Pseudo-vibration sensitivities for commercial laser vibrometers

Abstract

Pseudo-vibration sensitivities in laser vibrometry are the consequence of measurement noise generated by surface motions other than that on-axis with the incident laser beam(s), such as transverse and tilt vibrations or rotation. Concentrating solely on commercial laser vibrometers to maximise value to the vibrometer user, this paper quantifies pseudo-vibration sensitivities for five single beam instruments (for translational vibration measurement) and two parallel beam instruments (for angular vibration measurement) across a range of surface treatments and roughnesses. Transverse, tilt and rotation sensitivities are quantified for the single beam instruments. In-plane rotation, rotation and tilt sensitivities are quantified for the parallel beam instruments. Estimates of sensitivities for parallel beam instruments based on related quantifications for single beam instruments are shown to offer order-of-magnitude agreement. Further confirmation is provided of the benefits of smaller laser beams for measurements on tilting or rotating surfaces and of larger beams for measurements on surfaces with transverse motion. For rotor vibration applications, lower sensitivities are found for a focus location on the shaft rotation axis rather than its surface. Based on experimental evidence, refinements to the test methods are suggested with a view towards the formation of a Standard. These comprise placing limits on surface flatness and rotor out-of-roundness, inclusion of agreed ranges for target displacement amplitude and rotor diameter, and acknowledgement of the importance of focus location for rotation sensitivities.