A magnetic bearing system using capacitive sensors for position measurement

A capacitive sensor system for use in a bearingless induction motor is presented. Details of the circuit design are presented, and a theoretical step response is calculated and compared with experimental results. It is shown that one-dimensional position stability of an induction machine rotor can be obtained by controlling the currents in diametrically opposed conventional stator windings. In the case considered, the weight of the rotor was supported by a state current of 33% of the rated value. A margin is therefore available for the addition of a rotating component of current. The rotor of this machine has been reduced during development of the position sensor. It is anticipated that the system could be implemented with a smaller airgap and consequently a smaller steady-state current. The position signal was satisfactorily obtained using a simple airgap-mounted position-sensing system. An approximate model of the nonlinear system is sufficient for predicting the closed-loop step response for small perturbations