Yet Another Tutorial of Disturbance Observer: Robust Stabilization and   Recovery of Nominal Performance
  This paper presents a tutorial-style review on the recent results about the disturbance observer (DOB) in view of robust stabilization and recovery of the nominal performance. The analysis is based on the case when the bandwidth of Q-filter is large, and it is explained in a pedagogical manner that, even in the presence of plant uncertainties and disturbances, the behavior of real uncertain plant can be made almost similar to that of disturbance-free nominal system both in the transient and in the steady-state. The conventional DOB is interpreted in a new perspective, and its restrictions and extensions are discussed. 
Hyungbo Shim, Gyunghoon Park, Youngjun Joo, Juhoon Back, and Nam Hoon   Jo
cs.SY math.OC