Improvement of hole pressing roller device and design of the control system for vegetable seedling's seeding assembly line

Abstract: An automatic seedling machine line has been widely used for the seedling of vegetables and fruits in an intensive and efficient farming. It is a high demand for higher accuracy, production efficiency, and quality of seeding in the current vegetable seeding line. In this study, an improved active hole pressing roller device was designed with a new control system for a vegetable seeding line. The existing 2BSL-320 seeding assembly line was also optimized, including the soil paving, soil sweeping, hole pressing, seeding, soil covering, and transmission devices. The specific procedure was as followed. First of all, the seedling tray was filled with the soil at the soil paving device, where the brush was used to sweep the excess culture soil. The low accuracy of the original passive hole pressing roller was attributed to the uneven quality of the roller during the swing operation, where the seedling tray hit the baffle to rotate. Therefore, a new baffle was designed to replace the indenters in the device, resulting in the fact that the rotation was controlled by a stepping motor. Specifically, the arc length of two adjacent indenters was equal to the distance between the two centers of adjacent holes in the seedling tray. As such, 16 rows of indenters were installed for one revolution of the roller. The active hole pressing roller was then used to accurately press out the seed holes. Taking the Arduino Mega2560 as the control core, the control system of the seeding line mainly included the speed detection and control of the conveyor belt, while the initialization and control of the hole pressing and seeding device. Furthermore, a sensor system was selected, including the speed encoder, angle sensor, and a travel switch. A real-time measurement was made on the speed of the conveyor belt, the angular displacement of the hole pressing and seeding drum, as well as the position of the seedling tray on the conveyor belt. The actuator included the stepping and AC geared motor driven by stepper motor driver and AC motor frequency converter, respectively. Finally, the white powdery nursery soil and "Qinyou No. 2" rapeseed were taken to optimize the bench test. According to the requirement of technical standards, the seeding pass rate was greater than 90% when the production efficiency was 800 trays/h. After that, the speed control accuracy of the conveyor belt was verified under the given production efficiency of 100-800 trays/h. A comparison was made to evaluate the performance of the hole pressing with/without the closed-loop control. The open-loop controlled hole pressing roller was offset the seed hole center by 9.8-12.5 mm. A closed-loop control was also added. It was found that the hole pressing precision of the closed-loop control system was greatly improved, where each pressure head was pressed in the center of the seed cavity. Moreover, the highest production efficiency reached 800 trays/h in the seeding test. The results showed that the seeding pass rate of the seeding assembly line was greater than 90.8%, the repeat seeding rate was less than 3.9% and the empty rate was less than 5.3%, meeting the test requirements. Consequently, an automated seeding assembly line was developed to improve the traditional one that was controlled by a single-chip microcomputer and driven by a stepping motor. The control accuracy was also greatly improved under a higher production efficiency. The finding can offer a strong reference for the automatic vegetable seedlings.