JPH0136898B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in microwave moisture meters.
Generally, microwave absorption of water is extremely large in the microwave band. For this reason, it has been widely practiced in the past to measure the amount of water in an object to be measured from the amount of attenuation of microwaves by utilizing the microwave absorption of water.

Fig. 1 is an explanatory diagram of a conventional example configuration explaining such a conventional microwave moisture meter. An isolator that allows almost no microwave to pass through the microwave in the opposite direction, 3
5 is a transmitter horn connected to the isolator 2 via a first waveguide (or coaxial cable, hereinafter simply referred to as "waveguide") 4, and 5 is a receiver disposed at a predetermined distance l from the transmitter horn 3. Horn, 7 is second
A transmitting horn 8 is connected to the receiving horn 5 via a waveguide 6, and a receiving horn 8 is disposed at a predetermined distance l, which is the same as the distance between the transmitting and receiving horns 3 and 5, from the transmitting horn 7. Reference numeral 9 denotes an object to be measured made of, for example, a sheet of paper, placed in the space formed by the transmitting horns 3, 7 and the receiving horns 5, 8;
12 is a detection section that is made of, for example, a crystal diode, and detects the output of the receiving horn 8 via the third waveguide 10; This is a calculation unit that calculates the amount.

In the conventional example having the above configuration, the microwave transmitted from the oscillator 1 is projected onto the object to be measured 9 from the transmission horn 3 via the isolator 2 and the first waveguide 4. Further, the microwave that has passed through the object to be measured 9 and has been attenuated by water molecules is received by the receiving horn 5, passes through the second waveguide 6, and is projected onto the object to be measured 9 again from the transmitting horn 7. Ru. The microwaves that have passed through the object to be measured and are attenuated by water molecules again are received by the reception horn 8 and then passed through the third waveguide 10 and detected by the detection unit 11. Based on the output of the detection section, the amount of water in the object to be measured 9 can be determined by arithmetic processing performed in the calculation section 12.

However, in the above conventional example, the object to be measured 9
Since it is in the form of a sheet, the object to be measured tends to fluctuate, and the standing waves generated by such fluttering have the disadvantage that the pass line characteristics become a major source of measurement error.

The present invention has been made in view of these drawbacks, and its purpose is to reduce the effects of flapping of a sheet-like object to be measured in a microwave moisture meter that uses microwaves to measure the amount of moisture in an object to be measured. It is an object of the present invention to provide a microwave moisture meter capable of measuring the amount of moisture in a measured object with high precision without being subjected to large effects.

A feature of the present invention is that in a microwave moisture meter that uses microwaves to measure the amount of moisture in an object to be measured, the microwaves projected from a transmitting horn and transmitted through the object to be measured are transmitted to first and second receiving horns. and the difference in distance between these receiving horns and the object to be measured is nλ+λ/4.

Hereinafter, the present invention will be explained in detail using figures. FIG. 2 is an explanatory diagram of the configuration of an embodiment of the present invention,
In the figure, the same symbols as in FIG. 1 are used with the same meaning, and repeated explanation will be omitted here. A first receiving horn 13 is placed at a predetermined distance _l1 from the object to be measured 9 and receives the microwave transmitted through the object to be measured 9; and 14 is placed at a predetermined distance _l2 from the object to be measured 9. A second receiving horn 15 that receives the microwave transmitted through the object to be measured 9 is connected to receiving horns 13 and 14 via second and third waveguides 16 and 17, respectively, and receives input from these receiving horns 13 and 14. The magic T 18, which equally divides and sends out the microwave in two directions, is a non-reflection terminating element that receives the microwave sent in the direction of the magic T 15 and terminates the transmission line. Note that the microwaves sent in the direction of the magic T15 are detected by a detection section 11 made of, for example, a crystal diode. Further, the distances between the first and second receiving horns 13 and 14 and the object to be measured 9 are respectively l ₁ ,
l ₂ , the difference l ₃ is the difference between the first and second receiving horns 13, 14 so that the following formula (1) holds.
is placed. Furthermore, l ₁ = l ₂ - l ₁ ≒nλ±λ/4 (1) (where n: natural number, λ: wavelength) The microwaves received from the reception horns 13 and 14 are processed into signals and sent to the detection unit 11. The three-dimensional circuit to be reached is not limited to the embodiment shown in FIG. 2, and can be modified in various ways.

The operation of the embodiment of the present invention having the above configuration will be described below. In FIG. 2, microwaves sent out from an oscillator 1 pass through an isolator 2 and a second waveguide 4, and then from a transmission horn 3 to an object under test 9.
is projected on. In addition, the microwaves transmitted through the object to be measured 9 and subjected to attenuation and phase rotation by water molecules,
The signal is received by the first and second receiving horns 13 and 14. The outputs of these receiving horns 13 and 14 reach the magic T 15 via second and third waveguides 16 and 17, respectively, and are equally distributed and transmitted in both directions. The microwaves sent in this direction reach the non-reflection termination element 18 and are extinguished. Further, the microwaves sent in the direction are detected by the detection unit 11, and the detection signal is processed by the calculation unit 12 to calculate the amount of water in the object to be measured 9.

By the way, in the conventional example shown in Fig. 1, the voltage waveform along the line is expressed by the following equation (2) using the wave equation.
be guided as follows.

V=V ⁺ e− ^j 〓 ^x + ΓV ⁺ e ^j 〓 ^x (2) However, Γ: Reflectance, V ⁺ : Peak value of the sine wave representing voltage, β: Phase, x: Distance from the reference position The above formula ( From 2), the voltage amplitude |V| is derived as shown in equation (3) below.

｜V｜=｜V ⁺ ｜｜(1+Γe ^j2 〓 ^x) /e ^j 〓 ^x ｜ =｜V ⁺ ｜｜1+Γe ^j2 〓 ^x ｜ =｜V ⁺ ｜{1+Γcos2βx) ² +
Γ ² sin ² 2βx} ^1/2 = |V ⁺ | {(1+Γ) ² −2Γ(1−
cos2βx)} ^1/2 = |V ⁺ |{(1+Γ) ² −4Γsin ² βx} ^1/2 (3) From the above equation (3), the maximum value of voltage amplitude |V| |V _nax |
By finding the minimum value |V _nio | and taking their ratio, the following equation (4) is obtained.

|V _nax |/|V _nio |=1+Γ/1−Γ (4) On the other hand, in the embodiment of the present invention shown in FIG. It is arranged so that Therefore, as the amplitude |V| of the voltage waveform along the line, in addition to the above equation (3), the following equation is used, which is the signal of the part whose phase is separated from the above equation (3) by λ/4 (that is, 90 degrees). (5) is also detected by the detection unit 11. The object to be measured 9 is made of cardboard or the like, and moves parallel to each transmitting/receiving horn 3, 13, 14 (that is, each transmitting/receiving horn 3, 1
3 and 14, the object to be measured 9 moves vertically in parallel on the paper surface of FIG. 2 without wavering). Therefore, in the arithmetic unit 12, the above equation is simply
By simply taking the arithmetic average of (3) and the following equation (5), the waveguide 16,
The phase fluctuation based on the distance difference of 17 can be considered, and the maximum value |V _nax | and the minimum value |V _nio | of the average value (i.e., {Equation (3) + Equation (5)}/2) can be After calculating the ratio, the following formula (6) is obtained.

|V|=|V ⁺ |{(1+Γ) ² −4Γsin ² (βx+
90゜)} ^1/2 = |V ⁺ |{(1+Γ) ² −4Γcos ² βx} ^1/2 (5) In the above equations (4) and (6), since 0<Γ<1, 1+Γ/1-Γ>√1+ ² holds true. In addition, the above equation (4) shows the amplitude of the output signal when the path line of the device under test 9 changes in the conventional example, and the above equation (6) shows the amplitude of the output signal when the path line of the device under test 9 changes in the embodiment of the present invention.
(In other words, the ratio between the maximum value and the minimum value of the voltage amplitude is the amplitude of the output signal when the path line of the object to be measured 9 changes.) ). Therefore,
From the comparison of the above equations (4) and (6), it can be seen that the pass line characteristics of the object to be measured 9 are better in the embodiment of the present invention shown in FIG. 2 than in the conventional example shown in FIG. 1. I understand.

FIG. 3 is a graph showing the effect of the above-described embodiment of the present invention. In the graph, the vertical axis shows the received voltage fluctuation after passing through the object to be measured, and the horizontal axis shows the variation in the received voltage after passing through the object to be measured and the receiving horn. (i.e., the path line of the object to be measured). In Figure 3, (a) shows the value when the reflectance Γ is 0.1 using the above equations (3), (5), and (3) + (5)/2, and (b) is the reflectance Γ
The value at 0.2 is shown. The characteristic curves of equation (3) and equation (3)+(5)/2 in FIG. 3 correspond to the pass line characteristics of the conventional example and the embodiment of the present invention, respectively. Therefore, by comparing these, it can be seen that according to the embodiment of the present invention, the pass line characteristics of the object to be measured 9 are greatly improved.

According to the embodiment of the present invention as described in detail above, the transmitting/receiving horns 3, 13, and 14 are arranged so that the above formula (1) is satisfied, so that it is different from the conventional example. This has the advantage that it is less susceptible to the effects of standing waves caused by the fluttering of the object to be measured. It is also conceivable to mechanically move the receiving horns 13 and 14 up and down by λ/4, but even compared to such a method, according to the embodiment of the present invention, there is no need to worry about failure of moving parts, etc. It also has the advantage of having a long product life.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating the configuration of a conventional microwave moisture meter, FIG. 2 is a diagram illustrating the configuration of an embodiment of the present invention, and FIG. 3 is a graph showing the effects of using the embodiment of the present invention. 1... Oscillator, 2... Isolator, 3, 7... Transmission horn, 5, 8, 13, 14... Receiving horn, 4,
6, 10, 16, 17... waveguide, 9... object to be measured,
DESCRIPTION OF SYMBOLS 11... Detection part, 12... Arithmetic part, 15... Magic T, 18... Non-reflection termination element.

Claims (1)

[Claims] 1. In a microwave moisture meter that uses microwaves to measure the amount of moisture in an object to be measured, there is a transmission horn that projects the microwaves emitted from the microwave source onto the object to be measured, and a microwave that passes through the object. comprising first and second receiving horns that receive waves, and a distance between the receiving horns and the object to be measured.
A microwave moisture meter characterized in that the difference l ₃ between l ₁ and l ₂ is configured such that l ₃ =nλ+λ/4 (where n: natural number, λ: wavelength).