Analog compensation of inductive sensors

A magnetic bearing assembly comprising:

• a plurality of active magnetic bearings configured for levitating a rotatable member, the rotatable member rotatable about a central axis;
• a plurality of inductive sensors each associated with a respective one of the magnetic bearings, each of the inductive sensors positioned radially outwardly from the central axis and spaced apart from the rotatable member and configured for sensing an air gap between the inductive sensor and the rotatable member;
• a plurality of drive circuits electrically connected to the inductive sensors, each of the drive circuits configured for providing an input signal to a respective one of the inductive sensors such that the inductive sensors each generate an output signal representative of the air gap;
• an interconnection electrically connecting the inductive sensors to each other and combining the output signals therefrom to output a displacement signal, wherein the output signals from the inductive sensors cancel each other at the interconnection when the rotatable member is centered on the central axis, and wherein a non-zero displacement signal at the interconnection when the rotatable member is centered on the central axis represents a displacement error;
• a signal conditioning circuit electrically connected to the interconnection, the signal conditioning circuit receiving and responsive to the displacement signal and including an offset generator for adding an analog offset to compensate for the displacement error, the signal conditioning circuit configured for generating a position signal representative of a radial position of the rotatable member relative to the central axis as a function of the displacement signal with the offset;
• and
• a magnetic bearing controller receiving and responsive to the position signal for controlling the active magnetic bearings to minimize radial displacement of the rotatable member from the central axis.

A method of controlling a magnetic bearing assembly, the magnetic bearing assembly including a plurality of active magnetic bearings configured for levitating a rotatable member, the rotatable member rotatable about a central axis, the method comprising:

• providing input signals to a plurality of inductive sensors, wherein each of the inductive sensors is associated with a respective one of the magnetic bearings and positioned radially outwardly from the central axis and spaced apart from the rotatable member;
• sensing an air gap between each of the inductive sensors and the rotatable member;
• receiving an output signal representative of the sensed air gap from each of the inductive sensors;
• combining the output signals from each of the inductive sensors at an electrical interconnection to output a displacement signal, wherein the output signals from the inductive sensors cancel each other at the interconnection when the rotatable member is centered on the central axis, and wherein a non-zero displacement signal at the interconnection when the rotatable member is centered on the central axis represents a displacement error;
• receiving the displacement signal and adding an analog offset thereto to compensate for the displacement error;
• generating a position signal representative of a radial position of the rotatable member relative to the central axis as a function of the compensated displacement signal with the offset;
• and
• controlling the active magnetic bearings to minimize radial displacement of the rotatable member from the central axis as a function of the position signal.