A simple PID controller for a magnetic bearing with four poles and interconnected magnetic flux

The conventional structures of active magnetic bearings to support rotors dispose of eight poles operating in such a way that two adjacent poles share the same magnetic flux. In this work, a different electromagnetic structure is presented which is based on the so named bearingless motor with split windings. While this device is originally supplied by alternate electrical currents, in this case, as the purpose is to generate only radial forces, the windings are supplied by continuous currents forming a magnetic flux with four equivalent poles. To test this approach, a conventional induction motor was used and its rotor was appropriately modified. It will be shown that although the magnetic fluxes are coupled, for low speed rotation, all the controlled plant can be modeled as an unstable second order system, so PID controllers can be appropriately designed. In order to test this approach, a workbench consisting of displacement sensors, signal conditioners and two microcontrollers development boards was implemented. For different radial loads and changes of displacement references, the experimental results were well succeeded.