Research On Mid-Range Wireless Power Transfer In The Near-Field Region

The mid-range wireless power transfer(WPT)in near-field region is investigated in this paper.According to the comparison between transfer distance and coils' size,the WPT can be grouped into three categories: short-range transmission,namely transfer distance is far less than the coils' s size;long-range transmission,namely transfer distance is far more than the coils' s size;and mid-range transmission,namely transfer distance is several times greater than the coils' s size.Comparing with short-range and far-range WPT transfer,many problems remain to be done on the technologies of mid-range transfer,therefore,mid-range WPT is becoming a new hotspot in academia.Two technological ways,resonant coupling tansminssion in reactive near-field region(Re NFR)and high radiated efficiency antennas transmission in radiating near-field region(Ra NFR),for efficient mid-range WPT are investigated in this paper.The first way involving three aspects,analysis and comparing study in theory,efficient ways to extend the transfer distance,and implementation methodologies for predetermined-goals power transmission,are the main content in this Ph D thesis.The second way focus on designing electrically small antenna with high radiated efficiency and oveall efficiency,and investigating the transfer characteristics of the WPT system comprised of the proposed antennas.(1)The main research and innovations of mid-range WPT based on resonant coupling tansminssion way in Re NFR are summarized as follows:(a)The analysis and comparing study in theory: fist,the two theories for Re NFR coupling,coupled-mode theory(CMT)and circuit theory(CT),are comparatively studied;the two theories are equivalent when the WPT system reaches a steady state;second,the characteristics of inductive coupled method for shor-range transmission and resonant coupled method for mid-range transmission are analyzed based on CT,the conclusion is that theoretical basis of the two method are the same,while the manifestations of them are different;finally,WPT system is analyzed comparatively between Kirchhoff's voltage law(KVL)and band-pass filter theory(BPFT),it can be concluded that two constraints for WPT must exist according to BPFT,and analysis based on BPFT is a specialized case of that based on KVL,i.e.source-loaded Q-factor of sending coil equals load-loaded quality factor of receiving coil.However,comparing the analysis based KVL,the analysis based on BPFT is simpler due to the two constraints.(b)Efficient ways to extend the transfer distance: the efficient coupled mode region can be enlarged in 4-coil WPT system by reducing the coupled strength between feeding/load loop and sending/receiving coil,and then using frequency tracking method in the coupled region,the high power transfer efficiency(PTE)can be achieved over far greater distances;inserting relay coils between sending and receiving coil is the other approach to extend the energy transfer distance,generalised mistuning factor is introduced creatively for precisely analyzing the frequency splitting situation of multi-relay-coil system,especially of inserting one relay coil system.(c)Implementation methodologies for predetermined-goals power transmission: the charging voltage of a certain electric equipment,corresponding to power delivered to load(PDL)for the certain appliance,is a definite value,on the other hand,the PTE need to maximize in WPT system,therefore,a predetermined amount of PDL transfer at maximum PTE is one of the goals of this paper.Using 4-coil WPT system,the method of maximizing PTE using load-matching loop and controlling PDL using source-matching loop is first presented in this paper.The propsed approach,comparing with the method for controlling PDL by adding peripheral regulation circuits,greatly reduces the system complexity and power consumption introduced by the regulation circuits.In relay-coil system,in order to maintain the transfer distance unchanged,the coupled strength between relay coil and sending/receiving coil is adjusted by controlling the angle of sending/receiving coil,and then the goals of maximizing PTE or PDL can be obtained via combining basic algorithms in numerical software and the constructed optimal objective function.However,the PDL is low when optimal goal of maximizing PTE is achieved,and while the PTE is low when optimal goal of maximizing PDL is achieved.In order to obtain relatively large values of PTE and PDL simultaneously,the tradeoff optimisation method is proposed.The ratio of PTE and PDL can be controlled via regulating the the weighting factor.(2)The main research and innovations of mid-range WPT based on high radiated efficiency antennas transmission in Ra NFR are summarized as follows:The conclusion that the PTE of WPT system in Ra NFR is proportional to the radiated efficiencies of sending and receiving antennas is deduced by J.Lee via spherical mode theory(SMT),therefore the design of high efficienct electrically small planar antenna plays the key role for implementing such type of mid-range WPT.In this paper,The metamaterial-based efficient electrically small antennas theory that spherical shells of homogenous,isotropic negative permittivity(ENG)material surrounds a tranditional electrically small dipole to create an electrically small resonant antenna system,is used to design an approximate 50 ? input impedance and high efficient electrically small planar antenna for WPT application.The bending conductor strip as the inductive element plays the role of ENG in theory.The WPT system composed of the designed antenna with high radiated efficiency can obtain higher PTE in Ra NFR.