Hierarchical Hybrid Steering Control for Four-Wheel-Steering Vehicles Considering System Delays

This paper proposes a hierarchical hybrid steering control scheme for four-wheel-steering vehicles to deal with steer-by-wire system time delays and vehicle nonlinear characteristics. First, a variable steering ratio is developed considering the influence of the front and rear wheels' angles on the vehicle's steering characteristic. Then a hybrid feedforward controller combining with steady-state control, dynamic compensation, and oblique steering compensation is established to enhance vehicle dynamics stability in extreme driving conditions, taking into account the reference yaw rate and road adhesion constraints. Meanwhile, an adaptive linear quadratic regulator-based feedback controller with a piece-wise affine tire model is put forward to account for the influence of vertical load, road adhesion condition, and yaw rate tracking error. The controller adopts an adaptive weighting coefficient mechanism to satisfy various working conditions based on different steady-state gains of the front and rear wheel angles over the yaw rate. Finally, the effectiveness of the proposed scheme is experimentally verified under comprehensive testing scenarios through Hardware-in-the-Loop tests. The results show that the proposed scheme has superior performance relative to the state-of-the-art methods.