This paper introduces a new nonlinear model for microswitches based on the modified couple stress theory. The microswitch includes a microbeam which is connected to the clamped support from one side and attached to an electrostatically driven proof mass with a large gap from the other side. The microswitch operates in the pull-in instability with large deformation. The effects of fringing field and large curvature as well as size dependency are considered in the modeling. With regard to the size-dependent model, the equations of motion are obtained using Hamilton’s principle and solved by both numerical and analytical approaches. Consequently, dynamic pull-in instability is investigated based on the analytical and numerical solutions for dynamic conditions. The results depict that the dynamic deflection predicted by the modified couple stress theory is smaller than that obtained by the classical theory. The classical theory underestimates the pull-in instability voltage of the microswitches especially when the beam’s thickness is in the order of material length scale parameter. Furthermore, it is shown that neglecting nonlinearity due to large deflection leads to significant errors in the pull-in instability of the microstructures and these errors are calculated. The novelty of this paper is to provide a nonlinear size-dependent model for microswitches and to investigate the nonlinearity and instability of microswitches based on this model using the analytical and numerical methods.
