Adaptive slip vectoring for speed and yaw-rate control in electric vehicles with four in-wheel motors

A non model-based control architecture, called adaptive slip vectoring, is presented to regulate speed and yaw-rate in a fully electric vehicle equipped with four in-wheel induction motors. The actuators redundancy is dealt with by a Slip Vectoring strategy which, given a driving force and a driving yaw-moment as control constraints, allocates optimal slip references to each tire, so that four speed references are provided to the four in-wheel motors on the basis of vehicle accelerations, speed and yaw-rate regulation errors. In order to drive independently each in-wheel motor speed to its reference, a decentralized feedback control loop is designed which leads to exponentially convergent load torque estimates so that the torque-slip operating conditions can be monitored online and the load torque requirements are matched for each tire. As a result four decentralized current-pair reference inputs are provided to each motor. Structural sufficient conditions for local exponential stability of the speed and yaw-rate regulation are obtained for uniform rectilinear references. A new overall stability index is introduced and monitored online, and an automatic longitudinal speed reduction can be performed with the goal of keeping such a stability index within a predetermined range so that the structural sufficient conditions are met. Realistic CarSim simulations are presented: (i) a snowy uphill path illustrates the non model-based speed reduction driven by the stability index monitoring; (ii) standard moose-test maneuvers on dry and wet asphalt illustrate uniform yaw-rate tracking performance due to the continuous differential actions of the four in-wheel torques.