Audio Frequency Actuation of SMA Wires

The shape memory effect in nickel-titanium-based shape memory alloys (SMA) is due to a crystallographic reversible phase transformation between the low-temperature martensitic and the high-temperature austenitic phase of the material. Actuation frequencies of SMA wires are typically limited to a few Hertz, primarily due to the time required for cooling. Thus, not many are aware that SMA wires can also generate forces at audio frequencies of up to several thousand Hertz. This paper systematically investigates the factors that influence the ability of SMA wires to generate forces at high operating frequencies. Preliminary experimental results show that mean SMA wire stress and temperature, current amplitude, and frequency all have a significant impact on force amplitude. Measured force amplitudes varied between 10 mN at an actuation frequency of 100 Hz down to 0.2 mN at 7.5 kHz.