Steering Feedback Transparency Using Rack Force Observer

Closed-loop electric power assisted steering and steer-by-wire systems are non-transparent towards the environment dynamics, i.e. the tire-road interaction. To achieve the driver-environment transparency, an estimate of the environment interaction through the rack force is required. With an introduction of the dynamic rack force observer feedback, a closed-loop interconnection is formed. Hence, stability must to be ensured. At first, a driver coupled stability condition is derived to obtain the maximum achievable transparency for a passive environment port in different control architectures. Then, a non-linear rack force observer is proposed and implemented for transparency. Two estimation methods are compared: (a) using the body motion signals from the inertial measurement unit sensor; and (b) using the steering system sensors. Experiments indicate that the former estimation method has an inferior performance due to vehicle inertial properties. Whereas with the latter approach, the inclusion of higher order dynamics and a non-linear estimation algorithm shows a noticeable improvement. Finally, experiments on a real electric power assisted steering hardware illustrate the proposed steering feedback transparency, when driving at the limits of handling.