COTSIM-based Gain Optimizer for Feedback Control of Advanced Scenarios in NSTX-U

An optimizer has been developed to tune the gains of fixed-structure scenario controllers in NSTX-U. As indicated by their name, fixed-structure controllers have a well-defined structure but are functions of to-be-tuned gains. This is the case, for instance, of the widely used Proportional-Integral-Derivative (PID) controller, which is usually empirically tuned. As alternative, a more systematic approach to gain tuning is proposed in this work by selecting the gains based on a predefined, user-defined, optimization criterion. This criterion is expressed in terms of a to-be-minimized cost function weighing different metrics associated with closed-loop control performance such as tracking overshoot/undershoot, rise times, disturbance rejection, and steady-state tracking error. This optimization approach is enabled by closed-loop simulations carried out by using the Control Oriented Transport SIMulator (COTSIM) for NSTX-U, which is in turn wrapped around an optimizer based on the Sequential Quadratic Programming (SQP) technique. SQP is capable of solving arbitrary nonlinear optimization problems while handling constraints on the to-be-optimized parameters. The effectiveness of the method is illustrated for several scenario-control objectives.