Handling plant-model mismatch in Koopman Lyapunov-based model predictive control via offset-free control framework

Koopman operator theory enables a global linear representation of a given nonlinear dynamical system by transforming the nonlinear dynamics into a higher dimensional observable function space where the evolution of observable functions is governed by an infinite-dimensional linear operator. For practical application of Koopman operator theory, various data-driven methods have been developed to derive lifted state-space models via approximation to the Koopman operator. Based on approximate models, several Koopman-based model predictive control (KMPC) schemes have been proposed. However, since a finite-dimensional approximation to the infinite-dimensional Koopman operator cannot fully represent a nonlinear dynamical system, plant-model mismatch inherently exists in these KMPC schemes and negatively influences the performance of control systems. In this work, we present offset-free Koopman Lyapunov-based model predictive control (KLMPC) framework that addresses the inherent plant-model mismatch in KMPC schemes using an offset-free control framework based on a disturbance estimator approach and ensures feasibility and stability of the control system by applying Lyapunov constraints to the optimal control problem. The zero steady-state offset condition of the developed framework is mathematically examined. The effectiveness of the developed framework is also demonstrated by comparing the closed-loop results of the proposed offset-free KLMPC and the nominal KLMPC.