Adaptive observers for nonlinearly parameterized systems subjected to   parametric constraints
  We consider the problem of adaptive observer design in the settings when the system is allowed to be nonlinear in the parameters, and furthermore they are to satisfy additional feasibility constraints. A solution to the problem is proposed that is based on the idea of universal observers and non-uniform small-gain theorem. The procedure is illustrated with an example. 
I.Yu. Tyukin, P.A. Rogachev, H. Nijmeijer
math.OC