Method and device for photoelectric identification of a material web

Abstract: Method and device for identification of a moving (v) material web (W;F;FF). A beam of light (I₁) is directed at the material web by means of a transmitter device (21), said beam of light (I₁) producing a beam of light (I₂) reflected from the face of the material web (W;F;FF) to be identified. The latter beam of light (I₂) is converted by a receiver device (31) into an electric signal (U₂), on whose basis the presence, the quality, the condition, and/or the position of the material web (W;F;FF) is/are identified. The intensity of the beam of light (I₁) transmitted from the transmitter (21) of light is regulated on the basis of the intensity of the reflected beam of light (I₂). The reference level or levels of the electric identification signal derived from the reflected beam of light (I₂) is/are adapted in compliance with the environment of operation so as to optimize the identification and to minimize interference from the environment. The device comprises a microprocessor (40), to which an analog signal is passed through an A/D converter (39). The microprocessor (40) controls the regulation unit (34) for the intensity of the light to be transmitted, said unit controlling an adjustable voltage source (23), from which a regulated operation voltage (U₁) is supplied to the transmitter (21) of light.

Description: The invention concerns a method for identification of a moving material web, wherein a beam of light is directed at the material web by means of a transmitter device, said beam of light producing a beam of light reflected from the face of the material web to be identified, the latter beam of light being converted by a receiver device into an electric signal, on whose basis the presence, the quality, the condition, and/or the position of the material web is/are identified. Such a method is known from US 3,496,365. In paper machines and equivalent, in which a continuous material web is manufactured or employed, it is necessary to identify the presence of the material web or the location of its edge in various stages of the process. For these purposes, as a rule, photoelectric means of identification are used. The prior art devices of identification usually operate so that the source of light and the photocell are placed at opposite sides of the web to be monitored, and a break of the web and/or a shifting of the edge has the result that the photocell receives the beam of light, transmits an impulse further, which results in the alarm and possibly in other action. Also, various devices of identification are known which are based on reflection of light taking place from the material to be monitored and on changes occurring in said reflection. As an example of such devices, reference is made to the US Patent No. 4,146,797, wherein a device for identification of the location of the edge of a material web is described, which device comprises a source of light and a detector of light. The source of light in this device directs a spot of light at the lateral area of the web to be monitored, the position of said lateral area being monitored and detected on the basis of changes taking place in the intensity of the reflected light. Further examples of prior-art methods and apparatuses for monitoring webs and similar objects are found in US 4,924,406, GB 1,479,603, EP 0379281 and US 3 739 177 The prior-art photoelectric devices of identification do not operate adequately under all conditions, but disturbance occurs in them, and they require constant supervision, frequent calibration and cleaning. For example, paper machines are an operational environment that imposes very high requirements because of high temperature, moisture, and impurities, which produce disturbance in the prior-art photoelectric means of identification. With increasing speeds of paper machines, said problems have increased further. For example, in a paper machine, a false break alarm with the resulting operations usually causes a standstill of at least about one hour, because the restarting of the paper machine requires a number of steps, including the threading of the web through the machine. Thus, with the prior-art devices, false or missing alarms cause considerable economic losses and lowering of the degree of operation of the paper machine. A particular problem is produced by the areas of single-wire draw in the drying sections of paper machines, wherein the web is constantly supported by a drying wire. In such a case, by means of the device of identification, it is necessary to be able to distinguish the web from the drying wire, and a source of light and a photocell placed at opposite sides of the web and the wire cannot be used, but it is necessary to resort to the light reflected from the object to be examined. Moreover, contamination of the wire and variations in the colour and the moisture content of the paper web to be detected cause changes in the intensity of the reflected beam of light, making the identification by means of the prior-art devices uncertain. The operation of the prior-art web-identification devices is also disturbed by background light and its variations, such as oscillations of fluorescent lamps arising from the mains frequency. In the environment of a paper machine, there is also a considerable amount of infrared radiation, to which most photocells are sensitive, which also causes disturbance in photoelectric means of identification. The object of the present invention is to provide such a novel method for identification of the presence of a moving material web and, in special cases, also of the presence or location of the edge of a web that the drawbacks discussed above can be substantially avoided. It is a particular object of the invention to provide such a method for identification of a web suitable for a paper machine or paper finishing machines as is more reliable in operation than the prior-art devices so that false alarms do not arise and, on the other hand, correct alarms are not omitted, the objective being to increase the degree of operation of the paper machine. The further objectives and purposes of the invention come out from the following description. In view of achieving the objectives stated above, the invention is mainly disclosed by the characterizing features of claim 1. In the method and device in accordance with the invention, preferably operation controlled by a microprocessor is applied. As the light of identification, preferably visible light is used which penetrates through the paper as little as possible and whose reflection from the web and from the wire is different; the frequency of said light should be sufficiently far from the frequency range of the spectrum of a fluorescent tube. An example of a suitable wavelength of the light is λ = 670 nm. Identification in accordance with the invention can be employed advantageously in a number of different objects of use. The advantages of the invention come out with particular emphasis in paper machines and in paper finishing machines in indication of the presence or of the edge of a paper web, exceptionally in the indication of the position of the wire or paper. In a paper machine, typical objects of use of the invention are monitoring of the presence of the paper web on the face of a drying wire or a roll in the paper machine, monitoring of the position or condition of the paper web and/or the wire edge in several different positions, or monitoring of the presence of the paper web at free draws of the web in a paper machine. Besides paper machines, advantageous areas of application of the invention are various paper finishing machines, such as calenders, coating devices, winders and slitter-winders, and printing machines. In some cases, the method in accordance with the invention are also suitable for use in processes other than paper machines, such as, for example, processes that manufacture or process various material flows, such as plastic films or equivalent. In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being by no means strictly confined to the details of said embodiments. Figure 1 is a central sectional view of such a measurement head used in the invention in connection with which an electronic card is fitted. Figure 2 shows a block diagram of a device as shown in Fig. 1. Figure 3 shows a measurement head to which the light signal is passed by means of a fibre cable and, at the same time, Fig. 3 is a sectional view taken along the line A-A in Fig. 4. Figure 4 is a sectional view taken along the line B-B in Fig. 3. Figure 5 shows a block diagram of a system of identification. Figure 6A shows different locations of an identification measurement head in the former, press section, and the initial end of the drying section in a paper machine. Figure 6B is a continuation of Fig. 6A and shows alternative locations of a measurement head in and after the final end of the drying section in a paper machine. As is shown in Fig. 1, the measurement head comprises a cylindrical protective casing 11, to which a plane bottom part 12 has been attached by means of screws 12a. Inside the casing 11, there is an electronic card 20, to which an electric cable 13a passes through a lead-in 14. To the electronic card 20, a light transmitter 21, e.g. a LED, is connected, which emits a beam I₁ of light through an opening 16a in the casing 11 to the web W, which moves at the velocity v and whose presence and breaks are being monitored. From the area of the spot S lighted by the beam I₁ of light, a beam I₂ of light is reflected back to the measurement head 10, said latter beam of light being detected through an opening 16b in the casing on the light receiver 31, e.g. a light diode, on the electronic card 20. The transmitter 21 receives its control voltage through the cable 13a, and by means of the cable 13a the voltage signal of the light receiver 31 is passed to the system of identification 30. As is shown in Fig. 2, the transmitter 21 receives its regulated voltage U₁ from the voltage source 23, and, in a corresponding way, the output voltage U₂, which is proportional to the reflected beam I₂ at the light receiver 31, is passed through the units 32 and 33 to the identification system 30, whose functions are controlled by a microprocessor 40. A more specific exemplifying embodiment of the operational construction of the system 30 will be described in detail later, mainly with reference to Fig. 5. In Figs. 3 and 4, such an embodiment of the invention is shown in which the light signal I₁ is passed into the casing part 11 of the measurement head 10 by means of the light cable 13b by making use of the cable portion 13b1. The beam I₁ of light is focused through the optics 15a and the opening 16a onto the web W to be monitored, from whose light spot S, from its part S₁, a beam I₂ of light is reflected, which is received through the opening 16b by means of the optics, being passed further through the light cable portion 13b2 to the system 30, which comprises a light transmitter 21 and a light receiver 31. The light cable 13b is passed into the system 30 through a lead-in 14. The lenses 15a and 15b are connected to the casing, in connection with the bottom flange 12, by means of a holder 18. To keep the casing clean, pressurized air is passed through lead-in openings 17, which are also provided in connection with the measurement head shown in Figs. 1 and 2, into the casing 11, said air being discharged through the lenses of the optics 15a,15b and through the openings 16a,16b, thereby keeping the optics and the openings 16a,16b clean of paper pulp, dust, and other impurities. Fig. 3 illustrates the invention in a mode of operation in which the location of the edge W R of the web W, which moves at the velocity v, is detected. The web W moves on support of the wire F. In a corresponding way, it is possible to monitor the presence of the web W only, and not at all its edge. It is also possible to monitor the location of the edge F R of the wire F, or the condition of the lateral area of the wire. Measurement of the position of the edge W R of the web W and/or of the edge F R of the wire F is based thereon that the mutual proportions of the portions S₁ and S₂ of the light spot S that reflect in different ways vary and thereby influence the intensity of the reflected beam I₂. In monitoring of the wire F edge F R , the portion S₂ of the spot S is outside the wire F, from where no reflecting takes place. Fig. 5 shows an exemplifying embodiment of the identification system 30. The system 30 receives a voltage signal U₂, which is proportional to the light signal I₂ reflected from the web W to be monitored, from the light receiver 31 of the measurement head 10. The signal U₂ is passed to the signalprocessing part, which comprises an amplifier 32 and a high-pass filter 33, through which a signal spectrum which is higher than a certain limit frequency is passed through the gate circuits 34 and through the microprocessor 40a to the A/D converter 39. The microprocessor 40 is connected with a voltage reference 36, a reset circuit 37, and with a display/keyboard 38. By means of the circuits 34, samples are taken from the signals processed in the units 32 and 33 out of the voltage signal U₂ of the light receiver 31, which samples are passed to the A/D converter 39 of the microprocessor. The taking of samples in the circuits 34 is controlled by the sample-control circuit 35, which is controlled by the microprocessor 40. Further, by means of the circuit 35, the switch 24 of the unit 25 is controlled, which switch pulses the beam I₁ of the light transmitter 21 of the electronic system 20 of the measurement head by means of its control voltage U c . The unit 25 further includes an adjustable voltage source 23, which is controlled by the intermediate of the circuit 43 of regulation of the intensity of light so that the light transmitter 21 receives a suitable input voltage U₁ in accordance with the control of the system and an intensity of the identification beam I₁ in accordance with said input voltage. To the microprocessor 40, an external processor bus 41 is connected, which is again connected with input and output circuits 42. Through the circuit 42, the signals C OUT passing out from the system 30 are obtained, such as the alarm signals or measurement signals, e.g., concerning the position of the web W or wire F edge. In a corresponding way, from an external system, e.g. from the process computer of a paper machine, the necessary control signals C IN are obtained for the system 30. The unit 25 further includes a power source 27 and a serial interface circuit 26. Figs. 6A and 6B show some advantageous locations of a measurement head 10 a paper machine. All of the measurement heads 10 shown in Figs. 6A and 6B are not necessarily needed at one time. According to Fig. 6A, a measurement head 10 is placed in position 1 to monitor the location and/or the condition of the edge of the forming wire FF. In position 2, a measurement head 10 is placed after the centre roll KT in the press section to monitor the presence of the web W on the guide roll JT1, whereupon the web W runs into the separate last nip N in the press section. In position 3, a measurement head 10 is placed after the last nip N to monitor the paper web W on the face of the guide roll JT2, whereupon the web W is passed into the drying section, in connection with whose leading cylinder KS, in position 4, a measurement head 10 monitors the web W. The same operation takes place in position 5 of the measurement head 10. In the upper row, the drying section comprises heatable drying cylinders KS, and in the lower row leading cylinders TS, over which the drying wire F is passed along a meandering path so that, on the drying cylinders KS, the web W reaches direct contact with the heated faces of the drying cylinders KS, and on the leading cylinders TS the web is outside. In Fig. 6B, which is a continuation of Fig. 6A, in position 6, a measurement head 10 is placed to monitor the paper web W on the face of the drying cylinder KS. The measurement head 10 in position 7 has a corresponding function. In Fig. 6B, the cylinder groups in the drying section are cylinder groups provided with twin-wire draw, wherein there are two rows of drying cylinders KS, one row placed above the other, as well as an upper wire FY and a lower wire FA as fitted so that the web W runs as a free draw WP between the rows of cylinders. In position 8, the measurement head 10 monitors the paper web after the drying section in the free gap W₀. Besides the positions shown in Figs. 6A and 6B, the invention can also be applied in various positions in paper finishing devices. Even though, above, the invention has been described expressly in relation to paper machines and to paper finishing devices, the invention can also be applied to other, corresponding monitoring functions, e.g. in connection with various material webs, such as plastic webs or paper webs moving in printing machines. As the source of light 21, for example, super-bright LEDs are used which operate in the range of visible light (e.g., λ = 670 nm). Light is modulated under control by the processor 40 so that the length of one measurement cycle is, e.g., 1 ms and the length of a light pulse is 50 µs. When modulated light is used, when the light of the environment is measured, and when programmed signal processing is employed, the effect of the light in the environment on the measurement is eliminated. Particular attention has been paid to the frequency ranges of 50 Hz to 60 Hz and to their multiples. The light level can be regulated within a wide range, depending on the mode of operation, either by means of an external command or automatically. It is preferable to keep the level of the signal I n received from the paper web W invariable by regulating the intensity of the transmitted light I₁. The source of light has been chosen so that, when the optical system is clean, for example, about 20 % of the available light capacity is in use. As a result of the pre-treatment of the signal, for processing by the microprocessor 40, a signal is received whose level is proportional to the amount of light (I₂) collected out of the light pulse of the transmitter 21, from the face of the object W to be monitored, by means of the optics. When the mirror reflections are eliminated in the installation stage, the signal level is proportional to the reflectivity, i.e. brightness, of the face to be examined. The signal is compared with the reference levels set in the system 30, and, on the basis of the results of the comparison, the decision is taken, "web on" or "web off". Owing to changes in the conditions, e.g. contamination, the system must be calibrated from time to time. The making of the decisions and the calibration take place in different ways depending on the mode of operation of the device, of which there are two: 1. MANUAL mode and 2. AUTO mode, which will be described in the following. 1. MANUAL mode In this mode of operation, the device is calibrated by means of an external command C IN , e.g. by means of an external knob. The calibration is carried out when the web W is on. During calibration, the device adjusts the signal level U₂ of the receiver 31 to its set value by altering the light capacity of the transmitter 21. Further, it recalculates the reference level of the decision-making in accordance with the set percentage value and with the factual signal level. In the MANUAL mode it is also possible to choose an operation mode in which the device performs the above calibration operations independently at preset intervals if it interprets that the web W is on. The purpose of the calibration is to compensate for attenuation of the signal U₂ as a result of contamination of the measurement head and to adapt the detector to altered measurement conditions, e.g. for papers of different colour. The decision "web on/off" is taken on the basis of the reference level calculated during calibration, i.e. when the signal level is lower than the reference level, a web break is concerned. The output C OUT is two relay outputs "web on" and "web off", which operate as of alternating phases. 2. AUTO mode In this mode of operation, there is no separate calibration sequence, but the signal level U₂ is kept invariable all the time irrespective of whether the web W is on or not. In the AUTO mode, two reference levels are used, one of which is placed by a preset percentage below the signal level and the other one, correspondingly, above said signal level. The reference levels are formed by filtering the momentary value of signal level. The filtering coefficient is adjustable. In this mode of operation, the device detects rapid changes in the signal level, but it does not report the absolute state "web on/off". When the paper web W is broken, the signal level is lowered rapidly. From this situation, a pulse-shaped "web off" relay output is obtained. The pulse length is determined by the preset filtering coefficient. The "web on" output operates in a corresponding way when the paper web is passed on. In a static state "web on" or "web off", both of the relay outputs are in the state OFF. The operation mode is chosen by means of a mini jumper placed on the electronics module. For the other settings, e.g. the reference level, the electronics module has a display 38 and press knobs. The set values are stored in a memory protected from electricity failures (EEPROM). Measurement devices are not needed for tuning of the device. For example, the signal level and the light intensity value can be seen from the display 38, and so also the cause code of error alarm. In the following presentation, the numerals with no dimensions are examples, and these can be varied within certain limits. The set values are 0...99, with the exception of the guide value 0...255 of the signal level. The reference levels are given as percentages, the other values either as coefficients or as a numerical value directly corresponding to a part of the measurement signal. After the device has been installed mechanically, its use can be started. The device is given a guide value of the signal level, e.g., within a range of 0...255, which corresponds to a voltage of 0...5 V. In a normal situation, the signal level is set at about 100. Tolerance levels are given for the signal level, ± 5. A reference level is given as a percentage, e.g. 30 %. The filtering coefficients and the lower alarm limit of signal, e.g. 06, are given. The calibration is carried out as follows. When the web W to be monitored is at the measurement point S, the device is given an external calibration command. The device attempts to adjust the voltage of the transmitter LEDs 21 so that the receiver receives an amount of light equalling the preset signal level, consideration being given to the tolerances, i.e. 95...105. Hereupon the device calculates the reference level as equal to the given percentage value, i.e., when the measurement value is 100, the reference level is 30 % less than a hundred = 70. If the web W to be monitored reflects less light than the background material does, a reference level 2 is used in a way corresponding to the reference level 1. The measurement and the making of decision are carried out as follows. When the measurement is on and after the signal level has been stabilized after calibration, e.g., at 100, the internal clock of the device starts counting. After the web W has been on, e.g., for 2 h, the device performs calibration automatically if the signal has been lowered slowly below 95 or gone up slowly above 105 (100 - 5 or 100 + 5). If the signal is in the range of 95...105, no calibration is performed. After the calibration, the device calculates new reference levels. When the signal is lowered to below the reference level and stays below for the time of the operational delay, the decision is taken, whereby it is ascertained that the web W to be monitored is no longer present. The measurement signal can be stabilized by subjecting the measurement to a filtering coefficient with the coefficient 1. The filtering takes place, e.g., with steps of 20 ms. If the measurement signal is lowered to a level below the alarm limit of signal (06), the electronic system gives an error alarm, and during that period the device constantly states that the material W to be monitored is present at the light spot S. Hereby the effect of false alarms is prevented. The lowering of the signal may have the following reasons: the light fibres are broken, the receiver is out of order, impurities or foreign particles in front of the light spot S, in which case the beams of light are not reflected to the receiver 31. After the device has been in operation for a long time, the fibres in the light cable 13b may be aged, in which case their light carrying capacity is lowered, or the ends of the fibres may be contaminated, in which case the voltage U₁ of the transmitter LEDs 21 must be increased. When the voltage U₁ cannot be increased further, an alarm is given, which states that the adjustment of light is at the maximum. The device operates normally in spite of this. When the signal level is lowered, e.g. 100 → 80, correspondingly the reference level is lowered 70 → 56. In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from the details stated above for the sake of example only.

Claims: Method for identification of a moving (v) material web (W;F;FF), wherein a first beam of light (I₁) is directed at the material web by means of a transmitter device (21), said first beam of light (I₁) producing a second beam of light (I₂) emitted from the face of the material web (W;F;FF) to be identified, the second beam of light (I₂) being converted by a receiver device (31) into an electric signal (U₂), on whose basis the presence, the quality, the condition, and/or the position of the material web (W;F;FF) is/are identified, and wherein the intensity of the first beam of light (I₁) transmitted from the transmitter (21) of light being regulated on the basis of the intensity of the second beam of light (I₂), characterized in that the second beam of light is reflected from the material web, that the electric signal is compared with at least one reference level to identify the presence, the quality, the condition, and/or the position of the material web, and that each reference level is determined as a fixed percentage of the electric signal so as to optimize the identification and to minimize interference from the environment. Method as claimed in claim 1, characterized in that the light transmitter (21) is controlled so that it emits pulsed light (I₁), that during the invervals between the light pulses, the light in the environment is detected by means of the light receiver (31), and that the effect of the light in the environment on the measurement is eliminated by means of programmed processing of the signal. Method as claimed in claim 1 or 2, characterized in that, in the method, programmed processing (32, 33, 34) of the measurement signal controlled by a microprocessor (40) is used, and that, under control by the microprocessor (40), by means of a unit (43) for regulation of the intensity of the light, the input voltage (U₁) of the light transmitter (21), preferably a LED, is regulated. Method as claimed in any of claims 1 to 3, characterized in that the intensity of the beam of light (I₂) reflected from the material web (W;F; WW) to be identified is kept substantially invariable by adjusting the intensity of the transmitted beam of light (I₁), and that the light transmitter (21) and the other system related to it are arranged in such a way that, when the optical system is clean and free of disturbance, a fraction, preferably 10...30%, of the available total light capacity of the light transmitter (21) is in use. Method as claimed in any of claims 1 to 4, characterized in that the system is fitted, under control by a microprocessor (40), to perform the operations of calibration of the system independently at preset intervals when the system interprets that the material web (W;F;FF) is on. Method as claimed in any of claims 1 to 4, characterized c h a r a c t e r i z e d in that the signal level of the electric signal (U₂) given by the light receiver (31) is kept substantially invariable irrespective of whether the material web (W;F;FF) to be identified is on or not, that in this mode of operation two reference levels are employed, one of which is placed a fixed percentage below the signal level of the electric signal (U₂) and the other one correspondingly above said signal level, that the momentary electric signal (U₂) is filtered, preferably by means of a high-pass filter, and that, when the material web (W) is broken, the level (U₂) of the electric signal is lowered rapidly, whereby a, preferably pulse-formed, "web off" identification signal (C OUT ), preferably a relay output, is given.

Inventors: AHOLA JUHANI, ERKKILAE KARI, KOIRANEN JUKKA, PELLINEN KARI, VAEHAETALO HARRI, AHOLA, JUHANI, ERKKILAE, KARI, KOIRANEN, JUKKA, PELLINEN, KARI, VAEHAETALO, HARRI, Erkkilä, Kari, Vähätalo, Harri
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1991
Main classification: G01N21
IPC codes: D06H3, G01N21, G01V8, B65H26, G01B11, B65H23