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In this paper, two different track-follow controllers are proposed for read-only memory-type scanning probe microscopy-based data-storage systems: mode switching control and dual-rate master-slave control. The mode switching control has a simple structure but a large position offset due to misalignment between the stage and medium. Since only the position error signal is used during track following, the sampling rate is limited and robustness against disturbances is poor. To overcome this problem, a dual-rate master-slave control is proposed. It reduces the position offset effectively because the reference signal to the slave controller is updated every sync pattern. From the experimental results, we observed that both control methods have good repeatability in seeking and track following. The mode switching method has an offset of 92.3 nm, while the dual-rate master-slave controller reduced the offset by 93%. In addition, standard deviations of the two controllers were very similar. With a 400-nm track pitch, one sigma of position error signal was 23.8 nm for the mode switching and 22.8 nm for the dual-rate master-slave method. This is because the feedback-loop gain using the position error signal is the same. |
