Shape Memory Alloy (sma) Coil Actuators for Use in Controllable Mechanical Counter-pressure (mcp) Space Suits

Mechanical counter-pressure (MCP) space suits, which apply pressure to the wearer using tight-fitting materials rather than pressurized gas, offer numerous mobility and mass advantages over traditional gas-pressurized suits [1]. However, MCP design challenges related to pressure uniformity, pressure magnitude, and donning/doffing remain unsolved [2]. Previous MCP efforts have focused on passive compression technologies, but an ideal MCP garment would incorporate actively controllable elements designed to dynamically change the garment’s shape, which would enable real-time and high-resolution modification of compression profiles to address these issues [3]. We are investigating the use of shape changing materials (sometimes referred to as artificial muscles) in a wearable garment to generate controllable counter-pressure for MCP applications [4]. Specifically, we describe the development and characterization of coiled actuators made from 0.1016 mm (0.004”) and 0.3048 mm (0.012”) diameter shape memory alloy (SMA) wire for integration in textile architectures to produce counter-pressure. Production and shape setting of the coiled actuators, as well as experimental test methods, are described [5]. Force vs. length relationships for multiple coil actuators are reported, and the data are compared to both their predicted values and to the known characteristics of human muscle [6]. Preliminary results demonstrate spring-like behavior: the actuators exhibit a highly linear (R 2 > 0.99) relationship between isometric force generation and coil displacement (isometric test length fully actuated length), which is consistent with current literature [7]. Maximum forces generated per coil are measured to be 8.94 N (2.01 lbs) at a coil displacement of 16.51 cm (the physical limit of the test stand). Both the time evolution of generated forces and the average power consumption per coil are dependent on applied voltage: higher voltages lead to both higher power consumption and faster force responses. For each actuator, a critical voltage exists above which the induced current causes excessive joule heating, which partially resets the coil memory effect. This causes a drastic reduction in spring constant (and thus force generation) at a given length. The critical voltage value is unique to each coil, and is hypothesized to be dependent on both length and diameter of the SMA wire that comprises the actuator. Spring constants are found to be stable across tests for a given coil (when kept below the critical voltage), but were found to vary between coils, which is likely due to variations in coil packing density introduced during the initial coiling process. Finally, MCP concepts under development incorporating the experimental SMA actuators are presented.