Application of lasers for non-contact excitation and measurement of vibration

摘要

This work is part of a research devoted to the development of a non-intrusive modal analysis procedure based on laser techniques both for excitation and for measurement. The attention is focused on the experimental evaluation of uncertainty sources in modal parameter measurement, when high energy laser pulses are used to excite the vibration of the structure. The tests were performed on a cantilever beam excited firstly with a hammer and then with laser pulses from a Nd-YAG source (532 nm, 100 mJ/pulse). The problem due to the lack of knowledge on laser-induced input force is here considered. This, in fact, makes the precise quantitative measurement of the mobility functions and of the damping ratio difficult. To this aim, an "equivalent" input force was estimated by solving an inverse problem: the "equivalent" force is useful to determine the features of the laser-induced impulsive excitation. THe achieved results are in complete agreement with other characterisations presented in previous studies, in particular in terms of specific impulse, time duration and characteristic frequencies. Finally, several suggestions are given in order to minimise the influence of the problems connected with laser pulse excitation, in particular concerning the limited energy given to the structure under investigation.