This paper presents H-infinity and H-2 techniques for the design of a
certain class of prefilters. In order to avoid high order prefilters w
e use a particular structure in which both prefilter and feedback cont
roller share the same dynamics. The suboptimal H-infinity design probl
em is shown to be equivalent to an linear matrix inequality (LMI) cons
traint, and the solution of the H-2 optimal design may be obtained by
a simple formula. The H-2 technique developed will be illustrated on a
helicopter example and compared with classical techniques.