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Elastic actuation taps into elastic elements' energy storage for dynamicmotions beyond rigid actuation. While Series Elastic Actuators (SEA) andVariable Stiffness Actuators (VSA) are highly sophisticated, they do not fullyprovide control over energy transfer timing. To overcome this problem on thebasic system level, the Bi-Stiffness Actuation (BSA) concept was recentlyproposed. Theoretically, it allows for full link decoupling, whilesimultaneously being able to lock the spring in the drive train via aswitch-and-hold mechanism. Thus, the user would be in full control of thepotential energy storage and release timing. In this work, we introduce aninitial proof-of-concept of Bi-Stiffness-Actuation in the form of a 1-DoFphysical prototype, which is implemented using a modular testbed. We present ahybrid system model, as well as the mechatronic implementation of the actuator.We corroborate the feasibility of the concept by conducting a series ofhardware experiments using an open-loop control signal obtained by trajectoryoptimization. Here, we compare the performance of the prototype with acomparable SEA implementation. We show that BSA outperforms SEA 1) in terms ofmaximum velocity at low final times and 2) in terms of the movement strategyitself: The clutch mechanism allows the BSA to generate consistent launchsequences while the SEA has to rely on lengthy and possibly dangerousoscillatory swing-up motions. Furthermore, we demonstrate that providing fullcontrol authority over the energy transfer timing and link decoupling allowsthe user to synchronously release both elastic joint and gravitational energy.This facilitates the optimal exploitation of elastic and gravitationalpotentials in a synergistic manner.

Optimally Controlling the Timing of Energy Transfer in Elastic Joints: Experimental Validation of the Bi-Stiffness Actuation Concept