Experimental Research On Defrosting Of Cold Storage System Based On Frost Layer Visualization And Fuzzy PID Control

With the development of social economy,the demand for cold storage is increasing.Due to the influence of frost and cold storage temperature fluctuation during the operation of cold storage,the problem of energy consumption is very serious.The cold storage has become a big energy consumer.In order to reduce the damage of frost layer and the energy loss caused by the fluctuation of the cold storage temperature,the research and analysis are carried out in this subject from the improvement of the defrosting control method and the improvement of the automatic control system of the cold storage.In terms of reducing the damage of frost layer: this subject first expounds the formation mechanism of frost.And points out the complexity and difficulty of indirect measurement of frost layer in defrost control,and the defrost control method that based on direct measurement will be the inevitable trend of research and development in the future.Based on the existing continuous defrosting cold storage system,a camera device which is connected to the Raspberry Pi is installed at the evaporator fin to realize frost monitoring and image acquisition.Then,the structure of the test bench is completed.The electronic control design with the MCGS touch screen as the upper computer,and the Mitsubishi PLC as the lower computer,and based on the Modbus communication protocol and the acquisition module and the sensor.The real-time communication between the system and each module is completed.And realize the start-stop control,parameter setting,monitoring screen display,fault alarm and alarm information viewing,communication results viewing of each device,temperature data real-time/historical viewing,temperature data historical curve viewing,data export and deletion and other related functions of the experiment system.It can monitor and view the system operation in real time,that the human-computer interaction is great.Based on the traditional defrosting control method-differential pressure method,a defrosting experiment was carried out,which proved the existence of the problem of "false defrosting" in the traditional method.Then proved the rationality of using image processing method to obtain frost thickness parameter as the cut-in criterion for defrosting is discussed by sorting,processing and analyzing the obtained experimental data and images.However,due to this method is subject to the influence of some objective factors,it is difficult to realize the criterion for the end of defrosting.So the experiment was re-experimented for the end of defrosting.Optimized defrost end temperature by sorting out the temperature change curve of the inlet and outlet of the defrosting evaporator and combining the frost layer image.A new defrost control logic is proposed that based on the frost thickness H greater than or equal to 2900 μ m measured by image processing as the defrost start condition,and the defrost evaporator outlet temperature T greater than or equal to 18℃ as the defrost end condition.Next,defrost experiments are carried out based on the optimized new defrost control logic.The results show that the optimized defrost control logic is reliable and accurate.In terms of reducing the fluctuation of cold storage temperature: this subject designed a fuzzy PID controller for the cold storage system of this experiment,and established a mathematical model of the evaporator of the cold storage.Then established a simulation model with the help of the SIMULINK environment in MATLAB,and carried out a simulation test.The simulation simulation control effect is good.Next,the fuzzy PID control on the actual PLC is realized by using the look-up table method.Then the fuzzy PID control was used to replace the compressor start-stop control of the original system,and experiments were carried out under 4 sets of cold storage temperature conditions set at-5°C,-8°C,-12°C and-18°C,respectively.The results show that: after the improvement of the control method,the cold storage temperature fluctuations in the refrigeration mode operation are significantly reduced which means the system operation is more stable in the4 groups of experiments with different storage temperatures.When in the defrost mode,the recovery of the cold storage temperature is reduced by 0.6～0.9℃.So the application effect is great.