A graph approach to uniform observability of nonlinear multi-output systems

The sufficient conditions for uniform observability of multi-output nonlinear systems (observability "for any input") are revisited. A uniform observability graph is defined on a "canonical" form of the system. The absence of cycle in this graph provides a sufficient condition of uniform observability. The synthesis of a high gain observer requires a supplementary condition. In this sense, these results are a refinement of those already known. Meanwhile, the graph approach makes easier the understanding of the uniform observability condition in the multi-output case.