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Design and Characterization of a Dual-DOF Soft Shoulder Exosuit with Volume-Optimized Pneumatic Actuator

Rui Chen, Domenico Chiaradia, Daniele Leonardis, Antonio Frisoli

TL;DR

This work presents a volume-optimized dual-DOF soft shoulder exosuit that combines a curved abduction actuator (CAA) and a pouch-motor-based horizontal adduction actuator (HAA). It introduces the spindle-shaped angled actuator (SSAA) to reduce actuator volume by $35.7\%$ to $357\ \mathrm{mL}$ while maintaining $94.2\%$ of torque and achieving up to $35.2\%$ faster response, enabling portable operation. The exosuit demonstrates up to $59\%$ EMG reduction during abduction and up to $63.7\%$ during flexion in healthy users, with limited incremental gain from adding HAA during flexion, suggesting task-aware, selective activation may be optimal. Overall, the design provides a practical path toward home-based, multi-DOF shoulder assistance with quantified actuator contributions and guidance for future clinical validation.

Abstract

Portable pneumatic systems for 2 degree-of-freedom (DOF) soft shoulder exosuits remain underexplored, and face fundamental trade-offs between torque output and dynamic response that are further compounded by the need for multiple actuators to support complex shoulder movement. This work addresses these constraints through a volume-optimized spindle-shaped angled actuator (SSAA) geometry: by reducing actuator volume by 35.7% (357mL vs. 555mL), the SSAA maintains 94.2% of output torque while achieving 35.2% faster dynamic response compared to uniform cylindrical designs. Building on the SSAA, we develop a curved abduction actuator (CAA) based on the SSAA geometry and a horizontal adduction actuator (HAA) based on the pouch motor principle, integrating both into a dual-DOF textile-based shoulder exosuit (390 g). The exosuit delivers multi-modal assistance spanning shoulder abduction, flexion, and horizontal adduction, depending on the actuation. User studies with 10 healthy participants reveal that the exosuit substantially reduces electromyographic (EMG) activity across both shoulder abduction and flexion tasks. For abduction with HAA only, the exosuit achieved up to 59% muscle activity reduction across seven muscles. For flexion, both the single-actuator configuration (HAA only) and the dual-actuator configuration (HAA,+,CAA) reduced EMG activity by up to 63.7% compared to no assistance. However, the incremental benefit of adding the CAA to existing HAA support was limited in healthy users during flexion, with statistically significant additional reductions observed only in pectoralis major. These experimental findings characterize actuator contributions in healthy users and provide design guidance for multi-DOF exosuit systems.

Design and Characterization of a Dual-DOF Soft Shoulder Exosuit with Volume-Optimized Pneumatic Actuator

TL;DR

This work presents a volume-optimized dual-DOF soft shoulder exosuit that combines a curved abduction actuator (CAA) and a pouch-motor-based horizontal adduction actuator (HAA). It introduces the spindle-shaped angled actuator (SSAA) to reduce actuator volume by to while maintaining of torque and achieving up to faster response, enabling portable operation. The exosuit demonstrates up to EMG reduction during abduction and up to during flexion in healthy users, with limited incremental gain from adding HAA during flexion, suggesting task-aware, selective activation may be optimal. Overall, the design provides a practical path toward home-based, multi-DOF shoulder assistance with quantified actuator contributions and guidance for future clinical validation.

Abstract

Portable pneumatic systems for 2 degree-of-freedom (DOF) soft shoulder exosuits remain underexplored, and face fundamental trade-offs between torque output and dynamic response that are further compounded by the need for multiple actuators to support complex shoulder movement. This work addresses these constraints through a volume-optimized spindle-shaped angled actuator (SSAA) geometry: by reducing actuator volume by 35.7% (357mL vs. 555mL), the SSAA maintains 94.2% of output torque while achieving 35.2% faster dynamic response compared to uniform cylindrical designs. Building on the SSAA, we develop a curved abduction actuator (CAA) based on the SSAA geometry and a horizontal adduction actuator (HAA) based on the pouch motor principle, integrating both into a dual-DOF textile-based shoulder exosuit (390 g). The exosuit delivers multi-modal assistance spanning shoulder abduction, flexion, and horizontal adduction, depending on the actuation. User studies with 10 healthy participants reveal that the exosuit substantially reduces electromyographic (EMG) activity across both shoulder abduction and flexion tasks. For abduction with HAA only, the exosuit achieved up to 59% muscle activity reduction across seven muscles. For flexion, both the single-actuator configuration (HAA only) and the dual-actuator configuration (HAA,+,CAA) reduced EMG activity by up to 63.7% compared to no assistance. However, the incremental benefit of adding the CAA to existing HAA support was limited in healthy users during flexion, with statistically significant additional reductions observed only in pectoralis major. These experimental findings characterize actuator contributions in healthy users and provide design guidance for multi-DOF exosuit systems.
Paper Structure (17 sections, 7 equations, 8 figures, 2 tables)

This paper contains 17 sections, 7 equations, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Design and Characterization of the Shoulder Exosuit
  3. Exosuit Design
  4. Abduction Actuator Design
  5. Abduction Actuator Fabrication and Characterization
  6. Horizontal Adduction Actuator Fabrication and Characterization
  7. User Study
  8. Experimental Protocol and Setup
  9. Control Implementation and Actuation System
  10. Data Acquisition and Processing
  11. Use Study Results
  12. Discussion
  13. Spindle-Shaped Actuator Design Methodology
  14. System Integration and Characterization
  15. Functional Characterization in Healthy Users
  16. ...and 2 more sections

Figures (8)

  • Figure 1: Conceptual overview of the shoulder exosuit. (a) Shoulder abduction assistance. (b) Exosuit in deflated state. (c) Actuators of the exosuit. (d) Shoulder flexion assistance.
  • Figure 2: Comparison of uniform cylindrical angled actuator (UCAA) and spindle-shaped angled actuator (SSAA). (a) Illustration and dimensions of the two actuators. (b) Moment-angle relationship of the two actuators under different pressures. (c) Step response of the two actuators at 50 kPa and 90 kPa. (d) Rise time and (e) fall time comparison of the two actuators.
  • Figure 3: curved abduction actuator characterization. (a) Fabrication process of the curved abduction actuator. (b) Experimental setup for moment measurement. (c) Moment-angle relationship of the abduction actuator at various pressure levels. (d) Moment-pressure relationship of the abduction actuator. (e) Bode plot of the actuator at different pressure levels. (f) Force output stability over 2000 inflation cycles.
  • Figure 4: Fabric horizontal adduction actuator characterization. (a) Fabrication and dimensions of a pouch motor. (b) Sewing two pouch motors together to create the horizontal adduction actuator. (c) Parameters of the pouch motor. (d) Experimental setup for pouch motor characterization. (e) Parameters for pouch motor modeling. (f) Validation of the force--height model against experimental measurements at various pressure levels. (g) Geometric parameters for torque--angle modeling of the horizontal adduction actuator. (h) Comparison of modeled and experimentally derived torque as a function of horizontal adduction angle.
  • Figure 5: Subject testing protocol. (a) Setup for flexion experiments. (b) Communication between system and subjects. (c) Movement and actuator pressure conditions. (d) Procedure of the subject test.
  • ...and 3 more figures