Book excerpt: In this dissertation, the design of servo control algorithms is investigated to produce high-density and cost-effective hard disk drives (HDDs). In order to sustain the continuing increase of HDD data storage density, dual-stage actuator servo systems using a secondary micro-actuator have been developed to improve the precision of read/write head positioning control by increasing their servo bandwidth. In this dissertation, the modeling and control design of dual-stage track-following servos are considered. Specifically, two track-following control algorithms for dual-stage HDDs are developed. The designed controllers were implemented and evaluated on a disk drive with a PZT-actuated suspension-based dual-stage servo system. Usually, the feedback position error signal (PES) in HDDs is sampled on some specified servo sectors with an equidistant sampling interval, which implies that HDD servo systems with a regular sampling rate can be modeled as linear time-invariant (LTI) systems. However, sampling intervals for HDD servo systems are not always equidistant and sometimes, an irregular sampling rate due to missing PES sampling data is unavoidable. With the natural periodicity of HDDs, which is related to the disk rotation, those HDD servo systems with missing PES samples can be modeled as linear periodically time-varying (LPTV) systems. For the control synthesis of HDD servos with irregular sampling rates, an explicit optimal H_infinity control synthesis algorithm for general LPTV systems is first obtained by solving discrete Riccati equations. Then, the optimal H_infinity track-following control for irregular-sampling-rate servos is synthesized. Simulation and experiment studies, which have been carried out on a set of actual single-stage hard disk drives, demonstrate that the proposed control synthesis technique is able to handle irregular sampling rates and can be used to conveniently design a track-following servo that achieves the robust performance of a desired error rejection function for disturbance attenuation. Moreover, experiment results show that compared to the currently-used methodology for irregular sampling rates, the proposed control algorithm has significantly improved the servo performance. In addition, the feedback signal in HDD servos is generated from the servo patterns that must be pre-recorded using servo track writing process before the HDD can be used. Thus, the quality of the servo track writing process is also crucial to the accuracy of positioning read/write head. Recently, self-servo track writing has been developed in order to improve the quality of the written servo patterns and reduce the cost of servo track writing process. This dissertation considers two novel controller synthesis methodologies employing a feedforward control structure for performing concentric self-servo track writing in hard disk drives. Simulation results confirm that the two proposed control synthesis methodologies prevent error propagation from the previously written tracks and significantly improve servo track writing performance.