Development and Implementation of Torque Feedback for Steer-by-wire systems

Abstract

Steer-by-wire (SbW) has gained significant attention in recent years, revolutionizing vehicle steering by eliminating the mechanical connection between the steering wheel and steering rack. However, by eliminating the connection, the external forces acting on the steering rack are lost and no informative torque feedback will be given to the driver. A solution to this problem is to simulate the corresponding forces by implementing torque feedback using a Force Feedback (FFb) actuator. The focus of this master’s thesis was to develop and implement torque feedback for an SbW system. The objective was to create a steering feel model that simulated a base torque profile and incorporate a set of fundamental features such that the steering feel resembled a conventional steering system with a mechanical connection. Throughout the thesis, a hardware setup was available, integrated as a testing rig and in a testing vehicle for implementation, testing, and evaluation purposes. The system consisted of a steering wheel, a planetary gearbox, an FFb actuator, an Electronic control unit (ECU), and a steering rack. A closed-loop model was utilized to simulate the hardware setup in Simulink. The model incorporated the torque input from the driver, FFb actuator, and a reference generator i.e. the steering feel model, and was used for both implementation and testing purposes. After acceptable simulation results, the steering feel model was implemented in the ECU for real-life testing and evaluation of the torque feedback in the testing rig and vehicle. The initial step of designing the torque profile was by designing the base torque behavior. This was accomplished by deriving a set of system equations based on torque feedback measurements obtained from tests performed on a vehicle equipped with a conventional steering system. The equations contained a set of tunable parameters that were fine-tuned to accurately emulate the measured base torque. Some torque feedback features, such as End-stop were subjectively tuned as they were not based on the reference measurements. The development and implementation resulted in a steering feel model that closely resembled the behavior of the reference steering system, particularly for mid-range vehicle speeds. The steering feel for steering directions away from the center position was found to be satisfactory for all vehicle speeds for real-life testing on the testing rig and in the testing car. However, future work is required to refine the model and achieve hysteresis that fully emulates a conventional steering system for all vehicle speeds. Although, in order to improve the hysteresis, the implementation of a more efficient FFb actuator could minimize the significant influence of the cogging torque in the system. Future work on the features emulating understeer and oversteer is also required. Overall, this thesis contributed to advancing the understanding and implementation of torque feedback in SbW systems, providing insights for improving the steering feel and driving experience for such systems.