Stretchable Pneumatic Sleeve for Adaptable, Low-Displacement Anchoring in Exosuits

TL;DR

The paper addresses the challenge of low-displacement anchoring for upper-limb exosuits by introducing a stretchable sleeve built from fabric pneumatic artificial muscle (fPAM) bands. It systematically compares nine fPAM bands, an SPM band, and three sleeves (hook-and-loop, SPM, and fPAM-based) in terms of compressing/holding forces and mounting-point stiffness, and validates performance on an fPAM-actuated exosuit. Key findings show that wider and shorter resting-band geometries increase force, that the fully stretched fPAM band can match the SPM sleeve’s performance, and that inflatable sleeves reduce mounting-point displacement relative to the hook-and-loop sleeve, with the fPAM sleeve tolerating higher pressures before failure. The results suggest that the fPAM sleeve offers a simple, low-cost, and adaptable anchoring solution with potential advantages in comfort, durability, and ease of donning for upper-limb exosuits, while noting safety and comfort considerations for future work.

Abstract

Despite recent advances in wearable technology, interfacing movement assistance devices with the human body remains challenging. We present a stretchable pneumatic sleeve that can anchor an exosuit actuator to the human arm with a low displacement of the actuator's mounting point relative to the body during operation. Our sleeve has the potential to serve as an adaptable attachment mechanism for exosuits, since it can adjust its pressure to only compress the arm as much as needed to transmit the applied exosuit forces without a large displacement. We discuss the design of our sleeve, which is made of fabric pneumatic artificial muscle (fPAM) actuators formed into bands. We quantify the performance of nine fPAM bands of various lengths and widths, as well as three sleeves (an fPAM sleeve, a series pouch motor (SPM) sleeve as in previous literature, and an off the shelf hook and loop sleeve), through the measurement of the compressing force as a function of pressure and the localized pulling force that can be resisted as a function of both pressure and mounting point displacement. Our experimental results show that fPAM bands with smaller resting length and/or larger resting width produce higher forces. Also, when inflated, an fPAM sleeve that has equivalent dimensions to the SPM sleeve while fully stretched has similar performance to the SPM sleeve. While inflated, both pneumatic sleeves decrease the mounting point displacement compared to the hook and loop sleeve. Compared to the SPM sleeve, the fPAM sleeve is able to hold larger internal pressure before bursting, increasing its possible force range. Also, when not inflated, the fPAM sleeve resists the pulling force well, indicating its ability to provide anchoring when not actuated.

Figures (10)

• Figure 1: Our stretchable pneumatic sleeve anchored to the upper limb. (a) When integrated in an exosuit designed to move the human wrist, our pneumatic sleeve provides low-displacement anchoring for the exosuit's linear actuator on the upper arm. (b) Our pneumatic sleeve can adapt its compressing force by changing its internal pressure to only squeeze as hard as is needed to resist a desired pulling force.
• Figure 2: Schematic of the states of the fabric pneumatic artificial muscle (fPAM) band that makes up our sleeve. (a) In the fully stretched state, the fPAM band is uninflated, and the band fabric is stretched to reach its maximum length $l_0$ and minimum width $w_0$. (b) In the resting state, the fPAM band is uninflated and unstretched, with length $l_{rest.}$ and width $w_{rest.}$. These resting parameters are used to describe the width and length of the fabricated bands. (c) In the inflated state, the fPAM band is inflated, and its length $l(\epsilon)$ and width $w(\epsilon)$) are functions of the contraction ratio $\epsilon$.
• Figure 3: The series pouch motor (SPM) band (top) and the two equivalent fPAM bands tested. The fPAM(r) band (middle) has the same length and width as the SPM band when resting, and the fPAM(s) band (bottom) has the same dimensions when it is fully stretched as shown. All the bands have a push-to-connect pneumatic fitting to allow inflation and have an overlapping area of 4 cm where the ends of the band are sewn together. In the middle of the overlapping area, a thread is attached for the pulling force and holding force measurement.
• Figure 4: The three sleeves tested, all placed on a cylinder of 7.3 cm diameter: (a) the off the shelf hook and loop sleeve, (b) the SPM sleeve, and (c) the fPAM-based sleeve designed to be equivalent to the SPM sleeve when fully stretched.
• Figure 5: Measurement setup for compressing force, holding force and mounting point displacement measurement. The setup consists of a test cylinder around which the tested band or sleeve is wrapped, as well as an elevated platform for positioning the external force sensor, which can measure the pulling force or holding force ($F_h$) at the mounting point. A closed-loop pressure regulator maintains the desired pressure in the band or sleeve. The displacement of the mounting point is measured by a motion capture marker (top left corner), and the compressing force applied by the band ($F_c$) is measured by the force sensor in the center of the cylinder (bottom right corner). The interlocking elements at the ends of the cylinder can be added or removed depending on which force measurement is currently taking place.