Patent ID: 8662624

Claim:
A foreign object detection device comprising: a signal generator configured to generate an electrical signal; a transmitter configured to convert the electrical signal from the signal generator and to generate a pressure wave in a liquid; a receiver configured to receive a reflection wave from the liquid and to convert it to an electrical signal; a storage section configured to store information on a state of absence of a foreign object in the liquid; and a detection section configured to detect at least one of presence or absence of the foreign object and size of the foreign object based on difference between the electrical signal converted from the reflection wave and an electrical signal based on the stored information, wherein the signal generator generates the electrical signal having a frequency satisfying an equation given by f ≈ 1 2 ⁢ π ⁢ ⁢ Ro ⁢ 3 ⁢ γ ⁢ ⁢ Po ρ ⁢ where f is the frequency, Ro is maximum bubble size, Po is equilibrium pressure of the liquid, ρ is density of the liquid, and γ is specific heat ratio.