ANALYSIS OF SWEPT-SINE RUNS DURING MODAL SURVEYAND QUALIFICATION TESTS

摘要

Broad-band excitation like swept-sine excitation is of-ten used in modal survey tests of aerospace structuresin order to identify their structural dynamic proper-ties. The standard procedure includes the computa-tion of frequency response functions (FRFs) from thetime domain data which can be analysed in a modalanalysis to obtain the modal parameters of the struc-ture like eigenfrequency, modal damping and modeshape. Swept-sine excitation is also a common exci-tation type in qualification tests, in most cases car-ried out in terms of base excitation on vibration ta-bles. These data can also be used to identify dynamicmodels of structures. However, recent tests broughtup that reliable modal data from swept-sine test runsdepend on a proper data processing. The paper eluci-dates the strategy of modal analysis based on swept-sine excitation. The theoretical background of swept-sine testing is expounded with particular emphasis tothe transition through structural resonances. The ef-fect of different standard procedures of data process-ing like tracking filter, fast Fourier transform (FFT),and data reduction via averaging are investigated withrespect to their influence on the FRFs and modal pa-rameters. Particular emphasis is given to FRF distor-tions evoked by unsuitable data processing. All dataprocessing methods are applied on numerical exam-ples in order to investigate and demonstrate their ca-pabilities. Recommendations about the proper esti-mation of frequency response functions from slowlyswept-sine excitation are given in order to enable theoptimisation on swept sine application for future vi-bration tests.The load level of a structure during a swept-sine rundoes not only depend on the input force (or base exci-tation level) and the structural dynamics of the struc-tures (eigenfrequencies, generalised masses, damping),but also on the sweep rate. The paper explains how theexcitation level can be determined in case of swept-sineexcitation.