Design of a Compact Rotary Series Elastic Actuator with Nonlinear Stiffness for Lower Limb Exoskeletons*

This paper presents a compact rotary Series Elastic Actuator (rSEA) with nonlinear stiffness for the drive of lower limb exoskeletons. The rSEA can change its stiffness from zero to a large value with respect to the external load. A sensitive force control around the zero-torque equilibrium and a good performance in the high external load can thus be expected. The rSEA is suitable for the application of lower limb exoskeletons where a light-weight actuator with high force range and high force fidelity is required. The working principle of this rSEA is elaborated. Output torque and stiffness modeling and analysis are carried out, upon which a closed-loop control law has been developed. Simulation and preliminary experimental results are included to validate the design features.