Asher's Method in Normal Mode Testing

Abstract

The conventional method used in the dynamic testing of large structures is based on the Normal Mode Testing technique, in which the structure is excited at the resonant frequency by a set of monophase forces distributed over the structure, and the amplitude of the forces adjusted manually (appropriated) until the velocities at all points on the structure are in phase with the forces. Manual iterative procedures are carried out whilst observing Lissajous figures, where closing of the ellipses not only indicate proper force appropriation, but also confirm excitation at the resonant frequencies. Difficulties arise, however, when a large number of forces have to be adjusted to isolate modes in frequency ranges of high modal density. Therefore to automate the force appropriation task, different methods have been put forward. The one that will be described in this paper is due to Asher, who proposed a quantitative method, which detects the natural frequencies as well as provides the force ratios necessary for isolating the modes at each of these frequencies, using only experimental transfer accelerance data as input. A narrow frequency sweep is now carried out around each of the resonance frequencies with the respective appropriate forces calculated, and the complex power plotted from which the modal parameters are determined. To illustrate the effectiveness of Asher's method, results obtained from a theoretical model will be illustrated. Instrumentation required for these tests will be described as well as experimental results obtained from measurements on a practical structure using this system will be presented.