Torque Steer Mitigation Using Electric Power Steering (EPS) System

Abstract

Torque Steer is an inherent phenomenon for the Front Wheel Driven vehicles. It is a torque applied on the steering wheel when the vehicle is accelerated, produced by suspension compliance, asymmetries in the suspension and steering geometry and road irregularities. This torque is not desired to be felt by the driver. The aim will be study the factors producing torque on the steering wheel, calculate them under straight and cornering scenarios and distinguish which portion is wanted or unwanted. This information may be used in an EPS controller, which can counteract this torque. A model designed in Matlab/Simulink has been made in order to calculate the factors, with a previous calculation of new positions in geometrical points, the longitudinal, lateral and normal force and where they are applied, the torque flow and the self aligning torque. For that, a list of available sensors installed on most of the vehicle was used and represents the inputs to the model. Some assumptions were made in order to simplify calculations, considering the obtained results did not differ considerably from the real ones. For straight acceleration it was found the torque in the steering wheel was very small (peak of 0.5Nm) and mainly due to S.A.T. and the effect of the Secondary Couples. As S.A.T. is assumed to be wanted, Secondary Couples is the factor producing torque steer. In a real straight acceleration, the roll angle is not completely zero, thus it creates asymmetry from left to right side which produces this Secondary Couples effect. When accelerating in a corner, the torque in the steering wheel is due to S.A.T., the Secondary Couples and Normal Force factors. Torque steer is only due to Secondary Couples and has a peak of 8Nm. It is increasing with roll, and in this case the geometry is totally asymmetric. There is also friction in the differential, due to the different rotational speed of the inner and outer wheels of the curve. To get more accurate results, it is needed a better estimation of the lateral velocity, the friction coefficient, and the suspension travel.