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Wireless power transfer (WPT) from a transmitter resonator on the ground to an electrically powered miniature heli-copter was attempted to demonstrate WPT using magnetic resonance coupling to an object moving in 3D space. The transmission efficiency was optimized by automatic impedance matching for different flight attitudes: a maximum flight altitude of 590 mm was achieved. Furthermore, an estimation method of transmission efficiency using only the properties on the transmitter side was proposed, with transmission power regulated as constant against the change in the coupling coefficient.

Since wireless power transfer (WPT) was proposed by Nikola Tesla, WPT applications have increasingly attracted interest. Types of WPT are categorized as radiative and non-radiative forms, such as microwaves, lasers, electromagnetic induction, and magnetic resonance coupling (MRC). Earlier research shows that WPT via MRC has the considerably higher transmission efficiency than microwave transmission at the same distance as the transmission coil diameter. In 2007, an MIT group demonstrated WiTricity, the first MRC system, which conducted 1 m distance transmission with 90% efficiency [

To develop WPT technology that is applicable to mobile devices, it is first necessary to manufacture a small, light, and highly efficient receiver for use with the transmission system. Furthermore, automatic impedance- matching and load-power control systems are needed because the relative positions of the transmitter and receiver coils can change. For an impedance matching technique, some researchers have used a circuit with a tuning capacitor and inductor, controlling several condensers in the circuit [

For our previous study, a small helicopter with a light-receiving resonator (3.8 g) was developed using a resonance frequency ω of 40.68 MHz [

For this study, a flight attitude of 590 mm was achieved by optimizing the transmitter designing. In addition, a method for estimating the transmission efficiency was proposed to maintain the transmission power as constant against the change in the coupling coefficient.

To achieve a higher flight altitude than that reported from an earlier study, the transmission system was redesigned. An open-source software package of moment method, 4NEC2, was used to estimate the coupling coefficient between the coils [_{c} and wavelength λ is expected to satisfy λ/l_{c} < 0.15. However, it is difficult to satisfy this relation at ω = 40.68 MHz. Therefore, ω was changed to 13.56 MHz. The transmitter resonator was enlarged to 600 mm diameter while the receiver size remained the same. The method of one-side impedance matching on the transmitter side was the same as that used in our previous study [_{R} of a circular resonator is the same as that of 124 mm rectangular resonator. Because the voltage in the excitation coil and the resonator increases rapidly with the transmission distance, open-type coils were used as the excitation coil and the transmitter resonators. D_{T} is the transmitter resonator diameter, which is an open spiral coil like that shown in _{T} was set to about 5 W, which is 25% of the nor-

. Specifications of the resonators.

TYPE | Transmission | Receive |
---|---|---|

Resonance frequency, MHz | 13.561 | 13.561 |

Size, mm | D_{T} = 600 | 110 × 110 D_{R} = 62.124 |

Number of turns | 2.25 | 1 |

Quality factor | 197 | 313 |

Capacitance of a capacitor, pF | Open | 430 |

Weight, g | - | 3.11 |

Diameter of coils, mm | 10 | 3 |

Wireless power transfer system for the small helicopter

mal rated power of the helicopter’s motor. The nominal transmission distance l was set at 550 mm, which is twice as large as the average resonator diameter

The optimum impedance ratio r_{T} is a function of Q_{T}, Q_{R}, r_{R}, and the coupling-coefficient k as

where Q is a quality factor defined as Q = ωL/R. Subscripts T and R respectively denote the transmitter side and receiver side. Impedance ratios r_{T} and r_{R} are defined respectively as r_{T} = Z_{src}/R_{T} and r_{T} = Z_{load}/R_{R}. Subscripts src and the load respectively denote the power source and external load. At l = 0, k approaches unity and r_{T,opt} increases to 37. In preliminary testing, the impedance ratio of transmission side r_{T} was measured to confirm that the excitation coil can satisfy Equation (2). r_{T} is tunable by adjusting the coupling coefficient between the excitation coil and the transmitter resonator k_{ET}, as [

Subscript E denotes the excitation coil. To design the excitation coil, the voltage and the current between the excitation coil and the transmitter resonator were analyzed using LTspice [_{T} = 5 W. In this study, the receiver impedance was matched at l = 500 mm and fixed. Here, k_{ET} was measured in the same manner as k. _{T} satisfied the relation of Equation (2) within the movable range of the coil. Finally,

Measured r_{T} on the transmitter side as a function of distance between the centers of each coil

Measured and computed η with and without impedance matching as a function of dimensionless transmission-distance l'

. Specifications of the pick-up and excitation coil.

Type of coil | Pick-up | Excitation |
---|---|---|

Resonance frequency, MHz | 13.561 | 13.561 |

Size on a side, mm | 50 × 50 | 150 ×300 |

Diameter of wire, mm | 3.0 | 1.0 |

Capacitance of a capacitor, pF | 1100 | Open |

Material | Copper | Copper |

Number of turns | 1 | 5 |

Unloaded Q | - | 500 |

Resistance, Ω | - | 1.25 |

measured and computed power transmission efficiency η with and without impedance matching as a function of l'. Measured values showed good agreement with the computed values. In addition, the present transmission system achieved high transmission efficiency in the low-altitude region up to 45% by appropriate impedance matching.

Without impedance matching, η has a peak around the designed nominal altitude. It is difficult for the helicopter to hover at a certain altitude where η increased with l'. In this sense, impedance matching is unavoidable for the helicopter to conduct a safe liftoff and landing. r_{T,opt} was obtained by minimizing the reflection power P_{R}. This control method is extremely simple.

A small electric helicopter with the receiver system was used in the same manner as in our previous study. As

Automatic impedance-matching system for a flight demonstration

Flow of the automatic-control system

(PWM) signal is sent from the micro-computer to the actuator via a motor-driver circuit. An RF power source (T161-5613 HA; Thamway, Corp.) with maximum power of 400 W and RF frequency of 13.56 MHz was used. It enables us to monitor the input and the reflection as well as external control of the output power.

As _{T} = 8 W and P_{R} = 3 W. The excitation coil position was out of control. When impedance matching started at 10 s, P_{R} was 0.2 W.

When r_{R} is given, η can be estimated using parameters on the transmitter side. Considering the energy losses in excitation and pickup coils, k is expressed as

Automatic impedance-matching system for a flight demonstration

History of reflection power. At t = 10 s impedance matching started

where

light bulb. k is expressed with

Substituting Equation (5) into Equation (4), η yields

Actually, η_{load} is usually approximately unity. Also η_{src} and r_{T} are given. Consequently, η can be estimated by monitoring and r_{T} and S_{11}.

_{T} and R_{R} were pre-optimized at l = 300 mm. The mea- sured impedance ratio r_{R} was 9.0.

Constant power-feeding demonstration system

Pick-up coil with the rectifier circuit and the light bulb

As _{11} during the demonstration. Estimated η agreed well with theoretical η within 10% error.

. Specifications of at the pick-up coil and a resonator.

Type | Pick-up coil | Resonator |
---|---|---|

Resonance frequency, MHz | 13.561 | 13.561 |

Diameter, mm | 1000 | 1250 |

Diameter of wire, mm | 0.5 | 1.0 |

Capacitance, pF | - | 200 |

Material | Copper | Copper |

Number of turns | 3 | 1 |

Q-factor | - | 356 |

Automatic impedance matching with WPT to a small electric powered helicopter was proposed and demonstrated. Results show that the power transmission efficiency was improved up to 45% in the high k region. The flight altitude of 590 mm was achieved with the average resonator diameter

Furthermore, the transmission efficiency estimation method was proposed using only the properties on the transmitter side. By sliding a light bulb from 0 mm to 600 mm distant from the transmitter resonator, a constant power supply of the power of 10 W was demonstrated.