Efforts to enhance hard disk drive areal densities via increases in tracks per inch (TPI) will ultimately be influenced by the ability of the positioning system to locate the head with increasing precision in order to accurately seek and follow targeted data tracks. Actuator servo control system design goals revolve around reduction of track misregistration (TMR) in the presence of non-repeatable mechanical disturbances which can cause head placement errors impairing data reliability; error sources can include externally induced vibrations, shock, unpredictable variations in rotation due spindle and actuator bearing wear, thermal effects, and aerodynamic forces associated with rotating disks. Figure 1 illustrates a typical disk stack and actuator assembly configuration with influences on head placement.

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