Many processes involving rotating machinery could benefit from the continuous feedback of force applied to the bearings that support the machinery. Such a system could be used to provide diagnostics for process monitoring in a manufacturing application or to provide information for machine health monitoring. Active Magnetic Bearings (AMBs) have the capability to act concurrently as a shaft force sensor and
support bearing. This capability stems from the AMB's control system, which is designed to maintain a specific rotor position, regardless of forces acting on the rotor. Researchers have demonstrated the force sensing ability of AMBs; current state of the
art methods typically rely on a direct measurement of magnetic flux density as provided
by a Hall probe inserted in the magnetic field. In this work, a system identification
approach to force measurement is proposed; the proposed approach is applicable to all
active magnetic bearings and does not require Hall probes.
Recent developments in system identification of bearing forces (Kasarda et al., 2000)
indicate that a different approach is feasible. In the work of Marshall (Marshall et al.,
2001), a variety of perturbations are applied to an AMB while the AMB controller signals
are interrogated, no outside instrumentation such as force transducers or Hall probes
are required. The work of Kasarda and Marshall is the starting point for the work
presented here.
The initial work was expanded to include a general characterization of air gap for any
rotor position. Although this characterization relies on static testing to identify system
parameters, the identified parameters can then be used in the measurement of dynamic
forces. The identification procedure provides a measurement of effective air gap length.
Effective gap length is used to infer the effective position of the rotor with respect to the stator. This measurement is made for several specific rotor locations. The relationship
between the effective rotor positions provided by the identification and the rotor positions
reported by the AMB system sensors establishes a coordinate transformation. The procedure is also applied at different shaft rotation angles. In this way rotor runout can be identified.
