A resonant parallel elastic actuator for biorobotic applications

In cyclic motions, e.g. during locomotion, part of total kinetic energy, which otherwise would be wasted during direction inversion, can be conveniently stored in elastic elements in order to be later reused. Such energy recovery strategy, which can be adopted in robotic systems to improve energetic autonomy, mimics biological solutions, where muscles, tendons and ligaments are exploited also as elastic energy buffers. The addition of compliant elements, in parallel or in series to conventional actuators, may result in complex assemblies. Conversely, compact solutions can be obtained if elasticity is embedded in the structure of the actuator. In this paper an electric rotary compliant actuator, integrating a magnetic non-linear torsion spring, is presented. The system is a parallel elastic actuator and it allows to efficiently produce oscillating motions, as needed in several biorobotic applications. The stator comprises mono-phase windings while the rotor includes permanent magnets in axial Halbach configuration. Additional magnets are advantageously located in the stator to embody the intrinsic parallel elasticity. A proof-of-concept prototype was developed, whose winding coils were designed based on magneto-static FEM simulations. The actuator was experimentally characterized and the present dynamic behavior was compared to the theoretical one calculated by considering a non-linear electro-mechanical model.