  Magnetic levitation positioning technology has attracted considerable
research efforts and dedicated attention due to its extremely attractive
features. The technology offers high-precision, contactless,
dust/lubricant-free, multi-axis, and large-stroke positioning. In this work, we
focus on the accurate and smooth tracking problem of a multi-axis magnetically
levitated (maglev) planar positioning system for a specific S-curve reference
trajectory. The floating characteristics and the multi-axis coupling make
accurate identification of the system dynamics difficult, which lead to a
challenge to design a high performance control system. Here, the tracking task
is achieved by a 2-Degree of Freedom (DoF) controller consisting of a
feedforward controller and a robust stabilizing feedback controller with a
prescribed sparsity pattern. The approach proposed in this paper utilizes the
basis of an H-infinity controller formulation and a suitably established convex
inner approximation. Particularly, a subset of robust stabilizable controllers
with prescribed structural constraints is characterized in the parameter space,
and so thus the re-formulated convex optimization problem can be easily solved
by several powerful numerical algorithms and solvers. With this approach, the
robust stability of the overall system is ensured with a satisfactory system
performance despite the presence of parametric uncertainties. Furthermore,
experimental results clearly demonstrate the effectiveness of the proposed
approach.
