自主驾驶车辆紧急避障的路径规划与轨迹跟踪控制

Path Planning and Tracking Control of Autonomous Vehicle for Obstacle Avoidance

为减少道路突发事故,提高车辆通行效率,需要研究车辆的紧急避障以实现自主驾驶。基于车辆点质量模型,设计了非线性模型预测控制(MPC)路径规划器;基于车辆动力学模型,设计了线性时变MPC轨迹跟踪器。在路径规划层引入避障功能函数,通过车辆与障碍物的距离调节函数值大小,综合避障函数权重和路径偏差权重,规划出一条既能避开障碍物又使路径偏差最小的临时轨迹。在轨迹跟踪层,利用该临时轨迹和航向角偏差作为车辆主动转向控制参考量,将线性时变MPC优化问题转化为二次规划问题,计算满足车辆动力学约束的前轮转向角最优解。结果表明:所设计的双层MPC紧急避障控制策略对低速(60 km/h)、中速(80 km/h)、高速(100 km/h)行驶车辆有很强的适应性,高速行驶时最大质心侧偏角不超过1.0°,最大航向角偏差不超过2.5°,车辆横向稳定性良好,随着车速增大,车辆避障响应时刻提前;在多车连续避障场景中,自主驾驶车辆的质心侧偏角和航向角偏差均能控制在较小范围内,在多目标连续避障的路径规划和轨迹跟踪问题上同样具有很好的控制效果。

Emergency obstacle avoidance is one of the key points for autonomous driving system.A path planning controller based on non-linear model predictive control and a path tracking controller based on linear time-varying model predictive control are designed.In path planning controller,an obstacle-avoiding function is used to adjust the distance between the intelligent vehicle and obstacles by calculating the value of obstacle-avoiding function.A new route is supposed to be planned,which can not only keep away from obstacles but also decrease the deviations from the global course,by considering the different weights of obstacle-avoiding function and the planning error.In path tracking controller,the solution of linear time-varying model predictive control is converted into a positive-definite quadratic program.And then the desired steering angles of front wheel are calculated by using the reference trajectory and orientation angle of vehicle center.The results show that the obstacle-avoiding controller has strong robustness and the path tracking controller has better performance in controlling accuracy and vehicle dynamics stability when a vehicle travels at 60 km/h,80 km/h,or 100 km/h.The maximal side-slip angle is no more than 1°,and the maximal deviation of yaw angle is less than 2.5°.Obstacle avoiding action occurs in advance with the increase in velocity of autonomous vehicle.In addition,the dual-MPC controller is adaptive to the circumstance of multi-obstacles avoidance as well,which can be easily demonstrated by the small deviation of yaw angles and small side-slip angles.