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Nano-scale positioning, control and motion planning in hard disk drives


In this dissertation, we focus on optimization of cross- track and vertical positioning of the read/write element over the data track. First, a data-based approach is presented for modeling and controller design of a dual- stage servo actuator in a hard disk drive. Based on discrete-time models, different dual-stage track-following controllers were designed using classic and H-infinity loop shaping techniques. The controllers were implemented in real-time. Next, an input shaping algorithm based on convex optimization techniques is presented for closed- loop discrete-time linear time-invariant (LTI) system. The proposed algorithm allows closed-loop signals to be subjected to linear constraints on amplitude and rate of change. As an illustrative example the seeking process in a hard disk drive is investigated and experimentally verified. To study the dependence of the read signal on cross-track and vertical motion, a straightforward analytical model for the read back signal is derived for perpendicular and longitudinal magnetic recording. The model captures the contribution of a single bit rather than the contribution of a bit transition which makes it applicable to patterned media as well as continuous media. In addition, a novel method of measuring the relative head -medium spacing based on the measurement of the read back signal from servo sectors is developed. The spacing measurement is tested experimentally on a spin stand where the flying height is varied using the resistance heater element in a thermal flying height control slider. In addition, voltage step response measurements were obtained for data based modeling. Finally, a dynamic model of the resistance heater in a thermal flying height control (TFC) slider is identified based on experimentally obtained step -input data. A generalized realization algorithm is used for identification of a discrete-time dynamic model of the resistance heater. Based on the identified model and convex optimization techniques, a computational scheme is proposed to obtain optimized feed forward input profiles to the heater element that minimize repeatable flying height variations. The optimized input signals were applied to the heater and greatly reduced flying height variations were observed in spinstand experiments

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