# Patent

## Title
Electrostatic-capacity-type acceleration sensor and acceleration measuring device therewith

## Abstract

A moving member having a plurality of moving electrodes is supported by support members at both ends thereof on a substrate surface in such a way that it can be subjected to displacement in a two-dimensional plane. A plurality of fixed electrodes are arranged to face the plurality of moving electrodes respectively, thus forming different facing areas therebetween when an input acceleration is zero. The facing areas formed between pairs of the electrodes facing each other are varied in response to the displacement of the moving member, whereby a capacitance caused by one pair of the electrodes whose facing area is relatively small is used to detect a relatively small input acceleration, and a capacitance caused by the other pair of the electrodes whose facing area is relatively large is used to detect a relatively large input acceleration.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This is a continuation of U.S. patent application Ser. No. 11/210,966, filed Aug. 25, 2005, which is a continuation of U.S. patent application Ser. No. 10/790,440, filed Mar. 1, 2004, issued Feb. 26, 2007 under U.S. Pat. No. 7,004,027, claiming priority on Japanese Patent Application No. 2003-055562, filed on Mar. 3, 2003, the entire contents of which are incorporated herein by reference.


BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrostatic-capacity-type acceleration sensors that detect accelerations based on variations in electrostatic capacity (or capacitance), and it also relates to acceleration measuring devices using electrostatic-capacity-type acceleration sensors.
This application claims priority on Japanese Patent Application No. 2003-55562, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, a typical type of an electrostatic-capacity-type acceleration sensor (as disclosed in Japanese Patent Application Publication No. Hei 7-260510) comprises a capacitance detector having a moving electrode and a fixed electrode, which are arranged opposite to each other such that a separating distance therebetween is varied in response to an input acceleration, wherein a switched-capacitor circuit converts capacitance variation due to displacement of the moving electrode into voltage variation so as to detect and output an acceleration signal, and wherein in order to broaden the detection range, a servo circuit is provided so as to hold the moving electrode at a neutral position by feeding back the acceleration signal.
Another type of the electrostatic-capacity-type acceleration sensor (as disclosed in Japanese Patent Application Publication No. Hei 10-206457) comprises a moving electrode and a fixed electrode, which are arranged on a substrate surface such that a facing area therebetween is varied in response to an input acceleration, wherein based on the capacitance realized by the moving electrode and fixed electrode, the capacitance variation due to the displacement of the moving electrode is detected so as to produce an acceleration signal.
The acceleration sensor having the servo circuit makes it possible to broaden the detection range compared with the acceleration sensor not equipped with the servo circuit because the movement of the moving electrode is limited in response to the output of the servo circuit. However, the range of the acceleration realized by the servo circuit stabilizing the moving electrode is limited by the electrostatic attraction and the weight of the moving electrode; hence, it is not easy to broaden the detection range.
The acceleration sensor in which the facing area between the moving electrode and fixed electrode is varied provides only a single kind of variation characteristic with regard to the facing area responding to the input acceleration; hence, it is not easy to broaden the detection range.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a brand-new electrostatic-capacity-type acceleration sensor whose detection range can be broadened with ease.
An electrostatic-capacity-type acceleration sensor of this invention comprises a plurality of capacitance detectors having pairs of moving electrodes and fixed electrodes that are arranged to face each other on the substrate surface, wherein facing areas are varied in response to an input acceleration. Herein, the capacitance detectors are constituted such that the facing areas between the moving electrodes and fixed electrodes differ from each other when the input acceleration is zero, so that each of the capacitance detectors produces an acceleration signal in response to the capacitance variation occurring between the paired electrodes thereof.
Since the acceleration sensor employs the aforementioned constitution adapted in each of the capacitance detectors in which the facing areas between the moving electrodes and fixed electrodes differ from each other so as to produce a variety of acceleration signals in response to capacitance variations occurring between the paired electrodes, it is possible to easily broaden the overall detection range for detecting the input acceleration by simply increasing the number of the capacitance detectors installed therein, wherein it is possible to provide each capacitance detector with a specific detection range suited thereto, thus realizing a high accuracy in the detection of the acceleration.
In the above, all of the moving electrodes provided for the capacitance detectors can be integrally formed together. Thus, it is possible to improve the integration of components and parts in the manufacturing of the acceleration sensor on the substrate surface by using the modern semiconductor manufacturing technology.
Specifically, each of the capacitance detectors comprises two pairs of moving electrodes and fixed electrodes, which are arranged in such a way that the facing area between the first moving electrode and first fixed electrode increases while the facing area between the second moving electrode and second fixed electrode decreases in response to the same input acceleration, whereby it produces an acceleration signal in response to a ratio or a difference between the first capacitance actualized between the first moving electrode and first fixed electrode and the second capacitance actualized between the second moving electrode and second fixed electrode. Thus, it is possible to noticeably improve the detection sensitivity in each of the capacitance detectors because the acceleration signal can be precisely produced in response to the ratio or difference between the two types of the capacitance.
In addition, the capacitance detectors can be constituted such that one capacitance detector has a relatively small facing area between the paired electrodes so as to produce an acceleration signal in response to a relatively small input acceleration, and the other capacitance detector has a relatively large facing area between the paired electrodes so as to produce an acceleration signal in response to a relatively large input acceleration. Thus, it is possible to detect the input acceleration with a high sensitivity in a relatively broad detection range.
An acceleration measuring device of this invention comprises an electrostatic-capacity-type acceleration sensor including first and second capacitance detectors each having a pair of a moving electrode and a fixed electrode in which the facing area between the paired electrodes in the second capacitance detector is set to be larger than the facing area between the paired electrodes in the first capacitance detector when an input acceleration is zero, a first detection circuit for producing a first acceleration signal in response to capacitance variation occurring between the moving electrode and fixed electrode in the first capacitance detector, a second detection circuit for producing a second acceleration signal in response to capacitance variation occurring between the moving electrode and fixed electrode in the second capacitance detector, and a selector for selectively outputting the first acceleration signal as long as the first acceleration signal does not exceed a threshold level determined in advance and for selectively outputting the second acceleration signal when the first acceleration signal exceeds the threshold level.
The aforementioned acceleration measuring device can measure acceleration at a high accuracy in a relatively broad detection range. The threshold value set to the selector can be determined to exclude a sharp varying region in a capacitance varying characteristic established with respect to variations of the input acceleration; hence, it is possible to reliably avoid occurrence of error detection due to noise.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, aspects, and embodiments of the present invention will be described in more detail with reference to the following drawings, in which:
FIG. 1A shows a schematic arrangement of electrodes in a capacitance detector in which facing areas between moving electrodes and fixed electrodes are decreased in response to acceleration;
FIG. 1B shows a schematic arrangement of electrodes in a capacitance detector in which facing areas between moving electrodes and fixed electrodes are increased in response to acceleration;
FIG. 2 is a graph showing curves representing relationships between the input acceleration and capacitance varying ratio (CU/CD) with respect to various electrode overlap values (L);
FIG. 3 is a plan view showing a layout of parts constituting an electrostatic-capacity-type acceleration sensor in accordance with a preferred embodiment of the invention; and
FIG. 4 is a block diagram showing an example of an acceleration measuring circuit using the electrostatic-capacity-type acceleration sensor.



DESCRIPTION OF THE PREFERRED EMBODIMENT
This invention will be described in further detail by way of examples with reference to the accompanying drawings.
FIGS. 1A and 1B show examples of arrangements of electrodes in capacitance detectors used for an electrostatic-capacity-type acceleration sensor in accordance with the preferred embodiment of the invention, wherein FIG. 1A shows an arrangement of electrodes in the facing area decreasing side, and FIG. 1B shows an arrangement of electrodes in the facing area increasing side. With reference to FIGS. 1A and 1B, the operating principle of the electrostatic-capacity-type acceleration sensor will be described.
In FIGS. 1A and 1B, each of moving electrodes M1 to M4 has a rectangular shape in which the length is set to ‘2L’ and the width is set to a certain value. Herein, the moving electrodes M1 and M3 are arranged along a dashed line L1 in parallel with a prescribed distance therebetween such that the centers in their length directions match the dashed line L1. Similarly, the moving electrodes M2 and M4 are arranged along a dashed line L2 in parallel with a prescribed distance therebetween such that the centers in their length directions match the dashed line L2. The dashed lines L1 and L2 are drawn in parallel with each other. In FIG. 1A, the moving electrodes M1 and M2 are arranged in parallel with an electrode pitch ‘4L’ therebetween. In FIG. 1B, the moving electrodes M3 and M4 are arranged in parallel with an electrode pitch ‘4L’ therebetween. Each of the moving electrodes M1 to M4 has a deadweight function, so that they can be collectively subjected to displacement in left-right directions with respect to the drawing sheet of FIGS. 1A and 1B in response to an input acceleration while maintaining the aforementioned relationship of arrangement.
In FIG. 1A, fixed electrodes S1 and S2 are respectively arranged under the moving electrodes M1 and M2, wherein the fixed electrode S1 is arranged to oppositely face the left-half portion (whose length is ‘L’) of the moving electrode M1, and the fixed electrode S2 is arranged to oppositely face the left-half portion (whose length is ‘L’) of the moving electrode M2. For the sake of convenience, the fixed electrodes S1 and S2 are drawn not to overlap with the moving electrodes M1 and M2 in FIG. 1A. Similarly, in FIG. 1B, fixed electrodes S3 and S4 are respectively arranged under the moving electrodes M3 and M4, wherein the fixed electrode S3 is arranged to oppositely face the right-half portion (whose length is ‘L’) of the moving electrode M3, and the fixed electrode S4 is arranged to oppositely face the right-half portion (whose length is ‘L’) of the moving electrode M4. For the sake of convenience, the fixed electrodes S3 and S4 are drawn not to overlap with the moving electrodes M3 and M4 in FIG. 1B.
In the above, there are provided four pairs of the fixed electrodes and the moving electrodes oppositely facing each other, namely, M1-S1, M2-S2, M3-S3, and M4-S4, each pair of which realizes a certain capacitance. At 0 G where the input acceleration is zero, each pair realizes the same capacitance ‘C0’. Next, suppose that the input acceleration of 1 G is applied on the moving electrodes M1 to M4, all of which are moved rightwards as indicated by arrows in FIGS. 1A and 1B, and each of which is thus subjected to displacement in a distance ‘d’, the “paired” electrodes (i.e., M1-S1, and M2-S2) decrease the facing areas therebetween so that their capacitance decrease from C0 to CD in FIG. 1A, while the other “paired” electrodes (i.e., M3-S3, and M4-S4) increase the facing areas therebetween so that their capacitances increase from C0 to CU. Therefore, when the input acceleration acting on the moving electrodes M1 to M4 becomes greater than 1 G, the capacitance CD further decreases, and the capacitance CU further increases.
FIG. 2 is a graph showing relationships between the input acceleration and the capacitance varying ratio CU/CD with respect to various electrode overlap values, which are related to the electrode facing area ‘L’. That is, the graph of FIG. 2 provides eight curves K1, K2, K3, K4, K5, K6, K7, and K8, each of which shows the relationship between the input acceleration and the capacitance varying ratio CU/CD with regard to the electrode overlap value L, which is set to 0.5 μm, 0.75 μm, 1.0 μm, 1.25 μm, 1.5 μm, 2.0 μm, 2.5 μm, and 5.0 μm respectively.
According to the graph of FIG. 2, as the electrode overlap value L becomes greater, it becomes possible to detect ‘greater’ input acceleration. For this reason, the acceleration sensor of the present embodiment comprises a plurality of capacitance detectors having different electrode overlap values, each of which is adequately constituted to detect input acceleration in a certain detection range suited thereto. That is, a relatively small input acceleration can be detected using a capacitance detector whose electrode overlap value is relatively small, and a relatively large input acceleration is detected using a capacitance detector whose electrode overlap value is relatively large. Hence, it is possible to actualize a high sensitivity in detecting acceleration in a relatively broad range of detection.
With respect to each of the aforementioned curves (e.g., K1), in order to avoid error detection due to noise, it is preferable not to use a certain region in which the capacitance varying ratio CU/CD becomes large so as to make variation sharp.
FIG. 3 shows an electrostatic-capacity-type acceleration sensor 10 in accordance with the preferred embodiment of the invention.
In the acceleration sensor 10, a moving member MB having a deadweight function is arranged on the surface of a silicon substrate and is supported by four support members H1 to H4 at both sides thereof such that it can be subjected to displacement in a direction DS in parallel with the substrate surface. The ends of the support members H1 to H4 are fixed at prescribed positions by fixing members P1 to P4 respectively. Four moving electrodes M11, M21, M24, and M14 are arranged on one side of the moving member MB and are projected in parallel with the substrate surface. In addition, four moving electrodes M13, M23, M22, and M12 are arranged on the other side of the moving member MB and are projected in parallel with the substrate surface. All of the moving electrodes M11 to M14 have the same length projecting from the moving member MB, and all of the moving electrodes M21 to M24 have the same length projecting from the moving member MB. In addition, all of the moving electrodes M11, to M14 have the same width, and all of the moving electrodes M21 to M24 have the same width that is greater than the width of the moving electrodes M11 to M14.
All of the moving member MB, the moving electrodes M11-M14 and M21-M24, and the support members H1-H4 are integrally combined together as a single assembly. Such an assembly can be formed by patterning conductive layers, which are made of a semiconductor or a metal and are deposited on an insulating film covering the substrate surface, wherein after the patterning, the insulating film is removed to allow movements of the moving member MB, the moving electrodes M11-M14 and M21-M24, and the support members H1-H4, for example. In addition, four fixing holes are formed at four fixing positions on the insulating film covering the substrate surface and are filled with conductive plugs, made of a semiconductor or a metal, thus forming the four fixing members P1 to P4.
Four fixed electrodes S11 to S14 are arranged opposite to the moving electrodes M11 to M14 with relatively small facing areas therebetween, thus actualizing capacitances C11 to C14 having substantially the same value when the input acceleration is zero. In addition, four fixed electrodes S21 to S24 are arranged opposite to the moving electrodes M21 to M24 with relatively large facing areas therebetween, thus actualizing capacitances C21 to C24 having substantially the same value when the input acceleration is zero. Suppose that the moving member MB is subjected to displacement in the direction DS as indicated by an arrow in FIG. 4 (i.e., a forward direction in parallel with the sheet of FIG. 4), wherein the fixed electrodes S11, S12, S21, and S22 are associated with the moving electrodes M11, M12, M21, and M22 such that the facing areas therebetween are reduced so as to decrease the capacitances C11, C12, C21, and C22 respectively, while the fixed electrodes S13, S14, S23, and S24 are associated with the moving electrodes M13, M14, M23, and M24 such that the facing areas therebetween are increased so as to increase the capacitances C13, C14, C23, and C24 respectively. In FIG. 4, the capacitances C11-C14 and C21-C24 are associated with arrows, wherein a downward slanted arrow indicates that the corresponding capacitance is decreased, and an upward slanted arrow indicates that the corresponding capacitance is increased.
The aforementioned fixed electrodes S11-S14 and S21-S24 are formed by impurity-doped regions in which conductive impurities are selectively doped onto the substrate surface. Alternatively, they are formed by patterning conductive layers, made of a semiconductor or a metal, which are deposited on an insulating film covering the substrate surface.
Wiring layers W11-W14 and W21-W24 are respectively extended from the fixed electrodes S11-S14 and S21-S24. The wiring layers W11 and W12 are interconnected with a detection line Ta via connection members Q11 and Q12 respectively. The wiring layers W13 and W14 are interconnected with a detection line Tb via connection members Q13 and Q14 respectively. The wiring layers W2, and W22 are interconnected with a detection line Tc via connection members Q21, and Q22 respectively. The wiring layers W23 and W24 are interconnected with a detection line Td via connection members Q23 and Q24 respectively. The support member H1 is connected with a detection line Te.
The wiring layers W11-W14 and W21-W24 are formed by patterning such that their widths are reduced to be as small as possible in order to reduce parasitic capacitances thereof; the distances with the moving parts (i.e., MB, M11-M14, M21-M24) are increased to be as large as possible; and the electrode overlap values associated with the moving parts are reduced to be as small as possible. Similar to the fixed electrodes S11-S14 and S21-S24, the wiring layers W11-W14 and W21-W24 are formed by impurity-doped regions or conductive layers subjected to patterning. Eight connection holes are formed at prescribed positions on the insulating film covering the substrate surface and are filled with conductive plug, made of a semiconductor or a metal, thus actualizing the connection members Q11-Q14 and Q21-Q24. Incidentally, the connection members (e.g., Q11) can be formed by adopting the aforementioned treatment for use in the formation of the fixing members (e.g., P1).
For example, after the wiring layers W11-W14 and W21-W24 are formed as impurity-doped regions, a first wiring layer forming process is performed on the insulating layer covering the substrate surface so as to form the fixed electrodes S11-S14 and S21-S24; then, a second wiring layer forming process is performed on the insulating film covering the fixed electrodes S11-S14 and S21-S24 so as to form the moving member MB, the moving electrodes M11-M14 and M21-M24, the support members H1-H4, and the detection lines Ta-Td. Thus, it is possible to increase distances between the wiring layers W11-W14 and W21-W24 and the moving parts (i.e., MB, M11-M14, M21-M24); hence, it becomes possible to reduce parasitic capacitances.
FIG. 4 shows an equivalent circuit of the acceleration sensor 10 shown in FIG. 3; specifically, it shows an example of an acceleration measuring circuit using the acceleration sensor 10 shown in FIG. 3. The acceleration measuring circuit of FIG. 4 is designed to measure the input acceleration in a range between 0 G and 8 G as shown in FIG. 2, wherein the curve K7 is used for the measurement of a relatively small input acceleration that ranges from 0 G to 4 G, and the curve K8 is used for the measurement of a relatively large input acceleration that ranges from 4 G to 8 G Specifically, in a range R1 wherein 0 G≦input acceleration≦4 G, compared with the curve K8, the curve K7 provides a greater value of the capacitance varying ratio CU/CD, which in turn actualizes a high accuracy in measurement. In a range R2 where 4 G<input acceleration≦8 G, the curve K7 includes a measurement-incapable region and is not used in measurement, whereas the curve K8 is suitable in measurement. In order to avoid error detection due to noise, it is necessary not to use a sharp varying region of the curve K7 with respect to the range R1, and it is necessary not to use a sharp varying region of the curve K8 with respect to the great input acceleration over 8 G, wherein these regions are excluded from the measurement.
A detection circuit 12A is connected with the detection lines Ta, Tb, and Te; and a detection circuit 12B is connected with the detection lines Tc, Td, and Te. Suppose that the capacitance varying characteristic of the capacitance detector, including the capacitances C11 to C14, against the input acceleration is shown by the curve K7 shown in FIG. 2, while the capacitance varying characteristic of the capacitance detector, including the capacitances C21 to C24, against the input acceleration is shown by the curve K8 shown in FIG. 2, for example. Herein, the capacitance varying ratio CU/CD is calculated using C11-C14 and C21-C24 as follows:




  
    
      
        C
        13
      
      +
      
        C
        14
      
    
    
      
        C
        11
      
      +
      
        C
        
          12
          ⁢
          
            
          
        
      
    
  
  ⁢
  
    
  
  ⁢
  
    
      
        C
        23
      
      +
      
        C
        24
      
    
    
      
        C
        21
      
      +
      
        C
        22
      
    
  




With reference to the capacitance varying characteristic of the curve K7, the detection circuit 12A produces an acceleration signal AS1 representing the voltage related to a ratio (or a difference) between (C13+C14) and (C11+C12). With reference to the capacitance varying characteristic of the curve K8, the detection circuit 12B produces an acceleration signal AS2 representing the voltage related to a ratio (or a difference) between (C23+C24) and (C21+C22). The acceleration signal AS1 is increased in level as the input acceleration increases from 0 G. The acceleration signal AS2 is set such that it substantially matches the acceleration signal AS1 in level at the input acceleration of 4 G, and it is further increased from that level as the input acceleration increases from 4 G.
A selector 14 has terminals A and B for receiving signals as well as a select terminal SB. Herein, the selector 14 selectively outputs the acceleration signal SA1 when SB is low (or at ‘0’), while it selectively outputs a signal SO2, which is output from a selector 18, when SB is high (or at ‘1’). A comparator 16 compares the acceleration signal AS1 with reference voltage VR1 so as to produce a comparison output CO1, which is supplied to the select terminal SB of the selector 14. That is, the comparator 16 outputs CO1=0 when AS1≦VR1; and it outputs CO1=1 when AS1>VR1.
The prescribed voltage related to the capacitance varying ratio CU/CD, which is read from the curve K7 in FIG. 2 when the input acceleration is 4 G; is given as the reference voltage VR1 supplied to the comparator 16. As a result, the selector 14 provides a selection output SO1 corresponding to the acceleration signal AS1, which is produced in response to the input acceleration in the range R1 where 0 G≦input acceleration≦4 G.
The selector 18 has terminals A and B for receiving signals as well as a select terminal SB. That is, the selector 18 selectively outputs the acceleration signal SA2 when SB is low (or at ‘0’), while it selectively outputs an out-of-range signal ASx representing that the input acceleration is out of the measurement range when SB is high (or at ‘1’). A comparator 20 compares the acceleration signal AS2 with a reference voltage VR2 so as to provide a comparison output CO2 to the select terminal SB of the selector 18. That is, the comparator 20 outputs CO2=0 when AS2≦VR2; and it outputs CO2=1 when AS2>VR2.
The prescribed voltage related to the capacitance varying ratio CU/CD, which is read from the curve K8 in FIG. 2 when the input acceleration is 8 G is given as the reference voltage VR2 supplied to the comparator 20. As a result, the selector 18 provides a selection output SO2 corresponding to the acceleration signal AS2, which is produced in response to the input acceleration in the range R2 where 4 G<input acceleration≦8 G In this case, the selector 14 is set so as to select the terminal B for receiving the selection output SO2 of the selector 18 in response to the comparison output CO1 of the comparator 16; hence, the selector 14 provides the selection output SO1 corresponding to the acceleration signal AS2.
When the acceleration signal AS2 exceeds the reference voltage VR2 (where AS2>VR2), the comparison output CO2 of the comparator 20 turns to ‘1’; hence, the selector 18 provides the selection output SO2 corresponding to the out-of-range signal ASx. In this case, the selector 14 is set so as to select the terminal B for receiving the selection output SO2 of the selector 18; hence, it provides the selection output SO1 corresponding to the out-of-range signal Asx. An acceleration display (not shown) is provided to display an acceleration based on the acceleration signal AS1 or AS2 and to also display a message that the input acceleration is out of the measurement range in response to the out-of-range signal ASx.
The aforementioned acceleration measuring circuit of FIG. 4 is designed to perform measurement at a high accuracy with respect to a relatively broad range of 0 G≦input accelerations 8 G; which is realized by the combination of the aforementioned ranges R1 and R2. Herein, the measurement regarding the input acceleration is performed by excluding sharp varying regions of the curves K7 and K8 shown in FIG. 2; hence, it is possible to reliably avoid occurrence of error detection due to noise. The acceleration measuring circuit of FIG. 4 is designed selectively using two curves K7 and K8 within the eight curves K1 to K8 shown in FIG. 2. Of course, it is possible to easily modify the acceleration measuring circuit using three or more curves.
As described heretofore, an acceleration sensor of this invention is designed such that a plurality of capacitance detectors including pairs of moving electrodes and fixed electrodes in which facing areas therebetween differ from each other when the input acceleration is zero are arranged on the substrate surface, wherein each of the capacitance detectors is set to a prescribed measurement range of acceleration. Hence, by simply increasing the number of capacitance detectors, it is possible to actualize broadening of the overall detection range of acceleration for an acceleration measuring circuit; thus, it is possible to realize acceleration detection at a high sensitivity and in a relatively broad range.
As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the claims.

## Claims

1. An electrostatic-capacity-type acceleration sensor comprising:
a support member fixed at a prescribed position of a substrate;
a moving member that is supported by the support member;
a first moving electrode and a second moving electrode, which are fixed to the moving member;
a first fixed electrode arranged to face with the first moving electrode; and
a second fixed electrode arranged to face with the second moving electrode,
wherein an overlap value (L1) representing a length of overlap between the first moving electrode and the first fixed electrode when an input acceleration is zero is less than an overlap value (L2) representing a length of overlap between the second moving electrode and the second fixed electrode when the input acceleration is zero.

2. The electrostatic-capacity-type acceleration sensor according to claim 1, wherein an acceleration (A1) detected by the first moving electrode and the first fixed electrode is equal to or smaller than an acceleration A2 detected by the second moving electrode and the second fixed electrode, where A1≦A2.

3. An electrostatic-capacity-type acceleration sensor comprising:
at least one moving member;
at least one moving electrode that is fixed to the moving member and is subjected to displacement in synchronization with a displacement of the moving member; and
at least one fixed electrode arranged to face with the moving electrode, wherein a plurality of the moving electrodes and the fixed electrodes are respectively paired to form a plurality of capacitors (C1-C4),
wherein in response to a displacement of the moving member, the capacitors (C1, C2) are mutually varied in terms of capacitance, thus realizing a electrostatic capacity variation ratio (CU1/CD1), and the other capacitors (C3, C4) are mutually varied in terms of capacitance, thus realizing an electrostatic capacity variation ratio (CU2/CD2), so that the electrostatic capacity variation ratio (CU1/CD1) differs from the electrostatic capacity variation ratio (CU2/CD2).

4. An electrostatic-capacity-type acceleration sensor comprising:
at least one moving member;
at least one moving electrode that is fixed to the moving member and is subjected to displacement in synchronization with a displacement of the moving member; and
at least one fixed electrode arranged to face with the moving electrode, wherein a plurality of the moving electrodes and the fixed electrodes are respectively paired to form a plurality of capacitors (C1-C4),
wherein in response to a displacement of the moving member, the capacitors (C1, C2) are mutually varied in terms of capacitance, thus realizing a electrostatic capacity variation ratio (CU1/CD1), and the other capacitors (C3, C4) are mutually varied in terms of capacitance, thus realizing an electrostatic capacity variation ratio (CU2/CD2), so that the electrostatic capacity variation ratio (CU1/CD1) differs from the electrostatic capacity variation ratio (CU2/CD2);
wherein each pair of capacitors differs from each of the other capacitors in terms of the electrostatic capacity variation.

5. An acceleration measurement device comprising:
a plurality of moving electrodes, each having a same potential, which are fixed to a moving member; and
a plurality of fixed electrodes, which are respectively paired with the plurality of moving electrodes so as to form a plurality of capacitors,
wherein an overlap value (L1) representing a length of overlap between a first moving electrode and a first fixed electrode when an input acceleration is zero is less than an overlap value (L2) representing a length of overlap between a second moving electrode and a second fixed electrode when the input acceleration is zero,
wherein each of the fixed electrodes is connected with an detection circuit used for detection of a potential thereof, so that at least one detection circuit reads differential potentials of the capacitors, which are varied in response to the input acceleration,
wherein at least one capacitor produces a differential potential in response to the input acceleration, which is different from a differential potential produced by the other capacitor.

6. An electrostatic-capacity-type acceleration sensor comprising:
at least one moving member;
at least one moving electrode that is fixed to the moving member and is subjected to displacement in synchronization with a displacement of the moving member; and
at least one fixed electrode arranged to face with the moving electrode, wherein a plurality of the moving electrodes and a plurality of the fixed electrodes are respectively paired to form a plurality of capacitors,
wherein in response to a displacement of the moving member, a first pair of capacitors realizes a first electrostatic capacity variation ratio with respect to each other, and a second pair of capacitors realizes a second electrostatic capacity variation ratio with respect to each other, so that the first electrostatic capacity variation ratio differs from the second electrostatic capacity variation ratio.

7. The electrostatic-capacity-type acceleration sensor of claim 6 wherein each pair of capacitors differs from any other pair of capacitors in terms of the electrostatic capacity variation ratio.