Fault-tolerant homopolar magnetic bearings

This paper summarizes the development of a novel magnetic suspension that improves reliability via fault-tolerant operation. The suspension is suitable for flywheels used in satellites and space stations for attitude control and energy storage. Specifically, we show that flux coupling between poles of a homopolar magnetic bearing can deliver desired forces even after termination of coil currents to a subset of "failed poles". Linear, coordinate-decoupled force-voltage relations are also maintained before and after failure by bias linearization. We determined current distribution matrices that adjust the currents and fluxes following a pole set failure for many faulted pole combinations. We used one-dimensional magnetic circuit models with fringe and leakage factors derived from detailed, three-dimensional finite-element field models to obtain the current distribution matrices and the system response. Reliability is based on the success criterion that catcher bearing-shaft contact does not occur after pole failures. The magnetic bearing reliability is improved by increasing the number of the radial poles. An advantage of our method over other redundant approaches is a significantly reduced requirement for backup hardware such as additional actuators or power amplifiers.