Hardware Reference
In-Depth Information
of actuator, actuator position, properties of the enclosure, air pressure inside
the enclosure, mechanical features built inside the enclosure are the factors that
determine the nature of the flow and therefore the characteristics of the airflow
induced disturbances [91]. The spectrum of these disturbances is also related
to the spindle speed. With high spindle speed (7, 200−15, 000 RPM) in high
performance disk drives, these disturbances have become critical. However,
the models available for these disturbances are mostly empirical and their
properties are very sensitive to small changes. As a result, solving the problem
of flow-induced disturbances remains to be a challenge.
Media noise at the domains of servo bursts introduces a significant amount
of noise in the servo loop. The servo burst patterns are nothing but a sequence
of magnetic transitions on the media. These transitions, though depicted ear-
lier using straight lines, contain magnetization fluctuations near the recorded
transitions. These fluctuations make the readback waveform noisy; this noise
is known as transition noise. Media also introduces a noise, called particulate
or granularity noise, caused by random dispersion of the grains in magnetic
media. When a read head scans the servo burst patterns, the media noise af-
fects the burst signals. The burst signal is also affected by the MR head noise
and the noise from the electronics. It is further aggravated by demodulation,
the process to extract PES from the burst signals.
2.6 Track Seek Controller
There are two distinct and often competing objectives to be achieved by the
head positioning servo controller in HDD.
• Track Seek: track-to-track maneuver of the heads must be performed
in as short a time as possible, and
• Track Following: the position of the head must be regulated above
the center of the track with minimum variance during reading or writing
of data.
It may be appropriate to point out here that, on the contrary to the popular
use of the term tracking control in optimal control literature to represent con-
trollers that track a signal, the track-following in HDD is essentially a regulator
problem. Seek control, on the other hand, is a point-to-point control.
It is impossible to meet both the requirements of minimum-time seek and
minimum-variance track following using a linear controller when the magni-
tude of control signal is upper bounded. A linear controller can be optimized
to meet the specifications of disturbance rejection and good tracking perfor-
mance. Gains of the controller required to meet these specifications when the
position error is low, typically below 10% of the track pitch, cause actuator
saturation when the reference command is several tracks. Performance of the
