Note: Descriptions are shown in the official language in which they were submitted.
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Dt:~J~IlABl.E STRUCTUR ~ AR~A~GEMENT
~:~NlCA~ FIE~D
This invention relates to a deformable structural
arrangement, an assembly incorporating the arrangement,
an assembly to provide force upon activation, a method
of effecting a trans~ormation between ~orce and
displacement, and to a method to ampli~y e~iciently,
small displacements of force producing elements; the
invention is more especially concerned with an actuator
or shock absorber.
BACKGROUND ART
Miniature robotic systems have a need ~or power-
~ull, compact, lightweight actuators. Conventional
techniques such as electric, hydraulic, and pneumatic
actuators, suf~er ~rom a drastic reduction o~ the
amount o~ power they can deliver as they scale down in
size and weight.
Different actuator technologies, based on strain
developing in certain materials have been investigated.
In particular, Shape Memory Alloys (SMA) have a high
strength to weight ratio which makes them ideal ~or
miniature applications. A SMA ~iber can achieve a
pulling stress of 200 MPa. Comparing this to an
electro-magnetic actuator, which can only achieve 0.002
MPa, this represents a 105 increase in strength for a
given cross sectional area.
Thin fibers of shape memory alloy can accomplish
actuation by being pretreated to contract upon heating.
The contraction is a result of the fiber undergoing a
phase transition between its martensitic and austenitic
phases. When in the cool phase (martensitic) the alloy
is malleable and can easily be deformed by applying
external stress. The original pretrained shape can
then be recovered by simply heating the fiber above its
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phase transition temperature. Also, since the alloy is
resistive it can easily be heated electrically.
The high strength to weight ratio o~ shape memory
alloys is accompanied by several limitations. Shape
memory alloys cannot sustain shape recovery a~ter
strains o~ more than a ~ew percent, about 5% ~or a
working li~e o~ thousands or millions o~ cycles.
Activation is achieved by heating and cooling. Thus, a
primary disadvantage o~ previously proposed actuators
is that the displacement which can be achieved is
small, and second the speed o~ displacement is
moderate. They can however still be controlled through
the use o~ feedback and other control techniques. The
main physical limitation that needs to be overcome is
the absolute percent strain. Shape memory alloys can
achieve a workable strain o~ 5 percent. Many o~ the
designs o~ actuators using shape memory alloys depend
on mechanically ampli~ying the displacement either
through the use of long straight ~ibers, through the
ZO use of spring coils, or through bistable devices.
DISCLOSURE OF T~E lNv~N~llON
This invention seeks to provide a de~ormable
structural arrangement ~or e~fecting a trans~ormation
between ~orce and displacement or distance.
This invention also seeks to provide an actuator,
more especially an actuator ~or e~ecting
transformation between ~orce and displacement or
distance, or ~or trading e~iciently ~orce with
displacement.
Still ~urther the invention seeks provide a shock
absorber.
Still ~urther this invention seeks to provide a
device incorporating the actuator o~ the invention.
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Still ~urther this invention seeks to provide a
method of ef~ecting a transformation between ~orce and
displacement.
Still ~urther the invention seeks to provide such
an actuator which is lightweight.
The invention also seeks to provide a de~ormable
structural arrangement capable o~ e~ecting a high
variation in displacement, especially ~rom moderate
variations in displacement o~ primary contractile or
expanding elements.
Still further the invention seeks to provide
actuation with high variation in displacement from thin
~iber or ~ibers which thus can be activated rapidly by
heat, or other means.
Still further the invention seeks to provide an
actuator assembly which is compact.
In accordance with one aspect of the invention
there is provided a de~ormable structural arrangement
comprising: active element means operatively associated
with passive support means, said active element means
having a major axis adapted to change in length under
activation, one o~ said active element means and said
passive support means de~ining a cage o~ crossing
lengths in symmetrical array, said cage surrounding an
inner zone bounded by said active element means and
said passive support means.
In accordance with one particular aspect o~ the
invention, there is provided an actuator comprising at
least one ~iber which shortens under activation,
entrained between at least ~irst and second support
members, said support members being in opposed, spaced
apart relationship, the entrained at least one ~iber
de~ining a cage o~ crossing lengths o~ ~iber in
symmetrical array, said cage surrounding an inner zone
between said support structure members.
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In accordance with another aspect o~ the invention
there is provided an actuator for development o~ a
displacement ~rom a force, comprising at least one
~iber which shortens under activation, entrained under
strain between at least first and second support
members in a double helicoidal array, said support
members being in opposed, spaced apart relationship,
said double helicoidal array being e~ective to balance
all radial components o~ tension ~orces o~ the at least
one ~iber.
Suitably the actuator may include means to urge
the support members apart into the opposed, spaced
apart relationship with the at least one ~iber under
strain.
In accordance with still another aspect of the
invention, there is provided an assembly comprising a
component to be displaced and an actuator to ef~ect
displacement o~ the component, the actuator being an
actuator o~ the invention as described hereinbe~ore.
The component is operably connected to the second
support member such that displacement o~ the second
member relative to the ~irst member produces a
corresponding displacement o~ the component.
In accordance with another aspect o~ the invention
there is provided a structural arrangement comprising
active elements made of at least one fiber which
shortens under activation or stretches under stress,
entrained between at least ~irst and second support
members made o~ compression members, the support
members being in opposed, spaced relationship, the
active elements defining a cage of crossing lengths in
symmetrical array ~orming a double helical counter
rotating pattern, the cage surrounding an inner zone
free o~ interferences.
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According to yet another aspect of the invention
there is provided a method of developing a displacement
from a force comprising providing at least one fiber
which shortens under activation, entrained between at
least i~irst and second support members, the entrained
at least one fiber defining a cage of crossing lengths
of fiber in symmetrical array, and activating said at
least one ~iber to shorten said ~iber lengths such that
said second support is displaced towards said first
support member.
In another aspect o~ the invention there is
provided an actuator for development of a displacement
from a force or a shock absorber for the elimination of
a displacement with a ~orce comprising active elements
made of at least one fiber which shortens under
activation or stretches under stress, entrained under
stress between at least ~irst and second support
members made of compression members in a double,
counter-rotating helicoidal array, the support members
being in opposed, spaced apart relationship, the
helicoidal array being effective to balance all radial
components of forces in the at least one fiber.
In still another aspect of the invention there is
provided a method of eliminating a displacement with a
force comprising providing active elements o~ at least
one fiber which stretches under stress, entrained
between at least first and second support members, the
entrained at least one fiber defining a cage of
crossing lengths of fiber in symmetrical array, and
stressing the at least one fiber to stretch the fiber
lengths thereby displacing the second support member
away from the first support member to eliminate a
displacement adjacent the second support member.
In yet another aspect of the invention there is
provided a structural arrangement comprising active
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members which expand under activation or compress under
stress, attached between at least ~irst and second
restraining harnesses or loops, the restraining
harnesses or loops being in opposed, spaced
relationships, the active members de~ining a cage o~
crossing lengths in symmetrical array forming a double
helical counter-rotating pattern, the restraining
harnesses or loops being made o~ tensile members
disposed according to a star or polygonal regular
con~iguration or made of disks, the cage surrounding an
inner zone free o~ interferences.
In another aspect of the invention there is
provided a method of realizing a device with magnified
superelastic properties which can provide ~uasi
constant force under large strain deformation of the
active elements o~ at least one fiber which stretches
under stress, entrained between at least ~irst and
second support member, the entrained at least one fiber
de~ining a cage o~ crossing lengths of fiber in
symmetrical array, and stressing the at least one fiber
to stretch the fiber lengths thereby displacing the
second member away ~rom the ~irst support member to
counteract a displacement adjacent the second support
member.
BRIEF DESCRIPTION OF DRAWINGS
FIGS. lA and lB illustrate schematically in front
and top elevation, respectively, an actuator of the
invention;
FIGS. 2, 2A, 2B, 2C and 2D illustrates schemati-
cally geometry which underlies the principle ofoperation of deformable structural arrangements of the
invention;
FIG. 3 is a graphical plot of displacement gain
against weave pitch angle achieved by means o~ the
invention;
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FIGS. 4A and 4B illustrate schematically in top
and ~ront elevations the variables in the design o~ an
actuator o~ the invention;
FIG. 5 is a plot demonstrating how displacement
gain may be selected ~y a choice o~ tensile element
length and compression member separating distance in
the invention;
FIG. 6 illustrates the cage ~ormation in an
actuator o~ the invention;
FIG. 7 is a simpli~ied representation o~ a four
disk actuator with the disks unraveled;
FIGS. 8A and 8B, 9A and 9B, lOA and lOB, llA and
llB, 12A and 12B, 13A and 13B, 14A and 14B, 15A and
15B, 16A and 16B, 17A and 17B, 18A and 18B, and l9A and
l9B, and 20A and 2~B, show top views o~ a completed
fiber weave and the corresponding unraveled disk,
respectively, o~ an actuator o~ the invention; and
FIG. 21 is a simplified representation o~ two
actuators of FIGS. lA and lB in antagonistic working
relationship, and
FIG. 22 is a simpli~ied representation o~ an
actuator employing unequal actuation o~ active elements
o~ a unit.
MODES FOR CARRYING OUT THE lNv~NllON
The invention is particularly described with
re~erence to the embodiments in which the active
elements are tensile elements, more especially a ~iber
or fibers of a shape memory alloy, which ~ibers shorten
when heated, and the passive support is provided by
compression members in the ~orm o~ disks with notches
for restraining the fiber or fibers under tension. It
will be understood that other active tensile elements
may be employed in the invention which may be shortened
by an activation, ~or example, a piezo electric e~ect,
magnetostriction e~ect, thermally expanding vessels or
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made of contractile polymers activated by electricity
or light.
The fibers of shape memory alloy may typically
have a diameter of less than 1 mm. In general, the
~ibers will have a diameter o~ at least 2 microns and
typically at least 20 microns. Suitably the ~ibers
will have a diameter o~ 5 to 1000 microns, generally 5
to 150 microns, and pre~erably about 100 microns.
The fibers o~ shape memory alloy may suitably be
NiTi ~ibers which shorten in length during transition
between martensitic and austenitic phases o~ the alloy
upon being heated.
The actuator of the invention achieves mechanical
motion ampli~ication that is more compact than a long
straight length o~ fiber, and more e~ficient than using
spring coils. With further reference to FIGS. lA and
lB, an actuator 10 comprises end supporting disks 12
and 26 and intermediate supporting disks 14, 16, 18,
2Q, 22 and 24 therebetween, a cell 34 is defined
between each pair of disks, for example, disks 12 and
14 and twelve thin NiTi fibers Z8 entrained in a
counter rotating helical pattern around and between end
supporting disks 12 to 26 by engagement with notches
32. The disks 12 to 26 are separated by preloading
springs 30 that keep the fibers 28 under tension when
relaxed. When the ~ibers 28 are heated, they contract
pulling the disks 12 to 26 together. The weave pattern
o~ the fibers 28 accomplishes an e~ficient displacement
amplification. The abundant force of the alloy is
being traded off for a displacement gain. This
transformation between ~orce and displacement is highly
efficient since the only loss in work is due to the
slight bending o~ the ~ibers 28. Unlike shape memory
alloy coils, the entire cross section o~ the ~ibers 28
is performing work in the contraction. Coils su~er
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g
from the debilitating drawback o~ requiring a diameter
larger than necessary. This is especially negative,
when considering the response, since the response time
iS directly related to fiber diameter.
The response of the actuator 10 is limited by the
cooling rate o~ the NiTi ~ibers 28, which directly
depends on the surface area to volume ratio o~ fibers
28. The higher this ratio the more rapidly a fiber 28
will cool.
A great deal of the material is wasted in SMA
coils since, during the shape memory effect, only the
skin o~ the coil is actually contracting at the maximum
amount. The internal diameter of the coil is acting
both as a heat capacitance and as a source of an
opposing ~orce to the desired motion.
The weave pattern also results in an ideal
"tensegrity" structure, with all compression members
being passive and all tension members active, resulting
in an optimal use of the material. Loosely speaking,
this has a biological analogy seen in the skeletal
arrangements o~ creatures with endo-skeletons, where
the muscles are the active tension members, and the
bones are passive compression members.
The displacement amplification can best be seen by
considering the simplified case consisting of two beams
and two fibers as shown in FIG. 2A.
In FIG. 2A, there is shown disks 40 and 42 with
fibers 44 and 46, under tension, therebetween.
The variables are as ~ollows:
L = diameter of disks
d = separating distance
a = angle o~ pitch
s = length of ~ibers
As the fibers 44 and 46 contract, the disks 40 and
3S 42 are pulled together. The displacement gain, Ad/
== ~
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~s is defined as the change in stroke along the
separating distance, divided by the change in the fiber
length. Since ideally the motion is constrained along
d:
52 = ~2 !l;,Z
s = s
~52 _ ~2
buts= L/cvs~,so,
!iS5 ~ --COSC~'~ 5i7~(x
The displacement gain is inversely proportional to
the sine of the weave pitch. As the disks 40 and 42
get closer together the displacement gain dramatically
increases as seen in FIG. 3, asymptotically approaching
infinity.
The helicoidal weave pattern of the actuator in
FIGS. lA and lB achieves a displacement amplification
for each cell of the actuator. All the radial
components of the tension forces of the twelve fibers
28 cancel, leaving only a common axial force component.
In this manner the displacement gain allows the
actuator to have an overall strain greater than 5%,
while the force attenuation is compensated by using
several fibers pulling collectively. The displacement
gain also allows the fibers to operate at reduced
percent strain, and since the cycle lifetime of the
fibers increases dramatically at a lower than absolute
strain, the cycle lifetime is also increased
correspondingly. FIG. lA represents only one
configuration of the possible parameters o~ actuator
10. The supporting disk size and spacing, the number
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o~ ~ibers, and the displacement gain are all adjustable
parameters.
~ FIGS. 4A and 4B de~ine the variables involved,
highlighting only one o~ the ~ibers in a single
actuator cell.
With further re~erence to FIGS 4A and 4B, the
variables are as ~ollows:
L = length o~ ~iber along disk
r = disk radius
y = o~set angle between successive disks
s = length o~ fiber
d = interdisk separation
a = weave pitch angle
E~uation(l) shows that the displacement gain is
inversely proportional to the sine o~ the weave pitch
The weave pitch in turn is dependent on the ~iber weave
pattern and the radius and spacing o~ the supporting
disks From FIGS. 4A and 4B, it can be seen that
trigonometry gives us the ~ollowing equation ~or the
weave pitch:
~ =urcl~7l(L)
The weave pattern is determined by the number of
notches around the disk, and the relative alignment o~
successive disks The o~set angle, ~, is the angle
between notches o~ successive disks in the actuator.
The length along the disk can be ~ound by the
~ollowing:
L = 2r * sin( Y2)
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Putting all this together results in the ~ollowing
equation ~or the displacement gain:
5(l
5s ~ 7 c l a 7 ( 2 r 5 ~1~ ( ) ) )
The displacement gain can with respect to L and d
be given by:
S(I ~/L~ + .1~ I L2
~ 2 -~
FIG. 5 shows the displacement gain plotted against
the separation distance d, and the length along the
disk L, with a normalized radius.
The displacement gain can be augmented by
increasing the o~set angle, or by decreasing the
inter-disk distance. There are o~ course limits on
both o~ these parameters. As the~ o~fset angle
approaches 180 degrees, the ~ibers approach the axis o~
the disks. This causes the structure to become less
stable and reduces the available space in the center
~or the placement of the springs and/or a position
sensor, (an ideal place ~or a sensor). The radius o~
the inner bounding cylinder, shown in FIG. 6, can be
~ound by trigonometry to be ri= r*cosy where r is the
disk radius and y is the of~set.
As illustrated in FIG. 6, a cage 35 o~ the
entrained ~ibers 28 is ~ormed, with an inner zone 37
within and surrounded by cage 35.
Decreasing the distance in between the disks
dramatically increases the displacement gain but limits
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the amount of stroke per cell If the disks begin
their motion very close to one another they can only
move a small distance before they come in contact with
one another. The available stroke per cell can be
increased by either increasing the o~fset angle or
increasing the disk radius.
The weave pattern of the actuator determines how
many ~ibers are to be used collectively, and a~ects
the displacement gain through the choice of the offset
angle. Numerous configurations result in a stable
weave pattern that will operate much like the actuator
10 illustrated in FIG. lA and lB.
For the actuator 10 in FIGS lA and lB, eight
supporting disks 12 to 26 were chosen with 6 notches,
32 per disk, each spaced apart. A prototype actuator
10 was constructed by aligning the disks vertically so
that each successive disk was o~set by 30 degrees.
The weave pattern was obtained by threading a single
fiber 28 along the notches 32 o~ the eight disks 12 to
26. Adjacent disks 12 to 26 were connected by the
fiber 28 through notches 32 that were separated by an
offset angle of 90~. The two end disks 12 and 26 were
woven along successive notches as shown in FIG. lB.
To get a better idea of how the fibers are woven,
imagine the disks o~ the actuator rolled out so that
they are flat. FIG. 7 shows a four disk actuator with
the disks 12, 14, 16 and 18 unraveled. The fiber weave
would begin at an end disk 12 and pass through the
successive points 1 through 5. The fiber 28 would then
continue going back and forth between the two end disks
12 and 18 until it arrived back at its starting
position. The final result is twelve tensile elements
made of a single fiber 28 woven in counter helical
rotations such that all radial forces cancel out upon
contraction
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The completed weave or cage o~ ~ibers in a top
view and in an unraveled disk is illustrated in FIGS.
8A and 8B, respectively.
Other completed weaves in top view and in an
unraveled disk are illustrated in FIGS. 9A and 9B, lOA
and 10B, llA and llB, 12A and 12B, 13A and 13B, 14A and
14B, 15A and 15B, 16A and 16B, 17A and 17B, 18A and
18B, and l9A and l9B, and 20A and 20B, respectively.
The ~orce generated by the actuator can be
adjusted by choosing the number and size o~ ~ibers used
in the weave. The more ~ibers that are acting
collectively, the larger the ~orce generated. Again
there is a limitation here on the number o~ ~ibers that
can be used. As the number o~ ~ibers increases so does
the ~iber inter~erence in the weave. Fibers with a
larger diameter can be chosen, but at the expense o~
response as cooling times will increase. To obtain a
~ast response, one hundred micron ~ibers were chosen
~or the actuator prototype. Twelve 100 micron ~ibers
acting collectively, allow rapid cooling in ambient air
without compromising strength. Table 1 shows a number
o~ actuator con~igurations. The e~ect on the
displacement gain is given by the length L, with a
normalized radius.
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Nolclles # Or libers O~rscl l~llglc Lellglll
llUlll allglC ~ L
8 45 lG G7.5 1.111
1.
11'1.5 1.(;G~
7 57.5 14 8G.2 1.3G7
115 l.G87
G GU 11 G0 1.000
'~t) 1.'11~
1.73'1
72 10 7'1 1.17G
108 l.Gl~
4 ~U 8 ~U 1.414
1:~5 ~ 1.8~18
'l'ablc 1: 'l'able o~ aclua~or co~lrlguratiolls
The configurations in Table 1 are illustrated in
FIGS. 8A, 8B; 9A, 9B; lOA, lOB; llA, llB; 12A, 12B;
13A, 13B; 14A, 14B; 15A, 15B; 16A, 16B; 17A, 17B; 18A,
18B; l9A, l9B; and 20A and ZOB.
The numerous con~igurations available result in a
rich design space. Table 2 summarizes the various
tradeoffs in designing a shape memory alloy actuator.
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5Desire~l prol)erty llow 'l'radc-olr
Illerease clispl~celllell~ ill illcrease ~lislc r~lius illcre.~se in size
<lecre.~se ~1~Ieere.lse ill slrol~e l)er cell
lllcre~se Ç~rce ~ illcrease lil~er ~ .slower res~ol-se
illcrease iil)er #illcrease ill liber illlerlerellce
Illcrease slroke illcre~se we~ve l~ilcll ~Iecrei~selili clisl~l~celllelll g;~
illcre~se ~lisk r~liusillcre~se ill si~e
illcrcasc ;~ ~r eellsillcrease il~ size
Illcrease resl)ollse ~Ieerease ~ er <liallleler ~lecrease ill force
Decrease ill size (lec~ease ~lislc r.~lius ~Iecre~se ill ~lisl71~cclllell~ g~
~Iecrease ~ ~f cells~lecrease i~l slrolce
Tal)lC 2: I'dl~lC 0~ ~lCSi~ r;L(lC~rs
The actuator prototype o~ FIGS. lA and lB is hand
woven. The supporting disks 12 to 26 all have a
threaded center so that they can be mounted on a
threaded sha~t. The disks 23 to 26 are placed on the
shaft alternately with the preloading springs 30. The
proper alignment o~ successive disks 12 to 26 was
accomplished via guideholes drilled in the disks
corresponding to the desired o~set angle. For the
actuator prototype 10, four guide holes were required
o~set by 90~. Once the support disks were mounted and
the proper separation distance d, determined for the
desired displacement gain, the disks were fixed to the
center sha~t by two nuts at each end of the actuator.
The weave was then achieved by rotating the center
shaft as the fiber 28 was woven from end disk 12 to end
disk 26. In this manner it was possible to
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mechanically connect many tensile elements
collectively, quickly and securely. After the weave
~ was completed the two ends of the ~iber were merely
tied in a knot. This also provided a secure mechanical
connection as most of the stress on the fiber occurs at
the notches 32. If the fibers in the actuator only
exhibit the one way shape memory e~ect, it is
necessary to ~orce bias individual actuators so that
they will return to their original length when cooled.
This can easily be accomplished by using biasing
springs or by using actuators in an antagonistic
fashion. Shape memory alloys are especially suited to
antagonistic arrangements since the force required to
deform the alloy is much less than the force generated
bythe phase transformation. Using the actuators in an
antagonistic fashion also results in improved system
response. The response time of the actuator system
will then strongly depend on heat activation, which can
be tuned according to the input current amplitude.
As illustrated in the drawings, for example, FIGS.
lA and lB and 6, a single fiber or multiplicity of
fibers 28 are suitably entrained between a plurality of
compression or support members such as disks lZ to 26,
the plurality typically being greater than 2. The
compression or support members are urged into spaced
apart relationship by preloaded springs 32 which are
typically disposed within inner zone 37 of cage 35
illustrated in FIG. 6.
The compression or support member suitably has a
radial symmetry such as is provided by a disk, however,
star-shaped members or polygonal members having radial
symmetry are also appropriate.
The compression or support members are desirably
lightweight and electrically non-conducting, for
example, they may be of anodized aluminium or aluminium
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having an electrically insulating coating. Low thermal
capacitance and low thermal conductivity are also
desirable properties, so the members may be made o~ ~
heat resistant plastics, ceramics, or other materials
having these properties.
The shape memory alloy ~ibers 28 are heated in
order to e~fect the phase trans~ormation, and such
heating may be achieved by passage o~ an electrical
current through the ~ibers. In order to achieve this
the actuator 10 may include electrical connection means
~or conduction o~ electricity into the ~ibers 28 at
disk 12 and out o~ ~ibers 28 at disk 26.
Thus, ~or example, electrically conductive contact
plates may be mounted on or serve as disks 12 and 26 to
establish electric contact with ~ibers 28, so that a
source o~ electricity may be electrically connected to
the contact plate on disk 12 with the contact plate on
disk 26 connected to the ground or to the electrical
source to complete an electrical circuit.
Alternatively the 2 ends of the ~iber can be
electrically connected resulting in a serial
connection. This has the advantage o~ increasing the
resistance and lowering the re~uired current.
In the pre~erred embodiment in which the cage 35
o~ lengths o~ ~iber de~ines a helicoidal array that is
symmetrical so that radial components o~ tension ~orces
in the ~iber or ~ibers of the cage 35, balance to zero
leaving only an axial component o~ tension ~orces o~
the ~iber or ~ibers.
Applications ~or the actuator o~ the invention
include toys, camera shutters ~or aerospace, micro
manipulators, biomedical devices, and appliances and
indeed any assembly or device wherein there is a need
~or e~ecting a displacement o~ a component.
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As shape memory alloys are capable o~ absorbing
great quantities o~ mechanical energy upon de~ormation
resulting from an impact, they can be used to realize
compact shock absorbers, accomplishing the reverse
~unction o~ an actuator, that o~ absorbing rather than
generating mechanical energy The invention described
in a pre~erred embodiment may lend its advantageous
properties o~ optimal use o~ materials to realize shock
absorbers having great e~iciency and compactness.
These absorbers will be subject to exactly the same
design principles and rules that govern the design o~
the actuators.
As shape memory alloys are capable of undergoing
large de~ormations before breaking opposing a
relatively constant ~orce against strain, an e~ect
termed the superelastic e~ect, they can be used to
realize superelastic ~ixture, attachments or clamps.
The invention described in a pre~erred embodiment may
lend its advantageous properties o~ optimal use of
materials to realize superelastic fixtures, attachments
or clamps having the superelastic domain ampli~ied many
~old. These ~ixtures will be subject to exactly the
same design principle and rules that govern the design
o~ the actuators.
Alternate structures subject to the same design
principles and rules may be reused replacing all active
tensile elements by active compression members and
compression members by tensile elements.
Variations o~ the basic unit illustrated in FIG. 2
are illustrated in FIGS. 2A, 2B, 2C and 2D.
In FIG. 2B the crossing lengths de~ining the cage
are active elements in the ~orm o~ expansion members
144 and 146, which may be, ~or example, piston and
cylinder units or thermal expansion vessels and the
passive support members 140 and 142 are ~iber elements.
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On expansion of members 144 and 146, the displacement d
increases.
In FIG. 2C the crossing lengths defining the cage
are passive support members 240 and 242 and the active
elements are tensile fibers 244 and 246 which shorten
under activation.
On contraction of fibers 244 and 246 the
displacement d increases.
In FIG 2D the crossing lengths defining the cage
are passive support members in the form of fibers 340
and 342 and the active elements are expansion members
344 and 346, ~or example, piston and cylinder units.
On activation o~ the expansion members 344 and 346 the
displacement d, decreases.
In FIG. 2B the passive members 140 and 142 may be
in the form of restraining harnesses or loops of the
fiber elements.
Thus for each structure described hereinbe~ore
there corresponds a dual structure subject to the same
design principles and rules realized by replacing the
active tensile elements by active compression elements
and the passive compression members by passive tensile
elements. Upon expansion of such active compression
members the entire structure will expand with a
displacement amplification that follows the rules
described above. Such a structure will have the same
advantages of efficiency and optimal use of materials.
This will apply equally to the realization of actuators
and shock absorbers.
Thus in the present invention an actuator develops
a large displacement from tensile elements which
shorten by a small amount under activation; the tensile
elements can be a fiber or fibers, for example, shape
memory fibers, or other active tensile elements
undergoing shortening strain under activation. The
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fiber or fibers are entrained between opposed, spaced
apart compression members typically a stack of spaced
apart disks, the entrained ~ibers define a cage of
crossing lengths of fiber in symmetrical array
typically a double helicoidal array; the cage surrounds
an inner zone free of interferences which can be used
to lodge, if needed, springs to urge the compression
members apart and keep the structure stable when not in
use. The inner zone can also be used to contain
another concentric actuator, etc. Activation of the
~ibers shortens the fiber lengths producing an
amplified relative displacement of the complete
structure which can be translated to a component which
is to be displaced, and to which the actuator is
operably connected.
The displacement ampli~ication is accomplished
e~iciently, which is a most unusual and important
feature o~ the arrangement.
The ~ibers shorten by a small amount which the
structure amplifies by a large ~actor in an ef~icient
manner; the structure is simple, optimally light and
compact; this ~eature overcomes the limitations of
strain-based mechanical transducers (shape memory
alloys, magnetostrictive alloys, piezo electric
materials, contractile polymers) employed to
manu~acture actuators.
If the tensile elements are replaced by active
compression members that expand instead o~ contract and
the compression members by tensile members (strings,
cables, etc.), a dual structure is created which will
accomplish the same ampli~ication ef~ect and have the
same efficiency advantages but will expand instead of
contract.
In practicing the invention it is possible to
carry out the activation so that not all o~ the active
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elements are activated at the same time, or by the same
degree. Thus, for example, active elements of opposed
sides of the cage might be activated in an
intermittent, alternating relationship. to produce a
bending motion with alternating periodicity Such a
bending motion for an actuator o~ the type illustrated
in FIG. lA is shown in FIG. 22.
