Optimal Feedback Control for HVAC Systems: An Integral Sliding Mode Control Approach Based on Barrier Function

Efficient control of air-handling units (AHUs) in heating, ventilating, and airconditioning (HVAC) systems is crucial for maintaining comfortable conditions while minimizing energy consumption. This study focuses on a multi-input multi-output (MIMO) control design for a nonlinear dynamic model of an AHU in a single thermal zone featuring variable air volume (VAV) properties in cooling mode. The goal is to develop decoupling controllers for the AHU by manipulating the airflow rate and cold water flow rate. An integral sliding mode control based on barrier function is proposed for regulating the humidity ratio of the thermal zone according to the desired characteristics. Subsequently, an integral sliding mode control based on barrier function is combined with an optimal feedback controller using a linear quadratic regulator (LQR) to manage indoor temperature. Additionally, an approximate classical sliding mode differentiator (ACSMD) is designed to estimate unmeasurable states that are used to construct the sliding variable of the second controller. The performance of the proposed control is evaluated through numerical simulations. Results demonstrate the ability of the controllers to guide the humidity and temperature of the thermal zone toward the required values without prior knowledge of the upper bounds on parameter variation, reducing chattering and yielding an optimal robust integral sliding mode control/LQR controller.