Patent Publication Number: US-2022228937-A1

Title: Triaxial force sensor

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to a triaxial force sensor. 
     Related Art 
     Conventionally, a triaxial force sensor described in JP 2014-55985 A is known. The triaxial force sensor is configured to detect forces in orthogonal triaxial directions including distributed loads, and is equipped with a large number of planarly arranged stress detection elements. Each of the stress detection elements is of the piezoelectric element type, and is equipped with a pressing force detection element, one pair of shearing force detection elements, and the other pair of shearing force detection elements. The pair of shearing force detection elements and the other pair of shearing force detection elements are arranged in the shape of a cross in plan view so that the pressing force detection element is located at the center of the cross. The pressing force detection element and the shearing force detection elements are composed of a pair of upper and lower electrodes or the like. 
     In the stress detection element, the pressing force is detected by the pressing force detection element, the shearing force acting in the arrangement direction of the one pair of shearing force detection elements is detected by the one pair of shearing force detection elements, and the shearing force acting in the arrangement direction of the other pair of shearing force detection elements is detected by the other pair of shearing force detection elements. Then, distributed loads are detected by the large number of stress detection elements. 
     SUMMARY 
     According to the conventional tactile sensor described above, in order to detect forces in orthogonal triaxial directions, it is necessary to planarly arrange a large number of stress detection elements each composed of a single pressing force detection element and four shearing force detection elements. As a result, the sensor is increased in size, and a large number of electrodes are required, so that the number of components is increased, leading to an increase in manufacturing cost. 
     The present invention has been made to address the above problem, and an object of the present invention is to provide a triaxial force sensor that can be reduced in the number of components and size, thereby allowing a reduction in manufacturing cost, in the case of detecting forces in orthogonal triaxial directions. 
     In order to achieve the above object, according to an aspect of the present invention, a triaxial force sensor for detecting forces in orthogonal triaxial directions on the basis of changes in capacitance between electrodes includes: a flexible member having dielectric properties and elasticity; a plurality of first electrodes attached to the flexible member, the first electrodes being spaced apart from each other along a predetermined plane; and a plurality of second electrodes spaced apart from each other along the predetermined plane, disposed in spaced face-to-face relation to the plurality of first electrodes, and attached to the flexible member with the flexible member interposed between the second electrodes and the first electrodes, the second electrodes being configured to detect capacitances between the second electrodes and the first electrodes. The plurality of first electrodes includes one pair of the first electrodes and the other pair of the first electrodes different from the one pair of first electrode. The one pair of first electrodes are along a first of two straight lines extending along the predetermined plane while being orthogonal to each other, and are arranged so that when a force in a direction along the first straight line acts on the flexible member, the area of each of the one pair of first electrodes overlapping the second electrode when viewed from the second electrode side changes. The other pair of first electrodes are along a second of the two straight lines, and are arranged so that when a force in a direction along the second straight line acts on the flexible member, the area of each of the other pair of first electrodes overlapping the second electrode when viewed from the second electrode side changes. 
     In this triaxial force sensor, the first electrodes are attached to the flexible member, spaced apart from each other along a predetermined plane, and the second electrodes are spaced apart from each other along the predetermined plane, disposed in spaced face-to-face relation to the first electrodes, and attached to the flexible member with the flexible member interposed between the second electrodes and the first electrodes. Thus, when the force in the direction orthogonal to the predetermined plane acts on the flexible member, the distances between the first electrodes and the second electrodes change, and the force can be detected on the basis of the resulting changes in capacitance. 
     Further, the plurality of first electrodes includes one pair of the first electrodes and the other pair of the first electrodes different from the one pair of first electrode. The one pair of first electrodes are along a first of two straight lines extending along the predetermined plane while being orthogonal to each other, and are arranged so that when a force in a direction along the first straight line acts on the flexible member, the area of each of the one pair of first electrodes overlapping the second electrode when viewed from the second electrode side changes. The other pair of first electrodes are along a second of the two straight lines, and are arranged so that when a force in a direction along the second straight line acts on the flexible member, the area of each of the other pair of first electrodes overlapping the second electrode when viewed from the second electrode side changes. Thus, when the force in the direction along the first straight line acts on the flexible member, the capacitance between each of the one pair of first electrodes and the second electrode changes as a result of the change in the overlapping area. As a result, the force in the direction along the first straight line, that is, the shearing force, can be detected on the basis of the changes in capacitance. 
     Furthermore, the other pair of first electrodes are along a second of the two straight lines, and are arranged so that when the force in the direction along the second straight line acts on the flexible member, the area of each of the other pair of first electrodes overlapping the second electrode when viewed from the second electrode side changes. Thus, when the force in the direction along the second straight line acts on the flexible member, the capacitance between each of the other pair of first electrodes and the second electrode changes as a result of the change in the overlapping area. As a result, the force in the direction along the second straight line, that is, the shearing force, can be detected on the basis of the changes in the capacitance. As described above, it is possible to detect the shearing forces in the orthogonal biaxial directions and the force in the direction orthogonal to the predetermined plane (that is, the forces in the orthogonal triaxial directions) using the eight electrodes. Thus, as compared with a conventional case where four shearing force detection elements and a single pressing force detection element (that is, a total of 10 electrodes) are required to detect forces in orthogonal triaxial directions, the number of components can be reduced and the sensor can be downsized, so that manufacturing cost can be reduced accordingly. 
     In the present invention, the plurality of first electrodes is preferably provided with a first central electrode located in a center when the plurality of first electrodes is viewed from the second electrode side, and a plurality of first peripheral electrodes arranged around the first central electrode and including the one pair of first electrodes and the other pair of first electrodes. The plurality of second electrodes is preferably provided with a second central electrode located in a center when the plurality of second electrodes is viewed toward the first electrodes and disposed to face the first central electrode, and a plurality of second peripheral electrodes arranged around the second central electrode and including four second electrodes that face the one pair of first electrodes and the other pair of first electrodes. 
     With this triaxial force sensor, it is possible to detect the force in the direction orthogonal to the predetermined plane on the basis of the change in capacitance between the first central electrode and the second central electrode. Further, it is possible to detect forces in the orthogonal triaxial directions on the basis of the changes in capacitance between the one pair of first electrodes, the other pair of first electrodes, and the four second electrodes that face the one pair of first electrodes and the other pair of first electrodes. That is, using a total of ten electrodes, the force in the direction orthogonal to the predetermined plane can be detected at five locations, and at the same time, the shearing forces in the orthogonal biaxial directions can be detected. As a result, the detection area of the force in the direction orthogonal to the predetermined plane can be increased as compared with the conventional case where the pressing force can be detected only at one location using a total of ten electrodes. 
     In the present invention, the plurality of second electrodes is preferably provided with a second central electrode located in a center when the plurality of second electrodes is viewed toward the first electrodes, and a plurality of second peripheral electrodes arranged around the second central electrode. The plurality of first electrodes is preferably provided with the one pair of first electrodes and the other pair of first electrodes partially overlapping the second central electrode when the plurality of first electrodes is viewed from the second electrode side, and a plurality of first peripheral electrodes facing the plurality of second peripheral electrodes. 
     With this triaxial force sensor, the force in the direction orthogonal to the predetermined plane can be detected at four locations using the single second central electrode and the four first electrodes, and at the same time, the shearing forces in the orthogonal biaxial directions can be detected. As a result, the detection area of the force in the direction orthogonal to the predetermined plane can be increased as compared with the conventional case where the pressing force can be detected only at one location using a total of ten electrodes. Additionally, the force in the direction orthogonal to the predetermined plane can be further detected at a plurality of locations using the plurality of second peripheral electrodes and the plurality of first peripheral electrodes, so that the detection area of the force in the direction orthogonal to the predetermined plane can be further increased. 
     In the present invention, the plurality of second peripheral electrodes preferably includes a pair of the second peripheral electrodes, and the pair of second peripheral electrodes are preferably arranged along the second straight line with the first straight line between the pair of second peripheral electrodes when viewed toward the plurality of first peripheral electrodes. 
     In this triaxial force sensor, the plurality of second peripheral electrodes includes the pair of the second peripheral electrodes, and the pair of second peripheral electrodes are arranged along the second straight line with the first straight line between the pair of second peripheral electrodes when viewed toward the plurality of first peripheral electrodes. Thus, on the basis of the changes in capacitance between the pair of second peripheral electrodes and the first peripheral electrodes facing the pair of second peripheral electrodes, it is possible to detect the force in the direction orthogonal to the predetermined plane acting on one of the pair of second peripheral electrodes, and the force in the direction orthogonal to the predetermined plane acting on the other of the pair of second peripheral electrodes. As a result, the moment acting on the flexible member can be detected on the basis of the difference between the two forces and the distance from the first straight line. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view illustrating a configuration of a triaxial force sensor and the like according to a first embodiment of the present invention; 
         FIG. 2  is a cross-sectional view taken along line I-I in  FIG. 1 ; 
         FIG. 3  shows changes in capacitance when a force acts on the triaxial force sensor; 
         FIG. 4A  is a plan view illustrating a positional relationship between first and second electrodes when a shearing force does not act on the triaxial force sensor; 
         FIG. 4B  is a plan view illustrating a positional relationship between the first and second electrodes when a shearing force acts on the triaxial force sensor; 
         FIG. 5  shows a modification of the triaxial force sensor; 
         FIG. 6  shows the triaxial force sensor provided with a shield layer; 
         FIG. 7  shows the triaxial force sensor provided with an electrode for detecting the approach of an object; 
         FIG. 8  shows the triaxial force sensor provided with two electrodes for detecting the approach of an object; 
         FIG. 9  is a plan view illustrating a configuration of a triaxial force sensor and the like according to a second embodiment; 
         FIG. 10  is a cross-sectional view taken along line in  FIG. 9 ; 
         FIG. 11  is a cross-sectional view taken along line in  FIG. 10 ; 
         FIG. 12  is a plan view illustrating the arrangement of first and second electrodes; 
         FIG. 13  is an explanatory diagram of a moment acting on the triaxial force sensor; 
         FIG. 14  shows a modification of the triaxial force sensor; 
         FIG. 15  shows the triaxial force sensor provided with a shield layer; 
         FIG. 16  shows the triaxial force sensor provided with an electrode for detecting the approach of an object; and 
         FIG. 17  shows the triaxial force sensor provided with two electrodes for detecting the approach of an object; 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a triaxial force sensor according to a first embodiment of the present invention will be described with reference to the drawings. The triaxial force sensor  1  shown in  FIG. 1  according to the present embodiment is connected to a force detection device  40  via an electric wire (not shown). As will be described later, the force detection device  40  detects the forces in orthogonal triaxial directions acting on the triaxial force sensor  1  on the basis of the detection results of capacitances C 1  to C 9  by the triaxial force sensor  1 . 
     As shown in  FIGS. 1 and 2 , the triaxial force sensor  1  according to the present embodiment includes an electrode support  10 , nine first electrodes  11  to  19 , and nine second electrodes  21  to  29 . 
     Note that, in the following description, for convenience, the left side of  FIG. 1  is referred to as “left”, the right side of  FIG. 1  is referred to as “right”, the lower side of  FIG. 1  is referred to as “front”, the upper side of  FIG. 1  is referred to as “rear”, the front side of  FIG. 1  is referred to as “upper”, and the back side of  FIG. 1  is referred to as “lower”. 
     The electrode support  10  is formed from translucent silicon rubber having dielectric properties and elasticity (or flexibility), and has a solid rectangular parallelepiped shape. It should be noted that in the present embodiment, the electrode support  10  corresponds to a flexible member. 
     Each of the nine first electrodes  11  to  19  is a plate-shaped flexible electrode, and is connected to the force detection device  40  via a flexible electric wire (not shown). Further, each of the nine first electrodes  11  to  19  is attached to the electrode support  10  in a state flush with the lower surface of the electrode support  10  (see  FIG. 2 ). 
     The five first electrodes  11 ,  13 ,  15 ,  17 , and  19  are formed into squares having the same size in plan view, and the remaining four first electrodes  12 ,  14 ,  16 , and  18  are each formed into a rectangle having half the size of the first electrode  11  in plan view. In addition, the nine first electrodes  11  to  19  are arranged such that their respective sides are parallel to each other in plan view. 
     Further, as for the five first electrodes  11 ,  13 ,  15 ,  17 , and  19 , in plan view, the centers of the four first electrodes  11 ,  13 ,  17 , and  19  form a square, and the first electrode  15  is located at the center of the square. It should be noted that in the present embodiment, the first electrode  15  corresponds to a first central electrode, and the first electrodes  11  to  14  and  16  to  19  correspond to first peripheral electrodes. 
     Meanwhile, the first electrode  12  is placed at the center position between the two first electrodes  11  and  13  in a state in which the front edge thereof coincides with a straight line that passes through the centers of the two first electrodes  11  and  13 . In addition, the first electrode  14  is placed at the center position between the two first electrodes  11  and  17  in a state in which the right edge thereof coincides with a straight line that passes through the centers of the two first electrodes  11  and  17 . 
     Furthermore, the first electrode  16  is placed at the center position between the two first electrodes  13  and  19  in a state in which the left edge thereof coincides with a straight line that passes through the centers of the two first electrodes  13  and  19 . Meanwhile, the first electrode  18  is placed at the center position between the two first electrodes  17  and  19  in a state in which the rear edge thereof coincides with a straight line that passes through the centers of the two first electrodes  17  and  19 . 
     As described above, the triaxial force sensor  1  is configured so that the straight line connecting the centers of the pair of front and rear first electrodes  18  and  12  is orthogonal to the straight line connecting the centers of the pair of left and right first electrodes  14  and  16 , and the distance between the centers of the pair of front and rear first electrodes  18  and  12  and the distance between the centers of the pair of left and right first electrodes  14  and  16  are the same. It should be noted that in the present embodiment, either the pair of front and rear first electrodes  18  and  12  or the pair of left and right first electrodes  14  and  16  corresponds to one pair of first electrodes, and the other corresponds to the other pair of first electrodes. 
     Meanwhile, each of the nine second electrodes  21  to  29  is a plate-shaped flexible electrode, and is connected to the force detection device  40  via a flexible electric wire (not shown). Each of the nine second electrodes  21  to  29  is formed into a square smaller than the first electrode  11  in plan view, and embedded in the electrode support  10  in an orientation parallel to the upper surface of the electrode support  10  at a position below the upper surface of the electrode support  10  by a predetermined distance (see  FIG. 2 ). 
     The five second electrodes  21 ,  23 ,  25 ,  27 , and  29  are arranged concentrically with the five first electrodes  11 ,  13 ,  15 ,  17 , and  19 , respectively, in plan view. That is, the centers of the four second electrodes  21 ,  23 ,  27 , and  29  form a square, and the second electrode  25  is located at the center of the square. 
     Further, in plan view, the second electrode  22  is placed at the center position between the two second electrodes  21  and  23  in a state in which the rear half thereof overlaps the first electrode  12 . Furthermore, in plan view, the second electrode  24  is placed at the center position between the two second electrodes  21  and  27  in a state in which the left half thereof overlaps the first electrode  14 . 
     Meanwhile, in plan view, the second electrode  26  is placed at the center position between the two second electrodes  23  and  29  in a state in which the right half thereof overlaps the first electrode  16 . In addition, the second electrode  28  is placed at the center position between the two second electrodes  27  and  29  in a state in which the lower half thereof overlaps the first electrode  18 . 
     As described above, the triaxial force sensor  1  is configured so that the straight line connecting the centers of the two second electrodes  22  and  28  is orthogonal to the straight line connecting the centers of the two second electrodes  24  and  26 , and the distance between the centers of the two second electrodes  22  and  28  and the distance between the centers of the two second electrodes  24  and  26  are the same. It should be noted that in the present embodiment, the second electrode  25  corresponds to a second central electrode, and the second electrodes  21  to  24  and  26  to  29  correspond to second peripheral electrodes. 
     Meanwhile, the force detection device  40  is configured by combining an electric circuit and a microcomputer. In the force detection device  40 , by applying a voltage between the two first electrode  11  and second electrode  21 , the capacitance C 1  between the electrodes  11  and  21  is detected, and by applying a voltage between the two first electrode  12  and second electrode  22 , the capacitance C 2  between the electrodes  12  and  22  is detected. 
     Furthermore, by the same method as described above, the capacitance C 3  is detected as the capacitance between the two first electrode  13  and second electrode  23 , the capacitance C 4  is detected as the capacitance between the two first electrode  14  and second electrode  24 , and the capacitance C 5  is detected as the capacitance between the two first electrode  15  and second electrode  25 . 
     Additionally, the capacitance C 6  is detected as the capacitance between the two first electrode  16  and second electrode  26 , the capacitance C 7  is detected as the capacitance between the two first electrode  17  and second electrode  27 , the capacitance C 8  is detected as the capacitance between the two first electrode  18  and second electrode  28 , and the capacitance C 9  is detected as the capacitance between the two first electrode  19  and second electrode  29 . 
     The capacitances C 1  to C 9  change as shown in  FIG. 3  when forces in the orthogonal triaxial directions act on the triaxial force sensor  1 . It should be noted that in  FIG. 3 , an increase in capacitance is referred to as “Up”, and a decrease in capacitance is referred to as “Down”. As shown in the figure, when a downward force (that is, a load) acts on the triaxial force sensor  1 , all of the nine capacitances C 1  to C 9  increase. 
     Further, when a leftward force (leftward shearing force) acts on the triaxial force sensor  1 , the capacitance C 4  increases and the capacitance C 6  decreases. This is due to the following reasons. That is, if the leftward force Fx acts on the triaxial force sensor  1  as shown in  FIG. 4B  from the state shown in  FIG. 4A  in which no force acts on the triaxial force sensor  1 , the overlapping area of the two first electrode  14  and second electrode  24  aligned in the vertical direction increases by the amount shown by the hatching in  FIG. 4B , and accordingly, the capacitance C 4  increases. 
     At the same time, the overlapping area of the two first electrode  16  and second electrode  26  aligned in the vertical direction decreases by the amount shown by the hatching in  FIG. 4B , and accordingly, the capacitance C 6  decreases. In addition, the overlapping area of two first and second electrodes aligned in the vertical direction other than the above does not change, resulting in a state in which the capacitance does not change. 
     Meanwhile, when a rightward force (rightward shearing force) acts on the triaxial force sensor  1 , the capacitance C 4  decreases and the capacitance C 6  increases, contrary to the case where the leftward force acts. 
     Furthermore, when a forward force (forward shearing force) acts on the triaxial force sensor  1 , the overlapping area of the two first electrode  12  and second electrode  22  aligned in the vertical direction decreases, and the overlapping area of the two first electrode  18  and second electrode  28  aligned in the vertical direction increases. As a result, the capacitance C 2  decreases and the capacitance C 8  increases. 
     In addition, when a rearward force (rearward shearing force) acts on the triaxial force sensor  1 , the overlapping area of the two first electrode  12  and second electrode  22  increases, and the overlapping area of the two first electrode  18  and second electrode  28  decreases. As a result, the capacitance C 2  increases and the capacitance C 8  decreases. 
     On the basis of the above principle, in the force detection device  40 , the nine capacitances C 1  to C 9  are detected using the triaxial force sensor  1 , and the downward force (load) acting on the triaxial force sensor  1 , the shearing force acting in the left-right direction, and the shearing force acting in the front-rear direction are calculated by an arithmetic expression (not shown) on the basis of the nine capacitances C 1  to C 9 . That is, the forces in the orthogonal triaxial directions acting on the triaxial force sensor  1  are detected. 
     As described above, the triaxial force sensor  1  according to the first embodiment is configured by the nine first electrodes  11  to  19  being attached to the electrode support  10  in a state flush with the bottom surface of the electrode support  10 , and the nine second electrodes  21  to  29  being attached to the electrode support  10  in an orientation parallel to the nine first electrodes  11  to  19 . Therefore, when a downward force acts on the electrode support  10 , the distances between the nine second electrodes  21  to  29  and the nine first electrodes  11  to  19  change, and on the basis of the resulting changes in the capacitances C 1  to C 9 , the force acting on the electrode support  10  can be detected at a total of nine locations. That is, even when a distributed load acts on the electrode support  10 , the distributed load can be detected. 
     In addition, when leftward and rightward shearing forces act on the triaxial force sensor  1 , the capacitance C 4  between the two first electrode  14  and second electrode  24  and the capacitance C 6  between the two first electrode  16  and second electrode  26  change. Similarly, when forward and rearward shearing forces act on the triaxial force sensor  1 , the capacitance C 2  between the two first electrode  12  and second electrode  22  and the capacitance C 8  between the two first electrode  18  and second electrode  28  change. Therefore, it is possible to detect the shearing forces in the front-rear direction and the left-right direction on the basis of the changes in the capacitances C 2 , C 4 , C 6 , and C 8 . 
     Furthermore, the forces in the orthogonal triaxial directions can be detected using a total of eight electrodes (the first electrodes  12 ,  14 ,  16 , and  18  and the second electrodes  22 ,  24 ,  26 , and  28 ). Therefore, as compared with the conventional case where a total of ten electrodes are required, the number of components can be reduced and the sensor can be downsized, so that manufacturing cost can be reduced accordingly. 
     Additionally, with the triaxial force sensor  1 , it is possible to detect a moment around the straight lines extending in the front-rear direction and the left-right direction through the center of the second electrode  25  according to the principle described later. 
     It should be noted that the first embodiment is an example in which the triaxial force sensor  1  is provided with the solid electrode support  10 , but alternatively, as shown in  FIG. 5 , a triaxial force sensor  1 A may be provided with a hollow electrode support  10 A (flexible member). The triaxial force sensor  1 A has a plurality of voids  10   a  formed in the interior of the electrode support  10 A, and configured so that the first electrodes  11  to  19  and the second electrodes  21  to  29  are not short-circuited when a downward force is applied. 
     Since the triaxial force sensor  1 A has the voids  10   a  formed in the electrode support  10 A, when a shearing force acts on the electrode support  10 A, the displacement in the left-right direction and the front-rear direction between the two first and second electrodes aligned in the vertical direction is more likely to occur than the electrode support  10  of the first embodiment. Thus, the triaxial force sensor  1 A allows a further improvement in the detection sensitivity of shearing force than the triaxial force sensor  1  according to the first embodiment. 
     Further, a triaxial force sensor  1 B may be configured as shown in  FIG. 6 . The triaxial force sensor  1 B is provided with a shield layer  30 , and the shield layer  30  is embedded in the electrode support  10 , above the second electrode  21  to  29 . With the triaxial force sensor  1 B, the effect of the shield layer  30  can suppress the influence of external noise and improve the accuracy of triaxial force detection. 
     Furthermore, a triaxial force sensor  1 C may be configured as shown in  FIG. 7 . The triaxial force sensor  1 C is also provided with an electrode  31  above the shield layer  30 . The electrode  31  is configured to detect the approach of an object such as a finger Q, and is connected to an oscillation circuit and a force detection device (both not shown). The electrode  31  generates an electric field when driven by the oscillation circuit. The force detection device detects the approach of an object on the basis of the change in capacitance of the electrode  31  when the object enters the electric field generated by the electrode  31 . As described above, with the triaxial force sensor  1 C, it is possible to detect the approach of an object in addition to detecting the triaxial forces. 
     Further, a triaxial force sensor  1 D may be configured as shown in  FIG. 8 . The triaxial force sensor  1 D is also provided with two electrodes  32  and  33  above the shield layer  30 . The electrodes  32  and  33  are configured to detect the approach of an object such as a finger Q, and are connected to a force detection device (not shown). The force detection device applies a voltage to the electrodes  32  and  33 , and detects the approach of the object on the basis of the change in capacitance between the electrodes  32  and  33 . As described above, with the triaxial force sensor  1 D, similarly to the triaxial force sensor  1 C described above, it is possible to detect the approach of an object in addition to detecting the triaxial forces. 
     It should be noted that although the first embodiment is an example in which the electrode support  10  formed from silicon rubber is used as a flexible member, the flexible member in the present invention is not limited thereto, but may be anything that has dielectric properties and elasticity. For example, a dielectric such as a conductive resin such as a thiophene-based conductive polymer or PSS, PVC gel, polyvinylidene fluoride (PVDF), polydimethylsiloxane (PDMS), a silicon-based resin, a urethane-based resin, or epoxy-based resin, or a composite material of any combination thereof may be used as a flexible member. 
     Further, in the first embodiment, the rectangular parallelepiped electrode support  10  is used as the flexible member, but alternatively, flexible members having various shapes, such as an oval coin shape and a cylindrical shape, may be used. 
     Meanwhile, the first embodiment is an example in which the five first electrodes  11 ,  13 ,  15 ,  17 , and  19  and the five second electrodes  21 ,  23 ,  25 ,  27 , and  29  are square in plan view, but alternatively, polygonal ones other than square or circular ones in plan view may be used. In that case, it is sufficient that the overlap area between the five second electrodes  21 ,  23 ,  25 ,  27 , and  29  and the five first electrodes  11 ,  13 ,  15 ,  17 , and  19  does not change when a shearing force acts on the electrode support  10 . 
     Furthermore, the first embodiment is an example in which the four first electrodes  12 ,  14 ,  16 , and  18  have a rectangular shape in plan view, but alternatively, shapes having a symmetrical shape when viewed in plan view may be used. 
     In addition, the first embodiment is an example in which the four second electrodes  22 ,  24 ,  26 , and  28  are square in plan view, but alternatively, rectangular ones other than square in plan view may be used, as long as the shape has symmetry in plan view. 
     Meanwhile, the first embodiment is an example in which nine first electrodes are used as a plurality of first electrodes, but alternatively, eight or fewer or ten or more first electrodes may be used as a plurality of first electrodes. Further, the eight first peripheral electrodes are used as a plurality of first peripheral electrodes in the example, but alternatively, seven or fewer or nine or more first peripheral electrodes may be used as a plurality of first peripheral electrodes. 
     In addition, the first embodiment is an example in which nine second electrodes are used as a plurality of second electrodes, but alternatively, eight or fewer or ten or more second electrodes may be used as a plurality of second electrodes. Moreover, the eight second peripheral electrodes are used as a plurality of second peripheral electrodes in the example, but alternatively, seven or fewer or nine or more second peripheral electrodes may be used as a plurality of second peripheral electrodes. 
     Hereinafter, a triaxial force sensor according to a second embodiment of the present invention will be described with reference to the drawings. As shown in  FIGS. 9 to 12 , the triaxial force sensor  100  according to the second embodiment differs from the triaxial force sensor  1  according to the first embodiment in that the triaxial force sensor  100  is provided with twelve first electrodes  111  to  122  and nine second electrodes  201  to  209  in place of the first electrodes  11  to  19  and the second electrodes  21  to  29 , and therefore the first electrodes  111  to  122  and the second electrodes  201  to  208  will be mainly described below. The same components as those of the first embodiment are denoted by the same reference signs, and a description thereof will not be given here. 
     Each of the twelve first electrodes  111  to  122  is a plate-shaped flexible electrode, and is connected to the force detection device  40  via a flexible electric wire (not shown). Further, as shown in  FIG. 11 , the twelve first electrodes  111  to  122  are formed into squares having the same size as each other in plan view, and are arranged such that their respective sides are parallel to each other. In addition, the twelve first electrodes  111  to  122  are arranged at positions above the bottom surface of the electrode support  10  by a predetermined distance, and are embedded in the electrode support  10  in an orientation parallel to the bottom surface of the electrode support  10  (see  FIG. 10 ). 
     Among the twelve first electrodes  111  to  122 , the four first electrodes  111  to  114  are arranged in the vicinity of the central portion of the electrode support  10  in plan view such that a straight line L 1  connecting the centers of the pair of front and rear first electrodes  111  and  112  is orthogonal to a straight line L 2  connecting the centers of the pair of left and right first electrodes  113  and  114  at the center point of the electrode support  10 , and the distance between the centers of the front and rear first electrodes  111  and  112  and the distance between the centers of the left and right first electrodes  113  and  114  are the same (see  FIG. 12 ). 
     The four first electrodes  111  to  114  are arranged in such a manner that the second electrode  201  overlaps their respective halves in plan view. That is, the second electrode  20  is disposed in such a manner as to overlap the rear half of the first electrode  111 , the front half of the first electrode  112 , the right half of the first electrode  113 , and the left half of the first electrode  114 . 
     Further, the remaining eight first electrodes  115  to  122  surround the four first electrodes  111  to  114  and are arranged so as to be point-symmetric with respect to the center point of the electrode support  10  in plan view. More specifically, the two first electrodes  115  and  116  are separated leftward from the straight line L 1  by a predetermined distance L, and are arranged at equal intervals in the front-rear direction with respect to the straight line L 2 . In addition, the two first electrodes  117  and  118  are arranged in line symmetry with the two first electrodes  115  and  116  with the straight line L 1  therebetween. 
     Furthermore, the two first electrodes  119  and  120  are separated forward from the straight line L 2  by a predetermined distance L, and are arranged at equal intervals in the left-right direction with respect to the straight line L 1 . In addition, the two first electrodes  121  and  122  are arranged in line symmetry with the two first electrodes  119  and  120  with the straight line L 2  therebetween. It should be noted that in the present embodiment, either the pair of front and rear first electrodes  111  and  112  or the pair of left and right first electrodes  113  and  114  corresponds to one pair of first electrodes, the other corresponds to the other pair of first electrodes, and the first electrode  115  to  122  correspond to first peripheral electrodes. 
     Meanwhile, each of the nine second electrodes  201  to  209  is a plate-shaped flexible electrode, and is connected to the force detection device  40  via a flexible electric wire (not shown). In addition, the nine second electrodes  201  to  209  are arranged at positions below the upper surface of the electrode support  10  by a predetermined distance, and are embedded in the electrode support  10  in an orientation parallel to the upper surface of the electrode support  10  (see  FIG. 10 ). 
     Furthermore, as shown in  FIG. 9 , the nine second electrodes  201  to  209  are formed into squares in plan view, and are arranged in such a manner that their respective sides are parallel to each other and also parallel to the sides of the first electrodes  111  to  122 . 
     Among the nine second electrodes  201  to  209 , the second electrode  201  is centrally placed and has a larger size than the other second electrodes  202  to  209 . As described above, the second electrode  201  is disposed in such a manner as to overlap the halves of the four first electrodes  111  to  114  in plan view. 
     Furthermore, the eight second electrodes  202  to  209  are disposed so as to surround the second electrode  201  in plan view, have the same size as the first electrode  111  to  122 , and are arranged concentrically with the eight first electrodes  115  to  122 . That is, the eight second electrodes  202  to  209  are arranged so as to entirely overlap the eight first electrodes  115  to  122  in plan view. It should be noted that in the present embodiment, the second electrode  201  corresponds to a second central electrode, and the second electrodes  202  to  209  correspond to second peripheral electrodes. 
     As described above, the triaxial force sensor  100  according to the second embodiment is configured by the twelve first electrodes  111  to  122  being attached to the electrode support  10  in an orientation parallel to the upper surface of the electrode support  10 , and the nine second electrodes  201  to  209  being attached to the electrode support  10  in spaced face-to-face relation to the twelve first electrodes  111  to  122 . Furthermore, the single second electrode  201  is disposed in such a manner that the halves of the four first electrodes  111  to  114  overlap the second electrode  201  in plan view. 
     Therefore, when a downward force acts on the electrode support  10 , the distances between the nine second electrodes  201  to  209  and the twelve first electrodes  111  to  122  change, and on the basis of the resulting changes in capacitance, the downward force acting on the electrode support  10  can be detected at a total of twelve locations. That is, even when a distributed load acts on the electrode support  10 , the distributed load can be detected. 
     Further, in plan view, the four first electrodes  111  to  114  are arranged such that the straight line L 1  connecting the centers of the pair of front and rear first electrodes  111  and  112  is orthogonal to the straight line L 2  connecting the centers of the pair of left and right first electrodes  113  and  114  at the center point of the electrode support  10 , and the distance between the centers of the front and rear first electrodes  111  and  112  and the distance between the centers of the left and right first electrodes  113  and  114  are the same. 
     Therefore, when a shearing force acts on the electrode support  10 , as the electrode support  10  is elastically deformed, the overlapping area of the surfaces facing each other between the four first electrodes  111  to  114  and the second electrode  201  changes, resulting in changes in capacitance. Thus, the shearing forces in the orthogonal biaxial directions can be detected on the basis of the changes in capacitance. 
     Furthermore, the forces in the orthogonal triaxial directions can be detected using a total of five electrodes (the first electrodes  111  to  114  and the second electrode  201 ). Therefore, as compared with the conventional case where a total of ten electrodes are required, the number of components can be reduced and the sensor can be downsized, so that manufacturing cost can be reduced accordingly. 
     Further, with the triaxial force sensor  100 , when a straight line extending in the front-rear direction through the center of the second electrode  201  in the left-right direction and the vertical direction and a straight line extending in the left-right direction through the center of the second electrode  201  in the front-rear direction and the vertical direction are assumed, it is also possible to detect moments around the two straight lines. Hereinafter, the principle will be described. 
     First, a principle of detecting the moment Mx around the x axis with the center O of the second electrode  201  in the left-right direction and the vertical direction as the center of rotation in cases where the x, y, and z axes are set such that the depth direction is on the positive value side of the x axis, the right direction is on the positive value side of the y axis, and the upper direction is on the positive value side of the z axis as shown in  FIG. 13  will be described. 
     Here, when the capacitance between the second electrode  203  and the first electrode  116  is C_left, the dielectric constant is ε, the electrode area is S, and the inter-electrode distance is Z_left, the following equation (1) holds. 
         C _left=ε·( S/Z _left)   (1)
 
     Similarly, when the capacitance between the second electrode  205  and the first electrode  118  is C_right, the dielectric constant is ε, the electrode area is S, and the inter-electrode distance is Z_right, the following equation (2) holds. 
         C _right=ε·( S/Z _right)   (2)
 
     Further, when the downward force acting on the second electrode  203  side is Fx_left, the proportional coefficient is k, and the amount of change in capacitance C_left caused by the downward force Fx_left is ΔC_left, the following equation (3) holds. 
         Fx _left= k·ΔC _left   (3)
 
     Similarly, when the downward force acting on the second electrode  205  is Fx_right, the proportional coefficient is k, and the amount of change in capacitance C_right caused by the downward force Fx_right is ΔC_right, the following equation (4) holds. 
         Fx _right= k·ΔC _right   (4)
 
     Furthermore, since the length of the arm from the center O to the point of action is the above-described value L, the moment Mx can be calculated/detected by the following equation (5). 
         Mx=L ·( Fx _left− Fx _right)   (5)
 
     Similarly to the above, according to the principle, it is also possible to calculate/detect the moment around the y-axis with the center of the second electrode  201  in the front-rear direction and the vertical direction as the center of rotation. Therefore, with the triaxial force sensor  100  according to the present embodiment, it is possible to detect moments around two axes in addition to the triaxial forces. 
     It should be noted that although the description is omitted, the triaxial force sensor  1  according to the first embodiment can also detect moments around two axes in addition to the triaxial forces by the same principle as described above. 
     Further, the triaxial force sensor  100  according to the second embodiment is provided with the solid electrode support  10 , but alternatively, as shown in  FIG. 14 , the triaxial force sensor  101  may be provided with the hollow electrode support  10 A similarly to the triaxial force sensor  1 A described above. The triaxial force sensor  101  is configured so that the first electrodes  111  to  122  and the second electrodes  201  to  209  are not short-circuited when a downward force acts. The triaxial force sensor  101  allows a further improvement in the detection sensitivity of shearing force than the triaxial force sensor  100  according to the second embodiment for the reasons described above. 
     Further, a triaxial force sensor  102  may be configured as shown in  FIG. 15 . Similarly to the triaxial force sensor  1 B described above, the triaxial force sensor  102  is provided with the shield layer  30 , the effect of which can suppress the influence of external noise and improve the accuracy of triaxial force detection. 
     Furthermore, a triaxial force sensor  103  may be configured as shown in  FIG. 16 . Similarly to the triaxial force sensor  1 C described above, the triaxial force sensor  103  is also provided with the electrode  31  above the shield layer  30 . Thus, with the triaxial force sensor  103 , similarly to the triaxial force sensor  1 C, it is possible to detect the approach of an object in addition to detecting the triaxial forces. 
     Further, a triaxial force sensor  104  may be configured as shown in  FIG. 17 . Similarly to the triaxial force sensor  1 D described above, the triaxial force sensor  104  is also provided with the two electrodes  32  and  33  above the shield layer  30 . Thus, with the triaxial force sensor  104 , similarly to the triaxial force sensor  1 D, it is possible to detect the approach of an object in addition to detecting the triaxial forces. 
     It should be noted that the second embodiment is an example in which the eight first electrodes  115  to  122  and the eight second electrodes  202  to  209  are square in plan view, but alternatively, polygonal ones other than square or circular ones in plan view may be used. 
     Further, the second embodiment is an example in which the four first electrodes  111  to  114  are square in plan view, but alternatively, the four first electrodes  111  to  114  that have a symmetrical shape in plan view may be used. 
     Furthermore, the second embodiment is an example in which the distance from the straight line L 1  to the two second electrodes  202  and  203  and the distance from the straight line L 1  to the two second electrodes  204  and  205  are the same, but these distances may be configured differently, and even in this case, the moment around the x-axis can be detected. 
     Similarly, in this example, the distance from the straight line L 2  to the two second electrodes  206  and  207  and the distance from the straight line L 2  to the two second electrodes  208  and  209  are the same, but these distances may be configured differently, and even in this case, the moment around the y-axis can be detected. 
     In addition, the second embodiment is an example in which eight first electrodes are used as a plurality of first peripheral electrodes, but alternatively, seven or fewer or nine or more first electrodes may be used as a plurality of first peripheral electrodes. 
     Furthermore, the second embodiment is an example in which eight second electrodes are used as a plurality of second peripheral electrodes, but alternatively, seven or fewer or nine or more second electrodes may be used as a plurality of second peripheral electrodes. 
     REFERENCE SIGNS LIST 
     
         
           1  Triaxial force sensor 
           1 A to  1 D Triaxial force sensor 
           10  Electrode support (flexible member) 
           10 A Electrode support (flexible member) 
           11  to  14  First electrode (first peripheral electrode) 
           15  First electrode (first central electrode) 
           16  to  19  First electrode (first peripheral electrode) 
           21  to  24  Second electrode (second peripheral electrode) 
           25  Second electrode (second central electrode) 
           26  to  29  Second electrode (second peripheral electrode) 
           100  Triaxial force sensor 
           101  to  104  Triaxial force sensor 
           111  to  114  First electrode (one pair of first electrodes and other pair of first electrodes) 
           115  to  122  First electrode (first peripheral electrode) 
           201  Second electrode (second central electrode) 
           202  to  209  Second electrode (second peripheral electrode) 
         L 1  First straight line 
         L 2  Second straight line