Design and experiments of the cam swing rod intra-row weeding device for lettuce farm

Abstract: An intra-row weeding is highly required for the lettuce that is transplanted in a greenhouse. However, the general weeding machine cannot fully access the greenhouse environment in recent years. Particularly, the normally soft and loose leaves are also easy to damage, due to the typically higher planting density of lettuce in the greenhouse. Therefore, the 2-3 intra-plant weeding has posed a great challenge on the large-scale lettuce production. It is also necessary to realize automatic weeding operations with low damage and low power consumption, rather than manual weeding at present. In this study, a lightweight electric weeding device was developed for the intra-row weeding in a lettuce farm using a cam swing mechanism. The rotation speed was also real-time calculated for each working section of the cam, according to the vehicle speed and the crop protection radius. A motor was then utilized to drive the reciprocating movement of the pendulum rod for the precise rotation of the cam, thereby controlling the opening and closing of a pair of weeding blades. As such, an accurate separation was realized for the seedlings and weeding between lettuce plants. Furthermore, a machine vision system was also selected to accurately locate the lettuce seedlings, further to determine the protection area of the operation, particularly for the less damage to the seedlings, while the higher coverage rate of weeding. Alternatively, the cam structure directly determined the moving trajectory of the weeding shovel. Therefore, the dynamics of cam swing was analyzed to determine the outline size of the cam, including the farthest or nearest dwell angle, and the moving angle for rising or returning travel of the cam. Then, a weeding device was simulated to validate the combined motion of the weeding shovel, in terms of virtual assembly and kinematics using Solidworks software. The simulation demonstrated the motion trajectories and the maximum speed of the tool tip. The selection of motor was also derived from the input torque and power after simulation. Finally, field experiments were carried out, where the forward speed of the weeding device, the speed of the cam in the pushing section, and the depth of weeding shovel in soil were taken as the experimental factors, whereas, the injury rate on the seedling, the weeding rate, the power consumption of intra-row weeding unit were taken as experimental indexes. The response surface method (RSM) was also adopted to carry out a combined field test of three factors and three levels, with emphasis on the interaction of various factors on the performance indicators of the operation. The results showed that the depth of the weeding shovel in the soil presented the most significant effect on the weeding rate (P<0.01), while the forward speed presented the most significant effect on the damage rate to seedlings (P<0.01), and the cam speed and the depth of weeding shovel presented the most significant effect on the average power consumption (P<0.01). An optimal combination of operating parameters was achieved, where the cam rotation speed of 242 r/min, the soil penetration depth of 12.8 mm, and the forward speed of 0.56 m/s. In this case, all performance indicators were essentially satisfied the functional requirements of lettuce weeding, where the weeding rate achieved 93.22%, while the seedling damage rate was 2.87%, and the average power consumption of single seedling avoidance was 55.2 W. Taking the single-ridge and four-row lettuce as an example, the four rows of weeding were carried out, where the distance between two ridges was 170 cm. Correspondingly, the operating efficiency reached 0.346 hm2/h, when the whole machine was operated at a speed of 0.56 m/s (about 2 km/h). The feasible device can also effectively relieve the burden of weeding in a lettuce farm.