Apparatus and method for low cost control of shape memory alloy actuators
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.
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 INVENTIONThis 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 INVENTIONThermoelastic 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.
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.
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 INVENTIONThe 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.
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.
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 FIGURESThe invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which:
Like reference numerals refer to corresponding parts throughout the several views of the drawings.
DETAILED DESCRIPTION OF THE INVENTION
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.
The operation of the device of
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.
Dashed line 304 in
The apparatus 300 depicted in
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.
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.
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.
Referring to
Vcol=Vpos−Rsma*HFE*Ibase (Equation 1)
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
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).
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.
The apparatus 500 depicted in
The apparatus depicted in
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.
The technique associated with
According to another embodiment of the present invention, the process associated with
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.
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.
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.
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.
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.
Claims
1. A method of controlling a shape memory alloy actuator, comprising:
- applying power to a shape memory alloy actuator;
- obtaining a measured actuation parameter from said shape memory alloy actuator;
- deriving an operational characteristic parameter based upon said power and said measured actuation parameter; and
- identifying an actuation state parameter from said operational characteristic parameter, said actuation state parameter for use in modifying the control of said shape memory alloy actuator.
2. The method of claim 1 wherein applying power to said shape memory alloy actuator includes applying current to said shape memory alloy actuator.
3. The method of claim 1 wherein obtaining a measured actuation parameter includes obtaining a measured actuation parameter selected from voltage, time, and location limit.
4. The method of claim 1 wherein deriving an operational characteristic parameter includes deriving an operational characteristic parameter selected from resistance and position.
5. The method of claim 1 wherein identifying an actuation state parameter includes identifying an actuation state parameter selected from position and cycle duration.
6. The method of claim 1 wherein applying power to said shape memory alloy actuator includes applying current to said shape memory alloy actuator and obtaining a measured actuation parameter includes obtaining a transistor voltage, said method further comprising mapping said current and said transistor voltage to a selected resistor value of a plurality of disparate resistor values.
7. The method of claim 6 further comprising identifying, based upon said selected resistor value, an actuation state parameter in the form of a shape memory alloy actuator position value.
8. The method of claim 1 wherein applying power to said shape memory alloy actuator includes applying current to said shape memory alloy actuator and obtaining a measured actuation parameter includes obtaining a time value, said method further comprising mapping said current and said time value to a selected resistor value of a plurality of disparate resistor values.
9. The method of claim 1 wherein obtaining a measured actuation parameter includes obtaining a first location limit value and a second location limit value, said method further comprising identifying, based upon said first location limit value and said second location limit value, an actuation state parameter in the form of actuation cycle duration.
10. The method of claim 9 wherein identifying includes processing said first location limit value, said second location limit value, and a take-off time value.
11. The method of claim 10 wherein processing includes processing a take-off time value derived from a transistor resistor characteristic.
12. A mechanical actuator, comprising:
- a shape memory alloy;
- a controller connected to said shape memory alloy, said controller being adapted to apply power to said shape memory alloy; derive an operational characteristic parameter based upon a measured actuation parameter; identify an actuation state parameter from said operational characteristic parameter; and alter the application of power to said shape memory alloy based upon said actuation state parameter.
13. The mechanical actuator of claim 12 wherein said controller applies current to a transistor connected to said shape memory alloy.
14. The mechanical actuator of claim 13 wherein said controller maps a power value in the form of a current value and a measured actuation parameter in the form of a transistor voltage to a selected resistor value of a plurality of disparate resistor values.
15. The mechanical actuator of claim 14 wherein said controller identifies, based upon said resistor value, an actuation state parameter in the form of a shape memory alloy position.
16. The mechanical actuator of claim 12 wherein said controller maps a power value in the form of a current value and a measured actuation parameter in the form of a measured time value to a selected resistor value of a plurality of disparate resistor values.
17. The mechanical actuator of claim 16 wherein said controller identifies, based upon said resistor value, an actuation state parameter in the form of a shape memory alloy position.
18. The mechanical actuator of claim 12 wherein said controller identifies, based upon a first location limit value and a second location limit value, an actuation state parameter in the form of actuator cycle duration.
19. The mechanical actuator of claim 18 wherein said controller identifies said actuation state parameter using a take-off time value.
20. The mechanical actuator of claim 19 wherein said controller derives said take-off time value from a transistor resistor characteristic.
21. The mechanical actuator of claim 12, wherein said controller includes a fixed current supply to provide stable control, without feedback, during load fluctuations.
Type: Application
Filed: Sep 3, 2004
Publication Date: Mar 9, 2006
Inventors: Shawn Everson (Fremont, CA), Ali Ghorbal (Antioch, CA), Henry Nash (Old Amersham), Andrei Szilagyi (Danville, CA), Roderick MacGregor (Antioch, CA), Kurt Kuhlmann (San Jose, CA)
Application Number: 10/934,037
International Classification: F01B 29/10 (20060101);