1. Introduction
Seedling tray technology emerged in the mid-1970s as a modern cultivation method designed for efficient factory production, pioneered by European and American countries [
1]. Compared with the traditional seedling production method, it boasts several advantages [
2]: high seedling emergence rates, orderly growth patterns, increased transplant survival rates, shorter growth cycles, reduced incidence of pests and diseases, seed and energy conservation, and facilitation of variety promotion and standardized production management [
3]. This technology plays a crucial role in mechanizing and scaling the production of numerous vegetable and cash crops, ensuring their sustainable and efficient development [
4].
As vegetable planting mechanization progresses, the industry has transitioned from manual to semi-automatic and fully automatic transplanting machines [
5,
6]. There is no seedling tray conveying and picking device for the semi-automatic transplanter, and these are the most critical technologies when compared with the fully automatic transplanter [
7]. The cultivated neat seedlings need to be transported to the picking device and planting mechanism for planting [
8].
Seedling tray transport technology in developed countries started earlier. The XT616 transplanter for tea seedling trays produced by Transplant Systems in Australia [
9] is equipped with various types of manipulator end actuators to adapt to the transplanter of different specifications. The PF2R fully automatic vegetable transplanter made by Yanma in Japan [
10] can control the horizontal feeding plate through the double screw mechanism, and realizes the longitudinal feeding of seedlings through the linkage of the ratchet mechanism and chain transmission mechanism. To ensure the continuous and accurate transmission of the seedlings, a special winding seedling tray is used to realize the non-gap placement of adjacent seedling trays, and the side push rod of the chain and designed placement are used in the device. A nursery tray conveying mechanism was developed by Saramin in Sri Lanka [
11] to feed the seedlings to the planting mechanism, along with a proximity sensor, which can stop at the designed position. The above mechanisms in these countries use special seedling trays that are flexible or made of foam [
12,
13], which means the conveying devices are not universal.
In response to these challenges, Chinese research institutes and universities have carried out relevant research. Chuanhua et al. [
14] devised an electric seedling tray conveying device equipped with positioning profiling blocks. Sensors are installed at the lowest point of each set of profiling blocks to detect the position of the seedling tray. This method can accurately position the seedling tray, but it needs to add profiling blocks to each placed seedling tray, which is relatively low in practicability. Qizhi et al. [
15] designed an automatic transplanter tray shifting control method. The stepper motor is used to drive the chain transmission mechanism, and then to drive the seedling tray to carry out longitudinal feeding. The photoelectric sensor installed on the side of the seedling tray conveying device is used to obtain the position information of the seedling tray, and then the transmission and positioning of the seedling plate are realized by controlling the pulse signal output of the motor. While the reliability of this device is relatively low, once the seedling tray is damaged or there is a gap on the side of the tray, it will affect the accuracy of positioning [
16]. The above conveying devices are all transported in one direction [
17], which may reduce the efficiency of conveying the seedlings and further lower the speed of picking and planting.
A device was designed by Gaohong et al. [
18] for moving the seedling tray and fixing the picking device; the ratchet pawl is driven by the motor to realize the longitudinal intermittent movement of the seedling plate, and the double helix shaft is driven by the motor to realize the transverse reciprocating movement. This method has a complex mechanical structure and processing errors will lead to inaccurate positioning. Changjie [
19] designed a seedling tray feed mechanism for the hanging basket transplanter, which also uses a stepper motor to control the shift cylinder for reciprocating linear motion and a rocker rod–ratchet mechanism to control the seedling plate for longitudinal intermittent feeding. The devices generally control the special mechanical structure for movement and the structure is also complex. Furthermore, these devices are inclined to place the seedling tray, whose own gravity will affect the longitudinal positioning accuracy. Xin et al. [
20] designed a seedling tray conveying mechanism, with transverse and longitudinal seedling feeding functions, and photoelectric sensors were used for positioning. This device is easier to use and more effective than other mechanisms that move in two directions.
In summary, three types of positioning methods are utilized for the fully automatic transplanter’s seedling tray [
21]. Firstly, mechanical mechanisms like the crank lever, gap rotating plate rack, or incomplete gear systems achieve longitudinal movement for pot seedling plates. Although accurate, these methods are singular in their positioning approach and may involve complex design structures. Adjusting the movement distance is challenging, limiting adaptability to specific pot and seedling tray sizes. Secondly, the DC motor or stepping motor is used to drive the seedling tray conveying mechanism, the photoelectric switch positioning mode is accurate and the greatest advantage is that the spacing change is flexible. Thirdly, the conveying device with visual system positioning can enable two-dimensional positioning of the seedling tray [
22], which has a specific function. However, it is more sensitive to the vibration of the transplanting device and it is too expensive to be applied.
To enhance positioning accuracy, improve low efficiency of conveying and accommodate various specifications, this study introduces a dual-axis positioning tray conveying device integrated with a dual-sensor system and precise servo-control system. It employs a dual-sensor positioning algorithm and variable displacement method to optimize parameter combinations within the control system and its transfer function. The step-controlled bench was used for experimental validation with different tray specifications to validate the efficacy of this approach. This research lays crucial technological groundwork for seedling tray transportation and retrieval in fully automated transplanting machinery.
4. Conclusions
To enhance the versatility of the seedling tray conveying device used in fully automatic transplanting machines across various specifications, and improve the positioning accuracy of commonly used PVC/PC seedling trays in China, a dual-axis positioning tray conveying device was developed. This device supports seedling trays ranging from 21 to 288 holes, enabling X-axis tray conveyance and Y-axis platform movement. It employs a dual-sensor positioning algorithm and a variable displacement positioning method to achieve precise initial positioning and intermittent movement in both the X and Y directions.
The device utilizes a servo motor control system with a three-closed-loop control and a PID algorithm to achieve precise positioning. The schematic of the precise positioning control system and its transfer function were validated using Simulink software to optimize the parameter combinations. The results showed that = 30, = 10 and = 10 performed optimally, producing an ideal step response output curve with an integral time = 3 and differential time = 0.33.
The initial positioning and intermission conveying on the X-axis and Y-axis were tested on a bench with single-step debugging; nine different sizes of seedling trays were used to measure the deviations. The data showed that the initial positioning deviation of the seedling tray on the X-axis was up to 1.34 mm, the maximum deviation in intermission conveying was 0.85 mm and the maximum coefficient of variation was 26.28%. The positioning deviation on the Y-axis was smaller than that on the X-axis, the initial positioning deviation was up to 0.99 mm, the deviation in intermission moving was up to 0.98 mm and the coefficient of variation was up to 19.33%. The experimental results show that the deviation in seedling tray positioning and conveying can meet the requirements of accurate conveying, and is accurate for a fully automatic transplanting machine.