Friction Compensation Control for Power Steering

The effects of friction are critical to the dynamics of electric power steering (EPS). On the one hand, friction contributes to the stability of the system and filters some of the disturbances (road vibration, etc.). On the other hand, it negatively affects the driving feel and refrains from accurate positioning of the steering wheel. In addition, for steering manufacturers, friction hinders the development and tuning of the assistance strategy. Therefore, controlling or eventually suppressing friction in EPS is a real challenge. In this paper, a control strategy for the active compensation of friction in a column-assist-type EPS is presented. The assist motor and the electronic control unit are used to cancel the friction effect in order to imitate the behavior of an ideal frictionless system. The feasibility of this strategy is demonstrated on the power column (the upper part of the steering system) using the same information (signals) as that available in an actual product. The proposed control is based on a model of the power column including slip-and load-dependent friction forces. For this purpose, a detailed simulation model, developed and validated in a previous work, is reduced to a lower order model to enable real-time computation. The LuGre model is used to compute both the static and dynamic friction forces with continuous formulation. The control architecture is composed of two cascaded feedback loops. The internal loop estimates the internal friction of the system and compensates for it through the motor input. The external loop contains a frictionless reference model used as a trajectory planner and a linear controller, which attempts to minimize the error between the plant and the reference responses. The stability and the robustness of the control strategy are formally analyzed. Specifically, it is shown that the limit error between the plant and the reference responses can be made arbitrarily small with appropriate values of the gains. Experimental results demonstrate that the control strategy is successful in tracking the frictionless reference trajectory, and confirm the robustness against inaccurate friction parameters.