PWM Based Sensing and Control of Magnetic Bearings

This paper develops a PWM based position sensing method for motion control of active magnetic bearings. A practical method to determine the rotor position in terms of rotor-stator magnetic properties was investigated. An analog signal processing circuit which conditions the PWM voltage and current signals is used to implement the method. Analog biquad band pass filters, as well as low pass filters, demodulators, isolators and dividers were built. Preliminary tests were conducted using a four pole homopolar magnetic bearing by manually varying the rotor position and measuring the effectiveness of the signal processing module. The closed loop magnetic bearing motion control based on PWM sensing module was simulated using Matlab/Simulink. The simulation model includes 4-axis radial magnetic bearings with the dynamics of a high speed flywheel. A PD controller along with notch filters and lead compensators were employed in the closed loop control system. The simulation results demonstrated converged orbit trajectories of the rotors, which proved the effectiveness of the sensing scheme and the control strategy