Design and Control of Hybrid-type Three-pole Active Magnetic Bearings Using Redundant Coordinates

Recently, the design of active magnetic bearings (AMBs) for miniaturized industrial applications puts much emphasis on small size and low-power consumption as well as low-cost system production. In this paper, a novel hybrid type three-pole AMB system is proposed, which is divided into two bearing parts: a three-pole radial AMB and an axial permanent magnet bearing. The radial AMB consists of three active main poles with three coil windings, three sub-poles with radial permanent magnets, generating bias flux, and three Hall diodes, measuring the rotor position. The permanent magnet bearing is made of two facing axially polarized ring-type permanent magnets for passive stability in the axial and angular motion. Compared with the conventional four- or eight-pole AMB, the hybrid three-pole AMB system reduces the power consumption with fewer power amplifiers and is suitable for smaller-sized AMBs. The primary problem in modeling and control of the three-pole AMB in the conventional Cartesian coordinates is the strong coupling in magnetic flux generated by three main and sub-poles. Consequently, the system becomes strongly nonlinear with respect to control inputs as well as states. To resolve this difficulty in treating the strong nonlinearity in the conventional Cartesian coordinates, we introduce the redundant coordinates along with three-pole configuration for modeling and design of controller. Based on the redundant coordinates, three identical decoupled proportional-derivative controllers can be applied, which is very simple compared with previous nonlinear control algorithms developed for three-pole AMBs. Experiments are also carried out with a prototype AMB system to validate the proposed design and control concept based on the redundant coordinates.