Classification of two-degree-of-freedom underactuated mechanical systems

A new classification for two-degree-of-freedom mechanical systems is introduced, based on structural control properties. The largest feedback linearisable subsystem with internal stability is considered for such systems. The classification is depending on the closed-loop system that can be achieved under the constraint of internal stability. It is shown that the generalised conjugate momenta corresponding to each degree of freedom play a crucial role for the considered mechanical systems.