Torque Responsive Metamaterials Enable High Payload Soft Robot Arms

TL;DR

Soft robot arms often cannot simultaneously support large payloads and their own weight against gravity. This work introduces a four-layer metamaterial soft arm built from Handed Shearing Auxetics (HSA) and Bendable Extendable Torque resistant (BETR) shafts to enable gravity-resilient, high-torque operation. The authors characterize HSAs, evaluate BETRs for torque transmission, fabricate and assemble the arm, and validate performance with vertical lifting up to 2.3 kg, horizontal payloads around 0.6 kg, active grasping, and pipe-inspection tasks. The results show a repeatable, compliant arm capable of operating in non-hanging configurations, with clear potential for manipulation and inspection in constrained environments.

Abstract

Soft robots have struggled to support large forces and moments while also supporting their own weight against gravity. This limits their ability to reach certain configurations necessary for tasks such as inspection and pushing objects up. We have overcome this limitation by creating an electrically driven metamaterial soft arm using handed shearing auxetics (HSA) and bendable extendable torque resistant (BETR) shafts. These use the large force and torque capacity of HSAs and the nestable torque transmission of BETRs to create a strong soft arm. We found that the HSA arm was able to push 2.3 kg vertically and lift more than 600 g when positioned horizontally, supporting 0.33 Nm of torque at the base. The arm is able to move between waypoints while carrying the large payload and demonstrates consistent movement with path variance below 5 mm. The HSA arm's ability to perform active grasping with HSA grippers was also demonstrated, requiring 20 N of pull force to dislodge the object. Finally, we test the arm in a pipe inspection task. The arm is able to locate all the defects while sliding against the inner surface of the pipe, demonstrating its compliance.

Figures (8)

• Figure 1: We present a soft Handed Shearing Auxetics (HSA) robot arm that is able to support meaningful forces and torques while holding itself up against gravity. Subfigure (a) shows the assembled soft HSA arm, (b) shows the soft HSA arm lifting a 2.3 kg payload vertically, (c) shows the inspection task setup with the mounted webcam, and (d) shows the different pipe defects (holes, gash, scratches from top left) captured by the HSA arm during the inspection task.
• Figure 2: A System overview is presented for the HSA Arm. The arm is made from four layers, with each driven by a different HSA. Proximal layers use HSAs printed further along the auxetic trajectory culminating in a thinner distal HSA. HSAs on level two through four are driven by nested Bendable Extendable Torsionally Rigid (BETR) shafts. A rendered version of the arm is shown on the left and the realized construction can be seen on the right.
• Figure 3: Interpolated HSA force and torque characterization is presented here for each of the four layers of HSA in the soft arm. Thick- and thin-walled HSAs printed at 0% of way along the auxetic trajectory are presented with just expansion data (layer 3, 4 respectively) and HSAs printed beyond that (layer 1, 2) are shown with both expansion and compression regions.
• Figure 4: Plot of torsion and extension tests for the soft soft torque transmission elements in the robot arm. We characterize elements for layer 2 (Small: Short), layer 3 (Large BETR), and layer 4 (Small: Long). We also characterize the single non-extendable flex shaft for complete individual component level analysis. Torque characterization was done when in the extended (dashed) and rest states (solid). The X intercept represents the slack/deadband in the material and the slope represents the torsional stiffness. Forces required to extend the elements in the arm are also shown in the extension test plot. These values oppose HSA extension.
• Figure 5: Here we present the maximum load supported when the arm is positioned horizontally (b). First the arm's default resting position is recorded with a laser level. Then the arm is moved up to its maximum extension. Weights are added to the distal tip until the arm returns to its starting position. The arm is able to support 600 g when horizontally extended, resulting in 0.33 Nm of resisted torque. Subfigures (c) and (d) show the HSA arm before and after lifting a 603-g mustard bottle from the YCB object dataset.