Real-time identification of tire-road friction conditions

Tire-road friction characteristics are deeply interlaced with all vehicle dynamics control systems, as road conditions strongly affect the control schemes behaviour. This work aims at the real-time estimation of the wheel slip value corresponding to the peak of the tire-road friction curve, in order to provide anti-lock braking systems (ABS) with reliable information on its value upon activation. Different techniques based on recursive least squares and the maximum likelihood approach are used for friction curve fitting and their merits and drawbacks thoroughly examined. In addition, since one of the main issues in slip-based friction estimation during braking is vehicle speed estimation, an effective algorithm for addressing this task is developed. The proposed peak slip value estimation strategy is analysed and tested both in simulation and on data collected on an instrumented test vehicle. In the latter case, the vehicle speed estimation algorithm is used, and the estimated vehicle speed provided as input for friction estimation. Practical applicability constraints posed by typical ABS systems are also considered.