Design Of Soil Moisture Measurement System Based On Dielectric Constant

Soil moisture is an important part of water and energy exchange between the surface and atmosphere,and also a major factor in crop cultivation.With the development of information technology,modern agriculture is changing from traditional irrigation mode to precision irrigation,and the realization of precision irrigation is inseparable from excellent soil moisture observation technology.Multi depth soil moisture monitoring can provide data support for crop root growth environment observation,and has important research significance and wide application value for scientific precision irrigation,drought and flood disaster prevention and early warning.Based on the in-depth analysis of several common methods to obtain soil permittivity,a new ring electrode probe is designed by using frequency domain reflection technology,and a non-contact multi depth soil profile moisture measurement system is realized.Through the establishment of laboratory calibration environment,the performance of the sensor is verified and analyzed.The main contents of this paper are as follows(1)The types of soil moisture and the different expressions and calculation methods of soil moisture content were deeply analyzed,and the physical properties of soil and the theory of soil dielectric model were studied.(2)Several common methods of soil dielectric constant acquisition are compared,including time domain reflectometry,time domain transmission and standing wave ratio.The measurement principles of these methods are analyzed,and their advantages and disadvantages are discussed.Based on these comparisons,frequency domain reflection(FDR),which is more suitable for field environment measurement,is selected.(3)The measurement principle of soil moisture sensor based on frequency domain reflection technology is analyzed,the corresponding mathematical model is established,and a non-contact soil moisture sensor with ring electrode structure is designed for multi depth fixed-point measurement in field environment.The three-dimensional model of the sensor probe is established in ANSYS Maxwell.The soil environment with different water content is simulated by changing the dielectric constant of the material outside the sensor probe,and the corresponding boundary conditions are set and brought into the solver.The electric field distribution,capacitance change and sensor sensitivity of the sensor sensitive element are calculated.The influence of the sensor probe size variable process on the measurement performance is analyzed systematically.The design of the sensor probe size variable is carried out,and the best design scheme of the probe size is obtained.(4)The hardware circuit of the sensor is designed and built,and the function of measuring the output frequency and temperature of the acquisition circuit is completed,including the sensor acquisition circuit and the sensor control circuit.The water content signal acquisition circuit is the key to accurately measure the soil water content.A resonant circuit is formed by connecting a copper ring electrode with an inductor and a parallel voltage controlled oscillator.The abstract water content signal is converted into a frequency signal in the circuit.After a series of frequency dividing and shaping circuits,the analog signal is converted into a signal that can be recognized by a single chip microcomputer.In the sensor control part,STM3F411 MCU is selected to complete the power output control,signal frequency measurement,temperature measurement and data output.(5)Based on the completion of the hardware circuit,the embedded program is designed based on keil platform,including the frequency and temperature measurement module and the control module.At the same time,the upper computer display interface is designed by using graphical programming language,which realizes the display and storage of sensor measurement data.(6)The experimental environment was set up,and the sandy loam soil in Wuxi,Jiangsu Province was used to test and analyze the two sensor nodes of the soil moisture measurement system.According to the international standard drying method,soil samples with different soil moisture content were prepared,and the moisture sensor was calibrated,and the quadratic relationship between soil moisture content and measured signal frequency was established.The repeatability,stability and consistency of the soil moisture sensor were tested.The experimental results show that the quadratic regression models of sensor nodes 1 and 2 have good fitting effect,and the determination coefficients are R12 = 0.991 and R22 = 0.993,and they also have good stability,repeatability and consistency in performance test,which can meet the long-term fixed-point continuous soil moisture measurement in the field environment.The results show that the sensor prototype has relatively reliable measurement performance,and can realize the fixed-point multi depth measurement of soil.