Hardware Reference
In-Depth Information
Chapter 5
Servo Track Writer
5.1
Introduction
Servo track writing (STW) is the process that de
fi
nestracksandsectorson
the disk platters by creating the servo patterns so that the HDD servo system
can later use them to identify tracks and sectors, and to measure the relative
position of the read head relative to the center of the track. Accuracy and
precision required in the HDD servomechanism have been emphasized, and
the associated challenges have been explained in chapter 2 and chapter 3.
Performance of the HDD servomechanism is affected by the precision at which
the position feedback signal can be generated. The STW process, therefore,
plays an important role in making the continuously growing trend of track
density a reality.
The HDD servomechanism generates position feedback using the servo pat-
terns as reference, which are written on the disks when no reference is present
onthedisksorinsidetheHDD.Thereareseveralmechanismsusedtoprovide
the reference while servo track writing is performed. Each of these methods
has its own pros and cons in respect to factors such as cost, throughput, relia-
bility etc. These methods will be discussed in this chapter. For the time being,
we refer to servo track writing as a key stage in the process of manufactur-
ing of HDD that employs precision mechatronics and control. Although there
exists servo writing method that makes use of the mechanics available inside
the HDD, the common practice so far is to use an ultra-precision equipment,
known as Servo Track Writer (STW) or Servo Writer.
There are two critical control problems in the process of servo writing with
desired accuracy:
1. All the patterns required to de
fi
ne the tracks and sectors must be placed
in a concentric fashion. The STWs that are commonly in use controls
the position of the write head with respect to the redial position on the
307
