On the way to highly automated and autonomous driving, a robustly designed steering system is a key component. Therefore, the proposed article will present a new control approach for modern steer-by-wire systems. The novel approach consists of a true multivariable control for the driver´s steering torque and the rack position simultaneously using the requested torques of the downstream (SRU) and upstream (SFU) motor as control variables. Here, the plant model is a detailed model of a steer-by-wire system with nine degrees of freedom (see Fig.1). For the control design, an optimal reduced model is derived. The reduced plant model is linearized and augmented by linear models for the reference and disturbance environment of the steer-by-wire system and by a linearized model for the complete feeling generator computing the requested steering torque. The feeling generator consists of a rack force feedback, an active damping and an active steering wheel return. For this augmented model, a multivariable linear optimal static state space controller (LQR) design is performed. So, the whole environment of the real steering system is considered in the control design. Due to the multivariable approach and the augmented model containing all subsystems and dominant characteristics of the real system, the resulting control system shows excellent robustness characteristics. In contrary, currently used control approaches perform separate single-input single-output control designs for the steering feel and the rack position. Putting the corresponding separately designed control systems together via the feeling generator causes shifts of the eigenvalues and changes of the dynamic behavior (time and frequency responses), so that the supposed high robustness achieved by the respective design can no longer be guaranteed. These deficiencies are avoided by the new multivariable approach described in this contribution leading to improved and guaranteed robustness of the controlled steer-by-wire system. The presented control fulfills all the requirements of a modern steering system regarding robustness and can be adapted to different driving situations, for example by gain scheduling.
Session:
DRIVING & VEHICLE SAFETY
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| 16:30 - 17:00