Browse Publications Technical Papers 2019-01-1236

Development of Evaluation Methods for Electric Power Steering Loss of Assist 2019-01-1236

Over the last 10 years Electric Power Steering (EPS) has become the most popular steering technology for passenger vehicles due to energy efficiency, customer performance, and driving features. In parallel with this technological shift to EPS has been a similar shift in industry and governmental perspectives regarding the severity of the steering loss of assist (LoA) state. In many first and second generation EPS systems removal of power assist was viewed as a ‘safe state’ and was the default diagnostic action taken with the detection of any moderate to severe system fault. Today LoA is viewed as a Hazard in many operational scenarios, but despite this increased attention there currently are no published standards, metrics, or test protocols for the evaluation of vehicle performance in a Loss of Assist situation. The first part of this paper will examine the reasons prompting changes in the industry mindset regarding acceptability of LoA, review common assist failure causes in the EPS system, and analyze situational factors impacting human vehicle controllability during a LoA event. The second part of this paper will illustrate and propose a LoA subjective evaluation driving course and corresponding objective test with metric. These evaluation methods simultaneously allow consistent performance evaluation of existing production vehicles and enables prediction of vehicle characteristics early in the vehicle development cycle using Computer Aided Engineering tools. The final section of this paper provides recommendations for how steering LoA objective metrics can be applied in vehicle development as part of a system safety engineering process to classify the ASIL (Automotive Safety Integrity Level) of the Loss of Assist hazard then take appropriate design and manufacturing controls to improve overall system robustness and minimize potential field incidents.


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