This paper describes an ultrasonic ranging system (URS) that can be used to measure vibratory displacements in structures. A pair of ultrasonic transducers is used in a transmit-receive mode to acquire the motion of a point on a flexible structure. The structure's oscillating motion modulates the phase angle between the transmitted and received acoustic signals. A simple phase detector and lowpass filter combination demodulate the phase signal, thus extracting information about the motion of the point on the structure. The results reported here concentrate on some design issues of the URS and its use as a potential displacement measuring device for flexible structures. The URS accurately distinguished four vibration frequencies of a simple cantilevered aluminum beam, the highest frequency being near 50 Hz. A 3-D version of the system will be an effective, simple, and low-cost vibration sensor to be used in flexible structures' feedback control schemes. © 1991 IEEE