Research On Systems And Mechanisms Of Wireless Power Transfer Based On Nonlinear Controlled Voltage Source And Nonlinear Resonators

At present,magnetically coupled wireless power transfer systems are mainly based on linear LC circuits comprising a linear inductor and a linear capacitor.The linear LC circuit on the transmitting side is coupled with the linear LC circuit on the receiving side through the magnetic field to realize power transfer.The essence of system dynamics is the forced oscillation of coupled two-degree-of-freedom linear resonators.In fact,the LC circuit can also be composed of a nonlinear inductor and a nonlinear capacitor,and the LC circuit on the transmitting side can be powered by a controlled source as well.One typical example is the parity-time(PT)symmetric wireless power transfer proposed in recent years.By using a negative resistance controlled by the voltage across the capacitor on the transmitting side,the PT-symmetric wireless power transfer system can achieve constant output power and transfer efficiency within a certain range of transfer distance.Therefore,the problem is addressed to a certain extent that the transfer efficiency and output power change dramatically against alteration of transfer distance in existing magnetically coupled wireless power transfer systems.Compared to linear circuits,nonlinear circuits or systems have unique oscillation behaviors and very rich highly-efficient coupling mechanisms.For example,a nonlinear LC circuit forms a nonlinear resonator,whose natural frequency is changed with the variation of oscillating amplitude,and the internal resonance phenomenon occurs in nonlinear coupled systems.These shed new light on the further development of the magnetically coupled wireless power transfer technology.By introducing nonlinear components into existing magnetically coupled wireless power transfer systems,this thesis attempts to construct magnetically coupled wireless power transfer systems based on nonlinear circuits,explore energy transfer mechanisms in coupled nonlinear resonators,and then develop novel nonlinear wireless power transfer mechanisms.It is expected to break through the existing wireless power transfer mechanism based on linear circuits,improve the performance of wireless power transfer,and promote the practical versatility of magnetic coupling wireless power transfer systems.This thesis specifically studies magnetically coupled wireless power transfer systems based on the nonlinear controlled voltage source and the nonlinear resonator.The following work has been carried out:(1)The energy transfer characteristics between coupled two-degree-of-freedom nonlinear resonators without energy exchange with their surroundings are studied,and thus complete energy transfer conditions for highly-efficient coupling between nonlinear resonators are investigated.After a nonlinear voltage source controlled by the current across the resonator is introduced into the highly-efficient coupled resonators,the influence of the nonlinear controlled voltage source on the energy transfer between the transmitting and receiving resonators is analyzed based on the coupled-mode theory,thereby revealing that the nonlinear controlled voltage source can make the system automatically track the eigen-oscillation at the eigenfrequency.Besides,by replacing the linear resonator with a nonlinear resonator composed of a nonlinear capacitor and a linear inductor,whose natural frequency is related to the energy stored in the resonator,the effect of the nonlinear resonator on the energy transfer is further investigated.(2)A power electronic converter is used to construct the nonlinear controlled voltage source,and then magnetically coupled wireless power transfer system based on the nonlinear controlled voltage source is implemented.Using the coupled-mode theory,the mathematical model of the proposed system is derived.The relationship between the transfer efficiency as well as the output power and the parameters of the system is studied,and simulations and experiments are carried out.The theoretical and experimental results show that,compared with the existing parity-time symmetric wireless power transfer system,the proposed system can not only maintain constant transfer efficiency and output power within a certain transfer distance,but also overcome the limitation of transferred power due to saturated voltage of the negative resistance,and thus increasing the output power level from milliwatts to practical power level.Because the nonlinear controlled voltage source is implemented by a power electronic converter,the capacity of the proposed system depends on the power electronic converter.Therefore,the system can also be promoted to medium and high power applications.(3)A magnetically coupled wireless power transfer system based on the nonlinear resonator is constructed,and the resonance condition of maximum output power is derived based on the coupled-mode model.It is found that different from the magnetically coupled resonant wireless power transfer system based on linear resonators,the resonance condition of the proposed system depends on the energy stored in the resonator.As a result,narrow bandwidth for linear resonators of high-quality factors can be alleviated,and then the output power will not be changed dramatically with the increase of frequency detuning.Meanwhile,the frequency characteristic of the output power has a completely different constraint relationship with system parameters,thereby suppressing the frequency splitting phenomenon of the output power.(4)A magnetically coupled wireless power transfer system based on both a nonlinear controlled voltage source and a nonlinear resonator only on the transmitting side is constructed.The coupled-mode model of the system is established,and then the effects of coupling variations,frequency shifting variations,and power supply voltage variations on transfer performance are analyzed.It is found that the internal resonance mechanism enables the system to exhibit the amplitude saturation phenomenon in the transmitting resonator,which offsets the adverse effects of coupling changes and frequency shifting changes on the transfer characteristics.At the same time,a nonlinear capacitor construction method is proposed,and experiments further verify that the system can achieve stable output power and transfer efficiency within a range of transfer distance despite a certain shifting of the resonant frequency.(5)A magnetically coupled wireless power transfer system based on a nonlinear controlled voltage source and nonlinear resonators on both the transmitting and receiving sides is constructed.The coupled-mode model of the proposed system is established,and its transfer performance is analyzed under both resonance matching and resonance unmatching.The study shows that,compared to the wireless power transfer system based on an only nonlinear controlled voltage source,the proposed system maintains constant power transfer within a certain range of transfer distance as well as a greater tolerance for shifting of the linear resonant frequency,which enables the transfer performance not to change sharply as the frequency shifting increases.