Impacts of Dynamic Toe Angle Variations on Four-Wheel Independent Steering Control and their Optimization Strategies

Compared to traditional vehicles, four-wheel independent drive and four-wheel independent steering (4WID-4WIS) vehicles have gained significant attention from researchers due to their enhanced control flexibility and superior handling performance. The steering angle deviation caused by dynamic toe angle changes in two-wheel steering (2WS) systems is often minimal and hence overlooked. However, the impact becomes notably significant in 4WIS systems. This article contrasts the tire slip angle differences between 2WS and 4WIS, and delves into the effects of dynamic toe angle variations on 4WIS control. Solutions are proposed both in terms of steering angle control and suspension design. Firstly, a dynamic model for the 4WID-4WIS vehicle is established. Secondly, a hierarchical tire force distribution strategy is designed for trajectory tracking. The upper layer utilizes a sliding mode controller and PID controller to determine the total required longitudinal, lateral forces, and yaw moment for tracking. The middle layer allocates these combined forces and moments to individual tires based on constraint optimization, while the lower layer determines vehicle torques and steering angles according to the longitudinal and lateral forces exerted by each tire. In terms of steering control, feedforward control with bump steer compensation is implemented to improve wheel steering precision and lateral tire force control accuracy. Considering the toe angle variations during cornering, critical hardpoint coordinates are identified and optimized through sensitivity analysis in the suspension design, aiming to reduce dynamic toe angle changes. Lastly, the effectiveness of these proposed strategies is validated under constant radius and slalom scenarios using the co-simulation of Carsim and Matlab/Simulink. Simulation results highlight that toe angle changes due to suspension kinematic characteristics play a significant role in 4WIS control. The strategies proposed in this article notably improved tire slip angle errors, demonstrating superior yaw rate responses and lateral tracking accuracy during trajectory following.