Vibration measurements using continuous scanning laser vibrometry: advanced aspects in rotor applications

This paper builds on previous work concerned with the development of a comprehensive velocity sensitivity model for continuous scanning Laser Vibrometry. This versatile model predicts the measured velocity for arbitrary mirror scan angles and arbitrary target motion and it has been especially valuable in revealing the sources of additional components seen in continuous scanning and tracking measurements on rotors. The application to vibration measurements on rotors is the particular focus of this paper which includes, for the first time, a three dimensional consideration of the incident point on the target and validation of the DC component of measured velocity leading to evaluation of the individual components of the small but inevitable misalignments between the rotor and optical axes. This has not previously been possible. Misalignments in the region 0.5mm and 0.5° were found and the model shows how additional components of the order 10-20mm/s result for typical measurements. Such levels are significant as they are comparable with vibration levels likely in real applications and, if unexpected, may lead to data misinterpretation. The first thorough analysis of laser speckle effects in scanning Laser Vibrometer measurements on rotors is presented in the form of a speckle repeat map, together with experimental data quantifying the dramatic reduction in speckle noise found in tracking measurements. Finally, the velocity sensitivity model and the description of laser speckle effects are used to enable confident interpretation of data from a series of measurements on a rotating bladed disc.