The present invention relates to a capacitance acceleration sensor for detecting the magnitude of an acceleration acting in a specific direction on the basis of the change in an electrostatic capacitance caused by the variations in the distances between a movable electrode and stationary electrodes.
As disclosed in, for example, Japanese Utility Model Laid-Open Publication No. Hei 7-36065, Japanese Patent Laid-Open Publications Nos. Hei 5-333052 and Hei 918017, there has been conventionally known a capacitance acceleration sensor in which a movable electrode to be displaced in a specific direction in response to acceleration and stationary electrodes facing the movable electrode in the specific direction are disposed in a frame constituting the sensor body so that the magnitude of an acceleration acting in the specific direction is detected on the basis of the change in an electrostatic capacitance caused by the variations in the distances between these electrodes.
In this type of acceleration sensor: the movable electrode acts as a mass for use in the detection of acceleration; the movable electrode (the mass) swings when an acceleration is exerted in the specific direction; and the magnitude of the swing (the magnitude of the acceleration) is detected on the basis of the change in an electrostatic capacitance caused by the variations in the distances between the movable electrode and the stationary electrodes, and is outputted as an electric signal. In the capacitance acceleration sensor, it has been found that the displacement of the movable electrode remaining parallel with the stationary electrodes would provide a linear output and increase the stability on the detection accuracy.
The thickness of beam(s) supporting the movable electrode as the mass is typically set so small as to allow the quantity of the swing of the mass caused by an acceleration to be as large as possible to improve the detection accuracy and the sensitivity.
In the sensor disclosed in Japanese Utility Model Laid-Open Publication No. Hei 7-36065, the movable electrode (the referential mass) is supported with an elastic hinge by the frame, so that an acceleration causes an angular displacement of the movable electrode about the hinge. That is, it is difficult to ensure the linearity of the outputs of the sensor because the movable electrode pivoting about the hinge is incapable of remaining parallel with the stationary electrodes when the movable electrode is displaced by an acceleration.
In the sensors disclosed in Japanese Patent Laid-Open Publications Nos. Hei 5-333052 and Hei 9-18017, the movable electrode is supported by the frame with a plurality of beams, which are provided so as to extend in a plane orthogonal to the specific direction (the direction in which the movable electrode is displaced).
Such sensors as shown in FIG. 7 or 8 are known as sensors in which the movable electrode is supported with one or more beams provided within a plane parallel with the specific direction of the displacement of the movable electrode.
In the sensor shown in FIG. 7 (hereinafter referred to as a prior art 1), a movable electrode 63 is supported with a beam 64 by a frame 61. As shown in FIG. 9 in detail, when an acceleration acts in the direction of the arrow, the movable electrode 63 is tilted and displaced in the direction of the arrow and is incapable of remaining parallel with the stationary electrodes 62 (see the dashed lines in FIG. 9). As shown in the dot-and-dash curve in FIG. 4, for example, the linearity of the sensor outputs resulted from the variations in the distances between the electrodes is impaired relative to the variation in the magnitude of acceleration, and it is therefore difficult to obtain a sufficient reliability of the detection accuracy.
In the sensor shown in FIG. 8 (hereinafter referred to as a prior art 2), a movable electrode 73 is supported by a frame 71 with two beams 74 extending from both sides of the electrode 73 and the two beams 74 are disposed in different positions (in diagonal corners of the movable electrode 73) with respect to the specific direction in which the movable electrode 73 is displaced.
As shown in FIG. 10 in detail, when an acceleration acts on the sensor in the direction of the arrow, the movable electrode 73 is tilted and displaced in the direction of the arrow, as shown in the dashed lines in FIG. 10, and is incapable of remaining parallel with the stationary electrodes 72. This tendency becomes more noticeable particularly in the case that the center "Gv" of gravity of the movable electrode 73 is apart from the symmetrical center of the electrode 73. As a result, it has been impossible to ensure the linearity of the outputs of the sensor and it has been difficult to sufficiently increase the stability on the detection accuracy.