Retro-reflective Tracking And Focusing Technique For Microwave Power Transmission

In recent years,with the abundant appearance of mobile / portable electronic devices,how to provide power for these devices conveniently and efficiently becomes a hot research topic.As an alternative to the traditional power supply,the wireless power transmission technique gains more and more academic and industrial attention.Microwave power transmission delivers power via radiative electromagnetic waves,which makes it one of the promising solutions to wireless charging in the mid-to-long range.Nevertheless,there are still several technical challenges in realistic applications of this technique,including transmission efficiency,electromagnetic interference and tracking of mobile targets.Aiming at these challenges,this paper investigates a wireless power transmission scheme based on retro-reflective focusing.A deep analysis of the performance of retro-reflective arrays in focusing electromagnetic waves in the near and far field is given.An experimental scheme of tracking and focusing electromagnetic power based on pilot signal analysis is developed.Feasibility and performance of the retro-reflective focusing scheme in microwave power transmission are verified by experimental results.Specifically,the work accomplished includes the follows:(1)The far-field radiation characteristics between retro-directive array and retro-reflective focusing array is analyzed and calculated by numerical simulation.It is observed that,different from the retro-directive array,the retro-reflective array features “focusing in the near-zone without high gain in the far-zone”.To reduce the maximum far-zone gain of retro-reflective focusing array without affecting the near-zone focusing performance,irregular array arrangements(“V” shaped and perturbed)is proposed and their performance is verified by simulation and experimental results.(2)A system design of microwave power transmission system based on retro-reflective focusing technology is presented.In this design,the transmitter aperture is divided into multiple sub-arrays.Phase shifters are employed and controlled based on the pilot signal analysis,which provide the needed conjugate phases in feeding the antenna array.Different frequencies are adopted by the power carrier and the pilot signal so that they can exist simultaneously.Conjugate-phase feeding is achieved by proper scaling the phase information of the pilot signals.A heterodyne receiving scheme is used to reduce the cost of retrieving the phase information from pilot signals.(3)Based on the above system design,a microwave power transmission experimental system at 5.8GHz is implemented.The system includes two power transmitting and pilot signal receiving channels.Each receiving and transmitting channel includes a dual-channel planar-array antenna with high far-zone gain,a pilot signal analyzing circuit at the frequency of 2.9GHz,a power amplifying and phase-shifting control circuit at the frequency of 5.8GHz.There are 8×8 elements in the array where the center element is pilot-signal receiving antenna with the frequency of 2.9GHz and the other are power-transmission antenna array with the frequency of 5.8GHz.(4)The feasibility of tracking and powering mobile target based on analyzing polite signal is verified through the comparison of three groups of experiments.Experimental demonstration of the wireless powering to a mobile device 3.5m away is successfully accomplished with the accurate tracking capability.