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.


Nonlinear Structural Coupling: Experimental Application
Kalaycioglu, Taner; Özgüven, Hasan Nevzat (2014-02-06)
In this work, the nonlinear structural modification/coupling technique proposed recently by the authors is applied to a test system in order to study the applicability of the method to real structures. The technique is based on calculating the frequency response functions of a modified system from those of the original system and the dynamic stiffness matrix of the nonlinear modifying part. The modification can also be in the form of coupling a nonlinear system to the original system. The test system used i...
Adaptive Harmonic Balance Methods, A Comparison
Sert, Onur; Ciğeroğlu, Ender (2016-01-28)
Harmonic balance method (HBM) is one of the most popular and powerful methods, which is used to obtain response of nonlinear vibratory systems in frequency domain. The main idea of the method is to express the response of the system in Fourier series and converting the nonlinear differential equations of motion into a set of nonlinear algebraic equations. System response can be obtained by solving this nonlinear equation set in terms of the unknown Fourier coefficients. The accuracy of the solution is great...
Nonlinear 3D Modeling and Vibration Analysis of Horizontal Drum Type Washing Machines
Baykal, Cem; Ciğeroğlu, Ender; Yazıcıoğlu, Yiğit (2020-01-01)
In this study, a nonlinear 3-D mathematical model for horizontal drum type washing machines is developed considering rotating unbalance type excitation. Nonlinear differential equations of motion are converted into a set of nonlinear algebraic equations by using Harmonic Balance Method (HBM). The resulting nonlinear algebraic equations are solved by using Newton’s method with arc-length continuation. Several case studies are performed in order to observe the effects of orientation angles of springs and damp...
Nonlinear resonances of axially functionally graded beams rotating with varying speed including Coriolis effects
Lotfan, Saeed; Anamagh, Mirmeysam Rafiei; Bediz, Bekir; Ciğeroğlu, Ender (2021-11-01)
The purpose of the current study was to develop an accurate model to investigate the nonlinear resonances in an axially functionally graded beam rotating with time-dependent speed. To this end, two important features including stiffening and Coriolis effects are modeled based on nonlinear strain relations. Equations governing the axial, chordwise, and flapwise deformations about the determined steady-state equilibrium position are obtained, and the rotating speed variation is considered as a periodic distur...
FRF decoupling of nonlinear systems
Kalayclogiu, Taner; Özgüven, Hasan Nevzat (2018-03-01)
Structural decoupling problem, i.e. predicting dynamic behavior of a particular substructure from the knowledge of the dynamics of the coupled structure and the other substructure, has been well investigated for three decades and led to several decoupling methods. In spite of the inherent nonlinearities in a structural system in various forms such as clearances, friction and nonlinear stiffness, all decoupling studies are for linear systems. In this study, decoupling problem for nonlinear systems is address...
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.