HIGH-PRECISION, SHORT-RANGE ULTRASONIC SENSING BY MEANS OF RESONANCE MODE-LOCKING

This report describes a novel method of continuous-wave operation of u ltrasonic transducers, which enables short-range measurement with a re solution much finer than the sound wavelength. The transducer is locke d into a particular resonant mode of the transducer-gap-reflector syst em by a resonance-locking phase-locked loop. The resonant frequency de pends on the relative coherence of the transmitted and reflected waves , so that a change in the position of the reflecting surface will requ ire a corresponding change in the resonance frequency if the resonant mode is to be retained. A simple system of this type has been implemen ted using a cheap, commercially available transducer with far-from-opt imum characteristics. The system has been found to operate successfull y in a range well below the typical ring-down range of a corresponding pulse-echo system, and has displayed a resolution much finer than the sound wavelength for the transducer.