Instantaneous Angular Speed: comparisons between Torsional Laser Vibrometer and Encoder-Counter estimations

In recent years, some issues of the condition monitoring of rotating machinery have been tackled with the Instantaneous Angular Speed (IAS) analysis. In fact, from the variation of the IAS during the machine loads’ cycle it’s possible to identify defects and faults. The current work focuses on the estimation of the IAS through the Elapsed Time (ET) method and compares the results with the measurements of a B&K torsional laser vibrometer type 2523, allowing to highlight the differences between the methods and the influences of the processing parameters. The ET is based on the measurement, with a counter, of the time elapsed between the pulses of an encoder. Obviously, both encoder’s and counter’s characteristics, (the number of pulses/revolutions and the counter frequency), influence the range of applicability and the quality of the estimation. The B&K torsional laser vibrometer instead relies on the lasers’ signal correlation and the angle resampling technique. The measurements are performed on an unloaded asynchronous four poles electrical motor. This is driven by an adjustable frequency AC drive system to simulate different working speed conditions. The IAS is estimated between the 150rpm and the 1500rpm range. The study shows the advantages and disadvantages of the two measurements approaches, reports their limitations and suggests a novel procedure for deriving the IAS form encoder/counter signals elaboration.