Cascaded control of superheat temperature of an hvac system via super twisting sliding mode control

In this paper two separate cascaded control approaches to control superheat temperature, and length of two-phase flow in an evaporator of Heating Ventilation Air Conditioning Systems(HVAC) are presented. In the first approach, superheat temperature is controlled by inlet mass flow rate of the evaporator, while in the second approach, length of two-phase flow is controlled by outlet mass flow rate. Two inner loop and outer loop of the cascaded controller are designed using sliding mode along with feedback linearization. Superheat temperature, T_sh (first approach), and the length of two-phase flow, l (second approach) are controlled in the outer loop, and evaporating temperature of refrigerant, T_e, is controlled in the inner-loop for both approaches. To improve performance, a second order super twisting method has been utilized. The controller is equipped with a generalized super-twisting sliding mode observer to estimate the length of two phase flow of the refrigerant inside the evaporator. The robustness of the suggested control strategy against disturbance and parameter uncertainties is illustrated through simulation in MATLAB/Simulink environment.