悬挂式电动柔性疏花机控制系统设计与试验

为提高机械柔性疏花机对果树品种、树冠形状和大小的自适应性，设计了一种悬挂式电动柔性疏花机。基于仿形疏花的工作要求，确定了仿形参数关联式；采用超声波冠形探测方法，开发了嵌入式仿形疏花控制系统。田间试验测得仿形疏花执行机构的位置伺服控制误差较小，平移丝杠在位移目标值4.7～36 cm内的平均误差为0.17 cm，最大误差为0.9 cm；仿形丝杠在位移目标值5.7～15.3 cm内的平均误差为0.07 cm，最大误差为0.4 cm。结果表明所设计的仿形控制系统能满足疏花机平面位置伺服控制的精度要求。该研究可为果园疏花机械的结构设计和运行控制提供参考。

Design and experiment of control system for suspended electric flexible thinner

Abstract: Thinning is required in fruit tree production to ensure high quality fruit and proper fruit size, which is one of the most labor-intensive practices. Mechanical blossom thinning is an environmental friendly alternative to chemical blossom thinning. Good tree training techniques are required to get high-level blossom removal for mechanical thinning. Electric flexible thinner can realize blossom thinning by rotating flexible strings. There is a challenge in mechanical thinning due to the varying shape of tree canopy. To improve the adaptability of the electric flexible thinner on fruit tree variety, canopy shape and size, it is important to develop a profiling control system. In this paper, the structure of the actuators for profiling action of the electric flexible thinner was proposed. The flexible thinner was suspended on the rear three-point hitch of a tractor and consisted of a vertical rotating spindle attached with several segments of nylon cords in horizontal direction. The spindle was rotated by a brushless direct current motor and its spatial position could be adjusted by translational screw and profile adjusting screw. The ball screws were driven and controlled via stepper motors, which could control the offset into the trees and the angle of the thinner. With the ball screws, the thinning spindle could be stretched and tilted to conform to the height and inclination of the tree canopy. The range measuring rod could map the canopy as it passed and was placed ahead of the spindle to avoid sensor collision with rotating cords. Two pairs of ultrasonic sensors suspended on the range measuring rod could provide the angle and lateral offset sensing of the flexible thinner position by sending infrared laser pulses out and measuring the round-trip flight time to the trees. The spindle rotation speed, screw displacement and thinning angle were adjusted by the electric motors. During the blossom thinning process, the centrifugal force on the panicles of the fruit tree was used to cause rotating nylon cords to thin blossom. The intensity of thinning could be controlled by changing the spindle rotation speed, tractor speed and string arrangement and other parameters. To get the key parameters of profiling control, the geometric relationship between flexible thinner and canopy edge was analyzed. Using the modular structure, the hardware and software of profiling control system were developed. The controller performed the planning task and calculated the number of pulses for stepper motors of ball screws between the current position of the thinner and the currently measured canopy. All field tests with the electric flexible thinner were conducted at a tractor speed of 1.93 km/h. The field test results of single litchi plant illustrated the position servo control performance of two ball screws of the electric flexible thinner. The measured dynamic mean errors of the translational screw and profile adjusting screw were 0.17 and 0.07 cm respectively. To improve the performance of profiling controller, it required to move the thinner quickly without affecting thinning speed and to reduce the control latency to acceptable levels. The experimental results show that the actuators have good position adjusting accuracy and the proposed profiling control system can meet the requirements of real-time control.