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A robust non-linear output-feedback controller by the K-filter approach is proposed for position-tracking problem of a magnetic levitation system, where only the position measurement is available for control. Instead of the popular adaptive control techniques, a disturbance observer (DOB) is merged into the K-filter-based output-feedback controller to compensate the external disturbance and model mismatch, so that a much simpler controller is constructed. Rigorous stability of the non-linear control system is established. Experimental results are included to show the excellent performance of the designed controller.