In recent years, the cultivation of dragon fruit has expanded significantly due to its high nutritional value and growing consumer demand. With increasing production scales, manual harvesting has become a labor-intensive and costly process, highlighting the need for automated solutions. Robotic harvesting systems, particularly the end effector—the component that directly interacts with the fruit—play a crucial role in determining efficiency and success rates. This article presents the design, simulation, and experimental validation of a novel end effector for dragon fruit picking robots. The end effector employs an occlusive elliptical trajectory mechanism to achieve rapid shearing based on the fruit’s leaf edges as a positioning reference, aiming to minimize damage while matching manual cutting precision. Through comprehensive testing, the end effector demonstrates high performance in terms of success rate, speed, and reduced shear length, offering a promising solution for automated dragon fruit harvesting.
The design of the end effector is centered on mimicking manual harvesting techniques, where workers typically shear the fruit stem twice to create a V-shaped cut. Dragon fruits, specifically the red-fleshed variety, exhibit a unique morphology with棱角 (ridges) and逆向生长 (reverse growth) patterns. Key parameters such as fruit mass (165–460 g), longitudinal diameter (66–102 mm), and transverse diameter (55–80 mm) were measured to inform the end effector’s dimensions. The core mechanism involves a shearing机构 composed of a double-rocker linkage system, which drives upper and lower blades in an elliptical咬合 (occlusive) motion. This elliptical trajectory reduces the shear length on the leaf edges compared to circular alternatives, potentially lowering the risk of plant disease infection. The end effector integrates a pressure film sensor for feedback control, a dual-axis cylinder as the power source, and a wrist connector for attachment to a robotic arm. The overall structure ensures lightweight and efficient operation, with the shearing mechanism designed to exert sufficient force while minimizing vibration and interference.
The shearing mechanism’s motion trajectory was simulated to approximate an ellipse, with the standard equation given by:
$$ \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1 $$
where \(a = 73.11 \, \text{mm}\) and \(b = 45.43 \, \text{mm}\). Given the leaf edge width \(w\) ranging from 6 to 8 mm, the calculated shear length \(L\) falls within 36.34 to 41.64 mm, as derived from geometric analysis. This elliptical path enables precise cutting with minimal contact area, aligning with manual harvesting standards. To determine the required shearing force, single-blade tests were conducted on dragon fruit stems at varying loading speeds (5, 20, 100, and 200 mm/s). The results, summarized in Table 1, indicate that the shearing force decreases with increasing speed, with peak forces ranging from 66.2 N to 205.1 N. For the end effector design, a conservative estimate of approximately 50 N was adopted to ensure reliable cutting under dynamic conditions.
| Test Number | Fruit Mass (g) | Loading Speed (mm/s) | Stem Diameter (mm) | Peak Shearing Force (N) |
|---|---|---|---|---|
| 1 | 168.02 | 5 | 2.96 | 137.3 |
| 2 | 364.43 | 5 | 3.72 | 165.7 |
| 3 | 352.41 | 5 | 3.45 | 148.5 |
| 4 | 200.32 | 5 | 3.08 | 121.4 |
| 5 | 371.84 | 5 | 3.96 | 205.1 |
| 6 | 201.01 | 20 | 3.01 | 116.7 |
| 7 | 363.32 | 20 | 3.83 | 174.9 |
| 8 | 375.38 | 20 | 4.10 | 217.6 |
| 9 | 231.71 | 20 | 3.12 | 102.1 |
| 10 | 360.14 | 20 | 3.64 | 128.8 |
| 11 | 336.17 | 100 | 3.42 | 109.1 |
| 12 | 167.52 | 100 | 2.98 | 87.4 |
| 13 | 170.24 | 100 | 2.99 | 80.6 |
| 14 | 345.41 | 100 | 3.57 | 121.3 |
| 15 | 377.37 | 100 | 4.02 | 127.4 |
| 16 | 176.52 | 200 | 2.95 | 66.2 |
| 17 | 308.21 | 200 | 3.32 | 78.5 |
| 18 | 334.27 | 200 | 3.49 | 82.3 |
| 19 | 378.07 | 200 | 4.12 | 75.4 |
| 20 | 372.31 | 200 | 3.89 | 72.7 |
Force analysis of the shearing mechanism was conducted to size the power source. During the咬合 (occlusion) process, the forces acting on the linkage system can be modeled using static equilibrium equations. Let \(F_1\) represent the cylinder thrust, \(F_2\) the force on the连杆 (link), \(F_3\) and \(F_4\) intermediate forces, and \(F_5 = 50 \, \text{N}\) the shearing resistance. With几何 parameters \(a = 44 \, \text{mm}\), \(b = 18 \, \text{mm}\), \(\alpha = 33.4^\circ\), and \(\beta = 45^\circ\), the following equations are derived:
$$ F_1 = 2F_2 $$
$$ F_3 = F_2 \cos \alpha $$
$$ a F_4 = (a – b) F_3 $$
$$ F_5 = F_4 \cos \beta $$
Solving these yields \(F_2 = 143.59 \, \text{N}\) and \(F_1 = 287.18 \, \text{N}\). A dual-axis cylinder (model TN20×20S) was selected, providing a theoretical output force of 314.2 N at 0.5 MPa, ensuring adequate power while keeping the end effector lightweight. This cylinder enables high-speed operation up to 500 mm/s, which is beneficial for reducing采摘时间 (picking time).
Kinematic simulation of the end effector was performed using ADAMS software to validate the design and analyze motion characteristics. The virtual prototype, imported from SolidWorks, included all components of the shearing mechanism. Constraints were applied to simulate the cylinder’s extension, driving the double-rocker linkage and blade movement. The simulation results, shown in Figure 4 and Figure 5 (referenced generically), depict the displacement, velocity, acceleration, and angular parameters of the upper blade during the咬合 process. The curves indicate smooth motion without abrupt changes or interference, confirming the mechanism’s stability. Specifically, the blade velocity peaks at approximately 0.5 m/s, and acceleration remains within safe limits, ensuring minimal vibration and reliable shearing. The elliptical trajectory was verified to achieve full occlusion within 20 mm of cylinder stroke, with the shear length consistent with theoretical calculations. This simulation underpins the end effector’s robustness for real-world applications.
The control system for the end effector is designed for seamless integration with a robotic harvesting platform. It centers on an STM32F103C8T6 microcontroller, which processes signals from a pressure film sensor to trigger the shearing action. The sensor, an FSR resistive type, is set with a threshold force of 0.2 N. When the end effector approaches the dragon fruit, the sensor contacts the leaf edge, and upon exceeding the threshold, it outputs a high-level signal (3.3 V). This signal activates a relay, closing the circuit to a 24 V electromagnetic valve that controls the dual-axis cylinder. The cylinder extends, driving the linkage to perform the shearing motion, and retracts after completion to reset the blades. The control flowchart emphasizes simplicity and reliability, with the visual system—a ZED stereo camera and NVIDIA Jetson Xavier NX for image processing—guiding the robotic arm to position the end effector accurately. This closed-loop control ensures precise fruit targeting and reduces the risk of missed picks or damage.
To evaluate the end effector’s performance, a prototype was constructed and tested in both laboratory and field settings. The试验平台 (experimental platform) comprised the end effector mounted on a six-axis robotic arm, with the vision system for fruit detection. In laboratory trials, simulated dragon fruit stems were used to replicate orchard conditions. The air pressure supplied to the cylinder was varied from 0.4 to 0.7 MPa as the primary factor, with single fruit picking time, success rate, and shear length as metrics. Each pressure level was tested 20 times, totaling 80 trials on fruits with stem diameters of 3–5 mm. Success was defined as undamaged fruit detachment, and shear length was measured using calipers. The results, presented in Table 2, highlight that 0.5 MPa yielded the optimal balance: a 90% success rate, average picking time of 0.46 s, and shear length of 36.69 mm. Higher pressures reduced time but increased vibration and shear length, while lower pressures slightly decreased success rates.
| Air Pressure (MPa) | Success Rate (%) | Average Picking Time per Fruit (s) | Stem Diameter Range (mm) | Average Shear Length (mm) |
|---|---|---|---|---|
| 0.4 | 85 | 0.59 | 3–5 | 39.68 |
| 0.5 | 90 | 0.46 | 3–5 | 36.69 |
| 0.6 | 85 | 0.33 | 3–5 | 40.99 |
| 0.7 | 80 | 0.18 | 3–5 | 42.55 |
For comparative analysis, a circular trajectory end effector—a previous design—was tested under identical conditions. The results, shown in Table 3, demonstrate that the elliptical end effector significantly reduces shear length. At pressures from 0.4 to 0.7 MPa, the circular end effector produced average shear lengths of 74.13, 70.34, 79.91, and 82.40 mm, respectively. In contrast, the elliptical end effector achieved reductions of 46.47%, 47.84%, 48.70%, and 48.36% in shear length. This reduction is critical for minimizing plant wounding and potential disease entry, showcasing the advantage of the elliptical trajectory. Both end effectors exhibited similar picking times, but the elliptical design’s shorter shear length aligns better with manual harvesting practices, enhancing overall orchard health.
| Air Pressure (MPa) | Success Rate (%) | Average Picking Time per Fruit (s) | Stem Diameter Range (mm) | Average Shear Length (mm) |
|---|---|---|---|---|
| 0.4 | 85 | 0.56 | 3–5 | 74.13 |
| 0.5 | 85 | 0.51 | 3–5 | 70.34 |
| 0.6 | 80 | 0.37 | 3–5 | 79.91 |
| 0.7 | 75 | 0.23 | 3–5 | 82.40 |
Field trials were conducted in a dragon fruit orchard to validate the end effector under real-world conditions. Using the optimal pressure of 0.5 MPa, 40 fruits with stem diameters of 2.8–3.1 mm were randomly harvested in two groups of 20 trials each. The results, summarized in Table 4, show a success rate of 85%, average picking time of 0.43 s, and shear length of 39.80 mm. Slight decreases in performance compared to laboratory tests are attributed to environmental complexities, such as fruit orientation and foliage interference. Nonetheless, the end effector demonstrated robust functionality, with shear lengths consistently below 40 mm, meeting the goal of mimicking manual cutting. The field validation confirms the design’s practicality and readiness for integration into automated harvesting systems.
| Air Pressure (MPa) | Success Rate (%) | Average Picking Time per Fruit (s) | Average Shear Length (mm) | Stem Diameter Range (mm) |
|---|---|---|---|---|
| 0.5 | 85 | 0.43 | 39.80 | 2.8–3.1 |
| 0.6 | 80 | 0.36 | 41.90 | 2.8–3.1 |
In conclusion, the elliptical trajectory end effector for dragon fruit picking robots represents a significant advancement in automated harvesting technology. By leveraging an occlusive shearing mechanism based on an elliptical path, this end effector achieves rapid and precise fruit detachment with minimal damage to the plant. The design process incorporated thorough measurements of fruit morphology, force analysis, and kinematic simulation, ensuring mechanical reliability. Experimental results from both laboratory and field settings validate the end effector’s performance, with optimal operation at 0.5 MPa yielding high success rates, short picking times, and reduced shear lengths compared to circular alternatives. The integration of a pressure sensor-based control system enhances accuracy and adaptability. Future work could focus on scaling the end effector for different fruit varieties or optimizing the vision system for better target recognition in dense orchards. Overall, this end effector contributes to addressing labor shortages in agriculture and promoting sustainable farming practices through robotics.