Mathematical modeling and nonlinear controller design for a novel electrohydraulic power-steering system

This paper is concerned with the mathematical modeling and nonlinear controller design of a novel electrohydraulic closed-center power-steering system. After a short introduction to the requirements of modern power-steering systems, the construction of the power-steering system under consideration will be described. The main advantage of this construction is the enhancement of the overall energetic efficiency of the steering system and the possibility of a variable steering assistance while keeping the good steering feeling of traditional hydraulic steering systems. Based on a detailed mathematical model of the essential components of the system, it is shown how some of the parameters of the system can be chosen to achieve an optimal dynamic behavior of the closed-loop system. The controller design proceeds in two steps: first, a nonlinear controller for the assistance force based on the differential flatness property of the system is designed, and afterwards, a steering torque controller using an impedance matching design is derived. Finally, measurement results of a test stand show the good performance and the robust behavior of the proposed control strategy.