Vibration damping system

A vibration damping system adapted for use on top of a structure, comprises: a weight disposed on the top of the structure in a manner such that it can undergo simple harmonic oscillation; an acceleration sensor provided at the structure top for detecting the structure vibration in terms of acceleration; a weight driving mechanism for actuating the weight into the simple harmonic oscillation; and a phase and amplitude controller assembly for performing integration twice on the signal coming from the acceleration sensor, once to obtain the velocity signal, then to obtain the displacement signal, for reversing the sign of the velocity signal when the velocity signal exceeds a predetermined value, and for controlling the weight driving mechanism on the basis of the sign-reversed velocity signal so as to attenuate the vibration of the structure by giving the energy of the weight performing simple harmonic oscillation under control of the phase and amplitude controller assembly to the structure in optimum timing and with an optimum magnitude.