Controlling the Process

Open-loop stability. By defult, most controllers assume that a finite change in the process input will have a finite effect on the process output. This is not the case for processes that are themselves unstable or for processes like motors that integrate their inputs.

Time Frame. No matter what curve fitting technique it uses, a controller can only consider a finite collection of input and output samples as it attempts to identify the process model. It has to assume that all of the behaviour it needs to see for modelling purposes is contained in that interval and that it is taking measurements fast enough to see it. If the process moves significantly fast or significantly slower than expected, essential data may be overlooked.

Inverse Response. Processes that have a non-minimum phase or inverse respose will react to a control action by first moving in the opposite direction before reversing course. This is a relatively rare phenomenon, so most controllers don't expect it. A controller faced with an unexpected inverse response will start back-pedalling only to find that the process had already changed course on its own.

This situation often leads to closed-loop instability.