Modal testing using a scanning laser Doppler vibrometer

A number of vibration mode-shape measurement techniques are described in which the measurement point of a laser Doppler vibrometer (LDV) is continuously scanned over the surface of a sinusoidally excited structure. The LDV vibration output is an amplitude-modulated sine wave and mode shapes, defined along the scan line, may therefore be established by demodulation. Alternatively, in the frequency domain, the LDV output is a line spectrum, with sidebands centred on the excitation frequency and spaced at the scan frequency. The sideband amplitudes give series coefficients for the mode shape defined 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 point on a structure: a small-diameter circular scan will give the angular vibration of the structure at the centre of the circle; a conical scan, produced with a short-focus lens, will measure the translational vibration at the point of focus, in magnitude and direction. (C) 1999 Academic Press.