Research On Internal Model Control Strategy Of Compression Refrigerating System

Compression refrigeration systems are mostly designed and modeled on the basisof the maximum cold and heat load. However, in operation, due to the externalenvironment, the systems working under the off-rating conditions make the coolingcapacity generated greater than that actually required, thus resulting in cooling waste.To ensure the refrigeration system can make a supply of cooling capacity in demand,the Internal Model Control structure based refrigeration system control strategy wasdesigned. Through the simulation, the effectiveness of the strategy was finallyverified.At first, according to experimental data provided by the operation of therefrigeration system, an analysis was made on the system’s internal characteristics inthree aspects, including influencing factors and changing trends of the system’s mainparameters, the coupling relationship between the input and output variables of therefrigeration system’s components, and the phenomenon and reasons of the part of theoutput variables response hysteresis. In addition, the sixth-order nonlinear modelcharacteristics of the refrigeration system were analyzed to illustrate its advantagesover the previous ones, thereby reflecting the system’s main features and providing afoundation for the design of refrigeration system control strategy.Secondly, to eliminate the adverse effect brought by coupling and hysteresis to therefrigeration system, the refrigeration system internal model decoupling controlstrategy was designed. In the process, the online identification was made on thesecond order linear mathematical model with time delay and analysis was made tofind out whether the mode was steady state non-singular matrix and whether themodel inverse has complex Right Half Plane (RHP) poles. According to the aboveanalysis, the Internal Model Control structure was established and the ability ofdecoupling was considered. Through the application of the minimum superheat curve,superheat set point was optimized, thereby improving the internal model decouplingcontrol strategy. With the robustness analysis, the conditions were provided for therobust stability guaranteed by the refrigeration system strategy. In the simulationexperiment, under the condition of joining the step input signal and the stepdisturbance signal, comparison was made among three control strategies, the internalmodel decoupling control strategies, dual-loop PID control strategy and Smithpredictor decoupling control strategies, thus showing that the refrigeration internal model decoupling control strategy has the advantage of suppressing disturbance anddecoupling variables. In the above simulation assumptions, with the addition of threeuncertainty, comparison was made once more among the three strategies in terms ofthe refrigeration system output response curve, which verified that the refrigerationsystem decoupling control strategy is superior to other strategies in aspect of theimprovement of the robustnessFinally, sixth-order non-linear model was used for the design of the refrigerationsystem non-liear internal model control strategy, as the linear model adopted by therefrigeration system internal model decoupling control strategy cannot completelyreflect the complexity of the cooling system. In the procedure, the model wassimplified to sixth-order nonlinear affine form, and then through the use of feedbacklinearization method it was transformed into a pseudo linear form. On this basis, aninternal model control structure was built to complete the nonlinear internal modelcontrol strategy design. Theoretically, through the analysis of system performance androbustness analysis, the effectiveness of the strategy was not only verified, but theconditions for robust stability were also provided. Meanwhile, the simulation hasproved that the strategy has a strong decoupling capability, noise immunity androbustness.