Robust on-line relay automatic tuning of PID control systems

In this paper, a robust on-line relay automatic tuning method for PID control systems is developed which expand on the application domain of Astrom's renowned relay autotuning method. In the proposed configuration, a relay is applied to an inner loop of a controller-stabilised process in the usual manner. Using the induced limit cycle oscillations from the closed-loop system, the controller settings may be re-tuned non-iteratively to achieve enhanced performance without disrupting closed-loop control. Two control tuning methodologies are developed -- a direct and an indirect method based on an explicit process model. Simulation examples and a real-time experiment are provided to illustrate the practical appeal and potential advantages of the proposed method over the basic one.