Modal testing using a scanning laser Doppler vibrometer

A number of vibration mode-shape measurement techniques are described in wh ich the measurement point of a laser Doppler vibrometer (LDV) is continuous ly scanned over the surface of a sinusoidally excited structure. The LDV vi bration output is an amplitude-modulated sine wave and mode shapes, defined along the scan line, may therefore be established by demodulation. Alterna tively, in the frequency domain, the LDV output is a line spectrum, with si debands centred on the excitation frequency and spaced at the scan frequenc y. The sideband amplitudes give series coefficients for the mode shape defi ned along the scan line-Fourier coefficients for a circular scan or, via a simple transform, polynomial coefficients for a sinusoidal straight line or area scan. Useful measurements can also be made, directed at a single poin t on a structure: a small-diameter circular scan will give the angular vibr ation of the structure at the centre of the circle; a conical scan, produce d with a short-focus lens, will measure the translational vibration at the point of focus, in magnitude and direction. (C) 1999 Academic Press.