Capacitance Sensor And System Design For Soil Profile Moisture Detection

Water is the origin of all life activities,but with the development of human society,water resources have become a scarce resource.Although China's total water resources are rich,the huge water consumption makes China one of the most water deficient countries in the world.The agricultural water use only takes up 2/3 of the total water consumption in the country,but the agricultural water consumption rate is generally not high.Therefore,the effective acquisition of agricultural water saving,soil moisture information(especially soil profile water)and the spatial distribution of soil moisture are particularly important,which is of great significance to the realization of precision agriculture in agricultural production.In view of the single point measurement of soil moisture sensor and the high cost,this paper designs a low cost soil profile moisture sensor based on touch capacitance induction chip MPR121 to realize the multi-point measurement of soil moisture profile.In this paper,based on the soil moisture as the research object,based on the difference of the dielectric constant of soil,water and air,the relationship between soil moisture and capacitance value is explored;and the change law of the performance and output of the sensor circle probe structure under the different soil moisture(dielectric constant)is clarified;and the mechanism of soil moisture can be characterized by capacitance;A relatively reliable soil profile moisture sensor and system design scheme were established;Finally,the field test of the system is carried out,which proves the feasibility of the scheme.The main work of this paper is to complete the following four aspects:1.Based on the demand of multi-point soil profile moisture testing,on the basis of consulting a large number of literature on the advantage and disadvantage of different soil moisture detection technology at home and abroad are classified,designed a sensor based on soil profile detection system,and the detection mechanism of touch capacitive sensing chip MPR121 to conduct a comprehensive analysis,to ensure the feasibility of realization of soil profile detection.2.Ansoft Maxwell Software is used to explore and analyze the sensor ring probe.Respectively from the probe diameter,probe the axial length and axial spacing between two aspects,the research of sensor probe sensitive degree and detection range.Eventually determine the sensor ring probe 30 mm axial length,axial spacing between 1 mm and a diameter of 20 mm.This provides scientific theory basis for circle probe design.3.Calibration and calibration of self-made sensors through standard drying method experiments.Based on the different output capacitance values under different soil volumetric water content,the relationship between sensor output capacitance and soil moisture content was established as y=ln(0.9879+0.0012x),and the correlation coefficient R~2 was 0.996.This indicates that the correlation between the output of the sensor and the volumetric water content of soil is good.In addition,the consistency,repeatability and stability of the sensor are further tested by testing.Consistency test results show that the linear fit between the sensors is higher,the correlation coefficient is above 0.99,which fully shows that the sensor has higher consistency;the repeatability test shows that the sensor output has little difference,and the maximum error is 2.6%.It is better to describe the repeatability of the sensor;the stability test shows that the maximum error of the sensor output is 4.1%,and the standard deviation is0.02899.Within the error range,the sensor has good stability in the soil moisture retention stage.Then the performance of the designed sensor is compared with the drying method and the standing wave rate(SWR)sensor.The results show that the maximum error of the sensor and the standing wave rate sensor is 3.11%,and the maximum error of the measurement is 2.77%.This indicates that the performance of the self-made sensor is similar to that of standing wave type sensor and drying method,indicating that it has certain reliability.Finally,a preliminary test of the multi-nodes of the soil profile sensor was performed.The experimental results showed that the maximum error between the data of the three-node profile sensors was 3.2%,indicating that the performance of the sensor nodes was good.4.In order to further verify the performance of the sensor,the sensor was installed in Mohe,Heilongjiang,and field tests were carried out for half a year to achieve soil moisture measurements.The experiment acquired a large amount of data and based on this,the characteristics of temporal and spatial changes in soil water content were analyzed.