Automated modal analysis by scanning laser vibrometry: Problems and uncertainties associated with the scanning system calibration

The use of scanning laser Doppler vibrometry (SLDV) offers many opportunities to increase the investigation capability of experimental vibration testing. Nevertheless, its employment introduces a number of new problems with respect to traditional measurement techniques. In this work, after a brief introduction to laser vibrometry, the attention is focussed on the calibration of the scanning system. The scanner is used to move the measurement laser spot along the structure with respect to a 3-D (three dimensional) co-ordinate system, in such a way as to automate the measurements for modal analysis. Problems and uncertainty sources are investigated, both from a theoretical and an experimental point of view. Several calibration tests are performed and some suggestions on how to plan an optimal calibration are given. In the second part of the paper, the modal analysis is used to address these accuracy problems on a case study. Two modal tests on a turbine blade are performed with two different scanner calibrations and the results are compared, in order to highlight the differences in the extracted modal parameters influenced by the scanning system calibration and by the laser spot positioning errors. It is shown that low accuracy in the scanner calibration may cause uncertainty in the modal parameter extraction, in which case the use of SLDV may become erroneous. Some estimates, such as the mode complexity, can be used to verify if the defined nodes coincide with the scanned grid. (C) 2001 Academic Press.