Adaptive-twist Soft Finger Mechanism for Grasping by Wrapping

TL;DR

This work tackles the challenge of grasping densely packed objects with soft robotic fingers by enabling adaptive-twist deformation in both in-plane and out-of-plane directions to achieve wrapping grasping under a single actuation source. A variable stiffness mechanism interlocks a 2-DOF exoskeleton when the internal oil pressure rises, providing rigidity in tangential directions to preserve the twist during wrapping while remaining compliant otherwise. Finite element analysis guided the design, predicting a maximum moment capacity of $M_{ ext{max}} \approx 1.2$ Nm at $P=1.5$ MPa, with a required moment of $M_{ ext{required}} \approx 0.6$ Nm for grasping by wrapping; experimental validation confirmed these figures and demonstrated rapid hydraulic response with a time constant of $0.3$ s. The three-finger soft hand could grasp objects up to $3$ kg and successfully pick a cabbage from densely packed cabbages, illustrating practical applicability to agricultural tasks and cluttered object manipulation. Overall, the approach enables wrapping-based grasping with a compact, single-source actuation suitable for soft-hand manipulation in tightly packed environments.

Abstract

This paper presents a soft robot finger capable of adaptive-twist deformation to grasp objects by wrapping them. For a soft hand to grasp and pick-up one object from densely contained multiple objects, a soft finger requires the adaptive-twist deformation function in both in-plane and out-of-plane directions. The function allows the finger to be inserted deeply into a limited gap among objects. Once inserted, the soft finger requires appropriate control of grasping force normal to contact surface, thereby maintaining the twisted deformation. In this paper, we refer to this type of grasping as grasping by wrapping. To achieve these two functions by a single actuation source, we propose a variable stiffness mechanism that can adaptively change the stiffness as the pressure is higher. We conduct a finite element analysis (FEA) on the proposed mechanism and determine its design parameter based on the FEA result. Using the developed soft finger, we report basic experimental results and demonstrations on grasping various objects.

Figures (18)

• Figure 1: (a) A soft finger that only allows in-plane deformation. As shown, it is sometimes difficult to insert all fingers into unequal gap spaces among densely contained objects. (b) A soft finger that also allows out-of-plane directions can be easily inserted into gap spaces by adaptively-twisting deformation.
• Figure 2: Concept of grasping by wrapping and the developed soft finger.
• Figure 3: (a) Developed soft finger and (b) its structure. The finger consists of a rubber tube and an exoskeleton that converts internal pressure into desired finger movements, as shown in (c).
• Figure 4: 2-DOF joint unit in the exoskeleton, which enables adaptive-twist deformation in both in-plane and out-of-plane directions.
• Figure 5: Motion range of the soft finger in (a) in-plane and (b) out-of-plane directions.