Operational Modal Analysis with a 3D Laser Vibrometer Without External Reference

Abstract

The use of Scanning Laser Doppler Vibrometers (SLDV) is a common practice in the context with modal analysis. 3D SLDV have been used with classical experimental modal analysis for several years. In this case, three laser heads, which simultaneously point to the same location, are used and an additional input reference signal is required to combine the results of consecutive records. In operational modal analysis the input reference signal is replaced by an output reference signal, which may be obtained, for example, from an additional vibrometer or accelerometer.

Alternatively, if a 3 dimensional laser vibrometer is used, one of the three laser heads can be used to provide a reference signal for the other two heads, which can then position their laserbeams independently. However, this requires an appropriate post-estimation method for the transformation of 3D local skew coordinates since the common, built-in routines of commercially available 3D SLDV perform the respective coordinate transformation directly on the simultaneously acquired time series.

In the presented study a respective algorithm has been developed and applied to measured vibration data. For the modal identification the SSI cov/ref algorithm and three different merging strategies have been employed.