An optimal traction, braking, and steering coordination strategy for stability and manoeuvrability of a six-wheel drive and six-wheel steer vehicle

This paper describes an optimal traction, braking and steering coordination to improve vehicle lateral stability and manoeuvrability of a six-wheel driving/six-wheel steering (6WD/6WS) vehicle. The optimal coordination controller consists of an upper and lower level controller. The upper level controller determines front, middle steering angle, desired net yaw moment, and longitudinal net force according to the reference velocity and steering angle corresponding to a manual driver. The desired yaw moment is calculated by sliding-mode control theory. Based on the desired longitudinal net force, yaw moment, and tyre force information as inputs from the upper level controller, the lower level controller determines distributed lateral tyre forces and longitudinal tyre force on each wheel in proportion to the size of the friction circle of each wheel. The size of a friction circle is estimated using longitudinal/lateral velocity, yaw rate, wheel torque, and wheel angular velocity. Vehicle–driver–controller closed-loop simulations have been conducted to investigate the improved performance of the proposed optimal coordination controller over a conventional direct yaw moment controller (DYC) of the vehicle equipped with a mechanical drive system.