TECHNICAL FIELD 
     The present invention relates to an axial force sensor, and particularly concerns an axial force sensor to be used in bolt tightening management and a suspension axial force measurement device and the like. 
     BACKGROUND ART 
     Tightening management of bolts, screws, etc., has been commonly performed by managing torque when tightening. However, for preventing these from loosening, it is desired to directly measure tightening axial force rather than the tightening torque for management, and axial force sensors are therefore required. 
     Axial force sensors have other uses such as suspension test apparatuses and electric disk brakes. For example, Patent Literature 1 describes an axial force sensor to be used for an electric disk brake. This axial force sensor is configured with a pair of pressing plates and a plurality of crystal piezoelectric elements held sandwiched by the pair of pressing plates, and can, when a compressive force is applied to the pair of pressure plates, detect an axial force thereof by the crystal piezoelectric elements. 
     Also, Patent Literature 2 describes a suspension axial force measurement apparatus. For this axial force measurement apparatus, an axial force measuring section is provided over a suspension, and the suspension is supported by a mount via the axial force measuring section. As the axial force measuring section, a 6-component dynamometer is used, so that an axial force of the suspension can be measured as six forces components. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Published Unexamined Patent Application No. 2011-80586 
     Patent Literature 2: Japanese Published Unexamined Patent Application No. 2010-197276 
     SUMMARY OF INVENTION 
     Technical Problem 
     Meanwhile, axial force sensors are roughly divided into ones using piezoelectric elements, ones using strain gauges, and ones that use ultrasonic waves. The ones using piezoelectric elements and the ones that use ultrasonic waves have a problem that the costs are very expensive. On the other hand, the ones using strain gauges are low cost, but have a problem of bulk thickness. When described specifically, for the conventional axial force sensor using a strain gauge, it has been necessary because of its configuration to have the strain gauge attached to a part to be deformed, so that the strain gauge has been arranged parallel to its measurement direction. For this reason, there has been a problem that the axial force sensor has a thickness greater by at least the size of the strain gauge, and is thereby increased in size. 
     The present invention has been made in view of such circumstances, and an object thereof is to provide a low-cost, compact, high-precision axial force sensor. 
     Solution to Problem 
     In order to achieve the above-mentioned object, a first aspect of the invention provides an axial force sensor configured including a pair of pressing plates and a strain gauge arranged in parallel, and sandwiching the strain gauge with the pair of pressing plates. 
     The axial force sensor of the present invention has a sandwiched structure in which the strain gauge is sandwiched between the pair of pressing plates, and is very thin. According to this axial force sensor, when the pair of pressing plates are pressed from both sides, because the strain gauge is pressed to be deformed, the pressing force can be detected by detecting a change in resistance value of a sensitive element. In addition, for the conventional axial force sensor using a strain gauge, the strain gauge is arranged parallel to its measurement direction, whereas for the axial force sensor of the present invention, the strain gauge is disposed parallel to the pressing plates (that is, in a direction orthogonal to the measurement direction), and can thus be made very thin. 
     A second aspect of the invention is the first aspect of the invention, in which the strain gauge includes a plurality of sensitive elements consisting of resistors, and is provided with a transmitting and blocking mechanism that transmits a pressing force from the pair of pressing plates to at least one of the plurality of sensitive elements and blocks the pressing force with respect to the rest of the sensitive elements. 
     According to the present invention, when the pair of pressing plates are pressed, the pressing force is transmitted to a part of the sensitive elements, and the pressing force is not transmitted to the rest of the sensitive elements. In the sensitive element to which the pressing force is transmitted, various forces (e.g., forces including a bending stress and the like) including the pressing force are detected, and forces other than the pressing force are detected in the sensitive element to which the pressing force is not transmitted. Thus, by using a difference in detection values between the two types of sensitive elements, only the pressing force can be determined. In addition, the axial force sensor having a sandwiched structure in which the strain gauge is sandwiched with the pair of pressing plates has a drawback that it is easily affected by forces other than a pressing force, and the precision of measuring the pressing force easily degrades, but according to the present invention, the forces other than a pressing force can cancel each other out, so that the pressing force can be measured at high precision. 
     A third aspect of the invention is the second aspect of the invention, in which the transmitting and blocking mechanism is a plate-like spacer having an opening portion, and is arranged between the pair of pressing plates together with the strain gauge, and the sensitive elements of the strain gauge are respectively arranged at a position where the spacer makes contact and a position of the opening portion. 
     According to the present invention, a pressing force is transmitted to the sensitive element at the position where the spacer makes contact and the pressing force is blocked with respect to the sensitive element at the position of the opening portion. Thus, the pressing force can be detected based on a difference in detection values between the two types of sensitive elements. Also, according to the present invention, the axial force sensor has a sandwiched structure in which the plate-like spacer is sandwiched together with the strain gauge between the pair of pressing plates, and is very thin. 
     A fourth aspect of the invention is the third aspect of the invention, in which the spacer is formed in a ring shape, and has the opening portions arranged at constant angular intervals in a circumferential direction, and the strain gauge has the sensitive elements arranged at constant angular intervals at positions of the opening portions and between the opening portions at constant angular intervals. 
     According to the present invention, the sensitive elements to which a pressing force is transmitted and the sensitive elements with respect to which the pressing force is blocked are arranged alternately at constant angular intervals. Thus, forces other than an axial force can reliably cancel each other out, and the axial force at a center position can be precisely measured. 
     A fifth aspect of the invention is any one of the first to fourth aspects of the invention, in which a Wheatstone bridge circuit is formed for which the sensitive elements to which a pressing force is transmitted are arranged on opposite sides to each other and the sensitive elements with respect to which the pressing force is blocked are arranged on opposite sides to each other. According to the present invention, forces other than an axial force cancel each other out, and only the axial force can be detected. 
     Advantageous Effects of Invention 
     According to the axial force sensor of the present invention, which has a sandwiched structure in which the strain gauge is sandwiched between the pair of pressing plates, when the pair of pressing plates are pressed from both sides, because the strain gauge is pressed to be deformed, the pressing force can be detected by detecting a change in resistance value of the sensitive element. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing an axial force sensor of the present embodiment. 
         FIG. 2  is an exploded perspective view showing a configuration of the axial force sensor in  FIG. 1 . 
         FIG. 3  is a view showing an arrangement of the strain gauge in  FIG. 1 . 
         FIG. 4  is a circuit diagram of the strain gauge in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, preferred embodiments of an axial force sensor according to the present invention will be described in accordance with the accompanying drawings.  FIG. 1  is a perspective view showing an axial force sensor  10  applied with the present invention, and  FIG. 2  is an exploded perspective view showing an internal configuration of the axial force sensor  10 .  FIG. 3  is a plan view of a strain gauge  14 , and shows the positions of opening portions  16 A to  16 D and pressing portions  16 E to  16 H in a spacer  16 . 
     As shown in these figures, the axial force sensor  10  is configured with the pair of pressing plates  12 , the strain gauge  14 , and the spacer  16 , and has a sandwiched structure in which the strain gauge  14  and the spacer  16  are sandwiched with the pair of pressing plates  12 . 
     The pressing plate  12  is formed of a metal plate such as stainless steel in a ring shape. It suffices to set the size of the pressing plate  12  according to the measuring object, and for example, in the case of a use for tightening management of bolts etc., the inner diameter of the pressing plate is set to such a size so as to allow insertion therethrough of a shaft part of the bolt. 
     The spacer  16  is formed of a metal plate such as stainless steel in a ring shape. The material of the spacer  16  is not particularly limited, but for preventing the occurrence of an unnecessary strain or stress by the spacer  16 , a spacer of the same material as that of the pressing plates  12  is preferably used. Similarly, the size (inner diameter, outer diameter, thickness) of the spacer  16  is not particularly limited, but for preventing the occurrence of an unnecessary strain or stress by the spacer  16 , the spacer  16  is preferably designed similarly to the pressing plate  12 . Because the spacer  16  accordingly has substantially the same characteristics as those of the pressing plate  12 , the occurrence of an unnecessary strain or stress can be suppressed. In addition, the pressing plate  12  and the spacer  16  are separated from each other in the present embodiment, but the present invention is not limited thereto, and unevenness may be formed on the back surface of the pressing plate  12  in place of providing the spacer  16 . Also, in the present embodiment, the single spacer  16  is provided on one side of the strain gauge  14 , but two spacers  16  may be provided on both sides of the strain gauge  14 . 
     In the spacer  16 , four opening portions  16 A,  16 B,  16 C, and  16 D are formed in a manner of penetrating from the front to back surfaces. The four opening portions  16 A to  16 D are arranged at equal intervals (90 degree intervals), and formed, as their sizes (angles), with angles (approximately 45 degrees) for which the whole circumference is divided to be approximately ⅛. Thus, between the opening portions  16 A to  16 D, metal-plate parts (hereinafter, referred to as pressing portions)  16 E,  16 F,  16 G, and  16 H of the spacer  16  are arranged. The spacer  16  accordingly has, in its circumferential direction, the opening portions  16 A to  16 D and the pressing portions  16 E to  16 H arranged alternately at equal angular intervals. 
     The strain gauge  14  includes a base  20  formed in a ring shape, and on the base  20 , eight sensitive elements  22 A,  22 B,  22 C,  22 D,  22 E,  22 F,  22 G, and  22 H called grids are provided. The base  20  is made of an insulating material such as a polyimide resin, and the sensitive elements  20 A to  22 H are formed of a metal foil having a thickness of a few microns adhered onto that base  20 . The sensitive elements  20 A to  22 H have a pattern developed by multiple circumferentially arranged linear parts (gauge grids) connecting to other linear parts by folding tabs, and are connected to two connecting tabs arranged on the outer peripheral side. By connecting lead wires to the connecting tabs, a bridge circuit is formed as to be described later. In addition, a pattern so as to connect the respective sensitive elements  22 A to  22 H may be formed by a metal foil on the base  20 . Also, the material of the base  20  and the sensitive element  22  is not particularly limited, and for example, a base  20  made of a polyvinyl resin or polyphenol resin may be used. Further, the pattern shape of the sensitive elements  22 A to  22 H is not limited to the above, and variously shaped patterns may be appropriately selected. An appropriate selection can be made from various patterns including, for example, a pattern for which multiple linear parts are radially arranged, and these linear parts are alternately connected by folding tabs on the inner peripheral side or outer peripheral side, and a pattern linear parts of which are formed in whorls. 
     As shown in  FIG. 3 , the sensitive elements  22 A to  22 H are arranged at constant angular intervals (that is, intervals of 45 degrees) in the circumferential direction. Also, the sensitive elements  22 A to  22 H are arranged at positions to overlap the opening portions  16 A to  16 D or the pressing portions  16 E to  16 H when the strain gauge  14  and the spacer  16  are stacked one on top of the other. Specifically, the sensitive elements  22 A to  22 D are arranged at positions to overlap the opening portions  16 A to, respectively, and the sensitive elements  22 E to  22 H are arranged at the positions of the pressing portions  16 E to  16 H. In the case of such arrangement, in the sensitive elements  22 A and  22 D, because spaces (opening portions  16 A to  16 D) are arranged with respect to the pressing plate  12 , a pressing force from the pressing plate  12  is blocked by the opening portions  16 A to  16 D, and is not transmitted to the sensitive elements  22 A to  22 D. On the other hand, in the sensitive elements  22 E to  22 H, because the pressing portions  16 E to  16 H of the spacer  16  are arranged with respect to the pressing plate  12 , a pressing force from the pressing plate  12  is transmitted to the sensitive elements  22 E to  22 H. 
       FIG. 4  shows a circuit diagram of the strain gauge  14  in a simplified manner. As shown in the same figure, the sensitive elements  22 A to  22 H are connected so as to form a full-bridge circuit. Specifically, the sensitive elements  22 A to  22 D arranged at positions to overlap the opening portions  16 A to  16 D are arranged on opposite sides to each other and the sensitive elements  22 E to  22 H arranged at positions to overlap the pressing portions  16 E to  16 H are arranged on opposite sides to each other. 
     Next, the operation of the axial force sensor  10  configured as above will be described in an example of performing bolt tightening management. 
     First, a shaft portion of a bolt (not shown) is inserted through the axial force sensor  10 . Then, the bolt is tightened to perform a measurement by the axial force sensor  10 . As a result of the bolt being tightened, the pair of pressing plates  12  receives a force in the direction of pressure by sandwiching. This pressing force is transmitted to the sensitive elements  22 E to  22 H of the strain gauge  14  via the pressing portions  16 E to  16 H of the spacer  16 . Thus, the sensitive elements  22 E to  22 H are changed in resistance values according to the pressing force. On the other hand, in the sensitive elements  22 A to  22 D arranged in the opening portions  16 A to  16 D, because the pressing force is blocked by the opening portions  16 A to  16 D, the pressing force is not transmitted. Thus, the sensitive elements  22 A to  22 D are not changed in resistance values depending on the pressing force. 
     Meanwhile, the sensitive elements  22 A to  22 H receive a force other than the pressing force, for example, a bending stress and the like, and are also changed in resistance values depending thereon. For example, when a large compressive force is applied to an outer peripheral portion of the pair of pressing plates  12 , a large bending stress acts on an inner peripheral portion of the strain gauge  14 , and the sensitive elements  22 A to  22 H may receive an influence greater than the compressive force. 
     Therefore, in the present embodiment, the sensitive elements  22 E to  22 H to which a pressing force is transmitted and the sensitive elements  22 A to  22 D to which a pressing force is not transmitted are provided, and a bridge circuit is formed with these sensitive elements. For this reason, influences other than the pressing force cancel each other out, so that only the pressing force is detected. 
     As such, according to the axial force sensor  10  of the present embodiment, despite a sandwiched structure in which the strain gauge  14  is sandwiched with the pressure plates  12 , an axial force can be detected with high precision. Also, according to the present embodiment, because of being a sandwiched structure in which the strain gauge  14  and the spacer  16  are sandwiched with the pressing plates  12 , the axial force sensor  10  is very thin, and compact. 
     In addition, eight sensitive elements  22 A to  22 H are provided for the embodiment described above, but the number of sensitive elements is not limited thereto. However, the larger the number of sensitive elements, the more precisely the influences other than the pressing force can cancel each other out. Also, the arrangement of the sensitive elements is not limited to the ones described above, but it is preferable to arrange sensitive elements to which a pressing force is transmitted and sensitive elements to which a pressing force is not transmitted, alternately at equal intervals. 
     Also, for the embodiment described above, the opening portions  16 A to  16 D are formed in substantially fan shapes surrounded by two concentric circles different in diameter and two radiuses, but the shape of the opening portions is not limited thereto, and it suffices that the spacer  16  is formed so as to become out of contact with the sensitive elements  22 A and  22 D. Thus, the shape of the opening portions may be, for example, a circular shape, an elliptic shape, a rectangular shape, and the like. 
     REFERENCE SIGNS LIST 
       10  . . . axial force sensor,  12  . . . pressing plate,  14  . . . strain gauge,  16  . . . spacer,  16 E to  16 H . . . pressing portion,  16 A to  16 D . . . opening portion,  20  . . . base,  22 A to  22 H . . . sensitive element
A low-cost, compact, high-precision axial force sensor is provided. The axial force sensor includes a pair of parallel pressing plates and a strain gauge sandwiched therebetween. The strain gauge includes a plurality of strain-sensitive resistive elements around its periphery, and is provided with a spacer that transmits a pressing force from the pressing plates to some of the strain-sensitive elements but blocks the pressing force to the rest of the strain-sensitive elements. The output signal of the strain-sensitive elements blocked from the pressing force provide an accurate baseline to compare the output signal of the strain-elements subjected to the pressing force. The spacer has a uniform pattern of open and closed portions. On the strain gauge, the strain-sensitive elements are provided at both the open and closed portions of the spacer.