A capacitance type acceleration sensor according to a prior art is disclosed in U.S. Pat. No. 6,151,966. The sensor includes a movable electrode and a fixed electrode, each of which has a plurality of teeth. The movable electrode faces the fixed electrode so that they form a capacitor. When acceleration is applied to the sensor, a movable portion having a weight portion and the movable electrode in the sensor is displaced. Then, the capacitance of the capacitor changes in accordance with the displacement of the movable electrode. This capacitance change is measured so that the acceleration is detected. Here, the movable electrode is integrated with the weight portion, and the displacement direction of the movable electrode is perpendicular to a facing surface of the movable electrode. The facing surface of the movable electrode faces the fixed electrode. When the movable electrode moves toward the fixed electrode and the distance between the movable and fixed electrodes becomes small, the capacitance becomes larger. When the distance between the movable and fixed electrodes becomes large, the capacitance becomes smaller.
When the movable electrode is displaced, a squeeze damping effect is generated by viscosity of air disposed between the movable electrode and the fixed electrode. Particularly, the squeeze damping works between the facing surfaces of the movable and fixed electrodes. Therefore, when the sensor detects the acceleration near a resonant frequency of the movable portion, the displacement of the movable electrode and the capacitance change in accordance with the displacement become small, so that fine acceleration may not be detected. Namely, the sensor sensitivity is decreased.
USPTO Backgrounds
USPTO Backgrounds