果园巡检机器人长臂抖动抑制方法

针对果园巡检机器人长臂因抖动而严重制约图像采集效率的问题,该研究提出基于臂体上3个部位的仰角构建单一反馈量进行抖动抑制的方法。首先基于臂体外伸段的有限元模态分析结果,通过固有频率和静变形的等效,建立三杆两扭簧等效机构的动力学模型。并基于微分平坦理论将3个倾角传感器读数合成为一个系统输出量,以平坦输出量作为被控量,采用PID控制器进行抑振试验,再利用自抗扰控制(Active Disturbance Rejection Control, ADRC)思想中对广义扰动的处理方法,将被控模型等效为二阶系统形式,并将模型信息代入到扩张状态观测器中进行对比试验。试验结果表明,基于本文构建的方法均能起到快速抑制长臂抖动的作用。在冲击作用致使末端产生10°幅值的抖动情况下,PID控制器实现了7 s内抑制大幅抖动,自抗扰控制器实现了8 s内快速抑制大幅抖动的效果。PID控制器的输出出现3次饱和现象且小幅较高频率抖动不易消除,而自抗扰控制器仅出现一次饱和,具有抖动抑制过程平滑的优点。本文的控制算法简洁适于在单片机等微型计算机系统中应用,系统结构简单成本低,可为现代农业装备研发与应用中解决长臂类结构抖动的主动抑制问题提供理论参考。

Suppression method for the long flexible arm vibration of orchard inspection robots

Orchard inspection robot has widely been one of the most important equipment in modern orchard management using information technology. The robot with walking chassis and long arm is more suitable for this work than UAV, in cases where long-term monitoring is required, especially when it must be used in severe weather conditions. But the arm body is prone to shake, when the robot is walking in wind and rain or on uneven ground. If the long arm structure with strong rigidity is used, it will cause the overall bulkiness and the danger of overturning the whole vehicle easily, because of the high center of gravity. Therefore, the low stiffness arm body is beneficial to the safety of vehicle. Nevertheless, the lightweight long flexible arm tends to low-frequency vibration, due mainly to the low stiffness. Such low-frequency vibration can also cause another trouble, that is, high-quality image information cannot be captured during the vibration, and the process of waiting for vibration to subside is too long, which seriously restricts the acquisition efficiency of image information. If the stability control can be realized, this flexible characteristic will be conducive to the overall stability. Alternatively, Finite Element Method (FEM) has widely been one type of reliable calculation, when the analytical dynamic model cannot be obtained, due to the existence of concentrated mass or non-single section in the long flexible arm. FEM modal analysis can be utilized to greatly simplify the control model, where the cantilever part of long flexible arm was equivalent to a two degree of freedom pseudo rigid body. Particularly, this simplification can be carried out under the condition of low-frequency vibration. Therefore, this study aims to effectively suppress the long-flexible arm shaking of orchard inspection robot, further to improve the efficiency of image acquisition. Correspondingly, a vibration suppression control system was also proposed using the synthesis and feedback of elevation angles from three parts of arm. Firstly, a dynamic model of equivalent three-bar two torsion spring was established using the FEM modal analysis on the external extension of arm, where the equivalence of natural frequency and vibration modes depended mainly on the static equilibrium. The readings of three inclination sensors were then synthesized into a system output, according to the Differential Flatness theory. As such, the significant vibration of long flexible arm was rapidly suppressed within 9 s under the control of differential flat output as feedback. Specifically, the amplitude at the end of arm reached 10° decreasing to less than 2° within three control cycles, but the small amplitude and high frequency vibration were difficult to eliminate in the later stage, particularly when the PID controller was used. Fortunately, the profile of system output was relatively soft, and the curved of torque output was saturated twice, significantly less than that of PID, when ADRC controller was adopted. Although large vibration was effectively suppressed after five cycles, it was not easy to occur high-frequency jitter in the later stage. This control system can be expected to serve the long-flexible arm mechanism with impact disturbance or arm body jitter under moving conditions, particularly where the active vibration suppression is needed