Nonlinear system identification and nonlinear experimental modal analysis by using response controlled stepped sine testing

Karaağaçlı, Taylan
In this work, two novel nonlinear system identification methods are proposed in both the modal and spatial domains, respectively, based on response-controlled stepped-sine testing (RCT) where the displacement amplitude of the excitation point is kept constant throughout the frequency sweep. The proposed nonlinear modal identification method, which is also a nonlinear experimental modal analysis technique, applies to systems with several nonlinearities at different (and even unknown) locations (e.g. joint nonlinearities) and/or with continuously distributed (geometrical) nonlinearities, provided that no internal resonance occurs. This method identifies nonlinear modal parameters as functions of modal amplitude by applying standard linear modal analysis techniques to measured frequency response functions (FRFs) which come out in quasi-linear form by RCT. In the case of multiple sensors, nonlinear normal modes can be determined from the identified modal constants. Furthermore, near-resonant constant-force FRFs can be calculated from the identified modal parameters or can be directly extracted from the experiment by using a novel concept proposed in this work, namely the harmonic force surface (HFS). The key feature of the HFS is its ability to accurately extract the unstable branches and the turning points of constant-force FRFs, which makes it possible to extract the backbone curves of strongly nonlinear systems as well. Coming to the proposed nonlinear spatial identification method, it extends the classical describing function method to make the identification of localized nonlinearities nonparametric and to determine the frequency dependence of nonlinearity. The validation of the proposed methods is demonstrated with various numerical and experimental case studies including complex engineering systems such as a real missile structure with considerable damping nonlinearity due to bolted joints and a real control fin actuation mechanism with complex and strong nonlinearity due to backlash and friction.


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Citation Formats
T. Karaağaçlı, “Nonlinear system identification and nonlinear experimental modal analysis by using response controlled stepped sine testing,” Ph.D. - Doctoral Program, Middle East Technical University, 2020.