1. Field of the Invention
This invention relates to a vibratory gyroscope in which vibrational components based on a Coriolis for produced in reference to an angular velocity are detected by piezoelectric effects.
2. Description of the Related Art
FIG. 12A is a perspective view showing a conventional vibratory gyroscope. FIG. 12B is a cross-sectional view of the vibratory gyroscope shown in FIG. 12A.
In this type of vibratory gyroscope, a driven piezoelectric device 32a is affixed to an upper surface 31a of a prismatic vibrator 31 formed of a constantly elastic alloy such as elinvar or the like and whose cross-section is square. When drive power is applied to the piezoelectric device 32a, the vibrator 31 produces a bending vibration with a Y direction as an amplitude direction.
When the vibrator 31 producing the bending vibration is placed within a rotational system and an angular velocity .omega. about a Z axis is given, a Coriolis force extending in a direction (corresponding to an X direction) intersecting the vibrational direction (corresponding to the Y direction) acts on the vibrator 31. Thus, a vibration occurs in the vibrator 31 based on the Coriolis force with the X direction defined as the amplitude direction. As a result, the vibrator 31 produces an elliptical vibration obtained by adding the drive vibration in the Y direction and the vibration in the X direction based on the Coriolis force on a vector basis.
Assuming now that the mass of the vibrator 31, the velocity of a vibrational component in the Y-axis direction and the angular velocity about the Z axis are defined as m, v (vector value) and .omega. (vector value) respectively, the Coriolis force F (vector value) is given by the following equation: EQU F=2m(v.times..omega.))(x: vector product) [Equation 1]
The Coriolis force F is proportional to the angular velocity .omega..
In order to detect the vibrational component based on the Coriolis force, a detecting piezoelectric device 33 is normally attached to a side face 31c facing the amplitude direction (X direction) of vibration of the Coriolis force as shown in FIG. 12B.
FIG. 11 shows an output produced from a surface electrode of the piezoelectric device 33 shown in FIG. 12B in the form of a vector diagram.
When no angular velocity .omega. is given in a state in which the drive power is supplied to the piezoelectric device 32a and the vibrator 31 is vibrating with the Y direction as the amplitude direction, the output produced from the surface electrode of the piezoelectric device 33 becomes F0. Since the vibrator 31 is vibration driven in the Y direction, this output F0 is power converted by the piezoelectric device 33 based on the vibration driving. This output F0 will hereinafter be referred to as "null output". The null output F0 is, for example, 90.degree. out of phase with the drive signal supplied to the piezoelectric device 32a.
When the angular velocity .omega. is given and the vibrational component in the X direction based on the Coriolis force is produced, a Coriolis output F.omega. converted by the piezoelectric device 33 is produced according to the vibrational component based on the Coriolis force. Thus, a detected output F1 obtained from the surface electrode of the piezoelectric device 33 results in the sum of the null output F0 and Coriolis output F.omega. represented in vectors. The Coriolis output F.omega. indicated by a solid line in FIG. 11 corresponds to an output converted when the angular velocity .omega. about the Z axis is in the clockwise direction. When the angular velocity .omega. is counterclockwise, the Coriolis output F.omega. results in a vector direction indicated by a dotted line in FIG. 11.
A known method of determining the output component based on the Coriolis force from the detected output F1 obtained from the surface electrode of the piezoelectric device 33, involves determining the difference W in amplitude between F1.multidot.cos .theta. and the null output F0 and determining a change .theta. in phase of the detected output F1.
The method of determining the difference in amplitude is incapable of accurately detecting the difference W in amplitude between the F1.multidot.cos .theta. and the null output F0 when the null output F0 varies. In the vibratory gyroscope wherein the piezoelectric device is attached to the vibrator formed of the constantly elastic alloy, for example, the amplitude of the null output F0 is apt to vary due to a change in temperature. Thus, a temperature drift is produced when the component of the Coriolis force is detected.
Further, the method of detecting the change .theta. in phase difference has been disclosed in Japanese Patent Application Publication No. 4-14734, for example. According to the method disclosed in the same Publication, a feedback signal obtained from a feedback piezoelectric device 32b provided at a lower surface 31b of the vibrator 31 is 90.degree. phase-shifted to form a compare signal identical in phase to the drive signal. On the basis of the compare signal, a change .theta. in phase of the detected output F1 obtained from the piezoelectric device 33 is determined from an EX-OR circuit (Exclusive OR gate circuit).
In the method of detecting the phase change .theta. to thereby determine the vibrational component of the Coriolis force, the influence of the temperature drift is less reduced as compared with the aforementioned amplitude detecting method because the method is unaffected by a change in amplitude of the null output due to the change in temperature.
Since, however, the amplitude of the Coriolis output F.omega. is smaller than that of the null output F0, the amount of change .theta. in phase is not so great and hence high-accuracy detection falls into difficulties. Since high-accuracy detection was difficult to perform, the accuracy of the circuit needs to be improved in order to detect a fine change in phase. This results in an increased in manufacturing cost. When the small phase change .theta. is detected with high sensitivity, the gain of an analog portion in the circuit must be increased and the influence of the circuit drift due to the change in temperature becomes great.