Inferential control of a wafer stage using disturbance observers

In high-precision motion systems it is often not possible to directly measure at the location where performance is required. Therefore, performance variables need to be inferred from non-collocated sensor measurements. If flexible behavior is negligible, a static sensor transformation is used to find a rigid-body (RB) approximation of the performance variable. However, for next-generation motion systems positioning accuracy is ever-increasing, leading to a situation where flexible dynamics are not negligible. As a result, traditional single degree-of-freedom (DOF) controllers are inadequate [1]. The aim of this research is to control the unmeasured performance variable while taking flexible behavior into account, through 2-DOF controller structures and disturbance observers.