New Results on the Robustness of Selfsensing Magnetic Bearings

This paper shows the design and new results of a self-sensing homopolar active magnetic bearing (AMB). The considered magnetic bearing provides the bias flux using permanent magnets. Only the control flux for the stabi- lization of the system is provided by electro-magnets. This so called hybrid magnetic bearing (HMB) minimizes the energy losses and shows a high linearity. Therefore, this structure is a good choice for industrial applications which uses position sensors for closing the control loop. However, the self-sensing control of such bearing types require a position dependent inductance. Many homopolar structures share the same flux at two opposite poles and the coils of these poles are connected in serial. Hence, the inductance is almost independent of the rotor position. The proposed bearing type shows a way where also a hybrid homopolar magnetic bearing can be controlled with- out external position sensors. The self-sensing method is based on inductance measurement by applying voltage pulses to the system. The current slope is measured directly using an induction ring. A differential evaluation of the current slopes increases the position dependent part of the signal and decreases the errors due to eddy currents and the leakage flux. The last part of the paper shows the measured sensitivity function of the system at standstill and rotational speed.