The layout of a vehicle steering system has to resolve a compromise. While it is important for lateral vehicle control to feel steering torque feedback of lateral tire to ground interaction, disturbing forces shall not be present in the feedback steering torque. These disturbing forces result from road irregularities, wheel rotor imbalance, suspension asymmetry caused by production tolerances, wear or impacts, and additional vehicle internal forces, e.g. the steered wheels also driven by the engine or braked. In general these disturbances are reduced by an optimization of the suspension geometry to decrease the impact of the unintended forces on the steering system. The remaining disturbance is controlled to an acceptable level via force feedback sensitivity calibration of the steering system, what in return influences the intended driver sensitivity to feel lateral tire forces.
Modern electrical assisted steering systems allow overlay of additional assist torques to compensate for disturbance torques. This paper describes a function intended to counteract disturbances resulting from longitudinal tire forces, usually called Torque-Steer during acceleration and Brake-Pull under deceleration. The developed system identifies the disturbances and decides on the appropriate amount of compensation torque, which is applied by the steering system. To limit the cost of implementation Steering Torque Disturbance Rejection (STDR) function has been realized in software with emphasis on not requiring sensors on top of what is already commonly available in today’s vehicles.
STDR consists of two models calculating rackforce. While a vehicle based model is used to determine the force suitable for the driving situation, a steering system based model ascertains the force currently present. The presence of Torque-Steer and Brake-Pull is detected by comparing both forces during substantial acceleration or deceleration. The disturbance rejection torque request is based on the difference of both calculation models.