JP2003294662A - Method for measuring degree of wettability of soil and optical fiber bragg grating wettability sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for measuring a degree of wettability of soil which accurately measures in a short period of time and a FBG wettability sensor suitable for the method and having a simple structure. <P>SOLUTION: A FBG 2 is buried in the soil as the FBG 2 is fixed to a thermosensitive member 1. The temperature of the thermosensitive member 1 is detected from the reflection wavelength of the FBG 2. The degree of wettability of the soil is found from the temperature. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soil wetness measuring method for measuring soil wetness and a wetness sensor therefor, and particularly to a soil wetness measuring method and method capable of highly accurate measurement in a short time. Fiber Bragg Grating (Fiber Bragg G
rating; hereinafter referred to as FBG) wetness sensor.

[0002]

2. Description of the Related Art As a conventional technique for measuring soil wetness, there is a measuring method using a soil thermal resistance meter in which a heater and a thermocouple are combined. In this method, the heater and thermocouple are buried in the soil, and the soil is heated by the heater,
The temperature rise of the soil is measured with a thermocouple.
Since the temperature rise of soil depends on the amount of water contained in the soil, it is possible to derive the degree of wetness (the amount of water contained in a certain amount of soil) from the temperature rise measured by a thermocouple. it can.

Further, a method of measuring a wetness distribution using an optical fiber (Japanese Patent Laid-Open No. 11-304739) is known. In this method, an optical fiber is housed in a SUS tube that serves as a heater, and this SUS tube is wound around a cylinder to convert the distance resolution in the temperature distribution measurement in the longitudinal direction of the optical fiber into the distance resolution in the soil depth direction. Is. In this method, the SUS tube is energized to provide heat to the surrounding soil, and the degree of temperature rise in the SUS tube at that time is measured by measuring the temperature in the longitudinal direction of the optical fiber wound around the cylinder by the optical fiber temperature distribution measuring device. The change in distribution is measured, and the wetness distribution in the soil depth direction is obtained from the change in temperature distribution. The optical fiber temperature distribution measuring device utilizes the temperature dependence of the backward Raman scattered light intensity of the optical fiber.

[0004]

However, in the measuring method using the soil thermal resistance meter, electric power is supplied to the heater,
In addition, there is a drawback that it is necessary to draw a commercial power supply to the measurement site (embedded place) in order to detect the voltage of the thermocouple. In addition, a device (measuring device) for collecting and processing data from the soil thermal resistance meter is required, and when performing soil wetness measurement at multiple points, use multiple devices connected to each soil thermal resistance meter. It is not economical because it is essential to build a station building to house it. Also, the measuring instruments have weak lightning resistance and are relatively fragile. In bad weather that requires soil wetness measurement, it is often a thunderstorm, and if it is broken by lightning at that time, it is unreliable and impractical.

In the measuring method according to the above-mentioned publication, the power supply for temperature measurement (thermocouple power supply) does not have to be drawn into the measurement site. However, since the optical fiber is wound around the cylinder, the length of the optical fiber becomes long, and the distance for measuring with the optical fiber temperature distribution measuring device becomes long. Further, since the S / N of the optical fiber temperature distribution measuring device is determined by the number of times of averaging processing of backscattered light, the number of sensors to be installed cannot be increased so much. As the number of sensors installed increases, the length of the optical fiber becomes longer, and the number of averaging processes must be increased in order to maintain the measurement temperature accuracy, and the measurement cycle becomes longer. That is, the measurement cycle for calculating the wetness is long (currently about 1 to 2 hours), which is not practical. Further, if the length of the optical fiber is increased, the length of the heater is also increased, so that the capacity of the heater power source must be increased. Further, since the optical fiber needs to be wound around a cylinder so that transmission loss due to bending does not occur, the sensor diameter becomes large (currently about 20 cmφ).

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned problems and provide a soil wetness measuring method capable of performing highly accurate measurement in a short time, and an FBG wetness sensor suitable for the method and having a simple structure. .

[0007]

In order to achieve the above object, the soil wetness measuring method of the present invention is to embed FBG fixed in a heat sensitive member in the soil and to measure the heat sensitive member from the reflection wavelength of the FBG. The temperature of the soil is detected, and the wetness of the soil is calculated from the temperature.

It is also possible to embed a heat source by electric power in the vicinity of the heat-sensitive member embedded in the soil and obtain the degree of wetness of the soil from the temperature change of the heat-sensitive member when power is supplied to the heat source.

The temperature of the heat-sensitive member may be raised or lowered by periodically turning on and off the power of the heat source.

The wettability may be obtained from the rising or falling width of the temperature of the heat sensitive member.

The wetness may be obtained from the transient response characteristic of the temperature of the heat sensitive member.

Further, in the FBG wetness sensor of the present invention, the FBG is fixed to the heat sensitive member, and the reflection wavelength of the FBG changes according to the wetness of the soil in which the heat sensitive member is buried.

The FBG may be coated with a metal material, the heat-sensitive member may be made of metal, and the coating may be welded and fixed to the metal heat-sensitive member.

The FBG may be coated with a metal material, the heat sensitive member may be made of metal, and the metal heat sensitive member may be solder-fixed to the coating.

A heat source may be attached to the heat-sensitive member.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

As shown in FIG. 1, the FB according to the present invention.
The G-wetness sensor uses the FBG on the heat-sensitive member 1 made of a material having a high thermal conductivity such as a metal or a polymer plastic material.
2 is fixed. The heat-sensitive member 1 is formed as a rectangular parallelepiped having upper and lower surfaces having a relatively large area, side surfaces and end surfaces having a relatively small thickness, and has a plate-like appearance. This heat sensitive plate 1 has a concave space facing the upper surface formed inside the thickness thereof, and after accommodating the FBG 2 in this concave space, the concave space is filled with a resin mold (not shown). Optical fiber protection tubes 4 communicating with the concave space are provided on both end surfaces of the heat sensitive plate 1. The FBG 2 is formed in a part of the optical fiber 3 in the longitudinal direction, the optical fiber 3 is passed through the optical fiber protection tube 4, and the FBG 2 is fixed to the bottom of the concave space.

When the FBG wetness sensor is embedded in soil, the lower surface of the heat sensitive plate 1 is in contact with the soil to be measured. That is, the lower surface of the heat sensitive plate 1 is the measurement surface.
The FBG 2 is arranged immediately behind the measurement surface.

The method of fixing the FBG 2 to the heat sensitive plate 1 may be adhesion. For example, polyimide-coated FBG2
It is advisable to bond and fix with epoxy resin. Also, FBG
In order to fix 2 to the heat sensitive plate 1, the optical fiber 3 may be coated with a metal material, while the heat sensitive plate 1 may be made of metal. The coating of the optical fiber 3 is fixed to the metal heat sensitive plate 1 by welding such as solder welding.

Thus, the FBG wetness sensor is
Is integrated with the heat sensitive plate 1. The heat-sensitive plate 1 is a heat-sensitive member having a temperature corresponding to the degree of soil wetness by exchanging heat with the soil, or FB by thermally expanding itself.
It is a heat-sensitive member that transmits temperature information to G2. Further, the heat sensitive plate 1 also serves as a case for protecting the FBG 2 from an impact or bending due to an external force. The FBG 2 is distorted by the thermal linear expansion (including contraction) of the heat sensitive plate 1 to change its reflection wavelength, or the temperature of the heat sensitive plate 1 changes its reflection wavelength.

Although not shown, at one end of the optical fiber 3,
It is assumed that a wetting degree measuring device for introducing the light source light into the optical fiber 3 and taking out and receiving the reflected light from the FBG 2 is installed. A specific method for obtaining the wettability from the optical quantity will be described later.

A method for measuring soil wetness using the FBG wetness sensor shown in FIG. 1 will be described.

The FBG wetness sensor is buried in the soil. The measurement surface corresponding to the lower surface of the heat sensitive plate 1 comes into contact with the soil to be measured. Heat exchange occurs between the heat-sensitive plate 1 and the soil, and the temperature of the measurement surface of the heat-sensitive plate 1 and the temperature of the soil are in equilibrium. The temperature itself of the heat sensitive plate 1 or the strain due to the linear thermal expansion of the heat sensitive plate 1 is measured by the FBG 2 immediately behind the measurement surface.

The temperature or strain can be measured from the transition amount (wavelength shift amount) of the central wavelength of the FBG reflection spectrum. Since the FBG reflection spectrum changes by about 10 pm (pico meter) per 1 ° C. of temperature, the temperature of the heat sensitive plate 1 can be accurately measured by measuring the wavelength shift amount. The relationship between the wavelength shift amount and the temperature of the heat sensitive plate 1 is as follows when the sensor is manufactured (initial setting)
The error is ± 0.1 ° C for each sensor by calibrating to
It can be measured with a degree of accuracy.

The temperature of the heat sensitive plate 1 buried in the soil varies depending on the amount of water (wetness) contained in the soil. Measuring the reflection wavelength of the FBG 2 is performed by the heat sensitive plate 1
It is nothing but measuring the temperature of. Therefore, the measured reflection wavelength of the FBG 2 differs depending on the wetness. F
By constantly measuring the reflection wavelength of BG2, it is possible to monitor the wetness in real time.

FIG. 2 shows an FBG wetness sensor according to another embodiment. This FBG wetness sensor is based on the FBG of FIG.
A heat source 5 is added to the wetness sensor. Heat sensitive member 1
Is referred to as the heating plate 1 here. As shown in the figure, a heater 5 is inserted through the optical fiber protection tube 4 together with the optical fiber 3. The heater 5 is fixed to the bottom of the concave space of the heat generating plate 1.

Although not shown, a power source is installed at one end of the heater 5. This power source may be an intermittent power source that periodically turns on and off the power to the heater 5 at desired time intervals.

In this embodiment, the heater 5 is attached to the heating plate 1, the heating plate 1 is heated by the heater 5, and the strain due to the linear expansion of the heating plate 1 or the temperature itself of the heating plate 1 is measured by the FBG 2. You may. Even if the amount of heat given to the heater 5 by the electric power is the same, the temperature rise width of the heating plate 1 differs depending on the soil wetness. This temperature rise width is FBG2
By monitoring the shift amount of the reflected wavelength of
The soil wetness can be clearly understood.

The electric power supplied to the heater 5 may be turned on and off periodically to intermittently perform heating and natural cooling, and the degree of wetness may be determined from the rise or fall width of the temperature of the heat generating plate 1 due to this.

The wettability may be obtained from the transient response characteristic when the temperature rises or falls, for example, the time constant of temperature change.

As described above, according to the present invention, the temperature or the strain caused by the temperature is read from the reflection wavelength of the FBG, so that the temperature can be measured in real time. Further, the FBG can be arbitrarily formed in the middle of the optical fiber, and since the FBG is only fixed to the heat sensitive member, the structure of the wetness sensor is simple.

Compared with the conventional technique using a soil thermal resistance meter, since the present invention uses an optical fiber, it is not necessary to pull in a commercial power source to the measurement site and the lightning resistance is high.
Further, if a plurality of FBGs are formed in one optical fiber connected to the wetness measuring device, soil wetness can be measured at a plurality of points, which is economical.

Compared with the prior art utilizing backscattered light, the present invention does not wind an optical fiber around a cylinder, so the optical fiber length is shortened and the size of the sensor is reduced. In addition, the wettability can be measured in real time without having to take a measurement cycle.

[0034]

The present invention exhibits the following excellent effects.

(1) Since the temperature or the strain caused by the temperature is read from the reflection wavelength of the FBG, the temperature can be measured in real time.

(2) Since the FBG is only fixed to the heat sensitive member, the structure of the wetness sensor is simple.

[Brief description of drawings]

FIG. 1 is a perspective view of an FBG wetness sensor according to an embodiment of the present invention.

FIG. 2 is a perspective view of an FBG wetness sensor according to an embodiment of the present invention.

[Explanation of symbols]

1 Heat-sensitive member (heat-sensitive plate, heat-generating plate) 2 FBG 3 optical fiber 4 Optical fiber protection tube 5 Heat source (heater)

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Claims (9)

[Claims] 1. An optical fiber Bragg grating is fixed to a heat-sensitive member and embedded in soil, the temperature of the heat-sensitive member is detected from the reflection wavelength of the optical fiber Bragg grating, and the degree of wetness of the soil is determined from the temperature. A method for measuring soil wetness, comprising: 2. A heat source by electric power is buried in the vicinity of the heat sensitive member buried in the soil, and the degree of wetness of the soil is obtained from the temperature change of the heat sensitive member when power is supplied to the heat source. The soil wetness measuring method according to claim 1. 3. The soil wetness measuring method according to claim 2, wherein the electric power of the heat source is turned on and off periodically to raise and lower the temperature of the heat sensitive member. 4. The soil wetness measuring method according to claim 3, wherein the wettability is obtained from an increase width or a decrease width of the temperature of the heat sensitive member. 5. The soil wetness measuring method according to claim 3, wherein the wetness is obtained from the transient response characteristic of the temperature of the heat sensitive member. 6. An optical fiber in which an optical fiber Bragg grating is fixed to a heat sensitive member, and the reflection wavelength of the optical fiber Bragg grating changes according to the degree of wetness of the soil in which the heat sensitive member is buried. Bragg grating wetness sensor. 7. The optical fiber Bragg grating is coated with a metal material, the heat sensitive member is made of metal, and the metal heat sensitive member is welded and fixed to the coating. The optical fiber Bragg grating wetness sensor according to claim 6. 8. The optical fiber Bragg grating is coated with a metal material, the heat-sensitive member is made of metal, and the coating is soldered to the metal heat-sensitive member. The optical fiber Bragg grating wetness sensor according to claim 6 or 7. 9. The optical fiber Bragg grating wetting sensor according to claim 6, wherein a heat source is attached to the heat sensitive member.