Active Control and Seismic Isolation

An active control system is designed to induce artificial damping to the isolation structure of a building without increasing its stiffness. The control forces needed are exerted only on the base of the structure, and the system is quite practical. This control system can operate efficiently if the time delay that exists in the actual system does not exceed its critical value. If the delay is long, the steady‐state response will be resonant periodically at its critical delay times, and the transient response will become unstable. The critical delay time depends on both the fundamental frequencies of the structure and the artificial damping introduced by the control system. The higher modes of the system will suffer more of a stability problem than the lower modes, unless the nondimensional damping introduced to them is less than a specific value. A strategy of time‐delay compensation is established in this paper. The instability of the system can be efficiently prevented by a proper choice of artificial damping and artificial stiffness.