Vehicle Pure Yaw Moment control using differential tire slip

Direct yaw moment control generated by differential friction forces on an axle has been proved to be effective in improving vehicle lateral yaw stability and in enhancing handling performance. It consists of two levels of control tasks: calculating a yaw moment command at vehicle level and regulating the tire slip to deliver the moment at wheel level. Advanced powertrain with electrical in-wheel-motor makes fast wheel level control possible. This paper proposes an adaptive tire slip controller for pure yaw moment generation, which yields the maximal axle yaw moment by asymmetric axle friction force with no effect on vehicle longitudinal speed. Since the maximal friction is limited by the tire-road contact, control constraints at various vehicle speeds and on different surface conditions has to be taken into account. This algorithm can generate the optimal longitudinal slip ratio at the presence of lateral tire force based on a 2D Modified-LuGre tire model. One major difficulty of such type controllers is the unknown surface condition. A nonlinear adaptive braking/traction torque control is proposed to regulate the tire slip ratio with the estimation of surface condition. Simulation studies show that feeding back the estimate into the slip control makes the delivered friction force and yaw moment adaptive to surface conditions.