Non-contact time-of-flight fuel level sensor using plastic optical fiber

Abstract

The present invention relates to a non-contact time-of-flight fuel level sensor using plastic optical fiber. The fuel sensing system utilizes non-contact plastic optical fiber (POF) to optically sense the level of liquid fuel in a fuel tank. In one implementation, the fuel level sensing system includes the following elements: (i) a high-speed and high-power red laser diode; (ii) an ultra-high sensitivity photon counting avalanche photodiode; and (iii) a graded-index POF with a large diameter and a large numerical aperture. The fuel level is sensed when the avalanche photodiode first detects the incident light reflected by the POF end face and then detects the incident light reflected by the fuel surface in response to the laser pulse emitted by the red laser diode. The time delay detection circuit calculates the time interval separating the corresponding arrival times. The fuel level calculator calculates the fuel level based on the time interval provided by the time delay detection circuit.

Description

Technical Field

[0001] The present disclosure generally relates to systems and methods for measuring the level of a liquid in a storage tank, such as a storage tank or other container. More specifically, the present disclosure relates to systems and methods for using an optical sensor for liquid level measurement. Background Art

[0002] Continuous measurement of the level of a liquid is required in many commercial and military applications. For example, liquid level sensors are commonly used in the fuel tanks of aircraft, automobiles, and trucks. Liquid level sensors are also used to monitor the liquid level within storage tanks for fuel dispensing, wastewater treatment, chemical storage, food processing, and the like.

[0003] One existing solution is to use a capacitance probe placed within the fuel tank of an aircraft. This solution is bulky and expensive because the capacitance probe is metallic and is supported within the fuel tank by a metallic mounting structure. Additionally, the capacitance probe requires current to pass through the fuel tank, which is a characteristic that requires the use of highly shielded cables to feed through the fuel tank, thereby increasing the weight of the aircraft.

[0004] Another solution is to use an optical capacitance probe, where a capacitance signal caused by a fuel level change is transmitted through a fuel tank by a glass optical fiber (GOF). A laser is used to remotely power a solar cell within the tank, which in turn powers a capacitance sensing element and an optical transmitter. Thus, the optical capacitance probe has current generated within the fuel tank, which requires highly shielded cables and an enclosure for the solar cell and the optical transmitter within the fuel tank, increasing the weight. Additionally, using GOF at the fuel tank location has maintenance issues due to GOF breakage and results in labor-intensive repairs at the fuel tank location of the aircraft.

[0005] According to a typical plastic optical fiber-based fuel level sensor design, a plastic optical fiber sensing element (hereinafter referred to as a "POF sensor") is immersed within a fuel tank. The immersed POF sensor has the problem of long-term degradation of the POF sensor due to a chemical reaction with aircraft fuel. Additionally, when the POF sensor is immersed in aircraft fuel, fuel sludge deposited on the POF sensor degrades the optical transmission characteristics of the POF sensor, making optical fuel level measurement non-repeatable and inaccurate.

[0006] There is room for improvement in systems and methods for optically sensing the level of liquid fuel in a fuel tank using plastic optical fibers. Summary of the Invention

[0007] The subject matter disclosed herein relates in part to a plastic optical fiber fuel level sensor that can perform high-resolution aircraft fuel level sensing without contacting the fuel in a fuel tank using the free space optical time-of-flight principle. Since the POF sensor is not immersed in the fuel in the fuel tank, the non-contact POF sensor is neither prone to degradation due to the influence of "muddy substances" deposited on the POF sensor nor affected by changes in the refractive index of the fuel caused by changes in fuel quality. The non-contact POF sensor proposed herein also eliminates the influence of long-term degradation of the sensing element due to chemical reactions with the fuel. The non-contact POF sensor disclosed in more detail below enables highly accurate, repeatable, and safe fuel level measurement.

[0008] The systems and methods disclosed herein use non-contact plastic optical fibers (POFs) to optically sense the level of liquid fuel in a fuel tank. According to one embodiment, a fuel level sensor system uses a graded-index POF with a large core diameter and a large numerical aperture to guide light into the fuel tank. The graded-index POF with a large core diameter and a large numerical aperture has a much larger diameter than a glass fiber and is more ductile and flexible, which eliminates fiber breakage and maintenance problems in fuel tank designs. The fuel level sensor system also includes an ultra-high-sensitivity single-photon detector to detect the light reflected from the end face of the POF and the light reflected from the surface of the fuel. (As used herein, the term "single-photon detector" refers to a detector capable of detecting a single photon.) The system uses the time of flight of the light propagating from the end face of the POF to the reflecting surface and back to the end face of the POF to measure the distance between the end face of the POF and the fuel surface. This distance is then used to calculate the height of the liquid fuel in the fuel tank. This time-of-flight technique enables the system to measure the fuel level without the POF contacting the fuel inside the fuel tank. This eliminates concerns about fuel mud deposition and long-term degradation of the POF by the fuel. The fuel level sensing systems and methods disclosed herein produce highly accurate fuel level measurements and do not require electrical feedthroughs through the fuel tank. The design proposed herein is also lighter in weight since the installation of the POF sensor is simple and does not require highly shielded cables.

[0009] According to one embodiment, the fuel level sensing system includes the following elements: (i) a high-speed and high-power red laser diode; (ii) an ultra-high-sensitivity photon-counting avalanche photodiode; and (iii) a graded-index POF with a large diameter and a large numerical aperture. The fuel level is sensed when the avalanche photodiode first detects the incident light reflected from the end face of the POF and then detects the incident light reflected from the fuel surface in response to a laser pulse emitted by the red laser diode. A time-delay detection circuit calculates the time interval separating the respective arrival times. A fuel level calculator calculates the fuel level based on the time interval provided by the time-delay detection circuit.

[0010] An avalanche photodiode (APD) is a semiconductor photodetector that uses the photoelectric effect to convert light into electricity and provides gain through avalanche multiplication. By applying a high reverse bias voltage, the APD exhibits an internal current gain effect (about 100) due to impact ionization (avalanche effect).

[0011] Although various embodiments of systems and methods for optically sensing the level of liquid fuel in a fuel tank using a non-contact POF will be described in detail below, one or more of these embodiments are characterized by one or more of the following aspects.

[0012] One aspect of the subject matter disclosed in detail below is a method for measuring the level of a liquid contained inside a storage tank, the method comprising: storing the liquid in the storage tank while a space above the surface of the liquid is occupied by a gas; at a first instant in time, emitting a laser pulse of photons from an end face of a plastic optical fiber towards the surface of the liquid; at a second instant in time after the first instant in time, detecting an impact of a first photon from the emitted laser pulse on a photodetector, the first photon being reflected at the end face of the plastic optical fiber; at a third instant in time after the second instant in time, detecting an impact of a second photon from the emitted laser pulse on the photodetector, the second photon being reflected back into the plastic optical fiber at the surface of the liquid; detecting a time delay between arrival times of the first photon and the second photon; and determining the level of the liquid in the storage tank by processing data representative of the time delay. The method may further comprise: storing data representative of the geometry of the storage tank; measuring the density of the liquid in the storage tank; calculating the mass of the remaining liquid in the storage tank based on data representative of the geometry of the storage tank, the density of the liquid, and the level of the liquid; and displaying a gauge indicating the calculated mass of the liquid in the storage tank.

[0013] According to one embodiment of the method described in the immediately preceding paragraph: (1) the plastic optical fiber is a graded-index plastic optical fiber having a numerical aperture of at least 2.9; (2) the photodetector is an avalanche photodiode operating in Geiger mode; and (3) the laser pulse is generated by a red laser diode operating around 650 nm, generating a laser pulse having a wavelength including 650 nm and a width of 100 psec or less.

[0014] Another aspect of the disclosed subject matter is a system for measuring the level of a liquid in a storage tank, the system comprising: a 1×2 optical fiber coupler disposed outside the storage tank; a laser device optically coupled to the 1×2 optical fiber coupler and configured to emit a laser pulse of photons propagating towards the 1×2 optical fiber coupler; a graded index plastic optical fiber having one end optically coupled to the 1×2 optical fiber coupler and the other end having an end face disposed within the internal space of the storage tank; a photodetector optically coupled to the 1×2 optical fiber coupler and configured to generate electrons in response to the impact of photons on the surface of the photodetector; a time delay detection circuit operatively coupled to receive electrons from the photodetector and configured to generate a time delay signal indicative of the time delay between the impact of a first photon on the surface of the photodetector at a first time and the impact of a second photon on the surface of the photodetector at a second time after the first time; and a computer system operatively coupled to receive the time delay signal from the time delay detection circuit and configured to calculate an estimated level of the liquid in the storage tank based on the time delay. The system further includes an airtight and light-impervious fiber feedthrough connector mounted in a hole formed in the wall of the storage tank and an adhesive for fixing the distal end of the graded index plastic optical fiber inside the airtight and light-impervious fiber feedthrough connector, wherein the airtight and light-impervious fiber feedthrough connector is configured such that the end face of the graded index plastic optical fiber is exposed.

[0015] According to one embodiment of the system described in the immediately preceding paragraph: the graded index plastic optical fiber has a numerical aperture of at least 2.9; the photodetector is an avalanche photodiode configured to operate in Geiger mode; and the laser device includes a red laser diode operating around 650 nm with a pulse width of 100 psec or less.

[0016] The system may further include a display device communicatively coupled to the computer system, wherein the computer system is further configured to perform the following operations: (a) storing data representing the geometry of the storage tank; (b) receiving data representing a measurement of the density of the liquid in the storage tank; (c) calculating the mass of the remaining liquid in the storage tank based on the geometry of the storage tank, the density of the liquid, and the estimated level of the liquid; and (d) outputting an electrical signal representing the calculated mass of the liquid in the storage tank to the display device.

[0017] Another aspect of the disclosed subject matter is an aircraft that includes a fuel tank and a fuel level sensing system for measuring the level of liquid in the fuel tank, wherein the fuel level sensing system includes: a 1×2 fiber optic coupler disposed outside the fuel tank; a laser device optically coupled to the 1×2 fiber optic coupler and configured to emit a laser pulse of photons propagating towards the 1×2 fiber optic coupler; a graded index plastic optical fiber having one end optically coupled to the 1×2 fiber optic coupler and the other end having an end face disposed within the interior space of the fuel tank; a photodetector optically coupled to the 1×2 fiber optic coupler and configured to generate electrons in response to the impact of photons on the surface of the photodetector; a time delay detection circuit operatively coupled to receive electrons from the photodetector and configured to generate a time delay signal indicative of the time delay between the impact of a first photon on the surface of the photodetector at a first time and the impact of a second photon on the surface of the photodetector at a second time after the first time; and a computer system operatively coupled to receive the time delay signal from the time delay detection circuit and configured to calculate an estimated level of the liquid in the fuel tank based on the time delay. The aircraft further includes a display device communicatively coupled to the computer system, wherein the computer system is further configured to perform the following operations: store data representing the geometry of the fuel tank; receive measured data representing the density of the liquid in the fuel tank; calculate the mass of the remaining liquid in the fuel tank based on the geometry of the fuel tank, the density of the liquid, and the estimated level of the liquid; and output an electrical signal representing the calculated mass of the liquid in the fuel tank to the display device.

[0018] According to one embodiment of the aircraft described in the immediately preceding paragraph, the fuel level sensing system further includes an airtight and light-impermeable fiber feedthrough connector and an adhesive, the airtight and light-impermeable fiber feedthrough connector being mounted in a hole formed in the wall of the fuel tank, the adhesive fixing the distal end of the graded index plastic optical fiber within the airtight and light-impermeable fiber feedthrough connector, wherein the airtight and light-impermeable fiber feedthrough connector is configured such that the end face of the graded index plastic optical fiber is exposed. In one exemplary embodiment, the graded index plastic optical fiber has a numerical aperture of at least 2.9, the photodetector is an avalanche photodiode configured to operate in Geiger mode, and the laser device includes a red laser diode.

[0019] Other aspects of systems and methods for optically sensing the level of liquid fuel in a fuel tank using non-contact POF are disclosed and claimed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The features, functions, and advantages discussed in the foregoing section can be implemented independently in various embodiments or can be combined in other embodiments. For purposes of illustrating the foregoing and other aspects, various embodiments will be described hereinafter with reference to the accompanying drawings. The figures briefly described in this section are not drawn to scale.

[0021] Figure 1 is a diagram showing components of a time-of-flight fuel level sensing system according to one embodiment, the time-of-flight fuel level sensing system using non-contact POF to measure the level of liquid fuel in a fuel tank.

[0022] Figure 2 is showing for Figure 1 a cross-sectional view (hatching omitted) of an airtight light-impervious POF connector design for the time-of-flight fuel level sensing system partially depicted in

[0023] Figure 3 is a graph showing an example response of a photon-counting avalanche photodiode to successive impacts of light that is emitted once by a source but later reflected from respective surfaces A and B at different times.

[0024] Figure 4 is a block diagram identifying some components of a system for providing an indication of an estimated amount of fuel remaining in a fuel tank to a pilot.

[0025] Figure 5 is a flowchart identifying steps of a method for displaying a gauge indicating an amount of liquid in a storage tank according to one embodiment.

[0026] Reference will be made hereinafter to the accompanying drawings, wherein like elements in different drawings have the same reference numerals. Detailed Description

[0027] Illustrative embodiments of systems and methods for optically sensing the level of liquid fuel in a fuel tank using non-contact POF will be described in detail below. However, not all features of the actual implementation are described in this specification. Those skilled in the art will understand that in the development of any such actual implementation, many implementation-specific decisions must be made to achieve the developer's specific goals, such as meeting system-related and business-related constraints, which will vary from one implementation to another. In addition, it should be understood that such development work may be complex and time-consuming, but it is merely routine for those of ordinary skill in the art who benefit from this disclosure.

[0028] The POF fuel level sensor disclosed herein senses the level of liquid fuel in a fuel tank using the free space optical time-of-flight principle without contacting the liquid fuel. Since the POF sensor is not immersed in the fuel in the fuel tank, the non-contact POF sensor is not prone to deterioration due to chemical reactions with the fuel and is not affected by changes in the refractive index of the fuel caused by changes in the fuel quality. The non-contact POF sensor disclosed in more detail below enables highly accurate, repeatable, and safe fuel level measurement. The proposed POF fuel level sensor also eliminates the need for cables and power in the fuel tank and eliminates EMI and lightning problems in the fuel tank. In addition, it reduces the size, weight, and power of the fuel level sensors used in current commercial aircraft. However, the technology disclosed herein can be applied to other types of liquid storage tanks and is not limited to use in fuel tanks on aircraft.

[0029] Figure 1 is a diagram showing components of a time-of-flight fuel level sensing system according to one embodiment that uses a POF 14 to measure the level of a liquid 2, such as liquid fuel 2, in a storage tank 10, such as a fuel tank. The fuel 2 has a fuel surface 3. The space above the fuel surface 3 is occupied by a gas 4. In this embodiment, the fuel tank has a bottom wall 10a, side walls 10b, and a top wall 10c. The cross-sectional profile of the side walls 10b can have any shape (e.g., circular, rectangular, trapezoidal, triangular, etc.). The distance of the fuel surface 3 from the inner surface of the bottom wall 10a of the fuel tank determines the Figure 1 fuel level L shown in

[0030] The end of the POF 14 extends into the interior of the fuel tank. The POF 14 is fed through a hole in the top wall 10c of the fuel tank by means of a sealed (meaning airtight) and light-tight fiber feedthrough connector 12 (hereinafter referred to as "airtight and light-tight fiber feedthrough connector 12"). As used herein, "light-tight" means that ambient light outside the fuel tank cannot enter the fuel 10 through the hole in the top wall 10c. The end face 1 at the distal end of the POF 14 (see Figure 2 ) is located at the height H seen in Figure 1 . The light 6 leaving the POF 14 in the form of photons 6 is a conical beam with an increasing diameter, indicated by three bold arrows in Figure 1 . The left and right arrows indicate the envelope of the conical beam with an angle equal to 2θ. (It should be understood that including the three arrows is for illustration only and should not be construed as meaning that three separate beams or three separate photons are emitted from the end face of the POF 14). The emitted light 6 impinges on the fuel surface 3 in a circular area. Figure 1 The dashed arrow in Figure 2 indicates the light 8 reflected back to the end face 1 from the fuel surface 3 (see

[0031] The proximal end of POF 14 is connected and optically coupled to one side of a low crosstalk 1×2 fiber optic coupler 16. The other side of the 1×2 fiber optic coupler 16 is connected and optically coupled to a pair of POFs 22 and 24. The waveguides within the 1×2 fiber optic coupler 16 are configured such that POFs 22 and 24 are optically coupled to POF 14. According to a proposed implementation, POF 14 is a graded index plastic optical fiber with a large diameter (about 1 mm) and a large numerical aperture (large NA). Ishigure et al. disclose a suitable large NA (e.g., NA = 0.29) graded index POF in an article titled: "High Bandwidth, High Numerical Aperture Graded Index Polymer Optical Fibers", Journal of Lightwave Technology, Vol. 13, No. 8, August 1995, pp. 1686 to 1691. According to a proposed implementation, POF 14 is a large NA graded index POF composed of a poly(methyl methacrylate) tube filled with methyl methacrylate doped with diphenyl sulfide. POF 14 (including the cladding surrounding the core) is embedded in an opaque avionics-grade fiber optic jacket (not shown in the figure) for light shielding and environmental protection.

[0032] Referring again to Figure 1 , one of the two optical ports on the transceiver side of the 1×2 fiber optic coupler is connected to a high peak power laser device 28. The laser device 28 is connected to a laser driver 26 having a high-speed pulse generator. The laser device 28 also has an automatic power control (APC) circuit to stabilize the optical power of the laser pulses during temperature variations. According to a proposed implementation, the laser device 28 is a red laser diode. The red laser diode operates at a wavelength of approximately 650 nanometers (nm), at which the loss of POF 14 described in the previous paragraph is minimal. The reason for selecting the graded index POF is that the graded index POF has a bandwidth suitable for the transmission of narrow laser pulses. The laser device 28 is controlled by the laser driver 26 to emit laser pulses 18 having a width of approximately 0.1 nanoseconds (nsec) or less.

[0033] The other of the two optical ports on the transceiver side of the 1×2 optical coupler is connected to a photodetector 32 that is configured to detect the returned laser pulse 20 (the returned laser pulse 20 may consist of one or more photons) reflected from the fuel surface 3 and the end face of the POF 14. The returned laser pulse 20 is guided by the POF 14 and 24 back to the photodetector 32. According to one proposed implementation, the photodetector 32 is a high-sensitivity photon-counting avalanche photodiode. As previously mentioned, an avalanche photodiode is a semiconductor photodetector that converts impinging photons into electrons. An opaque shield for the POF 14 is required to prevent foreign photons from being detected by the photodetector 32.

[0034] The emitted light pulse 18 propagates through the POF 22, the 1×2 fiber optic coupler 16, and the POF 14 and enters the fuel tank of the aircraft. The end face of the POF 14 is held at a height H (measured from the inner surface of the bottom wall 10a of the fuel tank) by means of an airtight and light-opaque fiber feedthrough connector 12 that penetrates the top wall 10c of the fuel tank. The red light pulse emerging from the POF 14 propagates downward and illuminates the fuel surface 3. Because the aforementioned graded-index POF has a large numerical aperture, the emitted light propagates at large angles. Moreover, due to the large NA design, the POF 14 also has a large acceptance angle for light (photons) reflected from the fuel surface 3. The large acceptance angle improves the detection efficiency when detecting the fuel level L. The large NA design of the POF 14 is very important for time-of-flight fuel level detection because the optical reflectivity of the fuel is not very high compared to other types of reflective materials.

[0035] The weak laser return pulse 20 received by the POF 14 at end face 1 (see Figure 2 ) will propagate back to the photodetector 32 through the 1×2 fiber optic coupler 16 and the POF 24. According to one proposed implementation, the photodetector 32 is a silicon avalanche photodiode operating in Geiger mode. An important advantage provided by a Geiger-mode avalanche photodiode is its extremely high sensitivity to detect single photons. Another advantage of a Geiger-mode avalanche photodiode is its high dynamic range because it is not easily saturated by an appropriate quenching resistor in series. Thus, a Geiger-mode avalanche photodiode will respond to weak or strong optical pulses having the same peak pulse response and the same narrow pulse width (100 picoseconds (psec) or less). These characteristics are advantageous for time-of-flight detection of the fuel level. By using a Geiger-mode avalanche photodiode, no additional amplifier circuit is required in the receiver, thus simplifying the design of the receiver. The silicon Geiger-mode avalanche photodiode also has a maximum responsivity at a wavelength of 650 nm, which matches well with the wavelength of the red laser diode of the transmitter.

[0036] In response to the detection of the reflected optical pulse, the photodiode 32 outputs a narrow electrical pulse having a pulse width of 100 psec or less to the time delay detection circuit 30. The time delay detection circuit 30 determines the difference (ΔT D ) between the arrival time of the pulse reflected by the end face 1 and the arrival time of the pulse reflected by the fuel surface 3. Then, the fuel level calculator 34 determines the fuel level L based on the difference ΔT D output by the time delay detection circuit 30. The difference ΔT D is equal to the flight time of the photons from the end face 1 to the fuel surface 3 and then back to the end face 1, and each leg of the optical path is equal to the distance D( Figure 1 as shown). Since the speed of light is known, the distance D can be calculated from the difference ΔT D . The fuel level L can then be calculated from the height H and the distance D.

[0037] Figure 1 The system shown in also includes a fuel quantity optical data concentrator 36 (hereinafter referred to as "FQODC 36"). Digital data representing the fuel level calculated by the fuel level calculator 34 is input to the FQODC 36, and the FQODC 36 also monitors the operation of the laser driver 26 to ensure the correct generation of the optical signal pulses required for the time-of-flight fuel level measurement. The FQODC 36 includes a computer or processor configured to process sensor data (including the fuel level data received from the fuel level calculator 34 and other data as described below with reference to Figure 4 ), and a non-transitory tangible computer-readable storage medium for storing the processed sensor data.

[0038] Figure 1 The system described in also includes a fuel quantity processing unit 38 (FQPU), and the fuel quantity processing unit 38 provides a signal to the fuel gauge 42 so that the pilot can see the aircraft fuel level presented on the cockpit display. Fuel level information and other information required to calculate the fuel quantity (such as fuel density) are output from the FQODC 36 to the fuel quantity processing unit 38. The fuel quantity processing unit 38 includes a computer or processor configured to calculate the fuel quantity and a non-transitory tangible computer-readable storage medium for storing digital data representing the calculated fuel level. The fuel quantity processing unit 38 sends the digital data representing the fuel quantity to the fuel gauge 42 for display.

[0039] In the case where the photodetector 32 is a high-sensitivity Geiger-mode avalanche photodiode, it is very important to prevent ambient light from entering the POF 14. To provide light-tightness, the POF 14 is sealed with a light-tight sheath. Since the fuel tank is completely enclosed during flight, there is no need to worry about unwanted light being coupled into the POF 14 inside the fuel tank. However, feeding the POF 14 until the end face 1 is at Figure 1 the position shown (at height H) involves using a gas-tight and light-tight fiber feedthrough connector 12.

[0040] Figure 2 FIG. is a cross-sectional view (with hatching omitted to avoid clutter) showing a gas-tight and light-tight fiber feedthrough connector 12 according to one embodiment. The gas-tight and light-tight fiber feedthrough connector 12 includes a fiber ferrule 44a in the form of a cylindrical tube and an annular flange 44b projecting radially outward from the fiber ferrule 44a. In one proposed implementation, the fiber ferrule 44a and the annular flange 44b are integrally formed and made of stainless steel. An O-ring seal 45 is placed between the annular flange 44b and the outer surface of the top wall 10c of the fuel tank. Then, the resulting sandwich structure is fastened together using a plurality of nuts and bolts (e.g., four). Figure 2 Only two bolts 48a and 48b and two nuts 49a and 49b (threadedly engaging the threaded shafts of the bolts 48a and 48b respectively) are shown in. The O-ring seal 45 is used to seal the gas-tight and light-tight fiber feedthrough connector 12 to the top wall 10c. The distal end of the POF 14 is fed until the end face 1 is at height H inside the fuel tank (see Figure 1 ) and is tightly attached to the fiber ferrule 44a using an adhesive 46 such as a high-quality opaque epoxy resin 46 to prevent light leakage into the fuel tank. For safety compliance, the gas-tight and light-tight fiber feedthrough connector 12 also prevents fuel vapor from leaking from the fuel tank.

[0041] According to one configuration, an aircraft can be equipped with three fuel quantity optical data concentrators respectively mounted in the left and right wings and near the wing box inside the fuselage, while the fuel quantity processing unit is located in the cockpit and communicates with all three fuel quantity optical data concentrators. The electronic components included in Figure 1 the fuel level sensing system shown are located at a safe distance from the fuel tank. The length of the plastic optical fiber optically coupling each laser device / photodetector group to the corresponding fuel compartment should be a length that does not produce excessive optical loss of the laser pulse optical signal.

[0042] According to one proposed implementation, the POF 14 is a large-diameter, large-NA graded-index plastic optical fiber (as described above), and this graded-index plastic optical fiber has a minimum NA approximately equal to 0.29. The numerical aperture determines Figure 1The angle θ shown, where the angle θ is equal to half of the light emission and acceptance angle of POF 14. In the case of NA = 0.29, the angle θ is approximately 17 degrees. Thus, the full acceptance angle of POF 14 is approximately 34 degrees. By modifying the design of POF 14, a larger numerical aperture is feasible. If it is desired to measure a specific location on the fuel surface, it is also feasible to add a collimating lens at the end of POF 14.

[0043] As Figure 1 shown, the end face 1 of POF 14 (best seen in Figure 2 ) is at a distance D from the fuel surface 3. D is the distance to be detected by the time-of-flight measurement system. H is the distance from the end face 1 of POF 14 to the inner surface of the bottom wall 10a of the fuel tank, where H is a constant and H depends on the vertical height of the fuel tank in the wing of the aircraft. The fuel level L = H - D.

[0044] In the case where the transmitter generates a 0.1 nanosecond optical pulse from the laser device 28, the photodetector 32 will see multiple reflections from POF 14. Figure 3 Shows the relationship between the reflected peak signal detected by the photodetector 32 and time. The last two reflected peaks are due to the plastic / gas interface ( Figure 2 the end face 1 of POF 14 shown) and the gas / fuel interface ( Figure 2 the fuel surface 3 shown). The time difference (ΔTD) between these two peaks is calculated as follows:

[0045]

[0046] where c is the speed of light in free space and D is derived from Equation (1) as follows:

[0047]

[0048] where ΔTD is a parameter measured by the peak detection circuit within the time delay detection circuit 30 of the time-of-flight fuel level sensor.

[0049] The fuel level L is determined as follows:

[0050] L = H - D (3)

[0051] where H is a constant related to the height of the fuel tank.

[0052] For a receiver with a Geiger mode avalanche photodiode (hereinafter referred to as "Geiger mode APD receiver"), which can detect individual photons at 650 nm (= 0.65 microns), the receiver sensitivity is calculated according to the energy of one photon at 650 nm as follows:

[0053]

[0054] where h is Planck's constant and v is the frequency of the photon at a wavelength of 0.65 m.

[0055] In the case of a pulse width of 0.1 nanoseconds, assuming one photon per pulse, the sensitivity of the Geiger-mode APD receiver is

[0056]

[0057] The sensitivity in dBm is calculated as follows:

[0058]

[0059] The single-photon detection sensitivity is valid for the time-of-flight fuel sensor because the reflectivity of aircraft fuel is not very high compared to other types of reflective surfaces such as metal surfaces or surfaces with reflective coatings. For a transmitter where the laser device 28 is a red laser diode, the red laser diode is operated with an optical pulse having a peak power equal to 100 mW, where there is 1% reflection at the fuel surface and 1% of the reflected light is collected by the POF 14, and the number of photons detected by the Geiger-mode avalanche photodiode can be calculated as described below.

[0060] For example, assume that the length of the optical path from the laser device 28 to the end face 1 of the POF 14 is 16 meters, and the maximum loss of the optical fiber cable (including the POF 18 and the POF 14) is 0.18 dB / m. Then the optical fiber loss can be estimated as 16 m × 0.18 dB / m = 2.88 dB. Further assume that the insertion loss of the 2×1 optical fiber coupler 16 is approximately 5 dB. Therefore, the total loss of the laser pulse optical signal at the end face 1 of the POF 14 will be (2.88 + 5) dB = 7.88 dB or approximately 8 dB.

[0061] Assuming a peak optical power of 100 mW (20 dBm), the laser pulse optical signal at the end face 1 of the POF 14 is (20 - 8) dBm = 12 dBm = 16 mW (approx.). Assuming a minimum reflection of 1% at the fuel surface 3 and a minimum collection efficiency of 1% for the POF 14, the total reflected peak optical power collected at the end face 1 is 16 mW × 0.01 = 1.6×10 -3mW = -28 dBm. In the case of an 8 dB optical loss due to the optical path from the transceiver to the end face 1, the power received at the Geiger mode APD is (-28 - 8) dBm = -36 dBm. The sensitivity of the Geiger mode APD is approximately -55.14 dBm as shown in Equation (6); thus, the Geiger mode APD has a margin of 19.14 (55.14 - 36) dB above its minimum sensitivity. This large optical detection margin makes the measurement of the time-of-flight fuel level sensor very robust and reliable.

[0062] Using a laser pulse with a pulse width of approximately 0.1 nanoseconds (100 picoseconds), the fuel level detection resolution of the time-of-flight sensor is calculated as follows:

[0063]

[0064] where d r is the fuel level resolution, c is the speed of light, and t pw is the width of the laser pulse. Assuming a laser pulse width of 0.1 nanoseconds, the resolution of the time-of-flight sensor is

[0065]

[0066] A resolution better than 1.5 cm can be obtained using a laser pulse with a width shorter than 0.1 nanoseconds (100 picoseconds).

[0067] To ensure the safe transmission of the optical pulse into the fuel tank, the average optical power can be calculated and compared with a threshold. When irradiating the fuel surface with a peak optical power of 16 mW, the laser transmitter is designed to emit a pulse with a 100 picosecond optical pulse width at 1 millisecond (repetition rate of 1 millisecond per pulse). Under these conditions, the average optical power irradiating the fuel surface is calculated as follows:

[0068]

[0069] where P avg is the average optical power at the fuel surface. Since the average optical power of 1.6 nW is much lower than the allowable safety limit of the optical power for fuel irradiation (approximately 4 mW), the time-of-flight fuel level sensor disclosed herein can be designed to meet safety requirements.

[0070] The above fuel level sensor can be installed in a fuel tank on an aircraft together with a temperature sensor and a densitometer. Then, the fuel level and fuel density data of the fuel tank, as well as the geometry, can be used to calculate the estimated amount (i.e., mass) of fuel in the fuel tank. (To measure the mass of fuel for engine consumption and range calculations, the system can use measurements of fuel level and fuel density.) Additionally, an aircraft can be retrofitted by removing existing electrical fuel level sensors and installing optical fuel level sensors in their place. According to one fuel level sensor configuration, the height of end face 1 of POF 14 is determined by the positions of the corresponding sensors in the wing tanks and center tank of the aircraft. In a baseline configuration, each electrical sensor will be replaced one-for-one by an optical sensor, eliminating the weight of the wiring and support brackets and eliminating electromagnetic effects from lightning, short circuits in the wires, and wear. Using optical fibers instead of wires also eliminates any safety hazards caused by electrical fault conditions.

[0071] Figure 4 is a block diagram showing components of a system for measuring the amount of fuel in a fuel tank according to one embodiment. The system includes a fuel level sensor 52 that outputs an electrical signal representing the level of fuel in the fuel tank in the manner described above. According to one embodiment, Figure 4 the fuel level sensor 52 identified in Figure 1 includes the following components identified in

[0072] : POF 14, 1×2 fiber optic coupler 16, POF 22 and 24, laser device 28, laser driver 26, photodetector 32, time delay detection circuit 30, and fuel level calculator 34. Thus, the fuel level data output by the fuel level calculator 34 is the output of the fuel level sensor 52. Figure 4 Additionally,

[0073] the system partially depicted in Figure 4In the embodiment shown, the FQODC 36 communicates with the Fuel Quantity Processing Unit (FQPU) 38 via a multi-master serial bus called the CAN bus 58. To this end, the FQODC 36 and the fuel quantity processing unit 38 can each incorporate a controller and a transceiver of the type used in a Controller Area Network (CAN). Such a CAN controller and CAN transceiver are referred to herein as a "CAN node". The FQODC 36 has different dedicated analog circuits to measure the temperature, density, and level of the fuel respectively. The analog values of these parameters are converted into digital values, packed into a data field, and transmitted via the CAN bus 58 to the fuel quantity processing unit 38. The ARINC 845 CAN bus is an example of a simple avionics digital data bus that can be used, but alternatively, any other digital data bus, such as ARINC 425 or ARINC 664, can be used.

[0074] According to the CAN communication protocol, each CAN node is capable of sending and receiving messages, but not simultaneously. A message or frame mainly consists of an identifier representing the priority of the message and a plurality of data bytes. The message is serially transmitted by the CAN transceiver onto the CAN bus 58 and can be received by all CAN nodes. Each CAN node connected to the CAN bus 58 waits for a specified inactive time before attempting to send a message. If a collision occurs (i.e., if two nodes attempt to send messages simultaneously), the collision is resolved by bit-by-bit arbitration based on the pre-programmed priority of each message in the identifier field of the message. The message containing the highest priority identifier always wins bus access.

[0075] The sensor data obtained by the fuel level sensor 52, the densitometer 54, and the temperature sensor 56 is formatted according to the CAN communication protocol to form a CAN message, which is broadcast onto the CAN bus 58 and received by the fuel quantity processing unit 38. The fuel quantity processing unit 38 is configured to estimate the mass of the fuel remaining in the fuel tank (or its compartment) based on the measured fuel density, the known geometry of the fuel tank (or its compartment), and the measured fuel level H. For example, the volume of the remaining fuel can be calculated based on the known geometry and the measured fuel level, and then the mass of the remaining fuel will be equal to the product of the volume and the density. An electrical signal representing the estimated mass of the remaining fuel is output from the fuel quantity processing unit 38 to the fuel gauge 42. The fuel gauge 42 can take the form of a display device having a display processor that is programmed to display the measurement results (e.g., the fuel level or the fuel quantity) graphically and / or alphanumerically on a display screen.

[0076] The fuel quantity processing unit 38 can be a computer or part of a flight control system located on an aircraft. When identifying the quantity of fuel present in an irregularly shaped fuel tank, the fuel quantity processing unit 38 can execute various routines to calculate the quantity of fuel present based on optical power data received from a plurality of fuel level sensors 52 optically coupled to respective compartments of the fuel tank. The fuel information processing software can include routines that take into account the shape of the fuel tank to determine the quantity of fuel remaining in the fuel tank. The fuel information processing software can also include routines for a calibration process to establish a baseline or maintain the accuracy of the fuel readings prior to first use. The readings provided by the fuel quantity processing unit 38 to the fuel gauge 42 can be integrated or averaged prior to display and can be provided at different time intervals.

[0077] The wing fuel tank system using electrical sensors can be retrofitted by replacing the optical sensors disclosed herein. The double-shielded wires for the electrical sensors can be replaced with lightweight and flexible plastic optical fibers, eliminating the weight of the wires and support brackets and eliminating electromagnetic effects from lightning, short circuits, and wire wear.

[0078] In summary, a non-contact POF aircraft fuel level sensor using the time-of-flight principle has been disclosed. The fuel level sensor is capable of detecting the fuel level with high resolution, high accuracy, good long-term stability, and high safety operation. Using a graded-index POF with a large diameter and large NA, the time-of-flight fuel level sensor has a low maintenance cost because it eliminates the fiber breakage problems encountered in glass fiber fuel sensors. The time-of-flight fuel level sensor does not come into contact with the fuel, thus eliminating the problem of sensor degradation due to fuel sludge deposition and chemical reactions between the fuel and the POF sensor.

[0079] The fuel level calculator 34 can include one or more dedicated microprocessors or one or more general-purpose computers and can calculate the measured level (i.e., height) of the fuel by using a look-up table, a calibration curve, or by appropriately solving an equation. The fuel gauge 42 displayed in the cockpit can be controlled to indicate the quantity of fuel present in the fuel tank based on the sensor data received by the FQODC 36.

[0080] Each optical fiber is a flexible, optically transparent or translucent fiber made of extruded plastic. It can be used as a waveguide or an optical conduit to transmit light between the two ends of the fiber. An optical fiber typically includes a transparent or translucent core having a relatively high refractive index, which is surrounded by a transparent or translucent cladding material having a relatively low refractive index. Light is kept in the core by total internal reflection. This enables the optical fiber to be used as a waveguide.

[0081] In Figure 1 and Figure 4In the example embodiment shown, a plastic optical fiber is used to measure the level of fuel in a fuel tank. In other embodiments, the same device can be used to detect other liquids. For example, the system described above can be used to detect the presence of water in a container or hydraulic fluid in a storage tank of a hydraulic system. The illustration of detecting fuel in a fuel tank is presented for illustrative purposes and is not meant to limit the ways in which the system shown in Figure 1 and Figure 4 can be used.

[0082] More generally, Figure 5 is a flowchart showing the steps of a method 100 for identifying a gauge for displaying an indication of the amount of liquid in a storage tank according to one embodiment. Initially, data representing the geometry of the storage tank is stored in a computer-accessible non-transitory tangible computer-readable storage medium (step 102). A liquid is stored in the storage tank, where the space above the surface of the liquid is occupied by a gas (step 104). At a first instant in time, a laser pulse of photons is emitted from the end face of the plastic optical fiber towards the surface of the liquid (step 106). At a second instant in time after the first instant in time, an impact of a first photon from the emitted laser pulse on a photodetector is detected, the first photon being reflected at the end face of the plastic optical fiber (step 108). At a third instant in time after the second instant in time, an impact of a second photon from the emitted laser pulse on the photodetector is detected, the second photon being reflected back into the plastic optical fiber at the surface of the liquid (step 110). Then the time delay between the arrival times of the first and second photons is determined (step 112). Then the level of the liquid in the storage tank is determined by processing the data representing the time delay (step 114). In addition, the density of the liquid in the storage tank is measured (step 116). Then the mass of the remaining liquid in the storage tank is calculated based on the data representing the geometry of the storage tank, the density of the liquid, and the level of the liquid (step 118). A gauge indicating the calculated mass of the liquid in the storage tank is displayed (step 120).

[0083] Although systems and methods for optically sensing the level of liquid fuel in a fuel tank using a non-contact POF have been described with reference to various embodiments, those skilled in the art will appreciate that various changes can be made and elements thereof can be replaced with equivalents without departing from the teachings herein. In addition, many modifications can be made to adapt the concepts and teachings disclosed herein to a particular situation. Accordingly, it is intended that the subject matter covered by the claims not be limited to the disclosed embodiments.

[0084] As used in the claims, the term "computer system" should be broadly construed to include a system having at least one computer or processor, and which may have multiple computers or processors communicating via a network or bus. As used in the previous sentence, the terms "computer" and "processor" both refer to a device having a processing unit (e.g., a central processing unit) and some form of memory for storing a program readable by the processing unit (i.e., a non-transitory tangible computer-readable storage medium). For example, Figure 1 the fuel level calculator 34, the fuel quantity optical data concentrator 36, and the fuel quantity processing unit 38 shown in Figure 1 may be separate processors or computers communicatively coupled to form a "computer system".

[0085] In addition, the present disclosure includes embodiments according to the following clauses:

[0086] Clause 1. A method for measuring the level of a liquid contained inside a storage tank, the method comprising: storing the liquid in the storage tank while the space above the surface of the liquid is occupied by a gas; at a first instant in time, emitting a laser pulse of photons from an end face of a plastic optical fiber towards the surface of the liquid; at a second instant in time after the first instant in time, detecting the impact of a first photon from the emitted laser pulse on a photodetector, the first photon being reflected at the end face of the plastic optical fiber; at a third instant in time after the second instant in time, detecting the impact of a second photon from the emitted laser pulse on the photodetector, the second photon being reflected back into the plastic optical fiber at the surface of the liquid; detecting the time delay between the arrival times of the first photon and the second photon; and determining the level of the liquid in the storage tank by processing data representative of the time delay.

[0087] Clause 2. The method according to clause 1, wherein the plastic optical fiber is a graded-index plastic optical fiber having a numerical aperture of at least 2.9.

[0088] Clause 3. The method according to clause 1 or 2, wherein the photodetector is an avalanche photodiode operating in Geiger mode.

[0089] Clause 4. The method according to any one of clauses 1 to 3, wherein the laser pulse is generated by a red laser diode.

[0090] Clause 5. The method according to clause 4, wherein the red laser diode operates around 650 nm.

[0091] Clause 6. The method according to any one of clauses 1 to 5, wherein the laser pulse has a wavelength including 650 nm and a width of 100 psec or less.

[0092] Clause 7. The method as described in any one of Clauses 1 to 6 further includes: storing data representing the geometry of the storage tank; measuring the density of the liquid in the storage tank; calculating the mass of the remaining liquid in the storage tank based on the data representing the geometry of the storage tank, the density of the liquid, and the level of the liquid; and displaying a gauge indicating the calculated mass of the liquid in the storage tank.

[0093] Clause 8. The method as described in any one of Clauses 1 to 7, wherein the liquid is fuel and the storage tank is a fuel tank on an aircraft.

[0094] Clause 9. A system for measuring the level of a liquid in a storage tank, the system comprising: a 1×2 optical fiber coupler disposed outside the storage tank; a laser device optically coupled to the 1×2 optical fiber coupler and configured to emit a laser pulse of photons propagating towards the 1×2 optical fiber coupler; a graded-index plastic optical fiber having one end optically coupled to the 1×2 optical fiber coupler and the other end having an end face disposed within the internal space of the storage tank; a photodetector optically coupled to the 1×2 optical fiber coupler and configured to generate electrons in response to the impact of photons on the surface of the photodetector; a time delay detection circuit operably coupled to receive electrons from the photodetector and configured to generate a time delay signal representing the time delay between the impact of a first photon on the surface of the photodetector at a first time and the impact of a second photon on the surface of the photodetector at a second time after the first time; and a computer system operably coupled to receive the time delay signal from the time delay detection circuit and configured to calculate the level of the liquid in the storage tank based on the time delay.

[0095] Clause 10. The system as described in Clause 9, the system further comprising an airtight and light-tight fiber feedthrough connector mounted in a hole formed in the wall of the storage tank and an adhesive for fixing the distal end of the graded-index plastic optical fiber inside the airtight and light-tight fiber feedthrough connector.

[0096] Clause 11. The system as described in Clause 10, wherein the airtight and light-tight fiber feedthrough connector is configured such that the end face of the graded-index plastic optical fiber is exposed.

[0097] Clause 12. The system as described in any one of Clauses 9 to 11, wherein the graded-index plastic optical fiber has a numerical aperture of at least 2.9.

[0098] Clause 13. The system as described in any one of Clauses 9 to 12, wherein the photodetector is an avalanche photodiode configured to operate in Geiger mode.

[0099] Clause 14. The system as described in any one of Clauses 9 to 13, wherein the laser device includes a red laser diode.

[0100] Clause 15. The system as described in any one of Clauses 9 to 14, wherein the laser device operates around 650 nm.

[0101] Clause 16. The system as described in any one of Clauses 9 to 15, wherein the laser device is capable of emitting laser pulses having a width of 100 psec or less.

[0102] Clause 17. The system as described in any one of Clauses 9 to 16, the system further includes a display device communicatively coupled to the computer system, wherein the computer system is further configured to perform the following operations: storing data representing the geometry of the storage tank; receiving measurement data representing the density of the liquid in the storage tank; calculating the mass of the remaining liquid in the storage tank based on the geometry of the storage tank, the density of the liquid, and the level of the liquid; and outputting an electrical signal representing the calculated mass of the liquid in the storage tank to the display device.

[0103] Clause 18. The system as described in any one of Clauses 9 to 17, wherein the liquid is fuel and the storage tank is a fuel tank on an aircraft.

[0104] Clause 19. An aircraft, the aircraft includes a fuel tank and a fuel level sensing system as described in any one of Clauses 9 to 18 for measuring the level of the liquid in the fuel tank.

[0105] Clause 20. An aircraft, the aircraft including a fuel tank and a fuel level sensing system for measuring the level of liquid in the fuel tank, wherein the fuel level sensing system includes: a 1×2 optical fiber coupler disposed outside the fuel tank; a laser device optically coupled to the 1×2 optical fiber coupler and configured to emit a laser pulse of photons propagating towards the 1×2 optical fiber coupler; a graded index plastic optical fiber having one end optically coupled to the 1×2 optical fiber coupler and the other end having an end face disposed within the interior space of the fuel tank; a photodetector optically coupled to the 1×2 optical fiber coupler and configured to generate electrons in response to the impact of photons on the surface of the photodetector; a time delay detection circuit operably coupled to receive electrons from the photodetector and configured to generate a time delay signal representing the time delay between the impact of a first photon on the surface of the photodetector at a first time and the impact of a second photon on the surface of the photodetector at a second time after the first time; and a computer system operably coupled to receive the time delay signal from the time delay detection circuit and configured to calculate an estimated level of the liquid in the fuel tank based on the time delay.

[0106] Clause 21. The aircraft according to Clause 20, wherein the fuel level sensing system further includes an airtight and light-impermeable fiber feedthrough connector and an adhesive, the airtight and light-impermeable fiber feedthrough connector being installed in a hole formed in the wall of the fuel tank, the adhesive fixing the distal end of the graded index plastic optical fiber within the airtight and light-impermeable fiber feedthrough connector, wherein the airtight and light-impermeable fiber feedthrough connector is configured such that the end face of the graded index plastic optical fiber is exposed.

[0107] Clause 22. The aircraft according to Clause 20 or 21, wherein the graded index plastic optical fiber has a numerical aperture of at least 2.9, the photodetector is an avalanche photodiode configured to operate in Geiger mode, and the laser device includes a red laser diode.

[0108] Clause 23. The aircraft according to any one of Clauses 20 to 22, the aircraft further including a display device communicatively coupled to the computer system, wherein the computer system is further configured to perform the following operations: store data representing the geometry of the fuel tank; receive measured data representing the density of the liquid in the fuel tank; calculate the mass of the remaining liquid in the fuel tank based on the geometry of the fuel tank, the density of the liquid, and the estimated level of the liquid; and output an electrical signal representing the calculated mass of the liquid in the fuel tank to the display device.

[0109] Clause 24. A method for measuring the level of a liquid contained inside a storage tank, the method comprising: storing the liquid in the storage tank while the space above the surface of the liquid is occupied by a gas; at a first instant in time, emitting a laser pulse of photons from an end face of a plastic optical fiber towards the surface of the liquid; at a second instant in time after the first instant in time, detecting an impact of a first photon from the emitted laser pulse on a photodetector, the first photon being reflected at the end face of the plastic optical fiber; at a third instant in time after the second instant in time, detecting an impact of a second photon from the emitted laser pulse on the photodetector, the second photon being reflected back into the plastic optical fiber at the surface of the liquid; detecting a time delay between the arrival times of the first photon and the second photon; and determining the level of the liquid in the storage tank by processing data representative of the time delay.

[0110] Clause 25. The method according to clause 24, wherein the laser pulse is generated by a red laser diode, preferably wherein the red laser diode operates around 650 nm.

[0111] Clause 26. The method according to clause 24 or 25, the method further comprising: storing data representative of the geometry of the storage tank; measuring the density of the liquid in the storage tank; calculating the mass of the remaining liquid in the storage tank based on data representative of the geometry of the storage tank, the density of the liquid, and the level of the liquid; and displaying a gauge indicating the calculated mass of the liquid in the storage tank.

[0112] Clause 27. A system for measuring the level of a liquid in a storage tank, the system comprising: a 1×2 fiber optic coupler disposed outside the storage tank; a laser device optically coupled to the 1×2 fiber optic coupler and configured to emit a laser pulse of photons propagating towards the 1×2 fiber optic coupler; a graded-index plastic optical fiber having one end optically coupled to the 1×2 fiber optic coupler and an opposite end having an end face disposed within the interior space of the storage tank; a photodetector optically coupled to the 1×2 fiber optic coupler and configured to generate electrons in response to an impact of a photon on the surface of the photodetector; a time delay detection circuit operatively coupled to receive electrons from the photodetector and configured to generate a time delay signal representative of a time delay between an impact of a first photon on the surface of the photodetector at a first time and an impact of a second photon on the surface of the photodetector at a second time after the first time; and a computer system operatively coupled to receive the time delay signal from the time delay detection circuit and configured to calculate the level of the liquid in the storage tank based on the time delay.

[0113] Clause 28. The system as described in Clause 27, the system further comprising an airtight and light-impermeable fiber feedthrough connector and an adhesive, the airtight and light-impermeable fiber feedthrough connector being installed in a hole formed in the wall of the storage tank, the adhesive fixing the distal end of the graded-index plastic optical fiber within the airtight and light-impermeable fiber feedthrough connector.

[0114] Clause 29. The system as described in Clause 28, wherein the airtight and light-impermeable fiber feedthrough connector is configured such that the end face of the graded-index plastic optical fiber is exposed.

[0115] Clause 30. The method or system as described in any one of Clauses 24 to 29, wherein the graded-index plastic optical fiber has a numerical aperture of at least 2.9.

[0116] Clause 31. The method or system as described in any one of Clauses 24 to 30, wherein the photodetector is an avalanche photodiode configured to operate in Geiger mode.

[0117] Clause 32. The system as described in any one of Clauses 27 to 31, wherein the laser device includes a red laser diode.

[0118] Clause 33. The system as described in any one of Clauses 27 to 32, wherein the laser device operates around 650 nm.

[0119] Clause 34. The method or system as described in any one of Clauses 24 to 33, wherein the laser device is capable of emitting laser pulses having a width of 100 psec or less.

[0120] Clause 35. The system as described in any one of Clauses 27 to 34, the system further comprising a display device communicatively coupled to a computer system, wherein the computer system is further configured to perform the following operations: storing data representing the geometry of the storage tank; receiving measurement data representing the density of the liquid in the storage tank; calculating the mass of the remaining liquid in the storage tank based on the geometry of the storage tank, the density of the liquid, and the level of the liquid; and outputting an electrical signal representing the calculated mass of the liquid in the storage tank to the display device.

[0121] Clause 36. The method or system as described in any one of Clauses 24 to 35, wherein the liquid is fuel and the storage tank is a fuel tank on an aircraft.

[0122] Clause 37. An aircraft, the aircraft comprising a fuel tank and a fuel level sensing system as described in any one of Clauses 27 to 36, the fuel level sensing system for measuring the level of the liquid in the fuel tank.

[0123] Clause 38. The aircraft as described in Clause 37, wherein the fuel level sensing system further includes an airtight and light-impermeable fiber feedthrough connector and an adhesive, the airtight and light-impermeable fiber feedthrough connector is installed in a hole formed in the wall of the fuel tank, and the adhesive fixes the distal end of the graded index plastic optical fiber inside the airtight and light-impermeable fiber feedthrough connector, wherein the airtight and light-impermeable fiber feedthrough connector is configured such that the end face of the graded index plastic optical fiber is exposed.

[0124] Clause 39. The aircraft as described in Clause 37 or 38, wherein the graded index plastic optical fiber has a numerical aperture of at least 2.9, the photodetector is an avalanche photodiode configured to operate in Geiger mode, and the laser device includes a red laser diode.

[0125] The method claims set forth herein should not be construed as requiring that the steps recited therein be performed in alphabetical order (any alphabetical order in the claims is for reference purposes only of the steps recited previously) or in the order in which they are described, unless the language of the claim expressly provides or states conditions indicating a particular order in which some or all of these steps are to be performed. Method claims should also not be construed as excluding any portion of two or more steps that are performed simultaneously or alternately, unless the language of the claim expressly states conditions precluding such an interpretation.

Claims

1. A method (100) for measuring the level of a liquid (2) contained in a storage tank (10), the method (100) comprising: storing the liquid (2) in the storage tank (10) while a space above the surface (3) of the liquid (2) is occupied by a gas (4); at a first instant in time, emitting a laser pulse (18) of photons (6) from an end face (1) of a graded-index plastic optical fiber (14) towards the surface (3) of the liquid (2), wherein the graded-index plastic optical fiber (14) has a numerical aperture of at least 2.9 and the laser pulse (18) is generated by a red laser diode; at a second instant in time after the first instant in time, detecting an impact on a photodetector (32) of a first photon reflected at the end face (1) of the plastic optical fiber (14) from the emitted laser pulse (18), wherein the photodetector (32) is an avalanche photodiode configured to operate in Geiger mode; at a third instant in time after the second instant in time, detecting an impact on the photodetector (32) of a second photon reflected at the surface (3) of the liquid (2) back into the plastic optical fiber (14) from the emitted laser pulse (18); detecting a time delay between arrival times of the first photon and the second photon; and determining the level of the liquid (2) in the storage tank (10) by processing data representative of the time delay.

2. The method (100) according to claim 1, the method further comprising: storing data representative of the geometry of the storage tank (10); measuring the density of the liquid (2) in the storage tank (10); calculating the mass of the remaining liquid (2) in the storage tank (10) based on data representative of the geometry of the storage tank (10), the density of the liquid (2), and the level of the liquid (2); and displaying a meter (42) indicative of the calculated mass of the liquid (2) in the storage tank (10).

3. A system (50) for measuring the level of a liquid (2) in a storage tank (10), the system (50) comprising: a 1×2 fiber coupler (16) disposed outside the storage tank (10); a laser device (28) optically coupled to the 1×2 fiber coupler (16) and configured to emit a laser pulse (18) of photons (6) propagating towards the 1×2 fiber coupler (16), wherein the laser device (28) includes a red laser diode; a graded-index plastic optical fiber (14) having one end optically coupled to the 1×2 fiber coupler (16) and the other end having an end face (1) disposed within the interior space of the storage tank (10), wherein the graded-index plastic optical fiber (14) has a numerical aperture of at least 2.9; A photodetector (32) optically coupled to the 1×2 fiber coupler (16) and configured to generate electrons in response to the impact of the photons (6) on the surface of the photodetector (32), wherein the photodetector (32) is an avalanche photodiode configured to operate in Geiger mode; A time delay detection circuit (30) operatively coupled to receive electrons from the photodetector (32) and configured to generate a time delay signal representing the time delay between the impact of a first photon on the surface of the photodetector at a first time and the impact of a second photon on the surface of the photodetector at a second time after the first time; and A computer system operatively coupled to receive the time delay signal from the time delay detection circuit (30) and configured to calculate the level of the liquid (2) in the storage tank (10) based on the time delay.

4. The system (50) according to claim 3, the system (50) further comprising an airtight and light-impermeable fiber feedthrough connector (12) and an adhesive (46), the airtight and light-impermeable fiber feedthrough connector being mounted in a hole formed in the wall (10c) of the storage tank (10), the adhesive fixing the distal end of the graded-index plastic optical fiber (14) within the airtight and light-impermeable fiber feedthrough connector (12).

5. The system (50) according to claim 4, wherein, the airtight and light-impermeable fiber feedthrough connector (12) is configured such that the end face (1) of the graded-index plastic optical fiber (14) is exposed.

6. The system (50) according to claim 3, wherein, the laser device (28) operates at 650 nanometers (nm).

7. The method (100) according to claim 1 or the system (50) according to claim 3, wherein, the laser device (28) is capable of emitting laser pulses (18) having a width of 100 picoseconds (psec) or less.

8. The system (50) according to claim 3, the system (50) further comprising a display device communicatively coupled to the computer system, wherein, the computer system is further configured to perform the following operations: Store data representing the geometry of the storage tank (10); Receive measurement data representing the density of the liquid (2) in the storage tank (10); Calculate the mass of the remaining liquid (2) in the storage tank (10) based on the geometry of the storage tank (10), the density of the liquid (2), and the level of the liquid (2); and Output an electrical signal representing the calculated mass of the liquid (2) in the storage tank (10) to the display device.

9. The method (100) according to claim 1 or the system (50) according to claim 3, wherein, the liquid (2) is fuel and the storage tank (10) is a fuel tank on an aircraft.

10. An aircraft, the aircraft comprising a fuel tank and the system (50) according to claim 3, the system being a fuel level sensing system (50) for measuring the level of the liquid (2) in the fuel tank.

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