Design of two lightweight, high-bandwidth torque-controlled ankle exoskeletons

Kirby Ann Witte,Rachel W Jackson,Juanjuan Zhang,Steven H Collins

doi:10.1109/icra.2015.7139347

Kirby Ann Witte, Rachel W Jackson + Show 2 more

Open Access

https://doi.org/10.1109/icra.2015.7139347

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Abstract

Lower-limb exoskeletons capable of comfortably applying high torques at high bandwidth can be used to probe the human neuromuscular system and assist gait. We designed and built two tethered ankle exoskeletons with strong lightweight frames, comfortable three-point contact with the leg, and series elastic elements for improved torque control. Both devices have low mass (< 0.88 kg), are modular, structurally compliant in selected directions, and instrumented to measure joint angle and torque. The exoskeletons are actuated by an off-board motor, and torque is controlled using a combination of proportional feedback and damping injection with iterative learning during walking tests. We tested closed-loop torque control by commanding 50 N·m and 20 N·m linear chirps in desired torque while the exoskeletons were worn by human users, and measured bandwidths greater than 16 Hz and 21 Hz, respectively. During walking trials, we demonstrated 120 N·m peak torque and 2.0 N·m RMS torque tracking error. These performance measures compare favorably with existing devices and with human ankle musculature, and show that these exoskeletons can be used to rapidly explore a wide range of control techniques and robotic assistance paradigms as elements of versatile, high-performance testbeds. Our results also provide insights into desirable properties of lower-limb exoskeleton hardware, which we expect to inform future designs.

Highlights

Lower-limb exoskeletons have the potential to aid in rehabilitation [1], assist walking for those with gait impairments [2], reduce the metabolic cost of normal [3] and load-bearing walking [4, 5], improve stability [6] and probe interesting questions about human locomotion [4]
The Alpha exoskeleton featured increased compliance in uncontrolled directions, inexpensive manufacturing, and lighter construction due to the use of leaf springs as both series elastic elements and lever arms. This design includes larger medial and posterior protrusions than the Beta device, which may result in less natural gait
The Beta device featured strain gauges which proved to be lighter, more accurate, and less expensive than the load cell used on the Alpha device

Summary

Introduction

Lower-limb exoskeletons have the potential to aid in rehabilitation [1], assist walking for those with gait impairments [2], reduce the metabolic cost of normal [3] and load-bearing walking [4, 5], improve stability [6] and probe interesting questions about human locomotion [4]. The challenges of designing effective lower-limb exoskeletons may be simplified by focusing on a single joint. The ankle joint may prove an effective location for application of assistance. Though the most effective mechanical method to assist the ankle remains unclear, the process of designing and testing our devices has produced several guiding principles for exoskeleton design.

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