Direct reduced order discretization of continuous-time controller

In this paper, we investigate the problem of closed-loop approximation of a continuous-time controller by a low-order discrete-time controller with sample and hold devices and an anti-aliasing filter. The problem is first reduced to an open-loop problem of sampled-data controller reduction with continuous-time weights. The operator representing the error in approximation can be approximated (arbitrarily closely) by a time-invariant discrete-time system resulting from applying fast sampling and lifting. We propose a method for obtaining a low-order discrete-time controller which makes small the approximation error based on recasting the approximation problem as a four-block H_∞ problem. The latter can be solved efficiently by the method proposed by Glover et al. We also give a numerical example to verify the effectiveness of the method.