Passivity framework for nonlinear state observer

We solve the general problem of reconstructing the state of a plant only with the input and output information of the plant, referred to the observer problem. Our strategy of observer design is the output feedback passification to the error dynamics. In order to describe the passivity of the error dynamics effectively, the standard passivity is reformed for the augmented error dynamics which also includes the plant dynamics. The proposed framework includes the precise definition of Passivity-based State Observer (PSO) and the design scheme of PSO. It is also shown that a PSO has its potential robustness to the measurement disturbance and that the framework of PSO unifies the earlier works in the literature.