Cylindrical cam mechanism for unlimited subsequent spring recruitment in Series-Parallel Elastic Actuators

Series-Parallel Elastic Actuators (SPEA) enable variable recruitment of parallel springs and variable load cancellation. In previous work, we validated a MACCEPA-based SPEA prototype with a self-closing intermittent mechanism, to reduce motor load and improve energy efficiency. However, the mechanism only allowed for 4 parallel springs and a limited equilibrium angle range, which limits the variable load cancellation and operation range. Therefore, we developed a novel cylindrical cam mechanism for unlimited spring recruitment. This paper describes and validates the working principle of the cylindrical cam mechanism. Furthermore, the latest MACCEPA-based SPEA is presented with a maximum output torque of 40Nm and variable stiffness. Additive and traditional manufacturing techniques go hand in hand to overcome the actuator's complexity. The experiments endorse the working principle, demonstrate the variable stiffness, and prove the motor torque can be reduced to 5Nm while an output torque of 40Nm can be achieved.