Multivariable robust control of an air-handling unit: A comparison between pole-placement and H∞ controllers

Control of air-conditioner units is essential to achieve satisfactory comfort conditions in buildings while keeping low energy consumption and operation costs. In this paper, a nonlinear multiinput & multioutput model (MIMO) of an air-handling unit (AHU) is considered. After linearization of the state equations around the operating points, an observer and a regulator are designed for the estimation of state variables and disturbance rejection, respectively. A realistic uncertain model is considered which constitutes various uncertainties associated with complex nonlinear model of AHU. In the presence of model uncertainties, a H∞-robust controller is designed to guarantee robust performance of the air-handling unit. In addition, a simple controller based on pole-placement approach is developed. Indoor temperature and relative humidity are controlled via manipulation of valve positions of air and cold water flow rates. Achievement of tracking objectives is investigated through various desired commands of indoor temperature and relative humidity (including a sequence of steps and ramps-steps). According to results, the H∞-robust controller guarantees the robust performance of the AHU in tracking of desired set-paths (while using the simple pole-placement controller leads to high oscillatory behavior of the output variables and control efforts). Moreover, using H∞-robust controller results in less energy consumption in comparison with the pole-placement controller.