SMAFO: Stiffness modulated Ankle Foot Orthosis for a patient with foot drop

Weight and power consumption of variable stiffness actuation mechanism are critical for its implementation in a portable assistive device combined with an actuator, gearhead, elastic element, and batteries. Therefore, we present a portable stiffness modulation mechanism for an Ankle Foot Orthosis (AFO) utilizing pneumatic stiffness. The mechanism consists of two units: a driving unit that controls an air pump and an AFO unit that performs free motion and stiffness modulation functions. The actuation from the driving unit to the AFO unit via pneumatic hose makes the effective weight of the device light compared to the direct driven device at the ankle joint. We derived a mathematical model of the stiffness modulation to determine the pressure required. In order to control the required stiffness according to the gait phases, a threshold-based gait phase detection algorithm was implemented, employing ground reaction force sensor signals from heel and forefoot. The effectiveness of the proposed AFO was validated by the experiments in a patient with foot drop, performing walking with and without AFO actuation.