Design of Two-stage Actuator Using Shape Memory Alloy Wires with Different Transformation Temperatures

Inhalt:
Shape memory alloys (SMAs) are a class of metallic alloys that can exhibit the phase transformation phenomenon known as the shape memory effect (SME) when subjected to a temperature gradient or magnetic field. The primary application of these materials is the linear actuation systems because the SMAs can be readily contracted or recovered to its original form. The actuating displacement or force could be achieved by controlling temperature, which is beyond a certain threshold temperature by internal Joule heating. SMAs possess two different phases with three different crystal structures (i.e., twinned martensite, detwinned martensite, and austenite). Typically, the austenite phase is stable at high temperature, while the martensite phase is stable at lower temperatures. Upon SMA is heated beyond onset transformation temperature, it begins to transform into the austenite structure, resulting in recovering (i.e., contracting) into its original shape. During cooling, the transformation starts to reverse to the martensite. SMAs are particularly attractive for actuator applications because their strain (length change or stroke) during the contraction phase is relatively larger than in other smart materials, such as piezoelectric transducers, and it is typically 4%–5% of their initial length. If the contraction is constrained, large block forces can be generated. Compared to other actuation systems, high work output along with lower weight (i.e., high power density) can be achieved with SMAs. In this study, we design a two-stage actuator by simply connecting two SMA wires with different transformation temperatures (e.g. Flexinol(TM) wire φ = 0.15 mm, 90deg C and 60deg C) in series for implementing input shaping. We will experimentally demonstrate the effectiveness of proposed SMA-based two-stage actuator through the laboratory level test-bed.