Experimental analysis of a prestressed mechanism with multi-antagonistic and hysteretic SMA actuation

This paper presents the experimental analysis of a multi-antagonistic actuation system involving nickel-titanium (NiTi) shape memory alloy (SMA) wires that can be thermally activated individually, one or more at a time. In particular, the study focuses on the hysteretic effects that occur during the thermal activation sequences. A planar structure involving six NiTi SMA wires has been designed as a first step. Particular attention was paid to the tightening at the ends of the wires. A solution to reduce slippage has been proposed by using a specific thermal cycle during the tightening operation. Then, an experimental setup involving infrared and visible range cameras was developed to measure the movement of the mobile platform and the SMA wire temperatures, respectively. Finally, important points of the actuation have been studied through different tests, particularly the “attraction” effect of any heating step on the following steps. The study also highlights system initialization challenges (resetting the reference position of the mobile platform and initial prestressing), leading to shifts with respect to the positions obtained by finite element simulations. The paper shows that the multi-antagonistic and hysteretic character of actuation systems involving more than two NiTi SMA wires can be successfully managed.