Design and Performance Validation of a Variable-Span Arch (VSA) End-Effector for Dragon Fruit Harvesting
Abstract
1. Introduction
- (1)
- Introducing the VSA geometry for the first time to achieve linear and compliant adjustment of clamping forces, thereby alleviating stress concentration issues inherent to rigid grippers with limited contact points.
- (2)
- Employing a multi-layer redundant design that combines material and structural compliance, which significantly improves adaptability to irregular fruit geometries and reduces the risk of fruit damage.
2. Dragon Fruit Characteristics and Fruit Detachment Principle
2.1. Morphological and Structural Characteristics of Dragon Fruit
2.2. Stem Structure and Detachment Principle
3. Case Study and Analysis of a Preliminary End-Effector Design
3.1. Structure and Limitations of an Underactuated Three-Jaw Flexible End-Effector
- (1)
- Insufficient gripping force. Due to the highly elastic material of the jaws, the overall structure was prone to significant deformation when external forces were applied. This was particularly evident during the twisting motion required to detach the dragon fruit from its stem, where the jaw’s insufficient torsional stiffness prevented the effective transmission of torque, often resulting in slippage or grip failure (as shown in Figure 3a).
- (2)
- Poor adaptability to off-Center Fruits. In practical operations, due to factors such as visual positioning errors and the randomness of fruit postures, the central axis of the end-effector is often difficult to align perfectly with the stem. Under such non-ideal poses, the three-jaw structure, with its linear and non-enveloping contraction, is unable to form a stable, all-encompassing contact. This leads to an uneven distribution of clamping force, causing the fruit to shift or be dropped (as shown in Figure 3b).
- (3)
- Insufficient envelopment redundancy. The contact area during the gripping pro cess of this structure is limited, relying primarily on localized force at the tips of the jaws to complete the grasp. This results in concentrated stress on the fruit’s surface, which can easily cause indentations or internal damage. Concurrently, the three-jaw structure has inherent limitations in terms of its coverage area and gripping redundancy, making it unable to effectively adapt to fruits of varying sizes and postures (as shown in Figure 3c).
3.2. Design and Performance Evaluation of an Underactuated Four-Jaw Gripping Structure
4. Design of the Enveloping End-Effector
4.1. Overall Structural Design
4.2. Force Analysis of the Flexible Arch Structure
4.2.1. Static Compression Test and Force Response Analysis
4.2.2. Static Modeling of Fruit Gripping
- is the clamping force exerted by the VSA on the dragon fruit (N).
- is the coefficient of friction.
- H is the frictional torque generated by the VSA on the fruit’s surface (N·cm).
- is the component of an individual force (where = 1, 2, 3, or 4) projected onto the x-axis in the coordinate plane.
- is the resultant force of and along the x-axis.
- is the component of an individual force fi projected onto the y-axis in the coordinate plane.
- is the resultant force of and along the y-axis.
- is the moment of an individual force fi about point .
- is the resultant moment of and about point .
- is the angle between the lines of action of the VSA forces.
- is the torque, in units of kgf·cm.
- is the support force of the VSA, in Newtons (N).
- is the coefficient of friction.
- is the radial diameter of the contact area between the fruit and the VSA, in meters (m).
4.2.3. Determination of the Coefficient of Friction Between Dragon Fruit and TPU Material
4.2.4. Calculation of Total Torque for the Multi-Layer Gripping Structure
4.3. Motor Selection and Parameter Rationale
5. Field Gripping and Harvesting Trials and Analysis
5.1. Experimental Method
5.1.1. Experimental Platform, Samples, and Environment
5.1.2. Operational Workflow and Damage Assessment
- Level 0 (No Damage): Fruit skin intact, stem detachment point smooth, no pulp bruising.
- Level 1 (Slight Damage): Slight indentations on the fruit skin but no breakage.
- Level 2 (Moderate Damage): Fruit skin broken or obvious tearing at the stem.
- Level 3 (Severe Damage): Severe damage to the pulp tissue.
6. Results
6.1. Harvesting Success Rate and Efficiency
6.2. Assessment of Static Pressure Bruising and Torsional Harvesting Damage
7. Discussion
7.1. Challenges and Design Limitations in Field Trials
7.2. Future Outlook
- Structural Optimization and Miniaturization: Addressing the accessibility challenges identified, a primary task for future work will be to optimize and miniaturize the end-effector’s structure. This aims to reduce its overall size and weight without compromising its enveloping performance.
- Enhanced Perception and Adaptability: Integrating flexible tactile sensors to achieve closed-loop adaptive control of gripping force. Concurrently, visual algorithms will be improved to enhance robustness in complex environments.
- Expanded Application Scope: Extending the design concept to the harvesting of other irregularly shaped, high-value fruits, such as mangoes and avocados. Furthermore, its scalability and application in large-scale industrial harvesting robots will be explored.
8. Conclusions
- (1)
- Significantly Improved Gripping Reliability and Reduced Fruit Damage Rate: Benefiting from the VSA structure’s linear force control and multi-point flexible envelopment features, the end-effector effectively overcomes the damage and slippage issues caused by stress concentration and off-center gripping, which are common in traditional rigid jaws. Field trials showed that its optimal harvesting success rate reached 95%, with the pulp of harvested samples remaining completely undamaged, achieving genuinely low-damage harvesting.
- (2)
- Achieved More Efficient Harvesting Operations: The average harvesting time per fruit is approximately 15 s, which is a significant improvement in operational efficiency compared to traditional harvesting structures reported in the literature. The successful application of the 10 Nm motor, while ensuring the completion of harvesting tasks for the largest fruit samples, also validates the rational matching of the structure in terms of energy efficiency and lightweight design, ensuring stability during continuous operation.
- (3)
- Demonstrated Broad Application Potential: The principles of adaptive gripping and force control based on the VSA, as proposed in this study, are not limited to dragon fruit. This design concept can provide an important technical reference and an efficient, reliable solution for the automated harvesting of other high-value fruits with irregular shapes and fragile skins (such as mangoes, avocados, etc.).
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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No. | Torsional Separation Angle (°) | Maximum Torque (kgf·cm) | No. | Torsional Separation Angle (°) | Maximum Torque (kgf·cm) |
---|---|---|---|---|---|
1 | 360 | 4.6 | 9 | 540 | 11.2 |
2 | 540 | 14.1 | 10 | 720 | 9 |
3 | 720 | 13.2 | 11 | 540 | 8.2 |
4 | 270 | 9.2 | 12 | 640 | 9.5 |
5 | 720 | 10 | 13 | 540 | 10.5 |
6 | 480 | 11 | 14 | 480 | 11.8 |
7 | 540 | 7.8 | 15 | 540 | 9.4 |
8 | 540 | 8.8 | 16 | 720 | 12.3 |
VSA Span (mm) | Maximum Force (N) | Deformation at Maximum Force (mm) | Stiffness (N/cm) |
---|---|---|---|
55 | 24.610 | 37.399 | 60.937 |
90 | 20.026 | 23.704 | 4.305 |
70 | 9.326 | 6.825 | 39.622 |
64 | 11.827 | 7.066 | 45.342 |
No. | Inclination Angle (°) | No. | Inclination Angle (°) | No. | Inclination Angle (°) |
---|---|---|---|---|---|
1 | 22.2 | 6 | 19.6 | 11 | 19.1 |
2 | 18.7 | 7 | 21.3 | 12 | 21.0 |
3 | 20.9 | 8 | 20.1 | 13 | 19.4 |
4 | 19.5 | 9 | 19.8 | 14 | 22.8 |
5 | 21.3 | 10 | 20.9 | 15 | 20.3 |
Layer No. | Cross-Sectional Diameter (mm) | Deformation (mm) | Support Force (N) |
---|---|---|---|
1 | 55.2 | 9.98 | 14.1 |
2 | 84.32 | 24.54 | 7.7 |
3 | 92 | 28.38 | 4.4 |
4 | 84.32 | 24.54 | 7.7 |
5 | 55.2 | 9.98 | 14.1 |
Dimensional Parameter | Minimum Value (mm) | Maximum Value (mm) | Average Value (mm) | Standard Deviation (mm) |
---|---|---|---|---|
Axial Length | 75 | 87.8 | 81.9 | 3.2 |
Maximum Radial Width | 74.5 | 88.1 | 81.6 | 3.4 |
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Zhu, L.; Chen, Y.; Lv, Q.; Zhang, S.; Feng, X.; Kong, S.; Fu, G.; Chen, T. Design and Performance Validation of a Variable-Span Arch (VSA) End-Effector for Dragon Fruit Harvesting. AgriEngineering 2025, 7, 327. https://doi.org/10.3390/agriengineering7100327
Zhu L, Chen Y, Lv Q, Zhang S, Feng X, Kong S, Fu G, Chen T. Design and Performance Validation of a Variable-Span Arch (VSA) End-Effector for Dragon Fruit Harvesting. AgriEngineering. 2025; 7(10):327. https://doi.org/10.3390/agriengineering7100327
Chicago/Turabian StyleZhu, Lixue, Yipeng Chen, Qiuhui Lv, Shiang Zhang, Xinqi Feng, Shaoting Kong, Genping Fu, and Tianci Chen. 2025. "Design and Performance Validation of a Variable-Span Arch (VSA) End-Effector for Dragon Fruit Harvesting" AgriEngineering 7, no. 10: 327. https://doi.org/10.3390/agriengineering7100327
APA StyleZhu, L., Chen, Y., Lv, Q., Zhang, S., Feng, X., Kong, S., Fu, G., & Chen, T. (2025). Design and Performance Validation of a Variable-Span Arch (VSA) End-Effector for Dragon Fruit Harvesting. AgriEngineering, 7(10), 327. https://doi.org/10.3390/agriengineering7100327