Research On Array Capacitive Sensor For Detecting Soil Moisture

Soil moisture is a basic form of storage of water on the surface of the earth,restricting the balance between land and atmosphere and affecting the distribution of climate.More importantly,soil moisture is an important prerequisite for plants to grow well.If plants are to grow healthily,they must actively absorb the water,organic compounds,etc.At present,the measurement of soil moisture is mainly based on probe type,which is directly inserted into the soil to achieve a single point measurement of soil moisture.The length of the probe is typically 30 cm,and if deeper soil level moisture measurements are to be made,the soil needs to be excavated and the sensor inserted vertically into the soil layer.Although this method can measure the moisture content of the soil profile layer in a certain way,the measurement is not only time-consuming and laborious,but also destroys the soil quality of the soil.Therefore,this paper proposes an array type capacitive sensor,which can measure the soil moisture of different soil profiles on the basis of traditional single point measurement of soil moisture.This paper first analyzes the principle of capacitive sensor for measuring soil moisture.According to the dielectric model of soil,the level of soil moistu,re directly affects the magnitude of dielectric constant,which causes the change of capacitance.Based on this principle,array capacitors are designed.The sensor is used to measure the moisture of the soil pro file.Based on the designed sensor structure,a mathematical model of the sensor is established,and the finite element analysis method is used to solve the capacitance value measured by the sensor.The simulation model of the sensor was established by Maxwell.The relationship between the capacitance value and the dielectric constant was obtained.The influence of different sensor parameters on the capacitance measurement and sensitivity was analyzed.The optimal structural parameters of the sensor were designed.According to the optimal structural parameters of the sensor,the overall structural design of the array capacitive sensor is completed.The scheme of the capacitor signal detection system is designed.The micro-capacitor detection chip AD7746 is used as the core firmware of the measurement system,and the capacitor signal is collected by the STC89C52 single-chip microcomputer.At the same time,the upper computer was designed by Labview to realize the display and preservation of data.The experimental system was built and the measurement performance of the sensor was tested and analyzed.A standard sample of soil moisture was prepared by the drying method,and the calibration coeff-icient was calibrated to establish a functional relationship between soil moisture and sensor signals.The soil moisture was repeatedly measured by the sensor,and the stability and measurement accuracy of the measurement were analyzed.The experimental results show that the maximum absolute error of soil moisture measured by self-made sensor and drying method and FDR sensor is 4%and 3%respectively in the effective linear range of the sensor,that is,the soil moisture content is between 40%and 80%.Finally,the soil profile moisture was tested with an array capacitive sensor.The maximum absolute error of the test results compared with the FDR sensor was 3%,indicating that the array capacitive sensor has the ability to measure the moisture of the soil profile.