JPH0629866B2 - Microwave moisture analyzer - Google Patents

Description

TECHNICAL FIELD The present invention relates to a device for measuring the moisture content of a sample by detecting the amount of attenuation of microwave energy by the sample.

2. Description of the Related Art There are many devices that detect the amount of attenuation of microwave energy by a sample and measure the water content of the sample. Among them, the structure in which the sample is arranged in the microwave propagation path between the transmitting side antenna and the receiving side antenna and the microwave transmits the sample is disclosed in JP-A-55-42096 and JP-A-56-9243.
5, gazette 59-197842 gazette, or gazette 6
No. 0-135752 and the like are disclosed.

In order to obtain a more accurate measurement result in the moisture measuring device having this structure, it is important to treat the microwave reflection during the measurement. That is, most of the traveling microwave (primary traveling wave) projected from the transmitting antenna reaches the receiving antenna after passing through the sample, but a part of the traveling microwave (primary traveling wave) reaches the receiving antenna (sample placement unit) and The reflected wave is reflected by the device on the receiving side, and this reflected wave is further reflected by the sample placement unit and the device on the transmitting side to become a secondary traveling wave, which interferes with the primary traveling wave, and the reception microwave energy is detected by the receiving antenna. Disturb the value.

Since the degree of this disturbance is due to the interference of the secondary traveling wave with the primary traveling wave, it is affected by the change of the position of the sample and the thickness of the sample (change of the sample arrangement part) in the microwave propagation path.

Actually, as shown in FIG. 2, the transmission horn 1 as a transmission antenna and the reception horn 2 as a reception antenna are separated by a distance L (= 1.
61 mm) facing each other to form a propagation path, and the acrylic plate 3 so as to substantially cross the main part of the propagation path.
We made an experimental device in which (200 × 200 × 5 mm) was arranged so as to be orthogonal to the traveling direction of the microwave, and 9.4 GHz (λ
= 32 mm) while projecting the reception horn 2 and the acrylic plate 3
When the distance l is moved at intervals of 2 mm, the data in the column in the table of FIG. 3 (numerical values are received voltage values at the receiving horn 2) are obtained.

The reception voltage before inserting the acrylic plate 3 is set to 1.35 V (reference voltage) by an attenuator (not shown), and the circuit for detecting the reception voltage outputs as the microwave energy attenuation increases. The receiving voltage is configured to be large.

In the table of FIG. 3, there are some places where the numerical values are below the reference voltage, but it is considered that this is because the interference of the secondary traveling wave due to the reflected wave acts in such a direction that the primary traveling wave is amplified more than the attenuation due to the acrylic plate.

The reflection coefficients of the transmitting horn 1 and the receiving horn 2 and the reflection coefficient of the acrylic plate were the values shown in the table of FIG. The values in this table are based on the well-known standing wave measurement method, and are calculated from the voltage standing wave ratio (VSWR).

The vertical axis of the data in FIG. 3 is the voltage, and the horizontal axis is the distance l.
Taking this as a graph, as shown in FIG. 5A, the received voltage shows a substantially sine curve with a period of about 16 mm as the distance 1 changes.

Therefore, assuming that the acrylic plate 3 is used as the sample placement part, this experimental result shows that when the sample placement part moves up and down by 8 mm in width from the reference position set, the received voltage is the same even though the sample has the same water content. It means that there is a variation in the range of about 0.47V. Further, this means that even if the acrylic plate is assumed as a sample, and the thickness of the sample changes vertically by 8 mm with respect to the average thickness, a large variation occurs in the received voltage. Further, in actual measurement, both the change in the position of the sample and the change in the thickness of the sample are combined, but it is often difficult to maintain the position of the sample and the sample thickness constant.

The microwave moisture meter disclosed in Japanese Patent Laid-Open No. 59-197842 focuses on the measurement error due to the reflection from the object to be measured, and the means for solving the problem is to set the interval between the first transmitting and receiving horns to l1. And the interval between the second transmitting and receiving horns is l2
The first and second transmission / reception horns are arranged so that l1-l2 ≠ nλ + 1 / 4λ, and the voltage amplitude value detected by the first reception horn and the voltage amplitude value detected by the second reception horn are The arithmetic mean is adopted as the detection value.

For this reason, this moisture meter requires at least two detecting portions each consisting of a transmitting horn and a receiving horn, and has the drawback that the occupied space of the moisture meter becomes large and the structure becomes complicated. Further, the reflection of the transmitted microwave is not limited to the sample.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The present invention has a configuration in which a sample placement portion is provided in a microwave propagation path between a microwave transmission antenna and a reception antenna, and a transmission side device, a reception side device generated in the propagation path, An object of the present invention is to provide a moisture measuring device using microwaves, which suppresses the influence of secondary traveling waves caused by the reflected microwaves from the sample placement part on the detected value and can obtain more accurate measurement results.

Means for Solving the Problems A sample placement part is provided in a microwave propagation path between a microwave transmission antenna and a reception antenna, and microwave absorption is performed between the reception antenna and the sample placement part or between the transmission antenna and the sample placement part. Position your body.

The absorber is arranged with the surface having a low reflection coefficient as the sample side.

Action The absorber absorbs and attenuates the reflected wave of the primary traveling wave and the energy of the secondary traveling wave due to the reflected wave.

Embodiment FIG. 6 is shown in FIG. 2 which is provided with a transmission horn 1 as a microwave transmission antenna, a reception horn 2 as a reception antenna, and an acrylic plate 3 as a sample placement portion located between them. An embodiment is shown in which a microwave absorber 4 is arranged between the receiving horn 2 and the acrylic plate 3 of the moisture measuring device.

The transmitting horn 1 and the receiving horn 2 are arranged to face each other, and a microwave propagation path 5 is formed between them. The transmission horn 1 includes a microwave oscillating unit 7 mainly composed of a Gunn diode connected to a power supply unit 6, and a microwave is transmitted from the horn through an attenuator 8. The oscillation frequency of the Gunn diode is 9.4 GHz, and the attenuator 8 has the ability to adjust the reception voltage at the reception horn 2 to 1.350 V when there is nothing in the propagation path 5.

The reception horn 2 is provided with a reception unit 9 mainly composed of a detection diode, and the reception voltage, that is, the detection value, is a detection value processing unit 10.
Be transmitted to.

The absorber 4 is an absorber sheet (ECCOSORB ... product name,
Grace Japan Co., Ltd. is cut out, and the reflection coefficient is different between the front and back, and the cut surface of the receiving horn 2 is attached with the surface having a low reflection coefficient as the sample side surface.
The values of the reflection coefficient for the front and back of the absorber 4 are as shown in the table of FIG. Further, the reflection coefficient of the receiving horn 2 with the absorber 4 attached is as shown in the same table (the measuring method is the same as above).

When a sample whose water content is to be measured is placed on the acrylic plate 3 and a microwave is projected from the transmission horn 1, the microwave becomes a traveling wave (primary traveling wave) and passes through the sample and the acrylic plate, so that the absorber 4 It passes and reaches the reception horn 2. During this time, the primary traveling wave is absorbed by the sample, the acrylic plate 3, and the absorber 4 and attenuated by the reflection by these and the receiving horn 2, and a secondary traveling wave (secondary traveling wave) generated by further reflection of the reflected wave is generated. Wave) affected by interference with.

The received microwave energy is detected as a received voltage by the detection diode, and the detection value processing unit 10 uses the detected value as a basis for a known "correlation between moisture and the attenuation amount of microwave energy" and the like. Calculate and output the water content of the sample.

In this case, the absorber 4 is placed between the sample placement section and the receiving horn 2.
By arranging, the main part of the secondary traveling wave generated by the reflected wave passes through the absorber 4 at least twice (reciprocating) and its energy is greatly reduced, so the influence on the primary traveling wave is greatly suppressed. To be done.

Furthermore, since the absorber has the surface with the lower microwave reflection coefficient arranged as the sample side, the initial energy itself of the reflected wave is reduced, and the effect of the secondary traveling wave generated by the reflected wave is thereby reduced. Is also suppressed.

Hereinafter, this point will be described with reference to FIGS.
FIG. 1A shows the case where the absorber 4 is not arranged.
FIG. 6B shows a case according to the embodiment of the present invention in which the absorber 4 is arranged.

Microwave reflection coefficient of the sample ... a1 Transmittance of the absorber 4 (transmitted amount / projected amount) Sample side surface → reception horn side surface …… b1 Sample side surface ← reception horn side surface …… b2 Reception horn reflection coefficient …… a2 And

First, in FIG. 1 (a), a part of the primary traveling wave M0 transmitted from the transmitting horn 1 and transmitted through the sample placement portion is reflected by the receiving horn 2 to become a reflected wave R1, and this reflected wave R1 is further reflected. Becomes a secondary traveling wave K. This secondary traveling wave K interferes with the primary traveling wave M0.

At this time, assuming that the energy of the primary traveling wave M0 is m0, the reflected R1 energy becomes [a2 × m0] and the energy of the secondary traveling wave K becomes [a1 × a2 × m0], and the primary traveling wave M0 and the secondary traveling wave The energy ratio of the wave K is: 1: [a1 × a2] ... (A).

Since the degree of the interference varies depending on the position of the sample placement portion, it causes variations in the detected value and causes a decrease in measurement accuracy.

On the other hand, in the case of FIG. 1B, the primary traveling wave M0 transmitted from the transmission horn 1 and transmitted through the sample placement portion is partially reflected by the absorber 4 and transmitted while being absorbed, and transmitted primary traveling. The wave M1 is generated, and a part of the traveling wave M1 is reflected by the reception horn 2 to be a reflected wave R2.
Reference numeral 2 denotes a transmitted reflected wave R while being partially reflected and absorbed by the absorber 4.
3, the reflected wave R3 is further reflected by the sample disposition portion to become a secondary traveling wave S, and the secondary traveling wave S is partially reflected and absorbed by the absorber 4 and is transmitted to form a transmitted secondary traveling wave K. Become. Then, the secondary traveling wave S interferes with the primary traveling wave M0, and the transmitted secondary traveling wave K interferes with the transmitted primary traveling wave M1.

At this time, assuming that the energy of the primary traveling wave M0 is m0, the energy of the transmitted traveling wave M1 ... [b1 × m0] the energy of the reflected wave R2 ... [a2 × b1 × m0] the energy of the reflected wave R3 ... [a2 Xb1 xb2 xm
0] Energy of secondary traveling wave S ... [a1 × a2 × b1 × b
2 × m0] Energy of transmitted secondary traveling wave K ... [a1 × a2 × b1
Xb1xb2xm0].

Therefore, the ratio of the energies of the primary traveling wave M0 and the secondary traveling wave S and the ratio of the energies of the transmitted traveling wave M1 and the transmitted secondary traveling wave K are both 1: [a1 × a2 × b1 × b2]. ), The energy of the secondary traveling wave (S, K) that interferes with the primary traveling wave is [b1 × b2] as compared with (a) above.
It is reduced by a factor of two, and the influence of interference is reduced accordingly.

There are other waves that interfere with the primary traveling wave, but these do not affect the measurement accuracy. For example, the secondary traveling wave S1 formed by the reflected wave R2 'in the figure being reflected by the absorber 4
Interferes with the transmitted traveling wave M1, but since the distance between the absorber 4 and the receiving horn 2 is always constant, the degree of interference is also constant, which does not cause variations in measurement results. Also,
The traveling wave M0 is reflected by the absorber 4, and the secondary traveling wave S2 formed by being reflected by the sample disposing portion interferes with the primary traveling wave M0, but since the reflection coefficient of the absorber 4 toward the sample side is low, Interference is less than when there is no body 4.

7 (a) and 7 (b) show another configuration example of the transmitting side horn 1.
And the case where the absorber 4 is not arranged between the sample placement section and the sample placement section, for the purpose of comparison and description. In this case, the primary traveling wave M0 is a microwave that does not pass through the sample placement section. However, the same result can be obtained by the process and the reason similar to the above.

Further, as still another configuration example, if the absorber 4 is inserted both between the receiving horn 2 and the sample and between the transmitting horn 1 and the sample, the synergistic effect in each of the above cases can be obtained.

In order to grasp the reduction of the influence of interference as a numerical value, the same experiment as in FIG. 2 was conducted by moving the sample placement part in FIG. 1 (b) and FIG. 7 (b). The reference voltage of 1.350 V in this case was determined with the absorber 4 arranged and the sample arrangement part removed. The results are tabulated in FIG. A graph of this is shown in FIGS. 5B and 5C.

According to this, even if the absorber 4 is arranged between the sample disposition unit and the reception horn 2 or between the sample disposition unit and the transmission horn 1, the position change of the sample disposition unit (including the change of the sample thickness) causes reception. The period in which the voltage changes is the same as when the absorber 4 is not arranged, but the amplitude is about 0.21 V when the absorber 4 is arranged between the sample and the receiving horn 2.
Is 1/2 or less of that when the absorber 4 is not arranged, and when the absorber 4 is arranged between the sample and the transmission horn 1, it is about 0.32 V and about 2 / when the absorber 4 is not arranged. It is clear that the number is reduced to 3.

As a result, the influence of the secondary traveling wave, that is, the reflected wave on the primary traveling wave is suppressed.

The transmitting / receiving horns 1 and 2 are generally transmitting / receiving.
It is a receiving antenna, and the reflected wave is generally generated by absorption at the fixed side parts (not shown in the figure) such as those on the transmitting side device and receiving side device facing the sample side in the microwave propagation path. The body is placed between the sample placement part and the device.

Further, this moisture measuring device has a possibility of being installed in the tea kneading machine, and the reflectance of tea leaves in the tea kneading machine is shown in FIG.

Middle-rubbing of tea leaves is a process in which the tea leaves after steaming are roughly rubbed, then the tea leaves put into the drum are exposed to hot air while being rubbed while being rotated, and dried until a certain moisture value is reached. It is necessary to constantly monitor the condition and control the temperature and volume of hot air.

Then, the tea leaf sample is taken out from the drum at the time of measuring the water content and continuously supplied to the sample placement section of the water content measuring device by a vibration conveyor or the like, but the flat shape peculiar to the tea leaf and the water content still contained in a large amount. Becomes a nodule for
It is difficult to make the thickness uniform during supply.

Effect of the Invention The received voltage at the receiving antenna, that is, the variation in the detected value is small, and the water content in the sample can be measured more accurately.

[Brief description of drawings]

1 (a) and (b) are front views shown for explanation, FIG. 2 is a front view showing an experimental apparatus, FIG. 3 is a table showing experimental results,
FIG. 4 is a table showing the reflection coefficient of each member, and FIG. 5 (a).
(B) and (c) are graphs of the table of FIG. 3, FIG. 6 is a front view of the moisture measuring apparatus according to the present invention, and FIG. 7 (a).
(B) is a front view for explaining another configuration example. 1 ... reception horn, 2 ... reception horn, 3 ... acrylic plate, 4
... Absorber, 5 ... Propagation path, 6 ... Power supply section, 7 ... Microwave oscillation section, 8 ... Attenuator, 9 ... Reception section, 10 ... Detected value processing section. M0 ... primary traveling wave, M1 ... transmitted traveling wave, R2 ... reflected wave,
R3: transmitted reflected wave, S: secondary traveling wave, K: transmitted secondary traveling wave.

Claims (2)

[Claims] 1. A microwave propagation path is formed by opposing a microwave transmission antenna and a reception antenna, and a sample placement section is provided on the propagation path, and a microwave absorber is provided between the reception antenna and the sample placement section. The microwave moisture measuring device is characterized in that the surface having a low microwave reflection coefficient is arranged as the sample side. 2. A microwave propagation path is formed by facing a microwave transmission antenna and a reception antenna, a sample placement section is provided on the propagation path, and a microwave absorber is provided between the transmission antenna and the sample placement section. The microwave moisture measuring device is characterized in that the surface having a low microwave reflection coefficient is arranged as the sample side.