Design and optimisation of load-adaptive actuator with variable stiffness for compact ankle exoskeleton

To overcome the limitations of linear series elastic actuators (SEAs) with constant stiffness, a novel load-adaptive actuator with variable stiffness is proposed for actuating ankle exoskeletons through an inverted slider-crank mechanism. Using the proposed actuator, the stiffness of the exoskeleton can be adjusted passively based on the external load and joint angle. To achieve compactness, a novel nonlinear spring mechanism with user-defined load–deflection behaviour is designed by combining a cam mechanism with parabolic beams; this proposed mechanism is used as the series elastic element of the proposed actuator. Dimensional optimisation of the ankle exoskeleton, including the actuator, is subsequently performed for maximising the energy efficiency based on gait biomechanics and considering the constraints of the inverted slider-crank mechanism, electric motor, and the nonlinear spring mechanism. Simulation and experimental results show that the energy efficiency is improved by using the proposed load-adaptive actuator-powered ankle exoskeleton compared with using an exoskeleton driven by linear SEAs.