The "Pac-Man'' Gripper: Tactile Sensing and Grasping through Thin-Shell Buckling

TL;DR

This work tackles the challenge of universal grasping with embedded tactile sensing by introducing the Pac-Man gripper, a soft, thin-shell hemispherical actuator that buckles into a two-lobe mode to encapsulate fragile or slippery objects using a single fluidic input. The authors integrate analytical modeling, finite-element simulations, and extensive experiments to show that the critical buckling pressure scales cubically with slenderness, $P_c=\frac{E}{4(1-\nu^2)}\left(\frac{h}{R}\right)^3 \csc^3{\theta}$, and that adding a thin equatorial film enables a controllable, enclosed fluid cavity suitable for dexterous grasping. They characterize a three-regime design space (destructive, constructive, film deformation) and demonstrate electronics-free tactile sensing by monitoring pressure-volume responses to detect contact, buckling, and release in complex environments, including confined openings. The Pac-Man gripper achieves stable grasping of diverse objects with a high payload-to-weight ratio (up to $11.70\pm1.45$) and is scalable across sizes, offering a promising path toward haptic soft robotics for medical, agricultural, space, and underwater applications.

Abstract

Soft and lightweight grippers have greatly enhanced the performance of robotic manipulators in handling complex objects with varying shape, texture, and stiffness. However, the combination of universal grasping with passive sensing capabilities still presents challenges. To overcome this limitation, we introduce a fluidic soft gripper, named the ``Pac-Man'' gripper, based on the buckling of soft, thin hemispherical shells. Leveraging a single fluidic pressure input, the soft gripper can encapsulate slippery and delicate objects while passively providing information on this physical interaction. Guided by analytical, numerical, and experimental tools, we explore the novel grasping principle of this mechanics-based soft gripper. First, we characterize the buckling behavior of a free hemisphere as a function of its geometric parameters. Inspired by the free hemisphere's two-lobe mode shape ideal for grasping purposes, we demonstrate that the gripper can perform dexterous manipulation and gentle gripping of fragile objects in confined environments. Last, we prove the soft gripper's embedded capability of detecting contact, grasping, and release conditions during the interaction with an unknown object. This simple buckling-based soft gripper opens new avenues for the design of adaptive gripper morphologies with applications ranging from medical and agricultural robotics to space and underwater exploration.

Figures (15)

• Figure 1: Bioinspiration and Function of the "Pac-Man" Gripper.A) Folding shape and feeding behavior of the predatory Benthic tunicate represented in a schematic gage_deep-sea_1999 and in images from The Blue Planet documentary fothergill_blue_2001. Analogy with the hemispherical gripper that mimics the shape and function of the deep sea tunicate to gently manipulate objects. B) The buckling-induced grasping mechanism of the hemispherical gripper based on the change in volume $\Delta \overline{V}=\Delta V/V_0$ of the fluidic cavity with initial volume $V_0$. C) Experimental snapshots of the hemispherical gripper grasping, protecting, and releasing wet hydrogel spheres.
• Figure 2: Buckling Instability in Free Thin-Shell Domes.A) 3D numerical model: underformed shape and first buckling mode of the free hemispherical shell with radius $R$, thickness $h$, cap angle $\theta$, and applied internal pressure $P$. B) Two-dimensional analytical model of the hemispherical shell based on the boundary edge of the spherical dome. C) and D) Analytical and numerical results for the normalized critical buckling pressure with respect to slenderness ratio $\bar{h }$ (C) and normalized cap angle $\theta/\sqrt{\bar{h}}$ (D). The squares represent the classical buckling pressure for a full sphere under uniform pressure zoelly_ueber_1915.
• Figure 3: Design Space and Experimental Validation.A) Schematic of the hemispherical soft pneumatic actuator. B) Evolution of the three behavioral regimes (i.e. 'destructive' buckling, 'constructive' buckling, and film deformation regime) as a function of the slenderness ratio and normalized film thickness $\bar{h}$ and $\bar{t}$, respectively. The stars indicate the geometrical parameters of the experimental samples. C), D) and E) Numerical and experimental comparison of the normalized pressure-volume curves and deformed shape for a design in Regime 1, Regime 2, and Regime 3, respectively.
• Figure 4: Experimental Grasping Trials.A) and B) Experimental snapshots and normalized pressure-volume curves of the "Pac-Man" gripper grasping an electronic chip and a strawberry, respectively. The dashed lines represent the response of the gripper without grasping an object. C) Experimental snapshots and normalized pressure-time response of the "Pac-Man" gripper operating through a constrained opening to manipulate a cherry tomato. (I) Contact, (II) Buckling, and (III) Release key points are labeled in all the experimental curves.
• Figure 5: FE analyses of free hemispheres. Undeformed and buckling mode shape of free hemispheres for different cap angles $\theta$.