Reduced Complexity Nonlinear H∞ Controllers: Relation to Certainty Equivalence

This paper considers the problem of constructing reduced complexity controllers for output feedback nonlinear H∞ control. Conditions are obtained under which such controllers achieve the closed loop performance requirements. These controllers are non-optimal in general. However, in case optimality holds, they are in fact the certainty equivalence controllers. Conditions under which certainty equivalence holds are simplified and linked to the solvability of a functional equation.