A note on nonlinear ℋ∞ control of two-block interconnected systems

Nonlinear ℋ_∞ control for a class of two-block interconnected systems is investigated. The situation where the regular nonlinear ℋ_∞ suboptimal control problem is solvable for one of the blocks is considered. An auxiliary nonlinear system is defined from the original two-block system, and it is shown that a feedback solution to the nonlinear ℋ_∞ suboptimal control problem for this auxiliary system, also applies to the nonlinear ℋ_∞ suboptimal control problem for the original two-block system. The advantage of the auxiliary problem over the original problem is that the auxiliary penalty variable has lower dimension than the original penalty variable