Soft Wrist Exosuit Actuated by Fabric Pneumatic Artificial Muscles

TL;DR

This work tackles the challenge of providing safe, strong, and lightweight assistance for wrist motion using soft robotics. It introduces a planar 2D torque model for a four-fPAM wrist exosuit, couples it with a design-optimization workflow to place mounting points, and validates the approach through torque, ROM, and trajectory-tracking experiments. Key results show a peak torque of $3.3\,\mathrm{Nm}$ and improved torque prediction when accounting for endpoint stretching, though ROM is limited by fPAM contraction; planar and 2-DOF trajectory tracking demonstrate the device's potential for home rehabilitation with further control and hardware enhancements. The study demonstrates the practicality of fabric-based actuators for wearable exosuits and provides a pathway to personalized, low-cost assistive devices, while outlining necessary improvements in ROM, portability, and advanced control for real-world use.

Abstract

Recently, soft actuator-based exosuits have gained interest, due to their high strength-to-weight ratio, inherent safety, and low cost. We present a novel wrist exosuit actuated by fabric pneumatic artificial muscles that has lightweight wearable components (160 g) and can move the wrist in flexion/extension and ulnar/radial deviation. We derive a model representing the torque exerted by the exosuit and demonstrate the use of the model to choose an optimal design for an example user. We evaluate the accuracy of the model by measuring the exosuit torques throughout the full range of wrist flexion/extension. We show the importance of accounting for the displacement of the mounting points, as this helps to achieve the smallest mean absolute error (0.283 Nm) compared to other models. Furthermore, we present the measurement of the exosuit-actuated range of motion on a passive human wrist. Finally, we demonstrate the device controlling the passive human wrist to move to a desired orientation along a one and a two-degree-of-freedom trajectory. The evaluation results show that, compared to other pneumatically actuated wrist exosuits, the presented exosuit is lightweight and strong (with peak torque of 3.3 Nm) but has a limited range of motion.

Figures (14)

• Figure 1: Overview of our soft wrist exosuit. (a) Palmar view of the wrist exosuit with four fabric pneumatic artificial muscles (fPAMs) (blue) uninflated. (b-c) Palmar view showing that the fPAMs at the two sides of the hand promote radial (top) and ulnar (bottom) deviation. (d-e) Radial view showing that the fPAM at the dorsal side promotes wrist extension (top) and at the palmar side promotes wrist flexion (bottom).
• Figure 2: Prototype of the wrist exosuit. The wearable components include the glove (1) with restricting bands (2), the four fPAMs (3) with push-to-connect pneumatic fittings (4), the elbow band (5) and two IMUs (6). The off-board elements include the pressure regulators (7), the microcontroller (8), the signal conditioning circuit (9), the solenoid valve (10), and a safety switch (11).
• Figure 3: Various methods to attach the ends of the fPAM to the exosuit. The first row (a-b) shows the methods that were used for the final exosuit design and the second row (c-d) demonstrates the alternatives for attachment.
• Figure 4: 2D geometric models of the torque applied by an fPAM to the wrist. (a) When the actuator does not wrap around the wrist, the fPAM runs in a straight line (e.g., the configuration of the fPAM on the palmar side of the hand at wrist flexion). (b) When the wrist is rotated in the opposite direction, the actuator wraps around the wrist. Model parameters and introduced notations are indicated in the figure.
• Figure 5: Measurement setup for the peak biological wrist torque and the applied exosuit torque over wrist flexion/extension. (a) Side view of the torque measurement setup (without the human arm). The main components include the hand plate with adjustable angle (1), the torque sensor (2), the static structural elements (3,4), and additional support structures (5,6,7) to fix the arm. (b) Placement of the three motion capture markers used to measure the wrist angle.