Design of a Novel Compact Adaptive Ankle Exoskeleton for Walking Assistance

Series elastic actuators (SEAs) are commonly used in ankle exoskeletons for friendly human-robot interaction and high power efficiency. However, most ankle exoskeletons face a common performance limitation due to the use of fixed stiffness series springs. In this paper, we present an adaptive ankle exoskeleton for walking assistance. A novel compact variable stiffness SEA with a non-linear spring, which is able to passively change the spring stiffness as a function of output load, is developed to overcome the limitation of the conventional SEAs. The predefined nonlinear elasticity of the proposed passively variable stiffness SEA (pVS-SEA) is achieved with a cam mechanism and leaf springs, which result in a compact design. Furthermore, a variable transmission mechanism is adopted to modulate the physical exoskeleton stiffness as a function of ankle joint angle. The exoskeleton mechanism is optimized based on the human gait data by employing a genetic algorithm. The results show that the presented ankle exoskeleton is adaptable under different walking conditions, and the energy efficiency of the system is improved compared with the conventional ones.