Methods for the quantification of pseudo-vibration sensitivities in laser vibrometry

2012-04-11T13:44:19Z (GMT) by Peter Martin Steve Rothberg
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. On rougher surfaces, laser speckle is the cause but similar noise is observed in measurements from smoother surfaces. This paper's principal aim is to introduce two experimental methods for quantification, including dedicated data processing, to deliver sensitivities in three forms: a spectral map, a mean level per order and a total rms level. Single and parallel beam vibrometers and different surface roughness or treatment are accommodated, with sensitivities presented for two commercial instruments (beam diameters 90 and 520 mu m). For transverse sensitivity, a total rms level around 0.05% is found for the larger beam, a quarter of the level for the smaller beam. For tilt sensitivity, advantage shifts to the smaller beam with a total rms level around 0.45 mu m s(-1)/deg s(-1), less than one-third of that for the larger beam. Levels hold fairly constant across the rougher surfaces, reducing only for a polished surface. For rotation sensitivities (radial vibrations), advantage remains with the smaller beam with a total rms level around 2 mu m s(-1)/deg s(-1), compared to 5 mu m s(-1)/deg s(-1) for the larger beam, while sensitivity reduces with diminishing roughness. These sensitivities are especially valuable to vibrometer users in instrumentation selection and data analysis.