Composite Steering Strategy for 4WS-4WD EV Based on Low-Speed Steering Maneuverability

A composite steering control strategy, which combines four-wheel steering (4WS) and differential steering, is proposed in this paper, to optimize steering maneuverability in the conditions where the vehicle speed is below 15 Km/h, mainly for U-turning and parking conditions. A dynamic model is developed for the steering system and the tire system. Taking different steering wheel inputs into consideration, a 4WS control strategy proportional to the front wheel steering angle is quoted to improve the steering maneuverability in the low speed conditions and guarantee the manipulability by controlling the side slip of the vehicle. Based on the 4WS system, this paper explores the possibility of further improving the low-speed maneuverability of the vehicle through differential steering. And the differential steering control strategy is developed, including four hub-motor output modes. A composite steering controller is designed based on the 4WS-4WD electric vehicle platform. Through the real vehicle calibration tests, the output torque distribution coefficient of the hub motor in the differential steering control strategy is obtained, and the composite steering control strategy optimal for maneuverability is determined by comparing the turning diameters of the low-speed tests under the four modes. The experimental results show that compared with the single 4WS system, the composite steering system has an improved effect on the low-speed maneuverability of the vehicle, which can improve the handling performance of the vehicle under the low-speed condition.