Global adaptive observers for nonlinear systems via filtered transformations

The problem of determining global adaptive observers for a class of single-output nonlinear systems which are linear with respect to an unknown constant parameter vector is addressed. Sufficient conditions are given to observe asymptotically an equivalent state without persistency of excitation. Under additional geometric conditions the original state can be observed as well. The results obtained are based on a nonlinear change of coordinates driven by auxiliary filters (filtered transformations)