Realisation of two-degrees-of-freedom compensators

This paper presents two alternative realisations for two-degrees-of-freedom (2-DOF) compensators. The characterisation of 1-DOF and 2-DOF compensators is first presented in terms of the set of allowable designs: the advantages of a 2-DOF compensator over a 1-DOF are also reviewed. Despite these advantages, 2-DOF compensators are not used as expected. One possible reason being the lack of design methodologies for such compensators. This may be the reason the different approaches found in the literature on 2-DOF compensator design are mainly based on optimisation procedures. The principal aim of the paper is to provide a framework for 2-DOF design where well-known classical design methods apply. Two design examples are presented. The first one presents a PID-based 2-DOF compensator to show the application of the realisation in a classical setting. The second example shows how the step response of an existing feedback controller can be improved