Capability of ADRC for minimum-phase plants with unknown orders and uncertain relative degrees

The capability of ADRC is studied for linear time-invariant SISO minimum-phase systems with unknown order and unknown relative degrees. It is proved that ADRC can achieve stable tracking and zero-frequency gain being 1 for the plants with unknown but bounded relative degrees. And the knowledge of the sign of GAC is proved necessary to assure the close-loop stability. Then, it is demonstrated by numerical examples that one ADRC with fixed parameter is able to control several plants of different orders, relative degrees and parameters.