Abstract:
Shape memory alloy (SMA) actuating elements are commonly simpler and of lower mass than alternative actuator designs and may find particular application in the transportation industry. Such SMA-powered devices are usually reliable and long-lived but the phase transformations which occur in the SMA alloy and are responsible for its utility are not totally reversible. This irreversibility, a consequence of irrecoverable strain, may progressively degrade the long-term actuator performance as the irrecoverable strain accumulates over many operating cycles. Methods and devices for compensating for these effects and extending the useful cycle life of SMA actuators are described.

Description:
TECHNICAL FIELD 
     Many mechanical devices incorporate mechanisms designed to repeatedly cycle between a first configuration and a second configuration. One means of accomplishing these actions is to use mechanisms which incorporate an elongated shape memory alloy element, such as a wire, as the mechanical actuating device. But such wires or other elements may undergo undesired extension and reduced performance under extensive use. This invention pertains to methods of automatically restoring device performance by compensating for any in-use increase in the length of an elongated shape memory alloy member. 
     BACKGROUND OF THE INVENTION 
     Many mechanical devices may adopt one of two substantially fixed configurations and are repeatedly cycled between two operating positions. These positions may be designated as off/on or open/closed or extended/retracted or engaged/disengaged or some similar terminology. Exemplary automotive examples of such devices include a retractable air dam, latches and clutches among others. Other mechanical devices may adopt, in addition to ‘open’ and ‘closed’ positions a range of positions intermediate between these limits. Exemplary automotive examples of such devices are a set of louvers for controlled passage of air which may be ‘closed’, ‘open’ 50% open, 85% open etc., a fluid flow valve, rearview or side mirrors or visors or shades. Both types of devices may be operated using mechanisms incorporating an elongated shape memory alloy (SMA) actuator. 
     SMA actuated devices find particular application in vehicles where their low mass coupled with their reliability and relative simplicity makes them attractive replacements for electromechanical devices such as electric solenoids or motors. This is particularly so when the stroke or range of operation of the device is limited. 
     SMA actuators rely for their operation on the useful property of SMA alloys that they may forcibly shrink or shorten in length when heated. The force generated by such SMAs is significant and may be powerful enough to operate a device even when some mechanical or other resistance is encountered. 
     A device may incorporate various components and mechanisms to achieve a desired range of motions but the key components of the SMA actuator are a preselected length of SMA alloy arranged in series with a biasing element which creates a biasing force to reset the device and prepares it for re-use. A common form of a biasing element is a spring but other approaches such as dead weights or hydraulic cylinders, among others, may be used. The SMA may suitably be in the form of wire, tape, chain, cable, braid or any other elongated form of SMA capable of sustaining a tensile load. The spring is commonly attached at one end to a support and at its other end to one end of the SMA wire, with the second end of the SMA wire attached to a second support. The workpiece or component to be moved is positioned between the wire and the spring. Mechanical stops may be employed to enforce only an intended range of motion. 
     SMAs derive their useful properties from their ability to exist in two crystalline phases, a first, lower modulus, phase stable at lower temperatures, and a higher modulus, higher temperature phase of a different crystal structure. The transition from one phase to the other may, by appropriate choice of alloy system, alloy composition, heat treatment or applied stress, be selected to occur over a temperature span of from −100° C. up to about +150° C. or so. But, many useful SMA alloys exist in their lower temperature or martensite phase, at, or slightly above, about 25° C. or so, and transform to their higher temperature, or austenite, phase at temperatures ranging from about 60°-80° C. or so. These characteristics substantially assure that the SMA will be in its martensite phase at ambient temperature but may be readily transformed to its austenite phase with only modest heating. 
     SMA actuator wires, or similar, are first shaped, in their austenite phase to the desired form, then cooled to ambient temperature, resulting in their adopting the martensite crystal structure. While in their martensite phase the wire is stretched and deformed to its intended predetermined length. The deformation exceeds the maximum allowable elastic strain which may be imposed on the actuator, and is often termed pseudo-plastic deformation. These pseudo-plastically-deformed martensite wires are in the appropriate starting condition for the actuator. 
     Generally the stretch or strain, that is, the change in length of the wire divided by its original or base length, applied during such pseudo-plastic deformation does not exceed 7% and more commonly may be 5% or less. Importantly, the base length, to which all length changes are referred, is the length of the wire in its high temperature, austenite phase. 
     After being suitably deformed in their martensite phase, the SMA wires may, when heated and transformed to austenite, spontaneously revert to their original undeformed shape. In changing shape, the wire will contract by an amount substantially equal to the pseudo-plastic strain previously applied when it was in its martensite phase. So, by suitable choice of wire length, any desired displacement may be achieved. As an example, a 10 inch or so length of wire, prestrained to about 5% strain, may enable a total displacement of about one-half inch or so. The force applied during heating may be increased by increasing the wire diameter, or, more commonly, to facilitate prompt cooling of the device, by arranging multiple smaller diameter wires in parallel. 
     Actuator action may be reversed by stopping heating and allowing the wire to cool to about ambient temperature and revert to its martensite crystal structure. During cooling the SMA wire will not spontaneously change its length to its initial deformed length but, in its martensite phase, it may be readily stretched again to its initial predetermined length. The spring, or other biasing element, in series with the SMA wire is selected to deform the SMA when the SMA is in its less strong martensite phase. So, on cooling, when the austenite wire reverts to its martensite phase it is stretched, by the spring, to its initial length so that the cycle may be repeated. Provided the transition in crystal structure is fully reversible this cycle of extending and contracting the wire by application of suitable stimulus may continue indefinitely. 
     In practice however, the phase transitions and the accompanying cyclic length are not completely reversible, and some irrecoverable deformation occurs. These cycle-by-cycle irreversibilities accumulate over repeated cycles to permanently extend the wire. This permanent extension of the wire introduces slack into the initially-taut wire and both reduces the stroke obtainable from a device and renders its operation non-linear. These effects may be sufficient to impair the operation of the device or render it ineffective. In such a circumstance the device may need to be replaced. 
     There is therefore a need to mitigate the effects of irreversible transformation of SMA actuators in devices to extend the useful cycle life of such devices. 
     SUMMARY OF THE INVENTION 
     This invention is directed to mechanical tensioner devices for attachment to the SMA wire and adapted to automatically sense and remove the effects of any irrecoverable deformation which has occurred in the SMA wire. The irrecoverable deformation which results from some combination of irreversible deformation processes will extend the length of the SMA wire and develop slack in an initially-tensioned wire. In operation the tensioner will readily displace the SMA wire in a direction suitable for re-tensioning the SMA wire while opposing any motion in the opposite or reverse direction which would introduce yet further slack in the SMA wire. The tensioner may be capable of continuous motion or of motion in only discrete increments. Thus slack may be removed substantially continuously or allowed to first accumulate to some preset amount and only then be removed. The tensioner may be powered by any suitable power source including, for example an electric motor, but power is preferably provided by a stored energy device, for example, a spring in one its many variants. 
     The spring may apply tension to the SMA and may be attached directly to the SMA wire, or more typically to a sleeve, crimped or otherwise permanently secured to the SMA wire. In some embodiments, the tensioner may be a two piece device. In these embodiments, one piece of the tensioner is attached to the SMA wire on one end and to the spring on an opposing end. The second piece is attached to a support. The spring will displace the two tensioner pieces relative to one another while simultaneously tensioning the SMA. The strength of the tensioner spring should be chosen to freely move the tensioner pieces and modestly tension the SMA wire but should not be capable of excessively deforming the SMA wire. In such two-piece embodiments the first and second tensioner pieces are constructed and arranged to offer minimal resistance to motion under the urging of the spring but oppose motion in the opposing direction. Thus, once tensioned, the SMA wire will retain the applied tension until further irrecoverable deformation accumulates after additional use. 
     In a first embodiment the first and second tensioner pieces are like-shaped racks with asymmetric teeth. The racks are arranged in opposition, vertically and horizontally so that their teeth remain in contact and engaged. When moved in a first direction the rack teeth will engage and interfere and when moved in the opposing direction the tooth ramps of each rack will slide over one another to permit relative motion of the racks. Either rack may be positioned between, and connected at its ends, to the SMA wire and a spring. The remaining rack is permanently attached to a suitable support. 
     In a second embodiment, the first and second tensioner pieces are a toothed rack and a ratchet adapted to engage the rack. The rack may be attached to a support and the ratchet may be incorporated into a holder or frame which is attached, on one end, to the SMA wire and, on the other end to a spring. The ratchet may pivot about a pin in a manner which permits it to retract into its holder or frame such that it will not engage with the rack. Thus, when moved in a first direction, the tooth ramp of the rack will urge the ratchet to rotate about the pin and into a configuration which will avoid engagement with the teeth of the rack. Under relative motion in the opposite direction the rack, unable to rotate, will remain deployed and engage the rack and prevent further motion. 
     In a third embodiment, a hold open washer may be used to maintain tension in the SMA wire. By attaching the SMA wire on a tab, or outwardly-extending angled section, of the generally annular washer, a moment is created whenever the SMA wire applies tension to the washer. Such tension causes the washer, whose inner annular surface is in sliding engagement with a post, to tilt or pivot and enable the washer to engage the post. When tension is relieved, the washer rotates out of engagement with the post and may be displaced and slid along the post by a spring, to once again tension the SMA wire, rotate the washer, and secure the SMA wire in its tensioned state. 
     In a fourth embodiment a spring is attached to the end of the SMA wire and the wire is secured between a pair of releasable grippers. In this embodiment only a portion of the wire length, the portion between the grippers and the workpiece, is active in operating the device. When the grippers are released, the spring may re-tension the entire length of SMA wire so that when the grippers re-engage or re-grip the wire, the active length of the wire is similarly re-tensioned. In this embodiment, unlike those previously described, re-tensioning does not inherently occur during cycling of the SMA actuator. Rather re-tensioning is a process divorced from the normal operation of the actuator and requiring the additional operations of disengaging and re-engaging the grippers from the wire. Thus re-tensioning must be programmed or scheduled, but may be conducted as frequently as required, including up to after every cycle. 
     In a fifth embodiment one end of the wire is secured in a threaded plug which engages a threaded socket secured to a fixed support permitting rotational motion while preventing motion in other directions. By rotating the socket using a helical coil spring the socket may be advanced into the plug to tension the wire. 
     A further embodiment uses motors and brakes, to tension an SMA wire supported on pulleys, and further, provides a spool for storage of additional wire. This embodiment further enables refreshing the complete length of SMA wire with new and unused wire if re-tensioning of the SMA wire is so frequent and extensive that it exhausts the capabilities of the wire. 
     Other objects and advantages of practices of the invention will be apparent from the following descriptions of illustrative embodiments of the invention. In some of these descriptions reference is made to drawing figures which are described in the following section of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show, in section, an SMA device for deployment of a vehicle air dam. 
         FIGS. 2A and 2B  show, in top view, a louver device for controlled flow of air. In  FIG. 2A  the device is in its undeployed or ‘closed’ configuration and serves to effectively block passage of air through the louvers. In  FIG. 2B  the louvers are rotated to their ‘open’ position enabling generally free passage of air. 
         FIGS. 3A-D  illustrate an operating cycle for a representative SMA wire actuator. 
         FIG. 4  shows the SMA wire geometry of the actuator shown in  FIGS. 3A-D  after repeated cycles. 
         FIG. 5A  shows the SMA actuator of  FIG. 4  further incorporating a tensioner embodiment shown in  FIG. 5B  to illustrate the tensioner location relative to the other components of the actuator.  FIGS. 5B-D  illustrate alternate embodiments of the invention incorporating ratchets for attaching the SMA wire to the support  74  and removing the slack developed in the SMA wire. 
         FIG. 6  shows another embodiment for removing the slack from an SMA wire incorporating a hold open washer. 
         FIG. 7  shows yet another embodiment for removing slack from an SMA wire incorporating a selectively releasable clamp which may act on either the SMA wire or a crimp or similar structure rigidly attached to the SMA wire. 
         FIG. 8  shows a further embodiment for removing the slack from an SMA wire. This embodiment employs pulleys and selectively releasable clamps to grip the wire. Provision is also made for feeding wire from a spool to completely replace wire which is incapable of providing the intended design displacement. 
         FIG. 9  shows another embodiment for removing slack employing a threaded SMA wire crimp or similar structure. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following description of the embodiment(s) is merely exemplary in nature and is not intended to limit the invention, its application, or uses. 
     Automobiles may use shape memory alloy-actuated, or -powered, devices to operate devices which require a limited range of linear or rotary motion. Such devices are relatively simple and of low mass making them attractive substitutes for fractional horsepower electric motors or similar electromechanical devices. The devices take advantage of the ability of such shape memory alloys to repeatedly cycle between two extreme positions as, for example in a retractable air dam, a latch and a clutch, or to repeatedly cycle over a range of positions intermediate between predetermined limits, as for example in an adjustable louver array, a rearview or side mirror or a sun visor. 
     Shape memory alloys (SMAs) are particular alloys which undergo substantially reversible transformation between two crystal phases—a low temperature phase known as martensite and a high temperature phase known as austenite. The particular phase transformation temperature varies with alloy system, but generally ranges from between about −100° C. to about +150° C. or so. Shape memory behavior has been observed in a large number of alloy systems including Ni—Ti, Cu—Zn—Al, Cu—Al—Ni, Ti—Nb, Au—Cu—Zn, Cu—Zn—Sn, Cu—Zn—Si, Ag—Cd Cu—Sn, Cu—Zn—Ga, Ni—Al, Fe—Pt, Ti—Pd—Ni, Fe—Mn—Si, Au—Zd, and Cu—Zn but only a few of these alloys are commercially available. Nitinol, an alloy of nickel and titanium in near-equiatomic proportion, enjoys the widest use. 
     In many applications the SMA is preformed into a wire or similar elongated form such as a tape, chain, cable and braid among others, but, for convenience, only the term wire will be used in subsequent sections. Associated with the change in crystal structure experienced by SMAs is a change in shape, most obviously manifested as a change in the length of the wire. The magnitude of this change in length is characteristic of the specific alloy system and may range up to about 7% or so in some systems such as the Ni—Ti system. As the SMA element seeks to change its length it may apply appreciable force to overcome any mechanical drag or opposition. With appropriate design, mechanical devices may be fabricated to harness and utilize the force resulting from transformation to operate or actuate mechanisms or similar mechanical devices. 
       FIGS. 1A and 1B  show a representative example of a linear mechanical device representative of one automotive application and intended to cycle between fixed end-points, an SMA-deployed retractable air dam. Air dams are generally fitted beneath the front bumper of an automobile and extend into the under-vehicle airflow. Air dams may improve the handling and control of the motor vehicle, increase fuel economy, and also improve the routing of air flow for cooling/heat exchange in the vehicle engine compartment. The effectiveness of air dams is greatest at higher vehicle speeds and when they extend almost to the roadway but this configuration renders them most vulnerable to impact with roadway obstacles even at low speeds. Thus the geometry of fixed air dams necessarily represents a compromise between aerodynamic effectiveness and avoiding collision of the air dam with obstacles or road hazards. A better compromise may be made by using a retractable air dam. Such a retractable air dam offers the opportunity of deploying the air dam only at high speed and retracting the air dam at low speeds to minimize the likelihood of a damaging air dam collision when the air dam is least effective. 
     In the sectional view of  FIG. 1A  the air dam system  10  includes air dam  22 , housing  12  and an SMA actuation system comprising SMA wire  30 . The air dam is shown in its retracted position located out of airflow  38 . Air dam  22  is generally L-shaped with a longer portion  24  intended for insertion into air flow  38 . The shorter section  26  of air dam  22  has opposing surfaces  23 ,  25  and is mechanically attached to the SMA wire and responsive to its movement. Surface  23  is secured to an end of SMA wire  30  and surface  25  to an end of tension spring  28 . The opposing end of spring  28  is attached to the underside of cover  14 . SMA wire  30  is secured at its other end at mount  32  and routed around pulleys  34  to enable a more compact device. The device is contained within housing  12  formed of opposing, generally vertical walls  18 , generally horizontal cover  14  and opposing closure  16 . Closure  16  includes a slotted opening  20  with compliant flap seals  21  which sealingly engage to deny access of road splash and debris to housing  12  when air dam  22  is retracted. 
     In  FIG. 1B , SMA wire  30  has been actuated, preferably by utilizing the mechanical connectors on the wire ends as electrical connectors and passing an electric current along the length of the wire (details not shown). Wire  30 , prior to heating of the wire by the electric current, or other applicable means, was in its low temperature, lower strength martensite state. When wire  30  is in its low modulus, martensite state, it may be deformed and elongated by spring  28 , retracting air dam  22  into housing  12  as shown in  FIG. 1A . Heating SMA wire  30  results in its transformation into its higher strength austenite state, shown as wire  30 ′ in  FIG. 1B . Associated with its transformation to higher strength austenite, wire  30  seeks to shrink to a length appropriate to austenite wire  30 ′. As it does so it applies sufficient force to overcome the force exerted by spring  28 , causing spring  28  to extend and forcing portion  24  of air dam  22  through opening  20  where it extends into airflow  38 . Seals  21  are deflected into contact with the opposing surfaces of portion  24  of the air dam, so that they may continue to exclude moisture and debris from housing  12 . 
     On cessation of heating, wire  30 ′ will cool and transform to martensite wire  30 . The lower strength martensite wire  30  may be deformed by spring  28 , enabling spring  28  to contract and, because of its connection to surface  25  of air dam  22 , retract air dam  22  within housing  12  as shown at  FIG. 1A . 
     In this application, the SMA actuator mechanism is intended to operate in fixed displacement mode so that the air dam will extend beyond the housing by some predetermined extension. This, relatively simple, operating scheme may be implemented, for example, by progressively ramping the applied current passed through the wire until the design displacement is achieved and then applying only a terminal current to maintain the desired deployment. Similar mechanisms may be used to operate other devices including latches and clutches. 
     An alternative application of an SMA actuator is shown in  FIGS. 2A and 2B . In this application, a plurality of louvers  42 , arranged for control of airflow, are connected to a common bar  58  by linkages  48  at pivots  46 . In turn, bell crank  56  is also connected to bar  58  and SMA wire  52  and spring  54  arranged in opposition on its respective arms. In  FIG. 2A  the louvers are depicted in their closed or ‘airflow blocking’ configuration. SMA wire  52  is in its martensite phase and is stretched to a predetermined extent. In the configuration shown it may be noted that abutting louvers interfere with one another when in the closed position and so limit or control the extent of stretch. For other louver geometries, where such interference may not occur, a separate stop (not shown) may be used to limit stretch. In  FIG. 2A  SMA actuator  50  is shown with SMA wire  52  in its martensite phase and so capable of being stretched by spring  54 . Under the influence of spring  54 , bar  58  is moved in the direction of arrow  61 , orienting link  48  at approximately a 2 o&#39;clock position and closing the louvers. On heating and actuating SMA wire  52 ′, as shown in  FIG. 2B , bell crank  56  is rotated counterclockwise about pivot  60  displacing rack  58  in the direction of arrow  62  and so rotating link  48  counterclockwise about its pivot  46  to an approximately 10 o&#39;clock position. As link  48  rotates it, in turn drives each of louvers  42  to pivot about respective pivots  44  and orienting the louvers in their ‘open’ position. 
     It will be appreciated that because the transformation from martensite to austenite occurs over a narrow temperature range rather than at a specific temperature, the louver orientation may, under appropriate temperature control, be set to any position intermediate between the ‘closed’ and ‘open’ configurations shown in  FIGS. 2A and 2B  respectively. Other automotive devices which may employ similar position-controllable SMA actuators include rearview and/or side mirrors, visors and shades. 
       FIGS. 3A-D  illustrate a representative or generic SMA actuator mechanism  70 . In  FIG. 3A  SMA wire  80 , attached at one end to fixed support  74  and at a second end to a face of disc  82 , is at a temperature below its transformation temperature and so is substantially entirely in its martensite phase. In this lower strength phase, wire  80  is readily deformed by spring  76  and is extended to a maximum, predetermined length. The length is established by the separation between support  74  and stop  86 , and is enforced by interference of disc  82  with stop  86 . It will be appreciated that even though disc  82  contacts stop  86 , wire  80  is under tension. Disc  82  is rigidly attached to wire  80  at one face and to one end of spring  76  at an opposing face. A second end of spring  76  is attached to fixed support  72 . In  FIG. 3B  SMA wire  80 ′ is being heated, for example by passage of electrical current along its length (electrical circuit and connections not shown) and has partially transformed to the austenite phase, partially extending spring  76 ′. A later stage, corresponding to further transformation and contraction of wire  80 ″ and extension of spring  76 ″ is shown in  FIG. 3C , while the wire in its fully contracted length  80 ′ and the fully extended spring  76 ′″ are shown in  FIG. 3D . 
     When heating is discontinued, wire  80 ′ will cool, typically at a rate dictated by ambient conditions although forced cooling may be employed if rapid cooling is desired. As the wire cools it will transform, over a particular temperature range to martensite, and be deformed by spring  76  so that the actuator will revert to the configuration of  FIG. 3A . With the actuator now in the configuration of  FIG. 3A  the process may be repeated. Ideally such contraction and extension of the SMA wire would be fully reversible so that it may be subjected any number of repeated cycles without incurring any change in properties. 
     In practice, some, almost imperceptible, irrecoverable deformation occurs on every cycle due to some irreversibility of the deformation mechanism(s). These irreversibilities accumulate with continued cycling, promoting a gradual increase in the length of the SMA wire and a gradual reduction the tension in wire  80 , until, after some number of cycles, slack develops in the SMA wire. This is the condition represented in  FIG. 4 . For clarity the extension of wire  180  and its curvature between supports  74  and  82  has been exaggerated. This excess length, or slack, functions as a lost motion device, reducing the actuator stroke or requiring that the wire be heated more aggressively to develop the design stroke. However, more aggressive heating can lead to accelerated accumulation of irreversibilities and an associated accelerated loss of device function or premature material failure. In progressive operation devices, such as the louvers of  FIGS. 2A and 2B , this lost motion at the beginning of the stroke renders the response of the louvers to heating highly non-linear and unpredictable. By re-tensioning wire  180 , the ‘lost’ motion may be recovered and full device functionality maintained with a more limited stroke than was required for the un-tensioned wire. A number of embodiments of suitable tensioner devices are described below. 
       FIG. 5A  shows a first embodiment of the invention, illustrating its placement relative to support  74 , SMA wire  180  and the other SMA elements illustrated in  FIG. 4 . As will become apparent, the illustrated configuration is broadly applicable to many of the embodiments of the invention. In this first embodiment, better shown in  FIG. 5B , wire  180  is crimped or otherwise secured in sleeve  90  which incorporates a rack  101 , complementary to, and arranged in opposition to, rack  98 , which is secured to tension spring  92 . Rack  101  has teeth  100 , shaped and sized to engage rack  98  of post  94 . Teeth  100  are shaped to easily slide along ramp  97  of rack  98  as wire  180  is urged in the direction of arrow  102  by spring  92 , but to engage with steeply sloped edges  99  when tension is applied to the wire. Thus during transformation of wire  180 ′ from martensite to austenite, or during deformation of wire  180  by spring  76  while in its martensite form, a robust connection is established between SMA wire  180  and support  74  through post  94 . 
     But as slack develops in wire  180  the wire may be re-tensioned under the urging of a suitable stored energy device, here shown as spring  92 , which serves to advance tooth  100  along rack  98  in the direction of arrow  102 . Spring  92  serves both to detect the reduced tension and to re-establish tension. To be effective spring  92  must be pre-tensioned and operate as a stored energy device. If, or when, the loss of tension in the wire is minimal, spring  92  will be passive and continue to store energy, but once a predetermined loss of tension occurs, a portion of the stored energy will be expended to re-tension the wire. 
     The stored energy device of  FIGS. 5A and 5B  has been represented as a coil spring under tension. Of course any device, such as an electric motor, capable of moving the different parts of the tensioner relative to one another may serve the same purpose, but a spring-based device is simple and substantially maintenance-free. It will also be appreciated that, under different tensioner configurations, a coil spring in compression, a torsion spring, a leaf spring, a clock spring, a gas spring, a wave spring, or a urethane (or other compliant polymer) compression spring may be equally or more suitable than the depicted tension spring. Also, where only a small displacement of the SMA wire end is required to re-tension the wire, such as on a small-displacement mechanism utilizing only a short length of SMA wire, a suitable spring action may be achieved using one or more spring washers such as a Belleville washer, a wave washer or a clover dome washer. 
     Typically spring  92  should be selected to establish a tension in wire  180  which is generally equal to the initial tension imparted by spring  76 . It will be appreciated that for proper functioning, rack  98  and tooth  100  must remain engaged at all time. Suitably this may be achieved by application of modest pressure P in the direction of arrow  96 . One method of applying such pressure is through leaf spring  106  supported by hollow sleeve  104 . Hollow sleeve  104  allows access to rack  98  and is sized to accommodate crimp  90  while modestly flexing spring  106  which ensures consistent engagement of tooth  100  with rack  98  under all relative positions of tooth and rack. But other approaches to ensuring continued tooth engagement such as spring-loading tooth  100  may also be employed provided sleeve  104  is sized appropriately to the range of tooth motion. 
       FIG. 5C  illustrates a similar ratcheting rack device mounted on a post  194 , here using opposing racks  198 , each with ramp portion  97  and steeply sloped edge  99 . Opposed teeth  200 ,  200 ′ are attached to wire  180  through crimp  190  and intended to selectively engage racks  198  to impart tension to the wire. Opposed teeth  200 ,  200 ′ are arranged to rotate about pivot  201  and to be spring loaded (details not shown) so that, if unopposed, they will return to their extended configuration  200  but if opposed may adopt compact configuration  200 ′. Thus contact between ramp  97  and the complementary surface  297  of tooth  200  will induce tooth  200  to rotate inwardly about pivot  201 . When the rotation is a maximum, tooth  200  will adopt the configuration of tooth  200 ′, shown in ghost, clearing each of opposing racks  198 . Hence, if slack develops, crimp  190  may be moved toward support  74  under the urging of spring  92 , tensioning wire  180 , while continuing to oppose motion in the direction of arrow  108  during operation of the actuator. It will be appreciated that although spring  92  has been depicted as a coil spring other spring geometries such as leaf spring  192  shown in  FIG. 5D  may also be used. More generally, any means of imparting a directed force to crimp  190  to tension wire  180 ′, including a motor, solenoid, hydraulic cylinder or others known to those skilled in the art, may be used without limitation. Also, opposed racks are shown as aligned, that is the ramps and edges of each rack are arranged opposite one another. It will be appreciated that the effective tooth pitch may be reduced by offsetting one rack from the other provided engagement of a single tooth is sufficient to resist the transformation loads. Further, whether or not the racks are aligned or offset, crimp  190  may incorporate multiple teeth  200 ,  200 ′ as required to sustain the operating loads on the interacting rack and teeth. 
       FIG. 6  illustrates a second embodiment of the invention capable of continuously tensioning wire  180 ′. Here, a hold open washer  112  comprising washer section  110  with an opening sized to engage but slide freely on post  116 , and an angled solid extension  114 , is connected to crimp  290  by connector  124 . Washer section  110  of hold open washer  112  is acted upon by compression spring  118  encircling post  116  and supported by stop  120 . When SMA wire  180 ′ is slack, the hold open washer will be oriented as shown in ghost at  112 ′. In this orientation the hold open washer may be urged along post  116  toward support  74 . As it does so, it applies tension to SMA wire  180 ′ connected to angled extension  114 , rotating the hold open washer in the direction of arrow  122  to the inclined configuration shown at  112 . In this configuration, edge  126  of the opening of washer section  110  engages post  116  to resist motion of washer  112  in the direction of arrow  128  in operation of the SMA device. 
       FIG. 7  illustrates another embodiment of the invention. In this embodiment the SMA wire is clamped between two clamps  130 ,  132 . Clamps  130 ,  132  may be clamped and released by reversible motion of one or both of the clamps in the direction of arrow  134  and secured under an applied pressure P. Either or both of clamps  130 ,  132  may have features or protrusions, for example, and without limitation, as shown at  136 , for enhancing the gripping force. The clamp may generate only a frictional restraining force, or mechanically engage the wire, for example by incorporating shaped teeth or like features which may embed themselves into the wire. The clamps may act directly on SMA wire  180 ′, as illustrated, or on a crimp or sleeve surrounding the wire (not shown). Where mechanical engagement is intended, the clamp should engage with a crimp or sleeve, preferably made of a soft and readily deformed metal or alloy, to avoid damaging the SMA wire during the gripping process. Clamps  130 ,  132  are secured to support  140  (not shown) so that they may sustain the loads applied to wire  180 ′ during phase change or during tensioning by spring  76 . The attachment may not interfere with the ability of clamps  130 ,  132  to reversibly move relative to one another to engage and disengage SMA wire  180 ′. In this embodiment, an end of wire  180 ′ is not secured to fixed support  140  but rather wire  180 ′ passes through support  140  at opening  141  so that its end may be secured at moveable support  174 . 
     In operation, the SMA actuator may be exercised until some predetermined slack develops in the SMA wire. Then one or both of clamps  130 ,  132  may be withdrawn to separate the clamps and release pressure on wire  180 ′. With wire  180 ′ now released from clamps  130 ,  132  compression spring  292  acting against fixed support  140  displaces support  174 , attached to wire  180 ′, tensioning wire  180 ′. Clamps  130 ,  132  may then be brought into contact with the wire  180 ′ to secure wire  180 ′ and again render the actuator operable. Although in this embodiment tensioning was facilitated indirectly using compression spring  292  positioned between fixed support  140  and movable support  174 , tensioning may also be accomplished directly using tension springs as in the arrangements of  FIGS. 5B ,  5 C and  5 D. 
       FIG. 8  illustrates yet another embodiment of a tensioner. It will be appreciated that although re-tensioning the SMA wire may be effective in restoring device performance, such re-tensioning may not be continued indefinitely. After some number of operations, even a full and complete transformation of the wire volume to austenite may not generate sufficient displacement for satisfactory device operation. In this circumstance complete wire replacement is the only appropriate remedial action. The tensioner shown in  FIG. 8  is an embodiment which both offers opportunity to tension wire  280  and also to replace active SMA wire length  280  if re-tensioning alone will not be adequate to restore device performance. A length of SMA wire  280  is fed from supply spool  144 , around idler pulleys  149 ,  149 ′ to take-up spool  146 . In operation, wire may be withdrawn from supply spool  144  by rotation of take-up spool  146  in the direction indicated by arrow  152  after first releasing each of brake pads  140 ,  140 ′ by displacing them outwardly in the direction of arrows  142 ,  142 ′ respectively. Supply spool  144  is not driven but is rotated in the direction of arrow  150  by the rotation of take-up spool  146  transmitted by wire  280 . After withdrawal of a suitable wire length the wire may be tensioned in one of two ways. In a first embodiment brake shoe  140 , may be directed inwardly in the direction of arrow  142  to lock wire  280  against pulley  149  while continuing to power take-up spool  146  to rotate it in the direction of arrow  152 . The tension in wire  280  may be determined, for example, by using an electric motor to power spool take-up spool  146  and recording motor current. When the wire tension reaches a predetermined value then brake  140 ′ may be applied to pulley  149 ′ by displacing it inwardly along the direction of arrow  142 ′, locking the tensioned wire  280  between pulleys  149  and  149 ′. In an alternative embodiment SMA wire  80  may be tensioned by powering supply spool  144  in a direction indicated by arrow  154  while simultaneously powering take-up spool  146  in the direction of arrow  152  with both of brakes  140  and  140 ′ released. Because the two spools are acting in opposition wire  280  is tensioned. So when a predetermined tension is achieved, both of brakes  140  and  140 ′ may be applied generally simultaneously to maintain wire tension in the wire segment between pulleys  149  and  149 ′. Analogously to the actuator configuration represented in  FIGS. 3A-D , pulley  149  is secured to support  274 , and pulley  149 ′ is secured to disc  282 . 
     The configuration of  FIG. 8  enables both feeding and tensioning of wire. It will be appreciated that tensioning alone may be accomplished by securing the ends of wire  280  passing around pulleys  149  and  149 ′ to each of pulleys  149  and  149 ′. One or other of pulleys  149  and  149 ′ may be powered, for example using a clock spring, to apply tension to the wire. Tension may be maintained using a pawl (not shown) adapted for engagement with a ratchet (not shown) attached to the pulley and coaxial with the pulley axis. 
       FIG. 9  shows an embodiment for tensioning wire  180 ′ employing a helical spring  392 , shown in ghost and partial cutaway for clarity, encircling a receptacle  402  with a threaded, parallel sided interior cavity  408  extending partially through the receptacle. Interior cavity  408  is sized and shaped to engage threads  410  on the exterior of generally cylindrical crimp  412  which secures SMA wire  180 ′. Receptacle  402  engages support  374  in a manner which prohibits motion in the direction of arrow  414  but enables rotation about axis ‘A’. As shown, this is achieved by forming on the base of receptacle  402  a structure  406  comprising a shaft  407  attached to a surface of a disc  405  and locating structure  406  in a close-fitting cavity of complementary shape  404  in support  374 . The dimensions of structure  406  and cavity  404  should enable rotation of receptacle  402  under the urging of helical spring  392 . 
     Helical spring  392  is secured at one end  418  to support  374  and at a second end  416  to receptacle  402 , so that, when tensioned, spring  392  may rotate receptacle  402 . If threads  410  of crimp  412  engage threaded cavity  408 , then rotation of receptacle  402  will draw crimp  412  into cavity  408  and tension wire  180 ′. In tensioning wire  180 ′ disc  405  will be pulled into contact with a surface of cavity  404 , generating a frictional force which, with appropriate choice of spring tension and thread pitch will enable a predetermined tension in wire  108 ′ without overstressing. It will be appreciated that the particular association of the interior and exterior threads with specific components of the tensioner is illustrative and not limiting and may be reversed from that depicted without prejudice to the operation of the tensioner. 
     A tensioning mechanism may also employ gears. A rack and pinion device with the rack attached to a wire crimp and a driven pinion gear attached to a fixed support would enable tensioning without applying torque or torsional loads to the wire. However such a rack and pinion mechanism would require that power be maintained during operation or require the addition of a brake to avoid backdriving the pinion. A worm gear mechanism, with appropriate choice of gear parameters, may eliminate backdriving even under power-off operation. However some means of decoupling the wire from the driven gear may be necessary to avoid torquing the wire. 
     While preferred embodiments of the invention have been described as illustrations, these illustrations are not intended to limit the scope of the invention.