Abstract
A device for measuring small displacements as an application of the frequency modulation technique has been widely used for determination of the mode of vibration. However, there are many factors to be discussed associated with the device such as the limit of the air gap between vibrating body and pick up pole, the maximum or minimum measurable displacement, the amount of error, and so on. In this paper these points are discussed and the following results have been obtained; 1) the ratio of air gap (D) to radius of pick up pole (b) has to be selected at 0. 1〜0. 6, 2) the lower limit of measurable displacement is 2×10^<-5> m/m (at S/N=20dB), and the upper limit is 0. 6αb (where α is determined from the allowable maximum distortion), and 3) the maximum total error is about 6% (at b=1. 5 m/m).
