6. Graphical Description of Temperature Controlled Actuation of SMA Wires

Figure 6.8 shows two ways of arranging the 350 mm SMA wire. Top option looks at using a long SMA wire that makes the mechanism unusually long and also aesthetically/practically inconvenient from an application standpoint. Another option shown in bottom half involves bending wires around guide pins to better package the long wires (referred commonly as “Spooling” technique). Based on the application and packaging constraints, one can choose either of the two options or use shorter wires with lever arrangements for stroke amplifications. One can find several mechanisms for stroke/force amplifications in any “Mechanism design” texts that could potentially be very useful in designing SMA actuators especially from a packaging perspective (more on this in Chap. 8 later). However, it must be highlighted that such spooling technique can result in localized pockets of untransformed regions at the locations where the wires is bent (especially over guide pins). These sharp bends can result in accumulation of local plastic/residual strains over thermal cycling (phase transformation event) that can result in stroke degradation over repeated thermal cycling. It is thus advised to avoid sharp gradients especially during spooling around guide pins to minimize these pockets of untransformed regions.