Compensation for encoder geometry and shaft speed variation in time interval torsional vibration measurement

Abstract

Digital time interval techniques for the measurement of torsional vibrations in rotating shafts have been used in a variety of applications. The techniques use high-speed timers to detect the passage times (often referred to as “zero crossings”) in a carrier signal from a multiple pulse per revolution encoder on the shaft. Next, a reference array is created by computing the passage times for an encoder with geometrically uniform segments rotating at a constant speed. The measured times are compared against the reference array and the time difference processed to yield the shaft torsional vibration. If the reference array differs from the encoder passage times, the calculated torsional vibration will be subject to bias errors manifested as high-level order content. There are two potential causes for inconsistency in the reference signal: (1) geometric variation in angular encoding device and (2) a non-constant shaft speed. Procedures to handle these situations are addressed. An in situ encoder calibration method is presented that creates a reference array representative of the actual encoder angular intervals. The method uses time synchronous averaging of the encoder zero crossing over many shaft revolutions. The process causes the time passage variations induced by the torsional vibration to average to zero, ultimately yielding the time stamps caused by the passage of each respective encoder segment. The synchronous averaged encoder passage times are then used to correct the reference array to calculate the torsional vibration. Variation of the encoder geometry and changes in shaft running speed cause the torsional vibration be sampled on a non-uniform time interval basis. The non-uniform time sampling would cause errors if used with fixed sampling interval algorithms such as the DFT. A resampling algorithm is applied to create an array sampled with a constant time interval basis to minimize the error. The background and implementation details of time interval torsional vibration measurement enhancements are first presented. The capabilities are then demonstrated with experimental results from several pieces of rotating equipment. The tests show the successive improvement in the torsional vibration data as the proposed compensation methods are sequentially applied.