PCB Integrated Differential Current Slope Measurement for Position-Sensorless Controlled Radial Active Magnetic Bearings

This paper discusses a current slope measurement principle for self-sensing active magnetic bearings (AMBs). So called sensorless or self-sensing principles avoid external position sensors in the AMB and use the inductance dependency on the rotor position of the AMB itself to determine the rotor position for feedback control. By the help of a transformer core a combined evaluation of both opposite AMB coil inductances is possible. This differential approach determines the sensorless rotor position with high accuracy which allows a robust self-sensing AMB operation. The disadvantage of applying a wound transformer core can be overcome by a novel planar coil arrangement integrated in the printed circuit board of the AMB inverter. Different yoke materials for an optimal flux closure of the novel current slope sensing unit are investigated regarding to dynamic response and a high position sensitivity. Finally, experimental results of high permeable materials with well high frequency properties as ferrites or SMC confirm the sensing principle and allow a position-sensorless control of a three phase AMB.