Nonlinear Disturbance Observers Design And Applications To Euler-Lagrange Systems

Estimation of unknown inputs and/or disturbances has been a topic of constant interest in the control engineering for the past several decades. Disturbance observers (DOB) are a special class of unknown input observers that were introduced for robust motion control applications in the early 1980s and have found numerous industrial applications since then. In this article, a type of nonlinear disturbance observer (NDOB) structure is investigated and some properties of this NDOB class such as semi/quasi-passivity property, which have not been discussed in the previous NDOB literature, are presented. We show that a previous NDOB, which was first proposed for serial robotic manipulators, can be used for the more general class of Euler-Lagrange systems. Moreover, we show how the proposed NDOB can be used along with well-established control schemes such as passivity-based control. Finally, the proposed NDOBs are incorporated into the framework of the 4-channel teleoperation in order to achieve full transparency in the presence of unknown disturbances.