Nonlinear adaptive observer for side slip angle and road friction estimation

The side slip angle of a vehicle as well as the tire-road friction coefficient are important inputs for vehicle dynamics control system and automated driving modules. However measurement of these parameters are difficult and costly in mass production vehicles and need to be reliably and accurately estimated. We address the observer design problem for simultaneously estimating side slip angle and tire-road friction utilizing information from vehicle Electric Power Steering System (EPS). A key observation is that the vehicle dynamics can be transformed into a lower-triangular form. For non-affine parametrized systems in such a form we propose a nonlinear adaptive observer and prove the uniform exponential stability of the estimation error by constructing a strict Lyapunov function. The design procedure is subsequently applied to the vehicle observer design problem. Simulations demonstrate the robustness of the proposed observer against modeling error and measurement noise.