This paper shows a comparison based on dynamic behaviour of a rotating shaft when it is suspended by 4-axis
active magnetic bearings under several control systems. The control systems used are μ-synthesis, loop shaping design
procedure and Sub(H∞) robust control with the introduction of uncertainties on position and current gains of the actuators.
Each of these controllers is characterized by four input signals and four output signals and the introduction of uncertainties
on displacement gain and current gain is due to torn and worn of the components during the time, which can lead the entire
system to instability phenomena. The comparison of the performances is obtained through the introduction of same weighting
function for all three control systems. All simulations and results are performed by MATLab
