NREL-Exo: A 4-DoFs wearable hip exoskeleton for walking and balance assistance in locomotion

In this paper, we presented a high-power, self-balancing, passively and software-controlled active compliant, and wearable hip exoskeleton to provide walking and balance assistance. The device features powered hip abduction/adduction (HAA) and hip flexion/extension (HFE) modules to provide assistance in both sagittal and frontal planes. Each module's actuation unit employs a Series Elastic Actuator (SEA) to achieve passive compliance. The hip exoskeleton can work in two basic operation modes: human-in-charge and robot-in-charge. Both modes are integrated into the low-level controller based on the admittance control, making transitions smooth and stable. A new balance controller based on the “extrapolated center of mass” (XCoM) concept is presented for real-time control hip abduction/adduction to keep the center of mass (CoM) within the support polygon. The exoskeleton controller is designed to encourage participation in walking instead of overriding users' intrinsic behavior to achieve effective assistance and training. Our preliminary experiments on a healthy subject using the hip exoskeleton demonstrated the potential effectiveness of the device and controller in assisting locomotion.