Method and device for monitoring of the edge of a moving web

Abstract: Method and device for monitoring of a moving web, such as a wire (10) in a paper machine, a felt, or a material web (W), such as a board or paper web. A radiation bar (L) is directed at the face of the web (10;W) to be monitored by means of a series of transmitters (20₁...20 N ), which radiation bar is transverse to the direction of movement (v) of the web (10;W). Radiation reflected from the web (10;W) and from the background is detected by means of a series of receivers (30₁...30 M ). The measurement signals obtained in this way are passed to an electronic unit (40), in which a quantity that characterizes the monitoring of the web is detected from said measurement signals. The approximate location of the edge (10a), the edges (10a,10b), or of corresponding discontinuities of the web (10;W) to be monitored is detected "digitally" on the basis of the radiation beam (L n ) of the radiation transmitter (20 n ) in whose area the greatest change takes place in the received radiation. The precise location of the edge or equivalent of the web (10;W) to be monitored is determined analogously by, with one another, comparing the amplitudes of the signals of the adjacent receivers (30 n-1 ,30 n ) placed closest to the last-mentioned radiation transmitter (20 n ).

Description: The invention concerns a method for monitoring of a moving web, such as a wire in a paper machine, a felt or a material web, such as a board or paper web, in which method a radiation bar is directed at the face of the web to be monitored by means of a series of transmitters, which radiation bar is transverse to the direction of movement of the web, and radiation reflected from the web and from the background is detected by means of a series of receivers, the measurement signals obtained from said series of receivers being passed to an electronic unit, by whose means the quantity that characterizes the monitoring of the web is detected from said measurement signals. Further, the invention concerns a device intended for monitoring of an edge or equivalent of the moving web, according to the preamble of claim 11. Such a device is known from US 4,924,106. In paper machines and equivalent, in which a continuous material web is manufactured and/or closed fabric loops are employed, it is necessary to identify the location and/or the presence of the edge of the material web or of the fabric in various stages of the process. For these purposes, as a rule, photoelectric means of identification are commonly used. The prior art devices of identification usually operate so that a source of light and a photocell are placed at opposite sides of the fabric or web to be monitored, and a break of the web and/or a shifting of the edge of the web or fabric 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. In the monitoring of the edge of a wire in a paper machine, it is known in prior art to use three photocells placed side by side, each of said cells operating by the on/off principle. The precision of this monitoring device and/or the width of the area of monitoring has/have, however, proved unsatisfactory in several applications. Also, in prior art, 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 the position of the edge of the web being measured on the basis of changes taking place in the intensity of the reflected light. The prior-art photoelectric devices of identification do not operate adequately in all respects, 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. The increasing running speeds of paper machines are also influential in this connection thereby that ever higher requirements are imposed on the systems of regulation that control the transverse positions of the fabrics in the paper machine, said requirements being applied in particular to the precision and rapidity of the measurement of the position of a fabric. The operation of the prior-art web-monitoring devices is also disturbed by the background light and by the changes in same, such as oscillations in fluorescent tubes 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 results in disturbance in photoelectric means of identification. With respect to the prior art most closely related to the present invention, reference is made to the US Patent 4,146,797 and to the applicant's FI Pat. Appl. 910571 (equivalent to US Pat. Appl. 719,762 and corresp. EP Appl. 91 850 153.7 EP-A-0 516 913). In the last-mentioned applications, a method and a device are described 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 is 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. The intensity of the beam of light transmitted from the transmitter of tight is regulated on the basis of the intensity of the reflected beam of light. The reference level or levels of the electric identification signal derived from the reflected beam of light 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, to which an analog signal is passed through an A/D converter. The microprocessor controls the unit for regulation of the intensity of the light to be transmitted, which unit controls an adjustable voltage source, from which a regulated operation voltage is supplied to the transmitter of light. The object of the present invention is further development of the prior art coming out from the papers cited above as well as to provide a novel system for monitoring of a fabric or of a material web in particular for paper machines, which system is reliable in operation and insensitive to disturbance as well as more versatile in its operation as well as more accurate and quicker as regards the detection of the position of the web. It is a non-indispensable further object of the invention to provide a method and a device in which it is possible to use two or even more measurement heads, which can be placed at the same edge of the web to be monitored and/or at the opposite edges. In the latter case, the method and the device of the invention can also be integrated with measurement of the width of the web. When two or more measurement heads placed at the same edge of the fabric or web are used, the monitoring system in accordance with the invention can also be used for detecting the web speed and/or any edge defects in the web. In view of achieving the objectives stated above and those that will come out later, the method of the invention is mainly characterized in that the approximate location of the edge, edges, or of equivalent points of discontinuity of the web to be monitored is detected "digitally" on the basis of the radiation transmitter in the area of whose radiation beam the greatest change takes place in the received radiation, and that the precise location of the edge or equivalent of the web to be monitored is determined analogously by comparing the amplitudes of the signals of the nearest receivers adjacent to the last-mentioned radiation transmitter with each other. On the other hand, the device in accordance with the invention is characterized in that a web-monitoring quantity can be formed under application of the analog-digital principle of measurement. In the invention, the analog-digital principle of measurement is applied so that the approximate position of the edge or any other point of discontinuity of the web to be monitored in the direction transverse to the running direction of the web is determined by the digital principle, and the precise position is determined analogically by, with each other, comparing the signal levels of the two adjacent radiation receivers between which the radiation transmitter is placed facing the edge of the web, whereby, in the analogical determination, it is preferably possible to apply a linear model. In the invention, as the measurement radiation, preferably pulsed (λ ≈ 620 nm) light is used so that the measurement pulses are controlled alternatingly and successively to sweep over the different radiation transmitters, during whose light pulses, at a precisely specified point of time, the reading of the receiver placed between said transmitters is carried out. Said series of measurement pulses are repeated at sufficiently short intervals so that the necessary detection resolution is obtained. In the measurement system, preferably a separate electronic unit is used, in which the measurement results are computed and processed in any other way as well as the measurement sequences are controlled. In order to keep the capacity of the data transfer channel between the electronic unit and the measurement head reasonable, it is preferable to place a certain amount of control electronics and "intelligence" in the measurement head itself. In a preferred embodiment of the invention, the row of radiation transmitters/receivers is extended over an area wider than the area of measurement itself, so that, in the monitoring of the edge of the web, seen from outside the web, the first transmitter always directs its beam at the background, and the last transmitter always at the face of the web to be measured. It should be emphasized that even though, above and in the following, "light" is spoken of, it is not necessarily visible light that is meant, but in some cases it is also possible to use an electromagnetic radiation placed outside the wavelengths of visible light, in particular infrared radiation, which has suitable properties of reflection from the object to be measured as well as little disturbance. The scope of the invention also includes applications in which an induction measurement based on an electromagnetic phenomenon is used, in which case the web to be identified is a web made of a conductive material or a band made of a conductive material, so that the principle of eddy current can be applied in the measurement. In the following, the term web will be used for the object to be monitored, which term refers to a moving material web in general, such as a paper or board web or any other material web manufactured in process industry, or fabrics that form a closed loop, such as a forming wire, drying wire, or a press felt in a paper machine. 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 schematic illustration of the web monitoring system in accordance with the invention, partly as a block diagram. Figure 2 is a central axial sectional view of a measurement head in accordance with the invention taken along the line II-II denoted in Fig. 3. Figure 3 is a vertical sectional view taken along the line III-III in Fig. 2. Figure 4 illustrates the electric connections of the measurement-head unit and the central unit and the mutual fitting together of these units. Figure 5 shows the electronic unit of the monitoring system in accordance with the invention as a block diagram more detailed than that shown in Fig. 1. Figure 6 illustrates advantageous different positions of location of measurement heads in accordance with the invention in a paper machine. Figure 7 shows an arrangement of measurement heads in accordance with the invention in connection with opposite edges of the web to be monitored. Figure 8 shows, in a way corresponding to Fig. 7, the fitting of two measurement heads in accordance with the invention one after the other in connection with the same edge of the web to be monitored. Figure 9 shows an application of measurement heads in accordance with the invention as a paper-web break detector in a group with single-wire draw in the dryer section of a paper machine. Figure 10 shows an application of a measurement head in accordance with the invention as a break detector in a press section. Fig. 1 shows a block diagram of the monitoring system in accordance with the invention and an exemplifying embodiment of principle. By means of the system, the location of one edge 10a of a paper machine wire 10 in the transverse direction is monitored. The web to be monitored may also be other than a wire, for example a felt or an edge or the edges of a paper or board web. The position of the edge 10a of a web 10 is regulated by means of an alignment roll 11, the position of one 11a of the axle journals of said roll being adjusted by means of a mechanical actuator, which is represented schematically by the arrow 12. At the proximity of the edge 10a of the web 10, two measurement heads 15A and 15B are placed one after the other, the measurement heads being fixed preferably to vertical arms 14 so that their height positions can be adjusted, and which arms 14 are fixed at the tending side of the paper machine on support of its frame beam 13. According to Fig. 1, the monitoring system includes an electronic unit 40, which is connected with the measurement heads 15A and 15B by the intermediate of electric cables 26. The electronic unit 40 comprises a controller 45, a microprocessor 46, a display monitor 41, a RAM 48, and an EPROM 47, all of which are connected with each other. The measurement signals are passed from the measurement heads 15A and 15B through the cables 26 to preamplifiers 42, which are connected to a control logic 43, which is again connected to the controller 45 and to the measurement computing unit 44. The units 43 and 45 control the measurement sequences of the measurement heads 15a and 15b, which are repeated at certain intervals. Further, the measurement system includes a mechanical control unit 50, which is connected with the controller 45 by means of a cable 51. By the intermediate of the connection 52, the unit 50 passes a control signal to the actuator 12 that displaces one of the axle journals of the alignment roll 11. The electronic unit 40 is connected, by means of a series cable 54 (RS-485) and/or by means of I/O-control wires 55a and 55b, to a PLC or equivalent process or automation system 53, which controls the operation of the entire process, such as a paper machine. The electronic unit 40 is connected to a control desk 56, by whose intermediate, e.g., the web monitoring system and the transverse position of the web 10 can be controlled manually. Manual control can also be accomplished by means of an automation system 53 by the intermediate of the series cable 54. The system shown in Fig. 1 operates in principle as follows. The measurement heads 15A and 15B direct a set of light beams L at the area of the edge 10a of the web 10, a part of said set of light beams L being reflected from the web 10 and a part from the background. From the measurement heads 15A, 15B, the measurement signals are transferred by the intermediate of the cables 26 to the electronic unit 40, which processes the measurement signals in a way that will come out in more detail later. The controler 45 and the control logic 43 in the electronic unit 40 give control signals by the intermediate of the cables 26, by means of which signals the operation sequences of the measurement heads 15A and 15B are controlled, which sequences are repeated at certain intervals. Thus, the data transfer in the cables 26 takes place in two directions. A certain amount of electronics and "intelligence" can also be placed in the measurement heads 15A and 15B themselves. The electronic unit 40 gives a control signal to the unit 50 by the intermediate of the cable 51, which unit 50 again gives a regulation signal to the actuator 12 of the alignment roll 11 so that the edge 10a of the web 10 is guided and kept, by means of the feedback-connected regulation system, in the position determined by the set value. Moreover, the electronic unit 40 is connected to the automation system 53 proper of the paper machine or equivalent so that the electronic unit 40 can receive control signals by the intermediate of the cables 54, 55a and 55b and, in a corresponding way, it can give various signals to the system 53. In the following, with reference to Figs. 2 and 3, a preferred exemplifying embodiment of the construction and operation of the measurement head 15 in accordance with the invention will be described. The measurement head 15 comprises a cylindrical box part 16, at one of whose ends there is a closed end wall 16a and at the opposite end a second end wall 16b, which is provided with connections for the cable 26 and for the supply A in of cooling and/or cleaning air. In the measurement head 15, inside the box 16, there are six transmitter LEDs 20 1 ...20 6 and five receiver diodes 30 1 ...30 5 for the measurement light. The transmitters 20 and the receivers 30 are placed as evenly spaced in a straight line so that the transmitters 20 are placed at both sides of each receiver 30, for example, so that the components 20 and 30 are placed at uniform distances of about 20 mm from each other. Each receiver 30 sees (view sector R) one half of the areas illuminated by the light beams L of both of the transmitters 20 placed at its sides. The transmitters 20 are switched on by the electronic unit 40 alternatingly in a given sequence so that there are always two adjacent transmitters 20 on, with the exception of the transmission of a reference signal. Simultaneously, the reading of the receiver 30 placed between said transmitters 20 takes place. The duration of the radiation pulses of the transmitters 20 is, e.g., 10...100 µs, preferably about 50 µs. The receiver 30 is always read at a certain moment during the radiation pulses of said transmitters. The two extreme transmitters 20 1 and 20 6 in the measurement head are reference transmitters, whose beams L always arrive, the inner one exclusively on the face of the object 10;W to be monitored, and the outer one exclusively outside the object 10;W. However, the transmitters 20 1 and 20 6 are also used for formation of the measurement signal with the receivers 30 1 and 30 5 . It is assumed that the edge 10a of the object 10;W always remains between said extreme beams L. Thus, the measurement beams L proper are the beams 2...5. Each beam L illuminates an area of, for example, 40...50 mm. The width of the measurement area proper is, as a rule, about 100...200 mm, preferably about 160 mm. The beams L overlap each other partly, and thereby guarantee that no shadow areas remain between the beams L. The beams L of the transmitters 20 are formed by means of a lens 21 and a shade 21a out of the light of the LED. Attempts are made to make the brightness of the beams L as uniform as possible over the entire area of reflection. For this purpose, in the electronic unit 40, correction factors are computed for each transmitter in order that the computed brightness should be equal. In front of the receivers 30, there are no lenses, but there are filters 32, by whose means most of the interference produced by the outside lighting is eliminated. The transmitters 20 and the receivers 30 are attached to the optical frame 25 by means of various holders, which have a little allowance for adjustment for precise alignment of the beams L. The control electronics of the transmitters 20 and of the receivers 30 are fitted on a card 22, which is attached directly to the frame 23 by means of spacer bushings. The frame 23 and the electronics card 22 are protected by a double box 16,16C or by one box 16 and insulation material 17. Between the inner box 16C and the outer box 16, there is thermal insulation 17 to reduce the heat transferred from outside. The cooling of the measurement head 15 takes place by means of blowing of air A in -A out , which air is removed A out through a nozzle 19 that forms the light opening 19a. The function of the exhaust air flow A out is to prevent access of contamination particles into the interior of the nozzle 19 and onto the face of the glass 19b that protects the light opening 19a. The cooling air is passed (A in ) from the inner box 16c into the equalizing chamber of the nozzle through the holes closed by covering gates, the function of said gates being to close the holes if the air blowing is stopped or if washing water attempts to flow in through the holes. The function of the equalizing chamber of the nozzle 19 is to equalize the flow at different points in the nozzle. The nozzle 19 is made of a material with low thermal conductivity so as to prevent condensing of water. The outer box 16 is a smooth acid-proof tube, which operates as a support frame proper and as a mechanical shield as well as a fastening arm for the measurement head 15. The width a of the sector of the beams of light L in the direction of running v of the object 10;W is, as a rule, in the range of a ≈ 10...30 mm, preferably about 20 mm, and the corresponding width of the sector of the beam of light L in the perpendicular direction is b ≈ 35...45 mm, preferably about 40 mm. The angular width c of the sector of view of the receivers 30 is, as a rule, in the range of c ≈ 70...90 mm, preferably about 80 mm. The distance H of the measurement head from the object 10;W is, as a rule, in the range of H = 150...250 mm and, if necessary, it can be arranged to be adjustable by varying the position of the measurement head 15 on its support arm 14. As was stated above, the number of transmitters 20 is preferably one higher than the number of receivers 30 ( N = M - 1 ). The number N of the transmitters 20 is, as a rule, in the range of N = 4...10, preferably N = 5...7. The number N of the transmitters 20 depends on the width of the necessary area of measurement, and in some special applications it is possible to use even several dozens of transmitters, for example, when the width of the web is measured, in which case the measurement head may extend across a major part of the width of the web to be measured. The operation of the measurement system in accordance with the invention is based on the circumstance that the light emitted by the transmitters 20 is reflected in different ways from the object 10;W and from the background. The light transmitters 20 are switched on preferably so that a "sweeping" illumination is produced. When the light meets the object 10;W, it is reflected more intensively than when it arrives outside the edge 10a of the object 10;W, and in the signal obtained from the different receivers 30 it is possible to notice a clear change at the edge 10a. The amount of light reflected from the background is usually lower than that reflected from the object 10;W, because the distance to and from the background is considerably larger. In the invention, the approximate location of the edge 10a of the object 10;W is found out from the transmitter beam L n in whose area the greatest change takes place, and the precise location can be determined by comparing the values of the signals of the receivers 30 n-1 and 30 n adjacent to said transmitter beam L n with each other. In the invention, the precise location of the edge 10a can be determined advantageously by using a linear model, whose starting point is the difference between the values of the signals of the last-mentioned receivers 30 n-1 and 30 n and the sign of said difference. Thus, in the invention, the digital and the analogical principles of measurement have been combined in a novel way so that the approximate position of the edge 10a is determined digitally, and the precise position is determined by the analogical principle. In this way, it is possible to carry out a particularly precise measurement free of interference, which is also suitable for measurement of the width of the web 10;W and, in a special case, also for measurement of the web speed, for example, by making use of the correlation technique. The basic comparison and calibration take place in comparison with the signals obtained from the areas of the reference transmitters 20 1 and 20 6 . The reference transmitters 20 1 and 20 6 are placed at both ends of the light bar, in which case the beam of one of them always meets the face of the object 10;W, whereas the beam of the other one arrives outside the object. In the following, with reference to Fig. 4, the electrical wirings and the joining together of the measurement head 15 and the central unit 40 will be described. The microprocessor 49 of the electronic central unit 40 transmits a control signal to the multiplexing and demultiplexing circuits 80,81 of the electronic system of the measurement head 15 along the control signal cable c. After the control signal has been given, the transmitter LED 20 1 transmits a light pulse, and the receiver 30 1 measures the reflected light, transfers the signal through the preamplifier 42 to the MUX circuit 80 and from there through the line adapter 57a to the signal cable and further to the electronic unit 40, in which the signal is converted to digital form, filtered by means of a digital filter 82, and passed further to the microprocessor 49 for computing. In this way, the first reference level is obtained. Next, the electronic unit 40 switches the LEDs 20 1 and 20 2 as well as the receiver 30 1 on, the unit 40 forming the first measurement signal from the signal of the receiver 30 1 . These steps are repeated until the second reference transmitter 20 6 is reached, whose control is arranged in the same way as that of the first reference transmitter 20 1 . Moreover, as the last one, a sample is taken from a NTC detector (not shown), and on the basis of its signal the data are formed concerning the temperature of the measurement head 15. The voltage supply for the measurement head is passed through the wires 26 from the electronic unit 40. The brightness of the transmitter LEDs is regulated on the basis of the reference signal by means of the current-regulation unit 83 so that any contaminations on the faces of the receivers 30 do not produce distortion of the signal. In the following, with reference to Fig. 5, the more detailed construction and the substantial operation of the electronic unit 40 will be illustrated as a block diagram. The environment of the processor 49 includes a clock oscillator 65, a RESET logic 66, a bus controller 59, and RS adapters 67 and 68 as integral parts. The clock oscillator 65 times the CPU 49 and, at the same time, it times the frequency of taking samples from the detectors. The RESET logic 66 supervises the operation of the CPU 49. The CPU 49 transmits a command to the line controller 57c to control the measurement operations of the measurement head 15. The line controller 57c transmits a clock pulse and a RESET pulse to the measurement head, by means of which pulses the transmitter LEDs 20 and the receiver diodes 30 are timed. The measured detector signal S is passed along the wire 84 to the line adapter and to the amplifier 57a, being passed further to the CPU 49, in which the data are received and the output is transferred further to the RAM memory 48, from which it is then again brought back for computation to the CPU 49. In the NVRAM 57, the case-specific tuning parameters are stored, and in the EPROM 47, the program proper is placed, from which the CPU 49 seeks the necessary parameters and programs. The LCD display and the operating switches are connected to the bus controller 59 of the processor bus through the user interface 58. The I/O's are brought to the isolators 69 and 70, from which they are passed through the gates 60 and 61 to the processor 49. The analog I/O's are passed through the I/U converters 71 and isolators 62 to the CPU 49, and through the D/A converters 64, isolators 63, and U/I converters from the CPU 49. If the computation has noticed that the measurement head 15 is slightly contaminated, the CPU 49 controls the control signal C for the LEDs 20 to a higher level by the intermediate of the amplifier 57b, and in this way the brightness of the light of the LEDs 20 is increased. In the electronic system 40, there are detector-specific components 57 for two measurement heads. Fig. 6 is a schematic illustration of a paper machine and of preferred locations of measurement heads 15 in accordance with the invention in its connection. The paper machine, which is shown highly schematically, comprises a wire section 70, a press section 71, a dryer section 73, and a reel 74. The paper web W is transferred from the forming wire 10A at the pick-up point P onto the press felt, which carries it through the press nips N 1 and N 2 , whereupon the web W is transferred into the third press nip N 3 and from it further onto the lower felt 10D of the fourth separate nip N 4 . From the upper roll of the fourth nip N 4 the web W is transferred as a free draw W 0 onto the drying wire 10E and further through the dryer section 73. Hereupon the web W is transferred as a free draw W 1 to the reel 74. Fig. 6 shows measurement heads 15 in accordance with the invention in the position 1 to monitor the edge of the forming wire 10A, of the press felt 10B, 10C, and 10D as well as of the drying wire 10E. Further, a measurement head 15 in accordance with the invention is shown as fitted in the position 2 to monitor the presence of the web W on the free draw W 0 and W 1 , i.e. to detect a break and/or the location of the edge of the web W and/or the width T of the web. In the position 3, in the dryer section 73, the measurement head 15 is shown as monitoring the presence of the web W running on support of the drying wire 10E and/or the location of the edge of the web and/or the width of the web. Fig. 7 shows measurement heads 15A and 15C in accordance with the invention as placed at both edges 10a and 10b of the wire 10 or the web W, by means of which measurement heads the positions of the edges 10a and 10b are monitored. By means of joint processing of the signals of the measurement heads 15A and 15C, it is also possible to monitor and to measure the transverse width T of the web W when the distance T 0 between the measurement heads is known precisely. In this way, by means of the method of the invention, it is possible to monitor, e.g., the drying shrinkage of the web W as it runs through the dryer section 73 and as it shrinks at the same time in the transverse direction during, and as a result of, the drying. Fig. 8 shows two measurement heads 15A and 15B placed one after the other at a certain distance M from one another. The measurement heads 15A and 15B may operate "in parallel" so that they ensure the operation of each other so that one measurement head 15A, 15B may become inoperative, and it can even be removed for servicing, without deterioration of the monitoring. It is a further possibility of joint operation of the measurement heads 15A and 15B that, besides the location of the edge 10a of the wire 10 or web W, they are also used for measurement of the speed v of the wire 10 and/or the web W by determining the time t 0 that the wire 10 and/or the web W takes when it runs the distance M (v = M/t 0 ) between the measurement heads 15A and 15B. The time t 0 can be determined by processing the electric signals obtained from the measurement heads 15A and 15B by means of the correlation technique. The edge of a wire 10 or a web W is always to some extent "alive" (the edge 10a is not a straight line), which produces a corresponding high-frequency oscillation in the signals obtained from the measurement heads 15A and 15B, whereby the time t 0 can be determined by means of the correlation technique in a way in the other respects in itself known. The frequency range of the oscillations on whose basis the speed v is determined is considerably higher, at least by one order higher, than the frequency of change in the position of the web 10;W. Thus, said signals of change can be separated from each other, e.g., by means of filters. The principles of correlation technique have been described in respect of the measurement of the flow velocity of the pulp suspension in a paper machine, for example, in the FI Patent No. 67,627 (equivalent to US Pat. 4,484,478). According to Fig. 9, measurement heads 15 in accordance with the invention are placed at the proximity of the paper web W running on the drying wire 10E to operate as detectors of web W breaks in the dryer section 73 as the drying wire 10E runs over the drying cylinders 73a and the reversing suction rolls 73b so that the web W to be dried is pressed by the drying wire 10E into direct contact with the heated faces of the cylinders 73a and, on the reversing suction rolls 73b, at the side of the outside curve, in which case the presence of the web W and/or the location of its edge 10a is detected by means of a measurement head 15 in accordance with the invention. In the same connection, it is also possible to detect the locations of both edges 10a, 10b of the web W and thereby, by means of the principle described in Fig. 7, the width T of the web W, for example, when it is desirable to monitor the drying shrinkage of the web W as it runs through the dryer section 73, being dried at the same time. Fig. 10 shows a measurement head 15 as placed after the last nip N 4 in the press section on the free draw W 0 of the web W as it is passed onto the drying wire 10E. Also in this connection, besides a web W 0 break, it is also possible to monitor the location of one or both of its edges 10a, 10b, and thereby, if necessary, it is also possible to measure the width of the web W. 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 what has been stated above by way of example only.

Claims: Method for monitoring of a moving web, such as a wire (10) in a paper machine, a felt, or a material web (W), such as a board or paper web, in which method a radiation bar (L) is directed at the face of the web (10;W) to be monitored by means of a series of transmitters (20 1 ...20 N ), which radiation bar is transverse to the direction of movement (v) of the web (10;W), and radiation reflected from the web (10;W) and from the background is detected by means of a series of receivers (30 1 ...30 M ), the measurement signals obtained from said series of receivers being passed to an electronic unit (40), in which a quantity that characterizes the monitoring of the web is detected from said measurement signals, characterized in that the approximate location of the edge (10a), edges (10a,10b), or of corresponding discontinuities of the web (10;W) to be monitored is detected "digitally" on the basis of the radiation beam (L n ) of the radiation transmitter (20 n ) in whose area the greatest change takes place in the received radiation, and that the precise location of the edge or equivalent of the web (10;W) to be monitored is determined analogously by, with one another, comparing the amplitudes of the signals of the adjacent receivers (30 n-1 ,30 n ) placed closest to the last-mentioned radiation transmitter (20 n ). Method as claimed in claim 1, characterized in that the radiation transmitters (20) are switched on alternatingly one after the other so that a "sweeping" radiation bar (L) is formed, and, synchronously with each transmitter pulse, the reading of the receiving pulses is carried out during the transmitter pulses at a specified point of time, and that the measurement sequences are repeated at intervals sufficiently short in view of the measurement resolution by means of the control of the electronic unit (40). Method as claimed in claim 2, characterized in that the radiation transmitters (20) are controlled by means of the electronic unit (40) to be switched on alternatingly in a certain sequence so that, at one time, two adjacent transmitters (20 n , 20 n+1 ) are on and, at the same time, the reading of the receiver (30 n ) placed between said transmitters (20 n ,20 n+1 ) is carried out. Method as claimed in any of the claims 1 to 3, characterized in that, in the method, two reference transmitters (20 1 and 20 N ) are employed, which are placed outside the measurement area proper so that the radiation beam (L 1 ) of the first transmitter (20 1 ) always arrives outside the edge (10a) or equivalent to be monitored and the radiation beam (L N ) of the last transmitter (20 N ) always arrives completely in the area of the web (10;W) to be monitored. Method as claimed in any of the claims 1 to 4, characterized in that the adjacent sectors (R) of view of the radiation receivers (30) overlap each other in their lateral areas. Method as claimed in any of the claims 1 to 5, characterized in that the measurement heads (15A,15C) that apply the method are placed substantially at the same position at both edges (10a,10b) of the web (10;W) to be monitored, and that, besides, or in stead of, the location of the edge (10a) or edges (10a,10b) of the web (10;W) to be monitored, the transverse width (T)of the web is also determined on the basis of the signals obtained from the measurement heads (15A,15C) (Fig. 7). Method as claimed in any of the claims 1 to 6, characterized in that, in the method, measurement heads (15A,15B) operating in accordance with the method are used, which are placed at the same edge (10a) of the web (10;W) to be monitored at a certain mutual distance M from each other and which operate "in parallel" ensuring the operation of each other and/or on the basis of the signals obtained from which the speed v = M/t 0 of the web to be monitored is established by determining the running time t 0 of the web over the distance M between said measurement heads (15A,15B) by making use of the correlation technique. Method as claimed in any of the claims 1 to 7, characterized in that a measurement head (15) operating in accordance with the method is additionally used for detecting a break in, or the presence of, the paper or board web (W) as the web (W) runs on support of a fabric, such as a drying wire (10E) and/or as a free draw (W 0 ,W 1 ). Method as claimed in any of the claims 1 to 8, characterized in that the measurement signals obtained from a measurement head (15A) or from measurement heads (15A,15B,15C) operating in accordance with the method are used as regulation signals in a feedback-connected regulation system (40,50,52), by whose means the device that controls the transverse position of the web (10) to be monitored, such as an alignment roll (11), is regulated (Fig. 1). Method as claimed in any of the claims 1 to 9, characterized in that the method is applied in a paper machine so that the measurement signals of a measurement head or heads (15A,15B,15C) are passed to the electronic unit (40), which is connected by means of a series cable (54) and/or by means of corresponding I/O control wires (55a,55b) to the process or automation system (53) of the paper machine, and that the measurement sequence of the measurement heads is controlled by means of the control logic (43) of the electronic unit (40) (Fig. 1). Device intended for monitoring of an edge or equivalent of a moving web (10;W), comprising a measurement head (15), which is fitted free of contact at the proximity of the web (10,W) to be monitored, and which measurement head (15) is provided with a series of radiation transmitters (20 1 ...20 N ) and with a series of radiation receivers (30 1 ...30 M ), which receivers receive radiation that is derived from said radiation transmitters (20) and that is reflected from the web (10;W) to be monitored and from the background, on the basis of which radiation it is possible to form a measurement signal, which represents the position of an edge (10a) or of the edges (10a,10b) of the web (10;W) and/or the distance between said edges and/or a point of discontinuity in the web, wherein radiation transmitter (20) and radiation receivers (30), alternating with one another, have been placed in the measurement head (15) of the device in a row or in a corresponding formation, whose substantial direction is transverse to the running direction (V) of the web (10;W) to be monitored, that each radiation receiver (30 n ), which is placed between two successive radiation transmitters (20 n + 20 n+1 ), is arranged to detect radiation that is derived from both of said radiation transmitters and that is reflected from the web (10;W) to be monitored and/or from the background, and that the device comprises an electronic unit (40), which includes a control logic (43), which controls the measurement sequences of the device, as well as a signal-transfer unit (44), which is fitted to transfer the signals derived from the different receivers (30) successively to said electronic unit (40), characterized in that a web (10;W) monitoring quantity can be formed by means of application of an analog-digital principle of measurement such that the approximate location of the edge (10a), edges (10a,10b), or of corresponding discontinuities of the web (10;W) to be monitored can be detected "digitally" on the basis of the radiation beam (L n ) of the radiation transmitter (20 n ) in whose area the greatest change takes place in the received radiation, and that the precise location of the edge or equivalent of the web (10;W) to be monitored can be determined analogously by, with one another, comparing the amplitudes of the signals of the adjacent receivers (30 n-1 ,30 n ) placed closest to the last-mentioned radiation transmitter (20 n ). Device as claimed in claim 11, characterized in that the device comprises a measurement head (15), in which there is a box (16) and an oblong window (19a) for transmission and receiving of the radiation, and that in the interior of said box (16), radiation transmitters (20) and receivers (30) are fitted alternatively in a row. Device as claimed in claim 11 or 12, characterized in that the radiation transmitters (20) consist of LEDs and the radiation receivers (30) of photodiodes, which are placed in a row as uniformly spaces so that there is a transmitter (20) at both sides of each receiver (30). Device as claimed in any of the claims 11 to 13, characterized in that the outermost two radiation transmitters (20 1 ,20 6 ) are reference transmitters, whose radiation beams (L) are always applied so that the beam of one of them meets exclusively the face of the web (10;W) to be monitored, whereas the beam of the other one arrives completely outside said web. Device as claimed in any of the claims 11 to 14, characterized in that the measurement head (15) is fitted in a box, in which there are two casings (16, 16C), one placed inside the other, between which casings there is thermal insulation (17), and that said box is connected with circulation (A in -A out ) of cooling and/or cleaning air. Device as claimed in any of the claims 11 to 15, characterized in that the measurement head (15) comprises N pcs. of transmitters, N = 4...15, preferably N = 5...8, and/or that the radiation transmitters (20) and receivers (30) are placed alternatingly in a straight line at a mutual distance that is in the range of 15...30 mm, preferably about 20 mm, and/or that the perpendicular distance H of the measurement head (15) from the plane of the web (10;W) to be monitored is in the range of H = 150...250 mm, and/or that the width of the measurement area is in the range of about 100...200 mm, and/or that the duration of the radiation pulses of the radiation transmitters (20) is in the range of 10...100 µs, preferably about 50 µs.

Inventors: KOIRANEN JUKKA, PELLINEN KARI, SARKKINEN OLLI, KOIRANEN, JUKKA, PELLINEN, KARI, SARKKINEN, OLLI
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1993
Main classification: B65H23
IPC codes: B65H23