Abstract:
A micro accelerometer including a plurality of proof masses is provided to detect the acceleration of the first axis, the second axis and the third axis. The disclosed micro accelerometer has the advantages of close loop control, mechanical decoupling, and not relying on high aspect ratio manufacturing technology.

Description:
[0001]     This Non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 093116696 filed in Taiwan, R.O.C. on Jun. 10, 2004, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     The invention relates to an accelerometer, and, in particular, to an accelerometer that is manufactured by Micro Electro Mechanical System (MEMS) technology and has the capability of sensing one axis, two axes or three axes acceleration.  
         [0004]     2. Related Art  
         [0005]     U.S. Pat. No. 6,201,284 discloses an accelerometer which adopts proof masses to detect the acceleration of X, Y and Z axes. The proof mass detecting Z axis acceleration only moves along the Z axis, however, the motion of the proof mass detecting X axis may be coupled from the X and Y axes, and the motion of the proof mass detecting Y axis may be coupled from the X, Y and Z axes. The mechanical coupling in this structure is such that the sensing signals of different axes interfere with each other. The performance is thus affected seriously. The performance also relies on the high aspect ratio manufacturing technology. This is because the signals of the X and Y axes are detected by electrodes, which are co-planar with the proof mass. Only high aspect ratio manufacturing technology may satisfy the requirement of high sensitivity and small distance between the proof masses and the electrodes.  
         [0006]     Structure improvements and control methods regarding accelerometers is disclosed in U.S. Pat. Nos. 5,756,901, 5,817,942, 5,900,550, 6,223,598, 5,777,227, 6,128,956, 6,149,190, 5,487,305, and 6,018,998.  
         [0007]     The acceleration sensing method in the prior art adopts a co-planar electrode structure, as in U.S. Pat. Nos. 5,756,901, 5,817,942, 5,900,550, and 6,223,598. U.S. Pat. No. 5,487,305 has the drawbacks of insufficient intensity of signal sensing and complexity of signal processing circuits. Therefore, there is need for a new accelerometer structure.  
       SUMMARY OF THE INVENTION  
       [0008]     The object of the invention is to provide a micro accelerometer to substantially solve the foregoing problems and drawbacks of the related art.  
         [0009]     According to the object of the invention, one embodiment of the micro accelerometer for sensing the acceleration of a signal axis comprises a first proof mass, a second proof mass and a third proof mass arranged at each of the two opposite sides of the first proof mass, connected to the first proof mass by two flexible supporters and having a plurality of slots formed thereon, wherein the second proof mass and the third proof mass move along a first axial direction parallel to the first proof mass, and a plurality of electrode plate sets arranged relative to the second proof mass and the third proof mass respectively, for forming a plurality of sensing electrode sets at the first axial direction together with the second proof mass and the third proof mass, wherein the sensing electrodes are configured in differential circuits. Furthermore, the second proof mass and the third proof mass are arranged with a close loop control electrode for controlling the second proof mass and the third proof mass to move back to the original position when the second proof mass and the third proof mass generate displacement caused by acceleration.  
         [0010]     According to the object of the invention, another embodiment of the micro accelerometer for sensing the acceleration of X and Z axes or Y and Z axes comprises a first proof mass moving in the axial direction vertical to the first proof mass, a second proof mass and a third proof mass arranged at each of the two opposite sides of the first proof mass, connected to the first proof mass by two flexible supporters and having a plurality of slots formed thereon, wherein the second proof mass and the third proof mass move along a first axial direction parallel to the first proof mass, a lower electrode arranged relative to the first proof mass for forming a sensing capacitor together with the first proof mass, and a plurality of electrode plate sets arranged relative to the second proof mass and the third proof mass respectively, for forming a plurality of sensing electrode sets at the first axial direction together with the second proof mass and the third proof mass, wherein the sensing electrodes are configured in differential circuits. Furthermore, the second proof mass and the third proof mass are arranged with a close loop control electrode for controlling the second proof mass and the third proof mass to move back to the original position when the second proof mass and the third proof mass generate displacement caused by acceleration.  
         [0011]     According to the object of the invention, another embodiment of the micro accelerometer for sensing the acceleration of X and Y axes comprises a first proof mass, a second proof mass and a third proof mass arranged at each of the two opposite sides of the first proof mass, connected to the first proof mass by two flexible supporters and having a plurality of slots formed thereon, wherein the second proof mass and the third proof mass move along a first axial direction parallel to the first proof mass, a fourth proof mass and a fifth proof mass arranged at each of the two opposite sides of the first proof mass, connected to the first proof mass by two flexible supporters and having a plurality of slots formed thereon, wherein the fourth proof mass and the fifth proof mass move along a second axial direction parallel to the first proof mass, a plurality of electrode plate sets arranged relative to the second proof mass, the third proof mass, the fourth proof mass and the fifth proof mass respectively, for forming a plurality of sensing electrode sets at the first axial direction together with the second proof mass and the third proof mass and forming a plurality of sensing electrode sets at the second axial direction together with the fourth proof mass and the fifth proof mass, wherein the sensing electrodes are configured in differential circuits. Furthermore, the second proof mass and the third proof mass are arranged with a close loop control electrode, for controlling the second proof mass and the third proof mass to move back to the original position when the second proof mass and the third proof mass generate displacement caused by an acceleration. The fourth proof mass and the fifth proof mass are arranged with a close loop control electrode, for controlling the fourth proof mass and the fifth proof mass to move back to the original position when the fourth proof mass and the fifth proof mass generate displacement caused by an acceleration.  
         [0012]     According to the object of the invention, another embodiment of the micro accelerometer for sensing the acceleration of X, Y and Z axes comprises a first proof mass moving in the axial direction vertical to the first proof mass, a second proof mass and a third proof mass arranged at each of the two opposite sides of the first proof mass, connected to the first proof mass by two flexible supporters and having a plurality of slots formed thereon, wherein the second proof mass and the third proof mass move along a first axial direction parallel to the first proof mass, a fourth proof mass and a fifth proof mass arranged at each of the two opposite sides of the first proof mass, connected to the first proof mass by two flexible supporters and having a plurality of slots formed thereon, wherein the fourth proof mass and the fifth proof mass move along a second axial direction parallel to the first proof mass, a lower electrode arranged relative to the first proof mass for forming a sensing capacitor together with the first proof mass, and a plurality of electrode plate sets arranged relative to the second proof mass, the third proof mass, the fourth proof mass and the fifth proof mass respectively, for forming a plurality of sensing electrode sets at the first axial direction together with the second proof mass and the third proof mass and forming a plurality of sensing electrode sets at the second axial direction together with the fourth proof mass and the fifth proof mass, wherein the sensing electrodes are configured in differential circuits. Furthermore, the second proof mass and the third proof mass are arranged with a close loop control electrode, for controlling the second proof mass and the third proof mass to move back to the original position when the second proof mass and the third proof mass generate displacement caused by an acceleration. The fourth proof mass and the fifth proof mass are arranged with a close loop control electrode, for controlling the fourth proof mass and the fifth proof mass to move back to the original position when the fourth proof mass and the fifth proof mass generate displacement caused by an acceleration.  
         [0013]     According to the object of the invention, another embodiment of the micro accelerometer for sensing the acceleration of X or Y axes comprises a base, a proof mass arranged on the base and having a plurality of slots formed thereon, and moving in the axial direction parallel to the base, and a plurality of electrode plate sets arranged relative to the proof mass for forming a plurality of sensing electrode sets at the axial direction, wherein the sensing electrodes are configured in differential circuits. Furthermore, the proof mass is arranged with two close loop control electrodes positioned at the two sides of the proof mass, for controlling the proof mass to move back to the original position when the proof mass generates displacement caused by acceleration.  
         [0014]     According to the object of the invention, another embodiment of the micro accelerometer for sensing the acceleration of X, Y, and Z axes comprises a base, a first proof mass arranged on the base and having a plurality of holes formed thereon, and moving in the axial direction vertical to the base, a second proof mass arranged on the base and having a plurality of slots formed thereon, and moving in a first axial direction parallel to the base, a third proof mass arranged on the base and having a plurality of slots formed thereon, and moving in a second axial direction parallel to the base, a lower electrode plate arranged relative to the first proof mass for forming a sensing capacitor together with the first proof mass, and a plurality of electrode plate sets arranged relative to the second proof mass and the third proof mass, for forming a plurality of sensing electrode sets at the first axial direction together with the second proof mass, and forming a plurality of sensing electrode sets at the second axial direction together with the third proof mass, wherein the sensing electrodes are configured in differential circuits. Furthermore, the second proof mass and the third proof mass are arranged with two close loop control electrodes positioned at the two sides of the related proof mass, for controlling the second proof mass and the third proof mass to move back to the original position when the second proof mass and the third proof mass generate displacement caused by an acceleration.  
         [0015]     According to the principle and object of the invention, the plurality of electrode plate sets are arranged relative to the proof mass for forming a plurality of sensing electrode sets. The sensing electrodes are configured in differential circuits to increase the sensing capacitance.  
         [0016]     According to the principle and object of the invention, the disclosed micro accelerometer has the advantage of mechanically decoupling. The motion of the X and Y axes is in a decoupling direction, such that the signals do not interfere with each other. However, the sensing proof mass of Z axis detects three-axis motion, but only the areas of the sensing electrodes may be covered by the Z axial proof mass, the sensing signals of Z axis are not affected by the motion of X axis and Y axis.  
         [0017]     According to the principle and object of the invention, the disclosed micro accelerometer has the advantage of not being affected by the high aspect ratio manufacturing technology. The sensing electrodes of the three axes are arranged under the proof mass. The sensing electrodes of the X and Y axes detect the overlapped area variation between the proof mass and the electrodes. The sensing electrodes of the Z axis detect the distance variation between the proof mass and the electrode. Therefore, performance is not affected by the high aspect ratio manufacturing technology.  
         [0018]     According to the principle and object of the invention, the disclosed micro accelerometer has the advantage of close loop control to increase the precision and bandwidth effectively.  
         [0019]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0021]      FIG. 1A  is a schematic diagram of the first embodiment of the micro accelerometer in accordance with the invention;  
         [0022]      FIG. 1B  is a schematic diagram of the first embodiment of the micro accelerometer in accordance with the invention;  
         [0023]      FIG. 1C  is a schematic diagram of the first embodiment of the micro accelerometer in accordance with the invention;  
         [0024]      FIG. 1D  depicts the differential sensing circuit of the first embodiment of the micro accelerometer in accordance with the invention;  
         [0025]      FIG. 2  is a schematic diagram of the second embodiment of the micro accelerometer in accordance with the invention;  
         [0026]      FIG. 3  is a schematic diagram of the third embodiment of the micro accelerometer in accordance with the invention;  
         [0027]      FIG. 4A  is a schematic diagram of the fourth embodiment of the micro accelerometer in accordance with the invention;  
         [0028]      FIG. 4B  is a schematic diagram of the fourth embodiment of the micro accelerometer in accordance with the invention;  
         [0029]      FIG. 4C  is a schematic diagram of the fourth embodiment of the micro accelerometer in accordance with the invention; and  
         [0030]      FIG. 5  is a schematic diagram of the fifth embodiment of the micro accelerometer in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     Reference will now be made in detail to the embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
         [0032]     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.  
         [0033]     Refer to  FIGS. 1A  to  1 C depicting schematic diagrams of the first embodiment of the micro accelerometer, and  FIG. 1D  depicting the differential sensing circuit of the first embodiment.  
         [0034]     In the first embodiment, the micro accelerometer is composed of a first proof mass  10 , a second proof mass  20 , a third proof mass  30 , a fourth proof mass  40 , and a fifth proof mass  50 . The proof masses are manufactured by conductive material. The second proof mass  20  and the third proof mass  30  are arranged at one of the two opposite sides of the first proof mass  10 , while the third proof mass  40  and the fifth proof mass  50  are arranged at the other one of the two opposite sides of the first proof mass  10 . The proof mass  20 ,  30 ,  40 , and  50  are connected to the first proof mass  10  through a plurality pairs of flexible supporters  62   a ,  62   b ,  63   a ,  63   b ,  64   a ,  64   b    65   a , and  65   b  respectively.  
         [0035]     The plurality of flexible supporters  62   a ,  62   b ,  63   a ,  63   b ,  64   a ,  64   b    65   a , and  65   b  enable the proof masses to move along a first axis (e.g., the x axis), a second axis (e.g., the y axis) parallel to the surface of the first proof mass  10 , and/or a third axis (e.g., the z axis) vertical to the surface of the first proof mass  10 . The inertia mass body composed of the first proof mass  10 , the second proof mass  20 , and the third proof mass  30  moves along the first axis parallel to the first proof mass  10 , while the inertia mass body composed f the first proof mass  10 , the fourth proof mass  40 , and the fifth proof mass  50  moves along the second axis parallel to the first proof mass  10 .  
         [0036]     According to the principle of the invention, the aforementioned five proof masses form a three-axial accelerometer or a two-axial accelerometer. Furthermore, combination of any of the proof masses may compose accelerometers with desired axes. In one embodiment, for example, a one-axial or a two-axial accelerometer may be provided by combining the first proof mass  10 , the second proof mass  20  and the third proof mass  30 , or the first proof mass  40 , the fourth proof mass  10  and the fifth proof mass  50 .  
         [0037]     The first proof mass  10  includes a frame  11  and an upper electrode plate  12 , which are connected together by a plurality of flexible supporters  13  arranged at the sides of the upper electrode plate  12 . Each of the flexible supporters  13  has a connecting portion to connect the flexible supporters  13  and the upper electrode plate  12 . A plurality of holes is formed on the upper electrode plate  12 . Besides the shape and the quantity as shown in the figure, any variation or design may be performed. The main propose of the holes is to reduce damping occurring when the electrode plate  12  moves along the third axis.  
         [0038]     The proof mass  10  and the lower electrode plate  14  arranged under the base  100  form a sensing capacitor for the third axis. When the third-axial displacement of the proof mass  10  occurs due to the third-axial acceleration, the capacitance of the third-axial sensing capacitor varies with the distance change between the upper electrode plate  12  and the lower electrode plate  14 . Therefore, the third axial acceleration is detected through the capacitance variation. In one embodiment, a capacitor may be further provided in the close loop circuits as a reference capacitor for the acceleration sensing signal. The capacitance is the same as the initial value formed by the upper electrode plate  12  and the lower electrode plate  14 . Thus, close loop control is performed such that the first proof mass  10  may return back to its original position.  
         [0039]     The second proof mass  20  is connected to the frame  11  through a pair of flexible supporters  62   a  and  62   b , and is fixed on the supporting anchors  71 ,  72 ,  81 , and  82  through the flexible supporters  711 ,  721 ,  811  and  821 . The third proof mass  30  is connected to the frame  11  through a pair of flexible supporters  63   a  and  63   b , and is fixed on the supporting anchors  73 ,  74 ,  83 , and  84  through the flexible supporters  731 ,  741 ,  831  and  841 . The fourth proof mass  40  is connected to the frame  11  through a pair of flexible supporters  64   a  and  64   b , and fixed on the supporting anchors  71 ,  73 ,  85 , and  86  through the flexible supporters  712 ,  732 ,  851  and  861 . The fifth proof mass  50  is connected to the frame  11  through a pair of flexible supporters  65   a  and  65   b , and fixed on the supporting anchors  72 ,  74 ,  87 , and  88  through the flexible supporters  722 ,  742 ,  871  and  881 .  
         [0040]     Through the aforementioned supporting structure, the second proof mass  20  and the third proof mass  30  are restricted to only move along the first axis, while the fourth proof mass  40  and the fifth proof mass  50  are restricted to only move along the second axis. It is clear from the structure of the invention that the motion of the two axes do not interfere with each other. Thus, mechanical decoupling of the two axes is achieved in accordance with the object of the invention, and cross-axis sensitivity may be reduced effectively.  
         [0041]     A plurality of slots  21  is formed on the proof mass  20 . Similarly, a plurality of slots  31 ,  41 , and  51  are formed on the relative proof masses  40 ,  40 , and  50  respectively. Each of the slots is arranged with electrodes for forming a plurality of sensing electrodes, which are configured in differential circuits. Detailed description is given as follows. Refer to  FIGS. 1C and 1D . A plurality of rectangular electrode plates  23  and  24  interleaving with each other are arranged under the slots  21  of the proof mass. Each of the slots  21  covers a portion of the electrodes  23  and  24  for forming the sensing capacitors. Let the initial value of the capacitor be C 0 . When the proof mass  20  generates displacement along the first axis owing to the acceleration of the first axis, the effective area covered by the hole  21  and the electrode plates  23  and  24  changes accordingly, the capacitance of which becomes C 0 −ΔC and C 0 +ΔC, respectively. In one embodiment, in which N slots  21  and N electrode plates  23  and  24  are arranged relative to one another, the capacitance becomes N (C 0 −ΔC) and N (C 0 +ΔC) respectively. The total capacitance variance becomes 2NΔC after being detected by differential circuits. Thus, the acceleration of the first axis is obtained through detecting the capacitance variation.  
         [0042]     According to the principle of the invention, the third proof mass  30  has slots  31  and relative to electrode plates  33  and  34  for forming a plurality of sensing electrodes, which are configured in differential circuits. When the first axial displacement of the third proof mass  30  caused by the first-axial acceleration occurs, the capacitance value changes with the variation of the effective areas covered by the slots  31 . Thus, the first axial acceleration is obtained by sensing the capacitance variation. It is clear that the arrangement of the two sets of proof masses increases not only the inertia mass of inertia motion, but also the intensity of the sensing signals through.  
         [0043]     According to the principle of the invention, the fourth proof mass  40  and the fifth proof mass  50  are arranged with slots  41  and slots  51  respectively, and relative to electrode plates  43 ,  44 ,  53 , and  54  for forming a plurality of sensing electrodes, which are configured in differential circuits. When the fourth proof mass  40  and the fifth proof mass  50  generate the second axial displacement caused by the second-axial acceleration, the capacitance changes with the variation of the effective areas covered by the slots  41  and  51 . Thus, the second-axial acceleration is obtained by sensing the capacitance variation.  
         [0044]     A close loop control electrode  22 , which may be a rectangular electrode in one embodiment, is provided at one side of the second proof mass  20 . The first axial acceleration detected through the variation of the capacitance is fed back to the close loop control electrode  22  through feedback circuits (not shown) such that the second proof mass  20  may return to its original central position. According to the principle of the invention, the close loop control electrodes  32 ,  42 , and  52 , arranged relative to one side of the other proof masses, function the same as the close loop control electrode  22 . The three-axis accelerometer of the first embodiment of the invention includes three accelerometers independent from one another, each of which detects acceleration of a single axis without the influence of the other two accelerometers. Deletion of unnecessary accelerometer/s and maintaining accelerometer/s of the desired axis may be performed for sensing one-axis or tow-axis acceleration.  
         [0045]     Refer to  FIG. 2 , which depicts a schematic diagram of the second embodiment of the micro accelerometer in accordance with the invention. In the second embodiment, the close loop control electrodes are not the same as those in the first embodiment, while the other structures and functions are similar to those in the first embodiment.  
         [0046]     According to the principle of the invention, in the second embodiment, the close loop control electrode of the second proof mass  20  is an electrode plate  25  arranged at the exterior of the frame  11  and between the flexible supporters  62   a  and  62   b . Thus, the proof mass  20  may return to the original position when displacement occurs owing to acceleration. Similarly, the close loop control electrode of the third proof mass  30  is an electrode plate  35  arranged at the exterior of the frame  11  and between the flexible supporters  63   a  and  63   b . The close loop control electrode of the fourth proof mass  40  is an electrode plate  45  arranged at the exterior of the frame  11  and between the flexible supporters  64   a  and  64   b . The close loop control electrode of the third proof mass  50  is an electrode plate  55  arranged at the exterior of the frame  11  and between the flexible supporters  65   a  and  65   b . Thus, the proof masses  30 ,  40 , and  50  may return to their original positions when displacement occurs owing to acceleration.  
         [0047]     Refer to the  FIG. 3 , which depicts a schematic diagram of the third embodiment of the micro accelerometer in accordance with the invention. The close loop control electrodes are distinctive to this embodiment, while the other structures and functions are similar to those in the first embodiment.  
         [0048]     According to the principle of the invention, in the third embodiment, the close loop control electrode of the second proof mass  20  includes a first comb-like electrode  26  and a second comb-like electrode  27 . The first comb-like electrode  26  is connected to the second proof mass  20 . The comb fingers of the first comb-like electrode  26  and the second comb-like electrode  27  are arranged to interleave with each other. Thus, the proof mass  20  may return to its original position when displacement occurs owing to acceleration. The close loop control electrode of the third proof mass  30  includes a first comb-like electrode  36  and a second comb-like electrode  37 . The close loop control electrode of the fourth proof mass  40  includes a first comb-like electrode  46  and a second comb-like electrode  47 , while the close loop control electrode of the fifth proof mass  50  includes a first comb-like electrode  56  and a second comb-like electrode  57 . The structures and functions of the comb-like electrodes are the same as those of the second proof mass  20 .  
         [0049]     Refer to  FIG. 4 , which depicts the fourth embodiment of the accelerometer. In the fourth embodiment, the accelerometers of the first axis, the second axis and the third axis are arranged respectively, and their shape, structure and function are similar to the aforementioned embodiments, but not the same.  
         [0050]     The accelerometer in the fourth embodiment has a first proof mass  91  sensing the third axial acceleration vertical to a base, a second proof mass  92  sensing the first axial acceleration parallel to the base and a third mass  93  sensing the second axial acceleration parallel to the base. The acceleration sensors of the three axes are formed on the base. The first proof mass  91  is supported on four supporting anchors  912  through four flexible supporters  913 , while the second proof mass  92  is supported on four supporting anchors  922  through four flexible supporters  921 . The third proof mass  93  is supported on four supporting anchors  932  through four flexible supporters  931 .  
         [0051]     Refer to  FIG. 4B . A plurality of slots  923  is formed on the second proof mass  92 , while a plurality of slots  933  is formed on the third proof mass  93 . Electrode plates are placed under each slot for forming a plurality of sensing electrodes, which are configured in differential circuits. Detailed description is given as follows.  
         [0052]     Refer to  FIG. 4C . A plurality of rectangular electrode plates  924 ,  925 ,  934 , and  935  are interleaved with each other and arranged under each hole. Each slots  923  and  933  covers a portion of the electrode plates  924 ,  925 ,  934 , and  935  for forming a plurality of sensing electrodes, which are configured in differential circuits. Thus, when the second proof mass  92  generates displacement of the first axis caused by the first-axial acceleration, the capacitance changes with the variation of the effective areas covered by the slots  923 . The second axial acceleration is obtained in similar way.  
         [0053]     The acceleration sensor of the third axis, which is the vertical axis, includes an upper electrode plate  911  and a lower electrode plate  915 . The upper electrode plate  911  is connected through a plurality of flexible supporters  913  arranged at the four sides of the upper electrode plate  911 . The flexible supporter  913  has a connecting portion  913 A for forming the flexible supporters  913  and the electrode plate  911 . A plurality of holes  914  is formed on the upper electrode plate  911 . Besides the shape and the quantity as shown in the figure, any variation or design may be performed. The main propose of the holes is to reduce damping occurring when the upper electrode plate  913  moves along the third axis.  
         [0054]     The upper electrode plate  911  and the lower electrode plate  915  form the sensing capacitor of the third axis. When the first proof mass  91  generates displacement along the third axis owing to the third axial acceleration, the capacitance of the sensing capacitor of the third axis varies with the distance change between the upper electrode plate  911  and the lower electrode plate  915 . Thus, the third axial acceleration is obtained by detecting the capacitance variation. In one embodiment, a capacitor may be further provided in the close loop circuits as a reference capacitor for the acceleration sensing signal. The capacitance is the same as the initial value formed by the upper electrode plate  911  and the lower electrode plate  915 . Thus, close loop control is performed such that the first proof mass  91  may return back to its original position.  
         [0055]     The close loop control electrode of the second proof mass  92  has two electrode plates  926  and  927 , which are arranged at the two sides of the second proof mass  93  such that the proof mass  92  may return back to its original position when displacement occurs due to acceleration. The close loop control electrode of the third proof mass  93  has two electrode plates  936  and  937 , which are arranged at the two sides of the third proof mass  93  such that the proof mass  93  may return back to its original position when displacement occurs due to acceleration. Various changes or modifications of the close loop control electrodes formed by the electrode plates are possible as the second and/or the third embodiment.  
         [0056]     Refer to the  FIG. 5 , which depicts a schematic diagram of the fifth embodiment of the micro accelerometer in accordance with the invention. In the fifth embodiment, the close loop control electrodes are not the same as those in the fourth embodiment, while the other structures and functions are similar to those in the fourth embodiment.  
         [0057]     The second proof mass  92  is provided with two close loop control electrodes, each of which includes first comb-like electrodes  928 A and  929 A and second comb-like electrodes  928 B and  929 B. The first comb-like electrodes  928 A and  929 A are connected to the second proof mass  92 . The comb-like electrodes  928 B and  929 B are arranged to interleave with each other. Thus, the proof mass  92  may return to its original position when displacement occurs owing to the acceleration.  
         [0058]     Similarly, the third proof mass  93  is provided with two close loop control electrodes, each of which includes first comb-like electrodes  938 A and  939 A and second comb-like electrodes  938 B and  939 B. The first comb-like electrodes  938 A and  939 A are connected to the third second proof mass  93 . The comb-like electrodes  938 B and  939 B are arranged to interleave with each other. Thus, the proof mass  93  may return to its original position when displacement occurs owing to acceleration.  
         [0059]     The three-axis accelerometer of the fourth and fifth embodiments includes three accelerometers independent from one another, each of which detects acceleration of a single axis without the influence of the other two accelerometers. Deletion of unnecessary accelerometer/s and maintaining accelerometer/s of desired axes may be performed for sensing one-axis or two-axis acceleration.  
         [0060]     According to the object and the principle of the invention, the plurality of electrode plates arranged relative to the poof mass for forming a plurality of sensing electrodes, which are configured in differential circuits, may increase the sensing capacitance effectively.  
         [0061]     According to the object and the principle of the invention, the motion of the proof mass of the X and Y-axes is in a decoupling direction such that the sensing signals do not interfere with each other. Although the proof mass of the Z axis includes three-axes motion, only the areas of the sensing electrodes may be covered by the Z axis proof mass at any time, thus the sensing signals are not affected by the X and Y axes.  
         [0062]     According to the object and the principle of the invention, the sensing electrodes of the three axes are arranged under the relative proof masses. The sensing electrodes of the X and Y axes detect the covered area variation of the proof mass and the relative electrodes, while the sensing electrodes of the Z axis detect the distance variation of the proof mass and the relative electrodes.  
         [0063]     Although the invention has been explained by the embodiments shown in the drawings described above, it should be understood to the person skilled in the art that the invention is not limited to these embodiments, but rather various changes or modifications thereof are possible without departing from the spirit and scope of the invention. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents.