Robert J. McNab

A study of radial error propagation and self-servowriting in disk drives

In this paper, the present state of the art in self servowriting of disk drives is discussed. Servowriting is a process whereby the position burst information of the track is written onto the disk by means of a apparatus known as a servowriter. Since servowriters are expensive machinery, it is desired to maximize throughput on the factory floor by minimizing the time needed for servowriting the drive. Self servowriting is a process whereby this is accomplished without a servowriter by using the servo information from a previously written track as a reference (seed) for writing the burst information on the remaining tracks. An obstacle to this process is radial error propagation, whereby the tracking errors buildup causing distortion from circularity and variable track spacing. The problem and potential solutions in this process are discussed and verified by means of simulation.

An iterative learning controller for reduction of repetitive runout in disk drives

This paper considers the design of an iterative learning controller for the reduction of repeatable runout (RRO) in disk drives. The advocated controller is an add-on controller that does not require modification to the existing track following controller. A design methodology is provided for the selection of the parameters of the update law to ensure convergent learning. It is shown that the update law to ensure convergent learning is equivalent to stabilization of a multi-input multi-output system using static output feedback. Experimental results using a 3.5-in Western Digital drive show that a 50% reduction in the sigma of the RRO is achieved in ten iterations when the levels of repetitive and nonrepeatable runout in the position error signal are comparable.

Modeling and control for self-servowriting in hard disk drives: a repetitive process approach

This paper considers modeling and correction signal design for self servo writing in disk drives. Self-servowriting is a process whereby the position and timing burst information are written on the disk surface by using the servo information from a previously written track as a reference (seed) for writing the burst information on a subsequent track, and repeating the process. An obstacle to this process is radial error propagation, whereby the distortion in the successive written tracks builds up. In this paper, a reference correction signal is designed which when added to the position error signal (PES) prevents the radial error propagation. The design of the correction signal is carried out through a repetitive systems approach and does not require several revolutions of track following in order to compute it. The analysis is verified by means of simulations which show the containment of radial error propagation