Nonlinear responses of spherical pendulum vibration absorbers in towerlike 2DOF structures

This paper investigates the vibration control of a flexible towerlike structure using a single spherical pendulum vibration absorber (SPVA). The SPVA is attached to the structure, which is modeled as a two-degree-of-freedom system, subjected to horizontal, harmonic excitation. In the theoretical analysis, the nonlinearity of the restoring moment of the SPVA is considered, and new coordinates are implemented to avoid the non-integrability of the equations of motion due to a singularity. Van der Pol's method is used to determine the frequency response curves for the structure and SPVA. When the excitation direction is , the structure is only excited in the x-direction; however, the structure vibrates in both the x- and y-directions due to nonlinear coupling between the structure and SPVA, known as autoparametric resonance. Clockwise and counterclockwise whirling motions appear in the structure and SPVA depending on the excitation frequency. On part of the branches of the response curves, Hopf bifurcations occur followed by amplitude-modulated motions, and combination resonances are also observed in the numerical simulations. When , the structure is equally excited in the x- and y-directions; however, the vibrations in these directions appear at different amplitudes in the frequency response curves. Such phenomena also occur in the SPVA. The influence of the spring constant of the structure in the y-direction on the frequency response curves is also investigated. Experiments were conducted, and their results confirmed the validity of the theoretical analysis.