Design and characterization of a variable-stiffness ankle-foot orthosis powered by twisted and coiled artificial muscles

An ankle foot orthosis (AFO) is an assistive device that is intended to aid in the gait of a user. There is a need for a research-grade, variable-stiffness AFO that can be used outside of a lab setting. The PowAFO is a variable-stiffness, lightweight active AFO that utilizes the high power to weight ratio and low input voltage requirements of twisted and coiled artificial muscles (TCAMs) to achieve this goal. The orthosis is constructed of a carbon fiber shell with a spring module imbedded at the ankle to apply tension to the TCAMs which run along the back of the lower leg and contract when a voltage is applied. The force generated and the ankle angle are measured and an ℒ₁ adaptive controller is used to control the voltage applied to TCAMs to generate the desired stiffness value. Torque tracking was conducted to prove the feasibility of this design and was determined that the device can match a desired torque at a given angle. Future work includes further testing and validation to prove the feasibility of the stiffness matching capabilities in both benchtop and human-subject experiments.