MEMS chip for wind speed measurements

Abstract

A MEMS chip for wind speed measurements is provided. The chip integrates one or multiple embedded channels and a pressure sensor. The pressure sensor consists of a sensing membrane with a cavity beneath it. Each channel has one end connects to the cavity while the other end opens on the edge of the chip. To measure the wind speed, the membrane faces the wind and the air stagnates onto it while the channel connects the cavity to the static pressure. And the membrane deforms according to the wind pressure. The wind speed is then derived from the measured wind pressure.

Claims

1. A MEMS chip for wind speed measurements including a pressure sensor and an embedded channel that is sealed inside the MEMS chip except one opening end on an edge of the chip; the pressure sensor includes a substrate with a cavity, two parallel electrodes, and a pressure sensing membrane sitting on top of the substrate over the cavity; one of the two parallel electrodes is on an inner surface of the pressure sensing membrane inside the cavity of the substrate, and the other electrode is on the bottom of the cavity of the substrate; the embedded channel has one end connected to the cavity and another end opening on the edge of the chip; the pressure sensing membrane faces the wind and the air is stagnated onto the pressure sensing membrane; the embedded channel connects the cavity to the static pressure P.sub.s; the pressure sensing membrane deforms according to P.sub.t-P.sub.s, wherein P.sub.t is the stagnation pressure; the maximum membrane deformation is in a center of the cavity and a deformation w at other locations of the membrane is a function of x and y, wherein (x, y) is a two dimensional coordinate system on the surface of the membrane; the capacitance C between two electrodes after the deformation is calculated as $C=\mathrm{.Math.}\underset{\mathrm{electrode}}{}\frac{dxdy}{d-w\left(x,y\right)},$ where is the permittivity and d is the distance between two electrodes without deformation, the wind speed V is derived from a formula $\sqrt{\frac{2\left(P_t-P_s\right)}{}},$ wherein is the air density.

2. A MEMS chip for wind speed measurements including a pressure sensor and at least three embedded channels sealed inside the MEMS chip except that each channel has one opening end on an edge of the chip; the pressure sensor includes a substrate with a cavity, two parallel electrodes, and a pressure sensing membrane sitting on top of the substrate over the cavity; one electrode is on an inner surface of the pressure sensing membrane inside the cavity of the substrate, and the other electrode is on the bottom of the cavity of the substrate; each embedded channel has one end connected to the cavity and another end opening on each edge of the chip; the pressure sensing membrane faces the wind and the air is stagnated onto the pressure sensing membrane; each embedded channel connects the cavity to the static pressure P.sub.s; the pressure sensing membrane deforms according to P.sub.t-P.sub.s, wherein P.sub.t is the stagnation pressure; the maximum membrane deformation is at the center of the cavity and a deformation w at other locations of the membrane is a function of x and y, wherein (x, y) is a two dimensional coordinate system on the surface of the membrane; the capacitance C between the two electrodes after the deformation is calculated as $C=\mathrm{.Math.}\underset{\mathrm{electrode}}{}\frac{dxdy}{d-w\left(x,y\right)},$ where is the permittivity and d is the distance between two electrodes without deformation; the wind speed V is derived from a formula: $V=\sqrt{\frac{2\left(P_t-P_s\right)}{}},$ wherein is the air density.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 shows the front view of the MEMS chip with a single channel.

(2) FIG. 2 shows the section view of the MEMS chip with a single channel.

(3) FIG. 3 shows the front view of the MEMS chip with three channels.

(4) FIG. 4 shows the wind speed corresponding to the capacitance when membrane thickness is 1 m, 2 m, or 4 m.

(5) FIG. 5 shows the wind speed corresponding to the capacitance when membrane thickness is 8 m, 16 m, or 32 m.

REFERENCE NUMERALS IN THE DRAWINGS

(6) 1. channel

(7) 2. cavity

(8) 3. membrane

(9) 4. substrate

(10) 5. electrode

DETAILED DESCRIPTION OF THE INVENTION

(11) As FIG. 1 and FIG. 2 show, the MEMS chip has a membrane 3 sitting on top of the substrate 4. The substrate 4 has a cavity 2. The cavity 2 is beneath the membrane 3. There are two parallel electrodes 5. One electrode is on the surface of the membrane 3 inside the cavity 2 and the other one is on the bottom of the cavity 2. The channel 1 is embedded inside the chip. One end of the channel 1 is connected to the cavity 2. And the other end of the channel 1 opens on the edge of the chip as FIG. 2 shows.

(12) The membrane 3 faces the wind and the air is stagnated onto it during the measurement. Bernoulli's equation also tells that the edge of the chip is under the static pressure. So the cavity 2 is connected to the static pressure through the channel 1. And the membrane 3 deforms according to the pressure difference between the stagnation pressure and the static pressure, i.e. the wind pressure. This deformation is then transformed into measurable capacitance changes by the electrodes 5.

(13) The whole chip size is 4 mm4 mm and the cross section of the channel 1 is 200 m5 m. The size of the cavity 2 is 900 m900 m. The size of the electrodes 5 is 810 m810 m. Without deformation, the distance between two electrodes 5 is 1 m and the capacitance between them is calculated as:

(14) $C=\mathrm{.Math.}\frac{A}{d}=8.8542{10}^{-12}\frac{0.000810.00081}{1{10}^{-6}}5.81\mathrm{pico}\text{-}\mathrm{Farad}$
Where:

(15) C is the capacitance between two parallel electrodes 5;

(16) is the permittivity of the space between two electrodes 5;

(17) A is the surface area of the electrodes 5;

(18) d is the distance between two electrodes 5 without deformation;

(19) When a uniform wind pressure P.sub.w deforms the membrane 3, the maximum deformation w.sub.0 is at the center of the cavity 2. And the relationship between P.sub.w and w.sub.0 is,

(20) $P_w=\left(\frac{4.06h^2}{1-v^2}+\frac{1.994\left(1-0.271v\right)w_0^2}{1-v}\right)\frac{E^2}{1-v^2}\frac{{\mathrm{hw}}_0}{a^4}$
Where:

(21) h is the thickness of the membrane 3;

(22) E is the Young's modulus of the membrane 3;

(23) is the Poisson ratio of the membrane 3;

(24) a is the half length of the edge of the cavity 2;

(25) In a two dimensional coordinate system (x, y) on the surface of the membrane 3, the origin of the coordinate system is at the center of the cavity 2. The axis x and the axis y are parallel to the edges of the surface of the cavity 2. The deformation w of the other locations of the membrane 3 is a function of x and y,

(26) $w={w_0\left(1-\frac{x^2}{a^2}\right)}^2{\left(1-\frac{y^2}{a^2}\right)}^2\left(1+1.1\frac{x^2+y^2}{a^2}\right)$
Then the capacitance between two electrodes 5 after the deformation is calculated as,

(27) $C=\mathrm{.Math.}\underset{\mathrm{electrode}}{}\frac{\mathrm{dxdy}}{d-w\left(x,y\right)}$
The wind pressure changes the separation distance between two electrodes 5, and the capacitance changes accordingly.

(28) FIG. 4 and FIG. 5 depict the wind speed corresponding to the capacitance of the chip. And in the calculation, the Young's modulus of the membrane is 169 GPa; the Poisson ratio of the membrane is 0.3; the density of the air is 1.23 kg/m3.

(29) FIG. 3 shows another chip, where three channels 1 are used. Each channel has one end connect to the cavity 2. And the other end opens on the edge of the chip. This embodiment reduces the resistances for the air entering or leaving the cavity 2. Then the chip responds faster.

(30) It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.