Algorithms for real-time estimation of individual wheel tire-road friction coefficients

It has long been recognized in the automotive research community that knowledge of the real-time tire-road friction coefficient can be extremely valuable for active safety applications, including traction control, yaw stability control and rollover prevention. Previous research results in literature have focused on estimation of average friction coefficient for the vehicle or on average friction coefficient for both drive wheels of the vehicle. This paper explores the development of algorithms for reliable estimation of friction coefficient at each individual wheel of the vehicle. Three different algorithms are proposed based on the types of sensors available-one that utilizes engine torque, brake torque and GPS measurements, one that utilizes torque measurements and an accelerometer and one that utilizes GPS measurements and an accelerometer. These algorithms are first evaluated in simulation and then evaluated experimentally on a Volvo XC90 sport utility vehicle. Experimental results demonstrate that friction coefficients at the individual wheels and road gradient can both be estimated reliably. Individual wheel friction measurements are expected to be more valuable for active safety systems than average friction measurements.