Development of Evaluation Methods for Steering Loss of Assist

Loss of power steering assist (LoA) is viewed as a potential hazard in certain vehicle operational scenarios. Despite the importance of this steering failure mode, few published test protocols for the objective or subjective evaluation of vehicle performance in a loss of assist situation exist.

The first part of this paper examines five of the key steering failure modes that can result in LoA and discusses why LoA persists as a key industry challenge. The second part analyzes the situational dynamics affecting vehicle controllability during a LoA event and proposes a subjective evaluation driving course that facilitates evaluations in various LoA scenarios. A corresponding objective test procedure and metric is also proposed. These evaluation methods support consistent performance evaluation of physical vehicles while also enabling the prediction of vehicle characteristics early in the vehicle development process (VDP). The final section of this paper gives an overview of a steering system design method aimed at first classifying the level of risk associated with any LoA induced hazard (Automotive Safety Integrity Level - ASIL) and then taking appropriate software, electronic, mechanical, or manufacturing design controls to improve system robustness and minimize the likelihood of LoA field occurrences.