The bias magnetic field of the bias coil driven by bias current and small signal test current , results in the induced signal of the control coil . the terminal voltage of the control coil is detected by the test circuit . then the signal containing the information of rotor displacement is obtained , from which we can get the dc voltage signal proportional to the rotor displacement through half - wave rectification circuit and low pass circuit . this dc signal is put into a pid controller to get the control signal of the rotor displacement