Piezoelectric Springs for Novel Geophone Sensors

A.1. Abstract Helically coiled ceramic piezoelectric springs have recently been developed at the University of Birmingham for use as potential vibration sensors. The purpose of this programme of work was to investigate in more detail than had been possible previously, the behaviour of such devices, in terms of their response to vibrations in the range of frequencies commonly used in geophysical applications, i.e. from ten to several hundred Hertz. As part of this programme, a vibration analysis apparatus was set up, to determine accurately the frequency response of various piezoelectric springs. Results have been obtained which illustrate the main features of the behaviour of these springs. In brief, the typical electrical output behaviour of a light multi-turn spring with a small seismic mass is described by a strong fundamental longitudinal resonance, followed at higher frequencies by many separate overtones, resulting in a very crowded frequency spectrum. This type of output is undesirable for seismic applications, where more flat frequency responses are required. However, by altering the design of the springmass system the undesired overtones can be moved up the frequency spectrum away from the measurement area. This design approach may be useful in developing a suitable piezoelectric geophone to compete with conventional electromagnetic types.