Hardware Reference
In-Depth Information
Figure 2.21: Effect of runout: deviation of true track from the ideal circular
2.5.1 Repeatable and Non-repeatable Runout
Some internal sources give rise to disturbing forces that are repetitive in nature.
Most of the lateral and vertical movements of the fast-spinning disk platters
appear as repetitive in the sense that they have definite temporal pattern and
contributed mostly by mechanical factors. For example, disturbances caused
by misalignment between center of the spindle shaft and the disk center, wob-
bling of the disk platters, vibration modes of the disk platters, defects in the
inner and outer races of the ball-bearing etc, repeat with revolution. The mis-
alignment between the disk-center and shaft center makes the shape of a track
elliptical and not circular. Wobbling and vibration of the disk cause the track
move away or come closer to the head slider affecting the PES. A defect in
the inner or the outer race of bearing produces a lateral force whenever a ball
hits that defect. All these disturbing forces occur in every revolution making
them repetitive. However, if there is a defect in one ball of the bearing, the
lateral force is created only when that defect comes in contact with the in-
ner or the outer race. Occurrence of such event is rather random. Similarly
flutter caused by the aerodynamic interaction of the fast flowing air with the
mechanical components is also random in nature. These internal disturbances,
whether repeatable or not, are always present in the HDD servomechanism.
As a result, if a track is created by holding the write head steady while the
disk spins, the point below the head forms a trajectory that is not perfectly
circular but a wavy circle as shown in Figure 2.21.
The mechanical disturbances mentioned above, such as bearing defects or
spindle-disk misalignment, are also present during the process of Servo Track
Writing (STW) when the servo sectors, i.e., the reference marks for the tracks,
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