Model estimation and controller reduction: dual closed loop identification problems

Algorithms for direct controller reduction by identification in closed loop have been previously proposed (Landau and Karimi, 2000, and Karimi and Landau, 2000). In this paper it is shown that the plant model identification in closed loop using closed loop output error identification algorithms and the direct estimation in closed loop of a reduced order controller feature a duality character. Basic schemes, algorithms and properties of the algorithms can be directly obtained by interchanging the plant model and the controller. In the last part of the paper the interaction between plant model identification in closed loop and direct controller reduction is emphasized