On the Design and Control of Highly Backdrivable Lower-Limb Exoskeletons: A Discussion of Past and Ongoing Work

Lower-limb exoskeletons are external mechanical structures that support and assist human users during locomotion. The earliest studies on exoskeletons date back to the 1960s, whereas, over the previous decade, research on powered lower-limb exoskeletons has substantially expanded [1]. Exoskeletons with different architectures have been developed to achieve various goals. Typically, lower-limb exoskeletons can be classified into two broad categories based on their intended use: assisting people who have pathological gaits and augmenting able-bodied users. The first type of exoskeleton is designed to aid individuals with neurological conditions, for example, stroke or spinal cord injury (SCI). With the help of an exoskeleton, these people can complete different tasks that they cannot complete without assistance. For example, the bilateral hip-knee exoskeletons ReWalk [2] and Ekso Bionics [3] enforce predefined reference trajectories determined by a finite-state-machine structure to assist individuals with SCI. The bilateral Wandercraft exoskeleton adopts a hybrid dynamicsbased controller to stabilize dynamically feasible periodic gaits for users with SCI, while allowing them to actively control the exoskeleton speed through upper-body posture.