Reinforcement Learning is Direct Adaptive Optimal Control

Control problems can be divided into two classes: 1) regulation and tracking problems, in which the objective is to follow a reference trajectory, and 2) optimal control problems, in which the objective is to extremize a functional of the controlled system´s behavior that is not necessarily defined in terms of a reference trajectory. Adaptive methods for problems of the first kind are well known, and include self-tuning regulators and model-reference methods, whereas adaptive methods for optimal-control problems have received relatively little attention. Moreover, the adaptive optimal-control methods that have been studied are almost all indirect methods, in which controls are recomputed from an estimated system model at each step. This computation is inherently complex, making adaptive methods in which the optimal controls are estimated directly more attractive. Here we present reinforcement learning methods as a computationally simple, direct approach to the adaptive optimal control of nonlinear systems.