Position estimation in magnetic bearings using inductance measurements

A signal processing technique is presented by which the position of a rotor supported in magnetic bearings may be deduced from the bearing current waveform. The bearing currents are presumed to be developed with a bi-state switching amplifier which produces a substantial high frequency switching ripple. The amplitude of this ripple is a function of the power supply voltage, switching duty cycle, and bearing inductance. The inductance is predominantly a function of bearing air gap or, equivalently, rotor position while the duty cycle is primarily a function of developed bearing force. Ideally, the sensor signal processor should exactly extract the rotor position information while perfectly reflecting the bearing force information. When the bearing is a perfect inductor, these functional relationships are easily established and the gap dependence is monotonic. Since the voltage and duty cycle are both easily measured, the relationships can be inverted with a non-linear parameter estimator to extract the rotor position. One method of implementing this estimator is presented and its performance is evaluated by simulation. The method is demonstrated to produce a fairly wide bandwidth sensor with acceptably low feed-through of the bearing force.