Self-Synchronous Detection Method for Magnetic Suspension Digital Control

This paper deals with a detection method for picking up the amplitude component from a differential signal between two inductive type sensors in magnetic suspension control. The amplitude detection, which corresponds to measuring the displacement of the suspended body, is automatically completed in the software of a digital controller because the differential signal is synchronized with the sampling cycle and can be directly fed to the A/D converter. Such a digital synchronous detection scheme is embodied for a magnetic suspension device in which an electromagnet itself is used as an inductive type sensor. The inductive sensing is performed with a carrier current superimposed on the original exciting current of electromagnet, but the cartier frequency becomes much lower than that chosen for usual inductive type sensors. Thus a deterioration of the frequency response in the displacement measurement is inevitable. Besides, since the original exciting current itself varies the inductance of the electromagnet somewhat, a lowering of the measurement accuracy is inevitable as well. The present approach, which is named the serf-synchronous detection method, holds the response delay to a minimum and reduces the measurement error remarkably. As the result, stable control with a crossover frequency of 25 FIz is achieved with a carrier frequency of only 2 kHz.