Proposal of equivalent friction control on Electric Power Steering (EPS)

The research discussed in this paper proposed a control method that imparts equivalent friction to electronic power steering (EPS) systems in order to improve the steering feel. In order to impart friction stably, the proposed method uses motor torque rather than mechanical elements to impart friction. First, the effect of friction on the steering system was considered theoretically using simple harmonic motion models, and it was demonstrated theoretically that steering responsiveness could be increased by imparting appropriate friction force to the steering system. Based on the results of this analysis, a target value for equivalent friction force to be imparted by means of EPS control system was formulated. A method of linear approximation of the friction force necessary to realize equivalent friction force based on this target value was proposed. The method was applied in an actual vehicle, and the EPS system configuration necessary for the desired effect to be obtained was examined. It was considered that a system configuration incorporating a steering torque sensor with high torsional rigidity would be necessary, and a control method for imparting equivalent friction force was proposed. Finally, the effectiveness of the proposed method was verified in simulations using a vehicle model and in tests involving an actual vehicle. The vehicle model simulations demonstrated that the application of the proposed method increased steering responsiveness. The vehicle tests verified that the proposed method made it possible to obtain a stable steering feel, and was also effective in improving steering feel.