Modelling and control of a 2 D.O.F. high-precision positionning system

We present a hierarchical controller for a set of two linear synchronous motors with permanent magnets moving in two orthogonal directions for a two degree of freedom high-precision positionning system developed by Newport-France. This system is supposed to realize relatively fast linear displacements from 1 count (4 nm) to 400 mm. A linear displacement of 25mm must be achieved in less than 0.2 s with maximum acceleration of 1 g and current and voltage limitations, and with an error in the final position less than 1 count. A nonlinear model of the plant is obtained, consisting of two independent subsystems, representing the two linear drive dynamics. Flatness based motion planning and tracking design are proposed. Each motor´s tracking controller is then realized in a hierarchical way by a high level force control loop and a low level current loop.