New results on the controllability and observability of general composite systems

Using Rosenbrock\´s frequency domain approach [1], necessary and sufficient conditions are obtained for a general series-parallel or feedback composite system to be controllable or/and observable. The conditions are given in terms of the state equations of the composite system and are very simple to compute. Using these results, it is shown that a general series-parallel feedback cascade system is controllable or/and observable for "almost all" interconnection matrices, provided some mild conditions on the subsystems are imposed.