Angle domain cepstral comb and notch liftering—A method to separate concurrent gear and bearing faults under variable speed conditions

Abstract

In recent years, in particular because of the importance of wind turbines in renewable power generation, there is now a demand for signal processing techniques to cope with the variable speed. It is also important to distinguish between gear and bearing faults which are both critical in wind turbine monitoring. This paper describes a pair of interrelated techniques, which make use of the fundamental differences between gear and bearing fault signals to separately diagnose them, even when both are present at the same time, andfor speed variations up to ± 20%. The signals are order tracked to the (rotation) angle domain, after which the real cepstrum is calculated. The deterministic gear signals appear only at discrete quefrencies, which can be extracted by comb liftering, which results in signals virtually independent of speed and speed variation, and which are very close to the static transmission error (STE) of each gear. For bearing faults, the discrete quefrency components are instead removed by a comb notch lifter, leaving a signal dominated by the bearing fault information. The paper gives numerous examples from gear test rigs with concurrent gear faults (root crack and developing pitting) and bearing faults, separating and diagnosing the two types of fault sources while excluding background noise.