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Estimation and Decomposition of Rack Force for Driving on Uneven Roads

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Document pages: 23 pages

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 electricpower steering (EPS) systems. Estimates of rack force can be used by EPS toattenuate road feedback and reduce driver effort. Further, estimates of thecomponents of rack force (arising, for example, due to steering angle and roadprofile) can be used to separately compensate for each component and therebyenhance steering feel. In this paper, we present three vehicle and tiremodel-based rack force estimators that utilize sensed steering angle and roadprofile to estimate total rack force and individual components of rack force.We test and compare the real-time performance of the estimators by performingdriving experiments with non-aggressive and aggressive steering maneuvers onroads with low and high frequency profile variations. The results indicate thatfor aggressive maneuvers the estimators using non-linear tire models producemore accurate rack force estimates. Moreover, only the estimator thatincorporates a semi-empirical Rigid Ring tire model is able to capture rackforce variation for driving on a road with high frequency profile variation.Finally, we present results from a simulation study to validate thecomponent-wise estimates of rack force.

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