Design and test of the canopy wrap-around profiling-to-target sprayer for orchards

Precision spraying technologies have been widely applied in modern agriculture, such as profiling, targeting, and variable spraying. But there are still great challenges, in terms of uneven distribution of canopy sap, low sap utilization, high dosage of sap, and serious environmental pollution. Especially, the uneven spraying volume can be found in the cross-sectional shape of the tree canopy caused by the dynamic changes in the spacing between the nozzle and the canopy. It is a high demand for a high effective space area in the spraying volume of liquid. In this study, a novel canopy wrap-around profiling-to-target spraying was proposed to draw on the manual spraying actions, particularly with the nozzle completing dynamic surround profiling of each fruit tree during the advance of the sprayer and targeting the inside of the canopy for spraying. A spraying machine of wind-fed fruit trees consisted of five subsystems: the power platform, profiling-to-target mechanism, wind-fed spraying system, navigation control system, and canopy detection system. There were also environment perception, automatic navigation, and profiling-to-targeting functions. A symmetrical mechanism was also designed with cross-arranged, two-degree-of-freedom, and radial profiling, according to the nozzle motion requirements. The forward motion of the sprayer was combined with the position and attitude adjustment of the nozzle in plane space. Besides, the kinematic model of the nozzles was established to accurately control the lateral and rotational movement position and speed using the two-loop PID controllers. Then, the semicircular and polygon trajectory profiling control of the nozzle was achieved to match the motion speed of the sprayer. At the same time, the nozzle was rotated dynamically to keep the spray aligned with the center of the canopy, in order to reduce the drift of the liquid in the air. After that, trunk identification and localization were proposed using DBSCAN density clustering and 3-point trunk shape fitting, where the planar LIDAR scanned the horizontal plane of trunk height in real time during the operation of the sprayer. The 2-D LiDAR was employed and mounted horizontally on the front of the applicator to scan and identify the tree trunk in advance. The localization of the nearest trunk to the sprayer was sent to the control system in real time, in order to enable the trunk position tracking for the wrap-around profiling control. The results showed that the positioning errors of the longitudinal and lateral trunks were 9.44 and 1.74 cm, respectively. The total control system of the applicator was equipped with an STM32 processor using the FreeRTOS real-time operating system. The dynamic servo profiling control of the nozzle was realized using LiDAR trunk position information via the multi-task distribution and efficient processing of real-time trunk position information sensing, dynamic nozzle profiling trajectory control, data communication, and operation interaction. Finally, the quantitative indexes were taken as the droplet deposition, droplet density, droplet coverage, and median diameter of droplet volume. 10 test points were also arranged in the different areas of the canopy. Furthermore, the six sets of comparison tests were carried out between the wrap-around profiling and fixed-spacing spraying mode. It was found that the average droplet density of the wrap-around spraying mode was 72.2 droplets/cm2, the average deposition volume was 1.99 μL/cm2, and the droplet coverage rate was 47.5%. More importantly, the droplet deposition and density increased by 36.3%, and 58.3 %, respectively, compared with the fixed-spacing spraying mode. Therefore, a higher liquid utilization rate was achieved during this time. The coefficient of variation of droplet deposition was 27.7% for the wrap-around application, which was 60% lower than that in the fixed-spacing spraying mode. There was a significant improvement in the canopy application uniformity. In summary, the wrap-around profiling spraying mode can be expected to effectively improve the utilization rate and uniformity of precision spraying.