A system (hereinafter, referred to as a “wireless power feeding system”) using wireless power transmission in which power is supplied to an electric apparatus in a wireless manner without using a power cord or the like is being put into practical use. For example, an electromagnetic induction type wireless power feeding system using electromagnetic induction between antennae (coils) which are disposed to be separated from each other, or a magnetic resonance type wireless power feeding system using electromagnetic field resonance coupling is known. In addition, near field communication (NFC) is known as a standard regarding a wireless communication technique for transmitting information in a wireless manner, and IC cards or small portable terminal apparatuses conforming to the NFC standard also start to being widespread.
The magnetic resonance type wireless power feeding system is implemented by using a resonance circuit including coils and capacitors. The magnetic resonance type wireless power feeding system can have a longer transmission distance between a power transmission coil and a power reception coil than in an electromagnetic induction type of the related art by increasing a Q value of the coil, and has a feature of being strong to positional deviation of the power transmission coil and the power reception coil.
In addition, the magnetic resonance type wireless power feeding system has a feature of being hardly influenced by a foreign substance present between a power transmission side and a power reception side. However, since an amount of transmission power absorbed by a foreign substance is increased depending on the kind of foreign substance, power transmission efficiency is reduced, and there is also a concern that the foreign substance may be damaged due to heat generation caused by the power absorbed by the foreign substance.
The following PTLs 1 and 2 disclose techniques for detecting a foreign substance or an abnormality in a power feeding target in the magnetic resonance type wireless power feeding system.
For example, PTL 1 discloses a technique for detecting a foreign substance present between a power transmission side and a power reception side in a magnetic resonance type wireless power feeding system in which a power transmission coil is provided on a road, and a power reception coil is mounted on a vehicle, so that the vehicle is charged in a wireless manner. Specifically, according to the configuration disclosed in PTL 1, an impedance value is estimated by detecting a terminal voltage of a battery which is mounted in the vehicle and is charged on the basis of received power, and a notification of the presence of a foreign substance is sent and power feeding is also stopped in a case where a difference between the estimated value and an actually measured impedance value exceeds a threshold value.
In addition, PTL 2 discloses a technique for detecting an abnormality in a vehicle by detecting an increase in reflected power in a magnetic resonance type wireless power feeding system in which a power transmission coil is provided on a road, and a power reception coil is mounted on the vehicle, so that the vehicle is charged in a wireless manner. Specifically, according to the configuration disclosed in PTL 2, if a suspicious person gets in the vehicle which is being charged, impedance mismatching occurs due to a change in a vehicle height, thus reflected power increases, and a notification of an abnormality in the vehicle is sent when the increase in the reflected power is detected.