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Analysis and auto-tuning of supply air temperature PI control in hot water heating systems
Proportional and integral (PI) control, which is designed for a linear control system, is the most widely used control in HVAC systems. However, HVAC systems are nonlinear systems. Many efforts are made to tune the PI control loop. In practice, a lot of PI loops are still poorly tuned. A poorly-tuned loop affects indoor comfort, wastes energy, and increases maintenance cost. Manual tuning a PI loop is cumbersome and time-consuming, and cannot accommodate changes in system dynamics. Therefore, autotuning PI control loops based on the non-linearity of controlled systems is imperative. ^ The objective of this study is to develop easy-to-use and model-identification-free auto-tuning algorithms for the supply air temperature control system using a hot water coil. To achieve this goal, a thorough investigation is conducted on such temperature control systems. The ratio of the P gain to I gain is deduced through model reduction. ^ The characteristics of each component in a typical temperature control system and their impacts on the control system dynamics are also analyzed. The theoretical analysis demonstrates that the supply air temperature control system is a purely nonlinear system when the coil overall heat transfer coefficient is not constant. ^ This study finds that the controller gain ratio is associated with the valve position when the loop pressure is constant or known. The relationship between the valve position and controller gain ratio are developed for different types of valves to link system operations with controller tuning. ^ Three tuning algorithms, one-dimensional, two-dimensional and integrated tuning, are provided. One-dimensional method uses a fixed controller ratio to tune the loop. Two dimensional tuning are developed based on the Hook-Jeeves pattern search method. Integrated tuning method makes use of the relationship between the controller gain ratio and control valve position to integrate system operations with control loop tuning to overcome the disadvantages of the one- and two-dimensional tuning methods. Implementation of the algorithms provides satisfactory tuning results. Especially, the integrated tuning method is superior to other methods. ^ The analysis and the implementation of this study demonstrate that integrating system operations with controller tuning is the best solution on auto-tuning PI loops.^
Zheng, Bin, "Analysis and auto-tuning of supply air temperature PI control in hot water heating systems" (2007). ETD collection for University of Nebraska - Lincoln. AAI3243742.