Force Profiling of a Shoulder Bidirectional Fabric-based Pneumatic Actuator for a Pediatric Exosuit

TL;DR

This study experimentally profiles the contact forces of a single-cell fabric-based pneumatic shoulder actuator on an infant-scale exosuit rig, varying anchoring points and a fixed elbow pose. Using load cells and an encoder, it reveals that anchoring at $\frac{2}{3}$ of the upper arm length with a $90^{\circ}$ elbow yields the largest shoulder ROM while minimizing torso and upper-arm loads, though the system exhibits nonlinear, hysteretic force responses due to actuator unfolding. Comparisons with adult devices indicate substantially lower peak forces, highlighting safety considerations for pediatric applications. The findings offer actionable guidance on anchor placement and joint configurations and point to design improvements to achieve smoother, safer force transmission in infant exosuits. Overall, the work advances understanding of force profiles in pediatric soft actuators and informs practical guidelines for device safety and effectiveness.

Abstract

This paper presents a comprehensive analysis of the contact force profile of a single-cell bidirectional soft pneumatic actuator, specifically designed to aid in the abduction and adduction of the shoulder for pediatric exosuits. The actuator was embedded in an infant-scale test rig featuring two degrees of freedom: an actuated revolute joint supporting shoulder abduction/adduction and a passive (but lockable) revolute joint supporting elbow flexion/extension. Integrated load cells and an encoder within the rig were used to measure the force applied by the actuator and the shoulder joint angle, respectively. The actuator's performance was evaluated under various anchoring points and elbow joint angles. Experimental results demonstrate that optimal performance, characterized by maximum range of motion and minimal force applied on the torso and upper arm, can be achieved when the actuator is anchored at two-thirds the length of the upper arm, with the elbow joint positioned at a 90-degree angle. The force versus pressure and joint angle graphs reveal nonlinear and hysteresis behaviors. The findings of this study yield insights about optimal anchoring points and elbow angles to minimize exerted forces without reducing the range of motion.

Figures (5)

• Figure 1: Experimental setup employed in this work for force profiling of a fabric-based pneumatic shoulder actuator when the actuator anchoring points and elbow (locked in place) angle vary.
• Figure 2: Changes in shoulder joint angle over time where shaded area represents standard deviation. The panels at left and right represent anchoring at $\frac{2}{3}$ and $\frac{1}{2}$ the length of the upper arm, respectively.
• Figure 3: Forces applied by the actuator on the torso (Top) and upper arm (UA) (Bottom) for the different experimental conditions listed in Table \ref{['tab:conditions']}. Shaded zones reflect one standard deviation (30 trials for each condition).
• Figure 4: Actuator force changes on the torso (Top) and upper arm (Bottom) as the internal pressure of the actuator varies.
• Figure 5: Actuator force changes on the torso (Top) and upper arm (Bottom) as the shoulder joint angle increases.