Model-based Rack Force Estimation for Electric Power Steering

Abstract

In modern cars, hydraulic power steering (HPS) is continuously substituted by the electric power steering (EPS), active front steering (AFS) or steer-by-wire (SbW). In general, the main task of the EPS/HPS is the support of the driver in controlling the lateral position of the car and compensating the tyre aligning torques. These torques result in a steering rack force, which reliable long-term measurement induces disadvantages e.g. high sensor costs. Since the knowledge of the steering rack force is useful to improve various automotive control applications, the estimation of the steering rack force with real-time capable algorithms is in the focus of this research. First, a non-linear dynamic 4-mass model is given and validated by a prototype EPS. Second, an algorithm for steering rack force estimation is introduced using a non-linear friction compensation module and a linear disturbance observer. Finally, the estimation algorithm is analysed by means of validated numerical EPS model, a steering test bench and a real prototype car. The results state the excellent performance of the estimation algorithm, even under real operation conditions.