This paper presents a novel actuation solution to address the drop foot disorder. The proposed actuator consists of a superelastic Nitinol rod with a variable torsional stiffness that is adjusted by the controlled application of an axial load. The superelastic SMA element enables the AFO to provide sufficient torque during dorsiflexion to raise the foot. The provided torque at the ankle joint assists the patient in walking more naturally and subsequently prevents further issues such as muscle atrophy.
By appraising experimental data of the human gait, ankle stiffness is assessed in order to compare ankle behavior for various walking speeds during the swing phase. The adjustable compliance concept for the AFO is then explained, followed by a description of the actuation mechanism and complex loading configuration. Numerical modeling is also presented for the superelastic element of the AFO under specified multiaxial torsion-tension loading. Simulations are performed in MATLAB and variable stiffness results are compared with experimental data for verification.