Smith predictor-based nonlinear internal model control for water temperature regulation of combi-boiler systems

This paper proposes a reliable and practical water temperature control scheme for combi-boiler systems working in domestic hot water mode. In practice, the water temperature regulation process of a combi-boiler is strongly affected by the nonlinearities of actuators, the variation of system parameters caused by the fluctuation of water flow rate, external disturbances, and large, variable time delays. Although there are some results that can address one or two of these factors, a comprehensive and concise control scheme with consideration of all these factors is lacking. This paper aims to change this situation. First, based on the modeling and identification of real combi-boiler systems, the feedback linearization method is adopted in the controller synthesis to solve the effects of actuator nonlinearity. Next, a Smith predictor-based control structure is designed to eliminate system time delays. Finally, based on the internal model control (IMC) theory, an IMC-based parameter tuning method is proposed for the parameter selection of feedback controllers, in which the model parameters used in the controller synthesis are calculated in real-time based on the system identification results. As a result, the influence of model uncertainties and external disturbances can be effectively attenuated, and only one parameter is required to be tuned for the desired performance. The feasibility and effectiveness of the proposed control scheme are verified by simulations.