Modeling and Control of Non-Laminated Active Magnetic Thrust Bearings

Non-laminated magnetic thrust bearings exhibit reduced dynamic performance, compared to laminated radial bearings, due to eddy current effects. Segmented thrust bearing stators have been introduced to increase actuator performance by disrupting eddy current paths in the same manner as laminations in radial magnetic bearings. However, due to manufacturing limitations, thrust bearing stators cannot be easily segmented to the extent that they would be considered fully laminated. Therefore, eddy currents continue to affect their dynamic performance significantly. This work aims to improve the performance of non-laminated thrust active magnetic bearings through improved modeling and control design.
Currently, accurate modeling of segmented stator performance relies on finite element analysis, which is a time consuming process. In this work, an analytic model of cylindrical segmented electromagnetic actuators, including eddy currents effects, is developed. The model is an extension of the analytic model for C-type electromagnetic actuators developed by Zhu et al., [1, 2]. Zhu's work on cylindrical magnetic actuators, [3, 2] is also continued in order to develop an analytic model for cylindrical electromagnetic actuators with a center hole, i.e. non-laminated active magnetic thrust bearings. All analytic models developed in this work are verified via finite element analysis.
Based on analytic and finite element modeling, it is found that thrust bearing stator segmentation results in dramatic improvements in dynamic performance. With six stator cuts, and depending on the specific geometry, bandwidth from current input to force output is typically improved by more than 2-fold.
The potential for dynamic performance improvements for non-laminated magnetic thrust bearings by non-linear and dynamic compensation is also investigated. Simulations using a detailed non-linear model suggest that compensation, combined with proportional-integral-derivative control, will improve performance consistency and disturbance rejection for non-laminated thrust bearings compared to performance with PID alone.