Inline measurement and closed loop control method in printing machines

Spectral, densitometric, or color measured values are detected on sheet printing materials during the printing process in a sheet-fed printing press. The measured values are determined on sheets as they are moving through the printing press and the measured values are used in real-time by a computer to control parameters for controlling the printing process in the sheet-fed printing press.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method for detecting spectral, densitometric or color measured values on printing materials during the printing process in a printing press.

During every printing operation, the printer attempts to achieve a maximum accord between the printed copies and the original print. To this end, complicated quality control and monitoring of the printed printing materials by the printing personnel is required in a printshop operation. According to the prior art, this is carried out by means of visual assessment by the operating personnel and by the employment of optical measuring instruments, which measure either densitometrically or spectrally. For this purpose, in the case of sheet-fed offset printing presses, a sheet has to be removed from the delivery and is usually placed on a sheet supporting desk. On this desk, the sheet is illuminated with a standardized source of illumination and is measured with the aid of optical measurement technology or assessed visually. However, this process takes time, and, in addition, is made more difficult by the fact that the printing press continues to print during the quality control and, under certain circumstances, rejects arise if the assessed sheet does not correspond to expectations. Since, after each interruption to the printing process, the printing press needs a certain number of sheets until the printing process has reached a stable state again, rejects cannot be prevented either by shutting down the printing press quickly during the printing material inspection. Furthermore, in order to assess the printing sheet, printing personnel are needed who, during the quality control, are not available for other activities. Since, during the setup phase of a printing press, many possible adjustments have to be made, in particular in the inking unit area, rejects of between 150 and 400 sheets normally occur. This is made even more difficult by the fact that the printing process can generally be reproduced only with difficulty, since the printing result depends on very many parameters such as ink, temperature, water, paper, printing speed, rubber blanket, condition of the printing plate, etc. All these parameters normally change in some way from print job to print job, and it is therefore not sufficient to store the setting of a print job and to retrieve it in the same way for repeat jobs since, for example, the air temperature or atmospheric humidity could have changed in the meantime, so that, even for the same print job, new settings have to be made because of changed environmental conditions.

In the case of web-fed offset printing presses, the printed (newspaper) webs cannot simply be removed from the machine. Accordingly, there exist measuring systems which attempt to measure the quality of a printed web spectrally or densitometrically. A method for operating a sensing device for optical density measurement is disclosed in German published patent application DE 100 23 127 A1. There, the printed web which leaves the last printing unit in a web-fed offset printing press is guided over a deflection roll, a sensing device for optical density measurement, color measurement or spectral measurement being fitted parallel to the deflection roll. In this way, the quality of the printed web can be determined. In the description of the exemplary embodiments, it is indicated that the method disclosed in the application can also be applied during printing on sheet printing materials. However, an accurate description of how this is actually to be done cannot be gathered from the application, in particular the problem that, in the case of sheet printing materials, the guidance of the sheet printing materials over a deflection roll as in DE 100 23 127 A1 is not possible at all, is not solved, since sheet printing materials have to be held at least one point by a holding device such as grippers or the press nip of the printing unit. For this reason, the device disclosed in DE 123 127 A1 is not suitable for the quality assessment of sheet printing materials during the printing process in sheet-fed offset printing presses.

Furthermore, Ifra Special Report 3.35 describes inline measuring systems for web-fed rotary printing presses which operate with a closed control loop, that is to say the measured values registered by the inline measurement for assessing the printing quality of the printing material web are passed on directly to a computer of the web-fed rotary printing press and are processed there. The computer then corrects any deviations automatically and changes settings of the printing press. However, this method also inherently has the disadvantage that it is possible only to correct deviations which are permitted by the control system of the printing press. In particular, corrections to the color profile are not automatically possible in this way, since these can be made only in conjunction with the data from the prepress stage. Furthermore, in the case of the known inline measurements, only the data from a single print job, namely the specifically current print job, is taken into account when correcting the settings in the printing press.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method for inline measurement and closed-loop control processing in printing machines which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which enables automatic correction of deviations in the printing press over a plurality of print jobs.

With the foregoing and other objects in view there is provided, in accordance with the invention, a method for detecting spectral, densitometric or color measured values on sheet printing materials during a printing process in a sheet-fed printing press wherein sheets are moved through the printing press. The method comprises the following steps:

determining the measured values on sheets moving through the printing press; and

processing the measured values in a computer and using the processed values as control parameters for controlling the printing process of the sheet-fed printing press.

By way of the registration of measured data on sheets transported through the printing press, the current state of the system comprising the printing press can always be determined and, in this way, corrections can be made immediately and in real-time by a control system, which is otherwise not possible in sheet-fed printing presses. This control can be carried out during the setup phase but also during continuous printing. During continuous printing, however, corrections are necessary substantially more rarely, since here the behavior of the printing press is more stable. Therefore, in continuous printing it is not necessary to carry out so many measurements, for which reason the measuring strategy can be adapted to the respective state of the printing press. This is described in more detail further below in the text.

In an advantageous refinement of the invention, during the printing process in the printing press, not only are spectral, densitometric or color measured values registered continually on the printing materials that are being produced, but the measured values are evaluated in a computer of the printing press or a separate computer and at least those deviations which cannot be avoided accurately by changing the settings on the printing press are passed on to the control system in the prepress stage. This can be brought about relatively simply, in particular in what is known as computer to plate technology (CtP), since these digital prepress stages likewise have computers which are able to receive the corresponding data from the computer of the printing press. In this way, a closed control loop started from the finished printing material via the printing press and the prepress stage and back to the printing press again is closed. The measured values transmitted by the printing press and their assessment can thus be taken into account in the prepress stage during the production of the printing plates and it is therefore also possible to correct deviations which cannot be compensated for in the printing press on its own. It should be noted that color measured values are understood to be values in color spaces such as the Lab, the RGB or other unambiguous color spaces. Even over a plurality of print jobs, measured values can thus be taken into account during the creation of printing plates, so that, over many print jobs, a continuous improvement process takes place in the entire production chain from the scanner in the prepress stage as far as the end product in the printing press. In this way, it is possible to carry out an improvement process without having to register special test forms in a complicated process. Since, in a digital workflow as is most usual nowadays, the prepress stage with the scanners, plate exposers, raster image processors and the printing press are linked to one another, this data can also be interchanged without additional hardware and with little additional expenditure.

In a first refinement of the invention, provision is made for the measured values registered to be supplied to a computer and for the computer to use the measured values to create or correct a color profile when driving inking units of a printing press. For color reproductions that are true to an original, it is imperative to link the color profile of the printing press with the color profile of the prepress stage, in order in this way to keep deviations between the printed original and the printed end product as small as possible. By means of the data obtained by inline measurement and sent to the prepress stage, it is possible to relate the color profiles of printing press and prepress stage to one another and, in the event of any deviations, to correct the color profile of the printing press. Therefore, the color profile of the printing press is monitored and, if necessary, adapted continually and automatically without any action by the printing personnel.

In a further or alternative refinement of the invention, provision is made for there to be sensors for recording the measured values and for color calibration to be carried out at specific time intervals by means of a calibration device. Since, in the case of an inline measuring method, measured values are determined continuously, it is absolutely necessary to ensure that these measured values are comparable with one another. For such an accurate measurement, therefore, in addition to a single calibration during commissioning, regular system calibration is necessary in order to be able to take into account any heat-induced or wear-induced changes in the measured values, and aging-induced changes of illumination sources or contamination. For this purpose, the inline measuring device present in the printing press has a calibration device, which is set operating at specific intervals. In this way, it is ensured that the inline measuring system is continually recalibrated and the operation-induced deviations are avoided.

Provision is further made that, as a reference value for the calibration device, there is a calibration surface with associated color measured values which are stored in the computer. For this purpose, the measuring heads present in the inline measuring system for the spectral, densitometric or color measurement are aimed at a calibration surface at specific time intervals and recalibrated. In the measuring system, the color value of the calibration surface is known, so that the value determined by the measuring head can be compared computationally with the stored color value. If deviations occur, then the measuring electronics of the measuring head are recalibrated appropriately, that is to say a correction is made in such a way that the measured value is made equal to the color value stored in the computer. By means of this calibration, even contaminated measuring heads are able to supply measured results that can still be used at least over a relatively long time period while, without calibration, even after a relatively short time, cleaning of the entire measuring device or replacement of an aging illuminating device would be necessary.

Provision is advantageously made for the calibration surface to be white. For calorimetric reasons, the calibration measurement should ideally be carried out on a standardized white surface, for which reason the calibration surface is implemented in precisely this hue.

Provision is further made for one or more calibration surfaces to be arranged in the channel of a press cylinder in extension of the press cylindere surface. Since the inline measuring system has a plurality of measuring heads, preferably eight measuring heads in the case of 32 inking zones distributed over the width of the printing material, all the measuring heads must be set and monitored by means of calibration surfaces. However, since the lateral mobility of the measuring heads is restricted, it is not possible to move all the measuring heads to a calibration surface fitted at the side. Furthermore, it is important that the distance between calibration surface and measuring head correspond exactly to the distance between measuring head surface and printing material surface. In order to be able to fit the calibration surfaces for all the measuring heads over the entire width of the printing material, these are arranged in the channel of a press cylinder in extension of the press cylinder surface. As a result, the calibration surfaces have exactly the same spacing with respect to the measuring heads as the surface of the printing material and are not in the way during the printing operation.

In an alternative embodiment of the invention, provision is made for at least one calibration surface arranged laterally outside the press cylinder surface to be located between side wall and press cylinder. Calibration surfaces which are located in the printing channel have the greatest disadvantage that they contaminate during the printing process. On the other hand, if the calibration surface is outside the press cylinder surface, for example in the region of the side wall, it is subjected less to contaminants there. As a result, frequent cleaning operations of the calibration surface are avoided.

In a particularly advantageous refinement of the invention, provision is made for the sensors to be measuring heads and the calibration values determined by the calibration of one measuring head to be converted by means of the computer into calibration values for further measuring heads. This method is also designated transfer calibration, since all the measuring heads are not calibrated on individual calibration surfaces; instead one calibration surface arranged outside the cylinder surface, for example between side wall and press cylinder, is sufficient. This calibration surface can, however, be performed by only one of the measuring heads covering the edges of the printing material, since only these measuring heads can be moved laterally beyond the limits of the press cylinder. The other measuring heads are calibrated by means of a transfer calibration, by the entire measuring beam being moved further by a movement travel which corresponds to the spacing of the measuring heads from one another. Therefore, only a single measuring head in the edge region has to be calibrated on the calibration surface, while in the next step the measuring beam is moved by the spacing of the measuring heads, so that this first calibrated measuring head is able to register the zone of the second measuring head. This also applies in an analogous way to the further measuring heads, that is to say each measuring head then registers the measuring zone of the measuring head located beside it. During this calibration measurement, the measuring heads are aimed either at a white printing material or at a colored printed material. However, this plays no part in the progress of the calibration measurement. For instance, if the second measuring head beside the first measuring head which has been calibrated over the calibration surface is currently registering a specific blue shade, then this blue shade is registered by the first calibrated measuring head in the next step. The measured values from the first and second measuring head are then compared with one another and, if necessary, the values of the second measuring head are corrected. Therefore, the transfer calibration to the second measuring head has been concluded, and it is possible for the possibly corrected measured values from the second measuring head to be compared with the measured values from a third measuring head. This is done in exactly the same way for all further measuring heads in an iterative method, so that only a single measuring head has to be calibrated by means of a calibration surface, while all the others are calibrated in one step by means of computational comparisons.

Furthermore, provision is made for at least one calibration surface to be closed by means of a cover. By means of such a cover, the calibration surface can be protected reliably against contamination during the printing process. The cover is opened only when a calibration operation has to be carried out. Thus, the otherwise always repeated necessary cleaning of the calibration surface is dispensed with.

It has proven to be advantageous for the calibration to be carried out with the aid of an external measuring instrument. Since all the parts accommodated in the machine are susceptible to contamination and disruption, the transfer calibration can also be carried out by means of an external measuring instrument. For this purpose, on the operating desk there is a permanently installed measuring instrument or handheld measuring instrument which has its own incorporated calibration surface, calibrates itself to this surface at regular intervals and with which the printing material currently being printed is measured. Since this printing material has previously been measured by the inline measuring device and its measuring heads and removed from the printing press, the values determined thereafter with the handheld measuring instrument can be passed on directly to the measuring electronics in the measuring beam, and in this way the appropriate calibration can be carried out. Of course, the printing material can also be measured first in the unprinted state, that is to say as paper white, by using the handheld measuring instrument and then measured in the printing press by means of the measuring heads of the inline measuring device. In this way, the transfer calibration can also be carried out by using an external measuring instrument. The calibration can particularly advantageously be carried out in the print-free region directly after the grippers, since here the sheet is guided ideally and, in addition, there is always paper white present. This edge region usually has an unprinted area of 6-12 millimeters and is completely adequate for the measurement.

However, the external handheld measuring instrument can also be used for another purpose. For many reasons, the sheet is measured in the machine with the aid of a polarizing filter, which means that all the measured values are registered in a polarized manner. However, the regulation of the printing press operates with unpolarized values, since the information from the prepress stage is present only in unpolarized form, that is to say the measured values registered must be converted into unpolarized values. For this purpose, a computational relationship between polarized and unpolarized values must be stored in the printing press. This relationship can be produced with the aid of the handheld measuring instrument, which measures unpolarized. Thus, a sheet is measured once polarized with the inline measuring device in the printing press and once unpolarized and polarized outside the machine by means of a handheld measuring instrument. If this measurement is carried out over a plurality of sheets, a relationship between the polarized and the unpolarized measured values can be detected. This relationship is then stored in the computer of the printing press as a correction function, so that the values can be converted into one another at any time.

In a further refinement of the invention, provision is made for specific color values to be stored in the computer for each measuring head, the ratios between these color values being stored in the computer and a signal being output if there is a change in the stored measured value ratios. By means of such a device, the contamination of the inline measuring system is detected. Each spectrometer has a white measured value as an initialization parameter, for example when delivered. These white measured values belonging to the respective measuring heads are stored in terms of their ratios to one another for all the measuring heads. During the printing process, paper white measurements are carried out continually and the measured value ratios determined in the process are compared with the values stored in the measuring electronics. As soon as these ratios change, it being possible for certain tolerance bands to be set, this is judged to be a signal of contamination. In this case, and acoustic or visual signal is displayed to the operating personnel, whereupon cleaning of the measuring heads must be carried out.

Furthermore, provision is made for a first measuring head to register its own color zone and the color zone of a second measuring head located beside it, and for the second measuring head likewise to register its own zone and that of the first measuring head, and for the measured values registered to be compared with one another. In this way, a cross comparison between the individual measuring heads of the measuring modules of a beam-like inline measuring device in the printing press is made possible. Firstly, all the measuring heads measure a color zone on a printing material simultaneously, then the entire measuring beam is moved laterally to such an extent that each measuring head can then measure the measuring location of its neighbor. In the event that calibration is carried out correctly, these measured values must not differ from one another or differ only within quite narrow tolerance limits. However, if the measured values exhibit deviations, then it is possible as a result to conclude that there is contamination on the optics of the measuring heads.

A further possible way of discovering contamination on the measuring system results from the fact that, on at least one color zone of a measuring head, measurements are carried out on a light/dark edge, the measuring head being moved in uniform steps from one side on the other side of the light/dark edge over the light/dark edge until it is on the side on this side of the light/dark edge, and the intensity measured values registered in the process being compared with the known structure of the measuring head. Such a light/dark edge represents, for example, the transition from paper white to the colored region. This measuring region then has to be run through by a measuring head as follows. Firstly, the measuring head measures on the side of the light/dark edge which shows the paper white. The measuring beam is then, for example, moved over the width of the measuring area of the light/dark edge in 10 steps, 10 measurements being carried out. This means that the last measurement is carried out completely in the colored region of the measuring area. During the evaluation of these measurements, the intensity measured in each case is plotted against the local offset, it being necessary for the distance between the white value measured last and the color value measured first to correspond to the measuring range of the spectrometer of the measuring head, given exact optical imaging of the known structure width. This comparison is carried out by means of the measuring electronics and the values stored there of the structure of the measuring range of the spectrometer. If there is a deviation here, this is likewise an indicator of contamination.

Furthermore, provision is made for there to be an illuminating device, for a dark measurement to be carried out before the actual measurement by a measuring head and for the measured value registered in the process to be subtracted from the color measurement carried out with the illuminating device switched on. In order to be able to sense the surface of the printing material, the latter must be illuminated by using an illuminating device in the vicinity of the measuring head. However, since there is a distance of several centimeters between the printing material and the measuring beam, external light can also fall into the region between printing material and measuring head/illumating device. This falsifies the measured results and must be compensated for accordingly. One possibility is to perform a dark measurement, that is to say the illuminating device is first switched off and the measurement is carried out with the illuminating device switched off. The illumination is then switched on and the measurement is made with the illuminating device switched on. In this case, the order does not play any part, since for the purpose of correction it is merely necessary for the measured value registered during the dark measurement to be subtracted from the measured value registered with the illumination switched on. Scattered light or external light sources are, for example, slots in the machine through which the ceiling illumination of a print shop or daylight can fall, but there are also light sources in the machine itself, such as UV/IR dryers or other sensors which operate with light and whose light disrupt the measuring process. By means of a small change, it is also possible to compensate for periodically operating external light sources. For example, a dark measurement is carried out first, the influence of external light being registered for the first time, a light measurement is then carried out and then, once more, a dark measurement, during which only the influence of external light is again registered. If the external light source changes, the measured values from the two dark measurements differ from one another and, by comparing the two measured values, the computer can detect whether the external light has to be added or subtracted during the light measurement, since it is able to compare the measured values before and after. It is therefore possible for the gradient of the external light change to be determined, so that the influence of external light from the light measurement can also be computed out reliably in the event of changing, in particular periodic, external light.

A further possibility for correction in the event of incidental external light is that, at the same time as the color measurement from a first measuring head, by means of a second measuring head a measured value is registered on a white background of a printing material and the white reference value determined as a result is used to correct the color measured values determined by the first measuring head. To this end, the second measuring head must be accommodated so as to be separated physically from the first measuring head, which must always carry out the measurement on paper white. This can be, for example, the edge region of the printing material. The white reference value determined with the second measuring head is included in the calculation of the color or density values and in this way the influence of the external light is compensated for.

There is still a further possibility for external light compensation, namely that, during the registration of measured values on the printing material by means of one or more measuring heads, any light sources present are switched off, masked out or dimmed down to a non-critical level. In this case, the measuring electronics of the measuring heads are linked to the computer of the printing press, so that light sources in the printing press are switched off during the measuring operation. For example, the influence of the external light from a UV dryer is avoided during the measurement by the dryer being switched off briefly during the measurement and then switched on again. Another possibility is to mask out the external light source, by a shutter being fitted in front of the external light source. This shutter then covers the external light source as long as the measuring operation is being carried out. It is also possible to filter out specifically spectral values of the external light source which lie within the spectral range of the measuring device, by a filter being fitted which filters out the spectrum of the external light source. A similar effect is achieved by means of computational interpolation. Since the spectrum of the external light source is known, spectral values corresponding to the measuring spectrum are not used and, instead, by means of the adjacent values, the unusable values are interpolated over the spectrum of the external light source. Thus, peaks caused by the external light source in the measured spectrum can be computed out.

In order to compensate for external light, the following possibility is also provided, namely that the registration of measured values by measuring heads with any fluctuations of light sources are coordinated over time by means of at least one sensor which registers the fluctuations, or by means of a control signal of the fluctuating light source. In this case, too, information about the time behavior of the external light source must be available, that is to say these values must either be stored in a computer or the external light source supplies the information online to the computer via sensors. In this case, the measurements are coordinated by the computer in such a way that measurements are always made when the external light source is switched off or exhibits a minimum.

Furthermore, provision is made for a plurality of measuring heads to be distributed at equal intervals over the width of a printing material and to register the color zones simultaneously. In the large format (102 cm sheet width) in sheet-fed machines, 32 color zones extend over the entire printing material width; the result in the case of 6 printed colors is thus 192 measuring areas which have to be registered by the measuring electronics and the measuring heads. In this case, measuring cycles over at least 192 sheets are required at a single spectral measuring head, which is not sufficient for good regulation. For this reason, a plurality of measuring heads which are capable of measuring in parallel and simultaneously are needed. Since, after each measuring operation, the measuring heads are offset laterally by one color zone, in particular 8, 16 or 32 measuring heads are ideally suitable for the parallel measurement. In the case of 32 measuring heads and 32 color zones and also 6 printed colors, it is accordingly necessary for 6 measuring operations to be carried out on 6 printed sheets. After these 6 measuring steps, the adjustment to the settings of the printing press can then be made if necessary, in that corrected values are set with new inking zone setting on the printing press. In addition to the aforementioned measuring strategy, the measuring heads can also be moved in a way wherein the same color is always registered first over a plurality of sheets, so that this color can be readjusted well and only then are the measuring heads positioned to the next color, which is then likewise readjusted. Since different measuring strategies can be employed, the measuring device must store the measured values with a timestamp and a location marking in the computer of the printing press, so that the correct references can be produced at any time in order to be able to compare the actually comparable measured values correctly with one another. Then, the measuring strategy no longer plays any role and the measured values can be assigned correctly at any time.

In a refinement of the invention, provision is additionally made that, during printing operation, after the printing start-up phase, the measuring heads are positioned in such a way that they register a plurality of colors simultaneously. Since the mechanics and the drive motor of the measuring beam having the measuring heads are highly stressed by frequent measurement, what is known as lean operation increases the lifetime. However, since the values still change to a great extent during the start-up phase as a result of the process, frequent measurements have to be made continuously there while, in the continuous printing phase, another procedure can be selected since, during the continuous printing phase, the color values remain virtually constant as seen over time, so that it is possible to position the measuring heads over mixed areas. As soon as an excessively high tolerance deviation is detected, the measuring beam then begins its frequent measurements again as in the start-up phase, which measurements register all the areas and all the zones. As a result, the reason for the deviation can be measured and the regulation of the printing press can be activated appropriately.

The measuring device is also able to change its measuring strategy as a function of the measured values registered. For example, colored areas which exhibit low noise are not measured as often as colored areas with high noise. This means that each color is registered with a different measuring strategy, so that highly noisy colors are measured more frequently. If the noise in the case of these colors decays, the measuring strategy is also changed, so that the frequent measurements are reduced. The measuring strategy can also be carried out as a function of the printed image and the settings of the printing press itself. Since the data from the printed image from the prepress stage can be transmitted to the computer, the measuring system is also able to calculate an appropriate measuring strategy, since critical color areas in the printed image are previously known with their position and the hue.

In a further refinement of the invention, provision is made for the computer to store the position coordinates of print control strips applied to a printing material. The measurements on the color zones in printing presses are normally carried out in the region of the print control strips. In order that these measurements can be carried out reliably, the position of the print control strip on the printing material must be known to the measuring beam of the in-line measuring system. One possibility is for the printer to measure the position of the print control strip on the printing plates manually and to enter the position coordinates of the print control strip into the computer of the machine control system. Furthermore, the position coordinates from the prepress stage in a linked workflow system can also be transmitted to the computer of the printing press and used there. In both possibilities, however, there is the risk that, when the printing plates are clamped in the printing press or as a result of a register adjustment, the position of the print control strip on the printed sheet relative to the measuring heads is changed. However, by using the predefined rough position, the search area for an exact position determination can be restricted, which means that the work is made easier for the automatic position detection system.

Provision is also made for a sensor to be provided for determining the position of the print control strip on the printing material. By means of a two-dimensional sensor, for example a CCD image converter, the position of the print control strip can be determined. A pattern of the print control strip is installed in the machine control system and is compared with the image from the images registered by the CCD camera. As soon as the camera detects equivalence, the computer is able to calculate the position of the print control strip relative to the measuring beam and to send an appropriate starting signal to the latter in order that the measurement starts exactly when the print control strip comes to lie underneath the measuring heads. The use of a one-dimensional sensor is also suitable for the position detection of a print control strip if a detection segment, for example a bar code, precedes the print control strip. As soon as this bar code is detected by a barcode reader, it is known to the system that the print control strip then follows at a specific time interval. Therefore, the measuring operation can be triggered at the correct time. The position detection is necessary only at the start of the printing operation, since here still greater local deviations are to be expected. In the continuous printing phase, the local position of the markings is stable, so that here the detection segments have to be scanned only at long time intervals for the purpose of monitoring.

A particularly advantageous refinement of the invention is distinguished by the fact that, after each measurement, the measured values determined by the measuring heads are subjected to a plausibility test. In the case of in-line measurement with a closed control loop, it is particularly important to detect and separate out erroneous measured values automatically, since otherwise the inking zone control system sets the wrong values and rejects are produced unnecessarily, without the operating personnel being informed about this. For this reason, an in-line measuring system with closed control loop should subject the measured values to a plausibility test in order to be able to separate out implausible measured values. Such a check is carried out, for example, by means of the correlation between the stored original of the print control strip and the values from the measuring beam registered during each measuring operation. This also ensures that the measuring beam always moves to the correct measuring areas. The choice of the correct print control strip type may be checked by means of a further algorithm, wherein a sensor registers a coding area within the print control strip and checks the data encoded herein. Furthermore, during each measuring operation, a plausibility check on the measured values is carried out both in the space domain and in the time domain. To this end, limiting values for deviation, for example in the density range, are defined, which two successive or locally adjacent values lying together must not exceed. Here, the plausibility test is based on the fact that, in the offset process, the printing units in normal operation only permit continuous changes in the color values, so that jumps in the color density which exceed a specific order of magnitude can be attributed immediately to defects in the measuring system. In addition, a display can be provided which provides information about the state of the printing process. If the measuring system registers no deviations or only small tolerable deviations and controls them out by means of the machine control system, the OK state is displayed to the printing personnel on a display. If the machine is not in this stable state, this can be detected on the display and the printing personnel know that rejects are being produced.

The measuring method can also be used for the indirect moisture measurement of the sheet. In order to measure the moisture, the damping solution is usually reduced until, in the halftone print on the sheet, what is known as “scumming” occurs. According to experience, this scumming is first manifested at the start of the sheet, at the lateral edge of the sheet and in the halftone areas having 70%-90% area coverage. The moisture value is then increased again by a specific fixed percentage value. For the in-line measurement, a 70%-90% halftone area is introduced on the sheet in the print control strips or at positions for each color specifically arranged on the sheet at the sheet edge. From the knowledge of the area coverage of this area and the printed color density, slight scumming can thus be registered reliably by the measuring heads. Therefore, the ink-water balance can be set and monitored.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first, particularly, toFIG. 1thereof, there is shown a sheet-fed rotary printing press1having a sheet feeder module2and a sheet delivery module3and also four printing units4,5arranged between them. It will be readily understood by those of skill in the art that this configuration of a sheet-fed rotary printing press1is but an exemplary embodiment, since the number of printing units4,5between sheet feeder2and sheet delivery3is of no import with regard to the invention. The printing units4,5are connected to one another via transport cylinders9, so that printed sheets705stacked in the sheet delivery2are conveyed through the individual printing units4,5to the delivery3and can be printed in the printing units4,5. The last printing unit5seen in the sheet running direction differs from the other printing units4in that it has a measuring beam6as a sensing device for assessing the printing quality of printed sheets. The measuring beam6is therefore accommodated in the last printing unit5, since here all the colors applied in the printing operation are present on the printed sheets705, and therefore the final state of the printed sheet is present. In this connection, the term printing unit4,5is to be understood more widely, since of course one or more of the printing units4,5can also be varnishing units, sealing units or other sheet-processing units. Even if these other units are present in the printing press1, it is expedient for the measuring beam to be fitted in the last unit5, in order to be able to monitor the sheet705with all the varnish layers. All the printing units4,5have an impression cylinder7and a blanket cylinder8, which form the press nip100of a printing unit4,5. Furthermore, each printing unit4,5is equipped with an inking unit13. The cylinders7,8and the inking unit13are mounted in the side walls14of the printing press1and are driven by motors and gearboxes present there.

The press nip100between the printing cylinders7,8can be seen more clearly in the enlargement inFIG. 1. The enlargement of the surroundings of the press nip100in the last printing unit5together with the measuring beam6additionally shows the approximate size relationships of the cross section of the measuring beam6as compared with the diameter of the press cylinders7,8. Also fitted to the impression cylinder7are sheet grippers101, which guide the sheet705around the impression cylinder7, accept it from the transport cylinder9and transfer it to the delivery3. During the measuring operation by means of the measuring beam6, the printed sheet705is held firstly at its rear end by the press nip100and secondly at its leading end by the sheet gripper101. This ensures that the sheet705can move only minimally during the measuring operation, which is of importance to the measuring operation in as much as the distance between sheet705and measuring beam6should if possible not vary during the measurement. The dimensions of the cross section of the measuring beam6inFIG. 1in the case of a printing press1of 102 cm sheet format are 102 mm in width and 69 mm in height at its end face. Furthermore, the measuring beam6is inclined slightly with respect to the horizontal, so that it runs parallel to the surface of a sheet705when the latter is being guided by the sheet gripper101and the press nip100. Fixed to the measuring beam6is a sensor15, but this can also be integrated into the measuring beam6. This sensor15is an optical sensor, for example a camera, which is able to detect markings on the printed sheet705. In addition, the sensor15can be used for the purpose of observing external light sources800and triggering the measuring operation by the measuring beam6. To this end, the sensor15is linked to the measuring electronics201and the computer200of the printing press1. Thus, the measuring operation can be controlled by the sensor15in such a way that measurements are made only when no external light800is falling on the measuring area or directly into the sensing device6. The sensor15can comprise a combined sensor or a plurality of separate sensors. It is also possible for a plurality of sensors15distributed over the entire length of the measuring beam6to be fitted. The sensors15can also be integrated into the measuring beam6.

FIG. 2shows a sheet-fed rotary printing press1which, as distinct fromFIG. 1, is equipped with a sheet turning device10, so that, in the event of perfecting in the first four printing units4,5, one side of a sheet705can be printed and the other side can be printed in the second four printing units4,5. For this reason, the printing press1inFIG. 2has two printing units5to which measuring beams6are fitted, since both the front and the rear of a sheet must in each case be monitored by a measuring beam6. In order to be able to assess the final state of a printed sheet705both in relation to the front and to the rear here as well, the measuring beams6are located in the last printing unit5before the turning device10and in the last printing unit5before the sheet delivery3. As a special feature, the sheet-fed printing press1inFIG. 2has the possibility of displacing the measuring beam6. This means that the measuring beam6is configured such that it can be removed easily and can also be installed in another printing unit4. For this purpose, connections are also fitted to the printing units4preceding the two printing units5inFIG. 2. The printing units5,4designed to accommodate a measuring beam6are provided with electrical connections for this purpose, which are in each case connected to measuring electronics201. When the measuring beam6is plugged into the respective printing unit5,4, the measuring electronics201is automatically notified via appropriate encoding as to the printing unit5,4wherein the measuring beam6is currently located. The measuring electronics201are in turn connected to the control desk and computer200of the printing press1, so that all the measured values can be displayed there to the operating personnel of the printing press1. In addition, the settings of the printing press1can be changed on the operating desk200in order to control the printing quality. The computer200of the printing press1is additionally connected to prepress devices11via a cable-bound or wire-free connection12, for example also via an Internet connection; such devices11are in particular plate exposers for producing printing plates for offset printing presses. As a result of the connection12to the prepress stage11, it is possible to use the data originating from the measurements of the measuring beam6for changing the production process in the prepress stage11as well. Therefore, further-reaching changes in the printing process can be made than would be possible by means of simple changes to the settings of the printing press1. In addition, the production of the printing plates can be optimized. It is also possible for a hand-held measuring instrument202, which can be used for calibration purposes of the measuring modules603, to be connected to the computer200of the printing press1.

The interior of the measuring beam6is depicted inFIG. 3, the measuring beam6being constructed in such a way that it can be fixed in the printing unit5,4, while a movable measuring carriage605is arranged in the interior of the measuring beam6. The measuring beam6extends over the entire width of a printed sheet, in order to be able to monitor the edge regions of the printed sheet reliably. The measuring carriage605can be moved in the interior of the measuring beam6for this purpose, in order likewise to be able to measure over the entire width of the sheet. In order to register the surface of the printed sheet, the measuring carriage605inFIG. 3has eight measuring modules603having 8 measuring heads622, it being possible for the measuring carriage605to be moved in a plurality of steps or continuously, so that, in the case of 4 colors, after 16 measurements all 32 inking zones of a plurality of printed sheets705have been measured. For this movement operation, the measuring carriage605is mounted in a guide rail606, being driven by a linear motor604. For the purpose of simple maintenance of the measuring carriage605, the latter can be removed laterally from the measuring beam6by the side walls601being removed. For this purpose, the side walls601are configured so as to be easily removable, that is to say they are fixed to the housing of the measuring beam6by a plurality of screws.

The measuring beam6substantially comprises a U profile which is open on the side facing the printed sheet. In order to prevent the penetration of dirt and, in particular, printing ink, the open side of the U profile is closed by a removable base615, which additionally has transparent parts616made of glass, so that the measuring modules603on the measuring carriage605are able to sense the printing material located underneath through the base616of the measuring carriage615. Besides the measuring modules603together with their electronics, there is further equipment on the measuring carriage605. Since the measuring modules603also have illumination modules623in addition to the spectral measuring heads622, the measuring carriage605must be provided with a source of illumination610. The source of illumination constitutes a flash lamp610, which is supplied with electrical power by a mains power unit612located on the measuring carriage. The mains power unit612in turn and electronics of the measuring modules603are connected to the housing of the measuring beam6via flexible electric cables618. The end of the flexible electric cable618fixed to the housing of the measuring beam6ends in an electric plug connector619, by means of which the measuring beam6is connected to the electrical power supply of the printing press1and the measuring electronics201. In this case, the connection of electrical power and signal transmission can be carried out by means of a plug-in or rotatable combination plug. All the electrical components, including the measuring modules603, are fitted on one or a few circuit boards631, in order to ensure short current and signal paths in a small space.

Since there is only one flash lamp610on the measuring carriage605, its flash light must be transported to the individual illuminating modules623by means of injection optics611and following optical waveguides614. In addition to the mains power unit612of the flash lamp610, there are also flash capacitors607on the measuring carriage605in order to provide the necessary energy. In addition, the measuring carriage605contains a distributor device620for distributing electric energy to the individual electrical loads and for distributing the electric signals of the components networked with one another in the measuring carriage605. However, the sensing device6is not only capable of measuring the surface of a printed sheet spectrally, but it is also used for registering register marks and for evaluating the same. To this end, the measuring carriage605has a right-hand register sensor608and a left-hand register sensor613. It is therefore possible to register the register marks in the edge regions of a printed sheet. There can also be further register sensors, for example each measuring module603can include a register sensor, in order that a plurality of register marks over the entire width of the printing material705can be measured.

Since all of the electronics in the measuring carriage605are accommodated into a very small space, for example 70 percent of the volume of the measuring carriage605is filled with components, a great deal of waste heat is produced in a relatively small space. In order to be able to carry away the waste heat and in particular to prevent damage to and influence on the measuring modules603, the interior of the measuring beam6is liquid-cooled. A closed cooling circuit is produced by a plurality of ducts621in the interior of the measuring beam6and the side walls601, this cooling circuit being closed via coolant ducts617in the side walls601. The coolant ducts621,617are supplied with coolant via a coolant connection602on the outside of the measuring beam6. A pump for circulating the coolant therefore does not have to be fitted in the interior of the measuring beam6itself, but can be connected externally.

The side view of the measuring beam6, shown inFIG. 4, shows, in addition to the substantially U-shaped profile of the measuring beam6, the coolant ducts621running in the U profile, which are connected to the closed circuit at the two end faces of the measuring beam6by the coolant ducts617in the side walls601. Furthermore, the glass cover615in the base of the measuring beam can be seen, which protects the sensitive measuring modules603on the measuring carriage605against contamination. The U-shaped housing of the measuring beam6, the side walls601and the measuring beam base615with its glass inserts616are connected to one another via seals, so that no dust or liquids can get into the interior of the measuring beam6. Furthermore, on the outside of the base615there is a dirt-repellant surface628, over which there extend webs629located transversely with respect to the longitudinal extent of the measuring beam. The webs629hold the printing material705at a distance when it is being measured and, in this way, avoid direct contact between printing material705and base615. The webs629can also be coated in a dirt-repellant manner.

FIG. 5shows a view of the measuring beam6from below, it being possible to see the measuring beam base615well here. The measuring carriage605has eight measuring modules603, which each comprise the actual measuring heads623and illuminating modules623. In order to be able to measure the entire width of a printed sheet having 32 inking zones, after each measuring operation the measuring carriage605is moved laterally by one or more measuring areas. The distance between the measuring modules603is thus four inking zones, so that the measuring modules603measure exactly each fourth inking zone in parallel. Following four sensing operations, the sheet has then been measured over all 32 inking zones of a color. If printing is carried out with four colors, 16 sensing operations are accordingly necessary. Furthermore, a movable shutter627, which is able to cover a measuring module603, can be seen inFIG. 5. The shutter627can be present on every module603and is driven electrically or mechanically, but a common shutter627for all the modules603can also be used. InFIG. 5, the shutter627can be moved in the sheet transport direction, transversely with respect to the measuring beam6, and protects the optics of the measuring modules603against damage between the measuring operations; it can also cover all of the underside of the measuring beam6between the individual measuring operations. For this purpose, the drive of the shutter627is coupled to the computer200of the printing press.

Arranged at one end601or else at both ends inFIG. 5is a calibration surface801, to which the outer measuring modules603can be moved. If a measuring module603is positioned above the calibration surface801, then this standardized surface is measured. The surface is a white tile which corresponds to paper white. By means of measuring the tile801, a measuring module603can be calibrated at any time between two measurements on the printing material705. The measuring modules603which cannot move to the tile801are calibrated by means of transfer calibration from the adjacent measuring modules603. In order to protect the tile801against contamination, it can likewise be closed by means of a cover802that can be moved laterally. Thus, the tile801is always kept covered by the cover802between the calibration measurements.

Webs629which are dirt-repellent and hold the sheet at a distance can also be seen inFIG. 5. These webs629are connected to the cover615of the measuring beam6. The measuring beam is sealed off by a glass layer616located under the cover615. For the purpose of cleaning the glass layer616, the cover616having the webs629and the cut-outs for the clear view of the measuring modules603can be folded away onto the sheet705or removed, so that all of the area of the glass layer616can easily be cleaned.

In addition to the possibility, illustrated inFIG. 3, having light sources610arranged on the measuring carriage605, it is also possible, according to the arrangement inFIG. 6, to fit the flash lamp610outside the measuring carriage605and even outside the measuring beam6. In this case it is necessary to use flexible optical waveguides614, which connect the non-moving parts of the measuring beam6and the measuring carriage605. However, the flexible waveguides614can also be used when the lamp610is located on the carriage605, as inFIG. 3. In this case, the optical waveguides614can be led separately to each measuring module603, as inFIG. 6, but it is also possible to bundle the optical waveguides614at one point and to lead them to the respective measuring module603via longer paths in the interior of the measuring carriage605. If all the measuring modules603receive the light from a single light source610, it is ensured that all the measuring modules603use the same light during the measurement and therefore the measuring conditions for all the modules603are the same. It is also possible for an additional optical waveguide614to be connected to the lamp610and to open on the other side in a light reference measuring head632. This light reference measuring head632has the task of measuring the light from the lamp610and, in the event of a change, of outputting a signal relating to maintenance and inspection. Thus, a defective lamp610or one no longer equipped with sufficient illuminating power as a result of aging can be detected in good time.

As an alternative to flexible optical waveguides614as inFIG. 6, as shown inFIGS. 7A and 7Bthe principle of the optical trombone can also be used. In this case, the optical waveguides of the measuring carriage605and of the measuring beam6in each case end at the end faces625,626of the same, so that they are always located and aligned accurately with respect to one another. Between the end faces626of the optical waveguides of the measuring carriage605and the end faces625of the measuring beam6there is an optical interspace624which, as shown inFIGS. 7A and 7B, has a different size depending on the position of the measuring carriage605. The optical interspace624between the optical waveguides can be bridged by it being silvered. By means of this silvering, the light beams emerging from the optical waveguides of the measuring beam6can be coupled into the optical waveguides in any position of the measuring carriage605. Such an optical trombone is less susceptible to wear than flexible optical waveguides614, which is of enormous importance in view of million-fold measuring operations. This is because it has transpired that flexible optical waveguides614tend to break after relatively few measuring operations and then have to be replaced.

FIGS. 8A and 8Beach show the measuring beam6seen from below, with two different arrangements of measuring heads622and illuminating modules623. In the arrangement according toFIG. 8Athe measuring heads622and the illuminating modules623are aligned so as to cross over one another, so that the light which is reflected from the printing material is not sensed by the measuring head622located directly opposite, but is crossed over like a cross. Such an arrangement permits the disposition of many measuring heads in a small space, since here the distance between the measuring heads622and the opposite illuminating modules623can be smaller as compared with an arrangement according toFIG. 8B, wherein the measuring heads622sense the reflected light from exactly opposite illuminating modules623. The smaller space inFIG. 8Aresults from the diagonal crossing, since the distance between the illuminating modules623and the associated measuring heads622cannot be reduced arbitrarily. The distance is defined by the beam path from the illuminating modules623to the printing material and back to the measuring head622. With the crossover solution, the width of the measuring beam6and the measuring carriage605respectively can be reduced. Since, given the restricted space in the vicinity of the press nip100of a printing unit4,5, the space required is a decisive criterion, the arrangement according toFIG. 8Ais better suited to this case.

InFIG. 9, a print control strip700on a printed sheet705is illustrated. The print control strip700and the actual printed image are printed onto the sheet705in the printing units4,5of the printing press1. After the last printing unit5, the sheet705and the print control strip700are complete and can be measured by the measuring beam6. The sheet705here is present in what is known as the medium format, that is to say with a sheet width of 74 cm, and has 23 inking zones701,703. Each inking zone701,703comprises 6 color measuring areas702and four further measuring areas704. These inking zones701,703are measured by the measuring modules603of the measuring beam6. Normally, only one of the measuring areas702,704per color separation and inking zone701,703on a sheet705is measured by a measuring module603. In the case of 23 inking zones701,703, six measuring modules603and 10 measuring areas702,704per inking zone, this results in 40 measuring operations on 40 printed sheets705before all the measuring areas701,703have been registered once. For more measurements on fewer sheets, more measuring modules603have to be provided. Furthermore, a plurality of print control strips700can also be applied to a sheet, for example one at the sheet start and one at the center of the sheet or the end of the sheet. Alternatively, during continuous printing operation, that is to say when the printing press1is running at production speed and all the measuring areas702,704have reached their desired state, the measuring modules603can also be placed over specific measuring areas702,704which contain color information about a plurality or all of the colors. The measuring modules603then even do not have to be moved at all or much more rarely, since the color information is present in locally compact form in one measuring area. In the event of changes within the specific measuring areas, then the measuring mode is changed again, and all the measuring areas702,704are measured again as in the start-up phase.FIG. 10shows a similar embodiment to that ofFIG. 5; in both embodiments a measuring carriage605that can be moved laterally is located in an encapsulated, sealed measuring beam6. However, inFIG. 10the measuring beam has a continuous glass cover634which closes the underside of the measuring beam6. On the outside of the measuring beam6, over the continuous glass cover634, there is also a sheet guide plate for sheet guidance633, which bears two slots639in the longitudinal direction. Through these slots639and the glass cover634, the measuring modules603comprising measuring head622and illuminating module623in the measuring carriage605are able to measure a printing material705running through under the sheet guide633. In addition, there are webs629arranged on the outside of the glass cover634and within the slots639. The webs629prevent the printing material705touching the glass cover634and therefore soiling the latter. Since the webs629formed as inFIG. 10can under certain circumstances be in the beam path of the measuring module603, because the measuring carriage605must measure over the entire width of the printing material, a compensation device is provided which compensates for the influence of the webs629in the beam path of the measuring modules603. Such a compensation device has already been described at another point in this application.

An alternative embodiment toFIG. 10is shown byFIG. 11. Here, too, a measuring carriage605that can be moved is located in a measuring beam6, but the measuring beam is open at the bottom, for which reason the measuring carriage605is closed by a base635. For this purpose, the measuring carriage605has a base635made of sheet metal, which is additionally provided with glass viewing openings636. The glass openings636are positioned exactly under the beam paths of the measuring modules603. Therefore, inFIG. 11with 8 measuring modules603on the measuring carriage605, exactly 16 glass viewing openings636are provided underneath the 8 measuring heads622and 8 illuminating modules623. The glass openings636can be circular, as inFIG. 11, but can also be oval, rectangular or configured in another shape. In addition to the glass viewing openings636, in the base635of the measuring carriage there are also small blast air ducts637, through which blast air can escape from the interior of the measuring carriage605. This blast air is used for the purpose of keeping the printing material705at a distance from the base635, in order to avoid contact with the sheet705and therefore contamination of the glass openings636. At the same time, by means of the positive pressure produced in the interior of the measuring carriage605by the blast air, foreign bodies are prevented from penetrating into the interior of the measuring carriage605from outside. Blast air is applied to the blast air ducts637by means of a blast air source638, for example a small compressor or fan in the interior of the measuring carriage605.

FIGS. 12A,12B,12C and12D show various possible ways of fixing the printing material705during the measuring operation by the measuring beam6in a sheet-fed rotary printing press1. In addition to the possibility known fromFIG. 1inFIG. 12A, of fixing the printing material705at its one end by means of a sheet transport gripper101and at its other end by the press nip100between impression cylinder7and blanket cylinder8, there are further possible ways of fixing the sheet705even when it is not in the press nip100. According toFIG. 12B, a sheet705is held at both ends by transport grippers101on a transport cylinder9and in this way is fixed under the measuring beam6during the measurement. Instead of at least the transport gripper101trailing in the sheet transport direction, a blowing device16can also be installed above the transport cylinder9, as inFIG. 12C, which presses the free end of the sheet705not fixed in a gripper onto the transport cylinder9and thus fixes it. Furthermore, a solution according toFIG. 12Dcan also be employed. In this solution, the sheet705is fixed on the transport cylinder9substantially by means of vacuum. To this end, on the cylinder surface which comes into contact with the sheet705, the cylinder9has a plurality of air openings18which are connected to a vacuum chamber17in the interior of the cylinder9. The vacuum fixes the sheet705on the cylinder in this way, which can additionally be assisted by a transport gripper101, but does not have to be. The vacuum chamber17can be constituent part of a suction pump in the interior of the cylinder9or can be connected to a suction pump outside the cylinder9.

FIG. 13explains, how the measuring beam6is mounted in a printing unit of a printing press1. In the plan view of the installation location in the printing press1, it can be seen that the measuring beam6is in principle installed transversely with respect to the sheet transport direction19, between the side walls14of the printing press1. Since the intention is that the measuring beam6can also be retrofitted in already existing machines, the mounting is made via two lateral mounting plates20, which can in principle be installed in any printing press1as long as there is the necessary space. The mounting plates20can also compensate for different distances between the side walls14, by being designed to be of different thicknesses. The mounting plates20are fixed to the side walls14by means of mounting screws21and carry the mounting for the measuring beam6. At both its ends, the measuring beam6has covers22in each case, which enclose the measuring beam6and carry bearings23. These bearings23support the measuring beam6with respect to the mounting plates20and reduce vibrations which the printing press1would transmit to the measuring beam6. The covers22can be configured in such a way that the measuring beam6can be removed simply from the covers22.