Model Reduction of LTI Discrete-Time Multivariable Systems using Pade Approximation and Stability Equation Method

A mixed method is presented for reducing the order of higher-order discrete-time multivariable systems. The reduced model denominator is achieved by stability equation method. The reduced model numerator coefficients in w-domain are chosen to fit a number of initial time-moments and Markov parameters of the higher-order system (HOS). The w-domain reduced order model (ROM) is then transformed back to the z-domain. This method approximates both low and high magnitude poles to give better matching in both the steady state and transient regions of step response. The method is illustrated by an example.