Patent Publication Number: US-8966996-B2

Title: Force sensor

Description:
RELATED APPLICATION INFORMATION 
     This application is a 371 of International Application PCT/JP2011/067714 filed 27 Jul.2011 Entitled “Force Sensor”, which was published on 31 Jan. 2013, with International Publication Number WO2013/014803 A1, the content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a force sensor and in particular to a thin-type force sensor which is suitable for detecting force in the direction of each coordinate axis and moment around each coordinate axis in a three-dimensional orthogonal coordinate system. 
     BACKGROUND ART 
     Various types of force sensors have been used to control motions of robots and industrial machines. Also, a downsized force sensor has been incorporated as a man-machine interface of an input device for electronics. In order to reduce dimensions and cost, a force sensor to be used in the above-described applications is required to be simple in structure as much as possible and also to independently detect force for each coordinate axis in a three dimensional space. 
     At present, generally-used multi-axis force sensors are categorized into such a type that a specific directional component of force exerted on a mechanical structure is detected as displacement occurring at a specific site and such a type that the force is detected as a mechanical strain occurring at a specific site. A force sensor using a capacitive element is a representative sensor of the former displacement detection types. This force sensor has a capacitive element constituted by a pair of electrodes and detects displacement occurring at one of the electrodes by the force exerted on the basis of a capacitance value of the capacitive element. A multi-axis force sensor using a capacitive element has been disclosed, for example, in Japanese Unexamined Patent Publication No. 2004-325367 (U.S. Pat. No. 7,219,561) and Japanese Unexamined Patent Publication No. 2004-354049 (U.S. Pat. No. 6,915,709). 
     On the other hand, a strain gauge-type force sensor is a representative sensor of the latter type which detects a mechanical strain. This strain gauge-type force sensor detects a mechanical strain resulting from the force exerted as a change in electric resistance of strain gauges and others. The strain gauge-type multi-axis force sensor has been disclosed, for example, in Japanese Unexamined Patent Publication No. 8-122178 (U.S. Pat. No. 5,490,427). 
     However, in any of the multi-axis force sensors disclosed in the above-described Patent Documents, a mechanical structure is inevitably made thick. Therefore, it is difficult to make thin the sensor in its entirety. 
     On the other hand, in the fields of robots, industrial machines, input devices for electronics and others, it is desired to develop a thinner-type force sensor. Thus, an object of the present invention is to provide a force sensor which is simple in structure and can be made thinner. 
     DISCLOSURE OF INVENTION 
     (1) The first feature of the present invention resides in a force sensor which detects force or moment with regard to at least one axis, among force in a direction of each coordinate axis and moment around each coordinate axis, in an XYZ three-dimensional orthogonal coordinate system, 
     the force sensor comprising: 
     a force receiving ring which is arranged on an XY-plane so that a Z-axis is given as a central axis and receives exertions of force or moment to be detected; 
     a detection ring which is arranged on the XY-plane so that the Z-axis is given as a central axis and also arranged so as to be inside or outside the force receiving ring, at least a part of the detection ring undergoing elastic deformation by exertions of force or moment to be detected; 
     a supporting substrate which has an upper surface parallel to the XY-plane and is arranged at certain intervals below the force receiving ring and the detection ring; 
     a connection member which connects the detection ring to the force receiving ring at a predetermined exertion point; 
     a fixing member which fixes the detection ring to the supporting substrate at a predetermined fixing point; 
     a detection element which electrically detects elastic deformation of the detection ring; and 
     a detection circuit which detects a detection value of force in a predetermined coordinate axis direction or moment around a predetermined coordinate axis which is exerted on the force receiving ring, with the supporting substrate being fixed, on the basis of detection result of the detection element; wherein 
     a projection image of the exertion point on the XY-plane and a projection image of the fixing point on the XY-plane are formed at mutually different positions. 
     (2) The second feature of the present invention resides in a force sensor having the first feature described above, wherein 
     n number of plural exertion points and n number of plural fixing points are alternately arranged on an annular channel along a contour of the detection ring, and 
     the detection element electrically detects elastic deformation of the detection ring in vicinity of a measurement point defined between an exertion point and a fixing point adjacently arranged. 
     (3) The third feature of the present invention resides in a force sensor having the second feature described above, wherein 
     two exertion points and two fixing points are arranged on the annular channel along the contour of the detection ring in an order of a first exertion point, a first fixing point, a second exertion point and a second fixing point, and 
     individually defining a first measurement point arranged between the first exertion point and the first fixing point on the annular channel, a second measurement point arranged between the first fixing point and the second exertion point on the annular channel, a third measurement point arranged between the second exertion point and the second fixing point on the annular channel, and a fourth measurement point arranged between the second fixing point and the first exertion point on the annular channel, the detection element electrically detects elastic deformation of the detection ring in vicinities of the first to the fourth measurement points. 
     (4) The fourth feature of the present invention resides in a force sensor having the third feature described above, wherein 
     the first exertion point, the first fixing point, the second exertion point, and the second fixing point are arranged respectively at a positive domain of an X-axis, a positive domain of a Y-axis, a negative domain of the X-axis and a negative domain of the Y-axis, 
     vicinity of the first exertion point on the detection ring is connected to the force receiving ring via a first connection member extending along the positive domain of the X-axis, and vicinity of the second exertion point on the detection ring is connected to the force receiving ring via a second connection member extending along the negative domain of the X-axis, and 
     the detection element electrically detects elastic deformation of the detection ring in vicinities of the first measurement point, the second measurement point, the third measurement point and the fourth measurement point arranged respectively at a first quadrant, a second quadrant, a third quadrant and a fourth quadrant of the XY-plane. 
     (5) The fifth feature of the present invention resides in a force sensor having the fourth feature described above, wherein 
     defining a V-axis which passes through an origin O in the XYZ three-dimensional orthogonal coordinate system, with a positive domain positioned at the first quadrant of the XY-plane and with a negative domain positioned at the third quadrant of the XY-plane and forms 45 degrees with respect to the X-axis, and a W-axis which passes through the origin O in the XYZ three-dimensional orthogonal coordinate system, with a positive domain positioned at the second quadrant of the XY-plane and with a negative domain positioned at the fourth quadrant of the XY-plane and is orthogonal to the V-axis, the first measurement point, the second measurement point, the third measurement point and the fourth measurement point are arranged respectively at the positive domain of the V-axis, the positive domain of the W-axis, the negative domain of the V-axis and the negative domain of the W-axis. 
     (6) The sixth feature of the present invention resides in a force sensor having the first feature described above, wherein 
     n number of plural exertion points and n number of plural fixing points are alternately arranged on an annular channel along a contour of the detection ring, 
     vicinities of n number of the exertion points on the detection ring constitute diaphragms thinner in thickness than other parts, 
     n number of plural connection members are connected respectively to the diaphragms, and 
     the detection element electrically detects elastic deformation of the diaphragms. 
     (7) The seventh feature of the present invention resides in a force sensor having the sixth feature described above, wherein 
     four exertion points and four fixing points are arranged on the annular channel along the contour of the detection ring in an order of the first exertion point, the first fixing point, the second exertion point, the second fixing point, the third exertion point, the third fixing point, the fourth exertion point and the fourth fixing point, 
     vicinity of the first exertion point on the detection ring constitutes a first diaphragm, vicinity of the second exertion point on the detection ring constitutes a second diaphragm, vicinity of the third exertion point on the detection ring constitutes the third diaphragm, vicinity of the fourth exertion point on the detection ring constitutes the fourth diaphragm, and 
     the detection element electrically detects elastic deformation of the first to the fourth diaphragms. 
     (8) The eighth feature of the present invention resides in a force sensor having the seventh feature described above, wherein 
     the first exertion point, the second exertion point, the third exertion point and the fourth exertion point are arranged respectively at a positive domain of an X-axis, a positive domain of a Y-axis, a negative domain of the X-axis and a negative domain of the Y-axis, and the first diaphragm, the second diaphragm, the third diaphragm and the fourth diaphragm are positioned respectively at the positive domain of the X-axis, the positive domain of the Y-axis, the negative domain of the X-axis and the negative domain of the Y-axis, 
     defining a V-axis which passes through an origin O in the XYZ three-dimensional orthogonal coordinate system, with a positive domain positioned at a first quadrant of the XY-plane and with a negative domain positioned at a third quadrant of the XY-plane, and forms 45 degrees with respect to the X-axis, and a W-axis which passes through the origin O in the XYZ three-dimensional orthogonal coordinate system, with a positive domain positioned at a second quadrant of the XY-plane and with a negative domain positioned at a fourth quadrant of the XY-plane and is orthogonal to the V-axis, the first fixing point, the second fixing point, the third fixing point and the fourth fixing point are arranged respectively at the positive domain of the V-axis, the positive domain of the W-axis, the negative domain of the V-axis and the negative domain of the W-axis, and 
     the first diaphragm is connected to the force receiving ring via a first connection member extending along the positive domain of the X-axis, the second diaphragm is connected to the force receiving ring via a second connection member extending along the positive domain of the Y-axis, the third diaphragm is connected to the force receiving ring via a third connection member extending along the negative domain of the X-axis, and the fourth diaphragm is connected to the force receiving ring via a fourth connection member extending along the negative domain of the Y-axis. 
     (9) The ninth feature of the present invention resides in a force sensor having one of the first to eighth features described above, wherein 
     a lower surface of the detection ring is connected to the upper surface of the supporting substrate via the fixing member. 
     (10) The tenth feature of the present invention resides in a force sensor having one of the first to eighth features described above, wherein 
     both rings are arranged so that the force receiving ring is outside and the detection ring is inside, 
     a fixed assistant body whose lower surface is fixed onto the upper surface of the supporting substrate is provided further inside the detection ring, and 
     an inner circumferential surface of the detection ring is connected to an outer circumferential surface of the fixed assistant body via the fixing member. 
     (11) The eleventh feature of the present invention resides in a force sensor having one of the first to tenth features described above, comprising; 
     a force receiving substrate which is provided with an upper surface parallel to the XY-plane and arranged at certain intervals above the force receiving ring and the detection ring, wherein 
     a lower surface of the force receiving substrate is partially connected to an upper surface of the force receiving ring, and 
     a predetermined clearance is formed between the lower surface of the force receiving substrate and an upper surface of the detection ring. 
     (12) The twelfth feature of the present invention resides in a force sensor having the eleventh feature described above, wherein 
     an inclusive tubular body which includes the force receiving ring and the detection ring is connected to an outer circumference of the lower surface of the force receiving substrate, and a clearance is formed between a lower end of the inclusive tubular body and an outer circumference of the supporting substrate, and 
     the clearance is set dimensionally in such a manner that the lower end of the inclusive tubular body is brought into contact with the outer circumference of the supporting substrate, thereby restricting displacement of the force receiving substrate, when force or moment exceeding a predetermined tolerance level is exerted on the force receiving substrate. 
     (13) The thirteenth feature of the present invention resides in a force sensor having one of the first to twelfth features described above, wherein 
     a vertically penetrating through-hole is formed at a predetermined site on the force receiving ring, and a groove larger in diameter than the through-hole is formed at a position of the through-hole on an upper surface of the force receiving ring, 
     a displacement control screw which is inserted through the through-hole whose leading end is fixed to the supporting substrate and whose head is accommodated into the groove is additionally provided, and a clearance is formed between the displacement control screw and inner surfaces of the through-hole and the groove, and 
     the clearance is set dimensionally in such a manner that the displacement control screw is brought into contact with the inner surface of the through-hole or the inner surface of the groove, thereby restricting displacement of the force receiving ring, when force or moment exceeding a predetermined tolerance level is exerted on the force receiving ring. 
     (14) The fourteenth feature of the present invention resides in a force sensor having one of the first to thirteenth features described above, wherein 
     the force receiving ring is composed of a rigid body which does not substantially undergo deformation as long as exerting force or moment is within a predetermined tolerance level. 
     (15) The fifteenth feature of the present invention resides in a force sensor having one of the first to fourteenth features described above, wherein 
     both the force receiving ring and the detection ring are circular rings arranged on the XY-plane so that the Z-axis is given as a central axis. 
     (16) The sixteenth feature of the present invention resides in a force sensor having one of the first to fifteenth features described above, wherein 
     a detection circuit substrate which packages electronics constituting the detection circuit is provided on the upper surface of the supporting substrate. 
     (17) The seventeenth feature of the present invention resides in a force sensor having one of the first to sixteenth features described above, wherein 
     the detection element electrically detects displacement of the detection ring at a predetermined measurement point. 
     (18) The eighteenth feature of the present invention resides in a force sensor having the seventeenth feature described above, wherein 
     the detection element electrically detects a distance between a measurement target surface in vicinity of the measurement point on the detection ring and a counter reference surface facing the measurement target surface of the force receiving ring. 
     (19) The nineteenth feature of the present invention resides in a force sensor having the seventeenth feature described above, wherein 
     both rings are arranged so that the force receiving ring is outside and the detection ring is inside, 
     a fixed assistant body whose lower surface is fixed onto the upper surface of the supporting substrate is provided further inside the detection ring, and 
     the detection element electrically detects a distance between a measurement target surface in vicinity of the measurement point on an inner circumferential surface of the detection ring and a counter reference surface positioned on an outer circumference of the fixed assistant body and facing the measurement target surface. 
     (20) The twentieth feature of the present invention resides in a force sensor having the seventeenth feature described above, wherein 
     the detection element electrically detects a distance between a measurement target surface positioned in vicinity of the measurement point on a lower surface of the detection ring and a counter reference surface positioned on the upper surface of the supporting substrate and facing the measurement target surface. 
     (21) The twenty-first feature of the present invention resides in a force sensor having one of the eighteen to twentieth features described above, wherein 
     the detection element is constituted by a capacitive element having a displacement electrode provided on the measurement target surface and a fixed electrode provided on the counter reference surface. 
     (22) The twenty-second feature of the present invention resides in a force sensor having the twenty-first feature described above, wherein 
     the detection ring is composed of a flexible conductive material, and a surface of the detection ring is used as a common displacement electrode to constitute capacitive elements. 
     (23) The twenty-third feature of the present invention resides in a force sensor having one of the eighteenth to twentieth features described above, wherein 
     at least the measurement target surface on the detection ring is composed of a conductive material, and the detection element is constituted by an eddy current displacement sensor provided on the counter reference surface. 
     (24) The twenty-fourth feature of the present invention resides in a force sensor having one of the eighteenth to twentieth features described above, wherein 
     at least the measurement target surface on the detection ring is composed of a magnet, and the detection element is constituted by a Hall element provided on the counter reference surface. 
     (25) The twenty-fifth feature of the present invention resides in a force sensor having one of the eighteenth to twentieth features described above, wherein 
     the detection element is constituted by 
     a light beam irradiator which is fixed on the counter reference surface to irradiate a light beam obliquely with respect to a measurement target surface, 
     a light beam receiver which is fixed on the counter reference surface to receive the light beam reflected on the measurement target surface, and 
     a measurement circuit which outputs a measured value of distance on the basis of a position at which the light beam is received by the light beam receiver. 
     (26) The twenty-sixth feature of the present invention resides in a force sensor having one of the first to sixteenth features described above, wherein 
     the detection element electrically detects a mechanical strain in vicinity of a predetermined measurement point of the detection ring. 
     (27) The twenty-seventh feature of the present invention resides in a force sensor having the twenty-sixth feature described above, wherein 
     the detection element is constituted by a strain gauge attached onto a surface of the detection ring in vicinity of the measurement point so that a direction along an annular channel along a contour of the detection ring is given as a detection direction. 
     (28) The twenty-eighth feature of the present invention resides in a force sensor having the fifth feature described above, wherein 
     both the force receiving ring and the detection ring are circular rings arranged on the YY-plane so that the Z-axis is given as a central axis, 
     said rings are arranged so that the force receiving ring is outside and the detection ring is inside, 
     a cylindrical fixed assistant body whose lower surface is fixed on the upper surface of the supporting substrate, with the Z-axis being given as a central axis, is provided further inside the detection ring, the detection element comprises: 
     a first capacitive element including a first displacement electrode arranged in vicinity of the first measurement point on an inner circumferential surface of the detection ring and a first fixed electrode arranged at a position facing the first displacement electrode on an outer circumferential surface of the fixed assistant body, 
     a second capacitive element including a second displacement electrode arranged in vicinity of the second measurement point on the inner circumferential surface of the detection ring and a second fixed electrode arranged at a position facing the second displacement electrode on the outer circumferential surface of the fixed assistant body, 
     a third capacitive element including a third displacement electrode arranged in vicinity of the third measurement point on the inner circumferential surface of the detection ring and a third fixed electrode arranged at a position facing the third displacement electrode on the outer circumferential surface of the fixed assistant body, 
     a fourth capacitive element including a fourth displacement electrode arranged in vicinity of the fourth measurement point on the inner circumferential surface of the detection ring and a fourth fixed electrode arranged at a position facing the fourth displacement electrode on the outer circumferential surface of the fixed assistant body, 
     a fifth capacitive element including a fifth displacement electrode arranged in vicinity of the first measurement point on a lower surface of the detection ring and a fifth fixed electrode arranged at a position facing the fifth displacement electrode on the upper surface of the supporting substrate, 
     a sixth capacitive element including a sixth displacement electrode arranged in vicinity of the second measurement point on the lower surface of the detection ring and a sixth fixed electrode arranged at a position facing the sixth displacement electrode on the upper surface of the supporting substrate, 
     a seventh capacitive element including a seventh displacement electrode arranged in vicinity of the third measurement point on the lower surface of the detection ring and a seventh fixed electrode arranged at a position facing the seventh displacement electrode on the upper surface of the supporting substrate, and 
     an eighth capacitive element including an eighth displacement electrode arranged in vicinity of the fourth measurement point on the lower surface of the detection ring and an eighth fixed electrode arranged at a position facing the eighth displacement electrode on the upper surface of the supporting substrate, 
     a projection image of one of a pair of electrodes constituting said capacitive elements projected on a surface on which the other of said pair of electrodes is formed is included in the other electrode, and 
     when a capacitance value of the first capacitive element is given as C 1 , a capacitance value of the second capacitive element is given as C 2 , a capacitance value of the third capacitive element is given as C 3 , a capacitance value of the fourth capacitive element is given as C 4 , a capacitance value of the fifth capacitive element is given as C 5 , a capacitance value of the sixth capacitive element is given as C 6 , a capacitance value of the seventh capacitive element is given as C 7 , and a capacitance value of the eighth capacitive element is given as C 8 , the detection circuit outputs detection values of force Fx in a direction of the X-axis, force Fy in a direction of the Y-axis, force Fz in a direction of the Z-axis, moment Mx around the X-axis, moment My around the Y-axis and moment Mz around the Z-axis on the basis of the following arithmetic expressions:
 
 Fx =( C 1 +C 4)−( C 2 +C 3)
 
 Fy =( C 3 +C 4)−( C 1 +C 2)
 
 Fz =−( C 5 +C 6 +C 7 +C 8)
 
 Mx =( C 7 +C 8)−( C 5 +C 6)
 
 My =( C 5 +C 8)−( C 6 +C 7)
 
 Mz =( C 2 +C 4)−( C 1 +C 3).
 
     (29) The twenty-ninth feature of the present invention resides in a force sensor having the fifth feature described above, wherein 
     both the force receiving ring and the detection ring are circular rings arranged on the XY-plane so that the Z-axis is given as a central axis, 
     said rings are arranged so that the force receiving ring is outside and the detection ring is inside, 
     a cylindrical fixed assistant body whose lower surface is fixed on the upper surface of the supporting substrate, with the Z-axis being given as a central axis, is provided further inside the detection ring, 
     with the first exertion point given as a fifth measurement point and the second exertion point given as a sixth measurement point, the detection element comprises: 
     a first capacitive element including a first displacement electrode arranged in vicinity of the first measurement point on an inner circumferential surface of the detection ring and a first fixed electrode arranged at a position facing the first displacement electrode on an outer circumferential surface of the fixed assistant body, 
     a second capacitive element including a second displacement electrode arranged in vicinity of the second measurement point on the inner circumferential surface of the detection ring and a second fixed electrode arranged at a position facing the second displacement electrode on the outer circumferential surface of the fixed assistant body, 
     a third capacitive element including a third displacement electrode arranged in vicinity of the third measurement point on the inner circumferential surface of the detection ring and a third fixed electrode arranged at a position facing the third displacement electrode on the outer circumferential surface of the fixed assistant body, 
     a fourth capacitive element including a fourth displacement electrode arranged in vicinity of the fourth measurement point on the inner circumferential surface of the detection ring and a fourth fixed electrode arranged at a position facing the fourth displacement electrode on the outer circumferential surface of the fixed assistant body, 
     a fifth capacitive element including a fifth displacement electrode arranged in vicinity of the fifth measurement point on a lower surface of the detection ring and a fifth fixed electrode arranged at a position facing the fifth displacement electrode on the upper surface of the supporting substrate, and 
     a sixth capacitive element including a sixth displacement electrode arranged in vicinity of the sixth measurement point on the lower surface of the detection ring and a sixth fixed electrode arranged at a position facing the sixth displacement electrode on the upper surface of the supporting substrate, 
     a projection image of one of a pair of electrodes constituting said capacitive elements projected on a surface on which the other of said pair of electrodes is formed is included in the other electrode, and 
     when a capacitance value of the first capacitive element is given as C 1 , a capacitance value of the second capacitive element is given as C 2 , a capacitance value of the third capacitive element is given as C 3 , a capacitance value of the fourth capacitive element is given as C 4 , a capacitance value of the fifth capacitive element is given as C 9  and a capacitance value of the sixth capacitive element is given as C 10 , the detection circuit outputs detection values of force Fx in a direction of the X-axis, force Fy in a direction of the Y-axis, force Fz in a direction of the Z-axis, moment My around the Y-axis and moment Mz around the Z-axis on the basis of the following arithmetic expressions:
 
 Fx =( C 1 +C 4)−( C 2 +C 3)
 
 Fy =( C 3 +C 4)−( C 1 +C 2)
 
 Fz =−( C 9 +C 10)
 
 My=C 9 −C 10
 
 Mz =( C 2 +C 4)−( C 1 +C 3).
 
     (30) The thirtieth feature of the present invention resides in a force sensor having the twenty-eighth or twenty-ninth feature described above, wherein 
     the detection ring is composed of a flexible conductive material and a surface of the detection ring is used as a common displacement electrode to constitute each of the capacitive elements. 
     (31) The thirty-first feature of the present invention resides in a force sensor having the fifth feature described above, wherein 
     both the force receiving ring and the detection ring are circular rings arranged on the XY-plane so that the Z-axis is given as a central axis, 
     said rings are arranged so that the force receiving ring is outside and the detection ring is inside, 
     the detection element includes a plurality of strain gauges attached onto surfaces in vicinity of each of the first to the fourth measurement points of the detection ring so that a direction along an annular channel along a contour of the detection ring is a detection direction, 
     when one of an inner circumferential surface and an outer circumferential surface of the detection ring is defined as a laterally arranged surface and one of an upper surface and a lower surface of the detection ring is defined as a longitudinally arranged surface, each of the plurality of strain gauges is constituted by any one of strain gauges having the following eight attributes: 
     a strain gauge having a first attribute which is attached in vicinity of the first measurement point on the laterally arranged surface, 
     a strain gauge having a second attribute which is attached in vicinity of the second measurement point on the laterally arranged surface, 
     a strain gauge having a third attribute which is attached in vicinity of the third measurement point on the laterally arranged surface, 
     a strain gauge having a fourth attribute which is attached in vicinity of the fourth measurement point on the laterally arranged surface, 
     a strain gauge having a fifth attribute which is attached in vicinity of the first measurement point on the longitudinally arranged surface, 
     a strain gauge having a sixth attribute which is attached in vicinity of the second measurement point on the longitudinally arranged surface, 
     a strain gauge having a seventh attribute which is attached in vicinity of the third measurement point on the longitudinally arranged surface, and 
     a strain gauge having an eighth attribute which is attached in vicinity of the fourth measurement point on the longitudinally arranged surface, and 
     a detection circuit outputs: 
     a detection value of force Fx in a direction of the X-axis by a Wheatstone bridge circuit in which a strain gauge having the first attribute and a strain gauge having the fourth attribute are given as first opposite sides, and a strain gauge having the second attribute and a strain gauge having the third attribute are given as second opposite sides, 
     a detection value of force Fy in a direction of the Y-axis by a Wheatstone bridge circuit in which a strain gauge having the first attribute and a strain gauge having the second attribute are given as first opposite sides, and a strain gauge having the third attribute and a strain gauge having the fourth attribute are given as second opposite sides, 
     a detection value of moment Mx around the X-axis by a Wheatstone bridge circuit in which a strain gauge having the fifth attribute and a strain gauge having the sixth attribute are given as first opposite sides, and a strain gauge having the seventh attribute and a strain gauge having the eighth attribute are given as second opposite sides, 
     a detection value of moment My around the Y-axis by a Wheatstone bridge circuit in which a strain gauge having the fifth attribute and a strain gauge having the eighth attribute are given as first opposite sides, and a strain gauge having the sixth attribute and a strain gauge having the seventh attribute are given as second opposite sides, and 
     a detection value of moment Mz around the Z-axis by a Wheatstone bridge circuit in which a strain gauge having the first attribute and a strain gauge having the third attribute are given as first opposite sides, and a strain gauge having the second attribute and a strain gauge having the fourth attribute are given as second opposite sides. 
     (32) The thirty-second feature of the present invention resides in a force sensor having the fifth feature described above, wherein 
     both the force receiving ring and the detection ring are circular rings arranged on the XY-plane so that the Z-axis is given as a central axis, 
     said rings are arranged so that the force receiving ring is outside and the detection ring is inside, 
     the detection elements include a plurality of strain gauges attached onto surface in vicinity of each of the first to the fourth measurement points of the detection ring so that a direction along an annular channel along a contour of the detection ring is a detection direction, 
     when one of an inner circumferential surface and an outer circumferential surface of the detection ring is defined as a laterally arranged surface, and one of an upper surface and a lower surface of the detection ring is defined as a first longitudinally arranged surface and the other of them is defined as a second longitudinally arranged surface, each of the plurality of strain gauges is constituted by any one of strain gauges having the following 12 different attributes: 
     a strain gauge having a first attribute which is attached in vicinity of the first measurement point on the laterally arranged surface, 
     a strain gauge having a second attribute which is attached in vicinity of the second measurement point on the laterally arranged surface, 
     a strain gauge having a third attribute which is attached in vicinity of the third measurement point on the laterally arranged surface, 
     a strain gauge having a fourth attribute which is attached in vicinity of the fourth measurement point on the laterally arranged surface, 
     a strain gauge having a fifth attribute which is attached in vicinity of the first measurement point on the first longitudinally arranged surface of the detection ring, 
     a strain gauge having a sixth attribute which is attached in vicinity of the second measurement point on the first longitudinally arranged surface of the detection ring, 
     a strain gauge having a seventh attribute which is attached in vicinity of the third measurement point on the first longitudinally arranged surface of the detection ring, 
     a strain gauge having an eighth attribute which is attached in vicinity of the fourth measurement point on the first longitudinally arranged surface of the detection ring, 
     a strain gauge having a ninth attribute which is attached in vicinity of the first measurement point on the second longitudinally arranged surface of the detection ring, 
     a strain gauge having a tenth attribute which is attached in vicinity of the second measurement point on the second longitudinally arranged surface of the detection ring, 
     a strain gauge having an eleventh attribute which is attached in vicinity of the third measurement point on the second longitudinally arranged surface of the detection ring, and 
     a strain gauge having a twelfth attribute which is attached in vicinity of the fourth measurement point on the second longitudinally arranged surface of the detection ring, and 
     the detection circuit outputs: 
     a detection value of force Fx in a direction of the X-axis by a Wheatstone bridge circuit in which a strain gauge having the first attribute and a strain gauge having the fourth attribute are given as first opposite sides, and a strain gauge having the second attribute and a strain gauge having the third attribute are given as second opposite sides, 
     a detection value of force Fy in a direction of the Y-axis by a Wheatstone bridge circuit in which a strain gauge having the first attribute and a strain gauge having the second attribute are given as first opposite sides, and a strain gauge having the third attribute and a strain gauge having the fourth attribute are given as second opposite sides, 
     a detection value of force Fz in a direction of the Z-axis by a Wheatstone bridge circuit in which a serial connection side of a strain gauge having the fifth attribute with a strain gauge having the sixth attribute and a serial connection side of a strain gauge having the seventh attribute with a strain gauge having the eighth attribute are given as first opposite sides, and a serial connection side of a strain gauge having the ninth attribute with a strain gauge having the tenth attribute and a serial connection side of a strain gauge having the eleventh attribute with a strain gauge having the twelfth attribute are given as second opposite sides, 
     a detection value of moment Mx around the X-axis by a Wheatstone bridge circuit in which a strain gauge having the fifth attribute and a strain gauge having the sixth attribute are given as first opposite sides, and a strain gauge having the seventh attribute and a strain gauge having the eighth attribute are given as second opposite sides, 
     a detection value of moment My around the Y-axis by a Wheatstone bridge circuit in which a strain gauge having the fifth attribute and a strain gauge having the eighth attribute are given as first opposite sides, and a strain gauge having the sixth attribute and a strain gauge having the seventh attribute are given as second opposite sides, and 
     a detection value of moment Mz around the Z-axis by a Wheatstone bridge circuit in which a strain gauge having the first attribute and a strain gauge having the third attribute are given as first opposite sides, and a strain gauge having the second attribute and a strain gauge having the fourth attribute are given as second opposite sides. 
     (33) The thirty-third feature of the present invention resides in a force sensor having the eighth feature described above, wherein 
     both the force receiving ring and the detection ring are circular rings arranged on the YY-plane so that the Z-axis is given as a central axis, 
     said rings are arranged so that the force receiving ring is outside and the detection ring is inside, 
     a cylindrical fixed assistant body whose lower surface is fixed on the upper surface of the supporting substrate, with the Z-axis being given as a central axis, is provided further inside the detection ring, 
     the detection element comprises: 
     a first capacitive element group including a plurality of capacitive elements constituted by a first displacement electrode group including a plurality of displacement electrodes arranged at a first diaphragm on an inner circumferential surface of the detection ring and a first fixed electrode group including a plurality of fixed electrodes arranged at positions facing the respective displacement electrodes of the first displacement electrode group on an outer circumferential surface of the fixed assistant body, 
     a second capacitive element group including a plurality of capacitive elements constituted by a second displacement electrode group including a plurality of displacement electrodes arranged at a second diaphragm on the inner circumferential surface of the detection ring and a second fixed electrode group including a plurality of fixed electrodes arranged at positions facing the respective displacement electrodes of the second displacement electrode group on the outer circumferential surface of the fixed assistant body, 
     a third capacitive element group including a plurality of capacitive elements constituted by a third displacement electrode group including a plurality of displacement electrodes arranged at a third diaphragm on the inner circumferential surface of the detection ring and a third fixed electrode group including a plurality of fixed electrodes arranged at positions facing the respective displacement electrodes of the third displacement electrode group on the outer circumferential surface of the fixed assistant body, and 
     a fourth capacitive element group including a plurality of capacitive elements constituted by a fourth displacement electrode group including a plurality of displacement electrodes arranged at a fourth diaphragm on the inner circumferential surface of the detection ring and a fourth fixed electrode group including a plurality of fixed electrodes arranged at positions facing the respective displacement electrodes of the fourth displacement electrode group on the outer circumferential surface of the fixed assistant body, 
     a projection image of one of a pair of electrodes constituting said capacitive elements projected on a surface on which the other of said pair of electrodes is formed is included in the other electrode, and 
     the detection circuit outputs a detection value on the basis of a capacitance value of each of the capacitive elements. 
     (34) The thirty-fourth feature of the present invention resides in a force sensor having the thirty-third feature described above, wherein 
     the first capacitive element group includes an on-axis capacitive element of the first group arranged on the X-axis, a first capacitive element of the first group arranged in a positive direction of the Y-axis adjacent to the on-axis capacitive element of the first group, a second capacitive element of the first group arranged in a negative direction of the Y-axis adjacent to the on-axis capacitive element of the first group, a third capacitive element of the first group arranged in a positive direction of the Z-axis adjacent to the on-axis capacitive element of the first group, and a fourth capacitive element of the first group arranged in a negative direction of the Z-axis adjacent to the on-axis capacitive element of the first group, 
     the second capacitive element group includes an on-axis capacitive element of the second group arranged on the Y-axis, a first capacitive element of the second group arranged in a positive direction of the X-axis adjacent to the on-axis capacitive element of the second group, a second capacitive element of the second group arranged in a negative direction of the X-axis adjacent to the on-axis capacitive element of the second group, a third capacitive element of the second group arranged in the positive direction of the Z-axis adjacent to the on-axis capacitive element of the second group, and a fourth capacitive element of the second group arranged in the negative direction of the Z-axis adjacent to the on-axis capacitive element of the second group, 
     the third capacitive element group includes an on-axis capacitive element of the third group arranged on the X-axis, a first capacitive element of the third group arranged in the positive direction of the Y-axis adjacent to the on-axis capacitive element of the third group, a second capacitive element of the third group arranged in the negative direction of the Y-axis adjacent to the on-axis capacitive element of the third group, a third capacitive element of the third group arranged in the positive direction of the Z-axis adjacent to the on-axis capacitive element of the third group, and a fourth capacitive element of the third group arranged in the negative direction of the Z-axis adjacent to the on-axis capacitive element of the third group, and 
     the fourth capacitive element group includes an on-axis capacitive element of the fourth group arranged on the Y-axis, a first capacitive element of the fourth group arranged in the positive direction of the X-axis adjacent to the on-axis capacitive element of the fourth group, a second capacitive element of the fourth group arranged in the negative direction of the X-axis adjacent to the on-axis capacitive element of the fourth group, a third capacitive element of the fourth group arranged in the positive direction of the Z-axis adjacent to the on-axis capacitive element of the fourth group, and a fourth capacitive element of the fourth group arranged in the negative direction of the Z-axis adjacent to the on-axis capacitive element of the fourth group, and 
     when a capacitance value of the first capacitive element of the first group is given as C 11 , a capacitance value of the second capacitive element of the first group is given as C 12 , a capacitance value of the third capacitive element of the first group is given as C 13 , a capacitance value of the fourth capacitive element of the first group is given as C 14  and a capacitance value of the on-axis capacitive element of the first group is given as C 15 , when a capacitance value of the first capacitive element of the second group is given as C 21 , a capacitance value of the second capacitive element of the second group is given as C 22 , a capacitance value of the third capacitive element of the second group is given as C 23 , a capacitance value of the fourth capacitive element of the second group is given as C 24 , and a capacitance value of the on-axis capacitive element of the second group is given as C 25 , 
     when a capacitance value of the first capacitive element of the third group is given as C 31 , a capacitance value of the second capacitive element of the third group is given as C 32 , a capacitance value of the third capacitive element of the third group is given as C 33 , a capacitance value of the fourth capacitive element of the third group is given as C 34 , and a capacitance value of the on-axis capacitive element of the third group is given as C 35 , and 
     when a capacitance value of the first capacitive element of the fourth group is given as C 41 , a capacitance value of the second capacitive element of the fourth group is given as C 42 , a capacitance value of the third capacitive element of the fourth group is given as C 43 , a capacitance value of the fourth capacitive element of the fourth group is given as C 44 , and a capacitance value of the on-axis capacitive element of the fourth group is given as C 45 , 
     the detection circuit outputs detection values of force Fx in a direction of the X-axis, force Fy in a direction of the Y-axis, force Fz in a direction of the Z-axis, moment Mx around the X-axis, moment. My around the Y-axis, and moment Mz around the Z-axis, on the basis of the following arithmetic expressions: 
     
       
         
           
             
               
                 
                   Fx 
                   = 
                     
                   ⁢ 
                   
                     
                       - 
                       
                         ( 
                         
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             12 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             13 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             14 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             15 
                           
                         
                         ) 
                       
                     
                     + 
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           31 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           32 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           33 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           34 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           35 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     or 
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       - 
                       
                         ( 
                         
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             12 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             13 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             14 
                           
                         
                         ) 
                       
                     
                     + 
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           31 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           32 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           33 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           34 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     or 
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       
                         - 
                         C 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       15 
                     
                     + 
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       35 
                     
                   
                 
               
             
           
         
       
       
         
           
             
               
                 
                   Fy 
                   = 
                     
                   ⁢ 
                   
                     
                       - 
                       
                         ( 
                         
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             21 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             22 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             23 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             24 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             25 
                           
                         
                         ) 
                       
                     
                     + 
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           41 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           42 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           43 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           44 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           45 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     or 
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       - 
                       
                         ( 
                         
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             21 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             22 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             23 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             24 
                           
                         
                         ) 
                       
                     
                     + 
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           41 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           42 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           43 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           44 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     or 
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       
                         - 
                         C 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       25 
                     
                     + 
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       45 
                     
                   
                 
               
             
           
         
       
       
         
           
             Fz 
             = 
             
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     23 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     33 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     43 
                   
                 
                 ) 
               
               - 
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     14 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     24 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     34 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     44 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             
               M 
               ⁢ 
               
                   
               
               ⁢ 
               x 
             
             = 
             
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     23 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     44 
                   
                 
                 ) 
               
               - 
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     24 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     43 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             My 
             = 
             
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     14 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     33 
                   
                 
                 ) 
               
               - 
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     34 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             Mz 
             = 
             
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     21 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     32 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     42 
                   
                 
                 ) 
               
               - 
               
                 
                   ( 
                   
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       12 
                     
                     + 
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       22 
                     
                     + 
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       31 
                     
                     + 
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       41 
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     (35) The thirty-fifth feature of the present invention resides in a force sensor having the thirty-third or thirty-fourth feature described above, wherein 
     at least the diaphragms of the detection ring are composed of flexible conductive material, and a surface of a diaphragm is used as a common displacement electrode to constitute each of capacitive elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view (the upper part of the figure) and a side view (the lower part of the figure) showing a basic structure of a force sensor according to a basic embodiment of the present invention. 
         FIG. 2  is a cross sectional view (the upper part of the figure) of the basic structure shown in  FIG. 1  which is cut along an XY-plane and a longitudinal sectional view (the lower part of the figure) of the basic structure which is cut along an XZ-plane thereof. 
         FIG. 3  is a top view (the upper part of the figure) of a supporting substrate  300  and fixing members  510 ,  520  of the basic structure shown in  FIG. 1  and a longitudinal sectional view (the lower part of the figure) in which the basic structure is cut along a YZ-plane. 
         FIG. 4  is a cross sectional view (the upper part of the figure) on the XY-plane and a longitudinal sectional view (the lower part of the figure) on the XZ-plane, each of which shows a deformed state when force +Fx in the positive direction of the X-axis is exerted on a force receiving ring  100  of the basic structure shown in  FIG. 1 . 
         FIG. 5  is a longitudinal sectional view on the XZ-plane showing a deformed state when force +Fz in the positive direction of the Z-axis is exerted on the force receiving ring  100  of the basic structure shown in  FIG. 1 . 
         FIG. 6  is a longitudinal sectional view on the XZ-plane showing a deformed state when moment +My which is positive rotation around the Y-axis is exerted on the force receiving ring  100  of the basic structure shown in  FIG. 1 . 
         FIG. 7  is a cross sectional view on the XY-plane which shows a deformed state when moment +Mz which is positive rotation around the Z-axis is exerted on the force receiving ring  100  of the basic structure shown in  FIG. 1 . 
         FIG. 8  is a top view (the upper part of the figure) and a side view (the lower part of the figure), each of which shows an embodiment in which the fixed assistant body  350  for detecting displacement is added to the basic structure shown in  FIG. 1 . 
         FIG. 9  is a cross sectional view (the upper part of the figure) in which the basic structure shown in  FIG. 8  is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which the basic structure is cut along a VZ-plane. 
         FIG. 10  is a top view showing distance measurement sites in the basic structure shown in  FIG. 8 . 
         FIG. 11  is a table which shows changes in distances d 1  to d 8  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the basic structure shown in  FIG. 10 . 
         FIG. 12  is a top view showing a modification example of the distance measurement sites in the basic structure shown in  FIG. 8 . 
         FIG. 13  is a table showing changes in distances d 1  to d 4 , d 9 , d 10  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the basic structure shown in  FIG. 12 . 
         FIG. 14  is a cross sectional view (the upper part of the figure) in which a force sensor according to an embodiment using capacitive elements is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which the force sensor is cut along the VZ-plane. 
         FIG. 15  is a perspective view showing a dimensional relationship of a counter electrode of each of the capacitive elements used in the force sensor shown in  FIG. 14 . 
         FIG. 16  is a table showing changes in capacitance values of capacitive elements C 1  to C 8  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force sensor shown in  FIG. 14 . 
         FIG. 17  shows arithmetic expressions for determining force in the direction of each coordinate axis and moment around each coordinate axis which are exerted on the force sensor shown in  FIG. 14 . 
         FIG. 18  is a circuit diagram showing detection circuits used in the force sensor shown in  FIG. 14 . 
         FIG. 19  is a table showing changes in capacitance values of capacitive elements C 1  to C 4 , C 9 , C 10  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force sensor of the modification example shown in  FIG. 12 . 
         FIG. 20  shows arithmetic expressions for determining force in the direction of each coordinate axis and moment around each coordinate axis which are exerted on the force sensor of the modification example shown in  FIG. 12 . 
         FIG. 21  is a cross sectional view (the upper part of the figure) in which a force sensor according to a modification example which uses a detection ring  200  itself constituted by an electrically conductive material as a plurality of displacement electrodes is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which the force sensor is cut along the VZ-plane. 
         FIG. 22  is a cross sectional view in which a force sensor according to an embodiment using strain gauges is cut along the XY-plane (the respective strain gauges G 1  to G 8  shown in the figure are actually constituted by a plurality of strain gauges in parallel with each other). 
         FIG. 23  is a table showing changes in electric resistance of the strain gauges G 1  to G 8  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force sensor shown in  FIG. 22 . 
         FIG. 24  is a table showing specific measured values of stress (unit: MPa) applied to the strain gauges G 1  to G 8  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force sensor shown in  FIG. 22 . 
         FIG. 25  is a top view of a force sensor according to a modification example in which strain gauges G 9  to G 12  are additionally added to the embodiment shown in  FIG. 22 . 
         FIG. 26  shows circuit diagrams, each of which shows a detection circuit for detecting force in the direction of each coordinate axis in the force sensor of the modification example shown in  FIG. 25 . 
         FIG. 27  shows circuit diagrams, each of which shows a detection circuit for detecting moment around each coordinate axis in the force sensor of the modification example shown in  FIG. 25 . 
         FIG. 28  is a longitudinal sectional view on the XZ-plane showing a state in which a force receiving substrate  600  is added to the basic structure shown in  FIG. 1 . 
         FIG. 29  is a top view showing a modification example in which a method for fixing the detection ring  200  to the basic structure shown in  FIG. 1  is changed. 
         FIG. 30  is a top view showing an example in which a displacement control structure is added to the basic structure shown in  FIG. 1 . 
         FIG. 31  is a longitudinal sectional view in which the example shown in  FIG. 30  is cut along the XZ-plane. 
         FIG. 32  is a top view showing a force sensor according to a practical embodiment using capacitive elements. 
         FIG. 33  is a longitudinal sectional view in which the force sensor shown in  FIG. 32  is cut along the XZ-plane. 
         FIG. 34  is a longitudinal sectional view in which the force sensor shown in  FIG. 32  is cut along the VZ-plane. 
         FIG. 35  is a cross sectional view (the upper part of the figure) in which a basic structure of an embodiment having diaphragms is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which the basic structure is cut along the XZ-plane. 
         FIG. 36  is a longitudinal sectional view in which the basic structure shown in  FIG. 35  is cut along the VZ-plane. 
         FIG. 37  is a cross sectional view in which a force sensor constituted by adding capacitive elements to the basic structure shown in  FIG. 35  is cut along the XY-plane. 
         FIG. 38  is a table showing an electrode constitution of each of the capacitive elements used in the force sensor shown in  FIG. 37  (showing a state when viewed respectively from view points of e 1  to e 4 , with the positive direction of the Z-axis taken upward). 
         FIG. 39  is a table showing changes in capacitance values of capacitive elements C 11  to C 45  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force sensor shown in  FIG. 37 . 
         FIG. 40  is a longitudinal sectional view on the XZ-plane showing a deformed state when force +Fz in the positive direction of the Z-axis is exerted on the force sensor shown in  FIG. 37  (for the sake of convenience of explanation, each part is depicted with deformation). 
         FIG. 41  is a cross sectional view on the XY-plane showing a deformed state when moment +Mz which is positive rotation around the Z-axis is exerted on the force sensor shown in  FIG. 37  (for the sake of convenience of explanation, each part is depicted with deformation). 
         FIG. 42  shows arithmetic expressions for determining force in the direction of each coordinate axis and moment around each coordinate axis which is exerted on the force sensor shown in  FIG. 37 . 
         FIG. 43  is a front view showing a modification example of the diaphragm of the force sensor shown in  FIG. 37 . 
         FIG. 44  is a plan view showing modification examples of electrode groups used in the force sensor shown in  FIG. 37  (hatching is made to clarify the shape of each electrode and not for showing the cross section). 
         FIG. 45  is a cross sectional view in which a force sensor according to a modification example using an eddy current displacement sensor/a Hall element/a light beam distance meter is cut along the XY-plane. 
         FIG. 46  shows a perspective view and a block diagram for explaining a principle of measuring a distance by using the eddy current displacement sensor. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, the present invention will be described based on illustrated embodiments. 
     &lt;&lt;&lt;Section 1. Basic Structure and Principle of Force Sensor&gt;&gt;&gt; 
     First, a description will be given about a constitution of a basic structure of a force sensor according to the present invention and a principle of detecting force and moment by utilizing the basic structure. A force sensor according to the present invention has functions to detect force or moment at least on one axis among force in the direction of each coordinate axis and moment around each coordinate axis in an XYZ three-dimensional orthogonal coordinate system. Therefore, hereinafter, a description will be given of a constitution of the basic structure of the force sensor which is arranged in the XYZ three-dimensional orthogonal coordinate system. 
       FIG. 1  is a top view (the upper part of the figure) and a side view (the lower part of the figure) for showing the basic structure of the force sensor according to a basic embodiment of the present invention. In the top view, the X-axis is placed on the right side in the figure, the Y-axis is placed upward in the figure, and the Z-axis is placed in the front-side direction perpendicular to the sheet surface of the figure. On the other hand, in the side view, the X-axis is placed on the right side, the Z-axis is placed upward in the figure, and the Y-axis is placed in a depth direction perpendicular to the sheet surface of the figure. As shown in the figure, the basic structure is constituted by a force receiving ring  100 , a detection ring  200 , a supporting substrate  300 , connection members  410 ,  420  and fixing members  510 ,  520 . 
     The force receiving ring  100  is a circular plate-shaped (washer-shaped) ring arranged on the XY-plane so that the Z-axis is given as a central axis, and both the outer circumferential surface and the inner circumferential surface thereof are constituted so as to form a cylindrical surface. The force receiving ring  100  has functions to receive exertions of force or moment to be detected. More particularly, it has functions to transmit the force or moment to be detected to the detection ring  200 . 
     On the other hand, the detection ring  200  is a circular plate-shaped (washer-shaped) ring arranged on the XY-plane, as with the force receiving ring  100 , so that the Z-axis is given as the central axis. Both the outer circumferential surface and the inner circumferential surface are constituted so as to form a cylindrical surface. In the example shown here, the detection ring  200  is arranged inside the force receiving ring  100 . That is, the force receiving ring  100  is an outer ring arranged on the XY-plane, while the detection ring  200  is an inner ring arranged on the XY-plane. Here, the detection ring  200  is characterized in that elastic deformation is caused at least partially due to exertions of the force or moment to be detected. 
     The connection members  410 ,  420  are members for connecting the force receiving ring  100  with the detection ring  200 . In the example shown in the figure, the connection member  410  connects the inner circumferential surface of the force receiving ring  100  with the outer circumferential surface of the detection ring  200  at a position along a positive domain of the X-axis. The connection member  420  connects the inner circumferential surface of the force receiving ring  100  with the outer circumferential surface of the detection ring  200  at a position along a negative domain of the X-axis. Therefore, as shown in the figure, a clearance H 1  is secured between the force receiving ring  100  and the detection ring  200 , and a clearance H 2  is secured inside the detection ring  200 , as shown in the figure. 
     As is apparent from the side view, the force receiving ring  100  and the detection ring  200  are equal in thickness (dimension in the direction of the Z-axis). In the side view, the detection ring  200  is completely hidden inside the force receiving ring  100 . Although both of the rings are not necessarily required to be equal in thickness, it is preferable to make both the rings equal in thickness in view of realizing a thin-type sensor (sensor that is dimensionally reduced in the direction of the Z-axis as much as possible). 
     A supporting substrate  300  is a disk-shaped substrate, the diameter of which is equal to an outer diameter of the force receiving ring  100 , having an upper surface in parallel with the XY-plane and being arranged at certain intervals below the force receiving ring  100  and the detection ring  200 . The fixing members  510 ,  520  are members for fixing the detection ring  200  to the supporting substrate  300 . In the side view, the fixing member  510  is hidden behind the fixing member  520  and does not appear. The fixing members  510 ,  520  have functions to connect the lower surface of the detection ring  200  with the upper surface of the supporting substrate  300 . As depicted by broken lines in the top view, fixing member  510 ,  520  are arranged along the Y-axis. 
       FIG. 2  is a cross sectional view (the upper part of the figure) in which the basic structure shown in  FIG. 1  is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which it is cut along the XZ-plane. An origin O of the XYZ three-dimensional orthogonal coordinate system is shown at the center of the cross sectional view which is cut along the XY-plane. Here,  FIG. 2  clearly shows a state that the detection ring  200  is connected to the force receiving ring  100  via connection members  410 ,  420  arranged at two sites on both sides. 
       FIG. 3  is a top view (the upper part of the figure) showing the supporting substrate  300  and the fixing members  510 ,  520  of the basic structure shown in  FIG. 1  and a longitudinal sectional view (the lower part of the figure) in which the basic structure is cut along the YZ-plane. The top view of  FIG. 3  corresponds to a state that the top view of  FIG. 1  is rotated counterclockwise by 90 degrees, with the Y-axis taken on the left side. Further, in the top view of  FIG. 3 , the position of the detection ring  200  is depicted by the broken lines. On the other hand, the longitudinal sectional view of  FIG. 3  clearly shows a state that the detection ring  200  is fixed by the fixing members  510 ,  520  above the supporting substrate  300 . 
     Then, a description will be given of a principle of detecting force and moment by utilizing the basic structure. First, consideration will be given to a situation found at the basic structure when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed. 
     As described above, in the example shown here, both the force receiving ring  100  and the detection ring  200  are circular rings arranged on the XY-plane in such a manner that the Z-axis is given as the central axis. However, the detection ring  200  is required to undergo at least partially elastic deformation resulting from exertions of force or moment to be detected. In other words, the detection ring  200  is required to be at least partially flexible. This is because the force sensor according to the present invention is to detect force or moment which is exerted on the basis of elastic deformation occurring on the detection ring  200 . 
     On the other hand, as described above, the force receiving ring  100  is a constituent which has functions to transmit the force or the moment exerted on the detection ring  200 . The force receiving ring  100  may be in principle constituted by an elastic body which undergoes elastic deformation or may be composed of a rigid body which does not undergo elastic deformation. However, in practice, it is preferable that the force receiving ring  100  is composed of a rigid body which does not undergo substantial deformation, as long as exerting force or moment is within a predetermined tolerance level. This is because the exerting force or moment is transmitted to the detection ring  200  efficiently as much as possible. 
     In the present invention, each part of the basic structure can be constituted by any given material. With commercial usage taken into account, it is preferable to constitute each part by using generally-used industrial materials such as metals (aluminum alloy and iron-based metal, for example) and plastics. A member composed of a generally-used industrial material is usually available as an elastic body or a rigid body, depending on a form thereof. For example, in the case of a metal, a metal block behaves like a rigid body, while a thin plate metal behaves like an elastic body. Therefore, the force receiving ring  100  and the detection ring  200  are able to perform any given functions by changing the respective forms, even when they are constituted by the same material. 
     For example, even where the force receiving ring  100  and the detection ring  200  are constituted by the same aluminum alloy, as shown in the cross sectional view of  FIG. 2 , the force receiving ring  100  is able to function as a rigid body which is practically free of elastic deformation by being made relatively great in width. The detection ring  200  is able to function as an elastic body which undergoes elastic deformation substantially as a whole by being made relatively small in width. That is, the detection ring  200  can be used as a ring which is flexible as a whole. 
     As a matter of course, when force and moment are applied to the force receiving ring  100 , specifically, the force receiving ring  100  itself undergoes some elastic deformation. The force receiving ring  100  undergoes elastic deformation and is negligible if the elastic deformation is only slight as compared with the elastic deformation of the detection ring  200 . And, the force receiving ring  100  is to be considered as a practically rigid body. Therefore, hereinafter, a description will be given on the assumption that the force receiving ring  100  is a rigid body and elastic deformation resulting from force and moment occurs exclusively on the detection ring  200 . 
     First, consideration will be given to a change of the basic structure when force in a direction of the X-axis is exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed.  FIG. 4  is a cross sectional view (the upper part of the figure) on the XY-plane and a longitudinal sectional view (the lower part of the figure) on the XZ-plane, each of which shows a deformed state when force +Fx in the positive direction of the X-axis is exerted on the force receiving ring  100  of the basic structure shown in  FIG. 1 . The supporting substrate  300  is fixed and not movable, while the force receiving ring  100  moves to the right hand side in the figure by the force +Fx in the positive direction of the X-axis. As a result, the detection ring  200  undergoes deformation as shown in the figure. In addition, the broken lines in the figure show the positions of these rings before movement or deformation. 
     Here, for the sake of convenience of description of the deformation mode, consideration will be given to two fixing points P 1 , P 2  and two exertion points Q 1 , Q 2 . The fixing points P 1 , P 2  are points defined on the Y-axis and correspond to positions of the fixing members  510 ,  520  shown in  FIG. 1 . That is, the detection ring  200  is fixed to the supporting substrate  300  by the fixing members  510 ,  520  at the fixing points P 1 , P 2 . On the other hand, the exertion points Q 1 , Q 2  are points which are defined on the X-axis, and the detection ring  200  is connected to the force receiving ring  100  by the connection members  410 ,  420  at the exertion points Q 1 , Q 2 . 
     As described above, in the present invention, the exertion points are positions to which the connection members are connected, and the fixing points are positions to which the fixing members are connected. Then, it is important that the exertion points and the fixing points are arranged at different positions. In the example shown in  FIG. 4 , the fixing points P 1 , P 2  and the exertion points Q 1 , Q 2  are arranged at different positions on the XY-plane. This is because when the exertion points and the fixing points occupy the same positions, the detection ring  200  undergoes no elastic deformation. In addition, in the above example, the fixing points P 1 , P 2  and the exertion points Q 1 , Q 2  are all defined on the XY-plane. However, the exertion points and the fixing points are not necessarily defined on the XY-plane. Irrespective of whether the exertion points and the fixing points are on the XY-plane or not, only if an orthographic projection image of the exertion points on the XY-plane and an orthographic projection image of the fixing points on the XY-plane are formed at different positions, elastic deformation necessary for the present invention is allowed to occur on the detection ring  200 . 
     When the force +Fx in the positive direction of the X-axis is exerted on the force receiving ring  100 , as shown in  FIG. 4 , the force on the right-hand side in the figure is applied to the exertion points Q 1 , Q 2  of the detection ring  200 . However, since the fixing points P 1 , P 2  of the detection ring  200  are fixed, the detection ring  200  which is flexible is deformed from a reference circular state into a deformed state as shown in the figure (figures which show a deformed state in the present application are depicted to some extent in an exaggerated manner for emphasizing the deformed state and they do not necessarily show the deformation mode precisely). To be more specific, as shown in the figure, between the point P 1  and the point Q 1  as well as between the point P 2  and the point Q 1 , a tensile force is exerted on both ends of a quadrant of the detection ring  200 , by which the quadrant shrinks inwardly. Between the point P 1  and the point Q 2  as well as between the point P 2  and the point Q 2 , a pressing force is exerted on the both ends of the quadrant of the detection ring  200 , by which the quadrant swells outwardly. 
     Where force −Fx in the negative direction of the X-axis is exerted on the force receiving ring  100 , a phenomenon occurs in which the left and right sides are reversed to that shown in  FIG. 4 . Further, where force +Fy in the positive direction of the Y-axis and force −Fy in the negative direction of the Y-axis are exerted on the force receiving ring  100 , a phenomenon occurs in which a deformed state at the upper part of  FIG. 4  is rotated by 90 degrees. 
     Next, consideration will be given to a change of the basic structure when force in a direction of the Z-axis is exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed.  FIG. 5  is a longitudinal sectional view on the XZ-plane which shows a deformed state when force +Fz in the positive direction of the Z-axis is exerted on the force receiving ring  100  of the basic structure shown in  FIG. 1 . Although the supporting substrate  300  is fixed and therefore not movable, the force receiving ring  100  moves upward in the figure due to the force +Fz in the positive direction of the Z-axis. As a result, the detection ring  200  undergoes deformation as shown in the figure. Here, the broken line in the figure depicts a position of each ring before movement or deformation. 
     In this case as well, the deformation mode is fundamentally based on the fact that positions of the two fixing points P 1 , P 2  (positions fixed by the fixing members  510 ,  520 ) are not movable and positions of the two exertion points Q 1 , Q 2  move upward. The detection ring  200  gradually undergoes deformation from the positions of the fixing points P 1 , P 2  to those of the exertion points Q 1 , Q 2 . Further, where the force −Fz in the negative direction of the Z-axis is exerted on the force receiving ring  100 , the force receiving ring  100  moves downward in the figure. As a result, the detection ring  200  shows a deformation mode which is upside down as compared with the deformation mode shown in  FIG. 5 . 
     Next, consideration will be given to a change of the basic structure, when moment around the Y-axis is exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed.  FIG. 6  is a longitudinal sectional view on the XZ-plane showing a deformed state when moment+My which is positive rotation around the Y-axis is exerted on the force receiving ring  100  of the basic structure shown in  FIG. 1 . By the way, in the present application, a symbol of moment exerting around a predetermined coordinate axis is determined so that a rotation direction in which a right-hand screw rotates for allowing the screw to advance in a positive direction of the coordinate axis is given as a positive direction. For example, the rotation direction of moment +My shown in  FIG. 6  is a rotation direction in which the right-hand screw is allowed to advance in the positive direction of the Y-axis. 
     In this case as well, although the supporting substrate  300  is fixed and therefore not movable, the force receiving ring  100  receives the moment +My which is positive rotation around the Y-axis and rotates clockwise around the origin O in the figure. As a result, the exertion point Q 1  moves downward, while the exertion point Q 2  moves upward. The detection ring  200  gradually undergoes deformation from the positions of the fixing points P 1 , P 2  (positions fixed by the fixing members  510 ,  520 ) to the positions of the exertion points Q 1 , Q 2 . Where the moment −My which is negative rotation around the Y-axis is exerted on the force receiving ring  100 , a phenomenon occurs in which the left and right sides are reversed to that shown in  FIG. 6 . Further, where the moment +Mx which is positive rotation around the X-axis and moment −Mx which is negative rotation around the X-axis are exerted on the force receiving ring  100 , a phenomenon occurs in which the deformed state shown in the top view is rotated by 90 degrees. 
     Finally, consideration will be given to a change of the basic structure when moment around the Z-axis is exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed.  FIG. 7  is a cross sectional view on the XY-plane showing a deformed state when moment +Mz which is positive rotation around the Z-axis exerted on the force receiving ring  100  of the basic structure shown in  FIG. 1 . In this case as well, although the supporting substrate  300  is fixed and therefore not movable, the force receiving ring  100  receives the moment +Mz which is positive rotation around the Z-axis and rotates counterclockwise around the origin O in the figure. 
     As a result, counterclockwise force as shown in the figure is applied to the exertion points Q 1 , Q 2  on the detection ring  200 . However, since the positions of the fixing points P 1 , P 2  on the detection ring  200  are fixed, the detection ring  200  which is flexible is to undergo deformation from a reference circular state into a deformed state, as shown in the figure. To be more specific, as shown in the figure, between the point P 2  and the point Q 1  as well as between the point P 1  and the point Q 2 , a tensile force is exerted on both ends of a quadrant of the detection ring  200 , by which the quadrant shrinks inwardly. Between the point P 1  and the point Q 1  as well as between the point P 2  and the point Q 2 , a pressing force is exerted on the both ends of the quadrant of the detection ring  200 , by which the quadrant swells outwardly and undergoes deformation into an oval shape as a whole. On the other hand, where moment −Mz which is negative rotation around the Z-axis is exerted on the force receiving ring  100 , the force receiving ring  100  rotates clockwise around the origin O in the figure. A deformed state occurs in which the state shown in  FIG. 7  is reversed. 
     A description has been so far given of the deformation modes occurring on the detection ring  200  where force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force receiving ring  100 , with the supporting substrate  300  of the basic structure shown in  FIG. 1  being fixed. The deformation modes are different from each other and also different in extent of deformation depending on a magnitude of the force and moment which are exerted. Therefore, an elastic deformation of the detection ring  200  is detected to collect information on the mode and extent thereof, thus making it possible to detect the force in the direction of each coordinate axis and the moment around each coordinate axis individually and independently. This is a basic principle of the force sensor according to the present invention. 
     In order to conduct detection based on the above-described basic principle, the force sensor according to the present invention is provided, in addition to the basic structure shown in  FIG. 1 , with detection elements for electrically detecting elastic deformation of the detection ring  200  and detection circuits for outputting detection values of force in a direction of a predetermined coordinate axis or moment around a predetermined coordinate axis which is exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed, on the basis of detection results of the detection elements. A detailed description will be given of examples of specific constitutions of the detection elements and the detection circuits in Section 3 and subsequent sections. 
     As will be described later, the detection elements and the detection circuits can be incorporated into the basic structure shown in  FIG. 1 , by which the size of the force sensor according to the present invention can be made substantially equal to that of the basic structure. As apparent from the side view shown at the lower part of  FIG. 1 , the basic structure is structurally suitable for being made thin. That is, an entire thickness of the basic structure (dimension in the direction of the Z-axis) is a sum of the thickness of the force receiving ring  100  (thickness of the detection ring  200 ), that of the fixing members  510 ,  520  and that of the supporting substrate  300 . Here, the detection ring  200  may be set to be sufficiently thick in arranging detection elements to be described later. The fixing members  510 ,  520  may be set to be sufficiently thick so as not to inhibit downward deformation of the detection ring  200 . And, the supporting substrate  300  may be set to be sufficiently thick so as to support other constituents. 
     Therefore, the force sensor according to the present invention has functions to detect individually and independently force in the direction of each coordinate axis and moment around each coordinate axis in an XYZ three-dimensional orthogonal coordinate system. Further, this force sensor can be made simpler in structure and thinner in thickness than a conventional force sensor. 
     The basic structure shown in  FIG. 1  is an example in which the force receiving ring  100  is arranged outside and the detection ring  200  is arranged inside. However, a positional relationship between the rings can be exchanged. That is, a constitution in which the force receiving ring  100  is arranged inside and the detection ring  200  is arranged outside can be adopted. However, when the detection ring  200  in a deformed state comes into contact with an external object, disturbance is added to a deformation mode. This may result in a failure of obtaining a correct detection value. Therefore, in practice, it is preferable that the force receiving ring  100  is arranged outside and the detection ring  200  is arranged inside, as shown in the above example, thereby preventing the detection ring  200  from coming into contact with the external object. 
     &lt;&lt;&lt;Section 2. Detection of Displacement&gt;&gt;&gt; 
     As described above, in the force sensor according to the present invention, a mode and a magnitude of elastic deformation occurring on the detection ring  200  are detected, thereby determining a direction and a magnitude of exerted force or moment. Now, as a method for detecting a mode and a magnitude of elastic deformation, a method for detecting displacement at specific sites on the detection ring  200  (here, referred to as measurement points) will be described. In the embodiment shown here, as a detection element, used is an element having functions to electrically detect displacement of the detection ring  200  at predetermined measurement points. 
       FIG. 8  is a top view (the upper part of the figure) and a side view (the lower part of the figure), each of which shows an embodiment in which a fixed assistant body  350  for detecting displacement is added to the basic structure shown in  FIG. 1 . As shown in the figure, the detection ring  200  is arranged inside the force receiving ring  100  and the fixed assistant body  350  is arranged further inside in the basic structure. The fixed assistant body  350  is a cylindrical object having the Z-axis as a central axis, the lower surface of which is fixed on the upper surface of the supporting substrate  300 . An outer circumferential surface of the fixed assistant body  350  faces an inner circumferential surface of the detection ring  200 , with a clearance H 2  held therebetween. 
       FIG. 9  is a cross sectional view (the upper part of the figure) in which the basic structure shown in  FIG. 8  is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which the basic structure is cut along the VZ-plane. Here, the V-axis is an axis which passes through the origin O of the XYZ three-dimensional orthogonal coordinate system and forms 45 degrees with respect to the X-axis, wherein its positive domain positions at a first quadrant of the XY-plane and its negative domain positions at a third quadrant of the XY-plane. Further, the W-axis is an axis which passes through the origin O of the XYZ three-dimensional orthogonal coordinate system and is orthogonal to the V-axis, wherein its positive domain positions at a second quadrant of the XY-plane and its negative domain positions at a fourth quadrant of the XY-plane. The cross sectional view at the upper part of  FIG. 9  is depicted so that the positive direction of the V-axis is taken on the right-hand side and the positive direction of the W-axis is taken upward. This figure corresponds to a figure in which the fixed assistant body  350  is added to the basic structure shown at the upper part of  FIG. 2  and rotated clockwise by 45 degrees. Further, the longitudinal sectional view at the lower part of  FIG. 9  is a longitudinal sectional view in which the basic structure is cut along the VZ-plane and, therefore, the right-hand side is the positive direction of the V-axis. 
     As described in Section 1, two fixing points P 1 , P 2  are arranged on the Y-axis and two exertion points Q 1 , Q 2  are arranged on the X-axis on the detection ring  200 . Here, four measurement points R 1  to R 4  are also defined. As shown in the figure, a first measurement point R 1 , a second measurement point R 2 , a third measurement point R 3  and a fourth measurement point R 4  are respectively arranged at the positive domain of the V-axis, the positive domain of the W-axis, the negative domain of the V-axis and the negative domain of the W-axis. As a result, when an intermediate circle positioned at a midpoint between an outer-circumference contour circle and an inner-circumference contour circle of the detection ring  200  is defined in the cross sectional view at the upper part of  FIG. 9 , individual points, Q 1 , R 1 , P 1 , R 2 , Q 2 , R 3 , P 2 , R 4  are to be arranged at an equal interval on the intermediate circle in accordance with the above-described order. The reason for defining the four measurement points R 1  to R 4  at the above-described positions is that displacement resulting from elastic deformation of the detection ring  200  becomes most prominent at these positions. 
     Displacement in a radial direction of these four measurement points R 1  to R 4  may be detected by measuring distances d 1 , d 2 , d 3 , d 4  indicated by the arrows shown in the cross sectional view at the upper part of  FIG. 9 . Each of the distances d 1 , d 2 , d 3 , d 4  is a distance between a measurement target surface positioned in the vicinity of each of the measurement points R 1 , R 2 , R 3 , R 4  on the inner circumferential surface of the detection ring  200  and a counter reference surface positioned on an outer circumference of the fixed assistant body  350  and facing the measurement target surface. An increase in the distance indicates that the vicinity of the measurement point swells in a radial direction. A decrease in the distance indicates that the vicinity of the measurement point shrinks in a radial direction. Therefore, a detection element capable of electrically detecting the distances is made ready, by which it is possible to measure an extent of deformation in a radial direction of the vicinity of each measurement point. 
     Alternatively, it is possible to adopt a method for measuring distances d 1 ′, d 2 ′, d 3 ′, d 4 ′ indicated by the arrows shown in the cross sectional view at the upper part of  FIG. 9 . Each of the distances d 1 ′, d 2 ′, d 3 ′, d 4 ′ is a distance between a measurement target surface positioned in the vicinity of each of the measurement points R 1 , R 2 , R 3 , R 4  on the outer circumferential surface of the detection ring  200  and a counter reference surface positioned on an inner circumference of the force receiving ring  100  and facing the measurement target surface. However, the force receiving ring  100  in itself also undergoes displacement. Therefore, a measured value of each distance indicates a difference in displacement between the force receiving ring  100  and the detection ring  200 . As a result, some correction processing is needed in order to determine displacement at each measurement point. In practice, it is preferable to measure the distances d 1 , d 2 , d 3 , d 4 . 
     It is not necessary to provide the fixed assistant body  350  if measurement of the distances d 1 ′, d 2 ′, d 3 ′, d 4 ′ are made. A distance between the force receiving ring  100  and the detection ring  200  may be measured by using a detection element to electrically detect a distance between a measurement target surface positioned in the vicinity of each of the measurement points R 1 , R 2 , R 3 , R 4  on the detection ring  200  and a counter reference surface facing the measurement target surface of the force receiving ring  100 . Thus, there is eliminated a necessity for providing the fixed assistant body  350 . Further, if measurement of the distances d 1 ′, d 2 ′, d 3 ′, d 4 ′ are made, it is applicable that the force receiving ring  100  is arranged inside and the detection ring  200  is arranged outside. 
     On the other hand, displacement in a vertical direction (the direction of the Z-axis) of the four measurement points R 1  to R 4  may be detected by measuring distances d 5 -d 8  indicated by the arrows shown in the longitudinal sectional view at the lower part of  FIG. 9  (Although the distances d 6 , d 8  are not illustrated, the distance d 6  is a distance immediately under the measurement point R 2  positioned behind the fixed assistant body  350 , and the distance d 8  is a distance immediately under the measurement point R 4  positioned in front of the fixed assistant body  350 ). Each of the distances d 5 , d 6 , d 7 , d 8  is a distance between a measurement target surface positioned in the vicinity of each of the measurement points R 1 , R 2 , R 3 , R 4  on the lower surface of the detection ring  200  and a counter reference surface positioned on the upper surface of the supporting substrate  300  and facing the measurement target surface. An increase in the distance indicates that the vicinity of each measurement point undergoes upward displacement, while a decrease in the distance indicates that the vicinity of each measurement point undergoes downward displacement. Therefore, a detection element capable of electrically detecting the distances is made ready, by which it is possible to measure an extent of vertical deformation in the vicinity of each measurement point. 
     As described above, displacement in a radial direction and displacement in a vertical direction of the four measurement points R 1  to R 4  can be measured, by which it is possible to understand an overall deformation mode of the detection ring  200  and an extent of the deformation. It is, thus, possible to detect six-axis-components of force in the direction of each coordinate axis and moment around each coordinate axis in an XYZ three-dimensional orthogonal coordinate system. 
       FIG. 10  is a top view showing distance measurement sites necessary for detecting the six-axis-components. That is, in this example, as described above, with respect to the first measurement point R 1 , the distance d 1  (radial displacement) and the distance d 5  (vertical displacement) are to be measured, with respect to the second measurement point R 2 , the distance d 2  (radial displacement) and the distance d 6  (vertical displacement) are to be measured, with respect to the third measurement point. R 3 , the distance d 3  (radial displacement) and the distance d 7  (vertical displacement) are to be measured, and with respect to the fourth measurement point R 4 , the distance d 4  (radical displacement) and the distance d 8  (vertical displacement) are to be measured. 
       FIG. 11  is a table which shows changes in distances d 1  to d 8  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed, at the basic structure shown in  FIG. 10 . In the table, [+] indicates an increase in distance, [−] indicates a decrease in distance, and [0] indicates no variation in distance. The above results will be easily understood when consideration is given to the specific deformation mode of the detection ring  200  described in Section 1. 
     For example, when force +Fx in the positive direction of the X-axis is exerted on the force receiving ring  100 , as shown in  FIG. 4 , the detection ring  200  undergoes such deformation that a quadrant between the point P 1  and the point Q 1  and a quadrant between the point P 2  and the point Q 1  shrink inwardly, and a quadrant between the point P 1  and the point Q 2  and a quadrant between the point P 2  and the point Q 2  swell outwardly. Therefore, the distances d 1 , d 4  decrease, while the distances d 2 , d 3  increase. At this time, since the detection ring  200  undergoes no vertical deformation, the distances d 5  to d 8  do not change. The row +Fx in the table of  FIG. 11  shows the above-described results. For the same reason, the results are obtained shown in the row +Fy in the table of  FIG. 11  upon exertion of force +Fy in the positive direction of the Y-axis. 
     Further, when force +Fz in the positive direction of the Z-axis is exerted on the force receiving ring  100 , the detection ring  200  undergoes such deformation as shown in  FIG. 5 , by which the distances d 5  to d 8  increase. At this time, since the detection ring  200  undergoes no radial deformation, the distances d 1  to d 4  do not change. The row +Fz in the table of  FIG. 11  shows the above-described results. 
     Then, when moment +My which is positive rotation around the Y-axis is exerted on the force receiving ring  100 , the detection ring  200  undergoes such deformation as shown in  FIG. 6 , that is, the right half in the figure undergoes downward displacement, and the left half in the figure undergoes upward displacement. Thus, the distances d 5 , d 8  decrease and the distances d 6 , d 7  increase. At this time, since the detection ring  200  undergoes no radial deformation, the distances d 1  to d 4  do not change. The row +My in the table of  FIG. 11  shows the above-described results. For the same reason, the results are obtained shown in the row +Mx in the table of  FIG. 11  upon exertion of moment +Mx which is positive rotation around the X-axis. 
     Finally, when moment +Mz which is positive rotation around the Z-axis is exerted on the force receiving ring  100 , the detection ring  200  undergoes such deformation as shown in  FIG. 7 . That is, the detection ring  200  undergoes deformation in such a manner that a quadrant between the point P 1  and the point Q 1  and a quadrant between the point P 2  and the point Q 2  swell outwardly, while a quadrant between the point P 1  and the point Q 2  and a quadrant between the point P 2  and the point Q 1  shrink inwardly. Therefore, the distances d 1 , d 3  increase and the distances d 2 , d 4  decrease. At this time, since the detection ring  200  undergoes no vertical deformation, the distances d 5  to d 8  do not change. The row +Mz in the table of  FIG. 11  shows the above-described results. 
     Although the table of  FIG. 11  shows the results obtained upon exertion of force in a positive direction and moment with positive rotation, upon exertion of force in a negative direction and moment with negative rotation, the results are obtained in which [+] and [−] are reversed. As a result, patterns of change in distances d 1  to d 8  differ depending on individual cases on which six-axis-components are exerted, respectively. Further, the larger the force and moment which are exerted are, the larger the variance in distance is. Therefore, the detection circuits are used to make a predetermined operation on the basis of measured values of the distances d 1  to d 8 , thus making it possible to output independently detection values of the six-axis-components. Specific arithmetic expressions will be described in detail by referring to the embodiment of Section 3. 
     Where there is no need for obtaining detection values of all the six-axis-components, it is not always necessary to measure the distances in eight different ways. For example,  FIG. 12  is a top showing a modification example of distance measurement sites in the basic structure shown in  FIG. 8 . In the example shown in  FIG. 10 , the distances d 1  to d 8  are measured in eight different ways. However, in the example shown in  FIG. 12 , it is sufficient to measure the distances d 1  to d 4 , d 9 , d 10  in six different ways. 
     Here, the distances d 1  to d 4  are the same as those of the above-described example. That is, the distance d 1  indicates displacement in a radial direction for the first measurement point R 1 , the distance d 2  indicates displacement in a radial direction for the second measurement point R 2 , the distance d 3  indicates displacement in a radial direction for the third measurement point R 3 , and the distance d 4  indicates displacement in a radial direction for the fourth measurement point R 4 . On the other hand, no measurement is made for displacement in a vertical direction for the four measurement points R 1  to R 4 . In other words, no measurement is made for the distances d 5  to d 8  in the above-described example. Instead, the exertion point Q 1  is given as a fifth measurement point to measure the displacement thereof in a vertical direction. The second exertion point Q 2  is given as a sixth measurement point to measure the displacement thereof in a vertical direction. 
     That is, the distance d 9  shown in  FIG. 12  is a distance between a measurement target surface positioned in the vicinity of the first exertion point Q 1  (the fifth measurement point) on the lower surface of the detection ring  200  and a counter reference surface positioned on the upper surface of the supporting substrate  300  and facing the measurement target surface. The distance d 10  shown in  FIG. 12  is a distance between a measurement target surface positioned in the vicinity of the second exertion point Q 2  (the sixth measurement point) on the lower surface of the detection ring  200  and a counter reference surface positioned on the upper surface of the supporting substrate  300  and facing the measurement target surface. 
       FIG. 13  is a table which shows changes in distances d 1  to d 4 , d 9 , d 10  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed, in the basic structure shown in  FIG. 12 . In this case as well, [+] indicates an increase in distance, [−] indicates a decrease in distance, and [0] indicates no variation in distance. As described above, the above-described results are to be obtained for the distances d 1  to d 4 . 
     On the other hand, a fact that the above-described results are obtained for the distances d 9 , d 10  will be easily understood, when consideration is given to the specific deformation mode of the detection ring  200  described in Section 1. That is, even upon exertion of force +Fx in the positive direction of the X-axis or force +Fy in the positive direction of the Y-axis, the exertion points Q 1 , Q 2  do not vary in a vertical direction. Therefore, the rows +Fx and +Fy are [0] for the distances d 9  and d 10 . However, upon exertion of force +Fz in the positive direction of the Z-axis, the exertion points Q 1 , Q 2  undergo upward displacement, by which the row +Fz gives [+] for the distances d 9  and d 10 . 
     Further, upon exertion of moment +Mx which is positive rotation around the X-axis, the exertion points Q 1 , Q 2  positioning on the X-axis are points on the rotation axis and, therefore, do not vary in a vertical direction. Thereby, the row +Mx gives [0] for the distances d 9 , d 10 . However, upon exertion of moment +My which is positive rotation around the Y-axis, the exertion point Q 1  undergoes downward displacement, while the exertion point Q 2  undergoes upward displacement. Therefore, the row +My gives [−] for the distance d 9  and gives [+] for the distance d 10 . Finally, upon exertion of moment +Mz which is positive rotation around the Z-axis, the exertion points Q 1 , Q 2  do not vary in a vertical direction. Therefore, the row +Mz gives [0] for the distances d 9 , d 10 . Upon exertion of force in a negative direction and moment with negative rotation, the results are obtained in the table of  FIG. 13  in which [+] and [−] are reversed. 
     Consequently, patterns of change in distances d 1  to d 4 , d 9 , d 10  differ depending on individual cases on which the six-axis-components are exerted. Further, the larger the force and moment which are exerted become, the larger the variance in distance is. However, the row +Mx in the table of  FIG. 13  gives [0] for all the distances. This indicates that measurement of the distances d 1  to d 4 , d 9 , d 10  do not provide information on moment Mx around the X-axis. However, information is obtained on five-axis-components excluding the information on the moment Mx. Therefore, detection circuits are used to carry out a predetermined operation on the basis of measured values of the distances d 1  to d 4 , d 9 , d 10 , thereby making it possible to output independently detection values of the five-axis-components excluding the moment Mx. Specific arithmetic expressions will be described in detail by referring to the embodiments of Section 3. 
     Accordingly, the force sensor according to the present invention is able to detect independently six-axis-components, Fx, Fy, Fz, Mx, My, Mz. However, where there is no need for having detection values of all the six-axis-components, detection elements can be omitted, whenever necessary, to reduce costs. 
     Further,  FIG. 9  shows an example where two exertion points Q 1 , Q 2  and two fixing points P 1 , P 2  are arranged. In the force sensor according to the present invention, the number of the exertion points and the number of the fixing points are not necessarily limited to two. That is, in a fundamental embodiment to be described here, it is acceptable that the n number of plural exertion points and the n number of plural fixing points are alternately arranged on an annular channel along a contour of the detection ring  200 , and elastic deformation of the detection ring in the vicinity of each of the measurement points defined at positions between the exertion points and the fixing points arranged adjacent to each other is electrically detected by detection elements. 
     Therefore, it is acceptable that n is set to be equal to 3, for example, and three exertion points and three fixing points are alternately arranged. In this case, the detection ring  200  is to be connected to the force receiving ring  100  via connection members at the positions of three exertion points and fixed to the supporting substrate  300  via fixing members at the positions of three fixing points. 
     However, in practice, the example shown in  FIG. 9  is efficient. That is, two exertion points and two fixing points are arranged in the order of the first exertion point Q 1 , the first fixing point P 1 , the second exertion point Q 2  and the second fixing point P 2  on the annular channel along the contour of the detection ring  200 . Then, a first measurement point R 1  is defined between the first exertion point Q 1  and the first fixing point P 1  on the annular channel, and a second measurement point R 2  is defined between the first fixing point P 1  and the second exertion point Q 2 , a third measurement point R 3  is defined between the second exertion point Q 2  and the second fixing point P 2 , and a fourth measurement point R 4  is defined between the second fixing point P 2  and the first exertion point Q 1 . And, detection elements are used to electrically detect elastic deformation of the detection ring  200  in the vicinities of the first to the fourth measurement points R 1  to R 4 . 
     In particular, in the example shown in  FIG. 9 , the basic structure is structured to be in plane symmetry about the XZ-plane and also in plane symmetry about the YZ-plane. Therefore, an extent of displacement indicated by [+] or [−] in the tables shown in  FIG. 11  and  FIG. 13  becomes symmetrical, by which a relatively simple detection circuit can be used to obtain a detection value of force in the direction of each axis and moment around each axis. 
     Therefore, as shown in the example of  FIG. 9 , in practice, the following is preferable. That is, the first exertion point Q 1 , the first fixing point P 1 , the second exertion point Q 2  and the second fixing point P 2  are respectively arranged at the positive domain of the X-axis, the positive domain of the Y-axis, the negative domain of the X-axis and the negative domain of the Y-axis, the vicinity of the first exertion point Q 1  on the detection ring  200  is connected to the force receiving ring  100  by the first connection member  410  extending along the positive domain of X-axis, the vicinity of the second exertion point Q 2  on the detection ring  200  is connected to the force receiving ring  100  by the second connection member  420  extending along the negative domain of the X-axis, and the detection elements electrically detect elastic deformation of the detection ring  200  in the vicinities of the first measurement point R 1 , the second measurement point R 2 , the third measurement point R 3  and the fourth measurement point R 4  arranged respectively at the first quadrant, the second quadrant, the third quadrant and the fourth quadrant on the XY-plane. 
     Each of the measurement points R 1  to R 4  is not necessarily defined on the V-axis or the W-axis. However, as described above, displacement resulting from elastic deformation of the detection ring  200  becomes most prominent on the V-axis or the W-axis. Therefore, in view of increasing detection sensitivity, as shown in the example of  FIG. 9 , it is preferable that the first measurement point R 1 , the second measurement point R 2 , the third measurement point R 3  and the fourth measurement point R 4  are defined respectively in the positive domain of the V-axis, the positive domain of the W-axis, the negative domain of the V-axis and the negative domain of the W-axis. 
     &lt;&lt;&lt;Section 3. Embodiment Using Capacitive Element&gt;&gt;&gt; 
     Here, the detection element will be described by referring to an embodiment using a capacitive element. As described above, a capacitive element type multi-axis force sensor detects a specific directional component of force exerted on the mechanical structure as displacement occurring at a specific part. That is, a principle is adopted in which a capacitive element is constituted by a pair of electrodes and displacement occurring on one of the electrodes due to the force which is exerted is detected on the basis of a capacitance value of the capacitive element. 
     Therefore, here, a description will be given of an embodiment using capacitive elements which detect eight different distances d 1  to d 8  at the basic structure shown in  FIG. 10 .  FIG. 14  is a cross sectional view (the upper part of the figure) in which the force sensor according to the embodiment is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which the force sensor is cut along the VZ-plane. The force sensor shown in  FIG. 14  is constituted by adding sixteen electrodes E 11  to E 18 , E 21  to E 28 , and a predetermined detection circuit to the basic structure shown in  FIG. 9 . Eight sets of capacitive elements constituted by the sixteen electrodes function as detection elements for measuring the above-described eight different distances d 1  to d 8 . 
     As shown in the cross sectional view at the upper part of  FIG. 14 , the displacement electrodes E 21  to E 24  are provided respectively in the vicinities of the four measurement points R 1  to R 4  (measurement target surfaces) on the inner circumferential surface of the detection ring  200 . Further, the displacement electrodes E 25  to E 28  (shown by the broken lines in the figure) are provided respectively in the vicinities of four measurement points R 1  to R 4  (measurement target surfaces) on the lower surface of the detection ring  200 . These eight displacement electrodes E 21  to E 28  are such electrodes that develop displacement resulting from deformation of the detection ring  200 . 
     On the other hand, the eight fixed electrodes E 11  to E 18  are provided at the positions (counter reference surfaces) facing the eight displacement electrodes E 21  to E 28 , respectively. The eight fixed electrodes E 11  to E 18  are such electrodes that are directly or indirectly fixed to the supporting substrate  300  and always keep constant positions, irrespective of deformation of the detection ring  200 . To be more specific, the fixed electrodes E 11  to E 14  are provided at the positions facing the displacement electrodes E 21  to E 24  on an outer circumferential surface of a fixed assistant body  350  in a cylindrical shape. The electrodes are indirectly fixed onto the supporting substrate  300  via the fixed assistant body  350 . Further, the fixed electrodes E 15  to E 18  are directly fixed at the positions facing the displacement electrodes E 25  to E 28  on the upper surface of the supporting substrate  300 . Although only the displacement electrodes E 15 , E 17  appear in the longitudinal sectional view at the lower part of  FIG. 14 , the displacement electrode E 16  is positioned behind the fixed assistant body  350  and the displacement electrode E  18  is positioned in front of the fixed assistant body  350 . 
     Consequently, in the above-described embodiment, eight sets of capacitive elements are constituted by eight sets of displacement electrodes E 21  to E 28  provided on measurement target surfaces positioned in the vicinities of the measurement points R 1  to R 4  on the inner circumferential surface and the lower surface of the detection ring  200  and eight sets of fixed electrodes E 11  to E 18  provided on counter reference surfaces defined at the positions facing individual measurement target surfaces on the outer circumferential surface of the fixed assistant body  350  and the upper surface of the supporting substrate  300 . These eight sets of the capacitive elements function as detection elements of the present invention and electrically detect elastic deformation of the detection ring  200 . 
     Here, for the sake of convenience of description, the capacitive elements constituted by the fixed electrodes E 11  to E 18  and the displacement electrodes E 21  to E 28  facing thereto are individually referred to as capacitive elements C 1  to C 8 . Capacitance values thereof are expressed as C 1  to C 8  by using the same symbols. In general, a capacitance value of a capacitive element decreases with an increase in distance of a pair of counter electrodes constituting the capacitive element, and the value increases with a decrease in distance. Therefore, electrical measurement of capacitance values of the capacitive elements C 1  to C 8  make it possible to determine the distances d 1  to d 8  shown in  FIG. 10  and also to detect force or moment which is exerted on the basis of the table shown in  FIG. 11 . 
     However, a capacitance value of the capacitive element varies depending on an effective counter area of a pair of counter electrodes. A capacitance value increases with an increase in effective counter area and decreases with a decrease in effective counter area. Therefore, in order to accurately measure the distances d 1  to d 8  on the basis of the above-described principle, it is necessary that any displacement of the detection ring  200  does not result in change in effective counter area of each capacitive element. Therefore, a projection image in which one of the electrodes constituting each of the capacitive elements C 1  to C 8  is projected on a surface on which the other electrode is formed is included in the other electrode. 
       FIG. 15  is a perspective view showing a dimensional relationship of counter electrodes of each of the capacitive elements C 1  to C 8  used in the force sensor shown in  FIG. 14 . The figure shows a state that an electrode Ea and an electrode Eb are arranged so as to face each other. In this example, both of the electrodes Ea, Eb are rectangular plate-shaped electrodes. The electrode Ea is made dimensionally larger than the electrode Eb in longitudinal and horizontal dimensions. Further, both of the electrodes Ea, Eb are arranged at such positions that the respective center points face each other. Therefore, a projection image A (orthographic projection image) in which the electrode Eb is projected on a surface of the electrode Ea is included within the electrode Ea. In brief, the electrode Ea is substantially larger than the electrode Eb. 
     If a relationship of one pair of counter electrodes constituting each of the capacitive elements C 1  to C 8  used in the force sensor shown in  FIG. 14  corresponds to a relationship shown in  FIG. 15 , displacement developed on the detection ring  200  does not change an effective counter area of each of the capacitive elements, as long as the displacement is within a predetermined tolerance level. That is, in the example shown in  FIG. 15 , an internal area of the projection image A shown by the broken line is an effective counter area of the capacitive element. Even where both of the electrodes Ea, Eb undergo displacement in a direction parallel to the electrode surface, the effective counter area remains constant as long as the projection image A is included in the electrode Ea. 
     In the force sensor shown in  FIG. 14 , for one pair of the counter electrodes which constitute each of the capacitive elements C 1  to C 8 , each of the displacement electrodes E 21  to E 28  is set to be substantially larger than each of the fixed electrodes E 11  to E 18 . It is, however, acceptable that each of the fixed electrodes E 11  to E 18  is set to be substantially larger than each of the displacement electrodes E 21  to E 28 . Further, in the force sensor shown in  FIG. 14 , a relationship of one pair of the counter electrodes is that in which in a normal state where force or moment to be detected is not exerted in any way, a projection image in which one of the electrodes is projected on a surface on which the other electrode is formed (for example a projection image projected in a direction parallel to a line connecting both the center points of the electrodes) is included in the other electrode. 
     A dimensional difference between both of the electrodes is a parameter which determines a tolerance level of displacement of the detection ring  200  (a range in which the effective counter area is kept constant). Therefore, in order to set detection values in a wide dynamic range, it is necessary that a dimensional difference between both the electrodes is made large and a blank region outside the projection image A in  FIG. 15  is set to be wide. Where displacement of the detection ring  200  is within the tolerance level, an effective counter area of each of the capacitive elements C 1  to C 8  is kept constant. Therefore, variance in capacitance values C 1  to C 8  exclusively indicates variance in distances d 1  to d 8 . 
       FIG. 16  is a table showing changes in capacitance values of the capacitive elements C 1  to C 8  upon exertion of force in the direction of each coordinate axis and moment around each coordinate axis on the force sensor shown in  FIG. 14 . In this table, [+] indicates an increase in capacitance value, [−] indicates a decrease in capacitance value, and [0] indicates no variation in capacitance value. Symbols in parentheses in cells C 1  to C 8  in the table indicate one pair of counter electrodes which constitute each of the capacitive elements. For example, (E 11 &amp;E 21 ) in the cell of C 1  indicates that the capacitive element C 1  is constituted by a pair of counter electrodes E 11 , E 21 . 
     The table shown in  FIG. 16  can be obtained by exchanging [+] and [−] in each cell in the table of  FIG. 11 . The table shown in  FIG. 11  indicates an increase and decrease in distances d 1  to d 8 , while the table shown in  FIG. 16  indicates an increase and decrease in capacitance values C 1  to C 8 . A capacitance value decreases with an increase in distance between electrodes of one pair of counter electrodes constituting a capacitive element, and a capacitance value increases with a decrease in distance between the electrodes. Therefore, it will be easily understood that the table shown in  FIG. 16  can be obtained from the table shown in  FIG. 11 . 
     As apparent from the table shown in  FIG. 16 , patterns of change in capacitance values C 1  to C 8  differ from each other, depending on individual cases on which six-axis-components are exerted. Further, the larger the force and moment exerted, the larger the variance in capacitance value becomes. Thus, the detection circuits are used to conduct a predetermined operation on the basis of measured values of the capacitance values C 1  to C 8 , thus making it possible to output independently detection values of the six-axis-components. 
       FIG. 17  is a view which shows specific arithmetic expressions for determining force in the direction of each coordinate axis, Fx, Fy, Fz, and moment around each coordinate axis, Mx, My, Mz, which are exerted on the force sensor shown in  FIG. 14 . Reasons for obtaining individual detection values by the arithmetic expressions will be easily understood by referring to the table shown in  FIG. 16 . For example, reference of the row +Fx in the table of  FIG. 16  reveals that a detection value of +Fx is obtained from a difference between a sum of C 1  and C 4  indicated by [+] and a sum of C 2  and C 3  indicated by [−]. This is also true for other detection values. 
     Further, where force in the negative direction, −Fx, −Fy, −Fz, and moment which is negative rotation, −Mx, −My, −Mz, are exerted, [+] and [−] in the table of  FIG. 16  are reversed. Thus, the arithmetic expressions shown in  FIG. 17  are used as they are, by which it is possible to obtain individual detection values as negative values. Since the arithmetic expressions for the six-axis-components shown in  FIG. 17  are free of any interference of other axis components, it is possible to obtain independently individual detection values of the six-axis-components. For example, C 1 , C 2  decrease and C 3 , C 4  increase upon exertion of +Fy. In the arithmetic expression for Fx, the decreasing and increasing components are cancelled with each other, thereby excluding any possibility that a component of Fy is included in a detection value of Fx. 
     As shown in  FIG. 17 , the arithmetic expressions other than force Fz are those in which a difference is obtained between sums of two sets of capacitance values. Even when the basic structure swells or shrinks due to change in temperature environment to result in errors in which distances between counter electrodes vary, the errors can cancel each other out. Thus it is possible to obtain accurate detection results free of disturbance components. Where the difference detection is desired to be conducted for Fz as well, the following procedures may be conducted. That is, a displacement electrode is additionally provided on the upper surface of the detection ring  200 , an auxiliary substrate fixed to the supporting substrate  300  is provided thereabove, a fixed electrode is provided on the lower surface of the auxiliary substrate, capacitive elements are added for measuring a distance between the upper surface of the detection ring  200  and the auxiliary substrate, and a difference is obtained between capacitance values of these capacitive elements and capacitance values of the capacitive elements C 5  to C 8 . 
       FIG. 18  is a circuit diagram showing a detection circuit used in the force sensor shown in  FIG. 14 . This detection circuit is a circuit which outputs detection values of forces Fx, Fy, Fz and moments Mx, My, Mz as voltage values on the basis of the arithmetic expressions shown in  FIG. 17 . First, capacitance values C 1  to C 8  of eight sets of capacitive elements C 1  to C 8  are converted by C/V converters  11  to  18  respectively into voltage values V 1  to V 8 . Next, computing elements  21  to  30  are used to determine a sum of voltage values of two sets respectively, and computing elements  31  to  35  are also used to determine a difference. Thereby, they are output respectively as detection values of Fx, Fy, Mz, My, Mx. Further, a computing element  36  is used to determine a sum of voltage values of four sets, by which a value whose symbol is reversed is output as a detection value of Fz. 
     As a matter of course, the detection circuit shown in  FIG. 18  is one example. Any circuit may be used, as long as detection results can be output in principle on the basis of the arithmetic expressions shown in  FIG. 17 . For example, when one pair of capacitive elements are connected in parallel, a capacitance value of the pair of capacitive elements after connection is given as a sum of capacitance values of individual capacitive elements. Therefore, in the circuit diagram shown in  FIG. 18 , for example, where the capacitive elements C 1  and C 4  are connected in parallel, a capacitance value of the pair of capacitive elements after connection is given as C 1 +C 4 . It is, thus, possible to omit the computing element  21 . In a similar manner, it is also possible to omit the computing elements  22  to  30 ,  36 . 
     Further,  FIG. 18  is a view showing a detection circuit using an analog computing element. As a matter of course, operations shown in  FIG. 17  can be made by digital operation. For example, an A/D converter is connected to a latter part of the C/V converters  11  to  18 , by which the capacitance values C 1  to C 8  can be taken individually as digital values. Therefore, a digital circuit such as a micro computer is used to make such operations as shown in  FIG. 17 , by which each of the detection values can be output as a digital value. 
     Here, features of the force sensor using the capacitive elements shown in  FIG. 14  will be summarized as follows. The force sensor is characterized in that the basic structure is constituted with two circular rings in which both the force receiving ring  100  and the detection ring  200  are arranged on the XY-plane so that the Z-axis is the central axis, both of the rings being arranged so that the force receiving ring  100  is outside and the detection ring  200  is inside, and the cylindrical fixed assistant body  350  whose lower surface is fixed onto the upper surface of the supporting substrate  300  and the Z-axis being given as the central axis is provided further inside the detection ring  200 . 
     Then, the following eight sets of capacitive elements are provided as the detection elements: 
     (1) a first capacitive element C 1  including a first displacement electrode E 21  arranged in the vicinity of the first measurement point R 1  on an inner circumferential surface of the detection ring  200  and a first fixed electrode E 11  arranged at a position facing the first displacement electrode E 21  on an outer circumferential surface of the fixed assistant body  350 , 
     (2) a second capacitive element C 2  including a second displacement electrode E 22  arranged in the vicinity of the second measurement point R 2  on the inner circumferential surface of the detection ring  200  and a second fixed electrode E 12  arranged at a position facing the second displacement electrode E 22  on the outer circumferential surface of the fixed assistant body  350 , 
     (3) a third capacitive element C 3  including a third displacement electrode E 23  arranged in the vicinity of the third measurement point R 3  on the inner circumferential surface of the detection ring  200  and a third fixed electrode E 13  arranged at a position facing the third displacement electrode E 23  on the outer circumferential surface of the fixed assistant body  350 , 
     (4) a fourth capacitive element C 4  including a fourth displacement electrode E 24  arranged in the vicinity of the fourth measurement point R 4  on the inner circumferential surface of the detection ring  200  and a fourth fixed electrode E 14  arranged at a position facing the fourth displacement electrode E 24  on the outer circumferential surface of the fixed assistant body  350 , 
     (5) a fifth capacitive element C 5  including a fifth displacement electrode E 25  arranged in the vicinity of the first measurement point R 1  on a lower surface of the detection ring  200  and a fifth fixed electrode E 15  arranged at a position facing the fifth displacement electrode E 25  on the upper surface of the supporting substrate  300 , 
     (6) a sixth capacitive element C 6  including a sixth displacement electrode E 26  arranged in the vicinity of the second measurement point R 2  on the lower surface of the detection ring  200  and a sixth fixed electrode E 16  arranged at a position facing the sixth displacement electrode E 26  on the upper surface of the supporting substrate  300 , 
     (7) a seventh capacitive element C 7  including a seventh displacement electrode E 27  arranged in the vicinity of the third measurement point R 3  on the lower surface of the detection ring  200  and a seventh fixed electrode E 17  arranged at a position facing the seventh displacement electrode E 27  on the upper surface of the supporting substrate  300 , and 
     (8) an eighth capacitive element C 8  including an eighth displacement electrode E 28  arranged in the vicinity of the fourth measurement point R 4  on the lower surface of the detection ring  200  and an eighth fixed electrode E 18  arranged at a position facing the eighth displacement electrode E 28  on the upper surface of the supporting substrate  300 . 
     Here, a projection image in which one of the electrodes constituting each of the capacitive elements C 1  to C 8  is projected on a surface on which the other electrode is formed is to be included in the other electrode. An effective counter area of each of the capacitive elements is kept constant, as long as displacement of the detection ring  200  is within a predetermined tolerance level. 
     On the other hand, the detection circuits of the force sensor have functions to output detection values of force Fx in the direction of the X-axis, force Fy in the direction of the Y-axis, force Fz in the direction of the Z-axis, moment Mx around the X-axis, moment My around the Y-axis, moment Mz around the Z-axis on the basis of the following arithmetic expressions which are obtained when capacitance values of the individual capacitive elements C 1  to C 8  are indicated as C 1  to C 8  by using the same symbols:
 
 Fx =( C 1 +C 4)−( C 2 +C 3)
 
 Fy =( C 3 +C 4)−( C 1 +C 2)
 
 Fz =−( C 5 +C 6 +C 7 +C 8)
 
 Mx =( C 7 +C 8)−( C 5 +C 6)
 
 My =( C 5 +C 8)−( C 6 +C 7)
 
 Mz =( C 2 +C 4)−( C 1 +C 3).
 
     A description has been above given of the detection principle of the force sensor shown in  FIG. 14  and the detection circuits used in the sensor. The force sensor shown in  FIG. 14  is to measure eight different distances d 1  to d 8  shown in  FIG. 10  by using capacitive elements. Next, a brief description will be given of a force sensor which measures the six different distances d 1  to d 4 , d 9 , d 10  shown in  FIG. 12  by using capacitive elements. In this case, as similar to the force sensor shown in  FIG. 14 , the capacitive elements C 1  to C 4  (fixed electrodes E 11  to E 14  and displacement electrodes E 21  to E 24 ) are provided for measuring the distances d 1  to d 4 . However, since no measurement is made for the distances d 5  to d 8 , it is not necessary to provide the capacitive elements C 5  to C 8  (fixed electrodes E 15  to E 18  and displacement electrodes E 25  to E 28 ) used in the force sensor shown in  FIG. 14 . 
     Instead, capacitive elements C 9 , C 10  are provided for measuring the distances d 9 , d 10  shown in  FIG. 12 . Here, the capacitive element C 9  is a capacitive element which is constituted by a displacement electrode E 29  arranged in the vicinity of the exertion point Q 1  (functions as the fifth measurement point) on the lower surface of the detection ring  200  and a fixed electrode E 19  arranged at a position facing the displacement electrode E 29  on the upper surface of the supporting substrate  300 . The capacitive element C 10  is a capacitive element which is constituted by a displacement electrode E 30  arranged in the vicinity of the exertion point Q 2  (having functions as the sixth measurement point) on the lower surface of the detection ring  200  and a fixed electrode E 20  arranged at a position facing the displacement electrode E 30  on the upper surface of the supporting substrate  300 . In this case as well, the following device is provided, that is, such a relationship is maintained that a projection image in which one of the electrodes constituting each of the capacitive elements C 9 , C 10  is projected on a surface on which the other electrode is formed is included in the other electrode and an effective counter area is kept constant. 
       FIG. 19  is a table which shows changes in capacitance values of the capacitive elements C 1  to C 4 , C 9 , C 10  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force sensor. In this table as well, [+] indicates an increase in capacitance value, [−] indicates a decrease in capacitance value, and [0] indicates no variance in capacitance value. Symbols in the parentheses added in each cell C 1  to C 4 , C 9 , C 10  in the table indicate one pair of counter electrodes constituting each of the capacitive elements. As described above, the distance and the capacitance value are reversed in terms of increase and decrease. Thus, the table shown in  FIG. 19  can be obtained by exchanging [+] and [−] of individual cells in the table shown in  FIG. 13 . 
     As shown in the table of  FIG. 19 , patterns of change in capacitance values C 1  to C 4 , C 9 , C 10  differ depending on individual cases on which the six-axis-components are exerted. Further, the larger the force and moment which are exerted become, the larger variance in capacitance value is. Thus, the detection circuits are used to conduct a predetermined operation on the basis of measured values of the capacitance values C 1  to C 4 , C 9 , C 10 , thus making it possible to output independently a detection value of each axis component. However, with respect to moment Mx, as mentioned in Section 2, the results are obtained that all capacitance values are [0] (no variation). Thus, this force sensor is unable to detect the moment Mx. 
       FIG. 20  is a view showing specific arithmetic expressions for determining forces Fx, Fy, Fz and moments My, Mz which are exerted on the force sensor. Reasons why individual detection values can be obtained by the arithmetic expressions will be easily understood by referring to the table shown in  FIG. 19 . In order to output detection values of the five-axis-components on the basis of these arithmetic expressions, detection circuits similar to that of  FIG. 18  may be provided. As a matter of course, one pair of capacitive elements may be connected in parallel, by which some of the computing elements can be omitted. Further, each of the capacitance values C 1  to C 4 , C 9 , C 10  is taken as a digital value by using an A/D converter, by which each of the detection values can be output as a digital value as a result of digital operation. 
     Consequently, the force sensor is structured in such a manner that the capacitive elements C 5  to C 8  of the force sensor shown in  FIG. 14  are omitted and, instead, a fifth capacitive element C 9  and a sixth capacitive element C 10  are provided, with the first exertion point Q 1  given as the fifth measurement point and the second exertion point Q 2  given as the sixth measurement point. Here, the fifth capacitive element C 9  is constituted by a fifth displacement electrode E 29  arranged in the vicinity of the fifth measurement point. Q 1  on the lower surface of the detection ring  200  and a fifth fixed electrode E 19  arranged at a position facing the fifth displacement electrode E 29  on the upper surface of the supporting substrate  300 . The sixth capacitive element C 10  is constituted by a sixth displacement electrode E 30  arranged in the vicinity of the sixth measurement point Q 2  on the lower surface of the detection ring  200  and a sixth fixed electrode E 20  arranged at a position facing the sixth displacement electrode E 30  on the upper surface of the supporting substrate  300 . 
     Further, a projection image in which one of the electrodes which constitutes each of the capacitive elements C 1  to C 4 , C 9 , C 10  is projected on a surface on which the other electrode is formed is included in the other electrode. As long as displacement of the detection ring  200  is within a predetermined tolerance level, an effective counter area of each of the capacitive elements is kept constant. 
     Then, the detection circuits of the force sensor have functions to output detection values of force Fx in a direction of the X-axis, force Fy in a direction of the Y-axis, force Fz in a direction of the Z-axis, moment My around the Y-axis and moment Mz around the Z-axis on the basis of the following arithmetic expressions which are obtained when a capacitance value of each of the capacitive elements C 1  to C 4 , C 9 , C 10  is given as C 1  to C 4 , C 9 , C 10  by using the same symbols,
 
 Fx =( C 1 +C 4)−( C 2 +C 3)
 
 Fy =( C 3 +C 4)−( C 1 +C 2)
 
 Fz =−( C 9 +C 10)
 
 My=C 9 −C 10
 
 Mz =( C 2 +C 4)−( C 1 +C 3).
 
     Finally, a description will be given of a practical device which can be applied to a force sensor using capacitive elements.  FIG. 21  is a cross sectional view (the upper part of the figure) in which a force sensor according to a modification example in which the practical device is given to the force sensor shown in  FIG. 14  is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which the force sensor is cut along the VZ-plane. The modification example is characterized in that the detection ring  200  is composed of a flexible conductive material (for example, a metal such as an aluminum alloy) and the capacitive elements C 1  to C 8  are constituted by giving the surface of the detection ring  200  as a common displacement electrode E 0 . If at least a surface layer of the detection ring  200  is constituted by a conductive material, the entire surface is exerted as one common displacement electrode E 0 . Thus, the surface can be used as an electrode which functions as eight displacement electrodes E 21  to E 28  of the force sensor shown in  FIG. 14 . 
     As apparent from  FIG. 21 , no separate displacement electrode is formed on the surface of the detection ring  200 . However, for example, on the outer circumferential surface of the detection ring  200 , a part of the region facing the fixed electrode E 11  functions as the displacement electrode E 21  shown in  FIG. 14 , by which the capacitive element C 1  is formed. Similarly, on the lower surface of the detection ring  200 , a part of the region facing the fixed electrode E 15  functions as the displacement electrode E 25  shown in  FIG. 14 , by which the capacitive element C 5  is formed. 
     As a matter of course, when a common displacement electrode E 0  (the surface of the detection ring  200 ) is used in place of the eight displacement electrodes E 21  to E 28 , the displacement electrodes E 21  to E 28  shown in the circuit diagram of  FIG. 18  are mutually short-circuited. This does not, however, adversely affect the operation of the detection circuit in any way. That is, C/V converters  11  to  18  are able to convert individually and independently the capacitance values C 1  to C 8  between each of grounded nodes thereof and each of the fixed electrodes E 11  to E 18  to voltage values V 1  to V 8 , with all the displacement electrodes E 21  to E 28  being grounded. Therefore, no problem is posed, where the displacement electrodes E 21  to E 28  to be grounded are replaced by the common displacement electrode EU. 
     Consequently, electrodes which are necessary from a practical standpoint in the modification example shown in  FIG. 21  are four fixed electrodes E 11  to E 14  provided on the outer circumferential surface of the fixed assistant body  350  and four fixed substrates E 15  to E 18  provided on the upper surface of the supporting substrate  300 . Thereby, the sensor is made quite simple in entire structure. Further, the common displacement electrode E 0  is the entire surface of the detection ring  200 , functioning as the electrode Ea shown in  FIG. 15 . Thus, the electrode E 0  also meets the condition that a projection image in which one of the electrodes which constitutes each capacitive element is projected on a surface on which the other electrode is formed is included in the other electrode. On the other hand, the fixed electrodes E 11  to E 18  are required to be electrically insulated with each other. Thus, at least a part of the surface of the fixed assistant body  350  (a part at which the fixed electrodes E 11  to E 14  are located) and at least a part of the upper surface of the supporting substrate  300  (a part at which the fixed electrodes E 15  to E 18  are located) are required to be constituted by an insulation material. However, other members can be constituted by an electrically conductive material such as a metal. 
     &lt;&lt;&lt;Section 4. Embodiment Using Strain Gauge&gt;&gt;&gt; 
     The force sensors according to the present invention are commonly characterized in that elastic deformation of the detection ring  200  is detected for its mode and magnitude to determine a direction and magnitude of force and moment which are exerted. In Section 2, a description has been given of the method for detecting displacement of measurement points as a method for detecting the mode and magnitude of elastic deformation. In Section 3, the embodiment using capacitive elements has been described by referring to a specific method for detecting displacement. Here, in Section 4, a description will be given of a method for electrically detecting a mechanical strain in the vicinity of each of predetermined measurement points R 1  to R 4  on the detection ring  200  by using detection elements, as another method for detecting a mode and magnitude of elastic deformation. In particular, a description will be given of a specific embodiment which uses a strain gauge as a detection element for detecting strain. 
       FIG. 22  is a cross sectional view in which the force sensor according to the embodiment using a strain gauge is cut along the XY-plane. The basic structure of the force sensor is identical to the basic structure shown in FIG.  1  and also identical to the basic structure according to the embodiment using capacitive elements shown in  FIG. 14 . However, this embodiment of the force sensor differs from that shown in  FIG. 14  in that the strain gauge is used as a detection element in place of the capacitive element to detect a mechanical strain in place of displacement of measurement points. Four measurement points R 1  to R 4  shown in  FIG. 22  are identical to the four measurement points R 1  to R 4  shown in  FIG. 14 , and are points arranged on the V-axis or the W-axis. 
     In the above-described force sensor, a total of eight different strain gauges G 1  to G 8  are used to electrically measure strain in the vicinity of each of the four measurement points R 1  to R 4 . That is, as shown in  FIG. 22 , the strain gauges G 1  to G 4  are attached respectively in the vicinities of the measurement points R 1  to R 4  on the inner circumferential surface of the detection ring  200 , and the strain gauges G 5  to G 8  (depicted by the broken lines in the figure) are attached respectively in the vicinities of the measurement points R 1  to R 4  on the lower surface of the detection ring  200 . 
     Here, each of the strain gauges G 1  to G 8  is attached in the vicinity of each of the measurement points R 1  to R 4  on the surface of the detection ring  200  in such a manner that a direction along an annular channel along a contour of the detection ring  200  is in a longitudinal direction (a direction in which stress is detected). Therefore, it is possible to detect a mechanical strain occurring on the surface on which each of the strain gauges G 1  to G 8  is attached with respect to a direction along the circumference of the detection ring  200  as a change in electric resistance with respect to the longitudinal direction of each of the strain gauges. 
       FIG. 23  is a table showing changes in electric resistance of the strain gauges G 1  to G 8  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force sensor shown in  FIG. 22 . In this table, [+] indicates an increase in electric resistance, [−] indicates a decrease in electric resistance, and [0] indicates a negligible variation in electric resistance. The above-described results are obtained by analysis of a specific deformation mode of the detection ring  200  described in Section 1. 
     For example, where force +Fx in the positive direction of the X-axis is exerted on the force receiving ring  100 , the detection ring  200  undergoes such deformation as shown in  FIG. 4 . Therefore, when consideration is given to stress which is applied in the longitudinal direction with respect to each of the strain gauges G 1  to G 4  affixed onto the inner circumferential surface of the detection ring  200 , the stress is applied to the strain gauges G 1 , G 4  in an extending direction, thus resulting in an increase in electric resistance. And, the stress is applied to the strain gauges G 2 , G 3  in a shrinking direction, thus resulting in a decrease in electric resistance. On the other hand, the strain gauges G 5  to G 8  affixed onto the lower surface of the detection ring  200  indicate a negligible change in electric resistance (specific values will be described later). The row +Fx in the table of  FIG. 23  shows the above results. For the same reason, when the force +Fy in the positive direction of the Y-axis is exerted, the results are obtained shown in the row +Fy in the table of  FIG. 23 . 
     Further, when force +Fz in the positive direction of the Z-axis is exerted on the force receiving ring  100 , the detection ring  200  undergoes such deformation as shown in  FIG. 5 . At this time, the strain gauges G 1  to G 4  affixed onto the inner circumferential surface of the detection ring  200  indicate a negligible change in electric resistance. However, stress is applied to the strain gauges G 5  to G 8  affixed onto the lower surface of the detection ring  200  in an extending direction, thus resulting in an increase in electric resistance. The row +Fz in the table of  FIG. 23  shows the above results. 
     Then, when moment +My which is positive rotation around the Y-axis is exerted on the force receiving ring  100 , the detection ring  200  undergoes such deformation as shown in  FIG. 6 , and the right half of the figure undergoes downward displacement, while the left half of the figure undergoes upward displacement. At this time, the strain gauges G 1  to G 4  affixed onto the inner circumferential surface of the detection ring  200  indicate a negligible change in electric resistance. However, of the strain gauges G 5  to G 8  affixed onto the lower surface of the detection ring  200 , the strain gauges G 5  and G 8  are given stress in a shrinking direction, thus resulting in a decrease in electric resistance and the strain gauges G 6  and G 7  are given stress in an extending direction, thus resulting in an increase in electric resistance. These results are shown in the row +My in the table of  FIG. 23 . For the same reason, the results are obtained shown in the row +Mx in the table of  FIG. 23 , when moment +Mx which is positive rotation around the X-axis is exerted thereon. 
     Finally, when moment +Mz which is positive rotation around the Z-axis is exerted on the force receiving ring  100 , the detection ring  200  undergoes such deformation as shown in  FIG. 7 . At this time, the strain gauges G 5  to G 8  affixed onto the lower surface of the detection ring  200  indicate a negligible change in electric resistance. Of the strain gauges G 1  to G 4  affixed onto the inner circumferential surface of the detection ring  200 , the strain gauges G 2  and G 4  are given stress in an extending direction, thus resulting in an increase in electric resistance. The strain gauges G 1  and G 3  are given stress in a shrinking direction, thus resulting in a decrease in electric resistance. The row +Mz in the table of  FIG. 23  shows the above results. 
     Although the table of  FIG. 23  shows the results upon exertion of force in a positive direction and moment of positive rotation, the results are obtained in which [+] and H are reversed upon exertion of force in a negative direction and moment of negative rotation. Consequently, patterns of change in electric resistance of eight different strain gauges G 1  to G 8  differ depending on individual cases on which six-axis-components are exerted. Further, the larger the force and moment which are exerted become, the larger variance in electric resistance is. Thus, the detection circuits are used to conduct a predetermined operation on the basis of measured values of electric resistance, thus making it possible to output independently detection values of six-axis-components. 
       FIG. 24  is a table which shows specific analysis results obtained by Finite Element Method in which analysis is performed for stress (unit: MPa) applied to each of the strain gauges G 1  to G 8  when force in the direction of each coordinate axis and moment around each coordinate axis are exerted on the force sensor shown in  FIG. 22 . The table shown in  FIG. 23  is prepared on the basis of the table shown in  FIG. 24 . As described above, the cells [0] in the table of  FIG. 23  do not show that variation in electric resistance is zero but show that the variation is smaller than the variation in other cells of the table and negligible as a measured value. 
     As a matter of course, it depends on accuracy required for a force sensor whether the measured values are negligible or not. A force sensor for which only low accuracy is required can be handled so that these measured values may be negligible. However, in a force sensor for which high accuracy is required, the measured values cannot be negligible. In reality, when a variation in value of electric resistance occurs in the cells [0] in the table of  FIG. 23 , interference with other axis components is generated to result in a failure of obtaining highly accurate detection values. In this case, it is preferable that a micro-computer or the like is used to conduct correction operation, whenever necessary. 
     Where it is possible to deal with the variation in electric resistance of the cells [0] in the table of  FIG. 23  as being approximate to zero, detection values of force Fx, Fy and moment Mx, My, Mz exerting on the force sensor can be output by using detection circuits composed of a simple Wheatstone bridge. Further, in order that a detection value of force Fz is also obtained by using a simple Wheatstone bridge and all the six-axis-components are output from the detection circuits composed of the Wheatstone bridges, strain gauges G 9  to G 12  may be added to the upper surface of the detection ring  200 . 
       FIG. 25  is a top view of a force sensor according to a modification example in which the strain gauges G 9  to G 12  are added to the embodiment shown in  FIG. 22 . Though the strain gauges G 5  to G 8  shown in  FIG. 22  are provided on the lower surface of the detection ring  200 , the strain gauges G 9  to G 12  shown in  FIG. 25  are provided on the upper surface of the detection ring  200 . To be more specific, the strain gauges G 9  to G 12  are gauge resistances identical to the strain gauges G 5  to G 8  in shape and dimension and are respectively arranged immediately above the strain gauges G 5  to G 8 , respectively. Therefore, an increase and decrease in electric resistance of the strain gauges G 9  to G 12  are reversed in terms of an increase and decrease in electric resistance of the strain gauges G 5  to G 8  shown in the table of  FIG. 23 . 
     As described above, a total of twelve strain gauges G 1  to G 12  are used, by which all detection values of the six-axis-components can be output by detection circuits composed of Wheatstone bridges.  FIG. 26  shows circuit diagrams, each of which shows a detection circuit for detecting force Fx, Fy, 
     Fz in the direction of each coordinate axis in the force sensor of the modification example shown in  FIG. 25 .  FIG. 27  shows circuit diagrams, each of which shows a detection circuit for detecting moment Mx, My, Mz around each coordinate axis in the force sensor according to the modification example shown in  FIG. 25 . Each of the detection circuits is a circuit composed of a Wheatstone bridge which works upon supply of power from a power source  40 . 
     Reference of the row +Fx in the table of  FIG. 23  reveals that G 1 , G 4  are given as [+] and G 2 , G 3  are given as [−]. It will be, thus, easily understood that an output voltage VFx resulting from the Wheatstone bridge shown at the upper part of  FIG. 26  is given as a detection value of force Fx in the direction of the X-axis. For the same reason, an output voltage VFy resulting from the Wheatstone bridge shown at the middle part of  FIG. 26  is given as a detection value of force Fy in the direction of the Y-axis. Further, reference of the row +Fz in the table of  FIG. 23  reveals that G 5  to G 8  are given as [+], while G 9  to G 12  are given as H. Thus, it will be easily understood that an output voltage VFz resulting from the Wheatstone bridge shown at the lower part of  FIG. 26  is given as a detection value of force Fz in the direction of the Z-axis. 
     On the other hand, reference of the row +Mx in the table of  FIG. 23  reveals that G 5 , G 6  are given as [−] and G 7 , G 8  are given as [+]. It will be, therefore, easily understood that an output voltage VMx resulting from the Wheatstone bridge shown at the upper part of  FIG. 27  is given as a detection value of moment Mx around the X-axis. For the same reason, an output voltage VMy resulting from the Wheatstone bridge shown at the middle part of  FIG. 27  is given as a detection value of moment My around the Y-axis. Further, reference of the row +Mz in the table of  FIG. 23  reveals that G 1 , G 3  are given as [−] and G 2 , G 4  are given as [+]. It will be, thus, easily understood that an output voltage VMz resulting from the Wheatstone bridge at the lower part of  FIG. 27  is given as a detection value of moment Mz around the Z-axis. 
     As described above, detection values of six-axis-components, that is, the forces Fx, Fy, Fz and the moments Mx, My, Mz exerting on the force sensor, are output by the detection circuits, each of which is composed of a simple Wheatstone bridge. In order that symbols of output values are to be symbols in a correct axial direction and around a correct axis rotation, it is necessary that the polarity of the power source  40  connected to each Wheatstone bridge is set to an appropriate direction. 
     Where the detection circuits composed of Wheatstone bridges shown in  FIG. 26  and  FIG. 27  are assembled, it is necessary that each of the Wheatstone bridges is to constitute an electrically independent bridge. Thus, even a strain gauge indicated by the same symbol in the circuit diagram is required to be a different strain gauge which is electrically independent. For example, G 1  indicated in the circuit at the upper part of  FIG. 26 , G 1  indicated in the circuit at the middle part thereof and G 1  indicated in the circuit at the lower part of  FIG. 27  are all indicated by the same symbol. However, in reality, they are individually independent strain gauges. 
     Therefore, in reality, the strain gauge G 1  shown in  FIG. 22  or  FIG. 25  indicates a bundle of plural strain gauges electrically independent (the number of strain gauges necessary for constituting an individually independent Wheatstone bridge). In other words, the above-described strain gauges G 1  to G 12  indicate twelve different kinds of strain gauges, each kind of which has a different arrangement attribute. An actual force sensor is constituted by arranging strain gauges having any one of the twelve different kinds of attribute by the number necessary for constituting the Wheatstone bridges shown in  FIG. 26  and  FIG. 27 . 
     As a matter of course, where detection of five-axis-components Fx, Fy, Mx, My, Mz excluding force Fz is sufficient, the strain gauges G 9  to G 12  to be arranged on the upper surface of the detection ring  200  can be omitted and only the strain gauges G 1  to G 8  having eight different attributes can be used to constitute the force sensor. 
     In the example shown in  FIG. 22 , the strain gauges G 1  to G 4  are arranged on the inner surface of the detection ring  200 . On the other hand, they may be arranged on the outer surface of the detection ring  200 . Similarly, in the example shown in  FIG. 22 , the strain gauges G 5  to G 8  are arranged on the lower surface of the detection ring  200 . On the other hand, they may be arranged on the upper surface of the detection ring  200 . A direction in which stress is applied is reversed on the inner surface and the outer surface or on the upper surface and the lower surface. Therefore, [+] and [−] shown in the table of  FIG. 23  are reversed. However, individual detection values are obtained the same as before by the above-described Wheatstone bridges. 
     Consequently, features of the force sensor having the functions to detect five-axis-components will be summarized as follows. First, the basic structure is constituted with two circular rings in which both the force receiving ring  100  and the detection ring  200  are arranged on the XY-plane so that the Z-axis is given as the central axis and arranged so that the force receiving ring  100  is outside and the detection ring  200  is inside. 
     Then, as the detection elements, used are a plurality of strain gauges G 1  to G 8  which are attached in the vicinities of the first to the fourth measurement points R 1  to R 4  on the surface of the detection ring  200  in such a manner that a direction along an annular channel along a contour of the detection ring  200  is given as a detection direction. Here, when one of the inner circumferential surface and the outer circumferential surface of the detection ring  200  is defined as a laterally arranged surface (the inner circumferential surface is the laterally arranged surface in the example of  FIG. 22 ) and one of the upper surface and the lower surface of the detection ring  200  is defined as a longitudinally arranged surface (the lower surface is the longitudinally arranged surface in the example of  FIG. 22 ), each of the detection elements is constituted by any one of strain gauges having the following eight respective attributes: 
     (1) the strain gauge G 1  having a first attribute which is attached in the vicinity of a first measurement point R 1  on the laterally arranged surface, 
     (2) the strain gauge G 2  having a second attribute which is attached in the vicinity of a second measurement point R 2  on the laterally arranged surface, 
     (3) the strain gauge G 3  having a third attribute which is attached in the vicinity of a third measurement point R 3  on the laterally arranged surface, 
     (4) the strain gauge G 4  having a fourth attribute which is attached in the vicinity of a fourth measurement point R 4  on the laterally arranged surface, 
     (5) the strain gauge G 5  having a fifth attribute which is attached in the vicinity of the first measurement point R 1  on the longitudinally arranged surface, 
     (6) the strain gauge G 6  having a sixth attribute which is attached in the vicinity of the second measurement point R 2  on the longitudinally arranged surface, 
     (7) the strain gauge G 7  having a seventh attribute which is attached in the vicinity of the third measurement point R 3  on the longitudinally arranged surface, and 
     (8) the strain gauge G 8  having an eighth attribute which is attached in the vicinity of the fourth measurement point R 4  on the longitudinally arranged surface. 
     Then, as shown at the upper part of  FIG. 26 , the detection circuit outputs a detection value VFx of force Fx in the direction of the X-axis by a Wheatstone bridge circuit in which the strain gauge G 1  having the first attribute and the strain gauge G 4  having the fourth attribute are given as the first opposite sides and the strain gauge G 2  having the second attribute and the strain gauge G 3  having the third attribute are given as the second opposite sides. Then, as shown at the middle part of  FIG. 26 , the detection circuit outputs a detection value VFy of force Fy in the direction of the Y-axis by a Wheatstone bridge circuit in which the strain gauge G 1  having the first attribute and the strain gauge G 2  having the second attribute are given as the first opposite sides and the strain gauge G 3  having the third attribute and the strain gauge G 4  having the fourth attribute are given as the second opposite sides. 
     On the other hand, as shown at the upper part of  FIG. 27 , the detection circuit outputs a detection value VMx of moment Mx around the X-axis by a Wheatstone bridge circuit in which the strain gauge G 5  having the fifth attribute and the strain gauge G 6  having the sixth attribute are given as the first opposite sides and the strain gauge G 7  having the seventh attribute and the strain gauge G 8  having the eighth attribute are given as the second opposite sides. As shown at the middle part of  FIG. 27 , the detection circuit outputs a detection value VMy of moment My around the Y-axis by a Wheatstone bridge circuit in which the strain gauge G 5  having the fifth attribute and the strain gauge G 8  having the eighth attribute are given as the first opposite sides and the strain gauge G 6  having the sixth attribute and the strain gauge G 7  having the seventh attribute are given as the second opposite sides. As shown at the lower part of  FIG. 27 , the detection circuit outputs a detection value VMz of moment Mz around the Z-axis by a Wheatstone bridge circuit in which the strain gauge G 1  having the first attribute and the strain gauge G 3  having the third attribute are given as the first opposite sides and the strain gauge G 2  of the second attribute and the strain gauge G 4  having the fourth attribute are given as the second opposite sides. 
     If the force sensor for detecting six-axis-components including the force Fz is to be constituted, as shown in  FIG. 25 , the strain gauges G 9  to G 12  arranged on the upper surface of the detection ring  200  may be additionally affixed. In this case, even if strain gauges arranged on the upper surface of the detection ring  200  are named G 5  to G 8  instead of G 9  to G 12 , and strain gauges arranged on the lower surface of the detection ring  200  are named G 9  to G 12  instead of G 5  to G 8 , so that names of the strain gauges formed on the upper surface and that on the lower surface are changed, detection can be properly made same as before by using the Wheatstone bridge circuits shown in  FIG. 26  and  FIG. 27 . Therefore, when one of the inner circumferential surface and the outer circumferential surface of the detection ring  200  is defined as a laterally arranged surface, and one of the upper surface and the lower surface of the detection ring  200  is defined as a first longitudinally arranged surface and the other of them is defined as a second longitudinally arranged surface, each of the detection elements is constituted by any one of the strain gauges having the following twenty different attributes: 
     (1) the strain gauge G 1  having a first attribute which is attached in the vicinity of a first measurement point R 1  on the laterally arranged surface, 
     (2) the strain gauge G 2  having a second attribute which is attached in the vicinity of a second measurement point R 2  on the laterally arranged surface 
     (3) the strain gauge G 3  having a third attribute which is attached in the vicinity of a third measurement point R 3  on the laterally arranged surface, 
     (4) the strain gauge G 4  having a fourth attribute which is attached in the vicinity of a fourth measurement point R 4  on the laterally arranged surface, 
     (5) the strain gauge G 5  having a fifth attribute which is attached in the vicinity of the first measurement point R 1  on the first longitudinally arranged surface of the detection ring  200 , 
     (6) the strain gauge G 6  having a sixth attribute which is attached in the vicinity of the second measurement point R 2  on the first longitudinally arranged surface of the detection ring  200 , 
     (7) the strain gauge G 7  having a seventh attribute which is attached in the vicinity of the third measurement point R 3  on the first longitudinally arranged surface of the detection ring  200 , 
     (8) the strain gauge G 8  having an eighth attribute which is attached in the vicinity of the fourth measurement point R 4  on the first longitudinally arranged surface of the detection ring  200 , 
     (9) the strain gauge G 9  having a ninth attribute which is attached in the vicinity of the first measurement point R 1  on the second longitudinally arranged surface of the detection ring  200 , 
     (10) the strain gauge G 10  having a tenth attribute which is attached in the vicinity of the second measurement point R 2  on the second longitudinally arranged surface of the detection ring  200 , 
     (11) the strain gauge G 11  having an eleventh attribute which is attached in the vicinity of the third measurement point R 3  on the second longitudinally arranged surface of the detection ring  200 , and 
     (12) the strain gauge G 12  having a twelfth attribute which is attached in the vicinity of the fourth measurement point R 4  on the second longitudinally arranged surface of the detection ring  200 . 
     Then, as shown at the upper part of  FIG. 26 , the detection circuit outputs a detection value VFx of force Fx in the direction of the X-axis by a Wheatstone bridge circuit in which the strain gauge G 1  having the first attribute and the strain gauge G 4  having the fourth attribute are given as the first opposite sides and the strain gauge G 2  having the second attribute and the strain gauge G 3  having the third attribute are given as the second opposite sides. As shown at the middle part of  FIG. 26 , the detection circuit outputs a detection value VFy of force Fy in the direction of the Y-axis by a Wheatstone bridge circuit in which the strain gauge G 1  having the first attribute and the strain gauge G 2  having the second attribute are given as the first opposite sides and the strain gauge G 3  having the third attribute and the strain gauge G 4  having the fourth attribute are given as the second opposite sides. As shown at the lower part of  FIG. 26 , the detection circuit outputs a detection value VFz of force Fz in the direction of the Z-axis by a Wheatstone bridge circuit in which a serial connection side of the strain gauge G 5  having the fifth attribute with the strain gauge G 6  having the sixth attribute and a serial connection side of the strain gauge G 7  having the seventh attribute with the strain gauge G 8  having the eighth attribute are given as the first opposite sides, and a serial connection side of the strain gauge G 9  having the ninth attribute with the strain gauge G 10  having the tenth attribute and a serial connection side of the strain gauge G 11  having the eleventh attribute with the strain gauge G 12  having the twelfth attribute are given as the second opposite sides. 
     Further, as shown at the upper part of  FIG. 27 , the detection circuit outputs a detection value VMx of moment Mx around the X-axis by a Wheatstone bridge circuit in which the strain gauge G 5  having the fifth attribute and the strain gauge G 6  having the sixth attribute are given as the first opposite sides, and the strain gauge G 7  having the seventh attribute and the strain gauge G 8  having the eighth attribute are given as the second opposite sides. As shown at the middle part of  FIG. 27 , the detection circuit outputs a detection value VMy of moment My around the Y-axis by a Wheatstone bridge circuit in which the strain gauge G 5  having the fifth attribute and the strain gauge G 8  having the eighth attribute are given as the first opposite sides, and the strain gauge G 6  having the sixth attribute and the strain gauge G 7  having the seventh attribute are given as the second opposite sides. As shown at the lower part of  FIG. 27 , the detection circuit outputs a detection value VMz of moment Mz around the Z-axis by a Wheatstone bridge circuit in which the strain gauge G 1  having the first attribute and the strain gauge G 3  having the third attribute are given as the first opposite sides, and the strain gauge G 2  having the second attribute and the strain gauge G 4  having the fourth attribute are given as the second opposite side. 
     &lt;&lt;&lt;Section 5. Devices Suitable for Packaging&gt;&gt;&gt; 
     Here, a description will be given of several devices suitable for packaging of the above-described force sensors in robots, industrial machines and others. 
     &lt;5-1. Force Receiving Substrate&gt; 
       FIG. 28  is a longitudinal sectional view on the XZ-plane which shows a state in which a force receiving substrate  600  is added to the basic structure shown in  FIG. 1 . In reality, detection elements such as capacitive elements and strain gauges or the like are to be added to the basic structure. Here, for the sake of convenience of description, only the basic structure will be shown. 
     As shown in the figure, the force receiving substrate  600  is a substrate arranged at certain intervals above the force receiving ring  100  and the detection ring  200  and provided with an upper surface in parallel with the XY-plane. Here, a part of the lower surface of the force receiving substrate  600  (the vicinity of the outer circumference in the example shown in the figure) is connected with the upper surface of the force receiving ring  100 . Further, a predetermined clearance H 3  is formed between the lower surface of the force receiving substrate  600  and the upper surface of the detection ring  200 . This is due to an inner part of the lower surface of the force receiving substrate  600  being smaller in thickness than an outer part thereof. The above-described clearance H 3  is provided thereby eliminating such a possibility that displacement of the detection ring  200  is prevented by the force receiving substrate  600 . 
     The above-described embodiments are those of detecting force and moment which are exerted on the force receiving ring  100 , with the supporting substrate  300  being fixed. However, in the example shown in  FIG. 28 , force and moment exerted on the force receiving substrate  600  are detected with the supporting substrate  300  being fixed. In other words, force and moment received by the force receiving substrate  600  are transmitted to the force receiving ring  100 . 
     As described above, the force sensor providing the force receiving substrate  600  can be easily packaged, when it is applied to a relay portion such as a robot arm. For example, a hand portion of a robot is jointed on the upper surface of the force receiving substrate  600  and an arm portion of the robot is jointed on the lower surface of the supporting substrate  300 , by which the force receiving sensor can be packaged into a wrist of the robot. Thus, it is possible to detect the force and moment which are applied to the hand portion. 
     &lt;5-2. Another Method for Fixing Detection Ring&gt; 
       FIG. 29  is a top view showing a modification example in which a method is changed for fixing the detection ring  200  to the basic structure shown in  FIG. 1 . In this case as well, in reality, detection elements such as capacitive elements and strain gauges or the like are to be added to the basic structure. However, for the sake of convenience of description, only the basic structure is shown. 
     In the basic structure shown in  FIG. 1 , the detection ring  200  has been fixed to the supporting substrate  300  by the fixing members  510 ,  520 . In this case, the fixing members  510 ,  520  have functions to connect the lower surface of the detection ring  200  with the upper surface of the supporting substrate  300 . Meanwhile, in the basic structure shown in  FIG. 29 , the detection ring  200  is fixed to the fixed assistant body  350  by the fixing members  515 ,  525 . 
     In order to adopt the above-described fixing method, it is a precondition that both the rings are arranged so that the force receiving ring  100  is outside and the detection ring  200  is inside, and the fixed assistant body  350  whose lower surface is fixed onto the upper surface of the supporting substrate  300  is provided further inside the detection ring  200 . Each of the force sensors shown in  FIG. 14  and  FIG. 21  is provided with the basic structure having the above-described precondition. The force sensor shown in  FIG. 29  is similar to the force sensors shown in  FIG. 14  and  FIG. 21  in that the force receiving ring  100  and the detection ring  200  are connected by the connection members  410 ,  420  along the X-axis. However, in the force sensor shown in  FIG. 29 , in order to fix the detection ring  200  onto the supporting substrate  300 , the inner circumferential surface of the detection ring  200  is connected to the outer circumferential surface of the fixed assistant body  350  by the fixing members  515 ,  525 . 
     Since the fixing members  515 ,  525  are arranged along the Y-axis, the fixing points P 1 , P 2  and the exertion points Q 1 , Q 2  defined on the detection ring  200  are not different in positions from those of the embodiments which have been described above. Therefore, although the method different from the above-described embodiments is adopted, this force sensor has the same basic motions as those of the embodiments which have been described above. The detection ring  200  is to be fixed to the supporting substrate  300  via the fixing members  515 ,  525  and the fixed assistant body  350 . Since a member to be jointed on the upper surface of the supporting substrate  300  is only the fixed assistant body  350 , this force sensor is slightly simplified in assembly steps as compared with the embodiments which have been described above. Packaging can be done more easily, in particular where the fixed assistant body  350  and the supporting substrate  300  are jointed together at a final stage to package the force sensor. 
     &lt;5-3. Displacement Control Structure (1)&gt; 
       FIG. 30  is a top view which shows an example in which a displacement control structure is added to the basic structure shown in  FIG. 1 .  FIG. 31  is a longitudinal sectional view in which the example is cut along the XZ-plane. As shown in  FIG. 31 , vertically penetrating through-holes  105 ,  107  are formed in the vicinity of the both right and left ends of the force receiving ring  100 . Grooves  101 ,  103  larger in diameter than the through-holes  105 ,  107  are formed at the positions of the through-holes  105 ,  107  on the upper surface of the force receiving ring  100 . 
     Then, displacement control screws  111 ,  113  are fitted so as to be inserted through the through-holes  105 ,  107 . The displacement control screws  111 ,  113  are those in which the leading ends thereof are fixed into threaded holes formed on the upper surface of the supporting substrate  300  and the heads thereof are accommodated into the grooves  101 ,  103 . Further, clearances are formed between the displacement control screw  111  and the inner surface of the through-hole  105  and the inner surface of the groove  101 . Clearances are also formed between the displacement control screw  113  and the inner surface of the through-hole  107  and the inner surface of the groove  103 . 
       FIG. 31  is a sectional view cut along the XZ-plane and, therefore, shows only two displacement control screws  111 ,  113  and surrounding structures thereof. However, in reality, as shown in the top view of  FIG. 30 , four displacement control screws  111 ,  112 ,  113 ,  114  are arranged at predetermined sites (in this case, a total of four sites, that is positive and negative sites on the X-axis and positive and negative sites on the Y-axis) and they are all similar in structure around them. Heads of the four displacement control screws  111 ,  112 ,  113 ,  114  are accommodated respectively into the respective grooves  101 ,  102 ,  103 ,  104 , and a clearance is formed between each of the displacement control screws and each of the grooves. This means that each of the displacement control screws  111  to  114  does not function to fix the force receiving ring  100 . 
     Each of the displacement control screws  111  to  114  is that in which the leading end thereof is fixed into the threaded hole formed in the upper surface of the supporting substrate  300 . Therefore, the displacement control screws themselves are fixed in a state of being erected perpendicularly with respect to the supporting substrate  300 . Meanwhile, since the force receiving ring  100  is not fixed by the displacement control screws  111  to  114 , displacement takes place upon exertion of force or moment. However, where an extent of displacement increases, the displacement control screws  111  to  114  come into contact with the inner surfaces of the through-holes  105  to  108  or the inner surfaces of the grooves  101  to  104 , thereby restricting displacement of the force receiving ring  100 . 
     Here, if a clearance is set in such a dimension so that the force receiving ring  100  is restricted for displacement by the displacement control screws  111  to  114  where force or moment exceeding a predetermined tolerance level is exerted on the force receiving ring  100 , it is possible to restrict excessive displacement of the force receiving ring  100 . By providing the above-described control structure for restricting displacement of the force receiving ring  100 , it is possible to prevent the basic structure from being mechanically damaged upon exertion of excessive force or moment. In particular, the detection ring  200  is a flexible member and undergoes elastic deformation upon exertion of force within a predetermined tolerance level. However, exertion of force exceeding the tolerance level may damage the detection ring  200 . Therefore, in practice, as shown in the example, it is preferable to provide a certain displacement control structure. 
     &lt;5-4. Displacement Control Structure (2)&gt; 
     Here, a description will be given of an example of another structure for controlling excessive displacement of the force receiving ring  100 . The example shown here realizes a displacement control structure by utilizing a force receiving substrate  600  arranged above the force receiving ring  100 . That is, this example is a force sensor obtained by improving the embodiment using the capacitive elements described in Section 3 so as to be more suitable for packaging. The example is identical in basic structure to the force sensor shown in  FIG. 21 . 
       FIG. 32  is a top view of the above-described force sensor.  FIG. 33  is a longitudinal sectional view in which the force sensor is cut along the XZ-plane.  FIG. 34  is a longitudinal sectional view in which the force sensor is cut along the VZ-plane. Here, the V-axis and the W-axis are the same as those defined in the embodiment shown in  FIG. 21 . They are coordinate axes which are included in the XY-plane and inclined at 45 degrees with respect to the X-axis and the Y-axis. 
     As shown in  FIG. 32 , the upper part of the force sensor is covered with a disk-shaped force receiving substrate  600 A. The broken lines in  FIG. 32  depict a position of the force receiving ring  100 A. As shown in the figure, circular grooves  601 ,  603 ,  605 ,  607  are formed at four sites above the X-axis and the Y-axis of the force receiving substrate  600 A. And, through-holes are formed at the centers thereof into which fixing screws  611 ,  613 ,  615 ,  617  are inserted. The fixing screws  611 ,  613 ,  615 ,  617  have functions to fix the force receiving substrate  600 A to the force receiving ring  100 A. 
       FIG. 33  clearly shows a state that the fixing screws  611 ,  615  are fixed into threaded holes formed on the upper surface of the force receiving ring  100 A. The heads of the fixing screws  611 ,  615  are accommodated into the grooves  601 ,  605 . The fixing screws  613 ,  617  are also fixed in a similar manner. In addition, threaded holes  301 ,  303 ,  305 ,  307  are formed respectively immediately under the fixing screws  611 ,  613 ,  615 ,  617  on the lower surface of the supporting substrate  300 A. These threaded holes have functions to fix the supporting substrate  300 A to an object below (for example, an arm portion of a robot). 
     On the other hand, as shown in  FIG. 32 , circular through-holes  602 ,  604 ,  606 ,  608  are formed at four sites above the V-axis and the W-axis of the force receiving substrate  600 A, and cylindrical spacers  622 ,  624 ,  626 ,  628  are fitted thereinto. And fixing screws  612 ,  614 ,  616 ,  618  are inserted through further inside. The fixing screws  612 ,  614 ,  616 ,  618  have functions to fix the force receiving ring  100 A to an object above the force receiving substrate  600 A (for example, a hand portion of a robot). 
       FIG. 34  clearly shows the fixing screws  612 ,  616  and structures around them. The head of the fixing screw  612  is accommodated into the groove  121  formed on the lower surface of the force receiving ring  100 A, and the leading end thereof projects upward via a through-hole formed in the force receiving ring  100 A and a through-hole  602  formed in the force receiving substrate  600 A. The cylindrical spacer  622  is fitted between the fixing screw  612  and the inner wall surface of the through-hole  602 . The head of the fixing screw  616  is similarly accommodated into the groove  126  formed on the lower surface of the force receiving ring  100 A, and the leading end thereof projects upward through a through-hole formed in the force receiving ring  100 A and a through-hole  606  formed in the force receiving substrate  600 A. The cylindrical spacer  626  is fitted between the fixing screw  616  and the inner wall surface of the through-hole  606 . The fixing screws  614 ,  618  are also similar in structures around them. 
     Threaded holes which screw with the leading ends of the fixing screws  612 ,  614 ,  616 ,  618  are formed on the lower surface of an object above the force receiving substrate  600 A (for example a hand portion of a robot), by which the object is firmly fixed to the upper ends of the spacers  622 ,  624 ,  626 ,  628  by the fixing screws. 
     As shown in  FIG. 33  and  FIG. 34 , an inclusive tubular body  650  (a cylindrical object in this example) which includes the force receiving ring  100 A and the detection ring  200 A is connected to an outer circumference of the lower surface of the force receiving substrate  600 A (in the example shown in the figure, the force receiving substrate  600 A and the inclusive tubular body  650  are unified into a structural body). Further, clearances H 4 , H 5  are formed between the lower end of the inclusive tubular body  650  and the outer circumference of the supporting substrate  300 A. That is, in this example, a step is provided at a circumferential edge of the supporting substrate  300 A, and the clearance H 4  is formed between the lower end surface of the inclusive tubular body  650  and the step. The clearance H 5  is also formed between the inner wall surface of the inclusive tubular body  650  and the circumferential edge inside the step of the supporting substrate  300 A. 
     Here, the clearances H 4 , H 5  are set in such a dimension that, when force or moment exceeding a predetermined tolerance level is exerted on the force receiving substrate  600 A, the lower end of the inclusive tubular body  650  is brought into contact with the outer circumference of the supporting substrate  300 A, thereby restricting displacement of the force receiving substrate  600 A. Therefore, the force sensor is also able to prevent the basic structure from being mechanically damaged upon exertion of excessive force or moment. 
     In the force sensor, a detection circuit substrate  380  which packages electronics constituting a detection circuit is provided on the upper surface of the supporting substrate  300 A. In the example shown in the figure, the detection circuit substrate  380  is a washer-shaped circuit substrate which surrounds a fixed assistant body  350 A. As shown in  FIG. 34 , fixed electrodes E 15 , E 16 , E 17 , E 18  are arranged on the upper surface of the detection circuit substrate  380 . In the figure, although the electronics packaged on the detection circuit substrate  380  are omitted, the electronics are components which constitute, for example, the circuits shown in  FIG. 18 . Wiring is provided between the electronics, a common displacement electrode E 0  (that is, the detection ring  200 A) and each of the fixed electrodes E 11  to E 18 . 
     As described above, the detection circuit substrate  380  is provided on the upper surface of the supporting substrate  300 A and components of the detection circuits are packaged, thus making it possible to assemble internally all constituents necessary for the force sensor. Therefore, the sensor in its entirety can be saved for space and made thin. 
     &lt;&lt;&lt;Section 6. Diaphragm Forming Embodiment&gt;&gt;&gt; 
     In the embodiments described above, a circular ring which is entirely flexible is used as the detection ring  200 . However, a detection ring  200  used in the present invention is not necessarily flexible as a whole but a ring which undergoes at least partial elastic deformation will do. Here, a description will be given of an embodiment in which a diaphragm is formed at a part of the detection ring  200  so that the diaphragm has functions to undergo elastic deformation. 
       FIG. 35  is a cross sectional view (the upper part of the figure) in which a basic structure of the embodiment having diaphragms formed on the detection ring is cut along the XY-plane and a longitudinal sectional view (the lower part of the figure) in which the basic structure is cut along the XZ-plane. In this case as well, the V-axis and the W-axis are the same as those defined in the embodiments described above, and they are coordinate axes which are included in the XY-plane and inclined at 45 degrees with respect to the X-axis and the Y-axis. 
     The basic structure shown here is constituted so that a detection ring  200 B is arranged inside the force receiving ring  100 B, a fixed assistant body  350 B is arranged further inside and a supporting substrate  300 B is arranged below them. A clearance H 6  is secured between the force receiving ring  100 B and the detection ring  200 B, and a clearance H 7  is secured between the detection ring  200 B and the fixed assistant body  350 B. Then, the force receiving ring  100 B and the detection ring  200 B are supported at a position suspended upward with respect to the supporting substrate  300 B. Therefore, the detection ring  200 B is able to undergo deformation and displacement within a predetermined tolerance level. 
     The force receiving ring  100 B and the detection ring  200 B are both circular rings and arranged on the XY-plane, with the Z-axis given as a central axis. Further, the supporting substrate  300 B is provided with an upper surface parallel to the XY-plane and arranged at certain intervals below the force receiving ring  100 B and the detection ring  200 B. Then, the fixed assistant body  350 B is a cylindrical structure which is arranged, with the Z-axis given as a central axis. The lower surface of the fixed assistant body  350 B is jointed onto the upper surface of the supporting substrate  300 B. These features are common to those of the basic structure shown in  FIG. 8 . 
     However, in the embodiment shown here, the detection ring  200 B is not a geometrically perfect circular ring. As shown in the cross sectional view at the upper part of  FIG. 35 , four sets of diaphragms D 1  to D 4  are provided at a part of the detection ring  200 B. Then, each of four connection members  431  to  434  connecting the force receiving ring  100 B with the detection ring  200 B is a cylindrical member, with the X-axis or the Y-axis given as a central axis. And, the inner end thereof is connected to the center position of each of the diaphragms D 1  to D 4 . Therefore, the center position of each of the diaphragms D 1  to D 4  functions as each of the exertion points Q 11  to Q 14 . In practice, the force receiving ring  100 B, the detection ring  200 B and the connection members  431  to  434  can be manufactured as a unified structure by using the same material such as a metal. 
     On the other hand, as shown in the figure, fixing points P 11  to P 14  are defined on the V-axis and the W-axis of the detection ring  200 B. Then, the positions of the fixing points P 11  to P 14  on the detection ring  200 B are fixed on the upper surface of the supporting substrate  300 B by the fixing members  531  to  534 . Fixing members  531  to  534  depicted by the broken lines in  FIG. 35  are arranged on the lower surface of the positions of the fixing points P 11  to P 14  of the detection ring  200 B.  FIG. 36  is a longitudinal sectional view in which the basic structure shown in  FIG. 35  is cut along the VZ-plane. This figure clearly shows a state that the lower surface in the vicinity of the fixing point P 11  of the detection ring  200 B is fixed on the upper surface of the supporting substrate  300 B by the fixing member  531 , and the lower surface in the vicinity of the fixing point P 13  of the detection ring  200 B is fixed on the upper surface of the supporting substrate  300 B by the fixing member  533 . 
     Consequently, as shown in the cross sectional view at the upper part of  FIG. 35 , the detection ring  200 B is to receive force at four sites of exertion points Q 11  to Q 14  in a state of being fixed to the supporting substrate  300 B at four sites of the fixing points P 11  to P 14 . The diaphragms D 1  to D 4  are parts thinner in thickness than other parts formed on the detection ring  200 B and flexible. Therefore, when force is exerted on each of the exertion points Q 11  to Q 14  from the force receiving ring  100 B via the four connection members  431  to  434 , elastic deformation exclusively occurs at the diaphragms D 1  to D 4 . As a matter of course, specifically, where each part of the detection ring  200 B is constituted by the same material, elastic deformation occurs to some extent at parts other than the diaphragms D 1  to D 4 . However, in practice, elastic deformation concentrates on the diaphragms D 1  to D 4 . 
     Therefore, in the embodiment shown here, elastic deformation occurring at the diaphragms D 1  to D 4  is to be detected electrically by detection elements. To be more specific, as with the embodiment described in Section 3, a method is adopted in which capacitive elements are used to measure displacement occurring at the diaphragms D 1  to D 4 , thereby recognizing a mode of elastic deformation on the basis of the measurement result. As will be described later, when forces Fx, Fy, Fz in the direction of each coordinate axis and moments Mx, My, Mz around each coordinate axis are exerted on the force receiving ring  100 B in a state that the supporting substrate  300 B is fixed, deformation modes of the diaphragms D 1  to D 4  differ individually. It is, thus, possible to detect six-axis-components independently by measuring displacement occurring at the diaphragms D 1  to D 4 . 
       FIG. 37  is a cross sectional view in which a force sensor constituted by adding capacitive elements to the basic structure shown in  FIG. 35  is cut along the XY-plane. In the force sensor, a total of 40 electrodes are provided to constitute a total of 20 sets of capacitive elements. In this case, for the sake of convenience, a description will be given of a case where five electrodes arranged adjacent to each other are given as one electrode group to constitute a total of eight sets of electrode groups. In  FIG. 37 , the electrode groups indicated by symbols T 10 , T 20 , T 30 , T 40  are displacement electrode groups, each of which is constituted by five displacement electrodes, and arranged at the respective positions of diaphragms D 1 , D 2 , D 3 , D 4  on the inner circumferential surface of the detection ring  200 B. On the other hand, the electrode groups indicated by symbols U 10 , U 20 , U 30 , U 40  are fixed electrode groups, each of which is constituted by five fixed electrodes and arranged at the respective positions facing the displacement electrode groups T 10 , T 20 , T 30 , T 40  on the outer circumferential surface of the fixed assistant body  350 B. 
     Since it is difficult to show the eight sets of electrode groups (40 electrodes) in detail in  FIG. 37 , a shape and arrangement of each of the electrodes is shown in the table of  FIG. 38 . In this table, cells given as T 10 , T 20 , T 30 , T 40  show plan views of the displacement electrode groups T 10 , T 20 , T 30 , T 40  shown in  FIG. 37 . Cells given as U 10 , U 20 , U 30 , U 40  show plan views of the fixed electrode groups U 10 , U 20 , U 30 , U 40  shown in  FIG. 37 . Each of the plan views shows a state that the electrode group is viewed from the respective positions of view points e 1 , e 2 , e 3 , e 4  shown in  FIG. 37 . In order to clarify a positional relationship with respect to the XYZ coordinate system, each of the plan views shows the direction of each of the X, Y, Z axes. In any of the plan views, the Z-axis is placed upward. Broken lines in the plan view of each of the fixed electrode groups U 10 , U 20 , U 30 , U 40  depict projection images (projection images in the direction of the X-axis or in the direction of the Y-axis) of displacement electrodes belonging to the displacement electrode groups T 10 , T 20 , T 30 , T 40  facing thereto. 
     For example, the cell [T 10 ] in  FIG. 38  shows a plan view of five displacement electrodes T 11  to T 15  belonging to the displacement electrode group T 10 . This plan view is obtained by observing the electrodes in the positive direction of the X-axis from the position of view point e 1  shown in  FIG. 37 . All the displacement electrodes T 11  to T 15  are arranged at the diaphragm D 1  on the inner circumferential surface of the detection ring  200 B. Therefore, in practice, each of the electrodes is to be formed on the curved surface. However, in  FIG. 38 , for the sake of convenience of description, the electrodes are assumed to be arranged on a flat surface. The displacement electrodes T 11  to T 14  are trapezoidal electrodes identical in dimension, while the displacement electrode T 15  is a square electrode. The displacement electrode T 15  is arranged so that the center point is on the X-axis. The displacement electrodes T 11  to T 14  are arranged so as to surround the displacement electrode T 15 . In the plan view shown here, the displacement electrode group T 10  is symmetrical about the Y-axis and the Z-axis as well. 
     On the other hand, the cell [U 10 ] in  FIG. 38  shows a plan view of five fixed electrodes U 11  to U 15  belonging to the fixed electrode group U 10 . This plan view is obtained by observing in the negative direction of the X-axis from the position of the view point e 1  shown in  FIG. 37 . The fixed electrodes U 11  to U 15  are arranged at the respective positions facing the displacement electrodes T 11  to T 15 . In the plan view shown here, the fixed electrode group U 10  is symmetrical about the Y-axis and the Z-axis as well. As described above, the broken lines in the figure depict projection images (projection images in the direction of the X-axis) of the displacement electrodes T 11  to T 15  facing thereto. 
     The five displacement electrodes T 11  to T 15  respectively face the five fixed electrodes U 11  to U 15 , thereby forming five sets of capacitive elements C 11  to C 15 . Further, as shown in the cell [U 10 ] in  FIG. 38 , the fixed electrodes U 11  to U 15  depicted by the solid lines are respectively included in the projection images of the displacement electrodes T 11  to T 15  depicted by the broken lines. This means that a pair of opposing electrodes are in a relationship of the electrodes Ea, Eb shown in  FIG. 15 . That is, the capacitive elements C 11  to C 15  are capacitive elements which meet such a condition that a projection image in which one of the electrodes is projected on a surface on which the other electrode is formed is included in the other electrode. As long as displacement of the diaphragm D 1  is within a predetermined tolerance level, an effective counter area of each of the capacitive elements C 11  to C 15  is kept constant. 
     This is entirely true for the displacement electrode groups shown in the cells [T 20 ], [T 30 ], [T 40 ] and the fixed electrode groups shown in the cells [U 20 ], [U 30 ], [U 40 ] in  FIG. 38 . Consequently, the displacement electrodes T 11  to T 15  and the fixed electrodes U 11  to U 15  constitute the capacitive elements C 11  to C 15  (hereinafter, referred to as a capacitive element group C 10 ). The displacement electrodes T 21  to T 25  and the fixed electrodes U 21  to U 25  constitute the capacitive elements C 21  to C 25  (hereinafter, referred to as a capacitive element group C 20 ). Further, the displacement electrodes T 31  to T 35  and the fixed electrodes U 31  to U 35  constitute the capacitive elements C 31  to C 35  (hereinafter, referred to as a capacitive element group C 30 ). Still further, the displacement electrodes T 41  to T 45  and the fixed electrodes U 41  to U 45  constitute the capacitive elements C 41  to C 45  (hereinafter, referred to as a capacitive element group C 40 ). In this case, the capacitive element groups C 10 , C 20 , C 30 , C 40  have the functions to detect respectively displacement (elastic deformation) of the diaphragms D 1 , D 2 , D 3 , D 4 , as detection elements. 
     Then, in the force sensor, assessment will be made for changes in capacitance values of the capacitive element groups C 10 , C 20 , C 30 , C 40 , when forces +Fx, +Fy, +Fz in the direction of each coordinate axis and moments +Mx, +My, +Mz around each coordinate axis are exerted on the force receiving ring  100 B, with the supporting substrate  300 B being fixed.  FIG. 39  is a table which shows changes in capacitance values of the capacitive elements C 11  to C 45  when the force in the direction of each coordinate axis and the moment around each coordinate axis are exerted on the force sensor shown in  FIG. 37 . In this table, [+] shows an increase in capacitance value, [−] shows a decrease in capacitance value and [0] shows no variation in capacitance value. The fact that the above results are obtained will be easily understood when consideration is given to a specific deformation mode of the detection ring  200 B. 
     For example, in  FIG. 37 , when force +Fx in the positive direction of the X-axis is exerted on the force receiving ring  100 B, the diaphragm D 1  is stretched rightward and the diaphragm D 3  is pushed out rightward. As a result, a distance between the electrodes of the capacitive element group C 10  spreads to decrease the capacitance value. A distance between the electrodes of the capacitive element group C 30  narrows to increase the capacitance value. At this time, although the capacitive element groups C 20 , C 40  deviate in the direction of the X-axis, no substantial change in distance between the electrodes or in effective counter area occurs. Thus, the capacitance value does not change. The row +Fx in the table of  FIG. 39  shows the above-described results. For the same reason, upon exertion of force +Fy in the positive direction of the Y-axis, the results are obtained shown in the row +Fy in the table of  FIG. 39 . 
     Then, consideration will be given to a case where force +Fz in the positive direction of the Z-axis is exerted on the force receiving ring  100 B. 
       FIG. 40  is a longitudinal sectional view on the XZ-plane which shows a deformed state upon exertion of the force +Fz. For the sake of convenience of explanation, only necessary constituents are extracted and each part is shown in a deformed manner. Therefore, shapes of individual parts do not precisely show an actual deformation mode. As shown in the figure, upon upward movement of the force receiving ring  100 B, upward force is transmitted to the center part of each of the diaphragms D 1 , D 3  via connection members  431 ,  433 . As a result, the diaphragms D 1 , D 3  are inclined as shown in the figure. In the figure, the diaphragms D 1 , D 3  are depicted as flat plates. In reality, they assume a complicated configuration including torsion. However, when consideration is given to variation in distance with respect to an outer circumferential surface of the fixed assistant body  350 B, a distance d (up) at the upper part decreases, while a distance d (down) at the lower part increases, as shown in the figure. 
     As a result, in the table shown in  FIG. 38 , increased are capacitance values of the capacitive elements C 13 , C 23 , C 33 , C 43  composed of counter electrodes arranged above. And, decreased are capacitance values of the capacitive elements C 14 , C 24 , C 34 , C 44  composed of counter electrodes arranged below. Regarding counter electrodes arranged at the center or on the right and left sides, those at the upper half decrease in distance but those at the lower half increase in distance. Thus, the decrease in distance at the upper half is offset by the increase in distance at the lower half, and there is no change in capacitance values of the capacitive elements composed of the counter electrodes. The row +Fz in the table of  FIG. 39  shows the above results. 
     Then, consideration will be given to a case where moment +My which is positive rotation around the Y-axis is exerted on the force receiving ring  100 B. In this case, in  FIG. 37 , the left end of the force receiving ring  100 B moves in the positive direction of the Z-axis (a direction in which it is lifted up from the sheet surface). Therefore, the diaphragm D 3  is inclined as with that shown in  FIG. 40 , and a distance d (up) at the upper part decreases, while a distance d (down) at the lower part decreases. Thus, the capacitive element C 33  increases in capacitance value, and the capacitive element C 34  decreases in capacitance value. On the other hand, the right end of the force receiving ring  100 B moves in the Z-axis negative direction (a direction in which it moves to the back of the sheet surface). Therefore, the diaphragm D 1  is inclined in a direction reverse to that shown in  FIG. 40 , and a distance d (up) at the upper part increases, while a distance d (down) at the lower part decreases. As a result, the capacitive element C 13  decreases in capacitance value, while the capacitive element C 14  increases in capacitance value. The other capacitive elements do not change in capacitance value. The above results are shown in the row +My in the table of  FIG. 39 . For the same reason, the results shown in the row +Mx in the table of  FIG. 39  are obtained upon exertion of moment +Mx which is positive rotation around the X-axis. 
     Finally, consideration will be given to a case where moment +Mz which is positive rotation around the Z-axis is exerted on the force receiving ring  100 B.  FIG. 41  is a cross sectional view on the XY-plane which shows a deformed state upon exertion of the moment +Mz around the Z-axis. In this figure as well, for the sake of convenience of explanation, only necessary constituents are extracted and each part is shown in a deformed manner. Therefore, shapes of individual parts do not precisely show an actual deformation mode. As shown in the figure, upon exertion of the moment +Mz, the force receiving ring  100 B rotates counterclockwise, by which the diaphragm D 1  is inclined as shown in the figure. In this case as well, the diaphragm D 1  is depicted as a flat plate. However, in practice, it assumes a complicated configuration including torsion. When consideration is given to variation in distance with the outer circumferential surface of the fixed assistant body  350 B, as shown in the figure, a distance d (left) on the left side (the upper side in  FIG. 41 ) decreases when viewed from the view point e 1 , and a distance d (right) on the right side (the lower side in  FIG. 41 ) increases. As a result, the capacitive element C 11  increases in capacitance value, while the capacitive element C 12  decreases in capacitance value. For the same reason, the capacitive elements C 21 , C 32 , C 42  increase in capacitance value, while the capacitive elements C 22 , C 31 , C 41  decrease in capacitance value. The other capacitive elements do not change in capacitance value. The row +Mz in the table of  FIG. 39  shows the above results. 
     The table in  FIG. 39  shows the results obtained upon exertion of force in a positive direction and moment which is positive rotation. The results in which [+] and [−] are reversed are to be obtained upon exertion of force in a negative direction and moment which is negative rotation. Consequently, patterns of change in capacitance values of 20 sets of the capacitive elements C 11  to C 45  differ depending on individual cases upon exertion of six-axis-components. The larger the exerting force and moment become, the larger the variance is. Thus, detection circuits are used to conduct a predetermined operation on the basis of measured values of the capacitance, thus making it possible to independently output detection values of the six-axis-components. 
       FIG. 42  is a view which shows specific arithmetic expressions for determining forces Fx, Fy, Fz in the direction of each coordinate axis and moments Mx, My, Mz around each coordinate axis which are exerted on the force sensor shown in  FIG. 37 . Each of C 11  to C 45  in the expressions shows a capacitance value of each of the capacitive elements C 11  to C 45  indicated by the same symbol. Reasons for obtaining individual detection values by the arithmetic expressions will be understood by referring to the table shown in  FIG. 39 . For example, a first expression on Fx shows a difference between a sum of capacitance values of five sets of capacitive elements which are indicated by [+] in the row +Fx in the table of  FIG. 39  and a sum of capacitance values of five sets of capacitive elements which are indicated by [−]. Second and third expressions are expressions where, of five sets of capacitive elements, only four sets or one set is used. This is also true for other detection values. 
     Further, [+] and [−] shown in the table of  FIG. 39  are reversed upon exertion of forces Fx, Fy, Fz in a negative direction and moments Mx, My, Mz, which is negative rotation. Therefore, the arithmetic expressions shown in  FIG. 42  are used as they are, by which individual detection values can be obtained as negative values. The arithmetic expressions of six-axis-components in  FIG. 42  are free of interference with other axis components, thus making it possible to obtain individual detection values of the six-axis-components independently. Still further, any of the arithmetic expressions can be used to calculate a difference. Therefore, even where a change in temperature environment causes the basic structure to swell or shrink, resulting in errors in which distances between counter electrodes vary, the errors can cancel each other out. Thus, it is possible to obtain accurate results free of disturbance components. 
     Here, examples of detection circuits for outputting detection values of the six-axis-components are omitted on the basis of the arithmetic expressions shown in  FIG. 42 . It is possible to constitute the detection circuits according to the circuit diagrams shown in  FIG. 18 . As a matter of course, a plurality of capacitive elements are connected in parallel, thus making it possible to omit computing elements for arithmetic addition. Further, A/D converters are used to take the capacitance values C 11  to C 45  individually as the digital values, by which each of the detection values can be output in terms of a digital value as a result of digital operation. 
     Still further, the structure of the force sensor shown in  FIG. 37  is one example, a detailed specification of which can be changed, whenever necessary, in view of design. For example, the diaphragm D 1  of the force sensor shown in  FIG. 37  is that in which, as shown in  FIG. 43(   a ) (a view in which the direction of origin O is observed from the positive direction of the X-axis), the contour is rectangular and a connection member  431  (the cross section of which is shown) is jointed at the center thereof. The above-shaped diaphragm D 1  can be formed by cutting a part of the detection ring  200 B and manufactured in a relatively simple process. However, the diaphragm is not necessarily restricted to the rectangular shape. 
     The example shown in  FIG. 43(   b ) (also a view in which the direction of origin O is observed from the positive direction of the X-axis) is an example in which formed is a diaphragm Dr having a circular contour. This example is similar in that a connection member  431  is jointed at the center of the diaphragm Dr. However, the diaphragm Dr is formed by grooving in a circular shape on a detection ring  200 B and a periphery thereof in its entirety is surrounded by thick parts of the detection ring  200 B. 
     On the other hand,  FIG. 44  is a plan view which shows modification examples of electrode groups used in the force sensor shown in  FIG. 37  (hatching is made to clarify the shape of each electrode and not for showing the cross section). In the force sensor shown in  FIG. 37 , as shown in the plan view of  FIG. 38 , one set of electrode groups is constituted by five electrodes consisting of one square electrode and four trapezoidal electrodes. The number, shape and arrangement of the electrodes which constitute each electrode group are not restricted to those of the example shown in  FIG. 38 .  FIG. 44(   a ) is an example in which one set of electrode groups is constituted by four isosceles-triangle electrodes. Further,  FIG. 44(   b ) is an example in which one set of electrode groups is constituted by five electrodes consisting of one square electrode and four L-letter shaped electrodes. Still further,  FIG. 44(   c ) is an example in which one set of electrode groups is constituted by four square electrodes. As a matter of course, in addition to this example, various constitutions of electrodes are also available. 
     A detailed description will be omitted here about a method for detecting six-axis-components where the constitutions of electrodes shown as modification examples in  FIG. 44  are adopted. In any case, a table according to  FIG. 39  can be prepared. This table can be used to define arithmetic expressions according to  FIG. 42 . 
     Finally, features of the embodiment described in Section 6 will be summarized as follows. First, as shown in  FIG. 35 , four exertion points Q 11  to Q 14  and four fixing points P 11  to P 14  are alternately arranged on an annular channel along the contour of the detection ring  200 B, and vicinities of the four exertion points Q 11  to Q 14  on the detection ring  200 B constitute diaphragms D 1  to D 4  which are thinner in thickness than other parts. Then, four connection members  431  to  434  are connected to the diaphragms D 1  to D 4  respectively at the exertion points Q 11  to Q 14 , and four fixing members  531  to  534  are connected to the lower surface of the detection ring  200 B at the fixing points P 1  to P 4 . 
     As a matter of course, the number of exertion points and that of fixing points are not necessarily restricted to four and can be set to any given plural number of n. In brief, it is acceptable that n number of plural exertion points Q 11  to Qin and n number of plural fixing points P 11  to Pln are alternately arranged on an annular channel along the contour of the detection ring  200 B, the vicinities of n number of exertion points Q 11  to Qln on the detection ring  200 B constitute diaphragms D 1  to Dn which are thinner in thickness than other parts, n number of connection members are connected to the individual diaphragms, and the fixing points P 11  to Pin are fixed individually on the supporting substrate  300 B by n number of fixing members. Then, detection elements for electrically detecting elastic deformation of each of the diaphragms D 1  to Dn may be added to the above-constituted basic structure. 
     However, in practical use, as described in the above embodiment, detection can be made in practice very efficiently in the following manner, that is, four exertion points and four fixing points are arranged on the annular channel along the contour of the detection ring  200 B in the order of the first exertion point Q 11 , the first fixing point P 11 , the second exertion point Q 12 , the second fixing point P 12 , the third exertion point Q 13 , the third fixing point P 13 , the fourth exertion point Q 14  and the fourth fixing point P 14 , then, the first diaphragm D 1  is constituted in the vicinity of the first exertion point Q 11  on the detection ring  200 B, the second diaphragm D 2  is constituted in the vicinity of the second exertion point Q 12 , the third diaphragm D 3  is constituted in the vicinity of the third exertion point Q 13 , the fourth diaphragm D 4  is constituted in the vicinity of the fourth exertion point Q 14 , and detection elements are used to electrically detect elastic deformation of the first to the fourth diaphragms D 1  to D 4 . 
     In particular, the above-described embodiment is the most efficient constitution in realizing a force sensor which is capable of independently detecting six-axis-components, that is, forces Fx, Fy, Fz in the direction of each coordinate axis and moments Mx, My, Mz around each coordinate axis, in an XYZ three-dimensional orthogonal coordinate system. To be more specific, regarding exertion points, the first exertion point Q 11 , the second exertion point Q 12 , the third exertion point Q 13  and the fourth exertion point Q 14  are arranged respectively at a positive domain of the X-axis, a positive domain of the Y-axis, a negative domain of the X-axis and a negative domain of the Y-axis. Further, the first diaphragm D 1 , the second diaphragm D 2 , the third diaphragm D 3  and the fourth diaphragm D 4  are to be positioned respectively at the positive domain of the X-axis, the positive domain of the Y-axis, the negative domain of the X-axis and the negative domain of the Y-axis. 
     Then, the first diaphragm D 1  is connected to the force receiving ring  100 B via a first connection member  431  extending along the positive domain of the X-axis, the second diaphragm D 2  is connected to the force receiving ring  100 B via a second connection member  432  extending along the positive domain of the Y-axis, the third diaphragm D 3  is connected to the force receiving ring  100 B via a third connection member  433  extending along the negative domain of the X-axis, and the fourth diaphragm D 4  is connected to the force receiving ring  100 B via a fourth connection member  434  extending along the negative domain of the Y-axis. 
     On the other hand, regarding the fixing points, it is acceptable that the V-axis is defined so as to pass through the origin O in an XYZ three-dimensional orthogonal coordinate system, a positive domain being positioned at a first quadrant of the XY-plane and a negative domain being positioned at a third quadrant of the XY-plane and to form 45 degrees with respect to the X-axis and the W-axis is defined so as to pass through the origin O in an XYZ three-dimensional orthogonal coordinate system, a positive domain being positioned at a second quadrant of the XY-plane and a negative domain being positioned at a fourth quadrant of the XY-plane and to be orthogonal to the V-axis, the first fixing point P 11 , the second fixing point P 12 , the third fixing point P 13  and the fourth fixing point P 14  are defined to be arranged respectively at the positive domain of the V-axis, the positive domain of the W-axis, the negative domain of the V-axis and the negative domain of the W-axis, and the detection ring  200 B is fixed to the supporting substrate  300 B at each of the fixing points. 
     Here, elastic deformation of each of the diaphragms D 1  to D 4  may be electrically detected by a method in which capacitive elements or others are used to measure displacement of each part or by a method in which strain gauges or others are used to measure a mechanical strain caused at each part. The force sensor shown in  FIG. 37  adopts the former method, and is provided with the following device in order to measure displacement efficiently by using the capacitive elements. 
     First, both the force receiving ring  100 B and the detection ring  200 B are given circular rings arranged on the XY-plane so that the Z-axis is a central axis, and these rings are arranged so that the force receiving ring  100 B is outside and the detection ring  200 B is inside. Then, in order to support the fixed electrodes, a cylindrical fixed assistant body  350 B whose lower surface is fixed onto the upper surface of the supporting substrate  300 B, with the Z-axis being a central axis, is provided further inside the detection ring  200 B. 
     Then, the detection elements are constituted by the following four capacitive element groups. That is: 
     (1) a first capacitive element group C 10  composed of a plurality of capacitive elements C 11  to C 15  constituted by a first displacement electrode group T 10  composed of a plurality of displacement electrodes T 11  to T 15  arranged at a first diaphragm D 1  on the inner circumferential surface of the detection ring  200 B and a first fixed electrode group U 10  composed of a plurality of fixed electrodes U 11  to U 15  arranged at positions facing individual displacement electrodes of the first displacement electrode group T 10  on the outer circumferential surface of the fixed assistant body  350 B, 
     (2) a second capacitive element group C 20  composed of a plurality of capacitive elements C 21  to C 25  constituted by a second displacement electrode group T 20  composed of a plurality of displacement electrodes T 21  to T 25  arranged at a second diaphragm D 2  on the inner circumferential surface of the detection ring  200 B and a second fixed electrode group U 20  composed of a plurality of fixed electrodes U 21  to U 25  arranged at positions facing individual displacement electrodes of the second displacement electrode group T 20  on the outer circumferential surface of the fixed assistant body  350 B, 
     (3) a third capacitive element group C 30  composed of a plurality of capacitive elements C 31  to C 35  constituted by a third displacement electrode group T 30  composed of a plurality of displacement electrodes T 31  to T 35  arranged at a third diaphragm D 3  on the inner circumferential surface of the detection ring  200 B and a third fixed electrode group U 30  composed of a plurality of fixed electrodes U 31  to U 35  arranged at positions facing individual displacement electrodes of the third displacement electrode group T 30  on the outer circumferential surface of the fixed assistant body  350 B, and 
     (4) a fourth capacitive element group C 40  composed of a plurality of capacitive elements C 41  to C 45  constituted by a fourth displacement electrode group T 40  composed of a plurality of displacement electrodes T 41  to T 45  arranged at a fourth diaphragm D 4  on the inner circumferential surface of the detection ring  200 B and a fourth fixed electrode group U 40  composed of a plurality of fixed electrodes U 41  to U 45  arranged at positions facing individual displacement electrodes of the fourth displacement electrode group T 40  on the outer circumferential surface of the fixed assistant body  350 B. 
     Here, there is found such a relationship that a projection image in which one of the electrodes constituting each of the capacitive elements C 11  to C 45  is projected on a surface on which the other electrode is formed is included in the other electrode. Then, detection circuits are used to output a predetermined detection value on the basis of a capacitance value of each of the capacitive elements C 11  to C 45 . 
     As shown in  FIG. 44 , electrodes of individual capacitive elements which are members of each of the capacitive element groups may be available differently as to the number, shape and arrangement pattern thereof. In the force sensor adopting the embodiment shown in  FIG. 38 , each of the capacitive element groups is available in the following constitution. 
     (1) The first capacitive element group C 10  is provided with an on-axis capacitive element C 15  of the first group arranged on the X-axis, a first capacitive element C 11  of the first group arranged in the positive direction of the Y-axis adjacent to the on-axis capacitive element C 15  of the first group, a second capacitive element C 12  of the first group arranged in the negative direction of the Y-axis adjacent to the on-axis capacitive element C 15  of the first group, a third capacitive element C 13  of the first group arranged in the positive direction of the Z-axis adjacent to the on-axis capacitive element C 15  of the first group, and a fourth capacitive element C 14  of the first group arranged in the negative direction of the Z-axis adjacent to the on-axis capacitive element C 15  of the first group, 
     (2) The second capacitive element group C 20  is provided with an on-axis capacitive element C 25  of the second group arranged on the Y-axis, a first capacitive element C 21  of the second group arranged in the positive direction of the X-axis adjacent to the on-axis capacitive element C 25  of the second group, a second capacitive element C 22  of the second group arranged in the negative direction of the X-axis adjacent to the on-axis capacitive element C 25  of the second group, a third capacitive element C 23  of the second group arranged in the positive direction of the Z-axis adjacent to the on-axis capacitive element C 25  of the second group, and a fourth capacitive element C 24  of the second group arranged in the negative direction of the Z-axis adjacent to the on-axis capacitive element C 25  of the second group, 
     (3) The third capacitive element group C 30  is provided with an on-axis capacitive element C 35  of the third group arranged on the X-axis, a first capacitive element C 31  of the third group arranged in the positive direction of the Y-axis adjacent to the on-axis capacitive element C 35  of the third group, a second capacitive element C 32  of the third group arranged in the negative direction of the Y-axis adjacent to the on-axis capacitive element C 35  of the third group, a third capacitive element C 33  of the third group arranged in the positive direction of the Z-axis adjacent to the on-axis capacitive element C 35  of the third group, and a fourth capacitive element C 34  of the third group arranged in the negative direction of the Z-axis adjacent to the on-axis capacitive element C 35  of the third group. 
     (4) The fourth capacitive element group C 40  is provided with an on-axis capacitive element C 45  of the fourth group arranged on the Y-axis, a first capacitive element C 41  of the fourth group arranged in the positive direction of the X-axis adjacent to the on-axis capacitive element C 45  of the fourth group, a second capacitive element C 42  of the fourth group arranged in the negative direction of the X-axis adjacent to the on-axis capacitive element C 45  of the fourth group, a third capacitive element C 43  of the fourth group arranged in the positive direction of the Z-axis adjacent to the on-axis capacitive element C 45  of the fourth group, and a fourth capacitive element C 44  of the fourth group arranged in the negative direction of the Z-axis adjacent to the on-axis capacitive element C 45  of the fourth group. 
     In the embodiment which uses the above-constituted capacitive element groups, capacitance values of the individual capacitive elements C 11  to C 45  are expressed as C 11  to C 45  by using the same symbols, by which the detection circuits are able to detect detection values of force Fx in the direction of the X-axis, force Fy in the direction of the Y-axis, force Fz in the direction of the Z-axis, moment Mx around the X-axis, moment My around the Y-axis and moment Mz around the Z-axis on the basis of the following arithmetic expressions: 
     
       
         
           
             
               
                 
                   Fx 
                   = 
                     
                   ⁢ 
                   
                     
                       - 
                       
                         ( 
                         
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             12 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             13 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             14 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             15 
                           
                         
                         ) 
                       
                     
                     + 
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           31 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           32 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           33 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           34 
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           35 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                         
                     
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                     or 
                   
                 
               
             
             
               
                 
                   = 
                     
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                         ( 
                         
                           
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                             ⁢ 
                             
                                 
                             
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                   ⁢ 
                   
                     
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                           31 
                         
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                           ⁢ 
                           32 
                         
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                           33 
                         
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                           34 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                         
                     
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                     or 
                   
                 
               
             
             
               
                 
                   = 
                     
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                       ⁢ 
                       
                           
                       
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                       35 
                     
                   
                 
               
             
           
         
       
       
         
           
             
               
                 
                   Fy 
                   = 
                     
                   ⁢ 
                   
                     
                       - 
                       
                         ( 
                         
                           
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                             21 
                           
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                             22 
                           
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                             23 
                           
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                             24 
                           
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                             25 
                           
                         
                         ) 
                       
                     
                     + 
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       ( 
                       
                         
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                           41 
                         
                         + 
                         
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                           42 
                         
                         + 
                         
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                           43 
                         
                         + 
                         
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                           44 
                         
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                           45 
                         
                       
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                     ⁢ 
                     
                         
                     
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                     or 
                   
                 
               
             
             
               
                 
                   = 
                     
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                         ( 
                         
                           
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                             22 
                           
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                             ⁢ 
                             
                                 
                             
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                             24 
                           
                         
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                           41 
                         
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                           42 
                         
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                           ⁢ 
                           44 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     or 
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       
                         - 
                         C 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       25 
                     
                     + 
                     
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                       ⁢ 
                       
                           
                       
                       ⁢ 
                       45 
                     
                   
                 
               
             
           
         
       
       
         
           
             Fz 
             = 
             
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     23 
                   
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                     ⁢ 
                     
                         
                     
                     ⁢ 
                     33 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     43 
                   
                 
                 ) 
               
               - 
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
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                     14 
                   
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                     24 
                   
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                     ⁢ 
                     
                         
                     
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                     34 
                   
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                     ⁢ 
                     
                         
                     
                     ⁢ 
                     44 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             
               M 
               ⁢ 
               
                   
               
               ⁢ 
               x 
             
             = 
             
               
                 ( 
                 
                   
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                     ⁢ 
                     
                         
                     
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                     44 
                   
                 
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                     24 
                   
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                     43 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             My 
             = 
             
               
                 ( 
                 
                   
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                     ⁢ 
                     
                         
                     
                     ⁢ 
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                   + 
                   
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                     ⁢ 
                     
                         
                     
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                     33 
                   
                 
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               - 
               
                 ( 
                 
                   
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                     13 
                   
                   + 
                   
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                     ⁢ 
                     
                         
                     
                     ⁢ 
                     34 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             Mz 
             = 
             
               
                 ( 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   + 
                   
                     C 
                     ⁢ 
                     
                         
                     
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                     21 
                   
                   + 
                   
                     C 
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                     ⁢ 
                     32 
                   
                   + 
                   
                     C 
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                     42 
                   
                 
                 ) 
               
               - 
               
                 
                   ( 
                   
                     
                       C 
                       ⁢ 
                       
                           
                       
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                       12 
                     
                     + 
                     
                       C 
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                       22 
                     
                     + 
                     
                       C 
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                       31 
                     
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                       41 
                     
                   
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                 . 
               
             
           
         
       
     
     Also in the embodiment shown in  FIG. 37 , at least the diaphragms D 1  to D 4  on the detection ring  200 B are composed of a flexible conductive material, by which the surface of each of the diaphragms can be given as a common displacement electrode E 0  to constitute each of the capacitive elements C 11  to C 45 . In practice, the detection ring  200 B in its entirety is constituted by a metal such as an aluminum alloy so that the diaphragms D 1  to D 4  thinner in thickness can be flexible. In this case, the diaphragms D 1  to D 4  of the detection ring  200 B have functions as the displacement electrode groups T 10 , T 20 , T 30 , T 40 , thereby eliminating a necessity for providing a displacement electrode as a separate body. 
     &lt;&lt;&lt;Section 7. Other Modification Examples&gt;&gt;&gt; 
     Finally, a description will be given of modification examples applicable to the embodiments described above. 
     &lt;7-1. Modification Example on Detection of Displacement&gt; 
     In Section 3 and Section 6, a description has been given of the embodiments in which capacitive elements are used to detect displacement at predetermined sites on the detection ring. The detection elements used in detecting the displacement are not necessarily limited to capacitive elements, and any detection elements may be used as long as they are, in general, elements capable of detecting a distance. 
       FIG. 45  is a cross sectional view in which a force sensor according to a modification example having distance detection elements M 1  to M 4  on the V-axis and the W-axis on an outer circumferential surface of a fixed assistant body  350  is cut along the XY-plane in order to measure distances d 1  to d 4  at the basic structure shown in  FIG. 10 . Here, although an inner structure of each of the distance detection elements M 1  to M 4  is not shown, any elements can be used as long as it is a generally used element for detecting a distance. 
     For example, at least a measurement target surface of the detection ring  200  (an inner circumferential surface in the vicinities of the V-axis and the W-axis in the example shown in the figure) is constituted by a conductive material, by which an eddy current displacement sensor can be used as the distance detection elements M 1  to M 4  provided on the reference surface facing thereto.  FIG. 46  is a view which shows a basic principle of measuring a distance by an eddy current displacement sensor. The eddy current displacement sensor is a device which is constituted by a high-frequency oscillation circuit  71  and a coil  72 , having functions to electrically detect a distance between the coil  72  and a conductive detection object  73  arranged adjacent to the coil  72 . 
     The following is a basic principle of detecting a distance by the eddy current displacement sensor. First, when high-frequency current is allowed to flow from the high-frequency oscillation circuit  71  to the coil  72 , the coil  72  generates a high-frequency field  74 . Next, eddy current  75  flows to the conductive detection object  73  due to electro-magnetic induction of the high-frequency field  74 . Then, the eddy current  75  causes a change in impedance of the coil  72 , thus resulting in a change in oscillation of the high-frequency oscillation circuit  71 . The impedance change of the coil  72  depends on a distance between the coil  72  and the detection object  73 . Therefore, a circuit for detecting a change in oscillation is provided on the high-frequency oscillation circuit  71 , by which it is possible to electrically detect a distance between the coil  72  and the detection object  73 . 
     Further, at least a measurement target surface of the detection ring  200  is constituted by a magnet, by which a Hall element can be used as distance detection elements M 1  to M 4  provided on a reference surface facing thereto. The Hall element is an element having functions to detect a magnetic field by the Hall effect. Therefore, as long as the measurement target surface of the detection ring  200  is constituted by a magnet, it is possible to detect displacement of the measurement target surface on the basis of variation in a magnetic field. To be more specific, the detection ring  200  in its entirety may be constituted by a magnet (in this case, it is necessary to use a magnet having a mechanical strength resistant to deformation) or a magnet may be firmly attached onto the inner circumferential surface of the detection ring  200 . The magnetic field exerted on the Hall elements arranged as the distance detection elements M 1  to M 4  change in strength depending on displacement of the measurement target surface. Therefore, it is possible to use a detection value of the magnetic field of each of the Hall elements as a measured value of distance. 
     It is also possible to use a distance meter using a light beam as the distance detection elements M 1  to M 4 . It is acceptable that, for example, a light beam irradiator for irradiating a light beam obliquely to a measurement target surface (in the example shown in the figure, an inner circumferential surface in the vicinities of the V-axis and the W-axis) and a light beam receiver for receiving the light beam reflected on the measurement target surface are fixed on a counter reference surface facing the measurement target surface (in the example of the figure, an outer circumferential surface of the fixed assistant body  350 ) and there is also provided a measurement circuit for outputting a measured value of distance on the basis of a light receiving position of the light beam by the light beam receiver. When the measurement target surface of the detection ring  200  (the inner circumferential surface) undergoes displacement, the light beam is changed in position of irradiation and the reflected beam is changed in direction of emission. Thereby, the light beam detected by the light beam receiver is also changed in light receiving position. Thus, the measurement circuit is able to output a measured value of distance on the basis of the light receiving position. 
     &lt;7-2. Modification Example on Shape and Arrangement of Each Part&gt; 
     In the embodiments which have been described above, circular ring members are used as the force receiving ring  100  and the detection ring  200 . These rings are not necessarily a circular ring body but may be a loop-shaped member having an opening. They may be annular rings which are, for example, in a right octagonal, right hexagonal or square shape. They may be also annular rings in any given shape. 
     However, in practice, a circular ring body is preferably used as shown in the embodiments described above. Circular ring members are used as the force receiving ring  100  and the detection ring  200  and arranged concentrically, with the Z-axis being the central axis, by which the basic structure is structured to be in plane symmetry about both the XZ-plane and the YZ-plane. Here, each of the detection elements is arranged so as to be in plane symmetry about both the XZ-plane and the YZ-plane as the embodiments described above, thus making it possible to simplify signal processing by detection circuits. 
     For example, in the arithmetic expressions shown in  FIG. 17 ,  FIG. 20 ,  FIG. 42 , each term indicating a capacitance value is not multiplied by a coefficient. This is due to the fact that the above-described symmetry is secured. Where the detection ring  200  is constituted by an annular body in any given shape, it is necessary to multiply each term of the arithmetic expressions by a predetermined coefficient. As a result, the actual detection circuit ends up being complicated in constitution. 
     Further, shapes of the displacement electrode and the fixed electrode constituting a capacitive element are not limited to those shown in the embodiments described above. There may be adopted electrodes in any shape. However, as described above, in order to simplify the arithmetic expressions for obtaining detection values of individual axis components, it is preferable that an electrode forming pattern is such that which is in plane symmetry about both the XZ-plane and the YZ-plane. 
     &lt;7-3. Omission Of Constituents Unnecessary for Detection&gt; 
     Each of the embodiments which have been described above is such a force sensor that mainly detects six-axis-components. However, it is sufficient for the force sensor according to the present invention to have functions to detect at least force or moment with regard to one axis, among force in the direction of each coordinate axis and moment around each coordinate axis in an XYZ three-dimensional orthogonal coordinate system. Therefore, the present invention is applicable to a force sensor for detecting only force Fx exerted in the direction of the X-axis or a force sensor for detecting only moment My exerted around the Y-axis. 
     Therefore, in manufacturing a force sensor specialized in detecting only a particular component, it is possible to omit constituents which are not necessary for detection. For example, the force sensor shown in  FIG. 37  is provided with functions to detect all the six-axis-components. However, where only force Fx exerted in the direction of the X-axis is required for detection, at least, the capacitive elements C 15  and C 35  will be sufficient as shown in the cell of Fx in  FIG. 42 . Similarly, where only moment My exerted around the Y-axis is required for detection, it is sufficient to provide only the capacitive elements C 13 , C 14 , C 33 , C 34  as shown in the cell of My in  FIG. 42 . 
     &lt;7-4. Combination of Disclosed Technical Ideas&gt; 
     A description has been above given of technical ideas of the present invention by referring to various embodiments and also given of technical ideas on various modification examples. Many technical ideas described in the present application can be combined freely, unless special restrictive reasons are found. For example, a technical idea that an inside/outside positional relationship between the force receiving ring  100  and the detection ring  200  are reversed is applicable to the various embodiments which have been described above. Further, it is possible to use a strain gauge for detecting elastic deformation of the basic structure shown in  FIG. 35  (in this case, no fixed assistant body  350 B is required). It is also possible to fix the detection ring  200 B via fixing members to the fixed assistant body  350 B at the basic structure shown in  FIG. 35 . 
     The present application does not describe embodiments covering all and any possible combinations on the basis of various technical ideas which have been disclosed. However, a person skilled in the art is able to freely design an embodiment which is not directly disclosed in the present application by combining various technical ideas which have been disclosed in the present application. 
     INDUSTRIAL APPLICABILITY 
     The force sensor according to the present invention is optimally used in detecting force and moment for controlling motions of a robot and an industrial machine. The force sensor can be also used as a man/machine interface of an input device of electronics. This is in particular usable as a thin-type force sensor which detects six-axis-components which are force in the direction of each coordinate axis and moment around each coordinate axis in an XYZ three-dimensional orthogonal coordinate system.