Measurement of real normal modes with adapted resolution by means of a continuously scanning laser vibrometer

Abstract

This article presents a method of measuring mode shapes for a sinusoidally excited structure using a continuously scanning laser Doppler vibrometer (LDV). The LDV output signal is an amplitude-modulated vibration in which the excitation signal is modulated by the complex deflection of the excited structure. The mode shape can be derived along a scan path by the envelope of the modulated vibration using an applicable demodulation. For an optimised identification of the tuned mode shape, it is possible to adapt the scan path to the characteristics of the selected mode shape. Therefore, the laser beam is driven with varying instead of constant scan frequencies over the surface of the test object during one scan. An algorithm is presented to derive an applicable function for the mirror drive signals from the measured deflection along the scan path. The algorithm calculates the gradient of the measured responses and derives an optimised scan path. Hence the resolution and sampling rate can be adapted to the characteristics of the tuned mode