Research Papers

Fringing Electrostatic Field Actuation of Microplates for Open Air Environment Sensing

[+] Author and Article Information
Avinoam Rabinovich

Faculty of Engineering,
School of Mechanical Engineering,
Tel Aviv University,
Tel Aviv 69978, Israel
e-mail: avinoamr@post.tau.ac.il

Assaf Ya'akobovitz

Faculty of Engineering,
School of Mechanical Engineering,
Tel Aviv University,
Tel Aviv 69978, Israel
e-mail: assafy@umich.edu

Slava Krylov

Faculty of Engineering,
School of Mechanical Engineering,
Tel Aviv University,
Tel Aviv 69978, Israel
e-mail: vadis@eng.tau.ac.il

Contributed by the Technical Committee on Vibration and Sound of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received October 6, 2013; final manuscript received April 25, 2014; published online May 28, 2014. Assoc. Editor: Jeffrey F. Rhoads.

J. Vib. Acoust 136(4), 041013 (May 28, 2014) (10 pages) Paper No: VIB-13-1346; doi: 10.1115/1.4027559 History: Received October 06, 2013; Revised April 25, 2014

In the present study, we tested the feasibility of actuation of microplates by fringing electrostatic fields, i.e., field lines between plates and the sidewalls supporting them. Unlike the common close-gap actuation mechanism usually used in these kinds of devices, we present an alternative operational principle based on an electrostatic fringe field for the actuation of micro electromechanical (MEMS) plates, which is especially beneficial for open air environment operation. In order to validate the actuation principle, a circular MEMS plate was considered and an analytical model was built. The electrostatic force applied to the plate was extracted from a solution of a steady boundary value problem of a cylinder and was validated numerically using finite element simulation. This was followed by a solution of the plate governing equation of motion using an expansion theorem. Devices of two different geometries were fabricated and operated. Actuation of the plates by means of the fringing field was demonstrated experimentally. The proposed architecture and actuation principle is advantageous and overcomes many of the difficulties encountered in microplates electrostatically actuated by a close-gap electrode. Due to the absence of a small gap, the device is not prone to pull-in instability and stiction and is not subjected to squeeze-film damping. Moreover, since the actuation is separated from the front side of the device, open air contaminations, such as humidity or dust, cannot cause operational failure. In addition, the device is especially beneficial for mass sensing in an open environment, as well as flow senors where a flush-mounted smooth surface is important.

Copyright © 2014 by ASME
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Fig. 1

(a) Schematics of the device under discussion. (b) Air gap (electrical domain). Boundary conditions are shown. For the sake of demonstration, only a symmetric half of the device and air gap are shown. Field lines are shown as red lines in the case of (c) fringe field lines (originated from the sidewalls) and (d) parallel plate.

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Fig. 2

Contours of the maximal nondimensional plate displacement |w*(2)| as a function of H* and h* under resonance excitation. The axes are in logarithmic scale.

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Fig. 3

Eigen functions (modal shapes) obtained from FE simulation for device 2. (a) First axisymmetric mode. (b) First antisymmetric mode. (c) Second axisymmetric mode. Corresponding frequencies are listed in Table 2.

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Fig. 4

Steady component of the electrostatic pressure, P(1) (Eq. (7)), versus the normalized radial coordinate, obtained for (a) device 1 and (b) device 2. Pressure extracted from FE simulation for actuation voltage of 100 V is shown as circles and demonstrates good agreement with the analytical prediction of Eq. (7). Inset: potential distribution obtained from FE simulation.

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Fig. 5

Calculated frequency response. Plate maximal amplitude of deflection (for r = 0) is shown in the vertical axis for (a) device 1 and (b) device 2. Inset: the deformed shape shown by the amplitude of deflection of the plate when excited in resonance frequency.

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Fig. 6

Main steps of the fabrication process. Silicon is shown in gray color and silicon dioxide is shown as semi-transparent purple. For the sake of demonstration, the fabrication process of only a symmetric half of the device is shown. (a) Starting material–SOI wafer. (b) Front side partial etching of the silicon (DRIE). (c) Back side silicon etch (DRIE). (d) BOX silicon dioxide etching (HF acid).

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Fig. 7

Optical images of device 1 (Table 1) acquired using a confocal microscope. (a) Front side of the device. (b) Back side of the device.

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Fig. 8

Schematic illustration of the experimental setup. For the sake of demonstration only a symmetric half of the device is shown. A time dependent voltage was amplified and applied to the device. The motion was detected using LDV (focused laser beam is shown as a red cone). The FFT was calculated using a real time spectrum analyzer and data processing was completed by computer processing.

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Fig. 9

Measured frequency response for (a) device 1 and (b) device 2. Inset: the relation between the measured displacement amplitude and the amplitude of P(2) (Eq. (7)) at resonance frequency. The solid line represents a linear fit.

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Fig. 10

Schematics of an array of sensors, each sensors aim to detect a different target analyte. A cross section view of one of the sensors is shown in the inset. The device is attached to a back plane, where all the electronics can be located. The actuation space and the electronic can be sealed and separated from the outside environment.




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