Abstract:
A sensor provides a persistent indication that it has been exposed to temperatures below a certain critical temperature for a predetermined time period. An element of the sensor made from shape memory alloy changes shape when exposed, even temporarily, to temperatures below the Austenitic start temperature A s  and well into Martensite finish temperature M f  off the shape memory alloy. The shape change of the SMA element causes the sensor to change between two readily distinguishable states. The sensor includes a one-way stop element that creates a persistent indication of the temperature history, allowing the sensor to be manufactured and stored at temperatures above the Austenitic temperature without causing the indication of an over-temperature event.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This invention claims priority under 35 U.S.C. 120 to U.S. patent application Ser. No. 10/005,403, titled “Shape Memory Alloy Temperature Sensor,” incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to temperature sensors, specifically shape memory alloy temperature sensors that provide persistent indication once their temperature reaches, exceeds or goes below a critical value.  
           [0003]    Exposure to temperatures above or below a critical temperature can damage many important materials. Food products such as frozen dairy products and frozen meats can spoil when exposed to thawing temperatures for even a short time. Products that need to be kept cool but unfrozen, such as pharmaceutical drugs, vaccines, and serums can spoil if frozen temporarily and then warmed up to normal but cool temperatures. Frozen medical products such as blood and certain pharmaceuticals can be unsafe once exposed to thawing or other high temperatures, even if the temperature later returns to a safe value. Low temperatures can also compromise important properties of some rubber and rubber-like materials. The damage is often unseen, and can persist even if the temperature returns to an acceptable level. This situation can arise in transportation, where a frozen product temporarily experiences high temperatures due to improper handling or cooling equipment malfunction or a cooled product temporarily experiences a freezing temperature due to improper handling or cooling equipment malfunction.  
           [0004]    Many conventional temperature sensors do not provide a persistent record of temporary temperature deviations. Conventional temperature sensors, such as common thermometers, indicate the current temperature only. They provide a continuous indication of the current temperature of the material. They do not provide a permanent indication of out-of-range temperatures without additional permanent recording apparatus. Accordingly, there is a need for sensors that provide a persistent record of temporary out-of-range temperatures.  
           [0005]    Shape memory alloys (SMAs) have properties that can be useful in developing the needed sensors. An SMA can be trained to have a certain shape in its Austenitic state or at temperatures above the SMA&#39;s Austenitic finish temperature A f . The SMA moves in a certain fashion to a second shape, its Martensitic state, which is a softer state for the material, when the temperature drops below the Austenitic finish temperature A f  and eventually reaches below the Martensite start temperature M s . The SMA will not return to the Martensite shape without additional external force even if the temperature subsequently falls below the Austenitic temperature A f . SMAs are used in a variety of applications, such as those described in “Design and Modeling of a Novel Fibrous SMA Actuator,” Proc. SPIE Smart Materials and Structures Conference, vol. 2190, pp. 730-738 (1994), and “A Phenomenological Description of Thermodynamical Behavior of Shape Memory Alloys,” Transactions of the ASME, J. Appl. Mech., vol. 112, pp. 158-163 (1990). SMAs have been suggested for use in persistent temperature indicators. See Shahinpoor, U.S. Pat. No. 5,735,607, incorporated herein by reference. The sensors suggested by the U.S. Pat. No. 5,735,607, however, can require that the apparatus be kept below the threshold temperature during assembly and storage. This requirement can complicate manufacture and handling. There is a need for temperature indicators that can be manufactured, stored, and handled at arbitrary temperatures, then enabled to provide a persistent record of temporary temperature deviations.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention can provide a freeze indicator or an indicator of lower critical temperatures reached from higher temperatures. The present invention comprises a sensing element mounted with a body. The sensing element comprises a portion made with a shape memory alloy stressed by a resilient body such as a spring or an elastic flap. The sensing element mounts with the body, fixedly at a first end. At the second end, the sensing element mounts with a forcing element, which in turn mounts with the body. The forcing element exerts a force on the sensing element tending to elongate the shape memory alloy element once the freezing temperature or the lower critical temperature is reached. The force exerted is more than that required to elongate the shape memory alloy element when it is in its softened Martensitic state at the lower critical temperature, but less than that required to elongate the shape memory alloy element when it is in its contracted state. The sensing element, in one embodiment, mounts with the body in a unidirectional restraining relationship, where the restraining relationship allows the sensing element to elongate responsive to the forcing element, but, once a sufficient motion has occurred, substantially prevents shortening of the sensing element by means of one-way stops or locking mechanisms.  
           [0007]    In operation, the apparatus can be assembled at temperatures above the critical temperature of the shape memory alloy element, causing the sensing element to be at a length less than that required to engage the restraining relationship. As long as the apparatus does not experience temperatures below the critical temperature, the shape memory alloy element will overcome the forcing element and the sensing element will not engage the restraining element. If the temperature drops below the critical temperature, then the shape memory alloy element will soften, allowing the forcing element to move the sensing element into the restraining relationship. Subsequent temperature elevation above the critical temperature will not return the sensing element to the original configuration, since the restraining element now prevents contraction of the shape memory alloy element by means of one-way stops. By making the positioning of the sensing element within the restraining relationship perceptible, the apparatus provides a persistent indication of even transitory temperature excursions into the region where the shape memory alloy element is in its softened state.  
           [0008]    The present invention also comprises a variety of body, shape memory alloy element, sensing element, forcing element, and restraining element configurations.  
           [0009]    Advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
       
    
    
     DESCRIPTION OF THE FIGURES  
       [0010]    The accompanying drawings, which are incorporated into and form part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0011]    FIGS.  1 ( a,b ) is an illustration of an apparatus according to the present invention.  
         [0012]    FIGS.  2 ( a,b ) is an illustration of an apparatus according to the present invention.  
         [0013]    FIGS.  3 ( a,b ) is an illustration of an apparatus according to the present invention.  
         [0014]    FIGS.  4 ( a,b ) is an illustration of an apparatus according to the present invention.  
         [0015]    FIGS.  5 ( a,b ) is an illustration of an apparatus according to the present invention.  
         [0016]    FIGS.  6 ( a,b ) is an illustration of an apparatus according to the present invention.  
         [0017]    FIGS.  7 ( a,b,c ) is an illustration of an apparatus according to the present invention.  
         [0018]    [0018]FIG. 8 is an illustration of an apparatus according to the present invention.  
         [0019]    FIGS.  9 ( a,b,c ) is an illustration of an apparatus according to the present invention.  
         [0020]    FIGS.  10 ( a,b,c ) is an illustration of an apparatus according to the present invention.  
         [0021]    [0021]FIG. 11 is an illustration of an apparatus according to the present invention.  
         [0022]    FIGS.  12 ( a,b,c ) is an illustration of an apparatus according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The present invention comprises a sensing element mounted with a body. The sensing element comprises a portion made with a shape memory alloy. The sensing element mounts with the body, fixedly at a first end. At the second end, the sensing element mounts with a forcing element, which in turn mounts with the body. The sensing element exerts a force to resilient forcing element if the temperature is above the critical temperature. On the other hand the forcing element exerts a force on the sensing element tending to elongate the shape memory alloy element if the temperature is below or equal to the lower critical temperature (freezing temperature). The force exerted is more than that required to elongate the shape memory alloy element when it is in its softened Martensitic state, but less than that required to elongate the shape memory alloy element when it is in its contracted Austenitic state. The sensing element mounts with the body in either a unidirectional or rotatory restraining relationship, where the restraining relationship allows the sensing element to elongate responsive to the forcing element, but, once a sufficient motion has occurred, substantially prevents shortening of the sensing element by means of built in one-way no-return stops.  
         [0024]    In operation, the apparatus can be assembled at temperatures above the critical temperature of the shape memory alloy element, causing the sensing element to be at a length less than that required to engage the restraining relationship. As long as the apparatus does not experience temperatures below the critical temperature, the shape memory alloy element will overcome the forcing element and the sensing element will not engage the restraining element. If the temperature drops below the critical temperature, then the shape memory alloy element will soften, allowing the forcing element to move the sensing element into the restraining relationship. Subsequent temperature elevation above the critical temperature will not return the sensing element to the original configuration, since the restraining element now prevents contraction of the shape memory alloy element. By making the positioning of the sensing element within the restraining relationship perceptible, the apparatus provides a persistent indication of even transitory temperature excursions into the region where the shape memory alloy element is in its softened state.  
         [0025]    The present invention also comprises a variety of body, shape memory alloy element, sensing element, forcing element, and restraining element configurations.  
         [0026]    Example Embodiment  
         [0027]    FIGS.  1 - 6  are schematic illustrations of various states of an example embodiment of the present invention. The apparatus generally comprises a body  1 , a sensing element  2  mounted with the body  1 , and a forcing or resilient element  3  mounted with the body  1  and the sensing element  2 . Additional elements, and their interaction to achieve the desired functionality, are described below.  
         [0028]    FIGS.  1 ( a ) and  1 ( b ) are the side view and the top view of a needle version of an apparatus according to the present invention suitable for persistent indication of low temperature events. Sensing element  2  is made at least in part with and SMA. In FIG. 1( a ) the SMA wire  2  is its Austenitic contracted state and pulls the resilient element  3  (e.g., a spring) open and tensioned. A first indicator  4 , for example a green circle, will be visible through a window  5  in the body  1 , since an obscuring indicator  6  is pulled out of an obscuring relationship by the contracted SMA element  2 .  
         [0029]    FIGS.  2 ( a ) and  2 ( b ) are a close up side view and a top view of the example embodiment shown in FIGS.  1 ( a,b ). The SMA element  2  is its Austenitic contracted state and pulls the resilient body  3  open and tensioned. A first indicator such as a green surface  4  will be visible through a window  5  in the body  1 . Locking mechanism  7 , mounted with the body  1 , is configured such that it allows motion of the obscuring indicator  7  and an associated carrier  8 .  
         [0030]    FIGS.  3 ( a ) and  3 ( b ) are an isometric view and a close up isometric view of the example embodiment shown in FIGS.  1 ( a,b ), with the apparatus at a temperature just below the Austenite finish temperature of the SMA wire  2 . The SMA wire  2  begins to soften as it approaches its soft Martensitic state at lower temperature from its Austenitic contracted state. The resilient body  3  stretches the SMA wire  2  and pulls the obscuring indicator  6 , e.g., a red circle, to a position where the obscuring indicator  6  partially covers the first indicator  4 . The window  5  in the body will show part of each indicator  4 , 6 .  
         [0031]    [0031]FIG. 4( a ) and  4 ( b ) are a side view and a top view of the example embodiment shown in FIG. 1( a,b ), with the apparatus at a temperature below the critical lower or freezing temperature at which the SMA wire is in its soft Martensite state. FIGS.  5 ( a ) and  5 ( b ) are a close up side view and a top view of the apparatus in the same temperature condition. FIGS.  6 ( a ) and  6 ( b ) are an isometric view and a close up isometric view of the apparatus in the same temperature condition. The SMA wire  2  softens as it reaches its soft Martensitic state at lower temperature from its Austenitic contracted state and the resilient body  3  stretches the SMA wire  2  and pulls the obscuring indicator  6  to completely cover the first indicator  4 . The locking mechanism, comprising flaps  7 , engage carrier  8  of the obscuring indicator  6 , preventing it from moving to reveal the first indicator  4  even if the temperature goes back up to normal from the lower critical or freezing temperature. The indicator will consequently show, for example, a persistent red circle through the indicator window  5  if the apparatus ever experiences a temperature below the critical temperature, even if the temperature subsequently rises above the critical temperature. Various implementations of the restraining relationship are suitable for use with the present invention. For example, a pin can engage a slot or depression at the appropriate position. As another example, sawtooth or ratchet structures can allow motion in only a single direction. Other variations will be apparent to those skilled in the art.  
         [0032]    Example Embodiment  
         [0033]    FIGS.  7 - 9  are schematic illustrations of an example embodiment of the present invention. The apparatus generally comprises a body  11 , a sensing element  12  mounted with the body  11 , and a forcing or resilient element  13  mounted with the body  11  and the sensing element  12 . Additional elements, and their interaction to achieve the desired functionality, are described below.  
         [0034]    FIGS.  7 ( a ),  7 ( b ) and  7 ( c ) comprise a front view, top view, and side view of the flat square embodiment of an apparatus according to the present invention. Sensing element  12  comprises at least a portion made with an SMA wire. In the figure, the SMA wire  12  is its Austenitic contracted state and pulls the resilient body  13  open and tensioned. The apparatus accordingly will show a first indicator  14 , for example a green circle  14 , through an indicator window  15 . The SMA wire prevents a second indicator  16 , for example a red circle, from moving to where it would obscure the first indicator  15 . The second indicator  16  mounts with an element that is pivotably mounted with the body  11 . The pivotable mounting can comprise a pivoting plug  19 , with a stepped keyway  18  that engages a no-return stop  17  to prevent the second indicator from returning to the position shown in the figure once it has moved to a position obscuring the first indicator  15  and also engaging the no-return stops  17 .  
         [0035]    [0035]FIG. 8 depicts an isometric version of the example embodiment shown in FIG. 7. The SMA wire  12  is its Austenitic contracted state and pulls the resilient body  13  open and tensioned. The figure shows a pivoting plug, no-return stops, and stepped keyway. The desired functionality can also be achieved with other restraining relationship mountings. For example, the second indicator  16  or corresponding element can be configured to engage the first indicator  14  or the window  15  by, as an example, fitting into a recess or over a protrusion thereon. Keys, plugs, latches, and bendable legs (as in the previous example embodiment) can also be used in various combinations to accomplish the desired restraining relationship when the second indicator moves to the appropriate position.  
         [0036]    FIGS.  9 ( a ),  9 ( b ) and  9 ( c ) are the front view, top view and the side view of the example embodiment of FIGS.  7 - 8 , shown at a temperature below the critical lower or freezing temperature at which the SMA wire  12  is in its soft Martensite state. The SMA wire  12  softens as it reaches its soft Martensitic state at lower temperature from its Austenitic contracted state and the flap spring or the resilient body  13  stretches the SMA wire and rotates the second indicator  16  (e.g., red circle) to completely cover the first indicator  14  (e.g., green circle). The one-way no-return stops—flaps  17  on the pivoting plug  19  engages the edges  18  of the second indicator&#39;s associated element to prevent it from moving responsive to subsequent contraction of the SMA wire if the temperature goes back up to normal from the lower critical or freezing temperature. The apparatus can show a persistent red circle through its indicator window  15  indicating that the apparatus has experienced the lower critical temperature.  
         [0037]    Example Embodiment  
         [0038]    FIGS.  10 - 12  are schematic illustrations of various states on an example embodiment of the present invention. The apparatus generally comprises a body  21 , a sensing element  22  mounted with the body  21 , and a forcing or resilient element  23  mounted with the body  21  and the sensing element  22 . Additional elements, and their interaction to achieve the desired functionality, are described below.  
         [0039]    FIGS.  10 ( a ),  10 ( b ) and  10 ( c ) comprise a front view, top view, and side view of an example embodiment of the present invention at temperatures above the critical (freeze or lower) temperature. FIG. 11 depicts an isometric view of the same example embodiment. Sensing element  22  comprises an SMA wire  22 . The SMA wire  22  is its Austenitic contracted state and rotates the resilient body or the spring  23  open and tensioned. Thus the apparatus will show a first indicator  24  (e.g., a green circle) through an indicator window  25  while a second indicator  26  (e.g., a red circle) is prevented by the contracted SMA wire  22  from obscuring the first indicator  24 . The second indicator  26  mounts with the body  21  with a pivotable mounting comprising one-way no-return stops  27  on the pivoting plug  29  with a step keyway  28 , similar to that discussed in the previous embodiment.  
         [0040]    FIGS.  12 ( a ),  12 ( b ) and  12 ( c ) are the front view, top view and the side view of the circular flat version of the freeze (or lower critical temperature) indicator  21  at temperatures below the critical lower or freezing temperature at which the SMA wire  22  is its soft Martensite state. Thus the SMA wire  22  softens as it reaches its soft Martensitic state at lower temperature from its Austenitic contracted state and the flap spring or the resilient body  23  stretches the SMA wire and rotate the red circle assembly  26  to completely cover the green circle assembly  24 . The one-way no-return stops (flaps  27  on the pivoting plug  29  engages the edges  28  of the indicator to prevent it from contraction of the SMA wire if the temperature goes back up to normal from the lower critical or freezing temperature. Thus the indicator will show a persistent red circle through its indicator window  25  indicating that the package has experienced the lower critical temperature.  
         [0041]    Materials  
         [0042]    The present invention can sense a wide range of temperatures when made with appropriate SMAs. Those skilled in the art know of many suitable SMAs, including Ag—Cd, Au—Cd, Cu—Al—Ni, Cu—Sn, In—Ti, Ni—Al, Ni—Ti, Fe—Mn—Si, Cu—Zn-A, Cu—Al—Ni, alloys thereof, and shape memory polymers such as polyurethanes. These materials typically possess Austenitic temperatures from −200° C. to 110° C. The addition of excess nickel, iron, chromium, and copper to the equiatomic alloy is common to adjust its physical properties (including its Austenitic finish temperature A f ). These materials exhibit a rather abrupt solid phase shape change, due to solid phase transformation between the Martensite and the Austenite state, when they experience temperatures above or below such transformation temperatures.  
         [0043]    The particular sizes and equipment discussed above are cited merely to illustrate particular embodiments of the invention. It is contemplated that the use of the invention may involve components having different sizes and characteristics. It is intended that the scope of the invention be defined by the claims appended hereto.