[0001]     Shape memory alloys (or SMAs) are alloys that can exist in two different solid phases at different temperatures. Shape memory alloys typically can change shape upon heating above a solid-solid phase-transition temperature and to return to a certain alternate shape upon cooling. This property makes many shape memory alloys suitable for use as actuators. For many applications, heating and cooling of shape memory alloys depend on ambient conditions. This can lead to difficulties for applications where a shape memory alloy will be exposed to a variety of ambient conditions. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0002]      FIG. 1  is a block diagram of an embodiment of an actuator system, according to an embodiment of the present disclosure.  
         [0003]      FIG. 2  is an embodiment of an actuator, according to another embodiment of the present disclosure.  
         [0004]      FIG. 3  is an exemplary resistance versus temperature curve of an embodiment of an actuator, according to another embodiment of the present disclosure. 
     
    
     DETAILED DESCRIPTION  
       [0005]     In the following detailed description of the present embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice disclosed subject matter, and it is to be understood that other embodiments may be utilized and that process, electrical or mechanical changes may be made without departing from the scope of the claimed subject matter. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the claimed subject matter is defined only by the appended claims and equivalents thereof.  
         [0006]      FIG. 1  is a block diagram of an actuator system  100 , according to an embodiment. For one embodiment, system  100  includes a power loop  110  that includes a power supply  120  electrically connected to a shape memory alloy actuator  130 . System  100  also includes a control loop  140  that includes a current sensor  150  electrically connected in power loop  110  between power supply  120  and shape memory alloy actuator  130 . For one embodiment, a controller  160  of control loop  140  is electrically connected between current sensor  150  and power supply  120 . Power supply  120  may be a constant-current or a constant-voltage power supply or a power supply that can operate in constant-current or constant-voltage mode to provide, respectively, substantially constant current over a range of voltage or substantially constant voltage over a range of current. For one embodiment, the current and/or voltage supplied by power supply  120  can be varied. For another embodiment, a voltage sensor  170  may be connected across shape memory alloy actuator  130 . For some embodiments, an analog-to-digital (A/D) converter  180  is included in control loop  140  between current sensor  150  and/or voltage sensor  170 . For other embodiments, A/D converter  180 , current sensor  150 , and/or voltage sensor  170  are integral portions of controller  160 . Note that A/D converter  180  converts analog signals received thereat from current sensor  150  and/or voltage sensor  170 , converts them to digital signals, and transmits to controller  160 .  
         [0007]     For another embodiment, controller  160  is adapted to perform methods in accordance with embodiments of the present disclosure in response to computer-readable instructions. These computer-readable instructions are stored on a computer-usable media  190  of controller  160  and may be in the form of software, firmware, or hardware. In a hardware solution, the instructions are hard coded as part of a processor, e.g., an application-specific integrated circuit (ASIC) chip. In a software or firmware solution, the instructions are stored for retrieval by controller  160 . Some additional examples of computer-usable media include static or dynamic random access memory (SRAM or DRAM), read-only memory (ROM), electrically-erasable programmable ROM (EEPROM or flash memory), magnetic media and optical media, whether permanent or removable. Many consumer-oriented computer applications are software solutions provided to the user on some removable computer-usable media, such as a compact disc read-only memory (CD-ROM).  
         [0008]     For one embodiment, shape memory alloy actuator  130  is a wire or a block, as shown in  FIG. 2 , e.g., of a nickel titanium alloy, such as Nitinol (Nickel-Titanium Naval Ordnance Laboratory).  FIG. 3  shows that when shape memory alloy actuator  130  is heated while in a first solid phase (solid phase I), its resistance increases as its temperature increases until point A of  FIG. 3 , and its shape remains substantially unchanged. When the temperature is increased past a solid-solid phase-transition temperature of shape memory alloy actuator  130 , shape memory alloy actuator  130  changes shape when it changes from the first solid phase to a second solid phase (solid phase II). For one embodiment, the change in shape is characterized by a decrease in length L and an increase in the cross-sectional area A of shape memory alloy actuator  130 , as shown in  FIG. 2 . The decrease in length and the increase in cross-sectional area act to reduce the resistance of shape memory alloy actuator  130 . Further increases in temperature cause the resistance of shape memory alloy actuator  130  to increase while in its second solid phase, i.e., after point B of  FIG. 3 , while the shape remains substantially unchanged.  
         [0009]     In operation, control loop  140  determines the electrical resistance of shape memory alloy actuator  130  as it is heated by dissipating current supplied thereto by power supply  120 . Controller  160  monitors the resistance during heating. After the resistance starts to decrease, as a result of the phase transition and associated change in shape of shape memory alloy actuator  130 , and subsequently just starts to increase, signaling that the change in shape is complete, controller  160  sends a signal to power supply  120  instructing it to stop supplying power to shape memory alloy actuator  130 . Use of the term “complete” in the specification with reference to the degree to which a change in shape of the shape memory alloy has occurred includes a degree of change such that change in shape of the shape memory alloy is at least substantially complete. The precision with which the time when the resistance of shape memory alloy just begins to increase can be determined will affect the range within which the determination of the time when the change in shape is substantially complete will be of the time when the change in shape is fully complete. And, the uncertainty associated with determining when the resistance of shape memory alloy, such as in shape memory alloy actuator  130 , just begins to increase will be affected by the measurement tolerances associated with techniques and/or hardware selected for performing the resistance measurement, such as for controller  160 , voltage sensor  170 , analog-digital converter  180 , power supply  120 , and current sensor  150 . That is, when a slope of a curve of the resistance of shape memory alloy actuator  130  versus the temperature of shape memory alloy actuator  130  (or the heating time) transitions from negative to positive (i.e., at about point B of  FIG. 3 ), controller  160  instructs power supply to stop heating shape memory alloy actuator  130 , e.g., by stopping the flow of current to the shape memory alloy actuator  130 . This acts to adjust the heating time according to changing ambient conditions and thus acts to reduce the likelihood that shape memory alloy actuator  130  will be over or under heated as a result of changing ambient conditions that could occur when the heating time is fixed.  
         [0010]     For other embodiments, controller  160  instructs power supply to reduce heating shape memory alloy actuator  130 , e.g., by reducing the flow of current to the shape memory alloy actuator  130  to a level where the resistance of shape memory alloy actuator  130  is maintained at about that of point B. For some embodiments, the controller  160  can maintain shape memory alloy actuator  130  alternately in one of two states. In a first state, the actuator  130  is be maintained at about point A, e.g., at about the highest temperature in  FIG. 3  at which shape memory alloy actuator  130  is still elongated. That is, where the slope of the curve of the resistance of shape memory alloy actuator  130  versus the temperature of shape memory alloy actuator  130  transitions from positive to negative. In a second state, shape memory alloy actuator  130  is maintained at about point B, e.g., at about the lowest temperature in  FIG. 3  at which shape memory alloy actuator  130  is contracted. Maintaining the actuator at point A or B acts to reduce the time used to change to the other state. Note that the power supplied to shape memory alloy actuator  130 , and thus the amount of heat, is increased to change from phase I to phase II and to activate shape memory alloy actuator  130 . Note further that the amount of power (or heat) used to maintain shape memory alloy actuator  130  at point A is less than that used to maintain shape memory alloy actuator  130  at point B.  
         [0011]     For one embodiment, the resistance of shape memory alloy actuator  130  is measured by measuring the current flow through power loop  110  using current sensor  150  and the voltage drop across shape memory alloy actuator  130  using voltage sensor  170  and subsequently applying Ohms Law to compute the resistance from the measured current and voltage drop. This enables the resistance to be computed at a plurality of times during heating of shape memory alloy actuator  130 . For another embodiment, where power supply  120  is a constant-current power supply, the current is set and may be input into controller  160  by a user. For this embodiment, current sensor  150  would not be used, since the voltage drop from voltage sensor  170  can be used with the set value of the current to determine the resistance. For other embodiments, controller  160  may used to set a current or voltage output of power supply  120  via user inputs.  
         [0012]     For some embodiments, where power supply  120  is a constant-voltage power supply, the voltage is set and may be input into controller  160  by a user. For these embodiments, voltage sensor  170  would not be used, since the current flow from current sensor  150  can be used with the set value of the voltage to determine the resistance. Note that this may involve an accounting of other resistances in power loop  110  or may presuppose that these resistances are negligible compared to the resistance of shape memory alloy actuator  130 .  
         [0013]     For one embodiment, current sensor  150  may be a calibrated sense resistor having a predetermined resistance value connected in series with shape memory alloy actuator  130 , and the current through current sensor  150  and thus through power loop  110  is determined by measuring a voltage drop across the sense resistor and using Ohms law with the measured voltage drop and the predetermined resistance value.  
         [0014]     For other embodiments, failure of shape memory alloy actuator  130 , e.g., a break in shape memory alloy actuator  130 , may be detected by there being no current through power loop  110  or an effectively infinite resistance determined across shape memory alloy actuator  130 .  
       CONCLUSION  
       [0015]     Although specific embodiments have been illustrated and described herein it is manifestly intended that the scope of the claimed subject matter be limited only by the following claims and equivalents thereof.
An embodiment of a method includes determining when a shape memory alloy completes changing shape based on measurements of a resistance of the shape memory alloy and reducing power supplied to the shape memory alloy after determining when the shape memory alloy completes changing shape.