James Larminie

Estimation of slip ratio and road characteristics by adding perturbation to the input torque

Electric Vehicles (EVs) present several characteristics which can greatly enhance the performance of the vehicle stability control. The torque response is fast and precise, the wheel speed can be easily estimated and the small dimensions of the motors allow mounting of multiple motors on the same vehicle, each one driving a single wheel. Taking advantage of such characteristics will allow a standard motor controller to integrate feature such as traction-control system (TCS), anti-lock braking system (ABS) and direct yaw control (DYC). This paper presents a novel technique to estimate both the slip ratio and the road friction characteristic without any hardware outside a standard motor controller. Such technique is unaffected by external forces acting on the vehicle. It works by adding a small oscillation to the set torque and then measuring the gain and the phase shift of the wheel speed oscillation. These values can be correlated to the transfer function obtained linearising the vehicle model around an operating point, which is function of the slip ratio and the road characteristics. A SIMULINK simulation is used to validate the method.