Estimating Rack Force Due to Road Slopes for Electric Power Steering Systems

Abstract

The net force generated by tire moments on the steering rack of a vehicle, called the rack force, holds a significant amount of information about the tire-road angle, the surface profile of road, and the speed of the vehicle. As a result, many electric power steering control applications rely on the estimation of rack force. Current methods of rack force estimation are inaccurate because they either do not account for road profile in the rack force estimation or are susceptible to disturbances acting in the steering system. In this paper we overcome both of these limitations and develop two real-time capable models to estimate the rack force for driving on longitudinal and lateral road slopes using vehicle and tire dynamics models. We demonstrate the accuracy of our models by analyzing the results from actual driving experiments performed on sloped roads under normal and aggressive driving situations. We also compare the performance of the two models with a model existing in the literature that does not account for road slopes.