Order Tracking Under Run-Up and Run-Down Conditions

In recent years there has been considerable interest in the analysis of machine vibration signals in the presence of speed variations, sometimes over wide ranges. Computed order tracking based on phase demodulation to obtain a map of phase (rotation angle) versus time is limited to a maximum speed range of 2:1. This paper shows how analysis over a much greater speed range, for example a run-up or run-down, can be achieved by dividing the signal into overlapping segments, in each of which the speed range is less than 2:1. In the order domain, the highest frequency to be retained is proportional to the speed, and so at lower speeds the required sampling frequency can be reduced accordingly, as long as higher frequency components are first removed to avoid aliasing. The paper explains how this can be done in the most efficient way.