Method for operating a printing system, printing system and computer program product

Abstract

A method is specified for operating a printing system (2) which has a first printer (4) and a second printer (6), wherein a web (10) runs through the printing system (2) at a web speed, wherein a first print image (8) is printed onto the web (10) by the first printer (4) and subsequently the first print image (8) is overprinted with a second print image (12) by the second printer (6), wherein the first print image (8) undergoes longitudinal distortion between the first printer (4) and the second printer (6), wherein the first printer (4) is assigned a first sensor (18) which generates a first sensor signal (G1) which is dependent on the web speed of the web (10) in the first printer (4), wherein the second printer (6) is assigned a second sensor (22) which generates a second sensor signal (G1) which is dependent on the web speed of the web (10) in the second printer (6), wherein on the basis of the two sensor signals (G1,G2) a scaling parameter (P) is determined with which the longitudinal distortion is compensated.,

Description

The invention relates to a method for operating a printing system, to such a printing system and to a computer program product in connection therewith.In a printing system, a web, for example a paper web, is printed with a print. The print is regularly composed of a plurality of printed images printed one after the other. This is also referred to as "overprinting" or "overprinting". For example, a colored printed image is selectively overprinted with a varnish. In the case of such overprinting, the most correct possible alignment (registration) of the various printed images relative to one another is desirable.The various print images are each printed using a separate printer. Between the various printers, the web traverses a certain path during which the web changes its dimension due to drying and / or moistening, namely its length in the longitudinal direction (longitudinal distortion) and its width in the transverse direction (transverse distortion). Accordingly, a printed image printed with a printer may be distorted and then will not be printed correctly with a subsequent printed image downstream of the next printer. The problem is exacerbated when material parameters (e.g., grammar) and / or operating parameters (e.g., web speed) change dynamically.Against this background, it is an object of the invention to improve the overprinting of a first printed image with a second printed image. The overpressure should be as precise as possible. In particular, a dynamic compensation of the distortion which the first printed image experiences until the overprinting with the second printed image should also be possible. For this purpose, an improved method for operating a printing system is to be specified. In addition, a corresponding printing system and a computer program product are to be specified.The object is achieved according to the invention by a method with the features according to claim 1, by a printing system with the features according to claim 13 and by a computer program product with the features according to claim 14. The explanations in connection with the method also apply analogously to the printing system and the computer program product and vice versa. If steps of the method are implicitly or explicitly specified below, advantageous configurations for the printing system result from the fact that it is designed to carry out one or more of these steps. For this purpose, the printing system has, in particular, a correspondingly designed control unit.The method is used for operating a printing system which has a first printer and a second printer. In operation, a web passes through the printing system at a web speed. The web speed varies as the case may be along the printing installation. A first printed image is printed onto the web with the first printer and subsequently the first printed image is overprinted with a second printed image (subsequent printed image) with the second printer. This is referred to as "overprint", a fixed relationship (target position) of the two print images relative to one another being predefined. The two print images together form a print.Preferably, both printers are digital printers. Preferably, the first printer is a multi-color digital printer, in particular a CMYK inkjet printer, and the second printer is a digital lacquer printer. Such a configuration is also assumed below without limiting generality. A plurality of different colors are printed with the multicolor printer; an in particular transparent varnish is printed with the varnish printer. In both printers, printing is performed in particular position-selectively (digitally), i.e. in particular in the lacquer printer, not only a simple full-area application takes place, but the lacquer is printed only selectively, in particular where at least one colour has already been printed by the multicolor printer. The actual printing of the respective printed image in the respective printer takes place at a so-called printing location (first printing location of the first printer and second printing location of the second printer). The printing location is the location in the printer at which the ink / lacquer is applied to the web to produce the printed image. The printing point therefore does not denote a point on the web, but rather a point, in particular a fixed point, within the printing installation, through which the web is conveyed.The web is in particular a paper web. Both printers are arranged in-line, i.e. the web is guided from the first printer without interruption to the second printer and optionally also additionally processed between the two printers. The web generally has a longitudinal direction which corresponds to a conveying direction of the web and is also referred to as "machine direction". The two printers are in particular fundamentally independent and / or separate machines. The two printers are in particular spatially separated from one another, especially spaced apart from one another in the longitudinal direction.The first printed image experiences a longitudinal distortion, i.e. a distortion in the longitudinal direction, between the first printer and the second printer. This also results in an actual position which deviates from the desired position. Typically, the printed image also experiences a transverse distortion, which is regularly also greater than the longitudinal distortion. In the present case, however, the focus is on the longitudinal distortion, which can be up to 1 mm / m, for example. Distortion is generally understood to mean shrinkage or stretching. In the following, longitudinal shrinkage is assumed without limiting generality. In an expedient embodiment, the longitudinal distortion results in particular in that at least one drying and / or moistening unit is arranged between the first printer and the second printer, with which unit the web (and thus also the first printed image) is dried and / or moistened.A first sensor is assigned to the first printer, which generates a first sensor signal, which is a measure of the web speed of the web in the first printer. Similarly, a second sensor is assigned to the second printer, which generates a second sensor signal, which is a measure of the web speed of the web in the first printer. "Assigned" is preferably understood to mean that the respective sensor is part of the respective printer. Each printer accordingly has a sensor with which a parameter is measured which is indirectly or directly dependent on the web speed in order to output a corresponding sensor signal as a function thereof.In principle, a configuration is suitable in which the web speed is measured directly, for example optically by means of a laser sensor or a camera, or mechanically or in some other way. A configuration is particularly advantageous in which the first printer is assigned a first roller which guides the web, wherein the first sensor is a first encoder which measures a rotation of the first roller and generates the first sensor signal in the process, and wherein, analogously to the second printer, a second roller is assigned which guides the web, wherein the second sensor is a second encoder which measures a rotation of the second roller and generates the second sensor signal in the process. In this embodiment, the web speed is thus measured by means of transmitters on rollers for guiding the web. A respective roller is therefore also referred to as a measurement point. The encoder is expediently a rotary encoder, for example an encoder. The way in which the measurement is carried out exactly and which measurement principle is applied is rather inferior, however. More importantly, the sensor effectively measures the web speed, i.e., the longitudinal speed of the web, in the particular printer. In the mentioned embodiment with encoders, the web speed at the respective roller is then measured by measuring the rotation of the respective roller. The encoder signal is thus a measure of the web speed at the respective roller. Without limiting generality, the aforementioned embodiment with encoders on rollers is assumed below; the two sensor signals are then also referred to as encoder signals. The explanations also apply analogously, however, to any other measuring principle with which two sensor signals are generated as a respective measure for the web speed at two different measuring points in the printing installation.A scaling parameter (also a scaling factor) is now determined on the basis of the two sensor signals, more precisely a value of the scaling parameter at a given point in time. The scaling parameter is then used to compensate for the longitudinal distortion, i.e. at least partially or completely reduce it. The scaling parameter is determined, for example, as the difference or ratio of the two sensor signals or is derived therefrom, for example, by means of a table or a function. There are various possibilities for compensating the longitudinal distortion by means of the scaling parameter, some of which are explained in detail below; it is first of all more important that, in the present case, instead of markings on the web, for example, the two sensor signals are used to correct the longitudinal distortion. The scaling parameter is actually also a measure of the longitudinal distortion, so that this is also measured in particular.A core idea of the invention is in particular the use of sensors, preferably encoders, of the printing system for correcting (compensating) the longitudinal distortion experienced by the first printed image on its path to the second printer. This path is regularly long and is e.g. in the range from 10 m to 100 m. Without correction of the longitudinal distortion, the second printed image would be printed on for a longer or shorter time than the first printed image, so that a non-optimum overlap results. For example, with a longitudinal distortion of 0.5 mm / m (=0.05%), with a print 5 m long and with an optimum overlap at the beginning of the two print images at their end, there would be an offset of 2.5 mm. In the present case, it has now been recognized that the longitudinal distortion also results in a difference in the web speed in the first printer and in the second printer, i.e. the web speed varies along the printing system. In the case of longitudinal shrinkage, the web speed is lower in the second printer than in the first printer. The web speed can be measured with a corresponding sensor (e.g. transmitter on one of the rollers of the printing system). As a result, in particular a longitudinal distortion control is implemented at least on the basis of two sensor signals at two different measurement points. Expediently, the respective sensor is located as close as possible to the respective printing location, i.e. is at least assigned to the respective printer. The closer the respective measurement point is to the respective printing point, the more accurately the longitudinal distortion can be compensated. In principle, however, it is sufficient first if as little longitudinal distortion as possible or only little longitudinal distortion takes place between the measuring point and the respective printing point and if in any case there is in particular neither drying nor moistening of the web.The two rollers are in each case in particular also driven, i.e. each have its own drive. The two drives are independent of each other. The respective sensor in the form of a transmitter is then seated, for example, on the drive of the corresponding roller or on a central axis or rotational axis of the roller. Optionally, both drives and thus also the web speed on a respective roller are controlled by a web tension control. In this case, the printing system has a measuring roller between two driven rollers in each case, with which the web tension is measured in order then to control one of the driven rollers in a suitable manner as a function thereof. The invention can also be used in a printing system without web tension control.Preferably, the first sensor and the second sensor are each coupled to the web without slippage (also: "connected"). The respective sensor signal is thus a particularly accurate measure of the web speed. Especially in the case of two rollers and two encoders, the web is accordingly guided with the first roller and with the second roller in each case free of slippage. This improves the accuracy of the compensation of the longitudinal distortion, since it is now ensured that the web speed is measured as exactly as possible. It is particularly expedient if the web is also guided between the printing point and the respective measuring point overall free of slippage. A slip-free coupling of the encoder to the web is realized, for example, in that the roller has a correspondingly suitable coating. Alternatively or additionally, the wrap of the web around the roller is selected such that self-locking results. For this purpose, in particular a wrap of more than 180°, in particular of at least 270°, is suitable.A configuration is preferred in which the first roller is a printing cylinder of the first printer and / or the second roller is a printing cylinder of the second printer. The printing cylinder is that roller of the printer over which the web is guided during the printing of the printed image. The measurement point and the printing point are then identical, which gives maximum accuracy in compensating for the longitudinal distortion. Alternatively, however, it is also possible to use a different roller instead of the printing cylinder, which roller is, however, expediently located within the printer and at least lies along the web as close as possible to the printing cylinder and thus as close as possible to the printing point. A suitable roller is, for example, one which defines a wrap of the web around the printing cylinder. Such a roller is arranged immediately before and after the printing cylinder in order to bring the web upstream of the printing cylinder to the latter and to remove it from the latter again downstream thereof and thereby fix the wrap (defined by an angle) of the printing cylinder. Quite generally, however, any roller is suitable which is situated as close as possible to the printing point along the web and which is also coupled as smoothly as possible to the web as far as the printing point.Each of the printers preferably has a number of printing nozzles in order to print the respective printed image. The printing nozzles are arranged in particular in an arc shape around the printing cylinder of the respective printer. By "a number of" is meant "one or more", typically a respective printer has a plurality (about 10 to 500) of print nozzles. The printing nozzles are then controlled in such a way that they print the respective print image line by line, for which purpose the printing nozzles are activated ("fired") in a specific cycle which is dependent on the web speed in order to be able to print the respective print image always with the same length at different web speeds. Therefore, a sensor, in particular a transmitter, is typically already present on the respective printing cylinder, with which the timing of the printing nozzles is then controlled accordingly. The sensor signal of this sensor accordingly determines at which points in time the printing nozzles are activated in each case in order to generate a single point of the printed image. This sensor is now also advantageously used to compensate for the longitudinal distortion as described, so that no additional hardware is required for this purpose and already present components of the printing system are used to implement the invention. In a correspondingly suitable configuration, a timing of printing nozzles for printing on the first printed image is then controlled with the first sensor signal and a timing of printing nozzles for printing on the second printed image is controlled with the second sensor signal.For the timing of the printing nozzles, a precision transducer is expediently used as sensor, for example with an accuracy in the range from 1 / 10 mm to 1 / 100 mm. Such a high accuracy is not required, however, for the compensation of the longitudinal distortion described here, a lower accuracy is sufficient.Advantageously, the scaling parameter modifies a control signal with which a number of printing nozzles (in particular the printing nozzles already mentioned above) of one of the two printers is controlled. As a result, the corresponding printed image is printed with distortion in the longitudinal direction, in particular corresponding to the longitudinal distortion of the first printed image, so that ultimately an optimum overprint results. The control signal controls the printing nozzles in a time-dependent manner such that they print the printed image line by line on the web running past. The control signal thus determines the timing of the printing nozzles already described above.The control signal is, on the one hand, in particular dependent on the sensor signal which specifies the timing, i.e. when a respective line is printed, and is adapted to the web speed. On the other hand, the control signal is also dependent in particular on print data (image data) which define the print image, i.e. which print nozzle is activated in a given line of the print image in order then to print a corresponding pixel. The control signal (first control signal) for the print nozzles of the first printer is thus formed from the first sensor signal and from the print data for the first print image. Analogously, the control signal (second control signal) for the printing nozzles of the second printer is formed from the second sensor signal and from the print data for the second print image. It is also conceivable that only one of the two printers has printing nozzles, since it is ultimately sufficient if one of the two print images is scaled.To modify the control signal, the scaling parameter may be applied at one or more different locations. In a first suitable embodiment, the control signal is generated starting from one of the two sensor signals, which is scaled with the scaling parameter. The scaling parameter is therefore applied to one of the sensor signals, e.g. the second sensor signal, in order to scale this sensor signal and thus ultimately also the control signal. This has the advantage that the print data can remain unchanged. Expediently, however, the sensor signal is only scaled after the scaling parameter has been determined on the basis thereof. In a second suitable embodiment, at least one of the print images is defined by print data (as already mentioned), which are then scaled with the scaling parameter and by means of which the control signal is then generated. The scaling parameter is thus applied to the print data and the sensor signal can thus remain unaffected. The current longitudinal distortion can thus be immediately detected on the print data, which are also displayed in parallel in real time, for example. The first and the second configuration can in principle also be combined, for example by the sensor signal and the print data being in each case only partially scaled in order then to optimally compensate for the longitudinal distortion in combination. An embodiment is also conceivable and suitable in which the sensor signal and the print data are retained unchanged per se and only the control signal formed from these is scaled with the scaling parameter.In the present case, the scaling parameter is used in particular to adapt the lengths of the two print images to one another in order to compensate for the longitudinal distortion. Any fixed longitudinal offset, on the other hand, i.e. a constant displacement in the longitudinal direction by a value fixed along the path, can therefore in particular not be corrected. For this purpose, a mark is expediently used which is printed, for example, with the first printer and is used by the second printer as a start signal (trigger signal) for printing on the second printed image. The sensors, especially in one embodiment as a transmitter, are, on the other hand, too inaccurate for generating a start signal, especially because of the long path for the web between the two printers. By means of the mark, a somewhat optimum overprint is ensured only at the beginning of the printing. The scaling parameter then prevents the two print images from moving apart progressively along the print, i.e. so to speak an increasing longitudinal offset over the print. The mark itself is typically too small to detect longitudinal distortion, but detection and corresponding compensation of transverse distortion is possible with the mark rather because the transverse distortion is typically a factor of 3 to 10 greater than the longitudinal distortion.Suitably, depending on at least one material parameter of the web and / or at least one operating parameter of the printing system, pre-scaling is predefined for one of the two print images, preferably the first print image, so that this print image is printed in a pre-scaled manner. This is based on the consideration that the necessary value for the scaling parameter can be estimated roughly in advance as a function of the material parameters and / or operating parameters and then, starting from this, a dynamic modification of the scaling parameter as a function of the two sensor signals is significantly simpler. The prescaling is therefore a rough presetting (default value) for a subsequent fine control. The pre-scaling is determined depending on one or more material parameters and / or one or more operating parameters, e.g. on the basis of a database in which corresponding empirical values or test results are stored. Suitable material parameters are, for example, paper type or grammar of the paper. Suitable operating parameters are, for example, web speed or temperature and / or moisture with which the web is exposed.Expediently, the scaling parameter is determined repetitively, in particular continuously, so that the longitudinal distortion is dynamically compensated during operation. This reacts in particular to rapid changes during the operation of the printing system, which arise, for example, from a temperature and / or moisture control, by means of which the web is acted upon with variable temperature and / or moisture between the two printers. A dynamic change in the web speed, for example due to a web tension control or upon a change in order, is also reacted accordingly. Overall, a longitudinal distortion control, in particular longitudinal shrinkage control, is therefore realized, in which the longitudinal distortion which results between the two printers is measured and compensated for by suitable control of the first printer and / or of the second printer.A printing system according to the invention is designed to carry out a method as described above and has, for this purpose, in particular a correspondingly designed control unit. In particular, the two sensors and the printing nozzles (if present) are connected to the control unit. In particular, the print data for the two print images are also transmitted to the control unit, which then derives corresponding control signals for the print nozzles from these print data and the sensor signals. The control unit also determines the scaling parameter and preferably modifies the control signal with the scaling parameter.The computer program product according to the invention has instructions which cause the printing system to execute a method as described above, as described above.Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. The schematic figures show: FIG. 1 shows a method, FIG. 2 shows a printing system, FIG. 3a shows a first printed image at a first printing location, FIG. 3 bshows the first printed image from FIG. 3 aat a second printing location, FIG. 3 cshows the first printed image from FIG. 3 band a second printed image at the second printing location according to FIG. 3 b, FIG. 4 shows a part of the printing system from FIG. 2.FIG. 1 shows an exemplary embodiment of a method for operating a printing system 2. An exemplary printing system 2 with which the method is carried out is shown in FIG. 2. The printing system 2 has a first printer 4 and a second printer 6. A web 10 runs at a web speed through the printing system 2. a first printed image 8 is printed on the web 10 (here a paper web) in a first step S 1 using the first printer 4, and the first printed image 8 is subsequently overprinted with a second printed image 12 (subsequent printed image) using the second printer 6 in a second step S 2. This is referred to as "overprint", a fixed relationship (desired position) of the two print images 8, 12 relative to one another being predefined. The two printed images 8, 12 together form a print.In the exemplary embodiment shown, both printers 4, 6 are digital printers, specifically the first printer 4 is a digital multicolor printer, here a CMYK inkjet printer, and the second printer 6 is a digital lacquer printer for spot coating. A plurality of different colors are printed with the multicolor printer, and a transparent varnish is printed with the varnish printer. Position-selective (digital) printing is carried out in both printers 4, 6. The actual printing of the respective print image 8, 12 in the respective printer 4, 6 takes place at a so-called printing location D 1, D 2 (first printing location D 1 of the first printer 4 and second printing location D 2 of the second printer 6). The printing location D1, D2 is the location in the printer 4, 6 at which the ink / lacquer is applied to the web 10 to produce the printed image 8, 12.Both printers 4, 6 are arranged in-line, i.e. the web 10 is guided from the first printer 4 without interruption to the second printer 6 and optionally additionally processed between the two printers 4, 6. The web 10 generally has a longitudinal direction L which corresponds to a conveying direction of the web 10 and is also referred to as "machine direction".The first printed image 8 experiences a longitudinal distortion between the first printer 4 and the second printer 6, that is to say between the first step S 1 and the second step S 2, that is to say a distortion in the longitudinal direction L. This is shown by way of example in FIGS. 3 aand 3 b, wherein FIG. 3 ashows the first printed image 8 at the first printing location D 1 and FIG. 3 bshows the longitudinally distorted first printed image 8 at the second printing location D 2 (solid line) in comparison with the first printed image 8 at the first printing location D 1 (dashed line). If the second printed image 12 were now printed on the first printed image 8 at the first printed position D 2 in a matching manner, namely corresponding to the dashed line in FIG. 3 b, then a faulty overlap would result. In the example shown in FIGS. 3 a, 3 b, the first printed image 8 experiences a longitudinal shrinkage, the second printed image 12 would be printed too long without correction. This is shown by way of example in FIG. 3 c, which shows both printed images 8, 12 one above the other. In the exemplary embodiment shown, the longitudinal distortion arises in that at least one drying and / or moistening unit 14 is arranged between the first printer 4 and the second printer 6, with which unit the web 10 (and thus also the first printed image 8) is dried and / or moistened.Typically, the printed image 8 also experiences a transverse distortion (not shown), which is regularly also greater than the longitudinal distortion. However, the cross-distortion is not the subject matter here and is therefore not further observed here.In the exemplary embodiment shown here, the first printer 4 is assigned a first roller 16 which guides the web 10, and a first sensor 18, here a transmitter which measures a rotation of the first roller 16 and generates a first sensor signal G 1 (first transmitter signal). Similarly, a second roller 20 is assigned to the second printer 6, which guides the web 10, as well as a second sensor 22, here also a sensor, which measures a rotation of the second roller 20 and generates a second sensor signal G 2 (second sensor signal). Instead of measuring the rotation of the rollers 16, 20 with sensors 18, 22 designed as encoders, however, other measurement concepts are also conceivable and suitable in principle. It is important in particular that a sensor signal G 1, G 2 is generated in each case by the sensors 18, 22, which sensor signal is a measure of the web speed of the web 10 in the respective printer 4, 6.The measurements are carried out continuously and in parallel with steps S 1 and S 2. The respective roller 16, 20 is in the present case part of the respective printer 4, 6. Each printer 4, 6 accordingly has a roller 16, 20, the rotation of which is measured by a respective sensor 18, 22 in order to output a corresponding sensor signal G 1, G 2 as a function thereof. A respective roller 16, 20 is therefore also referred to as a measurement point. The sensor 18, 22 is, for example, a rotary encoder, e.g., an encoder. The two sensors 18, 22 then effectively also measure a web speed, i.e. a speed of the web 10 in the longitudinal direction L, at the respective roller 18, 22. The respective sensor signal G 1, G 2 is thus a measure of the web speed at the respective roller 16, 20.Based on the two sensor signals G 1, G 2, a scaling parameter P (also a scaling factor) is now determined in a third step S 3, more precisely a value of the scaling parameter P at a given point in time. The scaling parameter P is then used in a fourth step S 4 to compensate for the longitudinal distortion, i.e. to reduce it at least partially or completely. An exemplary embodiment for this is shown in FIG. 4, and this and variants thereof are explained in more detail further below. In the exemplary embodiment shown, the scaling parameter P is determined as the difference Δ between the two sensor signals G 1, G 2 or is derived therefrom by means of a table or a function (not explicitly shown). The scaling parameter P is actually also a measure of the longitudinal distortion, so that it is effectively also measured in the present case.In the present case, the sensors 18, 22 of the printing system 2 are accordingly used to correct (compensate) the longitudinal distortion experienced by the first printed image 8 on its way to the second printer 6. This path is regularly long and is, for example, in the range from 10 m to 100 m. The longitudinal distortion results in a difference in the web speed in the first printer 4 and in the second printer 6. The web speed is measured by a corresponding sensor 18, 22 on one of the rollers 16, 20 of the printing system 2. Starting from two sensor signals G 1, G 2 at two different measurement points, a longitudinal distortion control is then implemented. The respective sensor 18, 22 is located as close as possible to the respective printing location D 1, D 2, i.e., is at least associated with the respective printer 4, 6. The closer the respective measuring point (roller 16, 20) is to the respective printing point D1, D2, the more accurately the longitudinal distortion can be compensated. In principle, however, it is sufficient first of all if as little longitudinal distortion as possible or if only little longitudinal distortion takes place between the measuring point and the respective printing point D1, D2, and if at least neither drying nor moistening of the web 10 takes place there.In the exemplary embodiment of FIG. 2, the first roller 16 is a printing cylinder of the first printer 4 and the second roller 20 is a printing cylinder of the second printer 6. Alternatively, however, it is also possible to use a different roller instead of the printing cylinder, which roller is, however, expediently located within the printer 4, 6 and at least is located along the web 10 as close as possible to the printing cylinder and thus as close as possible to the printing point D 1, D 2. A suitable roller is, for example, one which defines a wrap around the printing cylinder by the web 10. In FIG. 2, such a roller 24 is arranged immediately before and after the printing cylinder in order to bring the web 10 upstream of the printing cylinder to the latter and to remove it again downstream of the latter and thereby fix the wrapping of the printing cylinder. Quite generally, however, any roller is suitable which is located along the web 10 as close as possible to the printing point D 1, D 2 and which is also coupled to the web 10 as far as the printing point D 1, D 2 as free of slippage as possible.The two rollers 16, 20 are in the present case also driven, i.e. each have its own drive (not shown). The two drives are independent of each other. The respective sensor 18, 22 is then seated, for example, on the drive of the corresponding roller 16, 20 or on a central axis or rotational axis A of the roller 16, 20. Optionally, both drives and thus also the web speed on a respective roller 16, 20 are regulated by a web tension regulator (not shown).In the present case, the first sensor 18 and the second sensor 22 are each coupled to the web 10 without slipping (also: "connected"). The web 10 is accordingly guided with the two rollers 16, 20 in each case free of slippage. A slip-free coupling of the sensor 18, 22 to the web 20 is realized, for example, in that the roller 16, 20 has a correspondingly suitable coating. In the exemplary embodiment of FIG. 2, for slip-free guidance, the winding of the roller 16, 20 with the web 10 is selected in such a way that self-locking results.Each of the printers 4, 6 shown here has a number of printing nozzles 26, 28 for printing the respective printed image 8, 12. By "a number of" is meant "one or more.". The printing nozzles 26, 28 are then controlled in such a way that they print the respective printed image 8, 12 line by line, for which purpose the printing nozzles 26, 28 are activated ("fired") in a specific cycle which is dependent on the web speed, in order to be able to print the respective printed image 8, 12 always with the same length at different web speeds. Therefore, a sensor 18, 22 (specifically a transmitter) is already present on the respective printing cylinder, with which the timing of the printing nozzles 26, 28 is then controlled accordingly. The sensor signal G 1, G 2 of this sensor 18, 22 accordingly determines at which points in time the printing nozzles 26, 28 are each activated in order to generate a single point of the printed image 8, 12. This sensor 18, 22 is now also used to compensate for the longitudinal distortion as described. The first sensor signal G 1 is then used to control a timing of (first) printing nozzles 26 for printing on the first printed image 8, and the second sensor signal G 2 is used to control a timing of (second) printing nozzles 28 for printing on the second printed image 12.In the present case, at least one of two control signals S 1, S 2 is modified with the scaling parameter P, with which the printing nozzles 26, 28 of the two printers 4, 6 are controlled. As a result, the corresponding printed image 8, 12 is printed with distortion in the longitudinal direction, corresponding to the longitudinal distortion of the first printed image 8, so that ultimately an optimum overprint results. The printing nozzles 26, 28 are controlled in a time-dependent manner with the control signal S1, S2 in such a way that they print the respective printed image 8, 12 line by line on the web 10 running past. The control signal S1, S2 thus determines the timing of the printing nozzles 26, 28.As is also shown in FIG. 4, the respective control signal S 1, S 2 is on the one hand dependent on one of the sensor signals G 1, G 2, which presets the clocking, i.e. when a respective line is printed. On the other hand, the respective control signal S 1, S 2 is also dependent on print data X 1, X 2 (image data) which define the respective print image 8, 12, i.e. which print nozzle 26, 28 is activated in a given line of the print image 8, 12 in order then to print a corresponding pixel. The control signal S1 (first control signal) for the print nozzles 26 of the first printer 4 is therefore formed from the first sensor signal G1 and from the print data X1 to form the first print image 8. Analogously, the control signal S2 (second control signal) for the print nozzles 28 of the second printer 6 is formed from the second sensor signal G2 and from the print data X2 to form the second print image 12. It is also conceivable that only one of the two printers 4, 6 has printing nozzles 26, 28, since it is ultimately sufficient if one of the two print images 8, 12 is scaled.To modify the control signal S1, S2, the scaling parameter P may be applied at one or more different locations. Four possible locations are indicated by dashed lines in FIG. 4. Of these, exactly one is then used in the present case, but in principle a combination is also possible. In a first embodiment, the control signal S 1, S 2 is generated on the basis of one of the two sensor signals G 1, G 2, which is scaled with the scaling parameter P. The scaling parameter P is therefore applied to one of the sensor signals G 1, G 2 in order to scale the sensor signal and thus ultimately also the control signal S 1, S 2 derived therefrom. The print data X1, X2 can remain unchanged. In a second embodiment, the print data X 1, X 2 defining the print images 8, 12 are scaled with the scaling parameter P and the respective control signal S 1, S 2 is then generated by means of these scaled print data X 1, X 2. It is sufficient if the scaling parameter P is applied either to the print data X 1 or to the print data X 2. The sensor signal G 1, G 2 can thus remain unaffected. A configuration which is not explicitly shown is also conceivable, in which the sensor signal G 1, G 2 and the print data X 1, X 2 are retained unchanged per se and only the control signal S 1, S 2 formed from these are scaled with the scaling parameter P.In the present case, the lengths (measured in the longitudinal direction L) of the two print images 8, 12 are matched to one another with the scaling parameter P in order to compensate for the longitudinal distortion. Any fixed longitudinal offset, on the other hand, i.e. a constant displacement in the longitudinal direction L by a fixed value along the path 10, cannot be corrected with this, however. For this purpose, a mark 30 is used in the present case, which is printed with the first printer 4 and is used by the second printer 6 as a start signal for printing on the second printed image 12. An exemplary marker 30 is shown in Figs. 3a-3c. By means of the mark 30, a somewhat optimum overprint is ensured only at the beginning of the printing. The scaling parameter P then prevents the two print images 8, 12 from moving apart progressively along the print, i.e. so to speak an increasing longitudinal offset over the print.In the present case, optionally also as a function of at least one material parameter of the web 10 and / or at least one operating parameter of the printing system 2, a prescale 32 is predefined for one of the two print images 8, 12, so that this print image 8, 12 is printed in a prescaled manner. This is based on the consideration that the necessary value for the scaling parameter P can be estimated roughly in advance as a function of the material parameters and / or operating parameters and then, starting from this, a dynamic modification of the scaling parameter P as a function of the two sensor signals G 1, G 2 is significantly simpler. The pre-scaling 32 is accordingly a rough pre-setting (default value) for a subsequent fine control. The pre-scaling 32 is determined depending on one or more material parameters and / or one or more operating parameters, e.g. on the basis of a database in which corresponding empirical values or test results are stored. Examples of material parameters are paper grade or grammar of the paper (i.e., web 10); examples of operating parameters are web speed or temperature and / or humidity applied to web 10.In the present case, the scaling parameter P is determined recurrently, especially even continuously, so that the longitudinal distortion is dynamically compensated during operation. This reacts to rapid changes during the operation of the printing system 2, which result, for example, from a temperature and / or moisture control, by means of which the web 10 is acted upon with variable temperature and / or moisture between the two printers 4, 6. A dynamic change in the web speed, for example due to a web tension control or upon a change in order, is also reacted accordingly. Overall, a longitudinal distortion control is therefore realized, in which the longitudinal distortion which results between the two printers 4, 6 is measured and compensated by suitable control of the first printer 4 and / or of the second printer 6.To carry out the method as described above, the printing system 2 has a correspondingly designed control unit 34, which is shown in FIG. 4. The two sensors 18, 22 and the printing nozzles 26, 28 are connected to the control unit 34. The print data X 1, X 2 for the two print images 8, 12 are also transmitted to the control unit 34, which then derives corresponding control signals S 1, S 2 for the print nozzles 26, 28 from these print data X 1, X 2 and the sensor signals G 1, G 2. The control unit 34 also determines the scaling parameter P and modifies one or both control signals S 1, S 2 with the latter.List of reference characters2 Printing system 4 First printer (CMYK) 6 Second printer (lacquer) 8 First printed image 10 Web (paper web) 12 Second printed image 14 Drying and / or moistening unit 16 First roller 18 First sensor (first encoder) 20 Second roller 22 Second sensor (second encoder) 24 Roller 26 (first) Printing nozzles 28 (second) Printing nozzles 30 Mark 32 Pre-scaling 34 Control unit A Axis of rotation D1 First printing point D2 Second printing point G1 First sensor signal (first encoder signal) G2 Second sensor signal (second encoder signal) L Longitudinal direction P Scaling parameter S1 First step (first printed image print) S2 Second step (second printed image print) X1 (first) Print data (for the first printed image) X2 (second) (for the second printed image) Δ Difference (of the sensor signals)

Claims

Method for operating a printing system (2) which has a first printer (4) and a second printer (6), a. wherein a web (10) runs through the printing system (2) at a web speed, b. wherein a first printed image (8) is printed onto the web (10) with the first printer (4) and the first printed image (8) is subsequently overprinted with a second printed image (12) with the second printer (6), c. wherein the first printed image (8) experiences a longitudinal distortion between the first printer (4) and the second printer (6), d. wherein the first printer (4) is assigned a first sensor (18) which generates a first sensor signal (G1) which is a measure of the web speed of the web (10) in the first printer (4), e. wherein the second printer (6) is assigned a second sensor (22), which generates a second sensor signal (G1), which is a measure of the web speed of the web (10) in the second printer (6), f. wherein a scaling parameter (P) is determined on the basis of the two sensor signals (G1, G2), with which scaling parameter the longitudinal distortion is compensated.Method according to claim 1, wherein a first roller (16) is assigned to the first printer (4) which guides the web (10), wherein the first sensor (18) is a first encoder which measures a rotation of the first roller (16) and generates the first sensor signal (G1) in the process, wherein a second roller (20) is assigned to the second printer (6) which guides the web (10), wherein the second sensor (22) is a second encoder which measures a rotation of the second roller (20) and generates the second sensor signal (G2) in the process.The method of claim 2, wherein the first sensor (16) and the second sensor (20) are each coupled to the web (10) in a slip-free manner.Method according to claim 2 or 3, wherein the first roller (16) is a printing cylinder of the first printer (4) and / or the second roller (20) is a printing cylinder of the second printer (6).Method according to one of Claims 1 to 4, wherein the first sensor signal (G1) is used to control a timing of printing nozzles (26) for printing on the first printed image (8), and the second sensor signal (G2) is used to control a timing of printing nozzles (28) for printing on the second printed image (12).Method according to one of Claims 1 to 5, wherein the scaling parameter (P) is used to modify a control signal (S1, S2), with which a number of print nozzles (26, 28) of one of the two printers (4, 6) is actuated.Method according to Claim 6, wherein the control signal (S1, S2) is generated on the basis of one of the two sensor signals (G1, G2), which is scaled with the scaling parameter (P).Method according to claim 6, wherein at least one of the print images (8, 12) is defined by print data (X1, X2) which are scaled with the scaling parameter (P) and by means of which the control signal (S1, S2) is generated.Method according to one of Claims 1 to 8, wherein, depending on at least one material parameter of the web (10) and / or at least one operating parameter of the printing system (2), pre-scaling (32) is predefined for one of the two print images (8, 12), such that this print image (8, 12) is printed in a pre-scaled manner.Method according to one of Claims 1 to 9, wherein the scaling parameter (P) is determined recurrently, in particular continuously, such that the longitudinal distortion is dynamically compensated during operation.Method according to one of claims 1 to 10, wherein between the first printer (4) and the second printer (6) at least one drying and / or moistening unit (14) is arranged, with which the web (10) is dried and / or moistened.Method according to one of claims 1 to 11, wherein the first printer (4) is a digital multi-colour printer, in particular a CMYK inkjet printer, and wherein the second printer (6) is a digital lacquer printer.Printing installation (2) which is designed to carry out a method according to one of Claims 1 to 12.A computer program product comprising instructions for causing the printing system (2) according to claim 13 to carry out the method according to any one of claims 1 to 12.

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