On the automatic tuning and adaptation of PID controllers

A simple approach to the automatic tuning of PID process controllers is proposed. Like the relay-based autotuner, its objective is to attain a design-point on the Nyquist diagram. By injecting sinewaves and employing a phase/frequency estimator, closed-loop adaptive tuning is possible and there is exact convergence to the design-point without the approximations of describing-function theory. The variant discussed here achieves a required phase margin and imposes a carefully chosen constraint on the controller parameters, leading to consistent behaviour for a wide variety of generic test-cases. A real-life demonstration on a non-linear flow rig is provided.