Method and apparatus for adjusting printing units

The invention relates to a method and to an apparatus for adjusting the position of a plurality of printing units arranged in succession in a machine for decorating ceramic objects, in particular tiles.

Machines are known for decorating ceramic tiles, comprising a conveying line along which the tiles are conveyed in an advance direction. Along the conveying line there is arranged a plurality of printing units, each of which includes an applying roller for applying to the tiles a ceramic glaze, i.e. a ceramic colour, according to any printing principle. Whilst the tile moves in the advance direction, each printing unit applies a respective colour to the tile. From the combination of the colours applied by the single printing units the desired decoration is obtained.

Before starting to produce decorated tiles, it is necessary to position the applying rollers of the single printing units correctly in relation to one another. This ensures that each ceramic colour is printed in an appropriate position with respect to the other colours so as to create a good-quality decoration on the finished tile.

In order to adjust the position of the applying rollers, a method is known according to which the first applying roller and the second applying roller positioned along the conveying line print respectively a first reference mark and a second reference mark on a test tile. The first reference mark comprises generally a square frame, whilst the second reference mark has the shape of a cross. When the first applying roller and the second applying roller are correctly positioned in relation to one another the cross is printed in the centre of the square frame.

If, on the other hand, the cross is printed on the test tile in a decentralised position compared with the square frame, it is necessary to perform certain adjusting operations for adjusting the position of the second applying roller with respect to the first applying roller. In this case, by

looking at the position of the cross with respect to the square frame, the operator, also according to his experience, understands how to modify the position of the second applying roller. After appropriately modifying the position of the second applying roller, the square frame and the cross are again printed on another test tile. If the cross is positioned in the centre of the square frame, the machine is ready for producing decorated tiles. Otherwise, the position of the second applying roller is modified further, until a satisfactory result is obtained.

The procedure disclosed above is repeated for each applying roller, which has to be positioned correctly with respect to the first applying roller before starting decorating. A drawback of the method disclosed above is that, generally, many attempts are required to position each applying roller correctly. In fact, it is difficult for the operator to be able to ensure, through a single positioning procedure, that the cross printed by the second applying roller is in the centre of the square frame printed by the first applying roller .

A further drawback of known methods is that the time that is necessary for correctly positioning the applying rollers largely depends on the competence and the attention of the operator. In fact, the operator may commit errors in deducing, from the relative positions of the cross and of the square frame, the adjustments to be made to the applying roller. Further, the operator may make a mistake in implementing the thus deducted adjustments to the applying roller.

Consequently, the operations of preliminary adjustment of the printing machine require rather a long time, which diminishes productivity. An object of the invention is to improve known methods for adjusting printing units arranged for printing a decoration on a ceramic object.

A further object is to reduce the number of attempts necessary for correctly positioning the various printing units in relation to one another.

Another object is to provide a method and an apparatus that enables also non-expert operators to position correctly the printing units of a machine for decorating ceramic tiles. In a first aspect of the invention, there is provided a method for adjusting the relative position of a first printing unit and of a second printing unit, said method comprising the steps of: acquiring the image of a ceramic object having a first reference mark printed by said first printing unit and a second reference mark printed by said second printing unit; - determining the virtual displacement required to place said second reference mark, on said image, in a desired position with respect to said first reference mark; obtaining from said virtual displacement at least an adjusting parameter to be applied to said second printing unit for adjusting the position thereof.

In a second aspect of the invention, there is provided an apparatus for adjusting the relative position of a first printing unit and of a second printing unit, comprising: a vision system programmed for acquiring the image of a ceramic object having a first reference mark printed by said first printing unit and a second reference mark printed by said second printing unit, computer means programmed for determining at least an adjusting parameter to be applied to said second printing unit, on the basis of the virtual displacement required for placing said second reference mark, on said image, in a desired position with respect to said first reference mark .

Owing to the invention, it is possible to calculate automatically the adjusting parameters on the basis of which the position of the second printing unit has to be modified

to be centred with respect to the first printing unit. This enables the operations of adjusting the position of the second printing unit to be accelerated as the number of attempts required for positioning this unit correctly is drastically reduced. Operator errors are also substantially eliminated, which decreases the downtime required for setting up the decorating machine. Lastly, as the adjusting parameters are calculated automatically, even inexpert operators can perform the adjusting operations. The method and the apparatus according to the invention can also be used with reference to printing units arranged downstream of the second printing unit, in order to position the printing units correctly with respect to the first printing unit. The invention can be better understood and implemented with reference to the attached drawings that illustrate some embodiments thereof by way of non-limiting example, in which : Figure 1 is a schematic side view of a decorating machine for decorating ceramic tiles;

Figure 2 is a schematic frontal view of an apparatus for measuring the relative position of a first printing unit and of a second printing unit of the machine in Figure 1; Figure 3 is a plan view of a portion of a ceramic tile on which there were printed a first reference mark and a second reference mark in a mutual non-correct position; Figure 4 is a view like the one in Figure 3, showing the first reference mark and the second reference mark printed in a correct position; Figure 5 shows the screen of a computer during a step of a method for determining the correct position of the second printing unit with respect to the first printing unit in the machine in Figure 1; Figure 6 shows the screen of the computer during a step subsequent to that in Figure 5;

Figure 7 shows the screen of the computer during a step subsequent to that in Figure 6;

Figure 8 is a view like the one in Figure 4, showing four reference marks according to an alternative version; Figure 9 is a schematic side view of a decorating plant comprising a decorating machine and including an apparatus for measuring the relative position of a first printing unit and of a second printing unit of the decorating machine. Figure 1 shows a decorating machine 1 for printing a decoration on a surface 3 of a ceramic object, for example a tile 4. The decorating machine 1 comprises a plurality of printing units 2 each of which is arranged for applying a ceramic glaze to the surface 3 of the tile 4. The printing units 2 are arranged in succession along a conveyor belt 5 that moves the tiles 4 in an advance direction F.

Each printing unit 2 comprises an applying roller 6 that is rotatable around a longitudinal axis thereof so as to interact, during rotation, with the surface 3 of the tile 4 moved by the conveyor belt 5 and apply to the tile 4 the corresponding ceramic glaze according to a preset decorative pattern .

In the example shown, the printing units 2 are gravure printing units of known type. Each applying roller 6 has an external surface 7 on which there is obtained a multiplicity of cavities, arranged according to the decorative pattern that it is desired to form on the tile 4. A doctor blade 20 distributes the ceramic glaze inside the cavities and keeps clean the zones of the external surface 7 interposed between adjacent cavities. During decorating, the tiles 4 are moved by the conveyor belt 5 along the advance direction F so as to interact in succession with all the applying rollers 6, each of which prints the corresponding decorative pattern on the surface 3. The desired decoration is obtained on the tile 4 from the combination of decorative patterns applied by each applying roller 6.

It is possible to use the adjusting procedure disclosed below in order to ensure that each printing unit 2 transfers the corresponding ceramic glaze to the tile 4 in a correct position, so that a good quality decoration is obtained from the combination of the ceramic glazes applied by each printing unit 2.

The first applying roller 6a of the sequence of applying rollers 6 arranged along the advance direction F applies to a test tile 4a a first ceramic colour that defines a first reference mark. As shown in Figures 3 and 4, the first reference mark may have the shape of a polygonal frame, for example a square frame 8.

Subsequently, the test tile 4a is advanced by the conveyor belt 5 until it interacts with the second applying roller 6b, which prints a second reference mark with a second ceramic colour, different from the first ceramic colour. In the example shown, the second reference mark has the shape of a cross 9. The square frame 8 and the cross 9 are printed on the test tile 4a by respective peripheral portions of the first applying roller 6a and of the second applying roller 6b. The peripheral portions of the first applying roller 6a and of the second applying roller 6b that print the square frame 8 and the cross 9 do not interact with the tiles 4 during normal production. Consequently, the square frame 8 and the cross 9 are printed only on the test tiles, but not on the tiles 4 during normal production.

If, on the test tile 4a, the cross 9 is printed in a centred position C with respect to the square frame 8, so that the cross 9 divides the square frame 8 into four substantially equal quadrants, as shown in Figure 4, the second applying roller 6b is correctly positioned with respect to the first applying roller 6a. If on the other hand, as shown in Figure 3, the cross 9 is printed on the test tile 4a in a position N that is not centred with respect to the square frame 8,

the position of the second applying roller 6b with respect to the first applying roller 6a has to be adjusted. In order to indicate to the operator how to modify the position of the second applying roller 6b, an apparatus 11 shown in Figure 2 is used. The apparatus 11 comprises a vision system 12 for detecting an image of a portion of the test tile 4a on which the square frame 8 and the cross 9 were printed. The apparatus 11 further comprises a resting plane 10 positioned below the vision system 12 for supporting the test tile 4a.

The vision system 12 comprises a camera 14 facing the resting plane 10 and an illuminating device that is not shown that is suitable for illuminating the test tile 4a. The camera 14 can be a black and white camera, whilst the illuminating device may comprise three or more monochrome light emitters, for example a red LED, a green LED and a blue LED. The black and white camera enables high-definition images to be obtained and is not very sensitive to slight variations in the tone of the colours with which the square frame 8 and the cross 9 are made. The black and white camera further enables the costs of the apparatus 11 to be maintained limited, being cheaper than colour cameras. It is nevertheless also possible, if desired, to use a colour camera. Following an operator command, the camera 14 acquires the image of the test tile 4a. In particular, the camera 14 performs three acquisitions, illuminating the test tile 4a first with the red LED, then with the green LED, and lastly with the blue LED. The apparatus 11 further comprises a display device including a screen 13, for example of the liquid-crystal type, on which the image of the test tile 4a acquired by the camera 14 is displayed. In this image the cross 8 and the square frame 9 are visible, just as they were printed on the surface of the test tile 4a. This is shown in Figure 5.

The image of the test tile 4a displayed on the screen 13 essentially consists of three colours: a first colour with which the square frame 8 was printed, a second colour with which the cross 9 was printed and a background colour. The vision system 12 comprises computer recognition means for recognising the square frame 8 and the cross 9. For this purpose, the computer recognition means enables all the points of the image to be grouped together that have the first colour, all the points of the second colour to be grouped together and lastly all the points of the background colour to be grouped together.

The apparatus 11 comprises manually controlled interface means, which are not shown in the figures, to enable an operator to modify the relative position of the square frame 8 and of the cross 9 on the screen 13. The manually controlled interface means may comprise a mouse, or a plurality of directional switches, or also a control lever

(joystick) connected to the apparatus 11.

During operation, by means of a cursor 21 the operator selects the cross 9 on the screen 13, which cross 9 is arranged initially in the non-centred position N with respect to the square frame 8, as shown in Figure 5. The outline of the cross 9 is thus highlighted on the screen 13, as shown in Figure 6. Subsequently, by means of the manually controlled interface means the operator can move the cross 9 on the screen 13 so as to place the cross 9 in the centred position C with respect to the square frame 8, as shown in Figure 7. The apparatus 11 further comprises computer means such as to detect the virtual displacement applied to the cross 9 to take the cross 9 from the non-centred position N to the centred position C on the screen 13. The computer means is programmed so as to convert the virtual displacement of the cross 9, generally measured in number of pixels along a theoretical X axis and a hypothetical Y axis, in an actual movement of the second applying roller 6b, expressed, for

example, in millimetres. This is for example possible owing to a preliminary calibrating operation, in which the computer means has established a correspondence between the number of pixels and unit of measurement used to adjust the position of the printing units 2 of the decorating machine 1. For this purpose, in the preliminary calibrating operation the vision system 12 has acquired the image of an object of known dimensions. In this manner one or more adjusting parameters are determined that indicate how the position of the second applying roller 6b has to be modified so that the cross 9 is printed in the centred position C that is centred with respect to the square frame 8. The adjusting parameters calculated as disclosed above may include a movement along the X axis and a movement along the Y axis of the second applying roller 6b, both the movements being expressed, for example, in millimetres.

The values of the adjusting parameters can be displayed on the screen 13, as shown in Figure 7. In this example, the second applying roller 6b has to be moved -0.180 mm along the X axis and -0.270 mm along the Y axis.

The operator, after reading the adjusting parameters on the screen 13, modifies the second applying roller 6b accordingly. In this manner the second applying roller 6b is positioned correctly with respect to the first applying roller 6a.

The procedure disclosed above is repeated for all the applying rollers 6 included in the decorating machine 1. In the example shown, the adjusting procedure is also repeated for the pair formed by the first applying roller 6a and by the third applying roller and by the further pair formed by the first applying roller 6a and by the fourth applying roller . Naturally, it is possible to use the procedure disclosed above also when the number of applying rollers is other than

four, for example when the decorating machine 1 comprises five applying rollers.

As explained above, the apparatus 11 enables the adjusting parameters to be calculated in a completely automatic manner, eliminating the errors that, in known methods, occurred when the operator had to deduct the adjusting parameters simply by looking at the test tile. Further, the apparatus 11 enables the second applying roller 6b to be positioned correctly with respect to the first applying roller 6a with a single adjusting operation, without having to resort to the numerous attempts required by the known methods. Lastly, the extreme simplicity of the apparatus 11 enables it to be used successfully also by non-expert operators . In an alternative embodiment, the apparatus 11 can be connected to a printer that enables the values of the adjusting parameters calculated according to the procedure disclosed above to be printed on a sheet of paper, possibly with reference to all the applying rollers 6 included in the decorating machine 1. This enables the possibility of operator error to be reduced further, the operator having to simply make the adjustments to the printing units 2 corresponding to the adjusting parameters printed on the sheet that he has in his hand. The adjusting parameters can also be saved to an appropriate file for subsequent processing. In this file other types of data can be added, for example relating to the environmental conditions . In a further alternative embodiment, the adjusting parameters can be saved to a removable data carrier, for example a CD-ROM, a diskette or a USB pen that is readable by a reading device of the decorating machine 1. The adjusting parameters read by the reading device can be displayed on the decorating machine 1, so as to enable the operator to perform the necessary adjustments manually, or can be used by a control device, included in the decorating

machine 1, to adjust the position of the printing units 2 automatically. In the latter case, the risks of operator error are practically eliminated.

In another alternative embodiment, it is possible to connect the apparatus 11 to the decorating machine 1, for example by a RS-232 port, Ethernet, a LAN network or a wireless connection, in such a manner that the apparatus 11 transmits the adjusting parameters directly to a control unit of the decorating machine 1. The control unit will then, in a completely automatic manner, adjust the position of the printing units 2 accordingly.

In a further alternative embodiment, after the image of the test tile 4a has been acquired by the vision system 12, the computer means can be programmed to move the cross 9 automatically so as to place the cross 9 approximately in the centre of the square frame 8, displaying the result on the screen 13. After that, the operator may be asked to confirm whether the position of the cross 9 with respect to the square frame 8 identified by the computer means and shown on the screen 13 is to be considered to be satisfactory. If the answer is affirmative, computer means calculates the adjusting parameters as disclosed previously. If the answer is negative, the operator centres the cross 9 manually with respect to the square frame 8 on the screen 13, after which the adjusting parameters are calculated.

The apparatus 11 can be portable, so that it is possible to move the apparatus 11 easily from one decorating machine to another to perform adjusting operations. The apparatus 11 can be connected to the mains power supply by a plug 22 or can be supplied directly by a suitable battery.

After all the printing units 2 of the decorating machine 1 have been correctly positioned, it is possible to start the production step to produce a batch of decorated tiles 4. In order to shorten further the time for preparing the decorating machine 1, it is possible to calculate simultaneously the adjusting parameters of all the printing

units 2. For this purpose, the test tile 4a is advanced below all the printing units 2, so that each printing unit 2 applies a respective reference mark to the test tile 4a. By simultaneously analysing the positions of each reference mark with respect to the one printed by the first applying roller 6a, it is possible to determine simultaneously the adjusting parameters of all the printing units 2. For this purpose, it is possible to use an arrangement of reference marks of the type shown in Figure 8. The arrangement in Figure 8 comprises first reference marks in the shape of first squares 108 having a first colour, that were printed by the first applying roller 6a. Second reference marks are further provided in the shape of second squares 109 having a second colour different from the first colour. The second squares 109 were printed on the test tile 4a by the second applying roller 6b. The arrangement shown in Figure 8 further comprises third reference marks including a plurality of third squares 110 printed with a third colour by a third applying roller, arranged downstream of the second applying roller 6b. Lastly, there are provided fourth reference marks comprising a plurality of fourth squares 111 printed with a fourth colour by a fourth applying roller, arranged downstream of the third applying roller . When the printing units 2 are correctly positioned with respect to one another, the reference marks printed by each printing unit are arranged in a regular manner, as shown in Figure 8. If, on the other hand, the reference marks of one colour are in a position that is not centred with respect to the reference marks of another colour, then the position of the corresponding printing unit has to be modified on the basis of the adjusting parameters calculated by the apparatus 1 following a procedure that is similar to the one disclosed above. It is also possible to use arrangements of the reference marks that are different from the one shown in Figure 8. In

general, each colour printed by a preset printing unit has to be positioned, in the arrangement printed on the test tile, such that the position thereof can be compared, both horizontally and vertically, with the position of all the other colours printed by the remaining printing units.

Although in the preceding description reference has always been made to printing units operating according to a gravure printing principle, it is possible to use the apparatus 1 also for adjusting the mutual position of printing units of other types, for example flexographic, offset or silkscreen printing units.

Further, the reference marks can have any desired geometrical shape. Lastly, the first printing unit and the second printing unit can be arranged according to any order along the advance direction F. The adjectives "first" and "second" are used only to distinguish the two printing units, but the first printing unit can be positioned downstream of the second printing unit or can print the first reference mark on the tile after the tile has already interacted with the second printing unit.

Figure 9 shows a decorating plant 100 comprising a decorating machine 101 which is structurally and functionally configured in a similar way to the decorating machine 1 disclosed with reference to Figure 1.

In the version shown in Figure 9, the decorating machine 101 comprises four printing units, but in another version a different number of printing units may be provided, according to the requirements of the desired decorative patterns to be obtained.

In this case, the decorating machine 101 comprises a first applying roller 106a, a second applying roller 106b, a third applying roller 106c and a fourth applying roller 106d, arranged in sequence along the advance direction F. The decorative plant 100 includes an apparatus 111 provided with a processing unit which is used to indicate to an

operator how to modify the relative positions of the applying rollers, the apparatus 111 being of the same type as the apparatus 11 disclosed with reference to Figure 2. The apparatus 111 is arranged downstream of the fourth applying roller 106d for acquiring images of a tile 104, after all the reference marks have been printed on the tile 104.

The tile 104 is conveyed along the advance direction F by a conveying line including a belt 105. A position sensor 130 is provided between the apparatus 111 and the fourth printing unit 106d. The position sensor 130 is configured for detecting the tile 104 approaching the apparatus 111 so that the latter can be activated for acquiring the images of the tile 104 at the right time. In this way, the apparatus 111, in response to a signal generated by the position sensor 130, acquires a succession of images of the tile 104 with a suitable frequency. In each image only a small region of the surface of the tile 104 is shown. Nevertheless, if all the acquired images are considered, the whole surface of the tile 104 is shown.

In the decorating machine 101, each end region of the four applying rollers may be configured for printing a reference mark onto the tile 104. In other words, a first end of each applying roller prints a reference mark and a second end of the applying roller, opposite to the first end, prints a further reference mark. The reference marks may be of the type shown in Figures 3 and 4 or of the type shown in Figure 8. In order to acquire images containing all the reference marks that are printed on the tile 104, the apparatus 111 can be slidably mounted on a shaft 131 which is arranged transversely to the advance direction F, in particular parallel to the applying rollers. The apparatus 111 can slide along the shaft 131 between a first position, in which a camera 114 included in the apparatus 111 acquires the reference marks printed by the

first ends of the applying rollers, and a second position in which the camera 114 acquires the reference marks printed by the second ends of the applying rollers.

The movement of the apparatus 111 along the shaft 131 may be suitably controlled by a control unit.

In an alternative embodiment, that is not shown, two stationary apparatuses 111 can be provided at opposite ends along the transversal direction T. In this configuration, each stationary apparatus is intended to acquire images corresponding to a respective end of the applying rollers and there is no the need for moving the apparatuses, which simplifies functioning of the decorating plant. The tile 104 may be conveyed by the belt 105, for example, at a speed equal about to 600 mm/sec. The apparatus 111 may acquire a succession of images of the tile 104 with an acquiring frequency for example equal to 30 frames/sec. A small region of the surface of the tile 104, for example having dimensions of about 60x45 mm, may be shown in each image. Therefore the camera 114, during operation, acquires a succession of images that are partially overlapped. In other words, two or more images of the succession may contain the same reference marks. The processing unit can be programmed so as to choose, among a plurality of images containing the same reference marks, the image that is most suitable for being processed in order to determine the adjusting parameters of the corresponding roller .

By positioning the apparatus 101 directly in the decorating plant 100, the relative position of the applying rollers may be easily controlled at any desired time.

In one embodiment, the conveying line of the decorating machine 101 may comprise a deflecting device located upstream of the first applying roller 106. At selected times, the deflecting device is activated in order to deviate a tile from the centreline of the belt 105 towards the periphery thereof. The deviated tile thereby interacts

with one of the ends of the applying rollers, which prints on the tile the corresponding reference mark. The position of each reference mark is then evaluated by the apparatus 111. The deflecting device enables the position of each applying roller to be periodically evaluated and, if necessary, adjusted. It is therefore possible to remedy undesired changes in the position of the applying rollers which may occur during normal production. The deflecting device, when activated, may send to a control unit of the apparatus 111 a signal which triggers the acquisition of tile images by the camera 114. In this case, there is no need to use the position sensor 130 to prompt image acquisition by the camera 114.