BISMAC control using SMA resistance feedback

Recently, bio-inspired shape memory alloy composite (BISMAC) actuators have been shown to be promising for the design of medusae rowing propulsion. BISMAC actuators were able to recreate bell deformation of Aurelia aurita by controlling shape memory alloy (SMA) deformation that allowed matching the contraction-relaxation deformation profile. In this study, we improve upon the control system and demonstrate feedback control using SMA wire resistance to decrease contraction time and power consumption. The controller requires the knowledge of threshold resistance and safe current inputs which were determined experimentally. The overheating effect of SMA wires was analyzed for BioMetal Fiber (BMF) and Flexinol 100 μm diameter wires revealing an increase in resistance as the wires overheated. The controller was first characterized on a SMA wire with bias spring system for a BMF 100 using I_hi = 0.5 A and I_low = 0.2 A, where hi corresponds to peak current for fast actuation and low corresponds to the safe current which prevents overheating and maintains desired deformation. A contraction of 4.59% was achieved in 0.06 s using the controller and the deformation was maintained for 2 s at low current. The BISMAC actuator was operated using the controller with I_hi = 1.1 A and I_low = 0.65 A achieving a 67% decrease in contraction time compared to using a constant driving current of I_low = 0.2 A and a 60% decrease in energy consumption compared to using constant I_hi = 0.5 A while still exceeding the contraction requirements of the Aurelia aurita.