Estimation of road frictional force and wheel slip for effective antilock braking system (ABS) control

The introduction of electric braking via brake-by-wire systems in electric vehicles) has reduced the high transportation delays usually involved in conventional friction braking systems. This has facilitated the design of more efficient and advanced control schemes for antilock braking systems (ABSs). However, accurate estimation of the tire-road friction coefficient, which cannot be measured directly, is required. This paper presents a review of existing estimation methods, focusing on sliding-mode techniques, followed by the development of a novel friction estimation technique, which is used to design an efficient ABS control system. This is a novel slip-based estimation method, which accommodates the coupling between the vehicle dynamics, wheel dynamics, and suspension dynamics in a cascaded structure. A higher-order sliding-mode observer-based scheme is designed, considering the nonlinear relationship between friction and slip. A first-order sliding-mode observer is also designed based on a purely linear relationship. A key feature of the proposed estimation schemes is the inclusion of road slope and the effective radius of the tire as an estimated state. These parameters impact significantly on the accuracy of slip and friction estimation. The performance of the proposed estimation schemes are validated and benchmarked against a Kalman filter (KF) by a series of simulation tests. It is demonstrated that the sliding-mode observer paradigm is an important tool in developing the next generation ABS systems for electric vehicles.