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A Soft Wrist with Anisotropic and Selectable Stiffness for Robust Robot Learning in Contact-rich Manipulation

Steven Oh, Tomoya Takahashi, Cristian C. Beltran-Hernandez, Yuki Kuroda, Masashi Hamaya

TL;DR

A novel soft wrist mechanism that addresses limitations through a simple yet effective design using two orthogonal leaf springs and rotary joints with a locking mechanism, demonstrating its potential to enable robust robot learning in contact-rich domains.

Abstract

Contact-rich manipulation tasks in unstructured environments pose significant robustness challenges for robot learning, where unexpected collisions can cause damage and hinder policy acquisition. Existing soft end-effectors face fundamental limitations: they either provide a limited deformation range, lack directional stiffness control, or require complex actuation systems that compromise practicality. This study introduces CLAW (Compliant Leaf-spring Anisotropic soft Wrist), a novel soft wrist mechanism that addresses these limitations through a simple yet effective design using two orthogonal leaf springs and rotary joints with a locking mechanism. CLAW provides large 6-degree-of-freedom deformation (40mm lateral, 20mm vertical), anisotropic stiffness that is tunable across three distinct modes, while maintaining lightweight construction (330g) at low cost ($550). Experimental evaluations using imitation learning demonstrate that CLAW achieves 76% success rate in benchmark peg-insertion tasks, outperforming both the Fin Ray gripper (43%) and rigid gripper alternatives (36%). CLAW successfully handles diverse contact-rich scenarios, including precision assembly with tight tolerances and delicate object manipulation, demonstrating its potential to enable robust robot learning in contact-rich domains. Project page: https://project-page-manager.github.io/CLAW/

A Soft Wrist with Anisotropic and Selectable Stiffness for Robust Robot Learning in Contact-rich Manipulation

TL;DR

A novel soft wrist mechanism that addresses limitations through a simple yet effective design using two orthogonal leaf springs and rotary joints with a locking mechanism, demonstrating its potential to enable robust robot learning in contact-rich domains.

Abstract

Contact-rich manipulation tasks in unstructured environments pose significant robustness challenges for robot learning, where unexpected collisions can cause damage and hinder policy acquisition. Existing soft end-effectors face fundamental limitations: they either provide a limited deformation range, lack directional stiffness control, or require complex actuation systems that compromise practicality. This study introduces CLAW (Compliant Leaf-spring Anisotropic soft Wrist), a novel soft wrist mechanism that addresses these limitations through a simple yet effective design using two orthogonal leaf springs and rotary joints with a locking mechanism. CLAW provides large 6-degree-of-freedom deformation (40mm lateral, 20mm vertical), anisotropic stiffness that is tunable across three distinct modes, while maintaining lightweight construction (330g) at low cost ($550). Experimental evaluations using imitation learning demonstrate that CLAW achieves 76% success rate in benchmark peg-insertion tasks, outperforming both the Fin Ray gripper (43%) and rigid gripper alternatives (36%). CLAW successfully handles diverse contact-rich scenarios, including precision assembly with tight tolerances and delicate object manipulation, demonstrating its potential to enable robust robot learning in contact-rich domains. Project page: https://project-page-manager.github.io/CLAW/
Paper Structure (23 sections, 3 equations, 9 figures, 2 tables)

This paper contains 23 sections, 3 equations, 9 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Large and multi-directional deformation
  3. Anisotropic and adjustable stiffness
  4. Lightweight and durable construction
  5. Low cost and simple implementation
  6. Related Work
  7. CLAW DESIGN CONCEPT
  8. Large and multi-directional deformation
  9. Anisotropic and adjustable stiffness
  10. Lightweight construction
  11. Low cost and simple implementation
  12. Deformation Principle of Compliant Module
  13. Deformation analysis of Compliant Module
  14. Convex-tape
  15. Lock Mechanism
  16. ...and 8 more sections

Figures (9)

  • Figure 1: The CLAW soft wrist: (a) A novel soft wrist featuring two orthogonal leaf springs to achieve anisotropic stiffness and enabling 6-DoF compliance. (b) CLAW’s extensive evaluation across a range of challenging contact-rich manipulation tasks using imitation learning.
  • Figure 2: Mechanical design of the CLAW: Orthogonal leaf springs and rotary joints provide 6-DoF compliance with anisotropic stiffness. A simple locking mechanism enables mode-switchable stiffness while preserving low weight and structural simplicity.
  • Figure 3: Deformation characteristics of the CLAW compliant module: Two orthogonal looped leaf springs with rotary joints and coil spring preload enable 6-DoF anisotropic compliance.
  • Figure 4: Length and range of motion of the leaf spring: (a) Fixed section of the leaf spring and dimensions of each part, (b) Maximum deformation state in the XY direction.
  • Figure 5: Locking mechanism of the CLAW: A single actuator moves four pins with different profiles on a carrier to engage grooves in joint links. This system enables three stiffness modes: (a) Free mode with all joints rotatable, (b) Half-lock mode restricting motion along the X-axis by locking two joints, and (c) Full-lock mode locking all joints to increase stiffness in X, Y, and yaw while retaining compliance in other directions.
  • ...and 4 more figures