ANALYSIS OF COMPUTED ORDER TRACKING

Abstract

Vibration analysis of rotating machinery is an important part of industrial predictive maintenance programmes, so that wear and defects in moving parts can be discovered and repaired before the machine breaks down, thus reducing operating and maintenance costs. One method of vibration analysis is known as order tracking. This is a frequency analysis method that uses multiples of the running speed (orders), instead of absolute frequencies (Hz), as the frequency base. Order tracking is useful for machine condition monitoring because it can easily identify speed-related vibrations such as shaft defects and bearing wear. To use order tracking analysis, the vibration signal must be sampled at constant increments of shaft angle. Conventional order tracking data acquisition uses special analog hardware to sample at a rate proportional to the shaft speed. A computed order tracking method samples at a constant rate (i.e. uniform Δt), and then uses software to resample the data at constant angular increments. This study examines which factors and assumptions, inherent in this computed order tracking method, have the greatest effect on its accuracy. Both classical and computed methods were evaluated and compared using a digital simulation. It was found that the method is extremely sensitive to the timing accuracy of the keyphasor pulses and that great improvements in the spectral accuracy were observed when making use of higher-order interpolation functions.