The mechanization of vegetable transplanting represents a significant challenge in modern agriculture. While manual and semi-automatic methods dominate, they are labor-intensive and inefficient. The development of a reliable, fully automatic transplanter is hindered by the performance of its core component: the seedling pickup device, specifically its end effector. The end effector is the part that directly interacts with the seedling plug (or “pot body”), and its design critically impacts success rates, seedling health, and overall operational speed. Among various pickup principles, the jacking-clamping method, where a plug is first pushed out from below and then clamped from the sides, shows promise due to its adaptability to standardized seedling trays. However, common failures such as unsuccessful jacking, excessive deformation of the soft plug during clamping, and plug damage remain significant bottlenecks. This article delves into the mechanical analysis, simulation-based optimization, and experimental validation of a novel jacking-clamping end effector designed to address these issues and meet the demands of high-speed, automatic leafy vegetable transplanting.
The proposed end effector system consists of two primary, coordinately acting units: a jacking unit and a clamping unit. The jacking unit features an array of rods, each comprising a long pusher and a shorter central needle. These rods pass through the drainage holes in the bottom of a standard 128-cell tray. The pushers, arranged with alternating lengths, engage the bottom of the plugs to eject them. The central needle, which protrudes slightly, penetrates the base of the plug upon ejection. This needle serves a crucial dual purpose: it anchors the plug, preventing it from toppling or shifting, and by creating a staggered “stair-step” pattern of ejected seedlings, it provides the necessary physical clearance for the clamping end effector to access each plug laterally. This design enables simultaneous, whole-row pickup, a key advantage for efficiency.
The clamping unit itself is a V-shaped gripping mechanism. It consists of two symmetrical jaws actuated by a pneumatic cylinder. The V-shaped configuration, as opposed to parallel grippers, is designed to apply a more uniform pressure gradient across the plug’s side, theoretically requiring less force to achieve a secure grip and thus inducing less compressive strain. The complete pickup cycle involves: (1) the jacking rods ejecting and stabilizing a full row of seedlings, (2) the V-shaped clamping end effector moving laterally to engage the plug, (3) the jaws closing to a preset distance to grip the plug firmly, (4) the jacking rods retracting, (5) the clamping end effector transporting the seedling to the drop point, and (6) the jaws opening to release the plug into a delivery tube.
The success of this operation hinges on the mechanical interaction between the end effector components and the viscoelastic, fragile seedling plug. To guide the design, a detailed quasi-static force analysis was performed for both the jacking and clamping phases.
For the jacking phase, a force balance model on the plug was established. Before ejection, as the central needle inserts, the plug is in equilibrium. The forces include the resultant normal force from the tray cell wall ($F_{ni}$), the frictional force from the needle ($f_{dz}$), the frictional force from the tray wall ($f_{ni}$), the plug’s weight ($G$), and the adhesive bonding force from the tray ($F_{nj}$). The equilibrium is given by:
$$F_{ni} + f_{dz} – f_{ni} = G + F_{nj}$$
Successful needle insertion without premature plug release requires careful management of the changes in $F_{ni}$, $f_{dz}$, and $f_{ni}$. During the acceleration phase of ejection, the pusher applies force ($F_{dg1}$) to overcome inertia ($F_{I1}=-ma_1$), adhesion, and friction. Applying d’Alembert’s principle:
$$F_{dg1} – F_{I1} = G + f_{ni} + F_{nj} – F_{ni}$$
A higher inertial force $F_{I1}$ is desirable to minimize the relative displacement between the fast-moving pusher and the slower-responding plug, which prevents the pusher from “digging into” and damaging the plug bottom. This suggests the jacking speed should be sufficiently high. Finally, when the jacking motion stops, the plug decelerates under gravity and needle friction ($f_{dz}$). The force balance during deceleration is:
$$F_{dg3} + F_{I2} – G – f_{dz} = 0$$
where $F_{I2} = -ma_2$. A larger needle friction $f_{dz}$ helps create a larger deceleration inertia $F_{I2}$, preventing the plug from sliding off the needle. The needle friction is a function of the normal pressure and the coefficient of friction: $f_{dz} = \mu_{dz} F_{Ndz}$. The normal pressure is related to the volume of the needle inserted. Therefore, the key design parameters for the jacking end effector identified from this analysis are: needle diameter, needle length, and jacking velocity.
For the clamping phase, the force analysis focuses on the V-shaped end effector. The gripping force required to hold the plug against gravity is derived from a balance of normal ($F_{mj}$) and frictional ($f_{mj}$) forces from the two jaws, resolved along the vertical axis. For a symmetrical grip with a V-angle $\beta$, the minimum required normal force per jaw is:
$$F_{mj} \ge \frac{G}{2\sin(\beta/2) + \mu_{mj}\cos(\beta/2)}$$
However, the plug is not a rigid body. It exhibits viscoelastic behavior, characterized by a compression-relaxation curve. The force required to achieve a certain grip is directly related to the compression distance (or loading distance) applied by the jaws. Excessive compression leads to permanent deformation, particle rearrangement, and structural damage to the plug’s matrix. Therefore, the optimal clamping compression distance is a critical parameter for the clamping end effector, balancing grip security with plug integrity.
To optimize these parameters without costly and time-consuming physical prototyping, Discrete Element Method (DEM) simulation was employed. The plug was modeled as a composite of particles representing its primary constituents: peat, vermiculite, and perlite. Key physical properties of real 25-day-old lettuce plugs were measured and used to calibrate the simulation model.
| Property | Value |
|---|---|
| Density | 0.7055 g/cm³ |
| Cohesion, C | 20.75 kPa |
| Internal Friction Angle, $\phi$ | 19.55° |
| Poisson’s Ratio, $\mu$ | 0.4 |
| Elastic Modulus, E | 4.47 x 10³ kPa |
| Shear Modulus, G | 1.596 x 10³ kPa |
The EEPA contact model in EDEM software was selected to simulate the cohesive, moist nature of the plug material. Material properties for the simulation are summarized below.
| Component | Property | Value |
|---|---|---|
| Plug Particles | Density | 0.7055 g/cm³ |
| Poisson’s Ratio | 0.4 | |
| Shear Modulus | 1.596e6 Pa | |
| End Effector (Steel) | Density | 7.85 g/cm³ |
| Poisson’s Ratio | 0.269 | |
| Shear Modulus | 7.9e10 Pa | |
| Tray (Polystyrene) | Density | 1.9 g/cm³ |
| Poisson’s Ratio | 0.42 | |
| Shear Modulus | 1.06e9 Pa |
A Box-Behnken experimental design was used for the jacking simulation, with needle diameter (A: 1-3 mm), needle length (B: 10-30 mm), and jacking velocity (C: 0.2-0.6 m/s) as factors. The evaluation index was the coordinate difference (H) between the plug bottom and the pusher face at the end of the stroke. A positive H indicates the needle did not fully anchor the plug; a negative H indicates the pusher caused compression damage. The goal was H=0. The experimental design and results are shown below.
| Run | A: Diameter (mm) | B: Length (mm) | C: Velocity (m/s) | H (mm) |
|---|---|---|---|---|
| 1 | 1 | 10 | 0.4 | -2.4 |
| 2 | 3 | 10 | 0.4 | -2.6 |
| 3 | 1 | 30 | 0.4 | 0.6 |
| 4 | 3 | 30 | 0.4 | 12.6 |
| 5 | 1 | 20 | 0.2 | -0.4 |
| 6 | 3 | 20 | 0.2 | 7.7 |
| 7 | 1 | 20 | 0.6 | -3.7 |
| 8 | 3 | 20 | 0.6 | -1.3 |
| 9 | 2 | 10 | 0.2 | -1.4 |
| 10 | 2 | 30 | 0.2 | 10.4 |
| 11 | 2 | 10 | 0.6 | -5.9 |
| 12 | 2 | 30 | 0.6 | 2.3 |
| 13-17 | 2 | 20 | 0.4 | -0.3 to -0.6 |
Analysis of variance (ANOVA) showed all factors and their interactions were highly significant (p<0.01). The response surface analysis revealed complex interactions. For instance, with velocity fixed at 0.4 m/s, a combination of small diameter and long length, or large diameter and long length, led to positive H (poor anchoring). The optimal solution for H=0, obtained through numerical optimization, was: Needle Diameter = 1.9 mm, Needle Length = 18 mm, and Jacking Velocity = 0.3 m/s.
Using these optimized jacking parameters, clamping simulations were run for different compression distances: 2, 4, 6, and 8 mm. The simulation visualized internal force chains and contact points. At 2 mm compression, force chains were strong but grip contact was sparse. At 8 mm, grip contact was excellent but force chains were weak and the plug exhibited significant lateral expansion, indicating structural loosening. At 4 mm compression, a strong network of force chains was maintained within the plug, and the contact points between the plug and the end effector jaws were uniformly distributed, suggesting a secure grip with minimal detrimental deformation. Therefore, the optimal clamping compression distance was selected as 4 mm.
Based on the DEM optimization, a physical prototype of the end effector system was built and tested. The jacking unit was configured with the optimized parameters (1.9 mm needle, 18 mm length, 0.3 m/s velocity). The V-shaped clamping end effector was set to close to a 4 mm compression distance. Performance validation tests were conducted on 25-day-old lettuce seedlings at a pickup frequency of 100 plants/min. The success rate (S) was calculated based on failures in jacking ($S_1$), clamping/dropping during transport ($S_2$), and mis-drop at release ($S_3$).
$$S = \frac{N – N_1 – N_2 – N_3 – N_4 – N_5}{N} \times 100\%$$
Where $N$ is the total number of pickups, $N_1$ is failed ejection, $N_2$ is plugs falling off the needle, $N_3$ is plugs crushed by the clamp, $N_4$ is plugs dropped during transport, and $N_5$ is misaligned drops. The test results from 252 pickups were: $N_1=10$, $N_2=2$, $N_3=3$, $N_4=0$, $N_5=2$. This yields a jacking failure rate $S_1$ of 4.76%, a clamping/dropping failure rate $S_2$ of 1.25%, and a mis-drop rate $S_3$ of 0.84%. The overall pickup and delivery success rate $S$ was 93.25%.
| Metric | Calculation | Result |
|---|---|---|
| Jacking Failure Rate ($S_1$) | $(10+2)/252 \times 100\%$ | 4.76% |
| Clamping/Drop Failure Rate ($S_2$) | $3/(252-10-2) \times 100\%$ | 1.25% |
| Mis-drop Failure Rate ($S_3$) | $2/(252-10-2-3) \times 100\%$ | 0.84% |
| Overall Success Rate (S) | $(252-10-2-3-0-2)/252 \times 100\%$ | 93.25% |
In conclusion, the systematic design and optimization of the jacking-clamping end effector have proven highly effective. The mechanical analysis correctly identified the critical parameters: needle geometry, jacking speed, and clamping compression distance. The use of DEM simulation provided a powerful tool to visualize and optimize the interaction between the end effector and the delicate plug, leading to an optimized parameter set. The physical validation test confirmed that this optimized end effector can achieve a high success rate (93.25%) at an operational frequency of 100 plants/min while maintaining good plug integrity. This performance meets the practical requirements for automatic dryland transplanting of leafy vegetable plug seedlings. The integration of the central needle for stabilization and the staggered-row concept is a key innovation that enables reliable, whole-row pickup. This work demonstrates a successful methodology for the development of high-performance end effector systems for agricultural robotics, combining theoretical mechanics, advanced numerical simulation, and experimental validation.