Optimized Variable Gear Ratio Steering System with Reduced EPAS Motor Size achieving Performance Targets
2022-01-1061
03/29/2022
- Content
- 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 & turning circle diameter 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 in such a way that the steering system demands lower power from the EPAS motor during the static full steering operation. By focusing on vehicle dynamics and thermal performance, several digital iterations were performed for optimizing VGR of steering system using full vehicle simulation in AMESIM® software. Based on simulation results, VGR gear was manufactured and installed in actual vehicle. Physical testing results related to driving dynamics and thermal performance 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.
- Citation
- Kulkarni, P., Salunkhe, S., Iqbal, S., Joshi, N. et al., "Optimized Variable Gear Ratio Steering System with Reduced EPAS Motor Size achieving Performance Targets ," SAE Technical Paper 2022-01-1061, 2022, .