Development of a scanning laser Doppler vibrometer for imaging the surface velocity of a submerged vibrating object of arbitrary geometry

Abstract

An underwater scanning laser Doppler vibrometer (USLDV) was developed as part of a system for imaging the vibrations of submerged surfaces of arbitrary geometry. USLDV allows for measurement of in-plane and out-of-plane velocity components with scan resolution limited typically by the resolution of the positioning system (< 1 mm). The Georgia Tech system features a compact underwater probe mounted on an acoustically transparent mounting truss whose position and orientation is computer controlled. The system was designed with special consideration of the measurement of compliant surfaces underwater. Characteristically low impedance and locally reacting, compliant surfaces are sensitive to field perturbations, such as those which may be generated by the measurement apparatus. Small scattered fields can generate sizable local surface motion, leading to measurement error. The design of the underwater system components was performed in consideration of the perturbation sensitivity of compliant surfaces as well as such issues as sound-induced modulation of the index of refraction, and vibration of the probe head due to incident sound and/or the motion of the positioning truss. [Work supported by ONR.]