Mass Normalized Mode Shapes Using Impact Excitation and Continuous-Scan Laser Doppler Vibrometry

Conventional scanning laser Doppler vibrometer (LDV) systems cannot be effectively employed with impact excitation because they typically measure a structure's response at only one point at a time. This necessitates exciting the structure at multiple points to create a multi-input-single-output modal test data base.. which is not only tedious, but prone to errors due to variations in the impact characteristics from one point to the next. Previous works [1, 2] have demonstrated that an LDV can be used to measure the mode shapes of a structure over a surface by scanning the laser spot continuously as the structure's response decays. The author recently presented a procedure [3] that allows one to post-process continuous-scan LDV (CSLDV) measurements of the free decay of a structure using standard modal parameter identification techniques. Using this approach, one can find the natural frequencies, damping ratios and mode shapes of a structure at hundreds of points simultaneously from a few free responses. The procedure employs a novel resampling approach to transform the continuous-scan measurements into pseudo-frequency response functions, fits a complex mode model, and then accounts for the time delay between samples to obtain the mode shapes. This paper extends the previous work by presenting an algorithm that uses the input force spectrum, measured by an instrumented hammer, to mass normalize the mode shapes obtained using the continuous-scan LDV process. Other issues such as the effect of the scan frequency on the procedure and on speckle noise are also briefly addressed.