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UBC Theses and Dissertations

Application of feedback to electromagnetic seismometers Meldrum, Robert David

Abstract

Negative feedback can be applied to an electromagnetic seismometer through the coil terminals using a Maxwell impedance bridge to bypass the coil impedance. The application of feedback by this method obviates the need for a second transducer, and permits direct calibration of the feedback seismometer. Feedback of a simulated ground acceleration proportional to the acceleration of the suspended mass effectively increases the mass, lengthening the natural period of the seismometer. Displacement feedback effectively stiffens the spring, decreasing the resonant period, while the seismometer damping can be increased with velocity feedback. Experimentally determined sensitivity curves obtained with a Willmore Mk.I. seismometer demonstrate the capabilities of negative feedback in altering the response of the seismometer. A resonant period of 17 sec. corresponding to an equivalent suspended mass of 1230 kg. is readily obtained. A discussion of instrument noise in the feedback seismometer is included.

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