Patent Publication Number: US-2018038714-A1

Title: System and method for determining a position of a moveable device

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to a control system, and more particularly relates to determining a position of a moveable device within a system. 
     BACKGROUND 
     Complex systems, such as aircraft, automobiles, spacecraft or other complex machinery include numerous parts which move. Aircraft, for example, have rudders, flaps, valves, landing gear, and numerous other parts which move. The position of the moveable parts within the complex system often needs to be known for the accurate operation of the complex system. 
     Accordingly, it is desirable to provide improved systems and methods for determining a position of a moveable device, which more accurately and efficiently determines the position. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background 
     BRIEF SUMMARY 
     In one embodiment, for example, a system for determining a position of a moveable device is provided. The system may include, but is not limited to, a variable displacement transformer, including but is not limited to, a shaft mechanically coupled to the moveable device, a primary coil arranged on a first side of the shaft, a first secondary coil arranged on a second side of the shaft, and a second secondary coil arranged on the second side of the shaft, a digital to analog converter electrically connected to the primary coil of the variable displacement transformer, the digital to analog converter configured to provide a sine wave signal to the primary coil of the variable displacement transformer having a predetermined frequency and a predetermined amplitude, and a processor electrically coupled to the first secondary coil and the second secondary coil, the processor configured to calculate, for each sample corresponding to a voltage induced by the primary coil onto the first secondary coil and a voltage induced by the primary coil onto the second secondary coil during a frequency period of the alternating current signal, Fourier Transform components by multiplying the voltage induced by the primary coil onto the first secondary coil by a first predetermined value and storing the results in a first accumulator, multiplying the voltage induced by the primary coil onto the first secondary coil by a second predetermined value and storing the results in a second accumulator, multiplying the voltage induced by the primary coil onto the second secondary coil by the first predetermined value and storing the results in a third accumulator, and multiplying the voltage induced by the primary coil onto the second secondary coil by the second predetermined value and storing the results in a fourth accumulator, the first predetermined value and the second predetermined value being unique to each frequency period of the sine wave signal, and determining, when a predetermined number of samples have been calculated, a sum of the values of the first buffer, a sum of the values of the second buffer, a sum of the values of the third buffer and a sum of the values of the fourth buffer, and determining the magnitude of the first secondary coil signal by calculating the square root of the sum of the squares of the first and second accumulators and storing the results in a first magnitude buffer, determining the magnitude of the second secondary coil signal by calculating the square root of the sum of the squares of the third and fourth accumulators and storing the results in a second magnitude buffer, and determining the position of the movable device based upon determined magnitude of the first secondary coil signal and the magnitude of the second secondary coil signal. 
     In another embodiment, for example, a method for determining a position of a moveable device is provided. The method may include, but is not limited to, generating, by an digital to analog converter, a sine wave signal having a predetermined frequency and a predetermined amplitude, outputting, by the digital to analog converter, the generated sine way signal to a primary coil of a variable displacement transformer, the variable displacement transformer comprising a shaft mechanically coupled to the movable device, outputting, by a first secondary coil of the variable displacement transformer, a voltage induced by the primary coil on the first secondary coil to a processor, outputting, by a second secondary coil of the variable displacement transformer, a voltage induced by the primary coil on the second secondary coil to a processor, calculating, by the processor, for each sample corresponding to a voltage induced by the primary coil onto the first secondary coil and a voltage induced by the primary coil onto the second secondary coil during a frequency period of the alternating current signal, Fourier Transform components by multiplying the voltage induced by the primary coil onto the first secondary coil by a first predetermined value and storing the results in a first accumulator, multiplying the voltage induced by the primary coil onto the first secondary coil by a second predetermined value and storing the results in a second accumulator, multiplying the voltage induced by the primary coil onto the second secondary coil by the first predetermined value and storing the results in a third accumulator, and multiplying the voltage induced by the primary coil onto the second secondary coil by the second predetermined value and storing the results in a fourth accumulator, the first predetermined value and the second predetermined value being unique to each frequency period of the sine wave signal, and determining, when a predetermined number of samples have been calculated, a sum of the values of the first buffer, a sum of the values of the second buffer, a sum of the values of the third buffer and a sum of the values of the fourth buffer, and determining, by the processor, the magnitude of the first secondary coil signal by calculating the square root of the sum of the squares of the first and second accumulators and storing the results in a first magnitude buffer, determining, by the processor, the magnitude of the second secondary coil signal by calculating the square root of the sum of the squares of the third and fourth accumulators and storing the results in a second magnitude buffer, and determining, by the processor, the position of the movable device based upon determined magnitude of the first secondary coil signal and the magnitude of the second secondary coil signal 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  illustrates a system for determining a position of a moveable device, in accordance with an embodiment; 
         FIG. 2  is a block diagram illustrating yet another system for determining a position of a moveable device, in accordance with an embodiment; 
         FIG. 3  is a flow chart illustrating an exemplary method for operating a system, in accordance with an embodiment; 
         FIG. 4  illustrates an example of the shape of the bandpass attenuation of a DFT calculated by correlation; 
         FIG. 5  is a flow diagram illustrating an exemplary method for compensating the position determined by the method illustrated in  FIG. 3 ; and 
         FIG. 6  is a flow diagram illustrating a method for compensating the system  100  for vibration. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Thus, any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. All of the embodiments described herein are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description. 
       FIG. 1  illustrates a system  100  for determining a position of a moveable device  110 , in accordance with an embodiment. The system  100  may integrated within or added to another system. The other system  105  may be, for example, a vehicle such as an aircraft, a spacecraft, an automobile, a watercraft, or the like. In other embodiments, the other system  105  an industrial system such as a factory, a refinery, or any other industrial, commercial or residential system where knowledge of a position of a part is needed. 
     The moveable device  110  may be any component of a vehicle, industrial, commercial or residential system which moves. In an aircraft, for example, the moveable device  110  may be any of various control surfaces such as a flap, an aileron, a rudder, a flaperon, a valve, any of various pilot inceptors including a control stick, a trim wheel, a rudder pedal and a steering yoke, any of various engine controls including a bleed valve, a thrust reverser and an inlet control vane, or the like. 
     The system  100  further includes a variable displacement transformer (VDT)  120 . The VDT  120  may be a linear variable displacement transformer (LVDT) or a rotary variable displacement transformer (RVDT), depending upon whether the moveable device  110  uses linear motion or rotary motion. The VDT  120  converts a position of the moveable device  110  into a proportional electrical signal containing a polarity, which indicates a direction relative to a zero reference, and an amplitude representative of a distance from the zero reference. 
     In one embodiment, for example, the VDT  120  may include a primary coil  122 . The primary coil  122  receives an input an alternating current (AC) signal from a Digital to Analog Converter (DAC)  130 , as discussed in further detail below. The VDT  120  may further include two secondary coils  124  and  126  and a shaft  128 . The shaft  128  is mechanically coupled to the movable device  110  in a manner that enables the VDT to accurately determine the position of the moveable device. The shaft  128  is positioned between the coils  122 ,  124  and  126 . When the moveable device  110  is moved, the shaft  128  moves a corresponding amount, either rotationally in the case of a RVDT or linearly in the case of a LVDT. When the primary coil  122  receives the input AC signal from the DAC  130 , the primary coil  122  induces an AC sine wave current on the secondary coils  124  and  126  proportional to the position of the shaft  128 . Accordingly, the secondary coils  124  and  126  output an AC sine wave voltage which can accurately be converted into a position of the moveable device  110 , as discussed in further detail below. 
     The DAC  130  may be composed of an analog filter that allows a digital version of the sine wave signal to be generated within the processor. There are several methods for generating a sine wave signal digitally. Two advantageous ways to generate the sine wave signal in the processor include pulse width modulation and pulse density modulation. In pulse width modulation the amplitude of the resulting analog signal is determined by the duty cycle of the digital signal into the analog filter. In pulse density modulation the periods of the digital pulse are fixed and the amplitude is determined by the quantity of the ‘logic one’ outputs versus the quantity of the ‘logic zero’ outputs. Because the fundamental frequency used in pulse density modulation can be higher than for a pulse width signal for a given processor clock and analog filer combination, pulse density modulation can produce a higher fidelity analog sine wave signal. In both modulation schemes, the serial stream of bits used for the signal generation can be calculated prior to circuit operation and stored in memory  180 . The system  100 , however, does not require either of these sine wave generation methods, and can work with other methods of providing a sine wave generation to the VDT sensor  120 . 
     Each secondary coil  124  and  126  is coupled to an anti-aliasing filter  140 . The anti-aliasing filters  140  prevent unwanted electrical noise from causing aliasing errors in the conversion of the signal data into position information. These anti-aliasing filters can be any of a number of filter types including passive filters built with combinations of resistors capacitors and inductors or a subset of these, active filters built with a combination of the passive elements previously mentioned and amplifiers. Anti-aliasing filters are typically low pass filters because the aliasing threats to the signal conditioning are typically higher that the signals of interest. The anti-aliasing filters  140  can also be bandpass or band rejection filters when the threats are such that these filters remove the unwanted aliasing frequency components. 
     The anti-aliasing filters  140  are coupled to the analog to digital converters (ADCs)  150 . In one embodiment, for example, the ADCs  150  may be delta-sigma, ADCs which are well suited to the signal processing because they provide a good combination of DC accuracies and high resolution. Another benefit of delta-sigma ADCs is that the serial stream of conversion data from the modulator front end allows for efficient, high order, digital low pass filter implementations. In one embodiment, for example, the delta-sigma, analog to digital converters may include a digital low pass filter  160  included on the same device. The digital filters  160  can provide very effective aliasing noise rejection because the transition from the pass band to the rejection band is typically very steep with rejection levels of 100 decibels (dB) or more and with band transitions of hundreds of dB per decade of frequency change. The phase performance of the digital filters is typically very linear and predictable due to the digital nature of the calculations. However, in other embodiments, separate digital filters  160  may be coupled to the output of the ADCs  150 . 
     As seen in  FIG. 1 , the outputs of the digital filters  160  are coupled to the one or more processors  170 . The processor(s)  170  may be one or more field programmable gate arrays, digital signal processors (DSPs), applications specific integrated circuits (ASICs), microprocessors, central processing units (CPUs), or the like, or any combination thereof. In the embodiment illustrated in  FIG. 1 , a single processor  170 , such as a FPGA, is configured to perform the positional analysis from both data paths (i.e., the voltage output by each secondary coil  124  and  126  through the analog-to-digital converters  150 ). However, any number of processors  170  may be utilized in the system  100 . In one embodiment, for example, the processor(s)  170  may be dedicated to the system  100 . However, in other embodiments, the processor(s) could be shared by other systems (e.g., another system in an aircraft or the like). While any number of processors  170  could be utilized in the system, the processor(s)  170  will hereinafter simply be referred to as the singular processor  170 . 
     The processor  170  determines the position of the movable device  110  by processing the data output from the ADCs  150 , and filtered by the digital filters  160  as discussed in further detail below. In one embodiment, for example, the processor may also control a frequency of ADCs  150  conversions and/or the number of ADC  150  conversions to use for each discrete Fourier transform (DFT) calculated in the processor  170  and/or the frequency of the DAC  130  to modify DFT transfer characteristics or to modify a signal-to-noise ratio, as discussed in further detail below. In the embodiment illustrated in  FIG. 1 , a single processor  170 , such as an FPGA, is provided to processes the data from the data paths and to control the DAC  130 . However, in other embodiments a separate processor  170  could be used to process the data output from the VDT  120  and a separate processor  170  may be used to control the ADCs  150  conversions and/or the DAC  130 . 
     The system  100  may further include a memory  180  communicatively coupled to the processor  170 . The memory may  180  store non-transitory computer readable instructions for implementing the system  100  as discussed in further detail below. The memory  180  may also be used to store pre-calculated data and/or operate as an accumulator as discussed in further detail below. 
     VDT sensors, such as the VDT  120 , are typically made using various metals including ferrites, copper and stainless steel. Metals have significant thermal coefficients of expansion (TCEs). The TCEs of the various metals frequently do not match, which results in variations in the mechanical alignments of the moving parts of the VDT  120 . Due to this phenomena, the performance of the VDT  120  as a position measurement device is compromised. Instead of having a linear relationship between the displacement of the shaft and the ratio of the secondary winding voltages, a more complex relationship results over temperature variations. To compensate for this phenomena, the system  100  further includes a temperature sensor  190  mounted on the VDT  120  and an analog to digital converter  195  coupled to the output of the temperature sensor  190 . The temperature of the VDT  120  is measured by the temperature sensor  190 . The Temperature sensor  190  could be of various types including a platinum resistance temperature device (RTD), a thermocouple, or a thermistor, or the like. The VDT sensor  120  behavior as a function of temperature can be determined prior to installation by careful measurements. These measurements provide a characterization of a given VDT  120  that allows a more accurate position measurement to be made when the deviations due to temperature are used to adjust the results of the position calculation based on VDT temperature and the characterization stored in the memory  180 . 
       FIG. 2  is a block diagram illustrating yet another system  100  for determining a position of a moveable device  110 , in accordance with an embodiment. In the embodiment illustrated in  FIG. 2 , the system  100  is identical to the system  100  illustrated in  FIG. 1 , however, the system  100  is configured to determine the temperature of the VDT  120  without the addition of a separate temperature sensor. In this embodiment, the temperature of the VDT  120  is determined by the processor  170  by causing the DAC  130  to apply a DC current such as a predetermined pulse width modulation or pulse density modulation, to the primary winding  122  of the VDT  120 . Each end of the primary winding  122  of the VDT  120  is coupled to the ADC  195  via electrical connections  200  and  210 . Each signal from electrical connections  200  and  210  is converted to a digital signal by the ADC  195  and output to the processor  170 . The processor  170  then determines the resulting voltage across the primary windings  122  of the VDT  120  by comparing the resulting signals. In this method, the resistance of the primary winding  122  can provide a temperature measurement of the VDT  120  because the primary winding is made of metal, typically copper and the resistance of copper is a function of temperature. Measuring the DC voltage induced in the primary winding due to the precision DC current applied enables an Ohm&#39;s law calculation within the processor  170  that determines the resistance of the primary winding. The resistance measured provides for a temperature measurement by means of an equation or a look-up table in memory  180 . The primary winding of the VDT  120  serves as an intrinsic resistance temperature detector (RTD) sensor for the purposes of VDT temperature measurement, which allows for the elimination of the temperature sensor  190  from the system, which saves cost and improves reliability of the system. 
     Because the modulation of the sine wave signal and the demodulation of the sensor signals are done in the same processor  170  using the same clock source for both, there is no opportunity for frequency or phase drift between the these components which provides for greater accuracy. 
       FIG. 3  is a flow chart illustrating an exemplary method  300  for operating a system  100 , in accordance with an embodiment. The system  100  is first initiated. (Step  310 ). In other words, the components of the system  100  are started and configured to begin the generation of positional data. As discussed above, the DAC  130  outputs a sine wave signal having a fixed frequency to the primary coil  122  of the VDT  120 . Upon initiation, the VDT  120  output signals from the coils  124 ,  126  are passed through the analog anti-aliasing filters  140 , the ADCs  150  and the digital filters  160  as discussed above. The filtered digital representation of the voltage output by the VDT  120  is then received at the processor  170 . As discussed in further detail below, the data generated by the VDT  120  may be used to calculate a correlation DFT as well as a fast Fourier transform (FFT). 
     Upon receiving the signal corresponding to a sample of the VDT  120 , the processor  170  then calculates the DFT for the sample by correlation, using the data received from the ADCs  150  and digital filters  160  and stores time samples and the DFT results in the memory  180 . (Step  320 ). The data output from the ADCs  150  and the digital filters  160  is a set of sequential time domain data that is input to the processor  170 . The data contains frequency and amplitude information about the position of the movable device  110  over the period of time when the signals from  124  and  126  were sampled. The DFT converts the data received from the digital filter  160 , from the time domain into the frequency domain. The correlation method of performing the DFT reduces the amount of math to be performed by the processor  170  when the magnitude of signals is desired for only one frequency when compare to the other methods of calculating a DFT, which necessarily requires calculations for multiple frequency components. 
     In one embodiment, for example, the processor  170  may use Equation 1 to perform the DFT by correlation. 
     
       
         
           
             
               
                 
                   
                     
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     ReX[k] and ImX[k] are the two dimensional, rectangular, vector components of the magnitude of the secondary signals from the secondary coils  124  and  126 . In the correlation method of calculating the DFT, “i” is effectively incremented through N values to determine the ReX and ImX vector components of each secondary coil&#39;s voltage magnitude. The ‘k’ in the equation is chosen to align with the frequency of excitation of the VDT; the frequency of the DAC  130 . The value of x[i] corresponds to the digitized voltage level input to the processor  170  from the ADCs  150  and digital filters  160  at each sampled point in time i. The value of N corresponds to the number of samples in the correlation DFT calculation. N may be selected to be any integer value only when the for the correlation DFT calculation. N may be selected such that N times the sample interval is equal to an integer multiple of the number of the sensor sine wave cycles received from the secondary coils  124  and  126  for the correlation method. Therefore, the processor  170  may select N based upon an optimal frequency for the sensor  120  and for the ADC  150  and for the filters  140  and  160 . For example, it may be determined that the best frequency to operate the VDT sensor has been determined to be 1800 Hz for reasons other than the DFT calculation such as optimal sensor operation and system timing constraints. Additionally, in this example, the optimal rate at which the ADC  150  operates may be determined to be 126,000 data samples per second, again for reasons other than the DFT calculations such as for rejecting aliasing threats. In this example appropriate values of N would be 70, 140, 210, 280 or even 2870, because these values of N divided by the sample data rate equals an integer number of 1800 Hz sine wave periods. As discussed in further detail below, when the FFT method of calculating the DFT is used, N must be chosen to be an integer power of two such as 32, 64, 128 or 256. 
     One benefit of the method  300  is that the method  300  does not require that N be an integer power of two to perform the DFT by correlation. In contrast, typical full Fourier transform calculations require the number of samples N to be an integer power of two. For the example above, the optimal sampling rate determined to be 126,000 data samples per second and the sensor frequency of 1800 Hz sine wave, does not result in a value of N that equals an integer power of two. Either the Sine wave frequency needs to change or the sample rate would need to change to satisfy the ‘integer power of two’ requirement, which would cause the implementation to use less optimal settings for the sensor and the analog to digital converter. By allowing N to be any integer value, the precision of the position determined by the system  100  and the overall performance of the system  100  can be optimized. In other words, rather than requiring the operations of the sensor  120  and the analog to digital converter  150  to be less than optimal for non-Fourier transform calculation reasons, the correlation process is more accommodating than other DFT methods. 
     Another example of the benefit of the correlation method over the other methods of DFT calculation is demonstrated when one considers the DFT&#39;s ability to attenuate unwanted signals.  FIG. 4  illustrates an example of the shape of the bandpass attenuation of a DFT calculated by correlation. As seen in  FIG. 4 , components with frequencies higher and lower than the carrier frequency are attenuated. When the VDT  120  experiences vibration at low frequencies, the VDT amplitude (what provides this amplitude?) modulates that vibrational motion into side band frequency components in the VDT secondary winding signals.  FIG. 4  also shows a beneficial situation where the unwanted frequency components are attenuated substantially optimally. When the vibration in the VDT  120  occurs such that the side band frequencies occur at frequencies away from the notches the design would be less optimal, attenuating the unwanted components by a lesser amount. As discussed in further detail below, By using an FFT of a portion of the time samples in addition to the correlation calculation, the frequency and amplitude of the vibrations can be determined. 
     Yet another benefit of the method  300  is that the values of the cosine and sine function can be calculated in advance and stored in the memory  180 , for example, in a look-up table, as they are fixed values. As seen above, 
     
       
         
           
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     from the real and imaginary components of equation 1 are values which can be calculated in advance as each contain variables which are known in advance. When compared to the FFT method, the number of sine and cosine values that need to be generated and stored is significantly less in the correlation method, thereby requiring a less memory usage. Furthermore, unlike the FFT equations, the processor  170  can perform incremental data calculations as the samples from the analog to digital converter arrive, before having all of the data samples, which produces the result sooner, reducing latency. In other words, the processor  170  can multiply each received value x[i] by the respective pre-calculated cosine and sine values stored in the memory  180  and then accumulate the DFT results, ReX[k] and ImX[k], in two memory  180  locations. Because all of the sampled x[i] values are multiplied by the factors above and the results of the multiplications are accumulated immediately after each individual multiplication, the results are available one multiplication and one addition after the final x[i] is received by the processor  170 . 
     The processor  170  continues to process the data from the VDT  120  and perform the DFT calculation by correlation described above until all of the N samples of the DFT have been calculated. (Step  330 ). 
     The processor  170  then calculates a magnitude of the real and imaginary components of the DFT by correlation for each of the secondary windings  124  and  126  of the VDT  120 . (Step  340 ). In one embodiment, for example, the processor may calculate the magnitude according to Equation 2: 
       Mag winding =√{square root over (( ReX[k]   2   +ImX[k]   2 ))}   Equation 2
 
     The processor  170  may then determine the position of the moveable device  110  by performing the calculation described in Equation 3. (Step  350 ). 
     
       
         
           
             
               
                 
                   
                     VDT 
                     position 
                   
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                       - 
                       MagB 
                     
                     
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                       + 
                       MagB 
                     
                   
                 
               
               
                 
                   Equation 
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     Where MagA is the magnitude of the data from secondary winding  124  and MagB is the magnitude of the data from secondary winding  126  calculated in Step  340 . 
     The calculated position of the moveable device is then used by the processor  170  or otherwise output to another relevant system which requires the position information. 
     In one embodiment, for example, the processor may immediately return to step  310  to initiate a next position determination. However, in another embodiment, for example, the processor may determine if a new frame has started. (Step  360 ). In this embodiment, the system  100  may utilize a a fixed update frame rate and then maintain that frame rate by only calculating the position of the moveable device  110  once per each frame. This is done for many practical reasons. One of the main reasons is that some system stability analysis is determined based on predictable update frame rates. Aircraft systems, for example, are dependent on multiple position sensors and other sensors. Another consideration in stability has to do with the latency of the data. Knowing when the samples are taken with respect to the other control calculations also aides in the stability of a system. 
     As discussed above, the VDT  120  may be made using various metals having different coefficients of expansion (TCEs). Because the components of the VDT  120  expand and contract at different temperatures, the mechanical alignments of the moving parts of the VDT  120  may be compromised. 
       FIG. 5  is a flow diagram illustrating an exemplary method  500  for compensating the position determined in method  300  for temperature fluctuations. Accordingly, the processor  170 , after determining the position of the moveable device  110  in Step  350 , may determine the temperature of the VDT  120 . (Step  510 ). As discussed above, a temperature sensor  190  may be coupled to the VDT  120  to sample a temperature of the VDT  120  as seen in  FIG. 1 . In another embodiment, illustrated in  FIG. 2 , a DC signal may be applied to the primary winding  122  of the VDT  120  by the processor  170  through the DAC  130 . The processor  170 , in this embodiment, may then determine the temperature of the device by measuring the voltage across the primary windings  122  of the VDT  120 , as discussed above. 
     The processor  170  may then compensate the position calculated in Step  350  according to the determined temperature in Step  510 . (Step  520 ). As discussed above, the behavior of the VDT  120  as a function of temperature can be determined prior to use by taking measurements of the VDT  120  at various temperatures. These measurements provide a temperature characterization of a given VDT  120  that allows a more accurate position measurement to be made. This temperature characterization can be stored in the memory  180 . Accordingly, the processor  170  in Step  520  modifies the position of the moveable device  110  calculated in Step  350  by adjusting the position according to the temperature characterization of the VDT  120 . 
     In one embodiment, for example, the processor  170  may modify the determined position of the moveable device via the method described in  FIG. 5  each frame. In other words, the processor  170  may modify the position of the moveable device  110  determined in step  350  each time the position is calculated. However, in other embodiments, the processor  170  may perform the temperature compensation less frequently. For example, the processor  170  may perform the steps  510  and  520  periodically (e.g., once every 5 frames, 10 frames, 50 frame, 100 frames, etc.) or upon demand by a user of the system  100 . 
     In one embodiment, for example, the processor  170  may also compensate the system  100  for vibration.  FIG. 6  is a flow diagram illustrating a method for compensating the system  100  for vibration. In this embodiment, for example, the processor  170  may then continue to receive data from the VDT  120  from Step  320  until enough samples are received to perform a FFT on the data. (Steps  610  and  620 ). As discussed above, a FFT requires a number of samples which is a power of 2 (i.e., 32, 64, 128, 256, etc.). Accordingly, in this embodiment, the processor  170  may continue to receive data from the VDT  120  rather than stopping after step  330  until the number of samples received from the VDT  120  is the lowest power of 2 greater than the value of N from the DFT by correlation calculations of Step  320 . 
     Once enough samples are received from the VDT  120 , the processor  170  performs a FFT on the data. (Step  630 ). By performing a FFT of the data from the VDT  120  the processor  170  can detect a vibration of the VDT  120  and determine a frequency of the vibration which may be corrupting the position measurement. By transforming the time domain data output from the VDT  120  into a frequency domain utilizing a FFT calculation, the processor  170  can determine which one or more frequency bins of the FFT include a vibration component. 
     The processor  170  then determines if any FFT bin magnitudes are greater than current correction limits. (Step  640 ). In other words, the processor  170  determines if the magnitude of a frequency bin of the FFT calculation corresponding to a vibration frequency is greater than a current correction limit. In one embodiment, for example, a correction limit may be chosen to be, for example, 2% of the full scale amplitude of the position sensor. 
     When a magnitude of one or more frequency bins is greater than the current correction level limit, the processor  170  determines if the largest bin is aligned with the current configuration. (Step  650 ). In other words, the processor  170  determines if the number of samples N from step  330  alignes a notch of the correlation calculation with the largest bin. Adjusting N is tha easiest way to move the notches of greater attenuation. However, setting other than the number N can also be part of the configuration. For example, a frequency of sensor excitation could alternatively be modified to move the attenuation notches. 
     When the largest bin is not aligned with the current configuration (from Step  650 ) or when the magnitude of a bin is less than or equal to the current correction level limit (from Step  640 ), the processor  170  selects a configuration stored in the memory  180  that best attenuates the frequency of vibration of the largest bin from the FFT calculation of Step  630 . (Step  660 ). In other words, the frequency and amplitude information of unwanted signal components determined by the FFT performed in the processor  170  can be used to select from a set of VDT and DFT configurations stored in the memory  180  to achieve an optimal filter. The configurations would include optional combinations of sine wage generator frequencies, the value N for the correlation DFT calculation and the frequency of ADC  150  conversions. 
     When the largest bin is aligned with the current configuration (from Step  650 ), or after selecting a stored configuration in Step  660  the processor  170  then returns to Step  610  to begin a next vibration correction determination and optimal configuration selection. In one embodiment, for example, the method  600  could be performed once per each frame of the correlation calculation of the method  300  as each aircraft or other vehicle housing the moveable device  110  is operating. As the correlation method allows for more choices for the value of N and for the number of samples used in each DFT calculation, the ability of the system to adapt to threat conditions is increased above that for DFT methods that use only correlation or only FFTs to determine the position of the moveable device  110 . However, in other embodiments, for example, the method  600  could be performed periodically, upon request of a user of the system  100  or even just once for each aircraft type certificate. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.