Control of an AMB to Zero Static Force

Someactivemagneticbearing(AMB)applicationsrequirethatthemean, orstaticforceexertedbytheAMBiszero. Examples include hybrid fluid film – AMB bearing pairings, rotor midspan dampers, and statically indeterminant systems with stiff rotors and more than two radial AMBs. When the position of the rotor relative to magnetic center is not known precisely, this zero force requirement can be hard to meet. A low frequency periodic biasing scheme is developed which enables a controller to detect non-zero static force by sensing rotor motion at the bias carrier frequency. A theoretical basis establishes feasibility but also the potential for coupling between control signals and the periodic bias. Simulation demonstrates that an ad–hoc bias adaptation scheme can successfully drive the static component to zero while permitting otherwise conventional control. The simulation results also show some spurious harmonic content associated with the sum and difference between a control signal frequency and the bias carrier frequency. Experimental results on a small, single DOF test rig further demonstrate the ability to drive the AMB static force to zero while adding damping to the system.