Characterization and Analysis of a Flexural Shape Memory Alloy Actuator

Richard Dauksher,Zachary Patterson,Carmel Majidi

doi:10.3390/act10080202

Abstract

Shape memory alloys (SMAs) are popular as actuators for use in soft robots due to their high work density and compatibility with miniaturized on-board batteries and power electronics. However, because SMA actuators are activated through electrical Joule heating, they exhibit poor energy efficiency and low actuator frequencies that arise from long cool-down times. Moreover, in the case of SMA wires that are subject to flexural loading, their load capacity and mechanical work output decrease exponentially with decreasing cross-sectional area. In this study, we perform analytic and numerical analyses to examine the thermal and structural design space around a particular class of flexural SMA wire actuators with the intention of increasing actuator operating frequency and actuation forces. Measurements obtained through experimental testing are consistent with theoretical studies of actuator force output and provide additional insight into the efficiency of electrical-to-mechanical energy conversion. Together, the theoretical and experimental studies provide insights that have the potential to inform SMA wire design and usage in soft robotic applications.

Highlights

Within soft robotics and related fields, many actuators have been developed to create mechanical compliant systems that can safely and stably interact with their environment with limited dependence on external control
Pneumatic actuators such as McKibben and pneumatic artificial muscle actuators are especially popular in soft robotics but require the supply of pressurized air from a compressor, which makes untethered designs challenging to implement [8,9]
A third approach that has been popularly adopted in both rigid [11,12,13,14] and soft [14,15,16] applications involves the use of shape memory alloys [17,18]

Summary

Introduction

Within soft robotics and related fields, many actuators have been developed to create mechanical compliant systems that can safely and stably interact with their environment with limited dependence on external control. A third approach that has been popularly adopted in both rigid [11,12,13,14] and soft [14,15,16] applications involves the use of shape memory alloys [17,18] These alloys undergo a phase change at low temperature, through which they are able to generate sufficiently large forces in a short time interval and can be controlled and powered with relatively lightweight and portable electronics [19,20]. We examine the thermal and structural design space around a particular class of flexural SMA wire actuators through a combination of analytic, computational, and experimental studies The purpose of this analysis is to determine how design factors like the diameter and number of SMA wires influence actuator properties like operating frequency and blocking force. These analyses and tests describe how design factors affect actuator performance for this class of flexural actuators

Actuator Design and Fabrication

Blocking Force

Conclusions