A Large-Range Ultrasonic Phase Method for Oil Film Thickness Measurement Considering the Temperature Influence

The oil film thickness is a key parameter for indicating the lubrication condition in tribological elements. However, limited by the confined location space and the tiny scale of the oil film, it is a big challenge for the accurate oil film thickness measurement. For the traditional ultrasonic oil film thickness measurement method, which utilizes the amplitude of the reflected signal from the oil layer, there is normally a blind zone when using a single ultrasonic sensor which severely limits the applications in industry. In this paper, a large-range ultrasonic oil film thickness measurement method based on the phase of the reflected signal from the oil layer was developed, this method was able to eliminate the measurement blind zone between the spring model measurement zone and the resonance measurement zone, and achieved a wide range of oil film thickness measurement. The influence of the temperature on the measurement accuracy of this method was analyzed and the compensation methods were proposed. The temperature could affect the measurement accuracy of the oil film thickness phase measurement method by changing both the phase of the reference signal and the acoustic parameters of the mediums. It was found that the phase of the reference signal change linearly as the temperature and a linear curve which was fitted by the phase of the reference signals with different temperatures could be constructed to compensate and eliminate the influence of the temperature on the phase of the reference signal. For the influence of the change of the acoustic parameters induced by the temperature, the measurement errors caused by the change of sound velocity in mediums were much bigger that than caused by the change of medium density. And among the three mediums constructed the oil film, the measurement errors caused by the change of the oil density and the sound velocity were the biggest, which reached 3.8% and 11.4% respectively. A calibration rig which consisted of ultrasonic measurement module, oil film formation module, oil film calibration module, and temperature control module was built to verify the accuracy of the proposed method under constant temperature and varying temperatures. The measured results was compared with both the set values and that obtained by the capacitance sensor. The test results indicated that the measurement error of the developed method was less than 8% under constant temperature, the temperature change would deteriorate the measurement precision and the measurement precision could be improved effectively with the temperature compensation method developed in this paper, decreasing the maximum measurement error down to 20% under varying temperatures. Furthermore, the proposed measurement method exhibited a good robustness under various temperatures. This paper could provide an effective ultrasonic measurement method for large range oil film thickness with single ultrasonic sensor. It also could enrich the ultrasonic oil film thickness measurement method and provide a reference for engineering applications.