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Modeling and Simulation of A Surface Micromachined Triaxial Accelerometer

L. Jiang, W. N. Carr
New Jersey Institute of Technology, US

Keywords: Accelerometer, Triaxial, MEMS, FEA

We present the modeling and simulation results in designing a surface micromachined, capacitive triaxial accelerometer. Electrical FEA simulations calculate the nominal values of the sensing capacitors and compared with parallel-plate analytical model. The mechanical static and modal simulations in ANSYS 5.7 were carried out to study the static sensitivity and resonant frequencies of the accelerometer. It was shown that the fringing capacitance must be considered in x- and y- axial devices due to the small area overlap. From the static mechanical simulation, it shows that the four folded beams make z-axis accelerometer sensitive only in z-axis. However, the limited planar aspect ratio makes the springs of x, y- devices also compliant in z-axis. The poor off-axial rejection is improved with fully differential readout. The capacitance variations caused by z-axial acceleration appeared as common-mode signal on x, y- readout and was canceled first orderly. It was shown that the accelerometer has sensitivity of 0.21fF/G in z-axis and 0.14fF/G in x, y-axes. MemDamping simulation program was used to study the damping effects. It uses a hybrid Navier-Stokes-Reynolds solver, taking into consideration of edge and vent hole effects. The squeeze-film damping dominates in z-axis. For the lateral moving proof mass, a slide-film damping resulted in small damping force compared to squeeze-film damping. It was shown that at the same pressure, the damping coefficient of z-axis is almost four orders higher than that for x,y-axes. The smaller damping will result in higher quality factor in the lateral axes and further improve the off-axial rejections.

NSTI Nanotech 2003 Conference Technical Program Abstract

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