This thesis presents controller parameter tuning using LOOP-PRO software and implements programmable logic controller (PLC) logic for Proportional-Integral-Derivative (PID) controllers in the cascade control of solution density, as part of a refinery distributed control system. Tests and responses for parameter tuning were obtained by recording PLC input and output signals with simulated refinery behaviour. An overview of the process control system is also provided, describing the interaction between sensors and actuators, the control logic, and the monitoring interface. This forms a broader picture of how the cascade PID structure fits within the complete process, from signal acquisition to final control element actuation.
The PID loops were implemented as part of the PLC code using PlantPAx software, Rockwell Automation’s process control library. This software also enabled supporting functions such as anti-windup protection, bumpless transfer, and output saturation handling, which are important for reliable operation in practice. During testing of the cascade control, the PID/PI structure gave similar results to the PI/PI structure, which was ultimately adopted as the primary structure. Solutions were selected so that the existing system will facilitate future improvements when the logic and control are transferred to the real refinery system.
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