Control of electrical power assist systems: HN design, torque estimation and structural stability

In this paper, the HN method is employed to design controllers for improved performance and robustness of electrical power assist steering systems (EPAS). EPAS systems are nonlinear MIMO systems with multiple objectives, including fast response to the driver torque command, good driver feel, and attenuation of load disturbance and sensor noises. Since the EPAS system has nonlinear frictions, component deviations and load disturbances, its linearized model is subject to significant modeling errors and external disturbances. Consequently, EPAS controllers must provide substantial robustness. In this paper, a control design method is introduced which employs the boost curve to form a feedforward control and an MISO HN optimal feedback to ensure refined performance, robustness, and disturbance attenuation. Since the driver’s torque command cannot be directly measured, due to packaging and cost issues, a torque estimator is introduced to generate the driver’s torque command by using other measured signals which are normally available in the vehicle. Introduction of the torque estimator leads to an additional feedback loop, affecting system robust stability and performance in a fundamental manner. Robust stability under this circumstance is established. The controllers are evaluated via simulation on both linearized systems and original nonlinear systems, and verified on vehicle testing data. r 2001 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.