Abstract: A novel recursive subspace method is developed based on fixed length moving window (FLMW) projection approximation used for estimating the modal parameter of linear time-varying structural system. The projection approximation subspace tracking (PAST) method with FLMW is developed to replace a singular value decomposition in order to decrease the computa-tional complexity. A 3 degree-of-freedom model with time-varying stiffness and a manipulator with moving mass are used to testify these two methods. The simulation results show that these two methods can avoid the influence of the old data efficiently, and the novel recursive method can estimate the parameter of linear time-varying system as efficiently as the recursive subspace method with FLMW, with the computational complexity decreasing at the same time.

Key words: Linear time-varying system, Parameter estimation, Projection approximation with moving window, Recursive subspace method