Design of A Differential Braking Controller Based on Road Reaction Force Estimation to respond to Steering System Failure in Autonomous Driving Situations

As the electrification of chassis systems accelerates, the demand for fail-safety strategies is increasing. In the past, the steering system was mechanically connected, so the driver could respond directly to some extent. However, the Steer-By-Wire system is composed of the column and rack bar as electrical signals, so the importance of response strategies for steering system failure is gradually increasing. When a steering system failure occurs, a differential braking control using the difference in braking force between the left and right wheels was studied. Recently, some studies have been conducted to model the wheel reaction force generated during a differential braking. Those studies linearly modeled the reaction force generated at the kingpin axis. Since actual tires and road surfaces are nonlinear and cause large model errors, model-based control methods have limited performance. Also, in previous studies assumed that the driver normally operates the steering wheel in a failure situation. However, if limited to a situation such as autonomous driving, the driver's handling may become abrupt. So, the information from the steering wheel angle sensor is difficult to trust for calculating the target yaw rate. Therefore, this paper proposes a new fail-safety strategy to prepare for steering failure in an autonomous driving situation. In this study, the target yaw rate was calculated using a camera sensor instead of steering wheel angle sensor, when the steering fails. And we estimated the road surface reaction force generated during a differential braking by using a disturbance observer. And the braking controller was configured to compensate for the estimated road reaction force. The proposed differential braking controller was designed by using MATLAB/CarSim. We confirmed that the method proposed in this study was more effective than the existing method in situations where road surface reaction force increased.