Servo Track Writer - Hard Disk Drive: Mechatronics and Control - page 310

market, there has been a focused interest over the years in improving the
precision of writing by STW, its throughput, and less usage of cleanroom
space [155], [21], [191], [198], [222].
This chapter presents brie fl y the principles of operation and other impor-
tant aspects of few schemes of servo track writing, namely,
1. the scheme with external positioner using a mechanical push pin,
2. the scheme using optical measurement based positioning system,
3. self servowriting, and
4. multidisk servo writing and its variants.
The fi rst two schemes are HDA-based (Head Disk Assembly) servo writing,
i.e., they use heads and actuators of the HDD to create the servo patterns
on its disks. But they use external mechanism to control both radial and
tangential positions of written patterns. The self servo writing scheme also
uses the HDD's heads and actuator, but no external mechanism is required.
Thisschemewritesonetrackandusethesignalfromthistrackasthereference
for creating an adjacent track. Multidisk servo writing uses a dedicated STW
to write servo patterns on a stack of disks. These servo written disks are later
assembled in different HDDs. Besides these 4 major schemes, we also introduce
brie fl y the process of creating servo patterns using printing. Through these
discussions, we explain how the position and timing accuracies are controlled
to achieve the desirable servo writing quality at an acceptable clean room
throughput.
5.2 HDA Servowriting
The HDA servowriting employs the components of the assembled HDA to cre-
ate servo patterns on the disks with the help of external sensors for radial and
tangential positions, and the control loops. The external sensors are mounted
on vibration-free platform using rigid fi xtures. The patterns are magnetically
written on the disk using the heads of the HDA, and the disks are spun by
the HDA's own spindle motor. However, the position of the actuator arm is
identi fi ed and displacement is measured using external mechanism - either a
mechanical fi xture called push-pin or an optical push-pin. The location of the
pattern along the track is determined by another mechanism external to the
HDA.
5.2.1 Control of Tangential Position using Clock Head
In order to align the servo patterns on different tracks, the servo patterns must
be placed at appropriate locations along the track and, therefore, the process