Low-Power, Minimal-Heat Exposure Shape Memory Alloy (SMA) Actuators for On-Body Soft Robotics

Abstract

In the world of soft-robotic medical devices, there is a growing need for low profile, non-rigid, and lower power actuators for soft exoskeletons and dynamic compression garments. Advanced compression garments with integrated shape memory materials have been developed recently to alleviate the functional and usability limitations associated with traditional compression garments. These advanced garments use contractile shape memory alloy (SMA) coil actuators to produce dynamic compression on the body through selective heating of the SMA material. While these garments can create spatially- and temporally-controllable compression, typical SMA materials (e.g., 70°C Flexinol) consume considerable power and require considerable thermal insulation to protect the wearer during the heating phase of the SMA actuation. Alternative SMA materials (e.g., NiTi #8 by Fort Wayne Metals, Inc.) transform below room temperature and do so using no applied electrical power and generate no waste heat. However, these materials are challenging to dynamically control and require active refrigeration to reset to material. In theory, low-temperature SMA actuators made from materials like NiTi #8 may maintain additional dynamic actuation capacity once equilibrated to room temperature (i.e., the material may not fully transform), as the SMA phase transformation temperature window expands when the material experiences applied stress. This paper investigates this possibility: we manufactured and tested low-temperature NiTi coil actuators to determine the magnitude of the additional force that can be generated via Joule heating once the material has equilibrated to room temperature. SMA spring actuators made from NiTi #8 consumed 84% less power and stabilized at significantly lower temperatures (26.0°C vs. 41.2°C) than SMA springs made from 70°C Flexinol, when actuated at identically fixed displacements (100% nominal strain) and when driven to produce equal forces (∼3.35N). This demonstration of low-power, minimal-heat exposure SMA actuation holds promise for many future wearable actuation applications, including dynamic compression garments.

Modelling of a shape memory alloy actuator for feedforward hysteresis compensator considering load fluctuation

“…Length of wire-type SMA (SMA wire) actually decreases when heated by Joule heating with application of an electric current. Shape memory alloy wire has many applications, and has been specifically used in artificial muscles [2,3], robots [4][5][6], and medical equipment [7,8]. These are some examples of how such strain is used in industrial equipment.…”

A Hydraulic Soft Microhand with an Application of Live Insect Manipulation

“…These actuators have the problem of miniaturization and assembling for the smaller size of actuator [38]. They use tubing to energize the actuator that remove the risk of wiring, high voltage, and high temperature damages to the live tissues [39]. One of the main benefits of this family of actuators is their compatibility with soft materials [40].…”