The absolute gravimeter newly developed in the Earthquake Research Institute, University of Tokyo, uses a small-size cat's-eye as a falling object. As a relative accuracy of 1×10
-9 is required for an absolute gravimetry, the angle of rotation of cat's-eye around its center of gravity is limited to about 15 seconds. In order to realize a non-rotating free fall, TSUBOKAWA et al. (1976) proposed a method of making use of a ceramic base as a cat's-eye supporter. The advantage of this method lies in the fact that, in a moment of separating from the supporting beam, the cat's-eye stays instantaneously in the air due to a rapid piezoelectric contraction of a ceramic base. Such a condition may be realized by controlling the delay time of separating the supporting beam from the electromagnet. In this experiment, in order to shorten the delay time, the starting motion of the supporting beam is accelerated by a spring and a reverse excitation current is fed immediately after cutting off the excitation current. It is found as a result, however, that about forty percent of trials has been unsuccessful due to unexpected elastic oscillations of the supporting beam. This percentage later has much improved by making a strong spring for accelerating the beam motion.
View full abstract