A bistable shape memory microswitch with high energy density

A novel concept and various layouts of a bistable microswitch are presented, that combine an antagonistic shape memory alloy (SMA) actuation mechanism with two end positions, and magnetic components to maintain the end positions in the power-off state. The antagonistic SMA actuation mechanism consists of two mechanically coupled freely suspended TiNi microbridges of 20 µm thickness that are pre-strained with respect to each other. The electro-thermo-mechanical response of the coupled microbridges is investigated for various degrees of pre-strain to determine the maximum actuation stroke, minimum required magnetic retention force and maximum switching force. Different layout variants consisting of hard- and soft-magnetic microstructures are compared and evaluated for magnetic retention. The optimization of the ferromagnetic attraction force of a specific layout is shown by finite element simulations. First demonstrator microswitches with outer dimensions of about 7 × 7 × 4 mm3 are presented that show large work outputs featuring strokes and contact forces up to 84 µm and 30 mN, respectively.