CN105682928B

CN105682928B - Sensor for optically detecting markings on a running material track

Info

Publication number: CN105682928B
Application number: CN201480050135.7A
Authority: CN
Inventors: F.韦斯特霍夫
Original assignee: Windmoeller and Hoelscher KG
Current assignee: Windmoeller and Hoelscher KG
Priority date: 2013-09-12
Filing date: 2014-09-12
Publication date: 2020-04-21
Anticipated expiration: 2034-09-12
Also published as: WO2015036565A2; WO2015036565A3; CN105682928A; DE102013015036A1; EP3043998A2

Abstract

The invention relates to a sensor (101, 201, 301, 401) for optically detecting markings on a running material track (105). In order to provide a sensor that can be reliably used for all possible combinations of printing colors and backgrounds without a loss in achievable accuracy, the sensor comprises: a light source (109) for generating a light spot (113) on a running material track, wherein the intensity of the light spot is controllable by a driver (110); a light receiver (114) for receiving light reflected from an aspect of the spot; a signal processing unit (116) for analyzing an output signal of the optical receiver; and a switching unit for switching between a teaching mode in which the intensity of the light spot is changed by the driver and a detection mode in which the intensity of the light spot is kept constant by the driver.

Description

Sensor for optically detecting markings on a running material track

Technical Field

The invention relates to a sensor for optically detecting markings on a running material track.

Background

Such sensors are known, for example, from EP 1050843 a2 or from EP 2278361 a 1.

Sensors for optically detecting markings on a running material path are used, for example, in printing presses in order to measure and, if appropriate, adjust the so-called register accuracy between successive printing units.

In printing technology, the term "register accuracy" refers to the positional accuracy of the printed monomer layer relative to a nominal position. Accordingly, the respective color layers of a plurality of successively arranged color members must be printed precisely on top of one another in order to produce a finished printed image having the desired color impression. Otherwise, the finished printed image appears blurred and inferior in quality. Thus, depending on the respective disturbance variable, for example, a peripheral register or a lateral register is involved.

In order to automatically calibrate register deviations, so-called register settings are used in printing presses and in particular in intaglio printing presses. For this purpose, the position of the respective mark on the running material path is optically detected in each printing unit, i.e. from the second printing unit, by means of a sensor.

To identify the register mark, a light beam is directed onto the track by a light source. A part of the light scattered back by the track is conducted back through the lens to the light receiver, which is able to detect the corresponding color change on the track and the corresponding sign edge as a function of time.

If a triangular shape is selected for the register marks, for example, the time difference between the straight mark edges of two register marks of the same color is a measure for longitudinal register, while the time difference between the mark edges of a single register mark is a measure for lateral register.

In a next step, an adjustment command is calculated in the controller from the sensor signal of the light receiver, which adjustment command is supplied to the respective register adjustment motor. By adjusting via the register adjustment motor, the path of the path between the two printing units is then shortened or lengthened in the case of peripheral register and shifted to the respective side in the case of side register.

Despite the use of these sensors in register adjustment, register deviations in the finished printed image can always be determined under certain conditions. The important reason for this is that, due to the diversity of the variations of the available printing colors and backgrounds, it becomes increasingly difficult to guarantee a high contrast and thus reliable register measurement for all combinations of printing colors and backgrounds.

Disclosure of Invention

The object of the present invention is therefore to provide a sensor which can be used reliably for all possible combinations of printing colors and backgrounds without any loss in terms of achievable accuracy.

This object is achieved by a sensor according to the invention.

The sensor according to the invention for optically detecting markings on a running material track comprises: a light source for generating a light spot on a running material track, wherein the intensity of the light spot is controllable by a driver; a light receiver for receiving light reflected from an aspect of the light spot; a signal processing unit for analyzing an output signal of the optical receiver; and a switching unit for switching between a teaching mode in which the intensity of the light spot is changed by the driver and a detection mode in which the intensity of the light spot is kept constant by the driver.

The invention is based on the recognition that an increased diversity of variations in background and printed color leads to an increased dynamic range at the light receiver, which dynamic range can no longer be reliably evaluated by the subsequent signal processing unit. In the case of weak received signals, the signal-to-noise ratio is therefore no longer sufficient for reliable analysis, whereas in the case of strong received signals the output signal of the optical receiver becomes saturated and therefore likewise cannot be used for contrast comparison.

However, if a teaching mode is provided in the sensor according to the invention, in which the intensity of the light spot is varied by the drive, the dynamic range of the received signal can be compensated in a targeted manner and the achievable reliability of the sensor is therefore increased to approximately 100%.

Alternatively, or in addition, a regulating amplifier for optimum level control may be connected between the optical receiver and the signal processing unit.

A regulated amplifier in the sense of the present invention is an amplifier with a variable, externally controllable amplification factor.

A controllable driver in the sense of the present invention is here a power module with variable, externally controllable power delivered to the light source.

In the teaching mode, the intensity of the light spot is preferably changed by the driver such that the output signal of the light receiver is in the optimal control range. The teaching mode can be performed within the register mark with a currently available clock frequency, e.g. 500 kHz. Here, a clock frequency of 500kHz corresponds to a clock period of 2 μ s. If further assume that: the average printed mark has a length of 5mm and a typical track speed is 600m/min, the spot of the sensor passes the printed mark within 500 μ s. That is, in the case of a2 μ s clock cycle, 250 different clocks can be passed in teaching mode within one printed mark in order to change the intensity of the light spot step by step and adjust it to an optimum level.

But normally also in the detection mode the changes in the output signal of the optical receiver should be detected step by step. However, with the switch position of the switching unit according to the invention, the signal processing unit can recognize: whether the sensor is currently operating in the teaching mode or the detection mode. Since the teaching mode expires as explained above only in a short time period of a few μ s or within a short distance of a few μm, it is even possible for the signal processing unit to deduce the determination signal which should actually be recognized during the teaching mode. For this purpose, the signal processing unit can use other system parameters, such as a travel sensor of the track cylinder or the track speed currently stored in the system.

The principle of optimum level control and/or optimum intensity control is analogous in some ways to the use of audio compressors or audio expanders in acoustic technology, for example for limiting or expanding the dynamic range during sound recording in the case of pre-given technical limits of the recording medium.

However, audio compressors are also used, for example, for the targeted processing of individual sound components in a sound map. Thus, for example, a person's singing voice naturally has a high degree of dynamics, which in unprocessed form makes it problematic to have singing appear in the foreground relative to the rest of the track. These level fluctuations can be equalized with the aid of the audio compressor, as a result of which a continuously high average level and thus a significantly improved signal presence are achieved.

In so-called downward audio compressors, in acoustic technology, the audio input signal of an amplifier is amplified from a certain level with a small amplification factor. In so-called up audio compressors, audio input signals of the amplifier below a certain level are amplified with a high amplification factor. Of course, a combination of the two principles and a non-linear characteristic curve of the amplification factor is also conceivable, wherein in any case a compression of the dynamic range of the audio input signal takes place.

The opposite situation exists when the audio input signal has a too small dynamic range. In this case, an audio extender is used to increase the dynamic range. That is, this means that the light channels are amplified more slightly, while the loud channels are amplified more loudly.

The regulating amplifier according to the invention and/or the controllable driver according to the invention can be used both as a compressor and as an expander.

According to a preferred embodiment, the light receiver is mounted such that the directly reflected light of the light spot falls on the light receiver. If in this case the control range of the light receiver is adapted to the surface of the extinction of the running material path, the light receiver is excessively overshot in the case of a reflective surface of the running material path. Conversely, if the control range of the light receiver is coordinated with the reflective surface, the signal of the surface that is extinguished disappears in the noise. That is, there are typical cases of excessively high dynamic ranges, so that the compression method described above is used.

According to a further preferred embodiment, the light receiver is mounted such that the diffusely scattered light of the light spot falls on the light receiver. If the control range of the light receiver is matched to the extinction surface of the running material track in this case, a smaller signal level is obtained at the light receiver on the reflective surface of the running material track, depending on the installation angle of the light receiver, but this signal level may be hardly distinguishable from the extinction surface with respect to the color change. Therefore, there are cases where the dynamic range is too low, so that the extension method described above is used.

The analysis of the directly reflected light of the light spot and the analysis of the diffusely scattered light of the light spot have advantages and disadvantages, respectively. Therefore, according to a further preferred embodiment, provision is made for: the directly reflected light and the diffusely scattered light are analyzed in two separate receiving channels and the receiving channel providing a more reliable signal is then used for the analysis, respectively.

Different schemes occur when actually determining the optimal level control and/or the optimal intensity excitation.

One solution consists in supplying a feedback signal to the signal processing unit for determining an optimum level control and/or for an optimum intensity excitation. This means that the output signal of the regulating amplifier is observed and the power of the controllable driver and the amplification factor of the regulating amplifier are adjusted according to the resulting envelope curve such that the desired control occurs. This method can be applied very easily, but has the disadvantage of a certain dead time, so that in the case of small contrast differences it is not possible to still identify the transition.

Another solution, in particular for time-critical applications, consists in that a feed-forward signal can be supplied to the signal processing unit for determining an optimum level control and/or for an optimum intensity excitation. For this purpose, a further light receiver is connected upstream of the original light receiver. Depending on the resulting envelope curve of the preceding optical receiver, an adjustment of the power of the controllable driver and of the amplification factor of the regulating amplifier is then carried out such that the desired control occurs at the following optical receiver. If it is dependent on the reliable recognition of edge-like contrast differences of the marking on the running material path, it must furthermore be ensured in this method that: the adjustment of the power of the controllable driver and of the amplification factor of the regulating amplifier is not performed exactly in the time in which the contrast difference is presumed. Due to this requirement, it is also conceivable to connect a plurality of optical receivers before the original optical receiver, so that a plurality of feed-forward signals are obtained in different stages, which feed-forward signals yield an increase in accuracy step by step. Thus, for example, in the first stage of the dynamic range, optimization can be carried out according to the compression method and/or according to the expansion method, while in the second stage the time is determined for which no adjustment of the power of the controllable driver and of the amplification factor is allowed, so that then in the last stage all required parameters for an optimal level control and/or for an optimal intensity excitation are present. Furthermore, it is likewise conceivable to connect different feed-forward stages not in series, but in parallel. Combinations of series and parallel connection of different feed forward stages are also contemplated.

It has been shown that at least four cases should be distinguished in the case of an optimum level control and/or in the case of an optimum intensity excitation in order to optically detect marks on a running material track in a reliable manner. Here, it is assumed hereinafter: the directly reflected light of the light spot falls on the light receiver. Furthermore, similar considerations apply to the case where the diffusely scattered light of the spot falls on the light receiver.

The background of the running material track has a matt surface and the mark itself is also made of a matt colour. In this case, it is recommended to first adjust the amplification factor of the booster to an intermediate value and then to select the intensity excitation such that the control at the output of the booster is likewise in the intermediate range in the case of an extinction background. The label-based color change (either level-up or level-down) can then be reliably recognized at the light receiver.

The background of the running material track has a reflective surface and the indicia is comprised of a matte color. In this case, it is recommended to first adjust the amplification factor of the booster to a middle value and then to select the intensity excitation such that the control at the output of the booster is in the upper range in the case of a reflective background. The change to the extinction color of the marking then results in a strong level drop which can be reliably recognized at the light receiver.

The background of the running material track has a matt surface and the indicia is comprised of a reflective colour. In this case, it is recommended to first adjust the amplification factor of the booster to an intermediate value and then to select the intensity excitation such that the control at the output of the booster is in the low range in the case of an extinction background. The change to the reflective color of the mark then results in a strong level increase, which can be reliably recognized at the light receiver.

The background of the running material track has a reflective surface and the mark itself is also made of a reflective colour. In this case, it is recommended to first adjust the amplification factor of the booster to a middle value and then to select the intensity excitation such that the control at the output of the booster is likewise in the middle range in the case of a reflective background. The label-based color change (either level-up or level-down) can then be reliably recognized at the light receiver.

A general observation of the four described cases shows that a mark on a running material track can lead to both a level increase and a level decrease at the light receiver. In order to be able to reliably detect whether a level increase or a level decrease is assigned to a contrast change from background to marking or conversely to a contrast change from marking to background, the time window already mentioned above can be used again, in which the power to the controllable driver and the change in the amplification factor of the regulating amplifier are blocked. The time window is determined by the feed forward stage provided for it, so that the sensor originally used to detect the marks on the running material track can employ the following logic:

a flag if a level up and a level down or conversely a level down and a level up occur within the time window.

Whereas if multiple level rises and/or multiple level drops occur within the time window, these levels cannot be unambiguously assigned to a mark. The corresponding error report is then forwarded to the upper level system.

According to a further preferred embodiment, it is provided that the light source is a high-power LED (LED stands for light-emitting diode or light-emitting diode). High-power LEDs are now available for light intensities in the range of 50 candela (units of light intensity, symbol: cd) and up to 100 lumen (units of luminous flux, symbol: Im) already at a few hundred milliamp of diode current. High power LEDs can be used as monochromatic LEDs with white light color and a spectrum of approximately uniform distribution in the visible range. But alternatively so-called multicolour LEDs, which can emit light in a plurality of colours, can also be used. For example, 3 LED chips of the basic colors red, green and blue or even 4 LED chips of the basic colors red, green and blue and white can be mounted on an SMD carrier.

The LED driver may be implemented, for example, in the form of two superimposed pulse width modulations. The effective diode current, and thus the brightness of the diode, is adjusted using pulse width modulation, while the second pulse width modulation is responsible for the clocking of the respective LED. As is known from the field of optical waveguide transmission technology, clock frequencies in the range of 1 mhz can be realized without problems in the case of LEDs. The clocking of the LEDs furthermore has the advantage that in each pulse a dark value (LED off) is available in addition to the brightness value (LED on). After the LED amplifier, the brightness value can then be subtracted from the dark value, so that interference effects can thereby be eliminated.

According to a further embodiment, it is provided that the light receiver is a color-to-voltage converter. The color-voltage converter may be formed, for example, by 3 photodiodes, which are preceded by color filters of the basic colors red, green and blue, respectively. The photocurrent is a measure of the light incidence over the corresponding wavelength range. Then, a current-to-voltage converter is connected after each photodiode, so that the output voltage is finally a measure for the light incidence over the respective wavelength range.

According to the invention, a regulating amplifier for optimum level control is now connected after this voltage.

When measuring the three basic colors red, green and blue, therefore, three adjusting amplifiers are then correspondingly required.

A particularly preferred application of the sensor according to the invention is as a measuring element in a register of a printing press.

Drawings

Further details and advantages of the invention are described in relation to the figures. Wherein:

fig. 1 shows a sensor according to the invention according to a first embodiment;

fig. 2 shows a sensor according to the invention according to a second embodiment;

fig. 3 shows a sensor according to the invention according to a third embodiment; and

fig. 4 shows a sensor according to the invention according to a fourth embodiment.

Detailed Description

Fig. 1 shows a sensor 101 according to the invention according to a first embodiment, which is arranged behind a printing member 102 of an intaglio printing press. In the printing member 102, a printing cylinder 103 (impression roller) and a forming cylinder 104 are schematically shown. The individual printing members of the intaglio printing press are arranged one after the other, with the material track 105 passing uninterrupted through each printing member. The embossing roller 103 is driven in a force-fitting manner by the forming cylinder 103 in contact with the material path 105. To facilitate color transfer from the forming drum 104 onto the material track 105, the material track may be electrostatically charged shortly before reaching the impression roller 103.

Each printing member prints indicia (also known as register marks) onto the material track. Three markers are shown in FIG. 1-namely marker 106, marker 107, and marker 108-. This means that the material track 105 has passed three printing members. That is, for example, indicia 106 originates from a first print, indicia 107 originates from a second print, and indicia 108 originates from a third print.

A high-power LED 109 with white light emission is provided as a light source in the sensor 101. The LEDs 109 are supplied with power by an energizable LED driver, wherein the power can be controlled by a control and analysis unit.

The light beam generated by the LED 109 passes through a semi-permeable mirror 111 and leaves the sensor 101 via a lens 112, and generates a light spot 113 on the material track. Depending on the surface of the material track, the light of the spot 113 is reflected according to a radiation pattern characteristic of the respective surface. A portion of the reflected light is detected by the lens 112 and conducted by the semi-permeable mirror to the light receiver 114.

The light receiver 114 is in principle formed by three photodiodes with preceding filters of the basic colors red, green and blue, respectively. The photocurrents of the three photodiodes are conducted to the conditioning amplifier 115 through a current-to-voltage converter.

Since the three photocurrents must be amplified, the conditioning amplifier 115 is actually three separate amplifiers whose amplification factors can also be controlled separately by three separate level conditioners. The outputs of the three amplifiers are also analyzed separately by the control and analysis unit 116. Furthermore, the control and evaluation unit 116 also controls the level controller separately. But for simplicity the entire assembly will be referred to hereinafter collectively as the conditioning amplifier 115.

The output signal 117 of the control amplifier 115 is supplied to a control and evaluation unit 116. The control and evaluation unit 116 in turn has three outputs, namely a first control output 118 for controlling the amplification factor of the regulator amplifier 115, a second control output 119 for controlling the LED driver, and a signal output 120, from which signal output 120 the sensor signal, i.e. the measurement result, is forwarded to the upper-level register via a field bus, for example a power over ethernet (Powerlink).

For optimal level control and/or for optimal intensity excitation, the control and analysis unit 116 analyzes the level of the output signal 117 of the regulating amplifier 115. If the level overshoots, the control and evaluation unit reduces the amplification factor of the control amplifier by 10dB by controlling the output 118. If there is an overshoot after this reduction, the amplification factor of the control amplifier is reset again to the previous value and the power of the LED driver 110 is then reduced by 10dB via the control output 119. This process repeats until output signal 117 no longer overshoots. The output signal 117 is then optimally controlled via the control output 118 by correspondingly adjusting the amplification factor of the regulating amplifier 115.

That is, the optimal level control and/or the optimal intensity excitation according to fig. 1 is performed according to the feedback principle. However, in real-time critical applications, the feedback principle has the disadvantage of an inherent dead time. This disadvantage can be avoided in the case of a configuration according to the feed forward principle as shown in fig. 2.

Fig. 2 shows a sensor according to the invention according to a second embodiment. Only the principle differences from fig. 1 are explained below. The rest is referred to the description according to fig. 1.

The sensor according to fig. 2 now consists of a feed-forward stage 201 and an original measuring stage 203. The measurement stage 203 gets a feed forward signal via the feed forward line 202, which contains a rough estimate of the level information and position information about the marker. This information is then used by the measurement stage 203 for optimal level control and/or for optimal intensity excitation. The sensor signal, i.e. the measurement result, is then transmitted via the signal output 204 via a field bus (e.g. an ethernet power line) to the superordinate register controller.

The mode of action of the feed-forward stage 201 is in principle equivalent to the mode of action of the sensor 101 according to fig. 1, i.e. the optimum level control and/or the optimum intensity excitation of the feed-forward stage 201 is based on the feedback principle described in the first embodiment. Due to the dead time, the feed forward stage 201 is therefore able to determine only rough estimates of the level information and position information about the markers, but these estimates are sufficient for the metrology stage 203 to guarantee optimal level control and/or optimal intensity excitation. In addition, reference is made to the above description at the beginning of the description for further modes of action of the feed forward principle.

Fig. 3 shows a sensor according to the invention according to a third embodiment. Only the principle differences from fig. 1 are explained below. The rest is referred to the description according to fig. 1.

In the case of the sensor 301 according to fig. 3, the difference with respect to the sensor 101 according to fig. 1 is that the beam path of the light beam now extends such that the use of a semi-permeable mirror can be dispensed with. This has the advantage that losses caused by the semi-permeable mirror can be avoided. To achieve this, the optical axis 302 of the light source and the optical axis 303 of the light receiver are tilted such that the angle of incidence of the optical axis 302 corresponds exactly to the angle of emergence of the optical axis 303.

Fig. 4 shows a sensor according to the invention according to a fourth embodiment. Only the principle differences from fig. 3 are explained below. The rest is referred to the description according to fig. 1 and according to fig. 3.

In the case of the sensor 401 according to fig. 4, the difference with respect to the sensor 301 according to fig. 3 is that the angle of incidence of the optical axis 402 no longer corresponds to the angle of emergence of the optical axis 403. This results in that direct reflections from the material track side are avoided. Rather, only the diffusely scattered light of the light spot is reflected to the light receiver, so that the light receiver is less overshot in the case of highly reflective surfaces of the material track. Sometimes, it is also conceivable to combine the two arrangements according to fig. 3 and according to fig. 4, as is described for example in EP 2278361 a 1.

Claims

1. A sensor for optically detecting a mark on a running material track, comprising:

a light source for generating a light spot on a running material track, wherein the intensity of the light spot is controllable by a driver;

a light receiver for receiving light reflected from an aspect of the light spot;

a signal processing unit for analyzing an output signal of the optical receiver; and

a switching unit for switching between a teaching mode in which the intensity of the light spot is changed by the driver and a detection mode in which the intensity of the light spot is kept constant by the driver,

wherein in the teaching mode the intensity of the light spot is changed by the driver such that the output signal of the light receiver is within an optimal control range, and

wherein in the detection mode, changes in the output signal of the light receiver are detected step by step.

2. A sensor according to claim 1, wherein a conditioning amplifier for optimal level control is connected between the light receiver and the signal processing unit.

3. A sensor according to any one of claims 1 to 2, wherein a light receiver is mounted such that directly reflected light from the spot falls onto the light receiver.

4. A sensor according to any one of claims 1 to 2, wherein a light receiver is mounted such that diffusely scattered light from the spot falls onto the light receiver.

5. Sensor according to one of claims 1 to 2, wherein a feed forward signal can be fed to the signal processing unit for optimally controlling the intensity of the light spot.

6. Sensor according to one of claims 1 to 2, wherein a feed forward signal can be fed to the signal processing unit for optimal level control.

7. Sensor according to one of claims 1 to 2, wherein the light source is a high power LED with white light emission.

8. The sensor according to one of claims 1 to 2, wherein the light receiver is a color-to-voltage converter.

9. Register control of a printing press with a sensor according to one of claims 1 to 8.