A treatise on highly twisted artificial muscle : thermally driven shape memory alloy yarn and coiled nylon actuators

摘要

A new perspective in the field of actuators was opened by the demonstration of multiwall carbon nanotube based actuators by Foroughi et al, in 2011. The approach involves applying a high degree of twist to create large torsional actuation in carbon nanotube yarns, and more recently in coiled nylon filaments. In this thesis torsional actuation is further studied in nylon, and extended to shape memory alloys (SMA). Torsional actuation is demonstrated using 25 μm diameter micro strands of shape memory alloy (SMAs) that are twisted together. These form yarns with Young’s modulus of 13.5 GPa in the Martensitic phase and 18 GPa in the Austenite state. In torsion, the SMA yarns show more than 8,000 rpm peak rotational speed with 11 reversible rotations for an 8 cm long yarn. This is observed upon applying 0.47 W/cm electrical input power. Providing more than 5 N.m/kg torque, SMA yarns may be of interest in biomedical and other applications. The mechanical behaviour of coiled nylon actuators is studied by testing elastic modulus and by investigating tensile stroke as a function of temperature. Loads that range from 35 MPa to 155 MPa were applied. For the nylon and the coiling conditions used, active thermal contraction totals 19.5 % when the temperature is raised from -40 ⁰C to 160 ⁰C, with most contraction above the glass transition temperature. Introducing various cooling methods was shown to enable increased rate of actuation up to several Hertz. Nylon coiled actuators potentially provide affordable and viable solutions for driving mechanical devices as recently demonstrated in robotic hands and arms. A new biomimetic arrangement of the nylon actuator is presented that imitates the human pennate muscle in structure, including the ability to vary stiffness by a factor of 9 and to increase isometric force from 19 N to 37 N by recruiting additional fibers.