Inductive Sensing System for Active Magnetic Suspension Control

Inductive sensors discussed in this paper are divided from a magnetically suspended body by a thin wall made from non-magnetic stainless steel. Therefore the carrier applied to the sensors is set to a relatively low frequency, but an influence of eddy cuirents on the metal wall can not be neglected. The low-frequency carrier produces a deterioration of the frequency characteristics in the inductive sensing. In addition, the eddy currents cause a decline of the sensor's sensitivity. Such undesirable circumstances are somewhat mitigated by a synchronous detection system proposed here. This system can perform the good sample-hold operation with twice of the carrier frequency in principle. The performances as well as the frequency characteristics are examined using a tested device. The analysis may bring us an idea of digital self-synchronous detection newly.