Abstract:
A linear actuator includes a plurality of sub-modules disposed in adjacent array and adapted to translate reciprocally parallel to a common axis. A plurality of shape memory alloy wires extend generally linearly and parallel to the axis, and are each connected from one end of a sub-module to the opposed end of an adjacent sub-module. The SMA wires are connected in a circuit for ohmic heating that contracts the SMA wires between the sub-modules. The sub-modules are linked by the SMA wires in a serial mechanical connection that combines the constriction stroke displacement of the SMA wires in additive fashion to achieve a long output stroke. Moreover, the sub-modules are assembled in a small volume, resulting in an actuator of minimal size and maximum stroke displacement. The sub-modules may be rods or bars disposed in closely spaced adjacent relationship, or concentric motive elements, with the serial mechanical connection extending from each motive element to the radially inwardly adjacent motive element, whereby the innermost motive element receives the sum of the translational excursions of all the motive elements concentric to the innermost element. The SMA linear actuator includes a restoring spring assembly having a restoring force that decreases with increasing displacement to minimize residual strain in the SMA components. The SMA wires are connected for ohmic heating in various series and parallel circuit arrangements that optimize force output, cycle time, current flow, and ease of connection.

Description:
BACKGROUND OF THE INVENTION 
     Linear actuators find widespread applications in industrial, commercial, vehicular, and domestic settings, in uses ranging widely from electric door locks and windshield wipers in automobiles to pin pullers and shutter controllers in mechanical designs. Generally speaking, linear actuators comprise solenoid devices in which an electromagnet is used to translate an armature, and the retraction or extension of the armature is operatively connected in a mechanism to perform useful work. Such devices are commodity items that are manufactured in many sizes, force/stroke outputs, and AC or DC operation. 
     Despite their widespread adoption, electromagnetic linear actuators have several important drawbacks that require design accommodations in mechanical systems. Due to the use of electromagnetism as the motive force, these devices necessarily require ferromagnetic materials to define the armature as well as a magnetic flux circuit to maximize the stroke force. Such materials are typically dense, and their use results in devices that are rather large and heavy, particularly in comparison to their stroke/force output characteristics. Moreover, the multiple turns of wire that comprise an electromagnet, typically hundreds or thousands, add another substantial mass to the device. 
     Another drawback of electromagnetic linear actuators is also due to the use of electromagnetism as the driving force. Typically, as the armature is extended from the electromagnetic, increasing portions of the armature are removed from the influence of the electromagnetic field, and the driving force is concomitantly reduced. As a result, the force versus stroke displacement characteristics of these devices generally exhibit high initial force values that decline rapidly with increase in stroke displacement. In many mechanisms it is desirable to deliver a constant force linear stroke, and it is necessary to design additional mechanisms to make use of the negatively sloped force/displacement characteristic. 
     In recent years much interest has been directed toward shape memory alloy (hereinafter, SMA) materials and their potential use in linear actuators. The most promising material is nickel titanium alloy, known as Nitinol, which, in the form of a wire or bar, delivers a strong contraction force upon heating above a well-defined transition temperature, and which relaxes when cooled. Assuming the Nitinol wire is heated ohmically or by extrinsic means, there is no need for the ferromagnetic materials and numerous windings of the prior art electromagnetic linear actuators, and there is the promise of a lightweight linear actuator that delivers a strong actuation force. Moreover, the force versus displacement characteristic of SMA is much closer to the ideal constant than comparable electromagnetic devices. 
     Despite the great interest in SMA actuators and many forms of SMA actuators known in the prior art, no practical SMA actuator mechanism has proven to be reliable over a large number of operating cycles. It has been found that Nitinol wire requires a restoring force to assist the material in resuming its quiescent length when its temperature falls below the material&#39;s transition temperature. Many prior art SMA actuator designs have made use of common spring assemblies, such as helical or leaf springs, to exert the required restoring force. These spring assemblies typically deliver a spring force that varies linearly with displacement, (F=kx), and the restoring force in most cases is a maximum at maximum stroke. It has been found that the SMA component responds poorly to this force/displacement characteristic, and the useful life of the SMA actuator is severely limited by such a restoring force. To overcome this problem, prior art designers have attempted to use simple weights depending from pulleys to exert a constant restoring force on the SMA component. Although more effective, this expedient results in a mechanism that is not easily realized in a small, widely adaptive package. 
     Another drawback inherent in known SMA materials is the relatively small amount of contraction that is exerted upon heating past the transition temperature. The maximum contraction is about 8%, and the useful contraction for repeated use is about 6%. Thus, to achieve a direct displacement stroke from the SMA component of about one inch, the SMA component must be over sixteen inches long. This material limitation results in a minimum size that is too large for many applications. Some prior art designs overcome this problem by wrapping the SMA wire about one or more pulleys to contain the necessary length within a shorter space. However, the SMA wire tends to acquire some of the curvature of the pulleys as it is repeatedly heated and cooled, and loses too much of its ability to contract longitudinally. The result is failure after a few number of operating cycles. Other prior art designs employ lever arrangements or the like to amplify the SMA displacement, with a concomitant reduction in output force. 
     It is evident that the prior art has failed to fully exploit the full potential of shape memory alloy, due to the lack of a mechanism that capitalizes on the useful material characteristics of SMA. 
     SUMMARY OF THE INVENTION 
     The present invention generally comprises a linear actuator that employs a shape memory alloy component to deliver a relatively long stroke displacement and reiterative operation over a large number of cycles. 
     In one aspect, the invention provides a plurality of SMA sub-modules, each capable of displacement upon heating of the respective SMA component. The sub-modules are linked in a serial mechanical connection that combines the stroke displacement of the sub-modules in additive fashion to achieve a relatively long output stroke. Moreover, the sub-modules may be assembled in a small volume, resulting in an actuator of minimal size and maximum stroke displacement. 
     The sub-modules may be fabricated as rods or bars adapted to be disposed in closely spaced adjacent relationship, each rod or bar linked in serial mechanical connection to the adjacent rod or bar. Alternatively, the sub-modules may comprise concentric motive elements, with the serial mechanical connection extending from each motive element to the radially inwardly adjacent motive element, whereby the innermost motive element receives the sum of the translational excursions of all the motive elements concentric to the innermost element. For all the sub-module embodiments, the serial links therebetween are provided by one or more shape memory alloy wires, each wire connected at opposed ends of adjacent sub-modules to apply contractile force therebetween. 
     In another aspect, the invention provides an SMA linear actuator assembly employing a spring assembly that is designed to apply a restoring force tailored to optimize the longevity of the SMA component. In one embodiment of the spring assembly, a roller/band spring (hereinafter, rolamite) is connected to the output shaft of the linear actuator assembly. The rolamite spring exerts a restoring force characterized by a decrease in force with increasing displacement, so that the SMA components are returned to their quiescent form with a minimum of residual strain. In a further embodiment, the spring assembly is comprised of a bar or rod connected to the output shaft of the SMA actuator assembly and confined in a channel for longitudinal translation therein. The bar includes shaped cam surfaces extending longitudinally therealong, and a cam follower extends from the channel and is resiliently biased to engage the cam surfaces. As the bar is translated by actuation of the SMA linear actuator assembly, the cam follower exerts a restoring force that is a function of the slope of the cam surface and the magnitude of the resilient force on the cam follower. By appropriate shaping of the cam surface, the assembly exerts on the SMA linear actuator assembly a restoring force characterized by a decrease in force with increasing displacement, whereby the number of cycles of operation is maximized. 
     In a further aspect, the invention includes a housing in which a plurality of drive rods are arrayed in generally parallel, adjacent relationship and supported to translate freely in their longitudinal directions. One end of each drive rod is connected to the opposed end of an adjacent drive rod by an SMA wire, defining a series of drive assemblies connected in additive, serially linked chain fashion. At one end of the chain, the drive assembly is joined by an SMA wire to the housing, and at the other end of the chain, the housing is provided with an opening through which an actuating rod may extend. Also secured in the housing is a spring, such as a rolamite roller/band spring, having one end connected to the housing and the other end connected to the actuator rod. The spring is designed to exert a restoring force having a constant or negative force versus displacement relationship. 
     Each SMA wire is connected in an electrical circuit, in one of several arrangements of series or parallel connections, so that ohmic heating may be employed to heat the SMA wires beyond their phase transition temperature. In the chain-connected series of SMA drive assemblies, the resulting contraction of the SMA wires is cumulative and additive, and the actuating rod is driven to extend from the housing with a high force output. When the current in the circuit is terminated, the SMA wires cool below the transition temperature, and the spring restores the SMA wires to their quiescent length by urging the actuating rod to translate retrograde and (through the chained connection of assemblies) to apply sufficient tension to re-extend all the SMA wires. 
     It may be appreciated that the SMA wires remain in substantially linear dispositions throughout the contraction/extension cycle, so that flex-induced stresses are avoided. To assist in heat removal for high power applications, the housing may be filled with oil or other thermal absorber, which may be cooled passively or actively. To deliver additional force, two or more SMA wires may be connected between the drive assemblies, rather than one wire. To provide enhanced actuation and retraction times, the SMA wires may be thinner. 
     Although the invention is described with reference to the shape memory component comprising a wire formed of Nitinol, it is intended to encompass any shape memory material in any form that is consonant with the structure and concept of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic mechanical diagram depicting the fundamental components of the shape memory alloy actuator of the present invention. 
     FIG. 2 is a cross-sectional elevation of one embodiment of the shape memory alloy actuator of the present invention. 
     FIG. 3 is a cross-sectional end view of a negative force constant rolling band spring assembly of the shape memory alloy actuator of the present invention. 
     FIG. 4 is a plan view of one embodiment of the band spring of the rolling band spring assembly depicted in FIG.  3 . 
     FIG. 5 is a partially cutaway side elevation showing a further embodiment of the shape memory alloy actuator of the present invention. 
     FIG. 6 is a schematic view of a further embodiment of a negative force constant spring assembly of the shape memory alloy actuator of the present invention. 
     FIG. 7 is a graph depicting force versus displacement for different spring assemblies. 
     FIG. 8 is a perspective view of a further embodiment of the shape memory alloy actuator of the present invention. 
     FIG. 9 is a top view of the embodiment of the actuator invention depicted in FIG.  8 . 
     FIG. 10 is a side elevation of the actuator invention depicted in FIGS. 8 and 9. 
     FIG. 11 is a top view of the assembled drive rods of the shape memory alloy actuator depicted in FIGS. 8-10. 
     FIG. 12 is an exploded view of the drive rod assembly of the shape memory alloy actuator depicted in FIGS. 8-11. 
     FIG. 13 is an exploded view of the drive rod assembly of the shape memory alloy actuator depicted in FIGS. 8-12, with the drive rods in an extended disposition. 
     FIG. 14 is a partial perspective view of a drive rod connection to a shape memory alloy wire, in accordance with the present invention. 
     FIG. 15 is a perspective view of a further embodiment of a shape memory alloy actuator employing the drive rod connection assembly shown in FIG.  14 . 
     FIG. 16 is a cross-sectional end view of a further embodiment of a shape memory alloy actuator of the present invention. 
     FIG. 17 is a perspective view of one motive element of the shape memory alloy actuator shown in FIG.  16 . 
     FIG. 18 is a schematic depiction of one series electrical circuit arrangement for heating the SMA wires of the shape memory alloy actuator of the invention. 
     FIG. 19 is a schematic depiction of a series electrical circuit arrangement for heating paired SMA wires of the shape memory alloy actuator of the invention. 
     FIG. 20 is a schematic depiction of another series electrical circuit arrangement for heating paired SMA wires of the shape memory alloy actuator of the invention. 
     FIG. 21 is a schematic depiction of another series electrical circuit arrangement for heating paired parallel SMA wires of the shape memory alloy actuator of the invention. 
     FIG. 22 is a perspective view of another embodiment of a shape memory alloy actuator employing the drive rod connection assembly shown in FIG.  14 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention generally comprises a linear actuator that employs at least one shape memory alloy component as the driving element. The invention provides relatively long stroke displacement with high force, and delivers reiterative operation over a large number of cycles. 
     With regard to FIG. 1, one significant aspect of the invention is the provision of a plurality of stages or sub-modules  31 A- 31 D that form the linear actuator motor  30 . Each sub-module  31  includes a longitudinally extending rod  32 , and end brackets  33  and  34  secured to the lower end and upper end of the rod  32 , respectively. The sub-modules  31  are arranged to translate reciprocally in the longitudinal direction. Note that the brackets  33  and  34  are generally parallel and extend in opposed lateral directions. A SMA wire  36 A extends from the lower bracket  33 A to an anchor point  37 , SMA wires  36 B extends from the lower bracket  33 B of sub-module  31 B to the upper bracket  34 A of sub-module  31 A, and SMA wires  36 C and  36 D join sub-modules B to C, and C to D, to complete a serial chain connection. The SMA wires  36 A- 36 D are fabricated to undergo a phase transition upon heating to a predetermined temperature to contract approximately 4%-8%. The contractile force and excursion of each SMA wire, represented by arrows A-D, is applied between the sub-modules  31 A- 31 D, each pulling on the next adjacent one, whereby the contractile excursion of each SMA wire  36 A- 36 D is combined additively. Thus the sub-module  31 D undergoes the greatest translation when all SMA wires contract, as labeled in FIG. 1 as total displacement (stroke). Indeed, the effective length of SMA wire in the mechanism is substantially equal to the sum of the lengths of all the SMA wires  36 A- 36 D. This effective length is achieved in a compact mechanism, without resort to pulleys or other bending of the SMA wires. 
     The longitudinal rod  32 D may be provided with an extended distal end  38  to facilitate delivering the output of the actuator  30  to operate a mechanism or perform other useful work. The SMA wires may be heated by connecting them in an electrical circuit that directs a current through all the SMA wires for ohmic heating. The circuit may extend from a negative terminal to bracket  33 D, and thence through SMA wire  36 D to the adjacent sub-module  31 C, and so on to a positive connection at anchor point  37 . In this series connection all wires  36  are heated at the same time and, due to the same current passing through all wires  36 , to the same extent. 
     The linear actuator described thus far with respect to FIG. 1 will exhibit a limited useful life (one or a few cycles of contraction and extension), due to the fact that SMA wire will not relax fully when cooled below the phase transition temperature, unless a restoring force is applied in the extension direction. To provide a restoring force, a spring  39  is connected at one end to the bracket  34 D of sub-module  31 D, and the other end is secured to a fixed structural point. The spring  39  is arranged to be extended by outward movement of the bracket  34 D, thus undergoing extension that increases as the wires  36  contract. When the wires are cooled and contract, the spring restoring force applied to the bracket  34 D is applied equally through the linked sub-modules  31  to all the SMA wires  36 . This restoring force aids the SMA wires in returning substantially fully to their original length, thus greatly lengthening the useful life of the mechanism  30 . Preferred embodiments of the spring  39  are described in the following specification, although standard forms of coil, leaf, or elastomer springs will suffice for a limited useful life of the mechanism  30 . 
     With regard to FIG. 2, the invention may provide a block-like housing  41  for securing the sub-modules  31  in a compact assembly. The housing includes a plurality of passages  42  extending therethrough in generally parallel arrangement to permit the longitudinal rods  32 A- 32 D to extend therethrough. Likewise, a plurality of passages  43  extend parallel and interspersed with the passages  42 , to receive the SMA wires  36 A- 36 D therethrough. The passages  42  are dimensioned to permit freely translating motion without any significant lateral movement, and the passages  43  are dimensioned to receive the SMA wires with clearance to eliminate contact. The array of passages  42  and  43  is laid out to accept the sub-modules  31 A- 31 D in serial linked fashion, as described above, and this layout may be in a linear arrangement or in a curved plane that contains all the axes of the passages  42 , further foreshortening the outer dimensions of the housing  41 . 
     With regard to FIG. 5, a further embodiment of the invention comprises a linear actuator  51  having an outer shell-like housing  52  defined by front, rear, top, and bottom walls  53 - 56 , respectively, in a trapezoidal configuration, and side walls  57  (only one shown in the cutaway view) extending therebetween to form a closed interior space. A plurality of track elements  58  are supported on both side walls  57  in parallel arrays that define slots extending longitudinally in a parallel, vertically spaced arrangement. A plurality of drive bars  59  are provided, each supported in one of the slots defined by the track elements  58  and received therein in freely translating fashion in their longitudinal direction. The drive bars  59  are disposed in a vertically stacked array, and may extend distally or retract proximally along the slots in which they are supported. 
     A plurality of SMA wires  61  is provided, each extending between and connected to the proximal end of one drive bar  59  and the distal end of the vertically superjacent drive bar. At the top of the vertically stacked array of drive bars, the SMA wire  61  is connected at its distal end to an anchor point  62 . At the bottom of the vertically stacked array the drive bar  59 ′ is provided with an elongated distal end that is aligned with a window  63  in end wall  53 , through which it may extend. The SMA wires  61  may be heated to a temperature above the phase transition temperature to contract the wires  61 . (Electrical wire connections are not shown for simplification of the drawing.) Each drive bar  59  is advanced incrementally, as shown by the arrow at the distal end of each bar  59 , and, since each wire  61  is anchored in the superjacent moving bar, the incremental translation of each bar is applied to the subjacent bar. Consequently, the lowermost bar  59 ′ undergoes the greatest longitudinal translation, extending through the opening  63  to perform useful work. 
     The SMA wires undergo a contraction of approximately 4%-8%. In the embodiment of FIG. 5, the configuration of the SMA wires determines that the contractile force is exerted substantially along the longitudinal directions of the drive bars  59 , and that the angle of the force vector does not change appreciably between the contracted and extended states of the wires  61 . 
     A spring assembly  64  is disposed below the lowermost drive bar  59 ′, and is attached thereto to apply a restoring force to bar  59 ′ and thus to all the SMA wires  61 . The spring assembly  64  comprises a rolamite spring, known in the prior art and described fully in Sandia Laboratory Report no. SC-RR-67-656, and available from the Clearinghouse for Federal Scientific and Technical Information of the National Bureau of Standards. Briefly, the spring consists of a pair of rollers  65  retained within chamber  66 , and a band spring  67  that is passed about both of the rollers  65  in an S configuration. The band spring  67  includes a tongue  68  extending therefrom through opening  69  and secured to the drive bar  59 ′. The rolamite spring tongue exerts a specified, engineered restoring force on the bar  59 ′ to assure that all the SMA wires  61  return to their fully extended disposition when the wires  61  are cooled below their shape memory transition temperature. 
     As shown in greater detail in FIGS. 3 and 4, the band spring  67  preferably is provided with an internal cutout  71  in an extended U configuration to define the longitudinally extending tongue  68 . The chamber  66  is defined by upper and lower walls  72  and  73 , respectively, to constrain vertical movement of the rollers. Side walls  74  (only one shown) join the upper and lower walls, and constrain lateral movement of the rollers  65 , so that the rollers  65  may move only longitudinally in the chamber  66 . The band spring  67  is secured at a proximal end to the inner surface of the lower wall  72 , and is passed about the two rollers  65  in an S configuration, as evident in FIG.  3 . The distal end of the band spring  67  is secured to the inner surface of the upper wall  73 , and the tongue  68  diverges from the S configuration to extend through the window  69  to join the drive bar  59 ′. As the tongue  68  extends from the opening  69  it pulls the band spring  67  distally, causing the rollers to roll on their respective portions of the band spring as they translate distally. The spring return force exerted on the tongue  68  is directly related to the difference between the energy liberated as portions of the band unbend versus the energy required to bend other portions of the band when the two rollers translate longitudinally. By selectively varying the width of the band spring  67 , or selectively varying the width of the cutout  71 , it is readily possible to generate a spring return force that follows a predictable mathematical function. 
     As depicted graphically in FIG. 7, a typical prior art helical spring or leaf spring develops a restoring force F that varies generally linearly with displacement x, or, F=−kx. For a rolamite spring, the function that relates spring return force with displacement may differ significantly from a typical coil spring or leaf spring. In particular, for restoring the SMA linear actuator mechanisms described herein, it has been found that the optimal force for restoring the SMA wires to full extension is one having a negative force constant; i.e., the restoring force decreases as extension of the spring increases. This force characteristic preserves the shape memory effect to the maximum extent, and results in a useful working life (in terms of total number of cycles of operation) in the same range as typical prior art linear actuators. 
     In other words, the slope of the graph representing the spring function exhibits a negative slope in at least a portion of the spring excursion. If the negative slope is constant, the graph will be linear and parallel to line A of FIG.  7 . The negative slope may change at different spring sections, producing a graph B comprised of several contiguous linear segments. Or the negative slope may vary continuously, producing a smoothly curved graph of the spring function, as represented by graphs C and D. (The band spring may also be fashioned to define positive slope areas, discontinuous spring functions, detent and dwell portions, neutral spring force, and the like, as required to provide these desired mechanical functions.) 
     It should be noted that the contractile force of the SMA wire phase transition is substantially constant as contraction takes place. As a result, the force delivered by the linear actuators described herein is substantially constant throughout the outward excursion of the actuator. This desirable characteristic is in marked contrast to typical solenoid actuators, which produce maximum force at initial actuation and taper off significantly as translation progresses. 
     With regard to FIG. 6, a further embodiment of a return spring having a having a negative force constant; i.e., the restoring force decreases as extension of the spring increases. A bar or similar moving element  76  is disposed in a channel  77  and is constrained to translate longitudinally therein, as shown by arrow L. The element  76  includes a side surface  78  defined by contiguous surface portions  78 A- 78 C that comprise a camming surface. A cam follower  79  is comprised of a telescoping mounting for a roller and a spring for urging the roller to engage the camming surfaces  78 A- 78 C. The roller is mounted to roll along the camming surfaces as they translate along the channel in the longitudinal direction. On an opposed side of the element  76 , a rectangular cutout portion  81  defines a linear, longitudinal surface  82  engaged by a cam follower  79 ′. The cam follower  79 ′ is provided to apply a lateral force to the element  76  to counterbalance the lateral force imparted by cam follower  79 , so that the element  76  will avoid becoming jammed in the channel  77 . 
     It may be appreciated that the resilient force impinging cam follower  79  into camming surfaces  78  is resolved by classical mechanics techniques into vector forces exerted longitudinally and laterally on the element  76 . The lateral forces are offset by the follower  79 ′ and the channel constraints, so that the longitudinal force component urges the element  76  to translate longitudinally, thereby constituting a restoring force. For example, as the element  76  translates distally (to the right in FIG.  6 ), the cam follower  79  encounters the steeply angled cam surface portion  78 B, and exerts a strong, substantially constant longitudinal restoring force. When the cam follower  79  progresses and impinges on the camming surface portion  78 A, the restoring force is decreased to a lower constant due to the shallower slope. (The surface  78  may comprise any number of segments, curves, or other features.) As the element translates proximally under the urging of the cam follower  79 , the portion  78 C acts as a stop to prevent further proximal translation. The spring assembly is capable of generating any desired restoring force function. 
     With regard to FIGS. 8-13, a further embodiment of the linear actuator of the present invention includes a housing  91  having a generally rectangular exterior and defining a rectangular interior space  94  extending longitudinally therein. A bottom plate  93  and a top plate  92  close the opposed ends of the space  94 , and the output plunger  95  of the actuator extends longitudinally through the central hole  97  of the top plate. Within the space  94  a matrix of drive rods  96  is disposed in closely packed array, the dimensions of the space  94  and the close spacing of the rods  96  constraining the rods  96  to be translatable only in the longitudinal direction. The rods  96  are formed as rectangular parallelepipeds, with each longitudinally extending rectangular surface of each rod being adapted to receive and secure one SMA wire, as detailed below. This construction enables any two rods  96  in the matrix to be connected together, end to opposite end, whether they are laterally or vertically adjacent (as viewed in FIG.  11 . The matrix also includes a spring housing  98  occupying the space of one drive rod  96 , as shown in FIGS. 11 and 12, and enclosing any form of return spring described herein. The drive rod  96 G at the center of the matrix supports the output plunger  95 , and is connected to the spring within the housing  98 , so that the spring applies a restoring force to all the SMA wires sufficient to restore the wires to their original length when cooled. 
     Drive rod  96 A may be connected at its lower end to an SMA wire that is connected at its upper end to the housing  91 . The upper end of rod  96 A is connected to an SMA wire that extends to the lower end of rod  96 B. Likewise, rod  96 B is connected to rod  96 C, and so forth to rods  96 D- 96 G, which supports the output plunger  95 . When all the SMA wires are actuated, the drive bars  96 A- 96 G extend in additive fashion, as shown in FIG. 13, to push the plunger  95  longitudinally with a strong, constant force. Although the array of drive bars  96  is depicted as a [3×3] matrix, the arrangement may take the general form of any [M×N] array. 
     With regard to FIG. 14, the drive bars  96  include at least one of the longitudinally extending channels  101 - 104 , each disposed in one of the four longitudinally extending rectangular faces of the parallelepiped configuration. Each channel  101 - 104  is dimensioned to receive and secure one SMA wire  106 . The wire  106  is provided with a mounting die  107  crimped to each end thereof, and a retaining pin  108  extends across the end of the channel to pinch the die  107  between the pin  108  and the sloped bottom surface at the end of each channel. The opposed ends of each pin  108  are secured in a passageway  109  extending from opposed sides of the bar  96  and intersecting the channel  101 . The provision of the channels  101 - 104  on each face of the bar  96  enables the connection of any bar  96  to any adjacent bar  96 , whether vertically stacked or laterally adjacent. Each channel  101 - 104  may be prepared as described with reference to channel  101  to effect interconnection of the adjacent bars  96 . The channels  101 - 104  enable the wires  106  to extend between the opposite ends of adjacent impinging bars  96  without any contact or mechanical interference imparted to the wires by the bars. 
     The crimped die  107  is formed of a conductive metal, and the engagement of the pin  109  enables electrical connection to the wires  106  by the simple expedient of securing the connecting wires to the outer ends of the pins  109 . 
     With regard to FIG. 15, a further embodiment of the linear actuator of the invention makes use of a drive bar  96  as shown and described with reference to FIG.  14 . In this embodiment the bars  96  are provided with top and bottom channels  102  and  103 , and are vertically stacked to be linked in serial, additive fashion as described previously. The vertical stacks (two shown, but any number is possible) are supported by side panels  111  and  112 , the side panels supporting at least one circuit board  113  that controls the application of current to the SMA wires of the vertical arrays. Conductors  114  extend from each circuit board to the mounting pins  108  of the adjacent drive bar vertical stack to complete circuits through the SMA wires. Alternatively, the circuit board may provide brush contacts that engage sliding contact pads placed on the drive bars  96 . In this embodiment the topmost drive bar undergoes the additive translation of all the subjacent bars, as described previously. 
     A further embodiment of the return spring  39  is shown in FIG. 22 with reference to the embodiment depicted in FIG.  15 . However, this spring construction may be employed with any of the linear actuator embodiments described herein. Drive bar  96 ′ at the upper end of the vertically stacked array of drive bars  96  undergoes the maximum longitudinal displacement, and operates the output plunger (not shown) of the array. A base plate  121  joins the side panels  111  and  112  below the array of drive bars. A deflection pin  122  extends laterally outwardly from drive bar  96 ′, and an elastically deformable beam  123  extends upwardly from the base plate  121  adjacent to the vertically stacked array, with the upper end of the beam disposed to impinge on the deflection pin  122  when the actuator is retracted. When the SMA wires are heated and contract, the longitudinal translation of bar  96 ′ drives the deflection pin  122  to bend the beam  123  elastically, thereby exerting a restoring force on the bar  96 ′ and on the array of drive bars connected thereto. The beam  123  may be shaped with a non-uniform cross-section, or provided with other aspects that provide a return force function that approximates the spring functions A-D of FIG. 7 sufficiently closely to provide full return of the SMA wires to their elongated state, and also a high number of repetition cycles. 
     With reference to FIGS. 16 and 17, a further embodiment of the linear actuator of the invention includes a plurality of drive module  126 , each comprising a tubular member of rectangular cross-section, although circular and polygonal cross-sections are equally usable. The drive modules  126  are dimensioned to be disposed in concentric, nested fashion with sufficient clearance for telescoping translation therebetween. Each drive module  126  includes a plurality of longitudinally extending projections  127 , each projection  127  extending from a medial end portion of one side of the respective drive module  126 , as shown in FIG.  17 . (For a cylindrical tubular array, the projections are spaced at equal angles about the periphery of the end of each drive module.) 
     Each side of each drive module  126  is provided with longitudinally extending channels  101  and  103  on the outer and inner surfaces, respectively, the channels being constructed as described with reference to FIG.  14 . Each projection  127  supports a mounting pin  108  received in aligned holes  109  to retain the crimped die  107  of an SMA wire  106 , as described previously. The inner channel  103  provides clearance for the SMA wire of the nested drive module disposed concentrically within. The number of SMA wires used may vary; in the embodiment shown in FIG. 17, at least two SMA wires  106  are used at radially opposed sides of the nested modules to provide balanced contractile forces that resist binding of the telescoping elements. Four SMA wires per module may be used, one secured to each projection  127 , to provide maximum contractile force and maximum force to the actuating plunger. A return spring assembly, of any construction discussed herein, may be placed within the inner cavity of the innermost concentric element  126  and connected between the innermost and outermost elements  126 . 
     The SMA wires  106  of any contractile array described herein may be connected for ohmic heating by any of the circuit arrangements depicted in FIGS. 18-21. In these Figures, each drive element  140  may represent any of the drive bars or drive modules  32 ,  59 ,  96 , or  126  described previously. Single SMA wires  141  are connected at like ends of the elements  140  by extendable wires (or sliding brush contacts)  146  to form a continuous series circuit that includes all of the SMA wires  141 . The moving end of the series circuit is connected to lead wire  143  and the other end, which is fixed in anchor point  142 , is connected to lead  144  of the current source that actuates the array. This circuit arrangement assures that all wires carry the same current. 
     With regard to FIG. 19, a pair of SMA wires  141  are extended between each pair of drive elements  140 , thereby multiplying the force output. This arrangement is depicted in the embodiments of FIGS. 15-17 and  22 , although all embodiments may support multi-wire arrangements. The paired SMA wires are electrically isolated each from the other, and extendable wires  147  (or sliding brush contacts) are secured to the like ends of the paired SMA wires, so that each pair of SMA wires is connected in series. The series pairs are likewise connected in a series electrical circuit by extendable wires  148 , with lead wires  143  and  144  extending from the same end of the array. (It may be appreciated that the number of wires extending between adjacent drive stages may be any integer number other than two.) This arrangement provides multiplied force output using a series circuit to actuate the wires. 
     With regard to FIG. 20, a further embodiment of the electrical actuating circuit of the invention includes paired SMA wires  141  extending between adjacent drive elements  140 , the paired wires being electrically isolated each from the other. Extendable wires  151  interconnect the SMA wires so that each SMA wire of each pair is connected in series with one of the SMA wires of the adjacent drive element. Thus the circuit is comprised of two series branches that extend from the anchor point  142  to the proximal end of the output drive element  140 , where they are bridged by connection  152 . This connection arrangement provides multiplied output force and, most notably, both leads  143  and  144  from the power circuit are connected at the fixed anchor point  142 , so that the leads are not connected to a moving object. 
     Another embodiment of the actuating circuit, depicted in FIG. 21, also makes use of paired SMA wires  141  extending between adjacent drive elements  140 . In this arrangement each pair of SMA wires is connected in parallel, and the paralleled wires are connected by extendable wires  154  in a series circuit. Lead  144  connects to the anchor point of the array, and lead  143  is connected at the proximal end of the output drive element  140 . This circuit arrangement provides the multiplied force output from a current draw that is double that of the previous embodiments. 
     Previous embodiments, such as those shown in FIGS. 15 and 22, depict electrical power connections from the circuit board to each drive bar assembly. This feature permits any of the connection schemes described above, and also permits direct connection to each SMA wire for individual actuation thereof. Thus actuation of the SMA wires may be carried out simultaneously, or staged sequentially in individual or grouped actuations. 
     It is noted that there is a direct correlation between the diameter of the SMA wires and the recovery (relaxation) time of the mechanisms described herein. That is, finer wire yields shorter recovery times. Multiple fine wires between adjacent drive elements may be more advantageous (in terms of actuation and recovery times) than a single, heavier gauge SMA wire, while producing approximately the same thrust. All embodiments of the invention have the explicit or implicit capability to use multiple SMA wires between adjacent stages of the mechanism. 
     In the embodiments of the linear actuator described herein in which the drive elements are enclosed in a housing, the housing may be filled with a liquid such as oil, ethylene glycol anti-freeze, or similar liquid that is lubricious and heat conducting. Such fluid enhances the speed of cooling of the SMA wires by a factor of one or two orders of magnitude, thereby increasing the rate of contraction of the SMA wires and enabling a far faster actuation and cycle rate for the assemblies. The extension and retraction of the drive elements aids in circulating the fluid for cooling purposes. The fluid may be pumped through the housing for maximum cooling effect in high duty cycle situations. 
     Although the invention is described with reference to the shape memory component comprising a wire formed of Nitinol, it is intended to encompass any shape memory material in any form that is consonant with the structural and functional concepts of the invention. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching without deviating from the spirit and the scope of the invention. The embodiments described are selected to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as suited to the particular purpose contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.