Radial vibration measurements directly from polished-circular rotors using laser vibrometry: A new approach based on oblique incidence and retro-reflection

Laser Doppler vibrometry (LDV) offers an attractive solution when radial vibration measurement directly from a rotor surface is required. Research has demonstrated application on rough rotors (typically coated with retro -reflective tape) but a significant cross-sensitivity to the orthogonal radial vibration component occurs and post-processing is essential to resolve individual radial vibration components. Measurements on polished-circular rotors were then presented to overcome this problem. While these worked well, a limit on orthogonal vibration amplitude, as a proportion of laser beam diameter, was encountered, detracting from the practicality of the approach. This paper presents an alternative approach to measurements on polished-circular rotors in which incidence is switched from normal to oblique and a retro -reflector is incorporated into the beam path to deliver a measurement that is free from the problems encountered previously. Comprehensive theoretical and numerical analysis is presented, with experimental validation. Measurement error of around 1% was readily achieved, while cross-sensitivity to the orthogonal radial vibration component, at less than 3% for typical vibration amplitudes, was significantly lower than for alternative LDV approaches. This makes the proposed approach easily the preferred option for radial vibration measurements directly from polished-circular rotors.