Study On Cold Storage Evaporator Superheat Control Based On Fuzzy PID

In the refrigeration system, the superheat not only affects the heat exchange efficiency of the evaporator, and also affected the security of the system. If the superheat is too large, it will reduce the heat exchange efficiency of the evaporator. If the superheat is too small, it will cause the compressor of the "liquid hammer" phenomenon. The experimental platform is the small low-temperature cold storage. The target is the superheat at the outlet of the evaporator, designing the superheat automatic control system of the cold storage evaporator. Because the characteristics of the cold storage evaporator will change with the change of load and operating conditions, and the traditional PID doesn’t have adaptive capacity, it can’t automatically make the necessary adjustments based on these changes. Fuzzy PID controller has both dynamic response performance of fuzzy controller, and a PID controller with good steady-state performance. Therefore, the fuzzy PID controller is used to control the superheat of the refrigerant at the evaporator outlet.However, in the design of fuzzy controller, the controller parameters are determined mainly by the operator’s experience. It makes that the fuzzy controller has certain subjectivity and randomness, and it can’t guarantee quality of control be optimal even suboptimal. The control rules, membership function, quantization factor and scaling factor are optimized by genetic algorithm in this paper, and the optimized fuzzy PID controller is used in the superheat control system of the low-temperature cold storage evaporator by using S7-200 PLC as the controller. The main works are as follows.(1) Construction of low-temperature cold storage system and design of the evaporator superheat control system, the constant frequency control of the compressor and the fuzzy PID control of the superheat are coupled. Through experiment of the evaporator superheat on the step response of electronic expansion valve, the transfer function of the evaporator superheat on the step response of electronic expansion valve has been identified.(2) Genetic algorithm is used to optimize the fuzzy PID controller. To narrow the search space and reduce the blindness of genetic algorithm, genetic algorithm is used to optimize the nearby space of the parameters. The optimized control rules, membership function, quantization factor and scaling factor have a significant change. The result of MATLAB simulation shows that the fuzzy PID controller has the advantages of small overshoot and short regulation time.(3) The upper system and the lower system are designed. The optimized parameters are established as query table in PLC data block, and the program will get the corresponding control parameters through the pointer according to the real-time data acquisition. Self-tuning fuzzy PID controller is realized based on SIEMENS S7-200 PLC.(4) Under the same operating conditions, the optimized fuzzy PID and conventional PID are used to control the refrigerant superheat at the evaporator outlet. The experiments are used to verify the advantages of the fuzzy PID controller in low temperature refrigerator evaporator superheat controlling, and the feasibility of parameter self-tuning fuzzy PID program.The experimental results show that the fuzzy PID controller has better superheat control effect of the evaporator, the overshoot is small, and fluctuating within ±0.3℃ of the superheat setpoint. The electronic expansion valve opening changes smoothly, it is beneficial to reduce the wear of valve, and it has great value in engineering application.