Continuous scanning laser vibrometry for measurements on rotating structures

It is readily accepted that a Laser Vibrometer measures target velocity in the direction of the incident laser beam but it is essential that, for correct measurement interpretation, the target velocity be considered in terms of the various target motion components. This paper begins with a review of the theoretical description of the velocity sensed by a dual mirror scanning Laser Vibrometer incident in an arbitrary direction on a rotating target of flexible cross-section undergoing arbitrary vibration. This comprehensive velocity sensitivity model can be applied to any Laser Vibrometer measurement configuration on any target and is sufficiently versatile to incorporate time dependent beam orientation. This is described in this paper with particular reference to continuous circular scanning Laser Doppler Vibrometry. The velocity sensitivity model allows a detailed analysis of the form of the measurement that is obtained in actual scan configurations. For example, additional components occur in a circular scanning Laser Vibrometer measurement on a rotating target that can be shown to be due to a combination of instrument configuration and target misalignment. In this paper, the measured data obtained from a circular scanning measurement on a rotating target undergoing axial vibration is investigated as a means of demonstrating the usefulness of the comprehensive velocity sensitivity model.