Improved relay autotuning using normalized time delay

The relay autotuner provides a simple way of finding PID controllers of sufficient performance. By using an asymmetric relay function the excitation of the process is improved. This gives better models, and hence a better tuning, without increasing the time consumption or complexity of the experiment. Some processes demand more accurate modeling and tuning to obtain controllers of sufficient performance. These processes can be singled out by their normalized time delays and be subject to further modeling efforts. The autotuner proposed in this paper provides a simple way of finding the normalized time delay from the experiment, and uses it for model and controller selection. The autotuner has been implemented and evaluated both in a simulation environment and by industrial experiments.