Trajectory tracking control of steer-by-wire autonomous ground vehicle considering the complete failure of vehicle steering motor

Abstract

This paper investigates the trajectory tracking control problem of steer-by-wire autonomous ground vehicle considering the complete failure of vehicle steering motor. In order to achieve the trajectory tracking control of autonomous ground vehicle in presence of the complete failure of vehicle steering motor, the mechanical transmission mechanism of vehicle steering system is analyzed with the steering motor fault being considered, and the differential steering is used to actuate the turning of vehicle in case of emergency. A model predictive controller with the constraints of tire cornering angle and road adhesion is designed to compute the expected front-wheel steering angle for vehicle trajectory tracking control, and a nonsingular terminal sliding mode controller is proposed to track the expected front-wheel steering angle using differential-steering moment, in which a novel observer design method is presented to estimate the actual front-wheel steering angle. Two case studies are implemented in a CarSim-Simulink co-simulation platform, and the simulation results have verified that the proposed method have satisfactory trajectory tracking control effectiveness in presence of the complete failure of vehicle steering motor.

Introduction

In recent decades, the development and progress of society has contributed to the overall technical level and popularity of the automobile industry [1], [2], [3]. Intelligent vehicles can effectively improve the road utilization rate of vehicles and passenger comfort, and reduce the potential road traffic risks [4], [5]. The autonomous control ability of vehicles also meets the increasing demand of current consumers for driving and riding [6], [7]. It is one of the major development directions in the field of vehicle control. The intelligent control technology of autonomous ground vehicle benefits from the maturity and popularization of vehicle electronic control technology [8], [9], [10], and the increasingly perfect supply channel of parts and components from the middle and lower reaches also provide a reliable guarantee for the development and expansion of the entire industrial chain [11], [12], [13], [14]. Recently, it has received unprecedented attention and research and development investment from researchers and many manufacturers. Trajectory tracking (TT) control is a common scene in driverless driving process of autonomous vehicle. In the process of trajectory tracking, the trajectory tracking error and heading error is aimed to be reduced in real time through the steering motion control of the vehicle [15], [16], [17], so as to approach the referenced trajectory, and in order to solve the trajectory tracking problem in different situations, there are many related research results at present [18], [19], [20], [21], [22], [23].

In the trajectory-tracking process of autonomous ground vehicle, vehicle steering control is the direct factor to guide autonomous vehicle to track the referenced trajectory. Therefore, the accuracy and reliability of vehicle steering control directly affect the effect of trajectory tracking. In this case, the steer-by-wire system has attracted the attention of researchers with its unique advantages, and has become one of the recognized solutions in the field of unmanned vehicle steering control. In the steer-by-wire system, part of the mechanical connection between the hand steering wheel and the front-wheel of the vehicle has been cancelled, so that this mechanical driving structure can get rid of various limitations of the traditional steering system, which is conducive to further improving the maneuverability, comfort and safety of the vehicle [24], [25], [26], [27]. The technology of steer-by-wire realizes the decoupling of driver's operation and vehicle's movement, which is helpful to improve the accuracy of steering control and the safety of passengers in the face of emergency [28]. The steer-by-wire system of vehicle adopts the direct torque drive by steering motor control to realize the vehicle steering, this advantage makes it easier to communicate and integrate with other active safety control subsystems of the vehicle, so as to provide a good hardware basis for autonomous steering of unmanned ground vehicles [29], [30], [31]. Based on the above considerations, steer by wire system is recognized as one of the key components and effective solutions to achieve advanced automatic driving at present. Therefore, the related research of steer-by-wire system and its integrated application on autonomous vehicle become the research hotspot in the field of vehicle intelligent control technology.

However, once the steer-by-wire system fails, the above-mentioned control advantages will no longer exist, and the control performance of autonomous ground vehicles will not be guaranteed, and even great risks may occur. Therefore, highly intelligent and autonomous capability is one of the important directions of the development of unmanned ground vehicles [32], [33], [34], [35], and the adaptability to multiple working conditions and the control regulation ability in case of emergency is one of the important ways to improve the ability of vehicle autonomous control [36], [37], [38], [39]. In the process of vehicle trajectory tracking, if the steering motor of vehicle fails completely, it is difficult for the existing control methods to make appropriate fault-tolerant control behavior for the emergency. The research on this problem in the existing literature is also insufficient, and this problem has great research necessity and practical application value. For this situation, considering the complete failure of vehicle steering motor, the mechanical transmission mechanism of vehicle steering system is analyzed and the dynamic equation of vehicle system is derived with the motor fault being considered, in which the differential steering moment is used to impel the vehicle to steer as required in case of emergency.

According to the above problem description, an overall control strategy for trajectory tracking control of autonomous vehicle is designed, in which a model predictive controller (MPC) is developed to obtain the expected front-wheel steering angle, and a nonsingular terminal sliding mode controller (NTSMC) is designed to track the expected front-wheel steering angle by vehicle differential steering. The main contributions of this paper are summarized as follows: (a) a novel problem description for vehicle steering control is investigated, in which the mechanism of vehicle differential steering and its influence on vehicle steering movement is studied; (b) a more all-round overall vehicle trajectory tracking control strategy is presented, including trajectory tracking control and front-wheel steering angle tracking control; (c) considering that the actual front-wheel steering angle is hard to be measured directly, a novel front-wheel steering angle estimator is proposed using the design method of unknown input observer (UIO).

The rest of this paper is organized as follows. The background and related works are presented is Section 2. The trajectory tracking control method is shown in Section 3. The simulation results are provided in Section 4, followed by the conclusion in Section 5.