Robust control of HVAC plant I: Modelling

A simulation model of a discharge air temperature plant is developed using SIMULINK within the MATLAB environment. Step response results from an open-loop form of this model are correlated with experimental data with good agreement. Settings for a fixed-parameter PID controller are `benchmarked' from manufacturers' recommendations and optimized at local plant operating conditions using cost-function minimization. A closed-loop form of the plant model is simulated using these controller specifications, revealing significant lack of robustness due to plant nonlinearity when the manufacturers' recommendations were applied. The locally optimized PID controller performed well, but this would require extensive understanding of plant characteristics to be achieved in practice. In a second part of this paper, a method is developed that achieves robust control for a wide variety of plant operating conditions based on simple a priori data about the plant.