ALL FIXED-ORDER H-INFINITY CONTROLLERS - OBSERVER-BASED STRUCTURE ANDCOVARIANCE BOUNDS

This note obtains a parameterization of the set of all stabilizing con trollers of order less than or equal to the plant, which yields for th e closed-loop transfer matrix a specified Hr norm hound. The algebraic results of covariance control are applied to the H-infinity control p roblem to yield a parameterization in terms of the Lyapunov matrix, wh ich carries many system properties (such as H-2 performance, covarianc e bounds, system entropy at infinity etc.). All low-order H-infinity c ontrollers are shown to have observer-based structure for ''reduced-or der models'' of the plant and are characterized by two Riccati equatio ns with a coupling condition.