Wireless electric power transmission apparatus

1. A wireless electric power transmission apparatus comprising:

two antennas having the ability to transmit electric power by a non-contact method via resonant magnetic coupling, one of the two antennas being a series resonant circuit, of which the resonant frequency is fs, the other antenna being a parallel resonant circuit, of which the resonant frequency is fp;

an oscillator which is connected to one of the two antennas that transmits RF power; and

a control section which controls a transmission frequency according to the magnitude of the electric power to be transmitted from one of the two antennas to the other,

wherein fs/fp is set to be a value that is less than one.

2. The wireless electric power transmission apparatus of claim 1 , wherein the control section changes a relation between the transmission frequency and the electric power according to a standard value that depends on a coupling coefficient k between the two antennas and fs/fp.

3. The wireless electric power transmission apparatus of claim 2 , wherein in a situation where the fs/fp value is less than the standard value, the control section sets the transmission frequency to be a value that falls within a first frequency range if the electric power is smaller than a reference value P 1 but sets the transmission frequency to be a value that falls within a second frequency range, which is lower than the first frequency range, if the electric power is greater than the reference value P 1 .

4. The wireless electric power transmission apparatus of claim 2 , wherein in a situation where the fs/fp value is greater than the standard value, the control section sets the transmission frequency to be a value that fails within a first frequency range if the electric power is greater than a reference value P 1 but sets the transmission frequency to be a value that fails within a second frequency range, which is lower than the first frequency range, if the electric power is smaller than the reference value P 1 .

5. The wireless electric power transmission apparatus of claim 1 , wherein in a situation where the fs/fp value is greater than a standard value that depends on the coupling coefficient k, the control section sets the transmission frequency to be a value that falls within a first frequency range if the electric power is greater than a reference value P 1 but sets the transmission frequency to be a value that falls within a second frequency range, which is lower than the first frequency range, if the electric power is smaller than the reference value P 1 , and

wherein in a situation where the fs/fp value is less than the standard value, the control section sets the transmission frequency to be a value that falls within a first frequency range that is lower than the frequency fp if the electric power is greater than the reference value P 1 but sets the transmission frequency to be a value that falls within a second frequency range, which is higher than the first frequency range, if the electric power is smaller than the reference value P 1 .

6. The wireless electric power transmission apparatus of claim 5 , wherein if the fs/fp value agrees with the standard value that depends on the coupling coefficient k, the control section keeps the transmission frequency constant irrespective of the electric power to transmit.

7. The wireless electric power transmission apparatus of claim 2 , wherein the standard value is a monotonically decreasing function of the coupling coefficient k.

8. The wireless electric power transmission apparatus of claim 7 , wherein the standard value is less than one and its magnitude is between −0.7309×k 2 −0.0269×k+0.9862 and 0.08×k 2 −0.357×k+1.0944.

9. The wireless electric power transmission apparatus of claim 1 , wherein the control section measures transmission efficiency by transmitting electric power wirelessly with the transmission frequency changed in multiple different control patterns, thereby determining a transmission frequency control pattern that would maximize the transmission efficiency.

10. The wireless electric power transmission apparatus of claim 1 , wherein the control section estimates whether the transmission efficiency increases or decreases as the transmission frequency rises or falls with respect to at least one value of the electric power to transmit and determines a transmission frequency control pattern based on the result of the estimation.

11. The wireless electric power transmission apparatus of claim 1 , wherein the two antennas have a resonant frequency adjustment function to change at least one of fs and fp, and

wherein the control section changes the fs/fp value after the wireless electric power transmission apparatus has been installed.

12. The wireless electric power transmission apparatus of claim 11 , wherein after the wireless electric power transmission apparatus has been installed, the control section transmits electric power wirelessly with fs/fp set to be multiple different values, measures transmission efficiency, and then determines an fs/fp value for a normal operation based on the transmission efficiency.

13. A power transmitting device for use in the wireless electric power transmission apparatus of claim 1 , comprising:

one of the two antennas configured to transmit RF power; and

an oscillator that is connected to the antenna.

14. A power receiving device for use in the wireless electric power transmission apparatus of claim 1 , comprising one of the two antennas configured to receive RF power.

15. A controller for use in the wireless electric power transmission apparatus of claim 1 , configured to control the resonant frequency according to the magnitude of the electric power to be transmitted from one of the two antennas to the other.