Multivariable analysis and H∞ control of a one-stage refrigeration cycle

Abstract

In this paper the robust control of a one-stage refrigeration cycle is addressed. This is a MIMO process, where the expansion valve opening and the compressor speed are used as manipulated variables, whereas the control of both the outlet temperature of evaporator secondary flux and the superheating degree of refrigerant at evaporator outlet is intended. After the identification of the system at several operating points, a controllability analysis is carried out and a multivariable robust H∞ controller is designed, using the S/KS/T Mixed Sensitivity Problem approach. Simulation results concerning reference tracking, coupling measurement and disturbance rejection are included, comparing the H∞ controller to a decentralised PID one. Better closed-loop performance of the robust controller and reduced coupling between controlled variables are observed, as well as agreement with the controllability analysis conclusions regarding rise time bounds. A versatile experimental plant designed and set up by the authors is also described. As future work it is expected that identification and comparison between simulation and actual results can be achieved.