Novel observer based force control for active magnetic bearings

State of the art active magnetic bearings are controlled using a current reference, a current observer or the current as a state variable in a state-space controller. Using the current to express the state of a magnetic bearing has several disadvantages such as an unstable zero, hidden information such as stray flux, eddy currents, magnetization or hysteresis of the iron core material. Above all, it is in a nonlinear relation with both the generated force and the air gap. In this paper a novel method is presented to directly and linearly control the force of a magnet using a force controller that is based on a flux estimator. The method is verified with simulations as well as with measurements. It decouples the magnet and position controllers, eases the modeling of non-ideal parameters of magnetic bearings, reduces noise, has a larger range of displacement and behaves intrinsically linear.