Advanced process control and relay auto-tuning

Abstract

Advanced control schemes are necessary in many industrial control problems although the PID control remains a control strategy that has been successfully used over the years. A particularly interesting technique in the automatic tuning of PID controllers is due to Astrom and co-workers who successfully used a relay feedback technique in the development of the so-called auto-tuner for the PID controller. Motivated by the Astrom and co-workers, in this thesis, particular attention is devoted to the relay feedback method and it's application to several advanced control fields, such as identification of process critical point with an improved accuracy, assessment of robustness in the frequency domain, controller tuning method based on the assessment and finally for the control of nonlinear plant. From the simplicity and practical viewpoints, this thesis has contributed to improve the original relay feedback method. The proposed technique improves the accuracy of the conventional approach by boosting the fundamental frequency in the forced oscillations, using a preload relay which comprises of a normal relay in parallel with a gain. Robustness is one of the major design objective to achieve for control systems functioning under harsh practical conditions. In this thesis, the basic relay feedback approach is modified for the assessment of robustness in control systems. In practice, most processes are nonlinear and using only PID controller, it is very difficult to control a plant which is nonlinear to give good performance. In view of this, the thesis proposed two approaches for the tuning of PID controller for nonlinear system using relay feedback approach. The relay continues to be used in the control configuration, but in a new different way. The results presented in the thesis have very practical values as well as sound theoretical contributions. This is evidenced by numerous simulation examples and successful results from the real-time experiments conducted.