ASA 127th Meeting M.I.T. 1994 June 6-10

5aSA1. A sensor for measuring low-frequency surface vibration of a fluid-loaded compliant structure.

Alan D. McClearly

A. Mark Young

Patrick J. Klippel

Naval Res. Lab., Underwater Sound Reference Detachment, P. O. Box 568337, Orlando, FL 32856-8337

D. H. Trivett

Texas Res. Int., Austin, TX 78733

Presently, the most common method of measuring the surface vibration of a fluid-loaded structure is through the use of accelerometers mounted on the surface. When the surface of the structure consists of a low-density, compliant material it is necessary to use small, low-mass accelerometers to avoid mass loading of the surface. There are a variety of commercially available, low-mass accelerometers all of which have low sensitivities. For high-frequency measurements the low sensitivities are not a major problem and the accelerometers are generally acceptable. However, at low frequency, where the acceleration is small, the signal-to-noise ratio obtained with the low-sensitivity accelerometers is generally unacceptable. To address the problem of making low-frequency measurements of the vibration of a submerged compliant surface the Underwater Sound Reference Detachment of the Naval Research Laboratory has developed a class of neutrally buoyant capacitive displacement sensors. The dynamic mass of these sensors is equal to the mass of the displaced fluid, thus the sensor does not add any additional loading to the surface. Since the sensors respond to the displacement of the surface these sensors are ideally suited for low-frequency measurements. Both the theory of operation as well as experimental results will be presented. [Work supported by ONR Code 452.]