Torque estimation in electrical power steering systems

Currently, all Electric Power Assist Steering (EPAS) systems employ a pinion torque sensor, between the steering column and pinion, to determine the amount of the torque assist to the driver. This torque assist is calculated via a tunable nonlinear boost curve. Then, this signal is used as control command to the electric motor to achieve the desirable level of assist. In this paper, torque estimators are devised to generate accurate estimates of the driver torque input and eliminate the need of the pinion torque sensor. Together with measured signals such as steering wheel rotational position, these estimates provide signals for a robust controller that consists of a feedforward part vs. the boost curve and an Hinfin optimal feedback to ensure refined performance, robustness, and disturbance attenuation. Introduction of estimators results in additional feedback loops and ushers in new issues of robust stability and performance. The concept of structural stability is introduced to capture the requirements of robust stability under various system configurations due to component failures. The requirements will guarantee a level of fault tolerance in case of control failure during its operation. Conditions are provided under which such structural stability is maintained.