Patent Publication Number: US-2022221487-A1

Title: Inertial measurement unit

Description:
The present application is based on, and claims priority from JP Application Serial Number 2021-002129, filed Jan. 8, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an inertial measurement unit. 
     2. Related Art 
     An inertial measurement unit (IMU) having a plurality of sensors such as an angular velocity sensor and an acceleration sensor and used to measure an amount of displacement of a building, a structure or the like is known. In the related-art inertial measurement unit, a malfunction due to a vibration, and a vibration rectification error (VRE) due to a vibration at a non-measurement target frequency that is different from a measurement target frequency, may occur, posing a risk of inducing a measurement error in the measurement target frequency range. Therefore, an inertial measurement unit that restrains a vibration at a non-measurement target frequency by a mechanical filter is proposed. 
     For example, JP-A-2010-258734 discloses an oven controlled crystal oscillator having a gel bushing as a mechanical filter. According to JP-A-2010-258734, a gel bushing is provided in the four corners of a sub substrate where the oven controlled crystal oscillator is installed, thus restraining an unwanted vibration at a non-measurement target frequency. 
     However, the gel bushings in the four corners described in JP-A-2010-258734 are arranged with the axial directions thereof laid parallel to each other along the same direction. Therefore, for example, a vibration along the Z-axis direction, which is the axial direction, is restrained, whereas vibrations along the X and Y-axis directions, which are not the axial direction, are restrained to a lesser extent. In this case, mechanical filter characteristics for a measured acceleration value in the axial direction of the mechanical filter (for example, Z-axis) and for measured acceleration values in the other directions (for example, X and Y-axis directions) are different. Therefore, the combined filter characteristic of the plurality of mechanical filters is anisotropic, making it difficult to evaluate an acceleration using the three axes. There is also a problem in that the VRE restraining effect is lower in the directions in which the filter characteristic is lower (for example, X and Y-axis directions), increasing the vibration rectification error in the measured acceleration value. 
     SUMMARY 
     An inertial measurement unit includes: a sensor unit including an inertial sensor, a case accommodating the inertial sensor, and a first fixing part having the case fixed thereto; an elastic member having a first elastic member mainly damping a vibration at a predetermined frequency in a first direction and a second elastic member mainly damping a vibration at a predetermined frequency in a second direction that is different from the first direction; a second fixing part where the sensor unit and the elastic member are arranged; and a fixing member fixing the sensor unit and the elastic member to the second fixing part. The fixing member has a first fixing member penetrating the sensor unit and the first elastic member and pressing the first elastic member, and a second fixing member penetrating the sensor unit and the second elastic member and pressing the second elastic member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing a schematic structure of an inertial measurement unit according to a first embodiment. 
         FIG. 2  is a schematic side view of the inertial measurement unit shown in  FIG. 1 , as viewed from a negative X-axis direction. 
         FIG. 3  is a partial cross-sectional view showing peripheries of an elastic member provided in the inertial measurement unit. 
         FIG. 4  is a plan view showing a schematic structure of an inertial measurement unit according to a second embodiment. 
         FIG. 5  is a cross-sectional view of the schematic structure of the inertial measurement unit shown in  FIG. 4 , taken along a line A-A. 
         FIG. 6  is a plan view showing a schematic structure of an inertial measurement unit according to a third embodiment. 
         FIG. 7  is a cross-sectional view of the schematic structure of the inertial measurement unit shown in  FIG. 6 , taken along a line B-B. 
         FIG. 8  is a front view showing a schematic structure of an inertial measurement unit according to a fourth embodiment. 
         FIG. 9  is a plan view showing the schematic structure of the inertial measurement unit shown in  FIG. 8 . 
         FIG. 10  is a side view showing the schematic structure of the inertial measurement unit shown in  FIG. 8 . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of an inertial measurement unit will now be described with reference to the drawings. In the embodiments below, an X-axis, a Y-axis, and a Z-axis are illustrated as three axes orthogonal to each other. A direction along the X-axis is referred to as an “X-axis direction”. A direction along the Y-axis is referred to as a “Y-axis direction”. A direction along the Z-axis is referred to as a “Z-axis direction”. Along each of the X-axis, the Y-axis, and the Z-axis, the direction of an arrow head is referred to as a positive (+) direction, and a direction opposite to the direction of the arrow head is referred to as a negative (−) direction. An object viewed from the positive Z-axis direction is referred to as being viewed in a plan view. In the embodiments below, an exterior case and a control circuit or the like of the inertial measurement unit are not illustrated. 
     1. First Embodiment 
     An inertial measurement unit  1  will now be described with reference to  FIGS. 1 to 3 . 
     As shown in  FIGS. 1 and 2 , the inertial measurement unit  1  has a sensor unit  10 , an elastic member  30 , a second substrate  23  as a second fixing part where the sensor unit  10  and the elastic member  30  are arranged, and a fixing member  40  fixing the sensor unit  10  and the elastic member  30  to the second substrate  23 . 
     The sensor unit  10  includes an inertial sensor  11 , a case  12  accommodating the inertial sensor  11 , and a first substrate  13  as a first fixing part having the case  12  fixed thereto. The sensor unit  10  functions as a 6-axis motion sensor having a 3-axis acceleration sensor and a 3-axis angular velocity sensor. 
     The inertial sensor  11  has a 3-axis acceleration sensor and a 3-axis angular velocity sensor. The case  12  has an outer shape of a rectangular parallelepiped that is substantially square as viewed in a plan view. Inside the case  12 , the inertial sensor  11 , and a control IC and a connector or the like, not illustrated, for causing the inertial sensor  11  to function, are accommodated. 
     In the case  12 , a screw hole, not illustrated, is formed near two diagonally opposite vertices of the square. The case  12  is thus fixed to the first substrate  13  with a screw or the like. The method for fixing the case  12  to the first substrate  13  is not limited to using a screw. The case  12  can also be fixed to the first substrate  13  by welding, an adhesive, caulking or the like. 
     The first substrate  13  is a substrate in the shape of an equilateral triangle as viewed in a plan view. The first substrate  13  has three slope parts  13   a , and a planar part  13   b  having front and back surfaces along the XY plane. 
     The slope part  13   a  is a triangular area provided near the vertices of the first substrate  13 . The front surface of the slope part  13   a  and the front surface of the planar part  13   b  next to the slope part  13   a  form an angle smaller than 180°. The slope part  13   a  is sloped in relation to the planar part  13   b.    
     The planar part  13   b  is a hexagonal area excluding the slope parts  13   a  of the first substrate  13 . The planar part  13   b  has a flat plate-like shape along the XY plane and has the case  12  fixed thereto substantially at the center on the front surface side. 
     In the negative Z-axis direction of the first substrate  13 , the second substrate  23  is arranged with a predetermined space from the first substrate  13 . The second substrate  23  is a substrate in the shape of an equilateral triangular having longer sides than the first substrate  13 , as viewed in a plan view. The second substrate  23  has three slope parts  23   a , and a planar part  23   b  having front and back surfaces along the XY plane. 
     The slope part  23   a  is a triangular area provided near the three vertices of the second substrate  23 . The front surface of the slope part  23   a  and the front surface of the planar part  23   b  next to the slope part  23   a  form an angle smaller than 180°. The slope part  23   a  is sloped in relation to the planar part  23   b.    
     The planar part  23   b  is a hexagonal area excluding the slope parts  23   a  of the second substrate  23 . The planar part  23   b  has a flat plate-like shape along the XY plane. A vibration measurement surface of a measurement target can be attached to the back surface of the planar part  23   b , which is opposite to the front surface facing the first substrate  13 , by using a magnet, an adhesive or the like, and the inertial measurement unit  1  can thus be installed. 
     The elastic member  30  is arranged at the slope parts  13   a  of the first substrate  13  and the slope parts  23   a  of the second substrate  23 . The elastic member  30  is a mechanical filter that can restrain an unwanted vibration and damp a particular frequency. The mechanical filter is a so-called gel bushing formed of a silicone rubber and bonds the first substrate  13  and the second substrate  23  together via a flexible structure. The filter characteristic of the mechanical filter includes a low-pass filter characteristic that restrains frequencies out of a measurement frequency range of the acceleration sensor. 
     A configuration in which the first substrate  13  and the second substrate  23  are fixed, using the elastic member  30  and the fixing member  40 , will now be described with reference to  FIG. 3 . 
     In the first substrate  13 , an opening  13   c  vertically penetrating the first substrate  13  to the front and back surfaces substantially at the center of the slope part  13   a  is provided. In the opening  13   c , the elastic member  30  and the fixing member  40  are inserted. In the second substrate  23 , a screw hole  23   c  vertically extending from the front surface substantially at the center of the slope part  23   a  is provided. A spiral thread is formed on the inner surface of the screw hole  23   c.    
     The elastic member  30  has a first part  30   a  arranged at the front surface side of the slope part  13   a , a second part  30   b  arranged opposite the first part  30   a  and in contact with the slope part  23   a , and a third part  30   c  coupled to the second part  30   b  at one end and located inside the opening  13   c.    
     The third part  30   c  of the elastic member  30  is inserted in the opening  13   c , thus causing the first part  30   a  and the other end of the third part  30   c  to engage with each other. Thus, the elastic member  30  is arranged in such a way as to hold the slope part  13   a  from the front surface and the back surface via the opening  13   c . At a center axis part of the elastic member  30 , a penetration hole along the direction of extension of the third part  30   c  is provided. At the inner surface of the penetration hole, a cylindrical control member  30   d  is provided. As the control member  30   d  is arranged, the amount of pressing on the elastic member  30  by the fixing member  40  can be controlled. The control member  30   d  is formed of a member with a higher rigidity than the elastic member  30  and thus prevents the elastic member  30  from being largely deformed and deteriorating in anti-vibration capability. 
     The fixing member  40  includes a male screw-type screw  40   a  that penetrates the first substrate  13  of the sensor unit  10  and the elastic member  30  and has a distal end thereof spirally fitted in the screw hole  23   c  provided in the second substrate  23  while pressing the elastic member  30  and thus fixes the elastic member  30 , and a washer  40   b  that is arranged between a head part of the fixing member  40  and the elastic member  30  and can uniformly press the elastic member  30  when the screw  40   a  is tightened. 
     The screw  40   a  is not limited to a male screw type. A bolt or a rivet that penetrates the elastic member  30  and can fix the elastic member  30  to the second substrate  23  while pressing the elastic member  30  can be used. 
     As shown in  FIGS. 1 and 2 , the inertial measurement unit  1  according to this embodiment has a first elastic member  31 , a second elastic member  32 , and a third elastic member  33 , as the elastic member  30 , and has a first fixing member  41  penetrating and pressing the first elastic member  31 , a second fixing member  42  penetrating and pressing the second elastic member  32 , and a third fixing member  43  penetrating and pressing the third elastic member  33 , as the fixing member  40 . Therefore, the first substrate  13  and the second substrate  23  are fixed together via the first elastic member  31 , the second elastic member  32 , and the third elastic member  33 . 
     The elastic member  30  in the inertial measurement unit  1  according to this embodiment has an anisotropic filter characteristic F and can mainly damp a vibration at a predetermined frequency in a direction along center axes J 1  to J 3  of the fixing member  40  penetrating the elastic member  30 . 
     In the inertial measurement unit  1  according to this embodiment, the first elastic member  31  mainly damping a vibration at a predetermined frequency in a first direction along the center axis J 1  of the first fixing member  41 , the second elastic member  32  mainly damping a vibration at a predetermined frequency in a second direction along the center axis J 2  of the second fixing member  42 , and the third elastic member  33  mainly damping a vibration at a predetermined frequency in a third direction along the center axis J 3  of the third fixing member  43 , are arranged at the three slope parts  13   a , respectively. 
     The first direction, the second direction, and the third direction are different from each other. The slope parts  13   a ,  23   a  where the first elastic member  31  is provided are equivalent to a first area. The slope parts  13   a ,  23   a  where the second elastic member  32  is provided are equivalent to a second area. The planar parts  13   b ,  23   b  are equivalent to a third area. 
     The slope parts  13   a ,  23   a  are sloped in relation to the planar parts  13   b ,  23   b . Therefore, the center axis J 1  of the first fixing member  41 , the center axis J 2  of the second fixing member  42 , and the center axis J 3  of the third fixing member  43 , arranged at the slope parts  13   a ,  23   a , are sloped toward the center of the sensor unit  10  and intersect each other at a point of intersection P. The screw holes  23   c  in the slope parts  23   a , through which the center axes J 1 , J 2 , J 3  pass, are located at the vertices of the equilateral triangle, as viewed in a plan view. The elastic member  30  is arranged in such a way that the equilateral triangle and the center axes J 1 , J 2 , J 3  form a regular triangular pyramid. 
     This embodiment can achieve the effects described below. 
     The inertial measurement unit  1  includes: the sensor unit  10  including the inertial sensor  11 , the case accommodating the inertial sensor  11 , and the first substrate  13  as the first fixing part having the case  12  fixed thereto; the elastic member  30  having the first elastic member  31  mainly damping a vibration at a predetermined frequency in the first direction, the second elastic member  32  mainly damping a vibration at a predetermined frequency in the second direction, which is different from the first direction, and the third elastic member  33  mainly damping a vibration at a predetermined frequency in the third direction, which is different from the first direction and the second direction; the second substrate  23 , where the sensor unit  10  and the elastic member  30  are arranged; and the fixing member  40  fixing the sensor unit  10  and the elastic member  30  to the second substrate  23 . The fixing member  40  has the first fixing member  41  penetrating the sensor unit  10  and the first elastic member  31  and pressing the first elastic member  31 , the second fixing member  42  penetrating the sensor unit  10  and the second elastic member  32  and pressing the second elastic member  32 , and the third fixing member  43  penetrating the sensor unit  10  and the third elastic member  33  and pressing the third elastic member  33 . 
     In the elastic member  30  having the anisotropic filter characteristic F in different directions in this way, the first elastic member  31 , the second elastic member  32 , and the third elastic member  33  are arranged in such a way that the center axes J 1 , J 2 , J 3  form a regular triangular pyramid. In this configuration, a combined filter characteristic S is not anisotropic near the point of intersection P and an isotropic filter characteristic that is equal on the three axes of the X-axis, the Y-axis, and the Z-axis is achieved. Thus, the inertial measurement unit  1  damping a vibration at a predetermined frequency in three different directions can be provided. 
     Also, this filter characteristic is the same for the measured value on each axis of the 3-axis acceleration sensor. Therefore, the evaluation of an input vibration using 3-axis acceleration values becomes easier. 
     Moreover, since the isotropic filter characteristic that is equal on the three axes is achieved, the VRE restraining effect on each of the three axes is increased. The inertial measurement unit  1  having less vibration rectification error in the measured acceleration value can be provided. 
     Also, since the control circuit evenly restrains vibrations in all directions, the evaluation of malfunctions becomes easier. 
     2. Second Embodiment 
     An inertial measurement unit  1 A according to this embodiment will now be described with reference to  FIGS. 4 and 5 . 
     In the description of the inertial measurement unit  1 A, the same components as in the foregoing embodiment are denoted by the same reference signs and the description thereof may be omitted or simplified. 
     As shown in  FIGS. 4 and 5 , the inertial measurement unit  1 A according to this embodiment has a sensor unit  10 A, an elastic member  30 , a second substrate  24  as a second fixing part where the sensor unit  10 A and the elastic member  30  are arranged, and a fixing member  40  fixing the sensor unit  10 A and the elastic member  30  to the second substrate  24 . 
     The sensor unit  10 A includes an inertial sensor  11 , a case  12  accommodating the inertial sensor  11 , and a first substrate  14  as a first fixing part having the case  12  fixed thereto. The sensor unit  10 A functions as a 6-axis motion sensor having a 3-axis acceleration sensor and a 3-axis angular velocity sensor. 
     The inertial sensor  11  and the case  12  are similar to those in the first embodiment. In the case  12 , a screw hole, not illustrated, is formed near two diagonally opposite vertices of the square. The case  12  is thus fixed to the first substrate  14  with a screw or the like. 
     The first substrate  14  is a substrate in a square shape as viewed in a plan view. The first substrate  14  has four slope parts  14   a , and a planar part  14   b  having front and back surfaces along the XY plane. 
     The slope part  14   a  is a triangular area provided near the four vertices of the first substrate  14 . The front surface of the slope part  14   a  and the front surface of the planar part  14   b  next to the slope part  14   a  form an angle smaller than 180°. The slope part  14   a  is sloped in relation to the planar part  14   b.    
     The planar part  14   b  is an octagonal area excluding the slope parts  14   a  of the first substrate  14 . The planar part  14   b  has a flat plate-like shape along the XY plane and has the case  12  fixed thereto substantially at the center on the front surface side. 
     In the negative Z-axis direction of the first substrate  14 , the second substrate  24  is arranged with a predetermined space from the first substrate  14 . The second substrate  24  is a substrate in a square shape having longer sides than the first substrate  14 , as viewed in a plan view. The second substrate  24  has four slope parts  24   a , and a planar part  24   b  having front and back surfaces along the XY plane. 
     The slope part  24   a  is a triangular area provided near the four vertices of the second substrate  24 . The front surface of the slope part  24   a  and the front surface of the planar part  24   b  next to the slope part  24   a  form an angle smaller than 180°. The slope part  24   a  is sloped in relation to the planar part  24   b.    
     The planar part  24   b  is an octagonal area excluding the slope parts  24   a  of the second substrate  24 . The planar part  24   b  has a flat plate-like shape along the XY plane. A vibration measurement surface of a measurement target can be attached to the back surface of the planar part  24   b , which is opposite to the front surface facing the first substrate  14 , by using a magnet, an adhesive or the like, and the inertial measurement unit  1 A can thus be installed. 
     The elastic member  30  is arranged at the slope parts  14   a  of the first substrate  14  and the slope parts  24   a  of the second substrate  24 . The elastic member  30  bonds the first substrate  14  and the second substrate  24  together via a flexible structure. 
     The elastic member  30  is arranged in such a way as to hold the slope part  14   a  from the front surface and the back surface. The fixing member  40  penetrates the first substrate  14  of the sensor unit  10 A and the elastic member  30  and has a distal end thereof spirally fitted in the screw hole provided in the second substrate  24  while pressing the elastic member  30  and thus fixes together the slope part  14   a  of the first substrate  14  and the slope part  24   a  of the second substrate  24 . 
     The inertial measurement unit  1 A according to this embodiment has a first elastic member  31 , a second elastic member  32 , a third elastic member  33 , and a fourth elastic member  34 , as the elastic member  30 , and has a first fixing member  41  penetrating and pressing the first elastic member  31 , a second fixing member  42  penetrating and pressing the second elastic member  32 , a third fixing member  43  penetrating and pressing the third elastic member  33 , and a fourth fixing member  44  penetrating and pressing the fourth elastic member  34 , as the fixing member  40 . Therefore, the first substrate  14  and the second substrate  24  are fixed together via the first elastic member  31 , the second elastic member  32 , the third elastic member  33 , and the fourth elastic member  34 . 
     The elastic member  30  in the inertial measurement unit  1 A according to this embodiment has an anisotropic filter characteristic F and can mainly damp a vibration at a predetermined frequency in a direction along center axes J 1  to J 4  of the fixing member  40  penetrating the elastic member  30 . 
     In the inertial measurement unit  1 A according to this embodiment, the first elastic member  31  mainly damping a vibration at a predetermined frequency in a first direction along the center axis J 1  of the first fixing member  41 , the second elastic member  32  mainly damping a vibration at a predetermined frequency in a second direction along the center axis J 2  of the second fixing member  42 , the third elastic member  33  mainly damping a vibration at a predetermined frequency in a third direction along the center axis J 3  of the third fixing member  43 , and the fourth elastic member  34  mainly damping a vibration at a predetermined frequency in a fourth direction along the center axis J 4  of the fourth fixing member  44 , are arranged at the four slope parts  14   a , respectively. 
     The first direction, the second direction, the third direction, and the fourth direction are different from each other. The slope parts  14   a ,  24   a  where the first elastic member  31  is provided are equivalent to a first area. The slope parts  14   a ,  24   a  where the second elastic member  32  is provided are equivalent to a second area. The planar parts  14   b ,  24   b  are equivalent to a third area. 
     The slope parts  14   a ,  24   a  are sloped in relation to the planar parts  14   b ,  24   b . Therefore, the center axis J 1  of the first fixing member  41 , the center axis J 2  of the second fixing member  42 , the center axis J 3  of the third fixing member  43 , and the center axis J 4  of the fourth fixing member  44 , arranged at the slope parts  14   a ,  24   a , are sloped toward the center of the sensor unit  10 A and intersect each other at a point of intersection P. The screw holes in the slope parts  24   a , through which the center axes J 1 , J 2 , J 3 , J 4  pass, are located at the vertices of the square, as viewed in a plan view. Each two neighboring axes of the sloped center axes J 1 , J 2 , J 3 , J 4  intersect each other at an angle of 90°. The elastic member  30  is arranged in such a way that the square and the center axes J 1 , J 2 , J 3 , J 4  form a regular square pyramid. 
     The inertial measurement unit  1 A according to this embodiment can achieve the effects described below. 
     The elastic member  30  of the inertial measurement unit  1 A has the fourth elastic member  34  mainly damping a vibration at a predetermined frequency in the fourth direction, which is different from the first direction, the second direction, and the third direction. The fixing member  40  has the fourth fixing member  44  penetrating the sensor unit  10 A and the fourth elastic member  34  and pressing the fourth elastic member  34 . In this configuration, as the elastic member  30  having the anisotropic filter characteristic F in different directions, the first elastic member  31 , the second elastic member  32 , the third elastic member  33 , and the fourth elastic member  34  are arranged in such a way that the center axes J 1 , J 2 , J 3   m  J 4  form a regular square pyramid, and an isotropic filter characteristic that is equal on the three axes of the X-axis, the Y-axis, and the Z-axis is achieved. Thus, the inertial measurement unit  1 A can achieve effects similar to those of the inertial measurement unit  1  according to the first embodiment. 
     3. Third Embodiment 
     An inertial measurement unit  1 B according to this embodiment will now be described with reference to  FIGS. 6 and 7 . 
     The inertial measurement unit  1 B differs from the foregoing inertial measurement unit  1 A in having a sensor unit  10 B instead of the sensor unit  10 A. In the description of the inertial measurement unit  1 B, the same components as in the foregoing embodiments are denoted by the same reference signs and the description thereof may be omitted or simplified. 
     As shown in  FIGS. 6 and 7 , the inertial measurement unit  1 B according to this embodiment has the sensor unit  10 B, an elastic member  30 , a second substrate  24  as a second fixing part where the sensor unit  10 B and the elastic member  30  are arranged, and a fixing member  40  fixing the sensor unit  10 B and the elastic member  30  to the second substrate  24 . 
     The sensor unit  10 B includes an inertial sensor  11  and a case  120  accommodating the inertial sensor  11  and functions as a 6-axis motion sensor having a 3-axis acceleration sensor and a 3-axis angular velocity sensor. 
     The case  120  is square as viewed in a plan view and has four slope parts  12   a , and a planar part  12   b  having front and back surfaces along the XY plane. 
     The slope part  12   a  is a triangular area provided near the four vertices of the case  120 . The front surface of the slope part  12   a  and the front surface of the planar part  12   b  next to the slope part  12   a  form an angle smaller than 180°. The slope part  12   a  is sloped in relation to the planar part  12   b.    
     The planar part  12   b  is an octagonal area excluding the slope parts  12   a  of the case  120 . The planar part  12   b  has a flat plate-like shape along the XY plane. 
     In the negative Z-axis direction of the case  120 , the second substrate  24  is arranged with a predetermined space from the case  120 . The second substrate  24  is a substrate in a square shape having longer sides than the case  120 , as viewed in a plan view. 
     The elastic member  30  is arranged at the slope parts  12   a  of the case  120  and slope parts  24   a  of the second substrate  24 . 
     In the case  120 , an opening  12   c  vertically penetrating the case  120  to the front and back surfaces substantially at the center of the slope part  12   a  is provided. The elastic member  30  and the fixing member  40  are inserted in the opening  12   c . The elastic member  30  bonds the case  120  and the second substrate  24  together via a flexible structure. 
     The elastic member  30  has a third part  30   c  thereof inserted in the opening  12   c  and is arranged in such a way as to hold the slope part  12   a  from the front surface and the back surface. The fixing member  40  penetrates the case  120  of the sensor unit  10 B and the elastic member  30  and has a distal end thereof spirally fitted in a screw hole provided in the second substrate  24  while pressing the elastic member  30  and thus fixes together the slope part  12   a  of the case  120  and the slope part  24   a  of the second substrate  24 . 
     In the inertial measurement unit  1 B according to this embodiment, a first elastic member  31  mainly damping a vibration at a predetermined frequency in a first direction along a center axis J 1  of a first fixing member  41 , a second elastic member  32  mainly damping a vibration at a predetermined frequency in a second direction along a center axis J 2  of a second fixing member  42 , a third elastic member  33  mainly damping a vibration at a predetermined frequency in a third direction along a center axis J 3  of a third fixing member  43 , and a fourth elastic member  34  mainly damping a vibration at a predetermined frequency in a fourth direction along a center axis J 4  of a fourth fixing member  44 , are arranged at the four slope parts  12   a , respectively. 
     The first direction, the second direction, the third direction, and the fourth direction are different from each other. The slope parts  12   a ,  24   a  where the first elastic member  31  is provided are equivalent to a first area. The slope parts  12   a ,  24   a  where the second elastic member  32  is provided are equivalent to a second area. The planar parts  12   b ,  24   b  are equivalent to a third area. 
     The slope parts  12   a ,  24   a  are sloped in relation to the planar parts  12   b ,  24   b . Therefore, the center axis J 1  of the first fixing member  41 , the center axis J 2  of the second fixing member  42 , the center axis J 3  of the third fixing member  43 , and the center axis J 4  of the fourth fixing member  44 , arranged at the slope parts  12   a ,  24   a , are sloped toward the center of the sensor unit  10 B and intersect each other at a point of intersection P. The screw holes in the slope parts  24   a , through which the center axes J 1 , J 2 , J 3 , J 4  pass, are located at the vertices of the square, as viewed in a plan view. Each two neighboring axes of the sloped center axes J 1 , J 2 , J 3 , J 4  intersect each other at an angle of 90°. The elastic member  30  is arranged in such a way that the square and the center axes J 1 , J 2 , J 3 , J 4  form a regular square pyramid. 
     The inertial measurement unit  1 B according to this embodiment can achieve the effects described below. 
     The elastic member  30  of the inertial measurement unit  1 B has the fourth elastic member  34  mainly damping a vibration at a predetermined frequency in the fourth direction, which is different from the first direction, the second direction, and the third direction. The fixing member  40  has the fourth fixing member  44  penetrating the sensor unit  10 B and the fourth elastic member  34  and pressing the fourth elastic member  34 . In this configuration, as the elastic member  30  having an anisotropic filter characteristic F in different directions, the first elastic member  31 , the second elastic member  32 , the third elastic member  33 , and the fourth elastic member  34  are arranged in such a way that the center axes J 1 , J 2 , J 3   m  J 4  form a regular square pyramid, and an isotropic filter characteristic that is equal on the three axes of the X-axis, the Y-axis, and the Z-axis is achieved. Thus, the inertial measurement unit  1 B can achieve effects similar to those of the inertial measurement units according to the foregoing embodiments. 
     4. Fourth Embodiment 
     An inertial measurement unit  1 C according to this embodiment will now be described with reference to  FIGS. 8 and 10 . 
     As shown in  FIGS. 8 and 10 , the inertial measurement unit  1 C has a sensor unit  10 C, an elastic member  30 , a second substrate  25  as a second fixing part where the sensor unit  10 C and the elastic member  30  are arranged, and a fixing member  40  fixing the sensor unit  10 C and the elastic member  30  to the second substrate  25 . 
     The sensor unit  10 C includes an inertial sensor  11 , a case  12  accommodating the inertial sensor  11 , and a first substrate  15  as a first fixing part having the case  12  fixed thereto. The sensor unit  10 C functions as a 6-axis motion sensor having a 3-axis acceleration sensor and a 3-axis angular velocity sensor. 
     The inertial sensor  11  and the case  12  are similar to those in the first embodiment. In the case  12 , a screw hole is formed near two diagonally opposite vertices of the square. The case  12  is thus fixed to the first substrate  15  with a screw or the like. 
     The first substrate  15  has substrates  15   a ,  15   b ,  15   c . The first substrate  15  has an integrated configuration where sides of the substrates  15   a ,  15   b ,  15   c  are coupled to each other. 
     More specifically, the substrate  15   a  is a flat plate-like substrate having front and back surfaces along the XY plane. The substrate  15   b  is a flat plate-like substrate having front and back surfaces along the ZY plane. The substrate  15   c  is a flat plate-like substrate having front and back surfaces along the ZX plane. 
     The one side in the positive X-axis direction of the substrate  15   a  is coupled to the one side in the negative Z-axis direction of the substrate  15   b . The one side in the positive Y-axis direction of the substrate  15   a  is coupled to the one side in the negative Z-axis direction of the substrate  15   c . Also, the one side in the positive Y-axis direction of the substrate  15   b  and the one side in the positive X-axis direction of the substrate  15   c  are coupled together. 
     The first substrate  15  has the case  12  fixed substantially at the center on the front surface located in the positive Z-axis direction of the substrate  15   a . In the case  12 , a screw hole, not illustrated, is formed near two diagonally opposite vertices of the square. The case  12  is thus fixed to the substrate  15   a  with a screw or the like. 
     The second substrate  25  is arranged with a predetermined space from the first substrate  15 . 
     The second substrate  25  has substrates  25   a ,  25   b ,  25   c . The substrates  25   a ,  25   b ,  25   c  are coupled to each other at one side, forming an integrated configuration. 
     More specifically, the substrate  25   a  is a flat plate-like substrate having front and back surfaces along the XY plane. The substrate  25   a  and the substrate  15   a  are located via a predetermined space from each other. The substrate  25   b  is a flat plate-like substrate having front and back surfaces along the ZY plane. The substrate  25   b  and the substrate  15   b  are located via a predetermined space from each other. The substrate  25   c  is a flat plate-like substrate having front and back surfaces along the ZX plane. The substrate  25   c  and the substrate  15   c  are located via a predetermined space from each other. 
     In the second substrate  25 , the one side in the positive X-axis direction of the substrate  25   a  is coupled to the one side in the negative Z-axis direction of the substrate  25   b . The one side in the positive Y-axis direction of the substrate  25   a  is coupled to the one side in the negative Z-axis direction of the substrate  25   c.    
     In the first substrate  15 , an opening vertically penetrating the first substrate  15  to the front and back surfaces substantially is provided. In the opening, the elastic member  30  and the fixing member  40  are inserted. In the second substrate  25 , a screw hole corresponding to the opening and vertically extending from the front surface is provided. A spiral thread is formed on the inner surface of the screw hole. 
     The elastic member  30  is arranged at the first substrate  15  and the second substrate  25 . The elastic member  30  bonds the first substrate  15  and the second substrate  25  together via a flexible structure. 
     The elastic member  30  is arranged in such a way as to hold the first substrate  15  from the front surface and the back surface via the opening in the first substrate  15 . The fixing member  40  penetrates the first substrate  15  of the sensor unit  10 C and the elastic member  30  and has a distal end thereof spirally fitted in the screw hole provided in the second substrate  25  while pressing the elastic member  30  and thus fixes together the first substrate  15  and the second substrate  25 . 
     The inertial measurement unit  1 C according to this embodiment has elastic members  30   x ,  30   y ,  30   z  as the elastic member  30 . 
     The elastic member  30   z  is provided at four positions on the substrate  15   a  and has a filter characteristic F of mainly damping a vibration at a predetermined frequency in a third direction along the Z-axis direction. The elastic member  30   x  is provided at four positions on the substrate  15   b  and has a filter characteristic F of mainly damping a vibration at a predetermined frequency in a first direction along the X-axis direction. The elastic member  30   y  is provided at four positions on the substrate  15   c  and has a filter characteristic F of mainly damping a vibration at a predetermined frequency in a second direction along the Y-axis direction. 
     The inertial measurement unit  1 C according to this embodiment can achieve the effects described below. 
     The inertial measurement unit  1 C has the elastic members  30   x ,  30   y ,  30   z  and therefore can achieve a filter characteristic F on the X, Y, and Z-axes. Therefore, the combined filter characteristic S of the elastic member  30  is not anisotropic and an isotropic filter characteristic that is equal on the three axes is achieved. Thus, the inertial measurement unit  1 C can achieve effects similar to those of the inertial measurement units according to the foregoing embodiments.