Design of fully compliant, in-plane rotary, bistable micromechanisms for MEMS applications

Abstract

A fully compliant bistable micromechanism (hereafter identified as an in-plane rotary bistable micromechanism or IPRBM) is designed to perform in-plane rotary motion with two stable positions. The micromechanism consists of four identical bistable mechanisms arranged in a cyclic symmetry manner about a central proof mass. This new class of micromechanism can be used in several microelectromechanical system (MEMS) devices such as gate valve, optical shutter, and mechanical lock. Two types of IPRBMs have been developed in this paper, called as the outside IPRBM and the inside IPRBM. These two mechanisms differ by their relative orientation of bistable mechanisms with respect to the central proof mass. The micromechanisms are fabricated by electroplating a soft magnetic material—Permalloy (80% Ni, 20% Fe) into their photoresist mold. The minimum feature size is 4 μm, which corresponds to the width of compliant linkages used in the mechanism. The fabricated micromechanisms are tested for their torque-deflection response by using an image-based force sensing method. The test results are compared with simulation results. A pseudo-rigid-body model as well as a finite element model of IPRBM is developed to simulate the mechanical response of the mechanism. The micromechanisms are shown to reversibly undergo 10–20° of in-plane rotation and required a maximum torque of 1–2 μN m depending on the design. The experimental results showed good overall agreement with the design. The testing of each type of IPRBM is performed on three different design cases between which the tether width and aspect ratio was varied. The study showed a relative advantage of slender tethers with high aspect ratio in minimizing out-of-plane deflection. Also, the anchor distance between bistable mechanisms is significant.