【作者】 陈德玲；

【导师】 张建武；

【作者基本信息】 上海交通大学 ， 车辆工程， 2008， 博士

【摘要】 转向系统是控制汽车行驶路线和方向的重要装置,其性能直接影响到汽车的操纵性能和稳定性能。主动前轮转向(AFS-Active front steering)通过电机根据车速和行驶工况改变转向传动比。低、中速时,转向传动比较小,转向直接,以减少转向盘的转动圈数,提高转向的灵敏性和操纵性;高速时,转向传动比较大,提高车辆的稳定性和安全性。同时,系统中的机械连接使得驾驶员直接感受到真实的路面反馈信息。因此,主动前轮转向为车辆行驶的灵敏性、舒适性和安全性设定了新标准,代表着转向技术的发展趋势。现有主动前轮转向系统大多在液压转向系统结构的基础上,采用行星齿轮机构,实现主动转向。系统中,通过调节液压油的输入输出实现对转向盘手力的调节,但必须对液压油进行冷却,因而系统复杂且不可避免存在着液压转向系统的弊端。本文在电动助力转向机构的基础上,设计了新型的主动前轮转向系统结构,通过一套行星齿轮机构与助转角电机实现主动前轮转向。助转角电机(AFS执行器)用来实现可变传动比(VSR-Variable steering ratio)以及对车辆稳定性的控制;原助力转向系统中的助力电机(EPS执行器),根据转向盘转矩和当前车辆行驶情况,对驾驶员转向手力进行调整。因而,该转向系统通过助力电机和助转角电机,分别对转向力矩和前轮转角进行调节,实现主动转向。利用助力电机对转向手力调节避免了现有主动前轮转向系统中液压助力的缺点。控制系统是主动前轮转向系统的核心,控制策略的优劣直接影响主动前轮转向系统的性能。为对该主动前轮转向系统进行控制研究,文中在系统结构分析的基础上,建立主动前轮转向系统模型和车辆操纵动力学模型,从转向性能和整车操纵稳定性要求出发,设计主动前轮转向控制系统,制定相应的控制逻辑和控制策略。研究中,将系统解耦为EPS执行器和AFS执行器,分别进行控制研究。EPS执行器实现转向盘转向力矩的控制;AFS执行器实现可变传动比控制和车辆稳定性控制。车辆稳定性控制过程中,采用横摆角速度和估计的质心侧偏角共同对车辆进行控制,利用质心侧偏角的控制补偿单一横摆角速度控制所引起的行驶跟踪误差。实际行驶过程中,车辆会受到系统内部和外界的干扰,车辆参数产生波动;同时系统中还存在非线性摩擦、传感器噪声以及负载扰动等不确定因素的干扰。为较好地实现对目标车辆运动状态轨迹的跟踪控制,文中将鲁棒控制理论引入主动前轮转向的控制。根据控制系统的不同特点,对EPS执行器进行抵抗传感器噪声和外部干扰输入的标准H∞鲁棒控制器设计;对AFS执行器针对车辆行驶过程中侧偏刚度的摄动和外界侧向风干扰,进行混合LQR/H∞的鲁棒最优控制器设计,以提高主动前轮转向控制系统的稳定性能、鲁棒性能和抗干扰性能。本文最后,根据所设计的控制策略,开发主动前轮转向系统的控制软硬件,并进行控制器硬件在环台架试验。硬件在环台架试验中,通过非线性车辆动力学模型和转向系统实物,构建主动前轮转向系统的仿真试验平台,对所设计的主动前轮转向控制器进行控制性能验证。试验结果表明:本文设计的主动前轮转向系统具有较好的转向轻便性能、回正性能,同时能够有效地提高车辆的稳定性。更多还原

【Abstract】 Steering system is an important component for lane changing control of wheeled vehicles. Its performance influences vehicle steerability and stability directly. Active front steering (AFS) varies the steering ratio electronically in direct relation to the speed and road conditions. Under normal road conditions at low and medium speeds, the steering becomes more direct, requiring less steering effort of the driver, increasing the car’s agility and drivability. At high speed, the steering becomes less direct offering improved directional stability. In addition, road information can be fed by the mechanical link maintained between the front wheels and the steering wheel. As a result, the AFS system provides the vehicle with a new standard of driving agility, amenity and safety. It is the trend in development of the steering system.Most of the existing AFS systems are developed in association with hydraulic power units and a planetary gear set. Therefore, some disadvantages appear inevitably due to the servo system in the hydraulic power steering and its cooling.The AFS system is designed based on a commercial electric power steering (EPS) system. Active steering is realized with a planetary gear set and an angle assist motor. The angle assist motor (AFS actuator) is used to impose variable steering ratio (VSR) control and to improve vehicle stability. The original torque assist motor (EPS actuator) is applied to modify the driver steering torque according to the speed and driving conditions. Therefore, the AFS system can regulate the steering torque and steering angle by the EPS actuator and AFS actuator, respectively. The motor used to regulate the steering torque can avoid the disadvantages of using hydraulic system in the existing AFS system.As a core part in the AFS system, the controller plays an important role in the AFS’performance. For investigation to the AFS control, a simplified model and the vehicle model are developed. Then, the control logic and the strategy of the AFS system are designed based on steering performance and vehicle stability. In treatment of the AFS system, the controls of EPS actuator and AFS actuator are decoupled. The EPS actuator is controlled to modify the steering effort, while the AFS actuator is controlled to impose VSR for the vehicle stability enhancement. In control of the vehicle dynamic stability, both the yaw rate and estimated sideslip angle are applied. The sideslip angle control is utilized to compensate the tracking error caused by single yaw rate control.During the driving, the vehicle is subjected to internal and external disturbance of the system. In addition, uncertainties caused due to the nonlinear friction, sensor noise and load varieties also exist in the system. The theory of robust control is introduced to the AFS control. According to working conditions, a standard H∞control is applied in the EPS actuator control for the purpose of prevention from external disturbance and sensor noise, while a mixed LQR/H∞optimal control is applied to the vehicle stability control of AFS actuator to deal with the crosswind disturbance and perturbation of cornering stiffness. Therefore, the stability and robustness of the AFS system control to all kinds of disturbance can be ensured.The AFS controller is developed according to the control strategy and also tested by means of the hardware-in-the-loop (HIL) simulation method. For the HIL simulation, a nonlinear vehicle model is provided to be a simulation platform for the AFS system. Effectiveness of the present AFS controller is verified by the HIL simulation procedure. By test results, it is indicated that the present AFS system appears to be effective in modifying the steering effort and improving the vehicle stability.更多还原