Experimental nonlinear dynamics of a shape memory alloy wire bundle actuator

In this paper, the nonlinear dynamics of a new Shape Memory Alloy (SMA) actuator that possesses impressive payload lifting capabilities are presented. This actuator consists of 48 SMA wires mechanically bundled in parallel forming one powerful muscle. It was designed to lift up to 45.4 kg (100 Ibs), which is approximately 300 times its weight. This SMA actuator was tested in open-loop experiments with different loads and different inputs, such as step, ramp, sinusoid and half sinusoid, and its dynamic characteristics were evaluated. An important observation made during the dynamic analysis was the unpredictability of the actuator's response when low to moderate voltages were applied This characteristic suggests possible chaotic behavior of the actuator, which could affect the system design and cause control difficulties in fine and high accuracy tasks. An investigation into chaos was conducted using time histories, phase plots, Poincare maps, and power spectrum density plots. As shown in the diagrams presented in this paper, system response to sinusoidal inputs with a larger mean voltage is periodic, whereas lower mean voltages produce unpredictable responses that indicate chaotic behavior.