Linearizing magnetic bearing actuators by constant current sum, constant voltage sum, and constant flux sum

For each controlled axis, two identical electromagnets are usually used to provide a net scalar force of changeable sign. Each magnet has a winding on it and is mostly premagnetized, leading naturally to a configuration in which the currents through the two windings sum to a positive constant value while each individual one varies. Apart from this prevailing constant current sum (CCS) configuration, constant voltage sum (CVS) of the driving voltages and constant flux sum (CFS) of the resultant fluxes provide two additional constant-sum scenarios. CFS is shown to be realizable by a simple nonlinear state feedback of only displacement and current. It is also shown that with CFS the state equations of the plant are linear. CVS is derived by approximation to CFS and is shown to yield a more linear plant than CCS. These constant-sum configurations are compared in terms of closed-loop performance, where for every considered aspect of performance CVS is shown to be better than CCS and CFS is the best. As to the complexity, CVS is the least complex and CFS is the most complex. While CFS is shown to be a candidate for high-performance applications, it is suggested that the widely accepted CCS be replaced by CVS in practice because of its better performance and lower complexity. Finally, problems and additional choices in implementation are discussed.