A Precise Force Measurement in Magnetic Bearings for Diagnosis Purposes

The aim of the presented project sponsored by the DFG (German Research Council) is to use active magnetic bearings for the diagnosis of centrifugal pumps. Therefore, active magnetic bearings are integrated in a single-stage respectively four-stage pump substituting the conventional ball bearings. The diagnosis procedures are based on evaluating frequency response functions where the active magnetic bearings operate as actuators and as sensors to measure the displacements and the forces induced in the rotating structure. Simulation results of an exemplarily fault applied to the pump system are presented. For this type of diagnosis using transfer functions, an accurate force measurement is crucial. The paper compares force measurement results and achievable accuracies of a radial magnetic bearing using different measurement techniques over a large operating range. Furthermore, a finite-element model is used to investigate fringing and leakage effects to improve the different measurement techniques. The finite element model allows eccentric rotor positions and non-syrmnetrical coil current distributions with nonlinear material properties of the stator and rotor part of the magnetic bearing.