A Differential Brake-Actuated Steering System for Redundancy of Steer-by-Wire

Steer-by-Wire (SbW) system offers advantages for vehicle electrification and autonomous driving. To utilize SbW system, it is essential to prioritize safety and develop reliable backup systems. This paper introduces a backup system that employs longitudinal differential forces to address SbW system failures. When the SbW system malfunctions and cannot perform its intended function, the proposed system controls lateral motion instead. The proposed system achieves this control by utilizing the differential longitudinal forces between the left and right sides. The system model incorporates the lateral motion and wheel steering models to consider the effect of the differential longitudinal forces. Additionally, sensitivity analysis is conducted for the model representing the changes in the front wheel steering angle. As a result, the parameters are mapped with respect to the vehicle speed which has the most significant impact. A model-based state observer based on Kalman filter estimates the unobservable vehicle states. A controller based on a linear quadratic regulator is developed to accurately perform the steering motion of the vehicle according to the driver's intention. The proposed brake-actuated steering system is validated through simulations and real-world experiments. The results show that it enables the vehicle to perform critical operations, such as lane changes, when SbW fails.