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Wireless power feeding was examined with strongly coupled magnetic resonance for an object moving in 3-D space. Electric power was transmitted from the ground to an electrically powered toy helicopter in the air. A lightweight receiver resonator was developed using copper foil. High Q of greater than 200 was obtained. One-side impedance matching the transmitter side was proposed to cope with high transmission efficiency and the receiver’s weight reduction. Results show that the efficiency drop near the ground was drastically improved. Moreover, the measured efficiency showed good agreement with theoretical predictions. A fully equipped helicopter of 6.56 g weight was lifted up with source power of about 5 W to an altitude of approximately 10 cm.

Magnetic resonance power feeding, a unique wireless power transmission technology, is now in demand in various fields. In 2007 and 2008, an MIT group reported wireless power transmission theory based on optics and photonic crystal theories, explaining it as a phenomenon caused by near-field evanescent waves [1,2]. One feature of this technology is its high transmission efficiency at meter-order distance [

A formula for the transmission efficiency can be derived from electric circuit theory [

A remarkable feature of wireless power transmission with strongly coupled magnetic resonance is its effectiveness at mid-ranges, which is several times greater than the resonator diameter. This feature enables wireless power feeding to a mobile object moving freely in a three-dimensional space. This report describes a powerfeeding demonstration to an electrically powered helicopter. The objective is development of an efficient, compact, and lightweight resonator, with validation of the impedance-matching theory through the demonstration.

Impedance matching, adjustment of the impedance ratio, is conducted in antenna tuners using variable capacitor units and inductive transformers to maintain high transmission efficiency. In the helicopter application, the coupling coefficient k varies dynamically because of the helicopter’s altitude change; both very low Ohm loss and a wide range of impedance transformation are necessary for strongly coupled magnetic resonance.

Considering the power transmission from a resonator with quality factor Q_{S}, impedance Z_{0}, and resonance frequency ω_{0 }to_{ }another resonator with Q_{D}, Z_{0}, and ω_{0} at the AC frequency of ω, then the transmission efficiency can be derived using Kirchhoff’s second law as shown below [

Therein, r_{S} and r_{D} respectively represent ratios of the source’s and device’s impedance Z_{0S} and Z_{0D} to the resonator resistance R_{S} and R_{D}, defined as

and

In the ω-r_{S}-r_{D} domain, η reaches its maximum value under conditions of

and

Then, maximum efficiency is expressed as

A typical resonant coupling system with input and output inductive transformers is presented in _{S}_{ }and r_{D} are adjustable by changing their respective coupling coefficients k_{S} and k_{D}.

_{S}Z_{0S}/R_{S}. The device impedance ratio is transformed to k_{D}Z_{0D}/R_{D}.

One-side impedance matching is one means to simplify the receiver device. The transmitter takes the optimum impedance ratio, although the receiver impedance ratio is not controlled. The theoretical efficiency of oneside control η_{1} is expressed as [

The theoretical transmission efficiencies indicated in Equations (1), (5), and (6) are depicted in

Q_{S} = Q_{D} = 200. When the impedance ratio is matched, then the transmission efficiency is improved, especially at a short transmission distance.

A high Q, compact, and lightweight receiver resonator is necessary to make a helicopter fly without a battery. For this study, a resonator was fabricated consisting of a rectangular loop and a mica condenser. It was composed of a copper pipe with 4 mm outer diameter to reduce its weight. The loop side length and the mica condenser capacitance were selected for the resonator to have a resonance frequency exactly equal to 40.68 MHz, which is the power source AC frequency.