Performance Limitations of Non-laminated Magnetic Suspension Systems

Limitations on the closed-loop performance of magnetic suspension systems employing electromagnetic actuators that are not constructed from laminations are examined. Eddy currents induced within the iron by time-varying magnetic fields are shown to have a strong effect on the system dynamics and hence the achievable performance. To obtain the needed relations, the theory of performance limitations, specifically the sensitivity integral constraint result, is extended to fractional order systems. The unstable pole of the plant and the achievable closed-loop bandwidth are then analytically determined as roots of a quantic polynomial. The results indicate that the required control effort increases as the square of rotor mass for actuators with significant eddy currents, while the relation is linear for laminated actuators.