Patent Publication Number: US-2023159182-A1

Title: Optical level gauge apparatus and method, liquid tank assembly and vehicle employing the same

Description:
TECHNICAL FIELD 
     The disclosure herein relates to an optical level gauge apparatus and method. Furthermore, the disclosure herein relates to a liquid tank assembly and a vehicle. 
     BACKGROUND 
     Many current solutions for level gauging require the sensing element in direct contact with the liquid inside the tank, sometimes even requiring the use of electrical energy inside the tank. 
     In some cases, the sensing method is based on discrete level measuring points, where e.g. a diode triggers a response once in direct contact to the liquid. This solution requires a high amount of ports, one per each sensing point, increasing the number of cables/ports and harness for high resolution requirements. Consequently maintainability and/or installation is/are affected. 
     Usual fiber optics sensors also require direct contact with the liquid, as the measuring principle is based on based on changes in one or more optical properties of the fiber along its length. In some arrangements, such as the ones disclosed in U.S. Pat. No. 9,074,921 B1, the fiber is co-installed with a wire cable to heat the liquid and detect the liquid level based on the difference in time needed to cool-off for the wet and dry sections of the fiber. 
     For these types of approaches using the heat dissipation rate difference between the gas and liquid phase of the hydrogen level some liquid boiling is inevitably created, contributing to the vaporization of the fuel leading to inefficient leakage. For maintenance tasks, many current solutions require emptying the tank to be able to open it and get access to the sensing elements installed inside. 
     SUMMARY 
     It is an object of the disclosure herein to improve level gauging inside liquid tanks, preferably in the field of cryogenic liquids. 
     The object is achieved by the subject-matter and embodiments disclosed herein. 
     The disclosure herein provides an optical level gauge apparatus configured for gauging a liquid level in a tank volume defined by a tank wall of a tank, the apparatus comprising:
         a light source;   a sensor device having a plurality of optical sensors, each optical sensor configured for detecting an amount of light, and the sensor device is configured to output a plurality of measured light values that are indicative of the amount of light detected by the respective optical sensor;   a light conducting device configured to be optically coupled to the light source, the sensor device, and the tank volume, wherein, when optically coupled to the light source, the sensor device, and the tank volume, the light conducting device is capable of conducting outgoing light rays from the light source to the tank volume and incoming light rays, that were reflected or scattered at least once within the tank volume, from the tank volume to the sensor device, and the incoming light rays are conducted in such a manner that each optical sensor corresponds to a specific portion of the tank wall;   a liquid level determining device configured for determining the liquid level based on the measured light values.       

     Preferably, the light source is configured to emit one of multi-color light having a plurality of discretely separated wavelengths, visible white light, single wavelength light having visible, ultraviolet or infrared wavelength. Preferably, the light source comprises a laser source. 
     Preferably, the light conducting device includes an optical output member that is configured to output the outgoing light with a, preferably conical, light ray pattern such that, when optically coupled to the tank volume, the outgoing light is capable of impinging on a tank wall circumferential portion of the tank wall. Preferably, the light conducting device includes an optical input member that, when optically coupled to the tank volume, is configured to receive the incoming light rays after them being reflected or scattered by the tank wall circumferential portion at least once, preferably exactly once. 
     Preferably, the input member is capable of receiving the incoming light rays after them being reflected or scattered by the tank wall circumferential portion and then subsequently being refracted by a liquid surface in the tank volume. 
     Preferably, the output member and the input member are combined into an input/output member. 
     Preferably, the light conducting device includes an outgoing light conductor, the outgoing light conductor being configured for conducting light emitted by the light source towards the tank volume, preferably to the output member. 
     Preferably, the light conducting device includes an incoming light conductor, the incoming light conductor being configured for conducting light received, preferably by the input member, from the tank volume to the sensor device. 
     Preferably, the light conducting device includes a bidirectional light conductor configured for conducting the outgoing and incoming light rays to and from the tank volume, preferably by an input/output member. 
     Preferably, the light conducting device conducts the incoming light rays such that the specific portions are, preferably equidistantly, distributed along the circumferential direction of the tank wall. 
     Preferably, a first specific portion and a second specific portion that neighbor each other along the circumferential direction of the tank wall are distanced along the longitudinal direction of the tank volume due to a light ray received from the first specific portion being refracted by a liquid level in the tank volume while another light ray received from the second specific portion not being refracted by the liquid level. 
     Preferably, the light conducting device conducts the incoming light rays such that a first and second group of specific portions are each, preferably equidistantly, distributed along the circumferential direction of the tank wall and are distanced along the longitudinal direction so as to allow measuring a tilt of the tank about a tank tilt axis that is perpendicular to the longitudinal direction, such as a transversal tank axis. 
     Preferably, the sensor device has an imaging sensor that is formed by the optical sensors, and the sensor device is configured to output the measured light values as image data. 
     Preferably, the light conducting device is configured as an imaging light conducting device capable of conducting the incoming light rays such that the sensor device is able to image at least a portion of the tank volume, where the outgoing light rays fall, preferably to image the tank volume where outgoing light rays fall. 
     Preferably, the liquid level determining device is configured to determine the liquid level by processing the image data. 
     Preferably, the apparatus further comprises a pattern arrangement that is arrangeable within or projectable into the tank volume, wherein the incoming light rays are caused by the pattern arrangement. 
     Preferably, the pattern arrangement is configured to absorb a first wavelength emitted by the light source and configured to emit a second wavelength that is different from the first wavelength. 
     Preferably, the pattern arrangement is suitable to reflect the outgoing light rays, thereby generating the incoming light rays. 
     Preferably, the pattern arrangement comprises at least one stripe that is configured to reflect outgoing light. Preferably, the pattern arrangement comprises two stripes each configured to reflect outgoing light and arranged parallel to each other. 
     Preferably, the sensor device is configured to detect the second wavelength, but preferably not the first wavelength. 
     Preferably, the pattern arrangement is illuminatable or projectable by the outgoing light rays such that incoming light rays are only generated a portion of the pattern arrangement that is in the same region above or below the liquid level as the output member. 
     Preferably, the light source is tunable in wavelength. 
     Preferably, the sensor device has a spectrometer or spectral camera that is respectively formed by the optical sensors, and the sensor device is configured to output the measured light values as spectral data or spectral image data respectively. 
     Preferably, the liquid level determining device is configured to control the light source so as to emit at a measurement wavelength, that is determined from the spectral data or the spectral image data and that allows an increase of a signal-to-noise ratio. 
     The disclosure herein provides a liquid tank assembly, preferably a cryogenic liquid tank assembly, for a vehicle, preferably an aircraft, the liquid tank assembly comprising a tank having a tank wall defining a tank volume for storing a liquid and a preferred optical level gauge apparatus, wherein the light conducting device is optically coupled to the tank volume. 
     Preferably, the light conducting device is optically coupled to the light source, the sensor device, and the tank volume. 
     Preferably, the light conducting device is optically coupled such that outgoing light rays are conducted from the light source to the tank volume and incoming light rays, that were reflected or scattered at least once within the tank volume, are conducted from the tank volume to the sensor device. 
     Preferably, the incoming light rays are conducted in such a manner that each optical sensor corresponds to a specific portion of the tank wall. 
     Preferably, the light source is configured to emit one of multi-color light having a plurality of discretely separated wavelengths, visible white light, single wavelength light having visible, ultraviolet or infrared wavelength. Preferably, the light source comprises a laser source. 
     Preferably, an optical output member is disposed outside the tank volume on the tank wall. 
     Preferably, the output member is optically coupled to the tank volume such that the outgoing light is output with a, preferably conical, light ray pattern such that the outgoing light impinges on a tank wall circumferential portion of the tank wall. 
     Preferably, an optical input member is disposed outside the tank volume on the tank wall. 
     Preferably, the light conducting device includes an optical input member that is optically coupled to the tank volume so that incoming light rays are received after them being reflected or scattered by the tank wall circumferential portion at least once, preferably exactly once. 
     Preferably, the input member receives the incoming light rays after them being reflected or scattered by the tank wall circumferential portion and them subsequently being refracted by a liquid surface in the tank volume. 
     Preferably, the output member and the input member are combined into an input/output member. 
     Preferably, the light conducting device includes an outgoing light conductor, the outgoing light conductor being configured for conducting light emitted by the light source towards the tank volume, preferably to the output member. 
     Preferably, the light conducting device includes an incoming light conductor, the incoming light conductor being configured for conducting light received, preferably by the input member, from the tank volume to the sensor device. 
     Preferably, the light conducting device includes a bidirectional light conductor configured for conducting the outgoing and incoming light rays to and from the tank volume, preferably by an input/output member. 
     Preferably, the light conducting device conducts the incoming light rays such that the specific portions are, preferably equidistantly, distributed along the circumferential direction of the tank wall. 
     Preferably, a first specific portion and a second specific portion that neighbor each other along the circumferential direction of the tank wall are distanced along the longitudinal direction of the tank volume due to a light ray received from the first specific portion being refracted by a liquid level in the tank volume while another light ray received from the second specific portion not being refracted by the liquid level. 
     Preferably, the light conducting device conducts the incoming light rays such that a first and second group of specific portions are each, preferably equidistantly, distributed along the circumferential direction of the tank wall and are distanced along the longitudinal direction so as to allow measuring a tilt of the tank about a tank tilt axis that is perpendicular to the longitudinal direction, such as a lateral tank axis. 
     Preferably, the sensor device has an imaging sensor that is formed by the optical sensors, and the sensor device is configured to output the measured light values as image data. 
     Preferably, the light conducting device is configured as an imaging light conducting device capable of conducting the incoming light rays such that the sensor device is able to image the tank volume. 
     Preferably, the liquid level determining device is configured to determine the liquid level by processing the image data. 
     Preferably, the apparatus further comprises a pattern arrangement that is arranged on or projected onto the tank wall, wherein the incoming light rays are caused by the pattern arrangement. 
     Preferably, the pattern arrangement is arranged on the tank wall. Preferably, the pattern arrangement comprises a material that is configured to absorb a first wavelength emitted by the light source and configured to emit a second wavelength that is different from the second wavelength. Preferably, the material is a fluorescent material. 
     Preferably, the pattern arrangement is suitable to reflect the outgoing light rays, thereby generating the incoming light rays. 
     Preferably, the pattern arrangement comprises at least one stripe that is configured to reflect outgoing light. Preferably, the pattern arrangement comprises two stripes each configured to reflect outgoing light and arranged parallel to each other. 
     Preferably, the sensor device is configured to detect the second wavelength, but preferably not the first wavelength. 
     Preferably, the pattern arrangement is illuminated or projected by the outgoing light rays such that incoming light rays are only generated for portions of the pattern arrangement that are below the liquid level. 
     Preferably, the tank wall comprises at least one anti-sloshing baffle that protrudes along a radial tank direction into the tank volume. Preferably, the pattern arrangement is arranged on or projected onto the anti-sloshing baffle. 
     Preferably, the light source is tunable in wavelength. 
     Preferably, the sensor device has a spectrometer or spectral camera that is respectively formed by the optical sensors, and the sensor device is configured to output the measured light values as spectral data or spectral image data respectively. 
     Preferably, the liquid level determining device is configured to control the light source so as to emit at a measurement wavelength, that is determined from the spectral data or the spectral image data and that allows an increase of a signal-to-noise ratio. 
     The disclosure herein provides a vehicle, preferably an aircraft, having a preferred optical level gauge apparatus or a preferred liquid tank assembly. 
     The disclosure herein provides an optical level gauge method for gauging a liquid level in a tank volume defined by a tank wall of a tank, the method comprising: 
     a) with a light conducting device, conducting light rays, that were reflected or scattered at least once within the tank volume, from the tank volume to a sensor device having a plurality of optical sensors, wherein the light rays are conducted in a manner that each optical sensor corresponds to a specific portion of the tank wall; 
     b) with the sensor device, detecting an amount of light by each optical sensor and outputting a plurality of measured light values that are indicative of the amount of light detected by the respective optical sensor; and 
     c) with a liquid level determining device, determining the liquid level based on the measured light values. 
     Preferably, outgoing light rays are being conducted from the light source to the tank volume. 
     Preferably, incoming light rays, that were reflected or scattered at least once within the tank volume, are being conducted from the tank volume to the sensor device. 
     Preferably, the incoming light rays are being conducted in such a manner that each optical sensor corresponds to a specific portion of the tank wall. 
     Preferably, the light source emits one of multi-color light having a plurality of discretely separated wavelengths, visible white light, single wavelength light having visible, ultraviolet or infrared wavelength. Preferably, the light source comprises a laser source that emits outgoing light. 
     Preferably, the output member outputs outgoing light with a, preferably conical, light ray pattern such that the outgoing light impinges on a tank wall circumferential portion of the tank wall. 
     Preferably, the light conducting device includes an optical input member that receives incoming light rays after them being reflected or scattered by the tank wall circumferential portion at least once, preferably exactly once. 
     Preferably, the input member receives the incoming light rays after them being reflected or scattered by the tank wall circumferential portion and them subsequently being refracted by a liquid surface in the tank volume. 
     Preferably, the light conducting device includes an outgoing light conductor, the outgoing light conductor conducting light emitted by the light source towards the tank volume, preferably to the output member. 
     Preferably, the light conducting device includes an incoming light conductor, the incoming light conductor conducting light being received, preferably by the input member, from the tank volume to the sensor device. 
     Preferably, the light conducting device includes a bidirectional light conductor conducting the outgoing and incoming light rays to and from the tank volume, preferably by an input/output member. 
     Preferably, the light conducting device conducts the incoming light rays such that the specific portions are, preferably equidistantly, distributed along the circumferential direction of the tank wall. 
     Preferably, a first specific portion and a second specific portion that neighbor each other along the circumferential direction of the tank wall are distanced along the longitudinal direction of the tank volume due to a light ray received from the first specific portion being refracted by a liquid level in the tank volume while another light ray received from the second specific portion not being refracted by the liquid level. 
     Preferably, the light conducting device conducts the incoming light rays such that a first and second group of specific portions are each, preferably equidistantly, distributed along the circumferential direction of the tank wall and are distanced along the longitudinal direction so as to allow measuring a tilt of the tank about a tank tilt axis that is perpendicular to the longitudinal direction, such as a lateral tank axis. 
     Preferably, the sensor device has an imaging sensor that is formed by the optical sensors, and the sensor device outputs the measured light values as image data. 
     Preferably, the light conducting device is configured as an imaging light conducting device that conducts the incoming light rays such that the sensor device is able to image the tank volume. 
     Preferably, the liquid level determining device determines the liquid level by processing the image data. 
     Preferably, the apparatus further comprises a pattern arrangement that is arranged on or projected onto the tank wall, wherein the incoming light rays reflected or scattered by the pattern arrangement. 
     Preferably, the pattern arrangement is arranged on the tank wall and comprises a material that absorbs a first wavelength emitted by the light source and emits a second wavelength that is different from the second wavelength. Preferably, the material is a fluorescent material. 
     Preferably, the pattern arrangement is suitable to reflect the outgoing light rays, thereby generating the incoming light rays. 
     Preferably, the pattern arrangement comprises at least one stripe that is configured to reflect outgoing light. Preferably, the pattern arrangement comprises two stripes each configured to reflect outgoing light and arranged parallel to each other. 
     Preferably, the sensor device detects the second wavelength, but preferably not the first wavelength. 
     Preferably, the pattern arrangement is illuminated or projected by the outgoing light rays such that incoming light rays are only generated for portions of the pattern arrangement that are below the liquid level. 
     Preferably, the tank wall comprises at least one anti-sloshing baffle that protrudes along a radial tank direction into the tank volume. Preferably, the pattern arrangement is arranged on or projected onto the anti-sloshing baffle. 
     Preferably, the light source is tuned in wavelength. 
     Preferably, the sensor device has a spectrometer or spectral camera that is respectively formed by the optical sensors, and the sensor device outputs the measured light values as spectral data or spectral image data respectively. 
     Preferably, the liquid level determining device controls the light source so as to emit at a measurement wavelength, that is determined from the spectral data or the spectral image data and that allows an increase of a signal-to-noise ratio. 
     The disclosure herein provides a computer readable storage medium, a data carrier signal, or a computer program comprising instructions that, upon execution by a computer system operatively coupled to an optical level gauge apparatus, causes the computer system to perform at least one step of a preferred method. 
     The idea in general relates to the field of optical sensing, and in particular to liquid level measurement inside an enclosed tank or vessel. It is applicable for vehicles of all kinds, specifically aircraft, such as passenger and cargo aircraft as well as urban aerial vehicles. The vehicles are to be equipped to use hydrogen as a fuel with the liquid hydrogen stored in corresponding tanks. 
     Another aspect of the idea is to achieve an all-optical solution to measure the liquid level inside an enclosed tank/vessel. The examples described rely on transporting light, using a waveguide (e.g. fiber optics cables), to and from the tank. No other type of electromagnetic or electrical energy is required inside the tank. 
     The preferably all electrically powered equipment for controlling and processing the light to and from the tank can be installed completely outside the tank volume, preferably in a non-pressurized, non-temperature controlled environment. With this current mass produced components, such as laser/light sources, photodetectors, CMOS/CCD camera chips and microcontrollers/microprocessors may be used without hardening them for a cryogenic environment. Furthermore, the ideas disclosed herein offer the potential to reduce installation and maintenance times and costs. 
     In one embodiment, the fill level of liquid hydrogen (LH2) is determined inside a closed tank/vessel. Electrical energy is avoided inside the tank thereby mitigating fire and explosion risk. Furthermore, the electronics and active components need not withstand the cryostat environment. The sensing solution allows for maintenance without the need to open the tank. A preferred tank shape is a cylinder. It should be noted that other shapes are possible. Under nominal position, the tank Z-axis (height) is parallel to the ground. 
     One principle used in the disclosed ideas is that LH2 has different absorption spectra and a different index of refraction (IOR) compared to gaseous hydrogen (GH2). These are known, for example, from Grossle et al. “Minimal and complete set of descriptors for IR-absorption spectra of liquid H2-D2 mixtures”, AIP Advances 10, 055108 (2020) (https://doi.org/10.1063/1.5111000) and Diller “Refractive Index of Gaseous and Liquid Hydrogen”, J. Chem. Phys. 49, 3096 (1968) (https://doi.org/10.1063/1.1670554). 
     The apparatuses and methods described herein are applicable to basically any liquid that exhibits at least one absorption band in the optical frequencies of the electromagnetic spectrum (i.e. UV, VIS, IR), and/or an IOR of the liquid phase that is different from that in the gas phase. 
     In one embodiment the measurement system comprises a light source (e.g. a laser) that is guided through an optical fiber bundle, a photodetector system to sense light, a microprocessor to apply signal processing techniques and algorithms, and a fiber bundle having N fibers which are organized in an arrangement, preferably in combination with an optical element (lens, prism, grating, or other), that allows to transmit light rays inside the tank towards the tank side walls. 
     One end of the fiber bundle (preferably, where the optical element is to be installed) may be located at the center with respect to the tank base, for a symmetrical scenario. Other positions are possible, but accounting for the changed geometrical scenario is required by corresponding programming of the microprocessor. 
     Light rays launched into the tank from the fiber bundle, preferably the optical element, travel towards the tank walls. A certain amount of light is back reflected or scattered and coupled back into the fiber bundle by the optical element, for example. 
     The tank walls can be coated, machined, or modified in a way, in order to maximize its scattering response in the back direction. It is also possible to install an element inside the tank that can be using a backreflection surface, with an appropriate material, shape and/or coating. This element can be simultaneously used, for example, as an anti-sloshing baffle or be disposed on an anti-sloshing baffle. If the outgoing light ray crosses the LH2 interface, the ray is refracted. The laser source can be tuned to a wavelength with an appropriate attenuation coefficient which allows to observe a different total path absorption that is sufficient to apply signal processing techniques (e.g. denoising, curve fitting, gradient, etc.) to the measured light values. All subsystems may be located externally from the tank, and thus no electrical energy is required to be sent into the tank. 
     If the optical design is such that the angle of the outgoing rays with the surface of the liquid is so that total internal reflection occurs when the fiber bundle output is located below the liquids surface, the method still can determine the liquid level provided the algorithm for the microprocessor is adapted. The level then can be inferred from the highest measuring point from which a reflection can still be measured. 
     In order to improve the results of the signal processing, the light source should be capable of being tuned to a wavelength with an appropriate attenuation coefficient, which preferably allows to observe a difference in the total path absorption between light rays traveling only through liquid, only through gas, or partially traveling through liquid and gas. 
     In another embodiment the measurement system comprises an illumination source, e.g. visible white light, single lambda ultraviolet, visible or infrared, a camera compatible with the spectrum of the light source, a microprocessor to apply image processing techniques and algorithms, and an imaging fiber bundle which transmits passively an image from the inside of the tank. The fiber bundle may have an additional arrangement to transport the light from the light source into the tank, or be a separate fiber arrangement running in parallel. Some optics element allowing the distribution of light inside the tank, and the collection of images providing a certain field of view may be attached to the fiber bundle. The inside of the tank can be illuminated by light generated by the light source and transported inside the tank through the fiber bundle. The image of the illuminated scene can be passively transmitted, through the fiber bundle to a camera outside the tank. The tank wall may be painted/coated with a pattern that facilitates image processing tasks. The pattern may also be painted on other elements inside the tank, e.g. anti-sloshing baffles. The received image is processed by the microprocessor using image processing algorithms. 
     The information on the liquid level can be obtained from the recorded images and applying image processing techniques for preferably edge detection and feature extraction. In addition, an image with the tank empty can be used for reference, to detect any deviation with images taken at different fill levels. The recorded images may also aid in general tank inspection, e.g. leakage detection, corrosion detection. The pattern may also exhibit a spatially varying reflectance spectrum which can aid the signal processing. 
     Preferably, a spectrometer is added to the system. 
     Light rays that cross the gas-liquid interface may be refracted randomly by the liquid&#39;s surface and therefore lead to blurring, which may aid the signal processing. The method is applicable as long as there is a difference between the IOR of the liquid and gas phase of the substance being sensed. 
     Assuming that the liquid&#39;s IOR is about 1.08 while the gas phase IOR is about unity, the difference is sufficient to register a detectable discontinuity in the reference pattern, which allows for the detection of the liquid interface. The detection of the liquid interface of the substance being sensed may not only be detected due to a discontinuity, i.e. due to refraction. It is also possible to dye the liquid so that it exhibits a color contrast against the reference pattern. In this case, the level extraction can be obtained through detection of the boundary edges where there is a color change in the reference pattern. 
     The reference pattern can be painted on inside the tank or baffles, or can be generated from the outside and projected into the tank using the same passive image transmission fiber bundle technology used for image acquisition. The actively generated pattern functionality may be performed by an illumination source that is passed through a screen element (by reflection or transmission) with the desired pattern, or an active device, such as a spatial light modulator (SLM) or digital micromirror device (DMD), that can generate structured light with the desired pattern. 
     In case of an externally generated pattern, a dedicated imaging fiber bundle cable may be employed to transport the structured light into the tank. This fiber bundle is preferably separate from the one used to transmit images out of the tank. 
     Similarly this method may infer the liquid level from pattern recognition using a camera. The rear wall may be configured non-reflective, except for some (e.g. two) stripes. The stripes are arranged so as to detect yaw and to cover the tank from bottom to top with inclined strips. An optical system, e.g. realized by cylindrical lenses, may project two lines precisely onto the reflective stripes when the tank is full. When the tank level decreases, a change in the received pattern occurs for the rays above the surface liquid. As a result, less power will be received on the expected pixel because the illumination does not hit the stripe due to refraction. This effect will increase as the surface normal is not flat due vibration, i.e. rays that cross the surface will further be refracted randomly. 
     The aforementioned methods may be extended by using spectral analysis in order to further improve the level sensing and/or retrieve further properties of the liquid such as temperature and/or pressure. In one implementation, the measuring points may reflect only certain wavelengths. Preferably, using a white light source and a spectrometer added to the external unit, this may aid in determining the measuring point number. 
     In another implementation, a tunable laser source may be used in order to tune the wavelength to a specific point in the absorption spectrum of the medium and thereby improve total path absorption and signal-to-noise-ratio. 
     In yet another implementation, the tank walls may be colored, which may aid pattern recognition, preferably if used in conjunction with a spectral camera. 
     Regarding measurement of temperature and/or pressure, the absorption bands of the liquid shift depending on temperature and pressure. By using a white light source and a spectrometer or a tunable laser and a photodiode, the absorption spectrum can be recorded. 
     For all methods presented the end of the fiber bundle that is attached to the tank can be located outside the tank vessel containing the liquid behind a sealed optical window, or be inside the tank using a feed through tube installation with a sealed optical window termination. The required fiber bundle may be built in a flexible, rigid or semi-rigid assembly. Likewise, if a feedthrough protective tube is used to guide the fiber bundle this could have bending points, when the fiber bundle assembly is rigid or semi-rigid. 
     For installation/deinstallation purposes, the fiber bundle may be guided to its final position by sliding it inside a feedthrough, and/or guiding tube that is terminated in the necessary optical window. To guarantee correct alignment, a magnetic alignment flange may be installed at the end of the fiber bundle. On the other hand, the fiber bundle cable may be terminated with a magnetic alignment ring that directly mates with a counterpart. 
     In order to ensure a unique orientation in its final installation position, the magnetic alignment elements may be built with a non-symmetrical profile and/or built with non-symmetrical alternating North and South pole sections. 
     The magnetic properties advantageously prevent any misalignment during the expected operational environment, in particular with respect to vibrations, but should allow for the complete fiber bundle cable assembly to be slided out by a reasonable amount of pull force applied by an operator, either by direct pulling or by using an appropriate tool. 
     This will allow for easy installation and maintenance of the complete sensing apparatus. The only element, if damaged, that would require a major maintenance effort is the optical window and its bonding material/resin. Any other mechanical design that allows for precise alignment may be used. 
     In one embodiment, accuracy may be increased by assistance of an additional inertial measurement unit (IMU) that provides attitude information about the tank position in real time, e.g. information on roll/pitch/yaw. 
     The achievable resolution is directly related to the amount of rays being launched inside the tank. For example, for a given vertical resolution Δd the number of rays is given by N=2D/Ad, where D is the tank base diameter. For typical measurements of D=100 cm and Ad=0.5 cm about N=400 rays are required. The required number of rays can be relaxed, if an appropriate signal processing algorithm is used. 
     In imaging, the achievable resolution depends on the amount of fiber cores that can be put together in the imaging fiber optics bundle core, the field of view offered by the optics element, and the size of the reference pattern features. Assuming D=100 cm, and imagining the full tank base area through a properly dimensioned optics element then the imaging fiber bundle requires about 360,000 fiber cores (i.e. 600×600) to cover with 3 pixels a target resolution of Δd=0.5 cm. 
     It should be noted that while the disclosure herein is described with reference to discrete light rays, this is merely an idealization for the sake of explaining the disclosure herein. In other words, the term light ray should be understood to include continuous configurations such as a continuous ring of light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the disclosure herein are described in more detail with reference to the accompanying schematic drawings that are listed below. 
         FIG.  1    depicts an embodiment of an aircraft; 
         FIG.  2    depicts an embodiment of a tank; 
         FIG.  3    depicts diagrams illustrating the measurement principle of one method for determining a liquid level; 
         FIG.  4    depicts diagrams illustrating the measured values; 
         FIG.  5    depicts another embodiment of a tank; 
         FIGS.  6  and  7    depict images of tank volume; 
         FIG.  8    depicts a schematic inside view of the tank of  FIG.  5   ; 
         FIG.  9    depicts an optical coupler; and 
         FIGS.  10  through  12    depict schematically a tank wall portion; and 
         FIG.  13    depicts another measurement principle. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    depicts an aircraft  10 . The aircraft  10  comprises a fuselage  12 . A pair of wings  14  is attached to the fuselage  12 . The aircraft  10  also comprises a tail plane section  16 . The tail plane section  16  includes a horizontal and vertical tail plane. In addition, the aircraft comprises at least one engine  18  to propel the aircraft  10 . The engines  18  are suitable to use hydrogen as fuel. 
     The aircraft  10  comprises a liquid tank assembly  20 . The liquid tank assembly  20  is fluidly coupled to the engines  18  for supplying the engines  18  with fuel. The liquid tank assembly  20  is configured for storing cryogenic liquids, such as liquid hydrogen (LH2), at the corresponding cryogenic temperatures and, as the case may be, under pressure. 
       FIG.  2    depicts the liquid tank assembly  20  in more detail. The liquid tank assembly  20  comprises a liquid tank  22 . The liquid tank  22  has a tank wall  24  that encloses a tank volume  26  for storing a cryogenic liquid  28 . The cryogenic liquid  28  has a liquid level L within the tank volume  26 . 
     The liquid level L is to be gauged by an optical level gauge apparatus  30 . The optical level gauge apparatus  30  comprises a light source  32 . The light source  32  emits at least one outgoing light ray  34 . The outgoing light ray  34  may have a specific wavelength A. The light source  32  is a laser source, for example. 
     The optical level gauge apparatus  30  further comprises a light conducting device  36 . The light conducting device  36  may be configured as a fibre optic cable or a fiber bundle. The light conducting device  36  is optically coupled to the tank volume  26 , so as to conduct the outgoing light ray  34  from the light source  32  into the tank volume  26 . 
     The light conducting device  36  may comprise an output member  38 . The output member  38  may comprise optical elements, such as a lens, a prism, a polarizer and the like. 
     Preferably the output member  38  is configured to launch a plurality of the outgoing light rays  34  into the tank volume  26  towards a tank wall circumferential portion  40  of the tank wall  24 . 
     In one embodiment not depicted in detail, the light source  32  is configured to emit two different discrete wavelengths λ 1 , λ 2 . The output member  38  may be configured such that outgoing light rays  34  having different wavelengths λ 1 , λ 2  impinge on different tank wall circumferential portions  40  that are spaced apart along the tank longitudinal axis. With this configuration, it is possible to detect pitching of the liquid tank  22 , as explained later. 
     As exemplified in  FIG.  2   , a first outgoing light ray  42  propagates through gas without being refracted and impinges on the tank wall circumferential portion  40 . A second outgoing light ray  44  initially propagates through gas evaporating from the cryogenic liquid  28 , before entering the cryogenic liquid  28 , thereby being refracted at the liquid surface  46  and subsequently propagating through the cryogenic liquid  28  towards another tank wall circumferential portion  40 . 
     The first and second outgoing light rays  42 ,  44  are backscattered from the tank wall circumferential portion  40 , each forming an incoming light ray  48 . 
     The tank wall  24  may have an interior reflective coating that is arranged at the tank wall circumferential portions  40 , so as to improve backscattering. It is also possible that the tank wall  24  is structured so as to favor backscattering of the outgoing light rays  34 . An example for a suitable structure is a reflective grating. 
     The light conducting device  36  may further comprise an input member  50 . Similarly to the output member  38 , the input member  50  may include optical elements. The output member  38  and the input member  50  need not necessarily be separate units, they may be integrated into a single unit and designated input/output member. 
     The input member  50  preferably receives the incoming light rays  48  and couples the incoming light rays  48  into a single bidirectional light conductor  52  of the light conducting device  36 . It should be noted that the bidirectional light conductor  52  is also optically coupled to the light source  32 , so as to conduct the outgoing light rays  34  from the light source  32  towards the tank volume  26 . It is further noted, that incoming and outgoing light rays  34 ,  48  may be conducted within a separate outgoing light conductor and incoming light conductor, respectively, of the light conducting device  36 . 
     The optical level gauge apparatus  30  further comprises a sensor device  54 . The sensor device  54  is optically coupled to the light conducting device  36 , so as to be able to receive the incoming light rays  48 . The sensor device  54  comprises a plurality of optical sensors  56 . The optical sensors  56  may be individual sensors, such as photodiodes, phototransistors, etc., or may be integrated into an optical chip, such as a CCD-chip or other imaging chip. 
     The light conducting device  36  is optically coupled to the sensor device  54  in a manner so that each optical sensor  56  corresponds to a specific tank wall portion  58 . This may, for example, be achieved by individually routing optical fibres of the light conducting device  36  towards each optical sensor  56 . 
     The sensor device  54  is configured to output measured light values that are indicative of the amount of light being detected by each optical sensor  56 . The measured light values can be processed further by a data processing device, such as a microcontroller, microprocessor or other computing device. 
     The optical level gauge apparatus  30  further comprises a liquid level determining device  60 . The liquid level determining device  60  is configured to process the output of the sensor device  54  using signal processing techniques, that are known per se. After processing, the liquid level determining device  60  outputs a liquid level value, that is indicative of the liquid level L within the tank volume  26 . The liquid level determining device  60  may be realized by a suitable data processing device, such as a microcontroller or microprocessor, an embedded system or by a general-purpose computer. 
     Referring now  FIG.  3   , a measurement principle that allows the liquid level determining device  60  to determine the liquid level L within the tank volume  26  is described in more detail. 
       FIG.  3   , top left diagram, depicts a schematic view of the liquid tank  22  along its axial direction, i.e., its longitudinal axis. Furthermore, this diagram depicts the plurality of specific tank wall portions  58  that are distributed along the circumferential direction of the liquid tank  22  with an equal distance to each other. Each specific tank wall portion  58  corresponds to exactly one optical sensor  56 . The diagram also depicts the liquid level L and two of the specific tank wall portions  58  as reference points  62 , in order to facilitate the explanation. 
       FIG.  3   , top right diagram, depicts a schematic longitudinal cross section of the liquid tank  22 . The outgoing and incoming light rays  34 ,  48  are depicted. It should be noted that the angle of refraction is greatly exaggerated in order to facilitate understanding of the measurement principle. 
       FIG.  3   , bottom diagram, depicts on the x-axis the sensor index, i.e. the number of the optical sensor  56  counted from a starting point. The y-axis depicts the attenuation of the incoming light ray  48  in a decibel scale. The baseline for this scale is for example given by the outgoing light ray  34 . As a result, points further along the y-axis designate a smaller attenuation than points closer to the x-axis. 
     As shown in  FIG.  3   , the more the outgoing and incoming light rays  34 ,  48  propagate through the cryogenic liquid  28 , the higher the attenuation value. This results in the attenuation curve  64  shown in  FIG.  3   , bottom diagram. 
     The liquid level L can be determined by determining the sensor index of the optical sensors  56  which begin to show less attenuation, as indicated by the arrows in  FIG.  3   , bottom diagram. Since the specific tank wall portions  58  are uniquely related with the optical sensors  56  and the position of the specific tank wall portion  58  and the geometry of the tank volume  26  are known, the liquid level determining device  60  is able to calculate the position of the liquid surface  46  and hence determine the liquid level L in the tank volume  26 . 
       FIG.  4   , top diagram, depicts the total path absorption, i.e., an absorption value, depending on the sensor index of the optical sensor  56 . The total path absorption is shown in linear scale. Hence, points further along the y-axis designate a larger total path absorption than points closer to the x-axis, resulting in the absorption curves  65  for different fill levels I, II, . . . , X of the tank volume  26 . Fill levels I, II, . . . , V are below 50% fill level, while VI, VII, . . . , X are above 50% fill level. The light conducting device  36  is arranged at the 50% level. 
       FIG.  4   , middle diagram, depicts the gradient of the total path absorption depicted in  FIG.  4   , top diagram. With this it is easier to detect the sensor index, at which the absorption value increases/decreases so that the liquid level L may be determined more precisely. 
       FIG.  4   , bottom diagram, depicts absorption curves  65  for different degrees 0°, ±15°, ±30° of rotation of the liquid tank  22  about its longitudinal axis, i.e. if the aircraft  10  rolls. As illustrated, all absorption curves  65  reach the same maximum, however the sensor index of the optical sensors  56  at which the respective absorption curve  65  begins to increase/decrease changes depending on the angle of rotation about the longitudinal axis. 
       FIG.  5    depicts another embodiment of a liquid tank assembly  20 . This embodiment is described only insofar as it differs from the embodiment previously described. 
     The optical level gauge apparatus  30  comprises a light source  32  that is capable of emitting a plurality of wavelengths, specifically two or more discrete wavelengths, or a continuous spectrum such as white light. The light source  32  may be configured as a laser source. 
     The light conducting device  36  is configured as an imaging light conducting device that allows imaging the tank volume  26  by a corresponding sensor device  54 . 
     The output member  38  may be configured to output the outgoing light rays  34  so as to form a pattern arrangement  66 . The pattern arrangement  66  is projected onto the inner surface of the tank wall  24  and may be comprised of axial and circumferential stripes as exemplified in  FIG.  6    to  FIG.  8   . The light source  32  may be optically coupled to the tank volume  26  by the light conducting device  36  such that the tank wall  24  may be illuminated. It is also possible that the tank wall  24  has the pattern arrangement  66  formed thereon. Furthermore, the tank wall  24  may comprise at least one anti-sloshing baffle  68  and the pattern arrangement  66  is disposed on or projected onto the anti-sloshing baffle  68 . 
     The input member  50  is configured such that an image of the tank volume  26  is passively conducted towards the sensor device  54 , including backscattered or backreflected incoming light rays  48 . 
     In this embodiment, the light conducting device  36  preferably comprises an outgoing light conductor  70  that is optically connected to the light source  32  and the output member  38  and conducts the outgoing light rays  34  from the light source  32  into the tank volume  26 . 
     Preferably the light conducting device  36  also comprises an incoming light conductor  72  that is configured as an imaging light conductor. The input member  50  may include optical elements that improve or facilitate imaging of the tank volume  26 . The input member  50  is optically connected to the incoming light conductor  72 , which in turn is connected to the optical sensor  56  of the sensor device  54 . In this embodiment the optical sensors  56  form an imaging sensor, and the sensor device  54  is configured to output image data. 
     The liquid level determining device  60  includes a computer vision algorithm for evaluating the image data detected by the sensor device  54  and may specifically search and determine distortions due to refraction or color changes of the pattern arrangement  66  in view of the liquid surface  46 . It is possible to dye the liquid in the liquid tank  22  in order to improve contrast and measurement reliability. Through, for example, edge detection algorithms, and other computer vision algorithms that are known in a manner per se, the pattern arrangement  66  can be analyzed and compared to the pattern arrangement  66  in a reference setup. By comparing the measured and the reference pattern arrangement  66 , the liquid level L can be determined. 
     In addition to determining the liquid level L, this embodiment may also be used to inspect the tank volume  26  for defects by illuminating the tank wall  26  and imaging the tank wall  26  using the sensor device  54 . Again, computer vision algorithms may be used to detect leaks or other defects. 
       FIG.  6    exemplifies the image of the tank volume  26 , when the liquid tank assembly  20  is about half filled and in a neutral position, i.e., the liquid tank  22  is horizontally aligned. 
       FIG.  7    depicts a similar situation, in which the liquid tank assembly  20  is aligned horizontally, however, the liquid tank  22  is rolled about its longitudinal axis. 
       FIG.  8    depicts a situation, in which the aircraft  10  and consequently the liquid tank assembly  20  are pitched. 
     The image of the pattern arrangement  66  changes depending on the current situation, so that as a result not only the liquid level L, but also pitching and rolling can be detected. 
       FIG.  9    depicts an embodiment of an optical coupler assembly  74 . The optical coupler assembly comprises an optical window  76  that may be attached to or integrated into the tank wall  24 , preferably at a position, which allows the sensor device  45  to image a large portion of the tank volume  26 . The optical coupler assembly  74  comprises an alignment flange  78 , that is configured such that the light conducting device  36 , specifically the input/output member can only be inserted in a manner, that allows meaningful imaging of the tank volume  26 . The light conducting device  36  may comprise an alignment ring  80  that matches the alignment flange  78 . In addition, it is possible that the alignment flange  78  and the alignment ring  80  are configured such that they magnetically interact and hold firm the light conducting device  36 . Still the light conducting device  36  can be removed easily by a technician through exerting a reasonable amount of pull force. The optical coupler assembly  74  may further comprise a feedthrough guiding tube  82  that feeds the light conducting device  36  into the liquid tank  22  close to the tank volume  26 , such that the optical level gauge apparatus can perform its function. 
     Referring to  FIG.  10    through  FIG.  13   , another measurement principle is explained in more detail, which utilizes the imaging capability of the optical level gauge apparatus  30 . 
       FIG.  10    depicts schematically a tank wall portion  84  that is arranged opposite the light conducting device  36 . 
     The tank wall portion  84  comprises the pattern arrangement  66 , which in this instance is configured to include two parallel slanted stripes  86 . The pattern arrangement  66  includes a reflective material, that upon being illuminated by the light source  38  reflects the outgoing light rays  34  as incoming light rays  48  back towards the light conducting device  36  along the longitudinal axis of the tank  22 .  FIG.  10    shows the stripes without being illuminated. The back reflection may be diffuse or specular, whereas the latter case may be realized by reflection gratings. 
       FIG.  11    illustrates the tank volume  26  being completely filled, wherein the pattern arrangement  66  is illuminated by the light source  32  and the reflected incoming light rays  48  are detected by the sensor device  54 . 
       FIG.  12    and  FIG.  13    illustrate what happens when the liquid level L drops. 
       FIG.  12    depicts the positions of the outgoing light rays  34 . Due to the propagation within the cryogenic liquid  28 , the outgoing light rays  34  illuminate the pattern arrangement  66  in a bottom section  88  below the liquid level L. Due to refraction at the liquid surface  46 , however, the outgoing light rays  34  failed to illuminate the pattern arrangement  66  in a top section  90  and are instead hitting a tank wall section  92  next to the pattern arrangement  66 . 
       FIG.  13    illustrates the actual image, that would be recorded by the sensor device  54  in this case. Only the bottom section  88  is visible, and the liquid level L can be directly read of the illuminated pattern arrangement  66 . 
     In order to improve liquid level gauging inside liquid tanks, especially cryogenic liquid tanks, the disclosure herein provides an apparatus and method for determining the liquid level L. In light source emits outgoing light rays  34  that are back reflected by the tank wall  24  of the liquid tank  22 . The back reflected incoming light rays are received by a light conducting device  36 , which conducts the incoming light rays  48  towards a sensor device  54 . The sensor device  54  has optical sensors  56  that are coupled to the light conducting device  36  in such a manner that there is a one-two-one relationship between each optical sensor  56  and a specific tank wall portion  58 . Due to the change in the amount of light that is received by the optical sensor  56  after the outgoing light rays  34  were refracted by the liquid surface  46 , the liquid level determining device  60  is capable of determining the liquid level L. 
     The subject matter disclosed herein can be implemented in or with software in combination with hardware and/or firmware. For example, the subject matter described herein can be implemented in or with software executed by a processor or processing unit. In one example implementation, the subject matter described herein can be implemented using a computer readable medium having stored thereon computer executable instructions that when executed by a processor of a computer control the computer to perform steps. Example computer readable mediums suitable for implementing the subject matter described herein include non-transitory devices, such as disk memory devices, chip memory devices, programmable logic devices, and application specific integrated circuits. In addition, a computer readable medium that implements the subject matter described herein can be located on a single device or computing platform or can be distributed across multiple devices or computing platforms. 
     While at least one example embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the example embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a”, “an” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority. 
     LIST OF REFERENCE SIGNS 
     
         
         
           
               10  aircraft 
               12  fuselage 
               14  wing 
               16  tail plane section 
               18  engine 
               20  liquid tank assembly 
               22  liquid tank 
               24  tank wall 
               26  tank volume 
               28  cryogenic liquid 
               30  optical level gauge apparatus 
               32  light source 
               34  outgoing light ray 
               36  light conducting device 
               38  output member 
               40  tank wall circumferential portion 
               42  first outgoing light ray 
               44  second outgoing light ray 
               46  liquid surface 
               48  incoming light ray 
               50  input member 
               52  bidirectional light conductor 
               54  sensor device 
               56  optical sensor 
               58  specific tank wall portion 
               60  liquid level determining device 
               62  reference point 
               64  attenuation curve 
               65  absorption curve 
               66  pattern arrangement 
               68  anti-sloshing baffles 
               70  outgoing light conductor 
               72  incoming light conductor 
               74  optical coupler assembly 
               76  optical window 
               78  alignment flange 
               80  alignment ring 
               82  feedthrough guiding tube 
               84  tank wall portion 
               86  stripe 
               88  bottom section 
               90  top section 
               92  tank wall section 
             L liquid level 
             λ 1 , λ 2  wavelength