CA2007300C - Method and apparatus for determining the moisture content of a material - Google Patents
Method and apparatus for determining the moisture content of a material Download PDFInfo
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- CA2007300C CA2007300C CA 2007300 CA2007300A CA2007300C CA 2007300 C CA2007300 C CA 2007300C CA 2007300 CA2007300 CA 2007300 CA 2007300 A CA2007300 A CA 2007300A CA 2007300 C CA2007300 C CA 2007300C
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- 239000000463 material Substances 0.000 title claims abstract description 95
- 238000000034 method Methods 0.000 title abstract description 29
- 230000008859 change Effects 0.000 claims abstract description 22
- 238000005259 measurement Methods 0.000 claims description 41
- 238000007620 mathematical function Methods 0.000 claims description 3
- 239000002023 wood Substances 0.000 description 13
- 238000001208 nuclear magnetic resonance pulse sequence Methods 0.000 description 5
- 101000623895 Bos taurus Mucin-15 Proteins 0.000 description 3
- 230000008901 benefit Effects 0.000 description 3
- 230000003111 delayed effect Effects 0.000 description 3
- 230000010363 phase shift Effects 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 229920001131 Pulp (paper) Polymers 0.000 description 2
- 230000033228 biological regulation Effects 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 230000005855 radiation Effects 0.000 description 2
- 230000009471 action Effects 0.000 description 1
- 239000007900 aqueous suspension Substances 0.000 description 1
- 238000004590 computer program Methods 0.000 description 1
- 238000010924 continuous production Methods 0.000 description 1
- 238000005314 correlation function Methods 0.000 description 1
- 125000004122 cyclic group Chemical group 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000001956 neutron scattering Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 230000001702 transmitter Effects 0.000 description 1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N22/00—Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more
- G01N22/04—Investigating moisture content
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Abstract
In the method of the invention, a microwave signal is passed through a material whose moisture content is to be measured, the change in the velocity of the signal which has travelled through the material is measured and the moisture content is determined on the basis of the measured change and a known interdependence between the changes in the microwave signal velocity and the moisture content of the material. The apparatus comprises a transmitting device (1) which transmits a microwave signal through the material to be measured, and a detecting device {2) and a counting device (3) which measure the change of velocity of the signal transmitted through the material and determine the moisture content of the material on the basis of a known interdependence of the change of velocity of the microwave signal in question and change of moisture content of the material.
Description
METHOD AND APPARATUS FOR DETERMINING THE MOISTURE CON-TENT OF A MATERIAL
The present invention relates to a method and an apparatus for determining the moisture content of a material.
The measuring methods and equipment currently used for on-line determination of the moisture content of materials are generally based on the use of capaci-tance, conductance, neutron scattering, infrared radi-ation or microwave radiation.
Microwaves are radio waves in the frequency range 300 MHz ... 300 GHz. The action of microwave hygrometers is generally based on the measurement of transit attenuation or phase shift. Transit attenuation measurement is sensitive to interference from reflec-tions, and phase shift measurement is technically difficult to perform, especially in the case of thick material layers where the phase shift may exceed 360°.
The object of the present invention is to achieve a method and an apparatus - in the first place for application in the wood and paper industry - for on-line determination of the moisture content of materials and to enable the measurement results to be utilized for real-time regulation of processes.
The characteristic features of the method and apparatus of the invention are as stated in claims 1 and 10.
The invention is based on the fact that the speed of propagation of a microwave in a material depends on the dielectric properties of the material as shown by the following equation:
v = C ~ ~Er~ + ErI J/G ~1~
where IErI - Ert2 + Erl2 Er' - real part of the relative dielectric constant of the material ' ~ CA 02007300 1999-08-25 sr" - imaginary part of the relative dielectric constant of the material c - light velocity The dielectric constant of water is large in comparison with the dielectric constants of most other materials, which is why the velocity of a microwave passing through a wet material decreases more than the velocity of a wave passing through a dry material. This makes it possible to measure the moisture content of the material. The time delay b caused by the decrease in the velocity of the wave can be calculated from the formula (2) below:
8 = d~V = d~(C~ (Er'+ Er )~2 ) (2) where b = time delay d = distance travelled by the microwave in the material v = velocity of the microwave in the material Er' - real part of the relative dielectric constant of the material er - relative dielectric constant of the material In the method of the invention for determining the moisture content of a material, a microwave signal is passed through the material to be measured, the change in the velocity of the signal which has travelled through the material is measured and the moisture content of the material is determined on the basis of the measured change and a known interdependence between the changes in the microwave signal velocity and the moisture content of the material.
In an embodiment of the method, the change in the velocity of the microwave signal is measured by the aid of frequency modulation, the transit time of the microwave pulse, burst or pulse sequence, or noise correlation.
In an embodiment of the method, the transit time of the microwave pulse, burst or pulse sequence which has travelled through the material is measured in a constant measuring gap.
In an embodiment of the method, the transit time within the measuring gap is measured directly, using a clock, as the time interval between the transmitted and the received microwave pulse, burst or pulse sequence.
In an embodiment of the method, the transit time of a continuous or intermittent pulse-type or pulse-sequence type microwave signal which has travelled through the material to be measured is determined by using correlation techniques, i.e. by transmitting wide-band noise or by modulating the microwave signal with random noise or random digital signals and determining the transit time by the aid of the cross correlation function of the transmitted and the received signals.
In an embodiment of the method, the change occurring in the velocity of the microwave signal when travelling through the material to be measured is de-termined with the aid of frequency modulation, whereby a microwave signal is produced, the frequency of the microwave signal is varied in accordance with a math-ematical function from the lower limit fz to the upper limit f2 of its frequency range and/or vice versa within a certain period of time T, the microwave signal is divided into a first component and a second component, the first component is transmitted through the material at the measuring point, the first component which has travelled through the material is mixed with the second component, an intermediate frequency signal of corresponding to the delay is formed from the mixed signal thus obtained, and the moisture content of the material under measurement is determined from the signal ' ~ CA 02007300 1999-08-25 ef on the basis of a known interdependence between the moisture content and the intermediate frequency signal ef.
The intermediate frequency of can also be calculated as follows:
ef = B~d~((Er'+IErI)/2)Z /(T~C) (3) where B = f2-f1 = sweep width (width of the band within which the fequency varies) f1 = low limit of sweep band = lower frequency f2 = high limit of " " - higher frequency d = distance of advance of the microwave in the material Er' - real part of the relative dielectric constant of the material er - relative dielectric constant of the material T - sweep time c - light velocity Table 1 shows typical intermediate frequency ef values for certain materials as obtained from the formula (3). The parameters employed are B - 2 GHz, T = 10 ms, d = 30 cm and c = 3~108 m/s.
Material ~ Er' ~ er" ~ of (Hz)~
~ Water ~ 80 ~ 5 ~ 1788 Wet wood ~ 23 ~ 2.5 ~ 959 Dry wood ~ 3 ~ 0.1 I 346 Air ~ 1 ~ 0 ~ 200 Table 1. Calculated ef values for different materials.
It can be seen from the table that the ' ' CA 02007300 1999-08-25 dielectric constant for water is high in comparison with the dielectric constants of the other materials.
Therefore, the intermediate frequency is higher in the case of wet wood than in the case of dry wood.
The present invention relates to a method and an apparatus for determining the moisture content of a material.
The measuring methods and equipment currently used for on-line determination of the moisture content of materials are generally based on the use of capaci-tance, conductance, neutron scattering, infrared radi-ation or microwave radiation.
Microwaves are radio waves in the frequency range 300 MHz ... 300 GHz. The action of microwave hygrometers is generally based on the measurement of transit attenuation or phase shift. Transit attenuation measurement is sensitive to interference from reflec-tions, and phase shift measurement is technically difficult to perform, especially in the case of thick material layers where the phase shift may exceed 360°.
The object of the present invention is to achieve a method and an apparatus - in the first place for application in the wood and paper industry - for on-line determination of the moisture content of materials and to enable the measurement results to be utilized for real-time regulation of processes.
The characteristic features of the method and apparatus of the invention are as stated in claims 1 and 10.
The invention is based on the fact that the speed of propagation of a microwave in a material depends on the dielectric properties of the material as shown by the following equation:
v = C ~ ~Er~ + ErI J/G ~1~
where IErI - Ert2 + Erl2 Er' - real part of the relative dielectric constant of the material ' ~ CA 02007300 1999-08-25 sr" - imaginary part of the relative dielectric constant of the material c - light velocity The dielectric constant of water is large in comparison with the dielectric constants of most other materials, which is why the velocity of a microwave passing through a wet material decreases more than the velocity of a wave passing through a dry material. This makes it possible to measure the moisture content of the material. The time delay b caused by the decrease in the velocity of the wave can be calculated from the formula (2) below:
8 = d~V = d~(C~ (Er'+ Er )~2 ) (2) where b = time delay d = distance travelled by the microwave in the material v = velocity of the microwave in the material Er' - real part of the relative dielectric constant of the material er - relative dielectric constant of the material In the method of the invention for determining the moisture content of a material, a microwave signal is passed through the material to be measured, the change in the velocity of the signal which has travelled through the material is measured and the moisture content of the material is determined on the basis of the measured change and a known interdependence between the changes in the microwave signal velocity and the moisture content of the material.
In an embodiment of the method, the change in the velocity of the microwave signal is measured by the aid of frequency modulation, the transit time of the microwave pulse, burst or pulse sequence, or noise correlation.
In an embodiment of the method, the transit time of the microwave pulse, burst or pulse sequence which has travelled through the material is measured in a constant measuring gap.
In an embodiment of the method, the transit time within the measuring gap is measured directly, using a clock, as the time interval between the transmitted and the received microwave pulse, burst or pulse sequence.
In an embodiment of the method, the transit time of a continuous or intermittent pulse-type or pulse-sequence type microwave signal which has travelled through the material to be measured is determined by using correlation techniques, i.e. by transmitting wide-band noise or by modulating the microwave signal with random noise or random digital signals and determining the transit time by the aid of the cross correlation function of the transmitted and the received signals.
In an embodiment of the method, the change occurring in the velocity of the microwave signal when travelling through the material to be measured is de-termined with the aid of frequency modulation, whereby a microwave signal is produced, the frequency of the microwave signal is varied in accordance with a math-ematical function from the lower limit fz to the upper limit f2 of its frequency range and/or vice versa within a certain period of time T, the microwave signal is divided into a first component and a second component, the first component is transmitted through the material at the measuring point, the first component which has travelled through the material is mixed with the second component, an intermediate frequency signal of corresponding to the delay is formed from the mixed signal thus obtained, and the moisture content of the material under measurement is determined from the signal ' ~ CA 02007300 1999-08-25 ef on the basis of a known interdependence between the moisture content and the intermediate frequency signal ef.
The intermediate frequency of can also be calculated as follows:
ef = B~d~((Er'+IErI)/2)Z /(T~C) (3) where B = f2-f1 = sweep width (width of the band within which the fequency varies) f1 = low limit of sweep band = lower frequency f2 = high limit of " " - higher frequency d = distance of advance of the microwave in the material Er' - real part of the relative dielectric constant of the material er - relative dielectric constant of the material T - sweep time c - light velocity Table 1 shows typical intermediate frequency ef values for certain materials as obtained from the formula (3). The parameters employed are B - 2 GHz, T = 10 ms, d = 30 cm and c = 3~108 m/s.
Material ~ Er' ~ er" ~ of (Hz)~
~ Water ~ 80 ~ 5 ~ 1788 Wet wood ~ 23 ~ 2.5 ~ 959 Dry wood ~ 3 ~ 0.1 I 346 Air ~ 1 ~ 0 ~ 200 Table 1. Calculated ef values for different materials.
It can be seen from the table that the ' ' CA 02007300 1999-08-25 dielectric constant for water is high in comparison with the dielectric constants of the other materials.
Therefore, the intermediate frequency is higher in the case of wet wood than in the case of dry wood.
5 The moisture content of a given material is determined from the frequency of the intermediate frequency signal of either via calculation or by graphical means, provided that the interrelation between the moisture content of the material and the frequency of the intermediate frequency signal is known. This interrelation can be determined by performing several measurements using the method and apparatus of the invention on a material whose moisture content is known or is to be measured. Based on the measurement results, a function describing the interdependence of the two quantities is formed. This function is then utilized in measurements performed using the method and apparatus of the invention to determine or calculate the moisture content of materials. The interdependence function can be included in a computer program which computes the final value of the moisture content.
In an embodiment of the method, the frequency of the microwave signal is varied continuously and cyclically from the lower frequency f1 to the upper frequency fz and then from the upper frequency f2 to the lower frequency f1.
In an embodiment of the method, the material to be measured consists of a stream of material such as wood chips, a paper web or a water suspension, e.g.
chemical or mechanical wood pulp, which is passed through a measuring gap.
In an embodiment of the method, the determination of the moisture content is implemented as a continuous measuring process.
The apparatus of the invention comprises a transmitting device which transmits a microwave signal through the material to be measured, and a detecting device and a counting device which measure the change of velocity of the signal transmitted through the material and determine the moisture content of the material on the basis of a known interdependence of the change of velocity of the microwave signal in question and change of moisture content of the material.
In an embodiment of the apparatus of the in-vention, the transmitting device comprises an oscillator designed to produce a signal of a frequency in the microwave range and to vary the signal frequency from a lower frequency f1 to an upper frequency f~ and/or vice versa within a given period of time T in accordance with a mathematical function, an isolator designed to pass the signal in the oscillator circuit in only one direction, a directional coupler designed to divide the signal into a first component and a second component, and a transmitter (e. g. a transmitting antenna) designed to transmit the first signal component obtained from the directional coupler through the material to be measured: the detecting device comprises a receiver (e.g, a receiving antenna) designed to receive the first signal component after it has travelled through the material to be measured, a mixer provided with a first input gate, a second input gate and an output gate and designed to receive the first component from the receiver via its first input gate and the second component directly from the directional coupler via its second input gate and to mix the signals applied to the input gates, to produce from the signal thus formed an intermediate frequency signal corresponding to the delay and to output it through the output gate; and the counting device is designed to control the oscillator and to measure the frequency of the intermediate frequency signal obtained from the mixer output gate and to determine the moisture content value of the material under measurement from the intermediate frequency signal on the basis of a known interdependence of said signal and the moisture content.
In an embodiment of the apparatus, the transmitter and the receiver are arranged on different sides relative to the material under measurement.
In an embodiment of the apparatus, the transmitter and the receiver are on the same side relative to the material to be measured and a reflector plate is provided on the opposite side to reflect the microwave signal proceeding from the transmitter to pass it to the receiver after it has travelled through the material.
In an embodiment of the apparatus, the counting device is designed to control the oscillator in such a way that the frequency of the microwave signal is continuously varied from the lower frequency f1 to the upper frequency f2 and then from the upper frequency f2 to the lower frequency f1.
In an embodiment of the apparatus, the apparatus is provided with a first electric cable, a second electric cable and a third electric cable, of which the first electric cable conducts the microwave signal from the directional coupler to the second input gate of the mixer, the second electric cable conducts the signal from the directional coupler to the transmitter and the third electric cable conducts the signal from the receiver to the first input gate of the mixer, the lengths of the first, second and third electric cables being so chosen that the frequency of the intermediate frequency signal will be in a range that is technically easy to measure. The frequency of the intermediate frequency signal can also be adjusted to the desired level by altering the sweep width (B), the sweep time (T) or the distance (d) travelled by the microwave in the material under measurement.
In prior art, the so-called microwave frequecy modulation (FM-CW) technique has been employed in short-distance radar applications e.g. for the measurement of the level of a surface or the thickness of ice. Such applications are described e.g. in the following publications: "An FM-Radar for Accurate Level Measurements", 9th European Microwave Conference, Brighton 1979, pp. 712-715, and Jakkula P., Ylinen P., Tiuri M.: "Measurement of Ice And Frost Thickness with an FM-CW Radar", 10th European Microwave Conference, Warsaw 1980.
In these previously known radar applications, the distance between the object of measurement and the radar transmitter/receiver varies, whereas the material, usually air, between the transmitter/receiver and the reflecting object remains unchanged as far as the microwave is concerned.
According to the invention, the microwave frequency modulation technique (FM-CW technique) can be employed in a completely new area of application, i.e.
measurement of the moisture content of a material, where it has never been applied before.
The invention has the advantage that the moisture measurement can be performed very quickly and continuously, e.g. in the case of a moving stream of material. Thus, the measurement results can be used for real-time regulation of continuous processes.
A further advantage of the invention is that the output signal to be measured permits easy processing or consists of a burst signal, the measurement of whose frequency is simple and easy and does not necessarily impose very high requirements on the electronics used in the apparatus.
The invention makes it possible to utilize all the advantages of the FM-CW technique in connection with the measurement of the moisture content of materials.
In the following, the invention is described in detail by referring to the attached drawing, wherein Fig. 1 illustrates the circuit principle of the apparatus of the invention, Fig. 2 illustrates circuit the principle of another embodiment of the apparatus of the invention, Fig. 3 shows the oscillator frequency and the mixer output signal of the apparatus in Fig. 2 as functions of time, Fig. 4 shows a detail of a third embodiment of the apparatus of the invention, and Fig. 5 shows measurement results obtained with a fourth embodiment of the method and apparatus of the invention.
The measuring apparatus shown in Fig. 1 transmits a microwave signal through the material under measurement, whereupon it measures the change of velocity of the signal which has passed through the material and determines the moisture content of the material on the basis of a known interdependence between the change of velocity of the microwave signal in question and the change of moisture content of the material. The apparatus comprises a transmitting device 1, a detecting device 2 and a counting device 3. The measurement of the change of velocity can be based on frequency modulation or on the transit time or noise correlation of a microwave pulse, burst or pulse sequence.
In the other embodiment of the apparatus of the invention shown in Fig. 2, a microwave signal is similarly transmitted through the material, whereupon, using frequency modulation, the appparatus measures the change of velocity of the signal which has passed through the material and determines the moisture content of the material on the basis of a known interdependence between the change of velocity of the microwave signal and the change of moisture content of the material.
The apparatus comprises a transmitting device 1, a detecting device 2 and a counting device 3.
The transmitting device 1 consists of an oscillator 4, an isolator 5, a directional coupler 6 and a transmitter 7. The detecting device 2 consists of a receiver 8 and a mixer 9.
The oscillator 4 produces a microwave-frequency 5 signal. Controlled by the counter 3, the oscillator changes the signal frequency in a linear fashion through a certain range of frequencies within a certain period of time. Thereupon the signal frequency is again changed linearly from the upper limit of the range to its lower 10 limit. These cyclic variations are continued without interruption.
The isolator 5 takes care that the microwave signal is only allowed to travel in one direction in the oscillating circuit.
The directional coupler 6 divides the microwave signal into a first component I and a second component II. The first component I of the microwave signal is passed via the second electric cable 12 to the transmitter 7.
The transmitter 7 transmits the signal through the material under measurement.
The receiver 8 receives the microwave signal I
which has travelled through the material under measurement. The transmitter and the receiver are placed on opposite sides of the material. On its way through the material, the signal I is retarded and delayed as compared to the second microwave signal component II, which is used as a reference quantity in the mixer 9.
The mixer comprises a first input gate RF, a second input gate LO and an output gate IF. The second input gate LO of the mixer is fed by the second signal component II, which is supplied directly from the di rectional coupler via cable 11. The first input gate RF
of the mixer 9 is fed by the first signal component I, supplied by the receiver via cable 13.
In the mixer, the signals I and II applied to its input gates RF and LO are mixed. From the signal thus obtained, the mixer produces an intermediate frequency signal of, which is obtained from the output gate IF.
By suitably choosing the lengths of the electric cables 11, 12 and 13, the intermediate frequency signal of corresponding to the delay is adjusted to a level allowing technically easy measurement.
The counting device 3 measures the frequency of the intermediate frequency signal ef obtained from the output gate IF of the mixer. The moisture content value of the material under measurement can be determined from this frequency on the basis of a known interdependence between the intermediate frequency signal of and the moisture content. For a given material, this interdependence being known, the moisture content value is obtained either by calculation or by graphical means. The interdependence can be determined by performing several measurements with the method and apparatus of the invention on a material with known moisture content values. Based on the measurement results, a function describing the interdependence of the quantities is formed. This function is then utilized in the computation of the moisture content value of materials in measurements performed with the method and apparatus of the invention.
Fig. 3a shows a graph representing the frequency of the microwave signals at the first input gate RF and at the second input gate LO of the mixer 9.
The signal frequency changes from a lower frequency f~
to a higher frequency f2 during a period of time T, the slope of the change being constant. The signal I applied to the first input gate RF of the mixer is delayed by a length of time b as compared to the signal II applied to the second input gate LO. The continuous line represents the signal II at the second input gate LO
In an embodiment of the method, the frequency of the microwave signal is varied continuously and cyclically from the lower frequency f1 to the upper frequency fz and then from the upper frequency f2 to the lower frequency f1.
In an embodiment of the method, the material to be measured consists of a stream of material such as wood chips, a paper web or a water suspension, e.g.
chemical or mechanical wood pulp, which is passed through a measuring gap.
In an embodiment of the method, the determination of the moisture content is implemented as a continuous measuring process.
The apparatus of the invention comprises a transmitting device which transmits a microwave signal through the material to be measured, and a detecting device and a counting device which measure the change of velocity of the signal transmitted through the material and determine the moisture content of the material on the basis of a known interdependence of the change of velocity of the microwave signal in question and change of moisture content of the material.
In an embodiment of the apparatus of the in-vention, the transmitting device comprises an oscillator designed to produce a signal of a frequency in the microwave range and to vary the signal frequency from a lower frequency f1 to an upper frequency f~ and/or vice versa within a given period of time T in accordance with a mathematical function, an isolator designed to pass the signal in the oscillator circuit in only one direction, a directional coupler designed to divide the signal into a first component and a second component, and a transmitter (e. g. a transmitting antenna) designed to transmit the first signal component obtained from the directional coupler through the material to be measured: the detecting device comprises a receiver (e.g, a receiving antenna) designed to receive the first signal component after it has travelled through the material to be measured, a mixer provided with a first input gate, a second input gate and an output gate and designed to receive the first component from the receiver via its first input gate and the second component directly from the directional coupler via its second input gate and to mix the signals applied to the input gates, to produce from the signal thus formed an intermediate frequency signal corresponding to the delay and to output it through the output gate; and the counting device is designed to control the oscillator and to measure the frequency of the intermediate frequency signal obtained from the mixer output gate and to determine the moisture content value of the material under measurement from the intermediate frequency signal on the basis of a known interdependence of said signal and the moisture content.
In an embodiment of the apparatus, the transmitter and the receiver are arranged on different sides relative to the material under measurement.
In an embodiment of the apparatus, the transmitter and the receiver are on the same side relative to the material to be measured and a reflector plate is provided on the opposite side to reflect the microwave signal proceeding from the transmitter to pass it to the receiver after it has travelled through the material.
In an embodiment of the apparatus, the counting device is designed to control the oscillator in such a way that the frequency of the microwave signal is continuously varied from the lower frequency f1 to the upper frequency f2 and then from the upper frequency f2 to the lower frequency f1.
In an embodiment of the apparatus, the apparatus is provided with a first electric cable, a second electric cable and a third electric cable, of which the first electric cable conducts the microwave signal from the directional coupler to the second input gate of the mixer, the second electric cable conducts the signal from the directional coupler to the transmitter and the third electric cable conducts the signal from the receiver to the first input gate of the mixer, the lengths of the first, second and third electric cables being so chosen that the frequency of the intermediate frequency signal will be in a range that is technically easy to measure. The frequency of the intermediate frequency signal can also be adjusted to the desired level by altering the sweep width (B), the sweep time (T) or the distance (d) travelled by the microwave in the material under measurement.
In prior art, the so-called microwave frequecy modulation (FM-CW) technique has been employed in short-distance radar applications e.g. for the measurement of the level of a surface or the thickness of ice. Such applications are described e.g. in the following publications: "An FM-Radar for Accurate Level Measurements", 9th European Microwave Conference, Brighton 1979, pp. 712-715, and Jakkula P., Ylinen P., Tiuri M.: "Measurement of Ice And Frost Thickness with an FM-CW Radar", 10th European Microwave Conference, Warsaw 1980.
In these previously known radar applications, the distance between the object of measurement and the radar transmitter/receiver varies, whereas the material, usually air, between the transmitter/receiver and the reflecting object remains unchanged as far as the microwave is concerned.
According to the invention, the microwave frequency modulation technique (FM-CW technique) can be employed in a completely new area of application, i.e.
measurement of the moisture content of a material, where it has never been applied before.
The invention has the advantage that the moisture measurement can be performed very quickly and continuously, e.g. in the case of a moving stream of material. Thus, the measurement results can be used for real-time regulation of continuous processes.
A further advantage of the invention is that the output signal to be measured permits easy processing or consists of a burst signal, the measurement of whose frequency is simple and easy and does not necessarily impose very high requirements on the electronics used in the apparatus.
The invention makes it possible to utilize all the advantages of the FM-CW technique in connection with the measurement of the moisture content of materials.
In the following, the invention is described in detail by referring to the attached drawing, wherein Fig. 1 illustrates the circuit principle of the apparatus of the invention, Fig. 2 illustrates circuit the principle of another embodiment of the apparatus of the invention, Fig. 3 shows the oscillator frequency and the mixer output signal of the apparatus in Fig. 2 as functions of time, Fig. 4 shows a detail of a third embodiment of the apparatus of the invention, and Fig. 5 shows measurement results obtained with a fourth embodiment of the method and apparatus of the invention.
The measuring apparatus shown in Fig. 1 transmits a microwave signal through the material under measurement, whereupon it measures the change of velocity of the signal which has passed through the material and determines the moisture content of the material on the basis of a known interdependence between the change of velocity of the microwave signal in question and the change of moisture content of the material. The apparatus comprises a transmitting device 1, a detecting device 2 and a counting device 3. The measurement of the change of velocity can be based on frequency modulation or on the transit time or noise correlation of a microwave pulse, burst or pulse sequence.
In the other embodiment of the apparatus of the invention shown in Fig. 2, a microwave signal is similarly transmitted through the material, whereupon, using frequency modulation, the appparatus measures the change of velocity of the signal which has passed through the material and determines the moisture content of the material on the basis of a known interdependence between the change of velocity of the microwave signal and the change of moisture content of the material.
The apparatus comprises a transmitting device 1, a detecting device 2 and a counting device 3.
The transmitting device 1 consists of an oscillator 4, an isolator 5, a directional coupler 6 and a transmitter 7. The detecting device 2 consists of a receiver 8 and a mixer 9.
The oscillator 4 produces a microwave-frequency 5 signal. Controlled by the counter 3, the oscillator changes the signal frequency in a linear fashion through a certain range of frequencies within a certain period of time. Thereupon the signal frequency is again changed linearly from the upper limit of the range to its lower 10 limit. These cyclic variations are continued without interruption.
The isolator 5 takes care that the microwave signal is only allowed to travel in one direction in the oscillating circuit.
The directional coupler 6 divides the microwave signal into a first component I and a second component II. The first component I of the microwave signal is passed via the second electric cable 12 to the transmitter 7.
The transmitter 7 transmits the signal through the material under measurement.
The receiver 8 receives the microwave signal I
which has travelled through the material under measurement. The transmitter and the receiver are placed on opposite sides of the material. On its way through the material, the signal I is retarded and delayed as compared to the second microwave signal component II, which is used as a reference quantity in the mixer 9.
The mixer comprises a first input gate RF, a second input gate LO and an output gate IF. The second input gate LO of the mixer is fed by the second signal component II, which is supplied directly from the di rectional coupler via cable 11. The first input gate RF
of the mixer 9 is fed by the first signal component I, supplied by the receiver via cable 13.
In the mixer, the signals I and II applied to its input gates RF and LO are mixed. From the signal thus obtained, the mixer produces an intermediate frequency signal of, which is obtained from the output gate IF.
By suitably choosing the lengths of the electric cables 11, 12 and 13, the intermediate frequency signal of corresponding to the delay is adjusted to a level allowing technically easy measurement.
The counting device 3 measures the frequency of the intermediate frequency signal ef obtained from the output gate IF of the mixer. The moisture content value of the material under measurement can be determined from this frequency on the basis of a known interdependence between the intermediate frequency signal of and the moisture content. For a given material, this interdependence being known, the moisture content value is obtained either by calculation or by graphical means. The interdependence can be determined by performing several measurements with the method and apparatus of the invention on a material with known moisture content values. Based on the measurement results, a function describing the interdependence of the quantities is formed. This function is then utilized in the computation of the moisture content value of materials in measurements performed with the method and apparatus of the invention.
Fig. 3a shows a graph representing the frequency of the microwave signals at the first input gate RF and at the second input gate LO of the mixer 9.
The signal frequency changes from a lower frequency f~
to a higher frequency f2 during a period of time T, the slope of the change being constant. The signal I applied to the first input gate RF of the mixer is delayed by a length of time b as compared to the signal II applied to the second input gate LO. The continuous line represents the signal II at the second input gate LO
while the broken line represents the signal I at the first input gate RF. The time difference b arises from the fact that the microwave signal is delayed on its way from the transmitter 7 to the receiver 8. From signals I and II, the mixer produces an intermediate frequency signal of, which is proportional to the moisture content of the material under measurement.
The curve in Fig. 3b represents the amplitude of the intermediate frequency signal ef as a function of time.
Fig. 4 shows an embodiment in which the trans-mitter 7 and the receiver 8 are placed on the same side of the material under measurement, with a reflector plate 10 provided on the opposite side. The plate reflects the microwave signal transmitted by the transmitter 7 to the receiver 8.
Fig. 5 presents the results of measurements on wood chips. These measurements were performed by the method of the invention to determine the interdependence between the moisture content and the frequency of the intermediate freqeuncy signal. The measurement parameters were B = 0.9 GHz, T = 11,1 ms and d = 5 cm.
The measurements were performed on wood chips with six different known moisture content values, so that the corresponding six frequencies of the intermediate frequency signal of were obtained. Because the moisture content of wood chips was desired to be expressed in terms of percentages by weight at each measuring point, the frequency values were divided by the density of the sample under measurement, whereby the effects of different densities on the results were eliminated.
In the figure, the measurement results and the interdependence function obtained from them are presented in the form of a graph. The vertical axis represents the frequency of the intermediate frequency signal ef divided by the density of the material under measurement. The horizontal axis represents the moisture content of the wood chips in terms of percentages by weight. The data for wood in Table 1 discloses the class on magnitude relating to solid wood and is therefore not comparable with the results of Fig. 5 measured on wood chips.
The invention is not restricted to the embodiment examples described above, but instead several variations are possible within the scope of the idea of the invention as defined in the following claims.
The curve in Fig. 3b represents the amplitude of the intermediate frequency signal ef as a function of time.
Fig. 4 shows an embodiment in which the trans-mitter 7 and the receiver 8 are placed on the same side of the material under measurement, with a reflector plate 10 provided on the opposite side. The plate reflects the microwave signal transmitted by the transmitter 7 to the receiver 8.
Fig. 5 presents the results of measurements on wood chips. These measurements were performed by the method of the invention to determine the interdependence between the moisture content and the frequency of the intermediate freqeuncy signal. The measurement parameters were B = 0.9 GHz, T = 11,1 ms and d = 5 cm.
The measurements were performed on wood chips with six different known moisture content values, so that the corresponding six frequencies of the intermediate frequency signal of were obtained. Because the moisture content of wood chips was desired to be expressed in terms of percentages by weight at each measuring point, the frequency values were divided by the density of the sample under measurement, whereby the effects of different densities on the results were eliminated.
In the figure, the measurement results and the interdependence function obtained from them are presented in the form of a graph. The vertical axis represents the frequency of the intermediate frequency signal ef divided by the density of the material under measurement. The horizontal axis represents the moisture content of the wood chips in terms of percentages by weight. The data for wood in Table 1 discloses the class on magnitude relating to solid wood and is therefore not comparable with the results of Fig. 5 measured on wood chips.
The invention is not restricted to the embodiment examples described above, but instead several variations are possible within the scope of the idea of the invention as defined in the following claims.
Claims (7)
1. Apparatus for determining the moisture content of a material, comprising a transmitting device which is arranged to transmit a microwave signal through the material to be measured, and a detecting device and a counting device which are arranged to measure the change of velocity of the signal transmitted through the material and to determine the moisture content of the material on the basis of said measured change and a predetermined known interdependence between the changes in the microwave signal velocity and the moisture content of the material, wherein the transmitting device comprises an oscillator designed to produce a signal of a frequency in the microwave range and to vary the signal frequency from a lower frequency f1 to an upper frequency f2 within a given period of time T in accordance with a mathematical function, an isolator designed to pass the signal in the oscillator circuit in only one direction, a directional coupler designed to divide the signal into a first component (I) and a second component (II), and a transmitter designed to transmit the first signal component obtained from the directional coupler through the material to be measured, that the detecting device comprises a receiver designed to receive the first signal component (I) after it has travelled through the material to be measured, a mixer provided with a first input gate (RF), a second input gate (LO) and an output gate (IF) and designed to receive the first component (I) from the receiver via its first input gate (RF) and the second component (II) directly from the directional coupler via its second input gate (LO) and to mix the signals applied to the input gates (LO) and (RF), to produce from the signal thus formed an intermediate frequency signal .DELTA.f corresponding to the delay of the first signal component (I) as compared to the second signal component (II) and to output it through the output gate (IF); and the counting device is designed to control the oscillator and to measure the frequency of the intermediate frequency signal .DELTA.f obtained from the mixer output gate (IF) and to determine the moisture content value of the material under measurement from the intermediate frequency signal on the basis of a known interdependence of said signal .DELTA.f and the moisture content.
2. Apparatus according to Claim 1, comprising a clock device to determine the transit time of microwave signal travelled through the material in a constant measuring gap.
3. Apparatus according to Claim 1, comprising a modulator device to modulate the microwave signal to be transmitted through the material.
4. Apparatus according to Claim 1, wherein the transmitter and the receiver are located on different sides of the material under measurement.
5. Apparatus according to Claim 1, wherein the transmitter and the receiver are on the same side relative to the material to be measured and a reflector plate is provided on the opposite side to reflect the transmitted microwave signal to the receiver.
6. Apparatus according to Claim 1, wherein the counting device is designed to control the oscillator in such a way that the frequency of the microwave signal is continuously and cyclically varied from the lower frequency f1 to the upper frequency f2 and then from the upper frequency f2 to the lower frequency f1.
7. Apparatus according to Claim 1, the apparatus is provided with a first electric cable, a second electric cable and a third electric cable, of which the first electric cable conducts the microwave signal from the directional coupler to the second input gate (LO) of the mixer, the second electric cable conducts the signal from the directional coupler to the transmitter and the third electric cable conducts the signal from the receiver to the first input gate (RF) of the mixer, the lengths of the first, second and third electric cables being such that the frequency of the intermediate frequency signal of will be in a range that is technically easy to measure.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI890201 | 1989-01-13 | ||
| FI890201A FI84402C (en) | 1989-01-13 | 1989-01-13 | Method and apparatus for determining moisture content of the material |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2007300A1 CA2007300A1 (en) | 1990-07-13 |
| CA2007300C true CA2007300C (en) | 2000-09-26 |
Family
ID=8527725
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2007300 Expired - Lifetime CA2007300C (en) | 1989-01-13 | 1990-01-08 | Method and apparatus for determining the moisture content of a material |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JP3026223B2 (en) |
| CA (1) | CA2007300C (en) |
| DE (1) | DE4000925C2 (en) |
| FI (1) | FI84402C (en) |
| SE (1) | SE505241C2 (en) |
Families Citing this family (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP3160474B2 (en) * | 1994-09-12 | 2001-04-25 | 株式会社東芝 | Microwave densitometer |
| DE19725305A1 (en) * | 1997-06-14 | 1998-12-17 | Micas Elektronik Gmbh U Co Kg | Method of determining the moisture content in an object |
| FI104447B (en) * | 1998-07-10 | 2000-01-31 | Valmet Automation Inc | Method and instrument for measuring the gas content of a liquid substance |
| JP2000111499A (en) | 1998-10-02 | 2000-04-21 | Toshiba Fa Syst Eng Corp | Microwave concentration-measuring device |
| US6854322B2 (en) * | 2002-06-10 | 2005-02-15 | The Procter & Gamble Company | Directional coupler sensor |
| US7151380B2 (en) * | 2004-08-06 | 2006-12-19 | Voith Paper Patent Gmbh | Microwave water weight sensor and process |
| DE102004058641B4 (en) * | 2004-12-01 | 2007-03-01 | Agueeva, Olga, Dr. | Method for the continuous or discontinuous determination of the concentration of the constituents contained in a medium by means of evaluation of the microwaves permeating the medium |
| DE102006052789B3 (en) * | 2006-11-09 | 2008-07-10 | Festo Ag & Co | Method and position measuring device for determining a position of a measuring object |
| EP3014254B1 (en) * | 2013-06-28 | 2022-10-12 | Sensors & Software Inc. | System and method for measurement of material property using variable reflector |
| DE102014216466A1 (en) * | 2014-08-20 | 2016-02-25 | Bayerische Motoren Werke Aktiengesellschaft | Quantitative determination of the water content of materials |
| CN104977312B (en) * | 2015-06-18 | 2017-10-13 | 四川莱源科技有限公司 | The method of testing of radar type microwave water measurer device |
| DE102017202214B4 (en) | 2017-02-13 | 2019-06-27 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method and arrangement for measuring properties of a material web |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3851244A (en) * | 1973-12-18 | 1974-11-26 | Electronic Ass Of Canada Ltd | Microwave moisture measuring apparatus |
| NL8100250A (en) * | 1980-03-13 | 1981-10-01 | Halliburton Co | ACOUSTIC LOG SYSTEM WITH SWING ENERGY SOURCE. |
| DE3150202A1 (en) * | 1981-12-18 | 1983-06-23 | Philips Patentverwaltung Gmbh, 2000 Hamburg | ARRANGEMENT FOR MEASURING THE HUMIDITY |
| JPH0785062B2 (en) * | 1987-06-09 | 1995-09-13 | 建設省土木研究所長 | Method for measuring soil dry density by electromagnetic waves |
-
1989
- 1989-01-13 FI FI890201A patent/FI84402C/en not_active IP Right Cessation
-
1990
- 1990-01-08 CA CA 2007300 patent/CA2007300C/en not_active Expired - Lifetime
- 1990-01-09 SE SE9000065A patent/SE505241C2/en not_active IP Right Cessation
- 1990-01-12 JP JP2005402A patent/JP3026223B2/en not_active Expired - Lifetime
- 1990-01-15 DE DE4000925A patent/DE4000925C2/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| FI890201A0 (en) | 1989-01-13 |
| SE505241C2 (en) | 1997-07-21 |
| SE9000065L (en) | 1990-07-14 |
| CA2007300A1 (en) | 1990-07-13 |
| JPH02238348A (en) | 1990-09-20 |
| FI890201A (en) | 1990-07-14 |
| JP3026223B2 (en) | 2000-03-27 |
| DE4000925A1 (en) | 1990-07-19 |
| FI84402B (en) | 1991-08-15 |
| FI84402C (en) | 1991-11-25 |
| SE9000065D0 (en) | 1990-01-09 |
| DE4000925C2 (en) | 1998-01-22 |
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