CN103592338A - Tube-pin-type soil moisture content detection method and device based on frequency domain reflectometry - Google Patents

Abstract

The invention discloses a needle-type soil moisture content detection method and device based on a frequency domain reflection method, and relates to the technical field of soil moisture content detection. Using the high-frequency signal as the excitation signal, according to the principle of impedance matching in the electromagnetic wave transmission theory, measure the amplitude of the high-frequency attenuation signal to reflect the change of the soil to be tested and the impedance of the probe; The function relationship between the amplitude of the high-frequency attenuation signal, temperature and bulk density or the correction method of temperature and bulk density to the moisture content can accurately obtain the moisture content of the soil. The device based on this method includes a needle-type soil moisture content probe, a 50Ω coaxial connection line, a high-frequency signal excitation circuit, a voltage detection circuit, a temperature sensor, a quality sensor, a power supply module, a microprocessor and a display module. The invention can eliminate the influence of soil temperature and bulk density on the measurement of soil water content, and realize accurate and rapid measurement of soil water content with low cost.

Description

A needle type soil moisture content detection method and device based on frequency domain reflectometry

technical field

The invention relates to the technical field of soil moisture content detection, in particular to a needle-type soil moisture content detection method and device based on a frequency reflection method. the

Background technique

Soil is the basic means of production for agricultural production and also the carrier that provides nutrients for crops. Soil moisture content is very important for the growth of crops. First of all, crops must absorb enough water to meet their own growth needs; secondly, water dissolves nutrients in the soil into a solution, which is exchanged through the concentration of the solution inside and outside the root cells of the plant to complete the absorption of nutrients; finally, the osmosis of water can Bring artificially applied fertilizers into the soil to improve soil fertility. Soil moisture content is an important indicator to understand the condition of the farmland and the growth status of the crops. the

At present, the methods used in soil moisture content detection devices mainly include tensiometer method, resistance method, neutron method, ray method, optical method and dielectric method. In contrast, the dielectric method of measuring soil moisture content through soil dielectric properties is a fast, simple and effective measurement method. In 1976, Topp and Davis first introduced time-domain reflectometry into the research of rapid measurement of soil moisture, and in 1980 applied the theoretical classification method in numerical approximation theory in statistical mathematics to find out the relationship between soil moisture content and dielectric constant. The empirical equation of polynomial relationship, and experiments were carried out under different composition and types of soil conditions, which proved to have high measurement accuracy. Based on this, the method of measuring soil moisture content based on dielectric properties has been widely explored and applied. the

At present, the soil moisture content sensors developed based on the dielectric method in the domestic market are mainly time-domain reflectometry (TDR) and frequency-domain reflectometry (FDR). Among them, the time domain reflectometry (TDR) is based on the principle that electromagnetic waves propagate at different speeds in different media, and the soil moisture content is measured according to the time difference between the high-frequency electromagnetic wave in the soil and the time difference between the transmitted wave and the reflected wave. Due to the high propagation speed of electromagnetic waves, the time difference between the transmitted wave and the reflected wave is extremely short, requiring ultra-high-speed delay measurement technology. Therefore, the cost of this technology is relatively high, and it is difficult to apply it to low-cost agricultural production. the

The frequency domain reflectometry (FDR) is based on the principle that the apparent dielectric constant of the soil changes with the change of the soil moisture content to measure the soil moisture content. Soil moisture detection instruments based on frequency domain reflectometry (FDR) generally work in the range of tens to hundreds of megahertz, and can express the change of dielectric constant in voltage or other forms. The measurement results of the frequency domain reflectometry (FDR) soil moisture content detection instrument are easily affected by the texture, temperature and bulk density of the soil. The existing frequency-domain soil moisture detectors overcome the influence of soil texture and ambient temperature through experimental calibration and the introduction of temperature sensors, but most of them do not consider the influence of soil bulk density, resulting in low measurement accuracy. The composite soil moisture sensor designed by adding the soil compaction sensor plays an important role in reducing the influence of bulk density and improving the detection accuracy of moisture content, but it increases the cost of the instrument, and the use of stepping motors increases the complexity of sensor operation. Complexity. Therefore, it is necessary to develop a rapid, low-cost, easy-to-operate, and high-precision soil moisture detector. the

The purpose of the present invention is to overcome the deficiencies of the prior art and provide a quick, low-cost, easy-to-operate and high-precision measurement method and device for soil moisture based on frequency domain reflectometry (FDR).

The technical problems to be solved by the present invention: (1) How to simultaneously eliminate the influence of soil temperature and soil bulk density on the measurement results based on the frequency domain reflectometry (FDR) soil moisture measurement technology, and the measurement speed is fast, the operation is convenient, and the cost is low , high precision; (2) Based on frequency domain reflectometry (FDR), design a measurement probe and related measurement auxiliary system to realize the rapid, convenient and low-cost elimination of temperature and bulk density at the same time when measuring soil moisture. The impact of the measurement results, to obtain a higher measurement accuracy.

Contents of the invention

In view of the problems existing in the existing frequency domain reflectometry (FDR) soil moisture content instruments, the purpose of the present invention is to provide a needle type soil moisture content detection method based on the frequency domain reflectometry. It is characterized in that the high-frequency signal is used as the excitation source, and according to the principle of impedance matching in the electromagnetic wave transmission theory, the amplitude of the high-frequency voltage at the connection point between the 50Ω coaxial cable and the needle-type soil moisture content probe is measured to reflect the The change of soil and probe impedance; the soil to be tested is brought out by using the soil adhesion and the needle structure probe, and the bulk density of the soil is obtained according to the soil quality measured by the mass sensor and the volume of the soil area brought out by the needle structure probe. The sensor measures the temperature of the soil, and accurately calculates the soil moisture content according to the function relationship between the soil moisture content established under the experimental conditions and the voltage amplitude at the probe, soil temperature and bulk density, or the correction method of temperature and bulk density to the moisture content . the

In addition, the purpose of the present invention is to provide a needle-type soil moisture content detection device based on the frequency reflection method, which has a simple structure, is easy to operate, and can quickly measure the bulk density of soil. the

The device includes: needle type soil moisture content probe 1, 50Ω coaxial connecting wire 2, high frequency signal excitation circuit 3, voltage detection circuit 4, temperature sensor 5, quality sensor 6, power module 7, microprocessor 8 and display Module 9;

The needle type soil moisture content probe 1 comprises a measuring probe 10, a measuring tube 11 and a measuring handle 12; the measuring probe 10 is located at the center of the measuring tube 11, and the measuring probe 10 and the measuring tube 11 are fixed on the measuring handle 12; The temperature sensor 5 is embedded and installed on the outside of the measuring tube 11 of the needle type soil moisture content probe 1; the quality sensor 6 is installed at the lower end of the measuring handle 12 of the needle type soil moisture content probe 1, and the lower end surface of the quality sensor 6 is a plane , which can ensure that the needle-type soil moisture content probe (1) and its upper accessories stand upside down on this plane; the measuring handle 12 is symmetrically arranged with a handle 19, which is connected with the needle-type soil moisture content probe 1 and the temperature sensor. 5 and the cable 20 connected to the quality sensor 6 passes through the center of the handle 19 on one side;

One end of the central wire of the 50Ω coaxial connection line 2 is connected to the measuring probe 10 of the needle type soil moisture content probe 1, and its shielded metal mesh wire is connected to the measuring tube 11 of the needle type soil moisture content probe 1; The other end of the center conductor of the shaft connection line 2 is connected to the high-frequency signal excitation circuit 3;

The high-frequency signal excitation circuit 3 includes a high-frequency signal generating chip 16, a differential processing circuit 17, and an amplifier circuit 18 for generating high-frequency signals applicable to the needle-type soil moisture content probe 1;

The voltage detection circuit 4 includes a high-frequency filter 13, a high-frequency wave detection 14 and a signal processing module 15, which are used to detect the amplitude of the high-frequency voltage at the place where the 50Ω coaxial connection line 2 is connected to the needle type soil moisture content probe 1, To reflect the change of the soil to be measured and the impedance of the probe; the temperature sensor 5 and the quality sensor 6 are respectively used to collect the temperature and quality of the measured soil sample;

Described microprocessor 8 is connected with voltage detection circuit 4, temperature sensor 5 and quality sensor 6 respectively, is used for processing voltage detection circuit 4 output signals, temperature sensor 5 output signals and quality sensor 6 output signals, calculates soil moisture content;

Described display module 9 is connected with microprocessor 8, is used for displaying the calculation result of soil moisture content that microprocessor 8 outputs;

The power supply module 7 is respectively connected with the high-frequency signal excitation circuit 3, the voltage detection circuit 4, the temperature sensor 5, the quality sensor 6 and the microprocessor 8 to provide them with power.

The detection method of a kind of soil water content detection device based on the frequency domain reflection method that the present invention proposes comprises the following steps:

S1: Insert the needle-type soil moisture content probe 1 into the soil to be tested, and start the measurement;

S2: The high-frequency signal is generated by the high-frequency signal excitation circuit 3, and is transmitted to the needle-type soil moisture content probe 1 through the 50Ω coaxial connection line 2. 2 Impedance mismatch will generate a high-frequency attenuation signal lower than the output voltage amplitude of the high-frequency signal excitation circuit 3;

S3: The voltage detection circuit 4 performs high-frequency filtering 13, high-frequency detection 14 and signal processing 15 on the detected high-frequency attenuation signal to obtain an uncorrected soil moisture content signal;

S4: measure the temperature of the soil to be measured by the temperature sensor 5 to obtain the soil temperature;

S5: pull out the needle-type soil moisture content probe 1, and the needle-type soil moisture content probe 1 takes out the soil to be tested by using the soil adhesion;

S6: Stand the needle-type soil moisture content probe 1 vertically on a horizontal surface, with the measuring probe 10 on top and the measuring handle 12 on the bottom, and measure the needle-type soil moisture content 1 and the total mass of the soil taken out by the mass sensor 6, according to The method of calculating soil bulk density by soil mass is:

Among them, ρ is the soil bulk density, U is the output signal of the mass sensor 6, m ₁ is the mass of the needle-type soil moisture content probe and its attached temperature sensor and mass sensor, a is the proportional coefficient, V is the measurement probe 10 and The volume of the soil area that can be accommodated between the measuring tubes 11 .

S7: The microprocessor 8 calculates the soil moisture content according to the established functional relationship or correction method between the soil moisture content and the voltage, temperature and bulk density, and the measured voltage, temperature and bulk density, and outputs it to the display module 9 to display the final result . the

Beneficial technical effects:

The method and device of the present invention utilize the high-frequency signal generation chip 16 to generate a high-frequency signal as an excitation source, and measure the 50Ω coaxial connection line 2 and the needle type soil moisture content probe 1 at the connection point according to the principle of impedance matching in the electromagnetic wave transmission theory. The amplitude of the high-frequency voltage can reflect the change of the soil to be tested and the impedance of the probe; the voltage amplitude of the needle type soil moisture content probe 1 can reflect the change of the impedance of the soil to be measured, obtain the dielectric properties of the soil, and use the temperature sensor to measure the soil Temperature, according to the soil quality measured by the mass sensor and the volume of the area that can accommodate the soil between the measuring probe 10 and the measuring tube 11 of the needle type moisture content probe 1, the bulk density of the soil is obtained, and the temperature of the soil is measured by the temperature sensor, Calculate the soil moisture content based on the function relationship between the soil moisture content established according to the experimental conditions and the amplitude of the high-frequency voltage at the point where the 50Ω coaxial connection line 2 connects to the needle type soil moisture content probe 1, soil temperature and bulk density. and bulk density give the correction method for moisture content. The method and device of the invention can eliminate the influence of soil temperature and bulk density on the measurement of soil water content at the same time, and realize accurate and rapid measurement of soil water content with low cost.

(1) The measurement is accurate. The influence of soil temperature and bulk density on soil moisture measurement can be eliminated at the same time. the

(2) The measurement is convenient and fast. The integrated design of the needle-type soil moisture content probe, temperature sensor, and quality sensor facilitates the rapid completion of the entire measurement process on site in real time and obtains measurement results. the

(3) The installation cost is low. The bulk density of soil is obtained through a mass sensor, and the structure is simple and the cost is low. the

Description of drawings

Fig. 1 is a structural block diagram of a needle type soil moisture detection device based on the frequency domain reflection method of the present invention;

Fig. 2 is a schematic diagram of the configuration of the needle type soil moisture content probe, temperature sensor and quality sensor of the present invention;

Fig. 3 is the functional block diagram of the voltage detection circuit of the present invention;

Fig. 4 is a composition block diagram of the high-frequency signal excitation circuit of the present invention;

Fig. 5 is a flowchart of a method for detecting soil moisture content based on frequency domain reflectometry according to the present invention.

In the figure: 1. Needle-type soil moisture content probe; 2. 50Ω coaxial connection line; 3. High-frequency signal excitation circuit; 4. Voltage detection circuit; 5. Temperature sensor; 6. Quality sensor; 7. Power module; 8. , microprocessor; 9, display module; 10, measuring probe; 11, measuring tube; 12, measuring handle; 13, high-frequency filtering; 14, high-frequency detection; 15, signal processing; 16, high-frequency signal generation chip ; 17. Differential processing circuit; 18. Amplifying circuit; 19. Handle 20. Cable. the

Detailed ways

The present invention will be described in detail below with examples and in conjunction with the accompanying drawings to further illustrate the purpose and characteristics of the present invention, but the embodiments of the present invention are not limited thereto. the

The theoretical basis of the invention is to use the frequency domain reflection method to detect the dielectric properties of the soil to reflect the moisture content of the soil. This embodiment is to apply this theory to measure soil moisture content. In this embodiment, the medium is soil, and the needle-type soil moisture content detection device utilizes a high-frequency signal excitation circuit 3 to generate a high-frequency signal, which is transmitted to the needle-type soil moisture content probe 1 through a 50Ω coaxial connection line 2. The impedance of the soil and needle type soil moisture content probe 1 does not match the impedance of the 50Ω coaxial connection line 2 to generate an attenuation signal. The voltage amplitude of the attenuation signal is related to the dielectric constant of the soil. The soil dielectric constant reflects the soil moisture content. The temperature and bulk density of the soil have a certain influence on the attenuation signal. The relationship between the soil moisture content and the voltage amplitude, temperature and bulk density of the attenuation signal can be established through the fitting equation, or the correction method of the moisture content can be given according to the temperature and bulk density , so that the precise moisture content of the measured soil can be obtained by using the temperature signal, the bulk density signal and the attenuated voltage signal. the

As shown in Figure 1, a structural block diagram of a needle type soil moisture detection device based on the frequency domain reflection method of the present invention includes:

The high-frequency signal excitation circuit 3 is connected with the 50Ω coaxial connection line 2 and then connected with the needle-type soil moisture content probe 1. The high-frequency signal generated by the high-frequency signal excitation circuit 3 is sent to the pipe needle through the 50Ω coaxial connection line 2 During the process of the type soil moisture content probe 1, the impedance mismatch between the needle type soil moisture content probe 1 and the 50Ω coaxial connecting line 2 occurs, and an attenuation signal is generated;

The voltage detection circuit 4 is connected to the needle type soil moisture content probe 1 and the 50Ω coaxial connection line 2, and detects the attenuation signal at the connection between the 50Ω coaxial connection line 2 and the needle type soil moisture content probe 1;

The temperature sensor 5 is embedded and installed on the outside of the needle type soil moisture content probe 1 to measure the soil temperature;

The mass sensor 6 is fixed on the lower end of the needle-type soil moisture content probe 1 to measure the quality of the soil sample, according to the soil quality and the volume of the soil area that can be accommodated between the measuring probe 10 and the measuring tube 11 of the needle-type soil moisture content probe 1 Calculate the bulk density of the soil;

The power supply module 7 provides AC or DC power supplies of different amplitudes for the high-frequency signal excitation circuit 3, the voltage detection circuit 4, the temperature sensor 5, the quality sensor 6 and the microprocessor 8;

The microprocessor 8 receives and processes the output signals of the voltage detection circuit 4, the temperature sensor 5, and the quality sensor 6, and processes them;

The display module 9 receives the result signal from the microprocessor 8 and displays it.

Figure 2 is a schematic diagram of the arrangement of the needle-type soil moisture content probe 1, temperature sensor 5, and mass sensor 6 of the present invention. The needle-type structure can be used to take out the soil to be tested by using the soil adhesion force, including: measuring probe 10, Measuring tube 11 and measuring handle 12, measuring probe 10 and measuring tube 11 are fixed on the measuring handle 12, measuring probe 10 and measuring tube 11 are concentric structures, the area between measuring probe 10 and measuring tube 11 is to be measured Effective soil measurement area. the

The needle-type soil moisture content probe 1, the measuring probe 10 and the measuring tube 11 are made of stainless steel, the radius of the measuring probe 10 is 3 mm, the length is 70 mm, the inner diameter of the measuring tube 11 is 30 mm, and the wall thickness is 1.5 mm. mm, the length is 70mm. Of course, the radius of the probe, the radius of the measuring tube, the thickness and length of the tube wall are not limited to the above values, and can be changed as required, based on the fact that the soil can be brought out and the attenuation signal can reflect the change of soil dielectric properties. The 50Ω coaxial connection line 2 in this embodiment is composed of a central wire and a shielded metal network wire, the measuring probe 10 is connected to the central wire, and the measuring tube 11 is connected to the shielded metal network wire. The measuring handle 12 and the handle 19 thereon in this example are made of epoxy resin PVC material. the

The temperature sensor 5 is embedded and installed outside the measuring tube 11 of the needle type soil moisture content probe 1 . The mass sensor 6 is fixed on the lower end of the measuring handle 12 of the needle-type soil moisture content probe 1. The lower end surface of the mass sensor 6 is a plane, which can ensure that the needle-type soil moisture content probe (1) and its upper accessories are based on this plane. Stand on a headstand with support. the

Fig. 3 is the functional block diagram of the voltage detection circuit of the present invention, the signal is carried out band-pass filter by high-frequency filtering 13, filter non-measurement frequency interference signal, carry out high-frequency detection to the signal after filtering by high-frequency detection 14, through signal processing 15 After processing, it becomes a DC signal suitable for processing by the microprocessor 8 . the

Figure 4 is a block diagram of the composition of the high-frequency signal excitation circuit of the present invention. The high-frequency signal generation chip 16 generates two complementary high-frequency signals, and the interference in the high-frequency signal is filtered by the differential processing circuit 17 through the differential method, and the amplifying circuit 18 The high-frequency signal is amplified into a signal suitable for transmission by the 50Ω coaxial connection line 2 and detection by the voltage detection circuit 4 . the

The high-frequency signal generating chip 16 of this embodiment selects the AD9959DDS chip, and the differential processing circuit 17 selects the OPA2658 amplifier as the core chip. the

As shown in Figure 5, a kind of detection method of the soil moisture content detection device based on the frequency domain reflection method of the present invention is:

Insert the needle-type soil moisture content probe 1 into the soil to be measured, and start the measurement; the high-frequency signal is generated by the high-frequency signal excitation circuit 3, and is transmitted to the needle-type soil moisture content probe 1 through the 50Ω coaxial connection line 2, Due to the impedance mismatch between the needle-type soil moisture content probe 1 and the 50Ω coaxial connecting line 2, a high-frequency attenuation signal lower than the amplitude of the high-frequency signal excitation circuit 3 will be generated; the high-frequency attenuation signal is output to the voltage detection The circuit 4 performs high-frequency filtering 13, high-frequency detection 14 and signal processing 15 to obtain the original soil moisture content signal; the temperature sensor 5 measures the temperature of the soil to be tested to obtain the soil temperature; the needle-type soil moisture content probe 1 is pulled out , the needle-type soil moisture content probe 1 uses soil adhesion to take out the soil to be tested; the needle-type soil moisture content probe 1 is vertically inverted on a horizontal plane, the measuring probe 10 is on the top, and the measuring handle 12 is on the bottom. The sensor 6 measures the needle-type soil moisture content probe 1 and the total mass of the soil brought out, and sends the quality signal output by the mass sensor 6 to the microprocessor. According to the soil quality and the soil area that can be accommodated between the measuring probe 10 and the measuring tube 11 The bulk density of the soil is calculated from the volume of the mass sensor, and the method of obtaining the bulk density of the soil from the mass sensor signal is:

Among them, ρ is the soil bulk density, U is the signal of the mass sensor, m ₁ is the mass of the needle-type soil moisture content probe, a is the proportional coefficient, and V is the volume of soil that can be accommodated between the measuring probe 10 and the measuring tube 11.

Under the experimental conditions, the soil moisture content and the amplitude of the attenuated voltage signal, soil temperature, and soil bulk density are fitted and modeled, and the functional relationship equation between the four is obtained. Apply the microprocessor 8 to realize the operation of the functional relationship equation, obtain the measurement result of the soil moisture content, output to the display module 9 to display the final result, or correct the soil moisture content according to the established temperature and bulk density, and do not consider the temperature and volume The soil moisture content obtained under the influence of the density is corrected, and then the accurate soil moisture content is obtained, and the final result is displayed by the display module 9 . the

According to the above method and device, taking the soil in Yangling area as an object, under laboratory conditions, the absolute error of the measuring device for measuring soil is -0.129~0.016g/cm ³ .

Experiments have shown that the soil quality moisture content, soil bulk density and temperature, and the frequency of the signal source output by the high-frequency signal excitation circuit 3 are all related to the signal voltage at the probe end of the needle-type soil moisture content probe 1 (that is, the uncorrected soil moisture content signal) have a noticeable effect. The variance analysis of the mathematical models of the uncorrected soil moisture content signal and soil mass moisture content, temperature and soil bulk density under the established 50, 100 and 150MHz shows that the model built at 150MHz is better than the mathematical models at 50 and 100MHz. The model is :

　　（150MHz下所建模型）

(Model built at 150MHz)

In the formula: R _v —— signal voltage ratio, which is the ratio of the uncorrected soil moisture content signal voltage to the output voltage of the high-frequency signal excitation circuit 3, and the output voltage of the high-frequency signal excitation circuit 3 is a constant value in the test; m- _w — soil moisture content, in %; ρ — soil bulk density, in g/cm ³ ; T — soil temperature, in °C.

According to the model, the program of calculating soil mass moisture content based on temperature, bulk density and signal voltage ratio is written by using Newton iteration method and Matlab2010b software, and the measured soil moisture content can be obtained. the

In the range of 5-50℃ and soil moisture content of 2.58%-21.43%, the coefficient of determination of the model is R ² =0.8496. The verification experiment shows that the absolute error range between the signal voltage ratio and the actual voltage ratio calculated according to the model is -0.166~0.159, and the average absolute error is 0.0432; The absolute error range of soil moisture content was -3.440%~4.039%, and the average absolute error was 1.237%.

It should be pointed out that the above-mentioned embodiment is only to illustrate the technical scheme of the present invention and is not limiting; in addition, the illustrations have described this embodiment in detail, and those skilled in the art should understand that any measures taken according to the specific embodiment of the present invention Any deformation shall not deviate from the spirit of the technical solution of the present invention and the scope described in the claims. the

Claims (3)

1. A tube needle type soil moisture content detection method based on a frequency domain reflection method is characterized in that a high-frequency signal is used as an excitation signal, and the amplitude of high-frequency voltage at the connection position of a 50-ohm coaxial connecting line and a tube needle type soil moisture content probe is measured according to the impedance matching principle in the electromagnetic wave transmission theory so as to reflect the impedance changes of soil to be detected and the probe; the soil to be detected is brought out by utilizing the soil adhesion and the pipe needle type structure, the volume density of the soil can be obtained according to the soil quality measured by the quality sensor and the volume of the soil which can be accommodated between the measuring probe of the pipe needle type soil moisture content probe and the inner side of the measuring pipe, the temperature of the soil is measured by utilizing the temperature sensor, and the moisture content of the soil is accurately calculated according to the soil moisture content established under the experimental conditions, the amplitude of the high-frequency voltage at the joint of the 50 omega coaxial connecting line and the pipe needle type soil moisture content probe, the functional relation between the soil temperature and the volume density or the correction method of the temperature and the volume density on.

2. A tube needle type soil moisture content detection device based on a frequency domain reflection method is characterized by comprising a tube needle type soil moisture content probe (1), a 50 omega coaxial connecting line (2), a high-frequency signal excitation circuit (3), a voltage detection circuit (4), a temperature sensor (5), a quality sensor (6), a power supply module (7), a microprocessor (8) and a display module (9);

the needle type soil moisture content probe (1) comprises a measuring probe (10), a measuring tube (11) and a measuring handle (12); the measuring probe (10) is positioned at the center of the measuring pipe (11), and the measuring probe (10) and the measuring pipe (11) are fixed on the measuring handle (12); the temperature sensor (5) is embedded and arranged outside the measuring tube (11) of the needle type soil moisture content probe (1); the mass sensor (6) is arranged at the lower end of a measuring handle (12) of the tube needle type soil moisture content probe (1), the lower end face of the mass sensor (6) is a plane, and the tube needle type soil moisture content probe (1) and the upper accessory parts thereof can be supported by the plane to stand upside down stably; handles (19) are symmetrically arranged on the measuring handle (12), and a cable (20) connected with the tube needle type soil moisture content probe (1), the temperature sensor (5) and the mass sensor (6) penetrates out of the center of the handle (19) on one side;

one end of a central lead of the 50 omega coaxial connecting line (2) is connected with a measuring probe (10) of the tube needle type soil moisture content probe (1), and a shielding metal mesh line of the central lead is connected with a measuring tube (11) of the tube needle type soil moisture content probe (1); the other end of the center lead of the 50 omega coaxial connecting wire (2) is connected with a high-frequency signal excitation circuit (3);

the high-frequency signal excitation circuit (3) comprises a high-frequency signal generation chip (16), a differential processing circuit (17) and an amplifying circuit (18) and is used for generating a high-frequency signal applicable to the tube needle type soil moisture content probe (1);

the voltage detection circuit (4) comprises a high-frequency filtering module (13), a high-frequency detecting module (14) and a signal processing module (15), and is used for detecting the amplitude of high-frequency voltage at the joint of the 50-ohm coaxial connecting line (2) and the pipe needle type soil moisture content probe (1) so as to reflect the changes of the impedance of the soil to be detected and the probe; the temperature sensor (5) and the mass sensor (6) are respectively used for collecting the temperature and the mass of the soil sample to be measured;

the microprocessor (8) is respectively connected with the voltage detection circuit (4), the temperature sensor (5) and the quality sensor (6) and is used for processing the output signal of the voltage detection circuit (4), the output signal of the temperature sensor (5) and the output signal of the quality sensor (6) and calculating the water content of the soil;

the display module (9) is connected with the microprocessor (8) and is used for displaying the soil moisture content calculation result output by the microprocessor (8);

and the power supply module (7) is respectively connected with the high-frequency signal excitation circuit (3), the voltage detection circuit (4), the temperature sensor (5), the quality sensor (6) and the microprocessor (8) to provide power for the high-frequency signal excitation circuit, the voltage detection circuit, the temperature sensor and the quality sensor.

3. The tube needle type soil moisture content detection device based on the frequency domain reflection method as claimed in claim 2, wherein the step of detecting the soil moisture content by using the device comprises the following steps:

s1: inserting the tube needle type soil moisture content probe (1) into soil to be measured, and starting measurement;

s2: the high-frequency signal excitation circuit (3) generates a high-frequency signal, the high-frequency signal is transmitted to the tube needle type soil moisture content probe (1) through the 50 omega coaxial connecting line (2), and the high-frequency attenuation signal lower than the output voltage amplitude of the high-frequency signal excitation circuit (3) is generated due to impedance mismatching of the tube needle type soil moisture content probe (1) and the 50 omega coaxial connecting line (2);

s3: the voltage detection circuit (4) carries out high-frequency filtering (13), high-frequency detection (14) and signal processing (15) on the detected high-frequency attenuation signal to obtain an uncorrected soil moisture content signal;

s4: measuring the temperature of the soil to be measured by a temperature sensor (5) to obtain the temperature of the soil;

s5: pulling out the pipe needle type soil moisture content probe (1), and taking out the soil to be detected by the pipe needle type soil moisture content probe (1) by utilizing the soil adhesive force;

s6: the method comprises the following steps of vertically inverting a pipe needle type soil moisture content probe (1) on a horizontal plane, arranging a measuring probe (10) on the upper part and a measuring handle (12) on the lower part, measuring the total mass of the pipe needle type soil moisture content probe (1) and the soil taken out by a mass sensor (6), and calculating the bulk density of the soil according to the mass of the soil:

whereinρThe density of the soil volume is the density of the soil volume,Uis the output signal of the mass sensor (6),m ₁the mass of a tube needle type soil moisture content probe (1) and a temperature sensor (5) and a mass sensor (6) which are attached to the probe,ais a coefficient of proportionality that is,Vfor measuring the volume of the soil-receiving region between the probe (10) and the measuring tube (11);

s7: the microprocessor (8) calculates the soil moisture content according to the established functional relation or correction method of the soil moisture content, the voltage, the temperature and the bulk density and the measured voltage, temperature and bulk density, and outputs the result to the display module (9) for displaying the final result.

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