Planar flexure members and actuators using them

Abstract

A planar flexure member for resisting rotation about a central axis thereof includes, in various embodiments, a central portion comprising a plurality of attachment points; and at least one serpentine flexure arm extending from the central portion in a plane. The arm(s) terminate in an arcuate mounting rail that includes a series of attachment points. The rails are positioned in opposition to each other to partially define and occupy a planar circular envelope radially displaced from but surrounding the central portion of the flexure member. A portion of the serpentine arms may extend to (or substantially to) the envelope between the mounting rails.

Claims

1. A system for transmission of torque from a motor to a load, the system having an output axis and comprising: a motor configured for rotation about an actuation axis; and a planar flexure member having: a central portion having a plurality of first attachment points; and at least two serpentine flexure arms extending oppositely and symmetrically from the central portion in a plane, each of the arms terminating in an arcuate mounting rail having a plurality of second attachment points, the mounting rails being positioned in opposition to each other to partially define and occupy a planar circular envelope radially displaced from but surrounding the central portion, a portion of each of the serpentine arms extending substantially to the envelope between the mounting rails, wherein the planar flexure member receives torque from the motor along the actuation axis and causes transmission of torque to the output axis.

2. The system of claim 1, wherein the actuation axis of the motor is coaxial with the output axis.

3. The system of claim 1, wherein the actuation axis of the motor is parallel to and offset with respect to the output axis.

4. The system of claim 1, wherein the actuation axis of the motor is oblique with respect to the output axis.

5. The system of claim 1, wherein the serpentine arms have a varying thickness with a thinnest portion thereof at the envelope.

6. The system of claim 1, wherein the arms and the central portion have a unitary height, the height being at least equal to a width of the arms at a narrowest portion thereof.

7. The system of claim 6, wherein a ratio of the height to the width is at least 2.

8. The system of claim 1, wherein the arms and the central portion have a non-unitary height.

9. The system of claim 1, wherein the flexure member comprises titanium.

10. The system of claim 1, wherein at least a portion of each of the arms has an I-beam cross-section.

11. The system of claim 1, wherein at least a portion of each of the arms has voids along a neutral bending axis thereof.

12. The system of claim 1, wherein the arcuate mounting rails are mechanically coupled to the motor via the plurality of second attachment points, whereby the load mechanically couples to the central portion via the plurality of first attachment points.

13. The system of claim 1, wherein the central portion is mechanically coupled to the motor via the plurality of first attachment points, whereby the load mechanically couples to the arcuate mounting rails via the plurality of second attachment points.

14. The system of claim 1, further comprising a gearbox for translating torque between the motor and the flexure member, an output of the gearbox being mechanically coupled to the flexure member via the plurality of first attachment points or the plurality of second attachment points.

15. The system of claim 14, wherein the gearbox is integral with the motor.

16. The system of claim 14, wherein the gearbox is separate from the motor.

17. The system of claim 14, wherein the output of the gearbox comprises aluminum.

18. The system of claim 1, wherein the motor and the flexure member are disposed within a robotic appendage.

19. The system of claim 1, further comprising a load mechanically coupled to the flexure member via the plurality of first attachment points or the plurality of second attachment points, wherein at least a portion of the load in contact with the flexure member comprises aluminum.

20. A method of transmitting torque to a load, the method comprising: mechanically coupling a motor to a planar flexure member, the motor being configured for rotation about an actuation axis, wherein the planar flexure member comprises: a central portion having a plurality of first attachment points, and at least two serpentine flexure arms extending oppositely and symmetrically from the central portion in a plane, each of the arms terminating in an arcuate mounting rail having a plurality of second attachment points, the mounting rails being positioned in opposition to each other to partially define and occupy a planar circular envelope radially displaced from but surrounding the central portion, a portion of each of the serpentine arms extending substantially to the envelope between the mounting rails; mechanically coupling a load to the planar flexure member; and receiving, with the planar flexure member, torque from the motor along the actuation axis, thereby causing transmission of torque to an output axis of the load.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

(2) FIG. 1 schematically illustrates a representative actuator system employing an embodiment of the invention.

(3) FIGS. 2A and 2B illustrate, respectively, perspective and sectional views of a planar flexure member in accordance with embodiments of the present invention. The width w and length L of the flexure arms of the member are indicated on FIG. 2A, as is the neutral bending axis of the flexure arm (dashed line). The thickness h of the central portion is indicated on FIG. 2B, as is the thickness h.sub.a of the flexure arms.

(4) FIG. 2C is a sectional view of a planar flexure member in accordance with embodiments of the present invention in which the thickness h of the central portion is the same as the thickness h.sub.a of the flexure arms.

(5) FIG. 2D is a sectional view, taken along line 2D-2D in FIG. 2A, of a flexure arm having an I-beam cross section in accordance with embodiments of the present invention.

(6) FIGS. 3A and 3B are plan and sectional views, respectively, of a representative deployment of the flexure member shown in FIGS. 2A and 2B. Some components are omitted for clarity in FIG. 3A.

(7) FIGS. 3C, 3D, and 3E are sectional views of other representative deployments of the flexure member shown in FIGS. 2A and 2B.

DETAILED DESCRIPTION

(8) FIG. 1 illustrates the basic components of an actuator system 100 that incorporates a flexure element in accordance herewith. The system 100 includes a torque-generating device, i.e., a motor 110, which may optionally be geared via a gearbox 112 (which lightens the system by facilitating use of a smaller motor 110 operating at higher speeds). The gearbox 112 may be integral to or separate from the motor 110. A torsional spring element 114 is linked in series with the output of the gearbox 112, or if no gearbox is employed, directly with the motor 110. The load 116 to be acted upon by the actuator system 100 is linked in series to the other end of spring element 114. The spring element 114 thereby introduces at the interface between the actuator 100 and the load 116 a series elasticity that affords precise control of the force applied to the load. The spring element 114 may be linked to the load 116 through a low-backlash transmission element (not shown) if desired.

(9) In a robot environment, the axial distance between the actuator system 100 and the load 116 may be tightly constrained, limiting the thickness of the spring element 114. The radial extent of the actuator system 100 may also be highly constrained, limiting the envelope diameter of the spring element. Hence, it is essential to pack the desired degree of stiffness into a small spatial region, while at the same time providing for sufficiently secure mounting of the spring element 114 to the gearbox 112 and the load 116 (or other mechanical output) to avoid slippage and wander.

(10) A representative elastic element fulfilling these contradictory constraints is shown in FIGS. 2A and 2B. The flexure member 200 is a planar structure having generally flat opposed surfaces, the visible one of which is indicated at 205. A central portion 215 includes a plurality of attachment points 217i.e., mounting holes arranged in a generally circular configuration and typically spaced equidistantly apart. The attachment points 217 accommodate screws or other fasteners that secure the flexure member 200 to the actuator motor or gearbox as described earlier.

(11) Emanating from the central portion 215 are a pair of serpentine flexure arms 220a, 220b, which extend oppositely and symmetrically from the central portion 215 in a plane. Although two arms 220 are shown, it should be understood that configurations utilizing a single arm 220, as well as more than two arms 220, are within the scope of the invention. The width w of the arms 220 (which may change along the length of the arms), as well as the length L of the arms 220 are indicated in FIG. 2A. In the illustrated embodiment, each of the arms 220a, 220b terminates in an arcuate mounting rail 225a, 225b. Each of the mounting rails 225 includes a plurality of attachment points 217 (mounting holes, once again, in the illustrated embodiment) that facilitate attachment of the flexure member 200 to the load or the drive. As best seen in FIG. 3A, described in greater detail below, the mounting rails 225 are positioned in opposition to define, along with the outer curved segments of the flexure arms 220, a substantially circular outer envelope for stability and symmetry of rotative force transmission. Because the mounting rails 225 occupy only a portion of the circular envelope, the flexure arms may extend outwardly so that the outer curved edges 270 meet or approach the envelope. In this way, the lengths of the flexure arms 220 may be maximized within a limited circular areai.e., their lengths are not constrained to fit inside a fully circular mounting collar.

(12) With reference to FIG. 2B, the flexure member 200 has a height h, which depends, in various embodiments, on the size of the actuator. Furthermore, although the flexure member 200 is planar, the height h may varythat is, different regions of the flexure member 200 may have different thicknesses. For example, FIG. 2B depicts an embodiment in which the height h of the central portion 215 of flexure member 200 is greater than the height h.sub.a of the arms 200. A representative range of heights is 2.5 to 9 mm. Where the height varies, a typical configuration has the thinnest (lowest h.sub.a) portion of the arms 220 at the outer edges thereof. Where the arms 220, the central portion 215 and the rails 225 have a unitary height, that height may be at least equal to the width of the arms at a narrowest portion thereof, representatively indicated at 230 in FIG. 2A; in one illustrative implementation, the ratio of height to width is 2:1. FIG. 2C depicts an embodiment in which the height h of the central portion 215 of flexure member 200 is equal to the height h.sub.a of the arms 200, i.e., the central portion 215 and the arms 200 have a unitary height.

(13) The arms 220 provide the elasticity of the flexure member 200. That is, as the central portion 215 is rotated, rotary force is transmitted to the arms 220 and vice versa. In various embodiments, the central portion 215 of the flexure member 200 is attached to the gearbox 112 (or to the motor 110), and the arms 220 elastically deform to a degree dependent on the torque applied to the central portion 215 and the reaction force of the load. In other embodiments, the arms 220 (via mounting rails 225) are attached to the gearbox 112 (or to the motor 110) and elastically deform while the central portion 215 of the flexure member 200 is attached to the load. The elasticity of the flexure member 200 depends on the modulus of the material from which the flexure member is fabricated as well as the lengths and thicknesses of the arms 220. In particular, each of the arms 220 may be approximately modeled as a cantilever beam with a stiffness k given by

(14) $k=\frac{{\mathrm{Eh}}_a{w}^3}{4L^3}$
where E is the Young's modulus of the flexure member 200, w is the cross-sectional width (radial dimension) of the arm shown in FIG. 2A, h.sub.a is the height (z-axis) of the arm shown in FIGS. 2B and 2C, and L is the length of the arm (from the central portion 215 to the mounting rail 225) shown in FIG. 2A.

(15) Because of this relationship, z-axis arm thickness h.sub.a can be traded off against arm width w in the x-y plane of the flexure member 200. If thickness is constrained by space limitations or machinability, in other words, a given reduction in thickness can be compensated for by a cubic increase in arm width in order to maintain the same stiffness. Although the cubic relationship implies a large area-wise increase in the arm footprint to achieve a thickness reduction, in fact this increase is readily accommodated by the serpentine configuration, which leaves substantial open space within the envelope of the flexure member 200space that is further increased by the limited-circumference mounting rails 225, which allow the outer edges of the arms 220 to be maximally spaced from the central portion 215. Other weight-reduction strategies may also be employed. For example, the arms may be shaped with an I-beam cross-section, as shown in FIG. 2D, to reduce the amount of material needed to achieve a given stiffness, or material may be removed along the neutral axis of bending 250 (e.g., voids or holes 260 may be formed along the neutral axis 250, as shown in FIG. 2A).

(16) Indeed, wider arms can aid manufacturability, since narrow features can be difficult to fabricate. Typical approaches used in the manufacture of planar flexures include stamping, water-jet cutting, laser cutting, and machining. Stamped parts can exhibit inferior edge quality and therefore durability limitations, and it can be difficult to retain complex feature shapes following heat treatment; hence slender, curved arm segments may be incompatible with stamping as a fabrication option. Water jet/laser cutting generally has a low-end dimensional control of about 0.005 for materials suitable for flexure members as contemplated herein, and for flexures designed for small operating torques, this variation translates into very large stiffness variations, since stiffness varies with the cube of the dimensional error. Additionally, the cost of water jet/laser cutting is fairly high compared with processes like extruding and slicing, and does not ramp to volume production easily. If desired, a finishing technique may be employed to adjust the final mechanical properties of the flexure member 200. For example, peening (e.g., shot peening) is frequently used to introduce surface residual compressive stresses and thereby increase the durability of metal parts.

(17) In general, an extrusion process followed by slicing into planar flexure elements is cost-effective and well-suited to embodiments of the present invention. A preferred material for the flexure element 200 is titanium, particularly when the flexure element is affixed to an aluminum load and/or rotor. The coefficient of friction between aluminum and titanium is higher than between steel and aluminum, reducing the possibility that the bolted joint will slip. Although a titanium flexure requires more material, the volume offset does not outweigh the density reduction titanium offers, and the net result is a lighter flexure. Titanium has a natural endurance limit in the same way steel does (though unlike many other materials) and therefore is well suited to elastic applications. Titanium has 60% of the stiffness of steel, which means that the flexure arms need to be a bit thicker relative to steel, reducing their sensitivity to tolerance variation. It should be noted that more than one flexure in accordance herewith may be stacked in various configurations to achieve balanced loading and the required torque deflection.

(18) FIGS. 3A and 3B show the flexure element 200 coupled to a load in a representative mechanical environment. The load itself (not shown, but which may be, for example a robot arm) is coupled via bolts 315 passing through the central mounting holes 217 of the flexure element 200. A circular frame 320 is mechanically coupled to the central portion of the flexure element 200 via cross roller bearings or a similar system. As is well understood in the art, crossed roller bearings comprise outer rings, inner rings, and rolling elements; they can also be metal spacers. Due to the crossed arrangement of the rolling elements, such bearings can support axial forces from both directions as well as radial forces, tilting moment loads and combinations of loads with a single bearing position. The outer rails 325 of the flexure element 200 are secured to a source of rotary power, such as a harmonic drive 330 or other gearing driven by a motor 335. Thus, the flexure element 200 transmits torque from the motor 335 to the system output, acting as a spring therebetween. As shown in FIG. 3B, an output axis 340 of the system may be coaxial with the actuation axis of the flexure element 200 and parallel to, but offset from, an actuation axis 345 of the motor 335. Specifically, the output of the motor 335 engages with a gear 347 that transmits torque from the motor 335 to the harmonic drive 330 (and thence to the flexure element 200 and the load coupled thereto) via another gear 348.

(19) FIG. 3C depicts another embodiment in which the actuation axis 345 of the motor 335 is oblique to the output axis 340 of the system. For example, the motor 335 may interface with gearing configured to receive torque from the motor 335 in one direction and transmit torque to the flexure element 200 in a different direction. For example, the motor 335 may interface with bevel gears 350a, 350b to alter the direction of torque transmission. While FIG. 3C depicts the actuation axis 345 of the motor 335 as being substantially perpendicular to the output axis 340, in other embodiments, actuation axis 345 is oblique to output axis 340 without being perpendicular thereto. As known in the art, bevel gears (and or other types of gears) may be configured with the input and output axes thereof separated by virtually any angle.

(20) FIG. 3D depicts yet another embodiment of the present invention in which the actuation axis 345 of the motor 335 is coaxial with the output axis 340 of the system. As shown, the motor 335 is coupled to the outer rails 325 of the flexure element 200 via the harmonic drive 330. The load (not shown, but which may be, for example a robotic appendage such as a robotic arm) is coupled via bolts 315 passing through the central mounting holes 217 of the flexure element 200.

(21) FIGS. 3B-3D depict the source of rotary power being mechanically coupled to the outer rails 325 of the flexure element 200 and the load being mechanically coupled to the center portion 215 of flexure element 200. As detailed above, embodiments of the present invention deploy the flexure element 200 in the reverse configuration. In FIG. 3E, the motor 335 is mechanically coupled to the center portion 215 of the flexure element 200 via bolts 355 and intervening gearing. The outer rails 325 of the flexure member 200 are mechanically coupled to the load via bolts 360, 365 and angled brackets 370. As in the configuration depicted in FIG. 3B, the actuation axis of the motor 345 is parallel to and offset with respect to the output axis 340. As may be readily understood by those of skill in the art, configurations in which the source of rotary power is coupled to center portion 215 may also feature actuation axis 345 being oblique with respect to or coaxial with output axis 340 in the manner depicted in FIGS. 3C and 3D.

(22) The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. In particular, embodiments of the invention need not include all of the features or have all of the advantages described herein. Rather, they may possess any subset or combination of features and advantages. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.