Modal Testing using Multiple References

With the increased availability of multi-channel FFT analyzers, more emphasis has been placed in recent years on the development of new methods for modal testing using multiple references. In the case of shaker testing, this means the use of two or more shakers. In the case of hammer (impact) testing, this means the use of two or more reference transducers. This paper presents the results of our recent experiences in both the collection of FRF data, and the curve fitting of it to obtain the modal parameters of a structure. The key difference between these results and those of a more traditional mo dal test is that multiple rows or columns of the structure's FRF matrix are not only collected simultaneously, but are also curve fit in a simultaneous manner. In this paper, the advantages offered by multiple reference modal testing and curve fitting are addressed. The theory and usefulness of a frequency domain curve fitting algorithm are extended to account for the effects of “out-of-band ” modes, which always occur in practical applications. Verifications of the method and its implementation using both analytical and experimental FRF data are also presented in the paper