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Electric power steering (EPS) systems utilize an electric motor to assist the driver’s steering efforts. Recently, EPS is drawing attention as a mechanism for improving vehicle stability and maneuverability. This paper proposes a control method using EPS to improve steering maneuverability when driving on rutted roads. The disturbance torque generated by the grooves in rutted roads affects maneuverability. Conventional EPS cannot discriminate between torque originating from driver input and aligning torque generated by the road surface. Thus, it is difficult for conventional EPS to reduce the unstable effect caused by ruts without affecting the driver’s steering feel. The method proposed here detects the amount of disturbance torque caused by the ruts in the road, and only when disturbance torque is detected, the EPS system compensates accordingly. As a result, steering pull is reduced without interfering with the driver’s steering feel.

In this paper, we suggest a novel algorithm to distinguish semi-steady states from various steering patterns and to estimate the stability factor. The algorithm also estimates each stability factor in left and right turns because there could be a case where they differ based on uneven tire wear and so on. The stability factor, which is the turning characteristic of a vehicle, has been treated as constant for most vehicle control systems. However, in fact, it may change in some situations, for example when a vehicle is overloaded. So there is a chance that a driver may be aware of an unusual sensation when vehicle control is designed based on a constant stability factor. We have succeeded in developing an algorithm to estimate the stability factor accurately enough to be able to compensate for it and have confirmed the effectiveness of the algorithm by simulation and vehicle testing as well.