Regional Control Laws as Fallback Strategy for Nonlinear MPC: Application to Wind Turbine Control

We propose a method for determining fallback control laws for nonlinear MPC problems. A fallback strategy is needed whenever a nonlinear MPC problem cannot be solved in due time. In this paper, we use the concept of regional MPC to compute local control laws that mimic the solution expected from the original problem. In regional MPC, the piecewise affine structure of the solution of linear-quadratic optimization problems is exploited by computing the current polytope and control law online. We use the regional control law, which is naturally stabilizing for more states than its corresponding polytope, and dismiss its region. In every time step, we use the solution of the original MPC problem as a basis to determine the regional control law. If in one time step no solution can be found, the regional control law from the previous time step is used as a fallback strategy. We show the effectiveness with an example of a wind turbine.