In a magnetic recording apparatus such as a hard disk apparatus, a plurality of tracks are formed in a concentric manner on a disk-shaped magnetic recording medium so as to record data on the tracks. In this case, a servo signal is formed on each of the tracks to specify the center of the track so as to control the position of a magnetic head adapted to read and write data.
Techniques are capable of reducing repeatable runout (hereinafter referred to as “RRO”) caused by the rotation of a magnetic recording medium by using information adapted to compensate for RRO (RRO compensation information) that has been written in advance to part of the servo signal.
This RRO compensation information varies from one cylinder to another and from one head to another, thus resulting in an enormous amount of such information to be produced for the entire magnetic recording medium. Therefore, a method is desired for determining the RRO compensation information quickly and with high accuracy.
Conventionally, two methods are known to generate the RRO compensation information, namely, a method operable to estimate the RRO compensation information using a control target model and another using repetitive control. Of these, the method using a control target model requires designing a highly accurate model for each hard disk drive, thus resulting in a low productivity due to the need to perform calibration for each hard disk drive.
On the other hand, the method using repetitive control leads to a wider disturbance frequency range in which to compensate for RROs. This requires designing a phase-stable repetitive control system suitable for the characteristics of a servo system over a wide frequency range. However, it's not been developed yet that a theoretical method to get such a repetitive control system.