Energetic Advantages of Constant Torque Springs in Series Parallel Elastic Actuators

Abstract

In physical human-robot interaction, active compliance is often preferred above passive compliance, since it makes the control of the output impedance easier than with the use of passive elements (e.g. springs). However, passive compliance has some interesting advantages, e.g. low mechanical output impedance, tolerance to impact loads, increased peak power output, and passive mechanical energy storage. Series-Parallel Elastic Actuation (SPEA) concepts are under investigation to improve the efficiency and torque capacity of these elastic actuators, but they still suffer from torque loss which increases with the number of units. This leads to a decrease in efficiency. This paper proposes a novel concept to overcome the limitations of the SPEA by introducing the use of constant torque springs. This novel concept, called Series-Parallel Elastic Constant Torque Actuator (SPECTA) proposes to implement constant torque springs in the SPEA architecture in order to lower the energy losses, increase efficiency and boost the energy storage. Simulations have shown that the losses in the SPECTA are significantly lower than the state of the art actuators for a lifting task.