Abstract:
A method and apparatus for preventing the unintended activation of SMA devices by ambient temperatures that exceed the phase transition temperature of the SMA material itself. In one embodiment a passive actuator is coupled to an active actuator, each having identical arrangements of SMA wire, but connected in opposite directions to compensate for temperature drift that is not due to powered heating. A second embodiment consists of a passive SMA wire connected to a latch/release mechanism allowing the actuator itself to move rather than moving the load. In a third embodiment the passive wire is connected to a load coupling, so that the load itself is disconnected from the actuator when the passive wire reaches the phase transition temperature. The passive wire may be made of a lower-temperature wire than the active wires, so that the release action occurs long before the active wire begins to be moved by ambient temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. application Ser. No. 10/056,233, filed Dec. 3, 2001, now U.S. Pat. No. 6,762,515, which in turn is a continuation of application Ser. No. 09/566,446, filed May 8, 2000, now U.S. Pat. No. 6,326,707, issued Dec. 4, 2001, for which priority is claimed. 

   FEDERALLY SPONSORED RESEARCH 
   Not applicable. 
   SEQUENCE LISTING, ETC ON CD 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to linear actuators and, in particular, linear actuators that employ shape memory alloy (SMA) elements to provide their motive power. 
   2. Description of Related Art 
   A new category of linear actuators employing SMA wire was introduced in U.S. Pat. No. 6,326,707, issued Dec. 4, 2001. These linear motors achieve useful displacement with significant force through the use of a Displacement Multiplied SMA mechanism. It is expected that SMA actuators will gain widespread acceptance and use in the near future, due to the fact that they produce much higher output force relative to their weight than current technologies (largely motors and solenoids). Their compact size allows them to fit into much smaller envelopes than existing actuators, solving numerous ‘real-estate’ and engineering issues. SMA actuators are long-lasting, easily performing a hundred thousand cycles. They can be manufactured simply, and in large quantities, inexpensively. 
   Due to the fact that the SMA motive elements (generally wires) are activated by thermal cycling, these devices are inherently sensitive to ambient temperatures, and susceptible to spontaneous actuation when the ambient temperature exceeds the SMA transition temperature. Nitinol wire is available commercially in formulations that have phase transition temperatures of 70° C. (LT) and 90° C. (HT). Thus if the ambient temperature exceeds these phase transition temperatures, the device will actuate inadvertently, with unpredictable and perhaps unfortunate consequences. 
   The specifications for many products and mechanical assemblies have ambient temperature tolerances that may exceed the phase transition temperatures of commonly available shape memory materials. For example, automobile manufacturers have operation and safety margins that most often require survivability, and even operability, in the temperature range of 100° C. to 120° C., which is greater than the transition temperature of Nitinol wires known in the prior art. There is an unmet need in the prior art for SMA actuators that can operate normally (intentionally actuated by powered operation) yet are prevented from operating spontaneously when the ambient temperature exceeds the SMA phase transition temperature. This need exists even if SMA materials are improved to transition at higher temperatures, for there will always be some uses for SMA devices that push the temperature limits of the materials. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention generally comprises a method and apparatus for preventing the accidental activation of SMA devices by ambient temperatures that exceed the phase transition temperature of the SMA material itself. The invention introduces an Over-Temperature Release Device (OTRD). The passive OTRD is generally external to the active actuator, so that it is in good thermal contact with the ambient temperature (the inside of a door panel, for instance). When the ambient temperature exceeds the onset temperature of the powered actuator, the OTRD releases a latch that prevents the displacement multiplied SMA (DM-SMA) from performing the intended work. 
   One important feature of this release mechanism is that the intended work cannot be performed above the normal actuation temperature. In many cases, this is actually a benefit for the intended function, and in other instances, it is an acceptable mode of operation. In addition, the active wire is not harmed or damaged in any way during the temperature excursion, since it also is free to move unimpeded if it does experience similar temperatures as the exposed, passive wire. It is also significant that the OTRD resets, allowing normal operations when the temperature returns to the normal temperature range. 
   In addition to exposing the passive wire to the ambient environment, the active wire can be substantially shielded from the environment for long periods of time. Even simple plastic cases can provide a high degree of thermal insulation, protecting the inside of the case for long time-periods from temperature excursions outside of it. 
   There are three general embodiments of the OTRD temperature compensation devices. All three employ passively heated shape memory alloy wires (bathed in the ambient environment), and actively controlled SMA wire actuators. One embodiment consists of a passive actuator coupled to an active actuator, each having identical amounts of SMA wire, but connected so as to compensate exactly for any temperature drift that is not due to powered heating. That is, one actuator is connected to another in opposing directions, one powered, the other unpowered. A second general embodiment consists of a passive SMA wire that is connected to a latch/release mechanism allowing the actuator itself to move against the force of a return spring when released. In a third general embodiment the passive wire is connected to a load coupling, so that the load itself is disconnected from the actuator when the passive wire reaches the phase transition temperature. 
   In any of these embodiments, the passive wire may be made of a lower-temperature wire than the active wire, so that the release action occurs long before the active wire begins to be moved by ambient temperature. The “LT” wire may also be stressed so that it actuates at a slightly higher, and better-defined, temperature, and so release is further guaranteed. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a cross-sectional side elevation showing a typical prior art SMA linear actuator in the quiescent (retracted) disposition. 
       FIG. 2  is a side elevation as in  FIG. 1 , showing the linear actuator in the activated (extended) disposition. 
       FIG. 3  is a cross-sectional side elevation depicting one embodiment of the present invention in which two SMA linear actuators are coupled in a over-temperature compensation mechanism. 
       FIGS. 4A-4C  are a sequence of views depicting the possible actuation states of the embodiment shown in FIG.  3 . 
       FIG. 5  is a cross-sectional side elevation depicting another embodiment of the present invention in which a passive SMA wire operates an overtemperature release mechanism. 
       FIG. 6  is a side elevation as in  FIG. 5 , showing that embodiment set up to retract upon actuation. 
       FIG. 7  is a cross-sectional side elevation depicting a further embodiment of the invention in which a load coupling is operated by a passive SMA wire to decouple the load from the actuator during overtemperature conditions. 
       FIG. 8  is a side elevation as in  FIG. 7 , showing the load coupling disengaged. 
       FIG. 9  is a side elevation as in  FIGS. 7 and 8 , showing the load coupling disengaged and the actuator in the activated state. 
       FIGS. 10 and 11  are detailed views showing the quiescent state and activated state of the latching mechanism of the embodiments of  FIGS. 7-9 . 
       FIGS. 12-14  are a sequence of cross-sectional side elevations depicting the activation states of another embodiment of the invention in which an actuator housing provides an integral actuator release mechanism during overtemperature conditions. 
       FIG. 15  is a functional block diagram depicting the fundamental elements of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention generally comprises an over-temperature safety mechanism that prevents spontaneous, inadvertent actuation of a SMA actuator due to high ambient temperatures and the like. In this description, the term “safety” as used herein follows the definition “a device designed to prevent a mechanism from being operated unintentionally, for example, one that keeps a gun from being fired by accident or an elevator from falling.” 
   With regard to  FIG. 15 , the fundamental components of the invention include a SMA actuator  101  and a load  102  which is connected to receive useful work from the actuator  101  in the form of translation, rotation, displacement, or the like. A latch  103  is connected between the SMA actuator  101  and the load  102  to selectively disconnect the load from the actuator, or to disconnect the actuator from mechanical ground, or otherwise prevent the actuator from delivering an actuating stroke to the load. The latch is operated by a passive SMA element  104  which is exposed to ambient temperature more so than the actuator  101 , or which has a lower phase transition temperature. That is, the element  104  is designed to be activated by ambient temperatures exceeding its phase transition temperature before the actuator  101  is activated by ambient temperature exceeding the phase transition temperature of the actuator  101 . Thus the load cannot receive work from the actuator  101  if high ambient temperatures cause the actuator  101  to be activated unintentionally. 
   With regard to  FIG. 1 , an exemplary SMA linear actuator  20  known in the art includes a plurality of rods or links  21  disposed in longitudinally aligned, vertically stacked relationship, the link constrained to slide longitudinally with respect to vertically adjacent links by a housing  22 . The bottom link  23  of the stack is mechanically grounded to the case  22 , and the top link  24  is connected to an output rod  26  which is translatable reciprocally along its axis. A plurality of SMA wires  27 , formed of Nitinol™, Flexinol™, or similar phase transition shape memory material, are arranged so that each wire  27  is connected between one end of a link  21  and the opposite end of a vertically adjacent link  21 . When the wires are extended (at a temperature below the phase transition temperature) the links  21  are stacked vertically as shown in FIG.  1 . When the wires are heated and contracted, as shown in  FIG. 2 , the contracted wires cause each link  21  to slide to the right with respect to the subjacent link. Thus the displacement of the links is added, and the cumulative displacement is carried out by the translation of link  24  and associated output rod  26 . The wires may be heated by electric current applied by a control circuit, and the links return to the quiescent condition of  FIG. 1  upon cooling, due either to an Intrinsic Return Mechanism (described in copending U.S. application Ser. No. 10/200,672, filed Jul. 22, 2002), or a return spring, or both. 
   As noted in the discussion above, the SMA wires  27  may be actuated inadvertently by exposure to ambient temperatures exceeding the phase transition temperature of the SMA material. One embodiment of an overtemperature safety mechanism comprises a pair of SMA linear actuator assemblies  20 A and  20 B each formed substantially as shown in  FIG. 1  (housing omitted for clarity). Here actuator  20 B is effectively a combination of elements  103  and  104  of FIG.  15 . The actuators  20 A and  20 B are disposed in vertically stacked relationship, with the lowest link of the lower actuator  20 B mechanically connected to ground, and the lowest link of actuator  20 A being connected to the upper most link of actuator  20 B. It is significant that the actuator  20 B is not powered, and may be actuated only by exposure to excessive ambient temperature; whereas the actuator  20 A is connected to a controlled electrical source to be intentionally and selectively actuated by electrical power. Furthermore, the two actuators are arranged to operate in opposite directions, so that their reactions to exceeding the phase transition temperature occur in opposing directions. 
   Thus, as shown in  FIG. 4A , wherein both actuators  20 A and  20 B have been triggered by excessive ambient temperature, actuator  20 A has extended to the right as indicated by arrow  31 A, an action due merely to the SMA wires contracting after being heated past transition by the ambient temperature. However, the actuator  20 B, which extends in the opposite direction when activated, as shown by arrow  31 B, serves to counteract the extension of rod  26 A by moving retrograde with respect to the motion of rod  26 A. Given that the two actuators are substantially similar in physical configuration, the opposite motions are offsetting, and the net effect is that combined mechanism does not undergo an actuation stroke. Thus an overtemperature condition cannot cause a spontaneous, inadvertent activation of the linear actuator. 
   With regard to  FIGS. 4B and 4C , a further variant of the assembly of  FIG. 4A  provides actuators  20 A and  20 B as described previously, with the added factor being that the actuator  20 B is selectively powered in the same manner as actuator  20 A. Thus when both actuators are in a quiescent disposition ( FIGS. 3  or  4 A) the output rod  26 A is disposed at position Q. If actuator  20 B is selectively activated while actuator  20 A remains quiescent, as shown in  FIG. 4C , the output rod  26 A is translated to the −1 position. Conversely, actuator  20 A is selectively activated while actuator  20 B remains quiescent, the output rod  26 A is translated to the +1 position. The result is a three position device, similar to a single throw, double pole switch. It is significant that the overtemperature compensation protection remains effective, in that overtemperature causes the assembly to remain in the quiescent (Q) position. 
   With regard to  FIG. 5 , a further embodiment of the invention provides a linear actuator assembly  20 C substantially as described with regard to  FIGS. 1 and 2  (corresponding components are given the same reference numeral with the suffix “C”). The housing  22 C is constrained to translate in a direction parallel with the axis of the output rod  26 C, and when the house is translated to the left a spring  38  provides a resilient restoring force to return the housing to the rightward position of  FIG. 5. A  lever  32  is mounted adjacent to the housing  22 C and pivots about a fulcrum  33 . The right end of the lever  32  is connected to a passively operating SMA wire that extends to a mechanical ground. At the other end of the lever, a latch  37  engages a lip  39  on the housing to prevent leftward translation of the housing  22 C. 
   In typical operation, electrical heating of the SMA wires  27 C causes the device  20 C to be actuated as shown in  FIG. 2 , so that the output rod  26 C pushes the load to the right and does useful work. However, when the passively activated wire  34  is heated by ambient conditions beyond its phase transition temperature, the contracting wire  34  rotates the lever  32  CCW about fulcrum  33 , causing the latch  37  to release lip  38  and enabling the housing  22 C to be free to translate laterally to the left. If the wires  27 C become spontaneously activated by the ambient overtemperature condition, the device  20 C will actuate, but the housing  22 C is less constrained to move to the left that is the output rod to move the load to the right. Thus the housing translates leftward, and the load is unmoved. This overtemperature lockout condition persists until the passive wire  34  cools below the phase transition temperature, and the spring  36  returns the lever CW to the latched position, and spring  38  translates the housing  22 C to the right to re-engage the latch  37 . 
   It may be noted that the passive wire  34  may have a phase transition temperature that is below that of the wires  27 C, so that the latch  37  is certain to release before the wires  27 C are spontaneously activated by the ambient overtemperature condition. Additionally or alternatively, the wire  34  may be positioned to be exposed to any anticipated heat source, such as adjacent heat generating devices or objects, or the like. 
   With regard to  FIG. 6 , the output rod  26 C may be connected to extend leftward from the left end of the topmost link  24 C, so that the output rod  26 C retracts upon actuation of the device. The lever assembly and latch function and their overtemperature lockout function are substantially as described with reference to FIG.  5 . 
   A further embodiment of the invention, depicted in  FIGS. 7-9 , includes the SMA linear actuator  20 D, substantially as described previously. The housing  22 D of the actuator is placed within a opening  42  of a bracket assembly  43 , the actuator  20 D being constrained thereby to translate only in a direction parallel to the output link  26 D. The housing  22 D is pinned to a mechanical ground, as shown by reference numeral  41 , so that the actuator  20 D is immobilized and the bracket assembly may translate laterally. When the actuator  20 D is activated by applying electrical current through its SMA wires, the output rod  26 D extends to the right, as shown in  FIG. 9 , and translates the bracket assembly  43  rightward in concert therewith. 
   Integral with the bracket assembly  43  is an output link  44  extending parallel to the output rod  26 D. One end  45  of the link  44  is received within the opening  46  of a load connector  47 , which is joined to a load device. A pivoting latch arm  48  is secured to the end  45  and positioned to engage or disengage the connector  47 . A passive SMA wire  49  extends along the link  44 , one end connected to the latch arm  48  and the other end connected to the bracket assembly  43 . As shown in  FIG. 10 , the SMA wire  49  is connected to the latch arm at a point proximate to the latch arm pivot  52 , so that a small amount of contraction of the wire  49  will cause sufficient rotation of the latch arm  48  to achieve disengagement. The latch arm is maintained in an engaged position with the load connector  47 , as shown in  FIG. 7 , by a spring  51 , so that the load connector is normally engaged by the latch arm  48  and activation of the actuator  20 D drives the bracket assembly, output link  44 , and load connector  47  to the right, as viewed in FIG.  7 . 
   However, if the passive SMA wire  49  is exposed to a heat source having a temperature greater than its phase transition temperature, the wire  49  will contract spontaneously and pull the latch arm  48  from the engaged position of  FIGS. 7 and 10  to the disengaged position of  FIGS. 8 and 11 . Once the latch arm  48  is disengaged (FIG.  8 ), spontaneous activation of the actuator  20 D will cause the output rod  26 D to push the bracket assembly  43  to the right but, being uncoupled from the load, will not translate the load. Thus an overtemperature condition cannot cause unintentional movement of the load. 
   As noted previously, the passive wire  49  may have a phase transition temperature that is below that of the SMA wires  27 D of device  20 D, so that the latch  47  is certain to release before the wires  27 D are spontaneously activated by the ambient overtemperature condition. Additionally or alternatively, the wire  49  may be positioned to be exposed to any anticipated heat source. 
   Another embodiment of the invention, depicted in  FIGS. 11-13 , includes a plurality of sliding links  51  in a stacked array and having a plurality of SMA wires connected therebetween (not shown), as described previously with respect to  FIGS. 1 and 2 . An output rod  52  extends from the top link of the stack, which is disposed to retract upon activation of the actuator and translate to the right as viewed in FIG.  12 . The links  51  are secured within the interior space  53  of a housing  54 . The housing  54  has a horseshoe configuration, and the interior space  53  provides clearance for the links  51  to move to their activated position, as shown in FIG.  12 . 
   One end  56  of the horseshoe shaped housing comprises a latch that engages the stack of links  51  to prevent leftward movement as the output rod  52  retracts to the right. A lever  57  is pivotally secured to the other end of the housing  54  by a pin  58  extending through one end of the lever and into the housing. A passive SMA wire  61  is disposed at the exterior of the housing  54 , one end being secured to the lever  57  at a point that is proximate to the pivot pin  58 . The passive wire  61  extends in wraparound fashion about the outer surface of the horseshoe shaped housing  54  and is secured at anchor  62 . The curve of the horseshoe has a radius that is at least 10-100 times the diameter of the wire  61 , so that there is insufficient bending stress to detract from the expected behavior of the shape memory material. 
   At the other end of the lever  57 , a pin extends from the lever end into a curved slot  63  intruding into the housing  54  adjacent to the latch end  56 . The slot  63  acts as a cam surface interacting with the pin  64 . If the SMA wire  61  is heated by ambient conditions to a temperature greater than its phase transition temperature, the wire contracts and causes the lever  57  rotate CCW. The pin  64  is driven to translate along the slot  63 , and the cam effect of the slot  63  acting on the pin  64  causes the latch end  56  to flex and widen the horseshoe shape, as shown in FIG.  14 . The latch end  56  releases the stack of links  51 , thereby effectively preventing the unanchored stack from retracting the output rod  52 . Thus, as shown in  FIG. 14 , if the stack of links  51  does become activated by ambient overtemperature conditions, the stack will translate toward the load, rather that the load being translated toward the actuator. As a result, no work is done on the load, and no unintentional, spontaneous actuation of the load can occur do to overtemperature conditions. 
   Note that the natural resiliency of the horseshoe housing  54  provides a restoring force that tends to move the opposed horseshoe ends together again, thus urging the latch end  56  to once again engage the stack of links  51  when it cools and resumes its quiescent disposition (with or without the assist of a return spring), as shown in FIG.  12 . The same restoring force also applies some tension to the passive SMA wire  61  as it cools, thus urging the wire  61  to return to 100% length. 
   As noted previously, the passive wire  61  may have a phase transition temperature that is below that of the SMA wires connecting links  51 , so that the latch  56  is certain to release before the wires of the stack of links are spontaneously activated by the ambient overtemperature condition. The wire  61  wraps around the horseshoe exterior surface, and is displayed have wide ranging exposure on three sides of the horseshoe shaped housing  54 . Thus the placement of the wire  61  increased the likelihood that the wire  61  will overheat and activate the safety release latch  56  before the stack of links  51  can be activated by the overtemperature event. 
   It may be appreciated that all the embodiments described herein have in common the use of a passive SMA component to prevent the delivery of the actuating stroke from the actuator to the load. The mechanisms for achieving this prevention generally either decouple the load from the output rod, or release the actuator body from mechanical ground so the output rod cannot apply force to the load, or apply the passive SMA component in countervailing effect to the actuator to neutralize displacement caused by overtemperature conditions. 
   In the previous descriptions some embodiments include the use of springs to apply a restoring force to the SMA wires as they cool and expand. This expedient may be applied to embodiments herein in which restoring springs are not mentioned explicitly. 
   The foregoing description of the preferred embodiment 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 embodiment described is 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.