Vibration control using switchable stiffness
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Over the last decade, the switchable stiffness (SS) control strategy has seen renewed academic interest due to the need for effective vibration control techniques. The SS control strategy involves switching the system’s stiffness between at least two distinct states. The system switches to a high stiffness state when the mass is moving away from the equilibrium and a low stiffness state when the mass is moving towards the equilibrium. The SS strategy has been shown to be both theoretically and experimentally effective for shock isolation and residual vibration suppression. The research presented in this thesis investigates the theory and implementation of the SS control. An experimental apparatus with an electromagnet (EM) actuator as a switchable stiffness spring is presented for the testing and implementation of the SS control. The non-linear dynamics, properties, and parameters are characterized through experimental identification. A detailed analytical dynamic model of the system is derived and verified. A series of computer simulations reveal the mechanism, stability, and properties of the SS strategy. It is shown that potential energy is dissipated from the system through stiffness reduction. A relationship is developed between the stiffness ratio and the amplitude reduction. The simulations also show a possible instability problem due to time delays. A novel delayed SS strategy, involving the introduction of an intentional delay, is presented to overcome this problem. Simulations verify the effectiveness and limitations of the delayed SS strategy. The SS control strategy is implemented in real-time. The experiments verify the instability due to time delays and the efficacy of the delayed SS strategies despite the system non-linearities. The performance of the system is, however, shown to be severely limited by the dynamics of the EM. It is postulated that a SS actuator with low energy input, high stiffness variation, and fast-switching times will enhance the performance significantly. The SS control strategy for 2DOF systems is investigated. A 2DOF system made up of two SDOF systems coupled by a beam is introduced. The generalized 2DOF system equations and models are derived. Preliminary simulations verify that the direct SS control strategy is effective at suppressing the vibrations of linear and non-linear 2DOF systems. Simulations also show that the delayed SS strategy is effective at suppressing vibrations for symmetrical 2DOF systems with time delays.
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