Optimized Variable Gear Ratio Steering System with Reduced EPAS Motor Size Achieving Performance Targets

In electric power assisted steering system (EPAS), the steering assistance torque is provided by the electric motor. The motor rating is decided based on rack force requirement which depends on the vehicle weight, steering gear ratio, wheel angles etc. The load on the EPAS motor varies with respect to the steered angles of the road wheels. The motor experiences higher load towards the road wheel lock position. Most of the steering systems used on passenger cars has rack and pinion gear with constant gear ratio (C-factor). The constant gear ratio is decided to create right balance between vehicle handling behavior and steering effort. The constant gear ratio exerts higher steering load which the EPAS motor is required to support up to road wheel lock angles and hence EPAS motor size increases.

This paper presents variable gear ratio (VGR) steering system in which gear ratio varies from center towards end lock stroke of rack & pinion. The VGR is optimized for thermal performance through simulation in such a way that the steering system demands lower power from the column type EPAS(C-EPAS) motor during the static full lock operation. Simulation competency is developed in AMESIM® software for thermal performance prediction and then several digital iterations were performed for optimizing VGR of steering system. Based on simulation results, prototype vehicle was prepared with VGR rack and pinion with reduced size EPAS motor. Physical testing results are observed to be in-line with those predicted by digital model. The study resulted in reduction of EPAS motor size by optimizing the VGR without affecting the steering performance with additional benefit of system cost optimization.