Abstract
IN the course of an attempt to improve the response of an inertial seismometer to very-low-frequency frame movements, it appeared worthwhile to investigate the response of a spring-mass system subjected to parametric excitation1. The parametric excitation or ‘pumping’ consisted of modulating the restoring force on the mass by means of an electromechanical spring2 driven from a sinusoidal current source. In the initial work the restoring force was varied at twice the resonant frequency of the system as is usually done in a degenerate parametric device. It was found both from theory3 and experiment that this will increase the relative motion between mass and frame only for inputs with a frequency at or near the resonant frequency, and that the input must bear the proper phase relationship to the pump. There is no improvement for frequencies much below resonance.
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References
Blackwell and Kotzebue, Semiconductor-Diode Parametric Amplifiers (Prentice-Hall, Inc., 1961).
Gilman, R., Report on Some Experimental Long-Period Seismographs, Bull. Seism. Soc. Amer., 50, 553 (1960).
Rodgers, P. W., “Mechanical Parametric Amplifiers”, submitted to J. App. Mech.
McLachlan, N. W., Theory and Application of Mathieu Functions (Dover, 1964).
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RODGERS, P. Sub-resonant Response of a Mechanical System parametrically excited at its Resonant Frequency. Nature 207, 853 (1965). https://doi.org/10.1038/207853a0
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DOI: https://doi.org/10.1038/207853a0
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