Sensorless Sliding Mode Control of Magnetic Bearing Actuators Using Implicit Switching Surfaces

Sensorless sliding mode control of a single-axis pull-pull type magnetic bearing actuator is reported in this paper. The two electromagnets are driven by switching between a positive and a negative constant voltage. Two switching surfaces being indirectly available through the measurement of driving current and its change rate are constructed, but only one of which is available at any time. Sliding mode control using these two switching surfaces in turn is shown to be possible. Reaching condition and the stability of sliding dynamics are discussed. The sliding mode turns out to take place in a subregion of state space defined by s1(x)s2(x)=0 rather than on a surface defined by s(x)=0 as in most standard cases. The mathematical results are verified by simulation and experiment.