Theoretical Basics and Closed Loop Control Design of Stray-Flux-Based Measurement Systems for Magnetic Bearings

In this paper the theoretical basics of a measurement system are given in detail which uses magnetic stray fluxes for sensing the rotor position in magnetic bearings. Reluctance models for the most spread types of magnetic bearings are evolved to create the mathematic equations in the form of matrices. Based on this a description of the dependence of the magnetic stray flux that occurs and the air gap length respectively the rotor position by analytical and numerical calculations can be done. It will be shown that this behavior of stray flux depends on where it is measured. Additional the influence of the coil current and its transient behavior on this measurement system will be discussed. Furthermore the application in an eight pole heteropolar magnetic bearing will be presented. Therefore a signal flow diagram is given that clarifies in which way the measured HALL-signals should be processed to generate a position signal as input for an PID-controller that provides a stable levitated rotor in the bearing center. Last but not least the courses of the stray flux based position signals are presented compared with the signals of an auxiliary capacitive measurement system and approaches are proposed for a further improvement of this alternative measurement system using typically neglected magnetic stray fluxes.