Identification of mode shapes with an adapted resolution using a continuously scanning laser doppler vibrometer

The paper presents a method of measuring responses 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 response of the excited structure. The mode shape defined along a scan path can be derived by the envelope of the modulated vibration using an applicable demodulation. For an optimized 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 was developed to derive an application function for the mirror drive signals from the measured responses along the scan path. The algorithm calculates the gradient of the measured responses and derives an optimized scan frequency and scan path, respectively. The scan frequency here is inversely proportional to the gradient of the measured responses. Hence the resolution and sampling rate could be adapted to the characteristics of the tuned mode shape.