I. Introduction
For a correct control of a system it is necessary to know its behaviour to set the controller accordingly. There are several characteristics that may be measured to describe the system’s behaviour, such as the static characteristics and step response. Desirably, the system can be controlled while making these measurements, but that is not possible. To achieve at the same time a continued control of the system and description of its behaviour, the methods of relay feedback identification [1], [2] may be used.