Robust and Decoupling Approach to PID Control of Vapour-compression Refrigeration Systems

Abstract

This paper presents a combined approach to the design of PID control applied to vapour-compression refrigeration systems. The underlying controller consists of two decentralised PIDs, aiming to control both the reference on the temperature of the evaporator secondary fluid, imposed by the cooling demand, and the degree of superheating, by manipulating the compressor speed and the expansion valve. A partial decoupling matrix is calculated to reduce the high coupling between the controlled variables, while the PIDs are tuned applying affine parameterisation and considering the condition for robust stability given by the study on the uncertainty sources and the estimation of the uncertainty region around a nominal linear model. The designed controller is applied to the system proposed in the Benchmark process challenge and some comparative simulations are presented and discussed, while the performance indices of the proposed controller with respect to the reference one are analysed.