Research questions: Active rear wheel steering system (ARWS), because of its ability to improve vehicle handling stability and maneuverability, has been widely used in long wheel-base manned or autonomous driving areas. For the autonomous driving technology, the existing independent ARWS is helpful for the chassis dynamics control as its steering angle control is independent and flexible. However, its functional safety risk is high. And few existing ARWS notice the ability to improve braking stability. So, this paper focuses on proposing an Integrated Dual-Mode Active Rear Wheel Active Steering System (IDM-ARWS) and its control method which can realize the above functions. Methodology: First, the detailed structure of the proposed IDM-ARWS is discussed, which uses an integrated actuator to improve high and low speed steady-state steering characteristics and directional stability during braking. Then, from the perspective of vehicle dynamics, the dynamic mechanism of rear wheel toe-in control to improve the directional stability of the vehicle during braking is expounded. Second, based on the power bond graph method, the dynamic model of the rear wheel active steering system is established in Simulink. Based on that, a robust control strategy for the electromechanical actuator of the proposed IDM-ARWS is presented to suppress internal and external perturbations caused by sensor noise and parameter uncertainty. After that, for the vehicle application purpose, a hierarchical control strategy for IDM-ARWS is proposed to improve high and low speed steady-state steering characteristics and directional stability during braking considering the suspension K&C characteristic. Results: Functional feasibility of dual-mode rear wheel angle movement and effectiveness of the hierarchical control strategy are evaluated in both steady state and transient condition simulation. It can be determined that the stability of vehicle get improved during braking by applying rear wheel toe-in control. The results also show that by applying the hierarchical control strategy, the understeering characteristic is suppressed and the turning radius of the vehicle gets decrease when steering at low speed; the response of yaw rate and side slip angle also get reduced when steering at high speed. What’s more, the simulation results also indicate that when suffering from same lateral interference, active rear wheel toe-in control can make the car has less lateral displacement and less prone to lose stability. And by taking the influence of suspension K&C characteristic into account, the controller can further improve the stability of the vehicle during braking. Limitation of this study: Firstly, when developing the rear wheel angle control strategy, the influence of the suspension K&C characteristics on the actual side slip angle of the rear wheel is considered only under the braking condition. Then, this paper does not discuss in depth to optimize the control strategy especially for the commonly occurring complex conditions with both steering and braking. All these will be researched further to improve the proposed control strategy. What does the paper offer that is new in the field in comparison to other works of the author? So far, most of the research toward rear wheel active steering technology focused on the performance improvement of vehicle under steering condition. This paper pays attention to improve both the stability in cornering and in braking. In particular, the proposed dual-mode rear wheel steering system simultaneously realizes uniform-direction steering and opposite-direction steering for different driving condition by using only one actuator through switching mode, so the functional reliability is enhanced. What’s more, by taking the influence of the suspension K&C characteristics into account, the control effect is further improved. Conclusion: The proposed IDM-ARWS just uses one actuator to realize uniform-direction steering and opposite-direction steering for different driving condition, compared to the existing independent ARWS. Therefore, it can better meet the high functional safety requirements of autonomous driving technology. The proposed hierarchical control strategy can improve high and low speed steady-state steering characteristics and directional stability during braking. Meanwhile, in the presence of disturbances, the proposed robust controller can track the target rear wheel angle well. By taking the influence of suspension K&C characteristic into account, the vehicle performance gets further improved.
Mr. Wang Junnian, Professor, State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, China