Design and Test of Seedling-Picking Mechanism of Fully Automatic Transplanting Machine

Abstract

The seedling retrieval mechanism is a crucial component of fully automatic transplanting machines, significantly influencing the quality, reliability, and efficiency of the transplanting process. Nonetheless, the existing seedling retrieval mechanisms in current transplanting machines exhibit several deficiencies, including substantial damage to seedlings and inadequate retrieval accuracy. To overcome these challenges, we propose an integrated approach combining pneumatic and mechanical techniques to further improve performance. By employing a lower thimble elevation and clamping mechanism, alongside a mathematical model based on the seedling removal process, this method ensures precise seedling extraction and minimizes damage to the root system and substrate. The novelty of this study lies in its ability to reduce the adhesion between seedlings and the holes of the plug plate, thereby minimizing non-destructive extraction of the seedlings and preserving the integrity of the matrix, which is essential for ensuring healthy seedling growth. Moreover, the optimization of the seedling retrieval trajectory enhances the accuracy of the seedling retrieval mechanism while also meeting the requisite speed requirements. Experimental results indicate that at a rate of 72 seedlings per minute, the extraction success rate reached 94.90%, and the casting success rate was 98.53%. The seedling injury rate was only 1.95%, resulting in an overall success rate of 91.69%. These findings confirm that the device meets operational efficiency requirements and delivers effective performance.

1. Introduction

The agricultural practice of vegetable transplanting involves transferring seedlings from nurseries, where they have continued to develop, to the field. A crucial tool in this process is the transplanter, which is essential for timely cultivation and saving labor [1]. Machine transplanting is more efficient than direct sowing, because it requires less labor, resulting in more consistent vegetable yields. Numerous devices and tools have been specifically developed for this task [2]. Additionally, using transplanting instead of direct sowing mitigates the risks associated with natural disasters, such as low temperatures and cold spells in spring, yet also enhancing seedling survival and crop quality [3]. The seedling transfer device serves as a transportation bridge between the nursery unit and the planting unit [4]. Within this device, the component most prone to issues is the seedling-picking mechanism. A well-designed and efficiently operating seedling-picking device is essential for achieving full automation of the transplanter and enhancing operational efficiency [5]. Currently, transplanting machinery in China is predominantly characterized by semi-automatic transplanting technology, with seedling planting largely relying on manual processes [6]. Consequently, there is an urgent need to design an automatic seedling-picking device to replace manual seedling feeding [7].

The transplanters currently under study exhibit distinctive features and advantages tailored to the specific circumstances of each country. In Europe, the United States, and other nations, where land is more concentrated and comprises larger fields, research on automatic transplanters predominantly focuses on wide, high-speed, large-scale, and integrated mechanical–electrical–hydraulic–automation technologies [8,9]. For instance, FERRARI in Italy has developed the FUTURA fully automatic transplanter [10], which boasts a high degree of automation and employs an ejecting–clamping seedling-picking and dropping mechanism. This transplanter can achieve an impressive operational efficiency of up to 4500 crops per row per hour. However, its large size renders it unsuitable for smaller fields in China. In contrast, a two-degree-of-freedom five-bar seedling extraction device was developed in [11], which, yet simple, is not designed for high-speed seedling extraction. Additionally, a two-finger gripper for a wide range of vegetable seedlings was tested primarily on leeks and fennel [12]. In Japan, due to local constraints, transplanters tend to be compact, purely mechanical, and lack intelligent features [13], resulting in lower seedling extraction efficiency. It is important to note that foreign-designed seedling extraction mechanisms are typically developed for the specific conditions of their respective countries and may not be suitable for crop applications in China.

Currently, domestic seedling extraction mechanisms for transplanting machines primarily include two types [13,14,15]: pneumatic and mechanical types. The pneumatic seedling extraction mechanism exhibits relatively low efficiency due to the limitations of its actuators and other components. In contrast, mechanical seedling pickup mechanisms can compromise extraction accuracy due to wear and tear on their components. This study presents a whole-row automatic seedling extraction device characterized by a simple structure, enhanced efficiency, effective seedling extraction, and minimized damage to potted seedlings. Through simulation and platform construction, we conducted a performance test of the entire machine, confirming the device’s rationality [16]. However, the combined design of the seedling extraction device introduces a greater number of moving parts, necessitating higher machining and installation precision. Furthermore, the components must exhibit a high level of cooperation, which leads to slight jittering and jamming issues during the debugging process. In comparison to [8], the insertion of ejecting-type seedling extraction may negatively impact the growth of potting seedlings, as it can adhere to the needle and damage the substrate. Additionally, as noted in [5], both direct addition and extraction methods tend to cause damage to the stems of the seedlings.

This research focuses on the integration of agricultural machinery with agronomy, leveraging existing seedling removal technologies. It comprehensively considers factors such as plug tray specifications, operating environments, and user requirements while analyzing the advantages and limitations of various seedling removal methods. To facilitate the automatic removal of seedlings by transplanting machines, further research will be conducted on the transplanting device, culminating in the design of an automatic seedling-picking and dropping mechanism that combines electric and pneumatic systems to replace manual seedling-picking operations. The research includes theoretical verification, three-dimensional modeling, and simulation analysis of the designed mechanism. Additionally, a corresponding control system will be developed for seedling tray transportation, automatic picking, and dropping. This system aims to ensure accurate, rapid, and efficient seedling harvesting and transplanting operations, thereby enhancing the overall efficiency of transplanting processes. Ultimately, this research seeks to promote the automation of agricultural transplanting machinery, reduce labor intensity and costs, and improve agricultural production efficiency, providing vital technical support for the development of automated transplanting machines in our country.

2. Materials and Methods

2.1. Overall Structural Design

The overall structural design of the seedling-taking and casting device, as depicted in Figure 1, is grounded in the operating principles of each component and their corresponding action flows. The device is primarily composed of several parts, including the frame, seedling feeding mechanism, seedling extraction mechanism, and seedling cup mechanism [17]. The seedling feeding mechanism comprises a seedling topping device and a tray feeding device. The topping device mainly includes an electromagnetic top bar and a mounting box, which facilitates the horizontal arrangement of seedlings in hole trays for potting. The tray feeding device consists of two pairs of roller devices driven by four stepper motors; the rollers grip the fluent strip device and slide it along the seedbed to achieve longitudinal feeding of the burrow tray. The seedling extraction mechanism mainly comprises an electric actuator, connecting plate, slider slide device, electric rotary table, seedling extraction arm, variable distance slide, and flexible seedling extraction claw [18]. The electric actuator, powered by a servomotor, extends and contracts to enable the horizontal movement of the entire seedling extraction arm. The connecting plate links the motorized actuator to the slider and motorized rotary table. The slider slide device works in conjunction with the electric actuator to facilitate the overall horizontal movement of the seedling-picking arm. The motorized rotary table, controlled by a servo motor, allows for angular adjustments of the seedling-picking arm. The flexible seedling extraction claw, functioning as the seedling-picking actuator, is mounted on the variable distance sliding table slider and can adjust the distance between adjacent clamping jaws according to different workstation requirements. The variable-pitch slide and seedling-picking arm are integrated into a single unit, enabling simultaneous seedling-picking operations for an entire row of six holes. Furthermore, the spacing of the seedling claws can be adjusted according to the center distance between adjacent seedling cups during the seeding process, ensuring precise seedling placement [8].

2.2. Design of the Seedling-Taking Device

The seedling-taking and dropping device operates by extracting seedlings and launching entire rows of them. Accurate positioning of the seedling claws is essential during this process. To accommodate the overall structural dimensions of the device, a variable distance mechanism was designed and customized to adjust the relative positions of the seedling claws in real time. The variable pitch slide table is motor-driven, allowing the active camshaft to rotate both forward and backward. As illustrated in Figure 1 (5) below, a sliding rod is installed within the camshaft’s guide groove, connecting to each slide block. Driven by the right servo motor, the sliding rod moves within the guide groove, driving the slider, which in turn performs linear motion along the guide rail to facilitate changes in the distance between the slide blocks. The spacing can be categorized into two forms: synchronous equidistant spacing and synchronous unequal spacing. For this study, the synchronous equidistant spacing form was selected. The variable pitch slide table employs an intermediate positioning synchronous equal distance variable distance method, enabling high-speed, arbitrary point variable distance operations of multiple slide blocks within the variable range established by multi-station setups. Precise positioning of the slider is achieved through the collaboration of a photoelectric switch installed in the module and an internal induction wheel. To align with the spacing between seedlings as designed in this study, when the custom-made variable-pitch slide block contracts to the midpoint, the center distance between adjacent slide blocks is 42 mm, which corresponds with the center distance of adjacent plug grids. Conversely, when the slide blocks extend to the endpoint, the center distance between adjacent slide blocks is 80 mm, matching the center distance of adjacent seedling cups. The contraction and expansion of the slide block on the variable-pitch slide table drive the air claw connected to the slide block, enabling variable pitch adjustments to accommodate the varying working distances of different workstations, and the sliding table pitch variation is realized by the sliding table active camshaft driven by the motor, as shown in Figure 2a. In this paper, the variable pitch slide module customized by Sid Automation Technology Co., Ltd. (Dongguan, China) is used as the variable pitch mechanism of the claw, and the model of China Jili Pneumatic Equipment Co., Ltd. (Dongguan, China) is BMC-20006 straight-through pneumatic flexible gripper is used as the clamping device.

The seedling extraction device primarily relies on the clamping action of pneumatic seedling extraction claws to remove plug seedlings from the plug tray and transfer them to the seedling cup. However, the overall structure of the device introduces both horizontal and angular displacements. The horizontal displacement is primarily facilitated by two electric push rods, while the angular displacement is managed by electric rotary table servo motors located on both the left and right sides, as illustrated in Figure 1 (7). The electric push rod is LTW065-S400-B04-T10-BM2-C121 made by Litway Automation Technology Co., Ltd. (Wuxi, China), which meets the necessary stroke requirements. This electric push rod motor is equipped with an MC2H-M080775F high-inertia servo motor produced by Shanghai Ruineng Gaoqi Automation Co., Ltd. (Shanghai, China). Due to the misalignment of the seedling feeding device and the seedling cup device in the vertical plane, horizontal displacement is required. During the seedling removal and throwing operations, the seedling removal arm must be capable of horizontal movement to facilitate transitions between the two workstations. To achieve this functionality, this article employs an electric push rod in conjunction with a slider slide rail device, as depicted in Figure 1 (8). The servo motor utilized for the electric rotary table is the Y200RA100 of Luoyang Congxin bearing Co., Ltd. (Luoyang, China).

2.3. Design of Top Seedling Mechanism

When the plug tray is inserted into the seedling feeding mechanism, it initially passes through the seedling top area. To facilitate the subsequent seedling removal operation and to ensure both the efficiency and success rate of this process, it is essential to loosen the entire row of seedlings in the plug tray. This article utilizes an electromagnetic seedling lifting method for seedling lifting operations, which primarily consists of six electromagnetic push rods housed within a fixed push rod box. The electromagnetic push rods are installed inside the push rod box, with the spacing between adjacent push rods aligned with the specifications of the tray. For the 72-hole tray selected in this study, the distance between adjacent cells is 42 mm. Consequently, the center distance between adjacent electromagnetic ejector pins is also set to 42 mm to correspond with the tray dimensions. The fixed ejector box is installed and fixed under the seedbed, and the seedling top area of the seedbed is provided with a rectangular opening slot. The electromagnetic ejector selected in this article is a push–pull long-stroke electromagnet produced by Jiexiang Electric. By using a push–pull force tester, it was measured that the plug seedlings overcame the adhesion force between the seedlings and the inner wall of the plug grid and reached the maximum value when they were in a loose state. If the displacement is 14.34 mm, the length of the ejector rod should be greater than or equal to the maximum displacement. Taking into account factors such as the thickness of the mounting plate on the displacement, the strike of the electromagnetic ejector rod is set to 20 mm, as shown in Figure 2d.

2.4. Design of Seedling Delivery Device

The seedling-feeding device mainly consists of 4 parts, as shown in Figure 1 (3), which are the bracket, the seedbed, the electromagnetic seedling lifting mechanism, and the longitudinal shifting mechanism; among them, the seedbed is supported by the bracket and fixed on the frame, and the seedbed is A roller runner is installed, and the plug tray is placed between the two runner bars. Two stepper motors are installed on the left and right sides of the seedbed, with each motor equipped with a rubber roller. The rollers are connected to the motors via couplings. The distance between the two rollers on the same side is adjustable, and the drive mechanism is located on the same side. The two stepper motors rotate in opposite directions, which causes the rollers to rotate in opposing directions. This action squeezes the thin fluency bar with the smooth strip, allowing the thin fluency bar to move along the inclined direction of the seedbed under the influence of the extrusion force, as shown in Figure 2b. When the plug tray is tilted on the seedbed, it passes through the seedling lifting area before arriving at the predetermined seedling-picking point. Six electromagnetic seedling lifting devices are installed below the seedling lifting area, as illustrated in Figure 1 (7). The selected electromagnetic ejector rod is a push–pull long-stroke electromagnet produced by Jiexiang Electric (Suzhou, China). This device controls the current to the electromagnetic seedling lifting apparatus, enabling the electromagnetic seedling lifting rod to move up and down according to a preset displacement amount. This movement facilitates the adjustment of the plug seedling substrate and helps overcome the adhesion between the substrate and the inner wall of the plug grid. As a result, the knot strength can reach a loose state, thereby enhancing the efficiency and success rate of subsequent seedling clamping. After collecting the seedlings from the entire plug tray, the roller device gathers the hole trays on the runner, completing a seedling feeding cycle.

2.5. Gas Circuit Component Selection

The air circuit control system primarily manages the switching between the processes of catching and placing seedlings. An air compressor acts as the air source, providing the necessary air pressure to drive the air claw. The pneumatic triplet is tasked with processing and adjusting the air pressure. A 24 V 3.0 W two-position three-way solenoid valve is used, while the vacuum generator selected is the CV-10 model, which includes a silencer and is responsible for utilizing the positive pressure air source to generate negative pressure. Additionally, the precision pressure regulating valve plays a crucial role in adjusting the positive air pressure. The air claw is based on the model of China Jili Pneumatic Equipment Co., Ltd. (Dongguan, China) is BMC-20006P model pneumatic flexible gripper. The distance between the fingers G (0~20 mm) can be adjusted according to the working air pressure. The air claw connection module adopts a straight-through-type rod: one end of the rod is the vent end, the other end is the internal threaded hole for installing the flexible air claw, and the middle part is the threaded sleeve that can move up and down for elastic buffer, and the upper and lower two nuts are used to fix the relative position between the mounting plate and the air claw, as shown in Figure 2d.

2.6. Design of the Specific Process for Seedling Picking and Throwing

The specific process of seedling extraction and planting is illustrated in Figure 3 and is described as follows: (a) The plug seedling reaches the predetermined extraction position after being lifted. At this stage, the electric push rod guides the seedling extraction arm to approach the extraction point. The distance between adjacent sliders on the variable-distance slide table is set to 42 mm. The air control system engages the flexible clamping fingers of the air claw to securely grip the plug seedling. (b) The air claw fingers maintain their grip, while the left and right electric rotary tables rotate synchronously, driving the extraction arm to move the plug seedling away from the tray. During this process, the position of the electric push rod and the variable-distance slide table sliders remain unchanged. (c) The electric push rod retracts horizontally while maintaining the rotary table’s angle, causing the extraction arm to move backward. The variable-distance slide table sliders are controlled to spread apart, increasing the distance between adjacent clamping fingers to 80 mm, aligning with the spacing of the rotating seedling cups. (d) The electric push rod continues to retract horizontally, guiding the extraction arm to the predetermined planting position. The rotary table is controlled to rotate counterclockwise, positioning the plug seedling above the seedling cup. At this point, the six corresponding claws align precisely with the centers of the respective seedling cups. (e) In the seedling-dropping state, the clamping fingers are controlled to open, allowing the potted seedling to descend into the seedling cup under the influence of gravity. Simultaneously, the seedling delivery system transports the loosened seedlings from the lower row of the tray to the extraction position. (f) The seedling cup delivers the potted seedling to the planting device. Meanwhile, the extraction arm, driven by the rotary table, rotates clockwise back to the initial extraction angle. The clamping fingers are then controlled to close, adjusting the distance between them to 42 mm to match the spacing of the plug tray cells. Subsequently, the electric push rod moves the extraction arm horizontally toward the extraction position, completing the entire seedling extraction and planting operation.

3. Theoretical Calculation

3.1. Calculation of Seedbed Inclination Angle Design

To ensure the stability and accuracy of the hole tray conveyance throughout the entire seedling conveying operation, while also maintaining a relatively compact structure that aligns with the dimensions of the subsequent transplanting frame, it is essential to design the overall structure of the seedling bed and determine the appropriate angle of inclination [19]. This design will guarantee that the hole trays remain secure during the conveying process and do not disengage during seedling topping and picking. Additionally, analyzing the forces acting on the seedling substrate within the hole grid is necessary to ascertain the angle between the lower surface of the hole tray and the horizontal plane, which is the inclination angle between the seedbed of the seedling delivery mechanism and the horizontal plane $\alpha$ as shown in Figure 4 [20].

$$F_{\alpha }=G\times \cos \beta$$

(1)

where $F_{\alpha }$ denotes represents substrate gravity force along the horizontal direction of the inner wall of the cavity grid (N); $G$ denotes the gravity of the substrate (N); and $\beta$ denotes represents the angle between the gravitational force and its component force along the horizontal direction of the cave lattice.

Due to the cavity tray substrate gravity G > 0, in order to ensure that the cavity tray seedling does not slide along the direction of the large mouth of the cavity grid to fall, $F_{\alpha }$ ≤ 0 should be satisfied, i.e., cos $\beta$ < 0, and the cavity tray seedling substrate should not fall easily, establishing the equation:

$$\alpha ={180}^{\circ }-\arctan \left[{\displaystyle \frac{l}{n-m}}\right]$$

(2)

where m is the substrate bottom side width (mm); n represents the width of the top edge of the substrate (mm); and l denotes the substrate height (mm).

Bringing in the values of m, n, and l for the calculations yields that the angle of inclination $\alpha$ = 102°, when the angle between the bottom surface of the burrowing tray and the horizontal plane is greater than or equal to 102 degrees, there is no tendency for the seedlings in the burrowing tray to slide or fall toward the large opening of the burrowing compartment. Taking into account the limitations of the actual assembly space, machine operation vibrations, and other relevant factors, this paper adopts the specified value of $\mathsf{\alpha }$ as 120°.

3.2. Analytical Calculations of the Seedling Extraction Mechanism

During the seedling extraction process, the expansion and contraction of the electric actuator enable the forward and backward movement of the entire seedling extraction arm. Furthermore, the rotation of the electric rotary table enables the swinging motion of the seedling extraction arm. A mathematical model was established, as depicted in Figure 5, to derive the geometric relationships between the structural components of the seedling extraction system and the movement dynamics of the seedling claw [21]:

This model primarily focuses on the expansion and contraction processes of the electric actuator, as well as the rotational dynamics of the electric rotary table. We establish a coordinate system with point $B$, the center of the electric rotary table, as the origin. Point $A$ represents the installation location of the seedling-picking claw, allowing the seedling-picking arm $AB$ to rotate around point $B$. During the seedling extraction operation, the electric actuator extends to position. $D_1$, resulting in the seedling extraction arm being in the configuration $A_1{B}_1$. At this stage, the seedling extraction arm is rotated ${\gamma }_1$ degree clockwise, moving to position $A_2{B}_1$, after which the electric actuator retracts from $D_1{E}_1$ to position $DE$. When the seedling extraction arm reaches the seedling casting position horizontally, the entire arm rotates ${\gamma }_3$ degrees counterclockwise, positioning it at $A_{3}B$, with point $A_3$ directly above the seedling cup. In accordance with the established coordinate system, we define $A_1{B}_1$ = $A_2{B}_1$ = $AB$ = $A_{3}B$ = $a$, and $B_{1}B$ = $b$, where the angle between the seedling arm and the horizontal direction during extraction is denoted as $\gamma$.

$$\gamma ={180}^{\circ }-\alpha$$

(3)

where $\gamma$ is the initial angle of the seedling arm, and$\mathsf{\alpha }$ is the angle between the bottom of the cavity tray and the ground surface, Taking the seedling arm as the object of study, the coordinates of point $A_1$ can be expressed as:

$$\left\{\begin{array}{l}{x}_{A1}=l_1\cos \gamma +b\\ y_{A1}=l_1\sin \gamma \end{array}\right.$$

(4)

where $l_1$ denotes seedling arm length (mm). Taking the seedling arm rotation angle$\gamma$ the clamping jaws clamp the hole tray seedlings out of the hole tray, to reach the $A_2{B}_1$ position; at this time, $A_2$ coordinates can be expressed as:

$$\left\{\begin{array}{l}{x}_{A2}=l_1\cos {\gamma }_2+b\\ y_{A2}=l_1\sin {\gamma }_2\end{array}\right.$$

(5)

where ${\gamma }_2$ is the angle between taking the seedling arm and the horizontal direction after rotating the $\gamma$ angles. When the overall contraction of the seedling-picking arm is backward to reach the pre-projection position, the overall angle of the seedling-picking arm remains unchanged; at this time, the coordinates of point $A$ can be expressed as:

$$\left\{\begin{array}{l}{x}_A=l_1\cos \gamma \\ y_A=l_1\sin \gamma \end{array}\right.$$

(6)

when the seedling feeding operation is carried out, the seedling-picking arm turns the angle counterclockwise and reaches the $A_{3}B$ position; at which time, the $A_3$ coordinate can be expressed as:

$$\left\{\begin{array}{l}{x}_{A3}=l_1\cos \left({\gamma }_3-{\gamma }_2\right)\\ y_{A3}=l_1\sin \left({\gamma }_3-{\gamma }_2\right)\end{array}\right.$$

(7)

Knowing the displacement equation for the end pickup point of the pickup arm, the second-order derivatives of Equations (4)–(7) are carried out at time $t$, respectively, to obtain the expression for the acceleration at the end pickup point as:

$$\left\{\begin{array}{l}{\ddot{x}}_{A1}=-l_1\cos \gamma \cdot {\left(\dot{\gamma }\right)}^2-l_1\sin \gamma \cdot \ddot{\gamma }\hfill \\ {\ddot{y}}_{A1}=-l_1\sin \gamma \cdot {\left(\dot{\gamma }\right)}^2+l_1\cos \gamma \cdot \ddot{\gamma }\hfill \end{array}\right.$$

(8)

$$\left\{\begin{array}{l}{\ddot{x}}_{A2}=-l_1\cos {\gamma }_2\cdot {\left({\dot{\gamma }}_2\right)}^2-l_1\sin {\gamma }_2\cdot {\ddot{\gamma }}_2\hfill \\ {\ddot{y}}_{A2}=-l_1\sin {\gamma }_2\cdot {\left({\dot{\gamma }}_2\right)}^2+l_1\cos {\gamma }_2\cdot {\ddot{\gamma }}_2\hfill \end{array}\right.$$

(9)

$$\left\{\begin{array}{l}{\ddot{x}}_A=-l_1\cos \gamma \cdot {\left(\dot{\gamma }\right)}^2-l_1\sin \gamma \cdot \ddot{\gamma }\hfill \\ {\ddot{y}}_A=-l_1\sin \gamma \cdot {\left(\dot{\gamma }\right)}^2+l_1\cos \gamma \cdot \ddot{\gamma }\hfill \end{array}\right.$$

(10)

$$\left\{\begin{array}{l}{\ddot{x}}_{A3}=-l_1\cos \left({\gamma }_3-{\gamma }_2\right)\cdot {\left({\dot{\gamma }}_3-{\dot{\gamma }}_2\right)}^2-l_1\sin \left({\gamma }_3-{\gamma }_2\right)\cdot \left({\ddot{\gamma }}_3-{\ddot{\gamma }}_2\right)\hfill \\ {\ddot{y}}_{A3}=-l_1\sin \left({\gamma }_3-{\gamma }_2\right)\cdot {\left({\dot{\gamma }}_3-{\dot{\gamma }}_2\right)}^2+l_1\cos \left({\gamma }_3-{\gamma }_2\right)\cdot \left({\ddot{\gamma }}_3-{\ddot{\gamma }}_2\right)\hfill \end{array}\right.$$

(11)

According to the expression for acceleration, in conjunction with Newton’s second law, it follows that the force acting on an object is the product of its mass and acceleration. The length of the seedling’s length $l_1$, along with variations in the angle of the pick-up arm relative to the horizontal, the initial angle of pick-up, and other factors, influence the acceleration at the endpoint of the pick-up. This, in turn, affects the magnitude of the pick-up force. Therefore, these parameters can be adjusted and optimized to regulate the seedling extraction force.

4. Control System Scheme Design

4.1. Control System Composition

The demand analysis of the control system reveals that the control system discussed in this article comprises a controller, driver, host computer, actuator, and sensors, as shown in Figure 6. The controller employed is a Programmable Logic Controller (PLC) that manages the operation of the entire control system. It receives and processes information from external sensors, coordinates the control output to each actuator, and executes the designated work project. The host computer communicates with the PLC via a USB interface, enabling real-time monitoring of the system’s operational status. The stepper motor driver converts electrical pulses into angular displacement, thereby controlling the rotation angle and speed of the stepper motors. Additionally, the servo motor driver regulates position, speed, and torque, offering three methods to control the servo motor for achieving high-precision positioning within the transmission system. The actuator components consist of four stepper motors, five servo motors, six electromagnetic seedling lifting mechanisms, and six pneumatic clamping jaws, which work in coordination according to the operational process to facilitate the mechanical movement of the system. Sensors and relays play a critical role in detecting the position of each execution component and transmitting the collected information to the PLC for coordinated control [22,23,24].

4.2. Gas Path Control

During the seedling extraction process, precise regulation of the opening and closing of the seedling claw fingers is crucial to closely match the stem size of the plug seedlings. At this stage, the air path linked to the negative pressure solenoid valve is maintained in a closed state, while the positive pressure solenoid valve controls the positive pressure air path. As a result, the clamping fingers are adjusted to a precise degree of opening and closing under the influence of positive pressure. When the clamping operation requires the engagement of the air claw fingers, the positive pressure solenoid valve switches to the negative pressure air path, and the negative pressure solenoid valve opens. Driven by the vacuum generator, the positive pressure air source is converted to negative pressure, causing the clamping fingers to close around the seedling stems and maintain this state until the seedlings are placed, as shown in Figure 7. The seedling placement operation utilizes the same air control mechanism as the pre-clamping phase. Specifically, the clamping fingers are controlled to open, permitting the seedlings to descend freely under gravity.

4.3. Control System Programming Flow

The primary design of the seedling-picking and feeding device comprises several key components, including port alloseedbed of the seedling delivery cation, initialization parameters, program reset, control timing, and logic control for seedling picking and dropping. Figure 8 presents the flow chart illustrating the main program of the control system.

5. Simulation and Test

5.1. Simulation Post-Processing and Result Analysis

To explore the process of transferring seedlings to cast seedlings, we analyze the movement characteristics of the flexible clamping claw’s pinch finger apex, focusing on the changes in displacement trajectory, speed, and acceleration [25]. Employing ADAMS 2020 software for kinematic simulation and analysis, we establish fixed constraints and integrate various motion drive types and functions to simulate the seedling extraction process. This approach enables us to verify whether the relevant structural parameters of the seedling extraction meet the established evaluation indices. Furthermore, we derive optimal combinations of seedling extraction trajectories, speeds, and acceleration parameters from the simulation results [26].

The model was imported into ADAMS, where the displacement, velocity, and acceleration curves in the x, y, and z directions were analyzed at the tip of the seedling claw. To initiate image post-processing, the ‘ADAMS Postprocessor’ module was accessed. This involves fusing the plots of the three different types of curves—displacement, velocity, and acceleration—along the x-axis, y-axis, and z-axis, respectively. The resulting data points for the x-axis, y-axis, and z-axis, along with their corresponding time displacement, velocity, and acceleration curves, are illustrated in Figure 9a–c. Subsequently, within the ‘Results’ section, navigate to ‘Review’ and select the command to create the trajectory curve. This will yield the trajectory curve representing the movement of the seedling claw at its apex, as depicted in Figure 10a–c.

The analysis of the figure indicates that the displacement, velocity, and acceleration of the seedling claw tip along the x-axis exhibit relative smoothness. In contrast, the displacement and velocity curves along the y and z axes are also relatively smooth, showing no significant abrupt changes. However, the acceleration curve demonstrates notable sudden changes, primarily occurring at 0.2 s, 0.5 s, 1.0 s, 3.0 s, and 3.5 s due to the activation of the drive function. This suggests that the operation of the motorized actuator or the motorized rotary table introduces impact phenomena into the system during their expansion, contraction, or rotation. To mitigate the effects of shock and vibration on the seedling extraction process, it is advisable to incorporate a buffering device at the junctions of the electric actuator and the electric rotary table. This addition would enhance the stability and reliability of the seedling extraction and casting processes. Furthermore, the seedling trajectory diagram demonstrates that the path taken by the seedlings aligns with the requirements for both picking and casting. Notably, the movement of the seedling-picking arm does not interfere with the functioning of other components. The trajectory’s front end is designed for the seedling-picking operation to grasp the stalks, while the rear end facilitates the seedling-casting operation, indicating a well-considered design for the seedling-picking path.

5.2. Test Results and Their Analysis

Comparison between the actual trajectory with the kinematic simulation trajectory of the end movement of the seedling claw during the seedling-picking operation, as depicted in Figure 11, reveals that the actual trajectory exhibits a non-smooth curve with minor fluctuations. These fluctuations are primarily attributed to the overall vibration of the device, in addition to certain errors in the post-processing of the captured points, which result in discrepancies when compared to the simulated trajectories. Despite these discrepancies, the overall consistency between the trajectory of seedling extraction and the simulated trajectory as well as the state of the clip seedlings in Figure 12, supports the validity of the seedling extraction process design and demonstrates the practical feasibility of the seedling extraction device.

Initially, the seedling pickup speed of the delivery mechanism was set at 60 plants/min, 72 plants/min, and 84 plants/min, corresponding to a complete row pickup cycle time of 6 s/row, 5 s/row, and 4.3 s/row, respectively. Three trays of pepper seedlings were selected to establish the order for the pickup and casting tests. The evaluation metrics included the success rate of seedling pickup, the success rate of seedling casting, the substrate crushing rate, the overall success rate, and the seedling injury rate. The criterion for evaluating damaged seedlings is based on the stem compression characteristics test. By designing the compression displacement to be less than 50% of the stem diameter, it can be ensured that the stems of pepper plug seedlings experience only elastic deformation. This approach minimizes damage to the epidermal cells of the stems and does not adversely affect the subsequent growth of the seedlings [27].

Take-and-drop trials were conducted in accordance with the test protocol, with a total of nine sets of trials, recording the trial data, and organizing and analyzing them. The success rates of seedling taking, seedling casting, substrate fragmentation, and overall success were calculated individually. Subsequently, the mean value for each index was determined. The test results are presented in Figure 13 and Figure 14 below.

6. Conclusions

During the prototype trial production, we concentrated on the connection of non-standard parts for processing, the assembly of the entire test stand, and debugging, which ultimately enabled us to conduct the trajectory test and assess seedling performance. The trajectory test employed a high-speed camera to capture the endpoint of the seedling claw’s trajectory, allowing us to verify that the actual trajectory of the seedling was largely consistent with both the theoretical and simulated trajectories, thus confirming the validity of the theoretical design. We established three different seedling extraction rates of 60 plants/min, 72 plants/min, and 84 plants/min for the seedling extraction test, conducting nine groups of tests to record data based on evaluation indices such as the success rate of seedling extraction, the success rate of seedling insertion, the rate of substrate fragmentation, and the overall success rate. The test results showed that at a seedling extraction rate of 72 plants/min, the success rate of seedling extraction was 94.90%, the success rate of seedling insertion was 98.53%, the injury rate of seedlings was 1.95%, and the comprehensive success rate was 91.69%.

There was a small difference between 60 plants /min and 72 plants/min; however, when the rate was further increased, there was a big difference in the effect of taking seedlings. When the picking rate was 84 plants /min, the success rate of picking was 89.35%, the success rate of seeding was 91.20%, the injury rate was 5.18%, and the overall success rate was 77.26%. This is mainly because when the seedling-taking rate increases, the time required for the seedling single cycle becomes shorter, and the operation speed of each motor is accelerated, resulting in more jitter and impact on the whole machine, which will lead to an increase in the frequency of taking seedlings and taking seedlings during the process of taking seedlings; however, when the speed is 60~72 plants/min, speeding up the rate of taking seedlings has little effect on the overall effect of taking seedlings.

The test indicated that the primary cause for the unsuccessful retrieval of seedlings was the insufficient size of the opening and closing mechanisms of the seedling claws, along with considerable deviation in the stems of some seedlings. Furthermore, the rapid movement of the seedlings highlighted structural issues related to the structural design of the seedling removal device, which contains numerous moving parts that require high dimensional processing and installation accuracy. Moreover, the coordination between the seedling feeding device, the seedling lifting device, and the seedling-taking and throwing device is critically demanding. During the debugging process, some problems with the overall device emerged. To address issues such as jitter and jamming, it will be necessary to optimize the structural parameters of the entire device in the future, enhance the tightness of the device connections, and reduce friction resistance among the kinematic pairs to achieve improved operational performance.

This study highlights specific advantages of the developed transplanting mechanism over existing technologies. Unlike traditional transplanting machinery, our design incorporates a lifting mechanism combined with a hybrid pneumatic–electric system, which significantly reduces damage to seedlings and substrates. Enhancements to the lifting mechanism help maintain the structural integrity of the substrate, which is crucial for ensuring the healthy growth of seedlings. Furthermore, the integration of pneumatic and electric systems optimizes force distribution, thereby minimizing potential damage during mechanical operation. Firstly, the design of this mechanism enhances the precision of seedling extraction, directly impacting planting efficiency and the quality of plant growth. Secondly, the innovative mechanical design not only enhances operational reliability but also reduces the skill requirements for operators, a critical consideration for commercial applications.