Estimating structural deformations for inferential control: a disturbance observer approach

Increasingly stringent requirements for motion systems lead to a situation where the positioning performance can often not be measured directly and therefore has to be estimated. A typical example is a wafer stage, where the performance is desired at the point-of-exposure on the wafer but the sensors are located at the edge of the wafer stage. Increasingly stringent performance requirements necessitate taking structural deformations, caused by actuation or disturbance forces, into account. The aim of this paper is to develop a disturbance observer approach including an observer relevant model identification approach and to experimentally validate this approach on a prototype motion system. The experimental results confirm that the proposed disturbance observer approach leads to an improved estimation of the unmeasured performance variables