WO2001002841A1

WO2001002841A1 - Method of measuring consistency of a web, and a measuring device

Info

Publication number: WO2001002841A1
Application number: PCT/FI2000/000616
Authority: WO
Inventors: Pekka Jakkula; Ilkka DAHLSTRÖM; Esko Tahkola; Kari Luostarinen
Original assignee: Metso Paper Automation Oy
Priority date: 1999-07-06
Filing date: 2000-07-05
Publication date: 2001-01-11
Also published as: FI991548A; AU5832400A

Abstract

The invention relates to a method and a measuring device for measuring the consistency of a web (112) on a former of a paper machine, the web (112) being in contact with a wire (114). In the measurement, microwave radiation (122) is transmitted to the web (112) by directing the microwave radiation (122) to that surface of the web (112) which is opposite to the contact surface of the wire and the web (112), and a measurement frequency range is used, in which the web (112) attenuates the interference radiation penetrating the web (112). Microwave radiation is also transmitted to a reference measurement, and the intensity of the radiation reflected in the reference measurement is measured. The consistency of the web (112) is measured by comparing the intensity of the microwave radiation (124) reflected from the web with the intensity of the radiation reflected in the reference measurement.

Description

METHOD OF MEASURING CONSISTENCY OF A WEB, AND A MEASURING

DEVICE

FIELD OF THE INVENTION

The invention relates to a method of measuring the consistency of a web on a former of a paper machine, the web being arranged on a wire. The measurement is carried out using microwave radiation.

BACKGROUND OF THE INVENTION

In the manufacture of high-quality paper, the water or dry solids content, i.e. consistency, of a paper web has to be accurately measured and adjusted. If the consistency of a paper web is too high, the quality of the paper web does not remain homogenous, which impairs the quality of the paper to be manufactured. If again the consistency of a paper web is too low, the quality of the paper web does not remain homogenous, which also impairs the quality of the paper to be manufactured and, in addition, the drying stage con- sumes much extra energy. The consistency is usually measured on a former whose adjustment is nowadays most generally based on the measurement of the amount of water removed.

A known solution for measuring the consistency of a web is disclosed in Finnish Patent Application 971324, which is incorporated herein by reference. In this solution, the consistency of a web is measured by means of the intensity of the natural thermal microwave radiation of the web. However, the radiation of the device considerably interferes with the measurement.

Another known measurement of the consistency of a web is disclosed in Finnish Patent 84402, which is incorporated herein by reference. In the solution, microwave radiation is transmitted through the web and the wire, and the propagation time of the microwave signal, which depends of the consistency, is measured. The measurement can be performed on a relatively dry web, since in the measurement of a web on a wire, the wire would significantly interfere with the measurement. US Patent 5,826,458, which is incorporated herein by reference, also discloses a known measurement solution. In this solution, a sensing head under the web measures the moisture of the web, the resonance frequency of microwaves being measured in the measurement. The measurement is affected by the moisture content of both the web and the wire, which impairs the determination of the moisture content of the web. Furthermore, the measure- ment requires that the sensing head be in contact with the web, and, accordingly, excludes the measurement of a moist web. The thickness of the web affects the measured moisture content, and this causes measuring error.

BRIEF DESCRIPTION OF THE INVENTION It is thus an object of the invention to provide a method and a device for implementing the method so as to solve the above problems. This is achieved with a method of measuring the consistency of a web on a former of a paper machine, the web being in contact with a wire, in which method microwave radiation is transmitted to the web, and microwave radiation reflected by the web is measured. Furthermore, in the method, the microwave radiation is transmitted to the web by directing the microwave radiation to that surface of the web which is opposite to the contact surface of the wire and the web, and the intensity of the microwave radiation reflected from the web is measured; interfering background radiation is filtered off so that it does not interfere with the measurement by using a measuring frequency range in which the web attenuates the intensity of the interference radiation penetrating the web so as to render it unessentially low as regards the measurement; microwave radiation is transmitted to a reference measurement, and the intensity of the radiation reflected in the reference measurement is measured; and the consistency of the web is measured by comparing the intensity of the microwave radiation reflected from the web with the intensity of the radiation reflected in the reference measurement.

This is also achieved with a device for measuring the consistency of a web on a former of a paper machine, the web being in contact with a wire, and the measuring device being arranged to transmit microwave radiation to the web and to measure microwave radiation reflected by the web. The measuring device is further arranged to transmit the microwave radiation to the web by directing the microwave radiation to that surface of the web which is opposite to the contact surface of the wire and the web, and to measure the inten- sity of the microwave radiation reflected from the web; the web is arranged to filter off interfering background radiation penetrating the web so that it does not interfere with the measurement in such a way that the measuring device is arranged to use a measuring frequency range in which the web attenuates the intensity of the interference radiation penetrating the web so as to render it unessentially low as regards the measurement; the measuring device is ar- ranged to transmit microwave radiation to a reference and to measure the intensity of the radiation reflected from the reference; and the measuring device is arranged to measure the consistency of the web by comparing the intensity of the microwave radiation reflected from the web with the intensity of the ra- diation reflected from the reference.

The preferred embodiments of the invention are disclosed in the dependent claims.

The method and system of the invention provide a plurality of advantages. The measurement is not sensitive to interference radiation, and the measurement can be carried out on a former for a web supported by a wire. This allows the measurement of a web containing much water and having a low consistency. The thickness of the web does not affect the measurement result.

BRIEF DESCRIPTION OF THE FIGURES In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which

Figure 1A shows a measurement arrangement, Figure 1 B shows measurement and reference signals, Figure 1 C shows a measurement signal,

Figure 2 shows a measurement arrangement, Figure 3A shows a measurement arrangement, Figure 3B shows measurement and reference signals, Figure 4 shows a measurement arrangement, Figure 5 shows a measurement point on a former,

Figure 6 shows a measurement arrangement for a reference, and Figure 7 shows a paper machine in the area of the former.

DETAILED DESCRIPTION OF THE INVENTION

The solution of the invention is applicable to the measurement of the moisture content of a web on the former of a paper machine, the web being supported by a wire.

Let us first study the solution of the invention by means of Figure 1. In the measurement arrangement, a microwave oscillator 100 sweeps the frequency of the microwave signal to be transmitted over the desired frequency band. The solution preferably uses the FMCW method (Frequency Modulation Continuous Wave). The microwave signal of the oscillator 100 is preferably sinusoidal, but the inventive solution also works with other waveforms. The frequency preferably exceeds 20 GHz and the frequency band is preferably some hundreds of megahertz, e.g. 24 GHz to 25 GHz. The sweep may be im- plemented for example by the frequency of the microwave signal to be transmitted rising from an initial frequency f₀ to the highest frequency f_max, whereupon the frequency falls from the highest frequency f_max back to the initial frequency f₀. This frequency sweep is repeated during the measurement. The rise and fall in the frequency preferably take place linearly, but non-linear variations are also feasible. The frequency rises and falls preferably continuously, but the inventive solution can also be implemented using non- continuous frequency variation. The length of the sweep is not essential to the invention, but, in practice, the advantageous length of a frequency sweep is between 10 ms and 100 ms. An isolator 102 prevents the reflected radiation from returning to the oscillator 100. The microwave signal from the oscillator 100 propagates through the isolator 102 along a microwave conductor 104 to a coupler 106, by means of which the microwave signal is applied to a reference measuring part 108 or an antenna 110. The microwave conductor 104 is a known coaxial cable, waveguide or stripline conductor. The microwave cou- pier 106 is a prior art commercially available coupler. The antenna 110 is a prior art horn antenna or a printed circuit antenna, such as a slot, flat panel or patch antenna.

Let us now study the measurement of a web 112. The microwave radiation is directed from the antenna 110 to the web 112 on a wire 114. The measurement uses such a measuring frequency range that the intensity of interference radiation, penetrated the web 112 and reflected from the web/wire interface, is attenuated so as to be unessentially low as regards the measurement when penetrating the web 112. The web 112 thus acts as both filter and measuring target in the solution of the invention. From the web 112, micro- wave radiation 122 transmitted to it is reflected back to the antenna 110. Reflected microwave radiation 124 comprises reflected components, mainly from the surface of the web 112 and partly from inside the web 112. Hardly any microwave radiation is reflected to the antenna 110 from the lower surface of the web 112, since the measuring frequency is so selected that the web 112 filters off the radiation that has penetrated the web 112. The received reflected radiation 124 propagates via the antenna 110, the coupler 106 and a directional coupler 128 to a mixer 116. The isolator 102 prevents the reflected radiation from propagating to the oscillator 100. A directional coupler 130 is used to couple microwave signal also from the oscillator 100 to the mixer 116. The directional couplers 128, 130 are prior art microwave couplers. The mixer 116 multiplies the microwave signals with each other, and since the signals to be multiplied have delay difference, the signals also have frequency difference because of the frequency sweep. When two signals at different frequencies are multiplied with one another, two signals are generated, one of which has a sum frequency (f1 + f2) of the two signals to be multiplied and the other signal has the difference frequency (f1 - f2) of the signals to be multiplied. When the frequency is swept, both frequencies f1 and f2 change, but their difference remains constant if the frequency sweep is linear. In non-linear frequency sweep, the change in the frequency difference due to the non-linearity can be taken into account, since the non-linearity is previously known. A filter 118 fil- ters off at least the sum frequency signal (f1 + f2). The filter 118 is preferably a band-pass filter that passes the difference frequency so that a minimum of interference reaches a meter 120. The meter 120 measures the amplitude of the difference-frequency signal from the filter 118, the amplitude being used to determine the consistency or moisture content of the web 112. The difference- frequency signal is usually on an audio frequency, which facilitates the processing of the signal. Consistency and moisture content correspond to one another and are interdependent, so that when the consistency is X%, the moisture content is 100% - X%. That is, if the consistency is for example 15%, then the moisture content is 100% - 15% = 85%. Let us now study an alternative reference 108 measurement. The microwave signal to be transmitted is coupled with the coupler 106 to reference measurement. The reference 108 comprises an attenuator 1080 and a microwave conductor 1082 implemented in the same way as the microwave conductor 104. The reference 108 is shorted out to achieve reflection. The attenuator is used to attenuate the intensity of the microwave radiation reflected from the reference 108 to the same level as the intensity of the microwave radiation reflected from the web 112 in the actual measurement. The attenuator 1080 is preferably adjustable, allowing the attenuation of the attenuator to be adjusted suitable for different webs 112. The reference line is as long as the actual measurement line, i.e. the delay of the reflection is as long from the reference as it is from the web. Furthermore, the attenuation of the reference is such that the intensity of the radiation reflected from the web is at least largely equal to the intensity of the radiation reflected from the reference. In the inventive solution, the intensity of the microwave radiation reflected from the web 112 is compared with the intensity of the microwave ra- diation reflected from the reference 108. This allows even a small deviation in the intensity of the microwave radiation reflected from the web 112 to be detected. The intensity of the microwave radiation reflected from the web 112 is the function of the consistency of the web 112.

Figure 1 B shows the measurement according to the invention. Am- plitude A is the vertical axis and time T the horizontal axis. The measurement starts for example with measurements of the reference, which are denoted by the letter R. The web, denoted by the letter M, is then measured. The measuring signal, which is the output signal of the filter 118, shows oscillation, which is typically at an audio frequency. The strength of this audio-frequency signal can be measured by e.g. amplitude or power measurement, and the strength of the audio-frequency signal is naturally the same as the intensity of the reflected microwave radiation. Time TO used up by the measurement of the reference and the web preferably corresponds to the frequency sweep time of the oscillator, i.e. it is usually between 10 ms and 100 ms. In this measure- ment, the amplitude of each oscillation can be measured separately, or the mean signal strength can be measured at measuring time TO.

Figure 1C clarifies the inventive measurement. The vertical axis is the amplitude and the horizontal axis the frequency. Local amplitude maxi- mums 150 and 152 represent reflections from, for example, the coupler 106 and the antenna 110. Since frequency sweep is used, the frequencies of these reflections 150 and 152 deviate from the frequency 154 of the microwave radiation reflected from the web 112. Limiting the measuring frequency range B with the band-pass filter 118 to the microwave radiation 154 reflected from the web 112 (or, similarly, from the reference 108) allows the consistency meas- urement to be performed without interference caused by reflections.

Let us now study the solution of the invention by means of Figure 2. In other respects, the solution is similar to the solution in Figure 1 , but this version uses two antennas, of which a first antenna 200 acts as a transmitter antenna and a second antenna 202 as a receiver antenna. The coupler 106 cou- pies the microwave signal to be transmitted to the transmitter antenna 200. From the transmitter antenna 200 the microwave radiation propagates to the web 112 and is reflected back to the receiver antenna 202. From the receiver antenna 202 the microwave signal propagates to a coupler 206, which couples the received microwave signal to the mixer 116, in which the received reflected microwave signal is multiplied by a microwave signal arrived from the oscillator 100 via the directional coupler 130. After this, signal processing is similar to that in the case of Figure 1A. The couplers 106 and 206 are identical. The measurement of the reference is different from that in the solution of Figure 1. The coupler 106 couples a microwave signal arriving from the oscillator 100 via an amplifier 204 and the coupler 206 to the mixer 116. The mixer 116 mul- tiplies the microwave signal arrived directly from the oscillator by the signal arrived via the attenuator 204, generating the difference frequency of the reference signal in the same way as in the solution of Figure 1A.

Figure 3A shows a solution that is principally similar to the solution of Figure 1A, but particularly the microwave signal measurement arrangement differs. The reference 300 is also slightly different. This solution uses no mixer for processing the measuring signal, but the received microwave signal is detected in a diode detector 302. The operation of the detector is based in a previously known manner on a diode coupling that comprises, in addition to a diode, typically a coil, a condenser and a resistance (not shown in the figure) or a corresponding microwave coupling. The strength of the detected signal is measured with a measuring device 304. Since in this measurement, different reflections interfere with more than in the solution of Figure 1 , the reference 300 comprises parts 3002 and 3004 that cause reflections and that are screws driven in a transmission line, for example. The reflection part 3002 causes a reflection corresponding to the reflection from the front end of the antenna (e.g. reflection 150 in Figure 1 C) in the measurement of the web 112, and the reflection part 3004 causes a reflection corresponding to the reflection caused by the lower end of the antenna 110 (e.g. reflection 152 in Figure 1 C). This allows the effect of reflections on the measurement to be eliminated. Figure 3B shows the output signal of the detector. The vertical axis is amplitude A and the horizontal axis time T. The measurement starts for example with measurements of the reference, denoted by the letter R. The web is then measured; denoted by the letter M. The strength of the measuring signal, which is in inverse relation to the consistency of the web, can be meas- ured for example by amplitude or power measurement. Time TO taken up by the measurement of the reference and the web preferably corresponds to the time of the frequency sweep of the oscillator, i.e. it is usually between 10 ms and 100 ms. This measurement preferably measures the average signal strength at measuring time TO.

In the inventive solution, the intensity of the microwave radiation re- fleeted from the web 112 is compared with the intensity of the microwave radiation reflected from the reference 300. This allows even a small deviation in the intensity of the microwave radiation reflected from the web 112 to be detected. The intensity of the microwave radiation reflected from the web 112 is the function of the consistency of the web 112. Figure 4 shows another way to implement the inventive solution. In the solution, the reflected microwave signal 124 and a signal from the oscillator are correlated in a correlator 400 at different delays. The waveform of the signal leaving the oscillator 100 can be changed as a function of time, whereby correlation is strong when the delay is such that the waveforms of the signal reflected from the web and the signal arriving directly from the oscillator are similar. An example of such use of the waveform is noise correlation. In this case, the waveform of the signal of the oscillator 100 is noise, which is naturally different at different points of time. When the noise signals of a signal reflected in the correlator 400 at some delay and arrived directly from the oscil- lator are equal, the correlation is at its maximum. In the inventive solution, the intensity of the maximum correlation of a signal reflected from the web is compared with the intensity of the maximum correlation of a signal reflected from the reference, and the intensity of the maximum correlation of the signal reflected from the web is used to determine the consistency of the web. The in- tensity of the correlation corresponds to the strength of the signal (e.g. Figures 1 B, 1 C and 3B).

Figure 5 shows an advantageous way to measure the moisture of a web. Microwave radiation is directed from an antenna 506 towards a web 502, which is on a wire 504. The wire 504 and the web 502 are on a roll 500. When disposed on the roll 500, both the wire 504 and the web 502 are arched. This way the reflection from the web 502 back to an antenna 506 requires less alignment than obtaining reflection from an even web surface.

Figure 6 shows a way to measure the reference. This way to measure the reference does not require a coupler 106 or references 108 or 300. For example in the case of Figure 4 (as in the cases of Figures 1A and 3A), the reference can be measured this way. In the inventive solution, one side or both sides of the web 112 on the wire comprise a reference target 600. In this case the antenna traverses the web 1 12 and measures the consistency of the web 112 in a direction transverse to the grain direction of the web. In the inventive solution, the measurement is continued over the edges of the web, whereby the measurement is performed from the reference target 600. For example a metal plate or the surface of water may constitute the reference target 600. The method may also be used for calibrating a measuring device.

Figure 7 shows the position of the measuring device in a paper machine. In the inventive solution, this part of the paper machine typically com- prises a headbox 701 , a top wire 702, a bottom wire 703, a press part 705 and control means 706. A microwave meter 101 measures the consistency of the paper web on the former before the press part 705. From the headbox 701 , which controls the uniform quality and consistency of the paper web, a mass having a consistency of about 1 % is fed between the suction rolls of the for- mer. A former is that part of a paper machine which uses cylindrical forming rolls that support the wire and which is located before the press part. A former comprises suction boxes operating at negative pressure for drying the paper web disposed on the wire. In the control arrangement of a former, control means 706 adjust the suction effect (SUCK) suitable on the basis of the con- sistency measurement such that the desired consistency can be maintained in the paper web. If the measurement is implemented by sweeping the entire width of the web, the suction can also be adjusted in the transverse machine direction. The operation of the headbox can also, but not necessarily, be controlled on the basis of the consistency measurement. Although the invention has been described above with reference to the example according to the attached drawings, it is obvious that the invention is not limited thereto, but may be modified in a variety of ways within the scope of the inventive idea disclosed in the attached claims.

Claims

1. A method of measuring the consistency of a web on a former of a paper machine, the web being in contact with a wire, in which method microwave radiation (122) is transmitted to the web (112), and microwave radiation (124) reflected by the web (112) is measured, c h a ra cte rize d in that the microwave radiation (122) is transmitted to the web (112) by directing the microwave radiation (122) to that surface of the web (112) which is opposite to the contact surface of the wire and the web (112), and the intensity of the microwave radiation reflected from the web is measured; a measuring frequency range is used in which the web (112) attenuates the intensity of the interference radiation penetrating the web (112) so as to render it unessentially low as regards the measurement; microwave radiation is transmitted to a reference measurement, and the intensity of the radiation reflected in the reference measurement is measured; and the consistency of the web is measured by comparing the intensity of the microwave radiation (124) reflected from the web with the intensity of the radiation reflected in the reference measurement.

2. A method as claimed in claim 1, characterized in that the measured consistency result is used to control the former to control the consistency in the web.

3. A method as claimed in claim 1, characterized in that the frequency of a microwave signal to be transmitted is swept over the desired frequency range; a received microwave signal and the transmitted microwave signal are multiplied in a mixer (116); the output signal of the mixer (116) is filtered to obtain a noise-free difference-frequency signal; the consistency of the web is determined by means of the strength of the difference-frequency signal generated.

4. A method as claimed in claim ^characterized in that the correlation of a transmitted microwave signal and a received microwave signal is generated at different delays; the highest correlation is searched for; the intensity of the highest correlation is measured and the consistency of the web is determined by means of the highest (105) intensity of the correlation.

5. A method as claimed in claim 1, characterized in that the frequency of a microwave signal to be transmitted is swept over the desired frequency range; the received microwave radiation is detected and the consistency of the web is determined by means of the strength of the detected signal.

6. A method as claimed in claim 1, characterized in that the microwave signal is transmitted and received with one antenna (110).

7. A method as claimed in claim 1, characterized in that two antennas (200, 202) are used in the measurement in such a way that a microwave signal is transmitted with one antenna (200) and the radiation reflected from the web is received with the other antenna (202).

8. A method as claimed in claim ^characterized in that the strength of a received microwave signal reflected from the web is measured by comparing the strength of the received signal with the strength of a reference signal.

9. A method as claimed in claim 1, characterized in that a transmitted microwave signal is coupled alternately to the reference measurement and the web (112) measurement; the strength of a received microwave signal reflected from the web is measured by comparing the strength of the received signal with the strength of a reference signal.

10. A method as claimed in claim 1, characterized in that the reference measurement is performed in such a way that the interfering reflections in the actual web (112) measurement are taken into account in the reference measurement.

11. A measuring device for measuring the consistency of a web on a former of a paper machine, the web being in contact with a wire, and the measuring device being arranged to transmit microwave radiation (122) to the web (112) and to measure microwave radiation (124) reflected by the web (112), characterized in that the measuring device is arranged to transmit the microwave radiation (122) to the web (112) by directing the microwave radiation (122) to that surface of the web (112) which is opposite to the contact surface of the wire and the web (112), and to measure the intensity of the microwave radiation (124) reflected from the web (112); the measuring device is arranged to use a measuring frequency range in which the web (112) attenuates the intensity of the interference radia- tion penetrating the web (112) so as to render it unessentially low as regards the measurement; the measuring device is arranged to transmit microwave radiation to a reference (108, 300, 600) and to measure the intensity of the radiation reflected from the reference (108, 300, 600); and the measuring device is arranged to measure the consistency of the web (112) by comparing the intensity of the microwave radiation (124) reflected from the web (112) with the intensity of the radiation reflected from the reference (108, 300, 600).

12. A measuring device as claimed in claim 11, characterized in that the measuring device is arranged to control the former of the paper machine to control the consistency in the web.

13. A measuring device as claimed in claim 11, characterized in that the measuring device is arranged to sweep the frequency of a microwave signal to be transmitted over the desired frequency range; the measuring device comprises a mixer (116) for multiplying a received micro- wave signal by the transmitted microwave signal; the measuring device comprises a filter (118) for filtering the output signal of the mixer (116) to generate a noise-free difference-frequency signal; and the measuring device is arranged to determine the consistency of the web by means of the strength of the generated difference-frequency signal.

14. A measuring device as claimed in claim 11, characterized in that the measuring device comprises a correlator (400) for generating the correlation of a transmitted microwave signal and a received microwave signal at different delays; the measuring device is arranged to search for the highest correlation; the measuring device is arranged to measure the in- tensity of the highest correlation and to determine the consistency of the web by means of the intensity of the highest (105) correlation.

15. A measuring device as claimed in claim 11, characterized in that the measuring device is arranged to sweep the frequency of a microwave signal to be transmitted over the desired frequency range; the measuring device comprises a detector (302) for detecting the received microwave radiation and the measuring device is arranged to determine the consistency of the web by means of the strength of the detected signal.

16. A measuring device as claimed in claim 11, characterized in that the measuring device is arranged to transmit and receive the microwave signal with one antenna.

17. A measuring device as claimed in claim 11, character- ized in that the measuring device comprises a transmitter antenna (200) for transmitting a microwave signal (122) and a receiver antenna (202) for receiving the microwave radiation (124) reflected from the web.

18. A measuring device as claimed in claim 11, character- i z e d in that the measuring device is arranged to measure the strength of a received microwave signal (124) reflected from the web (112) by comparing the strength of the received signal with the strength of a reference signal.

19. A measuring device as claimed in claim 11, characterized in that the measuring device comprises a coupler (106) for coupling a microwave signal to be transmitted alternately to the reference measurement and to the web measurement; the measuring device is arranged to measure the strength of a received microwave signal reflected from the web by comparing the strength of the received signal with the strength of a reference signal.

20. A measuring device as claimed in claim 11, character- ized in that the measuring device is arranged to perform the reference measurement in such a way that the reference measurement arrangement comprises reflection parts (3002, 3004) for generating reflections similar to the interfering reflections in the actual web measurement.