Development of a Multi-mode Adaptive Controller and Investigation of Gain Variations with Speed and Balance Changes
Type of DegreeMaster's Thesis
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Magnetic bearings offer a number of advantages over conventional rolling element bearings. Magnetic bearings provide support for rotating systems through magnetic levitation rather than by mechanical contact, nearly eliminating the energy losses attributable to friction in standard bearings. Low power consumption is one characteristic of magnetic bearings that has encouraged their use in an increasing number of applications. Another is the ability to use the bearing itself as an actuator in a controller that can alter the orbit of the rotating system within the bearing to reduce or eliminate the detrimental effects of disturbances acting on the system. In addition, controller outputs can potentially be used as an indicator of the general health or integrity of the system. This work details the development of a multi-mode adaptive controller for a magnetic bearing system that is capable of suppressing disturbances acting at synchronous and asynchronous frequencies and caused by rotating imbalances and base motion. The work was based on an existing adaptive controller that formed part of the overall control system for a well sorted and well developed magnetically suspended rotor and flywheel. The development of the controller made extensive use of system modeling techniques and model-in-the-loop simulations. Development also required continual refinement of the system model and on-going reconfiguration of the operating environment since the ever increasing complexity of the controller often exceeded the real-time capabilities of the processor. The modes of the controller, or the methods used by it to determine the frequency of the disturbance acting on the system, include discrete Fourier transform, rotor speed and manual observation. The adaptive controller was shown to produce excellent disturbance rejection and vibration suppression in all of the three modes. The capabilities of the controller operating in the first mode were demonstrated with simulated disturbances and in the second and third modes with software simulations, simulated disturbances and physical changes in the balance of the rotor and flywheel. iii This work also details the efforts to evaluate the predictive capability of adaptive controller gains. The correlation between gain variations and balance state has been demonstrated, but a repeatable and unambiguous response of the gains to a synchronous disturbance undetectable by other means has not been well established. The sensitivity of the gains to variations in rotor speed increases the difficulty of this task. Software simulations of the adaptive controller operating in speed mode showed the potential of using the gains as an indicator of a change in the balance or health of the system, but actual tests conducted on the magnetic bearing system were not as encouraging.