1. Field of the Invention
The present invention relates generally to gyroscopes, and more particularly to an angular rate sensor using piezoelectric oscillating elements.
2. Description of the Prior Art
Conventionally, inertial navigation systems incorporating gyroscopes have been used to determine the bearing of a moving object such as an airplane or a ship. The gyroscopes include a sensor composed mainly of a spinning gyro of the mechanical type. The mechanical gyro is solely satisfactory in insuring a stable direction detection. However, it requires a relatively large and expensive structure and cannot, therefore, be applied to consumer equipment which should be small in size and of relatively low cost.
Another known angular rate sensor is of the vibratory or oscillating type, which includes a sensor element that oscillates in conjunction with an oscillating object or structure to which the sensor element is secured, for detecting the so-called Coriolis force. Most vibrational angular rate sensors have either a piezoelectric mechanism or an electromagnetic mechanism. These sensors are characterized by the motion of a mass constituting a gyro which is a vibratory or oscillating motion rather than a rotary motion of a constant angular rate When an oscillating mass is subjected to an angular rate or velocity, there is produced a force, known as the Coriolis force, in the form of a vibratory torque which is proportional to the angular rate or velocity of the oscillating mass. Thus, vibrational sensing of a torque caused by the Coriolis force provides a measurement of an angular rate. This is the principle of operation of the vibratory angular rate sensors stated above. Most vibratory angular rate sensors devised heretofore utilize piezoelectric members as described for example in the Journal of the Japan Society for Aerological and Space Sciences, Vol. 23, No. 257, pp. 339-350.
One such known angular rate sensor which operates based on the principle described above is shown in FIG. 1 of the accompanying drawings. The angular rate sensor generally comprises a pair of vibratory piezoelectric detecting elements 21 and a pair of vibratory piezoelectric drive elements 22 longitudinally aligned end to end, and joined, with the detecting elements 21, respectively, by a pair of joint members 26 to form a pair of sensor elements. An electrode block 23 joins the sensor elements at the free ends of the drive elements 22 to jointly constitute a tuning-fork structure. The angular rate sensor also includes a support rod 24 firmly supporting the tuning-fork structure on a base 25. In assembly, the drive elements 22 and the electrode block 23 are joined by soldering at their bonding surfaces. The bonding surfaces of the respective drive elements 22 and the bonding surfaces of the electrode block 23 have this same width. For soldering, after solder is disposed between one drive element 22 and the electrode block 23 and also between the opposite drive element 22 and the electrode block 23, the drive elements 22 are forced against the electrode block 23 while the electrode block 23 is heated.
The conventional angular rate sensor of the foregoing construction operates as follows. To oscillate the drive elements 22, an alternating drive signal is applied between the outer surfaces of the respective drive elements 22 with their inside surfaces electrically connected together to form a common electrode. The drive elements 22, thus excited, oscillates about the electrode block 23 at the same frequency and opposite phase in a manner generally known as the tuning-fork oscillation.
If the detecting elements 21 oscillating at a velocity v is given an angular motion having an angular rate or velocity w, a force, known as the Coriolis force, is produced on the detecting elements 21. The direction of the Coriolis force is perpendicular to the direction of the velocity v and the magnitude of the Coriolis force is 2mvw (m is an equivalent mass of the detecting elements 21 at their free ends). Since the tuning-fork structure undergoes tuning-fork oscillation, at the moment when one detecting element 21 is oscillating at a velocity v, the opposite detecting element 21 is oscillating at a velocity -v. Consequently the Coriolis force produced on the opposite detecting element 21 is -2mvw. Thus, the pair of detecting elements 21 are given the Coriolis forces of opposite directions and deform in opposite directions with the result that the surfaces of the pair of sensor elements are electrically charged by the piezoelectric effect. The sensor elements are electrically connected together in such a manner that electric changes produced by the Coriolis forces are added together.
If a tuning-fork oscillation velocity v which is produced by the tuning-fork oscillation is given by EQU v=v.sub.0 .multidot.sin w.sub.O t
where v.sub.O is the amplitude of the tuning-fork oscillation velocity, and w.sub.O is the angular cycle of the tuning-fork oscillation, the Coriolis force Fc can be written as EQU Fc=2m.multidot.v.sub.O .multidot.w.multidot.sin w.sub.O t.
Thus, the Coriolis force is proportional to the angular rate w and the tuning-fork oscillation velocity V.sub.O and acts in a direction tending to deform the respective detecting elements in the facewise direction. The quantity of surface electric charges on the detecting elements 21 is described by EQU Qc-v.sub.O .multidot.w.multidot.sin w.sub.O t.
If the tuning-fork oscillation velocity amplitude v.sub.O is controlled at a constant level, we can obtain EQU Qc-w.multidot.sinw.sub.O t.
Thus, the surface electric charge quantity Qc produced on the detecting elements 21 is obtained as an output proportional to the angular rate w.
If the electrode block 23 of the conventional angular rate sensor which serves as a resilient joint member is relatively thick (i.e. has a relatively large retaining depth), each of the drive elements 22 being oscillated is retained over a relatively large area. With this large retaining depth, the drive elements 22 are unlikely to oscillate smoothly with the result that the angular rate sensor has a relatively large resonance impedance. It is preferable that the resonance impedance should be low. A higher resonance impedance requires a higher driving voltage in order to provide a constant amplitude of oscillation which is attended with the need for a higher source voltage. Furthermore, the angular rate sensor having a high resonance impedance tends to produce an unstable final output.
If the thickness of the resilient joint member is excessively small, each drive element 22 is retained by the joint member only over a relatively small area and hence a sufficient soldering strength is difficult to obtain. With insufficient soldering strength, the drive elements 22 are retained unstably and insufficiently. An angular rate sensor having such unstably soldered drive elements has a high resonance impedance.