Written-In RRO and NRRO Characteristics in Mobile Hard Disk Drives

The characteristics of repeatable runout (RRO) and nonrepeatable runout (NRRO) of hard disk drives (HDDs) have to be determined for higher tracks per inch since the static positional accuracy has almost reached its limit. To break this limit, it is necessary to identify precisely these track misregistration characteristics. We experimentally estimated the actual component of disturbances in both servo-writing and the actual drive using a mixture of helium and air. In the study described in this paper, windage or flow-induced vibration in servo-writing was the dominant component as a torque disturbance in the drive measurement. We then found that written-in RRO was key to understanding the characteristics of disturbances. As a result, the most dominant component in NRRO and even in written-in RRO is torque disturbance followed by positional disturbance for mobile HDDs.