Influence of Boundary Conditions on the Dynamic Characteristics of Squeeze Films in MEMS Devices

Pandey, Ashok kumar and Pratap, Rudra and Chau, Fook Siong (2007) Influence of Boundary Conditions on the Dynamic Characteristics of Squeeze Films in MEMS Devices. In: Journal of Microelectromechanical Systems, 16 (4). pp. 893-903.

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Abstract

Micromechanical structures that have squeeze-film damping as the dominant energy dissipation mechanism are of interest in this paper. For such structures with narrow air gap, the Reynolds equation is used for calculating squeeze-film damping, which is generally solved with trivial pressure boundary conditions on the side walls. This procedure, however, fails to give satisfactory results for structures under two important conditions: 1) for an air gap thickness comparable to the lateral dimensions of the microstructure and 2) for nontrivial pressure boundary conditions such as fully open boundaries on an extended substrate or partially blocked boundaries that provide side clearance to the fluid flow. Several formulas exist to account for simple boundary conditions. In practice, however, there are many micromechanical structures such as torsional microelectromechanical system (MEMS) structures that have nontrivial boundary conditions arising from partially blocked boundaries. Such boundaries usually have clearance parameters that can vary due to fabrication. These parameters, however, can also be used as design parameters if we understand their role on the dynamics of the structure. We take a MEMS torsion mirror as an example device that has large air gap and partially blocked boundaries due to static frames. We actuate the device and experimentally determine the quality factor Q from the response measurements. Next, we model the same structure in ANSYS and carry out computational fluid dynamics analysis to evaluate the stiffness constant K, the damping constant D, and the quality factor Q due to the squeeze film. We compare the computational results with experimental results and show that without taking care of the partially blocked boundaries properly in the computational model, we get unacceptably large errors.

Item Type:	Journal Article
Publication:	Journal of Microelectromechanical Systems
Publisher:	IEEE
Additional Information:	Copyright 2006 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.
Keywords:	Computational fluid dynamics (CFD);Experimental quality factor;Microelectromechanical system (MEMS);Partially blocked boundaries;Squeeze-film damping;Torsional motion
Department/Centre:	Division of Mechanical Sciences > Mechanical Engineering
Date Deposited:	22 Oct 2007
Last Modified:	19 Sep 2010 04:40
URI:	http://eprints.iisc.ac.in/id/eprint/12324

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