Robust disturbance rejection for improved dynamic stiffness of a magnetic suspension stage

Magnetic suspension is an attractive approach in realizing ultraprecision multiple-degree-of-freedom actuation for precision engineering. However, improving the dynamic stiffness of magnetic-suspension systems is an engineering challenge due to their noncontact nature. Two disturbance rejection algorithms that improve the dynamic stiffness of a magnetic-suspension stage (MSS) are presented in this paper. For rejection of narrow-band disturbances with unknown frequencies, an internal model principle-based control together with a frequency estimation algorithm based on adaptive-notch filtering is proposed. A chattering-free sliding mode (CFSM) disturbance rejection algorithm is developed in order to reject wide-band disturbances. The CFSM disturbance rejection scheme includes a continuous approximation of the switching function to avoid chattering, an integral control term to reduce the switching magnitude, and a derivative control term to elevate the bandwidth of disturbance rejection. Experimental results verified that the disturbances can be effectively rejected with the two developed algorithms. Consequently, the dynamic stiffness of the MSS is greatly improved.