A nonlinear series elastic actuator for highly dynamic motions

Abstract

A novel revolute nonlinear series elastic actuator called the HypoSEA is presented. The actuator uses a hypocycloid mechanism to stretch a linear spring in a nonlinear way. The actuator is optimized for highly dynamic tasks such as running and jumping, as it features a 120Nm torque capability and more than 30J of passive energy storage. When combined with a suitable controller, using the spring as an energy buffer can greatly reduce the work done by the rotor during periodic motions. The design has exceptionally low reflected mechanical impedance, making it robust against repeated impact loads. The nonlinear stiffening spring is optimized for the nonlinearities typically found in revolute-jointed hopping robots, and may be adjusted offline using a pretensioning mechanism. Finally, the low effective stiffness around the zero-torque equilibrium allows for extremely sensitive force control.