Patent ID: 8890391

Claim:
An external force detection apparatus that detects an external force acting on a piezoelectric plate, the external force detection apparatus comprising: the piezoelectric plate that is cantilevered at one end thereof on a base within a container; a first excitation electrode and a second excitation electrode provided respectively on a first surface side and a second surface side of the piezoelectric plate in order to vibrate the piezoelectric plate; an oscillating circuit that is electrically connected to the first excitation electrode; a movable electrode for forming a variable capacitor that is provided on the other end of the piezoelectric plate and electrically connected to the second excitation electrode, a fixed electrode that is separated from the piezoelectric plate, provided so as to oppose the movable electrode, and connected to the oscillating circuit, the fixed electrode forming the variable capacitor upon a change in capacitance between the fixed electrode and the movable electrode due to bending of the piezoelectric plate; and a frequency information detecting unit for detecting a signal that is frequency information corresponding to an oscillating frequency of the oscillating circuit, wherein in the piezoelectric plate, a thickness of an area between an area sandwiched by the first and the second excitation electrodes and the movable electrode is less than a thickness of the area sandwiched by the first and the second excitation electrodes, an oscillation loop is formed beginning from the oscillating circuit, passing through the first excitation electrode, the second excitation electrode, the movable electrode, the fixed electrode, and then returning to the oscillating circuit, and the frequency information detected by the frequency information detecting unit is for evaluating the external force acting on the piezoelectric plate.