Design and Evaluation of a Lightweight Soft Exosuit for Gait Assistance

Abstract

In this paper we present and evaluate the performance of a soft lower-extremity robotic exosuit with hip, knee, and ankle joint actuation intended to provide gait assistance. Compared to previous robotic exoskeletons, its lightweight and soft features enable the mechanical impedance and inertia of the system to be minimized. The exosuit has custom made McKibben style pneumatic actuators that attach to the wearer through a network of soft, inextensible webbing triangulated to attachment points utilizing a novel approach we call the virtual anchor technique. This approach is designed to transfer forces to locations on the body that can accept load rather than attach rigidly at the point of actuation. Pneumatic actuation was chosen because it provides lightweight, soft actuation with a power source that can be off-board or mounted in a backpack. The suit itself (human interface and actuators) had a mass of 3500 g and with peripherals (excluding air supply) is 7144 g. In order to examine the exosuit's performance, a pilot study with one subject was performed which investigated the effect of the ankle plantar-flexion timing on the wearer's hip, knee and ankle joint kinematics and metabolic power when walking. Our results indicate that the virtual anchor concept was successful in comfortably transmitting joint torques to the user while not restricting mobility. Wearing the suit in a passive unpowered mode had little effect on hip, knee and ankle joint kinematics as compared to baseline walking when not wearing the suit. Engaging the actuators at the ankles at 30% of the gait cycle for 250 ms altered joint kinematics the least and also minimized metabolic power. The subject's average metabolic power was 386.7 W, almost identical to the average power when wearing no suit (381.8 W), and substantially less than walking with the unpowered suit (430.6 W).