Strain wave gear device having strain gauge-type torque detection device

Abstract

A strain wave gear device (1) is provided with a strain gauge-type torque detection device (5). A gauge grid (61) of a strain gauge (6) straddles a boundary line (36) between a boss inside end surface (34a) and a diaphragm inside end surface (33a) of an external gear (3), and is attached to the boss inside end surface (34a) and the diaphragm inside end surface (33a). Compared to a case where the entire gauge grid (61) is attached to the diaphragm inner end surface (33a), a strain gauge output having less rotational ripple and excellent linearity can be obtained, and torque detection can be performed with high accuracy.

Description

Wave gear device equipped with a strain gauge type torque detection device

[0001] The present invention relates to a wave gear device, and more particularly to a wave gear device equipped with a strain gauge type torque detection device that detects transmitted torque using a strain gauge attached to a flexible external gear having a cup shape or a silk hat shape.

[0002] As a torque detection device for a wave gear device provided with an external gear having a cup shape or a silk hat shape, a strain gauge type torque detection device is used. The strain gauge type torque detection device focuses on a flexible external gear and detects transmitted torque based on a highly sensitive output obtained from a strain gauge attached to its diaphragm.

[0003] In a wave gear device, since the wave generator rotates with the external gear bent into an elliptical shape, flexible parts such as the diaphragm of the external gear have each part in its circumferential direction repeatedly displaced with a certain amplitude. Due to the displacement of each part that occurs regardless of the transmitted torque, a periodic error component (rotation ripple) appears in the output of the strain gauge. In the torque detection devices described in Patent Documents 1 to 3, by appropriately setting the number of strain gauges attached to the diaphragm and the attachment angular position in the circumferential direction of the diaphragm, the error component is surely removed and the detection accuracy of the transmitted torque is improved.

[0004] Further, since the strain gauge attached to the diaphragm of the external gear is bent following the bending of the diaphragm, there is a possibility that a disconnection may occur in the gauge lead connected to the gauge tab drawn from the gauge grid in the strain gauge. Patent Document 4 proposes a method of attaching the gauge grid to the surface of the diaphragm that can be repeatedly bent and attaching the portion on the side of the gauge tab to which the gauge lead is connected to the surface of a boss that is not affected by the bending.

[0005] Japanese Patent No. 3644558, Japanese Patent No. 4569990, Japanese Unexamined Patent Application Publication No. 2004 - 45378, Japanese Patent No. 3512160

[0006] Thus, in a harmonic drive gear system, torque can be detected by attaching strain gauges to the diaphragm of the external gear. However, even when the torque is zero, elliptical distortion occurs due to the rotation of the wave generator, which not only causes rotational ripple in the detection signal but also negatively affects its linearity. To address this, multiple strain gauges are attached to the diaphragm at predetermined angular intervals, and these detection signals are combined to eliminate rotational ripple and improve linearity.

[0007] If a strain gauge output with minimal error components can be obtained, the transmitted torque can be detected with high accuracy, and the number of strain gauges required to maintain detection accuracy can also be reduced. However, the current situation is that sufficient consideration has not been given to the placement of strain gauges on external gears from this perspective.

[0008] The object of the present invention is to provide a harmonic drive gear device equipped with a strain gauge type torque detection device in which the attachment position of the strain gauge is set so as to obtain an output with a small error component, with attention to this point.

[0009] The wave drive gear of the present invention comprises a rigid internal gear, a flexible external gear having a cup or top hat shape, a wave generator that partially engages the internal gear by bending the external gear in an elliptical shape, and a torque detection device equipped with a strain gauge for detecting the transmitted torque transmitted through the external gear, wherein the external gear comprises a diaphragm extending radially from one end of a cylindrical body on which the external teeth are formed, and a disc-shaped or annular boss integrally formed on the inner or outer peripheral edge of the diaphragm, and the boss end face on one axial side of the boss and the diaphragm end face on the same axial side of the diaphragm are smoothly continuous, and the strain gauge comprises a gauge grid made of an electrical resistor that draws a predetermined wiring pattern, and a gauge lead connected to a gauge tab drawn out from the gauge grid, wherein, when viewed along the radial direction, at least a portion of the gauge grid is attached to the side of the boss end face adjacent to the boundary line between the diaphragm end face and the boss end face.

[0010] The inventors investigated the behavior of the portion of an external gear from the diaphragm to the boss, which is repeatedly deflected by a wave generator, in order to determine the appropriate position for attaching strain gauges to the cup-shaped external gear. It was thought that at the boundary between the rigid boss and the flexible diaphragm of the external gear, the shear strain caused by torque application to the boss side and the diaphragm side would decrease sharply from the diaphragm side to the boss side, with the boundary line between the diaphragm and the boss as the dividing line. Contrary to this, the shear strain decreased smoothly from the diaphragm side to the boss side, and it was confirmed that shear strain also occurred in the boss side, within a predetermined boundary-adjacent region adjacent to the boundary line.

[0011] Furthermore, it was confirmed that in the boundary-adjacent region on the boss side, the strain caused by the rotation of the wave generator was smaller compared to the diaphragm side, but the shear strain caused by torque application was similarly large. In addition, it was confirmed that there was no significant change in the magnitude of the generated shear strain in the predetermined width region from the boss side to the diaphragm side, including the boundary line.

[0012] Based on these findings, the strain gauge grid was attached to the boundary adjacent region on the boss side of the boundary between the boss and diaphragm of an external gear, and its output was verified. Furthermore, the strain gauge grid was attached to both the boss and diaphragm, straddling the boundary between the boss and diaphragm, and its output was verified. As a result, compared to attaching the strain gauge grid to the diaphragm, a strain gauge output with less rotational ripple and superior linearity was obtained without a decrease in detection sensitivity.

[0013] For example, if, when viewed radially, a ring-shaped region of a predetermined width along a boundary line where torque can be detected on the boss end face is defined as the boss-side boundary adjacent region, then a gauge grid is attached to the boss end face such that its center in the planar direction is located within the boss-side boundary adjacent region.

[0014] Alternatively, a portion of the gauge grid can be attached to the diaphragm end face, straddling the boundary line, and the remaining portion of the gauge grid can be attached to the boss end face. For example, the gauge grid can be attached so that, when viewed radially, its planar center lies on the boundary line.

[0015] Furthermore, it is desirable to position the strain gauge so that its gauge tab and gauge lead are located on the side of the boss end face. This way, the gauge lead connected to the gauge tab is hardly affected by the deflection caused by the rotation of the wave generator, unlike when it is attached to the diaphragm, thus suppressing the occurrence of wire breakage and improving the durability of the strain gauge.

[0016] Furthermore, in the case of a cup-shaped harmonic drive gear equipped with a cup-shaped external gear, the external gear comprises a diaphragm extending radially inward from one end of a cylindrical body, and a boss integrally formed on the inner periphery of the diaphragm. In this case, it is desirable that the boss end face and diaphragm end face to which the gauge grid of the strain gauge is attached are the inner end face of the boss and the inner end face of the diaphragm, which are located inside the cup-shaped external gear. When attaching the strain gauge to the outer end face of the diaphragm, there are components such as output bearings on the outside of the diaphragm, making it difficult to secure space for strain gauge wiring. The inside of a cup-shaped external gear offers ample space, and the attachment surface is often flat, which is advantageous in terms of workability and reliability of adhesion.

[0017] In the harmonic drive gear of the present invention, the gauge grid of the strain gauge is attached to the boss-side boundary adjacent region on the boss end face of the external gear, or to the diaphragm end face, straddling the boundary line between the boss end face and the diaphragm end face. This makes it possible to obtain a strain gauge output with low rotational ripple and excellent linearity.

[0018] (A) is an explanatory diagram showing a harmonic drive gear equipped with a strain gauge type torque detection device according to an embodiment of the present invention, and (B) is an explanatory diagram showing the meshing state of the internal gear and external gear of the harmonic drive gear. (A) is a schematic longitudinal section view of the external gear to which the strain gauge is attached, (B) is an end view showing the outer end face, which is the end face on one side in the axial direction of the external gear, (C) is an end view showing the inner end face, which is the end face on the other side in the axial direction of the external gear, and (D) is a circuit diagram showing a Wheatstone bridge circuit composed of strain gauges. (A) is a half longitudinal section view of the external gear to which the strain gauge is attached, (B) is a half end view showing its inner end face, and (C) is an explanatory diagram showing the attachment position of the strain gauge. (A) is a schematic longitudinal section view of the external gear showing another example of the attachment position of the strain gauge, and (B) is an end view showing the inner end face of the external gear. These are explanatory diagrams showing three examples of external gears of a harmonic drive gear to which the present invention is applied.

[0019] An embodiment of a harmonic drive gear equipped with a strain gauge type torque detection device to which the present invention is applied will be described below with reference to the drawings.

[0020] Figure 1(A) is an explanatory diagram showing a harmonic drive gear equipped with a strain gauge type torque detection device according to an embodiment, and Figure 1(B) is an explanatory diagram showing the meshing state of the internal gear and external gear of the harmonic drive gear. As shown in these figures, the harmonic drive gear 1 consists of a rigid internal gear 2, a cup-shaped flexible external gear 3 positioned inside the internal gear, and a wave generator 4 that flexes the external gear 3 radially to partially mesh with the internal gear 2 and moves the meshing position of both gears in the circumferential direction. The external gear 3 is flexed in an elliptical shape by the wave generator 4 with an elliptical contour, and the external gear 3 meshes with the internal gear 2 at both ends of the major axis L of the ellipse. When the wave generator 4 is rotated by a motor or the like, the meshing position of the internal gear 2 and the external gear 3 moves in the circumferential direction, and relative rotation occurs between the two gears 3 and 4 based on the difference in the number of teeth of the two gears 2 and 3. For example, an internal gear 2 is fixed, and an external gear 3 is used as an output element, from which reduced rotation is output.

[0021] The cup-shaped external gear 3 comprises a radially flexible cylindrical body 32 on which external teeth 31 are formed on its outer circumferential surface, a disc-shaped diaphragm 33 extending radially inward from the rear end of the cylindrical body 32, and a disc-shaped boss 34 integrally formed with the central portion of the diaphragm 33. The cylindrical body 32 and the diaphragm 33 are elastically deformable parts and are repeatedly deflected radially by the rotating wave generator 4, with the inner peripheral edge of the diaphragm 33 connected to the boss 34. The boss 34, which is the attachment part to other members, is a part with substantially no elastic deformation.

[0022] The strain gauge type torque detection device 5 includes a torque detection unit 7 equipped with multiple sets of strain gauges 6 attached to the cup-shaped external gear 3 of the wave drive gear device 1, and a signal processing unit 8 that calculates the transmitted torque by signal processing the output of the torque detection unit 7 and outputs it to the outside.

[0023] Figure 2(A) is a schematic longitudinal cross-sectional view of the external gear 3 with strain gauges 6 attached, Figure 2(B) is an outer end view of the external gear 3 as seen from one side in the axial direction, Figure 2(C) is an inner end view of the external gear as seen from the other side in the axial direction, and Figure 2(D) is a circuit diagram showing the Wheatstone bridge circuit composed of strain gauges 6 in the torque detection unit 7.

[0024] Multiple strain gauges 6 are attached to the same circumference to compensate for output fluctuations caused by the deflection of the external gear 3. In this example, eight sets of strain gauges 6 are attached to the boundary between the inner end face 33a of the diaphragm and the inner end face 34a of the boss at equal angular intervals of 45° in the circumferential direction. Eight bolt holes 35 for fastening bolts (not shown) for coaxially fastening and fixing a load-side member (not shown) are formed in the boss 34 at equal angular intervals along the circumferential direction. The strain gauges 6 and bolt holes 35 are positioned offset from each other in both the circumferential and radial directions so as not to interfere with each other.

[0025] In Figures 2(C) and 2(D), eight sets of two-axis orthogonal strain gauges 6 are denoted by the reference numerals 6(A1, A2), 6(B1, B2), 6(C1, C2), 6(D1, D2), 6(E1, E2), 6(F1, F2), 6(G1, G2), and 6(H1, H2), where the pair of reference numerals in parentheses represent the two orthogonal strain gauge elements in each strain gauge 6. The output of the Wheatstone bridge circuit 7A, composed of the eight sets of strain gauges 6, is output from the torque detection unit 7 to the signal processing unit 8.

[0026] Figure 3(A) is a half-longitudinal cross-sectional view of the external gear 3, Figure 3(B) is a half-end view showing its inner end face, and Figure 3(C) is an explanatory diagram showing the attachment position of the strain gauge 6. The attachment position of the strain gauge 6 will be explained in detail with reference to these figures.

[0027] In the external gear 3, the end face of the diaphragm 33 that connects to the inner circumferential surface 32a of the cylindrical body portion 32 is the inner diaphragm end face 33a. The end face of the boss 34 that smoothly connects to this inner diaphragm end face 33a is the inner boss end face 34a. In this example, the inner diaphragm end face 33a is a plane located on a perpendicular plane PL perpendicular to the central axis 3a of the external gear 3, except for the outer peripheral edge portion that connects to the inner circumferential surface 32a of the cylindrical body portion 32. The inner boss end face 34a in this example is also a plane located on the said perpendicular plane PL. The boundary line 36 between the inner diaphragm end face 33a and the inner boss end face 34a is the position where the extended circular outer peripheral surface 34b of the boss 34 intersects when the circular outer peripheral surface 34b is extended in the axial direction. In Figures 2(C), 3(B), and Figure 4(B) described later, the position of boundary line 36 is shown by a dashed line for convenience.

[0028] The two-axis orthogonal strain gauge 6, using strain gauges 6 (A1, A2) as an example, comprises two orthogonally arranged strain gauge elements A1 and A2. Each of the strain gauge elements A1 and A2 includes a gauge grid 61 made of an electrical resistor of metal foil formed in a predetermined pattern on the surface of the gauge base, a pair of gauge tabs 62 extending from the gauge grid 61, and two gauge leads 63 connected to each of the gauge tabs 62. The strain gauge 6 (A1, A2) is attached with the strain gauge elements A1 and A2 tilted 45° to the left and right with respect to the radius line r centered on the central axis 3a of the external gear 3, and the gauge leads 63 facing radially inward.

[0029] Furthermore, the strain gauges 6 (A1, A2) are arranged such that a portion 61a of the gauge grid 61 is attached to the inner end face 33a of the diaphragm, straddling the boundary line 36 between the inner end face 34a of the boss and the inner end face 33a of the diaphragm, while the remaining portion 61b of the gauge grid 61 is attached to the portion of the inner end face 34a of the boss adjacent to the boundary line 36. For example, when viewed along the radial direction, the gauge grid 61 of the strain gauges 6 (A1, A2) is attached to the inner end face 33a of the diaphragm and the boss-side boundary adjacent region 37 such that the center of the gauge grid 61 in the planar direction lies on the boundary line 36.

[0030] Here, Figures 4(A) and 4(B) show another example of the attachment position of the strain gauge 6. Figure 4(A) is a schematic longitudinal section view of an external gear with a strain gauge attached, and Figure 4(B) is an end view showing the inner end face of the external gear. In the examples shown in these figures, the gauge grid 61 is attached to a position shifted towards the inner end face 34a of the boss from the boundary line 36. Specifically, on the inner end face 34a of the boss, a ring-shaped region of a predetermined width set along the boundary line 36 is set as the boss-side boundary adjacent region 37. In Figure 4(B), this boss-side boundary adjacent region 37 is shown in gray for convenience. The boss-side boundary adjacent region 37 can be determined by actual measurement or FEM (finite element method) analysis, and is a boss-side region in which torque can be detected with a predetermined accuracy. The gauge grid 61 is attached to the inner end face 34a of the boss such that its center in the planar direction is located within the boss-side boundary adjacent region 37. For example, in the case of the external gear of the cup-type harmonic drive gear of model 14 manufactured and sold by the applicant, even at a position about 0.5 mm from the boundary line, on the inner end face side of the boss, although the torque sensitivity is reduced to about half compared to the inner end face of the diaphragm, the effect of rotational ripple is also reduced by a similar amount, so torque measurement is possible with the same signal-to-noise ratio as when attached to the diaphragm. In this case, a ring-shaped region with a width of about 0.5 mm from the boundary line is set as the boss-side boundary adjacent region. The strain gauge is attached, for example, so that the center of its gauge grid is located within this region.

[0031] The inventors investigated the detection output obtained from each strain gauge 6 in the following cases: when the gauge grid 61 of each strain gauge 6 is attached so as to straddle the boundary line 36 between the diaphragm 33 and the boss 34; when the center of the gauge grid 61 is located in the boss-side boundary adjacent region 37; and when the strain gauge 6 is attached so as to the conventional method so that the entire gauge grid 61 is located on the inner end face 33a of the diaphragm. The inventors confirmed that when the gauge grid 61 of each strain gauge 6 is attached to the inner end face 33a of the diaphragm and the inner end face 34a of the boss (boss-side boundary adjacent region 37) straddling the boundary line 36, and when the gauge grid 61 is attached to the boss-side boundary adjacent region 37, the periodic error component included in the detection output is significantly reduced, the linearity of the detection output is improved, and accurate transmission torque detection can be performed.

[0032] Furthermore, it was confirmed that even when the number of strain gauges 6 is reduced, for example, as shown in Figures 5(A1) to (A3) and (B1) to (B3) below, torque detection can be performed based on the detection output from three sets of strain gauges 6 attached at 120° angular intervals, with the same level of accuracy as when torque detection is performed based on the output from eight sets of strain gauges 6 attached at equal angular intervals so that the entire gauge grid 61 is positioned on the inner end face 33a of the diaphragm.

[0033] (Other Embodiments) The present invention is also applicable to external gears equipped with bosses of various shapes, and to external gears in which load-side members, etc., are fixed to the boss by fastening means other than bolts. For example, the cup-shaped external gear 3A shown in Figures 5(A1), (A2), and (A3) has threads cut into the outer surface of an annular boss 34A, with a hexagonal hole for inserting a jig formed in the center, and a load-side member (not shown) is attached to the boss 34A by screw fastening. The cup-shaped external gear 3B shown in Figures 5(B1), (B2), and (B3) is an external gear also called a wine glass type, in which a large-diameter flange 341 is integrally formed at the end of the boss 34B. Bolt holes for fastening bolts are formed in the flange 341 at equal angular intervals along the circumferential direction. In addition to bolt fastening and screw fastening methods, welding is also known as a means of fixing load-side members, etc., to the boss, and the present invention can also be applied to external gears in which load-side members, etc., are welded to the boss.

[0034] In the example above, a two-axis superimposed strain gauge is used as the two-axis orthogonal strain gauge. A two-axis planar arrangement is also known as a two-axis orthogonal strain gauge, and of course, this type of two-axis orthogonal strain gauge can also be used. Furthermore, a single-axis strain gauge can be used instead of a two-axis orthogonal strain gauge. For example, along the boundary line between the inner end face of the boss and the inner end face of the diaphragm, a single-axis strain gauge tilted at a 45° angle to one side with respect to the radius line and a single-axis strain gauge tilted at a 45° angle to the opposite side can be arranged alternately.

[0035] Furthermore, strain gauges equipped with a gauge grid extending in an arc shape with a constant width can be used. For example, as shown in Figures 5(C1), (C2), and (C3), two arc-shaped strain gauges 6C1 and 6C2 with a 180° angle are used. In this case as well, each of the strain gauges 6C1 and 6C2 is positioned such that the radial center of the gauge grid coincides with the boundary line between the inner end face of the boss and the inner end face of the diaphragm. Alternatively, each of the strain gauges is positioned such that the radial center of the gauge grid is located within a predetermined area inside or outside the boundary line in the radial direction.

[0036] Next, the above example described a strain gauge type torque detection device for a harmonic drive gear equipped with a cup-shaped external gear. The present invention is similarly applicable to a strain gauge type torque detection device for a harmonic drive gear equipped with a top hat-shaped external gear.

Claims

1. A harmonic drive gear comprising: a rigid internal gear; a flexible external gear having a cup or top hat shape; a wave generator that partially engages the internal gear by bending the external gear in an elliptical shape; and a torque detection device equipped with a strain gauge for detecting the transmitted torque transmitted through the external gear, wherein the external gear comprises a diaphragm extending radially from one end of a cylindrical body on which the external teeth are formed, and a disc-shaped or annular boss integrally formed on the inner or outer peripheral edge of the diaphragm, wherein the boss end face on one axial side of the boss and the diaphragm end face on the same axial side of the diaphragm are smoothly continuous; the strain gauge comprises a gauge grid made of an electrical resistor that draws a predetermined wiring pattern, and a gauge lead connected to a gauge tab drawn out from the gauge grid, wherein, when viewed along the radial direction, at least a portion of the gauge grid is attached to the part on the side of the boss end face adjacent to the boundary line between the diaphragm end face and the boss end face.

2. A harmonic drive gear according to claim 1, wherein a portion of the gauge grid is attached to the diaphragm end face, and the remaining portion of the gauge grid is attached to the boss end face, with the boundary line in between.

3. A harmonic drive gear device according to claim 1, wherein, when viewed along the radial direction, a ring-shaped region of a predetermined width along the boundary line on the boss end face is defined as the boss-side boundary adjacent region, and the gauge grid is attached to the boss end face such that its center in the planar direction is located within the boss-side boundary adjacent region.

4. A harmonic drive gear according to claim 1, wherein, when viewed along the radial direction, a ring-shaped region of a predetermined width along the boundary line on the boss end face is defined as the boss-side boundary adjacent region, a portion of the gauge grid is attached to the boss-side boundary adjacent region, and the remaining portion is attached to the diaphragm end face, and when viewed along the radial direction, the center of the gauge grid in the planar direction is located on the boundary line.

5. A harmonic drive gear according to claim 1, wherein the gauge tab and the gauge lead of the strain gauge are located on the side of the boss end face.

6. A harmonic drive gear according to claim 1, wherein the external gear has a cup shape in which the diaphragm extends radially inward from one end of a cylindrical body, and the boss is integrally formed on the inner circumferential edge of the diaphragm, the diaphragm end face is an inner end face of the diaphragm connected to the inner circumferential surface of the cylindrical body, and the boss end face is an inner end face of the boss smoothly connected to the inner end face of the diaphragm.

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