Controlling the Speed of a Magnetically-Suspended Rotor with Compressed Air
Type of Degreethesis
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Magnetic bearing research is generally concerned with the development of the automatic control systems required to levitate a rotor, driven by either an electric motor or air turbine. The regulation of rotor speed in research has often been accomplished by manual methods, turning a rheostat in the case of an electric motor and by adjusting a regulator or valve if the drive is by an air turbine. Automatic control of rotor speed would facilitate research using magnetic bearings. This control would assist the development of adaptive disturbance rejection techniques since rotor speed could be easily adjusted for any change in the desired rejection frequency. Automatic speed control could also be central to health and containment strategies for magnetically suspended flywheels used in the control of space structures. If cracks are detected in a flywheel through health monitoring, the speed of the rotor/flywheel could be automatically reduced to a level where the damaged flywheel could be temporarily operated. This work details the development and implementation of a control system to automatically and precisely regulate the speed of a magnetically suspended rotor and flywheel. Development began with the installation of an electronic flow control valve and all instrumentation needed to measure rotor response. Once all hardware was in place, a Simulink model of the entire system, actuator (electronic valve) and plant (air turbine, magnetic bearings, rotor and flywheel), was created. This model was developed using system identification techniques where a step input is applied to the plant and its response is measured. A transfer function of the plant was derived from these tests, and it relates volumetric flow rate of air to rotor speed and uses variable coefficients. The operation of the electronic valve was too complex to be described by differential equations or transfer functions. Thus, a model of it was written in software and included in Simulink using an embedded MATLAB function. The Simulink model was then used to develop a speed controller for the simulated system. Simulations established the type of controller and its gains. A proportional-derivative or PD controller was found to accurately regulate the speed of the simulated system. When complete, this controller was combined with the actual magnetic-bearing controller to create the software necessary for bearing and speed control. This software was then executed on dSPACE hardware to provide overall control of the system - actuator, bearings, rotor and flywheel. Numerous tests were conducted on this system to tune the gains of the speed controller. Once tuned, the PD controller developed in the simulated environment worked exceptionally well on the real system. The controller can maintain the actual speed of the rotor to within a few rpm of the desired for speeds ranging from 200 to over 6000 rpm. The final part of this work involved developing instrument panels from which to operate the bearings and control the speed of the rotor. Instrument panels similar to those found in automobiles were created using ControlDesk, an integrated software development environment provided with dSPACE. A series of panels were designed and created so that variables necessary to operate and precisely control the bearings and rotor could be monitored and easily adjusted.