Measurement and compensation of ankle stiffness using sensor-integrated pneumatic ankle foot orthosis

Abstract

Weight and power consumption of variable stiffness actuation mechanism are critical for its implementation in a portable assistive device. We present a portable stiffness compensation mechanism utilizing an air pump motor for an Ankle Foot Orthosis (AFO). Pneumatic transmissions have been utilized in many assistive devices owing to the high power-to-weight ratio and inherent compliance, whereas a required air compressors or a pressurized gas chamber was drawback of the pneumatic actuation for the use in ambulatory assistive device. Therefore, we utilized a combination of a portable electric power source and an air pump motor for portable driving unit. In order to enhance the late response of the air pump, an accumulator was implemented to the pneumatic circuit. The circuit consisted of two valves, a pressure sensor, and a low friction cylinder. The cylinder makes the device backdrivable without control effort. As a result, the stiffness was modulated during the gait, according to the gait phase. The sensor-integrated AFO equipped with two biaxial force sensors and angle sensor measured the exact ankle torque and angle to measure the ankle stiffness during walking and stair climbing. The control strategies were determined by comparing the measured stiffness with the reference stiffness. The effectiveness of the proposed stiffness-compensated AFO was validated by experiments with foot-drop patients, performing walking and stair climbing tasks.