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In probe-based data storage devices, microelectromechanical system-based microscanners are typically used to position the storage medium relative to the read/write probes. Global position sensors are employed to provide position information across the full scan range of these microscanners. However, to achieve repeatable positioning, it is also necessary to have medium-derived position information. Dedicated storage fields known as servo fields are employed to obtain this medium-derived position information. The servo-patterns on these servo fields have to be written using the global position sensors prior to the regular operation of the storage device by employing a scheme known as ldquoself-servo writerdquo process. During this process, subnanometer positioning resolutions, well below that provided by the global position sensors, are desirable. Such precise positioning at acceptable bandwidth requires the directed design of the closed-loop noise sensitivity transfer function so as to minimize the impact of sensing noise. This paper describes control architectures in which the impact of measurement noise on positioning is minimal while providing satisfactory tracking performance. It is estimated that the positioning error due to sensing noise is a remarkably low 0.25 nm. Experimental results are also presented that show error-free operation of the device at high densities.
Date of Publication: Sept. 2008