Identification and control of multi-evaporator air-conditioning systems

Modern multi-evaporator air-conditioners (MEACs) incorporate variable-speed compressors and variable-opening expansion valves as the actuators for improving cooling performance and energy efficiency. These actuators have to be properly feedback-controlled; otherwise the systems may exhibit even poorer performance than the conventional machines which use fixed-speed compressors and conventional expansion valves. In this paper, feedback controller design for the MEAC system is first addressed through experimental identification. The identification produces a low-order, linear model suitable for controller design. The feedback controller employed is multi-input–multi-output-based and possesses a cascade structure for dealing with the fast and slow dynamics in the system. To determine appropriate control parameters, conditions that establish the stability for the cascade design are given. Due to the deficiency in control inputs, the proposed control structure exhibits steady-state errors in the superheat responses which in turn can produce unacceptable steady-state superheats. To resolve this issue, the reference superheat settings are determined via an optimization procedure so that the resultant steady-state superheats become acceptable. The control experiments indicate that the proposed controller can successfully regulate the indoor temperatures and maintain the steady-state superheat temperatures at acceptable levels.