A Bistable Shape Memory Microswitch

A novel concept of a bistable microswitch is presented, which combines an antagonistic shape memory alloy (SMA) actuation mechanism with two end positions and magnetic components to maintain these positions in power-off state. The layout of the antagonistic SMA actuation mechanism consists of two mechanically coupled freely suspended TiNi microbridges of 20 μm thickness, which are prestrained with respect to each other. The electro-thermo-mechanical response of the coupled microbridges is investigated for various degrees of prestrain to determine the maximum actuation stroke, minimum required magnetic retention force and maximum switching force. For magnetic retention, the ferromagnetic attraction of different hard and soft magnetic microstructures is studied. By varying the diameter of a hard magnetic cylinder for a given diameter of a soft-magnetic disc and fixed thicknesses, the ferromagnetic attraction force displays a pronounced optimum. First demonstrator microswitches with outer dimensions of about 7×7×4 mm³ are presented, which show large work outputs, featuring strokes and contact forces up to 84 μm and 60 mN, respectively.