U-019

A positioning method for hard disk servowriter using an auto focus laser encoder

Authors: Weidong Zhou and Lilong Cai
Affiliation: Dept. of Mechanical Eng., Hong Kong University of Science and Technology. W. Zhou is also with Department of Measurement and Control, Sichuan University

Abstract
A new servowriter positioning device for hard disk drives is presented. The existing servowriters widely use laser interferometry as its position sensing. Therefore the servowriter is expensive, complicated and large in size. To avoid these problems we propose to use a laser incremental encoder with autofocus operation instead. In the device, an optical read head focuses three beams upon on a grating scale. The middle beam is used to keep the read head in focus and two side ones to readout. The grating scale has two sets of the reflective grating displaced relative to each other by one eighth of the pitch width to give 90° out-of -phase signals. The optical read head can be the modification of CD or DVD pickup using the 3-beam principle, and can be naturally designed into the system. Therefore, the cost and size of the servowiter are significantly reduced. At the same time, by error-correcting grating scale the accuracy can be maintained. The positioning device can also be used as track sensor for directly providing the track position signal of read/write (R/W) heads of HDA instead of conventional servo pattern.

Weidong Zhou
Dept. of Mechanical Engineering
Hong Kong University of Science and Technology
Hong Kong, China
mezhou@ust.hk