Abstract: The modal identification of large and dynamically complex structures
often requires a multi-point excitation. Sine sweep excitation runs are applied
when it is necessary to concentrate more energy on each line of the frequency
spectrum. The conventional estimation of FRFs from multi-point excitation
requires uncorrelated excitation signals. In case of multi-point (correlated)
sine sweep excitation, several sweep runs with altered excitation force
patterns have to be performed to estimate the FRFs. An alternative way, which
offers several advantages, is to process each sine sweep run separately. The
paper first describes the conventional method for FRF estimation in case of
multi-point excitation, followed by two alternative methods applicable in case
of correlated excitation signals. Both methods generate a virtual single-point
excitation from a single run with multi-point excitation. In the first method,
an arbitrary structural point is defined as a virtual driving point. This
approach requires a correction of the modal masses obtained from modal
analysis. The second method utilizes the equality of complex power to generate
virtual FRFs along with a single virtual driving point. The computation of FRFs
and the modal identification using virtual single-point excitation are
explained. It is shown that the correct set of modal parameters can be
identified. The application of the methods is elucidated by an illustrative
analytical example. It could be shown that the separate evaluation of symmetric
and anti-symmetric multi-point excitation runs yield obviously better and more
reliable results compared to the conventional method. In addition, the modal
analysis of the separate symmetric and anti-symmetric excitation runs is
easier, since the stabilization diagrams are easier to interpret. The described
methods were successfully applied during the Ground Vibration Tests on Airbus
A380 and delivered excellent results. The methods are highly advantageous and
may thus be established as a new standard procedure for testing aerospace
structures.
Keywords: Structural dynamics, experimental modal analysis, multi-point sine sweep excitation, measurement of FRF, identification of modal masses
