Abstract:
An SMA actuator includes an SMA element having a first end, a second end, and a central portion therebetween, the SMA element being oriented so that the first and second ends are adjacent one another and attached to one of an anchor interface or payload interface, and so that the central portion of the SMA element is attached to the other of the anchor interface or the payload interface. An electrical shorting device electrically connects the first and second ends, whereby electrical potential may be provided between a first point located between the first end and the central portion and a second point located between the second end and the central portion, thereby causing the SMA element to activate reducing the length of the SMA element. Related latches are disclosed including such actuator, or other simplified or more complex actuators.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims benefit of International Patent Application, Serial Number PCT/US2005/041503, titled “Shape-Memory Alloy Actuator and Latches Including Same”, filed Nov. 17, 2005, which claims priority to U.S. Provisional Patent Application No. 60/629,163, filed on Nov. 17, 2004, titled “High Speed Shape-Memory Alloy Actuator for Trunk and Fuel Tank Latches”, both of which are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates generally to actuators incorporating elongated shape memory alloys (SMA). More particularly it relates to designs for and use of such actuators in the field of automotive latches, such as trunk latches and fuel filler flap latches. 
       BACKGROUND OF INVENTION 
       [0003]    Shape memory actuators produce displacement or force by forcing a suitable actuator element to undergo a transition between a low and a high temperature phase (martensite and austenite, respectively), each phase having characteristic dimensions. The necessary energy is most commonly supplied by ohmic heating and removed by any convenient mechanism, such as conductive, convective or radiative heat transfer. The use of SMA-based actuators is steadily growing owing in large part to their propensity for being conveniently packaged into narrow form factors. This ability is related to the fact that shape memory alloys are shaped into wires of modest cross section and sufficiently long length to achieve useful stroke during their activation. The slender shape is what permits insertion of SMA wires into narrow spaces. 
         [0004]    As shown in  FIG. 1 , the simplest arrangement of an SMA actuator as a straight length of wire  10 , however, remains somewhat awkward since the application of ohmic heating would require making electrical contacts to wire ends  12 ,  14  located at some distance from each other. End  12  is illustrated at anchor interface  16  and end  14  is illustrated at payload interface  18 . Voltage potential is provided by wire  20  and ground is connected by wire  22 . 
         [0005]    Accordingly, folded geometries have been employed so as to allow both electrical contacts to be near one another. As shown in  FIG. 2 , SMA actuator wire  10 ′ is folded so that ends  12 ,  14  are near one another. An example of this folded geometry is given in  FIG. 5  of U.S. Pat. No. 3,634,803. Another reason to prefer this geometry is that for a single fold, the two sides of the wire on each side of the fold account for a doubling of the available force compared to a single strand of the same cross section. Unfortunately, this benefit is accomplished by a doubling of the electrical resistance. When powered from a voltage source, the doubling results in halving the electrical current and the Joule heating power, while the heating time is significantly increased. If slower activation is not acceptable, the designer has the option of restoring the resistance of its initial value. One way to do this is by shortening the actuator by a factor of two. This time, the drawback is a reduction in stroke by the same factor. 
         [0006]    As illustrated in  FIG. 3 , the other approach is to double the cross sectional area of the SMA wire  10 ″ by increasing the wire diameter by about 41%. This allows the designer to keep both the increased force and the previous stroke, while the activation time is actually improved, even with respect to the case depicted in  FIG. 1 . Unfortunately, this option also has disadvantages. The first is that—once again—the electrical contacts are inconveniently located at opposite ends  12 ,  14  of a long actuator. The second is that now the cooling time is significantly increased, by an amount even greater than the shortening of the contraction time. 
         [0007]    What is needed is an actuator which can incorporate the force benefit of  FIG. 2  and  FIG. 3 , the convenient electrical interface shown in  FIG. 2 , and the activation speed of the configuration shown in  FIG. 3 , while retaining the cooling speed of that shown in  FIG. 2 . 
       SUMMARY 
       [0008]    According to certain aspects of the present invention, an SMA actuator is disclosed. Another aspect of the present invention teaches an improved latch release mechanism including an SMA actuator. The latch release mechanism may be applied to various applications including trunk and fuel tank latches, with or without SMA actuators as well. 
         [0009]    According to certain aspects of the invention, an SMA actuator includes an SMA element having a first end, a second end, and a central portion therebetween, the SMA element being oriented so that the first and second ends are adjacent one another and attached to one of an anchor interface or payload interface, and so that the central portion of the SMA element is attached to the other of the anchor interface or the payload interface. An electrical shorting device electrically connects the first and second ends, whereby electrical potential may be provided between a first point located between the first end and the central portion and a second point located between the second end and the central portion, thereby causing the SMA element to activate reducing the length of the SMA element. Various options and modifications are possible. 
         [0010]    For example, the payload interface may be a portion of one of a trunk latch or a fuel tank latch. The electrical potential may be connected via wires each having a terminal attached thereto, the terminal being attachable to the SMA element. The first point and second point may be each generally half-way between the payload interface and the anchor interface. 
         [0011]    The payload interface may be a portion of a latch. The latch may be configured to that activation of the SMA element moves the portion of the latch so that the latch is openable by a user, and the latch may be configured so that upon reclosing of the latch the SMA element is not damaged by movement of the portion of the latch. 
         [0012]    According to other aspects of the invention, an SMA actuator latch includes a movable catch attached to a closeable element; a keeper movable between positions engaged with the catch or separated from the catch; a pawl movable between a first position holding the keeper in the engaged position or a second position allowing the keeper to move; and an SMA actuator for moving the pawl from the first position to the second position. Again various options and modifications are possible. 
         [0013]    For example, the SMA actuator may be connected to the pawl. Also, the latch may further include a hook member for selectively retaining the pawl in the first position, the SMA actuator being connected to the hook member. The latch may be one of a trunk latch or a fuel tank latch; the catch may be one of a rotatable member or a leaf spring; the pawl may be spring biased; and/or the catch may be spring biased. 
         [0014]    The keeper may reset the catch upon re-closing of the latch. Also, the SMA actuator may include an SMA element having a first end, a second end, and a central portion therebetween, the SMA element being oriented so that the first and second ends are adjacent one another and attached to one of an anchor interface or a portion of the latch, and so that the central portion of the SMA element is attached to the other of the anchor interface or the portion of the latch; and an electrical shorting device electrically connecting the first and second ends, whereby electrical potential may be provided between a first point located between the first end and the central portion and a second point located between the second end and the central portion, thereby causing the SMA element to activate reducing the length of the SMA element and moving the portion of the latch, to thereby move the pawl from the first position to the second position. If so, the electrical potential may be connected via wires each having a terminal attached thereto, the terminal being attachable to the SMA element. Also, the first point and second point may be generally half-way between the payload interface and the portion of the latch. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]      FIG. 1  is a diagrammatical representation of prior art SMA actuator arrangement having a basic geometry. 
           [0016]      FIG. 2  is a diagrammatical representation of alternate prior art SMA actuator arrangement having a folded geometry. 
           [0017]      FIG. 3  is a diagrammatical representation of another alternate prior art SMA actuator arrangement having a thickened geometry. 
           [0018]      FIG. 4  is a diagrammatical representation of an example of an SMA actuator according to certain aspects of the present invention including mid-wire electrical contacts. 
           [0019]      FIG. 5  is a perspective view of one example of a mid-wire connector for an SMA wire having two holes. 
           [0020]      FIG. 6  is a perspective view of an alternate example of a mid-wire connector for an SMA wire having notched holes. 
           [0021]      FIG. 7  is a perspective view of another alternate example of a mid-wire connector for an SMA wire having notched holes. 
           [0022]      FIG. 8  is a side diagrammatical view of one example of a trunk latch according to certain aspects of the invention, in a closed position. 
           [0023]      FIG. 9  is a side diagrammatical view of the trunk latch of  FIG. 8 , as opening begins. 
           [0024]      FIG. 10  is a side diagrammatical view of the trunk latch of  FIG. 8 , as opening continues. 
           [0025]      FIG. 11  is a side diagrammatical view of the trunk latch of  FIG. 8 , as opening continues further. 
           [0026]      FIG. 12  is a side diagrammatical view of the trunk latch of  FIG. 8 , in an opened position, ready for re-closing. 
           [0027]      FIG. 13  is a side diagrammatical view of the trunk latch of  FIG. 8 , as closing continues. 
           [0028]      FIG. 14  is a side diagrammatical view of one example of a fuel tank latch according to certain aspects of the invention, showing an opened and closed position. 
           [0029]      FIG. 15  is a side diagrammatical view of a portion of the fuel tank latch of  FIG. 12 , as opening begins. 
           [0030]      FIG. 16  is a side diagrammatical view of a portion of the fuel tank latch of  FIG. 12 , as opening continues. 
           [0031]      FIG. 17  is a side diagrammatical view of a portion of the fuel tank latch of  FIG. 12 , as reclosing begins. 
           [0032]      FIG. 18  is a perspective view of a portion of the fuel tank latch of  FIG. 12 , in a closed position. 
           [0033]      FIG. 19  is a perspective view of a portion of the fuel tank latch of  FIG. 12 , in an opened position. 
       
    
    
     DETAILED DESCRIPTION  
       [0034]    Detailed reference will now be made to the drawings in which examples embodying the present invention are shown. The drawings and detailed description provide a full and detailed written description of the invention, and of the manner and process of making and using it, so as to enable one skilled in the pertinent art to make and use it, as well as the best mode of carrying out the invention. However, the examples set forth in the drawings and detailed description are provided by way of explanation of the invention and not meant as a limitation of the invention. The present invention thus includes any modifications and variations of the following examples as come within the scope of the appended claims and their equivalents. 
         [0035]    The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. 
         [0036]    According to certain aspects of the present invention, an SMA actuator  100  is illustrated in  FIG. 4 . SMA actuator  100  can have relatively the same amount of SMA wire  110  as the prior art structure of  FIG. 2 , and structurally differs at least by its electrical interconnect. In the preferred embodiment, two SMA wire ends  112 ,  114  that had previously been electrically isolated from one another are now to be electrically joined by an electrical shorting device  124 . Such device  124  is illustrated only diagrammatically, and may take various forms already known in the art, including:
       (i) winding the wire ends  112 ,  114  on a common binding post to be secured by tightening a nut;   (ii) crimping separate terminals on each wire end  112 ,  114  later to be joined by soldering a short conducting wire between them; or   (iii) using a single crimping component fitted with two crimping areas, one for each of the two wire ends  112 ,  114 .
 
Accordingly, electrical shorting device  124  may take any of the above forms or others suitable for electrically connecting ends  112 ,  114 . Another example of an electrical shorting device  124  will be disclosed below in relation with other aspects of this invention.
       
 
         [0040]    It should be noted that the electrical shorting device  124  may be the same as (i.e., comprise a part of) the fixed mechanical anchor interface  116  securing the SMA wire  110  to a relatively immovable object such as an actuator housing or a nearby structure. In this case, the anchor interface  116  should be selected to be electrically conductive, so as to provide the electrical shorting path discussed above. 
         [0041]    Also, as shown in  FIG. 4 , electrical contacts  120 ,  122  for an exemplary device of a specific embodiment can be made near the middle of or at least spaced from the ends of, each SMA wire leg. Thus, voltage potential may be connected at point  126  and ground may be connected at point  128 , both points being spaced from anchor interface  116  and payload interface  118  located at central portion of wire  100 . As illustrated, such connections are generally centrally located between electrical shorting device  124  and bend  130 , thereby providing two somewhat equal paths between points  126  and  128 . However, it should be understood that numerous variations in the geometry diagrammatically illustrated in  FIG. 4  are possible. 
         [0042]      FIG. 5  illustrates how such contacts may be made according to a particular embodiment of the present invention. According to this embodiment, a sliding terminal  132  consisting of a conductive material (alloys such as beryllium-copper, nickel-silver, phosphor-bronze, etc.) is provided with two holes  134 , made for example by stamping or chemical etching, and adapted to threadingly receive the SMA wire  110 . The normal tension in the wire  110  tends to keep the wire straight, but threading it from one side of the terminal  132  to the other and then back again tends to bend the wire. The contradiction is resolved by compromise: the wire  110  acquires a small, local deflection, and the previously flat terminal  132  is also bent somewhat to an extent which minimizes the overall deformation energy. Since deformation energy cannot be eliminated, both the wire  110  and the terminal  132  acquire a residual stress which maintains them in firm contact regardless of SMA activation. When the terminal  132  has a flexible wire (electrical contact  120 , for example) soldered to it as illustrated in  FIG. 5  or crimped to it (see  FIGS. 6  or  7 ), the electrical contact can be maintained reliably without the terminal  132  sliding along the wire  110 , although its ability to slide may be used during assembly to position the contact in the most propitious location. The electrical contact is also enhanced by the proximity of the relatively sharp edges of the terminal holes  132  to the wire  110 , and is able to withstand significant variations in wire tension without loosening. 
         [0043]    Similar properties are enjoyed by terminal variants  132 ′,  132 ″ illustrated in  FIGS. 6 and 7 . These variants differ from the previous terminal example  132  by having lateral notches  136  opened into the walls  138  of holes  134 . These notches  136  permit assembly of the mid-length terminals onto the SMA wire  110  after the latter is already assembled into the actuator. Terminal versions  132 ′ and  132 ″ differ from each other in the orientation of these notches  136 . Placing the notches  136  proximally to the electrical wire pigtail  120  as in  FIG. 7  may result in slightly greater contact stability if the pigtail is expected to be under some mechanical tension (at right angles to the SMA wire  110 ). 
         [0044]    Applications of one or more of the concepts above are illustrated in  FIGS. 8-19 . The fast acting SMA actuator concepts discussed above are shown in the exemplary context of automotive latches, such as one used for trunks ( FIGS. 8-13 ) and for fuel filler flaps ( FIGS. 14-19 ). SMA actuators may be suitable for this family of latches due to various possible requirements on their actuator, namely relatively high force, high stroke, high speed, broad range of operating temperature, low weight, and slender package form factor. 
         [0045]    Although illustrations discussed below teach the combined use of a fast acting SMA actuator and certain latches, the resourceful practitioner of the art will recognize that the various inventive components may be separated under conditions where one or more of the performance requirements is relaxed. For instance, if the range of operating temperatures is reduced, it may be possible to operate either trunk latches or fuel flaps by simply substituting a fast acting SMA actuator for the conventional geared electric motor presently employed in the industry in these applications. Conversely, if rapid actuation and package weight are not essential, the latches described below might be operated with any convenient small motor. In the figures below, the SMA actuators are depicted in a very schematic way to reflect the choices available to the designer to choose a fast acting SMA actuator or any other convenient actuator based on the stringency of the specific requirement. 
         [0046]    Compared to prior art trunk and fuel filler flap latches, the latches of the instant invention allow their actuators to operate, for example, with reduced force and stroke output. This is accomplished by collecting a moderate amount of energy from the human user who already expects to fully open the trunk latch or tank flap by hand. 
         [0047]    The construction and operation of one suitable trunk latch mechanism  200  is best seen in  FIGS. 8-13 . The latch  202  of mechanism  200  resembles in many ways a conventional trunk latch where the latch is typically mounted near the moving edge of the trunk lid and a mating metal loop (or keeper)  204  is fastened near the rear edge of the trunk floor, although these locations could be reversed. When the trunk is locked, the catch  206  captures and holds the loop  204  securely. A conventional pawl  208  is spring biased so as to engage the catch  206 . As illustrated, springs  210  and  212  bias catch  206  and pawl  208  respectively. Thus, if a conventional actuator (a geared electric motor) were used, it would need to overcome not only the friction between the loaded catch  206  and the pawl  208  but also the resistance of the spring  212  of the pawl. 
         [0048]    In  FIG. 8  it is seen that the pawl  208  of the present invention is spring biased, but it is biased in a direction urging the pawl away from the engagement with the catch  206 . To prevent this from happening unintentionally, the bottom end  214  of the pawl  208  is restrained by a hook-like device  216 , biased by a spring  218  in a direction which favors engagement of the pawl bottom end. The hook can overcome this bias when the SMA actuator  100  is activated. Another difference from the prior art is the presence of a cam lobe  220  on the catch  206 . This lobe  220  allows the rotation of the catch  206  to be transmitted to the pawl  208  in a direction opposite to the spring bias of the pawl. A further difference in the construction of the pawl  208  is the presence of an elastically compliant section  222  between top portion  208   a  and bottom portion  208   b.  This permits the top and bottom portions of the pawl  208  to undergo a small amount of relative rotation under certain conditions such as re-closing that may be understood by following the operation of the device in  FIGS. 8-13 . 
         [0049]    The operation of the latch mechanism  200  may be understood by the positions shown in  FIGS. 8-13 . In  FIG. 8 , the latch mechanism  200  is closed and the trunk lid is locked. In  FIG. 9 , the SMA actuator  100  has been activated. In  FIG. 10 , the SMA actuator  100  has been further actuated. Accordingly, the hook  216  is moved until it releases bottom end  214  of the pawl  208 , the pawl then releases the catch  206 , and the catch pivots clockwise until cam lobe  220  contacts the pawl. In  FIG. 11 , further rotation of catch  206 , whether by spring force or an operator pulling on the trunk, frees loop  204 , opening the trunk. Cam lobe  220  causes pawl  208  to continue clockwise rotation, eventually returning the pawl bottom  214  to its original position re-engaging the hook  216 . Therefore, when the catch  206  is fully open, both the pawl  208  and the hook  216  have returned to their original position as in  FIG. 8 . This action can follow SMA activation relatively quickly. This means that when the hook  216  attempts to return to its initial position, the SMA actuator  100  may not have cooled sufficiently to allow the hook to return right away. Fortunately, while the SMA actuator  100  is cooling, the job of restraining the pawl  208  can be accomplished by the cam lobe  220  of the catch  206 . The combination of the spring tension biasing the catch  206  toward the open position and the design of the lobe  220  and its mating surface  224  allows the spring bias of the pawl  208  to overcome the friction of the loaded catch  206  during unlatching, and yet cannot overcome the bias of the catch  206  when the cam surfaces  220 ,  224  are in contact. If necessary, the SMA actuator  100  may have an overstress protection mechanism (not shown) installed near the built-in extremity of the actuator. Such mechanism may consist of a mechanically compliant interface which is normally immovable, but may deflect when tensile stress exceeds a predetermined value, thus protecting the longevity of the actuator. 
         [0050]    With regard to the re-latching action depicted in  FIGS. 12 and 13 , it is seen that the presence of the hook  216  shields the SMA actuator  100  from any variation in stress during the closing of the trunk. As shown, during closing, the catch  206  rotates counterclockwise until it contacts upper portion  208   a  of pawl  208  with tip  206   a.  Upper pawl  208   a  rotates counterclockwise but, due to presence of compliant section  222 , bottom end  214  of pawl  208  remains retained by hook  216 . Eventually, tip  206   a  of catch  206  passes upper pawl portion  208   a  and is retained therein. The upper pawl  208   a  then snaps back into position, returning the device to the locked position of  FIG. 8 . 
         [0051]    Although the pawl  208  is described as being of unitary construction with two mobile portions  208   a  and  208   b  connected by a compliant section  222 , the same functionality may be alternatively accomplished by a more conventional construction consisting of two rigid links with a common pivot point and with an additional spring bias adapted to urge the two portions to be positioned opposite from each other. 
         [0052]    Therefore, using mechanism  200 , one may activate a trunk lid latch mechanism using minimal actuator stroke and force, but using the manual reset action available when the user opens the lid after remote unlocking. The catch  206  may be moved to an openable position due to SMA actuator activation of the hook  216  and pawl  208 . Therefore, the user can lift the lid to cause additional rotation of catch  206 . Alternately the catch  206  may be spring driven to open fully upon SMA actuator moving the hook  216  and pawl  208 . If it is desired to open the catch  206  fully without user intervention, an overstress protection compliant element may be added at anchor  116  for the SMA actuator  100 . This way, the early closing of the hook  206  would not damage the SMA actuator  100 . 
         [0053]    The construction and operation of the fuel filler flap latch, also known as the tank latch is shown in  FIGS. 14-19 .  FIG. 14  shows a fuel flap latch mechanism  300  along with a fuel flap  302  pivotally mounted to a vehicle body  304 . Flap  302  includes a keeper  306  for engaging a catch  308 , in the form of a leaf spring. The keeper  306 —when released—is urged toward the open position by the leaf spring  308 , which also serves as a catch, as described below. The keeper  306  uses a step-like feature  310  to keep it engaged when the fuel tank is not being accessed. However, instead of a conventional actuated pin, the keeper  306  is restrained by the multi-function leaf spring  308 . 
         [0054]    The catch  308  is biased in a direction away from engagement with the keeper  306 . Thus, when the SMA actuator  110  rotates the pawl  312  out of engagement with the catch  308  (see  FIG. 15 ), the latter—under its own bias—snaps in an outward direction, and the keeper  306  is released and moves slightly while urged upward by a lower portion  314  of the multi-function leaf spring  308 . A shoe-like extending portion  318  is located near the end of the keeper  306 , and retaining tabs  320  are built into the upper part of the leaf spring  308 . The interaction between the rear of the shoe-like portion  318  and the tabs  320  causes the rising keeper  306  (lifted either by a user or by another spring bias not shown in these figures) to recharge and return the leaf spring upper portion  316  to the initial position (see  FIGS. 15 and 16 ). The actuated pawl  312 —in turn—is then free to return to its initial position under its own biased spring  322 . 
         [0055]    When the user returns the flap  302  to its closed position, the catch  308  has already been secured by the pawl  312 . The upper portion  316  of the multi-function leaf spring  308  is deflected by the advancing “shoe”  318  portion of the keeper (see  FIG. 17 ), until it snaps back behind the step-like projection  310  of the keeper  306  to bring it back into a latched condition (as in  FIG. 15 ). 
         [0056]    The use of a leaf spring  308  might be taken to imply a relatively weak mechanism, subject to being defeated by intruders. To prevent such access, stiff side arms  324  (see  FIG. 18 ) are located in close proximity to solid shoulder-like projections built into the flap housing (not-shown). In the event of attempted intrusion, the intruder would try to lift the flap  302  mechanically. The stiff side arms  324  would then come into contact with the nearby shoulders which would prevent any further movement, since the leaf spring  308  would not yet have moved from the position shown in  FIG. 14  to the position shown in  FIG. 16 . In normal operation, the clearance between the side arms  334  and the shoulders would be sufficient to prevent any additional friction. It should be understood that spring portions  314  and  316  may either comprise a unitary part or two separate parts. 
         [0057]    Activating a flap latch mechanism using minimal actuator stroke and force but using the manual reset action available when the user opens the flap manually can therefore be beneficial. Also, use of a multi-function leaf spring can keep the cost low. In case of attempted burglary, the keeper hook  310  forces side arms  324  upwards. The side arms  324  then deflect slightly upward until they contact fixed retaining features, built into the fuel flap enclosure. Use of short and stubby side-arms  324  can withstand high shearing forces. The bottom and upper leaf springs  314  and  316  are kept relatively weak to allow leaf (not shown, for moving flap  302  relative to body  304 ) to overcome frictional forces. 
         [0058]    A common and critical feature of both embodiments discussed above is the fact that the stroke and the force of the SMA actuator  100  has been decoupled from the force and stroke required to release the latch. Such a feature is absent from conventional trunk and tank latches. This brings about a number of benefits such as:
       The size, cost, weight, speed and power consumption of the actuator are all reduced.   Variations in the load presented to the catch are not reflected in the load seen by the actuator.   A more consistent load on the actuator increases its reliability and cyclic endurance.       
 
         [0062]    Although the various embodiments of the present invention have been presented in terms of trunk latches and fuel filler flap latches, the principles outlined above are clearly of wider applicability as may be readily discerned by the skilled practitioner of the art. Accordingly the present invention should not be construed as limited by the embodiments shown. The disclosed SMA actuator  100  may be used with other mechanisms, and the disclosed mechanisms  200  and  300  may be carried out with actuators other than the disclosed SMA actuator  100 , or with actuators other than SMA actuators in general.