  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
attenuate road feedback and reduce driver effort. Further, estimates of the
components of rack force (arising, for example, due to steering angle and road
profile) can be used to separately compensate for each component and thereby
enhance steering feel. In this paper, we present three vehicle and tire
model-based rack force estimators that utilize sensed steering angle and road
profile to estimate total rack force and individual components of rack force.
We test and compare the real-time performance of the estimators by performing
driving experiments with non-aggressive and aggressive steering maneuvers on
roads with low and high frequency profile variations. The results indicate that
for aggressive maneuvers the estimators using non-linear tire models produce
more accurate rack force estimates. Moreover, only the estimator that
incorporates a semi-empirical Rigid Ring tire model is able to capture rack
force variation for driving on a road with high frequency profile variation.
Finally, we present results from a simulation study to validate the
component-wise estimates of rack force.
