Fractional Order Flux Observer for Thrust Bearing Control under the Influence of Eddy Currents

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The present aspiration towards sustainable industry solutions further increase the impact of oil- and maintenance-free magnetic bearings. Decreasing costs for the required semiconductor electronics boost the technology’s attractiveness and lead to innovative new applications. A particular challenge is the highly dynamic control of magnetic thrust bearings, as used in e. g. modern tool spindle drives. Design restrictions prohibit to laminate the magnetic cores, which is why the force dynamic is highly impaired by the magnetic skin effect caused by the eddy currents. The problem was solved in the first project phase with the development of a fractional-order flux estimator to complement the conventional decentralized and cascaded position control. The estimator uses the measured coil current to calculate the force-related flux in real-time and therefore compensate the impairment originating from the eddy currents.The second project phase aims to extent the range of application of the flux estimator to state controls and observers. Although they are known in research for many years, their practical significance emerges only now due to the increasing availability of fast FPGA controllers. It is essential to consider the fractional system behavior modeled by the flux estimator in the beginning of this new trend. The estimator’s implementation is carried out with an efficient biquad-filter cascade, but the preceding offline calculation process is cumbersome. It requires to employ arbitrary-precision arithmetic and additional know-how to avoid the coefficient-quantization errors in the occurring high-order polynomials. That is why a new calculation process has to be developed, exclusively based on numerically uncritical transfer functions in a factorized pole-zero form. Using the new findings and an optimized experimental setup, the extended dynamic limits of magnetic thrust bearings shall be redefined.