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.