Device and method for measuring water content of soil body based on fiber bragg grating

Abstract

The invention discloses a soil moisture content measuring device and method based on a fiber bragg grating. The soil moisture content measuring device comprises a protective shell, a signal transmitting module, an induction coil module, a wavelength detection module and a signal processing module, the induction coil module and the wavelength detection module are packaged through the protective shell; high and low frequency signals are output through the signal emission module and are transmitted to the induction coil module and the signal processing module; an induction current is generated through the induction coil module; wavelength change is detected through a wavelength detection module; wavelength information and capacitance information are processed through a signal processing module, and the soil moisture content is output; according to the invention, the problems of low accuracy, difficulty in networking, weak anti-interference performance and the like in the prior art are solved, quasi-distributed measurement of the water content of the soil body can be realized, and the system and the method are not only suitable for indoor tests, but also suitable for on-site long-term monitoring.

Description

Technical Field

[0001] The present invention is used in the field of measurement and mainly relates to a soil moisture content measurement device and method based on fiber Bragg grating. Background Art

[0002] Soil moisture content has a significant impact on its mechanical properties and engineering performance. Excessive moisture content increases soil weight and reduces its shear strength, thereby increasing the risk of natural disasters such as landslides, mudslides, debris flows, and ground subsidence. Therefore, rationally controlling soil moisture content is crucial for mitigating the impact of natural disasters. In particular, in natural disaster prevention and risk assessment, the impact of soil moisture on disasters must be fully considered and appropriate preventive and control measures implemented accordingly. The key to solving this problem lies in effectively measuring changes in soil moisture content over the long term.

[0003] At present, there are many mature methods for measuring soil moisture content, such as drying method, tensiometer method, time domain reflectometry (TDR), neutron method, resistance method, ground penetrating radar method, remote sensing method and standing wave method. Among them, the drying method is relatively simple and convenient, but it destroys the soil structure and cannot measure the dynamic changes of soil moisture content; the tensiometer method has low stability and is easily affected by the physical and chemical properties of the soil, making it difficult to achieve continuous in-situ measurement; the time domain reflectometry (TDR) is susceptible to interference and has a complex structure and is expensive; the neutron method has errors in the calibration process and has radioactive hazards. The resistance method is easily affected by factors such as the salt texture and pore distribution of the soil, and the calibration process is complicated; the ground penetrating radar method is suitable for detecting moisture content in large areas, but the interpretation is difficult and is significantly affected by conductivity; the remote sensing method has little disturbance to the soil and is suitable for detecting shallow geological bodies, but its results are multi-solution and its accuracy is easily affected by environmental factors; when the standing wave method measures soil with high water content, the presence of water will affect the propagation of electromagnetic waves, resulting in large errors in the measurement results of the probe impedance, thereby affecting the measurement accuracy of the standing wave method.

[0004] In recent years, in response to these problems, some new methods have emerged in the field of soil moisture measurement. For example, Chinese patent document CN202111265407.3 discloses a high-frequency electric field charge induction soil moisture sensor, but this technical solution can only perform single-point measurement and cannot achieve quasi-distributed measurement. It cannot effectively shield the influence of external electric and magnetic fields on the measurement results, and the measurement accuracy is poor. On the contrary, with the help of fiber optic sensing technology, which has outstanding advantages such as small size, light weight, corrosion resistance, intrinsic safety, long transmission distance, and easy large-scale networking, Liu Xifeng et al. (2022) and Wang Jiamin et al. (2022) proposed an active heating fiber Bragg grating (AH-FBG) method to measure soil moisture content. However, the biggest disadvantage of this method is that heating will destroy the soil structure, especially for special soils such as loess, red soil and expansive soil, and the soil structure determines the water holding state of the soil. , so it is impossible to measure the soil quickly and effectively; in addition, the heating process requires an external power supply, which consumes a lot of power and cannot be completed in a short time, making it difficult to achieve real-time monitoring; Chinese patent document CN202111202903.4 discloses a soil moisture monitoring device and method based on ultra-weak fiber Bragg grating sensing technology. During measurement, this technical solution is that the external soil water vapor contacts the soil inside the device until water vapor equilibrium is reached, and the soil moisture content is measured by the change in the grating wavelength caused by the change in the internal soil mass. However, on the one hand, since the device is a non-closed system, the water vapor equilibrium time is relatively long; on the other hand, since the ultra-weak fiber itself is relatively fragile and has a small elongation deformation, the range of measuring the increase in soil moisture content is limited, and once it is elongated, it will not produce retraction deformation, that is, it will not be able to measure when the water content decreases, which makes it impossible to continuously detect changes in soil moisture content for a long time. In summary, the existing soil moisture measurement methods are still unable to effectively achieve the purpose of quickly, accurately, and comprehensively measuring and long-term monitoring of soil moisture content. Summary of the Invention

[0005] In order to solve the above problems, the present invention discloses a soil moisture content measuring device based on fiber Bragg grating.

[0006] In order to achieve the above objectives, the present invention is implemented through the following technical solutions:

[0007] In order to achieve the above technical effects, the technical solution of the present invention is:

[0008] A soil moisture content measuring device based on fiber Bragg grating includes a calculation unit, which is communicatively connected to a pore water dielectric constant measuring unit and a soil relative dielectric constant measuring unit; the pore water dielectric constant measuring unit includes a conductive elastic element 4-4, and the two ends of the elastic element 4-4 are respectively electrically connected to the two ends of an induction coil 4-3 to form a closed loop; an induction probe group 1-6 is provided in conjunction with the induction coil 4-3, and the induction probe group 1-6 includes a transmitting probe group and a receiving probe group; the transmitting probe group includes two transmitting probes, and the receiving probe group includes two receiving probes, and the transmitting probes and the receiving probes are correspondingly arranged; the transmitting probes are respectively electrically connected to the high-frequency signal source of the signal source generator 1-2, and the two sides of the induction coil 4-3 are respectively provided with The induction electrode plate 4-2 is electrically connected to the receiving probes respectively; the surface of the elastic element 4-4 is pasted with a strain grating 4-5, the strain grating 4-5 is connected to the temperature compensation grating 4-6, the strain grating 4-5 and the temperature compensation grating 4-6 are communicatively connected to the wavelength demodulator, and the wavelength demodulator is communicatively connected to the calculation unit; the soil relative dielectric constant measurement unit includes a capacitance probe 1-7, the capacitance probe 1-7 is a low-frequency signal source with two ends electrically connected to the signal source generator 1-2 at one end, and is communicatively connected to the calculation unit through a capacitance signal receiver; the pore water dielectric constant measurement unit is used to detect the dielectric constant of pore water in the soil to be measured; the soil relative dielectric constant measurement unit is used to detect the soil relative dielectric constant of the soil to be measured.

[0009] Further improvement, the relative dielectric constant ε of the soil to be tested r The calculation method is as follows:

[0010]

[0011] Where k represents the electrostatic force constant, which is 9.0×10 9 N·m 2 / C 2 ; C and d2 represent the capacitance and distance between the capacitance probes 1-7 respectively; l and b represent the length and width of the capacitance probes 1-7 respectively.

[0012] As a further improvement, the dielectric constant of the pore water in the soil to be tested is obtained as follows:

[0013] The initial temperature of the soil and the corresponding wavelength of the temperature-compensated fiber Bragg grating are recorded; then the high-frequency signal source transmits the high-frequency signal through the transmitting probe group, and the receiving probe group receives the high-frequency signal and forms an alternating magnetic field between the induction electrode plate 4-2, thereby causing the induction coil 4-3 to generate an induced current and generate heat, causing the wavelength of the temperature-compensated grating 4-6 to change;

[0014] According to the relationship between the strain change Δε of the elastic element 4-4 and the wavelength change of the optical temperature compensation fiber Bragg grating Δλ=K ε Δε+KT ΔT calculates the temperature change ΔT of the soil to be tested, and combines it with the initial temperature to get the current temperature of the soil. Query the temperature and dielectric constant comparison table of water to get the dielectric constant ε of the pore water. w , where Δλ represents the wavelength variation of the temperature-compensated fiber Bragg grating; K T Indicates the temperature sensitivity coefficient of the temperature compensation grating; K ε Represents the strain sensitivity coefficient of the strain grating; the strain change Δε of the elastic element 4-4 is obtained by the strain grating 4-5.

[0015] The principle is as follows: the signal source generator transmits high-frequency signals and low-frequency signals to the induction probe and capacitance probe respectively. The high-frequency signal is transmitted to the induction probe and converted into an alternating electric field signal by the induction probe. The alternating electric field signal is transmitted to the induction coil module and generates an induced current in the induction coil module. The induced current causes the elastic element to heat up and deform, thereby causing the grating pitch of the fiber Bragg grating (FBG) attached to the elastic element to change; the low-frequency signal is converted into a capacitance signal by the capacitance probe and transmitted to the capacitance signal receiver. Finally, the wavelength signal data and the capacitance signal data are transmitted to the computer and merged and processed into soil moisture content data.

[0016] As a further improvement, the water content of the soil to be tested is calculated as follows:

[0017]

[0018] Where K1, K2 and K3 are expressed as follows:

[0019]

[0020] Among them, θ represents the water content of the soil to be tested; soil saturation S r ; G s Indicates the specific gravity of soil particles, generally ranging from 2.6 to 2.75. s is the dielectric constant of soil particles, ε w is the dielectric constant of pore water; ω represents the angular frequency; E e is the elastic modulus of the elastic element; A e is the cross-sectional area of ​​the elastic element; l e is the length of the elastic element; μ0 represents the magnetic permeability; r is the area of ​​the induction coil; R e is the resistance of the elastic element; M represents the signal amplitude; ε0 represents the dielectric constant in vacuum, which is 8.854187817×10-12F / m; dt represents the power-on time increment; t1 and t2 are the start and end times of the wavelength signal stability segment, respectively.

[0021] As a further improvement, the induction coil 4-3, elastic element 4-4, strain grating 4-5, temperature compensation grating 4-6 and induction electrode plate 4-2 are all located in the protective inner shell 4-1; the induction probe group 1-6 and the capacitance probe 1-7 are both fixed outside the protective inner shell 4-1; and the calculation unit is a computer 1-14.

[0022] Further improvements, the sensing probe group 1-6 includes a first probe 2-1, a second probe 2-2; a third probe 2-3 and a fourth probe 2-4; the capacitance probe 1-7 includes a fifth probe 2-5 and a sixth probe 2-6; the signal source generator 1-2 includes a high-frequency signal source interface 1-9 and a low-frequency signal source interface 1-10; wherein the high-frequency signal source interface 1-9 is electrically connected to the first probe 2-1 and the fourth probe 2-4 through a transmission cable; the low-frequency signal source interface 1-10 is electrically connected to the second probe 2-2 and the third probe 2-3 through a transmission cable; the first probe 2-1 and the fourth probe 2-4 are a group of transmitting probes; the second probe 2-2 and the third probe 2-3 are a group of receiving probes.

[0023] A soil moisture content measurement method based on fiber Bragg grating, the measurement method using the above-mentioned soil moisture content measurement device based on fiber Bragg grating, specifically comprising the following steps:

[0024] Step S1, device connection: assemble the soil moisture content measurement device based on fiber Bragg grating;

[0025] Step S2: Drill a hole at a preset depth in the soil to be tested, take out the in-situ soil and test its density ρ and specific gravity G. s and the dielectric constant ε of soil particles s , inserting the induction probe group 1-6 and the capacitance probe 1-7 of the soil moisture content measuring device based on the fiber Bragg grating into the borehole and fixing them, and backfilling the soil with the same density;

[0026] Step S3, turning on the high-frequency signal source and the low-frequency signal source for a period of time respectively, and debugging the soil moisture content measuring device based on the fiber Bragg grating; after the debugging is completed, turning on the high-frequency signal source and the low-frequency signal source at the same time, and making the soil moisture content measuring device based on the fiber Bragg grating operate for a period of time until the calculation unit automatically intercepts the wavelength signal data and the capacitance signal data within the wavelength change stable section [t1, t2];

[0027] Step S4: The capacitance signal data is converted into Converted into the relative dielectric constant ε of the soil to be tested r , and combined with wavelength signal data, soil particle dielectric constant ε s and the pore water dielectric constant ε wData, using the relationship between wavelength change Δλ, soil temperature ΔT and water content θ Calculate the moisture content of the soil to be tested.

[0028] As a further improvement, the step S1 is specifically as follows:

[0029] Step S1.1, using transmission cables to connect the high-frequency signal source and the low-frequency signal source of the signal generator to the high-frequency signal source interface and the low-frequency signal source interface of the protective housing, respectively;

[0030] Step S1.2, connecting the wavelength demodulator to the fiber grating interface of the protective housing using an optical fiber lead;

[0031] Step S1.3, connecting the capacitance signal receiver to the capacitance probe using a transmission cable, and connecting the wavelength signal demodulator and the capacitance signal receiver to a computer;

[0032] Step S1.4: If multiple soil moisture content measuring devices based on fiber Bragg gratings are connected in series or parallel for multi-point measurement, repeat steps S1.1 to S1.3 to connect each device in sequence.

[0033] As a further improvement, the step S2 is specifically as follows:

[0034] Step S2.1: Mark corresponding measuring points or marks at the pre-buried measuring point locations, drill holes to a predetermined depth using a drilling rig, and extract the in-situ soil at the corresponding depths;

[0035] Step S2.2: Divide the in-situ soil into three parts, take representative soil samples using the quartering method, and test the density ρ and specific gravity G of each in-situ soil according to the "Standard for Geotechnical Test Methods". s and the dielectric constant ε of soil particles s , and take the average value of three in-situ soil masses as soil layer parameters, and convert the in-situ soil density according to the relationship between the three-phase indicators of soil mechanics;

[0036] Step S2.3: Insert the fiber Bragg grating-based soil moisture content measurement device into the borehole to a predetermined depth, lead the transmission wire out of the borehole, backfill the borehole with in-situ soil of the same density, and ensure that the fiber Bragg grating-based soil moisture content measurement device is in close contact with the surrounding soil to establish an effective hydraulic connection;

[0037] Step S2.4: If the soil moisture content of a single section or multiple sections is to be measured, repeat steps S2.1 to S2.3, and number the soil moisture content measuring devices at different positions in sequence.

[0038] As a further improvement, the step S3 is specifically as follows:

[0039] Step S3.1: Turn on the low-frequency signal source and debug the signal processing module until it can effectively record the capacitance signal, ensuring that the module is properly connected.

[0040] Step S3.2, turn off the low-frequency signal source switch, turn on the high-frequency signal source switch, and debug the signal source generator (1-2) and the wavelength demodulator until the wavelength signal can be effectively recorded;

[0041] In step S3.3, the high-frequency and low-frequency signal source switches are turned on simultaneously, and the soil moisture content measurement device based on the fiber Bragg grating is operated for a period of time until the computer can automatically intercept the wavelength signal data and capacitance signal data within the wavelength change stable section [t1, t2], record and save the data.

[0042] Beneficial effects of the present invention:

[0043] 1. The present invention adopts fiber Bragg grating sensing technology, which has the advantages of anti-electromagnetic interference, corrosion resistance and fast response speed. It is suitable for areas affected by magnetic fields and harsh environments, and can perform long-term measurements.

[0044] 2. The device of the present invention is provided with a temperature compensation optical fiber, which can avoid the influence of ambient temperature changes on the measurement results.

[0045] 3. The device of the present invention is small in size, has simple operation steps, and can measure the moisture content of in-situ soil in real time.

[0046] 4. The device of the present invention can be calibrated and packaged indoors and directly installed on site, which is simple to install and lay out;

[0047] 5. The device of the present invention can measure the moisture content of the surface and deep layers of the soil, and can also perform distributed measurement of the soil moisture content, and the measurement range of the soil moisture content is large. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a schematic diagram of the overall structure of a soil moisture content measuring device based on fiber Bragg grating (FBG) according to the present invention;

[0049] Figure 2 Schematic diagram of the protective housing of the soil moisture content measuring device based on fiber Bragg grating (FBG) of the present invention;

[0050] Figure 3 Schematic diagram of the internal structure of the soil moisture content measuring device based on fiber Bragg grating (FBG) of the present invention;

[0051] Figure 4 Schematic diagram of the local structure of the induction coil module of the soil moisture content measurement device based on fiber Bragg grating (FBG) of the present invention.

[0052] In the figure: 1-1, protective housing; 1-2, signal source generator; 1-3, high-frequency signal source; 1-4, low-frequency signal source; 1-5, transmission cable; 1-6, induction probe; 1-7, capacitance probe; 1-8, fiber Bragg grating interface; 1-9, high-frequency signal source interface; 1-10, low-frequency signal source interface; 1-11, optical fiber line; 1-12, wavelength signal demodulator; 1-13, capacitance signal receiver; 1-14, computer; 2-1, first probe; 2-2, second probe; 2-3, third probe; 2-4, fourth probe; 2-5, fifth probe; 2-6, sixth probe; 2-7, electric field signal partition; 3-1, wire trough; 3-2, fixing bolt; 3-3, optical line; 3-4, energized wire; 4-1, protective inner shell; 4-2, induction electrode plate; 4-3, induction coil; 4-4, elastic element; 4-5 strain grating; 4-6, temperature compensation grating.

[0053] Specific implementation methods

[0054] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0055] Reference Figures 1 to 4As shown, a soil moisture content measuring device based on fiber Bragg grating (FBG) is characterized in that it includes a protective shell, a signal transmitting module, an induction coil module, a wavelength detection module and a signal processing module. The protective shell encapsulates the induction coil module and the wavelength detection module; the signal transmitting module is composed of a signal source generator, a transmission cable, an induction probe and a capacitance probe; the signal source generator is composed of a high-frequency signal source and a low-frequency signal source; the induction coil module is composed of a protective inner shell, an induction electrode plate, an induction coil, a energized wire and an elastic element; the wavelength detection module is composed of an optical fiber line, a temperature compensation grating and a strain grating; the signal processing module is composed of a computer, a wavelength signal demodulator and a capacitance signal receiver; the induction probe and the capacitance probe are in direct contact with the soil to be measured, and one end is fixedly connected to the protective shell; the induction probe is composed of a first probe, a second probe, a third probe and a fourth probe, and is mainly used to generate an alternating electric field; the first probe and the fourth probe are connected to the high-frequency signal source through a transmission cable The second probe and the third probe are connected to the induction electrode plate through a powered wire; the capacitance probe is composed of a fifth probe and a sixth probe, which are mainly used to measure the capacitance of the soil medium; the fifth probe and the sixth probe are connected to the low-frequency signal source at one end of the transmission cable, and are connected to the capacitance signal receiver at the other end; an induction electrode plate and an induction coil are provided in the protective inner shell; an induction coil is placed in the induction electrode plate, and the induction coil is connected to the elastic element through a powered wire to form a conductive loop to generate an induced current; an optical fiber line is pasted on the elastic element, and a strain grating and a temperature compensation grating are provided on the optical fiber line, and the optical fiber line is connected to a wavelength signal demodulator through an optical fiber lead; the wavelength signal demodulator and the capacitance signal receiver are connected to a computer through a transmission cable, and the computer analyzes and processes the received wavelength data and capacitance data.

[0056] Reference Figure 2 、 Figure 3 As shown, the protective shell is made of magnetic shielding material to block the influence of external magnetic fields. Fiber optic Bragg grating interfaces for optical fiber lines are provided on the upper and lower sides of the protective shell. An induction probe and a capacitance probe are fixedly connected on the right side. A signal source interface for a signal source generator is provided on the left side. Specific wire grooves are provided inside to arrange transmission cables and powered wires, and are fixed with fixing bolts. The surface of the protective shell is coated with electromagnetic wave reflecting material.

[0057] Reference Figure 2 、 Figure 3As shown, the first probe, the second probe, the third probe, the fourth probe, the fifth probe and the sixth probe with a protective cap are arranged in sequence on the right side of the protective shell, wherein the first probe, the second probe, the third probe and the fourth probe are all arranged in the same column, and the second probe and the third probe are located between the first probe and the fourth probe, the fifth probe and the sixth probe are staggered in one column with the first probe, the second probe, the third probe and the fourth probe, and are separated by an electric field signal baffle to avoid mutual interference between high and low frequency signals.

[0058] Reference Figure 3 、 Figure 4 As shown, a certain distance is set between the strain grating and the temperature compensation grating, and the strain grating segment optical fiber line is fixed on the elastic element to measure the strain of the elastic element; the temperature compensation grating segment optical fiber line is in a free state to measure the soil temperature; the elastic element is fixed on the protective shell.

[0059] Reference Figure 2 、 Figure 3 As shown, the wavelength signal demodulator is connected to the fiber grating interface on the protective shell through an optical fiber lead; the high-frequency signal source is connected to the high-frequency signal interface on the protective shell through a transmission cable, and the low-frequency signal source is connected to the low-frequency signal interface on the protective shell through a transmission cable.

[0060] Reference Figure 3 、 Figure 4 As shown, the protective inner shell is made of electrostatic shielding material to block the influence of external electric fields. The protective inner shell is fixed in the protective outer shell, and the protective inner shell is made of metal material.

[0061] The second invention of the present disclosure is to provide a soil moisture content measurement method using a fiber Bragg grating (FBG)-based soil moisture measurement device as described above, comprising the following steps:

[0062] Step S1, device connection: connect the signal generator to the signal source interface corresponding to the protective shell, connect the wavelength demodulator to the fiber Bragg grating interface of the protective shell, connect the capacitance signal receiver to the capacitance probe, and connect the wavelength signal demodulator and capacitance signal receiver to the computer.

[0063] Step S2, device installation and parameter acquisition: After the connection is completed, drill a hole at a certain depth of the soil to be tested, take out the in-situ soil and test its density ρ, specific gravity G s and the dielectric constant ε of soil particles s , insert the soil moisture content measuring device into the hole and fix it, and backfill the hole with soil of the same density.

[0064] Step S3, device debugging and measurement: Turn on the high-frequency signal source and the low-frequency signal source switches for a period of time respectively, and debug each module of the soil moisture content measurement device until it is buried intact; turn on the high-frequency signal source and the low-frequency signal source switches at the same time, and let the device work for a period of time until the computer automatically intercepts the wavelength signal data and capacitance signal data within the wavelength change stable section [t1, t2].

[0065] Step S4, data processing: the capacitance signal data is processed by the relation Converted into the relative dielectric constant ε of the soil to be tested r , and combined with wavelength signal data, soil particle dielectric constant ε s and the pore water dielectric constant ε w The relationship between wavelength change Δλ, soil temperature ΔT and water content θ is used. Calculate the moisture content of the soil to be tested.

[0066] The initial temperature T needs to be given during the temperature compensation fiber Bragg grating calculation process of this device 0， The initial temperature of the device is set with reference to the reference temperature.

[0067] The above step S1 is specifically as follows:

[0068] Step S1.1, using transmission cables to connect the high-frequency signal source and the low-frequency signal source of the signal generator to the high-frequency signal source interface and the low-frequency signal source interface of the protective housing, respectively;

[0069] Step S1.2, connecting the wavelength demodulator to the fiber grating interface of the protective housing using an optical fiber lead;

[0070] Step S1.3, connecting the capacitance signal receiver to the capacitance probe using a transmission cable, and connecting the wavelength signal demodulator and the capacitance signal receiver to a computer;

[0071] Step S1.4: If multiple soil moisture content measurement devices based on fiber Bragg gratings are connected in series or parallel for multi-point measurement, repeat steps S1.1 to S1.3 to connect each device in sequence;

[0072] The above step S2 is specifically as follows:

[0073] Step S2.1: Mark corresponding measuring points or marks at the pre-buried measuring point locations, drill holes to a predetermined depth using a drilling rig, and extract the in-situ soil at the corresponding depths;

[0074] Step S2.2: Divide the in-situ soil into three parts, take representative soil samples using the quartering method, and test the density ρ and specific gravity G of each in-situ soil according to the "Standard for Geotechnical Test Methods". s and the dielectric constant ε of soil particles s, and take the average value of three in-situ soil masses as soil layer parameters, and convert the in-situ soil density according to the relationship between the three-phase indicators of soil mechanics;

[0075] Step S2.3: Insert the fiber Bragg grating-based soil moisture content measurement device into the borehole to a predetermined depth, lead the transmission wire out of the borehole, backfill the borehole with in-situ soil of the same density, and ensure that the fiber Bragg grating-based soil moisture content measurement device is in close contact with the surrounding soil to establish an effective hydraulic connection;

[0076] Step S2.4: If the soil moisture content of a single section or multiple sections is to be measured, repeat steps S2.1 to S2.3, and number the soil moisture content measuring devices at different positions in sequence.

[0077] The above step S3 is specifically as follows:

[0078] Step S3.1: Turn on the low-frequency signal source and debug the signal processing module until it can effectively record the capacitance signal, ensuring that the module is properly connected.

[0079] Step S3.2, turn off the low-frequency signal source switch, turn on the high-frequency signal source switch, and debug the signal source generator (1-2) and the wavelength demodulator until the wavelength signal can be effectively recorded;

[0080] In step S3.3, the high-frequency and low-frequency signal source switches are turned on simultaneously, and the soil moisture content measurement device based on the fiber Bragg grating is operated for a period of time until the computer can automatically intercept the wavelength signal data and capacitance signal data within the wavelength change stable section [t1, t2], record and save the data.

[0081] The above step S4 is specifically as follows:

[0082] Step S4.1: Combine the capacitance signal data and use the following relationship to calculate the relative dielectric constant ε of the soil to be tested: r :

[0083]

[0084] Where k represents the electrostatic force constant, which is generally taken as 9.0×10 9 N·m 2 / C 2 ; C, d2 represent the capacitance and distance between the fifth probe and the sixth probe, respectively; l, b represent the length and width of the probe, respectively.

[0085] Step S4.2: Combine the wavelength signal data and calculate the wavelength variation of the fiber Bragg grating (FBG) using the equation Δλ = K. ε Δε+K TΔT calculates the temperature ΔT of the soil to be tested and the strain of the elastic element Δε, and calculates the dielectric constant ε of the pore water by looking up the table in combination with the soil temperature w .

[0086] Where Δλ represents the wavelength change of the grating; Δε represents the strain of the elastic element; ΔT represents the soil temperature; K T Represents the grating temperature sensitivity coefficient; K ε represents the grating strain sensitivity coefficient; ε w Represents the dielectric constant of pore water.

[0087] Step S4.3, combining the wavelength change Δλ, soil temperature ΔT, and dielectric constant ε of soil particles s and the dielectric constant ε of pore water w , soil saturation S r The water content θ of the soil to be tested is calculated using the following relationship:

[0088]

[0089] Where K1, K2 and K3 are expressed as follows:

[0090]

[0091] Where θ represents the soil water content; soil saturation S r ; G s Indicates the specific gravity of soil particles, which is generally 2.6 to 2.75; ω indicates the angular frequency; E e is the elastic modulus of the elastic element; A e is the cross-sectional area of ​​the elastic element; l e is the length of the elastic element; μ0 represents the magnetic permeability; r is the area of ​​the induction coil; R e is the resistance of the elastic element; M represents the signal amplitude; ε0 represents the dielectric constant in vacuum, which is 8.854187817×10-12F / m.

[0092] The calculation and derivation process of the relationship between the soil moisture content and the wavelength signal data is as follows:

[0093] Let the length and width of the probe be l and b respectively, and the distance between the second and third probes be d1. For the soil to be measured between the first and second probes, according to the three-phase composition and proportion of the soil, it can be regarded as pore gas, pore water and soil particles, where the widths of pore gas, pore water and soil particles are b respectively. a 、b w and b s , and b=b a +b w +b s , then the pore gas Va 、Pore water V w and soil particles V s The volumes can be expressed as:

[0094] V a =ld1b a (2)

[0095] V w =ld1b w (3)

[0096] V s =ld1b s (4)

[0097] Where V a 、V w and V s They represent the volumes of soil pore gas, pore water and soil particles respectively.

[0098] From equations (2) to (4), we know that the soil porosity ratio e can be expressed as:

[0099]

[0100] Similarly, according to the conversion relationship between the three-phase indicators of soil, the soil saturation S r It can be expressed as:

[0101]

[0102] According to (5) to (6), b w and b s The relationship between them is:

[0103]

[0104] According to the propagation speed of electromagnetic waves in soil Two-way travel time The equivalent dielectric constant ε of the soil to be tested can be calculated based on the relationship between r for:

[0105]

[0106] Where c0 represents the electromagnetic wave velocity; ε a Indicates the dielectric constant of pore gas, generally taken as 1.00053F / m; ε w Represents the dielectric constant of pore water, which can be obtained by looking up Table 1 according to the ambient temperature; ε s It represents the dielectric constant of soil particles, generally taken as 5F / m.

[0107] Table 1 Dielectric constants of water at different temperatures

[0108] Temperature (℃) Dielectric constant (F / m) 0 87.9 10 82.04 20 76.58 30 74.85 41 69.88 50 65.25 62 58.2 71 55.58

[0109] The combined equations (2) to (8) can be expressed as follows:

[0110]

[0111] A high-frequency electric field penetrating the soil to be measured is generated between the first and fourth probes to which a high-frequency signal is applied. Due to the electrostatic induction phenomenon, induced charges are generated on the second and third probes located between the first and fourth probes. The charge q(t) is expressed as:

[0112] q(t)=ε r ε0AMsinωt (11)

[0113] Where M represents the signal amplitude; ε0 represents the dielectric constant in vacuum, which is generally 8.854187817×10-12F / m; ω represents the angular frequency; t represents the power-on time; and A represents the sensing area of ​​all probes.

[0114] From formula (11), we know that the induced current I generated by the soil is expressed as:

[0115]

[0116] Theoretically, the induced current generated in the soil is equal to the sum of the induced currents generated by the pore gas, pore water and soil particles as electrolytes. However, since the induced current generated by the pore gas as the dielectric is much smaller than that of the pore water and soil particles, the total induced current I generated by the soil to be tested is t =I w +I s , then from formula (12), the induced currents generated by pore water and soil particles as dielectrics are expressed as:

[0117] I w =ε w ε0A w Mωcosωt (13)

[0118] I s =ε s ε0A s Mωcosωt (14)

[0119] Where I w and I s Respectively represent the induced current generated by pore water and soil particles as dielectrics; A w and A s They represent the area occupied by pore water and soil particles on the probe, respectively, and are calculated using the following formula:

[0120] Aw =lb w (15)

[0121] A s =lb s (16)

[0122] According to the full current law L B q ·dl=μ0∫∫ s J D ·dS calculates magnetic induction intensity B q , which can be expressed as:

[0123]

[0124] Where B q represents the magnetic induction intensity; μ0 represents the magnetic permeability; r is the area of ​​the induction coil.

[0125] The induced electromotive force U generated by the induction coil under the influence of the change in magnetic flux is expressed as:

[0126]

[0127] Where φ is the magnetic flux in the induction coil.

[0128] The elastic element generates heat when current flows through it. The heat Q generated is calculated according to Joule's law:

[0129]

[0130] Where R e is the resistance of the elastic element; t1 and t2 are the start and end times of the wavelength signal stability segment respectively.

[0131] The strain energy accumulated by the elastic deformation caused by heat generation in the elastic element can be expressed as:

[0132]

[0133] Where E e is the elastic modulus of the elastic element; Δε is the strain change of the elastic element; A e is the cross-sectional area of ​​the elastic element; l e is the length of the elastic element.

[0134] Since the central wavelength of FBG changes linearly with temperature and strain, the change of FBG wavelength can be expressed as:

[0135] Δλ=K ε Δε+K T ΔT (21)

[0136] Where K TIndicates the temperature sensitivity of the grating; K ε represents the grating strain sensitivity; ΔT represents the temperature change.

[0137] According to the law of conservation of energy, i.e., Q = W, and by combining equations (19) to (21), the relationship between the water content θ of the soil to be measured and the wavelength change Δλ is expressed as:

[0138]

[0139] Where K1, K2 and K3 are expressed as follows:

[0140]

[0141]

[0142] The above embodiment is only a specific implementation of the present invention and is not intended to limit the present invention. Any simple improvements and replacements made thereto are within the scope of protection of the present invention.

Claims

1. A soil moisture content measuring device based on fiber Bragg grating, characterized in that: The invention comprises a calculation unit, wherein the calculation unit is communicatively connected to a pore water dielectric constant measurement unit and a soil relative dielectric constant measurement unit; the pore water dielectric constant measurement unit comprises a conductive elastic element (4-4), and two ends of the elastic element (4-4) are respectively electrically connected to two ends of an induction coil (4-3) to form a closed loop; an induction probe group (1-6) is provided in conjunction with the induction coil (4-3), and the induction probe group (1-6) comprises a transmitting probe group and a receiving probe group; the transmitting probe group comprises two transmitting probes, and the receiving probe group comprises two receiving probes, and the transmitting probes and the receiving probes are arranged correspondingly; the transmitting probes are respectively electrically connected to a high-frequency signal source of a signal source generator (1-2), and induction electrode plates (4-2) are respectively provided on both sides of the induction coil (4-3), and the induction electrode plates (4-2) are respectively provided on both sides of the induction coil (4-3). The electrode plate (4-2) is electrically connected to the receiving probes respectively; a strain grating (4-5) is pasted on the surface of the elastic element (4-4); the strain grating (4-5) is connected to the temperature compensation grating (4-6); the strain grating (4-5) and the temperature compensation grating (4-6) are communicatively connected to a wavelength demodulator; the wavelength demodulator is communicatively connected to a calculation unit; the soil relative dielectric constant measurement unit comprises a capacitance probe (1-7); the capacitance probe (1-7) is two low-frequency signal sources, one end of each of which is electrically connected to a signal source generator (1-2) at the other end, and is communicatively connected to the calculation unit via a capacitance signal receiver; the pore water dielectric constant measurement unit is used to detect the dielectric constant of pore water in the soil to be measured; the soil relative dielectric constant measurement unit is used to detect the soil relative dielectric constant of the soil to be measured.

2. The soil moisture content measuring device based on fiber Bragg grating according to claim 1, characterized in that: The relative dielectric constant ε of the soil to be tested r The calculation method is as follows: Where k is the electrostatic force constant, which is 9.0×10 9 N·m 2 / C 2 ; C, d2 represent the capacitance and distance between the capacitance probes (1-7), respectively; l, b represent the length and width of the capacitance probes (1-7), respectively.

3. The soil moisture content measuring device based on fiber Bragg grating according to claim 1, characterized in that: The method for obtaining the dielectric constant of the pore water in the soil to be tested is as follows: The initial temperature of the soil and the wavelength of the corresponding temperature-compensating fiber grating are recorded; then the high-frequency signal source transmits the high-frequency signal through the transmitting probe group, and the receiving probe group receives the high-frequency signal and forms an alternating magnetic field between the induction electrode plates (4-2), so that the induction coil (4-3) generates an induced current and generates heat, causing the wavelength of the temperature-compensating grating (4-6) to change; According to the relationship between the strain variation Δε of the elastic element (4-4) and the wavelength variation of the optical temperature compensation fiber grating Δλ=K ε Δε+K T ΔT calculates the temperature change ΔT of the soil to be tested, and combines the initial temperature of the soil to obtain the current temperature of the soil. The dielectric constant ε of the pore water is obtained by querying the temperature and water dielectric constant comparison table. w , where Δλ represents the wavelength change of the temperature-compensated fiber Bragg grating; K T Indicates the temperature sensitivity coefficient of the temperature compensation grating; K ε Represents the strain sensitivity coefficient of the strain grating; the strain change Δε of the elastic element (4-4) is obtained by the strain grating (4-5).

4. The soil moisture content measuring device based on fiber Bragg grating according to claim 1, characterized in that: The calculation method of the water content of the soil to be tested is as follows: Where K1, K2 and K3 are expressed as follows: Among them, θ represents the water content of the soil to be tested; soil saturation S r ; G s Indicates the specific gravity of soil particles, generally the size is 2.6~2.75; ε s is the dielectric constant of soil particles, ε w is the dielectric constant of pore water; ω represents the angular frequency; E e is the elastic modulus of the elastic element; A e is the cross-sectional area of ​​the elastic element; l e is the length of the elastic element; μ0 represents the magnetic permeability; r is the area of ​​the induction coil; R e is the resistance of the elastic element; M represents the signal amplitude; ε0 represents the dielectric constant in vacuum, which is 8.854187817×10-12F / m; dt represents the power-on time increment; t1 and t2 are the start and end times of the wavelength signal stability segment, respectively.

5. The soil moisture content measuring device based on fiber Bragg grating according to claim 1, characterized in that: The induction coil (4-3), elastic element (4-4), strain grating (4-5), temperature compensation grating (4-6) and induction electrode plate (4-2) are all located in the protective inner shell (4-1); the induction probe group (1-6) and capacitance probe (1-7) are all fixed outside the protective inner shell (4-1); and the calculation unit is a computer (1-14).

6. The soil moisture content measuring device based on fiber Bragg grating according to claim 1, characterized in that: The sensing probe group (1-6) includes a first probe (2-1), a second probe (2-2); a third probe (2-3) and a fourth probe (2-4); the capacitance probe (1-7) includes a fifth probe (2-5) and a sixth probe (2-6); the signal source generator (1-2) includes a high-frequency signal source interface (1-9) and a low-frequency signal source interface (1-10); The high-frequency signal source interface (1-9) is electrically connected to the first probe (2-1) and the fourth probe (2-4) via a transmission cable; the low-frequency signal source interface (1-10) is electrically connected to the second probe (2-2) and the third probe (2-3) via a transmission cable; the first probe (2-1) and the fourth probe (2-4) form a group of transmitting probes; and the second probe (2-2) and the third probe (2-3) form a group of receiving probes.

7. A soil moisture content measurement method based on fiber Bragg grating, characterized in that: The measuring method adopts the soil moisture content measuring device based on fiber Bragg grating according to any one of claims 1 to 6, and specifically comprises the following steps: Step S1, device connection: assembling a soil moisture content measurement device based on a fiber Bragg grating; Step S2: Drill a hole at a preset depth of the soil to be tested, take out the in-situ soil and test the density ρ and specific gravity G of the in-situ soil. s and the dielectric constant of soil particles ε s , inserting a sensing probe group (1-6) and a capacitance probe (1-7) of a soil moisture content measuring device based on a fiber Bragg grating into a borehole and fixing them, and backfilling the borehole with soil of the same density; Step S3, turning on the high-frequency signal source and the low-frequency signal source for a period of time, respectively, to debug the soil moisture content measuring device based on the fiber Bragg grating; After debugging is completed, turn on the high-frequency signal source and the low-frequency signal source switches at the same time, and make the soil moisture content measuring device based on the fiber Bragg grating work for a period of time until the calculation unit automatically intercepts the wavelength signal data and the capacitance signal data in the wavelength change stable section [t1, t2] time; Step S4: The capacitance signal data is converted into Converted into the relative dielectric constant ε of the soil to be tested r , and combined with the wavelength signal data, the soil particle dielectric constant ε s and the pore water dielectric constant ε w Data, using the relationship between wavelength change Δλ, soil temperature ΔT and water content θ Calculate the moisture content of the soil to be tested.

8. The soil moisture content measurement method based on fiber Bragg grating according to claim 7, characterized in that: The step S1 is specifically as follows: Step S1.1, using a transmission cable to connect the high-frequency signal source and the low-frequency signal source of the signal generator to the high-frequency signal source interface and the low-frequency signal source interface of the protective housing respectively; Step S1.2, connecting the wavelength demodulator to the fiber grating interface of the protective housing by using an optical fiber lead; Step S1.3, connecting the capacitance signal receiver to the capacitance probe using a transmission cable, and connecting the wavelength signal demodulator and the capacitance signal receiver to a computer; Step S1.4: if multiple soil moisture content measuring devices based on fiber Bragg gratings are connected in series or parallel to perform multi-point measurement, then steps S1.1 to S1.3 are repeated to connect each device in sequence.

9. The soil moisture content measurement method based on fiber Bragg grating according to claim 7, characterized in that: The step S2 is specifically as follows: Step S2.1, marking corresponding measuring points or marks at the pre-buried measuring point positions, and drilling holes to a predetermined depth using a drilling rig, and taking out the in-situ soil at the corresponding depths respectively; Step S2.2: Divide the obtained in-situ soil into three parts, use the quartering method to take representative soil samples, and test the density ρ and specific gravity G of each in-situ soil according to the "Standard for Geotechnical Test Methods". s and the dielectric constant of soil particles ε s , and take the average value of three in-situ soil bodies as soil layer parameters, and convert the in-situ soil density according to the relationship between the three-phase indicators of soil mechanics; Step S2.3, inserting the soil moisture content measuring device based on the fiber Bragg grating into the predetermined depth of the borehole, and leading the transmission wire out of the borehole, backfilling the borehole with in-situ soil of the same density, and making the soil moisture content measuring device based on the fiber Bragg grating closely contact with the surrounding soil to establish an effective hydraulic connection; Step S2.4: if the soil moisture content of a single section or multiple sections is to be measured, steps S2.1 to S2.3 are repeated, and the soil moisture content measuring devices at different positions are numbered in sequence.

10. The soil moisture content measurement method based on fiber Bragg grating according to claim 7, characterized in that: The step S3 is specifically as follows: Step S3.1, turn on the low-frequency signal source switch and debug the signal processing module until the signal processing module can effectively record the capacitance signal to ensure that the module is well connected; Step S3.2, turn off the low-frequency signal source switch, turn on the high-frequency signal source switch, and debug the signal source generator (1-2) and the wavelength demodulator until the wavelength signal can be effectively recorded; Step S3.3, turn on the high and low frequency signal source switches at the same time, so that the soil moisture content measuring device based on the fiber Bragg grating works for a period of time, until the computer can automatically intercept the wavelength signal data and capacitance signal data in the wavelength change stable section [t1, t2] time, record and save the data.

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