Positioning control method for the seedling tray of automatic transplanters based on interval analysis

Abstract: The seedling tray conveying is one of the most components in an automatic transplanting machine. However, a large error can be found to identify the position of the seedling tray using a single photoelectric sensor. The reason can be the unstable light reflectivity of the outer surface on the seedling tray, due mainly to the easy deformation of the standard plastic tray. It is a high demand to accurately and rapidly position the seedling tray. In this study, a seedling tray conveying device was designed with fusion positioning using the push-rod translational conveying and dual sensor. A precise positioning control system was also proposed for the seedling trays. Firstly, the dual sensors were used to identify the in-position information of the seedling tray and the push rod, and then to fuse the structural information of the conveying device. A calculation model was also established to transport from the initial position to the given position for the seedling picking. As such, a seedling tray and push rod were designed after calculation. Secondly, an accurate evaluation was performed on the placement category between the push rods. The interval of each category was determined through normalized data processing. After that, an optimal displacement was given to the seedling tray using each placement category. The seedling tray was transported to the given position for the seedlings picking. Finally, the servo motor was used to drive the precise control of seedling tray delivery. Taking the 128-hole standard PVC hard plastic seedling tray as the test object, a series of performance tests were carried out for the positioning and seedling of the seedling tray conveying device. The results show that the control system was accurately determined the specific category of the seedling tray that placed arbitrarily on the push rod of the conveyor chain. Specifically, the faster the motor running pulse frequency was, the greater the positioning deviation was under different placement categories and conveying speeds. When the motor running pulse frequency was 800 Hz, the maximum conveying positioning deviation was 1.35 mm, the minimum value was 0.79 mm, the maximum average value of the positioning deviation was 1.07 mm, and the maximum coefficient of variation of the positioning deviation was 14.1%. Under different conveying speeds, the success rate of picking seedlings with the claws and holes was 100%, fully meeting the requirements of precise conveying and positioning. Therefore, the improved device can be expected to avoid the inaccurate and unstable positioning of a single photoelectric sensor. The finding can provide a technical guarantee for the automatic transplanter suitable for the standard plastic seedling tray.