基于事件触发的采摘机械臂轨迹跟踪间歇控制

Event-triggered intermittent trajectory-tracking control for harvesting robotic manipulators

  • 摘要: 目前,机械臂广泛应用于农业采摘领域,成为农业自动化的关键组成部分. 但是,采摘机械臂的连续控制策略对信息实时性要求极高,增加了控制成本,为此本文针对采摘机械臂提出了一种基于事件触发的采摘机械臂轨迹跟踪间歇控制策略. 首先,利用拉格朗日力学,建立考虑外部扰动的双关节采摘机械臂的动力学模型. 利用径向基函数(RBF)神经网络估计采摘机械臂的未知动力学函数与外部干扰,基于反步法建立连续反馈轨迹跟踪自适应控制器设计方法. 采用上下确界技术建立控制区与休息区的量化关系,进而提出控制区间的事件触发控制器设计方案. 利用李亚普诺夫稳定性理论,证明采摘机械臂系统收敛到一个有界区域,并实现预期跟踪目标. 利用闭环信号的有界性原理,证明所设计的事件触发机制无Zeno行为. 最后,分别考虑周期性间歇和非周期性间歇,分析本文采摘机械臂控制策略的可行性与有效性. 在相同的控制输入总量条件下,10%休息区间的间歇事件触发策略相比于连续控制策略,控制器更新次数降低68.31%,相对于传统PID控制,关节1的整体均方根误差(RMSE)指标降低22.18%,关节2的整体RMSE误差指标降低33.63%.

     

    Abstract: Robotic manipulators are extensively used in agricultural harvesting operations and represent a pivotal advancement in agricultural automation systems. Harvesting robotic manipulators operate in harsh, unstructured environments such as orchards and greenhouses, where external disturbances and unknown dynamics of the manipulator must be addressed. To enhance the trajectory-tracking precision and disturbance-rejection capabilities, continuous control strategies—particularly adaptive neural network-based approaches—have been widely implemented in harvesting robotic manipulators. However, these conventional continuous control paradigms impose stringent real-time computational requirements; consequently, persistent high-frequency monitoring of system states and manipulator dynamics is necessitated for real-time control computation and transmission, thereby incurring prohibitively high control costs. Hence, this study proposes event-triggered intermittent trajectory-tracking control for harvesting robotic manipulators to balance between control performance and control costs. First, a dual-joint harvesting robotic manipulator dynamic model is rigorously developed based on Lagrangian mechanics principles. This model comprehensively accounts for the inertial properties, Coriolis effects, gravitational loading, and disturbance inputs. To address the challenges inherent in harsh operational environments, a radial basis function neural-network architecture is implemented to estimate the unknown nonlinear dynamics and external disturbances of the system. This compensation mechanism significantly enhances the overall system stability and operational robustness under practical field conditions. Subsequently, a continuous trajectory-tracking adaptive controller is systematically designed using the backstepping control methodology. The second-order nonlinear dynamic structure of harvesting robotic manipulators inherently circumvents the mathematical complexity associated with virtual control derivation, thus rendering backstepping particularly suitable for this application. By applying the supremum-infimum technique, the total operational period is partitioned into actively controlled intervals and dormant rest phases to establish a precisely quantified relationship between control and rest intervals. The implemented strategy operates under dual operational modes: during control intervals, the controller executes event-triggered actions, whereas all control computations and transmissions are suspended during the rest intervals. This control strategy effectively reduces the computational burden and communication bandwidth requirements while significantly extending operational endurance, thereby reducing the overall control cost. Stability analysis employing the Lyapunov theory shows that under the proposed control strategy, both trajectory-tracking and neural-network estimation errors converge to a bounded region, thus enabling the desired tracking objectives to be achieved within the specified error margins. A comprehensive theoretical examination further establishes the uniform boundedness of all closed-loop signals, thus definitively confirming the absence of Zeno behavior (i.e., infinite triggering events within finite time). This critical property is governed by a strictly positive minimum inter-event time threshold, which ensures that no infinite triggering occurs during any finite operational period. Finally, the feasibility and effectiveness of the proposed control strategy are validated through periodic and aperiodic intermittent operation modes. Under an identical total control input, the event-triggered intermittent control strategy with 10% rest intervals achieved a lower controller update frequency by 69.31% compared with continuous control. Meanwhile, compared with the conventional PID control, it improved the overall RMSE for Joints 1 and 2 by 22.18% and 33.63%, respectively. These results collectively verify the strategy’s exceptional ability to achieve an optimal balance among tracking accuracy, computational cost, and communication cost in harvesting robotic manipulators.

     

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