Torsion Resistant Strain Limiting Layers Enable High Grip Strength of Electrically-Driven Handed Shearing Auxetic Grippers

TL;DR

This work addresses the limited payload handling of soft grippers by introducing a torsionally rigid strain limiting layer (TR-SLL) that constrains out-of-plane bending while preserving in-plane compliance. The TR-SLL is integrated with Handed Shearing Auxetic (HSA) actuators, enabling a single-HSA gripper to achieve pinch forces up to $5.8\,\text{N}$ and planar caging forces up to $14.5\,\text{N}$, and to lift up to $5\,\text{kg}$ on a UR5 arm. A design-space study using FEA identifies triangulated TR-SLL geometries (optimal near equilateral triangles, with a 5-triangle configuration) and confirms substantial gains in torsional stiffness $\kappa$ over flat SLLs, along with acceptable stress levels. The system demonstrates an $86\%$ success rate on 43 YCB objects, highlighting the practical impact of TR-SLL in enabling robust, payload-capable soft grippers with relatively simple fabrication and integration. These results suggest TR-SLLs can be broadly applied to enhance the performance of soft-robotic grippers in manipulation tasks demanding higher torque resistance and reliable grasp stability.

Abstract

Soft grippers have demonstrated a strong ability to successfully pick and manipulate many objects. A key limitation to their wider adoption is their inability to grasp larger payloads due to objects slipping out of grasps. We have overcome this limitation by introducing a torsionally rigid strain limiting layer (TR-SLL). This reduces out-of-plane bending while maintaining the gripper's softness and in-plane flexibility. We characterize the design space of the strain limiting layer and Handed Shearing Auxetic (HSA) actuators for a soft gripper using simulation and experiment. The inclusion of the TR-SLL with HSAs enables HSA grippers to be made with a single digit. We found that the use of our TR-SLL HSA gripper enabled pinch grasping of payloads over 1 kg. We demonstrate a lifting capacity of 5 kg when loading using the TR-SLL. We also demonstrate a peak pinch grasp force of 5.8 N, and a peak planar caging force of 14.5 N. Finally, we test the TR-SLL gripper on a suite of 43 YCB objects. We show success on 37 objects demonstrating significant capabilities.

Figures (11)

• Figure 1: We create a new Torsion Resistant Strain Limiting Layer (TR-SLL) that can be added to an existing soft gripper. It increases the gripper's resistance to torsion, allowing the gripper to lift larger payloads. The skeleton of the TR-SLL can be loaded, dramatically increasing lifting capacity. Here we create a Handed Shearing Auxetic (HSA)-based gripper using the TR-SLL and two quasi direct drive motors. In (a), We show the HSA-based TR-SLL gripper lifting a 1 kg mass using a planar caging grasp. In (b), we lift a 5 kg dumbbell using the skeleton from the TR-SLL, the maximum payload capacity of the UR5 robot arm.
• Figure 2: We demonstrate the common failure modes for soft robotic grasping. On the right we can see a simplified gripper grabbing a cylinder using an antipodal grasp. Only the Strain Limiting Layer and soft gripping material between the SLL and object is shown (not to scale). On the left, three common failure modes are shown; slipping due to low normal forces, twisting due to torsional deformation in the SLL, and shearing of the soft material. The maximum payload capacity for a this gripper is governed by the minimum of the set of these conditions. By understanding these limits, we can better inform gripper design. By including the TR-SLL, we dramatically increase our soft grippers resistance to twist. This allows it to lift larger payloads while having a better understanding of where the object is in our grasp.
• Figure 3: Torsional bending setup for simulated angle measurement. The example shown here is a five-triangle TR-SLL. A moment M is applied about the longitudinal axis of the TR-SLL. Line AB is deformed to line AC forming the angle $\beta$, which is then calculated using Eq. \ref{['eq:angle-calculation']}. The number of triangles was varied across a span of 100 mm resulting in different triangle widths.
• Figure 4: Plot of (a) torsional stiffness, (b) bending stiffness, and (c) average stress during bending and torsion as a function of number of triangles in the TR-SLL. In (a), we see torsional stiffness of the TR-SLL calculated from out-of-plane deformation due to an applied moment of 5 Nmm. The most torsionally resistant design is the one with five triagnels. In (b), we see the bending stiffness calculated from in-plane deformation due to an applied force of 0.01 N. The design that uses the least amount of input force to achieve the same displacement is the two triangle design. The average stress data in (c) provides insights on stress concentrations in the TR-SLL. All stress values are well below the yield of the material tested.
• Figure 5: This presents the linear interpolated performance heat maps of the short and long HSAs. We measure the mean force and torque as a function of extension and rotation. Negative values represent the HSA pushing on the environment, and positive values represent it pulling on the environment. In the heatmap we see non-linear behavior with a maximum negative force occurring around 0 mm extension and 45$^\circ$ for both HSAs. Values start to shrink beyond this as the HSA experiences larger deformations. Torque values follow a similar trend. Slices of the heatmaps are plotted for the maximum HSA rotation. This corresponds to the angle used when lifting objects. In the sliced 2D plots, we see the force (vs) extension, and the torque (vs) extension data for the long and short HSAs at 120$^\circ$ rotation.