Strain wave gearing and robotic arm

Provided is a strain wave gearing having a high stiffness and no limitation imposed on rotation, and a robotic arm including the strain wave gearing. A strain wave gearing includes an electric motor and a strain wave gearing reducer. The strain wave gearing reducer includes: an outer ring member including a first internal gear; a pair of second internal gears each having internal teeth formed along an inner periphery thereof, and the pair of second internal gears differing from the first internal gear in number of teeth; a flexible gear; and a cam member, which distorts the flexible gear in a radial direction to cause the flexible gear to engage with the first internal gear and the pair of second internal gears. The pair of second internal gears is fixed to a pair of fixing plates coupled to each other by a shaft penetrating the cam member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a strain wave gearing including an electric motor and a strain wave gearing reducer for reducing a rotational speed of the electric motor, and also relates to a robotic arm including the strain wave gearing.

2. Description of the Related Art

There is known a strain wave gearing including an electric motor and a strain wave gearing reducer (so-called harmonic drive (registered trademark)). The strain wave gearing of this type is provided to each joint of links of a robotic arm so as to pivot and rotate the links. The strain wave gearing reducer includes a thin, cylindrical, flexible external gear having external teeth and called flex spline, an internal gear having internal teeth and called circular spline, and a cam member called wave generator (see Japanese Patent Application Laid-Open No. S63-053340, Japanese Utility Model Publication No. H04-048346, and Japanese Utility Model Application Laid-Open No. S61-011047). The wave generator is formed into an elliptical shape so that the flex spline is deformed into an elliptical shape and pressed against the circular spline.

FIGS. 13A,13B, and13C are sectional views of three strain wave gearing reducers. First, one of the strain wave gearing reducers is described with reference toFIG. 13A. A stator101of an electric motor is provided to a fixed shaft100, and a rotor102is arranged along an outer periphery of the stator101. A wave generator103is fixed to the rotor102. Along an outer periphery of the wave generator103, an open end of a flex spline104is mounted, and the other end of the flex spline104is supported so as to rotate freely relative to the fixed shaft100. Along an outer periphery of the flex spline104, a circular spline105fixed to a cover106is arranged. The flex spline104is distorted by the wave generator103into an elliptical shape to engage with the circular spline105at two positions, that is, both ends along the major axis of the ellipse. The rotation of the wave generator103constructed by the rotor102of the electric motor causes a relative rotation between the flex spline104and the circular spline105, and a rotation output is extracted by a flange107fixed to one end of the flex spline104.

Next, another strain wave gearing reducer is described with reference toFIG. 13B. The strain wave gearing reducer includes two circular splines111and112having different numbers of teeth. A flex spline113is provided on an inner side of the two circular splines111and112, and there is provided a wave generator115for distorting and deforming the flex spline113into an elliptical shape so that the flex spline113is rotated in the elliptical shape. The rotation of the wave generator115causes a change in engagement positions between the flex spline113and the two circular splines111and112. When the engagement positions change during one revolution, the circular spline111rotates relative to the circular spline112with the shift corresponding to the difference in number of teeth between the two circular splines111and112. In this strain wave gearing reducer, one of the two circular splines111and112is used as a fixed shaft while the other is used as an output shaft.

Next, still another strain wave gearing reducer is described with reference toFIG. 13C. The strain wave gearing reducer includes one elliptical wave generator121fixed to an input shaft120, and one flex spline122having a tooth form on an outer periphery thereof. The strain wave gearing reducer further includes a first circular spline123, which is fixed to an arm123aprovided in a part of an outer periphery thereof and engages with the flex spline122, and a pair of second circular splines125and126, which engage with the flex spline122on both sides of the first circular spline123. In this strain wave gearing reducer, the pair of second circular splines125and126are coupled to each other by a coupling bar124, and the pair of second circular splines125and126rotate relative to the first circular spline123. The coupling bar124is used as an output shaft.

An industrial robotic arm is structured by connecting multiple joints in series. In each joint, the strain wave gearing including the electric motor and the strain wave gearing reducer is disposed, and hence the mechanical model is a model in which stiffnesses of the respective strain wave gearing reducers are connected in series. Therefore, unless a strain wave gearing reducer having a sufficiently high stiffness is used, the stiffness of the entire robotic arm lacks. If the stiffness of the robotic arm is low, the natural frequency of the robotic arm decreases, resulting in decrease in accuracy, increase in stabilization period, decrease in maximum operation speed, and other such performance degradation. The use of the strain wave gearing reducer having a high stiffness is an important factor for the robotic arm.

In the strain wave gearing reducer illustrated inFIG. 13A, a rotational force applied to a portion between the circular spline105and the flange107is also applied to the flex spline104provided therebetween. When the rotational force is applied to the flex spline104, the flex spline104being a flexible elastic member is torsionally deformed by the rotational force, and the flange107serving as the output shaft rotates by the torsional deformation. In the strain wave gearing reducer illustrated inFIG. 13A, the flex spline104serves as the output shaft, and hence the flex spline104is likely to be torsionally deformed, which causes the decrease in stiffness of the strain wave gearing reducer.

In the strain wave gearing reducer illustrated inFIG. 13B, the circular spline112situated in one end portion of the flex spline113rotates relative to the circular spline111situated in the other end portion. Hence, forces are applied to the external teeth of the flex spline113in opposite directions in a part engaging with the circular spline111and in a part engaging with the circular spline112. Because the forces are applied to the respective end portions of the flex spline113in opposite directions, the flex spline113is likely to be torsionally deformed by the forces, which causes the decrease in stiffness of the strain wave gearing reducer.

In the strain wave gearing reducer illustrated inFIG. 13C, the two circular splines125and126situated in both end portions of the flex spline122rotate relative to the circular spline123situated in the center portion of the flex spline122. The two circular splines125and126rotate in the same direction, and hence forces are applied to the end portions of the flex spline122in the same direction. Accordingly, the direction of the force applied to the center portion of the flex spline122is opposite to that of the force applied to each end portion, but the directions of the forces applied to both the end portions are the same. Thus, the flex spline122is not likely to be torsionally deformed, which suppresses the decrease in stiffness of the strain wave gearing reducer. In this strain wave gearing reducer, the two circular splines125and126are coupled to each other by the coupling bar124extending outside. Hence, the coupling bar124hits against the arm123asupporting the circular spline123, and accordingly no further revolution can be made than one revolution. When the strain wave gearing including the strain wave gearing reducer having such structure is used for the robotic arm, the movable range of the links is reduced.

SUMMARY OF THE INVENTION

In view of the above, the present invention provides a strain wave gearing having a high stiffness and no limitation imposed on rotation, and provides a robotic arm including the strain wave gearing.

The present invention provides a strain wave gearing, including: an electric motor; an outer ring member for rotation output having a tubular outer ring portion, and a first internal gear which is provided so as to protrude into an inner side of the outer ring portion and has internal teeth formed along an inner periphery of the first internal gear; a pair of second internal gears arranged on the inner side of the outer ring portion and on both end sides of the first internal gear, the pair of second internal gears each having internal teeth formed along an inner periphery of the pair of second internal gears, and a number of the internal teeth of the each second internal gear differing from a number of the internal teeth of the first internal gear; a flexible gear having external teeth formed along an outer periphery of the flexible gear, the flexible gear formed of a tubular flexible member which is arranged on an inner side of the first internal gear and the pair of second internal gears; a tubular cam member configured to distort the flexible gear in a radial direction so as to partially engage the flexible gear with the first internal gear and the pair of second internal gears, the tubular cam member rotated by a rotational force transmitted from the electric motor so as to move engagement positions in a circumferential direction so that a relative rotation between the first internal gear and the pair of second internal gears is generated; a pair of fixing plates, to each of which a corresponding second internal gear of the pair of second internal gears is fixed; and a coupling member penetrating the tubular cam member and coupling the pair of fixing plates to each other.

Further, the present invention provides a strain wave gearing, including: an electric motor; an outer ring member for rotation output having a tubular outer ring portion, and an internal gear which is provided so as to protrude into an inner side of the outer ring portion and has internal teeth formed along an inner periphery of the internal gear; a flexible gear having an external gear which has external teeth formed along an outer periphery of the external gear and is arranged on an inner side of the internal gear, the flexible gear formed of a tubular flexible member which has a pair of flange portions arranged on both end sides of the external gear, a number of the external teeth of the external gear differing from a number of the internal teeth of the internal gear; a tubular cam member configured to distort the flexible gear in a radial direction so as to partially engage the external gear with the internal gear, the tubular cam member rotated by a rotational force transmitted from the electric motor so as to move engagement positions in a circumferential direction so that a relative rotation between the internal gear and the external gear is generated; a pair of fixing plates, to each of which a corresponding flange portion of the pair of flange portions is fixed; and a coupling member penetrating the tubular cam member and coupling the pair of fixing plates to each other.

According to the present invention, both the end portions of the flexible gear are supported by the pair of second internal gears fixed to the pair of fixing plates, and hence the outer ring member including the first internal gear rotates with respect to the pair of second internal gears. At this time, a force from the first internal gear of the outer ring member is applied to the center portion of the flexible gear. To both the end portions of the flexible gear, however, forces are applied in the same direction, which is opposite to that of the force applied to the center portion, with the result that the torsional deformation of the flexible gear is suppressed and the stiffness is enhanced. In addition, the pair of fixing plates are coupled to each other by the coupling member penetrating the tubular cam member, and hence the outer ring member does not hit against the coupling member due to the rotation of the outer ring member, with the result that more than one revolution can be made for the outer ring member through the first internal gear.

According to the present invention, both the end portions of the flexible gear are constructed by the pair of flange portions, and the pair of flange portions are fixed to the pair of fixing plates, respectively. Accordingly, the outer ring member including the internal gear rotates with respect to the flexible gear including the external gear. At this time, the flexible gear is fixed to the pair of fixing plates with a simply supported beam structure, with the result that the torsional deformation of the flexible gear is suppressed and the stiffness is enhanced. In addition, the pair of fixing plates are coupled to each other by the coupling member penetrating the tubular cam member. Accordingly, the outer ring member does not hit against the coupling member due to the rotation of the outer ring member, with the result that more than one revolution can be made for the outer ring member through the internal gear.

DESCRIPTION OF THE EMBODIMENTS

Hereinbelow, exemplary embodiments of the present invention are described in detail with reference to the attached drawings.

First Embodiment

FIG. 1is a schematic view illustrating schematic structure of a strain wave gearing according to a first embodiment of the present invention.FIG. 2is a schematic sectional view taken along the line2-2ofFIG. 1. Hereinbelow, structure of a strain wave gearing reducer is described. As illustrated inFIG. 1, a strain wave gearing50includes one electric motor51and a strain wave gearing reducer52. The strain wave gearing50is integrally structured by building the electric motor51into the strain wave gearing reducer52. The electric motor51is a brushless DC motor.

Hereinbelow, specific structure is described. The strain wave gearing50includes an outer ring member3serving as a so-called circular spline. The outer ring member3includes a tubular (in this embodiment, cylindrical) outer ring portion3a, and an annular (in this embodiment, toric) first internal gear3b, which is provided so as to protrude into an inner side of the outer ring portion3aand has internal teeth formed along an inner periphery thereof. The outer ring member3is formed of a rigid member. The outer ring member3is a rotation output member that rotates about a rotational axis (inFIG. 1, two-dot chain line z; hereinafter, referred to as “z-axis”). The outer ring member3reduces the speed of input, that is, rotation of the electric motor51and outputs.

A pair of second internal gears2aand2bis arranged on the inner side of the outer ring portion3aof the outer ring member3and on both end sides of the first internal gear3b. Each of the second internal gears2aand2bhas internal teeth formed along an inner periphery thereof. In other words, in a direction parallel to the z-axis direction, the one second internal gear2ais disposed on one end side of the first internal gear3band the other second internal gear2bis disposed on the other end side of the first internal gear3b. The pair of second internal gears2aand2bis formed into an annular (in this embodiment, toric) shape. The pair of second internal gears2aand2bis each formed so as to have a rotational axis concentric with that of the first internal gear3bat the z-axis and have the same radius of the inner periphery as the first internal gear3b. However, the pair of second internal gears2aand2bis different from the first internal gear3bin number of internal teeth formed along the inner periphery. For example, the first internal gear3bhas 100 internal teeth formed therearound while the second internal gears2aand2beach have 98 internal teeth formed therearound. The second internal gear2aand the second internal gear2bare set to have the same structure and the same number of teeth.

A flexible gear1serving as a so-called flex spline is arranged on an inner side of the first internal gear3band the pair of second internal gears2aand2b. The flexible gear1has external teeth formed along an outer periphery thereof. The flexible gear1is formed of a flexible member, and includes a thin cylindrical portion and external teeth formed along an outer periphery of the thin cylindrical portion. The flexible gear1may be distorted to be deformed in a radial direction r orthogonal to the z-axis direction. The flexible gear1is set so as to have substantially the same width as a sum of the width of the first internal gear3band the widths of the pair of second internal gears2aand2b. Further, the flexible gear1has its external teeth formed in parallel to the z-axis direction. Hence, the flexible gear1is arranged so that a center portion of the flexible gear1engages with the first internal gear3band both end portions of the flexible gear1engage with the pair of second internal gears2aand2b.

As illustrated inFIG. 2, a cam member10serving as a so-called wave generator is arranged on an inner side of the flexible gear1through the intermediation of an outer rim12and a bearing11. The cam member10has its outer periphery formed into an elliptical shape. Specifically, the bearing11is arranged on an outer side of the cam member10and the outer rim12is arranged on an outer side of the bearing11. The cam member10rotates relative to the flexible gear1through the intermediation of the bearing11and the outer rim12, and distorts the flexible gear1in the radial direction r, to thereby cause the flexible gear1to partially engage with the first internal gear3band the pair of second internal gears2aand2b. In other words, the flexible gear1is distorted by the cam member10into an elliptical shape to engage with the first internal gear3band the pair of second internal gears2aand2bat two engagement positions along the major axis of the ellipse. In this case, the cam member10to be manufactured has its aspect ratio and size adjusted so that the flexible gear1engages with the three internal gears2a,2b, and3bat the two positions, that is, in the direction of the major axis of the ellipse. The cam member10is formed of a tubular rigid member having its inner periphery formed into a circular shape. The bearing11includes multiple cylindrical rollers or multiple balls. The bearing11allows the outer rim12to rotate along the outer periphery of the cam member10, that is, along the ellipse.

The electric motor51is arranged on an inner side of the cam member10. Specifically, the electric motor51includes a stator51aand a rotor51b, and the stator51aand the rotor51bare arranged on the inner side of the cam member10. The stator51aincludes multiple (inFIG. 2, six) coils7arranged into an annular shape about the z-axis. The stator51ais formed into an annular shape. The rotor51bincludes multiple permanent magnets13aand13barranged on both sides of the stator51a(coils7), yokes14aand14bto which the permanent magnets13aand13bare fixed, respectively, and motor housings9aand9bto which the yokes14aand14bare fixed, respectively. The pair of motor housings9aand9bis formed into an annular shape, and their annular outer rim portions are coupled and fixed to the inner periphery of the cam member10. With this structure, the cam member10is fixed to the rotor51band a rotational force of the electric motor51is transmitted to the cam member10, with the result that the cam member10rotates about the z-axis integrally with the rotor51b. Because of the rotation of the cam member10, the engagement positions between the first internal gear3band the flexible gear1and between the pair of second internal gears2aand2band the flexible gear1move in the circumferential direction, which may cause a relative rotation between the first internal gear3band the pair of second internal gears2aand2bwith the shift corresponding to the difference in number of teeth. For example, the case where the first internal gear3bhas 100 internal teeth formed therearound while the second internal gears2aand2beach have 98 internal teeth formed therearound is described. In this case, one revolution of the cam member10causes the first internal gear3bto rotate relative to the pair of second internal gears2aand2bwith the shift of two teeth (difference in number of teeth), that is, a 2/100 revolution. The reduction ratio is 1:50.

The electric motor51generates a rotational force by causing a current corresponding to a rotation position to flow through the coils7. The rotation position of the rotor51bis detected by a method of detecting positions of the magnets using a Hall element (not shown) or a method of directly detecting the positions using an encoder (not shown).

In the first embodiment, the strain wave gearing includes a pair of disc-like fixing plates4aand4barranged on the inner side of the outer ring portion3a. In the first embodiment, the pair of fixing plates4aand4bis formed into a toric shape with a through-hole formed in their center portion. The pair of fixing plates4aand4bis formed so that an outside diameter thereof substantially equals the diameter of each of the second internal gears2aand2b. The one second internal gear2ais fixed to an outer circumferential portion of the one fixing plate4a, while the other second internal gear2bis fixed to an outer circumferential portion of the other fixing plate4b. Accordingly, to the pair of fixing plates4aand4b, the corresponding second internal gears2aand2bare fixed, respectively. The pair of second internal gears2aand2bare arranged between the pair of fixing plates4aand4b.

The pair of fixing plates4aand4bis coupled to each other by a shaft5serving as a coupling member. Specifically, the shaft5is coupled to the pair of fixing plates4aand4bby means of bolts or the like. The shaft5is formed into a tubular shape, and is arranged so that a shaft center thereof conforms to the z-axis. In other words, the shaft5couples to each other inner circumferential portions of the pair of fixing plates4aand4bformed into an annular shape. Therefore, the shaft5is arranged on the inner side of the cam member10so as to penetrate the tubular cam member10. In other words, the shaft5couples the pair of fixing plates4aand4bto each other through a space surrounded by the inner periphery of the cam member10. Because the shaft5is formed into the tubular shape, electric wiring and other members may be provided through the inner space of the shaft5. With the coupling structure of the pair of fixing plates4aand4band the shaft5, the pair of second internal gears2aand2bintegrally rotate about the z-axis relative to the first internal gear3b. In other words, the first internal gear3brotates about the z-axis relative to the pair of internal gears2aand2b. Specifically, the pair of second internal gears2aand2b, the pair of fixing plates4aand4b, and the shaft5constitute an inner ring member, and the outer ring member3rotates relative to the inner ring member. Bearings6aand6bare arranged between peripheral end surfaces of the pair of fixing plates4aand4band an inner peripheral surface of the outer ring portion3aof the outer ring member3. In other words, the outer ring portion3aof the outer ring member3is supported by the pair of fixing plates4aand4bthrough the intermediation of the bearings6aand6b. The pair of fixing plates4aand4bmay be fixed to a fixed object (not shown), and at least one of the pair of fixing plates4aand4bis fixed to the fixed object (not shown).

The stator51aformed into the annular shape is fixed to an outer periphery of the shaft5. Bearings8aand8bare arranged between the shaft5and the rotor51b(motor housings9aand9b) formed into the annular shape, and the rotor51brotates around the shaft5about the z-axis.

In the first embodiment, the strain wave gearing reducer52includes the cam member10, the flexible gear1, the outer ring member3including the first internal gear3b, the pair of second internal gears2aand2b, the pair of fixing plates4aand4b, and the shaft5. The strain wave gearing50is structured by building the electric motor51into the strain wave gearing reducer52.

In the above-mentioned structure, when power is supplied to the coils7of the stator51aof the electric motor51, the rotor51bincluding the permanent magnets13aand13brotates, and accordingly the cam member10fixed to the motor housings9aand9brotates. Because of the rotation of the cam member10, the two engagement positions in the direction of the major axis of the ellipse, between the flexible gear1and the first internal gear3band between the flexible gear1and the pair of second internal gears2aand2b, rotate. One revolution of the cam member10causes a relative rotation between the first internal gear3band the pair of second internal gears2aand2bwith the shift corresponding to the difference in number of teeth between the first internal gear3band the pair of second internal gears2aand2b. In a case where the fixing plate4a(4b) is fixed to the fixed object (not shown), the pair of second internal gears2aand2bcoupled thereto does not rotate with respect to the fixed object, but instead the first internal gear3brotates with respect to the fixed object at the reduction ratio that is set based on the difference in number of teeth. In a case where the outer ring member3including the first internal gear3bis fixed to the fixed object (not shown), on the other hand, the outer ring member3does not rotate with respect to the fixed object, but instead the pair of second internal gears2aand2brotates with respect to the fixed object.

Next, calculation of a stiffness based on forces applied to the flexible gear1is described.FIGS. 3A and 3Bare explanatory views for the stiffness calculation.FIG. 3Aillustrates an engaging state between the flexible gear1and the first internal gear3band between the flexible gear1and the pair of second internal gears2aand2barranged on both the sides of the first internal gear3b.FIG. 3Billustrates the external tooth of the flexible gear1.

The rotational force applied to the outer ring member3serving as an output shaft of the strain wave gearing reducer52is applied to the center portion of the flexible gear1engaging with the first internal gear3band to both the end portions of the flexible gear1engaging with the two second internal gears2aand2b, and acts in such a direction as to deform the flexible gear1. In this case, the flexible gear1is supported by the two second internal gears2aand2b, and as illustrated inFIG. 3B, forces F/2 act on both the end portions of the flexible gear1in the same direction, which is opposite to that of a force F applied to the center portion of the external tooth of the flexible gear1. The flexible gear1has so-called simply supported beam structure, and hence the stiffness is enhanced. Hereinbelow, the stiffness is specifically determined through calculation. The flexible gear1includes the thin cylindrical portion and the external teeth.

The stiffness of the strain wave gearing reducer is a ratio of the rotational force to a rotation angle according to the rotational force. Hereinbelow, the following symbols are used in mathematical expressions.

T: torque applied to the strain wave gearing reducer, R: radius of the flexible gear, F: force applied to the external tooth of the flexible gear (F=T/R), G: modulus of transverse elasticity of the flexible gear, E: modulus of longitudinal elasticity of the flexible gear, ν: Poisson's ratio of the flexible gear, a: thickness of the thin cylindrical portion of the flexible gear, b: tooth width of the flexible gear, c: tooth height of the flexible gear, L: width of the flexible gear (length of the external tooth), n: number of engaging teeth.

In this case, it is assumed that the width of the first internal gear3bin the center portion is L/2, and the width of each of the second internal gears2aand2bin both the side portions is L/4. When assuming that the center of the applied force corresponds to the center of the first internal gear3b, the forces are applied as reaction forces from the second internal gears2aand2bon both sides to positions that are spaced apart at an interval of 3L/4. Accordingly, it may be considered as a bending problem of a beam in which both end portions of the flexible gear1are supported simply. Hereinbelow, the stiffness of the strain wave gearing reducer is considered separately for the case of the thin cylindrical portion and the case of the external tooth portion.

(1) Stiffness of Thin Cylindrical Portion

First Embodiment

First, a torsional stiffness of a circular pipe having an outside diameter of 2(R+a), an inside diameter of 2R, and a length of 3L/8 is considered. The torsional stiffness is assumed to be on both sides, and hence a doubled value of the torsional stiffness corresponds to a stiffness Knew1 of the thin cylindrical portion.

In this case, the following expression is used.

(2) Stiffness of External Tooth

First Embodiment

The external tooth of the flexible gear1is considered as the bending problem of the simply supported beam of the interval of 3L/4, and hence a relationship expressed in the following expression is established when assuming that F represents an external force and x represents displacement.

When F is represented by the torque T applied in the case of the radius R and x is represented by a rotation angle θ in the case of the radius R, a stiffness Knew2 of one external tooth is expressed as follows.

When assuming that n teeth engage, the stiffness of the strain wave gearing reducer of the first embodiment may be expressed as follows.

Next, as a reference example, the stiffness is calculated for the strain wave gearing reducer illustrated inFIG. 13B, in which the two circular splines111and112having different numbers of teeth engage with the flex spline113.FIGS. 4A and 4Bare explanatory views for the stiffness calculation.FIG. 4Aillustrates an engaging state between the flex spline113and the two circular splines111and112.FIG. 4Billustrates the external tooth of the flex spline113. As illustrated inFIG. 4B, when assuming that the center of the force applied to the external tooth of the flex spline113corresponds to the center between the circular splines111and112, forces are applied in opposite directions to positions that are spaced apart at an interval of L/2. The rotational force applied to the strain wave gearing reducer is applied to a portion between the two circular splines111and112, and hence the flex spline113is deformed. In this case, the flex spline113has so-called cantilever structure, and hence has a low stiffness. Next, the stiffness is specifically determined through calculation. The stiffness of the strain wave gearing reducer is a ratio of a relative rotational force applied to the portion between the two circular splines111and112to a relative rotation angle due to the relative rotational force. As illustrated inFIG. 4A, the flex spline113includes a thin cylindrical portion having the thickness a, and external teeth each having the tooth width b and the tooth height c. The stiffness of the strain wave gearing reducer may be estimated by summing up the stiffness of the thin cylindrical portion and the stiffness of the external tooth portion.

(3) Stiffness of Thin Cylindrical Portion

Reference Example

A twist angle of a circular pipe having an outside diameter of 2(R+a), an inside diameter of 2R, and a length of L/2 may be calculated in terms of the strength of materials, and a stiffness Kold1 may be expressed as follows.

(4) Stiffness of External Tooth

Reference Example

The external tooth of the flex spline113receives forces from the internal teeth of the two circular splines111and112held into contact with the flex spline113. When assuming that the external tooth of the flex spline113is a simply supported beam of the length of L/2, the two forces applied to the flex spline113are on different lines of action, and hence there is no drag generated against the forces. Thus, it does not contribute to the stiffness of the strain wave gearing reducer.

In summary, a stiffness Kold of the strain wave gearing reducer in the reference example may be estimated by the following expression.

Hereinabove, Expressions 1 to 6 are derived, and the first embodiment is now compared to the comparative example. First, comparison is made on the stiffness of the thin cylindrical portion. The stiffness of the first embodiment is Knew1, which is expressed in Expression 1, while the stiffness of the comparative example is Kold1, which is expressed in Expression 5. When a stiffness ratio therebetween is determined, all the symbols are canceled. The determined stiffness ratio is expressed as follows.

Even considering the thin cylindrical portion alone, the stiffness is expected to be enhanced to a value about five times as high as that of the comparative example.

Thus, according to the first embodiment, both the end portions of the flexible gear1are supported by the pair of second internal gears2aand2bfixed to the pair of fixing plates4aand4b. Hence, the outer ring member3including the first internal gear3brotates with respect to the pair of second internal gears2aand2b. At this time, as illustrated inFIG. 3B, the force F from the first internal gear3bof the outer ring member3is applied to the center portion of the flexible gear1, while to both the end portions of the flexible gear1, the forces F/2 are applied in the same direction, which is opposite to that of the force applied to the center portion. As a result, the torsional deformation of the flexible gear1is suppressed even when the flexible gear1of the same size is used, and the stiffness can be enhanced to a value more than about five times as high as that of the comparative example. In addition, the pair of second internal gears2aand2bare fixed to the pair of fixing plates4aand4b, and the pair of fixing plates4aand4bare coupled to each other by the shaft5penetrating the tubular cam member10. Hence, the outer ring member3does not hit against the shaft5due to the rotation of the outer ring member3. As a result, more than one revolution can be made for the outer ring member3through the first internal gear3b.

Further, in the electric motor51, the stator51ais arranged between the pair of fixing plates4aand4band fixed to the shaft5. The rotor51bis arranged between the pair of fixing plates4aand4band fixed to the inner periphery of the cam member10. With this structure, the rotor51brotates integrally with the cam member10. In the first embodiment, the arrangement of the stator51aand the rotor51bis devised so that the stator51aand the rotor51bare housed in an efficient way between the pair of fixing plates4aand4b, with the result that the device structure is reduced in size. Thus, when the strain wave gearing50is applied to a robotic arm (not shown), a small-size robot can be manufactured.

Second Embodiment

A strain wave gearing according to a second embodiment of the present invention is described. The second embodiment is different from the first embodiment in the structure of the strain wave gearing reducer.FIG. 5is a schematic view illustrating schematic structure of the strain wave gearing according to the second embodiment of the present invention. In the second embodiment, the same components as those in the first embodiment are represented by the same reference symbols, and description thereof is omitted herein. As illustrated inFIG. 5, a strain wave gearing50A includes an outer ring member3A serving as a so-called circular spline. The outer ring member3A includes a tubular (in this embodiment, cylindrical) outer ring portion30a, and an annular (in this embodiment, toric) internal gear30b, which is provided so as to protrude into an inner side of the outer ring portion30aand has internal teeth formed along an inner periphery thereof. The outer ring member3A is formed of a rigid member. The outer ring member3A is a rotation output member that rotates about the rotational axis (inFIG. 5, two-dot chain line z, that is, z-axis). The outer ring member3A reduces the speed of input, that is, the rotation of the electric motor51and outputs.

A flexible gear1A serving as a so-called flex spline is arranged on an inner side of the outer ring member3A. The flexible gear1A is formed of a flexible member, and includes an external gear1a, which is formed in the center portion of the flexible gear1A and has external teeth, which are to be engaged with the internal teeth of the internal gear30b, formed along an outer periphery thereof. The external gear1ais set to have a different number of teeth from that of the internal gear30b.

As illustrated inFIG. 6A, the flexible gear1A includes a pair of thin plate-like deformable portions16aand16b, which are connected to both ends of the external gear1aand can be distorted to be deformed. The flexible gear1A further includes a pair of flange portions15aand15b, which are respectively connected to the other ends of the deformable portions16aand16bon a side opposite to the one ends at which the external gear1ais connected, and are thicker than the deformable portions16aand16b. The external gear1a, the pair of deformable portions16aand16b, and the pair of flange portions15aand15bare formed into a tubular shape.

The one deformable portion16ais formed between the external gear1aand the one flange portion15a, while the other deformable portion16bis formed between the external gear1aand the other flange portion15b. The one flange portion15aout of the pair of flange portions15aand15bis fixed to the one fixing plate4a, while the other flange portion15bout of the pair of flange portions15aand15bis fixed to the other fixing plate4b. Accordingly, as illustrated inFIG. 5, the flexible gear1A is fixed to the pair of fixing plates4aand4bin an unrotatable manner. The cam member10rotates relative to the flexible gear1A through the intermediation of the bearing11and the outer rim12to partially press the external gear1aof the flexible gear1A and partially distort the pair of deformable portions16aand16bto deform the pair of deformable portions16aand16b. As a result, the external gear1apartially protrudes in the radial direction r. Accordingly, the flexible gear1A can be distorted to be deformed in the radial direction r.

The cam member10distorts the flexible gear1A in the radial direction r through the intermediation of the bearing11and the outer rim12, to thereby cause the flexible gear1A to partially engage with the internal gear30b. In other words, the flexible gear1A is distorted by the cam member10into an elliptical shape so that the internal gear30bengages with the external gear1aof the flexible gear1A at two engagement positions along the major axis of the ellipse. In this case, the cam member10to be manufactured has its aspect ratio and size adjusted so that the external gear1aof the flexible gear1A engages with the internal gear30bof the outer ring member3A at the two positions, that is, in the direction of the major axis of the ellipse. The cam member10is formed of a tubular rigid member having its inner periphery formed into a circular shape.

The cam member10is fixed to the rotor51band the rotational force of the electric motor51is transmitted to the cam member10, with the result that the cam member10rotates about the z-axis integrally with the rotor51b. Accordingly, because of the rotation of the cam member10, the engagement positions between the internal gear30bof the outer ring member3A and the external gear1aof the flexible gear1A move in the circumferential direction, which may cause a relative rotation between the internal gear30band the external gear1awith the shift corresponding to the difference in number of teeth. For example, the case where the internal gear30bhas 100 internal teeth formed therearound while the external gear1ahas 98 external teeth formed therearound is described. In this case, one revolution of the cam member10causes the internal gear30bto rotate relative to the external gear1awith the shift of two teeth (difference in number of teeth), that is, a 2/100 revolution. The reduction ratio is 1:50. In this case, the number of teeth of the external gear1amay be smaller than the number of teeth of the internal gear30b. With this structure, the outer ring member3A rotates in the same direction as the rotation direction of the rotor51bof the electric motor51. In a case where the outer ring member3A may rotate in a direction opposite to the rotation direction of the rotor51bof the electric motor51, the number of teeth of the external gear1amay be larger than the number of teeth of the internal gear30b.

In the second embodiment, similarly to the first embodiment, the pair of fixing plates4aand4bare coupled by the shaft5. The electric motor51is housed between the pair of fixing plates4aand4bin the same way as in the first embodiment. Because the shaft5is formed into the tubular shape, electric wiring and other members may be provided through the inner space of the shaft5. With the coupling structure of the pair of fixing plates4aand4band the shaft5, the flexible gear1A including the external gear1a, which is coupled to the pair of fixing plates4aand4b, integrally rotates about the z-axis relative to the internal gear30b. In other words, the outer ring member3A including the internal gear30brotates about the z-axis relative to the flexible gear1A including the external gear1a. Specifically, the flexible gear1A, the pair of fixing plates4aand4b, and the shaft constitute an inner ring member, and the outer ring member3A rotates relative to the inner ring member. The pair of fixing plates4aand4bmay be fixed to a fixed object (not shown), and at least one of the pair of fixing plates4aand4bis fixed to the fixed object (not shown).

In the second embodiment, a strain wave gearing reducer52A includes the cam member10, the flexible gear1A, the outer ring member3A including the internal gear30b, the pair of fixing plates4aand4b, and the shaft5. The strain wave gearing50A is structured by building the electric motor51into the strain wave gearing reducer52A.

In the above-mentioned structure, when power is supplied to the coils7of the stator51aof the electric motor51, the rotor51bincluding the permanent magnets13aand13brotates, and accordingly the cam member10fixed to the motor housings9aand9brotates. Because of the rotation of the cam member10, the two engagement positions in the direction of the major axis of the ellipse, between the external gear1aof the flexible gear1A and the internal gear30bof the outer ring member3A, rotate. One revolution of the cam member10causes a relative rotation between the external gear1aand the internal gear30bwith the shift corresponding to the difference in number of teeth between the external gear1aand the internal gear30b. In a case where the fixing plate4a(4b) is fixed to the fixed object (not shown), the flexible gear1A coupled thereto does not rotate with respect to the fixed object, but instead the outer ring member3A (internal gear30b) rotates with respect to the fixed object. In a case where the outer ring member3A is fixed to the fixed object (not shown), on the other hand, the outer ring member3A does not rotate with respect to the fixed object, but instead the flexible gear1A and the fixing plates4aand4bcoupled thereto rotate with respect to the fixed object.

Description is given by referring to a reference example illustrated inFIG. 6B.FIG. 6Billustrates the flex spline104having cantilever structure corresponding toFIG. 13A. The rotational force applied to the strain wave gearing reducer is applied to a portion between the flange107and the circular spline105. Accordingly, the flex spline104is supported on one side, and receives the rotational force at an end on the other side. In this case, the flex spline104has the so-called cantilever structure, and hence the stiffness is low.

In contrast, in the flexible gear1A of the second embodiment, as illustrated inFIG. 6A, the flange portions15aand15bare formed integrally with the external gear1athrough the intermediation of the deformable portions16aand16b, respectively. As illustrated inFIG. 5, the flange portions15aand15bare fixed to the fixing plates4aand4b, respectively. Hence, the rotational force applied to the strain wave gearing reducer is applied to a portion between the two flange portions15aand15band the internal gear30b. Accordingly, the two end portions of the flexible gear1A are supported, and the flexible gear1A receives the rotational force in the center portion. In this case, the flexible gear1A has so-called simply supported beam structure, and hence the stiffness is enhanced. The fact that the stiffness is enhanced with the simply supported beam structure is as described in the calculation of the above-mentioned first embodiment.

As described above, according to the second embodiment, both the end portions of the flexible gear1A are constructed by the pair of tubular flange portions15aand15b, and the pair of flange portions15aand15bis fixed to the pair of fixing plates4aand4b, respectively. Accordingly, the outer ring member3A including the internal gear30brotates with respect to the flexible gear1A including the external gear1a. The flexible gear1A is fixed to the pair of fixing plates4aand4bwith the simply supported beam structure, with the result that the torsional deformation of the flexible gear1A is suppressed and the stiffness is enhanced. In addition, the pair of fixing plates4aand4bare coupled to each other by the shaft5penetrating the tubular cam member10. Hence, the outer ring member3A does not hit against the shaft5due to the rotation of the outer ring member3A. As a result, more than one revolution can be made for the outer ring member3A through the internal gear30b.

In the electric motor51, the stator51ais arranged between the pair of fixing plates4aand4band fixed to the shaft5. The rotor51bis arranged between the pair of fixing plates4aand4band fixed to the inner periphery of the cam member10. With this structure, the rotor51brotates integrally with the cam member10. In the second embodiment, the arrangement of the stator51aand the rotor51bis devised so that the stator51aand the rotor51bare housed in an efficient way between the pair of fixing plates4aand4b, with the result that the device structure is reduced in size. Thus, when the strain wave gearing50A is applied to a robotic arm (not shown), a small-size robot can be manufactured.

Third Embodiment

A strain wave gearing according to a third embodiment of the present invention is described. The third embodiment is different from the second embodiment in the structure of the deformable portions of the flexible gear, and other components of the third embodiment are the same as those in the second embodiment.FIGS. 7A and 7Bare explanatory views of a flexible gear of the strain wave gearing according to the third embodiment of the present invention.FIG. 7Ais a partial schematic view of the flexible gear.FIG. 7Bis a development view of the flexible gear. As illustrated inFIGS. 7A and 7B, deformable portions16aand16bof a flexible gear1B are thin plate-like members connecting the external gear1ato the flange portions15aand15b, respectively. In the plate-like portions, multiple slits22ato22eextending in a circumferential direction (arrow C direction) of the flexible gear1B are formed. The multiple slits22ato22eare through-holes. The slits22ato22eare formed in the deformable portions16aand16bso as to be arranged in a zigzag pattern in a direction orthogonal to the circumferential direction C and the radial direction r (direction parallel to the length direction of the external tooth of the external gear1a).

In the flexible gear1B, the flange portions15aand15bat both ends of the flexible gear1B are fixed to the fixing plates4aand4b, respectively (seeFIG. 5). Accordingly, the flexible gear1B is in the state of being fixed at both the end portions thereof. Hence, the center portion of the flexible gear1B is pressed by the cam member10(seeFIG. 5) and the deformable portions16aand16bare deformed. As a result, the flexible gear1B is deformed into an elliptical shape so as to partially protrude. Therefore, a repeated stress is applied to the deformable portions16aand16b.

In the third embodiment, the deformable portions16aand16bhaving the slits22ato22eformed therein are provided between both the end portions of the flexible gear1B and the center portion of the flexible gear1B, and hence a long beam approximately connects the portions therebetween, with the result that it is easy to deform in the radial direction r. In other words, the deformable portions16aand16bare easy to deform in an out-of-plane direction. According to the strength of materials, the deformation in the out-of-plane direction is bending deformation, and hence rapidly increases with the cube of the length. However, a deformation in an in-plane direction or a deformation in a tensile direction increases only with the first power of the length. Accordingly, the ratio between stiffness in the in-plane direction and stiffness in the out-of-plane direction of the flexible gear1B may be adjusted depending on the design of dimensions of the holes of the slits22ato22e. For example, the flexible gear may be designed to have low stiffness in the out-of-plane direction and high stiffness in the in-plane direction. The force for deforming the external gear1aof the flexible gear1B into an elliptical shape is a force in the out-of-plane direction, and the rotational force applied to the strain wave gearing reducer is a force in the in-plane direction of the flexible gear1B. Thus, by designing the flexible gear1B so as to have low stiffness in the out-of-plane direction and high stiffness in the in-plane direction, the repeated stress generated by the elliptical deformation can be alleviated with no penalty in the stiffness of the strain wave gearing reducer, with the result that a longer life is expected. When the slits22ato22eare formed in the deformable portions16aand16bof the flexible gear1B along a direction in which the rotational force is applied, that is, the circumferential direction C of the flexible gear1B, the deformation that may occur by the torsional torque decreases.

Note that, the number of slits to be formed may be one, in the third embodiment, multiple slits22ato22eare formed. By forming the multiple slits22ato22eas described above, the effect of alleviating the stress further increases.

Further, in the third embodiment, the flexible gear1B is supported with so-called simply supported beam structure, and hence a high stiffness can be achieved. Even when the slits22ato22eare formed, the stiffness decrease is slight. In other words, each of the slits22ato22eonly needs to be designed to have such a size that the superiority of the stiffness is not lost. Because the stress applied to the flexible gear1B can be alleviated, a small-size, long-life device can be realized.

As illustrated inFIG. 8, the flexible gear may be a flexible gear1C in which one spiral slit22aswirling in the circumferential direction C and extending in the z-axis direction is formed in each of the deformable portions16aand16b. Also in this case, the same effect as that in the third embodiment is obtained. The number of slits to be formed is not limited to one, and multiple slits may be formed.

Fourth Embodiment

A strain wave gearing according to a fourth embodiment of the present invention is described. The fourth embodiment is different from the first to third embodiments in the mounting structure of the electric motor of the strain wave gearing. Specifically, in the first to third embodiments, the electric motor is arranged between the pair of fixing plates, but in the fourth embodiment, the electric motor is arranged outside the pair of fixing plates.FIG. 9is a schematic view illustrating schematic structure of the strain wave gearing according to the fourth embodiment of the present invention. The same components as those in the first embodiment are represented by the same reference symbols, and description thereof is omitted herein.

In the fourth embodiment, as illustrated inFIG. 9, a strain wave gearing50B includes multiple (in this embodiment, two) electric motors53and53. The two electric motors53and53are symmetrically arranged across the z-axis. Each electric motor53is arranged outside the pair of fixing plates4aand4b. Specifically, each electric motor53is arranged in a region outside the region surrounded by the pair of fixing plates4aand4b. Further, each electric motor53is fixed to the one fixing plate4aout of the pair of fixing plates4aand4b. The pair of fixing plates4aand4bare coupled to each other by the tubular shaft5centered on the z-axis.

A rotation shaft54of each electric motor53penetrates the one fixing plate4aand protrudes into the region between the pair of fixing plates4aand4b. The rotation shaft54also penetrates the other fixing plate4b. A base end portion of the rotation shaft54is supported in a rotatable manner through the intermediation of a bearing17aarranged in the one fixing plate4a. A leading end portion of the rotation shaft54is supported in a rotatable manner through the intermediation of a bearing17barranged in the other fixing plate4b. The leading end portion of the rotation shaft54is pressed against the bearing17bby a pressing plate19.

A cam member10B serving as a so-called wave generator includes a tubular cam portion10ahaving its outer periphery formed into an elliptical shape and its inner periphery formed into a circular shape, and an internal gear10barranged in the center portion of the inner periphery of the cam portion10aand having internal teeth formed so as to protrude into the inner side thereof. A gear (pinion gear)55is fixed to the rotation shaft54of each electric motor53. The gear55engages with the internal teeth of the internal gear10bof the cam member10B. Further, a pair of bearings18aand18bare provided between the inner periphery of the cam portion10aof the cam member10B and the pair of fixing plates4aand4b, and the cam member10B is supported so as to be rotatable about the z-axis.

In the above-mentioned structure, when power is supplied to the electric motors53and53, a rotational force is generated, which causes the rotation shafts54and the gears55to rotate. The cam member10B engaging with the gears55rotates by the rotation of the gears55. The two engagement positions in the direction of the major axis of the ellipse, between the flexible gear1and the first internal gear3band between the flexible gear1and the pair of second internal gears2aand2b, rotate by the rotation of the cam member10B. One revolution of the cam member10B causes a relative rotation between the first internal gear3band the pair of second internal gears2aand2bwith the shift corresponding to the difference in number of teeth between the first internal gear3band the pair of second internal gears2aand2b. In a case where the fixing plate4a(4b) is fixed to the fixed object (not shown), the pair of second internal gears2aand2bcoupled thereto does not rotate with respect to the fixed object, but instead the first internal gear3brotates with respect to the fixed object at the reduction ratio that is set based on the difference in number of teeth. For example, one revolution of the cam member10B causes the first internal gear3bto rotate relative to the pair of second internal gears2aand2bwith the shift of two teeth (difference in number of teeth), that is, a 2/100 revolution. When assuming that the reduction ratio between the rotation shaft54of the electric motor53and the internal gear10bof the cam member10B is 10, a still higher reduction ratio of 1:500 can be realized as a whole. Specifically, in the fourth embodiment, the speed is reduced depending on the number of teeth of the gear55fixed to the rotation shaft54and the number of teeth of the internal gear10bof the cam member10B, and hence the still higher reduction ratio can be realized.

Because the cam member10B rotates by combining the torque generated by the multiple electric motors53and53, a high rotational force may be generated even when a small motor is used. The fourth embodiment has described the case where two electric motors are mounted, but the number of electric motors is not limited thereto. One or more electric motors are only necessary, and even when the number of electric motors is changed, the same effect as the strain wave gearing reducer is obtained. Mounting more electric motors enables each electric motor to be further downsized. Conversely, design of providing one electric motor is also applicable.

As described above, according to the fourth embodiment, the electric motors53and53are arranged outside the strain wave gearing reducer52, and hence a higher-power motor can be used as compared to the built-in electric motor used in the first to third embodiments. Further, the multiple electric motors53and53can be arranged, and hence still higher power can be output. In particular, as an application example, when it is applied to the first axis of a base of a robotic arm, a high-speed robotic arm can be provided.

The fourth embodiment has described the case where the structure of the strain wave gearing reducer is the same as that of the strain wave gearing reducer52in the first embodiment, but the structure may be the same as that of the strain wave gearing reducer52A in the second embodiment. Also in this case, the same effect is obtained.

Fifth Embodiment

A strain wave gearing according to a fifth embodiment of the present invention is described. The fifth embodiment is different from the first to fourth embodiments (in particular, fourth embodiment) in the transmission mechanism for transmitting the rotational force of the electric motor to the cam member. Specifically, the fourth embodiment has described the mechanism in which the rotational force of the gear fixed to the rotation shaft of the electric motor is directly transmitted to the internal gear of the cam member, but the fifth embodiment describes a case where a planetary gearing mechanism is used.FIG. 10is a schematic view illustrating schematic structure of the strain wave gearing according to the fifth embodiment of the present invention. The same components as those in the fourth embodiment are represented by the same reference symbols, and description thereof is omitted herein.

A strain wave gearing50C includes one electric motor53. The electric motor53includes the rotation shaft54that rotates about the z-axis, and is arranged outside a pair of fixing plates40aand40band fixed to the one fixing plate40aout of the pair of fixing plates40aand40b. The base end portion of the rotation shaft54is supported in a rotatable manner through the intermediation of the bearing17aarranged in the one fixing plate40a. The leading end portion of the rotation shaft54is supported in a rotatable manner through the intermediation of the bearing17barranged in the other fixing plate40b. The leading end portion of the rotation shaft54is pressed against the bearing17bby the pressing plate19.

In the fifth embodiment, the electric motor53is arranged on the z-axis, and hence a coupling member40cis formed integrally with the one fixing plate40aout of the pair of fixing plates40aand40b, at a position other than the z-axis. The other fixing plate40bis coupled to the coupling member40cby means of a bolt26. In other words, the pair of fixing plates40aand40bare coupled to each other by the coupling member40cpenetrating the cam member10B. In other words, the coupling member40ccouples the pair of fixing plates40aand40bto each other through a space surrounded by the inner periphery of the cam member10B.

A sun gear25is provided to the rotation shaft54of the electric motor53, and multiple planet gears24are provided between external teeth of the sun gear25and the internal teeth of the internal gear10bof the cam member10B. Each planet gear24engages with the external teeth of the sun gear25and the internal teeth of the internal gear10bof the cam member10B. Note that, FIG.10illustrates one planet gear24alone but does not illustrate the other planet gears.

Each planet gear24is fixed to a rotation shaft20. The rotation shaft20to which the planet gear24is fixed is supported by the fixing plates40aand40bin a state in which the rotation shaft20cannot revolve around the sun gear25, and further supported by the fixing plates40aand40bthrough the intermediation of bearings21aand21bin a state in which the rotation shaft20can rotate on its axis. In other words, the rotational force of the electric motor53is transmitted to the cam member10B through the sun gear25and the planet gears24, and accordingly the cam member10B rotates.

Similarly to the fourth embodiment, the cam member10B is supported in a rotatable manner by the fixing plates40aand40bthrough the intermediation of the bearings18aand18b. Further, the outer ring portion3aof the outer ring member3is supported in a rotatable manner by the fixing plates40aand40bthrough the intermediation of the bearings6aand6b.

When power is supplied to the electric motor53, a rotational force is generated, which causes the sun gear to rotate and thereby causes the planet gears24engaging therewith to rotate (on its axis). Then, the cam member10B engaging with the planet gears24rotates. In the fifth embodiment, the sun gear25, the planet gears24, and the cam member10B including the internal gear10bconstitute a planetary gearing reduction mechanism. Accordingly, the rotation of the electric motor53is transmitted to the cam member10B while the speed thereof is reduced, and thus a high reduction ratio can be realized. For example, when assuming that the reduction ratio between the flexible gear1serving as the flex spline and the strain wave gearing reducer portion including the internal gear is 1:50, and that the reduction ratio between the rotation shaft54of the electric motor53and the cam member10B is 10, a high reduction ratio of 1:500 can be realized.

Because the electric motor53is arranged outside the strain wave gearing reducer, a larger motor can be used. In particular, the first axis of an industrial robotic arm requires a large rotational force, and hence applying the strain wave gearing50C of the fifth embodiment is effective.

The fifth embodiment has described the case where the structure of the strain wave gearing reducer is substantially the same as that of the strain wave gearing reducer52in the first embodiment, but the structure may be substantially the same as that of the strain wave gearing reducer52A in the second embodiment. Also in this case, the same effect is obtained.

Sixth Embodiment

A robotic arm including the strain wave gearing according to a sixth embodiment of the present invention is described. The robotic arm of the sixth embodiment includes any one of the strain wave gearings according to the first to fifth embodiments.FIG. 11illustrates schematic structure of a robotic arm80. The robotic arm80includes multiple links81to87, and joints91to96coupling a pair of adjacent links to each other, in each of which any one of the strain wave gearings described in the first to fifth embodiments is arranged. In other words, any one of the strain wave gearings according to the first to fifth embodiments only needs to be applied depending on the structure of the joint. The robotic arm80is an example of robots frequently used for industrial application. InFIG. 11, rotation directions of the links81to87of the respective joints91to96are represented by J1to J6, and the links81to87are connected in series. Hence, the stiffnesses of the strain wave gearing reducers of the strain wave gearings in the respective joints are also connected in series, and therefore the stiffness at the leading end of the robot tends to be low. To address this, a high stiffness is necessary particularly for the joints91and92near the base of the robot. This is because the distance from the leading end portion of the robot is large and therefore even a disturbance force of the same level becomes large disturbance torque for the joints91and92near the base, especially for the joint91.

FIG. 12illustrates an example in which the strain wave gearings50B and50of the above-mentioned embodiments are applied to the joints91and92near the base, respectively. The structure of the strain wave gearing50B is as described in the fourth embodiment, and the structure of the strain wave gearing50is as described in the first embodiment. The strain wave gearing50B is provided in the joint91between the first link81and the second link82, and the strain wave gearing50is provided in the joint92between the second link82and the third link83.

The fixing plate4aof the strain wave gearing50B is fixed to the first link81. The outer ring member3of the strain wave gearing50B is fixed to a stay82A of the second link82, and the second link82can therefore rotate with respect to the first link81. In the above-mentioned structure, when the electric motors53of the strain wave gearing50B rotate, the rotation shafts54rotate and thereby the cam member10B engaging with the gears55rotates. The engagement positions between the flexible gear1and the internal gear3bof the outer ring member3change, which causes the outer ring member3to rotate with respect to the fixing plates40aand40bby the rotation of the cam member10B. As a result, the second (one) link82can rotate more than one revolution with respect to the first (other) link81. The strain wave gearing reducer of the strain wave gearing50B has a high stiffness, which leads to a stable rotational operation of the second link82and ensures more than one revolution.

Meanwhile, the fixing plates4aand4bof the strain wave gearing50are fixed to the stay82A of the second link82. The outer ring member3of the strain wave gearing50is fixed to the third link83, and the third link83can therefore rotate with respect to the second link82. In the above-mentioned structure, when power is supplied to the electric motor51of the strain wave gearing50, the cam member10rotates. The engagement positions between the flexible gear1and the internal gear3bof the outer ring member3change by the rotation of the cam member10, which causes the outer ring member3to rotate with respect to the fixing plates4aand4b. As a result, the third (one) link83can rotate with respect to the second (other) link82. The strain wave gearing reducer of the strain wave gearing50has a high stiffness, which leads to a stable rotational operation of the third link83.

In the sixth embodiment, the strain wave gearing50B having a higher reduction ratio than the strain wave gearing50is applied to the joint91which requires a large rotational force. Accordingly, the operation of the robotic arm80becomes stable. In particular, the strain wave gearing50B includes the multiple electric motors53and53, and thus a larger rotational force can be generated.

The present invention has been described based on the above-mentioned first to sixth embodiments, but the present invention is not limited thereto. The first to sixth embodiments have described the case where the contour of the cam member is an elliptical shape, but the shape does not exactly need to be an ellipse as long as the flexible gear can be deformed to be pressed against the internal gear of the outer ring member. For example, a track shape, in which two eccentric circles and their common tangents are connected, is also applicable.

The above-mentioned first to fourth embodiments have described the case where the shaft5serving as the coupling member is provided separately from the pair of fixing plates4aand4band coupled thereto by means of bolts or the like. Alternatively, the coupling member may be formed integrally with the fixing plates when coupled. Still alternatively, the fixing plates4aand4band the internal gears2aand2b, or the fixing plates4aand4band the flexible gear including the flange portions may be formed integrally. In other words, as described in the above-mentioned first to fourth embodiments, the members to be coupled and fixed to each other may be formed integrally.

In the above-mentioned fifth embodiment, the coupling member40cis formed integrally with the one fixing plate40a, but the present invention is not limited thereto. The coupling member40cmay be formed integrally with the other fixing plate40b. Further, the coupling member40cmay be formed separately from both the fixing plates40aand40band coupled thereto by means of bolts or the like.

This application claims the benefit of Japanese Patent Application No. 2010-087482, filed Apr. 6, 2010, which is hereby incorporated by reference herein in its entirety.