Investigation on the Applicability of Active Magnetic Bearings to High Speed Spindle Design
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A novel concept applicable to the control of spindles at high speed is developed by using active magnetic bearings (AMBs) that are non-contact and of low vibration. Extensive literature reviews explicate that the broad applications of AMBs are severely hampered by the incomplete description of the underlying electro-magnetic-mechanical dynamics. The thesis considers the gyroscopic effect inherent of a flexible rotor and explores the geometry coupling of the electro-magnetic actuators to the formulation of a comprehensive nonlinear AMB-rotor model. The model provides the basis for the creation of a novel time-frequency control algorithm whose derivation requires no linearization or mathematical simplification of any kind, thus allowing the model system to retain its true fundamental characteristics. Unlike proportional-integral-derivative (PID) controllers that are dominant in most if not all AMB configurations, the controller developed for the research is inspired by the wavelet-based nonlinear time-frequency control methodology that incorporates the basic notions of online system identification and adaptive control. Wavelet filter banks and filtered-x least-mean-square (LMS) algorithm are two of the major salient physical features of the controller design, with the former providing concurrent temporal and spectral resolutions needed for identifying nonlinear states of motion and the latter ensuring the dynamic stability of the AMB-rotor system at all operating speeds subjected to the presence of external disturbances. It is shown in the thesis that the vibration of the rotor is unconditionally controlled by maintaining the mandatory 0.55 mm air gap at 150,000 and 187,500 rpm subject to a tight spatial constraint (tolerance) of the order of 0.1375mm. System responses with and without considering the gyroscopic motion and geometry coupling are studied to demonstrate the negative impact on misinterpreting the AMB-rotor dynamics when the two effects are neglected. The case of an impact of 5,000m/s2 in magnitude and 0.001 seconds in duration at 187,500rpm is also investigated to establish the robustness of the controller design. The time responses of all the cases considered are both temporally bounded and spectrally bandwidth-limited, thus demonstrating the effectiveness of the wavelet-based time-frequency controller design in mitigating the inherent instability of the AMB-rotor system at extreme speeds.
Liu, Mengke (2015). Investigation on the Applicability of Active Magnetic Bearings to High Speed Spindle Design. Master's thesis, Texas A & M University. Available electronically from