BSA -- Bi-Stiffness Actuation for optimally exploiting intrinsic compliance and inertial coupling effects in elastic joint robots

TL;DR

This paper introduces Bi-Stiffness Actuation, a novel approach that enables precise control of elastic energy transfer in elastic joint robots by decoupling joint stiffness, improving dynamic performance and timing in energy release.

Contribution

The paper proposes a new Bi-Stiffness Actuation mechanism that allows full decoupling of joint stiffness, enabling optimal energy transfer and timing control in elastic joint robots.

Findings

Bi-Stiffness Actuation achieves performance comparable to variable stiffness actuators.

Decoupling mechanism allows direct control of energy transfer timing.

Method improves dynamic maneuvers like throwing in elastic joint robots.

Abstract

Compliance in actuation has been exploited to generate highly dynamic maneuvers such as throwing that take advantage of the potential energy stored in joint springs. However, the energy storage and release could not be well-timed yet. On the contrary, for multi-link systems, the natural system dynamics might even work against the actual goal. With the introduction of variable stiffness actuators, this problem has been partially addressed. With a suitable optimal control strategy, the approximate decoupling of the motor from the link can be achieved to maximize the energy transfer into the distal link prior to launch. However, such continuous stiffness variation is complex and typically leads to oscillatory swing-up motions instead of clear launch sequences. To circumvent this issue, we investigate decoupling for speed maximization with a dedicated novel actuator concept denoted Bi-Stiffness Actuation. With this, it is possible to fully decouple the link from the joint mechanism by a switch-and-hold clutch and simultaneously keep the elastic energy stored. We show that with this novel paradigm, it is not only possible to reach the same optimal performance as with power-equivalent variable stiffness actuation, but even directly control the energy transfer timing. This is a major step forward compared to previous optimal control approaches, which rely on optimizing the full time-series control input.