Abstract
Steering wheel torque is the fastest channel to trigger the reaction of the human driver and is consequently a key feature for steering feel. In order to predict steering wheel torque, a simulation model has been developed and validated for a hydraulic assisted steering system including the steering column. At first, a modular high order model was developed to model the whole system physically. A model with five degrees of freedom was chosen for the mechanical system. New advanced friction elements were developed to match the system dynamic behaviour. The pressure and speed dependence of the friction in the steering system was implemented in the friction elements. The high order hydraulic system was modelled with consideration to fluid inertia and compliance. The steering valve was modelled physically based on its geometry. In the second part of this paper it is shown how this highly complex high order model can be reduced to a simplified low order model suitable for vehicle handling simulation. To achieve this, all compliances were combined into a single compliance. The same advanced friction models as found in the high order system were implemented. The hydraulic assistance of the low order model was implemented with a look up table, whereas the pressure build-up was time delayed. Both models produce results in close agreement with the measurements, and it is only at high frequencies that the high order model is found to perform slightly better. However, the low order model continues to predict the steering wheel torque very accurately and in combination with a vehicle model, the low order model is perfectly suited for vehicle dynamics simulation.
Keywords: Hydraulic steering system, Steering feel, Friction modelling, Hydraulic modelling, Hydraulic systems