Two-level robust optimal control of large-scale nonlinear systems

Finding an optimal control strategy for a nonlinear uncertain system is a challenging problem in the area of nonlinear controller design. In this paper, a two-level control algorithm is developed for robust optimal control of large-scale nonlinear systems. For this purpose, using a decomposition/coordination framework, the large-scale nonlinear system is first decomposed into several smaller subsystems, at the first level, where a closed-form solution as a feedback of states and interactions is obtained to optimize each subsystem. At the second level, a substitution-type prediction method, as a coordination strategy, is used to compensate the nonlinear terms of the system and to predict the interaction between subsystems. The coordinator mainly evaluates the next update for the coordination parameters and continues to exchange information with the first level, so that the overall optimum solution is obtained. This approach is applicable to any large-scale nonlinear uncertain system with unstructured bounded uncertainties. The effectiveness and performance of the proposed approach are investigated through simulation of a nonlinear quadruple-tank process