Design of a Robust Optimal Multivariable Control for a Steer-by-Wire System

On the way to highly automated and autonomous driving, a robustly designed steering system is a key component. Therefore, this article presents a new control approach for modern steer-by-wire systems. The approach consists of a multivariable control for the driver´s steering torque and the rack position simultaneously, using the requested torques of the downstream and upstream motor as control variables. The plant model used in this approach is a detailed model of a steer-by-wire system with nine degrees of freedom. For the control design, an optimal reduced model is derived. The reduced plant model is linearized, and it is augmented by linear models for the reference and disturbance environment of the steer-by-wire system and by a linearized model for the feeling generator that computes the requested steering torque. For this augmented model, a multivariable linear optimal static state space controller is designed. Hence, the whole environment of the real steering system is considered in the control design. Due to the multivariable approach and the augmented model that contains all subsystems and dominant characteristics of the real system, the resulting control system shows excellent robustness characteristics. Therefore, the presented control fulfills all the requirements of a modern steering system regarding robustness and can be adapted to different driving situations.