Patent Publication Number: US-2023160766-A1

Title: Sliding discrete fourier transform (dft) bins for fuel quantity measurements

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Indian Patent application No. IN 202111054150, filed Nov. 24, 2021. The entire contents of this application are incorporated herein by reference in their entirety. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to pressure and temperature measurements, and more particularly to optical Fabry-Perot based pressure and temperature sensing. 
     2. Description of Related Art 
     Pressure sensing can be performed using optical pressure sensors. These sensors function on the Fabry-Pérot interferometry. As pressure on the sensor varies, an optical cavity changes dimensions. There are various techniques that interpret the reflected data from the optical cavity to compute fuel pressure. One application, among others, is for fuel level sensing, fuel quantity gauging, and temperature sensing. Existing techniques such as wavelength tracking, peak-to-peak tracking, and Fourier Transform based algorithms have been used, each with its own set of advantages and disadvantages. Generally, there is a tradeoff between accuracy on the one hand and on the other hand computation time and memory depth. 
     The conventional techniques have been considered satisfactory for their intended purpose. However, there is an ever present need for improved systems and methods for producing fuel measurements with optical sensors. This disclosure provides a solution for this need. 
     SUMMARY 
     A method includes receiving wavelength domain data for a first time step from an optical pressure sensor, performing a Discrete Fourier Transform (DFT) to transform the wavelength domain data into frequency domain data, e.g. for the entire band, identifying a frequency of interest in the frequency domain data, and selecting a limited set of frequency bins in the frequency domain data based on the frequency of interest. The method includes receiving wavelength domain data for a second time step, performing a DFT to transform the wavelength domain data for the second time step into frequency domain data for the second time step only for the limited set of frequency bins, calculating pressure based on the frequency domain data for the second time step, updating the frequency of interest and the limited set of frequency bins, and repeating receiving wavelength data for subsequent time steps, performing a DFT to transform the wavelength data for the respective subsequent time steps, calculating pressure for each subsequent time step, and updating the frequency of interest and limited set of frequency bins for each subsequent time step. The method includes and outputting pressure data based on calculating pressure for the subsequent time steps. 
     Receiving wavelength domain data can include receiving a complete reflected spectrum. Identifying a frequency of interest can include calculating the frequency of interest (F) as: F = M*Fs/N, wherein M is bin number of a bin with the highest value, Fs is sampling frequency, and N is DFT bin size. Updating the frequency of interest can include calculating the frequency of interest (F) as: F = M*Fs/N, wherein M is bin number of a bin with the highest value, Fs is sampling frequency, and N is the total number of DFT bins/points, which can range from +/- N/2 bins/points. 
     Selecting a limited set of frequency bins can include limiting the set of frequency bins to bins in a range from M-x to M+x, wherein x is an integer. Updating the limited set of frequency bins can include limiting the set of frequency bins to bins in a range from M-x to M+x, wherein x is an integer. It is contemplated that x can be selected based on maximum change in a sensor generating the wavelength domain data. 
     Receiving wavelength domain data can include converting sensor output into an interference waveform to produce the wavelength domain data. The method can include using the pressure data to calculate fuel quantity in a fuel tank. The fuel tank can be aboard an aircraft and the method can include changing one or more flight parameters of the aircraft, e.g. speed, altitude, heading, trim, distribution of fuel across multiple fuel tanks, or the like, based on fuel quantity calculated. 
     A method includes receiving wavelength domain data for a time step, performing a DFT to transform the wavelength domain data for the time step into frequency domain data for the time step only for the limited set of frequency bins associated with a frequency of interest, calculating pressure based on the frequency domain data for the time step, and updating the frequency of interest and the limited set of frequency bins. The method includes repeating receiving wavelength data for subsequent time steps, performing a DFT to transform the wavelength data for the respective subsequent time steps, calculating pressure for each subsequent time step, and updating the frequency of interest and limited set of frequency bins for each subsequent time step. The method includes outputting pressure data based on calculating pressure for the subsequent time steps. 
     A system includes an optical pressure sensor. A processor is operatively connected to the optical pressure sensor to receive output from the sensor. The processor includes or is operatively connected to machine readable instructions configured to cause the processor to perform methods as described above. 
     The optical pressure sensor can includes an optical cavity mounted inside a fuel tank. The fuel tank can be aboard an aircraft. The processor can be operatively connected to a display in the aircraft for displaying fuel level and/or quantity information based on pressure data from the processor. The processor can be operatively connected to avionics of the aircraft for changing at least one flight parameter, e.g. speed, altitude, heading, trim, distribution of fuel across multiple fuel tanks, or the like, of the aircraft based on fuel level and/or quantity information from the processor. 
     These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
         FIG.  1    is a schematic view of an embodiment of a system constructed in accordance with the present disclosure, showing the optical sensor pressure sensor in an aircraft fuel tank; 
         FIG.  2    is a schematic view of the optical pressure sensor of  FIG.  1   , showing the sensor in both a non-deflected state in the absence of pressure and in a deflected state in the presence of pressure; 
         FIG.  3    is a process diagram for a method in accordance with the present disclosure, showing portions of the process for converting sensor data into pressure data; and 
         FIG.  4    is a graph showing frequency domain data with a sliding window for bins to be used for pressure calculations in accordance with the process of  FIG.  3   . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a partial view of an embodiment of a system in accordance with the disclosure is shown in  FIG.  1    and is designated generally by reference character  100 . Other embodiments of systems in accordance with the disclosure, or aspects thereof, are provided in  FIGS.  2 - 4   , as will be described. The systems and methods described herein can be used to provide fuel quantity measurements based on optical pressure sensor data with reduced computational expense relative to traditional techniques. More generally, while discussed herein in the exemplary context of fuel level and fuel quantity measurement, those skilled in the art will readily appreciate that systems and methods as disclosed herein can be applied to any suitable measurement application such as temperature and/or pressure measurement in a sapphire Fabry-Perot sensor for use in an air management system, high temperature measurement in a gas turbine engine, pressure and temperature measurement in a gas turbine engine, outside air temperature measurement on an aircraft, pressure measurement in a pitot-static sensor for airspeed indication, hydraulic pressure measurement in an aircraft landing gear, strain measurement in the load-transmitting members of a helicopter flight control system, and the like. 
     The system  100  includes an optical pressure sensor  102  mounted inside a fuel tank  104  of an aircraft  10  or other moving vehicle. An optic fiber  106  connects the optical pressure sensor to an optical processor  108  that generates sensor data output. A processor  110  is operatively connected to the optical pressure sensor  102  and the optical processor  108  to receive output from the sensor  102 . The processor  110  includes or is operatively connected to machine readable instructions configured to cause the processor  110  to perform methods as described herein. The processor calculates fuel levels and/or quantities and outputs them to avionics  112  and/or a cockpit display  114  of the aircraft  10 . The display  114  can display fuel level and/or fuel quantity information based on pressure data from the processor  110 . The avionics  112  can change at least one flight parameter, e.g. speed, altitude, heading, trim, distribution of fuel across multiple fuel tanks, or the like, of the aircraft  10  based on fuel level and/or quantity information from the processor  110 . 
     With reference now to  FIG.  2   , the optical pressure sensor  102  includes an optical cavity  116 . As shown on the left in  FIG.  2   , when there is no pressure acting on the diaphragm  118 , there is a certain spacing L between the diaphragm and a primary reflector  120 . As shown on the right in  FIG.  2   , when there is a pressure acting on the diaphragm  118 , it deflects inward relative to the optical cavity  116 , reducing the spacing between the diaphragm  118  and the primary reflector  120  to L 1 . A laser generated in the optical processor  108  of  FIG.  1    is emitted from the end of the optic fiber  106  in  FIG.  2   . The reflections of the laser from the diaphragm  118  and primary reflector  120  are reflected back into the optic fiber  106  and can be detected by the optical processor  108  of  FIG.  1   . Variations in the spacing L/L 1  change how the reflections of the diaphragm  118  and main reflector  120  interfere with one another, which can be interpreted into a pressure reading indicative of the pressure acting on the diaphragm  118 . This type of sensor  102  can be useful in aircraft fuel tanks where it is desirable to keep electrical wiring to a minimum, since the sensor  102  itself has no electrical components and only requires the optic fiber  106  to pass into the interior of the fuel tank  104  to the optical processor  108  (shown in  FIG.  1   ) where the electrical components are located. 
     With reference now to  FIG.  3   , method  200  includes taking sensor output as indicated by box  202 , e.g. from sensor  102  and/or optical processor  108  of  FIG.  1   , and converting it into an interference waveform or wavelength domain data as indicated in  FIG.  3    by box  204 . The method  200  includes receiving the wavelength domain data, e.g. in the processor  110  of  FIG.  1   , for a first time step. Receiving wavelength domain data can includes converting sensor output into an interference waveform to produce the wavelength domain data either in the optical processor  108  or in the processor  110  of  FIG.  1   . 
     As indicated in  FIG.  3    by box  206 , the method  200  includes performing a Discrete Fourier Transform (DFT) to transform the wavelength domain data into frequency domain data. For the first time step, the DFT is performed for the complete spectrum signal of the wavelength domain data. 
     With continued reference to  FIG.  3   , as indicated by box  208 , the method includes identifying a frequency of interest in the frequency domain data, and selecting a limited set of frequency bins in the frequency domain data based on the frequency of interest. Referring to  FIG.  4   , which represents some frequency domain data distributed in bins along the horizontal axis, identifying a frequency of interest includes calculating the frequency of interest (F) as: 
     
       
         
           
             F = M* 
             
               
                 Fs 
               
               / 
               N 
             
             , 
           
         
       
     
      wherein M is bin number of a bin with the highest value, Fs is sampling frequency, and N is the total number of DFT bins/points, which can range from +/- N/2 bins/points. Selecting a limited set of frequency bins includes limiting the set of frequency bins to bins in a range from M-x to M+x, wherein x is an integer. It is contemplated that x is selected based on maximum change in a sensor generating the wavelength domain data, which is a property of the measured physical parameter (pressure, temperature etc.). This physical parameter should be a slow varying parameter with respect to time, where the technique disclosed herein can be applied. In  FIG.  4   , the window  400 , which is a sliding window as discussed below, includes the limited set of frequency bins. 
     As shown in  FIG.  3    by box  210 , the method  200  includes recalculating the DFT points with the limited set of frequency bins. Box  212  indicates calculating the DFT bins covering maximum change in the sensor at the time step of the sensor output (box  202 ). As indicated in  FIG.  3    with box  214 , the method includes configuring the resulting DFT bin values as sliding window values, sliding the window for the next time step based on the peak value calculated in the previous time step, as indicated in box  216 , and repeating the process for sensor data for each subsequent time step as indicated in box  218 , where the DFT in the subsequent time steps only has to be calculated or performed for the limited sliding window of bins. Finding the frequency of interest and the window can be done using the same formulae as in the first time step. The DFT need not be performed on the full spectrum for the subsequent time steps. 
     The method includes using the pressure data to calculate fuel quantity in a fuel tank, e.g. fuel tank  104  of  FIG.  1   . The avionics  112  or pilot can use the fuel quantity information to change one or more flight parameters of the aircraft as described above with reference to  FIG.  1   . The peak frequency and its amplitude will be proportional to the spacing L/L 1  (cavity length as discussed above with reference to  FIG.  2   ). The cavity length is proportional to the pressure being sensed, which can be calculated for each time step once the peak frequency is identified for that time step. 
     Potential benefits of systems and methods as disclosed herein include accuracy akin to exhaustive calculation techniques with a small fraction of the computational expense, e.g. two or three orders of magnitude fewer calculations needed per time step. This accuracy and reduction in computational complexity leads to a decrease in execution time to achieve a highly accurate reading, potentially enabling applications where quick acquisition times are advantageous. 
     Methods and systems of the present disclosure, as described above and shown in the drawings, provide for fuel quantity measurements based on optical pressure sensor data with reduced computational expense relative to traditional techniques. While the apparatus and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the scope of the subject disclosure.