Investigation Of Current-Based Dynamic Force Measurement With Active Magnetic Bearings

Many processes involving rotating machinery could benefit from the continuous feedback of the forces 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 force feedback in other devices. In order to stand up to the demands of an industrial or harsh field environment the force measurement system would ideally be robust, inexpensive, and readily applicable to any AMB (Active Magnetic Bearing) system. To meet these goals, dynamic load data would be provided using monitored currents and positions only, without the use of Hall probes. This paper presents the first step towards developing a technique and algorithm for achieving dynamic current-based force measurement with AMBs. The proposed measurement system is based on a simple force equation developed from magnetic circuit theory which has been modified to accommodate an “effective current” to account for errors due to both current and speed-dependent loss mechanisms. The modified model presented here is not physics-based but determined by curve fitting the magnetic-circuit force model to known force data for a single load and bias current case at multiple speeds. Known force data was obtained from force transducers mounted under a bearing. This empirically determined model was then used for comparison to force transducer data for other load, speed, and bias current cases. If all of the data is polled the average error is 4% with a worst-case error of 21%. System dynamics caused modeling problems for the data collected at 3000 RPM, if errors in this speed range are neglected, the average error is 2.8% with a worst-case of 6.8%. The results of this study demonstrate that a speed-dependent AMB force model is possible, and set the groundwork for future investigations.