An Objective Assessment of Steering System Parameters for Driver’s Torque Feedback

Alexander Hitti

The electric power steering system has drastically impacted the concept of steering feel. In a Column Motor Driven Power Steering (C-MDPS) system, the assist motor is mounted directly to the steering column. This assist motor applies a torque onto the steering column to provide a desired torque, hence indirectly impacting the steering torque feedback or feel. A bond graph model of this C-MDPS system is developed and connected to a full vehicle bicycle car model. The resulting state equations from this model are implemented into MatLab/Simulink for simulation. Higher order friction models are used in the worm gear, the steering rack, and the vehicle tires to increase fidelity. The sensitivity of the system to key parameters and frictions is studied through varying these parametric elements by percentages of their nominal value and observing the effect on system outputs such as Yaw Rate Gain and Parking Effort. The system performance is validated based on objective data. A test stand setup is used to study the characteristics of the torque on the steering column and the worm gear friction. The full vehicle model is compared to data from a test vehicle.

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