Nonlinear system identification framework of folding fins with freeplay using backbone curves

Due to wear and manufacturing tolerance, the freeplay is unavoidable in the hinges of folding fins, which exerts significant effects on the aerodynamic characteristics. This paper proposes a backbone-curve-based framework for the dynamical identification of folding fins containing the freeplay nonlinearity. With no need to measure the input force signal and the response signals of nonlinear related Degrees of Freedom (DOFs), the proposed method is more direct and elegant than most existing nonlinear identification approaches, and it contains three steps: Firstly, the underlying linear model of the folding fin structure is obtained through the modal test on its linear sub-parts, and then, the harmonic approximation solves the analytical expressions of the backbone curves of measurable DOFs. Secondly, response data measured from the sine-sweep test are used to extract the fitting points of backbone curves for these DOFs. Finally, the curve fitting approach is applied to identify the freeplay parameters. A series of numerical experiments verify the effectiveness of the proposed method. A real-life folding fin structure is also employed to illustrate how the method can be applied. These examples demonstrate that the identification framework can give an accurate dynamic model of the folding fin structure.