Autotuned fuzzy-proportional-integral-derivative controller for cascade load-sharing compressor

Abstract

This thesis focuses on a control scheme of a load sharing parallel compressors commonly utilised in process industries. Typical control systems for parallel compressors in the industry, consist of a pre-set anti-surge control and loading-sharing control scheme. In this parallel compressor system, the characteristics of individual compressors are never the same, i.e. a stronger compressor will tend to take in more flow, causing the other compressor to lose its compressibility and causes it to fall into a surge region. The challenge occurs due to the proven nature of the whole system which exhibits control interactions between the components. Therefore, in this thesis, an optimised double feedback control i.e. a self-tuner fuzzy cascade PID controller and automatic self-tuning fuzzy PID antisurge controller, is proposed. The implementation of PID controllers is due to its simplicity and robustness, and its wide applicability in the existing process industry. On the other hand, adapting fuzzy tuners on PID controllers is proven can improve the performance of the conventional PID, and since fuzzy logic control box can be embedded to an existing distributed control system (DCS) in the process industry, the implementation is imminent. For the proposed controller, the cascade PID controller can preserve the linear structure of the PID. Meanwhile, the fuzzy logic controller able to continuously tune each of the PID parameter to adapt to rapid changes of the load sharing and surge control of the parallel compressors. The performance of the proposed controller design is evaluated with respect to the conventional cascade PID controller. The performance evaluation is performed in terms of the system’s transient response i.e. rise time, overshoot and settling time as well as the system’s stability using Simulink. The simulation results showing that the proposed fuzzy logic cascade PID controller can respectively achieve 72%, 88.2% and 92.9% improvement in reducing rise time, settling time and percentage overshoot as compared to the conventional cascade PID. From the results obtained, it can be concluded that the proposed controller design has a better adaptation and performance on the parallel load sharing compressors compared to the conventional cascade PID control.