Active versus passive PI(D) loop monitoring

The argument is made for the use of active monitoring techniques alongside the conventional passive methods in the assessment of SISO control loops. By using a probing sinewave together with phase and gain estimators to detect changes in the frequency response, a performance index can be derived that compares favourably in sharpness to the popular Harris-type indices. Moreover the index is directional, in other words indicative of the way the controller should be adjusted to improve performance. The quickness of the scheme and the small level of the probing signal help dispel the common concern about active methods disturbing the process. Theoretical results showing the effectiveness of the technique are backed up by a demonstration on a plant simulator.