Experimental Study of Nitinol Wire Arrangements as Servo-Biomimetics for Facial Muscles

This paper examines the implementation of Nitinol wire as a complex-shape actuation source specifically targeted for low-power muscle biomimetics. Nitinol is a type of shape memory alloy (SMA) which recovers its original shape after experiencing large deformation when heated above an austenite finish temperature. Previous preliminary work by the authors demonstrated successful closed-loop force control (i.e., recovery stress) using a simple proportional controller. The work presented in this paper builds upon the previous work by demonstrating closed-loop position control of various wire arrangements in the presence of inertial loads. A predeformed NiTi (4% pre-strain) wire is energized via Joule heating (martensite to austenite) and de-energized by conductive cooling (austenite to martensite). The experimental setup consists of a horizontally arranged NiTi wire (or wire bundle) fixed at one end and connected to a hanging weight through a pulley on the opposite end. The angular displacement of the pulley is measured with a non-contact magnetostrictive angle sensor, thereby providing the control feedback signal for the wire displacement. Successful closed-loop position control is demonstrated, and the relative ease of control is assessed for increasing weights. Given the dynamic loading of the moving wire, a proportional controller alone is insufficient to obtain stabilized responses. Therefore, PID with anti-windup method is employed. Although PID requires some trial and error and is quite sensitive to varying conditions, it appears to be stable and sufficiently precise when properly tuned. The effect of bundling wires on the speed of response is experimentally characterized, and different bundling arrangements are designed and examined in order to increase the geometric rate of convective heat transfer. Increasing the rate of heat transfer is particularly important during the forward (austenite-to-martensite) transformation, since its speed relies solely on passive cooling of the wires. Limitations in controlled load capabilities are discussed in the context of wire diameter, bundle size and controller tuning. A repeatability study of a properly tuned PID controller is also carried out by comparing the first few and last few samples of a 50,000-cycle test. In addition, it is shown that identical wires, when swapped, do not require re-tuning of the PID gains. Finally, this paper shows some preliminary actuator designs that can mimic complex muscle movement. Various geometric arrangements of Nitinol wires are embedded into a curable elastomer with skin-like flexibility and durometer. The potential facial muscle movements from these arrangements are shown and discussed.