Time–frequency analysis for ultrasonic measurement of liquid-layer thickness

Lubricant film thickness is one of the most important parameters to indicate the operating condition of machine elements, such as mechanical seals and hydrostatic slideway. When ultrasonic waves illuminate the interfaces between the substrates and a lubricant film, it will be reflected due to the change of the material properties at the interfaces. These reflected ultrasonic waves contain information about film thickness. In this paper, wavelet transform modulus maximum method was explored to extract the film thickness from its reflection ultrasonic signals. The performance of different wavelet functions within various scale factors was experimentally investigated, and the optimal wavelet function with the optimal scale factor was pointed out. It has been shown that the measurement error is less than 5% when the thickness of liquid layer is within a certain range.

► Wavelet transform was used to pick up the transient points in ultrasonic signals.
► The performance of different wavelet functions and scale factors are experimental investigated.
► The optimal wavelet function and scale factor for liquid layer measurement are obtained.
► The proposed method is effective on thickness measurement of liquid layer in machine elements.