Temperature compensation strategy for ultrasonic-based measurement of oil film thickness

Due to non-destructive testing characteristics, ultrasonic-based measurements are regarded as potential strategies in real-time monitoring of varied lubricant film thickness for tribological systems. Different theoretical models have been recently developed to calculate the film thickness from ultrasonic echo waves. However, the temperature influence on ultrasonic systems (for example, the acoustic parameters of materials and piezoelectric elements), which is non-negligible in the continuously running equipment, especially for heavy-loaded lubricated systems, has scarcely been considered. In this paper, the sensitivity of different parameters to the temperature variation is investigated in classical ultrasonic models. With the theoretical error analysis, a comprehensive temperature compensation strategy that considering the acoustic speed, material density, and reference signal, is proposed and integrated into the ultrasonic measurement algorithms. It is worth noting that the amplitude attenuation, waveform expansion, and signal time shift are considered in the compensation of the reference signal. Experimental verification is finally carried out in a calibrated rig with swept lubricant film thickness and different ambient temperatures. Test results suggest that the ultrasonic measurements with the proposed strategy effectively compensates the temperature influence and enables accurate calculations of lubricant film thickness under varying temperatures.