# Patent

## Title
Acceleration measurement method using electrostatic-capacity-type acceleration sensor

## Abstract

An acceleration measurement method adapted to an electostatic-capacity-type acceleration sensor comprising a first capacitance detector and a second capacitance detector both of which have moving electrodes and fixed electrodes arranged on a substrate surface so that facing areas therebetween are varied in response to input acceleration, wherein said second capacitance detector sets the facing areas between the moving electrodes and the fixed electrodes to be larger than those of the first capacitance detector when the input acceleration is zero. If first acceleration information representing capacitance variations detected between the moving electrodes and fixed electrodes in the first capacitance detector exceed an upper limit value that is set in advance with respect to the first capacitance detector, it outputs second acceleration information representing capacitance variations detected between the moving electrodes and the fixed electrodes in the second capacitance detector. If not, it outputs the first acceleration information.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part application based on U.S. patent application Ser. No. 10/790,440 filed on Mar. 1, 2004, now U.S. Pat. No. 7,004,027, and priority is claimed on Japanese Patent Application Nos. 2003-055562 filed on Mar. 3, 2003 and 2004-058504 filed on Mar. 3, 2004, the contents of which are incorporated herein by reference.


BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to acceleration measurement methods using electrostatic-capacity-type acceleration sensors that detect accelerations based on variations in electrostatic capacity (or capacitance).
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 an acceleration measurement method using an electrostatic-capacity-type acceleration sensor having a broadened acceleration detection range.
An electrostatic- capacity-type acceleration sensor 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. 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.
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 acceleration 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 acceleration detection.
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 modem semiconductor manufacturing technology.
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 measurement method using an electrostatic-capacity-type acceleration sensor according to this invention is adapted to the system comprising 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 means for producing first acceleration information in response to capacitance variation occurring between the moving electrode and fixed electrode in the first capacitance detector, and a second detection means for producing second acceleration information in response to capacitance variation occurring between the moving electrode and fixed electrode in the second capacitance detector.
The acceleration measurement method of this invention comprises an input step for inputting the first acceleration information and the second acceleration information from the first detection means and the second detection means respectively, a decision step for making a decision as to whether or not the first acceleration information exceeds the upper-limit value of detection that is determined in advance with respect to the first capacitance detector, and an output step for outputting the first acceleration information when the decision step produces a negative result and for outputting the second acceleration information when the decision step produces an affirmative result.
The aforementioned acceleration measurement method can be implemented in the form of programs executed in a computer.
According to the acceleration measurement method of this invention, it is possible for the acceleration measurement device to actualize high-accuracy acceleration measurement in the relatively broad range of detection and to avoid occurrence of detection error 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 K1 to K8 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;
FIG. 4 is a block diagram showing an acceleration measuring circuit using the electrostatic-capacity-type acceleration shown in FIG. 3;
FIG. 5 is a graph showing curves J1 to J5 representing relationships between the input acceleration and capacitance varying ratio (CU/CD) with respect to various electrode overlap values (L);
FIG; 6 is a block diagram showing an acceleration measurement device using an electrostatic-capacity-type acceleration sensor having characteristics shown in FIG. 5; and
FIG. 7 is a flowchart showing steps implemented in acceleration measurement.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
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 by 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 pruned 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 M11to 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 S11to 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 W21 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 plugs, 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 VRR2 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 acceleration≦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.
FIG. 5 is a graph showing the relationship between the input acceleration and capacitance varying ratio CU/CD in other examples of electrostatic-capacitance-type acceleration sensors using various electrode overlap values.
The electrostatic-capacitance-type acceleration sensors having characteristics of FIG. 5 are characterized in that the electrode displacement is set to 0.5 μm when the input acceleration is 1 G, so that the electrode displacement d is varied linearly in response to the input acceleration. Herein, various electrode overlap values L are gradually increased in the order of L1, L2, L3, . . . Ln, wherein Ln=Ln−1+1 (where L0=0) [μm]. The graph of FIG. 5 present five curves J1 to J5, each showing the capacitance varying ratio CU/CD (=(L+d)/(L−d)), with respect to n=1 to n=5 in the input acceleration range of 0 G to 8 G. Specifically, the curves J1, J2, J3, J4, and J5 show variations of the capacitance varying ratio (L+d)/(L−d) with respect to L1=1 μm, L2=2 μm, L3=3 μm, L4=4 μm, and L5=5 μm respectively.
When the electrode displacement d approaches the electrode width, the capacitance varying ratio (L+d)/(L−d) rapidly increases as shown in dotted lines in FIG. 5 so as to diverge. Herein, a prescribed upper limit value is set to the electrode displacement d with respect to each curve, whereby acceleration measurement is performed by changing over characteristics from one curve to another in such a way that when the electrode displacement d exceeds the upper limit value of the curve J1, it proceeds to the next curve J2; then, when the electrode displacement d exceeds the upper limit value of the curve J2, it proceeds to the next curve J3. That is, prescribed upper limit values are set to electrode displacements such as d1=0.9 μm, d2=1.9 μm, d3=2.9 μm, . . . Herein, threshold Tn for changing over the capacitance varying ratio CU/CD from one curve to another is expressed as follows:




  
    T
    n
  
  =
  
    
      
        
          L
          n
        
        +
        
          d
          n
        
      
      
        
          L
          n
        
        -
        
          d
          n
        
      
    
    =
    
      
        T
        
          n
          -
          1
        
      
      +
      20
    
  




Using the aforementioned formula, it is possible to set the threshold Tn in relation to the electrode overlap value Ln and the upper limit value dn for the electrode displacement d. Specifically, FIG. 5 describes thresholds T1=19, T2=39, T3=59, and T4=79 being set with respect to the curves J1, J2, J3, and J4 respectively. Table 1 shows relationships between Ln, dn, and Tn.












TABLE 1








Upper Limit
Threshold



Electrode
Value Of
For Capaci-



Overlap
Electrode
tance Vary-



Value
Displacement
ing Ratio



[μm]
[μm]
CU/CD























L1
1
d1
0.9
T1
19



L2
2
d2
1.9
T2
39



L3
3
d3
2.9
T3
59



L4
4
d4
3.9
T4
79



.
.
.
.
.
.



.
.
.
.
.
.



.
.
.
.
.
.



Ln
Ln−1 + 1
dn
dn−1 + 1
Tn
Tn−1 + 20










FIG. 6 is a block diagram showing the constitution of an acceleration measurement device using an electrostatic-capacitance-type acceleration sensor having the characteristics of FIG. 5, wherein the acceleration measurement device is designed such that the measurement processing is controlled by a microcomputer, for example.
In FIG. 6, an acceleration sensor 10A is configured similar to the foregoing acceleration sensor 10, which is described in conjunction with FIGS. 1A, 1B, 2, 3, and 4. Specifically, the acceleration sensor 10A differs from the foregoing acceleration sensor 10 in that it has ‘n’ capacitance detectors respectively corresponding to the aforementioned characteristic curves J1 to Jn (e.g., n=5). In a manner similar to that of the foregoing detection circuits 12A and 12B, a plurality of detection circuits DT1 to DTn detect output voltages of the capacitance detectors of the acceleration sensor 10A, which are produced in response to capacitance varying ratios CU/CD in correspondence with the characteristic curves J1 to Jn respectively, as acceleration signals. The detection circuits DT1 to DTn are connected with detection lines TL1 to TLn extended from the capacitance detectors in correspondence with the characteristic curves J1 to Jn respectively. Each of the detection lines TL1 to TLn includes three detection lines, which correspond to the foregoing detection lines Ta, Tb, and Te in FIG. 4, whereas FIG. 6 merely shows a single line as the representation of each of the detection lines DT1 to DTn for the sake of convenience.
The detection circuits DT1 to DTn output analog acceleration signals a1 to an, which are respectively supplied to A/D converters (i.e., analog-to-digital conversion circuits) AD1 to ADn, which in turn produce ‘digital’ acceleration data A1 to An. The acceleration data A1 to An represent values of the acceleration signals a1 to an respectively.
The A/D converters AD1 to ADn are connected with a bus 30, which is also connected with a CPU 32, a ROM 33, a RAM 36, and a utilization device 38.
The CPU 32 executes measurement processes in accordance with programs stored in tie ROM 34. Details of the measurement processes will be described later with reference to FIG. 7. Other than programs, the ROM 34 also stores reference data AT1 to ATn, which represent reference acceleration values (i.e., upper limit values in detection) in correspondence with thresholds T1 to Tn set for the capacitance varying ratios CU/CD.
The RAM 36 contains storage areas that are used as registers when the CPU 32 performs measurement processes. As registers, there are provided registers RS1 to RSn in correspondence with the A/D converters AD1 to ADn respectively. The registers RS1 to RSn input the acceleration data output from the A/D converters AD1 to ADn respectively.
The utilization device 38 utilizes the acceleration data set to the registers RS1 to RSn, wherein it is the device that does not only detect existence of acceleration but also performs measurement on acceleration in a relatively broad range or performs highly accurate measurement on the acceleration ranging from a small acceleration to a large acceleration caused by impact or collision. Specifically, it is actualized by a display, an electronic device such as an electronic game terminal and a portable telephone (or a cellphone), and an air-bag drive device adapted to an automobile, for example.
FIG. 7 shows an example of the measurement process, which is started when a power switch (not shown) is turned on. First, the CPU 32 proceeds to step 50 so as to perform an initialization process, wherein ‘0’ is set to all of the registers RS1 to RSn.
Then, the CPU 32 proceeds to step 52 in which it inputs the acceleration data A1 to An from the A/D converters AD1 to ADn respectively, so that the acceleration data A1 to An are set to the registers RS1 to RSn respectively. In step 54, a decision is made as to whether or not the acceleration data A1 of the register RS1 is equal to or smaller than the reference acceleration designated by the reference acceleration value designated by the reference data AT1. If a decision result is ‘YES’ (Y), the CPU 32 proceeds to step 56 in which the acceleration data A1 of the register RS1 is sent to the utilization device 38. This realizes the characteristic curve J1 shown in FIG. 5.
If the decision result of step 54 is ‘NO’ (N), or when the step 56 is completed, the CPU 32 proceeds to step 58 in which a decision is made as to whether or not the acceleration designated by the acceleration data A2 of the register RS2 is greater than the reference acceleration designated by the reference data AT1 and is equal to or smaller than the reference acceleration designated by the reference data AT2. When a decision result is ‘YES’ (Y), the CPU 32 proceeds to step 60 in which the acceleration data A2 of the register RS2 is sent to the utilization device 38. This realizes the characteristic curve J2 shown in FIG. 5.
If the decision result of step 58 is ‘NO’ (N), or when the step 60 is completed, the CPU 32 proceeds to step 64 in which a decision is made as to whether or not the acceleration designated by the acceleration data A3 of the register RS3 is greater than the reference acceleration designated by the reference data AT2 and is equal to or smaller than the reference acceleration designated by the reference data AT3. When a decision result is ‘YES’ (Y), the CPU 32 proceeds to step 64 in which the acceleration data A3 of the register RS3 is sent to the utilization device 38. This realizes the characteristic curve J3 shown in FIG. 5.
Thereafter, in a manner similar to that of the aforementioned processing realizing the curve J3, steps similar to the steps 62 and 64 are performed to realize the curve J4. When n=5, in step 66, a decision is made as to whether or not the acceleration designated by the acceleration data A5 of the register RS5 is greater than the reference acceleration designated by the reference data AT4 and is equal to or smaller than the reference acceleration designated by the reference data AT5. When a decision result is ‘YES’ (Y), the CPU 32 proceeds to step 68 in which the acceleration data A5 of the register RS5 is sent to the utilization device 38. This realizes the characteristic curve J5 shown in FIG. 5.
When the decision result of step 66 is ‘NO’ (N), or when the step 68 is completed, the CPU 32 proceeds to step 70 so as to make a decision as to whether or not the power switch is turned off to issue an end instruction. When a decision result is ‘NO’ (N), the CPU 32 proceeds back to the step 52, thus repeating the aforementioned steps 52–68. When the decision result of step 70 becomes ‘YES’ (Y), the CPU 32 detects an end of processing.
According to the aforementioned measurement method, it is possible to perform highly accurate measurement on acceleration by use of the curves J1 to J5 in the relatively broad range of detection, i.e., 0 G≦(input acceleration)≦8 G. Herein, the present embodiment performs acceleration detection by not using the sharp rise portions of the curves J1 to J5; hence, it is possible to avoid occurrence of detection error due to noise. Incidentally, the present embodiment sets n=5, which is not necessarily limited and can be changed to n=6 or more, for example.
As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiments are 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 acceleration measurement method adapted to an electrostatic-capacity-type acceleration sensor comprising a first capacitance detector and a second capacitance detector both of which have moving electrodes and fixed electrodes arranged on a substrate surface so that facing areas therebetween are varied in response to input acceleration, wherein said second capacitance detector sets the facing areas between the moving electrodes and the fixed electrodes to be larger than those of the first capacitance detector when the input acceleration is zero, said acceleration measurement method comprising the steps of:
inputting first acceleration information representing capacitance variations detected between the moving electrodes and the fixed electrodes in the first capacitance detector;
inputting second acceleration information representing capacitance variations detected between the moving electrodes and the fixed electrodes in the second capacitance detector;
making a decision as to whether or not the first acceleration information exceeds an upper limit value that is set in advance with respect to the first capacitance detector;
if a decision result is ‘NO’, outputting the first acceleration information; and
if the decision result is ‘YES’, outputting the second acceleration information.

2. A computer-readable medium containing a program for controlling a computer to execute an electrostatic-capacity-type acceleration sensor comprising a first capacitance detector and a second capacitance detector both of which have moving electrodes and fixed electrodes arranged on a substrate surface so that facing areas therebetween are varied in response to input acceleration, wherein said second capacitance detector sets the facing areas between the moving electrodes and the fixed electrodes to be larger than those of the first capacitance detector when the input acceleration is zero, said program for acceleration measurement method comprising:
in puffing first acceleration information representing capacitance variations detected between the moving electrodes and the fixed electrodes in the first capacitance detector;
inputting second acceleration information representing capacitance variations detected between the moving electrodes and the fixed electrodes in the second capacitance detector;
making a decision as to whether or not the first acceleration information exceeds an upper limit value that is set in advance with respect to the first capacitance detector;
if a decision result is ‘NO’, outputting the first acceleration information; and
if the decision result is ‘YES’, outputting the second acceleration information.

3. The acceleration measurement method adapted to an electrostatic-capacity-type acceleration sensor according to claim 1, wherein the sensor is connected with a plurality of detection circuits, said acceleration measurement method further comprising the step of:
making a determination with respect to each of outputs of the detection circuits.