Bias current optimisation and fuzzy controllers for magnetic bearings in turbo molecular pumps

Magnetic bearings are commonly used in turbomolecular pumps because of their suitability for high speed and contamination free operations. A five-axis turbo molecular pump with two radial and one axial magnetic bearing is considered in this paper. The effect of the bias current on the energy consumption for both unidirectional and differential control current strategies is studied. It is shown that optimising the bias current according to operational conditions is beneficial. A differential current control gives better efficiency compared with a unidirectional approach when the bias current is variable. The optimum bias current is a function of the desired bearing stiffness and the rotor vibration amplitudes. The desired bearing stiffness is a function of rotor vibration levels or transmitted forces. A fuzzy logic controller is set up to resolve the conflicting control requirements. A simulation involving a non-linear magnetic bearing model is used to verify the suggested control and optimisation techniques.