JP2007187494A - Vibration detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce noise and improve frequency characteristics without increasing the cost. <P>SOLUTION: In the vibration detector, for detecting anyone of the displacement, velocity, and acceleration of the object to be measured, is provided with an oscillator 21 freely supported by an elastic part 22, and fixed electrodes 12 and 32 at the position opposite to the oscillator 21, based on the static capacitance formed by the oscillator 21, the fixed electrodes 12 and 32 the gap d<SB>0</SB>between the oscillator 21 and center parts 12c and 32c of the fixed electrode 12 and 32 are made larger than the peripheral parts. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a vibration detector, and more particularly to a low noise vibration detector suitable for use as an acceleration sensor, a speed sensor, or a displacement sensor, which is manufactured using MEMS technology.

  As described in Patent Document 1, as shown in FIG. 1, a movable electrode plate 21 that is movably supported by an elastic portion 22, and a fixed electrode plate 12 that is fixed at a position facing the movable electrode plate 21. , 32, and any one of the displacement, velocity, and acceleration of the measured object that displaces the movable plate 21 based on the capacitance of the capacitor formed by the movable plate 21 and the fixed plate 12, 32. A vibration detector for detecting is known. This vibration detector is used, for example, as an acceleration sensor for seismometers or automobile airbags.

  In the figure, 2 is a silicon substrate disposed in the center, 1 and 3 are glass substrates as two light-transmitting materials sandwiching the silicon substrate 2 from above and below, and 20 occupies most of the silicon substrate 2. A main body part in which an elastic part 22 formed in a plate spring shape and a circular movable pole plate 21 supported by the elastic part 22 so as to be displaceable in the thickness direction are integrally provided by etching; ˜26 are silicon islands for extracting electrodes, 27 is a receiving portion for receiving a getter G as a gas adsorbent, 11 and 31 are brake plates, and 13 to 17 are lead wires having tapered through holes. The attachment portions 11a and 31a are brake electrodes, and 12a is a detection electrode.

  In this state, when a vibration is applied to the detection unit from the outside, the movable pole plate 21 supported at the neutral position (parallel position) by the elastic unit 22 is caused by the vibration. Displacement in the plate thickness direction while being restrained by. As a result, the capacitance of the variable capacitor formed by the upper fixed plate 12 and the movable plate 21, and the capacitance of the variable capacitor formed by the lower fixed plate 32 and the movable plate 21. Increase or decrease from each other, and accordingly, the capacity difference between the two fluctuates. Then, a control unit (not shown) applies a voltage corresponding to the variation of the capacitance difference to the braking electrode plates 11 and 31 via the lead wires (not shown) bonded through the lead wire attaching portions 13 and 14. By applying (feedback), control is performed to cancel out the electrostatic force generated by each of the braking electrode plates 11 and 31 and the inertial force of the movable electrode plate 21. Thereby, the displacement of the movable electrode plate 21 due to external vibration can be minimized. In the process, the control unit detects the vibration (for example, acceleration) applied to the vibration sensor based on the magnitude of the voltage fed back to each of the braking electrode plates 11 and 31, and outputs it to the outside.

  Such a resolution of the vibration detector is caused by thermal noise due to the movement of gas molecules existing in the upper and lower gaps (submicron to about 5 μm) of the movable electrode plate (also referred to as a vibrator) 21.

  Therefore, Patent Document 1 describes that a getter G is provided to reduce the number of gas molecules.

  The thermal noise is expressed by the following equation.

_{a = √ (4k b λT)} / m ... (1)

Here, a is noise input referred vibration detector, k _b is the Boltzmann constant, lambda is the attenuation coefficient, T is the temperature, m is the mass of the oscillator 21.

  The equation (1) shows that the input conversion noise a of the vibration detector depends on the attenuation coefficient λ. This attenuation coefficient λ is a fixed plate (also called a fixed electrode) as shown in the following expression. ) 12 and 32 and the shape of the gap formed by the vibrator 21.

λ = (μ _eff S ² / d ₀ ³ ) α (2)

Here, μ _eff is an effective viscosity coefficient, S is an area, d ₀ is a gap, α is a coefficient depending on pressure, and is a constant in a low frequency range.

  In general, when the shape of the gap is determined so that the internal pressure when the vibrator 21 vibrates becomes low, the attenuation coefficient λ becomes small, and the input conversion noise a of the vibration detector caused by the thermal noise due to the movement of gas molecules is reduced. Can be small.

  Therefore, FIG. 7 of Patent Document 1 also describes that a hole is formed in the center of the vibrator 21 where the internal pressure becomes the largest.

JP 2004-12326 A (FIGS. 1 to 3 and FIG. 7)

  However, the former method of providing the getter G leads to an increase in cost. In the latter method of making a hole in the center of the vibrator 21, the mass m of the vibrator 21 is also reduced, so that the input conversion noise a of the vibration detector is reduced. There was a problem that the efficiency for reducing the size was not good.

  The present invention has been made to solve the above-described conventional problems, and an object of the present invention is to provide a low noise vibration detector without increasing the cost.

  The present invention includes a vibrator that is displaceably supported by an elastic portion, and a fixed electrode fixed at a position facing the vibrator, and a capacitance of a capacitor formed by the vibrator and the fixed electrode In the vibration detector that detects any one of the displacement, velocity, and acceleration of the measured object based on the above, the gap between the fixed electrode and the vibrator is made larger than the peripheral portion, whereby the problem is solved. It has been solved.

  Further, a concave portion or a through hole is provided in the central portion of the fixed electrode.

  Further, the gap between the vibrator and the fixed electrode at the center is larger than that at the periphery.

  Further, a concave portion is provided in the central portion of the vibrator.

  Further, a concave portion is provided in the central portion of both the fixed electrode and the vibrator.

  Further, a through hole is provided in the central part of both the fixed electrode and the vibrator.

  In addition, a concave portion is provided in the central portion of the vibrator of the fixed electrode, and a through hole is provided in the central portion of the vibrator.

  In addition, a through hole is provided in the central portion of the vibrator of the fixed electrode, and a concave portion is provided in the central portion of the vibrator.

  According to the present invention, since the gap with the vibrator in the central portion of the fixed electrode is made larger than that in the peripheral portion, when the vibrator vibrates, the internal pressure at the enlarged gap is reduced, and therefore the damping constant λ becomes smaller. As a result, a vibration detector with a lower noise level can be obtained, and the frequency characteristics can be improved. Since this method can reduce only the attenuation coefficient λ without reducing the mass of the vibrator as compared with the conventional method, the input conversion noise a can be effectively reduced.

  Note that not only the fixed electrode side but also the gap with the fixed electrode in the central portion of the vibrator is made larger than the peripheral portion, so that the internal pressure in the gap can be further reduced. In this case, input conversion noise can be further reduced.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

As shown in FIG. 2 (disassembled perspective view), FIG. 3 (plan view), and FIG. 4 (transverse sectional view taken along line IV-IV in FIG. 3), the first embodiment of the present invention is fixed as in the conventional example. the electrodes 12 and 32 and annular shape, the central portion 12c, by recessing the 32c, the central portion 12c, the gap _{d 0} of the transducer 21 at 32c, is obtained by a larger than the peripheral portion.

  As for other points, the wiring patterns 41 to 43 are provided only on the upper surface of the glass substrate 1, and the conventional example shown in FIG. 1 is used except that only two islands 23 and 24 are provided for wiring extraction. Therefore, the corresponding parts are denoted by the same reference numerals, and detailed description thereof is omitted.

  According to the present embodiment, when the fixed electrodes 12 and 32 are flat as in the prior art, the internal pressure as shown by the broken line A in FIG. 5 causes the central portions 12c and 32c to be recessed. As shown by the solid line B, the attenuation coefficient λ decreases, so that an acceleration sensor with a low noise level can be obtained. For example, if the gap of 2 μm is expanded to 4 μm, the internal pressure at the center can be reduced to 1/8.

  Next, a second embodiment of the present invention will be described with reference to FIG.

  In the present embodiment, through holes 12h and 32h are provided in the central part of the fixed electrodes 12 and 32, and the gap with the vibrator 21 in the central part is made larger than that in the peripheral part. Since other points are the same as those in the first embodiment, description thereof will be omitted.

  According to the present embodiment, the internal pressure at the central portion can be substantially 0 (referenced to the external atmospheric pressure), and is further lower than that in the first embodiment, so that an acceleration sensor with a lower noise level can be obtained.

  Next, a third embodiment of the present invention will be described with reference to FIG.

  In the present embodiment, not only the central portions 12c and 32c of the fixed electrodes 12 and 32 are recessed, but also the central portion 21c of the vibrator 21 is recessed as in the first embodiment. Since other points are the same as those in the first embodiment, description thereof will be omitted.

  According to the present embodiment, the gap can be increased as compared with the first embodiment.

  Next, a fourth embodiment of the present invention will be described in detail with reference to FIG.

  In the present embodiment, in the same acceleration sensor as in the first embodiment, a through-hole 21h is provided in the central portion of the vibrator 21. Since other points are the same as those in the first embodiment, description thereof will be omitted.

  According to this embodiment, the gap at the center can be made larger than in the first and second embodiments.

  Note that the method of increasing the gap at the center is not limited to the above embodiment, and a recess is provided only in the center 21c of the vibrator 21, as in the fifth embodiment shown in FIG. As shown in the sixth embodiment, through holes 12h, 32h, and 21h are provided in both the central portion of the fixed electrodes 12 and 32 and the central portion of the vibrator 21, or as in the seventh embodiment shown in FIG. While the through holes 12h and 32h are provided in the central part of the fixed electrodes 12 and 32, the concave part can be provided in the central part 21c of the vibrator 21.

  In the above embodiment, the fixed electrodes 12 and 32 are annular, but the shape of the fixed electrode is not limited to this. For example, as in the eighth embodiment shown in FIG. 32 may be rectangular. In addition, either one of the vibrator 21 and the fixed electrodes 12 and 32 may be circular, and the other may be rectangular, or may have another shape.

  In the above embodiment, since the fixed electrodes 12 and 32 are provided above and below the vibrator 21, the sensitivity and temperature characteristics are good. It is also possible to use only one of the upper and lower sides.

  In each of the above embodiments, the silicon substrate 2 is sandwiched between the glass substrates 1 and 3, but the silicon surface is insulated to replace the glass substrates 1 and 3, and the exposed silicon is converted into an electrode. All can be made of silicon.

  In the above description, the description has been made in the case of the acceleration sensor in which the frequency of the vibration to be detected is smaller than the natural frequency of the detector. Can be used as a velocity sensor, or a displacement sensor when the frequency of vibration to be detected is greater than the natural frequency of the detector.

An exploded perspective view showing a configuration of a conventional vibration detector described in Patent Document 1 The disassembled perspective view which shows the structure of 1st Embodiment of this invention. Same top view Similarly, a cross-sectional view along line IV-IV in FIG. Diagram showing the principle of the present invention Sectional drawing which shows the principal part structure of 2nd Embodiment of this invention. Sectional drawing which similarly shows the principal part structure of 3rd Embodiment Sectional drawing which similarly shows the principal part structure of 4th Embodiment Sectional drawing which similarly shows the principal part structure of 5th Embodiment Sectional drawing which similarly shows the principal part structure of 6th Embodiment Sectional drawing which similarly shows the principal part structure of 7th Embodiment The top view which similarly shows the principal part structure of 8th Embodiment

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 3 ... Glass base 2 ... Silicon base 12, 32 ... Fixed electrode 12c, 21c, 32c ... Center part 12h, 21h, 32h ... Through-hole 21 ... Vibrator 22 ... Elastic part

Claims (9)

A vibrator supported by the elastic portion so as to be displaceable, and a fixed electrode fixed at a position facing the vibrator, and is covered based on a capacitance of a capacitor formed by the vibrator and the fixed electrode. In a vibration detector that detects any of the displacement, speed, or acceleration of the measurement object,
The vibration detector according to claim 1, wherein a gap between the fixed electrode and the vibrator is larger than that of a peripheral portion.   The vibration detector according to claim 1, wherein a concave portion is provided in a central portion of the fixed electrode.   The vibration detector according to claim 1, wherein a through hole is provided in a central portion of the fixed electrode.   The vibration detector according to any one of claims 1 to 3, wherein a gap between the vibrator and the fixed electrode in a central portion of the vibrator is larger than that in a peripheral portion.   The vibration detector according to claim 4, wherein a concave portion is provided in a central portion of the vibrator.   The vibration detector according to claim 2, wherein a concave portion is provided in a central portion of both the fixed electrode and the vibrator.   The vibration detector according to claim 3, wherein a through hole is provided in a central portion of both the fixed electrode and the vibrator.   The vibration detector according to claim 2, wherein a concave portion is provided in a central portion of the fixed electrode, and a through hole is provided in a central portion of the vibrator.   6. The vibration detector according to claim 3, wherein a through hole is provided in a central portion of the fixed electrode, and a concave portion is provided in a central portion of the vibrator.

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