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Application of feedback to electromagnetic seismometers

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Title: Application of feedback to electromagnetic seismometers
Author: Meldrum, Robert David
Degree Master of Science - MSc
Program Geophysics
Copyright Date: 1965
Subject Keywords Seismometry; Feedback (Electronics)
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.
URI: http://hdl.handle.net/2429/37207
Series/Report no. UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/]
Scholarly Level: Graduate

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