Estimation and decomposition of rack force for driving on uneven roads

Abstract The force transmitted from the front tires to the steering rack of a vehicle, called the rack force, plays an important role in the function of Electric Power Steering (EPS) systems. Estimates of rack force can be used by EPS to reduce driver effort and enhance driving performance. Further, estimates of the components of rack force can be used to perform targeted compensation to enhance steering feel and vehicle safety. Existing rack force estimators either do not support the estimation of rack force components or are only applicable for driving on flat roads. This paper presents three vehicle and tire model-based rack force estimators that can estimate individual components of rack force as well as the total rack force for driving on uneven roads. The estimators use the same vehicle model but use tire models of differing complexities. The performance of the estimators was validated and compared by performing driving experiments with non-aggressive and aggressive steering maneuvers on roads with low and high frequency profile variations. The results show that the estimation accuracy increases with the complexity of tire model used, especially during aggressive steering maneuvers and high frequency road profile variations. Finally, results from a simulation study are presented to show that the estimators are also capable of estimating the components of rack force.