Lower Limb Exoskeleton Systems—Overview

Abstract Lower body wearable robots, or lower limb exoskeletons, have developed rapidly in the past decade. These devices can be separated into three different categories: assistive exoskeletons, rehabilitation exoskeletons, and augmentation exoskeletons. Exoskeletons from the first two categories are designed for use on people with neurologic conditions or the elderly. Assistive exoskeletons mainly focus on users that have permanently lost their mobility. These exoskeletons assist/replace the impaired parts of the users, allowing users to live/behave like healthy people while wearing the devices. For this type of exoskeleton, precise position control with electrical motors is the most common combination. Alternatively, rehabilitation exoskeletons are intended for use by patients with disabilities that can be recovered through therapy and training. The goal of these systems is to create interactions with the users to rehabilitate them so that they may eventually function normally without the device. This type of exoskeleton often uses impedance control to create resisting forces for the training. The third category, augmentation exoskeletons, mainly aids healthy users. The goal of these exoskeletons is to enable performance of tasks that an able-bodied individual normally would be incapable of, or to reduce the effort required for tasks (less metabolic cost). Positive feedback or force control are often seen in this category since the device needs to follow the user’s motion. Actuators with higher power density are also more common (hydraulic, pneumatic, serial elastic actuator). Many successful exoskeletons can be found in the literature, yet the overall weight, stability, and users’ intention estimation are still under development. Exoskeletons that are light, stable, and can seamlessly cooperate with users will be the direction of future exoskeletons.