Plant root position detection method, optical fiber sensor, and position detection apparatus

Abstract

To provide a method for detecting the position of a plant root, capable of accurately measuring the position of the plant root in soil, capable of avoiding the effect of rainfall or the like on measurement accuracy, and excellent in environmental robustness, and to provide an optical fiber sensor and a position detector used for the method for detecting the position of the plant root.SOLUTION: A method for detecting a position of a plant root in soil characterized in that a fiber optic sensor is embedded in the soil to detect the plant root below a 20cm of the fiber optic sensor, and the position of the plant root is identified by a change in strain and / or strain generated around the plant root due to rainfall and / or irrigation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for non-destructively visualizing the position of plant roots in soil, and to an optical fiber sensor and a position detection device used in the method. [Background technology]

[0002] Conventionally, methods for non-destructively visualizing the state of the ground, soil, and its interior have been studied. For example, Patent Document 1 (JP 11-166869 A) proposes a method for detecting ground deformation and collapse by wiring a stretchable optical fiber into the ground, the method comprising: (A) providing x- and y-axes in the ground; (B) wiring the optical fiber so that it has x-axis and y-axis components within a two-dimensional plane in the ground formed by the x- and y-axes; (C) emitting pulsed light from one end of the optical fiber; (D) detecting, by a coherent detection method, Brillouin scattered light and Rayleigh scattered light resulting from strain in the optical fiber caused by deformation of the ground, which return after a time period corresponding to the distance; (E) measuring the frequency shift distribution to detect the magnitude of the strain in the optical fiber; and identifying the location of the strain from the return time of the incident light, thereby detecting deformation and collapse of the ground.

[0003] The deformation and collapse detection method described in Patent Document 1 above states that "the movement of the ground beneath railway tracks can be measured continuously over time over a range of several tens of kilometers. Furthermore, ground deformation and collapse can be detected from abnormal changes in strain, and the location of the occurrence can also be identified."

[0004] Furthermore, in Patent Document 2 (JP 2023-003007 A), the present inventor has proposed "an apparatus for detecting changes in a minute measurement object in an attenuating medium, comprising an optical fiber sensor unit, an interrogator unit, a signal processing unit, and a determination unit, wherein the interrogator unit measures changes in the optical characteristics of the optical fiber sensor unit caused by strain occurring inside the optical fiber sensor unit to obtain a signal, the signal processing unit reduces signal noise to obtain a measurement object signal, and the determination unit separates the measurement object signal from a background signal, the optical fiber sensor unit is made of optical fibers arranged along a cylindrical shape, and the radius (r0) of a circle having the same area as a cross section parallel to the disc surface of the cylindrical shape of the minute measurement object and the radius (r1) of the cylindrical shape satisfy r1≦710r0."

[0005] In the device for detecting minute changes in an attenuating medium described in Patent Document 2, the radius (r0) of the circumscribed circle in a cross section substantially perpendicular to the direction of the change and / or displacement of a minute measurement object and the radius (r1) of the cylindrical shape are set to be "r1≦710r0", and thus, a measurement error of 1×10 -6 It is possible to apply a strain of 1×10 or more. -6 If the change in the optical properties of the optical fiber is caused by the above strain, it can be measured by the interrogator unit to obtain a highly reliable signal, and minute changes in the attenuating medium can be detected. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-166869 [Patent Document 2] Japanese Patent Publication No. 2023-003007 Summary of the Invention [Problem to be solved by the invention]

[0007] The method for detecting ground deformation and collapse described in Patent Document 1 above can detect relatively large changes over a wide area, but cannot visualize minute changes in the order of millimeters, such as when visualizing the condition of plant roots in the soil.

[0008] Furthermore, in the device for detecting minute changes in a damping medium described in Patent Document 2, the optical fiber is fixed to a cylindrical frame, and strain applied to one part of the frame affects other areas, making it difficult to distinguish between signals caused by the object to be measured and signals caused by other phenomena. More specifically, for example, if a certain point in the damping medium is a stress source, the measurement value of an optical fiber located a distance x from that point is affected by an external force that has propagated through the damping medium from the stress source to the distance x, and a shear force (internal force) over a distance Δx when the external force propagated through the damping medium reaches the frame at a distance x-Δx. Therefore, it is difficult to distinguish between strain directly caused by changes in the state of plant roots and indirect strain caused by the structure of the frame.

[0009] The above-mentioned combination of strain and / or stress hinders the measurement of creep flow and pressure distribution around plant roots, especially during rainfall or irrigation. The reason is that in conventional techniques assuming sunny weather, the direct external force caused by the plant roots exceeds the structural shear force, making it possible to ignore the influence of the structure. However, when measuring pressure distribution during rainfall or irrigation, there is no concentrated stress source, so the signal to be measured is small, and since the pressure distribution over the entire measurement area is required for analysis, the influence of the structure of the frame placed in that area is integrated.

[0010] In view of the problems in the prior art as described above, an object of the present invention is to provide a plant root position detection method that can accurately measure the position of plant roots in soil and that has excellent environmental robustness and can also measure creep flow during rainfall and irrigation, as well as an optical fiber sensor and position detection device to be used in the plant root position detection method. [Means for solving the problem]

[0011] In order to achieve the above-mentioned object, the inventors have conducted extensive research into the effects of rainfall on the signals obtained by optical fiber sensors and the structure of optical fiber sensors, and as a result have discovered that it is extremely effective to identify the position of plant roots by measuring changes in horizontal strain and / or stress that occur around the plant roots due to rainfall and / or irrigation, and have arrived at the present invention.

[0012] That is, the present invention provides: The optical fiber sensor is buried in the soil, Detecting plant roots at a distance of 20 cm or less from the optical fiber of the optical fiber sensor; Identifying the location of the plant roots based on changes in horizontal strain and / or stress around the plant roots caused by rainfall and / or irrigation; The present invention provides a method for detecting the location of plant roots in soil, characterized by:

[0013] If the distance between the optical fiber and the plant root is 20 cm or less, the optical fiber sensor can detect changes caused by plant roots with a diameter of about 10 cm. Furthermore, if the distance between the optical fiber and the plant root is 5 cm or less, the optical fiber sensor can detect changes caused by plant roots with a diameter of about 1 cm. In addition, by using the optical fiber sensor of the present invention, changes in horizontal strain and / or stress that occur around the plant roots due to rainfall and / or irrigation can be clearly observed, and the location of the plant roots can be more accurately identified.

[0014] With conventional optical fiber sensors, the structural shear force between adjacent optical fibers has a large effect on the relationship between the measurement value and the measurement position, making it difficult to grasp signal changes caused by plant roots.However, by using the optical fiber sensor of the present invention, the shear force in the depth direction is reduced, making it possible to accurately grasp the position of plant roots.

[0015] In addition, in the method for detecting the position of a plant root in soil of the present invention, it is preferable that the location where the strain and / or the stress is reversed in the vertical direction due to the rainfall and / or the irrigation is detected as the position of the plant root. Here, the position of the plant root can be detected more simply and accurately by evaluating the change in strain before and after the rainfall based on the time difference of data measured by the optical fiber sensor.

[0016] During rainfall or irrigation, a flow velocity (creep flow) occurs from the soil surface into the soil, and the horizontal strain and stress around plant roots are affected by this creep flow. More specifically, horizontal strain and stress are reversed depending on the height of the plant root (the soil surface side and the underground side), with strain and stress decreasing on the soil surface side and increasing underground. In other words, by finding the point where horizontal strain and stress are reversed up and down during rainfall, the location of plant roots can be accurately identified.

[0017] In the method for detecting the position of plant roots in soil of the present invention, the position of the plant roots is detected based on the difference in measured values ​​in the soil depth direction, so it is important that horizontal strain and stress do not change significantly due to causes other than creep flow caused by rainfall. Here, the optical fiber sensor of the present invention can alleviate horizontal strain and can extremely effectively suppress the effects of causes other than creep flow during rainfall.

[0018] The present invention also provides An optical fiber sensor used in the method for detecting the position of a plant root in soil of the present invention, Optical fibers are arranged on the surface of a frame made of a resin film or a metal thin film, The frame has a truss structure framework; Also provided is an optical fiber sensor characterized by:

[0019] The greatest feature of the optical fiber sensor of the present invention is that the frame for arranging the optical fibers has a truss structure. A truss structure is an arrangement of triangular frames. When stress is applied to a frame having such a structure, the external force is received at the nodes at each vertex of the triangle, thereby efficiently alleviating the bending moment in the members. On the other hand, when stress is applied to a rectangular frame, the external force is received by the entire frame, which tends to generate a uniform bending moment in a wide range of members, making it difficult to identify the stress distribution from the strain in the frame. Here, the term "truss structure" in the present invention refers to a structure composed of triangular frames, and is a concept that also includes those in which the nodes, which are the joints, are not mechanically fastened so that they can rotate.

[0020] In contrast, in the optical fiber sensor of the present invention, the optical fibers are arranged in a frame with a truss structure, so that the stress applied to the frame is balanced at the nodes, minimizing the effect of strain caused by the frame on the optical fiber sensor. As a result, strain acting directly on the optical fiber from the object to be measured can be efficiently detected.

[0021] Furthermore, in the case of rainfall or irrigation, the gravity and buoyancy of water are applied to the measurement area, making it difficult to accurately extract information attributable to plant roots. Buoyancy also relieves the stress on the soil caused by plant roots, eliminating the strain measured before rainfall due to plant roots. In contrast, the optical fiber sensor of the present invention applies a root detection algorithm based on creep flow, making it possible to identify the location of plant roots even in the case of rainfall or irrigation, ensuring good environmental robustness.

[0022] In the optical fiber sensor of the present invention, it is preferable that the truss structure be formed by regularly arranged triangular through holes in the resin film or the metal thin film. Forming triangular through holes in a resin film or a metal thin film is easy, and a frame having a truss structure can be manufactured more simply and inexpensively than in a manufacturing process that joins element members. In addition, by forming the truss structure using through holes, there is no need to pay attention to joining quality.

[0023] Furthermore, by attaching the optical fiber to the surface of a resin film or a metal thin film, the optical fiber sensor unit can be easily handled and placed in the soil. In addition, by using a resin film or a metal thin film as the substrate to which the optical fiber is attached, it is possible to minimize the decrease in sensitivity to strain introduced into the optical fiber due to the application of external stress. When an optical fiber sensor with an optical fiber attached to the surface of a resin film or a metal thin film is buried in the soil, the shape may be maintained using appropriate supports, etc., as necessary.

[0024] Furthermore, in the optical fiber sensor of the present invention, it is preferable that the frame has a rectangular or cylindrical shape. By making the frame rectangular or cylindrical, it is possible to easily obtain a frame that is easy to install an optical fiber and has sufficient strength. Furthermore, since the frame is hollow, plant roots can move and / or grow from the outside to the inside of the frame or from the inside to the outside.

[0025] Furthermore, the present invention provides The optical fiber sensor unit, the interrogator unit, the signal processing unit, and the determination unit are provided. a change in optical characteristics of the optical fiber sensor unit caused by strain occurring in the soil is measured by the interrogator unit to acquire a signal; The signal processing unit reduces noise in the signal to obtain a signal to be measured; The determination unit separates the measurement target signal from a background signal; the optical fiber sensor unit has optical fibers arranged on a surface of a frame made of a resin film or a metal thin film, and the frame has a framework of a truss structure; Also provided is a position detection device characterized by:

[0026] By applying a frame with a truss structure to the optical fiber sensor, the positions of plant roots in the soil can be measured in sections even when there is rainfall or watering from the ground surface.In addition, by using the optical fiber sensor of the present invention, horizontal strain at different depths in the soil can be accurately measured, allowing the positions of plant roots in the soil to be determined in detail.

[0027] Furthermore, in the position detection device of the present invention, changes occurring in the optical fiber sensor are accurately reflected as strain in the optical fiber, so that the changes in the optical properties of the optical fiber caused by the strain can be measured by the interrogator unit to obtain a highly reliable signal, making it possible to detect minute changes in the soil.

[0028] In the position detection device of the present invention, it is preferable that the truss structure is formed by regularly arranged triangular through holes in the resin film or the metal thin film. [Effects of the Invention]

[0029] According to the present invention, it is possible to provide a plant root position detection method that can accurately measure the position of plant roots in soil and has excellent environmental robustness, which can avoid the effects of rainfall and the like that can affect measurement accuracy, as well as an optical fiber sensor and a position detection device used in the plant root position detection method. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is a schematic diagram showing an embodiment of an optical fiber sensor of the present invention. [Figure 2] FIG. 1 is a schematic diagram showing one embodiment of a frame 4 having a framework of a truss structure. [Figure 3]FIG. 1 is a schematic diagram showing one aspect of a conventional frame. [Figure 4] FIG. 10 is a schematic diagram showing another aspect of a conventional frame. [Figure 5] 1 is a schematic diagram showing a state in which an optical fiber 6 is attached to the surface of a frame 4 having a truss structure. [Figure 6] 1 is a schematic diagram illustrating an embodiment of a position detection device of the present invention. [Figure 7] FIG. 1 is a schematic diagram showing a situation when studying the influence of creep flow around a tuberous root. [Figure 8] This is a heat map showing the horizontal stresses around the tuber caused by rainfall. [Figure 9] 1 is an example of an algorithm for detecting the location of a root system. [Figure 10] 1 is a photograph showing the appearance of the optical fiber sensor used in Example 1. [Figure 11] 1 is a photograph showing the state in which an optical fiber sensor is buried in soil in Example 1. [Figure 12] 1 is a graph showing the relationship between the change in measured strain over time and the amount of rainfall during the measurement. [Figure 13] The measurement results show the change in strain before (a) and after (b) the rainfall, indicated by the dotted line in Figure 12. [Figure 14] This is the time difference between (a) and (b) in Figure 13. [Figure 15] 10 is a photograph showing the appearance of the optical fiber sensor used in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0031] Representative embodiments of the plant root position detection method, optical fiber sensor, and position detection device of the present invention will be described in detail below with reference to the drawings, but the present invention is not limited to these. Note that in the following description, the same or equivalent parts will be designated by the same reference numerals, and duplicate explanations may be omitted. Furthermore, since the drawings are intended to conceptually explain the present invention, the dimensions and ratios of the components shown may differ from the actual dimensions.

[0032] 1. Optical fiber sensor A schematic diagram of the optical fiber sensor of the present invention is shown in Figure 1. The optical fiber sensor 2 is characterized in that optical fibers 6 are arranged on the surface of a frame 4 made of a resin film or a thin metal film, and the frame 4 has a truss structure framework.

[0033] A schematic diagram of a frame 4 having a truss structure framework is shown in Figure 2. For comparison, schematic diagrams of a conventional frame are shown in Figures 3 and 4. Here, Figures 2 to 4 show the state of the resin film before it is formed into a rectangular tube or cylindrical shape. In Figures 2 to 4, the black parts indicate through-holes, and the frame 4 in Figure 2 has an array of triangular through-holes 8 to give it a truss structure, the frame 4 in Figure 3 has an array of circular through-holes 8, and the frame 4 in Figure 4 has an array of square through-holes 8.

[0034] In the case of a triangular frame, the bending moment acting on the members is alleviated by slight changes in the angle of the vertices acting as sliding joints, suppressing horizontal deformation of the frame 4 located across the joints. On the other hand, in the case of a rectangular frame, the bending moment is transmitted through the joined joints, causing deformation of the entire frame 4. In other words, by applying a truss structure consisting of a triangular frame to the frame 4, it is possible to reduce the effect of frame deformation on optical fiber measurements.

[0035] In the optical fiber sensor 2 of the present invention, an optical fiber 6 is attached to the surface of a frame 4 having a truss structure. For example, as shown in Fig. 5, the optical fiber sensor 2 can be obtained by attaching the optical fiber 6 to a resin film or thin metal film having a truss structure and rolling it into a rectangular or cylindrical shape as needed. The optical fiber 6 is stretched horizontally while being shifted vertically, and is fixed so that the optical fiber 6 and the frame 4 are in close contact with each other. When placing the sensor in the soil, the shape of the frame 4 can be maintained using appropriate supports as needed.

[0036] The method of fixing the frame 4 to the support is not particularly limited as long as it does not impair the effects of the present invention, and can be selected appropriately depending on the material, desired joint strength, etc., and for example, joining by adhesive or welding can be used.

[0037] The truss structure of the frame 4 is formed by arranging through holes 8 in a resin film or thin metal film, and is not obtained by connecting rods together mechanically. Therefore, unlike when rods are assembled using mechanical fastening such as pin joints, it does not have a hinge action at the fastening points, but it is possible to alleviate applied stress by changing the angle of the vertex, and separate the upper and lower strains on the frame 4. In other words, the truss structure acts as a buffer, and the horizontal optical fibers 6 do not affect each other due to differences in elevation, making it possible to measure strain in the soil independently.

[0038] As long as the frame 4 can maintain its shape with the optical fiber 6 attached, it is preferable that the frame 4 does not prevent strain from being introduced into the optical fiber 6 due to application of external stress.

[0039] The resin film used for the frame 4 is not particularly limited as long as it does not impair the effects of the present invention, and various conventionally known resin films can be used, but it is preferable to use a PTFE film as the resin film. The use of a PTFE film can sufficiently ensure flexibility, weather resistance, chemical resistance, etc., and allows the optical fiber sensor 2 to be installed for a long period of time, even in soil that is affected by pesticides, rainwater, etc.

[0040] Furthermore, the metal thin film used for the frame 4 is not particularly limited as long as it does not impair the effects of the present invention, and various conventionally known metal thin films can be used, such as aluminum foil or titanium film. By using a metal thin film for the frame 4 and an optical fiber 6 with a metal coating, the frame 4 and the optical fiber 6 can be welded together. Furthermore, by using aluminum foil or titanium film as the metal thin film, flexibility, weather resistance, chemical resistance, and the like can be sufficiently ensured, and the optical fiber sensor 2 can be installed for a long period of time even underground, where it is subject to the effects of pesticides, rainwater, and the like.

[0041] The optical fiber 6 needs to be attached to the frame 4 without any gaps. There are no particular limitations on the method for attaching the optical fiber 6 to the frame 4 as long as it does not impair the effects of the present invention, but for example, a fluororesin-impregnated glass cloth semiconductive adhesive tape can be used.

[0042] 2. Position detection device A schematic diagram of a position detection device of the present invention is shown in Figure 6. The position detection device 10 comprises an optical fiber sensor unit 12, an interrogator unit 14, a signal processing unit 16, and a determination unit 18. The greatest feature of the position detection device of the present invention is that the optical fiber sensor (optical fiber sensor 2) of the present invention is used in the optical fiber sensor unit 12.

[0043] The change in the optical properties of the optical fiber 6 caused by strain occurring inside the optical fiber sensor unit 12 is measured by the interrogator unit 14 to obtain a signal, the signal processing unit 16 reduces noise in the signal to obtain a measurement target signal, and the determination unit 18 separates the measurement target signal from background signals, thereby enabling detection of changes in the soil. In addition, the position detection device 10 preferably includes a recording unit for recording the signal obtained by the interrogator unit 14.

[0044] It is preferable to use an interrogator with high resolution for the interrogator unit 14. However, since the optical fiber sensor unit 12 of the position detection device 10 can obtain clear signals, a commercially available interrogator can be used.

[0045] Because the optical fiber sensor unit 12 is highly sensitive, it is accompanied by a lot of noise. To reduce this noise, the signal processing unit 16 preferably performs averaging on the time axis and low-pass filtering on the spatial axis. Since dynamic information on the plant roots being detected is important for separating the signal to be observed from other signals, it is preferable to perform averaging on the time axis in short increments, for example, about 1 second. For low-pass filtering on the spatial axis, when the total length of the optical fiber 6 is L, the cutoff frequency is preferably set to L / 66 mm to L / 33 mm.

[0046] The detection of signals caused by the target plant roots is performed by the determination unit 18. The determination unit 18 performs threshold setting, machine learning, deep learning, etc. from the time-dependent strain records after noise reduction processing to extract the strain caused by the target plant roots. The position and amount of movement of micro-movements can be inferred from the distribution of the extracted strain and its changes over time, but by performing calibration in advance to relate these changes to strain, it becomes possible to measure dynamic phenomena as well.

[0047] 3. Method for detecting the position of plant roots The method for detecting the position of plant roots of the present invention is characterized by burying the optical fiber sensor 2 of the present invention in soil, detecting plant roots that are 20 cm or less away from the optical fiber 6 of the optical fiber sensor 2, and identifying the position of the plant roots based on changes in horizontal strain and / or stress that occur around the plant roots due to rainfall and / or irrigation.

[0048] By setting the distance between the optical fiber 6 and the plant root to 20 cm or less, changes caused by plant roots with a diameter of about 10 cm can be detected by the optical fiber sensor 2. Furthermore, if the diameter of the plant root is small, the distance from the optical fiber 6 can be shortened; for example, by setting the distance between the optical fiber 6 and the plant root to 5 cm or less, changes caused by plant roots with a diameter of about 1 cm can be detected by the optical fiber sensor 2. The plant roots may approach the optical fiber 6 from inside the optical fiber sensor 2, or may approach the optical fiber 6 from outside the optical fiber sensor 2.

[0049] The plant root position detection method of the present invention can non-destructively visualize the root system growth of root vegetables such as sweet potatoes, the root establishment of plants, and the recovery process of soil ecosystems during no-till cultivation in situations where rainfall or surface irrigation is present, and can be used for agricultural and horticultural purposes.

[0050] The method for detecting the position of plant roots in soil of the present invention utilizes the effect of rainfall on strain and stress measured by an optical fiber sensor 2, which normally results in measurement noise, to accurately detect the position of plant roots. With conventional optical fiber sensors, rainfall and / or irrigation have a large effect on measured values, making it difficult to grasp changes in signals caused by plant roots. However, by using the optical fiber sensor 2 of the present invention, measurement errors in the depth direction during rainfall and / or irrigation are reduced, making it possible to accurately grasp the position of plant roots. Below, the method for detecting plant roots using water flow and sediment flow generated during rainfall and / or irrigation will be described in detail using simulation results.

[0051] Let V be the flow velocity in the soil due to rainfall and / or irrigation. inf Let us consider the effect of creeping flow around a tuberous root with a radius of R. In the situation shown in Figure 7, the flow velocity V inf The horizontal stress applied by τ φφIf this is the case, the horizontal stress generated around the tuberous root due to rainfall and / or irrigation can be expressed as in Equation 1, and its spatial distribution is shown in the heat map in Figure 8. MATLAB (registered trademark) from MathWorks was used as the analysis software. The tuberous root is at the center of Figure 8, and the stress and strain at its top (soil surface side) are negative, while the stress and strain at its bottom (underground side) are positive. In this way, the flow velocity V inf Since stress and strain are reversed at high and low levels of the tuber, the location of the tuber can be accurately identified by finding the points where stress and strain are reversed during rainfall and / or irrigation.

[0052]

number

[0053] In the position detection method that utilizes the reversibility of stress and strain described above, it is important that the horizontal strain at the measurement position with a difference in elevation does not reverse for any reason other than creep flow. In contrast, the optical fiber sensor 2 of the present invention applies a truss structure consisting of a triangular framework to the frame 4, thereby reducing the influence of the vertical position on the horizontal strain and stress in the frame 4.

[0054] Furthermore, since the optical fiber sensor 2 and the position detection device 10 can also detect increasing changes in strain that accompany the growth of plant roots, the location of the root system can be determined more accurately by combining the detection results in the absence of rainfall and / or irrigation with the plant root position detection method of the present invention that utilizes the effects of rainfall and / or irrigation, as shown in Figure 9. Furthermore, since it is possible to detect the location of the root system regardless of the presence or absence of rainfall and / or irrigation, a detection method that is robust to environmental conditions can be established.

[0055] Representative embodiments of the present invention have been described above, but the present invention is not limited to these, and various design modifications are possible, all of which are included in the technical scope of the present invention. [Example]

[0056] Example 1 The condition of sweet potato tubers was detected using the optical fiber sensor shown in Figure 10. The optical fiber sensor was buried in the soil as shown in Figure 11, and the sweet potato was placed inside the optical fiber sensor.

[0057] The optical fiber used was a high-definition fiber optic strain sensor manufactured by LUNA (a polyimide-coated single-mode fiber with a bending radius of 15 mm and a diameter of 155 μm). As shown in Figure 10, the optical fiber was aligned horizontally on a 0.2 mm thick PTFE sheet manufactured by Tokyo Glass Instruments Co., Ltd., while being displaced vertically, and fixed with NITOFLON (registered trademark), a thin PTFE adhesive tape with a width of 5 mm and a thickness of 0.1 mm. The frame was supported by a round or square frame plate 3D-printed with polyacetal rods and polylactic acid filament so that it could stand upright in the air.

[0058] The interrogator unit measures changes in the optical properties of the optical fiber caused by strain generated inside the optical fiber sensor to obtain a signal, the signal processing unit reduces the signal noise to obtain the signal to be measured, and the determination unit separates the signal to be measured from the background signal, thereby detecting minute changes. The signal obtained by the interrogator unit is also recorded in the recording unit.

[0059] The recorded signal was averaged in the time axis and low-pass filtered in the spatial axis in the signal processing unit. Using an interrogator and measuring instrument (ODiSI6100) capable of recording strain at ~250 Hz, time averaging was performed every second, and a low-pass filter with a length equal to 50 mm of the total length of the optical fiber was applied in the spatial direction to reduce Gaussian noise.

[0060] The change in strain over time obtained from 2,500 hours of continuous measurement and the amount of rainfall during measurement are shown in Fig. 12. Fig. 12 shows that the strain is reset by rainfall, and that rainfall is undesirable for conventional measurement methods using optical fiber sensors.

[0061] The change in strain before and after rainfall, shown by the dotted line in Figure 12, is shown in Figure 13. Figure 13 shows the relationship between the position of the optical fiber and the measured strain, with (b) showing the state four hours after (a). Figure 13 shows that the measured strain changes due to rainfall.

[0062] The time difference between (a) and (b) in Figure 13 is shown in Figure 14. The areas where the strain is reversed (negative strain at the top and positive strain at the bottom) (e.g., areas marked with dotted circles) can be clearly seen, and it can be determined that sweet potato tubers are present in these areas.

[0063] Example 2 The state of the sweet potato tuber was detected in the same manner as in Example 1, except that the optical fiber sensor shown in FIG. 15 was used.

[0064] The optical fiber used in the optical fiber sensor is the same as in Example 1. The material of the frame is the same as in Example 1, and it is a 0.2 mm thick PTFE sheet manufactured by Tokyo Glass Instruments Co., Ltd. with many triangular through holes to form a truss structure.

[0065] By using a frame with a truss structure, the influence of differences in height on the optical fiber sensor was suppressed, and it was confirmed that it was easier to identify the position of the tuberous root than in Example 1. [Explanation of symbols]

[0066] 2. Optical fiber sensor, 4···frame, 6···Optical fiber, 8. Through hole, 10. Position detection device, 12. Optical fiber sensor unit, 14. Interrogator section, 16···Signal processing section, 18... Judgment department.

Claims

1. The optical fiber sensor is buried in the soil, Detecting plant roots at a distance of 20 cm or less from the optical fiber of the optical fiber sensor; Identifying the location of the plant roots based on changes in horizontal strain and / or stress around the plant roots caused by rainfall and / or irrigation; A method for detecting the location of plant roots in soil, comprising:

2. detecting a location where the strain and / or the stress is reversed in the vertical direction due to the rainfall and / or the irrigation as the location of the plant root; 2. The method for detecting the position of a plant root in soil according to claim 1 .

3. 3. An optical fiber sensor used in the method for detecting the position of a plant root in soil according to claim 1 or 2, Optical fibers are arranged on the surface of a frame made of a resin film or a metal thin film, The frame has a truss structure framework; An optical fiber sensor characterized by:

4. the truss structure is formed by regularly arranged triangular through holes in the resin film or the metal thin film; 4. The optical fiber sensor according to claim 3, wherein:

5. The frame has a rectangular or cylindrical shape.

4. The optical fiber sensor according to claim 3, wherein:

6. A position detection device used in the method for detecting the position of a plant root in soil according to claim 1 or 2, The optical fiber sensor unit, the interrogator unit, the signal processing unit, and the determination unit are provided. a change in optical characteristics of the optical fiber sensor unit caused by strain occurring in the soil is measured by the interrogator unit to acquire a signal; The signal processing unit reduces noise in the signal to obtain a signal to be measured; The determination unit separates the measurement target signal from a background signal; the optical fiber sensor unit has optical fibers arranged on a surface of a frame made of a resin film or a metal thin film, and the frame has a framework of a truss structure; A position detection device characterized by:

7. the truss structure is formed by regularly arranged triangular through holes in the resin film or the metal thin film; 7. The position detection device according to claim 6, wherein:

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