A Sine Wave-Shaped Spring to Enhance the Compactness of Series Elastic Actuators

In this paper, we discuss the design methodology of a novel torsional spring, embedded in a series elastic actuator (SEA) for portable upper-limb exoskeleton applications. Starting from a simple beam model, a class of design candidates is examined through theoretical formulation and finite element (FE) simulations. We found out that a sine wave shape is the best compromise to achieve the desired compliance for a safe use of the exos. Besides, this novel design reveals to reduce the encumbrance of the joint with respect to previous architectures, for fixed weight. The spring was manufactured in a 3D-printed metal with main dimensions - $\phi$ diameter 98 mm, 18 mm of width - and weight - 0.34 kg. Experiments have been performed on the SEA to validate the joint characteristic in the torque-angle plane. Results are promising since linearity is confirmed and the experimental stiffness coincides with the theoretical estimate. In particular, the joint is validated for operation up to a deflection of 13.75° under a nominal torque of 9 Nm. Therefore, the proposed elastic joint is suitable for integration in physical human-robot interactions.

I. Introduction

Granting safe physical human-robot interaction (pHRI) is a critical concern when dealing with robots in direct contact with humans. Portable upper-limb exoskeletons, for example, assists users through an actuation mechanism that transmits torque to the arms, relieving muscle activity during arm elevation and weight-lifting tasks [1]. Being wearable devices, these types of applications must ensure safe operations and accurate force control.