Experimental and theoretical stability analysis of damped auto-parametric pendulum

Abstract eng:
Engineering structures subjected to dynamic loading are often equipped with special devices proposed to suppress adverse motion. However, these installations are often designed with certain stereotyped approach, considering only simplified linear single-degree-of-freedom (SDOF) model which in practice is often not sufficient. Typical examples, where the practical results differ from theoretical design are dynamic absorbers on towers. These dampers are often designed as a pendulum supposed to be effective in one direction only, ceasing however its functionality due to neglect of some features. The article describes an experimental and theoretical treatment of the auto-parametric pendulum damper modelled as a double-degree-of-freedom (DDOF) system. It is strongly non-linear, with the harmonic excitation being applied in the suspension point and interpreted for instance as an excitation by wind. The stability of the motion in a vertical plane is analysed in the meaning of semi-trivial vibration. This effect of stability loss and possible stability regain is very important from practical point of view, because the motion of an damper beyond the stability limits may act negatively and thus endanger the structure itself. Special experimental frame was developed. It contains a pendulum supported by the cardan joint and excited by a shaker. There are two magnetic units attached to the frame and to the supporting axes of rotation. The are able to reproduce linear viscous damping for both degrees of freedom. The stability of the system is analysed experimentally and compared with theoretical results.

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