Servo Track Writer - Hard Disk Drive: Mechatronics and Control - page 318

components of the HDD. Usually relatively low cost (compared to those avail-
able as the state of the art technology) spindle motors with various imbalance
conditions are used in HDDs to keep the cost of the fi nal product competitive.
The demand for higher throughput without increasing the production cost
was the primary motivation for developing an alternative method of writing
servo tracks called the self-servo track writing (SSTW) which can be performed
in an area outside expensive clean room as this method does not require any
opening on the HDD's enclosure. Since the technique uses the mechanics of
the HDD itself, there is no need to design mechanical pushpin and the form
factor of the HDD is not an issue anymore. Mechanical jigs and fi xtures are
still required to clamp the HDD rigidly on a vibration-free platform when
servowriting is carried out.
5.4.1 Basic Concept
The self-servo track writing (SSTW) refers to a process of servowriting that
uses as references the radial (for servo) and tangential (for timing) positions
regenerated from information previously written on the medium and sensed
by the MR head of the HDD itself [38], [221]. The functional block diagram is
similar to that of Figure 5.1 except for the additional role of the data head as
the clock head. It can servo-write an HDD without any external push-pin or an
optical position sensor. The process of self-servowriting is shown in Figure 5.2
and it generally involves the following steps [222]:
Step 1: Write some tracks or at least one track called the seed track(s) (the
i th STW track). One possible way of creating seed track is to write servo
patterns after biasing the actuator of the HDD fi rmly against the crash-
stop [43]. Or the seed tracks can be pre-written using MSTW on disks
beforethedisksareassembledintheHDD[22].
Step 2: With the seed track written on disks, the head slider is moved away
from the center of the seed track using the VCM actuator in a con-
trolled way such that the read sensor of the MR head can still sense the
transitions in the servo wedges of the seed track. The readback signal
generated by scanning the transitions j, j+1, ···of the seed track is used
as feedback for the closed loop system controlling the MR head. While
the position of the slider is controlled, the write head writes an auxiliary
servo pattern or the fi nal servo pattern on the j +1,j+2, ··· wedge
positions to create the i +1 th STW track.
Step 3: If i + 1 is less than the number prede fi ned according to the ID-OD
span and desired TPI, then set the i +1 th track as the new seed track,
and go to Step 2. Else all the tracks have been written, and stop the
process.