In this paper, we investigate the problem of closed-loop approximation of a
continuous-time controller by a low-order discrete-time controller with sa
mple and hold devices and an anti-aliasing filter. The problem is first red
uced to an open-loop problem of sampled-data controller reduction with cont
inuous-time weights. The operator representing the error in approximation c
an be approximated (arbitrarily closely) by a time-invariant discrete-time
system resulting from applying fast sampling and lifting. We propose a meth
od 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-infinity problem. The latter can be solved efficiently by the meth
od proposed by Clover et al.(1) We also give a numerical example to verify
the effectiveness of the method, Copyright (C) 1999 John Wiley & Sons, Ltd.