The Phase Shift of an Ultrasonic Pulse at an Oil Layer and Determination of Film Thickness

Abstract

An ultrasonic pulse incident on a lubricating oil film in a machine element will be partially reflected and partially transmitted. The proportion of the wave amplitude reflected, termed the reflection coefficient, depends on the film thickness and the acoustic properties of the oil. When the appropriate ultrasonic frequency is used, the magnitude of the reflection coefficient can be used to determine the oil film thickness. However, the reflected wave has both a real component and an imaginary component, and both the amplitude and the phase are functions of the film thickness. The phase of the reflected wave will be shifted from that of the incident wave when it is reflected. In the present study, this phase shift is explored as the film changes and is evaluated as an alternative means to measure oil film thickness.

A quasi-static theoretical model of the reflection response from an oil film has been, developed. This model relates the phase shift to the wave frequency and the film properties. Measurements of reflection coefficient from a static model oil film and also from a rotating journal bearing have been recorded. These have been used to determine the oil film thickness using both amplitude and phase shift methods. In both cases, the results agree closely with independent assessments of the oil film thickness. The model of ultrasonic reflection is further extended to incorporate mass and damping terms. Experiments show that both the mass and the internal damping of the oil films tested in this work have a negligible effect on ultrasonic reflection.

A potentially very useful application for the simultaneous measurement of reflection coefficient amplitude and phase is that the data can be used to negate the need for a reference. The theoretical relationship between phase and amplitude is fitted to the data. An extrapolation is performed to determine the values of amplitude and phase for an infinitely thick layer. This is equivalent to the reference signal determined by measuring the reflection coefficient directly, but importantly does not require the materials to be separated. This provides a simple and effective means of continuously calibrating the film measurement approach.