Enhancing the Performance of Pneu-net Actuators Using a Torsion Resistant Strain Limiting Layer

TL;DR

This work tackles the limited payload capacity of soft Pneu-net grippers caused by out-of-plane deformation. It introduces a passive, triangulated Torsion Resistant Layer (TRL) that increases torsional stiffness while preserving in-plane flexibility, enabling heavier and more reliable antipodal grasps. Through finite-element analysis and experimental validation, the TRL achieves up to about 98% reduction in out-of-plane bending, lifts up to 5 kg with a UR5 arm, and delivers substantial gains in peak grip force and grip stiffness. The TRL demonstrates robust improvements across a variety of non-box YCB objects, highlighting its potential to broaden the practical adoption of soft grippers in manipulation tasks.

Abstract

Pneunets are the primary form of soft robotic grippers. 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 (TRL). 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 for a Pneu-net gripper using simulation and experiment and map bending angle and relative grip strength. We found that the use of our TRL reduced out-of-plane bending by up to 97.7% in testing compared to a benchmark Pneu-net gripper from the Soft Robotics Toolkit. We demonstrate a lifting capacity of 5kg when loading using the TRL. We also see a relative improvement in peak grip force of 3N and stiffness of 1200N/m compared to 1N and 150N/m for a Pneu-net gripper without our TRL at equal pressures. Finally, we test the TRL gripper on a suite of six YCB objects above the demonstrated capability of a traditional Pneu-net gripper. We show success on all but one demonstrating significant increased capabilities.

Figures (10)

• Figure 1: We create a new Torsion Resistant Strain Limiting Layer (TRL) that can be added to any existing Pneu-net based gripper. It increases their resistance to torsion, allowing the gripper to lift larger payloads. Additionally, we can load the skeleton of the TRL directly, dramatically increasing lifting capacity. We show the TRL gripper lifting a 5kg dumbbell using the skeleton, the maximum payload capacity of the UR5 robot arm.
• Figure 2: We demonstrate the common failure modes for Pneu-net 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 are shown. 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 Pneu-net gripper is governed by the minimum of the set of these. Our gripper increases the torsional resistance and normal force applied compared to a standard Pneu-net gripper. This allows our gripper to lift larger payloads while having a better understanding of where the object is in our grasp.
• Figure 3: FEA results of (a) in-plane bending simulation, (b) torsion test and (c) torsional bending setup for simulated angle measurement. A moment M is applied along the longitudinal axis of the TRL. Line AB is deformed to line AC forming the angle, $\beta$, which is calculated using Eq. \ref{['eq:angle-calculation']}. The number of triangles was varied across a span of 100mm resulting in different triangle widths.
• Figure 4: Plot of In-plane displacement (a) and Angular displacement (b) as a function of number of triangles in the Strain Limiting Layer. In (a), we see in-plane deformations from an applied load of 0.01N and in (b), we see the angular displacement from a 5Nmm torque. Both plots follow a swoosh pattern with local maxima at the extreme ends of the range (two and thirty triangles respectively). We choose the 30 triangle solution for our TRL gripper since there is little difference in the angular displacement compared to the increase from adding the triangulated elements to the SLL. This choice reduces the stress concentration within the SLL, expanding lifetime of the gripper.
• Figure 5: This figure shows the casting process for a TRL gripper. First, (a), we print the 100mm long torsionally resistant Layer (TRL), then in (b), we cast the Pneu-net chambers out of Ecoflex 00-30 following instructions from the Soft Robotics Toolkit. In (c), we insert the TRL into the sacrificial mold and mix Ecoflex. After 25 minutes, we pour until 2mm above the TRL. In (d), we add the Pneu-net chambers and seal them with a thin layer of Ecoflex. Finally in (e), we remove the TRL gripper from the mold, place it spikes side down in a 1mm thick layer of Ecoflex in the Pneu-nets Strain Limiting Layer mold, cure it, and remove.