WO1996038370A1

WO1996038370A1 - On-line monitoring system for web edges by means of line cameras

Info

Publication number: WO1996038370A1
Application number: PCT/EP1996/002302
Authority: WO
Inventors: Jörg Inhelder
Original assignee: Fms Force Measuring Systems Ag
Priority date: 1995-05-29
Filing date: 1996-05-29
Publication date: 1996-12-05
Also published as: DE19519607A1; AU6002996A

Abstract

A process for the on-line monitoring of web edges by means of line cameras to detect and direct the lateral position of web edges and/or marking lines in the processing of travelling material webs in which CCD elements referred to as pixels form a sensor line on which the reflection of the travel of the web edge to be monitored, on which a light source is directed, is imaged by means of an optical system. Its analog voltage signals trigger a correcting signal on deviating from a set reference, by means of which a setting signal is introduced to correct the position of the material web, in which the analog voltage signal of the CCD line is taken to a first input of at least one comparator, to the second input of which is applied an adjustable analog voltage threshold. At the output of the comparator a digital 0 or 1 signal is generated depending on whether the threshold is exceeded or not reached and taken to a controller and by which, with the aid of a beat counter, the position of the light/dark or dark/light transition at the web edge is taken to a downstream electronic control system as an actual value. If there is no signal jump over an entire CCD line, a correcting signal from the controller shifts the threshold at the second comparator input or the power of the light source is altered.

Description

description

Method for online control of web edges using a line camera

The invention relates to a method for online control of web edges by means of a line camera for the detection and alignment of the lateral position of web edges and / or marking lines when processing or processing running material webs of different types, such as textiles, paper, plastic and metal foils, sorzeile form in which CCD-elements, a _'Sen¬ from which by means of an optical system of the Re¬ flexion of the course which is to be controlled web edge, to which a light source is directed imaged, and 0 which analog voltage signals for deviations trigger a correction signal from a setpoint value, by means of which an actuating signal for correcting the position of the material web is initiated.

5 In the production and processing of strip materials, a large number of measurement variables on the goods passing through during the production process or in the accompanying production technology are to be determined and evaluated. Geometric sizes such as bandwidth, strip edge position and 0 web thickness as well as quality features such as scratches, stains,

Holes, impurities, printing and fabric defects and edge tears are recorded and assessed. Increasing production speeds and the need for line stoppages. To minimize belt failures or material defects or material damage for cost reasons, as early as the start-up phase of the systems, it is essential that continuous, quick and exact control of a combination of measurement and test tasks is guaranteed.

0 line scan cameras for non-contact detection, control

ORIGINAL DOCUMENTS and correcting relevant geometric sizes and quality features on continuous belts are known in industrial measurement technology. CCD elements are used as sensors, which in principle consist of a large number of light-sensitive elements arranged next to one another in a line, referred to as pixels (picture elements), such as "Bänder Bleche Rohre", 8/93, P. 35, column 2. The relevant geo etry variables and quality features are mapped onto the sensor line with the aid of an optical system, the light entities received from the individual pixels being converted into electrical charges and, as an analog video signal, an analog / digital converter for digital processing for digital processing ¬ be fed. The digitized image data (geometry data or quality features) can be evaluated by a computer and, based on this, manipulated variables can be generated and fed back into the production process, where the specifically regulating or correcting tasks are carried out, as can be seen from "strips, sheets, tubes", 8 / 93, p. 36, column 1 u. 2 results.

Such measurement and inspection tasks for line cameras include, on the one hand, the assessment of the position of the object edges (width and position measurement), and, on the other hand, the detection of disturbances in the homogeneous brightness curve

(Surface damage, hole search, contamination). To solve this problem, light-dark changes which are to be interpreted either as object edges or as defects are to be determined and evaluated within the video signal supplied by the CCD element. The technical concepts available in the prior art for solving the above-mentioned problem are associated with a high level of technical complexity, as is shown and described in "Bänder Bleche Rohr", 8/93, p. 36, column 3, paragraph 1.

In order to make these processes technically simplified and cost-effective, so-called intelligent line cameras used. In contrast to the known conventional line cameras, these are provided in a housing with a complete signal processing chain of conventional image processing systems in order to carry out all decisions and calculations directly. In principle, however, the system described at the outset for recording, evaluating and correcting image data remains. This significantly improved optical sensor system with built-in measurement technology has physical and technological performance limits, as can be seen in "Bänder Bleche Rohr", 8/93, p. 36, column 3, paragraph 2.

Such an advanced CCD sensor can also only detect and emit a certain number of signals per unit of time. In contrast to this, an increasing number of pixels that can be generated on an image sensor is countered by a limited readout speed of 30 to 40 MHz. Therefore, with increasing number of pixels, the frame rate, which is available for evaluation per second, decreases, which is evident from "Technical Review" 44/90, p. 45, column 3, below.

Furthermore, when carrying out the process with incident light, it is difficult to produce absolutely homogeneous brightness distributions on the strip surface because contaminations on the light source, incident external light, printed

Paths with different color densities or different material densities with different light absorption can be misinterpreted as differentiated coverage of the pixels and consequently as path deviation.

As a result of the described physical and technological limits of the known technical solutions, error and failure rates that cannot be optimized further and still occur when processing and processing running material webs, as described in "Technical Review" 44/90, p .45. Column 1 is detailed below. In the conventional procedure, before starting up a system, the measuring and control devices have to be set and stored to the respective requirements for the greatest brightness value and the largest dark value and, as a result, an average value. This setting is necessary and very time-consuming at the start of every new work process. Readjustment is also often necessary during operation, for example if an external light source occurs or the brightness values change, if only slightly. Such readjustment necessitates a temporary standstill of the plant and results in rejects and a loss of production.

The method described in EP 0 555 853 makes it possible, for example, to readjust the light-sensitive sensor before starting up the system and whenever the processed material is changed, taking into account any contamination on the sensor. However, this adjustment is only possible before the first start-up, and is necessary again when the material is changed and before each start of work. Readjustment with continuously running web control is not possible with this known method because the sensor has to be reset, which is only possible when production stops.

In the methods described in DE 35 11 474 A1 and in EP 0 567 762, to solve this problem, sensors which function separately according to the reflection principle and according to the light barrier principle are used, or else a plurality of pairs which are exactly opposite one another Sensors are provided, which are activated and queried in pairs, which are intended to eliminate falsifying influences. These known procedures require considerable technical effort without offering solutions which are satisfactory in terms of production technology. It is therefore an object of the invention to provide a cost-effective method for checking web edges and / or marking lines when processing or processing running material webs of different types and quality with greatly differing brightness values, with little technical effort, without interrupting the work process This is followed by an automatic, continuous control of the setting and a continuous, optimizing readjustment, and which can be used for various types of measurement and control tasks at high measuring speeds while avoiding misinterpretations in the detection, and which also has an automatic, continuous error correction rectification as well as hiding the causes of faults.

This object is achieved in a method for online control of web edges by means of a line camera for capturing and aligning the lateral position of web edges and / or marking lines during the processing or processing of running material webs of different types such as textiles, paper, plastic or Metal foils, in which CCD elements form a sensor line, on which the reflection of the course of the web edge to be checked, to which a light source is directed, is imaged by means of optics and whose analog voltage signals trigger a correction signal in the event of deviations from a setpoint value which an actuating signal for correcting the position of the material web is initiated, in that

a) the analog voltage signal of the CCD line is fed to a first input of at least one comparator, at the second input of which an adjustable analog voltage threshold value is applied,

b) at the output of the comparator, depending on whether the threshold value is exceeded or undershot, a digital 0 or 1

Signal pending and fed to a controller. c) by means of which the position of the light / dark or dark / light transition on the web edge and a downstream control electronics are supplied as an actual value with the aid of a clock counter,

d) if there is no signal jump over an entire CCD line by a correction signal from the controller, the threshold value at the second input of the comparator is shifted or the power of the light source is changed.

This novel design and development of the on-line control method for web edge control, which is known per se, ensures that the analog voltage threshold value first applied at the start of work by continuous comparison with the continuously detected light / dark or dark / light transitions if there is no jump in the signal, it is corrected or the power of the light source is changed. This automatic correction ensures a contrast which is sufficiently strong and correctly process-positioned for reliable signal evaluation, by means of which continuous compliance with the setpoint values set for the path control is achieved.

The invention also includes an important further development of this procedure, which consists in the fact that

a) the analog voltage signal of the CCD line is fed to the first input of three comparators arranged in parallel, at the second input of which different and independently adjustable analog voltage threshold values are applied,

b) the mean voltage threshold value representing the light / dark or dark / light transition at the web edge, and the outer voltage threshold on both sides len values are above or below a preview distance,

c) and in the absence of the light / dark or ^" dark / light transition representing the web edge at one or both of the outer voltage threshold values, the power of the light source is corrected by the controller via a control line.

This very progressive process design enables a previously unknown early detection of environmental influences changing during the running work process, and it causes a delay-free and automatic triggering of the signal changes required for correction. If there is no signal jump at the output of only one of the comparators on both sides and in particular if the middle comparator responds simultaneously, the power of the light source can also be corrected by the controller via a control line.

The illuminance is thus continuously optimized on the basis of the measured values of the CCD line, this optimization being carried out both when starting up and when the system is in continuous operation. With this control of the illuminance, this procedure enables on the one hand a permanent adaptability of the system to different brightness values of the railway materials as well as changing environmental influences, such as temperature changes, incidence of ambient light, fine dust in the ambient air or sensor contamination . Despite such influences, the process enables the CCD line to be optimized at any time by continuously adjusting the light source without interrupting the web travel, so that the system is largely immune to interference and reliability. The invention further includes a further improvement in the reliability of web running operation with the highest level of interference immunity even at extraordinarily high web running speeds in that the correction signals of the three comparators are supplied on the one hand to a data memory and on the other hand respectively assigned comparison gates with the respective direct preceding output signal n-1 stored in short-term memories are compared and sent to a logic module which only issues a memory command to the data memory when the signal jump occurs. This peculiarity enables advantageous data compression, by means of which a high dynamic of signal processing can be achieved with little hardware outlay, and which furthermore enables a real-time transfer rate on the interface between CCD line pre-evaluation and control electronics.

An additional, progressive embodiment of the method according to the invention can also be seen in the fact that at least two light / dark or dark / light transitions are interrogated in order to detect a marking line or an image edge applied to the material web and by means of a preliminary evaluation with the immediately preceding output signals either in the controller or in the downstream control electronics are compared and it is determined whether the line formation is valid or incorrect information due to printed marks or fonts.

This procedure also has the great advantage that the control on a line can be carried out much more robustly, since neither the detection area of the sensor has to be selected unnecessarily narrow nor special regulations about a required minimum distance of print marks from the line are required.

An exemplary embodiment for the application of the inventive methods for online control of the railroad The course of a belt system is shown schematically in the drawing and is described and explained in more detail below.

It shows

Fig. 1 is a block diagram of a first embodiment and

2 shows a block diagram of an improved and preferred embodiment of the invention;

3a shows a schematic diagram with the analog voltage values of the measuring points in the procedure according to FIGS. 2 and

3b the respective signal jump at the outputs of the three comparators.

1, a light source 10 is directed to a limited area on both sides of the edge 12 of a material web 11 to be checked. Light rays reflected from this area are projected through optics 13 onto a CCD line (charged coupled device) 14, the output of which is connected to a first input of a comparator Kl, which is connected at its second input by the microcontroller 15 via an 8- bit DA converter 16 receives an analog threshold value.

The light rays are reflected by the partial area lying on the material web 11; From the partial area lying in the beam path of the light source 10 but next to the material web 11, no or only a few light beams (for example external light) are projected onto the CCD line 14. The output of the comparator Kl is connected to the micro-controller 15, and this communicates via a serial bidirectional interface 17 and the connection 18 with control electronics (not shown in the drawing), through which, if necessary, an actuating signal for aligning the material web 11 is generated.

The CCD line 14 consists of a large number (for example 2048) of pixels arranged in a row transversely to the running direction of the material web 11, which form measuring points at which analog signals in the form of charge packets are generated by the incident light, and which are temporary get saved.

These charge packets are by the clock generator 19 supplied via the shift clock line 20 shift clock of z. B. 2 MHz cyclically shifted in the manner of a shift register in the CCD line 14 until they successively leave them at their output as analog output voltage signals and are fed to the first input of the comparator K1. There they are compared with the threshold value set via the D / A converter 16. The CCD line 14 thus scans the passing material web 11 line by line in its edge region and continuously delivers analog voltage signals in the shift cycle, i.e. with a shift clock of 2 MHz and 2048 measuring points or pixels on the CCD line 14, a line set of new values is available after every millisecond; the scanning is therefore carried out with a correspondingly high resolution.

The output signals of the CCD line 14 are digitized; The further evaluation is also carried out digitally, including the interface 17 to the central evaluation device. This results in a data compression from 8 bits to 1 bit already at the comparator K 1. All recorded information will be evaluated centrally and, as will be explained further below, the sensor can be parameterized by the evaluation device for the respective application.

For the measuring points at which the CCD line has 14 light beams 96/38370 PO7EP96 / 02302

11 -

from the partial area of the scanning line lying outside the material web 11, the output signals received on the CCD line 14 will be below the threshold value set on the comparator K 1 and a digital O signal will be present at the output of the comparator K 1; for the measuring points in the coverage area at which the CCD line 14 receives light rays reflected from the material web 11, the output signals obtained will reach or exceed this threshold value, and a digital 1 signal will be present at the output of the comparator K 1 and the micro- Controller 15 fed.

With the help of a clock counter, not shown in Fig. 1, it can be determined and recorded at which clock, i.e. At which measuring point of the CCD line 14 the transition took place, which then also determines the current position of the web edge 12. If this position value deviates from the predetermined target value, a controller (not shown in the drawing) triggers an actuating signal for correcting the lateral position of the material web 11.

The light rays do not strike the CCD line 14 in parallel, but rather the reflected light rays, which are imaged on the CCD line 14 by the optics 13. Therefore, the transition corresponding to the web edge 12 does not take place in a single, precise charge jump from one measuring point of the CCD line to the next or from one shifting cycle to the next, but as a result of a certain light scattering, the amount of charge changes in several steps over several measuring points. It is therefore advantageous to choose a focal length of the optics 13 at which the web edge 12 in its target position corresponding to the threshold value is mapped exactly to a specific measuring point on the CCD line 14 in order to obtain an exact signal jump at the comparator K1.

Should, for example, change the material and result from Given the changed reflection properties, initially no transition is found over the entire CCD line 14, the controller 15 changes the threshold value or the light intensity of the light source until an edge or also a marking line is found upon excitation via the interface 17. If this does not succeed, an error signal is sent via the interface 17. In the same way, for example, a shift in the voltage level of the individual CCD elements due to a temperature drift is automatically compensated for.

As is well known, halogen lamps have proven to be well suited as light source 10 because they emit relatively good efficiency and emit white light, on the frequency range of which a CCD reacts very sensitively, and because of their sluggish reaction mode, they emit a constant one even in clocked operation Light off. The power of the light source 10 is ^" controllable ^" by the controller 15 via a control line 21; if, for example, the incidence of light becomes weaker due to contamination of the optics 13, this causes a shift in the voltage level of the CCD elements; the digitally queried position of the The transition remains, however, and the level shift can be compensated for by a controlled increase in the power of the light source 10, so that the threshold value of the comparator K1 is again exactly at the

Point of transition is reached. Conversely, a shift in the voltage level due to incidence of extraneous light in the opposite direction can be compensated for by a controlled reduction in the light source power.

The procedure described here according to Fig. 1 makes it possible to detect the web edge or a e.g. imprinted marking line for the web edge position control and the correction of target deviations can be carried out with great reliability and interference immunity.

In the rare cases of sudden, severe changes If the light or reflection conditions change, the voltage amplitude at the output of the CCD line 14 for the measuring point representing the transition no longer reaches the threshold value previously set on the comparator K 1, the web edge control can fail briefly until the threshold value tracked by the controller 15, ie is newly set.

Such an interruption, even if only a short one, can be disadvantageous for high-speed machines. It is therefore desirable to obtain a preliminary indication of reaching the web edge 12 from both directions when scanning the web edge area line by line so that any necessary adjustment of the threshold value is initiated and an interruption of the control process is avoided.

This extended object of the invention is achieved by further developing the method, which is shown in the block diagram according to FIG. 2; same parts with the same reference numerals as in Fig. 1 are designated.

As can be seen, the power of the light source 10 can be controlled by the controller 15 via control electronics or the interface 17 by means of the control line 21: as in FIG. 1, it is directed towards the material edge to be checked. Their reflection acts on the CCD line 14 via optics (not shown) (the material web is also not shown in FIG. 2).

Via the shift clock line 20, the CCD line 14 is driven by the clock generator 19 with the shift clock of preferably 2 MHz. The CCD line 14 is connected to a sampling and holding device 22 which temporarily stores the analog voltage signals originating from the measuring points of the CCD line 14 and to the first input of three comparators K1, K2, which are located in parallel. K3 that over receive the D / A converter 16 at their second other input independently of each other, different threshold values continuously monitored by the controller 15. In the case of the comparator K1, as in the embodiment according to FIG. 1, this is the threshold value 1 which represents the target value of the lateral web edge position. The threshold values 2 and 3 of the other two comparators K2 and K3 lie around a safety and display distance below or above, as can be seen from Fig. 3a.

The three comparators provide at their outputs a digital 0 to 1 signal for each measuring point or each pixel of the CCD line 14, depending on whether the voltage signal currently being supplied exceeds or falls below the threshold value in question. During the light / dark transition from the material web to the background, the voltage signal supplied by the CCD cell 14 makes an intensity jump at the comparator inputs, which causes the comparator outputs to jump from 0 to 1 signal in quick succession; the corresponding happens with a dark / light transition in the opposite direction, i.e. at the comparator outputs, the signal jumps from 1- to 0-.

This is shown schematically in FIG. 3a. The measuring points or pixels of the CCD line 14 are plotted on the horizontal and the analog voltage values of the individual measuring points can be seen on the vertical, while the threshold values of the comparators K1, K2, K3 are indicated by 1, 2 and 3 . The area a on the horizontal corresponds to the partial area of the measuring line outside the material web, the area b marks the transition at the web edge in which the comparator thresholds are successively reached, the area c corresponds to the area of coverage of the material web and the CCD Line 14.

3b is that at the outputs of the comparators Kl, K2, K3 are shown when the threshold values in the transition range are reached.

The outputs of the comparators K1, K2, K3 are connected to a memory 23 in which the light / dark or dark / light transitions together with the position on the CCD line determined by a clock counter 24 with the aid of the shift clock 14 can be saved. It communicates with the controller 15 and this via the serial bidirectional interface 17 and the connection 18 with central control electronics.

In normal operation, only the light / dark transition at the web edge, which corresponds to the average threshold value 1 of the comparator Kl, is detected and its position is transmitted to the central control electronics, which in turn determines whether the position of the web edge corresponds to the target value ¬ speaks and generates a control signal in the event of a deviation. The two additional comparators, which can be set to the upper and lower threshold values, provide the control electronics with a preview of the impending web edge transition from one direction or the other. In this way it can be avoided that due to changing reflection conditions, e.g. due to material changes, incident external light, dirt or other influences due to the resulting shift in the voltage level at the measuring points of the CCD line 14, the light / dark transition necessary for the detection of the web edge suddenly fails and accordingly the regulation is temporarily suspended.

In the event of a signal level change, one of the two outer threshold values 2 and 3 is no longer reached first, and the comparator K2 or K3 in question therefore no longer indicates a transition. However, the control is maintained by the comparator K 1 with the average threshold value 1, and there is sufficient time to ben transition signal one of the two outer gates Kompara¬ K2 or K3, the threshold values by the central ^'Re¬ gelelektronik the changed conditions readjusted accordingly. The web edge control is not interrupted, but can be continued continuously.

The power of the light source can also be changed according to changed conditions, e.g. in the event of contamination on the optical system, be controlled before the web edge control fails. In addition, there is the possibility of making the user aware of the change via a suitable interface and, for. B. prompt for cleaning the optical system.

The following reaction options of the CCD sensor according to the invention are possible under changed conditions:

- The reflection from the material web is so much weaker that the comparator K3 with the upper threshold value 3 no longer finds a transition: the light intensity of the light source 10 is increased or the threshold value 3 is lowered via the controller until the comparator 3 can respond again;

- The reflection from the material web becomes so much stronger that the comparator K2 with the lower threshold value 2 no longer finds a transition; thereupon the light intensity emanating from the light source 10 is reduced or the threshold value 2 is raised via the controller 15 until the comparator K2 can respond again;

- The pollution on the optical system increases, the intensity of the reflected light consequently decreases, and the comparator K3 with the upper threshold value 3 no longer finds a transition; the light intensity of the light source 10 is increased or the threshold value 3 of the comparator K3 is lowered; - The CCD line 14 is overdriven by a high proportion of extraneous light, the comparator 2 with the lower threshold value 2 no longer finds a transition; the light intensity of the light source 10 is reduced or the threshold value 2 of the comparator K2 is raised.

As already mentioned with reference to FIG. 1, the CCD line 14 continuously delivers analog signals with the frequency of the shift clock (in the example 2 MHz), which all trigger a digital 0 or 1 signal at each comparator output K 1, K 2, K 3 . Of interest for web edge control is only the respective signal jump in the transition area of the web edge, not the constant signals in the area of coverage of the material web or in the background area. In order not to flood the memory 23 and the subsequent electronics with unnecessary information signals, according to the invention the information signals are pre-evaluated in the sensor. For this purpose, each digital signal coming from the comparators K1, K2, K3 is compared with the immediately preceding one. Each output signal n coming from one of the comparators K1, K2, K3 is fed to one input of a comparison gate 25, at the other input of which the immediately preceding output signal n-1 of the same comparator K1, K2, K3 is present, which is stored in a short-term memory 26 was saved for the duration of the following shift cycle. Only if a change in the output signal n to the previous output signal n-1 is found in the comparison gate 25, does the subsequent logic circuit 27 issue a memory command to the memory 23, in which the latter then switches from light to dark or from Signals that indicate (pre-) dark to light on the web edge together with the position signal from the clock counter 24 are stored and forwarded to the controller 15 and the interface 17 for further evaluation so that an actuating signal can be emitted in the event of a lateral deviation of the web edge. The information is thus compressed from, for example, 2048 to 3 units per image. key line instead (corresponding to the number of pixels of the CCD line 14), and only relatively little data has to be stored in the memory 23 and transmitted via the interface 17; this results in a correspondingly high dynamic for the control and allows the use of a memory with a smaller storage capacity.

Because of its high resolving power, the sensor according to the invention also allows the lateral position of a material web 11 to be controlled after an applied marking, e.g. a printed marking line or a picture edge. For this purpose, three transitions are recorded on comparators K1, K2, K3; this is shown schematically in FIGS. 3a and 3b. In addition to the detection of the transition at the web edge (area c of the diagram in FIG. 3), the sensor detects two further transitions on the marking line in each scanning line, one from light to dark and one from dark to light, because in the Be ¬ rich of the marking line, the incident light is reflected significantly less than from the rest of the surface of the material web 11. If the three transitions are found in a line at a constant distance from one another, this shows the lateral position of the marking line, and the control can then be carried out respectively. By including three transitions in succession along with the two transitions on the marking line itself, that of the web edge, it is ensured that the marking line has actually been detected and not a fault location in the material. Because of the high line frequency of the sensor according to the invention, as mentioned above, a new set of values may be available per millisecond, it does not affect the regulation of the material web position if the marking line has a short interruption or another error.

If the shift clock generated by the clock generator and applied to the CCD line via a shift clock line is a Frequency of 2 MHz and on the CCD line as measuring points in a row across the material web 2048 light-sensitive elements (pixels), a new line set of voltage signals may be available at the output of the CCD line per millisecond.

The control according to a marking line can preferably be parameterized in the control electronics or selected (at the push of a button).

The sensor according to the invention can be used universally; as explained, it can be adjusted to very different materials with very different reflection properties. The use of two sensors on both sides of the material web 11 allows the position of the center of the web to be regulated. The sensors can advantageously be connected to the control electronics via plug contacts, and this can be switched to right or left web edge control or web center control by pressing a button.

Claims

Expectations

1. Procedure for online control of web edges using

Line scan camera for capturing and aligning the lateral position of web edges and / or marking lines during processing or processing of running material webs, in which CCD elements, referred to as pixels, form a sensor line on which the reflection of the course of the material to be checked is controlled by means of optics Web edge to which a light source is directed is imaged and whose analog voltage signals trigger a correction signal in the event of deviations from a set desired value, by means of which a

Control signal for correcting the position of the material web is initiated,

characterized in that

a) the analog voltage signal of the CCD line (14) is fed to a first input of at least one comparator (Kl), to the second input of which an adjustable analog voltage threshold value is applied,

b) a digital 0 or 1 signal is present at the output of the comparator (K1), depending on whether the threshold value is exceeded or not, and is supplied to a controller (15),

c) by means of a clock counter, the position of the light / dark or dark / light transition on the web edge (12) and a downstream control electronics is supplied as the actual value.

d) if there is no signal jump over an entire CCD line (14) due to a correction signal from the controller (15), the threshold value at the second input of the comparator tors (Kl) shifted or the power of the light source (10) is changed.

2. The method according to claim 1, characterized in that

a) the analog voltage signal of the CCD line (14) is fed to the first input of three comparators (K1, K2, K3) arranged in parallel, at their second input different and independently adjustable analog voltage threshold values (1, 2, 3) are present,

b) where the mean voltage threshold value (1) represents the light / dark or dark / light transition at the web edge (12), and the outer voltage threshold values (2, 3) on both sides are above or below this at a preview distance ,

c) and in the absence of the light / dark or dark / light transition representing the web edge (12) at one or both of the outer voltage threshold values (2, 3), the power of the light source (10) from the controller (15) via a Control line (21) corrected.

3. The method according to claim 1, characterized in that the correction signals of the three comparators (K1, K2, K3) are supplied on the one hand to a data memory (23) and on the other hand respectively assigned comparison gates (25) with the respectively immediately preceding ones in short-term memories ( 26) stored output signal n-1 are compared and sent to a logic module (27), which only issues a memory command to the data memory (23) when a signal jump occurs.

4. The method according to claim 1 and 3, characterized in that for detecting a marking line or an image edge applied to the material web (11) little At least two light / dark or dark / light transitions are interrogated and compared by means of a pre-evaluation with the immediately preceding output signals either in the controller (15) or in the downstream control electronics, and a determination is made as to whether the line formation is valid or is misinformation due to print marks or fonts.