Adaptive ultrasound reflectometry for lubrication film thickness measurements

Adaptive ultrasound reflectometry methods for lubrication film thickness measurements is of great use for condition monitoring and prognostics of systems which have high repair cost and remote locations, such as off-shore systems, as it recursively calibrates the incident ultrasound wave. Typical manual calibration require a constant incident wave over the life-cycle of the system, or until a new manual calibration can be conducted. Auto-calibration accounts for the changes in the incident ultrasound wave, caused by changing environmental conditions, occurring over longer periods of time. The vision of adaptive ultrasound reflectometry methods is therefore increased robustness of lubrication film thickness measurements in a range of applications. In this article an adaptive scheme proposed. The scheme is based on a thin layer time-of-flight method for thickness determination, and an extended kalman filter for estimation of the incident wave spectrum. The adaptive scheme is experimentally tested and the feasibility of the algorithm is established, but serious issues regarding the robustness and reliability of the method is revealed by a disturbance analysis. However, the experiments and a theoretical layer phase-lag sensitivity analysis reveal that the estimation of the incident wave phase is of high importance for layer thickness above 20?m and for very thin layers below 1?m the estimation of the magnitude is dominating the measurement accuracy. This entail that the research in adaptive schemes should be directed towards the phase or magnitude tracking performance, depending on the working range of the layer thickness, such that sufficient robustness and reliability of the algorithms can be assured.