Robust estimation of road friction coefficient

Vehicle active safety systems stabilize the vehicle by controlling tire forces. They work well only when the commanded tire forces are within the friction limit. Therefore, knowledge of the tire/road friction is important to improve the performance of vehicle active safety systems. This paper presents two methods to estimate the friction coefficient: one based on lateral dynamics, and one based on longitudinal dynamics. The two methods are then integrated to improve working range of the estimator and robustness. The first method is a nonlinear observer based on vehicle lateral/yaw dynamics and Brush Tire model, the second method is a recursive least squares method based on the relationship between tire longitudinal slip and traction force. The performance of the estimation algorithm is verified using test data under a wide range of friction and speed conditions.