Deformable structural arrangement

An actuator develops a displacement from a force; the actuator employs active tension elements which comprise a fiber or fibers which shorten under activation, for example, shape memory alloy fibers; the fiber or fibers are entrained between opposed, spaced apart support members, typically a stack of spaced apart disks; the entrained fiber or fibers define a cage of crossing lengths of fiber in symmetrical array, typically a helicoidal array. Activation of the fibers shortens the fiber lengths producing a relative displacement of the support members which can be translated to a component which is to be displaced, and to which the actuator is operably connected, in another embodiment the active tension elements stretch under stress so that instead of an actuator there is formed a shock absorber which eliminates displacement with a force.

BACKGROUND OF THE INVENTION 
i) Field of the Invention 
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 transformation between force and 
displacement, and to a method to amplify efficiently, small displacements 
of force producing elements; the invention is more especially concerned 
with an actuator or shock absorber. 
ii) Brief Description of Prior Art 
Miniature robotic systems have a need for powerfull, compact, lightweight 
actuators. Conventional techniques such as electric, hydraulic, and 
pneumatic actuators, suffer from a drastic reduction of the amount of 
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 for miniature 
applications. A SMA fiber 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 10.sup.5 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 phase transition temperature. Also, since the alloy is resistive it 
can easily be heated electrically. 
The high strength to weight ratio of shape memory alloys is accompanied by 
several limitations. Shape memory alloys cannot sustain shape recovery 
after strains of more than a few percent, about 5% for a working life of 
thousands or millions of cycles. Activation is achieved by heating and 
cooling. Thus, a primary disadvantage of previously proposed actuators is 
that the displacement which can be achieved is small, and second the speed 
of displacement is moderate. They can however still be controlled through 
the use of 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 of 5 percent. Many of the 
designs of actuators using shape memory alloys depend on mechanically 
amplifying the displacement either through the use of long straight 
fibers, through the use of spring coils, or through bistable devices. 
SUMMARY OF TEE INVENTION 
It is an object of this invention to provide a deformable structural 
arrangement for effecting a transformation between force and displacement 
or distance. 
It is a particular object of this invention to provide an actuator, more 
especially an actuator for effecting transformation between force and 
displacement or distance, or for trading efficiently force with 
displacement. 
It is a further object of the invention to provide a shock absorber. 
It is a further object of this invention to provide a device incorporating 
the actuator of the invention. 
It is a still further object of this invention to provide a method of 
effecting a transformation between force and displacement. 
A still further object of the invention is to provide such an actuator 
which is lightweight. 
Still another object of the invention is to provide a deformable structural 
arrangement capable of effecting a high variation in displacement, 
especially from moderate variations in displacement of primary contractile 
or expanding elements. 
It is still another object of the invention to provide actuation with high 
variation in displacement from thin fiber or fibers which thus can be 
activated rapidly by heat, or other means. 
It is still another object of the invention to provide an actuator assembly 
which is compact. 
In accordance with one aspect of the invention there is provided a 
deformable 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 of said active element means and said passive support means defining a 
cage of 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 of the invention, there is 
provided an actuator comprising at least one fiber which shortens under 
activation, entrained between at least first and second support members, 
said support members being in opposed, spaced apart relationship, the 
entrained at least one fiber defining a cage of crossing lengths of fiber 
in symmetrical array, said cage surrounding an inner zone between said 
support structure members. 
In accordance with another aspect of the invention there is provided an 
actuator for development of a displacement from a force, comprising at 
least one fiber 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 effective to balance all radial 
components of tension forces of the at least one fiber. 
Suitably the actuator may include means to urge the support members apart 
into the opposed, spaced apart relationship with the at least one fiber 
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 
effect displacement of the component, the actuator being an actuator of 
the invention as described hereinbefore. The component is operably 
connected to the second support member such that displacement of the 
second member relative to the first member produces a corresponding 
displacement of the component. 
In accordance with another aspect of 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 first and second support members made of compression 
members, the support members being in opposed, spaced relationship, the 
active elements defining a cage of crossing lengths in symmetrical array 
forming a double helical counter rotating pattern, the cage surrounding an 
inner zone free of interferences. 
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 
first 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 fiber to shorten said fiber lengths such that 
said second support is displaced towards said first support member. 
In another aspect of 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 force comprising active elements made 
of at least one fiber which shortens under activation or stretches under 
stress, entrained under stress between at least first 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 of 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 members which expand under activation or 
compress under stress, attached between at least first and second 
restraining harnesses or loops, the restraining harnesses or loops being 
in opposed, spaced relationships, the active members defining a cage of 
crossing lengths in symmetrical array forming a double helical 
counter-rotating pattern, the restraining harnesses or loops being made of 
tensile members disposed according to a star or polygonal regular 
configuration or made of disks, the cage surrounding an inner zone free of 
interferences. 
In another aspect of the invention there is provided a method of realizing 
a device with magnified superelastic properties which can provide quasi 
constant force under large strain deformation of the active elements of at 
least one fiber which stretches under stress, entrained between at least 
first and second support member, 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 member away from the first support member to counteract a 
displacement adjacent the second support member.

DESCRIPTION OF PREFERRED EMBODIMENTS WITH REFERENCE TO THE DRAWINGS 
The invention is particularly described with reference to the embodiments 
in which the active elements are tensile elements, more especially a fiber 
or fibers of a shape memory alloy, which fibers shorten when heated, and 
the passive support is provided by compression members in the form of 
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, for example, a 
piezo electric effect, magnetostriction effect, thermally expanding 
vessels or made of contractile polymers activated by electricity or light. 
The fibers of shape memory alloy may typically nave a diameter of less than 
1 mm. In general, the fibers will have a diameter of at least 2 microns 
and typically at least 20 microns. Suitably the fibers will have a 
diameter of 5 to 1000 microns, generally 5 to 150 microns, and preferably 
about 100 microns. 
The fibers of shape memory alloy may suitably be NiTi fibers which shorten 
in length during transition between martensitic and austenitic phases of 
the alloy upon being heated. 
The actuator of the invention achieves mechanical motion amplification that 
is more compact than a long straight length of fiber, and more efficient 
than using spring coils. With further reference to FIGS. 1A and 1B, an 
actuator 10 comprises end supporting disks 12 and 26 and intermediate 
supporting disks 14, 16, 18, 20, 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 28 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 fibers 28 are 
heated, they contract pulling the disks 12 to 26 together. The weave 
pattern of the fibers 28 accomplishes an efficient displacement 
amplification. The abundant force of the alloy is being traded off for a 
displacement gain. This transformation between force and displacement is 
highly efficient since the only loss in work is due to the slight bending 
of the fibers 28. Unlike shape memory alloy coils, the entire cross 
section of the fibers 28 is performing work in the contraction. Coils 
suffer from the debilitating drawback of 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 of the NiTi 
fibers 28, which directly depends on the surface area to volume ratio of 
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 of 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 force 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 of creatures with 
endoskeletons, 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 follows: 
L=diameter of disks 
d=separating distance 
.alpha.=angle of pitch 
s=length of fibers 
As the fibers 44 and 46 contract, the disks 40 and 42 are pulled together. 
The displacement gain, .DELTA.d/.DELTA.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: 
##EQU1## 
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. 1A and 1B 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. 1A represents only one 
configuration of the possible parameters of actuator 10. The supporting 
disk size and spacing, the number of fibers, and the displacement gain are 
all adjustable parameters. 
FIGS. 4A and 4B define the variables involved, highlighting only one of the 
fibers in a single actuator cell. 
With further reference to FIGS. 4A and 4B, the variables are as follows: 
L=length of fiber along disk 
r=disk radius 
.gamma.=offset angle between successive disks 
s=length of fiber 
d=interdisk separation 
.alpha.=weave pitch angle 
Equation (1) shows that the displacement gain is inversely proportional to 
the sine of the weave pitch. The weave pitch in turn is dependent on the 
fiber weave pattern and the radius and spacing of the supporting disks. 
From FIGS. 4A and 4B, it can be seen that trigonometry gives us the 
following equation for the weave pitch: 
EQU .alpha.=arctan(d/L) 
The weave pattern is determined by the number of notches around the disk, 
and the relative alignment of successive disks. The offset angle, .gamma., 
is the angle between notches of successive disks in the actuator. The 
length along the disk can be found by the following: 
EQU L=2r*sin(.gamma./2) 
Putting all this together results in the following equation for the 
displacement gain: 
##EQU2## 
The displacement gain can with respect to L and d be given by: 
##EQU3## 
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 offset angle, or 
by decreasing the inter-disk distance. There are of course limits on both 
of these parameters. As the offset angle approaches 180 degrees, the 
fibers approach the axis of the disks. This causes the structure to become 
less stable and reduces the available space in the center for the 
placement of the springs and/or a position sensor, (an ideal place for a 
sensor). The radius of the inner bounding cylinder, shown in FIG. 6, can 
be found by trigonometry to be r.sub.i =r*cos.gamma. where r is the disk 
radius and .gamma. is the offset. 
As illustrated in FIG. 6, a cage 35 of the entrained fibers 28 is formed, 
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 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 offset angle or 
increasing the disk radius. 
The weave pattern of the actuator determines how many fibers are to be used 
collectively, and affects 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 FIGS. 1A and 
1B. 
For the actuator 10 in FIGS. 1A and 1B, 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 offset by 30 degrees. The weave pattern was obtained 
by threading a single fiber 28 along the notches 32 of 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.degree.. The two 
end disks 12 and 26 were woven along successive notches as shown in FIG. 
1B. 
To get a better idea of how the fibers are woven, imagine the disks of 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. 
The completed weave or cage of fibers 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, 10A and 10B, 11A and 11B, 12A and 12B, 13A and 13B, 
14A and 14B, 15A and 15B, 16A and 16B, 17A and 17B, 18A and 18B, and 19A 
and 19B, and 20A and 20B, respectively. 
The force generated by the actuator can be adjusted by choosing the number 
and size of fibers used in the weave. The more fibers that are acting 
collectively, the larger the force generated. Again there is a limitation 
here on the number of fibers that can be used. As the number of fibers 
increases so does the fiber interference in the weave. Fibers with a 
larger diameter can be chosen, but at the expense of response as cooling 
times will increase. To obtain a fast response, one hundred micron fibers 
were chosen for the actuator prototype. Twelve 100 micron fibers acting 
collectively, allow rapid cooling in ambient air without compromising 
strength. Table 1 shows a number of actuator configurations. The effect on 
the displacement gain is given by the length L, with a normalized radius. 
TABLE 1 
______________________________________ 
Table of actuator configurations 
Notches Offset Angle 
Length 
num angle # of fibers .gamma. L 
______________________________________ 
8 45 16 67.5 1.111 
90 1.414 
112.5 1.663 
7 57.5 14 86.2 1.367 
115 1.687 
6 60 12 60 1.000 
90 1.414 
120 1.732 
5 72 10 72 1.176 
108 1.618 
4 90 8 90 1.414 
135 1.848 
______________________________________ 
The configurations in Table 1 are illustrated in FIGS. 8A, 8B; 9A, 9B; 10A, 
10B; 11A, 11B; 12A, 12B; 13A, 13B; 14A, 14B; 15A, 15B; 16A, 16B; 17A, 17B; 
18A, 18B; 19A, 19B; and 20A and 20B. 
The numerous configurations available result in a rich design space. Table 
2 summarizes the various tradeoffs in designing a shape memory alloy 
actuator. 
TABLE 2 
______________________________________ 
Table of design tradeoffs 
Desired property 
How Trade-off 
______________________________________ 
Increase displace- 
increase disk radius 
increase in size 
ment gain decrease d decrease in stroke per cell 
lncrease force 
increase fiber diameter 
slower response 
increase fiber # 
increase in fiber interference 
lncrease stroke 
increase weave pitch 
decrease in displacement gain 
increase disk radius 
increase in size 
increase # of cells 
increase in size 
Increase response 
decrease fiber diameter 
decrease in force 
Decrease in size 
decrease disk radius 
decrease in displacement gain 
decrease # of cells 
decrease in stroke 
______________________________________ 
The actuator prototype of FIGS. 1A and 1B is hand woven. The supporting 
disks 12 to 26 all have a threaded center so that they can be mounted on a 
threaded shaft. The disks 23 to 26 are placed on the shaft alternately 
with the preloading springs 30. The proper alignment of successive disks 
12 to 26 was accomplished via guideholes drilled in the disks 
corresponding to the desired offset angle. For the actuator prototype 10, 
four guide holes were required offset by 90.degree.. 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 shaft 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 mechanically connect many 
tensile elements collectively, quickly and securely. After the weave was 
completed the two ends of the fiber 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 effect, it is necessary to force 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 by the 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. 1A and 1B and 6, a 
single fiber or multiplicity of fibers 28 are suitably entrained between a 
plurality of compression or support members such as disks 12 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 having an electrically insulating coating. Low 
thermal capacitance and low thermal conductivity are also desirable 
properties, so the members may be made of heat resistant plastics, 
ceramics, or other materials having these properties. 
The shape memory alloy fibers 28 are heated in order to effect the phase 
transformation, and such heating may be achieved by passage of an 
electrical current through the fibers. In order to achieve this the 
actuator 10 may include electrical connection means for conduction of 
electricity into the fibers 28 at disk 12 and out of fibers 28 at disk 26. 
Thus, for example, electrically conductive contact plates may be mounted on 
or serve as disks 12 and 26 to establish electric contact with fibers 28, 
so that a source of 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 fiber can be electrically connected 
resulting in a serial connection. This has the advantage of increasing the 
resistance and lowering the required current. 
In the preferred embodiment in which the cage 35 of lengths of fiber 
defines a helicoidal array that is symmetrical so that radial components 
of tension forces in the fiber or fibers of the cage 35, balance to zero 
leaving only an axial component of tension forces of the fiber or fibers. 
Applications for the actuator of the invention include toys, camera 
shutters for aerospace, micro manipulators, biomedical devices, and 
appliances and indeed any assembly or device wherein there is a need for 
effecting a displacement of a component. 
As shape memory alloys are capable of absorbing great quantities of 
mechanical energy upon deformation resulting from an impact, they can be 
used to realize compact shock absorbers, accomplishing the reverse 
function of an actuator, that of absorbing rather than generating 
mechanical energy. The invention described in a preferred embodiment may 
lend its advantageous properties of optimal use of materials to realize 
shock absorbers having great efficiency and compactness. These absorbers 
will be subject to exactly the same design principles and rules that 
govern the design of the actuators. 
As shape memory alloys are capable of undergoing large deformations before 
breaking opposing a relatively constant force against strain, an effect 
termed the superelastic effect, they can be used to realize superelastic 
fixture, attachments or clamps. The invention described in a preferred 
embodiment may lend its advantageous properties of optimal use of 
materials to realize superelastic fixtures, attachments or clamps having 
the superelastic domain amplified many fold. These fixtures will be 
subject to exactly the same design principle and rules that govern the 
design of 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 of the basic unit illustrated in FIG. 2 are illustrated in FIGS. 
2A, 2B, 2C and 2D. 
In FIG. 2B the crossing lengths defining the cage are active elements in 
the form of expansion members 144 and 146, which may be, for example, 
piston and cylinder units or thermal expansion vessels and the passive 
support members 140 and 142 are fiber elements. 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, for example, piston and cylinder units. On 
activation of 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 hereinbefore 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 fiber or fibers are entrained between opposed, 
spaced apart compression members typically a stack of spaced apart disks, 
the entrained fibers 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 fibers 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 amplification is accomplished efficiently, which is a most 
unusual and important feature of the arrangement. 
The fibers shorten by a small amount which the structure amplifies by a 
large factor in an efficient manner; the structure is simple, optimally 
light and compact; this feature overcomes the limitations of strain-based 
mechanical transducers (shape memory alloys, magnetostrictive alloys, 
piezo electric materials, contractile polymers) employed to manufacture 
actuators. 
If the tensile elements are replaced by active compression members that 
expand instead of contract and the compression members by tensile members 
(strings, cables, etc.), a dual structure is created which will accomplish 
the same amplification effect 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 of the active 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 of the type illustrated in FIG. 1A is shown in FIG. 
22.