  This work deals with the problem of designing disturbance decupled observers
for the estimation of a function of the states in nonlinear systems. Necessary
and sufficient conditions for the existence of lower order disturbance
decoupled functional observers with linear dynamics and linear output map are
derived. Based on this methodology, a fault-estimation scheme based on
disturbance decoupled observers will be presented. Throughout the paper, the
application of the results will be illustrated through a chemical reactor case
study
