Research Objective: Several vehicle manufacturers have recently developed a steer-by-wire system as a solution to reduce vehicle production cost and packaging issues. While this technology has gained attention, a failure in this system could pose a serious safety hazard for passengers. To overcome this problem, direct yaw moment control (DYC) using dual motors can be a solution. This paper aims to analyze the performance of DYC in the steering system failure situation by considering factors such as yaw rate and safety margin. Methodology: In order to evaluate the performance of the fault tolerant control using dual motor, a safety margin is proposed as a performance index. The safety margin is defined as the minimum distance to the front object where collision avoidance using DYC is possible. In this paper, a vehicle dynamics simulation is conducted to analyze DYC performance. In this simulation, the steering angle is fixed, and different torque commands are applied on each motor. The torque command is limited to ensure that the longitudinal and lateral tire forces are located within the grip circle. As a result of DYC, vehicle path is changed though steering angle fixed at zero. Consequently, considering the vehicle size and road width, the minimum distance in order to avoid collision is obtained. Results: In this paper, a vehicle model with rear dual motor of 153N maximum torque and 125kW power is utilized. The vehicle dynamic simulation is conducted on IPG Carmaker interface. In this simulation environment, DYC based collision avoidance maneuver is implemented from 30km/h to 110km/h. As a result of the simulation, the maximum torque that can be applied within the tire grip circle at a speed of 110km/h was derived as 98.3N. At this point, a maximum yaw rate of 0.151rad/s was generated. When the length of the vehicle is 5.005m and the width is 1.925m, the safety margin is derived as 41.9756m. Limitations: There are several limitations in this paper. Firstly, the fault tolerant control performance is evaluated in a simulation environment, which does not consider uncertainties such as irregularities of road surface, change in vehicle weight, and weight transfer. Second, the performance of DYC is analyzed on constant vehicle speed. It means that a fusion of regenerative braking and DYC was not used to derive the collision avoidance maneuver. Contributions: Despite the limitations mentioned above, this paper has the following contributions. First, a fault tolerant control strategy using the rear dual motor system is proposed to cope with failures of the steer-by-wire system. This domain-cooperative control architecture ensures that the vehicle system remains safe in the event of a steering system failure. Second, the performance of the proposed control strategy using the rear dual motor is evaluated at each speed, and the maximum performance is presented as the safety margin. Based on these evaluation results, fault tolerant control strategies or decision making process can be established for several driving situations. Conclusion: In conclusion, this study analyzes the performance of fault tolerant control for steer-by-wire system failure using the real dual motor system, taking into account vehicle dynamic properties such as tire grip circle are taken into consideration during the analysis. This research result can be utilized to design and evaluate fail-safe control architecture of the vehicle with steer-by-wire systems.
Dr. Eng. Kicheol Jeong, Senior Researcher, Korea Automotive Technology Institute