Patent Publication Number: US-2013239565-A1

Title: Spatially graded sma actuators

Description:
FIELD OF THE INVENTION 
     Exemplary embodiments of the invention are related to metallic shape memory alloy (“SMA”) actuators and, more specifically, to SMA actuators having unique thermal response characteristics. 
     BACKGROUND 
     Shape memory alloys are well-known in the art. Shape memory alloys are alloy compositions with at least two different temperature-dependent phases. The most commonly utilized of these phases are the so-called Martensite and Austenite phases. In the following discussion, the Martensite phase generally refers to the more deformable, lower temperature phase whereas the Austenite phase generally refers to the more rigid, higher temperature phase. When the shape memory alloy is in the Martensite phase and is heated, it begins to change into the Austenite phase. The temperature at which this phenomenon starts is often referred to as the Austenite start temperature (A s ). The temperature at which this phenomenon is complete is called the Austenite finish temperature (A f ). When the shape memory alloy is in the Austenite phase and is cooled, it begins to change into the Martensite phase, and the temperature at which this phenomenon starts is referred to as the Martensite start temperature (M s ). The temperature at which Austenite finishes transforming to Martensite is called the Martensite finish temperature (M f ). It should be noted that the above-mentioned transition temperatures are functions of the stress experienced by the SMA sample. Specifically, these temperatures increase with increasing stress. In view of the foregoing properties, deformation of the shape memory alloy is typically at or below the Austenite transition temperature (at or below A s ). Subsequent heating above the Austenite transition temperature causes the deformed shape memory alloy sample to revert back to its permanent shape. Thus, a suitable activation signal for use with shape memory alloys is a thermal activation signal having a magnitude that is sufficient to cause transformations between the Martensite and Austenite phases. 
     Due to their temperature-dependent shape memory properties, shape memory alloys are used or have been proposed for use as actuators or other elements requiring controlled movement in various mechanical and electromechanical devices or other applications such as air flow control louvers, reversibly deployable grab handles, portable insulin pumps, and computer media eject mechanisms, to name a few. One commonly-used configuration is that of an SMA wire with two ‘remembered’ lengths, where the wire is attached to an element or device component that is moved between different positions by transforming the wire between longer and shorter remembered lengths. Other configurations can be utilized as well, such as an SMA actuator that can be transformed between a straight and bent shape. The thermal stimulus to transform an SMA actuator between different states can be a direct external thermal stimulus, such as heat applied from a heat source like an infrared, convective, or conductive heating element. However, in the case of an SMA wire actuator, the thermal stimulus is often applied by simply running electrical current through the wire to cause it to heat up, and terminating the current so that the wire cools down by transferring heat to the surrounding cooler environment. 
     The temperature at which the shape memory alloy remembers its high temperature form when heated can be adjusted by slight changes in the composition of the alloy and through thermo-mechanical processing. In nickel-titanium shape memory alloys, for example, it can be changed from above about 100° C. to below about −100° C. The shape recovery process can occur over a range of just a few degrees or exhibit a more gradual recovery. The start or finish of the transformation can be controlled to within a degree or two depending on the desired application and alloy composition. The mechanical properties of the shape memory alloy vary greatly over the temperature range spanning their transformation, typically providing shape memory effect, superelastic effect, and high damping capacity. For example, in the Martensite phase a lower elastic modulus than in the Austenite phase is observed. Shape memory alloys in the Martensite phase can undergo large deformations by realigning the crystal structure arrangement with the applied stress, e.g., pressure from a matching pressure foot. The material will retain this shape after the stress is removed. 
     The transition of a shape memory alloy between Martensitic and Austenitic states as a function of temperature is depicted in the plot of  FIG. 1  where vertical axis ξ represents the fraction of the composition in the Martensite state and the horizontal axis T represents the temperature. The upper curve shown in  FIG. 1  with the accompanying arrow pointing downward and to the right depicts the transition from the Martensitic state to the Austenitic state caused by an increase in temperature, with the A s  and A f  temperatures denoted on the horizontal axis. The lower curve in  FIG. 1  with the accompanying arrow pointing upward and to the left depicts the transition from the Austenitic state to the Martensitic state caused by a decrease in temperature, with the M s  and M f  temperatures denoted on the horizontal axis. 
     For many shape memory alloys, the change between the Martensitic state and the Austenitic state and vice versa in response to thermal stimulus can occur relatively quickly. This may be due to various factors such as the composition having a narrow temperature range between the A s  and A f  temperatures and/or between the M s  and M f  temperatures. Other factors include the electrical characteristics of the shape memory alloy being such that the temperature of an SMA wire heats quickly through the A s  to A f  temperature range when current is applied. This can lead to a relatively rapid change between remembered shapes or lengths of an SMA actuator, which is undesirable in many circumstances where a slower actuation is desired for aesthetic and/or functional reasons. 
     Accordingly, it is desirable to provide a shape memory alloy element where the response can be tailored to meet target actuation rates in response to a thermal stimulus. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a shape memory alloy element is configured to undergo a graded thermal change along a dimension of the shape memory alloy element in response to thermal stimulus. This graded thermal change produces a change between the Martensitic and Austenitic states of the shape memory alloy that is graded along this dimension, which in turn produces a graded displacement response of the shape memory element. 
     In an exemplary embodiment of the invention, the graded thermal response of the SMA element is produced by a gradation, along a dimension of the element, in the ratio of surface perimeter to cross-sectional area in a plane perpendicular to that dimension. In another exemplary embodiment, the graded thermal response of the SMA element is produced by a gradation, along a dimension of the element, in cross-sectional geometrical configuration in a plane perpendicular to that dimension. In yet another exemplary embodiment, SMA element has a coating thereon, and the graded thermal response is produced by a gradation, along a dimension of the SMA element, in cross-sectional geometrical configuration in a plane perpendicular to that dimension, or in thickness. 
     In yet another exemplary embodiment, an actuator includes a shape memory alloy element that is configured to undergo a graded thermal change along a dimension of the shape memory alloy element in response to thermal stimulus. This graded thermal change produces a change between the Martensitic and Austenitic states of the shape memory alloy that is graded along this dimension, which in turn produces a graded displacement response along the dimension of the shape memory element. In exemplary embodiments, the graded thermal response is provided by gradations, along that dimension, in the configuration of the SMA element or in a coating on the SMA element, as described above. In another embodiment, the graded thermal response of the SMA element is provided by a gradation, along a dimension of the SMA element, in the cross-sectional geometry or thickness of a portion of the actuator in thermal communication with the SMA element. In yet another exemplary embodiment, the graded thermal response is provided by a gradation, along a dimension of the SMA element in convection to which the SMA element is subjected. 
     The above features, and advantages thereby provided, along with other features and advantages are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which: 
         FIG. 1  is a plot of phase change versus temperature of a typical shape memory alloy; 
         FIG. 2  depicts a longitudinal cross-section view of an embodiment where an SMA element has a continuous gradation in diameter; 
         FIG. 3  depicts a longitudinal cross-section view of an embodiment where an SMA element has a coating with a continuous gradation in thickness; 
         FIG. 4  depicts a longitudinal cross-section view of an embodiment where an SMA element has stepwise gradations in diameter; 
         FIG. 5  depicts a longitudinal cross-section view of an embodiment where an SMA element has a coating with stepwise gradations in thickness; 
         FIG. 6  depicts a longitudinal cross-section view of an embodiment where an SMA element has stepwise and continuous gradations in diameter; 
         FIG. 7  depicts a longitudinal cross-section view of an embodiment where an SMA element has a coating with stepwise and continuous gradations in thickness; 
         FIGS. 8A and 8B  depict an embodiment where an SMA element has gradation in cross-sectional geometry; 
         FIGS. 9A and 9B  depict an embodiment where an SMA element has a coating with a gradation in cross-sectional geometry; 
         FIG. 10  depicts a longitudinal cross-section view of an actuator where a portion of the actuator in thermal communication with an SMA element has a gradation in thickness; and 
         FIG. 11  depicts a perspective view of an actuator configured to provide an SMA element with a gradation in convection. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     In accordance with an exemplary embodiment of the invention, a shape memory alloy element is configured to undergo a graded thermal change along a dimension of the shape memory alloy element in response to thermal stimulus. By graded thermal change along a dimension of the SMA element, it is meant that at a point in time, the thermal energy level at one position along this dimension is different than the thermal energy level at a different position along the dimension. Since it is the addition or withdrawal of thermal energy from the shape memory alloy that induces the phase change back and forth between the Austenitic and Martensitic states, the ability to modify the timing of thermal change at different positions on the SMA element enables the modification of the timing of the phase change at different positions on the SMA element, thereby modifying the timing of the displacement response of the SMA element in response to thermal stimulus. SMA elements can be formed in a variety of configurations and, accordingly there is no particular limitation on the orientation of the dimension along which the SMA element exhibits a graded thermal change as long as it provides the desired displacement response of the SMA element. In an exemplary embodiment, the dimension is a linear dimension. In another exemplary embodiment, the SMA element is in the form of a shape memory alloy wire and the linear dimension is parallel to the longitudinal axis of the wire. 
     The graded thermal response along a dimension of the SMA element can be provided by a gradation, along that dimension, in the ability of the SMA element to absorb or dissipate heat. In one exemplary embodiment, the graded thermal response is provided by a gradation, along the dimension, in the ratio of surface perimeter to cross-sectional area in a plane that is perpendicular to that dimension. As the gradation is integrated along the dimension, the ratio of cross-sectional area to surface perimeter corresponds to a ratio of volume to surface area. At a given density, volume corresponds to mass, and thus to the quantity of thermal energy in the SMA element. At a given heat transfer coefficient for the SMA material, the surface area corresponds to the rate of heat transfer into or out of the SMA element through that surface. Thus a greater ratio of cross-sectional area to surface perimeter (area to perimeter ratio or “APR”) will indicate slower heat transfer between the SMA element and its surroundings while a higher ratio will indicate faster heat transfer. In the typical case of heat energy generated internally by application of electrical current to the SMA element, areas with a lower APR will dissipate that heat more readily than areas with a higher ratio. Not accounting for any effect of cross-sectional variations on the rate of electrical resistance heat generation, areas with a higher APR will heat up more readily in response to the application of electrical current and will cool down more slowly when the current is removed, compared to areas with a lower APR. In one exemplary embodiment, the graded thermal response can be utilized to provide a time-based gradation in the displacement response of the SMA element where higher APR portions of the element exhibit a faster response during heating to thermal stimulus and lower APR portions of the element exhibit a slower response during heating. The reverse holds for cooling after the current has been shut off. In another exemplary embodiment, the graded thermal response can be utilized to provide a controllable overall displacement in response to the application varying levels of electrical current. In this embodiment, a given current level generates an amount of heat sufficient to raise the temperature high enough in some (higher APR) portions of the element to induce a phase change from Martensite to Austenite, but not in some (lower APR) portions of the element. Progressively higher current levels will cause lower APR portions to reach temperature levels sufficient to induce a phase change, thereby producing greater overall levels of displacement in the element. In this fashion, controllable levels of actuation can be provided by varying the current. 
     In an exemplary embodiment, APR can be varied by varying thickness or diameter of an SMA element. Turning now to the figures, where the same numbers may be used to identify the same or like elements in different figures.  FIG. 2  depicts a longitudinal cross-sectional view of an SMA element  10  in the form of a round SMA wire. In  FIG. 2 , SMA element  10  has right end  12  and left end  14 , which may optionally be configured for attachment to external elements or components to be acted on by the SMA element. The element, which is formed from a shape memory alloy  15 , has a continuous gradation in diameter, the diameter continuously changing from a smaller diameter at the left end  14  to a larger diameter at the right end  12 . For a round wire, the cross-sectional area is equal to πr 2  and the surface perimeter is equal to 2πr, and thus the APR is πr 2 /2πr=r/2. Thus, the continuously varying diameter or radius of the SMA wire shown in  FIG. 2  provides a continuously varying APR, and thus a varying thermal change along the axial dimension of the wire. 
     In addition to varying the thickness or diameter of the SMA element itself, APR can be varied with a coating on the SMA element  10  of varying thickness.  FIG. 3  depicts a longitudinal cross-section view of an SMA element  10  comprising a round SMA wire formed from a shape memory alloy  15  having a coating  17  thereon. The coating  17  has a continuous gradation in thickness, from no coating at the left end  14  to a thick coating at the right end  12 . The use of the coating  17  may provide additional parameters for tailoring the thermal response characteristics of the SMA element  10 , as the coating  17  allows for thermal transfer properties to be varied with APR, while eliminating variance in electrical resistance heat generation caused by cross-sectional area of the SMA metal itself. The coating  17  can also have a different thermal conductivity and different heat capacity than the SMA material itself, providing further parameters for tailoring the thermal response of the SMA element  10 . For example, the composition of the coating  17 , and correspondingly its heat capacity and/or thermal conductivity, can be varied in a graded fashion along an axial dimension of the SMA element  10 . 
       FIGS. 2 and 3  depict exemplary embodiments where SMA elements exhibit a continuous gradation in APR. In another exemplary embodiment, an SMA element can include a stepwise gradation in APR.  FIGS. 4 and 5  depict exemplary embodiments of SMA elements with stepwise gradations. In  FIG. 4 , SMA element  10  having right end  12  and left end  14  is formed from shape memory alloy  15 . The SMA element  10  has stepwise gradations in diameter between section  20  having a first diameter, section  22  having a diameter larger than the first diameter, and section  24  having a diameter larger than the diameter of section  22 . In  FIG. 5 , an SMA element  10  having a constant diameter wire formed from shape memory alloy  15  has a coating  17  thereon. Coating  17  has stepwise gradations in diameter between section  20  having a first thickness, section  22  having a thickness larger than the first thickness, and section  24  having a thickness larger than the thickness of section  22 . The stepwise gradations can be abrupt as shown for example in  FIG. 6  or they can have a chamfered configuration as shown in  FIG. 4 . The chamfered configuration can help manage stress concentration in the SMA element  10 , potentially avoiding formation of cracks that could lead to premature failure of the SMA element  10 . 
       FIGS. 6 and 7  depict embodiments of SMA elements with both continuous and stepwise gradations. In  FIG. 6 , SMA element  10  having right end  12  and left end  14  is formed from shape memory alloy  15 . The SMA element  10  has stepwise gradations in diameter between section  20  having a first diameter, section  22  having a diameter larger than the first diameter, and section  24  having a diameter larger than the diameter of section  22 . Additionally, the diameter of the SMA element  10  undergoes a continuous gradation, becoming progressively larger moving from left end  14  toward right end  12 , in each of the sections  20 ,  22 , and  24 .  FIG. 7  depicts an SMA element  10  that includes a coating  17  having gradations thickness between section  20  having a first thickness, section  22  having a thickness larger than the first thickness, and section  24  having a thickness larger than the thickness of section  22 . Additionally, the thickness of the SMA element  10  undergoes a continuous gradation, becoming progressively thicker moving from left end  14  toward right end  12 , in each of the sections  20 ,  22 , and  24 . 
     The embodiments in  FIGS. 2-7  described above rely on a gradation in cross-sectional area to surface perimeter (“APR”) to provide a gradation of heat flow in and out of an SMA element, thereby producing a graded thermal change and concomitant graded displacement response of the SMA element. In another exemplary embodiment, a graded thermal change along a dimension of an SMA element can result from a gradation in cross-sectional geometry in a plane of the SMA element perpendicular to that dimension. The cross-sectional geometry of an SMA wire affects the pattern of conductive heat transfer within the SMA element, which in turn impacts the distribution of heat energy that causes the SMA phase transformation. Accordingly, a gradation in cross-sectional geometry produces a graded thermal change and concomitant graded displacement response of the SMA element. Although a gradation in cross-sectional geometry may often be accompanied by a gradation in APR, a gradation in cross-sectional geometry would impact heat flux and distribution of heat energy in the SMA element even if the cross-sectional gradation were implemented with configurations and overall thickness/diameter variations so as to hold APR constant.  FIGS. 8 and 9  depict embodiments of SMA elements having a gradation in cross-sectional geometry as illustrated by radial cross-sectional views from different positions along the length of an SMA wire.  FIGS. 8A and 8B  depict a radial cross-section view of an SMA element  10  formed from shape memory alloy  15  where the SMA element  10  has a gradation between a round cross-sectional geometry as shown in  FIG. 8A  and a more complex cross-sectional geometry as shown in  FIG. 8B .  FIG. 8B  depicts a complex cross-sectional geometry formed from shape memory alloy  15 , having peripheral lobe portions  32  connected to circular cross-sectioned central portion  34  by legs  36 . In such a configuration, the peripheral lobe portions  32  would transfer heat to and from the surrounding environment more rapidly than central portion  34 , thereby providing a variation in heat flux (compared to the circular cross-sectioned geometry shown in  FIG. 8A ) in the SMA element  10  when it is either heating up or cooling down.  FIGS. 9A and 9B  depict a gradation in cross-sectional geometry provided by a coating  17 , where the SMA element  10  has a gradation between a round cross-sectional geometry as shown in  FIG. 9A  and a more complex cross-sectional geometry as shown in  FIG. 9B .  FIG. 9B  depicts an SMA element  10  formed from a shape memory alloy  15  having a coating  17  thereon in a star-shaped configuration. 
     As discussed above, SMA elements such as SMA wires may be used as actuators for a variety of devices simply by attaching the ends of the wire to components the actuator is intended to act upon and subjecting the wire to thermal stimulus. SMA elements can also be integrated with other components to form an actuator. For example, an SMA wire may be encased in a sleeve for protection or to maintain its position or shape in a particular configuration. Any of the above-described SMA elements can be integrated with other components to form an actuator. Additionally, in some exemplary embodiments described herein, a portion of the actuator in thermal communication with the SMA element includes a gradation, along a dimension of the SMA element, in cross-sectional geometrical configuration in a plane perpendicular to that dimension, or in thickness. Such embodiments are similar to the coating embodiments described above in  FIGS. 3 ,  5 ,  7 , and  9 , except that the gradation is provided by a portion of the actuator in thermal communication with the SMA element instead of by a coating on the SMA element. An exemplary embodiment is illustrated in  FIG. 10 , where actuator  40  has an SMA element  10  such as an SMA wire having right end  12  and left end  14  formed from a shape memory alloy  15 . The SMA element  10  is slidably disposed in a sleeve member  42 . A tight tolerance between the outer diameter of the SMA element  10  and the inner diameter of the sleeve  42  promotes thermal communication between the SMA element  10  and the sleeve member  42 . Sleeve member  42  is shown having a continuous gradation in thickness, from minimal thickness at the left end  14  to a greater thickness at the right end  12 . 
     In another exemplary embodiment, a graded thermal change can be provided to an SMA element by varying the degree of convection to which the SMA element is subjected. This can be accomplished in various ways, such as by providing an actuator with a fan that directs a graded pattern of airflow over the SMA element, by providing an actuator sleeve or housing that has a graded pattern of openings, or both. Portions of the SMA element exposed to greater levels of convection will have a higher rate of heat transfer to or from the surrounding environment, thus creating a thermal gradation in the SMA element, thereby providing a graded displacement response. An exemplary embodiment is depicted in  FIG. 11 , in which SMA element  10  is slidably disposed in actuator housing  44 . Actuator housing  44  is shown with grille members or fins  46  having a graded spacing therebetween so as to form a graded pattern of openings. Grille members or fins  46  are shown with wider spacing (thus allowing for greater convection) toward the left end  14  of the SMA element  10 , and narrower spacing (thus allowing for less convection) toward the right end  12  of the SMA element  10 . 
     Suitable shape memory alloy materials for fabricating the conformable shape memory article(s) described herein include, but are not intended to be limited to, nickel-titanium based alloys, indium-titanium based alloys, nickel-aluminum based alloys, nickel-gallium based alloys, copper based alloys (e.g., copper-zinc alloys, copper-aluminum alloys, copper-gold, and copper-tin alloys), gold-cadmium based alloys, silver-cadmium based alloys, indium-cadmium based alloys, manganese-copper based alloys, iron-platinum based alloys, iron-palladium based alloys, and the like. The alloys can be binary, ternary, or any higher order. Selection of a suitable shape memory alloy composition depends on the temperature range where the component will operate. SMA elements typically must be worked or trained at different temperatures in order to remember different shapes between the Austenitic and Martensitic states. SMA elements may exhibit one-way or two-way shape memory depending on the application for which they are intended, and the embodiments disclosed herein may be used with either one-way or two-way SMA elements. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the present application.