Design and experiments of an arc expansion type automatic seedling taking and throwing device for vegetable plug seedlings

Abstract: Seedling transplanting has been one of the most important planting steps for vegetable plug seedlings raising in China. However, manual and semi-automatic transplanting cannot fully meet the large-scale transplanting operations in recent years, due to the high labor cost. Alternatively, mechanization can also be a benefit to modern agriculture. An automatic seedling taking and throwing device can be expected for the vegetable plug seedlings to fully meet the vegetable transplanting for the high efficiency of vegetable plug seedling planting and the low labor intensity. Therefore, this study aims at the problems of the automatic seedling taking and throwing device for the vegetable plug seedling, such as the large structural size, easy failure in taking and throwing seedlings. The transplanting operation was proposed to analyze the individual taking and throwing seedlings, where the taking seedlings in a straight shape, and throwing seedlings in an arc shape. An arc-type expansion automatic seedling taking and throwing device was also designed for the transplanting operation of vegetable plug seedlings. A systematic analysis was performed on the main structure and working principle of this device. Three components included a seedling clamping assembly, a guided groove plate, and a rotary seedling receiving mechanism. Among them, the seedling clamping assembly was firstly bent into an arc-shaped expansion through the hinge structure, and then the eight vegetable plug seedlings were taken away from the seedling tray, and finally thrown into the seedling cups at one time. Theoretical analysis was carried out to determine the parameter equation of the guided groove centerline, including the radius of the circle formed by the geometric center of the seedling cups, and the minimum distance between two adjacent seedling cups. Some limitations were also considered, such as the reduced size of the rotary seedling receiving mechanism. As such, the mechanism was easy to assemble without the interference of seedling cups each other. A risk of collision was found between the vegetable plug seedling and the seedling cup when the seedling was thrown into the seedling cup with a high average frequency of taking and throwing seedlings. For this reason, the seedling throwing scheme was introduced to automatically set an appropriate delay using the current average frequency of taking and throwing seedlings. The centers of the substrate and the seedling cup coincided without deviation, in order to avoid substrates loss and seedling throwing failure caused by collision when the vegetable plug seedlings were thrown into the seedling cups under this seedling throwing scheme. The PLC automatic control system was built with the pneumatic circuits of the execution units, in order to coordinate the movements of various mechanisms, complete efficient and regular seedling taking and throwing operations. An orthogonal test was carried out, where the influencing factors, average frequency, air supply pressure, and average moisture content of substrates were selected as the experimental factors, while the success rate of taking and throwing seedlings and the rate of wounded seedlings were the target objectives. The range and variance analyses were performed on the experimental data. A better combination of experimental factors was obtained: the average frequency of taking and throwing seedlings was 90 plants per min, the air supply pressure was 0.4MPa, and the average moisture content of substrates was 30%. The demonstration tests were conducted with a combination of experimental factors. The success rate of taking and throwing seedlings was 94.05%, and the rate of wounded seedlings was 1.19%. This finding can provide a sound reference to design the automatic seedling taking and throwing device.