Series-Parallel Elastic Actuation (SPEA) for Reduced Torque Requirements

Traditional stiff actuators have a high reflected inertia, which do not meet the safety requirements, are bad to absorb shocks and cannot store and release energy which are for different applications required. Initiated by the introduction of the Series Elastic Actuator (SEA) by Gill A. Pratt in 1995, a wide range of Variable Impedance Actuators (VIA) are developed, for which inspiration was found in the elastic properties of biological muscle. These compliant VIA’s benefit from the inherent series elasticity since the series elastic element can store and release energy for energy efficiency and decouple the inertia from one link to another. As a result, VIA’s significantly extended the possibilities of traditional stiff actuation. However, current actuators do not yet reach the torque and power requirements for many novel applications like exoskeletons, humanoids, agile locomotion, prostheses and manipulators. This abstract reports on the study of a novel compliant actuator: the Series-Parallel Elastic Actuator (SPEA). The difference with previous studies on parallel elastic elements in joint actuation is that the parallel springs in the SPEA are actively controlled and not passively used. Main inspiration source is again the biological muscle that consists of a large set of parallel and series muscle fibers. The difference is that not all parallel springs in the SPEA need to have their own force converter (mostly DC motor), but are tensioned by an intermittent mechanism actuated by a single motor. This abstract discusses the design and testing of a first Proof Of Concept prototype.