Gas-liquid distribution state detection method for low-temperature liquid fuel storage tank of spacecraft

Abstract

The invention discloses a gas-liquid distribution state detection method for a low-temperature liquid fuel storage tank of a spacecraft. The method comprises the following steps: dividing response signals into transmission signals, elliptical reflection signals and echo signals; obtaining a preliminary gas-liquid distribution reconstruction image; obtaining a gas-liquid distribution reconstruction image; dividing the gas-liquid distribution reconstruction image into different bubble region sets; obtaining a two-dimensional gas-liquid distribution image; and obtaining a three-dimensional gas-liquid distribution image. According to the method, under the scene of microgravity and a general storage tank, the position of a gas-liquid boundary and the general distribution of a gas-phase region are subjected to weighted reconstruction by combining back projection, elliptical reflection and a local curvature reconstruction method; a blind area in a large measurement area of the storage tank is reconstructed based on a rule that local curvature does not suddenly change, and a gas-liquid distribution image is processed by adopting morphological processing and ellipse fitting based on a prior feature that an air pillow area is configured as an elliptical revolving body. Simple measuring equipment is adopted to measure the gas-liquid distribution state in the storage tank, and the method has important significance in aerospace craft propellant management, attitude control and propellant surplus measurement.

Description

Technical Field

[0001] The invention belongs to the technical field of on-orbit propellant container detection for spacecraft, and in particular relates to a method for detecting the gas-liquid distribution state of a cryogenic liquid fuel tank of a spacecraft. Background Art

[0002] There are complex phenomena such as evaporation phase change, pressure change, heat transfer, and pool boiling in the cryogenic liquid propellant tank, which generates bubbles dispersed in the liquid phase and merges to form an air pillow area. The final configuration of the air pillow area under microgravity conditions is an elliptical rotating body. Factors such as the rupture of the air pillow area, the addition of inert gas, unstable pressure or temperature distribution, and tank oscillation will lead to complex changes in the gas-liquid distribution inside the tank. The uncertainty of the gas-liquid state and the difficulty in accurate detection bring many problems to the on-orbit operation of the spacecraft. First, direct depressurization of the tank when the gas distribution is unknown may cause the liquid propellant to be discharged by mistake, resulting in serious consequences. Second, changes in the gas-liquid distribution will directly affect the center of mass position of the spacecraft, and then affect attitude control. Third, the positioning of the gas-liquid interface helps to measure the remaining amount of propellant, which can accurately predict the end of the satellite's life.

[0003] For gas-liquid distribution measurement, the existing technologies include optical methods, electrical tomography methods and acoustic methods. Among them, the optical method and electrical tomography method are respectively limited by the non-transparent wall of the tank and the conductivity of the fluid, and are not suitable for the gas-liquid two-phase measurement scenario of the tank. The ultrasonic tomography method in the acoustic method can simultaneously collect the attenuation signal and the multi-path reflection signal during the excitation and reception process of the array through the cyclic excitation and reception of the ultrasonic transducer array, without the need to use other experimental devices. Unlike other process tomography methods, the sound wave will be attenuated to a large extent when it is transmitted from the tank wall to the liquid propellant. Part of the energy will be transmitted in the wall in the form of guided waves, and the other part of the acoustic signal will be transmitted, scattered attenuated and reflected when propagating in the two-phase medium. The fusion of transmission and reflection methods can realize the reconstruction of the gas-liquid interface morphology and position of the two-phase flow. However, no effective method has been found so far. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide a method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank, which can monitor the three-dimensional gas-liquid distribution inside the tank and is suitable for tank measurement scenarios with large measurement areas, non-circulation and microgravity.

[0005] In order to achieve the above object, the gas-liquid distribution state detection method of the spacecraft cryogenic liquid fuel tank provided by the present invention comprises the following steps performed in sequence:

[0006] 1) A multi-layer ultrasonic transducer array is built on the tank, ultrasonic transducers of different channels in the ultrasonic transducer array are excited in turn, and the response signals are divided into three groups: transmission, elliptical reflection and echo signals;

[0007] 2) The transmission, elliptical reflection and echo signals are inverted by back-projection, elliptical reflection method and local curvature reconstruction method respectively, and then the sensitivity matrices obtained by the three methods are weighted superimposed to obtain a preliminary gas-liquid distribution reconstructed image;

[0008] 3) Based on the morphological processing method, the protrusions and noise points in the preliminary gas-liquid distribution reconstruction image are removed and the missing pixels are supplemented, and then the measurement blind area reconstruction method is used to reconstruct the measurement blind area to obtain the gas-liquid distribution reconstructed image;

[0009] 4) dividing the gas-liquid distribution reconstructed image into different bubble area sets based on whether the gas-liquid interface is connected, and then counting and locating the bubbles in each bubble area set;

[0010] 5) After different bubble region sets are distinguished, different bubble cross sections in the gas-liquid distribution reconstructed image are fitted with an ideal two-dimensional elliptical model to obtain a two-dimensional gas-liquid distribution image;

[0011] 6) Based on the location and number of bubbles in the two-dimensional image, multiple frames of the above two-dimensional gas-liquid distribution image are superimposed on the z-axis to obtain a three-dimensional gas-liquid distribution image, thereby fitting the three-dimensional morphology of the gas-liquid interface.

[0012] In step 1), the specific method of building a multilayer ultrasonic transducer array on the tank, exciting ultrasonic transducers of different channels in the ultrasonic transducer array in turn, and dividing the response signals into three groups of transmission, elliptical reflection and echo signals is as follows:

[0013] An ultrasonic transducer array is arranged on multiple cross sections of a tank in which propellant is stored, and excitation signals are applied to ultrasonic transducers of different channels in the ultrasonic transducer array in turn, and response signals caused by the excitation are collected; for an ultrasonic transducer array with n ultrasonic transducers, the ultrasonic transducer of one channel in the ultrasonic transducer array is excited each time, and response signals of n channels including the ultrasonic sensor itself that has been excited are collected after a short delay, and the ultrasonic transducers of n channels are excited in turn within one cycle, and a total of n response signals are collected. 2 segment response signals; then the response signals collected within a period are grouped, the response signals received at the relative position are taken as transmission signals, the response signals sent and received spontaneously are taken as echo reflection signals, and the other received signals are taken as elliptical reflection signals.

[0014] In step 2), the back-projection, elliptical reflection method and local curvature reconstruction method are used to invert the transmission, elliptical reflection and echo signals respectively, and then the sensitivity matrices obtained by the three methods are weighted superimposed to obtain a preliminary gas-liquid distribution reconstructed image. The specific method is:

[0015] 2.1. Transmission signal inversion:

[0016] When there is a partial gas phase area occlusion in the transmission path, the calculation formula of the sensitivity matrix P(i,j) of the transmission method is:

[0017]

[0018] Among them, i and j are the channel numbers of the excitation signal, P tr0 is the calibration transmission signal when there is no obstruction, A0 is the diameter of the piezoelectric chip of the ultrasonic transducer, and A tr is the shielding area of ​​the gas phase region. The amplitude of the attenuated transmission signal can be used to infer the size of the shielding area.

[0019] 2.2. Inversion of elliptical reflection signal:

[0020] The sensitivity matrix S obtained by the elliptical reflection method el It mainly depends on the sound wave transmission time and the sound pressure attenuation amplitude, and the sensitive matrix S is located by the gas-liquid interface. ij and the divergence angle constraint matrix S δ OK, the formula is:

[0021]

[0022] The gas-liquid interface positioning sensitive matrix S ij The formula is:

[0023]

[0024] Among them, l e is the amplitude parameter of the elliptical reflection, P ij is the amplitude of the elliptical reflection signal, c is the speed of sound in the liquid, t ij is the sound wave transmission time, d ij is the transmission distance of the sound wave.

[0025] The angle between the sound wave reflection path in the elliptical reflection method and the straight line where the ultrasonic transducer is located should be smaller than the divergence angle δ; when a certain channel transmits sound waves and other channels receive sound wave signals, the divergence angle constraint matrix S δ The formula is:

[0026]

[0027] Among them, j is the number of the receiving channel and d is the transmission distance of the sound wave.

[0028] 2.3. Echo signal inversion:

[0029] The echo signal amplitude P that can be captured by the ultrasonic transducer b The calculation formula is:

[0030]

[0031] θ is the angle of the echo signal relative to the center of curvature at the gas-liquid interface, which can be calculated by the following formula:

[0032]

[0033] Where R is the local curvature radius of the gas-liquid interface, P0 is the reference amplitude of the echo signal, which is measured by arranging a total reflection interface near the ultrasonic transducer; d0 is the diameter of the piezoelectric chip of the ultrasonic transducer. Based on the measurement signal, the sound wave transmission time t can be extracted ii and the echo signal amplitude P b , thereby solving the local curvature radius of the gas-liquid interface; based on the local curvature radius of the gas-liquid interface, the sensitivity matrix S obtained by the local curvature reconstruction method is obtained rf The ideal fan-shaped reflection interface S needs to be calculated R (d1, R) and the straight line L(θ) constituting the reflective fan-shaped gas-liquid interface, the formula is:

[0034]

[0035] Where d1 is the shortest distance between the ultrasonic transducer and the reflecting gas-liquid interface;

[0036] After processing the transmission signal, elliptical reflection signal and echo signal respectively, the sensitivity matrices obtained by the above three methods are weighted superimposed to obtain a preliminary gas-liquid distribution reconstructed image; in the weight parameter allocation, the weight ratio of the transmission: elliptical reflection: echo inversion method is 1:4:4.

[0037] In step 3), the morphological processing method is used to remove protrusions, noise points and fill in missing pixels in the preliminary gas-liquid distribution reconstruction image, and then the measurement blind area reconstruction method is used to reconstruct the measurement blind area. The specific method for obtaining the gas-liquid distribution reconstruction image is:

[0038] Before morphological processing, the image needs to be binarized. Let V th represents the threshold for screening the gas-liquid two-phase region, M n×n (i, j) represents the preliminary gas-liquid distribution reconstruction image matrix, and the formula for the binarization process is:

[0039]

[0040] Mean filtering is required before binarization;

[0041] Morphological processing mainly includes dilation and erosion, in which the dilation operation is extended by morphology. By comparing the relationship between pixels, pixels are added around pixels that meet the conditions, thereby filling the missing pixels in the image. Let P and Q be the sets of the two-dimensional integer space Z2, and the set Q is morphologically expanded to obtain the set P. This process is defined as:

[0042]

[0043] Similarly, using set Q to corrode set P is to concentrate all the pixels contained in set P into set Q, and traverse the pixels in the preliminary gas-liquid distribution reconstruction image for translation; the corrosion operation is a shrinking process, which will remove the redundant noise points in the morphological image. The definition of set P corroded by set Q is:

[0044]

[0045] After morphological processing, the blind area needs to be reconstructed using the blind area reconstruction method. The curvature radius measured near the blind area is used to estimate the curvature radius at the blind area. When the implicit function f(x, y) = 0 is satisfied, the curvature calculation formula of a point on the ellipse boundary is:

[0046]

[0047] The characteristic that the gas-liquid interface approaches the boundary of the ellipsoid indicates that the local curvature of the gas-liquid interface will not change drastically. Let r1 and r2 be the radii of two circular arcs of known curvature, and the curvature calculation formula of the unknown curvature segment between the two is:

[0048]

[0049] Finally, the gas-liquid distribution reconstructed image M(x,y) is obtained.

[0050] In step 4), the gas-liquid distribution reconstructed image is divided into different bubble area sets based on whether the gas-liquid interface is connected, and then the number of bubbles in each bubble area set is counted and located in the specific method as follows:

[0051] Assuming that the current pixel in the gas-liquid distribution reconstruction image is M(p,q), p is the horizontal coordinate of the pixel, q is the vertical coordinate of the pixel, and the total number of bubbles is k, the process of distinguishing different gas-liquid interfaces is expressed as:

[0052]

[0053] Among them, M(i,j) k A set representing different gas-liquid interface images; the pixel point first identified as the gas-liquid interface is stored in a bubble area set. When traversing to a pixel point of a new gas-liquid interface, the pixel point is called the current pixel point, and the distance between the current pixel point and all the pixels in the bubble area set is calculated. If the distance between the current pixel point and all the pixels in the bubble area set exceeds the threshold, it means that the current pixel point is far away from the gas-liquid interface and belongs to other bubbles. Therefore, it is necessary to open a new bubble area set and store the current pixel point in it; when there is a pixel point in the existing bubble area set and the distance between the current pixel point and the pixel point is less than the threshold, it means that the two pixels are adjacent on the gas-liquid interface, and the current pixel point is included in the bubble area set; therefore, different bubbles can be distinguished by judging whether the gas-liquid interface is connected, and finally the number of bubbles in each bubble area set is counted and located.

[0054] In step 5), the specific method of fitting different bubble cross sections in the gas-liquid distribution reconstructed image with an ideal two-dimensional elliptical model to obtain a two-dimensional gas-liquid distribution image is:

[0055] Under the condition that the internal gas-liquid distribution configuration approaches an ellipse, the gas-liquid distribution reconstructed image after ellipse fitting processing is expressed as:

[0056]

[0057] Among them, M i (x, y) is the gas-liquid distribution image reconstruction matrix before ellipse fitting, M(x, y)′ is the gas-liquid distribution image reconstruction matrix after ellipse fitting, ρ(j) is the gas-liquid interface curvature solution model, E el Post-processing operation for ellipse approximation of gas-liquid distribution reconstruction image;

[0058] Let {M u (i, j)} is the set of bubble regions in the u-th bubble region, M1 and M2 are any two boundary points in the discrete matrix two-dimensional image, d(M1, M2) is the distance between the two boundary points, and the maximum value of the distance between any two boundary points is calculated as follows:

[0059]

[0060] Post-processing operation of the gas-liquid interface boundary by elliptical approximation of the gas-liquid distribution reconstructed image el After processing, it is expressed as:

[0061]

[0062] The uth bubble area M u (i,j) Post-processing operation E of the ellipse approximation of the image reconstructed by gas-liquid distribution elAfter processing, the range of continuous solutions is obtained. After the spatial discretization of the solution, two pairs of discrete gas-liquid boundary positioning points can be obtained. On this basis, the fitting continuous function approaching the ellipse in the u-th bubble region can be expressed as:

[0063] d 2 (M a1 ,M a2 )y 2 +d 2 (M b1 ,M b2 )=1

[0064] Among them, M a1 、M a2 and M b1 、M b2 Two sets of intersection points for locating the gas-liquid interface; the two-dimensional gas-liquid distribution image is composed of pixel points and is discrete. The continuous function curve needs to be converted into a discrete pixel point image matrix.

[0065] In step 6), the specific method of superimposing multiple frames of the above two-dimensional gas-liquid distribution images on the z-axis to obtain a three-dimensional gas-liquid distribution image, thereby fitting the three-dimensional morphology of the gas-liquid interface is:

[0066] A multi-layer array is arranged on the tank. Each layer of the array can reconstruct a frame of two-dimensional gas-liquid distribution image of the gas-liquid interface. For an array with w layers, the number of layers w is used as the z-axis. Multiple frames of two-dimensional gas-liquid distribution images are superimposed in three-dimensional space, and the three-dimensional morphological features of gas-liquid distribution are stacked through interpolation and filtering. The three-dimensional gas-liquid distribution image matrix at this time is expressed as:

[0067]

[0068] Among them, G(z1,z2,z3) is a three-dimensional Gaussian filter matrix.

[0069] In summary, the beneficial effects of the present invention are:

[0070] In the scenario of microgravity and large tank volume, the back-projection method, elliptical reflection method and local curvature reconstruction method are combined to weightedly reconstruct the position of the gas-liquid boundary and the approximate distribution of the gas phase area; the blind area in the large measurement area of ​​the tank is reconstructed based on the rule that the local curvature does not change suddenly, and the gas-liquid distribution image is processed by morphological processing and elliptical fitting based on the prior feature that the air pillow area is configured as an elliptical body of revolution. The gas-liquid distribution state in the tank is measured using simple measuring equipment, which is of great significance to the propellant management, attitude control and propellant remaining measurement of aerospace vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1A flow chart of a method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank provided by the present invention;

[0072] Figure 2 It is a schematic diagram of the ultrasonic transducer array arrangement and measurement system in the present invention;

[0073] Figure 3 Schematic diagram of gas-liquid interface reconstruction using elliptical reflection signals in an embodiment of the present invention;

[0074] Figure 4 Schematic diagram of gas-liquid interface reconstruction using echo reflection signals in an embodiment of the present invention;

[0075] Figure 5 Schematic diagram of gas-liquid distribution measurement blind area reconstruction in an embodiment of the present invention;

[0076] Figure 6 Schematic diagram of three-dimensional reconstruction of gas-liquid distribution in an embodiment of the present invention;

[0077] Figure 7 It is a schematic diagram of the three-dimensional reconstruction result of the simulation measurement device in the embodiment of the present invention. DETAILED DESCRIPTION

[0078] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0079] like Figures 1 to 6 As shown, the gas-liquid distribution state detection method of the spacecraft cryogenic liquid fuel tank provided by the present invention comprises the following steps performed in sequence:

[0080] 1) A multi-layer ultrasonic transducer array is built on the tank, ultrasonic transducers of different channels in the ultrasonic transducer array are excited in turn, and the response signals are divided into three groups: transmission, elliptical reflection and echo signals;

[0081] The specific method is:

[0082] An ultrasonic transducer array 2 is arranged on multiple cross sections of a tank 1 in which a propellant 4 is stored. Excitation signals are applied to ultrasonic transducers of different channels in the ultrasonic transducer array 2 in turn, and response signals caused by the excitation are collected. The ultrasonic transducer has the functions of transmitting excitation and capturing response signals. For an ultrasonic transducer array 2 with n ultrasonic transducers, an ultrasonic transducer of one channel in the ultrasonic transducer array 2 is excited each time, and response signals of n channels including the ultrasonic sensor itself that has been excited are collected after a short delay. The ultrasonic transducers of n channels are excited in turn within one cycle, and a total of n response signals are collected. 2 segment response signals; then the response signals collected within a period are grouped, the response signals received at the relative position are taken as transmission signals, the response signals sent and received spontaneously are taken as echo reflection signals, and the other received signals are taken as elliptical reflection signals.

[0083] 2) The transmission, elliptical reflection and echo signals are inverted by back-projection, elliptical reflection method and local curvature reconstruction method respectively, and then the sensitivity matrices obtained by the three methods are weighted superimposed to obtain a preliminary gas-liquid distribution reconstructed image;

[0084] Different types of signals are processed using different inversion methods to reconstruct the gas-liquid distribution image. The key to this process is to obtain the sensitivity matrix of the inversion method. The calculation process is:

[0085] 2.1. Transmission signal inversion:

[0086] The basic principle of the multi-channel transmission inversion method is the back-projection method, which uses the attenuation of the transmission signal to invert the distribution of the gas phase region in the sound wave transmission path and the obstruction of the sound wave path. The larger the area of ​​gas phase obstruction in the sound wave propagation path, the smaller the transmission signal received by the ultrasonic transducer. When there is partial gas phase obstruction in the transmission path, the calculation formula of the sensitivity matrix P(i,j) of the transmission method is:

[0087]

[0088] Among them, i and j are the channel numbers of the excitation signal, P tr0 is the calibration transmission signal when there is no obstruction, A0 is the diameter of the piezoelectric chip of the ultrasonic transducer, and A tr is the shielding area of ​​the gas phase region. The amplitude of the attenuated transmission signal can be used to infer the size of the shielding area.

[0089] 2.2. Inversion of elliptical reflection signal:

[0090] like Figure 2 As shown in the figure, the air-liquid interface that triggers the reflection can be drawn by the position of the self-receiving and self-emitting ultrasonic transducer and the time of the sound wave. The sensitivity matrix S obtained by the elliptical reflection method is elIt mainly depends on the sound wave transmission time and the sound pressure attenuation amplitude, and the sensitive matrix S is located by the gas-liquid interface. ij and the divergence angle constraint matrix S δ OK, the formula is:

[0091]

[0092] The gas-liquid interface positioning sensitive matrix S ij The formula is:

[0093]

[0094] Among them, l e is the amplitude parameter of the elliptical reflection, P ij is the amplitude of the elliptical reflection signal, c is the speed of sound in the liquid, t ij is the sound wave transmission time, d ij is the transmission distance of the sound wave.

[0095] In the actual transmission process of ultrasonic waves, the divergence of the sound beam is limited. The angle between the sound wave reflection path in the elliptical reflection method and the straight line where the ultrasonic transducer is located should be smaller than the divergence angle δ. When a certain channel transmits sound waves and other channels receive sound wave signals, the divergence angle constraint matrix S δ The formula is:

[0096]

[0097] Among them, j is the number of the receiving channel and d is the transmission distance of the sound wave.

[0098] 2.3. Echo signal inversion:

[0099] like Figure 3 As shown in the figure, when a sound beam with a certain width is transmitted to the gas-liquid interface, the sound wave will be reflected in other directions based on the normal line of the gas-liquid interface. Since the gas-liquid interface has a certain curvature, the echo sound beam will diverge, and the self-transmitting and self-receiving ultrasonic transducer can only capture a part of the energy in the initial excitation signal. The echo signal amplitude P that the ultrasonic transducer can capture is b The calculation formula is:

[0100]

[0101] θ is the angle of the echo signal relative to the center of curvature at the gas-liquid interface, which can be calculated by the following formula:

[0102]

[0103] Where R is the local curvature radius of the gas-liquid interface, P0 is the reference amplitude of the echo signal, which is measured by arranging a total reflection interface near the ultrasonic transducer; d0 is the diameter of the piezoelectric chip of the ultrasonic transducer. Based on the measurement signal, the sound wave transmission time t can be extracted ii and the echo signal amplitude P b , thereby solving the local curvature radius of the gas-liquid interface; based on the local curvature radius of the gas-liquid interface, the sensitivity matrix S obtained by the local curvature reconstruction method is obtained rf The ideal fan-shaped reflection interface S needs to be calculated R (d1, R) and the straight line L(θ) constituting the reflective fan-shaped gas-liquid interface, the formula is:

[0104]

[0105] Where d1 is the shortest distance between the ultrasonic transducer and the reflecting gas-liquid interface. The local curvature reconstruction method reconstructs the fan-shaped gas-liquid interface that triggers the sound wave reflection based on the echo signal amplitude and the sound wave transmission time information. The larger the radius corresponding to the curvature of the gas-liquid interface, the stronger the echo signal energy.

[0106] After processing the transmission signal, elliptical reflection signal and echo signal respectively, the sensitivity matrices obtained by the above three methods are weighted superimposed to obtain a preliminary gas-liquid distribution reconstructed image. The superposition weights of different reconstruction methods reflect the priority of each method. For example, when the transmission signal determines that there are some areas without bubbles, the priority of the back projection method will be higher than the elliptical reflection method and the local curvature reconstruction method. In the weight parameter allocation, the weight ratio of the transmission: elliptical reflection: echo inversion method is 1:4:4.

[0107] 3) Based on the morphological processing method, the protrusions and noise points in the preliminary gas-liquid distribution reconstruction image are removed and the missing pixels are supplemented, and then the measurement blind area reconstruction method is used to reconstruct the measurement blind area to obtain the gas-liquid distribution reconstructed image;

[0108] Morphological operations determine the image parameters of a pixel based on the relationship between a pixel and its surrounding pixels. This relationship can be compared to the convolution operator used to traverse a frame of an image. Before morphological processing, the image must be binarized. Let V th represents the threshold for screening the gas-liquid two-phase region, M n×n (i, j) represents the preliminary gas-liquid distribution reconstruction image matrix, and the formula for the binarization process is:

[0109]

[0110] The value of the binarization threshold is related to the numerical distribution of the parameters of the combined reconstruction probability matrix. Since the parameters of the superimposed local area of ​​the weight matrix may be too large, the area of ​​the binarized area is too small. Therefore, it is necessary to perform mean filtering before binarization.

[0111] Morphological processing mainly includes dilation and erosion, which are used to remove bulges, noise or fill missing pixels in the preliminary gas-liquid distribution reconstruction image; the dilation operation is extended by morphology, and by comparing the relationship between pixels, pixels are added around the pixels that meet the conditions, thereby filling the missing pixels in the image; let P and Q be the sets of the two-dimensional integer space Z2, and the set Q is morphologically expanded to obtain the set P. This process is defined as:

[0112]

[0113] Similarly, using set Q to erode set P is to concentrate all the pixels contained in set P into set Q, and traverse the pixels in the preliminary gas-liquid distribution reconstruction image for translation. The erosion operation is a shrinking process that removes redundant noise points in the morphological image. The definition of set P eroded by set Q is:

[0114]

[0115] like Figure 5 As shown in the figure, after morphological processing, the blind area needs to be reconstructed using the blind area reconstruction method, and the curvature radius at the blind area is estimated using the curvature radius measured near the blind area. When the implicit function f(x,y)=0 is satisfied, the curvature calculation formula of a point on the ellipse boundary is:

[0116]

[0117] The characteristic that the gas-liquid interface approaches the boundary of the ellipsoid indicates that the local curvature of the gas-liquid interface will not change drastically. Let r1 and r2 be the radii of two circular arcs of known curvature, and the curvature calculation formula of the unknown curvature segment between the two is:

[0118]

[0119] Finally, the gas-liquid distribution reconstructed image M(x,y) is obtained.

[0120] 4) dividing the gas-liquid distribution reconstructed image into different bubble area sets based on whether the gas-liquid interface is connected, and then counting and locating the bubbles in each bubble area set;

[0121] Assuming that the current pixel in the gas-liquid distribution reconstruction image is M(p,q), p is the horizontal coordinate of the pixel, q is the vertical coordinate of the pixel, and the total number of bubbles is k, the process of distinguishing different gas-liquid interfaces is expressed as:

[0122]

[0123] Among them, M(i,j) k A collection representing different gas-liquid interface images. The pixel point first identified as the gas-liquid interface is stored in a bubble region set. When a new gas-liquid interface pixel point is traversed, the pixel point is called the current pixel point, and the distance between the current pixel point and all the pixels in the bubble region set is calculated. If the distance between the current pixel point and all the pixels in the bubble region set exceeds the threshold, it means that the current pixel point is far away from the gas-liquid interface and belongs to other bubbles. Therefore, a new bubble region set needs to be opened and the current pixel point is stored. When there is a pixel point in the existing bubble region set whose distance from the current pixel point is less than the threshold, it means that the two pixels are adjacent on the gas-liquid interface, and the current pixel point is included in the bubble region set. Therefore, different bubbles can be distinguished by judging whether the gas-liquid interface is connected. Finally, the number of bubbles in each bubble region set is counted and located.

[0124] 5) After different bubble region sets are distinguished, different bubble cross sections in the gas-liquid distribution reconstructed image are fitted with an ideal two-dimensional elliptical model to obtain a two-dimensional gas-liquid distribution image;

[0125] The main measurement target inside the tank 1 is the air pillow area 3. The final configuration of the air pillow area 3 is similar to an elliptical body of revolution, and the cross-sectional shape is an ellipse. Therefore, the gas-liquid interface images of different bubble areas can be regarded as ideal ellipse models during imaging. Under the condition that the internal gas-liquid distribution configuration is close to an ellipse, the gas-liquid distribution reconstructed image after ellipse fitting processing is expressed as:

[0126]

[0127] Among them, M i (x, y) is the gas-liquid distribution image reconstruction matrix before ellipse fitting, M(x, y)′ is the gas-liquid distribution image reconstruction matrix after ellipse fitting, ρ(j) is the gas-liquid interface curvature solution model, E el Post-processing operation for ellipse approximation of gas-liquid distribution reconstruction image;

[0128] Let {M u (i, j)} is the set of bubble regions in the u-th bubble region, M1 and M2 refer to any two boundary points in the discrete matrix two-dimensional image, d(M1, M2) is the distance between the two boundary points, and the maximum value of the distance between any two boundary points is calculated as follows:

[0129]

[0130] Post-processing operation of the gas-liquid interface boundary by elliptical approximation of the gas-liquid distribution reconstructed image el After processing, it can be expressed as:

[0131]

[0132] The uth bubble area M u (i,j) Post-processing operation E of the ellipse approximation of the image reconstructed by gas-liquid distribution el After processing, the range of continuous solutions is obtained. After the spatial discretization of the solution, two pairs of discrete gas-liquid boundary positioning points can be obtained. On this basis, the fitting continuous function approaching the ellipse in the u-th bubble region can be expressed as:

[0133] d 2 (M a1 ,M a2 )y 2 +d 2 (M b1 ,M b2 )=1

[0134] Among them, M a1 、M a2 and M b1 、M b2 The two sets of intersection points are used to locate the gas-liquid interface. The two-dimensional gas-liquid distribution image is composed of pixels and is discrete. The continuous function curve needs to be converted into a discrete pixel image matrix.

[0135] 6) Superimposing multiple frames of the above two-dimensional gas-liquid distribution images on the z-axis to obtain a three-dimensional gas-liquid distribution image, thereby fitting the three-dimensional morphology of the gas-liquid interface.

[0136] like Figure 6 As shown, a multi-layer array is arranged on the tank 1, and each layer of the array can reconstruct a frame of a two-dimensional gas-liquid distribution image of the gas-liquid interface. For an array with w layers, the number of layers w is taken as the z-axis, and multiple frames of two-dimensional gas-liquid distribution images are superimposed in the three-dimensional space, and the three-dimensional morphological features of the gas-liquid distribution are stacked through interpolation and filtering. At this time, the three-dimensional gas-liquid distribution image matrix is ​​expressed as:

[0137]

[0138] Among them, G(z1, z2, z3) is a three-dimensional Gaussian filter matrix. The acquisition interval of each frame of the image is the same. The shorter the interval, the higher the density of the superposition of the three-dimensional gas-liquid distribution image, and the closer the three-dimensional morphology reconstruction result will be to the real gas-liquid interface.

[0139] like Figure 7As shown in the figure, in the ground experimental environment, the 3D reconstruction effect of the simulated measurement device is:

[0140] The airbag is fixed with a heavy object as the measurement target, replacing the bubbles to be measured in the tank, and water is used instead of liquid propellant. The transducer array is fixed, and the airbag is moved at a constant speed through the ultrasonic transducer array to obtain reconstructed images of different cross-sections of the airbag. 50 frames of images are superimposed on the z-axis, and the reconstruction results reflect the morphology of the airbag. The 3D reconstruction results, 3D reconstruction side, and reconstructed experimental airbag are shown in Figure 1. Figure 7 As shown in (a), (b) and (c).

[0141] The above description enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest range consistent with the principles and novel features disclosed herein.

Claims

1. A method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank, characterized in that: The method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank comprises the following steps performed in sequence: 1) A multi-layer ultrasonic transducer array is built on the tank, ultrasonic transducers of different channels in the ultrasonic transducer array are excited in turn, and the response signals are divided into three groups: transmission, elliptical reflection and echo signals; 2) The transmission, elliptical reflection and echo signals are inverted by back-projection, elliptical reflection method and local curvature reconstruction method respectively, and then the sensitivity matrices obtained by the three methods are weighted superimposed to obtain a preliminary gas-liquid distribution reconstructed image; 3) Based on the morphological processing method, the protrusions and noise points in the preliminary gas-liquid distribution reconstruction image are removed and the missing pixels are supplemented, and then the measurement blind area reconstruction method is used to reconstruct the measurement blind area to obtain the gas-liquid distribution reconstructed image; 4) dividing the gas-liquid distribution reconstructed image into different bubble area sets based on whether the gas-liquid interface is connected, and then counting and locating the bubbles in each bubble area set; 5) After different bubble region sets are distinguished, different bubble cross sections in the gas-liquid distribution reconstructed image are fitted with an ideal two-dimensional elliptical model to obtain a two-dimensional gas-liquid distribution image; 6) Superimposing multiple frames of the above two-dimensional gas-liquid distribution images on the z-axis to obtain a three-dimensional gas-liquid distribution image, thereby fitting the three-dimensional morphology of the gas-liquid interface.

2. The method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank according to claim 1, characterized in that: In step 1), the specific method of building a multilayer ultrasonic transducer array on the tank, exciting ultrasonic transducers of different channels in the ultrasonic transducer array in turn, and dividing the response signals into three groups of transmission, elliptical reflection and echo signals is as follows: Ultrasonic transducer arrays (2) are arranged on multiple cross sections of a tank (1) storing propellant (4) therein, and excitation signals are applied to ultrasonic transducers of different channels in the ultrasonic transducer array (2) in turn, and response signals caused by the excitation are collected; for an ultrasonic transducer array (2) having n ultrasonic transducers, each time an ultrasonic transducer of a channel in the ultrasonic transducer array (2) is excited, response signals of n channels including the ultrasonic sensor itself that has been excited are collected after a short delay, and the ultrasonic transducers of n channels are excited in turn within one cycle, and a total of n response signals are collected. 2 segment response signals; then the response signals collected within a period are grouped, the response signals received at the relative position are taken as transmission signals, the response signals sent and received spontaneously are taken as echo reflection signals, and the other received signals are taken as elliptical reflection signals.

3. The method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank according to claim 1, characterized in that: In step 2), the back-projection, elliptical reflection method and local curvature reconstruction method are used to invert the transmission, elliptical reflection and echo signals respectively, and then the sensitivity matrices obtained by the three methods are weighted superimposed to obtain a preliminary gas-liquid distribution reconstructed image. The specific method is: 2.

1. Transmission signal inversion: When there is a partial gas phase area occlusion in the transmission path, the calculation formula of the sensitivity matrix P(i,j) of the transmission method is: Among them, i and j are the channel numbers of the excitation signal, P tr0 is the calibration transmission signal when there is no obstruction, A0 is the diameter of the piezoelectric chip of the ultrasonic transducer, and A tr is the shielding area of ​​the gas phase region. The amplitude of the attenuated transmission signal can be used to infer the size of the shielding area. 2.

2. Elliptical reflection signal inversion: The sensitivity matrix S obtained by the elliptic reflection method el It mainly depends on the sound wave transmission time and the sound pressure attenuation amplitude, and the sensitive matrix S is located by the gas-liquid interface. ij and the divergence angle constraint matrix S δ OK, the formula is: The gas-liquid interface positioning sensitive matrix S ij The formula is: Among them, l e is the amplitude parameter of the elliptical reflection, P ij is the amplitude of the elliptical reflection signal, c is the speed of sound in the liquid, t ij is the sound wave transmission time, d ij is the transmission distance of the sound wave; The angle between the sound wave reflection path in the elliptical reflection method and the straight line where the ultrasonic transducer is located should be smaller than the divergence angle δ; when a certain channel transmits sound waves and other channels receive sound wave signals, the divergence angle constraint matrix S δ The formula is: Among them, j is the number of the receiving channel, and d is the transmission distance of the sound wave; 2.

3. Echo signal inversion: The echo signal amplitude P that can be captured by the ultrasonic transducer b The calculation formula is: θ is the angle of the echo signal relative to the center of curvature at the gas-liquid interface, which can be calculated by the following formula: Where R is the local curvature radius of the gas-liquid interface, P0 is the reference amplitude of the echo signal, which is measured by arranging a total reflection interface near the ultrasonic transducer; d0 is the diameter of the piezoelectric chip of the ultrasonic transducer. Based on the measurement signal, the sound wave transmission time t can be extracted ii and the echo signal amplitude P b , thereby solving the local curvature radius of the gas-liquid interface; based on the local curvature radius of the gas-liquid interface, the sensitivity matrix S obtained by the local curvature reconstruction method is obtained rf The ideal fan-shaped reflection interface S needs to be calculated R (d1, R) and the straight line L(θ) constituting the reflective fan-shaped gas-liquid interface, the formula is: Where d1 is the shortest distance between the ultrasonic transducer and the reflecting gas-liquid interface; After processing the transmission signal, elliptical reflection signal and echo signal respectively, the sensitivity matrices obtained by the above three methods are weighted superimposed to obtain a preliminary gas-liquid distribution reconstructed image; in the weight parameter allocation, the weight ratio of the transmission: elliptical reflection: echo inversion method is 1:4:

4.

4. The method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank according to claim 1, characterized in that: In step 3), the morphological processing method is used to remove protrusions, noise points and fill in missing pixels in the preliminary gas-liquid distribution reconstruction image, and then the measurement blind area reconstruction method is used to reconstruct the measurement blind area. The specific method for obtaining the gas-liquid distribution reconstruction image is: Before morphological processing, the image needs to be binarized. Let V th represents the threshold for screening the gas-liquid two-phase region, M n×n (i, j) represents the preliminary gas-liquid distribution reconstruction image matrix, and the formula for the binarization process is: Mean filtering is required before binarization; Morphological processing mainly includes dilation and erosion, in which the dilation operation is extended by morphology. By comparing the relationship between pixels, pixels are added around pixels that meet the conditions, thereby filling the missing pixels in the image. Let P and Q be the sets of the two-dimensional integer space Z2, and the set Q is morphologically expanded to obtain the set P. This process is defined as: Similarly, using set Q to corrode set P is to concentrate all the pixels contained in set P into set Q, and traverse the pixels in the preliminary gas-liquid distribution reconstruction image for translation; the corrosion operation is a shrinking process, which will remove the redundant noise points in the morphological image. The definition of set P corroded by set Q is: After morphological processing, the blind area needs to be reconstructed using the blind area reconstruction method. The curvature radius measured near the blind area is used to estimate the curvature radius at the blind area. When the implicit function f(x, y) = 0 is satisfied, the curvature calculation formula of a point on the ellipse boundary is: The characteristic that the gas-liquid interface approaches the boundary of the ellipsoid indicates that the local curvature of the gas-liquid interface will not change drastically. Let r1 and r2 be the radii of two circular arcs of known curvature, and the curvature calculation formula of the unknown curvature segment between the two is: Finally, the gas-liquid distribution reconstructed image M(x,y) is obtained.

5. The method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank according to claim 1, characterized in that: In step 4), the gas-liquid distribution reconstructed image is divided into different bubble area sets based on whether the gas-liquid interface is connected, and then the number of bubbles in each bubble area set is counted and located in the specific method as follows: Assuming that the current pixel in the gas-liquid distribution reconstruction image is M(p,q), p is the horizontal coordinate of the pixel, q is the vertical coordinate of the pixel, and the total number of bubbles is k, the process of distinguishing different gas-liquid interfaces is expressed as: Among them, M(i,j) k A set representing different gas-liquid interface images; the pixel point first identified as the gas-liquid interface is stored in a bubble area set. When traversing to a pixel point of a new gas-liquid interface, the pixel point is called the current pixel point, and the distance between the current pixel point and all the pixels in the bubble area set is calculated. If the distance between the current pixel point and all the pixels in the bubble area set exceeds the threshold, it means that the current pixel point is far away from the gas-liquid interface and belongs to other bubbles. Therefore, it is necessary to open a new bubble area set and store the current pixel point in it; when there is a pixel point in the existing bubble area set and the distance between the current pixel point and the pixel point is less than the threshold, it means that the two pixels are adjacent on the gas-liquid interface, and the current pixel point is included in the bubble area set; therefore, different bubbles can be distinguished by judging whether the gas-liquid interface is connected, and finally the number of bubbles in each bubble area set is counted and located.

6. The method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank according to claim 1, characterized in that: In step 5), the specific method of fitting different bubble cross sections in the gas-liquid distribution reconstructed image with an ideal two-dimensional elliptical model to obtain a two-dimensional gas-liquid distribution image is: Under the condition that the internal gas-liquid distribution configuration approaches an ellipse, the gas-liquid distribution reconstructed image after ellipse fitting processing is expressed as: Among them, M i (x, y) is the gas-liquid distribution image reconstruction matrix before ellipse fitting, M(x, y)′ is the gas-liquid distribution image reconstruction matrix after ellipse fitting, ρ(j) is the gas-liquid interface curvature solution model, E el Post-processing operation for ellipse approximation of gas-liquid distribution reconstruction image; Let {M u (i, j)} is the set of bubble regions in the u-th bubble region, M1 and M2 refer to any two boundary points in the discrete matrix two-dimensional image, d(M1, M2) is the distance between the two boundary points, and the maximum value of the distance between any two boundary points is calculated as follows: Post-processing operation of the gas-liquid interface boundary by elliptical approximation of the gas-liquid distribution reconstructed image el After processing, it is expressed as: The uth bubble area M u (i,j) Post-processing operation E of the ellipse approximation of the image reconstructed by gas-liquid distribution el After processing, the range of continuous solutions is obtained. After the spatial discretization of the solution, two pairs of discrete gas-liquid boundary positioning points can be obtained. On this basis, the fitting continuous function close to the ellipse in the u-th bubble region can be expressed as: d 2 (M a1 ,M a2 )y 2 +d 2 (M b1 ,M b2 )=1 Among them, M a1 、M a2 and M b1 、M b2 Two sets of intersection points for locating the gas-liquid interface; the two-dimensional gas-liquid distribution image is composed of pixel points and is discrete. The continuous function curve needs to be converted into a discrete pixel point image matrix.

7. The method for detecting the gas-liquid distribution state of a spacecraft cryogenic liquid fuel tank according to claim 1, characterized in that: In step 6), the specific method of superimposing multiple frames of the above two-dimensional gas-liquid distribution images on the z-axis to obtain a three-dimensional gas-liquid distribution image, thereby fitting the three-dimensional morphology of the gas-liquid interface is: A plurality of arrays are arranged on the tank (1), and each array layer can reconstruct a frame of a two-dimensional gas-liquid distribution image of the gas-liquid interface. For an array with w layers, the number of layers w is taken as the z-axis, and multiple frames of two-dimensional gas-liquid distribution images are superimposed in a three-dimensional space, and the three-dimensional morphological features of the gas-liquid distribution are stacked through interpolation and filtering. At this time, the three-dimensional gas-liquid distribution image matrix is ​​expressed as: Among them, G(z1,z2,z3) is a three-dimensional Gaussian filter matrix.

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