<title>Guide to translational and rotational vibration measurements on rotors using laser vibrometry</title>

Abstract

Laser Doppler Vibrometry is now a well established experimental technique. Single beam and parallel beam instruments for translational and rotational vibration velocity measurements are now commercially available and gaining in popularity. 30 years ago, one of the first reported uses of laser vibrometry concerned rotating turbine blade vibrations. Non-contact operation and inherent immunity to target shape still prompt instrument manufacturers to quote measurement from rotors as a key application area. Success in these early investigations actually relied on the existence of only a single vibration component while significantly more complex motions are encountered in practice. This paper sets out a comprehensive model of the velocity sensed by both a single laser beam and by a pair of parallel beams incident on a rotating shaft element requiring three translational and three rotational co-ordinates to describe its vibratory motion fully. While the intention is obviously to measure the six vibration components, the model reveals how it is instead only possible to measure six vibration `sets', each inseparable combinations of more than one of the vibration components. Arrangements for isolation for each `set' are set out and an essential post-processing technique to resolve translational and rotational vibration components is demonstrated. This paper will prove to be an invaluable guide to the engineer wishing to take advantage of the powerful benefits of laser vibrometry while avoiding the possibility of serious data misinterpretation.