Robust two degree-of-freedom add-on controller design for automatic steering

A robust 2-DOF add-on controller design based on the disturbance observer is presented in this paper for improved performance in vehicle automatic steering. The application example is the benchmark problem on automatic steering of a city bus with large variations in mass and speed and for which the reference maneuvers and specifications are available in the literature. The analytical formulation of the compensator is presented, followed by evaluation and demonstration of the enhanced model regulation and disturbance rejection properties achieved by its use. Improved steering dynamics can be achieved using yaw rate feedback without the need for a yaw rate sensor. Noting that the steering angle rate actuator saturation forms a major limitation of performance, especially in the presence of the integrating actuator used in the city bus example, the performance enhancement due to the disturbance observer-based add-on compensator is investigated in the presence of actuator saturation. Finally, a disturbance feedforward-based add-on compensator is also presented for well-defined reference trajectories like the entering a bus stop bay maneuver, enabling preview.