Title: Lower limit on controller gain for acceptable disturbance rejection
Author: Sigurd Skogestad

Presented at IFAC-symposium Adchem'2003, Hong Kong, Jan. 2004 (original: June 2003).

Abstract:
The objective of almost all controller tuning rules found in the literature, going
back to the classic PID rules of Ziegler an Nichols (1942), is to get the
``fastest'' possible closed-loop response, subject to maintaining stability
with reasonable robustness margins. This gives a maximum limit on the
controller gain. In practice, however, we often want control to be as smooth
and ``slow'' as possible, subject to satisfying some minimum performance
requirements. This gives a minimum limit on the controller gain, and the goal
of this paper is to derive this minimum limit, when the performance
requirements is to achieve a specified level of disturbance rejection.
Together with the more traditional tunings rules 
this results in a range  for the acceptable controller gain.