Development of a compact millimeter-scale bistable actuator using the snap-through effect

Abstract

Bistable structures have two stable states separated by a high-energy unstable state. They have the ability to withstand perturbations less than their critical trigger load and require no external energy to maintain their stable states. This gives them an advantage over many linear actuators that are required to produce an on-off or up-down movement. However, they require an actuation mechanism to move from one state to another. In this research, a compact actuation mechanism with less power consumption, simple and low cost is proposed and developed to actuate a buckled bistable beam. The proposed mechanism consists of two DC motors, a pair of parallel strings and a pin, serving as a moment arm. This twisted string actuation concept and the bistable structure constitute the millimeter-scale actuator for on-off driving of control fins. A post-buckling beam behavior is modelled by extending the modified two-strip model to a clamp-clamp free-free plate. The center strip behavior is assumed to be the same as that of the entire plate and modelled using the Euler beam theory. This modeling is validated by using ABAQUS. A parametric study is conducted by introducing three different geometric imperfections, measured as a percentage of the beam’s thickness. The analytical and numerical results show good agreement for imperfections less than twice the beam’s thickness. A snap-through behavior of a buckled beam is analytically formulated using the Euler-Bernoulli beam theory and virtual beam function. A simple solution algorithm is proposed to determine the displacement, trigger force and moment of the bistable buckled beam. The algorithm solves for the snapping sequence of the beam when moving from one state to another in the spatial form. A vision-based experiment is conducted to validate the snapping sequence. The analytical snapping sequence results are in good agreement with the experiment results for the off-axis actuation of the beam. The newly proposed actuation mechanism in this research consists of twisted string actuator concept. Twisted string mechanism produces a pulling force to the moment arm, which in turn induce bending moment on the bistable beam. An experiment is conducted to validate the analytical formulation and evaluate the performance of the integrated system using sine and square input voltage. The proposed compact bi-stable actuator can operate at 8 mm stroke and 5 Hz bandwidth. Furthermore, two configurations of this actuation concept are presented.