Zhang, D., Lin, B., Kirli, A., and Okwudire, C., "Reduction of Steering Effort in the Event of EPAS Failure using Differential Braking Assisted Steering," SAE Int. J. Trans. Safety 5(2):227-233, 2017, doi:10.4271/2017-01-1489.
Electric Power Assisted Steering (EPAS) is widely adopted in modern vehicles to reduce steering effort. It is probable that some EPAS systems will experience a shutdown due to reliability issues stemming from electrical and/or electronic components. In the event of EPAS failure, power assist becomes unavailable and the steering system reverts to a fully manual state, leading to excessive steering torque demands from the driver to maneuver the vehicle at lower speeds, i.e., under 30 mph. This situation has resulted in dozens of reported crashes and several OEM safety recalls in the past few years. Inspired by recent work which utilizes independent driving torque of in-wheel-motor vehicles to reduce steering torque, this paper proposes the use of Differential Braking Assisted Steering (DBAS) to alleviate steep increases in steering torque upon EPAS failure. DBAS requires software upgrades with minimal hardware modification to EPAS, which is preferable for a backup system. A preliminary evaluation of DBAS is carried out using simulations in CarSim. Results show that DBAS reduces steering torque to similar levels as EPAS in all of the steady state, transient and urban driving scenarios investigated. The results also show that DBAS triggers vehicle speed reduction and oversteer, the latter of which is in conflict with Electronic Stability Control (ESC) which is designed to inherently prevent oversteer. Since ESC is typically crucial at higher speeds, this conflict can be avoided by employing DBAS only at lower speeds where steering assist is most needed. This paper motivates further study of DBAS for emergency safety in the event of EPAS failure.