Feedback linearisation of magnetic bearing actuators for a uniform upper bound of force slew rate

In a single-axis active magnetic bearing (AMB) actuator a one-degree-of-freedom moving mass is driven by the tractive forces of two electromagnets, since only one electromagnet is not capable of providing a driving force of changeable sign. The displacement of the moving mass is thus controlled by two current inputs to the two electromagnets. Usually a constraint is imposed on the two inputs to maintain a single input in effect. Such a constraint is also required when this nonlinear plant is linearised with the feedback linearisation (FBL) technique. However, there are infinitely many such constraints that can yield the same linearised plant, and with some of them the linearised plant is prone to be nonlinear. In the paper, two constraints for FBL are derived based on the internal physics of the plant. The first constraint is for minimum copper loss and the second is for a uniform upper bound of force slew rate. Experiment and simulation show that with the first constraint the feedback linearised plant appears to be less linear than the original giant, where the validity of the FBL algorithm is found to be sensitive to sensor noise, while with the second constraint the linearised plant does become more linear.