Hybrid Robotic Meta-gripper for Tomato Harvesting: Analysis of Auxetic Structures with Lattice Orientation Variations

TL;DR

Delicate tomato harvesting requires gentle yet reliable grasping to minimize bruising and post-harvest losses. The authors design a hybrid soft-rigid gripper with six internal auxetic lattice fingers mounted on rigid links and conduct a systematic orientation study across $0^{\circ}$, $30^{\circ}$, $45^{\circ}$, and $60^{\circ}$ using 2D Digital Image Correlation, nonlinear finite element analysis, and force/torque measurements. The work provides orientation-dependent performance benchmarks, showing that $0^{\circ}$ offers the best conformability and gentle contact while $45^{\circ}$ optimizes curvature matching and cage-ability, and $60^{\circ}$ enables stable, energy-efficient grasping; these insights are integrated into a force-torque framework for motor sizing. Overall, the study demonstrates how metamaterial geometry can tailor gripper performance for precision agriculture, enabling automated, damage-conscious harvesting with adaptable soft-rigid architectures.

Abstract

The agricultural sector is rapidly evolving to meet growing global food demands, yet tasks like fruit and vegetable handling remain labor-intensive, causing inefficiencies and post-harvest losses. Automation, particularly selective harvesting, offers a viable solution, with soft robotics emerging as a key enabler. This study introduces a novel hybrid gripper for tomato harvesting, incorporating a rigid outer frame with a soft auxetic internal lattice. The six-finger, 3D caging-effect design enables gentle yet secure grasping in unstructured environments. Uniquely, the work investigates the effect of auxetic lattice orientation on grasping conformability, combining experimental validation with 2D Digital Image Correlation (DIC) and nonlinear finite element analysis (FEA). Auxetic configurations with unit cell inclinations of 0 deg, 30 deg, 45 deg, and 60 deg are evaluated, and their grasping forces, deformation responses, and motor torque requirements are systematically compared. Results demonstrate that lattice orientation strongly influences compliance, contact forces, and energy efficiency, with distinct advantages across configurations. This comparative framework highlights the novelty of tailoring auxetic geometries to optimize robotic gripper performance. The findings provide new insights into soft-rigid hybrid gripper design, advancing automation strategies for precision agriculture while minimizing crop damage.

Figures (17)

• Figure 1: CAD model of the proposed gripper with various parts: the outer rigid link mechanism attached with soft auxetic structures. The linkage drive is shown on the left of the figure, while the tomato separator mechanism is shown on the right side of the figure.
• Figure 2: Two-dimensional view of auxetic structure interaction with the tomato to have a high shape conformability when the rigid link mechanism of the gripper actuates (Fig. 2(a): open position and Fig. 2(b): closed position)
• Figure 3: Experimental setup inclusive all mechatronics components used for analyzing auxetic structure samples. The left-hand side shows the various components of the setup designed to measure interaction forces, while the right-hand side depicts the setup in action. This setup facilitates the collection of multimodal information for a comprehensive analysis.
• Figure 4: The figure illustrates the complete setup and schematic diagram used for the digital image correlation process in the experimental deformation analysis of auxetic structure samples.
• Figure 5: The figure shows the various parts of the experimental setup, which are as follows: The left-hand side of the figure shows the strain gauges mounted on the 3D printed structure, the middle of the figure shows the strain measurement system, while the right-hand side of the figure shows the speckle pattern created through paint spray on the surface of the auxetic structure to improve the accuracy of the image correlation