A Comparison of Electric Power Assist Steering with Hydraulic Power Assist Steering for Automotive Applications

Eric W. Barrett | MS | 2002

Abstract:

An emerging technology gaining popularity in near-future vehicles is the electronically controlled Power Assist Steering system (EPAS). To date, EPAS has been developed and implemented on small, low-volume vehicle applications. This technology offers attractive advantages over traditional hydraulic power steering systems including improved fuel economy, reduced assembly time, environmental benefits, and easier tuning. With large automotive manufacturers continually bei ng challenged to improve their products environmental impact to society, the EPAS system offers a tremendous opportunity by improving overall vehicle fueld economy and eliminating the need for environmentally harmful hydraulic steering fluid. The challenges of implementing this newtechnoolgy include matching (or improving) the steering dynamics and reliability of the traditional hydraulic power assisted steering (HPAS) systems of today’s vehicles.

There is a considerable amount of technical information describing the unique dynamic characteristics of EPAS systems, primarily performance tuning, dynamic disturbance characteristics, such as actuation disturbances (also called torque ripple), noise disturbances (from sensors and controllers), and road disturbances. These articles discuss methods manufacturers have developed to address these unique EPAS characteristics, as well as, elaborate on EPAS system tuning for steering events such as parking maneuvers, on-center fee, lane change, and returnability. This work will briefly elaborate on these studies and compare them to HPAS performance characteristics.

More importantly, this thesis will analyse a particularly common and highly undesirable steering symptom known as steering “nibble” and will assess whether EPAS will mitigate this symptom better than a traditional hydraulic system. In order to assess the impact of EPAS on steering nibble, a CAE model will be developed to predict system dynamic responses for nibble characteristics. In addition, prototype vehicle testing will be conducted to gain actual steering wheel accelerations. These data will be used to improve the accuracy of the CAE model and to demonstrate system functionality.

There is also technical literature describing the diagnostic software developed for EPAS systems and the fail-safe operations required to ensure safety and reliability of this emerging technology. Although new technology carries the inherent risks of unanticipated failure modes, engineers have developed impressive diagnostics and fail-safe features to monitor, correct, or mitigate worrisome and potentially harmful system failures. In addition, this work describes how an EPAS system resolves two of the more common ‘soft’ failures of today’s HPAS system, namely, steering noise and power steering fluid leaks, this offering the potential of greater reliability versus the traditional hydraulic systems.

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