Narrow Results

The stepped microbeams are typical low stiffness structures widely used in MEMS devices. Approximated solutions of the natural frequency and the pull-in voltage of stepped microbeams, including clamped-free (CF) and clamped–clamped (CC) boundary conditions are developed, and the unique pull-in behavior of the stepped microbeams is investigated. The stepped microbeam is viewed as a beam with a rectangular electrode pad at its tip for CF beam and at its center for CC beam. The motion equations are deduced based on the Euler–Bernoulli beam and the modified couple stress theory. The natural frequency and the pull-in voltage are extracted with a one-degree-of-freedom model. The present model correctness is validated by comparing with the finite element results, and the effect of the length ratio of the electrode pad to the beam and the width ratio of the beam to the electrode pad are discussed. The results show that both the natural frequency and the pull-in voltage monotonously increase with the increase of the width ratio, and first decrease and then increase with the increase of the length ratio. The minimum value of them does not appear at the same time, but is determined by the width ratio. The results can be used to design and improve the performance of MEMS devices.

The pull-in instability is the inherent property of a micro-electromechanical system (MEMS) when the voltage is larger than its threshold value. Recently, a fractal MEMS system was proposed to overcome the pull-in instability with great success, and it has opened a total new path for the so-called pull-in stability. This paper suggests a pull-in plateau, a novel concept for qualifying the pull-in stability. The plateau’s basic properties are elucidated, and the effect of the fractal dimensions on the plateau width is elucidated, and the paper concludes that there exists a critical condition for an ever pull-in stability when both the acceleration and the speed of the system equal zero.

In this study, dynamic response of a micro- and nanobeams under electrostatic actuation is investigated using strain gradient theory. To solve the governing sixth-order partial differential equation, mode shapes and natural frequencies of beam using Euler–Bernoulli and strain gradient theories are derived and then compared with classical theory. Galerkin projection is utilized to convert the partial differential equation to ordinary differential equations representing the system mode shapes. Accuracy of proposed one degree of freedom model is verified by comparing the dynamic response of the electrostatically actuated micro-beam with analogue equation and differential quadrature methods. Moreover, the static pull-in voltages of micro-beams found by one DOF model are compared with the reported data in literature. The main advantage of proposed method based on the Galerkin method is its simplicity and also its low computational cost in analyzing the dynamic and static responses of micro- and nanobeams. Additionally, effect of axial force, beam thickness and applied voltage are analyzed. The results obtained based on strain gradient theory, are compared with classical and modified couple stress theories which are the special cases of the strain gradient theory. It is shown that strain gradient theory leads to higher frequency and lower amplitude in comparison with two other theories.

Size-dependent characteristics have been widely observed in microscale devices. For the electrostatically actuated circular microplate-based MEMS, we propose a new model to predict the size-dependent pull-in instability based on the strain gradient elasticity theory. The model embeds three material length scale parameters (MLSPs), which can effectively predict the size-dependent pull-in voltage. The model can be reduced to the classical continuum model when MLSPs are ignored. The results show that the normalized pull-in voltage predicted by the present model increases nonlinearly with the decrease of the size scale of the plate, and the size effect becomes prominent if the characteristic dimension (plate thickness) is on the order of microns or smaller. The effects of the plate thickness and gap on the pull-in voltage are also investigated.