CN103592338A - Tube-pin-type soil moisture content detection method and device based on frequency domain reflectometry - Google Patents
Tube-pin-type soil moisture content detection method and device based on frequency domain reflectometry Download PDFInfo
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Abstract
The invention discloses a tube-pin-type soil moisture content detection method and device based on frequency domain reflectometry, and relates to the technical field of the detection of the moisture content of the soil. A high-frequency signal is used as an excitation signal, so that a high-frequency attenuation signal amplitude can be measured according to the impedance matching theory in the electromagnetic wave transmission theory, so as to reflect the changes of impedances of the soil to be detected and a probe; meanwhile, the volume density and the temperature of the soil are measured; therefore, the moisture content of the soil can be precisely obtained according to a function relation between the moisture content, and the high-frequency attenuation signal amplitude, the temperature and the volume density or by a correction method for the moisture content based on the temperature and the volume density. The device based on the method comprises a tube-pin-type soil moisture content probe, a 50-ohm coaxial connection wire, a high-frequency signal excitation circuit, a voltage detection circuit, a temperature sensor, a mass sensor, a power module, a micro processing unit and a display module. According to the method and the device, disclosed by the invention, the influence of the temperature and the volume density of the soil on the measurement of the soil moisture content can be eliminated, and the soil moisture content can be accurately and quickly measured; furthermore, the cost is low.
Description
Technical Field
The invention relates to the technical field of soil moisture content detection, in particular to a tube needle type soil moisture content detection method and device based on a frequency reflection method.
Background
Soil is the basic production data of agricultural production and is also a carrier for providing nutrients for crops. Soil moisture content is very important for the growth of crops. Firstly, crops must absorb enough water to meet the growth requirements of the crops; secondly, dissolving nutrient substances in the soil into solution by water, and exchanging the solution concentration inside and outside the cells of the plant roots to complete the absorption of nutrition; finally, the penetration of moisture can bring artificially applied fertilizer into the soil, so that the soil fertility is improved. The water content of soil is an important index for understanding the farmland condition and the growth condition of crops.
Currently, methods used in soil moisture content detection devices mainly include tensiometer methods, resistance methods, neutron methods, ray methods, optical methods, and dielectric methods. In contrast, the dielectric method for measuring the water content of soil through the dielectric property of soil is a quick, simple and effective measuring method. In 1976, Topp and Davis firstly introduce a time domain reflection method into the research of soil moisture rapid measurement, and in 1980, an empirical equation of a polynomial relation between soil moisture content and dielectric constant is found out by applying a theoretical classification method in a numerical approximation theory in statistical mathematics, and tests are carried out under the soil conditions of different components and types, so that the method is proved to have higher measurement accuracy. Based on the method, the method for measuring the water content of the soil based on the dielectric property is widely explored and applied.
At present, soil moisture content sensors developed on the domestic market based on a dielectric method mainly have two types, namely a time domain reflection method (TDR) and a frequency domain reflection method (FDR). The Time Domain Reflectometry (TDR) is based on the principle that the propagation speeds of electromagnetic waves in different media are different, and measures the soil moisture content according to the time difference between the transmitted wave and the reflected wave of the high-frequency electromagnetic wave in the soil. Because the electromagnetic wave propagation speed is very fast, the time difference between the transmitted wave and the reflected wave is extremely short, and an ultra-high speed delay measurement technology is needed, so that the technology is high in cost and difficult to apply to low-cost agricultural production.
The Frequency Domain Reflectometry (FDR) measures the water content of soil according to the principle that the apparent dielectric constant of soil changes along with the change of the water content of soil. Soil moisture content detection instruments based on Frequency Domain Reflectometry (FDR) generally operate in the range of tens to hundreds of megahertz, and the change of the dielectric constant can be expressed by voltage or other forms. The measurement result of the soil moisture content detection instrument of the Frequency Domain Reflectometry (FDR) is easily influenced by the texture, temperature and volume weight of the soil. The existing frequency domain method soil moisture content detector overcomes the influence of soil texture and environment temperature by calibrating experiments and introducing a temperature sensor, but mostly does not consider the influence of soil volume weight, so that the measurement precision is not high. The combined type soil moisture content sensor designed by increasing the soil compactness sensor plays an important role in reducing the volume weight influence and improving the moisture content detection precision, but increases the cost of the instrument, and the use of the stepping motor increases the complexity of the operation of the sensor. Therefore, it is necessary to develop a soil moisture content detector which is fast, low-cost, simple and convenient to operate and high in precision.
The purpose of the invention is as follows:the method and the device for measuring the soil moisture content based on the Frequency Domain Reflectometry (FDR) are quick, low in cost, simple and convenient to operate and high in precision.
The technical problems to be solved by the invention are as follows:(1) on the basis of a Frequency Domain Reflection (FDR) soil moisture measurement technology, the influence of soil temperature and soil bulk density on a measurement result can be eliminated at the same time, and the method has the advantages of high measurement speed, convenience in operation, low cost and high precision; (2) based on a Frequency Domain Reflectometry (FDR), a measuring probe and a related measurement auxiliary system are designed, so that the influence of temperature and bulk density on a measuring result can be eliminated quickly, conveniently and simultaneously at low cost when the soil moisture is measured, and higher measuring precision is obtained.
Disclosure of Invention
Aiming at the problems of the existing Frequency Domain Reflection (FDR) soil moisture content instrument, the invention aims to provide a tube needle type soil moisture content detection method based on the frequency domain reflection. The method is characterized in that a high-frequency signal is used as an excitation source, and the amplitude of high-frequency voltage at the joint of a 50-ohm coaxial connecting line and a pipe 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 the soil to be measured and the probe; the method comprises the steps of taking out soil to be detected by utilizing soil adhesion and a pipe needle type structure, obtaining the bulk density of the soil according to the soil quality measured by a quality sensor and the volume of a soil area taken out by a probe of the pipe needle type structure, measuring the temperature of the soil by utilizing a temperature sensor, and accurately calculating the water content of the soil according to the function relation between the water content of the soil and the voltage amplitude value at the probe, the soil temperature and the bulk density or the correction method of the water content by the temperature and the bulk density established under experimental conditions.
In addition, the invention also aims to provide the tube needle type soil moisture content detection device based on the frequency reflection method, which has the advantages of simple structure and convenience in operation and can quickly measure the volume density of the soil.
The device includes: the device comprises a pipe 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 pipe needle type soil moisture content probe 1 comprises a measuring probe 10, a measuring pipe 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 a measuring handle 12; the temperature sensor 5 is embedded and arranged outside the measuring tube 11 of the tube needle type soil moisture content probe 1; the mass sensor 6 is arranged at the lower end of the 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 at one side;
one end of a central lead of the 50 omega coaxial connecting wire 2 is connected with a measuring probe 10 of the tube needle type soil moisture content probe 1, and a shielding metal mesh wire 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 suitable for the tube needle type soil moisture content probe 1;
the voltage detection circuit 4 comprises a high-frequency filtering 13 module, a high-frequency detection 14 module and a signal processing 15 module, 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 impedance changes of 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;
the power 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 4, the temperature sensor 5, the quality sensor 6 and the microprocessor 8.
The invention provides a detection method of a soil moisture content detection device based on a frequency domain reflection method, which 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, and 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 because the impedance of the tube needle type soil moisture content probe 1 is not matched with that of the 50 omega coaxial connecting line 2, a high-frequency attenuation signal lower than the output voltage amplitude of the high-frequency signal excitation circuit 3 is generated;
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: the pipe needle type soil moisture content probe 1 is pulled out, and the pipe needle type soil moisture content probe 1 takes out the soil to be detected 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 pipe needle type soil moisture content 1 and the total mass of soil brought out by a mass sensor 6, and calculating the soil bulk density according to the soil mass:
whereinρThe density of the soil volume is the density of the soil volume,Uin order to be the output signal of the mass sensor 6,m 1is the quality of a tube needle type soil moisture content probe and an attached temperature sensor and a quality sensor thereof,ais a coefficient of proportionality that is,Vfor measuring the volume of the soil region that can be accommodated 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 and the voltage, the temperature and the bulk density and the measured voltage, the temperature and the bulk density, and outputs the result to the display module 9 for displaying the final result.
The beneficial technical effects are as follows:
the method and the device of the invention utilize a high-frequency signal generated by a high-frequency signal generating chip 16 as an excitation source, and measure the amplitude of high-frequency voltage at the joint of a 50 omega coaxial connecting line 2 and a pipe needle type soil moisture content probe 1 according to the impedance matching principle in the electromagnetic wave transmission theory so as to reflect the changes of the impedance of the soil to be measured and the probe; the voltage amplitude of the tube needle type soil moisture content probe 1 can reflect the impedance change of soil to be detected, the dielectric property of the soil is obtained, the temperature sensor is used for measuring the temperature of the soil, the bulk density of the soil is obtained according to the soil quality measured by the quality sensor and the volume of an area capable of containing the soil between a measuring probe 10 and a measuring tube 11 of the tube needle type soil moisture content probe 1, the temperature sensor is used for measuring the temperature of the soil, and the moisture content of the soil is calculated according to the function relation between the soil moisture content established under experimental conditions, the amplitude of the high-frequency voltage at the connection part of the 50 omega coaxial connecting line 2 and the tube needle type soil moisture content probe 1, the soil temperature and the bulk density, or the correction method for giving. The method and the device can simultaneously eliminate the influence of the soil temperature and the bulk density on the measurement of the soil moisture content, realize the accurate and rapid measurement of the soil moisture content and have low cost.
(1) The measurement is accurate. The influence of soil temperature and bulk density on soil moisture content measurement can be eliminated simultaneously.
(2) The measurement is convenient and fast. The pipe needle type soil moisture content probe is integrally designed with the temperature sensor and the quality sensor, so that the whole measuring process can be conveniently and rapidly completed on site in real time, and a measuring result can be obtained.
(3) The device has low cost. The soil bulk density is obtained through the mass sensor, the structure is simple, and the cost is low.
Drawings
FIG. 1 is a structural block diagram of a tube needle type soil moisture content detection device based on a frequency domain reflection method;
FIG. 2 is a schematic diagram of the arrangement of the probe for measuring the water content of soil and the temperature sensor and the quality sensor;
FIG. 3 is a functional block diagram of the voltage detection circuit of the present invention;
FIG. 4 is a block diagram of the high frequency signal excitation circuit of the present invention;
FIG. 5 is a flow chart of a soil moisture content detection method based on a frequency domain reflection method.
In the figure: 1. a pipe needle type soil moisture content probe; 2. a 50 Ω coaxial connection line; 3. a high-frequency signal excitation circuit; 4. A voltage detection circuit; 5. a temperature sensor; 6. a mass sensor; 7, a power supply module; 8. a microprocessor; 9. a display module; 10. a measurement probe; 11. a measurement tube; 12. a measuring handle; 13. high-frequency filtering; 14. high-frequency detection; 15. signal processing; 16. a high-frequency signal generating chip; 17. a difference processing circuit; 18. an amplifying circuit; 19. handle 20, cable.
Detailed Description
The objects and features of the present invention will be further illustrated by the following detailed description of the invention with reference to the accompanying drawings, but the embodiments of the invention are not limited thereto.
The theoretical basis of the method is to detect the dielectric property of the soil by using a frequency domain reflection method to reflect the water content of the soil. The embodiment is to apply the theory to measure the soil moisture content. In the embodiment, the medium is soil, the tube needle type soil moisture content detection device utilizes the high-frequency signal excitation circuit 3 to generate a high-frequency signal, transmitted to the needle type soil moisture content probe 1 through the 50 omega coaxial connecting wire 2, and generates an attenuation signal by utilizing the impedance mismatch between the soil and the needle type soil moisture content probe 1 under high frequency and the impedance of the 50 omega coaxial connecting wire 2, the voltage amplitude of the attenuation signal is related to the dielectric constant of the soil, the dielectric constant of the soil reflects the water content of the soil, the temperature and the volume density of the soil have certain influence on the attenuation signal, the relation between the soil moisture content and the voltage amplitude, the temperature and the bulk density of the attenuation signal can be established through a fitting equation, or a correction method for giving the water content according to the temperature and the bulk density is adopted, so that the accurate water content of the soil to be detected can be obtained by utilizing the temperature signal, the bulk density signal and the attenuated voltage signal.
As shown in fig. 1, the structural block diagram of the tube needle type soil moisture content 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 50 omega coaxial connecting line 2 and then is connected with the tube needle type soil moisture content probe 1, and when the high-frequency signal excitation circuit 3 generates a high-frequency signal and sends the high-frequency signal to the tube needle type soil moisture content probe 1 through the 50 omega coaxial connecting line 2, the tube needle type soil moisture content probe 1 and the 50 omega coaxial connecting line 2 generate impedance mismatch to generate an attenuation signal;
the voltage detection circuit 4 is connected with the tube needle type soil moisture content probe 1 and the 50 omega coaxial connecting line 2 and is used for detecting an attenuation signal at the joint of the 50 omega coaxial connecting line 2 and the tube needle type soil moisture content probe 1;
the temperature sensor 5 is embedded and installed on the outer side of the pipe needle type soil moisture content probe 1 and is used for measuring the soil temperature;
the mass sensor 6 is fixed at the lower end of the tube needle type soil moisture content probe 1, measures the mass of a soil sample, and calculates the bulk density of soil according to the mass of the soil and the volume of a soil accommodating area between a measuring probe 10 and a measuring tube 11 of the tube needle type soil moisture content probe 1;
the power supply module 7 is used for providing alternating current or direct current power supplies with 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 the display module 9 receives and displays the result signal of the microprocessor 8.
As shown in fig. 2, which is a schematic configuration diagram of the tube needle type soil moisture content probe 1, the temperature sensor 5 and the quality sensor 6 of the present invention, the tube needle type structure is adopted, and the soil to be detected can be taken out by using the soil adhesion force, and the method includes: measuring probe 10, survey buret 11 and measurement handle 12, measuring probe 10 and survey buret 11 and fix on measurement handle 12, and measuring probe 10 and survey buret 11 are the concentric structure, and the region between measuring probe 10 and the survey buret 11 is the effective measuring area region of soil that awaits measuring.
The pipe needle formula soil moisture content probe 1, measurement probe 10 and survey buret 11 are made by stainless steel material, and measurement probe 10's radius is 3mm, and length is 70mm, surveys 11 inboard diameters of buret and is 30mm, and the wall thickness is 1.5mm, and length is 70 mm. Of course, the probe radius, the measurement tube radius, the tube wall thickness and the length are not limited to the above values, and can be changed as required so as to bring out soil and change the dielectric property of soil reflected by the attenuation signal as a reference. The 50 Ω coaxial connecting wire 2 in this embodiment is composed of a center wire and a shielding metal mesh wire, the measuring probe 10 is connected to the center wire, and the measuring tube 11 is connected to the shielding metal mesh wire. The measuring handle 12 and the handle 19 thereon in this example are made of epoxy PVC material.
And the temperature sensor 5 is embedded and installed on the outer side of the measuring pipe 11 of the pipe needle type soil moisture content probe 1. The mass sensor 6 is fixed at the lower end of the 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 ensured to stand stably by taking the plane as a support.
Referring to fig. 3, which is a schematic block diagram of the voltage detection circuit of the present invention, the signal is band-pass filtered by the high-frequency filter 13 to remove non-measurement frequency interference signals, and the filtered signal is high-frequency detected by the high-frequency detector 14 and processed by the signal processor 15 to become a dc signal suitable for being processed by the microprocessor 8.
Fig. 4 is a block diagram of the high frequency signal excitation circuit of the present invention, in which a high frequency signal generating chip 16 generates two complementary high frequency signals, a difference processing circuit 17 filters interference in the high frequency signals by using a difference method, and an amplifying circuit 18 amplifies the high frequency signals into signals suitable for transmission through a 50 Ω coaxial connection line 2 and detection by a voltage detection circuit 4.
In the present embodiment, the high-frequency signal generating chip 16 selects an AD9959DDS chip, and the difference processing circuit 17 selects an OPA2658 amplifier as a core chip.
As shown in fig. 5, a detection method of a soil moisture content detection device based on a frequency domain reflection method according to the present invention is:
inserting the tube needle type soil moisture content probe 1 into soil to be measured, and starting measurement; 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 with the amplitude lower than that 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; outputting the high-frequency attenuation signal to a voltage detection circuit 4 to carry out high-frequency filtering 13, high-frequency detection 14 and signal processing 15 to obtain an original soil moisture content signal; measuring the temperature of the soil to be measured by a temperature sensor 5 to obtain the temperature of the soil; the pipe needle type soil moisture content probe 1 is pulled out, and the pipe needle type soil moisture content probe 1 takes out the soil to be detected by utilizing the soil adhesive force; 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 horizontal plane, arranging a measuring handle 12 on the lower part, measuring the pipe needle type soil moisture content probe 1 by a mass sensor 6, bringing out the total mass of soil, sending a mass signal output by the mass sensor 6 to a microprocessor, calculating the bulk density of the soil according to the mass of the soil and the volume of a soil-containing area between the measuring probe 10 and a measuring pipe 11, and obtaining the bulk density of the soil by the mass sensor signal comprises the following steps:
wherein,ρthe density of the soil volume is the density of the soil volume,Uin order to be able to measure the signal of the mass sensor,m 1the quality of the tube needle type soil moisture content probe,ais a coefficient of proportionality that is,Vfor measuring the volume of soil that can be accommodated between the probe 10 and the measuring tube 11.
And fitting and modeling the amplitude of the soil moisture content and the attenuated voltage signal, the soil temperature and the soil bulk density under experimental conditions to obtain a functional relation equation of the four. The microprocessor 8 is used for realizing the operation of the function relation equation to obtain a soil moisture content measurement result, the soil moisture content measurement result is output to the display module 9 to display a final result, or the soil moisture content obtained without considering the influence of the temperature and the bulk density is corrected according to the established method for correcting the soil moisture content by the temperature and the bulk density, so that the accurate soil moisture content is obtained, and the display module 9 displays the final result.
According to the method and the device, soil in the Yangling area is taken as an object, and under the laboratory condition, the absolute error of the soil measured by the measuring device is measured to be-0.129-0.016 g/cm3。
Experiments show that the soil mass moisture content, the soil volume weight and the temperature and the frequency of a signal source output by the high-frequency signal excitation circuit 3 all have obvious influence on the signal voltage (namely, an uncorrected soil moisture content signal) of the probe end on the pin-type soil moisture content probe 1. The variance analysis of the established mathematical models of uncorrected soil moisture content signals, soil mass moisture content, temperature and soil volume weight under 50MHz, 100MHz and 150MHz shows that the 150MHz established model is superior to the 50MHz mathematical model and the 100MHz mathematical model, and the model is as follows:
In the formula:R v the signal voltage ratio is the ratio of the uncorrected soil moisture content signal voltage to the output end voltage of the high-frequency signal excitation circuit 3, and the output end voltage of the high-frequency signal excitation circuit 3 is a constant value in the test;m- w -mass water content of soil, unit;ρsoil volume weight in g/cm3；T-soil temperature in ° c.
According to the model, a Newton iteration method and Matlab2010b software are applied to write a program for calculating the water content of the soil mass according to the temperature, the volume weight and the signal voltage ratio, and the water content of the measured soil can be obtained.
The determination coefficient R of the model is verified within 2.58-21.43% of the soil moisture content at 5-50 DEG C2= 0.8496. The verification experiment shows that the absolute error range of the signal voltage ratio calculated according to the model and the actual voltage ratio is-0.166-0.159, and the average absolute error is 0.0432; the absolute error range of the soil water content and the actual soil water content calculated according to the known signal voltage ratio, the temperature and the volume weight is-3.440% -4.039%, and the average absolute error is 1.237%.
It should be noted that: the above embodiments are merely illustrative of the technical solutions of the present invention and are not limiting; in addition, the present embodiment is described in detail by the drawings, and those skilled in the art will understand that any modification according to the embodiment of the present invention may be made without departing from the spirit of the technical solution of the present invention and the scope of the claims.
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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