Simplifying H∞ controller synthesis via classical feedback system structure

The H_∞-optimal controller synthesis problem has previously been shown to lead to the solution of two algebraic Riccati equations. The properties of this solution are explored in the framework of the classical cascade controller feedback structure. In addition to allowing effective tradeoff between significant closed-loop transfer functions, the system structure is shown to simplify controller computation substantially in the practically important case of open-loop stable plant and weights, because at most one Riccati equation needs to be solved