Direct Discrete Design of a Multivariable LQG Compensator with Combined Discretization applied to a Steer-by-Wire System

Abstract:
On the way to highly automated and autonomous driving, a robustly designed steering system is a key component. Therefore, this article presents a direct discrete control design for modern steer-by-wire systems. The novel approach consists of a true multivariable control for both the driver´s steering torque and the rack position simultaneously using the requested torques of the downstream (AU) and upstream (FU) motor as control variables. For the control design, an optimal reduced plant model is used. It is derived from a detailed model of a steer-by-wire system with nine degrees of freedom. The reduced plant model is augmented by linear models for the reference and disturbance environment of the steer-bywire system as well as discretized based on the characteristics of the input variables. For this augmented model, a direct discrete multivariable linear quadratic Gaussian (LQG) compensator design is performed. The proposed control design considers the entire environment of the real steering system. The direct discrete approach restores the good characteristics of the continuous control and ensures that the discretization does not have any adverse effects. As a result, the resulting discrete control system shows the same good dynamic characteristics as the continuous system and has excellent robustness characteristics. Hence, the presented control satisfies the requirements of a modern steering system and can be adapted to various driving situations.