A method to analyze closed-loop stability of position control in an Electric Power Steering System for ADAS/Autonomous Driving

Paper #:
  • 2018-01-0703

Published:
  • 2018-04-03
Abstract:
Traditionally, Electric Power Steering (EPS) uses column torque to generate an assist torque to help driver steer a vehicle. Hence, EPS becomes a closed loop torque control system. The stability for EPS system is ensured through modeling, and control design followed by experiments. In this case, experimentation is necessary to determine a “borderline” stability margin of the EPS system. Advanced Driving Assistant Systems (ADAS), such as park assist, and Autonomous Driving (AD) functionality requires steering system (EPS or Steer-by-wire) to operate in angle/position control mode. In such cases, steering system may receive a target steering angle/position from path planning module and an assist torque is generated to follow the target angle. Hence, it is important to study stability of EPS system for position control mode. The closed-loop stability of position control in an EPS system can affect the overall vehicle motion. In this paper, we present a method to analyze the closed-loop stability of position control strategies in an EPS system using system modeling and experiment based correlation techniques. In this method, we determine stability margins using the open-loop transfer function for position control. This is done for various system gains using EPS model. The correlation technique uses in-vehicle data to determine an acceptable system behavior for maximum gain. In the end, using modeling based margins and correlation results, a minimum, acceptable stability margin i.e. “borderline” stability margin is determined. This determined "borderline" stability margin is used to evaluate closed-loop stability of EPS position control.
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