Study of two different tunable vibration absorbers : variable stiffness absorber and variable damping absorber

Abstract

A structure or machine w ill vibrate at the frequency of excitation when excited by a harmonic force. Vibration of the structure can be suppressed using a vibration absorber by setting the absorber’s natural frequency equal to the exciting frequency. However, when the exciting frequency varies, vibration of the structure attached with the vibration absorber may increase significantly. A tunable vibration absorber is an adaptive-passive device that can overcome the shortcoming of a traditional vibration absorber. Such a device is capable of adjusting its parameter on-line based on a feedback loop. In this thesis, two different tunable vibration absorbers are studied. The first one is a variable stiffness vibration absorber that provides a means to vary the absorber stiffness to ensure that the absorber frequency follows the exciting frequency. The second one is a variable damping vibration absorber that provides a means to vary the absorber damping to ensure the best performance of the absorber. Both the vibration absorbers are used to suppress vibration of a beam structure. The developed variable stiffness absorber is a cantilever-beam attached by a mass at its free end. The absorber stiffness is varied by changing the beam length through a DC motor drive system. Various models of the beam-absorber system are derived. The proposed tuning algorithm is based on the frequency information extracted from spectrum of the response. Using a simplified 2-degree-of-freedom (D O F) model, the computer simulation tests the tuning algorithm ’s ability to track a step change and a linear change in the exciting frequency. Factors affecting the performance of the absorber are investigated. The developed variable damping absorber is an electromagnetic damper th a t is based on the eddy current principle. Analytical study shows that there exist two crossing frequencies which determine whether damping should or should not be added to the absorber system. An on/off tuning control strategy is developed. Using a 2-DO F system, the computer simulation tests the tuning strategy when the exciting frequency experiences a multi-step change. For both the absorber systems, the experiment study focuses on validation of the tuning algorithms and addresses some practical issues encountered in implementation.