Design and Testing of Miniaturized Electrically Driven Plug Seedling Transplanter

Abstract

To address the issues of bulky structure and complex transmission systems in current transplanters, a compact, electric-driven automatic transplanter was designed. Using pepper plug seedlings as the test subject, this study investigated plug tray dimensions and planting patterns. According to the design requirement that the width of the single-row transplanter must be less than 62.5 cm, a three-dimensional transplanter model was constructed. The transplanter comprises a coaxially installed dual-layer seedling conveying device and a sector-expanding automatic seedling picking and depositing device. The structural dimensions, drive configurations, and driving forces of the transplanter were also determined. Finally, the circuit and pneumatic system were designed, and the transplanter was assembled. Both bench and field tests were conducted to select the optimal working parameters. The test results demonstrated that the seedling picking and depositing mechanism met the required operational efficiency. In static seedling picking and depositing tests, at three transplanting speeds of 120 plants/min, 160 plants/min, and 200 plants/min, the success rates of seedling picking and depositing were 100%, 100%, and 97.5%, respectively. In the field test, at three transplanting speeds of 80 plants/min, 100 plants/min, and 120 plants/min, the transplanting success rates were 94.17%, 90.83%, and 88.33%, respectively. These results illustrate that the compact, electric-driven seedling conveying and picking and depositing devices meet the operational demands of automatic transplanting, providing a reference for the miniaturization and electrification of transplanters.

1. Introduction

Vegetable cultivation techniques can be divided into two primary systems: direct sowing and transplanting. The transplanting technique involves pre-cultivating seedlings prior to field planting, effectively reducing low-temperature stress during the seedling stage and offering some advantages, such as shortened growth cycles [1,2,3]. Standardized plug tray seedling production, as a key component of intensive cultivation practices, enhances both yield and quality in crops like peppers [4,5].

Transplanting working modes can be categorized into two implementation approaches: manual and mechanical. Manual transplanting faces challenges such as high labor intensity and poor operational outcomes, with inconsistent plant spacing and depth negatively impacting crop growth [6,7,8]. In contrast, mechanical transplanting significantly enhances both operational efficiency and quality, laying a foundational basis for mechanized field management in later crop stages [9]. Existing transplanters are classified into two types: semi-automatic and automatic. While semi-automatic models reduce labor intensity, they fail to minimize human resource input, and the operating speed is relatively low and tends to decrease as the operation continues. Fully automatic transplanters, by integrating automated seedling picking and depositing mechanisms, achieve a transformative leap in production efficiency, representing the primary developmental trend for modern vegetable transplanting machinery [10,11].

With the increasing promotion of fully automatic transplanters, their adoption rate has risen annually and become the primary method for pepper cultivation now [5]. The primary difference between fully automatic transplanters and semi-automatic ones is whether they are equipped with an automated seedling picking and depositing device [12]. Currently, the automated seedling picking and depositing mechanisms are predominantly based on single-seedling picking methods, as exemplified by the vegetable transplanter developed by Yu Gaohong et al. [13], which includes a rotary seedling pickup unit. This unit utilizes gear-driven motion to guide the seedling claw along a predefined trajectory, enabling single-seedling automated picking and depositing. This picking method typically requires coordination with an electric tray-feeding device to control tray movement, thereby achieving ordered single-seedling retrieval. Meanwhile, some automated seedling picking and depositing mechanisms adopt full-row seedling picking methods, such as the system designed by Han Changjie [14], which deposits eight seedlings per operational cycle and delivers them sequentially to the planting unit via a seedling conveyor mechanism. This approach provides higher operational efficiency.

Due to the plant dimensions and agronomic characteristics of pepper, the pepper transplanter is mainly of the hanging-cup type. Currently, a mainstream model is the automatic pepper transplanter developed by Han Changjie and others from Xinjiang Agricultural University [15]. The entire machine employs mechanical transmission and cylinder-driven mechanisms, featuring a high degree of automation with a planting speed of up to 123 plants/min. The 2ZB-2J fully automatic high-speed transplanter, developed by Bazhou Liangjia Agricultural Machinery Manufacturing Co., Ltd. in Xinjiang Uygur Autonomous Region, China [16], operates in a fully automatic mode. This model integrates an automatic seedling supply system and is equipped with a full-row automatic seedling picking and depositing mechanism. It can pick eight seedlings at a single time and intermittently distribute the seedlings to the planter through a seedling conveying mechanism. The entire system uses mechanical transmission, and its single-row planting efficiency can reach up to 130 plants/min.

The HD144 transplanter, designed by Transplant System Pty. Ltd. in Auckland, Australia [17], utilizes a plug-clawing seedling retrieval mechanism driven by a pneumatic cylinder to achieve automated seedling picking and depositing into the planting unit, thereby completing fully automatic transplanting operations. YANMAR Corporation’s PF2R fully automatic vegetable transplanter, in Osaka, Japan [18] uses a needle-type seedling pickup claw and a chute-type seedling retrieval mechanism, integrating automated seedling picking and depositing. This model achieves a transplanting speed of up to 120 plants/min, with operational efficiency twice that of conventional semi-automatic transplanters. However, most of the above-mentioned transplanters are two-row machines with large structural dimensions. Additionally, since they entirely utilize mechanical transmission, there are significant transmission clearances, and frequent adjustments are needed for the coordination of various mechanisms during operation.

As for the miniaturized transplanter developed by Md Nafiul Islam et al. [19] and the transplanter designed by Qizhi Yang et al. [20], although they have achieved miniaturization, they have compromised on the efficiency of the transplanter, with a transplanting speed of only 60 plants/min.

In general, current transplanters are predominantly large, two-row machines and use mechanical transmission systems. These systems are driven by land wheels, which drive the seedling conveying mechanism and the seedling picking and depositing mechanism. However, such transmission systems are characterized by complex structures, significant transmission clearances, and low planting success rates. Additionally, the excessive loading on land wheels often causes slippage, negatively impacting planting performance [21,22]. Therefore, designing a miniaturized, electrically driven seedling conveying mechanism and a full-row, high-efficiency seedling picking and depositing mechanism is crucial for improving the operational efficiency and precision of transplanters and for enabling multi-row configurations.

2. Materials and Methods

2.1. Plug Tray Specifications and Planting Pattern

In Xinjiang, pepper planting primarily relies on plug tray seedling cultivation and mechanized transplanting. The plug tray seedlings are cultivated in greenhouses, where pepper seeds are sown into plug trays of corresponding specifications for nurturing. The most widely adopted type is the 128-cell tray (8 rows × 16 columns) [23]. As shown in Figure 1, the length of the plug tray L_p is 535 mm, the width of the plug tray T_p is 280 mm, the height of the plug tray H_p is 45 mm, the center distance between horizontal adjacent plugs W_c is 31.75 mm, and the center distance between vertical adjacent plugs L_c is 31.75 mm.

During the early growth stage of peppers, plastic mulching film is applied to increase soil temperature and promote seedling growth [24]. The common planting pattern in the Xinjiang Uygur Autonomous Region is shown in Figure 2. The row spacing R_s is 60 cm, the plant spacing S is 16 cm, the film width W_p is 100 cm, the two drip irrigation tape widths W_d is 40 cm, and the ridge width W_r is 125 cm.

Therefore, to meet the miniaturization design requirements for multi-row arrangement, the overall mechanism dimensions must meet the following requirements: the mechanism width (L) ≤ 62.5 cm, and the seedling claw width (L_c) ≤ 3.175 cm.

2.2. Comparison of the Seedling Conveying Mechanism

The seedling conveying mechanism is designed with two groups of seedling components; a group of seedlings dropping off at the same time does not affect the other group of seedlings receiving, and each group of seedling components is installed with 8 seedling cups. Therefore, the seedling conveying mechanism can be arranged in a straight line, as shown in Figure 3a. The seedling cups are installed on the chain or synchronous belt, each side of the 8 seedling cups, and each group of seedling components is driven by one side of the sprocket or synchronous belt wheel alone.

Another way is circular arrangement, as shown in Figure 3b; each group of eight seedling cups is installed on a fan-shaped plate, each fan-shaped plate angle θ is 120°, and the two groups of fan-shaped seedling-splitting assemblies are coaxially installed and independently driven. Both seedling cup arrangements can realize the function of the seedling conveying mechanism; however, larger seedling cup sizes generally result in better seedling reception. The scheme of the seedling conveying mechanism is determined by calculating the width of the structure L under the two arrangements with the same seedling cup size.

The width of the mechanism L₁ is, for the linear arrangement, obtained as follows:

$$L_1=10l+2r_1$$

(1)

The structure of the chain drive, for example, to the commonly used minimum roller chain 08A series has a minimum number of teeth recommended value of 17 teeth (based on ANSI standards) [25]. Two times the radius of the sprocket transmission parts r₁ is approximately equal to the sprocket tooth top diameter, commonly used simplified formula for the diameter of the chain sprocket tooth apex circle:

$$D_a=D+0.25p$$

(2)

Sprocket pitch circle diameter calculation formula:

$$D={\displaystyle \frac{p}{\sin \left(\frac{\pi }{z}\right)}}$$

(3)

From Equations (1)–(3):

$$L_1=10l+{\displaystyle \frac{p}{\sin \left(\frac{\pi }{z}\right)}}+0.25p$$

(4)

where l is the width of the seedling cup, mm; p is the sprocket pitch, 08A chain pitch p is 12.7 mm, and z is the minimum number of teeth, taken as 17.

The width L₂ of the mechanism in circular arrangement is as follows:

$$L_2=2l_{OC}=2(l_{OB}+l_{BC})$$

(5)

$$l_{OB}={\displaystyle \frac{l_{AB}}{\tan \left(\frac{\alpha }{2}\right)}}={\displaystyle \frac{l}{2\tan \left(\frac{\alpha }{2}\right)}}$$

(6)

The designed seedling cup is square in the cross-section:

$$L_2=2\left({\displaystyle \frac{l}{2\tan \left(\frac{\alpha }{2}\right)}}+l\right)$$

(7)

where α is ∠AOB, because 8 seedling cups are installed in a circular array, and if the 2 fan-shaped plate angle θ is 120°, then α is 15°.

Take the seedling cup width l = 60 mm and substitute it into Formulae (4) and (7). When the seedling conveying mechanism is in a linear arrangement, the mechanism width L₁ is about 672.32 mm. When a circular arrangement is used, the mechanism width L₂ is approximately 581.54 mm. With the same seedling cup size, the circular arrangement of seedling cups results in a smaller overall width of the seedling conveying mechanism, which is more suitable for miniaturized designs.

Taking the installation position of the seedling cup width l as being 60 mm as an example, the plate radius r₂ is as follows:

$$r_2={\displaystyle \frac{l}{2\sin \left(\frac{\alpha }{2}\right)}}$$

(8)

The calculated value of r₂ is approximately 231 mm. Given that the width of the seedling conveying mechanism in this state is 581.54 mm, which is approximately 43.46 mm short of the required design width of 625 mm, the seedling cups are designed in the shape depicted in Figure 9. The lower section of the seedling cup has a cross-sectional dimension of 60 × 60 mm, while the top section extends bilaterally to form a flared opening. The cup mouth dimensions are 60 × 100 mm, resulting in an overall width increase of 40 mm. This modification still meets the design requirements. Additionally, the enlarged cup mouth dimensions enhance the success rate of seedling reception.

2.3. Transplanter Structure and Working Principle

The overall structure of the transplanter is shown in Figure 4. The transplanter consists of ground wheel mechanisms, a duckbill-type planter, a planting detection switch, a seedling conveying mechanism, a pneumatic tray-feeding mechanism, and a seedling picking and depositing mechanism. The planter is driven by the ground wheel via a chain, and the plant spacing is maintained at 16 cm by adjusting the transmission ratio. The seedling conveying mechanism, seedling picking and depositing mechanism, and tray-feeding mechanism are driven by a motor and cylinder. The transplanter operates in fully automatic mode. The function of the seedling picking and depositing devices is to pick seedlings from the tray-feeding mechanism and to deposit them into the seedling cups of the conveying mechanism. The function of the seedling conveying mechanism is to transfer the plug seedlings in the seedling cups, which then transplants them into soil. The synchronization between the seedling conveying mechanism and the planter is achieved through the planting detection switch.

The core mechanisms of the transplanter are the seedling conveying device and the seedling picking and depositing device, as illustrated in Figure 5. These two devices are connected via a frame to form an integrated unit, which enables convenient installation and use in multiple conditions. The seedling conveying device rotates clockwise, which is indicated by arrow V in Figure 5. The drop-off position for seedlings in the conveying mechanism is located at the opening in the base plate directly above the seedling guide component. The receiving position for seedlings in the conveying mechanism is on the side of the picking and depositing mechanism where seedlings are picked.

2.3.1. Seedling Conveying Mechanism

The seedling conveying mechanism is illustrated in Figure 6. Which consists of seedling cups, a fixed circular plate for seedling cup A (referred to as Plate A), a fixed circular plate for seedling cup B (referred to as Plate B), a seedling drop-off detection switch, a seedling receiving detection switch, and two stepper motors. Each stepper motor independently drives a circular plate on which eight seedling cups are installed. A triggering block is installed on the first seedling cup of each circular plate to activate the seedling receiving detection switch and the seedling drop-off detection switch. Two AB plate contact switches are installed on Plate A to detect the contact status of the seedling cups on the circular plate.

During operation, when Plate A rotates to the seedling-receiving position (i.e., when the triggering block on the seedling cup of Plate A contacts the seedling receiving switch), the seedling picking and depositing mechanism operates once to complete the action of picking seedlings from the plug tray and depositing them into eight seedling cups. After this, Plate A continues rotating until it activates the seedling drop-off detection switch, reaching the seedling drop-off position (it rotates one additional seedling cup distance, at which time the seedling cup lid opens, and the plug seedling falls from the cup into the duckbill of the planter). Plate A then waits for the duckbill-type planter to trigger the planting detection switch. Each time the planting detection switch is triggered, Motor A rotates the fixed circular plate for seedling cup A (Plate A) by the distance of one seedling cup, achieving orderly and spaced seedling drop-off.

While Plate A is rotating to the seedling drop-off position, Plate B simultaneously deposits seedlings. After depositing seedlings, Motor B continues rotating until Plate B contacts Plate A, at which point the AB plate contact switch is triggered, and Plate B stops moving. Once the AB plate contact switch disengages, Plate B resumes rotation until it triggers the seedling drop-off detection switch (reaching the drop-off position). At this point, Plate B drops seedlings in an orderly way under the triggering of the planting detection switch, while Plate A rapidly moves to the seedling-receiving position to receive new seedlings. This cyclic process continues.

The seedling conveying mechanism is driven by a closed-loop stepper motor, ensuring precise receiving and drop-off of seedlings. While one set of seedling cups remains stationary to receive seedlings, the other set performs intermittent seedling drop-off operations. During seedling reception, the seedling cups remain stationary, which significantly improves the success rate of seedling reception and reduces plug soil loss. Seedling drop-off is triggered by sensors, ensuring accurate depositing of seedlings into the planter. Since the seedling conveying mechanism is motor-driven and the ground wheel only drives the planter, the forces on the ground wheel are reduced. During field operations, this design reduces ground wheel slippage and enhances the stability of plant spacing.

2.3.2. Seedling Picking and Depositing Mechanism

The seedling picking and depositing mechanism, as shown in Figure 7, comprises a transverse cylinder, a slide-table cylinder, a lifting cylinder, a longitudinal cylinder, a lifting guide rail and slider module, a longitudinal guide rail and slider module, and a transverse light rod linear bearing module. Two lifting cylinders drive the vertical motion of the entire machine, while the longitudinal extension and retraction of the longitudinal cylinder drive the longitudinal movement of the seedling claws and other components. The transverse expansion cylinder and its associated transverse expansion hinges are designed to spread the claws at specific intervals. The transverse expansion cylinder is internally installed, ensuring that the overall width of the mechanism remains unchanged during cylinder extension. Finally, the slide table cylinder extends by a specified distance, spreading the eight seedling claws into a circular arc arrangement.

The operational process of the seedling picking and depositing mechanism, as shown in Figure 8, is as follows: During operation, the longitudinal cylinder of the mechanism extends, both the transverse cylinder and the slide table cylinder are retracted, and the lifting cylinder extends, driving the entire row of 8 seedling claws to extend upward and reach the position above the tray-feeding mechanism for seedling picking. Upon reaching the seedling-picking position, the seedling claws close. After a set delay to ensure complete clawing, the lifting cylinder retracts, lifting the claws vertically to detach the plug seedlings from the plug trays. Subsequently, the longitudinal cylinder extends, the transverse cylinder remains retracted, the slide table cylinder extends, and the tray-feeding cylinder (controlling the movement of seedlings in an entire row of the plug tray) adjusts, driving the 8 seedling claws to spread transversely, expand longitudinally into a circular arc arrangement, and move longitudinally to a position directly above the seedling cups of the conveying mechanism. Finally, the mechanism waits for the seedling cups of the conveying mechanism to reach the seedling-receiving position. At this point, the seedling cups trigger the seedling receiving detection switch, the seedling claws open, and the process of transferring seedlings from the plug trays to the seedling cups is completed, marking the end of one picking and depositing cycle. After the plug seedlings are fully deposited into the seedling cups, the seedling picking and depositing mechanism continues its operational sequence, awaiting the next trigger from the seedling receiving detection switch to initiate the next cycle. Thus, the efficiency of the seedling picking and depositing mechanism directly impacts the overall transplanting efficiency.

The seedling picking and depositing mechanism uses linear modules such as guide rails and slider assemblies, ensuring that all movements of the mechanism are linear. In addition, the pneumatic cylinders respond rapidly, resulting in high repeatability accuracy and operational stability.

2.3.3. Seedling Cup

The seedling cup consists of a cup body, a cup lid, a torsion spring, and a mounting bolt, as shown in Figure 9. The material of the seedling cup is polycarbonate (PC) plastic, and each cup weighs 215 g, achieving a balance between strength and lightweight. The lid remains open under spring force. When the cup passes through the opening in the base plate, the cup lid opens instantaneously. This design prevents delays in lid opening and vibration of the cup lid that could otherwise result in failed seedling falls.

2.4. Main Mechanism and Parameter Design

2.4.1. Design of the Seedling Picking and Depositing Mechanism

Since the force exerted on the cylinder during the seedling picking and depositing process is relatively small, only the stroke was considered when selecting the cylinder, and commonly used models were adopted. The seedling picking and depositing mechanism is a key device in the transplanter. When the lifting cylinder rises to pick seedlings from the plug tray, its movement distance should exceed the length of H_p shown in Figure 1, it should be greater than 45 mm. Consequently, the TN20 × 50 lifting cylinder model was chosen to meet the design requirements. During the extension of the longitudinal cylinder, it must drive the entire seedling claws to move above the pneumatic tray-feeding mechanism. During retraction, the seedling claws should be positioned above the seedling cups for seedling deposition. The movement distance of the longitudinal cylinder is influenced by the relative installation positions of the pneumatic tray-feeding mechanism and the seedling conveying mechanism. Through measurement, the movement distance of the longitudinal cylinder is determined to be 175 mm, and the selected model is MAL25 × 175. Additionally, the position of the longitudinal cylinder can be adjusted using a clawing device. Therefore, as long as the cylinder stroke exceeds the required length, it meets the operational requirements.

During the retraction of both the transverse expansion cylinder and the slide-table cylinder in the seedling picking and depositing mechanism, the seedling claws remain closed for picking. During extension, the claws expand laterally and arcuately to deposit the seedlings, and in this state, the seedling claws are positioned above the arcuately arranged seedling cups. The transverse and longitudinal movements of the eight seedling claws differ from each other. Since the eight seedling cups are arranged in a circular array, with four claws on each side, the movement distances are symmetrical. By calculating the central coordinates of the eight seedling cups when they are in the seedling depositing position, the required movement displacements for each seedling claw can be determined.

The coordinate calculations for the central points of each seedling cup at the seedling-receiving position are illustrated in Figure 10. Points A, B, C, and D are the central points of four seedling cups, with their corresponding seedling claw central points denoted as a, b, c, and d. During seedling picking, the seedling claws remain closed, with four claws equally spaced. The distance between every two seedling claws equals the center-to-center spacing (W_c) of the plug tray, which is 31.75 mm. The selected seedling claws have a width (W_h) of 30 mm. By controlling the width of the claw mounting base to be 31.75 mm, equidistant seedling picking from the plug tray can be achieved. During seedling depositing, the central points of the seedling claws are vertically aligned with the central points of the seedling cups in the seedling conveying mechanism. By calculating the values of x_x and y_x, the theoretical transverse and arcuate displacements required for the expansion of the claws can be determined.

As shown in Figure 10, A′ is the intersection point with the x-axis by making a plumb line in the direction of the x-axis from point A. α₁ is the angle ∠OAA′ between the line OA connecting the seedling cup A with the center of rotation of the seedling cup fixing disc and the line AA′.

During seedling picking, the seedling cups are arranged in a circumferential array and are symmetrically aligned along the y-axis. Since O′O is perpendicular to the x-axis, it is parallel to AA′, and the angle α₁ = ∠O′OA. Given that θ is 120° and each seedling cup occupies an angle α = 15° on the seedling cup fixing disc, the angle ∠O′OA can be approximated as ${\displaystyle \frac{\alpha }{2}}$.

Likewise, the angle can also be approximated as follows:

$${\alpha }_1=\angle O^{\prime }OB={\displaystyle \frac{3\alpha }{2}}$$

(9)

So

$${\alpha }_x={\displaystyle \frac{\alpha }{2}}(2n+1),n=0,1,2,3$$

(10)

The OD length can be obtained from Equation (6):

$$\left\{\begin{array}{c}{x}_x=\sin {\alpha }_x\left[{\displaystyle \frac{l}{2\tan \left(\frac{\alpha }{2}\right)}}+{\displaystyle \frac{l}{2}}\right]\\ y_x=\cos {\alpha }_x\left[{\displaystyle \frac{l}{2\tan \left(\frac{\alpha }{2}\right)}}+{\displaystyle \frac{l}{2}}\right]\end{array}\right.$$

(11)

Since the extension line of OO′ passes through O″ when the seedling claws are closed, and from Figure 10b we can obtain the following:

$${x_x}^{\prime }={\displaystyle \frac{W_c}{2}}(2n+1),n=0,1,2,3$$

(12)

Therefore, the theoretical displacements Δx and Δy required for transverse and arcuate unfolding of the seedling picking and depositing mechanism are taken using point O as the reference point:

$$\left\{\begin{array}{l}\Delta x=x_x-{x_x}^{\prime }\hfill \\ \Delta y=y_x\hfill \end{array}\right.$$

(13)

Through calculations, the theoretical motion displacements of the four seedling claws are approximately as follows: claw a moves 18 mm transversely and 258 mm longitudinally; claw b moves 51 mm transversely and 240 mm longitudinally; claw c moves 80 mm transversely and 206 mm longitudinally; and claw d moves 95 mm transversely and 159 mm longitudinally. Therefore, a motion displacement of 100 mm is selected for the transverse expansion range of the cylinder, and the chosen model is MAL25 × 100. The seedling claws can be expanded at specific intervals through the transverse expansion hinge. The size of the hinge can be obtained by calculating the relative displacement of each seedling claw, and the calculation method is shown in Figure 10c.

The arcuate expansion displacement of the seedling claws is calculated using different seedling cups as reference points. When point C of the seedling cup is chosen as the reference point, the displacements required for the arcuate expansion of each claw are minimized, as shown in

Table 1. The arcuate expansion displacement of the seedling claws.

Seedling Claw	Arcuate Expansion Displacement
a	52 mm
b	34 mm
c	0 mm
d	−47 mm

, and the maximum longitudinal displacement is 52 mm. The slide-table cylinder selected for the arcuate expansion is model HLQ6 × 50S, with a width of 30 mm. The cylinder’s movement length is 50 mm, which differs by only 2 mm from the maximum longitudinal displacement, having a negligible impact on the seedling depositing. By adjusting the extension direction of the slide-table cylinder and adding a movement-limiting component, the arcuate expansion process can be achieved.

2.4.2. Design of Driving Force for the Seedling Conveying Mechanism

(1)

Force Analysis of Seedling Cup

The seedling conveying mechanism comprises two plates for seedling cups, as illustrated in Figure 6 and Figure 9. During the process of receiving seedlings and moving to the seedling drop-off position, the lids of the seedling cups remain closed, with the lids in contact with the base plates under the action of torsion springs. Upon reaching the seedling drop-off position, the lids open by 90° under the action of the torsion springs, completing the seedling fall action. The relationship between the torque and the torsion angle of the seedling cup lids is calculated using the following formula [26]:

$$T_N={\displaystyle \frac{EI\theta }{L}}$$

(14)

where T_N is the torsion spring torque, N·mm; E is the material modulus of elasticity, MPa; I is the moment of inertia of the spring wire cross-section, mm⁴; θ is the torsion angle, rad; and L is the torsion spring effective length, mm.

When the moment of inertia of the spring section is given, the effective length of the torsion spring is calculated as follows:

$$\left\{\begin{array}{l}I={\displaystyle \frac{\pi d^4}{64}}\hfill \\ L=\pi DN\hfill \\ {\theta }_{\mathrm{rad}}={\displaystyle \frac{\pi {\theta }_{\mathrm{deg}}}{180}}\hfill \end{array}\right.$$

(15)

where d is the spring wire diameter, mm; θ_rad is the torsion arc, rad; θ_deg is the torsion angle, °; D is the spring center diameter, mm; and N is the torsion spring’s effective number of turns.

From Equations (14) and (15), the following can be obtained:

$$T_N={\displaystyle \frac{E{d}^4\theta }{64DN}}$$

(16)

The spring torsion spring material is 65Mn steel, its modulus of elasticity is about 2 × 10⁵ MPa [27], the spring wire diameter is 0.8 mm, the torsion angle is 90°, the effective number of coils is 8 coils, and the spring outer diameter is 6.50 mm.

The force exerted by the torsion spring at the bottom of the seedling cup, F_s, is as follows:

$$F_s={\displaystyle \frac{T_N}{L}}$$

(17)

where the L moment of force is 60 mm.

Thus, the force of friction between the seedling cup and the base plate can be calculated as follows:

$$F_f=\mu F_N$$

(18)

where F_f is the force of friction, N; μ is the coefficient of rolling friction; and F_N is the positive pressure of the seedling cup on the base plate, N.

The force exerted by the torsion spring at the bottom of seedling cup F_s can be obtained from Formulae (16) and (17), which is 0.735 N. The seedling cup is fixed in the circular plate, and when the seedling cup lid is completely closed, a gap remains between the lid and the base plate, so the forms of pressure exerted by the seedling cup on the base plate are as follows: the pressure exerted by torsion springs on the base plate, the seedling cup lid gravity, and the pressure exerted by the weight of the seedling. Due to the small weight of the seedlings and seedling cup lids, they are negligible. The seedling cup lid material is polycarbonate (PC) plastic, the base plate material is Q235 ordinary carbon steel, and the coefficient of rolling friction between the lid and the base plate was taken as 0.3 [28]. According to Equation (18), the force of friction between a single seedling cup lid and the base plate can be obtained as 0.22 N.

(2)

Driving Torque for Seedling Conveying Mechanism

Each set of fixed circular plates for seedling cups in the seedling conveying mechanism requires independent driving by a stepper motor. Therefore, it is necessary to calculate the torque required for driving the circular plates. The forces acting on the plate are illustrated in Figure 11.

The moment of inertia of the plate is calculated as follows [29]:

$$J={\displaystyle \frac{M{R}^2}{2}}$$

(19)

where J is the moment of inertia of the plate, kg·m²; M is the mass of the plate and the load, kg; and R is the radius of the plate after installing the seedling cups, m.

The torques generated by the angular acceleration and the force of friction acting on the edge of the plate are, respectively, as follows:

$$\left\{\begin{array}{l}{\tau }_1=J\alpha \hfill \\ {\tau }_2=F_{f}R\hfill \end{array}\right.$$

(20)

where τ₁ is the angular acceleration moment, N·m; τ₂ is the friction moment, N·m; and α is the angular acceleration, rad/s².

The total torque needs to overcome the force of friction and provide angular acceleration at the same time [30]; therefore,

$$W={\tau }_1+{\tau }_2$$

(21)

After measurement, the mass of a single seedling cup is 215 g, while the combined mass of a single circular plate and its central shaft is approximately 1900 g, resulting in a total mass M of approximately 3620 g. The planting speed is more than 120 plants/min·row⁻¹, with a drop-off time of 0.5 s per seedling cup. Consequently, it takes 4 s for all eight seedling cups to complete the release of seedlings. During this period, the other set of seedling cups must rotate by 120° to reach the seedling-receiving position and then continue to rotate by 120° to reach the seedling drop-off position. Taking the seedling-receiving time as 2 s as an example, the rotational speed of the fixed circular plates for seedling cups must rotate 240° in 2 s, which is equivalent to 20 rad/min. The acceleration time of the PLC-controlled motor is set to 0.1 s [31], resulting in an angular acceleration of the circular plates of 3.33 rad/s².

According to the above Equations (19)–(21), the torque required to drive the seedling cup fixing plate is 0.86 N·m. Therefore, the torque of the stepper motor must be greater than this value.

2.5. Control System Design

2.5.1. Control System and Workflow

The machine control system uses a Siemens PLC, the S7-200smart (ST40) model (Siemens AG, Munich, Germany) [32]. The PLC unit integrates I/O (including 24 inputs/16 outputs) and supports the expansion of up to 6 additional modules (such as signal boards and communication modules). The output terminal is capable of generating high-frequency pulses, allowing for precise control of two stepper motors. Moreover, this particular PLC model offers high cost-effectiveness and stability. The input and output signals are shown in

Table 2. PLC I/O allocation table.

Input	Details	Output	Details
I0.0	Seedling receiving detection switch SQ1	Q1.1	Electromagnetic relay KA1 (seedling claws closed)
I0.1	Seedling drop-off detection switch SQ2	Q1.2	Electromagnetic relay KA2 (seedling claws open)
I0.2	Planting detection switch SQ3	Q0.3	Electromagnetic relay KA3 (longitudinal cylinders and tray-feeder cylinders extension, slide-table cylinders, and transverse cylinder contraction)
I0.3	AB plate contact switch SQ4	Q0.4	Electromagnetic relay KA4 (longitudinal cylinders and tray feed cylinders contraction, slide-table cylinders, and transverse cylinder extension)
I0.4	BA plate contact switch SQ5	Q0.5	Electromagnetic relay KA5 (lift cylinder extension)
I0.5	Start button SB1	Q0.6	Electromagnetic relay KA6 (lift cylinder retraction)
		Q0.0	Drive A-PUL+ (Motor A-PWM control)
		Q0.2	Drive A-DIR+ (Motor A-PWM control)
		Q0.1	Drive B-PUL+ (Motor B-PWM control)
		Q0.7	Drive B-DIR+ (Motor B-PWM control)

. The inputs include buttons, a seedling receiving detection switch, a seedling drop-off detection switch, an AB plate contact switch, and a planting detection switch. The outputs consist of relays (for controlling solenoid valves) and a stepper motor driver.

The control diagram for the machine is shown in Figure 12. The SBR_0 subroutine corresponds to one operational cycle of the seedling picking and depositing mechanism. The control process can be summarized as follows: When pressing the button, Motor A and Motor B begin to rotate. When Plate A rotates to the seedling drop-off position, it halts and awaits triggering by the planting detection switch. Each time the planting detection switch is triggered, Motor A drives Plate A to rotate by 15°, resulting in the dropping of one seedling at intervals. During this period, Plate B rotates to the seedling-receiving position and stops. Once a single seedling picking and depositing operation is completed, Motor B continues to rotate. When the two plates come into contact (i.e., when the BA detection switch is triggered), Motor B halts. The BA plates separate, and Plate B continues to rotate, catching up with Plate A to await the next seedling drop-off. When Plate B reaches the seedling drop-off position, it halts and awaits triggering by the planting detection switch. Each time the planting detection switch is triggered, Motor B drives Plate B to rotate by 15°, resulting in the dropping of one seedling at intervals. During this period, Plate A rotates to the seedling-receiving position and stops. Once a single seedling picking and depositing operation is completed, Motor A continues to rotate. When the two plates come into contact (i.e., when the AB detection switch is triggered), Motor A halts. The AB plates separate, and Plate A continues to rotate, catching up with Plate B to await the next seedling drop-off. This cycle repeats continuously.

In the overall process, except for the seedling picking and depositing mechanism, where cylinder actions are controlled through time delays, all other steps are executed by triggering contact switches to initiate the next operation. This design ensures high overall stability, and all mechanisms can be manually reset in case of a power outage.

2.5.2. Gas Circuit Design

Since the seedling picking and depositing mechanism was driven by cylinders and controlled by solenoid valves, the gas circuit was designed, which is shown in Figure 13.

The seedling picking and depositing mechanism consists of six types of cylinders, with a total of 20 cylinders controlled by four three-position five-way solenoid valves. Some cylinders work in synchronization, controlled by a single solenoid valve. The working principle of the cylinders is depicted in Figure 8. Each branch of the pneumatic circuit is equipped with a one-way throttle valve to adjust the air pressure, thereby controlling the response speed of the cylinders. Due to the differing response speeds of cylinders B and C and the varying air pressures in their respective branches, two separate solenoid valves are required for their control. The solenoid valves DT2 and DT3 are synchronously controlled by two identical electromagnetic relays, KA3 and KA4.

The speed of seedling picking and depositing can be controlled by adjusting the response time of the air cylinders, and the cylinder operation timing diagram is shown in Figure 14. Taking a seedling picking and depositing time of 3 s as an example, when eight seedling claws operate simultaneously to pick up seedlings, the speed of seedling picking and depositing reaches 160 plants/min. By modifying the response time of the seedling picking and depositing cylinders in the program, seedling picking and depositing operations can be performed at different speeds.

2.6. Test Design

2.6.1. Bench Tests Methods

To assess the performance of the transplanting machine, the entire machine was assembled, and a bench test was conducted at Bazhou Liangjia Agricultural Machinery Manufacturing Co., Ltd. on 18 April 2025. The test used pepper plug seedlings that were grown in 128 plug trays, aged 60 days, of the variety “Honglong 23”. The seedling tray substrate is prepared using a standard mixture of peat moss, perlite, and vermiculite in a 2:1:1 ratio. The average height of the plug-cultivated seedlings was approximately 20 cm.

(1)

Static Seedling Picking and Depositing Test

Based on the calculated seedling falling time and cylinder actuation time, each seedling pickup and placement cycle can be completed within 2.5 s. To evaluate the performance of the seedling picking and depositing mechanism, static seedling picking and depositing tests were conducted. The tests controlled 8 claws to simultaneously pick and deposit entire rows of seedlings, completing the picking and depositing actions in 4 s, 3 s, and 2.4 s, respectively. Since the entire row is picked and deposited simultaneously, the corresponding seedling picking and depositing speeds are 120 plants/min, 160 plants/min, and 200 plants/min. These three speeds were used to conduct static seedling picking and depositing tests. Each test involved continuously picking and depositing 5 rows of seedlings, totaling 40 seedlings (to ensure no plantings in the plug trays), and the success rate of seedling picking and depositing was calculated.

The calculation formulae for the seedling picking success rate, seedling depositing success rate, and combined seedling picking and depositing success rate are as follows:

$$P={\displaystyle \frac{N_P}{N}}\times 100\%$$

(22)

where P is the picking success rate, N is the number of plug seedlings, and N_P is the number of picked seedlings.

$$D={\displaystyle \frac{N_D}{N_P}}\times 100\%$$

(23)

where D is the depositing success rate and N_D is the number of deposited seedlings.

$$Q_{PD}={\displaystyle \frac{N_D}{N}}\times 100\%$$

(24)

where Q_PD is the picking and depositing success rate.

(2)

Performance Test of the Transplanter

To test the performance of the transplanter, a test bench as shown in Figure 15 was constructed. The processed seedling picking and depositing device and seedling conveying device were installed on the chassis of the 2ZB-2J high-speed automatic transplanter, which was manufactured by Bazhou Liangjia Agricultural Machinery Manufacturing Co., Ltd. The machine is driven by ground wheels, which operate the duckbill-type planter, as described in the aforementioned working principle. The seedling conveying mechanism is motor-driven and triggered by a planting detection switch. The action of the seedling picking and depositing mechanism is initiated when the seedling conveying mechanism contacts the seedling receiving detection switch. Upon testing, it was observed that when the planting speed increased to 140 plants/min, the impact during the operation of the two sets of seedling cups intensified. Additionally, due to the motor’s direct-drive configuration without a gear reducer or brake device, the motor’s stopping was imprecise, leading to a deterioration in overall operational performance. Consequently, the planting frequencies selected for the experiments were 80 plants/min, 100 plants/min, and 120 plants/min.

During the test, the motor and speed controller were used to rotate the roller, which in turn drove the ground wheels. Consequently, the planter rotated under the drive of the ground wheels. By controlling the motor speed, the operational frequency of the planter was observed and measured. The planter frequency was set to 80 plants/min, 100 plants/min, and 120 plants/min. Three sets of tests were conducted at these three speeds. After every 5 rows of operation (equivalent to 40 seedlings), the tests at each speed were performed alternately, while the seedling picking and depositing speed was maintained at a fixed rate of 160 plants/min. The seedling picking success rate, seedling depositing success rate, seedling drop-off success rate, planting success rate, and overall transplanting success rate were observed.

The calculation formulae for the seedling drop-off success rate, planting success rate, and overall transplanting success rate are as follows:

$$L={\displaystyle \frac{N_L}{N_D}}\times 100\%$$

(25)

where L is the seedling dropping success rate, and N_L is the number of seedlings dropped.

$$P={\displaystyle \frac{N_P}{N_L}}\times 100\%$$

(26)

where P is the planting success rate, and N_P is the number of seedlings planted.

$$T={\displaystyle \frac{N_P}{N}}\times 100\%$$

(27)

where T is the overall transplanter success rate.

2.6.2. Field Tests Methods

To test the actual performance of the transplanter, a field transplanting test was conducted on the prototype machine on 19 April 2025, in Wuhaoqu Township, Yanqi Hui Autonomous County, Bazhou, Xinjiang. The test utilized 128 plug tray seedlings of pepper, with a seedling age of 61 days and the variety “Honglong 23”. The selected test seedlings exhibited uniform growth, with a 100% healthy seedling rate. The average plant height of the plug seedlings was approximately 20 cm. The weather was cloudy during the experiment, but the soil moisture content and wind speed were relatively low, resulting in minimal impact on the experimental results.

The field trial was conducted as shown in Figure 16, with the transplanter towed by a tractor. The operational frequency of the planter was observed and adjusted by rotating the tractor’s hand throttle to control the planter frequency to approximately 80 plants/min, 100 plants/min, and 120 plants/min. Single-row field transplanting tests were conducted at these three operating speeds, with 40 seedlings at each speed. The tests at each speed were alternately repeated three times, and the planting outcomes were recorded and analyzed.

3. Results and Discussion

3.1. Static Seedling Picking and Depositing Test

The test results are shown in

Table 3. Results of static seedling picking and depositing test.

Transplanting Speed	Seedlings Picked (Count)	Picking Success Rate (%)	Seedlings Deposited (Count)	Depositing Success Rate (%)	Picking and Depositing Success Rate/%
120 plants/min	40	100	40	100	100
160 plants/min	40	100	40	100	100
200 plants/min	40	100	39	100	97.50

. During the process in which the seedling cups remained stationary and only the seedling picking and depositing mechanism was controlled to pick and deposit at different speeds, the overall success rate was high. At picking and depositing speeds of 160 plants/min and below, the success rate approached 100%. Even at a speed of 200 plants/min, the seedling picking and depositing mechanism still achieved 97.5%. However, due to the loose substrate in some plug trays, substrate detachment occurred during the picking process. Additionally, the inconsistent height of the plug-cultivated seedlings, particularly those with taller heights and loose substrate, resulted in slower dropping speed. These seedlings failed to fully disengage from the claws, and the picking and depositing mechanism proceeded to the next cycle of picking and depositing, leading to depositing failures.

3.2. Performance Test of Transplanter

The results of the transplanter performance test are shown in

Table 4. Results of transplanter performance test.

Transplanting Speed	Seedlings Picked (Count)	Picking Success Rate (%)	Seedlings Deposited (Count)	Depositing Success Rate (%)	Seedlings Dropped (Count)	Dropping Success Rate (%)	Seedlings Planted (Count)	Planting Success Rate (%)	Overall Transplanting Success Rate (%)
80 plants/min	39	97.50	38	97.44	38	100.00	38	100.00	95.00
	40	100.00	39	97.50	39	100.00	39	100.00	97.5
	40	100.00	40	100.00	40	100.00	40	100.00	100.00
Average Value	39.67	99.17	39.00	98.31	39.00	100.00	39.00	100.00	97.50
100 plants/min	40	100.00	37	92.50	36	97.30	36	100.00	90.00
	39	97.50	38	97.44	38	100.00	38	100.00	95.00
	39	97.50	38	97.44	38	100.00	38	100.00	95.00
Average Value	39.33	98.33	37.67	95.79	37.33	99.10	37.33	100.00	93.33
120 plants/min	38	95.00	36	94.74	36	100.00	36	100.00	90.00
	38	95.00	36	94.74	36	100.00	36	100.00	90.00
	39	97.50	37	94.87	36	97.30	36	100.00	90.00
Average Value	38.33	95.83	36.33	94.78	36.00	99.10	36.00	100.00	90.00

. Under operational conditions. The frame of the machine exhibited increased vibration, leading to a slight decrease in the seedling picking success rate. At an operating speed of 120 plants/min, the seedling picking success rate was 95.83%. Meanwhile, the seedling conveying mechanism rotated to the seedling-receiving position and stopped for static seedling reception. However, due to the continued operation of the ground wheels and planter, as well as the complex growth conditions of the plug seedlings, the seedling depositing success rate also declined, with a success rate of 94.78% under 120 plants/min. The seedling cups, made of smooth, molded plastic with spring-triggered lids, allowed timely seedling dropping, resulting in excellent seedling dropping performance from the cups into the duckbill-type planter. In the bench test, the planter did not contact the soil, eliminating external disturbances such as clogging and wind, leading to a planting success rate close to 100%. The overall transplanting success rates were calculated as follows: 97.5% at 80 plants/min, 93.33% at 100 plants/min, and 90% at 120 plants/min.

As can be seen from Figure 17, the performance of each operational stage of the transplanter exhibits a declining trend as the planting speed increases. However, the impact on the seedling dropping and planting stages is low, with success rates still reaching a high level. The overall transplanting success rate is primarily influenced by the seedling picking stage and the seedling depositing stage. Inaccurate seedling picking and depositing at high operating speeds are the main design flaws of this transplanter.

Simultaneously, a variance analysis was conducted on the transplanting success rate under different transplanting speeds. The analysis results are presented in

Table 5. Variance analysis of a single-factor bench test on transplanting speed.

Source of Variation	Sum of Squares (SS)	Degrees of Freedom (df)	Mean Square (MS)	F-Value	p-Value
Between Groups	84.72	2	42.36	8.72	0.014
Within Groups	29.17	6	4.86
Total	113.89	8

, showing an F-value of 8.72 and a p-value of 0.014 (less than 0.05). This indicates a statistically significant difference among groups, meaning the experimental outcomes under varying transplanting speeds exhibit statistically significant differences. Furthermore, it was observed that as transplanting speed increased, the planting success rate demonstrated a declining trend.

3.3. Field Tests

The results of the field tests are shown in

Table 6. Field test results of transplanter.

Transplanting Speed	Number	Seedlings Planted (Count)	Overall Transplanting Success Rate (%)
80 plants/min	1	37	92.50
	2	38	95.00
	3	38	95.00
Average Value		37.67	94.17
100 plants/min	1	36	90.00
	2	37	92.50
	3	36	90.00
Average Value		36.33	90.83
120 plants/min	1	36	90.00
	2	35	87.50
	3	35	87.50
Average Value		35.33	88.33

. Under three operating speeds of 80 plants/min, 100 plants/min, and 120 plants/min, the planting success rates were 94.17%, 90.83%, and 88.33%, respectively, demonstrating relatively good operational performance. These results verify the feasibility of the mechanisms’ principles and the coherent coordination among mechanical, electrical, and pneumatic components. The miniaturized, electrically driven, high-speed automatic transplanter meets the design requirements; the operating speed of this transplanter is at a relatively high level. However, compared to the bench test, the planting performance in the field experiment declined. This was attributed to the complex working environment in the field, where issues such as vibration, wind, and blockage of the planter’s duckbill occurred. Given the multiple spatial transfer stages involved in a fully automatic transplanter for plug seedlings, any impact at a single stage can lead to a decrease in the overall success rate. Specifically, in the field experiment, the seedling receiving success rate was reduced due to tractor vibrations, and the planter’s planting success rate was diminished by soil blockage. These two stages exhibited significant impacts compared to the bench test.

Similarly, a variance analysis was performed on the transplanting success rate under different transplanting speeds. The results, as shown in

Table 7. Variance analysis of a single-factor field experiment on transplanting speed.

Source of Variation	Sum of Squares (SS)	Degrees of Freedom (df)	Mean Square (MS)	F-Value	p-Value
Between Groups	57.00	2	28.50	7.31	0.021
Within Groups	23.33	6	3.89
Total	80.33	8

, indicate an F-value of 7.31 and a p-value of 0.021 (less than 0.05), demonstrating statistically significant differences among the three experimental groups. This signifies that transplanting speed exerts a significantly different impact on the outcomes. Additionally, a declining trend in planting success rate was observed with increasing transplanting speed.

4. Conclusions

Based on the premise of miniaturization and modularization, an electrically driven, fully automatic transplanter has been designed. Under the pepper cultivation agronomy in Xinjiang, this transplanter can be arranged to handle any desired number of rows. Meanwhile, to enhance the seedling receiving success rate and minimize the loss of substrate from the pot seedlings, the seedling conveying mechanism has been innovatively designed with two sets of fan-shaped structures. While one set of seedling cups intermittently releases seedlings, the other set can remain stationary to receive seedlings. Furthermore, based on the characteristics of the fan-shaped arrangement of the seedling cups and to improve the speed of seedling picking and depositing, an innovative fan-shaped unfolding mechanism was designed for synchronous whole-row seedling picking and depositing, capable of retrieving 8 seedlings at once. This design achieves miniaturization of the mechanism while ensuring both operational quality and speed.

The transplanter utilizes a PLC as its control unit, using motors and cylinders to drive tray feeding, seedling picking and depositing, and seedling conveying operations. The system is characterized by its rapid response and high repetition accuracy. The synchronization of various mechanisms is achieved through the activation of limit switches, resulting in a more streamlined overall transmission system. Compared to the intricate mechanical movements of conventional transplanters, this design decreased inaccuracies in mechanism coordination caused by gear or chain backlash.

By improving the manufacturing process and selecting motors with better performance (such as high-torque servo motors with a braking function, which require high precision in rotational angles), the operational performance of the transplanter can be further enhanced. During the seedling conveying process, the inadequate precision and braking performance of the stepper motors employed led to inaccurate positioning of seedling receiving and drop-off, resulting in high pot seedling loss rates in certain segments and consequently low overall transplanting success rates. Additionally, some deficiencies in processing and assembly precision also contributed to the operational performance not reaching a higher standard. Moreover, during the experiment, the operational performance remained relatively stable under each individual speed setting, and only small-sample testing was conducted. In summary, this study provides insights for the electrification of transplanters and offers a design approach for multi-row transplanters under the cultivation practices of the Xinjiang Uyghur Autonomous Region.