Hardware Reference
In-Depth Information
are written on the disk. (More in-depth explanation of the process of servo
track writing is given and various mechatronic challenges associated with it
are discussed later in chapter 5.) The burst patterns created in the servo
sectors form the reference of the track center, and this reference is used by
the servo control loop of HDD during its normal operation. Because of the
disturbances present during servo track writing, the created reference for the
track-center deviates from being perfectly circular in shape. However, same
actuator and disk-spindle assembly are used in both STW and HDD servo
controller for embedded servo scheme. Mechanical imperfections manifested in
the form of repeatable and non-repeatable disturbances are present at the time
of STW, and therefore, are written-in on the tracks in the form of non-circular
shape of the track reference. The repeatable disturbances are the same on
all tracks. Non-repeatable disturbances are however, written-in differently on
different tracks. So a track created by the STW records the non-repeatable
disturbances present while that particular track was written.
If the head is positioned on any servo-written track, it follows naturally
the deviations from perfectly circular track caused by repeatable mechani-
cal disturbances present during STW. In other words, repeatable disturbances
present at the time of STW disappear during the operation of HDD servomech-
anism. Deviations caused by non-repeatable mechanical distortions at the time
of STW are already written-in and appear in the HDD servomechanism as dis-
turbances that are synchronized with the spindle speed. These deviations in
the track reference constitute the repeatable runout (RRO) for the head posi-
tioning servomechanism. The non-repeatable mechanical disturbances are still
present during the operation of HDD and they cause deviations of track-center
reference with respect to the position of the head slider in a non-repetitive
manner. These factors constitute the non-repeatable runout (NRRO) affecting
the performance of the head positioning servomechanism. The deviations in
two servo-written tracks are illustrated in Figure 2.22. The references of the
servo-written tracks are shown using solid lines in this figure. The head is
expected to follow these references. It should be noted that these mechanical
distortions of the track are to be followed, not rejected, by the head positioning
servomechanism. However, due to limited bandwidth and presence of random
noise and disturbances, the read head can not follow the track-center perfectly.
The envelope around the path followed by the head is shown by dashed lines
in Figure 2.22. Since the track-references created by the STW is not perfectly
circular and since the non-repeatable variations of the references of one track
may differ from that of the adjacent tracks, it is possible to have one track
encroaching another at some points. Track misregistration due to this factor
of track-to-track “squeeze” is known as write-to-write track Mis-Registration
or WWTMR. When a head tries to follow the center of a track defined by
the servo patterns written by STW, it wanders around the center of the track
because of various disturbances affecting the servo loop. This misregistration
between the reference and the read-write head is known as write-to-read track
Mis-Registration or WRTMR.
