Abstract
Keywords: dynamic model, nonlinear system, power steering, controller development
The hydraulic power steering systems, which are used in today´s series cars, support the driver only by reducing the amount of steering wheel torque necessary to steer a vehicle. For additional support as it would be required for the implementation of driving assist functions such as parking assistance, lane keeping, road disturbance compensation, etc. it is necessary not only to reduce but also to superpose the steering torque. For this purpose the conventional hydraulic power steering system that is described in this paper is functionally enhanced by the integration of an electric motor into the system´s steering column.
To effectively analyze this steering system, a complex power steering simulation model including models for power steering pump and hose is built and the modeling performance is shown by comparisons with measurements that were made in an actual test vehicle. The model is expanded by the simulative integration of the electric motor in the steering column. For a structured design of the controller algorithms of the steering system electric motor, knowledge of the power steering system dynamics and here especially of the nonlinear hydraulic flow over the power steering valve orifices has to be utilized.
Therefore, a reduced order model considering the hydraulic nonlinearities is built and the simulation results are compared with the complex model. In the following the transfer functions relevant for steering system design (steering angle / steering torque, steering torque / road disturbance) are derived and the conclusions as well as an exemplary controller for position control of the power steering system are presented. Control performance is shown with simulation runs where a virtual full vehicle environment including the complex power steering simulation model and a parking assistance system as reference generator for the position control loop of the steering system is used.