The structures and features of control system are discussed for active magnetic bearings (AMBs) with frequency type of displacement sensors. The control systems are classified according to the signal conversions performed there. One of them uses a counter circuit for converting the frequency to a digital format, and the converted signal for digital signal processing. It is also pointed out that self-sensing operation can be realized by using hysteresis amplifiers because their switching rates are sensitive to load impedance. A digital self-sensing AMB, which uses a counter circuit for interface and hysteresis amplifiers for energizing the electromagnets, is manufactured for experimental study. The experimental results show that accurate suspension can be achieved with the proposed digital control system.

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