Issues in identification, validation in closed-loop and control design

Identification in closed-loop is unavoidable in the case of plants unstable in open-loop, like is often the case in applications (integrator behavior, for example). There are a lot of theoretical results available on identification in closed-loop, but from the practical point of view, there are still interesting aspects to be explained. These aspects allow to obtain better models for control. Model validation for control is a recent topics, and it is shown in this paper that the output sensitivity function plays an important role.