SoftHand Model-W: A 3D-Printed, Anthropomorphic, Underactuated Robot Hand with Integrated Wrist and Carpal Tunnel

Abstract

This paper presents the SoftHand Model-W: a 3D-printed, underactuated, anthropomorphic robot hand based on the Pisa/IIT SoftHand, with an integrated antagonistic tendon mechanism and 2 degree-of-freedom tendon-driven wrist. These four degrees-of-acuation provide active flexion and extension to the five fingers, and active flexion/extension and radial/ulnar deviation of the palm through the wrist, while preserving the synergistic and self-adaptive features of such SoftHands. A carpal tunnel-inspired tendon routing allows remote motor placement in the forearm, reducing distal inertia and maintaining a compact form factor. The SoftHand-W is mounted on a 6-axis robot arm and tested with two reorientation tasks requiring coordination between the hand and arm's pose: cube stacking and in-plane disc rotation. Results comparing task time, arm joint travel, and configuration changes with and without wrist actuation show that adding the wrist reduces compensatory and reconfiguration movements of the arm for a quicker task-completion time. Moreover, the wrist enables pick-and-place operations that would be impossible otherwise. Overall, the SoftHand Model-W demonstrates how proximal degrees of freedom are key to achieving versatile, human-like manipulation in real world robotic applications, with a compact design enabling deployment in research and assistive settings.

Figures (10)

• Figure 3: Range of motion of the SoftHand Model-W in the ulnar (A) and radial (B) deviations, and in flexion (C) and extension (D). Corresponding degrees of freedom of the human wrist (E) are shown below (based on huang2017wearable).
• Figure 4: Overview of the SoftHand-W. A) Technical drawing showing the palmer side with finger, palm, and write dimensions in mm. B) CAD model of a finger (palmer side up), with flexor (red) and extensor (blue) tendons.
• Figure 5: Wrist mechanism and tendon routing. A) Tendon routing through the palm and into the wrist (routing on the back of the hand is similar). B) CAD model of the 2-DoF wrist mechanism, showing the internal bearings and axles as well as palm and forearm connectors.
• Figure 6: CAD model of the 'forearm' - an actuation box containing four Feetech STS3215 servos and tendon spools, which enable finger flexion, finger extension, radial/ulnar deviation, and flexion/extension of the wrist. Mounting for the UR5 arm is achieved via a tool flange adapter at the base.
• Figure 7: Experimental stimuli