On laser vibrometry of rotating targets: effects of torsional and in-plane motion

Vibration measurements on rotating surfaces are often referred to in ·the commercial literature as a major application of laser Doppler vibration transducers. This paper examines such use of these instruments and shows how the presence of a velocity component due to the rotation itself leads to spurious measurement dependence on both torsional vibration and motion perpendicular to the line of incidence of the laser beam. In addition, the scale of this dependence increases with both rotation speed and perpendicular distance between the line of incidence and a parallel line through the centre of rotation. These phenomena are investigated theoretically and excellent agreement is found when compared with experimental data. Two solutions are suggested; the first involves careful alignment of the laser beam whereas the second requires two simultaneous, orthogonal measurements to be made. If neither method is adopted it is entirely conceivable that the intended solid body vibration measurement may be masked at many frequencies of interest.