Vehicle state estimation for anti-lock control with nonlinear observer

• Observer design is made to fulfill the requirements of anti-lock braking control.
• Observability for state estimation is found to be lacking in some driving situations.
• A switched nonlinear observer is proposed in order to deal with this problem.
• Tire-road friction estimation based on the vehicle longitudinal motion is introduced.
• Stability properties of the observer are analyzed using a Lyapunov based method.

Vehicle state estimation during anti-lock braking is considered. A novel nonlinear observer based on a vehicle dynamics model and a simplified Pacejka tire model is introduced in order to provide estimates of longitudinal and lateral vehicle velocities and the tire-road friction coefficient for vehicle safety control systems, specifically anti-lock braking control. The approach differs from previous work on vehicle state estimation in two main respects. The first is the introduction of a switched nonlinear observer in order to deal with the fact that in some driving situations the information provided by the sensor is not sufficient to carry out state estimation (i.e., not all states are observable). This is shown through an observability analysis. The second contribution is the introduction of tire-road friction estimation depending on vehicle longitudinal motion. Stability properties of the observer are analyzed using a Lyapunov function based method. Practical applicability of the proposed nonlinear observer is shown by means of experimental results.