Controllability analysis and robust control of a one-stage refrigeration system

This paper describes the controllability analysis and the design of a multivariable robust controller for a one-stage refrigeration cycle, which is the most used system for cooling issues and whose energy management and control are key factors. The controlled variables are the superheating degree of refrigerant at the evaporator outlet and the outlet temperature of evaporator secondary flux, whereas the control variables are the electronic expansion valve opening and the compressor speed. The system is considered therefore as a MIMO one. Up to eight different operating points are identified in order to design the multivariable H∞H∞ controller based on the S/KS/T Mixed Sensitivity Problem. Simulation results comparing this controller to a decentralised PID and to a multivariable predictive one are shown, proving a better performance of the robust controller as well as accomplishment with the controllability analysis conclusions.