Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
