The rising demand for efficient and cost-effective agricultural practices has brought automation to the forefront, particularly in labor-intensive tasks like harvesting. Agaricus bisporus, commonly known as the button mushroom, presents a significant challenge for automation due to its delicate nature, clustered growth pattern, and susceptibility to bruising. Existing mechanical grippers often cause unacceptable levels of damage, while conventional vacuum suction cups struggle with maintaining a reliable seal on the mushroom’s curved, variable surface, leading to high drop rates. This paper addresses these critical issues by introducing a novel end effector based on granular jamming principles, designed specifically for the gentle and reliable single-unit harvesting of Agaricus bisporus.
Our design philosophy centers on maximizing contact area and seal quality while minimizing applied stress. The proposed end effector utilizes a flexible membrane filled with granular material. In its unjammed state, the granules flow like a fluid, allowing the membrane to conform perfectly to the unique, non-uniform geometry of each mushroom cap. Upon application of vacuum, the granules interlock and jam, transitioning the membrane into a rigid, shape-locked state that securely grips the object. This fundamental mechanism allows our end effector to achieve a form-fitting grasp without excessive pressure, effectively solving the sealing problem inherent to rigid suction cups. The remainder of this work details the systematic design, simulation, and experimental validation of this flexible profiling end effector.
System Requirements and Conceptual Design
The successful automated harvesting of Agaricus bisporus imposes specific requirements on any end effector. Market standards define optimal cap diameters between 25 mm and 50 mm. The mushrooms grow densely, necessitating a end effector with a compact form factor to avoid disturbing adjacent, unripe specimens. The connection to the substrate (mycelium) is weak, requiring only a small separation force, but the mushroom tissue is soft and easily damaged, mandating a gentle, distributed gripping force. Therefore, the primary design objectives for our end effector were: (1) High adaptability to size (25-50 mm) and ellipticity variations, (2) A grasping mechanism that avoids shear and point loads on the cap surface, (3) High reliability and success rate, and (4) Operational compatibility with standard robotic manipulators.
We selected a single-unit,吸附-based picking strategy as the most suitable approach. The core innovation lies in replacing a standard rigid-bell suction cup with a flexible, granular-jamming-based suction gripper. The conceptual operation involves positioning the end effector above a target mushroom, lowering it to make contact, activating the profiling phase, applying vacuum to achieve adhesion and jamming, performing the separation motion, transporting the mushroom, and finally releasing it by pressure reversal. The mechanical design of the end effector prototype is centered around this flexible profiling sucker assembly.
Mechanical Design of the End Effector
The mechanical assembly of the end effector consists of two main subsystems: the flexible profiling suction module and the supporting pneumatic attachment structure.
Flexible Profiling Suction Module
This is the core functional component. It comprises a flexible latex membrane forming an enclosed pouch, which is filled with granular material (e.g., fine quartz sand). The latex was chosen over silicone for its superior elasticity, higher surface hardness, and lower surface tackiness, which promotes better conformability and release. The pouch features a concave opening profile with an optimized angle of 60°, determined through preliminary tests to maximize initial contact and success rate. The outer diameter of the membrane is set at 35 mm, providing a compact footprint suitable for the dense growth environment while housing sufficient material for effective profiling.
The suction force $F_{ads}$ generated by this module is the sum of the pressure-driven adhesive force and the friction force at the contact interface:
$$F_{ads} = F_{vac} + F_{fric} = |p| \cdot A_{eff} + \mu \cdot (F_{N1}+F_{N2}) \cdot \sin\alpha$$
where $p$ is the pressure difference (vacuum), $A_{eff}$ is the effective sealed contact area, $\mu$ is the coefficient of friction, $F_{N1}$ and $F_{N2}$ are normal reaction forces, and $\alpha$ is the contact angle. The profiling action directly maximizes $A_{eff}$ and enhances the seal, thereby increasing both $F_{vac}$ and $F_{fric}$ for a given vacuum level.
Supporting Structure and Pneumatic Integration
A rigid housing contains the flexible module. It includes an end cap with dual air channels—one connected to the interior volume of the membrane for vacuum/pressure application, and another (optional) for controlling the jamming state if a separate chamber were used. A support structure and a clamp secure the neck of the latex membrane. A filtered air passage ensures vacuum is applied to the cavity between the membrane and the mushroom cap while preventing granular material from escaping into the pneumatic system. This assembly is designed for easy attachment to the wrist flange of a robotic manipulator.
Finite Element Analysis and Parameter Optimization
To guide the design and understand the mechanical behavior, we performed Finite Element Analysis (FEA) using ANSYS Workbench. The model simulated the interaction between the granular-filled membrane (modeled as a continuous hyperelastic body with Mooney-Rivlin properties) and a mushroom cap (modeled as polyethylene for simplicity).
Determining Optimal Opening Diameter
A key design parameter is the diameter of the suction module’s opening. Simulations were run for opening diameters of 20, 22.5, 25, and 27.5 mm against a standard 25 mm mushroom cap. The results quantified the effective sealing diameter $D_{eff}$.
| Opening Diameter (mm) | Effective Sealing Diameter $D_{eff}$ (mm) |
|---|---|
| 20.0 | 22.1 |
| 22.5 | 24.2 |
| 25.0 | 25.0 |
| 27.5 | 25.0 |
The data shows that $D_{eff}$ increases with opening diameter until it saturates at the mushroom cap’s diameter. An opening of 25 mm achieves full-cap contact for a 25 mm mushroom, maximizing $A_{eff} = \pi D_{eff}^2/4$. Therefore, 25 mm was selected as the optimal opening diameter for the target size range.
Stress Distribution During Grasping
The FEA of the adsorption phase revealed important stress characteristics. The Von Mises stress is predominantly concentrated in an annular ring at the contact periphery, with lower stress at the center of the cap. The stress within the granular core is higher than on the outer membrane surface, indicating effective load transfer and jamming. The stress profile along a diameter is parabolic and symmetric, confirming stable and even force distribution even for elliptical caps, which is crucial for minimizing damage. This analysis validated the end effector‘s ability to distribute grasping forces favorably.
Control System and Pneumatic Circuit Design
A dedicated control system was implemented to manage the harvesting sequence reliably. The system operates in an automated mode, integrated with a higher-level vision and robot control system.
The pneumatic circuit is designed for simplicity and robustness. It employs a single miniature vacuum pump capable of both suction and blowing. A negative pressure regulator allows precise control of the grasping vacuum. Two solenoid valves switch the end effector between three states:
- GRASP: Valve V1 opens, connecting the pump’s suction port to the end effector. Vacuum is applied, conforming the membrane and jamming the granules.
- HOLD: Both valves are closed, maintaining the vacuum and the rigid state.
- RELEASE: Valve V2 opens, connecting the pump’s pressure port to the end effector. A brief positive pressure pulse breaks the seal and actively detaches the mushroom.
The control logic, executed by a Programmable Logic Controller (PLC), follows the sequence: Robot positions end effector -> Lower onto mushroom -> Activate GRASP -> Wait for stabilization -> Robot performs separation motion -> Transport to container -> Activate RELEASE -> Return to start. A pressure sensor provides feedback for closed-loop vacuum control, ensuring consistent adhesive force.
Experimental Validation and Results
A prototype of the flexible profiling end effector was fabricated and subjected to a series of laboratory and field tests to evaluate its performance.
Pull-Off Force Characterization
The fundamental performance metric is the holding force, measured as the pull-off force. Experiments investigated the influence of key parameters: vacuum level $p$, mushroom diameter $D_m$, membrane thickness $t$, and granule size.
1. Effect of Membrane Thickness: Using fine quartz (200-mesh) and a 25 mm mushroom model, membranes of thickness 0.7, 0.9, and 1.1 mm were tested. The relationship was non-linear, with the 0.9 mm membrane yielding the highest pull-off force across vacuum levels. Thinner membranes lacked stability and wrinkled, while thicker membranes had reduced conformability.
2. Effect of Granule Size: With a 0.9 mm membrane, three fillers were tested: 200-mesh quartz, 20-mesh quartz, and 3 mm plastic beads. Pull-off force was highest for 200-mesh quartz, followed by plastic beads, and lowest for 20-mesh quartz. Finer granules provide better conformity and a denser packing that improves the jamming effect and seal quality.
$$F_{pull} \propto f(\text{Granule Size}) \quad \text{(Negative Correlation)}$$
3. Effect of Mushroom Diameter and Vacuum: Using the optimal 0.9 mm membrane with 200-mesh quartz, pull-off force was measured for mushroom models of 25, 35, and 45 mm diameter across a vacuum range of -10 to -70 kPa.
| Parameter | Relationship with Pull-Off Force $F_{pull}$ | Primary Effect |
|---|---|---|
| Vacuum $|p|$ | $F_{pull} \propto |p|$ (Linear, slope = $A_{eff}$) | Direct driving force |
| Mushroom Diameter $D_m$ | $F_{pull} \propto D_m$ (Positive correlation) | Increases effective contact area $A_{eff}$ |
| Membrane Thickness $t$ | Non-linear, optimal at t=0.9mm | Affects conformability and seal stability |
| Granule Diameter | $F_{pull} \propto 1/(\text{Granule Diameter})$ | Determines jamming quality and surface conformity |
The linear relationship with vacuum confirms the theoretical model $F_{vac} = |p|A_{eff}$. The increase in slope (effective area) with mushroom diameter visually demonstrates the profiling action: the end effector makes more extensive contact with larger caps.
Comparative Test Against Standard Suction Cup
A direct comparison was made with a standard 25 mm silicone vacuum suction cup. The flexible profiling end effector (configured with 20-mesh quartz for best conformability) consistently generated a higher pull-off force at the same negative pressure. More importantly, to achieve a benchmark holding force sufficient for harvesting (approx. 1.5 N), the standard cup required significantly higher vacuum, which increases the risk of cap damage due to concentrated stress. Our end effector achieved the same force at a lower, safer vacuum level due to its larger effective contact area.
Field Harvesting Trial
The final validation was a live harvesting test on Agaricus bisporus in a growth tray. The end effector was mounted on a 3-axis Cartesian robot. 200 mushrooms with diameters between 25-50 mm were attempted. For comparison, standard silicone suction cups of 25, 35, and 45 mm were also tested on separate batches of 200 mushrooms.
| End Effector Type | Pick Attempts | Successful Picks | Success Rate | Average Required Vacuum (kPa) | Observed Damage Rate* |
|---|---|---|---|---|---|
| Flexible Profiling End Effector | 200 | 197 | 98.5% | -9.2 | 2.5% |
| Standard Silicone Cup (25mm) | 200 | 181 | 90.5% | -10.3 | 20.5% |
| Standard Silicone Cup (35mm) | 200 | 185 | 92.5% | -9.8 | 18.0% (est.) |
| Standard Silicone Cup (45mm) | 200 | 172 | 86.0% | -11.7 | 22.0% (est.) |
*Damage assessed as visible bruising or indentation 24 hours post-harvest. Manual harvesting had a ~1% damage rate.
The flexible profiling end effector achieved the highest success rate (98.5%) and the lowest damage rate (2.5%), comparable to careful manual harvesting. Failures were primarily due to severely misshapen or already damaged mushrooms where a seal could not be formed. The standard cups showed visible circular bruising (“suction rings”) on many mushrooms, caused by high localized pressure at the rigid cup’s rim, and had higher failure rates due to seal loss on irregular surfaces.
Conclusion
This work presented the complete design, analysis, and experimental validation of a novel flexible profiling end effector for automated Agaricus bisporus harvesting. The core innovation is the application of granular jamming within a latex membrane to create a suction gripper that passively conforms to the target’s geometry, resulting in a large, secure sealed contact area. FEA simulations guided the optimization of the opening diameter and confirmed favorable stress distribution. Systematic pull-off tests quantified the influence of key design parameters, leading to an optimal configuration.
The field trials conclusively demonstrated the superiority of this end effector over conventional vacuum cups. It achieved a 98.5% picking success rate and reduced physical damage to levels near manual picking (2.5%), while operating at lower vacuum pressures. This performance satisfies the critical requirements for gentle, reliable, and adaptive automation of mushroom harvesting. The proposed end effector design is compact, mechanically simple, and readily integrable with commercial robotic arms, offering a practical solution to a significant agricultural automation challenge. Future work will focus on further miniaturization for even denser clusters and integration with high-speed machine vision systems for fully autonomous selective harvesting.