An Ankle-Foot Orthosis with a Variable-Resistance Ankle Joint Using a Magnetorheological-Fluid Rotary Damper

This paper describes a prototype intelligent ankle-foot orthosis (iAFO) having an ankle joint with an adjustable viscous resistance torque. It was evaluated in a walking experiment involving a hemiplegic patient. The ankle joint was constructed with a magnetorheological (MR)-fluid damper having a changeable rotational viscosity. The iAFO consisted of an ankle joint, sensors for detecting walking, and a control box that comprised a circuit including Atmel AVR microprocessor and battery. Based on the difference in ankle joint function required in each stage of gait, walking was classified into three different phases in this study: the initial stance, mid-stance, and terminal stance/swing phases. A preliminary experiment was used to determine a method of detecting the shift from one phase to the next using the shank angle and foot contact information. The default resistance torque for each phase was configured. Using the prototype iAFO with the proposed control rules, a hemiplegic subject performed a walking experiment. While walking with the iAFO using suitable control rules, the ankle joint was maintained in dorsiflexion during the swing phase, and heel contact was achieved. In this respect, walking was better than unaided walking or walking using the iAFO with a resistance torque fixed at the highest level. The feasibility of the iAFO was confirmed.