Integral Reinforcement Learning for Linear Continuous-Time Zero-Sum Games With Completely Unknown Dynamics

In this paper, we develop an integral reinforcement learning algorithm based on policy iteration to learn online the Nash equilibrium solution for a two-player zero-sum differential game with completely unknown linear continuous-time dynamics. This algorithm is a fully model-free method solving the game algebraic Riccati equation forward in time. The developed algorithm updates value function, control and disturbance policies simultaneously. The convergence of the algorithm is demonstrated to be equivalent to Newton's method. To implement this algorithm, one critic network and two action networks are used to approximate the game value function, control and disturbance policies, respectively, and the least squares method is used to estimate the unknown parameters. The effectiveness of the developed scheme is demonstrated in the simulation by designing an H-infinity state feedback controller for a power system. Note to Practitioners-Noncooperative zero-sum differential game provides an ideal tool to study multiplayer optimal decision and control problems. Existing approaches usually solve the Nash equilibrium solution by means of offline iterative computation, and require the exact knowledge of the system dynamics. However, it is difficult to obtain the exact knowledge of the system dynamics for many real-world industrial systems. The algorithm developed in this paper is a fully model-free method which solves the zero-sum differential game problem forward in time by making use of online measured data. This method is not affected by errors between an identification model and a real system, and responds fast to changes of the system dynamics. Exploration signals are required to satisfy the persistence of excitation condition to update the value function and the policies, and these signals do not affect the convergence of the learning process. The least squares method is used to obtain the approximate solution for the zero-sum games with unknown dynamics. The developed algorithm is applied to a load-frequency controller design for a power system whose parameters are not known a priori. In future research, we will extend the results to zero-sum and nonzero-sum differential games with completely unknown nonlinear continuous-time dynamics.