Achilles: An autonomous lightweight ankle exoskeleton to provide push-off power

This paper presents the Achilles exoskeleton, an autonomous ankle exoskeleton that can generate 52% of the positive plantarflexion power around the ankle of a 80 kg individual with only 1.5 kg of mass added around the ankle joint. The mass of the exoskeleton is lower and the power density is higher than that of existing autonomous exoskeletons. This high power density was achieved by designing a series elastic actuator that consists of an electric motor and ball-screw gear with a carbon fiber reinforced leaf-spring as lever-arm. A dynamic model that includes the motor and gear properties, spring stiffness, and exoskeleton geometry was used to optimize the design parameters for positive power injection. Doing this for multiple combinations of preselected motors and gears and comparing their support to weight ratio, revealed the best drive combination. The performance of the realized exoskeleton was assessed in several tests. The actuator can track the optimized actuator stroke trajectory with a following error that has a RMS of 2.3 mm, it can track force reference signals with amplitudes of 1 N to 100 N with a bandwidth between 8.1 Hz and 20.6 Hz, and it outputs a maximum mechanical power of 80.2 W. These results show that the device is suitable for fulfilling its purpose: reducing the metabolic cost of walking with an autonomous device.