A new representation of H∞ suboptimal controllers

Elementary state-space concepts are used to derive a novel representation of H_∞ suboptimal controllers in terms of pairs of parametric Riccati equations with a coupling constraint. Unlike the classical parametrization in terms of linear fractional transformation, this state-space representation has a homogeneous algebraic formulation which should help to make the most out of the suboptimal controller diversity. Potential applications include the design of reduced-order controllers and more generally the selection of suboptimal controllers to meet or optimize additional constraints.