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Research Papers

Characterization of the Dynamical Response of a Micromachined G-Sensor to Mechanical Shock Loading

[+] Author and Article Information
Daniel Jordy, Mohammad I. Younis

Department of Mechanical Engineering, State University of New York at Binghamton, Binghamton, NY 13902

J. Dyn. Sys., Meas., Control 130(4), 041003 (Jun 04, 2008) (9 pages) doi:10.1115/1.2936849 History: Received January 11, 2007; Revised November 28, 2007; Published June 04, 2008

Squeeze film damping has a significant effect on the dynamic response of microelectromechanical system (MEMS) devices that employ perforated microstructures with large planar areas and small gap widths separating them from the substrate. Perforations can alter the effect of squeeze film damping by allowing the gas underneath the device to easily escape, thereby lowering damping. By decreasing the size of the holes, damping increases and the squeeze film damping effect increases. This can be used to minimize the out-of-plane motion of the microstructures toward the substrate, thereby minimizing the possibility of contact and stiction. This paper aims to explore the use of the squeeze film damping phenomenon as a way to mitigate shock and minimize the possibility of stiction and failure in this class of MEMS devices. As a case study, the performance of a G-sensor (threshold accelerometer) employed in an arming and fusing chip is investigated. The effect of changing the size of the perforation holes and the gap width separating the microstructure from the substrate are studied. A multiphysics finite-element model built using the software ANSYS is utilized for the fluidic and transient structural analysis. A squeeze film damping model, for both the air underneath the structure and the flow of the air through the perforations, is developed. Results are shown for various models of squeeze film damping assuming no holes, large holes, and assuming a finite pressure drop across the holes, which is the most accurate way of modeling. It is found that the threshold of shock that causes the G-sensor to contact the substrate has increased significantly when decreasing the holes size or the gap width, which is very promising to help mitigate stiction in this class of devices, thereby improving their reliability.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Sensing and arming chip

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Figure 2

Layout and finite-element model of the G-sensor. All dimensions are given in micrometers.

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Figure 3

First four out-of-plane mode shapes of the G-sensor

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Figure 4

(a) The shock spectrum of a spring-mass system subjected to a half-sine pulse. (b) The shock spectrum of the G-sensor for a shock pulse of 10g and varying duration.

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Figure 5

Shock amplitude needed to cause contact with the substrate for a 2μm and a 4μm gap

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Figure 6

(a) Response of point A on the G-sensor to an out-of-plane shock of 10g at a duration of 0.1ms. The maximum displacement is 0.86μm. (b) Response of Point A on the G-sensor to an out-of-plane shock of 10g at a duration of 1.0ms. The maximum displacement is 0.85μm.

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Figure 7

Schematic of the various ways to represent the perforations in the squeeze film damping analysis. (a) The physical representation of two perforations. (b) Neglecting the holes (infinite resistance). (c) No-hole resistance. (d) Finite-hole resistance.

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Figure 8

Damping and stiffness coefficients calculated from a squeeze film damping analysis of a single degree-of-freedom model when the G-sensor is excited with harmonic excitation of varying frequencies

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Figure 9

Pressure distribution due to squeeze film damping—no boundary conditions on the perforations (no-hole model)

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Figure 10

Pressure distribution due to squeeze film damping assuming ambient pressure on the perforations (wide-hole model). Notice the lower pressure values compared to Fig. 9.

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Figure 11

Effect of the hole size on the damping ratio

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Figure 12

Effect of the hole size on the transient response of the G-sensor when subjected to a half-sine shock pulse of duration=0.3ms and amplitude=10g. The gap width is set to 2μm. Xi is the damping ratio corresponding to a particular hole size.

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Figure 13

Shock level that causes the G-sensor to contact the substrate

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