Torsional vibration encoder-based monitoring methods and their experimental validation

Excessive torsional vibration of turbine generator rotors and torsionally induced blade vibration result in fatigue cracks arising at the shaft surface or blade roots. If not early detected severe damage to the machine can occur. Torsional vibration, however, is not measurable with standard vibration sensors and thus a specialized measurement must be used. In this paper, two encoder-based measurement methods are discussed. The first method is using an optical sensor together with a reflexive zebra-tape encoder. The second method is using an eddy-current sensor together with a copper incremental encoder. For both methods the measured signal quality and its usability for torsional analysis is compared. Experimental data were obtained on a test rig as well as on a steam turbine generator. Both methods prove to be suitable for obtaining the times of arrival of encoder markers used for instantaneous angular velocity and torsional vibration evaluation. However, the eddy current method suits better for practical applications where dirt and contamination by oil is a concern