Layer ToF Methods for Ultrasonic Lubrication-film Thickness Measurements

Reliability of mechatronic systems is of great importance in offshore industries, which entails increasing interest for monitoring techniques. The tribological interfaces in such mechatronic systems are frequently the failing parts of the system. In consequence, non-invasive tribological monitoring systems are receiving increasing interest. In particular lubrication film thickness measurements are of interest in hydraulic pumping and motoring units. Conventional methods for lubrication film thickness measurements using ultrasound is the spring method and the resonance method. However, to obtain sufficient accuracy the spring method require thin films below 20 µm. Further, the resonance method requires the film thickness and layer speed of sound being such that the layer resonance frequency is in the transducer frequency band. The Time-of-flight (ToF) of ultrasonic waves in the lubrication film is the inverse of the resonance frequency and therefore can the resonance method be classified as a ToF method. In this paper this class of methods called Layer ToF methods is described and a discussion of appropriate choice of methods is given. In addition, a noise analysis comparing the spring model and the layer phase-lag method is presented.