Abstract:
A sensor provides a persistent indication that it has been exposed to temperatures exceeding a 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 above the Austenitic temperature of 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 removable element that prevents the shape change, allowing the sensor to be manufactured and stored at temperatures above the Austenitic temperature without causing the indication of an over-temperature event.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to temperature sensors, specifically shape memory alloy temperature sensors that provide persistent indication once their temperature reaches a critical value.  
           [0002]    Exposure to temperatures above 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. 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.  
           [0003]    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.  
           [0004]    Shape memory alloys (SMAs) have properties that might be useful in developing the needed sensors. An SMA has a certain shape, its Austenitic state, at temperatures below the SMA&#39;s Austenitic temperature A f . The SMA moves in a certain fashion to a second shape, its Martensitic state, when the temperature rises above the Austenitic temperature A f . The SMA will not return to the Austenitic 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  
         [0005]    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 two ends. The fixed mounting at one end, however, is configured so that it can be converted to allow a moveable relationship between the body and the sensing, element. While the mounting is fixed, the sensing element is prevented from indicating temperature deviations, and the apparatus can be stored and handled freely. After the mounting is converted to provide a moveable relationship, however, temporary excursion can cause the SMA portion to contract, providing a discernible change in the relationship between the sensing element and the body.  
           [0006]    The SMA portion can be a wire made with a SMA, of length equal to the distance between the two mounting portions in one state, and less than the distance in the other state. The wire can be passed through an opening such as a notch or hole in the body, and a bend or a thickened portion used to prevent the wire from moving back through the opening. After the apparatus is in place, and the temperature reduced below the critical temperature, then the bend or thickened portion can be removed. Subsequent temperature increases can initiate a shortening of the SMA wire; the difference in length relative to the body can provide an indication of an over-temperature condition. Even if the temperature excursion was only transitory, the SMA wire will not appreciably lengthen once shortened, absent application of external force. Discernment of the condition of the apparatus can be facilitated by highlighting the relationship between the sensing element and the body. As examples, colored regions and witness marks can be used to make changes in the length or shape of the SMA portion readily apparent.  
           [0007]    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  
       [0008]    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.  
         [0009]    FIGS.  1  ( a,b,c ) is an illustration of an apparatus according to the present invention.  
         [0010]    [0010]FIG. 2 is an illustration of an apparatus according to the present invention.  
         [0011]    FIGS.  3 ( a,b ) is an illustration of an apparatus according to the present invention.  
         [0012]    FIGS.  4 ( a,b,c,d ) is an illustration of an apparatus according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    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 two ends. The fixed mounting at one end, however, is configured so that it can be converted to allow a moveable relationship between the body and the sensing element. While the mounting is fixed, the sensing element is prevented from indicating temperature deviations, and the apparatus can be stored and handled freely. After the mounting is converted to provide a moveable relationship, however, a temperature excursion can cause the SMA portion to contract, providing a discernible change in the relationship between the sensing element and the body.  
         [0014]    Example Embodiment  
         [0015]    [0015]FIG. 1 a  is a sectional view of an embodiment of the present invention. A sensing element  110  mounts with a body  100 . A first end  112  of the sensing element  110  mounts fixedly with an attachment portion  101  of the body  100 . The sensing element comprises a SMA portion  111 . The sensing element further comprises a removable engagement portion  113  that engages a holding portion  102  of the body  100 . The engagement portion  113  prevents changes in length of the sensing element  100  relative to the body  100 . Conversion of the fixed relationship between the engagement portion  113  and the holding portion  102 , for example by cutting the engagement portion  113  from the sensing element  110 , allows the sensing element  110  to contract. Exposure to temperature above the critical temperature of the SMA portion  111  can cause the SMA portion  111  to contract (once the restraint of the engagement portion is removed), moving, for example, an indicating portion  114  relative to an indicator surface  103 . That movement can be perceived, for example by using different colors for the indicating portion  114  and the indicator surface  103 , allowing ready determination of whether the apparatus has been exposed to excessive temperatures.  
         [0016]    Once the SMA portion contracts, it does not return to its original length without the application of force. Consequently, the indication of an over-temperature condition persists, even if the temperature subsequently returns to below the critical temperature. The engagement portion allows the apparatus to be manufactured, stored, and handled at temperatures in excess of the critical temperature, since the engagement portion prevents contraction of the SMA portion. The apparatus can be installed in the temperature-critical environment, and the temperature reduced to below the critical temperature, before changing the fixed engagement. Subsequent temperature changes will provide the desired persistent indication of exposure to elevated temperature.  
         [0017]    The SMA portion can be a wire made with a SMA, of length equal to the distance between the two mounting portions in one state, and less than the distance in the other state. The wire can be passed through an opening such as a notch or hole in the body, and a bend or a thickened portion used to prevent the wire from moving back through the opening. After the apparatus is in place, and the temperature reduced below the critical temperature, then the bend or thickened portion can be removed. Subsequent temperature increases can initiate a shortening of the SMA wire; the difference in length relative to the body can provide an indication of an over-temperature condition. Even if the temperature excursion was only transitory, the SMA wire will not return to its original length once shortened, absent application of external force. Discernment of the condition of the apparatus can be facilitated by highlighting the relationship between the sensing element and the body. As examples, colored regions and witness marks can be used to make changes in the length or shape of the SMA portion readily apparent.  
         [0018]    Example Embodiment  
         [0019]    [0019]FIG. 2 is a schematic view of an embodiment of the present invention. A body  200  defines a channel having an end  201 , a groove  202 , an indicator region  203 , and terminating with the end of the body  204 . A sensing element  210  comprises a first end  211  shaped to fit in the end  201  of the channel, a length of SMA wire  212  sized to fit in the groove  202 , an indicator  213  (e.g., green), and an indicator  213 ′ (e.g., red), and a second end  214 , or engagement knob, sized to be too large to fit into the groove at the end of the body  204 . The length of wire  212  is, in its expanded or Austenitic state, approximately as long as the groove  202 , and, in its contracted or Martensitic state, shorter than the groove  202 .  
         [0020]    The sensing element can be installed into the body at a temperature above the Austenitic finish temperature A f , even though the length in the Martensitic state is less than the length of the groove. In the Martensitic state, the sensing element (SMA wire) is in a superelastic state. An end of the element can be pressed the end of the channel. The SMA wire can be stretched and the engagement knob placed beyond the end of the groove. Once the apparatus is in the desired temperature environment (e.g., frozen below the Austenitic finish temperature A f ), the sensing element can be altered, for example by cutting off the second end or the engagement knob. If the critical temperature is reached, then the SMA wire will contract and move the red indicator in front of the display window. The indicator&#39;s changed position can be highlighted with, for example, contrasting colors and windows in the body. Indicators having area of greater than 50 mm 2  can be suitable for some applications. The body in the figure is shown with one end of narrow cross-section to facilitate insertion in items whose temperature is to be monitored, for example into foodstuffs such as meat. The indicator in the figure is shown as a flat element; it could comprise a colored portion of the wire; or witness marks on the wire, the body, or both; or a differently shaped portion of the wire such as a flattened portion thereof.  
         [0021]    Example Embodiment  
         [0022]    FIGS.  3 ( a,b ) is an illustration of an embodiment of the present invention. A wire or ribbon is made of a shape memory alloy  310 . The wire or ribbon  310  mounts with a body  300 , extending from a first portion of the body to a second portion of the body. A first end of the wire or ribbon mounts with the first portion of the body such that movement of the wire or ribbon toward the second portion of the body is constrained, for example by pinning the wire or ribbon to the body. A second end of the wire or ribbon has a thickened section attached. The wire or ribbon passes through a portion of the body that allows passage of the wire or ribbon, but does not allow passage of the thickened section. Two indicators mount with the wire or ribbon. The body has a first end shaped as a point to allow easy insertion into meat or other items whose temperature is to be monitored. The body further comprises a window or other indication feature that allows discernment of movement of the indicators mounted with the wire or ribbon. For example, the body can comprise a window that is over the first indicator when the wire or ribbon is in its longest state. Contraction of the wire or ribbon can bring the second indicator into view in the window. Giving the indicators contrasting colors can allow for easy human discrimination between indicators. Changes in temperature will not cause a change in indicator as long as the thickened portion is attached since movement of the wire or ribbon, and hence movement of the indicators, is constrained. Removal of the thickened portion allows the wire or ribbon to contract when the temperature exceeds a critical value, moving the indicators so that a different indicator is now discernible. The movement constraint provided by the thickened portion can also be provided, for example, by a bend in the wire or ribbon, or an additional element added to the wire or ribbon such as a pin attached to the wire or ribbon.  
         [0023]    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 ).  
         [0024]    Example Manufacturing Process  
         [0025]    An example manufacturing procedure for a thaw/refrozen food sensor according to the present invention is briefly described here. This manufacturing method can be modified to accommodate other threshold temperature values and applications. The manufacturing procedure described provides for manufacture of the sensor at room temperature. The sensor can be inserted or mounted on a fresh food package before the package is frozen. The package can then be frozen, and the sensor used to indicate if the food package has ever been thawed, even if subsequently refrozen.  
         [0026]    As shown in FIG. 4 a,  manufacture can start in room temperature by placing an SMA wire assembly  411  in the sensor shell  400  with one end secured to a pin  412 . In this situation the SMA portion  411  is at room temperature which can be well above a threshold Austenitic finish temperature A f  of, for example, 1° C. (Celsius). Such an Austenitic temperature is slightly above a freezing temperature of 0° C. The SMA at room temperature is in a contracted state and is shorter than the total length of the shell  400 . Shell lengths between 5 cm and 15 cm can be suitable. The SMA wire (diameters of 250-500 microns can be used) is in a super-elastic state (Martensitic state):contracted and behaving like a rubber band. Thus, the red indicator  415  will be lined up with an indicator window  403 . At this stage of manufacturing the engagement knob  413  is grabbed and the wire  411  is stretched (superelastic) to reach the holding slot or portion  402  and the knob is placed securely in the notch or slot of holding portion or groove  402  as shown in FIG. 4 b.  The green indicator  414  now will be lined up with the indicator window  403  and the SMA wire will be in a stretched state. The cap  404  containing the indicator window  403  is now placed or firmly snapped on top of the sensor shell  400  and the sensor is ready to be used in a fresh unfrozen body of food package.  
         [0027]    The sensor can now be inserted into the body of foodstuff, for example chicken, sausage or turkey, to be frozen. As an example, limiting the body cross-sectional area to less than 27 mm 2  can accommodate insertion into foodstuffs, for example a 2 mm diameter needle shape can be suitable. The food package with the sensor inserted in it can be put in a freezer to be frozen below 0 degrees Celsius. Thus, the SMA wire becomes Austenitic and softens, losing its tension. The indicator window  403  still shows a green indicator  414  as shown in FIG. 4 b.  Having been frozen the food package can now be inspected at an inspection station before shipping or placement in cold storage compartments and the inspector can break-off or remove the engagement knob  413  before the frozen food package is placed in a freezer for storing or shipping purposes. Upon breakage or removal of the engagement knob  413 , as long as the temperature remains below the Austenitic temperature, the SMA wire will not contract and the green indicator  414  will show through the indicator window  403  as shown in FIG. 4 c.  If the food package ever experiences temperature above the Austenitic temperature, (for example, temperatures that would cause dangerous thawing of the foodstuff), the SMA element  411  will contract and the green  414  indicator will move to display the red indicator  415  through the indicator window  403  as shown in FIG. 4d. The indicator will not change to green, even if the sensor is refrozen, because the SMA wire does not lengthen to original length without an applied resilient force such as a linear spring.  
         [0028]    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.