Non-smooth active control of seismic vibrations

In order to prevent seismic damage in civil buildings and mechanical equipment, e.g. during transport, passive isolators and dampers are commonly used. In the present work, a non-smooth strategy for active seismic isolation is presented. The method provides isolation from base excitation by dynamically switching the stiffness of the foundation. The controller works as follows: when the seismic movement is active, the velocity of the moving mass is monitored as the reference velocity. When such reference velocity is positive, the stiffness is reduced; when it is negative, the stiffness is increased. Numerical investigations show that the controller is capable to filter seismic excitation close to the natural frequency of the controlled system, and to reduce the total seismic energy transfer up to 5 times. The role played by the gravity in the active vibration filtering is pointed out by showing that no filtering action can be observed in gravity-free simulations. Even though the controlled system is linear, the controller presents a non-smooth fluctuation of the stiffness, so that a limit cycle occurs within the system.