Strain element, strain element manufacturing method, and physical quantity measuring sensor

A strain element (10), which is configured such that a frame portion (11) and a central portion (12) are connected by arm portions (20) to (22), is masked except for the arm portions (20) to (22) where a strain gauge (A1) and the like are to be disposed, and then peening is carried out. With this, a compressive residual stress layer is formed on four sides of each of the arm portions (20) to (22). When the strain element (10) receives a load resulting from an external force, the arm portions (20) to (22) elastically deform; however, due to the compressive residual stress layer thus formed, the arm portions (20) to (22) are less prone to fatigue failure. When projection of a shot material is carried out as peening, the surface roughness of the arm portions (20) to (22) increases, the adhesion of strain gauges improves, detection accuracy improves, and stable measurement can be ensured.

TECHNICAL FIELD

The present invention relates to a strain element and a physical quantity measurement sensor including the strain element. In particular, the present invention relates to a strain element, a method of producing a strain element, and a physical quantity measurement sensor in each of which the strain element has improved resistance to fatigue failure and ensures a long-term stable use.

BACKGROUND ART

There have conventionally been physical quantity measurement sensors such as a force sensor, a torque sensor, a load cell, and the like. A physical quantity measurement sensor detects, through use of a plurality of strain gauges, strain associated with elastic deformation caused by an external load (external force), and, from the results of the detection, measures the values of physical quantities regarding the external force, moment, and the like. Such a physical quantity measurement sensor generally includes a metal strain element that elastically deforms under an external force, and detects strain by disposing a plurality of strain gauges on this strain element. Note that specific examples of the physical quantity measurement sensor are disclosed in Patent Literatures 1 to 5 and Non-patent Literature 1 below.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

A physical quantity measurement sensor is structured such that, as described earlier, a strain element elastically deforms under an external force; therefore, if the physical quantity measurement sensor is used over a long period of time, metal fatigue builds up in the elastically deformable area of the strain element. Therefore, there is a problem in that if the built-up metal fatigue passes a critical point, fatigue failure occurs in the strain element. Note that the strain element is produced generally by machining (for example, cutting). Depending on design specifications and the like, portions having a sharp shape and/or an acute angled corner shape may be generated. Such portions are likely to undergo stress concentration when receiving an external force, and, in such areas where stress concentrates, the risk of the foregoing fatigue failure is significant.

Furthermore, a strain gauge disposed on the strain element is, in some cases, bonded with an adhesive. In a case where the strain gauge is attached by bonding, depending on the degree of elastic deformation (e.g., compressive deformation or tensile deformation) of the strain element, the layer of the adhesive may not conform to the elastic deformation, and sliding may occur between the area where the strain gauge is attached and the strain gauge. If such sliding occurs, a problem arises in that strain associated with elastic deformation cannot be detected accurately by the strain gauge and that measurement accuracy decreases.

Moreover, a strain element employed in a force sensor disclosed in Patent Literature 1 includes a plurality of arm portions which connect a central portion with a frame portion. The strain element is configured such that elastic portions (flexures) are provided between the frame portion and the arm portions (see paragraphs 0019 and 0020 of Patent Literature 1), and that each of the arm portions has a plurality of strain gauges disposed thereon (see paragraphs 0024, 0025 and FIG. 1 and the like of Patent Literature 1). The manner in which the strain gauges are disposed, as shown in FIGS. 1 to 3 and FIGS. 7 to 10, 12 and 13 of Patent Literature 1, has a problem in that it is difficult to detect strain associated with deformation when an external force is exerted on the arm portions in specific directions. Specifically, there is a problem in accuracy of strain detection when an external force is exerted in directions Mz, Fx, and Fy shown in FIG. 11 of Patent Literature 1.

An aspect of the present invention was made in view of the above circumstances, and an object thereof is to provide a strain element, a method of producing a strain element, and a physical quantity measurement sensor in each of which the strain element has improved strength (resistance) to fatigue failure.

Another object of an aspect of the present invention is to provide a strain element, a method of producing a strain element, and a physical quantity measurement sensor in each of which, in a case where a strain gauge is bonded with an adhesive or the like, the strain gauge is good at conforming to elastic deformation of the strain element.

A further object of an aspect of the present invention is to provide a strain element, a method of producing a strain element, and a physical quantity measurement sensor in each of which, in a case where the strain element is configured such that a central portion and a frame portion are connected by arm portions and that elastic portions are provided between the frame portion and the arm portions, the accuracy of strain detection is ensured even if the arm portions receive an external force in directions such as the foregoing specific directions.

Solution to Problem

In order to attain the above object, an aspect of the present invention is directed to a strain element which is elastically deformable in response to a load and which is configured to have a strain gauge disposed thereon, the strain gauge being configured to detect strain associated with deformation, the strain element including a strain portion which corresponds to a region subject to strain and which includes an area for disposition of the strain gauge, the strain portion being provided with a residual stress layer having negative residual stress.

An aspect of the present invention is directed to a method of producing a strain element which is elastically deformable in response to a load and which is configured to have a strain gauge disposed thereon, the strain gauge being configured to detect strain associated with deformation, the method including the steps of: a) masking the strain element except for a strain portion which corresponds to a region subject to strain and which includes an area for disposition of the strain gauge; and b) projecting a shot material at the strain element which has been masked, the step b) including causing the shot material to collide with the strain portion and thereby producing the strain element in which the strain portion is provided with a residual stress layer having negative residual stress and in which the strain portion has a surface roughness rougher than a portion other than the strain portion.

Advantageous Effects of Invention

According to an aspect of the present invention, a residual stress layer having compressive residual stress is formed in a strain portion. It is therefore possible to improve the resistance to fatigue failure of a portion which elastically deforms and in which a strain gauge carries out detection. This makes it possible to achieve a long-term stable use of a physical quantity measurement sensor in which the strain element in accordance with an aspect of the present invention is employed.

According to an aspect of the present invention, a shot material is projected under the condition in which a strain element is masked except for the strain portion. Therefore, a residual stress layer can be formed in the strain portion which is left unmasked. Furthermore, it is also possible to increase the surface roughness of the strain portion. This makes it possible to efficiently produce a strain element which is highly resistant to fatigue failure and in which a strain gauge of a bonded type is better at conforming to deformation because of the anchor effect provided by an adhesive.

DESCRIPTION OF EMBODIMENTS

FIGS.1to3illustrate a force sensor1, which is a specific example of a physical quantity measurement sensor in accordance with Embodiment 1 of the present invention. (a) ofFIG.1is a front view of the force sensor1, (b) ofFIG.1is a back view of the force sensor1, (a) ofFIG.2is a side view of the force sensor1, (b) ofFIG.2is a cross-sectional view of the force sensor1, andFIG.3is a cross-sectional view of main parts of the force sensor1. The force sensor1illustrated in the drawings such asFIG.1is for application in an industrial robot arm. As illustrated in (a) ofFIG.2, the force sensor1has a structure in which three disk-shaped members are stacked together. Furthermore, as illustrated in (a) ofFIG.2, the force sensor1is configured such that a table block2is attached to a robot hand-side (front side) and a base block6is attached to a robot arm-side (back side), and such that the strain element10in accordance with Embodiment 1 of the present invention is sandwiched between the table block2and the base block6.

The following description discusses X axis, Y axis, and Z axis shown in the drawings such asFIGS.1and2. The X axis is an axis that is parallel to the horizontal direction (transverse direction) of the force sensor1. The Y axis, which is orthogonal to the X axis, is an axis that is parallel to the vertical direction (height direction) of the force sensor1. The Z axis, which is orthogonal to the X axis and the Y axis, is an axis that is parallel to the thickness direction of the force sensor1(the same applies to the following descriptions). The force sensor1in accordance with Embodiment 1 is capable of measuring values regarding external force in directions of the respective X, Y and Z axes and moments about the respective X, Y and Z axes through strain detection by a plurality of strain gauges provided on the strain element10(force sensor1corresponds to a six-axis force sensor).

As illustrated in (a) ofFIG.1, the table block2is in the shape of a circle when viewed from front side, and has many holes (through-holes) in a flat front face2acorresponding to the front side. Specifically, the table block2has, in the vicinity of the center of a circle, bolt through-holes3a,3b, and3c(countersunk through-holes for passage of bolts) which are arranged to form an equilateral triangle symmetrical with respect to center line Y1in the height direction. The table block2also has locating through-holes4a,4b, and4c(through-holes with fit tolerance) which are arranged to form an inverted equilateral triangle symmetrical with respect to the center line Y1. The table block2further has hand-attaching screw holes5a,5b, and5c(internally threaded holes) which are arranged in the vicinity of the outer circumference to form an inverted equilateral triangle. These hand-attaching screw holes5a,5b, and5care used for attachment to the robot hand.

Furthermore, the table block2has, on a back face2bopposite the foregoing front face2a, a doughnut-shaped groove2caround a center portion2dwhich has the foregoing bolt through-holes3ato3cand locating through-holes4ato4c(seeFIG.1and (b) ofFIG.2). The thickness (dimension along the Z axis direction) of the outer contour of the groove2cis slightly less than the thickness of the center portion2d, and thereby a shape in which the center portion2dslightly protrudes is provided. With this, the back face2bof the center portion2dof the table block2makes contact with a front face10aof the strain element10, when the force sensor1is in an assembled state.

On the other hand, the base block6is also in the shape of a circle when seen from back side, as illustrated in (b) ofFIG.1. The base block6has arm-attaching screw holes7a,7b, and7c(internally threaded holes) which are arranged in the vicinity of the outer circumference of a flat front face6acorresponding to the back side to form an inverted equilateral triangle. The arm-attaching screw holes7a,7b, and7cin the base block6are used for attachment to the robot arm. Furthermore, the back face6b, opposite the front face6a, has a hollow6cin the center portion as also illustrated in (b) ofFIG.2. The back face6bis provided with screw holes8a,8b, and8c(internally threaded holes) which are arranged in the vicinity of the outer circumference to form an equilateral triangle. The back face6balso has locating holes9ato9csuch that the locating holes9ato9care adjacent to the respective screw holes8ato8c(see holes represented by dashed lines in (b) ofFIG.1). Note that such table block2and base block6are each produced from a lightweight metal material (for example, aluminum-based material) in order not to greatly affect the weight capacity of an industrial robot arm to which the force sensor1is applied.

FIGS.4and5illustrate a front side and a back side of the strain element10in accordance with Embodiment 1 of the present invention. As illustrated in (a) ofFIG.2, the strain element10is a plate-like member which is smaller in dimension in the thickness direction (dimension in the Z axis direction) than the foregoing table block2and base block6, and has a circular circumferential outline when seen from the front side and the back side. The strain element10includes: a peripheral frame portion11; a central portion12which is located in a space defined by the frame portion11so as to be spaced apart from the frame portion11; and three arm portions20,21and22which connect the frame portion11and the central portion12. Note that, in this example, the arm portions20to22are included in strain portions which correspond to regions subject to strain associated with elastic deformation.

The arm portions20to22radially extend outward from the center of the strain element10, and are disposed along a circumferential direction of the strain element10having a circular outer circumference so as to be spaced apart from each other by 120 degrees. Each of such arm portions20to22is, because of the structure, less rigid than the frame portion11and the central portion12, and each of the arm portions20to22is configured to elastically deform in response to an external load or moment.

The central portion12has an outer circumference substantially in the shape of an equilateral hexagon, and has, within the outer circumference, screw through-holes13a,13b, and13c(internally threaded through-holes) which are arranged to form an equilateral triangle, and locating through-holes14a,14b, and14c(through-holes with fit tolerance) which are arranged to form an inverted triangle. The screw through-holes13ato13ccorrespond to the bolt through-holes3ato3cof the foregoing table block2, and the locating through-holes14ato14ccorrespond to the locating through-holes4ato4cof the foregoing table block2. Furthermore, the central portion12, whose outline is substantially in the shape of a hexagon, connects to the arm portions20,21, and22at middle portions of outer peripheral edge portions12c,12d, and12ewhich are adjacent to and correspond to the locating through-holes14a,14b, and14c.

The frame portion11has an outer contour in the form of a circle, and an inner contour in the form of a hexagon which is obtained by uniformly enlarging the contour of the foregoing central portion12. The frame portion11has bolt through-holes18a,18b, and18cwhich are arranged to from an equilateral triangle, and locating through-holes19ato19cwhich are arranged adjacent to the respective bolt through-holes18ato18c. The bolt through-holes18ato18ccorrespond to the screw holes8ato8cof the foregoing base block6, and the locating through-holes19ato19ccorrespond to the locating holes9ato9cof the foregoing base block6.

The frame portion11is connected to the arm portions20,21, and22at middle portions of inner peripheral edge portions11c,11d, and11elocated opposite the outer peripheral edge portions12c,12d, and12eof the foregoing central portion12. Because of the presence of such arm portions20,21, and22, the space between the frame portion11and the central portion12is divided into three, resulting in formation of a first space15, a second space16, and a third space17. The frame portion11further has three through-openings25,26, and27(each corresponding to first through-opening) in the junctions where the frame portion11connects to the respective arm portions20to22. These through-openings25to27are in the shape of straight lines along the inner peripheral edge portions11c,11d, and11eat the inner circumference in the form of a hexagon, and are equal to or slightly greater in length than the edges of the respective inner peripheral edge portions11c,11d, and11e. Each of the through-openings25to27has a width that is set to a value within the range of about ⅛ to ⅕ of its length (in Embodiment 1, set to about 1/6.5).

The strain element10has the through-openings25to27in the frame portion11, and is thereby arranged so that deformability in directions of stretch of the arm portions20to22is reduced and that strain of the arm portions20to22associated with elastic deformation, in directions other than the directions of stretch, is easily detected.

The arm portions20to22, which are elastically deformable, each have, disposed on its arm front face20a,21aor22aillustrated inFIG.4corresponding to the front side of the strain element10, a set of four strain gauges (strain gauges C1to C4, strain gauges B1to B4, or strain gauges A1to A4). Also, the arm portions20to22each have, disposed on its arm back face20b,21b, or22billustrated inFIG.5corresponding to the back side of the strain element10, a set of four strain gauges C1′ to C4′, strain gauges B1′ to B4′, or strain gauges A1′ to A4′.

Such strain gauges A1to C4′ carry out detection of strain associated with elastic deformation of the arm portions20to22. The strain is detected from an electric change in resistance that occurs when the arm portions20to deform. The strain gauges A1to C4′ change their resistance in response to deformation of the arm portions. Therefore, strain is detected based on a change in output voltage of a bridge circuit illustrated inFIG.9(described later) associated with a change in resistance in the bridge circuit. Furthermore, the strain gauges A1to C4′ are capable of detecting strain in respective predetermined directions (hereinafter “detection directions”). By arranging the strain gauges A1to C4′ so that their detection directions are oriented as desired, detection suitable for strain such as bending, shearing, and/or the like of the arm portions20to22is carried out (see explanations forFIGS.6and7provided later).

The strain gauges A1to C4′ are each composed of: a thin metal film containing Cu—Ni as a main material and including a pattern; and a flexible resin film (polyimide-and-epoxy-based resin) that covers the thin metal film. Note that the main material for the strain gauges A1to C4′ is not limited to the above mentioned main material. Besides the above-mentioned main material, also Cu, Ni, Al, Ti, Cr, Ge, Ni—Cr, Si semiconductor, Cr—O, Cr—N, and the like can be used as the main material. Furthermore, the strain gauges A1to C4′ used in Embodiment 1 are of a type in which a base material for a strain gauge is coated with a strain gauge main material.

FIG.6illustrates, with use the arm portion21parallel to the Y axis direction as an example, a manner in which strain gauges are disposed on an arm front face corresponding to the front side of the strain element10. The arm portion21has the strain gauges B1to B4disposed on the arm front face21a(corresponding to one face of the arm portion). On the arm portion21, the strain gauges B1to B4are disposed such that they are symmetrical with respect to center line Y10which extends along the direction of extension of the arm portion21(corresponding to a direction which connects the central portion12with the frame portion11) (line that is parallel to the Y axis on the arm front face21aand that passes through the center of the strain element10).

Specifically, the strain gauges B1and B2(corresponding to the first strain gauge and the second strain gauge), of the set of four strain gauges B1to B4, are disposed in an area close to the central portion12such that their detection directions K1and K2are parallel to the center line Y10. Note that, inFIG.6, square-shaped parts disposed vertically on the left-hand side of the strain gauge B1are connection parts B1aand B1b(positive and negative connection parts) for electrical connection to the strain gauge B1. These connection parts B1aand B1bhave a lead wire (not illustrated) connected thereto (the same applies to square-shaped parts adjacent to the other strain gauges B2to B4illustrated inFIG.6).

The strain gauges B3and B4(corresponding to the third strain gauge and the fourth strain gauge), of the set of four strain gauges B1to B4, are disposed in an area close to the frame portion11such that their detection directions K3and K4are at an angle to the center line Y10so as to diverge away from each other with decreasing distance to the central portion12. Note that, in Embodiment 1, the detection directions K3and K4are each at an angle of 45 degrees to the center line Y10. The manner of disposition has been discussed using the strain gauges B1to B4on the arm front face21aof the arm portion21as an example. However, the same applies to the disposition of the strain gauges C1to C4on the arm front face20aof the arm portion20and to the disposition of the strain gauges A1to A4on the arm front face22aof the arm portion22.

FIG.7illustrates, with use of the arm portion21parallel to the Y axis direction as an example, a manner in which strain gauges are disposed on an arm back face corresponding to the back side of the strain element10, similarly to the case ofFIG.6. The disposition illustrated inFIG.7is one obtained by flipping the disposition illustrated inFIG.6about center line Y11(center line of the arm back face, corresponding to the center line Y10illustrated inFIG.6).

Specifically, the strain gauges B1′ and B3′, of the four strain gauges B1′ to B4′, are positioned on the right-hand side of the center line Y11, and the strain gauges B2′ and B4′ of the four strain gauges B1′ to B4′ are positioned on the left-hand side of the center line Y11, such that the strain gauges B1′ and B3′ and the strain gauges B2′ and B4′ are symmetrical with respect to the center line Y11. Furthermore, the strain gauges B1′ and B2′ (corresponding to the first strain gauge and the second strain gauge) are disposed in an area close to the central portion12such that their respective detection directions K1′ and K2′ are parallel to the center line Y11. The strain gauges B3′ and B4′ (corresponding to the third strain gauge and the fourth strain gauge) are disposed in an area close to the frame portion11such that their detection directions K3′ and K4′ are at an angle to the center line Y11so as to diverge away from each other with decreasing distance to the central portion12. Note that the angle here is the same as that ofFIG.6, and is 45 degrees.

Note that square-shaped parts illustrated adjacent to each strain gauge, such as the strain gauge B1′, illustrated inFIG.7are electrical connection parts, as with the case ofFIG.6. The manner of disposition has been discussed using the strain gauges B1′ to B4′ on the arm back face21bof the arm portion21as an example. However, the same applies to the disposition of the strain gauges C1′ to C4′ on the arm back face20bof the arm portion20and to the disposition of the strain gauges A1′ to A4′ on the arm back face22bof the arm portion22.

(a) to (c) ofFIG.8illustrate a masked state in a surface processing step of a process of producing the strain element10on which the strain gauges A1to C4′ are to be disposed (such a process corresponds to a method of producing a strain element in accordance with Embodiment 1 of the present invention). The strain element10itself is made mainly from a lightweight, elastically deformable metal material. For example, by cutting an aluminum-based material (such as A5052) or a stainless-steel-based material (such as SUS304) by machining, it is possible to form the material into the shape illustrated in the foregoing drawings such asFIGS.4and5.

However, machining (cutting) alone is insufficient to avoid, for example, generation of burrs on peripheries of a processed product (unfinished strain element10), and portions shaped like corners (corner portions) are likely to undergo stress concentration when a load (external force) is exerted. To address this, the step of corner easing is carried out with respect to a machined, processed product, and thereby burrs and the like are removed from edges, corners, and the like, at each of which two or more faces meet, of the peripheries of the processed product. Note that such corner easing may be carried out by any of slight chamfering, chamfering, or filleting; however, it is preferable that the corners of the portions that are likely to undergo stress concentration are eased by filleting to eliminate sharp corners and thereby the occurrence of stress concentration is prevented as much as possible.

In Embodiment 1, areas enclosed by dot-dot-dash lines illustrated inFIGS.6and7are filleted. Specifically, the strain element10, which has a shape illustrated in the drawings such asFIGS.4and5, is structured such that the frame portion11and the central portion12are connected by the arm portions20to22. Corner portions (for example, corner portions corresponding to the areas enclosed by dot-dot-dash lines with signs21gand21hinFIGS.6and7) at edges of a junction where the frame portion11and the arm portion20,21, or22connect to each other (the junction is, for example, the area indicated by sign21dinFIGS.6and7) are portions where stress concentration is likely to occur. Also, corner portions (for example, corner portions corresponding to the areas enclosed by dot-dot-dash lines with signs21eand21finFIGS.6and7) of edges of a junction where the central portion12and the arm portion20,21, or22connect to each other (the junction is, for example, the area indicated by sign21cinFIGS.6and7) are portions where stress concentration is likely to occur. Therefore, these corner portions corresponding to the areas with the signs21e,21f,21g, and21h, enclosed by dot-dot-dash lines, are filleted and stress concentration is to be reduced. Note that the radius of filleting can be a value within the range of about 0.1 to 0.3 mm, and, because of, for example, the relationship in dimensions between the strain element10and the arm portions20to22, a value around 0.2 mm is preferred. Note that the radius of curvature (for example, the radius of curvature R corresponding to the sign21einFIGS.6and7) of each of the corner portions indicated by the signs21eto21hillustrated inFIGS.6and7can have a value within the range of about 1.5 to 3.5 mm, and, in one example, a value of about 2 mm can be employed.

After the foregoing machining and corner easing are carried out, surface processing is carried out. Before the surface processing step is carried out, the strain element10is masked as illustrated in (a) to (c) ofFIG.8. Note that the cross-hatched areas in (a) to (c) ofFIG.8correspond to masked areas. Also in the following descriptions, the cross-hatched areas correspond to masked areas.

In this masking step, the strain element10is masked by adhesive tape T except for the arm portions20to22which include areas for disposition of the strain gauges A1to C4′ (i.e., the frame portion11and the central portion12are masked). Note that side faces in the thickness direction, such as outwardly facing faces of the central portion12, inwardly facing faces of the frame portion11, and an outwardly facing face10cof the strain element10(frame portion11), are also masked. Therefore, with regard to each of the arm portions20to22which are left unmasked, an area extending from a corresponding one of central junctions20cto22c(where the arm portion connects to the central portion12) to a corresponding one of outer junctions20dto22d(where the arm portion connects to the frame portion11) of a corresponding one of the arm front faces20ato22ais exposed. Also, on each of the arm back faces20bto22b, an area extending from a corresponding one of the central junctions20cto22cto a corresponding one of the outer junctions20dto22d(where the arm portion connects to the frame portion11) is exposed. Furthermore, opposite side faces of each of the arm portions20to22are also exposed. As such, all four sides of each of the arm portions20to22are exposed.

Next, the strain element10which has been masked is inserted into a shot blasting machine (or shot peening machine), and the step of projecting a shot material at the strain element10and thereby causing the shot material to collide with the strain element10is carried out. Examples of the shot material include abrasive grains, steel shots, steel grids, cut wires, glass beads, and organic matter. In this step, the four sides of each of the arm portions20to22, which are left unmasked, are only struck directly with the shot material. Therefore, the four sides (surfaces) of each of the arm portions20to22undergo plastic deformation due to collision with the shot material, and a residual stress layer having compressive residual stress (negative residual stress) (compressive residual stress layer which will become a hardened surface layer) is formed. Also, the four sides (surfaces) of each of the arm portions20to22are given a surface roughness rougher than those of the masked areas.

In the above step, the shot material collides also with the masked areas; however, the force of the collision is weakened by the mask. Thus, the collision is indirect collision, and the residual stress occurring in the non-masked arm portions20to22is greater (in absolute value of the compressive residual stress) than those in the other masked portions. The main material for use in masking is preferably one that enables easy masking operation, like a tape material such as adhesive tape. However, any material that can cover the strain element and thereby alleviate the colliding force of the shot material can be employed (besides tape, various kinds of coating materials can be employed).

Note that the following description will discuss an example in which the projection was carried out (shot blasting was carried out) with respect to a stainless-steel-based member (SUS304) as the strain element10with use of abrasive grains as the shot material. On each of the arm portions20to22, a residual stress layer having a −938 MPa residual stress (negative residual stress) is formed. Also, the surface roughness, whose value before processing was Rz (maximum roughness depth)=1.020 μm, became Rz=7.682 μm after the processing. The surface roughness after the processing was about 7 times as much as that before the processing.

With regard to the strain element10which has undergone such surface processing, on each of the non-masked arm portions20to22, a residual stress layer having a negative residual stress greater in absolute value than those in the other masked portions was formed. With this, the arm portions20to22increase in fatigue strength against elastic deformation, fatigue life is prolonged, and this makes it possible to achieve a long-term stable use of the force sensor1(physical quantity measurement sensor).

Furthermore, the surface roughness of the arm portions20to22is rougher than those of portions other than the arm portions20to22. Therefore, in a case where the strain gauges A1to C4′ are attached (bonded) with an adhesive to the arm front faces20ato22aand the arm back faces20bto22bof the arm portions20to22in the foregoing manners after the step of projecting the shot material, because of the roughness of the arm front faces20ato22aand the arm back faces20bto22b, the bonded strain gauges A1to C4′ are well anchored, and adhesiveness becomes greater than conventional techniques. With this, the strain gauges A1to C4′ are more likely to conform to elastic deformation of the arm portions20to22, and the accuracy of detection by the strain gauges A1to C4′ improves.

Next, the following description will discuss how the foregoing table block2, base block6, and strain element10are assembled to form the force sensor1(see the drawings such asFIGS.1to4). First, the strain element10and the base block6are stacked together such that a back face10bof the strain element10(back face11bof the frame portion11) faces and makes contact with the back face6bof the base block6.

Before doing so, locating pins P are press-fit into the locating holes9ato9cof the base block6. The strain element10and the base block6are positioned so that the locating pins P in the respective locating holes9ato9care press-fit into the locating through-holes19ato19cin the frame portion11of the strain element10when the strain element10is placed on the base block6, and then the strain element10and the base block6are stacked together (seeFIG.3). When the strain element10and the base block6are stacked together after they are positioned like above, the bolt through-holes18ato18cin the frame portion11of the strain element10are brought into a condition in which they are in communication with the screw holes8ato8cof the base block6. Therefore, bolts N (hexagon socket head bolts) are put through the bolt through-holes18ato18cof the strain element10and fastened to the screw holes8ato8cof the base block6, and the strain element10is fixed to the base block6such that the strain element10is placed on the base block6(see (b) ofFIG.2).

Next, the strain element10and the table block2are stacked together such that a front face10aof the strain element10(front face12aof the central portion12) faces and makes contact with the back face2bof the table block2. Before doing so, locating pins P are press-fit into the locating through-holes14ato14cin the central portion12of the strain element10. When the table block2is placed on the strain element10, the table block2is placed on the strain element10such that the locating pins P in the respective locating through-holes14ato14care press-fit into the locating through-holes4ato4cin the table block2.

When the table block2and the strain element10are stacked together such that they are positioned like above, the bolt through-holes3ato3cin the table block2are brought into a condition in which they are in communication with the screw through-holes13ato13cof the strain element10. Therefore, bolts N (hexagon socket head bolts) are put through the bolt through-holes3ato3cof the table block2and fastened to the screw through-holes13ato13cof the strain element. The table block2is fixed to and attached to the strain element10in a state in which the table block2is placed on the strain element10, thereby completing the force sensor1.

The general shape of the finished force sensor1is in the form of a cylinder as shown in (a) ofFIG.2. On the other hand, the table block2is shaped such that, as described earlier, the center portion2dof the back face2bprotrudes more than the peripheral portion of the back face2b. Because of this, there is clearance S between the peripheral portion of the table block2and the front face10aof the strain element10(front face11aof the frame portion11).

Then, with regard to the finished force sensor1, the front face6aof the base block6is attached to an end face at an end portion of an industrial robot arm, and the front face2aof the table block2is attached to a back end face of a robot hand. Then, when the industrial robot arm operates and the robot hand grabs an object such as a workpiece, a load (external force) caused by an impact resulting from the grabbing or the like is transmitted from the robot hand to the table block2. The load having been transmitted to the table block2is transmitted to the central portion12, of the strain element10, which is in contact with the center portion2dof the table block2.

The central portion12of the strain element10is a rigid body having a predetermined rigidity, whereas the frame portion11of the strain element10is also fixed to the base block6. Therefore, the load having been transmitted to the central portion12as described above is exerted on the arm portions20to22, which are lower in rigidity than the central portion12and the frame portion11. With this, the arm portions20to22elastically deform. The manner in which the arm portions20to22elastically deform depends on the area of the central portion12of the strain element10to which the load is transmitted. For example, when the central portion12receives a depressing load at or near the junction where the central portion12connects to the arm portion20, the arm portion20elastically deforms in a manner such that its portion connected to the central portion12flexes toward the base block6. On the contrary, the other arm portions21and22elastically deform in a manner such that their portion connected to the central portion12flexes toward the table block2.

According to the strain element10in accordance with Embodiment 1 of the present invention, even if the arm portions20to22elastically deform repeatedly, each of the arm portions20to22has a residual stress layer formed on its four sides as described earlier. Therefore, the strain element10is less likely to undergo fatigue failure over a long period of time. Furthermore, as described earlier, in the strain element10, the strain gauges A1to C4′ disposed on the arm portions20to22are good at conforming to elastic deformation of the arm portions20to22.

The degree of flexion of the arm portions20to22, which are elastically deformable under a load, is detected by the strain gauges A1to C4′ disposed on the arm portions20to22. With this, at the force sensor1, forces and moments in respective directions (six-axis forces) exerted on the central portion12of the strain element10are measured. The six-axis forces measured include: force Fx in the X axis direction; force Fy in the Y axis direction; force Fz in the Z axis direction; moment Mx about the X axis direction; moment My about the Y axis direction; and moment Mz about the Z axis direction, which are exerted on the central portion12. Next, an electrical system for measurement of these six-axis forces is discussed.

FIG.9is a circuit diagram for a strain gauge circuit29, illustrating a manner in which the twenty-four strain gauges A1to C4′ disposed on the strain element10as described earlier are electrically connected. The strain gauge circuit29, in which the twenty-four strain gauges A1to C4′ are connected, includes six bridge circuits I to VI. Each of the first to third bridge circuits I to III is a bridge circuit constituted by strain gauges, which are disposed in areas close to the central portion12, of the strain gauges disposed on the arm portions20to22. Each of the fourth to sixth bridge circuits IV to VI is a bridge circuit constituted by strain gauges, which are disposed in areas close to the frame portion11, of the strain gauges disposed on the arm portions20to22.

Specifically, the first bridge circuit I is a bridge circuit in which the strain gauges A1and A2(which are disposed in the area close to the central portion12on the arm front face22aof the arm portion22) and the strain gauges A1′ and A2′ (which are disposed in the area close to the central portion12on the arm back face22bof the arm portion22) are connected together. In the first bridge circuit I illustrated inFIG.9, the manner of connection is such that the strain gauges A1and A2are opposite each other and the strain gauges A1′ and A2′ are opposite each other (the second bridge circuit II and the third bridge circuit III also employ similar manners of connection.)

The second bridge circuit II is a bridge circuit in which the strain gauges B1and B2(which are disposed in the area close to the central portion12on the arm front face21aof the arm portion21) and the strain gauges B1′ and B2′ (which are disposed in the area close to the central portion12on the arm back face21bof the arm portion21) are connected together. The third bridge circuit III is a bridge circuit in which the strain gauges C1and C2(which are disposed in the area close to the central portion12on the arm front face20aof the arm portion20) and the strain gauges C1′ and C2′ (which are disposed in the area close to the central portion12on the arm back face20bof the arm portion20) are connected together.

Furthermore, the fourth bridge circuit IV is a bridge circuit in which the strain gauges A3and A4(which are disposed in the area close to the frame portion11on the arm front face22aof the arm portion22) and the strain gauges A3′ and A4′ (which are disposed in the area close to the frame portion11on the arm back face22bof the arm portion22) are connected together. In the fourth bridge circuit I illustrated inFIG.9, the manner of connection is such that the strain gauges A3and A3′ are opposite each other and the strain gauges A4and A4′ are opposite each other (the second bridge circuit II and the third bridge circuit III also employ similar manners of connection.)

The fifth bridge circuit V is a bridge circuit in which the strain gauges B3and B4(which are disposed in the area close to the frame portion11on the arm front face21aof the arm portion21) and the strain gauges B3′ and B4′ (which are disposed in the area close to the frame portion11on the arm back face21bof the arm portion21) are connected together. The sixth bridge circuit VI is a bridge circuit in which the strain gauges C3and C4(which are disposed in the area close to the frame portion11on the arm front face20aof the arm portion20) and the strain gauges C3′ and C4′ (which are disposed in the area close to the frame portion11on the arm back face20bof the arm portion20) are connected together.

The above-described strain gauge circuit29is arranged such that an input power supply voltage Ein is applied to each of the bridge circuits I to VI. Then, during the application of this voltage, the first bridge circuit I outputs an output voltage signal CH-I through its output terminal. Similarly, the second bridge circuit II outputs an output voltage signal CH-II, the third bridge circuit III outputs an output voltage signal CH-III, the fourth bridge circuit IV outputs an output voltage signal CH-IV, the fifth bridge circuit V outputs an output voltage signal CH-V, and the sixth bridge circuit VI outputs an output voltage signal CH-VI.

FIG.10is a block diagram illustrating an internal configuration of main parts of a signal processing module30, which processes the output voltage signals CH—I to CH-VI outputted from the foregoing strain gauge circuit29illustrated inFIG.9. The signal processing module30includes an amplifier31, an A-D converter32, a processor33, a memory34, and a D-A converter35. The amplifier31is electrically connected to the output terminal of the strain gauge circuit29illustrated inFIG.9, and contains AMP-I to AMP-VI for individually amplifying the output voltage signals CH—I to CH-VI from the strain gauge circuit29, respectively.

Amplified signals (analog signals) amplified by the AMP-I to AMP-VI of the amplifier31, respectively, are converted into digital signals through the A-D converter32, and then inputted to the processor33. The processor33serves to carry out a process of calculating the foregoing six-axis forces (Fx, Fy, Fz, Mx, My, and Mz) exerted on the central portion12of the strain element10. The forces are the results of measurement by the force sensor1. The processor33carries out the calculation process based on the following equation (1) while referring to a calibration matrix C stored in the memory34.
F=C×E(1)

In the equation (1) above, F is a matrix of equation (2) below, which represents the foregoing six-axis forces (Fx, Fy, Fz, Mx, My, and Mz) exerted on the central portion12. C is a calibration matrix of equation (3) below. E is a matrix of values obtained by converting the output voltage signals CH—I to CH-VI of the strain gauge circuit29from analog to digital (see equation (4) below).

Note that, in the calibration matrix C represented by the above equation (3), values (pre-calculated values) specific to each force sensor are used. Specifically, specific values of the elements of the calibration matrix C are found from (i) conditions in which the six-axis forces (Fx, Fy, Fz, Mx, My, and Mz) are exerted on the force sensor and (ii) the results of detection by the strain gauges A1to C4′ associated with elastic deformation of the arm portions20to22in those conditions.

As shown in the foregoing equation (1), the processor33multiplies the calibration matrix C of the equation (3) by the matrix E of the A-D converted values which are based on the output voltage signals from the A-D converter32, and thereby finds F which is a matrix of the six-axis forces (Fx, Fy, Fz, Mx, My, and Mz). The processor33makes it possible to output the result of the calculation (corresponding to physical quantity corresponding to elastic deformation of the strain element10under a load) as a digital signal. The processor33also makes it possible to output the result of the calculation by analog signal through the D-A converter35. Such an output value (output value by digital or analog signal) serves as a physical quantity measured by the force sensor1.

Note that the signal processing module30illustrated inFIG.10is disposed in the hollow6cof the base block6of the force sensor1(see (b) ofFIG.2). Also, the signal processing module30is arranged such that lead wires for transmission of the output signal from the processor33and the output signal from the D-A converter35extend outward through a cutout6ein a peripheral wall6dof the base block6(see (a) ofFIG.2).

(a) to (f) ofFIG.11are tables for the respective bridge circuits I to VI, in each of which, with regard to the foregoing force sensor1, cases where an external force and/or a moment (Fx, Fy, Fz, Mx, My, and/or Mz) is/are applied to the table block2with the base block6fixed are compared with no-load conditions. The tables show how the resistances of the strain gauges A1to C4′ change and whether or not voltage values of the output voltage signals CH—I to CH-V from the bridge circuits I to VI have changed, that is, whether or not there is unbalanced output.

In the force sensor1, in a case where the external force Fy in the Y axis direction is exerted on the central portion12while the frame portion11is fixed, forces act on the arm portions20and22and the arm portions20and22become deformed; however, strain does not occur in the arm portion21because the junction where the arm portion21connects to the frame portion11, near the through-opening26, flexes. In a case where the external force Fx in the X axis direction is exerted on the central portion12while the frame portion11is fixed, forces act on the arm portions20to22, respectively, and strain occurs. In a case where the external force Fz in the Z axis direction is exerted on the central portion12while the frame portion11is fixed, the arm portions20to22flex in a uniform manner.

Furthermore, in the force sensor1, in a case where the moment My about the Y axis direction is exerted on the central portion12while the frame portion11is fixed, the arm portion21is merely twisted and does not flex, whereas moments act on the arm portions20and22and the arm portions20and22flex. In a case where the moment Mx about the X axis direction is exerted on the central portion12while the frame portion11is fixed, moments act on the arm portions20to22, respectively, and the arm portions20to22flex. In a case where the moment Mz about the Z axis direction is exerted on the central portion12while the frame portion11is fixed, the arm portions20to22flex in a uniform manner.

As has been described, according to the force sensor1in accordance with Embodiment 1, a compressive residual stress layer (layer having negative residual stress) is formed on surfaces (four sides) of each of the arm portions20to22of the strain element10, and therefore the force sensor1has improved metal fatigue strength associated with elastic deformation. Furthermore, since stress concentration is reduced in corner portions at edges of junctions where the respective arm portions20to22connect to the frame portion11and in corner portions at edges of junctions where the respective arm portions20to22connect to the central portion12, working life of the force sensor1as a whole as a sensor is longer than conventional sensors. Furthermore, in the force sensor1in accordance with Embodiment 1, the strain gauges A1to C4′ disposed on the arm portions20to22are good at conforming to the arm portions20to22when the arm portions20to22elastically deform. Therefore, when the arm portions20to22elastically deform, sliding is less likely to occur between the strain gauges A1to C4′ and the arm portions20to22, and thereby measurement accuracy is improved as compared to conventional sensors. In addition, since the strain gauges A1to C4′ are disposed in a special manner (see the disposition of the strain gauges B3, B4, B3′ and B4′ and the like inFIGS.6and7), the accuracy of detection and measurement concerning strain when the moment and external force in and about Mz, Fx, and Fy directions are exerted is improved as compared to conventional sensors. Note that the present invention is not limited to the foregoing statements in Embodiment 1, and various variations are available.

For example, the foregoing description discussed a case in which the force sensor1is attached to an industrial robot arm; however, needless to say, for example, besides the industrial robot arm, the force sensor1can be used in applications such as tactile sensing by a remote-controlled robot and detection of resistance/external force exerted on wind tunnel test model. Furthermore, according to the foregoing description, the base block6and the frame portion11of the strain element10of the force sensor1are fixed, whereas the table block2and the central portion12of the strain element10serve to receive an external force (load). However, such conditions may be reversed so that the table block2and the central portion12of the strain element10are fixed whereas the base block6and the frame portion11of the strain element10serve to receive an external force and thereby the force sensor1may be used in applications of measurement subjects. Moreover, as the outer circumference of the central portion12, besides the shape of substantially a hexagon, some other polygonal shape, circular shape, or the like shape may be employed.

Furthermore, the foregoing manner in which the strain gauges A1to C4′ are disposed on the arm portions20to22(seeFIGS.6and7) is an example, and, needless to say, some other manner of disposition can be employed. For example, with regard to the strain gauges A3and A4and the like disposed in the area close to the frame portion11, such strain gauges may be at an angle (at an angle of 45 degrees to the center line Y10) so as to diverge away from each other with decreasing distance to the frame portion11, instead of being disposed so as to diverge away from each other with decreasing distance to the central portion12(see FIGS. 1 to 3 and 7 to 10 of Patent Literature 1). Furthermore, in the foregoing descriptions, the strain gauges A1to C4′, which are of a type in which a strain gauge main material is deposited on a base material for a strain gauge, are bonded to the arm portions20to22with an adhesive. However, strain gauges made of a thin metal film may be formed by directly or indirectly forming a film by vacuum deposition, sputtering method, or the like on the front faces and the back faces of the arm portions20to22.

Moreover, a residual stress layer may be formed on the four sides of each of the arm portions20to22by, instead of shot peening by which a shot material is allowed to collide, laser peening by which laser light (laser beam) is applied. Also in a case of carrying out laser peening, masking for blocking laser is put on the areas illustrated inFIG.8(cross-hatched areas). However, in a case where an apparats capable of controlling the range (area) irradiated with laser is used, masking is not necessary, and laser peening is carried out by controlling the apparatus so that laser is applied only to the four sides of each of the arm portions20to22.

Furthermore, the three through-openings25,26, and27in the frame portion11may be omitted, provided that the condition is such that the arm portions20to22are likely to elastically deform near the frame portion11. The condition in which the arm portions20to22are likely to elastically deform is, for example, a case in which the dimensions of the arm portions20to22in the direction of their extension are long in relation to the diameter of strain element10, sizes of the first space15to the third space17between the frame portion11and the central portion12, and the like. Other examples include a case in which the width of each arm portion orthogonal to the direction of extension is small and a case in which the thickness of each arm portion is small.

(a) and (b) ofFIG.12illustrate a strain element10′ in accordance with a variation. The strain element10′ in accordance with the variation is the same in the main configuration and the like as the foregoing strain element10illustrated inFIGS.4and5, and includes a frame portion11′, a central portion12′, and arm portions20′ to22′ (strain gauges are disposed on arm portions). A difference is that the strain element10′ in accordance with the variation does not have the through-openings25,26, and27of the frame portion11illustrated inFIGS.4and5but has near-center through-openings50′ to52′ (corresponding to second through-openings) formed in the central portion12′.

The near-center through-openings50′ to52′, in the central portion12′, are provided between (i) locating through-holes14a′ to14c′ provided in areas corresponding to the extensions of the respective arm portions20′ to22′ extending toward the central portion12′ and (ii) central junctions20c′ to22c′ where the respective arm portions20′ to22′ connect to the central portion12′. Each of the near-center through-openings50′ to52′ is in the form of a straight line, and is in parallel to and slightly shorter than a corresponding one of outer peripheral edge portions12c′,12d′, and12e′ which form the outer contour of the hexagonal central portion12′.

In the strain element10′ in accordance with the variation, the central portion12′ has the foregoing near-center through-openings50′ to52′, and thereby deformability of the arm portions20′ to22′ in stretch directions corresponding to the directions of extension of the arm portions20′ to22′ is reduced. This makes it possible to ensure detection accuracy in the stretch directions. Also, the central junctions20c′ to22c′, where the respective arm portions20′ to22′ connect to the central portion12′, have a smaller rigidity than in the case of the strain element10illustrated in the drawings such asFIG.4, and are capable of sensitively detecting strain even when a load is small.

(a) and (b) ofFIG.13illustrate a strain element10″ in accordance with another variation. The strain element10″ in accordance with another variation is the same in the main configuration and the like as the strain element10illustrated inFIGS.4and5. Also, a frame portion11″ has through-openings25″ to27″ corresponding to the through-openings25to27of the frame portion11which are characteristic of the strain element10. Also, a central portion12″ has near-center through-openings50″ to52″ corresponding to the near-center through-openings50′ to52′ of the central portion12′ which are characteristic of the strain element10′ of Variation10illustrated in (a) and (b) ofFIG.12.

The strain element10″ has the through-openings25″ to27″ in the frame portion11″ and has the near-center through-openings50″ to52″ (second through-openings provided between locating through-holes14a″ to14c″ and junctions where the respective arm portions20″ to22″ connect to the central portion12″) in the central portion12″. This achieves both the advantage provided by the foregoing through-openings25to27of the strain element10and the advantage provided by the near-center through-openings50′ to52′ of the strain element10′. This is particularly preferable in a case where, for example, measurement of a small external force is carried out. This is because the strain element10″ is arranged such that: the deformability of the arm portions20″ to22″ in stretch directions corresponding to the directions of extension of the arm portions20″ to22″ is further reduced; and that the opposite ends of each of the arm portions20″ to22″ are relatively smaller in rigidity and elastically deform more sensitively in response to an external force (load).

In the strain element10illustrated inFIGS.4and5, in the strain element10′ illustrated inFIG.12, and in the strain element10″ illustrated inFIG.13, the three locating through-holes14ato14c, the three locating through-holes14a′ to14c′, and the three locating through-holes14a″ to14c″ are provided in the central portions12,12′, and12″, respectively. Note, however, that the number of locating through-holes can be reduced to two, depending on specifications (for example, in the strain elements10,10′, and10″, the locating through-holes14c,14c′, and14c″ can be omitted, respectively. See locating through-holes114aand114bof a strain element110in accordance with Embodiment 2 illustrated inFIGS.19and20described later). As such, when the number of locating through-holes is two, it is possible to, for example, reduce the number of processing areas and the number of man-hours for assembly. Note that, in a case where the number of locating through-holes of a strain element (for example, strain element10) is reduced, a locating through-hole (for example, locating through-hole4c) of the table block2corresponding to that omitted locating through-hole (for example, locating through-hole14c) is also omitted.

Moreover, in the above descriptions, the number of hand-attaching screw holes for attachment of a robot hand to the table block2is three in total (the hand-attaching screw holes5ato5c). However, in a case where it is necessary to attach the robot hand more firmly, four hand-attaching screw holes may be provided in a circumferential direction so as to be spaced apart from each other by 90 degrees (see hand-attaching screw through-holes105ato105dof a table block102of the force sensor101in accordance with Embodiment 2 illustrated inFIG.16described later). Similarly, also with regard to the three arm-attaching screw holes7ato7cfor attachment of the base block6to the robot arm, in a case where it is necessary to attach the base block6to the robot arm more firmly, four arm-attaching screw holes may be provided in a circumferential direction so as to be spaced apart from each other by 90 degrees.

(a) to (c) ofFIG.14illustrate a variation of masking of the strain element10, in which non-masked portions are broader than those of the masked strain element10illustrated in (a) to (c) ofFIG.8. Specifically, areas including the outer peripheral edge portions12c,12d, and12e, where the respective central junctions20cto22c(which are the central portion12-side ends of the respective arm portions20to22) connect to the central portion12, are also left unmasked. These areas including the outer peripheral edge portions12c,12d, and12e, where the respective central junctions20cto22c(which are the central portion12-side ends of the respective arm portions20to22) connect to the central portion12, are areas in the form of straight lines parallel to the edges corresponding to the outer peripheral edge portions12c,12d, and12e. Furthermore, areas including the inner peripheral edge portions11c,11d, and11e, where respective outer junctions20dto22d(which are the frame portion11-side ends of the respective arm portions20to22) connect to the frame portion11(such areas are areas in the form of straight lines parallel to the edges corresponding to the inner peripheral edge portions11c,11d, and11e), are also left unmasked.

Specifically, the central junctions20cto22cand the outer junctions20dto22d, from which the arm portions20to22extend, are likely to undergo stress concentration. Because of this, there is a tendency that the vicinities of the outer peripheral edge portions12c,12d, and12eof the central portion12, where the respective central junctions20cto22cconnect to the central portion12, and the vicinities of the inner peripheral edge portions11c,11d, and11eof the frame portion11, where the respective junctions20dto22dconnect to the frame portion11, are also subjected to a large burden due to stress concentration. To address this, the step of projecting a shot material at the strain element10may be carried out under a condition in which, as illustrated in (a) to (c) ofFIG.14, the areas including the outer peripheral edge portions12c,12d, and12eand the areas including the inner peripheral edge portions11c,11d, and11eare also left unmasked. With this, a residual stress layer having negative residual stress is formed also in the areas corresponding to these edge portions, and the resistance to fatigue failure resulting from elastic deformation can be further improved. Note that, in this variation, the linear areas parallel to the edges corresponding to the respective inner peripheral edge portions11cto11eand outer peripheral edge portions12cto12eare also included in strain portions in addition to the arm portions20to22.

(a) to (c) ofFIG.15illustrate another variation of masking of the strain element10, in which non-masked portions are even broader than the variation illustrated in (a) to (c) ofFIG.14. Specifically, areas including corners of edges of the outer peripheral edge portions12c,12d, and12e, where the respective central junctions20cto22cof the respective arm portions20to22connect to the central portion12, are also left unmasked. These areas including the corners of the edges of the outer peripheral edge portions12c,12d, and12e, where the respective central junctions20cto22cof the respective arm portions20to22connect to the central portion12, are areas each in the form of a straight line including the opposite corners of the edge of a corresponding one of the outer peripheral edge portions12c,12d, and12e. Furthermore, areas of the inner peripheral edge portions11c,11d, and11e(where the outer junctions20dto22dof the respective arm portions20to22connect to the frame portion11), corresponding to the regions in longitudinal directions of the respective through-openings25to27, are also left unmasked. These areas of the inner peripheral edge portions11c,11d, and11e(where the respective outer junctions20dto22dof the respective arm portions20to22connect to the frame portion11), corresponding to the regions in the longitudinal directions of the respective through-openings25to27, are areas in the form of straight lines corresponding to the regions in the longitudinal directions of the respective through-openings25to27, near the inner peripheral edge portions11c,11d, and11e.

As described earlier with regard to the variation of masking in (a) to (c) ofFIG.14, the vicinities of the outer peripheral edge portions12c,12d, and12eof the central portion12are structurally subjected to a large burden due to stress concentration. Similarly, the vicinities of the inner peripheral edge portions11c,11d, and11eof the frame portion11are likely to undergo stress concentration due to the presence of the through-openings25to27in the form of slots. To address this, the step of projecting a shot material at the strain element10may be carried out under a condition in which, as illustrated in (a) to (c) ofFIG.15, the areas including corners of edges of the outer peripheral edge portions12c,12d, and12eare also left unmasked. The step of projecting a shot material at the strain element10may be carried out under a condition in which also the areas including the regions of the inner peripheral edge portions11c,11d, and11ecorresponding to the longitudinal directions of the through-openings25to27are also left unmasked. With this, a residual stress layer having negative residual stress is formed also in these areas, and the resistance to fatigue failure resulting from elastic deformation can be further improved. Note that, in this variation, non-masked portions of the inner peripheral edge portions11cto11eand outer peripheral edge portions12cto12eare also included in strain portions.

FIGS.16to18illustrate a force sensor101, which is a specific example of a physical quantity measurement sensor in accordance with Embodiment 2 of the present invention. The force sensor101in accordance with Embodiment 2 is for application in an industrial robot arm, similarly to the force sensor1in accordance with Embodiment 1 illustrated in the drawings such asFIG.1. Note, however, that the force sensor101in accordance with Embodiment 2 is characterized as employing a strain element110(seeFIGS.17to20) which is a single member serving both as the strain element10and the base block6of the force sensor1in accordance with Embodiment 1. The force sensor101in accordance with Embodiment 2, which employs such a structure, thereby also achieves a reduction in parts count, a reduction in the number of man-hours for assembly, a reduction in dimension in the Z axis direction, improvement in mountability of an electrical system board, and a reduction in the number of man-hours for processing, as compared to the force sensor1in accordance with Embodiment 1. The following description will discuss the force sensor101in accordance with Embodiment 2 in detail. Note that the X, Y, and Z axis directions in Embodiment 2 are the same as those of Embodiment 1.

As illustrated in (a) and (b) ofFIG.17, the force sensor101is configured such that a table block102located on a robot-hand side (front side) and a strain element110located on a robot-arm side (back side) are stacked together.

The table block102is constituted by a disk-shaped member having a certain thickness, is in the shape of a circle when viewed from front side (see (a) ofFIG.16), and has many holes (through-holes) in a flat front face102acorresponding to the front side. Specifically, the table block102has, in the vicinity of the center of a circle, bolt through-holes103a,103b, and103c(countersunk through-holes for passage of bolts) which are arranged to form an equilateral triangle symmetrical with respect to center line Y2in the height direction. Also, the table block102has locating holes104aand104b(holes with fit tolerance) on the left-hand side of the bolt through-hole103aand below the bolt through-hole103a.

The table block102further has hand-attaching screw through-holes105a,105b,105c, and105d(internally threaded through-holes) which are arranged in the vicinity of the outer circumference to substantially form a square. These hand-attaching screw through-holes105a,105b,105c, and105dare used for attachment to the robot hand. The table block102further has hand-locating holes104dand104eon a horizontal line orthogonal to the center line Y2extending in the height direction such that the hand-locating holes104dand104eare symmetrical with respect to the center line Y2. These hand-locating holes104dand104eare used for positioning relative to the robot hand. Furthermore, the table block102is shaped such that, in the back face102d, there is a hollow102cwith a rim portion102fremaining (see (b) ofFIG.17).

On the other hand, the strain element110is, as also illustrated inFIGS.19and20, constituted by a disk-shaped member which is thicker than the strain element10in accordance with Embodiment 1. The strain element110has a circular outer circumference when seen front side (see (a) ofFIG.19) and from back side (seeFIG.20), similarly to the strain element10in accordance with Embodiment 1. The strain element110is structured such that the central portion112and the frame portion111surrounding the central portion112are connected by three arm portions120,121, and122(corresponding to strain portions) (see (a) ofFIG.19andFIG.20). Note that the configurations of the arm portions120to122are basically the same as those of Embodiment 1, and elastically deform in response to an external load or moment. The strain element110in accordance with Embodiment 2 is characterized in that the dimension in the Z axis direction (thickness) is greater than that of Embodiment 1 and that the central portion112protrudes from the front face110aof the strain element110. Another characteristic is, for example, the back face110bhas a cavity110ein the form of a recess in the central area excluding the peripheral area.

The central portion112has screw through-holes113a,113b, and113c(internally threaded through-holes) which are arranged to form an equilateral triangle and locating through-holes114aand114b(through-holes with fit tolerance) in the vicinities of junctions where the central portion112connects to the respective arm portions120and121. The screw through-holes113ato113ccorrespond to the foregoing bolt through-holes103ato103cof the table block102, whereas the locating through-holes114aand114bcorrespond to the locating holes104aand104bof the table block102. Furthermore, the central portion112, whose outline is in the shape of substantially a hexagon, is connected to the arm portions120,121, and122at middle portions of respective outer peripheral edge portions112c,112d, and112e, which are three of the six edges of the hexagon and which do not face the screw through-holes113a,113b, and113c.

The frame portion111has an outer contour in the form of a circle, and an inner contour in the form of a hexagon which is obtained by uniformly enlarging the contour of the foregoing central portion112. Furthermore, the frame portion111has bolt through-holes118a,118b,118c, and118dwhich are arranged to from a quadrangle. Furthermore, the frame portion111is connected to the arm portions120,121, and122at middle portions of respective inner peripheral edge portions111c,111d, and111elocated opposite the respective outer peripheral edge portions112c,112d, and112eof the foregoing central portion112. Because of the presence of such arm portions120,121, and122, the space between the frame portion111and the central portion112is divided into three, resulting in formation of a first space115, a second space116, and a third space117.

The frame portion111further has three through-openings125,126, and127(each corresponding to first through-opening) in the junctions where the frame portion111connects to the respective arm portions120to122. These through-openings125to127are in the shape of straight lines along the inner peripheral edge portions111c,111d, and111eat the inner circumference in the form of a hexagon, and are equal to or slightly greater in length than the edges of the respective inner peripheral edge portions111c,111d, and111e, similarly to the through-openings25to27of Embodiment 1.

Furthermore, as illustrated inFIG.20, the strain element110has a hollow110e, which is in a large circular shape, in the central area when seen from the back face110b. The hollow110ehas a dimension in a radial direction that is long enough to include the foregoing through-openings125to127. Therefore, in the strain element110, a ring-shaped area including the arm portions120to122and the through-openings125to127in the frame portion111has the smallest thickness (dimension in the Z axis direction). This thickness is the thickness from a bottom110dof the hollow110eto the front face110aof the strain element110. The second thinnest is the thickness of the central portion112. This thickness is from the bottom110dof the hollow110eto a raised face112aof the central portion. The thickest is the thickness of an outer circumference area (ring-shaped area) of the frame portion111. This thickness is from the front face110aof the strain element110(identical to frame portion's front face111a) to the back face110bof the strain element110.

The strain element110in accordance with Embodiment 2 has thicknesses as described above. This ensures, in the thinnest ring-shaped area including the arm portions120to122and the through-openings125to127of the frame portion111, the ability to easily elastically deform in response to an external load or moment. Furthermore, the second thickest central portion112ensures rigidity that is necessary to function as, when the force sensor101is combined with the foregoing table block102, a force receiver serving to receive an external force from the table block102. Moreover, the outer circumference area (ring-shaped area) of the frame portion111, which is the thickest, is a portion directly joined to the robot arm, and therefore has a thickness corresponding to a rigidity required for operation of the robot arm.

Note that the strain element110has, in the back face110billustrated inFIG.20, locating holes119aand119bfor the robot arm, in the outer circumference area of the frame portion111. Also, the strain element110has a recess110fand a groove110gfor passage of lead wires for connections of the electrical system board accommodated in the hollow110e. Moreover, a substrate constituting an electrical signal processing module (see signal processing module30in accordance with Embodiment 1 illustrated inFIG.10) is disposed in the hollow110eof the finished strain element110. Also, lead wires for external connection, running from the substrate, are accommodated and disposed in the groove110g, within the hollow110eof the finished strain element110. With this, as illustrated in (b) ofFIG.16, the hollow110eis closed by attaching a circular cover160to the hollow110e. Also, a recess cover107is attached to the recess110f, and thereby the recess110fand the groove110gare closed.

The strain element110configured as described above has been formed by machining (cutting) an elastically deformable metal material, similarly to Embodiment 1, and corner easing to removed burrs on peripheries, edges, corners, and the like of the strain element110has also been carried out. With regard to this corner easing, careful corner easing is carried out also with respect to the strain element110in accordance with Embodiment 2, similarly to Embodiment 1. The corner easing is carried out with respect to, for example, corners of edges of junctions where the frame portion11and the respective arm portions120to122connect together, corresponding to the areas enclosed by dot-dot-dash lines inFIGS.6and7of Embodiment 1. The corner easing is carried out also with respect to corners of edges of junctions where the central portion112and the respective arm portions120and122connect together. Note that the radius of curvature forming each of such corners, and the like, are also the same as those of Embodiment 1. Furthermore, when such machining and corner easing are carried out, also in Embodiment 2, surface processing (processing by projecting shot material) is to be carried out after masking.

FIG.21illustrates the strain element110(unfinished strain element) which has been masked for surface processing. In the masking step involving masking, as illustrated inFIG.21, the strain element110is masked by adhesive tape T except for the arm portions120to122including areas for disposition of strain gauges A11to C14′ (i.e., the strain element110is masked except for strain portions. In Embodiment 2, the frame portion111and the central portion112are masked). Note that side faces in the thickness direction, such as outwardly facing faces of the central portion112, inwardly facing faces of the frame portion111, and a outwardly facing face110cof the strain element110(frame portion111), are also masked. Therefore, on each of the arm front faces120ato122of the arm portions120to122which are left unmasked, an area extending from a corresponding one of central junctions120cto122c(where the arm portion connects to the central portion112) to a corresponding one of outer junctions120dto122d(where the arm portion connects to the frame portion111) is exposed. Also, on each of the arm back faces120bto122bof the arm portions120to122which are left unmasked, an area extending from a corresponding one of central junctions120cto122cto a corresponding one of outer junctions120dto122d(where the arm portion connect to the frame portion11) is exposed. Furthermore, opposite side faces of each of the non-masked arm portions120to122are also exposed. Therefore, all four sides of each of the arm portions120to122are exposed.

Then, similarly to Embodiment 1, the step of projecting a shot material at the strain element110which has been masked, illustrated inFIG.21, is carried out, and thereby a residual stress layer having compressive residual stress is formed in the non-masked portions (strain portions). Moreover, the strain portions in which such a residual stress layer is formed has a surface roughness rougher than those of the non-masked areas.

When the step of projecting a shot material is completed as described above, the masking material is removed, and then the strain gauges A11to C14′ are bonded with an adhesive to the arm front faces120ato122aand the arm back faces120bto122bof the arm portions120to122. The strain gauges A11to C14′ in accordance with Embodiment 2 correspond to the strain gauges A1to C4′ in accordance with Embodiment 1, and are the same as the strain gauges A1to C4′ in accordance with Embodiment 1 in terms of the manner in which the strain gauges A11to C14′ are disposed, the manner in which they are bonded, the manner in which they are electrically connected (see strain gauge circuit illustrated inFIG.9), and the like. Also, a signal processing module for processing output voltage signal from a strain gauge circuit constituted by connecting the strain gauges A11to C14′, in the force sensor101in accordance with Embodiment 2, is also the same as that used in Embodiment 1 (seeFIG.10).

With regard to the strain element110having gone through such production steps, as described earlier, a substrate of an electrical signal processing module is accommodated in the hollow110eand the lead wires extending from the substrate are also accommodated in the groove110g, and then the hollow110eand the recess110fare closed with the cover106and the recess cover107. Next, the following description discusses, with reference toFIGS.17and18, a procedure by which the force sensor101constituted by the strain element110and the table block102is attached to the industrial robot arm.

First, as illustrated in (a) and (b) ofFIG.18, the strain element110is attached to the robot arm-side of the industrial robot arm. Before doing so, locating pins P2are press-fit into the locating holes119aand119bin the back face110bof the strain element110. The tips of the locating pins P2are press fit into locating through-holes in the robot arm provided so as to correspond to the locating holes119aand119b, and the back face110bof the strain element110is placed on the end face at an end portion of the robot arm (see (b) ofFIG.18). Then, bolts N2(hexagon socket head bolts) are put through the four bolt through-holes118ato118din the front face110aof the strain element110, and fastened to screw holes which are provided in the robot arm so as to correspond to the bolt through-holes118ato118d. This fixes the strain element110to the robot arm (see (a) ofFIG.18).

Next, the table block102is attached to the strain element110to assemble the force sensor101. Specifically, locating pins P are press fit into the locating holes104aand104bin the table block102from the back face102d, in advance. The tips of the locating pins P are press fit into the locating through-holes114aand114bwhich are provided in the raised face112aof the central portion112of the strain element110so as to correspond to the locating holes104aand104b. The back face102dof the table block102is placed on the raised face112aof the central portion112of the strain element110(see (b) ofFIG.17). Note that, in the drawings such as (b) ofFIG.17, the covers106and107which cover the hollow110eand the recess110fin the strain element110are not illustrated.

Next, bolts N (hexagon socket head bolts) are put through the three bolt through-holes103ato103cin the front face102aof the table block102. The bolts N are fastened to the three screw through-holes113a,113b, and113cwhich are provided in the raised face112aof the central portion112of the strain element110so as to correspond to the bolt through-holes103ato103c. With this, the table block102is fixed to the strain element110, and thereby the force sensor101in accordance with Embodiment 2 is completed.

Note that, with regard to a procedure by which the robot hand-side of the industrial robot arm is attached to the force sensor101, locating pins P1are press-fit into locating holes in the robot hand in advance. The tips of such locating pins P1are press-fit into the hand-locating holes104dand104ein the front face102aof the table block102. The robot hand is placed on the front face102aof the table block102(see (b) ofFIG.18). Then, bolts Ni (hexagon socket head bolts) are put through the four bolt through-holes in the robot hand, and fastened to the four hand-attaching screw through-holes105a,105b,105c, and105din the front face102aof the table block102. This fixes the robot hand to the table block102.

According to the force sensor101in accordance with Embodiment 2 configured as described above, a load against the robot hand is first exerted on the table block102. The load exerted on the table block102is received, as an external force, by the central portion112of the strain element110whose raised face112ais in contact with the back face102dof the table block102. Upon receipt of the external force by the central portion112of the strain element110, the arm portions120to122, which connect the central portion112and the frame portion111fixed to the robot arm, elastically deform, and the strain gauges A11to C14′ carry out detection of strain associated with such elastic deformation.

Furthermore, also in the frame portion111included in the force sensor101in accordance with Embodiment 2, a residual stress layer having compressive residual stress is formed on each of the arm portions120to122. Therefore, metal fatigue strength associated with elastic deformation is improved. Moreover, since stress concentration is reduced in corner portions at edges of junctions where the respective arm portions120to122connect to the frame portion111and in corner portions at edges of junctions where the respective arm portions120to122connect to the central portion112, working life can be prolonged. Moreover, Embodiment 2, which employs the frame portion111configured as described above, thereby achieves, in addition to the effects which are the same as the above-mentioned effects of Embodiment 1, a reduction in parts count, a reduction in the number of man-hours for assembly, a reduction in dimension in the Z axis direction, improvement in mountability of electrical system board, and a reduction in the number of man-hours for processing.

Note that, also in Embodiment 2, the adhesiveness of the strain gauges A11to C14′ disposed on the arm portions120to122is increased, and therefore measurement accuracy is also improved as compared to conventional techniques. Note that, also in Embodiment 2, various variations described earlier in Embodiment 1 can be employed.

FIGS.22and23illustrate variations of masking of the strain element110in accordance with Embodiment 2. (a) to (c) ofFIG.22correspond to the variation of masking in accordance with Embodiment 1 described with reference to (a) to (c) ofFIG.14. (a) to (c) ofFIG.23correspond to the another variation of masking in accordance with Embodiment 1 described with reference to (a) to (c) ofFIG.15. Each variation shows a state in which each non-masked area illustrated inFIG.21has been broadened from the opposite ends of each of the arm portions120to122.

Specifically, in the variation of masking illustrated in (a) to (c) ofFIG.22, areas including the outer peripheral edge portions112c,112d, and112e, where the respective central junctions120cto122c(which are the central portion112-side ends of the respective arm portions120to122) connect to the central portion112, are also left unmasked. These areas including the outer peripheral edge portions112c,112d, and112e, where the respective central junctions120cto122c(which are the central portion112-side ends of the respective arm portions120to122) connect to the central portion112, are areas in the form of straight lines parallel to the edges corresponding to the outer peripheral edge portions112c,112d, and112e. Furthermore, areas including inner peripheral edge portions111c,111d, and111e, where respective outer junctions120dto122d(which are the frame portion111-side ends of the respective arm portions120to122) connect to the frame portion111, are also left unmasked. The areas including the inner peripheral edge portions111c,111d, and111e, where the respective outer junctions120dto122d(which are the frame portion111-side ends of the respective arm portions120to122) connect to the frame portion111, are areas in the form of straight lines parallel to the edges corresponding to the inner peripheral edge portions111c,111d, and111e.

The central junctions120cto122cand the outer junctions120dto122d, from which such arm portions120to122extend, are likely to undergo stress concentration. Because of this, there is a tendency that the vicinities of the outer peripheral edge portions112c,112d, and112eof the central portion112, where the respective central junctions120cto122cconnect to the central portion112, and the vicinities of the inner peripheral edge portions111c,111d, and111eof the frame portion111, where the respective junctions120dto122dconnect to the frame portion111, are also subjected to a large burden due to stress concentration. To address this, the step of projecting a shot material at the strain element110may be carried out under a condition in which, as illustrated in (a) to (c) ofFIG.22, the areas including the outer peripheral edge portions112c,112d, and112eand the areas including the inner peripheral edge portions111c,111d, and111eare also left unmasked. With this, a residual stress layer having negative residual stress is formed also in the areas corresponding to these edge portions, and the resistance to fatigue failure resulting from elastic deformation can be further improved. Note that, in this variation, the linear areas parallel to the edges corresponding to the respective inner peripheral edge portions111cto111eand outer peripheral edge portions112cto112eare also included in strain portions in addition to the arm portions120to122.

In the another variation of masking illustrated in (a) to (c) ofFIG.23, areas including corners of edges of the outer peripheral edge portions112c,112d, and112e, where the respective central junctions120cto122cof the respective arm portions arm portions120to122connect to the central portion112, are also left unmasked. These areas including the corners of the edges of the outer peripheral edge portions112c,112d, and112e, where the respective central junctions120cto122cof the respective arm portions120to122connect to the central portion112, are areas each in the form of a straight line including the opposite corners of the edge of a corresponding one of the outer peripheral edge portions112c,112d, and112e. Areas of the inner peripheral edge portions111c,111d, and111e(where the respective outer junctions120dto122dof the respective arm portions120to122connect to the frame portion111), corresponding to the regions in longitudinal directions of the respective through-openings125to127, are also left unmasked. These areas of the inner peripheral edge portions111c,111d, and111e, corresponding to the regions in the longitudinal directions of the respective through-openings125to127, are areas in the form of straight lines corresponding to the ranges in the longitudinal directions of the respective through-openings125to127, near the inner peripheral edge portions111c,111d, and111e.

As described earlier, the vicinities of the outer peripheral edge portions112c,112d, and112eof the central portion112are structurally subjected to a large burden due to stress concentration. Similarly, the vicinities of the inner peripheral edge portions111c,111d, and111eof the frame portion111are likely to undergo stress concentration due to the presence of the through-openings125to127in the form of slots. To address this, the step of projecting a shot material at the strain element110may be carried out under a condition in which, as illustrated in (a) to (c) ofFIG.23, the areas including corners of edges of the outer peripheral edge portions112c,112d, and112eare also left unmasked. The step of projecting a shot material at the strain element110may be carried out under a condition in which also the areas including the regions of the inner peripheral edge portions111c,111d, and111ecorresponding to the longitudinal directions of the through-openings125to127are also left unmasked. With this, a residual stress layer having negative residual stress is formed also in these areas, and the resistance to fatigue failure resulting from elastic deformation can be further improved. Note that, in this variation, non-masked areas of the inner peripheral edge portions111cto111eand the outer peripheral edge portions112cto112eare also included in strain portions.

(a) to (c) ofFIG.24illustrate a strain element55in accordance with Embodiment 3 of the present invention, which is for application to a load cell (load converter and force converter) as a physical quantity measurement sensor. The strain element55in accordance with Embodiment 3 is constituted by a cylindrical member56. The strain element55is configured such that, on its front side56aof the outer circumferential surface extending from top to bottom beyond center line X5parallel to the X axis direction (horizontal direction), areas each intersecting center line Y5parallel to the Y axis direction (vertical direction) (i.e., a front-side upper area56cand a front-side lower area56d) are used as areas for attachment of strain gauges. The strain element55is also configured such that, on its back side56bof the outer circumferential surface extending from top to bottom beyond the center line X5parallel to the X axis direction, areas each intersecting the center line Y5parallel to the Y axis direction (i.e., a back-side upper area56eand a back-side lower area56f) are used as areas for attachment of strain gauges. These areas for attachment of the strain gauges are four quadrangular regions cross-hatched in (b) and (c) ofFIG.24.

Note that each area (the front-side upper area56c, the front-side lower area56d, the back-side upper area56e, and the back-side lower area56f) is in the shape of a quadrangle (rectangle), and, while the longitudinal directions of the front-side upper area56cand the back-side upper area56eare parallel to the center line Y5, the longitudinal directions of the front-side lower area56dand the back-side lower area56fare parallel to the center line X5. Also in Embodiment 3, the main material for the strain element55, specifications of strain gauges, the manner in which the strain gauges are disposed, and the like are the same as those of Embodiment 1.

(a) to (c) ofFIG.25illustrate the strain element55in accordance with Embodiment 3 which has been masked. Also in the production of the strain element55in accordance with Embodiment 3, the foregoing cylindrical member56is made from a material by machining (including corner easing), and then, as illustrated in (a) to (c) ofFIG.25, the member56is masked. Specifically, assume that the quadrangular regions including the front-side upper area56c, the front-side lower area56d, the back-side upper area56e, and the back-side lower area56f, which are areas for attachment of strain gauges, are a front-side upper strain portion56r, a front-side lower strain portion56s, a back-side upper strain portion56t, and a back-side lower strain portion56u, respectively. These strain portions, which correspond to regions subject to strain associated with elastic deformation under a load (such strain portions are the front-side upper strain portion56r, the front-side lower strain portion56s, the back-side upper strain portion56t, and the back-side lower strain portion56u), are left unmasked, and portions other than these strain portions (the front-side upper strain portion56r, the front-side lower strain portion56s, the back-side upper strain portion56t, and the back-side lower strain portion56u) are masked. Note that the masked areas are the areas cross-hatched in (a) to (c) ofFIG.25.

The strain portions (the front-side upper strain portion56r, the front-side lower strain portion56s, the back-side upper strain portion56t, and the back-side lower strain portion56u), which are left unmasked, are regions obtained by uniformly enlarging, about 1.5- to 4-fold, the front-side upper area56c, the front-side lower area56d, the back-side upper area56e, and the back-side lower area56fwhich are areas for disposition of strain gauges, respectively. In this example, the strain portions which are left unmasked are about 2-fold enlarged regions. Note that the masked areas of the strain element55are the front side56aand the back side56bof the outer circumferential surface (side surface) and circular top and bottom faces56gand56hof the member56excluding the strain portions (the front-side upper strain portion56r, the front-side lower strain portion56s, the back-side upper strain portion56t, and the back-side lower strain portion56u).

With respect to the strain element55which has been masked, shot peening or laser peening is carried out. By such peening, a residual stress layer having negative residual stress is formed in each of the strain portions (the front-side upper strain portion56r, the front-side lower strain portion56s, the back-side upper strain portion56t, and the back-side lower strain portion56u). With this, even when the strain portions (the front-side upper strain portion56r, the front-side lower strain portion56s, the back-side upper strain portion56t, and the back-side lower strain portion56u) of the front side56aand the back side56belastically deform in response to an external force (load), the strain portions are resistant to fatigue failure resulting from metal fatigue. Furthermore, in a case where shot peening involving projecting a shot material is carried out as peening, the strain portions (the front-side upper strain portion56r, the front-side lower strain portion56s, the back-side upper strain portion56t, and the back-side lower strain portion56u) are given a surface roughness rougher than those of other portions. Therefore, even in a case where the strain gauges are disposed by bonding with an adhesive, the strain gauges have improved adhesiveness and become better at conforming to elastic deformation, and the accuracy of strain detection can be maintained. Note that, also in Embodiment 3, the variations described earlier in the embodiments such as Embodiment 1 may be used if applicable.

(a) and (b) ofFIG.26illustrate a strain element60in accordance with Embodiment 4 of the present invention, which is for application to a load cell (load converter and force converter) as a physical quantity measurement sensor similarly to the foregoing Embodiment 3. The strain element60in accordance with Embodiment 4 is constituted by a cube-shaped member61, and has a hollow extending through the member61from a front face61bto a back face61d. The hollow, when seen from the front face61billustrated in (b) ofFIG.26, is constituted by (i) a left through-opening62and a right through-opening63each in the form of a circle and (ii) a connecting through-opening64which connects the left through-opening62and the right through-opening63. Since the hollow constituted by such through-openings (the left through-opening62, the right through-opening63, and the connecting through-opening64) is formed, when a top face61aor a bottom face61cof the cube-shaped member61receives a load, the top face61aor the bottom face61celastically deforms. For carrying out detection of strain associated with such elastic deformation, the top face61aand the bottom face61care provided with areas for disposition of strain gauges.

Specifically, on the top face61aof the member61, a top left area65aand a top right area65b, which are arranged along center line X8parallel to the X axis direction (horizontal direction) and which correspond to the left through-opening62and the right through-opening63, are used as areas for attachment of strain gauges. Furthermore, on the bottom face61c, a bottom left area65cand a bottom right area65d, which are arranged along the center line X8parallel to the X axis direction and which correspond to the left through-opening62and the right through-opening63, are used as areas for attachment of strain gauges (four quadrangular regions cross-hatched in (a) and (b) ofFIG.26). Note that the areas (the top left area65a, the top right area65b, the bottom left area65c, and the bottom right area65d) are each in the shape of a quadrangle (rectangle), and their longitudinal directions are each parallel to the center line X8. Also in Embodiment 4, the main material for the strain element60, specifications of strain gauges, the manner in which the strain gauges are disposed, and the like are the same as those of Embodiment 1.

(a) to (c) ofFIG.27illustrate the strain element60in accordance with Embodiment 4 which has been masked. Also in the production of the strain element60in accordance with Embodiment 4, first, the cube-shaped member61having the hollow constituted by the through-openings (the left through-opening62, the right through-opening63, and the connecting through-opening64) is made from a material by machining (including corner easing). Then, as illustrated in (a) to (c) ofFIG.27, the member61is masked. Specifically, quadrangular regions including the top left area65a, the top right area65b, the bottom left area65c, and the bottom right area65d, which are areas for attachment of strain gauges and are referred to as a top left strain portion61r, a top right strain portion61s, a bottom left strain portion61t, and a bottom right strain portion61u, respectively, are left unmasked. Portions other than these strain portions, which correspond to regions subject to strain associated with elastic deformation under a load (such strain portions are the top left strain portion61r, the top right strain portion61s, the bottom left strain portion61t, and the bottom right strain portion61u), are masked (masked areas are the areas cross-hatched in (a) to (c) ofFIG.27).

The strain portions (the top left strain portion61r, the top right strain portion61s, the bottom left strain portion61t, and the bottom right strain portion61u), which are left unmasked, are regions obtained by uniformly enlarging, about 1.5- to 4-fold, the top left area65a, the top right area65b, the bottom left area65c, and the bottom right area65d, respectively, which are areas for disposition of strain gauges. In this Example, the strain portions which are left unmasked are about 2-fold enlarged regions. Note that the masked areas of the strain element60are outer surfaces of the member61excluding the foregoing strain portions (the top left strain portion61r, the top right strain portion61s, the bottom left strain portion61t, and the bottom right strain portion61u). The outer surfaces are the top face61a, the front face61b, the bottom face61c, the back face61d, a left side face61e, a right side face61f, and inner walls of the through-openings (the left through-opening62, the right through-opening63, and the connecting through-opening64) constituting the hollow.

With respect to the strain element60which has been masked, shot peening or laser peening is carried out. By such peening, a residual stress layer having negative residual stress is formed in each of the strain portions (the top left strain portion61r, the top right strain portion61s, the bottom left strain portion61t, and the bottom right strain portion61u). With this, even when the strain portions (the top left strain portion61r, the top right strain portion61s, the bottom left strain portion61t, and the bottom right strain portion61u) of the top face61aand the bottom face61celastically deform in response to an external force (load), the strain portions are resistant to fatigue failure resulting from metal fatigue. Furthermore, in a case where shot peening involving projecting a shot material is carried out as peening, the strain portions (the top left strain portion61r, the top right strain portion61s, the bottom left strain portion61t, and the bottom right strain portion61u) are given a surface roughness rougher than those of other portions. Therefore, even in a case where the strain gauges are disposed by bonding with an adhesive, the strain gauges have improved adhesiveness and become better at conforming to elastic deformation, and the accuracy of strain detection can be maintained. Note that, also in Embodiment 4, the variations described earlier in the embodiments such as Embodiment 1 may be used if applicable.

(a) to (c) ofFIG.28illustrate a strain element70in accordance with Embodiment 5 of the present invention, which is for application to a torque sensor as a physical quantity measurement sensor. The strain element70in accordance with Embodiment 5 includes, similarly to the strain element10in accordance with Embodiment 1, a frame portion71having a circular circumferential outline when seen from front, and includes a central portion72(having a circular outline when seen from front side) which is located in the space defined by the frame portion71such that there are spaces78ato78dbetween the frame portion71and the central portion72. The strain element70is characterized in that four arm portions73to76, which connect the frame portion71and the central portion72, are arranged in a circumferential direction so as to be spaced apart from each other by 90 degrees. Note that, in (a) to (c) ofFIG.28, bolt holes, locating through-holes, and the like illustrated in the drawings such asFIGS.4and5are not illustrated, for clear illustration of the configuration of main parts of the strain element70.

According to the foregoing strain element10in accordance with Embodiment 1, the thickness (dimension in Z axis direction) is substantially the same among the frame portion11, the central portion12, and the arm portions20to22. However, according to the strain element70in accordance with Embodiment 5, the thicknesses of the frame portion71and the central portion72are greater, because, for example, the strain element70is for application to a torque sensor. Therefore, the thicknesses of the arm portions73to76are less than those of the frame portion71and the central portion72(see (b) and (c) ofFIG.28).

Areas of the strain element70for disposition of strain gauges are opposite side faces73aand73bof the arm portion73extending parallel to the Y axis direction and opposite side faces75aand75bof the arm portion75extending parallel to the Y axis direction. With regard to the areas of the strain element70for disposition of strain gauges, specifically, the regions cross-hatched in (a) and (b) ofFIG.28are areas73e,73f,75e, and75ffor disposition of strain gauges. Note that, in Embodiment 5, the main material for the strain element70, specifications of strain gauges, the manner in which the strain gauges are disposed, and the like are the same as those of Embodiment 1.

(a) to (c) ofFIG.29illustrate the strain element70in accordance with Embodiment 5 which has been masked. Also in the production of the strain element70in accordance with Embodiment 5, with respect to the strain element70which has been made into the shape illustrated in (a) to (c) ofFIG.29from a material by machining (including corner easing), masking is carried out to mask the regions cross-hatched in (a) to (c) ofFIG.29. These regions are portions other than the opposite side faces73aand73bof the arm portion73extending parallel to the Y axis direction and the opposite side faces75aand75bof the arm portion75extending parallel to the Y axis direction including the foregoing areas73e,73f,75e, and75ffor disposition of strain gauges (the side faces73aand73band the side faces75aand75bcorrespond to strain portions corresponding to regions subject to strain).

Each of the non-masked opposite side faces73aand73bof the arm portion73is a region extending from a junction73cwhere the arm portion73connects to the frame portion71to a junction73dwhere the arm portion73connects to the central portion72in the direction of extension of the arm portion73(the direction parallel to the Y axis), and each of the non-masked opposite side faces75aand75bof the arm portion75is a region extending from a junction75cwhere the arm portion75connects to the frame portion71to a junction75dwhere the arm portion75connects to the central portion72in the direction of extension of the arm portion75(the direction parallel to the Y axis). Note that the masked areas are the whole circumferences of the frame portion71and the central portion72, four sides of each of the arm portions74and76extending parallel to the X axis direction, and surfaces of the arm portions73and75excluding the opposite side faces73aand73bof the arm portion73extending parallel to the Y axis direction and the opposite side faces75aand75bof the arm portion75extending parallel to the Y axis direction.

With respect to the strain element70which has been masked, shot peening or laser peening described in Embodiment 1 is carried out. By such peening, a residual stress layer having negative residual stress is formed on the opposite side faces73aand73bof the arm portion73and the opposite side faces75aand75bof the arm portion75including the areas73e,73f,75e, and75ffor disposition of strain gauges. With this, even when the opposite side faces73aand73bof the arm portion73and the opposite side faces75aand75bof the arm portion75elastically deform in response to an external force (load), these faces are resistant to fatigue failure resulting from metal fatigue.

Furthermore, in a case where shot peening involving projecting a shot material is carried out as peening, the opposite side faces73aand73bof the arm portion73and the opposite side faces75aand75bof the arm portion75are given a surface roughness rougher than those of other portions. Therefore, even in a case where the strain gauges are disposed on the opposite side faces73aand73band the opposite side faces75aand75bby bonding, the strain gauges become better at conforming to elastic deformation of the arm portions73and75, and the accuracy of strain detection can be maintained.

Note that, also in Embodiment 5, various variations are available. For example, depending on the purpose of use or the like, the number of arm portions can be more than four. Furthermore, arm portions can be unequally spaced apart from each other in the circumferential direction instead of being equally spaced. Moreover, the shape of the outline of the strain element may be a polygon such as a quadrangle instead of a circle. Note that, also in Embodiment 5, the variations described earlier in Embodiment 1 can be employed.

(a) to (c) ofFIG.30illustrate a strain element80in accordance with Embodiment 6 of the present invention, which is for application to a load cell as a physical quantity measurement sensor. The strain element80in accordance with Embodiment 6 is constituted by a frame part81having a circular (ring-shaped) outline when seen from front. The strain element80is arranged such that opposite ends81cand81dof an outer circumferential face81b, intersecting center line X10(horizontal line H10in side view) parallel to the X axis direction (horizontal direction) (such ends are hereinafter referred to as outer-circumferential horizontal ends81cand81d), and opposite ends81eand81fof an inner circumferential face81a, intersecting the center line X10(such ends are hereinafter referred to as inner-circumferential horizontal ends81eand81f), i.e., four areas in total, are areas for attachment of strain gauges (areas81g,81h,81i, and81j). The areas81g,81h,81i, and81jfor disposition of strain gauges are the regions cross-hatched in (a) to (c) ofFIG.30. Note that, also in Embodiment 6, the main material for the strain element80, specifications of strain gauges, and the like are the same as those of Embodiment 1. The manner in which the strain gauges are disposed is the same as those (including variations) described in Embodiment 1. The strain gauges can be disposed such that their detection directions are each parallel to the Y axis direction.

(a) to (c) ofFIG.31illustrate the strain element80in accordance with Embodiment 6 which has been masked. Also in the production of the strain element80in accordance with Embodiment 6, a material is made into the strain element80having the shape illustrated in (a) to (c) ofFIG.31by machining (including corner easing). With regard to the strain element80, the regions cross-hatched in (a) to (c) ofFIG.31are masked. These regions are portions other than quadrangular outer-circumferential strain portions81rand81sand inner-circumferential strain portions81tand81uincluding the respective areas81g,81h,81iand81jfor disposition of strain gauges at the outer-circumferential horizontal ends81cand81dintersecting the center line X10and the inner-circumferential horizontal ends81eand81fintersecting the center line X10.

The outer-circumferential strain portions81rand81sand the inner-circumferential strain portions81tand81u(corresponding to strain portions corresponding to regions subject to strain), which are left unmasked, are regions obtained by uniformly enlarging, about 1.5- to 4-fold, the areas81g,81h,81i, and81jfor disposition of strain gauges, respectively. In this Example, the strain portions which are left unmasked are about 2-fold enlarged regions. Note that the masked areas of the strain element80are all the faces of the frame part81excluding the foregoing outer-circumferential strain portions81rand81sand the inner-circumferential strain portions81tand81u.

With respect to the strain element80which has been masked, shot peening or laser peening is carried out. By such peening, a residual stress layer having negative residual stress is formed in the outer-circumferential strain portions81rand81sand the inner-circumferential strain portions81tand81uincluding the areas81g,81h,81i, and81jfor disposition of strain gauges. With this, even when the outer-circumferential strain portions81rand81sand the inner-circumferential strain portions81tand81uof the frame part81elastically deform in response to an external force (load), the strain portions are resistant to fatigue failure resulting from metal fatigue.

Furthermore, in a case where shot peening involving projecting a shot material is carried out as peening, the outer-circumferential strain portions81rand81sand the inner-circumferential strain portions81tand81uof the frame part81are given a surface roughness rougher than those of other portions. Therefore, even in a case where the strain gauges are disposed by bonding, the strain gauges become better at conforming to elastic deformation of the outer-circumferential strain portions81rand81sand the inner-circumferential strain portions81tand81uof the frame part81, and the accuracy of strain detection can be maintained. Note that, also in Embodiment 6, the variations described earlier in Embodiment 1 can be employed.

(a) to (c) ofFIG.32illustrate a strain element90in accordance with Embodiment 7 of the present invention, which is for application to a load cell as a physical quantity measurement sensor. In the strain element90in accordance with Embodiment 7, a protruding portion92protruding in the form of a cylinder is provided at the center of a top end face94of a short-length cylindrical base portion91. Also, the strain element90has a hollow93in the bottom face opposite the end face94. The strain element90is configured such that four areas on an inner face95, which is the ceiling of the hollow93and which is opposite the end face94, are used as areas96to99for disposition of strain gauges. The four areas are arranged along center line X20(in the X axis direction) and center line Y20(in the Y axis direction) each passing through a center95aof the inner face95, and are each distant from the center95a. The areas96to99for disposition of strain gauges are the regions cross-hatched in (a) to (c) ofFIG.32. Note that, also in Embodiment 7, the main material for the strain element90, specifications of strain gauges, the manner in which the strain gauges are disposed, and the like are the same as those of Embodiment 1.

(a) to (c) ofFIG.33illustrate the strain element90in accordance with Embodiment 7 which has been masked. Also in the production of the strain element90in accordance with Embodiment 7, a material is made into the strain element90having the shape illustrated in (a) to (c) ofFIG.33by machining (including corner easing). This strain element is masked except for quadrangular strain portions90ato90dincluding the respective areas96to99for disposition of strain gauges (masked areas are the regions cross-hatched in (a) and (b) ofFIG.33).

The strain portions90ato90d(corresponding to strain portions in accordance with an aspect of the present invention), which are left unmasked, are regions obtained by uniformly enlarging (about 1.5- to 4-fold) the areas96to99for disposition of strain gauges. In this example, the strain portions which are left unmasked are about 2-fold enlarged regions. Note that the masked areas of the strain element90are all the faces of the base portion91and the protruding portion92excluding the foregoing quadrangular strain portions90ato90d.

With respect to the strain element90which has been masked, shot peening or laser peening is carried out. By such peening, a residual stress layer having negative residual stress is formed in the quadrangular strain portions90ato90dincluding the respective areas96to99for disposition of strain gauges. With this, even when the end face94and the inner face95of the base portion91elastically deform in response to an external force (load), the faces are resistant to fatigue failure resulting from metal fatigue.

Furthermore, in a case where shot peening involving projecting a shot material is carried out as peening, the quadrangular strain portions90ato90dare given a surface roughness rougher than those of other portions. Therefore, even in a case where the strain gauges are disposed by bonding, the strain gauges become better at conforming to elastic deformation of the end face94and the inner face95of the base portion91, and the accuracy of strain detection can be maintained. Note that, also in Embodiment 7, the variations described earlier in Embodiment 1 can be employed.

An aspect of the present invention is directed to a strain element which is elastically deformable in response to a load and which is configured to have a strain gauge disposed thereon, the strain gauge being configured to detect strain associated with deformation, the strain element including a strain portion which corresponds to a region subject to strain and which includes an area for disposition of the strain gauge, the strain portion being provided with a residual stress layer having negative residual stress.

According to an aspect of the present invention, a residual stress layer having negative residual stress (compressive residual stress) has been formed in a strain portion which is in a region where the strain element elastically deforms and which includes an area for disposition of the strain gauge. Therefore, the resistance to fatigue failure in the portion that elastically deforms, in which the strain gauge carries out detection, increases. Since the resistance to fatigue failure increases like this, a physical quantity measurement sensor including the strain element can be used stably over a long period of time. Note that the residual stress layer may be formed by, for example, causing a shot material to collide with the surface of the strain portion or irradiating the surface of the strain portion with laser.

An aspect of the present invention is arranged such that the strain portion has a surface roughness rougher than a portion other than the strain portion.

According to an aspect of the present invention, the strain portion, which includes the area for disposition of the strain gauge, has a surface roughness greater than a portion other than the strain portion. Therefore, the surface area of the strain portion where an adhesive makes contact with the strain portion increases, and, in a case where the strain gauge is bonded to the strain element with an adhesive or the like, the adhesiveness to the surface of the strain element (surface of the strain portion) increases, and the strain gauge is firmly bonded. Because of this, even when the strain element elastically deforms, the strain gauge firmly bonded to the surface of the strain element with the adhesive better conforms to the deformation, and the accuracy of strain detection can be increased as compared to conventional techniques.

An aspect of the present invention includes: a frame portion; a central portion which is located in a space defined by the frame portion so as to be spaced apart from the frame portion; and an arm portion which connects the frame portion with the central portion and which corresponds to the strain portion, and is arranged such that the frame portion has a first through-opening in a junction where the frame portion connects to the arm portion, the arm portion has, disposed on one face thereof, four of the strain gauges consisting of a first strain gauge, a second strain gauge, a third strain gauge, and a fourth strain gauge, the first strain gauge and the second strain gauge are disposed in an area close to the central portion such that (i) the first strain gauge and the second strain gauge are symmetrical to each other with respect to a center line of the one face, the center line extending in a direction of extension of the arm portion, and (ii) detection directions of the first strain gauge and the second strain gauge are parallel to the center line, and the third strain gauge and the fourth strain gauge are disposed in an area close to the frame portion such that (i) the third strain gauge and the fourth strain gauge are symmetrical to each other with respect to the center line and (ii) detection directions of the third strain gauge and the fourth strain gauge are at an angle to the center line so as to diverge away from each other with decreasing distance to the central portion.

According to an aspect of the present invention, the strain element is arranged such that: the frame portion and the central portion are connected by the arm portion; the frame portion has the first through-opening facing the arm portion; the arm portion has, disposed on one face thereof, the four strain gauges consisting of a first strain gauge, a second strain gauge, a third strain gauge, and a fourth strain gauge, the third strain gauge and the fourth strain gauge are located in an area close to the frame portion such that (i) the third strain gauge and the fourth strain gauge are symmetrical to each other with respect to the center line in the direction of extension of the arm portion and (ii) detection directions of the third strain gauge and the fourth strain gauge are at an angle to the center line so as to diverge away from each other with decreasing distance to the central portion. Therefore, in a case where an external force that causes moment in the thickness direction of the strain element is exerted, the third strain gauge and the fourth strain gauge, which are disposed at an angle to the center line so as to diverge away from each other with decreasing distance to the central portion, easily detect strain associated with deformation that occurs when an external force is exerted on the arm portion in the foregoing specific directions (the foregoing Mz, Fx, and Fy directions). That is, the directions at an angle to the center line, in which the third strain gauge and the fourth strain gauge are disposed, are directions in which strain associated with deformation that occurs when an external force is exerted on the arm portion in the foregoing specific directions is detected well. This makes it possible to ensure highly sensitive measurement.

An aspect of the present invention includes: a frame portion; a central portion which is located in a space defined by the frame portion so as to be spaced apart from the frame portion; and an arm portion which connects the frame portion with the central portion and which corresponds to the strain portion, and is arranged such that the central portion has (i) a locating through-hole in an area corresponding to an extension of the arm portion and (ii) a second through-opening located between the locating through-hole and a junction where the central portion connects to the arm portion.

According to an aspect of the present invention, the strain element is arranged such that: the frame portion and the central portion are connected by the arm portion; the central portion has the locating through-hole; and there is the second through-opening between the locating through-hole and the junction where the central portion connects to the arm portion. Therefore, in the region corresponding to an area of the central portion from which the arm portion extends (in the region corresponding to the junction where the central portion connects to the arm portion), the vicinity of the locating through-hole increases in rigidity, whereas the vicinity of the second through-opening has a relatively low rigidity and is likely to flex. Because of this, the portion of the arm portion where the arm portion connects to the central portion is likely to elastically deform, and strain detection by the strain gauge becomes easy. Accordingly, the accuracy of measurement of values of physical quantities regarding external forces and moments improves.

According to an aspect of the present invention, the strain element is masked except for the strain portion, and then the shot material is projected. As such, the shot material directly collides with the strain portion. Because of such direct collision, a residual stress layer having negative residual stress is formed in the strain portion, and the strain portion is given a surface roughness rougher than a portion other than the strain portion. The residual stress layer results in an increase in resistance to fatigue failure, and a physical quantity measurement sensor including such a strain element can be used stably over a long period of time. Furthermore, since the shot material directly collides with the strain portion and thereby the strain portion is given a surface roughness rougher than a portion other than the strain portion, the strain gauge disposed on the strain portion becomes better at conforming to deformation because of the anchor effect provided by the adhesive, and the accuracy of strain detection improves.

An aspect of the present invention is arranged such that the strain element includes (i) a frame portion, (ii) a central portion which is located in a space defined by the frame portion so as to be spaced apart from the frame portion, and (iii) an arm portion which connects the frame portion with the central portion and which corresponds to the strain portion, and includes the step of corner easing comprising easing (i) a corner of an edge of a junction where the frame portion and the arm portion connect to each other or (ii) a corner of an edge of a junction where the central portion and the arm portion connect to each other.

According to an aspect of the present invention, in the strain element configured such that the frame portion and the central portion are connected by the arm portion, the corner at which the frame portion and the arm portion connect to each other or the corner at which the central portion and the arm portion connect to each other is eased. This reduces stress concentration that is likely to occur in such corners, and thereby further increases the resistance to fatigue failure.

A physical quantity measurement sensor in accordance with an aspect of the present invention includes the strain element described above, and measures a physical quantity corresponding to deformation of the strain element in response to a load.

According to an aspect of the present invention, a physical quantity measurement sensor including the foregoing strain element measures a physical quantity corresponding to deformation of the strain element in response to a load. This makes it possible to provide a physical quantity measurement sensor that maintains stable, highly accurate measurement over a long period of time.

According to an aspect of the present invention, a residual stress layer having compressive residual stress has been formed in a strain portion. Therefore, the resistance to fatigue failure in the portion that elastically deforms, in which the strain gauge carries out detection, can be improved. This makes it possible to achieve a long-term stable use of a physical quantity measurement sensor in which the strain element in accordance with an aspect of the present invention is employed.

According to an aspect of the present invention, the area for attachment of the stain gauge has a large surface roughness. Therefore, the strain gauge becomes better at conforming to the elastic deformation of the strain element because of the anchor effect provided by the adhesive. This makes it possible to achieve stable measurement accuracy of a physical quantity measurement sensor in which the strain element in accordance with an aspect of the present invention is employed.

According to an aspect of the present invention, in the strain element arranged such that the frame portion and the central portion are connected by the arm portion and that the frame portion has the first through-opening facing the arm portion, the third strain gauge and the fourth strain gauge disposed on the arm portion are disposed such that they are at an angle to the center line so as to diverge away from each other with decreasing distance to the central portion. This makes it possible to improve the accuracy of detection of strain associated with elastic deformation that occurs when an external force is exerted on the arm portion in the foregoing specific directions (the foregoing Mz, Fx, and Fy directions).

According to an aspect of the present invention, in the strain element arranged such that the frame portion and the central portion are connected by the arm portion, the locating through-hole and the second through-opening have been formed corresponding to the area of the central portion where the central portion connects to the arm portion. This achieves a structure in which the portion of the arm portion where the arm portion connects to the central portion is likely to elastically deform in response to a load. This makes it possible to increase measurement accuracy of a physical quantity measurement sensor in which the strain element in accordance with an aspect of the present invention is employed.

According to an aspect of the present invention, a shot material is projected under the condition in which the strain element is masked except for the strain portion. Therefore, a residual stress layer can be formed in the strain portion which is left unmasked. Furthermore, it is also possible to increase the surface roughness of the strain portion. This makes it possible to efficiently produce a strain element which is highly resistant to fatigue failure and in which a strain gauge of a bonded type is better at conforming to deformation because of the anchor effect provided by the adhesive.

According to an aspect of the present invention, in the strain element configured such that the frame portion and the central portion are connected by the arm portion, a corner at which the frame portion and the arm portion connect to each other or a corner at which the central portion and the arm portion connect to each other has been eased. This reduces stress concentration that is likely to occur in such corners, and thereby further increases the resistance to fatigue failure.

According to an aspect of the present invention, a physical quantity measurement sensor including the foregoing strain element measures a physical quantity corresponding to deformation of the strain element in response to a load. This makes it possible to ensure the condition in which stable, highly accurate measurement is available over a long period of time.

INDUSTRIAL APPLICABILITY

The prevent invention is suitable for use in applications in which a physical quantity measurement sensor including an elastically deformable strain element increases the resistance to fatigue failure and ensures long-term stable use.

REFERENCE SIGNS LIST