Is Iterative Control Wasting Information?

Iterative identification and control design schemes improve performance by identifying better models on the basis of closed loop identification with former designed controllers. Once a new model is identified the former one is utterly disregarded for control design purposes. However former models can be more accurate for some frequency range that the new one. If this is the case we are throwing away useful already taken (i.e. at no extra cost) information. In this paper it is presented by means of an example that iterative control indeed can increase performance for a wider class of perturbations if former models are taken into account for controller design. Moreover a procedure to manage old and new controllers is sketched. The objective is to achieve monotonicity on the plant information.