Nonlinear Disturbance Observers: Design and Applications to Euler?Lagrange Systems

The estimation of unknown inputs and/or disturbances has been a topic of constant interest in control engineering for the past several decades; see 1 for a detailed survey on the development of unknown input observers. Disturbance observers (DOBs) are a special class of unknown input observers that were introduced for robust motion-control applications in the early 1980s 2. Since their introduction in the literature, DOBs have been employed efficaciously in numerous robust control and fault-detection applications, including process control 3, 4 and mechatronics 5-9. Figure 1 depicts a DOB employed in a typical control system. The DOB is used to reconstruct, from the measured output variables and the known control inputs applied to the system, unknown disturbances that are acting on the plant. The output of the DOB, namely, the estimated disturbance, can then be used in feedforward compensation of disturbances or faults.