Fuzzy logic control of a stiffness-adaptable seismic isolation system

Traditional passive isolation systems have been shown to provide satisfactory seismic mitigation performance under typical far-field earthquake conditions. However, because of the untunable designated isolation period, the performance of these systems is still strongly affected by the low-frequency resonance phenomenon observed during near-fault earthquakes, which are usually dominated by a long-duration impulse. The use of semi-active isolation systems with variable stiffness, which are usually equipped with a customized control algorithm to determine the isolation stiffness or period in real time, could be a promising solution to overcome this problem. To realize this technology, this study develops a smart isolation system that combines the leverage-type stiffness-controllable isolation system (LSCIS) with a simple fuzzy logic control (FLC). Because the proposed FLC merely requires feedback from the velocity and displacement responses of the isolation base, it is quite easily implemented. Theoretical analysis shows that the extreme displacement of the isolation level caused by near-fault seismic waves can be mitigated by the proposed system with tolerable acceleration response variations compared with that of the traditional passive isolation system. The movement of the pivot point of the LSCIS can also be adjusted to address the inherent hardware constraint that limits its practical application. Experimental verification conducted on a shake table further demonstrates the feasibility of integrating the LSCIS system with FLC after proper consideration of the friction effect generated by the leverage mechanism and the inevitable time delay effect.