Development Of Liquid Nitrogen Transmission Control System For Cryogenic Machining

Cryogenic machining has many advantages,such as improving the machinability of materials,prolonging the tool life and increasing the workpiece surface quality.As the result,it has great potential in the machining of difficult-to-cut materials.As cooling medium,the flow parameters of liquid nitrogen such as the flow rate,the gas-liquid phase ratio have significant effects on the cryogenic machining.Thus the development of the related equipment becomes the key to the further research and popularization of cryogenic machining.Because of low boiling point and low latent heat of vaporization,it is easy for liquid nitrogen to vaporize and form gas-liquid two-phase flow in the pipeline,and the thermal flow process of gas-liquid two-phase flow is very complicated,which means it is very difficult to stabilize and control the flow of liquid nitrogen.This paper aims at developing a liquid nitrogen transmission and control system for cryogenic machining,and the conditions of steady transfer of low gas mass fraction nitrogen and the control methods of liquid nitrogen jet flow were studied.Cryogenic fluid will change the parameters such as heat transfer,pressure drop as the flow pattern changes when boiling flow happens in pipeline.To address this situation the suitable components and pipes with excellent insulation effect were selected,and the main pipeline part of the system was designed.Based on AMESim,a numerical simulation model of gas-liquid two-phase flow and heat transfer of heated pipes was established.The transmission of liquid nitrogen jet under different inlet pressure and valve opening was calculated by simulation and the main pipeline system was verified.After the control unit and power supply unit were designed,the liquid nitrogen transmission and control system for cryogenic machining were developed.The transfer function of the system was fitted according to the input and output signals of the system and the PID control parameters were adjusted.Base on it,fuzzy control rules were established to adjust the PID parameters online according to the input signal and a fuzzy PID controller using “algebraic product-addition-centroid” as anti-fuzzy method was designed.By liquid nitrogen stable transmission performance verification experiment,the simulation results and piping system were tested.The STM32F4 engineering processes ofPID and fuzzy PID controlling means were composed.Each of them was downloaded to the ARM board and liquid nitrogen transport control experiment was conducted to test the system performance.In this paper,a kind of liquid nitrogen transmission and control system was developed.The equipment can provide stable liquid nitrogen jet with low gas mass fraction for cryogenic machining and realize the automatic control function of jet flow.