Fabric Pneumatic Artificial Muscle-Based Head-Neck Exosuit: Design, Modeling, and Evaluation
Authors: Katalin Schäffer, Ian Bales, Haohan Zhang, Margaret McGuinness
Organizations: Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA. · Faculty of Information Technology and Bionics, Pázmány Péter Catholic University, 1083 Budapest, Hungary. · Department of Mechanical Engineering and Robotics Center, University of Utah, Salt Lake City, UT 84112 USA.
Wearable exosuits assist human movement in tasks ranging from rehabilitation to daily activities; specifically, head-neck support is necessary for patients with certain neurological disorders. Rigid-link exoskeletons have shown to enable head-neck mobility compared to static braces, but their bulkiness and restrictive structure inspire designs using "soft" actuation methods. In this paper, we propose a fabric pneumatic artificial muscle-based exosuit design for head-neck support. We describe the design of our prototype and physics-based model, enabling us to derive actuator pressures required to compensate for gravitational load. Our modeled range of motion and workspace analysis indicate that the limited actuator lengths impose slight limitations (83% workspace coverage), and gravity compensation imposes a more significant limitation (43% workspace coverage). We introduce compression force along the neck as a novel, potentially comfort-related metric. We further apply our model to compare the torque output of various actuator placement configurations, allowing us to select a design with stability in lateral deviation and high axial rotation torques. The model correctly predicts trends in measured data where wrapping the actuators around the neck is not a significant factor. Our test dummy and human user demonstration confirm that the exosuit can provide functional head support and trajectory tracking, underscoring the potential of artificial muscle-based soft actuation for head-neck mobility assistance.
Figures & tables
Fig. 1: Demonstration of our fabric pneumatic artificial muscle-actuated head-neck exosuit assisting flexion-extension movement and axial rotation movement. The exosuit is able to support the anatomically accurate head weight of the test dummy while moving the head along these two degrees of freedom. The exosuit can also assist with lateral deviation.
Name and Actuation
Active DoF
Structure and Compliance
Weight
fPAM-actuated exosuit (our design)
3
Seven fPAM actuators and separate base; design includes routing points and underactuation
1.2 kg (4.5 kg base)
Cable-driven exosuit [ 3 ]
3
Five cables connect between a shoulder vest and a head piece to apply a moment on the head; cables are remotely actuated using electric motors; compliance is from force control
1.5 kg
Rigid-link exoskeleton [ 5 ]
3
Three-legged revolute-revolute-spherical parallel rigid linkage mechanism optimized to mimic 3D human head-neck movements; actuated by three servomotors mounted on the shoulders; no built-in compliance
1.2 kg
Elastic neck exoskeleton [ 6 ]
2
Compliant rod mechanism supports the weight of the head; the physical compliance of the rod can be adjusted via motors; has a passive degree of freedom for axial rotation
2.5 kg
TABLE I: Summary of Wearable Robotic Head-Neck Device Designs
Fig. 2: Front and back view of the exosuit prototype, with key electrical and mechanical components highlighted. Insets to the front view show the neck joint and side tightening structure of the vest, structures that are hidden in the main pictures. (E1-3) Close-up pictures of the off-board base and the IMU sensors, the electrical components. (M1-3) Actuator head mounting, routing, and torso mounting points, respectively. (M4) Tube (originating from the base) connecting to the actuator.
Fig. 3: Tensile testing setup for characterizing the pressure input and force output relationship of the actuator. The test setup with the fPAM endpoint mounts is shown on the left. The plot on the right shows the measured force data with dashed lines and the modeled force (using Eqn. 1 ) with continuous lines. The colors correspond to various pressure levels ranging from 0 kPa to 103.4 kPa.
r0 [m]
α0
p0
p1
p2
p3
0.0136
37.0
12.3
-182.9
791.3
-1121.4
TABLE II: fPAM force parameters derived from tensile testing
Fig. 4: Modeled range of motion and workspace. (a) The principal rotation axes representing range of motion and the visual target workspace configurations in Euler angles (body-fixed x-y-z ). The colors illustrate whether the exosuit meets the defined evaluation conditions. (b) Illustration of the head orientations corresponding to the 2D visual target workspace, and the workspace plotted under each condition. (c) The workspace changes when enforcing various limits on the compression force ( Fc ).
FE RoM [ ∘ ] (and %)
LD RoM [ ∘ ] (and %)
AR RoM [ ∘ ] (and %)
Biological
-59.5,+73.7 (100%)
-40.9,+43.1 (100%)
-80.8/77.7 (100%)
Reachable
-48.7,+73.7 (92%)
-29.9,+30.4 (72%)
-74.4,74.5 (94%)
Gravity comp.
-51.4,+46.8 (74%)
-30.6,+31.2 (75%)
-31.2,31.3 (40%)
TABLE III: Modeled Range of Motion From Resting Configuration
Fig. 5: Modeled fPAM torques in (a) flexion-extension (FE), (b) lateral deviation (LD), and (c) axial rotation (AR) for various actuator configurations. The configurations are: (1) two front actuators, (2) one front actuator, (3) crossed back actuators anchored to the back of the vest, (4) upside-down “V” back actuators anchored to the back of the vest, and (5 and 6) the same placements as (3 and 4) but anchored to the front of the vest.
Fig. 6: Experimental setup for measuring the force applied by (a) the front actuators in lateral deviation (LD), and (b) the back actuators in axial rotation (AR).
1st (FE)
2nd (FE)
1st (LD)
2nd (LD)
3rd (AR)
4th (AR)
5th (AR)
6th (AR)
Torque Integr. [Nm]
630
649
1147
752
369
360
169
369
Angle Range [ ∘ ]
116
101
66
75
147
126
96
130
∥τ∥ at 0 ∘ [Nm]
9.7
9.7
19.2
12.3
3.9
1.9
0.0
4.0
Measured force [N]
-
-
22.6
17.7
18.0
5.6
23.2
8.9
TABLE IV: Model-Based and Experimental Design Evaluation of fPAMs in Various Configurations on the Right Side of the Head
Fig. 7: Results of trajectory tracking with a 20 ∘ amplitude and 25 s period sinusoidal reference signal. The tracking was performed along (a) flexion-extension and (b) axial rotation. The experiment was repeated with the exosuit placed on the test dummy (blue) and on a human user (red).
The authors are with the Institute of Mechanical Intelligence and Department of Excellence in Robotics & AI, Scuola Superiore Sant’Anna (SSSA), 56127 Pisa, Italy
Dept. of Mechanical Engineering, University of Utah, UT 84112, USA. · Robotics Center, University of Utah, UT 84112, USA. · Kahlert School of Computing, University of Utah, UT 84112, USA.