垄耕模式的无人四驱四转移动作业平台路径跟踪控制

Path tracking of farming platform with unmanned four-wheel independent driving and four-wheel independent steering in ridge tillage mode

为了提高垄耕模式的无人四驱四转(four-wheel independent driving and four-wheel independent steering,4WID-4WIS)移动作业平台路径跟踪控制精度和稳定性,该研究提出一种基于非线性干扰观测器(nonlinear disturbance observer,NOB)的路径跟踪控制策略。考虑到转弯区域跟踪误差较大,引入原地转向数学模型,设计基于原地转向的航向角PI控制与纯追踪控制的切换控制策略,以实现转弯路径精准跟踪。在此基础上,根据横向偏差和路径弯曲度,设计基于前视距离函数的纯追踪算法及模糊比例补偿器,构建基于NOB的前馈补偿器,以减小上线距离和位置超调。最后对所设计的跟踪控制策略进行仿真和试验验证,结果表明:与传统纯追踪控制相比,所设计的路径跟踪控制策略在3种初始横向偏差下,上线距离、超调、全线平均绝对误差分别减小了32.2%~43.4%、0~42.4%和27.7%~49.5%,且曲线区域平均绝对误差减小33.7%~39.5%;在颠簸硬石板、草地、农田翻耕路况下的稳态区平均绝对误差分别减小了6.25%、33.3%和41.7%。该路径跟踪控制策略有效提高了系统鲁棒性和作业路径跟踪精度,可为垄耕模式的无人四驱四转农业机械导航系统开发提供创新思路和技术参考。

The current farming platform has been widely equipped with unmanned four-wheel independent driving and fourwheel independent steering(4WID-4WIS).However,the control system of path tracking is required for high accuracy and sufficient stability under complex working conditions.There were also complex working conditions under crop ridge cultivation,such asΠtype target path,curves,initial pose deviation,various soil moisture,and bumpy ground landscapes.In this study,a control strategy of path tracking was proposed using a nonlinear disturbance observer(NOB).A mathematical insitu steering model was introduced for the relatively low tracking errors in the turn area of theΠtype path,compared with the traditional Ackerman steering model.Two steering methods were then used to realize the turn path tracking.Meanwhile,a switch control strategy was designed between yaw angle proportional integral control and pure pursuit control using in-situ steering.Furthermore,the curve and initial pose deviation shared a relatively significant impact on the accuracy of working path tracking.Moreover,the distance traveled by the 4WID-4WIS farming platform was reduced to reach the working path and the maximum lateral deviation.The tracking accuracy of the work paths was improved to design a pure pursuit control using a lookahead distance function,and a fuzzy proportional compensator using the lateral deviation,as well as the curvature of the foresight area in the work path.Besides,the feedforward compensator with NOB was designed to avoid the relatively large yaw speed disturbances from the complex soil moisture,bumpy ground landscapes,kinematic models,and measurement errors.The NOB was also constructed to achieve the precise observation of disturbance for the expected path of farming platforms.The steering compensation angle was then calculated for the feedforward compensator to counteract the disturbance.Finally,the simulation was carried out in the Ubuntu/ROS environment.The NOB strategy of path tracking effectively reduced the distance traveled by farming platforms to reach the working paths,the maximum lateral deviation,and curve tracking errors.The accuracy and stability of path tracking were achieved in the anti-interference performance,where the disturbance momentum was observed accurately.And,the outdoor experiments show that the switch control strategy performed a smaller error of turn tracking onΠtype target path,compared with the traditional pure pursuit control.The tracking performance was also effectively improved.The pure pursuit with the look-ahead distance function and fuzzy proportional compensator under different initial pose deviation states reduced the distance traveled by farming platforms to reach the working paths by 32.2%-43.4%.The maximum lateral deviation,mean absolute errors of the whole line and curved area were reduced by 0-42.4%,27.7%-49.5%,and 33.7%-39.5%,respectively,indicating the high accuracy of working paths tracking.The NOB-based feedforward compensator was reduced by 6.25%mean absolute error in the steady area under hard slate condition,33.3%under grassland condition,and 41.7%under farmland condition.This control strategy of path-tracking effectively improved the system's robustness and path-tracking accuracy.The finding can also provide innovative ideas and technical references for the navigation system of unmanned four-wheel drive and four-rotation agricultural machinery in ridge tillage.