| The application of sensor networks is the main approach to agricultural informatization,but the power supply of network nodes has gradually become one of the factors restricting the continuous expansion of network scale.Different from the traditional battery wired power supply,stable wireless power transmission has great advantages in the workplace and transmission characteristics,and it is also one of the ways to solve the long-term power supply problem of wireless sensor network nodes.At present,the magnetically coupled resonant wireless power transmission technology has the characteristics of long transmission distance,high efficiency,safety and reliability,which can provide an effective solution for the stable power supply of soil sensor network nodes.Aiming at the problem of stable power supply of soil sensor in soil sensor network,this thesis first analyzes the power demand of soil sensor,and puts forward the overall requirements of power wireless transmission system design.Then,with the help of COMSOL Multiphysics finite element simulation software,the simulation models of double coil and three coil wireless power transmission system are established to study the variation characteristics of coil mutual inductance,transmission distance and offset distance.Secondly,a two-coil wireless power transfer（WPT）system compensation network directly powered by soil sensors is designed.Based on the coupled inductance model,the transformer T-network model is established and optimized,and the equivalent transformation ratio n1 and the equivalent coupling coefficient kr are defined.A new design method of compensation network for two-coil WPT system with variable gain and constant voltage characteristics is proposed.Considering the direct influence of the parasitic resistance of the system on the transmission characteristics of the system,the voltage gain expression and transmission efficiency expression under coil offset and frequency oscillation are derived.The variation of voltage gain stability and transmission efficiency of WPT system with different equivalent parameters is analyzed,and the optimal equivalent parameter expression under coil offset and optimal transmission efficiency is obtained.Then,the compensation network of three-coil WPT system indirectly powered by soil sensor is designed.The equivalent circuits of two types of three-coil WPT system:one with relay coil and another with dual transmitter and single receiver are established.The expression of compensation network parameters of three-coil WPT system with target gain constant current and constant voltage characteristics is given.The relay coil placement of three-coil WPT system with constant current characteristics under optimal transmission efficiency is obtained.The transmission efficiency of WPT system under constant voltage/constant current and the voltage gain stability of two three-coil WPT systems under constant voltage are compared.Finally,according to the power demand of soil moisture sensor,the experimental platform of two-coil and three-coil power wireless transmission system is built to verify the correctness and effectiveness of the proposed compensation network design method and transmission characteristic analysis conclusion of two-coil/three-coil WPT system.The experimental results show that for the magnetically coupled resonant energy transmission system composed of planar spiral coils with an average diameter of 19.08 mm,it can ensure that the two-coil WPT system reaches stable 5.7V output voltage when the transmission distance is20 mm.When the transmission distance is 40/44mm,the three-coil WPT system can output 5.6V under constant voltage and 0.27A under constant current,which can meet the direct power supply and indirect power supply of the soil sensor. |
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