EP2639069B1 - Method for detecting errors in the alignment of printed images and printing station for carrying out the method - Google Patents

Description

The invention relates to a method for detecting errors in the alignment of printed images of at least two printheads and possibly for correcting the alignment of the printed images or the printheads according to the preamble of claim 1 and a printing station configured for carrying out the method.

Preferably, the printheads in each case in the printhead linearly, for example, at a distance of the desired printing resolution, i. in the so-called pixel pitch, arranged printing nozzles, which can preferably emit each ink of a color. The printheads are arranged according to the invention in a printing station for printing three-dimensional body, wherein the printheads are arranged distributed symmetrically or asymmetrically around the body and aligned in the printing direction perpendicular to the tangent of the surface of the body. The printing direction in the sense of this text is defined as the direction in which the ink or printing ink is applied to the body during printing from the printing nozzles. The direction of printing is therefore normal (i.e., perpendicular to a surface or its tangent surface) at the pressure point of the surface. In the case of rotationally symmetrical bodies, the alignment takes place in particular on a body center axis, which in the case of rotationally symmetrical bodies, such as round bottles in the cross section, is also the axis of symmetry of the body, without the invention being necessarily restricted to the preferred application in rotationally symmetrical bodies.

The pressure on the body according to the invention by at least two of the print heads together, by means of a suitable control of the print heads, ie the print nozzles in the print head either together, individually or in groups, during a relative movement of the Body between the printheads or the printheads over, in particular a rotation of the body relative to the stationary printheads. In principle, however, a movement of the printheads relative to the fixed body or a different movement of printheads and body is possible. For example, the axis of rotation of the body may preferably be oriented in the direction of the linear arrangement of the printing nozzles in a print head, ie perpendicular to the printing direction.

A control can be realized according to the invention in various ways. It is important that a suitable control just then gives a print command to the print head or the print nozzles of a print head to be triggered if the pixel to be printed on the surface is at the print point of the respective print head. There are various implementation options for this purpose, some of which are described below.

In a relative to the printheads rotating body, the respective angle of rotation can be determined by a finely resolved rotary encoder. In a known angular position or angular distance of the various print heads, the print command at the respective printheads corresponding, determined rotation angles can be carried out. This realization is technically particularly simple and reliable. Such a rotary encoder, which is a kind of clock and outputs according to a defined rotational progress in each case a distance corresponding to the completed Drehwinkelinkrement clock signal can also be used to adjust a uniform, known speed or regulate. In this case, a fixed print frequency for a print head can be specified. Furthermore, generally a temporal control of the print heads or printing nozzles is also conceivable after determining a suitable printing time, for example by means of a rotary encoder with a lower resolution. However, the invention is not limited to a specific form of suitable control limited and can be implemented with any appropriate in this sense control.

Furthermore, the distance of the print heads to the body, in particular in the printing direction, adjustable to accommodate different body dimensions, eg. Bottles of different diameters, in the printing station and can print. For a high-quality print image, the print nozzles must be arranged at a defined distance from the surface to be printed. This is possible for bodies of different dimensions only if the print heads with the printing nozzles, preferably in the printing direction, are adjustable.

The above-described printheads, also known as drop-on-demand printheads for ink jet printing, were originally designed for lithographic printing applications where the printheads are unidirectionally moved while advancing the substrate being printed in a direction perpendicular thereto, such as it is also the case with an office printer. A print station with drop-on-demand printheads usually has at least one print head for each color, unlike an office printer. This arrangement is also a preferred application of the present invention. In principle, however, the invention can also be implemented in printing stations in which the printing nozzles of individual print heads can print with different colors.

In particular, if there is a separate print head for each color, it is indispensable for printing a color image that the distance between the respective print heads is known. By means of a software-based control (control unit), this known distance is taken into account in order, during the relative movement of the substrate to be printed and the printhead, to mix over one another and / or different colors in the respective desired image pixels to produce a two-dimensional image to print.

Printing machines or stations for three-dimensional bodies, in particular three-dimensional hollow bodies, which according to the invention are to be printed on their circumference, likewise require a plurality of print heads in order to be able to print the different colors of the image with pixel precision. These printheads are now distributed around the circumference of the hollow body and aligned with their pressure nozzles toward the surface tangent of the body at the pressure point, i. such that the printing direction is arranged normal to the surface to be printed.

In order to be able to print on bodies of different dimensions, in particular hollow bodies (eg bottles) of different diameters, the heads have to be displaced radially from the center of the printing station or to the center of the printing station in order to position the print heads in the correct position for one re-fix other hollow body diameter. This requirement does not exist with the two-dimensional planographic printing applications.

With this radial displacement of the print heads on a straight line extending in the printing direction, for rotationally symmetrical bodies, in particular a straight line extending from the center of the printing press to another radius or diameter, the distance between the print heads distributed around the circumference of the hollow body also changes. Because of the radial displacement while the angle between the printheads remains constant, but not the distance corresponding to the corresponding angle section of the circumference. This new distance must be taken into account when controlling the printheads to produce the desired print image.

However, according to the invention, this can be done by the software used for controlling the print heads or the individual print nozzles of the print heads, since the nominal spacings of the print heads or of the print nozzles of the individual print heads can be determined for each nominal position.

The problem, however, is that with each adjustment of the printheads to adapt to a different body size tolerances occur that can occur clearly or significantly disturbing in view of high demands on the quality of the print in the printed image. This problem does not arise in the two-dimensional planographic printing applications, since the array of printheads used there is firmly bolted relative to each other, so that the distance between the individual printheads or the nozzles of the printheads regardless of the two-dimensional surface to be printed and their size always is equal to. This does not apply to printing stations for three-dimensional bodies.

A typical application for the present invention is the direct printing of glass or plastic bottles in, for example, a filling plant, wherein a plurality of printing stations can be arranged on a carousel. For example, at a resolution of 720 dpi (dots per inch, dots per inch) required for such applications, about 28 print dots per millimeter are printed (1 inch or 25.4 mm). The distance between two pressure points is thus about 3/100 mm. This is the distance at which the printing nozzles are arranged in their linear direction in the print head. This spacing or resolution must also be achieved by driving the printheads as the body to be imprinted is relatively moved past the printheads to maintain that resolution even in the direction transverse to the linear orientation of the printheads in the printhead.

After an adjustment of the print heads, the distance between the individual print heads must again be known with an accuracy that is clearly above the pixel resolution of 3/100 mm, for example, an order of magnitude better. Because of the mechanical play in the guides of the print heads for adjusting the radial distance relative to the center of the printing station or the body surface to be printed and other tolerances, for example. The printheads themselves, but can not achieve an accuracy of 1/1000 mm, as it would be useful or necessary for a high-quality print image with a resolution of 720 dpi.

Therefore, it is necessary to align the print images of the individual printheads after adjustment, for example. By calibration again to each other. However, this is comparatively complicated when a manual correction and adjustment of the printheads has to be carried out, in which first the deviations of the print images of the individual printheads are determined experimentally and then a corresponding fine adjustment has to be made. This time is to be reduced in particular when the printing press, for example, is integrated in a filling system for bottles.

A system for printing on containers with at least one adjustable printhead is from the DE 10 2009 033 810 A1 known. The print head is adjustable by means of an electrical control device, wherein a sensor detects the object to be printed and independently positions the print head due to this recognition. To improve the accuracy of a printed image, a mark can be printed on the container in the printing press, with which then takes place an alignment or positioning of the print head of another printing press. The marker can be detected by a camera which outputs a corresponding output signal to the print head to properly position it.

The US 2003/0174184 A1 describes a method for calibrating a printer in which grid patterns of different colors are printed on one page. If the patterns in the different colors do not lie exactly as desired on one another, interference effects occur (for example as a moiré pattern), which are evaluated in an image of the printer and allow conclusions to be drawn about a misalignment of the individual colors of the printed image.

When aligning the printheads of a postage meter disclosed the EP 0 702 334 A1 the application of a vernier.

Object of the present invention is therefore to provide an effective and automated way to detect errors in the alignment of printed images and - if possible - and automatic corrections.

This object is achieved by the method of the type mentioned by the features of claim 1. In this case, provision is made in particular for the at least two print heads, but preferably all or more selected print heads of the print station, to generate a print image that can be assigned to a print head in each case, that records the print images of the two print heads in an optical image, in particular taken with a digital camera of suitable resolution, and that the captured optical image is evaluated by image processing means to determine a deviation of the printed images of the at least two print heads to their nominal, ie predetermined by the control of the print heads, arrangement. This can be done by means of conventional evaluation methods in digital image processing.

The deviation of the print image of a print head can be determined in particular relative to the print image of a print head defined as a reference print head, which is then the other of the two aforementioned print heads. Since both printed images are digitized in the optical image, which may possibly also be composed of several individual images, a photogrammetric evaluation of the image can be carried out on the basis of known imaging properties of the camera in order to detect a deviation of the printed image from the reference printed image.

The deviation of the printed image is in particular an offset, a rotation and / or a distortion of the printed images relative to each other. These can be easily captured and evaluated by means of image processing. For this purpose, the printed image preferably has simple geometric relations, for example lines or bar patterns. Such patterns can be easily recognized and evaluated by an image analysis to display a necessary adjustment of the printheads and / or automatically or by a simple user control to enable a corrected control. When displaying the necessary adjustment can preferably be equal to the necessary adjustment measure displayed.

The adjustment dimension can also be displayed in pixel units on an operating screen, whereby a correction of the determined misalignment of the print images of individual print heads is possible instead of or in addition to an automatic correction via a pushbutton or software button on a control panel of the print station. In particular, on this A manual or manually triggered correction of individual printheads or assigned to these printheads colors by shifting by one or more pixel distances, for example, each time the push button or softkey, in the direction of the linear arrangement of the printing nozzles and / or transverse thereto in the circumferential direction of movement.

The term "linear arrangement" of the printing nozzles refers to an arrangement according to the invention of the printing nozzles in a straight line in the printing head, preferably transversely, i.e., in the direction of the printing head. at a 90 ° angle to the (relative) direction of movement of the body or the surface of the body to be printed. However, the invention is not necessarily limited to such an arrangement in a straight line. It is, for example, also possible for the pressure nozzles to be arranged at an angle to one another, wherein the measure of the distance can then be in particular a pixel spacing or a multiple thereof. Even with such an arrangement, the term "direction of the linear arrangement of the nozzles" is used for a direction transverse to the circumferential direction of movement, often oriented vertically in a printing station.

Although a preferred application of the present invention is in a printing station for rotationally symmetrical body, the spacing adjustment of the printheads may also be used for printing not rotationally symmetric body, the printheads then in the relative movement of the body to be printed and the printheads bspw. By a Electromotive control or the like. Be adjusted appropriately, so that the distance between the printing nozzles remains to be printed on the substrate surface at a suitable pressure interval.

According to an advantageous embodiment of the proposed method according to the invention, the printed image of at least one print head may contain at least one stroke, wherein from the bar preferably a scale for the captured optical image is derivable. This is preferably true for any printhead whose print image is to be scanned for print registration on the object, but especially a reference printhead that provides correct registration of the print image. In an easy way to implement this feature, the length of the printed stroke may be known. For example. For example, all or a predetermined number of printing nozzles arranged linearly in a straight line in the print head can be actuated to print the stroke. Since the distance between two linearly arranged print nozzles is known and preferably corresponds to the pixel pitch or a multiple of the pixel pitch (if several print heads of the same color would be offset relative to each other), the driver is aware of the length of the nominal bar in the print image. As a result, a calibration of the camera is at least possible in such a way that subsequently the distances of printed images can be determined. This immediately allows the determination of correction parameters for the adjustment of the various print images, which is then displayed for a manual correction or output to the control of the print heads for an automatic correction. A corresponding evaluation also applies to the following evaluations.

Another alternative or additional possibility is that the nominal distance of at least two bars in the printed image of a print head is known. In this case, one or more printing nozzles can be controlled for a certain time, which corresponds to a certain stroke length transversely to the linear arrangement of the printing nozzles, ie in the circumferential direction of movement of the body. In the direction of the linear arrangement of the printing nozzles, a defined distance between two lines can be generated by lying between two sections with controlled pressure nozzles, a section with non-driven pressure nozzles. According to the invention, both possibilities can also be combined with one another in order to produce a line pattern with a regular length dimension, similar to a scale or a folding rule. The length and / or the distance can be measured in metric units, for example. Millimeter, or in pixel sizes. The latter is particularly useful for a correction, since the alignment of the print heads or the print nozzles of the print heads must be made relative to the pixels of the print image. The deviations in the pixel size of the printed image are decisive for this. In this case, an absolute calibration of the camera can be omitted particularly well.

In principle, it is also possible to use, instead of a line, any geometrically defined structure of the printed image which permits an evaluation of the length and / or the extent of the printed images in order to produce a measure of the errors in the orientation of the printed images that can be used for a correction.

For this, the distance of two by different print heads in a nominal, i. determined by the control of the printheads, distance printed lines from the optical image and compared with the nominal distance, wherein the deviation of the printed images is given by the difference between the nominal distance and the determined distance, which is then directly usable as a correction value, the possibly lead to an automatic or manually triggered or carried out change in the control of the print nozzles of the print heads. If a correction by a changed control of the print nozzles of the print heads is not possible, the invention provides to indicate the adjustment error with the appropriate Fehleraß to facilitate manual correction.

The above-mentioned determination of a correction value is carried out according to the invention in the direction of the line of the print nozzles of the print head and transversely thereto, ie in the circumferential movement direction or direction of movement of the body relative to the pressure nozzles. In these two directions is by an appropriate control of the print nozzles of the print head of a print body usually an automatic correction of the errors of the alignment of the print images of the two print heads to each other simply by changing the control of the print nozzles of the print heads possible.

In order to obtain a measure of error, the camera detecting the optical image can, for example, as explained above, carry out a self-calibration from known dimensions of the printed image. Alternatively, a measurement of the distance of the camera to the printed surface and / or in space, for example. By measuring the camera done so that the camera is calibrated in advance.

Since a calibration of the camera both in the context of a self-calibration and in the context of an external calibration with other measuring means consuming, it is proposed according to the invention that the printed image of at least one of the printheads contains a vernier. A vernier is a length unit shortened in scale by the unit of length LE used, the scale being predefinable according to the invention. For this purpose, it is accordingly provided according to the invention that the print image of a first print head, in particular the reference print head, shows a certain number of preferably straight lines at a distance of predetermined length units LE at a defined image position. This means that an excellent line, for example the zero point, is located at a defined image position of the printed image and the further lines are arranged parallel thereto at a distance from the predetermined length unit LE. The printed image contained in the vernier shows, at the same nominal image position, ie the same image position controlled by the control of the print heads, but offset in the longitudinal direction of the strokes of the first print head, preferably immediately thereafter, the same number of strokes spaced apart by the length unit shortened by the vernier scale. This is meant that here too an excellent line, for example the zero point of the vernier, should lie at the same defined nominal image position and the further lines of the vernier are arranged parallel thereto in the length unit of the vernier.

The aforementioned features will be explained again below with reference to an example. If the scale of the vernier scale is 1:10 = 1/10, the length unit of the vernier thus corresponds to 9/10 of the predetermined length unit LE. In other words, the length unit of the vernier is thus 1/10 shorter than the length unit LE. Thus, ten lines in the length unit of the vernier thus correspond exactly to 9 predetermined length units LE, as is apparent from the following equation: $$10*\left(9/10\right)\mathrm{LE}=9\mathrm{LE}\mathrm{,}$$

Therefore, it makes sense according to the invention if the number of printed lines of both the vernier and the predetermined length dimension LE therefore corresponds to straight or at least just the reciprocal scale. At 1/10 scale, at least 10 dashes are printed at a time. According to a preferred embodiment, in each case the number of lines corresponding to the reciprocal scale can also be printed before and behind the defined image position in order to check a symmetry about the defined image position. However, it is also sufficient according to the invention if in each case half of the reciprocal scale of the vernier corresponding number of bars in front of and behind the defined image position, i. the excellent line, is available.

As part of the evaluation of the printed image contained the vernier, starting from the defined image position or the excellent stroke, the lines of the predetermined unit length LE counted until a line of the predetermined Length unit LE coincides with a bar of the vernier, and then determines the degree of displacement of the printed images. For example, when the coincidence is detected at the 7th stroke, the shift of the print image containing the vernier to the print image having the predetermined length units LE is even 7/10, wherein 10 in the above example is the scale of the vernier. The number of strokes is thus related to the scale of the vernier to indicate the deviation in the length units LE.

Therefore, this number of strokes can be used directly as a correction value if the length measure LE is known without having to measure the recorded image. It is then sufficient to simply determine the linear coincidence of two lines as part of an image recognition and to count a number of lines arranged at regular intervals. This is possible with simple means of optical image analysis, which the expert knows.

The application of the vernier presupposes that the distance between two pressure points of a print head (for the generation of the adjacent lines) can be specified in the scale of the vernier. This is possible in any case with a pressure occurring in the circumferential direction of the body to be printed, because the distance can then be effected freely by the appropriate control of the print heads relative to the current position of the body moved relative to the respective print head.

In the direction of the linearly arranged nozzles in a printhead, on the other hand, it is necessary that the pitch of the nozzles in the printhead be adjustable according to the desired scale, or that the number of nozzles correspond to the square of the reciprocal scale or a multiple thereof. An adjustability of the distance of the nozzles in the printhead comes in the Practice hardly before. At a resolution of, for example, 720 dpi, which is often used for the direct printing of bottles instead of labels to be determined later on the bottles, the second possibility, however, is quite an option for the application of a vernier also in this direction running transversely to the direction of movement. With a vernier scale of 1/10, the number of nozzles needed is 100 = 10 ² . This number is at 720 dpi so more than seven times in one inch, so that even a multiple of the reciprocal distance can be used. If one chooses successive printing nozzles for the given unit of length LE 10, then the necessary 10 lines of the predetermined unit of length LE can be printed. The length unit of the vernier then amounts to 9 pressure nozzles.

According to the embodiment of the printing station according to the invention, the linearly arranged printing nozzles of the printing head are transversely to the direction of movement of the body, i. the relative movement between body and printheads, aligned. In this case, it is possible that a distance of the printed images, in particular lines, possibly also lines of the vernier, the printheads in the circumferential direction of movement by a change in the control of one of the printheads, preferably the printhead to be adapted to the reference printhead, is corrected. Preferably, also here the print image of a print head is corrected relative to the reference print head, which defines the correct orientation of the image. The change in the control of the print heads corresponds to a given relative movement of body and printheads in the circumferential direction just the distance of the detected print images.

With this correction, the pressure points of the individual printing nozzles can be optimally superimposed for one pixel. This is particularly important for the so-called subtractive printing technique in which the primary colors cyan (C), magenta (M) and yellow (Y) are printed on top of each other to produce mixed colors. If all colors are printed on each other in maximum intensity, arises according to the theory of colors, the color black. Preferably, in addition to the inks C, M, Y, a black ink printhead (B) is also provided, which has a higher contrast than the black tone produced by overprinting the colors C, M, Y.

According to another possible correction by the present method, it is provided that a spacing of the printed images, in particular lines, possibly also lines of the vernier, of the print heads in the direction of the linear arrangement of the printing nozzles of the print head, i. transverse to the relative direction of movement of body and printheads, is corrected by a given the relative movement of the distance of the print images corresponding displacement of the driven in the linear array pressure nozzles. Again, preferably, an adaptation of the print image of a print head to the print image of the reference print head, which corrects the correct orientation of the print image.

In the above-described correction can be provided in particular that the selected pressure nozzle is selected, which has the largest overlap with the corresponding pressure nozzle of the other printhead, in particular the reference printhead, wherein according to the invention, such a correction, if necessary, only made if the overlap of the Pressure nozzles is greater than a predetermined overlap limit. Such an overlap limit is preferably between about 70% to 90%, more preferably the overlap limit of the two pixels is greater than 80%. In other words, this means that the offset of the pixels of one print head to the pixels of the other print head is less than 30% to 10%, particularly preferably less than 20%. Especially when using a CMY color system customary in such printing, a poor match of the pixel position of the individual pixels leads to a poor overlap of the colors and thus to poor print quality.

If the overlap limit value can not be met by changing the control of the print nozzles of a printhead, it may also be possible to provide an indication of the offset to be adjusted for the assembly correction, which can then be done manually or automatically by machine control. For a mechanical control of the mechanical displacement correction, a correspondingly necessary displacement device would be required.

In order to be able to print an image completely in the intended height (ie in the direction of the linear arrangement of the printing nozzles), it is proposed according to the invention that the linear arrangement of the printing nozzles in a printing head has a larger number of pixels than the image to be printed on the body, so that when the print heads are displaced in the longitudinal direction of the linear arrangement, a displacement of the driven print heads can take place without the image fringing or not being completely printed at the upper or lower edge. Preferably, the overhang of the pixels in each linear array at both the top and bottom edges (based on the best possible set reference printhead) is approximately equal to the tolerance in those directions as the printheads are adjusted.

Another possibility for error detection according to the invention is a rotation of two printheads relative to each other about an axis extending in the printing direction of the printing nozzle axis by determining an angle between a preferably straight line of a printhead, in particular the reference printhead, and another preferably straight line of the other printhead , In particular, to be aligned printhead determine. The determined angle can then be output for an assembly correction, which can be carried out automatically or manually, assuming a correspondingly controlled adjustment possibility of the printing station. In which Printing the straight lines to convey the rotation of two print heads may preferably be provided that all print nozzles of a print head are controlled.

The same applies to the possibility of determining, as described below, a tilting of a printhead about an axis perpendicular to a plane defined by the direction of the linear arrangement of the printing nozzles in the printhead and the printing direction, i. at a tilt of the print head in the direction of the body to be printed or away from this. In this determination, the stroke length of a preferably straight line and / or the position of the bar relative to the stroke of a non-tilted printhead, in particular of the reference printhead, and / or a pressure range provided on the body, for example, relative to the position of the Detection device or camera of the optical image is determined.

According to a further development of the proposed method can be provided that a reference print head, which defines the correct orientation of the printed image on the printed body is determined by each printhead with preferably all printing nozzles on the body prints a straight line, which to the straight lines of the other printheads are staggered in the printed image, and by evaluating the printed lines on the body in an optical image, checking the stroke length, the position of the stroke on the body, and / or the orientation of the stroke on the body. The print head with the desired setting to the next coming print image is selected as a reference print head, directed towards the all other printheads or from which the printed images of the other printheads are evaluated.

The invention further relates to a printing station for printing a three-dimensional body with a plurality of print heads each having printing nozzles, wherein the print heads are arranged distributed in the printing station around the body. At least two are provided, but preferably at least four or a multiple of four printheads are provided, each printhead for printing the body may be provided with one color. The color of the ink printed by the printing nozzles of the printing heads may preferably be cyan (C), yellow (Y), magenta (M) and black (B). According to the invention the distance of the print heads to the body is adjustable to allow the printing of different sized body in the printing station, wherein the printing nozzles are aligned in their printing direction to the center of the body.

The printing station also has in its center a receptacle for the body, which is designed to be movable relative to the printheads. In particular, the receptacle is designed to be rotatable about a preferably also coinciding with the symmetry axis of the body axis of rotation. For example. For example, the three-dimensional body to be printed may be a plastic or glass bottle.

Furthermore, the printing station has a control unit which controls the printing heads or pressure nozzles and has a computing unit which is set up to apply pressure to the three-dimensional body through at least two of the print heads together by means of a suitable control of the print heads or pressure nozzles during a rotation of the body relative to the printheads. The fact that the print heads of the image print together means that the print heads or the print nozzles of the various print heads are controlled in such a way that, when actuated, a print nozzle prints a pixel on the body and the image is composed by printing many pixels on the body, wherein, for example for producing mixed colors Pixels of printing nozzles of different colors can also be printed on top of each other.

According to the invention, it is provided that the printing station is assigned a recording device with optical image processing means. This receiving device can, for example, have a camera which is accommodated with the printing station or preferably has a camera arranged separately. Furthermore, the arithmetic unit of the printing station, which according to the invention also comprises the arithmetic unit of an integrated or separate recording device with the image processing means, is set up to carry out the above-described method or parts thereof.

The printing nozzles may be arranged in the printing heads linearly, preferably parallel to the axis of rotation of the receptacle for the body. Possibly. but can also be provided, for example, a staggered arrangement of the printing nozzles in a print head, wherein the distance of the printing nozzles both in the linear arrangement and in the stepped arrangement in the direction of movement as well as transversely to the direction of movement can each correspond to a pixel of the image.

Further advantages, features, applications of the present invention will become apparent from the following description of an embodiment and the drawing. All described and / or illustrated features alone or in any combination form the subject matter of the present invention, also independent of their summary in the claims or their back references.

Fig. 1

eine schematische Ansicht einer erfindungsgemäßen Druckstation in der Aufsicht von oben;

Fig. 2

schematisch eine Ansicht eines Druckkopfes von vorne mit der linear ren Anordnung der Druckdüsen;

Fig. 3a bis 3d

übereinander Druckbilder von Druckköpfen bei verschiedenen Fehlern in der Ausrichtung der Druckbilder jeweils zweier Druckköpfe;

Fig. 4a bis 4d

schematisch die fehlerhaften Anordnungen der Druckköpfe, welche zu den Druckbildern der Fig. 3a bis 3d führen und

Fig. 5

ein erfindungsgemäß ein einen Nonius enthaltendes Druckbild.

Show it:

Fig. 1

a schematic view of a printing station according to the invention in the top view;

Fig. 2

schematically a view of a printhead from the front with the linear ren arrangement of the printing nozzles;

Fig. 3a to 3d

on top of each other print images of printheads with different errors in the alignment of the print images of two printheads;

Fig. 4a to 4d

schematically the faulty arrangements of the printheads, which to the print images of the Fig. 3a to 3d lead and

Fig. 5

a printed image containing a vernier according to the invention.

In Fig. 1 is schematically shown in plan view from above a printing station 1, with a plastic bottle, for example. A PET bottle, as a three-dimensional body 2 - or another preferably, but not necessarily rotationally symmetrical body - can be printed. For this purpose, the bottle 2 is received in the center of the printing station in a receptacle 3 such that the symmetry axis of the bottle 2 and a rotational axis of the receptacle 3 coincide. Through the receptacle 3, the bottle 2 can be moved or rotated in the direction of rotation 4 on the print head 5 of the printing station over. During the rotation of the bottle 2 past the printheads 5, ink is selectively printed on the circumference of the bottle by a control of printing nozzles 6 of the printheads 5, which is formed by a conventional control device with a computing unit, so as to print a pixel image on the circumference of the bottle Bottle 2 to produce.

It will be understood that the three-dimensional body referred to as bottle 2 in the present example which is to be printed in the printing station 1 can also be another printable body.

In order to accommodate bottles 2 of different diameters in the printing station 1, the printheads 5 are adjustable in the direction indicated by an arrow adjustment 7 in the radial direction to the center of the receptacle 3 and the axis of symmetry of the bottle 2, so that the distance of the pressure nozzles 6 to the bottle 2 can each correspond to the required pressure interval. The print heads 5 and the print nozzles 6 of the print heads 5 are respectively aligned with the center of the receptacle 3 or bottle 2 so that they print in a rotationally symmetrical, round body 2 in the normal direction on the peripheral surface of the body 2 (printing direction). According to the invention, the printing direction therefore runs perpendicular to the tangent or tangential plane of the surface of the body 2 to be printed at the printing point (pixel).

In the in Fig. 1 In the illustrated printing station, there are provided two groups of four printheads 5 each printing one of cyan (C), yellow (Y), magenta (M) and black (B). By the colors cyan, yellow and magenta can be on the body 2 by a so-called subtractive print all colors in the print image produce when printed in one pixel, the printed by the printing nozzles 6 of the various printheads 5 colors are each printed according to a predetermined mixing ratio. The additional black ink (B) is used to display a high-contrast black tone. This printing principle is basically known from two-dimensional planographic printing applications.

Due to the adjustability of the print heads 5 in the adjustment direction 7, the problem arises in the three-dimensional printing application that the adjustment Tolerances, for example. Due to the game in guides the printheads, has, so that the printed images of the print heads 5 relative to each other on the bottle 2 may have a misalignment. This leads to a qualitatively poor overall print image.

In order to be able to detect such an incorrect orientation of the printed images, a camera 8 assigned to the printing station 1 is provided, which is arranged at a suitable point in the transport path of the bottle 2 or the body 2 as indicated schematically by the dotted arrow "Transport" and Print image of the printheads 5 on the bottle 2 receives. The assignment of the camera 8 to the printing station 1 means that it is known which bottle 2 or which body 2 originates from which of possibly several printing stations of a printing press. In principle, it would also be possible to integrate a camera into a printing station. However, this is disadvantageous for reasons of space and time. Therefore, it is preferably proposed to arrange the camera 8 at a suitable location in the transport path, at which the image printed on the bottle 2 or the body 2 in the printing station 1 can simply be optically detected.

The captured optical image is then evaluated by image processing means to detect a deviation of the print images from at least two printheads 5 to their nominal arrangement.

This will be explained in detail below, wherein first a preferred arrangement of the printing nozzles 6 of a printhead 5 with reference to Fig. 2 is explained, which shows a view of a print head 5 from the center of the receptacle 3 of the printing station 1 forth against the printing direction.

A printhead 5 has linearly at a distance of the desired printing resolution, ie arranged at pixel pitch P pressure nozzles 6, wherein in Fig. 2 the clarity For the sake of simplicity, only some of the actual pressure nozzles 6 are shown. At a resolution of 720 dpi, there are about 28 printing nozzles per millimeter, so that the pixel pitch P between two printing nozzles 6 is about 3/100 mm.

Due to the adjustability of the print heads along the adjustment direction 7 can basically in the Fig. 3a to 3d errors in the alignment of printed images 9 occur. In the Fig. 3a to 3d In each case, the print images 9 of two print heads 5 are printed one above the other, which, when the print images of all print heads 5 are correctly aligned, result in a high-quality image.

In Fig. 3a are each shown in the direction of the linear arrangement of the printing nozzles 6 in the print head 5 printed straight lines 10, 11 on the printed image, where line 10 is the printed image of a first printhead 5, which is referred to as a reference printhead, to which the other printheads 5 (FIG. respectively whose lines 11) are to be aligned.

The bar 10 is printed on the bottle 2 at the position A in the circumferential direction of movement of the printed image 8. Dash 11 of the other print head 5 is found in the print image 9 at position B, although the drive of the print heads 5 nominally assumes that the positions A and B coincide (or have a defined offset, which is not shown here). Here, therefore, there is an error ΔA of the orientation of the print images 9 of the reference print head and the other print head in the circumferential movement direction.

Fig. 3b shows a corresponding print image 9 with a first line 10 of the reference print head, which lies in the drawing upper end at the position C. The bar 11 of the other print head 5 lies in the direction of the linear Arrangement of the pressure nozzles 6, ie in Fig. 3b offset in the height direction at the position D. So here is a height offset Δh of the two printheads 5 before.

Due to a calibration of the camera 8, which has taken the print images 9, the height offset .DELTA.h and the error .DELTA.A in the orientation in the circumferential direction of movement can be determined quantitatively, either in metric units or optionally also pixel pitches.

In the in Fig. 3c illustrated print image 9, the stroke 11 of the other print head relative to the line 10 of the reference print head is rotated by the angle α. Such a printed image 9 results when two print heads 5 are rotated relative to each other about an axis extending in the printing direction of a pressure nozzle 6 axis. The angle α can also be determined quantitatively from the printed image.

Fig. 3d Finally, in the printed image 9 shows a line 10 of the reference print head and a shortened line 11 of the other print head, but should have nominally the same size. The corresponding change in length ΔL can also be determined quantitatively in the printed image. Such a print image 9 is formed when a print head 5 is tilted relative to the reference print head about an axis perpendicular to a plane defined by the direction of the linear array of the print nozzles 6 in the print head 5 and the print direction. As a result, the bar 11 of the other (tilted) print head 5 is shortened by the length change ΔL relative to the bar 10 of the reference print head.

In the printed images 9 according to Fig. 3a to Fig. 3d appropriate Fig. 4a to Fig. 4d are the corresponding mounting error of the printheads 5 relative to a reference print head marked with the term "reference".

Fig. 4a shows from an on the three-dimensional body rotated in the receptacle 3, an offset .DELTA.A of the print heads 5, which should have an offset of .DELTA.A = 0 according to the nominal control. This offset .DELTA.A thus corresponds to the difference in the control of the various print heads 5, the just the error .DELTA.A according to Fig. 3 generated. The offset is, for example, an angular offset of the print heads due to the existing tolerances. In the case of such an assembly error occurring in the circumferential direction, the absolute distance ΔA between two lines or lines 10, 11 produced by two different print heads 5 (cf. Fig. 3a ) detected by a camera. This value is compared with the nominal value with which the two lines 10, 11 were printed. The resulting difference ΔA gives the correction value with which the beginning of the printing of the second stroke has to be delayed or advanced in order to print the two printing dots in one image. The corresponding correction of the drive thus corresponds exactly to the offset ΔA. The camera image can thus be used with the corresponding evaluation to automatically calibrate the system after setting to a print diameter.

In Fig. 4b is a height offset .DELTA.h of a printhead 5, which the bar 11 of the other printhead in Fig. 3b has generated relative to the reference printhead. With a displacement of the print heads 5 in the direction in the linear arrangement of the printing nozzles 6 can from the Fig. 3b the height offset .DELTA.h be determined. This height offset can be compensated for by the fact that in the other print head 5 printing nozzles 6 of the print head 5 are actuated relative to the reference print head 5 at the printing of the image, which correspond precisely to the height offset .DELTA.h in their spacing in the linear arrangement or to this next come. This can be provided that this correction is made only if the difference between the measured distance Δh and the nearest multiple of a pixel pitch does not exceed an overlap threshold, which is preferably between 70 to 90% of a pixel size, to avoid over-skewing the pixel images.

This will be explained again below using an example. Since this height offset Δh is a tolerance compensation, it can be assumed that a tolerance of a few 1/10 mm can be maintained. 1/10 mm corresponds to 2.8 print dots at a print resolution of 720 dpi. The height offset Δh is taken from the image evaluation. The printed image is accordingly shifted, depending on the direction of the determined deviation, by the determined number of pressure points, which permits a match of the pressure of the other print head 5 with the reference print head 5. The shift can be done automatically electronically by moving the image down by 5 pixels in the control of the individual print nozzles with a deviation of 5 pixels downwards.

Then no longer print nozzles with the nozzle numbers 1 to n, as in the reference print head but, the print nozzles with the nozzle numbers 5 to n + 5. This has the consequence that the printed image 9 may not have the full possible width of all given by the linear arrangement of the printing nozzles 6 in the printheads 5 dimensions. However, with tolerance compensation of the order of a few 1/10 mm, this is tolerable because the advantage of automatic correction and automatic compensation for better image quality and time savings outweigh the workload for the operator.

Fig. 4c simulates the assembly error in a relative to the reference print head 5 twisted other printhead 5. In the twisting remains the length of Dashes 10, 11 on the body 2 get how Fig. 3c shows. However, line 11 is inclined in the circumferential direction on the surface of the body 2 to be printed by a small angle α. This angle can be determined by the printed image Fig. 3c be recorded and quantified so that the value to be set can be indicated to the operator. However, in this case, the alignment usually has to be done mechanically, since an automatic correction of the pressure points would only be possible if a staggered triggering of individual pressure nozzles 6 of the print heads 5 would be possible or the print head could be rotated automatically controlled. However, this is not the case with conventional printheads 5. However, there is still the advantage that the correct value of the necessary fine adjustment can not be determined by tests, but can be read directly from the correction.

Fig. 4d Finally, FIG. 1 shows a different printhead 5 tilted with respect to the reference printhead 5. Due to the tilt, the distance to the bottle 2 changes for the different print nozzles 6 of the tilted printhead 5, so that the bar 11 of the other printhead 5 relative to the bar 10 of the reference printhead 10 is shortened by a change in length ΔL. This shortening ΔL of the bar 11 can also be determined from the optical image according to FIG Fig. 3d can be determined, wherein the operator from the degree of reduction .DELTA.L and the direction of the shortening based on the stroke 10 of the reference printhead can easily make a correction. Also in this case, automatic correction with simple, linearly arranged pressure nozzles 6 in a print head 5 is not readily possible.

Fig. 5 shows a nonius 12-containing print image 9, in which the reference print head has printed lines 10 at a distance of a predetermined length unit LE at or starting from a defined image position 13 of the print image 9. Another printhead has nominal at the image position 13, but in reality offset from the error ΔA starting at the real image position 14, a vernier printed on a scale of 1:10 based on the defined unit of length LE. This means that the distance of the lines 11 of the other printhead or the vernier 12 is shortened by 1/10 LE or is only 9/10 LE.

In the printed image 9, it can be easily recognized that the real image position 14 does not coincide with the defined or nominal image position 13. The measure of the error ΔA is quantitatively but difficult to read. In the quantitative determination, even without calibration of the camera 8, the printed vernier 12, of which in Fig. 5 only one relevant part of the explanation is shown.

It can also be easily determined with simple image processing means that the lines of the vernier and the reference print head on the seventh stroke from the defined image position match 13th This means that the error ΔA is just 7/10 of the unit length LE (7 units in the 1/10 scale). Thus, knowing the nominal unit of length LE, for example a pixel pitch or a multiple of a pixel pitch, the error ΔA can be determined simply from the printed image.

LIST OF REFERENCE NUMBERS

1

printing station

2

three-dimensional body, bottle

3

admission

4

direction of rotation

5

printhead

6

pressure nozzles

7

adjustment

8th

camera

9

Print images or optical image of several print images

10

Line of the reference print head

11

Stroke of the other printhead

12

vernier

13

defined picture position

14

real picture position

P

pixel pitch

.DELTA.A

Error in alignment in direction of movement

.delta.h

Height offset in the direction of the linear arrangement of the pressure nozzles

.DELTA.L

change in length

α

Angle of a rotation of the printheads

Claims (9)

1. A method for determineing errors (ΔA, Δh, ΔL, α) in the alignment of print images (9) of at least two printing heads (5) in a printing station (1) for printing a three-dimensional body (2), wherein the printing heads (5) are arranged distributed about the body (2) and are aligned in the printing direction perpendicularly to the surface tangent of the body (2), wherein the printing on the body (2) takes place by at least two of the printing heads (5) together, by means of a suitable activation of the printing heads (5) during a movement of the bodies (2) relative to the printing heads (5), wherein the spacing of the printing heads (5) to the body (2) is adjustable, wherein the at least two printing heads (5) each create a print image (9) that can be assigned to a printing head (5), wherein the print images (9) of the two printing heads are captured in an optical image and wherein the captured optical image is evaluated through image processing means in order to determine a deviation of the print images (9) of the at least two printing heads (2) from their nominal arrangement, characterized in that the print image (9) of at least one of the printing heads (5) comprises a vernier (12)

   - wherein the print image (9) of a first printing head (5) shows a certain number of marks (10) at the spacing of predetermined length units (LE) in a defined image position (13) and the print image (9) of the other printing head (5) containing the vernier (12) shows the same number of marks (11) offset in the same nominal image position (13) in longitudinal direction of the marks (10) of the first printing head spaced by the length units (LE) shortened by the scale of the vernier (12) and

   - wherein starting out from the defined image position (13) the marks of the preset length unit (LE) are counted until a mark (10) of the predetermined length unit (LE) coincides with a mark (11) of the vernier (12), and from this the extent of the displacement of the print image (9) is determined, and in that a spacing of the print images (9) of the printing heads (5) in the direction of the linear arrangement of the printing nozzles (6) of the printing head (5) is corrected by a shift of the printing nozzles (6) activated in the direction of the linear arrangement corresponding to the spacing of the print images (9).
2. The method according to claim 1, characterized in that the print image (9) of at least one printing head comprises at least one mark (10, 11), wherein from the mark (10, 11) or the marks (10, 11) a scale for the image can be derived.
3. The method according to any one of the preceding claims, characterized in that the spacing of two lines (10, 11) printed by different printing heads (5) at a nominal spacing from the optical image is determined and compared with the nominal spacing, wherein the deviation of the print images (9) is given by the difference between the nominal spacing and the determined spacing, which is used as correction value.
4. The method according to any one of the preceding claims, characterized in that a spacing of the print images (9) of the printing heads (5) in circumferential movement direction is corrected by a change of the activation of one of the printing heads (5).
5. The method according to any one of the preceding claims, characterized in that during the shifting of the printing nozzles activated in the direction of the linear arrangement the particular activated printing nozzle (6) which has the greatest overlap with the corresponding printing nozzle (6) of the other printing head is selected.
6. The method according to any one of the preceding claims, characterized in that a rotation of two printing heads (5) relative to one another about an axis running in printing direction of a printing nozzle (6) is determined by determining an angle between a mark (10) of the one printing head and a mark (11) of the other printing head.
7. The method according to any one of the preceding claims, characterized in that a tilting of a printing head (5) about an axis standing perpendicularly on a plane which is defined by the direction of the linear arrangement of the printing nozzles (6) in the printing head (5) and the printing direction is determined by a measurement of the mark length of a mark (11) and/or the position of the mark (11) relative to the mark (10) of a non-tilted printing head (5) and/or a printing region provided on the body (2).
8. The method according to any one of the preceding claims, characterized in that a reference printing head, which defines the correct alignment of the print image (9) on the printed body (2) is determined, in that each printing head (5) with all printing nozzles (6) prints a mark (10, 11) spaced on the body (2) and the printed marks (10, 11) on the body (2) are evaluated in an optical image wherein the mark lengths, the position of the mark (10, 11) on the body (2) and/or the alignment of the marks (10, 11) on the body (2) are checked.
9. A printing station for printing a three-dimensional body with multiple printing heads (5) each comprising printing nozzles (6), wherein the printing heads (5) are arranged distributed about the body (2), wherein the spacing of the printing heads (5) to the body (2) is adjustable and wherein the printing nozzles (6) are aligned in their printing direction towards the centre of the body (2), and with a mounting (3) for the body (2) in the centre of the printing station (1), which can be moved relative to the printing heads (5), and with a control device with computer unit forming the activation of the printing heads (5) which is equipped in order to create the pressure on the body (2) through at least two of the printing heads (5) together by means of suitable activation of the printing heads (5) during a movement of the body (2) relative to the printing heads (5), characterized in that the printing station (1) is assigned a recording device (8) with optical image processing means and in that the computer unit of the printing station (1) is equipped for carrying out the method according to any one of the claims 1 to 8.

Images