Stable Control of Active Magnetic Bearings With Enhanced Natural Frequency Suppression Using a Digital Band-Stop Filter

Active magnetic bearings (AMBs) are widely applied in ship propulsion systems due to their advantages of frictionless rotation, accurate positioning, and improved operational efficiency. However, ensuring stable and efficient levitation in a 4-pole AMB system remains a challenge. This paper proposes a double closed-loop control strategy to improve system stability and responsiveness. The outer loop uses a displacement sensor to adjust the air gap displacement, and the inner loop adjusts the electromagnetic force of each pole to maintain a balanced levitation. To more efficiently suppress the natural frequencies distributed over a wide range, a digital band-stop filter (BSF) is introduced to the gap sampling loop to suppress unintended vibrations caused by the natural frequencies. The experimental results of the 4-pole AMB prototype show that the proposed strategy achieves a suppression rate of 99.4% for natural frequency vibrations, greatly improving system stability in both the stand by and rotating states. These results confirm that the proposed control method is effective in mitigating disturbances and unwanted vibrations while maintaining the stability of the magnetic levitation state.