Series elastic actuators

TLDR

Actuator–load interfaces are traditionally made stiff, but reducing stiffness improves shock tolerance, lowers reflected inertia, enhances force control, protects the environment, and enables energy storage, at the cost of lower zero‑motion force bandwidth. The authors argue that for natural tasks, zero‑motion bandwidth is not the sole criterion and that deliberately incorporating series elasticity into actuators is advantageous. They define series elasticity as a passive spring in the actuator and present a control system suitable for general force or impedance control after discussing its trade‑offs. Experimental tests on a revolute series‑elastic actuator for the MIT humanoid robot Cog and a small planetary rover confirm the approach’s viability.

Abstract

It is traditional to make the interface between an actuator and its load as stiff as possible. Despite this tradition, reducing interface stiffness offers a number of advantages, including greater shock tolerance, lower reflected inertia, more accurate and stable force control, less inadvertent damage to the environment, and the capacity for energy storage. As a trade-off, reducing interface stiffness also lowers zero motion force bandwidth. In this paper, the authors propose that for natural tasks, zero motion force bandwidth isn't everything, and incorporating series elasticity as a purposeful element within the actuator is a good idea. The authors use the term elasticity instead of compliance to indicate the presence of a passive mechanical spring in the actuator. After a discussion of the trade-offs inherent in series elastic actuators, the authors present a control system for their use under general force or impedance control. The authors conclude with test results from a revolute series-elastic actuator meant for the arms of the MIT humanoid robot Cog and for a small planetary rover.