## 1 Introduction  
In grounded vehicles, the steering system plays an important role in ensuring the motion trajectory of the car according to the driver's desires and road traffic. Hydraulic power steering systems powered by the engine were introduced to reduce driver torque on the steering wheel. In recent years, EPS (electric power steering) systems have been widely deployed in modern vehicles due to their noticeable advantages over hydraulic power steering systems, including higher efficiency, more compact structure, better maintainability and lower fuel consumption [1; 2; 3; 4; 5]. In EPS systems, the assist torque is generated by an electric motor, which is powered by the car battery. Thus, the control strategies of assist torque need to consider the torque adjustment in accordance with the driver input torque, driver steering feeling and road conditions. Several control algorithms of EPS systems have been investigated for EPS control, including PI and PID EPS controllers [6; 7; 8; 9]. Although PI-and PID-controlled EPS exhibited satisfactory performance, their performance can be further improved by considering model mismatching and disturbances, including friction and tire forces. To this end, EPS-based fuzzy controllers have been proposed to reduce the influence of friction and road disturbances [10; 11]. [PERSON] [12] designed a robust \(H_{\infty}\) output-feedback yaw control algorithm based on differential steering and the complete failure of the active front-wheel steering. [PERSON] [13] introduced a loop-shapingtechnique to control the steering-column torque, which is measured by the torque sensor and reference torque tracking neglecting the estimation of the EPS parameters. [PERSON] [14] proposed a fault-tolerant control strategy based on fault estimation and reconstruction. [PERSON] [15] proposed an active disturbance rejection approach for the EPS system, which can significantly reduce steering wheel vibration torque. [PERSON] [16] introduced a robust steering-assist torque control of EPS systems that achieved optimal steering wheel torque tracking performance. [PERSON] [17] presented a new control framework for vehicle EPS systems based on admittance control, which can augment the base steering feel and improve the road condition awareness. [PERSON] [18] proposed a nonlinear decoupling control method to improve the stability and maneuverability of EPS. The robustness of EPS control methods is especially important under high assist gain. In previous studies on this topic, the linear quadratic regulator (LQR) approach was adopted to attain high robustness and stability [19; 20], in turn achieving high phase and gain margins. Further optimal controllers based on the LQR-based EPS system were proposed in [21; 22; 23]. System identification of the control plant is essential in model-based control strategies. Meanwhile, the control performance will be notably improved if the control plant parameters are identified in a precise manner. Thus, system identification is important in the design of EPS control systems. Several methods have been proposed to identify and determine mathematical models for control plants, including the off-line identification method for identifying the brushless DC parameters [24]. Another system-identification strategy consists of the crack and bearing dynamic parameters, which was investigated in [25]. Recently, [PERSON] [26] designed a controller by implementing an observer based on the linear-parameter-varying (LPV) model. The seating-system model and a 5 degrees of freedom (5-DOF) model were developed in [27] for optimal vibration control. Furthermore, [PERSON] [28] investigated the uncertainty of scheduling parameters for practical applications. Most existing EPS controllers do not consider EPS parameter estimation with a pre-determined EPS mathematical model. As a result, their control performance is compromised in practical EPS implementation. The reason for this is that the practical model parameters vary due to road disturbances, measurement noise and other factors.