U.S. patent application number 14/806807 was filed with the patent office on 2017-01-26 for planar flexure members and actuators using them.
The applicant listed for this patent is Jonathan Bond, Matthew Knoll, Umberto Scarfogliero, Andrew Wallace, Robert White. Invention is credited to Jonathan Bond, Matthew Knoll, Umberto Scarfogliero, Andrew Wallace, Robert White.
Application Number | 20170023083 14/806807 |
Document ID | / |
Family ID | 56842595 |
Filed Date | 2017-01-26 |
United States Patent
Application |
20170023083 |
Kind Code |
A1 |
Knoll; Matthew ; et
al. |
January 26, 2017 |
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 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.
Inventors: |
Knoll; Matthew; (San
Francisco, CA) ; Bond; Jonathan; (Melrose, MA)
; White; Robert; (Roslindale, MA) ; Scarfogliero;
Umberto; (Boston, MA) ; Wallace; Andrew;
(Needham, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Knoll; Matthew
Bond; Jonathan
White; Robert
Scarfogliero; Umberto
Wallace; Andrew |
San Francisco
Melrose
Roslindale
Boston
Needham |
CA
MA
MA
MA
MA |
US
US
US
US
US |
|
|
Family ID: |
56842595 |
Appl. No.: |
14/806807 |
Filed: |
July 23, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F16F 1/027 20130101;
F16D 3/005 20130101; F16D 3/12 20130101; F16D 3/79 20130101; F16D
3/62 20130101; B25J 17/0225 20130101 |
International
Class: |
F16F 1/02 20060101
F16F001/02; F16D 3/12 20060101 F16D003/12 |
Claims
1. A planar flexure member for resisting rotation about a central
axis thereof, the flexure member comprising: a central portion
comprising a plurality of 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, the mounting rails each comprising a
plurality of attachment points and being positioned in opposition
to to each other to partially define and occupy a planar circular
envelope radially displaced from but surrounding the central
portion, a portion of the serpentine arms extending substantially
to the envelope between the mounting rails.
2. The flexure member of claim 1, wherein the serpentine arms have
a varying thickness with a thinnest portion thereof at the
envelope.
3. The flexure member 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.
4. The flexure member of claim 3, wherein a ratio of the height to
the width is at least 2.
5. The flexure member of claim 1, wherein the arms and the central
portion have a non-unitary height.
6. The flexure member of claim 1, wherein the flexure member is
made of titanium.
7. The flexure member of claim 1, wherein at least a portion of the
arms has an I-beam cross-section.
8. The flexure member of claim 1, wherein at least a portion of the
arms has voids along a neutral bending axis thereof.
9. (canceled)
10. A rotary actuator comprising: a motor configured for rotation
about an actuation axis; and a planar flexure member having a
central output portion mechanically coupled to a load 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 attachment points
for mounting to the motor, 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 the serpentine arms extending
substantially to the envelope between the mounting rails.
11. The actuator of claim 10, wherein the serpentine arms have a
varying thickness with a thinnest portion thereof at the
envelope.
12. The actuator of claim 10, 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.
13. The actuator of claim 10, wherein a ratio of the height to the
width is at least 2.
14. The actuator of claim 10, wherein the arms and the central
portion have a non-unitary height.
15. The actuator of claim 10, wherein the flexure member is made of
titanium.
16. The actuator of claim 10, wherein at least a portion of the
arms has an I-beam cross-section.
17. The actuator of claim 10, wherein at least a portion of the
arms has voids along a neutral bending axis thereof.
18. (canceled)
19. The actuator of claim 10, wherein the actuator has an actuation
axis coaxial with an output axis.
20.-21. (canceled)
Description
TECHNICAL FIELD
[0001] The present invention relates to elastic flexure elements
and actuators employing these elements for use, for example, in
robotic applications.
BACKGROUND
[0002] Industrial robots perform a variety of tasks involving the
movement and manipulation of various objects. A typical industrial
robot as used, e.g., in a manufacturing environment, may have one
or more arms equipped with grippers that allow the robot to pick
up, transport, and manipulate objects. A key mechanical requirement
for industrial is the ability to generate large but precise forces
and torques while maintaining overall control stability. These
torques and forces are generated by actuators, i.e., motors
responsive to control signals to apply a commanded torque, which is
transmitted mechanically to a load either directly (where
rotational actuation is required) or via a linear conversion
element, such as a lead screw (when linear force is required).
[0003] Stiff actuators can exert large forces from small joint
displacements, and permit high-bandwidth force control and precise
position control. But stiffness makes force control difficult.
Because of the importance of force control in robotic applications,
stiffness and the attendant bandwidth is typically sacrificed to
achieve better force control. One approach is to utilize an elastic
element in series with the actuator. Elasticity has the effect of
making the force control easier, as larger deformations are needed
to exert a given force relative to a stiff actuator. robot. In
effect, the elasticity allows force to be controlled via position
rather than directly, which improves accuracy and stability, and
reduces noise.
[0004] Designing series elastic elements for robotic applications
can be challenging due to space constraints, the need to withstand
large and repeated applied torques without slippage or wander, and
the need for repeatable but economical manufacture. In a rotational
elastic element, for example, the design must incorporate
components with sufficient length to provide the desired elasticity
(since stiffness varies inversely with the cube of a component's
length), but must also provide a secure mounting frame to avoid
slippage. Because the frame typically defines the outer envelope of
the elastic element, it imposes a limit on the amount of internal
length that may be employed.
SUMMARY
[0005] The present invention provides, in various embodiments, a
planar flexure member for resisting rotation about a central axis
thereof that affords greater compliance than conventional designs.
In various embodiments, the flexure member comprises a central
portion comprising a plurality of 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, the mounting rails each comprising a
plurality of attachment points and being positioned in opposition
to to each other to partially define and occupy a planar circular
envelope radially displaced from but surrounding the central
portion, a portion of the serpentine arms extending substantially
to the envelope between the mounting rails.
[0006] In some embodiments, the serpentine arms have a varying
thickness with a thinnest portion thereof at the envelope. The arms
and the central portion may have a unitary height at least equal to
the width of the arms at a narrowest portion thereof. For example,
the ratio of height to width may be at least 2. In other
embodiments, the arms and the central portion have a non-unitary
height.
[0007] The flexure member may be made of titanium or other suitable
metal (or other material). In some implementations, the arms (or
portion thereof) have an I-beam cross-section. The arms may
alternatively or in addition include voids along a neutral bending
axis thereof.
[0008] In another aspect, the invention pertains to a planar
flexure member for resisting rotation about a central axis thereof.
In various embodiments, the flexure member includes a central
portion comprising a plurality of attachment points; and at least
one serpentine flexure arm extending from the central portion in a
plane and terminating in an arcuate mounting rail having a
plurality of attachment points.
[0009] In still another aspect, the invention relates to a rotary
actuator. In various embodiments, the actuator comprises a motor
configured for rotation about an actuation axis; and a planar
flexure member having a central output portion mechanically coupled
to a load 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 attachment points for mounting to the motor, 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
the serpentine arms extending substantially to the envelope between
the mounting rails.
[0010] In some embodiments, the serpentine arms have a varying
thickness with a thinnest portion thereof at the envelope. The arms
and the central portion may have a unitary height at least equal to
the width of the arms at a narrowest portion thereof. For example,
the ratio of height to width may be at least 2. In other
embodiments, the arms and the central portion have a non-unitary
height. The flexure member may be made of titanium or other
suitable metal (or other material). In some implementations, the
arms (or portion thereof) have an I-beam cross-section. The arms
may alternatively or in addition include voids along a neutral
bending axis thereof.
[0011] In some embodiments, the actuator has an actuation axis
coaxial with an output axis. In other embodiments, the actuator has
an actuation axis parallel to and offset with respect to an output
axis, or oblique with respect to an output axis.
[0012] The term "substantially" or "approximately" means .+-.10%
(e.g., by weight or by volume), and in some embodiments, .+-.5%.
The term "consists essentially of" means excluding other materials
that contribute to function, unless otherwise defined herein.
Nonetheless, such other materials may be present, collectively or
individually, in trace amounts. Reference throughout this
specification to "one example," "an example," "one embodiment," or
"an embodiment" means that a particular feature, structure, or
characteristic described in connection with the example is included
in at least one example of the present technology. Thus, the
occurrences of the phrases "in one example," "in an example," "one
embodiment," or "an embodiment" in various places throughout this
specification are not necessarily all referring to the same
example. Furthermore, the particular features, structures,
routines, steps, or characteristics may be combined in any suitable
manner in one or more examples of the technology. The headings
provided herein are for convenience only and are not intended to
limit or interpret the scope or meaning of the claimed
technology.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The foregoing will be more readily understood from the
following detailed description of the invention, in particular,
when taken in conjunction with the drawings, in which:
[0014] FIG. 1 schematically illustrates a representative actuator
system employing an embodiment of the invention.
[0015] FIGS. 2A and 2B illustrate, respectively, perspective and
elevational views of a planar flexure member in accordance with
embodiments of the present invention.
[0016] 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.
DETAILED DESCRIPTION
[0017] 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.
[0018] 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.
[0019] 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 217--i.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.
[0020] 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. 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 flexture
member 200 to the load or the drive. As best seen in FIG. 3,
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 meet or approach the envelope. In this way,
the lengths of the flexure arms 220 may be maximized within a
limited circular area--i.e., their lengths are not constrained to
fit inside a fully circular mounting collar.
[0021] 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 vary--that is, different regions of the flexure
member 200 may have different thicknesses. A representative range
of heights is 2.5 to 9 mm. Where the height varies, a typical
configuration has the thinnest (lowest h) 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 one illustrative
implementation, the ratio of height to width is 2:1.
[0022] 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. The outside of the flexure member
200 is attached to the gearbox 112 (see FIG. 1). 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. 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
k = Ebh 3 4 L 3 ##EQU00001##
where E is the Young's modulus of the flexure member 200, h is the
cross-sectional width (radial dimension) of the arm shown in FIG.
2B, b is the height (z-axis) of the arm, and L is the length of the
arm (from the central portion 215 to the mounting rail 225).
[0023] Because of this relationship, z-axis arm thickness h can be
traded off against arm width in the xy 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 areawise 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 200--space 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 to reduce the amount of material needed to
achieve a given stiffness, or material may be removed along the
neutral axis of bending (e.g., voids or holes may be formed along
the neutral axis).
[0024] 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
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 faily high
compared with processes like extruding and slicing, and does not
ramp to volume production easily. If desired, a finishing technique
maybe 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.
[0025] 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
coeffiecient of friction between aluminum and titanium is higher
than between steel and aluminum, reducing the possibility that the
bolted joint will slip. Although a titanimum flexure requires more
material, the volume offset does not outweigh the density reduction
titanium offers, and the net result is a lighter flexure. Titanum
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.
[0026] 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 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. Thus, the flexure element 200 transmits torque from the drive
330 to the system output, acting as a spring therebetween.
[0027] 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.
* * * * *