Improvement of output stability in magnetic resonance wire-less power supply using Helmholtz type transmission coil

Wireless power transmission can be applied to a wide range of fields requiring electric power from smartphones to electric cars and is expected as a next generation innovative technology. Currently, the mainstream of practical applications is the electromagnetic induction method, and it is used as a power feeding to multifunctional small electronic devices typified by mobile terminals. However, this method is limited to charging in a very close state, and has a disadvantage that it is vulnerable to displacement between transmission and reception coils. In order to solve those problems, magnetic field resonance method was announced from MIT in 2006
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. In this system, high efficiency transmission over long distances is enabled, and since the influence of positional displacement is relative small, it is possible to construct a charging system with high usability. Our laboratory aims at establishment of a local high usability power supply area finally. Specifically, it is the construction of the “unnoticed charging area” in which the transmitting coil is embedded in the floor or wall, and the user's device is unconsciously charged by the receiving coil mounted on the device automatically. However, in order to maintain high efficiency in the resonance method, it is necessary to change the optimum driving frequency with respect to the transmission / reception distance. For that change, accurate grasp of the distance and frequency changing function are required for power supply equipment, so the system becomes complicated. For the above reasons, we tried to realize a stable power supply by using a Helmholtz type coil which can obtain a uniform magnetic field between the coils on the transmitting side. As a result, although it was inferior to the conventional type in terms of maximum efficiency, we succeeded in maintaining constant efficiency between the coils.