Nonlinear Disturbance observer design for Euler-Lagrange systems: An initial study

Recently, Disturbance observer(DOB) has been investigated a lot. But it has a defect that it cannot be applied to nonlinear systems. This paper proposes a novel Nonlinear Disturbance observer(NDOB). NDOB consists of internal loop that generates an auxiliary control input that compensates external disturbances. The auxiliary input inversely satisfies nonlinear H_∞ optimality and existence of the controller guarantees input-to-state stability of internal loop. Simulation has been performed to verify proposed observer and shows an expected result.