A Novel Digital Circuit for Position Sensor De-modulation Using Advanced FPGA, Including Field Experience with a 1.45MW Motor Driven Compressor

This paper describes a novel demodulation circuit which has been developed for use with inductive position sensors for Active Magnetic Bearing applications. The circuit is designed to operate with conventional bridge type inductive position sensors. This circuit is novel in that the implementation is fully digital and uses a mixture of FPGA and DSP to ensure noise immune phase sensitive de-modulation. We present the architecture and implementation of the new circuit. The techniques to perform the phase sensitive de-modulation are described, including the techniques necessary to handle the phase shifts due to cable capacitance. In conventional analogue based systems these phase shifts are compensated by field changes to component values. The experience of using this sensor on a 1.5MW motor driven hydrogen compressor is described. We present the results and experiences of commissioning the machine. The noise levels recorded during testing, and the overall response of the machine both whilst levitated (non-rotating) and during operation are included. We finish with a comparison of the two methodologies; the precision analogue circuitry versus the digital along with our conclusions.