Tuning Guidelines for Generalized Notch Filters used for Unbalance Compensation for Magnetic Bearings

Unbalance compensation in magnetic bearing control algorithms are state of the art in modern applications of active magnetic bearings. The compensation is vital to keep synchronous vibrations on the housing on a minimum level and to avoid actuator saturation. There exist different possibilities to implement the compensation in the control loop. One of it is the generalized notch filter [1]. This filter technology has been successfully used in magnetic bearings for more than 20 years. One drawback of this filter is the speed dependent parameter set. For radial bearings of a five axis rotor, the notch filter parameters are in general two dense 4x4 matrices for one rotational speed. This high demand on computational power and memory is not a problem any more for modern digital signal processors. With a well tuned parameter set the unbalance compensation can be turned on well before crossing the rigid body modes. The paper presents a novel observer based stability proof of the generalized notch filter and introduces two bode diagrams to visualize the notch filter performance and robustness. The results are verified with a simple example and on a small 5-axis test rotor system.