In view of the problems related to vehicle-handling stability and the real-time correction of the heading direction, nonlinear analysis of a vehicle steering system was carried out based on phase plane theory. Subsequently, direct yaw-moment control (DYC) of the vehicle was performed. A four-wheel, seven-degree-of-freedom nonlinear dynamic model that included the nonlinear characteristics of the tire was established. The stable and unstable regions of the vehicle phase plane were divided, and the stable boundary model was established by analyzing the side slip angle–yaw rate ($\beta -\gamma$) and side slip angle–side slip angle rate $(\beta -\stackrel{·}{\beta })$ phase planes as functions of the vehicle state variables. In the unstable region of the phase plane, taking the instability degree as the control target, a fuzzy neural network control strategy was utilized to determine the additional yawing moment of the vehicle required for stability restoration, which pulled the vehicle back from an unstable state to the stable region. In the stable region of the phase plane, a fuzzy control strategy was utilized to determine the additional yawing moment so that the actual state variables followed the ideal state variables. In this way, the vehicle responded rapidly and accurately to the steering motion of the driver. A simulation platform was established in MATLAB/Simulink and three working condition was tested, that is, step, sine with dwell, and sine amplification signals. The results showed that the vehicle handling stability and the instantaneous heading-direction adjustment ability were both improved due to the control strategy.

针对车辆操纵稳定性和航向实时修正等问题，基于相平面理论对车辆转向系统进行非线性分析。随后，执行车辆的直接横摆力矩控制（DYC）。建立了包含轮胎非线性特性的四轮七自由度非线性动力学模型。划分车辆相平面的稳定区和不稳定区，通过分析侧滑角-偏航率建立稳定边界模型（$\beta -\gamma$) 和侧滑角-侧滑角率 $(\beta -\stackrel{·}{\beta })$相平面作为车辆状态变量的函数。在相平面的不稳定区域，以不稳定程度为控制目标，利用模糊神经网络控制策略确定车辆恢复稳定所需的附加横摆力矩，将车辆从不稳定状态拉回至稳定区。在相平面的稳定区域，采用模糊控制策略确定附加偏航力矩，使实际状态变量遵循理想状态变量。通过这种方式，车辆对驾驶员的转向动作做出快速而准确的反应。在MATLAB/Simulink中搭建仿真平台，测试了步进、正弦带驻留信号和正弦放大信号三种工况。