SPIRO: A Compliant Spiral Spring–Damper Joint Actuator With Energy-Based Sliding-Mode Controller

Elasticity in actuators plays a crucial role in enabling flexible, efficient, and safe motion in robots. However, achieving such advanced capabilities also requires the development of sophisticated controllers. This article introduces “SPIRO,” a novel variable impedance actuator (patent-pending) capable of dynamically changing its stiffness and damping profiles to adapt to various tasks. We also propose a new energy-based sliding-mode controller that facilitates robust stiffness and adaptive damping for intelligent actuator execution. Through experimental examples, we demonstrate how the compliance of the actuator can be adjusted based on the amount of disturbance energy, resulting in safer and more efficient motion. Moreover, the proposed mechanism, coupled with the designed controller, allows for a tradeoff between accuracy and safety, enabling more dynamic link movements akin to human muscle. The unique capabilities of SPIRO make it promising for potential applications, such as exoskeleton systems and human–robot interactions.

I. Introduction

Actuators are integral component of operating robots providing the necessary power and motion capabilities. However, conventional electric motors have limitations when it comes to tasks that require flexibility, energy efficiency, and safe interaction with the load side. For example, achieving optimal robot locomotion requires a delicate balance of stiffness and damping [1], [2]. This challenge becomes particularly significant when actuators need to directly interact with humans, such as in manipulators for active task support [3], [4], [5] or as assistive mechanisms in human locomotion [6], [7], [8], [9]. This demand for a balance between safety and propulsion power/efficiency has led to the emergence of a new class of actuators known as variable impedance actuators (VIAs).