Nonlinear model predictive control for autonomous vehicle drifting
In this paper, nonlinear model predictive control (NMPC) is proposed for autonomous vehicle drifting, that is, stabilizing the vehicle at a desired unstable equilibrium point. Firstly, a three-degree-of-freedom vehicle model with a nonlinear tire model is introduced, and the equilibrium points are calculated. The relationship between the desired unstable equilibrium point and the lateral stability region is analyzed based on the phase plane method. Secondly, NMPC is designed to force vehicle states to stay around the desired unstable equilibrium point, that is, to keep the vehicle in sustained drifting. The terminal region and terminal constraint of NMPC are determined off-line to guarantee stability. Thirdly, Koopman operator theory and dynamic mode decomposition with control are introduced to obtain an approximately linear model, by which the nonlinear optimization problem is converted to a quadratic programming problem. Finally, comparative experiments are conducted by simulation, in which various model uncertainties are considered. The effectiveness of the proposed approach to achieve sustained autonomous drifting and to ensure vehicle safety is illustrated, and the efficient implementation of the proposed approach is also shown.