| In the petroleum,chemical,aerospace and other fields,the real-time monitoring and accurate measurement of the liquid level in a sealed container are important for the industrial automation and safety production.In the actual industrial production,an appropriate measurement method is selected to suit the specific environment and requirements of production safety.The traditional liquid level measurement methods are numerous,of which the detection technologies are stable and reliable,the measurement accuracy is high.These methods usually require some or all of the detection equipment and sensors to be installed inside the container in advance,for some special industry,especially as the container is stored with high temperature,high pressure,inflammable,explosive,highly corrosive and strong volatile liquid.It is easy to cause a leakage accident,the maintenance is difficult and costly after a failure.Ultrasonic detection technology can achieve a true sense of non-contact and nonimmersion measurement without damaging the physical structure and integrity of a container.Therefore,based on the ultrasonic impedance,a novel ultrasonic method for detecting the liquid level is proposed to meet higher performance and reliability requirements and provide a theoretical basis for those special fields.Firstly,by simulating the sound field of a round piston transducer with the Multi-Gaussian Beam Model,and analyzing the shape of ultrasound beams and propagation characteristics in a metal container wall,this study presents a model for calculating the echo sound pressure by using the Kirchhoff paraxial approximation theory,based on which and according to different ultrasonic impedance between gas and liquid media,a method for detecting the liquid level from outside of sealed containers is proposed.In MATLAB,three kinds of liquid media with different ultrasonic impedance are simulated as detected objects;the echo sound pressure is calculated by using the proposed model under conditions of four sets of different wall thicknesses.The changing characteristics of the echo sound pressure in the entire detection process are analyzed,and the effects of different ultrasonic impedance of liquids on the echo sound pressure are compared.Secondly,the main factors influencing the accuracy of the model are analyzed,and some effective and improvement methods to eliminate these adverse effects are put forward.In order to solve the unstable and unreliable problems caused by poor coupling between the ultrasonic transducer and the container wall,the research proposes two solutions.The first scheme is to improve the conventional transducer,so that the stress of the transducer surface can be kept uniform,and the coupling consistency between the transducer and the container wall is ensured throughout the detection process.Therefore,this scheme can improve the stability and reliability of the detection.The second scheme which is based on the proposed detection model,uses an improved multi-wafer(or multi-transducer)combination transducers and uses the balance principle of echo energy to solve the liquid level measurement problems when the coupling condition is poor,this study simulates and validates the feasibility and effectiveness of the solution through a series of MATLAB simulation experiments.This scheme can be complementary to the measurement methods in Chapter 3.Finally,according to the analysis and simulation experiment of the theoretical model,the general scheme of the hardware and software of the ultrasonic liquid level instrument is designed.On this basis,the prototype is produced and verified through a series of experiments that are divided into three parts.In the first part of the experiment,two kinds of improved single transducers with different diameters were used.The liquid level was measured under four different wall thickness conditions.Combined the sound field characteristics,the effect of the transducers on the calculation of the echo sound pressure and the detection resolution were discussed and analyzed.The experimental results show that under the static condition,the measurement accuracy of the measurement model discussed in this study is less than ± 5mm,which can meet the design requirements.In the second part of the experiment,the British Class company’s PLI-D liquid level instrument is used in the same test conditions,the measurement results in line with its own specification,the measurement error remained at 5~10mm range,slightly higher than that of the model proposed in this study.In the third part of the experiment,multi-wafer(or multi-transducers)combined transducers are used,and in the same four groups of different wall thickness conditions,the practicality of this scheme was verified.Two kinds of combined transducers with different arrangement were selected to detect the liquid level,and the accuracy and validity of the measurement results were discussed and analyzed in detail.The experimental results show that this scheme can effectively overcome the detection problems caused by poor coupling conditions,and the detection accuracy is less than ± 5mm,which satisfies the design requirements.This scheme reduces the coupling requirement of sensors,so that the energy of the incident beam cannot be kept in a continuous steady state during the whole detection process,and the detection operation is more convenient and flexible,the detection process is easier to control and the detection result is more stable,accurate,and reliable. |
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