Mechanism Modeling And Control Of Continuous Flow Liquid Helium Thermostat

Since the discovery of cryogenic superconducting technology,cryogenic superconducting based on liquid helium has been successfully applied in many fields.Among them,continuous flow liquid helium thermostat provides a low-temperature working environment for large-scale cryogenic applications,which is of great significance to the efficient and stable operation of the whole cryogenic system.Due to the thermophysical characteristics of liquid helium in practical engineering,liquid helium is extremely easy to evaporate,which will lead to the instability of liquid level and pressure in the liquid helium thermostat and the coupling effect,which will have a significant impact on the stability and reliability of the control performance of the liquid helium cryogenic thermostat.In order to make the liquid level and pressure inside the liquid helium cryostat operate stably,it is necessary to analyze the interior of the liquid helium cryostat theoretically,establish the mathematical model of the liquid helium cryostat,and adopt the decoupling method to reduce the coupling effect between the liquid level and pressure as much as possible.The main research contents of this paper are as follows:(1)Taking the continuous flow liquid helium thermostat as the research object,the characteristics of the liquid helium cryogenic thermostat are studied,the working principle of the continuous flow liquid helium thermostat is analyzed,and the influence of the thermodynamic change process between liquid helium and gas helium on the liquid level and pressure in the thermostat is studied.At the same time,the thermodynamic model of liquid helium cryogenic thermostat is established based on the relationship between mass conservation,energy conservation and pressure volume temperature.The obvious coupling effect between liquid level and pressure in the thermostat is confirmed by theoretical analysis and field data analysis.(2)For the regulation of liquid level and pressure of liquid helium cryogenic thermostat,this paper uses T-8800 ultra-low temperature regulating valve to regulate the liquid level and pressure in the thermostat,explains the working principle of T-8800 ultra-low temperature regulating valve,establishes the flow equation of the regulating valve according to Bernoulli equation,establishes the mathematical model of the regulating valve through the mechanism analysis of the regulating valve,and finally establishes the transfer function of liquid level and pressure with respect to the opening of the valve.(3)The accuracy of the model is verified through system dynamic simulation and data simulation,and the influence of the coupling effect between liquid level and pressure in the thermostat on the interior of liquid helium cryogenic thermostat is analyzed.Based on the characteristics of the strong coupling control system between liquid level and pressure of the finally obtained controlled object model,The performance of liquid level and pressure control system is optimized and improved by using the advantages of effective decoupling of feedforward matrix decoupling method and high steady-state accuracy of PID controller(proportional integral differentiation).At the same time,fuzzy PID is compared with traditional PID control.The simulation results show that the liquid helium cryogenic thermostat model established in this paper can accurately reflect the thermodynamic changes inside the thermostat.Through the comparison of the simulation results of fuzzy PID and traditional PID,the former has better anti-interference and robustness,can better control the strong coupling system effectively,is suitable for similar cryogenic control systems,and has certain theoretical research significance for the establishment of large-scale cryogenic systems in the future.