Research On Soil Moisture Measurement System Based On SAW Technology Combined With UAV

As a large agricultural country,China’s agricultural modernization level is backward,and water resources are scarce,the implementation of precision irrigation is the trend of China’s agricultural development.Field soil moisture measurement can provide source data and decision basis for precision irrigation,which is of great significance.Based on the above research background,in view of the passive wireless advantages of surface acoustic wave(SAW)technology and the characteristics of fast patrol speed of unmanned aerial vehicle(UAV),this paper achieved the acquisition of soil moisture data based on SAW technology and UAV.In terms of theoretical analysis,coupling of mode(COM)theory was used to model and simulate the impedance loaded SAW sensors.The COM model between the echo characteristics of sensor and the capacitance of sensitive element was established.The variation law of echo peak value and phase with the capacitance of sensitive element was simulated and analyzed.The simulation results show that the variation of peak value is more sensitive than the phase.In addition,the effects of the number of inter-digital transducer finger pairs,the number of reflector finger pairs and the matching circuit on the impedance load performance were further simulated,which laid a theoretical foundation for the optimal design of the soil moisture sensor.In terms of sensor design,based on the impedance load principle of SAW and the corresponding relationship between soil dielectric constant and soil moisture,a hybrid sensor was constructed by using a moisture sensitive probe as a sensing element and a SAW device as a wireless interface.For the design of the SAW devices,the reference reflector was setted up to eliminate the influence of measurement distance,and the bidirectional structure was adopted to improve the wireless measurement distance.Meanwhile,the collision problem of echo signals was avoided by the design scheme of time division multiple access,and the simultaneous measurement of soil moisture at different depth levels of the same surface position was realized.For the design of the sensing element,the moisture sensitive probe was designed by the means of covering copper on the printed circuit board,and the structural parameters of the probe were optimized by using ANSYS software.Meanwhile,the effective sensitive soil range of the probe was also analyzed.In terms of the design of the measurement system,the following measurement scheme was proposed: the sensors are arranged in the field according to certain topological rule,and the sensors are quickly queried by UAV carrying reader and reader antenna,so as to obtain soil moisture data.Firstly,the reader was designed,manufactured and tested,and a soil moisture query platform based on UAV was built,including reader,reader antenna,UAV and ground station.Later,the secondary developments of UAV and ground station were carried out,then the serial port communication between UAV and reader,and the Mavlink communication between UAV and ground station were realized.Finally,the measurement system was tested on ground and UAV,and realized the simultaneous measurement of soil moisture at two depth levels,and eliminated the influence of measurement distance effectively.The wireless measurement distance of the system can reach 2.3 meters,and the accuracy of soil moisture ground measurement is ±1.5%.The UAV carrying with a reader and an antenna patroled all sensor nodes successfully,and the maximum error is 2.58%.