Relevance of actions and measurements in control performances

In this paper, the relative relevance of input and output variables in achieving the control performance is analysed. In order to compare different control structures, the performances of the closed loop controlled system are expressed by the poles' position. Attached to each input (output), the control effort (control contribution) is defined as the inverse of the maximum admissible delay in its path without provoking instability. This delay, that can be roughly estimated by the shift of the open-to-closed loop poles under the variable connection, is defined as the variable relevance in the control structure. As digital control implies open-loop control in the intersampling time interval, there is always a time delay in the updating of the control. Small time delays in the most relevant variables may result in unstable behaviour.