Measurement of mechanical vibrations using eddy current transducers and simple digital demodulating techniques

A coil with a soft magnetic ferrite core and excited by an AC current is placed near a metallic vibrating object. The impedance of the coil is influenced by the vibrations, so the voltage across the impedance is composed of a carrier voltage signal modulated by the mechanical vibrations. The produced signal is processed through a 10 bit analog-to-digital converter, a RAM, and a transient recorder for storing the data. It is found that the calculation of the fast Fourier transform of the quadrature of the stored data of the signal gives a rapid and accurate digital demodulation technique. A Z-80 microcomputer is used for storing and processing the data. A small number of points at the frequency spectrum will give the required unknown values of the amplitude and frequency of the mechanical vibrations