A Lower-Body Soft Exosuit with a Single Actuator: Design, Impedance Adaptation, and Control

Soft cable-driven lower-body exosuits offer flexibility and low mass compared to rigid exosuits. Mass is further reduced in ankle-only exosuits, taking advantage of the fact that ankle plantar flexion contributes approximately half of positive mechanical power during normal human walking. In this paper we report two contributions to soft lower-body ankle-only exosuits. First, we describe the design of an exosuit with a single actuator that is time-shared between the two ankles. The elimination of an actuator lowers the total mass and cost of the exosuit by taking advantage of the time-separation between periods when each ankle contributes positive power during walking. Second, we describe a novel bio-inspired control strategy that adapts the impedance at the assisted ankle in response to unknown terrain, mimicking natural human ankle variable impedance, while simultaneously learning the dynamic parameters of the exosuit. Experiments show rapid adaptation and convergence to stable impedance and dynamic parameters while subjects walk on different types of terrain. Moreover, subjects’ muscular effort, as measured by EMG sensors, is reduced relative to the cases of no exosuit or an unpowered exosuit.