Measuring Cryogenic Liquid Level Based On Image Processing

The liquid level measurement can be archived by processing the image inside the tank.The liquid level measurement based on image processing is free from capillary action and fluid’s viscosity which affect the traditional liquid level measurement using tubes such as capacitive liquid level sensor.Without tubes,the measuring instrument is composed of the light source and the camera which are smaller,lighter and easier to installation.In addition,not only the liquid level can be obtained from the image,but the fluctuation and phase of liquid could also be learned.In summary,image processing has broad application prospects in the field of liquid level measurement.In this article,we propose a set of low temperature storage tank liquid level image-based measurement system,which is composed of image processing algorithm,camera,temperature control module and protective structure.In order to better detect the sloshing liquid level during the powered-flight phase of rocket,after analyzing and comparing various image liquid level measurement methods,a measurement algorithm based on liquid surface edge feature detection and liquid surface profile pattern recognition is adopted.This method is less computationally intensive,and has good real-time performance and feasibility.For collecting images stably in a low-temperature propellant environment,the impact of the thermal environment of the low-temperature storage tank on the camera,as well as the reliability and safety of the camera in an inflammable and explosive environment is comprehensively analyzed.And combined with related literature,the related temperature control components and protective structure are completed.The test results show that the measurement system can achieve millimeter-level measurement accuracy.In terms of software algorithm,the original image is grayed out first,and then the circular structure element is used for expansion.On this basis,the larger circular structure element is used for the corrosion operation to highlight the contour of the liquid surface and complete the image preprocessing.Considering the detection effect,running time and algorithm complexity,the Sobel operator is used to extract the contour of the liquid surface,and an adaptive threshold module is added,which is suitable for more environments.The traditional Hough transform needs to traverse the image elements many times in the process of detecting the circle.In order to shorten the calculation time,a priori information is added to reduce the number of non-zero points,and the threshold is increased to reduce the number of matching points,thereby improving the efficiency of the algorithm and obtaining the pixel coordinates of the liquid level.Finally,the relative coordinates of the measurement system and the storage tank and the system imaging model are used to obtain the spatial position information of the liquid level to complete the liquid level measurement.In terms of the camera system,according to the environmental characteristics of the cryogenic propellant tank,the thermal environment of the camera system is analyzed,and then the measurement system is thermally simulated by numerical simulation software.Finally,the temperature control and protection structure of the system is designed according to the simulation results.The system protective shell is made of stainless steel 1Cr18Ni9Ti,with a three-level design of quartz glass windows and indium wire sealing rings.The insulation layer is filled with polyurethane,and polytetrafluoroethylene and polyimide are used to connect the system and the shell to reduce heat conduction.In addition,the PT1000 resistor is used to monitor the operating temperature of the system,and the electric heating film is used to heat the electronic unit and optical system of the system to maintain the operating temperature of the system.The liquid level results under static and dynamic conditions are obtained by experimental design of flat and undulating liquid level in cryogenic propellant tank.The test results show that the method is basically consistent with the actual state.The method is suitable for static liquid level or dynamic liquid level,and the measurement error is within 5mm.