images read by the device 1 in a lighting mode which differs from that used previously. In this figure, it is easy to recognize the different lighting sequences that constitute the chosen lighting mode. The first sequence is formed by alternating a group 18 of three thin contiguous bands 8a with a group 28 of three thin contiguous strips 8b of another color. This succession of alternations is followed, in the central part of the registration mark 32, by a second sequence formed by a succession of alternations of the bands 8a and 8b taken one by one, before going back to the first sequence formed alternations groups 18 and 28. It can thus be seen that, depending on the shape and / or size of a registration mark, it may be advantageous to vary the frequency of the alternations of the lighting either in increasing or reducing the illumination time of the light sources 3, 4, or by acting on the aperture of a diaphragm at the optics 6 so that the surface of the image portions 8 varies by proportion.

In the case where the marking marks read simultaneously have identical or very similar colors, the reading device can of course do without the alternation of lighting colors and illuminate said marks with a light of one and the same color during all the time that their reading takes. It will also be noted that, as necessary, changes in intensity of lighting can easily replace the alternations of colors without the detection principle used by this device is changed. It goes without saying that the number of registration marks that can be read simultaneously by this device is not a priori limited. To do this, it suffices to adapt the number of different color light sources of the device 1, without, however, excluding all the possible color combinations given for example by activating both a blue light source and a yellow light source for obtain on the printed support a zone 5 illuminated by a green light. Although part of the same blocks that constitute the sources 3 and 4, it is clear that the LEDs 13, 14 could also emit a different color light. By extension, it will also be mentioned that the wavelengths emitted by said light sources 3, 4 are not necessarily limited to the range covered by the visible range. Finally, it could be envisaged that the pixels 17, which in large number constitute the photosensitive element 7, may have different sensitivities to the lengths relative to one another. waves they receive. For example, some pixels 17 may be more particularly sensitive to red-orange colors while others would be rather receptive to blue-green or yellow colors. To analyze the image read, it would be sufficient for example to use the appropriate color pixels to find the different color marks. This option would make it possible to replace the diversity of colored light sources 3, 4 by being limited to a single light close to sunlight, for example, or conversely to extend the sensitization of the reading device to a greater extent. wide color palette. Many improvements can be made to the object of this invention within the scope of the claims.

Claims (10)

1. Device (1) for scanning register marks (21, 22, 31, 32) printed on a substrate (2) travelling in a polychrome printing machine, comprising at least one light source (3, 4) illuminating on the substrate (2) a lighting area (5) crossed by the register marks (21, 22, 31, 32), an optic (6) for receiving on a photosensitive element (7), consisting of a plurality of pixels (17), the travelling images of said register marks in the form of a plurality of portions (8) that are successively scanned according to a predetermined scanning rate, as well as a microprocessor (9) controlling the illumination of the light source (3, 4) and processing the electric pulses generated by the pixels (17), characterized in that the light source (3, 4) illuminates the substrate (2) on a lighting area (5) with at least one intensity and/or color modulation during the simultaneous or sequential scanning of at least two register marks (21, 22, 31, 32).
2. Device according to claim 1, characterized in that the modulations related to the illumination of the lighting area (5) are executed in synchronism with the scanning rate of each portion (8) of images.
3. Device according to claim 1, characterized in that the modulations related to the illumination of the lighting area (5) are executed in synchronism with the scanning rate of each register mark (21, 22, 31, 32).
4. Device according to claim 1, characterized in that the number of modulations per time unit is the same as the number of portions (8) of images scanned at the same time interval.
5. Device according to claim 1, characterized in that a series of different modulations represents a lighting cycle programmed and controlled by the microprocessor (9), and in that the illumination of the lighting area (5) varies in accordance with a successive repetition of at least one lighting cycle.
6. Device according to claim 5, characterized in that during a lighting cycle, the number of different colors or intensity applied to the lighting area (5) is proportional to the number of register marks of different colors that are simultaneously scanned.
7. Device according to claim 5, characterized in that the contrast of the register mark (21, 22, 31, 32) with respect to the substrate (2) is improved by using for illuminating the area (5), in a same cycle and for each of the colors of the register marks, a light having a wavelength between 380 nm and 780 nm and/or an intensity between 5% and 100% of the maximum intensity.
8. Device according to claim 1, characterized in that the photosensitive element (7) consists of a plurality of pixels (17) which are sensitive to at least one wavelength of at least one printed color.
9. Device according to claim 1, characterized in that the said portions (8) that are successively scanned, cover, when placed side by side, at least the whole surface of the register marks (21, 22, 31, 32) that are simultaneously scanned.
10. Device according to claim 1, characterized in that the geometrical form of said portions (8) that are successively scanned, is similar to the form of strips (8a, 8b) having a width of 0.1 mm to 5 mm.

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