1. Field of the Invention
The present invention relates to a sensor that detects a physical quantity, such as an angular velocity or an acceleration.
2. Description of the Related Art
Oscillation angular velocity sensors that use micro electro mechanical systems (MEMSs) have been proposed. In this type of angular velocity sensor, an oscillator is caused to perform a reference oscillation with a certain amplitude, and a Coriolis force that is generated upon input of an angular velocity is detected in the form of a displacement of oscillation of the oscillator. The oscillator must be designed so that the oscillator can readily oscillate in a direction for the reference oscillation and in a direction for detection. However, according to this scheme, the reference oscillation could cause oscillation in the direction for detection (a type of oscillation noise), and this might degrade the precision of a detection signal. In a scheme that has been proposed in view of this problem, an oscillator for reference oscillation (reference oscillator) and an oscillator for detection (detecting oscillator) are provided separately (this scheme is called the double-frame scheme).
As an example of a double-frame angular velocity sensor, in an angular velocity sensor proposed in U.S. Pat. No. 6,374,672, a donut-shaped reference oscillator supporting a disk-shaped detecting oscillator is caused to perform a reciprocating rotational oscillation, and the magnitude (maximum inclination) of oscillation caused by an angular velocity of the detecting oscillator provided inner to the reference oscillator is detected. According to this configuration, since it becomes easier to separate oscillation of the reference oscillator and oscillation of the detecting oscillator, the problem of degradation in the precision of a detection signal due to oscillation of the detecting oscillator caused by oscillation of the reference oscillator is alleviated.
In the angular velocity sensor described above, an angular velocity is detected on the basis of inclination of the detecting oscillator about a rotation axis for detection. The degree of inclination of the detecting oscillator can be recognized by detecting change in capacitance between a surface of the detecting oscillator and an opposing surface. More specifically, on a surface opposing the detecting oscillator (denoted by 70 in U.S. Pat. No. 6,374,672), detecting electrodes (denoted by 104 and 106 in FIG. 3 of U.S. Pat. No. 6,374,672) having semicircular shapes are provided. The degree of inclination of the detecting oscillator can be detected by detecting capacitances between the detecting oscillator and the individual detecting electrodes.
FIGS. 12A to 12C show the configuration according to U.S. Pat. No. 6,374,672, in a simplified manner with the reference oscillator omitted. FIG. 12A is a top view in which the vicinity of a detecting oscillator in a double-frame angular velocity sensor is shown as enlarged. FIGS. 12B and 12C are sectional views taken along line XIIB-XIIB perpendicularly to the sheet. In FIGS. 12A to 12C, 401 denotes a detecting oscillator, 402 denotes an upper electrode, 403 denotes a first lower electrode, and 404 denotes a second lower electrode. Furthermore, 405 denotes a left-half upper electrode, 406 denotes a right-half upper electrode, 407 denotes a lower supporting substrate, 408 denotes a rotation axis of the detecting electrode, and 409 denotes supports for the detecting oscillator 401.
In FIG. 12A, the detecting oscillator 401 and the supports 409 are indicated by dotted lines, and the first lower electrode 403 and the second lower electrode 404 provided on the lower supporting substrate 407 are indicated by solid lines. The upper electrode 402 and a reference oscillation generator are not shown in FIG. 12A.
The detecting oscillator 401 has a disk-like shape, and is supported by the supports 409 from above and below as shown in FIG. 12A. The detecting oscillator 401 is designed so that the detecting oscillator 401 can readily perform a reciprocating rotational oscillation in a direction of an arrow R about the rotation axis 408. The detecting oscillator 401 detects a Coriolis force on the basis of the magnitude of inclination of the detecting oscillator 401. On the detecting oscillator 401, the upper electrode 402 is provided. Hereinafter, it is assumed that the upper electrode 402 is composed of the left-half detecting electrode 405 and the right-half detecting electrode 406 on either side of the rotation axis 408.
On the lower supporting substrate 407 facing the upper electrode 402, the first lower electrode 403 and the second lower electrode 404 are provided. The first lower electrode 403 and the second lower electrode 404 are disposed line-symmetrically with respect to a center line (the rotation axis 408) of the supports 409 for the detecting oscillator 401.
Now, a case will be considered where the degree of inclination of the detecting oscillator 401 is detected on the basis of capacitances between the upper electrode 402 on the detecting oscillator 401 and the lower electrodes 403 and 404 on the lower supporting substrate 407.
Next, a capacitance CL between the upper electrode 402 and the first lower electrode 403 in FIG. 12B will be considered. The capacitance CL can be considered as a combined capacitance of a capacitance C1 between the left-half upper electrode 405 and the first lower electrode 403 and a capacitance C3 between the right-half upper electrode 406 and the first lower electrode 403. The distance between the left-half upper electrode 405 and the first lower electrode 403 is shorter than the distance between the right-half upper electrode 406 and the first lower electrode 403. Thus, the capacitance C1 is larger than the capacitance C3.
When the detecting oscillator 401 is inclined about the rotation axis 408 as shown in FIG. 12C, the distance between the detecting oscillator 401 and the first lower electrode 403 increases, and the distance between the detecting oscillator 401 and the second lower electrode 404 decreases. Since the capacitance CL is proportional to the electrode area and is inversely proportional to the distance between electrodes, the value of the capacitance C1 decreases and the value of the capacitance C3 increases. Since the capacitance C1 is larger than the capacitance C3, the amount of decrease in the capacitance C1 is larger than the amount of increase in the capacitance C3. Thus, the value of the combined capacitance CL decreases.
That is, it is possible to detect inclination of the detecting oscillator 401 by detecting an increase in the distance between the left-half detecting electrode 405 and the first lower electrode 403 on the basis of a decrease in the combined capacitance CL. A capacitance CR between the upper electrode 402 on the detecting oscillator 401 and the second lower electrode 404 on the lower supporting substrate 407 can be considered similarly as a combined capacitance of capacitances C2 and C4.
In this specification, a state where no Coriolis force is exerted on a sensor so that the detecting oscillator 401 is not inclined, as shown in FIG. 12B, will be referred to as a “neutral position for detection”. The supports 409 for the detecting oscillator 401 are designed so as to form springs such that the detecting oscillator 401 can readily oscillate rotationally in response to even a small Coriolis force. Thus, the detecting oscillator 204 could be caused to oscillate by electrostatic attractive forces caused by signals applied for measurement of inclination of the detecting oscillator 401. If the detecting oscillator 401 is displaced from the neutral position for detection and is caused to oscillate when no physical quantity to be detected is input as described above, then the stability of zero-point output of a detection signal is degraded and thereby sensitivity is degraded.