Patent Publication Number: US-2015059430-A1

Title: Inertial force sensor

Description:
TECHNICAL FIELD 
     The present invention relates to an inertial force sensor for detecting inertial force, such as acceleration and an angular velocity, which is used in, e.g. vehicles and portable terminals. 
     BACKGROUND ART 
       FIG. 19  is a top view of conventional inertial force sensor  501 . Inertial force sensor  501  is an acceleration sensor for detecting acceleration. Frame  1  includes fixed parts  1   a  to  1   d  connected to each other to form a ring shape surrounding hollow region  2 . Beams  3  to  6  each having respective one ends connected to frame  1  extend to hollow region  2 . Plummet  7  extends obliquely from another end of beam  3 . Plummet  8  extends obliquely from another end of beam  5 . Plummet  9  is connected to another end of beam  4 . Plummet  10   a  is connected to another end of beam  6 . Strain-sensitive resistors  11  are provided on an upper surface of beam  3 . Strain-sensitive resistors  13  are provided on an upper surface of beam  5 . Strain-sensitive resistors  12  are provided on an upper surface of beam  4 . Strain-sensitive resistors  14  are provided on an upper surface of beam  6 . Strain-sensitive resistors  11  to  14  are electrically connected to each other with wirings to form a bridge circuit. In conventional inertial force sensor  501 , plummets  7  to  10  are displaced in vertical directions in response to acceleration applied thereto. The displacements of the plummets changes resistances of strain-sensitive resistors  11  to  14 . The acceleration is detected based on a signal output from the bridge circuit due to the change of the resistances. 
     A conventional inertial force sensor similar to inertial force sensor  501  is disclosed in, for example, PTL 1. 
       FIG. 20  is a sectional view of another conventional inertial force sensor  502 . Inertial force sensor  502  is an acceleration sensor for detecting acceleration. Inertial force sensor  502  includes fixed part  201  and counter substrate  208  provided on an upper surface of fixed part  201 . Fixed part  201  includes outer frame portion  203 , plummet  202 , and deformable portion  204  having one end connected to outer frame portion  203  and another end connected to plummet  202 . Counter substrate  208  is connected to outer frame portion  203  and faces plummet  202 . Inertial force sensor  502  includes self-diagnostic electrode  207  formed on an upper surface of plummet  202  and counter electrode  206  provided on a lower surface of counter substrate  208 . Counter electrode  206  faces self-diagnostic electrode  207  with a predetermined air gap between counter electrode  206  faces self-diagnostic electrode  207 . 
     When voltage Vd is applied between self-diagnostic electrode  207  and counter electrode  206  to apply electrostatic force Fd to plummet  202 , plummet  202  can be displaced as if acceleration is applied to plummet  202 . 
     It is possible to determine whether or not inertial force sensor  502  works normally. 
     A conventional inertial force sensor similar to inertial force sensor  502  is disclosed in, for example, PTL 2. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laid-Open Publication No. 2007-85800 
     PTL 2: Japanese Patent Laid-Open Publication No. 5-322925 
     SUMMARY 
     An inertial force sensor includes a fixed part, a beam connected to the fixed part, a plummet connected to another end of the beam and being displaceable due to an inertial force to cause the beam to deform, a conductive part provided at the plummet, a strain-sensitive resistor provided at the beam for detecting a deformation of the first beam, first and second fault diagnostic electrodes provided at the fixed part, a first fault diagnostic wiring connecting the first fault diagnostic electrode to the conductive part through the beam, and a second fault diagnostic wiring for connecting the second fault diagnostic electrode to the conductive part through the beam. 
     The inertial force sensor does not continue to output an erroneous output signal when a crack occurs in the plummet, thus having high reliability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top view of an inertial force sensor in accordance with Exemplary Embodiment 1. 
         FIG. 2  is a top view of the inertial force sensor in accordance with Embodiment 1. 
         FIG. 3  is a top view of the inertial force sensor in accordance with Embodiment 1. 
         FIG. 4A  is a top view of the inertial force sensor in accordance with Embodiment 1. 
         FIG. 4B  is a schematic diagram of a detection circuit of the inertial force sensor in accordance with Embodiment 1. 
         FIG. 4C  is a schematic diagram of the detection circuit of the inertial force sensor in accordance with Embodiment 1. 
         FIG. 4D  is a schematic diagram of the detection circuit of the inertial force sensor in accordance with Embodiment 1. 
         FIG. 5  is a circuit diagram of the inertial force sensor in accordance with Embodiment 1. 
         FIG. 6  shows an output voltage of a fault diagnosis circuit of the inertial force sensor in accordance with Embodiment 1. 
         FIG. 7  is a top view of an inertial force sensor in accordance with Exemplary Embodiment 2. 
         FIG. 8  is a circuit diagram of the inertial force sensor in accordance with Embodiment 2. 
         FIG. 9  is a top view of an inertial force sensor in accordance with Exemplary Embodiment 3. 
         FIG. 10  is a sectional view of the inertial force sensor at line  10 - 10  shown in  FIG. 10 . 
         FIG. 11A  is a schematic view of the inertial force sensor in accordance with Embodiment 3. 
         FIG. 11B  is a schematic view of the inertial force sensor in accordance with Embodiment 3. 
         FIG. 12  is a circuit diagram of the inertial force sensor in accordance with Embodiment 3. 
         FIG. 13  is a top view of a Comparative Example of an inertial force sensor. 
         FIG. 14  is a top view of an inertial force sensor in accordance with Exemplary Embodiment 4. 
         FIG. 15  is a sectional view of the inertial force sensor at line  15 - 15  shown in  FIG. 14 . 
         FIG. 16A  is a top view of the inertial force sensor in accordance with Embodiment 4. 
         FIG. 16B  is a circuit diagram of the inertial force sensor in accordance with Embodiment 4. 
         FIG. 16C  is a circuit diagram of the inertial force sensor in accordance with Embodiment 4. 
         FIG. 16D  is a circuit diagram of the inertial force sensor in accordance with Embodiment 4. 
         FIG. 17A  is a top view of the inertial force sensor in accordance with Embodiment 4 for illustrating an operation of the inertial force sensor. 
         FIG. 17B  is a circuit diagram of the inertial force sensor in accordance with Embodiment 4 for illustrating an operation of the inertial force sensor. 
         FIG. 17C  is a circuit diagram of the inertial force sensor in accordance with Embodiment 4 for illustrating an operation of the inertial force sensor. 
         FIG. 17D  is a top view of the inertial force sensor in accordance with Embodiment 4 for illustrating an operation of the inertial force sensor. 
         FIG. 17E  is a top view of the inertial force sensor in accordance with Embodiment 4 for illustrating an operation of the inertial force sensor. 
         FIG. 18  is a top view of another inertial force sensor in accordance with Embodiment 4. 
         FIG. 19  is a sectional view of a conventional inertial force sensor. 
         FIG. 20  is a sectional view of another conventional inertial force sensor. 
     
    
    
     DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS 
     Exemplary Embodiment 1 
       FIG. 1  is a top view of inertial force sensor  1001  in accordance with Exemplary Embodiment 1. Inertial force sensor  1001  is an acceleration sensor for detecting acceleration, an inertial force, applied thereto. Inertial force sensor  1001  includes frame  20 , beams  23   a  to  26   a  and  23   b  to  26   b  connected to frame  20 , and plummets  27  to  30  connected to beams  23   a  to  26   a  and  23   b  to  26   b  and coupled to frame  20  through beams  23   a  to  26   a  and  23   b  to  26   b.  Frame  20  includes fixed parts  21   a  to  21   d  connected to each other to form a rectangular ring shape surrounding hollow region  2 . Fixed parts  21   a  and  21   b  are sides of the rectangular ring shape of frame  20  facing each other while fixed parts  21   c  and  21   d  are other sides of the rectangular ring shape of frame  20  facing each other. Beams  23   a  to  26   a  and  23   b  to  26   b  extend from frame  20  to hollow region  22 . One end of each of beams  23   a  and  23   b  is connected to fixed part  21   a  of frame  20 . One end of each of beams  24   a  and  24   b  is connected to fixed part  21   b  of frame  20 . One end of each of beams  25   a  and  25   b  is connected to fixed part  21   c  of frame  20 . One end of each of beams  26   a  and  26   b  is connected to fixed part  21   d  of frame  20 . 
     Plummet  27  is connected to another end of each of beams  23   a  and  23   b.  Plummet  28  is connected to another end of each of beams  24   a  and  24   b.  Plummet  29  is connected to another end of each of beams  25   a  and  25   b.  Plummet  30  is connected to another end of each of beams  26   a  and  26   b.    
     Plummet  27  is displaced due to the acceleration, the inertial force, applied thereto to cause beams  23   a  and  23   b  to deform. Plummet  28  is displaced due to the acceleration to cause beams  24   a  and  24   b  to deform. Plummet  29  is displaced due to the acceleration to cause beams  25   a  and  25   b  to deform. Plummet  30  is displaced due to the acceleration to cause beams  26   a  and  26   b  to deform. Strain-sensitive resistors  31   a  and  31   b  are provided on upper surfaces of beams  23   a  and  23   b,  respectively. Strain-sensitive resistors  33   a  and  33   b  are provided on upper surfaces of beams  25   a  and  25   b,  respectively. Strain-sensitive resistors  32   a  and  32   b  are provided on upper surfaces of beams  24   a  and  24   b,  respectively. Strain-sensitive resistors  34   a  and  34   b  are provided on upper surfaces of beams  26   a  and  26   b,  respectively. Beams  23   a  and  23   b  extend in a direction of an X-axis. Plummet  27  is located in a negative direction of the X-axis from fixed part  21   a  while plummet  28  is located in a positive direction of the X-axis from fixed part  21   b.  Beams  25   a  and  25   b  extend in a direction of a Y-axis perpendicular to the X-axis. Plummet  29  is located in a negative direction of the Y-axis from fixed part  21   c  while plummet  30  is located in a positive direction of the Y-axis from fixed part  21   d.    
     Plummet  27  faces plummet  28 , and plummet  29  faces plummet  30 . Conductive parts  27   a,    28   a,    29   a,  and  30   a  are provided on plummets  27 ,  28 ,  29 , and  30 , respectively. In this configuration, plummet  27  is supported by beams  23   a  and  23   b  from only one direction (the negative direction of the X-axis). Plummet  28  is supported by beams  24   a  and  24   b  from only one direction (the positive direction of the X-axis). Plummet  29  is supported by beams  25   a  and  25   b  from only one direction (the negative direction of the Y-axis). Plummet  30  is supported by beams  26   a  and  26   b  from only one direction (the positive direction of the Y-axis). This configuration prevents transition of beams  23   a  to  26   a  and  23   b  to  26   b  to different buckling modes by the displacement of plummets  27  to  30 , hence suppressing variation of sensitivity of inertial force sensor  1001  and a change of the sensitivity with time. Power-supply electrode  35  for applying a voltage, output electrodes  36  and  37 , and GND electrode  38  to be grounded are provided on each of fixed parts  21   a  to  21   d.  Power-supply electrode  35 , output electrodes  36  and  37 , and GND electrode  38  to be grounded are electrically connected to strain-sensitive resistors  31   a  to  34   a  and  31   b  to  34   b  with wirings  41  as to constitute a bridge circuit. 
     Fault diagnostic electrode  39  for applying a voltage for fault diagnosis and a pair of fault diagnostic electrodes  40   a  and  40   b  are provided on each of fixed parts  21   a  to  21   d.    
       FIGS. 2 and 3  are enlarged top views of inertial force sensor  1001  for illustrating a peripheral portion of fixed part  21   a  and a peripheral portion of fixed part  21   b,  respectively. In the peripheral portion of fixed part  21   a  shown in  FIG. 2 , fault diagnostic wiring  48   c  extends from fault diagnostic electrode  39  provided at fixed part  21   a  and is branched into branch lines  148   c  and  248   c.  Branch lines  148   c  and  248   c  are connected to conductive part  27   a  through upper surfaces of beams  23   a  and  23   b,  respectively. Thus, fault diagnostic electrode  39  provided at fixed part  21   a  is connected to conductive part  27   a  via fault diagnostic wiring  48   c.  Fault diagnostic wiring  48   a  extends from fault diagnostic electrode  40   a  provided at fixed part  21   a  through the upper surface of beam  23   a  to be connected to conductive part  27   a.  Thus, fault diagnostic electrode  40   a  provided at fixed part  21   a  is connected to conductive part  27   a  via fault diagnostic wiring  48   a.  Fault diagnostic wiring  48   b  extends from fault diagnostic electrode  40   b  provided at fixed part  21   a  through the upper surface of beam  23   b  to be connected to conductive part  27   a.  Thus, fault diagnostic electrode  40   b  provided to fixed part  21   a  is connected to conductive part  27   a  via fault diagnostic wiring  48   b.  In the peripheral portion of fixed part  21   b  shown in  FIG. 3 , fault diagnostic wiring  48   c  extends from fault diagnostic electrode  39  provided at fixed part  21   b  and is branched into branch lines  148   c  and  248   c.  Branch lines  148   c  and  248   c  are connected to conductive part  28   a  through upper surfaces of beams  24   a  and  24   b,  respectively. Thus, fault diagnostic electrode  39  provided at fixed part  21   b  is connected to conductive part  28   a  via fault diagnostic wiring  48   c.  Fault diagnostic wiring  48   a  extends from fault diagnostic electrode  40   a  provided to fixed part  21   b  through the upper surface of beam  24   a  to be connected to conductive part  28   a.  Thus, fault diagnostic electrode  40   a  provided at fixed part  21   b  is connected to conductive part  28   a  via fault diagnostic wiring  48   a.  Fault diagnostic wiring  48   b  extends from fault diagnostic electrode  40   b  provided at fixed part  21   b  through the upper surface of beam  24   b  to be connected to conductive part  28   a.  Thus, fault diagnostic electrode  40   b  provided to fixed part  21   b  is connected to conductive part  28   a  via fault diagnostic wiring  48   b.    
     Similar to the peripheral portions of fixed parts  21   a  and  21   b,  in the peripheral portion of fixed part  21   c,  fault diagnostic wiring  48   c  extends from fault diagnostic electrode  39  provided to fixed part  21   c  and is branched into branch lines  148   c  and  248   c.  Branch lines  148   c  and  248   c  are connected to conductive part  29   a  through upper surfaces of beams  25   a  and  25   b,  respectively. Thus, fault diagnostic electrode  39  provided to fixed part  21   c  is coupled to conductive part  29   a  via fault diagnostic wiring  48   c.  Fault diagnostic wiring  48   a  extends from fault diagnostic electrode  40   a  provided at fixed part  21   c  through the upper surface of beam  25   a  to be connected to conductive part  29   a.  Thus, fault diagnostic electrode  40   a  provided at fixed part  21   c  is connected to conductive part  29   a  via fault diagnostic wiring  48   a.  Fault diagnostic wiring  48   b  extends from fault diagnostic electrode  40   b  provided at fixed part  21   c  through the upper surface of beam  25   b  to be connected to conductive part  29   a.  Thus, fault diagnostic electrode  40   b  provided at fixed part  21   c  is connected to conductive part  29   a  via fault diagnostic wiring  48   b.  In the peripheral portion of fixed part  21   d,  fault diagnostic wiring  48   c  extends from fault diagnostic electrode  39  provided at fixed part  21   d  and is branched into branch lines  148   c  and  248   c.  Branch lines  148   c  and  248   c  extend through upper surfaces of beams  26   a  and  26   b,  respectively, to be connected to conductive part  30   a.  Thus, fault diagnostic electrode  39  provided at fixed part  21   d  is connected to conductive part  30   a  via fault diagnostic wiring  48   c.  Fault diagnostic wiring  48   a  extends from fault diagnostic electrode  40   a  provided at fixed part  21   d  through the upper surface of beam  26   a  to be connected to conductive part  30   a.  Thus, fault diagnostic electrode  40   a  provided to fixed part  21   d  is connected to conductive part  30   a  via fault diagnostic wiring  48   a.  Fault diagnostic wiring  48   b  extends from fault diagnostic electrode  40   b  provided at fixed part  21   d  through the upper surface of beam  26   b  to be connected to conductive part  30   a.  Thus, fault diagnostic electrode  40   b  provided at fixed part  21   d  is connected to conductive part  30   a  via fault diagnostic wiring  48   b.    
       FIG. 4A  is a top view of inertial force sensor  1001 . Strain-sensitive resistors  31   a  and  31   b  provided at beams  23   a  and  23   b  constitute resistors R 2  and R 4 , respectively. Strain-sensitive resistors  32   a  and  32   b  provided at beams  24   a  and  24   b  constitute resistors R 1  and R 3 , respectively. Strain-sensitive resistors  33   a  and  33   b  provided at beams  25   a  and  25   b  constitute resistors R 7  and R 5 , respectively. Strain-sensitive resistors  34   a  and  34   b  provided at beams  26   a  and  26   b  constitute resistors R 8  and R 6 , respectively. Strain-sensitive resistors  49   a  and  49   b  provided at frame  20  constitutes resistors R 9  and R 10 , respectively. 
       FIG. 4B  is a schematic diagram of a detection circuit of inertial force sensor  1001  for detecting acceleration in a direction of the X-axis. As shown in  FIG. 4B , resistors R 1 , R 2 , R 3 , and R 4  are connected to constitute a bridge circuit. A voltage is applied between a pair of nodes Vdd and GND opposite to each other while a voltage between another pair of nodes Vx 1  and Vx 2 , thereby detecting the acceleration in direction of the X-axis. 
       FIG. 4C  is a schematic diagram of a detection circuit of inertial force sensor  1001  for detecting acceleration in a direction of the Y-axis. As shown in  FIG. 4C , resistors R 5 , R 6 , R 7 , and R 8  are connected to form a bridge circuit. 
     A voltage is applied between a pair of nodes Vdd and GND opposite to each other while a voltage between another pair of nodes Vy 1  and Vy 2  is detected, thereby, detecting the acceleration in the direction of the Y-axis. 
       FIG. 4D  is a schematic diagram of a detection circuit of inertial force sensor  1001  for detecting acceleration in a direction of a Z-axis perpendicular to the X-axis and the Y-axis. As shown in  FIG. 4D , resistors R 5 , R 10 , R 8 , and R 9  are connected to form a bridge circuit. A voltage is applied between a pair of nodes Vdd and GND opposite to each other while a voltage between another pair of nodes Vz 1  and Vz 2  is detected, thereby, detecting the acceleration in the direction of the Z-axis direction. 
     Upon being used for a long time, conventional inertial force sensor  501  shown in  FIG. 19  may have a crack in bases of plummets  7  to  10 . Such crack may change an amount of a displacement in the vertical direction of plummets  7  to  10 , and cause resistances of strain-sensitive resistors  11  to  14  to fluctuate. Therefore, a signal output from the bridge circuit composed of strain-sensitive resistors  11  to  14  may not necessarily reflect the acceleration, thus preventing the acceleration from being detected accurately. 
     In inertial force sensor  1001  in accordance with Embodiment, if excessive acceleration is applied repetitively during the usage of inertial force sensor  1001  for a long time, the amounts of displacements of plummets  27  to  30  increases repetitively. This may cause beams  23   a  to  26   a  and  23   b  to  26   b  to fatigue, and produce cracks in the beams. Inertial force sensor  1001  in accordance with Embodiment  1  can detect a fault in which a crack is produced in a beam out of beams  23   a  to  26   a  and  23   b  to  26   b.    
       FIG. 5  is a circuit diagram of fault diagnosis circuit  1002  of inertial force sensor  1001  for detecting the fault. Input voltage VF for fault diagnosis which has been amplified by amplifier  42  of fault diagnosis circuit  1002  is applied to fault diagnostic electrode  39  provided at fixed part  21   a,  and is input into non-inverting input terminal  44  of comparator  43 . Input voltage VF applied to fault diagnostic electrode  39  is applied to inverting input terminal  45  of comparator  43  via fault diagnostic wiring  48   c  (branch line  148   c ), conductive part  27   a,  fault diagnostic wiring  48   a,  and fault diagnostic electrode  40   a.  Fault diagnostic electrode  40   a  is configured to be connected to inverting input terminal  45  of comparator  43 , and grounded via grounding resistor R 45 . 
     Similarly, input voltage VF for fault diagnosis which has been amplified by amplifier  42  of another fault diagnosis circuit  1002  is applied to fault diagnostic electrode  39  provided at fixed part  21   a,  and is input into non-inverting input terminal  44  of comparator  43 . Input voltage VF applied to fault diagnostic electrode  39  is applied to inverting input terminal  45  of comparator  43  via fault diagnostic wiring  48   c  (branch line  248   c ), conductive part  27   a,  fault diagnostic wiring  48   b,  and fault diagnostic electrode  40   b.  Fault diagnostic electrode  40   b  is configured to be connected to inverting input terminal  45  of comparator  43  and grounded via grounding resistor R 45 . 
     Similarly, input voltage VF for fault diagnosis, which is amplified by amplifier  42  of still another fault diagnosis circuit  1002 , is applied to fault diagnostic electrode  39  provided to fixed part  21   b,  and further input into non-inverting input terminal  44  of comparator  43 . Input voltage VF applied to fault diagnostic electrode  39  is applied to inverting input terminal  45  in comparator  43  through fault diagnostic wiring  48   c  (branch line  148   c ), conductive part  28   a,  fault diagnostic wiring  48   a  and fault diagnostic electrode  40   a.  Fault diagnostic electrode  40   a  is configured in such a manner that it is coupled to inverting input terminal  45  of comparator  43  and grounded through grounding resistor R 45 . 
     Similarly, input voltage VF for fault diagnosis which has been amplified by amplifier  42  of still another fault diagnosis circuit  1002  is applied to fault diagnostic electrode  39  provided at fixed part  21   b,  and input into non-inverting input terminal  44  of comparator  43 . Input voltage VF applied to fault diagnostic electrode  39  is applied to inverting input terminal  45  in comparator  43  via fault diagnostic wiring  48   c  (branch line  248   c ), conductive part  28   a,  fault diagnostic wiring  48   b,  and fault diagnostic electrode  40   b.  Fault diagnostic electrode  40   b  is configured to be connected to inverting input terminal  45  of comparator  43  and grounded via grounding resistor R 45 . 
     Similarly, input voltage VF for fault diagnosis which has been amplified by amplifier  42  of further fault diagnosis circuit  1002  is applied to fault diagnostic electrode  39  provided at fixed part  21   c,  and input into non-inverting input terminal  44  of comparator  43 . Input voltage VF applied to fault diagnostic electrode  39  is applied to inverting input terminal  45  of comparator  43  via fault diagnostic wiring  48   c  (branch line  148   c ), conductive part  29   a,  fault diagnostic wiring  48   a,  and fault diagnostic electrode  40   a.  Fault diagnostic electrode  40   a  is configured to be connected to inverting input terminal  45  of comparator  43  and grounded via grounding resistor R 45 . 
     Similarly, input voltage VF for fault diagnosis which has been amplified by amplifier  42  of further fault diagnosis circuit  1002  is applied to fault diagnostic electrode  39  provided at fixed part  21   c,  and input into non-inverting input terminal  44  of comparator  43 . Input voltage VF applied to fault diagnostic electrode  39  is applied to inverting input terminal  45  of comparator  43  via fault diagnostic wiring  48   c  (branch line  248   c ), conductive part  29   a,  fault diagnostic wiring  48   b,  and fault diagnostic electrode  40   b.  Fault diagnostic electrode  40   b  is configured to be connected to inverting input terminal  45  of comparator  43  and grounded via grounding resistor R 45 . 
     Similarly, input voltage VF for fault diagnosis which has been amplified by amplifier  42  of further fault diagnosis circuit  1002  is applied to fault diagnostic electrode  39  provided at fixed part  21   d,  and input into non-inverting input terminal  44  of comparator  43 . Input voltage VF applied to fault diagnostic electrode  39  is applied to inverting input terminal  45  of comparator  43  via fault diagnostic wiring  48   c  (branch line  148   c ), conductive part  30   a,  fault diagnostic wiring  48   a,  and fault diagnostic electrode  40   a.  Fault diagnostic electrode  40   a  is configured to be connected to inverting input terminal  45  of comparator  43  and grounded via grounding resistor R 45 . 
     Similarly, input voltage VF for fault diagnosis which has been amplified by amplifier  42  of further fault diagnosis circuit  1002  is applied to fault diagnostic electrode  39  provided at fixed part  21   d,  and input into non-inverting input terminal  44  of comparator  43 . Input voltage VF applied to fault diagnostic electrode  39  is applied to inverting input terminal  45  of comparator  43  via fault diagnostic wiring  48   c  (branch line  248   c ), conductive part  30   a,  fault diagnostic wiring  48   b,  and fault diagnostic electrode  40   b.  Fault diagnostic electrode  40   b  is configured to be connected to inverting input terminal  45  of comparator  43  and grounded via grounding resistor R 45 . 
       FIG. 6  shows output voltage Vout of comparator  43  of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   a  provided at fixed part  21   a  of inertial force sensor  1001 . In  FIG. 6 , the vertical axis represents output voltage Vout of comparator  43 , and the horizontal axis represents time. As shown in  FIG. 6 , a crack does not occur in beam  23   a  until time point tp 1 , hence allowing inertial force sensor  1001  to detect acceleration normally. In this normal use, since voltage VF is applied to both fault diagnostic electrodes  39  and  40   a,  comparator  43  outputs a voltage of 0V. When a crack occurs in beam  23   a  at time point tp 1 , at least one of fault diagnostic wirings  48   a  and  48   c  (branch lines  148   c ) is disconnected and opens. Then, voltage VF is input into non-inverting input terminal  44  of comparator  43  while the non-inverting input terminal is grounded via grounding resistor R 45  to have a voltage of OV applied thereto, hence causing comparator  43  to output voltage VF. According to Embodiment 1, voltage VF is 12.5 V. Thus, output voltage Vout of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   a  provided at fixed part  21   a  allows occurrence of a crack in beam  23   a  to be detected. Similarly, output voltage Vout of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   b  provided at fixed part  21   a  allows occurrence of a crack in beam  23   b  to be detected. 
     Similarly, output voltage Vout of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   a  provided at fixed part  21   b  allows occurrence of a crack in beam  24   a  to be detected. Similarly, output voltage Vout of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   b  provided at fixed part  21   b  allows occurrence of a crack in beam  24   b  to be detected. 
     Similarly, output voltage Vout of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   a  provided at fixed part  21   c  allows occurrence of a crack in beam  25   a  to be detected. Similarly, output voltage Vout of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   b  provided at fixed part  21   c  allows occurrence of a crack in beam  24   b  to be detected. 
     Similarly, output voltage Vout of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   a  provided at fixed part  21   d  allows occurrence of a crack in beam  26   a  to be detected. Similarly, output voltage Vout of fault diagnosis circuit  1002  connected to fault diagnostic electrodes  39  and  40   b  provided at fixed part  21   d  allows occurrence of a crack in beam  26   b  to be detected. 
     Exemplary Embodiment 2 
       FIG. 7  is a top view of inertial force sensor  2001  in accordance with Exemplary Embodiment 2. Inertial force sensor  2001  is an acceleration sensor for detecting acceleration, an inertial force, applied thereto. In  FIG. 7 , components identical to those of inertial force sensor  1001  in accordance with Embodiment  1  shown in  FIG. 1  are denoted by the same reference numerals. 
     Inertial force sensor  2001  includes fault diagnostic electrodes  51  and  52  provided only at fixed part  21   a,  instead of four fault diagnostic electrodes  39 , four fault diagnostic electrodes  40   a,  and four fault diagnostic electrodes  40   b  of inertial force sensor  1001  in accordance with Embodiment  1  shown in  FIG. 1 . A fault diagnostic electrode is not provided at each of fixed parts  2  lb to  21   c.  Inertial force sensor  2001  includes conductive parts  54   a  and  54   b  provided on an upper surface of plummet  27  instead of conductive part  27   a,  includes conductive parts  55   a  and  55   b  provided on an upper surface of plummet  28  instead of conductive part  28   a,  includes conductive parts  56   a  and  56   b  provided on an upper surface of plummet  29  instead of conductive part  29   a,  and includes conductive parts  57   a  and  57   b  provided on an upper surface of plummet  30  instead of conductive part  30   a.  Inertial force sensor  2001  includes plural fault diagnostic wirings  53  instead of fault diagnostic wirings  48   a  to  48   c.  Fault diagnostic wirings  53  extends through  57   b  via beams  23   a  to  26   a  and  23   b  to  26   b  to electrically connect in series from fault diagnostic electrode  51  to fault diagnostic electrode  52  via conductive parts  54   a  to  57   a  and  54   b.    
     Inertial force sensor  2001  can detect acceleration in directions of the X-axis, the Y-axis, and the Z-axis similarly to inertial force sensor  1001  in accordance with Embodiment 1. 
       FIG. 8  is a circuit diagram of fault diagnosis circuit  2002  of inertial force sensor  2001 . In  FIG. 8 , components identical to those of inertial force sensor  1002  shown in  FIG. 5  are denoted by the same reference numerals. In fault diagnosis circuit  2002 , fault diagnostic electrode  52  is connected to inverting input terminal  45  of comparator  43 . Input voltage VF is applied to fault diagnostic electrode  51 , and to inverting input terminal  45  of comparator  43  via fault diagnostic wiring  53 , conductive parts  54   a  to  57   a  and  54   b  to  57   b,  and fault diagnostic electrode  52 . In fault diagnosis circuit  2002 , similarly to fault diagnosis circuit  1002  in accordance with Embodiment  1  shown in  FIG. 6 , a crack does not occur in any of beams  23   a  to  26   a  and  23   b  to  26   b  until time point tp 1 , and allows inertial force sensor  2001  to normally detect the acceleration. In this normal usage, voltage VF is applied to both fault diagnostic electrodes  51  and  52 , and allows comparator  43  to output a voltage of OV. When a crack occurs in at least one of beams  23   a  to  26   a  and  23   b  to  26   b,  fault diagnostic wiring  53  is disconnected and open. Then, while voltage VF is input into non-inverting input terminal  44  of comparator  43 , the non-inverting input terminal is grounded via grounding resistor R 45  to have a voltage of OV applied, hence causing comparator  43  to output voltage VF. According to Embodiment 1, voltage VF is 12.5 V. Thus, output voltage Vout of fault diagnosis circuit  1002  allows occurrence of a crack in beams  23   a  to  26   a  and  23   b  to  26   b  to be detected. 
     Exemplary Embodiment 3 
       FIG. 9  is a top view of inertial force sensor  211  in accordance with 
     Exemplary Embodiment 3.  FIG. 10  is a sectional view of inertial force sensor  211  at line  10 - 10  shown in  FIG. 9 . Inertial force sensor  211  is an acceleration sensor for detecting acceleration, an inertial force, applied thereto. 
     Inertial force sensor  211  includes fixed part  212 , plummet  213 , beams  214   a  and  214   b  having respective one ends connected to fixed part  212 , counter substrate  215  connected to fixed part  212  such that counter substrate  215  faces plummet  213 , plummet-displacement electrode  216  provided on an upper surface of plummet  213 , counter electrode  217  provided on a lower surface of counter substrate  215 , fault diagnostic electrode  218  provided at fixed part  212 , and fault diagnostic wiring  219  for electrically connecting fault diagnostic electrode  218  to plummet-displacement electrode  216 . Respective another ends of beams  214   a  and  214   b  are connected to plummet  213 . The lower surface of counter substrate  215  faces the upper surface of plummet  213 . Counter electrode  217  faces plummet-displacement electrode  216 . Detection unit  214   c  is provided on beam  214   a  while detection unit  214   d  is provided on beam  214   b.  Fault diagnostic wiring  219  extends through beams  214   a  and  214   b  to be connected to fault diagnostic electrode  218  is connected to plummet-displacement electrode  216 . 
     In this configuration, voltage Vd is applied between plummet-displacement electrode  216  and counter electrode  217  to apply an electrostatic force to plummet  213 , and displaces plummet  213  as if acceleration is applied to plummet  213 , thus providing a self-diagnostic function for determining whether inertial force sensor  211  normally operates or not. 
       FIG. 11A  is a schematic view of inertial force sensor  211  having beam  214   b  not broken but having beam  214   a  broken.  FIG. 11B  is a schematic view of inertial force sensor  211  having beam  214   a  not broken but having beam  214   b  broken. As shown in  FIG. 11A , when beam  214   a  is broken, fault diagnostic wiring  219  is disconnected at beam  214   a.  As shown in  FIG. 11B , when beam  214   b  is broken, fault diagnostic wiring  219  is disconnected at beam  214   b.  Thus, when any one of beams  214   a  and  214   b  is broken, fault diagnostic wiring  219  is disconnected, and fault diagnostic electrode  218  is electrically disconnected from plummet-displacement electrode  216  not. Therefore, even when voltage Vd is applied to fault diagnostic electrode  218 , voltage Vd is not applied between plummet-displacement electrode  216  and counter electrode  217 , and does not displace plummet  213 . Therefore, it can be determined that inertial force sensor  211  is in a fault state. 
     A configuration of inertial force sensor  211  will be detailed below. Fixed part  212 , plummet  213 , beams  214   a  and  214   b,  and counter substrate  215  may be made of, e.g. silicon, molten quartz, or alumina. They are preferably made of silicon to provide inertial force sensor  211  with a small size by using a micromachining technology. 
     Fixed part  212  may adhere to counter substrate  215  with, e.g. adhesive, metal junction, ambient temperature junction, of anode junction. The adhesives may be, e.g. epoxy resin or silicone resin. The adhesive made of silicone resin can reduce a stress generated by hardening of the adhesive. Detection units  214   c  and  214   d  can utilize, e.g. a strain resistance method or a capacitance method. In the case that piezoelectric resistors are used as strain-sensitive resistors for detection units  214   c  and  214   d,  the sensitivity of inertial force sensor  211  can be improved. Furthermore, as the strain resistance method, when a thin film resistance method using an oxide film strain-sensitive resistor is used for detection units  214   c  and  214   d,  temperature characteristics of inertial force sensor  211  can be improved. 
       FIG. 12  is a circuit diagram of inertial force sensor  211  when detection units  214   c  and  214   d  use a strain resistance method. Strain-sensitive resistor R 201  corresponds to detection unit  214   c.  Strain-sensitive resistor R 204  corresponds to detection unit  214   d.  Strain-sensitive resistors R 202  and R 203  are reference resistors provided at fixed part  212 . As shown in  FIG. 12 , strain-sensitive resistors R 201 , R 202 , R 203 , and R 204  are connected to form a bridge circuit. A voltage is applied between a pair of nodes Vdd and GND opposite to each other while voltage Vs between another pair of nodes V 201  and V 202  is detected, thereby detecting the acceleration applied to inertial force sensor  211 . 
     The self-diagnostic function of inertial force sensor  211  will be described with reference to  FIGS. 10 and 12 . To perform the self-diagnosis, voltage Vd is applied between plummet-displacement electrode  216  and counter electrode  217 , as shown in  FIG. 10 . According to Embodiment 3, voltage Vd is about 12V. Thus, an electrostatic force is generated between plummet-displacement electrode  216  and counter electrode  217 , and displaces plummet  213  such that counter substrate  215  attracts plummet  213 . The displacement of plummet  213  decreases resistances of strain-sensitive resistor R 201  corresponding to detection unit  214   c  and strain-sensitive resistor R 204  corresponding to detection unit  214   d.  This increases output voltage Vs of the bridge circuit, and it can be determined that inertial force sensor  211  normally operates. 
       FIG. 13  is a top view of fixed part  212  of a Comparative Example of inertial force sensor  511 . In  FIG. 13 , components identical to those of inertial force sensor  211  according to Embodiment 3 shown in  FIG. 9  are denoted by the same reference numerals. Inertial force sensor  511  of the 
     Comparative Example includes fault diagnostic wiring  210  instead of fault diagnostic wiring  219  shown in  FIG. 9 . One end of fault diagnostic wiring  210  is connected to fault diagnostic electrode  218 . Another end of fault diagnostic wiring  210  is branched into two branch lines. One of the branch lines is connected to plummet-displacement electrode  216  through beam  214   a  while the other branch line is connected to plummet-displacement electrode  216  through beam  214   b.  In this configuration, for example, even if inertial force sensor  511  is in a fault state in which one of beams  214   a  is broken due to, e.g. drop or shock, since beam  214   b  is connected, fault diagnostic wiring  210  provided at beam  214   b  is not disconnected. Therefore, although beam  214   a  is broken, inertial force sensor  511  cannot detect the fault by the self-diagnostic function. 
     In inertial force sensor  211  in accordance with Embodiment 3, as shown in  FIGS. 11A and 11B , when one of beams  214   a  and  214   b  is broken, voltage Vd is not applied between plummet-displacement electrode  216  and counter electrode  217 . Therefore, plummet  213  is not displaced, resistances of strain-sensitive resistors R 201  and R 204  are not changed, so that it can be determined that inertial force sensor  211  is in a fault state. 
     Exemplary Embodiment 4 
       FIG. 14  is a top view of inertial force sensor  221  in accordance with Exemplary Embodiment 4.  FIG. 15  is a sectional view of inertial force sensor  221  at line  15 - 15  shown in  FIG. 14 . 
     Inertial force sensor  221  includes fixed part  222  having a frame shape, beams  234   a  to  237   a  and  234   b  to  237   b  having respective one ends connected to fixed part  222 , plummets  223   a  to  223   d,  counter substrate  225  coupled to fixed part  222  such that counter substrate  225  faces upper surfaces of plummets  223   a  to  223   d,  plummet-displacement electrodes  226   a  to  226   d  provided on upper surfaces of plummets  223   a  to  223   d,  respectively, counter electrodes  227   a  to  227   d  provided on a lower surface of counter substrate  225 , fault diagnostic electrodes  228   a  to  228   d  provided at fixed part  222 , and fault diagnostic wirings  229   a  to  229   d  for electrically connecting fault diagnostic electrodes  228   a  to  228   d  to plummet-displacement electrodes  226   a  to  226   d,  respectively. The lower surfaces of counter electrodes  227   a  to  227   d  face the upper surfaces of plummet-displacement electrodes  226   a  to  226   d,  respectively. Detection units  234   c  to  237   c  and  234   d  to  237   d  are provided on the upper surfaces of beams  234   a  to  237   a  and  234   b  to  237   b,  respectively. 
     Fault diagnostic wirings  229   a  to  229   d  are connected to fault diagnostic electrodes  228   a  to  228   d,  respectively. Fault diagnostic wiring  229   a  extends from fault diagnostic electrode  228   a  through beams  234   a  and  234   b  to be connected to plummet-displacement electrode  226   a.  Fault diagnostic wiring  229   b  extends from fault diagnostic electrode  228   b  through beams  235   a  and  235   b  to be connected to plummet-displacement electrode  226   b.  Fault diagnostic wiring  229   c  extends from fault diagnostic electrode  228   c  through beams  236   a  and  236   b  to be connected to plummet-displacement electrode  226   c.  Fault diagnostic wiring  229   d  extends from fault diagnostic electrode  228   d  through beams  237   a  and  237   b  to be connected to plummet-displacement electrode  226   d.    
     In this configuration, voltage Vd is applied between plummet-displacement electrodes  226   a  to  226   d  and counter electrodes  227   a  to  227   d  to apply electrostatic forces to plummets  223   a  to  223   d  to displace plummets  223   a  to  223   d  as if acceleration is applied to plummets  223   a  to  223   d,  thus providing a self-diagnostic function for determining whether inertial force sensor  211  normally operates or not. 
     A configuration of inertial force sensor  221  will be detailed below. 
     Fixed part  222  has a rectangular frame shape having hollow region  222   a  at the center thereof viewing from above. Hollow region  222   a  may have a rectangular shape or a circular shape. 
     As shown in  FIG. 14 , the outer edge of hollow region  222   a  has an octagon shape having four longer sides  222   b  and four shorter sides  222   c  located alternately. Four longer sides  222   b  may preferably face four corner portions  222   d  of fixed part  222 . This configuration allows adhesive region  222   e  for adhesively bonding counter substrate  225  to fixed part  222  to be located in a region between each of four longer sides  222   b  and respective one of corner portions  222   d.  This configuration allows an area of counter substrate  225  to be smaller than an area of fixed part  222 . The small area of counter substrate  225  can expose an end portion of fixed part  222  from counter substrate  225 , and allows fault diagnostic electrode  228  to be provided at the end portion of fixed part  222  and to be coupled to a package or an IC easily. 
     Beams  234   a  to  237   a  and  234   b  to  237   b  are preferably connected to four shorter sides  222   c  of hollow region  222   a.  This configuration reduces the lengths of wirings between fault diagnostic electrodes  228   a  to  228   d  and detection units  234   c  to  237   c  and  234   d  to  237   d  provided at the end portion of fixed part  222 , accordingly preventing unnecessary noises from being mixed. Examples of a method for adhesively bonding fixed part  222  to counter substrate  225  include adhesively bonding with adhesives, metal junction, ambient temperature junction, and anode junction. Adhesives, such as epoxy resin and silicone resin, can be used. When the adhesives are heated to be hardened in the manufacturing process, since a stress is generated due to the hardening of adhesives and a difference of linear expansion coefficients of fixed part  222  and counter substrate  225 , this stress is accumulated in beams  234   a  to  237   a  and  234   b  to  237   b  as residual stress. In inertial force sensor  221  in accordance with Embodiment 4, since plummets  223   a  to  223   d  are supported by beams  234   a  to  237   a  and  234   b  to  237   b  from only one direction, it is possible to suppress transition of beams  234   a  to  237   a  and  234   b  to  237   b  to different buckling modes. Silicone resin as adhesives can reduce the stress due to the hardening of the adhesive. 
     As shown in  FIG. 14 , beams  234   a  to  237   a  and  234   b  to  237   b  having one end connected to fixed part  222  extend to hollow region  222   a.  A thickness of each of beams  234   a  to  237   a  and  234   b  to  237   b  is preferably smaller than a thickness of fixed part  222 , and smaller than a thickness of each of plummets  223   a  to  223   d.  This configuration allows beams  234   a  to  237   a  and  234   b  to  237   b  to easily warp, and increases sensitivity of inertial force sensor  221  to the acceleration. Plummet  223   a  is connected to respective another ends of beams  234   a  and  234   b.  Plummet  223   b  is connected to respective another ends of beams  235   a  and  235   b.  Plummet  223   c  is connected to respective another ends of beams  236   a  and  236   b.  Plummet  223   d  is connected to respective another ends of beams  237   a  and  237   b.  Each of plummets  223   a  to  223   d  has a projection. The projection of plummet  223   a  faces the projection of plummet  223   b  while the projection of plummet  223   c  faces the projection of plummet  223   d.  That is, the projections of plummets  223   a  to  223   d  preferably face each other across the center of hollow region  222   a.  This configuration locates four plummets  223   a  to  223   d  close to each other. This arrangement increases the weights of four plummets  223   a  to  223   d  as to increase the sensitivity of inertial force sensor  221  and decrease the size of inertial force sensor  221 . 
     Fixed part  222 , beams  234   a  to  237   a  and  234   b  to  237   b,  plummets  223   a  to  223   d,  and counter substrate  225  may be made of, e.g. silicon, molten quartz, or alumina. They are preferably made of silicon to provide inertial force sensor  221  with a small size by using micromachining technology. 
     Detection units  234   c  to  237   c  and  234   d  to  237   d  can utilize, e.g. a strain resistance method or a capacitance method. When piezoelectric resistors are used for the strain resistance method, the sensitivity of inertial force sensor  221  can be improved. As the strain resistance method, a thin film resistance method employing oxide film strain-sensitive resistors improves temperature characteristics of inertial force sensor  221 . 
       FIG. 16A  is a top view of inertial force sensor  221  for illustrating a method for detecting acceleration. Strain-sensitive resistors R 203  and R 201  are disposed as detection units  234   c  and  234   d  provided on the upper surfaces of beams  234   a  and  234   b,  respectively. Strain-sensitive resistors R 204  and R 202  are disposed as detection units  235   c  and  235   d  provided on the upper surfaces of beams  235   a  and  235   b,  respectively. Strain-sensitive resistors R 205  and R 207  are disposed as detection units  236   c  and  236   d  provided on the upper surfaces of beams  236   a  and  236   b,  respectively. Strain-sensitive resistors R 206  and R 208  are disposed as detection units  237   c  and  237   d  provided on the upper surfaces of beams  237   a  and  237   b,  respectively. Strain-sensitive resistors R 209  and R 210  are disposed on fixed part  222 . 
       FIG. 16B  is a circuit diagram of an X-axis detection circuit of inertial force sensor  221  for detecting acceleration in a direction of the X-axis. Strain-sensitive resistors R 201 , R 202 , R 203 , and R 204  are connected to form a bridge circuit. While a voltage is applied between a pair of nodes Vdd and GND opposite to each other, potential difference Vsx between another pair of nodes VxP and VxM (a difference obtained by subtracting a voltage at node VxM from a voltage at node VxP) is detected, thereby detecting the acceleration in the direction of the X-axis. 
       FIG. 16C  is a circuit diagram of a Y-axis detection circuit of inertial force sensor  221  for detecting acceleration in a direction of the Y-axis. Strain-sensitive resistors R 205 , R 206 , R 207 , and R 208  are connected to form a bridge circuit. While a voltage is applied between a pair of nodes Vdd and GND opposite to each other, potential difference Vsy between another pair of nodes VyP and VyM (a difference obtained by subtracting a voltage at node VyM from a voltage at node VyP) is detected, thereby detecting the acceleration in the direction of the Y-axis. 
       FIG. 16D  is a circuit diagram of a Z-axis detection circuit of inertial force sensor  221  for detecting acceleration in a direction of the Z-axis. Strain-sensitive resistors R 205 , R 210 , R 206 , and R 209  are connected to form a bridge circuit. While a voltage is applied between a pair of nodes Vdd and GND opposite to each other, potential difference Vsz between the other pair of nodes VzP and VzM (a difference obtained by subtracting a voltage at node VzM from a voltage at node VzP) is detected, thereby detecting the acceleration in the direction of the Z-axis. 
     Next, a self-diagnostic function of inertial force sensor  221  in accordance with Embodiment 4 will be described. Inertial force sensor  221  in accordance with Embodiment 4 preforms the self-diagnosis with three voltage-applying patterns  1  to  3 . 
       FIG. 17A  is a top view of inertial force sensor  221  for illustrating voltage-applying pattern  1 .  FIGS. 17B and 17C  are circuit diagrams of inertial force sensor  221  for performing the self-diagnosis with voltage-applying pattern  1 . In voltage-applying pattern  1 , predetermined voltage Vd is applied between plummet-displacement electrode  226   a  provided on the upper surface of plummet  223   a  and counter electrode  227   a  while predetermined voltage Vd is applied between plummet-displacement electrode  226   c  provided on the upper surface of plummet  223   c  and counter electrode  227   c.  A voltage is not applied between plummet-displacement electrode  226   b  provided on the upper surface of plummet  223   b  and counter electrode  227   b,  and a voltage is not applied between plummet-displacement electrode  226   d  provided on the upper surface of plummet  223   d  and counter electrode  227   d.  This pattern generates an electrostatic force to displace plummets  223   a  and  223   c  such that counter substrate  225  attracts plummets  223   a  and  223   c,  but not to displace plummets  223   b  and  223   d.  The displacement of plummets  223   a  and  223   d  decreases resistances of strain-sensitive resistors R 201 , R 203 , R 205 , and R 207 . As shown in  FIG. 17B , in the Y-axis detection circuit, since a voltage at node VyM increases and a voltage at node VyP decreases, potential difference Vsy between nodes VyP and VyM (a difference obtained by subtracting a voltage at node VyM from a voltage at node VyP) is a negative value. Furthermore, as shown in  FIG. 17C , in the Z-axis detection, a voltage at node VzM increases, and a voltage at node VzP is not changed. Therefore, potential difference Vsz between nodes VzP and VzM (a difference obtained by subtracting a voltage at node VzM from a voltage at node VzP) is a negative value. Thus, when both potential differences Vsy and Vsx output from the Y-axis detection circuit and the Z-axis detection circuit are the negative values, it can be determined that beams  234   a,    234   b  and  236   a,    236   b  are not broken and the sensor operates normally. 
       FIG. 17D  is a top view of inertial force sensor  221  showing voltage-applying pattern  2 . In voltage-applying pattern  2 , predetermined voltage Vd is applied between plummet-displacement electrode  226   b  provided on the upper surface of plummet  223   b  and counter electrode  227   b  while predetermined voltage Vd is applied between plummet-displacement electrode  226   d  provided on the upper surface of plummet  223   d  and counter electrode  227   d.  At this moment, a voltage is not applied between plummet-displacement electrode  226   a  provided on the upper surface of plummet  223   a  and counter electrode  227   a,  and voltage Vd is not applied between plummet-displacement electrode  226   c  provided on the upper surface of plummet  223   c  and counter electrode  227   c.  This pattern generates an electrostatic force to displace plummets  223   b  and  223   d  such that they counter substrate  225  attracts plummets  223   b  and  223   d,  but not to displace plummets  223   a  and  223   c.  The displacement of plummets  223   b  and  223   d  decreases resistances of strain-sensitive resistors R 202 , R 204 , R 206 , and R 208 . Therefore, in the Y-axis detection circuit shown in  FIG. 16   c,  since a voltage at node VyM decreases and a voltage at node VyP increases, potential difference Vsy between nodes VyP and VyM (a difference obtained by subtracting a voltage at node VyM from a voltage at node VyP) is a positive value. In the Z-axis detection shown in  FIG. 16D , a voltage at node VzM is not changed, and a voltage at node VzP decreases. Therefore, potential difference Vsz between nodes VzP and VzM (a difference obtained by subtracting a voltage at node VzM from a voltage at node VzP) is a negative value. Thus, when potential difference Vsy output from the Y-axis detection circuit becomes the positive value and potential difference Vsz output from the Z-axis detection circuit becomes the negative value, it can be determined that beams  235   a,    235   b,    237   a,  and  237   b  are not broken and the sensor operates normally. 
       FIG. 17E  is a top view of inertial force sensor  221  showing voltage-applying pattern  3 . In voltage-applying pattern  3 , predetermined voltage Vd is applied between plummet-displacement electrodes  226   a  to  226   d  provided on the upper surfaces of plummets  223   a  to  223   d  and counter electrodes  227   a  to  227   d.  This operation generates an electrostatic force to displace plummets  223   a  to  223   d  such that counter substrate  225  attracts plummets  223   a  to  223   d.  The displacements of plummets  223   a  to  223   d  decrease resistances of strain-sensitive resistors R 201  to R 208 . Therefore, in the Y-axis detection circuit shown in  FIG. 16C , voltages at nodes VyM and VyP are not changed, potential difference Vsy between nodes VyP and VyM (a difference obtained by subtracting a voltage at node VyM from a voltage at node VyP) becomes zero. In the Z-axis detection circuit shown in  FIG. 16D , since a voltage at node VzM increases and a voltage at VzP decreases, potential difference Vsz between the pair of the other nodes VzP and VzM (a difference obtained by subtracting a voltage at node VzM from a voltage at node VzP) is a negative value. Thus, when potential difference Vsy output from the Y-axis detection circuit becomes zero, and potential difference Vzx output from the Z-axis detection becomes the negative value, it can be determined that beams  234   a  to  237   a  and  234   b  to  237   b  are not broken and inertial force sensor  221  operates normally. 
     If any beam of beams  234   a  to  234   a  and  234   b  to  237   b  connected to plummets  223   a  to  223   d  is broken, the plummet connected to the broken beam is not displaced, and it can be determined by the above self-diagnostic function that an operation is in a fault state. 
       FIG. 18  is a top view of another inertial force sensor  221 A in accordance with Embodiment 4. In  FIG. 18 , components identical to those of inertial force sensor  221  shown in  FIG. 14  are denoted by the same reference numerals. In inertial force sensor  221  shown in  FIG. 14 , four fault diagnostic wirings  229   a  to  229   d  connected to plummet-displacement electrodes  226   a  to  226   d  on the upper surfaces of plummets  223   a  to  223   d  are connected to other fault diagnostic electrodes  228   a  to  228   b,  respectively. Inertial force sensor  221 A shown in  FIG. 18  does not include fault diagnostic electrodes  228   c  and  228   d,  and includes fault diagnostic wirings  239   a  and  239   b  connected to fault diagnostic electrodes  228   a  and  228   b,  respectively, instead of fault diagnostic wiring  229   a  to  229   d.  Fault diagnostic wiring  239   a  extends from fault diagnostic electrode  228   a  through beams  234   a  and  234   b  to be connected to plummet-displacement electrode  226   a  on the upper surface of plummet  223   a.  Fault diagnostic wiring  239   a  further extends from plummet-displacement electrode  226   a  through beams  236   a  and  236   b  to be connected to plummet-displacement electrode  226   c  on the upper surface of plummet  223   c.  Fault diagnostic wiring  239   b  extends from fault diagnostic electrode  228   b  through beams  235   a  and  235   b  to be connected to plummet-displacement electrode  226   b  on the upper surface of plummet  223   b.  Fault diagnostic wiring  239   b  further extends from plummet-displacement electrode  226   b  through beams  237   a  and  237   b  to be connected to plummet-displacement electrode  226   d  on the upper surface of plummet  223   d.  Inertial force sensor  221 A can perform a self-diagnosis with voltage-applying patterns  1  to  3  shown in  FIGS. 17A to 17E . The smaller number of the fault diagnostic electrodes reduces the size of inertial force sensor  221 A. The smaller number of the fault diagnostic electrodes reduces the number of bonding wires between the fault diagnostic electrode and a mount board having inertial force sensor  221 A mounted thereto, hence simplifying the manufacturing process. 
     Inertial force sensors  211 ,  221 , and  221 A in accordance with the embodiments are acceleration sensors for detecting acceleration, but may be different types of sensors, such as strain sensors. 
     In the above exemplary embodiments, terms, such as “upper surface” and “lower surface”, indicating directions merely indicate relative directions dependent only on the relative positional relation of components, such as plummets of inertial force sensors, but do not indicate absolute directions, such as a vertical direction. 
     As mentioned above, inertial force sensors  211 ,  221 , and  221 A in accordance with Embodiments 3 and 4 can diagnose fault by the self-diagnostic function even when only one beam is broken due to shock or the like and the other beam is not broken, thus having high reliability. 
     Therefore, the inertial force sensors are useful as sensors, such as an inertial force sensor and an angular velocity sensor, which are used for, e.g. vehicles, navigation devices, and portable terminals. 
     INDUSTRIAL APPLICABILITY 
     An inertial force sensor according to the present invention has high reliability, and is useful as an inertial force sensor used for, e.g. vehicles and portable terminals. 
     REFERENCE MARKS IN THE DRAWINGS 
       21   a  Fixed Part (First Fixed Part) 
       21   b  Fixed Part (Second Fixed Part) 
       23   a  Beam (First Beam) 
       24   a  Beam (Second Beam) 
       27  Plummet (First Plummet) 
       27   a  Conductive Part (First Conductive Part) 
       28  Plummet (Second Plummet) 
       28   a  Conductive Part (First Conductive Part) 
       31   a  Strain-Sensitive Resistor (First Strain-Sensitive Resistor) 
       32   a  Strain-Sensitive Resistor (Second Strain-Sensitive Resistor) 
       39  Fault Diagnostic Electrode (First Fault Diagnostic Electrode, Third Fault Diagnostic Electrode) 
       40   a  Fault Diagnostic Electrode (Second Fault Diagnostic Electrode, Fourth Fault Diagnostic Electrode) 
       43  Comparator (First Comparator, Second Comparator) 
       44  Non-Inverting Input Terminal 
       45  Inverting Input Terminal 
       48   a  Fault Diagnostic Wiring (Second Fault Diagnostic Wiring, Fourth Fault Diagnostic Wiring) 
       48   c  Fault Diagnostic Wiring (First Fault Diagnostic Wiring, Third Fault Diagnostic Wiring) 
       211 ,  221 ,  221   a  Inertial Force Sensor 
       212 ,  222  Fixed Part 
       213 ,  223   a  Plummet (First Plummet) 
       214   a,    234   a  Beam (First Beam) 
       214   b,    234   b  Beam (Second Beam) 
       216 ,  226   a  Plummet-Displacement Electrode (First Plummet-Displacement Electrode) 
       217 ,  227   a  Counter Electrode (First Counter Electrode) 
       218 ,  228 ,  228   a - 228   d  Fault Diagnostic Electrode 
       219 ,  229   a - 229   d  Fault Diagnostic Wiring 
       223   c  Plummet (Second Plummet) 
       226   c  Plummet-Displacement Electrode (Second Plummet-Displacement Electrode) 
       227   c  Counter Electrode (Second Counter Electrode) 
       236   a  Beam (Third Beam) 
       236   b  Beam (Fourth Beam)