Radial vibration measurements directly from rotors using Laser Vibrometry: uncertainty due to surface roughness

Radial vibration measurements taken directly from rotors using Laser Vibrometry are known to show a significant cross-sensitivity to the orthogonal radial vibration component. A process for resolving the individual components is now well established and is suitable for both radial measurements and pitch / yaw measurements which show an equivalent cross-sensitivity. All of the work conducted in the development of this system was done using surfaces treated with retro-reflective tape, a common surface treatment for Laser Vibrometer measurements. In this paper investigations have been conducted on untreated surfaces with roughness close to or less than the wavelength of light. These investigations have highlighted an inherent uncertainty for measurements in this roughness range related to changes in the effective centre of the incident laser beam. This uncertainty is shown to be influenced by surface roundness and incident beam diameter as well as by the surface roughness itself. At even lower surface roughness on rotors with low roundness error, it is possible that the cross-sensitivity may be negligible.