SAE International Objective Evaluation of Steering Rack Force Behaviour and Identification of Feedback Information 2016-01-9112

Electric power steering systems (EPS) are characterized by high inertia and therefore by a considerably damped transmission behaviour. While this is desirable for comfort-oriented designs, EPS do not provide enough feedback of the driving conditions, especially for drivers with a sporty driving style. The systematic actuation of the electric motor of an EPS makes it possible to specifically increment the intensity of the response. In this context, the road-sided induced forces of the tie rod and the steering rack force provide all the information for the steering system's response. Former concepts differentiate between use and disturbance information by defining frequency ranges. Since these ranges overlap strongly, this differentiation does not segment distinctively. The presented article describes a method to identify useful information in the feedback path of the steering system depending on the driving situation. By applying a suitable driving manoeuvre, useful information can be considered isolated. This is done exemplary in an understeering situation and characteristics are identified in the behaviour of the steering rack force. Based on this information, the steering torque is influenced by an excitation of the servo motor via a newly developed steering function in order to depict the current driving situation in the steering torque. With the presented method, the parts of the steering rack force associated with use information are made haptically noticeable in the steering torque.