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. As the reduced order model is nonlinear due to the hydraulic properties of the steering system, a Taylor series linearization leads to a family of linear state-space models, which are valid around the respective operating point and are especially useful for frequency domain analysis of the steering system. Knowledge of the so-called lead transfer function (steering angle vs. desired motor torque) can be used in the design of the steering angle control algorithm. Controller design is performed in the H-infinite framework and controller performance is shown at the example of an automated parking manoeuvre where a special trajectory planner generates the reference angle for the position control algorithm. Future work will investigate the robustness of the presented control and also the effect of other parameter uncertainties than the hydraulic nonlinearities (e.g. road friction, variations in tire side slip resistance, etc.).