An Improved Direct Displacement Control With Second-Order UDE for 12/4 Bearingless SRM to Enhance Disturbance Rejection Capability

The conventional levitation control strategies for bearingless switched reluctance motors (BSRMs) depend on accurate mathematical models and exhibit weak rejection of displacement disturbance. To solve this problem, this paper proposes an improved direct displacement control (DDC) with second-order uncertainty and disturbance estimator (UDE). Unlike most UDE applications in first-order systems, this work establishes a complete second-order UDE design framework. Firstly, the reference trajectory for displacement is established. Next, the lumped disturbances, including uncertainties and external disturbances, are estimated by a low-pass filter. The estimated value is fed forward into the control loop to directly adjust the differences of winding currents on the opposite poles through the displacement errors. Then, the differences are superimposed on the bias currents to obtain the desired currents of each winding. Therefore, the BSRMs, which possess disturbance rejection capability, only need to track the desired currents to gradually drive the displacement to converge to its reference trajectory. Finally, the demonstrated performance of the proposed strategy is verified on a test rig of 12/4 BSRM-type magnetic levitation system with four degrees of freedom (DOFs).