Modelling and control of a disk file head-positioning system

This paper overviews servo control technology in the field of motion control for disk file systems. In the hard disk drive (HDD) system, the head must be positioned on the desired track quickly and precisely, and this performance must be robust. Recent required positioning accuracy is about 50 nm (+/-3sigma). To meet these requirements, many servo control methods have been proposed and applied to the system. Major research subjects for fast movement are a reference trajectory design and a track-secking servo design. In particular, a two-degree-of-freedom (2 DOF) servo control with a multirate sampling period has been developed for the track-seeking servo. Two design methods of 2 DOF feedforward controller are proposed here. While reading/writing data, very precise positioning on a target data track is required. One of the most effective methods for precise positioning under various disturbances is to increase the servo bandwidth. This paper examines the use of loop shaping, state feedback and a dual-stage actuator servo to increase the servo bandwidth with sufficiently robust stability. Feedforward methods to cancel disturbances are also applied under the condition that the disturbances are either detectable or can be estimated, such as repeatable runout error, which is synchronous to disk rotation.