Abstract:
A method of controlling a shape memory alloy actuator includes applying power to a shape memory alloy actuator. A measured actuation parameter is obtained from the shape memory alloy actuator. An operational characteristic parameter is derived based upon the power and the measured actuation parameter. An actuation state parameter is identified from the operational characteristic parameter. The actuation state parameter is used to modify the control of the shape memory alloy actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/500,576, filed Sep. 5, 2003, entitled “Speed and Motion Control of Actuators Using End-of-Travel Sensors,” and United States Provisional Patent Application No. 60/506,127, filed Sep. 25, 2003, entitled “Low Cost Resistive Control for SMA Actuators”, the disclosures of which are incorporated herein by reference. 
     
    
     BRIEF DESCRIPTION OF THE INVENTION  
       [0002]     This invention relates generally to shape memory alloy actuators. More particularly, this invention relates to an apparatus and method for a low cost shape memory alloy actuator using measured actuation parameter feedback.  
       BACKGROUND OF THE INVENTION  
       [0003]     Thermoelastic properties of shape memory alloys (SMAs) have been known since the 1930s, but commercially viable uses for SMAs were not widespread until the 1990s. Today, SMA actuators are finding unique applications in a variety of industries. Many of these applications require precise control over SMA actuator position or actuation speed. Various applications also require control over other actuation properties, such as end-of-travel limit-stops or overheat protection to prevent damage to the SMA actuator. In addition, it is often desirable that the SMA actuator has a small footprint, so that it may be used for limited-space applications, which require miniature or reduced-sized actuation mechanisms.  
         [0004]     The variation of resistance of elongated SMA elements during actuation is well known in the art. As the SMA element is heated from the martensite phase (low temperature, usually extended position) to the austenite phase (high temperature, contracted position), the resistance of the SMA element changes in response to change in temperature. This change in resistance and temperature may be correlated to a position of the SMA actuator and used for actuation control according to techniques known in the art, such as the technique described in commonly owned U.S. Pat. No. 6,574,958, incorporated herein by reference. Similarly, additional information regarding SMA actuator characteristics, such as load on the SMA actuator or environmental conditions, may be extracted from knowledge of the resistance of the SMA element and used to control the SMA actuator.  
         [0005]     In view of the foregoing, what is needed is an apparatus and method for actuation control of a shape memory alloy element using feedback of actuation characteristics. More specifically, what is needed is an apparatus and method that can accurately and quickly determine actuation characteristics, such as resistance of an SMA element or actuation speed, for actuator control while minimizing the need for additional components so as to reduce cost and maintain a small SMA actuator footprint.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention includes a method of controlling a shape memory alloy actuator by applying power to the shape memory alloy actuator. A measured actuation parameter is obtained from the shape memory alloy actuator. An operational characteristic parameter is derived based upon the power and the measured actuation parameter. An actuation state parameter is identified from the operational characteristic parameter. The actuation state parameter is used to modify the control of the shape memory alloy actuator.  
         [0007]     The invention also includes a mechanical actuator. The mechanical actuator has a shape memory alloy. A controller is connected to the shape memory alloy. The controller is adapted to apply power to the shape memory alloy, derive an operational characteristic parameter based upon a measured actuation parameter, identify an actuation state parameter from the operational characteristic parameter; and alter the application of power to the shape memory alloy based upon the actuation state parameter.  
         [0008]     The invention relies upon components that are typically already present in a shape memory alloy actuator. Therefore, the invention can be implemented at a relatively low cost. The various techniques of the invention provide designers with a variety of strategies for optimizing a given low cost shape memory alloy actuator. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0009]     The invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0010]      FIG. 1  illustrates a shape memory alloy actuator implemented using control techniques of the present invention.  
         [0011]      FIG. 2  illustrates process operations performed in accordance with an embodiment of the invention.  
         [0012]      FIG. 3  illustrates an apparatus for actuation control of a shape memory alloy actuator in accordance with an embodiment of the invention.  
         [0013]      FIG. 4  illustrates the relationship between resistance, collector voltage, and base current; the apparatus illustrated in  FIG. 3  uses this relationship to derive an operational characteristic parameter used in accordance with an embodiment of the invention.  
         [0014]      FIG. 5  illustrates an apparatus for actuation control of a shape memory alloy actuator using temperature and voltage compensation according to an embodiment of the present invention.  
         [0015]      FIG. 6  illustrates an apparatus for actuation control of a shape memory alloy actuator according to another embodiment of the present invention.  
         [0016]      FIG. 7  illustrates the relationship between resistance, time, and collector current; the apparatus of  FIG. 6  uses this relationship to derive an operational characteristic parameter used in accordance with an embodiment of the invention.  
         [0017]      FIG. 8  illustrates the relationship between actuator position, input power, and position sensors utilized in accordance with another embodiment of the invention. 
     
    
       [0018]     Like reference numerals refer to corresponding parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 1  illustrates a block diagram for actuation control of a shape memory alloy element using actuation feedback according to the present invention. Initially, a control input  110  corresponding to a desired actuator output is input to control process  120 . Actuator  130  may comprise a single shape memory alloy (SMA) element and associated drive circuitry, or alternatively may comprise additional SMA elements and various configurations of those SMA elements, including stroke-multiplying configurations such as those configurations disclosed in commonly owned U.S. Pat. No. 6,574,958. The actuator output  150  may be any characteristic associated with the actuation process, including but not limited to actuator position, actuator speed, actuator force, SMA element resistance, transistor voltage, actuator temperature, or other actuator characteristic that is a function of the actuation of an SMA actuator. As shown in  FIG. 1 , the actuator output is fed back to the controller  120  as a measured actuation parameter  140 . This measured actuation parameter  104  is typically a subset of the overall actuator output  150  and therefore may be delivered through a signal path that is different than the actuator output signal path.  
         [0020]     As will be described below, the controller  120  processes the input  110  and the measured actuation parameter  140  to derive an operational characteristic parameter for the shape memory alloy actuator. The operational characteristic parameter may be a variety of values including SMA resistance and actuator position, depending upon the embodiment. The controller  120  further processes the operational characteristic parameter to identify an actuation state parameter. As discussed below, the actuation state parameter may be such values as the SMA position or the actuation cycle duration. The control of the shape memory alloy actuator is then modified based upon the actuation state parameter.  
         [0021]     The operation of the device of  FIG. 1  is more fully appreciated with reference to  FIG. 2 .  FIG. 2  illustrates processing operations that may be implemented on the device of  FIG. 1 . The first processing operation of  FIG. 2  is to apply power to a shape memory alloy actuator ( 200 ). Next, a measured actuation parameter is obtained ( 202 ). As previously indicated, the measured actuation parameter may be a variety of values. By way of example, embodiments below disclose measured actuation parameters in the form of transistor voltage, time, and SMA location limits. Next, an operational characteristic parameter is derived based upon the power and measured actuation parameter ( 204 ). Embodiments of the invention described below include operational characteristic parameters in the form of SMA resistance and SMA position. An actuation state parameter is then identified from the operational characteristic parameter ( 206 ). Embodiments of the invention described below include actuation state parameters in the form of SMA position and actuation cycle duration. Finally, the control of the shape memory alloy actuator is modified based upon the actuation state parameter ( 208 ).  
         [0022]     As will be more fully appreciated with the specific examples below, the invention relies upon standard measurements (i.e., measured actuation parameters) to make actuator control decisions. However, these measurements are not used directly, rather they are used to deduce additional parameters, such as operational characteristic parameters and then actuation state parameters, which are then used in the control process. As demonstrated below, the utilization of these additional parameters allows for superior control of a shape memory alloy actuator. Advantageously, these additional parameters may be computed using relatively small physical and computational resources, which are implemented at low cost. Thus, the invention affords enhanced signal processing, while still affording the benefits of low cost and a small form factor. These and other benefits of the invention will be more fully appreciated with reference to the following embodiments.  
         [0023]      FIG. 3  illustrates an apparatus  300  to measure the resistance of a shape memory alloy (SMA) element  350  for actuation control according to one embodiment of the present invention. SMA element  350  is connected in series between a positive voltage  310  and the collector  332  of an npn-type bipolar junction transistor  330 . The emitter  336  of bipolar junction transistor  330  is connected to ground  320 . The bipolar junction transistor  330  is controlled by microcontroller  305 , which provides a base current to base  334  of bipolar junction transistor  330  via output pin  309  using a programmable current source. Input pin  307  of microcontroller  305  is connected between SMA element  350  and collector  332 . Power for microcontroller  305  is not shown, but may be provided by positive voltage  310  and ground  320  or by another source.  
         [0024]     Dashed line  304  in  FIG. 3  corresponds to the controller  120  previously discussed in connection with  FIG. 1 . Similarly, dashed line  302  corresponds to actuator  130 , although it is to be appreciated that control process  120  and actuator  130  may comprise additional modules, components, or devices not reflected in  FIG. 3 , such as modules, components, or devices to convert an SMA element resistance to an actuator position, or additional SMA elements or configurations.  
         [0025]     The apparatus  300  depicted in  FIG. 3  operates in the following manner. First, a collector voltage signal from collector  332  is input to microcontroller  305  via input pin  307 . The collector voltage signal is compared by comparator module  315  to a pre-determined threshold voltage signal. The collector voltage signal is then converted by analog-to-digital converter (ADC) module  325  to a digital voltage signal using an analog-to-digital conversion technique. Observe that the ADC  325  may be omitted since the comparator  315  operating with DAC  345  and processor  335  can implement the same function. If the collector voltage signal is less than or equal to the threshold voltage signal, processor module  335  processes the digital voltage signal to determine the resistance of SMA element  350 . If the collector voltage signal is greater than the threshold voltage signal, processor  335  increases the base current to base  334  of bipolar junction transistor  330  via output pin  309  using a programmable current source. Increasing base current to the base  334  causes additional current to flow through bipolar junction transistor  330 . Additional current through bipolar junction transistor  330  results in additional current through SMA element  350 , and causes the voltage at collector  332  to drop in response. This new collector voltage signal is input to microcontroller  305  via input pin  307  and the abovementioned process is repeated until a new collector voltage signal is less than or equal to the threshold voltage signal.  
         [0026]     The programmable current source associated with output pin  309  comprises a digital-to-analog converter (DAC) module  345  integrated with microcontroller  305 , according to one embodiment of the present invention. In another embodiment, microcontroller  305  does not comprise integrated DAC module  345 , and a secondary digital-to-analog current converter of n bit-depth is implemented by connecting n output pins of microcontroller  305  through n resistors in parallel to the base  334  of bipolar junction transistor  330 . The secondary digital-to-analog current converter of n bit depth may also be used in conjunction with an integrated digital-to-analog current converter to provide hardware gain scaling or offset adjust. Alternatively, gain scaling or offset adjust may be provided in software by microcontroller  305 .  
         [0027]     In one embodiment of the present invention, the threshold voltage signal is the minimum voltage detectable by microcontroller  305  for input pin  307 . According to this embodiment, input pin  307  functions as comparator module  315 . In operation, microcontroller  305  reads and converts successive collector voltage signals to digital voltage signals until a collector voltage signal falls below the minimum voltage for input pin  307 . When this occurs, microcontroller  305  may process the value of the last collector voltage signal detected to determine the resistance of the SMA element  350 . Alternatively, the comparison may be performed by discrete components or combinations thereof external to microcontroller  305 , or performed in software by microcontroller  305 .  
         [0028]     Conversion of the collector voltage signal to a digital voltage signal by ADC module  325  may be performed using any analog-to-digital conversion technique. The analog-to-digital conversion technique may include, but is not limited to, a software successive approximation register (SAR) using an iterative binary search algorithm that compares the digital voltage signal to the collector voltage signal, and selectively adjusts the bits of the digital voltage signal to approximate the collector voltage signal. One advantage of using SAR analog-to-digital conversion is fast conversion time, which minimizes the time power must be applied to SMA element  350  in order to read the collector voltage signal. Another advantage of using SAR analog-to-digital conversion is that the digital voltage signal may easily be calculated to n-bits of resolution by making n compares to the collector voltage signal. Because making additional compares increases the conversion time, the resolution may be chosen to optimize both signal accuracy and conversion time for a specific application. It is to be appreciated that other analog-to-digital conversion techniques, such as flash or half-flash encoding, external hardware conversion, or other internal software conversion may similarly be utilized to convert the collector voltage signal to a digital voltage signal. Alternatively, ADC module  325  may be integrated with input pin  307 . According to this embodiment, the collector voltage signal can be compared to the threshold voltage signal and converted to a digital voltage signal using only input pin  307 . The comparison and conversion steps may also by separated, including but not limited to using an external comparator in conjunction with an integrated ADC module  325 .  
         [0029]      FIG. 4  illustrates the relationship between resistance, collector voltage, and base current of apparatus  300 . Recall that the base current is known, since it is generated by the microcontroller  305 . The collector voltage, a measured actuation parameter, is also known, since it was acquired using the previously described techniques. The relationship of  FIG. 4  is also known empirically and is available (e.g., as a look-up table) for this embodiment of the invention. With the base current and the collector voltage known, a resistance value can be derived based upon the information of  FIG. 4 . This resistance value, which is an operational characteristic parameter, can then be used to produce an SMA position value. Known techniques for mapping an SMA resistance value to an SMA position are implemented by the microcontroller  305  to produce an actuation state parameter (i.e., position in this example), which may then be used to alter the control of the SMA actuator, if necessary.  
         [0030]     Referring to  FIG. 3  and  FIG. 4 , it is noted that when there is no base current to bipolar junction transistor  330 , the collector voltages corresponding to three different resistance values of SMA element  350  are identical. When the base current to bipolar junction transistor  330  is increased, the voltage at collector  332  drops as current begins to flow through SMA element  350 . This drop in collector voltage depends upon the resistance of SMA element  350 . As is illustrated in  FIG. 4 , a high resistance of SMA element  350  yields a large collector voltage drop at a specific base current, while a low resistance yields a smaller collector voltage drop at the same base current. It is therefore possible to determine the resistance of SMA element  350  by comparing the collector voltage to the base current and referring to a graph or table based on the operational characteristics of bipolar junction transistor  330 , such as a table stored in memory in microcontroller  305 . The resistance of SMA element  350  may also be calculated from the generic equation for a bipolar junction transistor: 
 
 Vcol=Vpos−Rsma*HFE*Ibase   (Equation 1) 
 
         [0031]     Wherein Vcol is the voltage at the collector of the bipolar junction transistor, Vpos is the positive voltage applied to the SMA element, Rsma is the resistance of the SMA element, HFE is the gain of the bipolar junction transistor, and Ibase is the base current. Alternately, a more accurate equation may be used to determine resistance of SMA element  350 , taking into account operational characteristics of the specific type of bipolar junction transistor  330 , including but not limited to gain variation over the active range of the transistor. Also note in  FIG. 4  that when bipolar junction transistor  330  is saturated, the collector voltages corresponding to three different resistance values of SMA element  350  all drop to nearly zero. Consequently a collector voltage in the active range, such as the threshold voltage depicted in  FIG. 4 , is chosen so that the resolution of microcontroller  305  is sufficient to distinguish between corresponding base currents and respective SMA resistance values.  
         [0032]     The preceding embodiment of the present invention utilizes an npn-type bipolar junction transistor. However, it is to be appreciated that in an alternate embodiment the bipolar junction transistor can be of a pnp-type. Similarly, the apparatus  300  may be adapted to use a field-effect transistor, including but not limited to a p-channel or n-channel type metal-oxide silicon field-effect transistor (MOSFET).  
         [0033]     The operational characteristics of a transistor can vary due to environmental factors such as change in temperature or voltage supply. A simple technique for calibrating a transistor consists of measuring the resistance of the SMA element when it is fully expanded and fully contracted. These resistance values may then be used to set transistor calibration parameters to compensate for environmental factors.  
         [0034]      FIG. 5  illustrates an alternate apparatus  500  to measure the resistance of a shape memory alloy element for actuation control with temperature and voltage compensation according to another embodiment of the present invention. SMA element  550  is connected in series between a positive voltage  510  and the collector  532  of a first npn-type bipolar junction transistor  530 . The emitter  536  of the first bipolar junction transistor  530  is connected to ground  520 . A reference resistor  555  is connected in series between positive voltage  510  and the collector  537  of a second npn-type bipolar junction transistor  541 . Second npn-type bipolar junction transistor  541  is chosen to be identical to first bipolar junction transistor  530 . The emitter  543  of second bipolar junction transistor  541  is connected to ground  520 . Bipolar junction transistors  530  and  541  are controlled by microcontroller  505 , which provides a base current to base  534  and base  539  of bipolar junction transistors  530  and  541 , respectively, using a programmable current source via output pin  509 . In an alternate embodiment, separate programmable current sources may be used to provide base currents to bipolar junction transistors  530  and  541 . Input pin  507  of microcontroller  505  is connected between SMA element  550  and collector  532  of bipolar junction transistor  530 . Input pin  508  of microcontroller  505  is connected between reference resistor  555  and collector  537  of bipolar junction transistor  541 . Power for the microcontroller  505  can be provided by positive voltage  510  and ground  520  or be provided by another source.  
         [0035]     The apparatus  500  depicted in  FIG. 5  operates similarly to apparatus  300  described previously in connection with  FIG. 3 , with the following additional features. Microcontroller  505  is adapted to read the voltage at collector  537  of second bipolar junction transistor  541  concurrently with the voltage at collector  532  of first bipolar junction transistor  530 . The voltage at collector  537  of second bipolar junction transistor  541  is a function of the current through reference resistor  555 . Because the resistance of reference resistor  555  is known, it is possible to determine the gain of the second bipolar junction transistor  541  using Equation 1 discussed previously in connection with  FIG. 4 . The gain of the second bipolar junction transistor  541  is a function of environmental factors such as change in temperature or voltage supply. Since second bipolar junction transistor  541  is chosen to be identical to first bipolar junction transistor  530 , microcontroller  505  may use information regarding the gain of the second bipolar junction transistor  541  to calibrate first bipolar junction transistor  530  and compensate for environmental factors. In an alternate embodiment of apparatus  500 , the microcontroller  505  may be configured to switch positive voltage  510  between bipolar junction transistors  530  and  535 , whereby the voltage at collector  537  is not read concurrently with the voltage at collector  532 , but is instead read sequentially or occasionally depending upon the specific application. Microcontroller  505  can also be configured to switch between bipolar junction transistors  530  and  541  using an alternate circuit, including but not limited to using additional transistors to control current flow to one or both transistors  530  and  541 . It is to be appreciated that while the current embodiment is described utilizing npn-type bipolar junction transistors, other transistor types such as pnp-type bipolar junction transistors or field-effect transistors may also be implemented without departing from the scope of the invention.  
         [0036]      FIG. 6  illustrates an apparatus  600  that uses a measured actuation parameter in the form of time to derive an operational characteristic parameter in the form of resistance, which is then converted to an actuation state parameter in the form of position. SMA element  650  is connected in series between a positive voltage  610  and the collector  632  of an npn-type bipolar junction transistor  630 . The emitter  636  of npn-type bipolar junction transistor  630  is connected to ground  620 . A resistor  655  is connected in series between the base  634  of npn-type bipolar junction transistor  630  and the collector  637  of a pnp-type bipolar junction transistor  641 . The emitter  643  of pnp-type bipolar junction transistor  641  is connected to positive voltage  610 . A base resistor  660  is connected in series between the base  639  of pnp-type bipolar junction transistor  641  and the output pin  607  of microcontroller  605 . Output pin  607  is adapted to operate as an open current sink. A feedback capacitor  680  is connected between output pin  607  of microcontroller  605  and collector  632  of npn-type bipolar junction transistor  630 . Collector  632  of npn-type bipolar junction transistor  630  is further connected to input pin  609  of microcontroller  605 .  
         [0037]     The apparatus depicted in  FIG. 6  operates in the following manner. Initially, the voltages of output pin  607  and input pin  609  are high, and feedback capacitor  680  is discharged. While the voltage of output pin  607  is high, pnp-type bipolar junction transistor  641  is in an ‘off’ state. The npn-type bipolar junction transistor  630  is in an ‘off’ state because no current flows through pnp-type bipolar junction transistor  641  to the base  634  of npn-type bipolar junction transistor  630 . Microcontroller  605  triggers a measurement cycle by temporarily decreasing the voltage of output pin  607 . Consequently, base current flows from base  639  of pnp-type bipolar junction transistor  641  through base resistor  660  to output pin  607 , causing pnp-type bipolar junction transistor  641  to saturate. At the same time, feedback capacitor  680  is still discharged. The voltage of output pin  607  is then returned to a high impedance state, and microcontroller  605  starts a timer. In the instant embodiment of the present invention, the timer is integrated with microcontroller  605  as timer module  615 . However, according to other embodiments, timer module  615  can alternatively be implemented in software by microcontroller  605 , or implemented using discrete components or combinations thereof external to microcontroller  605 . The pnp-type bipolar junction transistor  641  remains saturated as base current continues to flow from the base  639  of pnp-type bipolar junction transistor  641  through base resistor  660  into the feedback capacitor  680 . While pnp-type bipolar junction transistor  641  is saturated, npn-type bipolar junction transistor  630  is similarly saturated, and collector current flows through SMA element  650 . Voltage at input pin  609  is low while npn-type bipolar junction transistor  630  remains saturated. As base current flows from the base  639  of pnp-type bipolar junction transistor  641  into feedback capacitor  680 , feedback capacitor  680  continues to charge. Connecting feedback capacitor  680  to collector  632  of npn-type bipolar junction transistor  630  results in positive feedback, causing feedback capacitor  680  to charge rapidly. Once the feedback capacitor  680  is charged, base current ceases to flow from base  639  of pnp-type bipolar junction transistor  641 , and consequently both bipolar junction transistors  630  and  641  return to an ‘off’ state. When npn-type bipolar junction transistor  630  returns to an ‘off’ state, voltage at input pin  609  goes high and is sensed by sensor module  625 . Timer module  615  is then stopped and processor module  625  uses the information in  FIG. 7  to determine the resistance of SMA element  650 .  
         [0038]      FIG. 7  illustrates the relationship between resistance, time, and collector current of apparatus  600  described above in connection with  FIG. 6 . Referring to  FIG. 6  and  FIG. 7 , at t=t 1  a measurement cycle is triggered, saturating npn-type bipolar junction transistor  630 . The saturated collector current is inversely proportional to the resistance of SMA element  650 . Consequently, when the resistance of SMA element  650  is high, the saturated collector current will be lower than when the resistance of SMA element  650  is low. Similarly, the charge time of the feedback capacitor  680  is inversely dependent upon the saturated collector current. As is depicted in  FIG. 7 , lowering the saturated collector current increases the length of time npn-type bipolar junction transistor  630  is in an ‘on’ state. It is therefore possible to determine the resistance of SMA element  650  by measuring the length of time it takes for npn-type bipolar junction transistor  630  to return to an ‘off’ state and referring to a graph or table, such as a table stored in memory in microcontroller  605 . Alternately, the resistance of SMA element  650  may be calculated using an equation taking into account specific characteristics of apparatus  600 . After the resistance is computed, position can be determined using standard techniques.  
         [0039]     Those skilled in the art will appreciate that there are various alternate embodiments consistent with the invention. For example, a measured actuation parameter in the form of a start of cycle and end of cycle location limiter may be used to derive operational characteristic parameters in the form of start and finish positions, which may then be used to identify an actuation state parameter, such as cycle duration.  
         [0040]      FIG. 8  illustrates the relationship between actuator position, input power, and start and finish position sensors. Tpon is the time when input power is applied to the SMA element; Tas is the time the actuator begins to move from a start position; Tae is the time the actuator reaches the end of travel; Tpoff corresponds to when input power is cut to allow the actuator to return; Tars is the time when the actuator starts returning; Tare corresponds to the time the actuator completes its return to the start position; P 0  is a start-of-travel sensor adapted to detect when the actuator begins to move; and P 1  is an end-of-travel sensor adapted to detect when the actuator reaches the end of travel. It is noted that although  FIG. 8  illustrates the actuator motion in a linear fashion, this motion is often non-linear for many actuators. In addition,  FIG. 8  illustrates an actuator that returns automatically when power is cut, although a similar figure could be shown for actuators that need some form of power to return. For most actuators, the various times at which point the actuator achieves the above labels for a given application depends on many factors, many of which are environmental. For example, if the input power source fluctuates (e.g. batteries that drain over time), then this may affect the times. In actuators that are highly affected by environmental temperature, such as SMA actuators, then variations in temperature or simply air flow (e.g. the air conditioner just switched on) can radically affect these times. Variation in friction and load on an actuator or other tolerance changes from one product to another can also affect these times. By measuring the state of the P 0  and P 1  pins, the control process can make adjustments to compensate for many of these factors so as to achieve a consistency of motion.  
         [0041]     The technique associated with  FIG. 8  may alternatively be implemented by measuring the time between providing power to the SMA element and complete actuation of the SMA element. This embodiment of the present invention requires the SMA element to go through a complete actuation cycle before calculating the actuation speed. Consequently, control of the actuation speed is limited by the time required to complete at least one actuation cycle. In contrast, the actuation speed based on the take-off time or fall-off time can be determined during an actuation cycle, so that control of the actuation speed can occur during the first actuation cycle, allowing the control process to quickly adjust the actuation speed so as to achieve a consistency of motion. In addition, the control process is able to respond more quickly to changes in environmental factors, such as temperature or power supply variation, to maintain the desired actuation speed. Where an application requires the actuator to move at a specific time, a new take-off time may be estimated from a previous take-off time measurement and used to adjust when power is supplied to the SMA element actuator during subsequent cycles so that the actuator moves at the specific time.  
         [0042]     According to another embodiment of the present invention, the process associated with  FIG. 8  can also be implemented by measuring the take-off time or fall-off time using the resistance of the shape memory alloy element, without the need of position sensors, such as the start-of-travel sensor P 0  and end-of-travel sensor P 1 . Since resistance of the SMA element varies with position, it is possible to determine Tas (the time the actuator begins to move) by monitoring the resistance of the SMA element over time until a maximum resistance value, corresponding to start of actuation after power is applied, is achieved. The time it takes to achieve this maximum resistance value can be used to calculate the take-off time without a physical start-of-travel sensor. In addition, it is possible to determine Tae (end of actuation) by monitoring the SMA resistance until a predetermined resistance value is achieved, without a physical end-of-travel sensor. This approach has the advantage of eliminating sensors and therefore facilitates low cost actuators. On the other hand, this approach has shortcomings, including variation of the end-of-travel resistance over the lifetime of an SMA element and variation from one SMA element to another. For instance, over time an SMA element loses the ability to fully actuate, and thus the predetermined resistance value corresponding to actuator end-of-travel initially may not correspond to actuator end-of-travel after extended use. As a result, a control process might continuously apply power to the actuator in order to achieve the predetermined resistance value until catastrophic failure of the SMA element. In another approach, a technique of time-domain resistance analysis of an SMA element, such as the technique described in commonly owned U.S. Pat. No. 6,574,958, is used to determine both start and end-of-travel by monitoring the rate of change of the SMA resistance rather than the absolute value of the SMA resistance. Monitoring the rate of change of the SMA resistance has the advantage of not being dependent upon variation of SMA resistance due to type or extended use, and yields additional information regarding actuation characteristics, such as load on the actuator, overall performance of SMA element, or damage sustained by the SMA element. Monitoring the rate of change of the SMA resistance may be accomplished using any of the techniques described herein to measure the resistance of an SMA element or using any other technique known in the art, and control of actuation speed or other actuation characteristics may be achieved according to the methods and embodiments for actuation control of a shape memory alloy element using actuation feedback according to the present invention.  
         [0043]     Additional advantages of some embodiments of the present invention include automatic compensation of fluctuation in actuation load. The use of resistance for position measurement depends upon the relationship between resistance and percentage of transformation of the SMA element. Transformation of the SMA element depends, in turn, on both the temperature and mechanical load of the actuator. As the load on the actuator increases, the resistance of the SMA element increases in response.  
         [0044]     Because the actuation load affects the resistance of the SMA element, the actuator power source can play a critical role in determining how the actuator will respond to load fluctuations. With a fixed voltage supply, the power drawn by the SMA element is inversely proportional to its resistance. In the absence of feedback control, when the resistance increases due to a fluctuation in load, the resulting power drop will cause a drop in actuator temperature, and the resistance will further increase. This is equivalent to positive feedback due to fluctuation in actuator load, and is an unstable condition, which can result in the actuator moving away from a desired position.  
         [0045]     Alternatively, with a fixed current supply, the power drawn by the SMA element is directly proportional to its resistance. In the absence of feedback control, when the resistance increases due to a fluctuation in load, power will increase and cause a rise in actuator temperature. Since SMA element resistance is inversely dependent upon its temperature, the resistance will drop. This amounts to stabilizing feedback of the actuator, as the resistance of the SMA element tends to return to its initial state in response to fluctuations in load. This stabilizing effect of the fixed current supply may be only partially compensating for variations in load, due to additional cooling loss at higher actuator temperatures. Consequently, the resistance of the SMA element may not completely return to its initial state prior to the variation in load. However, this stabilizing effect is a natural outcome of using a fixed current supply and may be implemented without any additional intervention from the control process. The stabilizing effect similarly occurs when the load on the actuator decreases. For applications that require further compensation for fluctuations in load, additional corrections can be implemented concurrently using discrete components, software, or combinations thereof by way of non-limiting example.  
         [0046]     An embodiment of the present invention relates to a computer-readable medium having computer code thereon for performing various computer-implemented operations. In particular, control strategies of the invention may be implemented in software associated with a processor. The media and computer code may be those specially designed and constructed for the purposes of the present invention, or they may be of the kind well known and available to those having skill in the computer software arts. Examples of computer-readable media include hardware devices that are specially configured to store and execute program code, such as application-specific integrated circuits (“ASICs”), programmable logic devices (“PLDs”) and ROM and RAM devices. Examples of computer code include machine code, such as produced by a compiler, and files containing higher-level code that are executed by a computer using an interpreter. For example, an embodiment of the invention may be implemented using Java, C++, or other object-oriented programming language and development tools. Another embodiment of the invention may be implemented in hardwired circuitry in place of, or in combination with, machine-executable software instructions.  
         [0047]     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.